Architectural Coatings: Paint Formulations and their Properties

There are many different types of paints and coatings for the various substrates they beautify and protect. The type of paint or coating also impacts its properties and applications.

In beautification formulations, the important features are one-coat, uniform hide, with application by roller with minimum spatter and a smooth finish. It should be able to require minimum effort for the applicator. A coating of this type has little resin and a low level of titanium dioxide (TiO₂), but high levels of extender pigment[1]. This provides dry hiding and is a relatively lower-cost paint compared to a paint for walls and trim.

If a coating’s purpose is solely to beautify, its properties are focused on ease of application, and regulatory compliance. Examples of this would be an interior architectural ceiling paint or a dry fall paint for a production setting.

A dry fall paint is one that is applied by spray gun in a commercial factory or warehouse, where the overspray dries before it settles on the floor or equipment so that it can be cleaned up without the use of solvents. Again, this is a lower-cost paint whose function is to hide and is more for aesthetics. The regulatory compliance refers to VOC (volatile organic compounds) and the level appropriate for the class of paint formulated.

Interior architectural paints

Interior architectural paints have many more performance requirements. Referencing Dave Fuhr’s waterborne coatings articles linked below, the paint has to match the means of application and intended use as well as performance for the coating.

As an example, if the paint is designated for kitchen/bathrooms, it is formulated to withstand high moisture levels to maintain substrate adhesion, mitigate mildew growth, yet be able to withstand scrubbing and to be washable to easily remove stains. This is accomplished with the correct choice of a quality resin with pigments and additives that create a water-insensitive coating. In dark colors, it is also necessary to ensure that the paint doesn’t exhibit water spotting.

The paints will contain the right balance of rheology modifiers so they flow and level whether brushed, rolled or sprayed, yet provide a sufficient level of “drag” to promote film build, without an excess of sagging.

Other performance properties include resistance to burnishing (a gloss/sheen change with abrasion from a dry cloth, for example) and block resistance, which occurs with painted doors or windows and their frames, particularly in hot and humid climates.

Exterior architectural paints

Exterior architectural paints are a step above interior paints with respect to performance requirements, although they are not under the type of daily scrutiny as interior paints. Exterior paints may have a warranty for longer than 25 years, have to withstand repeated climate changes, including freeze/thaw cycles, resistance to rain, hail, sunlight and mold/mildew, as well as dimensional instability of the substrate.

Although surface preparation is stressed before applying a topcoat, they are formulated to have excellent adhesion over contaminated and poorly-prepared surfaces. Most current premium products are low VOC yet achieve open time, freeze-thaw stability and low-temperature application to as low as 35°F (1.7°C) with the use of exempt solvents.

Pigments that are used in interior coatings are not always suitable for exterior use. Titanium dioxide for exterior use (or universal grades) is surface-modified organically and/or inorganically to provide UV stability for the TiO₂.

The use of calcium carbonate (CaCO₃) will lead to “frosting”, where acidic rain will convert the extender pigment into insoluble salts that are especially pronounced with dark paint. Care should be taken with other extenders to ensure that impurities are not at a sufficient level to cause a reaction with the environment.

Stains, clearcoats and other lightly-pigmented or clear coatings also utilize UV-A (ultra violet absorbers) and HALS (hindered amine light stabilizers). These were covered in a previous article on coatings additives, linked below. Nano materials, such as iron oxides or zinc oxide, are inorganic materials that also protect the coatings from UV-degradation.

Primers prepare the substrate for the topcoat. They provide adhesion, a consistent non-porosity for the topcoat to ensure a uniform appearance, reduce vapor and water transmission and - in the case of metals - prevent corrosion. Metal primers contain reactive pigments that can passivate the metals and platy pigments to create a better barrier than irregular or spherical pigments would provide.