FR2810917A1 - METHOD OF IMPREGNATION AT THE HEART OF PIECES OF WOOD BY BATH HEAT TREATMENT - Google Patents

Abstract

The invention concerns a method for treating wood with an impregnating product. Said method is characterised in that it comprises a step which consists in treating the wood to be heat-impregnated at a temperature higher than 220 DEG C, so as to decompose the hemicelluloses thereof, followed by a step which consists in immersing said wood in a liquid bath, consisting of the impregnating product, which is at a temperature lower than the temperature of the wood derived from the heating step.

Description

1i 2810917 The present invention relates to a method for

  to deeply impregnate pieces of wood, in particular pieces of wood previously treated with heat, and this by

  immersion of these parts in an impregnation liquid.

  In order to improve the properties of wood, and in particular its dimensional stability as well as its resistance to degradation, various treatment methods have been proposed. Most of these various processes use impregnation techniques using active liquid substances which are sometimes under pressure when

  you want to perform a deep treatment.

  Thus, in the Bethel process, which is one of the oldest and best known in this technique, the wood is placed in an autoclave in which, after achieving a high vacuum making it possible to empty its cells of the air and of the moisture which they contain, it is subjected to an impregnation with an active aqueous solution containing treatment salts, this operation being carried out under pressure. The treatment products used are in particular solutions based on copper, chromium or arsenic in the case of mineral products, or creosote in the case of organic products. These various preservatives are toxic compounds which are no longer accepted by the new standards relating to

the environment.

  It has also been proposed, in particular with a view to giving wood better mechanical properties, and in particular better hardness properties, to impregnate it using organic monomers, such as resins

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  polyester, using a vacuum / pressure technique, then carrying out an in situ polymerization of the monomer in order to solidify the resin and thus form a wood / plastic composite. It has also been proposed, in particular in patents WO-A-98/39104 and WO-A-99/65615, to impregnate the wood, using vacuum / pressure means, with products which are

  organic derivatives of fatty acids or mineral oils.

  However, the interface is not favorable due to the almost fundamental incompatibility which exists between wood, with hydrophilic properties, and the organic products used which have hydrophobic properties, so that it is necessary to use significant pressure to penetrate the treatment products into the heart of the wood, under penalty of performing only one

surface treatment.

  Finally, in recent years, a so-called "cross-linking" wood treatment process has been developed which does not require additional treatment products, the main part of this process consisting in subjecting the wood to a heat treatment intended to cause within it thermocondensation reactions at the level of lignocellulosic matter. It is known that the woods thus treated benefit from very improved physicochemical, biological and mechanical qualities and in particular from excellent dimensional stability. It has also been found that these woods have an improved hydrophobic character.

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  This controlled heat treatment of wood is usually done in ovens or reactors. If such an operating mode does not present any major difficulty when it is implemented at the laboratory level, it is no longer the same when this implementation is carried out at the industrial level. One of the problems encountered stems from the difficulty which exists in maintaining, for a determined time, an enclosure of high volume at a temperature which is both uniform in time and in space. To this is added a problem which is linked to a strong tendency to oxidation, even to the flammability, of the treated woods when they are found, at high temperature in the presence of oxygen. This problem is solved by keeping the wood to be treated, during the heating phases, in an oxygen-free atmosphere, which can in particular be obtained by pulsing, in the treatment enclosure, a gas

  inert and in particular carbon dioxide or nitrogen.

  These reactors must consequently be provided with means making it possible to control their internal atmosphere, which results on the one hand by a high manufacturing cost of these and on the other hand by a cost price of the treatment operation important because of the

  consumption of the inert gas used.

  Furthermore, it is generally known that the heat transfers which form between gases and solids are most of the time difficult. If, in the case of laboratory reactors, such a transfer does not present much difficulty, it is not the same when one passes on an industrial scale, and problems are encountered at the level in particular of the phase of cooling which requires a residence time

  particularly long wood in the reactor.

  This is why it has been proposed, in the prior art, by US-A-2,296,316, a method of

  treatment of wood to prevent swelling

  this when it is then in the presence of water. This process consists of immersing the wood in a liquid bath, consisting of a mixture of sodium and potassium acetate brought to a temperature of around 290 C for a period of around fifteen to thirty minutes. At the end of the treatment process, the wood is extracted from the bath to be cooled and cleaned on the surface. Under these conditions, it will be noted that the wood is then in contact with the oxygen in the air, so that it undergoes a brutal thermal shock during cooling, and the air penetrates inside its pores and oxidizes these. It follows that the qualities which have been acquired by the heat treatment in the bath, are

  lost during such a cooling process.

  It has moreover been observed that the woods thus treated exhibited weakened mechanical characteristics as well as a number of bursts and cracks. It is undoubtedly for this reason that this technique, despite its age, has never succeeded in establishing itself and that it is

  currently almost abandoned.

  We know, on the other hand, that the wood retification technique is particularly interesting in that it makes it possible to free the cells of the latter from plant materials.

  that they contain, such as in particular the hemi-

  celluloses, so that the woods thus treated have a more marked hydrophobic character, a slightly increased open porosity which makes this material more suitable for the adsorption of inert products or on the contrary of active products such as for example insecticides such as

  anti-termite products or flame retardant components.

  The object of the present invention is to propose a method of impregnating the wood following in particular a method of treating the wood by heat, and in particular a method known as cross-linking, making it possible to introduce into the mass of the wood, at the heart of this one, a treatment product intended to improve some of its characteristics. The present invention thus relates to a method of treating wood with an impregnation product, characterized in that it comprises the steps consisting in heating the wood to be treated at a temperature above 220 ° C., then in immersing it in a liquid bath, consisting of the impregnation product, which is at a temperature

  lower than the temperature of the wood leaving the heating.

  Preferably the difference between the temperature of the impregnation bath and that of the wood before its immersion

  in it will be of the order of 80 C to 200 "C.

  The process according to the present invention is particularly advantageous when it is carried out on a wood having previously undergone a high temperature treatment, such as in particular a crosslinking treatment. It can thus constitute a phase of this treatment and in particular the last phase, or cooling phase thereof. This will result in substantial energy savings. The method according to the invention then makes it possible to ensure control of this cooling phase which is a critical operation as explained above. The present process thus avoids all thermal shocks and all the oxidation reactions to which a particularly vulnerable wood is subjected since it is at high temperature. The process according to the invention makes it possible to ensure that the treated wood has a relaxation phase during its cooling, as well as a rebalancing of its internal stresses, which avoids the creation of defects

  such as in particular cracking or cracking.

  We can however, of course, carry out the impregnation operation on a wood having undergone a heat treatment some time before and which will then be raised

in temperature.

  In order to achieve a pseudo-continuous treatment of the wood, including its impregnation, the treatment of the wood at high temperature will also be carried out by immersion of this wood in one or more baths, at a temperature suitable for the treatment step carried out. In this embodiment, one can use, as an impregnation product, one of the products forming one of the baths

  used for high temperature treatment.

  It is understood, under these conditions, that the temperature of the wood decreasing in contact with the impregnation bath, the internal pressures existing inside the fibers and channels of the latter decrease according to the law of ideal gases, so that the pressure existing in the pores of the wood decreases and that these will "suck" the liquid bath

in which the wood is immersed.

  Of course the wood will be all the more capable of absorbing the liquid bath that it has previously undergone a treatment (in particular a crosslinking treatment) which will have had the effect of emptying the cells of this wood of the

  plants, such as the hemicelluloses they contained.

  l0 It is of course possible to use such an embodiment of the invention to charge the cells of a retified wood with a neutral product, such as in particular paraffin, if it is desired for example to improve the hygrophobic character of the wood treated by crosslinking, or adsorb fire retardant, mineral solutions which can then be used to mineralize the treated wood, or

an insecticide product.

  In the case where the impregnation product is a mineral solution, it is advantageous to use a lime solution on which carbon dioxide is made to act, after impregnation. We thus realize a

wood mineralization.

  According to the invention, therefore, an extremely easy treatment is carried out in addition to a high temperature treatment of the wood at the same time as the cooling thereof is ensured under optimal conditions. The present invention also has other advantages, in particular when it follows a treatment.

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  thermal known as cross-linking. We know that the cracking of hemicelluloses that occurs during it releases acetic acid. This organic component produces two harmful effects on the wood thus heat treated: on the one hand it gives the material a very marked acid character (pH of the order of 3 to 4) which risks creating corrosion of metallic materials in contact with treated wood and, on the other hand, it gives the latter a characteristic odor which takes a long time to disappear. Finally, acetic acid causes, by acid attack, a depolymerization of the cellulose which, depending on the importance of the treatment, contributes to reducing the mechanical characteristics of the wood. According to the present invention, it is possible, using an impregnation bath with basic characteristics, to neutralize the acetic acid thus generated during the heat treatment operation at high temperature and thus avoid

  the previously mentioned drawbacks.

  According to the present invention, it is advantageous to carry out the various known ideal stages of a retification process, namely drying, the passage through the glass transition period of the wood, the crosslinking step and finally the absorption step. of a determined product, by passing through successive liquid baths of

appropriate temperature.

  An embodiment of the present invention will be described below by way of non-limiting example, with reference to the attached drawing in which: FIG. 1 is a schematic representation of an installation making it possible to implement a method of

  cross-linking / adsorption according to the invention.

  FIG. 2 is a schematic representation of an alternative implementation of the method shown in the

figure 1.

  FIG. 3 is a diagram representing the variation of the temperature as a function of time of wood samples during a process of treatment of ratification by

bath according to the invention.

  In a first embodiment of the wood treatment method according to the invention which is shown in Figure 1, it is proposed to treat at high temperature three types of wood samples: beech, poplar and pine maritime, following a high temperature treatment process, or cross-linking, which is carried out in a bath, and ensuring the control of the cooling of these samples of treated wood in a

  impregnation bath consisting of paraffin.

  By way of comparison, tests have been carried out on identical wood samples which have been subjected to the same heat treatment but the cooling phase of which has been carried out according to the prior state of the art, that is to say that this cooling phase was carried out under gaseous medium, namely in an atmosphere

carbon dioxide.

  Two treatment tanks were used for each sample, namely a first tank 1 containing paraffin brought to a temperature T1 of about 150 C and which

2810917

  corresponds to the value of the glass transition temperature of the three wood samples, and a second tank 2 containing mineral oil brought to a temperature T2 of the order of 240 ° C., this temperature representing the crosslinking temperature. The samples consisted of cylindrical bars 50mm in diameter and 160mm in length,

or a volume of 314 cm3.

  These samples were subjected to a crosslinking treatment, the different known characteristic stages of which are shown in the diagram in FIG. 3. (Variation of the temperatures of the sample in

function of time).

  In a first stage of the ratification treatment, the wood samples were immersed in paraffin tank 1 at 150 ° C. and they were kept there for a period t2 so as to firstly make them undergo drying (part oa of the curve) and on the other hand bring their entire volume to the glass transition temperature (part ab of the curve). When we were sure that each part of the samples had reached this temperature T1 of 150-C, we extracted them from tank 1 to immerse them in the bath contained in tank 2 which is at the ratification temperature T2 of 240 C. The samples were kept in the bath of tank 2 (part cd of the curve) for the time necessary for the ratification then, as soon as this was completed, at time t3, we extracted the samples from tank 2 to immerse them in the paraffin bath at

   C (part d-e of the curve) until time t4.

  il 2810917 As mentioned previously, under the effect of the decrease in temperature, the internal pressure of the channels and of the wood cells decreased and, according to the law of perfect gases, the internal pressure of the wood cells also decreased, if although these cells then "sucked" the paraffin bath and thus filled with it. The samples were then extracted at time t4 from the paraffin bath and, after cooling

  total (time t5) we studied them.

  The results are reported in Tables 1, 2, 3 below. TABLE 1: Maritime pine Sample mass Change in mass (grams) Rate Before After (in grams) (in%) of impregnation Cooling

  conventional under gas 753.91 655.17 - 98.74 13.09 -

  _ I Cooling according to the invention 752.82 792.04 + 39.22 5.20 16.99% in the bath A second series of identical wood samples was subjected to the same retification process, but the cooling step ( part of the curve) was carried out under a gaseous atmosphere, namely under carbon dioxide. The results are reported in Tables 1, 2, 3

correspondents.

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  TABLE 2 Beech Sample mass Change in mass (grams) Before After rate (in grams) (in%) of impregnation Cooling

  conventional under gas 807.62 731.60 - 76.02 - 9.40 -

  Cooling according to the invention 805.70 849.43 + 43.73 + 5.42 14.22% in bath TABLE 3 Poplar Sample mass Change in mass (grams) Rate Before After (in grams) (in%) of impregnation Cooling

  conventional under gas 409.20 381.40 - 27.80 - 6.79 -

  Cooling according to the invention 441.90 537.95 + 96.05 - 32.45% in bath These tables thus represent: - the mass of the sample before and after treatment (in grams) - the variation in mass of the sample following treatment (in mass and in%)

  - the rate of impregnation of the treated wood.

  In the case of heat treatment with gas cooling according to the prior art, there is a loss of mass of the samples

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  of wood which results in a lowering of the density of the material and which results in a reduction of the intrinsic properties and in particular of the characteristics

  mechanical properties of the treated wood.

  On the contrary, in the case of heat treatment with bath cooling, there is an increase, for the three species studied, in the mass of the material and its density. This increase in weight corresponds to the paraffin adsorption carried out by the sample during its cooling. It varies from% for maritime pine to 21.7% for poplar, which clearly shows that the penetration of paraffin has

carried out to the heart of the wood.

  Tests carried out on the two series of samples have shown that the swelling resistance of the impregnated samples, when they are then placed in a humid environment, is much higher than that of the non-impregnated samples which, however, due to their treatment of

  ratification is of a very high standard.

  Such a treatment proves to be particularly advantageous due in particular on the one hand to its extreme ease of implementation and on the other hand to the increase in the swelling resistance of the samples thus treated when they are available in

humid atmosphere.

  The present invention lends itself particularly well to continuous wood treatment. An example of such treatment has thus been shown in FIG. 2. In this embodiment, there are three bins intended to receive

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  successively the samples to be treated. The first tank 1 contains mineral oil maintained at a temperature T1 of 150C, and this bath will be used to carry out both drying of the samples and maintaining them at the glass transition temperature as explained above. The second bath 2 is also constituted by mineral oil, or by any other liquid capable of being brought and maintained at a temperature T2 of the order of 240 C without undergoing significant chemical decomposition and which represents the retification temperature . The bath 2 will receive the samples once the phase of maintaining in the tank 1 at the glass transition temperature is finished, that is to say when the entire mass of the wood to be

  treat will have reached the temperature of 150 C.

  As for the third bath 3, which will be maintained at a temperature T3 lower than the ratification temperature T2, it will consist of the product with which the wood samples are to be impregnated. This product may in particular be an insecticide product, a fireproofing product,

  and will be maintained at the temperature T3 of the order of 160 "C.

  It is also possible, according to the invention, to carry out the impregnation step without moving the wood to be treated from one bath to another, and this by lowering the temperature of the bath used to carry out the crosslinking. In such an embodiment, the ratification bath will then consist of the product with which the wood is to be impregnated and it will thus have to be able to withstand the temperature of

  ratification without undergoing significant degradation.

  1 5 l2810917 Although previously described embodiments in which the high temperature treatment of the wood, prior to impregnation, was carried out by immersion in baths, such treatment could of course be carried out by classical methods i.e.

  by heating in furnaces or reactors.

  As mentioned above, the process according to the invention overcomes a drawback of the processes for treating wood at high temperature. Indeed when

  we know that the cracking of hemi-

  celluloses has the effect of releasing acetic acid, the presence of which in treated wood has undesirable effects on it. According to the invention, this acetic acid can be eliminated using a cooling bath 3

  containing a basic product capable of neutralizing it.

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Claims (10)

  1.- A method of treating wood with an impregnation product, characterized in that it comprises the steps consisting in heating the wood to be impregnated at a temperature above 220 ° C., then in immersing it in a liquid bath, consisting of the impregnation product, which is at a temperature lower than the temperature of the wood at the outlet heating means.   2.- Method according to claim 1 characterized in that the impregnation operation is carried out on a wood having previously undergone a so-called high temperature treatment. 3.- Method according to claim 2 characterized in that the wood heating phase consists of a   high temperature wood treatment phase.   4.- Method according to one of claims 2 or 3   characterized in that the treatment of the wood at high temperature is carried out by immersion of this wood in at   minus a bath of at least one treatment product.   5.- Method according to claim 4 characterized in that the treatment product consists of the impregnation product.   6.- Method according to one of claims   previous characterized in that the difference between the temperature of the impregnation bath and that of the wood before   its immersion in it is around 80 C to 200 C. 17 2810917   7.- Method according to one of claims   previous characterized in that the impregnation product is a basic product.   8.- Method according to claim 7 characterized in that the impregnation product is a product capable of neutralize acetic acid.   9.- Method according to one of claims 1 to 6   characterized in that one uses, as a product   impregnation, a mineral solution.   10.- Method according to claim 9 characterized in that the mineral solution is a lime solution, on which is made to act, after impregnation, carbon dioxide.