There are two aspects to the colour changes that occur in timber floors. One is the oxidisation of the timber. This occurs when the pigments, tannins and other resins in timber are exposed to air and UV light.
Over time this will cause yellowing in light timbers, while darker timbers usually develop rich brown or pink red tones and uncoated timbers will grey or silver.
Oxidization is a natural process, and the effect is noted in both coated and uncoated timber. The degree to which timber will change colour depends on the extent of its exposure to direct and indirect UV light and will vary from species to species.
While the chemicals in floor finishes are known to influence the oxidization process in timber. In their own right change colour over time. Solvent based polyurethanes, tung oil and wax type finishes are more prone to yellowing than their water based counterparts.
Polyurethane coatings are known for their durability, chemical resistance, abrasion resistance and film clarity. However a little information about their chemical make-up can go a long way towards helping us understand why some products yellow and others don’t.
The building blocks for polyurethane are polyols and isocyanates. These chemicals can be:
- Dissolved in solvents in unreacted form as the basis of solvent based floor coatings;
- Dispersed in water in reacted form as the basis of a waterborne floor coating.
There are two types of isocyanate used to form polyurethane:
- Aromatic isocyanates that contain double carbon bonds, which react with ultra violet radiation and cause yellowing to occur, and
- Aliphatic isocyanates which do not contain double carbon bonds and therefore do not react to ultra violet light.
Aliphatic isocyanates are much more expensive than their aromatic counterparts, hence the higher cost of non-yellowing floor finishes.
Like their solvent-based polyurethane counterparts, oil and wax type floor finishes tend to yellow over time. Even those that appear totally void of colour, like linseed oil. While imparting a gold or amber hue to the timber, oil and wax finishes. While generally considered less durable, given the need for more frequent refurbishment. Are often a viable solution for floors that experience excessive movement or where a more visible grain structure is a desirable aesthetic.
An experiment to observe the yellowing of polyurethane, a coat of solvent-based polyurethane was applied to a sheet of clear glass. In a relatively short period of time the coating material took on a warm yellow glow. This experiment can also be conducted by partially coating a white painted board. This is a great way to show to consumers the effect of age and UV light on polyurethane coated materials.
Historically, water-based finishes were thought to be less durable than solvent-based counterparts. As well as slower curing and more environmentally friendly.
But technology changes, and in a relatively small space of time the manufacturers of theses coatings have closed the gap. And are now offering products with comparable durability and super-quick drying times.
Some of the products currently available are dry enough for use 24 hours after application. All without altering the key characteristics; as well as being environmentally friendly and non-yellowing.
This is what made them popular in the first place, such as our Carbon Coat Polycarbonate Timber Coating.
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