Foaming Of The Polylactic Acid Blends With Supercritical Carbon Dioxide

The method of producing highly porous3D scaffolds through foaming biodegradablepolymers using supercritical CO2, which could avoid using the biological poisonousdissolvent, has become a research focus among scholars over the world. Because thescaffolds prepared by this method have a low interconnection, there is a main researchdirection to improving the interconnection. To resolve the problem above, in this paper, weproposed a method to make PLA foams which has a interconnected bubble structure byincorporating a water soluble polymer(PVOH, polyvinyl alcohol) into the biodegradablepolymer PLA.In this paper, a triple-screw extruder was successfully used to compound PLA andthermally sensitive polymer PVOH, with a processing temperature175℃(below the meltingpoint of PVOH192℃) to avoid the thermal degradation of PVOH. It found that PVOH wasmixed with PLA in solid form with high content(40wt%), small size(The dispersed phaseaverage size2.7μm) and high uniformity to meet the requirements of foaming preparedmaterials. The crystallization, melt strength and mechanical properties of the mixture ofPLA and PVOH were investigated. PVOH has similar effects with inorganic fillers addedinto PLAwhich can promote thecrystallization of PLA matrix. The crystallinity of PLA andmelt strength increased by148.72%and by141.10%, however tensile strength andelongation at break decreased by8.64%and24.83%respectively.PLA/PVOH foams with high cell density(1.5×10~9cell/cm~3), small size(average bubblediameter9μm), high PVOH content(30wt%) were fabricated in the high pressure vesselwith supercritical CO2.The effect of saturation pressure, soaking time, foaming temperature,cooling speed and the content of PVOH on the foaming of the blends were investigated. Theresults showed that PVOH existed in small solid particle among the setted temperaturerange, playing a role of foam nucleating agent, thus increased the bubble density of PLA/PVOH foams. The foams were post-processed using a method of combiningglycerine/solution soaking and ultrasonic vibration technology, which successfully washedthe PVOH dispersed phase embedded in the wall of the bubble out. Porous PLA materials with a content of interconnection were fabricated, which could become an effective way toproducing porous PLA scaffolds in the future.