Polyurethane Additives - Catalysts

In the polyurethane foam system, multiple chemical reactions occur, which can happen simultaneously or sequentially, making the process highly complex. These reactions include the reaction between isocyanates and polyols, water, and other active hydrogen-containing substances, as well as the self-polymerization of isocyanates (commonly seen in PIR rigid foam production).

Organic isocyanate compounds contain highly unsaturated isocyanate groups (-N=C=O), making them chemically highly reactive. The oxygen and nitrogen atoms in these groups have high electron cloud density, with oxygen being the most electronegative, acting as a nucleophilic center that attracts hydrogen to form hydroxyl groups. Conversely, the carbon atom has the lowest electron cloud density, making it strongly electropositive and susceptible to nucleophilic attack. The catalytic mechanism of catalysts in the reaction between isocyanates and active hydrogen compounds is complex. For tertiary amine catalysts, the mechanism involves the nucleophilic attack of basic tertiary amine compounds on the positively charged carbon ion (C⁺), forming an active intermediate compound. This intermediate readily reacts with active hydrogen compounds, releasing the basic catalyst, which then interacts with another isocyanate molecule to form a new intermediate.

In foam production, the primary reactions are between isocyanates and polyols, and isocyanates and water. These reactions release significant heat, raising the system's temperature and promoting foam curing. Among these, the reaction between isocyanates and polyols, also known as the gelation reaction or chain extension reaction, significantly increases the viscosity of the liquid material, leading to solidification.

The reaction between isocyanates and water, often referred to as the blowing reaction, generates carbon dioxide, which serves as the gas source for the cell structure in both soft and certain rigid polyurethane foams. This reaction produces carbon dioxide and substituted urea through a two-step process:

Formation of an unstable intermediate, carbamic acid.

Decomposition of carbamic acid into carbon dioxide and substituted urea.

The reaction rates of water with isocyanates and secondary hydroxyls with isocyanates are comparable, but the decomposition rate of carbamic acid increases significantly above 100°C.

In foam manufacturing, it is crucial to balance these two reactions to produce high-quality products. If the blowing reaction occurs too early and viscosity growth is slow, most gas will be generated before the gelation reaction completes, leading to bubble coalescence and gas escape. This can reduce foam strength and tear resistance, or even cause foam collapse, resulting in high-density, non-uniform solids. Conversely, if the gelation reaction significantly outpaces the blowing reaction, rapid viscosity growth will cause premature gelation before adequate gas formation, yielding high-density foam with unsatisfactory properties.

For optimal foam performance, these two primary reactions must balance so that when the blowing reaction completes, the foam network structure is strong enough to trap the bubbles. Catalysts are typically used to adjust the complex reactions to achieve this balance. They also accelerate reactions, reducing the production cycle time.

In molded foam production, material viscosity should initially increase slowly, allowing the material to flow into all mold cavities. When the material fully expands to fill the mold, the gelation reaction should be complete. This requires the gelation reaction to start slowly but finish quickly, where catalysts play a crucial role.

Catalysts are indispensable additives in polyurethane foam production. Regardless of the process, catalysts are necessary to regulate reaction speeds. This is because the reactions between isocyanates and polyether (or polyester) polyols, as well as between isocyanates and water, are relatively slow at room temperature. To ensure the blowing and gelation reactions achieve balance within a short timeframe, catalysts must be used to accelerate these reactions. Since different catalysts exhibit varying catalytic activities for different reactions, it is common to use two or more types of catalysts to control reaction levels, meeting the requirements of various production processes and product types.