1. What is polyurethane prepolymer?
Polyurethane prepolymer, also known as polyurethane prepolymer, is a polymer semi-finished product formed by the proportional reaction of polyisocyanate with polyols (polyethers or polyesters), in which the reactive functional groups remain the same at both ends of the molecular chain. It is not a final product, but an intermediate that can be further reacted with chain extenders, crosslinkers or other active substances to form the final polyurethane material. The prepolymer method (also known as two-step method) is the most mainstream synthesis route in the polyurethane industry. The molecular structure and mechanical properties of the prepolymer method are more controlled than that of the one-step method, in which all the raw materials react together.
Abstract: Polyurethane prepolymer is an active intermediate formed by partial reaction of isocyanate and polyol. It is at the heart the "prepolymer method (two-step) process, which requires extended chains or cross-links to obtain the final polyurethane product.
2, Core raw materials: selection of isocyanates and polyols.
The two basic raw materials for preparation of prepolymers are polyisocyanate and oligomeric polyols. Commonly used polyisocyanates include toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), etc. TDI and MDI are commonly used as final NCO prepolymers, while HDI and IPDI are widely used in high-end coatings and water-based systems due to their anti-yellowing properties. In the case of polyols, polyether polyols (such as polyethylene glycol, polypropylene glycol and tetrahydrofuran glycol) are used in soft systems, while polyester polyols (such as polybutylene adipate) are used in hard systems that require weather resistance and mechanical strength. The hydroxyl value, acid value, moisture content and metal ion content of raw materials directly affect the quality of prepolymer.
Abstract: Isocyanates determine the hardness and reactivity of prepolymers, polyols determine the properties of soft segments and end-uses, and purity and pretreatment of raw materials are the first step in preparing qualified prepolymers.
3, Reaction principle: gradually add isocyanate and hydroxyl groups
The synthesis of polyurethane prepolymers is basically a gradual addition reaction between isocyanate groups (-NCO) and hydroxyl groups groups (-OH) to form amino ester bonds (-NHCO-). This is an exothermic reaction in which the order of reaction activity is: primary alcohol-OH>secondary alcohol-OH>phenol-OH; phenol-OH; -NCO activity on aromatic rings>- NCO activity on cycloalkanes>- NCO activity on fatty hydrocarbons. Competitive reactions may occur in response to different functional groups. Therefore, it is necessary to guide the reaction direction by controlling the ratio of raw materials and technological conditions. The addition of catalysts, such as organotin compounds octate and stannous octoate and dibutyltin dilaurate, and triamine,such as triethylenediamine and triethylamine, can greatly accelerate the reaction.
The chemical reaction of synthetic prepolymers is the gradual addition of an exothermic stepwise addition reaction between -NCO and -OH to form amino ester bonds. Differences in group activity and catalyst selection are key factors controlling reaction direction and reaction rate.
4.Preparation of terminal NCO prepolymer (common use)
End NCO prepolymer is the most commonly used type in industry, and its preparation involves excess isocyanate to retain free-NCO groups at both ends of the reaction molecular chain. The typical process is to first dehydrate in a vacuum (usually 120 °C, vacuumed for 2-3 hours), cool, then slowly drip the polyalcohol into an excess of polyisocyanates, stirring under nitrogen protection. At the same time, remove the heat from reaction in time to keep the temperature between 40 and80 °C. NCO moles are generally controlled between 1.5:1 and 3:1, and the free NCO content final product is generally 3% to 12%. During the reaction, a small amount of solvent (e.g. acetone, ethyl acetate, toluene) can be added to regulate the viscosity, or polymerization inhibitor (e.g. benzene sulfonyl chloride) can be added to prevent gel formation. The reaction endpoint was determined by measuring the NCO content, and the reaction is stopped when the theoretical value is reached.
5, Preparation of End OH Prepolymer.
Unlike terminal NCO prepolymers, terminal OH prepolymers allow excess polyols to retain hydroxyl groups (-OH) at both ends of the reaction molecular chain. The method is similar to end NCO, except that the ratio of ingredients is reversed --too much polyols, not enough isocyanates, and the NCO the molar ratio controlled between 0.8:1 and 1:1. Because the reactivity of hydroxyl group is much lower than that of isocyanate groups, the terminal hydroxyl prepolymer has good storage stability and is not easy to self-polymerize. They are commonly used as the main or soft part of a two-component adhesive. In practical applications, terminal OH prepolymers is usually obtained by further extending the chain with a slight excess of diethylene glycol based on terminal NCO prepolymers.