Лако-красочные материалы — производство

As mentioned in the previous chapter, resins are polymeric materi­als capable of forming useful films after application of coatings. For good film formation, a high MW resin is necessary. In general, in the paint and coating industry, two types of resins are used:

(1) those with very high MW that are capable of forming useful films on their own after application of coatings, without any need for a chemical reaction, and

(2) those with low MW that require, after application of coatings, a chemical reaction to increase their MW and become capable of film formation.

These two fundamental film formation mechanisms form the basis of their classification. In general, all resins, and hence all coatings, can be classified into two distinct types, thermoplastic resins and thermosetting resins.

Thermoplastic resins are high MW polymers that are capable of forming useful films without requiring any chemical reactions during or after film formation. Thermoplastic resins are essentially linear or branched polymers. The film formation of coatings based on such polymers primarily involves only the evaporation of solvent. The dry films of such resins are essentially stacks of entangled long chain amorphous polymers physically interacting with each other through Van der Waals forces. Due to their long chains, the degree of entanglement and physical interactions are strong enough to provide some useful properties, but in general, they lack some desired mechanical properties and resistance to attack by solvents. As the viscosity of a polymer solution is dependent on its MW, the high MWs of thermoplastic resins give them high solution viscosi­ties. This requires use of higher amounts of solvents in their coa­tings to bring their viscosities down to acceptable levels. However, thermoplastic resin latexes (dispersions of thermoplastic resins in water) can afford coatings without necessarily using solvents. Such systems are very popular and form a major category of today’s waterborne coatings.

Thermosetting resins, the another important resin type, are poly­meric materials that must undergo some chemical reaction after coatings are applied to form useful films. These chemical reactions, called cross-linking reactions or curing reactions, essentially con­nect relatively smaller polymer chains, leading to an increase in MW and formation of a cross-linked or network polymer structure. Thus, the structure of the polymer in the cured films of thermo­setting systems, unlike thermoplastic ones, is formed by strong chemical linkages connecting polymer chains, making these films strong and resistant to chemical and solvent attack. In general, the binders for thermosetting coatings are comprised of relatively low

MW resins with reactive functional groups and an added cross­linking agent (cross-linker). After the coating is applied, under sui­table reaction conditions, cross-linkers react with functional groups of resins, forming a dry film with a cross-linked network structure. Some thermosetting resins have functional groups that are capable of cross-linking by reacting with themselves (self-cross-linking) or with the components of the atmosphere, such as moisture or oxygen, without the need for an added cross-linking agent. Thus, it follows that the structure of the film formed and the properties of the coatings based on thermosetting and thermoplastic resins are fundamentally different and find application in different end uses. Table 2.1 shows typical examples and applications of resin types.