Three classes of preservatives are commonly used for the pressure treatment of pine timber.
1. Water-borne– examples are:
Alkaline Copper Quaternary [ACQ]
2. Solvent-borne– examples are:
Light Organic Solvent Preservative [LOSP]
3. Oil-borne– examples are:
Pigment Emulsified Creosote [PEC]
Water-borne chemical solutions comprise a mixture of water soluble compounds of copper and other chemicals. In the treatment of rural fence posts, other chemicals can be included in the formulation to prevent glowing and smouldering of the wood after a fire.
Solvent based chemicals, such as LOSP, are solutions of fungicides, insecticides and in some cases water repellent chemicals. White spirit is the solvent commonly used for LOSP formulations.
Oil borne preservatives such as creosote and pigment emulsified creosote are a complex mixture of chemicals obtained from the distillation of coal tar. Being a liquid it is normally used without the addition of a solvent. Fuel oil however is sometimes added to facilitate the treatment of railway sleepers.
Comprehensive testing has established that these preservatives are highly toxic to decay-causing fungi and insects and remain active in the wood indefinitely. CCA and creosote or PEC are also extremely effective against marine borers. Treatment preservatives do not protect pine against weathering. The application of suitable surface finishes is necessary for weather protection.
Some comments on the most widely used treatment options are:
1. Copper Chrome Arsenic [CCA]
CCA water-borne preservative used to be by far the most widely used preservative for the treatment of pine timber. The copper acts as a fungicide while the arsenic is also an insecticide. Chrome is included to fix the copper and arsenic to prevent them being leached from the timber.
Wood treated with CCA is clean, odourless, has a slight green colour and is able to be glued and painted when dry. Variations in the shade and intensity of the green colour of CCA treated wood are due to varying degrees of exposure to sunlight immediately after treatment. Colour therefore, cannot be used as a reliable guide to the level of preservative treatment. Also, the absence of colour in some areas of treated timber is no indication that they are untreated; use of a standard chemical indicator is the only sure way of establishing the presence of preservative.
Pine which has been pressure treated with CCA preservative and re-seasoned after treatment to between 8 and 15 per cent moisture content, can be supplied as MGP or F-grade structural timber. (Specific re-drying and grading requirements should be met to ensure products are fit-for-purpose.)
CCA has been replaced by ACQ and TanE for all applications where frequent skin contact can occur, such as playgrounds and decking. It remains widely accepted as the best treatment option for other applications such as roof trusses in Asia, retaining walls and landscaping products.
2. Alkaline Copper Quaternary [ACQ]
Alkaline Copper Quaternary uses copper as an agent against fungal and insect attack and quaternary ammonium compounds as an added defence against fungi, termites and wood boring insects. ACQ also has a green to brown appearance that quickly weathers to brown with sun exposure. The treatment is a similar process to CCA and timber needs to be re-dried after treatment.
3. Copper Azole (CuAz, TanE)
Copper Azoles use copper as a fungicide and azole as a biocide/insecticide. Copper Azole has a green colour after treatment and will gradually weather to a honey brown. The treatment is a similar process to CCA and timber needs to be re-dried after treatment.
CuAz, TanE and ACQ are the most common used treatment options for H3/H4/H5 as they provide an economical and robust treatment solution for higher protection without avoiding the highly emotional CCA debate. Disadvantages are the corrosive nature and dimension changes with double drying.
SOLVENT-BORNE CHEMICALS
LOSP refers more to the solvent carrier in the treatment process than the actual preservatives, which comprise various solutions of organic fungicides and insecticides (such as copper naphthenates and synthetic pyrethroids).
LOSP treated pine provides long lasting protection against decay and insect attack in above-ground applications.
LOSP can be used for treating appearance or structural pine timber which has been sawn or profiled. LOSP is usually clear for H3 treatment class leaving the light coloured appearance of pine virtually unchanged, red for class H2 and blue for class H2-F.
LOSP treatment is the most used treatment for applications where dimension stability is important after machining (doors, windows) and where non-corrosive treatment is required (instead of hot-dipped galvanized for ACQ or TanE)
OIL-BORNE CHEMICALS
Creosote treated wood is highly water repellent and resistant to weathering and so, can be used in many highly hazardous situations. Maintenance is not necessary.
At pressures of about 1400 kPa creosote provides deep and permanent treatment of pine. Because it is non-aqueous, drying of treated wood is not required. It also does not alter the timber dimensions during treatment.
The surface of creosote treated timber is generally oily and black. It cannot be painted or glued. It remains oily to handle for some months after treatment and gives off a strong odour.
The timber has a dry, dark brown finish after treatment.
Because of occupational health and safety and environmental concerns this treatment is only used for the most hazardous applications, such as marine.
THE TREATMENT PROCESS
Prior to treatment, all timber is seasoned (air or kiln-dried) to remove moisture that would otherwise inhibit the uptake of the preservative chemicals.
For water-borne and oilborne treatments the wood is usually dried to a moisture content of about 20 per cent (where structural timber is to be machine stress graded prior to treatment, it is dried to a level not exceeding 15%).
Timber to be LOSP treated is machined to its final size and as far as practicable, all drilling, notching and end-trimming completed. LOSP treatment uses relatively low pressures. Timber distortion is minimised because the treatment is solvent based.
TREATMENT METHODS
The effectiveness of any timber treatment is dependent upon chemical penetration and retention.
Commercial scale preservative chemical treatment is carried out by vacuum-pressure methods in large specially designed treatment cylinders. This is the most common and has proven to be the most effective treatment method.
Initially a vacuum is applied to the cylinder to remove as much air as possible from the timber so that maximum absorption can be achieved. With the vacuum maintained, the preservative is pumped into the cylinder from a holding tank. Pressure is then applied until the required uptake of preservative has occurred. At this stage the unused solution is pumped from the cylinder and the cycle completed by application of a final vacuum to remove excess preservative.
Creosote is impregnated by a method in which no initial vacuum is applied and the air trapped in the wood under pressure is utilised to expel surplus preservative when the final vacuum is drawn. Treatment is carried out with the creosote heated to about 95°C. The vacuum-pressure method can be controlled to achieve the required preservative loading and degree of wood penetration for any specified end use.
WHERE TO USE TREATED RADIATA PINE
Preservative treated pine is intended for use wherever the wood is permanently or regularly exposed externally to weather, in contact with the ground, or subjected to high moisture conditions. This applies to both structural and non-structural applications.
Treatment is also recommended for pine used internally in buildings where humid or other continuously wet conditions exist such as indoor swimming centres, ‘wet’ factories and greenhouses.
Preservative treatment can be used for general framing in areas of high termite risk in lieu of, or in addition to, other physical and/or chemical barriers. The following are some of the more common uses for theavailable types of preservative treatments.
Water-borne preservative treated pine (CCA, ACQ, CuAz):
exterior wall cladding, fascia and joinery, underfloor timbers, stumps, piles and posts, roof beams exposed to the weather or interior humid conditions, pergolas, decks, retaining walls and landscaping timbers, playground equipment, fencing, farm buildings, vineyards and hop vine poles, glass house framing, cooling tower slats and marine structures.
Solvent-borne treated pine (LOSP):
exterior cladding, joinery, fascia, elevated decks, pergolas, garden furniture and glue laminated components; also interior linings in bathrooms and laundries, and structural components in areas of high humidity such as indoor swimming pools.
Oil-borne treated pine (creosote, PEC):
rural fencing, vineyard trellising and heavy duty uses such as marine piles, utility building poles, railway sleepers and bridge beams and decking.
LIFE EXPECTANCY
The service life of preservative treated pine products is dependent upon the level of treatment given and the degree of hazard presented by the conditions of use. Some applications can be guaranteed for up to 40 years in Asia.
FASTENERS
Metal fasteners in contact with some types of treated timber may exhibit a propensity to corrode. The presence of excessive moisture in the timber aids formation of an electrochemical cell (or electrical circuit) which sets off a corrosive reaction.
Moisture presence is the important factor. The higher the moisture content, the higher the more aggressive the corrosion.
The following recommendations may prevent corrosion:
• Plastic gaskets, washers and coatings are recommended for electrical insulation between metal and treated timber
• Oversizing bolt holes and coating bolts with grease will resist corrosion
• Use fasteners which are compatible with the treatment type and the method of connection required. Hot-dipped galvanised fixings are most often specified, however in severe environments such as in close proximity to the coast, stainless steel or other fixings may be required to provide an adequate service life.
HEALTH AND SAFETY
The following are a few simple safety precautions that should be adhered to when working with treated timber.
• Always check the material safety data sheet, which fully detail any possible hazards related to handling, cutting and disposal of treated timber products - before handling timber.
• Always wear a filter mask and eye protection when sawing, machining or sanding treated pine
• Wear gloves at all times and wash hands before eating and
• Brush or wash sawdust off skin and clothes immediately.
Please remember, treated pine timber should not be burnt as a method of disposal. It should definitely not be used as fuel for barbecues. The safest and recommended method of disposal is by burial.
Source: includes Pine Australia & Manufacturers Guidelines
