CA2098520A1 - Method and apparatus for forming composite wood products - Google Patents
Method and apparatus for forming composite wood productsInfo
- Publication number
- CA2098520A1 CA2098520A1 CA002098520A CA2098520A CA2098520A1 CA 2098520 A1 CA2098520 A1 CA 2098520A1 CA 002098520 A CA002098520 A CA 002098520A CA 2098520 A CA2098520 A CA 2098520A CA 2098520 A1 CA2098520 A1 CA 2098520A1
- Authority
- CA
- Canada
- Prior art keywords
- assembly
- pressure
- high frequency
- wood
- heating
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 239000002131 composite material Substances 0.000 title claims abstract description 85
- 239000002023 wood Substances 0.000 title claims abstract description 82
- 238000000034 method Methods 0.000 title claims abstract description 37
- 238000010438 heat treatment Methods 0.000 claims abstract description 121
- 238000003825 pressing Methods 0.000 claims abstract description 39
- 238000004519 manufacturing process Methods 0.000 claims abstract description 16
- 238000000280 densification Methods 0.000 claims abstract description 5
- 239000003292 glue Substances 0.000 claims description 30
- 238000000429 assembly Methods 0.000 claims description 26
- 230000000712 assembly Effects 0.000 claims description 26
- 230000000452 restraining effect Effects 0.000 claims description 24
- 238000009835 boiling Methods 0.000 claims description 12
- 238000007789 sealing Methods 0.000 claims description 12
- 230000000087 stabilizing effect Effects 0.000 claims description 11
- 239000000463 material Substances 0.000 claims description 7
- 229920001187 thermosetting polymer Polymers 0.000 claims description 7
- 229920005989 resin Polymers 0.000 claims description 4
- 239000011347 resin Substances 0.000 claims description 4
- 239000004020 conductor Substances 0.000 claims description 2
- 230000003750 conditioning effect Effects 0.000 abstract description 18
- 230000006835 compression Effects 0.000 abstract description 13
- 238000007906 compression Methods 0.000 abstract description 13
- 230000006641 stabilisation Effects 0.000 abstract description 8
- 238000011105 stabilization Methods 0.000 abstract description 8
- 229920005610 lignin Polymers 0.000 description 5
- 239000002184 metal Substances 0.000 description 5
- 229920006395 saturated elastomer Polymers 0.000 description 5
- 239000007788 liquid Substances 0.000 description 4
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 4
- 229920002522 Wood fibre Polymers 0.000 description 3
- 210000004027 cell Anatomy 0.000 description 3
- 239000000470 constituent Substances 0.000 description 3
- 230000005684 electric field Effects 0.000 description 3
- 238000001704 evaporation Methods 0.000 description 3
- 230000008020 evaporation Effects 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- 229920002488 Hemicellulose Polymers 0.000 description 2
- 238000010793 Steam injection (oil industry) Methods 0.000 description 2
- 230000008901 benefit Effects 0.000 description 2
- 210000002421 cell wall Anatomy 0.000 description 2
- 229920002678 cellulose Polymers 0.000 description 2
- 239000001913 cellulose Substances 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 230000001143 conditioned effect Effects 0.000 description 2
- 238000010924 continuous production Methods 0.000 description 2
- 239000011159 matrix material Substances 0.000 description 2
- 229920001568 phenolic resin Polymers 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 239000000523 sample Substances 0.000 description 2
- 239000004902 Softening Agent Substances 0.000 description 1
- 230000004075 alteration Effects 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 238000007596 consolidation process Methods 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000011437 continuous method Methods 0.000 description 1
- 238000006731 degradation reaction Methods 0.000 description 1
- 239000003989 dielectric material Substances 0.000 description 1
- 238000004049 embossing Methods 0.000 description 1
- 239000012530 fluid Substances 0.000 description 1
- SLGWESQGEUXWJQ-UHFFFAOYSA-N formaldehyde;phenol Chemical compound O=C.OC1=CC=CC=C1 SLGWESQGEUXWJQ-UHFFFAOYSA-N 0.000 description 1
- 239000002783 friction material Substances 0.000 description 1
- 230000009477 glass transition Effects 0.000 description 1
- 238000007731 hot pressing Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 239000012811 non-conductive material Substances 0.000 description 1
- 239000004014 plasticizer Substances 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 238000005381 potential energy Methods 0.000 description 1
- 230000002035 prolonged effect Effects 0.000 description 1
- 230000001902 propagating effect Effects 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 239000000758 substrate Substances 0.000 description 1
- 229920001169 thermoplastic Polymers 0.000 description 1
- 239000004416 thermosoftening plastic Substances 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
- B27M—WORKING OF WOOD NOT PROVIDED FOR IN SUBCLASSES B27B - B27L; MANUFACTURE OF SPECIFIC WOODEN ARTICLES
- B27M3/00—Manufacture or reconditioning of specific semi-finished or finished articles
- B27M3/0013—Manufacture or reconditioning of specific semi-finished or finished articles of composite or compound articles
- B27M3/0026—Manufacture or reconditioning of specific semi-finished or finished articles of composite or compound articles characterised by oblong elements connected laterally
- B27M3/0053—Manufacture or reconditioning of specific semi-finished or finished articles of composite or compound articles characterised by oblong elements connected laterally using glue
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
- B27M—WORKING OF WOOD NOT PROVIDED FOR IN SUBCLASSES B27B - B27L; MANUFACTURE OF SPECIFIC WOODEN ARTICLES
- B27M1/00—Working of wood not provided for in subclasses B27B - B27L, e.g. by stretching
- B27M1/02—Working of wood not provided for in subclasses B27B - B27L, e.g. by stretching by compressing
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
- B27N—MANUFACTURE BY DRY PROCESSES OF ARTICLES, WITH OR WITHOUT ORGANIC BINDING AGENTS, MADE FROM PARTICLES OR FIBRES CONSISTING OF WOOD OR OTHER LIGNOCELLULOSIC OR LIKE ORGANIC MATERIAL
- B27N3/00—Manufacture of substantially flat articles, e.g. boards, from particles or fibres
- B27N3/08—Moulding or pressing
- B27N3/18—Auxiliary operations, e.g. preheating, humidifying, cutting-off
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
- B27N—MANUFACTURE BY DRY PROCESSES OF ARTICLES, WITH OR WITHOUT ORGANIC BINDING AGENTS, MADE FROM PARTICLES OR FIBRES CONSISTING OF WOOD OR OTHER LIGNOCELLULOSIC OR LIKE ORGANIC MATERIAL
- B27N3/00—Manufacture of substantially flat articles, e.g. boards, from particles or fibres
- B27N3/08—Moulding or pressing
- B27N3/24—Moulding or pressing characterised by using continuously acting presses having endless belts or chains moved within the compression zone
Abstract
ABSTRACT OF THE INVENTION
This invention relates to a novel method and heating apparatus which are useful for the production of wood composite products. A
method of making a wood composite product, comprising: conditioning by high frequency, followed by compression and heat stabilization.
In the conditioning stage a composite mat restrained by external pressure is plasticized by combination of heat and moisture while under pressure and while being transported through the heating device. The plasticized mat is subsequently formed into densified product that may include densification by continuous pressing. The compressed mat is heat stabilized within the press or in a heating device following the pressing cycle. An apparatus comprising heating a composite assembly in a stationary heating apparatus while the composite assembly is under pressure and while the said composite assembly is being continuously transported through the heating apparatus.
This invention relates to a novel method and heating apparatus which are useful for the production of wood composite products. A
method of making a wood composite product, comprising: conditioning by high frequency, followed by compression and heat stabilization.
In the conditioning stage a composite mat restrained by external pressure is plasticized by combination of heat and moisture while under pressure and while being transported through the heating device. The plasticized mat is subsequently formed into densified product that may include densification by continuous pressing. The compressed mat is heat stabilized within the press or in a heating device following the pressing cycle. An apparatus comprising heating a composite assembly in a stationary heating apparatus while the composite assembly is under pressure and while the said composite assembly is being continuously transported through the heating apparatus.
Description
`~ 8~
METHOD AND APPARATUS FOR FORMING
CONPOSIT~ WOOD PRODUCTS
This invention relates to a method and apparatus for continuous making of compressed products from wood that involves conditioning by high frequency, followed by compression and heat stabilization. In the conditioning stage a composite mat restrained by external pressure is plasticized by com~ination of heat and moisture while under pressure and while being transported through 1~ the heating device. The plasticized mat is subsequently formed into densified product that may include moulding into a variety of shapes by continuous pressing. The compressed mat is heat stabilized within the press or in a heating device following the pressing cycle.
Wood composite produ~ts are being produced in a variety of shapes and sizes, ranging from the well known panel products to substitutes for large size structural lumber. These products have in many respects properties superior to solid wood. They have lower variability of strength properties and improved stability. The present art of wood composite manufacturing teaches a number of continuous methods which utilize high frequency heating in the production of simple flat or rectangular shapes. Examples of such methods are disclosed by Lamberts et al., US Patent No. 4,221,950, Neubauer et al, ~S Patent No. 4,420,3S7 and Churchland, US Patent No 4,456,498. Production of densified wood and products of more complex forms, such as in case of formed panels and deep embossing, can benefit from high degree of plasticization of the composite assembly before pressing. In the plasticized state the assembly is 3~ not only much easier to compress and mould, but very importantly, the possibility of seriously damaging the wood structure is substantially reduced. The apparatus and methods disclosed by the - -~ 2098~20 known art do not allow high plasticization of the wood assembly prior to pressing.
Components of a wood cell undergo characteristic changes when exposed to higher temperatures and moisture. These changes are of prime importance in the manufacture of formed wood composites. It is well kno~ln that the elastic properties of wood are dramatically modified as the wood components go through the glass transition phase and change from a stiffer to a softer state. The amount of plasticization is a function of moisture content, temperature, heating rate, and the presence of softening agents. The degree to which the individual components are plasticized at a particular set of conditions varies. The accepted values of temperatures at which the wood components can be considered plasticized are as follows:
Wood component Dry TemP. Temp. at Saturation Amorphous Cellulose 220 C Room Hemicellulose 180 C Room Lignin 160 C 100 C
The transition temperatures at conditions encountered in practical situations will be between the two extreme values indicated above.
In conventional wood composite manufacturing processes the wood components are at different stages of plasticization. The degree of plasticization may vary greatly throughout the elements of the assembly and the overall cross section. Non-uniformity of heating is an inherent characteristic of conventional manufacturing 3~ processes. In hot pressing the temperature is highest near the heated press platens and diminishes in the direction away from the platens. In the case of dielectric heating a temperature gradient 2 ~
will exist within the individual element as well as the whole cross-section because of preferential heating of the glue lines and ! wet areas.
When a composite assembly is compressed cold, or before the uniformity of temperature is achieved, the amount of deformation in any particular area of the assembly will depend on the stiffness of the region at that point in time. Before commencement of heating the dominating variable will be the stiffness of the wood fibre and the moisture content. When larger degree of densification is being performed, a portion of the energy needed to form the densified composite is stored in the cold compressed mat in the form of potential energy. This energy is released when the subsequent heating changes the elastic characteristics of the wood fibre. A
non-uniform heating lowers the stiffness of the heated areas and this allows some of the stored energy to be transferred from the cooler areas to the heated areas where it is relaxed through compression of the wood cells. Consequently, the areas that are receiving preferential heating will be subjected to a larger amount of compression. If the strength limit of the deformed cells is exceeded, cracks develop and further compression may result in their collapse.
Compression damage of unheated or non-uniformly heated wood fibre may arise from a differential responses of the various constituents of wood to moisture. ~s indicated previously, both amorphous cellulose and hemicellulose may be significantly plasticized at room temperature by high moisture contents. The softening temperature of lignin at saturation is around 100C. It can be expected that the wood has higher moisture content adjacent to the glue line when aqueous resins are used. Since lignin lacks the same degree of plasticity at temperatures below 100C as the 2 ~;9 ~
other constituents, the integrity of the cell wall matrix will be altered. If the wood is fully compressed at this state, the non-uniformity of plasticization can lead to structural damage to the cell walls and to crack propagation within the wall under stress.
From the point of view of quality of the product and the efficiency of the manufacturing process it is therefore very desirable to achieve a high and uniform plasticization of the entire wood matrix before subjecting it to high forming pressures. High frequency heating is useful for efficient heating and plasticization of wood composite assemblies. It produces heat by exciting water molecules in oscillating electric field. Its large advantage is due to its ability to evenly and uniformly heat the entire composite mass. The water molecules deep inside the composite billet are affected by the electric fields as much as the ones closer to the surface.
Since the method does not rely on heat transfer, the assembly does not have to be fully compressed when heated, as is the case with conventional heating. The high frequency heating method is especially suitable for products that use aqueous thermosetting glues, such as the commonly used phenol formaldehyde resins. Their application adds water in the glueline and increases efficiency of dielectric heating by increasing the conductivity of the gluelines.
Industrial high frequency heating methods are of two types:
Radio or Microwave frequency. The difference between the two types is in the frequency at which they operate and the method of applying the heating energy. Whereas radio frequency heating uses frequency typically between about 1 MHz and about 50 MHz, the microwave frequencies used for heating applications are between about 900 MHz and about 2500 MHz. A typical radio frequency heating apparatus is a pair of opposing stationary electrodes. The material to be heated is placed in the electric field between the two electrodes. Microwave heating is applied in form of propagating waves that are directed (beamed) at the material. Although radio frequency is used in illustrations of the present invention, it ; will be apparent to those skilled in the art that the microwave heating can be also used to practice the disclosed invention. Steam injection heating method may also be applied in practising the t present invention.
The method disclosed herein produces a high degree of 10 plasticization before forming. The manufacturing method combines conditioning, pressing and stabilizing steps in one manufacturing process. The apparatus disclosed herein is suitable for a continuous plasticization of wood composite assemblies and production of densified or formed wood composite products. ~
This invention pertains to a method of making a wood composite -product, comprising the steps of: a) preparing a composite assembly constructed of a plurality of wood elements and a wood bonding glue between the elements, the glue between the wood elements forming a 20 plurality of gluelines within the composite assembly; b) advancing the prepared composite assembly through a heating apparatus, the composite assembly being subjected simultaneously to a restraining external pressure and heat within the heating apparatus, heating the restrained composite assembly to the boiling temperature of the moisture within the assembly to plasticize the wood, the magnitude of the steam pressure generated by boiling the moisture within the assembly being controlled by the restraining external pressure acting on the assembly, the excess steam being allowed to migrate between and through the wood elements and to vent to the exterior ~0 of the assembly; c) advancing the heated and plasticized wood composite assembly through a continuous pressing means to achieve the desired shape, form and densification of the wood product; d) 2098~20 .
further heating the formed composite product to fully cure the glue and to stabilize the composite assembly. In the method according to the present invention the heating method can be high frequency heating method or steam injection method. The restraining external pressure in step b) can be imposed on the wood composite assembly by the heat applicators or it can be generated by gas pressure, such as steam or air, applied in the space between the heat ..
applicators and the restrained composite assembly.
. -' 10 The restraining external pressure acting on the composite assembly can be between zero to about 100 psi. The boiling temperature of the moisture within the assembly can be between about 100 C to about 150 C. The stabilizing temperature in the method according to the present invention can be between about 150 C to about 210 C. The wood bonding glue can be a thermosetting aqueous resin.
This invention further pertains to an apparatus for continuous heating of a moisture and glue containing wood composite assembly 2~ comprising: at least one high frequency heating device having upper and lower frame means, the upper frame means supporting an upper high frequency heat applicator assembly and the lower frame means supporting a lower high frequency heat applicator assembly, the upper and lower high frequency heat applicator assemblies facing each other, the wood composite assembly being located between the said upper and lower high frequency heat applicator assemblies, the high frequency applicator assemblies comprising: a) heat applicators connected to a generator of high frequency heating energy; b) lower and upper belts made from a material substantially 33 transparent to high frequency energy, the belts being located between the respective heat applicators and the wood composite assembly, the wood composite assembly being compressed between the upper and lower belts within the high frequency heating device while it is being transported through the said heating device, the compressing pressure being generated by pressure injected into the space between the belts and the heat applicators, the pressure in the space between the heat applicators and the corresponding belts forming a pressure cavity of a variable depth; d) means of pressure sealing the cavity between the heat applicators and the corresponding belts. In the apparatus according to the present invention at least one of the frame means can be movable in respect to the other frame means. The pressure sealing means in step c) can be provided by bearing blocks forming a continuous pressure tight enclosure on all sides around the applicators, the bearing blocks being slidable within the frame means in the direction perpendicular to the wide surface of the belt located between the frame means and the assembly, the bearing blocks being in continuous pressurized sliding contact with the belt, the pressurized sliding contact between the bearing blocks and the belt being achieved by gas pressure acting on the opposite faces of the bearing blocks. The pressure within the variable depth pressure cavity can be air pressure or steam pressure. The depth of the variable depth pressure cavity can be between zero and about 6" or preferably between zero and about 1". The high frequency heat applicators can be electrodes for radio frequency heating or microwave applicators.
This invention further pertains to an apparatus wherein the high frequency heating apparatus is followed by a continuous pressing apparatus, the continuous pressing apparatus compressing the heated composite assembly and providing further heating to the 3~ compressed composite assembly to cure the glue and to stabilize the compressed composite. In the apparatus according to the present invention the curing and stabilization heating of the compressed 8~
composite assembly can be done in a high frequency heating apparatus following the continuous pressing apparatus.
This invention further pertains to an apparatus for continuous heating of a wood composite assembly constructed of a plurality of wood elements and a wood bonding glue dispersed between the elements, said apparatus comprising: one or more high frequency heating devices having upper and lower frame means, the upper frame means supporting an upper electrode assembly and the lower frame means supporting a lower electrode assembly, the upper and lower electrode assemblies facing each other and at least one of the electrode assemblies being movable in respect to the other assembly, the gap between the upper and lower electrode assembly defining the working space of the heating device, the electrode assemblies comprising: a) electrodes connected to a generator of high frequency heating energy; b) endless belts made from a material substantially transparent to high frequency energy, the belts being located between the respective electrodes and the wood composite assembly, the composite assembly being compressed between ~0 the upper and lower belts within the working space of the heating device while it is being transported through the said heating device, the compressing pressure being generated by gas pressure injected into the space between the belts and the electrodes the pressure in the space between the electrodes and the corresponding belts forming a pressure cavity of a variable depth; c) means of pressure sealing the cavity between the electrodes and the belts.
This invention further pertains to an apparatus for continuous heating of a wood composite assembly constructed of a plurality of ~0 wood elements and a thermosetting glue dispersed between the elements, said apparatus comprising: one or more high frequency heating devices having upper and lower frame means, the upper frame 2 ~
means supporting an upper electrode assembly and the lower frame means supporting a lower electrode assembly, the upper and lower electrode assemblies facing each other and at least one of the electrode assemblies being movable in respect to the other assembly, the gap between the upper and lower electrode assembly defining the working space of the heating device, the electrode assemblies comprising: a) pressing plates supported by the frame means; b) endless belts made from electrically conductive material and being electrically connected to a generator of high frequency heating energy, the belts being located between the respective pressing plates and the wood composite assembly, the composite assembly being compressed between the belts of the upper and lower electrode assemblies within the working space of the heating device while the said composite assembly is being transported through the said heating device, the compressing pressure being generated by gas pressure injected into the space between the belts and the pressing plates, the pressure in the space between the pressing plates and the corresponding belts forming a pressure cavity of a variable depth; c) means of pressure sealing the cavity between the pressing plates and the belts. The pressure sealing means in step c) can be provided by bearing blocks forming a continuous pressure tight enclosure on all sides around the pressing plates, the bearing blocks being slidable within the frame means in the direction perpendicular to the wide surface of the belt located between the frame means and the assembly, the bearing blocks being in continuous pressurized sliding contact with the belt, the pressurized sliding contact between the bearing blocks and the belt being achieved by gas pressure acting on the opposite faces of the bearing blocks. In this embodiment of the present invention the 3~ pressing plates can be connected to the generator of high frequency energy. The connection between the generator of hi~h frequency heating energy and the electrically conductive belts can be by 2~9~
means of pressurized sliding contact between a conductive part of the bearing blocks and the endless belts, the conductive part of the bearing block being connected to the said generator of high frequency heating energy.
The substance and nature of thi~ invention in certain embodiments is illustrated in the following drawings. The drawings should not be interpreted as restricting the spirit or scope of the invention in any way:
Figure 1 is a schematic side view illustration of an apparatus for continuous production of wood composite product in accordance with the method of the present invention.
Figure 2 illustrates a schematic cross-section view of a heating apparatus according to one embodiment of the present invention.
Figure 3 illustrates a partial schematic cross-section view of a high frequency heating apparatus in accordance with the present invention.
To achieve a high and uniform plasticization, the conditioning should be carried out while the assembly is restrained and the ! restraining external pressure is closely contro-lled. Both moistureand temperature act as plasticizer for wood. Control of the temperature and moisture conditions within the assembly can be achieved by controlling the restraining external pressure acting on the assembly. Heating the moist wood assembly to near the plasticizing temperature in the absence of restraining external pressure results in a free evaporation of moisture from the mat and pre-cured gluelines. If, on the other hand, the mat is subjected to controlled restraining pressure, heating does not result in excessive loss of moisture from the mat until the internally formed steam pressure reaches levels at which it can overcome the restraining pressure and vent to the exterior of the assembly. In normal manufacturing processes the amount of moisture present within the assembly is usually sufficiently high to produce saturated steam conditions in the interior of the assembly. The relationship between saturated steam pressure and temperature is well known and documented. In the present invention this relationship is used to control the plasticizing conditions within the wood assembly. Since the external restraining pressure controls the pressure of the saturated steam within the assembly, it also controls the boiling temperature of the moisture within the assembly. During heating the temperature will be increasing until the corresponding steam pressure can overcome the restraining pressure and vent to the exterior at the rate that balances the rate at which the steam is generated. A balance boiling temperature of the moisture is established. The desired temperature range for plasticization according to the present method is between about 100 C and about 150 C. The preferred temperature range is between about 120 C and about 140 C. The relatively low restraining pressures are between zero and about 100 psi. In addition, since the rate of moisture evaporation is also controlled, the curing rate of the glue system is also controlled. The typical glue systems used in the production of wood composite products, which may be aqueous thermosetting resins such as phenol formaldehyde glues, generally require dissipation of moisture from the gluelines to complete the curing reactions. Delay of moisture evaporation retards curing of the glue.
The method according to the present invention has three stages. The first is a plasticizing and conditioning staye. The second is a compression and product forming stage. The third is a stabilization stage.
209852~
The conditioning is carried out under relatively low external pressure. During the initial phase the temperature of the restrained assembly is raised to the desired plasticizing temperature of between about 100 C and about 150 C. The mobility of the liquid glue is initially increased by temporally lowered viscosity, and is assisted by the elevated steam pressure. Glue penetrates into the fissures and flaws in the wood substrate so these can be sealed and repaired in the compression stage. The pressure and temperature of the saturated steam is determined by the magnitude of the external pressure and the heating rate that is applied. It should be apparent to those skilled in the art that to achieve saturated steam conditions and the desired level of control during this stage requires sufficient amount of moisture within the assembly. The total initial moisture content can be between about 8% and about 25~ and can be partly present in the wood and partly in the glueline.
As is well known, the boiling point of a liquid is a function of the vapor pressure and it is constant for pure liquids boiling at a constant vapor pressure. For complex liquids, such as aqueous glues, the boiling point may differ from that of pure water and does not necessary remain constant at constant vapor pressure because of the chemical and viscosity changes taking place throughout the heating process. However, an increase of temperature throughout the boiling stage will be at slower rate. Heating of the assembly should be at higher rate and should be uniform. As indicated previously the high frequency heating methods can achieve rapid and uniform heating.
The conditioning stage is terminated when the excess moisture has been dissipated at the plasticizing temperature and the glue system is about to undergo the curing reaction. The mat is :
2098~20 plasticized and the compression stage should follow with minimum delay.
In the compression stage the heated and plasticized mat is compressed to the final density and desired form. The pressures required for the final compression of plasticized wood are generally much lower than the pressures required in cold pressing processes. The high degree and the uniformity to which the composite assembly is plasticized prevents compression damage~
reduces variations in density and allows high levels of densification as well as forming into a variety of shapes.
Figure 1 schematically illustrates a continuous production system useful in practising the method according to the present invention. It should be obvious to those skilled in the art that this method can be also applied in a stationary, non-continuous pressing apparatus equipped with high frequency heating. Such an arrangements is considered within the scope of the present invention.
As shown in Figure 1 the composite assembly 11 is carried through the conditioning heating device 12-between the lower endless belt 13 and the upper endless belt 14. Within the conditioning device 12 the assembly is subjected simultaneously to restraining pressure and heat according to the above description of the conditioning method. The plasticized and conditioned mat exits the conditioning device and is without delay compacted and formed in the continuous pressing apparatus 15. The pressing apparatus can be heated to provide the heat stabilization following full compression. However, it may be more convenient to apply the stabilization in a after-heating device 16. Even if after-heating is used to heat-stabilize the composite, the continuous pressing o apparatus may be heated to about the exit temperature from the conditioning device. The dwell time in the press at the conditioning temperature will sufficiently pre-cure the glue in the billet. The product will retain its shape after exiting from the continuous press. The final stabilization may be carried out in the stabilizing heating device 16. Restraining pressure can be applied also during the stabilizing stage. It was found however, that the restraining pressure may not be needed in the stabilizing stage because large portion of the moisture has been dissipated from the billet in the conditioning stage and the steam pressures remain lower. Furthermore, a full glueline strength is achieved in the early stages of stabilization.
As indicated above, the stabilizing stage can be carried out at any time after mat consolidation is achieved within the press or following the pressing cycle. In the stabilizing stage the temperature of the billet is elevated above the curing temperature of the glue. The final temperature can be higher than the minimum needed for thermosetting the glue to further increase the stability of the product. Main wood components, and especially the glue-like polymer lignin, are thermoplastic. To achieve improved stability, lignin should~ attain a high degree of flowability during the stabilizing stage. The preferred range of temperatures used in the stabilizing stage is between about 150 C and about 210 C.
Temperatures over 210 C are not recommended because of the possibility of damaging thermal degradation of the wood constituents. Prolonged heating periods should also be avoided.
Applying constant and controlled pressure to the wood assembly moving through the heating and conditioning device 12 or 16 in Figure 1 is not practical with the existing high fre~uency heating equipment. As described by Lamberts et al. and Neubauer et al. in 2~98~2~
the previously mentioned US Patents, the existing art does not allow any measurable pressure between the high frequency heat applicators and the moving assembly or the transporting belts. The Lamberts and Neubauer's methods require either a gap between the electrode and the assembly, or the transporting belts are allowed to be only grazingly contacting the electrodes. The heated wood assembly can not be adequately restrained to carry out the method of the present invention. ~ measurable contact pressure would result in a large drag, making the transportation through the heating device difficult or impossible. The heating device according to the present invention allows heating of pressure restrained wood assembly, while it is being continuously transported through a heating device.
The schematic cross-section shown in Figure 2 is a depiction of one embodiment of the present invention. It shows a cross section of the heating and conditioning device according to the present invention having a lower and an upper electrode assemblies, the lower electrode assembly comprising frame 21, electrode 23 and lower belt assembly comprising lower belt 27 and bearing blocks 28.
The upper electrode assembly is indicated as being of similar construction as the lower assembly comprising-frame 22, electrode 24 and upper belt assembly comprising upper belt 29 and bearing blocks 30. The bearing blocks 28 and 30 form a continuous enclosure on all sides around the applicators 23 and 24. In this embodiment of the invention the belts 27 and 29 are made from a dielectric material which is substantially transparent to high frequency energy. Position of the belt assemblies is monitored by probes 31 shown schematically. The composite assembly 32 being conditioned is located between the upper belt 29 and lower belt 27. The area of contact between the composite material and the belt assemblies is defined as the heating area further defining the working space of 2098~2~
.he heating device. One or both electrode assemblies are movable in respect to each other such as to open or close the distance between the respective upper and lower electrodes. The electrodes 23 and 24 are connected to a source of high frequency heating energy.
Flexible sealing means 33 and 34, which may be located on the bearing blocks 28 and 30, are sealing the cavities 35 and 36 to allow their pressurization. The bearing blocks 28 and 30 are shown movable within their respective locating seats such that they remain in pressure contact with the belts 27 and 29 while the depth of the cavities 35 and 36 may vary between zero and a predetermined maximum value. The bearing blocks 28 and 30 can be made of electrically non-conductive material that has low friction characteristics. A number of such materials are available in the industry. Belts 27 and 29 are shown separated from the respective electrodes 23 and 24 by the pressurized cavities 35 and 36. The space 43 between the bearing blocks 28 and 30 and the respective frames 21 and 22 can be connected with the respective pressurized cavities 35 and 36 or can be pressurized independently to provide additional flexibility for the use of the apparatus.
During heating the cavities 35 and 36 are pressurized with air, steam or other gas or fluid. The pressure within the cavities functions as the restraining external pressure for controlling the plasticizing and stabilizing conditions within the assembly, as discussed previously. In addition, the gas pressure lowers or eliminates the direct contact pressure between the electrodes and the respective belts, thus making the movement of the belts in respect to the electrodes frictionless. A small amount of frictional drag will exist as a result of the contact between the bearing frames 30 and 28 and the respective belts. Naking the bearing frames from a suitable low friction material will minimize the frictional resistance to transporting motion. Allowing the bearing frames 28 and 30 to be movable in respect to the main frames 21 and 22 in the direction perpendicular to the wide face of the belts and the composite assembly will allow the external restraining pressure to remain constant even though the composite assembly may compress as it is heated. It may be important from the operational point o~ view to maintain the pressurized cavities 35 and 36 at a constant depth, which may be at or near zero, they can be adjusted by moving either of the frames 21 or 22 to a new position. This may be done without affecting the magnitude of the restraining pressure. The depth of the cavities can be monitored by the position probes 31.
Figure 3 shows an alternative embodiment of the present invention. It is a schematic and partial view of the upper belt assembly. Although the Figure 3 shows only partially the upper belt assembly, a similar arrangement can be illustrated for the lower assembly as well. In this embodiment of the invention the belt 41 is made of metal which is electrically conductive and is connected to a generator of high frequency heating energy by means of a contact bar 42. The metal belts function as the heating electrodes.
A sliding electrical connection to the metal belt 41 is shown provided by means of a contact bar 42. The pressure between the sliding electrode belt 41 and the respective contact bar 42 is generated by the gas pressure within the pressurized cavity 43. The electrode 24 in Figure 2 becomes pressing plate 44. However, the pressing plate 44 may also be directly connected to the high frequency generator, since there may be an occasional contact between the belts and the pressing plates.
In a further embodiment of the present invention the previous two embodiments are combined. In this arrangement one electrode assembly, preferably the high voltage electrode assembly, comprises 2098~20 an electrode 24 and a dielectric belt 29 transparent to high frequency heating energy as shown in Figure 2. The opposing electrode assembly, which can be the grounded electrode assembly, comprises a pressing plate 44 and a metal belt 41, as illustrated in Figure 3. The metal belt can serve as the main means of transportation through the heating device. The dielectric belt may not have to be driven. The operation of the heating device according to this embodiment of the invention is the same as described previously.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, ma~y alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
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METHOD AND APPARATUS FOR FORMING
CONPOSIT~ WOOD PRODUCTS
This invention relates to a method and apparatus for continuous making of compressed products from wood that involves conditioning by high frequency, followed by compression and heat stabilization. In the conditioning stage a composite mat restrained by external pressure is plasticized by com~ination of heat and moisture while under pressure and while being transported through 1~ the heating device. The plasticized mat is subsequently formed into densified product that may include moulding into a variety of shapes by continuous pressing. The compressed mat is heat stabilized within the press or in a heating device following the pressing cycle.
Wood composite produ~ts are being produced in a variety of shapes and sizes, ranging from the well known panel products to substitutes for large size structural lumber. These products have in many respects properties superior to solid wood. They have lower variability of strength properties and improved stability. The present art of wood composite manufacturing teaches a number of continuous methods which utilize high frequency heating in the production of simple flat or rectangular shapes. Examples of such methods are disclosed by Lamberts et al., US Patent No. 4,221,950, Neubauer et al, ~S Patent No. 4,420,3S7 and Churchland, US Patent No 4,456,498. Production of densified wood and products of more complex forms, such as in case of formed panels and deep embossing, can benefit from high degree of plasticization of the composite assembly before pressing. In the plasticized state the assembly is 3~ not only much easier to compress and mould, but very importantly, the possibility of seriously damaging the wood structure is substantially reduced. The apparatus and methods disclosed by the - -~ 2098~20 known art do not allow high plasticization of the wood assembly prior to pressing.
Components of a wood cell undergo characteristic changes when exposed to higher temperatures and moisture. These changes are of prime importance in the manufacture of formed wood composites. It is well kno~ln that the elastic properties of wood are dramatically modified as the wood components go through the glass transition phase and change from a stiffer to a softer state. The amount of plasticization is a function of moisture content, temperature, heating rate, and the presence of softening agents. The degree to which the individual components are plasticized at a particular set of conditions varies. The accepted values of temperatures at which the wood components can be considered plasticized are as follows:
Wood component Dry TemP. Temp. at Saturation Amorphous Cellulose 220 C Room Hemicellulose 180 C Room Lignin 160 C 100 C
The transition temperatures at conditions encountered in practical situations will be between the two extreme values indicated above.
In conventional wood composite manufacturing processes the wood components are at different stages of plasticization. The degree of plasticization may vary greatly throughout the elements of the assembly and the overall cross section. Non-uniformity of heating is an inherent characteristic of conventional manufacturing 3~ processes. In hot pressing the temperature is highest near the heated press platens and diminishes in the direction away from the platens. In the case of dielectric heating a temperature gradient 2 ~
will exist within the individual element as well as the whole cross-section because of preferential heating of the glue lines and ! wet areas.
When a composite assembly is compressed cold, or before the uniformity of temperature is achieved, the amount of deformation in any particular area of the assembly will depend on the stiffness of the region at that point in time. Before commencement of heating the dominating variable will be the stiffness of the wood fibre and the moisture content. When larger degree of densification is being performed, a portion of the energy needed to form the densified composite is stored in the cold compressed mat in the form of potential energy. This energy is released when the subsequent heating changes the elastic characteristics of the wood fibre. A
non-uniform heating lowers the stiffness of the heated areas and this allows some of the stored energy to be transferred from the cooler areas to the heated areas where it is relaxed through compression of the wood cells. Consequently, the areas that are receiving preferential heating will be subjected to a larger amount of compression. If the strength limit of the deformed cells is exceeded, cracks develop and further compression may result in their collapse.
Compression damage of unheated or non-uniformly heated wood fibre may arise from a differential responses of the various constituents of wood to moisture. ~s indicated previously, both amorphous cellulose and hemicellulose may be significantly plasticized at room temperature by high moisture contents. The softening temperature of lignin at saturation is around 100C. It can be expected that the wood has higher moisture content adjacent to the glue line when aqueous resins are used. Since lignin lacks the same degree of plasticity at temperatures below 100C as the 2 ~;9 ~
other constituents, the integrity of the cell wall matrix will be altered. If the wood is fully compressed at this state, the non-uniformity of plasticization can lead to structural damage to the cell walls and to crack propagation within the wall under stress.
From the point of view of quality of the product and the efficiency of the manufacturing process it is therefore very desirable to achieve a high and uniform plasticization of the entire wood matrix before subjecting it to high forming pressures. High frequency heating is useful for efficient heating and plasticization of wood composite assemblies. It produces heat by exciting water molecules in oscillating electric field. Its large advantage is due to its ability to evenly and uniformly heat the entire composite mass. The water molecules deep inside the composite billet are affected by the electric fields as much as the ones closer to the surface.
Since the method does not rely on heat transfer, the assembly does not have to be fully compressed when heated, as is the case with conventional heating. The high frequency heating method is especially suitable for products that use aqueous thermosetting glues, such as the commonly used phenol formaldehyde resins. Their application adds water in the glueline and increases efficiency of dielectric heating by increasing the conductivity of the gluelines.
Industrial high frequency heating methods are of two types:
Radio or Microwave frequency. The difference between the two types is in the frequency at which they operate and the method of applying the heating energy. Whereas radio frequency heating uses frequency typically between about 1 MHz and about 50 MHz, the microwave frequencies used for heating applications are between about 900 MHz and about 2500 MHz. A typical radio frequency heating apparatus is a pair of opposing stationary electrodes. The material to be heated is placed in the electric field between the two electrodes. Microwave heating is applied in form of propagating waves that are directed (beamed) at the material. Although radio frequency is used in illustrations of the present invention, it ; will be apparent to those skilled in the art that the microwave heating can be also used to practice the disclosed invention. Steam injection heating method may also be applied in practising the t present invention.
The method disclosed herein produces a high degree of 10 plasticization before forming. The manufacturing method combines conditioning, pressing and stabilizing steps in one manufacturing process. The apparatus disclosed herein is suitable for a continuous plasticization of wood composite assemblies and production of densified or formed wood composite products. ~
This invention pertains to a method of making a wood composite -product, comprising the steps of: a) preparing a composite assembly constructed of a plurality of wood elements and a wood bonding glue between the elements, the glue between the wood elements forming a 20 plurality of gluelines within the composite assembly; b) advancing the prepared composite assembly through a heating apparatus, the composite assembly being subjected simultaneously to a restraining external pressure and heat within the heating apparatus, heating the restrained composite assembly to the boiling temperature of the moisture within the assembly to plasticize the wood, the magnitude of the steam pressure generated by boiling the moisture within the assembly being controlled by the restraining external pressure acting on the assembly, the excess steam being allowed to migrate between and through the wood elements and to vent to the exterior ~0 of the assembly; c) advancing the heated and plasticized wood composite assembly through a continuous pressing means to achieve the desired shape, form and densification of the wood product; d) 2098~20 .
further heating the formed composite product to fully cure the glue and to stabilize the composite assembly. In the method according to the present invention the heating method can be high frequency heating method or steam injection method. The restraining external pressure in step b) can be imposed on the wood composite assembly by the heat applicators or it can be generated by gas pressure, such as steam or air, applied in the space between the heat ..
applicators and the restrained composite assembly.
. -' 10 The restraining external pressure acting on the composite assembly can be between zero to about 100 psi. The boiling temperature of the moisture within the assembly can be between about 100 C to about 150 C. The stabilizing temperature in the method according to the present invention can be between about 150 C to about 210 C. The wood bonding glue can be a thermosetting aqueous resin.
This invention further pertains to an apparatus for continuous heating of a moisture and glue containing wood composite assembly 2~ comprising: at least one high frequency heating device having upper and lower frame means, the upper frame means supporting an upper high frequency heat applicator assembly and the lower frame means supporting a lower high frequency heat applicator assembly, the upper and lower high frequency heat applicator assemblies facing each other, the wood composite assembly being located between the said upper and lower high frequency heat applicator assemblies, the high frequency applicator assemblies comprising: a) heat applicators connected to a generator of high frequency heating energy; b) lower and upper belts made from a material substantially 33 transparent to high frequency energy, the belts being located between the respective heat applicators and the wood composite assembly, the wood composite assembly being compressed between the upper and lower belts within the high frequency heating device while it is being transported through the said heating device, the compressing pressure being generated by pressure injected into the space between the belts and the heat applicators, the pressure in the space between the heat applicators and the corresponding belts forming a pressure cavity of a variable depth; d) means of pressure sealing the cavity between the heat applicators and the corresponding belts. In the apparatus according to the present invention at least one of the frame means can be movable in respect to the other frame means. The pressure sealing means in step c) can be provided by bearing blocks forming a continuous pressure tight enclosure on all sides around the applicators, the bearing blocks being slidable within the frame means in the direction perpendicular to the wide surface of the belt located between the frame means and the assembly, the bearing blocks being in continuous pressurized sliding contact with the belt, the pressurized sliding contact between the bearing blocks and the belt being achieved by gas pressure acting on the opposite faces of the bearing blocks. The pressure within the variable depth pressure cavity can be air pressure or steam pressure. The depth of the variable depth pressure cavity can be between zero and about 6" or preferably between zero and about 1". The high frequency heat applicators can be electrodes for radio frequency heating or microwave applicators.
This invention further pertains to an apparatus wherein the high frequency heating apparatus is followed by a continuous pressing apparatus, the continuous pressing apparatus compressing the heated composite assembly and providing further heating to the 3~ compressed composite assembly to cure the glue and to stabilize the compressed composite. In the apparatus according to the present invention the curing and stabilization heating of the compressed 8~
composite assembly can be done in a high frequency heating apparatus following the continuous pressing apparatus.
This invention further pertains to an apparatus for continuous heating of a wood composite assembly constructed of a plurality of wood elements and a wood bonding glue dispersed between the elements, said apparatus comprising: one or more high frequency heating devices having upper and lower frame means, the upper frame means supporting an upper electrode assembly and the lower frame means supporting a lower electrode assembly, the upper and lower electrode assemblies facing each other and at least one of the electrode assemblies being movable in respect to the other assembly, the gap between the upper and lower electrode assembly defining the working space of the heating device, the electrode assemblies comprising: a) electrodes connected to a generator of high frequency heating energy; b) endless belts made from a material substantially transparent to high frequency energy, the belts being located between the respective electrodes and the wood composite assembly, the composite assembly being compressed between ~0 the upper and lower belts within the working space of the heating device while it is being transported through the said heating device, the compressing pressure being generated by gas pressure injected into the space between the belts and the electrodes the pressure in the space between the electrodes and the corresponding belts forming a pressure cavity of a variable depth; c) means of pressure sealing the cavity between the electrodes and the belts.
This invention further pertains to an apparatus for continuous heating of a wood composite assembly constructed of a plurality of ~0 wood elements and a thermosetting glue dispersed between the elements, said apparatus comprising: one or more high frequency heating devices having upper and lower frame means, the upper frame 2 ~
means supporting an upper electrode assembly and the lower frame means supporting a lower electrode assembly, the upper and lower electrode assemblies facing each other and at least one of the electrode assemblies being movable in respect to the other assembly, the gap between the upper and lower electrode assembly defining the working space of the heating device, the electrode assemblies comprising: a) pressing plates supported by the frame means; b) endless belts made from electrically conductive material and being electrically connected to a generator of high frequency heating energy, the belts being located between the respective pressing plates and the wood composite assembly, the composite assembly being compressed between the belts of the upper and lower electrode assemblies within the working space of the heating device while the said composite assembly is being transported through the said heating device, the compressing pressure being generated by gas pressure injected into the space between the belts and the pressing plates, the pressure in the space between the pressing plates and the corresponding belts forming a pressure cavity of a variable depth; c) means of pressure sealing the cavity between the pressing plates and the belts. The pressure sealing means in step c) can be provided by bearing blocks forming a continuous pressure tight enclosure on all sides around the pressing plates, the bearing blocks being slidable within the frame means in the direction perpendicular to the wide surface of the belt located between the frame means and the assembly, the bearing blocks being in continuous pressurized sliding contact with the belt, the pressurized sliding contact between the bearing blocks and the belt being achieved by gas pressure acting on the opposite faces of the bearing blocks. In this embodiment of the present invention the 3~ pressing plates can be connected to the generator of high frequency energy. The connection between the generator of hi~h frequency heating energy and the electrically conductive belts can be by 2~9~
means of pressurized sliding contact between a conductive part of the bearing blocks and the endless belts, the conductive part of the bearing block being connected to the said generator of high frequency heating energy.
The substance and nature of thi~ invention in certain embodiments is illustrated in the following drawings. The drawings should not be interpreted as restricting the spirit or scope of the invention in any way:
Figure 1 is a schematic side view illustration of an apparatus for continuous production of wood composite product in accordance with the method of the present invention.
Figure 2 illustrates a schematic cross-section view of a heating apparatus according to one embodiment of the present invention.
Figure 3 illustrates a partial schematic cross-section view of a high frequency heating apparatus in accordance with the present invention.
To achieve a high and uniform plasticization, the conditioning should be carried out while the assembly is restrained and the ! restraining external pressure is closely contro-lled. Both moistureand temperature act as plasticizer for wood. Control of the temperature and moisture conditions within the assembly can be achieved by controlling the restraining external pressure acting on the assembly. Heating the moist wood assembly to near the plasticizing temperature in the absence of restraining external pressure results in a free evaporation of moisture from the mat and pre-cured gluelines. If, on the other hand, the mat is subjected to controlled restraining pressure, heating does not result in excessive loss of moisture from the mat until the internally formed steam pressure reaches levels at which it can overcome the restraining pressure and vent to the exterior of the assembly. In normal manufacturing processes the amount of moisture present within the assembly is usually sufficiently high to produce saturated steam conditions in the interior of the assembly. The relationship between saturated steam pressure and temperature is well known and documented. In the present invention this relationship is used to control the plasticizing conditions within the wood assembly. Since the external restraining pressure controls the pressure of the saturated steam within the assembly, it also controls the boiling temperature of the moisture within the assembly. During heating the temperature will be increasing until the corresponding steam pressure can overcome the restraining pressure and vent to the exterior at the rate that balances the rate at which the steam is generated. A balance boiling temperature of the moisture is established. The desired temperature range for plasticization according to the present method is between about 100 C and about 150 C. The preferred temperature range is between about 120 C and about 140 C. The relatively low restraining pressures are between zero and about 100 psi. In addition, since the rate of moisture evaporation is also controlled, the curing rate of the glue system is also controlled. The typical glue systems used in the production of wood composite products, which may be aqueous thermosetting resins such as phenol formaldehyde glues, generally require dissipation of moisture from the gluelines to complete the curing reactions. Delay of moisture evaporation retards curing of the glue.
The method according to the present invention has three stages. The first is a plasticizing and conditioning staye. The second is a compression and product forming stage. The third is a stabilization stage.
209852~
The conditioning is carried out under relatively low external pressure. During the initial phase the temperature of the restrained assembly is raised to the desired plasticizing temperature of between about 100 C and about 150 C. The mobility of the liquid glue is initially increased by temporally lowered viscosity, and is assisted by the elevated steam pressure. Glue penetrates into the fissures and flaws in the wood substrate so these can be sealed and repaired in the compression stage. The pressure and temperature of the saturated steam is determined by the magnitude of the external pressure and the heating rate that is applied. It should be apparent to those skilled in the art that to achieve saturated steam conditions and the desired level of control during this stage requires sufficient amount of moisture within the assembly. The total initial moisture content can be between about 8% and about 25~ and can be partly present in the wood and partly in the glueline.
As is well known, the boiling point of a liquid is a function of the vapor pressure and it is constant for pure liquids boiling at a constant vapor pressure. For complex liquids, such as aqueous glues, the boiling point may differ from that of pure water and does not necessary remain constant at constant vapor pressure because of the chemical and viscosity changes taking place throughout the heating process. However, an increase of temperature throughout the boiling stage will be at slower rate. Heating of the assembly should be at higher rate and should be uniform. As indicated previously the high frequency heating methods can achieve rapid and uniform heating.
The conditioning stage is terminated when the excess moisture has been dissipated at the plasticizing temperature and the glue system is about to undergo the curing reaction. The mat is :
2098~20 plasticized and the compression stage should follow with minimum delay.
In the compression stage the heated and plasticized mat is compressed to the final density and desired form. The pressures required for the final compression of plasticized wood are generally much lower than the pressures required in cold pressing processes. The high degree and the uniformity to which the composite assembly is plasticized prevents compression damage~
reduces variations in density and allows high levels of densification as well as forming into a variety of shapes.
Figure 1 schematically illustrates a continuous production system useful in practising the method according to the present invention. It should be obvious to those skilled in the art that this method can be also applied in a stationary, non-continuous pressing apparatus equipped with high frequency heating. Such an arrangements is considered within the scope of the present invention.
As shown in Figure 1 the composite assembly 11 is carried through the conditioning heating device 12-between the lower endless belt 13 and the upper endless belt 14. Within the conditioning device 12 the assembly is subjected simultaneously to restraining pressure and heat according to the above description of the conditioning method. The plasticized and conditioned mat exits the conditioning device and is without delay compacted and formed in the continuous pressing apparatus 15. The pressing apparatus can be heated to provide the heat stabilization following full compression. However, it may be more convenient to apply the stabilization in a after-heating device 16. Even if after-heating is used to heat-stabilize the composite, the continuous pressing o apparatus may be heated to about the exit temperature from the conditioning device. The dwell time in the press at the conditioning temperature will sufficiently pre-cure the glue in the billet. The product will retain its shape after exiting from the continuous press. The final stabilization may be carried out in the stabilizing heating device 16. Restraining pressure can be applied also during the stabilizing stage. It was found however, that the restraining pressure may not be needed in the stabilizing stage because large portion of the moisture has been dissipated from the billet in the conditioning stage and the steam pressures remain lower. Furthermore, a full glueline strength is achieved in the early stages of stabilization.
As indicated above, the stabilizing stage can be carried out at any time after mat consolidation is achieved within the press or following the pressing cycle. In the stabilizing stage the temperature of the billet is elevated above the curing temperature of the glue. The final temperature can be higher than the minimum needed for thermosetting the glue to further increase the stability of the product. Main wood components, and especially the glue-like polymer lignin, are thermoplastic. To achieve improved stability, lignin should~ attain a high degree of flowability during the stabilizing stage. The preferred range of temperatures used in the stabilizing stage is between about 150 C and about 210 C.
Temperatures over 210 C are not recommended because of the possibility of damaging thermal degradation of the wood constituents. Prolonged heating periods should also be avoided.
Applying constant and controlled pressure to the wood assembly moving through the heating and conditioning device 12 or 16 in Figure 1 is not practical with the existing high fre~uency heating equipment. As described by Lamberts et al. and Neubauer et al. in 2~98~2~
the previously mentioned US Patents, the existing art does not allow any measurable pressure between the high frequency heat applicators and the moving assembly or the transporting belts. The Lamberts and Neubauer's methods require either a gap between the electrode and the assembly, or the transporting belts are allowed to be only grazingly contacting the electrodes. The heated wood assembly can not be adequately restrained to carry out the method of the present invention. ~ measurable contact pressure would result in a large drag, making the transportation through the heating device difficult or impossible. The heating device according to the present invention allows heating of pressure restrained wood assembly, while it is being continuously transported through a heating device.
The schematic cross-section shown in Figure 2 is a depiction of one embodiment of the present invention. It shows a cross section of the heating and conditioning device according to the present invention having a lower and an upper electrode assemblies, the lower electrode assembly comprising frame 21, electrode 23 and lower belt assembly comprising lower belt 27 and bearing blocks 28.
The upper electrode assembly is indicated as being of similar construction as the lower assembly comprising-frame 22, electrode 24 and upper belt assembly comprising upper belt 29 and bearing blocks 30. The bearing blocks 28 and 30 form a continuous enclosure on all sides around the applicators 23 and 24. In this embodiment of the invention the belts 27 and 29 are made from a dielectric material which is substantially transparent to high frequency energy. Position of the belt assemblies is monitored by probes 31 shown schematically. The composite assembly 32 being conditioned is located between the upper belt 29 and lower belt 27. The area of contact between the composite material and the belt assemblies is defined as the heating area further defining the working space of 2098~2~
.he heating device. One or both electrode assemblies are movable in respect to each other such as to open or close the distance between the respective upper and lower electrodes. The electrodes 23 and 24 are connected to a source of high frequency heating energy.
Flexible sealing means 33 and 34, which may be located on the bearing blocks 28 and 30, are sealing the cavities 35 and 36 to allow their pressurization. The bearing blocks 28 and 30 are shown movable within their respective locating seats such that they remain in pressure contact with the belts 27 and 29 while the depth of the cavities 35 and 36 may vary between zero and a predetermined maximum value. The bearing blocks 28 and 30 can be made of electrically non-conductive material that has low friction characteristics. A number of such materials are available in the industry. Belts 27 and 29 are shown separated from the respective electrodes 23 and 24 by the pressurized cavities 35 and 36. The space 43 between the bearing blocks 28 and 30 and the respective frames 21 and 22 can be connected with the respective pressurized cavities 35 and 36 or can be pressurized independently to provide additional flexibility for the use of the apparatus.
During heating the cavities 35 and 36 are pressurized with air, steam or other gas or fluid. The pressure within the cavities functions as the restraining external pressure for controlling the plasticizing and stabilizing conditions within the assembly, as discussed previously. In addition, the gas pressure lowers or eliminates the direct contact pressure between the electrodes and the respective belts, thus making the movement of the belts in respect to the electrodes frictionless. A small amount of frictional drag will exist as a result of the contact between the bearing frames 30 and 28 and the respective belts. Naking the bearing frames from a suitable low friction material will minimize the frictional resistance to transporting motion. Allowing the bearing frames 28 and 30 to be movable in respect to the main frames 21 and 22 in the direction perpendicular to the wide face of the belts and the composite assembly will allow the external restraining pressure to remain constant even though the composite assembly may compress as it is heated. It may be important from the operational point o~ view to maintain the pressurized cavities 35 and 36 at a constant depth, which may be at or near zero, they can be adjusted by moving either of the frames 21 or 22 to a new position. This may be done without affecting the magnitude of the restraining pressure. The depth of the cavities can be monitored by the position probes 31.
Figure 3 shows an alternative embodiment of the present invention. It is a schematic and partial view of the upper belt assembly. Although the Figure 3 shows only partially the upper belt assembly, a similar arrangement can be illustrated for the lower assembly as well. In this embodiment of the invention the belt 41 is made of metal which is electrically conductive and is connected to a generator of high frequency heating energy by means of a contact bar 42. The metal belts function as the heating electrodes.
A sliding electrical connection to the metal belt 41 is shown provided by means of a contact bar 42. The pressure between the sliding electrode belt 41 and the respective contact bar 42 is generated by the gas pressure within the pressurized cavity 43. The electrode 24 in Figure 2 becomes pressing plate 44. However, the pressing plate 44 may also be directly connected to the high frequency generator, since there may be an occasional contact between the belts and the pressing plates.
In a further embodiment of the present invention the previous two embodiments are combined. In this arrangement one electrode assembly, preferably the high voltage electrode assembly, comprises 2098~20 an electrode 24 and a dielectric belt 29 transparent to high frequency heating energy as shown in Figure 2. The opposing electrode assembly, which can be the grounded electrode assembly, comprises a pressing plate 44 and a metal belt 41, as illustrated in Figure 3. The metal belt can serve as the main means of transportation through the heating device. The dielectric belt may not have to be driven. The operation of the heating device according to this embodiment of the invention is the same as described previously.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, ma~y alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
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Claims (25)
1. A method of making a wood composite product, comprising the steps of:
a) preparing a composite assembly constructed of a plurality of wood elements and a wood bonding glue between the elements, the glue between the wood elements forming a plurality of gluelines within the composite assembly;
b) advancing the prepared composite assembly through a heating apparatus, the composite assembly being subjected simultaneously to a restraining external pressure and heat within the heating apparatus, heating the restrained composite assembly to the boiling temperature of the moisture within the assembly to plasticize the wood, the magnitude of the steam pressure generated by boiling the moisture within the assembly being controlled by the restraining external pressure acting on the assembly, the excess steam being allowed to migrate between and through the wood elements and to vent to the exterior of the assembly;
c) advancing the heated and plasticized wood composite assembly through a continuous pressing means to achieve the desired shape, form and densification of the wood product;
d) further heating the formed composite product to fully cure the glue and to stabilize the composite assembly.
a) preparing a composite assembly constructed of a plurality of wood elements and a wood bonding glue between the elements, the glue between the wood elements forming a plurality of gluelines within the composite assembly;
b) advancing the prepared composite assembly through a heating apparatus, the composite assembly being subjected simultaneously to a restraining external pressure and heat within the heating apparatus, heating the restrained composite assembly to the boiling temperature of the moisture within the assembly to plasticize the wood, the magnitude of the steam pressure generated by boiling the moisture within the assembly being controlled by the restraining external pressure acting on the assembly, the excess steam being allowed to migrate between and through the wood elements and to vent to the exterior of the assembly;
c) advancing the heated and plasticized wood composite assembly through a continuous pressing means to achieve the desired shape, form and densification of the wood product;
d) further heating the formed composite product to fully cure the glue and to stabilize the composite assembly.
2. The method according to claim 1 wherein the heat is generated by high frequency heating method.
3. The method according to claim 1 wherein the restraining external pressure in step b) is imposed on the wood composite assembly by the heat applicators.
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4. The method according to claim 1 wherein the restraining external pressure in step b) is produced by steam pressure applied in the space between the heat applicators and the restrained composite assembly.
5. The method according to claim 1 wherein the restraining external pressure acting on the composite assembly in step b) is achieved by air pressure applied in the space between the heat applicators and the restrained composite assembly.
6. The method according to claim 1 wherein the restraining external pressure acting on the composite assembly in step b) is between zero to about 100 psi.
7. The method according to claim 1 wherein the boiling temperature of the moisture within the assembly is between about 100 °C to about 150 °C.
8. The method according to claim 1 wherein the stabilizing temperature in step d) is between about 150 °C to about 210 °C.
9. The method according to claim 1 wherein the wood bonding glue is a thermosetting aqueous resin.
10. An apparatus for continuous heating of a moisture and glue containing wood composite assembly comprising: at least one high frequency heating device having upper and lower frame means, the upper frame means supporting an upper high frequency heat applicator assembly and the lower frame means supporting a lower high frequency heat applicator assembly, the upper and lower high frequency heat applicator assemblies facing each other, the wood composite assembly being located between the said upper and lower - Page 2 of Claims -high frequency heat applicator assemblies, the high frequency applicator assemblies comprising:
a) heat applicators connected to a generator of high frequency heating energy;
b) lower and upper belts made from a material substantially transparent to high frequency energy, the belts being located between the respective heat applicators and the wood composite assembly, the wood composite assembly being compressed between the upper and lower belts within the high frequency heating device while it is being transported through the said heating device, the compressing pressure being generated by pressure injected into the space between the belts and the heat applicators, the pressure in the space between the heat applicators and the corresponding belts forming a pressure cavity of a variable depth;
d) means of pressure sealing the cavity between the heat applicators and the corresponding belts.
a) heat applicators connected to a generator of high frequency heating energy;
b) lower and upper belts made from a material substantially transparent to high frequency energy, the belts being located between the respective heat applicators and the wood composite assembly, the wood composite assembly being compressed between the upper and lower belts within the high frequency heating device while it is being transported through the said heating device, the compressing pressure being generated by pressure injected into the space between the belts and the heat applicators, the pressure in the space between the heat applicators and the corresponding belts forming a pressure cavity of a variable depth;
d) means of pressure sealing the cavity between the heat applicators and the corresponding belts.
11. An apparatus according to claim 10 wherein at least one of the frame means is movable in respect to the other frame means.
12. An apparatus according to claim 10 wherein the pressure sealing means in step c) is provided by bearing blocks forming a continuous pressure tight enclosure on all sides around the applicators, the bearing blocks being slidable within the frame means in the direction perpendicular to the wide surface of the belt located between the frame means and the assembly, the bearing blocks being in continuous pressurized sliding contact with the belt, the pressurized sliding contact between the bearing blocks and the belt being achieved by gas pressure acting on the opposite faces of the bearing blocks.
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13. An apparatus according to claim 10 wherein the pressure within the variable depth pressure cavity is air pressure.
14. An apparatus according to claim 10 wherein the pressure within the variable depth pressure cavity is steam pressure.
15. An apparatus according to claim 10 wherein the depth of the variable depth pressure cavity is between zero and about 6".
16. An apparatus according to claim 10 wherein the depth of the variable depth pressure cavity is between zero to about 1".
17. An apparatus in accordance with claim 10 wherein the high frequency heating apparatus is followed by a continuous pressing apparatus, the continuous pressing apparatus compressing the heated composite assembly and providing further heating to the compressed composite assembly to cure the glue and to stabilize the compressed composite.
18. An apparatus in accordance with claim 17 wherein the further heating of the compressed composite assembly is done in a high frequency heating apparatus following the continuous pressing apparatus.
19. An apparatus according to claim 10 wherein the high frequency heat applicators are electrodes for radio frequency heating.
20. An apparatus according to claim 10 wherein the high frequency heat applicators are microwave applicators.
21. An apparatus for continuous heating of a wood composite assembly constructed of a plurality of wood elements and a wood - Page 4 of Claims -bonding glue dispersed between the elements, said apparatus comprising: one or more high frequency heating devices having upper and lower frame means, the upper frame means supporting an upper electrode assembly and the lower frame means supporting a lower electrode assembly, the upper and lower electrode assemblies facing each other and at least one of the electrode assemblies being movable in respect to the other assembly, the gap between the upper and lower electrode assembly defining the working space of the heating device, the electrode assemblies comprising:
a) electrodes connected to a generator of high frequency heating energy;
b) endless belts made from a material substantially transparent to high frequency energy, the belts being located between the respective electrodes and the wood composite assembly, the composite assembly being compressed between the upper and lower belts within the working space of the heating device while it is being transported through the said heating device, the compressing pressure being generated by gas pressure injected into the space between the belts and the electrodes the pressure in the space between the electrodes and the corresponding belts forming a pressure cavity of a variable depth;
c) means of pressure sealing the cavity between the electrodes and the belts.
a) electrodes connected to a generator of high frequency heating energy;
b) endless belts made from a material substantially transparent to high frequency energy, the belts being located between the respective electrodes and the wood composite assembly, the composite assembly being compressed between the upper and lower belts within the working space of the heating device while it is being transported through the said heating device, the compressing pressure being generated by gas pressure injected into the space between the belts and the electrodes the pressure in the space between the electrodes and the corresponding belts forming a pressure cavity of a variable depth;
c) means of pressure sealing the cavity between the electrodes and the belts.
22. An apparatus for continuous heating of a wood composite assembly constructed of a plurality of wood elements and a thermosetting glue dispersed between the elements, said apparatus comprising: one or more high frequency heating devices having upper and lower frame means, the upper frame means supporting an upper electrode assembly and the lower frame means supporting a lower electrode assembly, the upper and lower electrode assemblies facing - Page 5 of Claims -each other and at least one of the electrode assemblies being movable in respect to the other assembly, the gap between the upper and lower electrode assembly defining the working space of the heating device, the electrode assemblies comprising:
a) pressing plates supported by the frame means;
b) endless belts made from electrically conductive material and being electrically connected to a generator of high frequency heating energy, the belts being located between the respective pressing plates and the wood composite assembly, the composite assembly being compressed between the belts of the upper and lower electrode assemblies within the working space of the heating device while the said composite assembly is being transported through the said heating device, the compressing pressure being generated by gas pressure injected into the space between the belts and the pressing plates, the pressure in the space between the pressing plates and the corresponding belts forming a pressure cavity of a variable depth;
c) means of pressure sealing the cavity between the pressing plates and the belts.
a) pressing plates supported by the frame means;
b) endless belts made from electrically conductive material and being electrically connected to a generator of high frequency heating energy, the belts being located between the respective pressing plates and the wood composite assembly, the composite assembly being compressed between the belts of the upper and lower electrode assemblies within the working space of the heating device while the said composite assembly is being transported through the said heating device, the compressing pressure being generated by gas pressure injected into the space between the belts and the pressing plates, the pressure in the space between the pressing plates and the corresponding belts forming a pressure cavity of a variable depth;
c) means of pressure sealing the cavity between the pressing plates and the belts.
23. An apparatus according to claim 22 wherein the pressure sealing means in step c) is provided by bearing blocks forming a continuous pressure tight enclosure on all sides around the pressing plates, the bearing blocks being slidable within the frame means in the direction perpendicular to the wide surface of the belt located between the frame means and the assembly, the bearing blocks being in continuous pressurized sliding contact with the belt, the pressurized sliding contact between the bearing blocks and the belt being achieved by gas pressure acting on the opposite faces of the bearing blocks.
24. An apparatus according to claim 22 wherein the pressing plates - Page 6 of Claims-are connected to the generator of high frequency energy.
25. An apparatus according to claim 23 wherein the connection between the generator of high frequency heating energy and the electrically conductive belts is by means of pressurized sliding contact between a conductive part of the bearing blocks and the endless belts, the conductive part of the bearing block being connected to the said generator of high frequency heating energy.
- Page 7 of Claims-
- Page 7 of Claims-
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002098520A CA2098520A1 (en) | 1993-06-16 | 1993-06-16 | Method and apparatus for forming composite wood products |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002098520A CA2098520A1 (en) | 1993-06-16 | 1993-06-16 | Method and apparatus for forming composite wood products |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2098520A1 true CA2098520A1 (en) | 1994-12-17 |
Family
ID=4151799
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002098520A Abandoned CA2098520A1 (en) | 1993-06-16 | 1993-06-16 | Method and apparatus for forming composite wood products |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2098520A1 (en) |
Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1415778A2 (en) * | 2002-11-04 | 2004-05-06 | Innotech-Betriebstechnikgesellschaft m b H | Press and method for press forming wood |
| CN101774196A (en) * | 2010-02-26 | 2010-07-14 | 李茂林 | Process method for manufacturing hard wood by high-frequency warming and solidifying |
| CN101863064A (en) * | 2010-06-04 | 2010-10-20 | 李茂林 | Process for simultaneously heating and pressing modified hard wood by using steam and high frequency double heat source |
| CN103029189A (en) * | 2012-12-03 | 2013-04-10 | 常乐融融 | High-frequency mould-pressing hot press machine |
| CZ304024B6 (en) * | 2008-12-02 | 2013-08-28 | Mendelova zemedelská a lesnická univerzita v Brne | Method of uniform areal compression of wood, especially spruce |
| EP1950016B1 (en) * | 2007-01-29 | 2016-05-04 | Gerhard Sauli | Method for manufacturing a wooden profile |
| CN110860443A (en) * | 2019-11-28 | 2020-03-06 | 安徽东平木业股份有限公司 | Splicing wood frame glue drying device |
| DE102019112634B3 (en) * | 2019-05-14 | 2020-10-15 | Siempelkamp Maschinen- Und Anlagenbau Gmbh | Device for the continuous heating of a pressed material mat |
-
1993
- 1993-06-16 CA CA002098520A patent/CA2098520A1/en not_active Abandoned
Cited By (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1415778A2 (en) * | 2002-11-04 | 2004-05-06 | Innotech-Betriebstechnikgesellschaft m b H | Press and method for press forming wood |
| EP1415778A3 (en) * | 2002-11-04 | 2008-02-20 | Innotech-Betriebstechnikgesellschaft m b H | Press and method for press forming wood |
| EP1950016B1 (en) * | 2007-01-29 | 2016-05-04 | Gerhard Sauli | Method for manufacturing a wooden profile |
| CZ304024B6 (en) * | 2008-12-02 | 2013-08-28 | Mendelova zemedelská a lesnická univerzita v Brne | Method of uniform areal compression of wood, especially spruce |
| CN101774196A (en) * | 2010-02-26 | 2010-07-14 | 李茂林 | Process method for manufacturing hard wood by high-frequency warming and solidifying |
| CN101863064A (en) * | 2010-06-04 | 2010-10-20 | 李茂林 | Process for simultaneously heating and pressing modified hard wood by using steam and high frequency double heat source |
| CN101863064B (en) * | 2010-06-04 | 2012-04-04 | 李茂林 | Process for simultaneously heating and pressing modified hard wood by using steam and high frequency double heat source |
| CN103029189A (en) * | 2012-12-03 | 2013-04-10 | 常乐融融 | High-frequency mould-pressing hot press machine |
| CN103029189B (en) * | 2012-12-03 | 2014-09-03 | 常乐融融 | High-frequency mould-pressing hot press machine |
| DE102019112634B3 (en) * | 2019-05-14 | 2020-10-15 | Siempelkamp Maschinen- Und Anlagenbau Gmbh | Device for the continuous heating of a pressed material mat |
| CN110860443A (en) * | 2019-11-28 | 2020-03-06 | 安徽东平木业股份有限公司 | Splicing wood frame glue drying device |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Discontinued |