EP0548758B1 - Board and method to manufacture it - Google Patents

Abstract

In a process for continuous or discontinuous manufacture of boards, a flat web (15) of chips, fibres or similar particles, which contain lignocellulose and/or cellulose treated with at least one added binder, is formed on a transport surface (11), wetted with a water/binder mixture (20) with a heat-curing binder and a reaction-accelerating component in the form of a mist immediately before pressing and then pressed under the action of heat between two pressing surfaces to give a board(14). <IMAGE>

Description

The invention relates to a method for producing plates according to the preamble of claim 1 and a plate according to the preamble of claim 7, as e.g. are known from document EP-A-8566.

In the field of the production of chipboard, wood fiber or similar boards, it is known (bison brochure "Endless fiberboard systems" 3000-5.89-W .; DE-OS 39 02 705) that the proportion of binder in a single-layer nonwoven structure is generally 8 -10% or approx. 11 to approx. 13% and the binder content in the case of a multi-layer nonwoven structure approx. 6 to approx. 8% in the middle layer and approx. 11 to approx. 13% in the top layer (s), in each case related on non-woven (absolutely dry) fleece particles.
It is also already known (F. Kollmann, Holzwerkstoffe, Springer Verlag, 1966, pp. 353 - 355) to spray the surface of nonwovens consisting of fine particles such as wood chips (also called chip cakes) with water in order to have smoother and harder surfaces during the final pressing to obtain. If the amount of water sprayed on is increased too much, the bulk density of the outer cover layer drops and thus the cross-tensile strength drops suddenly.

From DD-PS 56 627 a process for the production of surface-treated chip moldings is known, in which a chemically reactive substance is added to a mixture consisting of comminuted, organic fiber materials and a binder, and after a cold pre-compression of this mixture onto the surface of the pre-compact, a thermosetting plastic resin sprayed on or poured on, which is already in the cold condition is precondensed. This creates a gelatinous viscosity of the synthetic resin, and a barrier is created from the synthetic resin at the interface between the chip mixture and synthetic resin, which prevents the synthetic resin from flowing off into the absorbent surface of the molding during the instantaneous liquefaction of the synthetic resin at the start of the hot pressing process.

The disadvantage of this special plate production is, above all, that the boundary layers occurring as part of the barrier formation significantly impair the strength of the plate.

The invention has for its object to provide a method with which a wood chip, wood fiber or the like plate can be produced or to create a wood chip, wood fiber or the like plate that has a high bending strength, a smooth with a comparatively low binder content and has a closed and sealed surface and has no tendency to stick to the pressing surfaces.

To achieve this object, the features of claims 1 and 7 are provided.

It is particularly important for the process according to the invention that, in addition to water and a thermosetting binder, the binder mixture also contains a reaction-accelerating component which, in cooperation with the other two components, ensures rapid curing of the binder in the press nip and contributes to adhesive effects the pressing surfaces are avoided and an optimally sealed outer layer is obtained. It is of particular importance that the thermosetting binder containing a reaction accelerating component applied directly in front of the press nip in the form of a mist to the nonwoven surface, thereby avoiding that in the cold state precondensation and an undesirable boundary layer formation impairing the strength of the plates produced occur. Surprisingly, the total binder content can be reduced both in the case of a single-layer and in a multi-layer nonwoven structure, and at the same time a high flexural strength can be achieved.

Both the thermosetting binder in the three-component mixture and the reaction-accelerating component in this mixture can consist of a commercially available component, the use of a sulfonated polycondensation product based on melamine optimally fulfilling the particular requirements given in the use case according to the invention.

Preferably, the thermosetting binder contained in the water-binder mixture consists of the same binder that is already contained in the nonwoven, so that only a single type of binder is present across the plate, with a hardener introduced both with that previously in the nonwoven already contained as well as can react with the subsequently applied binder.

It can also be advantageous if the transport surface is wetted with the water-binder mixture before the fleece is applied.

Application amounts for the water-binder mixture and binder concentrations previously present in the nonwoven are specified in claim 9. With these design rules, increased flexural strength can be achieved with a reduced total binder content. The increased binder content in the outer layer or in the outer layers overcompensated thus the lower binder content inside the fleece.

A plate designed according to the invention consists of a uniformly formed and pressed under the influence of heat fleece, which only has a selectively higher binder content in one or both outer layers. The plate can be made from several layers of nonwoven. However, the outer layer (s) and the subsequent inner layer (s) are each assigned to one and the same nonwoven layer.

The invention can also be carried out in such a way that when the nonwoven is produced, the binder content is reduced to such an extent that the cohesion of the plate produced is subsequently guaranteed, but the flexural strength of such a plate would not be sufficient for practical use. However, by spraying on a suitable amount of a water-binder mixture before pressing, the proportion of binder in the surface area can be increased so that overall a significantly improved flexural strength of the plate is achieved. The further advantage provided by this is that the average binder content of the board is significantly reduced without loss of strength. This is of considerable economic importance.

Further advantageous refinements and features of the invention are specified in the subclaims.

Fig. 1

eine schematische Darstellung einer Vorrichtung zur Ausführung des Verfahrens zur Herstellung von Holzspan-, Holzfaser- oder dergl. Platten, und

Fig. 2

einen schematischen Querschnitt durch eine erfindungsgemäße Platte.

The invention is described below, for example with reference to the drawing; in this shows:

Fig. 1

is a schematic representation of an apparatus for performing the method for the production of chipboard, wood fiber or the like. Plates, and

Fig. 2

a schematic cross section through a plate according to the invention.

The surface of an endless conveyor belt 11 forming a transport surface is sprayed with a water-binder mixture 20 by means of a nozzle 16 connected to a storage vessel 21. However, this spraying of the conveyor belt is not necessary. In the direction of transport indicated by an arrow, the nozzle 16 is followed by a feed device 17 for fine particles 18, for example containing 5% binder, e.g. Shredded wood chips are provided, which are deposited from above as a fleece 15 onto the transport surface 11 moistened with the water-binder mixture 20.

After the feed device 18 and immediately before the press inlet, a further spray nozzle 19 is provided, which is connected to the same storage container 21 as the spray nozzle 16 which may be provided and converts the water-binder mixture into a mist form and thus wets the nonwoven surface immediately before the press inlet. The distance between the nozzle 19 and the press inlet is minimized, so that the time which elapses between the misting of the water-binder mixture on the fleece and the start of pressing in the heated press is very short.

It is understood that in the case of wider conveyor belts 11, a plurality of spray nozzles 16 and 19 are to be arranged next to one another in order to achieve uniform wetting with the water-binder mixture 20 over the entire width of the fleece 15.

In a similar manner, the feed device 17 must also extend over the entire width of the transport surface 11, so that a nonwoven 15 of uniform thickness over the entire width is deposited on the transport surface 11.

The spray nozzles 16, 19 do not have to spray vertically from above onto the transport surface 11 or the fleece 15, but can also be inclined at an angle of up to 60 ° to the vertical.

The fleece 15 thus wetted up and down with the water-binder mixture is introduced into a double belt press 22 and pressed therein with heating to form a plate 14 which, viewed in the direction of transport, emerges from the rear end of the double belt press 22 and rests on a flat base 23 .

The plate 14 has the structure shown in FIG. 2. On both sides of a middle layer 24 with a bulk density of 650 kg / m³ and a binder content, e.g. Urea resin, of 5% there are thin outer layers 25, 26 in which there is an increased binder content of, for example, 7.5% due to the spraying on of the water-binder mixture 20. Due to the stronger compression caused by the water content, the bulk density there is about 850 kg / m³.

The outer layers 25, 26 have a thickness of 0.4 mm, for example.

The spray nozzles 16, 19 can work with or without compressed air and should in any case ensure that the water-binder mixture is dissolved in a fine droplet of mist.

The transition from the lower binder concentration in the middle layer 24 to the higher concentration in the Outer layers 25, 26 are not abrupt, but are flowing or continuous, so that the formation of boundary layers which impair strength is avoided.

It has already been pointed out that, for example, urea resins, urea-melamine resins or mixtures thereof can be used as thermosetting binders. An alternative mixture of binders, which can also be used in the context of the invention, consists of water, a paraffin emulsion and a release agent, and good results are achieved if the ratio by weight percentages is approximately 90: 9: 1. Commercially available paraffin emulsions and release agents can be used.

Claims (8)

1. Method of manufacturing boards in which a planar mat (15) of chips or fibres containing lignocellulose and/or cellulose or like particles with at least one admixed hardenable binder such as, for example, urea resin, ureamellamine resin or their mixtures with another synthetic resin is formed on a transport surface and is sprayed with a water/binder mixture on the surface remote from the transport surface (11) and then pressed under the action of heat between two press-surfaces of a continuously operating press into a board (14), characterized in that the binder in the water/binder mixture comprises a sulfonated polycondensation product on the basis of mellamine, and in that the spraying on of the mixture takes place in the form of a mist onto at least the surface of the mat remote from the transport surface (11) directly in front of the press gap of the continuously operating press.
2. Method in accordance with claim 1, characterized in that the heat curable binder in the water/binder mixture is the same as is already contained in the mat (15).
3. Method in accordance with claim 1 or claim 2, characterized in that the water/binder mixture contains 2.5 to 50 %, preferably 20 to 40 % and in particular approximately 30 % of binder.
4. Method in accordance with one of the preceding claims, characterized in that, depending on the height of the mat and/or the thickness of the hard outer layer (s) to be generated, 5 g/m² to 300 g/m², in particular approximately 20 to 40 g/m² of water/binder mixture is applied onto the mat surface.
5. Method in accordance with one of the preceding claims, characterized in that the water/binder mixture has such a binder concentration and is applied in such a quantity to the mat surface that the proportion of solid binder present there amounts to more than 6 %, in particular to 7 % and to a maximum of 12 %, preferably to less than 10 % and in paricular to less than 8 %, and in particular to approximately 7.5 %.
6. Method in accordance with one of the preceding claims, characterized in that the binder concentration in the single layer mat (15) prior to the application of the water/binder mixture is greater than 4 % and smaller than 6 %, and is in particular approximately 5 %, and, in a multi-layer mat, is correspondingly selected in the middle layer prior to the application of the water/binder mixture and amounts in the covering layer or layers to between ca. 7.5 and ca. 13 %.
7. Board consisting of chips or fibres or like particles containing lignocellulose and/or cellulose with at least one admixed heat-curable binder which have been pressed together under the action of heat, characterized in that the proportion of binder in at least one outer layer 25, 26, and thus the final density, is increased relative to the interior having one or more layers, with the final density in the inner layer lying at approximately 550 to 750 kg per m³ and amounting to 750 to 950 kg per m³ in the outer layer, and in that the proportion of solid binder present in the outer layer 25, 26 lies in the range from ca. 7.5 to ca. 13 %, whereas the binder concentration in the interior is preferably greater than 4 % and smaller than 6 % and in particular amounts to about 5 %; and in that the proportion of solid binder in the outer layer consists of a sulfonated polycondensation product on a mellamine basis.
8. Board in accordance to claim 7, characterized in that both outer layers have an increased proportion of binder and an increased density and in that their bending strength is greater than 50 kg per mm², preferably lies in the region from 55 to 65 kg per mm² and amounts in particular to about 60 kg per mm².

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