ADHESIVES FOR FIBER BOARDS AND A PROCESS FOR THE PREPARATION THEREOF

Abstract: An adhesive binder for bonding lignocellulosic particles, fibers and flakes of vegetable origin for the preparation of particle boards, fiber boards, flake boards and similar wood-based products, and the preparation process thereof, and the prepared fiber board or other end product by using said adhesive binder, and the process used for the preparation of said product. Said adhesive binder comprises an oxidized water soluble fraction derived from mechanical or chemimechanical pulping of lignocellulosic raw materials, and can further contain, e.g., lignin isolated from chemical pulping, or a surfactant. The preparation of the adhesive binder composition comprises the steps of separating a liquid effluent from wood raw material and concentrating and oxidizing said liquid effluent in an aqueous suspension containing an oxidase and introducing oxygen-containing gas into said suspension.

Description: Adhesives for fiber boards and a process for the preparation thereof The present invention relates to the manufacture of fiber boards and similar wood-based products comprising finely divided lignocellulosic particles and fibers mixed with and bonded together with an adhesive binder. In particular the present invention concerns novel adhesive binders and a preparation process thereof as well as fiber boards manufactured by using said adhesive binders. The rapid increase in the production of particle boards, flake boards and fiber boards, especially medium density fiber boards (in the following also abbreviated MDF boards), during the last decades has created a demand for adhesives that are inexpensive, available in large quantities, and independent of crude oil. Lignin meets well these requirements, and it does not contain any formaldehyde, which traditionally has been considered a serious problem with conventional urea-formaldehyde (UF) adhesives. As a major wood component, native lignin is neither hygroscopic nor soluble in water. Because of its structure as a polyphenol, lignin as an adhesive should be similar to phenol-formaldehyde resins. This is 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. Furthermore, during technical pulping lignin becomes soluble in water, due to degradation and chemical changes. 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 particle board, plywood and fiber board instead of conventional PF or UF 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 fiber board manufacture is its hygroscopicity. For this reason it cannot really compete with other natural or synthetic adhesives. 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 enzymes for creating radical reactions have shown limited success so far. Fibers and wood chips used in the production of the fiber board contain 5 - 20 % water and the laccases used need some water to effectively catalyze the polymerization reaction needed for extensive bonding of the fiberboard. Kraft lignin like native lignin to its major part is, however, insoluble in water and thus two solid phases are formed on the production line. An uneven distribution of the solids cause spotting and major failure in the strength properties of the board formed in the pressing stage. 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 and to use these fibers as such without any additional binders for manufacturing wood fiber boards (cf. EP Patent Application No. 0 565 109). The main problem relating to said technology is the long incubation time required (up to seven days). Components derived from annual plant materials, such as feruioylarabinoxylans, can also be used as additives for adhesives in particle boards. Thus, according to Feldman et al. (WO 96/03546) wood fibers and chips can be bonded together using oxidized phenolic polysaccharides. These xylans occur only in annual plants, not in softwood or hardwood materials. They are not industrially available. The present invention aims at eliminating the problems relating to the prior art. In particular it is an object of the present invention to provide a novel lignin-based adhesive for the preparation of particle boards, fiber boards and similar wood-based products. It is another object of the present invention to provide new particle boards, fiber boards and similar wood-based products. It is a third object to provide a method for manufacturing the adhesive. These and other objects, together with the advantages thereof over known lignin-based adhesives and processes for the preparation thereof, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed. It is known in the art that during mechanical refining of chips, a part of the fibrous raw material is dissolved (about 1 % of the fiber weight). This fraction, which primarily contains the same chemical components as the fibers (carbohydrates, extractives and lignin) decontaminates the circulation waters and effluents of paper mills and increase the volumes of waste waters that have to be treated. Different filtration and concentration methods are being employed to separate the dissolved compounds from the process waters. As a result, a concentrated waste fraction without any practical use today is being obtained. Now it has surprisingly been found that this soluble lignin carbohydrate fraction is particularly useful as an additive or adjuvant for gluing of particles boards, fiber boards and other similar wood-based composite products. Particularly good gluing is achieved if this fraction is polymerized with laccase (or similar oxidase) enzyme(s). The results are on the same level as those obtainable with conventional phenol or urea formaldehyde resins. In short, according to the present invention, it has been found that the adhesive, usually externally produced, such as phenolformaldehyde or ureaformaldehyde, can be replaced by natural, wood-derived fractions, separated from the mechanical wood refining processes. The adhesive binder thus comprises an oxidized, water soluble fraction derived from mechanical or chemimechanical pulping of lignocellulosic raw materials. The additional benefit of this method is that the volumes of waste waters requiring external purification are diminished. Furthermore, MDF boards with excellent strength properties can be obtained. The invention also concerns a process for preparing an adhesive binder composition, which comprises the steps of - separating a liquid effluent from mechanical or chemimechanical pulping of wood raw material, - concentrating said effluent to increase its dry matter content, - forming an aqueous suspension containing said dry matter and an oxidase, and - oxidizing said dry matter in the presence of said oxidase by introducing oxygen- containing gas into the suspension. The invention will now be explained in more detail with the aid of the following detailed description and with reference to a number of working examples. Within the context of the present invention, the terms "adhesive", "adhesive binder" and "resin" designate a chemical composition which, in the wet stages of the manufacture of, e.g. particle and fiber boards, provides adhesion between the particles, fibers or flakes. After heat compression during board manufacture, the composition containing polymerized resin works as a binder which keeps the particles or fibers or flakes bonded together. The term "wood-based product" denotes any lignocellulose-based product, such as particle boards, fiber boards (including high and medium density fiber boards, i.e. hard boards and MDF boards), flake boards, plywood and similar products constituted by particles, fibers or flakes of vegetable origin, in particular derived from wood or annular or perennial plants mixed with and bonded together with adhesive binders. For polymerizing lignin and carbohydrates of soluble wood fractions, oxidative enzymes capable of catalyzing oxidation of phenolic groups can be used. These enzymes are oxidoreductases, such as peroxidases and oxidases. "Peroxidases" are enzymes which catalyze oxidative reaction using hydrogen peroxide as their substrate, whereas "oxidases" are enzymes which catalyze oxidative reactions using molecular oxygen as their substrate. Phenoloxidases (EC 1.10.3.2 benzenediol: oxygen oxidoreductase) catalyze the oxidation of o- and p-substituted phenolic hydroxyl and amino/amine groups in monomeric and polymeric aromatic compounds. The oxidative reaction leads to the formation of phenoxy radicals and finally to the polymerization of lignin and possibly the carbohydrate matter. In the method of the present invention, the enzyme used may be any of the enzymes catalyzing the biological radical formation and secondary chemical polymerization of low molecular weight lignins, such as laccase, tyrosinase, peroxidase or oxidase. As specific examples of oxidases the following can be mentioned: laccases (EC 1.10.3.2), catechol oxidases (EC 1.10.3.1), tyrosinases (EC 1.14.18.1) 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, Pleuroius, Fusarium and Trametes. Suitable peroxidases can be obtained from plants or fungi or bacteria. Preferred peroxidases are those which originate from plants, in particular horseradish peroxidase and soy bean peroxidase. The terms "surfactant" or "surface active agent" are synonymously used to designate compounds which have affinity to water and to hydrophobic materials, thus helping the hydrophobic materials to suspend in water. Soluble fraction derivable from mechanical pulping 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 fibres in processes which separate the individual fibres from each other. During the refining process, some (about 0.1 to 5 %, typically about 0.5 to 2 %) of the woody raw material is dissolved in the aqueous phase due to the prevailing conditions, such as the high temperature of the refining. These solubilized fractions are composed of the basic components of wood; cellulose, hemicellulose and lignin. The amounts depend on the wood species and the refining conditions used. Roughly, the process water of mechanical pulping of softwood chips contains some 40 to 70 % carbohydrates, 10 to 25 % lignin and 1 to 10 % extractives. In contrast, hardwood pulping process water contains some 20 to 60 % carbohydrates, 10 to 25 % lignin and 10 to 40 % extractives. The percentages are given by way of examples only and they have been calculated from the dry weight of the solid matter. The fiber fractions of the refining are collected and further used for manufacturing paper. The solubilized fractions are separated from the fibres or recirculated during the paper manufacture process. As mentioned above, in the existing processes, the formation of a soluble fraction during mechanical or chemimechanical pulping gives rise to environmental loading; the soluble fraction forms the major waste water stream which has to be purified by external waste water treatment plants. In addition, when recycled in the process, these fractions cause severe problems during paper manufacture due to the presence of dissolved and colloid substances. In connection with the present invention it has turned out that the various components of the soluble fraction of mechanical or chemimechanical pulping of wood are all reactive towards oxidation in the presence of oxidative enzymes. As evidenced by the results of Example 3, water extracts of MDF fibers, fractions solubilized during TMP and PGW preparation and MDF process water all can be oxidized by enzymes Although not wishing to be confined to any specific theory, it is suggested that there are formed various oxidized compounds containing, e.g., phenoxy radicals, which will provide adhesion to the lignocellulosic particles and fibers and which will take part in polymerization reactions. Manufacture of adhesive composition According to the present invention, the separated soluble fraction can be formulated into an adhesive binder by mixing it with an oxidase to provide oxidation and polymerization of the carbohydrates, lignin and extractives present. For this purpose, the soluble fractions are separated from the fibers and the aqueous phase is concentrated by filtration, ultrafiltration or evaporation or other suitable separation techniques. The dry matter content of the soluble fraction is usually less than 10 wt-%, often less than 5 wt-%. Consequently, it has to be concentrated to a much higher concentration before use. Typically, the dry matter content of the adhesive composition treated with enzymes, is about 20 to 80 wt-%. The soluble fraction used for the preparation of an adhesive binder comprises preferably about 10 to 70 % carbohydrates and about 1 - 30 % lignin calculated on basis of the dry weight of the water soluble fraction. The enzyme used can be any of the enzymes prior known for catalyzing the oxidation and polymerization of aromatic compounds of lignins, such as laccase, tyrosinase, or other oxidases. 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 mg protein/g of dry matter, preferably about 0,1 to 5 mg protein/g of dry matter. The activity of the oxidase is about 1 to 100,000 nkat/mg, preferably over lOO nkat/mg. In connection with the present invention it has been found that oxygen plays a decisive role in the enzymatic polymerization of carbohydrates, extractives and lignin of any origin. This is important in particular for the production of adhesives for the manufacture of fiber boards, particle boards and flake boards and other similar wood-based products. Thus, in addition to the carbohydrate or lignin material, also oxygen is needed in sufficient amounts. The oxidative reaction leads to the formation of oxidized radicals (e.g. phenoxy radicals) and finally to the polymerization of the material. In the known methods discussed 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 supply can be increased by various means, such as efficient mixing, foaming, introducing air enriched with oxygen or oxygen supplied by enzymatic or chemical means to the solution. Although any oxygen-containing gas can be used, it is preferred to use air, oxygen enriched air, oxygen gas or pressurized systems of these. Thus, according to an embodiment of the invention, the mixture comprising the soluble fraction and the enzyme is vigorously mixed in the presence of oxygen by e.g. aerating the mixture. The mixing time is usually about 1 min to 24 h, preferably about 5 min to 10 h. According to an alternative embodiment, the supply of oxygen is achieved by producing a foam of the adhesive binder by mixing the soluble fraction of lignin into water together to form a mixture and by bubbling a gas through the suspension to form bubbles having a medium diameter of 0.001 to 1 mm, in particular about 0.01 to 0.1 mm. The dispersion is preferably foamed to 1.2 to 10 times of the original volume. The foam is produced by using a surface active agent that can be anionic, cationic or non- ionic. Thus, the surfactant can be selected from the group consisting of alkylsulfonate or alkyl benzene sulfonate, Tween® and other commercial polysorbate compounds, fatty acid soaps, lignosulfonates, sarcosinates, fatty acid amines or amines or poly(oxyetylene alcohol)s and wood and plant extractives. Foam stabilizers and solid surfactants, such as CMC, gelatin, pectin, wood extractive and similar compounds, can be used to produce and enhance the foam stability. A small amount of the surface active compounds is needed, i.e. about 0.01 to 10 %, in particular about 0.05 to 5 %. The foam can be produced by foaming in a static foamer or in a turbulent foam cell by using known mixing technology. In both of the above embodiments, the adhesive binders can be produced separately from the fibers and chips, which are mixed with the adhesive binder by extruding or spraying the foam to the fibers or chips. The adhesive binder can also be produced simultaneously by mixing the fibers or chips with the components of the binder composition. The adhesive binder composition is added either before or after drying of the fibres of the board after refining. The composition is added to the particles, fibers or flakes in an amount of 0.1 to 30 %, preferably about 1 to 10 % of the total dry weight. According to a particularly preferred embodiment, lignin is admixed with the binder compositions before the enzymatic oxidation. The amount of lignin can amount to 1 to 99 % of the dry matter of the composition, preferably the composition contains 5 to 95 % lignin and 95 to 5 % soluble fraction (which already in itself contains some lignin). A part of the lignin can be added in the form of lignin-containing fibers. As the results of Examples 4 and 5 below show, particle boards and MDF boards manufactured by using adhesive binder compositions containing 10 % of the soluble fraction, 90 % isolated kraft lignin and an oxidase (laccase) provides boards having equal strength properties compared to boards prepared by conventional technology employing UF resins. By substituting 1 to 50 % of the isolated lignin with the soluble carbohydrate/lignin fraction described herein, it is possible significantly to reduce the costs of the natural resin binder. The following non-limiting examples will elucidate the invention. Example 1 Separation of the soluble fraction of mechanical pulping The soluble fraction was isolated from the softwood or hardwood fibres after the refining (TMP or PGW) process, and concentrated by evaporation to various dry weight concentrations, according to the needs. Example 2 Composition of the soluble fractions The composition of the soluble fraction of the TMP process, using softwood as raw material, obtained after refining, or a respective fraction from hardwood was analyzed for the major constituents (Table 1). Table 1. Composition of the TMP softwood process water Composition % of DW Lignin 18 Extractives 5 Carbohydrates 57 Reducing compounds 24 Identified sugars Arabinose 1.3 Galactose 1.9 Glucose 7.2 Xylose 0.7 Mannose 10.8 Sugar acids 0.5 As can be seen, the major constituents are lignin and glucomannans, in about equal amounts. In addition, however, several other components could be identified. Therefore, no conclusions can be made on the exact composition of the active glueing material. The respective solubilized fraction from hardwood contained about the same amount of lignin (22 %), but higher amount of extractives (22 %) and carbohydrates (40 %) with a much higher amount of glucose (20 %). Example 3 Reactivity of the soluble fraction towards enzymatic polymerization The reactivities of wood based soluble fractions isolated from different processes were analyzed for their capability to be oxidized by the enzyme used. The reactivity describes the relative value of oxidation potential, relative to the polymerization and further glueing efficiency. The reactivities are compared to the corresponding values of lignosulphonate and kraft lignin, the known wood based additives in glueing. The results are presented in Table 2. Table 2: Reactivity of solubilized fractions from PGW, TMP, spruce saw dust, pine saw dust, and MDF fibers as measured by enzymatic oxidation using laccase; enzyme dose 20 μkat/1 Solubilized fraction O, consumed n_2 / g substrate PGW 0.2 TMP 4 1 Water extract from spruce saw dust 12.8 Water extract from pine saw dust 2.8 Water extract from MDF fiber 7.5 Lignosulphonate 3.4 Kraft lignin 2.5 Example 4 Glueing of MDF fibres with the soluble fraction and soluble fraction mixture in addition to lignin in the enzyme catalyzed glueing reaction Soluble fractions produced from softwood TMP, as described in Example 1, were used as a binder in particle board and MDF test panel glueing. 4.0 g of the soluble fraction or the soluble fraction combined with kraft lignin was vigorously mixed and aerated for 30 min with 4.0 g of laccase concentrate (activity 4,000 nkat/g) in 2.0 g of 2 M sodium acetate buffer (pH 4,5). In case of MDF panels 5.5 g of the mixture was sprayed onto or mechanically mixed with 20 g of dry fibers. The fibers or chips had already been treated with 0.7 % of wax (Mobilex 54, 60 % emulsion- in water) of the dry weight of the fibers. The reference tests were performed without laccase (water was used instead) and using commercial ureaformaldehyde resins. For strength tests particle board panels of the size of 50 mm x 50 mm x 2 mm (weight about 5 g) were prepared by pressing 2 min in 30 kp/cm 2 of pressure and 190 °C of temperature and MDF panels of the size of 90 mm x 90 mm x 2 mm (weight about 22 g) were prepared by pressing 2 min in 50 kp/cm 2 of pressure and 190 °C of temperature. After pressing the panels were then cut into four pieces (50 mm x 12 mm x 2 mm). These pieces were tested for parallel tensile strength with Zwick tensile strength testing equipment Table 3. Results of the small scale glueing tests for MDF fibers Binder Tensile strength MPa Soluble fraction 22 ± 3 Soluble fraction + laccase 30 ± 4 Soluble fraction (10 %) + lignin 38 ± 2 (90 %) + laccase Water 15 ± 2 Reference UF resin 40 ± 5 Example 5 Glueing of particle boards with the soluble fraction and a mixture of the soluble fraction and lignin The glueing of particle board test panels was performed according to the procedure presented in example 5, except that in case of particle board panels, 1.4 g of the mixture was either sprayed onto or mechanically mixed with 4.4 g of particle board chips. For strength tests particle board panels of the size of 50 mm x 50 mm x 2 mm (weight about 5 g) were prepared by pressing 2 min at a pressure of 30 kp/cm 2 and a temperature of 190 °C. After pressing the panels were then cut into four pieces (50 mm x 12 mm x 2 mm). The results are shown in Table 4. Table 4. Results of the small scale glueing tests for particle boards Binder Tensile strength MPa Soluble fraction 3.8 ± 0.6 Soluble fraction + laccase 8.0 ± 0.4 Soluble fraction (10 %) + Indulin 11.8 ± 0.4 AT 90 % + laccase Water 3.0 ± 0.5 Reference UF resin 12.5 ± 0.7

Claims: WHAT IS CLAIMED IS: 1. A particle board, fiber board, flake board or a similar wood-based product, comprising lignocellulosic particles, fibers and flakes of vegetable origin bonded together with an adhesive binder, wherein the adhesive binder contains a water soluble fraction oxidized with an oxidase and derived from mechanical or chemimechanical pulping of lignocellulosic raw materials. 2. The product according to claim 1, wherein the adhesive binder contains 10 to 70 % carbohydrates and 1 - 30 % lignin calculated on basis of the dry weight of the water soluble fraction. 3. The product according to claim 1 or 2, wherein the adhesive binder in addition to the water soluble fraction contains lignin isolated from chemical pulping of lignocellulosic materials. 4. The product according to claim 3, wherein the adhesive binder contains 5 to 95 wt-% water soluble fraction and 95 to 5 wt-% lignin. 5. The product according to claim 3 or 4, wherein said lignin of the adhesive binder is polymerized. 6. The product according to any one of claims 1 to 4, wherein the adhesive binder composition contains a surfactant. 7. A process for preparing particle boards, fiber boards or similar wood-based products, wherein the particles, fibers and flakes of vegetable origin are bonded together with an adhesive binder containing a water soluble fraction oxidized with an oxidase and derived from mechanical or chemimechanical pulping of lignocellulosic raw materials. 8. A process according to claim 7, wherein the particles, fibers or flakes of vegetable origin are bonded together with an adhesive binder containing a mixture formed by an enzymatically oxidized water-soluble fraction derived from mechanical or chemimechanical pulping and polymerized lignin. 9. A process according to claim 7 or 8, wherein the adhesive binder composition is added to the fibers of the board before drying and after refining. 10. A process according to claim 7 or 8, wherein the adhesive binder composition is added after drying of the fibers of the board after refining. 11. A process according to any of the claims 7 to 10, wherein the adhesive binder composition is added to the particles, fibers or flakes in an amount of 1 to 10 % of the total dry weight. 12. A process according to any of the claims 7 to 10, wherein the preparation of the adhesive binder composition comprises the steps of - separating a liquid effluent from mechanical or chemimechanical pulping of wood raw material, - concentrating the liquid effluent to increase the concentration of the dry matter thereof, - forming an aqueous suspension containing said dry matter and an oxidase, and - introducing oxygen-containing gas into the supension in order to oxidize said dry matter in the presence of said oxidase. 13. The process according to claim 12, wherein the oxidase is laccase or a peroxidase. 14. The process according to claim 12 or 13, wherein the dry matter of said effluent is concentrated to a dry weight concentration of 20 - 60 % 15. The process according to any one of the claims 12 to 14, wherein the liquid effluent comprises the process water of refiner mechanical pulping, pressurized refiner mechanical pulping, chemimechanical pulping, thermomechanical pulping, groundwood or pressurized groundwood pulping or chemithermomechanical pulping. 16. The process according to any one of the claims 12 to 15, wherein the oxygen-containing gas comprises air, oxygen enriched air, oxygen gas or mixtures thereof. 17. The process according to claim 16, wherein the oxygen-containing gas is introduced by foaming the suspension. 18. The process according to claim 17, wherein the suspension is foamed to 1.2 to 10 times of its original volume.

Inventors: GAEDDA LARS, NIKU-PAAVOLA MARJA-LEENA, QVINTUS-LEINO PIA, TUOMINEN SIMO, VIIKARI LIISA, GAEDDA, LARS, NIKU-PAAVOLA, MARJA-LEENA, QVINTUS-LEINO, PIA, TUOMINEN, SIMO, VIIKARI, LIISA
Applicants: GAEDDA LARS, NESTE OY, NIKU PAAVOLA MARJA LEENA, QVINTUS LEINO PIA, TUOMINEN SIMO, VIIKARI LIISA, GAEDDA, LARS, NIKU-PAAVOLA, MARJA-LEENA, QVINTUS-LEINO, PIA, TUOMINEN, SIMO, VIIKARI, LIISA
Application year: 1998
Main classification: C09J197
IPC codes: C08L97, C09J197, D21J1