Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
  • B27N—MANUFACTURE BY DRY PROCESSES OF ARTICLES, WITH OR WITHOUT ORGANIC BINDING AGENTS, MADE FROM PARTICLES OR FIBRES CONSISTING OF WOOD OR OTHER LIGNOCELLULOSIC OR LIKE ORGANIC MATERIAL
  • B27N3/00—Manufacture of substantially flat articles, e.g. boards, from particles or fibres

Definitions

• the present invention relates to a process for the manu ⁇ facture of glued pressed products by drying of a humid raw material containing wood, wood fibre and/or cellulose, adding glue and pressing said raw material in order to form a glued product such as wood chipboard, plywood and such like.
• a humid raw material containing wood, wood fibre and/or cellulose
• glue and pressing said raw material in order to form a glued product such as wood chipboard, plywood and such like.
• the present invention also relates to an arrangement for cooling of a raw material containing wood, wood fibre and/or cellulose which is dried under heating in a non-oxidizing atmosphere.
• the manufacture of wood chipboard is carried out in several stages which can vary somewhat as to their arrangement in different factories.
• the main stages are the preparation of chips from the raw material, drying of the chips, screening and classification, preparation of glue, addition of glue to the chips, forming of the chips into a mat, pressing of the mat to a board and finishing of the board.
• the object of the first stage, the preparation of the chips is to obtain a material with the desired particle size and particle shape.
• the raw material which can be e.g. sawdust, low-grade timber, wood waste from plywood manufacture and such like
• the subdivision of the wood material is carried out by cutting or grinding chippers.
• the processing is per ⁇ formed at room temperature or at the temperature of the out ⁇ side air depending on the arrangement of the factory. With e.g.
• the temperature of the chips can rise appreciably because of the significant input of mechan ⁇ ical power.
• the chips are dried ' to a final mois ⁇ ture content of normally 1-5 % of the weight of the dry wood substance.
• a high dry-substance content is required partly in order to obtain good glue bonding and partly so that the pressing into boards can be carried out without the formation of steam bubbles in the board. Together with the glue a cer ⁇ tain amount of water is also added and this must be taken in ⁇ to account in determining a suitable moisture content for the dried chips.
• two different methods of drying are used ' .
• One method is to dry the chips by means of direct con ⁇ tact with hot combustion gases or with air which has been preheated in e.g.
• the chips are dried by means of the indirect supply of heat to the chips from heating surfaces.
• the moist chips are brought into good contact with the hot drying gas and are transported- mainly with the current through the drying apparatus itself which is commonly of a drum type.
• the drying gas and the dried .. . chips are separated in a subsequent cyclone separator.
• the temperature of the drying gas at the beginning of drying is normally over 200°C but in certain combustion-gas driers can rise as high as 700°C.
• the tem ⁇ perature level is normally restricted to a maximum of 150- 160°C.
• the effective area of contact between the chips and the heating surface e.g. a steam-heated tube packet
• the heating surface e.g. a steam-heated tube packet
• this type of drier operates as a direct hot-air drier but with the difference that the preheating of the air is incorporated in the drying section to form a single stage.
• the temperature of the chips is determined at the beginning of drying, when the chips still have a high moisture content, by the equilibrium with " the surrounding drying gas, i.e. primarily by the so-called wet-bulb temperature of the dry- . ing gas which is normally approx. 50-80°C.
• the tempera ⁇ ture of the chips rises considerably and approaches the temperature of the drying gas.
• the tempera ⁇ ture of the chips approaches the level of 150°C.
• the combustion gas generator of a combustion-gas drier must be operated with a large surplus of air which results in a considerable oxygen content in the combustion gases.
• the oxygen content of the combustion gases is therefore not appreciably different from the level of oxygen content in a hot-air drier.
• the water vapour evaporated from the chips becomes mixed with the drying gas which is thereby diluted.
• the amount of drying gas is large compared with the amount of evaporated water and the moisture ratio rarely rises above 0.3 kg H-O/kg of dry gas.
• the oxygen content is therefore in practice norm ⁇ ally above 10 % by volume in the drier. (The oxygen content of the air is 21 % by volume. ) This applies also to indirect driers.
• a classification of the chips is made normally into two fractions, i.e. an outer-layer fraction with fine material and a core-layer fraction with course material. These fractions are kept separate and processed separately up to the formation of the chip mat when the fractions form the outer and core layers of the board respectively. Classi ⁇ fication can also be carried out prior to drying in which case drying is carried out separately for each fraction.
• glue is added to the chip fractions with effective mixing so that the glue is uni ⁇ formly distributed over the surfaces of the chips.
• carbamide glue is the most usual. In connection with the addition of glue hardener for the glue and other possible ' additives are also added.
• the manufacture of plywood displays despite the superficial differences, several production phases corresponding to those in the manufacture of chipboard.
• the actual manufacturing process begins with the cutting of the veneer in which a long mat of veneer with a thickness of 1-3 mm, usually 1.5 mm, is peeled from a log in a veneer lathe. This is followed by dry ⁇ ing of the veneer. Drying may, however, be preceded by clip ⁇ ping of the veneer mat into sheets and sorting of the sheets.
• the drier is accordingly arranged either for drying long mats of veneer or for drying sheets of veneer.
• the first-named is generally arranged so that the veneer mat is transported be ⁇ tween metal nets in a back-and-forth motion through the dry ⁇ ing chamber, a so-called belt drier.
• a sheet drier the transport through the drier is performed with the aid of a pair of rotating cylinders, a so-called roll drier.
• the drying is carried out with hot air which is heated by steam radiators.
• the drying has been made more efficient by blowing hot air onto the veneer from nozzle boxes whereby an internal recirculation and reheating of the air is achieved.
• the drier has been designed as a combustion-gas drier.
• the temperature of the drying air is normally kept at 150-190°C.
• the temperature of the veneer follows the same pattern as the chip temperature in chip drying, i.e. approx. the wet-bulb temperature of the drying air, 60-85°C, at the beginning and up to 150°C at the end of drying.
• Very considerable amounts of leakage air enter the drying chamber and as a result the maximum moisture ratio of the drying air is in practice 0.8-1.0 kg H-O/kg dry air (on average considerably lower). This means that the oxygen content of the humid drying air is in practice normally above 10 % by volume.
• the drying of the veneer is followed by clipping into sheets, sorting of the sheets, joining of sheets and parts of sheets together at the edges, and glue spreading.
• the desired num ⁇ ber of glue-coated sheets are laid up so that the grain directions of adjacent layers are at right-angles to each other.
• This pile of veneer is then pressed between hot press plates whereon the glue bonds the sheets of veneer to each other. After edge trimming the final plywood produce is finished.
• the strength of the single veneers is good in bending at right-angles to the grain direction but very poor in ben- ding along the grain direction.
• the object of the present invention is accordingly to achieve a process for the manufacture of glued pressed products by drying of a humid raw material containing wood, wood fibre and/or cellulose, adding glue and pressing the raw material to form a glued product with improved properties compared with similar products which are dried, glued and pressed in a similar manner.
• the present invention is based on the surprising discovery that the above-named undesired changes in the wood can be reduced or completely eliminated by preventing contact with oxygen in the process stages in which the temperature of the wood is elevated.
• the prevailing atmosphere suitably contains less than 10 % by volume of oxygen, advantageously not more than 5 % by volume, e.g. less than 1 °/_ by volume of oxygen.
• the tem ⁇ perature is suitably maintained lower than 200°C and pre ⁇ ferably higher than 60°C. Further it is advantageous to add the glue to the raw material while the latter has an elevated temperature in order to avoid contact with oxygen between drying and the application of the glue.
• the inert atmosphere in the stage or the stages in which the temperature is elevated is suitably achieved and main ⁇ tained by isolating the raw material from the air by means of displacing the air with non-oxidizing gas.
• the raw material is heated by e.g. hot steam, after which the hot, dry raw material is fed through an air lock and brought into con ⁇ tact with cooled, substantially dehumidified non-oxidizing gas before discharge through an air lock into the surround ⁇ ings. Cooling can also be carried out at reduced pressure.
• wood chips and veneer examples of material suitably employed as raw material in the manufacture of glued pressed products according to the foregoing process.
• the invention also relates to an arrangement for cooling of raw material containing wood, wood fibre and/or cellulose, which is dried under heating in a non-oxidizing atmosphere.
• such an arrangement comprises a closed chamber with air locks for feeding in hot raw material and discharging cold raw material for the manufacture of said glued pressed products, an inlet for non-oxidizing gas, a cooling device cpmmunicating with said chamber, means for creating a flow of gas from said chamber to said cooling de ⁇ vice and back therefrom as well as an outlet for the conden ⁇ sate from said cooling device.
• Elevated temperatures can also occur in heat- treatment processes and e.g. grinding processes where the temperature rises as a consequence of the high input of mechanical energy. It should also be noted that when hot wood material is discharged from a process stage it easily comes into contact with the surrounding air.
• the oxygen content in the drying atmos ⁇ phere is at least 10 % in the apparatus currently employed for drying of wood chips and veneer.
• the oxygen content can be appreciably reduced if drying is carried out in a steam atmosphere in such a manner that the oxygen is displaced.
• Two possibilities can be distinguished. On the one hand in ⁇ direct drying in which the heat is introduced via a heating surface and in which the vapour phase is formed by the water evaporated from the wood.
• direct drying in which the heat is introduced in the form of superheated steam and the vapour phase is formed by the steam thus in ⁇ troduced together with the water evaporated from the wood.
• the drying apparatuses for these drying methods are known per se, e.g.
• FIG. 1 The figure illustrates diagram- matically and by way of an example an embodiment for chips which can, however, readily be modified for other materials.
• the material flows from the drying chamber via a cyclone separator 1 through a rotary-vane feeder 2 to a cooling chamber 3.
• the material at this stage has an elevated tem ⁇ perature and is accompanied by a certain amount of steam, partly as such and partly in the form of water which can be evaporated in the cooling chamber.
• an oxygen-free inert gas atmosphere is maintained which contains minor amounts of water vapour.
• the inert gas is circulated by the aid of a fan through a cooling battery 5 and is thereafter brought into good contact with the wood material in the cooling chamber 3.
• the inert gas is cooled by the battery 5 through which flows e.g. cold water and at the same time a part of the water vapour in the inert gas is condensed in said battery and the condensate is run out through a separate pipe 6.
• the cooled and dry wood material is discharged by a rotary-vane discharger 7.
• the cooling chamber 3 can be furnished with fittings to obtain a good contact with the inert gas and a sufficient transit time through the apparatus.
• Losses of inert gas are made up by addition of inert gas (e.g. nitrogen) via a pipe 8.
• a slight excess pressure is suitably maintained in -ehamber 3 in order • to prevent leakage of air into the chamber.
• a cooling of the wood material is obtained in an oxygen-free atmosphere without any appreciable rise in the moisture con ⁇ tent of said material since the partial pressure of the water vapour is kept continuously low by the dehumidifying effect of the cooling battery.
• the apparatus can also be arranged so that a reduced pressure is maintained in the cooling chamber by means of condensing out steam with the cooling battery and removing inert gas by a vacuum pump. The temperature of the chips is then de ⁇ termined by the equilibrium at the pressure in question and cooling can thus be obtained without contact with oxygen.
• process stages other than drying such as e.g. heat treat ⁇ ment and grinding in which elevated temperatures occur in the wood material it is possible to prevent contact with oxygen by displacing the oxygen with oxygen-free inert gas or steam.
• the apparatus can be arranged so that the water vapour formed during grinding displaces oxygen from the apparatus.
• Two samples of chips were taken at a wood chipboard factory from the same batch of chips prior to drying and were treated in different way ⁇ .
• One sample (the reference sample) was dried in a laboratory drier by hot air to a normal final moisture content and from the dried chips test chipboards were " made by the normal laboratory procedure and the pro ⁇ perties of these boards were tested.
• the other sample of chips was dried in a steam drier by means of superheated oxygen-free steam. The drying took place in the following manner.
• the chips are fed by a rotary-vane feeder without prewarming into a circulating stream of steam under pressure (in the present case approx. 0.2 MPa abs.) .in which the speed of the steam is sufficient to transport the chips through the drier.
• the steam-chip suspension thereon passes through tubes which are heated from the outside by steam (approx. 1.0 MPa abs.).
• the carrying steam is thus kept continuously superheated (in the present case the steam temperature was 150-170°C) whereby the water in the chips is evaporated and the chips dried.
• the chips are separated from the steam by a cyclone separator and discharged by a rotary-vane device. Most of the steam is recirculated by a fan but part, corresponding to the water evaporated from the chips, is removed from the apparatus.
• the temperature of the chips in the drier was thus initially (after the heating-up phase) equal to the boiling point of water at the pressure in question (approx. 120°C) and at the end of the drying equal to the temperature of the steam, i.e. approx. 150 C.
• the chip temperature was thus within the same range as in conventional drying.
• the oxygen content in the drying.apparatus was -very low.
• the small amounts of air which unavoidably accompanied the chips when they were fed in were removed in conjunction with the continual drawing off of steam.
• the oxygen content in the steam was therefore very consid ' erably under 1 %.
• the chips contained small quantities of water on discharge from the apparatus and part of this water was evaporated by the rapid reduction in pressure.
• the amount of glue in the board can be reduced without impairing the properties of the board compared with conven ⁇ tional production.
• the board can be pressed to a lower density without impairing the properties of the board compared to conven ⁇ tional technology.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Manufacturing & Machinery (AREA)
• Wood Science & Technology (AREA)
• Forests & Forestry (AREA)
• Chemical And Physical Treatments For Wood And The Like (AREA)
• Drying Of Solid Materials (AREA)
• Dry Formation Of Fiberboard And The Like (AREA)

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Publications (1)

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Family Applications (1)

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Family Cites Families (3)

• 1984
  • 1984-09-28 FI FI843834A patent/FI72075C/sv not_active IP Right Cessation
• 1985
  • 1985-09-13 EP EP19850904632 patent/EP0227675A1/en not_active Withdrawn
  • 1985-09-13 WO PCT/FI1985/000075 patent/WO1986002032A1/en unknown

Non-Patent Citations (1)

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