AU732714B2

AU732714B2 - Method for coating fibers with glue

Info

Publication number: AU732714B2
Application number: AU11423/99A
Authority: AU
Inventors: Christian Boehme; Andreas Michanickl
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 1997-09-16
Filing date: 1998-09-15
Publication date: 2001-04-26
Anticipated expiration: 2018-09-15
Also published as: BR9812455A; AU1142399A; CA2304232A1; DE59802663D1; WO1999014023A1; DE29880083U1; EP1017550B1; EP1017550A1; ATE211960T1; DE19740676C2; JP2001516657A; NZ502760A; DE19740676A1; CN1270549A

Abstract

The invention relates to a method for coating fibers with glue, whereby glue is applied to fibers with a humidity index lower than 100 % and the fibrous agglomerates remaining after that process are whirled in a fiber/air current until they dissolve. The glue-coated fibers are suited for producing derived timber products, including beaverboards and preforms. The invention also relates to a device for applying said method and a method for controlled glue application.

Description

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WO 99/14023 PCT/DE98/02736 Method for coating fibers with glue The invention relates to a method for coating fibers with glue, an apparatus for implementing the method and a method for controlling the application of glue.

In the following text, fiber designates all fibers which contain lignocellulose and which in particular are used for the manufacture of wood-based materials. Typical fibers are those which are produced from wood chips or from annual plants, from palms or grasses by the RMP (Refiner Mechanical Pulping), TMP (Thermomechanical pulping) or CTMP (Chemo-thermo-mechanical pulping) process, but fibers produced by chemical wood pulping are also. included here. The fibers coated with..glue can preferably be used for the manufacture of wood-based materials; they are primarily suitable for the production of fiberboards. An example which is typical but not the only one by a long way is the use of glue-coated lignocellulosic fibers for the manufacture of MDF boards (Medium-Density Fiberboards).

Fibers which are used for:the manufacture. of REPLACEMENT PAGE (RULE 26) 2 wood-based materials are manufactured from liquid-impregnated chips. Before being coated with glue, the fibers have to be dried to a fiber moisture content (mass of water related to the absolutely dry mass of fiber) below 100%, preferably between 1 and in order to ensure permanent glue coating, and in order to prevent so-called steam eruptions. These are cracks and disruptions in the fibrous structure of the wood-based material which arise as a result of steam bubbles which are produced during the glue-coating operation. "Dry fibers", as opposed to the liquid-impregnated fibers which are present following the defibering of chips, is therefore used below to designate fibers having a fiber moisture content below 100%. However, fibers with a fiber moisture content between 1 and 10% are particularly preferably concerned.

In order to manufacture wood-based materials from fibers, the mass of fiber has between 5 and glue added to it, based on the absolutely dry mass of fiber. The glues used are various synthetic resins, frequently, for example, phenol formaldehyde, melamine formaldehyde or, if waterproof glue-coating is not required, urea formaldehyde. The term glue is used below to designate all the natural glues and synthetic resins which are applied to fibers containing lignocellulose in order to manufacture wood-based 3 materials.

In the production of medium-density fiberboards (MDF), nowadays so-called "blow-line" glue coating is performed. In this process, fibers, directly following their production in the refiner at high temperatures and high pressure, are mixed with the glue in the so-called "blow line". The glue coating takes place as a result of the glue being sprayed into the blow line.

The blow line is located between the refiner and a fiber dryer. Because of the pressure difference between refiner and fiber dryer, the still moist and very hot fibers shoot through the blow line at a speed from 200 to more than 450 m/s. As a result of the turbulence which is produced in the blow line during this process, the glue which is sprayed in mixes with the fibers and, as a result, the fibers are coated with glue. This process is completed in fractions of a second. At the end of the blow line, the glue-coated fibers pass into the fiber dryer. There, they are dried to the desired moisture content and subsequently compressed to form board materials or moldings.

This type of glue coating is currently used in every MDF factory. It is technically simple to carry out, but conceals two significant disadvantages. On the one hand, during the drying of the fibers formaldehyde from the glue is released on the fiber surface. This passes into the atmosphere with the dryer waste air.

4 For this reason, complicated installations, such as waste-air scrubbers, electrostatic filters and the like for cleaning the dryer waste air from MDF fiber dryers are required or are prescribed by the respective licensing authorities. Waste-air cleaning systems are very expensive to procure and to operate.

On the other hand, the glue partially hardens during the drying process or else diffuses into the fibers and is then no longer available for the connection of fibers. As a rule, drying takes place from a fiber moisture content (fiber moisture content means mass of water related to the absolutely dry mass of fiber) from over 100% down to a fiber moisture content around 10%. The glue which is hardened in this process no longer contributes to fiber bonding. It is, so to speak, lost. It is assumed that about 20% of the glue used is no longer reactive following the drying.

Since glue is more expensive than fibers, this loss constitutes a significant cost factor.

As an alternative to the above-described wet glue coating, from time to time the dry coating of MDF fibers with glue has also been attempted on large industrial scale using glue mixers from the chipboard industry. For the purpose of dry coating fibers with glue, chip mixers have been used which are employed for coating wood chips with glue in chipboard production.

The construction of such mixers comprises a rotatable 5 cylinder, which is generally disposed horizontally and through which a central shaft passes. The shaft is provided with outlet openings for the glue. If necessary, conveying means for the chips are fitted in the cylinder or on the shaft. In this case, the energy consumption of chip mixers is considerable.

Further processes for dry coating with glue are described in US Patent 2,658,847 and DDR Patent 78 881.

The fibers are in each case separated before the application of glue, the glue is applied by spraying and the fibers are processed directly to form wood-based materials without further treatment. In this case, it is attempted to prevent the formation of fiber agglomerates if possible, but this cannot be avoided completely as has been shown in practice.

In the German laid-open specification 1 653 223, it is proposed to separate the dry glue-coated fibers again by means of picking rolls before further processing. However, this method has not proven itself since the glue-coated fibers block up the picking rolls, and because the picking rolls not only separate the fibers but also produce fiber agglomerates again as a result of the mechanical pressure exerted.

Dry glue-coating has primarily failed on the poor distribution of glue and on the formation of fiber agglomerates. These give rise to a nonuniform surface on the fiberboard and a heterogeneous structure in the -6bulk density of the board. The bulk density fluctuations caused by the fiber agglomerates can lead to damage to the press platens of modern continuous pressing installations.

The object of the invention is to propose a method for coating fibers with glue in which the use of glue is minimized. Furthermore, the object of the invention is to provide an apparatus for implementing this method, with which fibers are coated with glue in an optimum way with the lowest ,possible use of glue and o* energy. Finally, the object of the invention is to propose a method for controlling the application of glue, with which the glue-coating operation is carried out under optimized conditions., The above-described object. is, achieved, by a method for coating fibers,, which. have, previously .been *o set to a fiber moisture content below 100, with glue, glue being applied to the fibers, and the fiber agglomerates which may result from the glue-coating operation or are present after the glue-coating :o operation subsequently being subjected to turbulence in a fiber/air stream and being broken up again in this way by swirling.

Previously, all the measures with which attempts were made to coat fibers with a fiber moisture content below 100% with .glue .were. directed: to cRAL/configuring the glue-coating operation in such a way ,.IP 6a that no fiber agglomerates are produced. The method according to the invention also strives to achieve this, but also makes a separation in the steps of coating the fibers with glue and breaking up fiber.

It has been shown contrary to the prejudices 0 0* 0O 0 00 *000 0 0 *0 oo 00 oooooS ooo 7 to those skilled in the art that the uniform coating of the relatively dry fibers with glue is ensured, and that the fiber agglomerates can be broken up again without damaging the fibers, that is to say also without any loss in fiber strength. With the method as claimed in the main claim, it is in particular possible to coat dry fibers, that is to say fibers with a fiber moisture content below 100%, preferably between 10 and 1% fiber moisture content, with glue and to break up fiber agglomerates produced during the glue-coating operation again, by the fiber agglomerates being broken up again by means of swirling in a fiber/air stream.

The particular advantage of these measures resides in the fact that the dry, glue-coated fibers, after any fiber agglomerates which may be present have been broken up, can be fed immediately to the production of wood-based materials. Glue losses which are caused by the hardening of glue during the drying of fibers are avoided here.

The manner in which the fibers are fed to the glue-coating operation and subsequently to the operation of breaking up the fiber agglomerates can be configured in any desired way. Mechanical transport of the fibers, for example by means of screw feeders, can be imagined. However, it is particularly preferred for the fibers to be conveyed in an air stream, referred to below as a fiber/air stream. This type of transport can 8 be implemented particularly cost-effectively and simply. Complicated conveying means are dispensed with, and the fiber/air stream is simple to control. In conjunction with this description, air primarily means ambient air, but this designation also includes all other gases which can be used to transport the fibers and, if necessary, also to treat the fibers.

A preferred configuration of the method provides for the fibers to be dried before being coated with glue. In this case, the moisture content of the fibers is preferably dried to values below 20% or below depending on the requirements of the production which follows the glue-coating operation, even below to below 1% moisture content. During the drying operation, account is taken, if necessary, of the fact that the application of glue means that moisture is reapplied to the fibers.

Fiber agglomerates are preferably broken up in that the fibers and the fiber agglomerates, which are conveyed in a fiber/air stream, are caused to flow in a manner which is turbulent, separated and subjected to intense vortices. The vortices produced in turbulent flow exert shear forces on the fibers and fiber agglomerates, which are such that the fiber agglomerates are dispersed again into individual fibers, but such that the fiber itself remains undamaged. Since the glue adheres more firmly to the 9 fiber surface then the fibers adhere to one another, the separated fibers still have a uniform glue coating.

Breaking up fiber agglomerates following the glue-coating operation means that the particularly homogeneous and uniformly glue-coated fibrous material desired by the processors is provided, which is in particular a precondition for the uniform and trouble-free production of fiberboards.

In order to produce turbulent flow, in which fiber agglomerates are broken up, pieces of plant which are already known per se from chip and fiber manufacture and processing are particularly suitable.

According to the invention, refiners, defibrators or mills are operated in such a way that, counter to the originally intended purpose, the fibers and fiber agglomerates are not worked on mechanically, for example beaten to a higher degree of beating, as they pass through the refiner or the mill, but are merely swirled in a turbulent flow and, in the process, are separated, that is to say "fluffed".

When separating fiber agglomerates, for example, a refiner runs with a plate spacing of more than 0.2 mm, preferably more than 1.0 mm. At this opening width, no beating takes place, but the fiber/air stream is entirely caused to flow between the two plates of the refiner in a turbulent, separated flow subject to vortices. Refiners and mills are known 10 as devices with an extremely high power consumption, and for this reason there are inherently misgivings on cost grounds about the renewed, additional use of such pieces of plant, which are already used for manufacturing the fibers themselves as well. However, these misgivings may be refuted by the fact that the refiners or mills are not intended to exert any beating work here. The devices are run at idle, so to speak.

The power consumption for separating the fiber agglomerates is also extremely low, so that hardly any additional costs arise as a result of these devices being used.

Depending on the loading and size of the production plants, it is also possible for simpler apparatuses to be used for swirling the fiber/air stream. A rotor or propeller, which is disposed at the outlet of the apparatus for coating fibers with glue or in the following line of the fiber/air stream, is able to accelerate and/or swirl the fiber/air stream adequately, so that any fiber agglomerates present are broken up again.

It is readily conceivable to coat fibers with a high fiber moisture content with glue in the abovedescribed way and, if necessary, to separate them again, in order subsequently to dry them and process them further. However, it is particularly preferred to coat already dried fibers with glue and, following the 11 breaking up of fiber agglomerates, to feed these directly to a molding station in order to manufacture wood-based materials. Precisely this sequence of process steps ensures the economic use of power and glue, so that by comparison with the conventional sequence of process steps, considerable cost savings are achieved.

The fibers can be further processed immediately after they have been coated with glue and after the fiber agglomerates have been broken up. However, intermediate stores can also be provided, in order to collect the fibers before they are coated with glue or after the fiber agglomerates have been broken up, before they are fed to the glue-coating operation or to production.

As already explained, fibers are coated with glue in the blow line at speeds from more than 200 m/s to more than 450 m/s and subsequently dried. It is obvious that, both because of the imponderables of the drying process and also because of the conditions of the glue-coating process itself, which can be set only approximately, an excess of glue always has to be provided, which entails considerable costs. It is therefore viewed as a particular advantage of the method according to the invention that it is possible to coat fibers with glue at low speeds, since in this way the addition of glue can be metered precisely and 12 sparingly. The speed of the fibers during the glue-coating operation is in any case below 150 m/s, preferably below 50 m/s. Falling speeds in the range between 0.1 and 10 m/s are particularly preferable. At such low falling speeds, which can be set by appropriate control of the fiber/air stream, sufficient time remains to coat the fibers optimally with glue.

It is considered particularly advantageous if the fibers fall at a natural falling speed, following gravity, in a fiber/air stream. However, it may also be appropriate to subject the fibers to a certain countercurrent or, for example, to lead them through the glue-coating zone on a spiral path, in order to increase the residence time in the glue-coating zone or in order to condition fibers for further processing after the glue-coating process.

A further advantageous refinement of the method according to the invention is likewise associated with the low speed of the fibers when they are being coated with glue. During the glue-coating operation, according to the invention the climate, in particular the temperature and atmospheric humidity, in the vicinity of the fibers can be set in such a way as is most advantageous for the application of the glue and, for example, for preventing the glue from hardening. This measure therefore also contributes to the economical use of glue. Temperature and atmospheric humidity, as -13critical parameters for the surrounding climatic conditions, can be set simply, for example by adding temperature-controlled air at a predefined atmospheric humidity.

If necessary, the glue-coating operation can be carried out in the presence of inert or protective gases, for example if the presence of oxygen is injurious to the glue.

As a result of the lower speed of the fibers when being coated with glue, it is'possible to set the 06 .moisture content of the fibers deliberately within specific limits while they are being coated with glue and/or while the fiber agglomerates are being broken

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up. This means that it is possible, for example, to moisten overdried fibers slightly or to adjust fibers optimally again for further processing if they have too *e high a moisture content as a result of the application *e of a glue containing too much water.

o The method according to the invention may be implemented particularly simply and cost-effectively in I :the apparatus described below. An apparatus for applying glue to fibers with a fiber moisture content of less than 100% comprises,' in the simplest configuration, a tower which has a filling opening for the fibers to be coated with glue, means disposed in the tower or in its wall for applying glue, and an 13a outlet for the glue-coated fibers, an apparatus for breaking up fiber agglomerates by generating turbulence in a fiber/air stream being connected to this outlet.

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@00550 S S -14- The tower has any desired cross section, but is preferably of cylindrical shape. The filling opening and the outlet are ineach case fitted to opposite ends of the tower, preferably in the area of the ends. The filling opening can be constructed as a simple opening for introducing the fibers into the tower, but can also be constructed as a closable opening or as an air lock, for example when the mass of fiber to be introduced into the tower can be controlled by opening or closing the filling opening, or if defined climatic conditions 0O OS S. in the tower have to be maintained. In the same way as the filling opening, the outlet can be constructed as 0005 an opening, which is adjoined by a pipe, for example, o as a closable opening or as an air lock.. The apparatus 000000 for breaking up fiber agglomerates either adjoins the outlet directly or is connected to the outlet by a .05'0. connecting line, for example a pipe.

SeeS 0o0o S The fibers pass into the tower through the filling opening. The tower preferably stands vertically in relation to the longitudinal axis of the tower, but can also be disposed at an angle or even horizontally.

5505 The fibers are introduced into the tower mechanically or which is preferred in a fiber/air stream, and conveyed through the tower. If the tower is vertical, it is particularly advantageous if the fibers, 15 following the force of gravity, can sink through a glue-coating zone, in which the coating with glue is carried out, in the tower without any additional use of energy. The tower has been proven to be particularly suitable to carry out the glue-coating operation, since it has an adequately large volume to permit a throughput which, for example, can supply a fiberboard production system. In addition, a tower is simple to set up and is readily able to accommodate means for coating with glue, for feeding air or for swirling the fibers, which may be needed for implementing the method.

The apparatus for breaking up fiber agglomerates which has already been described in more detail above, is arranged at the outlet from the tower.

After being coated with glue, the fibers, especially if a refiner or a mill is disposed at the outlet of the tower, are drawn in by the suction which is generated by the rotor in the refiner or in the mill as the fiber/air stream is accelerated. As a rule, therefore, special conveying means between tower and refiner or mill are not required.

The means for applying glue, which are fitted in or on the tower, are preferably constructed as nozzles, which are connected to a supply container via feed lines. Nozzles are most suitable to atomize the glue finely and thus to apply it to the surface of the 16 numerous fibers which are conveyed through the tower.

In the simplest, most cost-effective and therefore most advantageous embodiment, the nozzles can be disposed in the wall of the tower. The tower and the nozzles for the glue have to be matched to one another in such a way that the nozzles do not spray the glue as far as the opposite wall of the tower, but on the other hand complete coating of the fibers with glue is also ensured. The disposition of the nozzles can be selected freely, it can be defined annularly, spirally or in any other suitable way. The area of the tower in which the nozzles for coating the fibers with glue are fitted is designated the glue-coating zone. The injection of the glue can either be carried out pneumatically in the air stream or air-free by injecting by means of static pressure.

In addition to the means for applying glue, according to a preferred development of the apparatus, means for introducing air or other gases are disposed in the tower or in the wall of the tower. In the simplest case, the means for introducing air or other gases are the openings of corresponding feed lines.

Preferably, however, these means for introducing air or other gases are also constructed as nozzles. The nozzles are either disposed distributed in the glue-coating zone and/or outside the glue-coating zone.

They can be used for various purposes. By means of 17 introducing air, for example, the course of the fiber/air stream and/or the speed of the fibers can be guided and influenced. The air can be cooled, warmed, moistened or dried, so that the climate in the tower, in particular in and after the glue-coating zone, can be set via the means for introducing air. As already described above, it is generally air which will be introduced into the tower. However, if an inert gas or protective gas atmosphere should be required for special applications, this can be implemented at any time with the aid of the means for introducing gases.

It is likewise possible to introduce gaseous reaction components, which are intended to interact with the glue, into the tower in this way. However, additives which are important for the finished wood-based product, such as fungicides, can also already be introduced here.

Finally, means for swirling and/or conveying fibers can be disposed in the tower or on the wall of the tower. These means can be constructed as guide plates, rotors, chicanes or the like. Since the conveying speed and the swirling of the fibers is critical for the quality and uniformity of the coating with glue, the provision of such swirling means is a measure which generally improves the result of the glue coating significantly.

The above-described means for applying glue, 18 for introducing air and for swirling and/or conveying fibers are preferably fitted on or in the wall of the tower. This is simpler in constructional terms and, from the point of view of maintenance, is also more beneficial than the insertion of such means in the interior of the tower. If, however, reasons require these means to be disposed in the tower, for example reasons of space, then this is readily possible. It is particularly preferable for the above-described means for applying glue, for introducing air and for swirling and/or conveying fibers to be constructed so as to be adjustable. If nozzles are used, these should be adjustable, in order still to be able to perform optimal alignment of the spray cones following installation, so that dead angles are avoided. In addition, it may be necessary to carry out changes to the setting, if the throughput of the fibers in the tower changes or if other glues are processed. It should also be possible to vary the spray cone of the nozzles and the throughout, for the abovementioned reasons.

The tower for implementing the method according to the invention is preferably disposed vertically, with the filling opening at the top and an outlet at the bottom end of the tower. With this disposition, the force of gravity is used to convey the fibers in the fiber/air stream, and expenditure of power is therefore 19 minimized. Depending on the arrangement and alignment of the production plant which is present, it may, however, be necessary to dispose the tower at an angle or lying down, that is to say horizontally. If the tower is not vertical then care has to be taken that the fiber/air stream runs approximately in the center of the tower. Deposits on the walls must be avoided if possible, in order to prevent "incrustation" of the tower. However, this can be implemented in a simple way by means of the appropriate disposition of nozzles.

In the simplest embodiment, the tower is cylindrical or approximately cylindrical. However, it is preferable for the tower to be of conical construction, at least in some sections. The tower should be of conical construction in particular in the area of the outlet after the glue-coating zone, in order, by means of the reduction in cross section, to effect an increase in the speed of the fiber/air stream, in order that the formation of vortices and turbulence is assisted. However, it is also possible to construct the tower conically overall or to join two truncated cones to form a tower in such a way that the maximum cross section of the tower is located between the filling opening and outlet. The swirling and the conveying speed of the fibers can be influenced effectively by the shape of the tower.

The means for breaking up fiber agglomerates 20 are preferably constructed as refiners or mills. These pieces of plant, known per se, have the common factor that they have at least one rotor, that is to say a rapidly rotating component. The geometry of the refiner or of the mill ensures that a fiber/air stream, which is moved on a fixed conveying path from the filling opening, through the glue-coating zone to the outlet and through the refiner or the mill, is caused in a defined way to follow a flow which is turbulent and subjected to vortices, without any mechanical damage to the fibers occurring. Breaking up the fiber agglomerates also therefore takes place in a very non-damaging way, since additional air is dragged into the fiber/air stream during the production of the vortices and turbulence.

The fiber agglomerates are broken up by being swirled in the fiber/air steam. The turbulent flow, which is subject to vortices, is preferably achieved by accelerating the fiber/air stream. In this case, a relatively low increase in the speed of the fiber/air stream by about 25% is sufficient, assuming that the fiber/air stream is swirled as intensely as possible during the acceleration. A greater increase in the speed, for example by about 50%, 100% or 200% or more, is equally suitable to eliminate fiber agglomerates as a result of swirling in the fiber/air stream. It is advantageous that it is sufficient to swirl the 21 fiber/air stream for a short time. Seconds or fractions of a second of the swirling action are sufficient to disperse the fiber agglomerates again into glue-coated individual fibers.

Fiber agglomerates produced during the glue-coating operation are also eliminated by swirling if the fiber/air stream is accelerated from a speed of about 10 m/s during the glue-coating operation to about m/s and is swirled. However, the same effect is also achieved by accelerating the fiber/air stream to about 150 m/s, without the fibers being damaged. Fiber agglomerates which have been produced as a result of glue coating at a speed of about 150 m/s are dispersed again by accelerating the fiber/air stream to about 200 m/s, without fibers being damaged.

In order to produce complete turbulence in the fiber/air stream with the lowest possible use of power it proves to be advantageous to provide screens, guide plates or chicanes in the region of the means for breaking up fiber agglomerates. Using these measures, the use of energy, which is in any case already low, to break up fiber agglomerates can be lowered further and, in addition, it can be ensured that fiber agglomerates are broken up completely.

According to an advantageous embodiment, the power of the means for breaking up fiber agglomerates can be adjusted. As already described further above, 22 although a low drive power is needed to subject the fiber/air stream as completely as possible to turbulence, it may be necessary to adapt the power of the refiners, mills or propellers or rotors in the event of any fluctuation in the throughput.

A particularly preferred development of the invention provides for the fiber/air stream, after the fiber agglomerates have been broken up, to be led into or through a sifter, in which excess air, which has been picked up during the generation of the turbulence, is separated out of the fiber/air stream. The introduction of a sifter means that the volume of the fiber/air stream is reduced to a considerably lower quantity again. The excess air is preferably circulated through lines which are disposed between the sifter and the means for introducing air and other gases. It is obvious that although it is possible to discharge excess air simply into the environment, it is advantageous inter alia because of the more favorable energy balance to close the circuit, at least to some extent. In particular if it is not air but other gases that are added to the tower, closing the circuit is recommended, in order to save purification plant, which would otherwise have to be installed if excess air were discharged into the environment.

According to a particularly preferred development of the apparatus for applying glue to 23 fibers, means with which the temperature and/or atmospheric humidity of the excess air can be set, before this air is fed back to the tower again, are fitted in or on the lines which are disposed between the sifter and the means for introducing air into the tower. If necessary, cleaning devices can also be used, which extract undesired materials in the circuit of the air stream to be carried. The abovementioned means for setting temperature and/or atmospheric humidity, that is to say cooling or heating devices or air moisteners or air dessicators, prove to be particularly advantageous if the climatic conditions in the glue-coating zone or in the tower overall are controlled in accordance with specific predefinitions.

As an independently inventive part which contributes to achieving the object of coating fibers with glue economically and avoiding excessive use of material and/or energy, a description is given below of a method of controlling the application of glue to the fibers in which, depending on the mass of fiber to be coated with glue and/or the proportion of glue to be applied per mass of fiber, the means for applying glue are set, individually, in groups or as a whole, such that they introduce into the tower a respectively predefined proportion of glue per unit time or per weight unit, referred to the mass of fiber to be coated with glue. As opposed to the prior art, according to 24 the method proposed here, it is possible to exert an influence on the application of glue to the fibers and to control the reaction conditions in a correspondingly detailed manner. As a result, the use of glue and energy can be optimized, and the uniformity of the application of glue to the fibers can be improved in a specific way.

An improved application of glue is also achieved by the means for introducing air or other gases into the tower being set such that in each case a predefined swirling and/or a predefined path or a predefined speed of the fiber/air stream is maintained.

With this measure, the path of the fiber/air stream through the tower is matched to the glue-coating zone in such a way that the glue introduced is deposited as completely as possible on the surface of the fibers.

If, in addition, the mass of fiber introduced into the tower per unit time and/or the fiber moisture content are registered, and if the temperature and/or the atmospheric humidity in the tower or in the glue-coating zone are set as a function of these values, then the fibers can be set optimally to the requirements on the glue placed by production, in particular to the requirements of the presses for compressing the fibers, for example to form MDF boards, which is not possible in accordance with the prior art.

According to an advantageous development of the 25 method, the temperature of the fibers as they enter the tower or in the environment is likewise registered as an important reaction parameter for the behavior of the fibers during the glue-coating operation and the hardening of the glue, and is included in the control system.

It can already be seen as a forward step if the temperature and atmospheric humidity, the addition of air or other gases, the transport speed and other parameters described above can be set overall in the tower. According to a particularly preferred development of the invention, however, a flow and/or climatic profile which is optimized in each case for specific regions or zones of the tower is moreover set.

Thus, for example in the region of the filling opening (the filling zone), the temperature of the fibers can be set optimally to the glue-coating operation by introducing temperature-controlled air for the fiber/air stream. In the glue-coating zone, the atmospheric humidity and temperature, but also the path and speed of the fiber/air stream, for example, can be predefined such that the application of glue proceeds without losses and with the minimum formation of fiber agglomerates. In the region of the outlet, by setting the temperature and atmospheric humidity, the fiber moisture content of the glue-coated fibers can be set such that the compression of the fibers proceeds 26 without interference. In order to set these profiles, the respective means for introducing glue or air or other gases, or the means for swirling or conveying fibers can in each case be driven individually or in groups.

In order to optimize the glue-coating operation, it is further proposed to set the consistency of the glue to be applied to the fibers as a function of the atmospheric humidity in the tower.

Since the moisture content of the fibers is a critical variable, from the point of view of the compression operation, and since additional moisture is applied to the fibers as a result of the application of the glue, precise setting of the consistency of the glue means improved setting of the glue-coated fibers to the compression operation.

Exemplary embodiments of the invention will be explained in more detail below using the example of Figures 1 to 3, in which: Fig. 1 shows a schematic illustration of an apparatus for coating fibers with glue, in longitudinal section; Figs. 2a-d show a cross section through the glue-coating zone of an apparatus according to Fig. 1; Fig. 2e shows a longitudinal section through the glue-coating zone of an apparatus according to 27 Fig. 1; Figs. 3a-c show a schematic illustration of various tower forms, in longitudinal section; and Fig. 4 shows a schematic illustration of a fiber inlet into an apparatus for coating fibers with glue.

Fig. 1 shows an apparatus 2 for coating fibers which contain lignocellulose with glue. A tower 4 is the heart of the apparatus 2. The tower 4 is produced from stainless steel and if necessary is coated on the inside with Teflon or a comparable material with an anti-adhesion action. The coating prevents the glue-coated fibers from adhering to the wall 6 of the tower. The tower 4 is about 10 m high and has a diameter of about 1.5 m, it is of essentially cylindrical construction. Let into the wall 6 of the tower 4 are glue application nozzles 8, whose spray cones are adjustable. The glue application nozzles 8 are connected to a glue supply container 12 via a feed line 10. Also let into the wall 6 of the tower 4 are air nozzles 14. The air nozzles 14 likewise have adjustable spray cones. The air sprayed in through the air nozzles 14 is supplied via feed lines 16.

Made at the upper end of the tower 4 is a simple filling opening 15, which can neither be closed, nor can its passage be varied. Other design forms of the filling opening, for example openings with adjustable diameters or air locks or the like can be 28 used if required. Fibers with a fiber moisture content below 100% are put into the tower 4 through the filling opening 18. The fibers are preferably fed either from a supply container or from a dryer.

Disposed at the opposite end of the tower 4 is an outlet 20. The outlet 20 connects the tower 4 to a refiner 22. The walls 24 of the outlet 22 are constructed to be conical toward the refiner 20. The outlet 20 opens approximately at the center of the refiner disks 26. The conical outlet 20 effects a considerable increase in the flow speed, as a result of the narrowing of the cross section, and as a result promotes the formation of turbulence, and, as a result of the acceleration, produces intensive swirling of the fiber/air stream.

The refiner 22 has two profiled disks 26, of which one disk remains at rest (stator) and one disk rotates at about 4000 revolutions per minute (rotor).

The disks are disposed at a distance of about 1.5 mm from each other. At this disk spacing, the rotor runs approximately at idle, it does not provide any beating work, since the passage between the stator and rotor is too great. The refiner 22 produces turbulence, which leads to fiber agglomerates being broken up. The refiner 22 is also produced from stainless steel and, if necessary, coated, for example with Teflon, so that the glue-coated fibers do not remain adhering to the 29 disks 26 or other parts of the refiner 22.

Connected downstream of the refiner 22 is a sifter 28, which is connected to the refiner 22 by a line 30. In the sifter 28, fibers and excess air are separated. The excess air is routed back into the tower 4 from the sifter via the line 16 and the air nozzles 14. From the sifter 28, the glue-coated fibers are fed either to a supply container or to a molding station or a press for producing wood-based materials.

The apparatus 2 depicted in Fig. 1 is particularly suitable for implementing the method according to the invention for coating fibers which contain lignocellulose and have a fiber moisture content below 100% with glue. The fibers are produced conventionally from chips, for example by RMP, TMP or CTMP processes. The defibering of the chips is carried out at a high moisture content, more than 100%, usually at more than 200% water content, referred to the absolutely dry mass of fiber. The fibers are subsequently dried and then fed either directly from the dryer or from a supply container to the glue-coating operating.

The fibers are conveyed into the tower 4 through the filling opening 18 in a fiber/air stream.

Moving the fibers in an air stream or another suitable gas stream is simple, energy-saving and cost-effective. The air stream, directed from the 30 filling opening 18 to the outlet 20, is preferably produced by the rotating refiner 22, which continually takes in the fiber/air stream. As an alternative or in addition to the refiner 22, the fiber/air stream can also be produced by the air nozzles 14. The speed of the fiber/air stream is by far less than in the previously described blow-line process. The fiber/air stream has a speed of less than 150 m/s, preferably below 50 m/s. In the exemplary embodiment, it is assumed that the fiber/air stream is moved through the tower 4 and the glue-coating zone at a speed which corresponds to the natural fall speed. In the fiber/air stream, the fibers are moved into the glue-coating zone illustrated hatched in Fig. 1. The glue-coating zone is the region of the tower 4 in which the glue application nozzles 8 are let into the wall 6 of the tower. In the glue-coating zone the fibers are sprayed with about 10 to 20% of glue, based on the absolutely dry mass of fiber. The glue is sprayed in finely distributed and is deposited virtually completely on the surface of the fibers.

In order to minimize glue losses, the air nozzles 14 which are arranged between the glue application nozzles 8 in the glue-coating zone are aligned in such a way that they align the fiber/air stream such that the fibers are exposed as uniformly as possible to the spray cone from the glue application 31 nozzles 8. Here, it is also entirely possible for the spray cone of the air nozzles 14 to be directed counter to the main direction of motion of the fiber/air stream.

Applying glue to the fibers with the apparatus described here already minimizes the formation of undesired fiber agglomerates. Previous known methods for the dry coating of fibers with glue used chip mixers with relatively small volumes and mechanical conveying devices which, although they are quite suitable for applying glue to chips, cannot be used at all for applying glue to fibers. Fibers have quite different dimensions, a far greater surface per unit weight and, consequently, a lower bulk density, but no stiffness worth mentioning.

The fiber agglomerates which are still formed, in spite of the improved application of glue, are broken up again in a subsequent method step. For this purpose, the fiber/air stream with the fiber agglomerates contained in it is fed to the outlet and from there out of the refiner 22. The outlet 20 is of conical construction, and the cross section tapering toward the refiner 22 effects a continuous increase in the speed of the fiber/air stream, and therefore an increase in the vortices and turbulence in the fiber/air stream. As it passes through the refiner 22, whose rotor runs at about 4000 revolutions per minute, 32 the speed of the fiber/air stream is increased to about m/s, and the fiber/air stream is made completely turbulent. In the turbulence of the separated flow, which is subject to vortices, the fiber agglomerates are broken up without the fibers being damaged. The spacing between the disks 26 of the refiner 22, at about 1.5 mm, is dimensioned such that no beating of the glue-coated fibers takes place. The rotational speed of the rotor and the diskspacing are to be understood as approximate statements. The respective settings are selected by those skilled in the art such that the desired result, breaking up the fiber agglomerates, is achieved with the lowest possible use of energy.

The glue-coated, now separated fibers are fed in the fiber/air stream from the refiner 22 to the sifter 28. Here, excess air which was taken in during the production of the turbulence is separated out again. The sifter, just like the pieces of plant already described (tower 4, refiner 22), is likewise made of stainless steel and, if necessary, coated. The same applies to the lines through which the fiber/air stream is led.

The sifter 28 is a component known per se for separating excess quantities of gas or air. The excess air is at least partly fed back into the tower 4 through lines 16. From the sifter 28, the fibers are 33 either conveyed into a storage container or transported to a molding station, which, for example, forms board blanks, which are compressed in a subsequent press to form fiber boards or moldings.

Further measures for optimizing and improved implementation of the method can be taken with an apparatus according to Fig. 1. For example, the air nozzles 18 can be used not only to control the fiber/air stream and to swirl the fibers, but together, in groups or individually can also be used for the injection of gases which influence the reactivity of fibers or glue; if appropriate the injected gas can also be a component of the glue.

Means for swirling the fibers are not illustrated in the tower 4 in Fig. 1. However, reference is made to the fact that such means, for example chicanes, guide plates or the like, can be fitted in the tower 4 or to the walls 6 of the tower 4, in order to ensure the optimum guidance of the fiber/air stream.

The application of glue is advantageously controlled by the injected quantity of glue in each case being metered as a function of the mass of fiber put into the tower 4. For this purpose, the throughput of the glue application nozzles 8 is increased or decreased, depending on the mass of fibers to be coated with glue per unit time.

34 It may be desirable to change the moisture content of the fibers before, during or after the glue coating, in order to adjust the fibers and, if necessary, also the glue as well as possible to the subsequent compression operation. The method according to the invention offers the possibility of predefining the climatic conditions in the tower 4, in particular the temperature and/or atmospheric humidity, in order in this way to influence the fiber moisture content specifically. It is possible, for example, for overdried fibers to be remoistened from the filling opening 18 as far as the entry to the glue-coating zone by injecting moist air, or for insufficiently dried fibers to be dried further by injecting dry air. The premature hardening of glue or the penetration of glue into the fibers can be prevented by setting a defined temperature and atmospheric humidity in the glue-coating zone. Thus, temperatures from about 20 to 800C, but mostly about 20 to 400C at an atmospheric humidity of about 40 to 95% are set, as being particularly suitable for the application of glue. The atmospheric humidity may also intrinsically be below but then measures have to be taken to prevent static charging of the fibers. Statically charged fibers have a tendency to deposit on the tower wall; they are no longer readily carried along in the fiber/air stream. Under these conditions, all the glues 35 used industrially, including PMDI isocyanate glues, can be used for coating the fibers with glue.

Following the glue-coating operation it is possible, for example, for the excess liquid picked up during the application of glue to be removed again, at least partly, by means of subsequent drying, in that dry air is injected through the air nozzles 14, which are disposed between the glue-coating zone and outlet According to the invention, such a climatic profile is readily set by the disposition of air nozzles 14 and glue application nozzles 8, by registering the mass of fiber put into the tower 4 and, if appropriate, the fiber moisture content, and by the control of the nozzles 8 and 14 as a function of the mass and moisture content of fibers. Furthermore, it is advantageous in addition to register the temperature of the fibers as they are put into the tower 4, and to set the temperature and, if appropriate, the atmospheric humidity in the tower 4 as a function of the temperature of the fibers and of the fiber moisture content. It is particularly preferred if the climatic conditions in the tower 4 before, in and after the glue-coating zone are in each case set as a function of the state of the uncoated or glue-coated fibers.

In order to dry or to moisten the air to be 36 injected into the tower, in each case as required, and in order to warm or cool the air, it is expedient to dispose appropriate dryers or air moisteners, heat exchangers or heaters or cooling devices in the line 16 which connects the sifter 28 to the air nozzles 14. The devices disposed in the line 16 and the air nozzles 14 can either be set separately from one another or a programmable control unit can be inserted, being provided with or connected to measurement points or sensors which register the temperature of the fibers and, if appropriate, their moisture content and/or the temperature and/or the atmospheric humidity in the tower 4. The application of glue is automatically controlled in this way. The predefinitions for controlling a specific plant can readily be registered by those skilled in the art, since on the one hand predefinitions for the processing climatic conditions of the glues are recommended by the manufacturers and since on the other hand the parameters temperature, atmospheric humidity and speed of the fiber/air stream can simply be set within the known, technically predefined limits.

Figs. 2a to e show a cross section through the tower 4 with glue applicator nozzles 8 and air nozzles 14 disposed in the wall 6 of the tower 4. The nozzles 8, 14 can either be disposed in separate planes, as illustrated in Fig. 2a, or they can be located in one 37 plane, as shown in Fig. 2b. The nozzles 8, 14 can spray radially into the tower 4 (see Figs. 2a, or the spray cone can be directed into the tower 4 at an angle a or at an angle P, as shown in Figs. 2c e. The angles a and P can in each case assume values between 0' and 900, based on the longitudinal axis of the tower 4.

Figs. 3a to c show, in addition to the form depicted in Fig. 1, further variants of a tower 4, in which fibers with a fiber moisture content below 100% can be coated with glue. Fig. 3a shows a tower 4 which tapers. conically from the filling opening 18 to the outlet 20. Here, outlet 20 and tower 4 merge into one another, since this conical form of the tower 4 permits an ideal transition to an apparatus for breaking up fiber agglomerates. As a result of the continuous tapering of the tower 4 from the filling opening to the outlet, the acceleration and swirling of the fibers is effectively assisted. Fig. 3b shows a tower 4 which widens conically from the filling opening 18 to the outlet 20. The advantageous factor here is that the fiber/air stream takes a particularly long path through the glue-coating zone so that the glue-coating operation can be controlled well. Fig. 3c shows a tower 4 which is widened conically in a top section, preferably from the filling opening 18 as far as the end of the glue-coating zone and which tapers 38 conically from the end of the glue-coating zone as far as the outlet 20. Although such a construction is complicated, it combines the advantages of the tower forms described using the [sic] in Figs. 3a and 3b. The dimensions of the tower 4 can vary considerably, in each case depending on the form of the tower, on the fiber throughput per unit time and on the predefinitions with regard to the climatic profile to be set. Overall heights from about 5 to 25 m and diameters from 0.75, preferably about 3.5 m to 6 m, can readily prove to be expedient.

Fig. 4 shows the upper section 32 of a tower 4 having an alternative embodiment of a filling opening 19. The cross section of the upper section 32 widens like a truncated cone from the filling opening 19 to the tower 4. Disposed in this upper section 32 is a distribution cone 34, so that the fiber/air stream put in through the filling opening 19 is distributed annularly and put into the tower 4.

A perforated screen 36 is fitted in the tower 4, underneath the distribution cone 34. Said screen covers the cross section of the tower 4. The hole size is between about 0.5 and 5 cm, preferably between about 0.8 and 3 cm. Fixed in or under the distribution cone 34 is a drive 38 for a distribution apparatus 40. The rotating distribution apparatus 40, constructed here as a doctor 42 with a number of arms, distributes the 39 fiber/air stream put in via the filling opening 19 and the upper section 32 of the tower 4 over the entire cross section of the tower. The fibers therefore fall into the tower 4, into the glue-coating zone, distributed uniformly over a large cross section.

Claims

2. Method according to Claim i, characterized in that before being coated with glue, the fibers are dried to a fiber moisture content below 100%, preferably to a fiber moisture content between 10 and 1%.
3. Method according to at least one of Claims 1 or 2, characterized in that the fiber agglomerates, after being coated with glue, pass through a refiner, a rotor or a mill to break up the fiber agglomerates.
4. Method according to one of the preceding claims, characterized in that after the fiber agglomerates have been broken up, the fibers are fed to a molding station to produce wood-based material products. Method according to Claim 4, characterized in that before or after agglomerates have been broken up, the fibers are fed to an intermediate store.
6. Method according to one of the preceding 41 claims, characterized in that the glue is applied to fibers which are moved in a fiber/air stream at a speed below 150 m/s, preferably below 50 m/s, in particular below 10 m/s.
7. Method according to Claim 6, characterized in that the glue is applied to fibers which are moved vertically or on a spiral path.
8. Method according to one of the preceding claims, characterized in that during the glue-coating operation, a predefined climate is set in the vicinity of the fibers.
9. Method according to Claim 8, characterized in that during the glue-coating operation, the temperature and/or atmospheric humidity is set in the vicinity of the fibers. Method according to one of the preceding claims, characterized in that the moisture content of the fibers is changed during the glue-coating operation and/or while the fiber agglomerates are being broken up.
11. Method according to one of the preceding claims, characterized in that after the glue-coating operation, the speed of the fiber/air stream is increased by at least 25% in order to break up the fiber agglomerates.
12. Method according to Claim 11, characterized in that after the glue-coating operation, the speed of the -42 fiber/air stream is increased by at least 100% in order to break up the fiber agglomerates.
13. Apparatus for implementing the method according to Claim 1, having a tower which has a filling opening (18) for the fibers to be coated with glue, means 10, 12) for applying glue, which are disposed in the tower or in its wall and an outlet for the glue-coated fibers, an apparatus (22, 26) for breaking up fiber agglomerates by generating turbulence in a fiber/air stream being connected to this outlet
14. Apparatus according to Claim 13, characterized in that the means for applying glue are constructed as nozzles. 0 Apparatus according to Claim 13, characterized in that means (14) for introducing air or other gases 0005w into the tower are disposed in the tower or in the wall of the tower.
16. Apparatus according to Claim 15, characterized in that the means (14) for introducing air or other gases into the tower are constructed as nozzles, which are disposed in the tower (14) or in the wall of the tower.
17. Apparatus according to Claim 13, characterized in that means for swirling and/or conveying fibers are __disposed in the tower or on the wall of the 4 2a
18. Apparatus according to one of Claims 13 to 17, *d 0 0 0@ S SG SOS S dS~S S SS*S~ h S sea, *055 S 5e J0 B~ 00 S 0 S OS S 0 ease.. S 43 characterized in that the means for applying glue, the means (14) for introducing air or other gases and/or the means for swirling and/or conveying fibers, in particular nozzles, are constructed so as to be adjustable.
19. Apparatus according to Claim 13, characterized in that the tower is disposed vertically, at an angle or horizontally. Apparatus according to Claim 13, characterized in that the tower is of conical construction, at least in some sections.
21. Apparatus according to Claim 13, characterized in that the means (22) for breaking up fiber agglomerates have a rotor.
22. Apparatus according to Claim 13, characterized in that the means (22) for breaking up fiber agglomerates have screens, baffle plates or the like.
23. Apparatus according to Claim 13, 21 or 22, characterized in that the power of the means (22) for breaking up fiber agglomerates can be adjusted.
24. Apparatus according to Claim 13, characterized in that a sifter through which the fibers pass in a fiber/air stream and in which excess air is separated out of the fiber/air stream, is disposed after the means (22) for breaking up fiber agglomerates. Apparatus according to Claim 24, characterized in that lines (16) are disposed between the sifter (28) 44 and the means (14) for introducing air into the tower through which lines the air separated out of the fiber/air stream is at least partly routed back into the tower again.
26. Apparatus according to Claim 25, characterized in that means for setting the temperature and/or means for setting the atmospheric humidity are disposed in or on the lines (16) which are fitted between the sifter (28) and the means (14) for introducing air into the tower
27. Method for controlling the application of glue to fibers with a fiber moisture content below 100% in an apparatus according to one or more of Claims 13 to 26, in which, depending on the mass of fiber to be coated with glue and/or the proportion of glue to be applied per mass of fiber, the means for applying glue are set, individually, in groups or as a whole, such that they introduce into the tower a respectively predefined proportion of glue per unit time.
28. Method for controlling the application of glue to fibers according to Claim 27, characterized in that means for introducing air into the tower and/or means for swirling fibers are set such that in each case a predefined swirling of the fibers and/or a predefined path of the fiber/air stream is maintained.
29. Method for controlling the application of glue to fibers according to Claim 27 or 28, in which the 45 mass of fiber introduced into the tower per unit time and the fiber moisture content are registered, and in which the temperature and/or the atmospheric humidity in the tower are set as a function of the fiber moisture content and the mass of fiber. Method for controlling the application of glue to fibers according to Claim 29, in which, in addition, the temperature of the fibers introduced into the tower is registered and taken into account in order to set the temperature and/or the atmospheric humidity in the tower.
31. Method for controlling the application of glue to fibers according to one of Claims 27 to characterized in that a temperature and/or atmospheric humidity profile is set in the tower, between the filling opening and outlet.
32. Method for controlling the application of glue to fibers according to one of Claims 27 to characterized in that the consistency and/or the temperature of the glue to be applied to the fibers is set as a function of the fiber moisture content and/or the temperature of the fibers to be coated with glue.
33. Method for controlling the application of glue to fibers according to one of Claims 27 to 32, characterized in that the consistency of the glue to be applied to the fibers is set as a function of the 46 atmospheric humidity in the tower.