Chemical Industry: Types and Principles of Polyurethane Curing Agents

TDI-TMP curing agent

A commonly used curing agent for two-component polyurethane coatings is TDI-TMP (toluene diisocyanate-trimethylolpropane) adduct. Excellent properties are widely used in two-component polyurethane coatings and adhesives.

Reaction principle

TDI-TMP addition reaction is a typical NCO/OH reaction with the following characteristics:

1. The reaction requires heating and is irreversible, and the products produced are stable and do not produce small molecule products;

2. The two NCO groups on the TDI molecule have certain activity differences due to different positions, but the three primary hydroxyl groups on the TMP molecule have no activity differences;

3. The reaction is a high degree of branching and multifunctional reaction;

4. Most of them are carried out in the form of solution reactions;

5. The quality of polyisocyanate with 3 NCO groups on each molecule affects its miscibility with paint resin, the gloss of the paint film and the strength of the paint film. have a greater impact on other properties, so the production process has higher requirements for product quality. In the production of TDI-TMP adducts, formula design and process conditions will have a great impact on the quality of the product. Slight fluctuations in process conditions or slight changes in the formula will cause defective products.

TDI trimer curing agent

Under the action of a catalyst, three molecules of TDI monomer undergo a nucleophilic reaction. The electron cloud density of nitrogen and oxygen in isocyanate is large and electronegative, while the electrons of carbon The cloud density is small and electropositive, so the carbon atom becomes an electrophilic center and is easily attacked by nucleophiles. Under the action of a catalyst, isocyanate groups add to each other and a polymerization reaction occurs. Lewis bases are usually used as catalysts in the reaction.

Reaction principle

The O=C-N-R structural unit on the TDI trimer ring has good thermal stability, and there are no unstable hydrogen atoms on the isocyanurate ring. Therefore, products containing isocyanurate rings have good thermal stability, and TDI trimer has become a widely used isocyanurate ring product in the coating, foam, and plastic stool industries.

The key to the TDI trimerization reaction is to select the appropriate catalyst. There are many types of catalysts for the TDI trimerization reaction. Commonly used catalyst types include alkaline metal carboxylates (sodium oxalate), phosphorus-containing compounds (tributylphosphorus), tertiary amines (DMP-30), organic metal compounds, etc.

HDI trimer curing agent

HDI trimer is an aliphatic curing agent. Since it does not contain easily oxidized groups such as benzene rings, it has good yellowing resistance. At the same time, the isocyanuric acid ring of HDI trimer has a stable structure and is not easily decomposed at high temperatures. Therefore, it has the advantages of good thermal stability, good wear resistance, and good corrosion resistance. It is often used as a polyurethane curing agent. Furniture, automotive industry, aviation industry and sports equipment fields.

Reaction principle

1. The difficulty in preparing HDI trimers is to find an efficient catalyst that can trimerize HDI monomers and a reaction that reduces the relative molecular mass distribution width of the polymerized product. control conditions.

2. According to literature reports, common catalysts include quaternary ammonium salts, alkali metal carboxylates, quaternary ammonium bases, organic phosphine catalysts, and organic metal oxides.

MOCA

3,3′-Dichloro-4,4′-diaminodiphenylmethane, commonly known as MOCA, is a kind of aromatic diamine and can be used as polyurethane Cross-linking agent and curing agent for (CPU) and epoxy resin, etc., widely used in the automotive industry, machinery manufacturing, sports facilities and other fields.

In industrial synthesis, MOCA uses o-chloroaniline as raw material, which is acidified with hydrochloric acid to form o-chloroaniline hydrochloride. Formaldehyde solution is further added dropwise for condensation. After the condensation is completed Add sodium hydroxide solution to neutralize, and after washing, recrystallization, and drying, MOCA solid is obtained.

Reaction principle

MOCA growth process flow chart