Page 56 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

1.42 CHAPTER 1

TABLE 1.10 Common Chemical Blowing Agents 56
Decomposition Gas yield,
Blowing agent temp., °C ml/g Polymer applications
Azodicarbonamide 205–215 220 PVC, PE, PP, PS, ABS,
PA

Modified azodicarbonamide 155–220 150-220 PVC, PE, PP, EVA, PS,
ABS
4,4’-Oxybis(benzene- 150–160 125 PE, PVC, EVA
sulfohydrazide)
Diphenylsulfone-3,3’- 155 110 PVC, PE, EVA
disulfohydrazide
Trihydrazinotriazine 275 225 ABS, PE, PP, PA
p-Toluylenesulfonyl 228–235 140 ABS, PE, PP, PA, PS
semicarbazide

5-Phenyltetrazole 240–250 190 ABS, PPE, PC, PA,
PBT, LCP
Isatoic anhydride 210–225 115 PS, ABS, PA, PPE, PBT,
PC

A wide range of thermoplastics can be converted into foams. Some of the most com-
mon materials include polyurethanes, polystyrene, and polyethylene. Polyurethanes are a
popular and versatile material for the production of foams and may be foamed by either
physical or chemical methods. In the physical reaction, an inert low-boiling chemical is
added to the mixture, which volatilizes as a result of the heat produced from the exother-
mic chemical reaction to produce the polyurethane (reaction of isocyanate and diol).
Chemical foaming can be done through the reaction of the isocyanate groups with water to
produce carbamic acid, which decomposes to an amine and carbon dioxide gas. 42
43
Rigid polyurethane foams can be formed by pour, spray, and froth. Liquid polyure-
thane is poured into a cavity and allowed to expand in the pour process. In the spray
method, heated two-component spray guns are used to apply the foam. This method is
suitable for application in the field. The froth technique is similar to the pour technique ex-
cept that the polyurethane is partially expanded before molding. A two-step expansion is
used for this method using a low-boiling agent for preparation of the froth and a second
higher-boiling agent for expansion once the mold is filled.
Polyurethane foams can also be produced by reaction injection molding or RIM. 44
This process combines low-molecular-weight isocyanate and polyol, which are accurately
metered into the mixing chamber and then injected into the mold. The resulting structure
consists of a solid skin and a foamed core.
45
Polystyrene foams are typically considered either as extruded or expanded bead. Ex-
truded polystyrene foam is produced by extrusion of polystyrene containing a blowing
agent and allowing the material to expand into a closed cell foam. This product is used ex-
tensively as thermal insulation. Molded expanded polystyrene is produced by exposing
46
polystyrene beads containing a blowing agent to heat. If the shape is to be used as loose-

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