EP4271879A1

EP4271879A1 - Method for producing products based on wood as a raw material

Info

Publication number: EP4271879A1
Application number: EP21831039.9A
Authority: EP
Inventors: Anton Friedl; Maximilian LEHR; Martin MILTNER
Original assignee: MM Board and Paper GmbH
Current assignee: MM Board and Paper GmbH
Priority date: 2020-12-29
Filing date: 2021-12-15
Publication date: 2023-11-08
Also published as: JP2024501377A; US20240076831A1; EP4023812A1; CN116897233A; EP4271880A1; CA3203411A1; WO2022144176A1; WO2022144377A1

Abstract

The invention relates to a method for producing products based on wood as a raw material, characterised in that wood in the form of wood particles is subjected to an extraction treatment using an extractant comprising one or more organic solvents or an organic aqueous mixture of the solvent or solvents with water, wherein the content of fatty acids in the wood particles is reduced by the extraction treatment of the wood particles using the solvent by at least 70% measured as hexanal content in wt.% after accelerated aging for 72 hours at 90°C, but the content of cellulose, hemicelluloses and lignin is kept substantially constant during this extraction treatment.

Description

Process for the manufacture of products based on wood as a raw material

The invention relates to methods for the manufacture of wood-based products, in particular methods for the pre-treatment of wood.

Wood as a raw material for industrial processing into wood-based products (such as wood fiber boards or cardboard) contains - in addition to the main components cellulose, hemicelluloses and lignin - many different low and high molecular weight substances such as fatty acids, resin acids, phenols and terpenes. These substances are summarized as so-called extractive substances (or extractives) because they can be extracted from wood with hot water and/or organic solvents (Koch, Raw Material for Pulp; in: Sixta (ed) Handbook of Pulp (2006) , Weinheim : Wiley-VCH Verlag GmbH & Co. KGaA, p 21-68) . Many of these extractive substances are "organoleptically relevant substances" since they can lead to odor and taste interactions and impairments in wood particle-based products with the environment relevant to the respective end use (e.g. food in the case of packaging cardboard or room air in the case of wood fiber boards). Extract substances, which naturally have characteristic odors as volatile hydrocarbons (e.g. terpenes), are primarily responsible for aldehydes (especially hexanal), which are formed by autocatalytic oxidation of fatty acids naturally occurring in wood (Schreiner et al., Resolving the smell of wood-identification of odor-active compounds in Scots pine (Pinus sylvestris L.) (2018) , Scientific Reports 8:8294).

Furthermore, wood particles contain resins that can coagulate into sticky particles during processing of the wood particles, which is intensified by the fats and waxes that also occur naturally in the wood. These resinous particles, also known as "pitch" or "stickles", also cause disruptive effects on the paper/cardboard surface, which can subsequently lead to problems in further processing of the paper/cardboard (e.g. printing). These substances can also have a disruptive effect on the production process by causing deposits on machine parts, rollers and clothing parts, etc. (Sixta et al., Chemical Pulping Processes; in: Sixta (ed) Handbook of Pulp (2006), Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA, p 109-509; Björklund Jansson et al., Wood Extractives; in: Ek et al.(eds) Wood Chemistry and Wood Biotechnology, 1 (2009) , Walter de Gruyter GmbH & Co. KG, , Berlin, pp 147-171) .

At present, fatty acids are not removed from wood particles for wood particle-based products such as cardboard or fiberboard. Instead, the auto-oxidation of these fatty acids is inhibited or delayed by binding the heavy metal ions, which act as catalysts, in the wood particle products by adding complexing agents such as ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA). However, EDTA and its metal complexes are only poorly and slowly degradable in waste water treatment and are therefore considered ecologically questionable today, which entails increasing problems, especially for waste water from production plants for wood particle-based products. In other processes that have been described to improve the organoleptic properties, bleaching (e.g. DE 10 2004 050 278 A1), delignifying (e.g. DE 28 18 320 A1), oxidizing or reducing (e.g. DE 33 44 239 C2, WO 2006/ 039914 A1, DE 10 2006 020 612 A1) chemicals are used.

It is therefore an object of the present invention to reduce the content of releasable odor- and taste-active aldehydes in wood particle-based products such as cardboard or wood fiber boards by auto-oxidation of fatty acids naturally occurring in wood. This should be done without the use or addition of complexing agents.

Another object of the present invention is a significant improvement in the organoleptic properties of the wood particles and aged wood particles that are produced by this method, so that the products produced with it have little or no odor pollution even without the use or addition of bleaching, have delignifying, oxidizing or reducing chemicals. In particular, with the method according to the invention, after aging of the wood particles (up to 6 months), an undesirable odor and taste changes occurring in the course of this aging in the foodstuffs coming into contact with them are to be prevented.

Finally, a preferred object of the present invention is to remove other ingredients from wood particles that are undesirable in the planned product, for example resin acids.

Accordingly, the present invention relates to a method for producing products based on wood as a raw material, in which wood in the form of wood particles is subjected to an extraction treatment with an extraction agent which comprises one or more organic solvents or an organic-aqueous mixture of the solvent(s) with water, the content of fatty acids in the wood particles being reduced by at least 70%, measured as hexanal, by the extraction treatment of the wood particles with the extraction agent content in wt. -% after accelerated aging for 72 h at 90 °C, but the content of cellulose, hemicelluloses and lignin is essentially preserved in this extraction treatment.

As has been shown in the course of the present invention, the inventive extraction of the organoleptically questionable substances from the raw materials can significantly improve the organoleptic properties of the wood particles without adding complexing agents, but rather by removing potential aldehyde sources, above all Fatty acids can be achieved. However , it is known that fatty acids as a source of aldehydes and subsequently as a source of organoleptic impairment of the wood particles can only be quantified satisfactorily with great effort . According to the invention, the hexanal content of the wood particles after aging was therefore used to assess the success of the extraction. This has proven to be a reliable parameter for the organoleptic properties of the products to be made from the raw material, such as paper, cardboard, wood fiber boards, etc. , which correlates excellently with the (very time-consuming) traditional aging tests (see the investigations and proofs in the example section below).

It was found that the method according to the invention can be used to extract a large part of the fatty acids and other disruptive substances (resin acids, etc.) without as a result, the substance of fuel (cellulose, hemicelluloses and lignin) is significantly impaired. While any improvement in the removal of fatty acids from the wood raw material has benefits, a significant reduction of at least 70% for suitable large scale commercial use is readily possible with the present process. However, the treatment according to the invention is preferably selected such that the desired reduction is achieved in any case, and a reduction of at least 70%, preferably at least 90%, in particular at least 95%, is achieved. Accordingly, the conditions to be used according to the invention can then be selected, for example on the basis of the nature of the wood or the extractant, such that the hexanal reduction according to the invention results in any case. In this way, preferred maximum values in absolute contents of hexanal can also be set. Although wood particles with a hexanal content of 2 mg/kg dry matter (DM) can also be used for certain applications, absolute values below 1 mg/kg DM are preferred. In the case of the particularly strict guidelines that were applied in the course of the examples of the present invention, a hexanal content of 0.5 mg/kgTM hexanal (based on dry wood) was marked as an empirically determined value below which experience has shown that organoleptic impairments are not sensory are more noticeable. Accordingly, a preferred embodiment of the method according to the invention relates to extraction of the wood particles to a hexanal content of 0.5 mg/kg DM or below. For hexanal contents of the wood particles > 0.5 mg/kgTM it could be determined that the risk of an organoleptic impairment of the wood particles also increased with increasing hexanal content. For the purposes of the present invention, the hexanal content can be determined in case of doubt using headspace gas chromatography (HS-GC) by measuring approx. 0.2 g of air-dried wood particles (90-95 wt% (= wt %) Dry matter content (TSG)) are filled into a headspace vial. In these vials, the sealed wood particles must then be aged for six months at room temperature (approx. 20 °C) in order to oxidize the fatty acids to hexanal. Since this takes a lot of time and therefore does not allow a timely assessment of the extraction success, the hexanal content is determined according to the present invention via accelerated aging according to DIN ISO 5630-2. This was used to determine the hexanal content in the experiments in the example section (unless explicitly stated otherwise); this method is also relevant in case of doubt for the determination of the hexanal content for the purposes of the present invention. The wood particles are sealed in HS-GC vials, aged for 72 h at 90 °C and then the hexanal content is determined using HS-GC.

The wood particles are preferably less than 5 cm in size, preferably in the form of fibers, chips, strands, wood chips or mixtures thereof. This particle feature (as "particle size") can preferably be determined by sieve analysis using a square mesh sieve. The particle feature of the particle size is therefore formulated in terms of the mesh size, preferably 5 cm (other sizes (depending on practical necessity), e.g. 4, 3, 2 or 1 cm (or possibly also higher) can thus also be set easily.) The particles according to this size definition thus correspond, according to this preferred embodiment, essentially to the undersize of the sieve with mesh size as the particle feature. The wood particles are preferably in the form of fibers, chips, strands, chips or mixtures thereof.

According to a preferred embodiment, the wood particles are broken down into fibrous wood, in particular wood fibers with mean fiber lengths between 0.5 and 2 mm and mean fiber diameters between 10 and 50 μm, by mechanical and/or thermal and/or chemical digestion. Average fiber length and average fiber diameter relate to the average length, determined by optical measurement of the suspended fibers. This optical measurement usually leads to uniform results (regardless of the chosen methodology and analysis device), but the devices PulpEye (http://www.pulpeye.com/products/pulpeye/) and in particular MorFi Fiber have proven to be particularly suitable Analyzer (http://www.techpap.com/fiber-and- shive-anal yzer-morfi-neo, lab-device, 31.html), proven.

The amount of organic compounds that can be extracted using the method according to the invention varies depending on the type of wood and part of the tree from which the wood particles are obtained (heartwood/sapwood). For example, pine heartwood contains up to 9% by weight of extractable material, whereas fir sapwood usually only has up to 1% extractable material (Björklund Jansson et al., Wood Extractives; in: Ek et al. (eds) Wood Chemistry and Wood Biotechnology, 1 (2009) , Walter de Gruyter GmbH & Co. KG, , Berlin, pp 147-171 Nisula Wood Extractives in Conifers: A Study of Stemwood and Knots of Industrially Important Species (2018) , Abo Akademi University Press, Abo ) . In particular, types of wood such as pine, birch, lime or poplar contain higher amounts of fatty acid esters (birch 2315 mg/kg, lime 7544 mg/kg or pine 5807 mg/kg of fatty acids; DE 10 2009 046 127 A1), which can be broken down into aldehydes and organic acids by the manufacture of products from these woods without this being able to be prevented by changing technological parameters. In addition, there are larger fluctuations in the content of fatty acids and triglycerides depending on the location, tree age, height section of the individual tree and between heartwood and sapwood z.

DE 10 2009 046 127 A1 describes a method for producing wood fiber materials, in which volatile organic compounds ("VOC"), aliphatic and aromatic aldehydes, in particular hexanal and furfural, are reduced, with wood having at least one compound to adjust a neutral to basic pH value (such as NaOH) and at least one complexing agent (such as EDTA or DTPA) It is emphasized that no bleaching, delignifying, oxidizing or reducing chemicals are required for such production are used and no squeezing of the liquid formulation takes place before the wood or wood chips are plasticized. However, the use of complexing agents such as EDTA and DTPA, as mentioned above, entails a serious waste water and environmental problem. The present invention brings with it thus decisive advantages compared to a mere chemical-thermal-me, which is known per se Mechanical digestion (CTMP), wherein wood chips are mixed with sodium sulfite and EDTA, both in terms of environmental aspects and in terms of the inventive effect in the ef fi cient depletion of fatty acids from the raw material.

US Pat. No. 5,698,667 discloses a pretreatment of a lignocellulosic material by extraction with an organic solvent (e.g. acetone) to remove wood extractives such as volatile organic compounds (VOC) and higher molecular weight pitch components without significantly affecting the integrity of the lignocellulosic components of the material. However, no industrially usable depletion of the undesirable ingredients could be achieved (the maximum depletion of pitch and VOCs was 54.4 and 65% (after two extractions for acetone in water (80/20) and 100% acetone), although there The methodology used cannot ensure that volatile components pass into the gas phase when the solvent is removed from the extract and that the depletions determined by the evaporation residue of the extract are therefore to be set even lower.

In contrast, in the method according to the invention with the help of an extractive pretreatment of wood particles (such as wood fibers) with organic solvents or organic-aqueous mixtures of the solvent or solvents with water, a large part of the fatty acids naturally occurring in the wood are already removed before further processing (for example to cardboard or wood fiber boards) away. In addition, other extractives such as resin acids, phenols and terpenes in the mechanical pulp are greatly reduced by this pre-treatment. According to the invention, the hexanal content has proven itself as an indicator for the auto-oxidation of naturally occurring fatty acids in wood, especially when it is determined after accelerated aging of the wood particles (72 h at 90° C.). According to the invention, this hexanal content is reduced by at least 70%, based on the potential of the starting raw material (as well as--in absolute contents), preferably down to below 0.5 mg/kg DM. This allows With the present invention, odor and taste interactions of wood particle-based products with the environment relevant to the respective end use (for example food in the case of packaging cardboard or room air in the case of wood fiber boards) can be greatly reduced. At the same time, the pretreatment does not significantly change the composition of the wood particles, i.e. cellulose, hemicelluloses and lignin are not reduced to any significant extent (at least by no more than 10%, preferably by no more than 6%, in particular by no more than 4% reduced) extracted and/or degraded, this reduction preferably being determined as the extracted solid mass, based on the starting material, the wood particles. In addition to reducing the proportion of organoleptically relevant components in the wood particles, the present invention results in other important product and process advantages, especially for paper and cardboard production:

Pitch (stickles, resins, etc.) are removed from the process and can therefore not have a disruptive effect on the production process, so that there are no or reduced deposits on machine parts, rollers and clothing parts, etc. comes.

As a result, with the present invention, disruptive effects on the paper/cardboard surface can also be reduced, as a result of which problems in the further processing of the paper/cardboard (eg printing) are also significantly reduced. Due to the significantly lower proportion of these components on the surface and also inside the paper/cardboard, there is also increased product quality and, surprisingly, an increase in mechanical strength. Due to the improved product properties mentioned of products that are made from the wood particles extracted according to the invention, new areas of application can also be opened up that can only be covered today with paper/cardboard whose fiber components are cellulose and/or treated BCTMP (bleached chemi -thermo-mechanical pulp ) (e.g. high-quality packaging boxes ) or for particularly delicate applications that can only be served with cellulose today.

In addition, the extractive pre-treatment according to the present invention also greatly reduces the COD loads of the waste water from the process, as a result of which larger production capacities are made possible with the same COD load in the waste water.

According to the present invention, the release of odor- and taste-active aldehydes in wood particle-based products such as cardboard or wood fiber boards by auto-oxidation of fatty acids naturally occurring in the wood can also be achieved entirely without the addition of complexing agents. The addition of a complexing agent such as EDTA or DTPA in the manufacture of wood particle-based products to reduce the odor and taste of these products becomes superfluous, which eliminates the EDTA/DTPA contamination of the waste water and thus the (reason) water-related environmental impacts of the production of wood particle-based products are reduced, and the EDTA/DTPA contamination of the products themselves is avoided. Furthermore, in addition to fatty acids, other extractives are also significantly reduced and thus removed from the process, which reduces the extractive loading of the process water in the production of the wood particles and thus lower demands on the waste water cleaning/treatment Has. The reduced extractive load of the process water also has the advantage that problems in the manufacturing process of wood particle-based products can be reduced and the quality of the products can be increased (eg avoidance of pitch/stickies problems in cardboard production). This variant is therefore a particularly environmentally friendly embodiment of the present invention.

In practice, for a certain type of wood or for certain heating materials for an extraction of at least 70% of the fatty acids or for establishing a maximum absolute value of hexanal of less than 2 mg/kg dry matter, preferably less than 1 mg/kg dry matter, in particular less than 0.5 mg/kg dry matter (measured in each case as hexanal content in % by weight of the extracted wood particles after accelerated aging for 72 h at 90°C), required specific process conditions (such as size, shape and dry matter content of the wood particles, choice of extractant, water content, temperature, treatment time, pH, amount of extractant (in relation to the wood), pressure, number of extraction stages, variants of contacting between extractant and solid, mode of operation, etc.) can be determined using the teaching disclosed herein, in particular taking into account the results presented in the example section, directly and without any further inventive step w earth, especially considering the (usually known) amounts of organic substances (such as fatty acids, resin acids, phenols, terpenes, etc.) of the wood raw material, the nature of the wood (heartwood, sapwood, mixtures, etc.) and the requirements for the final product (cardboard, paper, fiberboard, application in specific areas (food, pharmaceuticals, animal feed, etc.) .

In particular, the present invention can be relative easily integrated into existing production systems or operated with existing systems.

One or more of the objects set out above can be achieved according to the invention by a method having the features of patent claim 1 . Advantageous configurations with expedient developments of the invention are specified in the respective dependent claims.

The choice of the liquid phase of the extract according to the invention (referred to here as the extractant) also depends on the relevant type of wood (and its natural content of extractives). However, the solvent or the solvent mixture in the extraction agent must also be selected in such a way that there is no significant loss of cellulose, hemicelluloses and lignin and the treatment time is nevertheless not excessively long. Mixtures of ethanol, acetone and water according to the invention with 0-95% by weight ethanol, preferably 50-90% by weight ethanol, and 0-99% by weight acetone, preferably 30-90% by weight -% acetone, exposed as organic-aqueous solvents in the extractant. Further organic solvents which can be used instead of (or for certain purposes also together with) ethanol or acetone with regard to an industrial scale in the extractant are, for example, methanol, n-propanol and isopropanol.

In the process according to the invention, preferred ratios of extractant to solid/dry substance are 5:1-25:1 (w/w), preferably 8:1-17:1 (w/w).

With the ratios and concentrations in extractant given herein, the following considerations apply: With 100% extractant is always the total amount of extractant present after extraction, namely the extractant, including the materials extracted from the wood particles and the water contained in the wood raw material (in the wood particles). However, since the amounts of wood raw material are usually given here as "wood dry matter" and the substances extracted from the wood are usually less than 1% of the total mass of the extractant, the ratios before extraction essentially correspond (just + /- 1%) also the conditions after the extraction (the possibly existing water content in the wood starting material, which is usually given in the economic utilization, is accordingly always already included in the extraction agent). This results in the use of 100 wt. -% acetone concentration of the extractant at 1:10 (solid matter: extractant) thus 1 kg dry wood mass and 10 kg acetone. 70% by weight acetone concentration of the extractant at 1:10 (solid matter: extractant) would then be e.g. 1 kg wood dry mass, 7 kg acetone and 3 kg water The extracted material would, as mentioned, still be added, but experience has shown that the concentration is around ations well below 1% by weight in the extractant and is therefore negligible.

The extraction temperatures can also be determined based on the other wood and process parameters, in particular taking into account the energy consumption that higher temperatures require. According to the invention, the treatment preferably takes place at an extraction temperature of 20-150.degree. C., preferably 40-120.degree. C., in particular 50-110.degree.

According to a preferred embodiment, the invention is appropriate methods operated at atmospheric pressure; in certain cases, extraction under pressure can be advantageous (despite the additional energy expenditure of the pressure application). Therefore, according to a preferred embodiment, the treatment according to the invention takes place at an absolute extraction pressure of 1-5 bar, preferably 1-1.49 bar.

The duration of the extraction process according to the invention can also be determined on the basis of the other process parameters, which duration is necessary for the depletion of fatty acids to be achieved. The treatment according to the invention preferably takes place during an extraction time of 10 minutes to 8 hours, preferably 30 minutes to 7 hours, in particular 1 to 5 hours.

In principle, the present method is suitable for all wood-based products in which organoleptic properties play a role. The method according to the invention is particularly suitable for products that are in use for a long time, for packaging foodstuffs or are used indoors. Therefore, the method according to the invention is particularly suitable for the (large-scale) production of cardboard, paper, wood fiber boards, chipboard, commodities (for example (or in case of doubt) according to the definition of the Austrian Food Safety and Consumer Protection Act (LMSV, Federal Law Gazette I No. 13/ 2006, idFv 01.10.2020) ), medical products (for example (or in case of doubt) according to the definition of the Austrian Medical Devices Act (MPG, Federal Law Gazette No. 657/1996, idFv 01.10.2020) ), in particular for the production of cardboard.

Particularly advantageous process parameters for the extraction treatment according to the present invention are selected from: treatment with ethanol in a concentration of at least 65% by weight at at least 65°C for a period of at least 3 hours;

treatment with ethanol in a concentration of at least 65% by weight at at least 85°C for a period of at least 30 minutes;

treatment with ethanol in a concentration of at least 70% by weight at at least 105°C for a period of at least 30 minutes; or

Treatment with ethanol in a concentration of at least 45% by weight at at least 105°C for a period of at least 5 hours.

Treatment with acetone in a concentration of at least 50% by weight at at least 40°C for a period of at least 30 minutes.

In the context of the present invention, it has been found that batch as well as continuous and semi-continuous extractions are possible and can even have an advantageous effect on the extraction result, particularly if the partial residence time per extraction step is a maximum of 1 hour.

When determining the process parameters, it is also essential that conditions are selected in which there are no significant losses of the wood substance (i.e. the content of cellulose, hemicelluloses and lignin). It can therefore also be provided that during the treatment according to the invention the dry substance content of the wood particles used is reduced by less than 10%, preferably by less than 5%, in particular by less than 4%, this reduction preferably being related to the extracted solid mass on the starting material, namely the wood particles, is determined. As mentioned above, different types of wood vary in their VOC content. The specific process parameters according to the invention are also to be adjusted accordingly. However, since this is made possible with the disclosure of the invention contained herein for all industrially relevant types of wood, the wood particles can preferably be selected from the industrially relevant types of wood, ie, for example, from coniferous wood particles, preferably spruce wood particles, fir wood particles, pine wood particles, or larch wood particles; Deciduous wood particles, in particular beech wood particles, poplar wood particles, birch wood particles, or eucalyptus wood particles; or mixtures thereof.

According to a preferred embodiment, the wood particles are mixed with the extractant during the treatment.

As already mentioned above, a preferred embodiment of the method according to the invention is that the wood particles are pressed after treatment with the extractant in order to remove the extractant. After the (extraction) treatment with the extractant, the wood particles can preferably be cleaned one or more times with an extractant, more preferably with an organic-aqueous solvent with a similar or the same concentration as that of the extractant. The extractant used in the extraction and/or washing can be removed from the wood particles by repeated washing with water or drying, with drying being particularly preferred. Both the extraction agent and the washing water are preferably regenerated for reuse after the process according to the invention. As mentioned, the extractives, especially fatty acids, can be extracted from the extractant Resin acids, separated and used as by-products.

With the method according to the invention it is possible to prepare wood particles in such a way that the content of fatty acids in the wood particles is reduced in such a way that the product to be produced from the wood raw materials does not have any organoleptically disadvantageous properties. According to the invention, extraction of the wood particles with the extraction agent preferably reduces the content of fatty acids in the wood particles by at least 75%, preferably by at least 80%, in particular by at least 90%, measured as the hexanal content in % by weight of the wood particles in the starting material in comparison to the extracted wood particles after accelerated aging for 72 h at 90°C.

The wood particles obtained in this way preferably have a fatty acid content in the wood particles of less than 2 mg/kg dry matter, preferably less than 1 mg/kg dry matter, in particular less than 0.5 mg/kg dry matter, measured as hexanal content as mass fraction of the extracted wood particles after accelerated aging for 72 h at 90°C.

In addition to reducing the hexanal content, it is also possible with the method according to the invention to reduce the mechanical strength of the extracted wood particles, measured as Tensile Index of test sheets in Nm/g, by at least 10%, preferably by at least 15%, in particular by at least 25%. , whereby the freeness, measured in °SR, changes by less than 10%.

It has been shown according to the invention that, in addition to the extraction of fatty acids, resin acids (along with other VOCs) can also be extracted using the method according to the invention. As part of a preferred embodiment the fatty acids, resin acids and/or optionally other extractives extracted into the extraction agent are fed to a further purification process and can then be made available as by-products of the production in an extracted and optionally further purified form.

According to a preferred embodiment, no complexing agents are added to the wood particles, in particular complexing agents selected from polybasic and polyfunctional carboxylic acids, aminomethyl carboxylic acids, aminomethyl phosphonic acids and their compounds, EDTA, DTPA EGTA, EDDS and their salts, polyphenols, tannins, amino acids, Peptides, proteins, polycarboxylates, phosphates, polyphosphates, phosphonic acids, polyphosphonates, phosphated, phosphonylated, sulfated and sulfonated polymers, added during the extraction process, especially during the entire manufacturing process for the products made from the wood particles. In this embodiment, the method according to the invention represents an extremely advantageous and practice-relevant variant, if only for ecological reasons.

Further features of the invention result from the claims, the figures and the description of the figures. The features and feature combinations mentioned above in the description, as well as the features and feature combinations mentioned below in the description of the figures and/or shown alone in the figures, can be used not only in the combination specified in each case, but also in other combinations, without departing from the scope of the invention to leave . Therefore, the invention also includes and discloses embodiments that are not explicitly shown and explained in the figures, but are separated Combinations of features emerge from the explanations explained and can be generated. Versions and combinations of features are also to be regarded as disclosed which therefore do not have all the features of an originally formulated independent claim. Furthermore, embodiments and combinations of features, in particular through the embodiments presented above, are to be regarded as disclosed which go beyond or deviate from the combinations of features presented in the back references of the claims. In particular, the present invention is explained in more detail using the following examples and the figures, without of course being restricted thereto. Show it:

Fig. 1 the hexanal content at an extraction temperature of 70°C. The x-axis shows the extraction time in h; the y-axis the hexanal content of the extracted wood pulp in mg/kgTM;

Fig. 2 the hexanal content at 90°C extraction temperature. The x-axis shows the extraction time in h; the y-axis the hexanal content of the extracted wood pulp in mg/kgTM;

3 shows the hexanal content at an extraction temperature of 110°C. The x-axis shows the extraction time in h; the y-axis the hexanal content of the extracted wood pulp in mg/kgTM;

4 shows the reduction in hexanal content at an extraction temperature of 70°C. The x-axis shows the extraction time in h; the y-axis the reduction of the hexanal content in % based on dry starting material; 5 shows the reduction in hexanal content at an extraction temperature of 90°C. The x-axis shows the extraction time in h; the y-axis the reduction of the hexanal content in % based on dry starting material; and

6 shows the hexanal content reduction at 110° C. extraction temperature. The x-axis shows the extraction time in h; the y-axis the reduction of the hexanal content in % based on dry starting material.

Examples :

The aim of the process developed in this patent is a significant improvement in the organoleptic properties of the wood particles and aged wood particles produced by this process. The undesirable odor that occurs, especially after the wood particles have aged (up to 6 months), and the taste of the food that comes into contact with them, is mainly caused by aldehydes (especially hexanal), which are caused by autocatalytic oxidation of in the wood naturally occurring fatty acids are formed. As mentioned above, this autocatalytic oxidation is currently prevented or greatly slowed down industrially by complexing the metal ions present in the wood particles, which act as a catalyst, by adding complexing agents such as ethylenediaminetetraacetic acid (EDTA). The process of this patent achieves a significant improvement in the organoleptic properties of the wood particles without adding complexing agents, but by removing potential aldehyde sources, primarily fatty acids.

Because fatty acids as a source of aldehydes, and subsequently Source of organoleptic impairments of the wood particles, which can only be quantified satisfactorily with great effort, the hexanal content of the wood particles after aging was used to assess the extraction success of the tests of this patent. At this point it should be noted that 0.5 mg/kgTM Hexanal (relative to dry wood) marks the empirically determined value below which experience has shown that organoleptic impairments are no longer perceptible to the senses. For hexanal contents of the wood particles > 0.5 mg/kgTM the following applies: the higher the hexanal content, the greater the organoleptic impairment of the wood particles.

The hexanal content can be determined using headspace gas chromatography (HS-GC) by filling a headspace vial with approx. 0.2 g air-dried wood particles (90-95 wt% TSG). In these vials, the sealed wood particles must then be aged for six months at room temperature (approx. 20 °C) in order to oxidize the fatty acids to hexanal. Since this takes a lot of time and therefore does not allow a timely assessment of the extraction success, accelerated aging according to DIN ISO 5630-2 was carried out for the present experiments (unless explicitly stated otherwise). The wood particles were sealed in HS-GC vials, aged for 72 h at 90° C. and then the hexanal content was determined using HS-GC. Although this standard for accelerated aging has been withdrawn, the extraction tests carried out in Table 1 show that the method of accelerated aging delivers comparable values or that the hexanal values for accelerated aging are on average even higher and thus even provide security with regard to of extraction success. Table 1

The hexanal content was used for the extraction success of all experiments according to the present invention, since experience has shown that this is the main factor influencing the organoleptic impairment of wood particles. The extractive substance content using Soxhlet extraction according to the TAPPI standard T204 is too imprecise for this, as Table 2 shows. It should be noted at this point that the TAPPI standard T204 was not used to determine the extractive content of the wood particles in all of the tests in this patent, but rather the NREL method NREL/TP-510-42619, which is very similar to T204 and wood -Grind instead of wood flour was used as the starting material. Table 2

As can be seen in Table 2, the Soxhlet extractions with ethanol differ significantly from those with acetone in terms of the hexanal content, but not significantly in terms of the extractive content. This means that, for example, two differently extracted wood particles can have different organoleptic properties despite not having a significantly different extractive content. The hexanal content of the wood particles after aging is thus a much stronger and more accurate indicator of the organoleptic impairment than the extractive content, and was therefore used for the extraction success according to the present invention. In addition, the hexanal content of the starting material and the resulting reduction in the hexanal content were specified for all experiments, since the starting materials are snapshots and the hexanal content can therefore vary greatly from time to time.

Nevertheless, the extracted extractive mass (determined as the evaporation residue of the extract) is an important indicator for the solid mass loss of the extraction , since it - apart from a few very volatile compounds - includes almost the entire solid mass removed by extraction. Thus, the evaporation residue of the extract together with the hexanal content of the extracted wood particles is an important measure for assessing the selectivity of the extraction.

Experiment: Comparison of solvents based on Soxhlet extractions For the pre-selection of solvents, ground wood samples were extracted with three different solvents. In each case 3 g of air-dried wood pulp were extracted for 24 hours using the Soxhlet method according to the NREL procedure NREL/TP-510-42619. The results are shown in Table 3.

Table 3

Table 3 shows that even with higher Hexanal contents of the starting material ethanol are best extracted, followed by acetone. Cyclohexane extracts by far the worst, which means that completely non-polar solvents are unsuitable for the extraction of fatty acids. According to Reichardt and Welton (Reichardt and Welton, Solvents and Solvent Effects in Organic Chemistry ⁴ (2011), Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA, pp 550-552), the empirically determined polarity of cyclohexane is 0 (very non-polar) compared to 0.355 from acetone and 0.654 from ethanol. Nevertheless, extraction with cyclohexane was also able to reduce the hexanal content by 79%.

Experiment 1: Extraction of dry wood pulp

The influence of the parameters solvent concentration, temperature and extraction time was examined on the basis of the extraction of air-dried wood pulp with ethanol (EtOH). For this purpose, approx. 2 g of air-dried groundwood were placed in an ethanol-water mixture with a solids:extractant ratio of 1:10 w/w and ethanol concentrations of 50, 70 and 90 wt% at temperatures of 70, 90 and 110 °C for 0.5, 1, 2, 4 and 8 h in small autoclaves. After extraction, the groundwood was pressed, washed with ethanol, pressed again and washed again with deionized water before it was dried, aged and analyzed. In FIGS. 1 to 3, the hexanal contents achieved by the extraction at the different extraction temperatures and solvent concentrations are plotted over the extraction time.

Figures 1 to 3 show that the extraction success at 70 or 90 wt% ethanol is significantly improved compared to that at 50 wt% - especially at lower temperatures. Ethanol concentrations of 70 or 90 wt% ethanol reduce the hexanal content to a comparable level. With the extraction temperatures, it is apparent that 90 and 110°C reduce the hexanal content approximately equally, while it is not reduced as much at 70°C. With 90 wt% ethanol, the hexanal content can even drop below the 0.5 mg/kgTM mark from 90°C, which means that the groundwood extracted under these conditions no longer has any organoleptic impairments, according to experience. Based on the different extraction times, it can be seen that the hexanal content drops the most at the beginning of the extraction and especially in the first 4 hours. This can also be seen in FIGS. 4 to 6. These figures reflect the extraction successes shown in FIGS. 1 to 3, since the same batch of starting material was used for all experiments in this series of experiments. It is clearly evident that in the worst case (0.5 h with 50 wt% ethanol at 70 wt% the hexanal content could only be reduced by approx. 20%, while with 90 wt% ethanol from 90 °C it was over 95 % has been reduced.

Table 4 shows that despite the high hexanal content reductions achieved in this test series, the extracted solids mass is max. 7 wt% (based on the starting material) - but usually significantly lower. With a determined extractive content of the starting material of approx. 3 wt%, this means that neither hemicelluloses nor lignin are extracted to any significant extent. Table 4

Experiment 2: Extraction of wet wood pulp with ethanol under different conditions

The extraction of moist groundwood represents real conditions much better, especially in cardboard production, than the extraction of dry groundwood, as in test 1. In addition, significantly more sample mass was taken for these tests (factor 225) than for test 1 in order to get a more meaningful to get result. Approx. 450 gTM mechanically dewatered groundwood (approx. 25 wt% TSG) in an ethanol-water mixture with a solids:solvent ratio of 1:10 w/w and ethanol concentrations of 60 wt% at temperatures of 70 and 90 °C for 2 and 4 h in an autoclave. After extraction, the groundwood was pressed, washed with ethanol, pressed again and washed again with deionized water before it was dried, aged and analyzed. Table 5 shows the results of these extractions. Table 5

The tests show that - compared to the extraction of a smaller and, above all, air-dry wood-pulp mass - the tests with mechanically dewatered wood-pulp and more sample mass show higher hexanal contents in the extracted wood-pulp. Nonetheless, over 73% reduction in hexanal content was achieved in each setting.

Experiment 3: Extraction of wet wood pulp with three different organic solvents

Experiment 3 was carried out to test three different technically relevant solvents under real extraction conditions. There were approx. 450 gTM mechanically dewatered groundwood (approx. 25 wt% TSG) in a solvent-water mixture with a solids: extractant ratio of 1:10 w/w and a solvent concentration of 70 wt% at a temperature of 70 °C extracted in an autoclave for 4 h. After the extraction, the ground wood was pressed, washed with extraction agent, pressed again and washed again with deionized water before it was dried, aged and analyzed. Table 6 shows the results of these extractions. Table 6

As can be seen in Table 6, acetone extracts the unsaturated fatty acids responsible for hexanal formation significantly better than undenatured ethanol. However, the best extraction results could be achieved with ethanol, which is incompletely combined with butanone. was galled. The hexanal content of 2.77 mg/kgDM achieved with 70 wt%, 70°C and 4 h extraction time is still well above the 0.5 mg/kgDM limit, but corresponds to a reduction of 80%. With all extractions, the evaporation residue of the extract is only between 2.2 and 2.4 wt% based on the starting material dry mass, which means that with a determined extractive content of 3.6 wt%, the main wood components are cellulose, hemicelluloses and In fact, lignin was not attacked and the extractions were therefore very selective. The abietic acid content of the extractions in this experiment was reduced by 41-55%, based on the starting material. Since abietic acid was chosen here as the key substance for the content of resin acids, a reduction in the content by around 50% is an indication of the clear reduction in resin through the process of this patent.

Experiment 4: Extraction of moist wood pulp with acetone at different solids/extractant ratios

The influence of different solids/extractant ratios was examined in this experiment. Approx. 200 - 450 gDM (depending on the solids: extractant ratio) mechanically dewatered groundwood (approx. 25 wt% TSG) were mixed with a solvent-water mixture consisting of 70 wt% acetone and 30 wt% deionized water Solid:extractant ratios of 1:10, 1:15 and 1:25 w/w at a temperature of 50°C for 1, 2 and 4 hours in an autoclave. After extraction, the groundwood was pressed, washed with extraction agent, pressed again and washed again with deionized water before it was dried, aged and analyzed. The results of these extractions are listed in Table 7. Table 7 As shown in Table 7, the hexanal content in all

Extractions (with the exception of the solid:extractant ratio of 1:10 (w/w) with a starting material hexanal content of 14.07 mg/kgDM) reduced by over 80%. For extractant to solid ratios above 10:1 (w/w) the reduction of the hexanal content under the same extraction conditions is comparably high and always well over 70 1 despite different starting material hexanal contents .4 - 3.7 wtl is evidence for the quantitative preservation of the lignocellulosic components in this process. By reducing the abietic acid content by 31-54 liters, based on the starting material, this test also showed that the process of this patent can significantly reduce resin.

Experiment 5: Modification of the mechanical properties by the extractive treatment of groundwood according to the present invention

This experiment serves to investigate the effects of the extractive treatment on the mechanical properties of the extracted wood particles. For this purpose, approx. 300 - 450 gDM (depending on the solids:extractant ratio) mechanically dewatered groundwood (approx. 25 wtl TSG) in an extractant-water mixture with solids:extractant ratios of 1:10 and 1:15 w/w and a solvent concentration of 60 and 70 wtl at temperatures of 50 °C, 70 °C and 90 °C. Ethanol 96 fully undenatured (EtOH pure), ethanol 96 fully incompletely denatured with butanone (EtOH comp.) and acetone were used as solvents. After extraction, the ground wood was pressed, washed with extraction agent, pressed again and washed again with deionized water before sample sheets were formed from it, on the basis of which the mechanical properties were examined. Of the Mass loss results from the evaporation residue of the extract and is related to the starting material dry mass. Table 8 lists the results. Table 8

As can be seen in Table 8, the degree of grinding and thus the dewatering of the ground wood hardly changes as a result of the extraction, which has the advantage with regard to any further processing (e.g. to cardboard) that existing Production plants do not have to be modified or converted. The stiffness index also changes only slightly as a result of the extraction, whereas the tensile index, as a measure of the breaking force, rises sharply and reproducibly . Compared to this high increase, the mass loss averages between 25 and 41% very low at around 2 % due to the evaporation residue of the extract . This means that the wood particles gain disproportionately strong strength through the extraction with little mass loss, which is of great importance for the lightweighting trend in the packaging sector in particular.

Legend :

TM = dry matter

TSG = dry matter content

Preferred embodiments:

In view of the above description of the present invention, the following preferred embodiments of the invention are disclosed herein:

1 . Process for the manufacture of products based on wood as a raw material, characterized in that wood in the form of wood particles is subjected to an extraction treatment with an extraction agent which contains one or more organic solvents or an organic-aqueous mixture of the solvent or solvents Water comprises, wherein the content of fatty acids in the wood particles is reduced by at least 70% by the extraction treatment of the wood particles with the extractant, measured as the hexanal content in weight. -% after accelerated aging for 72 h at 90 °C, but the content of cellulose, hemicelluloses and lignin is essentially preserved in this extraction treatment.

2 . Method according to embodiment 1, characterized in that the wood particles are present with a size of less than 5 cm, the particle size preferably being by a sieving using a square mesh screen is determined, in particular using a square mesh screen with a mesh size of 5 cm or less.

3. The method according to embodiment 1 or 2, characterized in that the wood particles are in the form of fibers, chips, strands, wood chips or mixtures thereof.

4. The method according to one or more of embodiments 1 to 3, characterized in that the wood particles are fibrous wood by mechanical and/or thermal and/or chemical digestion.

5. The method according to one or more of embodiments 1 to 4, characterized in that the wood particles are wood fibers with average fiber lengths between 0.5 and 2 mm and average fiber diameters between 10 and 50 μm, with the average fiber length and the average fiber diameter the mean length , determined by optical measurement of the suspended fibers.

6. The method according to one or more of embodiments 1 to 5, characterized in that the solvent content of the organic-aqueous solvent mixture in the extractant, determined as the concentration of the liquid phase of the extract, consists of 0-95% by weight ethanol, preferably 50 - 90% by weight ethanol, 0-99% by weight acetone, preferably 30-90% by weight acetone, 0-70% by weight n-propanol, 0-85% by weight iso-propanol and/or or 0-99% by weight of methanol.

7. The method according to one or more of embodiments 1 to 6, characterized in that the ratio of extractant to solid dry substance is 5:1 - 25:1 (w/w), preferably 8:1 - 17:1 (w/w ) amounts to. 8. Process according to one or more of embodiments 1 to 7, characterized in that the extraction treatment is carried out at an extraction temperature of 20-150°C, preferably 40-120°C, in particular 50-110°C.

9. Process according to one or more of embodiments 1 to 8, characterized in that the extraction treatment is carried out at an absolute extraction pressure of 1-5 bar, preferably 1-1.49 bar.

10. The method according to one or more of embodiments 1 to 9, characterized in that the extraction treatment takes place during an extraction time of 10 minutes to 8 hours, preferably 30 minutes to 7 hours, in particular 1 to 5 hours.

11. The method according to one or more of embodiments 1 to 10, characterized in that the method is used for the production of cardboard, paper, wood fiber boards, chipboard, commodities, medical products, in particular for the production of cardboard.

12. The method according to one or more of embodiments 1 to 11, characterized in that the extraction treatment is selected from:

treatment with ethanol in a concentration of at least 65% by weight at at least 65°C for a period of at least 3 hours;

treatment with ethanol in a concentration of at least 70% by weight at at least 105°C for a period of at least 30 minutes; or Treatment with ethanol in a concentration of at least 45% by weight at at least 105°C for a period of at least 5 hours.

13. The method according to one or more of embodiments 1 to 12, characterized in that the treatment with the extractant is carried out as a batch, continuous or semi-continuous extraction, preferably with a maximum partial residence time of 1 hour per extraction step.

14. The method according to one or more of embodiments 1 to 13, characterized in that during the extraction treatment the content of cellulose, hemicelluloses and lignin is reduced by less than 10%, preferably by less than 5%, in particular by less than 4%, is reduced, this reduction preferably being determined as the extracted solid mass, based on the starting material, the wood particles.

15. The method according to one or more of embodiments 1 to 14, characterized in that the wood particles are selected from softwood particles, preferably spruce wood particles, fir wood particles, pine wood particles, or larch wood particles; Deciduous wood particles, in particular beech wood particles, poplar wood particles, birch wood particles, or eucalyptus wood particles; or mixtures thereof.

16. The method according to one or more of embodiments 1 to 15, characterized in that the wood particles are mixed during the treatment with the extractant. 17. The method according to one or more of embodiments 1 to 16, characterized in that the wood particles are pressed after treatment with the extractant in order to remove the extractant.

18. The method according to one or more of embodiments 1 to 17, characterized in that the wood particles are cleaned one or more times with an extractant after treatment with the extractant, preferably with an organic-aqueous solvent with a similar or the same concentration as that of the Extractant to be cleaned.

19. The method according to one or more of embodiments 1 to 18, characterized in that the extraction agent is removed from the wood particles by washing one or more times with water or drying, preferably by drying.

20. The method according to one or more of embodiments 1 to 19, characterized in that the content of fatty acids in the wood particles is reduced by at least 75%, preferably by at least 80%, in particular by at least 90%, by extraction of the wood particles with the extraction agent , measured as hexanal content in % by weight of the wood particles in the starting material compared to the extracted wood particles after accelerated aging for 72 h at 90°C.

21. The method according to one or more of embodiments 1 to 20, characterized in that the content of fatty acids in the wood particles by extraction of the wood particles with the extractant to a content of less than 2 mg / kg dry matter, preferably less than 1 mg / kg dry matter , in particular less than 0.5 mg/kg dry matter, is reduced, measured as hexanal content as a mass fraction of the extracted wood particles after accelerated aging for 72 h at 90 °C.

22 . Method according to one or more of embodiments 1 to 21, characterized in that resin acids are also extracted in addition to the fatty acids.

23 . Process according to one or more of embodiments 1 to 22, characterized in that the fatty acids, resin acids and/or optionally other extractives extracted with the extraction agent are fed to a further purification process.

24 . Method according to one or more of embodiments 1 to 23, characterized in that the wood particles no complexing agents, in particular complexing agents selected from polybasic and polyfunctional carboxylic acids, aminomethyl carboxylic acids, aminomethyl phosphonic acids and their compounds, EDTA, DTPA EGTA, EDDS and their salts , Polyphenols, tannins, amino acids, peptides, proteins, polycarboxylates, phosphates, polyphosphates, phosphonic acids, polyphosphonates, phosphated, phosphonylated, sulfated and sulfonated polymers, in the course of the extraction process, in particular in the course of the entire manufacturing process for the from the Hol products manufactured z particles , are added .

25 . Process according to one or more of embodiments 1 to 24, characterized in that the extraction agent and any washing liquids used, in particular water, are regenerated for reuse.

26 . Process according to one or more of the execution Forms 1 to 25, characterized in that the extraction treatment, in addition to reducing the hexanal content, also reduces the mechanical strength of the extracted wood particles, measured as Tensile Index of sample sheets in Nm/g, by at least 10%, preferably by at least 15%, in particular by at least 25%, the degree of grinding, measured in °SR, changing by less than 10%.

Claims

- 42 - Claims :

1. A method for producing products based on wood as a raw material, characterized in that wood in the form of wood particles is subjected to an extraction treatment with an extraction agent which comprises one or more organic solvents or an organic-aqueous mixture of the solvent or solvents with water , wherein the content of fatty acids in the wood particles is reduced by at least 70% by the extraction treatment of the wood particles with the extraction agent, measured as hexanal content in % by weight after accelerated aging for 72 h at 90°C, but the cellulose content , hemicelluloses and lignin is essentially preserved in this extraction treatment.

2. The method according to claim 1, characterized in that the wood particles are present with a size of less than 5 cm, the particle size preferably being determined by sieving using a square mesh screen, the wood particles preferably being in the form of fibers, chips, strands, wood chips or mixtures of which are available.

3. The method according to claim 1 or 2, characterized in that the wood particles by mechanical and / or thermal and / or chemical digestion to fibrous wood, in particular wood fibers with average fiber lengths between 0.5 and 2 mm and average fiber diameters between 10 and 50 pm , are, where the average fiber length and the average fiber diameter are based on the length average , 43 by means of optical measurement of the suspended fibers.

4. The method according to one or more of claims 1 to

3, characterized in that the solvent content of the organic-aqueous solvent mixture in the extractant, determined as the concentration of the liquid phase of the extract, consists of 0-95% by weight ethanol, preferably 50-90% by weight ethanol, 0-99 % by weight of acetone, preferably 30-90% by weight of acetone, 0-70% by weight of n-propanol, 0-85% by weight of isopropanol and/or 0-99% by weight of methanol.

5. The method according to one or more of claims 1 to

4, characterized in that during the extraction treatment the content of cellulose, hemicelluloses and lignin is reduced by less than 10%, preferably by less than 5%, in particular by less than 4%, this reduction preferably being expressed as extracted solid mass, based on the source material, the wood particles, is determined.

6. The method according to one or more of claims 1 to

5, characterized in that the wood particles are selected from softwood particles, preferably spruce wood particles, fir wood particles, pine wood particles, or larch wood particles; Deciduous wood particles, in particular beech wood particles, poplar wood particles, birch wood particles, or eucalyptus wood particles; or mixtures thereof.

7. The method according to one or more of claims 1 to

6, characterized in that the wood particles after treatment with the extractant once or several times 44 cleaned with an extractant, preferably with an organic-aqueous solvent having a similar or the same concentration as that of the extractant.

8. The method according to one or more of claims 1 to

7, characterized in that the content of fatty acids in the wood particles is reduced by at least 75%, preferably by at least 80%, in particular by at least 90%, by extraction of the wood particles with the extraction agent, measured as the hexanal content in wt. of the wood particles in the starting material compared to the extracted wood particles after accelerated aging for 72 h at 90°C.

9. The method according to one or more of claims 1 to

8, characterized in that the content of fatty acids in the wood particles by extraction of the wood particles with the extractant to a content of less than 2 mg / kg dry matter, preferably less than 1 mg / kg dry matter, in particular less than 0.5 mg / kg dry matter , measured as hexanal content as a mass fraction of the extracted wood particles after accelerated aging for 72 h at 90°C.

10. The method according to one or more of claims 1 to

9, characterized in that in addition to the fatty acids, resin acids are also extracted with the extraction.

11. The method according to one or more of claims 1 to

10, characterized in that the extracted with the extractant fatty acids, resin acids and / or given if other extractives are fed into a further purification process.

12 . Method according to one or more of claims 1 to

11, characterized in that the wood particles do not contain any complexing agents, in particular complexing agents selected from polybasic and polyfunctional carboxylic acids, aminomethyl carboxylic acids, aminomethyl phosphonic acids and their compounds, EDTA, DTPA EGTA, EDDS and their salts, polyphenols, tannins, amino acids, peptides, proteins , Poly carboxylates, phosphates, polyphosphates, phosphonic acids, polyphosphonates, phosphated, phosphonylated, sulfated and sulfonated polymers, in the course of the extraction process, in particular in the course of the entire manufacturing process for the products made from the wood particles, are added.

13 . Method according to one or more of claims 1 to

12, characterized in that the extraction agent and any washing liquids used, in particular water, are regenerated for reuse.

14 . Method according to one or more of claims 1 to

13, characterized in that the extraction treatment, in addition to reducing the hexanal content, also reduces the mechanical strength of the extracted wood particles, measured as Tensile Index of sample sheets in Nm/g, by at least 10%, preferably by at least 15%, in particular by at least 25%, whereby the degree of grinding, measured in ° SR, changes by less than 10%.