EP1799412B1 - Method for reducing the release of volatile organic compounds (voc) from wood and wood chip products and wood materials derived therefrom in particular particle boards - Google Patents

Abstract

The invention relates to a method for reducing the emissions of volatile organic compounds (VOC) from wood and wood chip products, such as wood chips, wood strands, wood fibres and wood veneers, in particular, made from soft wood and derived timber products made therefrom.

Description

The invention relates to a method for reducing the emission of volatile organic compounds (VOC) from wood and wood crushing products, such as wood chips, wood strands, wood fibers and wood veneers, in particular of softwood, and derived wood materials.

The volatile organic compounds are classified according to the boiling point or vapor pressure very volatile organic compounds (VVOC), volatile organic compounds (VOC) and semi-volatile organic compounds (SVOC). Volatile Organic Compounds (VVOCs) include compounds with a boiling point below 50 ° C. These include formic acid and formaldehyde. Volatile Organic Compounds (VOC) are subsumed to include those volatile organic compounds whose boiling points are between about 50 ° C and 260 ° C. The VOCs include acetic acid and terpenes.

The VOC are air pollutants, certain VOC cause eg odor nuisance and can have toxicological effects on living things. The VOCs are mainly derived from the ingredients (extractives) of the wood or its transformation products and, where appropriate, the degradation products of the main components of the wood (lignin, celluloses, hemicelluloses). The VOCs include wood components such as α-pinene, β-pinene, Δ ³ -caren, which mainly escape from the wood of the conifers. Modification products of the ingredients include pentanal, hexanal, etc. From the hemicelluloses, acetic and formic acid can be formed. In particular, softwoods contain ingredients (extractives) such as resins and fats, which lead to the formation of volatile organic terpene compounds and aldehydes such as pentanal and hexanal. Hardwoods release mainly acetic and formic acid. The chemical composition and amount of VOC depends on many factors, in particular the type of wood, the treatment of the wood, the storage period and the storage conditions of the wood or its shredded products, as well as from the drying conditions. Furthermore, in wood-based materials, the binder influences the emission of VOCs.

Volatile organic compounds can escape from the wood and from the wood chippings and derived wood materials. For wood materials made from wood chippings such as wood shavings, wood strands, wood fibers and wood veneers by known methods, volatile organic compounds are released during manufacture, after production and when used. In particular, wood-based materials such as Oriented Strand Boards (OSB), which are produced in Europe predominantly from unconstrained softwoods such as pine and Douglas fir, can still produce large amounts of volatile organic compounds after production.

As mentioned, the nature and extent of the emission of VOCs from wood-based materials is additionally determined, inter alia, by the type of wood and, in particular, by the production technology and the binder used. For example, the gluing of wood chips with alkaline-curing phenol-formaldehyde resins (PF resins) leads to increased formation of acetic acid, since the alkali splits off the acetyl groups of the hemicelluloses in the wood, which lead to the formation of acetic acid (cf. Roffael, E., Miertzsch, H. and Schröder, M. 1990: On the Mechanism of the Formation of Volatile Acids in Bonding with Alkali-Curing Phenol-Formaldehyde Resins. Wood Central Journal 116 (111): 1684-1685 ). Other industry-standard binders for the production of particleboard and fiberboard are acid-hardening urea-formaldehyde resins (UF resins), adhesives based on diisocyanates (PMDI) and tannin-formaldehyde resins (TF resins).

In particular, volatile organic compounds with unsaturated carbon-to-carbon bonds can react with the atmospheric oxygen under ambient conditions. By reaction of certain VOCs with oxygen (O ₂ ) or other oxidizing agents, particularly during the storage of the wood, the wood shredded products or the wood-based materials, odorous compounds may be formed arise. For certain wood materials such as Oriented Strand Boards (OSB), which are mostly made of softwood in Europe, the odor can be particularly unpleasant and concise. This is particularly true when the wood, as is usually the case, is used without prior storage for wood-based material production.

1. a) Behandeln von Rohspäne bzw. Strands mit einer geeigneten Chemikalie, wobei der pH-Wert des Holzes erhöht wird;
2. b) Trocknen der Holzspäne bzw. Strands
3. c) Mischen der Holzspäne bzw. Strands mit einem Bindemittel zum Beleimen; und
4. d) Pressen dieser Holzspäne bzw. Strands.

CA 1 269 602 A1 discloses a method for producing wood-based materials comprising the steps:

1. a) treating raw chips or strands with a suitable chemical, wherein the pH of the wood is increased;
2. b) drying the wood chips or strands
3. c) mixing the wood chips or strands with a binder for gluing; and
4. d) pressing these wood chips or strands.

For these reasons, it was the object of this invention to provide a process that reduces the amount of volatile organic compounds (VOC) from wood, wood chippings and wood-based materials in the manufacture of wood-based materials, such as chipboard.

The object is achieved by the method according to claim 1.

In this case, the adjusting by reaction with the wood moisture pH should be above 7, in particular above 9. Possible suitable substances which cause such a reaction, in particular an alkaline reaction, are bases such as sodium hydroxide (NaOH), basic salts such as sodium carbonate (Na ₂ CO ₃ ), sodium sulfite (Na ₂ SO ₃ ) and trisodium phosphate (Na ₃ PO ₄ ). Even gases such as ammonia, which lead to pH values higher than 7 after a reaction with water, can be used for this purpose. In the wood or in the aqueous extracts of the wood, the pH can be adjusted due to a reaction of the alkali with the ingredients or free or liberated wood acids below the theoretically expected by reaction of the alkali with the moisture content of wood. Of course, other agents well known to those skilled in the art may be used to effect an increase in the pH of the wood. The treatment of the wood or its shredded products can be carried out for the production of wood-based materials before or after storage of the wood. The lignocelluloses can be washed after the aforementioned treatment and before the gluing with water or with aqueous solutions. The treatment is usually carried out at room temperature and under atmospheric pressure.

The addition of alkali such as sodium hydroxide to the binder in the manufacture of wood-based materials made of lignocellulose-containing materials made with alkaline phenol-formaldehyde resins (PF resins) is known. For example, Schmidt-Hellerau (1968) investigated the influence of the addition of a 50% sodium hydroxide solution to the binder immediately before the gluing or after drying of the chips of the extra-rich sweet chestnut on the physical-technological properties of the particleboard ( Schmidt-Hellerau, Ch., 1968: Possibilities for increasing the quality of phenolic chipboards. Wood Central Journal 94: 1327 ). The addition of alkali in amounts of up to 20% (based on wood) to the already alkaline PF resins had the goal to increase the alkali balance of the binder to buffer acids in the highly acidic chestnut wood chips.

The results of Schmidt-Hellerau (1968) indicate that the addition of concentrated sodium hydroxide solution (50% solution) to the binder immediately prior to gluing effectively promotes the micaeability of the wood chips of certain extractive hardwoods. The addition of an excess of alkali, as is the case with Schmidt-Hellerau (1968), can not be compared with the method according to the invention, since according to the invention only one adjustment of the pH with the wood moisture is preferably to be achieved over 7.

According to the present method, the alkali is added to the wood or wood chips before drying and gluing and not to the binder (e.g., phenolic resin). The influence of the addition of alkali to the binder on the emissions of VOCs from chipboards was not determined in the investigations by Schmidt-Hellerau (1968).

While the use of alkali has a positive effect on the physico-technological properties and quality of particle board bonded with alkaline hardening PF resins, the treatment of beech wood chips with alkaline ammonia solutions before drying has a negative effect on the Physico-technological properties of acid-hardening urea-formaldehyde resins (UF resins) bonded chipboard ( Roffael, E. and Parameswaran, N., 1986: Influence of latent acidity of beech wood chips on their ability to be glued with urea formaldehyde resins. Wood as raw material 44: 389-393 ). The work also does not address the reduction of emissions of volatile organic compounds.

Roffael and Parameswaran (1984) have beech wood with 6% ammonia solution at 100 ° C for 3 hours in an autoclave and then washed the chips until neutral. This treatment led to an increase of the nitrogen content in the wood and to the buffering of the acids contained therein as well as to a partial degradation of the hemicelluloses ( Parameswaran, N. and Roffael, E., 1984: Knowledge and findings on the effect of ammonia on wood shavings. Wood as raw material 42: 327-333 ). The digestion of the wood chips under pressure in ammonia solution at 100 ° C resulted after washing the chips to reduce the release of volatile acids from the chips and from wood chipboards produced therefrom. Through this treatment, however, the chips experienced a profound and undesirable change in their physical properties (sorption behavior, compressibility) and chemical properties. Both the investigations of Schmidt-Hellerau (1968) and of Roffael and Parameswaran (1984, 1986) refer to hardwoods.

As stated above, according to the invention, it has been found that with alkaline treatment of wood and wood chippings before drying and gluing with the binder, increasing the pH of the materials to a pH greater than 7, the emission of VOCs from the significantly reduced from these materials produced wood materials.

The wood or wood crushing products may be both conifers and hardwoods. Also mixtures of these two types of wood are possible. Preferably, the wood shavings and strands of conifers come from.

The wood-based materials produced from the lignocellulose-containing materials can be chipboard, such as OSB. The plates produced can be one or more layers. Furthermore, the other layers may also contain other lignocelluloses such as straw or bark.

After drying the treated wood chips and strands they can be glued with the binder. Examples of typical binders include phenol-formaldehyde resin (PF resin), diisocyanate-based adhesive (PMDI), melamine-urea-phenol-formaldehyde resin (MUPF resin) and / or tannin-formaldehyde resin (TF resin), or a mixture thereof. However, other binders such as thermoplastic binders, e.g. Metacrylatreaktionsharze be used.

In multilayer wood materials different binders can be used in the different layers.

Furthermore, the wood-based materials produced from the treated lignocellulose-containing materials may additionally be post-treated with alkali, such as sodium hydroxide, sodium sulfite, trisodium phosphate and sodium carbonate and mixtures thereof after production. The aftertreatment can also be done by ammonia gas.

In addition to the alkali treatment before drying the wood or the wood comminution products, the method according to the invention can also include a further alkali treatment after drying. The treatment according to the invention of the wood or of the wood shredded products can take place before or after storage of the wood.

After the alkali treatment according to the invention, the wood or the wood shredded products may optionally be washed with water or aqueous solutions.

The alkali compounds such as sodium hydroxide, sodium sulfite or sodium carbonate used to increase the pH can be applied or impregnated by spraying or by other known methods on the wood or the wood crushing products. Preferably, the value of the incorporated compound to increase the pH in a range of 0.1% to 2% solids on atro chips.

Starting from the described prior art, the invention aims to develop a simple method for reducing the release of volatile organic compounds (VOC) from wood and wood chippings and from wood-based materials produced therefrom. The following examples are intended to illustrate the invention without limiting it.

example 1

From a 70-year-old pine trunk (Pinus sylvestris L.), core and sapwood boards (20 cm x 13 cm x 1.9 cm) were cut in and stored at 4 ° C until they were cut (13 days after impact). The core and sapwood boards were cut to strands (20 cm x 1.9 cm x 0.04 cm) with a disk chipper.

To reduce emissions of VOC, the core and sapwood strands were sprayed in one case with 2% sodium sulfite solution and in another case with 2% sodium hydroxide solution. The dosage of sodium sulfite or sodium hydroxide was in each case at 1.5% solids based on absolutely dry (atro) strands. The treated strands were air dried without being washed. As a reference served on the air dried untreated core and sapwood strands of pine.

The pH was determined on cold-water extracts of the untreated and treated core and sapwood strands. Furthermore, at the beaches, the release of volatile acids was determined on the basis of the bottle method (EN 717-3) (Table 1). The results show that, although the production of volatile organic acids is promoted by the alkali treatment, their release is nevertheless surprisingly reduced. This result was unpredictable insofar as the literature has hitherto not differentiated between the formation of volatile organic acids or VOCs and their release into the air.

Incidentally, by the formation of the volatile acids, the pH, as already mentioned, below the theoretically adjusting by reaction with the moisture content of the wood due to the buffering of the alkali. <b> Table 1: </ b> pH, formate and acetate ion content in cold aqueous extracts and volatile acid release from untreated and treated with sodium sulfite or sodium hydroxide (each 1.5% solids on dry beach) Core and sapwood strands of pine Beach type / treatment PH value Acidity (cold water extract) (mg / 100g atro beach) Acid delivery (bottle method, 24h) (mg / 100g atro beach) formate acetate formic acid acetic acid Heartwood / untreated 5.6 4.2 13.5 0.5 1.9 Heartwood / 1.5% Na ₂ SO ₃ 5.7 5.7 nn 0.3 1.7 Heartwood / 1.5% NaOH 9.5 52.6 924.2 0.1 0.7 Sapwood / untreated 5.8 nn nn 0.9 2.0 Sapwood / 1.5% Na ₂ SO ₃ 5.9 0.2 nn 0.5 1.8 Sapwood / 1.5% NaOH 9.4 85.4 1,120.1 0.1 0.6 nn: undetectable, since below detection limit

Further, at the untreated and treated strands, the emissions of certain volatile organic compounds (VOC) were determined after 5 h, 24 h and 48 h storage in a 23 liter test chamber at 23 ° C and 50% relative humidity. The loading level of the test chamber was 0.5 m ² / m ³ , the air exchange rate was 0.5 h ^-1 .

The results are summarized in Tables 2 and 3. It is clear from the tabulated data that the treatment of the strands with sodium sulfite (Table 2) or with sodium hydroxide (Table 3) leads to a reduction in the release of volatile organic compounds (VOC). It also shows that at the same concentration, the sodium hydroxide is more effective than the sodium sulfite in reducing the release of volatile terpene compounds. <b> Table 2: </ b> Influence of the treatment of pine core and sapwood strands (<i> Pinus sylvestris L. </ i>) with sodium sulphite (1.5% solids on dry beach) on concentrations ( μg / m <sup> -3 </ sup>) of volatile organic compounds (VOC) after 5 h, 24 h and 48 h in a 23 liter test chamber at 23 ° C. and 50% relative humidity (degree of loading: 0, 5 m <sup> 2 </ sup> / m <sup> 3 </ sup>, air exchange rate: 0.5 h <sup> -1 </ sup>) component type of beach untreated 1.5% sodium sulfite (Solid on atro beach) Test duration in the 23-liter test chamber 5 h 24 hours 48 h 5 h 24 hours 48 h α-pinene core 1316 924 693 325 237 155 cotter 501 299 186 98 56 34 camphene core 31 16 9 6 3 2 cotter - - - - - - β-pinene core 43 21 13 7 5 3 cotter 9 4 3 - - - myrcene core 97 47 26 22 12 7 cotter 17 7 4 - - - Δ ³ -Caren core 1047 711 512 434 288 184 cotter 370 192 112 94 52 30 limonene core 91 45 25 - - - cotter 14 6 3 - - - Terpenes * / α-pinene core 144 68 39 23 13 7 cotter - - - - - - Terpinolene / α- core 357 171 89 68 39 21 terpinolene cotter 55 20 11 - - - Sesquiterpenes * / core 92 41 19 11 6 2 longifolene cotter 35 10 2 - - - C4 Benzene * / core 40 21 13 - - - butylbenzene cotter - - - - - - α-terpineol core 47 22 8th 6 4 2 cotter 14 4 2 - - - * Quantification with the specified individual component component type of beach untreated 1.5% sodium hydroxide (Solid on atro beach) Test duration in the 23-liter test chamber 5h 24 hours 48 h 5 h 24 hours 48 h α-pinene core 1426 1008 601 197 126 81 cotter 143 86 54 274 200 84 camphene core 39 21 16 4 2 1 cotter - - - - - - β-pinene core 37 21 16 4 2 1 cotter 2 2 - - - - myrcene core 108 50 40 13 8th 4 cotter 4 2 - - - - Δ ³ -Caren core 1597 1105 661 198 126 80 cotter 107 63 37 175 110 48 limonene core 90 49 35 10 5 3 cotter 4 2 - 9 5 3 Terpenes * / α-pinene core 46 24 19 11 5 1 cotter - - - - - - Terpinolene / α- core 447 237 158 40 23 13 terpinolene cotter 10 6 3 - - - Sesquiterpenes * / core 55 31 24 7 2 - longifolene cotter 6 1 - - - - C4 Benzene * / core 60 35 25 8th 5 3 butylbenzene cotter 2 1 - 23 7 7 α-terpineol core 24 12 9 - - - cotter - - - - - - * Quantification with the specified individual component

Example 2

From a 70-year-old pine trunk (Pinus sylvestris L.), core and sapwood boards (20 cm x 13 cm x 1.9 cm) were cut. The core and sapwood boards were cut to strands (dimension 20 cm x 1.9 cm x 0.04 cm) with the aid of a disc chipper. The chips were dried in a drying oven at 70 ° C to a moisture content of about 5%.

After drying, part of the core and sapwood strands were treated with sodium hydroxide solution. The treatment was carried out by first loading the strands in a gluing drum with 7.5% sodium hydroxide solution (dosage 1.5% solids based on absolutely dry (atro) strands). Subsequently, the strands were dried in a drying oven at a temperature of 50 ° C to a moisture content of about 6%.

As a reference served core and sapwood strands, which were not treated with sodium hydroxide solution. Since it was expected that the above-mentioned drying step would have an influence on the OSB's VOC emissions, the untreated core or sapwood strands were respectively pre-exposed in a gluing drum with an amount of water equal to the above-mentioned. Dosage of sodium hydroxide solution corresponded. Subsequently, the strands were dried in a drying oven at a temperature of 50 ° C to a moisture content of about 6%.

From the untreated and NaOH treated beaches, Oriented Strand Boards (OSB) were made with PF resin as a binder. The binder content was 10% (solid resin based on atro strands). As a curing accelerator, 6% potassium carbonate (solid based on solid resin) was added. The top and middle layers of the OSB were oriented differently. The mass fraction of the outer layers was 25%, that of the middle layer was 75%. The gluing levels of the top and middle layers were the same. The pressing temperature was 200 ° C, the Press time factor was 15 sec / mm. The plates had a target density of 650 kg / m ³ , the plate thickness was 16 mm.

At OSBs prepared from untreated and sodium hydroxide treated strands, VOC emissions were determined after 24 hours of storage in a 1m ³ test chamber at 23 ° C and 50% relative humidity. The loading level of the test chamber was 0.5 m ² / m ³ , the air exchange rate was 0.5 h-1. <b> Table 4: Concentration (μg / m <sup> 3 </ sup>) of volatile organic compounds (VOC) of PF resin-bound Oriented Strand Boards (OSB) from untreated and 1.5 % NaOH (solid on atro strands) treated core and sapwood strands of pine (test duration 24 h) variant Substance ( μ g / m ³ ) heartwood heartwood sapwood sapwood treatment untreated beaches 1.5% NaOH (solid on atro strands) untreated beaches 1.5% NaOH (solid on atro strands) 1. Aromatic hydrocarbons Σ C4-benzenes 2 - - - 2. Terpene α-pinene 33 12 14 6 β-pinene 10 3 6 4 Δ ³ -Caren - 2 - - limonene 2 - - - 3. Alcohols n-pentanol - - 9 13 4. aldehydes n-pentanal 23 - 27 31 n-hexanal 50 21 62 59 n-octanal - - - 6 benzaldehyde 13 7 4 4 5. Ketones acetone 27 14 14 15 1-hydroxy-2-propanone - 5 - 3 6. acids acetic acid 107 57 231 126 7. Other Σ Other 8th 6 15 3 Σ VOC (TVOC) 275 127 382 270

The results in Table 4 show that the VOC emissions of PF resin-bound OSB from core and sapwood strands are reduced by treating the strands with sodium hydroxide. Furthermore, the formaldehyde release of the PF resin-bound OSB was lowered by treatment of the strands (Table 5).

Table 5: Formaldehyde release of PF resin bound OSB made from untreated and 1.5% NaOH (solid on strands) treated core and sapwood strands of pine Beach type / treatment PH value humidity
(%) formaldehyde release
(Bottle method) mg / 1000 g atro plate 3 h 24 hours Heartwood / untreated 7.5 6.7 0.7 8.0 Sapwood / untreated 8.3 6.7 1.6 13.5 Heartwood / 1.5% NaOH 9.3 5.9 0.2 2.9 Sapwood / 1.5% NaOH 9.6 6.3 0.3 3.9

Claims (20)

1. A method for the production of wood materials comprising the step:

   a) Treating raw chips or strands with a suitable chemical in a way that a reaction occurs and the pH-value of the wood is elevated on pH-value of higher pH 7;

   b) drying the chips or strands;

   c) mixing the chips or strands with a binder for binding and

   d) pressing said chips or strands.
2. The method according to claim 1, characterized in that the wooden material are chip boards.
3. The method according to claim 1 or 2, characterized in that the wooden materials are oriented strand boards.
4. The method according to any one of the preceding claims, characterized in that the chemical in step a) is selected from alkali hydroxide, alkali carbonate, alkali sulphite or alkali phosphate.
5. The method according to claim 4, characterized in that the chemical in step a) is selected from sodium hydroxide, sodium carbonate, sodium sulphite or tri-sodium phosphate.
6. The method according to any one preceding claims, characterized in that the treatment is with a mixture of alkali.
7. The method according to claim 6, characterized in that the mixture of alkali is a mixture of two or more of sodium hydroxide, sodium carbonate, sodium sulphite, and/or tri-sodium phosphate.
8. The method according to claim 1 to 3, characterized in that the chemical is gaseous ammonia.
9. The method according to any one of the preceding claims, characterized in that the amount of alkali is of from 0.1 % to 2 % solid material per atro chip.
10. The method according to any one of the preceding claims, characterized in that the treatment is in a way that the pH-value of the chips or boards is elevated over pH 9.
11. The method according to any one of the preceding claims, characterized in that the chips or strands obtained in step b) are treated additionally with alkali.
12. The method according to any one of the preceding claims, characterized in that the chips or strands are obtained from coniferous wood.
13. The method according to any one of the preceding claims, characterized in that the treatment of the wood or the chips or strands is before or after storage of the wood.
14. The method according to any one of the preceding claims, characterized in that the chips or strands are washed with water or watery solutions after step a).
15. The method according to any one of the preceding claims, characterized in that a solid or multilayered board is produced.
16. The method according to any one of the preceding claims, characterized in that the binder is a phenol-formaldehyde-resin, a binder on the basis of diisocyanates, a melamine-urea-phenol-formaldehyde-resin, a tannin- formaldehyde-resin or a mixture thereof.
17. The method according to any one of claims 15 or 16, characterized in that in different layers different binders are used.
18. The method according to any one of the preceding claims, characterized in that the boards obtained with the treated chips or strands are treated additionally with alkali after production of said board.
19. The method according to claim 18, characterized in that the alkali selected from sodium hydroxide, sodium carbonate, sodium sulphite and/or tri-natrium phosphate.
20. The method according to any one of the preceding claims, characterized in that the boards produced from the treated chips or strands are treated with gaseous ammonia after production.