Article Archive | Back To Current Issue

Fall 2006 - Article 2

From The Lab
Oil-Modified Urethane Coatings

Over the years we have published numerous articles
on Essential’s water-based coatings: chemistry, drying
mechanism and properties to name just a few. This article is our first in-depth look at our OMU technology – specifically, formulations and how they dry and cure once applied to a wood floor.

Oil-Modified Urethane (OMU) wood finishes and seals are fairly straightforward formulations. There is a slight
difference in lower solids coatings (30% – 40%) versus higher solids (50%+) VOC-compliant formulations.

Low solids’ finishes and seals contain a urethane resin as their backbone (solids left on floor). The resin is blended with an evaporative solvent (usually mineral spirits), a metal drier (usually cobalt-based) and an anti-skinning agent (keeps the finish from gelling in the bucket). These products range in VOC from 500 – 550 g/L.

Higher solids’ products replace some of the mineral
spirits with the addition of more resin to lower the VOC. Since they tend to dry slower (less evaporative solvent), an additional metal drier is added (usually aluminum-based) along with an accelerator. These coatings have VOC of <450 g/L.

Another class of OMU is the 350 VOC formulations. They achieve their lower 350 VOC through the incorporation of an exempt solvent called, PCBTF. The EPA has determined that PCBTF is not an ozone depleter so it is exempt from VOC calculations. The 350 coatings perform equivalent on the floor to standard 450 OMU’s.

OMU’s dry and cure through two different mechanisms. Drying takes place as the mineral spirits evaporates. Depending on the formulation and environmental factors such as temperature and humidity, this process may take from one to 12 hours. Curing takes place through an oxidative mechanism. Oxygen, along with the cobalt drier, reacts with the drying oil in the urethane resin to crosslink the coating. The faster this oxidative crosslinking occurs, the faster the film will cure and be ready to handle foot traffic.

Essential Industries has products that address the various VOC categories. Our OMU Wood Seal (#230) and 40% Gym Finish (#293) fit into the low solids category. Our High Solids Gym Finish (#234) is a 450 VOC-compliant finish. Lastly, Comply (#232) is our 350 VOC-compliant finish.