plastic blowing agent

Text/POLYMER

As a high-performance plastic, Styrofoam is widely used in people's daily lives. The plastics that can be used for foaming mainly include PP, PE, PA, PU, EVA, PS, etc. The gas in plastic foaming process usually comes from a blowing agent, which is a type of substance that can force polymeric materials such like plastics, form cells. According to principle of foaming process of blowing agent, it can be divided into two categories: physical blowing agent and chemical blowing agent.

Fig. 1. Foamed plastic

1. Physical blowing agent

Physical blowing agent does not undergo any chemical change during foaming process of plastic, but produces a large amount of gas by changing physical state to make foam. Processing methods include injection foaming, extrusion foaming, etc., which are commonly used. Preferred physical blowing agents include supercritical CO2, water, low boiling point liquid blowing agents, etc.

2.1 Supercritical CO2

Supercritical CO2 is a type of supercritical fluid. A supercritical fluid (supercritical fluid) is a fluid whose temperature and pressure are above its critical point. The liquid has both gaseous diffusion and liquid solubility, and has characteristics of low viscosity and low surface tension. Supercritical CO2 is colorless, odorless, non-toxic, cheap, and its critical conditions are easy to reach.

Figure 2. CO2 phase diagram dependence

2.2 water

The raw material and production process of water blown foam is under study. Currently, polyurethane (PU) is most widely used water-blown foam. The basic principle is that water and isocyanate react to release carbon. dioxide, which is actually chemical foaming. By adjusting internal chemical structure of polyurethane and isocyanate, it is possible to produce foams with fine cells, smooth surfaces and low heat generation. Typically, water is used as a foaming agent in production of heating pipe insulation, packaging foam and agricultural foam, among others, which do not require high insulation.

2.3 Low boiling liquid blowing agent

The boiling point of this type of blowing agent is below 70°C at normal pressure, as shown in Table 2-1. Among low boiling blowing agents, first two blowing agents are of low toxicity but flammable; last two blowing agents together are called freon.

Table 2-1 Commonly used low boiling point blowing agents

Second, chemical blowing agent

During foaming process, chemical blowing agent undergoes chemical changes and decomposes with release of gas (for example, CO2, N2 or NH3), causing polymer to foam. Chemical blowing agents are divided into inorganic chemical blowing agents and organic chemical blowing agents.

Table 2-2 Classification of chemical blowing agents

Inorganic chemical blowing agents are not easy to be uniformly dispersed in polymers, and resulting CO2 is easily dispersed through membrane wall, and it is difficult to control temperature range of decomposing and evolving gases, so they are often used as auxiliary blowing agents, Medium AC organic chemical blowing agents are most commonly used light -yellow crystalline powder, high gas formation and a wide choice of activators, which makes it difficult for resulting nitrogen and carbon dioxide to escape from melt.

Figure 3. Air Conditioner Foaming Agent and Gassing Performance

Third, choice of blowing agent

When choosing a blowing agent, most important principle is: under conditions of controlled speed and predictable temperature, gas released is not only large in quantity, but also has good reproducibility; gas and solid produced by reaction have no poison. In addition, more attention should be paid to decomposition temperature of blowing agent, amount of added blowing agent, ratio of blowing agent and blowing agent, and particle size of blowing agent.

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