| Poly(ethylene terephthalate) (PET) is a kind of thermoplastic aromatic polyesters with excellent properties. Lightweight of PET materials is necessary in order to reduce the cost and save resources, and foaming is the main method to realize this object. The melt strength and melt foamability of PET could be improved by decreasing the temperature, or increasing the molecular weight together with introducing long chain branch to backbone. This work focused on the effective preparation of foamable PET as well as optimal design and regulation of PET foaming process with supercritical CO2 as blowing agent, including supercritical CO2 assisted PET polycondensation, supercritical CO2 foaming solid state PET, and supercritical CO2 foaming melt state PET. PET polycondensation assisted by supercritical CO2, PET solid state foaming process, and PET melt foaming process. This work was aimed at facilitating the diffusion of volatile by-products in PET matrix during polycondensation process and increasing the molecular weight of PET via taking full advantage of the plasticization effect of supercritical CO2, shortening the CO2 saturation period in PET solid state foaming process and increasing the expansion ratio of PET foams by means of developing a new CO2 saturation method which could limit the induced crystallization effect of CO2, and determining as well as broadening the melt foaming temperature window of PET based on its rheological and crystallization properties.The PET polycondensation in both melt and solid states were conducted with high pressure CO2 and ambient pressure N2 sweeping, revealing that only the solid state polycondensation (SSP) of PET could be facilitated by high pressure CO2 due to the substantial increase of free volume and no detectable increase of interior diffusion path of volatile by-products at the lower polycondensation temperature. With the comparison of SSP processes of PET in dynamic and static CO2 supplying modes, a periodical supercritical CO2 renewing strategy for SSP of PET was proposed. The SSP process was promoted significantly and only six hours were required from the degree of polymerization of 100 to 150, while it takes nearly twenty hours in the industrial SSP process of PET. The effects of CO2 renewing period, CO2 pressure, reaction temperature, and initial molecular weight on the newly proposed SSP strategy were investigated systematically, and a semi-empirical kinetic model was applied to fit the experimental data.Crystalline PET foams with higher expansion ratio were prepared by the newly-designed solid state foaming process based on periodical CO2-renewing saturation strategy, which could limit the induced crystallization and thus increase the diffusivity as well as concentration of CO2 in PET matrix. As a result, the CO2 saturation period was shortened and the expansion ratio was raised. The PET saturated by the periodical CO2-renewing strategy was foamed at 100℃, and PET foams with the cell diameter between 5-22μm, the cell density between 2.42×108-2.93×109 cells/cm3, and expansion ratio betweenn 3-6 were prepared. Through the annealing of theses PET foams at 110-130℃, the crystallinity of PET foams increased to over 30%. The cell morphology was still well kept during the annealing process.PETs with different intrinsic viscosities, melt elasticities, and non-isothermal crystallization properties were prepared by reactive extrusion with pyromellitic dianhydride (PMDA). Rheological properties of PETs were measured to characterize its molecular structure evolution and viscoelasticity, which determined the highest melt foaming temperature. The non-isothermal crystallization from melt under compressed CO2 were characterized, and the onset crystallization temperature determined the lowest melt foaming temperature. And the rate of non-isothermal crystallization from melt related to cell stabilization. Based on the batch foaming process, it was confirmed that the melt foamability of PET was controlled by its melt elasticity and non-isothermal crystallization behavior under compressed CO2. PET modified by 0.5wt% with intrinsic viscosity of 0.89dL/g and 0.8wt% PMDA with intrinsic viscosity of 1.36dL/g got the melt foaming temperature windows of 40℃ and 70℃, respectively. PET foams with the expansion ratio between 10-50 times, the cell diameter between 15-37 μm and the cell density between 6.2 x 108-1.6 x 109 cells/cm3 could be controllably produced.In order to improve the melt foamability of PET, the layered nanoclay was introduced to PET matrix. PET/clay nanocomposites with different intrinsic viscosities, viscoelasticities, and crystallization properties were prepared by extrusion blending. PMDA was extruded together with PET and clay to introduce long chain branch to PET backbone and delaminate the clay layers. The melt foaming temperature window of PET/clay nanocomposites was also determined by their viscoelastic properties and non-isothermal crystallization properties under high pressure CO2. Although the molecular weights and viscoelastic properties of nanocomposites were much lower than those of foamable PMDA-modified PET, the melt foamability of nanocomposites was significantly improved by the well-dispersed clays due to the heterogeneous nucleation effect, enhanced non-isothermal crystallization rate, and so on. Foaming temperature windows of 20-60℃ were explored for PET/clay nanocomposites with intrinsic viscosity from 0.67 to 0.94 dL/g, in which nanocomposites foams with cell diameter between 29-53 μm, cell density between 6.5×107-6.9×108 cells/cm3, and expansion ratio between 10-50 times could be produced. |
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