(19)
(11) EP 2 889 112 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication:
01.07.2015 Bulletin 2015/27

(21) Application number: 13199729.8

(22) Date of filing: 27.12.2013
(51) International Patent Classification (IPC): 
B27K 1/00(2006.01)
F26B 3/02(2006.01)
B27K 5/00(2006.01)
F26B 21/00(2006.01)
(84) Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
Designated Extension States:
BA ME

(71) Applicant: "Latvian State Institute of Wood Chemistry" Derived public person
1006 Riga (LV)

(72) Inventors:
  • Andersone, Ingeborga
    LV1001 Riga (LV)
  • Andersons, Bruno
    LV1001 Riga (LV)
  • Cirule, Dace
    Riga LV1084 (LV)
  • Biziks, Vladimirs
    LV4000 Limbazi (LV)
  • Cirkova, Jelena
    LV1055 Riga (LV)
  • Grinins, Juris
    LV-3301 Kuldiga (LV)
  • Irbe, Ilze
    LV-1064 Riga (LV)
  • Kurnosova, Nina
    LV1035 Riga (LV)
  • Sansonetti, Errj
    LV1009 Riga (LV)

(74) Representative: Dolgicere, Nina 
Patent Agency KDK Dzerbenes Street 27
1006 Riga
1006 Riga (LV)

   


(54) Method for hydrothermal treatment of wood


(57) The present invention relates to a woodworking industry and may be used for hydrothermal treatment of wood, wood materials, and woodware The object of the present invention is to increase hydrophobicity of wood. According to the present invention the object is achieved by that wood is heated up to 120-160°C in a steam atmosphere, in which it is seasoned, and then wood is cooled up to room temperature, and heated during the first stage at a rate 0.16-0.22°/min up to 100°C, and then the heating rate of obtained steam is decreased up to 0.11-0.13 °/min until the temperature of 160°C is reached, and wood is seasoned at the temperature of 160°C during 1-3 hours at a steam pressure 0.6-0.7 MPa, and cooled up to the temperature of 20°C during 6-8 hours. the method of hydrothermal treatment of wood according to the present invention allows increasing hydropphobicity of wood by several times along with saving energy costs. Moreover, hydrophobicity values of treated wood are higher than those of impregnated wood.




Description

Technical Field



[0001] The present invention relates to a woodworking industry and may be used for hydrothermal treatment of wood, wood materials, and woodware.

Background Art



[0002] Wood is one of the most popular building materials. One of the main factors, which lower the competitiveness of wood compared to metals and synthetic materials, is a relatively short service life. The decisive factor of applicability of woodware is the effect of fungal infection because of natural moisture content therein. Therefore, the risk of fungal infection, which especially becomes apparent under the influence of weather conditions, and also because of moisture content of permanent or changing nature, shall be obligatory taken into account. However, the conventional methods of wood protection, for example, varnish-and-paint coating, have several disadvantages.

[0003] Natural wood hygroscopicity, which manifests itself as swelling and shrinkage thereof in a medium with changing moisture content, is the reason of tensioning and subsequent surface cracks, which may destroy protective varnish-and-paint coating, and hence, deteriorate hydrophobic properties and strength. The use of chemical protection in some cases may result in washing out of protective substances, lowers the effectiveness of protection as a whole and causes a negative influence on the environment.

[0004] It is known an antiseptic protection of wet wood and timber from rotting by applying the composition according to USSR Author's Certificate No. 489638, 03.12.73, IPC B27K 3/52, which comprises sodium pentachlorophenolate 38-42, thiourea 10-12, mineral oil 2.0-2.5, triphenyl phosphate 1.5-2.0 and sodium carbonate 41.5-48.5.

[0005] The disadvantage of this composition is that the activity thereof is manifested only in high concentration in water solutions. The antiseptic agent relates to the first class of hazard, i.e. it is environmentally toxic substance. Moreover, antiseptic properties of the composition are maintained only during constant feeding of working solutions, while wood quality is decreased that worsens the process of treatment thereof. It is known a method of wood treatment by applying impregnating liquid in the form of extracts of natural substances on wood surface (Method of Making a Mordant (Stain) for Wood. USSR Author's Certificate No. 52527, 1935, IPC C09D 15/00,). Such treatment allows making low value wood specie look like high value ones. The dye enters wood to a depth of approximately 1 mm. Compositions for wood staining modify only a surface appearance of wood that makes impossible a mechanical treatment of stained woodware.

[0006] The closest is the known method for hydrothermal treatment of wood of broadleaved tree species, wherein at the first stage wood is heated up to 130-165°C by hot air at a rate of 0.5-0.75°/min followed by the seasoning in obtained steam atmosphere at a temperature of 165°C during 0.5-1.75 hour. At the next stage wood is heated again up to 200°C in steam-air mixture during 4-8 hours, and then the product is treated by water steam at a temperature of 120-160°C (EP1696193).

[0007] The known method provides not only the modification of the appearance of wood but also increases hydrophobicity thereof and resistance to fungal microorganisms.

[0008] However, the method is a complicated one from technological point of view because it consists of many stages with their own temperature conditions, and requires significant energy consumption.

[0009] Moreover, these conditions of treatment while increasing hydrophobicity of wood simultaneously decrease its mechanical properties, namely, strength and plasticity, because the processes of torrefaction begin at a temperature higher than 160°C.

Summary of invention



[0010] The object of the present invention is to increase hydrophobicity of wood.

[0011] According to the present invention the object is achieved by that wood is heated up to 120-160°C in a steam atmosphere, in which it is seasoned, and then wood is cooled up to room temperature, and heated during the first stage at a rate 0.16-0.22°/min up to 100°C, and then the heating rate of obtained steam is decreased up to 0.11-0.13 °/min until the temperature of 160°C is reached, and wood is seasoned at the temperature of 160°C during 1-3 hours at a steam pressure 0.6-0.7 MPa, and cooled up to the temperature of 20°C during 6-8 hours. Features distinctive from those of a prior art in a proposed combination have not been found in the known information sources. A state of the art patent search revealed no solutions relating to proposed combination of conditions, which have effect on the surface of wood for improving its performance properties and decorative properties. The aforementioned proves that the invention meets the "inventive level" criterion because it does not clearly follow from the state of art.

[0012] The material of cell wall of lignocellulosic biomass represents interpenetrating space polymeric network, the main components of which are natural polymers of three types: cellulose, lignin and hemicelluloses. Intra- and intermolecular bond inside lignin-carbohydrate network is made by both chemical covalent bonds and hydrogen bonds due to polar hydroxyl groups in all components composing thereof. Conditions of treatment according to the present invention allow almost completely destroying hemicelluloses and, correspondingly, increasing microbiological resistance to fungal microorganisms, for which hemicellulose is a source of nourishment. At the same time due to the modification of the structure the hydrophobicity of wood is increased. Conditions of treatment according to the present invention do not result in heating of samples higher than 160°C, therefore, it fails to initiate the process of thermal destruction of wood and increase energy density thereof as the result of torrefaction.

Brief description of drawings



[0013] The present invention is illustrated, by way of example, in the accompanying drawings, wherein: Fig.1 is the curve of pressure and temperature change depending upon the time of treatment.Fig.2 are the curves of the change of hydrophobicity of wood depending upon the time; curve 1 - unprocessed wood, curve 2 - impregnated wood, curve 3 - hydrothermally treated samples of wood.

Industrial applicability



[0014] The method according to the present invention is carried out as follows.

[0015] Samples of wood are loaded into an autoclave filled with water, and the temperature is increased at a rate of 0.16-0.22°/min up to the temperature of 100°C, then the rate of heating of obtained steam is decreased up to 0.11-0.13°/min until the temperature of 160°C is reached. Such heating conditions prevent micro-cracking in cell membrane (fibrilla) of wood. When steam temperature of 160°C is reached the heating is stopped and wood is seasoned during 1-3 hours at a steam pressure 0.6-0.7 MPa.

[0016] Then during 6-8 hours the pressure is decreased and the temperature in the autoclave is decreased up to 20°C.

[0017] The following hardwood species were used for heat treatment: European grown Grey alder (Alnus incana), birch (Betula pendula) and aspen (Populus tremula).

[0018] For each timber species, at least 16 boards were treated for a set of process conditions. Standard cross section had a thickness of 28-30 mm and a width of 100-105 mm. The length of the boards was approx. 1.0 m. The moisture content of the boards was relatively low (10-12%).2 boards per wood species were selected and 5 specimens per board were used for the investigation of dimensional stability of hardwood after each thermal treatment. All samples before the tests were oven dried according to the test methodology.

[0019] The heat treatment was performed in one stage heat treatment process with three sub-stages, namely, (1) heating up to the modification temperature; (2) holding at the modification temperature and (3) cooling of the reactor. The treatment was done in a 540 litre WTT pilot scale autoclave, and the treatment temperature and holding time varied between 140 and 160°C and 1h and 3h, respectively. In this process, wood was treated in a saturated steam under superatmospheric conditions (0.6-0.7 MPa). The saturated steam was used as the heating medium to increase the heat transfer between the autoclave wall and the boards.

[0020] In closed systems during the heating process, water evaporated from the wood and from additionally filled water in the autoclave generates the high-pressure steam. To obtain the necessary amount of saturated steam in the autoclave, the amount of filled water was calculated by taking into account the initial moisture content of wood and the volume of the autoclave.

[0021] Cooling down of the autoclave was accomplished by a slow during 6-8 hours and controlled release of pressure and temperature to atmospheric conditions.

[0022] The hydrophobicity is determined as follows.

[0023] Contact angle has been measured using the sessile drop technique. It consists in a deposition of a drop of water on the sample surface and then the spread of the drop is recorded and the corresponding contact angle is obtained. The volume of the drop is 10 µl and one drop per second is recorded. According to the properties of the wood surface, the drop of water will stay for a short or long time and the contact angle will be large or small depending on the surface hydrophobicity.

[0024] The graph represents the contact angle value for impregnated wood samples, untreated control wood samples and thermally treated wood samples.

[0025] Samples have been selected choosing those with similar surface properties (without defects or knots, similar annual growth rings density) and then planed and sanded to obtain a smooth surface. Wood rapidly absorbs water drop during measurements. The contact angle rapidly decreases in few seconds for untreated wood, as is seen from the graph after 9 seconds it is not possible to measure further contact angle. Contact angle is slightly better for impregnated wood, but also in this case the time of measurement is 14 seconds. Thermally treated wood gives the largest values of contact angle, due to increased hydrophobicity thereof, in this case the drops remain on wood for a longer period of time before absorbing in the wood.

[0026] Therefore, the method of hydrothermal treatment of wood according to the present invention allows increasing hydropphobicity of wood by several times along with saving energy costs. Moreover, hydrophobicity values of treated wood are higher than those of impregnated wood.


Claims

1. A method of hydrothermal treatment of wood by heating up to the temperature 120-160°C in a steam atmosphere, seasoning at this temperature with the subsequent cooling up to room temperature, characterized in that at the first stage wood is heated at a rate of 0.16-0.22°/min up to 100°C, and then the rate of heating of obtained steam is decreased up to 0.11-0.13 °/min until the temperature of 160°C is reached, and wood is seasoned at the temperature of 160°C during 1-3 hours at a steam pressure 0.6-0.7 MPa, and cooled up to the temperature of 20°C during 6-8 hours.
 




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Cited references

REFERENCES CITED IN THE DESCRIPTION



This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description