Preservation of Timber

Preservatives are used to protect timber against deterioration due to fungi and attacks of termites, borers and marine organisms. Most efficient means of preserving timber are, good seasoning and free circulation of air. Protection against moisture is afforded by oil-paint provided that the timber is perfectly dry when first painted; otherwise the filling up of the outer pores only confines the moisture and causes rot. A prerequisite for satisfactory treatment is that the timber shall be seasoned so that the outer layers have a moisture content 

of less than 20 per cent. For exposed timber, the only remedy at present available is impregnation by substances poisonous to fungi, these substances being either of the oil or chemical types. 

The preservatives used, fall into two categories-Barrier  Preservatives and Diffusing preservatives. The former are generally copper based oils which may be applied by brush or spray or in emulsion form.  The latter are generally copper fluorohoron gels and solid rods. 

In the latest chemical products developed, copper a traditional timber barrier preservative is deleted as it is considered that it reduces the ability of the active constituents to diffuse. These diffusing preservatives are introduced by drilling holes (generally 16mm dia) in the timber at predetermined locations, installing the gel or rod and inserting a stopper which may be removed for renewal of the preservative. 

Barrier preservatives are applied to end grains of  all structural members which are subject to wetting and drying through rain. Spacing of holes in critical locations should not exceed 300mm. Holes are usually located only where decay is suspected. 

Although the chemicals used, have low toxicity, contact with the eyes can be painful and skin contact can produce burning or rashes.  Also some chemicals can stain or possibly damage clothing. 

Another problem involves the dripping of the preservatives.  Alternative water based products to be used are copper chrome arsenic preservatives. 

A product by name PRESCHEM ROD is with active ingredients Boron and fluorine. The rod is hygroscopic and swells to fill the drill hole tightly.  Diffusion is slower but no free liquid is involved so that dripping does not occur. 

Another problem is the incompatibility of the preservative with the paint systems. Barrier preservatives are applied once paint is removed and paint will not adhere to the surface once the preservative is applied. 

In the case of the preservative B.P.P.G (BUSAN POLE PRESERVATIVE GEL) a product based on (THIOCYANOMETHYLTHIO) BENZOTHIAZOLE, it was noticed that it virtually acted as a paint stripper from within. The problem was overcome using a solid rod form of diffusing preservative like PRECHEM rod. 

In the U.K., the two principal types of preservatives used for new building components: 

1. The water borne salt types are the formulations based on solutions of copper sulphate, sodium dichromate and arsenic pentoxide, they are applied by pressure and 
2. Organic solvent based types. The solutions consist of one or more organic fungicides in an organic solvent such as white spirit or a more refined solvent. Common fungicides are Pentachlorophenol, Tributyl Tin Oxide, Zinc Napthenale and Pentachlorophenyl Laurate.  There are many others. In addition an insecticide should be added if resistance to insect attack is required. It is quite normal to apply organic solvents by a double vacuum process in a pressure tank although some species can be treated for certain applications by immersion in an organic solvent solution. 

Use of tar oil type preservative e.g., creosote gives strong smell and can contaminate. Hence this is usually relevant to the preservation of fencing. 

Timber preserved by a diffusion process e.g., the boron process must be carried out on timber before it is dried. Several preservatives are formulated and designed for remedial work.  

Water Soluble Preservatives

Chemical Salts 

They are water borne preservatives and they are mostly salts dissolved in water.  The usual salq used are copper sulphate, mercury chloride, sodium fluoride and zinc chloride. 

Solutions are prepared from these salts and they are applied on timber surface. These preservatives are odourless and non-inflammable. The treated surface can be painted or varnished after drying. These preservatives have good penetration and timbers treated with these preservatives will show an immediate increase in weight of 240 to 480kg/cum. After drying, the net increase in wight will come down to about 5 to 30 kg/cum. 

Coal Tar 

Timber surface is coated with hot coal tar with the help of brush.  Coal tar becomes workable when heated.  The process is known as tarring.  Coal tar has unpleasant smell and appearance. It makes timber suitable for painting.  Hence tarring is adopted for frames of doors and windows, rough timber work, etc. and it is found to be most useful for parts embedded ground or masonry because of its cheapness and effective resistance.  Coal Lar is fie-resistant. 

Creosote Oil 

In this case, timber surface is coated with creosote oil. The process is known as creosoting. Creosote oil is obtained by the distillation of tar. Creosoting is carried out as follows: 

1. Timber is thoroughly seasoned and dried. 
2. It is then placed in an airtight chamber. 
3. Air is pumped out from the chamber. 
4. Creosote oil is then pumped under a high pressure of about 7 to  10 kg/cum and at a temperature of about 50 C. 
5. After a period of about 1 to 2 hours, when timber has sufficiently absorbed creosote oil, it is taken out of chamber. 

Creosote oil is one of the best antiseptic, i.e. substance poisonous for wood attacking fungi. It is a black or liquid, weakly affected by water, neither volatile nor hygroscopic, harmless to wood or metal, inflammable, with an unpleasaut odour and having low wood-penetrating ability to the extent of l mm to 2mm only. 

Creosotuig cyclically doubles the life of timber and it is generally adopted for piles, poles, railway sleepers, etc.  Depending upon the net retention and hype of timber, creosote treated  timber will normally increase in weight by 80 to 320 kg/cum. Creosote oil IS highly toxic in nature arid gives out highly unpleasant smell. Creosote oil should not be used for interior surfaces of dwelling houses, fwd stuff storage premises, in underground installations and near inflammable surfaces. 

Oil Paints 

Timber surface is coated with two or three coats of oil paint. Wood should be seasoned. Otherwise sap will be confined and it will lead to decay of timber. Oil paints preserve timber from moisture and thus make it durable. 

Solignum Paints 

These paints preserve timber from white ants as they are highly toxic in nature. They can be iluxed with colour pigments arid applied in hot state with the help of brush. Timber surface may, therefore. be given the desired colour or appearance.