NZ331785A - Process for preserving timber by allowing fixative into only the outer layer of the timber - Google Patents

Abstract

A process of accelerating fixation of a preservative, such as chromic acid, copper oxide or arsenic acid, in preservative-treated timber in which the timber cells are only partially filled with preservative. The process comprises: 1) contacting the timber with a fixiant solution at a pressure and for a time sufficient to allow penetration of the fixiant solution into the outer regions for the timber only, and the allow the fixiant to react with the preservative within the timber cells in those regions. The fixiant solution achieves a penetration of the outer regions of the timber of no more than about 15mm, and 2) removing excess fixiant solution from the timber.

Description

PATENTS FORM 5 PATENTS ACT 1953 COMPLETE SPECIFICATION Number 331785 Dated 8 SEPTEMBER 1998 TIMBER PRESERVATION We, QUANTUM TIMBER PRODUCTS (NZ) LIMITED, New Zealand company of 7-9 Fanshaw Street, Auckland, New Zealand, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement. (followed by, office! I '8 SEP 1999 | t—ggCEiypn J 138827 vl WGN* 138773 TIMBER PRESERVATION FIELD OF THE INVENTION This invention relates to the treatment of timber, and more particularly to the fixation of preservatives in timber.

BACKGROUND Timber is used in many applications for structural as well as decorative purposes where it faces the risk of decay. This, if not protected, significantly shortens its service life raising the service cost as well as the cost of replacement. To alleviate this, timber is routinely protected by application of preservative treatments which rely on chemicals toxic to decay fungi and insects. These need to be chemicals 15 that can be present in the timber in an insoluble or fixed form, but are still toxic to the attacking fungi or insect.

Current methodology is to use chemical combinations such as Chromic Acid, Copper Oxide and Arsenic Acid which exist in soluble form prior to treatment, but 20 on contact with air and organic matter present in timber, the soluble hexavalent chromium converts to insoluble trivalent chromium together with a lowering of acidity. This results then in a series of reactions forming in part the insoluble Copper Chromate and Chrome Arsenate amongst others. Copper is also bound to the lignin by ion exchange reactions. This process ultimately achieves greater 25 than 99% fixation of the otherwise available components but is slow, taking days to complete and is dependent on external factors such as temperature. Thus, unless fixation of the preservative is accelerated in some manner, the soluble toxic preservative may contribute to site pollution.

Other safer chemicals such as Copper Sulphate have been used in the past as preservatives but typically ultimately fail to protect from decay due to an inadequate degree of fixation. la 55 I o Methods of fixation are currently available using heat to accelerate this process. These currently use steam, hot water or hot air as the transport mechanism. The disadvantages are the significant time taken to raise the temperature of the timber, that the timber has to be filleted, every layer reducing the volume that can treated each charge, that the timber charge is not fixed through the timber for a significant period after the timber is removed from the cylinder due to the need for the heat to permeate through and fix the components, and that sludge forms within a block stacked packet if fixed in this form.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a process which will enable accelerated fixation of timber preservatives to occur to an extent that the product does not contribute to site pollution from the toxic compounds used to levels beyond those acceptable under current guidelines; or at least to offer the public a useful choice.

Accordingly, in a first aspect the present invention provides a process of accelerating fixation of the preservative in preservative treated timber in which the timber cells are only partially filled with preservative, the process comprising the steps of: (i) contacting the timber with a fixiant solution at a pressure and for a time sufficient to allow penetration of the fixiant solution into the outer regions of the timber only, and to allow the fixiant to react with the preservative within the timber cells in those regions wherein the fixiant solution achieves a penetration of the outer region of the timber of no more than about 15 mm; and (ii) removing excess fixiant solution from the timber.

Preferably, the timber is contacted with the fixiant solution at ambient pressure. lisJELLECTUAL PROPERTY OFFICE OF N.Z. " 7 MAR 2001 138773 Conveniently, the preservative is a solution containing chromic acid, copper oxide and arsenic acid. Alternatively, the preservative can be a solution containing copper sulphate.

Preferably, the fixiant solution is selected from reductants, complexing agents and sequestering agents.

More preferably, the fixiant solution is a reductant selected from the group 10 consisting of solutions containing hydrogen peroxide, sodium trimercapto-s-triazine and mixtures thereof.

Preferably, the timber has been treated with preservative using a process including the steps of applying a vacuum to timber within a treatment cylinder to 15 remove air from the timber cells, and subsequently flooding the cylinder with preservative solution, wherein the level of vacuum applied is such as to only partially remove air from the timber cells.

Preferably, the fixiant solution is contacted with the timber by applying a vacuum 20 to the treatment cylinder following the treatment with preservative to remove excess preservative, subsequently flooding the treatment cylinder with the fixiant solution, and bringing the pressure of the cylinder to ambient pressure.

In a further aspect the present invention provides timber when treated by a 25 process as defined above.

While the invention is broadly as defined above, it will be appreciated by those persons skilled in the art that it is not limited thereto and also includes embodiments of which the following description provides examples. 3 138773 DESCRIPTION OF THE INVENTION As defined above, in general terms the invention relates to a process of accelerating the fixation of preservative in timber, in which the timber cells are 5 only partially filled with the preservative. The process includes the steps of contacting the timber with a fixiant solution, at a suitable pressure and for a time sufficient to allow the fixiant solution to penetrate the outer regions of the timber only and not the entire volume of the timber. It is preferred that the fixiant solution penetrates the timber by no more than approximately 15 mm. The 10 contact time of the timber with the fixiant is also selected so that it is sufficient to allow the fixiant to react with the preservative to achieve the required fixation. The fixiant solution is then removed from the timber.

The process of the present invention thereby provides an envelope of timber 15 material in which the preservative is fixed. This will minimise the amount of available preservative which can be washed off the timber (for example by rainfall), and thereby minimise the level of any potential pollution from the preservative. The remainder of the preservative chemical present at some depth within the timber matrix is allowed to fix naturally.

The process of the present invention is performed on timber which has been treated with a chemical preservative using a so called "modified full cell" process. By "modified full cell" we mean that the process leaves the cells of the timber only partially full of preservative containing solution, in contrast to "full cell" 25 processes which leave the timber cells full or saturated with preservative containing solution. The use of a "modified full cell" process leaves room for a subsequent process to further fill the cells with a solution containing a reactive chemical to accelerate the fixation of the preservative.

Timber to be treated by a process according to the present invention is therefore first treated with an aqueous chemical preservative solution, which can conveniently be a combination of chromic acid, copper sulphate and arsenic acid, using a "modified full cell" process as outlined above. 4 138773 Most conveniently, this treatment will be carried out in a treatment cylinder, and a vacuum drawn and held to partially evacuate air from the timber cells. For example, a vacuum of about -35kPa relative to ambient pressure applied for 5 about 5 minutes is generally suitable to achieve the required partial evacuation of the timber cells. The preservative solution is then flooded into the cylinder under vacuum. The vacuum is then removed and pressure applied, typically a pressure of about 1400 kPa relative to ambient pressure, and held for a suitable period of time to allow saturation of the timber, typically about 15 minutes. The 10 preservative solution is then pumped back to its storage vessel, and a vacuum applied to strip the remaining preservative from the treatment cylinder.

Following the treatment with preservative, the timber is contacted with an aqueous fixiant solution. The fixiant can be selected from reductants such as 15 hydrogen peroxide, hydrazine, ferric chloride and sodium metabisulphate, complexing agents such as ethylene diaminetetra acetic acid, sequestering agents such as sodium sulphide, dithiocarbamate, sodium trithiocarbonate, and sodium trimercapto-s-triazine. In preferred embodiments of the invention, the fixiant is either a solution of hydrogen peroxide (at a concentration of about 2% 20 w/v), a solution of sodium trimercapto-s-triazine (at a concentration of about 0.3% w/v), or a mixture of both hydrogen peroxide and trimercapto-s-triazine (each present in the above concentrations).

The contacting of the timber with the fixiant may conveniently be carried out in 25 the treatment cylinder in which the preservative has been applied to the timber. Specifically, following the removal of the preservative solution under vacuum, the cylinder is flooded with fixiant solution while maintaining a vacuum (conveniently of about -75kPa). Once the cylinder is filled, the fixiant solution is held in contact with the timber at a suitable pressure for a time sufficient to enable both 30 (1) the fixiant to penetrate into the outer regions of the timber only and not its entire volume, and (2) reaction of the fixiant with the preservative (ie fixing) to take place. To achieve this, it is preferred that the fixiant solution is held in 138773 contact with the timber at ambient pressure, suitably for a period of around 10 minutes.

Following completion of the contact period, the fixiant solution is pumped back to 5 its storage tank and a vacuum applied to complete stripping of the fixiant solution.

In an alternative embodiment of the invention, if convenient, the preservative treated timber may simply be dipped in the fixiant solution.

The invention will now be further described with reference to the following non-limiting examples.

EXAMPLES The preservative solution contained Chromic Acid, Copper Oxide and Arsenic Acid in the optimum elemental ratios detailed in the,New Zealand and Australian Product Specification standards of MP 3640:1992 and AS 1604:1992 respectively. The timber is kiln dried to moisture contents in the 15-20% range. The timber 20 charge is block stacked and loaded into a treatment cylinder and the door closed and secured.

A vacuum of -35kPa relative to ambient pressure is drawn and held for 5 minutes. The preservative solution is then drawn into the treatment cylinder by 25 virtue of the vacuum, from the preservative store tank, this vacuum being maintained by the vacuum pump. On complete filling of the cylinder, the vacuum line is closed and solution pumped in to a pressure of 1400 kPa relative to ambient pressure. This pressure is maintained for 15 minutes then reduced steadily to ambient over ten minutes. The preservative solution is then pumped 30 back to the preservative store tank and a vacuum drawn on the treatment cylinder to -85 kPa relative to ambient pressure and held for 45 minutes. During this vacuum a separate pump strips solution from the bottom of the cylinder and flooding line returning it to the preservative store tank. 6 138773 At the end of this period and with no further preservative solution being stripped, a separate store tank containing fixiant is allowed to flood the cylinder while maintaining a vacuum of at least -75 kPa relative to ambient pressure. Once the 5 cylinder is completely filled, the flooding line valve is closed and the fixiant solution held in contact with the timber at ambient pressure for 10 minutes. Any oxygen gas pressure is relieved. On completion of the period, the fixiant solution is pumped back to the fixiant store tank. A vacuum is then drawn to -85kPa relative to ambient pressure. The stripping pump is also run in this period and 10 maintained until stripping has stopped or two minutes at -85kPa, whichever is longer. The vacuum is then relieved, the door opened and the charge removed.

The effectiveness of the process can be measured by a simulated rainfall test. This duplicates a typical rainfall event using an average volume of water sprayed 15 over an area of timber in a standard time. The resultant solution collected under the timber packet is analysed for the active components, then calculated as the weight of component washed off per volume of timber. By calculation of the expected volume of timber held on a given area of yard versus the expected rainfall over this area, the result can be calculated as potential soil 20 contamination or allowable concentration in storm water runoff.

Two charges using the fixiants of hydrogen peroxide (2%) and sodium trimercapto-s-triazine (0.3%) were completed using the above process aimed at preservative retentions of 3.6 kg/m3 and 7.0kg/m3 respectively. Both gave total 25 preservative wash off levels below lg/m3.

A further example of the use of the fixiant compounds was demonstrated in the following experiment. Short blocks of kiln dried wood were end sealed with resin and treated with preservative using a modified full cell process as described 30 above. These were then dipped in solutions of the following: 2% Hydrogen Peroxide 2% Hydrogen Peroxide and 0.3% Sodium Trimercapto-s-triazine 7 138773 0.3% Sodium Trimercapto-s-triazine for ten minutes. These were then allowed to dry for one hour and then dipped in water to duplicate wash off. All three solutions gave much lower wash off than 5 an unfixed control, with 0.3% sodium trimercapto-s-triazine giving extremely low results with Copper. It is these types of compound which are expected to enable the use of copper sulphate as a preservative.

INDUSTRIAL APPLICATION It is believed that a process according to the present invention will find acceptance as a process of accelerating the fixation of chemical preservations in timber, thereby reducing the potential for pollution of the environment by such 15 preservatives in their soluble, unfixed state.

One advantage of the process of the present invention is that it minimises the amount of fixiant required. This is because the fixiant is present only in the outer regions of the timber (which effectively minimises washoff), and the 20 preservative in the inner regions of the timber is allowed to fix naturally.

While the invention has been described with reference to particular embodiments, it will be appreciated by those persons skilled in the art that it is not limited thereto and that variations and modifications can be made without 25 departing from the scope of the invention as defined in the following claims.

Throughout the description and claims of the specification the word "comprise" and variations of the word, such as "comprising" and "comprises" are not intended to exclude other additives, components, integers or steps. 8 ■?? 1 ? '•ci %J/

Claims (13)

WHAT WE CLAIM IS:

1. A process of accelerating fixation of the preservative in preservative treated timber in which the timber cells are only partially filled with preservative, the process comprising the steps of: (i) contacting the timber with a fixiant solution at a pressure and for a time sufficient to allow penetration of the fixiant solution into the outer regions of the timber only, and to allow the fixiant to react with the preservative within the timber cells in those regions, wherein the fixiant solution achieves a penetration of the outer regions of the timber of no more than about 15 mm; and (ii) removing excess fixiant solution from the timber.

2. A process as claimed in claim 1 wherein the timber is contacted with the fixiant solution at ambient pressure.

3. A process as claimed in claim 1 or 2 wherein the preservative is a solution containing chromic acid, copper oxide and arsenic acid.

4. A process as claimed in claim 1 or 3 wherein the preservative is a solution containing copper sulphate.

5. A process as claimed in any one of the preceding claims wherein the fixiant solution is selected from the group consisting of reductants, complexing agents and sequestering agents.

6. A process as claimed in claim 5, wherein the fixiant solution is a reductant selected from the group consisting of solutions containing hydrogen peroxide, sodium trimercapto-s-triazine and mixtures thereof.

7. A process as claimed in claim 6, wherein the fixiant solution is a solution containing hydrogen peroxide, at a concentration of about 2% w/v. INTELLECTUAL PROPERTY OFFICE OF N.Z. 7 MAR 2001 9 El E0 Efl\7!ED

8. A process as claimed in claim 6, wherein the fixiant solution is a solution containing sodium trimercapto-s-triazine, at a concentration of about 0.3% w/v.

9. A process as claimed in claim 6, wherein the fixiant solution is a solution containing both hydrogen peroxide, at a concentration of about 2% w/v and trimercapto-s-triazine, at a concentration of about 0.3% w/v.

10. A process as claimed in any one of the preceding claims, wherein the timber has been treated with preservative using a process including the steps of applying a vacuum to timber within a treatment cylinder to remove air from the timber cells, and subsequently flooding the cylinder with preservative solution, wherein the level of vacuum applied is such as to only partially remove air from the timber cells.

11. A process as claimed in claim 10, wherein the fixiant solution is contacted with the timber by applying a vacuum to the treatment cylinder following the treatment with preservative to remove excess preservative, subsequently flooding the treatment cylinder with the fixiant solution, and bringing the pressure of the cylinder to ambient pressure.

12. Timber when treated by a process as claimed in any one of the preceding claims.

13. A process as defined in claim 1 of accelerating fixation of the preservative in preservative treated timber in which the timber cells are only partially filled with preservative, substantially as herein described with reference to the Examples. INTELLECTUAL PROPERTY OFFICE OF N.Z. - 7 MAR 2001