GB2078607A - The use of gumwood as a veneer in the manufacture of plywood - Google Patents

Abstract

A gumwood log is boiled in a solution containing a surface active agent and a latex solvent, the treated log is then cut into veneer sheets, the cut sheets are heated in a solution containing latex solvent in an autoclave under pressure and then at a lower pressure in a solution containing an insecticide. The veneer sheets are then dried and are subsequently formed into plywood in a known manner.

Description

SPECIFICATION A process for manufacture of gumwood plywood The present invention relates to a process for manufacture of plywood from gumwood. The gumwood is cultivated in various Southeast Asian countries such as Malaysia, Thailand, Indonesia and is recognized by many experts as wood material which can be easily collected and having excellent product qualities.

The gumwood belongs the the Euphorbiaceae family. Its scientific name is Hevea brasiliensis Muell-Arg. Generally, the gumwood is called the "yanpara". The gumwood normally ceases to produce latex at the age of about 20-25 years and in many cases, the trees are cut down, burnt and discarded.

At present, a very small quantity of such gumwood is used for fish boxes, chip materials and charcoal, but the gumwood has not been utilized to any great extent. The reasons are, for example: (1) The wood has a low degree of tolerance to discoloring bacteria and bacteria causing decay can reach an advanced state of decay in a very short time. The reason is the presence of latex in the capillaries and voids the wood's structure and this latex contains starch, saccharose and aromatic substances, which discolor the wood and which wood decaying bacteria thrive on.

(2) This latex fills the entire wood structure and after sawing or processing end products, the latex travels to their surfaces.

(3) Certain insects lay eggs in the gumwood and eat wood tissue. In case of the gumwood, complete insect control is difficult using the usual insect control treatments.

(4) Defects such as bending, warp, twist, break or the like occur frequently in the gumwood, and yield after processing is extremely low.

As described in the foregoing, the gumwood has an extremely low degree of tolerance to discoloring and decay caused by bacteria and insect decay is frequently encountered. The latex remaining in the wood structure is the biggest obstacle to utilization on a worldwide scale.

However, since the gumwood has the following features, if the foregoing problems were solved, the gumwood would, to the contrary, have extremely high utility value.

(1) Since the gum trees are planted systematically in geometric patterns on flat fields, cutting and hauling are not problems, and can be carried out at an extremely low cost compared with other species. In Thailand and Malaysia, planned cutting of gum trees and their utilization is strongly desired, and felling permits are not needed. Rather, a subsidy is given which facilitates this type of business.

(3) Gum trees are planted according to plan every year, and because of their enormous volume of growth and tremendous accumulated volume, gum trees can meet further quantitative demand.

Also, as the color of the gumwood is an attractive white which facilitates applications of unvarnished finishing processes to the gumwood, the wood is suitable for higher class products such as furniture, fittings, and fixtures or the like.

Taking the foregoing circumstances into consideration, the present inventor has succeeded in discovering an ideal process for utilization of the gumwood as a result of research over a long period of time. Namely, the method is comprised of treating the gumwood to produce a veneer sheet, and of manufacturing plywood consisting of these veneer sheets. However, as described in the foregoing, since the gumwood contains latex in its structure and has other special characteristics, it is not possible to manufacture the plywood by merely employing conventional processes. Rather, special methods utilizing the present invention become necessary.

The process which constitutes this invention requires the following constituent processes, namely, (a) pre-treating process, (b) veneer sheet manufacturing process, (c) veneer sheet treating process, and (d) plywood manufacturing process.

(a) Pre-treating process This process consists of placing the gumwood in a tank filled with hot water, adding a surface active agent having special penetration and diffusion characteristics and a latex solvent of suitable alkalinity to the hot water in the tank, and boiling the gumwood for several hours. The rubber latex filling the voids of the structure of the gumwood become soft and is easily dissolved.

Moreover, the wood structure becomes soft and clean. The results of the inventor's experiments indicate that it is preferable to heat the solution to about 600C before boiling and to soften the wood structure slowly for several hours, and then to raise the heat of the solution to boiling. If the wood is heated too rapidly, it becomes distorted.

The surface active agent penetrates and diffuses inside the gumwood, and shows high stability with regard to heat, chemicals, and hard water. Moreover, the surface active agent has a highly desirable cleaning action. Through the results of various experiments performed by the inventor, it has been demonstrated that anionic alkylnaphthalene sulphonic acid sodium of dodecylbenzene sulphonic acid sodium are most effective as surfactants. Also, as a rubber (latex) solvent, triethylamine, ethylbutylamine, diethylaminoethanol or soda ash with suitable alkalinity can be used.

The addition of the surface active agents promotes the penetration of the latex solvent deeply into the wood structure.

The surface active agent and the latex solvent together are mixed with water to make a solution the strength of which is as follows; the upper limit is in the range of 1%, the lower limit is 0.1%, and the most preferable level is 0.30.4%.

If the concentration of the chemicals is increased too much, there is a tendency for the wood structure to suffer damage.

(b) Veneer sheet manufacturing process The wood undergoing the foregoing treating process (a) is taken out, and is sawed to produce veneer sheets having a thickness of 0.3-3 millimetres by means of the well-known rotary race or slicer for used small diameter workpieces by means of which plywood material is manufactured.

(c) Veneer sheet treating process The veneer sheets are shifted to an autoclave.

The pressure in this autoclave can be raised or lowered. This autoclave is filled with the hot water, and the hot water contains the latex solvent referred to in the foregoing.

In this process, the pressure of the autoclave is raised at the first stages so that the process penetrates deeply into the gumwood structure.

namely, the hot water containing the solvent in the autoclave is subjected to pressure which causes the hot water to penetrate and diffuse deeply into the capillaries and voids of the wood structure. Simultaneously, since the hot water contains the solvent, the rubber latex contained inside the gumwood structure is uniformly dissolved. The major portion of the latex contained is removed by this latex dissolving action and with the use of such an alkaline solvent, the gumwood becomes slightly alkaline thereby preventing the adhesion or proliferation of bacteria. The heating time is adjusted depending upon the thickness of the veneer sheets. In case of a veneer sheet of about 3 millimetres in thickness, about 1 5-30 minutes at 1 200C is sufficient.

In order to promote the effects of the process, pressure is applied at 4-6 kg-cm2 by means of a compressor. By this heating and compressing process, the latex component and the resin component contained in the gumwood veneer sheets are dissolved, and the capillaries of the wood structure are cleaned.

After this treatment is finished, the solution is shifted to another autoclave and the wood is kept inside the first autoclave. The wood is then subjected to reduced pressure of 600-700 mm/ Hg for 1 5-40 minutes. By means of this pressure reduction, a boiling phenomenon occurs inside the wood, most of the latex remaining inside the wood structure is forced out of the wood structure, and the small amount of latex still remaining is spread uniformly through the wood structure.

When the pressure reduction process is finished, hot water containing an insecticide is injected into the autoclave. By using the compressor, a pressure of 4--6 kg/cm2 is applied for the required time, thereby allowing the sufficient penetration of the insecticide into the wood structure. Since the latex has been removed already from the gumwood and the wood structure has been cleaned, the insecticide can easily penetrate depply. If needed, pressure reduction can be used to effect a homogeneous penetration of the insecticide into the wood structure. For an insecticide, an alkaline type is preferable, and one can be prepared by mixing borax and boric acid at a low cost.

(d) Plywood manufacturing process The veneer sheets treated by the foregoing process are dried in the sun or by using a drier.

Then adhesives are applied and the directions of the grain of the sheets are varied as the sheets are stacked one on the other and are subjected to pressing by a hot press thereby forming sheets of plywood.

This plywood manufacturing process is the same as the conventional processes. The resulting plywood made from the gumwood has a white surface color and the luster is bright. It is a high quality plywood which has not been obtainable from conventional woods up to the present time such as, for lauan material. Accordingly, this plywood is an epoch-making material for highclass furniture, fittings, and fixtures.

Also, the unique strength of the gumwood is present, and because of such qualities, the wood has excellent clamping power for screws, bending strength, compressive strength.

The present invention becomes more concrete by referring to the following embodiments.

EXAMPLE A gum tree log the diameter of which was 40 cm and the length 2 metres was placed in a boiling tank the length of which was 4 metres and the depth of which was 2 metres. The tank was filled with hot water at about 60"C. Alkylnaphthalene sulphonic acid sodium at a concentration of 0.3% as a surface active agent in aqueous solution and soda ash at a concentration of 0.4% as a latex solvent were added to the hot water in the tank. The resulting solution was heated for 4 hours.

The temperature was then elevated to 800C-900C, and then the solution was boiled for 3-4 hours (pre-treating process).

The log was removed, and was sawed to produce veneer sheets the thickness of which was 0.5-3 millimetres by means of a rotary race or slicer (veneer sheet manufacturing process).

A set consisting of 1 O veneer sheets was bundled, and each bundle was enclosed in metal mesh. Before the temperature of the veneer sheets dropped, the bundle of veneer sheets was shifted to an autoclave the diameter of which was 1.5 metres and length 4 metres. The autoclave was filled with hot water at a temperature of 800C-900C. This hot water contained soda ash atx a concentration of 0.4% as a latex solvent. The temperature was raised to 1 200 C. Pressure was applied by means of a compressor and the pressure in the autoclave was maintained at 4 kg/cm6kg/cm2. This condition was maintained for about 1 5-30 minutes.

Then, the liquid in the autoclave was shifted to a separate container by opening a valve.

The valve was then closed again, and the pressure was reduced to 600-700 mm/Hg. After reducing pressure, hot water the temperature of which was 95-1 000C, consisting of a solution at a concentration of 5%, consisting of 50% borax and 50% boric acid as an insecticide, was put into the autoclave. The wood was treated for 1 5-30 minutes while applying a pressure of 4 kg/cm2.

The solution was drained at this stage in the process, and then pressure was reduced to about 700 mm/Hg for about 10 minutes (veneer sheet treating process).

The resulting veneer sheets were dried and coated with adhesives. The direction of the wood grain of the sheets were varied and several sheets were piled one on the other, and then were subjected to pressing by means of a hot press, thereby forming plywood.

The plywood obtained by this process contained in the pressure invention had beautiful white surface as described in the foregoing. In contrast, during waterproofing testing, the plywood manufactured from untreated veneer sheets had no commercial value at all since both surfaces of each sheet were discolored black and latex appeared on the surface in 20-30 days. In comparison with conventional plywood, the plywood manufactured by the process contained in the present invention did not show almost any change on the surface. Furthermore, 200-300 woodborers were placed in a mesh box containing treated plywood and untreated plywood. The untreated plywood was eaten to such a degree that the original shape of the plywood was indistinguishable within a week. On the other hand, the treated plywood was recognized to have effected to a depth of about 0.5-1.0 millimetre at the edge of the plywood, but even after 6 months, the damage from these insects did not proceed.

Claims (5)

1. A process for manufacture of gumwood plywood which comprises the steps of: a) boiling a gumwood log gradually in a solution containing a surface active agent and a latex solvent of suitable alkalinity, b) cutting the log treated with the foregoing boiling process to produce veneer sheets for plywood, c) heating the veneer sheets in a solution containing the latex solvent in an autoclave while applying pressure and, then, reducing the pressure, heating the veneer sheets in a solution containing an insecticide while applying the pressure, and d) drying the veneer sheets, stacking several veneer sheets and, by means of the adhesives and pressing the boards while applying the heat.

forming plywood.

2. A process for manufacture of gumwood plywood as claimed in claim 1 wherein the surface active agent is either anionic dodecylbenzene sulphonic acid sodium or alkylnaphthalene sulfonic acid sodium.

3. A process for manufacture of gumwood plywood as claimed in claim 1 wherein the latex solvent is either triethylamine, ethylbutylamine, diethylaminoethanol or soda ash with suitable alkalinity.

4. A process for manufacture of gumwood plywood as claimed in claim 1 wherein the insecticide is either borate soda, borax or boric acid.

5. A process as claimed in claim 1 and substantially as described in the specific example hereinbefore set forth.