基于力学性能和结晶行为探究不同成核剂对聚乳酸性能影响

Effect of various nucleating agents on mechanical properties and crystallizationbehavior of poly(lactic acid)

探究杨木纤维作为一种生物基成核剂对聚乳酸(PLA)力学和结晶性能的影响,并与常用的成核剂滑石粉和酰肼化合物对比。分别将杨木纤维(WF)(0.5wt%、1wt%、2wt%、4wt%)、滑石粉(Talc)(1wt%、2wt%、4wt%、8wt%)和酰肼化合物(TMC-300)(0.3wt%、0.5wt%、1wt%、2wt%)在不同含量条件下与PLA进行熔融共混挤出、模压制备复合材料,基于力学性能确定每种成核剂在PLA中的较优添加量,并进一步在较优添加量条件下研究3种成核剂对PLA结晶性能的影响,包括结晶行为、晶体形貌和结构。3种成核剂均能提高PLA的缺口冲击强度,与成核剂Talc和TMC-300相比,添加WF对PLA的拉伸性能和弯曲性能改善效果更为显著,在WF较优添加量条件下(1wt%),复合材料的断裂伸长率、拉伸强度和弯曲强度与纯PLA相比分别提高了27%、17%和18%。采用差示扫描量热法比较3种成核剂对PLA的结晶行为影响,结果表明添加1wt%WF提高了PLA非等温结晶的结晶度,但低于添加1wt%Talc和0.5wt%TMC-300的复合材料结晶度。根据等温结晶动力学分析,WF可以缩短PLA在等温结晶过程的半结晶时间,加快PLA的结晶速率,添加1wt%WF、1wt%Talc和0.5wt%TMC-300后PLA在110℃等温结晶的半结晶时间从23.6 min分别降低至7.2、2.7和1.4 min。热台-偏光显微镜观察结果表明,不同成核剂诱导PLA在110℃等温结晶时的晶体形貌不同,WF、Talc为PLA结晶提供了大量成核位点,促进PLA晶粒微细化,而TMC-300诱导PLA产生纤维束状晶体且结晶速率明显加快,这与等温结晶动力学分析结果一致。蚀刻后的冲击断面形貌也表明,不同形貌的晶体堆积是复合材料的力学性能存在差异的原因之一。X射线衍射结果表明,3种成核剂均能促进PLA产生有序的α晶,其中添加WF时α(110)/(200)晶的衍射峰强度最高,同时WF能显著降低PLA的晶粒尺寸。杨木纤维可以作为PLA的一种生物基成核剂,具有增强和成核的双重作用。本文为进一步优化WF对PLA的成核作用提供基础,也为促进木塑复合材料的绿色发展提供借鉴。

The aim of this study was to investigate the effect of poplar wood fiber as a bio-nucleating agent on the mechanical properties and crystallization behaviors of poly(lactic acid)(PLA),and compare with common nucleating agents talc powder and hydrazide compounds.Poplar wood fiber(WF)(0.5wt%,1wt%,2wt%,4wt%),talc powder(Talc)(1wt%,2wt%,4wt%,8wt%)and hydrazide compounds(TMC-300)(0.3wt%,0.5wt%,1wt%,2wt%)were blended with PLA to prepare composite at various contents by extrusion and molding process,respectively.The optimal content of each nucleating agent was determined based on mechanical properties of composites.The effect of WF,Talc and TMC-300 under optimal content on the crystallization properties including crystallization behaviors,crystal morphology and structure of PLA-based composites was compared.All the three types of nucleating agents can improve the notched impact strength of PLA.Compared with Talc and TMC-300,the addition of WF results in more significant improvement in tensile and flexural properties of PLA-based composite.Under the optimal addition content(1wt%)of WF,the elongation at break,tensile and flexural strength increase by 27%,17%and 18%in comparison with neat PLA,respectively.The effect of WF,Talc and TMC-300 on the crystallization behaviors of PLA was studied through differential scanning calorimetry.Results show that adding 1wt%WF can improve the crystallinity of PLA in the non-isothermal crystallization,but it is much lower than that of composite with 1wt%Talc and 0.5wt%TMC-300.According to the isothermal crystallization kinetic analysis,WF can also reduce the half-crystallization time of PLA matrix,and improve the crystallization rate.The half-crystallization time under isothermal crystallization at 110℃is reduced from 23.6 min(neat PLA)to 7.2,2.7 and 1.4 min when adding 1wt%WF,1wt%Talc and 0.5wt%TMC-300,respectively.Hot-stage polarized light microscope observation shows that the crystal morphology of PLA induced by various nucleating agents is different during 110℃isothermal crystallization.WF and Talc provide a large number of nucleation sites for PLA crystallization,which promotes the grain refinement of PLA.TMC-300 induces PLA to form fibrous bundle-like crystals accompanied with higher crystallization rate,which is consistent with isothermal crystallization kinetic analysis results.The SEM observation of impact facture morphology after etching treatment indicates that the accumulation of crystals with different morphologies is one reason for the difference of mechanical properties.Wide angle X-diffraction analysis shows that all the three types of nucleating agents can promote the generation of orderlyα-crystal.The diffraction peak intensity ofα(110)/(200)crystals is highest when adding WF.Besides,WF can significantly decrease the crystal size of PLA.This study demonstrates that poplar wood fiber can be used as a bio-nucleating agent for PLA,which plays dual effect of reinforcement and nucleation.This study provides a basis for optimizing the nucleation ability of WF for PLA,and also provides references for further promoting the green development of wood-plastic composite.