A range of different chemical treatments has been developed to enhance the durability and service life of timber. Such chemical treatments impregnate the wood cells, making them resistant to decay, insects, weather or fire.
While chemical treatments add to the cost of the timber, they can significantly increase its lifetime. When used in environments in which there are known biological hazards, it is cost-effective to specify treated timber and expect a longer service life or a lower cost maintenance schedule. Remember though that treatment of timber is not a fail safe solution that will prevent any biological degradation. Rather it inhibits the degradation when the timber is used in a normal environment. In abnormal environments (such as cooling towers and some horticultural applications where the timber is almost continually wet) specialist advice on detailing for durability should be sought.
The preservative treatment of timber or timber products involves the introduction of stable chemicals into the cellular structure of timber that protect the timber from hazards such as wood destroying organisms like fungi and insects. Preservative treatments may also include the introduction of chemicals that inhibit fire. A design guide on timber service life is available for download below.
Preservative treatment against decay and insect attack
To determine whether the use of preservative treated timber is required, consider the following issues:
• The presence of a hazard - moisture, insect, decay, fire etc.
• The degree of structural reliability required - degree of load sharing, cost of failure and risk of injury or death if failure occurs
• Expected service life of the structure
• Natural durability of the timber
• Presence of sapwood
• Type of design of the building or component
Preservative treatment of timber is primarily concerned with protection of the sapwood. It is not possible to effectively treat heartwood in most species, as heartwood cells contain resins and other extractives which prevent the uptake of preservative solutions. The wide sapwood bands of the major plantation softwoods (radiata, slash, hoop pine) can be effectively treated with preservatives.
Preservative treatment level is specified using the same hazard level scale used to indicate hazard level.
• Treatment Level H1 - suitable for use in H1 Hazard environments - protected indoors - no termite risk
• Treatment Level H2 - suitable for use in H2 Hazard environments - protected with termite risk (eg underfloor timbers)
• Treatment Level H3 - suitable for use in H3 Hazard environments - external above ground (eg fascias, pergolas, verandahs)
• Treatment Level H4 - suitable for use in H4 Hazard environments - ground contact, generally dry (eg posts in normal conditions)
• Treatment Level H5 - suitable for use in H5 Hazard environments - ground contact, aggressive (eg posts in high water table areas, bridge piles)
• Treatment Level H6 - suitable for use in H6 Hazard environments - sea water contact (eg jetty timbers)
A suite of standards made specifically for the timber treatment industry gives chemical retention rates and depth of penetration in order to satisfy requirements for the relevant H level of treatment.
Different processes of forcing the chemical into the wood give slightly different depth of penetration, but in general, the depth of penetration of the chemical may not change much with increasing levels of treatment using the same process. Increasing H levels have a requirement for increasing chemical retention rates (gms of chemical bonded to one kg of wood).
H1 treatment levels can be achieved with all methods. Higher levels of treatment are commonly achieved in practice using CCA treatment or LOSP application, and with higher salt concentration in the timber. Rarely does a higher level of treatment indicate that the treatment has penetrated further into the heartwood.
Common preservative treatments
The common preservatives can be sorted by solvent (the vehicle that carries preservatives into the timber), and by the active chemicals that provide the protection against hazards. Many preservatives treatments are applied by a process that drives the preservative and solvent into the timber under pressure. The solvent must then be removed before the timber is made available for use.
Boron salts- Boron provides effective protection against attack by insect borers. It has been used in Australia for more than 40 years to protect the sapwood of susceptible hardwoods against lyctid or ‘powder post' borers.
The treatment consists of soaking freshly sawn unseasoned timber in solutions of boron salts. The salts diffuse through the timber and after treatment, the timber is allowed to dry. Boron treatments are common in timber where an appearance grading is required. It is also easy to apply even by painting on, and doesn't change the colour or appearance of the wood. However, Boron is not fixed in the timber and can be leached. This restricts boron treated timber to interior uses such as flooring or joinery that is protected from the weather both in service and during construction.
CCA - (Copper, Chromium and Arsenic salts) impregnates the timber with water and salts. CCA is pressure impregnated and designed to react with the wood cell components so that the active elements, copper (Cu), chromium (Cr) and arsenic (As) are fixed into the wood's structure. The arsenic component protects the sapwood from insect attack, the copper and arsenic from fungal degrade, while the chromium component chemically locks the elements into the timber, offering high resistance to leaching. Because the treatment is resistant to leaching, it can be used on timber that will be exposed to the elements or used in-ground.
It generally penetrates sapwood and a small region of heartwood near the surface of the timber (deeper near the end-grain). However, most of the heartwood remains unprotected though surrounded by a thin case of CCA-treated timber. Generally, a species with appropriately durable heartwood should be chosen for each application. Where a decay-susceptible heartwood is used, then care should be taken to paint on some treatment (typically copper napthanate) to regions in which the protective CCA "case" has been breached by sawing or drilling. CCA treatment inhibits fungal and termite attack, but has no effect on weathering in sunlight. Some sealant should be applied if weathering of the timber is seen as a potential problem.
There are a number of different formulations of CCA commonly used by the Australian timber industry. Each has different production requirements, but all of them have equivalent performance for a given H classification. The treatment is free of odour and so is suitable for use in enclosed spaces or for indoor applications; however, the treated timber has a green appearance, which may detract from its appearance in some applications. The treated product has minimal release of volatile components and as the salts are "fixed" in the timber, they are safe to handle or work.
After treatment, the timber commonly has a moisture content that would class it as unseasoned. In order to give the treated timber dimensional stability, it must be re-dried. This process can decrease its strength a little, and invariably adds a little to the cost of the timber.
LOSP - (Light Organic Solvent-borne Preservative) uses a light organic solvent to take the preservative chemicals into the timber. It is drawn out in the last stages of treatment, leaving the preservative behind. These preservatives are mainly regarded as fungicides, with copper, tin, zinc, PCP's as major toxicants. Synthetic pyrethroids may be incorporated if an insect hazard is also present.
LOSP treated timber is unchanged in appearance. This makes it an appropriate treatment for appearance or quality applications, such as external joinery, e.g. windows, and for treating house framing. The treated timber does not swell with the treatment, so it is quite suitable for treatment of finished items such as mouldings and joinery.
Most LOSP contain fungicide, insecticide and wax to give the surface water repellent properties. LOSP-treated timber is suitable for painting with either acrylic or oil-based systems, and is most effective as a treatment when sealed. LOSP-treated timber is not suitable for in-ground use.
LOSP treatments are mainly factory applied using a pressure and vacuum system, but solutions are available for painting as part of a maintenance program. The moisture content of the timber is unchanged. Different formulations of LOSP are available. If a wax is included, the organic compounds can make the timber water repellent, which improves its durability, but can make it difficult for use with most common glues.
ACQ - (Alkaline Copper Quat) contains copper and a quaternary ammonium compound. It protects against decay fungi and insects. There are a number of different forms, and generally the form which is most suitable will be selected for any given species. Its colour can range from dark green to a brownish green, and it is applied using a pressure process as a water-borne preservative. It is free of arsenic and can be used to give up to an H4 treatment. It can be painted or stained when dry.
Copper Azole is one of a new generation of arsenic free preservative treatments that can be used in water-borne pressure treatment processes. It is a preservative that contains copper, boric acid and tebuconazole. It seems to be an appropriate treatment for radiata pine, and has been used in Australia already as a replacement for CCA for treatments up to H4. It is odourless and can be stained or painted when finished.
Synthetic pyrethrins, e.g. 'blue pine', a range of synthetic pyrethrins and pyrethroids (e.g. bifenthrin, permethrin) has been developed for use in treating timber. They are commonly dissolved in water or oil such as linseed oil and are applied by spraying or dipping to cover the timber in the preservative. Used primarily in framing timber South of the Tropic of Capricorn. Some proprietary products are available that are in a paint-on or spray-on formulation. Currently treatment to H3 level has been trialled. Many of these are similar chemicals to the ones used in flea collars for dogs and cats or for fly sprays, so already have community acceptance.
Creosote and PEC (pigment emulsified creosote) Creosote and PEC are commonly used oil-borne preservatives and are painted onto timber surfaces, but can also be applied in a pressure-based process for better penetration. They have volatile components and, hence, a characteristic odour. This makes them only suitable for use in external or industrial applications.
PEC is made by emulsifying high temperature creosote (HTC) with 30% water and additives, including a special micronised pigment. The function of the pigment is to lock the major HTC components into the timber so that they do not bleed into the surface, particularly in hot weather. The pigment also ensures that the volatile lower boiling point fractions of HTC remain in the wood. PEC is thus a clean version of HTC in which treated commodities are safe and easy to handle.
Creosote is typically used on railway sleepers, timber bridge components, power poles, marine structures and decking, and piles. It is frequently used in conjunction with CCA treatment to give a durable case to timber. Because it can be painted on, it is suitable for use in maintenance programs.
Pressure treatment process
Most common treatment processes are pressure treatments. The following points outline the basic steps involved in pressure treatment processes:
• Wood is placed in a pressure vessel for treatment. The vessel is designed to be able to give a positive pressure and a negative pressure (suction).
• The first step is generally a suction step and it literally sucks moisture out of the wood in the vessel. This takes some time to happen. (The suction also opens up pores in the wood that make it easier for the chemicals to be forced into the wood.)
• A mixture of treatment chemicals and a solvent that will carry them into the wood is added to the vessel. The vessel is placed under pressure and the chemicals and solvent are forced into the wood. Pressure can be varied during this time to get as much penetration as needed.
• The pressure is released, the chemicals are removed from the vessel, and can be used in the next treatment cycle. Once the chamber is drained, the wood is removed.
• The wood is left to stand on a hard pad and any chemicals and solvent that drains from the pack are trapped and used again. Once the packs have drained, then the charge is sent for redrying.
• The wood has a very high moisture content at the completion of the treatment, so the solvent must be removed in the same way that water was removed from newly milled green timber. The first steps are air-drying to bring the moisture content down to around the fibre saturation point. This product can then be sold as unseasoned timber, or further drying can be undertaken to produce a redried, seasoned product. Where redrying uses kilns, it has been found that lower temperatures must be used to prevent degrade of mechanical properties.
Improving timbers natural chemical resistance
Timbers natural chemical resistance can be enhanced further by impregnating with:
• Phenolic resins (improves acid resistance)
• Furfuryl alcohol (increases alkaline resistance)
• Monomeric resins followed by polymerisation (greatly improves chemical resistance)
Protection for marine piles or timber in contact with sea water
In addition to using species that have natural durability in these environments, timber can be treated to increase the service life of marine structures. Appropriate strategies may include:
• Impregnating timbers with chemical preservatives. Species with wide sapwood bands can be effectively treated to provide resistance to marine borers. CCA is effective against limnoria while creosote type preservatives are effective against teredo. Often both CCA and creosote are used to give H6 protection.
• Using chemical and/or mechanical barriers
Environmental impact of chemicals used in treatment
Preservative chemical manufacturers and wood treaters must adhere strictly to the established guidelines and codes of practice, and the treatments must be applied and handled correctly by the user to comply with Occupational Safety and Health legislation. The following specific comments have been drawn from recent work on the impact of chemically treated timber on the environment.
CCA (copper chrome arsenate) Most CCA-treated timber is produced by a process that accelerates the fixation mechanism and which leaves the timber surfaces clean and relatively dry. Hence, the potential for leaching of elements is reduced and the impact on the environment when the timber goes into service is minimised. Some published work has suggested that the concentrations of metal ions leaching from waterborne treated timber is low and similar to background soil levels. The new generation of water repellent CCAs are even less likely to be leached than the conventional, unmodified CCAs. As more and more of these modified CCA formulations are used the leaching debate will become less relevant.
However, other work has generated concern over the use of CCA in timber (particularly in the USA). There are still doubts about some of this research internationally, and it is fair to say that these concerns are not yet proved or resolved. At present, many countries (including Australia) are taking a conservative view and suggesting that CCA treated timber not be used where it comes into direct contact with children or food - specifically play equipment and picnic tables.
Creosote/PECs (pigment emulsified creosote) are oil-borne substances and are commonly used externally as power poles or bridge piers. Creosote is a very complex product containing 300 or more components and its effectiveness is a result of the combination of these components. The function of the pigment is to lock the major HTC (high temperature creosote) components into the timber so that they do not bleed into the surface, particularly in hot weather. The pigment also ensures that the volatile lower boiling point fractions of HTC remain in the wood. PEC is thus, a clean version of HTC in which treated commodities are safe and easy to handle. The use of barrier creams by workers can reduce the potential for skin irritation when handling creosote, and there is little evidence that these substances pose a risk to the environment. In fact, mobile fractions, which may migrate downwards in timber structures in service are readily degraded by soil micro-organisms.
LOSP (light organic solvent preservative). Any potential environmental impact of LOSP is mainly restricted to the treatment plant site, since the active ingredients in LOSP are carried into the timber by a hydrocarbon solvent; in Australia this is white spirit. Worker exposure to white spirit and escape of solvents into the atmosphere are the major concern from the environmental and worker health and safety point of view. The volatile organic compound emissions from LOSP plants to the environment are strictly controlled. Modern treatment plants are designed to minimise escape of, and operator exposure to fluids, mists, and vapours. In addition, much of the solvent is recovered at the plant during the closed-in part of the post-treatment cycle. Although small amounts of solvent may remain in the timber when it leaves the treatment yard, the volumes of LOSP-treated timber delivered to building sites does not present a solvent vapour hazard. Current LOSP formulations contain biocides that are relatively environmentally acceptable. The preservative chemicals used can be updated readily to comply with contemporary requirements.
Recycling and disposal of treated timber
The potential problem of dealing with treated timber at the end of its service life and the handling of off-cuts on a building site can be effectively tackled through appropriate recycling and disposal methods. It has proved possible to reuse these products by reducing them to particles or flakes for the manufacture of composite panel materials. Other alternatives being explored include encouraging the suppliers of treated timber to recover off-cuts for the purpose of recycling, which also avoids the possibility of mixing treated timber residues with other building wastes during site clean up, and simplifies subsequent disposal.
Disposal by industrial burning (approved incineration), is viewed as an acceptable option, particularly if there is some energy captured in the process. It is a particularly effective method using high temperatures, i.e. 800 1100 °C, for the recovery of the metals from the ashes of CCA-treated residues. Recent laboratory studies have also explored the possibility of extracting preservatives from spent treated timber through the process of biodegradation. It is known that some micro-organisms can remove or release heavy metals from wood wastes. There have been some encouraging results using both fungi and bacteria to release Cu, Cr, and As from treated wood samples. The resulting wood wastes, which are relatively free from heavy metals, can then be safely disposed of by incineration or composting.
These and other disposal methods are currently under research and development, but are starting to find commercial application in Australasia, Europe and North America.