To make plastics spongy and lightweight, pockets of air or gas are introduced into them. Such plastics are known as foamed plastics
(expanded plastics). Several methods are available to expand plastics:
for example, nitrogen gas is injected during molding, or cracked gas (nitrogen, carbon dioxide, etc.) is mixed by using a foaming agent whichbreaks down at molding temperatures.

Forms made of thermoplastic resins are durable and used widely for bedding,
cushions, leather, etc. Synthetic leather is produced by foaming vinyl chloride which has been made soft by the addition of a plasticizer.
Synthetic wood has been made to have a specific gravity equal to that of wood by adjusting the volume of gas introduced into the plastics. Polystyrene and rigid polyurethane, which contain gas and thus have the property of shutting off heat, are used as heat insulators for refrigerators, buildings, etc.