Successful formulation of coatings demands consideration of both the ease of application and the final surface finish. This surface finish is, in very many cases, a thin polymeric film, making polymers a key constituent of coatings products. Acrylics, polyesters, epoxy resins, polyurethanes, alkyd polymers and latexes are just some of the polymer types routinely applied to a wide variety of substrates.
The molecular weight and molecular weight distribution of these polymers are both key properties that influence ease of use and finish quality. The direct correlation of molecular weight with melting point, for example, means that for powder coatings, molecular weight must be optimized to achieve desirable flow and leveling properties during heating, without compromising the flow properties of the raw powder. Low-molecular-weight resins can soften or become sticky at relatively low temperatures, increasing the tendency to cake, which compromises spray application.
With solvent-miscible and suspension-based paints, and with varnishes, higher-molecular-weight polymers tend to be associated with a more durable protective surface, while lower molecular weights may give higher gloss and better penetration of the substrate. With solvent-miscible products based on alkyd polymers, such as gloss paints for domestic use, the incorporation of high-molecular-weight polymers is problematic because their inclusion increases product viscosity to unacceptable levels. Here, a possible solution is to introduce branched polymers: for a polymer of equivalent molecular weight, branching reduces the associated solution viscosity.