nanoclay composites, environmentally friendly sustainable technology: A review

chemical engineering research and design 8 6 ( 2 0 0 8 ) 1083–1093

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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd

Review

Wood/polymer/nanoclay composites, environmentally friendly sustainable technology: A review Max Hetzer a,b , Daniel De Kee a,b,∗ a b

Chemical and Biomolecular Engineering Department, Tulane University, New Orleans, LA 70118, United States Tulane Institute for Macromolecular Engineering and Science (TIMES), United States

a b s t r a c t A review with 71 references of recent literature regarding wood/polymer, polymer/clay and wood/polymer/clay nanocomposites. © 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Wood/polymer composites; Polymer/clay nanocomposites; Wood/polymer/clay nanocomposites; Mechanical properties; Thermal properties

Contents 1. 2.

3.

4. 5.

1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymer/clay nanocomposites (PCNs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Preparation and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Clay modification (compatibilization) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Methods for preparation of the nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Modeling polymer/clay systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wood/polymer composites (WPCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Modification of wood fibers and matrix for enhanced compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Rheological properties of wood/polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Mechanical properties of wood/polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wood/polymer/clay nanocomposites (WPCNs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction

Thermoplastic composites play an important role in our society, as uses of such composites range from cookware to components of the space shuttle. In the early 1990s, Toyota researchers produced a new group of polymer–clay composites which were termed polymer-layered silicate

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nanocomposites or polymer nanocomposites. The common applications of these composites are in the automotive door as well as in the airplane interior paneling. Since polyolefins are generally hydrophobic and layered silicates such as natural and synthetic clays are hydrophilic, either a filler or the

∗ Corresponding author at: Chemical and Biomolecular Engineering Department, Tulane University, New Orleans, LA 70118, United States. E-mail address: [email protected] (D. De Kee). Received 23 January 2008; Accepted 15 May 2008 0263-8762/$ – see front matter © 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2008.05.003

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polyolefin must be modified to increase the compatibility of all phases. Once the clays and the polymer are compatible, the properties conferred by the nanoclays to the polymer matrix are comparable or exceed those of traditional materials (Bhattacharya et al., 2008). In recent years, researchers developed numerous methods of preparation of composites composed of olefins and inorganic fillers such as clay and calcium carbonate. Lagaly (1999) describes the nature of fillers that are currently in use as well as the difference between conventional fillers and nanofillers. Conventional fillers are macroscopic inorganic materials considered to be spherical particles, fibers, or plates. Wood fibers have the advantage of being low cost reinforcing fillers. They are also characterized by low density and high resistance to breakage (Raj et al., 1989). A disadvantage of using wood as a filler resides with the thermal instability of wood above 200 ◦ C. The majority of thermoplastics exhibit melting points between 160 ◦ C and 220 ◦ C, which is in the range of thermal decomposition of wood. There are two major challenges to overcome in using wood fibers/flour as a filler: (1) the thermal decomposition of wood fibers/flour at the processing temperature and (2) the incompatibility of the wood fibers/flour and the polyolefin matrix. The difference between conventional composites and nanocomposites lies in the size of the filler, the different preparation approaches are shown in the Fig. 1. The size of conventional fillers ranges between 10 ␮m and 1 cm; whereas, the size of a nanofiller ranges between 1 nm and 500 nm. The high aspect ratio of nanoclays allows for the lower loading of filler to achieve the desired mechanical results. Typically 3–5 wt.% for nanoclay is used as opposed to greater than 30 wt.% for a conventional filler, to achieve similar results. There are two main classifications for fiber-reinforced composites: continuous fiber and discontinuous fiber composites. The advantages of using continuous fibers is the higher strength and stiffness of the final composite when compared to those of discontinuous fiber-reinforced composites. The major disadvantage of using continuous fibers is the increased processing time and cost of these composites (Mitchell, 2004). In the early work on clay composites by Toyota research teams (Usuki et al., 1993; Yoshitsugu et al., 1993), clay was originally added as filler to a Nylon 6 matrix in order to increase the strength of the panels to be used in the interior of Toyota cars in the early 1990s. Based on their pioneering work, numerous researchers have used clay and polymer to generate composite materials (Bozveliev et al., 1991; Galgali et al., 2004; Han et

Fig. 2 – Chemical structure of the dimethyl dihydrogenated tallow quaternary ammonium ion. al., 2001; Hasegawa et al., 1998, 2000a, 2003, 2000b; Hasegawa and Usuki, 2002, 2004; Hotta and Paul, 2004; Hyun et al., 2001; Jeon et al., 1998). It is understood that the high aspect ratio of the nanoclays allow for a greater interfacial area, enhancing reinforcement properties. Silicates such as montmorillonite, hectorite, and saponite have a layered structure that upon exfoliation leads to composites with very high stiffness and strength (Park et al., 2002).

2.

Polymer/clay nanocomposites (PCNs)

2.1.

Preparation and properties

2.1.1.

Clay modification (compatibilization)

Hydrophilic clays and hydrophobic polymers are not compatible in their virgin states and a modification of either the polymer or clay is necessary for dispersion, intercalation and/or exfoliation of clay tactoids in the polymer matrix. Surface modification of clay has commonly been used to achieve a greater compatibility of the clay and polymer. Ion exchange of Na+ or Ca2+ gallery cations in the mineral by alkylammonium ions is frequently chosen to modify the clay (Park et al., 2002). The structure of the dimethyl dihydrogenated tallow quaternary alkylammonium ion is shown in Fig. 2 (Lertwimolnun and Vergnes, 2005). Park et al. (2002) organically modified montmorillonite with dodecylammonium chloride to prepare polymer/clay nanocomposites (PCNs). This surface modification of the clay surface leads to an increase of approximately 75% (from ´˚ to 18.21 A) ´˚ of the distance between silicate interlayers. 10.33 A An improvement in the thermal stability of the nanocomposites is also achieved. The introduction of small molecular weight alkyl chain ions, changes the organophobic nature of the nanoclay into an organophilic one. The octadecyl ammonium cations are about 2.4 nm in length, fully extended, and replace the exchangeable cations in the galleries. The exchange rate of the cations by organic modifiers is 95–98%, and taking the surface area and the cation exchange capacity of the clays into account, this results in about one organic modifier for every square nanometer (Marchant, 2003). Jeon et al. (1998) modified silicate fillers with the dodecylamine ions. They observed that the clay gallery spaces were successfully intercalated with dodecylamine. The addition of dodecylamine to the silicate layers enhanced clay swelling in the HDPE matrix (Fig. 3).

2.1.2.

Fig. 1 – Different approach to factication of micro- and nano-com-composites (Gao, 2004).

Methods for preparation of the nanocomposites

There are several effective methods for the preparation of polymer/clay nanocomposites: (1) melt intercalation; (2) insitu polymerization; and (3) solution dispersion (Bhattacharya et al., 2008). The melt intercalation method is the most favored by the industry due to the continuous nature of the process and economic factors. The in-situ polymerization and the solution dispersion methods are well described elsewhere (Bhattacharya et al., 2008; Utracki, 2004a,b).

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Fig. 3 – Structure of 2:1 phyllosilicates (nanoclays) (Bhattacharya et al., 2008). Hasegawa et al. (2003) exploited the fact that the clay exfoliates in water to form clay dispersions to prepare Nylon 6/clay nanocomposites. The relative viscosity of the nanocomposites (2.78) was not too different from the relative viscosity of the Nylon 6 (2.61). The tensile modulus increased by 35% when compared to neat Nylon 6 and the increase in the strength of the composite was attributed to a complete dispersion (exfoliation) of clay in the polymer (see Fig. 4). The Nylon 6 matrix as well as the clay is hydrophilic, allowing for easy exfoliation of clay in the polymer matrix. When the clay was introduced to a hydrophobic matrix such as polyethylene (PE), polypropylene (PP), or polystyrene (PS) a compatibilizing agent such as maleic anhydride was necessary to induce polymer intercalation into the gallery spacings of the clay platelets. The alignment of clay particles in the polymer matrix resulted in more pronounced shear thinning (Kim et al., 2003). A linear viscoelastic region was observed for strain amplitudes below 5 wt.% clay, while for a clay loading exceeding 10 wt.%, a linear viscoelastic response was observed over a smaller strain amplitude (less than 2% strain). It was found that the regions of linear viscoelastic behavior (5 wt.%) for the uncompatibilized PP and PP/clay are very wide and insensitive to the presence of

Fig. 4 – Different types of clay morphology in a polymer/clay nanocomposites: (a) phase separated microcomposite; (b) intercalated nanocomposite; (c) exfoliated nanocomposite (Bhattacharya et al., 2008).

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organically modified montmorillonite (OMMT). The linear viscoelastic region of these materials extended to a strain of 20%. At low frequency, PP/OMMT melts exhibit higher storage (G ) and loss moduli (G ) as well as a higher complex viscosity (* ) than neat PP. There was a monotonic increase with increasing OMMT content. Clay exfoliation can be detected using X-ray diffraction (XRD), transmission electron microscopy (TEM) and to a certain degree, scanning electron microscopy (SEM). Fig. 5 illustrates two examples of the clay morphology in the polymer/clay nanocomposites, that of intercalated (Fig. 5a) and exfoliated morphology (Fig. 5b). Wang et al. (2001a) prepared maleated polyethylene clay nanocomposites using a Brabender type mixer. Full exfoliation of the clay was shown via XRD; the peak at 2 = 3.6◦ that is characteristic of nanoclay, was not observed in the maleic anhydride grafted polyethylene (MAPE)/clay nanocomposites. TEM scans confirmed the exfoliation of clay in the MAPE/clay nanocomposites. Galgali et al. (2004) attempted to quantify the relationship between the flow induced orientation and the tensile modulus of extruded PP/clay nanocomposites. A tape was extruded under controlled conditions of shear history and temperature. The clay orientation in the melt was quantified using 2D XRD studies. It was found that the orientation of clay in the compatibilized polymer matrix increased with increasing shear rate, while the orientation of clay in an uncompatibilized matrix was not influenced by shear rate. The driving force for the hydrogen bonding between the maleic anhydride (MA) groups (Kawasumi et al., 1997) and the oxygen groups on the silicates, provides a possible dispersion mechanism for polymer intercalation. Hasegawa and Usuki (2004) observed that spontaneous clay exfoliation occurs in MA modified PP (MAPP), however, no significant clay dispersion was observed until a twin-screw extruder was used. Added shear was found to disperse the silicates in the MAPP matrix. Only maleated olefins were found to intercalate in the galleries of the silicates at the first stage of the mixing process. If the miscibility of maleated olefins is sufficient to disperse clay at the molecular level, exfoliation will take place. Other authors (Hasegawa et al., 1998; Hasegawa et al., 2000a; Hotta and Paul, 2004) have shown that originally immiscible polymers intercalate clay galleries in the presence of MA. The use of twin-screw extruders for the preparation of PE/clay nanocomposites is a common procedure. Kato et al. (2003) used a twin-screw extruder with a residence time of 12 min to prepare PE/clay nanocomposites. Complete exfoliation of clay was observed in the maleated samples. It was confirmed via XRD studies that showed the interlayer distance of the ˚ No exfoliation was observed silicate layers exceeded 100 A. for non-maleated PE/clay composites. The tensile strength of the maleated PE/clay composites increased with clay loading from 102 MPa for neat PE to 157 MPa for PE/clay composites with 3 wt.% clay. The PE/clay composites also showed superior barrier properties; the permeability of PE/clay composites decreased over 30% with a clay loading of less than 5 wt.%. The crystallization rate of the polymer matrix has been shown to have an inverse effect on the intercalation and exfoliation of the polymer chains into the clay gallery spacing (Han et al., 2001). The interlayer spacing of the clay gallery increases with the crystallization temperature (Tc ). A higher interlayer spacing occurs in a high-temperature region (Tc > 70 ◦ C) for observed clay contents. At high Tc , the crystallization rate is sufficiently slow and the polymer chains have sufficient time to intercalate inside the silicate galleries.

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Fig. 5 – TEM images of polyethylene/clay nanocomposites: (a) intercalated structure; (b) exfoliated structure. Note: the scale bar is 20 nm. With an increase of crystallization temperature and a decrease of clay content in PP/clay nanocomposites, more and more chains find themselves within the silicate galleries, giving rise to higher intercalated species. The extent of intercalation strongly depends on the time the clay is exposed to the molten matrix. The interaction of polymer chains and filler strongly affects the rheological behavior of the composites. It was found that the rheological behavior of PS-co-MA/M6A nanocomposites at low frequency is very different from that of the PS/clay nanocomposites (Lim and Park, 2001). An enhancement of the storage modulus was found to be very large at an angular frequency of 10−1 rad/s (G = 89 Pa, G = 900 Pa for unmodified PS/clay and G = 90 kPa, G = 30 kPa for maleated PS/clay composites). At a higher angular frequency (ω = 1 rad/s) the moduli (G and G ) of maleated and unmaleated PS/clay composites converge. It is believed that the clay–clay interactions as well as a large interfacial area between polymer chains and clay are responsible for the rheological behavior of MAPE/M6A. The maleated PE/clay nanocomposites showed substantial enhancement of the storage modulus, possibly due to the exfoliated morphology of the nanocomposites. Exfoliated structures show not only a plateau-like behavior at low frequency, but also enhanced moduli at high frequency. For PS/organically modified montmorillonite (PS/OMMT) nanocomposites at low shear rates (), ˙ the viscosity () increases substantially and monotonically with OMMT content at a given ˙ (Kim et al., 2003). At high shear rates, the viscosity and degree of shear thinning for the PS/OMMT are comparable with those of the unfilled polymer as a result of the silicate layers or even anisotropic tactoids being parallel to the flow direction. There is a higher degree of shear thinning as well as an increase in relaxation time with increasing clay content. The increase in the degree of shear thinning is attributed to the alignment of clay layer structures with shear. A shear induced clay platelet orientation was observed for the PS/OMMT nanocomposites. Linear viscoelasticity was observed for strain amplitudes below 5% for PS composites with low clay content (<5 wt.%), while for the higher clay content (>10 wt.%) composites, the onset of the nonlinear viscoelastic region was observed at strains as small as 1%. Kim et al. (2003) observed a linear vis-

coelastic region up to 0.3% strain. The G and G values of the nanocomposites show a monotonic increase at all frequencies with clay loading. The higher the G and the smaller the slope, the more pronounced the interaction between the silicate sheets and their tendency to form a three dimensional superstructure. Interestingly, Gu et al. (2004) report regions of linear viscoelastic behavior for PP and PP/clay nanocomposites up to 20% strain. They also observed a gradual decrease in the power-law dependence of G and G with increasing clay content. Kawasumi et al. (1997) suggested a possible dispersion mechanism for polymer intercalation into the clay gallery spacings. Only maleated olefins were found to intercalate into the silicate layers at the first stage of the mixing process. The driving force of the intercalation is associated with the strong hydrogen bonding between MA and the oxygen groups on the silicates. If the miscibility of maleated olefins is such as to disperse clay at the molecular level, exfoliation will take place. The intercalation of PP into the clay gallery spacings was observed when PP/clay nanocomposites were prepared using a twin-screw extruder (Hasegawa et al., 1998). Complete exfoliation did not occur. It was found that the presence of maleic anhydride improved the dispersion of clay in the polypropylene matrix. A slightly different approach to preparation of PP/clay nanocomposites was suggested by Hasegawa et al. (2000a). The clay was mixed with maleated PP in a twinscrew extruder and then mixed with neat PP. This two-step process seemed to be successful at producing an exfoliated PP/clay nanocomposite. Maleic anhydride was found to be an effective compatibilizer for linear low density polyolefin/clay composites (Hotta and Paul, 2004). The dispersion process of clay in PP was also investigated by Hasegawa and Usuki (2004). Maleic anhydride was used as a compatibilizer and two processes were investigated: a no shear dispersion and a shear induced dispersion. It was found that polypropylene intercalates the clay gallery spaces spontaneously (without shear). However, the silicate layers did not disperse in the PP matrix until shear was induced. (Hasegawa et al., 2000b; Kato et al., 2003) observed that neat polyethylene does not exfoliate clay under any conditions, however, maleated polyethylene (MAPE) exfoliates clay readily. The tensile strength of the hybrids was higher than that

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Table 1 – Some properties of PE/clay hybrids (from Kato et al. (2003)) Sample

PE PCN1 (clay 1 wt.%) PCN2 (clay 2 wt.%) PCN3 (clay 3 wt.%)

Gas permeability coefficient × 1013 (cm (STP) s−1 Pa−1 )

Tensile modulus (GPa)

5.26 3.78 3.91 3.48

1.02 1.80 1.40 1.57

of the neat polymer and the strength and the modulus of the hybrids increased with clay content [see Table 1]. The PE/clay nanocomposites showed superior gas barrier properties; with the permeability coefficients of the composites decreasing with increasing clay content [see Table 1]. The rheological properties are highly affected by the introduction of nanofiller to the polymer matrix. Wang et al. (2001b) reported that the addition of 1 wt.% Cloistite 20A to a MAPE matrix resulted in a storage modulus increase of two decades at low frequency. The complex viscosity of the nanocomposites increased by an order of magnitude compared to the MAPE matrix complex viscosity. In another study by Wang et al. (2002), the nanoclay was found to be completely exfoliated in MAPE. They observed the nanoclay to be well-dispersed and oriented within the MAPE matrix in the flow direction. The MAPE/Cloisite 20A nanocomposites showed an improved tensile modulus (12 MPa) over the pure MAPE matrix polymer (9 MPa). The addition of MA to the polyethylene matrix enhanced the adhesion between the matrix and the clay. Since there were fewer cavities present in the nanocomposites, the tensile strength of the material was also enhanced. It is clear that the addition of maleic anhydride to the polymer matrix increases its hydrophilicity (Hotta and Paul, 2004; Wang et al., 2001a). Increased hydrophilicity of the polymer combined with the increased hydrophobicity of the organically modified clay leads to a well-dispersed/exfoliated clay structure in the nanocomposite. Exfoliation of clay tactoids within the polymer matrix leads to a dramatic enhancement of the mechanical properties with only a small (<5 wt.%) amount of the filler (Galgali et al., 2004; Han et al., 2001; Hasegawa et al., 2000a,b; Hasegawa and Usuki, 2004; Hotta and Paul, 2004; Kim et al., 2003; Wang et al., 2001a; Kawasumi et al., 1997; Kato et al., 2003; Wang et al., 2002; Hristov and Vasileva, 2004; LeBaron et al., 1999; Lee et al., 1999; Maiti et al., 2002; Tanoue et al., 2004; Tseng et al., 2001; Utracki and Kamal, 2002; Wang et al., 2001c).

2.2.

Modeling polymer/clay systems

Numerous work has been done on modeling composite materials using analytical (Yoon et al., 2002; Luo and Daniel, 2003; Fornes and Paul, 2003) and finite element methods (Zhang and Wang, 2005; Berger et al., 2005; Zhu and Narh, 2004; Sheng et al., 2004). Researchers were interested mainly in the mechanical response of the polymer/inorganic filler composites to applied stresses. Fornes and Paul (2003) examined the reinforcement of Nylon 6 by layered aluminosilicated (LAS) and glass fibers via the composite theories of Halpin-Tsai and Mori-Tanaka. The large reinforcing effect of exfoliated clay in the polymer matrix was predicted by both models and verified experimentally. The two models differ in the way they treat the filler: the Halpin-Tsai model treats fibers as fibers and disks as

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rectangular platelets, whereas the Mori-Tanaka model treats fibers and disks both as ellipsoidal particles. The theories of Halpin-Tsai and Mori-Tanaka capture the experimentally observed nanocomposite stiffness behavior. Both theories demonstrate that even higher levels of reinforcement could be achieved by higher levels of dispersion, larger platelet diameters and improved platelet orientation. Dispersing montmorillonite (MMT) within a nylon matrix results in a considerable increase in stiffness at all temperatures (Fornes and Paul, 2003). The reinforcement effect is greatest in the region above the glass transition temperature, Tg , of the matrix. This reinforcement is mainly due to the large difference in the mechanical properties of the filler and the matrix as the composite moves from the glassy to the rubbery state. Addition of clays increases the dampening effect at temperatures between Tg and Tm . Adding small amounts of filler increases the heat deflection temperature (HDT), defined as the temperature at which the composite under a constant stress of 1.82 MPa deflects by 0.2% strain, well above that of pure nylon, which occurs near the Tg . However, beyond 2 vol.% the rate of increase in HDT begins to diminish, which is expected since the HDT is approaching the melting point of the polymer. Since Nylon 6 is a semi-crystalline polymer, some level of stiffness is maintained beyond Tg and up to Tm . The Halpin-Tsai equations were used to predict storage moduli-temperature data for Nylon 6/clay nanocomposites (Fornes and Paul, 2003). It was assumed that the stiffness properties of MMT were completely elastic and constant over the temperature range tested. Thus, a direct substitution of a complex (dynamic) modulus for Young’s modulus in the equations was possible through elimination of the imaginary component. The resulting equation given below is the real component of the nanocomposite complex storage modulus: E =

1 [ag(2Em − 2Em ) − 2abEm Em + bgEm − bhEm ] f

(1)

where a, b, f, g, and h are functions of filler aspect ratio, volume fraction, Em , and/or Em . The model captures the shape of the E vs ω curve, but overpredicts the data since it assumes unidirectional alignment of the filler, whereas in general this is not the case. A large component of the overestimation arises from the disparity of the E (T) curves for nanocomposites versus pure Nylon 6 at temperatures beyond 80 ◦ C. The overestimation is more pronounced with filler concentration and is very sensitive to small changes in log (E ). Hetzer (2007) developed a modified Halpin-Tsai equation that predicts the Young’s moduli dependence of filled composites on the temperature. The modified Halpin-Tsai equation is expressed as follows: Eii () 1 + i ()Vf = (1 − ) E0 1 − i ()Vf

(2)

where  = 2(l/d) for i = 1;  = 2 for i = 2. This model was used to analyze the moduli obtained for the PCNs with respect to temperature. The aspect ratio of the nanoclay platelet decreases as a function of the stack number (N), i.e. the number of platelets per stack. This dependence is given by (Bharadwaj, 2001): ı(N) =

ı0 N + (N − 1)3.7

(3)

where ı(N) is the stack dependent aspect ratio, and ı0 is the aspect ratio of a clay platelet. The model predictions (see Fig. 6)

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Fig. 6 – E11 –T profiles of polymer/clay (Cloisite 20A) nanocomposites, high Mw compatibilizer, lines represent model predictions. (a) 1 wt.% nanoclay; (b) 3 wt.% nanoclay; and (c) 5 wt.% nanoclay (Hetzer, 2007). indicate that as the stack number increases, the mechanical properties of the composites decrease dramatically. It was also shown that the compatibilizer type plays a substantial role in controlling the stack number. The increase of high Mw content in the compatibilizer blend decreased the average stack number. The model predicts that upon addition of as little as 5 wt.% nanoclay to the polymer matrix, the modulus of the polymer matrix would increase by 350% assuming total exfoliation of the nanoclay within the polymer matrix. Since total exfoliation was not achieved, the experimental results (∼150%) fell short of the model prediction of 350%. The aspect ratio of the nanofiller has a significant effect on the bulk properties of the PCNs.

3.

Wood/polymer composites (WPCs)

3.1. Modification of wood fibers and matrix for enhanced compatibility Wood/polymer composites (WPCs) represent another example of a structured multiphase material. The WPC market is experiencing a substantial growth, averaging over 20% per year since 1998. Decking is the largest market for WPCs, and it is probable that the demand will increase dramatically due to EPA regulations banning current arsenic-containing treatments of construction materials. The modification of polyethylene (PE) with wood should provide an increased stiffness of the polymer and the PE should provide protection for the wood fibers from moisture absorption. However, wood and PE are naturally incompatible and a compatibilizer is needed to improve the ultimate performance of the WPCs. Maleic anhydride grafted polyethylenes (MAPEs) can be used as compatibilizers for wood/polyethylene composites. Sangalov et al. (2001) modified dispersed wood flour by oligoethoxysiloxine. It was found that after modification the wood exhibited hydrophobicity, resistance to biodegradation and flame. The compatibility of wood flour with the polymer matrix was also enhanced. The treatment of wood with different silane coupling agents as well as with

polymethylene-polyphenyl isocyanate (Kokta et al., 1989a) resulted in improved adhesion between the fiber and the matrix. It was shown that the stress at yield of the composites with silane-treated wood fiber increased from 10.9 MPa to 17.3 MPa at 30.0 wt.% fiber content. A tensile modulus increase of 800 MPa was observed in composites containing 30.0 wt.% of fiber modified with a silane compatibilizer. An increase in Young’s modulus of 56 MPa/wt.% with the use of 5 wt.% MA compatibilizer was also reported (Rodriquez et al., 2003). The tensile properties of the composites increased with increasing concentration of silane compatibilizer. The elongation at yield remained about constant (7 ± 0.8%) but the fracture energy showed an increase to a maximum of 1.6 × 10−4 kJ at 10 wt.% and then decreased with increasing fiber content. It was also observed that the method of preparation and composition of wood fiber can affect the ultimate properties of the composites. Pickering and Abdalla (2003) attempted to optimize the silane uptake using a controlled hydrolysis process in order to optimize the composite strength. Two silanes, gammaaminopropyl-triethoxysilane (GS) and dichlorodiethylsilane (DSC) were used in this study. The hydrolysis of silane to silanol occurs with the water bound in the wood fibers as follows: (CH3 CH2 )2 SiCl2 + 2H2 O → (CH3 CH2 )2 Si(OH)2 + 2HCl Following hydrolysis, the molecules of silanol can react with hydroxyl groups on the fiber surface forming ether bonds and producing ethyl groups at the wood fiber surface increasing the hydrophobicity of the fiber and increasing its compatibility with polyethylene. For GS, the amine groups become protonated due to the presence of acetic acid in the carrier. A reaction between the silicon (of the compatibilizer) and the hydroxyl group (of the fiber) will occur, increasing the compatibility of wood fibers and polyethylene. A surface treatment using silane coupling agents was found to improve the strength of composites containing 5 wt.% fibers. This finding suggests that silane is assisting in the interfacial bonding between the fibers and the polymeric matrix. The surface

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treatment resulted in little benefit in strength at 10 wt.% fiber content, and at higher fiber contents a reduction in mechanical strength was observed. Takatani et al. (2000) explored the effect of lignocellulosic materials on the flour (mesh 20 and 120) as well as newspaper flour and steam-exploded beech flour. The composites were prepared via a solution method followed by compression molding. They determined that the modulus of rupture, the density, and water resistance of the composites increased with wood loading up to 70 wt.%. The strengthening effect was also more pronounced for those composites with a higher wood content. This finding was in agreement with the literature (Hristov and Vasileva, 2004; Pickering and Abdalla, 2003; Coutinho et al., 1997; Balasuriya et al., 2001; Kokta et al., 1989b; Maldas and Kokta, 1991; Murphy et al., 2003; Myers et al., 1991; Simonsen et al., 1998). The compatibilizer also reduced the number of connected particles and allowed for improved mobility of the clusters/particles. In the case of non-compatibilized mixtures of 50 wt.% wood flour (WF), a large number of untreated wood flour particles or clusters may interact frequently and become mechanically entangled due to their irregular shapes and surfaces and/or physically linked through surface H-bonding. The relative movement of one particle/cluster with respect to other particles/clusters is hindered as a result of these links (Marcovich et al., 2004).

3.2. Rheological properties of wood/polymer composites The rheological properties of suspensions are greatly affected by the concentration of the particles in the mixture. Many equations have been proposed to describe the effect of filler concentration on the viscosity of suspensions. One of the equations describing the dependence of the zero-shear viscosity 0 , on the volume fraction of the particles  is given below (Marcovich et al., 2004):



0 = 0s 1 −

 m

−b (4)

where 0s is the zero-shear rate viscosity of the solvent,  is the volume fraction of the particles, m is the volume fraction at maximum packing and b is an exponent, equal to 2 for spherical particles. The Carreau-Yasuda model (Marcovich et al., 2004; Carreau et al., 1997) given by Eq. (5) is the popular model to describe non-Newtonian flows.  − ∞ a (n−1)/a = [1 + () ˙ ] 0 − ∞

(5)

where  is the shear viscosity, 0 is the zero-shear rate viscosity, ∞ is the limiting viscosity at large shear rates, ˙ is the shear rate,  is a relaxation time, n is the pseudoplastic parameter (power-law), and a is a parameter that is related to the curvature in the transition zone between 0 and the power-law region. Generally, a good fit can be found by setting a = 2 and ∞ = 0, which results in the Carreau model. Finally for highly concentrated materials a simple power for the complex viscosity law expression can represent observed behavior (Marcovich et al., 2004): n−1

∗ = h(ω)

(6)

where * is the complex viscosity, h is a constant, ω is the frequency (rad/s) and n is a power-law index.

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It was found that the linear viscoelastic (LVE) region is very small for PP/WF untreated composites. Strains smaller than 0.5% are necessary for the frequency scan tests to remain in the linear region. No LVE behavior was observed for 50 wt.% HDPE-WF composites at strains as low as 0.1% (Xiao and Tzoganakis, 2003). A large drop of the modulus with deformation has been reported by Marcovich et al. (2004). This drop has been attributed to the disruption of the initial structure of the filler in a PP melt. As the filler concentration increased, the value of the storage modulus in the linear viscoelastic region was found to increase by at least two orders of magnitude. At 50 wt.% wood loading no Newtonian region was observed. The pseudoplastic exponent n in the Carreu model decreased with increasing filler concentration. The reason for the shear thinning at low strains is related to the viscoelastic behavior of the matrix and polymer links between particles and/or clusters (Marcovich et al., 2004). The creep response of the 50 wt.% HDPE-WF composite materials showed a sudden decrease at 50 wt.% wood content. These composites behaved as elastic solids. Composites at lesses wood loading levels behaved as viscoelastic materials. The Cox-Merz relation could not be observed in the case of HDPE-WF materials. The melt elasticity was found to decrease with increasing wood content, and shear thinning was observed for all composites (Xiao and Tzoganakis, 2003). The rheological behavior of wood fiber/polyethylene composites made of corona treated fibers was investigated by Dong et al. (1992). They reported that composites filled with treated fibers exhibited decreased melt viscosity relative to the composites filled with untreated fibers. The flow behavior of the composites was non-Newtonian, exhibiting shear thinning. The onset of shear-thinning behavior occurred at lower values of ˙ with increasing wood fiber content and the degree of shear thinning increased with fiber concentration. Fiber orientation in the direction of flow is thought to cause the shear-thinning effect. The melt viscosity of the composites was observed to increase with fiber volume fraction (Dong et al., 1992; Maiti and Hassan, 1989). Perturbation of laminar flow of the polymer matrix by the stiffer particles (wood fibers) and hindering of the chain mobility in the polymer matrix is also thought to be related to the increased melt viscosity of the composites.

3.3. Mechanical properties of wood/polymer composites The toughness of a material can be improved by increasing its ability to deform plastically by initiation of a very large number of microscopic yield events (Hristov and Vasileva, 2004). This involves initiation of a large number of microcracks by inorganic particles or short fibers, initiation of small plastic zones induced by stress concentration at particles, and cavitation at the surface of (or inside) particles, with subsequent polymer stretching between the resulting microvoids. Small, well-dispersed and weakly adhering particles initiate a large number of small and stable voids, which permit further deformation of the polymer matrix between the particles. Maleated polyolefins were found to be the most suitable materials for improving the interfacial adhesion between wood fibers and a polymer matrix (Hristov and Vasileva, 2004; Rodriquez et al., 2003; Balasuriya et al., 2001; Kokta et al., 1989b; Maldas and Kokta, 1991; Oksman and Clemons, 1998). Hristov and Vasileva (2004) investigated the mechanical properties and deformation mechanisms of polypropylene/wood fiber composites modified with styrene–butadiene

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Table 2 – Clay interlayer diffraction peak and corresponding d-spacing of clay/HDPE and wood/clay/HDPE composites (Zhong et al., 2007) HDPE/MAPE/clay

2 (◦ )

˚ d (A)

Wood/HDPE/MAPE/clay

2 (◦ )

˚ Gallery spacing d (A)

88/10/2

1st extrusion pass 2nd extrusion pass

2.979 3.105

29.63 28.43

50/44/5/1

2.539

34.76

84/10/6

1st extrusion pass 2nd extrusion pass

3.002 2.995

29.40 29.48

50/42/5/3

2.696

32.74

80/10/10

1st extrusion pass 2nd extrusion pass

3.168 3.215

27.87 27.46

50/40/5/5

2.822

31.28

rubber and maleated polypropylene as impact modifier and compatibilizer, respectively. An enhanced yield stress of 23.5 MPa and a Young’s modulus (E11 ) of 1220 MPa as well as impact strength of 14.2 kJ/m2 were observed for a 10 wt.% wood content composite with 10 wt.% maleic anhydride grafted polypropylene (MAPP) loading. Generally, fillers with higher stiffness than the matrix can increase the modulus but cause a dramatic decrease in elongation at break. Data obtained by Hristov and Vasileva (2004) support this statement. Maleated polypropylene leads to a brittle-to-ductile transition characterized by intensive stress whitening during the deformation process. The deformation mechanism in unmodified PP/wood fiber composites is characterized by fiber pullout, debonding and cavitation of the matrix, resulting in brittle fracture. MAPP enhances the interfacial adhesion and assists the stretching of polymer fibrils from the wood fiber covering surface layer. Local plasticity is most likely caused by the debonding/cavitation in that layer (Hristov and Vasileva, 2004). Almost all of the elongation occurred in the matrix of the composite if the filler was rigid. If there is good adhesion between the filler and the matrix, a decrease of the elongation at break, even with small amounts of filler can be expected (Oksman and Clemons, 1998). If the adhesion is poor, the elongation at break may decrease more gradually. Oksman and Clemons (1998) reported that the PP/WF composite elongation at break was dramatically decreased compared to that of unfilled PP. The addition of MAPP further decreased the elongation at break. The Young’s modulus of the WF filled PP was 2.4 GPa which is a 70% increase, compared to the modulus of unfilled PP. A lack of voids around the filler particles in the composites with MA modified PP is indicative of improved adhesion between the wood particles and the matrix. Oksman and Clemons (1998) suggested that MAPP may also act as a dispersing agent between polar fillers and a non-polar matrix, resulting in an improved filler dispersion. Balasuriya et al. (2001) used a high shear twin-screw extruder to mix up to 70 wt.% of wood flakes with HDPE to produce wood polymer composites (WPC). High tensile and flexural strengths of twin-screw compounded medium melt flow index (MMFI) composites containing up to 50 wt.% wood flakes, indicated that the high shear melt mixing is a better processing method than mechanical blending. The high shear melt mixing resulted in improved flake wetting, and consequently improved mechanical properties of the composites. The flexural strength of the composites was found to be controlled by flake wetting and flake distribution in the matrix. The tensile strength was found to be more sensitive to the matrix properties. Rodriquez et al. (2003) observed a slight decrease in Young’s modulus, tensile strength and impact resistance with increased processing temperature of wood fiber-reinforced

composites. The best mechanical properties were found at a processing temperature of 150 ◦ C for PP. This temperature, however, is not appropriate for this polymer system because of the high torque measured on the screw of the extruder and a longer residence time of the composite in the mixer, which renders the process inefficient and uneconomical. It was found that the organic material acts as an efficient reinforcing agent, increasing the modulus of the compatibilized composite at a rate of 20 MPa/wt.% compared to that of the non-compatibilized composite.

4. Wood/polymer/clay nanocomposites (WPCNs) Zhong et al. (2007) incorporated nanoclay (Cloisite® 20A) into wood/polyethylene composites in the presence of maleic anhydride grafted polyethylenes (MAPEs). The observed that the addition of less than 5 wt.% nanoclay to the wood/polyethylene matrix resulted in an increased d-spacing of the nanoclay [see Table 2]. The viscosity of the nanoclay containing WPC was not significantly affected by the incorporation of the nanoclay. The coefficient of thermal expansion (CTE) decreased by 60% to 2.5 × 10−5 mm/mm ◦ C−1 for a 3 wt.% nanoclay loading and the heat deflection temperature (HDT) was increased by 10–93 ◦ C. The flexural strength decreased by 24% and the flexural modulus increased by 10% with the 3 wt.% nanoclay loading. This fact was attributed to the insufficient MAPE loading in the final composites. The mechanical properties increased by 30% when at least 7 wt.% MAPE was used in the final WPCNs. The addition of nanoclay to the WPC systems increased their thermal degradation onset temperature indicative of an enhanced thermal stability (Hetzer, 2007). The addition of the nanoclay to a WPC system increased Young’s modulus (E11 ) from 2.3 GPa to 3.8 GPa. Yeh et al. (2005) modifed the polypropylene matrix of the WPC with layered silicates. The MA grafted PP was used as a compatibilizer in this study. They determined that the addition of layered silicates increased the Young’s modulus of WPC from 4.2 GPa to 4.58 GPa with a 10 wt.% clay loading. They also observed that the addition of the layered silicates to the wood/polypropylene matrix resulted in poor adhesion between wood fibers and the polymer matrix. The authors speculated that addition of more compatibilizer may increase the adhesion between wood fibers and the polymer matrix. The modulus of WPC reinforced with layered silicates followed a modified mixing rule (Yeh et al., 2005): Ec = Ef 1 0 Vf + Em (1 − Vf )

(7)

where Ec , Ef , and Em are the moduli of the composite, fiber and matrix, respectively, Vf is the fiber volume frac-

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tion,  1 and  0 are length and orientation correction factors, respectively. Kumar and Singh (2007) mixed ethylene–propylene (EP), Cloisite 20A© , 5 wt.% maleic anhydride grafted EP (MEP) and 5–15 wt.% cellulose fibers, to form an intercalated composite (TC5–15). The clay gallery spacing (d) increased by an aver˚ in EP/MEP/clay composite. The addition of cellulose age of 6 A to EP/MEP/clay composite did not further significantly affect the gallery spacing of the nanoclay. The modulus of elasticity was observed to increase upon addition of nanoclay and cellulose fillers to a neat EP matrix (EP = 773 MPa, TC15 = 1622 MPa). The yield stress at break was not improved as significantly for ternary composites ( y TC = 20 MPa) as for cellulose only containing composites ( y cc = 31 MPa). Elongation at break was approximately the same for both types of composites at the maximum filler loading (15 wt.%). The cellulose only composites exhibited higher stress at break (∼28 MPa) at maximum filler loading than the ternary composites (∼18 MPa). The melting point of EP increased with the addition of nanoclay and cellulose. The maximum melting point for the composites was observed at maximum filler loading to be 170 ◦ C. The decomposition temperature increased to a maximum of 483.3 ◦ C at 10 wt.% cellulose loading, and the addition of cellulose to the nanocomposites increased the thermal degradation temperature by an average of 20 ◦ C from 460 ◦ C to 480 ◦ C. Also, the addition of the nanoclay to the cellulose containing composites resulted in a 15% decrease of water absorption by the composites.

5.

Conclusions

Polymer/clay nanocomposites and wood/polymer composites have been extensively studied. The major advantage of using a nanoclay filler in a polymer matrix is the dramatic increase in the mechanical properties with inclusion of only a small amount of nanofiller (<10 wt.%). When compared with a traditional filler, the addition of the nanoclay does not change the viscosity or the density of the system by all that much. The addition of wood filler to a polymer matrix is environmentally friendly. Wood flour from a sawmill process can be used as filler and the polymer matrix may be obtained from recycled polyethylene or polypropylene, thus reducing the amount of polymer that is being sent to landfills. Another positive aspect of using wood/polymer composites is the possible elimination of pressure treated wood for outdoor construction projects such as decking. The EPA (CCA, 2007) states that chromate copper arsenic that is used as a wood preservative and pesticide in the pressure treatment process can leach out of the wood to contaminate the water table. The use of wood/polymer composites will eliminate this problem. Wood/polymer composites are however not as strong as native lumber; they have a large coefficient of thermal expansion and require more support due to lower flexure strength. Addition of small amounts of nanoclay filler has been shown to decrease the coefficient of thermal expansion and increase the flexure modulus of wood/polymer composites. The processing conditions were not different from that of WPCs. The wood/polymer composites market has been growing an average of 20% per year for the past 8 years. It is possible that the innovations in this technology will lead to near total substitution of pressure treated lumber by wood/polymer composites. Other areas that stand to benefit from this technology include industries dealing with sporting goods, automotive, aerospace, etc. activities.

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References Balasuriya, P.W., Ye, L. and Mai, Y., 2001, Mechanical properties of wood flake-polyethylene composites. Part I: Effects of processing methods and matrix melt flow behavior. Composites: Part A, 32: 619–629. Berger, H., Kari, S., Gabbert, U., Rodriquez-Ramos, R., Guinovart, R., Otero, J.A. and Bravo-Castillero, J., 2005, An analytical and numerical approach for calculating effective material coefficients of piezoelectric fiber composites. Int J Solids Struct, 2005: 5629–5714. Bharadwaj, R.K., 2001, Modeling the barrier properties of polymer-layered silicate nanocomposites. Macromolecules, 34(26): 9189–9192. Bhattacharya, S.N., Kamal, M. and Gupta, R., (2008). Polymeric Nanocomposites: Theory and Practice. (Carl Hanser Verlag, Munich). Bozveliev, L.G., Kosfield, R. and Uhlenbroich, T., 1991, The relationship between the physico-mechanical properties and the filler concentration in the HD polyethylene-dispersed filler system. J Appl Polym Sci, 43: 1171–1180. Carreau, P.J., De Kee, D.C.R. and Chhabra, R.P., (1997). Rheology of Polymeric Systems: Principles and Applications. (Hanser, New York). Chromated Copper Arsenate (CCA). [Electronic] 2007 August, 2007 [cited 2007 November 16]; Available from: http://www.epa.gov/ oppad001/reregistration/cca/index.htm#content. Coutinho, F., Costa, T. and Carvalho, D., 1997, Polypropylene-wood fiber composites: effect of treatment and mixing conditions on mechanical properties. J Appl Polym Sci, 65: 1227–1235. Dong, S., Sapieha, S. and Schreiber, H.P., 1992, Rheological properties of corona modified cellulose/polyethylene composites. Polym Eng Sci, 32(22): 1734–1739. Fornes, T.D. and Paul, D.R., 2003, Modeling properties of Nylon 6/clay nanocomposites using composite theories. Polymer, 44: 4993–5013. Galgali, G., Agarwal, S. and Lele, A.K., 2004, Effect of clay orientation on the tensile modulus of polypropylene-nanoclay composites. Polymer, 45: 6059–6069. Gao, F., November, 2004, Clay/polymer composites: the story. Mater. Today, Gu, S., Ren, J. and Wang, Q., 2004, Rheology of poly(propylene)/clay nanocomposites. J Appl Polym Sci, 91: 2427–2434. Han, Y., Wang, Z., Li, X., Fu, J. and Cheng, Z., 2001, Polymer-layered silicate nanocomposites: synthesis, characterization, properties and applications. Curr Trends Polym Sci, 6: 1–16. Hasegawa, N. and Usuki, A., 2002, Influence of crystallization on intercalation, morphology, and mechanical properties of polypropylene/clay nanocomposites. Macromolecules, 35: 2042–2049. Hasegawa, N. and Usuki, A., 2004, Silicate layer exfoliation in polyolefin/clay nanocomposites based on maleic anhydride modified polyolefins and organophilic clay. J Appl Polym Sci, 93: 464–470. Hasegawa, N., Kawasumi, M., Kato, M., Usuki, A. and Okada, A., 1998, Preparation and mechanicla properties of polypropylene-clay hybrids using a maleic anhydride-modified polypropylene oligomer. J Appl Polym Sci, 67: 87–92. Hasegawa, N., Okamoto, H., Kato, M. and Usuki, A., 2000, Preparation and mechanical properties of polypropylene-clay hybrids based on modified polypropylene and organophilic clay. J Appl Polym Sci, 78: 1918–1922. Hasegawa, N., Okamoto, H., Kawasumi, M., Kato, M., Tsukigase, A. and Usuki, A., 2000, Polyolefin-clay hybrids based on modified polyolefins and organophilic clay. Macromol Mater Eng, 280/281: 76–79. Hasegawa, N., Okamoto, H., Kato, M., Usuki, A. and Sato, N., 2003, Nylon 6/Na-montmorillonite nanocomposites prepared by compounding Nylon 6 with Na montmorillonite slurry. Polymer, 44: 2933–2937.

1092

chemical engineering research and design 8 6 ( 2 0 0 8 ) 1083–1093

Hetzer, M., Ph.D. Dissertation, Tulane University, New Orleans (2007). Hotta, S. and Paul, D.R., 2004, Nanocomposites formed from linear low density polyethylene and organoclays. Polymer, 45: 7639–7654. Hristov, V.N. and Vasileva, S.T., 2004, Deformation mechanisms and mechanical properties of modified polypropylene/wood fiber composites. Polymer Compos, 25(5): 521–526. Hyun, Y.H., Lim, S.T., Choi, H.J. and Jhon, M.S., 2001, Rheology of poly(ethylene oxide)/organoclay nanocomposites. Macromolecules, 34: 8084–8093. Jeon, H.G., Jung, H.-T., Lee, S.W. and Hudson, S.D., 1998, Morphology of polymer/silicate nanocomposites. Polym Bull, 41: 107–113. Kato, M., Okamoto, H., Hasegawa, N., Tsukigase, A. and Usuki, A., 2003, Preparation and properties of polyethylene-clay hybrids. Polym Eng Sci, 43(6): 1312–1316. Kawasumi, M., Hasegawa, N., Kato, M., Usuki, A. and Okada, A., 1997, Preparation and mechanical properties of polypropylene-clay hybrids. Macromolecules, 30: 6333–6338. Kim, T.H., Lim, S.T., Lee, C.H., Choi, H.J. and Jhon, M.S., 2003, Preparation and rheological characterization of intercalated polystyrene/organophilic montmorillonite nanocomposite. J Appl Polym Sci, 87: 2106–2112. Kokta, B.V., Maldas, D. and Daneault, C., 1989, Use of wood fibers in thermoplastics. VII. The effect of coupling agents in polyethylene-wood fiber composites. J Appl Polym Sci, 37: 1089–1103. Kokta, B.V., Raj, R.G. and Daneault, C., 1989, Use of wood flour as filler in polypropylene: studies on mechanical properties. Polym Plast Technol Eng, 28(3): 247–259. Kumar, A.P. and Singh, R.P., 2007, Novel hybrid of clay, cellulose, and thermoplastics. I. Preparation and characterization of composites of ethylene-propylene copolymer. J Appl Polym Sci, 104(4): 2672–2682. Lagaly, G., 1999, Introduction: from clay mineral-polymer interactions to clay mineral-polymer nanocomposites. Appl Clay Sci, 15: 1–9. LeBaron, P., Wang, Z. and Pinnavaia, T.J., 1999, Polymer-layered silicate nanocomposites: an overview. Appl Clay Sci, 15: 11–29. Lee, J.Y., Baljon, A.R. and Loring, R.F., 1999, Spontaneous swelling of layered nanostructures by a polymer melt. J Chem Phys, 11(21): 9754–9760. Lertwimolnun, W. and Vergnes, B., 2005, Inlfuence of compatibilizer and the processing conditions on the dispersion of nanoclay in a polypropylene matrix. Polymer, 46(10): 3462–3471. Lim, Y.T. and Park, O.O., 2001, Phase morphology and rheological behavior of polymer/layered silicate nanocomposites. Rheol Acta, 40: 220–229. Luo, J. and Daniel, I.M., 2003, Characterization and modeling of mechanical behavior of polymer/clay nanocomposites. Compos Sci Technol, 63: 1607–1616. Maiti, S.N. and Hassan, M.R., 1989, Melt rheological properties of polypropylene-wood fiber composites. J Appl Polym Sci, 37: 2019–2032. Maiti, P., Nam, P.H., Okamoto, M. and Kotaka, T., 2002, The effect of crystallization on the structure and morphology of polypropylene/clay nanocomposites. Polym Eng Sci, 42(9): 1864–1871. Maldas, D. and Kokta, B.V., 1991, Influence of maleic anhydride as a coupling agent on the performance of wood fiber-polystyrene composites. Polym Eng Sci, 31(18): 1351–1357. Marchant, D., Ph.D. Dissertation, Michigan State University, East Lansing, MI (2003). Marcovich, N., Reboredo, M.M., Kenny, J. and Aranguren, M.I., 2004, Rheology of particle suspensions in viscoelastic media. Wood flour-polypropylene melt. Rheol Acta, 43: 293–303. Mitchell, B.S., (2004). An Introduction to Materials Engineering and Science: for Chemical and Materials engineers. (John Wiley & Sons, Inc, Hoboken, NJ).

Murphy, D.P., McNally, W.R. and Billham, M., 2003, The effect of coupling agents on the mechanical properties of wood-polymer composites. ANTEC, 2408–2412. Myers, G.E., Chahyadi, I.S., Coberly, C.A. and Ermer, D.S., 1991, Wood four/polypropylene composites: influence of maleated polypropylene and process and composition variables on mechanical properties. Int J Polym Mater, 15: 21–44. Oksman, K. and Clemons, C., 1998, Mechanical properties and morphology of impact modified polypropylene-wood flour composites. J Appl Polym Sci, 67: 1503–1513. Park, S., Seo, D. and Lee, J., 2002, Surface modification of montmorillonite on surface acid-base characteristics of clay and themal stability of epoxy/clay nanocomposites. J Colloid Interface Sci, 251: 160–165. Pickering, K.L. and Abdalla, A., 2003, The effect of silane coupling agents on radiata pine fibre for use in thermoplastic matrix composites. Composites: Part A, 34: 915–926. Raj, R.G., Kokta, B.V., Maldas, D. and Daneault, C., 1989, Use of wood fibers in thermoplastics. VII. The effect of coupling agents in polyethylene-wood fiber composites. J Appl Poly Sci, 37: 1089–1103. Rodriquez, C.A., Medina, J.A. and Reinecke, H., 2003, New thermoplastic materials reinforced with cellulose based fibers. J Appl Polym Sci, 90: 3466–3472. Sangalov, Y.A., Krasulina, N.A. and Jiasova, A.J., 2001, Some aspects of wood-polymeric composites production. Russian Polymer News, 6(2): 38–42. Sheng, N., Boyce, M.C., Parks, D.M., Rutledge, G.C., Abes, J.I. and Cohen, R.E., 2004, Multiscale micromechanical modeling of polymer/clay nanocomposites and the effective clay particle. Polymer, 45: 487–506. Simonsen, J., Jacobsen, R. and Rowell, R., 1998, Wood-fiber reinforcement of styrene-maleic anhydride copolymers. J Appl Polym Sci, 68: 1567–1573. Takatani, M., Ohsugi, S., Kitayama, T., Saegusa, M., Kawai, S. and Okamoto, T., 2000, Effect of lignocellulosic materials on the properties of thermoplastic polymer/wood composites. Holzforschung, 54: 197–200. Tanoue, S., Utracki, L.A., Garcia-Rejon, A., Tatibouet, J., Cole, K.C. and Kamal, M.R., 2004, Melt compounding of different grades of polystyrene with organoclay. Part 1: Compounding and characterization. Polym Eng Sci, 44(6): 1046–1060. Tseng, C., Wu, J., Lee, H. and Chang, F., 2001, Preparation and crystallization behavior of syndiotactic polystyrene-clay nanocomposites. Polymer, 42: 10063–10070. Usuki, A., Kojima, Y., Kawasumi, M., Okada, A., Fujushima, A., Kurauchi, T. and Kamigaito, O., 1993, Synthesis of Nylon 6-clay hybrid. J Mater Res, 8: 1179–1184. Utracki, L.A., (2004a). Clay-Containing Polymeric Nanocomposites (Rapra Technology, Shawbury). Utracki, L.A., (2004b). Clay-Containing Polymeric Nanocomposites (Rapra Technology Limited, Shawbury). Utracki, L.A. and Kamal, M.R., 2002, Clay-containing polymeric nanocomposites. Arabian J Sci Eng, 27(1C): 43–67. Wang, K.H., Choi, M.H., Koo, C.M., Choi, Y.S. and Chung, I.J., 2001, Synthesis and characterization of maleated polyethylene/clay nanocomposites. Polymer, 42: 9819–9826. Wang, K.H., Xu, M., Choi, Y.S. and Chung, I.J., 2001, Effect of aspect ratio of clay on melt extensional process of maleated polyethylene/clay nanocomposites. Polym Bull, 46: 499–505. Wang, K.H., Mingzhe, X., Choi, Y.S. and Chung, I.J., 2001, Effect of aspect ratio of clay on melt extensional process of maleated polyethylene/clay nanocomposites. Polym Bull, 46: 499–505. Wang, K.H., Chung, I.J., Jang, M.C., Keum, J.K. and Song, H.H., 2002, Deformation behavior of polyethylene/silicate nanocomposites as studied by real-time wide-angle x-ray scattering. Macromolecules, 35: 5529–5535. Xiao, K. and Tzoganakis, C., 2003, Rheological properties of HDPE-wood composites. ANTEC, 975–978.

chemical engineering research and design 8 6 ( 2 0 0 8 ) 1083–1093

Yeh, S., Ortiz, D., Al-Mulla, A. and Gupta, R., 2005, Mechanical and thermal properties of wood/layered silicate/plastic composites, In Proccedings of the 8th International Conference on Woodfiber-Plastic Composites. (society of plastic engineers, Wisconsin, MI) Yoon, P.J., Fornes, T.D. and Paul, D.R., 2002, Thermal expansion behavior of Nylon 6 nanocomposites. Polymer, 43: 6727–6741. Yoshitsugu, K., Arimitsu, U., Masaya, K., Akane, O., Yoshiaki, F., Toshio, K. and Osami, K., 1993, Mechanical properties of Nylon 6-clay hybrid. J Mater Res, 8: 1185–1189.

1093

Zhang, Y.C. and Wang, X., 2005, Thermal effects on interfacial stress transfer characteristics of carbon nanotubes/polymer composites. Int J Solids Struct, 42: 5399–5412. Zhong, Y., Poloso, T., Hetzer, M. and De Kee, D., 2007, Enhancement of wood/polyethylene composites via compatbilization and incroporation of organoclay partilces. Polym Eng Sci, 47(6): 797–803. Zhu, L. and Narh, K.A., 2004, Numerical simulation of the tensile modulus of nanoclay-filled polymer composites. J Polym Sci B: Polym Phys, 42: 2391–2406.