Polylactide-pine wood flour composites.
Plastics have been one of the most highly valued materials mainly because of their extraordinary versatility and low cost . However, the widespread use of plastics has become a significant concern due to their negative impact on the environment; specifically, the sources from which plastics are derived and their biodegradability. Almost all synthetic plastics are made from petroleum and its allied components. These natural resources take millions of years to form and are finite in quantity. In addition, plastics derived from fossil resources are largely nonbiodegradable.
The increased use in plastics over the years has resulted in an increase in plastic waste, which often is dumped as municipal solid waste. Thus, there is an immediate need to develop a non-petroleum-based and sustainable feedstock, and this has predominantly shifted the attention of many researchers toward biobased plastics.
Biobased plastics are sustainable, largely biodegradable and biocompatible [2-4]. They reduce our dependency on depleting fossil fuels and are [CO.sub.2] neutral. One of the most promising biobased polymers that has attracted the interest of many researchers is poly (lactide) (PLA), which is made from plants and is readily biodegradable. PLA, though discovered in the 1890s, is finding an edge in this new era of science and will soon replace many commodity plastics because of its biodegradability property and biobased nature.
PLA is a linear aliphatic thermoplastic polyester made from the ring opening polymerization of lactide, the cyclic dimer of lactic acid ([alpha]-hydroxy acid) [5-7]. PLA has high-modulus, reasonable strength, excellent flavor and aroma barrier capability, good heat sealability, and can be readily fabricated, thereby making it one of the most promising bio-polymers for varied applications . As such, PLA can become one of the most preferred commodity plastics in the future. Despite these desirable features, several drawbacks tend to limit its widespread applicability such as high cost, brittleness, and narrow processing windows. Polymer composites offer a convenient approach to tailor the materials cost and engineer the material properties.
Eco-friendly biocomposites, composed of natural fibers and biobased plastics, are of great importance to the material world, not only as a feasible solution to growing environmental threats, but also as a sustainable solution to the uncertainty of the world's petroleum supply . Natural fibers have several advantages over traditional inorganic fillers such as a renewable nature, low cost, low density, low energy consumption, high specific strength and stiffness, [CO.sub.2] sequestration, biodegradability, and less wear on machinery [9-10].
Of the many types of natural fillers, wood flour (WF) has attracted much attention [11-22] because it offers significant cost reduction as well as ease in processing. WF has been used as filler material in variety of polymers for property enhancement and to reduce costs [11-22]. Liber-Knec et al.  investigated the mechanical properties of polypropylene (PP)-WF composites by varying the size and percentage of WF. They concluded that the addition of WF to PP enhanced the mechanical properties of PP. Ichazo et al.  conducted a similar study but on natural rubber filled WF and found that the strength and elongation-at-break declined due to weak interfacial bonding. Lee and Ohkita  showed how the modification of fillers (WF) with a suitable compatibilizer (PCL-graft-maleic anhydride) enhanced the overall tensile properties of the WF filled polymers (PCL and polybutylenesuccinate-butylenecarbonate (PBSC)). Thus it is important to use a coupling agent in the processing of WF filled polymers to enhance their mechanical properties. In the present study, silane has been used as coupling agent to enhance the properties of PLA-pine wood flour (PWF) composites. Though studies in the past have reported that silane acts as a good coupling agent between WF and polymer [17, 19-20], to our knowledge, none of the studies used PLA as their matrix.
In this study, we compounded the PLA-PWF composites with different PWF loading levels via a kinetic-mixer (K-mixer) and prepared the testing specimens via an injection molding machine. Specimens were prepared with and without treating the PWF with a silane coupling agent. Various properties, including mechanical (static and dynamic) and thermal properties, were studied. The static mechanical properties reported here are tensile modulus, tensile strength, fracture toughness and strain-at-break, and the dynamic mechanical properties reported are storage modulus and tan [delta]. The crystallization behavior was investigated using differential scanning calorimeter (DSC). The morphologies of the fractured specimens captured using a scanning electron microscope were also investigated. Finally, an analytical model to predict the modulus of the composite system, developed by Halpin and Tsai , has been presented and was compared with the experimental results.
Polylactide (NatureWorks[TM] PLA 3001D) in pellet form was obtained from NatureWorks[R] LLC, Minnetonka, MN. It has a specific gravity of 1.24 and a melt flow index around 15 g/10 min (190[degrees]C/2.16 kg). Its glass transition temperature is 68-75[degrees]C and its melting temperature is 167[degrees]C. PWF (nominal 40 mesh/425 [micro]m) was supplied by American Wood Fibers, Schofield, WI, which has the following properties--moisture content, ~8%; bulk density, 13 lbs./cu. ft.; acidity (pH), 4.7; specific gravity, 0.4; ash content, 0.5%. GE Silicones--Silquest A-174[R] silane (gamma-Methacryloxypropyltrimethoxysilane), obtained from Witco Corporation, was used as the coupling agent. The chemical formula for silane is given below:
[CH.sub.2] = C(C[H.sub.3])C[O.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.2]Si (O[CH.sub.3])[.sub.3]
Processing PLA With PWF
An injection molding machine (Cincinnati Milacron 33-ton) was used to mold the tensile test samples. Prior to processing, both PLA and PWF were dried in an oven at 55[degrees]C and 105[degrees]C, respectively, to remove any moisture that had been absorbed during the storage. A kinetic mixer (K-mixer) (Vanzetti Systems Series 3009) was used to blend the PLA and PWF. First, the PWF was fed into the K-mixer and a few drops of silane (as per the percentage) were added. The K-mixer was turned on and once it reached 5000 rpm (revolutions per minute), it was switched off. This helped to disperse the silane within the PWF. The silane percentage employed in this study is a percentage relative to the PWF content in the formulation; thus, the higher the PWF content, the higher the silane content will be. Subsequently, the PLA pellets (weighed for appropriate proportion) were added to the mixing chamber and the machine was turned on again to raise the rpm to 5000. Mixing between the PWF and the molten PLA, resulting from the increased temperature due to the friction force, occurred inside the chamber. As the temperature increased to a preset value (204[degrees]C), the chamber door opened and the molten blend was dumped into the basket located below. The molten blend was cooled in a cold press, granulated and dried at 55[degrees]C for 1 h. The dry material was then fed into the hopper of the injection molding machine.
The injection molder was run via specifications of the PLA. The following temperatures were set at respective zones: 202[degrees]C near the feeder, 204[degrees]C in the middle zone(s), and 207[degrees]C at the injection tip. While operating, great care must be taken to prevent the PLA from staying in the barrel of the injection molder at these temperatures for too long because the material quickly degrades. A variable pack/hold pressure of 700-1000 psi was used depending on the weight percentage of the PWF (700 psi for 20% and up to 1,000 psi for 40%). A pack/hold time of 15 s also was used to ensure maximum material in the mold. Since PLA has a low glass transition temperature, a mold temperature of 19[degrees]C was maintained to allow adequate freezing in the allotted cooling time. A cooling time of 40 s per part was provided to ensure the part did not break upon die separation. Five samples have been prepared (PLA; PLA-20%PWF-0%Silane; PLA-20%PWF-0.5%Silane; PLA-40%PWF-0%Silane; PLA-40%PWF-0.5%Silane).
The static tensile properties (modulus, strength, toughness, elongation-at-break) were measured at room temperature (~25[degrees]C) and atmospheric conditions (relative humidity of ~50 [+ or -] 5%) with a 5 KN load cell on an Instron Model 5566 tensile tester. The cross-head speed was set at 0.2 in/min. The extensometer used was an MTS 634.31E-24 with a one-inch gauge length. All tests were carried out according to the ASTM standard (ASTM-D638); five specimens of each sample were tested and the average results were reported. All samples were tested after having been subjected to room temperature and atmospheric conditions for approximately two weeks.
Dynamic Mechanical Analysis
The dynamic mechanical spectra of the different specimens, cut from injection molded samples, were obtained using a dynamic mechanical spectrometer (TA instrument, model DMA Q800). The specimens were tested in a single-cantilever mode. Liquid nitrogen was used for cooling. They were heated at a rate of 3[degrees]C/min from 0[degrees]C to 85[degrees]C with a frequency of 1 Hz and strain of 0.02%, which is in the linear viscoelastic region as determined by a strain sweep.
Scanning Electron Microscopy
The fracture surfaces from the tensile tests were examined using a scanning electron microscopy (SEM) (Hitachi S-570) operated at 10 KV. All specimens were sputter coated with a thin layer of gold (~20 nm) prior to examination. The comparison between micrographs of different specimens was made at the same level of magnification.
Thermal analysis of injection molded specimens was carried out using a DSC (TA Instruments, Auto DSC-Q20). The sample used for testing was sliced from the injection molded specimen. The sample was first heated from 40[degrees]C to 180[degrees]C, kept isothermal for 3 min, cooled to 0[degrees]C, and finally reheated to 200[degrees]C. The ramp speed in all the heating-cooling processes was 10[degrees]C/min.
RESULTS AND DISCUSSION
Morphology of the PWF
The composition of the PWF reinforcement consists of many constituents (Cellulose, 42%; Lignin, 30%; Hemicellulose, 28%), which shows that the nature of the material is complex . An SEM image of PWF used in this study is shown in Fig. 1. As can be inferred from Fig. 1, PWF does not have a fixed geometry and varies in length and diameter (l/d ratio: approximately 3-5). Also, the rough surfaces seen on the outer structure of the PWF may benefit the interfacial adhesion provided that an appropriate coupling agent is used.
Morphology of the Fracture Surface
The morphology of the fracture surface of the injection-molded specimen resulting from the tensile testing experiments was investigated using SEM to understand the fracture behavior of PLA-PWF composites. It was assumed that the samples would fracture at the weakest point. For the purpose of recognizing the individual phases within the composite, PWF (see Fig. 1) and pure PLA (see Fig. 2) samples were scanned and SEM images were taken.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Figures 3 and 4 show the SEM images of fracture surfaces of the composite specimens at different PWF levels treated with and without silane. The effect of silane could not be predominantly identified in the SEM images; however, the increased PWF content can be well seen. A comparison of the images at the same magnification reveals that PWF is much more densely packed in higher percentage samples. As quoted in chapter-1 of Agarwal and Broutman , the higher the percentage of filler, the stiffer the composite will be. This has been confirmed in the tensile test results discussed in the subsequent section. By examining the images that give an understanding of the type of fracture that took place, we can infer that PLA showed a smooth fracture surface without obvious plastic deformation. Similar morphology can be observed in composite specimens (20% and 40% PWF), demonstrating that brittleness predominates the fracture mode. As shown in Figs. 3 and 4, an accurate distinction cannot be made at the interface between the PLA and PWF, indicating that good adhesion exists between the two. This has been confirmed later in the tensile test results discussed in the subsequent section, in which the strength of the composite remained the same irrespective of the PWF loading, which generally is not the case in PLA-WF composites .
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
All the specimens tended to follow brittle fracture mode as evident from the SEM images and stress-strain curves (see Fig. 5). The brittleness of a material is often gauged by material properties such as toughness and strain-at-break. Toughness, which is the energy-to-fracture per-unit volume of the specimen , is obtained by integrating the area under the stress-strain curve (Fig. 5 and Table 1). The toughness of the PWF filled samples decreased significantly when compared with pure PLA (Table 1) and decreased with the PWF content. An important feature associated with toughness is necking before fracture, which is not seen in any PWF-filled specimen (see Fig. 5). Only PLA exhibited necking in the stress-strain curve and had undergone stress-whitening during the tensile test. The stress-whitening phenomenon observed in the pure PLA samples is caused by crazing. Crazing is a phenomenon that frequently precedes fracture in some thermoplastic polymers [27, 28]. More crazes tend to result in higher strain-at-break and toughness values . The decrease in toughness may be attributed to the fact that the PWF particles acted as stress concentrators. The strain-at-break (%) as shown in Table 1 also decreased with the PWF content owing to the decreased deformability of the matrix (PLA in this case) due to the restriction offered by the rigid filler particles. This can be well identified in the SEM images, which show much denser PWF in the composites at higher PWF loadings.
The modulus of PLA-PWF composites increased with the PWF content (Table 1). This increment in the modulus is in agreement with the findings from the literature  that adding fillers to a polymer restrains the movement of its chains, thereby increasing the stiffness. As discussed in the morphology section, higher filler content showed a much higher density of PWF in the polymer, which resulted in a stiffer composite. The tensile strength of filled composites generally is found to be declining when compared with their virgin polymer . However, in this study, as can be seen from Table 1, consistent strength was found for different levels of PWF. This can be attributed to the good interfacial adhesion between the PWF and PLA, which might be due to the very nature (rough) of the PWF as discussed previously. Statistically, the effects of silane on the tensile properties reported in this section were not significant.
[FIGURE 5 OMITTED]
Halpin-Tsai Model for Young's Modulus
There exists a wide variety of theoretical models to predict the elastic properties of conventional fiber composites in terms of the properties of the constituent materials . In some special cases where specific fiber arrangements are considered, closed-form elastic solutions can be obtained. In this study we considered Halpin and Tsai (H-T) empirical equations , which hold good for short fiber composites and when the fibers are randomly oriented in the polymer matrix.
The Young's modulus, [E.sub.r], of a randomly oriented short fiber composite is given by Halpin-Tsai as,
[E.sub.r] = [3/8][E.sub.L] + [5/8][E.sub.T] (1)
where [E.sub.L] and [E.sub.T] are longitudinal and transverse moduli, given below
[E.sub.L] = [E.sub.m] [[1 + (2l/d)[[eta].sub.L][V.sub.f]]/[1 - [[eta].sub.L][V.sub.f]]] and [E.sub.T] = [E.sub.m] [[1 + 2[[eta].sub.T][V.sub.f]]/[1 - [[eta].sub.L][V.sub.f]]] (2)
The constants [[eta].sub.L] and [[eta].sub.T] are given as,
[[eta].sub.L] = [([E.sub.f]/[E.sub.m]) - 1]/[([E.sub.f]/[E.sub.m]) + (2l/d)] and [[eta].sub.T] = [([E.sub.f]/[E.sub.m]) - 1]/[([E.sub.f]/[E.sub.m]) + 2] (3)
In Eqs. 1-3, [E.sub.f], [E.sub.m], [V.sub.f], [V.sub.m], and l/d are defined as:
[FIGURE 6 OMITTED]
[E.sub.f] -- Young's modulus of reinforcement (PWF)
[E.sub.m] -- Young's modulus of polymer (PLA)
[V.sub.f] -- Volume fraction of reinforcement
[V.sub.m] -- Volume fraction of polymer i.e., 1- [V.sub.f]
l/d -- Aspect ratio of reinforcement
Figure 6 compares the model results predicted by Halpin-Tsai empirical relation for the modulus of the PLA-PWF composite with the experimental results obtained from the tensile testing. The properties of PWF and PLA used in the H-T model (Table 2) are obtained from literature  and experiments, respectively. Since no literature was found for 40-mesh PWF, properties of 30-mesh have been used in the model, which are believed to be close enough for computation and comparison. As the figure shows, the Young's modulus increases with the PWF content. Experimental results with and without silane have been furnished. Although the model and experimental results do not coincide, they have shown a similar trend. The observed mismatch between the theoretical and experimental results may be due to the following factors [31-33]:
1. The model does not account for the interface between the filler and polymer.
2. The model does not account for the variability in the modulus of the constituent materials, which is generally observed in materials.
3. Some of the PWF particles tend to break down during processing resulting in a significant variation in the aspect ratio of the PWF, which was not accounted for.
Additionally, as previously stated, there was no specific data available for the 40-mesh PWF employed for this study, which required us to consider properties of 30-mesh PWF in the model. Thus, the model can be developed further to minimize the mismatch between the experimental and theoretical results by accounting for the first three conditions and by obtaining sufficient data for the 40-mesh PWF. Although literature is available in the public domain that has accounted for interface and statistical variations in the prediction of modulus of fiber composites, a comprehensive work on these aspects is beyond the scope of this article.
[FIGURE 7 OMITTED]
Dynamic Mechanical Analysis
The viscoelastic properties of the PLA-PWF composites were studied using a dynamic mechanical analysis (DMA). A general declining trend of storage modulus (see Fig. 7) for all the curves is observed when the specimens go through higher temperatures with the most rapid reduction occurring at ~69[degrees]C, corresponding to the glass transition temperature of PLA. In general, the storage modulus appeared to increase with increasing PWF content, which is in agreement with the tensile modulus measurement (Table 1). Figure 8 plots the storage modulus at 25[degrees]C as a function of the PWF content for both silane treated and untreated samples. The linear curve fit clearly illustrates that the storage modulus increased with the PWF content for both treated and untreated samples. In addition, as clearly indicated in both Figs. 7 and 8, the addition of a small amount of silane (0.5% of the PWF) has improved the storage moduli of the composites at the same PWF loading level, particularly at 40% PWF content. The loss factor (tan [delta]) is shown in Fig. 9. The addition of PWF led to a reduction of the glass transition temperature ([T.sub.g]) as evidenced by a shift of the peak temperature towards the left. The values of the glass transition temperatures were listed in Table 3. This is in agreement with what was reported in the literature . The area integration under the tan [delta] curve decreased with the PWF content, indicting that the PLA-PWF composites exhibited more elastic behavior with increasing PWF, which is in accordance with the tensile test results.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Differential Scanning Calorimetry
The crystallization behaviors were studied using DSC. Figure 10 shows the thermograms obtained from the second heating cycle. Table 4 shows the numerical values of temperature and enthalpy obtained from the first cooling and second heating cycles and crystallinity for the PLA-PWF composites with different PWF and silane percentages.
As can be seen in Fig. 10, a double-melting peak was obtained for the PLA-PWF composites with silane-treated PWF, although it was most obvious at 20% PWF. For the PLA-20%PWF (treated with silane) composite, the two melting peaks were observed at 164.4[degrees]C and 169.6[degrees]C. Similarly for PLA-40%PWF (treated with silane), the two melting peaks were seen at 158.1[degrees]C and 167.4[degrees]C. It is common to observe a double-melting peak in polymers, which may be due to several reasons as outlined in . In the present study, the double-melting peak behavior observed in the PLA-PWF composites when the PWF was treated with silane may be attributed to two different types of crystalline structures (e.g., variation in thickness of the lamellar structure and size of the spherulites) obtained during the crystallization process due to the possible influence of silane treated PWF .
[FIGURE 10 OMITTED]
The cold crystallization temperature was decreased by about 14[degrees]C for a PLA-20%PWF (un-treated with silane) composite specimen when compared with pure PLA. Additionally, the cold crystallization peak vanished completely for all other three PLA-PWF samples, namely PLA-20%PWF (treated with silane) and PLA-40%PWF (treated and untreated with silane), indicating that there was no more amorphous region in those samples that had the ability to crystallize during the second heating process.
The crystallinity of PLA is computed using Eq. 4 [21, 36-37]:
[[chi].sub.c](% Crystallinity) = [[DELTA][H.sub.m]/[DELTA][H.sub.m.sup.0]] x [100/w] (4)
[DELTA][H.sub.m.sup.0] - 93.7 J/g
w - weight fraction of PLA in the sample
[DELTA][H.sub.m] is the enthalpy for melting, [DELTA][H.sub.m.sup.0] is the enthalpy of melting for a 100% crystalline PLA sample, and w is the weight fraction of PLA in the sample. To determine the crystallinity of the sample, the extra heat absorbed by the crystallites formed during heating (i.e., cold crystallization) had to be subtracted from the total endothermic heat flow due to the melting of the whole crystallites . Thus, the modified equation can be written as follows:
[[chi].sub.c](% Crystallinity) = [[[DELTA][H.sub.m] - [DELTA][H.sub.cc]]/[DELTA][H.sub.m.sup.0]] x [100/w] (5)
[DELTA][H.sub.cc] : Cold--Crystallization Enthalpy
As can be inferred from Table 4, the addition of PWF enhanced the crystallinity of PLA from 13% to 38% and 44%, respectively for PLA-20% and PLA-40% PWF (with no silane treatment) samples. Also, the crystallinity of PLA was higher for samples with the same PWF content but treated with silane. For instance, the crystallinity of PLA-20%PWF composites treated with silane was 12% higher than that of PLA-20%PWF composites without any silane treatment. Similarly, the crystallinity of PLA-40%PWF composites treated with silane was 7% higher than that of PLA-40%PWF composites without any silane treatment. In fact, the crystallinity of the PLA-20% PWF composite treated with silane is 6% more than that of the PLA-40% PWF composite without any treatment. This indicates that silane has played an important role in improving the crystallinity of PLA-PWF composites. This is in agreement with findings from the literature  that the surface chemistry of fibers is the governing factor for the crystal formation in PLA-based composites. Thus, it can be concluded that the addition of PWF increases the crystallinity by acting as nucleating agent; however, a treatment with a coupling agent such as silane further enhances the crystallinity.
PLA-PWF composites were prepared by K-mixer and injection molding machine. A morphological study of PWF via SEM showed a rough surface on the outer structure that was believed to provide good interfacial adhesion. Similar studies on PLA-PWF composites revealed good adhesion between PLA and PWF and good dispersion of PWF in the polymer. Pure PLA undergone crazing as is evidenced from the stress-whitening phenomenon and the SEM micrograph; however, crazing was not observed in the PLA-PWF composites.
The addition of PWF increased the modulus but decreased the toughness and strain-at-break; however, the strength remained the same. The effects of silane on the tensile properties were not statistically significant. The theoretical Young's modulus predicted by Halpin-Tsai empirical relation was found to increase with increasing PWF content; however; there exists a mismatch between the theoretical and experimental results due to the reasons discussed previously.
The storage modulus of the PLA-PWF composites increased with the PWF content. Further the effect of silane was clearly visible as the specimens treated with silane generally showed higher values of storage modulus than their un-treated counterparts. The addition of PWF to PLA reduced the glass transition temperatures slightly taken from the peak values in the tan [delta] curves.
The cold crystallization peak decreased with the addition of 20% PWF untreated with silane and disappeared for all other PLA-PWF composites investigated. The crystallinity of PLA was enhanced significantly by the addition of PWF. Additionally, PWFs treated with silane showed higher nucleating ability than untreated PWFs. The silane treated PWFs, when compared to untreated PWFs, increased the crystallinity of the PLA-PWF composites about 12% and 7% at 20% and 40% PWF loading level, respectively.
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Srikanth Pilla, (1) Shaoqin Gong, (1) Eric O'Neill, (2) Roger M. Rowell, (2) Andrzej M. Krzysik (2)
(1) Department of Mechanical Engineering, University of Wisconsin, Milwaukee
(2) Forest Products Laboratory, United States Department of Agriculture, Madison, Wisconsin
Correspondence to: Shaoqin Gong; e-mail: firstname.lastname@example.org
Contract grant sponsors: National Science Foundation (NSF DMI-0544729) and Wisconsin Department of Agriculture, Trade and Consumer Protection.
TABLE 1. Mechanical properties of PLA-pine wood flour composites. Property Fracture toughness Sample [J/[m.sup.3]] Strain at break Pure PLA 5.4E6 [+ or -] 1.0E6 0.2 [+ or -] 0.02 PLA-20%WF-0% Silane 2.3E6 [+ or -] 0.3E6 0.1 [+ or -] 0.01 PLA-20%WF-0.5% Silane 2.1E6 [+ or -] 0.3E6 0.1 [+ or -] 0.01 PLA-40%WF-0% Silane 1.3E6 [+ or -] 0.3E6 0.1 [+ or -] 0.01 PLA-40%WF-0.5% Silane 1.6E6 [+ or -] 0.2E6 0.1 [+ or -] 0.01 Property Young's Modulus Ultimate tensile strength Sample [MPa] [MPa] Pure PLA 638.9 [+ or -] 9.9 55.5 [+ or -] 0.6 PLA-20%WF-0% Silane 854.4 [+ or -] 12.3 55.5 [+ or -] 1.9 PLA-20%WF-0.5% Silane 861.8 [+ or -] 9.02 54.5 [+ or -] 4.5 PLA-40%WF-0% Silane 1191.1 [+ or -] 29.9 51.7 [+ or -] 6.2 PLA-40%WF-0.5% Silane 1180.4 [+ or -] 32.7 57.1 [+ or -] 3.1 TABLE 2. Properties of constituent materials of the composite used in Halpin-Tsai model. Property Value Unit [E.sub.f] 27,500 MPa [E.sub.m] 638.9 MPa [V.sub.f] 0-0.5 l/d 4 TABLE 3. The glass transition temperature of the PLA and PLA-pine wood flour composites. Material [T.sub.g] ([degrees]C) PLA 69.4 20% PWF-0% Silane 67.4 20% PWF-0.5% Silane 66.8 40% PWF-0% Silane 65.6 40% PWF-0.5% Silane 66.8 TABLE 4. Thermal characteristics of PLA and PLA-PWF composites. Melting temperature PLA:PWF: [T.sub.cc] [DELTA][H.sub.c] [T.sub.m1] Silane (%) ([degrees]C) (J/g) ([degrees]C) 100:0:0 108.7 25.0 169.7 80:20:0 94.7 4.1 168.5 79.5:20:0.5 -- -- 164.4 60:40:0 -- -- 169.1 59.5:40:0.5 -- -- 158.1 Melting temperature PLA:PWF: [T.sub.m2] [DELTA][H.sub.m] [T.sub.c] [[chi].sub.c] Silane (%) ([degrees]C) (J/g) ([degrees]C) (%) 100:0:0 37.1 92.4 13 80:20:0 32.7 95.8 38 79.5:20:0.5 169.6 37.5 111.5 50 60:40:0 24.6 94.6 44 59.5:40:0.5 167.4 28.4 98.2 51 [T.sub.cc], cold crystallization temperature; [DELTA][H.sub.c], cold crystallization enthalpy; [T.sub.m1], temperature of lower melting peak; [T.sub.m2], temperature of higher melting peak; [T.sub.c], melting crystallization temperature; [DELTA][H.sub.m], melting enthalpy; [[chi].sub.c], degree of crystallinity.
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|Author:||Pilla, Srikanth; Gong, Shaoqin; O'Neill, Eric; Rowell, Roger M.; Krzysik, Andrzej M.|
|Publication:||Polymer Engineering and Science|
|Date:||Mar 1, 2008|
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