Thermal Properties And Nonisothermal Crystallization Kinetics Of Polylactic Acid Composites

The current modification of polylactide(PLA)mainly focuses on petroleum-based fillers,which compromises the 100%degradability of PLA composites after end-of-life.Polyhydroxybutyrate pentanoic copolyester(PHBV),a fully degradable thermoplastic polyester derived from renewable resources,has a high degree of crystallinity and can be used as a crystallization promoter of PLA.However PHBV has some drawbacks,such as high production costs.In this case,filling low-cost natural cellulose fibers(CF)into PLA and PLA/PHBV blends to produce 100%bio-based degradable composite materials can not only improve the performance of PLA and expand its application field,but also reduce the production costs and achieve good economic benefits.In this thesis,CF was firstly ball milled to obtain suitable fillers,and then different proportions of CF/PLA and CF/PLA/PHBV composites were prepared by non-solution method.Differential scanning calorimetry(DSC)was used to characterize the glass transition,crystallization and melting behavior of the composites;X-ray diffraction(XRD)was used to test the crystal structure of the composites;and polarizing microscope(POM)was used to characterize Observe the crystal morphology of the samples;thermogravimetric analyzer(TGA)was used to characterize the pyrolysis of composites.The CF/PLA and CF/PLA/PHBV composites prepared by CF with the best ball milling time were found.And its non-isothermal cold crystallization kinetics was discussed by using Jeziorny method to modify Avrami equation,finally got the following conclusions.(1)CF can effectively promote the crystallization of PLA and PLA/PHBV.With the addition of CF,the T_c of PLA and PLA/PHBV decreased,the peak width of crystals decreased,and the crystallinity increased,but no new crystal form was introduced.The samples all have a pure spherulite structure,and the spherulites have a black cross extinction phenomenon.The addition of CF increased the nucleation points of PLA and PLA/PHBV,and accelerated the crystal growth rate.The effect of CF milling for 10min on the crystallization promotion of PLA and PLA/PHBV was most obviously.The crystallization ability of CF/PLA increases with the increase of CF content,but the crystallinity of CF/PLA/PHBV ternary composites is highest when the CF content is13%.CF slightly reduces the thermal stability of PLA,but can cause PHBV to begin thermal decomposition at higher temperatures.Ball milling reduced the thermal stability and crystallization performance of CF.The longer the milling time,the worse the CF's crystallization and thermal stability.After adding 25%PHBV,the T_g and T_c of PLA/PHBV composites were reduced by 13°C and 19°C,respectively,as well as the crystallinity was increased by 16%.Both PLA and PLA/PHBV crystals have black cross-matting,and the nucleation density of the composite becomes larger,the crystal growth speed becomes faster,and a new crystalline form?form is generated;and the PHBV slightly reduced the thermal stability of PLA.(2)The non-isothermal cold crystallization kinetics of a CF(ball milling for 10min)and PLA blends was studied.It was found that CF can effectively promote the crystallization of PLA and make its crystallization less affected by the heating rate.The Avrami indices of PLA and CF/PLA composites were 3.22-5.61,2.43-3.56,2.41-3.66,and 2.18-3.81,respectively.The addition of CF reduced the change in n value.At the same heating rate,the k_a value of CF/PLA composites is larger than PLA,and the ka value increases with the increase of CF content.CF can increase the crystallization rate of PLA,shorten the crystallization time,enhance the crystallization ability.The nucleation effect of CF to PLA is greater,when the CF content is greater.(3)The non-isothermal cold crystallization behavior and kinetics of CF(ball milling for 10 min)and PLA/PHBV composites were studied.The addition of CF can improve the cold crystallization capability of PLA/PHBV,and render the PLA/PHBV crystallization be less affected by the heating rate.The Avrami indices of PLA/PHBV and CF/PLA/PHBV composites are 2.72-3.88,2.55-3.83,2.65-3.26,and 2.2.81-3.81,respectively.At the same heating rate,after adding CF,the k_a value of the ternary composite is larger than PLA/PHBV,under the same conditions,the k_a value of sample17 is the largest.The addition of CF accelerates the crystallization rate of PLA/PHBV,shortens the crystallization time,improves the crystal regularity.The nucleation effect of CF to PLA/PHBV is the most obvious,when the CF content reaches to 13%.