Wood preservation

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.

History

As proposed by Richardson, [Richardson, B.A. Wood preservation. Landcaster: The Construction, 1978.] treatment of wood has been practised 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.

Hazards

Old timber industrially treated with approved preservative products should pose no greater threat to the public than untreated timber. 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.

Unfortunately previous poor practices in industry have left legacies of contaminated ground and water around wood treatment operation sites in a number of cases. However under currently approved industry practices and regulatory controls such as implemented in Europe, North America, Australia, New Zealand, Japan and elsewhere, environmental impact of these operations should be minimal.

Wood treated with modern preservatives is generally safe to handle given appropriate handling precautions and personal protection measures. However, treated wood may present certain hazards in some circumstances such as during combustion or where wood dust or other fine residues are generated or where treated wood comes into direct contact with food and agriculture.

Material safety data sheets and safe handling guidelines are required by law to be provided by suppliers of wood preservative chemicals and treated wood products. This information should be obtained and reviewed before handling and using wood preservative chemicals and treated wood products.

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 timber are classified as pesticides. Treating timber provides long-term resistance to organisms that cause deterioration. If it is applied correctly, it extends the productive life of timber 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-borne salts, Oil-borne preservatives, and Light Organic Solvent Preservatives (LOSPs). These are discussed in more detail below.

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, splitting and checking than alternatives.

Chromated copper arsenate (CCA)

An extremely common preservative originally developed in the 1930s at the Forest Research Institute in Dehra Dun, India, by Dr Sonti Kamesam. Dr Kamesam sold the patent rights for what he originally called ASCU salts to the Bell Telephone Company of the USA. The acronym "CCA" is believed to have come into popular use for this product in the 1950s. 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 timber, CCA is a preservative that was extremely common for many decades; however, it contained arsenic.

Pressure treatment with an aqueous solution of CCA salts is often known in the UK as Tanalisation. The chemicals are applied using a vacuum and pressure cycle, and the treated wood is then stacked to dry (see "Application Processes" below). During the process, the mixture of salts reacts to form insoluble compounds, helping with leaching problems. If the process is carried out correctly, very little preservative is left on the surface of the wood, and the safety hazard from surface toxins is minimised. Occasionally, a white powdery deposit may be seen on the surface; this is either hydrated-sodium sulphate, a harmless by-product of the CCA salts, or some resin that has exuded from the wood during treatment. Both these deposits are easily removed by scrubbing or brushing, and such treatment will also reduce the minimal amounts of arsenic, which may be present on the surface of the wood.

In the last decade concerns were raised that 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 the powerful US Consumer Product Safety Commission issued a report in 2002 stating that exposure to arsenic from direct human contact with CCA treated wood may be higher than was previously thought. On 1 January 2004 the Environmental Protection Agency (EPA) in a voluntary agreement with industry began restricting the use of CCA in treated timber 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 improve environmental safety, although the EPA were careful to point out that they had not concluded that CCA treated wood structures in service posed an unacceptable risk to the community. The EPA did not call for the removal or dismantling of existing CCA treated wood structures.

In Australia, the Australian Pesticides and Veterinary Medicines Authority (APVMA [http://www.apvma.gov.au/] ) restricted the use of CCA preservative for treatment of timber used in certain applications from March 2006. CCA may no longer be used to treat wood used in 'intimate human contact' applications such as children's play equipment, furniture, residential decking and handrailing. Use for low contact residential, commercial and industrial applications 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 [http://www.apvma.gov.au/chemrev/arsenic.shtml] found no evidence that demonstrated CCA treated timber posed unreasonable risks to humans in normal use. Similarly to the US EPA, the APVMA did not recommend dismantling or removal of existing CCA treated wood structures.

In Europe, [http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2003:004:0009:0011:EN:PDF Directive 2003/2/EC] restricts the marketing and use of arsenic, including CCA wood treatment. CCA treated wood is not permitted to be used in residential or domestic constructions. It is permitted for use in various industrial and public works, such as bridges, highway safety fencing, electric power transmission and telecommunications poles.

Alkaline copper quaternary

Alkaline copper quaternary (ACQ) is a preservative made up of copper, a fungicide, and a quaternary ammonium compound (quat), an insecticide which also augments the fungicidal treatment is a wood preservative that has come into wide use in the USA, Europe, Japan and Australia following restrictions on CCA. Its use is governed by national and international standards, which determine the volume of preservative uptake required for a specific timber end use.

Since it contains high levels of copper, ACQ-treated timber 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 timber. 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 non-arsenic containing wood preservatives for virtually all residential use timber in 2004.

Modern versions have been developed which offer improved performance to those mentioned above. It should be noted that the American Wood Protection Association (AWPA) standards for ACQ require a retention of 0.25 pounds per cubic ft (PCF) for above ground use and .40 pcf for ground contact. Check the label and beware that some dealers are offering ACQ products with sub-standard retentions, these sub-standard retentions should not be used in applications where AWPA standard products are specified.

Chemical Specialties, Inc (CSI, now Viance) received US EPA's Presidential Green Chemistry Challenge Award in 2002 for commercial introduction of ACQ. Its widespread use has eliminated major quantities of arsenic and chromium previously contained in CCA.

Copper azole

Copper azole preservative (denoted as CA-B under American Wood Protection Association (AWPA) standards) is the other major copper based wood preservative that has come into wide use in the USA, Europe, Japan and Australia following restrictions on CCA. Its use is governed by national and international standards, which determine the volume of preservative uptake required for a specific timber end use.

It is marketed exclusively under the "Wolmanized" brand in the US, and the "Tanalith" brand across Europe and other international markets. Many companies treating with ACQ treatments attempt to piggyback the success of this brand by describing ACQ treated timber as "Tanalised", which is a registered trade mark infringement.

The AWPA standard retention for CA-B is .10 pounds per cubic ft (pcf) for above ground applications and .21 pcf for ground contact applications. Recently a new version of copper azole, denoted as CA-C, has been introduced under the Wolmanized brand. CA-C has been submitted to the AWPA for evaluation and acceptance as a new AWPA standard is expected in due course.

The copper azole preservative incorporates organic triazoles such as tebuconazole or propiconazole as the co-biocide, which are also used to protect food crops. The general appearance of wood treated with copper azole preservative is similar to CCA with a green colouration.

Other copper compounds

These include copper HDO (CuHDO), copper chromate, copper citrate, acid copper chromate and ammoniacal copper zinc arsenate (ACZA). The CuHDO treatment is an alternative to CCA, ACQ and CA used in Europe and in approval stages for United States and Canada. AZCA is generally used for marine applications.

Micronized copper technology

Micronised copper preservative technology is now being used extensively in the USA. These systems provide a lighter color than dissolved copper systems such as ACQ and are often sold at lower cost. However, recent reports have raised questions about the long term preservative performance. Marketed as MicroPro it is the only technology to achieve Environmentally Preferable Product Certification. This "EPP" Certification was issued by Scientific Certifications Systems (SCS) in December 2007 and is based on a comparative Life Cycle Impact Assessment with an industry standard. However, as of February 2008 no report has been made public documenting the methods, assumptions and inputs used in the analysis. Micronized copper is a relatively new product whose life expectancy has not been tested for more than several years. It should be noted that micronized copper preservatives have not been submitted to the American Wood Protection Association (AWPA) for evaluation. Although there are some manufacturer claims of meeting AWPA standards, none of the micronized copper preservatives are AWPA standards and should not be used in applications where AWPA standards are specified.

Borate preservatives

Boric acid, oxides and salts (borates) are effective wood preservatives and are supplied under numerous brand names throughout the world. Borate treated wood is of low toxicity to humans, and does not contain copper or other heavy metals. However unlike most other preservatives, borate compounds do not become fixed in the wood and can readily be leached out. Therefore they should not be used where they will be exposed to rain, water or ground contact. Recent interest in low toxicity timber for residential use, along with new regulations restricting some wood preservation agents, has resulted in a resurgence of the use of borate treated wood for floor beams and internal structural members.

odium 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. One company, Timber Treatment Technology, LLC, has found that infusing timber with a chemical solution containing sodium silicate with a specified energy level applied yields wood that not only does not provide flake or layering on the wood, nor does it wash out as others have done in the past; and it provides processed timber that received a class A fire classification. Their processed wood also paints and stains as new wood does. TTT, LLC, sells these products under the name TimberSIL. Other uses include fixing pigments in paintings and cloth printing, and for preserving eggs.

Potassium silicate-based preservatives

There are a number of European "natural" paint fabricants that have developed potassium silicate (potassium waterglass) based preservatives. They are frequently include boron compounds, cellulose, lignin and other plant extracts. They are a surface application with a minimal impregnation for internal use.

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.

Fire retardant treated

This treated wood contains a fire retardant chemical that remains stable in high temperature environments and does not increase the corrosivity of metal hardware in contact with the wood. Also use baden

Oil-borne preservatives

These include pentachlorophenol and creosote. They are toxic, have an unpleasant odour and are generally not used in consumer products.

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. These days virtually all creosote is manufactured from the distillation of coal tar. It often collects inside chimneys and may cause 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.

Linseed oil

In recent years in Australia and New Zealand, Linseed has been incorporated in preservative formulations as a solvent and water repellent to 'envelope treat' timber. This involves just treating the outer 5mm of the cross-section of a timber member with preservative eg Permethrin 25:75, leaving the core-untreated. While not as effective as CCA or LOSP methods, envelope treatments are significantly 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.Linseed oil is used to preserve Wood fences, log cabins, and wood furniture.(Such woods as Willow, Pine, oak and exc.) The function of linseed oil as a preservative is believed to be related to its action as a water repellent and drying agent rather than a direct biocidal activity.A number of European companies have developed natural-oil-only-based treatments; no synthetic preservative such as permethrin is added. Menz Holz OHT use autoclave impregnation with linseed, sunflower and rapeseed oil for 6 to 8 hours.

Other emulsions

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 insecticide, 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.

New Technologies

Wood acetylation

Chemical modification of wood at the molecular level has been used to improve its performance properties. Many chemical reaction systems for the modification of wood, especially those using various types of anhydrides, have been published; however, the reaction of wood with acetic anhydride has been the most studied. [(Rowell et al, 2008)]

The physical properties of any material are determined by its chemical structure. Wood contains an abundance of chemical groups called "free hydroxyls". Free hydroxyl groups readily absorb and release water according to changes in the climatic conditions to which they are exposed. This is the main reason why wood’s dimensional stability is impacted by swelling and shrinking. It is also believed that the digestion of wood by enzymes initiates at the free hydroxyl sites - which is one of the principal reasons why, wood is prone to decay. [Roger M. Rowell, Bert Kattenbroek, Peter Ratering, Ferry Bongers, Francesco Leicher, and Hal Stebbins, “Production of Dimensionally Stable and Decay Resistant Wood Components Based on Acetylation,” presented at International Conference on Durability of Building Materials and Components. Istanbul, Turkey, 2008]

Acetylation effectively changes the free hydroxyls within wood into acetyl groups. This is done by reacting the wood with acetic anhydride, which comes from acetic acid (known as vinegar when in its dilute form). When free hydroxyl groups are transformed to acetyl groups, the ability of the wood to absorb water is greatly reduced, rendering the wood more dimensionally stable and, because it is no longer digestible, extremely durable. In general, softwoods naturally have an acetyl content between 0.5 to 1.5% and more durable hardwoods between 2 to 4.5%. Acetylation takes wood well beyond these levels with corresponding benefits. These include an extended coatings life due to acetylated wood acting as a more stable substrate for paints and translucent coatings. Acetylated wood is non-toxic and does not have the environmental issues associated with traditional preservation techniques.

The acetylation of wood was first done in Germany in 1928 by Fuchs. In 1946, Tarkow, Stamm and Erickson first described the use of wood acetylation to stabilize wood from swelling in water. Since the 1940’s, many laboratories around the world have looked at acetylation of many different types of woods and agricultural resources.

In spite of the vast amount of research on chemical modification of wood, and, more specifically, on the acetylation of wood, commercialization did not come easily. The first patent on the acetylation of wood was filed by Suida in Austria in 1930. Later, in 1947, Stamm and Tarkow filed a patent on the acetylation of wood and boards using pyridine as a catalyst. In 1961, the Koppers Company published a technical bulletin on the acetylation of wood using no catalysis but with an organic cosolvent [Goldstein et al. 1961, Dreher et al. 1964] In 1977, in Russia, Otlesnov and Nikitina came close to commercialization but the process was discontinued presumably because cost-effectiveness could not be achieved. In 2007 a London-based company, with production facilities in The Netherlands, achieved cost-effective commercialization and began large-scale production of acetylated wood. [Spencer, Sally. "Accoya off the blocks." Timber Trades Journal 31 May 2007. . ]

Natural preservatives

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 extremely slow growth 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 many 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. Redwood is commonly used for similar applications in the western United States.

It should be noted that the natural durability or rot and insect resistance of wood species is always based on the heartwood (or truewood). The sapwood of all timber species should be considered to be non-durable without preservative treatment.

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.

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.All process's use autoclave pressure + heat, and nitrogen or water vapour to control drying in a staged treatment process ranging from 24 to 48 hours at temperatures of 180 à 230°C depending on timber species.All increase the durability, dimensional stability and hardness by at least one class. The timber is darkened in colour and there is a reduction in the mechanical properties of the timber ; modulus of elasticity increased 0 to 10%, modulus of rupture diminished by 5 to 20% e.g. timber requires drilling for nailing otherwise it may split. Not all process's are equal in their reduction of mechanical properties.Timber used for cladding, flooring, furniture and windows are treated with this process.

Application processes

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.

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.

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.

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.

teeping

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.

Kyanizing

Named after John Howard Kyan, who patented this process in England in 1832, Kyanizing consists of steeping wood in a 0.67% mercuric chloride preservative solution.

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.

Osmosis process

Pressure processes

Pressure processes are the most permanent method around today in preserving timber life. 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 used 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.

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.

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.

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.

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.

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 "incidere", a compound of "in" and "caedere" (to cut). 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.

Unfortunately the impregnation of Spruce, the most important structural timber large areas in Europe has shown that unsatisfactory treatment depths have been achieved with impregnation. The maximum penetration of 2 mm (1/12 in.) is not sufficient to protect wood in weathered positions. The 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.)

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.

ee also

* Sodium silicate#Timber Treatment
* Koppers

References

External links

;Non-CCA Wood Preservatives
* [http://npic.orst.edu/hottopic/AltCCA.pdf Non-CCA Wood Preservatives: Guide to Selected Resources - National Pesticide Information Center]

;Arsenate
* [http://www.atsdr.cdc.gov/HEC/CSEM/arsenic Case Studies in Environmental Medicine - Arsenic Toxicity]
* [http://bitterplace.homeip.net:8080/modules.php?name=News&file=article&sid=127 CPSC Test coatings to reduce arsenic emissions from pressure treated wood]

;Borate
* [http://www.babb.com/boratelumber/whyborate.html Borate Treated Lumber in Modern Applications]
* [http://www.wolmanizedwood.com/hd/borate.pdf Analysis of Borate as an Insecticide in Wood]

;Sodium silicate
* [http://www.bartleby.com/65/so/sodiumsi.html Brief cutout from The Columbia Encyclopedia]
* [http://mineral.galleries.com/minerals/silicate/albite/albite.htm The Mineral Albite]