The invention relates to a method for manufacturing a strand woven bamboo product, comprising the processing steps of: providing bamboo strips, roughening the surface of the bamboo strips, at least partially glueing the roughened bamboo strips, and pressing the roughened and glued bamboo strips in the desired form to the strand woven bamboo product in a mould. The invention further relates to a strand woven bamboo product obtainable according to the method of the invention.

Bamboo is a versatile material which finds application in, among other areas, the manufacture of furniture, floors and walls. The great advantage is that bamboo is a fast- growing plant which can be used in many cases as wood-substitute material and can be a substitute particularly for slow-growing wood types. Bamboo hereby has an environmentally-friendly image. Although bamboo is already applied on large scale for indoor applications, there are various practical problems and drawbacks relative to traditional wood types in the case of use in an outdoor environment. The drawbacks relative to traditional wood types are, among others, in the field of stability of the product and durability. More specifically, weathering occurs more rapidly in bamboo under natural influences, particularly weather influences (water, sunlight) and attack by rot, fungi and other micro-organisms. These influences have the result that in the course of time the bamboo discolours, rots, becomes mouldy and has a reduced mechanical strength. Owing to these properties bamboo is seen as an unsuitable material for outdoor applications. Because of the environmental advantages of bamboo there is great interest in improving the properties of bamboo products so that the natural materials preferred for outdoor applications, such as tropical hardwood, can be replaced by bamboo.

A bamboo product with greatly improved properties is the strand woven bamboo product. The known method for manufacturing a strand woven bamboo product comprises the following processing steps of: - providing dried bamboo strips,

- roughening the surface of the bamboo strips (so-called crushing, wherein the surface area of the strips is enlarged),

- at least partially glueing the roughened bamboo strips, and - pressing the roughened and glued bamboo strips in the desired form into the strand woven bamboo product in a mould. The pressing can be performed at relatively low temperatures as well as at relatively high temperatures in order to accelerate curing of the glue, subject to the types of glue used.

The obtained strand woven bamboo product has an increased material density and improved mechanical properties compared to unprocessed bamboo. The strand woven bamboo products can be used for instance as indoor application for floors and other high-quality applications. The strand woven bamboo product known from the prior art is however less suitable for outdoor applications, since it is susceptible to, among other things, cracking and the development of surface fungi.

It is an object of the invention to provide a strand woven bamboo product which is better suited to outdoor applications than the known strand woven bamboo products.

The invention provides for this purpose a method for manufacturing a strand woven bamboo product of the type stated in the preamble, comprising at least one thermal preservation treatment, wherein bamboo is heated in a low-oxygen or oxygen-free atmosphere. Such a preservation treatment creates a product which has the favourable properties of a known strand woven bamboo product, i.e. an increased density and hardness, in combination with an improved durability due to an improved resistance to fungi and rot. In addition, the bamboo discolours to a colour which is darker right through, which can be aesthetically attractive. A thermal preservation treatment is a treatment wherein the bamboo is heated for a predetermined time. The oxygen content of the atmosphere must be relatively low to prevent combustion of the bamboo. This can be done for instance by displacing oxygen with an inert gas, such as nitrogen gas, by heating under vacuum or using steam. The temperature is preferably at least 120 ⁰ C. The treatment time depends on the temperature used and the method used. When steam is used to displace oxygen and when bamboo strips are being treated and at a temperature rising in stages to about 200 ⁰ C, the overall treatment time will generally amount for instance to between three and four hours. The thermal preservation treatment can for instance be performed on the stem from which the bamboo strips are manufactured, on the manufactured bamboo strips or on the strand woven bamboo product known from the prior art. An oxygen-free atmosphere is understood to mean an atmosphere in which (substantially) no oxygen is present. A low-oxygen atmosphere is understood to mean an atmosphere with an oxygen content of a maximum of 50%, preferably lower than 25% and more preferably lower than 10%. It is possible here to envisage applying preferably dried atmospheric air with an oxygen percentage of about 20%.

Thermal treatment of the normal, non-strand woven bamboo is per se known: it is not however known to apply a thermal preservation treatment in a strand woven bamboo product. Applying a thermal preservation treatment to a normal, non-strand woven piece of bamboo gives a product which has an improved stability, a good resistance to rot and mould, and a relatively good resistance to cracking under weather influences. In contrast to the strand woven bamboo product given a thermal preservation treatment according to the invention, a normal piece of bamboo which has been thermally treated has a relatively low density, is relatively soft and moreover has a brittle structure. Non- strand woven thermally treated bamboo is hereby suitable to only limited extent for outdoor applications.

The thermal preservation treatment should not be confused with a so-called carbonization. In carbonization bamboo is heated by steam under increased pressure to a temperature (significantly) lower than 120 ⁰ C, with the object of giving the bamboo material a darker colour. Carbonization does not generally take place in a low-oxygen environment. Such a treatment has no or relatively little effect however on durability (resistance to rot and fungi) and crack resistance of bamboo, this in contrast to the thermal treatment wherein the bamboo is heated in a low-oxygen atmosphere to a temperature of preferably higher than 12O ⁰ C. Carbonization is therefore not a preservation treatment.

The bamboo strips preferably undergo at least one thermal preservation treatment prior to glueing. This creates a product which, in addition to having improved mechanical properties, in particular an improved hardness, is also better able to withstand the action of micro-organisms and has a better stability. A thermal preservation treatment is understood to mean a temporary heating for a sufficiently long period of time to change the molecular structure of the bamboo material such that it is less easily attacked by micro-organisms. In another preferred treatment at least one thermal preservation treatment is performed at a temperature of at least 120 ⁰ C after pressing. The thermal preservation treatment is performed here on the already pressed strand woven product. It is important that the moisture content of the (already) pressed strand woven product is relatively low; preferably less than 4% w/w. This in order to prevent the possible occurrence of excessive changes in dimension due to drying which takes place during an initial phase of the thermal preservation treatment. When the strand woven product is heated at such a temperature for a determined time, the molecular structure of the bamboo is changed such that a better resistance to fungi and a greater stability of the product is realized. It is advisable to hold the already pressed strand woven product under mechanical pressure without the whole surface area of the already pressed strand woven product being covered. By not covering the whole surface area moisture which is possibly still present can still easily leave/escape from the product. The mechanical pressure will serve to limit deformation which can occur.

The sequence of the processing steps is important for the quality of the obtained end product. As described above, it is possible to envisage already performing the thermal preservation treatment on the stem from which the bamboo strips are manufactured. However, due to the heat treatment the stem becomes brittle and susceptible to cracking, whereby losses during manufacture of the bamboo strips are relatively large. It is also possible to envisage performing the thermal preservation treatment on the strand woven end product, such as finished, profiled terrace parts. Owing to the heat treatment however, moisture will always be extracted from the material during the treatment, whereby shrink and therefore deformation of the material occurs, and the end product will no longer meet its size specifications.

It is important that the bamboo strips used are sufficiently dried prior to the further processing. After the treatment steps, and particularly after the pressing, it is important to condition the products or half-products under controlled conditions for a predetermined time at a predetermined degree of air humidity and temperature.

The thermal preservation treatment preferably comprises of heating the bamboo strips for a predetermined time at a temperature between 120°C-300°C. At such a temperature a good preservation is realized within a relatively short time, wherein the strips largely retain their mechanical strength. It is particularly important at such a temperature that the heating is carried out in low-oxygen or oxygen-free manner. The thermal preservation treatment does have the result that the bamboo strips become slightly more brittle, particularly in the case of long treatment times and/or high temperatures. The brittleness of the strips is otherwise compensated in the end product by the compacting of the material during compression. The thermal preservation treatment is preferably performed at between 150-250 ⁰ C. At such temperatures the treatment time is relatively short, wherein the process takes place under optimal control.

In a preferred embodiment at least one thermal preservation treatment is performed prior to roughening of the bamboo strips. This has the advantage that roughening of the bamboo strips is found to be easier in the case of bamboo strips which have undergone a thermal preservation treatment compared to roughening of dried bamboo strips which have not yet undergone a thermal preservation treatment.

Conversely, it is advantageous that at least one thermal preservation treatment is performed after roughening of the bamboo strips. This is advantageous because, due to the enlarged surface area, a good preservation can be realized in a relatively short time compared to bamboo strips whose surface area has not yet been enlarged.

It is advantageous when the thermal preservation treatment comprises multiple steps, wherein each step comprises at least one period in which the temperature is increased, and at least one cooling period. It is possible to envisage performing a thermal preservation treatment both before and after the roughening.

In a preferred embodiment the bamboo strips are glued with a water-resistant glue. The glue is preferably a water-resistant glue for outdoor applications. The obtained strand woven bamboo product thus acquires improved durability. Particularly suitable glues are glues based on phenol. Such a glue provides exceptionally good adhesion of roughened bamboo strips which have undergone a thermal preservation treatment.

The invention also provides a strand woven bamboo product obtainable in accordance with the method according to the invention, wherein the bamboo product is preferably evidently suitable for outdoor applications. Such a product has an increased density, improved mechanical properties and improved resistance to the action of surface fungi, as well as resistance to rot and moulding from inside (durability) as discussed above. The strand woven bamboo product according to the invention has a relatively good resistance to rot and moulding (durability) and to surface fungi. The strand woven bamboo product according to the invention also has a relatively good form-retention. The strand woven bamboo product according to the invention moreover has a relatively good resistance to cracking. The bamboo product can for instance be a construction element.

In a preferred embodiment the construction element is adapted to cover floors, walls and/or ceilings, in particular for outdoor applications. Such a construction element can for instance be a panel, or a floor part or terrace part, optionally profiled.

In another preferred embodiment the construction element is a part for a piece of furniture. Particularly envisaged here are pieces of furniture wherein, in addition to the required mechanical strength, attack by weather influences and micro-organisms are also important, in particular garden furniture including tables and chairs.

The invention will now be elucidated on the basis of the following non-limitative examples.

Figures Ia-Ic show the starting material (bamboo strips), intermediate product and end product of the method according to the invention.

Example 1: Thermal preservation treatment at strip level (Preference 1)

The advantageous properties of bamboo products obtained with the method according to the invention are shown in the following comparative test. Prior art strand woven bamboo products are here compared to a bamboo product given a thermal preservation treatment according to the invention. The treatment steps used are as far as possible the same.

The strand woven bamboo product manufactured according to the prior art is obtained according to the following steps of: 1. Providing bamboo strips, to be obtained from a bamboo stem;

2. Drying the bamboo strips to a stable moisture content;

3. Roughening the strips, preferably in mechanical manner, wherein the surface area is enlarged; 4. Applying glue to the bamboo strips, preferably by immersion in a glue bath with a phenol-based glue suitable for outdoor applications;

5. Drying the glued bamboo strips;

6. Cold or hot pressing of the glued bamboo strips to a desired form such as a beam or plate with an increased density; 7. Conditioning the obtained strand woven product.

In the invention a thermal preservation treatment is performed during the process, wherein the strips are heated in a low-oxygen/oxygen-free environment, for instance using steam, at a temperature higher than 120 ⁰ C, preferably between 150-250 ⁰ C. This thermal preservation treatment is preferably performed after drying of the strips and before the glueing (step 4). This step can be taken before as well as after step 3, though preferably after enlarging the surface area for the most efficient possible method. If the thermal preservation treatment is already performed on the bamboo stem, it can become too brittle and this make further processing into bamboo strips more difficult. Thermal preservation treatment of the strand woven bamboo end product increases the danger of deformation and cracking.

The obtained products are tested for stability, density, hardness, brittleness, resistance to surface fungi, durability and cracking resistance. Stability is the stability of the form, particularly under changing weather influences such as air humidity and temperature changes. The stability (form-retention) is preferably tested in accordance with European standard EN 1910.

Density is the density (kg/m ³ ) of the finally obtained bamboo product. Hardness is the hardness (kg/mm ² ) of the surface of the material, and is preferably measured in accordance with EN 1534. Brittleness is the extent to which the obtained material breaks under the influence of external forces. A brittle material breaks suddenly at a determined load without preliminary plastic deformation.

The resistance to surface fungi is the degree to which fungi develop on the surface of the product after a certain period of exposure to air and moisture. A test herefor is described in EN 152.

The durability class is a measure of the extent to which the material can withstand biological attack, particularly by rot and mould. This is preferably determined in accordance with standard EN 350-1 combined with EN 113. The class in which the material belongs is determined here subject to the amount of weight decrease.

The resistance to cracking is the extent to which cracks occur in the material after prolonged exposure to weather influences, and is determined qualitatively. Results are summarized in Table I and briefly discussed.

Table I: Advantages and disadvantages of preservation methods in relation to normal bamboo* for outdoor applications

strongly negative effect negative effect

+/- no substantial positive or negative effect + positive effect

-H- strongly positive effect

Phyllostachys pubescens as basic material in sliced form (strips)

Appearance

The test is performed by placing the products close to each other for several months in an unprotected environment, wherein they are equally exposed to weather influences such as sun, wind and rain. During this period the appearance of the products is recorded optically by means of photography; optical differences are qualitatively assessed by a skilled person.

As a result of the thermal preservation treatment the strand woven bamboo end product acquires a deep brown colour right through which is substantially darker than the product obtained according to the prior art. The surface of the products manufactured according to the invention are also found to display less surface crack formation. The strand woven bamboo end product given the thermal preservation treatment (without further finishing such as varnishing) displays a more rapid greying compared to the prior art strand woven bamboo product if placed in an outdoor environment. If desired, this can be countered through the use of a suitable varnish or other treatment product.

This test also shows that the thermally treated strand woven bamboo products manufactured according to the invention can better withstand surface fungi, which is qualitatively visible and can be confirmed by microbiological tests in accordance with EN- 152.

In addition, the stability is tested in accordance with EN- 1910. The width/thickness/length of conditioned samples are measured at relative air humidity of 30-85%. The strand woven bamboo end product given the thermal preservation treatment according to the invention displays a better stability/form-retention than the prior art strand woven bamboo end product.

A bamboo strip 1 is shown by way of illustration in figure Ia. Such bamboo strips generally have a length varying between 1800-2500 mm, width between 10-25 mm and thickness between 1-6 mm.

Figure Ib shows a roughened bamboo strip 2 wherein a part of the surface is enlarged by roughening, so-called 'crushing'. A suitable water-resistant glue adheres better to the roughened part 3 of the bamboo strip than to a non-roughened surface 4. In the final product the whole surface of bamboo strip 2 is preferably roughened. These bamboo strips 1 , 2 are thermally treated before or after roughening, and subsequently glued.

In figure Ic a plurality of thermally treated, roughened bamboo strips 5 are glued and compressed to form a strand woven bamboo product 6 which has been given a thermal preservation treatment. The thermally preserved strand woven bamboo product 4 forms a substantially homogenous material in which the individual thermally preserved bamboo strips 3 can be identified by a skilled person: only a few individual bamboo strips are shown in figure for the sake of clarity. The thermally preserved strand woven bamboo product 4 has the favourable density and hardness of a normal strand woven bamboo product which has not been given a thermal preservation treatment, and the favourable stability (form-retention), resistance to rot and fungi and resistance to cracking of normal, non-compacted bamboo which has been undergone a dry thermal preservation treatment.

Example 2: Thermal preservation treatment of already pressed strand woven product (Preference 2)

The following method is performed making use of standard bamboo strips: 1. Providing bamboo strips, to be obtained from a bamboo stem in accordance with a known prior art method;

2. Drying the bamboo strips to a stable moisture content;

3. Roughening the dried strips, preferably in mechanical manner, wherein the surface area is enlarged; 4. Applying glue to the roughened bamboo strips, preferably by immersion in a glue bath with a phenol-based glue suitable for outdoor applications;

5. Drying the glued bamboo strips in order to cure the glue and to obtain a stable moisture content;

6. Hot pressing of the glued bamboo strips in a pre-shaped mould to a desired form such as a beam or plate with an increased density;

7. Conditioning the obtained strand woven product in an environment with constant temperature and moisture content;

8. Thermal preservation treatment at high temperature, wherein the parts are held under mechanical pressure.

Once the bamboo strip has been pressed to a strand woven product of the desired form, the parts are given an additional thermal preservation treatment at a temperature of preferably between about 150-250°. By performing the thermal treatment under mechanical pressure deformation resulting from the heat treatment is countered. The obtained result gives a product with an increased density, high form-retention and a greatly improved resistance to surface fungi and rot.

Test results are summarized in Table II. Table II is largely similar to Table I ₅ wherein small differences occur in density, hardness and brittleness. The material properties obtained according to the invention are always amply sufficient for most applications in the outdoor situation, such as in furniture, fencing and ground covering.

Table II: Advantages and disadvantages of preservation methods in relation to normal bamboo* for outdoor applications

strongly negative effect negative effect

+/- no substantial positive or negative effect + positive effect -H- strongly positive effect

* Phyilostachys pubescens as the basic material in sliced form (strips)

It will be apparent that the invention is not limited to the exemplary embodiments shown and described here, but that within the scope of the appended claims numerous variants are possible which will be self-evident to the skilled person in this field.