AU5847799A

AU5847799A - Method for producing medium density fibre panels

Info

Publication number: AU5847799A
Application number: AU58477/99A
Authority: AU
Inventors: Paul Buchholzer
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 1998-08-05
Filing date: 1999-07-03
Publication date: 2000-02-28
Also published as: EE200000407A; IL139932A0; JP2002522251A; CA2317513A1; HUP0101990A3; WO2000007785A1; BR9907063A; TR200001953T2; HUP0101990A2; DE19930800A1; KR20010033491A; CN1308572A

Description

WO 00/07785 PCT/DE99/02054 Method for producing medium density fiber boards The invention relates to a method for producing low, medium or high density fiber boards (LDF, MDF, HDF) by hot compression of a fiber board formed of glued fibers, in which, in order to produce the fibers, chips are softened with steam at superatmospheric pressure and then ground up into fibers, which are conveyed by steam pressure through a blow line into a tubular dryer and, after being dried, are fed to a distributing machine or the like. Conventionally, debarked wood and/or annual plants are chopped up into chips and subsequently cleaned by screening and washing. The cleaned chips then pass via a filling hopper and a plug screw into a digester, where they are softened with steam at superatmospheric pressure. The chips softened in this way are conveyed via a further screw into a disk refiner, where they are ground up into fibers. The fibers then pass with steam pressure through a blow line into a tubular dryer and, from there, via a cyclone separator into a screen and thereafter into a fiber bunker upstream of a distribut ing machine. In this case, the binder is sprayed into the blow line before the fiber mixture is dried. The distributing machine forms an endless fiber mat on a continuous belt and, after cold precompaction, said mat passes into a hot press and is compressed there to form WO 00/07785 - 2 - PCT/DE99/02054 the fiberboard strip. A problem in the production of MDF boards is gluing the fibers. At the beginning, this gluing was carried out in trough mixers, but this led to fiber agglomerates and felting. This resulted in nonuniform fiber gluing, but above all in the undesired formation of flecks of glue in the board surfaces. These disadvantages were eliminated with the introduc tion of blow-line gluing, which nowadays is the general prior art. In this case, the binder is sprayed onto the fiber stream downstream of the refiner, into the blow pipe which ends in the inlet region of the tubular dryer. It is therefore the still wet, hot fibers which are glued, and are only dried subsequently. This generally used gluing method has a disadvantage, specifically an increased consumption of glue which, at about 12-15% binder / absolute dry mass of fiber, is above the glue consumption of trough-mixer gluing, at about 10-11%. In spite of intensive investigations, the unequivocal reasons for this increased consumption of glue could never be completely explained. In blow-line gluing there is additionally the risk of blockage of the blow line. In order to eliminate this problem, extensive investigations have been carried WO 00/07785 - 3 - PCT/DE99/02054 out, and proposed solutions have been published (see, for example, DE 41 22 842 C2). Some of these proposed solutions recommend the injection of glue directly in the area where the fiber emerges from the blow line, or else as the fiber mixture enters a separator, before the fiber drying is carried out in the tubular dryer. According to another proposal, a blow-line cover is to be provided in the outlet area of the blow line through which cover air is blown in. The blow-line surface cooled in this way is intended then to lead to the formation of condensation on the inner surface of the pipe and thus prevent fiber deposits. Tapering the end of the blow line is also recommended in order to avoid deposits. From Derwent Publication AN 060 276, and SU-A-1 021 629, it is possible to glean the proposal that the injection of binder take place in a separator between the end of the blow line and the beginning of a dryer. According to WO 89/05 716, the binder can also be introduced between the stages of a two-stage dryer. In order to reduce undesirable binder precuring, buffering by means of the addition of alkali is recommended. However, control of the pH and buffer capacities of the fibers is absolutely necessary. In addition, the principle of adding phenolic resin in the gaseous state, with subsequent treatment with formal dehyde and ammonia, has proven to be uneconomic and very difficult to implement industrially. The outlay on WO 00/07785 - 4 - PCT/DE99/02054 machinery was too high; there was no saving in glue with respect to blow-line gluing. In order to reduce the consumption of glue, diluting the binder to a solids content of 35-45% was also recommended. By this means, allegedly an improved distribution of glue and lower precuring of the binder should take place, since the fibers are cooled down by the energy of evaporation of the water. However, this proposal did not lead to positive results in practice either. Concurrently, however, the binder is always applied to the wet and hot fibers; loss of binder, for example as a result of precuring and hydrolysis and as a result of fiber deposits in the dryer, was tolerated as obviously unavoidable. EP 0 728 562 A2 discloses a method for producing wooden material boards from wood fibers or, for example, MDF boards from wood fibers. Here, the previously prepared fibers are first fed to a dryer, from where they pass by means of air transport into a fiber bunker. A measurement of the moisture is carried out there, the output signal from which controls the dryer. From the fiber bunker, the fibers are fed into a delivery line, where they pass through a mass determining station before they reach a wetting zone. Arranged in the WO 00/07785 - 5 - PCT/DE99/02054 wetting zone is at least one gluing nozzle, which is connected to a high pressure pump and can inject glue into the delivery line. After being glued, the glued fibers pass through a screen and are subsequently fed pneumatically to a distributing bunker which is assigned to a forming station. There, the glued fibers are distributed to form a mat and are subsequently compressed into boards. Upstream of the wetting zone, the aforementioned delivery line tapers, as viewed in the conveying direction, in the manner of a Venturi nozzle to a reduced diameter, which opens in a diffuser section which initially expands in a bulbous shape as viewed in the flow direction, this widening then being followed by a taper to a smaller diameter, which corresponds to the initial diameter of the delivery line. In an area of the diffuser section in which the cross section increases, a number of glue nozzles distributed uniformly on the periphery are arranged in such a way that the fibers flowing past are wetted by the glue mist injected. In order to achieve the most uniform wetting possible, the spray cone of each glue nozzle and its alignment in relation to the horizontal mid-axis of the delivery line is in each case adjust able. The gluing -can take place at room temperature or else can be carried out in a preheated air stream at temperatures below 1000C, preferably 500 to 75 0 C, in order to introduce as much heat energy as possible into the fiber mat to be distributed.

WO 00/07785 - 6 - PCT/DE99/02054 The invention is again based on the object of reducing the binder consumption in the production of MDF boards. Starting from the method described at the beginning, according to the invention this object is achieved in that the end section of the tubular dryer is con structed as a glue wetting zone in which, by enlarging the tubular dryer flow cross section, the transport speed of the fiber mixture is reduced and turbulent flow is therefore produced, the turbulence of which is increased by injecting additional air which, at the same time as the binder is injected, is injected axially with the latter into the center of the wetting zone, whose length is about 5 to 10 times greater than its diameter. Here, it is expedient for the air injection to be carried out annularly around the binder injection and preferably at the same angle. Instead of the previously promulgated and generally used blow-line gluing, according to the invention, therefore, tubular-dryer gluing is to be provided, by means of which the previous disadvantages are avoided. As a result of gluing the already dried and largely cooled fibers according to the invention in the end section of the tubular dryer, binder precuring and hydrolytic decomposition of the binder in the tubular WO 00/07785 - 7 - PCT/DE99/02054 dryer are avoided. As a result of producing increased turbulence in the flow with the aid of additionally injected air, flow conditions are created which ensure a uniform binder distribution. By means of the axial injection of the binder, as referred to the wetting zone, wetting of the wall of the pipe and therefore caking on the inner wall of the pipe and flecking in the surface of the compressed fiber board are avoided. This is further advantageously assisted by performing the air injection annularly around the binder injection. In order to produce the desired high turbulence, it is advantageous if the transport speed of the fiber mix ture in the glue wetting zone is reduced by about 100% to 300% with respect to the transport speed in the dryer zone. In the blow line, the wet and hot fibers are at a temperature of about 160 - 170 0 C, have a speed of about 100 m/s and a residence time of about 0.1 - 0.3 s. In conventional methods, the dryer inlet temperature is at about 180 0 C and the dryer outlet temperature is at about 65 0 C, the fiber mixture being dried within about 4 - 6 s to a residual moisture of 10 - 12%. According to the invention, however, the moisture of the dried fibers must be lower, as compared with blow-line gluing, since as a result of the gluing the moisture of WO 00/07785 - 8 - PCT/DE99/02054 the fibers rises, but for technological reasons the final moisture of the glued fibers may not exceed 10 12%. According to the invention, provision is therefore made for the moisture of the fiber mixture to be reduced by the drying to less than 20%, preferably to 2-10%, so that the final moisture of the glued fiber mixture is less than 30%, preferably 8-12%. To the extent that this results in a higher dryer outlet temperature, it is expedient, in order to avoid binder precuring, if the fiber mixture is cooled down to a temperature of 50-704C directly before and/or after or during the injection of the binder (via one or more nozzles). This cooling can preferably be carried out by the atomization air used to inject the binder, and/or the binder, having their temperature controlled, it being advantageous for the fiber mixture in the wetting zone, as well as the binder injected here, to be at a temperature of about 50* to 700C, preferably about 60C. In addition, in this connection provision can be made for the air additionally injected to increase turbulence to have its temperature controlled and/or to be made moist. The glued fiber mixture is separated from its transport air in a cyclone connected downstream of the tubular dryer, it being possible for this cyclone to be connected directly to the end of the wetting zone.

WO 00/07785 - 9 - PCT/DE99/02054 The binder used can be all organic binders, such as UF, MUF, MUPF, MIUPF, PF and tannin resins, as well as PMDI adhesives. Rebuilding existing MDF plants with blow line gluing to the tubular-dryer gluing according to the invention is possible without difficulty, since only the end section of the modulated [sic] tubular dryer has to be replaced. The tube section to be inserted here has to be equipped with the necessary injection rings. Existing control and regulating units from blow-line gluing can continue to be used. By comparison with blow-line gluing, tubular-dryer gluing leads to considerably lower production costs and to high-quality MDF boards. During the production of MDF boards, the binder costs are about 19% of the production costs (without amortization and taxes) . They are therefore the second most important cost factor. Given a plant with an annual capacity of 250,000 M 3 , a reduction in the consumption of glue of 1% solid resin / absolute dry mass of fiber leads to savings of DM 1.5 million. Using the tubular-dryer gluing according to the invention, by comparison with the conventional method, up to 33% binder can be saved. If synthetic resin binders are used, this is not only economically but also ecologically practical, since by this means, resources are naturally protected and the ecological balance of the material is improved.

Claims

1. A method for producing low, medium or high density fiber boards (LDF, MDF, HDF) by hot compression of a fiber board formed of glued fibers, in which, in order to produce the fibers, chips are softened with steam at superatmospheric pressure and then ground up into fibers, which are conveyed by steam pressure through a blow line into a tubular dryer and, after being dried, are fed to a distributing machine or the like, characterized in that the end section of the tubular dryer is constructed as a glue wetting zone in which, by enlarging the tubular dryer flow cross section, the transport speed of the fiber mixture is reduced and turbulent flow is thereby produced, the turbulence of which is increased by injecting additional air which, at the same time as the binder is injected, is injected axially with the latter into the center of the wetting zone, whose length is about 5 to 10 times greater than its diameter.

2. The method as claimed in claim 1, characterized in that the air injection is carried out annularly around the binder injection and preferably at the same angle.

3. The method as claimed in claim 1 or 2, WO 00/07785 - 11 - PCT/DE99/02054 characterized in that the transport speed of the fiber mixture in the glue wetting zone is reduced by about 100% to 300% with respect to the trans port speed in the dryer zone.

4. The method as claimed in claim 1, 2 or 3, characterized in that the atomization air used to inject the binder, and/or the binder, have their temperature controlled.

5. The method as claimed in one of the preceding claims, characterized in that the fiber mixture in the wetting zone, as well as the binder injected here, are at a temperature of about 500 to 70*C, preferably about 600C.

6. The method as claimed in one of the preceding claims, characterized in that the air additionally injected to increase turbulence has its tempera ture controlled and/or is made moist.

7. The method as claimed in one of the preceding claims, characterized in that the moisture of the fiber mixture is reduced by the drying to less than 20%, preferably to 2-10%, so that the final moisture of the glued fiber mixture is less than 30%, preferably 8-12%. WO 00/07785 - 12 - PcT/DE99/02054

8. The method as claimed in one of the preceding claims, characterized in that the glued fiber mixture is separated from its transport air in a cyclone connected downstream of the tubular dryer.