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.
Introduction and background
Natural fiber reinforced polymer composites (NFPCs), as an important branch in the field of composite materials, have been studied for decades [1–11]. 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) [10–13]. These fibers are mainly made of cellulose, hemicelluloses, lignin and pectins, with a small quantity of extractives. The fiber constituents vary depending on their origination. Compared with conventional inorganic fillers such as glass fiber and carbon fibers, natural fibers provide many advantages: (1) abundance and therefore low cost, (2) biodegradability, (3) flexibility during processing and less resulting machine wear, (4) minimal health hazards, (5) low density, (6) desirable fiber aspect ratio, and (7) relatively high tensile and flexural modulus. Incorporating the tough and light-weight natural fibers into polymer (thermoplastic and thermoset) matrices produces composites with a high specific stiffness and strength . The renewable and biodegradable characteristics of natural fibers facilitate their ultimate disposal by composting or incineration, options not possible with most industrial fibers. The fibers also contain sequestered atmospheric carbon dioxide in their structure and are invariably of lower embodied energy compared to industrially produced glass fibers.
Although natural fibers can offer the resulting composites many advantages, the usually polar fibers have inherently low compatibility with non-polar polymer matrices, especially hydrocarbon matrices such as polypropylene (PP) and polyethylene (PE) [15,16]. The incompatibility may cause problems in the composite processing and material properties. Hydrogen bonds may form between the hydrophilic fibers, and thus the fibers tend to agglomerate into bundles and unevenly distribute throughout the non-polar polymer matrix during compounding processing [17,18]. There is also insufficient wetting of fibers by the non-polar polymer matrices, resulting in weak interfacial adhesion. As a result, the stress transfer efficiency from the matrix to the reinforcing fibers is reduced. The incompatibility may not be an issue when using polar polymers such as unsaturated polyester (UP) and epoxy resin as matrices; however, the resulting composites, similar to the composites with non-polar matrices, will be susceptible to moisture deterioration and fungal damage during outdoor service. The moisture absorption of the natural fibers may cause dimensional changes of the resulting composites and weaken the interfacial adhesion [19,20]. Mould and decay fungi may also grow on/in the composites, although more slowly than in the fibers alone, when they are utilized in the long-term under wet conditions . In addition, natural fibers are of limited thermal stability and, therefore, thermal degradation may take place during composite processing at a high temperature, especially in the cases of thermal extrusion and hot compression processes.
By this token, treatment of natural fibers is beneficial in order to improve the water resistance of fibers, enhance the wettability of the natural fiber surface by polymers (mainly non-polar polymers) and promote interfacial adhesion. The performance of fibers is critical to obtain the improved physical and mechanical properties of the resulting composites. Physical treatments (e.g. electronic discharge in the different media such as plasma and corona technologies [22–25]) may create a hydrophilic or hydrophobic fiber surface by changing the surface energy to consequently increase the compatibility of the treated fiber with the polymer matrices. These surface treatments only modify a very shallow surface of cell walls and thus do not change the hygroscopic characteristics of fibers. Chemical modification provides the means of permanently altering the nature of fiber cell walls : by grafting polymers onto the fibers [27,28], crosslinking of the fiber cell walls , or by using coupling agents . These modifying strategies have been generally reviewed recently [26,30]. The chemical modification may make the fiber cell walls more dimensionally stable, reduce water sorption, or increase resistance against fungal decay, but there may be an associated reduced dynamic strength such as impact strength due to embrittlement. A coupling agent is a chemical that functions at the interface to create a chemical bridge between the reinforcement and matrix. It improves the interfacial adhesion when one end of the molecule is tethered to the reinforcement surface and the functionality at the other end reacts with the polymer phase. Extensively used coupling agents for NFPCs are copolymers containing maleic anhydride such as maleated polypropylene (MAPP) or maleated polyethylene (MAPE) [18,31–33]. The anhydride groups of the copolymers may react with the surface hydroxyl groups of natural fibers forming ester bonds whilst the other end of copolymer entangles with the polymer matrix due to their similar polarities . Isocyanates have also been reported as the coupling agents used in NFPCs. Urethane links can be formed between the isocyanate functionality and the hydroxyl group of natural fibers [35,36], consequently blocking these hygroscopic hydroxyl sites .
Silanes are recognized as efficient coupling agents extensively used in composites and adhesive formulations . They have been successfully applied in inorganic filler reinforced polymer composites such as glass fiber reinforced polymer composites [e.g. 39,40] and mineral filled polymer composites [e.g. 41,42]. Silanes are also adhesion promoters in many adhesive formulations or are used as substrate primers, giving stronger adhesion . The bifunctional structures of silanes have also been of interest in applying them for natural fiber/polymer composites, since both glass fibers and natural fibers bear reactive hydroxyl groups, and extensive researches have accordingly been carried out to screen the varied silane structures for NFPC production
The aim of this paper is to review the recent progress in using silane coupling agents for the production of natural fiber reinforced polymer composites. The effects of treatments of natural fibers with silanes on the properties of fibers and the resulting composites are discussed and the interaction mechanisms between phases clarified.
Silane structures and hydrolysis processes
- 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(4n)ASiA(R0 X)n (n = 1,2) where R is alkoxy, X represents an organofunctionality, and R0 is an alkyl bridge (or alkyl spacer) connecting the silicon atom and the organofunctionality. In the past decades, various silane structures have been tested for coupling of inorganic reinforcements such as glass fiber and organic polymer matrices. The structures used to couple the natural fibers and polymer matrices are relatively limited. Most of the established silanes used for NFPCs are trialkoxysilanes. The organofunctionality of the silane interacts with the polymer matrices with their interaction modes depending on the functionality’s reactivity or compatibility towards the polymer. A nonreactive alkyl group of the silane may increase the compatibility with non-polar matrix due to their similar polarities; however, the reactive organofunctionality may covalently bond with as well as being physically compatible with the polymer matrices. These organofunctionalities of silanes are typically amino, mercapto, glycidoxy, vinyl, or methacryloxy groups. The most reported silanes and their applied target polymer matrices are listed in Table 1.
With regard to these silanes shown in Table 1, aminosilanes, especially c-aminopropyltriethoxysilane (APS), are most extensively reported in the literature as coupling agents between natural fibers and thermoplastics or thermosets. Vinyl- and acryl-silanes are coupling agents that are able to establish covalent bonds with polymeric matrices in the presence of peroxide initiators. Methacrylate–functional silanes can display high levels of reactivity with unsaturated polyester matrices , whilst azidosilanes can efficiently couple inorganic fillers with thermoplastic matrices [62,63]; however, there have been few reports of their use in natural fiber reinforced thermoplastic composites. The application and effects of these typical silanes on the NFPC’s properties will be presented below.
- Hydrolysis processes of silanes The alkoxysilanes have been demonstrated to be able to directly react with ASiAOH groups of silica thereby forming ASiAOASiA bonds [e.g. 43,67,68] without any requirement of prehydrolysis. However, silanes do not undergo the same reaction with the hydroxyl groups of cellulosic fibers even at high temperature . This has been attributed to lower acidity of cellulosic hydroxyl groups compared with silanol . In addition, cellulose is generally unreactive to many chemicals and the OH groups of the microfibrils have very low accessibility. Based on the fact, an optional strategy is to activate the alkoxysilane by hydrolyzing the alkoxy groups off thereby forming the more reactive silanol groups. As a result, the silanol may react with the hydroxyl groups of fibers or condense themselves on the surfaces of fibers and/or in the cell walls forming macromolecular network. Although the formed ASiAOACA bonds are eventually not stable towards hydrolysis, blocking the hydroxyl groups (reversible to hydrolysis) and the formation of macromolecular network (permanent) under heating condition facilitate an enhancement of interfacial adhesion of treated fibers and polymer matrices and of the properties of the resulting composites. To hydrolyze the alkoxy groups off, participation of water is essential. Even though the natural fibers under room condition contain bound water which may act to hydrolyze the silanes; however, additional water is required to achieve a complete hydrolysis of silanes [70–72].
- Effect of silane structures on the hydrolysis
Alkoxy groups associated with silane coupling agents prior to utilization can be hydrolyzed off thereby liberating the corresponding alcohols in the presence of water and generating reactive silanol groups. The alkoxy groups are usually ethoxy or methoxy. Under the same hydrolytic conditions the methoxy groups of trimethoxysilane hydrolyzes more rapidly than the ethoxy groups of triethoxysilane . The hydrolysis of trimethoxysilane producing methanol may be more environmentally problematic than the triethoxysilane releasing ethanol because methanol is more toxic than ethanol. The number of alkoxy groups will determine the amount of water used to fully hydrolyze them and influence the adhesion between silanes and filler. Di- and tri-alkoxy silanes produce stronger adhesion strength than mono-alkoxy silanes since they form more binding sites after they are hydrolyzed . If it is desired to deliver an alkoxy functional silane to the interior of the fiber this can be accomplished by dissolving the silane in the appropriate alcohol (e.g. ethanol for an ethoxy silane). Hydrolysis can then be achieved by subsequent water treatment