One area of coatings that often goes overlooked is high-film-build self-leveling coatings. These are coatings that can range from 25 mils (625 microns) to 1 inch (2.5 centimeters) thick. They are 100%-solids urethanes or epoxies and are designed to go over epoxy primers for flooring applications, such as industrial settings or car parks. The reason for the epoxy primer is to address the negative reaction of isocyanates with water, which causes foaming.

In past columns, we have covered epoxy coatings, and while epoxies make great flooring coatings, high-build urethanes can provide properties that even the best epoxies struggle with, such as very high abrasion resistance, crack bridging or, in the case of aliphatic urethane systems, extreme exterior durability.

Since this area covers a lot of material, I will break it down into multiple columns over several months. This month focuses on self-leveling floor coatings, their needs and the technologies available. In other columns, I will discuss isocyanates, polyols for urethane systems and finally, formulating tips for these systems. I will not delve too deeply into epoxies since they were covered in previous Formulating with Mike columns.

When pouring concrete for floors, it is nearly impossible to make them completely flat and level. While this is not an issue in many applications, low spots can cause water to pool, which is not ideal. A liquid that flows out, self-levels and then hardens can fill these low areas, creating a smooth, level floor. Some argue that concrete alone is sufficient and does not need coatings. In many cases, this is correct, but uncoated concrete has several deficiencies, such as being brittle compared to coatings, which leads to poor impact and abrasion resistance. Most concrete floors become dusty as the surface wears away, developing divots from impacts. With the increasing concern over airborne particulates in cities, concrete dust is considered a health issue. Additionally, concrete is porous, making it prone to stains and allowing moisture to pass through, leading to efflorescence and humidity problems (discussed in depth in previous Formulating with Mike columns).

We need a solution for bare concrete. There are several technologies for coating concrete, but few support high-build coatings. The two main technologies are epoxy mastic coatings and urethanes. High-build coatings need to be approximately 100% active to prevent defects from shrinkage and to achieve true self-leveling. While there are 100%-active UV-cured systems for flooring, limited UV penetration in pigmented coatings prevents the high film builds required for this application. Since an epoxy primer is necessary over concrete, many opt to stay with epoxy coatings, which is often acceptable, particularly in cases where very high chemical resistance is required.

The key advantage of urethanes over epoxy coatings is their ability to be both hard and flexible. A properly formulated aromatic urethane can match the compressive strength and Shore D hardness of an epoxy but with two to three times the flexibility and more than double the abrasion resistance. This results in longer service life in high-traffic areas such as warehouses and parking decks, and the flexibility to avoid cracking when the concrete moves, especially over small cracks (crack bridging). Additionally, coated floors offer an aesthetically appealing finish, contributing to the growing popularity of these systems.

What do we mean by self-leveling coatings? These are coatings that, when applied, flow into low spots and defects in the concrete, creating a smooth, level flooring system. They work well on horizontal floors but can be challenging on slopes such as ramps. Since water does not pool easily on ramps, achieving a completely smooth coating is less critical. The application process differs for epoxies and urethanes.

For epoxies, a primer is not always necessary if the concrete is in good condition, but it is recommended. High-build epoxy mastics do not always wet out and penetrate the concrete effectively, which is where lower-viscosity epoxy primers are beneficial. Most concrete coating failures stem from cohesive failure in the concrete itself, where the coating adheres but the concrete fails, causing delamination. A low-viscosity primer penetrates and reinforces the top layer of concrete, anchoring the coating to the substrate. For urethane systems, the primer also prevents moisture in the concrete from reacting with isocyanates, which can cause foaming in the coating.

Key aspects of self-leveling systems include adequate rheology and effective reactivity. The coating must have low enough viscosity to flow and build a high-level coating, but not so low that it pools on sloped floors. Effective reactivity means having a sufficient pot life to allow for application — such as pouring, squeegeeing or rolling — while still hardening within about eight hours for a reasonable return-to-service time. These coatings are often applied and cured during downtime (off-shift hours, evenings or weekends) to minimize disruption. If multiple coats are required (primer, self-leveling and possibly a topcoat), this timing can become challenging.

In next month’s column, I will explore isocyanates, their chemistries, differences and how to select the right one for this application. In the following column, I will cover polyol chemistries, including polyether, polyester, acrylic and natural oils for bio-based systems. In the final column, I will discuss formulating strategies and multilayer systems.

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