Page 360 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.40 CHAPTER 5

later postprocessing such as printing, decorating, and adhesive bonding. It has been sug-
gested that this is a problem mainly in dimethyl silicones and may be avoided by using
mixed methyl/aryl silicones.

5.6 FOAMING AGENTS

5.6.1 General Foaming Process
There are a great variety of foaming processes. They all involve these three basic steps:
1. Liquid state. This can be a polymer melt, low-MW thermosetting prepolymer, vinyl
plastisol, or latex.
2. Blow bubbles. This can be done with a permanent gas, hollow microspheres, a vol-
atile liquid, or a chemical reaction that produces a gas.
3. Solidify. A thermoplastic melt can be cooled, a thermosetting prepolymer can be
cured, a plastisol can be gelled, and a latex can be coagulated and/or evaporated.

5.6.2 Physical Foaming Agents

5.6.2.1 Permanent Gases. Air can be whipped into liquid plastisol or used to froth liq-
uid polyurethane formulations. In hot processing, where air could cause oxidative degra-
dation, nitrogen alone is used; typically, it is compressed into molten polymer during
molding or extrusion to make thermoplastic structural foam.
5.6.2.2 Hollow Microspheres. These may be glass or plastic (polyolefin, VDC/AN,
phenolic). They may contain air or a volatile liquid. They can be stirred into a liquid sys-
tem, such as epoxy, which is then cured. This is interesting for deep-sea and low-dielectric
products.

5.6.2.3 Volatile Liquids. These are chilled and mixed into the liquid polymer system at
room temperature. They provide temporary plasticization. Then, the heat of processing
volatilizes them, plasticization ceases, and the expanding gas bubbles produce the foam.

5.6.2.3.1 Pentane. Pentane (BP 34°C) is 10 percent soluble in polystyrene. It is low
in cost and nontoxic but somewhat flammable. Isopentanes, butanes, and hexanes, and
mixtures of these, are often used.
5.6.2.3.2 Fluorocarbons. Fluorocarbons were used in rigid polyurethane foam and
sometimes in flexible polyurethane and polystyrene as well. They are nonflammable and
nontoxic and give excellent foaming and thermal insulation. Unfortunately, it was discov-
ered that they accumulate in the stratosphere, destroy the ozone layer, which protects us
from UV radiation, and contribute to global warming. By international agreement, they
are being phased out in stages until they have been completely eliminated. The industry is
still searching for suitable substitutes.
5.6.2.3.3 Supercritical CO . This is liquefied at high pressure, forced into the molten
2
polymer, and then allowed to expand and form gas bubbles. This process has been suc-
cessful in thermoplastic structural foams.

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