Zhenhao Xi

Extrusion foaming of poly(ethylene terephthalate) with carbon dioxide based on rheology analysis

Although low density poly(ethylene terephthalate) foams take advantage of the combination of good performances and lightness and have a wide range of engineering applications, extrusion foaming of the poly(ethylene terephthalate) is still faced with many challenges, especially the inadequate rheological characteristics. In this work, the melt strength of four types of poly(ethylene terephthalate) was compared on the basis of the analysis on the dynamic complex viscosity, storage and loss modulus, relaxation spectrum, loss factor and extensional viscosity using rheometer. It was found that the two long chain branching poly(ethylene terephthalates) owned overall higher melt viscosity and greater elasticity than the other two linear ones. Furthermore, only one long chain branching poly(ethylene terephthalate), i.e. PET4, succeeded in the extensional viscosity characterization and showed the phenomenon of strain hardening. Thereafter, PET4 was successfully applied for the extrusion foaming with CO2 as the blowing agent. The effects of the foaming temperature, screw revolution speed, gas input and addition of nucleation agent were carefully investigated on the extrusion foaming and the resulted poly(ethylene terephthalate) foams properties. Under the optimization condition, the extruded poly(ethylene terephthalate) foams were produced with the volume expansion ratio of 9.5, average cell size of 265 µm, cell density of 4.58 × 105 cells/cm3 and a narrow cell size distribution.

Effect of pre-curing process on epoxy resin foaming using carbon dioxide as blowing agent:

A thermosetting epoxy resin system consisting of diglycidylether of bisphenol A (DGEBA) and m-xylylenediamine (MXDA) was successfully foamed by carbon dioxide (CO2) using two-step batch process. Isothermal curing kinetics of epoxy system was developed to help control the pre-curing degree of resin under different pre-curing conditions. Samples with different pre-curing degrees were prepared and then foamed via temperature-rising foaming process. It was found that the pre-curing degree was a crucial index for the foamability of epoxy resin. The effects of pre-curing conditions on curing reaction as well as further foaming results were investigated, and the results showed that the pre-curing degree from 37.7% to 46.3% was the proper foaming range for the chosen epoxy resin. With increasing pre-curing degrees from 37.7% to 51.6%, viscosity and elasticity of pre-cured resins increased, and correspondingly, average cell size of epoxy foams decreased from 329.8 µm to 60.8 µm while cell density increased from 1.4 × 105 cells/cm3 to 8.6 × 105 cells/cm3. Furthermore, the foamed samples with the same pre-curing degree had similar cell morphology regardless of pre-curing conditions.

Cell characteristics of epoxy resin foamed by step temperature-rising process using supercritical carbon dioxide as blowing agent

A temperature-rising batch foaming process with supercritical carbon dioxide (ScCO2) as blowing agent was used to prepare epoxy resin foams consisting of diglycidyl ether of bisphenol A and m-xylylenediamine. The dissolution and diffusion behaviors of CO2 in pre-cured epoxy resin were investigated, as well as the parameter effect of CO2 saturation step and foaming step on the cell characteristics. It was proved that closed-cells could be generated for CO2 unsaturated samples and the cell characteristics with the same dissolved CO2 concentration were similar. The merged and cracked bubble morphologies were usually obtained for CO2-saturated epoxy resin samples. With increasing CO2 concentration from 0.021 g CO2/g epoxy resin to 0.061 g CO2/g epoxy resin in the unsaturated samples, the cell size increased from 170.2 µm to 262.6 µm and the cell density decreased from 6.8 × 105/cm3 to 3.1 × 105/cm3. Bubble nucleation and growth occurred simultaneously with curing reaction in temperature-rising step. As the final foaming temperature increased from 60℃ to 120℃, the cell size of samples with dissolved CO2 concentration of 0.021 g CO2/g epoxy resin increased from 172.7 µm to 369.0 µm, while the cell density first increased from 6.8 to 7.3 and then decreased to 3.5. The cell size of samples with CO2 concentration of 0.031 g CO2/g epoxy resin increased from 145.3 µm to 180.5 µm with foaming time from 5 min to 20 min, but changed slightly when curing reaction almost finished and CO2 was depleted after 20 min.