Production of microporouspolyurethane Elastomers often use water as a foaming agent. In addition, trace amounts of water are inevitably present in polyols such as polyester and polyether and other raw materials. Therefore, the reaction between isocyanate and water is often encountered. Moreover, this reaction occurs simultaneously under the reaction conditions of isocyanate and polyol.
Chemist Wurtz believes that the reaction of isocyanate with water first produces unstable carbamic acid, and then quickly decomposes to produce amines and carbon dioxide. The released carbon dioxide can act as a foam to generate foam. The amine formed by the reaction is easy to enter in the presence of excess isocyanate--step reaction generates urea, which acts as a chain extender. The reaction of 1 mol of water with isocyanate to form urea can produce 22.4Lcarbon dioxideGas (under standard conditions).
Although in Water is commonly used as a foaming agent in the production of microcellular polyurethane elastomers, but in the production of prepolymers and non-foamed polyurethane elastomer products, water is very harmful. The presence of water not only easily generates bubbles, causing the product to be scrapped, but also reacts with isocyanate to form urea, which will increase the viscosity of the prepolymer. Moreover, urea can further react with isocyanate to form biuret branching or cross-linking, which significantly reduces the storage stability of the prepolymer. Therefore, when producing prepolymers, there are strict requirements on the moisture content and environmental humidity of raw materials such as polyols. It is usually necessary to heat and vacuum dehydrate the unsealed polyols such as polyester and polyether to reduce the moisture content to less than 0.05%, and the reaction is carried out under the protection of dry N2 to reduce the influence of air humidity.
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