Catalytic Systems for Flexible and Rigid Foams

In the polyurethane flexible foam molding process, the foam material requires good fluidity in the early stages and rapid gelation in the later stages. This necessitates selecting appropriate catalysts to balance the foaming and gelation reactions. Early polyester and prepolymer-based polyether flexible foams primarily used low-activity dimethylalkylamine catalysts, such as N,N-dimethylcyclohexylamine and N,N-dimethyldodecylamine.  

However, for one-step polyether flexible foams, these catalysts alone fail to meet the requirements due to the low viscosity of polyether polyols and the low reactivity of secondary hydroxyl groups at the ends of polyethers. The slow chain-growth reaction of the foam results in poor initial strength, leading to foam collapse. Therefore, a highly active catalyst like triethylenediamine (TEDA) is needed. Yet, using TEDA alone makes the process difficult to control.  

In 1958, the American Mobay Chemical Company first discovered organotin compounds as highly efficient gelation catalysts. Studies showed that dialkyltins had a catalytic efficiency for gelation reactions ten times higher than tertiary amines. Early one-step conventional polyether PU flexible foams primarily used a mixture of triethylenediamine and dibutyltin dilaurate. However, the latter accelerated oxidative degradation of polyether flexible foams at high temperatures (above 140°C) and was soon replaced by stannous octoate.  

For medium-density flexible foams, inexpensive dimethylethanolamine is commonly added to neutralize acidic impurities in the foam material, reducing the usage of more expensive catalysts.  

Currently, the most widely used catalysts in polyether flexible polyurethane foams are triethylenediamine, bis(dimethylaminoethyl) ether, stannous octoate, and dimethylethanolamine. In standard flexible foams, triethylenediamine primarily acts as a foaming catalyst and is used alongside organotin catalysts. In high-resilience flexible foams, it functions as a gelation catalyst in combination with A-1 (bis(dimethylaminoethyl) ether).  

In recent years, significant attention has been paid to developing reactive catalysts, particularly for applications like automotive foam seats and mattresses, which demand low odor and minimal fogging. Reactive catalyst molecules are designed to incorporate groups that react with isocyanates, thereby anchoring the catalyst to the polyurethane resin and reducing volatile components.  

Catalytic Systems for Rigid Foams  

Rigid polyurethane foams also typically use composite catalyst systems, often incorporating tertiary amine catalysts. Similar to flexible foam molding systems, delayed-action catalysts can also be employed. In PIR (polyisocyanurate) rigid foam formulations, trimerization catalysts are usually required. These catalysts include two main types: organometallic compounds and tertiary amines (quaternary ammonium salts).