Лако-красочные материалы — производство

Polyurethanes are an increasingly important type of resins finding applications in virtually all types of coatings due to their versa­tility and excellent performance properties. Polyurethane resins are polymers containing urethane (-NH-CO-O-) linkages, usu­ally made by reaction of compounds containing isocyanate groups (-NCO) and hydroxyl groups (-OH).

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R—N=C=0 + HO-R’ ————————— — R—NH—C-O—R’

Isocyanate Alcohol Urethane

Figure 2.59: Urethane formation

The reaction between isocyanates and alcohols, also called the urethane reaction, can occur at room temperature without any catalyst. The presence of urethane linkages in polyurethane resins confers unique and desirable properties to their coatings, mainly because — NH-CO-O — is a strong linkage that is resistant to acids, bases and hydrolysis by water. One unique feature of polyurethane resins is their ability to strongly interact through formation of strong interchain hydrogen bonding between urethane linkages that provi­des stiffness and resilience to polyurethane coatings, unmatched by most other resin systems.

One of the most important considerations in polyurethane chemi­stry and technology is the high reactivity of isocyanate compounds with a wide range of active H-compounds (generally, compounds containing a H atom attached to an O or N atom), including moisture in the air or human skin. This poses serious challenges of health and toxicity hazards as well as safety, handling and storage issues for isocyanate compounds. However, the currently available tech­nologies have made possible safe handling and use of isocyanate compounds for use in paints and coatings.

Reaction of isocyanates with different active H-compounds that form the fundamental basis for polyurethane coating systems are shown in Figure 2.60. The order of relative rates for these reactions is: -NH2 >> — OH > H2O > — COOH. Many types of catalysts are availa­ble that are used to control these reactions. It should be noted that while urethanes are produced only from reaction of isocyanates with alcohols, other reactions are also effectively controlled and used in different polyurethane coating systems.

The most important components of polyurethane resins, therefore, are isocyanates and polyols (compounds containing two or more than two hydroxyl groups); nevertheless, other additives and modi­fiers, such as catalysts and solvents, are also used.