Wood curing method

A method for heat-treating wood, including at least one step of maintaining the wood to be treated at a predetermined temperature in a treatment chamber in order to destroy at least partially the hemicellulose of the wood. The method comprises monitoring the current amount of at least one of the gases given off during hemicellulose decomposition throughout the treatment step, and stopping the treatment step once the amount begins to reach a substantially constant value.

FIELD OF THE INVENTION 
The present invention relates to an improvement in methods of treating wood 
at high temperature and in particular in so-called curing treatments. It 
also relates to a sensor enabling said method to be carried out. 
BACKGROUND OF THE INVENTION 
It is known that, in the natural state, wood or the wood fibers which are 
in contact with a humid atmosphere, tend to be logged with water, going as 
far as absorbing it up to 100% of their weight. Such water absorption is 
accompanied, on the one hand, by a swelling and, on the other hand, by a 
loss of the mechanical qualities and qualities of cohesion of the material 
which, in certain cases, can go as far as an advanced disintegration 
thereof. This is why the habit is taken, before every step of machining 
wood, of effecting a drying step which, by eliminating the water 
therefrom, improves its dimensional stability. 
Although the drying step enables the water to be eliminated from the wood, 
it in no way modifies the hydrophilic nature thereof, with the result that 
it is again likely to reabsorb the water eliminated during drying when it 
is again in a humid atmosphere. 
In order to decrease the hydrophilic character of the natural wood and thus 
to give it a long-lasting dimensional stability, different techniques of 
heat treatment at high temperature have been proposed. 
Among these techniques, it has been proposed to subject the natural wood to 
different steps of treatment including in particular a drying in open 
circuit followed by heating and maintenance at a temperature included 
between about 220.degree. C. and 300.degree. C. for a determined period. 
Such a technique of treatment, called curing, makes it possible to give 
the wood both a hydrophobic character and an excellent dimensional 
stability. 
However, it has been ascertained that the operation of curing had to be 
carried out with the greatest rigour, on pain of decreasing the mechanical 
characteristics of the wood treated. In fact, it is known that this curing 
step has for its object to partly destroy the hemicellulose of the wood 
without being detrimental to the structure thereof, in other words without 
destroying the lignin. 
Under these conditions, one of the major difficulties encountered during 
the curing treatment consists in determining the time during which the 
curing temperature must be maintained in order to destroy the 
hemicellulose without significantly destroying the lignin. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome this drawback by 
proposing a method for detecting the end of the treatment level of the 
curing phase. 
The present invention thus has for its object a method of treating wood by 
heating, comprising at least one step in which the wood to be treated is 
maintained at a determined temperature so as to destroy at least in part 
the hemicellulose of the wood, characterized in that it consists in 
monitoring, all along said step, the existing amount of at least one of 
the gases resulting from the decomposition of the hemicellulose, and in 
interrupting this step of treatment as soon as this amount begins to reach 
a substantially constant value. 
In one embodiment of the invention, the wood is disposed in a treatment 
chamber provided with a sensor sensitive to acetic acid and/or carbon 
dioxide and/or carbon monoxide. 
The present invention also has for an object a sensor intended for carrying 
out this method of treating wood, characterized in that it comprises a 
detector element constituted by a metallic oxide of the type allowing the 
detection of the reducing gases. Interestingly, said metallic oxide is a 
tin or titanium dioxide.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 thus represents the variation in temperature T (in .degree.C.) as a 
function of time t (in mins.), to which a chamber containing wood to be 
treated, constituted by ash, has been taken during a curing process. 
Such a process of treatment comprises three steps, namely a drying step A, 
a glass transition step B, and a curing step proper C. 
The first step of drying A is itself divided into two phases, a first phase 
A.sub.1 during which the temperature of the drying chamber containing the 
ash to be treated is progressively raised, at a temperature-rise speed of 
about 5.degree. C./min., from the ambient temperature up to a temperature 
T.sub.1 close to 100.degree. C., followed by a phase A.sub.2 during which 
the temperature of the chamber is maintained at the level value T.sub.1 
until the end of drying. 
During the second step B, the temperature of the chamber is progressively 
raised, at a temperature-rise speed close to the preceding one, from 
temperature T.sub.1 to a temperature T.sub.g of 170.degree. C. close to 
the glass transition temperature of the essence of wood in question, 
namely ash in the present case. The temperature T.sub.g is maintained at 
this level value for the time necessary for all the mass of wood treated 
to reach the glass transition temperature T.sub.g. It will be noted that 
the fact of extending the duration of this level is not translated by any 
detrimental consequence as far as the respect of the mechanical qualities 
of the product treated is concerned. 
During the third step C, the temperature of the chamber is progressively 
raised, during a phase C.sub.1, at a temperature-rise speed close to the 
preceding speed, from the glass transition temperature T.sub.g of 
170.degree. C. up to the curing temperature T.sub.r of 230.degree. C. and 
the temperature of the oven is maintained during a second phase C.sub.2 at 
this level value, until a large proportion of hemicellulose is decomposed. 
It is known that one of the difficulties of this specific phase resides in 
the fact that the temperature must be maintained for a sufficiently long 
time for a large percentage of hemicellulose to be decomposed, but that it 
is imperative not to exceed this time, on pain of beginning to destroy the 
lignin at the same time, which would then be translated by a drop in the 
mechanical characteristics of the wood treated. 
FIG. 2 represents a graph constituted by two series of curves which have 
been superposed. A first curve (reference I) represents the variation 
.DELTA.m/.DELTA.t of the loss of mass .DELTA.m of the treated wood as a 
function of time, during the complete process of curing. A second series 
of curves represents the variation of absorbance A as a function of time, 
in the domain of the infrared, during the same process of treatment, 
characteristic of a release of three gases coming from the decomposition 
of the hemicellulose, namely acetic acid (curve II.sub.a), carbon dioxide 
(curve II.sub.b) and carbon monoxide (curve II.sub.c). 
Concerning the variation of the mass of the treated wood represented by 
curve I, the presence of two peaks is observed, which are characteristic 
firstly of the loss of mass due to the drying of the wood and, secondly, 
of the loss of mass due to the decomposition of the hemicellulose. It is 
also observed in this Figure that there is coincidence of the second peak 
corresponding to the greatest mass drop (curve I) and the three peaks 
characteristic of the acetic acid (curve II.sub.a), carbon dioxide (curve 
II.sub.b) and carbon monoxide (curve II.sub.c) produced. 
According to the present invention, the amount of one or several of the 
gases produced by the decomposition of the hemicellulose is monitored, in 
order to detect the instant t.sub.a which corresponds to the moment when 
there is no longer any decomposition of the hemicellulose and which 
therefore indicates that the reaction of curing is terminated. 
Such monitoring may be effected by means of sensors of known type which, on 
the one hand, are able to detect the specific gases produced by the 
decomposition of the hemicellulose, and in particular acetic acid, carbon 
dioxide, or carbon monoxide and, on the other hand, are able to withstand 
the temperatures of the treatment. A plurality of sensors may also be used 
conjointly, which are each specifically sensitive to one of the gases and 
of which the signals are processed by electronic means, so as to make a 
possibly weighted mean of the measurements effected by each sensor. A 
sensor sensitive to all three gases mentioned above may preferably be 
used, which simplifies processing of the signal furnished. Of course, to 
effect monitoring, a gas analysis measuring chain, particularly by 
infrared spectrography, may also be employed. 
Applicants have established that a certain type of sensors is particularly 
interesting for carrying out the method of treatment according to the 
invention. The sensors of this type comprise a sensitive element which is 
constituted by a metallic oxide, and more particularly a metallic oxide of 
the type allowing detection of the reducing gases. Sensors will thus be 
more particularly retained whose sensitive element is constituted by tin 
dioxide. Sensors whose sensitive element is constituted by titanium 
dioxide or zinc oxide may also be used. 
FIG. 3 represents, on the same graph, on the one hand, the absorbance A of 
the acetic acid (curve I), of the carbon dioxide (curve II) and of the 
carbon monoxide (curve III) during a step of curing proper of pieces of 
beech and, on the other hand, the signal S in volts (curve IV) produced by 
a sensor of the prior state of the art which is disposed in the treatment 
chamber. It is observed in FIG. 3 that the maximum of the signal S 
furnished by the sensor coincides substantially with the maxima of the 
curves of absorbance of the acetic acid, carbon dioxide and carbon 
monoxide, with, however, a sight delay, which makes it possible for the 
user, when he detects the maximum of the signal S given by his sensor, of 
being sure that the hemicellulose is indeed decomposed. 
An operation of heat treatment of pieces of hornbeam wood which is carried 
out according to the invention, i.e. by monitoring the end of the phase of 
curing proper by detecting the moment when the greater part of the 
hemicellulose is destroyed, will be described hereinafter. 
The whole of the treatment comprises a drying step A which is itself 
followed by a glass transition step B and a curing step C which is 
monitored according to the invention with a sensor of the type described 
previously. 
In FIG. 4, only the step of curing C proper has been represented. There has 
thus been plotted on the same graph and as a function of time, on the one 
hand, the variation of the temperature T of the treatment chamber (in 
broken lines) and, on the other hand, the signal S (in volts) furnished by 
the sensor (in solid lines). In accordance with the invention, the curing 
phase proper is stopped at instant t.sub.a when the signal S of the sensor 
passes through a maximum, i.e. when the heating is interrupted to allow 
the temperature to drop. 
Measurements made on the hornbeam wood thus treated have confirmed that a 
large part of the hemicellulose is indeed decomposed, which guarantees the 
efficacity of the curing, and that the lignin is not yet attacked, which 
guarantees the conservation of the mechanical qualities of the wood 
treated. These measurements are grouped together in the Table hereinbelow. 
The pentosans which represent the majority of the hemicelluloses of this 
wood have been dosed and it has been observed that, from the natural state 
to the cured state, the percentage thereof passed from 25.6% to 15.9%, 
which represents a reduction of 37%. 
TABLE 
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Elementary analysis 
Carbon Hydrogen Oxygen Pentosans 
Lignin 
Density 
(%) (%) (%) (%) (%) (g/cm.sup.3) 
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HORNBEAM 
natural 
47.48 6.39 46.45 25.67 17.82 0.75 
cured 49.95 5.99 43.83 15.95 23.30 0.67 
PINE 
natural 
47.62 6.35 44.75 10.70 23.04 0.55 
cured 51.93 5.92 42.18 3.24 25.63 0.47 
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The lignins were also dosed and it was observed that there was no 
destruction thereof, which guarantees the maintenance of the mechanical 
qualities of the wood treated. It is also observed that the density 
decreases only slightly, passing from 0.75 to 0.67, which represents a 
reduction of the order of 10%. 
The same treatment was also effected on another essence of wood, namely 
pine, and the results are shown in the above Table. These results confirm 
in all points those obtained for the hornbeam. 
Interestingly, there may be associated with the sensor electronic signal 
analysis means which will detect any zero value or any reversal of the 
slope of the curve representative of the signal S produced during the 
treatment. 
Of course, the signal produced by the sensor during the curing phase is a 
function of the nature of the wood treated. FIG. 5 thus represents the 
variation of the signal S produced by the same sensor, in the case of a 
curing treatment effected on pine (curve a) and of the same treatment made 
on beech (curve b). 
It is observed in this Figure that, although the characteristic peak of the 
respective signals produced by the sensor is less marked in the case of 
pine than in the case of beech, it is nonetheless easy to detect the 
cancellation of the slope or its reversal defining the instant t.sub.a 
when the level of curing must be interrupted.