Study On Tunable Cell Structure Of Polystyrene Foam Using Upercritical Carbon Dioxide As Blowing Agent

Foaming polystyrene (PS) materials with excellent heat insulation, shock absorption, electrical insulation, and correction resisting, has drawn many attention in recent years. Normally, foaming PS materials were prepared by using Freon gas as blowing agent; but, with the growing concern of environment, using CO2 as green blowing agent becomes a major trend. However, many new problems come out in this transitional phase, such as the low solubility of CO2 in PS matrix, and the high diffusion rate; this will lead to a poor cellular structure induced by using CO2 as blowing agent. Thus, how to control the cellular structure for CO2 foaming is a vital problem for foaming PS industry.In foaming process, cell nucleation is an important phase since it mainly decide the cellular structure. Nucleation agent for cell, such as inorganic matter, was usually used to increase the cell density. However, inorganic matter possessed a poor compatibility with polymer matrix, and it greatly compromise the mechanical properties of foaming materials. In this study, the crosslinking points and dispersion phase in PS blends was used as cell nucleation site to improve the cell density, thus to control the cellular structure. Moreover, the crosslinking and dispersion morphology of the interface will affect the solubility and diffusion rate of CO2 in PS matrix; consequently, the cellular structure was evenly affected. The major research aspects were as following:(1) PS/crosslinked-PS blends were prepared by dissolution-polymerization method. Various monomer contents were added to adjust the crosslinking number and distribution. Subsequently, the PS/crosslinked-PS blends were foamed by solid-state foaming. The research results revealed that the cellular structure had a close relationship with the crosslinking number and distribution. The melt strength of PS blends increased firstly with the increasing monomer content; then decreased. The crosslink point could act as heterogeneous cell nucleation site. When adding 24 phr St into blends, its cell density could reach to 1.6x108 cell/cm.(2) PS/PETG blends were prepared by melt-mixing method. And their foams were prepared by solid-state foaming method. A two-steps pressure depression process was carried out to control the cell nucleation and cell growth. The SEM results showed that the PS/PETG ratio, processing temperature, and shear filed obviously influenced the blending morphology. And a blend with high dispersion-phase-density and finer dispersion-phase-size-distribution could improve the cell density and cell size distribution. With a proper process condition, the dispersion phase density of PS/PETG blend could reach to 3.32×1011 cell/cm3; its cell density increased to 2.93×109 cell/cm3, and cell size decreased to 7.65 μm. A bi-modal cellular structure with big cells of 220.8 μm in diameter and small cells of 58.4 μm was also obtained by two-steps pressure depression method. The cell density for big cells and small cells were 6.27×105 cell/cm3 and 2.62×107 cell/cm3, respectively.(3) PS/PLA blends were prepared by melt-mixing method. Reactive compatibilizer was used to improve the dispersion of PLA in PS matrix. The results showed that reactive compatibilizer could increase the dispersion density and change the crystallization behavior of PLA. The crystal polymer in PS matrix could act as cell nucleation agent to induce heterogeneous cell nucleation. Besides, lamellar crystal of PLA would hinder the diffusion of CO2; thus prevent the cell collapse. With a proper controlling of crystallization of PLA in PS matrix, a fine cell structure of PS/PLA blends foams with cell density of 9.27×107 cell/cm3, cell size of 66.64 μm, and density of 0.045 g/cm3 could be obtained.(4) PS/crosslinking-PMA blends and PS/crosslinking P(St-MA) blends were prepared by dissolution-polymerization method to study the effect of blending morphology on cellular structure. The results showed that MA monomer had a poor compatibility with PS. High MA monomer content will lead to high local crosslinking density in blends, which cause a poor dispersion. While adding St as monomer for copolymerization, the dispersion of MA and AIBN in PS matrix could be greatly improved. Because the high activity of MA, the degradation of linear PS obviously be reduced. The inducing of MA in PS matrix also greatly increased the cell nucleation effect due to the high surface tension of PMA. Moreover, the CO2 solubility had been enhanced by inducing MA. Consequently, a cell structure of PS blends foams with cell density of 1.01x 108 cell/cm, cell size of 98 μn, and density of 0.026 g/cm3 could be obtained with adding 9.6 phr MA into PS.