What the Architect Needs to Know


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     Wood can be a very beautiful material, either as trim or as an entire object.  Not only on boats, but around homes (doors, floors, counters, tables, etc.) clear coatings on wood have been used for thousands of years.  In ancient China in fact, lacquer was not only an article of commerce but was accepted by the government in payment of taxes.

     The high gloss and depth of the lacquer or varnish make the natural beauty of wood visible. Out of direct sunlight, varnish and lacquer maintained their beautiful appearance and high gloss, but with direct sun exposure the gloss rapidly failed, the surface turned dull and the coating cracked and peeled.

     Modern technology has developed clear coatings which have exceptional durability and resistance to degradation by weather. The modern two component polyurethanes now far exceed the gloss retention and scratch resistance of any one-part finish and even the best modern varnish holds its gloss far better than the varnish of a hundred or a thousand years ago.

     In order to understand how clear coatings can last for years when exposed to the elements, it is necessary to understand why coatings fail.  They fail in two different ways.  First, water may cause a chemical decomposition of the coating itself, or 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.

     Second, ultraviolet light (which is an invisible part of both natural sunlight and interior fluorescent light) can cause chemical decomposition of the substance of the coating itself, or can cause chemical decomposition of the wood beneath the coating, allowing the bond between the wood and the coating to fail. Ultraviolet light also burns away (oxidizes) the surface, causing the coating to lose its gloss.

     Clear coatings may fail by loss of flexibility with age, and this will manifest as a cracking, tearing, or peeling of the film.  Where the coating was applied to two adjoining pieces of wood and bridged over them, relative motion may tear the film loose from the substrate without the film itself failing.  The visual result of this is usually a whitish line appearing in the clear coating over the wood joint.

     Clear coatings may also fail by burnout of the Ultraviolet absorbers.  This will manifest as the underlying wood color bleaching lighter and finally to grey. This rarely happens with age: Most failed clear coatings are applied too thin and they fail too soon.

     Delamination of a clear coating from the substrate may also happen, usually around the time the substrate changes color to gray. This is caused by the wood being decomposed by the ultraviolet light; the coating now has nothing to stick to and of course delaminates, peeling and cracking as it comes loose. Clear coatings containing Ultraviolet absorbers must be applied to some predictable thickness, so that there is enough Ultraviolet absorber chemicals over the wood to afford some protection to the wood.  The design thickness of Five Year Clear TM Polyurethane is .010 inches, and this can be reliably obtained by controlling the usage rate so that the solids content of a certain size kit is applied over a specified area. This film thickness has been found sufficient to protect the color of the wood in latitude 28, around Ensanada, Mexico and San Diego, CA.

     Smith & Co. Five Year Clear Polyurethane is stabilized to resist water degradation by its molecular structure, and to resist ultraviolet degradation and the attendant loss-of-gloss in various ways that will be discussed in this essay.

     Ultraviolet light attacks almost everything.  All organic compounds, whether synthetic or natural, will eventually be attacked and broken down by ultraviolet light. Even the best urethane paints, called aliphatic (a chemist's name for a straight molecular chain) or linear (an ordinary person's name for the same thing) polyurethane will lose about half their gloss in two years of outdoor exposure.  It is not enough to make a clear coating which is not much degraded by ultraviolet light, as such a coating would simply transmit the ultraviolet light through to the wood underneath.  Wood consists of hollow fibers of cellulose (a kind of sugar, very tasty to fungus and termites) glued together by a material called lignin.  Lignin is a very hard, strong resin (a phenolic resin, chemically a half-brother to the resorcinol glue used to make plywood) which is very resistant to water, but is decomposed very quickly by ultraviolet light.  Therefore, ultraviolet absorbers were invented.  These chemical compounds act as magnets for ultraviolet light.  When a molecule of this absorber material captures a photon (light comes in small units; they are called photons) it converts the energy of the ultraviolet photon into heat.  When it does this, the molecule vibrates.  The phenomenon is very much like ringing a bell.  We know that if you strike a bell often enough, the bell will crack.  The molecules of ultraviolet absorber wear out in the same way.  Eventually they will die and no longer absorb ultraviolet light.  The more ultraviolet absorbers the manufacturer puts in the clear coatings, the longer the coatings will last, assuming that high-quality ingredients are used and the coating itself is correctly designed.  The very best ultraviolet absorbers in the world are made by Ciba-Geigy in Switzerland.  They are very expensive and we at Smith & Co. use a lot of them in our Five Year Clear polyurethane.

     Sometimes ultraviolet light will break a molecular chain in the urethane polymer.  When this happens the two molecular fragments (called free radicals) will glue themselves onto neighboring polymer chains, making extra cross-links.  These are extra branches in a chain, like rungs on a ladder.  As more cross-links are made, the coating loses its gloss and its elongation capability.   That is to say, it becomes stiffer and cannot stretch as much as the natural expansion of the wood, and eventually cracks and tears and flakes.  Polyurethanes, traditional varnish and, for that matter, any clear finish will get more brittle with age for this reason.

     There are special chemicals designed to trap and neutralize these free radicals before they can do their damage.  They are called antioxidants (something like vitamin-E, actually) and they work the same way your antioxidant vitamins work to keep you healthy.  The best antioxidants in the world are also made by Ciba-Geigy in Switzerland.  They are very expensive and we use a lot of them.

     Interestingly enough, conventional varnishes cure by a chemical reaction between the oil and the oxygen in the air.  This is called oxidation, and the addition of antioxidants to a conventional varnish would poison the curing reaction.  It is therefore impossible to add antioxidants to varnish and thus any varnish will lose its gloss and flexibility fairly quickly.   One of the reasons that two-component polyurethanes generally last longer than varnishes is that it is possible to use antioxidants in two-component polyurethanes to extend their life.  There are other additives (chemically similar to TeflonTM) which give the very high gloss.  They are made in the U.S.A. but are also quite expensive.  The Additive Package (as we paint chemists call it) is actually about double the cost of the basic urethane-resin-plus-solvent ingredients, which is why our clear polyurethane costs about triple what it costs to make any other similar products.  Any other competent paint manufacturer could make something similar to our Five-Year Clear, if they wanted to spend the money; all the chemists know who makes good ultraviolet absorbers.  Coating manufacturers who sell to distributors who sell to wholesalers who sell to retailers who sell to the end user must make a low-cost product so everyone in the chain can make a profit.  It is our policy to sell our Five Year Clear factory direct only to the end user, and thus give more value to the end user.

     The final step in obtaining a durable clear coating is to develop a stable chemical bond to the wood beneath the coating.  This can be done by saturating the wood surface first with Clear Penetrating Epoxy Sealer (CPES).  The sealer glues down the clear coating, while the ultraviolet absorbers in the clear coating protect the epoxy from the sunlight.

     Smith & Co. Clear Penetrating Epoxy Sealer impregnates the wood substrate with a hydrophobic resin system wihch bonds the wood surface fibers together and into the wood substrate, where there was open porosity. This gives a stronger surface, better attched to the bulk of the wood itself and thus promotes water resistance of the wood substrate as well as better topcoat adhesion.

     Sometimes it is desirable to stain wood before a clear coating is applied.  It is important that the stain dry and chemically cure quickly and completely, so that the solvents in the Clear Penetrating Epoxy Sealer do not redissolve the binder resins of the stain and rearrange the pigments of the stain, leaving brush marks of coloring.  Waterborne stains with acrylic resin binders are not recommended as epoxies do not adhere to them.  Stains that contain mineral spirits are likely to be compatible.  If one were able to obtain a transparent, oil-base stain, those are the types that have historically been used with the greatest success.  Your personal experience and evaluation will be necessary to find satisfactory stains.

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