Types and Catalytic Activity of Tertiary Amine Catalysts

Tertiary amine catalysts are one of the most commonly used types of catalysts in the production of polyurethane foam. Most tertiary amine catalysts can catalyze both the reaction between isocyanates and water and the reaction between isocyanates and polyols, which are the two main reactions in polyurethane foam synthesis. However, the catalytic activity varies among different amine compounds.

Classification of Tertiary Amine Catalysts

Tertiary amine compounds used as polyurethane catalysts can be categorized based on their chemical structure into aliphatic amines, alicyclic amines, and aromatic amines.

1. Aliphatic Amines

Common examples: N,N-Dimethylcyclohexylamine, Bis(2-dimethylaminoethyl) ether, Triethylenediamine, N,N,N’,N’-Tetramethyl-1,2-ethylenediamine, N,N,N’,N’’,N’’-Pentamethyldiethylenetriamine, Triethylamine, N,N-Dimethylbenzylamine, N,N-Dimethylhexylamine.

2. Alicyclic Amines

Common examples: Triethylenediamine, N-Ethylmorpholine, N-Methylmorpholine, N,N’-Diethylpiperazine, N,N’-Diethyl-2-methylpiperazine, N,N’-Bis(α-hydroxypropyl)-2-methylpiperazine, N-(2-Hydroxypropyl)-N,N-dimethylmorpholine.

3. Amino Alcohols

Examples: Triethanolamine, N,N-Dimethylethanolamine.

These are reactive catalysts that can be used in combination with other high-activity catalysts. Triethanolamine also acts as a crosslinking agent in molded foams.

4. Aromatic Amines

Examples: Pyridine, N,N’-Dimethylpyridine.

Catalytic Effect and Industrial Relevance

In the initial stages of the reaction, catalytic efficiency plays a crucial role in the industrial production of polyurethane foams. The manufacturing process for flexible polyurethane foam is highly dependent on catalysts due to the low functionality of the reactants and the lower crosslinking density of the products compared to rigid foams. Flexible foams require a longer time to reach sufficient strength in their cell network, and they exhibit a high open-cell rate.

Catalysts promote the NCO-OH and NCO-H₂O reactions, leading to a rapid increase in polymer molecular weight. Simultaneously, gas is generated, the polymer viscosity rises, and the foam expands and solidifies quickly.

The catalytic activity is typically evaluated based on the foam rise time and the heat released during the reaction.

Factors Affecting Catalytic Activity

Basicity

The catalytic activity of tertiary amines is significantly influenced by their basicity. Stronger basicity corresponds to higher catalytic activity. Electron-donating substituents on the nitrogen atom increase the electron cloud density, raising the basicity and catalytic activity. Conversely, electron-withdrawing substituents decrease the basicity and catalytic efficiency. Some researchers consider basicity a key metric for evaluating the activity of amine catalysts.

Molecular Structure

The spatial hindrance of substituents on the nitrogen atom greatly affects catalytic activity. Smaller substituents enhance activity, while bulkier groups hinder the approach of the nitrogen atom to the isocyanate's carbonyl group, reducing catalytic performance.

An early study abroad demonstrated this principle by examining the reaction between phenyl isocyanate and 1-butanol in toluene at 40°C. Despite having a higher dissociation constant pKa, N,N,N’,N’-Tetraethylmethylenediamine exhibited lower activity due to steric hindrance from its four ethyl groups. In contrast, triethylenediamine, with its unique cage-like structure and exposed nitrogen atoms, showed strong catalytic activity.

Catalyst Dosage and Economic Considerations

When the catalyst dosage is low, increasing it leads to a significant rise in reaction speed. However, beyond a certain point, further dosage increases have a diminishing effect on reaction speed. Given the relatively high cost of catalysts, it is essential to determine an optimal dosage based on the specific reaction requirements.