Despite early experiments with sodium bicarbonate—ordinary baking soda—the era of foamed plastics had to await the results of German work on hydrazine-based rocket propellents during World War II. Information on these azo compounds leaked out to Uniroyal in the U.S., so that by 1950, both this company and Bayer in Germany were using exothermic blowing agents in rubber.
Uniroyal commercialized its blowing agents under the Celogen name. They were based on hydrazine, hydrazide, or azodicarbonamide. The last became one of the most widely used exothermic chemical blowing agents (CBAs) for plastics. One of its first big uses was PVC plastisol flooring in the late 1950s to early 1960s. Another, which developed in the mid-’60s, was low-density crosslinked LDPE foams. Applications included cushioning materials for car door interior panels as well as packaging and athletic equipment padding. Injection molded PS, ABS, and PP foams also originated in the mid-’60s using azo CBAs.
A major early extrusion application in the early to mid-1970s was telephone wire and cable insulation, primarily of foamed HDPE. In the mid-1970s, structural foam—HIPS, HDPE, PP, and ABS—began to become prominent. Initially, structural foam makers had used direct injection of nitrogen gas, but that required a costly license and specially modified equipment. The availability of chemical blowing agents allowed all molders, including smaller ones, to produce both small and large parts economically.
Use of CBAs for rotomolding foamed parts began in the early 1980s. About that time, interest in foamed TPEs for soft-touch applications took off. Activated azos with reduced initiation temperatures were among those used.
In 1984, CBAs of the endothermic variety came to the U.S. from Europe. The first was Hydrocerol from Germany’s Boehringer Ingelheim (now a product of Clariant Masterbatches.) Ironically, they were based on sodium bicarbonate plus citric acid. These products were less sensitive to loading levels and were said to produce finer cell structures and smoother surfaces. Also, endothermics were “cleaner,” producing only CO2 and water, and left no deposits on screws or molds. Faster degassing of parts was also claimed, and they contributed less color to foamed parts.