1. Foaming catalyst
Polyurethane catalyst (foam) can Polymers are available in a wide variety of structures, thus addressing a wide range of commercial needs. The choice of catalyst type profoundly affects the gelling and foaming reactions of the foam. The choice of catalyst affects the overall response of the foam
system, as well as the selectivity for a specific reaction. The reaction selectivity of a foam system is measured by the storage time of the system, the shape of the product, and the demoulding (curing) time. The selectivity of the catalyst affects the reaction equilibrium, the type and sequence of formation of polymers with different linkage methods, and the fluidity of the foam system. Catalysts also affect the physical properties of the final resulting foam.
Amine catalysts mainly affect the reaction between water and isocyanate. They are often called foaming catalysts. However, they have varying degrees of activity and promote reactions between polyols and isocyanates. This property of amine catalysts
is special for polyols with high inherent reactivity. In some formulations, only amine catalysts are used. There are also some ultra-concentrated amine-free catalysts, such as some antimony compounds, that can be used as foaming catalysts.
The most commonly used catalysts for polyurethane foam are tertiary amines, quaternary amines, amine salts and metal carboxylates (such as Sn2+, Sn4+, or K+). Tertiary amines show high selectivity for foaming reactions and also influence gelation and cross-linking reactions. Amine salts
Salts of heat-sensitive amines, such as triethylenediamine, are mainly used to provide delayed reactions.
Tin compounds are a type of catalyst used to promote the reaction between polyols and isocyanates. Therefore, they are known as gel catalysts. As with amine catalysts, some formulations use only organotin catalysts. Metal carboxylates react strongly with gels. Divalent tin (Sn2+) compounds are low cost, but they are hydrolytically unstable. They are typically used in a process application where separate catalyst components can be provided, such as soft bulk bubbles. Tin compounds
(Sn4+) have greater resistance to hydrolysis and can be added to composite materials, such as flexible molded foams.
Tris(dimethylaminomethyl)phenol, 2,4,6-tris[3-(dimethylamino)propyl]hexahydrogen Triazines, quaternary ammonium salts and some potassium compounds have strong selectivity for trimerization reactions.
Equilibrium Catalyst
In the absence of a catalyst, the actual reaction rate between isocyanate and hydroxyl groups is too slow. Polyurethane foam can also be foamed without a catalyst, but the foam may not be completely cured and will not have excellent physical properties.
The reaction of urethane is mainly catalyzed by alkaline materials. For example: tertiary amines such as 1,3-propanediamine, tetramethylbutanediamine, diethanolamine and triethylamine are widely used.
The CO2 used in rigid polyurethane foaming systems is formed by the water-isocyanate reaction, the difference between the semiurethane reaction and the urethane reaction. A balance of relative rates is necessary. If the urethane reaction doesn’t happen quickly enough
the gas won’t be surrounded by cells and no foam will form. Conversely, if the urethane reacts too quickly and the polymer forms before gas is formed, the result will be a foam that is too dense. The gel reaction is more difficult to control through the use of catalysts than the carbamate reaction.
Tertiary amines can be used alone as catalysts, but for some applications such as spraying, faster gelation rates are required. Metal salts, especially tin salts, can be used alone or in combination with tertiary amine catalysts to accelerate the foam gel reaction. Stannous octoate and dibutyltin dilaurate are of greater importance for rigid polyurethanes. Stannous octoate will hydrolyze in the presence of an alkaline catalyst and will lose activity. When stannous octoate and water coexist at room temperature, there is only a few hours of stability.
Dibutyltin dilaurate can exist stably in the main resin system for several months. Therefore, for some packaging foam systems that require a certain storage period, it is preferable to consider using dibutyltin dilaurate catalyst.
Delayed catalysts have also been widely used. There are also some buffered amine catalysts, that is, the activity of the amine catalyst is inhibited in the presence of acidic substances. Acidic substances can be used to delay the urethane reaction. Hydrogen chloride and benzyl
Acid chlorides have been used in combination with amine catalysts to control reaction rates. A small amount of acid can increase the foaming time from 2.2 minutes to 6 minutes.
Temperature can also be used to control the polyurethane foaming reaction. The higher the temperature, the faster the foaming speed. Some rigid polyurethane foam systems that require a delay will have all raw materials mixed at extremely low temperatures (approximately −300°F)
When these systems are heated, the systems foam.
For CO2 foaming system, only tertiary amine can be used as catalyst. However, �For physical blowing agent systems, a higher activity catalyst is required due to the cooling effect of the physical blowing agent. There is a synergistic effect between tin catalyst and tertiary amine
Rigid foams of polyether or polyester one-time molding systems and prepolymer systems, because their cross-linking degree is large enough and the gel speed is fast enough, only tertiary amine catalysts can meet the foaming requirements. The structure of the tertiary amine has a great impact on its catalytic efficiency
and therefore also has a great impact on foam production. The catalytic activity of polyurethane catalyst increases as the basicity of the amine increases and increases as the steric hindrance of the amino nitrogen decreases .
