Pore Structure Control And Property Study On Polylactic Acid Microcellular Foams

Since supercritical fluid technology microcellular foaming was developed in 1980’s, it has been studied and developed extensively. With the improvement of the environmental protection consciousness of people and the development of green economy, researchers have been made microcellular foaming technology used in commercial production for injection molding, extrusion and hollow products. Meanwhile, researchers focus on the polymer materials with the features that less environmental pollution, lower energy consumption, biodegradable, and sources that can be found in nature world widely. Poly lactic acid(PLA) has became one of the hot research spots because of its advantages of green plant source and biodegradable. Polylactic acid foam materials can replace petroleum-based foam plastic and be used in packing and consumer goods areas. PLA is semicrystalline polymer, the crystallization rate is slow, its heat resistance is also very poor, melt strength is not high, foam shape is prone to foam collapse and merger during the foaming process. These are not conducive to the foam.In order to make a comprehensive and thorough research on the performance of PLA foam material, which is prepared with good pore structure of the PLA bubble form foam material, by adding bubble density, reducing the size of bubble holes and homogenization bubble size distribution of the holes. In this paper, PLA foamed samples were produced using homemade autoclave and molds through quick pressure relief methods. Firstly, the effect of microcellular foaming process conditions on cell structure was studied, at the same time, the mechanical properties and heat insulation property were also studied. It was established the relationship between performance and microcellular foaming process according to cell microstructure. The experimental results showed that average cell size were range of 6-280 μm by adjusting the foaming pressure. Compared to two kinds of PLA foamed samples with different brand, it was found that the cell size of PLA4032 D foamed samples was samller than that of PLA2003 D foamed samples. The compressive strength and compressive yield stress of the foaming material are increased and then decreased with increasing pressure. The tensile strength and elongation are also increased and then decreased with increasing the foaming pressure. Thermal conductivity decreased with increasing pressure, but at the high pressure, thermal conductivity increased instead. Since the polylactic acid plasticizing action of gas, crystallization improved under high pressure, the foaming temperature rises with the decrease of the degree of crystallinity, a larger size and higher regularity of a spherulite structure formed simultaneously. This would be detrimental to the formation of uniform cell structure, the solubility of the gas in a large area in spherulites depress, which occurs unfoamed area, but it may be appropriate to improve the crystallinity of the melt strength and reduce the cell size. The mechanical properties of studies have found that there is an optimum foaming temperature, compression strength at this temperature reaches a maximum. PLA foam sample increases with the foam space increases, cell orientation extent is larger, the cell walls in the growth direction stretches thin even rupture. The Compressive strength and tensile modulus increases with reducing foaming space.Dual cell structures with a bimodal pore distribution in overall plastic have an excellent performance, the experimental binary PLA prepared by two-step cell structure relief method, in the first step the pressure of the relief drop to a range with 2-8 MPa which can form binary cell structure.As the compression performance experiment shows, the compression strength of the foam decreases as the differential pressure of the first-step pressure relief increases. When rise the foam temperature, the mechanical property will be reduced as the appearrment of big cells merger and the cell structure burst. Also, the experiment compare the effect of three kinds of nanoparticle on the crystallization property of PLA, CNT has the best effect on the PLA crystallization, and the degree of crystallinity is high. On the plasticization of the high pressure saturated gas, the crystallinity of the PLA/CNT nanocomposite is the highest when 80 oC, and the spherulite is big, more regular, because the crystalline region cann’t absorb gas and cann’t foam. When the foam temperature is 100 oC, the crystalline region become small and uniform, causing the smaller cell and higher cell density. When the foam temperature is 120 oC, although the crystalline region has the heterogeneous nucleation effect, the cell will bigger because the melt viscosity decreases.At last, we studied the foam performance of PLA/PBS blend system, controlled the cell morphology by changing the percentage and foam temperature, because the foam temperature is closed to the melting point of PBS, the lower melting point PBS has effective effect on increasing the cell persistence ration, when the PBS is 20wt%, the cell persistence ration of the blend system is highest(96.2%), then propose the closed melting point cell open theory, and use it to the preparation of Bimodal distribution cell, then get the highly connected biodegradable dual cell structures whose persistence ration is up to 96%.