K. U. Kim, B. C. Kim, S. M. Hong, S. K. Park
May 27, 2013
Rheological and processing studies have been performed on the extrusion of neat and expandable polyvinylchloride (PVC) compounds. Processing additives greatly change both rheological and physical properties. In the normal PVC processing temperature range, the neat compound exhibits very small values of power-law index, which increase with temperature. The usual usage level of dioctylphthalate (DOP), a plasticizer, and Acryloid K-125, a processing aid based on polymethylmethacrylate, increases the power-law index, whereas BTA, an impact modifier based on terpolymer of methylmethacrylate-butadiene-styrene, decreases it. DOP normally decreases viscosity and first normal stress difference. On the other hand, Acryloid K-125 and BTA increase them. Plotting first normal stress difference against shear stress, however, eliminates the effect of DOP and BTA level. Inclusion of a blowing agent increases the powerlaw index, and produces a substantial reduction in viscosity. At high shear rates, all the viscosity data of the expandable PVC compounds fall onto the single viscosity curve of the neat PVC compound. Two blowing agents, azodicarbonamide (ADCA) and sodium bicarbonate (SBC), yield similar reduction of the torque on a screw, but ADCA brings about much larger viscosity reduction than SBC. Further, ADCA shows more rapid bubble expansion rate on leaving the die exit and produces more uniform cells. On the other hand, SBC requires induction time for bubble expansion after leaving the die exit and gives a wider cell size distribution. Particularly in the foaming process with SBC, skin formation is considerably promoted by incorporating small quantities of DOP along with BTA, suggesting a strong potential for manufacturing rigid PVC structural foams with thicker skins by a single extrusion process.