WO2002002288A1

WO2002002288A1 - Method of manufacturing fiberboards

Info

Publication number: WO2002002288A1
Application number: PCT/FI2001/000636
Authority: WO
Inventors: Liisa Viikari; Anneli Hase; Simo Tuominen; Pia Qvintus-Leino; Jaakko Laine
Original assignee: Dynea Chemicals Oy
Current assignee: Dynea Chemicals Oy
Priority date: 2000-07-05
Filing date: 2001-07-03
Publication date: 2002-01-10
Anticipated expiration: 2003-01-05
Also published as: FI20001612A7; FI20001612L; AU2001282161A1; FI20001612A0

Abstract

This invention concerns a method for manufacturing a compressed product. According to the method, a lignocellulosic material is contacted with an activating agent to produce modified lignocellulosic material containing free radicals, the modified material is formed into a layered structure, and said layer is pressed into a compressed product. According to the invention, the lignocellulosic material is contacted with the activating agent in fluffed state.

Description

METHOD OF MANUFACTURING FIBERBOARDS

Background of the Invention

Field of the Invention

The present invention relates to the manufacture of products which comprise fibrous lignocellulosic particles which are pressed and bonded together into a layered, compressed structure. In particular, the present invention concerns methods for producing fiberboards, such as medium density fiberboards, particleboards, fla eboards and strand boards by activating lignocellulosic material with an activating agent to produce modified lignocellulosic material containing free radicals, forming the modified material into a layer, and compressing said layer.

Description of Related Art

The rapid increase in the production of particleboards, flakeboards and fiberboards, particularly medium density fiberboards, 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. As a major wood component, native lignin is neither hygroscopic nor soluble in water. However, during pulping, lignin becomes soluble in water, due to degradation and chemical changes.

Structurally lignin is a polyphenol and, therefore, as an adhesive it should be similar to phenol-formaldehyde resins. This is also true for native lignin in wood, while technical lignins (lignosulphonate or kraft lignin) have been shown to have serious limitations due to their low reactivity (kraft lignin) or due to their high hygroscopicity (lignosulphonates). The use of spent sulphite 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 phenol- or urea- formaldehyde adhesives. Reference is made to DE Patents Nos. 3 037 992, 3 621 218, 3 933 279, 4 020 969, 4 204 793 and 4 306 439 and PCT Applications published under Nos. WO 93/25622, WO 94/01488, WO 95/23232 and WO 96/03546.

The main drawback of using SSL as an adhesive for fiberboard manufacture, however, is its hygroscopicity. For this reason it cannot really compete with other natural or synthetic adhesives. Components derived from annual plant materials, such as feruloylarabinoxylans, can also be used as additives for adhesives in particleboards. Thus, according to Felby et al. (WO 96/03546) wood fibers and chips can be bonded together using a phenolic polysaccharide and oxidising enzymes.

Curing of lignin is a crosslinking process, which leads to new carbon-carbon and ether bonds between different lignin molecules or within one macromolecule. Inter- as well as intramolecular crosslinking reactions decrease the solubility and swelling of lignin. Crosslinks in lignin can be achieved either by condensation or by radical coupling reactions. Further, it has been shown that laccases and peroxidases can be used as polymerisation or curing catalysts of lignin (DE Patent No. 3 037 992, WO 96/03546). However, the enzymes used for catalyzing radical formation have shown limited success so far. Fibers and wood chips used in the production of the fiberboard contain 5 - 20 % water and the laccases used need some water to effectively diffuse and catalyse the polymerisation reaction needed for extensive bonding of the fiberboard. However, kraft lignin 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 a large decrease in the strength properties of the board formed in the pressing stage. The enzymatic methods described in the art suffer also from difficult and expensive application methods and additional process stages, such as soaking of lignin or fibers with enzymes in water, or drum mixing of adhesive in the otherwise continuous board manufacturing process.

A further problem relating to the use of isolated lignin is the high price of kraft lignin which is near the limit for economical production of particleboards if the amount of lignin needed for good adhesion can not be limited to a very small amount, as is the situation in the techniques presented here.

For the above mentioned reasons, lignin-based board production processes have not, so far, led to any major practical applications.

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 that method, the middle lamella lignin is activated by "incubation", which comprises the step of contacting the lignocellulosic material, such as mechanically defiberised pulp, with laccase in aqueous phase. 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. In particular it is an object of the present invention to produce a high-quality fiberboard or a similar compressed structure composed of lignocellulosic particles by using technically practicable and economically inexpensive steps.

The present invention is based on the finding that the processing times can be significantly shortened by subjecting the lignocellulosic material to the action of an activating agent in 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.

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

The present invention provides considerable advantages. Good bonding of the lignocellulosic particles (e.g. the fibers) is obtained; in fact, the mechanical strength properties (e.g. IB, MOE and MOR) are on 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 on the same board density level can be achieved by the present invention. Further, one particularly valuable advantage obtained by using the technique described 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 technique. This will increase the production capacity of the existing production plants. The boards do not contain and emit any additional formaldehyde except for that naturally occurring in the fibers.

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 using turbulent gas flow conditions. Further features and advantages of the present invention will become apparent from the following description of the invention.

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 activating agent.

Detailed Description of the Invention

The invention will now be examined more closely with the aid of the following detailed description and with reference to a number of working examples.

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.

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.

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), thermomechamcal 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. If desired, it is possible to produce additional free radicals by separately adding water- soluble monomeric phenolic compounds ("booster" compounds) to the fibrous material. Suitable monomeric compounds capable of forming phenoxy radicals are, e.g., 1,2- catechol, 2,6-dimethoxyphenol and guaiacol.

Contacting of lignocellulosic material with activating agent

The contacting of the lignocellulosic raw material with the activating agent will now be examined more closely.

Generally, the oxidative enzyme and the chemical oxidising 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 10 to 100 % (relative humidity) is particularly suitable.

The enzyme used can be any of the enzymes known to catalyze the oxidation of lignins and other phenolic compounds for catalysing the oxidation and polymerisation of aromatic compounds such as lignins. Laccase and other oxidases are examples of these enzymes. 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/kg of dry matter, preferably about 0.1 to 5 g protein/kg of dry matter. The activity of the oxidase is about 1 to 100,000 nkat/g, preferably over 10 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 lignin- containing material. 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 phenolic 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. Oxygen or an oxygen-containing gas can be introduced into the enzyme solution or suspension before it is mixed with the raw material. In this way, a high level of activity can be ensured from the very start of the reaction.

Various spray-heads and nozzles and atomizers can be used for the introduction of the enzyme solution. Likewise, liquid chemicals can be sprayed or fed in any other conventional way into the raw material as long as proper mixing of the material is ensured. Gaseous activation chemicals, 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 lignocellulosic 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.

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 blowline

9 dryer

10 compressor

11 compressor/heater

12 cyclone

The basic sequence of the MDF fiberboard manufacturing process comprises the following main steps:

- refining wood-containing raw material to produce wood fibers,

- drying the fibers in a blowline-dryer system 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. 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 based on wet fibers.

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

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 semi commercial neutral Myceliophthora 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

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 in the presence of steam at a refiner pressure of 8 bar (corresponding to a temperature of about 170 °C). Activating agents were added to the fibers in a blowline so that the amount of laccase 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 to 5 % of dry fibers. Laccase addition temperatures varied between 99 and 170 °C.

Enzymatically treated fibers were then dried in the dryer to a moisture content in the range of5.5 to l4 %.

Dried fibers were formed to mats measuring 160 mm x 500 mm x 600. 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 3A. IB values for various activating agents/adhesives

NL = enzyme N + lignin NT = enzyme T + lignin n.a.: not assayed

In all biobinders 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 MDF panels with lignin containing additives, laboratory scale

Test panels were manufactured on a laboratory scale by mixing the laccase enzyme having an activity of 100 nkat/g dry fibers into the batch of 500 g of fibers for 10 min. Then the fibers were manually pre-pressed to a thickness of about 150 millimeters. The moisture content of the pre-pressed fiber mat was from 8 to 10 %. Panels of size 200 mm x 200 mm x 12 mm were pressed in a hot press (170 to 190 °C) using pressing times of 20 s/mm. Panels were tested according to EMB standard methods. Lignin-containing materials were tested as binder additives. Results for panels prepared using dry matter separated from process water of the TMP process and dry matter extracted from MDF fibers are presented in Table 4 below.

Table 4.

Example 5

450 g of dry spruce fibers (mc. 5.5 %) are sprayed with a 3.5 wt.-% solution of H₂O in distilled water. The solution also contains 45 mg of FeSO₄x7H₂O. The final moisture content of the fibers after spray treatment is 12 %. The moistened fibers are then pressed to the thickness of 12 mm in a hot press (hot press temperature 190 °C, pressing time 20 s/mm).

The properties of the panel prepared are presented in the table below.

Table 5.

Example 6

450 g of dry spruce fibers (mc. 5.5 %) are sprayed with an aqueous 5.65 wt.-% solution of H₂O₂. The solution also contains 7.2 mg of FeSO₄ 7H₂O. The final moisture content of the fibers after spray treatment is 12 %. The moistened fibers are then pressed to the thickness of 12 mm in a hot press (hot press temperature 190 °C, pressing time 20 s/mm). Table 6.

Example 7

450 grams of spruce fibers (mc. 5.5%) are treated with 100 kGy of γ-radiation. The dry fibers are formed to a mat of 200 mm x 200 mm x 100 mm and then pressed to the thickness of 12 mm in a hot press (press temperature 190 °C, press time 20 s/mm).

The properties of the panel are presented in the table below.

Table 7.

Example 8

450 grams of spruce fibers (mc. 5.5%) are treated with 100 kGy of γ-radiation. 22.5 grams of water is sprayed on the fibers and mixed thoroughly to the radiated fibers. The moistened fibers are formed to a mat of 200 mm x 200 mm x 100 mm and then pressed in a hot press to the thickness of 12 mm.

Table 8.

Example 9

Comparison between the strengths of panels glued with enzyme concentrate and panels glued with water

Test panels were manufactured on a laboratory scale by mixing the binder (laccase enzyme activity 100 nkat/g dry fibers) into the batch of 500 g of fibers for 10 min. Then the fibers were manually pre-pressed to a thickness of about 150 millimeters. The moisture content of the pre-pressed fiber mat was from 8 to 10 %. Panels of size 200 mm x 200 mm x 12 mm were pressed in a hot press (170 to 190 °C) using pressing times of 20 s/mm. Panels were tested according to EMB standard methods.

For comparison panels were manufactured also using the same protocol but water was used instead of enzyme concentrate. The results are presented in the table below.

Table 9.

Claims

WHAT IS CLAIMED IS:

1. A method of manufacturing a compressed product comprising

- providing lignocellulosic material, - contacting the lignocellulosic material with an activating agent to produce modified lignocellulosic material containing free radicals,

- forming the modified material into a layered structure, and

- pressing said layer into a compressed product, c h a r a c t e r i z e d by - contacting the lignocellulosic material with the activating agent in fluffed state.

2. The method according to claim 1, wherein lignocellulosic material is contacted with the activating agent in a mixing zone subjected to intensive turbulence.

3. The method according to claim 2, wherein the lignocellulosic material is contacted with the activating agent in a turbulent gas flow.

4. The method according to any of claims 1 to 3, wherein the activating agent is selected from the group comprising oxidative enzymes, chemical oxidising agents and radical- producing radiation.

5. The method according to claim 4, wherein the oxidative enzyme or chemical oxidising agent is mixed with the lignocellulosic material at a temperature of 25 to 70 °C, preferably 30 to 60 °C.

6. The method according to claim 4, wherein the oxidative enzyme or chemical oxidising agent is mixed with the lignocellulosic material having a moisture content of 30 to 100 % at a temperature in excess of 70 °C.

7. The method according to claim 4, wherein said radical-producing radiation comprises gamma-radiation or electron beam radiation.

8. The method according to claim 6, wherein said lignocellulosic material is subjected to radical-producing radiation at a temperature of 25 to 50 °C.

9. The method according to any of claims 1 to 8, wherein the lignocellulosic material is contacted with the activating agent during drying of the material.

10. The method according to any of the preceding claims, wherein the lignocellulosic material is selected from the group of wood particles in the shape of e.g. chips, shavings or fibers, saw dust, and fibers of annual or perennial plants.

11. The method according to any of the preceding claims, comprising the steps of

- defibrating woody raw material to produce wood fibers, - drying the fibers in a blowline-dryer system 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.

12. The method according to claim 11, wherein the woody raw material is refined in the presence of water steam.

13. The method according to claim 11 or 12, wherein the dried fibers are separated from the turbulent fluid flow in a cyclone and the dry fibers are formed into a mat and pre- pressed before final pressing to panels.

14. The method according to any of claims 11 to 13, wherein the fibers are contacted with the activating agent at a point before the formation of said mat.

15. The method according to any of the preceding claims, wherein the fibers are contacted with the activating agent in a blowline-dryer system interconnecting a refiner and a separation means for the fibers at a point near the refiner, in the middle of the blowline- dryer system or near the separation means.

16. The method according to any of the preceding claims, wherein the lignocellulosic material is dried to a moisture content in the range of 1 to 20 % after the addition of the activating agent.

17. The method according to claim 7, wherein the lignocellulosic material is dried to a moisture content in the range of 1 to 20 % before the treatment with radical-producing radiation.

18. The method according to claim 1, wherein the activating agent comprises an aqueous solution of an oxidase or a chemical oxidizing agent or it comprises an oxidizing gas. .

19. The method according to claim 18, wherein the activating agent is sprayed onto said fibrous lignocellulosic material.

20. The method according to any of the preceding claims, wherein the fibers are contacted with the activating agent at least 5 minutes before the fibrous mixture is formed into a layered structure

21. The method according to any of the preceding claims, wherein said activating agent comprises a mixture of an oxidase and organic matter selected from the group of lignin, lignin derivatives, and organic compounds, including other phenolic compounds.

22. The method according to any of the preceding claims, wherein the activating agent comprises an enzyme selected from the group consisting of oxidases and peroxidases.

23. The method according to claim 22, wherein the oxidase is selected from the group consisting of laccase, catechol oxidase, mono-oxygenase and bilirubin oxidase.

24. The method according to claim 23, wherein the laccase is obtained from a bacterial or fungal strain of the group consisting of Aspergillus, Neurospora, Podospora, Botrytis, Lentinus, Polyporus, Rhizoptonia, Coprinus, Coriolus, Phlebia, Pleurotus and Trametes.

25. The method according to any of the preceding claims, wherein oxygen is introduced into the mixture of the oxidase and fibrous lignocellulosic material in the form of an oxygen-containing gas.

26. The method according to claim 25, wherein the oxygen-containing gas comprises air, oxygen enriched air, oxygen gas or mixtures thereof.

27. The method according to claim 25 or 26, wherein the oxygen-containing gas comprises air, oxygen enriched air, oxygen gas or mixtures thereof used for drying of the fibers.

28. The method according to any of claims 25 to 27, wherein the oxygen-containing gas is heated to a temperature of 30 to 100 °C.

29. The method according to any of the preceding claims, wherein the fibrous lignocellulosic material is mixed with processing and/or performance aids.

30. The method acccording to any of the preceding claims, wherein the amount of oxidative agent added to the lignocellulosic material in the blowline-dryer system, calculated as enzyme activity per dry fiber mass, is 10 to 1000 nkat/g dry fiber.

31. The method to any of the preceding claims, wherein the amount of oxidative agent added to the fibrous lignocellulosic material, calculated as dry substance, is 0.1 to 10 % of the dry fibers.