EP0820371B1 - Method and device for the continuous production of panels of lignocellulose-containing particles - Google Patents

Abstract

PCT No. PCT/EP97/00529 Sec. 371 Date Oct. 8, 1997 Sec. 102(e) Date Oct. 8, 1997 PCT Filed Feb. 6, 1997 PCT Pub. No. WO97/28936 PCT Pub. Date Aug. 14, 1997A method for continuously producing panels (7) of lignocellulose-containing particles (2). According to the invention, binder is continuously applied to the particles (2) which are continuously shaped to form a mat (4). The mat (4) is continuously precompressed and at the same time continuously preheated by the effect of a high-frequency high-voltage field. The mat (4), which is guided in a plane, is compressed to form the panels (7) under the effect of further heat.

Description

The invention relates to a method for producing Sheets of particles containing lignocellulose, with binders is continuously applied to the particles, the Particles are continuously formed into a mat, the Mat is continuously pre-compacted, the mat being continuous by exposure to a high-frequency high-voltage field is warmed up and being guided in one plane is pressed under further heat to the plates. Furthermore, the invention relates to a device for Implementation of such a method with a gluing machine for the continuous application of binder on the Particles, with a mat former for continuous shaping of the Particles into a mat, with a pre-press for continuous Pre-compact the mat with an HF heater for continuous heating of the mat by the action of a high-frequency high-voltage field and with a hot press for Pressing the between two press plates in one plane guided mat under further heat to the plates.

The invention thus relates only to methods and devices which at least including the prepress and the HF heating work completely continuously, which means until then no intermittent facilities for Come into play.

Moreover, the invention relates only to methods and corresponding devices in which the in one plane guided mat is pressed hot to the plates. This closes the use of so-called calender presses, with which only Slabs of small thickness and made from certain materials are.

In contrast, the invention is not limited to a specific binder or a certain size and composition of the Lignocellulosic particles restricted. That is, it comes does not depend on whether the binder is for example a urea resin or a formaldehyde-free binder is. Nor is it decisive whether the manufactured boards around chipboard, MDF boards or OSB boards is. However, the invention is in the making certain plates with special advantages.

A method and an apparatus of the one described in the introduction Art are from "Proceedings 27th International Particleboard / Composite Materials Symposium W.S.U. 1993, pages 55 to 66: SUCCESS STORY: MODERN PARTICLEBOARD USING EASTERN HARDWOODS " known. There is a device for continuous production of chipboard described in the prepress for continuous pre-compression of the mat using an HF heater continuous heating of the mat by the action of a high-frequency high-voltage field is connected. The HF heater increases the temperature of the mat from room temperature around 40 ° C. It is reported that through the Use of the HF heating in front of the hot press increases the productivity of the Device could be increased significantly because of the warmed up Mat a much shorter pressing time in the hot press needed.

The heating power of an HF heater depends on the field strength of the effective alternating field. This means that to achieve the same heating output with double electrode spacing twice the AC voltage must be used. Size However, tensions are always with the special danger of Breakthroughs connected, causing serious damage to the HF heater being able to lead. In addition, those with the punctures accompanying electro-magnetic impulses also others damage electrical or electronic equipment. Ultimately, it can be in the manufacture of panels by the Breakthroughs also for igniting the mat or for damage come on the finished panels.

To take this problem into account, it is therefore known an HF heater for continuous heating of the mat Exposure to a high frequency high voltage field also behind the pre-press for continuous pre-compression of the mat to be arranged where the mat has only a reduced thickness and therefore a significantly smaller electrode spacing of the HF heater is possible. For example, in US 4216179 methods and devices described in which a mat Lignocellulose-containing particles are first pre-compressed to reduce the inlet thickness by about a third, then with With the help of an HF heater warmed up and finally continued is pressed to the final thickness of the mat reducing plate.

In the "Taschenbuch der Spanplattenentechnik, Deppe / Ernst, 3. Edition ", an HF pre-press is connected on page 175 mentioned with a calender system, in which the mat when Hot pressing not in one plane but by one in cross section round heating drum is led around. Calender systems are, as already mentioned, only for the production of thin plates suitable. One OSB board comes with one Calender system due to the springback properties of the underlying flat pieces of wood out of the question. how an HF pre-press is to be set up for a calender system, is neither from the immediate context nor from the this place in the pocketbook of chipboard technology cited publications. However, it can be assumed that it is the arrangement also known in calender systems is a HF press connected downstream of a pre-press is. Basically, the installation of an HF press in a Calender system relatively unproblematic because the mat for the so thin plates that can be produced are also only thin and so a small electrode spacing of the HF heating allows.

RF heaters are also found in batch work Plants for the production of boards made from lignocellulose Particle use. Firstly, there is the HF heating of the mat known in hot pressing in a stack press. Because stack presses with HF heating are technically very complex and the efficiency of the HF heating is limited, the economy applies of hot presses with HF heating as not given.

There are also discontinuous pre-presses with HF heating known for the mat during pre-compaction. in this connection are single-day presses that have a complicated have technical structure because the electrodes of the HF heating for uniform heating of the mat over the entire The length and width of the press must extend to the length can be of the order of 20 m. this means for example, that in the HF heating relatively large currents have to flow that can only be managed with great effort.

The invention has for its object the use of an HF heater in the continuous manufacture of panels Optimize lignocellulose-containing material.

According to the invention, this object is achieved in a method of Type described above solved in that the mat at continuous pre-compression by exposure to the high-frequency High voltage field is warmed. This means for a device of the type described that the RF heating is arranged within the pre-press. By the Summarize pre-compaction and heating of the mat with the RF heater in one place, the RF heater can have minimal electrode spacing, so that only a minimal AC voltage is used. That way not just the risk of breakdowns and the associated Reduces interference, but it also goes from the HF heating outgoing electromagnetic stray radiation. Basically, it is only a comparatively minor technical one Expenses for the HF heating to operate, since this only must be designed for comparatively low voltages. in the Contrary to a discontinuous pre-press HF heating, the expenditure on equipment is also very low because the HF heater in the invention theoretically only in one line must act on the continuous mat. that is, the area of the electrodes can be kept small and through the low voltage required in connection with the small ones Electrode surfaces only need relatively small currents in the HF heating flow. Overall, this also results in advantages the efficiency of the electrical energy used, because this is proportional to the product of voltage and current.

By installing the HF heating in the pre-press existing device for the production of plates Lignocellulose-containing material can clearly show their capacity be increased. This requires the integration of the HF heating no additional space in the pre-press, it is included comparatively low technical effort can be realized.

The high-frequency high-voltage field of the HF heater preferably acts on the mat where it has its smallest thickness reached during pre-compression. Here is the smallest electrode gap the HF heating can be implemented.

A big increase in productivity in the manufacture of Sheets made of lignocellulose-containing material are already there accessible if the mat is only at a temperature below 60 ° C, in particular between 45 and 55 ° C is heated. at there are no such comparatively low temperatures unwanted condensation of water or binder on the Prepress on even if the binders are not specially designed the new process has a coordinated composition.

Particularly large increases in the capacity of a device for Manufacture of sheets made of lignocellulosic material have resulted in plates with a thickness of 12 to 22 mm. With smaller and larger sheet thicknesses, the capacity advantage is not so pronounced, because there the hot pressing Mat in the hot press with transfer of contact heat Advantage of the heated mat due to the course of the temperature penetration curves cannot be fully exploited.

Heating the mat in the has a special advantage Pre-press using a high-frequency high-voltage field in the production of OSB panels from flat Pieces of wood. Thin OSB boards are due to the large Restoring forces of the flat pieces of wood used so far cannot be produced commercially continuously because the press belts would be too heavily loaded by continuous hot presses. By however, the HF heating in the pre-press becomes the lignin in the Plasticized mat and the binders already start to show adhesive properties, so that the restoring forces of the flat pieces of wood decrease sharply. The result is a very little rebound of the mat after the pre-press observed and the mat can also be in a continuous working hot press to be pressed to the OSB boards. The slight cracking of the mat after the pre-press is general a special feature of the method according to the invention.

With the new device, the electrodes of the HF heating on the backs of the plates that act on both sides Press belts of the pre-press can be arranged. Here are the Electrodes of the HF heater are preferably arranged where the Press belts have their smallest distance from each other.

One electrode of the RF heater can be grounded, which is the grounded electrode on the other side of the mat opposite Press belt is designed to be radio frequency resistant. If an electrode of an HF heater is grounded an asymmetrical RF feed. Here, the grounded Electrode also called a cold electrode. In the area of this cold electrode, the material stresses are less than on the "hot" electrode. It is therefore sufficient to convert one existing device for the production of plates from lignocellulose-containing Material if at least the press belt Prepress, which is assigned to the hot electrode, see above is converted that it is radio frequency resistant.

Figur 1

ein Flußdiagramm zur Durchführung des neuen Verfahrens,

Figur 2

den schematischen Aufbau einer Vorpresse bei der neuen Vorrichtung,

Figur 3

den schematischen Aufbau einer kontinuierlichen Heißpresse bei der neuen Vorrichtung,

Figur 4

ein Preßwegdiagramm einer diskontinuierlichen Heißpresse bei der neuen Vorrichtung,

Figur 5

zwei Temperatureindringkurven und zwei Vergleichskurven zu dem neuen Verfahren und die

Figuren 6 und 7

Auftragungen der Querzugfestigkeit für zwei Herstellungsbeispiele und Vergleichsbeispiele.

The invention is explained and described in more detail below with the aid of exemplary embodiments, and shows

Figure 1

a flow chart for the implementation of the new method,

Figure 2

the schematic structure of a pre-press in the new device,

Figure 3

the schematic structure of a continuous hot press in the new device,

Figure 4

a pressing path diagram of a discontinuous hot press in the new device,

Figure 5

two temperature penetration curves and two comparison curves for the new method and the

Figures 6 and 7

Cross-tensile strength plots for two manufacturing and comparative examples.

In the process outlined in FIG. 1, binder is first used in a gluing machine 1 continuously on lignocellulose-containing Particle 2 applied. Then be the particles 2 in a mat former 3 continuously into one Mat 4 shaped. In a pre-press 5, the mat 4 continuously pre-compressed. At the same time acts in the pre-press 5 an RF heater for continuous heating by a high-frequency high-voltage field on the mat 4. The warmed up and pre-compacted mat 4 is then in a hot press 6 continuously pressed into a plate 7, which then in individual plates can be cut.

The gluing machine used in the new process 1, like the mat former, is known in its construction. Here are compared to known devices for the production of Lignocellulosic particle plates no changes intended. However, the pre-press has a special structure 5, whose inner structure is shown schematically in FIG. 2 is. The inlet thickness 27 of the mat 4 is in the pre-press between two rollers 8 and 9 rotating press belts 10 and 11 reduced to a thickness of 12. Behind the press belts 10 and 11 the mat 4 jumps again to an outlet thickness 13 on. An HF heater is located in the area of the minimum thickness 12 of the mat 4 14 arranged. A possible location for one second HF heater 14 is indicated by a dashed line. The existing HF heater 14 has two behind each the press belts 10 and 11 arranged electrodes 15 and 16. The electrode 16 is grounded, so that the HF heating after Principle of asymmetrical feeding works. Accordingly the electrode 16 is also called the cold electrode and the electrode 15 referred to as the hot electrode. Because the RF heater 14 in Area of minimum thickness 12 acts on the mat 4 is sufficient a comparatively low voltage to the for the desired energy transfer to the mat 4 required To achieve field strength. This also eliminates the risk of Breakthroughs kept within narrow limits. At least the press belt 10 of the pre-press 5 is designed to be radio frequency resistant. At which the cold electrode 16 associated press belt 11, this is not absolutely necessary, but also recommended. Compared to pre-presses that are not equipped with an HF heater 14 are the outlet thickness 13 of the mat 4 after the pre-press 5 comparatively low because the restoring forces in the mat 4 can be reduced by the HF heating. This is on one Plasticizing lignin and taking effect of the binder due to the heating of the mat 4.

The hot press 6 outlined in FIG. 3 has the usual structure a continuously operating hot press, in which the Mat 4 between endless on rollers 17 and 18 supporting Press plates 19 and 20 is guided and under the action of heat is pressed to the plate 7. The corresponding Heating elements are not shown in Figure 3.

The vertical distance between the press plates 19 and 20 is about Length of a hot press is not constant, as in FIG. 4 can be seen in the for a discontinuous hot press this distance as the thickness d of the plate 4 in its path s through the hot press 6 is applied. Via a first section 21 the mat 4 is compressed, thereby heating the Cover layers of the mat 4 by the press plates 19 and 20th excess contact heat. In a subsequent section 22 the thickness of the plate d becomes somewhat larger kept constant, the contact heat of the press plates 19 and 20 penetrates into the middle of the plate. Then the Plate 4 in a section 23 to its smallest thickness d compressed to calibrate the plate and after the Calibrate to air. The plate then leaves the hot press.

In Figure 5, the temperature curve in the middle of the plate is Plate 4 in the hot press 6 for two examples of the invention Process and for two comparative examples over the absolute pressing time t. The empty triangles and the empty diamonds correspond to MDF boards with a nominal thickness of 16 or 30 mm, according to the invention using an HF heater 14 were produced in the pre-press 5. The filled squares and the filled circles, in contrast, correspond to comparative examples, where MDF boards with a nominal thickness of 16 or 30 mm without using the HF heater 14 were. With mats preheated to 50 ° C for 16 mm plates the temperature in the middle of the plate rises fairly quickly and reaches 80 ° C after only 60 seconds, d. H. about 45 seconds earlier than non-HF preheated mats with an initial temperature of approx. 30 ° C. In both cases the temperature was from the press plates 19 and 20 heating heating plates 227 ° C. An analog, but flatter temperature curve is in the 30th mm plates observed. At the initial temperature difference of approx. 20 ° C between the warmed and the not warmed Mats is the time difference to reach 80 ° C at 30 mm plates 75 seconds on the absolute pressing time related relative time difference is however smaller than in the 16 mm plates. Under the same starting conditions was between the types of gluing urea-formaldehyde resin and polyurethane resin no difference in the temperature profile can be determined.

The temperature penetration curves according to FIG. 5 belong to the following examples:

1.16 mm plates

Under the following boundary conditions, MDF boards with a nominal thickness of 16 mm were produced once with and once without heating the mats in the pre-press using a high-frequency high-voltage field: Species 100% softwood approx. 90 - 95% pine and 5 to 10% spruce original shape wood chips binder Urea resin (Leuna 5554) Beleimungsart blowline Rohdicke 17.8 / 17.5 mm Dick shrinkage approx. 0.3 mm after cooling density 770 kg per cubic meter solid resin 10% on atro fibers Harder without addition of hardener humidity approx. 8 - 10% Mattentemp. without HF about 30 ° C ' Mattentemp. with HF approx. 50 ° C Heating times without HF 10 s / mm Heating times with HF 7.5 - 5.5 s / mm pressing temperature 227 ° C (hot plates)

The transverse tensile strengths achieved were evaluated after EMB standard, with each measuring point in that shown in Figure 6 Diagram represents an average of 5 transverse tensile tests per plate. In Figure 6 are the right over the heating time of 10 s / mm Cross tensile strengths shown, which are without HF heating Heating of the mat resulted. On the left over the The heating time range from 5.5 - 7.5 s / mm are the transverse tensile strengths reproduced with HF heating. In the heating time range from 7 to 7.5 s / mm, which corresponds to a heating time reduction of 25 to 30%, the strength level with HF heating is significantly higher than the values without HF heating. The spreading areas are also significantly lower compared to the initial values without HF heating. With the heating time of 6.3 s / mm, the strength level is slightly lower but still above the Initial values without HF heating. With a heating time of 5.5 s / mm the strength level is approx. 20% compared to the initial values dropped, but is still above the EMB standard.

2. 30 mm plates

MDF boards with a nominal thickness of 30 mm were produced under the following conditions: Species 100% softwood approx. 90 - 95% pine and 5 - 10% spruce original shape wood chips binder Urea resin (BASF 570 / NESTE 36 75) nominal thickness 30 mm Rohdicke 32.0 / 32.6 mm Dick shrinkage approx. 0.6 mm after cooling density 750 kg per cubic meter solid resin 12% on atro fibers Harder without addition of hardener humidity approx. 10% Mattentemp. without HF approx. 30 ° C Mattentemp. with HF approx. 50 ° C Heating times without HF 13 s / mm Heating times with HF 11 to 8 s / mm pressing temperature 227 ° C (hot plates)

Determined in the same way as for the 16 mm plates Transverse tensile strengths are plotted in Figure 7. Right in figure 7 the strengths appear without over the heating time of 13 s / mm HF heating, the left over the heating times of 8 to 11 s / mm Strengths with HF heating. With a heating time of 11 s / mm the values with HF heating are still at a higher level than the initial values without HF heating. Below 11 seconds to 8 seconds becomes a downward trend below the starting level recognizable. HF heating at the limit of 50 ° C in the mat is not the case for thicker panels efficient as for panels with a thickness of 12 to 22 mm because the heat penetration curve is less strong for thicker plates being affected.

The HF heating, which was used in the examples according to the invention, has the following technical data: RF power output 15 kW at 100% duty cycle frequency 27.12 MHz + - 0.6% mains connection 400 V three-phase current + 6% - 10% 50 Hz control voltage 230 V / 50 Hz Power consumption at full load 32 kVA High voltage rectification silicon diodes transmitting tube - Make: ABB - Type: IQL 12-1 electrode plate - Length: 500 mm - Width: 800 mm

When the mat 4 is heated to temperatures using the HF heating below 60 ° C no condensation occurred through the Temperature difference between the heated mat and the cold one Prepress on without special binders or any Precautions had to be taken on the pre-press. Higher temperatures when heating the mat 4 are below Compliance with precautions regarding the press belts of the Prepress and the binder possible.

LIST OF REFERENCE NUMBERS

1 -

gluing machine

2 -

particle

3 -

Matt shaper

4 -

mat

5 -

pre-press

6 -

hot press

7 -

plate

8th -

role

9 -

role

10 -

press belt

11 -

press belt

12 -

thickness

13 -

exit thickness

14 -

RF heating

15 -

electrode

16 -

electrode

17 -

role

18 -

role

19 -

press plate

20 -

press plate

21 -

section

22 -

section

23 -

section

27 -

inlet thickness

Claims (8)

1. Method of manufacturing panels out of lignocellulose containing particles, wherein binder is continuously applied on to the particles, the particles are continuously formed to a mat, the mat during be pre-pressed is continuously warmed through application of a high-frequency high voltage field and the mat conveyed in one plane is pressed to the panels while applying further heat, characterized in that the high frequency voltage field after the mat (4) being pre-pressed, acts on a narrowest area of thickness (12), which is less than the out-feed thickness (13), to which the mat (4) springs back after pre-pressing, and wherein the mat (4) during the application of the high-frequency high voltage field is held between the pre-press conveyor belts (10, 11) substantially at the narrowest thickness (12).
2. Method according to claim 1, characterized in that the mat (4) is warmed to a temperature below 60 °C, especially between 45 and 55°C.
3. Method according to one of claims 1 or 2, characterized in that the panels (7) formed by pressing the mat (4) have a thickness from 12 to 22 mm.
4. Method according to one of claims 1 to 3, characterized in that the particles (2) are flat wood strands, out of which OSB-panels are continuously produced.
5. Device for the production of panels from cellulose containing particles comprising a gluing machine for continuously applying binder on to lignocellulose containing particles, a mat former for continuously forming the particles to a mat, a pre-press for continuously pre-pressing the mat, a HF-heating device for continuously warming up the mat by applying a high frequency high voltage field and with a hot-press for pressing the mat between two press-coils along one plane through additional heating to panels, characterized in that the HF- heating device (14) is in the pre-press (5) on an area where the mat (4) is pre-pressed to the narrowest thickness (12).
6. Device according to claim 5, characterized in that the electrodes (15, 16) of the HF- heating device (14) are located on the back side of the pre-press conveyor belts (10, 11) of the pre-press (5), which press the mat (4) from both front sides.
7. Device according to claim 6, characterized in that the electrodes (15, 16) of the HF- heating device (14) are located at that area, where the pre-press conveyor belts (10, 11) have the narrowest distance to each other.
8. Device according to claims 6 or 7, characterized in that one electrode (16) of the HF- heating device (14) is earth, and that the pre-press conveyor belt (10) on the side opposite the earth electrode (16) attaching the mat (4) is resistant to high-frequency.

Images