Protection of wood
Protection of wood
The two main types of protection for woods are barrier and chemical.
Barrier coatings can be applied to the wood to both preserve and seal the wood these coatings are usually made from epoxy resins and Oils. For example Tung oil penetrates the wood then hardens to form a hydrophobic layer up to 5mm thick.
Chemical protection can be classified into 3 different types of categories. These are;
- Oil Bourne preservatives – The most common oil-type preservatives are creosote, pentachlorophenol, and copper naphthenate. Occasionally, oxine copper and IPBC (3-iodo-2-propynyl butyl carbamate) also are used for aboveground applications. The conventional oil-type preservatives, such as creosote and pentachlorophenol solutions, have been confined largely to uses that do not involve frequent human contact. The exception is copper naphthenate, a preservative that was developed more recently and has been used less widely. Oil-type preservatives may be visually oily, or oily to the touch, and sometimes have a noticeable odour. However, the oil or solvent that is used as a carrier makes the wood less susceptible to cracks and checking. This type of preservative is suitable for treatment of glue-laminated stringers for bridges where cracks in the stringers could alter the bridges’ structural integrity.
- Water Bourne preservatives – Waterborne preservatives react with or precipitate in treated wood, becoming “fixed.” They resist leaching. Because waterborne preservatives leave a dry, paintable surface, they are commonly used to treat wood for residential applications, such as decks and fences. Waterborne preservatives are used primarily to treat softwoods, because they may not fully protect hardwoods from soft-rot attack. Most hardwood species are difficult to treat with waterborne preservatives. These preservatives can increase the risk of corrosion when metals contact treated wood used in wet locations. Metal fasteners, connectors, and flashing should be made from hot-dipped galvanized steel, copper, silicon bronze, or stainless steel if they are used with wood treated with waterborne preservatives containing copper. Aluminum should not be used in direct contact with wood treated with waterborne preservatives containing copper. Borates are another type of waterborne preservative. However, they do not fix in the wood and leach readily if they are exposed to rain or wet soil. Borate treatment does not increase the risk of corrosion when metals contact preservative-treated wood. Examples of water bourne preservatives are ; Chromated Copper Arsenate (CCA),Ammoniacal Copper Zinc Arsenate (ACZA), Alkaline Copper Quaternary (ACQ) Compounds, Copper Azoles (CBA-A and CA-B), Borates, Other Waterborne Preservatives
- Light organic solvent preservatives – This class of timber treatments use white spirit, or light oils such as kerosene, as the solvent carrier to deliver preservative compounds into timber. Synthetic pyrethroids are typically used as an insecticide, such as permethrin, bifenthrin or deltamethrin. While still using a chemical preservative, this formulation contains no heavy-metal compounds. With the introduction of strict volatile organic compound (VOC) laws in the European Union, LOSPs have disadvantages due to the high cost and long process times associated with vapour-recovery systems. LOSPs have been emulsified into water-based solvents. While this does significantly reduce VOC emissions, the timber swells during treatment, removing many of the advantages of LOSP formulations.
Most commonly used chemicals for Oil bourne preservatives is Creosote. Creosote is Coal-tar and is one of the oldest known wood preservatives. It is made by carbonising coal at high temperatures. Unlike most of the other oil preservatives Creosote is not dissolved in oil, but is naturally a mixture of organic molecules giving it its place in the oil bourne preservatives. The creosote gives wood a dark brown to black colour. Cresote However, creosote-treated wood has advantages to offset concerns with its appearance and odor. It has a lengthy record of satisfactory use in a wide range of applications and is relatively inexpensive. Creosote is effective in protecting both hardwoods and softwoods and improving the dimensional stability of the treated wood.
Most commonly used chemicals for Water bourne preservatives is CCA. CCA is Chromated Copper Arsenate can be used on wood above ground below ground and in contact with sea or freshwater.It usually has a greenish colour due to the chromium. It has a basic chemical composition of 47.5% Chromium trioxide 18.5 Copper oxide and 34%Arsenic pentoxide dissolved in water. Copper acts a primary fungicide, arsenic is a secondary fungicide and also an insecticide and Chromium acts as U.V resistance (as wood is a natural polymer). The risk of chemical exposure from wood treated with CCA is minimized after chemical fixation reactions lock the chemical in the wood. The treating solution contains hexavalent chromium, but the chromium reduces to the less toxic trivalent state within the wood (Hexavalent Chromium is carcinogenic, new regulations are enforcing that other alternatives need to be looked at). This process of chromium reduction also is critical in fixing the arsenic and copper in the wood.
Although largely undocumented, some preservatives may be more appropriate than others in sensitive ecosystems). For example, CCA has a much lower copper content than other waterborne preservatives (except the borates). Although there is no evidence at this time to suggest that any of the wood preservatives leach enough copper to harm terrestrial or freshwater ecosystems, CCA may pose less of a threat to aquatic ecosystems than preservatives with more copper. Similarly, preservatives without arsenic may pose less of a threat to mammals than those that do contain arsenic. Once again, there is no evidence that wood preservatives containing arsenic harm people or other mammals if they are used as intended. Wood treated with oil borne preservatives often produces an oily surface sheen when installed in stagnant freshwater environments. This may be unacceptable in some situations. Waterborne preservatives may be more appropriate when the treated wood will have extensive contact with freshwater
Selecting a Wood Preservative
The type of preservative that is most appropriate depends on the species of wood being treated, the type of structure, the cost, the availability of treated wood, and the specific area where the wood will be used. Generally, hem-fir (hemlock and fir) and southern pine can be treated adequately with any of the commercial wood preservatives, although copper napthenate has not been standardized for use with hem-fir. CCA is not recommended for treatment of Douglas-fir, which is more readily treated with oil-type or ammoniacal preservatives. CCA is not recommended for treating hardwoods that will contact the ground.