Fungi are microscopic organisms that consist of threadlike structures and depend on organic matter for food. There are three types of wood fungi:
• Mould - grows on the surface of wood utilising sugars and other carbohydrates. It gives the wood a 'woolly' or powdery appearance but does not affect timber strength.
• Sapstain - these fungi are pigmented and discolour or stain infected timber. Sapstain fungi do not affect timber strength but do make small holes in cell walls and membranes, making it more susceptible to rot.
• Decay - is the most destructive form of fungal attack. Three types occur: brown, white and soft rots. These utilise the cellulose and lignin in cell walls, quickly weakening timber strength.
Insects involved with wood destruction are generally either termites or beetles. However, wood wasps, moths and carpenter ants are significant in some areas around the world. Insects feed on the wood, damaging it by chewing and boring into it. Some insects do negligible structural damage while others, such as termites, can leave only a thin outer shell of wood, the inside being completely hollow.
Preservation Chemicals
In New Zealand, about half of the sawn timber production is 'treated' in some way to protect the wood from decay or biological attack. Globally, the annual loss due to biological destruction of timber is estimated in excess of US$10 billion. As most wood preservation efforts are aimed to counter biological degradation, only that will be discussed in this section.
The two principal organisms responsible for destroying wood are fungi and insects.
Timber preservation works by impregnating wood with toxic chemicals that poison or make it unpalatable to attacking organisms. The main wood preservative chemicals are:
Creosote - produced by the high-temperature distillation of bituminous coal. The distillate consists of solid aromatic hydrocarbons, and tar acids and bases. Creosote is toxic to fungi and insects, is relatively insoluble in water, and is generally low cost. The pungent odour, oily finish and its propensity to leak and stain surrounding material makes it less desirable in some situations. Creosote is used where timbers are in contact with the ground such as railway sleepers, fence palings and battens. It is also effective in marine structures.
PCP - or pentachlorophenol (C6CI5OH) was used for many years as a fungicide. It also affords good protection from insects when dissolved in heavy oil. It is ineffective against marine borers. Many countries, including New Zealand have banned the use of organochlorines such as PCP, because of their broad toxicity and persistence in the environment.
LOSP - light organic solvent preservative contains insecticides for internal use, and combinations of fungicides and insecticides for external use, and sometimes water repellents. LOSPs may also contain mouldicides. All of these preservative components are incorporated in a solvent carrier such as white spirit.
LOSPs are used to protect timber against insects, including termites, and decay, though they are not intended for use in ground-contact situations. In addition, LOSPs can provide a degree of weather protection when water repellents have been incorporated. LOSPs are preservatives that leave the treated timber dry after treatment, and do not cause it to swell and distort in any way.
CCA - or copper-chrome-arsenate is a broad spectrum preservative. Copper and arsenic are the fungicide and insecticide respectively, while the chromium fixes the chemical to the wood. The chemical penetrates wood well, is odourless, and surfaces can be painted once dried. CCA treatment is common in New Zealand for outdoor or ground contact uses. The chemical can be used in both terrestrial and marine environments.
However, recent public health scares – originating in the USA and Europe – have led to the restriction of CCA for use in domestic and public access applications. This is due to fears in some quarters that the public may be at risk of taking up arsenic from skin contact with treated wood. Applications of particular concern have been children’s playgrounds, picnic areas and decking. Scientific research on the actual risks involved is continuing.
Boron - salts are another broad spectrum preservative, effective against fungi and most insects with low mammalian toxicity. The main problem with boron treatment is that it is water-soluble and does not chemically bond to the wood. It can therefore be leached out and can only be used in low hazard environments such as house framing.
Preservation Processes
There are a number of types of treatment techniques utilised in wood preservation:
Pressure Impregnation - uses vacuum and pressure to obtain chemical penetration of permeable timbers, while controlling the amount of preservative retained. The timber must be free of stain and have a moisture content of less than 25%. Many treatment schedules are used, the pressure fluctuations and timing being distinctive in each.
The Bethell process is the most important of the treatment processes and achieves about 90% of the theoretical maximum uptake in radiata pine. The Lowry treatment is designed to achieve maximum penetration with a low retention of preservative. Retention is around the 60% of theoretical maximum. The Rueping process is used principally with preservative suspended in hot oil such as creosote and PCP where a low net retention is desired for some hazard categories. Net retention here is as low as 40-50%. The Alternating Pressure method utilises repeat applications of pressure and vacuum to force preservative into green wood.
Vapour Phase - utilises the fact that some Boron esters boil at low temperatures. The liberated gas can be drawn into timber where it reacts with water and condenses. For this reason, timber must be very dry (<5-6% moisture content) or only low penetration is achieved. Treatment of framing timber in the drying kiln is possible and can offer considerable cost savings.
Vacuum treatments - utilise volatile organic solvents to transport the preservative into the wood. It is designed to treat dry profiled or machined wood. TBTO is typically used, being introduced to the timber by either a double vacuum or low pressure cycle. The advantage of this technique is that there is no dimensional swelling as associated aqueous treatments and the wood can be painted within a couple of days of treatment.
Diffusion - is used to introduce boron salts to green timber. The timber is sprayed or dipped in the preservative solution and block stacked (without fillets). The wood is then tightly wrapped and left for a number of weeks during which the boron salts diffuse into the wood. Thicker timber may require a second dip to top up the salt levels. For this technique to be successful the timber must have a moisture content of over 50%. If even just the timber surface dries out, the process may not work at all, or be uneconomically slow. Another method using the diffusion process is called double diffusion. This works in the same manner except two successive chemical treatments are used. The second chemical treatment (Na2Cr2O7/Na2CrO4 and Na2HasO4) precipitates with the first (CuSO4) to form a non-leachable preservative.