The reaction product of isocyanate group and hydroxyl group is urethane. Studies have shown that the reaction between isocyanate and hydroxyl is a second-order reaction, and the reaction rate constant changes with the hydroxyl content and does not change with the isocyanate concentration.
Polyol and polyisocyanate form polyurethane (referred to as polyurethane).
If the isocyanate group in the reactant is excessive, that is, the molar ratio of isocyanate group to hydroxyl group is greater than 1, then a polyurethane preform with NCO terminal groups will be obtained. The preparation of polymers, elastomers, adhesives, coatings and even foam plastics all involve the preparation of polyurethane prepolymers.
If the amount of hydroxyl groups, isocyanate groups and other substances in the reaction mixture is increased, a polymer with infinite molecular weight can theoretically be produced. However, due to the influence of trace amounts of moisture, catalytic impurities and monofunctional impurities that may exist in the system, the molecular weight of polyurethane generally ranges from tens of thousands to more than 100,000.
If there is an excess of hydroxyl groups, the result will be hydroxyl-terminated polyurethane, which can be used as the main agent of polyurethane adhesives and raw rubber for mixing polyurethane elastomers.
The reactivity of isocyanates and hydroxyl compounds is affected by their respective molecular structures. As mentioned before, the reactivity of isocyanate is related to the type of core group, the type and position of the aromatic core substituent, and ultimately is affected by the molecular structure of alcohol or oligomer polyol.
In the reaction between isocyanate and hydroxyl compounds (alcohols), the order of reactivity of various hydroxyl groups is: primary hydroxyl groupsecondary hydroxyl group>tertiary hydroxyl group, The relative rates of their reactions with isocyanates are 1.0, 0.3, and 0.01 respectively. This is mainly caused by the steric hindrance effect and polar factors of hydroxyl compounds. The reactivity of alcohols of the same type (such as primary alcohols) is also different. This is due to the influence of factors such as the structure of the alcohol itself, concentration of reactants, isocyanate index, trace amounts of acids and bases, etc. during the reaction. For polyhydroxy compounds with the same functionality, the reaction rate is faster when the molecular weight is smaller; when the hydroxyl content is the same, the reaction rate is faster when the functionality is larger, and the viscosity of the reactants increases faster.
Due to the different reactivity of primary hydroxyl groups and secondary hydroxyl groups, the reaction rate of polyester diol and polytetrahydrofuran diol containing terminal primary hydroxyl groups is 10 times that of polyoxypropylene glycol with secondary hydroxyl groups at the terminal groups about.
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