What happens in Foamed Polyvinyl Chloride?-1

Foamed or Microcellular Polyvinyl Chloride (M-PVC) is formed by a process called nucleation. Homogenous microcellular foams, with an average cell size of the order of 10 microns, can be produced in PVC by the process of nucleation.

In this process, gas is dissolved under high pressure into the liquefied U-PVC as it's undergoing the extrusion process. This causes the uniform creation of billions of tiny bubbles that later form cells of densities ranging from l07 to l09 cells/cc. Carbon dioxide is used as the nucleating gas. A majority of the cell growth was found to occur in the early stages of foaming. The bubble nucleation density has an Arrhenius-type dependence on temperature.

Additionally, in contrast to microcellular foams, structural PVC foams are typically characterized by a large variation in foam density across the foam thickness. Also, nucleation can occur at temperatures significantly below the original glass transition temperature, and that a microcellular structure can be achieved within the range of foaming temperatures. The nucleation density increases with increasing foaming temperature, but the average cell diameter remains fairly constant over the range of foaming temperatures. The small cell size and high cell density in microcellular foams provide the possibility to foam thin-wall parts in the 0.5 to 2 mm range.

Some of the cellular structure of extruded PVC foam is produced by the exothermic thermal decomposition of chemical foaming agent (CFA). Azobisformamide (ABFA) is the most popular CFA for PVC. Its decomposition is dependent on PVC formulation components and mainly the thermal stabilizers which are acting as kickers. For the prediction of the processibility of a foamed rigid PVC compound, the temperature of decomposition (Td) of the ABFA must be adjusted within the range of PVC processing temperature. Ideally, the ABFA will not produce any gas until after the PVC has fused and can form a melt seal. This prevents gas from escaping out of the extruder hopper during processing at an industrial scale. It is an exothermic CFA that generates heat during the decomposition process.

ABFA is modified to alter its Td and many such CFAs are available in the market. The exothermic chemical foaming agents are organic in nature and give off N2 gas when they decompose. They liberate more gas per gram of foaming agent than endothermic agents and higher gas pressure (around 220cc/gm). The decomposition temperature of exothermic foaming agents can also be raised or lowered by the addition of kickers. In PVC foam formulations, stabilizers perform a dual role not only do they prevent thermal degradation of the polymer but they may also activate the decomposition of the chemical blowing agents like azodicarbonamide (ADC). PVC stabilizers containing lead, cadmium, and zinc are the most efficient activators (kickers). These additives depress the Td of ABFA and increase its rate of decomposition. Liquid mixed metal stabilizers containing the same cations show increased activation due to their solubility in plasticizers. The activating effect of the basic metal salts is derived from the instability of the corresponding Azodicarboxylates they are formed at the first step in the decomposition of ABFA. Since, the Azodicarboxylates of lead, cadmium and zinc decompose at lower temperatures than the diamide and these initiate the thermal breakdown of ABFA. ABFA is the most commonly used foaming agent. The activation of ABFA by metal salts is concentration-dependent and the rate of ABFA decomposition increases as the amount of metal salt in the compound increases.