Continuous Extrusion Production of Microcellular Rigid PVC

This study examined the effect of Poly[vinyl chloride] (PVC) formulation on the cell morphology and density of rigid PVC foamed with supercritical CO2 in a continuous extrusion process. Cell morphology and the density of foamed samples were controlled by blending two acrylic-based processing aids (all-acrylic foam modifier K-400 and acrylic-based impact modifier KM-334), using a mixture design. The effect of blend ratios on the fusion and die swell behaviors of PVC was investigated by means of a torque rheometer and on a single-screw extrusion capillary rheometer, respectively. Fusion was promoted as the relative amount of the all-acrylic foam modifier increased in the blends. Similarly, the elastic constant of PVC, derived from the linear relationship between the die swell and apparent shear stress, increased upon increasing the relative amount of the all-acrylic foam modifier in the blends, thus suggesting an increase in the melt elasticity of PVC. Microcellular rigid PVC foams with densities of approximately 0.15 g/cm(3) and a tenfold volume expansion were produced. An optimum ratio of impact modifier to all-acrylic foam modifier of 1:3 was found to maximize the foam expansion. Using impact modifier alone or all-acrylic foam modifier alone yielded expansions considerably lower than that achieved with the 1:3 blend. The experimental results indicated that fusion is not the only criterion to control the cell morphology and density achieved in microcellular rigid PVC foams. The melt must have a viscosity low enough to allow bubble formation and growth, as well as elasticity high enough to prevent cell coalescence. J. VINYL ADDIT. TECHNOL., 15:211-218, 2009. (C) 2009 Society of Plastics Engineers