A method for preserving wood using sodium silicate and sodium bicarbonate in first treating the wood with sodium bicarbonate and thereafter with sodium silicate.

BACKGROUND OF THE INVENTION

Sodium silicate is the common name for a compound sodium metasilicate, Na ₂ Si0 ₃ , also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and automobiles.

The use of sodium silicate for industrial wood impregnation has been described at least since 1920. For example, US1344891 describes a method for treating wood and lumber to make wood fire proof. The process consist of applying to the wood, a coat of fire proofing material as sodium silicate or water glass, applying thereto a protective coating of plastic material which hardens and is capable of receiving a water proofing paint to the plaster coating. A coating of saw dust may be applied to absorb the silicate to absorb the sodium silicate not taken up by the timber, the coating of silicate being sufficient to impregnate or saturate a layer of sawdust.

Water glass is also utilized for the protection of wood and wood products improving the resistance against fire, termite, rot and decay. It is also known that adding a coagulant to water glass such as sodium bicarbonate, improves wood protection. Sodium bicarbonate makes sodium silicate insoluble due to the releases of carbon dioxide (produced by sodium bicarbonate) to sodium silicate.

FR2560547 (1984) discloses a composition of sodium bicarbonate and silicate and a process for the protection of wood; particularly coconut wood. FR2560547 relates to the protection of wood against natural enemies such as fungi and insects. A method for treating wood is disclosed comprising the step of impregnation of wood by immersion in the solution or by application of the solution by spraying the composition comprising sodium silicate and a coagulant, sodium bicarbonate. The composition comprises at least 50% in volume of sodium silicate and 1 to 10% in volume of sodium bicarbonate. The wood in contacted for some time with the composition making the wood resistant against for example insects.

US4612050 (1984) also discloses a liquid sodium silicate composition for application of wood. According to US4612050 water glass penetrates into wood and stiffens to a jelly inside the wood, a method of imitation of petrified wood. Therein is used at least 15% by weight of sodium silicate. The method comprises also mixing mineral clay with water (allowing minerals to leach into the water), acidifying the solution to an incipient gelling condition; penetrates and gels and fixes the minerals from the composition in the wood.

US7297411 (2004) describes the transition of water glass from a water soluble form to a solid form by adding heat. Herein is disclosed a polymerized form of sodium silicate that is water insoluble. As a result of application of heat, the sodium silicate undergoes dehydration (loss of water) and a process of polymerization that forms increasingly larger moieties of (Si04)n ^"1 while still maintain an overall charge of -1 that forms an association with the free sodium. As the material polymerized the resultant material increases in size to the point that it is no longer able to dissolve in water, thus becoming insoluble. In conclusion, heat is applied to the water glass to foam and create a form which is insoluble in water.

US6586109 describes a method of protecting cellulose material comprising in order, the steps of heating a cellulose preservative to about 180°F spraying said preservative on the cellulose material, impregnating the cellulose material with a preservative, cooling the sprayed cellulose material to a temperature of about 112 °F and subsequently spraying sodium bicarbonate on cooled cellulose material. The preservative is applied through immersion and to apply the fire retardant sodium bicarbonate through spraying or vacuum and pressure treatment. Sodium silicate and sodium bicarbonate combine to form an insoluble gel. For this reason, in general, the sodium silicate spray is applied to the tie separately from the application of sodium bicarbonate. Mixing the two reagents in order to apply the combination to the tie in a single step immediately produces a gel within the mixture and thus fails to provide a satisfactory gel coating because gel formed in the mixture would block the penetration of the preservative into wood by suspending the preservative in a gel formed outside of the wood matrix rather than within and on the surface.

There is a definite need to develop better methods to get sufficient protection of the internal wood fiber at low production cost.

SUMMARY OF THE INVENTION

The problem of present invention may be seen as a provision of a method to achieve improved penetration of sodium silicate into wood for the protection of wood.

The solution as present inventor found out lies in the order of treating wood with sodium silicate and sodium bicarbonate; first impregnating the wood with coagulant sodium bicarbonate and then followed by impregnating the wood with sodium silicate, this results in an improved method of protection of wood. The protection of the internal part of wood using sodium silicate is difficult as sodium silicate does not penetrate easily into wood fiber at ambient or elevated temperature. Many other commercial non-environmental friendly wood impregnations liquids penetrate easily into the wood and also result in internal protection of the wood product. However, these preservatives are to be faded out in the coming years.

Such well known preservatives are for example used in building applications include; Chromated Copper Arsenate (CCA-C), Alkaline Copper Quat, (ACQ-C, ACQ-D, ACQ-D Carbonate),

Micronized Copper Quat (MCQ), Copper Azole (CA-B & CA-C, pCA-C) and Sodium Borates (SBX/DOT). These treatments are often referred to by trade names such as Wolmanized Natural Select™, (Copper Azole), Preserve and NatureWood®, (ACQ), MicroPro™, Smart Sense™ (MCQ), and Advance Guard® (Borate).

Known methods to treat wood in the prior art are the application of heat and high pressure and these to some extend increase the penetration of sodium silicate into the wood. Unfortunately, these methods are costly and also not efficient. Using a low concentration of sodium silicate solution also increases the penetration depth into the wood, but this also reduces the protection benefits. Mixing sodium silicate and sodium bicarbonate prior applying to the wood results in problems with timing and penetration as the solution is instable and easily such mixture gels. Furthermore unused mixture needs to be disposed and the mixture will also stick to equipment and tools, it is hereby hard to maintain a continuous process and keep the equipment durable and clean. Such problems are disturbing for an industrial process and increase cost of handling.

Any contamination of the sodium silicate by a coagulant (such as known sodium bicarbonate) will start gelling (fibrils) and it blocks the pit of the wood cells making any treatment difficult if not impossible. As said above, present method differs from other prior art methods by impregnating the wood product with coagulant (sodium bicarbonate) first and then secondly impregnate with sodium silicate.

This two steps impregnation process is than followed by optionally drying process with heat. Accordingly, a first aspect of the invention is a process for the protection of wood comprising the steps of; i) preparing a solution of sodium bicarbonate in water by steering at a temperature not higher than 100 °C;

ii) treat the wood with the sodium bicarbonate solution as obtained in step i);

iii) prepare a solution of at least 15% by weight of sodium silicate in water by pouring or pumping sodium silicate into water until the solution is homogeneous;

iv) treat the wood with the sodium silicate solution as obtained in step iii);

v) optionally dry the wood of obtained in step iv) by applying heat at a temperature of at feast 70"C, and

vi) obtain a protected wood. DEFINITIONS

The term treatment of wood is defined herein as contacting the chosen wood with a liquid to obtain preservation of the wood.

The term "protection" of wood is defined herein as protection against weathering, insect attacks, fungal and mold attacks and combination of such factors which affect the destruction of wood and its fibers to make it a worthless wood.

DETAILED DESCRIPTION OF THE INVENTION Present invention relates to a method for the treatment of wood using a composition comprising water glass (sodium silicate with formular Na ₂ 0(Si0 ₂ )) and sodium bicarbonate with formular (NaHC0 ₃ or CHNa0 ₃ ),

Provided is a detailed description of the method of the first aspect of present invention.

Sodium bicarbonate (99%) is commercial available as white powder. Sodium silicate is commercial available as powder and solutions.

Preferred is a sodium silicate with a high ratio such as 3.0 - 3.25 or more preferred a sodium silicate solution of 35 to 40% e.g. N44. Preparing a solution comprising sodium bicarbonate in water

Preferred is the preparation of a 10% by weight solution of sodium bicarbonate by steering until the sodium bicarbonate powder is dissolved. Keep the sodium bicarbonate solution at ambient temperature.

As it concerns the temperature of the solution comprising sodium bicarbonate it is preferred to keep the temperature of the solution under 50 °C, else the solution starts to release carbon dioxide and the efficacy of sodium bicarbonate declines. Preferred is a temperature between 20°C and 50°C. The preferred range of 20°C to 50°C has the advantage that more sodium carbonate can be added to the water. And as said above, at temperatures higher than 50C, more sodium bicarbonate disappeared (dissociated) - so preferred is to maintain the temperature between 20°C and 50°C.

There may be chosen any water sources for the preparation of the sodium bicarbonate water solution, it is known to the skilled person that calcium ions (so called "hard water" reacts with the sodium bicarbonate (sodium bicarbonate is also used as a water softener).

Generally, any water with any water hardness may be used and is not a special negative effect on the benefits of present invention. For convenience, softer water is easier to use e.g. moderate softer water of not more than a hardness of 10 DH (German water hardness scale). A degree of General Hardness (dGH or 'German degree (°dH, deutsche Harte)' is defined as 10 mg/L CaO or 17.848 ppm. Moderate soft is 3.72-6.75 hardness in dGH/°dH. Hard is 6.78 to 10.08 in dGH/°dH and very hard is higher than 10.14.

Steering or mixing is done by hand or using a motor driven stirrer. There are many mixing devices available and these are known to the skilled person.

Preferred is a solution of sodium bicarbonate in water, comprising sodium bicarbonate in an amount from 5% to 15% by weight. At a temperature of at least 0°C and not higher than 50°C

Preferred is to keep the temperature of the solution at 20 °C or higher but not higher than 50°C, else the sodium bicarbonate will not dissolve or will start to precipitate and more steering might be needed again. The chosen temperature of the water is dependent on the solubility of the sodium bicarbonate in water.

It is known to the skilled person that the solubility of sodium bicarbonate is dependent on the temperature, for example at 0°C, it is possible to solubilise about 7% sodium bicarbonate, at 20°C a maximum of 10% sodium bicarbonate solution can be reached and at 60"C, 16.5% sodium bicarbonate solution may be reached. Most preferred is a solution of sodium bicarbonate at 20 "C and not higher than 50°C at atmospheric pressure.

Treat wood with the sodium bicarbonate solution

The wood product is treated with the sodium bicarbonate solution using common industrial wood impregnation processes; as for example a pressure tank impregnation technique. Preferred may be to use an open cell impregnation technique as this leave cavities open for a subsequently treatment with the sodium silicate. Further, if pressure may be used to treat the wood with the sodium bicarbonate for example duration of 30 minutes or more is needed depending on the wood board dimensions and species of the wood chosen. It is preferred to target a solution weight uptake of 50%. This results in about 5% final uptake of sodium bicarbonate. It is not preferred to use vacuum after introducing sodium bicarbonate as vacuum stimulates sodium bicarbonate to release carbon dioxide and hereby becomes less active coagulant as ingredient for treating the wood.

For wood treatment ordinary containers may be used.

Notice; aluminium containers should be avoided for sodium silicate solutions as it react with aluminium. Also glass surfaces might be damaged by sodium silicate.

Dry the wood at a temperature not higher than 50°C

An intermediate drying at low heat under 50 °C may be used before subsequently introducing sodium silicate to the wood product. Intermediate drying is recommended if more uptake of sodium silicate to the wood product is desired.

Preferred is a drying temperature between 5°C and 50°C, more preferred between 25° and 45°C and most preferred is 45 °C.

Prepare a solution of sodium silicate in water while steering by pouring or pumping sodium silicate into water until the solution is homogeneous.

Pour or pump sodium silicate into water to prepare for example a 20% by weight solution of sodium silicate, while steering during the pouring, in such a way a homogenous solution of sodium silicate in water is obtained. It is preferred to pour the sodium silicate into the water during steering. Less preferred is to pour the water into the sodium silicate as the sodium silicate is heavy and will stay on the bottom of the tank/container and therefore will be harder to steer up. Preferred is to minimize the exposure to the air as sodium silicate tends to gel with air contact. Preferred is to keep the solution in an airtight container during long time storage and with a lid and covered during processing.

As it concerns sodium silicate and the type of water, preferred is moderate softer water of not more than 10DH as defined herein.

If very hard water as defined herein is used, it may be a solution to add 5-15% more sodium bicarbonate and sodium silicate in the two respective solutions. For mixing the sodium silicate into water, a hand or motor driven mixer may be used. If heat is applied, optionally, a container with heating options may be chosen.

The amount of sodium silicate as prepared is preferred from 5% to 30% by weight and most preferred is 20%.

Treat the wood with sodium silicate solution

Sodium silicate solution may be heated at a temperature from 20°C to 100°C. Most preferred heating temperature is 60 °C. In this way better balance of heating cost compared to the benefit from better penetration is achieved.

There may be applied pressure to the sodium silicate while treating the wood; to obtain a better penetration and a higher final percentage uptake of preferred 3 to 10 bar depend on the board dimensions and density of the wood. Wood can also be contacted with sodium silicate using common industrial wood impregnation processes as known to the skilled person and by optionally using the standard known pressure impregnation technique.

Types of wood

Many wood types may be chosen for the treatment of present invention, from over 100 species and are the same as wood types used for the treatment by preservatives (prior art) using pressure impregnation. A preferred wood type to be treated is herein is Radiata pine (also Pina radiate, family of Pinacea). Other preferred woods are for example rubber wood, bamboo wood, young grown plantation wood e.g. teak wood from thinning.

Selection of wood species for using this invention can be of various reasons; the wood is locally available at low cost, the wood species is easily treatable, the wood product may need further protection to extend its service life, the use of the wood product benefits from the treatment method of present invention (e.g. fire doors, outdoor furniture, wood in service at waterways and national parks (non-hazard chemicals)). Hard wood species in general do not accept present invented treatment method well; which is the same as for treatment using existing wood preservatives. The benefit efficiency ratio of present invention treatment method on hard wood are limited, as in the case for other (prior art) treatment preservatives. Incising may increase the uptake of impregnation liquid and make the treatment method interesting for hardwood e.g. beech. When hard wood is chosen incising is recommended.

In case, pressure is used, this may be applied by using known standard industrial available pressure tanks for wood. The wood may be treated by dipping the wood in the solution and using industrial available tanks.

As for the stirring speed, this may be chosen as such that the sodium silicate mixes with water before sodium silicate reaches the bottom of the mixing container; if the sodium silicate reach the bottom on the container it might be difficult to stir it up again due to higher density of sodium silicate. Dry the wood of obtained by ventilation and/or applying heat

An intermediate drying of the wood at low heat under 50 °C may be used before subsequently introducing sodium silicate by place to wood in a ventilated area; drying for 1 day or more may be useful depending on board dimensions and wood species. This is done to e.g. reduce the moisture to reach 25% weight increase.

Dry the wood by applying heat

The process of the first aspect comprises a step v) is dry the wood of obtained in step iv) by applying heat at a temperature of at least 70°C. Applied pressure to the sodium bicarbonate while treating the wood

Pressure may be applied to treat the wood in the sodium silicate for about one hour or more at a pressure of preferred 3 to 10 bar depend on the board dimensions, density and structure of the wood. Preferred is to target a solution weight uptake of 50%. As a known method, the density of the impregnation solution can be measured using a scale; and a level gauge will make it possible to monitor the weight uptake during the process, all depending on used equipment. By applying 3 to 10 bar and a 20% solution, such results in about 10% final uptake of sodium silicate. It is preferred to avoid contamination of sodium silicate solution with coagulant sodium bicarbonate before it reaches the wood product, as gelling (fibrils) will block the pit of the wood cells and/or generate waste of impregnation liquid. It might also disturb performance of pump, valve and pipes by clotting.

An example of a known pressure tank for the purpose of present invention may be a T10 Plant, T20 Plant or DN 1500*15000.

Obtain a protected wood of first aspect

After the steps of impregnation process the wood product is dried using common wood drying techniques e.g. Kiln dryer. The drying time is depending on; the chosen wood species, the board dimensions and on the equipment used as well as the final desired moisture content.

Wood drying temperature of over 70 °C is preferred during drying to secure and ease the reaction for the insolubility of the sodium silicate. Vacuum drying may be used to decrease the drying process time.

For drying, a ventilated drying place may be chosen. Drying the wood may be done by a kiln drier or other known wood drying equipment available and known to the skilled person.

Present invention of this method of first aspect results in the following advantageous features for wood.

The method of present invention allows a sodium bicarbonate buffer inside the wood to react with the subsequently sodium silicate and at the same time protect the internal of the wood against fungus attacks. Furthermore, the sodium bicarbonate supports the sodium silicate's fire protection of the wood product.

Sodium bicarbonate is an anti-fungi in and fire retardant substance; which solution is very diffusible and thus very effective wood preservation coagulant for sodium silicate wood impregnation systems when it used before the sodium silicate.

Sodium bicarbonate penetrates easier into the wood and gives internal protection of the wood product. By using the sodium bicarbonate first the sodium bicarbonate penetration into the wood fiber is easier and much deeper penetration can be achieved. To obtain the benefit of sodium bicarbonate improved penetration, it necessary to introduce the sodium bicarbonate to the wood product first, before any contamination from sodium silicate occurs. The two steps impregnation process gives no timing delays In the process as compared to a two component mixture process of prior art . Both of the solutions are stable; the sodium bicarbonate solution and sodium silicate solution may also be reused without any gelling issues in holding tanks, pressure tanks, pumps and valves; as long as the sodium silicate solution exposure to air is minimized as also explained herein.

Lower impregnation energy consumption (temperature, pressure and time) may be obtained with the present invention as expensive heating of the liquids are minimized herein. Ambient temperature will in most cases be sufficient during the treatment process. This sodium silicate impregnation method of present invention allows more flexibility in process parameters (the process window) than prior art sodium silicate impregnation methods. Present method allows in most cases to operate at ambient temperatures (temperature of the impregnation liquid and treatment method of the chosen wood). Pressure, timing and drying techniques may be chosen to match equipment capabilities. Even very simple equipment can be used if process time is less important.

A Wood obtained by the method of first aspect (steps i to iv) (and not step v)

As the treatment of present invention is a chemical reaction between sodium silicate and sodium bicarbonate, the obtained protected wood may thereafter be transported in a wet condition (without drying the treated wood of present invention - step v of first aspect) with the purpose to for example save cost in the process by not drying (saving energy cost), and may be useful for a specific purpose. The protected wood under wet conditions, and treated according of the first aspect has the improved characteristics as described herein as also in case of the drying step of step v) is not included in the process. The drying step of step v) is optional.

EXAMPLES

Example 1: Treated wood versus not treated wood

Two identical (1x2x6 inch) wooden boards of Radiata pine where prepared; one for the treatment according to present invention and one not treated. The non-treated wood board (the reference board) where kept under ambient conditions. One board was first treated with a 10% by weight sodium bicarbonate solution for (½) half an hour and a pressure of 3 bars applied at a constant temperature of 30 °C (ambient temperature) was applied. Secondly the wood was air dried in the sun during one day.

After air drying the wood (board) was treated with a Sodium Silicate 20% solution by weight for one hour and pressure was applied of 3 bars at 30 °C (ambient temperature). After the two steps of impregnation, the wooden board, where dried in a conventional oven at 100 °C for one day.

The two boards; one treated and one non-treated; where exposed to tropical weathering for three months and visually inspected to measure differences. The non-treated board turned black after a few weeks; whereas the treated board kept it bridge appearance long beyond the 3 months exposure.

Example 2: Method of verification of presence of sodium bicarbonate and sodium silicate preservative's penetration into treated wood.

Sliding up a treated board will make it possible to obtain a qualitative measurement of the penetration of the sodium bicarbonate and/or sodium silicate, either by laboratory analyse of the slides or by simple evaluate the boards visual e.g. using a pH-indicator to ease the visual appearance of the presence of sodium bicarbonate and sodium silicate in the wood slides. A simple pH indicator like phenol red will indicate the penetration qualitative by turning redish where sodium bicarbonate and sodium silicate is present and yellowish where only wood substance is present. wood treated according to the first aspect of present invention i.e. where sodium silicate is used first than followed by sodium bicarbonate (second), as compared to a prior art wood where sodium bicarbonate is used first; indicates a better penetration of sodium bicarbonate when it is introduced first to the wood product. Such better penetration may be measured easily by the method as described herein and results in an improved preserved wood. Conclusion

Figure 1 shows the result. Dark wood is wood which is rotten, under destruction, cellulose fibers to the wood are disintegrating. Light wood as shown in figure 1 , is wood which is intact and protected as discussed in present invention. The wood as obtained by present method of impregnation. Wood of treated by the invention can be recognised by improved penetration of sodium bicarbonate and may be measured according to the method as described in example 2.

DRAWINGS AND FIGURES

Fig.1. Figure 1 shows a picture of a board of wood (Radiata pine). The black coloured area is the non-treated wood according to the description of example 1. Dark wood is wood which is rotten, and under destruction. Light wood (not black), is intact and preserved.

REFERENCES

1. US Patent Nr. 1344891

2. US patent 4612050

3. US patent 7297411

4. Patent No. FR2560547A1 5. US Patent US6586109

6. US1016111A sodium carbonate sodium silicate

7. US6586109 gel formation problem two steps solved

8. US1591752A sodium carbonate and silica wood

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