Interfacial aspects of paints and coatings

Paints or surface coatings are complex multiphase colloidal systems that are appUed as a continuous layer to a surface [1], A paint usually contains pigmented materials to distinguish it from clear hlms that are described as lacquers or varnishes. The main purpose of a paint or surface coating is to provide aesthetic appeal as well as to protect the surface. For example, a motor car paint can enhance the appearance of the car body by providing color and gloss and it also protects the car body from corrosion. 

When considering a paint formulation one must know the specihc interaction between the paint components and substrates. These processes are governed by interfacial phenomena such as wetting, spreading, deposition and adhesion of the paint components to the substrate. The latter can be wood, plastic, metal, glass, etc. The interaction forces between the paint components and the substrate must be considered when formulating any paint. In addition, the method of application can vary from one substrate and another. 

The main objective of the present section is to consider the interfeicial and colloid phenomena involved in a paint system, its flow characteristics or rheology, its interaction with the substrate and the main criteria that are needed to produce a good paint for a particular application. 

To obtain the fundamental understanding of the above basic concepts one must consider first the paint components. Most paint formulations consist of disperse systems (solid in liquid dispersions). The disperse phase consists of primary pigment particles (organic or inorganic) which provide the opacity, color and other optical effects. These are usually in the submicron range. Other coarse particles (mostly inorganic) are used in the primer and undercoat to seal the substrate and enhance adhesion of the top coat. The continuous phase consist of a solution of polymer or resin which provides the basis of a continuous film that seals the surface and protects it firom the outside environment. Most modern paints contain latexes which are used as film formers. These latexes (with a glass transition temperature mostly below ambient temperature) 

The primary shape of pigmented particles is determined by their chemical nature, their crystalline structure (or lack of it) and the way the pigment is created in nature or made synthetically. Pigments as primary particles may be spherical, nodular, needle or rod-like, or plate-like (lamellar). This is illustrated in Fig. 4.1. 

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