A Study Based On The α And β Nucleating Agents Inducing Crystallization Of PLA

PLA is a low-carbon and environment-friendly biodegradable plastics. However, it also has many drawbacks, such as low elongation at break, poor toughness, and poor thermal stability. In general, it is a very effective way to regulate the crystallization of PLA, and then improve its properties though adding nucleating agents. In this thesis, we firstly fabricated the PLA composites by melt blending PLA with WBG-II, TMB-5000 and fine PPZN, then investigated the influence of WBG-II, TMB-5000 and fine PPZN on the microstructure,crystallization behavior and other properties of PLA.Firstly, the influence of WBG-II and TMB-5000 on the crystal structure, crystal morphology and crystallization behavior of PLA was studied. The results showed that the WBG-II and TMB-5000 not only can accelerate the nucleation rate and increase the crystallinity, but also can decrease the spherulite size of PLA. The nucleation of WBG-II is more effective, the t1/2 value of PLA/WBG-II composites after isothermal crystallization at110 °C was only 1.73 min. Further, the light transmittance of PLA/WBG-II was 10.3%higher than PLA/TMB-5000, which is due to the smaller nuclei and spherulite size of PLA/WBG-II. In addition, the incorporation of WBG-II and TMB-5000 induced the orientation of PLA molecular chain, and then formed β crystal. In general, β structure is somewhat less stable than α structure corresponding to a certain degree of disorder in the crystals.Secondly, we investigated the effect of WBG-II concentration on the crystal structure,crystal morphology and crystallization behavior of PLA, and further analyzed its non-isothermal crystallization kinetics. XRD results indicated that it helps to improve the proportion of β crystal in PLA though increasing the content of WBG-II, however, the proportion of β crystal was not linear increment with the increasing content, but existed an optimum concentration. DSC analysis revealed that the Xc and Tmc values increased with increasing content when the content of WBG-II is less than 0.25%, then decreased a little when the content is more than 0.25% and gradually levelled off at higher content. This again illustrates that it has reached a saturation concentration when the content is 0.25%. The optimum crystallization temperature was about 120 °C for PLA/WBG-II0.25. Besides, it will lead to the transformation of α crystal to β crystal if increasing the annealing time.Non-isothermal crystallization kinetics and POM analysis indicated the WBG-II can providea large number of nuclei, which greatly accelerated crystallization rate and thus accelerated the overall crystallization process. At the same time, the process of crystal growth and final crystal morphology of PLA had also undergone a significant change.Finally, the change of crystallinity, heat resistance, transparency and mechanical properties of neat PLA, PLA/PPZN/plasticizer A and PLA/composites containing organic nuclei after annealing for different time was studied. We can discover that the crystallization rate became faster, and the spherulites became much smaller in nucleated PLA. The crystallinity of PLA/PPZN/plasticizer A and PLA/composites containing organic nuclei after annealing at 80 °C for 20 s respectively reached 22.3% and 25.3%, and increased with the increasing annealing time. It is because of the increase of crystallinity of PLA/PPZN/plasticizer A and PLA/composites containing organic nuclei, thus improving their heat resistance. Their HDT after annealing at 80 °C for 60 s were separately increased to 80.9 °C and 95.7 °C. Although their heat resistance was significantly improved, the light transmittance after annealing at 80 °C for 60 s were respectively reduced to 18.9% and29.6%. In addition, the toughness was improved when the plasticizer was added in PLA matrix, so that the elongation at break and notched impact strength were increased obviously.Overall, the PLA/composites containing organic nuclei not only showed higher heat resistance, elongation at break and notched impact strength, but also presented better light transmission properties.