US3090411A - Method of simultaneous stabilization and drying of wood - Google Patents

Description

United States Patent Oil" 3,090,411 Patented May 21, 1963 3,090,411 METHGD F SIMULTANEGUS STAEILIZATIGN AND DRYING 8F WGQD Flemming G. .Iuncirer, Havndal, .lprgen Damgaard and Vagn Kingo Jacobson, Koge, Peter Honk, Heliernp and Eorgen Schmidt, Koge, Denmark, assiguors to Junckers Savvaerk A/S, Vaerftsvcj, Koge, Denmark No Drawing. Filed July 26, 1961, Ser. No. 126,894 Claims priority, application Denmark July 29, 1960 2 Claims. (Cl. led-320) It is known to reduce the tendency of wood to warp when the humidity of the air varies. This tendency to contraction and bulging is one of the greatest drawbacks involved by the use of wood. Since contraction and bulging are in different directions tangentially and radially to the annual rings of the wood and since further the moisture content in the wood may vary from spot to spot, variations in the moisture content of the wood will involve warpings and twistings of pieces already shaped.

In the course of time many proposals have therefore been made for treating Wood in order to reduce its tendency to work as a consequence of varying air humidity. A common feature of most stabilising methods proposed in literature is at some stage of the treatment to subject the wood to a rather long period of impregnation with various chemicals such as phenol resin. In fact, very substantial results of stabilisation have been obtained in this way, but the said methods have not been adopted to any wide extent in the wood working industry because they are rather costly and therefore mean an unreasonable increase in the price of the wood.

It has also been tried to reduce the moisture content of fresh wood to l0l2% by pressing the wood mechanically between two plates heated to 160' 250, using a pressure of 12-65 -kg./cm. :for %4 minutes. This drying is, however, not sufficient to prevent the wood from working, but drying beyond that range has been abandoned for fear that the wood should lose some of its content of resin which is presumed to melt at the said temperature. Furthermore, it is unavoidable that owing to hydrolysis there will be blotches in the outer layers of the fresh wood which face the pressing plates.

In order to avoid such blotches it has been found necessary in another case to air-dry wood veneer so much that it is subject to a linear contraction of 13% prior to the press drying, at which it has then been possible to reduce the water content to 1-5 When subsequently the wood is carefully moistened to contain up to 6-8%, it will be stabilised, but it has in fact not been possible to avoid contraction to obtain stabilization without marring blotches.

The present invention has for its object to make it possible in press drying without formation of blotches to avoid contraction of the wood in directions at right angles to the direction of pressing, even in the case of wood an inch thick or more.

According to the invention the wood, which is preferably available in a thickness of at least mm, and which has a water content of 30% or more in which it has preserved its full widths, is subjected to a pressure of 5-100 kg. per cm? at right angles to the direction of the fibres at a temperature of 110200 C. for a period of /25 hours until the water content of the wood has been reduced to less than 5% by weight. The said period of pressure ensures the said drying of the wood even at such low temperatures as those indicated, at which the risk of blotches is very small.

By this treatment the wood is, as mentioned, only stabilized in a direction at right angles to the direction of pressure, that is, not in the said direction itself. This is, however, less essential in several uses. In the case of Z parquetry iloors, it is, for example, only the contraction and bulging of the wood in one of the directions of a parquet block, namely the direction of width of the block, which involves drawbacks in practice in the mounted parquetry floor.

By treating wood in the manner dealt with according to the invention the stabilization of the Wood in directions at right angles to the direction of pressure will be maintained, if only the water content of the wood does not later exceed about 25%. This means in practice that the wood preserves its stability even though it is exposed to varying air humidity which may go as far as up to about of relative humidity. If, however, the wood gets into contact with water, it will later be capable of contracting and bulging like untreated wood.

When beechwood from being in equilibrium at 20% relative humidity is brought into equilibrium at 60% relative humidity, it will expand by 5.7% in tangential direction. Under identical conditions the Wood treated according to the invention and hot-pressed at right angles to the direction of its fibres will be capable of expanding by about 2.2%. As a result, an anti-contraction eilect has been obtained equal to:

In conventional hot drying of crude beech blocks, reducing their water content to less than 5%, 10-15% of the blocks have had to be rejected prior to press drying owing to warping, and the finished blocks have been produced in the running production with a width of 113 By stabilizing and drying wood of the identical crude block width by the method according to the invention it will be possible, owing to the eliminated contraction in width, to produce it in block widths of mm, which means an increase of 15% in area. With the use of the present invention considerable advantages are thus obtained in addition to a substantially improved economy, since there will be practically no warping nor twisting of the wood, nor will there be any perceptible decrease in width.

Other advantages of the method according to the invention will be apparent from the following examples.

A piece of beechwood, 40 cm. long, 7.5 cm. wide and 3 cm. thick, was pressed in its green condition for two hours, the pressure applied to the surface of the wood being 14 kg/cm. at a temperature of C. in the pressing plates.

When the wood was removed from the press, its water content had decreased to 1%. Its thickness had decreased by 0.9 c. and its width by 0.1 cm. Its length was unaltered.

A piece of ashwood :of identical dimensions was airdried to a water content of 30%. The wood was hotpressed under identical conditions and for the identical time, and its water content was found to have been reduced to 2 /2%. Its thickness had decreased by 0.7 cm. and its Width by 0.1 cm.; its length was unaltered.

In a third case a piece of beechwood of identical dimensions as mentioned in the foregoing, but with red heart, was air-dried to a water content of 30% The wood was subsequently subjected to the same hot-pressing as was used in the two preceding examples, and when removed from the press the wood was 'found to have a water content reduced to 3%. Its thickness had decreased by 0.7 cm. and its width by 0.1 cm. Its length was unaltered.

If by further proceeding according to the invention the wood is fed to the hot press in a boiled or thoroughly steamed condition, a reduced pressing time is sufiicient, just as the product obtained is without any essential internal stresses.

Boiling or steaming of the wood can be performed in easiest manner by introducing it into a steam oven or kiln which need not have any pressure substantially above atmospheric. The time the wood has to stay in the said oven depends on the thickness and state of the wood. In the case of fresh beechwood of a thickness of 3 cm., 14% hours will be suitable.

According to the invention the wood has further prior to the hot-pressing being subjected to pressing or rolling at temperatures of 100 C. and less and at pressures of 30-130 kg./cm. until the major part of the water contained in the pores of the wood has been squeezed out. By this preliminary treatment of the wood, which, Where fresh beechwood is involved, may appropriately be for a :few minutes, that is, 2-10 minutes, the advantage obtained is in addition to the aforesaid reduction of the pressing time a prompt removal of a substantial portion of the water-insoluble substances which would otherwise be apt to discolor the wood when it is press-dried.

Furthermore, according to the invention the wood may be fed to the hot press after prior absorption of ammonia in quantites of 1-3 percent by weight. The treatment of the wood with ammonia may take place at ordinary ternperature for up to 6 hours in a kiln filled with vapors of ammonia. This absorption of ammonia will soften the wood so that it can be compressed more at the hot pressing, and by means of the ammonia treatment it is further possible to regulate the hue of the wood to the desired degree. Incidentally, it is known to fume Wood by means of ammonia.

Prior to, during and after the absorption of ammonia the wood may be treated with aldehyde in a kiln in which the temperature is kept at around 110-150 C. By sup plementing the ammonia treatment with this aldehyde treatment the result obtained is that the stabilization aimed at by the hot pressing is further improved. Incidentally, the aldehyde will in known manner have a preserving efiect against fungi and insects.

Since several of thebest known and cheapest aldehydes, such as formaldehyde, only produce a stabilising efiect in acid condition, it may be advantageous when using such aldehydes in combination with the method accordingto the invention to add beforehand to the liquid or gaseous phase of the aldehyde small amounts of a suitable and such as hydrochloric acid, sulphuric acid, nitric acid or an acid salt such as ammonium chloride, zinc chloride or aluminium chloride.

Compared with the hot-air drying in drying chambers as usually adopted the hot pressing itself means a very substantial economy in space requirements, frequently up to 90%, owing to the fact that the hot pressing can be carried through much faster than air drying.

What we claim and desire to secure by Letters Patent is:

1. In treating wood to stabilize its tendency to warp consequent to varying air humidity, the improvement which comprises relieving the wood of its ability to contract tangentially tto its annual rings by applying pressure of 5 to 100 kg. per om. at a temperature of 110-200 C. across the width of the wood and only in a direction at right angles to its fibers for a period of /2 to 5 hours until the water content of the wood has been lowered to below 5% by Weight of the Wood, whereby the wood will be stabilized against contraction in directions at right angles to the direction of said hot pressing and will not warp with varying air humidity.

2 In treating Wood to inhibit its tendency to warp consequent to varying air humidity, the improvement which comprises relieving the wood of ability to contract tangentially to its annual rings by first adjusting the water content of the wood to comprise about by weight and thereafter applying pressure of 5 to 100 kg. per cm. at a temperature of 1l0200 C. across the width of the wood and only in a direction at approximately right angles to its fibers for a period of /2 to 5 hours until the water content of the wood has been lowered to below 5% by weight of the wood, whereby the wood will be stabilized against contraction in directions at right angles to the direction of said hot pressing and will not warp with varying air humidity.

References Cited in the file of this patent UNITED STATES PATENTS 1,356,015 Roy Oct. 19, 1920 1,697,248 Lauter Jan. 1, 1929 1,952,664 Esselen Mar. 27, 1934 2,064,965 Will Dec. 22, 1936 2,136,730 Sweetland Nov. 15, 1938 2,313,953 Loughborough .L Mar. 16, 1943 2,666,463 Heritage Jan. 19, 1954

Claims (1)

1. IN TREATING WOOD TO STABILIZE ITS TENDENCY TO WARP CONSEQUENT TO VARYING AIR HUMIDITY, THE IMPROVEMENT WHICH COMPRISES RELIEVING THE WOOD OF ITS ABILITY TO CONTRACT TANGENTIALLY TO ITS ANNUAL RINGS BY APPLYING PRESSURE OF 5 TO 100 KG. PER CM2. AT A TEMPERATURE OF 110-200* C. ACROSS THE WIDTH OF THE WOOD AND ONLY IN A DIRECTION AT RIGHT ANGLES TO ITS FIBERS FOR A PERIOD OF 1/2 TO 5 HOURS UNTIL THE WATER CONTENT OF THE WOOD HAS BEEN LOWERED TO BELOW 5% BY WEIGHT OF THE WOOD, WHEREBY THE WOOD WILL BE STABILIZED AGAINST CONTRACTION IN DIRECTIONS AT RIGHT ANGLES TO THE DIRECTION OF SAID HOT PRESSING AND WILL NOT WARP WITH VARYING AIR HUMIDITY.