Important Amine Catalysts and Their Applications

1. Triethylenediamine

Triethylenediamine, officially known as 1,4-diazabicyclo[2,2,2]octane, is commonly referred to as triethylene diamine in the industry. Its structural formula is N(CH₂CH₂)₃N, making it a tertiary amine catalyst with a bicyclic structure. In its normal state, it appears as a white crystalline solid with a relative density of 1.14 at 28°C. Its pure form has a melting point of 154°C and a boiling point of 174°C. It sublimes easily, is hygroscopic, and dissolves in various solvents. It has a slight amine odor, less pronounced than most tertiary amine catalysts. Its vapor pressure is 533 Pa at 50°C and 7.7 kPa at 100°C. With a pKa value of 5.4, it has high solubility in water, dissolving 46g in 100g of water at 25°C.

Triethylenediamine is one of the most significant tertiary amine catalysts, widely used in flexible, semi-rigid, and rigid polyurethane foams. Its importance is particularly notable in the one-step foaming process, whose success is closely linked to the discovery and development of this catalyst. It is highly active, requiring only small amounts, and effectively catalyzes both gelation and foaming reactions.

Its unique chemical structure—a cage-like compound with two nitrogen atoms connected by three ethylene groups—makes it highly symmetrical and dense. The nitrogen atoms lack bulky substituents, allowing easy access to their lone electron pairs. In foaming systems, it reacts with isocyanates to form unstable active complexes, which, upon urethane bond formation, are released to further catalyze the reaction. Although not a strong base, it exhibits high catalytic activity for isocyanate and active hydrogen compounds due to this mechanism.

In the one-step foaming process, its low dosage (usually less than 1%) effectively supports both chain extension and foaming reactions, promoting rapid initial viscosity increase and foam growth.

2. Bis(dimethylaminoethyl) Ether

Bis(dimethylaminoethyl) ether is another crucial amine catalyst, appearing as a pale yellow transparent liquid with a flash point of 64°C and a density of 0.85 g/cm³ at 25°C. It is water-soluble and exhibits high catalytic activity and selectivity for foaming reactions. Initially developed by Union Carbide Corporation under the trade name Niax A-1, it was later produced by Compton's Silicone (OSI) division under the same name. It is available as a 70% solution in dipropylene glycol, marketed as Dabco BL11 by Air Products. Domestically, A-1 catalyst also contains 70% bis(dimethylaminoethyl) ether in dipropylene glycol, with a viscosity of 4.0–5.5 mPa·s, a refractive index of 1.4346, and a density of 0.902 g/cm³ at 25°C.

A-1 catalyst is primarily used in the production of flexible polyurethane foams. Its strong catalytic effect on water reactions reduces foam density. Approximately 80% of its effect targets gas-generation reactions, while 20% supports gelation. This makes it close to the ideal amine catalyst model. Adjusting its dosage significantly impacts gas generation more than gelation. Within the tolerance of tin catalysts, their amount can remain unchanged while maintaining reaction balance, enabling high-quality foam production. Thus, Niax A-1 improves production tolerance, preventing quality issues from operational or metering errors.

3. Dimethylcyclohexylamine and Dimethylethanolamine

(1) Dimethylcyclohexylamine

This low-viscosity, medium-activity amine catalyst has a density of 0.85 g/cm³ at 25°C, a flash point of 48°C, and a viscosity of 2 mPa·s at 25°C. It is water-insoluble and mainly used in rigid polyurethane foams, providing balanced catalytic performance for foaming and gelation. However, it has a strong odor.

(2) Dimethylethanolamine

Commonly used in rigid foam production, dimethylethanolamine contains a hydroxyl group that reacts with isocyanates, incorporating it into the polymer structure, unlike volatile triethylamine. It is a liquid with a density of 0.85 g/cm³ at 25°C, a flash point of 44°C, a viscosity of 6 mPa·s, and a hydroxyl value of 638 mg KOH/g. Despite its low catalytic activity for foam rise and gelation, its strong alkalinity neutralizes trace acids in the system, particularly isocyanate-derived acids, protecting other organic amines. Its buffer-like low activity and strong neutralization are particularly beneficial when paired with triethylenediamine, allowing lower dosages of the latter to achieve desired reaction rates. High-ratio mixtures (e.g., 80% dimethylethanolamine and 20% triethylenediamine) are economically advantageous but are only suitable for rigid foams due to their limited catalytic effect on later-stage reactions.

4. N-Ethylmorpholine

N-Ethylmorpholine is a medium-strength tertiary amine catalyst, ideal for polyester-based flexible polyurethane foams. These foams are widely used in clothing, fabric laminates, footwear, and solvent-resistant vehicle padding. Due to the high reactivity and viscosity of polyester polyols, low-activity catalysts like N-ethylmorpholine or N-methylmorpholine are preferred. They strongly catalyze gelation after foam rise, maximizing cell expansion without significantly altering reaction rates across different concentrations. However, these catalysts have a strong odor.

5. Amine Trimerization Catalysts

Some amine catalysts are also excellent trimerization catalysts for isocyanates, used to produce isocyanurate foams. These foams offer superior flame retardancy and low smoke generation during combustion, making them ideal for building materials. Isocyanurate-modified foams are an effective solution for improving flame resistance.

Various amine catalysts include pentamethyldiethylenetriamine, a strong foaming catalyst, and tetramethylethylenediamine, a medium-activity catalyst suitable for flexible and semi-rigid foams. N,N’-diethylpiperazine and N,N'-diethyl-2-methylpiperazine are used in flexible and molded foams.