Biocomposites reinforced-with-natural-fibers-2000-2010 2012-progress-in-polymer-science

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Progress in Polymer Science 37 (2012) 1552– 1596

Contents lists available at SciVerse ScienceDirect

Progress in Polymer Science

j ourna l ho me p ag e: www.elsev ier .com/ locate /ppolysc i

iocomposites reinforced with natural fibers: 2000–2010

mar Faruka,d,∗, Andrzej K. Bledzkia,c, Hans-Peter Finkb, Mohini Saind

Institut für Werkstofftechnik, Kunststoff- und Recyclingtechnik, University of Kassel, Moenchebergstrasse 3, D-34109 Kassel, GermanyFraunhofer Institute for Applied Polymer Research, Geiselbergstrasse 69, D-14476 Potsdam-Golm, GermanyWest Pomeranian University of Technology, Institute of Material Science and Engineering, 19 Piastow Ave, 70310 Szczecin, PolandCentre for Biocomposites and Biomaterials Processing, University of Toronto, 33 Willcocks Street, Toronto, ON M5S3B3, Canada

r t i c l e i n f o

rticle history:eceived 3 August 2011eceived in revised form 19 April 2012ccepted 25 April 2012vailable online 2 May 2012

eywords:iocompositesatural fiberiopolymeranofibercetylationlkylationilanealeated coupling

nzyme treatmenthermoplastichermosetsnjection moldingompression moldingxtrusionultrusionesin transfer moldingheet molding compoundFT-D methodensile

a b s t r a c t

Due to environment and sustainability issues, this century has witnessed remarkableachievements in green technology in the field of materials science through the devel-opment of biocomposites. The development of high-performance materials made fromnatural resources is increasing worldwide. The greatest challenge in working with naturalfiber reinforced plastic composites is their large variation in properties and characteristics.A biocomposite’s properties are influenced by a number of variables, including the fibertype, environmental conditions (where the plant fibers are sourced), processing meth-ods, and any modification of the fiber. It is also known that recently there has been asurge of interest in the industrial applications of composites containing biofibers reinforcedwith biopolymers. Biopolymers have seen a tremendous increase in use as a matrix forbiofiber reinforced composites. A comprehensive review of literature (from 2000 to 2010)on the mostly readily utilized natural fibers and biopolymers is presented in this paper.The overall characteristics of reinforcing fibers used in biocomposites, including source,type, structure, composition, as well as mechanical properties, will be reviewed. More-over, the modification methods; physical (corona and plasma treatment) and chemical(silane, alkaline, acetylation, maleated coupling, and enzyme treatment) will be discussed.The most popular matrices in biofiber reinforced composites based on petrochemical andrenewable resources will also be addressed. The wide variety of biocomposite process-ing techniques as well as the factors (moisture content, fiber type and content, couplingagents and their influence on composites properties) affecting these processes will be dis-cussed. Prior to the processing of biocomposites, semi-finished product manufacturing isalso vital, which will be illustrated. Processing technologies for biofiber reinforced compos-ites will be discussed based on thermoplastic matrices (compression molding, extrusion,injection molding, LFT-D-method, and thermoforming), and thermosets (resin transfer
lexural
mpact molding, sheet molding compound). Other implemented processes, i.e., thermoset com-pression molding and pultrusion and their influence on mechanical performance (tensile,flexural and impact properties) will also be evaluated. Finally, the review will conclude withrecent developments and future trends of biocomposites as well as key issues that need tobe addressed and resolved.

Crown

∗ Corresponding author at: Centre for Biocomposites and Biomaterials Processanada. Tel.: +1 416 9781615; fax: +1 416 9783834.

E-mail address: [email protected] (O. Faruk).

079-6700/$ – see front matter. Crown Copyright © 2012 Published by Elsevier Lttp://dx.doi.org/10.1016/j.progpolymsci.2012.04.003

Copyright © 2012 Published by Elsevier Ltd. All rights reserved.

ing, University of Toronto, 33 Willcocks Street, Toronto, ON M5S3B3,

td. All rights reserved.

dx.doi.org/10.1016/j.progpolymsci.2012.04.003

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http://www.elsevier.com/locate/ppolysci

mailto:[email protected]

dx.doi.org/10.1016/j.progpolymsci.2012.04.003

O. Faruk et al. / Progress in Polymer Science 37 (2012) 1552– 1596 1553

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15542. Reinforcing fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1554

2.1. Fiber source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15542.2. Fiber types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1554

2.2.1. Flax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15542.2.2. Hemp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15552.2.3. Jute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15552.2.4. Kenaf. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15562.2.5. Sisal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15562.2.6. Abaca . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15572.2.7. Pineapple leaf fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15582.2.8. Ramie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15582.2.9. Coir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15582.2.10. Bamboo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15582.2.11. Rice husk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15592.2.12. Oil palm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15592.2.13. Bagasse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1559

2.3. Structure and chemical composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15592.4. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15602.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562

3. Modification of natural fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15623.1. Physical method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562

3.1.1. Corona treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15623.1.2. Plasma treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562

3.2. Chemical method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15633.2.1. Silane treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15633.2.2. Alkaline treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15643.2.3. Acetylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15653.2.4. Maleated coupling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15653.2.5. Enzyme treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1567

3.3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15674. Matrices for biocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1567

4.1. Petrochemical based . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15674.1.1. Thermoplastic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15684.1.2. Thermosets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1571

4.2. Bio-based . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15714.2.1. PLA (polylactide acid) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15714.2.2. PHB (polyhydroxybutyrate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15724.2.3. Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15744.2.4. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1574

4.3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15745. Processing techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1574

5.1. Factors influencing processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15745.1.1. Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15745.1.2. Fiber type and content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15755.1.3. Influence on composites properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1575

5.2. Semi-finished product manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15765.3. Processing technologies for natural fiber reinforced thermoplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1576

5.3.1. Compression molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15765.3.2. Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15765.3.3. Injection molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15765.3.4. Long fiber thermoplastic-direct (LFT-D) method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1577

5.4. Processing technologies for natural fiber reinforced thermosets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15785.4.1. Resin transfer molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15785.4.2. Sheet molding compound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1578

5.5. Other processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15795.5.1. Thermosets compression molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15795.5.2. Pultrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15795.5.3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1579

6. Performance of biocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1580
6.1. Tensile properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.2. Flexural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.3. Impact properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1580 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1581

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1581

1554 O. Faruk et al. / Progress in Polymer Science 37 (2012) 1552– 1596

6.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15827. Development and future trends of biocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15828. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . .

1

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used in the higher value-added textile markets. Nowadays,it is widely used in the composites area.

The static and dynamic mechanical properties of non-woven based flax fiber reinforced PP composites were

Table 1Commercially major fiber sources.

Fiber source World production (103 ton)

Bamboo 30,000Jute 2300Kenaf 970Flax 830Sisal 378Hemp 214Coir 100Ramie 100

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

There is a growing trend to use biofibers as fillers and/oreinforcers in plastics composites. Their flexibility duringrocessing, highly specific stiffness, and low cost (on a vol-metric basis) make them attractive to manufacturers. Thisentury has witnessed ever-increasing demands for the uti-ization of plastics as important raw materials, more than0% of which are thermoplastics. Biofiber reinforced plas-ic composites are gaining more and more acceptance intructural applications.

Technological development connected with consumeremands and expectations continues to increase demandsn global resources, leading to major issues of materialvailability and environmental sustainability. Over the lastew decades biofiber composites have been undergoing aemarkable transformation. These materials have becomeore and more sufficient as new compositions and pro-

esses have been intensively researched, developed andonsequently applied. The petroleum crisis made biocom-osites significantly important and biocomposites haveecome engineering materials with a very wide range ofroperties. However, like all materials, they are constantlynder competitive pressure from the global market, which

n turn, necessitates continuous research. The times ofimply mixing plastics with natural waste fillers and char-cterizing their main properties are gone.

The concept of using bio-based plastics as reinforcedatrices for biocomposites is gaining more and more

pproval day by day. The developments in emerging bio-ased plastics are spectacular from a technological point ofiew and mirror their rapid growth in the market place. Theverage annual growth rate globally was 38% from 2003 to007. In the same period, the annual growth rate was asigh as 48% in Europe. The worldwide capacity of bio-basedlastics is expected to increase from 0.36 million metricon (2007) to 2.33 million metric ton by 2013 and to 3.45

illion metric ton in 2020. The main product in terms ofroduction volumes will be starch-based plastics, PLA andHA [1].

The increasing number of publications during the recentears including reviews [2–9] and books [10–14] reflect therowing importance of these new biocomposites. Bledzkind Gassan have reviewed the reinforcement of the mosteadily used natural fibers in polymer composites up until999 in their review paper [15]. This paper aims to reviewore current reinforcement of natural fibers in polymer

omposites from 2000 to 2010. This paper will not addressatural fibers from animals (e.g., silk or wool) or cottonr man-made cellulosic fibers. This review also excludesood fiber or flour. Given the broadscope of this article, it
ill inevitably be incomplete, but will hopefully provide
sensible overview of the most popular natural fibers inolymeric composite materials in the last 11 years.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1584

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1584

2. Reinforcing fibers

An increased awareness that non-renewable resourcesare becoming scarce and our inevitable dependence onrenewable resources has arisen. This century could becalled the cellulosic century, because more and morerenewable plant resources for products are being discov-ered. It has been generally stated that natural fibers arerenewable and sustainable, but they are in fact, neither. Theliving plants are renewable and sustainable from which thenatural fibers are taken, but not the fibers themselves.

2.1. Fiber source

The plants, which produce natural fibers, are classifiedas primary and secondary depending on their utilization.Primary plants are those grown for their fiber content whilesecondary plants are plants in which the fibers are pro-duced as a by-product. Jute, hemp, kenaf, and sisal areexamples of primary plants. Pineapple, oil palm and coirare examples of secondary plants. Table 1 shows the mainfibers used commercially in composites, which are nowproduced throughout the world [16].

2.2. Fiber types

There are six basic types of natural fibers. They are clas-sified as follows: bast fibers (jute, flax, hemp, ramie andkenaf), leaf fibers (abaca, sisal and pineapple), seed fibers(coir, cotton and kapok), core fibers (kenaf, hemp and jute),grass and reed fibers (wheat, corn and rice) and all othertypes (wood and roots).

2.2.1. FlaxFlax, Linum usitatissimum, belongs to the bast fibers. It

is grown in temperate regions and is one of the oldest fibercrops in the world. The bast fiber flax is most frequently

Abaca 70Sugar cane bagasse 75,000Grass 700

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O. Faruk et al. / Progress in Po
studied considering the effect of zein coupling agent [17].Zein is a protein, extracted from corn and used as solution.Composites containing zein coupling agent were foundto possess improved mechanical properties. The storagemodulus of composites was found to increase with zeincoupling agent coating due to enhanced interfacial adhe-sion.

The tensile mechanical properties of flax fibers are esti-mated according to their diameter and their location inthe stems [18]. The large scattering of these properties isascribed to the variation of the fiber size along its longitu-dinal axis. The higher values of the mechanical propertiesof the fibers issued from the middle of the stems are associ-ated with the chemical composition of their cell walls. Themechanical properties of unidirectional flax fiber/epoxymatrix composites are studied as a function of their fibercontent. The properties of the composites are lower thanthose expected from single fiber characteristics.

Various investigations of flax fiber/polypropylene com-posites have been completed. These studies focus on manydifferent variables, including: comparison between NMT(natural fiber thermoplastic mat) and GMT (glass fiberthermoplastic mat) [19], the influence of fiber/matrix mod-ification and glass fiber hybridization [20], the effect of fibertreatment on thermal and crystallization properties [21],the influence of surface treatment on interface by glyceroltriacetate, thermoplastic starch, �-methacryl oxypropyltrimethoxy-silane and boiled flax yarn [22], comparison ofmatrices (PP and PLA) on the composite properties [23],the effects of material and processing parameters [24], andthe influence of processing methods [25]. Buttler [26] pre-sented the feasibility of using flax fiber composites in thecoachwork and bus industry.

The effect of bio-technical fiber modification [27], frac-ture behavior and toughness [28], the influence of alkalinefiber treatment on unidirectional composites [29], theeffect of processing parameters [30] on the consecutivedecortication stages of flax fibers (retting, scutching, andhackling) on the flax fiber reinforced epoxy compositeshave also been evaluated.

Flax fiber composites reinforced with polyester resinhave been evaluated for thermal degradation and fireresistance [31], chemical treatments on surface propertiesand adhesion [32], and the effect of chemical treat-ments on water absorption and mechanical properties [33].Three resins from soybean oil (methacrylated soybean oil,methacrylic anhydride modified soybean oil, and aceticanhydride modified soybean oil) were used also as matricesfor flax fiber reinforced biocomposites [34].

2.2.2. HempAnother notable bast fiber crop is hemp, which belongs

to the Cannabis family. It is an annual plant that growsin temperate climates. Hemp is currently the subject ofa European Union subsidy for non-food agriculture, anda considerable initiative in currently underway for theirfurther development in Europe.

Composites of PP with hemp fibers, which were func-tionalized by means of melt grafting reactions with glycidylmethacrylate (GMA) and prepared by batch mixing, wereexamined [35]. The modification of fibers and the PP matrix,

cience 37 (2012) 1552– 1596 1555

as well as the addition of various compatibilizers werecarried out to improve the fiber–matrix interactions. Com-pared to the unmodified system, a modified compositeshowed improved fiber dispersion in the PP matrix andhigher interfacial adhesion as a consequence of chemicalbonding between the fiber and the polymer (PP/Hemp).The thermal stability and phase behavior of the compositeswas largely affected by the fiber and matrix modification.Changes in the spherulithic morphology and crystallizationbehavior of PP were observed in the composites due to thenucleating effect of the hemp fibers. Moreover, a markedincrease in the PP isothermal crystallization rate (in therange 120–138 ◦C) was recorded with increasing content ofmodified hemp. All composites displayed a higher tensilemodulus (about 2.9 GPa) and lower elongation at break ascompared to plain PP; compatibilization with modified PP(10 phr) resulted in an increased stiffness of the compositesas a result of improved fiber–matrix interfacial adhesion.

Pickering and co-workers [36–38] investigated theeffects of chelator, white rot fungi, and enzyme treatmentson the separation of hemp fibers from bundles, and theimprovement of the interfacial bonding of the hemp fiberswith the PP matrix. The obtained results showed that theinterfacial shear strength of the treated fiber compositeswas higher than that for untreated fiber composites. Thisverifies that the hemp fiber interfacial bonding with PP wasimproved by the white rot fungi treatment. Compositesconsisting of hemp fibers treated with chelator concentratehad the highest tensile strength of 42 MPa, which equalsan increase of 19% compared to composites with untreatedhemp fiber.

Hemp fiber reinforced PP composites exhibited interest-ing recyclability [39]. The obtained results prove that themechanical properties of hemp fiber/PP composites remainwell preserved, despite the number of reprocessing cycles.The Newtonian viscosity decreases with cycles, indicating adecrease in molecular weight and chain scissions inducedby reprocessing. The decrease of fiber length with repro-cessing could be another reason for decrease of viscosity.

Epoxy resins, which were used as a matrix for hempfiber reinforced composites, were studied regarding theeffect of fiber architecture on the falling weight impactproperties [40], properties and performances of compos-ites for curved pipes [41], impact load performance of resintransfer molded composites [42], micro-mechanics of thecomposites [43], the influence of hybrid blends made ofsoybean oil and nanoclay [44], and the usefulness of unret-ted hemp as a source of fiber for biocomposites [45].

Kunanopparat et al. [46,47] studied the feasibility ofwheat gluten as a matrix for hemp fiber reinforced compos-ites regarding their thermal treatment and plasticizationeffect on the mechanical properties.

2.2.3. JuteJute is produced from plants of the genus Corchorus,

which includes about 100 species. It is one of the cheap-est natural fibers and is currently the bast fiber with the
highest production volume. Bangladesh, India and Chinaprovide the best condition for the growth of jute.
Ray and co-workers [48–50] extensively investigatedalkali treated jute fiber reinforced with vinyl ester resin.

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556 O. Faruk et al. / Progress in Po

n their studies, they regarded the dynamic, mechanical,hermal, and impact fatigue behavior compared to that ofntreated jute fiber–vinyl ester composites. Longer alkalireatment removed the hemicelluloses and improved therystallinity, enabling better fiber dispersion. The dynamic,echanical, thermal and impact properties were superior

wing to the alkali treatment, comprising treatment time,oncentration and conditions.

Mohanty et al. [51] investigated effects and influencef surface modification on the mechanical and biodegrad-bility of jute/Biopol and jute/PA (Poly Amide) composites.nhancements in tensile strength of more than 50%, 30%n bending strength and 90% in impact strength werebserved in the composites and are comparative to val-es achieved for pure Biopol sheets. Degradation studieshowed that after 150 days of compost burial more than0% weight loss of the jute/Biopol composites occurs.

The effects of hybridization [52] on the tensile prop-rties of jute–cotton woven fabric reinforced polyesteromposites were investigated as functions of the fiberontent, orientation and roving texture. It was observedhat tensile properties along the direction of jute rovinglignment (transverse to cotton roving alignment) increaseteadily with fiber content up to 50% and then show a ten-ency to decrease. The tensile strength of composites with0% fiber content parallel to the jute roving is about 220%igher than pure polyester resin.

Jute fiber reinforced PP composites were evaluatedegarding the effect of matrix modification [53], the influ-nce of gamma radiation [54], the effect of interfacialdhesion on creep and dynamic mechanical behavior [55],he influence of silane coupling agent [56,57], and the effectf natural rubber [58].

The properties of jute/plastic composites were studied,ncluding the thermal stability, crystallinity, modification,rans-esterification, weathering, durability, fiber orien-ation on frictional and wear behavior, eco-design ofutomotive components, and alkylation [59–66].

Polyester resin was used as matrix for jute fiber rein-orced composites and the relationship between waterbsorption and dielectric behavior [67], the elastic prop-rties, notched strength and fracture criteria [68], impactamage characterization [69], weathering and thermalehavior [70], and effect of silane treatment [71] werexamined.

.2.4. KenafKenafbelongs to the genus Hibiscus and there are about

00 species. Kenaf is a new crop in the United Statesnd shows good potential as a raw material for usagen composite products. Latest advances in decorticationsquipment which separates the core from the bast fiberombined with fiber shortages, have renewed the interestn kenaf as a fiber source.

Thermoforming has proven to enable the successfulabrication of kenaf fiber reinforced PP sheets into sheetorm [72]. The optimal fabrication method found for these

aterials was at the compression molding process, whichtilizes a layered sifting of a micro-fine PP powder andhopped kenaf fibers. The fiber content (30 and 40 wt%)rovided adequate reinforcement to increase the strength

cience 37 (2012) 1552– 1596

of the PP matrix. The kenaf–PP composites compressionmolded in this study proved to have superior tensile andflexural strength when compared to other compressionmolded natural fiber composites such as other kenaf, sisal,and coir reinforced thermoplastics. With the aid of theelastic modulus data, it was also possible to compare theeconomic benefits of using kenaf composites instead ofother natural fibers and E-glass. The manufactured kenaf-maleated PP composites have a higher modulus/cost and ahigher specific modulus than sisal, coir, and even E-glass.Thus, they provide an option for replacing existing mate-rials with a higher strength, lower cost alternative that isenvironmentally friendly.

Hybrid composites of wood flour/kenaf fiber and PPwere prepared to investigate the hybrid effect on the com-posite properties [73]. The results indicated that whilenon-hybrid composites of kenaf fiber and wood flour exhib-ited the highest and lowest modulus values respectively,the moduli of hybrid composites were closely related to thefiber to particle ratio of the reinforcements. With the helpof the hybrid mixtures equation it was possible to predictthe elastic modulus of the composites better than whenusing the Halpin–Tsai equation.

Variations of the mechanical and thermal propertiesinduced by the exposure of kenaf fiber reinforced com-posites with PP to electron beam radiation [74], thecomparison of kenaf/PP composites with feather fiber/PP,recycled kraft pulp fiber/PP, and recycled news pulpfiber/PP composites [75] and, surface modified kenaffibers with modified polyester resin as matrix [76], andinfluence of toughening by natural rubber on kenaffibers with polyester resin as matrix [77] were alsoevaluated.

2.2.5. SisalSisal is an agave (Agave sisalana) and commercially pro-

duced in Brazil and East Africa. Between 1998–2000 and2010, the global demand for sisal fiber and its productsis expected to decline by an annual rate of 2.3% as agri-cultural twine. The traditional market for fibers continuesto be eroded by synthetic substitutes and by the adop-tion of harvesting technologies that utilizes less or notwine.

Magnesium hydroxide and zinc borate were incorpo-rated into sisal/PP composites as flame retardants [78].Adding flame retardants into sisal/PP composites reducedthe burning rate and increased the thermal stability of thecomposites. No synergistic effect was observed when bothmagnesium hydroxide and zinc borate were incorporatedinto the sisal/PP composites. In addition, the sisal/PP com-posites exhibited insignificant differences in shear viscosityat high shear rates indicating that the types of flame retar-dants used in this study had no impact on the processabilityof the composites. The sisal/PP composites which flameretardants were added to, exhibited tensile and flexuralproperties comparable to those of the sisal/PP composites,
which flame retardants had not been added to.
Sisal /PP composites were investigated regarding theenvironmental effects on the degradation behavior [79],the influence of coupling agents on the abrasive wear

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0

1

2

3

4

5

6

Jute-PPFlax-PPAbaca-PP

Not

ched

cha

rpy

impa

ct

stre

ngth

[mJ/

mm

²]

5mm5mm+MAH-PP

Fig. 1. Comparison of notch Charpy strength of abaca/jute/flax fiber–PP

carried out by Thomas and co-workers [104,105] with spe-cial reference to the effects of fiber loading, frequencyand temperature. The storage modulus was found to behighest for composites with 40% fiber loading, indicating


properties [80], and the effect of ageing on the mechanicalproperties [81].

Zhang et al. [82] developed all plant fiber compositesby converting wood flour into thermoplastics using anappropriate benzylation treatment and compounding bothdiscontinuous and continuous sisal fibers to produce com-posites from renewable resources. The degradation testsdemonstrated that the prepared sisal/plasticized woodflour composites were fully biodegradable. To acceleratethe decomposition process, both cellulose and lignin in thecomposites should be considered. The hydrophobicity andflame resistance of composites is important when regard-ing practical applications. Molecular modification and/orincorporation of inorganic additives are proper measuresprovided that the composites biodegradability remainsunchanged.

Many other studies were carried out on sisal fiberreinforced polyester composites regarding their moisture-absorption properties [83], and fiber treatment withadmicellar [84]. Sisal fiber reinforced phenolic resin com-posites were investigated with regard to the chemicalmodification of such with lignins [85], the modificationusing hydroxy terminated polybutadiene rubber [86], theeffect of cure cycles [87], using glyoxal from naturalresources [88], and the effect of alkali treatment [89].Furthermore epoxy resin was used as a matrix for sisalfiber reinforced composites and examined regarding theinfluence of fiber orientation on the electrical properties[90], and the degree of reinforcement [91]. Investigationswere also carried out using cement as a matrix for sisalfiber reinforced composites focusing on their crackingmicro-mechanisms [92], and the effects of accelerated car-bonation on cementitious roofing [93].

Towo et al. [94,95] prepared the treated sisal fiber com-posites with an epoxy and polyester resin matrix. Fatigueevaluation and dynamic thermal analysis tests were per-formed. Composites containing alkali treated fiber bundlesproved to have better mechanical properties than thosewith untreated fiber bundles. Alkali treatment had thegreatest effect on the polyester resin matrices. An improve-ment in the fatigue lives of composites was observed forthe alkali treatment of sisal fiber bundles. Constant-life dia-grams of epoxy matrix composites with untreated or alkalitreated fiber bundles show the superiority of the alkalitreated fiber composites regarding low cycle fatigue. Epoxymatrix composites have a longer fatigue life than polyestermatrix composites. The effect of chemical treatment onthe fatigue life is significantly positive for polyester matrixcomposites but has much less influence on the fatigue lifeof epoxy matrix composites.

Sisal fibers were also investigated with other matrices,such as rubber [96,97], phenol formaldehyde [98], cellu-lose acetate [99], bio polyurethane [100], and polyethylene[101] regarding their mechanical, morphological, chemical,and Cure characteristics.

2.2.6. Abaca
The abaca/banana fiber, which comes from the banana
plant, is durable and resistant to seawater. Abaca, thestrongest of the commercially available cellulose fibers, isindigenous to the Philippines and is currently produced

composites with and without MAH–PP.Adopted from [103]. Copyright 2007. By permission from Budapest Uni-versity of Technology.

there and in Ecuador. It was once the preferred cordagefiber for marine applications.

Bledzki et al. [102,103] examined the mechanical prop-erties of abaca fiber reinforced PP composites regardingdifferent fiber lengths (5, 25 and 40 mm) and differentcompounding processes (mixer-injection molding, mixer-compression molding and direct compression moldingprocess). It was observed that, with increasing fiber length(5–40 mm), the tensile and flexural properties showed anincreasing tendency though not a significant one. Amongthe three different compounding processes compared,the mixer-injection molding process displayed a bettermechanical performance (tensile strength is around 90%higher) than the other processes.

When abaca fiber PP composites were compared withjute and flax fiber PP composites, abaca fiber compositeshad the best notched Charpy (Fig. 1) and falling weightimpact properties. Abaca fiber composites also showedhigher odor concentration (Fig. 2) compared to jute andflax fiber composites.

A dynamic mechanical analysis of, polarity parame-ters of banana fiber reinforced polyester composites were

Fig. 2. Comparison of odor emission concentration abaca/jute/flaxfiber–PP composites.Adopted from [103]. Copyright 2007. By permission from Budapest Uni-versity of Technology.

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hat the incorporation of abaca fiber in the polyesteratrix induces reinforcing effects at higher temperatures.

ncreased dynamic modulus values and low damping val-es verified improved interactions between the fiber andhe matrix.

Abaca fibers have been investigated with cement [106],olyurethane [107], aliphatic polyester resin [108], PP109], urea formaldehyde [110], PE [111], polyester [112],nd polyvinyl alcohol [113] as matrices, in order to evaluatehe composites properties.

.2.7. Pineapple leaf fiberPineapple (Ananas comosus) is a tropical plant native

o Brazil. Pineapple leaf fiber is rich in cellulose, rela-ively inexpensive and abundantly available. Furthermore,t has the potential for polymer reinforcement. At presentineapple leaf fibers are a waste product of pineap-le cultivation and therefore these relatively inexpensiveineapple fiber can be obtained for industrial purposes.

Pineapple leaf fiber was reinforced with polycarbon-te to produce functional composites [114]. The silanereated modified pineapple leaf fibers composite exhibitedhe highest tensile and impact strengths. The thermogravi-

etric analysis showed that the thermal stability of theomposites is lower than that of neat polycarbonate resin.n addition, the thermal stability decreased with increasingineapple leaf fiber content.

The thermal conductivity and thermal diffusivity ofineapple leaf fiber reinforced phenol formaldehyde com-osites were studied using the Transient Plane SourceTPS) technique [115]. It is found that the effective ther-

al conductivity and effective thermal diffusivity of theomposites decrease, as compared with pure phenolormaldehyde as the fraction of fiber loading increases.

The quality enhancement of pineapple leaf fiber haseen attempted using different surface modifications likeewaxing, alkali treatment, cyanoethylation and graftingcrylonitrile onto dewaxed fibers [116]. The mechanicalroperties reach an optimum at fiber load of 30 wt% andmong all modifications, 10% acrylonitrile grafted fibereinforced polyester composite exhibited the maximumensile strength (48.36 MPa). However, cyano-ethylatedber composites exhibited better flexural and impacttrengths, i.e., 41% and 27% more than the detergentashed composite respectively.

The influence of surface treatment and fiber content ofineapple leaf fiber was also investigated with PP [117] andatural rubber as matrix [118].

.2.8. RamieRamie belongs to the family Urticaceae (Boehmeria),

hich includes about 100 species. Ramie’s popularity as aextile fiber has been limited largely by regions of produc-ion and a chemical composition that has required morextensive pre-treatment than is required of the other com-ercially important bast fibers.Ramie fiber reinforced PP composites were fabricated

sing a hybrid method of melt-blending and injectionolding processes [119]. Different ramie fiber/PP com-

osites were fabricated by varying the fiber length, fiberontent and method of fiber pre-treatment. The results

cience 37 (2012) 1552– 1596

exhibited increases in fiber length and fiber content alsoshow increased tensile strength, flexural strength and com-pression strength noticeably in turn. Yet, they also result innegative influences on the impact strength and elongationbehavior of the composites.

Thermoplastic biodegradable composites consisting oframie fibers and a PLA/PCL matrix were manufacturedusing the in situ polymerization method [120]. The effectsof fiber length and content on the tensile and impactstrengths of this natural-fiber-reinforced biodegradablecomposite were discussed, including the influence of asilane coupling agent for improved interfacial adhesion.The results showed that the tensile strength and impactstrength were highest when a silane coupling agent wasemployed, the ramie fiber length was 5–6 mm and whenthe fiber content was 45 wt%.

Bulletproof panels were made from ramie fiber rein-forced composites by hand lay-up process with epoxy asa matrix [121]. These prototype bulletproof panels werebelieved to be lighter in weight and more economical thanconventional bulletproof panels. Conventional bulletproofpanels are made from ceramic plates, kevlar/aramid com-posites, and steel-based material, which are popular inmilitary standard antiballistic equipment today. The bul-let testing results showed that the panels could resist thepenetration of a high-impact projectile (level II) obtainingonly minimal fractures. Level IV ballistic testing showedthat all prototype panels could not resist the high-impactvelocity of the projectile. Therefore, tests proved that ramiefibers have sufficient breaking strength and toughness forlevel II bullet testing.

Ramie fibers were also reinforced using polyester[122,123], epoxy–bioresin [124], soy protein [125,126],epoxy [127] and PP [128] for the matrix.

2.2.9. CoirCoir husk fibers are located between the husk and

the outer shell of the coconut. As a by-product of theproduction of other coconut products, coir production islargely determined by demand. Abundant quantities ofcoconut husk imply that, given the availability of labor andother inputs, coir producers can adjust relatively rapidly tomarket conditions and prices. It is estimated that approx-imately 10% of all husks are utilized for fiber extraction,satisfying a growing demand for fiber and coir products.

Characterization and utilization of coir fiber in coir/natural rubber composites [129], dynamic mechanicalbehavior of coir/natural rubber composites [130], durabil-ity of coir/cement composites [131], the effect acetylationon the coir/epoxy composites [132], the influence oftreated coir fiber on the physico-mechanical properties ofcoir/PP composites [133,134], and the effect of fiber phys-ical, chemical and surface properties on the thermal andmechanical properties of coir/PP composites were investi-gated and evaluated [135].

2.2.10. Bamboo
Bamboo (Bambusa Shreb.) is a perennial plant, which
grows up to 40 m in height in monsoon climates. Generally,it is used in construction, carpentry, weaving and plaitingetc. Curtains made of bamboo fiber can absorb ultraviolet

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radiation in various wavelengths, making it less harmful tohuman body.

The development of composites for ecological pur-poses (eco-composites) using bamboo fibers and their basicmechanical properties were evaluated [136]. The steamexplosion technique was applied to extract bamboo fibersfrom raw bamboo trees. The experimental results showedthat the bamboo fibers (bundles) had a sufficient specificstrength, equivalent to that of conventional glass fibers.The tensile strength and modulus of PP based compositesincreased about 15 and 30% when using steam-explodedfibers. This increase was due to good impregnation and areduction of the number of voids, in comparison to com-posites using fibers that were mechanically extracted.

The mechanical and thermal properties of bamboo fiberreinforced epoxy composites [137], the effects of fiberloading, coupling and bonding agents on the mechani-cal properties of bamboo fiber/natural rubber composites[138,139], the isothermal crystallization kinetics of modi-fied bamboo cellulose/PCL composites [140], the influenceof environmental aging on the mechanical properties ofbamboo/glass fiber reinforced PP hybrid composites [141],and the effect of ceramic fillers on mechanical propertiesof bamboo fiber/epoxy composites [142] were evaluated.

2.2.11. Rice huskRice is only one of the large group of cereal grains that

can be used to produce hull fibers. Nowadays wheat, corn,rye, oats and other cereal crops are used to produce fibersand investigating to reinforce in composites area.

Ismail et al. [143–147] studied the mechanical prop-erties of rice husk filled polymer composites and theirrelation to fiber loading, coupling agent, processability,hygrothermal aging, and hybridization effect.

Other studies have focused on: flame retardant prop-erties of rice husk/PE composites [148], the using of ricehusk as filler for rice husk/PP composites [149], the ther-mogravimetric analysis of rice husk filled HDPE and PPcomposites [150], the enhancement of the processabil-ity of rice husk/HDPE composites [151], the effect of thepercentage of rice husk content, hydroxyl groups andsize on the flexural, tensile, and impact properties of ricehusk/polyurethane composites [152], nonlinear viscoelas-tic creep characterization of HDPE-rice husk composites[153], the effect of the rice husk size and composition onthe injection molding processability of rice husk/PE com-posites [154], photocatalytic performance of a carbon/TiO2composite with rice husk [155], the effect of different con-centrations and sizes of particles of rice husk ash-in themechanical properties of rice husk/PP composites [156],the effect of different coupling agent on rice husk/co-polymer PP composites [157], dimensional properties ofrice husk/unsaturated polyester composites [158], and car-bon/silica composites fabricated from rice husk by meansof binder-less hot-pressing [159].

2.2.12. Oil palm
Oil palms (Elaeis) comprise two species of the Arecaceae
or palm family. Nowadays, oil palm empty fruit bunchfibers possess potential as a reinforcement fiber for plastic.Thomas et al. [160,161] investigated the stress relaxation

cience 37 (2012) 1552– 1596 1559

behavior of phenol formaldehyde resins reinforced with oilpalm empty fruit bunch fibers and hybrid composites com-posed of oil palm fibers and glass fibers. The effects of fiberloading, fiber treatment, physical ageing and strain level onthe stress relaxation behavior were examined and the rateof relaxation at different time intervals was calculated inorder to explain gradual changes in relaxation mechanisms.

Abu Baker et al. [162,163] investigated the effect of oilpalm empty fruit bunch and acrylic impact modifier onmechanical properties and influence of oil extraction ofoil palm empty fruit bunch on the processability of PVCcomposites.

The effects of oil extraction, compounding techniquesand fiber loading [164], and the effect of matrix modi-fication [165] on the mechanical properties of oil palmempty fruit bunch filled PP composites were examined.In addition, a comparative study of oil palm empty fruitbunch fiber/PP composites and oil palm derived cellu-lose/PP composites [166] were studied. The influence ofchemical modification on oil palm/phenol formaldehydecomposites [167], a comparison of epoxy and polyestermatrixes reinforced with oil palm fibers [168], dielectricrelaxation properties of oil palm/polyester resin compos-ites [169], the effect of palm tree fiber orientation ondynamic electrical properties of palm tree fiber-reinforcedpolyester composites [170] have also been investigated.

2.2.13. BagasseBagasse is the fibrous residue which remains after

sugarcane stalks are crushed to extract their juice. It iscurrently used as a renewable natural fiber for the man-ufacture of composites materials.

The compression and injection molding processes wereperformed in order to evaluate which is the better mix-ing method for fibers (sugarcane bagasse, bagasse celluloseand benzylated bagasse) and PP matrixes [171]. The injec-tion molding process performed under vacuum proved towork best. Composites were obtained with a homogeneousdistribution of fibers and without blisters. Although, thecomposites did not have good adhesion between the fiberand the matrix according to their mechanical properties.

The effect of botanical components of bagasse on thesetting of bagasse/cement composites [172], design andoptimization of PHB/bagasse fiber composites [173], tribo-logical properties of bagasse/polyester composites [174],creep properties of bagasse fiber reinforced PVC and HDPEcomposites [175], bagasse/HDPE composites for radia-tion shielding [176], molecular mobility information ofbagasse fiber and their distribution in bagasse/EVA com-posites [177], silane treated bagasse fiber reinforcementfor cementitious composites [178], using polyurethanefrom castor oil [179], durability of bagasse/PP compositesexposed to rainbow fungus [180], and ecodesign and lifecycle assessment as strategy for automotive componentsfrom bagasse/PP composites [181] were examined.

2.3. Structure and chemical composition

Climatic conditions, age and the degradation processinfluence not only the structure of fibers, but also the chem-ical composition. The major chemical component of a living


Table 2Chemical composition of some common natural fibers.

Fiber Cellulose (wt%) Hemicellulose (wt%) Lignin (wt%) Waxes (wt%)

Bagasse 55.2 16.8 25.3 –Bamboo 26–43 30 21–31 –Flax 71 18.6–20.6 2.2 1.5Kenaf 72 20.3 9 –Jute 61–71 14–20 12–13 0.5Hemp 68 15 10 0.8Ramie 68.6–76.2 13–16 0.6–0.7 0.3Abaca 56–63 20–25 7–9 3Sisal 65 12 9.9 2Coir 32–43 0.15–0.25 40–45 –Oil palm 65 – 29 –Pineapple 81 – 12.7 –Curaua 73.6 9.9 7.5 –Wheat straw 38–45 15–31 12–20 –

5

tccocwTwsw

2

wmmmimafiri

dt

TP

Rice husk 35–45 19–2Rice straw 41–57 33

ree is water. However, on dry basis, all plant cell wallsonsist mainly of sugar based polymers (cellulose, hemi-ellulose) that are combined with lignin with lesser amountf extractives, protein, starch and inorganics. The chemi-al components are distributed throughout the cell wall,hich is composed of primary and secondary wall layers.

he chemical composition varies from plant to plant, andithin different parts of the same plant. Table 2 [182–184]

hows the range of the average chemical constituents for aide variety of plant types.

.4. Properties

The properties of natural fibers differ among citedorks, because different fibers were used, differentoisture conditions were present, and different testingethods were employed. The natural fiber reinforced poly-er composites performance depends on several factors,

ncluding fibers chemical composition, cell dimensions,icrofibrillar angle, defects, structure, physical properties,

nd mechanical properties, and also the interaction of aber with the polymer. In order to expand the use of natu-al fibers for composites and improved their performance,
t is essential to know the fiber characteristics.
The range of the characteristic values, as one of therawbacks for all natural products, is remarkably higherhan those of glass-fibers, which can be explained by

able 3hysico-mechanical properties of natural fibers.

Fiber Tensile strength (MPa) Young’s modulus (G

Abaca 400 12

Bagasse 290 17

Bamboo 140–230 11–17

Flax 345–1035 27.6

Hemp 690 70

Jute 393–773 26.5Kenaf 930 53

Sisal 511–635 9.4–22

Ramie 560 24.5

Oil palm 248 3.2

Pineapple 400–627 1.44

Coir 175 4–6

Curaua 500–1150 11.8

20 14–178–19 8–38

differences in the fiber structure due to the overall envi-ronment conditions during growth. Natural fibers can beprocessed in different ways to yield reinforcing elementswith different mechanical properties. Mechanical proper-ties of natural fibers can be influenced by many factors.Such as either fiber bundles or ultimate fiber is beingtested. Table 3 [182,183] presents the important physico-mechanical properties of commonly used natural fibers.

The hydrophilic nature of fibers is a major problem forall cellulose-fibers if used as reinforcement in plastics. Themoisture content of the fibers is dependent on the contentof non-crystalline parts and the void content of the fibers.Overall, the hydrophilic nature of natural fibers influencesthe mechanical properties. Table 4 [185] shows the equi-librium moisture content of some natural fibers.

The moisture content at a given relative humidity canhave a great effect on the biological performance of a com-posite made from natural fibers. For example, a compositemade from abaca fibers would have much greater moisturecontent than would a composite made from flax fibers.

The physical properties of each natural fiber are crit-ical, and include the fiber dimensions, defects, strengthand structure. There are several physical properties that
are important to know about for each natural fiber beforethat fiber can be used to reach its highest potential. Fiberdimensions, defects, strength, variability, crystallinity, andstructure must be taken into consideration.
Pa) Elongation at break (%) Density [g/cm3]

3–10 1.5– 1.25– 0.6–1.12.7–3.2 1.51.6 1.481.5–1.8 1.31.6 –2.0–2.5 1.52.5 1.525 0.7–1.5514.5 0.8–1.630 1.23.7–4.3 1.4


on of a sing.

strength increased from 12.75 MPa for the samples with-out transcrystallinity to 23.05 MPa for the transcrystallizedsamples according to the single fiber fragmentation test.

Fig. 3. IllustratiAdopted from [186] Copyright 2001. By permission from Springer Publish

Knowledge about fiber length and width is important forcomparing different kinds of natural fibers. A high aspectratio (length/width) is very important in cellulose basedfiber composites as it give an indication of possible strengthproperties. The fiber strength can be an important factor inselecting a specific natural fiber for a specific application.Changes in the physical properties can be due to differencesin fiber morphology. Major differences in structure such asdensity, cell wall thickness, length and diameter result indifferences in physical properties. It is also interesting tonote that the morphology of the land plant fibers is veryfrom different to that of water plant fibers.

In between fiber qualities, the cellulose content andspiral angle is differed from fiber to fiber and also in a sin-gle fiber design. Natural fiber is a composite of the threepolymers (cellulose, hemicelluloses and lignin), in whichthe unidirectional cellulose microfibrils constitute the rein-forcing elements in the matrix blend of hemicellulose andlignin. The structure of such a fiber was built as multi-ply construction with layers P, S1, S2 and S3 of cellulosemicrofibrils at different angles to the fiber axis (Fig. 3) [186].

Depending on the single fiber cell layer, the differentspiral angle of those layers will have a pronounced influ-ence on the properties of the fiber. The elastic modulusdependency on the spiral angle of the fiber is determined

Table 4The equilibrium moisture content of different natural fibers at 65% relativehumidity (RH) and 21 ◦C.

Fiber Equilibrium moisture content (%)

Sisal 11Hemp 9.0Jute 12Flax 7Abaca 15Ramie 9Pineapple 13Coir 10Bagasse 8.8Bamboo 8.9

ingle fiber cell.

(Fig. 4) assuming that the fibers (holocellulose fibers) werelignin-free with a cellulose content of 65 wt% (which is typ-ical cellulose content for natural fibers [186]). The relativethickness of the different layers were chosen to be P = 8%,S1 = 8%, S2 = 76%, and S3 = 8%. It is seen that the elastic mod-ulus decreases with increasing spiral angle.

Crystallinity values of natural fibers vary in differentparts of the plant. The crystallinity tends to decrease asthe plant matures, but the difference between bast andcore fibers is inconclusive. The permeability of kenaf coreis highest and is closely followed by cotton.

The effect of transcrystallinity on the interface in flaxfiber (green flax, dew-retted flax, Duralin-treated flaxand stearic acid-treated flax fiber)/isotactic PP compositeshas been investigated [187,188]. A significant strengthen-ing of the interface was observed when transcrystallinitywas present. Surface modification (stearation) of flaxfiber also induced transcrystallinity. The interfacial shear

Fig. 4. Correlation between spiral angle and elastic modulus of a naturalfiber.Adopted from [186]. Copyright 2001. By permission from Springer Pub-lishing.

1 lymer S

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2

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ivtsmpti

3

mfitiaattfw

3

mmsitpet


The chemical composition varies from plant to plant,nd within different parts of the same plant. It also variesithin plants from different geographical regions, ages, cli-ate and soil conditions.The chemical properties are influenced by the fiber

rowth time (days after planting), the botanical classi-cation of the fiber and the stalk height. The chemicalomposition can also vary within the same part of a plant.oth the root and stalk core have a higher lignin contenthan that of the fibers.

.5. Summary

There are thousands of different fibers in the world andn fact only few of these fibers have been studied. Mostesearch has been carried out to study the potential usef natural fibers for technical applications and this reviewaper has only covered the most widely studied and usedatural fibers in reinforced composites materials. Amonghe most popular natural fibers; flax, jute, hemp, sisal,amie, and kenaf fibers were extensively researched andmployed in different applications. But nowadays, abaca,ineapple leaf, coir, oil palm, bagasse, and rice husk fibersre gaining interest and importance in both research andpplications due to their specific properties and availabil-ty.

It should be mentioned that there are also shortcom-ngs; a lack of consistency of fiber qualities, high levels ofariability in fiber properties related to the location andime of harvest, processing conditions, as well as theirensitivity to temperature, moisture and UV radiation. Aulti-step manufacturing process is required in order to

roduce high quality natural fibers, which contributes tohe cost of high-performance natural fibers as well as themproved mechanical properties of the composites.

. Modification of natural fibers

The main disadvantages of natural fibers in reinforce-ent to composites are the poor compatibility between

ber and matrix and their relative high moisture absorp-ion. Therefore, natural fiber modifications are consideredn modifying the fiber surface properties to improve theirdhesion with different matrices. An exemplary strengthnd stiffness could be achieved with a strong interfacehat is very brittle in nature with easy crack propagationhrough the matrix and fiber. The efficiency of stress trans-er from the matrix to the fiber could be reduced with aeaker interface.

.1. Physical method

Physical methods include stretching, calendaring, ther-otreatment, and the production of hybrid yarns for theodification of natural fibers. Physical treatments change

tructural and surface properties of the fiber and therebynfluence the mechanical bonding of polymers. Physical
reatments do not extensively change the chemical com-osition of the fibers. Therefore the interface is generallynhanced via an increased mechanical bonding betweenhe fiber and the matrix.
cience 37 (2012) 1552– 1596

3.1.1. Corona treatmentCorona treatment is one of the most interesting tech-

niques for surface oxidation activation. This processchanges the surface energy of the cellulose fibers. Coronadischarge treatment on cellulose fiber and hydrophobicmatrix was found to be effective for the improvement ofthe compatibilization between hydrophilic fibers and ahydrophobic matrix.

Tossa jute fibers [189] were corona discharge treated toimprove the mechanical properties of natural fiber/epoxycomposites. Corona-treated fibers exhibited significantlyhigher polar components of free surface energy withincreasing treatment energy output. Owing to difficultiesin effective treatment of three-dimensional objects withcorona discharge, the increased polarity of treated yarns isrelatively small. Furthermore a decrease in the yarn tenac-ity was observed with an increasing corona energy level.

The mechanical properties of corona treated hempfiber/PP composites were characterized by tensile andcompressive stress–strain measurements [190]. The treat-ment of compounds (fibers or matrix) leads to a significantincrease in tensile strength. The modification of cel-lulosic reinforcements rather than PP matrix allowsgreater improvement of the composites properties with anenhancement of 30% of Young modulus. The observationof the fracture surfaces enables discussion about adhesionimprovement.

Flax and Hemp fiber mats were corona treated andthe optimum length of corona treatment to improve thecomposites’ (with a natural resin matrix) tensile modulusand strength was determined [191]. The composite ten-sile breaking strength reaches a maximum earlier, namelyafter only 5 min of corona treatment and declines rapidlyafterwards. The fiber appearance confirmed that relativelyshort periods of corona treatment are more than suffi-cient to improve adhesion in the fibers. In addition, it wasalso proven that there is no further improvement achievedwhen longer corona treatments are applied, because thesurface opening of the fiber does not improve further after15 min of treatment. However, the degradation of the fibersproceeds further.

3.1.2. Plasma treatmentPlasma treatment is another physical treatment method

and is, similar to corona treatment. The property of plasmais exploited by the method to induce changes on the sur-face of a material. A variety of surface modifications canbe achieved depending on the type and nature of the gasesused. Reactive free radicals and groups can be produced, thesurface energy can be increased or decreased and surfacecross-linking can be introduced.

Polyester composites reinforced with flax fibers, whichwere submitted to helium cold plasma treatment, wereinvestigated by means of water permeation measurementsand mechanical tests [192]. The analysis of the permeationand mechanical results showed that plasma treatment
improves the fiber/matrix adhesion and enhances themechanical property stiffness.
Martin et al. [193] prepared sisal–HDPE compositesand showed that some improvements of the mechanical

lymer S
properties of the composites are achieved due to theplasma treatments.

Jute fibers were subjected to oxygen plasma treatmentas a function of the plasma power in order to enhance inter-facial adhesion [194]. The interlaminar shear strength ofplasma treated jute fiber/HDPE composites, which wereexposed to plasma powers of 30 W and 60 W, increasedby approximately 32 and 47%, respectively compared tountreated composites. In turn, the flexural strength ofuntreated composites increases by only 45%. It can be con-cluded that exposure to plasma powers of 60 W for 15 minin the studied range is the most suitable parameter foroxygen plasma treatment of jute fibers and enables theachievement of the best interfacial adhesion with HDPE.

The influence of plasma treatment on the morphology,wettability, and fine structure of jute fibers and its impacton the interfacial adhesion of jute fibers/unsaturatedpolyester were investigated [195]. A scanning electronmicroscopic micrograph showed the rough surface mor-phology and degradation of fiber due to an etchingmechanism caused by the plasma. Plasma treatmentresulted in the development of hydrophobicity in fibers.However, among all treated fiber composites, the flexu-ral strength of composites prepared with fibers treated for10 min with plasma only showed an improved mechanicalstrength of approximately 14% in comparison to raw fibercomposites.

Jute fibers were treated indifferent plasma reactors(radio frequency “RF” and low frequency “LF” plasma reac-tors) using O2 for different plasmapowers to increase theinterface adhesion between the jute fiber and polyestermatrix. The interlaminar shear strength increased from11.5 MPa for the untreated jute fiber/polyester compositeto 19.8 and 26.3 MPa for LF and RF oxygen plasma treatedjute fiber/polyester composites, respectively. The tensileand flexural strengths improved of jute fiber/polyestercomposites for both plasma systems. It is clear that oxy-gen plasma treatment of jute fibers by using RF plasmasystem showed greater improvement on the mechanicalproperties of jute/polyester composites compared to usingLF plasma system [196].

3.2. Chemical method

Cellulose fibers, which are strongly polarized, are inher-ently incompatible with hydrophobic polymers due to theirhydrophilic nature. In many cases, it is possible to inducecompatibility in two incompatible materials by introducinga third material that has properties intermediate betweenthose of the other two. There are several coupling mecha-nisms in materials (e.g., weak boundary layers, deformablelayers, restrained layers, wettability, chemical bonding,and acid–base effect).

The development of a definite theory for the mecha-nism of bonding using coupling agents in composites is acomplex problem. The main chemical bonding theory alone
is not sufficient. So the consideration of other conceptsappears to be necessary. These include the morphologyof the interphase, the acid–base reactions in the interface,surface energy and the wetting phenomena.
cience 37 (2012) 1552– 1596 1563

Chemical modifications of natural fibers aimed atimproving the adhesion within the polymer matrix usingdifferent chemicals were investigated.

3.2.1. Silane treatmentThe surface energy of fibers is closely related to the

hydrophilic nature of the fiber. Some investigations areconcerned with methods to decrease hydrophilicity. Silanecoupling agents may contribute hydrophilic properties tothe interface, especially when amino-functional silanes,such as epoxies and urethanes silanes, are used as primersfor reactive polymers. The primer may supply much moreamine functionality than can possibly react with the resinin the interphase. Those amines, which could not react, arehydrophilic and therefore responsible for the poor waterresistance of bonds. An effective way to use hydrophilicsilanes is to blend them with hydrophobic silanes such aspheniltrimethoxysilane. Mixed siloxane primers also havean improved thermal stability, which is typical for aromaticsilicones.

The surface of the kenaf fiber was modified using a silanecoupling agent to promote adhesion with the PS matrix[197]. The fiber–matrix adhesion increased through a con-densation reaction between alkoxysilane and hydroxylgroups of kenaf cellulose. Due to the fiber modification,kenaf/PS composites exhibited higher storage modulus andlower tan ı than those with untreated fiber indicating agreater interfacial interaction between the matrix resin andthe fiber.

Silane treatment improved the storage modulus ofabaca fiber reinforced polyester composites [198]. Chemi-cal modifications with silane A174 (�-methacryl oxypropyltrimethoxy-silane) and also with NaOH resulted in max-imum increase of the modulus. The hydrogen-bonddonating acidity is found to be the lowest for fibers treatedwith silane A174, after pre-treatment with 0.5% NaOH. Onthe contrary, the highest value was noted for fibers treatedwith silane A151 (vinyl triethoxysilane). The overall polar-ity was found to be at a maximum for fibers treated withsilane A151 (vinyl triethoxysilane).

The effects of different chemical treatments on thefiber–matrix compatibility in terms of the surface energyand the mechanical properties of composites werereported [199]. The composites were compounded withtwo kinds of flax fibers (natural flax and flax pulp)and a PP matrix. The applied treatments were vinyltrimethoxy silane (VTMO), maleic anhydride (MA) andmaleic anhydride-polypropylene copolymer (MAPP). Thethree treatments reduced the polar component of thesurface energy of the fiber. Composites containing MAPP-treated fibers possessed the highest mechanical properties,while the MA and VTMO-treated fibers obtained similarvalues to those of the untreated ones.

Ismail et al. [200,201] treated oil palm empty fruit bunchand coir fibers with silane coupling agents and found thatwith the addition of silane, the scorch time and cure time
increased. Furthermore, the tensile strength, tensile mod-ulus, tear strength, fatigue life and hardness of oil palmempty fruit bunch and coir fiber reinforced polyester andnatural rubber composites were enhanced.


Table 5Variation of the structure and properties of hemp Yarn (hemp 10/1, Rohemp) due to mercerization (22% NaOH).

Mercerizationtemperature time (◦C)

Mercerization(min)

Stress at themercerization

Degree of conversioncell I in cell II (%)

Tensile strength(cN/tex)

Elongation atbreak (%)

Tension modulus(cN/tex)

Untreated – 29.8 3.15 11484 60 Isometric ≈50 28.5 2.34 1617

10 60 Isometric ≈50 24.9 2.45 121420 60 Isometric ≈50 28.2 2.27 170110 40 Free shrinkage ≈70 17.6 6.86 357

0

N

3

uwwtwbs

c[hpabntdcai

thfippmiStgawmhbtmm

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the fiber bundles. It was also observed that only the ten-sile modulus of elasticity exceeds the value of the originalunmodified material.

Table 6Influence of different fiber treatments on flexural strength of unidirec-tional flax–PP composites.

Fiber treatment Flexural strength (N/mm2)

Untreated 77Mercerizeda 115MAH–PPb 127

10 60 Constant load (100 g) ≈4

ote: Distance of clamped support = 500 mm, test speed = 250 mm/min.

.2.2. Alkaline treatmentAlkaline treatment or mercerization is one of the most

sed chemical method (removes a certain amount of lignin,ax and oils covering the external surface of the fiber cellall) for natural fibers when used to reinforce thermoplas-

ics and thermosets. The important modification achievedith alkaline treatment is the disruption of the hydrogen

onding in the network structure, thereby increasing theurface roughness.

The effect of alkali treatment on the wetting ability andoherence of sisal–epoxy composites has been examined202]. Treatment of sisal fiber in a 0.5 N solution of sodiumydroxide resulted in more rigid composites with lowerorosity and higher density. The treatment improved thedhesion characteristics, due to improved work of adhesionecause it increases the surface tension and surface rough-ess. The resulting composites showed improvements inhe compressive strength and water resistance. It was alsoiscovered that the removal of intracrystalline and inter-rystalline lignin and other waxy surface substances by thelkali substantially increases the possibility for mechanicalnterlocking and chemical bonding.

The effects of alkali treatment of pineapple leaf fiber onhe performance of the pineapple leaf fiber/PLA compositesave been shown [203]. It was found that alkali-treatedber reinforced composite offered superior mechanicalroperties compared to untreated fiber reinforced com-osites. This study also suggested that the appropriateodification of fiber surfaces significantly contributes to

mproving the interfacial properties of the biocomposites.ince improvement of the interfacial adhesion increaseshe impact performance, surface properties of fibers can bereatly modified in terms of different treatments as char-cterized by an increased surface energy, which improvesettability of the fiber when compounding with a PLAatrix. DMA showed that treated fiber composites have a

igher storage modulus, which indicates greater interfacialond strength and adhesion between the matrix resin andhe fiber. The alkalized fiber composites have high storage

odulus values, which correspond with their high flexuralodulus.The effect of alkylation on the tensile properties of

amie fiber was explored [204]. The load application tech-ique was employed during alkali treatment, in order to

mprove the mechanical properties of the fiber. The ramie
ber was alkali-treated with a solution containing 15%aOH and was subjected to applied loads of 0.049 and.098 N. The results showed that the tensile strength of thereated ramie fiber was by improved 4–18% more than for
29.2 2.60 1499

untreated ramie fibers. The treated fibers’ Young’s modu-lus decreased. It should be noted that fracture strains of thetreated ramie fiber drastically increased to 0.045–0.072.That equals twice to three times higher than those of theuntreated ramie fiber. It was considered that such prop-erty improvements upon alkylation were correlated witha change of the morphological and chemical structures inmicrofibrils of the fiber.

Jute fibers were subjected to alkali treatment with asolution containing 5% NaOH for 0, 2, 4, 6 and 8 h at 30 ◦C[205]. The modulus of the jute fibers improved by 12%, 68%and 79% after 4, 6 and 8 h of treatment, respectively. Thetenacity of the fibers improved by 46% after 6 and 8 h treat-ment and the percentage of breaking strain was reducedby 23% after 8 h treatment. For 35% of the composites withfiber treated for 4 h, the flexural strength improved by 20%from 199.1 to 238.9 MPa. The flexural modulus improvedby 23% from 11.89 to 14.69 GPa and the laminar shearstrength increased by 19% from 0.238 to 0.283 MPa.

Ramie fiber [206], green coconut fiber [207], sisal [208],jute [209] and coir [210] fibers were alkalized and theirproperties evaluated.

The variation of mechanical and structural propertiesof hemp yarn upon alkylation in dissimilar conditions wasinvestigated [211]. Alkylation with a rather high alkali con-centration (22%) never led to a complete lattice conversionof cellulose I in cellulose II (Table 5). As expected, the high-est degree of conversion (70%) was reached by a freelyshrinking yarn. Further, the alkylation of hemp yarn underisometric conditions or with constant fiber load (100 p) ata NaOH concentration of 22% results in a crystalline lat-tice conversion of only 40–50%. However, this occurs onlywhen combined with the highest strengths and moduli of

Mercerized + MAH–PP 149

Note: Lengthwise, fiber content: 36 vol%.a 29% NaOH at 20 ◦C, 20 min, isometric.b 1% Licomont AR 504 in toluene, 10 min.


Fig. 5. Influence of acetylation of flax fiber on the degree of polymeriza-tion and degree of crystallinity of cellulose.

Fig. 6. Influence of acetylation of flax fibers on the moisture absorption

PP matrix, Mishra et al. [222] reported that maleic anhy-

Adopted from [212]. Copyright 2008. By permission from Budapest Uni-versity of Technology.

Table 6 [211] explicitly shows that, besides the signif-icant effects of single alkylation and MAH–PP treatments,a combination of the isometric alkylation procedure (e.g.,increasing fiber strength) and coupling agents results inthe highest increase of the composite properties (90%) incomparison to a composite with untreated flax fibers forreinforcement.

3.2.3. AcetylationAcetylation is another method of modifying the surface

of natural fibers and making them more hydrophobic. Itdescribes the introduction of an acetyl functional groupinto an organic compound. The main idea of acetylationis to coat the OH groups of fibers which are responsiblefor their hydrophilic character with molecules that have amore hydrophobic nature.

The influence of acetylation on the structure and prop-erties of flax fibers were investigated. Modified flax fiberreinforced PP composites were also prepared [212]. Theeffect of acetylation on the degree of polymerization andcrystallinity of flax fiber is illustrated in Fig. 5.

It was observed that the degree of polymerizationslowly decreased while the degree of acetylation increaseduntil 18%. Once reaching an acetyl content of 18% acetylcontent, the degree of polymerization decreased rapidlydue to vigorous degradation of cellulose. It was also notablethat the degree of crystallinity increased a little bit regard-ing the degree of acetylation, because of the removal oflignin and extractibles. After that, the degree of crystallinityof the cellulose decreased with respect to the degreeof acetylation owing to the decomposition of acetylatedamorphous components on the cellulose surface.

The influences of the degree of acetylation of flax fiberson the moisture absorption properties are illustrated inFig. 6 [212]. At 95% RH, the 3.6%, 12%, 18% and 34% degreeof acetylated flax fibers showed about 14%, 18%, 27% and42% lower moisture absorption properties than untreatedflax fiber respectively. The moisture absorption propertiesdecreased proportionally with the increase of acetyl con-tent of fibers due to the reduction of hydrophilicity of thefibers.

Seena et al. investigated the effect of acetylation onabaca fiber reinforced phenol formaldehyde compositesand reported that the tensile strength, tensile modulus and

property at 95% relative humidity.Adopted from [212]. Copyright 2008. By permission from Budapest Uni-versity of Technology.

impact strength were found to improve compared to non-treated abaca fiber composites [213].

Tserki and co-workers [214] investigated the acety-lation of flax, hemp and wood fibers. A removal of thenon-crystalline constituents of the fibers, an alteration ofthe characteristics of the surface topography, a change ofthe fiber surface free energy and an improvement of thestress transfer efficiency in the interface took place.

The effects of acetylation on the biological degradationand shear strength of coir and oil palm fiber reinforcedpolyester composites were evaluated [215,216]. It wasreported that acetylated natural fiber reinforced polyestercomposites exhibit higher bio-resistance and less tensilestrength loss compared to composites with silane treatedfiber in biological tests.

Zafeiropoulos and co-workers extensively investigatedthe influence of acetylation on the engineering and char-acterization of the interface in flax fiber/PP composites[217,218]. Furthermore, they also investigated the applica-tion of Weibull statistics [219] and Gaussian statistics [220]to the tensile strength of flax fibers.

3.2.4. Maleated couplingNowadays, maleated coupling is widely used to

strengthen natural fiber reinforced composites. The funda-mental difference with other chemical treatments is thatmaleic anhydride is not only used to modify fiber surfacebut also the polymeric matrix to achieve better interfa-cial bonding in between fiber and matrix and improvedmechanical properties in composites. There are numerouspublished studies in which the effect of maleic anhydridegrafting on the mechanical properties of natural fibers hasbeen investigated and it is impossible to list all of themhere. Hence, only a few representative studies are dis-cussed here.

Mohanty et al. [221] used MAPP as a coupling agentfor the surface modification of jute fibers. It was foundthat a fiber loading of 30% with a MAPP concentrationof 0.5% in toluene and 5 min of impregnation time with6 mm average fiber lengths resulted in the best possibleoutcomes. An increase in flexural strength of 72.3% wasobserved for the treated composites. In addition to the

dride treatment reduced the water absorption to a greatextent in abaca, hemp and sisal fiber-reinforced novolaccomposites. Mechanical properties like Young’s modulus,


y (a) unA

flfid

fltlcBrao

HabppfisbchNr

Fig. 7. Micrograph of abaca fiber surface morphologdopted from [233]. Copyright 2010. By permission from Elsevier Ltd.

exural modulus, hardness and impact strength of plantber-reinforced composites increased after maleic anhy-ride treatment.

The effects of MAH–PP coupling agents on rice-huskour reinforced PP composites were evaluated [223]. Theensile strengths of the composites decreased as the filleroading increased, but the tensile properties were signifi-antly improved with the addition of the coupling agent.oth the notched and unnotched Izod impact strengthsemained almost the same with the addition of couplinggents. A morphological study revealed the positive effectf a compatibilizing agent on interfacial bonding.

Abaca fiber reinforced composites based onDPE/Nylon-6 blends were prepared [224]. Maleicnhydride grafted styrene/ethylene–butylene/styrene tri-lock polymers (SEBS-g-MA) and maleic anhydride graftedolyethylene (PE-g-MA) were used to enhance the impacterformance and interfacial bonding between the abacabers and the resins. By employing SEBS-g-MA, bettertrengths and moduli were obtained for HDPE/Nylon-6ased composites than for corresponding HDPE based
omposites. It was found that the addition of SEBS-g-MAad a positive influence on the reinforcing effect of theylon-6 component in the composites. Thermal analysis
esults showed that fractionated crystallization of the

modified (b) NDS modified (c) fungamix modified.

Nylon-6 component in the composites was induced by theaddition of both SEBS-g-MA and PE-g-MA. The thermalstability of both composite systems differed slightly,except for an additional decomposition peak, which isrelated to the minor Nylon-6 for the composites from theHDPE/Nylon-6 blends. In combination with SEBS-g-MA,the addition of Nylon-6 and the increased fiber loadinglevel led to an increase in the water absorption value ofthe composites.

The influence of MAH–PP on the fiber–matrix adhesionin jute fiber-reinforced PP composites and on the materialbehavior under fatigue and impact loadings was inves-tigated [225]. The fiber–matrix adhesion was improvedby treating the fibers’ surface with the coupling agentMAH–PP. It was shown that a strong interface is connectedwith a higher dynamic modulus and reduction in stiffnessdegradation with increasing load cycles and applied max-imum stresses. The specific damping capacity resulted inhigher values for the composites with poor bonded fibers.Furthermore, the stronger fiber–matrix adhesion reducedthe loss-energy for non-penetration impact tested com-
posites about roughly 30%. Tests, which were performedat different temperatures, showed higher loss energies forcold and warm test conditions compared to those car-ried out at room temperature. The post-impact dynamic

O. Faruk et al. / Progress in Polymer S

Fig. 8. Tensile and flexural strength of enzyme treated and unmodifiedabaca–PP composites.Adopted from [233]. Copyright 2010. By permission from Elsevier Ltd.

modulus roughly 40% after 5 impact events and was 30%lower for composites with poor and good fiber–matrixadhesion, respectively.

The effectiveness of MAH as coupling agent has dis-cussed also on thermal and crystallization properties ofsisal fiber/PP composites [226], tensile properties of hilde-gardia fiber/PP composites [227], wetting behavior of flaxfiber/PP composites [228], transcrystallinity of jute fiber/PPcomposites [229], surface properties and water uptakebehavior of flax, hemp fiber reinforced PP composites [230],effects of micro-sized cellulose based corn fibers as rein-forcement agents in PP composites [231], and dynamicmechanical properties of flax and hemp fiber/PP compos-ites [232].

3.2.5. Enzyme treatmentThe use of enzyme technology is becoming increasingly

substantial for the processing of natural fibers. Currently,the use of enzymes in the field of textile and natural fibermodification is also rapidly increasing. A major reason forembracing this technology is the fact that the applicationof enzymes is environmentally friendly. The reactions cat-alyzed are very specific and have a focused performance[233].

Bledzki et al. [233] investigated PP composites withenzyme treated abaca fibers. The surface morphologies ofenzyme treated and untreated abaca fibers are shown inFig. 7. In Fig. 7a, it was observed that the untreated fibersurface is rough, containing waxy and protruding parts.The surface morphology of treated fibers is observed inFig. 7b and c. If the waxy material and cuticle in the treatedsurface are removed, the surface becomes smoother. Fib-rillation is also known to occur when the binding materialsare removed from the surface of the treated fibers. Fibersurface damage was also observed for naturally digestedfibers which occur in natural digestion systems.

The effects of enzyme treatment of abaca fiber on thetensile and flexural strength are shown in Fig. 8. The tensilestrength of enzyme treated abaca composites was found tohave increased 5–45% due to modification. Natural diges-tion system (NDS) modified abaca fiber composites showedlittle improvement in comparison to unmodified abaca
fiber composites. The tensile strength of fungamix modifiedcomposites increased by 45% as compared to unmodifiedfiber composites. A conventional coupling agent (MA–PP)has a positive effect on the tensile strength. It was found
cience 37 (2012) 1552– 1596 1567

to improve by 40%. Enzymatic treatment was also investi-gated with hemp fiber [234,235], and flax fibers [236].

3.3. Summary

Though natural fiber reinforced composites have devel-oped significantly over the past few years because of theirlow cost, low relative density, and high specific strength,the interfacial adhesion between fiber and matrix is still anissue. Extensive research was carried out and reported inliterature and reviewed in this paper, showing the impor-tance of the interface and the influence of various types ofsurface modifications on the physical and mechanical prop-erties of biocomposites. The observed trend lied with thechemical modification compared to physical modification.It has also been shown that maleated and silane treat-ment is becoming a choice method due to beneficial results.Additive suppliers improved the additives with higheramounts of anhydride functional groups than previousgrades (used in 1980s and 1990s), which create more sitesfor chemical links, resulting in significant performanceimprovement at low additive contents. Using couplingagents reduced the water absorption of the compositesbut has not resulted in decreased long term performance.As described above that enzyme technology to modify thenatural fiber surface, is increasing substantially due to envi-ronment friendly. In addition, enzyme technology couldbe cost effective, improved product quality compared tomostly use maleated and silane modification.

4. Matrices for biocomposites

The composites’ shape, surface appearance, environ-mental tolerance and overall durability are dominatedby the matrix while the fibrous reinforcement carriesmost of the structural loads, thus providing macroscopicstiffness and strength. The polymer market is dominatedby commodity plastics with 80% consuming materialsbased on non-renewable petroleum resources. Govern-ments, companies and scientists are driven to find analternative matrix to the conventional petroleum basedmatrix through public awareness of the environment, cli-mate change and limited fossil fuel resources. Thereforebiobased plastics, which consist of renewable resources,have experienced a renaissance in the past few decades.Fig. 9 shows that the matrices currently used in bio fibercomposites depend on biobased or petroleum based plas-tics and also on their biodegradability.

4.1. Petrochemical based

The effects of the incorporation of natural fibersin petrochemical based thermoplastics and thermosetmatrixes were extensively studied. Polypropylene (PP),polyethylene (PE), polystyrene (PS), and PVC (polyvinylchloride) were used for the thermoplastic matrixes.
Polyester, epoxy resin, phenol formaldehyde, and vinylesters were used for the thermoset matrices and arereportedly the most widely used matrices for natural fiberreinforced polymer composites.


plasticA

44(dpiatTciatct

dtm

Fig. 9. Current and emergingdopted from [1].

.1.1. Thermoplastic

.1.1.1. PE (polyethylene). Several mechanical propertiesdeformation, fracture, thermal diffusivity, thermal con-uctivity, and specific heat) of flax fiber/HDPE biocom-osites were evaluated [237,238]. Strength and stiffness

mprovement combined with high toughness can bechieved by varying the fiber volume fraction and con-rolling the bonding between layers of the composite.he thermal conductivity, thermal diffusivity, and spe-ific heat of the flax/HDPE composites decreased withncreasing fiber content, but the thermal conductivitynd thermal diffusivity did not change significantly atemperatures in the range (170–200 ◦C) studied. The spe-ific heat of the biocomposites increased gradually withemperature.

Traditionally the mechanism of moisture absorption isefined by diffusion theory; but the relationship betweenhe microscopic structure-infinite 3D-network and the

oisture absorption could not be explained. Wang et al.

s and their biodegradability.

[239] introduced the percolation theory and a percolationmodel was developed to estimate the critical accessiblefiber ratio; the moisture absorption and electrical conduc-tion behavior of composites. At high fiber loading whenfibers are highly connected, the diffusion process is thedominant mechanism; while at low fiber loading close toand below the percolation threshold, the formation of acontinuous network is key. Hence, percolation is the dom-inant mechanism. The model can be used to estimate thethreshold value which can in turn be used to explain mois-ture absorption and electrical conduction behavior. Fig. 10shows the increment of electrical conductivity with theincreasing moisture content of the composite with 65% ricehull fiber loading. The composite started showing conduc-tivity after it absorbed approximately 50% of maximum
moisture. After this, conductivity increased quickly withfurther moisture absorption. The pattern of the incrementof electrical conductivity suggests a diffusion process ofmoisture absorption.

O. Faruk et al. / Progress in Polymer S

Fig. 10. The moisture absorption and electrical conductivity for

forced PP composites based on natural fiber mat thermo-

HDPE–rice hull composite with 65% fiber content.Adopted from [239]. Copyright 2006. By permission from Elsevier Ltd.

The hygrothermal weathering properties of rice hullreinforced HDPE composites were studied by Wang et al.[240]. The samples (50% rice hull and 50% HDPE) absorbed4.5% moisture after 2000 h of exposure to a relative humid-ity (RH) of 93% at 40 ◦C. The walls of the samples swelledsignificantly (7.1%) in thickness and ultimately developedabout 5 mm of longitudinal bowing (Fig. 11) based on 61 cmlong boards. Both expansion and bowing were partiallyrecovered after another 2000 h exposure to 20% RH at 40 ◦C.

Deformation results in increased moisture contents.Temperature also played a significant role by causing directthermal expansion or contraction and by affecting the rateand the amount of moisture adsorption.

Sisal fiber reinforced PE [241] and HDPE [242,243]composites were examined regarding their interfa-cial properties, isothermal crystallization behavior andmechanical properties. Sisal fiber treatment with stearicacid increased the interfacial shear strength by 23%compared to untreated fibers in sisal/PE composites. Per-manganate (KMnO4) and dicumyl peroxide (DCP) roughenthe fiber surface and introduced mechanical interlockingwith the HDPE. Sisal fiber reinforced HDPE shows a stablede-bonding process with Permanganate and DCP treat-ment. Silane treated sisal fiber reinforced HDPE exhibits anunstable debonding process. A high crystallization rate and
short crystallization half time were observed for sisal/HDPEcomposites compared to neat HDPE. This demonstrates thestrong nucleating abilities of sisal fibers. With increasing
Fig. 11. Partial recovery of bowing (a) before recovery: 5.00 mAdopted from [240]. Copyright 2005. By permission from Elsevier Ltd.

cience 37 (2012) 1552– 1596 1569

fiber content, the crystallization activation energy of thecomposites decreased.

Similar investigations (PE as matrix) were carriedout using soya powder [244], curaua [245], rape straw[246,247], hemp [248], rice straw [249], bagasse [250] ricehull fibers [251] and LDPE as matrixes with wheat straw[252], abaca, bagasse and rice straw fibers [253].

4.1.1.2. PP (polypropylene). The thermo-mechanicalbehavior of hemp fibers was studied with focus on man-ufacturing high performance natural fiber reinforced PPcomposites. Hemp fibers subjected to quasi-static tensileexperiments and temperature-controlled harmonic testsrevealed a complex behavior, involving several mecha-nisms [254]. The hemp fiber behavior was affected bytemperature and acted not only as an activation factor, butalso as a degradation factor with respect to the visco-elasticproperties of the fibers. It was discovered that PP–hempfiber composites with relatively high performance can beachieved with some specific mechanical properties.

The effect of sisal fiber degradation and aging on thePP/sisal fiber composites was investigated. The fiber matproduction method allowed the fibers to avoid much of theprocessing degradation, which is often encountered dur-ing conventional shear mixing processes [255]. The bestpossible mechanical properties for the sisal fiber/PP com-posites were achieved when the fiber length was greaterthan 10 mm and the fiber mass fraction was in the range of15–35%. The aged sisal fiber showed lower tenacity, break-ing strength and elongation with in comparison to freshsisal fibers. On the contrary, in sisal/PP composites, agedsisal fiber composites exhibited better mechanical proper-ties than fresh sisal fiber/PP composites [256].

Rana et al. [257] observed an increase in impact strengthin jute fiber/PP composites as a result of fiber loading. Itwas also found that both the impact and tensile propertiesshowed increasing trends with the addition of compati-bilizers. However, the opposite was true for the flexuralproperties. Improvements of physico-mechanical proper-ties achieved by post-treatment [258], and the interfacialand durability evaluation [259,260] of jute fiber/PP com-posites were studied.

The environmental performance of hemp fiber rein-

plastic (NMT) was evaluated by quantifying carbon storagepotential and CO2 emissions. The results were comparedwith commercially available glass fiber composites [261].

m of bowing (b) after recovery: 3.00 mm of bowing.


Fig. 12. CO emissions per ton composite and reduction in emissions bysA

bbclites

fdwatmca

c

A

2

ubstituting glass fiber with hemp fibers.dopted from [261]. Copyright 2003. By permission from Elsevier Ltd.

Fig. 12 shows CO2 emissions in ton/ton of composite foroth natural and glass fibers. These values are estimatedy converting energy into CO2 emissions using standardonversion factors for different fossil fuels. The heat energyiberated by incineration of hemp fiber and PP is also addedn non-renewable energy requirements to make calcula-ions simpler. The results demonstrate a net reduction inmissions of 3 ton CO2/ton of product if glass fibers are sub-tituted by hemp fibers.

The potential of wheat straw fibers to be used as rein-orcing additives for PP prepared with the influence ofifferent processes (mechanical and chemical processes)as investigated [262]. Fibers prepared by mechanical

nd chemical processes were characterized with respecto their chemical composition, morphology, physical,

echanical and thermal properties. The fibers prepared by
hemical processes exhibited better mechanical, physicalnd thermal properties.
The morphology of the wheat straw fiber surfaces pro-essed by mechanical and chemical means is shown in

Fig. 13. Surface of wheat straw fibers by different methods (adopted from [262]. Copyright 2006. By permission from Elsevier Ltd.

Fig. 14. TGA curves of wheat straw fibers prepared by different processes.Adopted from [262]. Copyright 2006. By permission from Elsevier Ltd.

the SEM photomicrographs (Fig. 13). The surface of thechemically processed fibers was more uniform and homo-geneous while the fibers processed mechanically exhibitmore surface irregularities. Mechanically processed fibersstill contained the cellular residues such as lignin and hemi-cellulose, which cement the fibers together.

The influence of chemical and mechanical processes onthe TGA of the wheat straw fibers is shown in Fig. 14.The onset of degradation of wheat straw fibers preparedby mechanical and chemical processes were 217 ◦C and242 ◦C respectively indicating the suitability of these fibersfor processing with thermoplastics, especially polyolefins.The higher onset of degradation temperature indicates theimproved thermal stability of the fibers prepared by chem-
ical processing.
Similar to PE, PP is also widely used as a matrix incombination with different natural fibers: such as hemp[263,264], flax [265–267], kenaf [268,269], oil palm [270],

) mechanical processing and (b) chemical processing.

lymer S
date palm [271], coir [272,273], bamboo [274,275], abaca[276,277], rice hull [278–280], jute [281,282], and wheatstraw [283].

4.1.1.3. PS (polystyrene) and PVC (polyvinyl chloride). Onlylimited studies were reported regarding the usage of PSand PVC as matrixes for natural fibers. The thermal anddynamic mechanical behavior [284], as well as the rheolog-ical [285] and mechanical properties [286] of PS compositesreinforced with sisal fibers were studied. The PS compos-ites reinforced with agave fiber were also studied regardingtheir fiber surface modification [287]. The impact proper-ties of PVC composites reinforced with bamboo fibers [288],the interfacial modification of bagasse fiber [289] and thethermal degradation of abaca fiber [290] reinforced PVCcomposites were also evaluated.

4.1.2. Thermosets4.1.2.1. Polyester. The possibility of using date palm fibersas reinforcement in polyester composites was investigated[291]. Flexural properties and impact strength of date palmfiber/polyester composites were found to be influenced byfiber content and fiber treatment method. Soda treatedfibers exhibited higher mechanical properties comparedto untreated fiber/polyester composites. The fiber frac-tion and fiber length for this system were optimized for9 wt% and 2 cm, respectively. Water absorption was lightlyaffected by surface modification of the fibers and was rel-atively low.

The hybrid effect on the mechanical properties of abacaand sisal fiber reinforced polyester composites was eval-uated [292]. A positive hybrid effect was observed forthe flexural properties. The tensile strength was foundto be increased when the volume fraction of banana wasincreased. A negative effect was observed for the impactproperties. Recent works regarding sisal and jute fiber[293], sisal [294], abaca [295], abaca and sisal [296], flax[297], jute fibers [298], sisal, abaca and bamboo fiber [299]reinforced polyester composites have been developed.

4.1.2.2. Epoxy. The excellent properties of epoxy (goodadhesion, mechanical properties, low moisture content,little shrinkage, and processing ease) make it one of thebest matrix materials for composites. The effect of fibertreatment on the mechanical properties of unidirectionalsisal/epoxy composites was reported [300]. Ganan et al.[301] evaluated the mechanical and thermal propertiesof sisal/epoxy composites as a function of fiber modifica-tion. Kenaf [302], hemp and flax [303], oil palm [304], sisal[305,306], flax [307], sisal and hemp [308], flax, hemp andkenaf [309], lantana camara fiber [310] and sugar palm fiber[311] were reinforced with an epoxy matrix. Their mechan-ical properties, processing methods, water absorption, andfracture toughness were evaluated.

4.1.2.3. Phenolic. Phenolic resins show superior fire resis-tance to other thermosetting resins. Sreekala et al.
[312–316] extensively investigated oil palm fiber rein-forcement in phenolic resins. The fiber surface modifi-cation [312], a comparison of mechanical properties toglass fiber/phenolic resin composites [313], the dynamic
cience 37 (2012) 1552– 1596 1571

mechanical properties regarding the fiber content andhybrid fiber ratio [314], the water absorption [315] andstress–relaxation behavior [316] of oil palm/phenolic resincomposites were evaluated. Bamboo [317], jute [318,319],date palm fiber [320], and grewia optiva fiber [321] werealso investigated with phenol formaldehyde resin as amatrix.

4.2. Bio-based

Public concern about the environment, climate changeand limited fossil fuel resources are important drivingforces, which motivate researchers to find alternatives tocrude oil. Bio-based plastics may offer important contri-butions by reducing the dependence on fossil fuels and, inturn the related environmental impacts. Biopolymers haveexperienced a renaissance in the recent years. Many newpolymers were developed from renewable resources, suchas starch, which is a naturally occurring polymer that wasre-discovered as a plastic material. Others are poly lacticacid (PLA) that can be produced via lactic acid from fer-mentable sugar and polyhydroxyalkanoate (PHAs), whichcan be produced from vegetable oils next to other bio-basedfeed stocks.

4.2.1. PLA (polylactide acid)Kenaf fiber reinforced PLA composites were studied

extensively. Investigations have been undertaken con-cerning chemical modifications such as alkylation [322]and silane treatments [323], the biodegradability [324]and the mechanical and dynamic mechanical proper-ties [325]. The alkali treated fiber reinforced composites,followed by silane-treated fiber reinforced composite pos-sessed significantly improved mechanical properties. Theheat deflection temperature (HDT) of the kenaf rein-forced PLA composites was significantly higher than thatof neat PLA resin. The biodegradability of kenaf/PLA com-posites corresponds to the weight of the composites,which decreased approximately 38% after four weeks ofdecomposition. Young’s modulus (6.3 GPa) and the ten-sile strength (62 MPa) of the kenaf/PLA composites (fibercontent = 70 vol%) were comparable to those of traditionalcomposites. In addition, the storage modulus of the com-posite remained unchanged until the melting point of PLAwas reached.

The interfacial characterization of flax fiber/PLA com-posites was performed on a micro-scale using themicrobond testing method [326]. The interfacial mecha-nisms of flax/PLA biocomposite were exposed to varyingthermal treatments (i.e., different cooling rates and anneal-ing to release thermal stress). The treatments resulted indifferent micro-structures and residual stress states insidethe material. Cooling kinetics is shown to modify the inter-facial properties and when the cooling rate is slow, theinterfacial properties improve. In addition, a high degreeof crystallinity was measured, including transcrystallinityaround the flax fiber. The higher crystalline morphology of
the PLA results in an improvement of the tensile and shearproperties and higher residual compressive stress.
Bledzki et al. [327] investigated PLA biocomposites withabaca and man-made cellulose fibers and compared these

1572 O. Faruk et al. / Progress in Polymer S

0

40

80

120

160

200

PP/cellulose PP/abaca PP PLA/cellulose PLA/abaca PLA

[MPa]

Tensile strength

Flexural strength

(a)

0

2000

4000

6000

8000

10000

PP/cellulose PP/abaca PP PLA/cellulose PLA/abaca PLA

[MPa]

Tensile modulus

Flexural modulu s(b)

FPA

tcctofRE1

pdalc1mbrsioifiatft2

b[bt

ig. 15. Tensile and flexural properties [(a) strength and (b) modulus] ofLA and PP composites (fiber content 30 wt%).dopted from [327]. Copyright 2009. By permission from Elsevier Ltd.

o PP composites. The composites were processed usingombined molding technology: first a two-step extrusionoating process was carried out and consecutively an injec-ion molding was completed. With man-made cellulosef 30 wt%, the tensile strength and modulus increased byactors of 1.45 and 1.75 times in comparison to neat PLA.einforcing with abaca fibers (30 wt%) enhanced both the-modulus and the tensile strength by factors of 2.40 and.20, respectively (Fig. 15).

The fibers’ length distribution in pellets after com-ounding and in samples after injection molding isepicted in Fig. 16. The measurement was evaluated usingpproximately 500 fibers. A significant decrease in the fiberength after compounding on the single-screw extruderan be observed, from the cut length of 15 mm to about–2 mm. Moreover, the injection molded samples showerely an insignificant drop in length, when put side

y side to pellets. This indicates to massive fiber lengtheduction during extrusion processes. Processing on theingle-screw extruder already affected the length signif-cantly. Compounding on twin-screw extruders, which isften carried out during composite processing, can exceed-ngly influence the fiber length even more. The minimalber length achieved during compounding is not beingffected during the injection molding process; however,he occurrence of the fiber length seems to be more uni-orm. As a result, most of the fibers, which remain longerhan 2 mm after compounding, are shortened less than

mm during injection molding.PLA as a matrix for biocomposites reinforced with

iofiber was extensively investigated for jute fiber328–331], flax fiber [332,333], kenaf [334,335], coir [336],amboo [337] and cellulose fibers [338,339]. Special atten-ion was paid to the influence of chemical treatments,

cience 37 (2012) 1552– 1596

processing techniques, coupling agents, micro-fibrillatedfiber bundles, and reinforcement using man-made cellu-lose fibers.

The mechanical properties of PLA reinforced with flaxfibers [340], the influence of the additives on the inter-facial strength [341], as well as the effects of recycling[342] and sea water aging [343] on the mechanical prop-erties was investigated. It was found that the compositestrength is about 50% better compared to similar PP/flaxfiber composites, which are used in many automotive pan-els today. The different additives significantly influencedthe interfacial strength of the PLA/flax fiber compositeswhile extraction with acetone had no effect compared tothe untreated fibers. The use of TDP (4,40-thiodiphenol)imparted the most significant increase in interfacial shearstrength whilst HBP (Hyper Branched Polyester) had anopposing effect. The repeated injection molding cycleswere influenced by many parameters; i.e., the reinforce-ment geometry, the molecular weight of PLA and, thethermal and rheological behavior. The tensile stiffness ofPLA is hardly affected by seawater at temperatures approx-imately of 40 ◦C, but the composite does loses stiffness andstrength. The interfacial adhesion weakens more due to wetaging. Cracks also appear in the PLA and flax fibers afterlonger periods of time.

Huda et al. [344,345] examined the effect of reinforc-ing recycled newspaper, the addition of silane and talcon the thermal and mechanical properties of PLA/recyclednewspaper fiber composites. The results indicated thatthe addition of fibers increased the thermal stability ofthe composites compared to neat PLA. The heat deflec-tion temperature of the PLA/recycled newspaper fibercomposites was found to be comparable to that of theglass fiber-reinforced PLA composites. The silane treatedtalc reinforced PLA/recycled newspaper fiber hybrid com-posites flexural and impact strength was found to besignificantly higher than that of those made withoutsilane treated composites. The silane treated hybridcomposites showed improved properties such as a flex-ural strength of 132 MPa and a flexural modulus of15.3 GPa, while the untreated composites exhibited flexu-ral strength and modulus values of only 77 MPa and 6.7 GPa,respectively.

PLA and PHBV were compounded with man-madecellulose; jute and abaca fibers and their mechanicalperformance were studied and compared to PP basedcomposites [346]. It is clearly seen in Table 7 that thetested biopolymer based composites clearly show betteror comparable mechanical characteristic values than the‘common’ natural fiber reinforced PP composites.

4.2.2. PHB (polyhydroxybutyrate)The biopolymer polyhydroxybutyrate (PHB) was also

reinforced with flax fiber. Barkoula et al. [347] investigatedthe effects of fiber and copolymer HV (hydroxyvalerate)contents, the biodegradability, and the influence of man-ufacturing methods (compression molding of non-woven
mats and injection molding of short fiber compounds) andprocessing conditions (cooling temperature and anneal-ing) on the mechanical properties of the composites. Itcan be concluded that the addition of flax fibers, along


PLA com
Fig. 16. The fiber length of abaca/Adopted from [327]. Copyright 2009. By permission from Elsevier Ltd.
with controlled processing conditions, is a convenient tech-nique to toughen the PHB matrix. The injection moldedcomposites exhibited lower impact strength than othermanufactured composites. The tensile strength droppedsignificantly in the initial stage of degradation and wasmore gradual in later stages of biodegradation.

The interfacial improvement of PHB/flax compositeswas also evaluated. The addition of TDP at various con-centrations (up to 10% v/v) brought advantageous changesin the dynamic flexural properties and thermal stability(favorable shift of degradation temperatures to higher val-ues). With increases of the TDP content, improvementswere observed in the fiber–matrix bonding and a change in
the matrix from brittle to ductile [348]. Zini et al. [349] illus-trated that composites prepared by compression moldingusing long fiber mats showed better mechanical properties
Table 7Mechanical properties of PLA, PHBV and PP based natural fiber reinforced compo

Matrix Fiber Tensile E-modulus (GPa)

PLA – 3.4 ± 0.23

Man-made cellulose 5.8 ± 0.15

Abaca 8.0 ± 0.34

Jute 9.6 ± 0.36

PHBV/ecoflex – 2.14 ± 0.07


Abaca 4.4 ± 0.06

Jute 7.0 ± 0.26

PP – 1.5 ± 0.03


Abaca 4.9 ± 0.11

Jute 5.8 ± 0.47

posites after different processes.

compared to composites obtained by batch mixing fiberswith molten polymer. Chemically modified (by acetylationor by short-chain-PEG grafting) fibers exhibit better adhe-sion and the best results were obtained with acetylatedfibers.

Several lines of transgenic flax were produced by over-expressing the PHB synthesis genes [350]. It was revealedthat the cellulose in fibers from the transgenic plants wasmore highly structured than in fibers from the controlplants and PHB was strongly bound to the cellulose of thefibers by covalent ester and hydrogen bonds. The PHB/flaxcomposite with fibers from transgenic plants was signifi-cantly stronger and stiffer than the composites with fibers
from the control plants.
Polyhydroxybutyrate-co-valerate (PHBV) bioplasticwas reinforced with hemp [351], coir [352], and bamboo

sites.

Tensile strength (MPa) Tensile elongation-at-break (%)

63.5 ± 0.4 3.3 ± 0.592.0 ± 4.7 1.9 ± 0374.0 ± 0.7 1.44 ± 0.181.9 ± 2.9 1.8 ± 0.027.3 ± 0.3 7.0 ± 1.141.7 ± 3.8 2.3 ± 1.028.0 ± 1.3 0.9 ± 0.135.2 ± 1.3 0.8 ± 0.029.2 ± 0.4 >5071.6 ± 2.7 3.5 ± 0.542.0 ± 0.5 1.7 ± 0.247.9 ± 2.7 1.4 ± 0.1

1 lymer Science 37 (2012) 1552– 1596

[m

4

ittmriTmr

m[[fi

4

taBnbfi(

4

tuircwippmdtttniddTbor

5

an


353] fibers. The mechanical, thermo-mechanical andorphological properties were evaluated.

.2.3. StarchAlvarez et al. [354–356] developed composites by mix-

ng biodegradable starch matrixes with sisal fibers. Thehermal, melt rheological and creep properties were inves-igated extensively. It was reported that the shear rate is the

ost influential processing condition regarding the mate-ial structure. The intercalation effectiveness of the matrixn the fibers is directly linked to the rheological behavior.he addition of sisal fibers to the polymeric matrix pro-otes a significant improvement of the composite creep

esistance and thermal properties.Investigations were carried out using starch as the

atrix of composites with kenaf, bagasse [357], curaua358], coir [359], bamboo [360], sisal [361], hemp362,363], abaca and bagasse [364] fibers, wheat strawbers [365], and kenaf fibers [366].

.2.4. OthersA soy based biodegradable resin matrix was applied

o cellulose from papers [367], flax [368], sisal [369]nd pineapple leaf [370] fiber reinforced composites.iodegradable resins have been tested, including cashewut shells [371,372] with hemp and kenaf fibers, poly-utylene succinate (PBS) with jute fibers [373,374] bamboobers [375], and hemp fibers [376], and polycaprolactonePCL) with flax fibers [377] and bamboo fibers [378].

.3. Summary

The petroleum derived thermoplastics PP and PE arehe two most commonly employed thermoplastics in nat-ral fiber reinforced composites. Day by day, there is great

nterest in developing biocomposites with a thermoplasticather than thermoset matrix, mainly due to their recy-lability. Also the choice of a thermoplastic matrix fits wellithin the eco-theme of biocomposites, but there are some

mportant limitations on the recyclability and mechanicalerformance of thermoplastics. Generally, the mechanicalroperties of thermosets exhibit higher than the ther-oplastic (lower modulus and strength). In addition, a

ramatic loss in properties is observed above the glassransition temperature, which leads to decrease in otherhermally sensitive properties such as creep resistance. Onhe contrary, thermoplastics show greater fracture tough-ess than thermosets and thus are more useful in resisting

mpact loads. Another remarkable change was the intro-uction of biopolymers in recent years with the aim ofecreasing reliance on petroleum-based thermoplastics.he availability and outstanding mechanical properties ofiopolymer PLA has led to this matrix system being onef the most thoroughly investigated in the biocompositesesearch area.

. Processing techniques

Generally, natural fiber reinforced plastic compositesre manufactured by using traditional manufacturing tech-iques (designed for conventional fiber reinforced polymer

Fig. 17. Water absorption of HDPE composites with and without compat-ibilizers.Adopted from [380]. Copyright 2007. By permission from Elsevier Ltd.

composites and thermoplastics). The processing tech-niques includes compounding, mixing, extrusion, injectionmolding, compression molding, and resin transfer mold-ing (RTM) and above mentioned techniques have beenwell developed and accumulated experience has proofedtheir successability for producing composites with control-lable quality. Innovative technologies and process solutionsshould be intensively researched to get the high strengthengineering composites which is related to their new appli-cations area.

5.1. Factors influencing processing

5.1.1. MoistureThe moisture content at a given relative humidity can

have a great effect on the biological performance of a com-posite made from natural fibers and therefore fiber dryingbefore processing is a significant step. A composite madefrom pennywort fibers would have much greater moisturecontent (57%) at 90% RH humidity than would a compositemade from bamboo fibers (15%). The pennywort productwould be much more prone to decay as compared to thebamboo product. Moisture content of natural fibers withtime under different conditions (relative humidity) and fordifferent fiber treatments were analyzed [379]. It was evi-dent that the moisture content as well as the rate at whichit accumulates is directly related to the ambient relativehumidity. Fiber treatment can reduce both the moisturecontent level and the rate of absorption very significantly.It should be noted that bio based polymers can be moresensitive to moisture than natural fibers.

Panthapulakkal and Sain investigated the effect of com-
patibilizer on the water absorption behavior of HDPEcomposites with different fillers (wheat straw, cornstalk,and corncob). This is shown in Fig. 17 [380]. Presence ofcompatibilizer had no effect on the water absorption of


moldinterials T

Fig. 18. Compounding methods for injectionAdopted from [383]. Copyright 2006. By permission from Institute for Ma

corncob filled composites. Wheat straw and cornstalk filledcomposites showed a decrease in the water uptake withthe incorporation of compatibilzers. This indicates that, theflaws and gaps at the interface of the filler and HDPE arethe main factors for moisture diffusion in the compositeswithout compatibilizer. As observed in mechanical prop-erties, the observed reduction in water uptake is higher inwheat straw filled composites compared to cornstalk filledcomposites. After 40 days of water absorption, percentagereduction in the water uptake of composites is 19%, 12%and 0% respectively for wheat straw, corn stalk and corncobfilled composites.

It is already seen that the enhanced water contentcan drastically affect mechanical properties (such as, com-pression, flexural and tensile) of the composites [381].
Nowadays new extruder screw design (higher L/D ratio)allows better degassing and, consequently, lower mois-ture content. In addition, the machine’s barrel must beredesigned.
Fig. 19. Influence of different natural fibers on the tensile and Charpy notched imAdopted from [387]. Copyright 2008. By permission from Woodhead Publishing L

g of natural fiber reinforced thermoplastics.echnology of the University of Kassel, Germany.

5.1.2. Fiber type and contentThe natural fiber type and content is generally essen-

tial for furthering the sustainability of the composite. Inaddition, natural fibers length (short or long), aspect ratio(length/diameter), chemical compositions (such as ricehusk contains higher silicates than flax or jute fiber) havean great influence on the processing and therefore process-ing parameters and accessories should be developed. Thecompounding process significantly influences the shorten-ing, fibrillation, as well as the thermal deterioration of thefibers in early stages; the final properties of the productare already determined at the beginning of the productionprocess [382]. Fig. 18 shows the different compoundingprocesses for injection molding of natural reinforced ther-moplastics [383].

5.1.3. Influence on composites propertiesAs described above, the natural fiber type has strongly

influence on the performance of the composite. There are

pact strength (+23 ◦C) of PLA biocomposites (fiber content 30 wt%).td.

1576 O. Faruk et al. / Progress in Polymer S

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umerous types of natural fibers, which differ regardingheir chemical structure [384–386]. The influence of theype of natural fiber on some mechanical parameters ofLA biocomposites is presented in Fig. 19 [387].

In general, increasing fiber content in the compositesncreases the composite’s stiffness significantly. Addition-lly its strength is increased through the addition of naturalbers [388,389]. Higher fiber content improves the impacttrength. Unfortunately, increased fiber content increaseshe composite’s odor (Fig. 20) as well as the water uptake387]. Moreover, the composite’s ductility can be affected.he fiber length and its geometry also play a decisive role inomposites. Usually, most mechanical properties of a fiberan be enhanced by increasing the aspect ratio [390].

In addition, the use of one appropriate additive (cou-ling agents, lubricants, light stabilizers, colorants, flameetardants, foaming agents, odor reduction agents, and bio-ides) in very small quantities (0.5–5%) can significantlymprove most of physical, chemical or mechanical proper-ies of natural fiber composite material [391].

.2. Semi-finished product manufacturing

Mat production is one of the initial steps in semi-nished product manufacturing. Fiber mats (fiber fleece)

rom natural fibers could be manufactured by cardingethod, aerodynamic fleece making, the fiber spreading

rocess, and wet fleece production. The other step is theroduction of slivers and fiber yarns. The fiber prepara-ion (opening, mixing, and carding) is similar to fleeceroduction. Granule production is the other importanttep in the manufacture of semi-finished products. Naturalber granules can be produced via pelleting, compounding,ultrusion and the pull-drill process regarding the homo-eneity of the granules and the desired fiber lengths.

.3. Processing technologies for natural fiber reinforcedhermoplastics

.3.1. Compression molding
The pressure method is very popular in the manu-
acture of natural fiber composites because of its higheproducibility and low cycle time. The two methods inse are compression and flow compression molding. The

cience 37 (2012) 1552– 1596

processes differ concerning the kind of semi-finished prod-uct used and its cutting. In the compression moldingprocess, flat semi-finished products or hybrid fleeces areusually used that are either larger than the form or are cutexactly to the size of the desired part.

Whole and split wheat straws with lengths up to 10 cmwere used with PP to make lightweight composites bymeans of the compression molding process [392]. Theeffects of the wheat straw concentration, length, and splitconfiguration (half, quarter, and mechanically split) onthe flexural and tensile properties of the composites wereinvestigated. Compared with whole wheat straws–PP com-posites, mechanically split wheat straw–PP compositeshave a flexural strength that is higher by 69%. They alsohave a 39% higher modulus of elasticity, 18% higher impactresistance properties, 69% higher tensile strength and a 26%higher Young’s modulus. Compared with jute–PP compos-ites, mechanically split wheat straw–PP composites havea 114% higher flexural strength, 38% higher modulus ofelasticity. In addition, they also have a 10% higher tensilestrength, 140% higher Young’s modulus, a 50% lower impactresistance and better sound absorption properties.

The compression molding process was tested for jutefiber with PP [393], an ethylene propylene co-polymer[394] and, baggase fibers with PET composites [395].Also the abrasive wear behavior, interfacial adhesion, anddegradability were evaluated.

5.3.2. ExtrusionThe extrusion process is used by the plastics industry

for the production of granules and also in the continuousproduction of semi-finished products or components. Sin-gle screw as well as twin-screw extruders that run eitherco- or counter-rotating may be used for this process. Singlescrew extruders are used when the mixing effect does nothave to be very high. Owing to the excellent mixing effect ofthe twin-screw extruder the natural fiber materials can behomogenously distributed and wetted in the thermoplasticmelt.

HDPE composites with bagasse [396] and curaua [397]fibers reinforced HDPE composites were obtained byextrusion process. The effects of coupling agents on themechanical and thermal properties of those compositeswere investigated and evaluated. Curaua fibers were alsoreinforced with PA-6 by using a co-rotating twin-screwextruder [398]. The fiber contents of 20 wt%, 30 wt% or40 wt% and fiber lengths of 0.1 or 10 mm were studied.Fibers were treated with N2 plasma or washed with NaOHsolution, to improve their adhesion to PA-6. The tensileand flexural properties of this composite are better thanunfilled, but are still lower than those of glass fiber rein-forced polyamide-6.

5.3.3. Injection moldingIt is possible to produce complex geometric compo-

nents with functional elements fast and also in greatnumbers by injection molding. It offers a number of advan-
tages (economics of scale, minimal warping and shrinkage,high function integration, use of recycled materials) ascompared to compression molding [399] and hardly anyfinishing needed.


Table 8Tensile strength and modulus of PP and PP/hemp fiber/glass fiber hybrid composites before and after aging (A. 0 wt% glass fiber, B. 5 wt% glass fiber, C.10 wt% glass fiber, D. 15 wt% glass fiber).

Sample designation Tensile strength (MPa) Tensile modulus (GPa)

Original Wet sample Retention (%) Original Wet sample Retention (%)

PP 30.1 ± 0.1 29.9 ± 0.5 99 1.10 ± 0.03 0.89 ± 0.03 81A 52.5 ± 0.5 34 ± 0.6 65 3.77 ± 0.05 1.64 ± 0.04 44B 53.7 ± 1.6 34.8 ± 1.6 65 4.07 ± 0.05 1.74 ± 0.03 43

05

C 57.9 ± 0.7 34.9 ± 0.9 6D 59.5 ± 0.9 35.5 ± 0.5 6

Pickering and his co-workers [400–403] extensivelyinvestigated hemp fiber reinforced PP composites usedin the injection molding process. Their investigationsincluded fiber treatments and modifications, model predic-tions of micro-mechanics and strengths, the optimizationof hemp fiber quality, and the influence of bag retting and,white rot fungal treatments.

The effect of surface treatment on the injection moldedpineapple leaf fiber reinforced polycarbonate compositeswas evaluated [404]. The modified pineapple leaf fiberscomposite produces enhanced mechanical properties. Theresults from the thermogravimetric analysis showed thatthe thermal stability of the composites is lower than thatof neat polycarbonate resin and that the thermal stabil-ity decreased with increasing pineapple leaf fiber content.Injection molded jute [405] and sisal [406] fiber reinforcedPP composites were also investigated regarding their ther-mal, hydrothermal and, dynamic mechanical behavior. Theintroduction of the pre-impregnation technique was alsoexamined.

Panthapulakkal and Sain investigated the influence ofwater absorption on the tensile properties of injectionmolded short hemp fiber/glass fiber-reinforced polypropy-lene hybrid composites [407]. Tensile tests were performedon wet samples after the saturation level (3624 h) isreached (Table 8). A significant reduction in strength andstiffness is observed for all composites because of thechanges to the fibers, mainly natural fibers, and the inter-face between the matrix and fiber, as the effect of waterabsorption on PP matrix is only secondary. Swelling of nat-ural fiber as a result of prolonged exposure to water, leadsto at reduction in the stiffness of the fibers. The loss instrength and modulus values of the hemp fiber compositesare believed to be the inability of the swelled natural fiberto carry the stress transferred from the matrix through the
disrupted interface as a result of water absorption. Despitethe reduction in the equilibrium moisture content in hybridcomposites, glass fibers did not alter the degradation of thehemp fiber composites.
Table 9Effect of filler type on mechanical strength performance of filledPP composites (4

Fiber type BKP Flax

Tensile (MPa) 50 42

Tensile modulus (GPa) 3.0 3.2

Flexural (MPa) 78 67

Flexural modulus (GPa) 3.3 3.4

Notched Izod (J/m) 40 44

U-notched Izod (J/m) 205 150

4.25 ± 0.04 2.04 ± 0.05 484.4 ± 0.01 2.32 ± 0.12 53

Injection molded flax, hemp, core hemp, bleached kraftpulp (BKP) and wood flour reinforced PP composites wereprepared and the effect of MAPP on the mechanical prop-erties was investigated [408]. Table 9 clearly indicated thatthe BKP–PP composites have the highest tensile and flex-ural strength and unnotched impact strength. Flax andhemp-filled PP composites have similar strength prop-erties. In comparison, wood flour-filled PP compositeshave the lowest mechanical strength. Such a trend can beattributed to the actual fiber length in the composites. It hasbeen observed that after compounding, the fiber length offlax and hemp decrease significantly, whereas that of BKPfibers dose not change much. This is possibly due to thefact that the flax and hemp bundles are prone to being torndown to very small size by the high-shear force from themelted PP. To preserve the fiber length, less severe treat-ment is necessary but at the cost of poor dispersion of fibersin the resin matrix. The poor strength of wood flour rein-forced composites is due to the low aspect ratio of woodflour particles, which is far below the critical fiber lengthrequired for reinforcement.

Bledzki et al. [409] investigated the different separationprocesses (mechanical, refiner and enzymatic separation)with injection molded hemp and partially with flax andwheat straw reinforced PP composites. It was found thatthermomechanical processed hemp fiber–PP compositespossessed better mechanical properties compared to otherprocess and composites.

5.3.4. Long fiber thermoplastic-direct (LFT-D) methodDifferent methods [410] to directly process natural

fibers with the LFT-D-process are shown (Fig. 21). Expressmethod, which is the basic thought of the combined extru-sion and press method to combine the natural fiber matswith the polymer melt directly in the pressing tool. A film
of molten polymer is placed into the pressing tool with thehelp of an adjustable extruder and a natural fiber fleeceis added to the molten mass and the layers are pressedtogether. Another possibility to process natural fibers with
0% filler, 2% compatibilizer).

Milled hemp Core hemp Wood hour

42 29 353.0 2.3 2.9

70 52 593.5 2.6 3.0

42 20 28145 100 105


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Table 10Mechanical properties of 20 vol% glass fiber, 20 vol% hemp fiber and35 vol% hemp fiber-reinforced unsaturated polyester composites.

20 vol%glass fibers

20 vol%hempfibers

35 vol% hempfibers

Fig. 21. Express method andopted from [410]. Copyright 2002. By permission from Institute for Ma

he LFT-D-method is to directly feed off fiber yarns or sliv-rs into either a twin screw extruder or directly into annjection molding machine. Research has shown that it isossible to retain the long fiber structure through an opti-al configuration of the screws; however, the handling of

livers is rather problematic in an industrial applicationextrusion with high throughput).

.4. Processing technologies for natural fiber reinforcedhermosets

.4.1. Resin transfer moldingResin transfer molding (RTM) is a method for the

roduction of component parts made of fiber-plastic com-osites. During the RTM procedure, dry semi-finished fiberarts are streamed and consequently soaked with reactionesin by a pressure gradient within a closed vessel. The fol-owing methods can be distinguished with regard to thedmission by the pressure gradient: high pressure injec-ion, twin wall injection, vacuum injection, and differentialressure injection.

Hemp fiber-unsaturated polyester composites wereanufactured using a resin transfer molding (RTM) pro-

ess [411]. RTM composites with various fiber contents,p to 20.6% by volume, were manufactured. The wettingf the fibers was very good. The resin injection time wasbserved to increase dramatically at high fiber contentsue to the low permeability of the mat. Keeping a constantold temperature is the key to obtain fast and homoge-

eous curing of the part. The cure of the resin in the moldas simulated. The results given by the modified model

re presented in Fig. 22. The predicted temperatures weren very good agreement with the experimental data. The

odel predicted the degree of cure and the rate of cure ofhe resin with time as well.

The hemp fiber composites manufactured with RTMrocess were found to have a very homogeneous structureith no noticeable defects [412]. The tensile, flexural and

mpact properties of these materials was found to increaseinearly with increasing fiber content (Table 10). It wasbserved that the optimum properties were not reached inhis study and that fiber content higher than 35 vol% shouldield better mechanical properties. When compared to a
lass fiber composite however these natural fiber compos-tes had much lower performances.
Hemp fiber reinforced polyester [413] and phenolicesin [414] composites were prepared using RTM process

Fig. 22. Comparison of RTM experiment and corrected model for a pureresin system.Adopted from [411]. Copyright 2004. By permission from Elsevier Ltd.

and their cure simulation, interfacial adhesion, and flowvisualization were evaluated.

The effects of fiber surface modification [415] wereevaluated. A comparison of the RTM process to the com-pression molding process [416] of sisal fiber reinforcedpolyester composites was made. Composites containingflax and hemp fiber with bio-resin [417], flax and jute fiberwith epoxy and polyester resin [418], and jute fiber withpolyester resin [419] were also prepared using the RTMprocess.

5.4.2. Sheet molding compoundThe sheet molding compound (SMC) material made of

natural fibers and a special resin based on vegetable oil

Tensile strength (MPa) 85.0 32.9 60.2Tensile modulus (GPa) 1.719 1.421 1.736Flexural strength (MPa) 175.9 54.0 112.9Flexural modulus (CPa) 7.74 5.02 6.38Impact strength (kJ/m2) 60.8 4.8 14.2


onent pterials T

Fig. 23. Thermoset mat compression process and ready compAdopted from [423]. Copyright 2006. By permission from Institute for Ma

parts [420]. The middle bumper part for a city bus wasmanufactured with standard tools. The risk potential forthe fabricator is lower, both during production and treat-ment of the prepregs which are made of natural fibers andvegetable oil. Emissions do not occur; hence, there is noneed for breathing protection [421]. Flax fiber reinforcedSMC material was developed and the processing param-eters regarding the physico-mechanical properties wereoptimized [422].

5.5. Other processes

5.5.1. Thermosets compression moldingThe thermosets compression process uses mats made of

natural fibers. As shown in Fig. 23, the mats are just sprayed,not moistened, with resin and compressed into their finalcontour in a hot tool; due to the air-permeability the partscan be covered easily in a vacuum covering process [423].

The dynamic and static mechanical properties of shortabaca/sisal hybrid fiber reinforced polyester compositeachieved using the thermoset compression molding pro-cess were determined [424]. Dynamic properties, such asthe storage modulus, damping behavior and static mechan-ical properties (i.e., tensile, flexural and impact properties)were investigated as a function of the total fiber volumefraction and the relative volume fraction of the two fibers.The storage modulus was found to increase with fiber vol-ume fraction above the glass transition temperature (Tg) ofthe matrix. A maximum value was obtained at a volumefraction of 0.40. The tensile modulus and flexural strengthwere found to be the highest at a volume fraction of 0.40,which indicates effective stress transfer between the fiberand the matrix. Sisal/polyester composites displayed thebest damping behavior and highest impact strengths com-pared to abaca/polyester and hybrid composites. However,maximum stress transfer between the fiber and the matrixwas obtained in composites with a volume ratio of bananaand sisal equal to 3:1. The tensile strength and flexuralmodulus reached their maximum at this volume ratio,while the impact strength reached its minimum.

The thermal conductivity, diffusivity and specific heatof thermoset compression molded polyester/natural fiber
(abaca/sisal) composites were investigated for several fibersurface treatments as functions of the filler concentration[425]. The thermo-physical behavior of hybrid pineap-ple leaf fiber (PALF) and glass fiber reinforced polyester
art; raw and covered by all functional and decorative details.echnology of the University of Kassel, Germany.

composites were also evaluated regarding the constanttotal fiber loading equal to 0.40 volume fraction by vary-ing the ratio of PALF and glass. The results show thatthe chemical treatment of the fibers reduces the com-posite thermal contact resistance. Hybridization of naturalfibers with glass allows a significantly better heat trans-port ability of the composite. The thermal conductivity,which is measured in the direction transverse to the planeof composite plate, could be well represented by a seriesprediction model.

5.5.2. PultrusionFibers are pulled from a creel through a resin bath and

then on through a heated die. The impregnation of the fibercontrols of the resin content and curing of the materials intotheir final shape is completed using the die. Although pul-trusion is a continuous process, which produces a profileof constant cross-sections, a variant known as pulformingallows for some variation to be introduced into the cross-sections.

Flax fiber reinforced PP composites were developed bymeans of the thermoplastic pultrusion process and theirphysic-mechanical properties were evaluated [426]. Thewater absorption behavior of pultruded jute fiber rein-forced unsaturated polyester composites was found tofollow so-called pseudo-Fickian behavior [427]. The flex-ural and compression properties were found to decreasewith the increase in water uptake in percentage. Theseflexural and compression behaviors were attributed to theplasticization of the matrix-fiber interface and the swellingof the jute fibers.

Pultruded kenaf fiber [428], jute fiber [429] and hybridjute/glass and kenaf/glass fiber [430] composites wereinvestigated and their flexural behavior using acousticemission, dynamic properties and degradation of compres-sive properties were evaluated.

5.5.3. SummaryIn recent years the processing and production tech-

nologies for biocomposites have also advanced. To date,injection molding, extrusion, compression molding, sheetmolding and resin transfer molding are the major man-
ufacturing processes for natural fiber reinforced plasticcomposites. But new downstream and auxiliary equipmenthas been designed. Such as: unique heating and singleor dual venting systems for in-line drying, high-intensity

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. Performance of biocomposites

It is important to be knowledgeable of certain mechan-cal properties of each natural fiber in order to be able toxploit the highest potential of it. Among these propertiesre the tensile, flexural, impacts, dynamic mechanical andreep properties. In general, natural fibers are suitable foreinforcing plastics, due to their relatively high strength,tiffness and low density.

.1. Tensile properties

The tensile properties are among the most widely testedroperties of natural fiber reinforced composites. The fibertrength can be an important factor regarding the selec-ion of a specific natural fiber for a specific application. Aensile test reflects the average property through the thick-ess, whereas a flexural test is strongly influenced by theroperties of the specimen closest to the top and bottomurfaces. The stresses in a tensile test are uniform through-ut the specimen cross-section, whereas the stresses inexure vary from zero in the middle to maximum in theop and bottom surfaces.

The comparison of the tensile properties of theDPE/hemp fiber composites showed that the silane

reatment and the matrix-resin pre-impregnation of theber produced a significant increase in tensile strength,hile the tensile modulus remained relatively unaffected

431,432]. The longitudinal tensile strength increased from1.8 MPa to 79.3 MPa for the silane treated fibers, whichquals an increase of approximately 10%. The increasen the transverse tensile strength from 2.75 MPa for thentreated fibers to 3.95 MPa for the treated fibers is higher,epresenting an increase of 43%. It is evident that theehavior is fiber dominated, as the longitudinal fiber direc-ion, is 1900% higher compared to transverse direction.iber surface modification has little impact. However, themprovement in the fiber–matrix interactions plays a moremportant role for the matrix and/or interphase dominatedehavior.

Rice husk reinforced PP composites with filler load-ngs of 10 wt%, 20 wt%, 30 wt% and 40 wt% were designed433]. Six test temperatures (−30, 0, 20, 50, 80 and 110 ◦C)nd five levels of crosshead speed (2, 10, 100, 500 and500 mm/min) were designed during tensile testing. Theensile strengths of the composites decreased slightly ashe filler loading increased. The tensile modulus improvedith increasing filler loading. As the crosshead speed

ncreased, the composite became more brittle. At lowerest temperatures (−30 and 0 ◦C), the composites exhib-ted strong and brittle properties similar to glass fiber, buthe tensile strength and modulus decreased due to the glass

cience 37 (2012) 1552– 1596

transition of the matrix polymer PP as the test temperaturewas increased from 0 to 20 ◦C.

A study on the effect of alkaline treatment on tensileproperties of sugar palm fiber reinforced epoxy compos-ites was investigated [434]. The treatment was carriedout using different concentrations of sodium hydroxide(NaOH) solutions and varying soaking periods. It wasobserved that as the alkali concentration and the length ofsoaking periods increased, the tensile strength decreased.However, the tensile modulus results are much higher thanthose of untreated fiber composite specimens, thus provingthe effectiveness of the treatment.

Composites consisting of aliphatic polyester (Bionolle)with flax fibers were prepared via batch mixing [435].The effects of processing conditions on the fiber lengthdistribution and the dependence of the composite ten-sile properties on the fiber content were investigated. Thetensile modulus changes with the amount of fiber con-tent according to the modified rule-of-mixture equation.The tensile strength of Bionolle/flax composites tends todecrease with fiber loading, showing that there is no adhe-sion between the matrix and the fibers. Flax fibers withchemically modified surfaces were also tested as reinforc-ing agents. An increase in tensile strength of 30% wasobserved when flax fibers (25 vol%) were substituted withfibers containing acetate groups. No significant strengthchanges were observed in composites containing fiberswith valerate groups or polyethylene glycol chains graftedat the surface.

The following investigations were completed with ther-moplastic matrices; the effect of moisture absorption ofsisal fiber/PP composites [436], the influence of surfacetreatment (NaOH solution) of coir fiber/PP composites[437], the surface esterification of bagasse/LDPE com-posites [438], the surface modification of olive husk/PPcomposites [439], the performance of hybrid jute/betel nutfiber reinforced PP [440], and the effects of surface treat-ments of luffa fiber/PP composites [441]. It was observedthat the tensile properties were influenced by all the men-tioned factors.

The tensile properties of natural fiber reinforced ther-mosets were investigated regarding the siloxane treatmentof jute fiber/polyester and epoxy composites [442], thetemperature and loading rate effects of kenaf fiber/epoxycomposites [443], the effects of a differing geometryof abaca fiber/epoxy composites [444], the influence ofmoisture absorption of bamboo/vinylester composites[445], the effect of oil palm fibers volume fraction ofoil palm/polyester composites [446], and the influenceof the fiber orientation and the volume fraction of alfafiber/polyester composites [447].

The effects of hybridization and the chemical modi-fication of oil palm/sisal fiber reinforced natural rubbercomposites [448,449], the effects of high temperature onramie fiber/biodegradable resin composites [450], effect ofbiodegradable matrix type (PLA, PHBV, PBS) on regeneratedcellulose fiber/biopolymer composites [451], the influence
of bio based coupling agent on bamboo fiber/PLA and PBScomposites [452], preparation of sandwich composite pan-els for building applications from jute/polyester [453], andthe structural and mechanical characterization of sugar

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beet pulp/PLA composites [454] on the tensile propertieswere evaluated.

Rao et al. [455] investigated the tensile properties ofbamboo, date palm, abaca, oil palm, sisal, coir and vakkafibers regarding their fiber cross sections, atmosphericmoisture content and density.

6.2. Flexural properties

The flexural stiffness is a criterion of measuringdeformability. The flexural stiffness of a structure is a func-tion based upon two essential properties: the first is theelastic modulus (stress per unit strain) of the material thatcomposes it; and the second is the moment of inertia, afunction of the cross-sectional geometry.

Zampaloni et al. [456] focused on the fabrication ofkenaf fiber reinforced PP sheets that could be thermo-formed for a wide variety of applications with propertiesthat are comparable to existing synthetic composites. Theoptimal fabrication method determined for these materi-als was determined to be a compression molding processwhich utilized a layered sifting of a microfine PP powderand chopped kenaf fibers. When comparing the flexu-ral strengths of the kenaf fiber/PP composite materials,40% kenaf/PP composites performed significantly betterthan the 30% kenaf/PP composites. The flexural strengthof 40% kenaf/PP was equivalent to the flax/PP, larger thanthe hemp/PP and almost doubles that of the coir/PP andsisal/PP composites. The flexural strength of 30% kenaf/PPcomposites showed results equivalent to those of the 40%hemp/PP composites, while also outperforming the coir/PPand sisal/PP composites.

Composites of PP and HDPE reinforced with 20 wt%curaua fibers were prepared and the effect of screw rotationspeed was evaluated by measuring the flexural propertiesof the composites [457]. The yield stress in flexural tests forthe composites with PP and HDPE matrixes, decreased withthe increase in screw rotation speed. For HDPE a higher rateof decrease was shown. However, the flexural modulus wasnot affected by the screw rotation speed. The elongationat break exhibited different behavior; this effect strength-ened HDPE at screw rotations above 300 rpm, and slightlyincreased for PP composites above 350 rpm.

The effect of acetylation [458] on the flexural proper-ties of bagasse/PP composites (properties decreased dueto acetylation), and the effect of different maleated cou-pling agents (flexural properties increased above 60% withoptimum loading of coupling agent) on jute and flax fiberreinforced PP composites [459] have been evaluated.

Composites were fabricated using abaca fiber with vary-ing fiber lengths and fiber loading [460]. The analysis ofthe flexural properties revealed the optimum length ofabaca fibers required for abaca fiber/phenol formaldehyderesin composites. The flexural strength and modulus val-ues increased with fiber length for composites with shortfibers. Furthermore, the maximum flexural strength and
modulus values were obtained for 40 mm fibers. Due tofiber loading up to 45%, the flexural modulus increasedto about 25%. The flexural strength also showed goodenhancement, because of increasing fiber loading.
cience 37 (2012) 1552– 1596 1581

The flexural properties of coir fiber/polyester compos-ites were evaluated [461]. Composites were prepared usingtwo molding pressures and with amounts of coir fiber upto 80 wt%. When the fiber amount equaled up to 50 wt%,rigid composites were obtained. Higher fiber amountsresulted in composite performances similar to that of flex-ible agglomerates. The results obtained for the flexuralstrength made it possible to compare the technical perfor-mance of the composites with that of other conventionalmaterials.

The flexural properties of thermoset resins, such as car-danol reinforced with hemp, ramie, jute and flax fibers[462], epoxy resin with coir fibers [463], and hemp, kenaf,flax and henequen fibers [464], polyester resin reinforcedbagasse fibers [465], coir and oil palm fibers [466], hempand kenaf fibers [467], were also investigated.

Biocomposites were fabricated from chopped hempfiber and cellulose ester biodegradable plastic via two dif-ferent processes: powder impregnation by compressionmolding and an extrusion process followed by an injec-tion molding process [468]. Fabricated through processextrusion followed by injection molding process of hemp(30 wt%) fiber reinforced cellulose acetate biocompositeexhibited flexural strength of 78 MPa and modulus of elas-ticity of 5.6 GPa as contrast to 55 MPa and 3.7 GPa for thecorresponding hemp fiber/PP based composite. The corre-sponding biocomposites showed superior flexural strengthand modulus values during the extrusion and ensuinginjection molding processes, as compared to biocompos-ites powder impregnated through compression moldingprocess.

Biodegradable composites were prepared with the meltblending technique using PCL and oil palm empty fruitbunch fiber [469]. Since oil palm fiber is not compati-ble with PCL, a binder (polyvinyl pyrrolidone), was usedto improve the interaction between PCL and the fibers.The composites produced were irradiated with the aidof an electron beam to improve the mechanical proper-ties. The flexural strength and modulus of the compositeswere improved by an additional 1% by weight of poly(vinylpyrrolidone) and irradiated with a 10 kGy electron beam.

6.3. Impact properties

Impact strength is the ability of a material to resistfracture under stress applied at high speed. Biofiber rein-forced plastic composites have properties that can competewith the properties of glass fiber thermoplastic compos-ites, especially concerning specific properties. However,one property, namely the impact strength is often listedamong the major disadvantages of biofiber reinforced com-posites. In recent years, the development of new fibermanufacturing techniques and improved composite pro-cessing methods along with enhancement of fiber/matrixadhesion has improved the current situation somewhat.

Jute fiber was treated with o-hydroxybenzenedi-
azonium salt (o-HBDS) in alkaline media [470]. Treated anduntreated jute fiber reinforced PP composites were pre-pared with different weight fractions (20, 25, 30, and 35%)of jute fiber. The increase of Charpy impact strength was

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ound to be exceptionally high (in some cases ∼200%) asompared to those of literature values.

Composite panels consisting of virgin and recycledDPE and four types of rice straw components includ-

ng rice husk, rice straw leaf, rice straw stem, and wholeice straw were made via melt compounding and com-ression molding [471]. Composite panels with rice huskxhibited better impact strength than composites panelsade of other straw fibers. Minor differences concerning

he impact properties existed among leaf, stem, and wholetraw fibers. The recycled HDPE resin and its compositesossessed a significantly better impact strength comparedo the virgin HDPE systems. This was attributed to the addi-ives used during initial processing.

Recycled HDPE composites were reinforced withagasse fibers and the influence of coupling agentypes/concentrations on the composite impact prop-rties were studied [472]. The addition of maleatedolyethylene (MAPE), carboxylated polyethylene (CAPE),nd titanium-derived mixture (TDM) improved the com-atibility between the bagasse fiber and recycled HDPE.he impact properties of the resultant composites wereomparable with those of virgin HDPE composites. The bet-er impact strength of the composites was achieved, when

MAPE coupling agent was used. The impact strengthncreased with increasing MAPE content.

Oil palm empty fruit bunch fiber reinforced compositeas studied. The effects of fiber loading and impact mod-

fiers, namely chlorinated polyethylene (CPE) and acrylic,n the impact properties of oil palm fiber filled-PVC com-osites were of particular interest [473]. The results fromhe impact test showed that the unfilled PVC samples ofoth impact modifiers change from brittle to ductile with

ncreasing impact modifier concentration. The incorpo-ation of fibers into unmodified PVC and modified PVCesulted in the reduction of the impact strength. As the fiberontent increased from 10 to 40 phr, the impact strengtheduced about 40% and 30% for acrylic-modified PVCnd CPE-modified PVC, respectively. The impact strengtheduction was only marginal for unmodified PVC compos-tes. The impact modifier enhanced the impact strength ofber reinforced PVC composites. However, the effective-ess decreased with increased fiber loadings. The impacttrength of CPE-modified PVC was higher than acrylic-odified PVC at fiber contents of 20 phr and higher.The impact properties regarding the influence of the

ber microstructure of sisal, abaca, jute and flax fiber rein-orced PP composites [474], the processing methods forax fiber/PP composites [475], and the effect of impactodifiers (natural rubber and ethylene propylene dieneonomer) on grass fiber/PP composites [476] were inves-

igated.Hemp fiber reinforced unsaturated polyester compos-

tes were subjected to low velocity impact tests in ordero study the effects of non-woven hemp fiber reinforce-

ent on their impact properties [477,478]. The compositepecimen containing 0, 0.06, 0.10, 0.15, 0.21 and 0.26
ber volume fractions (Vf) were prepared and their impactesponses were compared with samples containing anquivalent fiber volume fraction of chopped strand mat-glass fiber reinforcement. A significant improvement in
cience 37 (2012) 1552– 1596

the load bearing capability and impact energy absorptionwas found after the introduction hemp fiber as reinforce-ment. The results indicated a clear correlation betweenfiber volume fractions, stiffness of the composite lami-nate, impact load and total absorbed energy. Unreinforcedunsaturated polyester control specimens exhibited brittlefracture behavior with a lower peak load, lower impactenergy and less time until failure occurred than hemp rein-forced unsaturated polyester composites. The impact testresults showed that the total energy absorbed by 0.21 fibervolume fraction (four layers) of hemp reinforced specimensis comparable to the energy absorbed by the equivalentfiber volume fraction of chopped strand mat E-glass fiberreinforced unsaturated polyester composite specimens.

The effects of the bi-dimensional orientation of leaf stalkfibers from peach palms (peach palm powder and weave)on the impact strength behavior of polyester/fiber rein-forced composites were studied [479]. The Izod impact dataobtained for the composites with only woven material wasconsiderably superior to that of composites containing onlypowder, suggesting that the reinforcement in the form ofpowder is inefficient, independent of the size range of theparticles used.

The impact strength of the bamboo fiber/PLA com-posites decreased after addition of bamboo fiber [480].The impact strength of treated fiber reinforced compos-ites improved after the addition of treated fibers. Goodfiber/matrix adhesion provides an effective resistanceto crack propagation during impact tests. The impactstrength increased significantly (33%) for silane treatedbamboo fiber/PLA composites compared to untreated bam-boo fiber/PLA composites with 30 wt% fiber content.

The toughness of the short fiber reinforced compositescan be influenced by a number of factors, such as the intrin-sic properties of the matrix, the fiber volume fraction, andinterfacial bond strength. Therefore, strong interactionsbetween the hydroxyl groups of biofibers and the couplingagents are needed to overcome the incompatibility prob-lem. In doing so, the impact, tensile and flexural strengthsof biofiber reinforced composites can be increased.

6.4. Summary

Tensile, flexural and impact properties are the mostcommonly investigated mechanical properties of natu-ral fiber reinforced plastic composites. Impact strengthis one of the undesirable weak points of these materi-als in terms of mechanical performance. Besides thesetensile, flexural and impact properties, the long-term per-formance (creep behavior), dynamic mechanical behavior,compressive properties are also investigated for naturalfiber composites. To improve performance to the desiredlevel, still much work is to be done considering fiber pro-cessing, non-linear behavior, fiber–matrix adhesion, fiberdispersion, composite manufacturing with optimized pro-cessing parameters.

7. Development and future trends of biocomposites

The advanced natural fiber reinforced polymer com-posite contributes to enhancing the development of

lymer S
biocomposites in regards of performance and sustainabil-ity. Biocomposites have created substantial commercialmarkets for value-added products especially in automo-tive sector. However, in order to be able to expand intoother markets, such as commercial construction and con-sumer goods, composites need to achieve high-qualityperformance, serviceability, durability, and reliabilitystandards.

In recent years, the major advancement lies within theestablishment of nanotechnology (i.e., reinforcing as wellas producing nanocrystalline cellulose from natural fibers).Natural fibers consist of approximately 30–40% celluloseand about half of that is crystalline cellulose. The nanocrys-talline cellulose may be only one-tenth as strong as carbonnanotubes but it costs 50–1000 times less to produce.

Wood pulp has often been used as the starting materialfor research on nano cellulose production. In the literaturethere are reports of cellulose nano/microfibril extractionfrom diverse non-wood sources including hemp fibers[481,482], sugar beet pulp [483,484], potato pulp [485],swede root [486], bagasse [487–491], sisal [492,493], algae[494], stems of cacti [495,496], banana rachis [497], flaxfibers [498,499], plantain [500], water hyacinth [501], bam-boo [502], coir [503], pea hull [504], pineapple leaf [505],and wheat straw [506].

Alemdar and Sain investigated the reinforcing poten-tial of composites comprising a starch-based thermoplasticpolymer and cellulose nanofibers obtained from agro-residues [507]. Cellulose nanofibers were isolated fromwheat straw by a chemi-mechanical technique and deter-mined to have diameters in the range of 10–80 nm andlengths of several thousand nanometers. Fig. 24 showsthe structure of the wheat straw fibers after the chem-ical treatment. These images visually suggest the partialremoval of hemicelluloses, lignin and pectin after chemi-cal treatment, which are the cementing materials aroundthe fiber-bundles. It is clear from the image that the aver-age diameter of the fibers is about 10–15 �m, which islower than the average size of the fiber bundles, 25–125 �mbefore chemical treatment. TEM image (Fig. 24b) for thenanofibers obtained after the chemi-mechanical treat-ment. The mechanical treatment of the fibers resulted indefibrillation of nanofibers from the cell walls and theTEM image of the nanofibers shows the separation of thesenanofibers from the micro-sized fibers.

Alemdar and Sain [508] extracted MFC from wheatstraw and soy hulls via mechanical treatment involvingcryocrushing followed by disintegration and fibrillation.These authors found that almost 60% of the nanofibershad a diameter within a range of 30–40 nm and lengthsof several thousand nanometers. Cryocrushing is an alter-native method for producing nanofibers in which fibers arefrozen using liquid nitrogen and high shear forces are thenapplied. When high impact forces are applied to the frozenfibers, ice crystals exert pressure on the cell walls, caus-ing them to rupture and thereby liberating microfibrils.The cryocrushed fibers may then be dispersed uniformly
into water suspension using a disintegrator before high-pressure fibrillation.
A blend containing 10% cellulose nanofibers obtainedfrom various sources, such as flax bast fibers, hemp fibers,

cience 37 (2012) 1552– 1596 1583

kraft pulp or rutabaga and 90% poly(vinyl alcohol) wasused for making nanofiber reinforced composite materialby a solution casting procedure [509]. Both tensile strengthand Young’s modulus were improved compared to neatPVOH film, with a pronounced four- to five-fold increasein Young’s modulus observed.

Similarly, Wang and Sain [510,511] reported that soy-bean stock-based nanofiber-reinforced PVOH films (up to10% nanofiber content) demonstrated a two-fold increasein tensile strength when compared with films withoutfiller. With a higher loading of MFC, the relative enhance-ment of mechanical properties was even more remarkable.They also applied solid phase melt-mixing followed bycompression molding to produce MFC-reinforced compos-ites. In their study nanofiber was directly incorporated (upto 5 wt%) into PE and PP matrix using a Brabender mixer andan ethylene-acrylic oligomer emulsion was used as a dis-persant. This work demonstrated that coating MFC fiberswith the dispersant improved the cellulose dispersion inthese polymers. As a consequence, the mechanical proper-ties of PP and PE nanocomposites were slightly improvedwhen compared to pure matrices; however, this enhance-ment was not considered significant.

Sain and Bhatnagar [512] invented a method to obtainMFC from renewable feedstocks such as natural fibers androot crops via cryocrushing of the pre-treated pulp withliquid nitrogen. The pulp was hydrolyzed at a moderatetemperature of 50–90 ◦C and then both acidic and alkaliextractions were performed.

Taniguchi [513] utilized natural cellulose fibers derivedfrom different sources (such as cotton, hemp, wood pulp,seaweed, cereals, sea squirts, bacteria, etc.) for the produc-tion of MFC by fibrillating raw materials between rotatingtwin disks while adding shear stress in the vertical direc-tion of fiber long axes. Joseph et al. [514–516] investigatedthe effect of microfibrilation of oil palm fiber on thedynamic mechanical properties of oil palm fiber reinforcedrubber composites.

As technology improves for biocomposites reinforcedwith natural fibers to provide enhanced material and prod-uct characteristics, the products will become more diverseand enter markets that as of yet are unexplored. Today nat-ural fiber reinforced biocomposites are found extensivelyin automotive sectors [517–521].

By the time, biocomposite materials and associateddesign methods are sufficiently mature to allow theirwidespread use, issues related to construction materials.The development of methods, systems and standards couldsee biocomposite materials at a distinct advantage overtraditional materials. There is a significant research effortunderway to develop biocomposite materials and exploretheir use as construction materials, especially for load bear-ing applications.

Burgueno et al. manufactured cellular beams andplates as load bearing structural components from hemp,jute and flax fibers with unsaturated polyester resin[522–525] for housing panel applications. The material
and structural performance was experimentally assessedand compared with results from short-fiber compos-ite micro-mechanics models and sandwich analyses. Itwas demonstrated that cellular biocomposite components


EM imagA

ctacmwoua

udabfibbsiteimiocfpbmctokmfi

fnrac

Fig. 24. SEM images of the wheat straw microfibers (a), and Tdopted from [507]. Copyright 2008. By permission from Elsevier Ltd.

an be used for load-bearing components by improvingheir structural efficiency through cellular materialrrangements. Furthermore, it was verified that theyould compete with components made from conventionalaterials. This research should continue in conjunctionith development of conventional composite materials in

rder to provide a solution in the future to allow widerse of the bio composite materials by civil engineeringpplications.

In the future, these biocomposites will see increasedse in structural applications. Various other applicationsepend on their further improvements. But there are still

number of problems that have to be solved beforeiocomposites become fully competitive with syntheticber composites. Biocomposites are sustainable and coulde fully recyclable, but could be more expensive if fullyio-based and biodegradable and they are extremely sen-itive to moisture and temperature. If a proper matrixs used, biocomposites could be 100% biodegradable, butheir biodegradation can difficult to control. Biocompositesxhibit good specific properties, but there is high variabil-ty in their properties. Biocomposites showed non-linear

echanical behavior, poor long-term performance and lowmpact strength. Many of these weaknesses can and will bevercome with the development of more advanced pro-essing of natural fibers and their composites. Howeverully environmental superiority of biocomposites com-ared to synthetic fiber composites is still questionableecause of their relatively excessive processing require-ents, which in turn consume more energy. Therefore,

areful life-cycle assessment of biocomposites is essen-ial in order to retain the main advantage in the processf developing high performance biocomposites. New mar-ets will develop when these biocomposite products areore durable, dimensionally stable, moisture proof, and

re resistant.Nanotechnology shows numerous opportunities

or improving biocomposite products by providing
anotechnology-based coatings to increase water uptake,educe biodegradation and volatile organic compoundsnd even flame resistance. The use of nanocrystallineellulose is being explored for a variety of uses since it is
e (magnification 15,000×) of the wheat straw nanofibers (b).

stronger than steel and stiffer than aluminum. Nanocrys-talline cellulose reinforced composites could soon provideadvanced performance, durability, value, service-life, andutility while at the same time being a fully sustainabletechnology.

8. Conclusions

Biocomposites reinforced with natural fibers and/orbiopolymers have developed significantly over the pastyears because of their significant processing advantages,biodegradability, low cost, low relative density, high spe-cific strength and renewable nature. These composites arepredestined to find more and more application in the nearfuture, especially in Europe, where pressure from bothlegislation and the public is rising. Interfacial adhesionbetween natural fibers and matrix will remain the key issuein terms of overall performance, since it dictates the finalproperties of the composites. Many studies are examined,reviewed and highlighted in this review paper regardingthe importance of the interface, the influence of varioustypes of surface modifications, different types of matricesused for the composites, as well as fabrication methods,and the performance of composites.

Further research is required to overcome obstaclessuch as moisture absorption, inadequate toughness, andreduced long-term stability for outdoor applications. Inparticular, different weathering conditions, such as tem-perature, humidity, and UV radiation all affect the servicelife of the product. The major detrimental effect ofhygrothermal and UV exposure are property deterioration,discoloring and deformation.

Significant research is currently underway around theworld to address and overcome the obstacles mentionedabove. This effort to develop biocomposite materials withimproved performance for global applications is an ongo-ing process.

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