GB2183319A - Method of treating rubber tree wood - Google Patents

Abstract

The invention relates to treating rubber tree wood to prevent oxidation and bacterial derived discolouration of rubber tree wood, and to avoiding warping of the wood. A method of treating rubber tree wood is described in which newly lumbered rubber tree wood is dried in a forced air blowing tunnel to reduce surface percentage water content to 10-15% and, subsequently further dried to reduce the overall percentage water content to 15-30%. Thereafter, the rubber wood is heated above 130 DEG C at a pressure above 10 kg/cm<2> until the water content reduces to 3-5%. The wood can then be advantageously aired until the percentage water content attains 6-7%.

Description

SPECIFICATION Method of treating rubber tree wood by pressure This invention relates to the treatment of wood from rubber trees to make it suitable for manufacturing purposes.

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

Rubber tree wood is a uniformly white wood hav- ing an air dried specific gravity about 0.6 -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 twenty-five to thirty year cycle, supplies of rubber trees are easily available so that compared with, say, oaktrees, beach trees, birch trees, and other broad leafed trees around the world, rubber trees are particularly suitable economically for production of furniture wood since the wood is of good quality and there is a stable supply.However,woodfrom rubbertrees has not been exploited forthe following reasons: (1 ) Rubber tree wood contains latex (vegetable rubber, starch, refined oil, oils and fats, oleic acid, and othercompositions) in conduits or cavities ofthe structure. When rubber tree wood is lumbered, itis subject two damage in a veryshorttime by discoloring bacteria which rapidly and vigorously discoloursthe surface of the rubberwood blue thereby reducing its economic value. Deteriorating bacteria can also damagethewood structure in a relativelyshorttime.

Furthermore, the rubber tree wood has an acidic property of pH is 4 - 5, so that when in contact with air after lumbering or in contact with ultra-violet rays from the sun, the surface of the rubber tree wood is rapidlyoxidisedtocausea red discolouration. The discolouration by either bacteria or this oxidation effect can extend into the wood from its surface given sufficient time. Consequently, these factors greatly impairthevalueofthewood produced.

(2) 30-40% ofthe rubbertreewood 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 rubbertrees is good. However, when the log condition disappears during fabrication into board or angularwood, contraction rapidly occurs so that the rubbertreewood warps by bending or curving in the direction where the highest proportion of tension wood occurs or in the direction where the degree of contraction is significant.This warping of the rubbertree wood occurs rapidly even in the raw wood condition, howeverthewarping effect is enhanced by drying and is conspicuously evident even if the percentage water content drops below 12%. This property is hardly ever found in other ordinary broad leafed trees and so gives rubber wood a bad reputation as a difficu It wood.

Furthermore the high percentage oftension wood causes improper cutting, improper planing, im- proper coating and fabrication in addition to the occurrence ofwarp. Therefore, the warping effect is yet another aspect reducing the value of the rubber tree wood 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 the insects lay eggs in the wood. This problem of insects is constantly present even after the wood has been fabricated into sayfurniture, building materials and the like.

Indeed, the insect problem is so major that rubber tree wood has not hitherto been usedforpruposes except for low class use having a short life time, such as fish boxes.

(4) The latex ingredient includes a refined oil com ponentsothatwhentherubberwood is in a warm and heated condition it permeates relatively easily to the surface of the rubber 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 aims to prevent therecurrence of any of these drawbacks to allow thereby successful utilization of the excellent properties of the rubber tree wood forfurniture and building materials, orthe like.

According to the invention there is provided a method oftreating rubber tree wood comprising the steps of: (a) drying the rubbertreewood substantially immediately after lumbering to reduce percentage water content of the wood to 15 - 30%; (b) pressing and heating the wood at a pressure above 10 kg/cm2 and a temperature above 1 30"C which corrects completely warp such as bending, curving and the like generating on the rubberwood, and prevents the recurrence of such warp.

In thisway, step (a) prevents the discolouring and deteriorating bacteria having any substantial effect and reduces the oxidation problems. This is particularly if the surface of the wood is preferentially dried. Conveniently, the subsequent adjustment of the overall percentage water contentto 15 - 30% is an effective condition for complete removal of the warp issuing step (b) and recurrence thereof can be prevented by suitable operation of the heat and press variable.

The present invention wiil now be described with reference to a generalised method and two specific examples.

Generalised method The rubber tree wood is discoloured blue by deteriorating bacteria within several hours after lumbering or alternatively, red discolouration occurs as a result ofthe oxidation phenomenon. Consequently, it is necessary to apply a discolouration preventing treatment immediately after lumbering. Consequently, rubbertreewood after lumbering is immediately transferred to a tunnel furnace with a large size air blower using rapid velocity air of the order of 10 - 20 metres per second from inlet to outlet. The air is blown through the furnace as the wood is carried therethrough for a selected time. It will be apparent that during the dry season naturally dried air can be used, wherein the rainy season hotairmustbesupp- lied to the furnace.As a result, the water content on the surface ofthe wood is dispersed rapidly by the forced air blowing so that proliferation ofthe discolouring bacteria is reduced by destroying bacteria on the surface of thewood. Whilst atthe sametime the latex on the surface of the wood is regarded as the cause of the occurrence of the oxidation effect as it produces a starch and an acidic solution from dissolving saccharide. The oxidation effect is also rapidly eliminated by the forced air blowing thereby preventing the red discolouration.

The forced air blow drying rapidly lowers the percentage of the overall water content of the wood to about 40%. Indeed, most of the drying energy is employed to dry the wood. Consequently, the percent age ofwatercontent on the surface portion ofthe wood is reduced to about 10 - 15%. At this level of water content, the discolouring bacteria and deteriorating bacteria can no longer stay on the wood and in addition most of the saccharide solution, regarded as a cause of the oxidation effect, is eliminated. Thus, this initial drying process both accelerates the drying for the next process and also prevents the aforementioned discolouration and oxidations without using harmful chemicals.

However, if such dried wood is left for a long time, the water content from the interior ofthe wood permeates to the surface thereby again presenting conditions in which discolouration and oxidation can occur. Consequently, the rubber tree wood is shifted to the nexttreatment process within two orthree weeks.

in the next step, the drying process is extended to adjustthe overall water content of the wood to lower it belowthefibre saturation point, whilst the lowest percentage water content does not fall below substantially 15%. Preferably, the percentage water content is about 20 - 25%. In this way, the water content of the wood is adjusted to make it suitable forthe next step.

The wood having undergone the aforementioned adjustment drying is then shifted to a hot press having a temperature above 1 30"C and a pressure above 10kg/cm2. The heat and the pressure are preferably applied at a rate of 1 minute per millimetre thickness of wood. in this way, the wood is dried to a condition resembling almost absolutely dry wood wherein the percentage water content is 3 - 5%. Consequently, the problems of warp such as bending, curving and the like are corrected. Furthermore, the recurrence of such warp problems even afterfabrication are prevented. The imbalance of the water content which is a cause ofthe warp is eliminated by the high temperature and pressure.In addition,thestrong stresses resulting from the abnormal cellulose composition, in particular in the tension wood are reduced bythe high temperature and high pressure.

Moreover, the insect attractiveness of the latex, starchness of the structural ingredient of the rubber tree wood is eliminated by the treating process and atthe same time the decomposition products from the saccharide ingredient are effective to keep away insects.

The wood having undergonethe heat and pressure treatment above is then subjected to a curing treatment wherein it is stored for two weeks to two months, so that the percentagewatercontent returns to a stable 6-7% as a result of natural absorption of moisture.

As a result of the aforementioned treatments, discolouration, deterioration, damage by insects, occurrence of warp, improper cutting, improper planning, improper coating and the like which are drawbacks of hitherto known rubber tree wood are eliminated. Consequently, furniture wood and building materials can be successfully provided from rubber tree wood without spoiling the white wood colour immediately after lumbering. This treatment is particularly advantageous since no polluting chemicals are employed and also the drying process time is halved. Consequently, economic production of rubbertree wood can be achieved.

It will be apparent that the series of treatments outlined above allow successful treatment of rubber treewoodto make it suitable for utilization asfurniturewood and building materials. The preferred characteristics of rubber tree wood are retained during the treatment process making the production of rubber tree wood itself economically useful.

Although some of the techniques above are known, the series of steps outlined above have not been combined to treat rubbertree wood, the treatment of which has hitherto been considered as extremely difficult.

Example I A semi-circularthin iron sheetforced air blow dryins tunnel is employed which has a height of 2 metres, a width of 2 metres and a length of 30 metres with iron rails running from the inlet to outlet. The tunnel is equipped with a large moisture removal electric motor drive air exhaust blower, blowing hot air of 80"C at a velocity of 1 m/s-1 . Rubberwood immediately from lumbering and having a thickness of 25 mm, width 100 mm and length 1,300 mm with an 80% water content is stacked on a truck and is shifted from an inletto an outletofthetunnel byan automatictransfer system. The tunnel in question had four heating pipes of 50 mm diameter on the underside supplied by3 kg/cm2steam pressurefrom a750 kg/H boiler during the rainy season.In order to com plete the drying for the prevention ofdiscolouration and oxidation of the rubber tree wood, a period of approximately 12 hours forced air blow drying is required.

The overall eventual water content of the rubber tree wood is reduced to 40 - 50% whereas the water content of the surface of the wood (up to about 3 mm fromthesurface) is 10-12%. The rubbertreewood afterthisdrying is the samewhite colouras after lumbering. Discolouration does not occur even ifthe wood is leftoutfortwoweeks.

The next step is to adjust the drying of the rubber tree wood to make it suitable for the following hot press step. This adjustment step uses the forced air blow tunnel again and employs an airspeed and temperature similarto those of the discolouration prevention drying step. Once again, the drying time is about 12 hours, so asto reduce the overall percentage water content to approximately 20 - 25%. The treated rubber tree wood is then unloaded from the tunnel truck and mounted in a hot press of dimensions 120 cm by 240 cm for about 25 minutes.

Typically, the temperature of the press is 1 50"C and the pressure 25 kg/cm2. Up to 15 stages can be employed in the press. At the end of this process, the overall water content of the rubbertree wood is typically 4% so that the effects of warp of the rubber wood, which would have occurred before the heat and pressure process, are corrected.

After the pressure step, the rubbertree wood is stacked in a warehouse for a two week clearing time wherein the water content elevates to about 6.5%.

There remains no warp.

Cutting and planing of the rubbertreewood can take place after the curing process to utilize the wood forframewood of box like furniture. It is found that fluffy or uneven wood surfaces are not produced and the polished rubbertree wood is different from that found in conventional rubber tree wood and indeed canbefinishedto produceawood havingtheimage of a higher class wood. Coating of the rubbertree wood by undercoats and finishing coats is found to be homogenous and an extremely smooth finish can be obtained producing rubber tree wood furniture which is regarded as impossible by conventional techniques.

Example 2 In orderto produce wood for composite wood or frame wood for building orthe like, newly lumbering rubber tree wood having a thickness 36 mm, width 100 mm, length 2,400 mm and a water content percentage of 70% is stacked on the truck of the first ex ample. Thetruck is then charged into the drying furnace tunnel which blows warm air at a velocity of 15 metres per second, employing natural atmosphe ric air in the dry season. The truck is automatically transferred through the tunnel so that the air blow drying is carried out for about 24 hours. The water content percentage then reduces to 45 - 50% with the surface of the rubbertreewood having awatercontent percentage of 12 - 13%.Once again, such rubber tree wood does not produce discolouration even if left out for about two weeks and, the white newly lumbered rubbertreewood colour is maintained.

The adjustment drying step is carried outfortwo days at a temperature of 70 - 100 C with an air velocity of 3 - 5 metres per second between each wood, so as to reduce the water content percentage to 17-20%.

The heat and pressure step is carried out for 36 minutes using the hot press having a temperature of 150"C at a pressure of 35 kg/cm2. This reduces the water content percentage of the rubber tree wood to about 3% so that warp is eliminated. Athree week curing process in a warehouse raises the water con- tent percentage to about 6%.

This rubbertreewood can then be cut to angular wood with a width of 13 mm and a thickness of30 mm and planed to produce a composite piece of wood. During the finger and bonding processes app lied to produce a composite wood, it is found thatthe frameforan interiordoorcould be constructed. During the cutting, planing and fabrication stages, no occurrence of warp is found and no similar drawbacks, such as fluffing and uneven surfaces. In addition, bonding and coating processes are satisfactorily carried out.

It can be seen from the above results that the processes applied to the rubber tree wood allow itto be employed to produce an enhanced building material.

The pieces of wood used in examples 1 and 2were placed in a box fortesting damage by insects such as grain borer, wood engraver, and similarinsects. The damage produced by such insects was observed, but the insects merely licked the surface of the wood without producing any actual damage.

Itwill be apparent that absolute dried condition is to be considered as encompassing a water content of substantially 3 - 5% and water contents wherein the effects of warp such as bending, curving and the like are substantially corrected. It will also be apparent that the water content figures given above are preferred examples and the actual figures employed will reflectthe quality of wood ultimately produced and may vary with differentspecied of rubbertree.

Claims (8)

1. Amethodoftreating rubbertreewoodcomprising the steps of: (a) drying the rubber tree wood substantially immediately after lumbering to reduce percentage water content of the wood to 15 - 30%; (b) heating and pressing the wood at a tem perature above 130"C and at a pressure above 10 kg/ cm2 to reduce the percentage water content of the wood to resemble an absolute dried condition.

2. A method as claimed in claim 1 wherein the step (a) comprises the steps of drying preferentially the surface of the wood prior reducing the percent age water content of the wood to 15 - 30%.

3. A method as claimed in claim 2 wherein the percentage water content of the surface is lowered to 10 - 15% priorto reducing the percentage water con- tent of the wood to 15-30%.

4. A method as claimed in any preceding claim wherein the percentage water content of the absolutely dried condition is 3 - 5%.

5. A method as claimed in any preceding claim wherein the heating and pressing of the wood is app lied ata rate of 1 minute per 1 mm thickness of wood.

6. A method as claimed in any preceding claim further including the following step: (c) curing the rubbertree wood after the step (b) until the percentage water content of the rubber wood attainssubstantially6 -7%.

7. A method as claimed in any preceding claim wherein the reduced percentage water content in step (a) is 20 - 25%.

8. Amethodoftreatingrubbertreewoodsub- stantiallyas herein described.