The concept of coatings

The traditional Chinese name of coating is paint. Coating is a kind of liquid or solid material which is applied to the surface of an object through different application methods. It can form fixed, adhesive and continuous solid coating film with certain strength, having decoration, protection or other special functions to the object.

It is usually thick liquid made of resin, oil or emulsion, with or without pigments, fillers, additives and organic solvent or water. The definition of coating in Coating Process, an authoritative book in Chinese coating field, is:”Coating is a kind of material which can be applied to the surface of an object through different application method to form fixed, adhesive and continuous solid film with certain strength. The film formed in this way is generally called coating film, or paint film or coat.”

What we usually said as paint is just one kind of coatings. It refers to the kind of liquid or solid material which is applied to the surface of an object to form a film to have protective, decorative or other special functions (insulation, anti-rust, anti-mildew or heat resisting).

As early coatings usually use vegetable oil as the main raw material, so they are often called oil paint. Now, synthetic resin has largely or totally replaced vegetable oil and therefore it is called coating. Coatings are not all liquid, and powder coatings are also a major variety of coatings. Coatings are organic chemical polymer materials, and the coating film formed is a high-molecular compound. According to the modern classifications of chemical products, coatings belong to fine chemical products. Modern coating is gradually becoming a multi-functional engineering material, and is an important industry of chemical industry.

Composition of coatings

A coating is usually composed of four basic components: film-forming substance (resin), pigments (including extender pigments), solvent and additives.

1. The film-forming substance is the main component of coating film. It can independently form continuous film of a certain degree of strength, for example, all kinds of resin, drying oil, organic macromolecular substance and inorganic substance like water glass. It is essential component of coating film. It includes grease, grease processing product, cellulose derivative, natural resin and synthetic resin. The film-forming substance also include some non-volatile reactive diluents which is the main substance making the coating adhesive to the applied object to form continuous film, and the basis of coating which determines the basic characteristics of coating.

According to the main film-forming substance used in the coating, coatings can be divided into oil-based coating, fiber coating, synthetic coating and inorganic coating; according to the characteristics of the coating or the paint film, they can be divided into solution coating, latex coating, sol coating, powder coating, glazed coating, anti-glossing coating and colorful artistic coating.

2. There are usually two kinds of pigments/fillers: one is tinting pigment, for example, titanium dioxide and chrome yellow; the other is extender pigment which is also called filler, for example, calcium carbonate and talcum powder.

Pigments are substances which can dye an object. There are soluble pigment and insoluble pigment. Inorganic pigments are usually mineral substances. Human has known and used inorganic pigments long time ago when human uses colored soil and ore to draw on rock or apply them on their bodies. Organic pigments usually come from plants and marine animals, for example, Rubia blue, vine yellow and the purple extracted from shellfish in ancient Rome.

Filler is also called filling agent in chemical products and it refers to solid materials which are used to improve processing performance and mechanical property of products and/or reduce cost. In sewage treatment field, it is mainly used in contact oxidation process where the microorganism will accumulate on the surface of the filler to increase surface contact with sewage to make degradation treatment.

3. Solvent includes hydrocarbon (mineral spirit, kerosene, gasoline, benzene, Toluene, xylene, etc.), alcohols, ethers, ketone and esters. The main functions of solvent and water is to disperse the film-forming base stock to form viscous liquid. It helps application and improvement of some performances of the coating film.

4. Additives include defoamer, levelling agent and special functional additives like substrate wetting agent. The dosage of these additives is small. Usually they cannot help to film-forming, but have an important role in the film-forming process, determining the final performance of the coating film.

Main functions

There are four main functions: protection, decoration, hiding products defects and other special functions and improve product value.

Main difference

Definitions of coating and paint in the national standard

In the vocabulary-general terms (HotTag) for paints and varnishes in the national standard (GB 5206.1), the definition of coating is as follows:

It is a general name of liquid or solid material which is applied to the surface of an object to form solid film with protective, decorative or special performance (e.g. insulation, anti-corrosion, marking, etc.). Early coating uses vegetable oil as main material, so it is also called paint. Now, synthetic resin has largely or fully replaced vegetable oil, so it is called coating.

Based on the above definitions, we can get two points:

1. This kind of material is called coating rather than paint is the result of development of paint varieties and classifications. Because the word paint cannot cover all the product varieties in the industry, while coating can. Therefore, it is more accurate and scientific to use coating as its name.

For example, it is not appropriate to call powder coating paint, so coating and paint are not the same thing in this case. Therefore, the word “coating” can cover solid powder coatings and liquid paint, while the word “paint” cannot. So the explanation in the standard is not enough.

2. The note for the term “coating” in the standard, “in detailed coating names, the word “paint” is also used, for example, ready-mixed paint and paste paint”, is also not proper. Because the word “paint” is not only used in detailed coating varieties, but also in many coating classifications like acrylic paint and polyurethane paint, as well as widely used in general reference in terms of application fields like anti-corrosion paint, automobile paint and marine paint. Actually, except some individual cases, the two words can be exchanges and the word “paint” is not only used in some detailed coating varieties.