Timber & Wood Products: Preservatives, Binders, Fixing

Preservatives in Timber Products

While a comprehensive review is beyond the scope of this document, a wide range of treatments is available, with varying levels of human and environmental toxicity. Very toxic chemicals have historically been introduced into our homes and buildings in the cause of timber treatment such as DDT, Dieldrin, Pentachloraphenol, Lindane, Tributyltin oxide and Arsenic. Scientific understanding and public sentiment changes over time, and we are using increasingly sophisticated treatments. The US recently joined a growing list of countries in phasing out the use of CCA treated timber (copper chrome arsenate) for most applications (US EPA). Permethrin, organic zinc compounds, IPBC, Dichlofluanid and Propiconazole are all less toxic than the earlier treatments, although true health and environmental impacts will be apparent only after many years of use.

Preservatives Used in the treatment of Wood

Product	Use	Advantages	Disadvantages
(Boron/Fluorine products.)	Preservative, fungicide and insecticide. Dry pellet or liquid form. Pellets moisture activated. For H1-H4 applications*	Considered largely benign/ very low toxicity. Commonly available. Low initial cost.	As the products are water soluble, they leach out over timber and *Must be replenished regularly (often 2-5 years) Widely used in EU in exterior windows and similar applications, in Australia primarily a maintenance product e.g. fence posts.
LOSP using linseed oil carrier	Preservative, fungicide and insecticide for H1-H3 applications	avoids use of white-spirit solvent, a greenhouse gas intensive solvent.	May still use Tributyltin Oxide, often banned in marine preservatives due to aquatic toxicity
(LOSP) light organic solvent particles	Preservative, fungicide and insecticide for H1-H3 applications	Considered low-toxic, commonly available. Low initial cost. A new formulation is available ('T2') that uses linseed oil instead of white spirit, eliminating the greenhouse impact of white-spirit off-gassing.	Not suitable for in-ground applications. Oil-based primer may be required for painted applications (check with suppliers instructions).
ACQ (ammoniacal copper quaternary)	Preservative, fungicide and insecticide suitable for H1-H5 applications.	Highly effective. Contains no high toxicity arsenic/chromium compounds. Can be burnt and mulched.	Poor availability for most applications now improving slowly. Slightly higher initial dollar cost. Paintable.
Copper Azole	Preservative, fungicide and insecticide suitable for H1-H4 applications.	Highly effective. Contains no high toxicity arsenic/chromium compounds. Can be burnt and mulched.	Poor availability for most applications now improving slowly. Slightly higher initial dollar cost. Paintable.
CCA (copper crome arsenate solution) (tanalized) Available water/ oil base	Preservative, fungicide and insecticide suitable for H1-H6 applications.	Lowest initial dollar cost, paintable, highly effective. Oil based variant offers improved resistance to surface weathering of timber.	Cannot be safely disposed of except as landfill: highly toxic smoke from burning, cannot be mulched. Not presently reusable for timber products e.g. particleboards although research continuing. Any shavings and offcuts must be disposed of as landfill. Effectively prohibited in Japan and some European countries, phase-out in US. Some research shows significant toxicity from contact to treated timber to children under 6 years (Environmental Working Group 2001)
PEC (pigmented emulsified creosote)	Preservative, fungicide and insecticide suitable for in-ground applications.	Emulsified form of traditional Creosote. Highly effective, lower toxicity, may be disposed of by incineration.	Cannot be painted. Sill considered quite toxic, generally used for agricultural applications.
Creosote	Preservative, fungicide and insecticide suitable for H4-H6 applications.	High efficacy.	High toxicity and restricted use (eg for farm posts). A known carcinogen. Cannot be painted. Strong smelling.

*Applications are attributed 'Hazard' levels under Australian Standards. These range from 'H1' (above ground, indoors and protected) to 'H6', timber permanently in contact with salt water. Windows are typically an H3 application (outside but not in-ground), while fences (in ground contact) typically an H4.

Binder Toxicity

Many wood products today incorporate formaldehyde-based glues. The IARC has classified formaldehyde as a Category 1 'known human carcinogen' (IARC 2008). .

In many commonly used forms formaldehyde will offgas, particularly in new products and in high heat and humidity. Inhalation of formaldehyde vapors can cause fatigue, respiratory irritation, allergic skin reactions and is a probable carcinogen (IARC 2003).

Conventional interior grade products (plywoods, MDF's, particleboards) typically use urea-formaldehyde glues, which are relatively unstable and offgass consistentently over time. According to the Australian CSIRO this offgassing is not stopped for more than a matter of weeks by painting, varnishing or laminate surfaces (Brown, pers. comm. 2003). These materials are a primary contributor to formaldehyde emission concerns in buildings (Brown 2001).

Plywoods used for exterior and structural applications typically use phenol formaldehyde (PF) resin. PF is a more expensive, water-resistant glue, and a much more stable product that offgasses at a much lesser rate than UF glue.

The US Green Building Council's LEED Credit for Composite Wood on the other hand requires composite timber and agrifibre boards to be free of any added UF products..

Fixing

The reclaimation of demolition timbers for use and reprocessing as recycled timber is prevented due to the use in recent decades of adhesive fixings. While deconstructability is fundamental to reuse and high-value recycling, current glue & nail practices will ensure this is in most instances impractical, and repair difficult. It is critical wherever possible to use fixings that facilitate deconstruction and high-value reuse.