The Effect Of Annealing On Crystal And Heat Resistance Of Poly (Lactic Acid)

Recently, polylactic acid(PLA), a new biodegradable material, has drawn broad attention due to its superior properties, such as being plentiful, renewable, environmental friendly as well as only giving water and CO2 upon decomposition. PLA has been utilized in producing packing materials and medicinal-related materials, which results from its great transparency, biological compatibility and biodegradability. Although PLA possess the above advantages, it still shows certain drawbacks regarding its performance, such as the slow crystallization rate and poor heat resistance, as a consequence of preventing its further application. Researchers in this area have promoted the performance by adding the nucleating agents and adopting annealing in the process of PLA production, which, however, are still beyond the standards for PLA to be applied to the practical applications. Plus, annealing would also extend the producing cycle, which leads to more cost. In this article, a combined method will be discussed. We expect that applying the annealing to the nucleated PLA would enhance the crystallization rate and heat resistance of PLA. Meanwhile, the annealing temperature and time period would reduce by adding the nucleating agents.Modified lignin(ML) and organic bentonite(OBT) are chosen as the nucleating agents. PLA/ML and PLA/OBT mixtures are prepared as different percentages in composition through the melt blending method. The annealing processes are conducted on pure PLAs as well as nucleated PLAs under different annealing temperatures, times and mediums, which are monitored through Differential Scanning Calorimeter(DSC), Polarizing Microscope(POM), X-ray diffraction(XRD), Fourier Infrared Spectrometer(FTIR) and Thermal Deformation Temperature Meter. According to the tests performed, conclusions are as follows.POM pictures show that right-angled intersection extinction phenomenon occurs during thermally crystallization. Upon adding ML or OBT, the density of PLA nuclei materials increases, as well as the crystallization rate. Based on XRD results, ML and OBT are both efficient in promoting PLA crystallization without changing PLA crystal structures that possess a ? crystal form. Also, DSC characterization data show the maximum degree of crystallization happens when ML and OBT are at the percentage of 0.5% of the mass fraction, respectively.FTIR shows the molecular structure of PLA would not change upon annealing. However, annealing would increase the degree of crystallization for PLA, judging by the results from DSC and Thermal Deformation Temperature Meter, which, at the same time, can improve the heat resistance of PLA. In addition, adding the ML and OBT nucleating agents can also enhance the crystallization process and the heat resistance of PLA, and the annealing temperature and time can be decreased simultaneously. Further, annealing in dimethylsiloxan can achieve PLA crystallized at a relatively lower temperature and shorter crystallization time, and have an advanced heat resistance property, compared to those in the presence of air or water.