| Poly(L-lactic acid) foam shows great biodegradability, biocompatibility, mechanical property and has wide application in biological medicine, food packaging. However, PLLA belongs to crystalline polymer which has poor melt strength provided by the manufacturer. Therefore, the instability during the foaming process brings about mass cell collapse and mergence as PLLA foam can’t withstand the stable cell structure. The traditional foaming processes always lead to the problems such as large cellular size, uneven distribution of cells, foaming agent residues. The goal of this experiment is to prepare uniform distribution cellular poly(L-lactic acid) foam by environmentally friendly supercritical CO2foaming technology combined with simple and reliable irradiation crosslinking technology. The experiment is composed of the following three parts:(1)Preparation and Characterization of60Co-y irradiation modification PLLA. Two different irradiation dose PLLA with different content of polyfunctional monomer TAIC and TMPTMA via60Co-y irradiation method were prepared. Viscosity-average molecular weight, melt flow rate, gel content, differential scanning calorimeter analysis indicated that polyfunctional monomer TAIC and TMPTMA enhanced the irradiation crosslinking of PLLA effectively, thus improving the melt strength. In addition, the sensitization effect of TAIC was better than that of TMPTMA, PLLA with TAIC was more conducive to foaming process. Different polyfunctional monomer and irradiation dose did influence the crystallization behavior of PLLA.(2)Study of batch foaming apparatus via supercritical CO2. The lab apparatus for batch foaming was developed by conventional design method, including the design of moulding chamber, supercritical CO2delivery mechanism, temperature controlling monitor system, pressure measurement system, et al. To achieve the goal of studying foamig process conditions effect on the cellular microstructure, the technological parameters e.g. foaming temperature, saturation pressure, saturation time through rapid depressurization method was discussed and analyzed. (3)Study of PLLA foaming technological parameters via supercritical CO2process. Foaming process and technological parameters such as foaming temperature, saturation pressure, saturation time together with water absorption capability and degradation performance of PLLA foam were studied and analyzed. The apparent density revealed the trend first decreased then increased with the increased irradiation dose, and reached the minimum when irradiation dose was10kGy. The apparent density showed different variation with foaming temperature, saturation pressure, saturation time. The technological parameters have great impact on the cellular microstructure of PLLA foam. The cellular size and porosity increased and cell density and apparent density decreased with the increased foaming temperature. The comprehensive mechanics performance decreased sharply. However, the saturation pressure showed the reversed trend, and the cell distribution was more uniform with fine mechanics properties. As the saturation time increasing, the cellular mergence phenomenon occupied the most and cellular wall became thinner, which cause the increased cellular size and decreased cell density with the tensile strength, compressive strength and impact strength keep decreasing. The water absorption of PLLA foam first increased then decreased with the increased irradiation dose, and the higher the foaming temperature, the greater the water absorption. The buried degradation method indicated that the degradation process of PLLA foam was rather slow under the natural environment. The technological condition and irradiation dose did’t showed any regular variation with the degradation ratio. |
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