The present invention relates to a novel method of drying and optionally expelling resin from newly cut lumber. The invention also relates to apparatus for carrying out said method.

TECHNICAL DESCRIPTION Newly cut lumber contains large amounts of water, often more than 50% by weight.

This high water content causes problems in the further refinement of the lumber and must therefore be reduced. A traditional method of drying lumber is to first stack the lumber and then allow it to remain stacked until an acceptable moisture content has been reached. It can take several weeks to reach the dryness required by the furniture industry (normally about 6%). The lumber is vulnerable during this long dryingperiod and can be damaged both by crack formations and decay. Moreover, such long handling times are always allied with high handling costs, storage costs and similar costs.

There is therefore a need to develop better and, above all, quicker ways of drying lumber and timber.

The resin that is likely to be present on and in lumber that has been newly cut from conifers can also be problematic, by subjecting tools to wear and by impairing the appearance of the end product. There is thus also a need of methods of removing resin from lumber.

Attempts have been made to dry wood by heating the wood with the intention of vaporising or otherwise expelling the water present. Heating of wood in different forms of ovens and kilns is an expensive business and does not significantly lower

the drying time. RU 2034697 describes a method of shortening the drying time, by exposing the wood to microwaves. It is said that resin can also be expelled by this technique. Although the effective drying time is reduced, the method is based on the cyclic exposure of the wood to microwaves. It is necessary to move the wood to and from a microwave oven at regular intervals, which is effort demanding.

U. S. 4,971,840 describes a technique for drying and impregnating moist wood material. The moist wood material is immersed in an oil bath that contains a moisture absorbing substance (colloidal silicon dioxide), followed by a bath that contains urea-formaldehyde resin. The drying process is said to take at least one calendar day.

U. S. 4,027,401 describes a technique for drying a cut tree trunk. The trunk is placed in a container that includes two chambers. The trunk passes through the container dividing wall. Packings between the trunk and the wall prevent gas from passing adjacent the trunk. In order for gas to travel from one chamber to the other, it is forced to pass through the trunk itself. Gas pressure is applied between the two chambers, with the aid of a vacuum pump. Vapours of a volatile organic solvent may be added to the supply air on the pressure side. This is done in order to dissolve any resin in the log and to drain off the resin on the suction side.

The technique described in U. S. 4,027,401 has certain drawbacks. The use of a gaseous pressure medium (air) makes drying less effective. The gaseous pressure medium is sucked through the log, therewith preventing a pressure difference greater than 1 atmosphere being obtained. It is difficult to withdraw resin from the log solely by using solvent vapours.

There is thus a need for a method in which newly cut coniferous wood can be dried and its resin removed both quickly and easily.

SUMMARY OF THE INVENTION

It has now been found that a cut tree trunk that has front and back major cut surfaces and possibly further cut surfaces at which branches have been removed from the tree trunk can be dried by generating a pressure difference between two of said cut surfaces, wherewith liquid in the trunk is drained off at a cut surface that is subjected to a pressure which is lower than the pressure at the cut surface that is subjected to the highest pressure. By using as a pressure medium a water-miscible organic solvent that is capable of dissolving resin, it is possible to remove resin from the trunk in conjunction with the drying process.

DEFINITIONS The term"log"used in this document refers to a tree trunk that has been cut two times. The type of wood concerned is not critical and the invention can be applied, in principle, to all types of wood that are of interest commercially, such as fir, pine, larch and other coniferous trees, birch, beech, asp, lime, elm, maple, etc.

By"major cut surface"is meant the surface that is left when cutting a tree trunk. A log will thus have two major cut surfaces, because it is the result of cutting through a tree trunk twice. The surfaces created when cutting branches from the log are called minor cut surfaces. The major cut surface that lies nearest the root of the tree will normally be larger than the other major cut surface.

By"pressure medium"is meant a liquid medium that is used to apply pressure onto a cut surface. The pressure medium is comprised of a water-miscible organic solvent that is able to dissolve resin. It is important that the solvent is not excessively harmful to the environment. Examples of such solvents are lower alcohols, such as methanol, ethanol, propanol, isopropanol and propane-2-ol, and esters of lower carboxylic acids, such as methyl formate, methyl acetate, methyl propionate, ethyl formate, ethyl acetate and ethyl propionate.

By"pressure generator"is meant a device that can change the pressure in a given space. Compressors are examples of suitable pressure generators.

By"enclosure means"is meant a dome-shaped element that can enclose a cut surface. This element is comprised of material that is essentially impermeable to the pressure medium used. The enclosure element is anchored to the log around the cut surface such that the boundary surface or interface between the enclosure element and the log will also be essentially impervious to the pressure medium.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 is an explanatory sketch of a log and is used to support the definitions in this document; and Figs. 2 and 3 illustrate respectively two embodiments of methods and apparatus according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION This invention is based on the realisation that a tree trunk can be viewed simply as a collection of mutually parallel fine bores that contain water, and that the trunk can be dried by creating a pressure difference across these fine bores, so as to withdraw the water from the trunk. The resin present in the wood can be removed together with the water, by using a pressure medium that consists of a substantially water- free solvent that will dissolve resin and that can be mixed with water. The inventive method enables wood to be dried very quickly. For instance, a pine log having a

diameter of 20 cm and a length of 2 m can be dried to a moisture content of 6% in less than one hour. The method also results in homogeneous drying of the wood, therefore considerably reducing the risk of the wood cracking.

As a basis of further discussion of the invention, reference is made to Figure 1 in which different basic terms relevant to the present document are defined. A log 2 has two major cut surfaces 4,6, resulting from having cut the trunk at two places.

Branches that were found on the log had been removed conventionally, leaving minor cut surfaces 8.

As before mentioned, the invention is concerned with a method of drying cut lumber and the possible removal of resin therefrom, and also relates to apparatus for carrying out the method. Figures 2 and 3 illustrate examples of the inventive method and apparatus. It will be understood by the person skilled in this art that these two exemplifying embodiments are merely illustrations of the invention and that they are not intended to limit the scope of the invention.

Figure 2 illustrates a basic embodiment of the present invention. An enclosure element 12 has been attached to a major cut surface 6 of a log 2 in a manner to obtain an essentially impervious interface 10 between log 2 and enclosure element 12. It will be understood that the enclosure element is not restricted for use solely with a major cut surface, but that it can also be used in connection with a minor cut surface. However, it is important that the interface and enclosure element are both essentially impervious to the pressure medium used, even when pressure is applied.

The enclosure elements will suitably comprise elastic and gas/liquid impervious devices made from different types of rubber suitable for sealing purposes, and the enclosure element is affixed to the log with the aid of suitable fastener means. The skilled person will be aware of various suitable methods of affixing the enclosure element to the log in an essentially gas-tight fashion in different situations. The enclosure element 12 is connected to a pressure generator 16 via a connection 14.

When a suitable pressure, typically a pressure of 0.4-20 bar, is applied to the major cut surface 6, water will be released from the opposing major cut surface 4 and also from other minor cut surfaces 8 when found.

A typical pressure applied when practicing the present invention will be at least 0.4 bar and will preferably lie in the range of 0.5-20 bar. The skilled person will be able to chose a pressure that is suitable with respect to chosen conditions, such as the type of wood concerned and the length of the logs, by means of routine experiments.

The length of the logs is not critical, and may vary within wide limits. Short logs have the drawback of increasing the number of cuts that must be made on each trunk. Higher pressures are required when practicing the invention on longer logs.

Consequently, it is preferred that the logs will have a length of between 0.5-8 m, and preferably within the range of 1-5 m. Although the inventive method is normally carried out at room temperature, it may be beneficial to increase the temperature in some cases, in order to increase the viscosity of the water, resin and pressure medium. However, a temperature of 150°C should not be exceeded.

Figure 3 illustrates an alternative embodiment of the invention. In this embodiment, an enclosure element 12 has been connected to a major cut surface 4 on a log 2 in a manner to obtain an essentially tight interface 10 between log 2 and enclosure element 12. A second enclosure element 18 has been connected to the other major cut surface 6 such as to obtain an essentially impervious interface 30 between the enclosure element 18 and the log 2. A compressor 16 is used to raise the pressure of a liquid pressure medium, wherewith said pressure is conducted through a conduit means 14 to the major cut surface 4 and the enclosure element 12. Water and any dissolved resin will be exposed on the major cut surface 6 together with liquid pressure medium. These liquids or fluids are removed by suction through the medium of a suction device 22 and passed to an intermediate tank 26 through a conduit means 20. Water and/or resin is/are tapped from the tank into a vessel 34, through a conduit means 32. Liquid pressure medium may optionally be recycled through the compressor 16, via conduit means 28.