Classification of polyurethane catalysts and selection of catalysts for different polyurethane products
1. Introduction
Catalyst is a chemical substance that can change the rate of chemical reaction. The catalysts used in polyurethane synthesis can be broadly classified into tertiary amine catalysts, organotin compounds, non-tin organometallic compounds and other new catalysts.
2. Characteristics of various catalysts
Amine catalysts are good catalysts for NCO and water reaction; organotin and organomercury have high selectivity for NCO and OH reaction and are not sensitive to water and NCO reaction; some new catalysts such as temperature-sensitive catalysts have different catalytic efficiency at different temperatures and are widely used in some two-component polyurethane products.
3. Purpose of using catalysts
Usually we use catalysts for two main purposes.
1. to make the reaction proceed in the direction we expect.
2. To regulate the reaction rate, shorten the reaction time and improve the production efficiency.
4. Principles of choosing catalysts for different polyurethane products
Polyurethane products can be broadly divided into foam, elastomer and polyurethane resin. Here we make a brief analysis of catalyst selection for these three types of products respectively.
4.1 Catalyst selection for polyurethane foam Polyurethane foam products are mainly synthesized from polyether, isocyanate, blowing agent, catalyst and so on. The main process of foam formation is generally considered as follows.
1, the foam is generated in the polyurethane reaction system by physical and chemical methods, and the foam is allowed to be evenly dispersed in the reaction system, where the main foaming method is to use water and isocyanate to produce carbon dioxide for foaming.
2、The process of foam generation requires the viscosity of the reaction system to increase in order to stabilize the foam from escaping.
3. When the foam formation reaches the required number and size, the viscosity of the reaction system needs to increase continuously or even form a cross-linked system to stabilize the foam and shape the product. In this case, we need at least two kinds of catalysts to regulate the reaction. One catalyst is to promote the reaction between isocyanate and water, that is, to promote the foaming reaction, usually choose amine catalyst; the other is to promote the reaction between isocyanate and polyether or alcohol, that is, the reaction of polyurethane molecular chain growth and cross-linking, usually choose metal catalyst. Therefore, in the process of polyurethane foam synthesis, we generally choose amine catalysts and organometallic catalysts to be used together in order to produce synergistic effects to accomplish the best results. The specific types and ratios of amine catalysts and organometallic catalysts need to be experimentally or empirically adjusted according to different products, so we will not make too many statements here.
4.2 Polyurethane elastomer catalyst selection Polyurethane elastomer reaction to NCO and water should be strictly avoided, so this time generally choose metal-based catalysts, for example, organotin, organobism bismuth, even eliminated organomercury, etc.
4.3 Polyurethane resin catalyst selection Polyurethane resin synthesis process should also try to avoid the reaction of NCO and water, so the general choice of organic bismuth, organic tin more.
