In previous columns, we discussed self-leveling coatings, with a focus on 100% active polyurethane systems. In this month’s column, we will discuss formulating tips for these systems.

The advantage of 100% active self-leveling urethane coatings based on methylene diphenyl diisocyanate (MDI) is the low viscosity, good physical properties of the coatings and relatively low cost. These systems can be formulated to have the same compression strength and Shore D hardness of epoxy systems, but still have about three times the elongation at break and twice the Taber abrasion resistance. This allows them to be crack bridging. The disadvantage of these systems is that the MDI will readily react with water to form polyurea and CO₂. The evolution of the gas can cause foam issues in the coating.

Most self-leveling coatings go over concrete, and this concrete will normally contain moisture. The best way to prevent the urethane from foaming is to use a primer/sealer such as a two-component (2K) epoxy. Once the substrate is sealed to stop water from encountering the urethane, the next place for moisture to contact the coating is from raw materials in the coating. Therefore, it is necessary to use low water-containing urethane-grade raw materials. It is difficult to keep moisture out of some raw materials like pigments, so a moisture scavenger is added to the resin side of the 2K system. The most common is zeolite powder. Zeolite powder is nonreactive, has a high moisture adsorption capacity and works at low moisture levels, making it excellent for pigmented urethane systems. It is normally supplied as a powder or as a dispersion (normally ~50% in castor oil). Since it takes one to three days to adsorb the water through physical adsorption, you cannot make and use a coating right away and expect no bubbling. A good starting point is ~0.25–0.50% by weight of the resin side of the formula.

If the system is clear (such as for aliphatic topcoats) or you need to dewater raw materials for the isocyanate part of the 2K system, monomeric isocyanates can be used as moisture scavengers. Oxazolidine-based moisture scavengers react quickly with water, but like monomeric isocyanates, can have toxicological issues. Other liquid moisture scavengers include silanes that will also react with water.

Choice of the urethane catalyst (not normally needed for MDI, but required for aliphatic urethane systems) includes either gelling-type catalysts like triethylene diamine (TEDA) or organotin, or blowing catalysts like 2,2'-dimorpholinodiethylether (DMDEE). Gelling catalysts promote the urethane reaction over the urea reaction. Blowing catalysts promote the urea reaction over the urethane reaction. In coatings, only gelling catalysts are used since we want to minimize the water side reaction resulting in urea linkages.

For low-temperature application, catalysts can be used with MDI-based systems, but it is critical that the coating be at the low temperature during mixing to prevent too short a pot life. If longer pot life is needed, organic acids can help retard the urethane reaction.

Often polypropylene glycol (PPG) polyols are used with MDI, but there are compatibility issues between MDI and higher molecular weight PPGs. There are several ways to help compatibilize the two, including adding castor oil, ethylene diamine or toluene diamine-based PPG polyols, or by reacting the PPG polyol with ethylene oxide to create a polyethylene glycol end cap. As castor oil is often used in these systems, that is the preferred way to go.

Due to the low viscosity of these systems, pigments will easily settle out. Adding an antisettling agent will be necessary. The key is to prevent settling without increasing viscosity to the point that the coating loses its self-leveling nature. Often, fumed silicas or clays are used to accomplish this.

Lowering gloss in self-leveling coatings is difficult and takes a large amount of pigment. Normally, hydrophilic pigments like calcium carbonate should be avoided to limit water in the coating. There is only so much moisture scavengers can do. Fumed silica for antisettling and zeolite for moisture scavenging will help lower gloss in addition to the extender pigments. Adding pigments will increase tensile strength and hardness but can hurt flexibility and elongation.

As a final note, MDI-based, self-leveling urethane systems are great for areas that are dimensionally less stable (such as concrete with cracks) or where high abrasion resistance is needed (parking decks, warehouses with forklift traffic). If high chemical resistance is needed, epoxy systems can provide it. If UV resistance is needed, an aliphatic topcoat is required.

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