Seeking an Effective Finish?

Seeking an Effective Finish? Consider Chemistry and Surface Properties, First!

“Stump the Chemist” – sounds like a game show! But today, we’re answering real-life questions that clients have directed to us. The next time you have a conundrum, send it our way – we’d be happy to help. 

Question #1: Which coating is harder: Alkyd, urethane, or acrylic?

This is actually a trick question. Each class of chemistries can be configured to exhibit “soft and flexible” or “hard and rigid” properties. Another name for alkyds (that is actually a combination of the precursors used to make an alkyd: alcohol and acid), and the true chemical and modern designation, is “polyester.” However, the term “alkyd” is still widely used and marketed.

Alkyds are great for high gloss coatings with excellent clarity. There are both interior and exterior grades. They cure more slowly and will require a catalyst – sometimes internal in a 1K form or it may need to be added in a 2K form – and cure by an air oxidizing reaction.

Urethanes are composed of two types, aromatic and aliphatic. Aromatic urethanes are generally more mechanically durable but do not weather as well as aliphatic urethanes. Aliphatics are an excellent choice for exterior environments and have greater light stability for color sensitive applications.  They don’t generally exhibit quite the durability of their equivalent aromatic urethane counterpart but relative to other chemistries, they perform very well. Both aromatic and aliphatic urethanes can be elastomeric, soft and pliable, or hard and inflexible. Gloss values can be relatively high if desired with good clarity. Aromatic urethanes are great for flooring and aliphatic urethanes are good for exterior surfaces. 1K and 2K versions are available and solids can range from low to 100%.

Acrylics cover a wide class of materials. Some are simply the best for exterior exposure while others are only for interior usage. There are 1K and 2K versions of acrylics. Acrylics are great for cabinetry along with aliphatic urethanes, as they can give very good chemical resistance and anti-yellowing. The more advanced acrylics include 1K chemistry that is self-crosslinking to create 2K properties.

In short, the answer to your question is tricky and it depends what you are finishing and the overall properties you need it to exhibit!

Question #2: Is hardness affected by solvent? Or does it depend purely on resin?

Our rule of thumb is that the best solvent for the resins contained in the coating are the last ones to evaporate during drying, either by air or by oven. This way, the coating is assured to lay flat and be very uniform. Being the last solvent out and the best option for the resins, any retained or residual solvent will definitely soften the coating. 

The resin, upon full solvent evaporation, is then the determining factor of the hardness exhibited – along with the presence of any pigments or inorganic solids, such TiO2, silica, talc, calcium carbonate, barytes, etc. The inorganic solids and their content are definitely contributors towards exhibited finish hardness. With clear coats, it up to the resin.

Question #3: More pigmented colors seem to exhibit a softer surface after curing – is this an illusion?

This is not an illusion! You are observing the result correctly.

To create a dark color, we need to use very little white TiO2 pigment or none at all. Light colors – and especially whites – contain up to 20% TiO2 or even slightly more to create opacity and brightness. Much lower concentrations of other colorants are then added to create off whites, pastels, and other light colored finishes. 

TiO2 is a ceramic like substance of high hardness and abrasiveness. Primers contain considerable amounts of TiO2, along with other suspended inorganic solids. Those other solids are not as hard or abrasive as TiO2 but collectively, they effectively wear out spray tips such that replacement is necessary at various frequencies. 

Dark colors don’t contain those pigments or inorganics. Just a small amount of carbon black pigment (less than 10%) is more than enough to render the system opaque black. Therefore, the properties of the resin alone with minimal support of the dark colorants is responsible for the hardness of the finish. Formulators prefer to keep the chemistry of a paint scheme consistent and typically use the same resin base for clear, dark and light colored finishes. When necessary, silica can be an effective filler in the formulation of dark colors as it is clear when properly incorporated and will not influence gloss at the proper concentration.  It can support hardness accordingly.  The role of a formulator is one who can balance a number of factors!

Question #4: What has better adhesive properties, glossy or flat paint?

The gloss should not be a factor – but with improperly formulated finishes, flat or matte finishes can exhibit poorer adhesion. 

The gloss of a finish is due to micro-irregularities of the surface and surfaces that scatter more light appear to be flat or matted.  The surface gloss is usually adjusted with the addition of a micro-particulate additive to the coating formulation.  This additive is called a flatting aid, which is usually a very, very fine silica. The surface with all the micro-irregularities that result in low gloss due to high light scattering actually increases the surface area versus gloss surfaces that are very smooth and level. Greater surface area bonds tighter to adhesive tape because there is more surface to stick to. 

When pulling the tape from the surface, a low gloss surface will require more force to remove it and an improperly formulated finish will be pulled off more readily than its glossy counterpart.

Thank you for the excellent questions! We hope this blog helps you better understand how chemistry and surface properties are so important when considering effective finishes. Join our bi-weekly newsletter list to stay up-to-date with all of our blogs and latest news.