Process for upgrading low-quality wood

Description

[0001] The invention relates to a process for upgrading low-quality wood to high-quality wood in an environmentally sound way, and to high-quality wood obtained by means of this process. Such a process is known from EP 0373726.

[0002] According to this document a cellulosic fibrous aggregate is formed from a cellulosic fibrous material by a process which comprises: a softening stage comprising exposing a section of cellulosic fibrous material to the action of an aqueous softening agent at a temperature in the range of from 150 °C to 220 °C at a pressure of at least the equilibrium vapour pressure of the softening agent at the operating temperature, thereby at least partially disproportionating and hydrolysing the hemicellulose and lignin present in the cellulosic fibrous material; and a curing stage comprising drying the product of the softening stage at a temperature in the range of from 100 °C to 220 °C to yield a cross-linked cellulosic matrix.

[0003] This process uses traditional ways of heating and drying the wood. These methods rely on thermal conduction to raise the temperature of the wood and evaporate water contained therein. The poor thermal conductivity of wood and the sensitivity of the process chemistry to extended heating times, result in limitations on product thickness and quality for such process. Furthermore, it has been found that gradients in temperature, pressure and moisture concentration induce stresses in wood, which may result in the formation of cracks and consequent loss of mechanical strength. Hence it can be concluded that there is need for a process for upgrading low-quality wood which allows the processing of sizable sections of low-quality wood.

[0004] Surprisingly it has now been found that relatively large sections of low-quality wood can be upgraded to arrive at a final product which has good mechanical performance properties in addition to being moisture resistant, in a process wherein the aqueous medium, in the presence of which the softening of the low-quality wood is conducted, includes an aqueous pH buffer.

[0005] Accordingly the invention relates to a process for upgrading low-quality wood to high-quality wood, comprising:

a) a softening stage, wherein one or more sections of low-quality wood are heated to a temperature in the range of from 160 to 240 °C in the presence of an aqueous medium and at a pressure which is at least the equilibrium vapour pressure of said aqueous medium at the operating temperature, thereby at least partially hydrolysing the hemicellulose and disproportionating the lignin present in said wood,

b) a dewatering stage, and

c) a curing state, characterized in that the softening stage is conducted in the presence of a pH buffer, having a pH in the range of from 3.5 to 8.

[0006] The pH of the buffer is preferably in the range of from 4.0 to 6.5, and more preferably from 4.5 to 6.

[0007] The buffering agent preferably comprises a mixture of an acid or base and a salt, more preferably a mixture of an organic acid and a salt of an organic acid. The organic acid is suitably an optionally substituted alkanoic acid, especially an alkanoic acid containing up to 6 carbon atoms, or benzoic acid. The acid is preferably a mono-, di- or tri- carboxilic acid, substituted by up to three hydroxy groups, especially one hydroxy group as the sole substituent, or is not substituted at all. The buffering agent is especially a mixture of acetic acid and an ammonium or alkali metal salt thereof, or a mixture of citric acid and an ammonium or an alkali metal salt of citric acid. The alkali metal component of said salt is preferably sodium or potassium. Earth alkaline metal salts such as magnesium and/or calcium salts may also be used. Conveniently the buffering agent is dissolved in the aqueous medium wherein the concentration of the buffering agent is suitably in the range of from 0.01 to 2 mol/litre and preferably from 0.05 to 1.0 mol/litre. The concentration of buffer is considered to be the concentration of the acid (or base), together with the concentration of the salt.

[0008] Conveniently the aqueous medium is water. Advantageously air-dry low quality wood is impregnated with the aqueous medium containing the buffer in advance of the softening. Preferably the impregnation is carried out by immersion of the air-dry sections in the aqueous buffer solution at a temperature < 100 °C. Generally the air-dry wood has a moisture content in the range of from 5 to 25 %w, and preferably from 10 to 22.5 %w (i.e. 10 to 22.5 parts of moisture and 90 to 77.5 parts of solids). After impregnation the wood suitably contains more than 40 %w of moisture, preferably more than 45 %w and more preferably in the range of from 47 to 50 %w of moisture.

[0009] As described hereinbefore the buffering agent preferably comprises a mixture of an organic acid and salt of an organic acid. As an alternative to having both the acid and the salt component of the buffering agent available when starting the softening stage, it is also possible to start off with only the salt component, e.g. the ammonium or alkali metal salt of the organic acid, and using for the acid component the acid which is generated in situ during the hydrolysis of the hemicellulose, i.e. acetic acid. It will be appreciated that the buffering agent will be complete as soon as some of the generated acetic has dissolved in the aqueous medium containing the salt component of the buffering agent. Moreover it will also be appreciated that the initial pH of the aqueous medium will be governed by the concentration and nature of said salt component.

[0010] The softening of the low-quality wood is preferably conducted at a temperature in the range of from 170 to 220 °C, and at a pressure which higher than the equilibrium vapour pressure of the aqueous medium at the operating temperature.

[0011] In view of the aqueous nature of the medium in the presence of which the softening is conducted, steam is a preferred source of heat for use in the process of the present invention. Under these conditions it is preferred that the actual heating is effected by allowing steam to condense on the surface of the sections of the low-quality wood. Hence, at least initially, there will be a significant difference in temperature between the outside and the inner parts of a section. It will be appreciated that as a result of said difference in temperature there will also be a difference in the rate of the hydrolysis and disproportionation reactions between the inner and outer parts of the sections. In order to obtain a softened lignocellulosic material which has good overall properties it is preferred that the ultimate temperature difference between the outside and centre of a section is not more than 20 °C and preferably not more than 10 °C. The time required, to achieve such a temperature equilibrium will be largely determined by the dimensions of the sections i.e. the shortest distance to the centre of a section, over which the heat has to be transferred. For regularly shaped sections, e.g. sections having a circular or rectangular cross-section, said distance will correspond with 50% of the diameter or 50% of the shortest side of said cross-section respectively.

[0012] Without wishing to be bound to any theory it is supposed that during the hydrolysis of the hemicellulose and the disproportionation of the lignin, acidic compounds are formed in addition to a number of desirable compounds such as aldehydes and phenols, which acidic compounds may cause the catalysed degradation of cellulosic fibres. The formation of these acidic compounds did not create any problems when sections of relatively small dimensions were employed, as the time required to achieve the hereinbefore mentioned temperature equilibrium, was apparently short enough to prevent the cellulosic fibre degradation. However, when considerably larger size sections were employed, the time required to achieve said temperature equilibrium was much longer, and consequently said cellulosic fibre degradation could not be avoided, thus resulting in fibrous composites having poor mechanical performance properties. With the process of the present invention which is conducted in the presence of said pH buffer, any acidic compound formed within the lignocellulosic material, i.e. the low-quality wood will be neutralized, thereby preventing the acid catalysed degradation of the cellulosic fibres. Consequently the residence time of the low-quality wood sections at the softening temperature will be less critical and in general may be up to 2 hours longer compared to the situation in the absence of a pH buffer, while with very high concentrations of said buffer, it is anticipated that residence times far in excess of the hereinbefore mentioned 2 hours are also permissible.

[0013] Hence it will be appreciated that a process which allows considerably longer residence times at the softening temperature, offers the possibility to upgrade sections of low-quality wood which are considerably larger than would be possible in a similar process in the absence of a pH buffering system.

[0014] The low-quality wood which may be employed as starting material in the process of the present invention will generally comprise sections of lightwood, i.e. materials characterised by a low density, relatively poor mechanical performance properties and poor moisture resistance. The use of said lightwood starting material in the present process will result in a product showing significantly improved mechanical performance properties and moisture resistance. Examples of trees yielding such lightwood include spruce, poplar, willow beech pine and eucalyptus, i.e. trees which generally have a high growth rate. Sections of heavywood may suitably also be upgraded in the present process, however, with these materials the most important improvement will be found in the moisture resistance.

[0015] It will be appreciated that the maximum size of the smallest dimension of the sections which can be successfully upgraded in the process of the present invention, will be also dependent on the nature of the lignocellulosic material to be used, as it can be expected that the heat transfer through a low density wood from surface to centre will require less time than would be the case for a section of similar dimensions having a higher density. Hence the smallest dimension of a lightwood section for use in the present process may be considerably larger than for one based on heavywood. The starting materials are preferably trunks or boards of wood, i.e. elongated sections of wood. The minimum length is preferably at 25 cm, more preferably at least 50 cm, still more preferably least 1 m. Usually wood sections of at least 1.5 m will be used, up to 4, or even 6 meters long. The starting material usually will have a width and thickness of at least 2 x 2 cm (especially when heavy wood is used), preferably 3 x 3 cm, more preferably 4 x 4 cm. Preferably the length of the different pieces of starting material are the same or almost the same. The starting material may have a square or rectangular diameter, but also a circular, hemicircular or even irregular diameter is possible.

[0016] As the present process is eminently suited to be conducted on a larger scale, it can advantageously be used for industrial purposes. Hence it will be appreciated that a constant quality of the ultimate composite will be a primary requirement. Consequently it is preferred in the present process to employ not only sections based on the same type and source of lignocellulosic material but moreover also having the same shape and size.

[0017] Upon completion of the softening stage the reactor contents are cooled to temperature below 100 °C before the reactor is opened. Subsequently the softened material is submitted to a dewatering treatment to remove most of the aqueous medium, if not all. Dewatering may be effected, for example, by the application of pressure to the material by means of rollers and/or a press, by vacuum evaporative drying techniques or via chemical means, e.g. by contacting with a suitable adsorbent or absorbent. In such a dewatering stage it is preferred that the temperature should not exceed 100 °C and preferably not exceed 80 °C, in order to prevent premature cure or cross-linking occurring in the softened material. More preferably the dewatering stage is conducted after having cooled the softened material to a temperature below 10 °C. Under these conditions the reactive compounds formed during the hydrolysis of the hemicellulose and/or disproportionation of the lignin have a low solubility or are insoluble in the aqueous medium. This will thus reduce the loss of said reactive compounds during the dewatering stages, which compounds play a vital part in the subsequent curing stage.

[0018] It is a particularly advantageous feature of this invention that the product of the softening stage and the dewatering stage is a soft material capable of being easily moulded. Accordingly, a most convenient method of effecting the process of the invention is to cure the material being processed in a heated mould. This enables the aggregate product to be formed in any desired shape. Sufficient pressure is applied during curing in the mould to achieve a product of the required density and shape, such pressures typically ranging from 1 bar to 50 bar, often pressures in the range of from 3 to 20 bar being sufficient for most purposes. Curing is effected at a temperature in the range of from 100 °C to 220 °C, typically from 140 °C to 200 °C.

[0019] The duration of the curing stage will vary according to the material being cured and the prevailing temperature. Complete curing will require a residence time of from 10 minutes to, in some cases, up to 10 hours. In most cases a cure time in the range of from 1 to 3 hours will be sufficient to obtain a high-quality wood material.

[0020] Any aqueous medium present in the softened lignocellulosic material after the dewatering stage will almost certainly be removed via evaporation during the subsequent curing stage.

[0021] In the context of the present invention the term "mould", wherein the dewatered softened wood is to be cured, should be interpreted to also include a platen press equipped with spacers and further auxiliary equipment, wherein regularly shaped, softened sections are placed next to one another for curing. Should the dimensions of the ultimate desired composite be such that it can not be directly obtained from a single softened section, this can be remedied by employing a mould having the required dimensions and introducing therein a sufficient number of softened sections and cure them together to provide the desired composite.

[0022] Whenever possible it is advantageous to conduct one or more and preferably each stage substantial in the absence of oxygen, especially those stages which are conducted at elevated temperature. It has been found that the presence of oxygen can have a negative influence on one or more of the properties of the ultimate composite. An obvious way to achieve an oxygen-free environment is to avoid the introduction of air together with the sections of wood to be softened. This may conveniently be achieved by immersing the starting material in water, preferably at elevated temperature, especially up to 100 °C, before treatment. This has the dual effect of expelling any air trapped in the starting material and ensuring the material has the required moisture content for the softening stage, as discussed hereinbefore.

[0023] In addition to having considerably improved mechanical properties and moisture resistance, the sections of high-quality wood prepared according to the process of the present invention have maintained the typical wood-appearance characteristics of the starting material, i.e. the presence of a grain. The presence of said grain in the ultimate composites confirms that the elongate cellulosic structure of the starting material has been maintained, and allows the obtained composites to be worked by the same techniques as untreated wood, e.g. sawing and planing.

[0024] The invention will be further illustrated with the following example which should however not be construed to be a restriction of the present invention.

Example

[0025] 6 Sections of air-dry sawn poplar having the following dimensions: length 2 m, width 20 cm and thickness 5 cm, were soaked overnight in an aqueous solution containing 0,6 g or 6 g sodium acetate / litre water respectively, and having a temperature of 95 °C. Subsequently the soaked wooden sections were heated in a closed vessel by means of steam of 200 °C condensing on the surface of the sections, until the temperature in the centre of the sections was 185 °C while the temperature at the outside was 200 °C, which temperatures were reached in 1.5 hours. Subsequently the contents of the vessel were cooled to 10 °C before opening the vessel, whereupon the softened sections were transferred to a press and compressed for 5 minutes during which the pressure was gradually increased from 1 to 3 bar, to stimulate the removal of the aqueous phase.

[0026] The dewatered and softened sections were placed next to one another in a platen press, having a temperature of 195 °C, of which both plates were provided with a dewatering screen. The outside sections were supported with a piece of untreated light wood having a somewhat higher thickness than the softened sections, to prevent excessive deformation during the subsequent compression. Finally two stainless steel spacers having thickness of 3 cm were placed on the lower plate which thickness corresponded with the ultimate thickness of the desired composites (planks). The press was closed for which a pressure of 5 bar was required, and the samples were held at 195 °C for 1.5 hours. Subsequently the material was allowed to cool to ambient temperature before being evaluated. The evaluation results have been collected in Table 1 hereinafter.

Comparative experiment

[0027] The procedure as described in the Example was repeated with the exception that the buffering agent was omitted.

[0028] The evaluation results of these samples have been included in Table 1.

Table 1

Na acetate g/l	6	0.6	-
Initial pH	7.9	7	6.6
Ultimate pH	5	4	3
Density, g/cm³ (ρ)	0.7	0.7	0.7
Shore D hardness units	70	65	30
Bending strength, MPa (σ)	125	115	60
Specific bending strength (σ/ρ)	175	165	85

From the data collected in Table 1 it can be observed that the mechanical performance properties of the composites prepared according to the process of the present invention are far superior to those prepared according to a process outside the scope of the present invention.

Claims

1. A process for upgrading low-quality wood to high-quality wood, comprising:

a) a softening stage, wherein one or more sections of low-quality wood are heated to a temperature in the range of from 160 to 240 °C in the presence of an aqueous medium and at a pressure which is at least the equilibrium vapour pressure of said aqueous medium at the operating temperature, thereby at least partially hydrolysing the hemicellulose and disproportionating the lignin present in said wood,

b) a dewatering stage, and

c) a curing stage, characterized in that the softening stage is conducted in the presence of a pH buffer, having a pH in the range from 3.5 to 8.

2. A process as claimed in claim 1, wherein the pH of the buffer is in the range of from 4 to 6.5.

3. A process as claimed in claim 1 or 2, wherein the buffering agent is a mixture of an organic acid and a salt of an organic acid.

4. A process as claimed in claim 3, wherein the buffering agent is a mixture of acetic acid and an ammonium or alkali metal salt thereof, or a mixture of citric acid and an ammonium or alkali metal salt thereof.

5. A process as claimed in any one of claims 1 to 4, wherein the concentration of the buffer is in the range of from 0.01 to 2 mol/litre.

6. A process as claimed in any one claims 1 to 5, wherein one or more sections of air-dry low-quality wood are impregnated with the buffering agent-containing aqueous medium.

7. A process as claimed in any one of claims 3 to 6, wherein the acid component of the buffering agent is generated in situ during the softening stage, and the salt component is added to the aqueous medium in advance of said softening stage.

8. A process as claimed in any one of claims 1 to 7, wherein the softening is conducted at a temperature in the range of from 170 to 220 °C, and the curing is conducted at a temperature in the range of from 140 to 200 °C.

9. A process as claimed in any one of claims 1 to 8, wherein the the ultimate temperature difference between the centre and the outside of the sections in the softening stage is not more than 20 °C and preferably not more than 10 °C.

10. A process as claimed in any one of claims 1 to 9, wherein the softened sections are cooled to a temperature below 100 °C before being removed from the reactor.