Log in to view the full article
OMU, P/U, poly, polyurethane, urethane, water-base, waterbased polyurethane ... there are so many ways to call finish by another name that it gets confusing, even for experienced contractors. The following should help make sense of the many similar terms used for types of wood flooring finish.
Oil-Modified Polyurethane
OMU is the most commonly used urethane today. It is called "poly" by many, and just "urethane" by about an equal number of people. Technically, the description is backwards— it is actually a polyurethane-modified oil.
Polyurethane is a harder, more wear-resistant compound than the linseed oil and soya oil varnish bases that were used many years ago. Coatings companies reacted some of the polyurethane onto the oil base and formed what we know as oil-modified polyurethane. They are slow-drying but very forgiving and easy to use. They are usually targeted at residential use in today's market.
Traditionally, the carrier for OMU was mineral spirits or a close derivative, so sometimes these products are referred to as solvent-based products. More recently, there have been versions of this product that are using water as the carrier rather than mineral spirits: The OMU is emulsified into water. These dry faster and generally have higher molecular weights (making them tougher) than traditional OMU. They are not quite as forgiving during application as traditional OMU, but are more forgiving than most water-based systems.
Moisture-Cure Urethane
M/C finishes are the oldest of the true urethanes. They react with moisture in the air to cure, which is why they are called moisture-cure urethanes. The higher the humidity, the faster they react.
The byproduct of the reaction is carbon dioxide gas. In very humid areas, heavy films can be an issue, as they will skin over and trap gas, leaving the film with a foam-like appearance. In very dry climates, these finishes are too slow, as there is not enough humidity to react with them before they soak into the surface of the wood.
M/C urethanes are very tough and clear. They have a light color and are one of the most wear-resistant products. These M/C systems generally have strong solvents and cannot be used in some regions with VOC restrictions or where the customer has sensitivity to solvents.
Water-Based Finishes
For the last 20 years or so, these have been lumped together into a single category against solvent-based coatings. The market and the coatings have become so sophisticated that they need to be broken up into several new categories:
Acrylic: Acrylics are the base for most water-based sealers other than the OMU emulsions mentioned above. They are clear, light coatings with very fast dry times and a forgiving nature for the contractor. They are not particularly tough unless catalyzed/hardened/co-reacted. They have good sealing properties and add to film build for the appearance of depth.
Acrylic polyurethane: These systems are a blend of easy-to-use acrylics with some tougher polyurethane to add more wear-resistance. They come both catalyzed and uncatalyzed. These finishes are typically used in low-wear situations such as residential applications. When catalyzed, an aziridine catalyst is typically used.
Polyurethane acrylic: These finishes are blends as well, but smaller amounts of the easy-to-use acrylics are blended with more of the tougher but harder-to-use polyurethanes to improve the application properties. Most of these formulas are catalyzed with an aziridine catalyst and cost more. They are very tough and are often used in highwear situations. They are easy to mix and use and, when catalyzed, generally have a pot life of 24 hours or more.
Polyurethane: There are very few products in this category, as it is difficult to make their application easy and forgiving.
Polyurethane/Isocyanate: These have been evolving: Early ones were difficult to mix and apply evenly, and they had a short working pot life of two to three hours, depending on temperature. Over the last couple of years, they have improved considerably. They are easier to mix, and have longer working life of about 3 to 5 hours (some even say overnight in cool conditions). If properly mixed, they are very forgiving and easy to apply. They take a few days to come to 90 percent hardness. These ultimately are long-wearing, tough systems.
Aluminum Oxide
In prefinished flooring, the most common form of coating is the aluminum oxide boosted coatings. Manufacturers have added a long-wearing particle to the finish to prolong the life of the coating. These have improved considerably over the last few years: the particles have gotten smaller and smaller, and the mar- and scuff-resistance has improved measurably. Smaller particles equate to more clarity in the film. The coatings also have become more flexible to eliminate checking as the wood expands and contracts. Recoating is still a challenge, but it can be done when necessary.
Nano Technology
The newest buzzword in finishes is nano technology. "Nano" means "small." As with the aluminum oxide prefinished systems, smaller particles are being added to many coatings to increase the wear- and mar-resistance. Older coatings with particles had larger particles that would sink in the systems upon rest and become difficult to stir, resulting in uneven looks, sheens and spotty wear resistance. The new nano systems have such small particles that they are lighter and stay better-suspended in the system, making stirring no more difficult than mixing a satin finish.
There is a second form of nano coating in which the raw material suppliers attach the particle directly to the polymer chain in the coatings. This results in a stable, easy-to-use, high-wear version of the nano coating.
In our business of installing and coating wood floors, we must stay abreast of the changes available to us both for practical and environmental reasons. It's important that you understand the finishes on today's market and find the ones that will work the best for your business.