Wood industrially pressure-treated with approved preservative products poses a limited risk to the public, but should be disposed of properly. On December 31, 2003, the US wood treatment industry stopped treating residential lumber with arsenic and chromium (chromated copper arsenate, or CCA). This was a voluntary agreement with the United States Environmental Protection Agency. CCA was replaced by copper based pesticides, with exceptions for certain industrial uses. Industrial wood preservation chemicals are generally not available directly to the public and may require special approval to import or purchase depending on the product and the jurisdiction where being used. In most countries, industrial wood preservation operations are notifiable industrial activities that require licensing from relevant regulatory authorities such as EPA or equivalent. Reporting and licensing conditions vary widely depending on the particular chemicals used and the country of use.
Some wood treatment Methods
Heat treatments
There is ongoing research as to whether heat treatments can be used to make timber more durable. By heating timber to a certain temperature, it may be possible to make the wood-fibre less appetising to insects. Although unlikely to be as effective as chemical preservatives, anecdotal evidence suggests some consumers would prefer chemical-free timber preservation methods.
Heat treatment can also improve the properties of the wood with respect to water: lower equilibrium moisture, less moisture deformation, and weather resistance. It is weather-resistant enough to be used, unprotected, in facades or in kitchen tables, where wetting is expected.
There are 3 similar European heat treatments: Retiwood, developed in France, Thermowood, developed in Finland by VTT, and Platowood developed in The Netherlands. These processes autoclave the treated wood, subjecting it to pressure and heat, along with nitrogen or water vapour to control drying in a staged treatment process ranging from 24 to 48 hours at temperatures of 180 to 230 °C depending on timber species. These processes increase the durability, dimensional stability and hardness of the treated wood by at least one class; however, the treated wood is darkened in colour, and there are changes in certain mechanical characteristics: specifically, the modulus of elasticity is increased to 10%[citation needed], and the modulus of rupture is diminished by 5% to 20%[citation needed] ; thus, the treated wood requires drilling for nailing to avoid splitting the wood. Certain of these processes cause less of an impact than others in their the mechanical effects upon the treated wood. Wood treated with this process is often used for cladding or siding, flooring, furniture and windows.
Mud treatment
Wood and bamboo can be buried in mud to help protect it from insects and decay. This practice is used widely in Vietnam to build farm houses consisting of a wooden structural frame, a bamboo roof frame and bamboo with mud mixed with rice hay for the walls. While wood in contact with soil will generally decompose more quickly than wood not in contact with soil, it is possible that the predominantly clay soils prevalent in Vietnam provide a degree of mechanical protection against insect attack which compensates for the accelerated rate of decay.Also, since wood is only subject to bacterial decay under specific temperature and moisture content ranges, submerging it in water-saturated mud can retard decay by saturating the wood's internal cells beyond their moisture decay range
Brush and spray treatments
Brushing preservatives is a long-practised method and often used in today's carpentry workshops. Technological developments mean it is also possible to spray preservative over the surface of the timber. Some of the liquid is drawn into the wood as the result of capillary action, but this penetration is insignificant and not suitable for long-term weathering. By using the spray method, coal-tar creosote, oil-borne solutions and water-borne salts (to some extent) can also be applied. A thorough brush or spray treatment with coal-tar creosote can add 1 to 3 years to the lifespan of poles or posts. Two or more coats provide better protection than one, but the successive coats should not be applied until the prior coat has dried or soaked into the wood. The wood should be seasoned before treatment.
Dipping
Dipping consists of simply immersing the wood in a bath of creosote or other preservative for a few seconds or minutes. Similar penetrations to that of brushing and spraying processes are achieved. It has the advantage of minimizing hand labor. It requires more equipment and larger quantities of preservative and is not adequate for treating small lots of timber. Usually the dipping process is useful in the treatment of window sashes and doors. Treatment with copper salt preservatives is no longer allowed with this methodSteeping
In this process the wood is submerged in a tank of water-preservative mix, and allowed to soak for a longer period of time (several days to weeks). This process was developed in the 19th century by John Kyan. The depth and retention achieved depends on factors such as species, wood moisture, preservative and soak duration. The majority of the absorption takes place during the first two or three days, but will continue at a slower pace for an indefinite period. As a result, the longer the wood can be left in the solution, the better treatment it will receive. When treating seasoned timber, both the water and the preservative salt soak into the wood, making it necessary to season the wood a second time. Posts and poles can be treated directly on endangered areas, but should be treated at least 30 cm (0.98 ft) above the future ground level.The depth obtained during regular steeping periods varies from 5 to 10 mm (0.20 to 0.39 in) up to 30 mm (1.2 in) by sap pine. Due to the low absorption, solution strength should be somewhat stronger than that in pressure processes, around 5% for seasoned timber and 10% for green timber (because the concentration slowly decreases as the chemicals diffuse into the wood). The solution strength should be controlled continually and, if necessary, be corrected with the salt additive. After the timber is removed from the treatment tank, the chemical will continue to spread within the wood if it has sufficient moisture content. The wood should be weighed down and piled so that the solution can reach all surfaces. (Sawed materials stickers should be placed between every board layer.) This process finds minimal use despite its former popularity in continental Europe and Great Britain.
source :wikipedia
