Foaming agents are substances that can make porous materials by producing gas bubbles in a viscous liquid or a molten polymer. Foaming agents can be classified into three main categories: chemical foaming agents, physical foaming agents, and surfactants. Chemical foaming agents are compounds that decompose under heat and release gases such as carbon dioxide and nitrogen, which form fine pores in the polymer matrix. Physical foaming agents are compounds that change their physical state, such as compressing gas, evaporating liquid, or dissolving solid, to create gas bubbles. Surfactants are substances that have high surface activity and can reduce the surface tension of liquids, forming air bubbles that are surrounded by a double layer of molecules on the liquid film.
Foaming agents have various applications in different industries, such as rubber, plastic, concrete, paper, and food. Foaming agents can improve the properties of the materials, such as reducing weight, increasing thermal and acoustic insulation, enhancing strength and flexibility, and creating special textures and appearances. Foaming agents can also reduce the cost of production, as less material is needed to fill the same volume.
The earliest use of foaming agents was in the rubber industry in the 19th century. Sodium carbonate and other volatile liquids were used to make foaming agents, and a patent was applied by Hancock and others in 1846. Since then, carbonates have been widely used as foaming agents because of their safety, endothermic decomposition, and nucleating effect. In 1940, DuPont invented the first organic foaming agent, azobisformamide, which was toxic and polluting, but attracted attention because of its convenience. Later, better foaming agents were developed, which had no pollution, high foaming efficiency, and other advantages. Azodicarbonamide became the most popular foaming agent in the 1950s because of its non-pollution, odorlessness, and other advantages. Japan invented hydrazide foaming agents in the 1970s, which had stable electrical properties and were mainly used in electrical materials, especially wires and cables. These performances showed that organic foaming agents became the mainstream of foaming agents and received more and more attention. Nowadays, with the improvement of living standards, people hope to use more environmentally friendly foaming agents, so developing green foaming agents has become the trend of society.
Foaming agents work by producing gas bubbles when the external conditions change, such as heating, decompressing, etc. The gas bubbles fill the polymer melt and form a foam structure. According to the different foaming principles, foaming agents can be divided into chemical foaming agents and physical foaming agents. Chemical foaming agents are more widely used than physical foaming agents, because they have higher gas yield, lower cost, and better compatibility with polymers. However, chemical foaming agents also have some disadvantages, such as large decomposition heat, large residue, odor, and pollution. Physical foaming agents have the advantages of no residue, no pollution, and easy control of foaming process, but they have the disadvantages of low gas yield, high cost, and poor compatibility with polymers. Therefore, the choice of foaming agents depends on the specific requirements of the products and the processing conditions.
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