Study On The Effects Of Composite Nucleating Agent On Crystallization Behaviors And Kinetics Of Poly (Lactic Acid)

Poly lactide(PLA) is a kind of bio-degradable plastic with a wide prospect,however, the current applications of PLA is limited due to its low ability ofcrystallization. Based on this, the crystallization behaviors and kinetics of PLAaffected by a new sort of composite nucleating agent(CNA) were studied in this paper.Modified lignin(MZS-B) and activated bentonite(AB) were chosen and compoundedin10different proportions, then added into PLA matrix, therefore10kinds ofPLA/composite nucleating agents samples were prepared. Differential ScanningCalorimetry(DSC), Polarizing Microscope(POM) and Mechanical Properties Testswere used respectively to study the effects of these CNAs on the crystallization andmechanical properties of PLA and discussed the connection between them. Hereafter,samples with typical results were chosen for further studies including isothermaland non-isothermal crystallization behaviors of the melt-crystallization and cold-crystallization process, and the mechanisms and kinetics were studied.DSC results showed that the presence of the CNAs may improve the crystallinityand crystallization rate of PLA. The heating curves showed that adding the CNAs ledto an elevation about2-10%of the crystallinity of PLA, meanwhile, the coolingcurves showed that the presence of the CNAs may increase the area of the exothermicpeaks. These results indicated that the presence of the CNAs may improve thecrystallization ability and enhance the crystallization rate of PLA significantly,however, the range of the distribution of the PLA crystal structure was widensimultaneously. POM results showed the Maltese Cross of the samples, indicating thatPLA crystals are spherulites. The presence of the CNAs may increase the nucleationdensity, leading to the increasing of the spherulites, however, the size of thespherulites decreased and the structure became incomplete. Mechanical test resultsshowed that the presence of the CNAs made the tensile strength of PLA improved, butshowed no evident patterns. Meanwhile, the tensile modulus and the impact strengthdecreased.The studies of isothermal crystallization kinetics showed that adding CNAs ledto s significant enhancement of the rate of melt-crystallization and cool-crystallizationof PLA, but had no evident effects of the crystallization mechanism. During the earlyperiods, the crystallization behaviors showed an linear growth described as theAvrami model, while the late behaviors showed a significant deviation, indicating a more complicated mechanism. Furthermore, the heating curves of the samples afterthe experiences of cool-crystallization with different temperature was studied, whichshowed double melting peaks, i.e the melt-recrystallization phenomena. The studiesof non-isothermal crystallization kinetics showed that during the melt-crystallizationand cool-crystallization periods, the presence of CNAs may increase the area of theexothermic peaks. During the early periods, the crystallization behaviors showed anlinear growth matched the Avrami model, while the late behaviors showed asignificant deviation and fluctuation with no significant patterns, indicating a morecomplicated mechanism. Meanwhile, the presence of CNAs led to wider ranges ofexothermic peaks and double exothermic peaks phenomena, indicated that thestructure of the PLA crystals became incomplete. Another, for the cool-crystallizationprocess, with the increasing of the heating rates, double melting peaks were observed,i.e the melt-recrystallization phenomena.