Technical field

The invention relates to wood based powders. The invention relates to wood based powders suitable for use in composite materials. The invention relates to composite materials comprising wood based powder. The invention relates to methods for manufacturing wood based powders.

Background

Reactivity and bondability of materials correlate with their volume specific area. Therefore, in many applications, powders are used because of their high surface area in relation to their total volume. For environmental reasons, use of renewable materials is oftentimes preferred over fossil materials. Wood is a commonly used renewable material. In the papermaking industry, it has long been known to grind wood to obtain fibrous raw material . Typically, grinding results in fibrous material of small particle size, wherein the fibres have a very large length to width ratio, typically of the order of 100 - 10000. At a larger length scale, shredders and firewood processors have been used to chop wood material into smaller pieces. Such devices typically produce relatively large wood particles with a length to width ratio of at most 5. Such particles are also obtainable by sawing wood.

When using wood material in particular in composites, the lengthy fibres resulting from grinding are not optimal materials for all purposes. In some applications, wood material with a lower aspect ratio could be more easily mixed with the polymer matrix. Moreover, a particle size of the wood material should be sufficiently, but not too, small.

Furthermore, the composition of the wood material itself is not always optimal for use. Summary

It has now been found that the properties of the wood based powder itself can be affected by the method by which the wood material is manufactured. In particular, it has been found that when the piece of virgin wood, from which the wood based powder is manufactured, is soaked and heat treated before manufacturing the powder therefrom, the wood material itself contains less extractives than virgin wood. This is most likely on one hand due to the extractives dissolving from the virgin wood material to the aqueous bath during soaking and on the other hand these compounds flowing towards surfaces of the piece of wood during heat treatment, and also possibly partly evaporating from the wood material. The composition has been found beneficial for many applications. Moreover, it has been found that when the wood based powder is removed from the piece of wood by sanding, the size distribution of the powder is in a preferable range. Moreover, it has been found that when the wood based powder is removed from the piece of wood by sanding, the aspect ratio of the particles is in a preferably range, i.e. larger than obtainable by a shredder, and smaller than obtainable by grinding. This seems to happen at least when the moisture content of the veneer is at a suitable level during sanding. Moreover, a grain orientation of the surface that is sanded seems to play a role in the properties of the wood based powder. Finally, a direction of movement of a sanding surface relative to the grain orientation of the piece of wood may play a role in the properties of the wood based powder.

It has been found that such wood based material comprises volatile compounds to a lesser extent than virgin wood materials. This has an effect in uses involving odour and/or taste of the material. A lesser amount of volatiles implies less odours and a smaller effect on taste.

A method for manufacturing wood based powder is disclosed in more specific terms in the independent claim 23. Preferable embodiments are disclosed in the dependent claims 24 - 56. These and other embodiments are disclosed in the description.

A wood based powder as obtainable by the method is disclosed in more specific terms in the independent claims 1 and 2. Preferable embodiments are disclosed in the dependent claims 3 to 22. These and other embodiments are disclosed in the description.

Brief description of the drawings

Fig. 1 a shows a method for producing wood based powder, the method including soaking and heat treatment,

Fig. 1 b shows a method for producing wood based powder, the method including soaking a log, separation a primary piece of wood from the log, and heat treatment of the primary piece of wood,

Fig. 1 c shows sanding a surface of a primary piece of wood with a primary sanding surface in such a way that the primary sanding surface is moved perpendicularly to a grain orientation of the primary piece of wood,

Fig. 1d shows sanding a surface of a primary piece of wood with a primary sanding surface in such a way that the primary sanding surface is moved unidirectionally with a grain orientation of the primary piece of wood,

Fig. 2a shows an embodiment of a method for producing wood based powder,

Fig. 2b shows an embodiment of a method for producing first wood based powder and second wood based powder,

Fig. 2c shows pre-treatment of wood veneers according to an embodiment of a method for producing wood based powder,

Fig. 2d shows removing at least some large particles from an intermediate powder to form the wood based powder,

Fig. 2e shows fractionating the wood based powder to obtain first wood based powder and second wood based powder,

Fig. 2f shows an embodiment of a method for producing wood based powder from only high quality wood,

Fig. 2g shows an embodiment of a method for producing wood based powder by sanding two opposite surfaces of a plywood board,

Fig. 3a shows, in a side view, a plywood board,

Figs. 4a to 4d show methods for manufacturing composite material,

Fig. 5a shows a scanning electron microscope image of wood based powder, and Fig. 5b shows an optical microscope image of sawdust.

Detailed description Figure 1 a shows an embodiment of a method for manufacturing wood based powder 200. In the method, a primary piece of wood 110 is treated. The primary piece of wood 110 as such may be received or made e.g. from a log 910 (Fig. 2a) by separating therefrom. In order to affect the composition of the primary piece of wood 110, the primary piece of wood 110 is soaked. Referring to Fig. 1 a, the primary piece of wood 110, that has been made from a larger piece of wood, can be soaked. As an alternative, and with reference to Fig. 1 b, the primary piece of wood 110, when a part of a large piece of wood (such as a log 910), can be soaked, and the primary piece of wood 110 can be separated from the larger piece 910 (e.g. the log 910) later. Soaking (shown by the reference numeral 805 in Fig. 2a) is performed by arranging the primary piece of wood 110, optionally when a part of the larger piece 910 of wood, into a bath of aqueous solution 806. The aqueous solution 806 comprises water. During soaking, the temperature of the aqueous solution 806 is at least 15 °C. Preferably, the temperature of the aqueous solution 806 is from 15 °C to 70 °C or from 15 °C to 50 °C. Higher temperatures may affect the quality of the wood based powder 200. The aqueous solution 806 is heated, if necessary. For example, in winter, the aqueous solution may be heated, at least when soaking is done outdoors. Thus, an embodiment comprises heating the aqueous solution 806. When a log 910, from which the primary piece of wood 110 is separated, is soaked, the log 910 is soaked for at least six hours, preferably at least 12 hours, or more preferably at least 24 hours. Shorter times, such as at least 1 hour, may be sufficient, if the primary piece of wood 110 is separated from the log 910 (or, in general, from a larger piece 910) before soaking. Soaking increases the plastic properties of wood e.g. before cutting the primary piece of wood 110 from log 910. Soaking also dissolves some components of the wood to the aqueous solution 806. Thus, the aqueous solution 806 comprises also lignin, water-soluble wood, and/or bark matter. In other words, the aqueous solution 806 comprises extractives, and certain amount of solid particles. However, in order for the aqueous solution 806 to effectively dissolve the extractives from the wood material, the purity of the solution 806 should be sufficiently high. Therefore, in an embodiment, a solids content of the aqueous solution 806 is at most 3000 mg/I (i.e. at most 0.3 wt%). Solids content refers to the solids content as measured according to the standard SFS-EN 872 (2005), by using a Whatman GF/C filter. In addition or alternatively, a chemical oxygen demand (CODc _r ) of the aqueous solution 806 is preferably at most 10000 mg/I. Herein the chemical oxygen demand is indicative of the amount of organics in said solution 806. The chemical oxygen demand refers to the value thereof as measured by the dichromate method of the chemical oxygen demand discussed in the standard ISO 15705 (2002), and the photometric method COD-Cr therein described. Typically, the solids content is from 400 mg/I to 1600 mg/I and the chemical oxygen demand is from 3000 mg/I to 10000 mg/I.

Soaking affects an amount of extractives of the wood by dissolving some of them into the solution 806. Moreover, it has been found that some compounds of the wood are soluble to water, but the rate of dissolving them into water may be different. Therefore, soaking also selectively affects the content of different compounds of wood.

As indicated above, some compounds of the wood, when heated, flow towards surfaces of the piece of wood, and also possibly partly evaporate from the wood material . Therefore, in order to affect the composition of a primary surface 1 12 of the primary piece of wood 1 10, the primary piece of wood 1 10 is heat treated. It has been found that when the primary piece of wood 1 10 is heat treated, the heat treatment has the effect that some flowable materials, such as resins, of the primary piece of wood 1 10 flow towards surfaces of the primary piece of wood 1 10, the surfaces including a primary surface 1 12. In this way the content of such flowable materials near the primary surface 1 12 is increased. This happens at least when a temperature used in the heat treatment is suitably high. For these reasons, the method comprises heating the primary piece of wood 1 10 at a temperature of from 100 °C to 220 °C, preferably from 120 °C to 200 °C. The lower limit is sufficient for affecting the flowability of the compounds within the wood, and the higher limit ensures that the properties of the wood are not compromised due to too high temperature. Moreover, having a reasonably low temperature enables heating using low pressure steam. When a log 910 is soaked, it is energetically favourable to separate the primary piece of wood 1 10 from the log before said heat treatment. This significantly reduces the time needed for heat treatment. In an embodiment, the primary piece of wood 1 10, after having been separated from a log 910, is heat treated at a temperature of from 100 °C to 220 °C for at least

1 minute, preferably at least 2 minutes. In an embodiment, the primary piece of wood 1 10, after having been separated from a log 910, is heat treated at a temperature of from 120 °C to 200 °C for at least 1 minute, preferably at least

2 minutes.

Because of the heat treatment, or another heat treatment, the primary piece of wood 1 10 is dried. The primary piece of wood 1 10 is dried to a moisture content of at most 10 wt%. It is possible that the moisture content immediately after heat treatment is over 10 wt%, e.g. at most 15 wt%, and the primary piece of wood 1 10 is allowed to dry to the moisture content of at most 10 wt% at a lower temperature. Preferably the primary piece of wood 1 10 is dried to a moisture content of from 0.5 wt% to 10 wt% or from 1 .5 wt% to 10 wt%. Drying to an even lower moisture content would unnecessarily increase use of energy for drying. However, the temperature plays a role in the flowability of the flowable material of the primary piece of wood. Therefore, a preferable embodiment comprises heating and drying the primary piece of wood 1 10 at a temperature of from 180 °C to 190 °C to a moisture content of at most 15 wt%, and optionally allowing the primary piece of wood 1 10 to further dry at a lower temperature.

After the heat treatment, the primary surface 1 12 of the primary piece of wood 1 10 is sanded to form wood based particles 210 by said sanding, as indicated in Figs. 1 a and 1 b. The wood based powder 200 comprises these wood based particles 210.

However, the wood based powder 200 may comprise also other material than the wood based particles 210. Such other material include sand, which may separate from the sanding surface, and resin, which may have been used as an adhesive of the plywood, and of which a portion may be sanded off. Therefore, the wood based powder 200 comprises at least 75 wt% wood particles 210. More preferably, the wood based powder 200 comprises at least 90 wt% or at least 95 wt% wood particles 210.

It has been found that also a sanding direction relative to the grain direction of wood affects the properties of the wood based powder 200. In order to form a surface, in which the grain direction of wood fibres is preferable for production of wood based powder 200, in an embodiment, the primary piece of wood 1 10 is separated from the large piece 910 of wood material (e.g. log 910) in such a way that the primary piece of wood 1 10 comprises surfaces, of which one is a primary surface 1 12 and the grain direction of the wood on the primary surface forms an angle of from 45 to 135 degrees with the normal N of the primary surface 1 12. For example the grain direction of the wood on the primary surface 1 12 may be perpendicular to a normal N of the primary surface 1 12. The primary surface 1 12 may then serve as the surface that is sanded. Figs. 1 c, 1 d, and 3a show, as an example, a normal N of a primary surface 1 12. A grain direction D1 is also shown in Figs. 1 c and 1 d.

Moreover, is has been found that removing material by sanding from the dry primary surface 1 12 produces a more suitable sized distribution for the wood based powder 200 than sanding e.g. a moist surface. As a limiting moisture content, it has been found that when the moisture content during sanding is sufficiently low, e.g. at most 10 wt%, an optimal size distribution for the wood based powder can be obtained by sanding. The piece of wood 1 10 that is sanded need not be totally dry, whereby the moisture content during sanding may be e.g. from 0.5 wt% to 10 wt%, such as from 1 .5 wt% to 10 wt%.

Since the moisture content of the primary piece of wood 1 10 after the aforementioned heat treatment may be somewhat higher, the primary piece of wood may be allowed to dry to the aforementioned moisture content.

In order to form the wood based powder 200, the heat treated and dried primary piece of wood 1 10 is sanded. In particular, the primary piece of wood 1 10 is sanded at such a point of time that its moisture content of the surface that is sanded is at most 10 wt%, such as from 0.5 wt% to 10 wt% or from 1 .5 wt% to 10 wt%. Thus, the method comprises sanding the primary piece of wood 1 10, of which moisture content is at most 10 wt%, to separate the wood based powder 200 from the primary piece of wood 1 10. More preferably, the primary piece of wood 1 10, when sanded, is even drier. Preferably, when sanding the primary piece of wood 1 10, a moisture content of the primary piece of wood 1 10 is at most 7 wt%, such as from 2 wt% to 7 wt%, or at most 6 wt%, such as from 3 wt% to 6 wt%.

What has been said about the moisture content of the primary piece of wood 1 10 applies to the moisture content of the wood based powder 200. Thus, in an embodiment, a moisture content of the wood based powder 200 is at most 10 wt%. Other possible moisture contents of the wood based powder 200 include from 0.5 wt% to 10 wt%, at most 7 wt%, from 1 .5 wt% to 7 wt%, at most 6 wt%, and from 2 wt% to 6 wt%, as indicated above for the primary piece of wood 1 10. Such a substantially dry wood based powder has beneficial effects in various applications. As an example, the powder mixes well with many hardenable materials. As another example, the powder, if used to filter liquid materials, takes in some moisture and in this way swell, which densifies the filter and improves the filtering capacity.

Referring to Fig. 1 a, the primary piece of wood 1 10 may be sanded with a primary sander 250 having a primary sanding surface 252. In Fig. 1 a, the primary sander 250 is a belt sander, and the primary sanding surface 252 forms a belt. Naturally, when sanding, the primary sanding surface 252 faces the primary piece of wood 1 10, even if not explicitly indicated in Fig. 1 a for illustrative purposes. The primary sander 250 (or another sander 251 ) needs not be a belt sander. The primary sander 250 (or another sander 251 ) may be e.g. a random oribit sander, an orbital sander, or a piece of abrasive paper. A sander 250 may comprise a piece of sandpaper, which forms the primary sanding surface 252. The primary sanding surface 252 may be a surface of an object other than sandpaper. As a result of sanding, wood based powder 200 is formed.

With reference to Figs. 1 c and 1 d, the primary surface 1 12 of the primary piece of wood 1 10 can be sanded by a first sanding surface 252, such as abrasive paper, e.g. sandpaper. Advantageously, a primary sanding surface 252 is used whose roughness according to the standard ISO 6344 is from P20 to P220; more preferably from P60 to P220, or from P60 to P180. When sanding with the primary sanding surface 252, wood based powder 200, i.e. primary wood based powder 200A, is obtained. In a preferable embodiment, such a primary surface 1 12 of the primary piece of wood 1 10 is sanded that a normal N of the primary surface 1 12 forms an angle of from 45 to 135 degrees with, e.g. is perpendicular to, a grain direction D1 of the wood forming the primary surface 1 12.

Referring to Fig. 2b, advantageously, the sanding is carried out in two steps. First, a secondary sanding surface 254 of a secondary sander 251 is used to sand a primary surface 1 12 of the primary piece of wood 1 10. The secondary sanding surface 254 may have a roughness of from P20 to P100, more preferably from P40 to P80, according to the standard ISO 6344. Thereafter, a finer primary sanding surface 252 may be used; i.e. the secondary sanding surface 254 may be coarser than the primary sanding surface 252. In this embodiment, the primary sanding surface 254 may have a roughness of from P60 to P220, more preferably from P80 to P180, according to the standard ISO 6344. Thus, an embodiment comprises, before sanding the primary surface 1 12 with the primary sanding surface 252, sanding the primary surface 1 12 of the primary piece of wood 1 10 with a secondary sanding surface 254. When sanding with the secondary sanding surface 254, secondary wood based powder 200B is obtained. Correspondingly, when sanding with the primary sanding surface 252, primary wood based powder 200A is obtained.

With reference to Figs. 1 c and 1 d, in an embodiment such a primary surface 1 12 of the primary piece of wood 100 is sanded that a normal N of the primary surface 1 12 forms an angle of from 45 to 135 degrees with the normal N of the primary surface 1 12. For example, the normal N of the primary surface 1 12 may be perpendicular to grain direction D1 of the primary piece of wood 1 10 of the surface 1 12. It has been found that in such cases the size distribution of the wood based powder 200 is suitably narrow, suitably wide, and the particles are suitably small. Moreover, the wood particles may have a beneficial aspect ratio as detailed below. In particular, as indicated in Fig. 1 d, an embodiment comprises sanding the primary surface 1 12 of the primary piece of wood 1 10 in a direction R1 that forms an angle at of at least 30 degrees with the grain direction D1 . The species of wood that is sanded also affects the properties of the wood based powder 200. For the application detailed below, it seems that powder obtained from spruce, pine, beech, or birch functions well. Thus, in an embodiment, the primary piece of wood 1 10 comprises spruce, pine, beech, or birch. However, at least in plywood, the different veneers may be made from different wood species. Therefore, more specifically, in an embodiment, at least a part of the primary piece of wood 1 10 is made from spruce, pine, beech, or birch, and the part of the piece of wood 1 10 that is made from spruce, pine, beech, or birch is sanded to form the powder 200. Correspondingly in an embodiment, the wood particles 210 of the wood based powder 200 originate from at least one of spruce, pine, beech, or birch. In an embodiment, the wood particles 210 of the wood based powder 200 originate from spruce. In an embodiment, the wood particles 210 of the wood based powder 200 originate from birch. In an embodiment, the wood particles 210 of the wood based powder 200 originate from beech. In an embodiment, the wood particles 210 of the wood based powder 200 originate from pine. Different powders may be mixed. As a result of mixing, at least the following mixtures are possible:

- an embodiment, wherein a part of the wood particles 210 of the wood based powder 200 originate from birch and a part of the wood particles 210 of the wood based powder 200 originate from softwood(s), such as spruce and/or pine;

- an embodiment, wherein a part of the wood particles 210 of the wood based powder 200 originate from beech and a part of the wood particles 210 of the wood based powder 200 originate from softwood(s), such as spruce and/or pine;

- an embodiment, wherein a part of the wood particles 210 of the wood based powder 200 originate from birch and a part of the wood particles 210 of the wood based powder 200 originate from beech;

- an embodiment, wherein a part of the wood particles 210 of the wood based powder 200 originate from spruce and a part of the wood particles 210 of the wood based powder 200 originate from hardwood(s), such as birch and/or beech;

- an embodiment, wherein a part of the wood particles 210 of the wood based powder 200 originate from pine and a part of the wood particles 210 of the wood based powder 200 originate from hardwood(s), such as birch and/or beech; - an embodiment, wherein a part of the wood particles 210 of the wood based powder 200 originate from pine and a part of the wood particles 210 of the wood based powder 200 originate from spruce.

It has been found that the aforementioned process steps for manufacturing the wood based powder can be performed while simultaneously manufacturing plywood. In particular, plywood can also be manufactured such that a primary piece of wood 1 10, i.e. a first wood veneer 1 10, is obtained by peeling (i.e. turning) with a lathe and cutting the resulting veneer mat. Moreover, before the peeling and cutting, a log 910 can be soaked in the aforementioned aqueous solution 806. After peeling and cutting, the first wood veneer is dried in an oven at a reasonably high temperature, e.g. at the temperature indicated above. Moreover, finally, if the first wood veneer 1 10 is a surface veneer of a plywood board 100, the first wood veneer 1 10 may be sanded to finish the plywood board. In addition, in such a case, a normal N of the primary surface 1 12 of the first veneer 1 10 (i.e. a first surface 1 12 of the plywood board 100) is perpendicular to grain direction D1 of the first veneer (i.e. a primary piece of wood 1 10). Moreover, it has been found that separating a veneer from a log by peeling produces lathe checks to the veneer (i.e. the primary piece of wood). In addition, the lathe checks function as channels for some of the remaining volatile components to evaporate during the heat treatment, such as drying. In this way, it seems that separation by peeling has also an effect on the composition of the wood based powder. Thus, in an embodiment, the primary piece of wood 1 10 is peeled from the log 910, i.e. separated by using a lathe.

In this respect, Fig. 2a shows a method for manufacturing the wood based powder 200 and a plywood board 100, which method, as a last step comprises sanding, whereby wood based powder 200 is manufactured. The process starts by falling trees to obtain logs 910. After transfer to a production plant, the logs 910 are first soaked 805 in the aqueous solution 806. The logs are soaked for at least six hours, preferably at least 12 hours, more preferably at least 24 hours. What has been said above regarding the composition of the aqueous solution 806 applies. What has been said above regarding the temperature of the aqueous solution 806 applies. After soaking 805, wood material is separated from the log 910 by peeling 810. Thus, a veneer mat 920 is separated from the log 910. Thereafter, the process comprises cutting 820. The veneer mat 920 may be cut 820 with a blade 822 to form veneers 1 10, 120, 130, i.e. at least a first veneer 1 10, which is an example of a primary piece of wood 1 10. Typically also a second veneer 120, a third veneer 130, a fourth veneer 140, and a fifth veneer 150 is produced either from the same veneer mat 920 or from different veneer mats. Also the other veneers are examples of primary piece of wood that can be sanded. The process comprises drying 830. Typically, the veneers 1 10, 120, 130 are dried in an oven at a temperature T2. The temperature T2 within the oven may correspond to the aforementioned temperature of heat treatment. Thus, the temperature T2 may be from 100 °C to 220 °C, such as from 160 °C to 190 °C such as from 180 °C to 190 °C. In this way, drying can be seen as a primary way for heat treating the primary piece of wood 1 10. Even if not shown in the Figures, the veneer mat 920 may be dried 830 before cutting the veneers 1 10, 120, 130 from the veneer mat 920.

Thereafter, the process comprises application 840 of adhesive to at least some of the veneers 1 10, 120, 130. The adhesive may be applied e.g. on both sides of every second veneer using rollers 841 , 842. The adhesive may be applied e.g. on only one side of each veneer, except for a face veneer, e.g. by spraying or pouring. Thereafter, the process comprises hot-pressing 850. In hot pressing, the veneers 1 10, 120, 130, 140 are arranged in a stack such that adhesive is arranged in between veneers, and the stack is hot-pressed using a first pressing surface 851 and a second pressing surface 852. A temperature of at least one of the first pressing surface 851 and the second pressing surface 852 may be from 100 °C to 160 °C or from 1 10 °C to 160 °C; preferably from 120 °C to 150 °C. In this way, hot-pressing can be seen as a secondary way for heat treating the primary piece of wood 1 10. The stack may be pressed in between the first pressing surface 851 and the second pressing surface 852 by applying a pressure (i.e. a force per area) of from 0.5 MPa to 3.5 MPa, such as from 1 .0 MPa to 3.5 MPa. The temperature and the pressure, in combination, has the effect that the adhesive is hardened, and a solid plywood board 100 is formed. More preferable pressure ranges in hot-pressing include 1 .2 MPa to 3.5 MPa or 1 .5 MPa to 2.0 MPa. It seems that also the hot pressing has an effect on the chemical content of the wood material of a surface 1 12 of the plywood board 100. The plywood board 100 comprises the first veneer 1 10 as a face veneer (i.e. a veneer forming the primary surface 1 12 of the plywood panel 100) and a second veneer 120, which is arranged in between two face veneers.

Alternatively, any other heat treatment can be seen as a tertiary way for heat treating the primary piece of wood 1 10.

Thereafter, the process comprises sanding 860. As indicated above, the first veneer 1 10 (i.e. primary piece of wood 1 10) is sanded with a primary sander 250 having a primary sanding surface 252 to form wood based powder 200.

Referring to Fig. 3a, a plywood board 100 comprises a is a first veneer 1 10, a second veneer 120, a third veneer 130, and an adhesive layer in between the first veneer 1 10 and the second veneer 120. However, preferably, the adhesive layer is not sanded in order not to add dust of adhesive to the wood based powder 200. Thus, in an embodiment, only a part of the first veneer 1 10 is sanded off by said sanding. In other words, the primary surface 1 12 of the plywood board 100 is sanded to separate the wood based powder 200 from the first veneer 1 10 such that the adhesive layer is not sanded.

In general, it has been found that rates of dissolving and flow rates of different compounds of the wood during soaking and during heat treatment are different. Therefore, both soaking and heat treatment selectively affect the content of different compounds of the wood based powder obtainable by sanding.

Table 1 shows the content of some compounds in the following materials: spruce sawdust, which is an example of wood based material that has been neither soaked nor heat treated (i.e. virgin wood material); spruce veneer which is an example of wood based material that has been soaked and dried at elevated temperature; and spruce plywood sanding dust, which is an example of wood based material that has been soaked, dried, and hot- pressed. In addition, Table 1 shows the content of some compounds in the following materials: birch veneer which is an example of wood based material that has been soaked and dried at elevated temperature; and birch plywood sanding dust, which is an example of wood based material that has been soaked, dried, and hot-pressed. The results of Table 1 were determined by acetone extraction from the material using Soxhlet extractor and gas chromatography with flame-ionization detection analysis.

Table 1 : Content of some compounds in some wood based materials.

As indicated in Table 1 , soaking and heat treatment decrease in particular the total amount of extractives. In general, extractives are non-structural compounds present in wood that can be extracted using polar or non-polar solvents. They include compounds such as alkaloids, waxes, fats, proteins, phenolics, gums, pectins, resins, terpenes, and essential oils. Extractives represent a small proportion of wood, typically less than 10% of the dry mass. The composition of extractives varies widely from species to species, and the amount of extractives in a given species depends on the growth conditions. Variations exist also between tree parts (bark, stem, or branch). Some of the wood extractives are water soluble, which means that soaking process will leach some compounds of extractives from the logs. Water-soluble compounds comprise various phenol compounds, carbohydrates, glycosides, and soluble salts. Dissolving some of the extractives from the virgin wood material by soaking is beneficial, since some of the extractives are, in use, also volatile. This affects the odour of the wood based powder.

For example, wood based powder obtained from spruce that has been soaked and dried comprises less than 1.5 mg/g extractives. When compared to saw dust obtained from virgin spruce, which has 3.3 mg/g extractives in total, the total amount of extractives of the soaked wood is only 42 % of the total amount of extractives of the virgin wood. Herein the term“virgin” refers to wood that has not been heat treated or soaked. More precisely the term“virgin” refers to wood that has been only mechanically treated (e.g. sawn, cut, sanded, or lathed). As for birch, the total extractives of birch plywood sanding dust has been found to be 0.85 mg/g or 1 .1 mg/g, as indicated in Table 1 .

Thus, in an embodiment, a total amount of extractives of the wood based powder is at most 1 .5 mg/g. This applies at least to both birch and spruce.

Moreover, in an embodiment, a total amount of extractives of the wood based powder is at most 50 % of a total amount of extractives of wood the same wood species as the wood based powder. In an embodiment, a total amount of extractives of the wood based powder is at most 50 % of a total amount of extractives of dust (e.g. saw dust or sanding dust) obtainable from virgin wood of the same wood species as the wood based powder. This applies at least to both birch and spruce. The term “amount” refers to relative amount, i.e. concentration, as indicated by the unit mg/g.

Referring to Table 1 and with reference to spruce in particular, sterols and lignans may be preferable compounds in some applications; in particular, when the wood based powder is used as a filter material for liquid foodstuff and/or reinforcing material in a hardenable material. Therefore, in an embodiment, the wood based powder 200 comprises at least 0.25 mg/g or at least 0.3 mg/g sterols. This applies to both spruce and birch. In an embodiment, the wood based powder 200 comprises at least 0.90 mg/g or at least 1 .0 mg/g lignans. This applies to spruce. In an embodiment, the wood based powder 200 comprises powder of spruce or birch, and the sterol content of the wood based powder is at least 0.25 mg/g or at least 0.3 mg/g. In an embodiment, the wood based powder 200 comprises powder of spruce and the lignan content of the wood based powder 200 is at least 0.90 mg/g or at least 1 .0 mg/g.

Moreover, as indicated in Table 1 , the soaking decreases the amount of some compounds to a greater extent than the heat treatment increases the content of the compounds. Referring to Table 1 , such compounds include fatty acids, resin acids, betulinol, steryl esters, and triglycerides. In an embodiment, the wood based powder 200 comprises at most 0.1 mg/g or at most 0.05 mg/g triglycerides. This applies to spruce and birch. In an embodiment, the wood based powder 200 comprises at most 0.1 mg/g or at most 0.05 mg/g betulinol. This applies to spruce and birch. In an embodiment, the wood based powder 200 comprises powder of spruce or birch and the content of triglycerides of the wood based powder is at most 0.1 mg/g or at most 0.05 mg/g. In an embodiment, the wood based powder 200 comprises powder of spruce or birch and the betulinol content of the wood based powder 200 is at most 0.1 mg/g or at most 0.05 mg/g.

Furthermore, in an embodiment, the wood based powder 200 comprises from 0.15 mg/g to 0.40 mg/g, such as from 0.20 mg/g to 0.40 mg/g fatty acids. This applies to spruce and birch. In an embodiment, the wood based powder 200 comprises spruce and from 0.40 mg/g to 1.3 mg/g, such as from 0.50 mg/g to 0.80 mg/g resin acids.

Referring to Fib. 2b, sanding 860 of the primary surface 112 of the plywood board 100 may be done in two phases, as indicated above for sanding the primary piece of wood 110. This applies also more generally to sanding any primary surface 112 of any primary piece of wood 110. As a result, primary and secondary (200A, 200B) wood based powders are obtained. The properties may differ, since the grit sizes of the primary and secondary sanding surfaces (252, 254) are different from each other (see above).

Referring to Fig. 2c, in an embodiment, the process comprises, after cutting 820 and before drying 830, pre-drying 870 a veneer 110 by applying pressure. As an example, the veneers 110, 120, ... may be arranged in a pile, which is pressed with a first squeezing surface 861 and a second squeezing surface 862. By applying a large pressure, water is squeezed out of the veneers. The veneers may be squeezed one by one e.g. in between rollers (not shown). A pressure p2 in the range of from 5 MPa to 25 MPa may be used in the pre- drying 870. Referring to Fig. 2d, it has been found that the sanding 860 may produce a mixture 200’ (i.e. an intermediate powder 200’) comprising the wood based powder 200 and larger particles, e.g. splinters 205. As for characterizing splinters from the wood based powder 200, the splinters may have a length of at least 5 mm or at least 2 mm. Therefore, an embodiment comprises producing an intermediate powder 200’ by the sanding 860 and removing splinters from the intermediate product 200’. The splinters may be removed by a cyclone. More specifically, an embodiment comprises removing at least some of such particles that have a length of at least 5 mm from an intermediate powder 200’ to form the wood based powder 200. An embodiment comprises removing at least some of such particles that have a length of at least 2 mm from an intermediate powder 200’ to form the wood based powder 200. The splinters 205 are shown in Fig. 2d.

The sanding affects the size distribution of the resulting wood particles 210. In particular, the roughness (i.e. coarseness) of the sanding surface affects the size distribution. Preferable roughness was indicated above.

Table 2 shows volumetric percentiles of wood particles 210 obtained by sanding spruce or birch. It is noted that the wood based material 200 comprises the wood particles 210 and may comprise also other particulate material. Spruce was sanded with a coarse sanding surface (roughness P60) and a finer sanding surface (roughness P80). Birch was sanded with one or two sanding surfaces having the roughness/roughnesses of P80 and/or P100. Because of a finer sanding surface, the birch based powder is finer. The particle size distribution was measured using a particle size analyser (Beckman Coulter, model LS 13 320), of which operating principle is based on scattering of light. More specifically, a particle is illuminated by a laser, and from the scattering angle and/or scattered intensity, a size of the particle can be calculated using Fraunhofer diffraction theory and Mie scattering theory. The particle is assumed spherical, whereby only one size for a particle is obtained, even if the actual shape of the particle is more complex hereinafter, when considered feasible, this particle size is referred to as a spherical equivalent diameter (SED) or spherical equivalent size (SE size). The result from multiple particles is given as a volumetric SE size distribution, i.e. how large percentage, by volume, belongs to a certain SE size range. Table 2 shows some of the percentile values of such a distribution, i.e. volumetric percentile values. Thus, each N % volumetric percentile value indicates that N % by volume of the particles have a SE size of at most the N % volumetric percentile value. For the results of Table 2, splinters having a SE size of more than 2 mm were not measured, and the results of Table 2 concern only the other particles. Table 2 shows also a reference powder (“Spruce, ref.”), which was obtained by sanding a dry log of spruce in a manner similar to manufacturing plywood. However, the dry log was neither soaked nor heat treated. A similar grit size was used as for the sanding dust indicated by “Spruce, Fine”).

Table 2: volumetric percentile values of some wood based powders.

As indicated in Table 2, the volumetric median SE size (i.e. 50 % volumetric percentile value) of the wood particles ranged in between 108 mΐti and 157 mΐti. This value can be affected by roughness of the sanding surface. In an embodiment, the volumetric median SE size (i.e. 50 % volumetric percentile value) of the wood particles 210 of the wood based powder 200 is from 25 mΐti to 500 mΐti, such as from 50 mΐti to 300 mΐti, such as from 70 mΐti to 200 mΐti. As indicated by the reference sample, the particle size of the reference sample is less, even if the sanding was performed in a similar manner. This seems to be a result of shorter particles having substantially similar width. This shows as a lower aspect ratio, as detailed below in Table 3.

Typically, processes other than sanding, e.g. sawing, result in wood flour having a mass-based median particle size of at least 1 mm. By sieving, it was found that in a typical sawdust, 24.5 wt% of the particles were over 4 mm, 18.0 wt% of the were from 2 mm to 4 mm, and 57.5 wt% of the particles were less than 2 mm. In the other end, mechanical grinding, which is done in papermaking, is performed at a much higher moisture content, thereby producing a suspension of thin fibres having a length of e.g. from 1 mm to 4 mm. The relatively high length of the fibres seems to be due to the wet grinding, which separates the fibres without breaking them, in contrast to the sanding of dry wood to obtains wood based powder 200. The grain orientation D1 of the wood relative to the normal N of the surface that is sanded may also play a role. An angle a1 between the sanding direction R1 relative to the grain orientation D1 may also play a role.

Because of the relatively small particle size, the wood based powder 200 comprising the wood particles 210 is well suited for use with hardenable materials. For example, because of the small size, it mixes better with a matrix material than sawdust. Moreover, because of dryness, it mixes better with a matrix material than a suspension of e.g. wood fibres. Moreover, as detailed below, the aspect ratio of the wood particles is not too high, which also improves the mixing.

Moreover, even if the wood based powder of Table 2 is not a result of fractionating, the size distribution is not very wide. As an example, in the Table 2, a ratio of the 90 % volumetric percentile value to the 10 % volumetric percentile value is from 15 to 17. Correspondingly, in an embodiment, a ratio of the 90 % volumetric percentile value to the 10 % volumetric percentile value is at most 20. As detailed below, this value can be substantially decreased by fractionating, e.g. sieving, the powder. However, if coarse and fine powders are intermixed, this value would increase. As an example, in the Table 2, a ratio of the 75 % volumetric percentile value to the 25 % volumetric percentile value is from 3.7 to 4.0. Also this value can be decreased by fractionating and increased by mixing fine and coarse powders.

It has been found that the wood based powder 200 is utilizable as a filler or filtering material when the size distribution of the particles is not too narrow. When the wood based powder 200 comprises both large and small particles, the small particles fill up the spaces in between the larger particles thereby forming a reasonably dense packing. For this reason, in an embodiment, a ratio of the 75 % volumetric percentile value to the 25 % volumetric percentile value is at least 3.0 or at least 3.3.

However, in other terms, the aforementioned size distribution indicates that the powder 200 is, to some extent, homogeneous, which also indicates better mixing with a matrix material. Moreover, if used as a feedstock material for chemical processes, the reasonably small variation in size is beneficial from the point of view of process design (i.e. the design of the process, in which the powder 200 is used as feedstock). For this reason, in an embodiment, a ratio of the 75 % volumetric percentile value to the 25 % volumetric percentile value is at most 4.5 or at most 4.0.

However, it has also been found that wood based powder 200 may be utilizable more efficiently in some applications, if the size distribution of the wood based powder is not too wide. An even narrower side distribution can be obtained by fractionating 890 the wood based powder 200. Sieving is an example of a fractionation 890. With reference to Fig. 2e, an embodiment comprises fractionating the wood based powder 200 (or the primary wood based powder 200A or the secondary wood based powder 200B) to at least a first wood based powder 200a and a second wood based powder 200b. The powder 200 is fractionated such that an average particle size of the first wood based powder 200a is different from an average particle size of the second wood based powder 200b. An embodiment comprises fractionating the wood based powder 200 to at least a first wood based powder 200a, a second wood based powder 200b, and a third wood based powder 200c. The powder 200 is fractionated such that an average particle size of the first wood based powder 200a is different from an average particle size of the second wood based powder 200b; the average particle size of the first wood based powder 200a is different from an average particle size of the third wood based powder 200c; and the average particle size of the second wood based powder 200b is different from the average particle size of the third wood based powder 200c. For example, sieving results in the largest particles (e.g. of the third powder 200c) being arrested on top of a sieve, while the smallest particles (e.g. of the first powder 200a) can be collected from a bottom of the sieve (see Fig. 2e).

Thus, in an embodiment, a size distribution of the wood based powder 200 (or first wood based powder 200a) is reasonably narrow. Without showing any results, after fractionating, a size distribution of the fraction (e.g. first wood based powder 200a) may be such that a ratio of the 75 % volumetric percentile value to the 25 % volumetric percentile value is e.g. at most 2.5. E.g. fractionating by sieving may be done using such a sieve that the sieve size halves in between each two subsequent sieves. This corresponds to situation in which the size of the smallest particles of a fraction is roughly half of a size of the largest particles of the same fraction.

Referring still to Fig. 2e, the different fractions 200a, 200b, and 200c, can be conveyed to separate containers 240a, 240b, 240c for use without mixing the fractions. Thus, an embodiment comprises conveying the first wood based powder 200a to a first container 240a and conveying the second wood based powder 200b to a second container 240b. The powders may be conveyed e.g. pneumatically or by a screw.

With reference to Fig. 2b, even if the method does not comprise fractionating, two different fractions may be produced by subsequent sanding steps. Also in such an embodiment, the different powders 200A and 200B can be conveyed to separate containers 240A, 240B for use without mixing the fractions. Thus, an embodiment comprises conveying the primary wood based powder 200A to a primary container 240A and conveying the secondary wood based powder 200B to a secondary container 240B. The powders may be conveyed e.g. pneumatically or by a screw.

With reference to Fig. 2a the wood based powder 200 can be conveyed to a containers 240. The powder may be conveyed e.g. pneumatically or by a screw.

It has also been found that the sanding produces wood particles that have a reasonably low aspect ratio, however, still more than one. Generally, a length of a particle is the greatest one dimensional measure thereof, and a width is a one dimensional measure that is perpendicular to the length. Still further, an aspect ratio is generally defined as the length divided by the width. Such a definition is used hereinbelow, in particular, when the length and width are measured using a CCD camera. Flowever, it is noted that sometimes also a thickness is defined as the smallest one dimensional measure of the particle, the thickness being perpendicular to the length and the width. The CCD measurement technique does not distinguish between a“thickness” and a “width”. Flowever, in general it seems that there is not large difference in between a width and a thickness of the wood particles of the wood based powder.

Typical wood fibres have a very large aspect ratio of e.g. from 100 to 150 depending on wood species. Moreover, in general, on object having an aspect ratio of at least 50 may be called a fibre. However, the particles of the wood based powder 200 are not fibrous. Moreover, typically processes other than sanding, e.g. sawing, result in wood flour having an average aspect ratio of particle of the order of at most 5. Specifically, an aspect ratio of particles of sawdust is typically at most 5. However, for many applications, a too high an aspect ratio is not beneficial, because of problems with stirring. Moreover, a too low an aspect ratio is not beneficial, because of such particles does not adhere as well to matrix materials. Moreover, a surface to volume ratio can be increased by increasing aspect ratio. Thus, particles with a low aspect ratio do not strengthen a composite materials as much as particles with a higher aspect ratio.

It seems that sanding produces wood particles 210 of which aspect ratio somewhere in between the aforementioned limits. In order to quantify this, the size and shape of wood particles were measured. As detailed above, the SE size measurements based on diffraction only give indication of SE size, but not any indication of shape. In order to measure both size and shape, the technique detailed in the thesis“Utilization of tube flow fractionation in fibre and particle analysis” by Ossi Laitinen (ISBN 978-951 -42-9449-5) was used.

In brief, in the method, the particles to be analyzed (in this case the wood based powder 200) is mixed with deionized water to form a suspension. The suspension is conveyed within a tube in order to cause a flow therein. The flow has the tendency of moving large particles first, and smaller particles only later. In this way, the tube flow is a fractionation mechanism to separate large particles from smaller ones. This provides for accuracy of optical measurements.

The flow is passed by a CCD camera that takes images of the particles of the suspension. An image of a particle is a two-dimensional projection of the particle. From the images, the maximum one dimensional measure a (i.e. maximum Feret’s diameter, i.e. length) for each one of the particles is measured. In addition, at least one of (i) the area A of the two-dimensional projection and (ii) another one dimensional measure (i.e. another Feret’s diameter) i.e. width b is determined. If only the area is measured, from the area A, the width b of the particle can be determined, e.g. by assuming an oval shape. Therefrom an aspect ratio can be calculated as AR =a/b.

Measurements of several wood particles of spruce and birch (separately) and using different sanding surfaces indicated an aspect ratio of from 15 to 21 (see Table 3). Flaving an aspect ratio that is not too high facilitates the mixing the particles within a matrix e.g. by stirring. Stirring fibres oftentimes results in the fibres to stuck with the tool used for stirring.

To visualize the aspect ratio, Fig. 5a shows a scanning electron microscopy (SEM) image of some particles of the wood based powder 200. The lengths (i.e. maximum Feret’s diameters) have been shown for some particles. The widths of these particles are also observable from the figure. For comparative purposes, Fig. 5b shows an image of sawdust. As seen from these figures, an average aspect ratio of the particles of the wood based powder of Fig. 5a is higher than an average aspect ratio of the particles of the sawdust of Fig. 5b.

Table 3 shows average lengths and aspect ratios of some wood based particles as measured with the CCD measurements. In the Table 3, the length is a length-weighted average of the length, the width is an average width, and the aspect ratio is an average aspect ratio. In addition, Table 3 shows a percentage (by number) of fines. For results of Table 3, the fines are defined as particles having a length of less than 20 mΐti.

Table 3: Average length, width, and aspect ratio of some wood based particles and a reference. The results of Tables 2 and 3 are not easily comparable, since the Table 2 relates to a spherical equivalent diameter, whereas for Table 3 a different measurement technique for the length was used. These methods do not, in general, give exactly the same results.

To test the effect of the heat treatment of the properties of wood based powder, spruce, which was neither soaked nor heat treated, was sanded with a similar sanding surface as the sample“Spruce, Fine” in Table 3. Moreover, the length- weighted average of the length, the average width, and the average aspect ratio of such particles was measured using the CCD technique as described above. Some of the results are shown in Table 3 (“Spruce, ref.”). When compared to other results of Table 3, it seems that the soaking and heat treatment have the effect that the particles resulting from sanding are about twice as long as without the treatments, resulting in higher aspect ratio of the particles.

Based on the CCD measurements, an average width b of the particles 210 may be from 2 mΐti to 15 mΐti. Noteworthy to say, this applies both to the soaked and heat treated material as well as to the reference material. Moreover, in these measurements, a length weighted average length a of the wood particles was from 25 mΐti to 300 mΐti, such as 40 mΐti to 200 mΐti, or as indicated in the Table 3, from 52 mΐti to 147 mΐti. It is also noted that the length and/or the width as defined above, may differ, and differs, from the SE size as measured by the diffraction measurements. Typically, the diffraction measurements overestimate the size, when the particle is not spherical.

It seems that the aforementioned aspect ratio is at least to some extent a result of tearing the wood particles 210 from the substantially dry primary piece of wood 110 while sanding as detailed above. The tearing seems to break to wood fibres of the primary piece of wood 110, which results in somewhat shorter particles than e.g. grinding. Tearing occurs at least when the grain orientation of the surface that is sanded is proper, as detailed above. Shorter particles have a smaller aspect ratio. The tearing of the material also has the effect, that the surface area of the wood particles 210 is reasonable high. This also affects the usability of the wood based powder 200 as a filler material or a filter material or a reinforcing material. In an embodiment, an average aspect ratio of the wood particles is from 7 to 30. In an embodiment, an average aspect ratio of the wood particles is from 10.0 to 25. A factor affecting the usability of the wood based powder is the amount of very small particles comprised by the wood based powder. These very small particles are often called as fines. A threshold SE size (i.e. SE diameter, i.e. SED) for what is a fine and what is not may be e.g. 20 mΐti or 32 mΐti. Particles smaller than this threshold may be called fines. Table 4 indicates the volumetric percentage of the fines of the samples of Table 2. SE size measurements for results of Table 4 were performed using the Beckman Coulter LS 13 320 size analyser.

Table 4: Volumetric percentage of fines for two different fine threshold, and for four different samples of wood based powder. Results are not available for birch with fine threshold 20 mΐti. Results from a reference sample are also shown in Table 4. As indicated in Table 4, the wood based powder 200 comprises a significant amount of fines. More specifically, in an embodiment, the wood based powder comprises such wood particles 210 that at least 7 vol-% of the wood particles 210 are at most 32 mΐti in SE size. In a similar manner, in an embodiment, the wood based powder comprises such wood particles 210 that at least 5 vol-% of the wood particles 210 are at most 20 mΐti in SE size. Moreover, in an embodiment, the wood based powder comprises such wood particles 210 that at most 15 vol-% of the wood particles 210 are at most 32 mΐti in SE size.

It has been found that the lignin content of fines is oftentimes higher than a lignin content of lager wood particles. Thus, e.g. if such a wood based powder that contains a high amount of lignin is needed, the wood based powder may be fractionated. In particular, in an embodiment, the wood based powder is fractionated to fines and non-fines. In an embodiment, the wood based powder is fractionated to at least fines having a SE size of less than 20 mΐti or less than 32 mΐti. More specifically, at least 95 vol% of the fines may have a SE size of less than 20 mΐti or less than 32 mΐti. As for the remaining part, at most 1 vol% of the wood particles 210 of the remaining part may be smaller than 32 mΐti or smaller than 20 mΐti.

Moreover, the fines are typically more hydrophobic than other fractions of the wood based powder. Hydrophobicity may be another reason for fractionating the fines to obtain [a] wood based powder comprising mainly fines and [b] wood based powder substantially free from fines. Clearly, the wood based powder substantially free from fines would have a lower lignin content and would be more hydrophilic than the powder before fractionating. It has been found that lignin may cause colouring of a composite, when the powder is used as a part of a composite. It is also noted that in the method, primary piece of wood 1 10 comprises lignin. Correspondingly, the wood particles 210 of the wood based powder 200 comprise lignin. This is in contrast to e.g. a pulp suspension, of which manufacturing process involves removal of lignin.

Referring to Fig. 2f, the making of the wood based powder (200, 200A, 200B) in a plywood manufacturing process has the further benefit that the primary piece of wood 1 10, i.e. in case of plywood a first veneer 1 10, is of a higher quality than another veneer. Referring to Fig. 3a, the veneers 1 10, 120, 130, 140, 150 herein are numbered such that the first veneer 1 10 forms the primary surface 1 12 of the plywood panel. Moreover, the second veneer 120 is attached to the first veneer 1 10 such that no other veneer is arranged in between the first 1 10 and the second 120 veneers. Moreover, the second 120 veneer is arranged in between the first veneer 1 10 and the third veneer 130. Thus, the second 120 veneer does not form such a surface (1 12, 122) of the plywood board 100 that a normal of the surface (1 12, 122) is unidirectional with the thickness of the plywood board 100. Since the second veneer 120 is not visible from the plywood board 100, the quality of the second veneer 120 may be lower that the quality of the first veneer 1 10. The term quality refers to multiple features of a veneer 1 10, 120, 130. Applicable quality factors can be found from e.g. the standards ISO 2426-1 (2000-12-01 ), ISO 2426-2 (2000-12-01 ), and ISO 2426-3 (2000-12-01 ). ISO 2426-2 applies for veneers of hardwood, while ISO 2426-3 applies for veneers of softwood.

With reference to Fig. 2f, an embodiment comprises determining 835 that a quality of the second veneer 120 is less than a quality of the first veneer 1 10. The embodiment further comprises determining 835 that a quality of the second veneer 120 is less than a quality of another veneer, the other veneer not being the first veneer or the second veneer. Thus, the other veneer (i.e. the fifth veneer in Fig. 2f) may be used as the other face veneer, i.e. to form the secondary surface 122 (see Fig. 3a) of the plywood board, wherein the secondary surface 122 is opposite to the primary surface 1 12 and has a normal to the direction of thickness of the plywood board.

Therefore, in an embodiment, the primary piece of wood 1 10 is a first veneer 1 10, formed from wood by peeling. The embodiment comprises receiving a second veneer 120 formed from wood by peeling and a third veneer 130 formed from wood by peeling. The embodiment comprises determining 835 that a quality of the second veneer 120 is less than a quality of the first veneer 1 10.

The embodiment comprises attaching the second veneer 120 to the first veneer 1 10 and the third veneer 130 to the second veneer 120 by adhesive 190 in such a way that the first, second, and the third veneers (1 10, 120, 130) are arranged on top of each other and the second veneer 120 is arranged in between the first veneer 1 10 and the third veneer 130, to form a plywood board 100 having a primary surface 1 12 that is formed by the first veneer 1 10 and having the second veneer 120 as an intermediate veneer. In short, the embodiment comprises forming a plywood board 100 having a primary surface 1 12 that is formed by the first veneer 1 10. What has been said about the normal N of the primary surface 1 12 applies. The embodiment comprises, thereafter, sanding at least the primary surface 1 12 of the plywood board 100 to separate the wood based powder 200 from the first veneer 1 10. In this way, the second veneer 120 is not sanded. It is not sanded, because it is an intermediate veneer, i.e. arranged in between two other veneers 1 10, 130. In case the third veneer 130 is also an intermediate veneer (as in Fig. 2f, wherein only the veneers 1 10, 150 are face veneers) also the third veneer 130 is not sanded.

However, the embodiment may comprise sanding at least a secondary surface 122 of the plywood board 100 to separate the wood based powder 200 from the secondary surface. The third veneer 130 may be a face veneer (not shown), whereby a surface of the third veneer 130 may form the secondary surface 122, which may be sanded. Fig. 2g shows an embodiment, wherein both the primary surface 1 12 and the opposite secondary surface 122 are sanded. It is noted that in plywood, the first veneer 1 10 does not form the secondary surface 122.

Sanding only high quality veneers, such as the first veneer 1 10 has the effect that the wood particles 210 obtained by sanding have a high quality. In other words, the variation of composition and/or size of wood particles is diminished. According to the aforementioned standards, the quality of a veneer is related e.g. to an amount of knots in the veneer. In a high quality veneer, the number of knots is less than in a low quality veneer. Thus, by sanding only high quality veneers, a lesser number of knots will be sanded. It seems that the material composition and the resulting particle size obtainable by sanding knots differs from the composition and the resulting particle size obtainable by sanding knot-free wood.

Producing the wood based powder in such manner also has the effect that the surface veneers (at least 1 10), which may be of higher quality than the intermediate veneers, are often produced from sapwood. Correspondingly, typically heartwood is used mainly for the intermediate veneers (e.g. 120). Moreover, sapwood typically has better properties than heartwood. Thus, by producing the wood based powder by sanding plywood has the effect that a larger portion of the wood based powder is made from sapwood. This may be one reason for the wood based powder having the beneficial properties as described herein. Therefore, an embodiment comprises sanding the primary surface 1 12 of the primary piece of wood 1 10, wherein the primary surface 1 12 comprises sapwood. The sapwood may be sapwood of one of spruce, pine, beech, or birch.

Referring still to Fig. 2f, as indicated above, adhesive may be applied e.g. on both surfaces of every second veneer. For example, adhesive may be applied to the veneers 120, 140 that will be arranged next to the face veneers 1 10, 150 of Fig. 2f. This is indicated in Fig. 2f by introducing the veneers 120, 140 between the rollers 841 , 842. Correspondingly, adhesive need not be applied to the veneers 1 10, 130, 150 that are arranged next to the veneers of which both surfaces have been covered with adhesive. As an alternative each (e.g. 120, 13, 140, 150) but one (e.g. 1 10) of the veneer may be provided with adhesive on one side only (not shown).

Referring to Figs. 2a, 2b, and 2e, preferably, the wood based powder 200 or the different wood based powders (primary 200A, secondary 200B, first 200a, second 200b, third 200c as obtainable from different sanding phases and/or by fractionation) is/are conveyed 865 to a container 240, 240a, 240b, 240c. Referring to Fig. 2a, when only one fraction of wood based powder 200 is produced, the wood based powder 200 is preferably conveyed 865 to a container 240 with a conveyor 230. Referring to Fig. 2b, when two wood based powders 200, i.e. the primary wood based powder 200A and the secondary wood based powder 200B, are produced, the different fractions are preferably conveyed to separate containers. I.e. the primary wood based powder 200A is conveyed 865 to a primary container 240A using a primary conveyor 230A; and the secondary wood based powder 200B is conveyed 865 to a secondary container 240B using a secondary conveyor 230B. Flowever, even if not shown, when more than one fractions of wood based powder 200 are produced, the different fractions may be conveyed to the same container. Flowever, as a result, the different fractions are mixed, which may not be beneficial in all applications of the wood based powder, since this would widen the size distribution of the particles. Referring to Fig. 2e, when fractions 200a, 200b, 200c of wood based powder 200 are produced, the different fractions are preferably conveyed to separate containers 240a, 240b, 240c. Thus, an embodiment of the method comprises conveying 865 the wood based powder (200, 200a, 200b, 200c) to a container (240, 240a, 240b, 240c). The wood based powder 200, 200a, 200b, 200c is conveyed with a conveyor arrangement. Preferably the conveyor arrangement comprises a pneumatic conveyor, but it may comprise e.g. a screw conveyor. Thus, a preferably embodiment comprises pneumatically conveying 865 the wood based powder (200, 200a, 200b, 200c) to the container (240, 240a, 240b, 240c). In an embodiment, the first wood based powder 200a is conveyed 865 to a first container 240a using a first conveyor; and the second wood based powder 200b is conveyed 865 to a second container 240b using a second conveyor. In Fig. 2e, furthermore the third wood based powder 200c is conveyed 865 to a third container 240c using a third conveyor. Storing different fractions in different containers has the effect that the size distribution of the wood based powder remains reasonably narrow.

Each one of the fractions 200a, 200b, 200c of the wood based powder 200 are also wood based powders 200. It has been found that such wood based powder 200 (the term including the wood based powders 200A, 200B, 200a, 200b, 200c obtained by fractionating and/or subsequent sanding phases) has various applications. For example, the wood based powder may be used as a filter material. The wood based powder may be used as a filter material for filtering liquid suspension, most preferably aqueous suspension. Such aqueous suspensions are common in the food industry, including breweries and other fermenting processes. As another example, the wood based powder may be used as part of a composite material. The wood based powder may be used in composites e.g.

- to reinforce the composite and/or

- to replace some more expensive material (i.e. as a filler) and/or

- as a hardener and/or

- as an additive.

Thus, in an embodiment, the wood based powder 200 (or at least one of the primary wood based powder 200A, secondary wood based powder 200B, first wood based powder 200a, the second wood based powder 200b, and the third wood based powder 200c) is preferably utilized as part of a composite material 300. Figures 4a and 4b illustrate utilization of the wood based powder 200, 200A, 200B, 200a, 200b, 200c as part of a composite material 300. Referring to Figs. 4a and 4b, the embodiment comprises arranging available the wood based powder 200, 200A, 200B, 200a, 200b, 200c as discussed above. The wood based powder may be arranged available by manufacturing the wood based powder with a method as discussed above. The wood based powder may be arranged available by buying it from a supplier. The method comprises arranging available hardenable material 310.

Referring to Fig. 4a, in an embodiment, the hardenable material 310 comprises liquid that is hardenable, e.g. polymerizable. Examples include moldable thermoset polymers, including epoxy and phenol-formaldehyde; and thermoplastic polymers in the molten state. The liquid hardenable material 310 and the wood based powder 200 may be mixed to form a preform 305 for composite material. The preform 305 may be subsequently hardened to form the composite 300.

Referring to Fig. 4b, in an embodiment, the hardenable material 310 comprises granules of solid material. The granules may be e.g. granules of some thermoplastic material. The granules may comprise polyethylene (PE, including PE homopolymer and PE copolymer), polypropylene (PP, including PP homopolymer and PP copolymer), or polylactic acid (PLA), or a mixture of at least two of these. The granules of the hardenable material 310 and the wood based powder 200 may be mixed to form a preform 305 for composite material . In the embodiment of Fig. 4b, the hardenable material 310 of the preform 305 for composite material may be melted to form a viscous or liquid preform 305’ for composite material. Naturally, the wood based powder 200 may be mixed to liquid material obtained by melting thermoplastic material.

Preferably, a melting temperature of the thermoplastic material is at most 220 °C, such as from 105 °C to 190 °C or from 120 °C to 180 °C. This has the effect that melting temperature is not much higher that the temperature used to manufacture the wood based powder. In this way, substantially all such compounds of wood that would be volatile at the temperature of the melted thermoplastic have been volatilized already during the heat treatment, when preparing the wood based powder. In this way volatile compounds (VOC) are not present, or they are present at most to an insignificant amount, in the when mixed with the thermoplastic. Volatile compounds could e.g. impose porosity to the composite. Irrespective of the polymer matrix being thermoplastic or thermoset, the removal of volatile compounds (VOC) from the wood by heat treatment is beneficial. In general, the volatile compounds, if not removed, may have the effect of the material having a characteristic odor, which not necessarily preferable.

Referring to Figs. 4a and 4b, thereafter, the preform 305, 305’ for composite material is hardened to form the composite material 300. As indicated above, the preform 305 for composite material may be melted before hardened. The preform 305 may be hardened in a mould to form an object of the composite material 300 with a certain shape. As indicated in Figs. 4a and 4b, the composite 300 comprises hardened material 320 and the material 200’ of the wood based powder 200. Clearly, the material 200’ is no longer a powder, as it is bound by the hardened material 320. The hardened material 320 is obtained from the hardenable material 310 by hardening, optionally after liquefying, e.g. melting the hardenable material (see Figs. 4b and 4d).

In an embodiment, the hardenable material 310 is mixed with the wood based powder 200, 200A, 200B, 200a, 200b, 200c in such an amount that the composite material 300 comprises the wood based powder 200, 200A, 200B, 200a, 200b, 200c in an amount of at least 10 wt% or at least 15 wt%. Flowever, the benefits of using wood based powder may be observable also with a lesser amount. For example, in an embodiment, The hardenable material 310 is mixed with the wood based powder 200, 200A, 200B, 200a, 200b, 200c in such an amount that the preform 305 comprises the wood based powder 200, 200a, 200b, 200c in an amount of at least 10 wt% or at least 15 wt%. One benefit of using wood based powder in a composite is that by using such wood based powder is discussed above, the content thereof can be raised higher than in case some other type of particulate wood material would be used. Thus, in a preferable embodiment, hardenable material 310 is mixed with the wood based powder 200, 200A, 200B, 200a, 200b, 200c in such an amount that the preform 305 comprises the wood based powder 200, 200a, 200b, 200c in an amount of at least 50 wt%, at least 80 wt%, or at least 90 wt%.

The composite material may comprise the polymer matrix material e.g. from 3 wt% to 75 wt%, such as 4 wt% to 60 wt%, such as from 5 wt% to 50 wt%. Thus, in an embodiment, the hardenable material 310 is mixed with the wood based powder 200, 200A, 200B, 200a, 200b, 200c in such an amount that the preform 305 comprises the hardenable material within these limits. If a high proportion of the wood based powder 200 is used, a lesser proportion of the polymer matrix is needed. As conventional, the weight percentages (wt%) of the wood based powder 200, the hardenable material 310 and the other compounds (as detailed below) add up to 100 wt%.

The composite material may further comprise other materials including an auxiliary filler, coupling agent, or lubricant.

The auxiliary filler 312 may be an inorganic filler or an organic filler. An inorganic filler may comprise kaolin clay, ground calcium carbonate, precipitated calcium carbonate, titanium dioxide, wollastonite, talcum, mica, silica, or a mixture thereof.

As an organic auxiliary filler material 312 sawdust can be used. The auxiliary filler material 312 may be mixed with the hardenable material 310. Fig 4c shows an example, wherein the auxiliary filler material 312 and wood based powder 200 are added to liquid or viscous hardenable material 310. Fig 4d shows an example, wherein the auxiliary filler material 312 and wood based powder 200 are added to granules of solid hardenable material 310. Flowever, in an embodiment other material is not used. In this embodiment, the preform 305 consists of the hardenable material 310 and the wood based powder 200, 2000A, 200B, 200a, 200b, 200c. Correspondingly, the composite 300 consists of the hardened material 320 and the wood based powder 200, 200A, 200B, 200a, 200b, 200c.

The coupling agent for the composite material may be a polymeric coupling agent. The coupling agent may, in principle, be any chemical which is able to improve the adhesion between two main components. This means that it may contain moieties or components, which are reactive or compatible with matrix material and moieties or components, which are reactive or compatible with the wood based powder. Examples of coupling agent comprise or consists of anhydrides, preferably maleic anhydride (MA), polymers and/or copolymers, such as maleic anhydride functionalized FIDPE, maleic anhydride functionalized LDPE, maleic anhydride-modified polyethylene (MAHPE), maleic anhydride functionalized EP copolymers, maleated polyethylene (MAPE), maleated polypropylene (MAPP), acrylic acid functionalized PP, HDPE, LDPE, LLDPE, and EP copolymers, styrene/maleic anhydride copolymers, such as styrene-ethylene-butylene-styrene/maleic anhydride (SEBS-MA), and/or styrene/maleic anhydride (SMA), and/or organic-inorganic agents, preferably silanes and/or alkoxysilanes such as vinyl trialkoxy silanes, or combinations thereof.

The lubricant the composite material may be a wax or a wax based material. Lubrication is a technique of using a lubricant to reduce friction and/or wear in a contact between two surfaces. Typically lubricants contain 90% base oil and less than 10% additives. Due to their efficient lubricating effect, the lubricants significantly improve the flow characteristics and process behavior of compounds during extruding, molding, etc. Lubricants reduce viscosity, promote dispersion, shorten mixing times and lower mixing temperatures and energy requirements. Examples of lubricants suitable for the present applications include hydrocarbons, stearates, fatty acids, esters and amides, which may be modified with functional groups.

The term“composite material” may refer to the material before hardening or after hardening. Therefore, in an embodiment, a composite material 300 comprises the wood based powder 200 (the term herein including fractions 200a, 200b, 200c thereof and/or powders 200A, 200B obtained from different sanding phases) and [a] hardened plastic material 320 and/or [b] hardenable material 310. The form “and/or” is used here, since hardening of the hardenable material 310 is not necessarily perfect. Therefore, some of the hardenable material may remain in an unhardened state also after hardening. Moreover, in a preferable embodiment, the composite material comprises the wood based powder in an amount of at least 15 wt%. In a preferable embodiment, a total amount of the hardened plastic material 320 and/or hardenable material 310 is at most 50 wt% in the composite 300.

The preform 305 and/or the composite material 300 may be used for various purposes. Workability of the composite material 300 is improved, when the hardened material 320 of the composite material 300 is thermoplastic.