Wood preservation
From Wikipedia, the free encyclopedia
All measures that are taken to ensure a long life of wood fall under the definition wood preservation (timber treatment). Apart from structural wood preservation measures, there are a number of different (chemical) preservatives and processes (also known as timber treatment or lumber treatment) that can extend the life of wood, timber, wood structures or engineered wood. These generally increase the durability and resistance from being destroyed by insects or fungus.
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[edit] History
Treatment of wood has been practiced for almost as long as the use of wood itself. Some accounts reach back to the beginning of recorded history. For example the Bible in Genesis, 6:13-14 “And God said unto Noah… make thee an ark of gopher wood; rooms shalt thou make in the ark, and shalt pitch it within and without with pitch.” There are also records of wood preservation reaching back to ancient Greece during Alexander the Great’s rule, where bridge wood was soaked in olive oil. The Romans also protected their wood by brushing their ship hulls with tar. During the Industrial Revolution wood preservation became a corner stone of the wood processing industry. Inventors and scientists such as Bethell, Boucherie, Burnett and Kyan made historic developments in wood preservation, with the preservative solutions and processes.
[edit] Hazards
Unfortunately, many of the timber treatments have proven to be extremely hazardous both to the workers and the environment; as a result many treatment centers have been forced to close and undertake massive environmental restoration and remediation. With many of the chemicals having a long period through which they are highly dangerous, the legacy of the chemical cocktails can probably never be totally eliminated.
[edit] Chemical Preservatives
Timber or lumber that is treated with a preservative generally have it applied through vacuum and\or pressure treatment. The preservatives used to pressure-treat lumber are classified as pesticides. Treating lumber provides long-term resistance to organisms that cause deterioration. If it is applied correctly, it extends the productive life of lumber by five to ten times. If left untreated, wood that is exposed to moisture or soil for sustained periods of time will become weakened by various types of fungi, bacteria or insects.
Chemical preservatives can be classified into three broad categories: Water-bourne salts, Oil-bourne preservatives, and Light Organic Solvent Preservaives (LOSPs). These are discussed in more detail below.
[edit] Water-borne Preservatives
Water is the most common solvent carrier in preservative formulations due to its availability and low cost. Water-bourne systems do however have the drawback that they swell timber, leading to increased twisting, spliting and checking than alternatives.
[edit] Chromated copper arsenate (CCA)
An extremely common preservative developed in the 1930s. In CCA treatment, copper is the primary fungicide, arsenic is a secondary fungicide and an insecticide, and chromium is a fixative which also provides ultraviolet (UV) light resistance. Recognized for the greenish tint it imparts to lumber, CCA is a preservative that was extremely common for many decades, however it contained arsenic. The chemicals may leach from the wood into surrounding soil, resulting in concentrations higher than naturally occurring background levels. A study cited in Forest Products Journal found 12–13% of the chromated copper arsenate leached from treated wood buried in compost during a 12-month period. Once these chemicals have leached from the wood they are likely to bind to soil particles, especially in soils with clay or soils that are more alkaline than neutral. In the United States on 1 January 2004 the Environmental Protection Agency (EPA) began restricting the use of CCA in treated lumber in residential and commercial construction, with the exception of shakes and shingles, permanent wood foundations, and certain commercial applications. This was in an effort to reduce the use of arsenic and increase environmental safety.
In Australia, the Australian Pesticides and Veternary Medicines Authority (APVMA [1]) restricted the use of CCA treated timber in certain applications from March 2006. CCA may no longer be used in 'intimate human contact' applications such as children's play equipment, furniture, residential decking and handrailing. Use for commercial-use remains unrestricted, as does its use in all other situations. The APVMA decision to restrict the use of CCA in Australia was a precautionary measure, even though the report[2] found that no evidence could be found that demonstrated CCA timber was a danger to humans in normal use.
[edit] Alkaline copper quaternary
Alkaline copper quaternary (ACQ) is a preservative made up of copper, a fungicide, and quaternary ammonium compound (quat), an insecticide which also augments the fungicidal treatment. Since it contains high levels of copper, ACQ-treated lumber is five times more corrosive to common steel, according to American Wood Preservers Association (AWPA) test results. It is necessary to use double-galvanized or stainless steel fasteners in ACQ lumber. Use of fasteners meeting or exceeding requirements for ASTM A 153 Class D meet the added requirements for fastener durability. The U.S. began mandating the use of ACQ in end-consumer lumber in 2004.
[edit] Other copper compounds
These include copper azole (CA), copper chromate, copper citrate, acid copper chromate and ammoniacal copper zinc arsenate (ACZA). The CA treatment is an alternative to CCA and ACQ in the United States and Canada. AZCA is generally used for marine applications.
[edit] Borate preservatives
Borate treated wood is non-toxic to humans, and contains no coppers or other heavy metals. Borate taken into the body is excreted, rather than building up as heavy metals do. Unlike most other preservatives, borate compounds do not become fixed in the wood and can be washed out. Therefore they cannot be used where they will be exposed to standing water. Recent interest in low toxicity lumber for residential use, along with new regulations restricing wood preservation agents, has resulted in a resurgence of the use in borate treated wood for floor beams and internal structural members.
[edit] Sodium silicate-based preservatives
Sodium silicate is produced by fusing sodium with sand or heating both ingredients under pressure. It has been in use since the 1800s. It can be a deterrent against insect attack and possesses minor flame-resistant properties; however, it is easily washed out of wood by moisture, forming a flake-like layer on top of the wood. Other uses include fixing pigments in paintings and cloth printing, and for preserving eggs.
[edit] Bifenthrin spray preservatives
In Australia, a water-based bifenthrin preservative has been developed to improve the insect resistance of timber. As this preservative is applied by spray, it only penetrates the outer 2mm of the timber cross-section. Concerns have been raised as to whether this thin-envelope system will provide protection against insects in the longer term, particularly when exposed to sunlight for extended periods.
[edit] Oil-borne Preservatives
These include pentachlorophenol, copper naphthenate, and creosote. All of them are toxic and are generally not used in consumer products.
[edit] Coal-tar Creosote
Creosote is a tar-based preservative that has been commonly used for telephone poles and railroad ties. Creosote is one of the oldest wood preservatives, and was originally derived from a wood distillate. It often collects inside chimneys causing a fire hazard. Creosote is regulated as a pesticide and is not usually sold to the general public. It is still used for railway sleepers and utility poles.
[edit] Linseed Oil
In recent years in Australia and New Zealand, Linseed has been used as a solvent to 'envelope treat' timber. This involves just treating the outer 5mm of the cross-section of a timber member with preservative, leaving the core-untreated. While not as effective as CCA or LOSP methods, envelope treatments are significanly cheaper as they use far less preservative. Major preservative manufacturers add a blue dye to envelope treatments. There is an on-going promotional campaign in Australia for this type of treatment.
[edit] Other Emulsions
[edit] Light Organic Solvent Preservatives (LOSP)
This class of timber treatments use white spirit as the solvent carrier to deliver preservative compounds into timber. Synthetic pyrethroids are used as an intecticide, such as permethrin, bifenthrin or deltamethrin. In Australia and New Zealand, the most common formulations use Permethrin as an insecticide, and Propaconazole & Tebuconazole as fungicides. 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.
[edit] Natural Preservatives
[edit] Naturally rot-resistant woods
This includes Western Redcedar, Huon Pine, Merbau, Ironbark, many cypresses and Coast Redwood. These species are resistant to decay in their natural state, due to high levels of organic chemicals called "extractives", mainly polyphenols. Extractives are chemicals that are deposited in the heartwood of certain tree species as they convert sapwood to heartwood. However, many of these species tend to be prohibitively expensive for general construction applications.
Huon pine was used for ship hulls in the 19th century, but overharvesting and huon pine's tremendously slow grow rate makes this now a specialty timber. Merbau is still a popular decking timber and has a long life in above ground applications, but it is considered unsustainably logged (see Illegal logging) and is too hard and brittle for general use. Huon pine is so rot resistant that fallen trees from over 20000 years ago are still commercially valuable.
Ironbark is a good choice where available. It is harvested from both old-growth and plantation in Australia and is highly resistant to rot and termites. It is most commonly used for fence posts and house stumps.
Eastern red cedar and black locust have long been used for rot-resistant fence posts and rails in eastern United States, with the black locust also planted in modern times in Europe.
[edit] Tung oil
Tung oil has been known about for hundreds of years in China, where it was used as a preservative for wood ships. The oil penetrates the wood, then hardens to form an impermeable hydrophobic layer up to 5 mm into the wood. As a preservative it is effective for exterior work above and below ground, but the thin layer makes it less useful in practice. It is not available as a pressure treatment. Some manufacturers recommend tung oil as a stabiliser for CCA.
[edit] 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.
[edit] Application Processes
[edit] Introduction and History
Probably the first attempts made to protect wood from decay and insect attack consisted of brushing or rubbing preservatives onto the surfaces of the treated wood. Through trial and error the most effective preservatives and application processes where slowly determined. In the Industrial Revolution, demands for such things as telegraph poles and railroad ties helped to fuel an explosion of new techniques that emerged in the early 19th century. The sharpest rise in inventions took place between 1830 and 1840, when Bethell, Boucherie, Burnett and Kyan were making wood-preserving history. Since then, numerous processes have been introduced or existing processes improved. The goal of modern day wood preservation is to ensure a deep, uniform penetration with reasonable cost, without endangering the environment. The most widespread application processes today are those using artificial pressure through which many woods are being effectively treated, but several species (such as Spruce, Douglas Fir, Larch, Hemlock and Fir) are very resistant to impregnation. With the use of incising, the treatment of these woods has been somewhat successful but with a higher cost and not always satisfactory results. One can divide the wood-preserving methods roughly into either non-pressure processes or pressure processes.
[edit] Non-pressure Processes
There are numerous non-pressure processes of treating wood which vary primarily in their procedure. The most common of these treatments involve the application of the preservative by means of brushing or spraying, dipping, soaking, steeping or by means of hot and cold bath. There is also a variety of additional methods involving charring, applying preservatives in bored holes, diffusion processes and sap displacement.
[edit] Brush and Spray Treatments
Brushing preservatives is a long-practiced method and often used in today’s carpentry workshops. Through technology developments 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.
[edit] 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 method.
[edit] Steeping
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 (1 ft) above the future ground level.
The depth obtained during regular steeping periods varies from 5 mm to 10 mm (1/8 to 1/3 in.) up to 30 mm (1 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.
[edit] Kyanizing
Named after John Kyan, who patented this process in England in 1832, Kyanizing consists of steeping wood in a 0.67% mercuric chloride preservative solution.
[edit] Hot and Cold Bath
Patented by C. A. Seeley, this process achieves treatment by immersing seasoned wood in successive baths of hot and cold preservatives. During the hot baths, the air expands in the timbers. When the timbers are changed to the cold bath (the preservative can also be changed) a partial vacuum is created within the lumen of the cells, causing the preservative to be drawn into the wood. Some penetration occurs during the hot baths, but most of it takes place during the cold baths. This cycle is repeated with a significant time reduction compared to other steeping processes. Each bath may last 4 to 8 hours or in some cases longer. The temperature of the preservative in the hot bath should be between 60 to 110 °C (140 to 225 °F) and 30 to 40 °C (85 to 105 °F) in the cold bath (depending on preservative and treespecies). The average penetration depths achieved with this process ranges from 30 mm to 50 mm (1 to 12/3 in.). Both preservative oils and water-soluble salts can be used with this treatment. Due to the longer treatment periods, this method finds little use in the commercial wood preservation industry today.
[edit] Osmosis Process
In this process, first developed in Germany, the preservative is applied to the surface of green wood in the form of a cream or paste. The wood is then stacked in solid piles, which are covered securely with waterproof tarp to prevent moisture loss. The treated wood is left covered for 30 days (up to 90 days), as the water-soluble portions of the preservative diffuse into the water of the green wood. The osmosis process is often used in the United States and Canada for the treatment of fence posts, as well as the subsequent treatment of ground-line areas for standing poles. But because of its intensive time and labor consumption it is not used on a large scale basis.
[edit] Sap Displacement
Sap displacement takes place when one brings a preventative into or onto the sapwood of a living tree which carries it within the sap stream for long distance. Numerous attempts have been made to find a practical method of injecting preservatives (or dyes) into living trees. The idea of injecting treatment into a tree to repel fungal, parasite (mistletoe) or insect attacks has also been tested, with positive results in the domestic crop corn. The original Boucherie treatment was also based on this principle of sap displacement, not to be confused with the process now known under that name, which was a later development.
Through transpiration in the leaves or needles, water is evaporated into the air, causing a small vacuum within the branches. This vacuum sums within the tree trunk, bringing nutrient enriched water up through the sapwood. The travel of any treatment brought into the wood tends to be mainly vertical without horizontal spread. Therefore, to guarantee the uniform spread of treatment, it is necessary to sever most of the sapwood. Best results are obtained during periods of active transpiration. In the former Democratic Republic of East Germany, this method became popular to dye sapwood. Holes were bored into a standing tree, filled with dye, and plugged. This resulted in a partial coloration of the tree. There have been several different approaches on how to sever and prepare the sapwood for treatment. Levi S. Gardner (in 1910) patented his method of sawing in a deep cut around the base of a tree, sealing the outer edge of the cut, and then bringing dye solution into the hollow chamber within.
H. Renner took another approach (in 1929); his method consisted of boring a horizontal hole completely through the tree trunk near the base. A two-edged saw was then inserted, and the two opposite quadrants of the cross section were sawed through. This was repeated by a second hole at a right angle to first, and 30 cm (1 ft) above the first. The outer edge of the cut was also sealed and then fed with preservative from a container and hose hung from above. In 1934, Carl Schmittutz developed a method which involves peeling the bark around the base of a tree, brushing a preservative in paste form over the debarked area, and finally covering the treatment with a waterproof tarp or paper. During this method the preservative diffuses into the sapwood and is carried through the outer year rings upward. Other methods, such as felling the tree and then treating it with its branches still attached, have been suggested by U.S. Bureau of Entomology and Plant Quarantine. This can be done by either forming a sealed chamber around the base of the tree (using, for example, an old tire inner tube), and then filling it with preservative which is drawn into the tree. A smaller tree that is light enough to lift can be impregnated by propping the tree against another, with its stump in a barrel or other container that is filled with preservative. Generally, the benefits from such a treatment are the low energy costs and the low danger of leaching. Nevertheless, this process is not used on a major scale.
[edit] Pressure Processes
Pressure processes are those in which the treatment is carried out in closed cylinders with applied pressure and/or vacuum. These processes have a number of advantages over the non-pressure methods. In most cases, a deeper and more uniform penetration and a higher absorption of preservative is achieved. Another advantage is that the treating conditions can be controlled so that retention and penetration can be varied. These pressure processes can be adapted to large-scale production. The high initial costs for equipment and the energy costs are the biggest disadvantages. These treatment methods are used to protect ties, poles and structural timbers and find use throughout the world today. The various pressure processes that are used today differ in details, but the general method is in all cases the same. The treatment is carried out in cylinders. The timbers are loaded onto special tram cars, so called “buggies,” and into the cylinder. These cylinders are then set under pressure often with the addition of higher temperature. As final treatment a vacuum is frequently produced to extract excess preservatives. These cycles can be repeated to achieve better penetration.
LOSP treatments often use a vacuum impregnation process. This is possible because of the lower viscosity of the white-spirit carrier used.
[edit] Full-Cell Process
In the full-cell process, the intent is to keep as much of the liquid absorbed into the wood during the pressure period as possible, thus leaving the maximum concentration of preservatives in the treated area. Usually, water solutions of preservative salts are employed with this process but it is also possible to impregnate wood with oil. The desired retention is achieved by changing the strength of the solution. William Burnett patented this development in 1838 of Full-Cell Impregnation with water solutions. The patent covered the use of zinc chloride on water basis, also known as Burnettizing. Full-Cell Process with oils was patented in 1838 by John Bethell. His patent described the injection of tar and oils into wood by applying pressure in closed cylinders. This process is still used today with some improvements.
[edit] Empty-Cell Processes
The empty-cell process is used when a deep penetration with a limited final retention of liquid is desired. In the empty-cell processes, excess preservative in the timber is subsequently recovered, resulting in a coating of the cell rather than filling. The empty-cell processes are mainly used to impregnate wood with oil. There are two basic empty-cell processes and many other variations. Both the Rueping and the Lowry process are usually limited to the treatment of timber with creosote or other preservative oils, although they can also be used for injecting water solutions. The main area of application is the impregnation of such products as railway ties, poles, posts, lumber and many forms of construction timbers.
[edit] Rueping Process
The Rueping process was patented by Max Rüping of Germany in 1902. The main difference to other methods is the application of preliminary air pressure to the wood, before the insertion of the preservative oil.
[edit] Lowry Process
This empty-cell process was patented in 1906 by C. B. Lowry. The process is similar to that of Rueping but it does not require an initial air pressure. The preservative oil is impregnated into the timber without any preliminary treatment.
[edit] Fluctuation Pressure Process
Contrary to the “static” Full-Cell and Empty-Cell processes, the Fluctuation Process is a “dynamic” Process. By this process the pressure inside the impregnation cylinder changes between pressure and vacuum within a few seconds. There have been inconsistent claims that through this process it is possible to reverse the pit closure by Spruce. However the best results that have been achieved with this process by Spruce do not exceed a penetration deeper than 10 mm (1/3in.). Specialized equipment is necessary and therefore higher investment costs are incurred.
[edit] Boucherie Process
Developed by Dr. Boucherie of France in 1838, this approach consisted of attaching a bag or container of preservative solution to a standing or a freshly cut tree with bark, branches, and leaves still attached, thereby injecting the liquid into the sap stream. Through transpiration of moisture from the leaves the preservative is drawn upward through the sapwood of the tree trunk.
The modified Boucherie process consists of placing freshly cut, unpeeled timbers onto declining skids, with the stump slightly elevated, then fastening watertight covering caps or boring a number of holes into the ends, and inserting a solution of copper sulfate or other water- borne preservative into the caps or holes from an elevated container. Preservative oils tend to not penetrate satisfactorily by this method. The hydrostatic pressure of the liquid forces the preservative lengthwise into and through the sapwood, thus pushing the sap out of the other end of the timber. After a few days, the sapwood is completely impregnated; unfortunately little or no penetration takes place in the heartwood. Only green wood can be treated in this manner. This process has found considerable usage to impregnate poles and also larger trees in Europe and North America, and has experienced a revival of usage to impregnate bamboo in countries such as Costa Rica, Bangladesh, India and the state of Hawaii.
[edit] High pressure sap displacement system
Developed in the Philippines, this method (abbreviated HPSD) consists of a cylinder pressure cap made from a 3 mm thick mild steel plate secured with 8 sets of bolts, a 2-HP diesel engine, and a pressure regulator with 1.4 - 14 kg/m2 capacity. The cap is placed over the stump of a pole, tree or bamboo and the preservative is forced into the wood with pressure from the engine.
[edit] Incising
First tested and patented by Kolossvary, Haltenberger, and Berdenich of Austria in 1911 and 1912 (U.S. pats. 1,012,207 and 1,018,624) with several improvements from O. P. M. Goss, D. W. Edwards and J. H. Mansfield among others, this process consists of making shallow, slit-like holes in the surfaces of material to be treated, so that deeper and more uniform penetration of preventative may be obtained. The term "incising" or perforating comes from Latin foramen, meaning hole. This process can also be used to ensure a long durability of food through its packaging. The air exchange can be ensured with laser-incised holes within packaging that are so fine that moisture does not escape. Incisions made in sawed material usually are parallel with the grain of the wood. This process is common in North America (since the 1950s), where Douglas-Fir products and pole butts of various species are prepared before treatment. It is most useful for woods that are resistant to side penetration but allow preservative transport along the grain. In the region in which it is produced, it is common practice to incise all sawed Douglas fir 3 in. or more in thickness before treatment.
With the impregnation of Spruce, the most important structural timber in large areas of Europe, unsatisfactory treatment depths have been achieved. The maximum penetration of 2 mm (1/12 in.) is not sufficient to protect wood in weathered positions.
Present-day incising machines consist essentially of four revolving drums fitted with teeth or needles or with lasers that burn the incisions into the wood. Preservatives can be spread along the grain up to 20 mm (5/6 in.) in radial and up to 2 mm in (1/12 in.) tangential and radial direction.
In North America, where smaller timber dimensions are common, incision depths of 4 to 6 mm (1/6 to 1/4 in.) have become standard. In Europe, where larger dimensions are widespread, incision depths of 10 to 12 mm (1/2 in.) are necessary. The incisions are visible and often considered to be wood error. Incisions by LASER are significantly smaller than those of spokes or needles. The costs for each process type are approximately for spoke/conventional all-round incising €0.50 per m², by laser incising €3.60 per m² and by needle incision €1.00 per m². (Figures originate from the year 1998 and may vary from present day prices.)
[edit] Microwaving
An alternative method of increasing the permeability of timber involves using microwave technology. Ongoing research in this area is being conducted by the Cooperative Research Centre at the University of Melbourne, Australia.
[edit] See also
[edit] External links
- Arsenate
- Case Studies in Environmental Medicine - Arsenic Toxicity
- CPSC Test coatings to reduce arsenic emissions from pressure treated wood
- Borate
- Sodium silicate
