The purpose of organic paints and coatings is essentially twofold.
They can be required to provide a solution to aesthetic and/or protective problems.
For example, when painting the bodywork of a motorcycle, the coating is expected to enhance its shape thanks to its color and shine, as well as to protect it from corrosion, if the motorcycle is made of mild steel. In the case of a polymer matrix composite bodywork, this last aspect doesn’t matter and, therefore, the coating only has the purpose of conferring the required aesthetic appearance; not infrequently, economic motivations lead to the creation of plastic materials that are already dyed, in order to avoid the subsequent coating. Considering the nature of the paints, it is clear that there is a close relationship between the coating used and the substrate. The prerogatives of a wood varnish are obviously different from those of a varnish made for a metal support and the method of application, drying and polymerization are also different. In the formulation of a paint suitable for a particular purpose, it is, therefore, important to know both the materials to be painted, and the final physical and mechanical characteristics required, as well as the possibility of application and polymerization. For example, a paint meant for cast iron parts needs to resist impact, while the coating of a tin can must have a high degree of flexibility. Surfaces are rarely what they seem; with the exception of noble metals, all metal surfaces are covered with a layer of oxide and, moreover, the cleanliness and purity of a surface cannot be taken for granted. However, a large part of the final properties and behavior of the coating depends on the surface and, therefore, this is an important aspect. In general, the surfaces to be coated are “dirty” and it is necessary to produce and design coatings that adapt to non-ideal surfaces: coatings that tolerate highly heterogeneous surfaces are called “robust”. As for industrial coating processes, special pre-treatments suitable for every surface (especially in the processing of metal surfaces) are required and are included in the processing chain.
It is difficult, if not impossible, to merge all the desirable features into a single coating; the main properties that a coating may have are: opacity, re-covering, color, shine, smoothness, texture, adhesion to the substrate, specific physical and mechanical properties (hardness, resistance to wear and impact), resistance to chemical agents, corrosion protection and the all-encompassing durability. In practice, we often resort to the use of a protective “system”, consisting of several coatings layered in succession, each with a certain characteristic. The number of different layers required for a complete coating depends on the type of substrate and on the environment in which the product will be used. A typical protective system consists of a primer, an undercoat and a topcoat. Each one of these can have pigments in its composition, and for each one of these, several layers may be applied. The primer is often used to seal the substrate and to achieve good adhesion between the substrate and the undercoat; it can simply confer opacity or absorb the mechanical stresses between the substrate and the coating, for example, in case of embrittlement and hardening of the latter due to the exposure of the part, or if the thermal expansion of the substrate should occur due to daily or seasonal cycles. The undercoat has two basic purposes: it contributes significantly to protect the substrate and it provides a sufficiently smooth layer for the application of the topcoat. The topcoat is then applied to complete the coating and to confer the desired surface and aesthetic finish. Overall, it is what protects the wood or metal on which it is applied. Paint deterioration can be due to changes in the chemical nature of the film, with subsequent changes in its mechanical and physical properties; these changes are strictly and inevitably linked to environmental factors: simple solar radiation is responsible for polymer degradation and cross-linking. The development of new pigments can contribute to the improvement of durability, but the polymeric binder remains the weak point of the system. The development of coating systems for a specific end-purpose and for certain environmental conditions is an ideal approach, but it becomes feasible only when it is possible to overcome the economic compromises (cost-benefit ratio) that determine consumer choices.