Prior art It is known to impregnate wood with wood preservatives containing various oils.

For example, the following are used: 1) crude tall oil of varying composition: - rosin acids 0-70% - fatty acids 20-70% - remainder 10-30% 2) tall oil distillate, consisting either of rosin acids or fatty acids, 3) air-drying polyunsaturated oils, for example linseed oil, rapeseed oil, olive oil and sunflower oil, 4) fatty acids, i. e. straight-chain or branched monocarboxylic acids which have 8-25 carbon atoms and contain double and triple bonds. Examples of suitable fatty acids are oleic acid, linoleic acid and linolenic acid. Use is expediently made, in this connection, of a mixture of several fatty acids, for example SYLFATe 2S (Arizona Chemical, US) which contains 96% free fatty acids and which is produced from tall oil.

So as to ensure that the product has a dry surface or that the oil dries, it is always necessary to use some type of additive. As a rule, drying agents which accelerate the autooxidation of the oils by 02 are used. Drying agents are consequently catalysts. They are frequently oil-soluble alkoxide, soaps, complexes or organometallic compounds of Co, Mn, Zr, Ca and Ba. An example of a drying agent is that which is sold under the name Mixed Drier VX73 (Bjorn Fredlund AB, SE) and contains 6% Co and 9% Zr.

SE 0103075-8 describes a wood preservative which is a tall oil product which contains fatty acids and/or rosin acids which have been saponified with 1-10 percent by weight of a metal, such as Ca. The addition of the metal results in a dry surface being obtained.

SE 0103074-1 describes a wood preservative which is based on crude tall oil which is made smear-free by being enriched with rosin acids. In order to improve the result still further, the product can be after-treated by boiling it in water or by subjecting it to an extended treatment in the wood preservative at an elevated temperature.

Object of the invention The object of the invention is to produce a process for impregnating wood with an oil without there being any need for the oil to contain an additive which promotes drying, i. e. without there being any need to add conventional drying agents to oils in general or rosin acids to crude tall oil.

Another object is to produce a dry surface on an oil-impregnated product whether or not this oil contains an additive which promotes drying.

Brief description of the invention These objects are achieved by means of a process for impregnating a wood product with an oil-based wood preservative in which the oil consists of crude tall oil without any addition of rosin acids, of tall oil distillate, of air-drying polyunsaturated oils or of fatty acids, which process comprises the following steps: 1. pressure-treating the wood product in the wood preservative, 2. removing the pressure, 3. after-treating the product by heating the wood preservative containing the immersed product to a temperature of at least 80°C,

4. removing the product from the oil.

The after-treatment results in the wood product having a dry surface.

In the impregnation process according to the invention, it is possible to use a wood preservative without any drying agent and for the treated product to nevertheless have a dry surface.

The invention also relates to a process for producing a dry surface on a wood preservative-impregnated product with the wood preservative being based on an oil which consists of crude tall oil without any addition of rosin acids, of tall oil distillate, of air-drying polyunsaturated oils or of fatty acids, which process is characterized in that the product is after-treated for at least 15 minutes in the same wood preservative agent, or an agent having the same composition or having essentially the same composition, while the agent is heated to a temperature of at least 80°C. The after-treatment is expediently carried out for at least 15 minutes.

Detailed description of the invention The wood preservative which is used in the process according to the invention preferably consists of only oil, for example tall oil, tall oil distillate, fatty acid, or drying oil. While the oil can contain conventional additives such as fungicides, it expediently does not contain any additive, such as a drying agent, which promotes drying. While the oil is expediently undiluted, it can also, if so desired, contain diluents, for example turpentine or white spirit.

Especially good results are obtained with the present process when the wood preservative contains added acetic acid. In this case, the oil expediently contains 1-10% acetic acid, preferably 2-8% and then, especially, approx. 3-5%, calculated as 60 percent-strength acetic acid and on the weight of the wood preservative. Approx.

4% of 60 percent-strength acid has been found to be a suitable addition. The acetic

acid facilitates the impregnation, lowers the boiling point and enables the after- treatment temperature to be lowered. It also acts as a fungicide.

A customary course of treatment when impregnating a wood product in accordance with the invention is the following: Oil is stored in a working tank at a suitable temperature. The wood product is introduced into an autoclave and oil from the working tank is poured in so as to cover the product. If the oil has been at a higher temperature than the product, the oil then cools in connection with this. If the viscosity of the oil is too high, it is heated until a suitable viscosity is reached. Pressure is applied. At the end of the pressure phase, the oil has once again cooled somewhat. The pressure is removed and the product is after-treated by the oil being heated to the desired temperature by causing it to circulate through a heat exchanger. This after-treatment is carried out for at least 15 min, expediently at least 20 min, and then, in particular, for at least 30 min.

The after-treatment time includes the time which is required for heating the oil. The after-treatment can expediently be terminated when the oil has reached the desired temperature. In all the experiments, a total after-treatment time of 30 minutes has been found to be sufficient.

The simples way to carry out the after-treatment is to allow the pressure- impregnated product to remain in the wood preservative and to heat the preservative while it circulates round the product. However, it is also possible to first of all remove the product from the wood preservative, then allow the product to drain and, where appropriate dry and after that, where appropriate at a later time, immerse the product once again in the wood preservative and heat the latter while circulating it.

The after-treatment temperature for the oil is at least 80°C, expediently at least 90°C, preferably at least 100°C. Especially good results are obtained at a lowest temperature of 110°C. In certain cases, a temperature of 120°C can be advantageous. While higher temperatures can be used, they do not appear to afford any additional advantage. For economic reasons, therefore, preference is given to

the treatment temperature being at most 120°C.

Examples In the experiments, pinewood test specimens were impregnated with different oils as follows: 1) The oil is poured over test specimens in an autoclave.

2) Temperature equalization takes place between the oil and the test specimens, with the temperature of the oil falling.

3) Exchange of heat takes place for the purpose of heating to the desired impregnation temperature which is such as to ensure that the viscosity of the oil is sufficiently low for penetration to take place.

4) The pressure phase starts at this temperature.

5) No exchange of heat takes place during the course of the pressure phase and the temperature of the oil therefore falls.

6) The pressure phase is concluded by the pressure being allowed to fall to the ambient pressure.

7) The after-treatment is begun by the oil being subjected to heat exchange so as to heat it to the desired temperature.

8) In all the experiments, the after-treatment time is 30 minutes.

9) After that, the test specimens are removed from the oil and allowed to drain.

10) The tackiness is checked after 15 and 30 minutes.

11) After that, the tendency for the test specimens to bleed is examined by placing them under a heating lamp for 60 min.

The temperature at which bleeding begins is given in the following tables, together with other conditions and test results. All the test specimens which had a dry surface before the bleeding regained the same fine surface when they cooled down.

Results of tests carried out when using linseed oil which was at different temperatures which were employed for the purpose of<BR> achieving a dry surface affter impregnation<BR> In all the tests, the pressure time was 15 min with the overpressure being 8 kg. The test specimens were tested under a heating lamp for<BR> 60 min. All the values are means for 4 test specimens. The after-treatment time was 30 min in all the experiments. Temp. at Temp. at Temp. at end of after- Surface of test Surface of test Temp. when Absorption start of end of treatment, °C specimen after specimen after bleeding kg/m3 of pressure pressure 15 min. 30 min. occurs, °C sapwood phase, °C phase, °C 70 44 70 very smeary very smeary 50 245 80 48 80 Smeary 2 specimens dry 50 344 2 specimens smeary 90 64 90 Dry dry 55 289 quite a lot 100 66 100 Dry dry 75 420 (extremely little) Results of tests carried out when uwing tall oil which was at differnet temperatures which were employed for the purpose of<BR> achieving a dry surface after impregnation<BR> In all the tests, the pressure time was 15 min with the overpressure being 8 kg. The test specimens were tested under a heating lamp for<BR> 60 min. All the values are means for 4 test specimens. The after-treatment time was 30 min in all the experiments. Temp. at Temp. At Temp. at en of after- Surface of test Surface of test Temp. when Absorption start of end of treatment, °C specimen after specimen after bleeding kg/m3 of pressure pressure 15 min. 30 min. occurs, °C sapwood phase, °C phase, °C 30 25 30 very smeary very smeary 50 50 39 50 very smeary very smeary 50 139 60 44 60 very smeary very smeary 50 160 70 52 70 very smeary very smeary 50 126 (small dry patches) 80 64 80 Smeary smeary (small dry 50 154 patches) 90 66 90 Somewhat tacky somewhat tacky 50 310 (small dry patches) 100 71 100 Somewhat tacky sapwood side dry, 55 216 heart side tacky 110 77 110 slightly tacky dry 55 144 120 86 120 Dry dry 65 268 very slight Results of tests carried out when using tall oil in the added presence of 4% acetic acid, with the tall oil being at different<BR> temperatures which were employed for the purpose of achieving a dry surface after impregnation<BR> In all the tests, the pressure time was 15 min with the overpressure being 8 kg. The test specimens were tested under a heating lamp for<BR> 60 min. All the values are means for 4 test specimens. The after-treatment time was 30 min in all the experiments. Temp. at Temp. At Temp. at end of after- Surface of test Surface of test Temp. when Absorption start of end of treatment, °C specimen after specimen after bleeding kg/m3 of pressure pressure 15 min. 30 min. occurs, °C sapwood phase, °C phase, °C 80 57 80 Somewhat smeary somewhat smeary 50 290 90 61 90 Somewhat smeary, dry 55 329 heart side 100 68 100 Somewhat smeary, dry 60 416 heart side slight sweating 110 72 110 Dry dry 70 404 (extremely little) Results of tests carried out when using Sylfat ester 901E which was at different temperatures which were employed for the<BR> purpose of achieving a dry surface after impregnation<BR> In all the tests, the pressure time was 15 min with the overpressure being 8 kg. The test specimens were tested under a heating lamp for<BR> 60 min. All the values are means for 4 test specimens. The after-treatment time was 30 min in all the experiments. Temp. at Temp. at Temp. at end of after- Surface of test Surface of test Temp. when Absorption start of end of treatment, °C specimen after specimen after bleeding kg/m3 of pressure pressure 15 min. 30 min. occurs, °C sapwood phase, °C phase, °C 70 52 70 Smeary Smeary 55 350 80 56 80 Smeary Somewhat smeary 55 345 90 62 90 somewhat smeary, Dry 60 420 heart side Slight moisture slight sweating hyeart side 100 60 100 somewhat smeary, dry 65 375 heart side slight moisture slight sweating heart side 110 70 110 Dry dry 70 360 slight sweating

It is evident from the test results that a temperature of 80°C is sufficient for some test specimens which have been treated with linseed oil to be dry after a drying period of 30 min. At 90°C, all the test specimens are already dry after a drying period of 15 min.

A higher temperature or a longer drying period may be required in the case of tall oil.

The temperature which was required fell when 4% acetic acid was added. At the same time, the susceptibility to bleeding decreased. The increase in absorption which is recorded in the tables may depend on the varying composition (heartwood-sapwood) of the test specimens.

In the above-described experiments, smeariness and tackiness was checked after draining and drying for 15 and 30 minutes, respectively. However, in most cases, the test specimens were found to have a dry and tack-free surface almost immediately after having been taken up out of the oil.

In the experiments, the temperature which was used at the beginning of the pressure phase was the same as the temperature at the end of the after-treatment. However, this is not a requirement when carrying out the process but was simply a practical measure in connection with these experiments since the oil was heat-exchanged to the same temperature in both stages. At the beginning of the pressure phase, the wood preservative should be at a temperature which is conventional for its viscosity to be sufficiently low to enable it to penetrate into the wood product. This temperature is usually between 30 and 50°C.

The invention is defined in the following patent claims and is not to be regarded as being limited to the above implementation examples. All modifications and variations which come within the scope of the claims are possible.