Changing the properties of wood
The purpose of protecting wood is to preserve the good properties of wood and wood products and, at the same time, to prevent damage by decay, fungus, pests, etc. Usually, wood can keep untreated for long periods if the material receives adequate structural protection. A prerequisite for the long-term durability of wood is, among other things, to keep its moisture content constantly below 20%. If wood must be kept in conditions such that its protection is not possible merely by structural means, chemical protection can also be used. Such methods include spraying, coating with or dipping in a wood protection agent, or pressure- and vacuum impregnation. Before chemical methods started to be used, wetting the wood in a saline solution or charring its surface were used as means of protecting it, amongst other methods.
Sprayed and brush-applied wood protection agents usually only penetrate the surface of the wood to a depth of 1-2 mm, so their wood-protecting effect is minor, unless the agent is reapplied often enough. With the dipping method, the chemical agents can reach a depth of about 5 mm under the surface of the wood. New wood protection agents and coatings are being constantly developed, and their range of choices is quite large. Most wood protection agents contain pentachlorophenol, lacquers and water-repellent substances. Wood impregnation aims to protect the wood against biological destruction and pests. Wood impregnation classes are determined according to the stress that the target is exposed to. In vacuum impregnation, the surface of the wood is impregnated to a depth of 5-10 mm. In pressure impregnation, the impregnation agent can penetrate the whole of a pine tree with the exception of the heartwood. Spruce cannot be impregnated to a depth of more than 10 mm because of the aspiration (= closing of the cell walls) of the surface wood. Several dozen kilogrammes of impregnation agent are absorbed per cubic metre of wood, and the amounts vary depending on the class of impregnation.
There is much and partly contradictory information about surface treatment agents and coatings for the components of wooden buildings. Set at different ends of the spectrum have usually been industrially produced paint products and natural so-called traditional paints. For example, the use of paints in facade cladding that form dense films and the many kinds of damage that have ensued have ruined the reputation of wood as a durable material for facades. On the other hand, however, the cause of the damage has not always been only the paint, but also errors in design and construction together with poor-quality wood. From a perspective of the surface treatment of wooden facades, the key thing is that the structures are correctly made.
Wooden surfaces have traditionally been treated principally to achieve the desired colour rather than to improve the resistance of the wood to the weather. In choosing wood surface treatment agents and coatings, it is important to take into account how easy it is to take care of the surface and the right time for applying the treatment. The degree of roughness of the surface of façade cladding affects the durability of a painted surface. A finely-sawn wooden surface is usually best from a point of view of paint durability. Paint usually peels off planed surfaces exposed to tough weather conditions. On the other hand, excessively rough and uneven surfaces are difficult to treat and easily get dirty. For example, organic tar and red earth paints can experience wear from the effects of weather. Linseed oil chalks and wears in a similar way. Because of this, repainting neither makes the paint film thicker nor causes too dense a layer on the wood surface, so maintenance- and repainting can be done without the laborious removal of the old paint. In maintenance painting, typical errors are neglecting to do it at all or doing it too rarely, the wrong choice of paint and painting on a surface that is too wet or dirty.
The properties of wood can be changed and improved by means of different treatments. Wood that has been developed in accordance with requirements set for the purpose is called technical wood. Porous wood can be processed using pressure, heat or chemicals that impregnate the wood. The modification and chemical treatment of deciduous trees are somewhat easier than for conifers because of the difference in the trees’ cell structure. Using chemical agents such as maleic anhydride glycerol, it is possible to reduce the moisture dynamics of the wood and to improve its resistance to decay and fire. Many treatment agents are, however, relatively expensive, and some of them or the chemical agents created as a by-products of their processing have been proven to be harmful to the environment. Because of this, the chemical protection of wood is beginning to be shunned as general ecological values gain in importance. The density, strength and surface hardness of wood can be increased by compressing it. Deciduous wood can be compressed so that its volume is reduced by up to 50%. The corresponding figure for conifers is approximately 40%. Compressed wood, however, returns to its original volume under the influence of moisture, unless it is stabilised using chemical agents.
Wood can also be heat-treated, reducing its moisture dynamics and improving its biological durability. Wood always gets darker with heat treatment. Moreover, several different extracts are removed from wood, it gets lighter, its equilibrium moisture content declines and its heat insulation capacity increases by almost one-third. At the same time, however, its rigidity is reduced and its strength properties are impaired to a certain extent. Unless the heat treatment process is done correctly, the risk of the wood cracking is high. Originally, internal cracking was considered to be one of the worst faults of heat-treated wood when it was machined after treatment. The surface of heat-treated wood gets very compact, so glueing, for instance, becomes more difficult, and drying times are slower than usual when using normal PVAc glues that are absorbed into the wood. Paint, on the other hand, adheres better on heat-treated wood. Protecting the ends of the wood by painting or with batten strips is also important in heat-treated wood, to prevent water from being absorbed in the direction of the grain.