(1) Introduce air into raw material stirring
(2) Solvent volatilization during curing
(3) Side reactions
Generally, it is possible to change the solvent composition and improve the process conditions by reducing the moisture in the raw materials and the environment. Select the appropriate polyurethane catalysts to reduce or suppress the appearance of air bubbles.
The rapid development of the polyurethane industry is due to catalysts. At present, there are more than 200 kinds of catalysts used in the synthesis of polyurethane materials, which are basically divided into two categories: tertiary amine and its salt catalysts, and metal alkyl compounds. The former mainly includes triethylenediamine, dimethylethanolamine, and DMDEE; the latter mainly includes dibutyltin dilaurate, bismuth carboxylate, and stannous octoate. The principle of its use depends not only on the catalytic activity of the catalyst, but also on the selectivity of catalysis, chemical hazard, compatibility with other raw materials, price, stability, etc.
Tertiary amines, as the most widely used category of polyurethane catalysts, have strong basicity and high activity, and can rapidly catalyze the isocyanate-hydroxyl reaction, and also catalyze the reaction between isocyanate and water to generate carbon dioxide. It is widely used in the production of various polyurethane foams and the preparation of many microcellular elastomers. Its representative is triethylenediamine. Because it is inconvenient to use as a solid, manufacturers usually use it by diluting it into a 33% concentration solution with glycol.
But it cannot be ignored that if the polyurethane reaction is simply and roughly divided into foaming and gelation, the catalytic reaction efficiency of triethylenediamine on isocyanate and water accounts for about 60% of the total efficiency. That is to say, it has a strong catalytic foaming effect. When we cannot remove the water in the system and have strict requirements on product bubbles, amine catalysts should be avoided.
In terms of organometallics, lead, tin, mercury, bismuth, manganese, zinc, copper, titanium and other alkyl compounds have certain catalytic effects in polyurethane systems. However, in the production of foam-free products, the catalyst of choice is still organic mercury. This harmful catalyst has almost no catalytic activity to isocyanate water, but can catalyze the isocyanate-hydroxyl reaction. The use of this polyurethane catalyst will not cause bubbles due to trace moisture.