Choosing the Right Liquid Coating Application

Q. We are looking at various types of liquid coatings. What would be your recommendation?

A. An understanding of the composition and curing process of coatings provides insight for making a coating selection, helping finishers match the product to their performance, regulation and application requirements. The first decision in coating selection is often about the general type of technology preferred: solvent-borne, high-solids and waterborne. Choosing a specific technology type for finishing requires matching the application and performance attributes with end-use requirements, customer specifications and regulatory guidance.

Solvent-borne coatings are often referred to as the workhorses of the industry because of their reputation for providing long-lasting finishes that dry quickly and meet high production schedules. Available as total coating systems, they suit a wide range of coating needs with products that include fillers, primers, sealers and topcoats. The breadth of solvent-borne coatings has been formulated to meet the specific but varying demands for gloss, color retention and chemical resistance across industries and end uses.

The durability and long-lasting appearance characteristics historically provided by solvent-borne products has made them a popular choice for metal-finishing applications as diverse as military, industrial, agricultural, construction equipment, office furnishings and consumer electronics. In fact, solvent-borne dominates the listing of federal standards because of its long history, and the general comfort with the technology and the performance it provides to users. Finishers who want very durable coatings for plastic substrates have also turned to solvent-borne coatings, since they offer exceptional film durability, long-term performance and superior texturability over this substrate as well.

While past solvent-borne formulations were developed with performance and application needs in mind, advances in solvent-borne technologies and raw materials have helped make finishes more environmentally friendly as well. There are many coatings options that meet government regulations and the various specifications related to hazardous air pollutants (HAPs) and volatile organic compounds (VOCs).

High-solids coatings have their own advantages. Containing less solvent than conventional materials, these coatings result in fewer emissions and greater transfer efficiency. Available in single- or two-component systems, these types of coatings include polyesters, alkyds, epoxies, urethanes and acrylics. High-solids coatings are generally defined as those that have at least 60-percent solids content by volume, with some products even exceeding 90-percent solids content. The challenge for manufacturers in the development of these coatings has been to deliver a product that can be applied easily with conventional equipment. Solvents lower the viscosity of the coating in the container and provide the optimal application characteristics, so finding a way to maintain conventional applications while minimizing solvent use is an additional challenge in developing these products. By using unique raw materials, including polymers with lower molecular weight, and more efficient solvents, formulations with standard viscosities and properties similar to conventional technologies can be easily achieved.

The development of new cross-linking technologies has also led to gains with high-solid coatings. These technologies maximize the performance of polymers while reducing initial viscosity, making application easier. 

Waterborne coatings get their performance characteristics in part from more highly engineered raw materials: synthetic polymers that are used in combination with additives that give the coating a faster dry time and excellent adhesion to substrates. From resins to raw-material combinations, the technology for waterborne finishing has improved, and manufacturers have put more resources into creating these types of coatings options for finishers. Waterborne coatings can be applied to the full range of substrates—wood, metal, glass, masonry and plastic—using most types of application equipment, including spray, dip or flow-coating. Some waterborne coatings will air-dry, while others require baking to cure.

Waterborne finishing options also provide the significant advantage of lower VOCs, with most offering levels well below 2.0 lb/gallon, and zero HAPs, giving finishers the flexibility they need to meet evolving regulatory and consumer requirements. Additional advantages of waterborne finishes include lower costs related to reduction and cleanup, an improved work environment with lower odor, and potential reduction in insurance costs.

Waterborne coatings come in different types of formulations that contain one of three types of polymer resin technologies: water-reducible, water-solubilized and water-emulsion. Since these polymers have significant differences, each provides its own specific set of characteristics to the finished product.

Water-emulsion coatings contain circular-shaped particles that have a high molecular weight when dispersed in water. This increase in molecular weight provides improved performance without having an adverse affect on viscosity. Of the three types of waterborne formulations, water-emulsion coatings offer the most durable coating properties, including good chemical and water resistance. Advances in water-emulsion technology, and the addition of modified resins and additives also have produced good corrosion resistance. These newer water emulsion coatings also offer faster cure times, longer pot life, improved resistance to yellowing, enhanced application properties and better gloss retention. With these capabilities, water-emulsions have been a popular choice for the automotive and business-machine industries.

Water-soluble coatings are different from water-emulsion coatings in that the spherical particles in these formulations (called colloidal dispersion polymers) are smaller and swell in water. Colloidal-dispersion polymers have characteristics of both emulsions and solutions, resulting in a coating that is easy to apply and provides good gloss and durability, as well as water and chemical resistance. Because of these benefits, water-solubles are most often specified for industrial product finishing applications.