The term "low density foam" generally is applied to a cellular product which has a small cell structure and which has a density of 20 lbs/ft.sup.3 (0.32 g/cm.sup.3) or less.
Although low density foams can be produced from a variety of polymer compositions, chlorinated polyvinyl chloride polymers impart desirable properties to a low density foam that are not normally found in foams made from other types of polymers. Polyvinyl chloride homopolymers, however, normally will not produce satisfactory low density foams since they usually contain about 5 to 10 percent crystallinity. Such crystallites do not melt completely below about 215.degree. C. Because of the poor thermal stability of polyvinyl chloride homopolymers and other problems relating to the polymer physics of the system at processing temperatures of about 215.degree. C., it is not feasible to process the polymer at the melting temperature of the crystallites. If one attempts to extrude the polyvinyl chloride homopolymer at temperatures below 215.degree. C. to produce low density foam, a foam of poor quality results, since the crystallites act as physical crosslinks which restrict the growth of cells and cause a high density foam to be formed. Chlorinated polyvinyl chloride homopolymers, however, can be formed essentially free of crystallites and, as a result, are not subject to the aforesaid problem.
U.S. Pat. No. 3,366,580 to Kraemer, et al., the disclosure thereof being incorporated herein by reference, teaches that a foam product can be produced from post-chlorinated polyvinyl chloride resin. In accordance with the process described therein, a polyvinyl chloride polymer that has been post-chlorinated to a chlorine content of at least 60%, is impregnated with a halogenated hydrocarbon blowing agent (for example, trichloromonofluoromethane) and subsequently expanded into a rigid, predominantly closed-cell foam. The closed-cell structure of the foam contributes to the physical strength of the foam and enables the hydrocarbon blowing agent to become entrapped within the closed cells of the foam.
When a foam is intended to be used as a structural or insulation product, a low density foam consisting largely of closed cells of very small size normally would be preferred. Cellular structures with closed cells of less than 500 microns, desirably less than 350 microns, in diameter are desirable.
The use of a "nucleating agent" allows the formation of an increased percentage of closed cells in a foam and, thus, results in a better quality low density foam product. A nucleating agent in a thermoplastic polymer creates minute cell sites into which blowing agent gases can expand. The nucleating agent does not function as the blowing agent for the foam composition. Instead, a primary blowing agent is used in conjunction with the nucleating system. Nucleating agents may be finely-divided inorganic solids, a compound which decomposes to form nuclei cell sites within the thermoplastic polymer mass when heated to a temperature below that at which the polymer is objectionably affected, or a combination of materials which react in situ within the thermoplastic polymer mass to produce the desired nuclei cell sites. Examples of inorganic solids which have been described as suitable for use as nucleating agents are silica, talc, alumina, diatomaceous earth, calcium carbonate, calcium sulfate, and alkali metal or aluminum silicates. Examples of compounds which decompose or react together to form cell sites at acceptable temperatures are mixtures of alkali metal bicarbonates and solid organic acids such as citric acid or oxalic acid, and hydrated salts such as hydrated sodium citrate. A mixture of finely-divided silica aerogel, citric acid and sodium bicarbonate has been suggested as a nucleating agent that performs particularly well. While the aforementioned nucleating agents are reported to increase the number of closed cells produced, they have not been found to produce foams of the desired uniform small cell size in chlorinated polyvinyl chloride polymer foams (as will be illustrated hereinafter).
In the copending application of Nehmey and Summers (Ser. No. 301,906), filed Sept. 14, 1981), the disclosure thereof being incorporated herein by reference, a process is disclosed for producing a low density chlorinated polyvinyl chloride polymer foam using a halogenated hydrocarbon with 1 to 3 carbon atoms as a primary blowing agent and azodicarbonamide as a nucleating agent. The resulting foam is characterized as having a closed-cell structure, a cell size diameter of less than 500 microns and a density of less than 20 lbs/ft.sup.3 (0.32 g/cm.sup.3). A low density foam possessing such properties would have a low thermal conductivity and be particularly useful in insulating applications.
While a low density chlorinated polyvinyl chloride foam having many excellent foam characteristics is produced when azodicarbonamide is used as a nucleating agent, the use of azodicarbonamide does reduce the dynamic thermal stability of the foam, and, as a consequence, requires the use of larger quantities of stabilizer to achieve the desired level of thermal stability.