In the polyurethane rigid foam catalyst system, the required CO2 is obtained through the water-isocyanate reaction, so it is necessary to control the reaction balance between semicarbamate and urethane. If the urethane doesn’t react quickly enough, gas will escape and foam won’t form. On the other hand, if the urethane reacts too quickly, the polymerization reaction will complete the polymer before gas is formed, resulting in a foam that is too dense. Gel reactions are much less catalyzed than urethane reactions.
Tertiary amines can be used alone as catalysts. But for some applications that require faster reaction speeds, such as spraying, metal salts, especially tin salts, are needed to accelerate the gel reaction. They can be used alone or in combination with tertiary amine type catalysts. For rigid polyurethane, stannous octoate and dibutyltin dilaurate are important gel catalysts. However, stannous octoate will hydrolyze in the presence of an alkaline catalyst and lose its activity. Systems containing stannous octoate and water are stable for only a few hours at room temperature. The dibutyltin dilaurate system remains stable for several months. For this reason, dibutyltin dilaurate is preferred as a catalyst in some applications that require a certain storage period.
Many polyurethane rigid foam catalyst applications also require a delayed catalyst, that is, a buffered amine catalyst. In the presence of acid, its amine activity is weakened. Acidic substances can be used to delay the urethane reaction. Hydrogen chloride and benzoyl chloride have been used with amine catalysts to control reaction rates. Adding a small amount of acid extends the foaming time from 2.2 to 6 minutes. The concentration of the tertiary amine catalyst can significantly affect the foaming parameters. Table 8.5 shows the effect of catalyst concentration on foaming parameters such as foam time, tack-free time, foam density, and compressive strength.
