When preparing polyurethane foam, a compound called silicone surfactant is often used, commonly known as "silicone oil" or polyurethane foam leveling agent and polyurethane foam stabilizer. At present, such silicone surfactants on the market mainly use polyether-modified organosiloxane structure, usually based on silicon-carbon bond organosilicon surfactants, supplemented by silicon-oxycarbon-bonded organosilicon surfactants. Its structure is as follows:
In the example formula, m and n represent the average number of dimethicone and methyl polyether alkyl siloxane segments; x and y represent the average number of ethylene oxide and propylene oxide segments in the polyether molecule; Whereas, r denotes alkyl, acyloxy, aromatic group, or hydrogen atom with a number of carbon atoms ranging from 1 to 4.
The functions of silicone surfactants for polyurethane foams mainly include:
1.Emulsification and compatibilization: Because the main raw materials of polyurethane foam such as isocyanate, polyol, catalyst, chemical blowing agent, physical foaming agent, silicone surfactant and other additives are not completely miscible or incompatible in the system. Through the synergistic effect of the hydrophilic and hydrophobic segments of the polyether segments in the structure of the silicone surfactant, the raw materials with large differences in hydrophilicity and hydrophobicity in the system can be formed into a homogeneous mixed system.
2.Nucleation: According to the nucleation theory, under the condition of certain external energy and bubble radius, reducing the surface tension can reduce the Gibbs free energy required to form bubbles, and form more nucleation points, so as to obtain a uniform and fine cell structure.
3.Foam stabilization and porosity: In the early stage of foam formation, water reacts with isocyanates to produce carbon dioxide and polyurea, and a large amount of polyurea will promote the foam to form open pores. Silicone surfactants dissolve polyurea and distribute it evenly across the foam matrix, ensuring foam stability before the foam skeleton is formed. The solubility of organosilicon surfactants with different structures to polyurea is different, and appropriate selection can control the precipitation of polyurea, thereby affecting the opening of foam.