Interface engineering through matrix modification in natural fibre composites
Abstract:
The dependence of the mechanical performances and physical properties of natural fibre reinforced plastics on their interface properties is reviewed, with particular emphasis on low adhesion and the potential ways to overcome this problem via matrix modification. The focus is on polypropylene-based composites and maleic anhydride grafting. The way in which matrix modification improves the adhesion, as well as the parameters that influence the efficiency of bonding are considered. Finally, the effect of matrix modification on interfacial properties and on macroscopic properties of natural fibre composites is discussed. Two main composite categories are considered, long and short natural fibre reinforced polymer composites.
2.1 Introduction
As indicated by the title of this book and chapter, the scope of this chapter is to highlight the research performed in the area of matrix modification in natural fibre composites in order to engineer the interface of these systems and, in turn, to maximize their performance. It is outside the scope of this chapter to compile a full list of papers published regarding natural fibre composites, where matrix modification is performed. Rather, the focus is on systems that have been successfully used, from a scientific as well as from an industrial point of view, by modifying the most common matrices in natural fibre composites. In order to do so, we discuss briefly the motivation behind using natural fibre composites and the trends in their use over the years to allow the reader to understand which systems are the most important to be discussed next. The challenges in using natural fibre composites and the problem of low adhesion is also considered, together with a brief review of the potential ways to overcome this problem.
Next, we discuss the most successful ways to modify the matrix in natural fibre composites. In this section, the focus is on polypropylene (PP) based composites and maleic anhydride (MAH) grafting. Matrix modification of natural fibres composites with polyethylene (PE), poly(ethylene terephthalate) (PET), poly(butylene terephthalate (PBT) and biodegradable matrices is also briefly reviewed. The way matrix modification improves adhesion, as well as the parameters that influence the efficiency of bonding are considered, by employing the different theories of coupling mechanism, adhesion and chemical bonding. Finally, the effect of matrix modification on the interfacial properties and on the macroscopic properties of natural fibre composites is discussed. The synergy between the modification of the matrix and the fibres is discussed for two main categories of composites, i.e. long-fibre and short-fibre composites.
As aforementioned, the focus of this chapter is to review various ways of modifying the polymer matrix in order to enhance adhesion. It is important to keep in mind the difficulty in defining a clear boundary between fibre, matrix modification and coupling agents. In most cases materials used as coupling agents are at the same time modifiers of the fibre and/or the matrix.
2.2 Motivation behind using natural fibre composites and trends
The replacement of conventional glass fibre reinforced plastics (GFRP) by natural fibre ones has been boosted in recent years by ecological concern, environmental awareness and new rules and regulations, which require sustainability and eco-efficiency in technical applications. Lignocellulosic natural fibres are renewable and nonabrasive, and can be incinerated for energy recovery (Bledzki and Gassan, 1999). Furthermore, natural fibre reinforced composites, upon correct design and manufacturing, can be competitive in both mechanical performance and price compared with glass fibre reinforced ones (Berglund and Peijs, 2010; Bledzki and Gassan, 1999; Garkhail et al., 2000; Heijenrath and Peijs, 1996; Joshi et al., 2004; Peijs, 2000; Peijs, 2002; Peijs et al., 1998; Singleton et al., 2003; Stamboulis et al., 2000; Wambua et al., 2003).
The focus until very recently was on composites with polyolefin matrices and, more specifically, on PP, where the effect of natural fibre addition on the physical and mechanical properties has been discussed comprehensively (Barkoula, et al., 2010b; Bledzki and Faruk, 2006; Bledzki and Gassan, 1999, Bos et al., 2006; Cantero et al., 2003; Doan et al., 2006; Espert et al., 2004; Garkhail et al, 2009; 2000; Gassan, 2002; George et al, 2001a; 2001b; Heijenrath and Peijs, 1996; Hornsby et al., 1997; Jayaraman 2003; Keener et al., 2004; Peijs, 2000; Peijs et al., 1998; Singleton et al., 2003; Stamboulis et al., 2000; 2001; Van Den Oever et al., 2000; Zafeiropoulos et al., 2002a; 2002b). Natural fibre reinforced PP is relatively cheap, is thermally stable, and is extensively used in technical applications, e.g. in the automotive industry (Bledzki et al., 2006; Brouwer, 2001; Eisele, 1994; Karus et al., 2005; Mieck et al., 1996; Schlößer and Knothe, 1997; Schüssler, 1998).
Although polyolefin-based natural fibre composites are the most researched ones, during the last decade, biopolymers which originate from renewable raw materials, have been also proposed as matrices in natural fibre composites (Averous and Boquillon, 2004; Barkoula et al., 2010a; Bax and Muessig, 2008; Berglund and Peijs, 2010; Bodros et al., 2007; Mohanty et al., 2002; Nishino et al., 2003; Oksman et al., 2003; 2005; Vila et al., 2008). Generally, biopolymers are thermoplastic materials offering advantages such as low processing time, recyclability along with a feature of biodegradability, and additional recovery options, such as composting. Being fully integrated into natural cycles or carbon dioxide (CO2) neutral combustion, biocomposites also meet the steadily increasing environmental demands of legislative authorities. Recent reviews have been published on the potential of these materials in various applications (Chiellini et al., 2004; Gatenholm and Mathiasson, 1994; Gatenholm et al., 1992; Mohanty et al., 2000; 2002; Philip et al., 2007; satyanarayana et al., 2009; Shanks et al., 2004; Van De Velde and Kiekens, 2002; Yu et al., 2006). Among other biodegradable polymers, polyhydroxybutyrate (PHB) and its copolymers are of special interest, because PHB is a highly crystalline polymer and has a melting point, strength and modulus comparable to those of isotactic PP (Jiang et al., 2008).
2.3 Challenges in using natural fibre composites: the problem of low adhesion
Some of the difficulties in using natural fibres as reinforcement in polymer matrix composites are related to: (a) the relatively low thermal stability of natural fibres (up to 230 °C) (Bledzki and Gassan, 1999; George et al., 2001a; 2001b; Myers et al, 1990), and potential fibre degradation when used with matrices requiring processing at high temperatures, (b) their susceptibility to moisture (Bledzki and Faruk, 2004; Stamboulis et al., 2000; 2001), (c) the fibre decomposition during compounding with the polymer matrix (Barkoula et al, 2010b; Keller, 2003; Van Den Oever et al., 2008), which can lead to significant fibre breakage, influencing the morphology and final properties of the composite. Next to fibre breakage, compounding may also be used in an advantageous way to separate the technical cellulose fibres into elementary fibres (Berglund and Peijs, 2010; Snijder, et al. 1999).
However, one of the biggest problems in natural fibre reinforced plastics (NFRP), as in case of polymer matrix composites reinforced with inorganic fibres/fillers, is the low adhesion between the reinforcement and the matrix (Bledzki and Gassan, 1999; Bos, 2004; John and Thomas, 2008; Mieck et al., 1995a, 1995b; Mohanty et al., 2001; Nechwatal et al., 2005; Peijs et al., 1998; Saheb and Jog, 1999; Snijder and Bos, 2000). Bonding between the matrix and the fibre is dependent on the atomic arrangement and chemical properties of the fibre and on the molecular conformations and chemical constitution of the polymer matrix. Therefore, the interface is specific to each fibre/matrix system. Natural fibres are hydrophilic and polar in nature, whereas common thermoplastic matrices are hydrophobic and non-polar. The weak bonding between the non-polar matrix and the polar fibres is caused by the large difference in the respective surface energies (Belgacem et al., 1994; Myers et al., 1990) of the two materials linked to the existence of the hydroxyl groups and C–O–C links on the surface of the cellulose fibre. The only bonding mechanisms in this case are relative weak dispersion (chemical) and morphological (mechanical) bonding (Felix et al., 1993). This fact results in poor bonding between the fibres and the matrix, which, in turn, prevents the necessary wet-out of the fibres by molten polymer leading to poor dispersion of the fibres, insufficient reinforcement, and poor mechanical properties. More importantly, a weak interface means insufficient stress transfer from the matrix to the fibres through shear stresses at the interface.
The significance of the fibre/matrix interface for the macroscopic response of fibre reinforced composites is well known. Felix and Gatenholm, (1991) showed that a natural fibre composite with poor adhesion responded at small strain as a material with holes the shape of the filler. A strong interface allows effective stress transfer between the matrix and fibres. When the interface is too strong then brittle scission of fibres occur, whereas a too weak interface leads to pull-out of fibres. It is therefore not straightforward to find an ideal interface to optimize all the macroscopic properties simultaneously (Gamstedt and Almgren, 2007). Besides affecting the tensile strength of a composite, many other properties including fracture toughness, impact toughness, resistance to creep, fatigue and dimensional stability, environmental degradation, are also affected by the characteristics of the interface. In these instances, the relationship between properties and interface characteristics are generally complex (Gamstedt and Almgren, 2007). Based on the above, it can be concluded that in any composite system, interfacial properties need to be optimized rather than maximized, and this is particularly true for natural fibre composites, which often suffer from a lack of toughness, attributed to too strong interfacial interactions relative to the fibre’s strength.
Physical and chemical methods can be used to optimize the interface in natural fibre composites (Araújo et al., 2008; Arbelaiz et al., 2005a; Arbelaiz et al, 2005b; Bledzki and Gassan, 1999; Bledzki et al, 1996; 2004; Bos, 2004; Cantero et al., 2003; Dalväg et al., 1985; Doan et al., 2006; Erasmus and Anandjiwala, 2009: Felix and Gatenholm, 1991; Gamstedt and Almgren, 2007; Garkhail et al, 2000; Gassan, 2002; George et al., 2001b; Keener et al., 2004; kim et al., 2007; Li et al., 2007; Manchado et al., 2003; Mieck et al., 1995a, 1995b; Nechwatal et al., 2005; Nyström et al., 2007; Pickering et al., 2007; Roberts and Constable, 2003; Snijder and Bos, 2000; Van de Velde and Kiekens, 2001; Zafeiropoulos et al., 2002a; Zafeiropoulos et al., 2002b). These modification methods have varying efficiencies for the adhesion between matrix and fibre. The nature of adhesion in composites of modified cellulose fibres and PP has been discussed extensively over the past two decades, and numerous publications can be found on the subject (Arbelaiz et al., 2005b; Cantero et al., 2003; Doan et al., 2006; Erasmus and Anandjiwala, 2009; Felix and Gatenholm, 1991; Felix et al., 1993; George et al., 2001b; Roberts and Constable, 2003; Van de Velde and Kiekens, 2001). A detailed analysis is given by Felix and Gatenholm (1991) and George et al. (2001b), where the various physical and chemical methods for improved adhesion have been discussed. More recently a review article was published on the chemical treatments of natural fibre for use in polymer composites (Li et al., 2007). In summary adhesion can be promoted by:
Natural fibres have surfaces covered with accessible hydroxyl and also carboxyl end groups. The surfaces of carbon and glass fibres are relatively inert. Graphite and silica surfaces contain relatively few functional groups (Gamstedt and Almgren, 2007). Natural fibres therefore show more versatility because the interface may be engineered to achieve an optimal interface with regard to the desired property (Gamstedt and Almgren, 2007). The various surface chemical modifications of natural fibres such as treatment by alkali, silane, isocyanate or permanganate, latex coating, acetylation, or monomer grafting under UV radiation have achieved various levels of success in improving fibre strength and fibre/matrix adhesion in natural fibre composites. Methods for natural fibre treatment have been reviewed extensively and are the subject of the chapter 1 of this book.
The substance which promotes or establishes a stronger bond at the matrix/reinforcement interface is known as the coupling agent. Another term that describes the third interphase component, used to improve adhesion, is compatibilizer. The role of the coupling agent/compatibilizer is to interact with both the fibre and matrix, in order to bridge the properties of the two different systems (Felix and Gatenholm, 1991). In general, to enhance the compatibility between hydrophobic and hydrophilic components of a system, the compatibilizing agents have a functional group able to react with the hydroxyl groups of cellulose, an alkyl chain, which decreases the hydrophilicity of the fibre and, at the same time, makes its surface more compatible for good adhesion to the matrix. Lu et al. (2000) provided a review of the various coupling agents used to modify the fibre/matrix adhesion in natural fibre reinforced plastics.
The adhesion between the fibre and matrix in natural fibre composites can also be adjusted by modifying the matrix. For polyolefin matrices, one of the most appropriate ways to modify the matrix is by changing their chemistry by attaching polar groups onto the polymer backbone. Biopolyesters, like PHB, on the other hand are polar in nature and are expected to show better adhesion with natural fibres than polyolefins (Shanks et al, 2004). The problem of adhesion seems also to be less relevant when all-cellulose composites are considered (Duchemin et al., 2007; Eichhorn et al., 2010; Gea et al., 2007; 2010; Gindl and Keckes 2005; Gindl et al, 2006; Nishino and Arimoto, 2007; Nishino et al., 2004; Qin et al., 2008; Soykeabkaew et al., 2008; 2009a; 2009b). As well as modifying the matrix with additives, it has been suggested that the morphology of the matrix and more specifically the crystallization process could be controlled so as to create a transcrystalline layer between the polymer matrix and the cellulose fibre. Existence of a transcrystalline layer has been reported to have a significant effect on the fibre/matrix adhesion of natural fibre composites (Felix and Gatenholm 1994; Garkhail et al., 2009; Gray, 1974; Quillin et al, 1993; son et al, 2000; Zafeiropoulos et al., 2001a; 2001b; 2001c).
2.4 Matrix modification, coupling mechanism and efficiency of bonding
The most important and efficient group in improving interfacial adhesion between a natural fibre and a polyolefin matrix is based on maleic anhydride-grafted PP (MAH-PP) (Arbelaiz et al., 2005a; 2005b; Bledzki and Gassan, 1999; Bledzki et al, 1996; Cantero et al, 2003; Dalväg et al, 1985; Felix and Gatenholm, 1991; Feng et al., 2001; Fung et al., 2002; Garkhail et al., 2000; Gauthier et al., 1998; George et al, 2001b; Joly et al, 1996a; Joseph et al., 2003; Karmaker, 1997; Karmaker and Youngquist, 1996; Karmaker et al, 1994; Manchado et al, 2003; Mieck et al, 1995b; Mishra et al, 2000; Myers et al., 1991a; 1991b; Oksman and Clemons, 1998; Qiu et al, 2003; Rana et al, 1998; 2003; sanadi et al, 1994a; Sanadi and Caulfield, 2000; sanadi et al, 1995; Snijder and Bos, 2000; Van Den Oever et al, 2000).
MAH is used to modify the polymer matrix in the presence of a free radical initiator. It is then grafted on to cellulose fibres. The MAH groups of the polymer chain either react directly with the hydroxyl groups of the cellulose or interact by hydrogen bonding with the cellulose. The molecular chain of MAH is much shorter than that of the polymer matrix and cellulose fibres. Long chains of high molecular weight are obtained usually by grafting MAH with PE, PP, and polystyrene (Rowell et al., 1996). Some other systems reported for modifying polyolefins are maleated styrene–ethylene/butylenes-styrene triblock copolymers and ethylene–propylene random copolymers (Long and Shanks, 1996), polyethylene copolymer grafted with MAH (Sgriccia and Hawley, 2007), and treatment with acrylic acid, 4-pentanoic acid, 2,4-pentadienoic acid and 2-methyl-4-pentanoic acid (Erasmus and Anandjiwala, 2009). The formed copolymers (MAH-PE, MAH-PP, S-MAH), styrene–ethylene–butylene–styrene/MAH (SEBS-MAH) are used as coupling agents (Raj and Kokta, 1991) to improve interfacial properties of the interface. Besides the grafting of MAH with polyolefin matrices, it has been found to modify also PS matrix (Maldas and Kokta, 1990; 1991), while ethylene/n-butyl acrylate/glycidyl methacrylate (EBGMA) and ethylene methylacrylate (EMA) are copolymers that have been used to modify the adhesion between natural fibres and PET (Corradini et al, 2009).
The effectiveness of MAH is the result of a better compatibility (Arbelaiz et al., 2005a; 2005b; Erasmus and Anandjiwala, 2009) and the ability of MAH to decrease the amount of hydrogen bonding between the fibres (Kazayawoko et al., 1997), and to form covalent bonding between hydroxyl groups of the cellulosic fibres and the anhydride groups of the MAH (Rana et al, 1998).
Other important coupling agents for PPs are silanes. According to Mieck et al. (1994; 1995a; 1995b) and Mieck and Reußlmann (1995) the application of alkyl-silanes does not lead to chemical bonds between the cellulose fibres and the PP matrix. It seems, however, that the hydrocarbon chains do affect the wettability of the fibres and the chemical affinity of the PP with the natural fibres.
Although biopolyesters, such as poly(lactic acid) (PLA) and poly(hydroxybutyrate) (PHB) are more hydrophilic than polyolefins, research suggests that the interaction between PLA and PHB with inorganic fillers can be improved. For this reason, PLA matrices modified with MAH have been investigated (Zhang and Sun, 2004; Huneault and Li, 2007). Mitomo et al., (1995) reported the radiation graft behaviour of methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), acrylic acid (AA) and styrene (ST) onto PHB and its copolymers. Grondahl et al. (2005) grafted AA onto PHB for tissue engineering applications. Chen et al. (2003), reported the synthesis and characterization of MAH-PHB beyond surface grafting. For the composites based on thermoset matrices, such as epoxy, polyester, and phenol formaldehyde, very little is done to modify the thermoset matrices to increase adhesion. Here, the most common treatments to enhance adhesion are simple alkali treatments of the natural fibres, which cleans and dewaxes the fibre (Gassan and Bledzki, 1999b; Mishra et al., 2001; Mwaikambo and Ansell, 2003: Ray et al., 2001; 2002a; 2002b, 2002c).
In order to understand how the modification of the matrix improves the fibre/matrix adhesion in natural fibre composites it is necessary to explain why materials adhere to each other. For that purpose a large number of adhesion mechanisms have been proposed and recognised (Felix and Gatenholm, 1991; Oksman and Clemons, 1998; Voyutskii, 1963). These include (a) adsorption and chemical bonding, (b) diffusion, (c) electrostatic attraction and (d) mechanical interlocking. The main chemical bonding theory alone is not sufficient. So, the consideration of other concepts appears to be necessary, including the morphology of the interface, the acid–base reactions at the interface, surface energy and the wetting phenomena (Felix and Gatenholm, 1991; Maldas et al, 1989; Westerlind and Berg, 1988). Bledzki and Gassan (1999) proposed several mechanisms of coupling in natural fibre composites, viz (a) elimination of weak boundary layers, (b) production of a tough and flexible layer, (c) development of a highly cross-linked interphase region, (d) improvement of the wetting between polymer and substrate, (e) formation of covalent bonds with both materials and (f) alteration of acidity of substrate surface.
The exact mechanisms of the grafting reactions have been and continue to be debated in the literature (Gaylord and Mishra, 1983; Heinen et al., 1996). A schematic representation of the most accepted interaction between the MAH group and the cellulosic fibre is presented in Fig. 2.1. The MAH group or the AA groups grafted on the polymer matrix react with functional groups present on the surface of the cellulose fibre to form chemical bonds as primary interaction mechanism. A second type of interaction consists of co-crystallization of the high molecular weight tail with the molecular chains of the polymer matrix, giving physical entanglements. As aforementioned, the MAH group, present in the MAH-PP, not only provides polar interactions but can covalently link to the hydroxyl groups on the cellulosic fibre (Qingxiu and Matuana, 2003). The formation of ester linkages and hydrogen bonds between the MAH– and the –OH of cellulose has been indicated through Fourier transform infrared spectroscopy (FTIR) and electron spectroscopy for chemical analysis (ESCA) by Felix and Gatenholm, (1991). Whereas, the alkyl group molecules chain ends promote adhesion with the bulk polymer matrix through chain entanglements (Ranganathan et al., 1999). Also, the similarity of the additive and matrix structure can permit the occurrence of segmental crystallization, which is desirable for cohesive coupling between the copolymer and the PP matrix (Felix and Gatenholm, 1991). After the MAH-PP treatment, the surface energy of the fibres is increased to a level much closer to the surface energy of the matrix (Garkhail, 2002).
2.1 Schematic representation of the adhesion process between MAH-PP and the cellulose fibre. (Garkhail, 2002)
Lu et al. (2001) suggested a broader interfacial bonding consisting of covalent bonds, secondary bonding (such as hydrogen bonding and Van der Waals forces) and mechanical interlocking. The mechanical interlocking may occur between the fibre and MAH and the polymer and MAH. All of these bonding mechanisms may concurrently exist across the interface at varying degrees (Keener et al., 2004).
The proposed mechanisms for PP grafted with AA have been discussed in detail in Erasmus and Anandjiwala (2009). Briefly, grafting of AA onto PP is initiated by peroxide radicals. The peroxide grafting of the AA occurs at the tertiary carbons of the polymer chain or at the terminal unsaturated part of the chain. The radicals extract hydrogen atoms, preferably from the tertiary carbon of the polymer chain, leading to the creation of new reactive sites, which are expected to react with other monomers or, as in this instance, with AA. A drawback of this process is that, as the polymer is grafted with AA, the molecular weight is lowered owing to chain degradation which results in a reduction in viscosity. In the same study, chemicals that contain the two functional groups (AA, 4-pentanoic acid, 2,4-pentadienoic acid and 2-methyl-4-pentanoic acid) were used as coupling agent in the treatment of the PP. The double bond (of the coupling agent) acts as the anchoring point for the PP and the carboxylic acid (of the coupling agent) ultimately forms the ester linkage to the cellulose.
There are several theories on how silanes work as coupling agents (Plueddeman, 1982). One is the chemical reaction theory. It simply states that a covalent bond is formed between the filler and polymer. The silano group reacts with the surface of the filler, whereas the organofunctional group reacts with the polymer. Another mechanism is formation of an interpenetrating network, in which the silane molecules diffuse into the polymer matrix, forming an interphase network of polymer and silane. These two explanations are the most widely accepted, although neither one alone completely explains chemical coupling (Plueddeman, 1982).
The two most discussed properties of MAH-grafted polymers that could influence their effectiveness as coupling agents for cellulose fibre composites are molecular weight, which affects entanglement with the matrix chains, and acid number, which determines the functionality present in the coupling agent (Canche-Escamilla et al., 1999; Felix et al., 1993; Gassan and Bledzki, 2000; Olsen, 1991; Panthapulakkal et al, 2005; Rana et al, 1998). In addition to molecular weight and acid number, the choice of peroxide and reaction temperature can be controlled to create a polyolefin with the desired level of grafted MAH (Keener et al., 2004).
2.4.1 Molecular weight
The molecular weight of the MAH-polymer determines how well the grafted polymer molecules will diffuse into the polymer matrix, create entanglements and co-crystallize (Rana et al., 1998; Olsen, 1991). When polymer chains are very short, there is little chance of entanglement between chains and they can easily slide past one another (Neilsen, 1977). In addition, no co-crystallization is expected. When the polymer chains are longer, entanglement between chains can occur and chains are able to diffuse deeper into the matrix. Thus, MAH-polymer chains become more involved in inter-chain entanglements (Felix et al., 1993). A minimum chain length is necessary to develop these entanglements (Neilsen, 1977). If the molecular weight of the coupling agent is too high, it may entangle with the polymer molecules so that the polar groups on the coupling agent have difficulty ‘finding’ the –OH groups on the fibre surface (Rowell, 1996).
2.4.2 Acid number
Acid number measures the functionality present in the coupling agent, which is dependent upon MAH units grafted per polymeric chain. If the acid number is too high, then the MAH may be too close to the polar surface and thus, the interaction with the non-polar phase might not be sufficient (Keener et al., 2004).
The acid or anhydride functional group reacts with the functional groups present on the surface. Then grafting of acid or anhydride occurs on a single polymer molecule leading to adhesion which is localized and not sufficient. For enhanced bonding, numerous acid or anhydride groups are required to react at multiple sites. Although this is beneficial in terms of uniform interactions, it limits the co-crystallization ability with the matrix (Roberts and Constable, 2003).
There are a few studies that investigated the effects of MAH-PP or AA-PP on the interphase thickness of PP composites. In a study by Felix and Gatenholm (1991), the interphase thickness was determined for a group of cellulose-PP composites containing MAH with a range of molecular weights. It was found that the increase in the molecular weight led to an increase in the thickness of the interphase layer along with the mechanical properties. Other studies evaluated several different coupling agents consisting of MAH-PP with various molecular weights and acid contents (Olsen, 1991; Sanadi et al., 1994b). All MAH agents improved the mechanical properties. However, the coupling agents with both high molecular weight and high acid content produced the highest mechanical properties. Low molecular weight on the other hand is expected to act more as a dispersing agent instead of a true coupling agent (Lu et al., 2000).
As mentioned above, another way to control the fibre/matrix stress transfer is through the control of the morphology of the matrix material and, more specifically, of the crystallization process near the interface. Nucleation of crystals is the first stage of the crystallization process, and is the result of homogenous as well as heterogeneous processes. Heterogeneous nucleation occurs in the presence of foreign surfaces e.g. particles, fibres, impurities, dust or additives. For composites, when heterogeneous nucleation occurs with a sufficiently high density along a reinforcing fibre surface, the resulting crystal growth is restricted to the lateral direction so that a columnar layer develops around the fibre. This phenomenon is known as transcrystallization (Garkhail et al., 2009). First Gray, (1974) reported that cellulose fibres induced transcrystallinity in isotactic PP (iPP). He documented the growth of a transcrystalline layer under isothermal crystallization at 136 °C with purified wood fibres. Less purified fibres developed nucleation, but this required a longer time than purified fibres. Felix and Gatenholm (1994) conducted a more systematic study for highly purified cotton fibre/iPP. They suggested that under isothermal crystallization the iPP chains have sufficient time to adopt the most favourable conformation and, because the crystal structures of cellulose and iPP are matching, there is an increased Van der Waals interaction between the iPP and the cellulose. The effect of various conditions such as the crystallization temperature, time and cooling rates on the formation of transcrystallinity was investigated on dew retted and green flax/iPP systems by Zafeiropoulos et al. (2001a; 2001b; 2001c). The transcrystallinity phenomenon in the flax/PP system was affected not only by the different types of flax fibres but also by the different types of PP. The isothermal crystallization behaviour and mechanical properties of ionomer-treated sisal fibres reinforced high-density polyethylene (HDPE) composites was investigated by Choudhury (2008). It was concluded that the crystallization behaviour of HDPE was influenced by the presence and concentration of short sisal fibres. Also here the surfaces of sisal fibres acted as nucleating sites for the crystallization of the matrix polymer, promoting the growth and formation of a transcrystalline layer in the composites. The concentration of nucleating sites in the HDPE/sisal composites increased as the fibre content increased, which was reflected in the increase of the overall crystallinity of the composites. Finally, Garkhail et al. (2009) investigated the effect of flax fibre reinforcement on the crystallization behaviour of PP and found that flax fibres are good nucleating agents for PP matrix leading to the development of a well-defined transcrystalline interphase zone. After isothermal crystallization from the melt a transcrystalline layer was found having lamellar crystals grown perpendicular to the fibre axis. Figure 2.2 shows the transcrystalline zone grown around the flax fibre in a PP matrix at a constant crystallization temperature of 138 °C with increasing crystallization time. It is clear that the thickness of the transcrystalline layer or interphase increased with time and reached a maximum value after a certain time. The nucleation density on the fibre surface was higher than in the bulk matrix; hence, there was less competition from spherulitic nucleation and growth in the polymer melt. This led to further growth of the transcrystalline layers, which became thicker and more uniform, before they impinged with spherulites grown in the bulk matrix. The effect of MAH on the crystallization process has been also reported in the literature (Yin et al., 1999; Manchado et al., 2003). It was found that the addition of MAH-PP increased the nucleation capacity, and accelerated the crystallization process, leading to a transcrystalline region. PE modified with MAH and processed with wood flour showed an increase in overall crystallinity compared with the pure and unmodified PE. The hypothesis that explains this phenomenon is again the nucleation of crystallites (Villar and Marcovich, 2003). Similar results were reported by Mildner and Bledzki, (1999) who studied the systems jute fibre (untreated and alkali treated)/iPP and jute fibre/MAH-PP. They found that the thickness of the transcrystalline layer varied with the cooling rate, with the system jute/MA-PP having the thickest layer.
2.2 Effect of crystallization time on transcrystalline thickness in PP/flax system at a crystallization temperature of 138 °C. Cooling rate 10 °C min‒ 1; (a) 2 min, (b) 10 min, (c) 34 min. (Garkhail et al., 2009)
2.5 Effect of matrix modification on interfacial properties
It is beyond the scope of this chapter to review all potential methods to evaluate the interfacial adhesion after modification. One direct measure of fibre/matrix adhesion is the interfacial shear strength (IFSS). Although much has been reported on the effect of modification on the mechanical properties, much less is published on actual values of the interfacial shear strength after modification.
Previous studies showed that the modification with MAH led to a significant improvement in the apparent IFSS (Doan et al., 2006; Nechwatal et al., 2005; Manchado et al., 2003; Stamboulis et al., 1999; Van Den Oever et al., 2000). Stamboulis et al. (1999) used the pull out test to determine the IFSS of flax/PP and flax/PP/MAH-PP. For the same system, Garkhail (2002) used the micro-debond test. Van Den Oever and Bos (1998) calculated the IFSS of elementary and technical flax fibre with PP and MAH-PP. They calculated IFSS values of 13 MPa for the elementary fibres, which increased to 28 MPa with the addition of 1 wt% MAH in PP matrix. Similarly, in the case of a technical fibre, the addition of the MAH led to an increase from 8 to 12 MPa. Other studies showed that the addition of 2 wt% MAH-PP in jute/PP composites (with two different PP grades) led to IFSS values of 16.3 and 19.8 MPa, which were very close to the shear yield strength values of the pure PP matrices as calculated from the Von Mises criterion 
These results indicate a good interaction between the jute fibres and the PP matrix with MAH-PP (Doan et al., 2006). In a system of wood fibre with PP, the IFSS increased from 14 to 23 MPa, with the addition of MAH-PP, which means that here the addition of coupling agent improved adhesion between fibre and matrix by almost 65% (Nyström et al., 2007). These results are similar to the results of Van de Velde and Kiekens (2001) who report an increase in the interlaminar shear strength in a unidirectional PP/flax system from circa 10 to 24 MPa upon the addition of MAH-PP.
Garkhail (2002) comprehensively studied the effect of both fibre and matrix modification on the IFSS when MAH-PP was added to a PP/flax composite. Using the micro-debond test, the effect of the MAH-content on the IFSS, when the matrix is modified was documented. A photomicrograph of a PP/MAH-PP blend on a flax fibre is shown in Fig. 2.3. The effect of the MAH-content on the IFSS, when the matrix is modified is presented in Fig. 2.4.
2.3 Droplet of PP/MAH-PP blend (80:20) on flax fibre for the micro-debond test. (Garkhail, 2002)
2.4 Interfacial shear strength (IFSS) values of flax fibre with varying amounts of MAH-PP concentration in the PP matrix. (Garkhail, 2002)
As can be seen from this figure, the introduction of 5 wt% MAH-PP in the PP matrix resulted in a substantial improvement in the IFSS of the untreated fibres. With the amount of MAH-PP increasing to 10 wt%, the average IFSS reached nearly 14 MPa, which is an increase of 90% compared with the matrix without MAH-PP and is close to the shear yield stress of the matrix (~16 MPa). However, the IFSS declined with the further increase in the concentration of MAH-PP in the matrix. So the optimal (or critical) MAH-PP concentration for the system used, was 10 wt%. Other studies reported an optimum in adhesion, which existed at a specific concentration found through examining the influence of the composition of copolymers on bonding strength (Garkhail, 2002; Van Den Oever and Bos, 1998). Van Den Oever and Peijs (1998) stated that the critical concentration results from the variation of the secondary structure and molecular connectivity (i.e. the orientation and conformation) of the polymers within the interface. The IFSS depends on the mode of fracture i.e. adhesive type or cohesive type. When the amount of compatibilizing agent is low (less than the critical concentration, ϕc), the limiting factor is the adhesive strength. With increasing compatibilizing agent concentration, the adhesive strength is improved through an increase in polar interactions. However, at concentrations greater than ϕc, the cohesive strength (i.e. within the matrix) may be reduced owing to the dense attachment of the polar molecular chains to the solid substrate, leading to a reduction in chain entanglements of polar molecules with the bulk matrix (Fig. 2.5). As a result, the cohesive strength can be lower than the adhesive strength and the interface may fail cohesively instead of adhesively i.e. through the ‘interphase’ or bulk matrix, leading to a poor stress transfer (Garkhail, 2002).
2.5 Schematic representation of the transformation in the fibre/matrix interface/interphase on increasing the polar groups in the matrix. (Garkhail, 2002)
The effect of a transcrystalline region on composite interfacial and macromechanical properties has been also studied in various fibre reinforced thermoplastic systems with contradictory results (Folkes and Hardwick, 1987; 1990; Gati and Wagner, 1997; Pompe and Mader, 2000; son et al, 2000; Wagner et al., 1993; Wood and Marom, 1997; Zafeiropoulos et al., 2001a; 2001b; 2001c; Zhang et al., 1996). In some instances a positive effect of the transcrystalline layer on the interfacial properties has been reported, owing to some kind of physical coupling between the fibre and polymer matrix (Wagner et al, 1993; Zafeiropoulos et al., 2001a; 2001b; 2001c, Choudhury, 2008), whereas other studies report negative effects on both interfacial and macroscopic properties (Garkhail et al., 2009). By applying the single fibre fragmentation test (Felix and Gatenholm, 1994; Zafeiropoulos et al., 2001a; 2001b; 2001c) showed that there was an increase of the interfacial shear strength (IFSS) when transcrystallinity was present. They attributed this increase to improved interactions between the fibre and the matrix because of the transcrystalline layer. However, a number of studies reported no or even negative effects of transcrystalline interfaces on the interfacial properties of composites, especially for inorganic fibre reinforced composites (Folkes and Hardwick, 1987; 1990; Garkhail et al., 2009; Gati and Wagner, 1997; Pompe and Mader, 2000; son et al., 2000; Wagner et al., 1993; Wood and Marom, 1997; Zhang et al, 1996).
Garkhail et al. (2009) investigated two different cooling conditions (quench and slow cooling), for studying the effect of transcrystallization on the IFSS in micro PP/flax composites by the fibre pull-out test method. For the PP/flax average IFSS values of 7.8 and 9.8 MPa were measured for samples with and without a transcrystalline layer, respectively, without adding any coupling agent. Both values were relatively high IFSS compared with PP/glass, probably owing to the rougher surface of the natural fibres, which can result in additional physical coupling between fibre and matrix. As can be seen, the presence of the transcrystalline layer here had a negative effect on the IFSS and led to a reduction in the stress transfer capability from the matrix to the fibre. As aforementioned these findings were in contrast to those reported for a dew-retted flax/PP system by Zafeiropoulos et al. (2001a). They reported IFSS values of 12 and 23 MPa for samples without and with a transcrystalline layer, respectively, with a positive effect of the transcrystalline layer on the interface properties. Although this difference could be partly caused by differences in the type of flax, PP, sample processing and data reduction, the main difference is most likely the result of the very different micromechanical tests used.
2.6 Effect of matrix modification on macroscopic properties
It is well known that fibre/matrix adhesion is very significant for the macroscopic response of fibre-reinforced composites. The most important properties when a matrix modifier is used are the strength and modulus in tension and bending. The effect of matrix modification on properties, such as fracture toughness, impact toughness, resistance to creep and fatigue has been less studied. The dimensional stability and environmental degradation of modified natural fibre composites has received considerable attention and data are mostly available on MAH-PP modified natural fibre reinforced PP composites. In general, it can be stated that compatibilization can lead to significant improvements in strength and moisture resistance. However, maximizing the interfacial strength also hinders fibre pull-out which in turn leads to a reduction in toughness. A typical natural fibre used in most of today’s composites has a tensile strength of only one fourth that of glass (Garkhail et al., 2000). At similar fibre/matrix adhesion levels, for example, as a result of similar compatibilization schemes, fracture processes in natural fibre composites involve fewer energy-absorption mechanisms, such as debonding and fibre pullout, than in GFRPs, leading to a more brittle fracture. Hence, interfaces in natural fibre composites need to be weaker than in GFRPs in order to obtain similar levels of toughness. Moreover, unlike isotropic glass fibres, natural fibres are highly anisotropic, (Garkhail et al., 2000, Bos et al., 2002; Bos, 2004) and after optimization of the interface, internal adhesion within the fibre bundle structure often becomes the weakest link to initiate failure. As is shown by Bos et al. (2002), the lateral strength and thus the shear strength within the fibre cell wall is lower than the strength of the fibre in the length direction. Once fibre matrix adhesion becomes stronger than the lateral or shear strength of the secondary cell wall, the lateral strength of the cell wall becomes the limiting factor, and composite strength cannot be further increased by optimization of the compatibilizer (Van Den Oever et al., 2000). This has been well illustrated on the example of a flax stem by Van Den Oever et al. (2000) (see Fig. 2.6).
2.6 Photographs of a cross section of a flax stem (a) with the bast fibres marked and (b) with spots marked where weak technical fibre bonding is expected. (Van Den Oever et al., 2000)
Most of the studies on the effect of matrix modification in natural fibre composites with approximately 2–5 wt% MAH-PP report a substantial increase in the strength of the modified systems, both in tension and flexure, whereas the stiffness is less influenced (Arbelaiz et al., 2005a; Arbelaiz et al., 2005b; Barkoula et al, 2010b; Bledzki et al, 2004; Bos, 2004; Bos et al, 2006; Dalväg et al., 1985; Doan et al., 2006; Garkhail et al., 2000; Gassan and Bledzki, 1997; 1999a; Karmaker, 1997; Karmaker and Youngquist, 1996; Karmaker et al., 1994; Keener et al., 2004; Kim et al., 2007; Manchado et al., 2003; Pickering et al., 2007; Snijder et al., 1998; Van De Velde and Kiekens, 2001; 2003). Depending on the system studied, the amount, molecular weight and MAH functionality, improvements in composite strength of 20–100% were reported. In terms of stiffness both in tension and flexure, an increase in the range of 20% was documented. Young’s modulus reflects the capability of both fibre and matrix material to transfer the elastic deformation in the case of small strains without interface fracture. Therefore, it is not surprising that the Young’s modulus is less sensitive to variations in interfacial adhesion than strength which is strongly associated with debonding and interfacial failure (Doan et al., 2006).
Gassan and Bledzki (1997; 1999a) showed that the impact energy decreases owing to the lower energy absorption in the interface of PP/jute composite. Gassan and Bledzki, (1999a and 1999b) investigated the influence of MAH-PP on the impact behaviour of PP/jute composites and found that damage initiation is shifted to higher loads in the case of a strong fibre/matrix adhesion. The dissipation factor was also higher for MAH-PP modified systems. It was explained that the absence of a coupling agent results in impact energy being absorbed by debonding and frictional effects at the fibre/matrix interface.
kim et al. (2007) on the other hand showed increased Izod impact values with the addition of various types of MAH-PP modifiers. Dalväg et al. (1985) also presented the effect of different MAH-PP content on the impact properties of cellulosic thermoplastic composites, and found a remarkable improvement. Oksman and Clemons (1998) studied the optimization of the mechanical properties of PP/wood flour composites by means of the addition of various compatibilizers, in particular the impact behaviour of the composites, and they succeeded in doubling the unnotched impact strength (from 86 to 167 J m‒ 1) and notched Izod impact strength (from 26 to 54 J m‒ 1) while retaining a tensile strength of 30 MPa and a modulus of 1.9 GPa, by adding a maleated styrene–ethylene/butylenes–styrene triblock copolymer (SEBS-MA). These seemingly inconsistent results on the effect of matrix modification on the impact performance can often be explained on the basis of the role of fibre length on the energy absorption mechanisms during impact. As mentioned earlier, too strong an interface leads to brittle failure, with no energy absorption, whereas too weak an interface leads to fibre pull-out without significant effort and energy dissipation. For short fibre composites with fibre lengths well below the critical fibre length, such as wood fibre composites, improved adhesion may lead to improved energy absorption through an increase in pull-out or debonding stress. Composites on the other hand based on longer fibres such as jute fibres may show embrittlement because improved adhesion leads to a reduction in pull-out length.
Sain et al. (2000) showed only a marginal improvement in creep properties by maleic and maleimide interfacial modification in wood fibre PP composites. A stronger effect of the interface is expected in cyclic loading under fatigue conditions, where an imperfect interface can lead to repeated frictional sliding, which induces temperature increase and energy dissipation. Gassan and Bledzki (1997) measured a significant decrease in energy dissipation during cyclic loading for jute fibre reinforced PP with improved interface from MAH-PP. Doan et al. (2006) reported improved thermomechanical behaviour of MAH-PP modified PP/jute, with an increase in the storage modulus values. Gassan and Bledzki (1998) created a graph (see Fig. 2.7) to provide an overview of the influence of MAH-PP coupling agent on the most discussed mechanical properties of natural fibre reinforced composites on the example of jute/PP composites (Gassan and Bledzki, 1998).
2.7 Influence of coupling agent (MAH-grafted PP) on the mechanical properties of jute PP composites (fibre content = 37 vol. %). (Gassan and Bledzki, 1998)
A comprehensive study on the effect of AA as a matrix modifier on the response of natural fibre composites is performed by Erasmus and Anandjiwala (2009). Similarly to the addition of MAH, AA addition leads to enhancement of the strength when used as a matrix modifier. Both tensile strength and modulus increase with increasing AA content up to 2 wt% where a maximum is achieved, and then decreases with a further increase in AA content. Addition of 1 wt% AA causes a decrease in flexural strength, but as the amount of AA increases, there is an increase in flexural strength. However, as the amount of AA increases the benefits of fibre/matrix coupling come into play and the stress is transferred from the matrix to the fibre more effectively. The drop in modulus and tensile strength at the higher concentrations could possibly be attributed to damage caused to the fibre, instead of being caused by coupling. Another possible explanation is the increased use of peroxide during the 4 wt% AA modification, which causes an increase in β-scission. A decrease in molecular weight of the PP could lead to a reduction in composite strength (Erasmus and Anandjiwala, 2009). Again, the addition of AA to the matrix has a negative effect on the Charpy impact strength. As the amount of AA increased, the composites showed a reduction in impact strength. This result is consistent with the other findings reported, namely that a too strong interaction between fibre and matrix leads to a poor impact strength (Sain et al., 2005). This might indeed suggest that AA is effective in improving interfacial interaction (Erasmus and Anandjiwala, 2009). Son et al. (2000) found that the tensile strength of macrocomposites decreased with the presence of a transcrystalline layer and also with increasing isothermal crystallization time. To investigate the effect of transcrystallinity on the macromechanical properties of the PP/flax composites, two different cooling conditions (quench and slow) were used and reported by Garkhail et al. (2009). PP/flax composites had a lower modulus and strength with the presence of a transcrystalline interphase. This was attributed to two causes: first, the weaker fibre/matrix interface, as also measured by the IFSS test; second, the different morphology of the PP matrix itself in composites processed using different cooling conditions (fast quench cooled versus slowly cooled).
Several studies looked at the effect of matrix modification on the environmental response of natural fibre reinforced plastics. As mentioned before, one of the major restrictions in the applicability of these composites comes from the fact that these materials are very susceptible to moisture. This results in water uptake and dimensional instability (swelling), and the fibre/matrix interface is one of the weakest links in this process, in addition to the hydrophilic nature of the natural fibres. Typically, the cellulosic natural fibres swell more than the polymer matrix. Improved interfacial bonding through matrix modification resulted in reduction in the water uptake and diffusion coefficient in natural fibre composites as stated in MANOJvarious publications (Arbelaiz et al., 2005c; Avella et al., 1995; Bailie et al., 2000; Espert et al., 2004; Gauthier et al, 1998; George et al, 1998; Joly et al, 1996b; Joseph et al, 2002; Marcovich et al, 2005; Naik and Mishra, 2006; Peijs et al, 1998; Rana et al. 1998; Stamboulis et al, 2000; Thwe and Liao, 2003). From the above studies, the reasons for this improvement can be summarized as: (a) better wetting of the fibre, (b) fewer gaps at the fibre/matrix interface, and (c) ester linkages from the reaction of MAH with the –OH groups of the fibre, resulting in a less hydrophilic fibre surface. All these effects are interrelated and lead to less moisture uptake since capillary absorption through narrow channels along the interfaces is suppressed.
2.6.1 Short versus long fibre composites
In most engineering applications natural fibres are found in the form of short or long discontinuous reinforcements in the polymer matrix. Two manufacturing routes are mainly utilized, (i) the mat technology, where non-woven natural-fibre mats are compression moulded with the polymer matrix to produce random natural-fibre mat reinforced thermoplastic composites (NMT) (Garkhail et al, 2000; Heijenrath and Peijs, 1996; Jolly and Jayaraman, 2006; Nechwatal et al., 2005; Peijs et al., 1998; Stamboulis et al., 2000; Van Den Oever et al., 2000; Wielage et al., 2003) and (ii) the granule technology where natural fibre reinforced polymer granules are injection moulded or extrusion compression moulded (ECM) to produce mainly short fibre reinforced composites (Arbelaiz et al., 2005a; 2005b; Aurich and Mennig, 2001; Bos et al., 2006; Cantero et al, 2003; Hull, 1981; Li and Sain, 2003; Nyström et al, 2007; Wielage et al., 2003). In discontinuous fibre reinforced composites, the parameters that influence the macroscopic response are the intrinsic fibre properties, their length and diameter, and the fibre/matrix adhesion (Hull, 1981; Nechwatal et al., 2005; Van Den Oever and Bos, 1998). There is a critical fibre length below which no effective reinforcement can be expected. This critical fibre length can be predicted based on micromechanical models and is different for stiffness, strength and impact properties (Thomason and Vlug, 1996; Thomason et al., 1996; Nechwatal et al., 2005).
The critical fibre length is interrelated with the fibre/matrix adhesion, and more specifically is dependent on the IFSS. The critical fibre length (Lc) can be calculated using the Kelly–Tyson theory (Kelly and Tyson, 1965):
where σf is the fibre strength, d is the fibre diameter, and τ is the interfacial shear strength.
It is obvious that the improvement of the fibre/matrix adhesion with matrix modification results in lower critical fibre lengths required for improvement of the composite’s macroscopic response. The influence of interface compatibilizer was studied in compression moulded PP/flax, where no significant improvements on the mechanical properties (Garkhail et al., 2000; Nechwatal et al., 2005) were found with the addition of compatibilizers. On the other hand, for injection-moulded PP/flax composites, positive effects of coupling agents on final mechanical properties were reported in numerous studies (Arbelaiz et al., 2005a; 2005b; Bos et al., 2006; Cantero et al., 2003; Li and Sain, 2003; Nechwatal et al., 2005; Nyström et al., 2007; Wielage et al., 2003). These studies conclude that an adhesion promoter based on MAH-PP behaves as a true coupling agent, i.e. improves the mechanical performance of the PP/flax composites (Arbelaiz et al., 2005a; 2005b; Bos et al, 2006; Cantero et al., 2003; Li and Sain, 2003; Nechwatal et al., 2005; Nyström et al., 2007). This is because NMTs contain fibres that are long enough to provide reinforcement (Lf > Lc), which is not the case in injection moulded composites, where normally shorter fibres are found compared with compression moulded composites (Lf < Lc). Therefore, fibre/matrix adhesion becomes more critical.
In a recent study by Barkoula et al. (2010b) the influence of MAH-PP concentration on flax/PP composites, which were manufactured through injection moulding (compounding through kneading), was investigated. The materials investigated consisted of composites with 28 wt% of short flax fibres. As reference a pure PP matrix was used.
The tensile modulus of the injection moulded MAH-PP/flax composites is given in Fig. 2.8 which shows that the modulus results are approximately constant, 6.2 and 1.7 GPa for the 28 wt% flax series and the reference PP material, respectively. As expected no influence of fibre/matrix adhesion on composite stiffness was observed.
2.8 Tensile modulus of injection moulded flax/PP/MAH-PP composites with varying amount of MAH-PP content in the matrix (28 wt% flax fibres). (Barkoula et al., 2010b)
The tensile strength values of the injection moulded flax/PP/MAH-PP composites are given in Fig. 2.9. The matrix tensile strength shows a constant value of around 30 MPa. On increasing the amount of MAH-PP in the flax/PP/MAH-PP composite a maximum tensile strength was observed for 20 wt% MAH-PP. This behaviour is in line with a previous observation on the effect of MAH-PP on the mechanical properties of GFRPP (Van Den Oever and Peijs, 1998), where first an increase in strength with compatibilizer is found, followed by a subsequent decrease at higher fractions of MAH-PP. An optimum in interfacial shear strength was also observed for MAH-PP/flax systems. However, here an optimum was found at 10 wt% MAH-PP content in microcomposites, measured through micro-debond tests (Mieck et al., 1995b). The observed optimum in micro-debond tests is not directly reflected in the macro-composite properties, which may be because of effects related to fibre volume fractions (single fibre versus bulk composite). However, such high weight fractions of MAH-PP are not economical when a balance in price and property enhancement is desired. Such balance can, however, be obtained for 3–5 wt% addition of MAH-PP in PP.
2.9 Tensile strength of injection moulded flax/PP/MAH-PP composites with varying amount of MAH-PP content in the matrix (28 wt% flax fibres). (Barkoula et al., 2010b)
A comparison was also made with results on long fibre NMTs published previously (Garkhail et al., 2000). Figures 2.10 and 2.11 show the stiffness and strength of compression moulded NMT (Garkhail et al., 2000) and injection moulded (compounding done through kneading) flax/3 wt% MAH-PP composites, respectively. In spite of a reduction in fibre length during injection moulding it can be seen that injection moulded samples showed tensile properties similar to those of NMT (fibre length ~25 mm) compression moulded composites. This could be because of reasons like: (a) improved fibre efficiency because of dimensional changes, i.e. a reduction in fibre diameter through fibre opening during compounding and (b) changes in fibre orientation along the direction of polymer flow in the case of injection moulded composites. Dimensional changes can play an important role since the flax fibres used for composite reinforcement are often actually fibre bundles (so-called technical fibres) consisting of fibre cells (so-called elementary fibres) (Garkhail et al., 2000; 2009). Also, it has been shown that the fibre tensile strength is strongly dependent on the fibre length (Thomason et al., 1996) and therefore a reduction in fibre length could have led to a separation of these fibre bundles into fibre cells, hence leading to improved fibre efficiency through enhanced fibre tensile properties (of fibre cells) or improved fibre aspect ratio.
2.10 Tensile modulus of PP/flax composites manufactured through compression moulding of NMT (Garkhail et al., 2000) and injection moulding as a function of fibre volume fraction. Also shown is glass fibre based GMT. (Garkhail et al., 2000)
2.11 Tensile strength of PP/flax composites manufactured through compression moulding of NMT (Garkhail et al., 2000) and injection moulding as a function of fibre volume fraction. Also shown is glass fibre based GMT. (Garkhail et al., 2000; Barkoula et al., 2010b)
The above claims can be better understood via the calculation of the critical fibre length for technical and elementary flax fibres. For this calculation, strength values are taken from the literature (technical fibre: 800 MPa (Heijenrath and Peijs, 1996), elementary fibre: 1500 MPa (Heijenrath and Peijs, 1996), for fibres with 80 and 12.5 mm diameter, respectively). The IFSS are those stated above, i.e. 8 MPa for PP/flax (Garkhail, 2002; Garkhail et al, 2000; 2009; Mieck et al, 1995b) and 16 MPa (Garkhail et al, 2000) for MAH-PP/flax. Using these values the critical fibre length was calculated as shown in Table 2.1 (Barkoula et al., 2010b). Based on this data a critical fibre length of approximately 2 mm was found in the case of MAH-PP/PP with technical flax fibres. To attain short fibre composite strengths which are equivalent to 90% of the tensile strength of a continuous fibre composite, fibre lengths > 5Lc are required (Thomason and Vlug, 1996). This means that, for effective strength improvement, fibre lengths of approximately 10 mm are required for MAH-PP/flax and of approximately 15 mm for PP/flax. These results are in agreement with Nechwatal et al. (2005) where optimum strength for MAH-PP/flax is obtained for approximately 8 mm fibre lengths. If elementary fibres are assumed in combination with MAH-PP matrix then 5Lc is equal to 3 mm. A considerable fraction of the fibres can be above 3 mm after injection moulding, and this could be the reason why injection moulded composites can have similar strength to NMTs.
Table 2.1
Critical fibre lengths for PP/flax and MAH-PP/flax systems with elementary and technical fibres (Barkoula et al., 2010b)
In another publication (Stamboulis et al., 2000) the environmental response with regard to MAH-PP addition was evaluated on three different types of NMTs: (i) treated Duralin™ flax/PP, (ii) treated Duralin™ flax/MAH-PP and (iii) untreated green flax/PP as a reference. The Duralin™ treatment, which consisted of a steam or water-heating step of the flax fibre above 160 °C for 30 minutes and a drying/curing step above 150 °C for 2 hours, led to improved moisture resistance of cellulose fibres. The results of the moisture content as a function of the exposure time are presented in Fig. 2.12. As shown in Fig. 2.12, green flax fibre based composites are more sensitive to moisture than the other two types of composites. Also, the diffusivity of Duralin™ flax fibre composites (Table 2.2), as calculated from the initial slope of the moisture uptake curves, is much lower than that for NMTs based on green flax fibre mats. It is interesting to note that the use of MAH-PP as a compatibilizer lowers the diffusivity even further. Clearly, the initial moisture uptake in this composite system is taking place at a lower rate than for the PP system without compatibilizer. The maximum moisture content level is, however, similar for both PP and MAH-PP based Duralin™ flax systems. The higher diffusivity for the PP system without compatibilizer indicates that initially a fair amount of moisture uptake takes place along the fibre/matrix interface.
Table 2.2
Maximum moisture content and diffusivity of PP/flax composites (Stamboulis et al., 2000)
| Composite | Max. moisture content, Mm,c (%) |
Diffusivity, Dc (cm2 s‒1) |
| PP/green flax | 18.0 | 1.3 × 10‒2 |
| PP/Duralin™ flax | 12.8 | 7.8 × 10‒3 |
| MAH-PP/Duralin™ flax | 13.5 | 5.0 × 10‒3 |
2.12 Moisture content as a function of time for green flax/PP, Duralin/PP and Duralin/MAH-PP NMT composites. (Stamboulis et al., 2000)
2.7 Future trends
The most recent trends in natural fibre composites comprise the introduction of all-cellulose composites and cellulose nanofibres for nanocomposites. Although research in the area of cellulose nanofibres and nanocomposites has shown that these materials have an exciting potential as reinforcement in nanocomposites (Berglund and Peijs, 2010; Eichhorn et al., 2010), they do not offer any particular solution to the problem of low fibre/matrix adhesion. Moreover, it can be said that since the surface area increases substantially when fibres are of nanosize, the importance of fibre/matrix adhesion and particularly dispersion is even greater in these materials. Moreover, so far most cellulose nanofibres that have been produced exhibit fairly low aspect ratios, implying that interface engineering in these materials will become even more important in order to achieve high reinforcement efficiencies. On the other hand all-cellulose composites might offer alternative solutions towards better fibre/matrix adhesion.
Recently, a completely new route to cellulose-based composites with good interfacial adhesion was proposed by the groups of Nishino and Peijs. (Nishino et al., 2004; Nishino and Arimato, 2007; Qin et al., 2008; Soykeabkaew et al., 2008; 2009a; 2009b). They focused on approaches following self-reinforced polymer concepts for thermoplastic fibres (Alcock et al., 2006a; 2006b; 2007). Such an approach was followed to create high-strength all-cellulose composites, having the essential advantages of being fully biobased and biodegradable materials. These composites exhibited excellent mechanical properties, e.g. a tensile strength of approximately 500 MPa which compared very favourably to more traditional unidirectional natural fibre based composites (Heijenrath and Peijs, 1996; Van Den Oever et al., 2000; Oksman et al., 2002; Liu et al., 2005). Moreover, because both fibre and matrix are composed of the same material they show excellent interfacial compatibility. During composite preparation, the surface layer of the cellulose fibres is partially dissolved to form the matrix phase of ‘all-cellulose’ composites. The developed surface dissolution method results not only in very high fibre volume fractions, but also in a gradual interphase or interfacial region, which minimizes voids and stress concentrations, and can even lead to transparent composites.
2.8 Sources of further information and advice
Dalväg et al. (1985) carried out one of the first studies on the efficiency of cellulosic fillers in common thermoplastics and the effect of processing aids and coupling agents. Felix and Gatenholm (1991) published a seminal study on the nature of adhesion in composites of modified cellulose fibres and PP. Mieck et al., (1995a, 1995b) studied fibre/matrix adhesion in flax fibre reinforced thermoplastics; one study was on the effect of silane modification and the second on the use of functionalized PP. Bledzki and Gassan (1999) provided a review on natural fibre reinforced plastics and their potential application. Lu et al. (2001) published a review of coupling agents and treatments for natural fibre composites. George et al. (2001b) and Mohanty et al. (2001) have compiled reviews on interfacial modifications in natural-fibre composites and their effects on the composite properties. Another important study is the one by Nechwatal et al. (2005) on the processing technologies used for the addition of the MAH in natural fibre reinforced composites. Important studies conducted by Bos and coworkers are summarized by Bos (2004). Current international research into cellulose composites is well documented by Berglund and Peijs (2010) and Eichhorn et al. (2010). Interesting books and book chapters include the following:
(a) Handbook of polypropylene and polypropylene composites, ed. Karian HG, second edition, Chapter 3: Chemical coupling agents for filled and grafted polypropylene composites (Roberts and Constable, 2003), which provides a good review of coupling agents for PP and the chemistry behind grafting.
(b) Properties and performance of natural-fibre composites, ed. K Pickering, Woodhead Publishing Limited, which provides an overview of the types of natural fibres used in composites, a discussion of the fibre-matrix interface and how it can be engineered to improve performance, an examination of the increasing use of natural-fibre composites in automotive and structural engineering and the packaging and energy sector and, finally, a consideration of the methods used to assess the general mechanical performance of natural fibre composites.
More useful information can be obtained at the website http://www.nova-institut.de/.
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