Method for producing a wooden composite material board, in particular an MDF or HDF board

Abstract

The disclosure relates to a method for producing a wooden composite material board, in particular an MDF or HDF board, wherein a cake made of wood chips or wood fibres wetted with a binder is spread, which is subsequently pressed in a hot press to form a board of a desired thickness, wherein, before the pressing, a mixed powder made of colour pigments and a second binder is spread onto an upper side of the cake as a primer, wherein the cake is sprayed with water after the spreading of the mixed powder.

Claims

1. A method for producing a wood-based board by scattering a mat of wood fibers or wood chips wetted with a first binder, which is subsequently pressed in a hot-press to form a board of desired thickness, the pressing being preceded by the scattered application, to a top side of the mat, of a powder mixture of color pigments and a second binder as priming agent, wherein the mat, after the scattered application of the powder mixture, is sprayed with water in an amount of 10 to 30 g/m.sup.2, on the powder mixture of color pigments and the second binder, prior to the pressing to form the wood-based board, wherein a steam pulse is developed during the pressing due to the spraying of the water in an amount of 10 to 30 g/m.sup.2, and the mat of wood fibers or wood chips wetted with the first binder is subjected to a pre-pressing to carry out a precompaction prior to the scattered application and the spraying of the water.

2. The method as claimed in claim 1, wherein the first binder and the second binder are at least substantially identical.

3. The method as claimed in claim 1, wherein the powder mixture comprises 80 to 90 wt % of pigments and 10 to 20 wt % of the second binder.

4. The method as claimed in claim 2, wherein the pigments are white.

5. The method as claimed in claim 1, wherein pulverulent melamine resin, urea resin, acrylate, or ethylene-vinyl acetate copolymers are used as first and/or second binder.

6. The method as claimed in claim 1, wherein the powder mixture is scattered on in an amount of between 5 and 120 g/m.sup.2.

7. The method as claimed in claim 3, wherein the pigments comprise pulverulent pigments comprising an individual pigment or a mixture of different pigments.

8. The method as claimed in claim 4, wherein the pigments comprise titanium oxide, calcium carbonate, or barium sulfate.

9. The method as claimed in claim 4, wherein the white pigments are admixed with iron oxide pigments.

10. The method as claimed in claim 1, wherein the wood-based board is MDF or HDF board.

11. The method as claimed in claim 1, wherein the water is sprayed prior to hot pressing to form the wood-based board.

12. The method as claimed in claim 1, wherein the scattered application comprises two scattering processes onto the mat of wood fibers, prior to the pressing in a hot press.

13. The method as claimed in claim 12, wherein the two scattered processes comprises a first scattering process with a first powder mixture comprising first pigments used with a leading scattering apparatus in a transport direction and a second scattering process with a second powder of a second pigment used with a last scattering apparatus.

14. The method as claimed in claim 1, wherein the wood fibers or wood chips wetted with the first binder are dried prior to the scattering application.

15. The method as claimed in claim 14, wherein residual moisture content is removed from the wood fibers or wood chips and the first binder is dried.

16. The method as claimed in claim 15, wherein the dried wood fibers or wood chips and the first binder are scattered into a conveyor system.

17. The method as claimed in claim 16, wherein the dried wood fibers or wood chips and the first binder are subjected to the precompaction, prior to the scattering of the powder mixture of color pigments and the second binder onto the top side of the mat.

18. The method as claimed in claim 17, wherein the powder mixture comprises 80 to 90 wt % of pigments and 10 to 20 wt % of the second binder, and is scattered in an amount of between 5 and 120 g/m.sup.2.

19. The method as claimed in claim 18, wherein the powder mixture is controlled through rotational speed of a roller with indentations and a doctor blade drawing over the indentations of the roller to maintain a constant width during production.

20. The method as claimed in claim 19, further comprising brushing the powder mixture out of the indentations by a brush travelling through the indentations such that the powder mixture falls onto a screen which oscillates transversely to a transport direction which evens out inhomogeneities in the scattering application on the top side of the mat.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Exemplary embodiments of the invention are briefly elucidated below with the aid of a drawing, in which:

(2) FIG. 1—shows an operating scheme for producing a wood fiber board,

(3) FIG. 2—shows a schematized partial representation of the manufacturing operation,

(4) FIG. 3—shows the schematic sectional representation of a scattering apparatus transverse to the transport direction, and

(5) FIG. 4—shows the section along the line IV-IV according to FIG. 3.

DETAILED DESCRIPTION

(6) The method of the invention is elucidated using FIG. 1. Wood 10 provided in log form is first comminuted to wood chips HS in a chipper 100, which are washed and boiled in a downstream boiler 101. The wood chips HS thus made ready are then transferred into the refiner 102, in which they are defibrated to form wood fibers HF. In transport direction T, the wood fibers HF are resinated in a resinating apparatus 103 with a first binder, examples being melamine resin, urea resin, acrylate, and ethylene-vinyl acetate copolymer. The resinated wood fibers HF are transported on and then dried in a drier 104. The drier 104 consists of a preliminary drier 104.1 and a secondary drier 104.2. In the drier 104, any residual moisture content is removed from the wood fibers HF, and the first binder is dried. After the drier 104, the wood fibers HF are transported for classification into the classifier 105, from which they pass into the first scattering apparatus 106. This scattering apparatus 106 then scatters the wood fibers HF onto a conveyor belt 111, which is not shown in any more detail here but which circulates in the transport direction T, to form a mat 1. This fiber mat 1 then enters the preliminary press 107, which carries out precompaction of the mat 1. Downstream of the preliminary press 107 in the transport direction T there is at least one second scattering apparatus 108, in which the priming agent 3 is provided and is scattered onto the top side 1.1 of the mat 1 by means of the second scattering apparatus 108. There are preferably two or three second scattering apparatuses 108 arranged one after another in the transport direction T. The priming agent is a powder mixture composed of pigments and a second binder. The powder mixture consists to an extent of 80 to 90 wt % of pigments and of 10 to 20 wt % of the second binder, and is scattered on in an amount of between 5 and 120 g/m.sup.2. The amount scattered on is dependent on the desired hiding power of the primer. When the powder mixture has been scattered onto the mat 1, the top side 1.1 is sprayed with water via the spraying device 109, the amount sprayed on being about 10 to 30 g/m.sup.2. The mat 1 is subsequently transported into the hot-press 110, which is preferably a continuous press, where it is pressed under high pressure and high temperature to form a wood-based board 2 of desired thickness.

(7) The construction of the scattering apparatus 108 can be taken schematically from FIG. 2. It consists of a hopper 108.1 for receiving the powder mixture, a roll 108.2, which is arranged beneath the hopper 108.1 and is provided with indentations 108.3 over its periphery, a doctor 108.4 which brushes over the indentations 108.3, a brush 108.5 which travels through the indentations 108.3, and a screen 108.6, which is arranged beneath the roll 108.2 and is driven in oscillation. The distance between the screen 108.6 and the top side 1.1 of the mat 1 is approximately between 5 and 10 cm.

(8) The powder mixture for scattering onto the mat 1 is introduced into the hopper 108.1. The powder mixture falls into the indentations 108.3 in the rotating roll 108.2. The amount of powder mixture applied is controlled through the rotational speed of the roll. Via the doctor 108.4, the amount applied over the production width is kept constant, by the drawing of the doctor 108.4 over the indentations 108.3. The brush 108.5, which is arranged in the turning direction D of the roll 108.2, brushes the powder mixture out of the indentations 108.3 by travelling through them. The powder mixture then falls onto the screen 108.6, which oscillates transversely to the transport direction T and so evens out inhomogeneities in the scattering on the top side 1.1 of the mat 1.

(9) The scattered amount varies, depending on the required hiding power and on the pigment used, between 5 and 120 g of powder mixture per square meter. In the case of a target value of L according to the CIELAB color system of around 90, the amount required of a powder mixture consisting only of titanium oxide and binder is from 12 to 16 g/m.sup.2. When calcium carbonate is used as pigment, the applied amount of the powder mixture is 60 to 80 g/m.sup.2. After passage through the continuous press 110, the major part (>90%) of the powder mixture is fixed on the top side 1.1 of the mat 1. The remainder is located on the circulating pressing belt of the continuous press 110, and will be deposited on the top side 1.1 during the next circulation. This operation makes the surface more uniform and/or reduces the cloudiness of the primer. Through the use of more than two second scattering apparatuses 108, powder mixture with more inexpensive pigments can be used in the leading scattering apparatus 108 in transport direction T, and high-value titanium dioxide pigment can be employed in the last scattering apparatus 108.

(10) Of course, only part of the desired primer can be applied even by the scattered application of the powder mixture. In spite of scattering onto the porous mat 1, trials found no significant penetration of the powder mixture into the mat 1 (the board). In contrast to applicator rolls, a scattering apparatus has significantly smaller fluctuations in application. As a result it is possible to achieve a reduction in the amount of priming agent applied. The problems which may occur on wet application of primers, through changes in viscosities due to temperature fluctuations, evaporation of venting agents, or aging phenomena of the liquors, are eliminated by the application of the powder mixture. A switch in production from primed boards to unprimed boards is possible within a few minutes, requiring only the removal from operation of the scattering apparatuses 108 and possibly the spraying apparatus 109. Moreover, a change in the hiding power is achievable rapidly and readily by varying the amount of the powder mixture applied. Remnants of liquid primers need not be disposed of. Plants do not have to be cleaned, and even the pressing belt of the continuous press 110 is pigment-free again after one circulation.

WORKING EXAMPLE 1

(11) Ahead of the continuous press 110, the scattered mat 1 of wood fibers HF, having a mat thickness of around 10 cm, was scattered by means of two scattering apparatuses 108 with a powder mixture of calcium carbonate (85 wt %) and melamine resin powder (15 wt %) in an amount of 50 g/qm. Each of the two scattering apparatuses 108 scattered 25 g/m.sup.2 of powder mixture. A third scattering apparatus 108 contained a powder mixture in which the calcium carbonate had been replaced by titanium oxide. This powder mixture was scattered in an amount of 5 g/m.sup.2 onto the mat 1 with the calcium carbonate-melamine resin powder mixture. Then, in the spraying apparatus 109, water was sprayed in an amount of 30 g/m.sup.2 onto the powder mixture. The mat 1 was subsequently pressed in the continuous press 110 to form an HDF 8 mm thick. After the press, a sample was taken and a determination made of the value of L according to the CIELAB system. A value of 89.3 was measured. The wood-based boards 2, which in spite of one-sided coating exhibited no warping, were freed from loose constituents on the surface after cooling in a pressure section by a cleaning abrasion, after which they were provided with a synthetic resin primer (melamine resin, about 20 g/m.sup.2, solids content about 65 wt %). The primer was then dried in a forced-air drier, given a finishing primer, and dried again. Thereafter it was printed by gravure with a decoration and provided with a masking/transport layer, and finally it was dried and cooled. The board 2 was then pressed to a laminate with an overlay and a counterbalancing layer in a further production step in a short-cycle press.

WORKING EXAMPLE 2

(12) Ahead of a continuous press 110, the top side 1.1 of a mat 1 consisting of wood fibers HF and having a thickness of around 13 cm was scattered, using two scattering apparatuses 108, with an amount of a mixture of titanium oxide (85 wt %) and melamine resin powder (15 wt %) of 15 g/m.sup.2, with each scattering apparatus 108 scattering 7.5 g/m.sup.2. Water was then sprayed onto the scattered powder mixture in an amount of 15 g/m.sup.2 by way of the spraying apparatus 109. The mat 1 was subsequently pressed in a continuous press 108 to form an HDF board 10 mm thick. After the continuous press 110, a sample was taken and a determination made of the value of L according to the CIELAB system. A value of 90.1 was measured. The wood-based boards 2, which in spite of one-sided coating exhibited no warping, were freed from loose constituents on the surface after cooling in a pressure section by a cleaning abrasion, after which they were provided with a synthetic resin primer (melamine resin, about 20 g/m.sup.2, solids content about 25 wt %). The primer was then dried in a forced-air drier, given a finishing primer, and dried again. Thereafter it was printed by digital with a decoration and provided with a masking/transport layer, and finally it was dried and cooled. The wood-based board 2 was then pressed to a laminate with an overlay and a counterbalancing layer in a further production step in a short-cycle press.

WORKING EXAMPLE 3

(13) Ahead of a continuous press 110, a mat 1 of wood fibers HF with a thickness of around 13 cm was scattered by means of three scattering apparatuses 108 with a mixture of calcium carbonate (85 wt %) and melamine resin powder (15 wt %) in an amount of 75 g/m.sup.2, with each scattering apparatus (108) scattering 25 g/cm.sup.2. 30 g of water per m.sup.2 were then sprayed onto the scattered powder mixture by means of the spraying apparatus 109. The mat 1 was subsequently pressed in the continuous press 110 to form an HDF board 10 mm thick. After the continuous press 110, a sample was taken and a determination made of the value of L according to the CIELAB system. A value of 86.1 was measured. The wood-based boards 2, which in spite of one-sided coating exhibited no warping, were freed from loose constituents on the surface after cooling in a pressure section by a cleaning abrasion, after which they were coated with a synthetic resin primer (melamine resin: about 20 g/m.sup.2, solids content: about 65 wt %). The primer was then dried in a forced-air drier, given a finishing primer, and dried again. Thereafter it was printed by digital with a decoration and provided with a masking/transport layer, and finally it was dried and cooled. The wood-based board 2 was then pressed to a laminate with a liquid overlay and a liquid counterbalancing layer in a further production step in a short-cycle press.