Process for the production of OSB wood-based boards with reduced emission of volatile organic compounds (VOCs)

Abstract

A process for the production of OSB wood-based panels including: a) producing wood strands from suitable wood logs; b) treating at least part of the wood strands with steam at a temperature between 80 C. and 120 C. and a pressure between 0.5 bar and 2 bar; c) drying the steam-treated wood strands; d) gluing the steam-treated and dried wood strands and, optionally, gluing the non-steam treated wood strands with at least one binder; e) scattering the glued wood strands onto a conveyor belt; and f) pressing the glued wood strands into an OSB wood-based board. The steam treatment takes place after the wood strands have been produced and made available, or after the wood strands have been sifted and separated according to the use of the wood strands for the middle and top layers of the panel. Also, an OSB wood-based panel made using the process.

Claims

1. A process for the production of an OSB wood-based panel, with reduced emission of volatile organic compounds (VOCs) in a production line, comprising: a) producing wood strands from suitable wood; b) treating at least part of the wood strands with steam, wherein the steam is passed over the wood strands at a temperature between 80 C. and 120 C. and a pressure between 0.5 bar and 2 bar, where the wood strands have been chipped and made available, or the wood strands have been sifted and separated for use as wood strands for middle and top layers of the panel before steam treatment; c) drying of the wood strands treated with the steam; d) gluing of the steam treated and dried wood strands and optionally gluing of non-steam treated wood strands with at least one binder; e) spreading the glued wood strands onto a conveyor belt; and f) pressing of the glued wood strands into an OSB wood-based panel.

2. The process according to claim 1, wherein the steam treatment takes place outside a production line in which the spreading and pressing take place.

3. The process according to claim 2, wherein the wood strands are steam treated in a process that is separate from a process in which the drying, gluing, spreading, and pressing are conducted.

4. The process according to claim 2, wherein the steam treated and dried wood strands are stored before introduction into the production line.

5. The process according to claim 1, wherein the steam treatment of the wood strands is integrated into a production line in which the spreading and pressing take place.

6. The process according to claim 1, wherein the steam treatment is carried out in at least one steam treatment unit.

7. The process according to claim 6, wherein the steam treatment unit is operated as a batch system or continuously.

8. The process according to claim 1, wherein the steam treatment of the wood strands is carried out over a period of 5 to 30 min.

9. The process according to claim 1, wherein the steam treated and optional non-steam treated wood strands are glued with a polymer adhesive as a binder selected from the group consisting of formaldehyde adhesives, polyurethane adhesives, epoxy resin adhesives, and polyester adhesives.

10. The process according to claim 1, wherein a polyurethane adhesive based on aromatic polyisocyanates is used as a binder.

11. The process according to claim 10, wherein the steam-treated and optional non-steam treated wood strands are glued with a binder amount of 1.0 to 10 wt % based on a total amount of wood strands.

12. The process according to claim 1, wherein identical binders or different binders are used for top and middle layers of the panel.

13. The process according to claim 1, wherein the glued steam-treated and/or non-steam treated wood strands are spread onto the conveyor belt to form a first cover layer along a transport direction, then to form a middle layer transverse to the transport direction, and finally to form a second cover layer along the transport direction.

14. The process according to claim 1, wherein steam treatment of the wood strands is conducted in an oxygen-free or low-oxygen atmosphere; the steam-treated wood strands are separated into wood strands suitable for use as a middle layer and a top layer; and the glued steam treated wood strands are spread on the conveyor belt in an order of a first lower cover layer, a middle layer, and a second upper cover layer prior to pressing of the glued wood strands into the OSB wood-based panel.

15. The process according to claim 1, wherein the wood strands are sifted and separated into wood strands suitable for use as a middle layer and a top layer prior to steam treatment; treatment of the wood strands is conducted in an oxygen-free or oxygen-poor atmosphere; and the glued steam treated and optional non-steam treated wood strands are scattered onto the conveyor belt in an order of a first lower cover layer, a middle layer, and a second upper cover layer prior to pressing of the glued wood strands into the OSB wood-based panel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in more detail below with reference to the figure in the drawing provided as an example.

(2) FIG. 1 shows a schematic representation of an embodiment of a device for the steam treatment of wood strands;

(3) FIG. 2 shows a schematic representation of a first implementation form of the inventive process for the production of OSB boards; and

(4) FIG. 3 shows a schematic representation of a second form of implementation of the procedure according to the invention.

DESCRIPTION OF THE INVENTION

(5) FIG. 1 shows a variant of a device 10 for steam treatment of wood strands. The device comprises a (thermally insulated) tubular body 11 with a perforated conveyor belt 12 running through the tubular body 11. Above the conveyor belt 12 a supply line 13 for the steam is provided, the supply line 13 having a plurality of nozzles 14 for spraying the steam onto the wood strands located on the conveyor belt 12. Below conveyor belt 12, a collecting medium 15 is provided for the steam condensate that forms.

(6) The present device 10 enables a steam treatment of the strands even before the drying of the strands in the drum dryer. All or parts of the strands, which are intended for the later production of the OSB, can be treated. The treatment takes place at normal pressure and achieves the homogeneous treatment of the strands with steam by using the conveyor belt 12 made of metal mesh.

(7) The conveyor belt is dimensioned so that the strands cannot fall through the gaps. The strands are transferred directly after production to the conveyor belt 12, which runs through the tube body 11. The strands are distributed on conveyor belt 12 in such a way that steam can flow homogeneously through the strands. Separation stations positioned at regular intervals ensure the dissolution of existing or emerging strand piles.

(8) Since the conveyor belt 12 is cooled by condensate formed when the strands are heated and has a relatively high mass compared to the strands, the belt must be heated before the strands are scattered. This accelerates the heating of the strands and thus reduces the treatment time with steam. The heating can take place by resistance heating or by radiation.

(9) Steam is then applied to the strands from above through nozzles 14. The steam has a temperature of approx. 100 C. The insulation of the metal tube 11 ensures that the heat losses are as low as possible. The condensate formed is collected under the transport device 12, freed of suspended particles and returned to the system after a cleaning step to remove dissolved substances.

(10) The residence time of the strands in the saturated steam atmosphere is 5 to 15 minutes. At regular intervals the progress of the strand heating is determined by thermal sensors. The temperature of the strands should be close to 90 C. at the end of the treatment.

(11) The first embodiment of the process according to the invention shown in FIG. 2 describes the individual process steps starting with the provision of the wood starting product up to the finished OSB wood-based board.

(12) Accordingly, suitable wood starting material for the production of the wood strands is first provided in step 1. All softwoods, hardwoods or mixtures of these are suitable as wood starting materials.

(13) The debarking (step 2) and chipping (step 3) of the wood starting material takes place in suitable chippers, whereby the size of the wood strands can be controlled accordingly. After shredding and provision of the wood strands, they may be subjected to a pre-drying process, where a moisture content of 5-10% with respect to the initial moisture content of the wood strands is set (not shown).

(14) In the case of the embodiment shown in FIG. 2, the wood strands are introduced into a steam treatment device (step 4). The steam treatment of the wood strands takes place in a temperature range between 80 and 120 C. at a pressure between 0.5 bar and 2 bar. The resulting condensate can be collected and the substances washed out of the wood strands (terpenes, aldehydes) can be used for further applications.

(15) After completion of the steam treatment, which in this case takes approx. 10-20 minutes, the steam-treated wood strands are dried (step 9), classified and separated (step 5).

(16) Separation takes place in wood strands for use as a middle layer (step 6a) or as a top layer (step 6b) with respective gluing.

(17) The glued, steam-treated wood strands are scattered on a conveyor belt in the sequence first lower top layer, middle layer and second upper top layer (step 7) and then pressed into an OSB wood-based board (step 8).

(18) In the second embodiment shown in FIG. 3, the wood starting material is first provided (step 1), debarked (step 2) and chipped (step 3) in analogy to FIG. 1. If necessary, the wood strands are subjected to a pre-drying process, whereby a moisture content of 5-10% is set in relation to the initial moisture content of the wood strands (step 3a).

(19) In contrast to the embodiment variant of FIG. 2, the optional drying is followed by separation in wood strands for use as a middle layer or as a top layer (step 5).

(20) This is followed by steam treatment of the wood strands intended for the middle layer (step 4a) and/or steam treatment of the wood strands intended for the top layer(s) (step 4b) in a suitable steam treatment device. The steam treatment of the wood strands is carried out in a temperature range between 80 and 120 C. at a pressure between 0.5 bar and 2 bar. The resulting condensate can be collected and the substances washed out of the wood strands (terpenes, aldehydes) can be used for further applications.

(21) It is also possible that only the wood strands for the middle layer are subject to steam treatment, while the wood strands for the top layers remain untreated.

(22) After completion of the steam treatment, which in this case takes approx. 10-20 minutes, the steam-treated wood strands are dried (steps 9a, 9b) and glued (steps 6 a, b).

(23) The glued, water-treated wood strands are scattered on a conveyor belt in the sequence first lower top layer, middle layer and second upper top layer (step 7) and then pressed into an OSB wood-based board (step 8).

(24) In the finishing process, the OSB wood-based board obtained is assembled in a suitable manner.

Embodiment Example 1

(25) Strands are produced from pine trunks (length: max. 200 mm, width: 20 mm, thickness: max. 1 mm, humidity max. 50%) and treated in a continuous process with steam at a temperature of about 100 C. The strands are then heated to a temperature of about 100 C. During the treatment, the strands are loosely piled up on a conveyor belt which has perforations and thus permits the passage of steam after passing through the strands. The steam treatment was preferably carried out from top to bottom. The conveyor belt is guided through a tubular body. Preferably, nozzles are installed above the conveyor belt with the loosely poured strands, which distribute the steam evenly over the strands. The treatment with steam takes about 15 minutes. The steam treatment is carried out with the exclusion of oxygen as far as possible, so that one can speak of reductive process control.

(26) The tube body has a diameter of 50 cm and a length of 3 m to achieve a residence time of about 15 minutes. The conveyor belt was moved through the tube body at a speed of about 2 m/10 minutes. In the feed direction, the tube body was slightly angled upwards (2 to 10 degrees), so that the condensate formed could easily be trapped. This is a test plant with which the effect should be proven. For a production line, it can be enlarged and easily optimized by a specialist in terms of transport speed and quantity.

(27) The strands are then dried in a conventional drum dryer. The energy requirement of the drum dryer is significantly reduced, as the strands already have a temperature of around 90 C. when they enter the dryer. Then they are glued in a coil with adhesive, preferably with PMDI (approx. 3 wt % on atro wood).

(28) The glued stands are scattered as top and middle layers in a standard OSB plant. The percentage distribution between the middle and top layers is preferably 70% to 30%. The strands are pressed into boards with a bulk density of about 570 kg/m.sup.3. After a storage period of one week, the test plate together with a standard plate of the same thickness was tested for VOC release in a microchamber.

(29) Chamber parameters: Temperature: 23 C.; Humidity: 0%; Air flow rate: 150 ml/min; Air change: 188/h; Loading: 48.8 m.sup.2/m.sup.3; Sample surface: 0.003 m.sup.2; Chamber volume: 48 ml. The values of the most important parameters in terms of quantity are shown in Table 1.

(30) TABLE-US-00001 TABLE 1 Experimental plate Standard plate parameter [g/m.sup.2 h] [g/m.sup.2 h] hexanal 194 1474 3-carene 208 626 -pinene 181 925 Pentanal 155 -pinene 285 2-octenal 60 115

(31) As can be seen from the results, the emissions of the quantitatively most important parameters are significantly reduced. Some parameters could no longer be detected. Surprisingly, this also applies to the saturated and unsaturated aldehydes, which, according to the assumed correct and plausible path of formation, should only form in the press at high temperatures. This means that either the previously assumed mechanism for the formation of aldehydes is incorrect or that the aldehyde precursors are chemically converted during steam treatment in such a way that the formation of aldehydes is only possible to a limited extent. Originally it was expected that steam treatment would only reduce terpene emissions. These would be expelled in the process as in steam distillation.

Embodiment Example 2

(32) Corresponds to example 1, but in contrast only strands for the middle layer (about 70% of OSB) were treated with steam, with the following result of the VOC test:

(33) Chamber parameters: Temperature: 23 C.; Humidity: 0%; Air flow rate: 150 ml/min; Air change: 188/h; Loading: 48.8 m2/m3; Sample surface: 0.003.sup.m2; Chamber volume: 48 ml. The values of the most important parameters in terms of quantity are shown in Table 2.

(34) TABLE-US-00002 TABLE 2 Experimental plate Standard plate Parameter [g/m.sup.2 h] [g/m.sup.2 h] hexanal 243 1474 3-carene 299 626 -pinene 178 925 pentanal 155 -pinene 285 2-octenal 61 115

(35) Here, too, a significant reduction in emissions can be observed, although the top layer consists of standard strands.