Review
Silane coupling agents used for natural fiber/polymer composites: A review

https://doi.org/10.1016/j.compositesa.2010.03.005Get rights and content

Abstract

Natural fiber reinforced polymer composites (NFPCs) provide the customers with more alternatives in the material market due to their unique advantages. Poor fiber–matrix interfacial adhesion may, however, negatively affect the physical and mechanical properties of the resulting composites due to the surface incompatibility between hydrophilic natural fibers and non-polar polymers (thermoplastics and thermosets). A variety of silanes (mostly trialkoxysilanes) have been applied as coupling agents in the NFPCs to promote interfacial adhesion and improve the properties of composites. This paper reviews the recent progress in using silane coupling agents for NFPCs, summarizes the effective silane structures from the silane family, clarifies the interaction mechanisms between natural fibers and polymer matrices, and presents the effects of silane treatments on the mechanical and outdoor performance of the resulting composites.

Section snippets

Introduction and background

Natural fiber reinforced polymer composites (NFPCs), as an important branch in the field of composite materials, have been studied for decades [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Natural fibers have different origins such as wood, pulp, cotton, bark, nut shells, bagasse, corncobs, bamboo, cereal straw, and vegetable (e.g., flax, jute, hemp, sisal, and ramie) [10], [11], [12], [13]. These fibers are mainly made of cellulose, hemicelluloses, lignin and pectins, with a small

Silane structures

To effectively couple the natural fibers and polymer matrices, the silane molecule should have bifunctional groups which may respectively react with the two phases thereby forming a bridge in between them. Silane coupling agents have a generic chemical structure R(4−n)single bondSisingle bond(RX)n (n = 1,2) where R is alkoxy, X represents an organofunctionality, and R′ is an alkyl bridge (or alkyl spacer) connecting the silicon atom and the organofunctionality. In the past decades, various silane structures have been

Fiber surface coating and cell wall modification

In the present literature, there are several different means reported to apply silanes to natural fibers. These can be divided into fiber surface treatment and cell wall modification. Spraying is a relatively easy way to treat the fiber surface with a silane solution. The silanes are dissolved into certain organic solvents or solvent/water mixtures and the prepared solution is directly sprayed onto the fibers. If the solution is water-free, the sprayed silanes can be partly hydrolyzed via the

Inter-phase compatibility

The interaction mode between the silane-treated fiber and the polymer matrix is a crucial factor for the mechanical properties of the resulting NFPCs. Physical blending of the silane-treated fibers and the thermoplastic matrices enhances their mutual adherence via inter-molecular entanglement, or acid–base interactions (ABI) [43]. The interfacial shear strength (ISS) between jute fibers and PP, determined by a microdroplet micromechanical test, was improved by treating jute fibers with a 0.5%

Outdoor performance

The moisture sorption of NFPCs measured according to standard ASTM D-1037 [137] generally indicates very low moisture contents (in a range of ca. 2%), although this depends upon the fiber contents. This has been attributed to the effect of the encapsulation of fibers by polymers [138]; however, with the NFPCs in service there often appears deleterious effects such as surface colour fading and erosion, warpage, mold growth, fungal decay, and strength loss after a long-term exposure to an

Summary

Most established silanes used for natural fiber/polymer composites are trialkoxysilanes bearing a non-reactive alkyl or reactive organofunctionality. Silane is hydrolyzed forming reactive silanols and is then adsorbed and condensed on the fiber surface (sol–gel process) at a specific pH and temperature. The hydrogen bonds formed between the adsorbed silanols and hydroxyl groups of natural fibers may be further converted into covalent bonds by heating the treated fibers at a high temperature,

Acknowledgement

The author Dr. Yanjun Xie would like to thank the German Academic Exchange Service (DAAD) for a research grant support.

References (148)

  • M. Bengtsson et al.

    Silane crosslinked wood plastic composites: processing and properties

    Compos Sci Technol

    (2006)
  • S.M.B. Nachtigall et al.

    New polymeric-coupling agent for polypropylene/wood–flour composites

    Polym Test

    (2007)
  • M. Abdelmouleh et al.

    Short natural-fibre reinforced polyethylene and natural rubber composites: effect of silane coupling agents and fibres loading

    Compos Sci Technol

    (2007)
  • L.A. Pothan et al.

    The role of fibre/matrix interactions on the dynamic mechanical properties of chemically modified banana fibre/polyester composites

    Compos Part A – Appl Sci

    (2006)
  • H. Ismail et al.

    The effects of a silane coupling agent on curing characteristics and mechanical properties of bamboo fibre filled natural rubber composites

    Eur Polym J

    (2002)
  • K.L. Pickering et al.

    The effect of silane coupling agents on radiata pine fibre for use in thermoplastic matrix composites

    Compos Part A – Appl Sci

    (2003)
  • M. Abdelmouleh et al.

    Modification of cellulosic fibres with functionalised silanes: development of surface properties

    Int J Adhes Adhes

    (2004)
  • M. Castellano et al.

    Modification of cellulose fibres with organosilanes: under what conditions does coupling occur?

    J Colloid Interf Sci

    (2004)
  • H.J. Kang et al.

    NMR studies of the hydrolysis and molecular motion of aminopropylsilane

    Mater Sci Eng A – Struct

    (1990)
  • A.C. Miller et al.

    Effect of silane coupling agent adsorbate structure on adhesion performance with a polymeric matrix

    Compos Part A – Appl Sci

    (2003)
  • B. Riegel et al.

    Kinetic investigations of hydrolysis and condensation of the glycidoxypropyltrimethoxysilane/aminopropyltriethoxy-silane system by means of FT-Raman spectroscopy I

    J Non-Cryst Solids

    (1998)
  • M. Pantoja et al.

    Analysis of hydrolysis process of γ-methacryloxypropyltrimethoxysilane and its influence on the formation of silane coatings on 6063 aluminum alloy

    Appl Surf Sci

    (2009)
  • C.H. Chiang et al.

    The structure of γ-aminopropyltriethoxysilane on glass surfaces

    J Colloid Interf Sci

    (1980)
  • M.W. Daniels et al.

    Silane adsorption behavior, microstructure, and properties of glycidoxypropyltrimethoxysilane-modified colloidal silica coatings

    J Colloid Interf Sci

    (1998)
  • N. Nishiyama et al.

    Adsorption behavior of a silane coupling agent onto a colloidal silica surface studied by 29Si NMR spectroscopy

    J Colloid Interf Sci

    (1989)
  • K.C. Vrancken et al.

    The role of silanols in the modification of silica gel with aminosilanes

    J Colloid Interf Sci

    (1995)
  • A. Valadez-Gonzalez et al.

    Effect of fiber surface treatment on the fiber-matrix bond strength of natural fiber reinforced composites

    Compos Part B – Eng

    (1999)
  • A. Valadez-Gonzalez et al.

    Chemical modification of henequen fibers with an organosilane coupling agent

    Compos Part B – Eng

    (1999)
  • P.J. Herrera-Franco et al.

    A study of the mechanical properties of short natural-fiber reinforced composites

    Compos Part B – Eng

    (2005)
  • C.A.S. Hill et al.

    Effect of fiber treatments on mechanical properties of coir or oil palm fiber reinforced polyester composites

    J Appl Polym Sci

    (2000)
  • P. Zadorecki et al.

    Future prospects for wood cellulose as reinforcement in organic polymer composites

    Polym Compos

    (1989)
  • J.Z. Lu et al.

    Chemical coupling in wood fiber and polymer composites: a review of coupling agents and treatments

    Wood Fiber Sci

    (2000)
  • S. Mukhopadhyay et al.

    Interface behavior in polypropylene composites

    J Thermoplast Compos

    (2003)
  • J. George et al.

    A review on interface modification and characterization of natural fiber reinforced plastic composites

    Polym Eng Sci

    (2001)
  • M. Narkis et al.

    Review of methods for characterization of interfacial fiber–matrix interactions

    Polym Compos

    (1988)
  • H. Ishida

    A review of recent progress in the studies of molecular and microstructure of coupling agents and their functions in composites, coatings and adhesive joints

    Polym Compos

    (1984)
  • H. Jiang et al.

    Development of poly(vinyl chloride)/wood composites. A literature review

    J Vinyl Addit Technol

    (2004)
  • T. Sabu et al.

    Cellulose fibre reinforced polymer composites

    (2009)
  • Z. Xiao et al.

    Review for development of wood plastic composites

    J Northeast Forest Univ

    (2003)
  • R.B. Seymour

    Cellulose-filled polymer composites

    Pop Plast

    (1978)
  • A.K. Bledzki et al.

    Wood-filled thermoplastic composites

    Mech Compos Mater

    (1998)
  • R.G. Raj et al.

    Compounding of cellulose fibers with polypropylene: effect of fiber treatment on dispersion in the polymer matrix

    J Appl Polym Sci

    (1989)
  • M. Kazayawoko et al.

    Matuana LM surface modification and adhesion mechanisms in woodfiber–polypropylene composites

    J Mater Sci

    (1999)
  • B. Singh et al.

    Influence of fiber surface treatment on the properties of sisal–polyester composites

    Polym Compos

    (1996)
  • A. Schirp et al.

    Influence of fungal decay and moisture absorption on mechanical properties of extruded wood–plastic composites

    Wood Fiber Sci

    (2005)
  • D. Sun

    Investigating the plasma modification of natural fiber fabrics-the effect on fabric surface and mechanical properties

    Text Res J

    (2005)
  • M.N. Belgacem et al.

    Effect of corona modification on the mechanical properties of polypropylene/cellulose composites

    J Appl Polym Sci

    (1994)
  • I. Sakata et al.

    Activation of wood surface by corona treatment to improve adhesive bonding

    J Appl Polym Sci

    (1993)
  • C.A.S. Hill

    Wood–plastic composites: strategies for compatibilising the phases

    J Inst Wood Sci

    (2000)
  • C. Daneault et al.

    Grafting of vinyl monomers onto wood fibers initiated by peroxidation

    Polym Bull

    (1988)
  • Cited by (1790)

    View all citing articles on Scopus
    View full text