Adding some filler to the plastic can improve certain properties of the plastic and is therefore more suitable for certain specialized applications. In order to reduce the density and hardness of the plastic, or to enhance its thermal insulation or sound insulation, the most desirable filler is the void. Plastics containing voids or cells, classified as foam. As the degree of foaming, that is, the difference in the volume fraction of the foam caused by the voids, the properties of the foam may differ considerably from the base plastic. A blowing agent is a chemical that can be added to a plastic to release a gas at a suitable time during the process to form cells in the plastic.
The formation of plastic foam can generally be divided into four stages:
In the first stage, the blowing agent must be completely uniformly dispersed in the polymer, and the polymer is usually in a liquid or molten state. The blowing agent can now form a true solution in the polymer or simply disperse uniformly in the polymer to form a two phase system.
In the second stage, after a large number of individual bubbles are formed, the system is transformed into a system in which the gas is dispersed in the liquid. At this time, a nucleating agent is often added to promote the formation of a large number of small bubbles. The nucleating agents are generally very fine inert particles which provide a site for the formation of a new gas phase.
In the third stage, the initially formed cells are constantly increasing because more gas diffuses and penetrates the cells through the polymer. If this period of time is long enough, the individual cells will come into contact with each other. If the walls separating the individual cells are broken, then by this coalescence, larger cells are formed. If the foam is formed mainly by cell interconnection, it is called an open cell foam. If it is a foam formed by cells that are not connected to each other, it is called a closed cell foam. If cell agglomeration is allowed to proceed without limitation, the foam collapses because the gas is completely separated from the polymer.