GB2183807A - High speed drying of rubber tree wood - Google Patents

Abstract

Rubber tree wood is dried in a manner which prevents oxidation and bacterial derived discolouration of the wood and warping of the wood. Immediately lumbered rubber tree wood is located in a hot press applying a temperature higher than 140 DEG C and at a pressure higher than 2 kg/cm<2>. The heating and pressurization are applied at a rate of 2-6 minutes per 1 mm of thickness of rubber tree wood. The pressure may be released at timed intervals, typically after visual evidence of boiling of the water in the wood. The dried wood is stacked in face to face contact with previously dried wood to allow stabilization of the water content in the wood.

Description

SPECIFICATION High speed drying of rubber tree wood This invention relates to the rapid drying of rubber tree wood.

The collection of rubber solution from rubber trees becomes economically difficult after 25-30 years growth so that fresh saplings must be planted. Before these fresh saplings are planted the old waste trees are bulldosed flat or cut down so that they can be burned where they lay.

Rubber tree wood is a uniformly white wood having an air dried specific gravity about 0.6 to 0.7. The wood is ideal as furniture wood because of its bending strength and hardness properties. As rubber trees are cyclically replanted in a 25-30 year cycle, supplies of rubber trees are readily available so that compared with, say, oak trees, beech trees, birch trees and other broad-leafed trees around the world, rubber trees are particularly suitable economically for the production of furniture wood since the wood is of good quality and there is a stable supply.

However, wood from rubber trees has not been exploited for the following reasons: (1) Rubber tree wood contains latex (vegetable rubber, starch, refined oil, oils and fats, oleic acid, and other compositions) in conduits or cavities of the wood structure. When rubber tree wood is lumbered, it is subject to damage in a very short time by discolouring bacteria which rapidly and vigorously discolours the surface of the rubber tree wood thereby reducing its economic value. Deteriorating bacteria can also damage the wood structure in a relatively short time. Furthermore, the rubber tree wood has an acidic property of pH 4-5 so that when in contact with air after lumbering or in contact with ultraviolet rays from the sun, the surface of the rubber tree wood is rapidly oxidised to cause a red discolouration.The discolouration by the bacteria or this oxidation effect can extend into the wood from its surface given sufficient time. Consequently, these factors greatly impair the value of the wood produced.

(2) 30-4096 of the rubber tree wood is generally composed of a tension wood which preserves the balance of stress when the tree is in a log condition. This is an unusual feature compared with other broad-leafed trees so that the quality and quantity of the wood available from rubber trees is good. However, when the log condition disappears during processing into board or angular wood, contraction rapidly occurs so that the rubber tree wood warps by bending or curving in a direction where the highest proportion of tension wood occurs and in the direction where the degree of contraction is significant. This warping of the rubber tree occurs rapidly even in the raw wood condition, however the warping effect is enhanced by drying and is conspicu ously evident even if the percentage water content drops below 12%.This property is hardly ever found in other ordinary broad-leaf trees and so gives rubber wood a bad reputation as a difficult wood. Furthermore, the high percentage of tension wood causes improper cutting, improper planeing, improper coating and fabrication in addition to the occurrence of warp. Therefore, the warping effect is yet another aspect reducing the value of the rub ber tree for products in which broad-leafed trees of the world are already employed.

(3) The aforementioned latex ingredient attracts insects by its unique fragrance so that insects lay eggs in the wood. This problem of insects is constantly present even after the wood has been fabricated into, say, furniture, building materials and the like. Indeed, the insect problem is so major that rubber tree wood has not hitherto been used for purposes except for low class use having a short lifetime, such as fish boxes.

(4) The latex ingredient includes a refined oil component so that when the rubber tree wood is in a warm and heated condition,the oil permeates relatively easily to the surface of the rubber tree wood after a time so that any coatings thereon tend to peel off increasing the likelihood of attack by the discolouring bacteria, deteriorating bacteria and insects.

It is an object of the present invention to eliminate the foregoing drawbacks and also aims to prevent the reoccurrence of any of these drawbacks to allow thereby successful utilisation of the excellent properties of the rubber tree wood for furniture and building materials, or the like.

According to the invention there is provided a method of drying rubber tree wood compris ing the steps: (a) placing newly lumbered rubber tree wood into a hot press; (b) actuating the hot press to apply high temperature and high pressure to the rubber tree wood to force dry the rubber tree wood until the rubber tree is in an absolutely dry condition. In this way the foregoing four drawbacks can be substantially eliminated so -that the quality of the rubber tree wood produced is improved.

Preferably, the heating is at a temperature higher than 140"C and the pressure is higher than 2 Kg/cm2. As a result, the generation of the red and blue discolourations during the conventional natural drying is prevented and also the deterioration of the wood product quality resulting from discolourations fre quently occuring in the preparatory stage of artificial drying and low temperature artificial drying has been prevented so that the white wood quality of rubber tree wood can be retained to thereby maintain the colour of the fresh wood from just after lumbering.

Although desirable to dry rubber tree wood rapidly, hitherto the occurrence of warp such as curling, bending and the like has been impossible to prevent so that a desirable high speed drying process has not been possible.

By using the present invention and applying it to untreated rubber tree wood, the wood can be dried into an absolute dry condition in a short time of 30 to 150 minutes according the thickness and wood water content. At the same time, the effects of warp are substantially eliminated.

Consequently, the present invention achieves a shortening of drying time and an improvement on dry wood yield without the drawbacks of warp generation.

In a prefered embodiment of the invention the pressure is released at 5-15 minute periods according to the water content and thickness of the rubber tree wood. By releasing the pressure from the rubber wood at a period of 5-15 minutes, preferably after the appearance of water boiling from the end of the wood, a further acceleration of drying is possible. The present invention succeeds in rapid drying as a result of the high heat conductivity of the water content retained by untreated rubber tree wood. Consequently, the high temperature from the hot press is thermally conducted straight to the center of the wood in a short time to prevent generation of strong stresses by the known contraction of the tension wood in rubber tree wood.Rubber tree wood is mostly tension wood and a major cause of warp is abnormal contraction of the fibre consituting the tension wood and the unique composition of the cell wall which is at least 50% of a gelatinous layer. By use of the hot press the cellulose of the tension wood is completely fixed and the gelatinous layer accelerating the generation of warp is fixed so that warp is eliminated and warp after fabrication is prevented. Furthermore, the high temperature and pressure decomposes the fragrance releasing substances in the wood that attracts insects and the substances are decomposed into compositions disliked by insects. Consequently, the invention advantageously overcomes the problem associated with insects without use of any chemical agents having strong pollution properties or expensive chemical agents.

Preferably, after drying, the rubber tree wood is stacked in face to face alignment with previously dried rubber tree wood. Conveniently, the end of rubber tree wood can be enclosed by a heat insulated water-proof sheet. In this way, the wood can be left out until it reaches equilibrium with the atmospheric temperature. Thereafter, the rubber tree wood can be left out in the atmosphere for 1-2 weeks before use. The purpose of this is that when the wood immediately removed from the press is allowed to contact air, the high heat is dispersed rapidly from the surface of the air and air of relatively high moisture content is concentrated around the surface of the rubber tree wood. Consequently, the rubber tree wood can absorb the moisture of the air to generate a strong stress gradient on the surface of the rubber tree wood.There is also a water gradient between the center portion of the wood and the surface portion of the rubber tree wood which could generate a slight stress.

Consequently, to avoid this the immediately dried wood is stacked in face to face contact with previously dried wood.

By employing the invention, the rubber tree wood becomes a useful wood material on account of its increased hardness, improved cutting, surface polishing properties and improved coating properties. This allows the wood to be utilised for furniture, fittings, fixtures and building woods without the aforementioned drawbacks normally associated with rubber tree wood.

An example of the present invention will now be described.

A hundred sheets of rubber tree wood of thickness 25 mm, width 100 mm and length 2 meters and in the untreated condition immediately after lumbering were inserted into a hot press having a hot plate of width 900 mm, length 4 meters. To start the drying process, the press was operated to apply a heated temperature of 150"C and a pressure of 2 kg/cm2 to the rubber tree wood.

The wood in the press started to gush out bubbles of water from the butt ends thereof as a result of the interior vapor pressure about 7 minutes after the start of processing.

Thereafter, the pressure of the press was released for a time, the first pressure release was typically carried out for 3 minutes after 15 minutes from start. A second pressure release was carried out for 2 minutes 30 minutes after the start, and a third pressure release was carried out for one minute 45 minutes after the start. The processed wood was removed from the hot press 60 minutes from the start of processing and were stacked in face to face contact with previously processed wood. Heat insulated waterproof sheet were provided to enclose the butt ends of the stacked wood. This wood was then left out in a warehouse.

The atmospheric temperature in the warehouse was kept for 48 hours at a temperature of 25-26"C at night and a temperature during the day of 30-33"C. Consequently, the surface temperature of the wood at the center portion of the stacked wood reached equilibrium with the atmospheric temperature. The sheets were then removed and the wood was left out for 2 weeks.

The water content of the wood was typically 85% in the untreated wood before the heating and pressurisation.

Immediately after processing and the water content of the surface portion was 0-3%, and 3-6% for the center portion. It will be apparent that absolutely dry condition is to be construed to encompass at least these water content figures. These figures are prefered examples only and the actual figures employed in the method will reflect the quality of the wood ultimately produced and may vary with different species of rubber tree. The percentage water content after 48 hours was typically 5-6% for both the surface portion and the center portion, and the percentage water content after two weeks was 6-7% for both the surface portion and the center portion.

Furthermore, the thickness of the untreated wood was 25 mm initially and 23.4 mm at the completion time of heating and pressurisation so there was a 1.6 mm or 6.4% contraction. The contraction was almost identical with the contraction with conventional artificial drying.

After the two days of covering with a sheet, and the two weeks wihtout the cover the aforementioned measurements of water content were taken on the 15th day. At the same time, the following, points were examined, the warp of the rubber wood, curving and bending and the like, warp of the wood upper surface, center portion and lower surface (performed by dividing the thickness into three equal parts), and warp of the wood on the left side, center portion and right side (performed by dividing the width of 100 mm into three equal parts). The results showed 8 sheets of the wood with extremely small warp with respect to the upper surface and lower surface but such warp did not prevent the sheets from being used for a practical purpose. Thus it can be seen that the generation of the stress producing the warp such as curving, bending and the like is substantially completely removed.

Furthermore, cutting and polishing of the wood was satisfactorily carried out, and there were no fluffy nor uneven surfaces or the like and a gloss was produced on the cut surface which was not found in the conventional artificial dried wood.

Moreover, in the coating process, undercoat, middle coat and upper coat produced an extremely smooth finish coat of homogeneous type. Additional touching up required for conventional coatings on wood was not needed, and the final coat required no additional coating.

The processed wood of this invention produced no drawback when used in cabinet type furniture, plywood material for tables, chair material, interior door frame material, plywood material for staircases, and floor material.

Claims (7)

1. A method of drying rubber tree wood comprising the steps: (a) placing newly lumbered rubber tree wood into a hot press; (b) actuating the hot press to apply high temperature and high pressure to the rubber tree wood to force dry the rubber tree wood until the rubber tree wood is in an absolutely dry condition.

2. A method as claimed in Claim 1 wherein the hot press is operated at a temperature higher than 140"C and at a pressure higher than 2 Kg/cm2.

3. A method as claimed in Claim 2 wherein the application of heat and pressure to the rubber tree wood occurs at a rate of between 2-6 minutes per 1 mm thickness of rubber tree wood.

4. A method as claimed in Claim 3 wherein the pressure is released at 5-15 minute periods according to the water content and thickness of the rubber tree wood.

5. A method as claimed in Claim 4 wherein the first pressure release occurs in response to visual appearance of boiling water condition from the rubber tree ends.

6. A method as claimed in any preceeding claim wherein the absolutely dry wood is removed from the hot press and stacked in face to face contact with previously dried rubber tree wood and the ends of the dried rubber tree wood are heat insulated.

7. A method of drying rubber tree wood substantially as herein described.