EP0820371A1

EP0820371A1 - Method and device for the continuous production of panels of lignocellulose-containing particles

Info

Publication number: EP0820371A1
Application number: EP97902338A
Authority: EP
Inventors: Jürgen Dr. Kramer
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-02-08
Filing date: 1997-02-06
Publication date: 1998-01-28
Anticipated expiration: 2017-02-06
Also published as: US5913990A; DE19604574A1; AU1601597A; WO1997028936A1; ATE218956T1; DE59707471D1; EP0820371B1; CA2217654A1

Abstract

The invention concerns a 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

Method and device for the continuous production of plates from lignocellulose-containing particles

The invention relates to a process for the production of plates from lignocellulose-containing particles, binder being applied continuously to the particles, the particles being continuously formed into a mat, the mat being continuously pre-compressed, the mat being continuous Exposure to a high-frequency high-voltage field is heated and the mat guided in one plane is pressed to the plates under the action of further heat. Furthermore, the invention relates to a device for carrying out such a method with a gluing machine for the continuous application of binder to the particles, with a mat former for the continuous shaping of the Particles into a mat, with a pre-press for continuous pre-compression of 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 mat guided between two press plates in one plane with further heat the plates.

The invention therefore relates only to methods and devices which, at least including the pre-press and the HF heating, operate completely continuously, and in which no abruptly operating devices have been used until then.

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

In contrast, the invention is not restricted to a specific binder or a specific size and composition of the lignocellulose-containing particles. That is, it does not matter whether the binder is, for example, a urea resin or a formaldehyde-free binder. Nor is it decisive whether the boards produced are particle boards, MDF boards or OSB boards. However, the invention is associated with particular advantages in the production of certain plates.

A method and a device of the type described at the outset are known from "Proceedings 27th International Particleboard / - Composite Materials Symposium WSU 1993, pages 55 to 66: SUCCESS STORY: MODERN PARTICLEBOARD USING EASTERN HARDWOODS". A device for the continuous production of chipboard is described there, in which the pre-press for an RF heater for continuous heating of the mat by the action of a high-frequency high-voltage field is connected upstream of the continuous pre-compression of the mat. The HF heating increases the temperature of the mat from the room temperature by about 40 ° C. It is reported that the use of the HF heater before the hot press significantly increased the productivity of the device because the heated mat requires a considerably shorter pressing time in the hot press.

The heating power of an HF heater depends on the field strength of the effective alternating field. This means that in order to achieve the same heating output with twice the electrode spacing, twice as large an alternating voltage must be used. However, high voltages are always associated with the particular risk of breakdowns, which can lead to serious damage to the HF heating. In addition, the electro-magnetic pulses accompanying the breakdowns can also damage other electrical or electronic devices. Ultimately, the breakthrough in the manufacture of panels can also ignite the mat or cause damage to the finished panels.

To take this problem into account, it is therefore known to arrange an HF heater for continuously heating the mat by the action of a high-frequency high-voltage field behind the pre-press for continuously pre-compressing the mat, where the mat only has a reduced thickness and therefore a significantly smaller electrode spacing HF heating is possible.

In the "Taschenbuch der Spanplatteentechnik, Deppe / Ernst, 3rd edition", an HF pre-press is mentioned on page 175 in connection with a calender system, in which the mat is not round in one plane but hot in cross-section during hot pressing Heating drum is led around. Calender plants are, as already mentioned, only suitable for the production of thin plates. The production of OSB panels with a calender system is out of the question because of the springback properties of the underlying flat pieces of wood. How an HF pre-press for a calender system is to be constructed does not emerge from the immediate context or from the documents cited at this point in the pocketbook of chipboard technology. However, it must be assumed that the arrangement is also known in the case of calender systems, in which an RF heater is connected downstream of a pre-press. Basically, the installation of an HF press in a calender system is relatively unproblematic because the mat for the thin plates that can be produced with it is also only thin and therefore allows a small electrode spacing for the HF heating.

HF heaters are also used in discontinuous systems for the production of plates made of particles containing lignocellulose. On the one hand, the HF heating of the mat during hot pressing in a stack press is known. However, since stack presses with HF heating are technically very complex and the efficiency of HF heating is limited, the economy of hot presses with HF heating is not considered to be given.

Discontinuous pre-presses with an HF heating for the mat during pre-compression are also known. These are single-day presses, which have a complicated technical structure, because the electrodes of the HF heating must extend over the entire length and width of the press in order to heat the mat evenly, the length being of the order of 20 m. This means, for example, that relatively large currents must flow in the HF heating, which can only be managed with great effort.

The object of the invention is to optimize the use of an HF heater in the continuous production of plates made of material containing lignocellulose. According to the invention, this object is achieved in a method of the type described in the introduction in that the mat is heated during the continuous pre-compression by the action of the high-frequency high-voltage field. For a device of the type described at the outset, this means that the HF heater is arranged inside the pre-press. By combining the pre-compression and the heating of the mat with the HF heating in one place, the HF heating can have a minimal electrode spacing, so that only a minimal AC voltage is used. In this way, not only is the risk of breakdowns and the associated disturbing influences reduced, but also the electromagnetic stray radiation emanating from the HF heating system is reduced. In principle, only a comparatively low technical outlay for the HF heating is to be carried out, since this only has to be designed for comparatively low voltages. In contrast to a discontinuously working pre-press with HF heating, the outlay on equipment is also very low, since with the invention the HF heating theoretically only has to act in one line on the continuously passing mat. This means that the area of the electrodes can be kept small and, due to the low voltage required in connection with the small electrode areas, only relatively small currents have to flow in the HF heater. This also results in advantages in terms of the efficiency of the electrical energy used, since this is proportional to the product of voltage and current.

By installing the HF heating in the pre-press of an existing device for the production of plates from material containing lignocellulose, its capacity can be increased significantly. The integration of the HF heating in the pre-press does not require any additional space, it can be implemented with comparatively little technical effort.

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. The smallest electrode spacing of the HF heating can be realized here.

A great increase in productivity in the production of plates made of lignocellulose-containing material can already be achieved if the mat is only heated to a temperature of below 60 ° C., in particular between 45 and 55 ° C. At these comparatively low temperatures, there are also no undesirable condensations of water or binder on the pre-press, even if the binders do not have a composition that is specifically tailored to the new process.

Particularly large increases in the capacity of a device for producing plates from material containing lignocellulosic material have resulted in plates with a thickness of 12 to 22 mm. With smaller and larger plate thicknesses, the capacity advantage is not so pronounced, because when the mat is hot-pressed in the hot press with the transfer of contact heat, the advantage of the heated mat due to the course of the temperature penetration curves cannot be fully utilized.

Heating the mat in the pre-press by the action of a high-frequency high-voltage field has a particular advantage in the production of OSB panels from flat pieces of wood. Due to the large resetting forces of the flat pieces of wood used, thin OSB panels have not been able to be produced commercially continuously for a long time, since the press belts would be subjected to excessive loads from continuous hot presses. Due to the HF heating in the pre-press, however, the lignin in the mat is plasticized and the binders already begin to show adhesive properties, so that the restoring forces of the flat pieces of wood decrease sharply. As a result, a very slight re-opening of the mat after the pre-press is observed and the mat can also be pressed to the OSB boards in a continuously operating hot press. The slight cracking of the mat after the prepress is general a special feature of the method according to the invention.

In the new device, the electrodes of the HF heater can be arranged on the back of the press belts of the pre-press acting on the plate on both sides. The electrodes of the HF heating are preferably arranged where the press belts are at their smallest distance from one another.

One electrode of the HF heating system can be grounded, with the press band opposite the grounded electrode on the other side of the mat being designed to be radio frequency resistant. If an electrode is an HF heater grounded, one speaks of an asymmetrical HF feed. The grounded electrode is also referred to as the cold electrode. In the area of this cold electrode, the material stresses are lower than on the "hot" electrode. When converting an existing device for the production of plates made of lignocellulose-containing material, it is therefore sufficient if at least the press belt of the pre-press, which is assigned to the hot electrode, is retrofitted so that the high-frequency resistance is formed.

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

FIG. 1 shows a flow chart for the implementation of the new method,

FIG. 2 shows the schematic structure of a prepress in the new device,

FIG. 3 shows the schematic structure of a continuous hot press in the new device,

FIG. 4 shows a pressing path diagram of a continuous hot press in the new device, Figure 5 shows two temperature penetration curves and two comparison curves for the new method and

FIGS. 6 and 7 plots of the transverse tensile strength for two production examples and comparative examples.

In the method outlined in FIG. 1, binder is first applied continuously to lignocellulosic particles 2 in a gluing machine 1. The particles 2 are then continuously formed into a mat 4 in a mat former 3. The mat 4 is continuously pre-compressed in a pre-press 5. At the same time, an RF heater acts on the mat 4 in the pre-press 5 for continuous heating by means of a high-frequency high-voltage field. The heated and pre-compressed mat 4 is then continuously pressed in a hot press 6 to form a plate 7 which can subsequently be divided into individual plates.

The gluing machine used in the new process

I just like the mat former known in its structure. There are no changes compared to known devices for producing plates from particles containing lignocellulose. However, the pre-press 5 has a special structure, the internal structure of which is shown schematically in FIG. The inlet thickness 27 of the mat 4 is in the pre-press between two press belts 10 and

II bis reduced to a thickness 12. Behind the press belts 10 and 11, the mat 4 jumps up again to an outlet thickness 13. An HF heater 14 is arranged in the area of the minimum thickness 12 of the mat 4. A possible location for a second HF heater 14 is indicated by a dashed line. The existing HF heater 14 has two electrodes 15 and 16 respectively arranged behind the press belts 10 and 11. The electrode 16 is grounded, so that the HF heating works on the principle of asymmetrical feeding. Accordingly, the electrode 16 also becomes the cold electrode and the electrode 15 referred to as the hot electrode. Because the HF emission 14 acts on the mat 4 in the area of the minimum thickness 12, a comparatively low voltage is sufficient to achieve the field strength required for the desired energy transfer to the mat 4. This also keeps the risk of breakdowns within narrow limits. At least the press belt 10 of the pre-press 5 is designed to be radio frequency resistant. With the press belt 11 assigned to the cold electrode 16, this is not absolutely necessary, but is also recommended. Compared to pre-presses that are not equipped with an HF heater 14, the run-out thickness 13 of the mat 4 after the pre-press 5 is comparatively small because the restoring forces in the mat 4 are reduced by the HF heater. This is due to plasticizing lignin and the binding agent becoming active by heating the mat 4.

The hot press 6 sketched in FIG. 3 has the usual structure of a continuously operating hot press, in which the mat 4 is guided between endless press plates 19 and 20, which are supported on rollers 17 and 18, and is pressed to the plate 7 under the influence of heat. The corresponding heating elements are not shown in Figure 3.

The vertical distance between the press plates 19 and 20 is not constant over the length of a hot press, as can be seen from FIG. 4, in which, for a discontinuous hot press, this distance is plotted as the thickness d of the plate 4 on its way s through the hot press 6. The mat 4 is compressed by means of a first section 21, the cover layers of the mat 4 being heated up by contact heat transmitted from the press plates 19 and 20. In a connecting section 22, the thickness of the plate d is kept constant to a somewhat greater extent, the contact heat of the press plates 19 and 20 penetrating into the middle of the plate. Then the plate 4 is compressed in a section 23 to its smallest thickness d in order to calibrate the plate and after Calibrate to air. The plate then leaves the hot press.

5 shows the temperature profile in the middle of the plate of the plate 4 in the hot press 6 for two examples of the method according to the invention 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 and 30 mm, respectively, which were produced according to the invention using an HF heater 14 in the pre-press 5. In contrast, the filled squares and the filled circles correspond to comparative examples in which MDF boards with a nominal thickness of 16 or 30 mm were produced without the use of the HF heater 14. In the case of mats for 16 mm plates preheated to 50 ° C., the temperature in the middle of the plate increases fairly quickly and reaches 80 ° C. after only 60 seconds. H. approx. 45 seconds earlier than with non-HF preheated mats with an initial temperature of approx. 30 ° C. In both cases, the temperature of the heating plates 19 and 20 heating plates was 227 ° C. An analog, but flatter temperature curve is observed for the 30 mm plates. With the initial temperature difference of approx. 20 ° C between the heated and the non-heated mats, the time difference to reach 80 ° C for 30 mm plates is 75 seconds, but the relative time difference related to the absolute pressing time is smaller than for the 16 mm Plates. Under the same initial conditions, there was no difference in the temperature profile between the types of gluing urea-formaldehyde resin and polyurethane resin.

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: Type of wood: 100% softwood approx. 90 - 95% pine and 5 to 10% spruce

Initial form: Wood chips Binder: Urea resin (Leuna 5554) Glueing type: Blower tube gluing Raw thickness: 17, 8/17, 5 mm

Thickness shrinkage: approx. 0.3 mm after cooling Bulk density: 770 kg per cubic meter of solid resin. 10% on atro fibers Hardener: without addition of hardener Moisture: approx. 8 - 10%

Mat temp. without HF approx. 30 ° C 'mat temp. with HF: approx. 50 ° C heating times without HF: 10 ε / mm heating times with HF: 7.5 - 5.5 s / mm press temperature: 227 ° C (heating plates)

The transverse tensile strengths achieved were evaluated in accordance with the EMB standard, with each measuring point in the diagram shown in FIG. 6 representing an average of five transverse tensile specimens per plate. FIG. 6 shows the transverse tensile strengths to the right over the heating time of 10 s / mm, which were obtained without HF heating for heating the mat. The transverse tensile strengths with HF heating are shown on the left side above the heating time range of 5.5 - 7.5 s / ram. 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 scatter ranges are also significantly smaller in comparison with 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. At the heating time of 5.5 s / mm, the strength level dropped by about 20% compared to the initial values, 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:

Type of wood: 100% softwood approx. 90 - 95% pine and 5 - 10% spruce

Initial form: wood chips

Binder: urea resin (BASF 570 / NESTE 36 75)

Nominal thickness: 30 mm

Raw thickness: 32.0 / 32.6 mm

Thickness shrinkage: approx. 0.6 mm after cooling

Bulk density: 750 kg per cubic meter

Solid resin: 12% on dry fibers

Hardener: without hardener addition

Humidity: approx. 10%

Mat temp. without HF: approx. 30 ° C

Mat temp. with HF: approx. 50 ° C

Heating times without HF: 13 s / mm

Heating times with HF: ll to 8 s / mm

Press temperature: 227 ° C (hot plates)

The transverse tensile strengths determined in the same way as for the 16 mm plates are plotted in FIG. 7. The strengths without 'HF heating appear above the heating time of 13 s / mm on the right in FIG. 7, and the strengths with HF heating above the heating times of 8 to 11 s / mm. 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. A downward trend below the initial level is already discernible in less than 11 seconds to 8 seconds. The RF heating at the limit of 50 ° C in the mat is therefore not as efficient for the area of thicker sheets as for sheets with a thickness of 12 to 22 mm, because the heat penetration curve is less influenced by thicker sheets. The HF heating, which was used in the examples according to the invention, has the following technical data:

HF useful power: 15 kW at 100% duty cycle Frequency: 27.12 MHz + - 0.6%

Mains connection: three-phase current 400 V

+ 6% - 10% 50 Hz control voltage: 230 V / 50 Hz power consumption at full load: 32 kVA

High voltage rectification: silicon diodes

Transmitter tube: - Make: ABB

- Type: IQL 12-1 electrode plate: - Length: 500 mm

- Width: 800 mm

When the mat 4 was heated with the HF heating to temperatures below 60 ° C., no condensation occurred due to the temperature difference between the heated mat and the cold pre-press, without special binders or any precautionary measures having to be taken with the pre-press. Higher temperatures when heating the mat 4 are possible while observing precautionary measures with regard to the press belts of the prepress and the binder.

B E Z U G S Z E I C H E N L I S T E

1 dressing machine

2 particles

3 mat formers

4 mat

5 prepress

6 hot press

7 plate

8 roll

9 roll

10 press belt

11 press belt

12 thickness

13 outlet thickness

14 HF heating

15 electrode

16 electrode

17 roll

18 roll

19 press plate

20 press plate

21 section

22 section

23 section

27 inlet thickness

Claims

PATENT CLAIMS:

1. Process for the production of plates made of lignocellulose-containing particles, binder being continuously applied to the particles, the particles being continuously formed into a mat, the mat being continuously precompressed, the mat being continuously acted on by a high-frequency high-voltage field is heated and the mat guided in one plane is pressed under further heat to the plates, characterized in that the mat (4) is heated during the continuous pre-compression by the action of the high-frequency high-voltage field.

2. The method according to Anεpruch 1, characterized in that daε high-frequency high-voltage field acts on the mat (4) where it reaches its smallest thickness (12) during precompression.

3. The method according to claim 1 or 2, characterized in that the mat (4) to a temperature below 60 ° C, in particular between 45 and 55 ° C, is heated.

4. The method according to any one of claims 1 to 3, characterized gekenn¬ characterized in that the plates (7) to which the mat (4) is vεrpreßt have a thickness of 12 to 22 mm.

5. The method according to any one of claims 1 to 4, characterized gekenn¬ characterized in that the particles (2) are flat pieces of wood, which are continuously produced OSB boards.

6. Device for the production of plates from lignocellulose-containing particles according to the method according to one of claims 1 to 5, with a gluing machine for the continuous application of binder to the particles, with a mat former for continuously shaping the particles into a mat with a pre-press for continuous pre-compression of 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 mat guided between two press plates in one plane with further heat exposure to the plates, characterized in that the HF heating (14) is arranged within the prepress (5).

7. The device according to claim 6, characterized in that

Electrodes (15 and 16) of the HF heater (14) are arranged on the rear sides of press belts (10 and 11) of the pre-press (5) which act on the mat (4) on both sides.

8. The device according to claim 7, characterized in that the electrodes (15 and 16) of the HF heater (14) are arranged εind where the press belts (10 and 11) are at their smallest distance from each other.

9. Apparatus according to claim 7 or 8, characterized in that an electrode (16) of the HF heater (14) is grounded and that daε of the grounded electrode (16) on the other side of the mat

(4) opposing press belt (10) high-frequency resistant.