Composite Catalysts and Delayed Catalysts

In the study and production of foams, it has been clearly observed that the catalytic effect is significantly enhanced when two catalysts are used together. This phenomenon, known as the "synergistic effect" between catalysts, allows for a more effective catalytic process. To achieve a balance in various reactions during production, it is common to use two or more catalysts to form a composite catalytic system. This approach maximizes the catalytic effect while minimizing the amount of catalyst required. Manufacturers can adjust the dosages of different catalysts based on raw materials, processes, and production equipment to produce high-quality products. The "synergistic effect" is demonstrated in the table below.

Not only do tin and amine catalysts exhibit this synergistic effect, but even different amines, such as tetramethylbutanediamine (TMBDA) and triethylenediamine (TEDA), can exhibit similar interactions.

Synergistic Effect of Amine and Organotin Catalysts (on NCO-OH Reaction)

Based on the principles of catalyst synergy and to facilitate usage for producers, specialized catalyst manufacturers have developed various composite catalyst products.

In certain foaming reactions, especially for polyurethane foams with complex molded shapes, it is crucial to ensure a high yield. During the initial phase, the viscosity of the materials should not increase too rapidly to allow them to fill every corner of the mold cavity. Delayed catalysts are often used to extend the cream time of the foam, enabling the material to fully expand and fill the mold. These catalysts exhibit significant catalytic activity only when the foam material's temperature has risen to a certain level.

Domestic and international catalyst manufacturers, as well as some foam producers, have developed various delayed catalysts, typically based on tertiary amines. These delayed-action amine catalysts generally belong to the quaternary ammonium salt category. Some or all of the amine catalysts are neutralized with acids to form quaternary ammonium salts, which are inactive at low temperatures. In the mid-to-late stages of foam formation, the heat released by the system decomposes the quaternary ammonium salts, and the tertiary amines begin to exhibit catalytic activity, accelerating the latter stages of the reaction. Isocyanates can also react with carboxylic acids to promote the release of amines. Common acids used to neutralize tertiary amines include formic acid, acetic acid, and HCl.

Another type of rare thermosensitive amine catalyst, such as DBU and its salts, shows very low activity at room temperature but significantly increases in catalytic activity as the temperature rises.

To improve production efficiency, it is desirable to use highly active catalysts or increase catalyst dosage to shorten the curing time of foam plastics. However, this often results in excessively short cream times and poor material flowability. To address this issue, delayed catalysts are also employed.