Such fiberboards contain lignocellulosic fibres which are bonded together in the presence of an inorganic salt having a multivalent metal cation and shaped into a compressed product having a density of at least 750 kg/m3. The fibres are bonded together essentially in the absence of synthetic polymer resins.

The wood panel industry produces a wide variety of products from plywood to oriented strandboard, from laminated veneer lumber to engineered wood products, from hardwood plywood to particleboard and medium density fiberboard. Usually, the wood particles, which concept in the present context covers both wood fibres, saw dust, wood strands and wood sheets, are bonded together using various industrial resins, including phenolic, melamine and urea resins.

Wood panels, such as fibre boards, have high strength and provide excellent insulation properties. However, they are sensitive to moisture because wood fibres have a tendency to swell upon contact with water. Therefore, there is a need to provide new panels with improved dimensional stability.

Westin et al. [Westin, M. , Simonson, R. and Ostman, B. , Holz als Roh-und Werkstoff 58 (2001) 393-400, Springer-Verlag] have described the effect of kraft lignin addition to wood chips or board pulp prior to fiberboard production and shown that it is possible to improve the properties of fiberboards by adding kraft lignin to wood chips prior to defibration and alternatively to fibres. Westin et al. also discuss the addition of aluminum salts together with the kraft lignin. The use of aluminum salts as such is disclosed in some of the examples, but the authors conclude that without the addition of lignin, only a small amount of metal can be fixed and very little stabilization effect is gained with the metal treatment alone. The boards manufactured by Westin et al. are rather thin (having a thickness of 3 mm) and the fibers studied have been exclusively derived from softwood.

Even if kraft lignin is a by-product of kraft pulping, it is commercially available at only

slightly lower prices than the traditional synthetic resins which it resembles. Thus, the actual economical benefit of using kraft lignin instead of convention resins is rather small.

There is, therefore, still a need for a method of manufacturing wood panels, which would provide products having improved dimensional stability at reduced costs.

It is an aim of the present invention to eliminate the problems of the prior art and provide a novel panel and a method for the production thereof.

The present invention is based on the idea of producing fiberboards comprising lignocellulosic fibers, which consist essentially of hardwood fibers. According to the present invention, such fibers are bonded together using a ferric salt, such as ferric sulfate or ferric nitrate. To obtain sufficient bonding strength, the concentration of the metal cation, i. e. the Fe (III) ion, in the board should be at least 0.3 wt-% of the dry matter of the fibers.

More specifically, the fiberboard or similar wood-based product according to the invention is mainly characterized by what is stated in the characterizing part of claim 1.

The process according to the invention is characterized by what is stated in the characterizing part of claim 11.

Considerable advantages are obtained by means of the invention. Thus, by using iron (III) salts, good bonding strengths are obtained without the use of traditional synthetic polymeric resins, such as phenolic resins or urea formaldehyde or melamine urea formaldehyde resins. Iron (III) salts are well soluble in water which makes it possible to use them at very strong concentrations, which decreases the amount of water incorporated into the fibrous mixture before it is compressed into a rigid structure. As a result of the decreased moisture contained in the fibrous mat, pressing times are reduced and pressing can be carried out at increased temperatures without risking blistering of the boards.

Panels, which are produced using metal salts for bonding, are free from adhesive resins and can be recycled with other recyclable wood. Further, as far as the overall economics of the process is concerned it should be pointed out that, calculated by weight, less bonding agent needs to be transported, because the metal salt can be shipped dry, whereas adhesive resins

are usually transported in the form of dispersions. Also the storability is advantageous for metal salts compared to adhesive resins. Adhesive resins become more viscous over time and do not have a long shelf life and do not tolerate long shipping distances. Metal salt bonding agent can be stored dry and the source is not dependent on plant location as is the case for formaldehyde resins.

Next the invention will be examined more closely with the aid of a detailed description and with reference to some working examples.

Figure 1 shows the Now scheme of an MBF fiber treatment and panel processing process.

According to the present invention, fiberboards are produced from fibrous lignocellulosic material by mixing the material with a ferric salt, shaping the mixture into a layer and compressing the layer into a compressed sheet-formed product using increased temperature and pressure.

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 which primarily comprises said fibers mixed with a suitable adhesive. It should be pointed out that the present products can be called"layered structures"which include 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 layers of fibers.

Generally, the particles have sizes in the range of 0.01 to 10 mm. Particularly advantageous properties are obtained with wood fibers having a fiber length distribution, in which at least 50 % are smaller than 0.249 and preferably at least 35 % are smaller than 0.125 mm, as will be explained below in greater detail. A distribution of this kind will reduce swelling of the compressed product.

According to a preferred embodiment, the fiberboards comprise a in addition to the iron salt a second inorganic salt rendering properties of resistance against unwanted microbiological reaction to the board.

Even if the ferric salt provides good bonding strength as such, there are a number of ways of further improving the strength properties of the boards. Thus, it is possible to generate free radicals in the fibers during the manufacture of the board. Such free radicals will activate the lignin present in the fibers and the activated lignin will work as a bonding

agent. The free radicals can be generated by the use of radiation, or chemically or enzymatically. The chemical activating agent can be selected from typical free radical forming agents, such as hydrogen peroxide, organic peroxides, potassium permanganate, ozone, chlorine dioxide etc.

The decomposition of hydrogen peroxide generates oxygen radicals, particularly hydroxyl radicals (HO') and superoxide ariion radicals (O2). Peroxide decomposition is strongly catalyzed by polyvalent metal ions (applied in the form of, e. g. , ferrous sulfate). Hydroxyl radicals are powerful electrophiles, which in contact with wood fibers preferably attack the electron-rich lignin component rather than the carbohydrates. Lignin model compound smdics have shown that they are able to abstract a hydrogen from a phenolic hydroxyl group or a benzylic alcohol group, whereby phenoxy and benzyl radicals, respectively, are formed. Other reactions between lignin and hydroxyl radicals include aromatic ring hydroxylation and demethoxylation. Phenoxy radicals can be formed also from originally non-phenolic aromatic units, which have undergone oxidative hydroxylation or demethoxylation. The coupling of mainly phenoxy radicals generates covalent bonds between lignin units resulting in lignin polymerization. In this respect, activation with a chemical agent is similar to activation with an oxidative enzyme.

According to a particularly preferred embodiment, free radicals are generated by a combination of hydrogen peroxide and ferric salts. Generally, the dosage of peroxide can be in the range of 0.1 to 10 % from the dry matter of the fibers. Preferably the dosage is about 0. 3-5 %.

Another preferred embodiment comprises the use of ferric salt with oxidative enzymes, such as laccase. As a result of the latter embodiment, the panel produced will contain some minor residues of the oxidative enzyme used for generating free radicals in the fibers during the manufacture of the board. The amount of enzyme used varies depending on the activity of the enzyme and on the amount of 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.

Still a further alternative comprises using the ferric salt together with isolated lignin, such as kraft lignin.

The amount of the ferric salt can vary depending on whether it is use alone or in combination with a radical generating agent. Usually, if employed alone, the amount of

ferric salt-calculated as metal (i. e. metal ion) per dry substance of fibrous material is preferably from about 0.9 % by weight up to 2 % or even more. When used in conjunction with a radical generating agent, such as laccase or hydrogen peroxide, the amount can be reduced to about 0.5 to 0.9 wt-% Fe (III) ions of the dry fiber matter.

The ferric salt can be mixed with the fibrous raw material in wet state or in dry state.

According to a preferred embodiment the ferric salt is mixed with the fibers in fluffed state, which means using air or another gas as the main medium for the The ferric salt is preferably selected from the group of ferric sulfate and ferric nitrate.

The board contains a second salt selected from the group of inorganic salts of bi-or trivalent metal cations. The second salt can be selected, e. g. , from the group of inorganic salts of aluminum, copper and calcium.

As mentioned above, it is particularly preferred to implement the invention using hardwood fibers. Such fibers are selected from the group consisting of beech, birch, aspen, alder, poplar, eucalyptus, maple and mixed tropical hardwood fibers.

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 having a fines proportion of >50 % of fines (<0.249 mm).

It appears, this is, however, just one possible explanation, that the fine fiber fraction acts as a filler in the voids between longer fibers. When the voids 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 % of fines <0. 249 mm, the higher the fines content in fiber matrix, and 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 %.

The present boards are prepared by a method comprising the following steps: - providing a lignocellulosic fiber material, - contacting the fiber material with an inorganic salt having a multi-valent metal cation in the absence of a synthetic polymeric resin to form a mixture of fibres and salt, - forming the mixture into a layered structure, and pressing the layered structure into a compressed product having a density of at least 750 kg/m3.

The characterizing steps of the invention are two, viz. refining chips of a hardwood tree species to provide the lignocellulosic material and using an Fe (III) salt as the inorganic salt and mixing the salt with the fiber material in an amount sufficient to provide a concentration of Fe (III) ions in the compressed product of at least 0. 3 wt-% calculcated from the dry fiber matter.

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 pointed out that the salt addition according to the present invention lowers the vapor pressure. Another way of reducing vapor pressure is to use wire mats between the preformed fiber mat and aluminum plates.

Generally, the layered structure is pressed at a temperature sufficient to provide an inner temperature of the layered structure of at least 90 °C. However, much higher temperatures can be used in practice, i. e. the layered structure can be compressed at a temperature of at least 140 °C.. Thus, with the present invention panel delamination can be practically eliminated and panels can be pressed at 190 °C even without wire mats. The present panels have very low swelling values (from 5 to 10 %).

It would appear that during pressing salts of the bivalent metals used form complexes with hydroxyl groups of the fibers and improves panel moisture resistance, but this is just one possible explanation.

Even then as low swelling values as 10-13 % can be obtained.

The lignocellulosic fibers are mixed with ferric sulfate or ferric nitrate and optionally w with a salt selected from the group consisting of inorganic salts of aluminum, copper and calcium.

Examples The following examples illustrate the invention. The examples were carried out at laboratory conditions using the process depicted in Figure 1. Fibers made of beech by refining in a TMP refiner at 10 bar were used. The gap between then refining discs was 0.4 mm and the production rate 65 kg/h. The moisture content of the fibers was 5.5 to 7 % before the addition of the ferric salt. The first step 1 of the process comprises verification of the quality of the fibers to determine fiber size distribution and moisture content of the fibers. In the next stage 2, fibers and chemicals were mixed in a Forberg laboratory mixer and chemical was sprayed using a Wagner sprayer. Mixing time was 10 min.

The chemicals optionally comprise an enzyme, which creates radical in the fibers. The radicals formed were determined 3 by assessing the intensity of the red color of the treated fibers, which value was compared with the initial value determined in step 1.

Fiber pressing was carried out in steps 4 and 5, whereby first a mat was formed of the fibers and the mat formed was then pressed. A fixed pressure press was used. Panel size was 22. 5x22. 5 cm. Press temperature was maintained with hot oil circulation. Maximum press pressure was 1700 psi for the 10 cm press cylinder, being ca 18 bar for the panel.

Press closing time was max 10 s. Panel thickness was fixed with the press closing stoppers.

For each panel, four samples of the size 50x50 mm were made. IB and swelling values of the panels were measured according to standard methods. The bits were sanded with a band sander.

The results of the produced panels are given in Table 1 below.

Table 1. Properties of panels Example Metal salt, % Metal, % Panel Press Panel Panel Panel thickness, mm temperature, density, strength, swelling C kg/m3 IB, MPa % 1 FENO 10% Fe 1.4% 11 180 830 0.47 18 2 FENO 10% Fe 1.4% 11 230 810 0.59 18 3 FENO 10% Fe 1.4% 11 180 830 0.47 18 4 FENO 10% Fe 1.4% 11 230 810 0.59 18 5 FeNO 10% Fe 1.4% 11 140 940 0.95 18 6 FENO 6.2 % Fe 0.9% 11 180 920 0.66 25 7 FENO 4.2%+ezsol2% Fe0.6% 11 140 910 0.84 21 8 FENO 7% Fe 1% 12 180 830 0.4 18 9 FESO 3% Fe 0.9% 11 180 790 n.a. 17 10 FESO 5.9% Fe 1.6% 11 180 850 n.a 10 11 FESO 3.6% Fe 1% 4.2 180 760 0.12 16 12 CANO 10.6% Ca 1.8% 11 180 960 1.03 29 FENO: ferric nitrate<BR> FESO: ferric sulfate<BR> CANO: calcium nitrate<BR> ezsol: laccase solution

From the results in Table 1 it can be seen that by using a non-adhesive resin based MDF production system, panels can be made with a fiber metal-salt bonding agent without any formaldehyde resins. As will appear from the table, the requirements for low swelling of the MDF panels (below 20 %) were reached when using a metal salt as the fiber bonding agent. In many of the panels a good panel strength value of 0.6 MPa or more was reached and even the moderate values of several other test panels were obtained in combination with good dimensional stability properties (low-swelling). Also good MDF panel properties were seen as a result of the combination of a metal salt with laccase enzyme for fiber bonding.

The tests illustrate some of the particular advantages which can be obtained when using the metal salt based MDF fiber bonging agent instead of traditional adhesive resin binders.

Thus, a lower panel press temperature can be used when compared to common practices in today's adhesive resin based panel pressing systems as seen in some of the results in Table 1. The result can lead to better MDF-plant press economics and shorter production cycle times. Also more dry material can be dissolved in the fiber metal salt bonding agent solution compared to adhesive resins, resulting in less water in the fiber mass. This is a clear advantage in the press economics.

When comparing Fe-salt and Al-salt bonding agents, the use of Fe-salt is more advantageous, because more Fe-salt can be diluted into water and less water needs to be removed during pressing, resulting in shorter MDF panel press cycles, decreased energy need and steam emissions.