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.