Light Organic Solvent Preservatives (LOSPs)

What are Light Organic Solvent Preservatives (LOSPs)?

Light Organic Solvent Preservatives (LOSPs) are preservatives that contain insecticides for internal use, and combinations of fungicides and insecticides for external use, and sometimes water repellents. LOSPs may also contain mouldicides. All of these preservative components are incorporated in a solvent carrier such as white spirit.

LOSPs are used to protect timber against insects, including termites, and decay, though they are not intended for use in ground-contact situations. In addition, LOSPs can provide a degree of weather protection when water repellents have been incorporated. LOSPs are preservatives that leave the treated timber dry after treatment, and do not cause it to swell and distort in any way.

LOSPs have traditionally been colourless, and although it is now possible to obtain green and brown-coloured LOSPs, such colour that is imparted to the timber by these coloured variants is short-term. Also, it should be noted that some of the dyes used in coloured LOSPs might migrate through paint films that are later applied to the surface of the LOSP-treated timber.

The colour or general visual appearance of a piece of treated timber is not a reliable guide for identifying the actual preservative used. Users should check the marking (or branding) on the treated timber, or ask the supplier if in doubt. For example, some LOSP is tinted green, but this should not be confused with the green copper chrome arsenic (CCA) used in ground contact applications, for which LOSPs are unsuitable.

LOSPs and other preservatives like CCA are intended to provide adequate protection to a wide range of commodities but they may not necessarily offer directly equivalent performance over the broad range of applications. Most types of preservative-treated timber are subject to the effects of weathering when used in exposed outdoor situations. Any timber product used in such a situation will benefit from the application of surface coatings that provide additional protection against moulds, discolouration, checking and dimensional movement. LOSP-treated timber is generally more prone to these affects than CCA-treated timber, and it should always be given a protective coating after treatment (see below).

Where are Light Organic Solvent Preservatives used?

Light Organic Solvent Preservatives (LOSPs) are used for high value building components such as external joinery, e.g. windows, and for protecting house framing against termites (H2 level). The commodities treated include windows, doors, external joinery, house frames, cladding, outdoor garden furniture, feature fences, etc. In recent times, a range of engineered timber products including laminated posts and beams are also being treated with LOSPs. A number of timber species are used for these commodities, mainly radiata and slash pine of local origin, but also imported species from north America and south east Asia, i.e. western red cedar, oregon, and meranti.

In Australia, LOSPs were originally designed to provide relatively short-term protection for building timbers and were applied by dipping processes, giving a temporary water repellent protection. These days, LOSPs are impregnated into the timber to provide long-term protection to high value commodities for use out of ground contact.

How are Light Organic Solvent Preservatives applied?

A modern, fully enclosed LOSP treatment plant in South Australia

LOSPs are impregnated into the timber, generally in its final shape and form, by sophisticated vacuum pressure treatment plants running computer-controlled treatment cycles. The exact preservative penetration and retention requirements are set out in the AS/NZS 1604 Standard series. It is preferable that the integrity of this preservative treatment should not be breached, e.g. by machining, drilling, and cutting on site. If this is unavoidable, it is essential that some form of brush-on treatment be applied to the newly exposed cut surfaces (see also the following section on post-treatment care).

Post-treatment Care for Light Organic Solvent Preservatives

LOSP-treated timber should never be used in ground contact. Furthermore, as with any treated timber product used in weather-exposed applications, a post-treatment protective coat should be applied, such as paint or a similar film-forming finish. This is essential for optimum performance of LOSPs. Note that some manufacturers guarantee the performance of their LOSP-treated timber only if such post-treatment applications are made. For best results, the post-treatment application should be re-applied from time to time, as a regular maintenance practice.

LOSP treatment may interact with some surface coatings. For example, residual solvent from the LOSP treatment process may interfere with both adhesion and drying of the surface coating. In addition, water repellent agents in the LOSP may affect the adhesion or spreading of some coatings. LOSP-treated timber suppliers may provide guidelines for painting, but you should always follow the surface coating manufacturer's instructions. The Australian Paint Manufacturers Federation can also be contacted for advice.

It is essential that all exposed surfaces resulting from cutting, notching or drilling of the timber during installation are resealed with some form of supplementary preservative. Suitable 'in can' preservative solutions can be obtained from most timber, paint and hardware suppliers.

Using Light Organic Solvent Preservatives

LOSPs have been registered and approved for use in Australia for some years. They are specified in national Standards for specific applications that may be summarised as follows:

HAZARD LEVEL H1	Inside, above ground and completely protected from the weather Powder post beetles	Lyctus susceptible hardwood framing, flooring, internal joinery
HAZARD LEVEL H2	Inside, above ground and protected from wetting Termites	House Framing
HAZARD LEVEL H3	Outside, above ground, subject to periodic wetting Decay, termites and wood boring insects	Feature fences, fascias, pergolas, window joinery

LOSPs will protect timber and other wood-based products against deterioration, as listed above. Remember though that LOSPs should not be used other than as described above. Where LOSP-treated products are used in weather-exposed situations, they should have some form of protective coating that may need to be maintained during the product's service life.

LOSPs are non-corrosive to nails, bolts and metal fixings. However, galvanised steel or other corrosion resistant fixings are recommended for outdoor, weather-exposed situations.

The environment and Light Organic Solvent Preservatives

Modern LOSP treatment plants are fully enclosed so that emissions from the solvent used in the formulation are well controlled. In addition, the plants are designed to minimise escape of, and operator exposure to fluid, mists, and vapours. Although much of the solvent is recovered at the plant during the closed-in part of the post-treatment cycle, small amounts of solvent stay in the timber when it leaves the treatment yard. However, the volumes of LOSP-treated timber delivered to building sites do not present an environmental hazard.

LOSP treatment plants produce drip-free treated timber and this virtually eliminates the risk of chemicals run-off in the treatment yard. Since timber is preferably treated with LOSPs in its final shape and form, there is little treated waste generated by this type of treatment. The LOSPs used in Australia today contain relatively environmentally benign biocides, such as permethrin for insect control (mainly termites in house framing).

Further information about Light Organic Solvent Preservatives

The Timber Preservers Association of Australia (TPAA) is the preservation industry's peak advisory body. The TPAA provides technical advice on all preservatives and preservative-treated timber, including LOSPs.

Contact the Timber Preservers Association of Australia (TPAA)