A coating is a polymer-based layer that is applied to the surface of a product. Coatings are used to protect the underlying material from all kinds of environmental effects, such as humidity, corrosion, chemical substances, fouling or wear. Besides protection, coatings are also applied for decoration purposes.
A good adhesion of the coating to the underlying material is very important. Therefore, it is often necessary to pre-treat the surface of a material and/or to add adhesion promoters to the coating formulation.
Generally a coating consists of:
Polymer (also called binder or resin)
The binder is the most important component of a coating. It determines to a large extent the properties of the coating. The binder forms the sealing layer, adheres to the surface of the underlying substrate material and is the matrix for all the other components of the coating. Properties like pot life, application behavior, endurance, UV-resistance, resistance to chemicals, susceptibility to hydrolysis, adhesion, toughness and flexibility are determined by the binder. Many binders can be used in coatings. The most widely used binders and their properties are presented in the overview below.
The polymeric binders can be divided in two groups:
These polymers have a relatively low molecular weight (Mn = 1,000 - 10,000) and are dissolved in a solvent, resulting in good flow behavior. The polymers contain reactive groups, which react after application of the coating and form a dense insoluble polymer network (chemical drying). Physical drying also takes place due to evaporation of the solvent. The properties of the coating are determined by the chemical structure of the polymer and its crosslink density.
These polymers have a high molecular weight (Mn > 10,000) compared to thermosetting polymers. Coatings based on thermoplastic polymers contain large quantities of solvent or are dispersed in water (e.g. latex). Drying is purely physical and is based on the evaporation of the solvent and/or water. The properties of the coating are determined by the chemical structure and the physical properties of the polymers.
Good price/quality ratio
Large variations in flexibility
Good chemical resistance
Properties strongly depend on the kind of monomers used
Pigments, either organic or inorganic, are the color agents in the coating.
Additives are added to a coating to improve processability or certain properties of a coating. Examples of additives in coatings are thickeners, rheology modifiers, anti-foam agents, drying agents, surfactants or anti-skin agents.
Organic solvents lower the viscosity of the coating and are used to facilitate application by brush, sprayer or roller. Nowadays, coatings which contain little solvent are popular, since solvents have been proven to cause health and environmental problems.
Ecologically sound coatings are UV-curable coatings, high-solid coatings (less than 30% solvent), powder coatings and water-borne coatings (no or only a very small amount of solvent).