DE2536540A1 - Wood chip board prodn by hot pressing - in which heating is under program control to produce required density - Google Patents

Abstract

Wood chip board is formed by pressing wood chip mats containing adhesive between electrically heated presser plateS, and during pressing the heating power is under programme control to adjust the density of the wood chips or fibres nd so control the overall density of the resulting wood chip board. The resulting board has uniform density and stiffness with a good surface finish. Pref. heating is controlled by controlling the current or voltage applied to the heaters in the presser plates and pref. the pressure applied to the mats is also under programme control. Pref. for a constant pressure the temperature is gradually increased to a desired level and then decreased. Pref. the temperature gradient in the board is below 0.50 degrees C/S.

Description

Process for the production of chipboard and fibreboard with a given Density profile.

The invention relates generically to a method of production of chipboard and fiberboard, - being provided with a hardenable binder Preßgutmatten arranged between electrically heatable press plates, by means of unheated Press plates of plate presses are subjected to a pressing pressure for the duration of the pressing and exposed to the influence of a heating power after the pressure has been built up. It is doing this with the usual Raw materials worked. The binders are especially precondensed synthetic resin binders. But you can also use other binders or awaken and use wood constituents as binders. Press duration denotes a Time span, namely the pressing time of the individual Preßgutmatten, starting with the Build-up of the pressing pressure to the end with pressing pressure again zero. Designated heating power the heating energy supplied in the time unit. In other words, it is the heating energy the time integral of the heating power.

The press plates are unheated and, if necessary, cooled.

In known generic methods (DT-OS 23 35 015) are subject the supply of the heating power and the pressing pressure no special program. The generated Although products are superior to those used in classic plate presses with steam-heated, heavy press platens are produced, but the products are like the invention has recognized with regard to the setting of a special density profile and with regard to the surface quality can still be improved. Density profile denotes the over the cross-section of a finished-pressed chipboard, fiberboard or the like from the surface Measured over the middle area to the other surface and therefore as a curve Density that can be applied across the cross-section. Surface quality means, in particular, strength and hardness of the surface layers, but also porosity and roughness. Density and Strengths are related and more or less proportional to each other.

When pressing pressed material mats with plate presses, the heavy steam-heated Having press plates, it is basically known (from Bismarck "To optimize chipboard pressing in Holz-Zentralblatt, July 5, 1974), the Compaction process, d. H. the build-up of the pressing pressure from zero to a predetermined one Value to control or regulate, especially influencing the compression speed will. In the context of these known measures you control the pressure z. B. in the sense a maximum pressure limitation. The heating power remains unaffected and can because of the great inertia of the steam-heated press plates during the press period as well are not significantly influenced.

Rather, permanently heated press plates are used on a regular basis. All of this has more or less proven itself, depending on how the process is carried out, however the pressing time is in the order of 1000 seconds. It is true that you have yourself in the frame these measures known in plate presses with heavy steam-heated press plates time and again to improve the product quality in terms of flexural strength and transverse tensile strength endeavored, but decisive improvements have not succeeded.

Also the surface quality of the finished pressed chipboard, fiberboard and the like is in need of improvement, in particular

if it concerns panels without coating layers and if work is carried out without recooling.

The invention is based on the object of specifying a method with which it is possible in the manufacture of chipboard and fibreboard over the cross-section defined density profiles (in the sense of the definition above) can be influenced and to produce reproducibly, both with thick plates and with thin plates. At the same time, the surface quality and the absolute values should in terms of flexural strength and transverse tensile strength the finished Plates are improved. - Thin plates means, within the scope of the invention, finished pressed ones Boards and fibreboards with a thickness of up to 8 mm. Close thick panels to this limit upwards.

The invention is based on the generic method and the belonging to the invention that the way of supplying the Heating power during the pressing period has a considerable influence on the formation of the density profile Has.

The solution to the above problem is that during the Pressing time on the one hand the pressing pressure and on the other hand the heating power as a function can be controlled and regulated by the time according to a predetermined program and as a result, the density profile of the finished chipboard or fiberboard is set. According to a preferred embodiment of the invention, the program for the compression pressure on the one hand, the program for the heating power on the other hand coordinated and coupled. If the pressing pressure is kept constant during the pressing time, so it is within the scope of the invention to readjust it continuously or in stages or to readjust.

This is preferably done so that the program for the heating power and that for the pressing pressure, plotted as curves over time, with essentially horizontal curve branches and, on the other hand, steep flanks are driven. the Coordination of the programs for baling pressure on the one hand and heating power on the other happens according to a preferred embodiment of the invention via the pressing temperature. To this end, the invention initially teaches that at an essentially constant pressing pressure the heating power is supplied as an energy pulse from practically constant power, until the pressing temperature in the material to be pressed (in the middle of the cross-section or in selected Layers) has reached a target value, and that afterwards the heating output is reduced to zero or withdrawn to the extent that it is will that for the remainder of the press time only the pressing temperature is reached or maintained. Optimal product quality is achieved when the heating power of the energy pulse on a gradient of The pressing temperature applied over time is set, which is above 0.50 OC / sec lies. It is preferable to work with a temperature gradient that corresponds to the 2nd OC / sec and more. The procedure described comes with an energy pulse therefore of particular importance. - The set temperature of the press temperature is z. B. the temperature prescribed for the hardening of the binder or one opposite reduced, - as will be explained.

During the production of chipboard and fibreboard, the pressed material experiences known as plasticization under the influence of pressure and temperature, from the interaction of wood chips and steam before the hardening of the binder begins or has ended. In this sense plasticized pressed material is under the Influence of the pressing pressure more compacted than non-plasticized. The plasticization begins in the method according to the invention in contact with the electrically heated Press plates and flows into the pressed material over time.

It is straightforward in the context of the method according to the invention possible, the progress of this plasticization from the surface areas to Central to be controlled via special programs for baling pressure and heating power. That then enables the setting of special density profiles, e.g. B.

and especially the production of particle board and parabolic fibreboard Density distribution, d. H. with high surface density and, in contrast, reduced Density in the middle area, but also the manufacture of chipboard and fiberboard with a density that is substantially homogeneous over the cross-section.

A method according to the invention for the production of opposite the state of the art quite new chipboard and fiberboard with high surface density and, on the other hand, reduced density in the middle area (i.e. inside) is thereby characterized in that during a first portion of the pressing time over the pressing plates such a large heating power is supplied that the surface areas of the to be pressed Preßgutmatte experience a plasticization, but this portion of the pressing time is so limited that in the central area of the mat of material to be pressed, no or at least no no significant plasticization yet occurs, and that afterwards when or after it is reached this first part of the pressing time the pressing pressure is reduced and additional, compared to that of the energy pulse, reduced heating power for complete curing is fed. The invention uses the fact emphasized above that the plasticization of the material to be pressed from the surfaces in contact with the heated press plates to the inside with defined, measurable resp. determinable speed advances. Anyway, towards the end of the Pressing duration the reduced pressing pressure in general continuously down to zero dismantle, but terminate the supply of heating power before this dismantling. The press pressure on the other hand, it can also be gradually reduced at the end of the pressing period. The gradual one or gradual dismantling is recommended to start with the opening and closing of the associated Plate press too large movements of air and possibly water vapor from the pressed material out to prevent. However, it also serves to be uncontrolled To prevent changes in the finished pressed, but still hot pressed material mats, associated with a sudden open of the associated platen press could. Taking into account the fact that there is also moisture regularly in the pressed material is in the form of water, which is unset in steam during the pressing process, recommends the invention to proceed in such a way that, towards the end of the pressing period, the pressing pressure is initially applied until the vapor pressure is further reduced and after a pause for evaporation. The vapor pressure results according to the usual rules of thermodynamics from the pressing temperature by a mat of pressed material, which is subjected to the pressing pressure, for the determination of the Has steam pressure as it were as a pressure vessel. The steam escapes from the pressed mat as soon as the pressing pressure is lower than the steam pressure.

In the context of the process according to the invention, the above specified measures not only a defined density profile with in this embodiment high surface density and thus high strength and, on the other hand, reduced Density in the middle range, but also high surface quality in itself and for themselves. Depending on the raw material and binder, the pressing temperature is chosen so high that as it is possible, but care must be taken that there is a combustion of the pressed material mat to be pressed or the synthetic resin binder in the pressed material mat not yet entered. That can be achieved without further ado and always in such a way that any no further finishing of the surfaces is necessary. this fact is, moreover, of great importance to be prevented. However, it also serves to be uncontrolled To prevent changes in the finished pressed, but still hot pressed material mats, which could be associated with a sudden opening of the associated platen press. Taking into account the fact that there is also moisture regularly in the pressed material is in the form of water, which is converted into steam during the pressing process, recommends the invention to proceed in such a way that, towards the end of the pressing period, the pressing pressure is initially applied until the vapor pressure is further reduced and after a pause for evaporation. The vapor pressure results according to the usual rules of thermodynamics from the pressing temperature by one a Preßgutmatte, which is subjected to the pressing pressure, for the determination of the Has vapor pressure as it were as a pressure vessel. The steam escapes from the pressed Press mat as soon as the press pressure is lower than the steam pressure.

In the context of the process according to the invention, the above specified measures not only a defined density profile with in this embodiment high surface density and thus high strength and, on the other hand, reduced Density in the middle range, but also high surface quality in itself and for themselves. Depending on the raw material and binder, the pressing temperature is chosen so high that as it is possible, but care must be taken that there is a combustion of the pressed material mat to be pressed or the synthetic resin binder in the pressed material mat not yet entered. That can be achieved without further ado and always in such a way that any no further finishing of the surfaces is necessary. this fact is of great importance, when it comes to manufacturing is about chipboard and fiberboard, which is subsequently given a special surface coating experienced, for example, painted or printed.

To produce chipboard and bevel slats, one across the cross-section have substantially homogeneous density, the invention teaches that during the Pressing time on the press plates so large a heating power is supplied that the Preßgutmatte due to the heat flowing in from the surface towards the center everywhere the same plasticization experiences, and that during the pressing period with the progressive plasticization decreasing pressure again and again - in stages or is continuously readjusted or readjusted. Here, too, one gets towards the end the pressing time the pressing pressure initially down to the steam pressure for the purpose of evaporation and then, if necessary after a pause for evaporation, continuously or in stages reduce to zero.

The control or regulation of the heating power can in principle on done different ways. The easiest way to get the result is when the press plates can be switched as ohmic resistances and the heating power via the current intensity is controlled or regulated as a parameter. The heating power is then the square proportional to the current.

The advantages achieved can be seen in the fact that when implemented the method according to the invention for the first time in the history of the production of Particleboard and fibreboard panels can be made that are adjustable and reproducible have a defined density profile, wherein even significant fluctuations in the raw material do not interfere with the results. It goes without saying that there are significant fluctuations in the raw material for the pressed material mats required, on the one hand the program for the press pressure and on the other hand the program for the supply of heating power to adapt to the raw material, which can be done without difficulty through operational adjustment of the process parameters after simple experiments can be done. Of particular advantage is the fact that the finished pressed Boards always have a high surface quality.

This is based on the fact that first the pressure is built up and then the Heating power is supplied, with very steep temperature gradients for the pressing temperature are adjustable because practically no inertia with ohmic heating of the press plates is being worked on. Nonetheless, burns can be caused in the pressed material mats or in their surface layers or in the binder. Without difficulty the surface density and thus the strength in the area of the surfaces Choose and adjust so that the strength is greater than the vapor pressure inside a plate when the plate press is opened or when the pressure is reduced. Kick the so-called bursters therefore no longer on. The absolute values of transverse tensile strength and flexural strength are considerably higher than, based on the same raw materials and binders, achievable with classic processes that work with steam-heated press plates is. Of particular advantage is the fact that the pressing time is only 2 of the order of magnitude from 10 sec. is, while it is with classical procedures, under comparable conditions with regard to the volume of the pressed material in the order of 103 sec. amounts to.

The method of the invention can be used when it is it is about individual pressed material mats in a plate press to press, the press plates are then no longer heated because the press heat over the electrically heated press plates is fed. However, you can use the invention Realize the process also in the so-called stacking package pressing, i.e. in the press plate, Press mat, press plate, press mat and so on can be stacked to form a package. In addition, the method according to the invention includes both the production of tempered Chipboard and fibreboard as well as those that do not receive any compensation. Compensation refers to any surface treatment, especially the application of coating layers made of synthetic resin-impregnated papers, but also the application of papers that do not contain any synthetic resin. Even other pay layers, for example photographic layers, lacquers and paints from a carrier film relocated to the surface of a chipboard or fibreboard, fall under this term. It is of particular importance that all these remuneration measures be carried out by providing the pressed material mats with appropriate supports before the Preßgutmatten placed between the press plates and pressed. The pressed material mats can be raw mats, as they are produced by a spreading process generated, but it can also be pre-compacted mats, which are subsequently subjected to the process according to the invention. Regularly one works within the scope of the invention, however, with raw mats, which immediately after the spreading process can be further treated in the manner described. Because of the high Surface qualities of the panels produced can also easily especially without grinding.

In the following, the invention is illustrated by means of a few exemplary embodiments explained to which graphic representations of programs on heating power and pressure belong.

Fig. 1 is a process scheme in the production of chipboard with over the cross-section of homogeneous density, FIG. 2 shows a process diagram for the production of chipboard with high surface density and low density in the middle area, and FIG. 3 shows a process scheme for the production of chipboard with a defined Residual moisture.

Example 1 For the production of chipboard with over the cross section homogeneous density was made with glued chips from 30% spruce, 40% oak, 30% green fleece worked, which had a moisture content of 11.5%. The binder content of the glued Chips was 5%. It was a precondensed synthetic resin binder. This Synthetic resin binder required the target temperature for complete curing a pressing temperature of 140 C.

The procedure was carried out as that shown in FIG graphic representation describes. The time t is plotted as the abscissa. The ordinates are plotted pressing pressure P, pressing temperature T and heating power L, the latter proportional to the square of the current strength. The corresponding Curves over time were denoted by P, T and L, respectively. One takes from the graphic First representation that first the pressure P completely built up and then the heating power L is supplied. The heating power L is for a section the pressing time (10 to 140 seconds) is kept at an essentially constant value, until the pressing temperature T has reached its setpoint. That is in this embodiment the temperature of 140 ° C, which is necessary for the complete curing of the synthetic resin binder is required. During this part of the pressing time, the pressing pressure is also increased P held constant. Thereafter, the heating power L is reduced, namely in Embodiment completely, because the pressing temperature T for the relatively small remainder of the pressing time holds by itself at its target value. The result was the pressing pressure after the first part of the pressing time to the end of the pressing time held constant at a substantially constant pressing temperature T, it was thereafter continuously reduced to zero. The result is ready-pressed chipboard with a board weight of 3 1020 kg / m3, board thickness 4 mm. The density is accordingly homogeneous density distribution constant over the entire cross-section. The transverse tensile strength of the finished pressed chipboard was 14 kg / cm2, the flexural strength 450 kg / cm2. During the pressing process, the heating voltage was a maximum of 33 volts.

The pressing was carried out in the rest of the so-called stacking package, fourteen pressed material mats, alternating with press plates, in a stack.

The homogeneous density profile achieved is also shown at the top right in FIG. 1 has been entered, namely in the cross section shown there through a finished pressed Chipboard, the scale of which has been greatly enlarged compared to nature.

Example 2 For the production of chipboard with in the surface areas high density, comparatively low in the area of the middle, over the cross-section parabolic density distribution was glued with 20% spruce, 10% pine, 20% beech, 50% gate chip worked. The moisture content of the glued chips was 11.5 %. The binder content was 8% and a precondensed synthetic resin was used. This required a pressing temperature as the setpoint temperature for complete curing 0 of 160 C.

From the graph of FIG. 2 it can be seen that the heating power L has already been withdrawn here before the target temperature resulting from the binder of 160 OC was reached. The heating power L was namely after plasticizing the Surface layers of the pressed material mats withdrawn before the plasticization reached the center of the pressed material mat. The press temperature at this point was about 110 OC. At the same time or approximately at the same time as the heating power L was also the Compression pressure P reduced and then the Preßgutmatten at reduced Heating power L and reduced pressing pressure P finished pressed until the end of the pressing period. That leads to a parabolic density profile, which is shown in Fig. 2 at the top right with a greatly enlarged scale compared to nature as a cross-section has been represented by a ready-pressed chipboard. You can see that the Density in the surface areas is around 1100 kg / m3, but in the middle only at around 3 750 kg / m3. The overall transverse tensile strength was the finished 2 pressed plates at 8 kg / cm, the flexural strength at 385 2 kg / cm. - The maximal The heating voltage during the pressing process was 44 volts.

Here, too, the pressing took place in the so-called stacking package.

Example 3 To produce chipboard with a defined residual moisture content, was initially proceeded as has been explained for example 1. That is true So especially for chip material, binder, heating temperature and pressing in the stacking package. The graphic representation is again constructed as explained for Example 1 has been, d. H. with the curves P, L and T for pressing pressure, heating power and pressing temperature. It can be seen that at the end of the pressing time, the pressing pressure P is reduced to a fraction of its Setpoint and up to below the value set according to thermodynamic laws Steam pressure in the pressed material was reduced.

At this reduced pressing pressure P, there was then a reduced heating output L subsequently supplied as setting energy pulse E for the residual moisture. The achievable Residual moisture has been shown as a parameter on vertical lines. It amounts to selectable and depending on the energy pulse E, for example 5%, 3% or less.

In the exemplary embodiment, the residual moisture was in accordance with the process scheme at 2%, the transverse tensile strength and flexural strength were no different from the example 1.

Claims (10)

Expectations : f \ process for the production of chipboard and fibreboard, - With pressed material mats provided with a hardenable binder between electrically heatable press plates arranged, by means of press plates of plate presses for the pressing time is subjected to a pressing pressure and after the pressing pressure has been built up for adjustment the pressing temperature are exposed to the influence of a heating power, d a d u r c h e k e n n n n z e i c h n e t that on the one hand the pressing pressure during the pressing time and on the other hand the heating power as a function of the time after a predetermined one Program controlled or regulated and thereby the density profile of the finished Particleboard or fiberboard is set. 2. The method according to claim 1, characterized in that the program for the heating power and that for the pressing pressure, plotted as curves over the Time, branches with essentially horizontal curves and, in contrast, steep flanks be driven. 3. The method according to any one of claims 1 or 2, characterized in that that at an essentially constant pressing pressure the heating power as an energy pulse is supplied with practically constant power until the pressing temperature in the material to be pressed (in the middle or in selected surface layers) has reached a target value has, and that then the heating power is reduced to zero or so far that reached the pressing temperature for the remainder of the pressing time or only the pressing temperature is held. 4. The method according to claim 3, characterized in that the heating power of the energy pulse on a gradient of the pressing temperature plotted against time is set, which is over 0.50 OC / sec. lies. 5. The method according to any one of claims 1 to 4, in the embodiment for the production of chipboard and dry fibreboard with a high surface density and, on the other hand, reduced density in the middle cross-sectional area, characterized in that that during a first part of the pressing time over the press plates such a large one Heating power is supplied that the surface areas of the mat to be pressed experience plasticization, but this portion of the pressing time is limited is that in the central area of the pressed material mat no or at least none significant plasticization occurs. and that afterwards on or after reaching the At the end of this section, the pressing pressure is reduced and additional, compared to the of the energy pulse, reduced heating power is supplied for complete curing will. 6. The method according to any one of claims 1 to 5, characterized in that that towards the end of the pressing period, the reduced pressing pressure continuously down to zero is dismantled. 7. The method according to claim 6, characterized in that towards the end the pressing time the pressing pressure initially down to the vapor pressure of the material contained in the pressed material Water vapor and then gradually further degraded. 8. The method according to any one of claims 1 to 4 in the embodiment for the production of chipboard and fibreboard with on the Cross-section of substantially homogeneous density, characterized in that during the pressing time is supplied with such a large heating power via the press plates that the pressed material mat through the heat flowing in from the surface towards the center undergoes continuous plasticization, and that during the pressing period with the progressive plasticization decreasing pressure again and again - in stages or is continuously readjusted or readjusted. 9. The method according to claim 8, characterized in that towards the end the pressing time the pressing pressure at least up to about the steam pressure and then it is continuously reduced to zero. 10. The method according to any one of claims 1 to 9, characterized in, that the pressure plates are connected as ohmic resistors and the heating power is controlled or regulated via the current intensity as a parameter.