Process for producing wood-based OSBs having reduced emission of volatile organic compounds (VOCs)

Abstract

A process for treating wood strands suitable for producing OSBs includes treating the wood strands in water at a temperature in the range from 50 C. to 100 C. The present invention likewise relates to a process for producing wood-based OSBs, in particular wood-based OSBs having reduced emission of volatile organic compounds (VOCs), including: a) production of wood strands from suitable timbers; b) treatment of at least part of the wood strands with water; c) drying of the wood strands which have been treated with the water; d) coating of the wood strands which have been treated with water and dried and coating of wood strands which have not been treated with water with at least one binder; e) scattering of the glue-coated wood strands on a conveyor belt; and f) pressing of the glue-coated wood strands to give a wood-based OSB.

Claims

1. A process for producing wood-based OSBs comprising: a) production of wood strands from suitable timbers; b) treatment of at least part of the wood strands with water having a temperature in the range from 50 C. to 100 C. for a time of from 10 to 60 minutes; c) drying of the wood strands which have been treated in the water; d) coating of the wood strands which have been treated with water and dried and optionally coating of wood strands which have not been treated with water with at least one binder; e) scattering of the binder-coated wood strands on a conveyor belt; and f) pressing of the binder-coated wood strands to give a wood-based OSB.

2. The process according to claim 1, wherein water-treated wood strands or a mixture of water-treated wood strands and wood strands which have not been treated with water are used as middle layer and/or covering layer of the wood-based OSB.

3. The process according to claim 1, wherein the step of treatment of the wood strands with water is carried out separately from the production process for the OSBs or is integrated into the production process for the OSBs.

4. The process according to claim 1, wherein the wood strands which have been treated with water and have not been treated with water are coated with an amount of binder of from 1.0 to 5.0% by weight based on the total amount of the wood strands.

5. The process according to claim 1, wherein the binder-coated wood strands are pressed at temperatures in the range from 200 to 250 C. to give the wood-based OSB.

6. The process according to claim 1, wherein the wood strands which have been treated with water and have not been treated with water are coated with an amount of binder of from 2 to 4% by weight based on the total amount of the wood strands.

7. The process according to claim 1, wherein the binder-coated wood strands are pressed at temperatures in the range from 220 to 230 C. to give the wood-based OSB.

8. The process according to claim 1, wherein treatment of the wood strands are treated with water having a temperature in the range from 60 to 90 C.

9. The process according to claim 1, wherein treatment of the wood strands are treated with water having a temperature in the range from 70 to 90 C.

10. The process according to claim 1, wherein the treatment of the wood strands with water is carried out for a time of from 15 to 45 minutes.

11. The process according to claim 1, wherein the treatment of the wood strands with water is carried out for a time of from 20 to 30 minutes.

12. The process according to claim 1, wherein the wood strands are covered completely by water during the treatment.

13. The process according to claim 12, wherein the wood strands are completely immersed in the water.

14. The process according to claim 1, wherein the wood strands are conveyed continuously on at least one transport device through an apparatus containing the water for the water treatment.

15. The process according to claim 14, comprising discharging the water from the apparatus after passage of the wood strands.

16. The process according to claim 15, comprising purifying the water discharged from the apparatus and recirculating the purified water into the apparatus.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be illustrated below with reference to the figures of the drawing for a working example. The drawing shows:

(2) FIG. 1 a schematic depiction of one embodiment of an apparatus for treatment of wood strands with water;

(3) FIG. 2 a schematic depiction of a first embodiment of the process of the invention for producing OSBs, and

(4) FIG. 3 a schematic depiction of a second embodiment of the process of the invention.

DESCRIPTION OF THE INVENTION

(5) FIG. 1 shows one variant of an apparatus (water contact unit) 10 for treatment of wood strands with water. The apparatus comprises an immersion tank 11 filled with water and a transport facility in the form of three transport rollers 12.

(6) The present apparatus 10 allows treatment of the strands with water before drying of the strands in the drum dryer. Here, all or part of the strands which are provided for later production of the OSB can be treated.

(7) The immersion tank 11 has a water inlet 13 and a water outlet 14. The water discharged through the water outlet 14 can be subjected to a purification step (e.g. filtration or precipitation of impurities) in a purification apparatus 15, heated at the same time and recirculated via the water inlet 13 back into the immersion tank.

(8) The use of a holder-down 16 ensures that the wood strands are completely under water and thus completely surrounded by water.

(9) The introduction and discharge of the OSB strands into and from the immersion tank 11 are carried out in a suitable way via slopes 17, 18 which are provided at a suitable angle in the immersion tank 11. The perforated metal plates make it possible for water to be separated off from the strands.

(10) The first embodiment of the process of the invention, as shown in FIG. 2, describes the individual process steps commencing with the provision of the wood starting material through to the finished wood-based OSB.

(11) Accordingly, suitable wood starting material for producing the wood strands is firstly provided in step 1. All coniferous timbers, timbers from broad-leaved trees or else mixtures thereof are suitable as wood starting material.

(12) The debarking (step 2) and the shredding (step 3) of the wood starting material is carried out in shredders suitable for this purpose, with the size of the wood strands being able to be controlled appropriately. After comminution and provision of the wood strands, these are optionally subjected to a predrying process, with a moisture content of 5-10% of the initial moisture content of the wood strands being set (not shown).

(13) In the case of the embodiment shown in FIG. 2, the wood strands are introduced into a water treatment apparatus (step 4). The water treatment of the wood strands is carried out using hot water having a temperature in the range from 70 C. to 90 C.

(14) The washing water arising here can be discharged from the process and, after one or more purification steps, fed back to the water treatment and thus be recycled.

(15) After conclusion of the water treatment, which in the present case takes about 15-30 minutes, the water-treated wood strands are dried (step 9), screened and separated (step 5).

(16) Separation is carried out into wood strands for use as middle layer (step 6a) or as covering layer (step 6b) with coating with glue in each case.

(17) The glue-coated water-treated wood strands are scattered onto a conveyor belt in the order of first lower covering layer, middle layer and second upper covering layer (step 7) and subsequently pressed to give a wood-based OSB (step 8).

(18) In the second embodiment shown in FIG. 3, the wood starting material is, in a manner analogous to FIG. 1, firstly provided (step 1), debarked (step 2) and shredded (step 3). The wood strands are optionally subjected to a predrying process, with a moisture content of 5-10% of the initial moisture content of the wood strands being set (step 3a).

(19) In contrast to the variant shown in FIG. 2, separation into wood strands for use as middle layer or as covering layer (step 5) occurs as early as after the optional drying.

(20) This is followed by the water treatment of the wood strands provided for the middle layer (step 4a) and/or water treatment of the wood strands provided for the covering layer(s) (step 4b) in a suitable water treatment apparatus in each case. The treatment of the wood strands with water is carried out in a temperature range from 70 C. to 90 C.

(21) It is also possible for only the wood strands for the middle layer to be subjected to a treatment with water, while the wood strands for the covering layers remain untreated.

(22) After conclusion of the water treatment, which in the present case takes about 15-30 minutes, the water-treated wood strands are dried (step 9a, 9b) and coated with glue (steps 6a, b).

(23) The glue-coated water-treated wood strands are scattered onto a conveyor belt in the order first lower covering layer, middle layer and second upper covering layer (step 7) and subsequently pressed to give a wood-based OSB (step 8).

(24) In the final processing step, the wood-based OSB obtained is in each case finished in a suitable manner.

Working Example 1

(25) Strands (length: max. 200 mm, width: 20 mm, thickness: max. 1 mm, moisture content: max. 50%) are produced from pine logs and treated with water having a temperature of 90 C. in a continuously operating process. The treatment is carried out in a vessel through which the strands are slowly transported. Holders-down ensure that the strands are completely under water. The treatment time/residence time in the tank is about 15 min.

(26) The water present in the tank is continuously discharged from the process and freed of suspended materials and fine particles by filtration. Furthermore, the dissolved constituents are precipitated and the water is subsequently returned to the process. The discharged water had a COD (chemical oxygen demand) of 700 mg/l. It is therefore only moderately polluted with organic compounds and can be purified relatively easily.

(27) The strands are then dried in a drum dryer. The energy consumption of the drum dryer is reduced significantly since the strands already have a temperature of about 70 C. on entry into the dryer. They are then coated in a coil with PMDI (about 3% by weight of glue based on dry matter of strands).

(28) The glue-coated strands are scattered as covering layer and middle layer in an OSB plant. The percentage distribution between middle layer and covering layer is at least 70% to 30%. The strands are pressed to give boards which have a bulk density of about 570 kg/m.sup.3. After a storage time of about one week, the test board was tested together with a standard board having the same thickness for release of VOCs in a microchamber.

(29) Chamber Parameters:

(30) Temperature: 23 C.; humidity: 0%; airflow: 150 ml/min; air change: 188/h; loading: 48.8 m.sup.2/m.sup.3; specimen surface area: 0.003 m.sup.2; chamber volume: 48 ml.

(31) The values for the most important parameters in terms of amount are shown in the following table.

(32) TABLE-US-00001 Test board Standard board Parameter [g/m.sup.2 h] [g/m.sup.2 h] Hexanal 304 510 3-Carene 220 461 -Pinene 361 588 Pentanal 64 119 -Pinene 214 425 Acetone 52 95

(33) As can be seen from the table, a significant reduction in the emission is observed for the most important parameters. The values are reduced to about half.

Working Example 2

(34) Strands (length: max. 200 mm, width: 20 mm, thickness: max. 1 mm, moisture content: max. 50%) are produced from pine logs and treated with water having a temperature of 70 C. in a continuously operating process. The treatment is carried out in a vessel through which the strands are slowly transported. Holders-down ensure that the strands are completely under water. The treatment time/residence time in the tank is about 30 min.

(35) The water present in the tank is continuously discharged from the process and freed of suspended materials and fine particles by filtration. Furthermore, the dissolved constituents are precipitated and the water is subsequently returned to the process. The discharged water had a COD (chemical oxygen demand) of 545 mg/l. It is therefore only moderately polluted with organic compounds and can be purified relatively easily.

(36) The strands are then dried in a drum dryer. The energy consumption of the drum dryer is reduced significantly since the strands already have a temperature of about 60 C. on entry into the dryer. They are then coated in a coil with PMDI (about 3% by weight of glue based on dry matter of strands).

(37) The glue-coated strands are scattered as covering layer and middle layer in an OSB plant. The percentage distribution between middle layer and covering layer is at least 70% to 30%. The strands are pressed to give boards which have a bulk density of about 590 kg/m.sup.3. After a storage time of about one week, the test board was tested together with a standard board having the same thickness for release of VOCs in a microchamber.

(38) Chamber Parameters:

(39) Temperature: 23 C.; humidity: 0%; airflow: 150 ml/min; air change: 188/h; loading: 48.8 m.sup.2/m.sup.3; specimen surface area: 0.003 m.sup.2; chamber volume: 48 ml.

(40) The values for the most important parameters in terms of amount are shown in the following table.

(41) TABLE-US-00002 Test board Standard board Parameter [g/m.sup.2 h] [g/m.sup.2 h] Hexanal 198 510 3-Carene 171 461 -Pinene 154 588 Pentanal 58 119 -Pinene 17 425 Acetone 52 95

(42) A significant reduction in the most important parameters is observed as a result of lengthening the treatment time with hot water. This is particularly prominent in respect of the terpenes. In the case of these, the emission is reduced by up to 95%.