Лако-красочные материалы — производство

Earlier we discussed two-component polyurethane systems for sol — ventbased or high-solid systems. In general, any waterborne coating system requires resins and cross-linkers that are water dispersible. To confer water dispersibility, resins and cross-linkers are generally modified with a sufficient degree of hydrophilic groups or struc­tures. When such modified materials are added to water under suf­ficient shear, they break down into discrete droplets or particles and are stabilized by the hydrophilic moieties. Thus, two-component waterborne polyurethane systems can be prepared using water — dispersible polyols (component I) and water-dispersible polyisocy­anates (component II). See Figure 2.67 presents an example of the structure of water-dispersible polyisocyanates. Such compon­ents are commercially available.

Component I is generally supplied as a polyol dispersion in water and component II (supplied free from water) is added to component I just prior to application.

Once the two components are mixed and the coating applied, the curing reaction begins. It is important to note that during curing, despite the presence of a large excess of water, the predominant curing reaction is between isocy­anate and polyol compounds. This is mainly because of a signifi­cantly lower reaction rate of isocyanate with water compared to that
with polyols. Nevertheless, a certain degree of reaction between isocyanate and water does take place, and the cured film always contains a distribution of urethane and urea linkages.

Two-component polyurethane systems have become very popu­lar for many commercial applications, as they can offer products with low VOC content while maintaining high performance. Such polyurethane systems are increasingly used in industrial coating applications.