This TIP is a commentary on thermal-cured lacquers used for basecoating in vacuum metallizing. The role lacquers play in vacuum metallizing is as important as the metallizing itself for many applications. Knowledge of the functions, limitations, and proper usage of lacquers is essential to the metallizer who wishes to use these lacquers. Be aware that there are other types of basecoats available, such as ultraviolet-cured and water-based coatings. Although some of these are becoming more popular because of solvent emission control regulations, solvent-based lacquer coatings are still the most widely used. This discussion will be limited to these types of coatings.
The basecoat is a lacquer which is applied to the surface of a part before metallization. Basecoats serve two primary functions for the vacuum metallizer:
- Basecoats level out small imperfections on the substrate, allowing for a smooth, mirrorlike surface.
- Basecoats also promote adhesion of the metallizing to the substrate.
All lacquer-type coatings are composed of three main ingredients:
- First is the resin or solids of a coating. The resin is a plastic material and is what remains of a coating when it dries. Which plastic resin is in the coating depends on a number of factors, including what plastic the substrate is made of. The percentage of solids in the liquid coating will affect the method of application and the cost per gallon of the coating. Usually, the higher the percentage of solids in a coating, the more expensive it is.
- The second type of ingredient is solvents. Solvents are used to carry and disburse the resin onto the parts. Multiple solvents may be combined to achieve the exact properties required for a particular job. Solvents in the coating can be matched to the method of application and the substrate material and can be adjusted to optimize wettability and adhesion of the coating.
- The final ingredients in a coating are additives. These are a variety of materials used in small quantities to fine-tune a coating’s properties. Additives can affect the drying time, the wettability, the hardness, adhesion, and other properties of the coating.
Some basecoats are better than others for a specific job. Some of the desirable properties of a basecoat would be that it is as environmentally safe as possible, can be applied by a variety of methods, dries quickly and avoids runs, adheres well to an untreated substrate, promotes adhesion of the metallized layer, and provides gloss. Not all basecoats provide all of the properties equally. As an end-user, it is your responsibility to decide which of these factors are most important and choose your product accordingly. The cost is an important factor as well. To assist is evaluating basecoat properties, the table on the reverse side of this page lists properties and variables which control them.
Even after carefully choosing a basecoat for an application, problems can arise. The best course of action is usually to contact your coating supplier. Listed here, however, are some of the more common coating problems and some suggested solutions:
Use additives to promote adhesion, reduce solids to thin the coating, increase bake time, check substrate surface, change coatings.
Increase solids, increase evaporation rate, change the solvent mix to prevent runoff.
Reduce viscosity, use additive or retarder to reduce evaporation rate, use additive (surfactant) to reduce surface tension.
Sags & Runs
Solvent too slow or viscosity too high, use temperature different from mix temperature, the coating applied too heavily or rotated inadequately.
Low-density area in a substrate, contaminated coating/solvent, solvent attacking substrate, basecoat too thin.
Silicone or other contaminants on a substrate or in the coating, over-reduced paint, use of epoxy coatings.