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Winter 2007 - Article 2

From the Lab
Oil-Modified Urethane Coatings

In the last issue of Essentially Speaking, we looked at how OMU’s are formulated and how they dry and cure once applied to floors. In this issue, we’ll dig a little deeper into the chemistry of OMU’s and look at how the resins used in formulating the finish are made.

We know from the previous article that OMU’s generally consist of four main ingredients: resin, evaporative solvent (usually mineral spirits), metal drier and an anti-skinning agent. The resin is obviously the key ingredient since this is what is left on the floor once the product dries and cures.

The resin used in formulating OMU’s is made by reacting an isocyanate (usually toluene diisocyanate or TDI) with a hydroxyl-bearing drying oil (hence the term oil-modified urethane) such as linseed oil. The urethane content, oil content and oil type all can impact the performance of these coatings. Linseed oil is commonly used in floor coatings since it provides fast dry time, good hardness and flexibility. It does tend to amber more than other oils, which is something all of us are accustomed to seeing on gym floors. If a lighter color is needed (varnishes used for furniture would be an example), then drying oils such as soya and sunflower can be used in place of linseed. Though lighter in color, these oils don’t provide the hardness of linseed.

The generic structure of an OMU is as follows: