Why Wood Epoxy?

Wood in its natural state is one of the best building materials in the world. It has a good weight strength ratio, it doesn't fatigue, and is visually pleasing so the structures themselves can be part of the decorative system. In this respect wood is unique among materials. And it requires no special production methods to process the raw material. The downside is that it absorbs moisture vapour from the atmosphere.

The common misconception is that wood absorbs water as a liquid. This is largely untrue. As an example, think of a thin walled structure like a garden shed. When it rains, water doesn't get through the walls. The wood is waterproof. But if you stand in that garden shed on a very humid day, the walls would be very wet inside; the reason being that wood absorbs moisture vapour - that is, water in a gaseous form in the atmosphere.

Let's look at what happens to wood when it absorbs water vapour. In its natural or green state, when timber is felled, it has a moisture content of about 50%. This is because a tree is a living thing, drawing water up through its cellular structure to provide and store nutrients. Left to dry naturally, the moisture content will fall to about 16%. This is called seasoned or air dried timber. If dried in a kiln, the moisture content is reduced to 12% or less.

When air or kiln dried wood is exposed to the atmosphere on a humid day, it gains moisture; on a dry day, it loses moisture. As it gains moisture, wood swells. As it loses moisture, it shrinks. It is this swelling and shrinking that causes timber structures and parts to move and to cast off their coating systems, because these systems are insufficiently flexible to cope with the amount of movement.

We are all aware of the results of swelling and shrinking. The paint on our houses flakes off windows, especially where two parts are joined. Doors and windows jam in humid weather and become free in dry weather. These are mechanical disadvantages.

But there is a decay problem as well. For timber to decay there has to be both oxygen and water. The moisture content has to be at least 20%, which is exactly the situation with normal untreated timber structures in the open air, or in the ground. Submerge timber is water, but in an oxygen-free environment and it won't rot. And if you expose timber to oxygen but keep the moisture content below 20% it won't rot either.

There is a third factor, which is strength. Between 50% and 12% moisture content, timber gets stronger the drier it is. So at 12% moisture content, a piece of timber is substantially stronger than the same piece at 50%. In addition, as timber becomes drier, it becomes lighter.

Timber is a great structural material. It is plentiful, it is beautiful, it is renewable, it is low tech, and it is environmentally friendly. If we can reliably keep the moisture content at 12%, it will be dimensionally stable, so paint coatings won't fall off and joints will maintain structural integrity and stability. It won't rot or decay, and we shall be using the material in its lightest, strongest state. This means we can build lighter smaller structures for the same strength.

Traditional boat building methods have to overcome the four problems of timber as a structural material which, as we have seen, are dimensional instability, decay, variable weight and variable strength. As a result, only certain timber species are suitable. Suitable timber, such as teak, pitch pine and oak, are resistant to decay because of the presence of natural resins oils and tannins. And these timbers tend to shrink and swell least, with varying moisture content. These traditional timbers are also relatively heavy.

Because the structures have to be designed to allow for movement of swelling and shrinking, traditional boat building joints are designed to allow for some movement while retaining structural integrity. And structural sizes or scantlings have to be designed to allow for the material in its weakest state, that is, wettest state. So the resulting structure is heavier, larger and clumsier than optimum.

Few adhesives are viable with the traditional boatbuilding timbers, because of the oils and resins; thus timber pieces have to be fastened together by mechanical fastenings and joints, generally producing a less rigid structure. And paint coatings don't stay on very well either, giving an on-going maintenance task.

In regard to sailboats, when the force developed by the sails is applied to the structure through the spars and rigging, part of the effort is lost or absorbed in moving the structure about. Thus, a lower proportion of the effort goes into moving the sailboat forward.

But all these problems can be overcome if the moisture content can be reduced, say to 12%, and held reliably at that reduced level.

That is exactly what the modern wood epoxy system does. A good epoxy system such as the WEST™ System will hold the moisture content of the timber to within 1/2 of 1%. This means that timber can be used in its lightest strongest condition, and the structures can be engineered to take advantage of this without having to consider the problems of the timber absorbing moisture or becoming heavier or weaker.

The epoxy coatings are both waterproof and more importantly, vapour proof, so that even in conditions of high humidity, like a steam bath, the moisture content does not rise significantly. Epoxies are also first-class adhesives. Pieces of epoxy coated timber can be effectively glued together structurally. An epoxy hull becomes a mono-structure, instead a lots of separate parts mechanically fastened together.

Every part of the mono-structure contributes to the totality of the load bearing structure. A further advantage is that the epoxy resin coating migrates into the outer cells of the timber, displacing the air that would normally be there, and this substantially increases the ability of the timber to withstand compressive loads.

Contrary to popular belief, when a piece of timber such as a beam ruptures, it does so because the cells on the side where the load is applied fail in compression, rather than the cells on the other side failing in tension. The application of an epoxy coating significantly strengthens timber as well as allows the timber to be used in its naturally strongest condition.

Because of the excellent adhesive qualities, joint structures are much simplified. Because of the low moisture content, and lack of exposure to oxygen, the timber will not decay. This allows for the use of many more species of timber, heretofore unavailable to boat building for their lack of durability. Indeed, the more traditional timber such as teak, pitch pine and oak are not suitable for epoxy resins as the presence of resins, oils, and tannins impairs adhesion.

We can use woods such as Douglas fir, larch, mahogany, ash, sycamore, and most soft woods, many of which are grown reliably and verifiably in an environmentally sustainable way.

Because it doesn't gain moisture it doesn't move, so coatings are viable for much longer. This gives a modern wood epoxy timber structure much the same maintenance profile as other modern engineering materials, such as aluminium and glass fibre.

A single disadvantage is that the coating will degrade in sunlight. This can be fixed by mixing a pigment in the resin or applying a paint system over it. If a clear finish is required, an ultra-violet resistant varnish is applied over the wood epoxy coating and maintained regularly. Having said all that it is reckoned that an epoxy coating system left uncovered will last seven years in full tropical sunlight.

Summing up, wood epoxy provides an elegant, lightweight, strong, low-maintenance, and long-lived structure with assured mechanical properties.

© George Whisstock. This article is for information only and may not be comercially reproduced in any form or used in any way without permission.