Multi-coat systems

Paint primers are the first coat of a multi-coat system to be applied to a surface. On wood, for example, an important function is to provide adequate bonding between the substrate and subsequent finishing coals, especially under damp conditions. 

On metals, while promotion of adhesion of the multi-coat system is equally important, an added requirement is the control of corrosion. In metal primers, anti-corrosive properties are achieved by incorporating either pigments that are anodic to the metal substrate or pigments, such as zinc phosphate, that inhibit corrosion by other mechanisms. 

Coatings are usually applied as multi-layered systems that are composed of primer and topcoat. However, in some cases - for example automotive coating systems -this may vary from four to six layers. Each coating layer is appHed to perform certain specific functions, though its activities are influenced by the other layers in the system. The interactions among different layers and the interfadal phenomenon play an important role in the overall performance of the multi-coat systems [5]. Different properties of coatings are typically associated with specific parts of a coating system (Fig. 1.1) [6]. 

Thermoplastic acrylics are used in a variety of applications for metal, both directly (e.g. in Light Industrial) and as part of a multi-coat system (e.g. in Off-shore and Marine). Fast drying, water-white colour and good compatibility with other polymers as well as exterior durability are the properties which attract their use in coatings for metal protection. 

Nanoparticles are frequently used as a suspension in some kind of solvent. This is a two phase mixture of suspended solid and liquid solvent and is thus an example of a colloid. The solid doesn t separate out as a precipitate partially because the nanoparticles are so small and partially because they are stabilised by coating groups that prevent their aggregation into a precipitate and enhance their solubility. Colloidal gold, which has a typical red colour for particles of less than 100 nm, has been known since ancient times as a means of staining glass. Colloid science is a mature discipline that is much wider than the relatively recent field of nanoparticle research. Strictly a colloid can be defined as a stable system of small particles dispersed in a different medium. It represents a multi-phase system in which one dimension of a dispersed phase is of colloidal size. Thus, for example, a foam is a gas dispersed in a liquid or solid. A liquid aerosol is a liquid dispersed in gas, whereas a solid aerosol (or smoke) is a solid dispersed in a gas. An emulsion is a liquid dispersed in a liquid, a gel is liquid dispersed in a solid and a soils a solid dispersed in a liquid or solid. We saw in Section 14.7 the distinction between sol and gel in the sol gel process. 

Coating by polymer film can be a multi-step process, e.g., to produce multi-layered systems or to further functionalize and to improve usability. 

For better understanding of the CVD phase diagrams, examples based on single component system such as tungsten carbide coating and multi-component system such as mullite coating are discussed below for various process conditions. 

An essential feature of this new design is a better controlled gas flow pattern in the coating zone. This is achieved by application of a guiding system in which the gas flow is accelerated, stabilized, and given a precise amount of swirl, which eliminates slugging, often seen in traditional coaters, and stabilizes multi-tube systems. Particles are... 

Especially in the automotive industry, the appearance of a painted surface becomes more and more important. An automotive finish is a multi-layer system - E-coat, Primer, Topcoat, clear coat. Each layer can have several characteristics that influence the appearance of the final finish. The two major objectives of the paint manufacturer are to optimize the physical/chemical properties of the coating as well as to achieve an optimum paint appearance (wet, smooth look). The end use of the paint (trucks versus prestige cars), then determines the necessary degree of achievement. 

These materials can be applied as homogeneous coatings or as multi-layers of different materials for a given application. Examples of multi-layer systems are... 

Another approach is based on multi-component systems [97-101]. It has been pursued, in particular, by the group of Professor Dalton. In many cases they have used a two components system formed by a polyurethane NLO oligomer with isocyanate end groups that is reacted with a cross-linking agent, triethanolamine, as example [102, 103]. In this approach, the oligomer (or prepolymer) is prepared first then a solution of the oligomer and of the cross-linker is spin-coated on the substrate (Fig. 2.25). 

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