WO2002051558A1

WO2002051558A1 - Treatment of wood surfaces

Info

Publication number: WO2002051558A1
Application number: PCT/NZ2001/000298
Authority: WO
Inventors: Bernard Stanley Winton Dawson
Original assignee: New Zealand Forest Research Institute Limited
Priority date: 2000-12-22
Filing date: 2001-12-24
Publication date: 2002-07-04
Also published as: NZ509133A

Abstract

A method of coating wood comprising photostabilising a surface of the wood using delignification of the surface including removal/destruction of photoabsorbent lignin fragments, and subsequently coating the surface with a transparent coating. Suitable delignification agents include chemical delignifying agents, lignin-degrading radiation and lignin-degrading enzymes. Suitable delignification agents include peracetic acid or peroxymonosulphuric acid.

Description

TREATMENT OF WOOD SURFACES

TECHNICAL FIELD

This invention relates to a treatment for protecting the surface of wood including from exposure to outdoor conditions.

BACKGROUND ART

Clear varnishes of a variety of types are well known for interior applications. For example clear varnishes are commonly used with good results on fttmiture and flooring. The use of varnish allows surface protection while retaining a natural wood colour. Furthermore the use of varnish also allows viewing of the wood grain through the varnish for aesthetic appeal to consumers and users of the varnished articles.

While the use of varnishes has been widespread for indoor applications, that has not applied in outdoor settings. Most varnishes do not suiΥive on wood in exterior situations. The use of varnishes on wood cladding materials will typically result in failure within a 12-18 month period and will usually result in a highly disfigured appearance. One exception is the marine spar varnishes that have a high content of ultraviolet absorbers and typically require a high number of coatings (8-10). Spar varnishes have not become more widely used because of the cost of the ultraviolet absorbers and the need for the large number of coatings. The usual approach to this problem is to avoid varnishes and to use pigmented paint or stain systems. These are cost effective but mask the natural appearance and grain otherwise apparent on the wood surface. When stains are used, these usually require remedial work including coating reapplication every year or every other year.

It is therefore an object of the present invention to provide a method for treating wood to provide it with a transparent coating suitable for use in applications outdoors and/or to provide the public with a useful choice.

DISCLOSURE OF THE INVENTION

In one aspect, the invention provides a method of coating wood comprising photostabilising a wood surface using delignification of the surface including removal/destruction of photoabsorbent lignin fragments, and subsequently coating the surface with a transparent coatmg. The method may be applied to one surface (or part of one surface) or to more than one surface or to all surfaces. Generally it is preferred to remove or deactivate the delignifying agent after the delignifying step.

The invention uses delignification at the surface of wood, preferably down to a depth of about 2-80 cells, preferably 5-70 cells, more preferably 20-40 cells (about 1 mm). The delignifying agents which can be used include chemical delignifying agents such as peracids (especially peracetic acid and peroxymonosulfuric acid), lignin-degrading radiation (especially UV radiation) and lignin-degrading enzymes. The delignification treatment photostabilises the wood surface. The extent of delignification is such that the result is not a pulped cellulosic wood surface.

The reaction of the wood surface with delignifying agents such as peracetic acid, results in the formation of lignin or lignin fragments with increased hydrophilic nature rendering it more soluble. Extensive lignin fragment dissolution takes place on alkali treatment following peracetic acid treatment. The alkali treatment is one example of a treatment for the dissolution of lignin fragments; more generally a wide range of aqueous solutions can be employed. The water washing following the alkali treatment is also important to neutralise any remaining acidity or alkalinity.

In a preferred embodiment, the delignification agent is an oxidative chemical delignifying agent, preferably peracetic acid or peroxymonosulfuric acid. Other oxidising agents such as acidified sodium chlorite may also be used. Once the delignification has been completed to the appropriate depth, the wood surface is extracted soaked in neutral or alkaline solution, preferably a dilute sodium hydroxide solution to solubilise any lignin fragments on or trapped in the wood surface and to remove the unreacted peracid. The surface is finally rinsed or soaked with water from a bath to return the pH towards a normal wood pH value and the rinse solution to a neutral pH. Next, fungicidal protection is applied by soaking in a solution of antisapstain solution. Sentry (Chemcolour Ltd) is a preferred fungicidal solution but other effective antisapstain solutions are also useful. Preferred active ingredients include Preventol A5, copper 8 quinolinolate, N-methyl pyrrolidone, benzalkonium chloride, methylene bis thiocyanate, and 2-n-octyl-isothiazolin-3-one. After the wood is dried (with care not to introduce drying stresses which would lead to deep checking) a polymer pretreatment may be applied to the wood surface to penetrate and react with the wood to stabilise the wood. Following this, varnishes (possibly coloured) are applied to build up a surface film of varnish which will prevent water from entering into the wood. This results in a photostable wood surface with fungicidal protection and hydrophobicity suitable to combat the exterior elements of sunlight, rain and the microbial activity.

Application of the delignifying agent to the wood can be carried out in a variety of ways provided there is an adequate period of contact with active chemical, preferably at elevated temperature for a period of at least 10 minutes, for example by spraying or dipping.

Treatment in a bath of delignifying agent is currently preferred. However it is important that whatever method is used, the contact with the delignifying agent is sufficient to produce delignification to the required depth. Simply applying by brushing onto surface will generally not be effective because of dilution of the delignifying agent through penetration into the wood surface. Once the sufficient delignification has occurred, it is preferred to stop the delignification reaction and/or remove delignifying agent and lignin fragments. The methods of carrying this out will depend on a number of factors including the delignifying agent is used. Where a peracid is used, washing with an alkali especially sodium hydroxide and subsequent water washing is favoured after the major effect of lowering the temperature to ambient. Alkali treatment and water washing is also useful with other delignifying agents. Where enzymatic delignification is used however further methods such as use of an enzyme inhibitor may also be used.

Wood treated with chemical delignifying agents may also be advantageously treated with supercritical fluids (especially supercritical C0₂) or cryogenic liquids (especially liquid CO₂). These may be used with or without modifiers for extracting chemicals from biomass. If methanol is used as an additive in supercritical CO₂, extraction of lignin fragments and other chemicals is particularly effective. These types of extractions may also be used when lignin breakdown is by other methods. For example supercritical CO₂ extraction of wood treated with UV radiation is useful and has the advantage that it can be done without water spraying thus avoiding wetting and subsequent redrying of wood.

It is preferred to endseal before delignification. This may be achieved using coatings, typically three heavy applications of epoxy or polyurethane allowing curing time after each application. Alternative means of end sealing may also be used, for example, a mechanical barrier such as a close fitting plastic cap.

When a peracid is used as the delignification agent, elevated temperatures are preferably used, preferably 30-90 °C. Most preferred are temperatures of the order of 50-80 °C. Typically with radiata pine, the treatment uses concentrations of up to 11% peracetic acid and a treatment time of 10-300 minutes, preferably 30-120 minutes. In another preferred embodiment the delignifying agent is UN radiation. The UN radiation may be from sunlight or artificially produced. Particularly if sunlight is the UN radiation source, it is preferred to first treat the wood with a fungicide as the process typically takes weeks. Preferred fungicides are those which are penetrative and slowly release fungicide.

It is preferred to use this method with alternating UV radiation and washing steps. The washing removes photoabsorbent lignin fragments. Use of 400 hours of UV irradiation combined with water spraying 15% of the time is effective. A typically useful cycle is 102 minutes of UN radiation followed by 18 minutes of water washing. However the times for radiation and washing may be varied greatly.

The invention has wide applicability to wood from different species of trees. Particularly preferred for use in the invention are softwoods used for cladding of housing and outdoor furniture, for example radiata pine weatherboards, verandah cladding, joinery, doors and an assortment of garden furniture including tables and chairs. It has particular usefulness for non- horizontal surfaces which do not pool water or suffer abrasive action.

Many different coatings may be used in the invention to coat the delignified wood. After the delignification treatment, washing and gentle drying, the preferred first coating application is of a penetrating nature, whether water-based or oil-based. This should be reactive towards the photostablised wood surface, should penetrate up to 10 mm and be able to form chemical bonds with the wood. Examples of these reactive pretreatments are isocyanates and melamine derivatives. The preferred top-coatings are flexible rather than rigid to avoid splitting due to wood movements. Preferred coatings include a wide range of transparent non-pigmented varnishes such as polyurethane, aliphatic methanes, acrylics, fluoropolymers, epoxides, hybrid polymer formulations such as acrylic-polyurethanes. Currently the most preferred coatings include acrylics and aliphatic urethanes. Generally preferred coatings are clear and transparent. The invention also includes coatings which are transparent but also have some colour.

Coating application can be by a variety of means including spraying, brushing, curtain coating and rolling. An unbroken polymer film of up to 200 microns dry film thickness is suitable after the reactive precoating treatment has cured.

The coatings may include additives if desired. For example fungicidal agents may be included in the coating formulations. Semi-transparent pigments such as superfine iron oxide may be included for aesthetic reasons especially for outdoor uses.

In another aspect of the invention, there is provided a method of exposing wood to sunlight wherein the surface is protected by a coatmg as described above. The wood may be used as a cladding material.

In another aspect the invention provides a method of forming a wood-polymer surface composite zone on at least one surface of the wood comprising delignifying a wood surface including removal/destruction of photoabsorbent lignin fragments, and subsequent coating of the surface with a transparent coating.

In another aspect the invention comprises a coated product of a method of the invention.

The coatings can cure in the ambient conditions or by reaction, for example with atmospheric moisture or oxygen or by curing with the aid of radiation such as infrared or ultraviolet. Also coatings may be of a thermoplastic nature which form a film by coalescence on solvent evaporation.

BRTEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a FT-IR spectrum for a peroxymonosulphate-treated sample and a control sample of wood where delignification is evidenced by removal of the 1510 wavenumber band.

Figure 2 shows Del b values derived from the L and b colour space for (a) pine with/without peracetic acid treatment (b) pine with brushed chlorite treatment, (c) spruce with brushed chlorite treatment, (d) spruce without brushed chlorite treatment and (e) pine with weatherometer treatment. Each is plotted against hours in the weatherometer. The numerals indicate different subtreatments. Figure 3 shows Del a values for (a) pine with/without peracetic acid treatment (b) pine with brushed chlorite treatment, (c) spruce with brushed chlorite treatment, (d) spruce without brushed chlorite treatment and (e) pine with weatherometer treatment. Each is plotted against hours in the weatherometer. The numerals indicate different subtreatments. Figure 4 shows Del L values for (a) pine with/without peracetic acid treatment (b) pine with brushed chlorite treatment, (c) spruce with brushed chlorite treatment, (d) spruce without brushed chlorite treatment and (e) pine with weatherometer treatment. Each is plotted against hours in the weatherometer. The numerals indicate different subtreatments.

Figure 5 shows the extent of delignification of a 20 mm thick wood sample in cross-section after the peracetic acid treatment.

Figure 6 shows the extent of delignification of a 20 mm thick wood sample in cross-section after the brushed chlorite acid treatment.

Figure 7 shows the extent of delignification of _a 20 mm thick wood sample in cross-section after the weatherometer treatment.

Figure 8 shows the extent of delignification of a 20 mm thick wood sample in cross-section after the peracetic acid treatment (higher magnification).

Figure 9 shows a 20 mm thick control wood sample in cross-section (higher magnification). EXAMPLES

The following examples further illustrate practice of the invention.

EXAMPLE 1 Delignification of the surface region of solid Pinus radiata wood using peroxymonosulphate (Ps).

Sample Preparation

Solid wood. Radiata pine was used as solid wood which for the preliminary treatments was cut and dressed to 99 x 63 x 10 mm. The wood had an approximate density of 450 kg m and a moisture content of about 10%. For the scale up, it was cut and dressed to 230 x 60 x 10 mm (weatherometer size). The average density of this wood was 456 kg/m³ and the average moisture content was 10%. In both cases the majority of the samples were quarter sawn with a small number flatsawn for separate analysis of early wood and late wood. The preliminary treatment samples were not end sealed as would normally be the case. This was because the aluminium primer available would not only increase the metal ion problem that the use of the EDTA was to overcome. However, once the treatments were shown to work, an alternative polyurethane marine varnish was successfully tried and used for the scale-up treatments.

Equipment

Preliminary trials. The samples were placed in plastic bags (1 sample/bag), sufficient to hold 100 ml of treatment solution around the sample. The bag of sample and solution was then held in a wire mesh frame in order to keep the sample immersed in the solution and the bag immersed in the water bath which provided the heat source.

Scale up. The samples were held in a plastic mesh racking system (30 at a time) which was placed into a treatment bath. - a plastic lined stainless steel basket was used to provide maximum rigidity and heat transfer. A glass stirrer was used to maintain a continuous flow of solution, and therefore an even temperature and concentration around the samples. The temperature requirement of up to 90 °C was provided by a water bath. NB all equipment in contact with the Ps treatment was non-metallic.

Treatment Schedule

The peroxymonosulphate used for this treatment was OXONE as supplied by Du Pont Company, USA and the treatment method consisted of three stages plus drying as above:

Pre-treatment. The samples were washed in a solution of EDTA, 25 g/1 in distilled water (adjusted to pH 5-6 with concentrated H₂SO ), at 90 °C for 60 min and then washed in distilled water for 60 min. This pre-treatment was used to remove transition metal ions which would otherwise have reduced the selectivity of the Ps, thus reducing the effect of the treatment and also leading to potential degrade of the cellulose as well as the lignin. Main treatment. This was 10% Ps (2KHSO₅, KHSO₄, K₂SO , MW 614.7) by weight, in a buffer solution of 3.0M acetic acid (CH₃COOH - mw 60.05):0.2 M sodium acetate (CH₃COONa.3H₂O - mw 136.8), made up with distilled water. The samples were treated in this solution for 120 min at 75 °C and then washed in distilled water for 60 min. A solution concentration of 5% Ps was also used during the preliminary trials.

Post treatment. After the main treatment, the samples were washed in a solution of 1.0 M sodium hydroxide (NaOH mw 40.0) for 60 min at 50 °C. The samples were then washed for 60 min as follows: 10 min in distilled water and 5 min in 0.1 M hydrochloric acid, repeated twice, and then 15 min in distilled water.

The solutions were added at the rate of 100 ml/sample for the preliminaries and 12.5 1/treatment for the scale up. The solutions for the scale up were preheated before the sample rack was added, EDTA - 85 °C, Ps - 69 °C and NaOH - 42 °C.

Treatment Control

Progress during treatments, solution stability and chemicals consumed, were monitored by titrating sample solutions against a 0.1 M solution of sodium thiosulphate (Na₂S₂O₃.5H₂O - mw 248.0). The sodium thiosulphate was prepared and standardised according to Vogel (1961) A text book of Quantitative Inorganic Analysis 3rd Ed. Longmans, London. Analysis of all preliminary all preliminary treatment samples and selected scale up treatment samples was carried out using Light microscopy, FT-IR and L a b colour measurements (initial preliminary trials only) as reported below.

Light microscopy. Hand cut sections were prepared from each sample. Phloroglucinol 1% (in 95% EtOH), with concentrated HC1, was used to stain the sections for lignin. A Zeiss Photo Microscope 2 was used to view and photograph the sections.

FT-IR. A small area from the surface of each sample was filed off, to limit analysis to the surface only. This was then ground up with potassium bromide for analysis by infra-red spectroscopy. A Digilab FTS60 was used for the FT-IR.

Colour measurements. A Minolta Chroma Meter was used to record L a b measurements before and after treatment. Delta a, Delta L and Delta b values were monitored for change as an indication of colour change following timber treatment.

Results

Buffers and stability. A number of buffers were tried in order to find one suitable for efficient reaction of the Ps with the wood but which produced no reaction between the buffer itself and either the Ps or the wood. The buffer chosen was the 3.0 M acetic acid: 0.2 M sodium acetate, as above. The buffer had no effect on the wood and was stable when heated. Chemical titrations. The titration results showed that consumption of reagent chemical was consistent for both the preliminaries and the scale up treatments. The times used for each treatment were those deemed most effective based on the initial preliminary work. Effectiveness was based on the delignification achieved for a given Ps concentration and treatment time.

Light microscopy. The control sections were stained throughout, indicating full lignin presence, ie no delignification. For the treated samples, the preliminary Ps treatments produced delignification up to 5 cells deep without a NaOH wash up to 8 cells deep with the NaOH wash. At the scale up level (first and second scale-ups), the delignification was up to 5 cells deep. Delignification was not always uniform but showed a distinct gradient between the delignified and non-delignified areas.

FT-IR. The controls showed no delignification as expected while the treated samples showed up to 50% delignification without a NaOH was hand up to 80% delignification with a NaOH wash (Table 1). The spectra presented in Figure 1 and Table 1 were analysed quantitatively, and indicate that 60-70% delignification has been achieved. The Ps treatments produces a reduction in the carbonyls present (Fig 1) due to the inclusion of the NaOH post-treatment wash.

TABLE 1 - Delignification depth with Ps treatment

Chemical analysis

Klason lignin analysis of treated veneers showed a 50% removal of lignin on treatment. Noticeable colour changes occurred during the treatment processes. The Ps treatments bleached the radiata pine wood surface during the main Ps stage and then darkened the surface during the NaOH wash stage.

Discussion

The above results show that the Ps treatments have been successful in removing at least some of the lignin groups from the surface region of radiata pine wood. The FT-ER results indicate that up to 70%, or more, of the lignin in the surface region has been removed. The light microscopy results agree with this in that guaiacyl is the predominant lignin group in softwoods, and the principle component of guaiacyl is coniferyl alcohol which is readily stained with phloroglucinol and HC1. Therefore, no staining of the wood cell walls indicates removal of the guaiacyl lignin. The fact that the deliginified radiata pine wood showed little or no lifting of fibres from the surface is also in agreement with a partial delignification.

Delignification of solid radiata pine wood has been successfully achieved using peroxymonosulphate to oxidise the lignin content in the surface region of the wood.

EXAMPLE 2 - Use of Peracetic Acid

Materials

Kiln dried, clear radiata pine sapwood, size 125 x 25 mm was used. The random lengths of timber were machined down to 20 mm, then both sides sawn off to 100 mm . Boards with a length of 400 mm were cut out, avoiding knots and other irregularities. A total of 120 boards were needed for this trial. All four lengthwise corners were rounded off with a router and an 8 mm hole drilled at the centre of either end. The two surfaces (front and back) were sanded on the large belt sander to remove any machine marks and surfaces were cleared of any dust.

Before treatment all boards were tightened together in four groups of 32, each with two 8 mm PVC rods through the holes at each end, each board with a 6 mm thick PVC spacers between them. At the end of each rod a PVC spacer and nut was placed.

The Delignification Treatment

Pretreatment: To remove any trace of metal ions, 4.5 kg Ethylenediamine tetraacetic acid .2Na (EDTA.2Na) was dissolved in 240 L water at 40 °C. Resulting acidity: pH 5. Added 100 g NaOH which brought the pH to 5.5. Heated solution to

50°C, added samples, kept samples submerged with extra weights for 2 hours. Emptied bath and filled with 240 L distilled water for washing the samples the rest of the day and overnight. Treatment: Removed samples from bath, drained and rinsed bath. Filled bath with 150 kg acetic acid (135 L). Diluted 27 L of hydrogen peroxide 50% (H₂O₂> with 18 L distilled water, added this to bath, slowly and carefully. Used skin protection gear and breathing mask with filters.

Started heating to target temperature of 70 °C whilst taking regular samples to monitor the formation of peracetic acid. Heated at 70 °C for 5 hours, left heating on reduced power overnight. Next morning added distilled water to 240 L mark. Final peracetic acid concentration was 4.4% at time of sample immersion. Monitored PA concentration by sampling during the treatment time of 2.5 hours. A bath of 120 boards were treated. PA concentration was down to 2.1% after 2.5 hours, at which time samples were removed from bath and submerged into two other baths with rinse water. Refreshed rinse water several times.

Extraction: Extracted reaction products from samples with 0.1 M sodium hydroxide (NaOH) at 50 °C for one hour at 50°C. Rinsed samples twice with 240 L water (10 minutes) followed by a one hour acetic acid wash (150 mL per 150 L water). A second acetic acid wash (5 minutes) was performed as the boards were a little slimy to touch, possibly caused by residual caustic. Rinsed three times with 150 L water. Anti sapstain

Treatment: Half the peracetic treated samples were treated with anti-sapstain solution called SENTRY. Followed recommended dilution of 3.2 L. SENTRY (concentrate) into 80 L water. Boards were dipped into this solution and submerged for 1 minute. Weighed when drip dry.

Drying: Placed all samples in fύmehood at room temperature (20°C-25 °C) for mild drying conditions. Previous kiln drying at elevated temperatures caused checking, therefor a mild drying process was chosen. The samples were left in blocks of 32, held together with the PVC rods. Air flow was approx. lm/sec through the gap between boards. Two samples were weighed separately to monitor weight loss (drying rate). After 4 weeks these two samples were back on their original weight. After almost 3 weeks the small, weatherometer samples were already slightly below their original weight.

Waste disposal: The peracetic acid in the reaction liquid was neutralised with iron filings. Caustic solution added until neutral (pH6-7).

It was noted that a lot of checking on numerous boards had already occurred, even before drying. This is probably due to the high absorbency of both treatment solutions and rinse water. Samples at the treatment stage had a density of approximately 1. SAMPLE IDENTIFICATION

All samples were labelled after air drying in the fumehood.

All samples were identified according to:

1. Control or Peracetic treatment

2. Coating system (see Table 2)

3. Followed by sequential sample number in treatment system

4. Half were dipped in an antisapstain solution after the peracetic acid treatment process TABLE 2

The preliminary results show improved resistance to the outdoors environment, for the samples receiving the peracetic acid treatment.

EXAMPLE 3 - Comparative Trial

SAMPLES

18 large boards (standard size) and 19 small boards (weatherometer size), as well as veneer samples were prepared for peracetic acid treatment and a similar amount was left as control and reference samples

EDTA TREATMENT

A selection of samples was pre-treated with EDTA (ethylenediamine tetraacetic acid disodium salt). Using a medium size glass container, 500 g EDTA was dissolved in 20 L water and pH adjusted with 1 M sodium hydroxide (NaOH) from pH 4 to pH 5.5

The boards were submerged and heated in the solution at 50 °C in a thick walled glass chromatography development container. The glass container stood in a water bath with a water level slightly above the solution level in the glass reaction container. The water bath was heated with two non-metal heaters. After 2 hours the samples were removed from the bath and thoroughly rinsed with de-ionised water. Note: the size of the glass container and its wall thickness requires that temperatures of water bath and the liquid content of the glass container be the same at the start of the heating cycle to prevent cracks.

PERACETIC ACID TREATMENT

A large glass container, placed in a 400 L PVC water bath, was used to prepare the peracetic acid and delignification of the samples in one batch.

Two heaters, 3 kW each and controlled by thermostats, were used in the water bath to obtain and maintain required temperatures.

The delignification chemical, peracetic acid, was generated in-situ by addition of 10.5 L 30% Hydrogen peroxide solution to 25.9 L glacial acetic acid. The acetic acid was preheated to 35 °C. The exothermic reaction, causing the temperature to rise, was stabilised by the surrounding water bath. The reaction forms peracetic acid and this was monitored by titration at regular intervals. The peracetic acid content was measured at 2.3%, half an hour after start. At that time the temperature was maintained at approx. 35 °C and left overnight. Next morning (8.30am) the temperature had slightly increased to 44 °C and peracetic acid was measured to be 8.4%. This concentration was already above the target, so the heaters were turned to full to reach and maintain 70 °C for the delignification to take place. At 9.45am temperature 48 °C, peracetic 8.7%. At 11.15am temperature 70 °C and peracetic 10.7%

Samples were submerged at 11.25am, after which temperature in reaction chamber started to rise. At 11.45am peracetic acid measured at 9.2%. At 12.05pm the peracetic measured only 0.11%), reaction and foaming had stopped. At 1pm samples were removed from the reaction vessel and rinsed with water.

The veneer samples were virtually disintegrated and unidentifiable. All other samples were well done, showing no visual difference between EDTA pre-treated and non-freated samples.

ALKALI EXTRACTION

The delignification products were extracted from the wood surface by submerging in a 0.1 M solution of sodium hydroxide (caustic soda) at 50 °C for 60 minutes. This was followed by a 5 minute water wash then an acetic acid wash (0.1 M), with thorough rinsing with water to neutral pH.

After this step, selected boards were immersed in a 4% Sentry solution for 30 seconds. All boards were dried at room temperature in the fume hood. (Air flow approx. 1 m/sec) It was noted that all boards still had a rather strong acetic acid smell.

The peracetic acid freatment of boards proceeded very fast since it was very exothermic. In fact the temperature of the glass reaction chamber sitting inside the water bath was raised to 70 °C by heating the water bath to about 80-85 °C. Most of the reaction probably occurred in the first 25 minutes. The alkali extraction step went smoothly. The water washing was gentle with the good result of little fuzzy fibre seen. The veneers samples in the reaction bath were disintegrated with the treatments and lost.

COMPARATIVE METHOD (BRUSHED CHLORITE METHOD)

Acidified sodium chlorite solution was used to treat both radiata and spruce samples. The radiata and spruce boards for treatments were treated with a 6 % (m/v) NaClO₂ solution containing a sodium acetate/acetic acid buffer. The application method was a liberal brushing to the bark side surface.

The samples were thus immediately transferred to an oven at 60 °C for one hour. Samples were filleted with 6 mm fillets for heating. Following heating, the samples were immediately washed by immersion in the sink containing cold tap water, for 60 seconds. (This method differs from the invention in the use of insufficient lignin breakdown and/or insufficient washing to remove lignin fragments).

The boards were then sorted into two sets for either Sentry treated or immediate drying. The sentry was immersion in a 5 % sentry solution in the decon bath for 30 s. These boards were then taken to a fume cupboard, along with the non-sentry treated boards, for ventilation for a couple of days. Finally the boards were conditioned in the lab for a further few days. WEATHEROMETER TREATMENT METHOD

Standard size (14; large; 380 x 100 x 20 mm) and weatherometer size (14; 230 x 60 x 10 mm) boards were exposed in the Atlas Ci65 weatherometer to the standard light and water spray exposure conditions for 440 hours. At this time the boards were withdrawn, as the surfaces were now just white in colour. Standard size samples were held in position by means of a screw in the rear side of the board. Weatherometer panels were held in the normal metal frames.

Note that while the weatherometer is a sterile environment due to the UN radiation, microbial populations will grow on boards once the weatherometer is turned off if the sample panels have a moisture content high enough to support their activity.

FURTHER TREATMENT METHOD-SENTRY WITHOUT ANY PRETREATMENT Following the observation of bleaching of sentry treated boards on exposure, a set of standard size boards were treated with a 5 % sentry solution, dried and prepared for exterior exposure without any coating. These boards were brought into the lab and weighed after one month, two months and then every three months. Three boards were also sampled at each time for coating all round with A665 PU before being continuing their exterior exposure. The final sampling were after 24 months. Boards were conditioned back to their a moisture content of about 14 to 18 % before being coated.

STAINING FOR LIGNIN

Colour development of cross-sections from treated wooden panels is effected by using a chemical stain which colours lignin containing areas purple leaving lignin depleted areas unstained or wood coloured. Concentrated hydrochloric acid was brushed over the cross- section of wood and then a saturated aqueous solution of phloroglucinol was immediately brushed on the same surface. Colour development was immediate sand stable for several hours.

COATING TYPES

• Pretreatment (reactive isocyanate was the pre-coating used in the results reported here but a few poly vinyl butyral coated samples gave similar results) • Topcoats (Bayer PU Desmophen A665 and Nuplex acrylic 6882)

OVERVIEW AND DETAIL ON SAMPLES

• Key to trial design and sample numbers

The trial design for the standard size boards and weatherometer size boards include peracetic acid treated, brushed chlorite treatment method and weatherometer treatments of boards.

• Weighing

Weighing of boards was done immediately before and after each coating application.

• Coating application rates

Set coating application rates were not targeted since the heterogeneity of the samples was large. The peracetic acid treated boards sucked up large amounts of coatings, as did the weatherometer treated boards. We therefore applied the coatings relevant to the boards with the more porous surfaces having heavier applications. • Sanding

Before each coating the surface was sanded with 240 grit sanding using a hand held sanding machine. The quality of the coating systems applied reflected this preparation,

• Coating features

Two reactive primers were used. The main one was the Uroxsys Durocoat reactive isocyanate primer. This reactive primer with react with the wood cellulose and hemicellulose hydroxyl groups and would also react with the topcoats if the topcoat is applied before full primer cure.

A few boards were prepared by application of a 5 % (m/v) poly vinyl butyral (PVB) solution in butanol with brushing to refusal. The PVB is a reacted polymer and dries simply by solvent removal. Isopropanol dissolves the PVB readily but brushing properties in butanol were better.

• End painting

At the completion of coating (5 July 2001), all standard size boards had 25 and 50 mm regions masked with masking tape so that they could be painted with white enamel paint following the guidelines of EN927, Part 3. COLOUR MEASUREMENT

The colour of panels was measured with a Minolta Chroma Meter CR- 200 at three positions along the boards using a positioning template at 0, 500 and 1000 hours weatherometer running time. The Lab colour space was used. The coordinates are:

Lightness, L: 0 -black to 100-white a greener = 0 < a > 0 = redder b bluer = 0 < b > 0 = yellower

For varnishes, the coordinates L and b are most useful. L reflects lightness of the board and b the yellowness.

The data that are plotted in Figures 2, 3 and 4, are the averages of the three boards for each set in the design. Control samples are not plotted due to clutter, but in almost all cases their colour change was minor.

RESULTS

The change in each of the coordinates Del L, Del a and Del b, were calculated and plotted against time in the weatherometer. The pine samples that had been treated with peracetic acid or the weatherometer had least colour change on accelerated weathering. The brushed chlorite treated pine sample performed the same as untreated pine samples with no improvement in photostability. The brushed chlorite treated spruce samples had a small improvement over untreated spruce samples. A smaller second order effect on discolouration was that acrylic coating tended to be less changed in Del L, a and b values than PU coating.

Effects of weathering outdoors normally exhibits as cracking, peeling, mould growth, discolouration and loss of adhesion. In a weatherometer mould growth is restrict due to UV radiation. If samples are not exhibiting peeling or checking, discolouration is the best indicator of photodegradation occurring. Photodegradation results in a change in wood structure at the wood surface resulting in a loss of wood surface strength, adhesion of coating and with the dimension change is the wood as a results of moisture change, delamination and coating rupture.

After 1000 hours of artificial weathering, the peracetic treated and weatherometer treated samples were performing with better photostability than the brushed chlorite treated samples. Representative pool results are shown in Figures 2-4. The treatment codes are shown in Table 3.

The results from the staining for (Figs 5-9) were consistent with a relationship between delignification and photostability. The peracetic acid treatment produced and an envelope of about 2 mm around the treated boards. The brushed chlorite treatment method resulted in no loss of stain around the samples indicating no loss of lignin. The weatherometer treated samples showed a thin envelope of delignification. The control samples showed no delignification.

The control sample and brushed chlorite method both gave no observable delignification. The peracetic acid treated samples were very effectively delignified which means that the reactive chemicals penetrated into the wood and the conditions were suitable for a reaction with the wood to take place. The thinner envelope for weatherometer samples is consistent with the UV radiation in the weatherometer penetrating the surface region to less than 0.5 mm. The water in the weatherometer effectively washed away the lignin fragments produced on the irradiated wood surface.

TABLE 3

PA = peracetic acid br chlorite = brushed chlorite

W'om = Weatherometer pretreatment

h further experiments (data not shown) chlorite treatment was found to be effective in the invention when applied from a bath, rather than simply by brushing.

The above Examples are illustrations of the practice of the invention, it will be appreciated by those skilled in the art that the invention can be carried out with numerous modifications and variations. For example, the chemicals and concentrations used may be varied.

Claims

CLAIMS:

1. A method of coating wood comprising photostabilising a surface of the wood using delignification of the surface including removal/destruction of photoabsorbent lignin fragments, and subsequently coating the surface with a transparent coating.

2. A method as claimed in claim 1 wherein the deligmfication agent is selected from a chemical delignifying agent, lignin-degrading radiation and lignin-degrading enzymes.

3. A method as claimed in claim 2 wherein the delignifying agent is selected from chemical delignifying agents, UV radiation and lignin-degrading enzymes.

4. A method as claimed in claim 3 wherein the delignifying agent is selected from chemical delignifying agents and UV radiation.

5. The method as claimed in claim 4 wherein the delignifying agent is a chemical delignifying agent chosen from peracids.

6. A method as claimed in claim 5 wherein the peracid is selected from peracetic acid or peroxymonosulphuric acid.

7. A method as claimed in claim 6 wherein the peracid is peracetic acid.

8. A method as claimed in claim 4 wherein the delignifying agent is UN radiation.

9. A method as claimed in any one of claims 1-8 wherein the wood surface is delignified to a depth of about 2-80 cells.

10. A method as claimed in any one of claims 1-9 wherein after treatment with the delignifying agent, photoabsorbent lignin fragments are removed using extraction/soaking in an alkaline solution or by supercritical fluid extraction.

11. A method as claimed in claim 10 wherein photoabsorbent lignin fragments are removed using extraction soaking in an alkaline solution.

12. A method as claimed in claim 10 or 11 wherein the surface is rinsed with water to return the pH of the rinse solution to a normal pH value.

13. A method as claimed in any one of claims 1-12 wherein prior to coating of the wood with a transparent coating, the surface is treated with an antifungicidal solution.

14. A method as claimed in any one of claims 1-13 wherein the wood is coated with a transparent non-pigmented varnish.

15. A method as claimed in claim 14 wherein the coating is selected from polyurethane, aliphatic urethanes, acrylics, fluoropolymers, epoxides and acrylic-polyurethane and other hybrid polymer formulations.

16. A method as claimed in claim 15 wherein the coating is selected from acrylic and aliphatic urethanes.

17. A method as claimed in any one of claims 1-16 wherein the coating is transparent but has some pigmentation/colour.

18. A method as claimed in any one of claims 1-16 wherein the coatmg is clear and transparent.

19. A method as claimed in any one of claims 1-18 wherein prior to applying the transparent coating, the wood surface is treated with an agent which penetrates the photostablised wood surface and reacts with it.

20. A method as claimed in claim 19 wherein the prefreating agent is an isocyanate or melamine derivative.

21. A method as claimed in any one of claims 1-20 wherein all the surfaces of the wood are treated.

22. A product of the method of any one of claims 1-21.