Each composite has its own unique chemical properties which is attributable to the filler type and method of preparation which in turn affects the composite’s thermal properties.26,49,80,81 For example, DDGS contains a higher concentration of protein («26%) than found in most wood and lignocellulosic fiber particles («1.5-7%). The DSC thermal properties of the DDGS, PW and PINEW composites are shown in Table 13.7. Only single endothermic (melting) temperature and exothermic (crystal­lization) peaks were observed in the DSC curves for all formulations conducted. Little difference was found between the melting points (Tm) of neat HDPE (i. e., 130.4°C) and the original DDGS formulations (HDPE-25DDGS and HDPE — 25DDGS-MAPE). Chemical modification (A or AM) treatments caused DDGS for­mulations to exhibit slightly higher Tms than HDPE or the nonchemically treated — DDGS composite controls. This trend was duplicated with the PW formulations given the same chemical modification treatments. Generally, biocomposites will exhibit a slightly higher Tm compared to that the neat thermoplastic resin.49,80 The increase in Tm in the composite is probably due to the disruption of the HDPE crystal lattice network by the presence of the filler particles. This is demonstrated by the HDPE-40PINEW formulation, which contains 40% Pine wood filler and subsequently exhibits the highest Tm value (133.5°C) recorded in this study (Table 13.7). Interest­ingly, the HDPE-10STDDGS/30PINEW, which also contains 40% filler had a lower Tm value (131.8°C). Although both of these formulations contained 40% filler, de­cidedly different mechanical and physical properties were exhibited by them (Tables 13.2-13.5). Considerably more research needs to be conducted to determine how the mixing fillers of different chemical composites interact and affect their resulting thermal properties.

The addition of fillers to the HDPE results in composites with lower crystalliza­tion enthalpy (DHc) and melting enthalpy (DHm) values compared to neat HDPE (Table 13.7). It has been suggested that that wood fillers absorb more heat energy in the melting of composites which results in their lower DHm values when compared to neat thermoplastic resins.43

The degree of crystallinity (xc) varied considerably depending on the filler type employed in the composite. Other investigators have also observed a variation in the degree of crystallinity values associated with various LPC.3049 The degree of crystal­linity values for DDGS, PW and PINEW biocomposites were higher when MAPE was included in the formulation (Table 13.7). This situation may also be related to the HDPE resin employed. For example, PW blended with a different HDPE source showed distinctly lower x values than neat HDPE employed in this study.30 When the concentration of PINEW is increased to 40%, x values decreased markedly be­low that of neat HDPE. One explanation for this phenomenon for the reduction in X values is due to the amount of free volume occurring between the polymer chains capable of allowing filler to be intermixed.81 As the volume of the filler increases less resin polymer intermolecular free volume is available for dissipating the filler material.81

The thermal properties of biocomposites need to be determined because the pro­cessing temperatures (extrusion and injection molding) may often exceed 200°C. Commercial HDPE products are often made with high melt temperature resins. The thermogravimetric curves for the various composites are plotted in Fig. 13.7 and these results are summarized in Table 13.8. The degradation of neat HDPE em­ployed in this study, occurs in a single stage, beginning at 461.7°C, with a maximum decomposition peak occurring at 478.3°C. HDPE degradation was 99.7% complete at end of this stage. Similarly, the HDPE-MAPE blend mimicked these parameters, although exhibiting somewhat lower degradation and peak maximum temperatures. In contrast, there are several earlier degradation peaks for the DDGS composites. Examining the HDPE-25DDGS formulation reveals a major degradation tempera­ture (Td) for the DDGS flour occurring at ~ 242.2°C which subsequently results in maximum peak temperature at 322°C. Minor degradation peaks also occur and are the decomposition of low molecular weight components such as hemicellulose which degrades between 225 to 325°C.782 Next a larger second higher degradation peak occurs with a maximum at 321°C, which is corresponds to the decomposi­tion of cellulose which degrades in the 300 to 400°C.82 Further, a third degradation peak corresponds to lignin decomposition which is reported occurring near 420°C; however it is not readily seen in this study.82 This peak was obscured by the de­composition of the HDPE. The DDGS composite has a residual weight of 5.9% ash residue from the heterogeneous ingredients associated with the filler. Differences among the DDGS composite Td’s are due to the association of the filler material and the plastic resin. Higher Td’s and maximum peak temperatures occurred for STD — DGS composites compared to the DDGS composites; this can be attributed to the occurrence of higher levels of low-molecular-weight organic compounds in DDGS composites compared to that found in the STDDGS composites. Similarly, other investigators report that addition of extractables (clay) caused a decrease in Td val­ues to occur.12 The results presented here confirm a previous study using DDGS formulations blended with a different HDPE resin.30 The addition of the coupling agent MAPE had a somewhat complex influence on the decomposing behavior the DDGS composites. In some cases, inclusion of MAPE in the DDGS and STDDGS formulations (HDPE-25DDGS-MAPE and HDPE-25STDDGS-MAPE) resulted in occurrence of lower degradation temperatures (1st Td) compared to formulations without MAPE (HDPE-25DDGS and HDPE-25STDDGS) (Table 13.8). Chemi­cally modified DDGS formulations (HDPE-25 STDDG/A) showed considerably higher 1st peak degradation initiation temperatures (Td) and 1st degradation maxi­mum peaks compared to the untreated DDGS formulations (HDPE-25DDGS). The chemical modification treatments (A and AM) improves the thermal stability of for­mulations. This phenomenon has been reported by other investigators where the 1st decomposition temperatures of HDPE-acetylated-WF were reported to higher when compared to HDPE-WF formulations.51 Likewise, chemically treated PW formula­tions (HDPE-25STPW/A and HDPE-25STPW/AM) exhibited higher 1st peak deg­radation initiation temperatures (Td) and 1st degradation maximum peaks compared to the untreated PW formulations (HDPE-25PW).

“Mixed” filler formulations (HDPE-12.5 STDDGS/12.5PINEW) and HDPE- 12.5DDGS/12.5PINEW-MAPE) exhibited much higher 1st degradation initiation temperatures (Td) and 1st degradation maximum peaks than the nonmixed DDGS filler formulations (HDPE-25STDDGS and HDPE-25STDDGS-MAPE). This is at­tributed to the presence of PINEW in the mixture, which probably masks the pres­ence of DDGS particles in these formulations. Perhaps, mixed filler composites containing PINEW flour and DDGS could be considered more “thermally stable” than the DDGS formulations tested (Table 13.8). More study is necessary to prove this contention. However, these “mixed” fillers composites were not as stable as em­ployment of formulations containing PINEW alone. Based on the TGA analysis and since the processing temperatures did not exceed 210°C the DDGS, PW and DDGS — PINEW composites were thermally stable for the temperatures in which they were subjected to in this study.

13.4 CONCLUSIONS

The mechanical properties of two potential lignocellulosic material reinforcements, DDGS and PW, for use in commercial LPC was conducted in this study. Further, the benefit of “mixing” chemically dissimilar fillers, DDGS with PINEW, was assessed. The tensile, flexural, impact strength, environmental durability (soaking responses), and thermal properties of injection molded test specimens were measured. Compari­son of the composites to neat HDPE that was processed with the same conditions was conducted to determine the relative merits of using a filler against a control. Using DDGS subjected to solvent extraction (STDDGS) produces a composite with superior mechanical properties (HDPE-25 STDDGS) compared to the composite made with the original DDGS material (HDPE-25DDGS). Further, formulations of STDDGS with MAPE (HDPE-25STDDGS-MAPE) exhibited slightly lower tensile and flexural moduli but slightly higher ultimate stresses than similar formulations made without MAPE. The flexural properties and the tensile modulus of a solvent extracted DDGS with MAPE exceeded those of neat HDPE. The PW composites in general exhibited greater tensile and flexural properties than the DDGS com­posites made with similar formulations. Chemical modification by acetylation and malation of DDGS and PW fillers prior to compounding had mixed effects on im­proving the mechanical properties of the composites studied when compared to the untreated controls. In fact, the tensile and flexural moduli of composites containing chemically modified fillers were slightly lower than the baseline solvent treated filler with MAPE. The mixing of PINEW and STDDGS resulted in formulations (HDPE-12.5DDGS/12.5PINEW and HDPE-12.5DDGS/12.5PINEW-MAPE) with
improved mechanical properties compared to the STDDGS formulations (HDPE- 25STDDG or HDPE-25STDDGS-MAPE). All DDGS and PW composites soaked in water for 872 h exhibited weight gain, color changes, and some alteration in their mechanical properties, especially El%. The thermal stability of LPC formulations can be improved by mixing with wood filler and employing chemical modification.

13.5 ACKNOWLEDGEMENTS

The authors acknowledge Kimberly Pelphrey for technical assistance and Dr. N. Joshee for Paulownia wood material. Mention of a trade names or commercial prod­ucts in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Ag­riculture. USDA is an equal opportunity provider and employer.