Effects Of Isothermal Crystallization On Crystallization And Properties Of Wood Fiber/Polylactic Acid Composites

Polylactic acid(PLA)is a biodegradable crystalline polymer.It is widely applied in medical treatment,health products,textile and other fields for its excellent biocompatibility and mechanical properties.The natural wood fiber is added to PLA to prepare composites which is not only green,but also possesses the advantages of both materials has become an important direction of PLA composites.PLA products' process would affect aggregation structure and physical-mechanical properties of PLA.Therefore,it is necessary to concern about the effect of the preparation of WF/PLA composites on its properties.The study will improve the crystallization rate and crystallinity of PLA,and provide basis for improving the properties of the composites.In this paper,the composite combined poplar wood fibers and PLA was prepared by hot-pressing.The effects of isothermal crystallization including cold crystallization and crystallization from melt cooling at five differernt temperatures(90?,100?,110?,120?,130?)on crystallization properties and physical-mechanical properties of both materials were analyzed.Differential scanning calorimeter(DSC)was used for testing crystallinity and melting characteristics of pure PLA panels and WF/PLA composites.The isothermal crystallization process were observed by polarizing microscope(POM).In addition,the effect of isothermal crystallization on crystallization properties of PLA were analyzed combining isothermal crystallization kinetics.And the study analyzed the cause of differences of mechanical properties combining crystallization characteristics with mechanical properties by scanning electron microscope(SEM)and gel chromatography(GPC).The results are described as follows:(1)The crystallinity of pure PLA panels and composites increased after isothermal crystallization.The crystallinity of pure PLA panels and composites increased with cold crystallization temperature increased,and increased from 4.6% to 46.5%,and from 15.1% to 38.6%,respectively.A large number of crystals formed in pure PLA,and the crystallization rate and crystals size increased as the cold crystallization temperature was higher.The crystallization rate of PLA in WF/PLA composites at initial crystallization was faster than that of pure PLA.While the addiation of wood fibers reduced the crystallization rate of PLA and increased the time required to complete the entire crystallization.After crystallization from melt cooling,the crystallinity of pure PLA panels and composites increased from 4.6 % to 24.8 %,and 15.1 % to 40.5 %,respectively.Pure PLA mainly homogeneous nucleated during the crystallization.The maltase phenomenon unique to spherulites appeared at 110?.Meanwhile the crystalline dimension grew to the biggest and crystallization rate was the slowest.Moreover,wood fiber had a heterogeneous nucleating ability to transcrystallize at PLA matrix.The crystallization rate of PLA in composites reduced duing to the hinder between wood fibers and crystals.(2)Cold crystallization and crystallization from melt cooling negligibly changed the density of both materials.The water resistance of pure PLA panels didn't change while the water resistance of the composites got worse after isothermal crystallization.(3)WF/PLA composites had higher flexural modulus than pure PLA panels,but flexural strength was lower.After cold crystallization,the flexural strength of both materials decreased with increasing temperature and increasing crystallinity.The flexural modulus of pure PLA panels increased with increasing temperature,while the composites did the opposite.The flexural strength of both materials were greater than untreated materials at 90?,which indicated that cold crystallization could improve the flexural strength of PLA materials in a certain temperature range.Compared with untreated materials,the flexural strength and tensile strength of pure PLA panels increased after crystallization from melt cooling.However,the flexural strength and tensile strength of composites increased only at 130? isothermal crystallization process.In addition,the effect of cold crystallization and crystallization from melt cooling on tensile strength was consistent with the effect on the flexural strength.(4)The prime reason for the decline in mechanical properties of composites was the worse interface compatibility rather than the decrease of molecular weight.In addition,spherulite defects in crystalline PLA and oversized crystals also had an adverse effect on mechanical properties.This study showed that cold crystallization and crystallization from melt cooling had a significant effect on crystallization properties and physical-mechanical properties of PLA materials.The crystallinity of PLA in WF/PLA composites can be regulated by cold crystallization and crystallization from melt cooling.