METHOD OF PRODUCING COMPRESSED LAYERED STRUCTURES Background of the Invention Field of the Invention The present invention relates to the manufacture of fiberboards and similar wood-based products. In particular, the present invention concerns the manufacturing of such products by a process comprising the steps of providing a lignocellulosic material, contacting the lignocellulosic material with an activating agent to produce a modified lignocellulosic material containing free radicals, forming the modified material into a layered structure, and pressing the layered structure into a compressed product.

Description of Related Art The rapid increase in the production of particleboards, flakeboards and fiberboards, particularly medium density fiberboards and especially MDF board production during the last decades demands an adhesive that is cost efficient, available in large quantities, easy to use and independent of crude oil availability. Lignin meets well these demands, and it does not contain formaldehyde, which traditionally has been considered a serious problem with urea-formaldehyde (UF) adhesives.

The use of spent sulfite liquor (SSL) as an adhesive for paper, wood and other lignocellulosic materials is well known in the art, and a large number of patent applications has been filed during the last three decades for the use of lignin products as adhesives for particleboard, plywood and fiberboard instead of conventional PF or urea- formaldehyde adhesives. The main drawback of using SSL as an adhesive for fiberboard manufacture is, however, its hygroscopicity. For this reason, it cannot really compete with other natural or synthetic adhesives.

Curing of lignin is a cross-linking process, which leads to new carbon-carbon and ether bonds between different lignin molecules or within one macromolecule. Inter-as well as intramolecular cross-linking reactions decrease the solubility and swelling of lignin.

Cross-links in lignin can be achieved either by condensation or by radical coupling reactions. Further, it has been shown that laccase enzymes and other peroxidases can be used as polymerization or curing catalysts of lignin (DE Patent No. 3 037 992, WO 96/03546). However, the use of the enzymes for catalyzing radical formation has shown limited success so far. Fibers and wood chips used in the production of the fiberboard contain 5-20 % water, and the laccases need some water for efficient diffusing

into the material and for catalyzing the polymerization reaction, which is necessary for extensive bonding of the fiberboard. Kraft and native lignins are mostly insoluble in water and, thus, two solid phases are formed on the production line. An uneven distribution of the solids causes spotting and large reduction in the strength properties of the board formed during the pressing stage. The enzymatic methods described in the art suffer also from fact that the application methods are difficult and expensive and require additional process stages, such as soaking of lignin or fibers with enzymes in water. Another problem is formed by drum mixing of adhesive in the otherwise continuous board manufacturing process.

Instead of lignin-based adhesives, it has been suggested to activate the lignin of wood fibers with laccase alone, and to use these fibers as such without any additional binders for manufacturing wood fiberboards (cf. EP Patent No. 0 565 109). In the known method, the middle lamella lignin is activated by"incubation", which comprises the step of contacting the lignocellulosic material, such as mechanically defiberized pulp, with laccase in aqueous phase over prolonged periods of time. One of the main problems relating to said technology is the extremely long incubation time required (up to seven days), which makes the prior art method economically unattractive. Further, the strength properties of the panels described have been unsatisfactory.

Summary of the Invention The present invention aims at eliminating the problems relating to the prior art. It is in particular an object of the present invention to produce fiberboard of high quality by using technically practicable and economically inexpensive steps.

The present invention is based on the finding that the internal bonding strength is strongly dependent on the actual number of free radicals in the lignocellulosic material.

Furthermore, it appears that the phenolic backbone of the lignin in the lignocellulosic material is capable of significantly stabilizing the radicals and to increase their half-life.

As a result, by generating a large amount of radicals in the lignocellulosic material during defibering and any subsequent treatment step with an activating agent it is possible to obtain a modified lignocellulosic material in which the activated fibers and other lignocellulosic particles will strongly bond to each other when they are intimately contacted during a succeeding pressing step.

Thus, according to the present invention the lignocellulosic material is processed in such a way that it can be activated by means of an activating agent in order to produce a modified lignocellulosic material containing an increased number of free radicals/g of

lignocellulosic material. Depending on the wood species used, the actual minimum number varies somewhat. However, we have found that for softwood fibers the level is 1 x 1018 free radicals/g of dry lignocellulosic material and for hardwood fibers 3 x 1018.

By pressing the layered structure it is possible to form a compressed product, which has an internal bonding of at least 0.7 MPa.

More specifically, the present invention is mainly characterized by what is stated in the characterizing part of claim 1.

The present invention provides considerable advantages. Good bonding of the fibers is obtained; in fact, the bonding strength is at the same level or better than that obtainable with conventional phenol or urea formaldehyde resins. Also absolute strength properties exceeding the existing standard values obtainable by conventional phenol or urea formaldehyde resins at the same board density level can be achieved by the present invention. Further, one particularly valuable advantage obtained by using the technique presented herein is that a shorter pressing time can be used after the mat-former compared to the times used in the conventional PF or UF resin techniques. This will increase the production capacity of the existing production plants.

Furthermore, in comparison to the above-mentioned in situ gluing of lignin, the contacting times are much shorter, since incubation is not needed and pressing times are also greatly reduced. Contacting of lignocellulosic material with the activating agents can be further improved by keeping the lignocellulosic material in a fluffed state. In such a state the lignocellulosic material can be vigorously mixed so that the individual particles of the material are evenly subjected to the action of the activating agent. This will provide a modified lignocellulosic material, which contains homogeneously distributed free radicals.

Compressed products will have high and consistent strength properties. Since the particles are not suspended in a liquid phase, their moisture content can rapidly be reduced, which facilitates the mechanical formation of a layered structure.

Brief Description of the Drawing Figure 1 gives an outline of the first part of a MDF process scheme showing the addition points of the lignin-based adhesive

Detailed Description of the Invention This invention is presented in more detail below. Several examples of the work done are provided.

Definitions The term"fibrous lignocellulosic material"denotes finely divided particles of vegetable origin, in particular derived from wood or annual or perennial plants. Preferably the material is in the form of fibers, fibrils and similar fibrous particles.

"Fiberboard"or"fibrous panel"is a product for, e. g., constructional uses including insulation purposes and for use in boarding, flooring and furniture applications. It primarily comprises lignocellulosic fibers mixed with a suitable adhesive. It should be pointed out that the present products can be called"layered structures"which term includes both the above-mentioned boards and panels as well as compressed structures of any shape. They do not necessarily need to be flat or laminar but they can have any form as long as they contain several adjacent layers of fibers.

The present invention can also be employed for the manufacture of particleboards, flakeboards and similar structures.

The"lignocellulosic"material can comprise any lignin-containing material and it is preferably selected from the group of finely-divided raw materials, including woody materials, such as wood particles (e. g in the form of wood chips, wood shavings, wood fibers and saw dust), and fibers of annual or perennial plants. The woody raw material can be derived from hardwood or softwood species, such as birch, beech, aspen, alder, eucalyptus, maple, mixed tropical hardwood, pine and spruce. Nonwood plant raw material can be provided from straws of grain crops, reed canary grass, reeds, flax, hemp, kenaf, jute, ramie, sisal, Abaca, coir, bamboo and bagasse.

A suitable finely-divided raw material can be provided by any process producing a comminuted material from lignin-containing starting materials. Refining, grinding and milling can be mentioned as examples of applicable processes. Particularly preferred processes are those which produce particles which are lignin-covered. Disc refining in the presence of steam is a suitable process for producing fibers suitable for fiberboard manufacture. The TMP process with an optional chemical pretreatment can be mentioned as a specific example of such processes.

Generally, the particles have sizes in the range of 0.01 to 50 mm. Particularly advantageous properties are obtained with wood fibers having a fiber length distribution, in which at least 50 % are shorter than 0.249 and preferably at least 35 % are shorter than 0.125 mm. A distribution of this kind will reduce swelling of the compressed product.

For the purpose of the present invention, the expression"in fluffed state"is used to designate a state in which the finely-divided or comminuted lignin-containing material is dispersed in gas phase and in which the lignin-containing material is free-flowing because it contains only small amounts of free water, if any. Such fluff material can be fluidized in a stream of a gas or a gas mixture, such as oxygen or air. The gas may optionally contain suspended matter in the form of solids or droplets.

According to a preferred embodiment, the activation of the fiber is carried out in fluffed state. As used herein, "in fluffed state"is synonymous with"in gas dispersion". By using a turbulent gas flow, it is possible to achieve vigorous mixing of the material in gas phase and, thus, even distribution of an activating agent (or the action of an activating agent).

Typically, the gas passes through the solids at a velocity sufficient to fluidize the material.

Although it is preferred to accomplish fluidization of the fluffy material in a gas medium, e. g. in a turbulent gas flow, for the purpose of the invention it is generally sufficient to mix the material vigorously during the contacting with the activating agent. Thus, mixing of the fluffy material can be carried out mechanically.

The lignin-containing material can be brought into fluffed state for example in a plug flow pipe or in a mixing vessel. The gas flow through the flow pipe can be turbulent and the pipe can optionally be provided with static mixers. The mixing vessel can be provided with a mechanical mixing means, such as a rotating impeller.

Examples of suitable process equipment include conventional pneumatic conveyor flash tube dryer systems, drum dryers and fluidized bed dryers in which drying is effected by heated air. The dryer systems may comprise drying in multiple stages. The temperature in multiple stage dryers is generally lower than in a one-stage dryer, and lower temperatures are beneficial for the stability of the radicals formed.

Within the context of the present invention the term"activating agent". designates any means capable of generating free radicals within the lignocellulosic material used as a raw material of board manufacturing. The free radicals are generally phenoxy radicals stabilized by their several resonance forms, increasing their half-life from several hours up to several days. The activating agent can be a chemical, such as hydrogen peroxide, an

enzyme, such as laccase, or it can comprise physical means, such as gamma-radiation, capable of generating radicals within the lignin matrix of the lignocellulosic material.

It should be noted that the activating agent can comprise a single agent or a mixture of several agents. Thus, for example, the action of an enzyme, such as laccase, can be complemented with radical-producing radiation. The activating agent can also be used together with a conventional resin or lignin-based glue. The conventional resin can be any known phenol-or urea-formaldehyde based adhesive. A particularly interesting combination is formed by mixing oxidizing enzymes and lignin suspensions (such as kraft lignin or lignin-containing fractions from wood or non-wood processing). In these mixtures, the oxidizing enzyme will produce radicals in the added phenolic material as well as in the lignocellulosic board raw material.

According to a first embodiment, the activating agent comprises oxidative enzymes capable of catalysing the oxidation of phenolic hydroxyl groups. These enzymes are often called phenoloxidases and they catalyze the oxidation of phenolic hydroxyl groups in monomeric, dimeric, oligomeric or polymeric phenolic compounds. The oxidative reaction leads to the formation of phenoxy radicals and finally to the polymerization of lignin. The phenoloxidases include peroxidases and oxidases."Peroxidases"are enzymes, which catalyse oxidative reaction using hydrogen peroxide as their substrate, whereas "oxidases"are enzymes which catalyse oxidative reactions using molecular oxygen as their substrate. In the method of the present invention, the enzyme used may be any of the enzymes catalyzing radical formation in lignin and other phenolic substances present, such as laccase, tyrosinase or peroxidase.

As specific examples of oxidases the following can be mentioned: laccases (EC 1.10. 3.2), catechol oxidases (EC 1.10. 3.1), monophenol mono-oxygenase (E. C. 1.14. 99.1) and bilirubin oxidases (EC 1.3. 3.5). Laccases are particularly preferred oxidases. They can be obtained from bacteria and fungi belonging to, e. g. , the following strains: Aspergillus, Neurospora, Podospora, Botrytis, Lentinus, Polyporus, Rhizoctonia, Coprinus, Coriolus, Phlebia, Pleurotus and Trametes. Suitable peroxidases can be obtained from plants, fungi or bacteria.

The enzymes can be used as such, preferably in the form of aqueous solutions or, as mentioned above, mixed with oxidizable organic material. Such material comprises for example isolated lignin and soluble pulp fractions. During industrial refining of wood by, e. g. , refiner mechanical pulping (RMP), pressurized refiner mechanical pulping (PRMP), thermomechanical pulping (TMP), groundwood (GW) or pressurized groundwood (PGW) or chemithermomechanical pulping (CTMP), the woody raw material, derived from

different wood species, is refined into fine fibers in processes which separate the individual fibers from each other. In chemical pulping processes, lignin is solubilised by chemicals in sulphite (SI) or sulphate (kraft) processes. In all types of processes, lignin- containing fractions can be isolated. Depending on the type of process, these solubilised fractions are composed in different ratios of the basic components of wood ; lignin, cellulose and hemicellulose. The relative amounts depend on the wood species and the process conditions used. Roughly, the process water of mechanical pulping contains some 20 to 70 % carbohydrates, 10 to 40 % reducing compounds, 10 to 25 % lignin and 1 to 10 % extractives. The material dissolved in the spent liquids is mainly lignin. These soluble fractions can be used in the adhesive agent compositions of the invention.

A chemical activating agent can be selected from typical free radical forming agents, such as hydrogen peroxide, Fenton's reagent, organic peroxides, potassium permanganate, ozone and chlorine dioxide.

According to a preferred embodiment, the decomposition of hydrogen peroxide in the presence of the lignocellulosic material is controlled by using a salt. Examples of such salts are inorganic transition metal salts, in particular salts of sulfuric acid, nitric acid and hydrochloric acid. Ferrous sulfate is a suitable compound, which will form a two- component system with hydrogen peroxide. In the presence of ferrous sulfate, hydrogen peroxide will first yield hydroxyl and other oxygen radicals (= Fenton's reagent). These radicals will then react with phenolic groups present in the lignocellulosic material to produce phenoxy radicals. The amount of ferrous sulfate needed for controlling the reaction is usually about 0.001 to 1 %, preferably 0.005 to 0.1 %, based on the dry matter of the raw material. The ferrous sulfate or other transition metal salt can be added together with the hydrogen peroxide or it can be admixed with the raw material before it is contacted with hydrogen peroxide.

The radical-producing radiation comprises gamma-radiation or electron beam radiation or any other high-energy radiation capable of forming radicals in a lignocellulosic (lignin- containing) raw material.

Activation of the lignocellulosic material The quality of fiberboards can be characterized primarily by two parameters, viz. internal bond (IB) and the 24 h thickness swelling.

We have found that the development of mechanical strength properties (characterized by the IB) is dependent on the reactivity of the fibers towards the activating agent. The

reactivity of the fibers is, again, to a large extent dependent on the native wood species used as the raw material. However, according to the present invention, the reactivity can be enhanced by properly selecting the defiberization conditions. The higher the temperature and the refiner pressure the more reactive the fibers will be. Generally, by increasing the concentration of water-soluble lignin fractions, more radicals can be formed on the fibers. Although we do not want to be bound to any specific theory, it appears that the content of water-extractable aromatic compounds in the fibers increases progressively with increasing temperature of defibration. The water-extractable material generated is enriched in glucomannan and other hemicelluloses and very low in cellulose. Water- extracts contain aromatic compounds rich in phenolic hydroxyl groups and low in ß-0-4 linkages. The amount and hemicellulose content of the water-extracts increase with increasing defibration temperature.

Thus, in order to increase the amount of fibrous surface lignin, it is preferred to carry out the refining of the wood-containing raw material at high temperatures in the presence of water steam. According to a particularly preferred embodiment of the invention, the temperature of the refining is in excess of 150 °C, preferably about 160-210 °C, in order to significantly increase the proportion of lignin-covered fibers. Then, free radicals are produced in the lignin-covered fibers by contacting the fibers with a suitable activating agent.

When wood is defibrated at sufficiently high temperatures, fiber lignin is plasticized and wood failure occurs in the lignin-rich middle lamella fiber region. As a result, the lignin concentration of the fiber surface is higher than the bulk lignin content of the wood.

Fiber surface analyses by ESCA indicated that for fibers produced at conditions of high refining temperatures, lipophilic extractives cover more than 60 % of the fiber surfaces while the surface lignin content of extractives-free fibers roughly doubled their bulk lignin content and decreased with increasing defibration temperature. The high content of surface lignin suggests that wood failure has occurred in the lignin-rich middle lamella of the fibers during refining.

As a result of this treatment, a lignocellulosic material is obtained in which the fibers are more susceptible of generating free radicals on the surface. Treatment of these fibers e. g. with hydrogen peroxide and a metal salt to promote peroxide decomposition to oxygen radicals or with laccase generates phenoxy radicals on the fiber surfaces. When peroxide- treated fibers are pressed together at high temperatures e. g. to make fiberboard, interfiber covalent bonds are formed involving at least partly coupling of the phenoxy radicals on fiber surfaces. This results in an adhesive effect to which also other reactive groups on fiber surfaces may contribute.

It should be noted that when refined at the same conditions, hard wood lignin contains more potential sites for radical formation than soft wood lignin. As a result, according to a preferred embodiment, the conditions of the hard wood refining are slightly milder than for softwood: for soft wood an advantageous refiner temperature range is 196 °C to 202 °C and for hardwood from 171°C to 186 °C. For every species the higher the temperature and pressure the better the IB values have been.

In order to produce fiberboards having high internal bond, hardwood is preferably defiberized at a pressure of at least 12 bars (188 °C) and softwood at a pressure of at least 16 bars (202 °C).

If desired, it is possible to produce additional amounts of free radicals by separately adding water-soluble phenolic material to the fibrous material.

According to one embodiment, the water-soluble phenolic material comprises aromatic monomeric compounds capable of forming stable free radicals. As a specific example, 1, 2-catechol, 2, 6-dimethoxyphenol and guaiacol can be mentioned.

It is also possible to add the water-soluble phenolic material in the form of a water-soluble fraction which contains phenolic compounds and is obtained from mechanical wood processing.

Generally, the oxidative enzyme and the chemical oxidizing agent can be mixed with the lignocellulosic material at any moisture content. For the action of the enzyme it is preferred to treat fibers containing some moisture. Thus, a moisture content of about 30 to 100 % is particularly suitable.

The enzyme used can be any of the enzymes known to catalyze the oxidation and polymerization of aromatic compounds such as lignins, such as laccase, or other oxidases.

The amount of enzyme used varies depending on the activity of the enzyme and on the dry matter content of the composition. Generally, the oxidases are used in amounts of 0.001 to 10 g protein/g of dry matter, preferably about 0.1 to 5 g protein/g of dry matter. The activity of the oxidase is about 1 to 100,000 nkat/g, preferably over 100 nkat/g.

As discussed above, a separated lignin fraction can be formulated and used as an adhesive binder by mixing it with an oxidase to provide oxidation and polymerization of the additives. Typically, the dry matter content of the adhesive composition treated with enzymes is about 2 to 50 wt-%. This fraction may be added in an amount ranging from 0

to 20 % of the fibers. The amount of any monomeric aromatic compounds can be of the same order.

The enzyme along with any lignin fraction or isolated lignin product is preferably introduced in the form of an aqueous solution or suspension and fed into the raw material under vigorous mixing. Various spray-heads and nozzles and atomizers can be used for the introduction of the enzyme solution. Likewise, liquid-form chemical agents can be sprayed or fed in any other convenient way into the raw material as long as proper mixing of the material is ensured. Gas-phase chemical agents, such as ozone and chlorine dioxide can be conducted into the raw material by means of a turbulent gas flow.

The other activation chemicals are dosed in proper amounts to produce radical levels corresponding to those generated by the above-mentioned laccase dosages. Thus, to mention an example, hydrogen peroxide can be employed in amounts of 0.001 to 10 %, preferably about 0.01 to 5 %, of the dry substance of the lignin-containing raw material.

During the feed of the activating agent, the raw material is kept in fluffed state so that there is immediate and efficient mixing of the raw material with the activating agent. Even when activation is achieved by radical-producing radiation, good mixing is preferred so that homogeneous distribution of radicals throughout the material can be obtained.

The radiation dose of gamma-radiation is typically in the range of 10-1000 kGy.

The contacting of the lignin-containing raw material with the activating agent can take place at any point between the provision of a suitable finely-divided raw material and the forming of a final product from the raw material by pressing. Further, the contacting can take place once or several times. Thus, the total amount of the activating agent can be divided into several portions and admixed with the lignin-containing material a plurality of times during the drying of the refined raw material and/or during the forming of the layered structure. Similarly, the calculated radiation dose can be applied to the material in several portions. In the following, three embodiments are described in more detail. It should be noted that the activation can comprise any of these embodiments or a combination of two or three of them.

According to a first embodiment, the raw material is contacted with the activating agent before it is pressed into a final shaped product. Even if the phenoxy radicals produced appear to be fairly stable, by generating the radicals shortly before pressing, it seems that improved mechanical properties are obtained due to increased internal bonding within the pressed product. "Shortly"stands for short time intervals of, typically, less than 180

minutes, preferably less than 30 minutes although the actual time depends on the particular process configuration and, in particular, on the temperature. Generally, in the case of enzymes, it is preferred to allow a certain space of time for the formation of radicals before pressing the layered structure. Pressing is typically carried out at elevated temperatures which will destroy residual enzyme activity. As will be discussed below in more detail, when contacting is carried out at low temperature (ambient up to 70-80 °C) the contacting time can be several hours (e. g. 0.5-5 hours) and the contacting can take place several hours (0.5-5 hours) before pressing.

Usually, the contacting is mainly carried out before pressing. However, by activating the raw material during pressing the surface properties of the product can be modified and, e. g. , the surface strength and the smoothness increased. Within the scope of this embodiment it is possible to contact the surface of the product with the activating agent during the pressing, for example during or after pre-pressing but before pressing the product into final thickness.

According to a second embodiment, the lignocellulosic material is contacted with activating agents, such as enzymes or chemical activating agents, during drying of the refined fibers. This contacting takes suitably place in a drying system comprising a blowline and a dryer (referred to as a blowline-dryer system), which interconnects a refiner used for producing a defiberized material and a separation means for the fibers (a cyclone). According to this embodiment a woody raw material is refined to produce wood fibers, the fibers are dried in a blowline-dryer system in a turbulent flow of air, steam or a similar fluid, the activating agent is mixed with the fibers in the blowline-dryer system in a zone of turbulence, the dried fibers are formed to a mat and the mat is pressed into a panel. The activating agent (s) can be added at any point, e. g. near the refiner, in the middle of the blowline-dryer system or near the separation means.

The activating agents are introduced into the blowline and/or into the dryer via an inlet tube or a set of tubes connected to the blowline and/or the dryer. Normal inlet lines used for feeding conventional resins can be used.

Typically, the oxidative enzyme or chemical oxidizing agent is mixed with the ligno- cellulosic material at a temperature of 25 to 70 °C, preferably 30 to 60 °C.

An alternate embodiment is based on the finding that laccases and similar oxidative enzymes retain their catalytic activity at temperatures far exceeding the boiling temperature of water under the favorable reaction conditions and the particular process

outline developed. This enables extremely fast polymerization of lignin and related rapid formation of adhesive bonds in the product.

Thus, according to a third embodiment, fiberboards are produced by mixing fibrous lignocellulosic raw materials, such as wood fibers, with aqueous solutions of laccase enzymes at very high temperatures to produce a fiber/enzyme mixture. The fibers are then formed into a mat or similar fibrous layer, which is compressed at an elevated temperature to a panel of suitable thickness.

The temperature can be over 80 °C, even up to the boiling point of water or higher. The enzymes are employed as such or together with lignin suspensions, such as kraft lignin or lignin-containing fractions from wood or nonwood plant processing.

The third embodiment of the invention can be carried out in a blowline, and/or in a dryer or during any other drying operation. However, it generally comprises the steps of : - providing fibrous lignocellulosic material, - mixing the lignocellulosic material at a temperature of at least 80 °C with an activating agent comprising an oxidative enzyme to produce a fibrous mixture, forming the fibrous mixture into a layer, and - pressing said layer into a panel.

In this embodiment, the oxidative enzyme is mixed with said fibrous material at a temperature of 85 to 180 °C, in particular about 99-170 °C.

When the activating agent comprising an oxidative enzyme is applied on the fibers at elevated temperatures, near or above the normal boiling point of water, excellent quality fiberboards are obtained and the pressing times shortened. Although we do not wish to be limited to any specific theory, it would appear that the lignin substance in the adhesive or naturally present in the fibers protects the catalyzing enzyme during the time needed for lignin polymerization to form high strength bonds within the fiber-adhesive matrix in the subsequent pressing process. The activating agent in this application can be formed by the enzyme solution as such with or without additives. The protecting mechanism would appear to be similar in both cases.

In connection with the present invention it has been found that oxygen plays a decisive role in the enzymatic polymerization of lignin of any origin. This is important in particular for the production of radicals in lignocellulosic material used for the manufacture of fiberboards, particleboards and flakeboards and other similar wood-based products. Thus, in addition to the lignin containing material, also oxygen is needed in

sufficient amounts. The oxidative reaction leads to the formation of radicals (e. g. phenoxy radicals) and finally to the polymerization of the material.

In the known methods discussed in above, crosslinking was only partially achieved because of apparent limitations on the availability of oxygen. The limitation of the reaction by oxygen manifests itself in the long reaction times used, and in the poor strength properties obtained, thus impairing the result of the enzyme-aided polymerization. Oxygen can be supplied by various means, such as efficient mixing, foaming, air enriched with oxygen or oxygen supplied by enzymatic or chemical means, such as peroxides to the solution. Although any oxygen-containing gas can used, it is preferred to use ambient air, oxygen enriched air, oxygen gas, pressurized systems of these or oxygen releasing chemicals. The oxygen-containing gas can be heated to a temperature of, e. g. , 30 to 125 °C when simultaneously used for drying of the fibers.

According to a preferred embodiment, the oxygen-containing gas comprises air, oxygen enriched air, oxygen gas or mixtures thereof used for drying of the fibers. Thus, the oxygen contained in the air flow in a blowline and a dryer may be sufficient for provide the oxygen needed in the reaction. Oxygen-containing gas can separately be introduced into the blowline or the dryer, if the normal oxygen content of the air flowing through the blowline or the dryer is insufficient.

According to an alternative embodiment, mainly applicable to the case in which the oxidative enzyme is mixed with an isolated lignin or a soluble fraction containing carbohydrates and lignin, oxygen is supplied by foaming the activating agent binder. This can be achieved by mixing the soluble fraction lignin with water after which gas is bubbled through the suspension or the suspension is agitated mechanically to form bubbles having a medium diameter of 0. 001 to 1 mm, in particular about 0. 01 to 0. 1 mm.

The lignocellulosic material is subjected to radical-producing radiation at a temperature of 25 to 50 °C. Preferably the lignocellulosic material is dried to a moisture content of less than 20 % before contacted with radical-producing radiation. The raw material can be treated with the radical-producing radiation in a blowline, in a dryer or in a separate mixing vessel or anywhere from the blowline to the press as long as fluff state and good mixing conditions are prevailing. It is generally preferred to carry out the radiation treatment immediately before the mat forming.

Acceptable IB values seem to be quite easily obtained with hardwood as well as softwood panels when the refining conditions are properly adjusted.

In addition to varying the production parameters, the IB values of softwood panels can be adjusted by regulating the density of the panel. This applies to softwood panels in particular. Thus, for spruce fibers produced by refining at 16 bars (202 °C), acceptable IB values can be obtained when densities are above 900 kg/m3. For birch and aspen refined at 12 bars (186 °C) good IB values can be obtained with normal MDF densities around 700- 800 kg/m3.

According to a preferred embodiment, for softwood fibers the densities of the compressed structures are in particular in excess of 880 kg/m3, preferably about 900 to 950 kg/m3. At these densities, contacting of the fibers is enhanced and chemical bonding between adjacent fibers will take place.

It has been found that for hardwood fibers, which are much shorter than softwood fibers, the density of the panels is not critical for strength development. According to another preferred embodiment, for hardwood panels good IBs are reached by adjusting the densities to a value in the range of 650 to 750 kg/m3.

Reduction of thickness swelling As discussed above, another feature of importance for the performance of the boards is the thickness swelling. In connection with the present invention we have found that the fiber size distribution of the refined fibers influences the 24 h thickness swelling of the compressed articles. Generally, by increasing the fines content (dry sieve analysis, Bauer McNett analysis) in the fiber matrix it is possible to reduce swelling values of the 24 h thickness swelling test. According to a particularly preferred embodiment, a thickness swelling of less than 25 %, in particular less than 20 %, is obtained by using a lignocellulosic raw-material containing >50 % of fines (<0.249 mm).

Possible explanations for this include that the fines fraction may act as a filler in the gaps between longer fibers. When the gaps are blocked with fillers, the fillers prevent the water from penetrating between the fibers and no swelling can happen. According to the present invention, in order to reduce thickness swelling it is preferred to refine the wood raw- material at high temperatures and high pressures and to aim at a fines content of at least 50% (<0. 249 mm), the higher the fines content in fiber matrix, the better the water resistance of the panel.

Some of the swelling may further be eliminated with wax addition. In general the amount of added wax can be about 0.1 to 2, preferably 0.2 to 1.5 %.

Pressing of fiberboards According to a preferred embodiment, at least a part of the individual fibers are brought within a distance of 2 A from each other during pressing to allow for inter-fiber bonding.

As regards pressing of the fiberboards it should be noticed that on a laboratory scale delamination problems have been solved by salt addition to lower the vapor pressure and also by the use of wire mats between the preformed fiber mat and aluminium plates. With an addition of 0.1 to 10 %, preferably 2 to 5 %, of CaCl2 båsed on the fiber dry weight, panel delamination can be eliminated and panels can be pressed at 190 °C without wire mats and no delamination happens. These panels have very low swelling values (from 5 to 10 °/O}. During pressing salts of bivalent metals form complexes with hydroxyl groups of the fibers which improves panel moisture resistance. When salt is not added, wire mats can be used to eliminate the vapor pressure formation inside the panel and panel delamination after press opening, when pressing is done at 190 °C. Swelling values even as low as 10-13 % can be obtained.

Process outline The processing steps are illustrated by the enclosed process layout for the first part of an MDF plant. In the drawing the following reference numerals are used: 1 debarker 2 chipper 3 screen 4 washer 5 conveyor 6 feed hopper 7 refiner 8 blow-line 9 cyclone 10 dryer The basic sequence of the MDF fiber manufacturing process comprises the following main steps: - refining a wood-containing raw material to produce wood fibers, - drying the fibers in a blow-line in turbulent fluid flow, - contacting the fibers with the activating agent, -forming said dried fibers to a mat, and - pressing said mat into a panel.

Before refining, the debarked wood is transferred to a chipper 2. After chipping the chips are screened 3 and washed 4 to remove mineral impurities from the chips. After washing the chips are preheated and, via a conveyor 5 and a feed hopper 6, conducted to a refiner 7 in which they are defiberized.

Mechanical defibering is carried out, for example, in a disc refiner 7 in the presence of water steam having a pressure of 12 to 20 bar and temperature in excess of 160 bar.

Retention time is about 1 to 20 min, for example about 4 min. After refining, the fibers contain some 30 to 70 %, typically about 50 %, moisture.

Drying takes place in a blowline-dryer system 8,9, 11 in turbulent flow of air or another fluid. Since the blowline 8 connecting the refiner to the dryer 9 is kept at non-pressurized conditions, water will evaporate efficiently during fluid flow transportation already when the pressure is released. Further drying is carried out in the dryer 9, which is also typically operated at non-pressurized conditions. In a blowline-dryer system the moisture content of the fibers is typically reduced from 30-70 % to about 1-20 %, in particular about 5 to 15 %. Heated air is introduced to the dryer 9 from compressor/heater 11. Typically, in a one- stage dryer, the temperature of the drying air is about 170 °C. In a two-stage dryer, the temperature of the drying air can be considerably lower, e. g. about 120 °C. After drying, fiber separation is performed in cyclones. The temperature of the dried fibers fed to the cyclone is about 50 to 80 °C. After the cyclone the dry fibers are first formed into a mat, then pre-pressed and finally pressed into panels. The pressing can be carried out by a continuous press. As mentioned above, the present invention provides for short pressing times in the range of 10 to 25 s/mm.

After the drying and the recovery of the fibers they can be collected and kept in a fiber bin which forms an intermediate storage.

According to the invention, the fibers are contacted with the activating agent at any point before the formation of said mat. A prerequisite is that the mixing is sufficiently efficient.

Thus, the fibers can be contacted with the activating agent at any point along the blowline- dryer system, for example, in the blowline at a point near the refiner, in the middle of the blowline-dryer system and/or near the separation means, depending on the specific panel manufacturing process. Some alternative addition points are indicated with arrows 8a, 9a and 9b in Figure 1. In a two-stage (or multistage) dryer, the contacting can be effected between the drying stages. The advantage of this contacting point is that the temperature is rather low which provides for enhanced stability of the radicals. The fibers can also be contacted with the activating agent in the fiber bin.

Generally, the moisture content of the fibrous lignocellulosic material is already somewhat reduced before the activating agent is added. The lignocellulosic material can, however, be dried to a moisture content in the range of 1 to 20 % either after the addition of the activating agent or before the treatment with radical-producing radiation.

It is particularly preferred that the fibers are contacted with the activating agent at least 5 minutes before the fibrous mixture is formed into a layered structure to allow for radical formation. At low temperatures, typically in the range of about 20 to 80 °C, the contacting. time and the time allowed for radical formation before the fibrous mixture is formed into a layered structure can be even up to several hours. At high temperatures, typically in excess of 100 °C, radicals are more rapidly terminated, and it is preferred to proceed to pressing within less than about 60 minutes, As mentioned above, it is also possible to contact the layered structure with some activating agent during the actual pressing for example to increase the surface strength of the structure. This activation step can take place after the pre-pressing step.

The fibrous lignocellulosic material is preferably mixed with processing and/or performance aids before it is pressed into a panel.

The major advantage of the invention is the preferable, small amount of the activating agent that is needed for good board properties, which means reductions in the production costs. Another advantage is the possibility to use existing board manufacturing machinery in the production of new type of solely wood-based, high-quality fiberboard. The pressing times are short, typically less than 15 s/mm.

The following non-limiting examples elucidate the invention.

Example 1 Stability of the laccase enzymes at high temperatures Different laccases were studied for their temperature stability. Laccases were added to the fibers under conditions similar to those used in the MDF process. Enzyme treated fibers were maintained at 100 °C for different time periods. The residual activities present in the fiber material were measured and compared with residual activities of enzymes in solution without fibers. The results are presented in Table 1.

Table 1. Enzyme activities remaining after treatment at 100 °C for 1-5 minutes with and without the fibers Enzyme Residual activity, % Residual activity, % Residual activity, % After 1 min with After 5 min with fibers after 1 min without fibers fibers Laccase 1 35 30 19 Laccase 2 54 n. a. 26 n. a.: not assayed As can be seen from the table, the residual activities were surprisingly high, when fibers were present. These results show that the enzymes are able to act under the conditions of the MDF process, even when added at high temperatures.

Example 2 Composition of the activating agents/adhesives 1,2, 3 and 4 As activating agent 1, a semicommercial neutral lyeeliophthora thermophila laccase (abbreviated "N"). having a pH optimum of 7 and an activity range of 1000 to 15000 nkat/ml was used. Activating agent 2 was an enzyme culture concentrate of Trametes hirsuta produced on pilot scale having an activity in the range from 1000 to 4500 nkat/g (abbreviated"T"). The pH optimum was 4.5. Adhesives agents 3 and 4 comprised premixed and aerated suspensions of enzymes N and T with Indulin AT kraft lignin, which contained 1000 nkat of enzyme activity per gram of lignin.

Table 2. Composition of activating agents/adhesives 1,2, 3 and 4 Activating/Enzyme Preparation technique adhesive material 1. Enzyme N Semi-commercial concentrate 2. Enzyme culture T Pilot scale concentrate 3. Enzyme-lignin N Laboratory scale premixing and aeration suspension 4. Enzyme-lignin T Laboratory scale premixing and aeration suspension

Example 3 Manufacture of fiberboards Fiber material for fiberboards was manufactured in a pilot scale facility using the enzymes of Example 2. The production rates were 65 to 75 kg/h both for birch chips and pine chips.

Chips were defiberized at three different refiner pressures: 8,12 and 14 bars (the corresponding temperatures being about 170,185 and 190 °C, respectively). Adhesives were added to the fibers in the blow-line so that the amount of adhesive calculated as enzyme activity per dry fiber mass was from 100 to 400 nkat/g dry fiber or as dry substance amounting to 0.3-5 % of dry fibers. Resin addition temperatures varied between 99 and 170 °C.

Resinated fibers were then dried in the dryer to a moisture content in the range of 5.5 to 14%.

Dried fibers were formed to mats measuring 160 mm x 500 mm x 600 mm. The weight of each mat was about 3.5 kg.

Panels were pressed at temperatures between 100 and 190 °C. Pressing times varied from 11 to 30 s/mm.

Table 3. IB values for different fibers and adhesives Adhesive\fiber A (170 °C) A (185 °C) A (190 °C) B (170 °C) B (185 °C) type \ IB/MPa IB/MPa IB/MPa IB/MPa IB/MPa max. max. max. max. max. 1. enzyme 0.39 0.67 0. 90 n. a. 0.58 concentrate N 2. enzyme culture 0.40 1.1 n. a. 0.39 0. 55 concentrate T' 3. premixed n. a. n. a. n. a. n. a. 0.56 biobinder NL 4. premixed n. a. 0. 83 n. a. n. a. n. a. biobinder TL A: birch B: pine

Defibration temperature in parenthesis, °C All panels were pressed at a temperature of 190 °C using pressing times from 11 to 30 s/mm. In all biobinders used the enzyme activities were from 100 to 400 nkat/g. In premixed biobinders the amount of lignin used was 5 % based on the dry fibers.

Example 4 Gluing of fibers for particle board, small scale Table 4. Results of small scale gluing tests for mdf Glue-material Lignin Tensile content % strength MPa of dw TMP process water 18 8. 0 MDF water extract 21 10. 3 Example 5 The radical concentration (absolute spins) were assayed for fibers produced from various wood species and activated with laccase (100-400 nkat/g of Myceliophtora thermophila). The results will appear from Table 5 below.

Table 5. Radical concentration of fibers using DPPH (1.5 x 1021 spins/g) in toluene as standard Fiber Radicals in untreated fiber (spins/g) Radicals in treated fiber (spins/g) Aspen 8 bars6. 1x101. 7xl0is Aspen 12 bars 4.731017 3.7#1018 Birch 8 bars 7.4#1017 3. 2 #1018 Birch 12 bars 7.4#1017 9.2#1018 Birch 14 bars 6.431017 9.6#1018 Eucalypt 8 bars 1.1#1018 3. 5 x 1018 Pine 8 bars 6. 2 x 1017 8. 3 x 1017 Pine 12 bars 4.7#1017 9. 0 x 1017 Spruce 8 bars 1. 1 x 1017 5. 1 x 1017 Spruce 12 bars 2. 6 x 1017 8. 0 x 1017 Spruce 14 bars 3.6#1017 9.7#1017 Spruce 16 bars. 3. 3#1017 1.3#1018

Example 6 Using 100 to 400 nkat/g of Myceliophtora thermophilia laccase at pH 7, the relative radical levels were compared with IB values for panels produced from wood fibers fiberized at different pressures (temperature). The results are given in Table 7.

The panels were produced by following the procedure of Example 3.

Table 7. Glued panels Fiber Radicals IB/MPa Swe ! l/% Density Press Fines content Production True disc formed w. M. kg/m3 temp./°C (<50mesh), % rate, kg/h clearance, mm thermophila SERIES 1 aspen, 8 bars 18 0. 70 >100 802 170 42 73 0. 65 aspen, 12 bars 39 1. 0 36. 5 789 170 48 73 0. 63 birch, 8 bars 33 0. 57 34 797 190 44 80 0. 57 birch, 8 bars 33 0. 30 >100 724 170 26 n. a. n. a. birch, 12 bars 95 1. 1 37 735 190 44 1. 00 70 birch, 14 bars 100 0. 70 44 695 190 29 n. a. n. a. eucal t, 8 bars* 37 0. 45 350 882 pine, 8 bars 9 0. 39 >100 790 190 7 75 0. 62 pine, 12bars90. 58 >100 777 190 27 72 0. 63 SERIES 2 spruce, 8 bars 39 0. 09 spruce, 8 bars* 39 0. 06 450 897 spruce, 12 bars 53 n. a. so 61 1. 3 58 822 170 39 65 0. 6 spruce, 14 bars* 61 0. 71 52 1023 spruce, 16 bars 100 1. 1 21 909 190 37 67 0. 6 spruce, 16 bars 100 2. 0 29 1090 170 48 62 0. 72 1. 3 27 990 170 48 62 0. 72 EBM standard (12 mm) 0. 6 12 *produced with another pressing apparatus

Table 6 shows that good results are obtained using enzyme and refining pressures of 14 bar or more for spruce and 12 or even 10 bar for hardwood, i. e. birch, aspen and beach.

Based on the experimental data of Tables 5 and 6, the radical amounts should be after the activation treatment 0.7x1018 or more, preferably at least 1x1018 free radicals/g of lignocellulosic material for softwood fibers, and 3x1018 or more, preferably 3.5x1018 free radicals of lignocellulosic material for hardwood fibers. In some cases, radical levels of over 5x1018 free radicals/g of lignocellulosic material can be obtained for birch. For pine over 8.8x1018 free radicals/g of lignocellulosic material are possible.

Generally in comparison to wood fibers produced at normal pressures employed today in refiners, by raising the refiner pressure to 12 or 14 bar, respectively from 8 or 10 bar, an increase of at least 10 %, preferably at least 15 %, in particular at least 20 %, typically about 25 to 50 % (in case of birch even more than 100 %, or even more than 150 %) in the radical amounts can be obtained. The temperatures of the refining carried out according to the invention are determined by the pressure (use of steam), and are generally 10 to 15 degrees higher than the conventional temperatures of the refiners. Thus, for spruce, the temperature can be over 180 °C, in particular 185 to 210 °C.

The improvement in IB values of the panels/boards is at least about 10 %, preferably at least 15 % and in particular at least 20 % compared to panels/boards manufactured from conventional fibers.