METHOD OF MANUFACTURING LIGNOCELLULOSIC BOARD

Abstract: A method for the continuous manufacture of board from lignocellulosic material, where the material is disintegrated to particles and/or fibers, dried, glue-coated and formed to a mat and pressed to a finished board. The formed mat is heated with steam and thereafter compressed to near final thickness, whereafter it is pressed to a manageable board in a calibration section.

Description: Method of manufacturing lignocellulosic board This invention relates to a method for the continuous manufacture of board from lignocellulosic material. Methods of manufacturing board from raw material based on lignocellu- lose are well-known and widely used in practice. The manufacture com¬ prises the following steps: disintegration of the raw material to part¬ icles and/or fibers of a suitable size, drying to a definite moisture ratio and glue-coating the material prior to or after the dying, form¬ ing the glue-coated material to a mat, which can be built-up of several layers, possibly cold pre-pressing, pre-heating, nozzle-spraying of the faces etc,, as well as a hot pressing at the simultaneous application of pressure and heat in a discontinuous or continuous press to a finished board. At conventional hot pressing the pressed material is heated substant¬ ially by thermal conduction from adjacent heating plates or steel belts, which have a temperature of 150-250 C, depending on the type of pressed product, used glue type, desired capacity etc. The moisture of the mat¬ erial closest to the heat sources is hereby vaporized, whereby there a dry layer develops,and a steam front moves successively inwardly to the board core from each side as the pressing proceeds. When the dry layer develops, a temperature of at least 100 C prevails in this layer, which initiates normal glues to cure. When the steam front has arrived at the core, at least a temperature of 100 C has been reached there, and the board commences there also to harden, whereafter the pressing can be finished within seconds. This applies to the use of conventional urea- -formaldehyde glues (UF) and the like, such as melamine-reinforced (MUF) glues. When using other glues with higher curing temperature, a higher temperature and a higher steam pressure must be developed in the board before hardening can take place. In order to bring about desired board properties, a press must be cap¬ able to apply high face pressures at high temperatures. This is per se no problem for a discontinuous press which, however, has other dis¬ advantages, such as inferior thickness tolerances etc. At continuous presses the requirement of high face pressures at simultaneously high temperatures has made necessary expensive precision solutions as reg¬ ards the roller bed between steel belt and underlying heating plate. The method of introducing heat to the board via thermal conduction also has the effect that the heating takes a relatively long time, which re¬ sults in long press lengths (large press surfaces) . Presses up to a length of 40 have been delivered. Besides, at known continuous press¬ es it is practically not possible to make the heating plates sufficiently flexible, so that density profiles cannot be formed as freely as at dis¬ continuous pressing. Another method of board manufacture based on the introduction of steam between the heating plates in a discontinuous press has also been used to a restricted extent. Since at the supply of steam the material is heated in seconds, the heating time is shortened radically. Besides, the compression resistance of the material is reduced considerably when steam has been supplied. This is a positive feature, which implies that the press could be designed with less press power and substantially shorter length (smaller press surface) . In order to obtain desired prop- ertiesαf a board manufactured according to this method, however, it was necessary to apply conventional pressing technique with high surface pressures and thermal conduction from conventional heating plates at the beginning of the press cycle, whereby after a long heating time a face layer with high density was obtained. It was first thereafter poss¬ ible to blow in steam for heating the core portion of the board. This has involved problems, because the steam had to be blown through the newly formed face layer with high density , and because the pressing time has been extended considerably during the period of high pressure and thermal conduction. Due to all this, a steam press operating acc¬ ording to this concept has a substantially lower capacity, alternatively requires a larger press surface and higher press power than would be necessary when uniform density is desired, At all manufacturing methods mentioned, a soft face layer with lower strength, unacceptable paintability etc. is obtained, which requires this layer to be ground off. The resulting material loss amounts to 5-15%, depending on board type, thickness etc. The present invention has the object to offer a novel manufacturing process for the continuous pressing of board of material based on lignocellulose, by which method it is possible to utilize the advant¬ ages of steam heating, which implies that then the equipment can be designed with a substantially smaller press surface and lower press power, i.e. less expensive, and, besides, without heating plates, which renders the equipment still less expensive, and thereby substant¬ ially producing a board with uniform density profile, which can be used in this state or be further refined. According to the invention, the pressing at a basic embodiment is carr¬ ied out so that the matfarmed is heated with steam and thereafter com¬ pressed to near final thickness, whereafter it is pressed to a manage¬ able board with uniform density profile or with slightly increased face density. According to one entooctLrreπt, the mat is compressed to modest density, whereafter steam is supplied. The mat is thereafter compressed further to above final density, whereafter the mat is allowed to expand slightly and harden to such a degree that a manageable board is obtained. At a preferred embodiment of the process, the mat coming from the form¬ ing (which can be not pre-pressed or cold pre-pressed in a separate belt pre-press if it is desired to better clear belt transitions and to be able to more easily indicate possible metal) is first compressed in a press inlet to a roller press provided with wires to a density 3 of 150-700 kg/m , whereafter gas or steam of controlled pressure and overheating degree is supplied through the faces via steam chest(s) and/or steam roller(s). The mat is hereafter compressed successively to less than final thickness by means of roll pairs, whereafter it is allowed to expand in additional roll pairs, whereafter the board hardens. The roller press should also be heated to prevent condensation at the steam supply. The object of the compression to a thickness less than the final one is to compress the mat strongly, so that smaller loads are obtained in subsequent roll pairs. This method is desirable in order to reduce the loads on the machine, but is not necessary for the process. The compression of the mat is of importance for the density profile of the board pressed. By adjusting the mat density, at which steam is supplied, the face density of the board can be controlled. At increas¬ ing mat density, the density of the pressed board changes from a uniform density profile to a density profile with increased face density. Such an increase in the mat density, however, implies an increase in com¬ pression work in the inlet zone of the mat. At an alternative embodiment of the invention, the mat is heated in the way described above, but continued compression in a calibration sect¬ ion does not take place longer than to near final thickness, whereafter the board is exposed to high heat and line loads in a hot calendering section. Hereby a board with increased face density is obtained. At this embodiment, the mat is compressed in the inlet wedge to modest density, whereafter steam is supplied in a similar way as described above. The mat is hereafter compressed further to near final thickness and is allowed to partially harden in a calibration section, whereby the board becomes sufficiently stable for continued transport to a hot calendering section, where the board is compressed between roll pairs at supplied heat and pressure to high density, whereafter it is allowed to spring back to final thickness in the outlet. Contrary to all previously known presses for manufacturing board based on lignocellulose, it was found that from a process-technological point of view it is possible to obtain board with good properties even at high densities in spite of the absence of heating plates. At the application of the method according to the invention, the steam is supplied continuously. When a small excess of steam above tne amount required for heating the mat is suσplied, this ensures that all air enclosed in the mat is pressed rearwarα m the inlet, whereby it is further ensured that all parts of the mat are heated. The characterizing features of the invention are apparent from the att¬ ached claims. The invention is described in greater detail in the following, with ref¬ erence to the accompanying drawings illustrating an application of the invention. Fig. 1 shows a heated belt press witn steam supply. Fig. 2 shows the density in the thickness αirection of a board. Figure 1 is a lateral view of an equipment according to the invention, comprising a belt press 1 and a hot calendering section 13. The belt press 1 is in known manner provided with drive rollers 2, drawing, rollers 3, guide rollers 4 an adjustable inlet portion 5 with inlet rolls 6, at least one steam roller 7, at least one compression roll 8, calibrat¬ ing rolls 9 in a calibration section 10 and surrounding wire 11, alter¬ natively perforated steel belt with wire. The mat is compressed in the 3 inlet portion 5 to a predetermined density in the range 150-700 kg/m , 3 preferably in the range 250-500 kg/m ,at the passage past the steam roller 7, whereby steam of 1-6 bar is injected into a sector in contact with the wire in an amount sufficient to heat the mat to 100°C and drive out all included air. The compression resistance of the mat is hereby reduced significantly, and continued compression at the compression roll 8 and in the calibration section 10 can be carried out with very small forces. At an alternative embodiment, a conventional steam chest can be used at the beginning of the calibration section in order to ensure a sufficiently high temperature during the hardening of the board (depending on board type, etc.). Due to the use of only rolls, excess steam is free to flow off through the wire, and therefore normally no vacuum suckiπg-off zone is re¬ quired at the end of the calibration section. At an alternative embod¬ iment, a vacuum box can be installed in order to facilitate the control of residue moisture and de-flashing of excess steam. As an alternative or complement to the steam roller 7, one or several conventional steam chests can be used. When it is desired to improve the density and tightness etc. of the face layer and/or to fine calibrate the thickness measure of the board and/or to provide the board with a suitable pattern or face structure, the board can pass through a section with one or several hot calender¬ ing rolls 13 with high surface temperature, possibly preceded by a sect¬ ion 12 where suitable steam, gas or liquid can be supplied as pre-prepar- ation. The hot calendering rolls, at an alternative, can be surrounded by an endless steel belt. A uniform density profile, as mentioned, can be obtained by supplying the steam at low or modest mat density, and without additional treat¬ ment by hot calendering rolls. In Fig. 2 a density profile is shown, which can be brought about at thin board (for example 1 mm) , substant¬ ially by passing the board through said hot calendering rolls. Still higher face density tops can be achieved by a hot calendering section with roll pairs enclosed by hot steel belt, whereby the board in the hot calendering section is compressed to a face density slightly higher than the desired final face density at high temperature (150-300°C) and be passed through a number of roll pairs and thereafter be expanded to final thickness. The invention is not restricted to the embodiments described, but can be varied within the scope of the invention idea.

Claims: Claims 1. A method for the continuous manufacture of board from lignocell¬ ulosic material, where the material is disintegrated to particles and/ or fibers, dried, glue-coated and formed to a mat and pressed to a finished board, c h a r a c t e r i z e d i n that the formed mat is heated with steam and thereafter compressed to near final thickness, whereafter it is pressed to a manageable board in a calibration section. 2. A method as defined in claim 1, c h a r a c t e r i z e d i n that the density is controlled of the formed mat before it is heated with steam for controlling the face density of the pressed board. 3. A method as defined in claim 1 or 2, c h a r a c t e r i z e d i n that the mat after heating with steam is compressed to below final thickness and thereafter is allowed to expand to final thickness when it enters the calibration section. 4. A method as defined in any one of the claims 1 - 3, c h a r a c t e r i z e d i n that the mat is compressed with rolls, and the steam is added in an amount above that required for the heat¬ ing, whereby air included in the mat is pressed rearward in the mat. 5. A method as defined in any one of the claims 1 - 4, c h a r a c t e r i z e d i n that the pressing in the calibration section is carried out with press rolls, and residue steam in the mat is allowed to escape between the press rolls. 6. A method as defined in any one of the claims 1 - 5, c h a r a c t e r i z e d i n that additional steam is supplied in the beginning of the calibration section in order to ensure a suff¬ iciently high temperature during the hardening of the board. 7. A method as defined in any one of the claims 1 - 4, c h a r a c t e r i z e d i n that the board after the calibration section is exposed to hot calendering for fine calibration and/or modi¬ fication of the face density of the board. 8. A method as defined in claim 7, c h a r a c t e r i z e d i n that the board at the hot calendering is compressed to a face density slightly above the desired final face density.

Inventors: LUNDGREN GOERAN, SCHEDIN KURT, SISLEGAARD LARS-OTTO, THORBJOERNSSON SVEN-INGVAR, LUNDGREN, GOERAN, SCHEDIN, KURT, SISLEGAARD, LARS-OTTO, THORBJOERNSSON, SVEN-INGVAR
Applicants: LUNDGREN GOERAN, SCHEDIN KURT, SISLEGAARD LARS OTTO, SUNDS DEFIBRATOR IND AB, THORBJOERNSSON SVEN INGVAR, LUNDGREN, GOERAN, SCHEDIN, KURT, SISLEGAARD, LARS-OTTO, SUNDS DEFIBRATOR INDUSTRIES AB, THORBJOERNSSON, SVEN-INGVAR
Application year: 1996
Main classification: B27N3
IPC codes: B27N3