The Study On Compounding Factors Of Wood Fiber/Poly(Lactic Acid) Bio-composites

Wood fiber/poly(lactic acid) (WF/PLA) bio-composite based on renewable wood material and biodegradable poly(lactic acid) made from corn starch is a fully biodegradable and environmental friendly bio-composite and it is one of the important research area of bio-composites. The WF/PLA bio-composites would be of great importance to the material world, not only as a solution to growing environmental threat but also as a solution to the uncertainty of petroleum supply. The compounding factors and mechanisms influencing the structures and properties of the WF/PLA bio-composites were analyzed in this thesis. It was hoped to provide the basal data for the development of WF/PLA bio-composites.The effects of three representative compounding mode, including extrusion, high-speed mixing and physical mixing, compounding temperatures and the moisture content of wood fiber on aggregated state structure, physical-mechanical and thermal properties of the WF/PLA composites were studied. Then the influencing mechanisms of wood fiber content, modified poly(lactic acid) and compatilizers on the structures and properties of the WF/PLA composites were analyzed systemically. The property endurance and the degradation of the WF/PLA composites were investigated and evaluated too.The main results are as follows:(1) The influences of compounding modes on the structures and properties of WF/PLA composites were significant.The wood fiber and poly(lactic acid) in the WF/PLA composites experienced different thermal and shear stress reactions during compounding. The influences of compounding modes on the structures and properties of WF/PLA composites were significant. The poly(lactic acid) degraded arose from different compounding modes was the key that affected the properties of WF/PLA composites.In extrusion mode, components experienced a long thermal history and stronger shearing stress. The poly(lactic acid) molecular in WF/PLA composites degraded seriously and the weight-average molecular weight ((M|-)w) and number-average molecular weight ((M|-)n) of the poly(lactic acid) in the WF/PLA bio-composites were just 13.5% and 14.6% of that of the raw material. The melt temperature decreased significantly and the flexural strength of the WF/PLA composites were the lowest. But the density and water resistance of the WF/PLA composite were the highest and the best each, because the poly(lactic acid) and wood fiber compounded thoroughly. In the physical mixing mode, wood fiber and poly(lactic acid) were just mixed physically and the poly(lactic acid) in WF/PLA composites degraded lesser. The (M|-)w and (M|-)n of poly(lactic acid) reserved 69.9% and 67.3% of that of raw poly(lactic acid). The flexural strength of the composites is highest, but the density was the lowest and water resistance was the poorest. In high-speed mixing, the thermal and shearing stress were moderate. WF/PLA composites made by high-speed mixing mode got preferable general properties. The (M|-)w and (M|-)n of poly(lactic acid) in WF/PLA composites were 51.0% and 51.9% of the raw poly(lactic acid). The flexural strength of the composite was 45.12MPa. Low density, better water resistance and highest flexural modulus were got for the WF/PLA composites by high-speed mixing mode.(2) The effects of compounding temperatures on the structures and properties of WF/PLA composites were significant, but the effects of wood fiber moisture contents were not significantDuring the preparing of WF/PLA composites by kneading, the (M|-)w of poly(lactic acid) in WF/PLA composites decreased significantly when the compounding temperature increaed from 180℃to 200℃. The (M|-)n changed a little between 180℃and 190℃, but it decreased significantly when the compounding temperature increased from 190℃to 200℃. At compounding temperature 190℃, the melt temperature of the poly(lactic acid) was the lowest.(3) The correlativity between wood fiber content and the structures and properties of WF/PLA composites was obviousWith the wood fiber content increased from 30% to 60%, the density and flexural modulus of the composites increased, but flexural strength and water resistance decreased. When the wood fiber content increased from 70% to 90%, the density, flexural strength, flexural modulus, and water resistance all decreased. The flexural modulus of the composites were increased by compounding wood fiber with poly(lactic acid).The molecular weight decreased seriously when wood fiber compounded with poly(lactic acid). With the wood fiber content increased, the (M|-)w and (M|-)n of poly(lactic acid) decreased obviously, and all the cold crystal temperature, glass transition temperature, melting temperature and decomposition temperature decreased.(4) The structures and properties of WF/PLA composites made of different modified poly(lactic acid) and compatilizers were different.The poly(lactic acid) degraded and the molecular decreased significantly when different modified poly(lactic acid) compounded with wood fiber. The smaller the poly(lactic acid)'s molecular was, the lower the mechanical strength and the poorer of water resistance of the composites were.The effects of adding compatilizers on the properties of the WF/PLA composites were determined by the degraded extent of poly(lactic acid) caused by the compatilizers. The important role of the compatilizers in WF/PLA composites were to protect the poly(lactic acid ) molecular not to degrade during compounded.Compatilizer maleic polypropylene CA60 increased the flexural strength of the composites, but led the poly(lactic acid) of the composites degraded seriously, and the water resistance of the composites was the poorest. The (M|-)w and (M|-)n of poly(lactic acid) in composites with compatilizer maleic polypropylene M300 or H1100P were not increased significantly compared with the composites without compatilizer. They could improve the water resistance of the WF/PLA composites obviously. The (M|-)w of WF/PLA composites with compatilizer fatty acid amide AD281 was similar to that of without compatilizer. Polyterrafluoroethylene coated with acrylate polymer A3000 could improve the compatibility of the components, and increase molecular weight of poly(lactic acid) in composites. The result of AD281 and A3000 to improve the water resistance of the composites were obvious.(5) The biodegradability of WF/PLA composites was excellentAfter placed in room for 24 months, the WF/PLA composites still preserved 75% flexural strength and 85% flexural modulus compared to original mechanical properties. The results indicated that the endurance of WF/PLA composites can meet the need of use. The biodegradable WF/PLA composites are much more suitable used in the case of short term and requirement of biodegradation properties for the products.For the WF/PLA composites with different formulations, molecular degradation of poly(lactic acid)was the important character of the composites degraded. After buried in soil for 6 months, the biodegradation of WF/PLA composites was different. (M|-)w and (M|-)n of the composites WF/PLA002, WF/PLA022, WF/PLA/AD281 were decreased much more after buried in soil for 6 months. At the same time, there weren't changes for the modifier and compatilizer in the composites. For WF/PLA012, WF/PLA003 and WF/PLA002/A3000 composites, the molecular weight of poly(lactic acid) in composites did not decreased, and the molecular curves of modifiers or compatilizers changed or disappeared.The flexural strength and flexural modulus of the WF/PLA composites, crystal and non-crystal areas of poly(lactic acid) all indicated that the composites degraded obviously after buried in soil for 6 months. The poly(lactic acid) in WF/PLA composites were easily degraded than raw (lactic acid) itself.In conclusion, the effects of compounding mode, temperature, wood fiber content and different poly(lactic acid) on the structure and properties of the WF/PLA composites were significant. The correlativity between the thermal, physical-mechanical properties and the molecular weight of poly(lactic acid) of the composites is very obvious too. The degradation of poly(lactic acid) was promoted when it was compounded with wood fiber. But the property endurance and the degradation properties of WF/PLA composites were excellent. WF/PLA composites are the environmental friendly bio-composites.