A chemical blowing agent is a chemical that produces gases, usually nitrogen, carbon dioxide, water, and other small amounts of other gases. The gas produced is nitrogen as the best, low permeability, odorless and non-toxic, can produce good performance, uniform foam.

At the appropriate time in the processing process, it will release gas, so that the formation of plastic bubble holes. The formation of foam can generally be divided into four stages.

In the first stage, the foaming agent must be completely and evenly dispersed within the polymer, which is usually in a liquid or molten state. At this point, the blowing agent can form a true solution in the polymer, or it can be uniformly dispersed in the polymer to form a two-phase system. The mixture of blowing agent and polymer material is usually added in the form of close or open chain. When choosing blowing agent, it is necessary to consider that the rubber mixing temperature is lower than the decomposition temperature of blowing agent.

The second stage, after the formation of a large number of individual bubbles, the system is transformed into a gas dispersed in the liquid system. Nucleating agents are often added to the system to promote the formation of a large number of small bubbles. Otherwise, extremely uneven bubble holes will be created. Nucleating agents are generally very fine inert particles, and additives such as calcium carbonate and kaolin carbon black can be used as nucleating agents.

In the third stage, the initial formation of the cell increases in size as more gas diffuses and enters the cell through the polymer. If this period is long enough, the individual pores will come into contact with each other and fuse. If the foam is formed mainly through the interconnecting of the cell holes, it is called an open-cell foam. If the foam is formed by unconnected bubble holes, it is called closed cell foam. If the cavity fusion is allowed to continue without limit, the foam will collapse.

In the fourth stage, when the viscosity of the polymer increases, the foaming agent is basically completely decomposed, and the bubble can no longer grow, the foam will stabilize and become a styling product. The viscosity of the polymer can be increased by cooling, crosslinking or other methods. The last three stages of the foaming process can be as short as a fraction of a second, or no longer than a few seconds.

The quality of foam is good or bad, and the decomposition of foaming agent and the gel of polymer material in the production process (the viscosity of polymer material) is closely related. It is usually required that the decomposition process of the blowing agent is consistent with the gel process of the polymer material, or the gel is slightly earlier than the decomposition of the blowing agent.

If the above two processes do not match, you will not be able to obtain a foam with good performance. If the gel process lags behind the foaming process, the viscosity of the bubble wall formed is too low, which will cause the foam to be soft, and can not support the foam material and collapse in serious cases; On the contrary, the strength of the bubble wall is too high, so that the internal pressure of the bubble is too high, so that the hole wall tears, there will be a large area of cracking in the center of the foam.