Лако-красочные материалы — производство

Polyurethane adhesives are formed by the reaction of various types of isocyanates with polyols. The polar urethane group enables bonding to various surfaces. Depending on the raw materials used, glue lines with either rubberlike behavior or elastic-to-brittle hard behavior can be achieved. The end groups determine the type of the adhesive, whether it is a reactively or a physically hardening adhesive.

Table 13 Advantages and Disadvantages of PMDI Compared to Other Adhesives, Especially UF Resins

Advantages Higher storage stability

Formaldehyde-free gluing, despite the fact that formaldehyde is used in the production of MDI/PMDI Higher reactivity Higher bonding strength Higher tolerance against humidity Lower consumption of adhesive

Disadvantages Higher price, but this is compensated by the low adhesive consumptions and sometimes shorter press times

Adhesion to all other surfaces, e. g., also press platens. This imposes the use of (i) special internal or external release agents [219], (ii) special types of PMDI [220] or (iii) the use in the board surface of adhesives other than isocyanates The necessary use of special emulsifiers (EMDI) or special dosing and gluing systems

Greater worker protection requirements due to the toxicity and the low but nevertheless existing vapor pressure of monomeric MDI, which need special precautions during use

One-component isocyanate adhesive systems consist of chains with isocyanate groups on chain ends or on branching sites. These isocyanate groups can react with the moisture content of the surfaces to be bonded, and a hardened system forms from this addition reaction. Thus, at least one of the two surfaces must contain the amount of water necessary for hardening. Due to the high viscosity of these adhesives, dilution with organic solvents or higher temperatures are necessary. Additionally, the adhesive may contain various other components, such as flowing agents, fillers, antioxidants, bactericides, or dyes. The bondline reaches the necessary green strength within a few hours and hardens over a few days. During the reaction of the isocyanate group with the moisture content of the wood, CO2 is formed, which causes some foaming of the bondline. The bondlines themselves are more or less resistant against humidity and water.

The two-component systems consist of (i) a polyol or polyamine and (ii) an iso­cyanate. The hardening starts with the mixing of the two components. Due to the low viscosities of the two components they can be used without addition of solvents. The weight ratio between the two components determines the properties of the bondline. Linear polyols and low amounts of isocyanates give flexible bondlines, whereas branched polyols and high amounts of isocyanates lead to hard and brittle bondlines. The pot life of the two-component systems is determined by the reactivity of the two components, the temperature, and the addition of catalysts, and can vary between 0.5 and 24 h. At room temperature hardening occurs within 3 to 20 h.