According to the source of foaming gas, blowing agents can be divided into the following two categories: physical blowing agents and chemical blowing agents. As the name suggests, the physical blowing agent produces gas through a physical change of state – generally from liquid to gaseous; On the other hand, the gas produced by chemical blowing agents is through a chemical reaction. The latter is a heat-sensitive chemical, when heated, the blowing agent undergoes a decomposition reaction to produce two decomposition products, gaseous and solid, and the above two types of blowing agents can be divided into many subcategories. Physical blowing agent Theoretically, any material that can be mixed with a polymer and subsequently vaporized during processing can be used as a blowing agent. But in practice, it is best for physical blowing agents to liquefy under the right conditions and to vaporize at (or below) normal plastic processing temperatures to foam plastics. These blowing agents must be soluble in polymers under the right conditions, but not excessively soluble. The ability of the blowing agent gas to penetrate the polymer is important, as is the volume of gas released per unit weight of the blowing agent. The latter is known as blowing agent efficiency, which is an important indicator for all types of blowing agents, and the standard for efficient blowing agents is to release at least 150-200cc. of gas per gram (in standard condition).
The most commonly used physical blowing agents today are chlorofluorocarbons, hydrocarbons, and compressed gases such as nitrogen and CO2. The requirements for the use of this type of blowing agent are that the temperature and pressure must ensure that the blowing agent is mixed with the plastic in liquid form. When the temperature and/or pressure is changed, the blowing agent is vaporized and foaming begins. For example, if a low-boiling hydrocarbon blowing agent is pumped into an extruder filled with molten plastic, the high pressure in the extruder will keep the hydrocarbon blowing agent in liquid form to dissolve in the polymer, and when the polymer melt leaves the extruder, a sudden pressure drop will force the blowing agent to vaporize, resulting in foam. Simple gases, such as nitrogen and CO2, require much higher pressures than hydrocarbons or chlorofluorocarbons to achieve this result. Nucleating agents can promote the formation of a uniform cell structure.
For the production of ultra-low density foams (densities as low as about 10 lb/cu.ft.), the use of chlorofluorocarbons and hydrocarbon blowing agents is very effective. There are two main reasons for this. One is that the amount of blowing agent required is very large (sometimes up to 20% or more of the weight of the base resin), and this type of blowing agent is cheaper than chemical blowing agent, which compensates for the higher cost of the equipment; Second, it is related to the thermodynamics of foam stabilization. As discussed earlier in this article, in order for a foam to stabilize, the viscosity of the polymer melt must be changed before it collapses. The most widely used hydrocarbon and chlorofluorocarbon blowing agents can actually reduce the melt viscosity of the polymers they are used in. When they vaporize and detach from the polymer solution, the viscosity of the melt increases, reducing the cooling costs required to stabilize the foam. This is especially important for ultra-low-density foams.
Despite the inherent benefits of blowing agents, they are not without problems in their use. Most of these blowing agents pollute the natural environment and continue to get worse. In particular, chlorofluorocarbons. Hydrocarbons also have this problem, but to a lesser extent.