Wood preserving formulations based on copper salts, for instance on copper arsenates and/or copper chromates, are commonly used to preserve wood in contact with the ground. These inorganic preservatives are often used in preference to organic preservatives and creosote. Copper- Chromium-Arsenate is one such preservative which has been successful. It contains copper 11 ions, chromate and arsenate ions. The weight ratio of the metals (based on CuO, CrO3 and Asz05) are preferably in the range (17 to 21): (44.5 to 50.5): (30 to 38) (to total 100).

An aqueous solution of the wood preservative is impregnated into timber in a process in which the timber is first subjected to reduced pressure to open the fibres, the timber is then immersed in the aqueous composition under raised pressure, for instance up to 12 bar or more, excess composition is then drained and the timber is dried under reduced pressure.

Excess composition which does not become impregnated into the timber is collected and returned to the reservoir. The composition should be stable in the reservoir, physically and chemically, for extended periods of at least one month, for instance six months or more.

It would be desirable to improve wood preservatives based on copper salts, especially CCA products, by providing them with hydrophobing properties.

According to the present invention there is provided an aqueous composition comprising a copper-based wood preservative and a wax emulsion.

The novel composition is usually acidic, although alkaline (ammoniacal) compositions may also be suitably hydrophobised by including the wax emulsion. The pH of an acidic composition is preferably less than 5, more preferably less than 3, for instance around 2.

The composition generally comprises a polymeric stabiliser. The stabiliser should be stable under the pH conditions. Suitable acid-stable polymers useful as stabilisers are xanthan gums.

The composition should generally comprise an emulsifier for the wax emulsion. The emulsifier should, again, be stable under the pH conditions in the composition. Preferably the emulsifier comprises, or, most preferably, consists of a non-ionic compound. Most preferably the emulsifier is a polyethoxylated compound, most preferably a polyethoxylated alcohol, for instance of a C12 24-alkanol, having 5 to 20 ethoxy groups, most preferably 10 to 15 ethoxy groups. Other suitable non-ionic emulsifiers are polyglycosylated alcohols or acids. The emulsifier is preferably soluble in water (no cloud point according to DIN 53917), has a cloud point in saline of less than 60°C (DIN 53917) and HLB in the range 12 to 15.

The wax is preferably a refined paraffin wax. Preferably the wax should have an oil content of less than 1%, and a melting point of at least 57°C, for instance in the range 57 to 75°C, most preferably in the range 57 to 60°C. The wax should be in the form of a dispersed phase having substantially all particles having a size less then 1 um The wood preservative is generally a Copper-Chromium-Arsenate (CCA) compound.

The composition preferably contains wood preservative and wax in weight ratios in the range 1: 2 to 5: 1, more preferably in the range 1: 1 to 2: 1, for instance about 3: 2.

The composition is found to be adequately chemically and physically stable such that it may be used in pressure treatment systems, in which a reservoir of composition is reused and intermittently replenished, over a period of at least one month, more preferably at least six months. According to a further aspect of the invention therefore there is provided a new process in which timber is impregnated with the novel aqueous composition.

Preferably the process comprises the steps of: a) subjecting the timber to reduced pressure in air (i. e. dry); b) contacting the timber by immersing it in the aqueous composition under raised pressure; and c) removing excess composition and drying the timber.

Generally the excess composition is recovered and re-used to impregnate further batches of timber. Suitably the recovered composition is stored in a reservoir, maintaining its chemical and physical stability for a period of at least one month, preferably at least six months.

In the process the initial reduced pressure treatment step is preferably carried out for a period of at least 1 minute, up to about 30 minutes. The immersion step is preferably carried out at ambient temperature for a period of at least 15 minutes, for instance at least 30 minutes, up to 2 hours, preferably about one hour at a pressure of at least 5 bar, more preferably 10 to 15 bar. The drying step is usually carried out at ambient temperature and reduced pressure, for instance of 5 minutes or more, up to half an hour.

Alternatively or additionally the treated timber may be heated to dry it.

The emulsion and wood preservatives are generally supplied to the wood treatment establishment in the form of concentrates, preferably each component being provided as separate concentrates. According to a further aspect of the invention there is provided a package containing a copper- based wood preservative concentrate and a wax emulsion concentrate.

Preferably the wax emulsion concentrate has a solids content of at least 10% by weight, most preferably at least 20% by weight, for instance in the range 25 to 40% by weight, based on wax in total emulsion concentrate.

The emulsion concentrate comprises emulsifier, as defined above, and, preferably, stabiliser, again as defined above. The ratio of wax to emulsifier is preferably in the range 1: 40 to 1: 5, most preferably in the range 1: 20 to 1: 8. The ratio of wax to stabiliser is preferably in the range 20: 1 to 100: 1.

The copper based wood preservative concentrate is generally provided in substantially solid form, for instance in the form of a powder.

Preferably the wood preservative is a copper chromium arsenate compound.

It is convenient for the package further to contain acid, generally inorganic acid, suitable for adjusting the pH of the acidic aqueous composition which is the preferred form of the wood preservative and wax containing aqueous composition.

The wax emulsion comprising the preferred xanthan stabiliser and preferred non-ionic emulsifier, is believed to be novel. According to a further aspect of the invention there is provided a novel wax emulsion comprising a refined paraffin wax having oil content less than 1% and a melting point of at least 57°C, a stabiliser which is a xanthan gum, and a non-ionic emulsifier which is soluble in water, has a cloud point in saline of no more than 75°C (according to DIN 53917), and has HLB in the range 12 to 15.

The novel wax emulsion should preferably be formed of wax having a melting point in the range 57 to 75°C, preferably was having a melting point specification in the range 57 to 60°C.

The emulsifier should preferably be an ethoxylated alkanol, as described above. The weight ratios of emulsifier, stabiliser and wax, as well as the total concentration and the particle size of wax in the emulsion (concentrate) should be as mentioned above.

The present invention provides an aqueous composition which has good stability in pressure treatment apparatus, such that the composition may be recovered and re-used, remaining chemically and physically stable in the reservoir for at least one month, for instance six months or more. The composition provides improved properties regarding water resistance, inhibiting water ingress into the treated timber in use. Selection of an appropriate wax emulsion, however, allows moisture egress from the timber, to allow drying. The wax appears not to prevent or reduce the rate of impregnation of the wood preservative to an undesirable extent. The stability of the bath is superior to that of other hydro-phobed CCA compositions.

The invention is illustrated further in the accompanying examples.

Example 1-Preparation of Emulsion A fully refined paraffin wax having oil content less than 1 % and a melting point specification in the range 57 to 60°C is heated to 95°C. Water is heated to 85 to 90°C and 0.5% by weight of xanthan gum, which is acid stable, is added. To the stabiliser solution, 2.25% by weight of a 12 mole ethoxylate of C16-alkanol supplied as an 80 weight % aqueous mixture, is added. The heated wax is added to the aqueous phase to provide a mixture containing 27.25% by weight wax (based on total composition). The pre- emulsion, maintained at a temperature in the range 90 to 92°C, is recycled through a homogeniser under suitable conditions of pressure and number of passes to give an emulsion having particle size less than 1 um. The emulsion is cooled.

Example 2-Composition Make Up A commercially available copper chromium arsenate wood preservative compound, Celcura AO, and wax emulsion are dissolved into water with acid to provide a composition having pH 2 containing 2% wax emulsion and 3% CCA. The composition is added to the reservoir of the pressure treatment apparatus. Timber is treated in the apparatus in a process in which it is first subjected to a vacuum to open the wood fibres.

Subsequently the acidic aqueous composition is contacted with timber under a pressure of 12 bar for a period of 1 hour at ambient temperature. Excess composition is recovered and recycled to the reservoir, whilst the timber is dried by reducing the pressure for a period of about 30 minutes. The timber is subjected to performance tests to compare properties of the preservative, and to determine water run off after application. Furthermore the emulsion itself is tested for its quality using a qualitative shear test. The stability of the aqueous composition is determined after 48 hours and after three months in the reservoir.

The composition formed according to Example 1 showed good properties of the emulsion itself. The emulsion was compatible with the CCA in the acidic aqueous composition after 48 hours and after three months in the apparatus reservoir. The CCA's preservative action was retained. The composition provided good hydrophobing as judged by water beading on the timber surface after treatment.

In a series of comparative tests, a wax emulsion containing half the quantity of stabiliser compared to that of example 1 and 40% wax was tested. However the emulsion was not stable in the aqueous composition in the reservoir during the three month testing period.

A further comparative example was made up in which the wax emulsion contained 40 weight % of wax but which contained the same level of stabiliser as that of example 1. This emulsion, however, had poor quality as judged by a shear test prior to addition to the CCA.