A Classic Engineering Problem
Engineering is simply the practice of applying physics and chemistry to technical problem solving. And all problem solving starts with a problem statement. The problem statement is derived from an understanding of why things fail. Thus, in our pursuit of the best clear coat system for wood, we will start with an analysis of the failure mechanisms involved.
Why Clear Coatings Fail
To more effectively develop clear wood coating systems that will last under prolonged exposure to the elements, first requires a thorough understanding of how and why such coatings fail. There are six key mechanisms for the failure of a clear wood coating:
- The coating is attacked by UV
- The coating outgases its plasticizers with time
- The coating is attacked by water
- The underlying wood is attacked by water
- The wood is attacked by UV
- The coating is mechanically damaged by the movement of the wood substrate
Each of these factors manifests a different effect. Taken together the most apparent visible effects include yellowing, loss of gloss, cracking and flaking of the coating. The wood progressively loses its originally attractive color and ultimately bleaches to gray.
1. Ultraviolet Light Attack—A Loss of Flexibility
Exposure to ultraviolet radiation slowly breaks the molecular chains in the coating. These molecular fragments (called “free radicals”) attach themselves to adjacent polymer chains making extra cross-links.
As more cross-links are made, the coating loses its elongation capability—it becomes stiffer and less able to stretch to comply with the natural expansion and contraction of the wood. It eventually develops cracks and tears away from the substrate.
Traditional varnish, lacquer, shellac, and even modern polyurethanes—any clear organic finish will get more brittle with age. You can see the effect by noticing that aged, flaking varnish curls outwards. This is because the outer surface becomes shorter than the inner surface due to the extra surface cross-linking.
2. Plasticizer Evaporation
Evaporation of plasticizers is another reason coatings lose flexibility with age. Plasticizers are non-reactive chemicals which some manufacturers add to their resins to make them less brittle. With exposure to UV and heat, and with incorrect selection, the plasticizer will evaporate with age, as it diffuses out of the material. Some lower-end epoxy products have volatile plasticizers like this (sometimes called fugitive plasticizers). Perusal of the associated MSDS will reveal they contain benzyl alcohol or benzyl butyl phthalate—two of the most common fugitive plasticizers.
3. Coating Is Attacked By Water
Water causes a loss of film strength in the coating allowing it to tear more easily, and stretch less before it fails. The effects of water involve chemical reactions between water and some kinds of plastics (acrylics and polyesters, for example) which deteriorate the coating. Fiberglass boats, for example, develop gel-coat blisters as a result of water attacking the polyester resin.
4. Wood Is Attacked By Water
Water may cause a chemical decomposition or swelling of the wood beneath the coating, allowing the bond between the wood and the coating to fail. Obviously, a coating that is too permeable to water will accelerate this attack.
5. Wood Is Attacked By UV
Wood can be thought of as the original composite structure. It consists of hollow fibers of cellulose, a form of sugar which are glued together by a material called lignin. Lignin is a very hard, strong resin, chemically related to the resorcinol glue used to make plywood. While being highly water resistant, lignin is adversely affected by ultraviolet light. With prolonged UV exposure the lignin breaks down allowing the wood fibers to separate.
6. Wood Substrate Movement
Where the coating is applied to two adjoining pieces of wood (as in the case of mahogany planking on a classic wood boat), relative motion between these pieces may tear the film loose from the substrate without the film itself failing. As the coating loses its flexibility, it becomes stiffer, and resists stretching to comply with the wood movement. Eventually the force required to stretch the coating over the joint exceeds the shear strength of the adhesive bond between the wood and the coating, and the coating tears away from the surfaces near the joint.