Since polyurethane production is actually a process of chemical reaction control, its reaction rate is very important. Different products, different processes and different formulations have different requirements for the reaction rate. Taking a reasonable response rate that adapts to needs plays an important role in ensuring and improving product quality and quality.
1. Ratio of OH and NCO
When the ratio of OH to NCO is closer to 1:1, the reaction rate is faster. When NCO:OH is greater than or less than 1:1, the reaction rate slows down. When NCO:OH>1:1, it will increase the product hardness, thermal stability, elasticity, mechanical strength and other physical properties, but reduce the elongation and tensile strength, and affect the storage life of some liquid polyurethane products. When NCO:OH<1:1, it will improve the soft feel, elongation and peel strength of the product, but it will reduce some physical properties such as hardness and wear resistance.In addition, the reactivity of isocyanates and hydroxyl compounds is also affected by their respective molecular structures. The reactivity of various hydroxyl compounds is: primary hydroxyl>secondary hydroxyl>tertiary hydroxyl.
2. The pH value of raw materials
For the polyurethane reaction, the acid-base value of the raw material will affect the reactivity with isocyanate. As far as the raw materials of polyether polyol and polyester polyol are concerned, the acid value is the amount of residual carboxyl groups. It reacts with isocyanate to form amide and release carbon dioxide, which will not only cause chain termination, but also easily form bubbles. The acid also has a bad effect on reaction catalysis and reduces the hydrolysis resistance of the product. Keeping the reaction system reasonably alkaline is beneficial to the smooth progress of the reaction. Too high a base number will result in poor control of the reaction rate, which will seriously affect the production process and product quality.
3. Catalyst
For the same type of catalyst, adding a reasonable amount of catalyst is conducive to controlling a reasonable reaction rate. Excessive catalyst will also sacrifice product quality, and certain organotin catalysts will affect the hydrolysis resistance of products.
4. moisture
Although many polyurethane formulations have added moisture, it does not mean that moisture is the raw material that should be used in polyurethane production. But for the polyurethane reaction, the moisture content of the mixture in the reaction system must be strictly controlled. Because as the amount of water increases, the reaction rate will increase. Especially in the presence of a catalyst, the reaction between isocyanate and water can be accelerated. Although the reactivity of water with isocyanates is lower than that of primary hydroxyl groups, it is comparable to secondary hydroxyl groups. In the production of polyurethane products other than polyurethane foam, water must be strictly controlled, because water reacts with isocyanate to generate unstable carbamic acid, which is easily decomposed into carbon dioxide and amine. In addition, in the prepolymer, moisture will reduce the NCO content in the prepolymer.
5.functional groups
The larger the functional group, the faster the reaction rate and the higher the viscosity of the material.
6. molecular weight
Under the same conditions such as functional groups, the smaller the molecular weight, the higher the reactivity.
7.diols and diamines
The reaction rate of diol and diamine is very different, and the activity of diol is much lower than that of diamine. In particular, primary aliphatic amines react with isocyanates very quickly and are generally difficult to control, so aromatic diamines with relatively low activity are commonly used.
8. Temperature
In general, the higher the reaction temperature, the higher the reaction rate. But in practice, the polyurethane reaction temperature is controlled between 60-100 ℃. Because the temperature exceeds 130 ℃, especially linear molecular chain reactions are prone to undesirable problems, resulting in branching and cross-linking, and affecting the regularity between molecules. Below 60 ℃, the reaction rate is very slow, and the reaction is unfavorable.
9. Other
The greater the polarity of the solvent, the slower the reaction rate is affected, and vice versa; the lower the solid content, the smaller the reaction rate, and vice versa; the higher the viscosity of the material, the slower the reaction rate; the faster the stirring speed, the faster the reaction; The higher the amount), the faster the reaction rate;
