1) INTRODUCCION2) PLASTICOS Y DESARROLLO SOSTENIBLE 2.1) Impacto ambiental 2.2) Producción de matrices plásticas a partir de recursos

renovables3) MATERIALES COMPUESTOS DE BAJO IMPACTO AMBIENTAL4) MATRICES PARA MATERIALES COMPUESTOS DE BAJO IMPACTO AMBIENTAL 4.1) Almidón y mezclas almidón-acetato de celulosa 4.2) Polilactida (ácido poliláctico, PLA)5) MATERIALES COMPUESTOS DE BAJO IMPACTO AMBIENTAL CONTENIENDO REFUERZOS OBTENIDOS A PARTIR DE MATERIALES LIGNOCELULÓSICOS

5.1) Objetivos y tipos de pretratamientos5.2) Propiedades de materiales compuestos constituidos por

matrices renovables y refuerzos derivados de materiales lignocelulósicos

6) CONSIDERACIONES FINALES Y CONCLUSIONES6.1) Aplicaciones y tendencias de mercado6.2) Perspectivas

7) REFERENCIAS

MATERIALES COMPUESTOS DE BAJO IMPACTO AMBIENTAL PRODUCIDOS A PARTIR DE RECURSOS NATURALES

1.- INTRODUCCIONLos plásticos tienen naturaleza polimérica. Algunos polímeros naturales (como ámbar, lacas y

gutapercha) se conocen desde la antigüedad. El celuloide, obtenido desde el siglo XIX, es un ejemplo de un plástico artificial producido a partir de materias primas naturales (deriva de la celulosa, y se emplea en películas fotográficas). La tecnología del plástico cambió dramáticamente a principios del siglo XX, cuando aparecieron los productos derivados del petróleo, que dominaron el mercado debido a su bajo coste y a su estabilidad química. La II Guerra Mundial produjo un gran aumento de la producción de plásticos, definiendo la situación actual del mercado.

Algunos polímeros se producen sintéticamente, mientras que otros (como almidón o celulosa) se encuentran en la naturaleza. Aunque los plásticos se producen a partir de muchos compuestos, el 90% del total se fabrican a partir de cinco polímeros, todos ellos sintéticos.

Plásticos habituales (nomenclatura)

Usos habituales

Polietileno (PE) Bolsas de supermercado, botellas plásticas

Polipropileno (PP) Envases, apliques, piezas automóvil

Poliestireno (PS) Espuma de empaquetado, envases alimentos, vasos desechables, cajas de CD

Poliestireno de alto impacto (HIPS) Capas de recubrimiento en frigoríficos, envases alimentos, vasos desechables

Acrilonitrilo-butadieno-estireno (ABS) Carcasas de equipos electrónicos

Polietilen-tereftalato (PET) Botellas para bebidas carbonatadas, recipientes, películas plásticas, envases para calentamiento por microondas

Poliéster (PES) Fibras, materiales textiles

Poliamidas (PA) Fibras, sedal de pesca, moldes para piezas de automóvil

Cloruro de polivinilo (PVC) Tuberías y canaletas, cortinas de duchas, marcos de ventanas, pavimentos

Poliuretanos (PU) Espumas para amortiguación y aislamiento térmicos, recubrimiento de superficies, rodillos de impresión

Policarbonato (PC) CD, gafas, escudos antidisturbios, ventanas seguridad, luces tráfico

Cloruro de polivinilideno (PVDC) Envases alimentarios

Hoy, la industria de los plásticos es un componente importante de las economías occidentales. Por ejemplo, la industria de los plásicos en USA incluye más de 20.000 fábricas que producen o distribuyen materiales o productos, dando empleo a 1.5 millones de trabajadores y facturando 300.000 millones de dólares por año.

Se estima que en el mundo se producen más de 50 millones de toneladas métricas de plásticos por año. Se prevé que el consumo de plástico crezca aproximadamente al 5% por año, hasta alcanzar 250 millones de kg en 2010. En Europa, el consumo de plástico anual por persona es de 60 kg, en comparación con 80 kg en USA. Adicionalmente, en torno al 20% de los residuos urbanos son de naturaleza plástica. De acuerdo con BASF, el consumo de plásticos anual per capita en USA aumentará desde 101 kg en 2001 hasta más de 130 kg en el 2010. Se espera un aumento continuo de la demanda de materiales plásticos en el futuro inmediato.

La figura muestra la distribución mundial de la demanda de materiales plásticos

(fuente: http://www.fuji-keizai.com/e/report/bio_plastic_e.html)

2) PLÁSTICOS Y DESARROLLO SOSTENIBLE

En la fabricación de plásticos, el polímero aún no procesado se manipula (incluyendo extrusión, moldeo por inyección y moldeo por compresión) para obtener los productos deseados, que pueden presentar conformaciones muy distintas (desde películas hasta sólidos de configuración tridimensional).

El polímero no procesado puede mezclarse con distintos aditivos (por ejemplo, plastificantes o colorantes, véase la Figura adjunta), con el fin de modificar las propiedades del producto final (por ejemplo, resistencia mecánica, flexibilidad, fragilidad y apariencia).

La sostenibilidad de la industria del plástico puede considerarse desde dos puntos de vista diferentes:

a) Su impacto ambiental (incluyendo la biodegradabilidad)

b) La posibilidad de que el proceso productivo emplee materias primas renovables.

Polímero no procesado

Aditivos

.Plastificantes

.Pigmentos, colorantes

.Agentes para “crosslin- king”

.Promotores de adhesión

.Conservantes

.Espesantes

.Desmoldantes

MEZCLA Y MOLDEO

Producto final

2.1 Impacto ambiental

La utilización de plásticos es muy habitual debido a su facilidad de producción, bajo coste y durabilidad. Sin embargo, esas mismas propiedades contribuyen a ocasionar un gran problema ambiental: dado que el plástico es barato, se tira a la basura fácilmente, y luego permanece en el ambiente debido a su resistencia a los procesos biológicos de degradación.

El plástico se amumula en el ambiente a un ritmo aproximado de 12 millones de toneladas métricas por año. La mayor parte de este material no es biodegradable y causa graves efectos en el ambiente terrestre y marino, como por ejemplo:

.oclusión de canales de desagüe, facilitando inundaciones locales (como sucedió en Mumbai, India en 1998).muerte de animales (ganado, animales marinos) por acumulación de plásticos en el estómago o porque los animales se enredan en ellos. Algunos ejemplos son la muerte de tortugas (que confunden plásticos transparentes que flotan con medusas, y los ingieren), mamíferos marinos (más de 100.000 mamíferos marinos mueren anualmente en los océanos por los problemas antes mencionados) y las aves marinas (que comen plásticos, y que mueren de inanición al permanecer éstos en el estómago impidiendo la digestión).

Estos datos confirman que la producción de plásticos degradables comercialmente competitivos es una cuestión de gran importancia.

Los plásticos degradables pueden clasificarse en las categorías siguientes:.Plásticos fotodegradables, que poseen grupos sensibles a la luz directamente incorporados como aditivos en la estructura del polímero. Los plásticos fotodegradables se vuelven débiles y quebradizos cuando se exponen a la luz de sol durante períodos prolongados. Los grupos fotosensibles incluyen di-cetonas, derivados del ferroceno (aminoalquilferroceno) y especies con grupos carbonilo. Estos plásticos se degradan a través de un proceso en dos etapas: inicialmente, la luz ultravioleta rompe algunos enlaces, reduciendo el peso molecular y volviendo quebradizo el plástico; luego, éste se degrada por esfuerzos físicos (como acción de olas o rozamiento contra rocas). La Figura muestra un esquema simplificado de la degradación de polietileno (PE) por peroxidación. .Plásticos semibiodegradables, tales como mezclas de almidón y polietileno..Plásticos totalmente biodegradables, por ejemplo los constituidos por almidón o poliésteres.

PE HC−OOH HC−O • + • OH

Calor o esfuerzos mecánicosO2

Calor o luz ultravioleta

2.2 Producción de matrices plásticas a partir de recursos renovables

Debido al constante aumento del precio de los combustibles fósiles, a su creciente escasez y a su contribución al efecto invernadero, se han redescubierto los polímeros producidos a través de fuentes renovables, a los que se ha prestado una atención creciente a lo largo de las últimas dos décadas.

La biomasa vegetal generada por biosíntesis (véase la Figura adjunta) es la materia prima renovable más importante para el desarrollo sostenible.

De modo general, los polímeros que proceden de fuentes renovables pueden clasificarse en tres grupos:

.polímeros naturales, como el almidón y la celulosa,

.polímeros sintéticos producidos a partir de monómeros naturales, como la polilactida (PLA).polímeros producidos por fermentación microbiana, como los polihidroxialcanoatos (PHA), que distintos microorganismos almacenan como fuentes internas de carbono y energía como mecanismo de supervivencia. El PHA puede ser sintetizado por bacterias a partir de distintas fuentes renovables, es completamente biodegradable y biocompatible, y tiene carácter termoplástico. Se conocen más de 90 tipos distintos de PHA, formados por varios monómeros, y su número no ha parado de crecer.

Nutrientes

Procesos metabólicos

Biopolímeros (lignina, celulosa, hemicellulosas)

6CO2 + 6H2O + luz solar

Fotosíntesis

C6H12O6 + 6O2

Luz solarOxígeno

GlucosaAgua

CO2

En resumen, el desarrollo de procesos para la producción de plásticos ecológicos biodegradables (plásticos “verdes”) a través de procesos que produzcan un impacto ambiental limitado es una de las líneas de actuación que es preciso emprender. Este tipo de compuestos es hoy una realidad, y la mejora de las tecnologías de procesamiento que se alcancen en un futuro inmediato permitirán aumentar el éxito de este empeño.

Como ejemplo, la Figura adjunta muestra los principios generales en que podría basarse un proceso de fabricación de bioplásticos basado en la utilización de biomasa como materia prima.

FraccionamientoBiomasa

Agua o disoluciones ácidas

monómeros (azúcares)

Polímeros (celulosa, almidón) o Hidrólisis y fermentación

Bioplásticos (PHA) o monómeros para polimerización (PLA) Producto final

Basura Reciclado o compostaje

Procesamiento

Nutrientes

3) MATERIALES COMPUESTOS DE BAJO IMPACTO AMBIENTAL Los materiales compuestos (“composites”) están constituidos por dos o más materiales con propiedades químicas o físicas significativamene diferentes, que permanecen separados y distinguibles en el producto final. Las propiedades de un determinado material compuesto deben ser distintas de las de sus componentes.

En la Biblia, el Éxodo proporciona ejemplos de materiales compuestos primitivos, tales como adobes hechos de paja y barro, o el material de construcción de la cuna de Moisés (hecha de juncos, brea y fango), que podría considerarse como un tipo de material compuesto reforzado con un material fibroso.

Los constituyentes de los materiales compuestos son: -la matriz polimérica, que rodea y soporta los materiales de

refuerzo, -el refuerzo, que se halla inmerso en la matriz y mejora las

propiedades físicas de ésta.

Este trabajo se centra en materiales compuestos de bajo impacto ambiental (“environmentally friendly composites”) consitituidos por matrices biodegradables y refuerzos obtenidos a partir de recursos renovables. Estos tipos de materiales suelen denominarse “materiales compuestos ecológicos (“eco-composites”) o “materiales compuestos verdes” (“green composites”). Se presta atención particular al procesamiento por extrusión e inyección por moldeo (técnicas empleadas en el Proyecto NATURPLAS II).

4) MATRICES PARA MATERIALES COMPUESTOS DE BAJO IMPACTO AMBIENTAL Las matrices poliméricas biodegradables que se producen a partir de materias renovables pueden clasificarse en las siguientes categorías:

-Matrices sintéticas biodegradables, correspondientes a polímeros presentes en materias primas naturales. Ejemplos típicos de este grupo son el almidón y los polihidroxialcanoatos. -Matrices biosintéticas modificadas, como los derivados de celulosa. Compuestos químicos derivados de la celulosa (por ejemplo, el 2-5 acetato de celulosa, que tiene propiedades termoplásticas y mantiene la biodegradabilidad) abren nuevas rutas para este tipo de aplicaciones.-Polímeros semi-biosintéticos. Las unidades monoméricas se producen de modo natural o por vía fermentativa, y luego se polimerizan por vías sintéticas clásicas. Un ejemplo representativo de este tipo de polímeros el PLA (polilactida/ácido poliláctico). -Polímeros producidos por síntesis química, tipo al que pertenecen los poliésteres. Algunos ejemplos representativos son la policaprolactona (PCL), así como amidas de poliésteres y ésteres del ácido poli-itacónico. Estos compuestos tienen en común el poseer un grupo éster muy reactivo, que desempeña un papel importante en la producción de los materiales compuestos.

AMILOSA AMILOPECTINA

COMPONENTES DEL ALMIDÓN

Enlace (1-4)Enlace (1-6)

En la producción de PLA, los isómeros D(-) y L(+) del ácido láctico pueden producirse por fermentación bien como enantiómeros puros o como una mezcla de isómeros, para luego ser polimerizados a PLA.

Las propiedades de las matrices citadas pueden mejorarse mezclándolas con determinados componentes. Por ejemplo, mezclas de almidón y acetato de celulosa pueden mejorar las propiedades como matriz termoplástica del almidón puro.

Las matrices consideradas en el Proyecto NATURPLAS han sido almidón, mezclas de almidón y acetato de celulosa y PLA. Las secciones siguientes resumen información sobre estas matrices.

Ácido L-Láctico

Ácido D-Láctico

PROPIEDADES DEL ÁCIDO LÁCTICO

Nombre químico: Ác. 2-hidroxipropanoico

Fórmula química: C3H6O3

[50-21-5] L: [79-33-4]

Número CAS : D: [10326-41-7] D/L:[598-82-3]

L: 53 ºCPunto de fusión D: 53 ºC

D/L: 16.8 ºC

4.1) Almidón y mezclas almidón-acetato de celulosa

El almidón es un polisacárido constituido por unidades de glucosa, que se encuentra por ejemplo en el arroz, maíz, patatas, trigo y tapioca. La producción anual de almidón es de más de 35.000 millones de kg. Una parte importante de esta cantidad se utiliza para fines distintos de los alimentarios.

La Figura resume algunas ventajas e inconvenientes de los bioplásticos a base de almidón.

-Biodegradabilidad

-Estabilidad térmica

-Abundancia

-Bajo precio (es el biopo- límero más barato)

-Carácter renovable

-Procesabilidad por distintas tecnologías(incluyendo extrusión e inyección por moldeo)

VENTAJAS

INCONVENIENTES

-Solubilidad en agua

-Naturaleza quebradiza

- Carácter hidrofílico (pobre compatilidad con otros polímeros/refuerzos)

-Absorción de agua y sensibilidad a la humedad

-Resistencia mecánica limitada (tests de impacto)

-Durabilidad escasa (descomposición rápida)

Las propiedades negativas pueden contrarrestarse, al menos parcialmente, de los siguientes modos:

.Modificando químicamente el almidón (usualmente por esterificación parcial o completa de los grupos hidroxilo de las cadenas laterales) para mejorar su compatibilidad con otros componentes de la formulación. Por ejemplo, se han propuesto tratamientos superficiales del almidón con mónomeros de caprolactona o valerolactona, a fin unir estas moléculas al almidón por medio de enlaces covalentes. Alternativamente, el almidón puede procesarse para “injertar” copolímeros con propiedades termoplásticas (“graft copolymerization”). .Mezclando almidón con otros polímeros, como el acetato de celulosa. Las mezclas de almidón soluble con acetato de celulosa mejoran determinadas propiedades del almidón, como reducción en la afinidad por el agua, resistencia mecánica o disminución de la susceptibilidad a la degradación. Las mezclas conteniendo almidón se biodegradan a velocidades que dependen de la composición y de la cristalinidad.

OO

OHOH

OH

OHO

O

O

OH

OHHO

O

O

OH

O

O

OHOH

OH

OH

CELULOSAGrupos hidroxilo reactivos que pueden esterificarse por reacción con ácido

acético y anhidrido acético en presencia de ácido sulfúrico para dar unidades de anhidroglucosa mono-, di- o tri- sustituidas

4.2 Polilactida (ácido láctico, PLA)

El ácido láctico puede producirse a gran escala a través de la fermentación de sueros lácteos, azúcares, materiales amiláceos o sustratos celulósicos (algunos de ellos de origen residual). Algunos residuos típicos que pueden utilizarse para la fabricación de ácido láctico son los desperdicios de la fabricación de patatas fritas o los efluentes de la industrial del queso. La Figura adjunta muestra los pasos involucrados en la producción de ácido láctico a partir de almidón.

Como se explicó anteriormente, dependiendo de la tecnología empleada y de las condiciones experimentales, la fermentación puede conducir a la producción de mezlcas de isómeros de ácido láctico [D(-) y L(+)] o a isómeros puros. El ácido L-láctico puro tiene un precio de mercado casi 3 veces mayor que las mezclas de isómeros D(-) y L(+).

Almidón Dextrosa (sin refinar)

Fermentación

Producción de monómeros

Producción de polímero

Ácido poliláctico

Modificación para clientes

Aplicaciones

Ácido láctico

Lactida

Debido a la naturaleza quiral del ácido láctico existen dos lactidas:

.Poli-DL-lactida (PDLLA), obtenida por polimerización de una mezcla rácémica del L- y D- lactidas, de caracter amorfo, que puede degradarse rápidamente..Poli-L-lactida (PLLA), que resulta de la polimerización de la L,L-lactida (también denominada L-lactida), que tiene una cristalinidad en torno al 37%, una temperatura de transición vítrea entre 50 y 80 ºC y una temperatura de fusión de 173-178 ºC. La PLLA funde a 170-180 ºC, y tiene un punto de reblandecimiento de 58 ºC, una resistencia a la tracción de 700 kg/cm y una transparencia del 94%. Puede procesarse en películas de espesores entre 10 y 500 μm de espesor, y admite procesamiento de moldeo por inyección. La PLLA es usualmente dura y quebradiza.

Los progresos en biotecnología han permitido ell desarrollo de procesos comerciales para la producción de D-LA (por PURAC), permitiendo la producción de poli-D-lactida (PDLA), que se mezcla generalmente con PLLA para mejorar las propiedades mecánicas, debido a la fuerte interacción entre las cadenas de PLLA y PDLA. Así, los estereocomplejos de mezclas PLLA/PDLA tienen una temperatura de fusión de 220–230 ºC, del orden de 50 ºC más altas que las de PLLA y PDLA.

LD-lactidaLL-lactida

Las tecnologías de reacción citadas anteriormente permiten la obtención de una extensa gama de productos de diferente peso molecular y cristalinidad, que resultan útiles para distintos tipos de aplicaciones. El PLA se produce habitualmente por polimerización a través de la apertura del anillo de la lactida (véase Figura) en medios catalizados.

El brillante futuro del PLA lo coloca al frente de las industrias emergentes de los plásticos biodegradables. Algunas compañías que producen PLA son: -NatureWorks LLC (una filial de Cargill Corporation, U. S. A.) -Toray Industries Inc. (Japón) -Galactic (Bélgica) -PURAC (una filial de CSM, Países Bajos)

Catalizador, calor

Lactida Polilactida

Polimerización de lactida a polilactida

En comparación con otras matrices plásticas derivadas del petróleo, el PLA muestra buenas propiedades mecánicas. Por ejemplo, el PLA presenta ventajas sobre el prolipropileno en cuanto a resistencia a la tracción, módulo de tracción y resistencia la flexión.

La Figura adjunta proporciona datos comparativos sobre la interrelación entre el módulo de tracción y la elongación de rotura entre distintos tipos de matrices plásticas. Los resultados muestran que el PLA tiene una elongación de rotura pequeña (uno de sus principales inconvenientes). La segunda desventaja del PLA reside en su limitada resistencia al impacto.

Comparación de polímeros

Mód

ulo,

MPa

Elongación de rotura, %

En términos de impacto ambiental, el PLA es totalmente degradable y susceptible al compostaje (por ejemplo, los embalajes de PLA son biodegradables, y se descomponen en menos de 60 días en instalaciones comerciales de compostaje, o en alrededor de 180 días en instalaciones de compostaje comerciales o municipales).

La Figura muestra otras ventajas y desventajas del PLA. Parte de estos problemas pueden reducirse o evitarse mezclando dos o más polímeros o refuerzos con propiedades químicas y físicas diferentes.

-no tóxico

-estabilidad térmica limitada

-excelenteprocesabilidad

-posibilidad de producción a partir de residuos

-propiedades mecánicas regulables por mezcla con productos obtenidos a partir de distintos isómeros en distintas proporciones

-posibilidad de producción a partir de materias primas renovables

OTRAS VENTAJAS OTRAS DESVENTAJAS

-mayor precio que otros productos derivados del petróleo -alta velocidad de

permeación de agua

-pobre compatibilidad con otros polímeros/refuerzos

-alta sensibilidad a la humedad

-resistencia limitada en tests de impacto

-descomposición rápida

Los términos “materiales compuestos de bajo impacto ambiental”,“materiales compuestos ecológicos” o “materiales compuestos verdes” se emplean habitualmente para describir los materiales compuestos que presentan ventajas ambientales y ecológicas respecto a los convencionales.

En este trabajo, la atención se ha centrado en materiales compuestos que contienen las matrices poliméricas biodegradables que se han descrito en secciones previas, así como refuerzos derivados de materiales lignocelulósicos. La Figura adjunta ilustra la estructura de un sustrato lignocelulósico típico.

5) MATERIALES COMPUESTOS DE BAJO IMPACTO AMBIENTAL CONTENIENDO REFUERZOS OBTENIDOS A PARTIR DE MATERIALES LIGNOCELULÓSICOS

Lignina

Hemicelulosas

Celulosa

Célula vegetal

Pared de la célula

El introducir refuerzos derivados de materiales lignocelulósicos en matrices plásticas permite una serie de mejoras, incluyendo: .Reemplazar productos químicos caros por productos naturales baratos .Mejorar las propiedades tecnológicas .Obtener un beneficio ambiental, derivado del carácter renovable y biodegradable de los refuerzos. .Obtener mejoras sociales y económicas, que surgen del aumento esperado de la demanda.

La Figura adjunta muestra algunas fuentes típicas de materiales lignocelulósicos

.Bosques (>60% del total de generación de materiales lignocelulósicos) .Agricultura (pajas, hierbas, subproductos agrícolas).Otros (plantas y hierbas que crecen en tierras improductivas, etc.).

.Industria, particularmente subproductos o residuos sólidos (bagazo, serrín, etc.).Residuos sólidos urbanos (papel de periódico y papeles usados, cartones)

Fuentes primarias (materias primas nativas) Fuentes secundarias (materias primas procesadas)

FUENTES DE MATERIALES LIGNOCELULÓSICOS

Los materiales lignocelulósicos son heterogéneos, y poseen una naturaleza química compleja. Sus componentes puede clasificarse como sigue:.Componentes estructurales, de naturaleza polimérica, que incluyen:

..Celulosa, un homopolímero constituido por unidades de beta-glucosa, con estructura parcialmente cristalina y notable resistencia mecánica.

..Hemicelulosas, un heteropolímero que puede estar constituido por distintos azúcares (glucosa, manosa, xilosa, galactosa, arabinosa) y sustituyentes (como grupos urónicos, grupos fenólicos esterificados y grupos acetilo).

..Lignina, un polímero amorfo constituido por unidades fenil-propano..Constituyentes no estructurales, que incluyen extractos, cenizas y proteínas.

En los materiales lignocelulósicos nativos, la celulosa se halla inmersa en una matriz de lignina y hemicelulosas. El contenido en celulosa de los materiales lignocelulósicos depende de la materia prima considerada, yendo desde el 45% en madera resinosas hasta más del 70% en hojas fibrosas.

OH

OR

OO

O

OHOH

OH

O

OO

O

OHOH

RO

OH

OH

Estructura del xilano, un polímero hemicelulósico típico

Se han empleado como refuerzos para materiales compuestos tanto lignocelulósicos nativos como procesados. Se han publicado estudios sobre materiales compuestos reforzados con materiales celulósicos obtenidos a partir de:

.Maderas, incluyendo pino, haya, abeto y eucalipto.

.Pasta de celulosa procedentes de factorías kraft, incluyendo fibras residuales.

.Distintos materiales naturales distintos de las maderas, como bambú, fibra de coco, algodón, sisal, cáñamo, lino, yute, esparto, cáscaras, ramio, palmas, bananas, zuros de maíz, hierbas y pajas.

.Subproductos industriales y residuos de las industrias alimentarias (como el bagazo), instalaciones de industrias textiles, etc.

.Residuos urbanos de naturaleza celulósica, como papel usado y cartones.

Los materiales de refuerzo obtenidos a partir de materiales lignocelulósicos corresponden al tipo de “fibras duras”, y que incluyen a las fibras obtenidas de hojas y de maderas.

Pinus pinaster, el pino más común en el Noroeste de la Península Ibérica

Eucalyptus globulus, el eucalipto más común en el Noroeste de la Península Ibérica

Los efectos de refuerzo causados por materiales derivados de materias primas de naturaleza celulósica dependen de distintos factores, incluyendo:

.El origen y la naturaleza del material considerado (tipo, edad, condiciones de crecimiento…), que determinan la estructura interna y la composición de las fibras..El grado de polimerización de la celulosa, el tamaño de fibra y la distribución de tamaños de las fibras (usualmente, las fibras de mayor longitud permiten mayores efectos de refuerzo)..La relación de aspecto (“aspect ratio”), o cociente entre longitud y diámetro. Las fibras con mayor relación de aspecto suelen mejorar la resistencia y las propiedades de tracción..El contenido en fibra de los materiales compuestos,.Las alteraciones en las fibras causadas por tratamientos físicos y químicos,.La compatibilidad interfacial entre la matriz y los refuerzos.

La Figura adjunta muestra otras ventajas de los refuerzos derivados de materiales lignocelulósicos

-Carácter renovable-Posibilidad de producción a partir de residuos -Biodegradabilidad-No contribuye a las emisiones de CO2

-Producción limitada de cenizas

-Gran disponibilidad-Geográficamente dispersos-Menor coste que las matrices plásticas-Menor coste que otros refuerzos

-Buenas propiedades mecánicas específicas-Alta resistencia-Area superficial relativamente reactiva-Posibilidad de funcionalización-Facilidad de procesamiento (alta flexibilidad, naturaleza no abrasiva) -Buena resistencia química-Baja densidad aparente-Buena capacidad de aislamiento acústico-Procesabilidad por distintas tecnologías (incluyendo moldeo por inyección)

Económicas y prácticas

Ambientales

Ventajas de los refuerzos derivados de materiales lignocelulósicos

Tecnológicas

Alternativamene, algunos de los inconvenientes de los refuerzos derivados de materiales lignocelulósicos son:

.Adhesión interfacial inadecuada

.Elevado carácter hidrófilo

.Escasa resistencia a la humedad

.Estabilidad térmica limitada (no se recomiendan temperaturas mayores de 175 ºC durante tiempos de procesamiento prolongados)

Estas desventajas pueden paliarse (al menos parcialmente) de las siguientes formas:.Utilizando matrices plásticas polares.Modificando las fibras por procedimientos químico-fisicos a fin de: .. Obtener mejor compatibilidad interfacial, mejando el anclaje entre matriz y refuerzo. .. Aumentar el tamaño de poro y la rugosidad de la superficie, lo que redunda en la

mejora de las propiedades mecánicas.

5.1 Objetivos y tipos de pretratamientos

Cuando se pretende obtener un refuerzo basado en fibras de celulosa para materiales compuestos a partir de un determinado material lignocelulósico, el principal objetivo de los tratamientos químicos a implementar es eliminar fracciones no-celulósicas. Esta estrategia puede implicar:

-La eliminación de componentes no estructurales (extractos, ceras).-La eliminación de los componentes estructurales distintos de la celulosa (lignina y hemicelulosas), a fin de aumentar el contenido del sustrato en celulosa, ya que es ésta quien proporciona los efectos de refuerzo que se buscan.

La Tabla adjunta presenta datos sobre las propiedades de fibras celulósicas procedentes de distintas materias primas.

1117200.53792020 Carbono alta resistencia8.225501.32001820 Carbono11.927602.51241440 Aramid (K49)

 45804.6852500 Vidrio (S) 34003.4712550 Vidrio (E)      8401.81001520Lino 9201.7701520Cáñamo

2008602.0601520 Yute20-80413-16271.635-821440 Piña

50-300530-6403-710-221450 Sisal25-40980  1500Abacá

 200-8006-12271520Algodón100-450131-17515-404-61150Coco (cáscara)

Diámetroμm

ResistenciaMPa

Elongación%

MóduloGPa

Densidadkg/m3

  

Fibr

as n

atur

ales

Fibr

as a

rtifi

cial

es

Propiedades de las fibras

En relación con los tratamientos poco agresivos, la bibliografía ha considerado tanto el procesamiento acuoso como el procesamiento alcalino.

El procesamiento acuoso incluye:

- autohidrólisis (con agua caliente comprimida) para eliminar extractos, extraer materiales tipo cera y solubilizar las hemicelulosas, que se convierten en oligómeros solubles

- explosión con vapor (“steam explosion”), que además altera la estructura del sustrato, provocando la rotura de las paredes celulares y liberando la celulosa de la matriz lignocelulósica.

Equipamiento de explosión con vapor (source: http://www.biogasol.dk/2me2.htm)

La oxidación húmeda (“wet oxidation”) es una tecnología relacionada con las anteriores, en que se añade oxígeno al medio acuoso en que se procesa la materia prima para facilitar la descomposición de la lignina y de las hemicelulosas.

En función de las condiciones de operación, los tratamientos alcalinos pueden causar distintos efectos sobre los materiales lignocelulósicos nativos, incluyendo: .eliminación de extractos, ceras y otros materiales no estructurales.conversión de los haces de fibras de celulosa en otros de menor tamaño y en fibras aisladas.hinchamiento, con reducción de la intensidad de los enlaces de hidrógeno en la red celulósica.reducción del diámetro de la fibra, con aumento de la relación de aspecto.aumento del área accesible, con una topografía rugosa, que favorece la interpenetración.alteración de las propiedades fisicoquímicas de la celulosa (reducción de cristalinidad y grado de polimerización),.modificación de la orientación de las zonas cristalinas de la celulosa.aumento de la facilidad de mojado, mejorando la reactividad.eliminación de los componentes poliméricos no celulósicos de la pared vegetal (celulosa, lignina)

Algunos procesos de pasteo suponen un caso extremo de tratamiento alcalino, donde el objetivo principal es eliminar la lignina. Esto puede conseguirse empleando NaOH (proceso a la sosa) o mezclas NaOH/Na2S (proceso kraft). Las fibras kraft y los sustratos que las contienen (como papel de periódico o cartones) se han empleado como refuerzos para materiales compuestos.

Alternativamente, los tratamientos alcalinos en condiciones suaves pueden llevarse a cabo en presencia de compuestos oxidantes (como H2O2). Esta estrategia permite una deslignificación selectiva (debido a la susceptibilidad de la lignina a reacciones de oxidación), y puede emplearse para que las fibras de celulosa que se encuentran en la superficie del material lignocelulósico se vuelvan accesibles a los reactivos químicos o a las matrices plásticas.

Una posibilidad alternativa es llevar a cabo distintos tratamientos para modificar las propiedades de las fibras naturales (por ejemplo, su carácter hidrófilo, lo que reduciría los efectos de la humedad sobre el material compuesto y mejoraría la compatibilidad interfacial). Como ejemplo, la Figura muestra el esquema de la reacción de carboximetilación de la celulosa, que puede emplearse para introducir grupos carbonilo.

CARBOXIMETILCELULOSA (Sustitución en C6 ≥ Sustitución en C2 >>

Sustitución en C3

Monosubstituted unit

Isopropanol - ClH2C-COOH

123

4 5

6

123

4 5

6

123

4 5

6

123

4 5

6 n

CH2

CO O

-

O

HOH

HH

H

H

OOH

O

O

HO

HH

H

H

OOH

OH

O

HOH

HH

H

H

OO

O

O

HOH

HH

H

H

OH

O

CH2 CO

O-

CH2 CO

O-

CH2

COO

-

CH2 CO

O-

123

4 5

6

123

4 5

6

123

4 5

6

123

4 5

6

O

HOH

HH

H

H

OOH

OH

O

HOH

HH

H

H

OOH

OH

O

HOH

HH

H

H

OOH

OH

O

HOH

HH

H

H

OH

OH n

Disubstituted unit

CELULOSA

Los agentes químicos empleados para la modificación de fibras celulósicas deben enlazarse a éstas de forma covalente, a fin de cambiar sus propiedades.

Las reacciones de modificación química de la celulosa deben cumplir distintas condiciones, como:.deben transcurrir a pH moderado (medio neutro o débilmente ácido/alcalino).la cinética debe ser suficientemente rápida para asegurar una conversión razonable cuando se opere en condiciones de utilidad práctica,.deben evitarse temperaturas superiores a 150 ºC, a fin de evitar reacciones de degradación.los derivados deben ser estables.los reactivos deben poder penetrar en la red celulósica, lo que puede exigir el hinchamiento de la celulosa.las propiedades tecnológicas del derivado (color, características mecánicas, estabilidad térmica, sensibilidad hacia la humedad) deben ser iguales o mejores que las del sustrato sin derivar.

Además, la derivación debe ser económica para asegurar la viabilidad práctica de la reacción considerada.

La bibliografía ha propuesto distintas alternativas para la obtención de derivados, incluyendo acetilación, cianoetilación, blanqueo, tratamientos con silano, benzoilación, tratamientos con peróxido e isocianato, acrilación, recubrimiento con látex, etc. De modo adicional o alternativo, la matriz plástica puede modificarse para mejorar su compatibilidad con los refuerzos de naturaleza celulósica.

La copolimerización (“graft copolymerization”) y la inserción de grupos de acoplamiento (“coupling groups”) son formas usuales de llevar a cabo las reacciones de derivación de la celulosa. La copolimerización se ha empleado para unir polietileno a las fibras celulósicas en una reacción inducida por peróxido, mientras que se han empleado como grupos de acoplamiento el silano, anhidrido maleico, titanato y triazima.

Introducción de un monómero (M) en cadenas de celulosa y celulosa acetilada

a) Formación del radical de la macromolécula

b) Unión del monómero

Inicio

Propagación

Terminación

Inserción de un agente de acoplamiento

H2NR

Fibra celulósica

Fibra celulósica

5.2) Propiedades de materiales compuestos constituidos por matrices renovables y refuerzos derivados de materiales lignocelulósicos Los materiales compuestos reforzados con fibras naturales proporcionan una alternativa única a los materiales compuestos convencionales formados por polipropileno o poliésteres insaturados reforzados con fibra de vidrio, admitiendo una amplia diversidad de aplicaciones prácticas. Los materiales compuestos formados por almidón reforzado con fibras de celulosa son un ejemplo típico de un material compuesto formado por polímeros naturales, presentando ventajas como su naturaleza renovable, carácter biodegradable, abundancia y bajo coste. En la bibliografía se ha propuesto emplear como refuerzos del almidón una serie de materiales celulósicos como sisal, algodón, bambú, yute, pajas, kenaf, madera y pasta kraft. Al igual que muchos otros materiales compuestos reforzados con fibras, el objetivo prinicpal de introducir refuerzos celulósicos en matrices de almidón es la mejora de las propiedades mecánicas. Así, el almidón termoplástico reforzado presenta hasta cuatro veces mejores propiedades mecánicas que la matriz sin reforzar. En materiales compuestos a base de almidón conteniendo fibras de lino en disposición unidireccional y cruzada, obtenidas por prensado en caliente por el procedimiento de apilamiento de capas, las propiedades mecánicas dependen del contenido en fibras y de la disposición de éstas. La resistencia a la tracción aumenta con el contenido en fibras (hasta el 40%). En el mejor caso, la resistencia a la tracción del material compuesto fue tres veces mayor que el de la matriz de almidón puro, mientras que la presencia de fibras aumentó el módulo en varios órdenes de magnitud. La Sección 7 presenta referencias sobre este tipo de materiales compuestos.

Los materiales compuestos reforzados con acetato de celulosa son otra de las alternativas consideradas en la bibliografía. Estos materiales compuestos son biocompatibles, lo que permite aplicaciones de tipo biológico. Algunos trabajos publicados sobre este tema han examinado aspectos como citotoxicidad, adhesión celular y estabilidad térmica, así como la aplicación de técnicas experimentales que evitan etapas de procesamiento termomecánico (que podrían ocasional una degradación prematura de las fibras). Las mezclas de almidón y acetato de celulosa presentan mejoras de propiedades en el intervalo 52-64%. En el caso de materiales compuestos almidón-acetato de celulosa-fibras de celulosa, los productos presentan una ductilidad reducida en comparación con la matriz sin reforzar. La Sección 7 presenta referencias sobre este tipo de materiales compuestos.

El PLA es un polímero que puede establecer un nuevo modelo de desarrollo industrial, ya que los países desarrollados son totalmente dependendientes de los recursos fósiles para obtener los combustibles, polímeros y productos químicos que se necesitan en el mundo actual. Los expertos y analistas han concluido que las nuevas materias primas jugarán un papel significativo en un mundo que se halla ante el reto de los problemas interrelacionados del agotamiento de los recursos fósiles y de los aumentos de las emisiones con efecto invernadero, de los agentes contaminantes y de los residuos sólidos. En este contexto, el PLA presenta propiedades mecánicas comparables a las de los termoplásticos tradicionales, y puede fabricase a partir de materias primas que contengan polisacáridos (incluyendo tanto materiales amiláceos como lignocelulósicos). En base a las ideas anteriores, la “sostenibilidad” del PLA puede contemplarse desde distintos puntos de vista: .Sostenibilidad económica, como un negocio rentable, capaz de producir beneficios a lo largo de la cadena de valor (incluyendo nuevos mercados para los productos agrícolas, nuevas oportunidades profesionales para científicos y tecnólogos, y otros beneficios económicos a los inversores y a la sociedad)..Sostenibilidad ambiental, relacionada con la producción de bienes útiles, que pueden desarrollar funciones positivas en el mercado y en la sociedad, con menor impacto ambiental que las alternativas empleadas hoy en día..Sostenibilidad social, que se traduce en responsabilidad social, e incluye conceptos como igualdad de oportunidades para todos los agentes involucrados en la cadena de valor. Todas estas ideas puede aplicarse (e incluso ampliarse) a los materiales compuestos conteniendo refuerzos lignocelulósicos. Estos materiales compuestos abren el camino a la nueva generación de materiales, productos y procesos sostenibles, con bajo impacto ambiental y altamente eco-eficientes. Este tipo de materiales compuestos se han citado como uno de los productos más atrayentes del siglo XXI, constituyendo una alternativa ventajosa siempre que las propiedades puedan mejorarse manteniendo los costes suficientemente bajos y conservando la biodegradabilidad.

Uno de los factores cruciales que deben considerarse en el reforzamiento de materiales compuestos con fibras celulósicas es la proporción de éstas. Usualmente, la resistencia aumenta con el contenido en refuerzo hasta un valor umbral, que depende del caso estudiado. Se han citado valores de contenido en refuerzo de hasta un 50%, si bien el grado de reemplazamiento más habitual está en torno al 30%. De hecho, contenidos en fibras mayores de este límite suelen conducir a mezclas muy viscosas, que resultan difíciles de procesar en equipos convencionales de moldeo por inyección. En los ensayos de tracción, se espera que el módulo de los materiales compuestos conteniendo PLA y refuerzos celulósicos aumente significativamente respecto a la matriz pura de PLA, mientras que la resistencia debe variar poco, y la elongación se reduce. Por otra parte, la presencia de los refuerzos celulósicos en matrices de PLA suele provocar un empeoramiento a la resistencia al impacto, ya que este parámetro depende estrechamento de la adhesión entre matriz polimérica y refuerzo. Como en otros materiales compuestos, la interfase PLA-refuerzo juega un papel crítico a la hora de asegurar que las propiedades de cada componente contribuyen óptimamente a las propiedades del producto final.

La adhesión interfacial se ve afectada por distintos factores, entre los que se encuentran la naturaleza de las interfases, las condiciones de operación (que pueden originar degradación térmica) y cambios químicos superficiales que pueden perjudicar las propiedades globales.

La Sección 7 muestra información sobre propiedades de materiales compuestos a base de PLA.

6) CONSIDERACIONES FINALES Y CONCLUSIONES

La sostenibilidad de un determinado producto o proceso debe evaluarse utilizando los métodos adecuados. Con este fin, la “International Standards Organization” (ISO) ha venido desarrollando desde 1993 programas de análisis de ciclos de vida (“Life Cycle Assessment”, LCA) que proporcionan las herramientas necesarias para hacer un análisis e inventario de los aportes y excedentes de materiales y energía (“input/output”) asociados a un determinado producto. La primera iniciativa en el desarrollo del análisis de ciclo de vida fue establecer líneas de actuación para los próximos años que definiesen cómo desarrollar y difundir herramientas prácticas para evaluar las oportunidades, riesgos e intercambios asociados a productos y servicios a lo largo de todo su ciclo de vida, de cara a alcanzar un desarrollo sostenible. A través del análisis de ciclo de vida resulta posible comparar los impactos ambientales de varios plásticos “verdes” entre sí y con poliolefinas convencionales (que suponen más del 90% de la producción actual de plásticos).

En comparación con los polímeros de almidón, los beneficios ambientales derivados de la utilización de PLA (que representa el 10-15% de la producción de plásticos “verdes”) o de la utilización de polímeros biodegradables producidos a partir de fuentes no renovables (que suponen aproximadamente el 10% del total), parecen ser menores, aunque conservan ventaja sobre los polímeros tradicionales.

Para los poliésteres de origen microbiano, la ventaja ambiental (de existir) parece pequeña, pero las tecnologías de producción fermentativa se han desarrollado recientemente, y tanto el método como la escala de producción pueden afectar a las evaluaciones de los efectos ambientales globales.

T.U. Gerngross y S.C.Slater han publicado un análisis de ciclo de vida (“How Green Are Green Plastics?” Scientific American, August 2000) que ha recibido mucha atención, pero su estudio se enfoca únicamente hacia los poliésteres microbianos, por lo que su trabajo responde mejor a la pregunta “hasta qué punto son verdes los poliésteres microbianos?”).

Para analizar el efecto (o el impacto) de un determinado producto en el ambiente, han de considerarse los distintos aspectos mostrados en la Figura adjunta. Aunque el análisis de ciclo de vida de los plásticos está todavía en sus etapas iniciales de desarrollo, comienzan a aparecer determinadas tendencias. Una revisión de 20 estudios de análisis de ciclo de vida de polímeros biodegradables (M. Patel, presentación en la “7th World Conference on Biodegradable Polymers and Plastics”, Pisa, Italia, Junio 2002) indica que el almidón, el mayor componente de aproximadamente el 75% de los “plásticos verdes”, ofrece importantes beneficios ambientales en comparación con polímeros convencionales.

Extracción de materias primas

Procesos Reciclado de productos

Vertedero

Utilización de productos

6.1) Aplicaciones y tendencias de mercado

El número de aplicaciones de los plásticos biodegradables y materiales compuestos está aumentando enormemente. Esta tendencia está impulsada por las tecnologías modernas, que proporcionan herramientas poderosas para determinar microestructuras a diferentes niveles, y para comprender las relaciones entre estructuras y propiedades. Estos nuevos niveles de comprensión proporcionan oportunidades para desarrollar materiales para nuevas aplicaciones. El PLA es un ejemplo representativo en este campo. La Figura adjunta muestra un diagrama que muestra el amplio campo que abarcan las aplicaciones genéricas del PLA

En otros lugares de esta web puede encontarse más información sobre aplicaciones de materiales compuestos en la industria del automóvil, muebles, etc. Como ejemplos representativos de desarrollos recientes, la Tabla adjunta lista aplicaciones propuestas para un PLA comercial (NatureWorksTM).

Tipo de negocio Aplicaciones

– Recipientes para frutas frescas y vegetales– Bandejas para alimentos

-Termoformados rígidos – Envases opacos (yogures)– Contenedores para panadería, hierbas frescas y

caramelos – Embalajes de pantallas y material electrónico– Materiales desechables y vasos para bebidas frías

– Envoltorios enrollables (caramelos)– Cajas para envoltorio y material de embalaje transparente – Películas laminadas – Exhibidores de regalos

-Películas con – Tapas para mercancías situadas en exhibidores orientación biaxial – Cuños para etiquetado

– Envoltorios para flores– Cintas – Bolsas capaces de sostenerse – Bolsas para tartas, cereales y pan

– Leche de consumo rápido-Botellas – Aceites comestibles

– Agua embotellada

6.2 Perspectivas

El brillante futuro de los plásticos verdes y de los correspondientes materiales compuestos se basa en tres pilares: nuevos materiales, nuevos procesos y nuevos productos (o aplicaciones).

Por ejemplo, en el caso del PLA, el producto obtenido a partir de almidón (también llamado PLA1) se espera que sea reemplazado por el PLA de segunda generación (denominado PLA2), basado en la lignocelulosa. En este caso, se espera que las materias primas sean residuos de cosechas (tallos, paja, cáscaras y hojas), y que tanto la celulosa como las hemicelulosas se conviertan en azúcares para fermentación en las llamadas “bio-refinerías”. La fracción residual, rica en lignina, puede quemarse o gasificarse para producir vapor, que suministraría energía para los distintos procesos de conversión.

El concepto de bio-refinería hace máximo el valor añadido de las materias primas lignocelulósicas, obteniendo distintos productos, valorizando subproductos y co-productos, mejorando el balance producción/consumo de energía y optimizando recursos y excedentes, incluyendo el tratamiento de residuos. En relación con la producción de ácido láctico, el proceso se adaptará para permitir la utilización de azúcares derivados de materiales lignocelulósicos, y se optimizará para reducir el consumo de materias primas, entre otras mejoras.

Fibra de vidrio

Polímeros proced. petróleo

HOY

MAÑANA

Nanofibras celulósicas

Bio-polímeros

Componentes moldeados

Componentes moldeados

Las nuevas tecnologías de producción de materiales compuestos abarcan la utilización de nano-refuerzos, que pueden permitir grandes mejoras en las propiedades mecánicas con bajos grados de reemplazamiento. Este tipo de refuerzos es particularmente importante para polímeros obtenidos a partir de fuentes renovables, dado que en su mayor parte tienen como desventajas sus bajas temperaturas de reblandecimiento y módulos.

Los materiales compuestos a base de PLA y nano-refuerzos de naturaleza celulósica constituyen un ejemplo representativo de productos con bajo impacto ecológico. Su desarrollo y utilización a gran escala dependerá del coste de los nano-refuerzos, que debe ser competitivo con los de otras alternativas existentes en el mercado. La Sección 7 incluye una lista de referencias que trata de materiales compuestos conteniendo nano-refuerzos.

7) REFERENCIAS

7.1) Referencias con información general sobre materiales compuestos reforzados con fibras naturales

7.2) Referencias sobre materiales compuestos a base de almidón y/o acetato de celulosa

7.3) Referencias sobre materiales compuestos a base de PLA

7.4) Referencias sobre la utilización de sustratos celulósicos (nativos, pretratados o sometidos a modificación química) para la producción de materiales compuestos

7.5) Referencias sobre la biodegradabilidad/compostaje/análisis de ciclo de vida de matrices plásticas y materiales compuestos

Bismarck, Alexander; Mishra, Supriya; Lampke, Thomas.Plant fibers as reinforcement for green composites. Mohanty, Amar K.; Misra, Manjusri; Drzal, Lawrence T. Natural Fibers, Biopolymers, and Biocomposites (2005), 37-108. A review discussing the variety, structure and mech. properties of plant fibers and advantages of their use as reinforcement for plastics. The bast fibers (flax, kenaf, nettle, hemp, jute, ramie), leaf fibers (sisal, henequen, pineapple, abaca, oil palm), seed fibers (cotton), fruit fibers (coconut husk or coir), and stalk fibers (cereal straw) are considered. The increasing environmental awareness, growing global waste problems, and continuously rising high crude oil prices motivate development of the environmentally and economically viable materials using renewable resources. Composites with moderate strength can be used in many noncrit. structural applications in automotive, electronic, packaging, and building industries. Green composites made entirely from renewable agricultural resources offer a unique alternative to the commonly used synthetic reinforcing fibers, such as carbon, glass or aramid, because of their low d., good mech. properties, abundance, and easy recycling. Some drawbacks related to the use of plant fibers as reinforcement for polymers are high moisture absorption, low microbial resistance, and low thermal stability. Bledzki, A. K.; Gassan, J. Composites reinforced with cellulose based fibres. Progress in Polymer Science (1999), 24(2), 221-274. This review article, with refs., concerning natural and man-made cellulose fiber-reinforced plastics, introduces possible applications of this material group. The phys. properties of natural fibers are mainly detd. by the chem. and phys. compn., such as the structure of fibers, cellulose content, angle of fibrils, cross-section, and by the d.p. Only a few characteristic values, but esp. the specific mech. properties, can reach comparable values of traditional reinforcing fibers. This phys. structure can be modified by using alkali treatment and acetylation processes. The application of natural fibers as reinforcements in composite materials requires, just as for glass-fiber reinforced composites, a strong adhesion between the fiber and the matrix, regardless of whether a traditional polymer (thermoplastics or thermosets) matrix, a biodegradable polymer matrix or cement is used. Further this article gives a survey about phys. and chem. treatment methods which improve the fiber matrix adhesion, their results and effects on the phys. properties of composites. These different treatments change among others the hydrophilic character of the natural fibers, so that moisture effects in the composite are reduced. To bring about hydrophobic properties to natural fibers, a special treatment, termed acetylation, can be used. The effectiveness of this method is strongly influenced by the treatment conditions used. The mech. and other phys. properties of the composite are generally dependent on the fiber content, which also dets. the possible amt. of coupling agents in the composite. The influence of such treatments by taking into account fiber content on the creep, quasi-static, cyclic dynamic and impact behavior of natural fiber-reinforced plastics are discussed in detail. For special performance requirements, hybrid composites made of natural and conventional fibers can be prepd. with desired properties. The processing conditions play, next to the mech. properties of natural fibers, an important role for the industrial use of these materials. Natural fibers seem to have little resistance towards environmental influences. This can be recognized in the composite and can be advantageously utilized for the development of biol. degradable composites with good phys. properties. Eichhorn, S. J.; Baillie, C. A.; Zafeiropoulos, N.; Mwaikambo, L. Y.; Ansell, M. P.; Dufresne, A.; Entwistle, K. M.; Herrera-Franco, P. J.; Escamilla, G. C.; Groom, L.; Hughes, M.; Hill, C.; Rials, T. G.; Wild, P. M. Current international research into cellulosic fibers and composites. Journal of Materials Science (2001), 36(9), 2107-2131. The following paper reviews with 101 refs a no. of international research projects being undertaken to understand the mech. properties of natural cellulose fibers and composite materials. In particular the use of novel techniques, such as Raman spectroscopy, synchrotron x-ray and half-fringe photoelastic methods of measuring the phys. and micromech. properties of cellulose fibers is reported. Current single fiber testing procedures are also reviewed with emphasis on the end-use in papermaking. The techniques involved in chem. modifying fibers to improve interfacial adhesion in composites are also reviewed, and the use of novel fiber sources such as bacterial and animal cellulose. It is found that there is overlap in current international research into this area, and that there are complementary approaches and therefore further combining of these may make further progress possible. In particular a need to measure locally the adhesion properties and deformation processes of fibers in composites, with different chem. treatments, ought to be a focus of future research.

7.1) Referencias con información general sobre materiales compuestos reforzados con fibras naturales

Garlotta, Donald. A literature review of poly(lactic acid). Journal of Polymers and the Environment (2002), Volume Date 2001, 9(2), 63-84. A literature review is presented regarding the synthesis and physicochem., chem., and mech. properties of poly(lactic acid) (PLA). PLA exists as a polymeric helix, with an orthorhombic unit cell. The tensile properties of PLA can vary widely, depending on whether or not it is annealed or oriented or what its degree of crystallinity is. Also discussed are the effects of processing on PLA. Crystn. and crystn. kinetics are examd. Soln. and melt rheol. is also discussed. Four different power-law equations and 14 different Mark-Houwink equations are presented. NMR, UV-visible, and FTIR spectroscopy of PLA are briefly discussed. Finally, research conducted on starch-PLA composites is introduced. Mohanty, A. K.; Misra, M. Studies on jute composites - A literature review. Polymer-Plastics Technology and Engineering (1995), 34(5), 729-92. A review with 327 refs. on the title material. Mohanty, A. K.; Misra, M.; Drzal, L. T. Sustainable Bio-Composites from Renewable Resources: Opportunities and Challenges in the Green Materials World. Journal of Polymers and the Environment (2002), 10(1/2), 19-26. A review. Sustainability, industrial ecol., eco-efficiency, and green chem. are guiding the development of the next generation of materials, products, and processes. Biodegradable plastics and bio-based polymer products based on annually renewable agricultural and biomass feedstock can form the basis for a portfolio of sustainable, eco-efficient products that can compete and capture markets currently dominated by products based exclusively on petroleum feedstock. Natural/Biofiber composites (Bio-Composites) are emerging as a viable alternative to glass fiber reinforced composites esp. in automotive and building product applications. The combination of biofibers such as kenaf, hemp, flax, jute, henequen, pineapple leaf fiber, and sisal with polymer matrixes from both nonrenewable and renewable resources to produce composite materials that are competitive with synthetic composites requires special attention, i.e., biofiber-matrix interface and novel processing. Natural fiber-reinforced polypropylene composites have attained com. attraction in automotive industries. Natural fiber-polypropylene or natural fiber-polyester composites are not sufficiently eco-friendly because of the petroleum-based source and the nonbiodegradable nature of the polymer matrix. Using natural fibers with polymers based on renewable resources will allow many environmental issues to be solved. By embedding biofibers with renewable resource-based biopolymers such as cellulosic plastics; polylactides; starch plastics; polyhydroxyalkanoates (bacterial polyesters); and soy-based plastics, the so-called green bio-composites are continuously being developed. Saheb, D. Nabi; Jog, J. P. Natural fiber polymer composites: A review. Advances in Polymer Technology (1999), 18(4), 351-363. A review with 115 refs.; natural fiber-reinforced composites is an emerging area in polymer science. These natural fibers are low-cost fibers with low d. and high specific properties. These are biodegradable and non-abrasive. The natural fiber composites offer specific properties comparable to those of conventional fiber composites. However, in development of these composites, the incompatibility of the fibers and poor resistance to moisture often reduce the potential of natural fibers and these drawbacks become crit. issue. This review presents the reported work on natural fiber-reinforced composites with special ref. to the type of fibers, matrix polymers, treatment of fibers and fiber-matrix interface. Yu, Long; Dean, Katherine; Li, Lin. Polymer blends and composites from renewable resources. Progress in Polymer Science (2006), 31(6), 576-602. A review. This article reviews recent advances in polymer blends and composites from renewable resources, and introduces a no. of potential applications for this material class. In order to overcome disadvantages such as poor mech. properties of polymers from renewable resources, or to offset the high price of synthetic biodegradable polymers, various blends and composites have been developed over the last decade. The progress of blends from three kinds of polymers from renewable resources-(1) natural polymers, such as starch, protein and cellulose; (2) synthetic polymers from natural monomers, such as polylactic acid; and (3) polymers from microbial fermn., such as polyhydroxybutyrate-are described with an emphasis on potential applications. The hydrophilic character of natural polymers has contributed to the successful development of environmentally friendly composites, as most natural fibers and nanoclays are also hydrophilic in nature. Compatibilizers and the technol. of reactive extrusion are used to improve the interfacial adhesion between natural and synthetic polymers.

Ammar, I.; Ben Cheikh, R.; Campos, A. R.; Cunha, A. M.; Campos, A. R. Injection molded composites of short alfa fibers and biodegradable blends. Polymer Composites (2006), 27(4), 341-348. Fully biodegradable composites made from two polymer blend matrixes (SEVA-C: starch and a copolymer of ethylene vinyl alc.; and SCA: starch and cellulose acetate) and short Alfa fibers were developed and processed by conventional injection molding into std. tensile specimens. For each kind of matrix, the influence of the reinforcement load was evaluated, using fiber amts. from 0 to 30% (wt/wt). An optimization study was carried out for the composite SEVA-C with 10% Alfa fiber. The obtained results establish that the produced biodegradable composites present a significant improvement in stiffness for both matrixes. Improvements in the tensile strength were obsd. only for the Alfa fiber reinforced SEVA-C. However, for both matrixes, the reinforcement causes a significant loss in the material ductility. Results from design of expts. (Hadamard plans) were used to explain the influence of the injection molding conditions on the mech. behavior of the obtained composites, mainly on the stiffness values. Cunha, A. M.; Campos, A. R.; Cristovao, C.; Vila, C.; Santos, V.; Parajo, J. C..Sustainable materials in automotive applications. Plastics, Rubber and Composites (2006), 35(6/7), 233-241. The potential use of sustainable polymer based composites, obtained from renewable resources, is revised in terms of properties and envisaged applications. Using the typical specifications required within the automotive industry as ref., the potential of these composites is evaluated and some alternative routes for property improvement are discussed. Specific examples of the development of biodegradable polymeric composites are presented including the use of different types of matrixes, e.g. starch based blends and poly(lactic acid), and two types of natural reinforcements, i.e. fibers from pine and cellulose. This paper also presents and compares the mech. properties and morphologies obtained on injection molding composite parts made with different combinations of biodegradable matrixes and fiber reinforcements subjected to different phys. and chem. treatments. The damage caused to the fibers during the compounding and injection molding processing stages was studied too.Endres, Hans-Josef; Pries, Andreas.. Mechanical properties of starch -filled polymer compounds. Starch/Staerke (1995), 47(10), 384-93. Important mech. properties of native potato, maize and wheat starch granules were detd. With these results and under precondition of a sufficient interfacial quality the resulting mech. properties of composites reinforced with starch granules could be predicted theor. These theor. calcd. mech. parameters have been verified by tensile tests of the different composite materials. For evaluation of the interfacial quality a model for failure of unidirectional reinforced materials has been used, applied to the investigated composites and discussed in detail using some biodegradable composites as examples. The mech. properties of the matrix materials could be improved by their reinforcement with starch granules. At the same time the final costs of the composite materials could be reduced in consequence of the low costs of native starch of about 1 DM/kg and the degrdn. behavior could also have been accelerated by an increasing amt. of starch. The max. amt. of starch has been limited to 40%. Up to this filling ratio the examd. materials allowed processing almost without complications.

7.2) Referencias sobre materiales compuestos a base de almidón y/o acetato de celulosa

Garlotta, Donald; Doane, William; Shogren, Randal; Lawton, John; Willett, J. L. Mechanical and thermal properties of starch-filled poly(DL-lactic acid)/poly(hydroxy ester ether) biodegradable blends. Journal of Applied Polymer Science (2003), 88(7), 1775-1786. The mech., structural, and thermal properties of injection-molded composites of granular cornstarch with poly(DL-lactic acid) (PDLLA) and poly(hydroxy ester ether) (PHEE) were investigated. The composites have tensile strength 17-66 MPa at starch loading 0-70 wt%. SEM micrographs of fracture specimens reveal good adhesion of the starch granule with the polymer matrix. The starch/matrix adhesion is greatest when the matrix PDLLA/PHEE ratio is 0-1. At PDLLA/PHEE ratio <1, as the starch content increases, the tensile strength and modulus increase, while the elongation at break decreases. The effects of starch on the mech. properties of the PDLLA composites shows that as the starch content increases, the tensile strength and elongation to break decrease, while the Young's modulus increases. In contrast, the tensile strength of the PHEE composites increases with increasing starch content. Gattin, Richard; Copinet, Alain; Bertrand, Celine; Couturier, Yves. Biodegradation study of a starch and poly(lactic acid) co-extruded material in liquid, composting and inert mineral media. Biodegrdn. of a co-extruded starch/poly(lactic acid) polymeric film was studied in liq., inert solid, and composting media. Main mech. properties of this film were Young's modulus: 2340 MPa; elongation at break: 50%; and contact angle: 118°. Mineralization of the material C content was followed using appropriate exptl. methods of the International Std. Organization. Whatever the biodegrdn. medium used, the percentage of mineralization was better than the required 60% value for definition of a biodegradable material. Moreover, re-partitioning of material C between various degrdn. products was quantified throughout the duration of exptl. runs. The presence of starch facilitated biodegrdn. of the polylactic component, esp. in liq. media. Hokens, D.; Mohanty, A. K.; Misra, M.; Drzal, L. T. Environment-friendly " green " biodegradable composites from natural fiber and cellulosic plastic. Polymer Preprints (American Chemical Society, Division of Polymer Chemistry) (2001), 42(2), 71-72. To produce biodegradable materials of com. value, an expensive biopolymer (cellulose acetate) was mixed with an inexpensive natural fiber (henequen leaf) as a filler and a biocomposite was fabricated using powder impregnation processing. Thermal and mech. properties of the resulting biocomposite were examd. Surface modification (5% aq. NaOH, 1 h) improved fiber-matrix adhesion and thus the performance of the biocomposite. Lee, Sang Hwan; Lee, Sang Yool; Nam, Jae Do; Lee, Youngkwan. Preparation of cellulose diacetate/ramie fiber biocomposites by melt processing. Polymer ( (2006), 30(1), 70-74. Plasticized cellulose diacetate (CDA) was prepd. by homogenizing cellulose diacetate (CDA), triacetin (TA), and epoxidized soybean oil (ESO) in a high-speed mixer, then the CDA mixt. was mixed with ramie fiber to produce a green composite material. In DMA anal., the glass transition temp. of plasticized CDA and the composite was obsd. at 85 °C and 140 °C, resp. A composite reinforced with alkali treated ramie fiber exhibited significantly higher mech. properties, such as 15% increase in tensile strength as well as 41% increase in Young's modulus when compared with com. polypropylene. In the SEM image anal., much enhanced adhesion between plasticized CDA and alkali treated ramie fiber (AlRa) was obsd.

Mayer, Jean M.; Elion, Glenn R.; Buchanan, Charles M.; Sullivan, Barbara K.; Pratt, Sheldon D.; Kaplan, David L.Biodegradable blends of cellulose acetate and starch : Production and properties. Plastics Engineering (New York) (1995), 29(Degradable Polymers, Recycling, and Plastics Waste Management), 183-93. Blends of cellulose acetate (CA, 2.5 degree of substitution) and starch (St) were melt processed and evaluated for mech. properties, biodegradability during composting, and marine and soil toxicity. Formulations contg., on a wt. basis, 57% CA, 25% corn St and 19% propylene glycol (PG) had mech. properties similar to polystyrene. Increasing plasticizer or St content lowered tensile strength. Simulated municipal composting of CA alone showed losses of 2-3 and 90% dry wt. after 30 and 90 days, resp. CA/St/PG blends in both soil burial and composting expts. indicate that PG and St are degraded first. Extended incubations are required to detect losses from CA. Marine toxicity tests using polychaete worms and mussels showed no toxicity of CA or St. High doses had an adverse effect due to oxygen depletion in the marine water due to rapid biodegrdn. of the polymers. Preliminary plant toxicity tests of the CA/St blends showed no neg. impact on growth and yield for sweet corn, butternut squash, and plum tomatoes. The results indicate that CA/St blends have acceptable properties for injection-molded applications and are biodegradable and nontoxic. Onteniente, Jean-Paul; Safa, Laurent Haidar; Abbes, Boussad. Process for making new biofragmentable materials : comparative study of the physical properties of wheat starch and cellulose acetate blends and those of wheat starch and starch acetate blends. Bio-fragmentable starch blends were obtained by extrusion, using either wheat starch and cellulose acetate dry blends, in which the degree of substitution (DS) of cellulose acetate was close to 2.3, or wheat starch and starch acetate dry blends in which the DS of starch acetate was around 1.5. After prepn., the dry blends were stored for one week at 20° and 65% relative humidity (RH) before being injection molded to produce standardized samples for tensile tests. Tensile strength tests (ISO/R527) and Charpy shock tests (ISO/R179) were carried out on the injection molded samples after having been stored under the same conditions of temp. and RH. The dimensional stability of the samples was estd. by measuring the volumetric shrinkage after storage. The hydrophobic behavior of the blends was evaluated by measuring the development of the contact angle of a drop of water placed on the samples. Starch was not compatible with 2.3 DS cellulose acetate. The samples prepd. with starch and 2.3 DS cellulose acetate blends had a thin surface layer composed of cellulose diacetate; this layer increased the hydrophobicity. The samples prepd. with starch and 1.5 DS starch acetate blends were globally homogeneous and did not exhibit any surface layer, but the blends were less hydrophobic than the 2.3 DS cellulose acetate blends.

Viana P.; Vallo, Claudia; Kenny, Jose M.; Vazquez, Analia. Effect of chemical treatment on the mechanical properties of starch-based blends reinforced with sisal fiber. Journal of Composite Materials (2004), 38(16), 1387-1399. The effect of chem. treatment of sisal fibers on mech. properties of fiber-reinforced biodegradable plastics (biocomposites) was investigated. The biocomposites were produced by extrusion of polycaprolactone/starch blends (Mater-Bi-Z ZF 03) as the matrix with washed or sodium hydroxide- or acetic acid-treated sisal fibers as a reinforcement. The alk. or acetylation treatment of the fibers was carried out to enhance adhesion and compatibility between the fibers and the matrix. Tensile properties of the biodegradable composite were improved by the presence of the fibers. The untreated fibers showed better reinforcing properties than the acetylated or alkali-treated fibers. This was attributed to an impairment of the mech. properties of the acetylated fibers and incompatibility of the alkali-treated fibers. The results were supported by SEM anal. of the fibers and the composite materials. Wollerdorfer, Martina; Bader, Herbert. Influence of natural fibers on the mechanical properties of biodegradable polymers. Industrial Crops and Products (1998), 8(2), 105-112. Fiber reinforced plastics are used whenever there is the need for very high mech. properties combined with low wt. In that respect natural fibers are of basic interest since they not only have the functional capability to substitute the widely used glass fibers but they also have advantages from the point of view of wt. and fiber-matrix adhesion, specifically with polar matrix materials. They have good possibilities in waste management due to their biodegradability on the one hand and their much lower prodn. of ash during incineration on the other. The influence of plant fibers such as flax, jute, ramie, oil palm fibers and fibers made from regenerated cellulose on the mech. properties of biodegradable polymers was investigated using thermoplastics like polyesters, polysaccharides and blends of thermoplastic starch. The composites were produced by extrusion compounding with a co-rotating twin screw extruder. The pellets obtained were further processed into tensile test bars by injection molding. Depending on the kind of polymer, a fiber content of 20-35% could be achieved. Generally a considerable tensile strength improvement of polyesters could not be obsd. However the chem. similarity of polysaccharides and plant fibers, which consist mainly of cellulose, resulted in an increased tensile strength of the reinforced polymers. For reinforced thermoplastic wheat starch, it was four times better (37 N/mm2) than without fibers. The reinforcement of cellulose diacetate and starch blends caused a stress increase of 52% (55 N/mm2) and 64% (25 N/mm2), resp.

Ben, Goichi; Kihara, Yuichi. Development and evaluation of mechanical properties for Kenaf fibers /PLA composites. Key Engineering Materials (2007), 334-335(Pt. 1, Advances in Composite Materials and Structures), 489-492. A new type of composite used biodegradable resins and natural fibers is now being developed and this new type of composites is designated as a green composite. This paper presents a fabrication method and mech. properties of the green composite used Kenaf fibers as the reinforcement and PLA as the matrix. In order to obtain the higher tensile strength, various kind of surface treatments were executed on the Kenaf fiber and some parameters were changed during the process of fabrication. Then, these effects on the strength of green composite are also reported. Chen, Chien-Chung; Chueh, Ju-Yu; Tseng, How; Huang, Haw-Ming; Lee, Sheng-Yang Preparation and characterization of biodegradable PLA polymeric blends. Biomaterials (2003), 24(7), 1167-1173.The purpose of this study was to fine-tune the mech. properties of high mol.-wt. poly-l-lactic acid (PLLA), esp. to increase its toughness without sacrificing too much of its original strength. Besides of its long degrdn. time, PLLA is usually hard and brittle, which hinders its usage in medical applications, i.e., orthopedic and dental surgery. Some modifications, such as the addn. of plasticizers or surfactants/compatibilizers, are usually required to improve its original properties. PDLLA can degrade quickly due to its amorphous structure, thus shortening the degrdn. time of PLLA/PDLLA blends. Blends of biodegradable poly-l-lactic acid (PLLA) and poly-dl-lactic acid (PDLLA) or polycaprolactone (PCL), in addn. to a third component, the surfactant-a copolymer of ethylene oxide and propylene oxide, were prepd. by blending these three polymers at various ratios using dichloromethane as a solvent. The wt. percentages of PLLA/PDLLA (or PCL) blends were 100%/0%, 80%/20%, 60%/40%, 50%/50%, 40%/60%, 20%/80% and 0%/100%, resp. Phys. properties such as the cryst. m.p., glass transition point (Tg), phase behavior, degrdn. behavior, and other mech. properties were characterized by thermogravimetric anal., DSC, IR spectroscopy, gel permeation chromatog., and dynamic mech. anal. (DMA). DSC data indicate that PLLA/PDLLA blends without the surfactant had two Tg's. With the addn. of the surfactant, there was a linear shift of the single Tg as a function of compn., with lower percentages of PLLA producing lower glass transition temps. indicating that better miscibility had been achieved. DMA data show that the 40/60 PLLA/PDLLA blends without the surfactant had high elastic modulus and elongation, and similar results were obsd. after adding 2% surfactant into the blends. The 50/50 PLLA/PDLLA/2% surfactant blend had the highest elastic modulus, yield strength, and break strength compared with other ratios of PLLA/PDLLA/2% surfactant blends. The elongation at break of 50/50 PLLA/PDLLA was similar to that of PLLA. Again, the elongation at break of 50/50 PLLA/PDLLA/2% surfactant was almost 1.2-1.9 times higher than that of 50/50 PLLA/PDLLA and PLLA. Elongation of PLLA increased with the addn. of PCL, but the strength decreased at the same time. In conclusions, adding PDLLA and surfactant to PLLA via soln.-blending may be an effective way to make PLLA tougher and more suitable to use in orthopedic or dental applications.

7.3) Referencias sobre materiales compuestos a base de PLA

Cunha, A. M.; Campos, A. R.; Cristovao, C.; Vila, C.; Santos, V.; Parajo, J. C..Sustainable materials in automotive applications. Plastics, Rubber and Composites (2006), 35(6/7), 233-241. The potential use of sustainable polymer based composites, obtained from renewable resources, is revised in terms of properties and envisaged applications. Using the typical specifications required within the automotive industry as ref., the potential of these composites is evaluated and some alternative routes for property improvement are discussed. Specific examples of the development of biodegradable polymeric composites are presented including the use of different types of matrixes, e.g. starch based blends and poly(lactic acid), and two types of natural reinforcements, i.e. fibers from pine and cellulose. This paper also presents and compares the mech. properties and morphologies obtained on injection molding composite parts made with different combinations of biodegradable matrixes and fiber reinforcements subjected to different phys. and chem. treatments. The damage caused to the fibers during the compounding and injection molding processing stages was studied too.Garlotta, Donald. A literature review of poly(lactic acid). Journal of Polymers and the Environment (2002), Volume Date 2001, 9(2), 63-84. A literature review is presented regarding the synthesis and physicochem., chem., and mech. properties of poly(lactic acid) (PLA). PLA exists as a polymeric helix, with an orthorhombic unit cell. The tensile properties of PLA can vary widely, depending on whether or not it is annealed or oriented or what its degree of crystallinity is. Also discussed are the effects of processing on PLA. Crystn. and crystn. kinetics are examd. Soln. and melt rheol. is also discussed. Four different power-law equations and 14 different Mark-Houwink equations are presented. NMR, UV-visible, and FTIR spectroscopy of PLA are briefly discussed. Finally, research conducted on starch-PLA composites is introduced. Garlotta, Donald; Doane, William; Shogren, Randal; Lawton, John; Willett, J. L. Mechanical and thermal properties of starch-filled poly(DL-lactic acid)/poly(hydroxy ester ether) biodegradable blends. Journal of Applied Polymer Science (2003), 88(7), 1775-1786. The mech., structural, and thermal properties of injection-molded composites of granular cornstarch with poly(DL-lactic acid) (PDLLA) and poly(hydroxy ester ether) (PHEE) were investigated. The composites have tensile strength 17-66 MPa at starch loading 0-70 wt%. SEM micrographs of fracture specimens reveal good adhesion of the starch granule with the polymer matrix. The starch/matrix adhesion is greatest when the matrix PDLLA/PHEE ratio is 0-1. At PDLLA/PHEE ratio <1, as the starch content increases, the tensile strength and modulus increase, while the elongation at break decreases. The effects of starch on the mech. properties of the PDLLA composites shows that as the starch content increases, the tensile strength and elongation to break decrease, while the Young's modulus increases. In contrast, the tensile strength of the PHEE composites increases with increasing starch content.

Gattin, Richard; Copinet, Alain; Bertrand, Celine; Couturier, Yves. Biodegradation study of a starch and poly(lactic acid) co-extruded material in liquid, composting and inert mineral media. International Biodeterioration & Biodegradation (2002), 50(1), 25-31. Biodegrdn. of a co-extruded starch/poly(lactic acid) polymeric film was studied in liq., inert solid, and composting media. Main mech. properties of this film were Young's modulus: 2340 MPa; elongation at break: 50%; and contact angle: 118°. Mineralization of the material C content was followed using appropriate exptl. methods of the International Std. Organization. Whatever the biodegrdn. medium used, the percentage of mineralization was better than the required 60% value for definition of a biodegradable material. Moreover, re-partitioning of material C between various degrdn. products was quantified throughout the duration of exptl. runs. The presence of starch facilitated biodegrdn. of the polylactic component, esp. in liq. media. Hu, Ruihua; Lim, Jae-Kyoo .Fabrication and mechanical properties of completely biodegradable hemp fiber reinforced polylactic acid composites. Journal of Composite Materials (2007), 41(13), 1655-1669. Biodegradable composite materials can be produced by the combination of biodegradable polymers and natural fibers. In this study, a new biodegradable composite of hemp fiber reinforced polylactic acid (PLA) was fabricated using the hot press method. Mech. properties of composites with different fiber vol. fractions were tested. The optimum fiber content was detd. according to the test results. Effects of alkali treatment on the fiber surface morphol. and the mech. properties of the composites were investigated. Test results show that the composite with 40% vol. fraction of alkali treated fiber has the best mech. properties. The tensile strength, elastic modulus, and flexural strength of the composite with 40% treated fiber are 54.6 MPa, 8.5 Gpa, and 112.7 MPa resp., which are much higher than those of PLA alone. The composites have lower densities, which were measured to be from 1.19 g/cm3 to 1.25 g/cm3. Specific strengths were also calcd. Surface morphologies of fiber and fracture surfaces of the composites were obsd. using the SEM method. Huda, M. S.; Mohanty, A. K.; Drzal, L. T.; Schut, E.; Misra, M. "Green" composites from recycled cellulose and poly(lactic acid): Physico-mechanical and morphological properties evaluation. Journal of Materials Science (2005), 40(16), 4221-4229. "Green"/biobased composites were prepd. from poly(lactic acid) (PLA) and recycled cellulose fibers (from newsprint) by extrusion followed by injection molding processing. The physico-mech. and morphol. properties of the composites were investigated as a function of varying amts. of cellulose fibers. Compared to the neat resin, the tensile and flexural moduli of the composites were significantly higher. This is due to higher modulus of the reinforcement added to the PLA matrix. Dynamic mech. anal. (DMA) results also confirmed that the storage modulus of PLA increased on reinforcements with cellulose fibers indicating the stress transfers from the matrix resin to cellulose fiber. Differential scanning calorimetry (DSC) and thermogravimetric anal. (TGA) showed that the presence of cellulose fibers did not significantly affect the crystallinity, or the thermal decompn. of PLA matrix up to 30 wt% cellulose fiber content. Overall it was concluded that recycled cellulose fibers from newsprint could be a potential reinforcement for the high performance biodegradable polymer composites.

Lunt, James. Large-scale production, properties and commercial applications of polylactic acid polymers. Polymer Degradation and Stability (1998), 59(1-3), 145-152. A review with 6 refs. on lactic acid polymers (PLA). Recent developments in the capability to manuf. the monomer economically from renewable feedstocks have placed these materials at the forefront of the emerging biodegradable plastics industry. Increasing realization of the intrinsic properties of these polymers, coupled with a knowledge of how such properties can be manipulated to achieve compatibility with thermoplastics processing, manufg., and end-use requirements has fueled technol. and com. interest in PLA products. This paper discusses the various technologies being used to produce polylactic acids. In addn., attention is drawn to how monomer stereochem. can be controlled to impart targeted utility in the final polymers. Specific applications are described to illustrate further the range of properties that can be developed by utilizing both the basic monomer/polymer chemistries in combination with post-modification techniques. Finally, the biodegrdn. mechanism of polylactic acids will be discussed and contrasted with other biodegradable polymers. Mohamed, Abdellatif A.; Finkenstadt, V. L.; Palmquist, Debra E. Thermal properties of extruded-injection molded poly (lactic acid) and milkweed composites. Proceedings of the NATAS Annual Conference on Thermal Analysis and Applications (2007), 35th 26#769/1-26#769/7. Currently, most polymer composites utilize petroleum-based materials that are non-degradable and difficult to recycle or incur substantial cost for disposal. Green composites can be used in nondurable limited applications. In order to det. the degree of compatibility between Poly (lactic Acid) (PLA) and different biomaterials, PLA was compounded with milkweed fiber. Milkweed is a new crop oil seed. After oil extn., the remaining cake retained approx. 10% residual oil and 47% protein. The pressed seed cake (10% moisture) was ground and passed through a 300 mm screen. The fiber was added at 85:15 and 70:30 PLA:Fiber. The composites were blended by extrusion (EX) followed by injection molding (EXIM). Differential Scanning Calorimetry (DSC) and Thermogravimetric Anal. (TGA) were used to analyze the EX and the EXIM composites. The effect of the fiber on the enthalpic relaxation (ER) of PLA was detd. by aging. After melting in the DSC sealed pans, composites were cooled by immersion in liq. nitrogen and aged (stored) at room temp. for 0, 7, 15, and 30 days. After storage, samples were heated from room temp. to 180 °C at 10°C/min. The pure PLA showed a glass transition (Tg) at 59°C and the corresponding DCp was 0.464 J/g/°C. The PLA glass transition was followed by crystn. and melting transitions. The ER of neat PLA and composites steadily increased as a function of storage time. Although the presence of fiber has little effect on ER, injection molding reduced ER. The percentage crystallinity of neat unprocessed PLA dropped, as a result of EX, by 95% and by 80% for the EXIM. The degrdn. was done in air and nitrogen environment. The degrdn. Activation Energy (Ea) of neat PLA exhibited a significant drop in nitrogen environment, while increased in air, indicating PLA resistant to degrdn. in the presence of oxygen. Overall, injection molding appeared to reduce Ea of the composite. Milkweed significantly reduced Ea values in a nitrogen environment, while in air Ea exhibited increased values. Enzymic degrdn. of the composites showed a higher degrdn. rate for the EX samples vs. EXIM, while 30% milkweed exhibited higher wt. loss compared to the 15%.

Nishino, Takashi; Hirao, Koichi; Kotera, Masaru; Nakamae, Katsuhiko; Inagaki, Hiroshi. Kenaf reinforced biodegradable composite. Composites Science and Technology (2003), 63(9), 1281-1286. Mech. properties of environmentally friendly composites made of kenaf fiber and poly-l-lactic acid (LACEA) resin were investigated. The Young's modulus (6.3 GPa) and tensile strength (62 MPa) of the composites (fiber content 70 vol.%) were comparable to those of conventional fiber composites. These properties were higher than those of the kenaf sheet and the LACEA film themselves. This is considered to attribute to the strong interaction between the fiber and LACEA. In addn., the storage modulus of the composite remain unchanged up to the LACEA m.p. The effects of the polymer mol. wt. and the fiber orientation on the mech. properties of the composite were also investigated. It was found that kenaf fiber can be a good candidate for the reinforcement fiber of high performance biodegradable polymer composites. Oksman, K.; Skrifvars, M.; Selin, J.-F Natural fibres as reinforcement in polylactic acid (PLA) composites. Composites Science and Technology (2003), 63(9), 1317-1324. The focus in this work has been to study if natural fibers can be used as reinforcement in polymers based on renewable raw materials. The materials were flax fibers and poly(lactic acid) (PLA). PLA is a thermoplastic polymer made from lactic acid and has mainly been used for biodegradable products, such as plastic bags and planting cups, but in principle PLA can also be used as a matrix material in composites. Because of the brittle nature of PLA triacetin was tested as plasticizer for PLA and PLA/flax composites in order to improve the impact properties. The studied composite materials were manufd. with a twin-screw extruder having a flax fiber content of 30 and 40 wt.%. The extruded compd. was compression molded to test samples. The processing and material properties were studied and compared to the more commonly used polypropylene-flax fiber composites (PP/flax). Preliminary results show that the mech. properties of PLA and flax fiber composites are promising. The composite strength is about 50% better compared to similar PP/flax fiber composites, which are used today in many automotive panels. The addn. of plasticizer does not show any pos. effect on the impact strength of the composites. The study of interfacial adhesion shows that adhesion needs to be improved to optimize the mech. properties of the PLA/flax composites. The PLA/flax composites did not show any difficulties in the extrusion and compression molding processes and they can be processed in a similar way as PP based composites.

Pan, Pengju; Zhu, Bo; Kai, Weihua; Serizawa, Shin; Iji, Masatoshi; Inoue, Yoshio. Crystallization behavior and mechanical properties of bio-based green composites based on poly(L-lactide) and kenaf fiber. Bio-based polymer composite was successfully fabricated from plant-derived kenaf fiber (KF) and renewable resource-based biodegradable polyester, poly(L-lactide) (PLLA), by melt-mixing technique. The effect of the KF wt. contents (0, 10, 20, and 30 wt %) on crystn. behavior, composite morphol., mech., and dynamic mech. properties of PLLA/KF composites were investigated. It was found that the incorporation of KF significantly improves the crystn. rate and tensile and storage modulus. The crystn. of PLLA can be completed during the cooling process from the melt at 5°C/min with the addn. of 10 wt % KF. It was also obsd. that the nucleation d. increases dramatically and the spherulite size drops greatly in the isothermal crystn. with the presence of KF. In addn., with the incorporation of 30 wt % KF, the half times of isothermal crystn. at 120 °C and 140°C were reduced to 46.5% and 28.1% of the pure PLLA, resp. Moreover, the tensile and storage modulus of the composite are improved by 30% and 28%, resp., by the reinforcement with 30% KF. SEM observation also showed that the crystn. rate and mech. properties could be further improved by optimizing the interfacial interaction and compatibility between the KF and PLLA matrix. Overall, it was concluded that the KF could be the potential and promising filler for PLLA to produce biodegradable composite materials, owing to its good ability to improve the mech. properties as well as to accelerate the crystn. of PLLA. Serizawa, Shin; Inoue, Kazuhiko; Iji, Masatoshi. Kenaf-fiber-reinforced poly(lactic acid) used for electronic products. Journal of Applied Polymer Science (2006), 100(1), 618-624. High-performance biomass-based composites were fabricated with poly(lactic acid) (PLA) and kenaf fibers, which fixate CO2 efficiently. The composites show good thermal stability (distortion temp. under load) and modulus and the fibers enhance crystn. of PLA, to facilitate molding of the material. The effect of recycling on the composites was also assessed. Eliminating short particles from kenaf fibers resulted in improved impact strength of the composites. Kenaf fibers without particles are practically comparable to glass fibers. The mech. strength of the composites was further improved by using a plasticizer of a copolymer of lactic acid and an aliph. polyester. The composites (PLA/kenaf fiber and PLA/kenaf fiber/plasticizer) show good practical characteristics for use as housing materials of electronics, comparable to petroleum-based composites such as glass fiber - acrylonitrile-butadiene-styrene (ABS) resin.

Shibata, Mitsuhiro; Ozawa, Koichi; Teramoto, Naozumi; Yosomiya, Ryutoku; Takeishi, Hiroyuku. Biocomposites made from short abaca fiber and biodegradable polyesters. Macromolecular Materials and Engineering (2003), 288(1), 35-43. Natural fiber-reinforced biodegradable polyester composites were prepd. from biodegradable polyesters and surface-untreated or -treated abaca fibers (length .apprx.5 mm) by melt mixing and subsequent injection molding. Poly(butylene succinate)(PBS), polyester carbonate (PEC)/poly(lactic acid)(PLA) blend, and PLA were used as bio-degradable polyesters. Esterifications using acetic anhydride and butyric anhydride, alkali treatment, and cyanoethylation were performed as surface treatments on the fiber. The flexural moduli of all the fiber-reinforced composites increased with fiber content. The effect of the surface treatment on the flexural modulus of the fiber-reinforced composites was not so pronounced. The flexural strength of PBS composites increased with fiber content, and esterification of the fiber by butyric anhydride gave the best result. For the PEC/PLA composites, flexural strength increased slightly with increased fiber content (0-20 wt.-%) in the case of using untreated fiber, while it increased considerably in the case of using the fiber esterified by butyric anhydride. For the PLA composite, flexural strength did not increase with the fiber reinforcement. The result of soil-burial tests showed that the composites using untreated fiber have a higher wt. loss than both the neat resin and the composites made using acetylated fiber. Tanaka, Chiaki; Vu, Minh Duc; Okubo, Kazuya; Fujii, Toru. Development of green composites using micro-fibrillated bamboo fibers -application to inject molding. Bamboo Journal (2007), 24 17-26. Green composites using micro-fibrillated cellulose fibers extd. from bamboo (MFB) for injection molding were developed in conjunction with PLA. PLA has a function to give viscosity to the compds. while MFB assure the high mech. performance at not only room but also elevated temps. MFB was mixed with water based PLA since MFB contained a lot of water for handling. No micro/nano fibers can be utilized if dried MFB is powd. Less than 50% MFB was included in PLA based compds. and the bending and impact strengths were measured at room temp. The test results confirmed that injection molding was achieved when MFB wt. % content was 30% or lower at 170, 180 and 190°C when MFB was mixed with PLA in water. The bending strength and elastic modulus of MFB/PLA composites increased by 1.8 times and 1.5 times as high as the original PLA in the case of 30 wt% MFB. The impact strength of MFB/PLA composites evaluated by the Izod impact test was also improved in the case of 10 wt% MFB while it decreased in the case of 30 wt% MFB. Vink, Erwin T. H.; Rabago, R.; Glassner, David A.; Springs, Bob; O'Connor, Ryan P.; Kolstad, Jeff; Gruber, Patrick R. The sustainability of Nature Works polylactide polymers and Ingeo polylactide fibers: An update of the future. Macromolecular Bioscience (2004), 4(6), 551-564. A review on Cargill Dow company activities in the area of green chem. with focus on polylactide polymer chem. The following topics are discussed: NatureWorks polylactide prodn. process, today's and future products from NatureWorks polylactide, Ingeo fibers, other materials from lactic acid, the pursuit of sustainability, the "ideal" sustainable material, biomass utilization and biorefineries, poly(lactic acid) (PLA) life cycle inventory impacts of wind power renewable energy certificates to offset net (residual) greenhouse gases emissions, future price trends on PLA, waste management options, composting, chem. recycling, and what makes NatureWorks PLA a more sustainable polymer.

Wang, L.; Ma, W.; Gross, R. A.; McCarthy, S. P.Reactive compatibilization of biodegradable blends of poly(lactic acid) and poly(e -caprolactone). Polymer Degradation and Stability (1998), 59(1-3), 161-168. Poly(e-caprolactone) (PCL) was reactively blended with poly(lactic acid) (PLA) using three catalysts/coupling agents in a twin screw mixing chamber. Ph3PO3 showed the most promising results as a coupling or branching agent. PLA and PCL were also melt blended without any catalyst/coupling agents in order to make a comparison of the properties. The transesterification reaction was monitored by measuring torque values as a function of time. 1H NMR was used to characterize the structure of the reactive and phys. blend products. The blend samples were characterized for phys. properties and biodegrdn. Mech. property measurements indicated that the elongation of the PLA/PCL blends improved significantly when compared to pure PLA esp. for the reactively compatibilized blends, and that a synergism was obsd. for certain compns. (PLA/PCL = 80/20 or 20/80). Degrdn. studies showed that the enzymic degrdn. rates (or normalized wt. loss) of the reactively compatibilized blends were much higher than that of pure PLA and PCL, while the degrdn. rates of phys. blends are between those of pure PLA and PCL. Wong, S.; Shanks, R. A.; Hodzic, A. Effect of additives on the interfacial strength of poly(L-lactic acid) and poly(3-hydroxy butyric acid)-flax fiber composites. Composites Science and Technology (2007), 67(11-12), 2478-2484. The interfacial shear strength (IFSS), evaluated by single fiber pull-out tests was quantified for various biopolymer-flax fiber composites that were modified with additives. The additives included a plasticizer (glycerol triacetate) (GTA) absorbed onto/into the fibers, 4,4'-thiodiphenol (TDP) that is capable of forming hydrogen bonds between the matrix and cellulose from the fibers, and a hyperbranched polyester (HBP) to impart improved fracture toughness. Fibers were washed with acetone to remove the surface impurities and dried under vacuum before absorption of plasticizer and adsorption of thiodiphenol. It was found that the different additives significantly influenced the IFSS for the biopolymer-flax fiber systems while extn. with acetone had a no effect on the IFSS compared with the untreated fibers. The use of TDP imparted the most significant increase in IFSS while the HBP had an opposing effect. The use of ESEM corroborated with the findings of the single fiber pull-out tests. Yu, Long; Petinakis, Steven; Dean, Katherine; Bilyk, Alex; Wu, Dongyang. Green polymeric blends and composites from renewable resources. Macromolecular Symposia (2007), 249/250(Advanced Polymers for Emerging Technologies), 535-539. Currently new blends and composites are extending the utilization of polymers from renewable resources into new value-added products. This paper briefly discusses the development in this area, and introduces our research, in particular the starch-based nanocomposites, biodegradable polyester/starch blends and cellulose-reinforced PLA composites. It can be seen that hydrophilic character of natural polymers has contributed to the successful development of environmentally friendly composites, as most natural fibers and nanoclays are also hydrophilic in nature. On the other hand, hydrophobic property and moisture sensitivity of biodegradable polyesters are challenge to be reinforced by the natural materials.

Alessandro; Botaro, Vagner; Zeno, Elisa; Bach, Sylvie. Activation of solid polymer surfaces with bifunctional reagents. Polymer International (2001), 50(1), 7-9. An original procedure is described which calls upon the heterogeneous grafting of bifunctional mols. onto a reactive polymeric surface, leaving 1 of the functions available for further exploitation. The principle of this strategy is to use reagents characterized by a rigid planar structure bearing the 2 active moieties at opposite sites of the mol. The examples provided include the reactions of lignocellulosic fibers with 1,4-phenylene diisocyanate and 1,2,4,5-benzenetetracarboxylic anhydride, which give rise to their surface activation through the incorporation of covalently bound isocyanate or anhydride functions. Ammar, I.; Ben Cheikh, R.; Campos, A. R.; Cunha, A. M.; Campos, A. R. Injection molded composites of short alfa fibers and biodegradable blends. Polymer Composites (2006), 27(4), 341-348. Fully biodegradable composites made from two polymer blend matrixes (SEVA-C: starch and a copolymer of ethylene vinyl alc.; and SCA: starch and cellulose acetate) and short Alfa fibers were developed and processed by conventional injection molding into std. tensile specimens. For each kind of matrix, the influence of the reinforcement load was evaluated, using fiber amts. from 0 to 30% (wt/wt). An optimization study was carried out for the composite SEVA-C with 10% Alfa fiber. The obtained results establish that the produced biodegradable composites present a significant improvement in stiffness for both matrixes. Improvements in the tensile strength were obsd. only for the Alfa fiber reinforced SEVA-C. However, for both matrixes, the reinforcement causes a significant loss in the material ductility. Results from design of expts. (Hadamard plans) were used to explain the influence of the injection molding conditions on the mech. behavior of the obtained composites, mainly on the stiffness values. Bana, Ruchi; Banthia, A. K. Green Composites : Development of Poly(Vinyl Alcohol)-Wood Dust Composites. Polymer-Plastics Technology and Engineering (2007), 46(9), 821-829. As part of an ongoing research on biodegradable composites, which can be aptly termed as green composites, the present article reports on the incorporation of wood industry waste material, wood dust, as org. filler in film matrix based on Poly(Vinyl Alc.) as continuous phase. In this study as filler, dust of Piyasal wood was used. In order to improve the compatibility between wood dust and plastic material, different amts. of crosslinking agents, such as glutaraldehyde, were used and the effect of these on water absorption and biodegradability was studied. The as-synthesized PVA-Wood dust composite materials are typically characterized by Fourier-Transformation IR (FTIR) spectroscopy and, wide-angle x-ray diffraction. The mech. anal. of the composite material was studied by tensile test. The morphol. image of as-synthesized materials was studied by optical microscope (OM). Bledzki, A. K.; Reihmane, S.; Gassan, J. Properties and modification methods for vegetable fibers for natural fiber composites. Journal of Applied Polymer Science (1996), 59(8), 1329-36. A review with 45 refs. on the existing phys. and chem. modification methods, e.g., plasma treatment or graft copolymn., which are used to enhance adhesion of natural fibers, e.g., jute, flax, hemp, ramie, sisal, and coir, in polymer composites. Properties of the composites are also reviewed with respect to coupling methods and fiber. Modified cellulose fiber-polymer interaction mechanisms are complex and specific to every definite system. By using an coupling agent, like silanes or stearin acid, the Young's modulus and the tensile strength increases, dependent on the resin, until 50%. Simultaneously, the moisture absorption of the composites decreases for about 60%. With other surface modifications, similar results are obtained.

7.4) Referencias sobre la utilización de sustratos celulósicos (nativos, pretratados o sometidos a modificación química) para la producción de materiales compuestos

Buschle-Diller, G.; Zeronian, S. H. Enhancing the reactivity and strength of cotton fibers. Journal of Applied Polymer Science (1992), 45(6), 967-79. Both NaOH soln. of mercerizing strength and anhyd. methylamine (I) are suitable pretreatments for enhancing the reactivity of cotton cellulose. Favorable results are achieved by maintaining the fiber material in the never-dried state after the swelling treatment. Extn. by org. solvents is preferred over water-washing in order to remove the swelling agent. When cotton is swollen with either aq. NaOH or anhyd. I and then washed and dried, its crystallinity, as detd. by x-ray diffraction is not lowered as much as it is if it is acetylated to an acetyl content of .apprx.9% before drying. The greatest modifications of the crystal structure of cotton were on I treatment followed by chloroform and pyridine washing and acetylation in the never-dried state (MeCP product), as well as by alc. mercerization followed by ethanol and pyridine washing and acetylation in the never-dried state (AMEP). As detd. by moisture regains, no significant differences are apparent between the accessibility of samples of low acetyl content (ca. 9%) prepd. by either the AMEP or by the MeCP treatment. The DTA curves of I-treated cotton with an acetyl content close to that of com. diacetate and the com. product are dissimilar. It was concluded from the DTA curve of the deacetylated product prepd. from this MeCP sample that it has a highly disordered structure. The tensile properties of the acetylated products of low acetyl content are considerably improved if acetylation is preceded by mercerization with subsequent solvent exchange, and less so if it is preceded by I followed by solvent exchange. Incorporation of acetyl groups significantly enhances the breaking strength and extensibility of mercerized solvent-washed materials. Cassano, Roberta; Trombino, Sonia; Bloise, Ermelinda; Muzzalupo, Rita; Iemma, Francesca; Chidichimo, Giuseppe; Picci, Nevio. New Broom Fiber (Spartium junceum L.) Derivatives: Preparation and Characterization. Journal of Agricultural and Food Chemistry (2007), 55(23), 9489-9495. In the past decade interest in biopolymers has increased. Attempts were made to prep. new composite systems from biopolymers by binding different synthetic polymers to a biopolymer backbone. This paper reports the synthesis and characterization of derivatized broom fibers to prep. composites with either degradability or fireproofing properties. Synthetic strategies are described for the introduction of polymerizable functional groups or fluorine atoms on the glucose of cellulose chains of broom. The fibers contg. polymerizable groups were copolymd. with dimethylacrylamide and styrene and, after that, investigated by optical polarizing microscopy (OPM) and SEM anal. (SEM). The materials contg. fluorine were submitted to thermogravimetric anal. (TGA) and differential scanning calorimetry (DSC) for the purpose of verifying the fireproofing. Such derivatized biomaterials could be successfully used for applications in agriculture and in the packaging area.

Corrales, F.; Vilaseca, F.; Llop, M.; Girones, J.; Mendez, J. A.; Mutje, P. Chemical modification of jute fibers for the production of green - composites. Journal of Hazardous Materials (2007), 144(3), 730-735. Natural fiber reinforced composites is an emerging area in polymer science. Fibers derived from annual plants are considered a potential substitute for non-renewable synthetic fibers like glass and carbon fibers. The hydrophilic nature of natural fibers affects neg. its adhesion to hydrophobic polymeric matrixes. To improve the compatibility between both components a surface modification is proposed. The aim is the chem. modification of jute fibers using a fatty acid derivate (oleoyl chloride) to confer hydrophobicity and resistance to biofibers. This reaction was applied in swelling and non-swelling solvents, pyridine and dichloromethane, resp. The formation of ester groups, resulting from the reaction of oleoyl chloride with hydroxyl group of cellulose were studied by elemental anal. (EA) and FTIR spectroscopy. The characterization methods applied has proved the chem. interaction between the cellulosic material and the coupling agent. The extent of the reactions evaluated by elemental anal. was calcd. using 2 ratios. Electron microscopy was applied to evaluate the surface changes of cellulose fibers after modification process. Cunha, A. M.; Campos, A. R.; Cristovao, C.; Vila, C.; Santos, V.; Parajo, J. C..Sustainable materials in automotive applications. Plastics, Rubber and Composites (2006), 35(6/7), 233-241. The potential use of sustainable polymer based composites, obtained from renewable resources, is revised in terms of properties and envisaged applications. Using the typical specifications required within the automotive industry as ref., the potential of these composites is evaluated and some alternative routes for property improvement are discussed. Specific examples of the development of biodegradable polymeric composites are presented including the use of different types of matrixes, e.g. starch based blends and poly(lactic acid), and two types of natural reinforcements, i.e. fibers from pine and cellulose. This paper also presents and compares the mech. properties and morphologies obtained on injection molding composite parts made with different combinations of biodegradable matrixes and fiber reinforcements subjected to different phys. and chem. treatments. The damage caused to the fibers during the compounding and injection molding processing stages was studied tooGeorge, Jayamol; Sreekala, M. S.; Thomas, Sabu. A review on interface modification and characterization of natural fiber reinforced plastic composites. Polymer Engineering and Science (2001), 41(9), 1471-1485. A review on phys. and chem. treatment methods for improving the fiber-matrix interfacial adhesion in natural fiber-reinforced plastic composites.Gomes, Alexandre; Matsuo, Takanori; Goda, Koichi; Ohgi, Junji. Development and effect of alkali treatment on tensile properties of curaua fiber green composites. Composites, Part A: Applied Science and Manufacturing (2007), 38A(8), 1811-1820. This paper describes development and improvement of mech. properties of a so-called green composite that was fabricated by reinforcing a cornstarch-based biodegradable resin with high-strength natural fibers extd. from a plant named curaua. Two fabrication methods are proposed, in which stretched slivers of curaua fibers are prepd. as reinforcement to increase the composite strength. Moreover, highly concd. alkali treatment was applied to curaua fibers to improve mech. properties of green composites. Tensile test results showed that alkali-treated fiber composites increased in fracture strain twice to three times more than untreated fiber composites, without a considerable decrease in strength. This result proves that appropriate alkali treatment is a key technol. for improving mech. properties of cellulose-based fiber composites.

Gou, M.; Corrales, F.; Vilaseca, F.; Llop, M. F.; Mutje, P Chemical modification of cellulose in order to increase the wettability and adhesion in composites. Afinidad (2004), 61(513), 393-395. A two steps process was proposed to turn the hydrophilic character of surface cellulosic substrates into hydrophobic. Initially pure cellulose was chem. modified with methacrylic anhydride and subsequently coated with polystyrene by co-polymn. process. Although the first step yielded modified cellulose with hydrophobic surface character, the polymn. process increased that hydrophobic behavior. IR spectra, surface energy, contact angle, and enzymic degrdn. of the copolymer were tested. Hokens, D.; Mohanty, A. K.; Misra, M.; Drzal, L. T. Environment-friendly " green " biodegradable composites from natural fiber and cellulosic plastic. Polymer Preprints (American Chemical Society, Division of Polymer Chemistry) (2001), 42(2), 71-72. To produce biodegradable materials of com. value, an expensive biopolymer (cellulose acetate) was mixed with an inexpensive natural fiber (henequen leaf) as a filler and a biocomposite was fabricated using powder impregnation processing. Thermal and mech. properties of the resulting biocomposite were examd. Surface modification (5% aq. NaOH, 1 h) improved fiber-matrix adhesion and thus the performance of the biocomposite. Hu, Ruihua; Lim, Jae-Kyoo .Fabrication and mechanical properties of completely biodegradable hemp fiber reinforced polylactic acid composites. Journal of Composite Materials (2007), 41(13), 1655-1669 . Biodegradable composite materials can be produced by the combination of biodegradable polymers and natural fibers. In this study, a new biodegradable composite of hemp fiber reinforced polylactic acid (PLA) was fabricated using the hot press method. Mech. properties of composites with different fiber vol. fractions were tested. The optimum fiber content was detd. according to the test results. Effects of alkali treatment on the fiber surface morphol. and the mech. properties of the composites were investigated. Test results show that the composite with 40% vol. fraction of alkali treated fiber has the best mech. properties. The tensile strength, elastic modulus, and flexural strength of the composite with 40% treated fiber are 54.6 MPa, 8.5 Gpa, and 112.7 MPa resp., which are much higher than those of PLA alone. The composites have lower densities, which were measured to be from 1.19 g/cm3 to 1.25 g/cm3. Specific strengths were also calcd. Surface morphologies of fiber and fracture surfaces of the composites were obsd. using the SEM method. Ibrahim, N. A.; Yunus, W. M. Z. Wan; Abu-Ilaiwi, F. A. F.; Rahman, M. Z. A.; Ahmad, M. B.; Dahlan, K. Z. M. Optimized condition for grafting reaction of poly(butyl acrylate) onto oil palm empty fruit bunch fiber. Polymer International (2003), 52(7), 1119-1124. Prepn. of poly(Bu acrylate)-grafted oil palm empty fruit bunch fiber (OPEFB) has been successfully carried out using H2O2/Fe2+ as a redox initiator in aq. soln. The effects of reaction temp. and period, as well as the amt. of monomer, initiator and nitric acid, on the percentage of grafting were investigated. The percentage of grafting increases with amt. of monomer and can be controlled by setting the appropriate reaction conditions. The max. percentage of grafting (about 265%) was obtained when the reaction was carried out without acid under optimum conditions (reaction period: 2 h, reaction temp.: 55 °, amt. of H2O2: 5.88 mmol) with 42.2 mmol of monomer. Mechanisms of grafting of Bu acrylate onto OPEFB were proposed. Characterization of the grafted OPEFB was done by Fourier-transform IR spectroscopy and SEM. The thermal properties were studied by thermogravimetric anal.

Joseph, P. V.; Rabello, Marcelo S.; Mattoso, L. H. C.; Joseph, Kuruvilla; Thomas, Sabu. Environmental effects on the degradation behaviour of sisal fibre reinforced polypropylene composites. Composites Science and Technology (2002), 62(10-11), 1357-1372. The environmental degrdn. behavior on the phys. and mech. properties of short sisal/PP composites has been studied with special ref. to the influence of ageing conditions like treatment with water and UV radiation. The dependence of water uptake on the sorption characteristics of sisal/PP composites was evaluated by immersion in distd. water with respect to fiber loading, temp., and chem. treatment. It is found that water uptake increases with increased fiber loading due to the increase in cellulose content. The raising the temp. to 70°C is accompanied by an increase of the rate and extent of sorption. Chem. treatments such as TDI-PPG polyurethane, PAPI, and maleated PP were applied to sisal fiber. All chem. modified fiber composites showed lower uptake than the unmodified composites. This is because all chem. treatments reduce the fiber hydrophilicity thereby favoring strong interfacial adhesion between the fiber and the PP matrix. The influence of water uptake on the tensile properties of sisal/PP composites was studied. Tensile properties decreased with water uptake, time of immersion, and fiber loading. The behavior was strongly dependent on the chem. treatment and fiber orientation. The influence of the tensile properties of sisal/PP composites exposed to UV radiation was studied. The tensile properties were found to decrease with increasing irradn. time. The redn. in properties is due to chain scission and degrdn. occurring to PP mols. as a result of photooxidn. promoted by UV radiation. As a result of UV irradn., surface cracks can be seen on neat PP and PP composites which can be evidenced from SEM photos. The retention of tensile properties increased with increase of fiber loading. Karnani, Rajeev; Krishnan, Mohan; narayan, Ramani. Biofiber-reinforced polypropylene composites. Polymer Engineering and Science (1997), 37(2), 476-483. Biofibers, natural lignocellulosics, have an outstanding potential as a reinforcement in thermoplastics. This study deals with the prepn. of lignocellulosic composites by reactive extrusion processing in which good interfacial adhesion is generated by a combination of fiber modification and matrix modification methods. PP matrix was modified by reacting with maleic anhydride and subsequently bonded to the surface of the modified lignocellulosic component, in-situ. The fiber surface was modified by reacting it with a silane in a simple and quick aq. reaction system, similar to that employed for glass fibers. The modified fibers are then extruded with the modified polymer matrix to form the compatibilized composite. The various reactions between the lignocellulosic fiber/filler and modified polymer chains, is expected to improve the interfacial adhesion significantly as opposed to simple mixing of the two components, since new covalent bonds between the fiber surface and matrix are created in the former case. These composite blends were then injection molded for mech. characterization. Typical mech. tests on strength, toughness and Izod impact energy were performed and the results are reported. These findings are discussed in view of the improved adhesion resulting from reactions and enhanced polar interactions at phase boundaries.

Keller, Andreas. Compounding and mechanical properties of biodegradable hemp fibre composites. Composites Science and Technology (2003), 63(9), 1307-1316. Up to now the reinforcing potential of hemp fibers has not been exhausted, as the fibers are bundled and, therefore, a homogeneous distribution of fibers and matrix has not been possible. In the present study the fiber bundles used for the composites were degummed by means of biol. processes and steam explosion. The degummed fibers, sepd. into single cells, were integrated into the brittle poly (3-hydroxybutyrate-co-hydroxyvalerate) matrix and into the ductile co-polyester amide matrix by means of a co-rotating twin-screw extruder. Composites with a fiber vol. fraction of up to 42% could be achieved. The tensile strength of the ductile material was almost doubled by the reinforcement with 27% of fibers to 30 MPa, the E-modulus was quadrupled to 3.5 GPa. No improvement of the tensile strength of the brittle matrix could be achieved, whereas its E-modulus was increased up to 6 GPa. As the composite behavior is detd. by the matrix, the fiber reinforcement is accompanied by a redn. of the impact strength. Khan, Mubarak A.; Hinrichsen, G. Surface modification of jute and its influence on performance of biodegradable jute-fabric/Biopol composites. Composites Science and Technology (2000), 60(7), 1115-1124. Surface modifications were made of two varieties of jute fabrics, i.e. hessian cloth (HC) and carpet backing cloth (CBC), involving dewaxing, alkali treatment, cyanoethylation, and grafting, to est. their use as reinforcing agents in composites based on a biodegradable polymeric matrix, Biopol. The chem. treated fabrics are characterized by Fourier-transform IR spectroscopy and thermogravimetric anal. The effects of different fiber surface treatments and amts. of fabrics on the performance of the resulting composites are investigated. Mech. properties such as tensile strength, bending strength and impact strength increase in comparison to pure Biopol as a result of reinforcement with jute fabrics. More than 50% enhancement in tensile strength, 30% in bending strength and 90% in impact strength of the composites relative to pure Biopol sheets are obsd. under the present exptl. conditions. SEM investigations show that surface modifications improve the fiber/matrix adhesion. Degrdn. studies show that after 150 days of compost burial more than 50% wt. loss of the jute/Biopol composite occurs. Li, T. Q.; Li, R. K. Y. A fracture mechanics study of polypropylene-wood flours blends. Polymer-Plastics Technology and Engineering (2001), 40(1), 1-21. Two polypropylene resins, i.e., EPS30R and a reclaimed com. resin from recycled polypropylene bottles, were blended with 4 types of wood flour in a co-rotating twin-extruder and the extruded pellets were dried and injection molded into void-free plates. The effects of a maleated polypropylene additive, i.e., Epolene E-43, filler content and wood flour particle size on impact resistance were studied using notched Izod impact and Charpy tests. The notched Izod impact strength of composites with a relatively brittle resin matrix exceeded that of the neat resin at higher filler loading when the composites contained maleated polypropylene. The Izod strength of composites with a tough matrix increased with the content of the maleated polypropylene interfacial modifier and was higher in composites with coarser fillers. Fracture mechanics analyses of instrumented drop-wt. Charpy test results were performed to study the nature of these increases in impact fracture resistance. Both the fracture toughness, Kc, and the crit. strain release energy, Gc, increased with filler content in composites contg. maleated polypropylene. In composites without maleated polypropylene, however, Gc decreases slightly with filler content while Kc increases less significantly. The increases of Gc with maleated polypropylene and with increasing filler particle size were also obsd. for the composites with a tougher polymer as matrix.

Li, Y.; Mai, Y.-W.; Ye, L. Sisal fiber and its composites: a review of recent developments. Composites Science and Technology (2000), 60(11), 2037-2055. Sisal fiber is a promising reinforcement for use in composites on account of its low cost, low d., high specific strength and modulus, no health risk, easy availability in some countries and renewability. In recent years, there has been an increasing interest in finding new applications for sisal-fiber-reinforced composites that are traditionally used for making ropes, mats, carpets, fancy articles and others. The properties of sisal fiber itself, the interface between sisal fiber and matrix, and the properties of sisal-fiber-reinforced composites and their hybrid composites are discussed. Suggestions for future work are also presented. Madsen, Bo; Hoffmeyer, Preben; Thomsen, Anne Belinda; Lilholt, Hans.Hemp yarn reinforced composites - I. Yarn characteristics. Composites, Part A: Applied Science and Manufacturing (2007), 38A(10), 2194-2203. Publisher: Elsevier Ltd. The potential of plant fibers in structural materials components is explored by applying textile hemp yarn for fabrication of composites with aligned fibers. This first paper presents a detailed characterization of the hemp yarn. It is shown that the hemp yarn has a high cellulose content (about 90%), the fibers are evenly dispersed within the yarn with a mean twisting angle of about 11°, the moisture sorption capacity is relatively low (e.g. moisture content of about 8% at 65% RH), and the mech. properties are good (e.g. tensile ultimate stress of about 660 MPa). These findings show that textile hemp yarn is well suited for composite reinforcement. The accompanying second paper is addressing the mech. properties of the composites. Megiatto, Jackson D., Jr.; Hoareau, William; Gardrat, Christian; Frollini, Elisabete; Castellan, Alain. Sisal Fibers: Surface Chemical Modification Using Reagent Obtained from a Renewable Source; Characterization of Hemicellulose and Lignin as Model Study. Journal of Agricultural and Food Chemistry (2007), 55(21), 8576-8584. Sisal fibers have one of the greatest potentials among other lignocellulosic fibers to reinforce polymer matrixes in composites. Sisal fibers have been modified to improve their compatibility with phenolic polymer matrixes using furfuryl alc. (FA) and polyfurfuryl alcs. (PFA) that can be obtained from renewable sources. The modification corresponded first to oxidn. with ClO2, which reacts mainly with guaiacyl and syringyl units of lignin, generating o- and p-quinones and muconic derivs., followed by reaction with FA or PFA. The FA and PFA modified fibers presented a thin similar layer, indicating the polymer character of the coating. The untreated and treated sisal fibers were characterized by 13C CP-MAS NMR spectrometry, thermal anal., and SEM. Furthermore, for a better understanding of the reactions involved in the FA and PFA modifications, the sisal lignin previously extd. was also submitted to those reactions and characterized. The characterization of isolated lignin and hemicellulose provides some information on the chem. structure of the main constitutive macrocomponents of sisal fibers, such information being scarce in the literature.

Megiatto, Jackson D., Jr.; Oliveira, Francieli B.; Rosa, Derval S.; Gardrat, Christian; Castellan, Alain; Frollini, Elisabete. Renewable resources as reinforcement of polymeric matrices: composites based on phenolic thermosets and chemically modified sisal fibers. Macromolecular Bioscience (2007), 7(9-10), 1121-1131. Lignocellulosic materials can significantly contribute to the development of composites, since it is possible to chem. and/or phys. modify their main components, cellulose, hemi-celluloses and lignin. This may result in materials more stable and with more uniform properties. It has previously been shown that chem. modified sisal fibers by ClO2 oxidn. and reaction with FA and PFA presented a thin coating layer of PFA on their surface. FA and PFA were chosen as reagents because these alcs. can be obtained from renewable sources. In the present work, the effects of the polymeric coating layer as coupling agent in phenolic/sisal fibers composites were studied. For a more detailed characterization of the fibers, IGC was used to evaluate the changes that occurred at the sisal fibers surface after the chem. modifications. The dispersive and acid-base properties of untreated and treated sisal fibers surfaces were detd. Biodegrdn. expts. were also carried out. In a complementary study, another PFA modification was made on sisal fibers, using K2Cr2O7 as oxidizing agent. In this case the oxidn. effects involve mainly the cellulose polymer instead of lignin, as obsd. when the oxidn. was carried out with ClO2. The SEM images showed that the oxidn. of sisal fibers followed by reaction with FA or PFA favored the fiber/phenolic matrix interaction at the interface. However, because the fibers were partially degraded by the chem. treatment, the impact strength of the sisal-reinforced composites decreased. By contrast, the chem. modification of fibers led to an increase of the water diffusion coeff. and to a decrease of the water absorption of the composites reinforced with modified fibers. The latter property is very important for certain applications, such as in the automotive industry. Misra, M.; Tripathy, S. S.; Nayak, S. K.; Mohanty, A. K. Graft copolymerization of acrylonitrile on chemically modified sisal fibers. Macromolecular Materials and Engineering (2001), 286(2), 107-113. Graft copolymn. of acrylonitrile (AN) on chem. modified sisal fibers was studied using a combination of NaIO4 and CuSO4 as initiator in an aq. medium in the temp. range of 50-70°C. Effects of reaction medium, variation of time and temp., concn. of CuSO4, NaIO4 and AN, and the amt. of sisal fiber on the percentage of graft yield have been investigated. Water absorption (%) and tensile properties such as tensile strength, Young's modulus and extension at break of untreated, chem. modified and AN-grafted sisal fibers were evaluated and compared. FTIR spectroscopy and SEM of the chem. modified and AN-grafted sisal fibers have been carried out. Mohamed, Abdellatif A.; Finkenstadt, V. L.; Palmquist, Debra E. Thermal properties of extruded-injection molded poly (lactic acid) and milkweed composites. Proceedings of the NATAS Annual Conference on Thermal Analysis and Applications (2007), 35th 26#769/1-26#769/7. Currently, most polymer composites utilize petroleum-based materials that are non-degradable and difficult to recycle or incur substantial cost for disposal. Green composites can be used in nondurable limited applications. In order to det. the degree of compatibility between Poly (lactic Acid) (PLA) and different biomaterials, PLA was compounded with milkweed fiber. Milkweed is a new crop oil seed. After oil extn., the remaining cake retained approx. 10% residual oil and 47% protein. The pressed seed cake (10% moisture) was ground and passed through a 300 mm screen. The fiber was added at 85:15 and 70:30 PLA:Fiber. The composites were blended by extrusion (EX) followed by injection molding (EXIM).

Mohanty, Amar K.; Drzal, Lawrence T.; Ferguson, Ken J.; Dale, Bruce E.; Misra, Manjusri; Schalek, Richard.Explosion treatments of corn stalk fibers and their characterizations: A new look at value-added applications. 224th ACS National Meeting, Boston, MA, United States, August 18-22, 2002 (2002), POLY-411. As a part of our ongoing project on developments of corn stalk fibers, and corn stover (corn stalk with corn leaves) to find their high value-added applications in emerging bio-composites and green nanocomposites arena, this paper reports the thermal and morphol. characterizations of exploded corn stalk fibers. Interest in the use of green technol. in lignocellulosic biomass conversion to fuels and chems. has increased in recent years. One particularly attractive material that fits easily and naturally within bio-refinery based on lignocellulose, is microcryst. cellulose "nanowhiskers". Explosion treatment (ammonia fiber explosion, carbon dioxide explosion) of biomass is utilized with a long-range objective in designing microcryst. nanowhiskers. Various pretreatments like alkalization treatment at various pH conditions followed by explosion, with subsequent appropriate biochem. treatments are planned for future bio-based nanowhisker research. Pretreatment with alkali followed by explosion exhibit extensive defibrillation of corn fiber. Nam, Sunghyun; Netravali, Anil N. Green composites I. Physical properties of ramie fibers for environment-friendly green composites. Fibers and Polymers (2006), 7(4), 372-379. The surface topog., tensile properties, and thermal properties of ramie fibers were investigated as reinforcement for fully biodegradable and environmental-friendly 'green' composites. SEM micrographs of a longitudinal and cross-sectional view of a single ramie fiber showed a fibrillar structure and rough surface with irregular cross-section, which is considered to provide good interfacial adhesion with polymer resin in composites. An av. tensile strength, Young's modulus, and fracture strain of ramie fibers were measured to be 627 MPa, 31.8 GPa, and 2.7 %, resp. The specific tensile properties of the ramie fiber calcd. per unit d. were found to be comparable to those of E-glass fibers. Ramie fibers exhibited good thermal stability after aging up to 160°C with no decrease in tensile strength or Young's modulus. However, at temps. higher than 160°C the tensile strength decreased significantly and its fracture behavior was also affected. The moisture content of the ramie fiber was 9.9 %. These properties make ramie fibers suitable as reinforcement for 'green' composites. Also, the green composites can be fabricated at temps. up to 160 °C without reducing the fiber properties. Netravali, A. N. Interfacial and mechanical properties of environment-friendly "green" composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin. Journal of Materials Science (1999), 34(15), 3709-3719. Phys. and tensile properties of pineapple fibers were characterized. Tensile properties of pineapple fibers, like most natural fibers, showed a large variation. The av. interfacial shear strength between the pineapple fiber and poly(hydroxybutyrate-co-valerate) (PHBV) was 8.23 MPa as measured by the microbond technique. SEM photomicrographs of the microbond specimens revealed an adhesive failure of the interface. Fully degradable and environment-friendly "green" composites were prepd. by combining pineapple fibers and PHBV with 20 and 30% wt. content of fibers placed in a 0°/90°/0° fiber arrangement. Tensile and flexural properties of these "green" composites were compared with different types of wood specimens. Even though tensile and flexural strength and moduli of these "green" composites were lower than those of some wood specimens tested in grain direction, they were significantly higher than those of wood specimens tested in perpendicular to grain direction.

Nishino, Takashi; Hirao, Koichi; Kotera, Masaru; Nakamae, Katsuhiko; Inagaki, Hiroshi. Kenaf reinforced biodegradable composite. Composites Science and Technology (2003), 63(9), 1281-1286. Mech. properties of environmentally friendly composites made of kenaf fiber and poly-l-lactic acid (LACEA) resin were investigated. The Young's modulus (6.3 GPa) and tensile strength (62 MPa) of the composites (fiber content 70 vol.%) were comparable to those of conventional fiber composites. These properties were higher than those of the kenaf sheet and the LACEA film themselves. This is considered to attribute to the strong interaction between the fiber and LACEA. In addn., the storage modulus of the composite remain unchanged up to the LACEA m.p. The effects of the polymer mol. wt. and the fiber orientation on the mech. properties of the composite were also investigated. It was found that kenaf fiber can be a good candidate for the reinforcement fiber of high performance biodegradable polymer composites. Oksman, K.; Skrifvars, M.; Selin, J.-F. Natural fibres as reinforcement in polylactic acid (PLA) composites. Composites Science and Technology (2003), 63(9), 1317-1324. The focus in this work has been to study if natural fibers can be used as reinforcement in polymers based on renewable raw materials. The materials were flax fibers and poly(lactic acid) (PLA). PLA is a thermoplastic polymer made from lactic acid and has mainly been used for biodegradable products, such as plastic bags and planting cups, but in principle PLA can also be used as a matrix material in composites. Because of the brittle nature of PLA triacetin was tested as plasticizer for PLA and PLA/flax composites in order to improve the impact properties. The studied composite materials were manufd. with a twin-screw extruder having a flax fiber content of 30 and 40 wt.%. The extruded compd. was compression molded to test samples. The processing and material properties were studied and compared to the more commonly used polypropylene-flax fiber composites (PP/flax). Preliminary results show that the mech. properties of PLA and flax fiber composites are promising. The composite strength is about 50% better compared to similar PP/flax fiber composites, which are used today in many automotive panels. The addn. of plasticizer does not show any pos. effect on the impact strength of the composites. The study of interfacial adhesion shows that adhesion needs to be improved to optimize the mech. properties of the PLA/flax composites. The PLA/flax composites did not show any difficulties in the extrusion and compression molding processes and they can be processed in a similar way as PP based composites. Pan, Pengju; Zhu, Bo; Kai, Weihua; Serizawa, Shin; Iji, Masatoshi; Inoue, Yoshio. Crystallization behavior and mechanical properties of bio-based green composites based on poly(L-lactide) and kenaf fiber. Journal of Applied Polymer Science (2007), 105(3), 1511-1520. Bio-based polymer composite was successfully fabricated from plant-derived kenaf fiber (KF) and renewable resource-based biodegradable polyester, poly(L-lactide) (PLLA), by melt-mixing technique. The effect of the KF wt. contents (0, 10, 20, and 30 wt %) on crystn. behavior, composite morphol., mech., and dynamic mech. properties of PLLA/KF composites were investigated. It was found that the incorporation of KF significantly improves the crystn. rate and tensile and storage modulus. The crystn. of PLLA can be completed during the cooling process from the melt at 5°C/min with the addn. of 10 wt % KF. It was also obsd. that the nucleation d. increases dramatically and the spherulite size drops greatly in the isothermal crystn. with the presence of KF. In addn., with the incorporation of 30 wt % KF, the half times of isothermal crystn. at 120°C and 140°C were reduced to 46.5% and 28.1% of the pure PLLA, resp.

Pasquini, Daniel; Teixeira, Eliangela de Morais; Curvelo, Antonio Aprigio da Silva; Belgacem, Mohamed Naceur; Dufresne, Alain. Surface esterification of cellulose fibres: Processing and characterisation of low-density polyethylene/ cellulose fibres composites. Composites Science and Technology (2008), 68(1), 193-201. Low-d. polyethylene was filled with cellulose fibers from sugar cane bagasse obtained from organosolv/supercrit. carbon dioxide pulping process. The fibers were also used after chem. modification with octadecanoyl and dodecanoyl chloride acids. The morphol., thermal properties, mech. properties in both the linear and nonlinear range, and the water absorption behavior of ensuing composites were tested. The evidence of occurrence of the chem. modification was checked by X-ray photoelectron spectrometry. The degree of polymn. of the fibers and their intrinsic properties (zero tensile strength) were detd. It clearly appeared that the surface chem. modification of cellulose fibers resulted in improved interfacial adhesion with the matrix and higher dispersion level. However, composites did not show improved mech. performances when compared to unmodified fibers. This surprising result was ascribed to the strong lowering of the degree of polymn. of cellulose fibers (as confirmed by the drastic decrease of their zero tensile strength) after chem. treatment despite the mild conditions used. Rajan, Akhila; Abraham, T. Emilia. Coir fiber-process and opportunities: part 2. Journal of Natural Fibers (2007), 4(1), 1-11. Coir is a versatile lignocellulosic fiber obtained from coconut trees (Cocos nucifera) and is available in large quantities, in the order of 5 million tons a year globally. The best way to bring the existing coir industry to a higher level is the development of new value-added coir products. Coir-based composites, depending on their specific characteristics, could find a position within the wide scale of domestic and com. applications and products. Chem. prodn. of whiter coir fiber by the removal of lignin with sodium hydroxide and subsequent bleaching with acid produces a weak thin fiber having reduced strength and the treatment adversely affects the spinning properties. Biol. treatment to produce partially delignified, whiter fiber will be a better and milder alternative to this problem. Biosoftened coir fibers are spinnable and can be blended with natural fibers for producing furnishing fabrics, textiles, and so on. Sain, M. M.; Kokta, B. V.; Imbert, C. Structure-property relationships of wood fiber-filled polypropylene composite. Polymer-Plastics Technology and Engineering (1994), 33(1), 89-104. The effects of the addn. of interface modifiers on the structure and properties of wood fiber-filled isotactic polypropylene (I) are studied. Although an increase in the wood fiber loading reduces the overall cost of composites, the mech. properties of such composites are very poor in the absence of a suitable interface modifier. The poor mech. and thermal properties of the unmodified composite are attributed to increased heterogeneity induced in the system with increased concn. of wood fiber. A significant improvement in tensile strength and thermal resistance of these composites in the presence of a suitable interface modifier such as maleated I, itaconic anhydride, or bismaleimide-modified I is due to the improved dispersity of wood fibers in the I matrix as well as to the development of a stable interface effected by chem. interaction. In general, all modifiers affect the crystallinity of I in composites, and an improvement of tensile strength is marked by a lowering of the crystallinity of I.

Sanadi, Anand R.; Caulfield, Daniel F.; Jacobson, Rodney E.; Rowell, Roger M. Renewable Agricultural Fibers as Reinforcing Fillers in Plastics: Mechanical Properties of Kenaf Fiber-Polypropylene Composites. Industrial & Engineering Chemistry Research (1995), 34(5), 1889-96. Kenaf fibers and polypropylene (PP) were blended in a thermokinetic mixer and then injection molded, with the fiber wt. fractions varying to 60%. A maleated polypropylene was used to improve the interaction and adhesion between the nonpolar matrix and the polar lignocellulosic fibers. The specific tensile and flexural moduli of a 50 wt.% of kenaf-PP composite compare favorably with a 40 wt.% of glass fiber-PP injection-molded composite. Kenaf fibers are a viable alternative to inorg./mineral-based reinforcing fibers as long as the right processing conditions are used and they are used in applications where the higher water absorption is not crit. Shibata, Mitsuhiro; Ozawa, Koichi; Teramoto, Naozumi; Yosomiya, Ryutoku; Takeishi, Hiroyuku. Biocomposites made from short abaca fiber and biodegradable polyesters. Macromolecular Materials and Engineering (2003), 288(1), 35-43. Natural fiber-reinforced biodegradable polyester composites were prepd. from biodegradable polyesters and surface-untreated or -treated abaca fibers (length .apprx.5 mm) by melt mixing and subsequent injection molding. Poly(butylene succinate)(PBS), polyester carbonate (PEC)/poly(lactic acid)(PLA) blend, and PLA were used as bio-degradable polyesters. Esterifications using acetic anhydride and butyric anhydride, alkali treatment, and cyanoethylation were performed as surface treatments on the fiber. The flexural moduli of all the fiber-reinforced composites increased with fiber content. The effect of the surface treatment on the flexural modulus of the fiber-reinforced composites was not so pronounced. The flexural strength of PBS composites increased with fiber content, and esterification of the fiber by butyric anhydride gave the best result. For the PEC/PLA composites, flexural strength increased slightly with increased fiber content (0-20 wt.-%) in the case of using untreated fiber, while it increased considerably in the case of using the fiber esterified by butyric anhydride. For the PLA composite, flexural strength did not increase with the fiber reinforcement. The result of soil-burial tests showed that the composites using untreated fiber have a higher wt. loss than both the neat resin and the composites made using acetylated fiber. Vaca-Garcia, C.; Thiebaud, S.; Borredon, M. E.; Gozzelino, G. Cellulose esterification with fatty acids and acetic anhydride in lithium chloride/N,N-dimethylacetamide medium. Journal of the American Oil Chemists' Society (1998), 75(2), 315-319. Homogeneous esterification of cellulose (I) with satd. fatty acids (n-octanoic to n-octadecanoic) was accomplished with acetic anhydride co-reactant in LiCl/N,N-dimethylacetamide (LiCl/DMAc) medium. I mixed triesters (CMT) were obtained after 5 h at 130° with an av. of 2.2 acetyl groups and 0.8 fatty substituents per anhydrodroglucose unit. A mixed acetic-fatty anhydride, formed in situ, accounted for the grafting of the fatty moiety. The purified products were characterized and compared to the analogous I simple fatty triesters (CST) that were synthesized from fatty acid chlorides in pyridine medium. Dynamic contact angle with water, glass transition, and storage moduli were correlated with the length of the fatty substituents. The CMT proved to be highly hydrophobic and more mech. resistant than the CST.

Wambua, Paul; Ivens, Jan; Verpoest, Ignaas. Natural fibres: can they replace glass in fibre reinforced plastics? Composites Science and Technology (2003), 63(9), 1259-1264. In this work, natural fibers (sisal, kenaf, hemp, jute, and coir)-reinforced polypropylene composites were processed by compression molding using a film stacking method. The mech. properties of the different natural fiber composites were tested and compared. A further comparison was made with the corresponding properties of glass mat-reinforced polypropylene composites from the open literature. Kenaf, hemp, and sisal composites showed comparable tensile strength and modulus results but in impact properties hemp appears to out-perform kenaf. The tensile modulus, impact strength, and ultimate tensile stress of kenaf-reinforced polypropylene composites were found to increase with increasing fiber wt. fraction. Coir fiber composites displayed the lowest mech. properties, but their impact strength was higher than that of jute and kenaf composites. In most cases the specific properties of the natural fiber composites were found to compare favorably with those of glass. Wang, Z.; Xiao, H.; Sain, M. Poly (butyl acrylate)-modified cellulose fibres for toughening wood polymer composites. Society of Automotive Engineers, [Special Publication] SP (2007), SP-2115(Advances in Plastic Components, Processes and Techniques), 43-49. One of the key challenges of the wood polymer composites (WPC) is the inadequate toughness partly due to the incompatibility of the natural fibers and PP matrix. In this work, we performed the surface modification of the natural fiber by either in-situ grafting polymn. of Bu acrylate (PBA) or adsorbing matrix-compatible cationic PBA latex on the fiber surfaces. The results indicated that the mech. properties of the polypropylene (PP) composites contg. the modified fibers, unnotched Izod impact strength in particular, have been improved significantly. The influencing factors and the mechanism of toughening process have also been preliminarily investigated. Witt, Michael; Anderson, Ross; Pauly, Simon; Lee, Brendan. Effects of soaking and freezing on composites made from wood-based fillers and biodegradable plastics. Polymer Composites (2006), 27(4), 323-328. Biodegradable plastic composites were subjected to prolonged soaking and freezing treatments to assess the effects on the mech. performance. Radiata pine flour and thermomech. pulp fibers were used as fillers at various addn. levels in three different com. polymer matrixes. Two were bioderived, one oil-derived, each with different hydrophobicities. Depending on the nature of the biodegradable polymer matrix, the rates and extents of water uptake were found to be either enhanced or reduced by the wood-derived fillers. Although the higher aspect ratio of the pulp fibers improved mech. performance, relative to the wood flour, water uptake was also significantly enhanced in some cases.

Dornburg, Veronika; Lewandowski, Iris; Patel, Martin. Comparing the land requirements, energy savings, and greenhouse gas emissions reduction of biobased polymers and bioenergy. An analysis and system extension of life-cycle assessment studies. Journal of Industrial Ecology (2004), Volume Date 2003, 7(3-4), 93-116. This study compares energy savings and greenhouse gas (GHG) emission redns. of biobased polymers with those of bioenergy on a per unit of agricultural land-use basis by extending existing life-cycle assessment (LCA) studies. In view of policy goals to increase the energy supply from biomass and current efforts to produce biobased polymers in bulk, the amt. of available land for the prodn. of nonfood crops could become a limitation. Hence, given the prominence of energy and greenhouse issues in current environmental policy, it is desirable to include land demand in the comparison of different biomass options. Over the past few years, numerous LCA studies have been prepd. for different types of biobased polymers, but only a few of these studies address the aspect of land use. This comparison shows that referring energy savings and GHG emission redn. of biobased polymers to a unit of agricultural land, instead of to a unit of polymer produced, leads to a different ranking of options. If land use is chosen as the basis of comparison, natural fiber composites and thermoplastic starch score better than bioenergy prodn. from energy crops, whereas polylactides score comparably well and polyhydroxyalkaonates score worse. Addnl., including the use of agricultural residues for energy purposes improves the environmental performance of biobased polymers significantly. Moreover, it is very likely that higher prodn. efficiencies will be achieved for biobased polymers in the medium term. Biobased polymers thus offer interesting opportunities to reduce the utilization of nonrenewable energy and to contribute to GHG mitigation in view of potentially scarce land resources.Dornburg, Veronika; Lewandowski, Iris; Patel, Martin. Comparing the land requirements, energy savings, and greenhouse gas emissions reduction of biobased polymers and bioenergy. An analysis and system extension of life-cycle assessment studies. Journal of Industrial Ecology (2004), Volume Date 2003, 7(3-4), 93-116. This study compares energy savings and greenhouse gas (GHG) emission redns. of biobased polymers with those of bioenergy on a per unit of agricultural land-use basis by extending existing life-cycle assessment (LCA) studies. In view of policy goals to increase the energy supply from biomass and current efforts to produce biobased polymers in bulk, the amt. of available land for the prodn. of nonfood crops could become a limitation. Hence, given the prominence of energy and greenhouse issues in current environmental policy, it is desirable to include land demand in the comparison of different biomass options. Over the past few years, numerous LCA studies have been prepd. for different types of biobased polymers, but only a few of these studies address the aspect of land use. This comparison shows that referring energy savings and GHG emission redn. of biobased polymers to a unit of agricultural land, instead of to a unit of polymer produced, leads to a different ranking of options. If land use is chosen as the basis of comparison, natural fiber composites and thermoplastic starch score better than bioenergy prodn. from energy crops, whereas polylactides score comparably well and polyhydroxyalkaonates score worse. Addnl., including the use of agricultural residues for energy purposes improves the environmental performance of biobased polymers significantly.

7.5) Referencias sobre la biodegradabilidad/compostaje/análisis de ciclo de vida de matrices plásticas y materiales compuestos

Gattin, Richard; Copinet, Alain; Bertrand, Celine; Couturier, Yves. Biodegradation study of a starch and poly(lactic acid) co-extruded material in liquid, composting and inert mineral media. International Biodeterioration & Biodegradation (2002), 50(1), 25-31. Biodegrdn. of a co-extruded starch/poly(lactic acid) polymeric film was studied in liq., inert solid, and composting media. Main mech. properties of this film were Young's modulus: 2340 MPa; elongation at break: 50%; and contact angle: 118°. Mineralization of the material C content was followed using appropriate exptl. methods of the International Std. Organization. Whatever the biodegrdn. medium used, the percentage of mineralization was better than the required 60% value for definition of a biodegradable material. Moreover, re-partitioning of material C between various degrdn. products was quantified throughout the duration of exptl. runs. The presence of starch facilitated biodegrdn. of the polylactic component, esp. in liq. media.Gerngross, T. U.; Slater ,S. C. How Green are Green Plastics? Scientific American Aug00. Summary not available. Full article available at http://www.mindfully.org/Plastic/Biodegrade/Green-PlasticsAug00.htmHu, Ruihua; Lim, Jae-Kyoo .Fabrication and mechanical properties of completely biodegradable hemp fiber reinforced polylactic acid composites. Journal of Composite Materials (2007), 41(13), 1655-1669. Biodegradable composite materials can be produced by the combination of biodegradable polymers and natural fibers. In this study, a new biodegradable composite of hemp fiber reinforced polylactic acid (PLA) was fabricated using the hot press method. Mech. properties of composites with different fiber vol. fractions were tested. The optimum fiber content was detd. according to the test results. Effects of alkali treatment on the fiber surface morphol. and the mech. properties of the composites were investigated. Test results show that the composite with 40% vol. fraction of alkali treated fiber has the best mech. properties. The tensile strength, elastic modulus, and flexural strength of the composite with 40% treated fiber are 54.6 MPa, 8.5 Gpa, and 112.7 MPa resp., which are much higher than those of PLA alone. The composites have lower densities, which were measured to be from 1.19 g/cm3 to 1.25 g/cm3. Specific strengths were also calcd. Surface morphologies of fiber and fracture surfaces of the composites were obsd. using the SEM method. Joshi, S. V.; Drzal, L. T.; Mohanty, A. K.; Arora, S. Are natural fiber composites environmentally superior to glass fiber reinforced composites. Composites, Part A: Applied Science and Manufacturing (2004), 35A(3), 371-376. A review of comparative life cycle assessment studies of plant fiber- and glass fiber-plastic composites. The key drivers of their relative environmental performance are identified. Plant fiber composites are likely to be environmentally superior to glass fiber composites in most cases for the following reasons: (1) plant fiber prodn. has lower environmental impacts compared to glass fiber prodn.; (2) plant fiber composites have higher fiber content for equiv. performance, reducing more polluting base polymer content; (3) the light-wt. plant fiber composites improve fuel efficiency and reduce emissions in the use phase of the component in auto applications; and (4) end of life incineration of plant fibers results in recovered energy and carbon credits.

Khan, Mubarak A.; Hinrichsen, G. Surface modification of jute and its influence on performance of biodegradable jute-fabric/Biopol composites. Composites Science and Technology (2000), 60(7), 1115-1124 . Surface modifications were made of two varieties of jute fabrics, i.e. hessian cloth (HC) and carpet backing cloth (CBC), involving dewaxing, alkali treatment, cyanoethylation, and grafting, to est. their use as reinforcing agents in composites based on a biodegradable polymeric matrix, Biopol. The chem. treated fabrics are characterized by Fourier-transform IR spectroscopy and thermogravimetric anal. The effects of different fiber surface treatments and amts. of fabrics on the performance of the resulting composites are investigated. Mech. properties such as tensile strength, bending strength and impact strength increase in comparison to pure Biopol as a result of reinforcement with jute fabrics. More than 50% enhancement in tensile strength, 30% in bending strength and 90% in impact strength of the composites relative to pure Biopol sheets are obsd. under the present exptl. conditions. Patel, Martin; Bastioli, Catia; Marini, Luigi; Wuerdinger, Eduard.Life-cycle assessment of bio-based polymers and natural fiber composites. Biopolymers (2003), 10 409-452. A review. Results from life-cycle assessment studies for the com. most important bio-based polymeric materials, including starch polymers, polyhydroxyalkanoates, polylactides, lignin-epoxy resins, epoxidized linseed oil, and composites reinforced with natural fibers such as flax, hemp and china reed are discussed. The products include primary plastic materials, loose-fill packaging material, films, bags, mulch films, printed circuit boards, thickeners for lacquers, panels for automobiles, and transport pallets. Pietrini, Matteo; Roes, Lex; Patel, Martin K.; Chiellini, Emo.Comparative Life Cycle Studies on Poly(3-hydroxybutyrate)-Based Composites as Potential Replacement for Conventional Petrochemical Plastics Biomacromolecules (2007), 8(7), 2210-2218. A cradle-to-grave environmental life cycle assessment (LCA) of a few poly(3-hydroxybutyrate) (PHB) based composites was performed and was compared to commodity petrochem. polymers. The end products studied are a cathode ray tube (CRT) monitor housing (conventionally produced from high-impact polystyrene, HIPS) and the internal panels of an av. car (conventionally produced from glass-fibers-filled polypropylene, PP-GF). The environmental impact is evaluated on the basis of nonrenewable energy use (NREU) and global warming potential over a 100 years time horizon (GWP100). Sugar cane bagasse (SCB) and nanoscaled organophilic montmorillonite (OMMT) are used as PHB fillers. The results obtained show that, despite the unsatisfying mech. properties of PHB composites, depending on the type of filler and on the product, it is possible to reach lower environmental impacts than by use of conventional petrochem. polymers. Tsuji, Hideto.Polylactides. Biopolymers (2002), 4 129-177. A review on the basic aspects of synthesis, processing, structures, phys. properties, hydrolysis and biodegrdn. of poly(lactic acids) (PLAs). The phys. properties and hydrolysis and biodegrdn. behavior of PLAs are controllable by varying mol. characteristics, highly ordered morphol., and material shapes, and also by polymer blending. The most appropriate biodegradable polymer for the resp. end uses can be selected from the PLA based polymeric materials having different phys. properties, enzymic and non-enzymic hydrolytic behavior, and cost/performance. PLAs were regarded as biomedical materials and matrixes in the forms of rods, plates, films, meshes, and microspheres for tissue engineering and drug delivery systems.