TABLE OF CONTENTS Blowing agents are solid materials that decompose to release gases at particular temperatures matching the appropriate melt viscosity necessary to retain a foam structure. There are two main types: (1) azodicarbonamide (H 2NCON=NCONH 2), which is exothermic in nature and decomposes at ~215 C to give primarily nitrogen gas, but also NH 3, CO, and CO 2, and (2) sodium bicarbonate (NaHCO 3), which is endothermic and so absorbs heat as it releases carbon dioxide gas over a wide temperature range, which coincides with the processing temperature window of PVC-U foam. Various grades are available to suit particular applications and both have well-controlled particle size distribution.
The azo compounds, particularly suitable for PVC-P formulations for flooring, wall covering, leather cloth, and so on, must be specially formulated to include an activation agent, which catalyses the earlier decomposition, and so increases the total gas evolved at a lower temperature to suit the melt viscosity. Occupational exposure to azodicarbonamide dust should be avoided and granular forms improve handling. Plasticiser damped versions are also available. The foam formation depends on the gelation rate and the rheological characteristics of the gelled/fused plastisol, the rate of gas formation during the gelation phase, and the rate of heat transfer during the foaming of the plastisol. Food contact approval (sealing gaskets) for azodicarbonamide (ACDM) is being withdrawn by the EC. This is based on the presence (below 25 parts per billion ppb) of semicarbazide, a decomposition product, of...
