Page 362 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.42 CHAPTER 5

5.6.3.3.7 Why So Many Azo Compounds? Theoretically, the gas must be released at
a critical melt viscosity that permits cell expansion but is strong enough to prevent burst-
ing of cell walls. Each polymer and each process reaches this critical viscosity at a differ-
ent temperature.

5.6.3.3.8 Concentrations Used in Processing. The amount of chemical foaming
agent used depends mainly on the process (see Table 5.31).

TABLE 5.31 Concentrations of Foaming Agents Used in Processing

Process %
Soft plastisol 2–4
Extrusion of structural foam 0.2–1.0

Injection molding of structural foam 0.3–0.5
Elimination of sink marks in injection molding 0.05–0.1

5.6.4 Combinations of Foaming Agents
Processors have always had some interest in using combinations of foaming agents, such
as combinations of volatile hydrocarbons or combinations of fluorocarbons. The effort to
replace fluorocarbons has led to much greater study of such combinations, between classes
as well as within classes of foaming agents. Typical combinations are as follows:
• Air + isocyanate/water
• Hydrocarbon + carbon dioxide
• Volatile liquid + chemical foaming agent

5.7 FLAME RETARDANTS

Conventional materials (wood, fabric, paper, paint) burn. In a fire, steel melts, and con-
crete crumbles. This causes much damage to property and loss of life. People have had to
cope with this for thousands of years, and they simply accept it. When new synthetic poly-
mers burn in a fire, people are very upset, and they demand that we remedy the problem.
Aside from simple burning, fires produce several related problems that may be even
more serious. When fires produce smoke, people caught in the fire may be unable to see
the escape route. Indoor fires in a limited air supply produce carbon monoxide, and fires
involving halogens will produce halogen and hydrogen halide gases, whose toxicity may
rob people of the will to escape. Sophisticated electronic equipment can be corroded by
acid combustion gases, resulting in huge replacement costs. Thermoplastic ceilings may
melt and drip hot molten burning plastic on firemen trying to fight the fire.

5.7.1 Testing
Testing is a major problem. Many experts consider all laboratory-scale tests to be unrealis-
tic and irrelevant. Large-scale tests require larger batches of material than are available in

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