Surface coating is the process by which items are covered in a thin film, such as paint or a protective coating. The additional coating can be functional or decorative and can add, enhance or disguise certain features of the material.

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Many surface coatings are synthetic polymers, which are made from large molecules and provide a tough and flexible film. These coatings can also add pigment, opacity or other special properties. The most common type of coating is paint, which can enhance the colour and protect the base material.

Polymers in surface coatings

Polymers provide the right properties for a surface coating, consisting of pigment particles that can adhere to the base material. The polymers allow the coating to stick well to the material and enable properties such as thermal, mechanical or gloss to work.

Some important factors in a polymer used in surface coating include the weight distribution of the molecules. Also important are the solubility, the glass transition temperature and the structure, which can be linear, networked or branched.

Film-forming processes

When used in industry, surface coatings undergo film formation. The liquid coating is applied to the material and cured to a high molecular weight. The film thickness can be applied in a range of thicknesses; however, it is always less than 0.5mm.

Film forming processes were potentially toxic before the 1960s, as they emitted chemicals into the atmosphere. Solvent levels are now much lower and improved processes mean there are three main types: solvent, evaporation, and carrier liquid. For some materials, it might be best to use a combination of these three types of processing techniques.

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The NHS publishes guidelines on how to reduce toxicity related to surface coating Today, a surface engineering company such as will use the most modern and safe processes that carry the lowest levels of toxicity.

Solvents have not always been used. In the Middle Ages, for example, drying oils were often applied to create enhanced properties of materials without a solvent. In this way, the paint worked to form a film coating by the process of oxidative cross-linking. It is the oxygen in the air that helps network polymers to provide decorative or functional properties. Drying oils can also be used with natural solvents, such as tree gum, to create varnishes.