Boitechnology

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.

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.[3]

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. [4]

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[5] 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.[6]

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

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.

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.

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.

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 ne

Particulate (micronised or distributed) copper preservative technology has recently been introduced in the USA and Europe. In these systems, the copper is ground to micro sized particles and suspended in water rather than being dissolved in a chemical reaction as is the case with other copper products such as ACQ and Copper Azole. There are currently two particulate copper systems in production. One system uses a quat biocide system (known as MCQ) and is a take-off of ACQ. The other uses an azole biocide (known as MCA) and is a take-off of copper azole.

Proponents of the particulate copper systems make the case that the particulate copper system perform as well or better than the dissolved copper systems as a wood preservative. None of the particulate copper systems have been submitted to the American Wood Protection Association (AWPA) for evaluation, thus the particulate systems should not be used in applications where AWPA standards are required. However, all of the paticulate copper systems have been tested and approved for building code requirements by the International Code Council (ICC). The particulate copper systems provide a lighter color than dissolved copper systems such as ACQ or Copper Azole.

One of the particulate copper systems, marketed as MicroPro, is the only preservative technology to achieve Environmentally Preferable Product Certification. The “EPP” Certification was issued by Scientific Certifications Systems (SCS), and is based on a comparative Life Cycle Impact Assessments with an industry standard.

The copper particle size used in the micronized copper products ranges from 1 to 700 nm with an average under 300 nm. Larger particles do not adequately penetrate the wood cell walls. It is claimed by the proponents of micronized copper products that the copper nano particles which escape the wood will bond readily with organic matter and become biologically inactive.

Copper azole preservative (denoted as CA-B and CA-C under American Wood Protection Association (AWPA) standards) is a 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.

Copper Azole is similar to ACQ with the difference being that the dissolved copper preservative is augmented by an Azole co-biocide instead of the quat biocide used in ACQ. The Azole co-biocide yields a Copper Azole product that is effective at much lower retentions than required for equivalent ACQ performance.

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. The AWPA standard retention for CA-C is .06 pounds per cubic ft (pcf) for above ground applications and .15 pcf for ground contact applications.

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.