Page 55 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

INTRODUCTION TO POLYMERS AND PLASTICS 1.41

FIGURE 1.41 The rotational molding process. 40

1.6.6 Foaming
The act of foaming a plastic material results in products with a wide range of densities.
These materials are often termed cellular plastics. Cellular plastics can exist in two basic
structures: closed-cell or open-cell. Closed-cell materials have individual voids or cells
that are completely enclosed by plastics, and gas transport takes place by diffusion
through the cell walls. In contrast, open-cell foams have cells that are interconnected, and
fluids may pass easily between the cells. The two structures may exist together in a mate-
rial so that it may be a combination of open and closed cells.
Blowing agents are used to produce foams, and they can be classified as either physical
or chemical. Physical blowing agents include

• Incorporation of glass or resin beads (syntactic foams)
• Inclusion of an inert gas, such as nitrogen or carbon dioxide into the polymer at high
pressure, which expands when the pressure is reduced
• Addition of low boiling liquids, which volatilize on heating, forming gas bubbles when
pressure is released
Chemical blowing agents include
• Addition of compounds that decompose over a suitable temperature range with the evo-
lution of gas
• Chemical reaction between components
The major types of chemical blowing agents include the azo compounds, hydrazine de-
41
rivatives, semicarbazides, tetrazoles, and benzoxazines. Table 1.10 shows some of the
common blowing agents, their decomposition temperatures, and primary uses.

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