EP0936961B1 - Method for treating wood by impregnation - Google Patents

Abstract

A method for treating wood in which the wood to be treated is placed in a chamber in which a vacuum is produced, where the chamber is filled with a hardening product, in particular a monomer, so as to impregnate the wood by causing the product to penetrate the space between its fibers, where the product is hardened, in particular by polymerization. This method is characterized in that the wood impregnating operation is preceded by a step of controlled thermal treatment.

Description

The present invention relates to a improvement of wood treatment processes in which we impregnate it with a product curable.

We know that in its natural state, wood or wood fibers which are in contact with an atmosphere wet tend to soak up water, up to absorb 100% of their weight from it. This water absorption is accompanied by swelling, characteristic of a decrease in the qualities of cohesion of the material, which can range, in certain cases, until an advanced disintegration of it. It is why is it our habit to precede any wood shaping operation of a drying step which, by removing water from it, improves its dimensional stability.

If the drying step removes water from the wood it does not change anything against the character hydrophilic of it so that it is again able to reabsorb the water eliminated during drying, when it is again in a humid atmosphere.

In order to reduce the hydrophilic character of the wood natural, and thus give it stability dimensional dimension, we proposed different high temperature heat treatment techniques.

Among these techniques, it has been proposed to subject with natural wood different stages of treatment including in particular open circuit drying followed heating and maintaining a temperature between about 220 ° C and 300 ° C during a specified period. Such a processing technique, so-called retification, gives the wood, the both hydrophobic and excellent dimensional stability.

We know from patent FR-A-1 406 137 a wood treatment process, in which the latter, after heating which may bring it to a temperature at most equal to 177 ° C., is subjected to a impregnation at a pressure different from that of the atmospheric pressure, namely, a depression or overpressure.

We also proposed, in particular in order to improve the mechanical properties of wood, to make use of treatment techniques which consist of impregnate it with a monomer then, this impregnation having been carried out, to polymerize it in situ, using different techniques, such as in particular the action of a gamma radiation or the action of heat.

Treatment methods used in the state prior of the technique consist in placing the wood to be treated in an enclosure in which the empty, to fill it with the impregnating monomer, and to apply a strong pressure in this enclosure of so as to penetrate the monomer into the wood. He ... not then remains to polymerize the monomer.

If these techniques prove satisfactory on the plan to improve the mechanical properties of treated wood, especially hardness, they are much less so on that of stability volume of wood in a humid atmosphere. So we have noticed that the woods thus impregnated presented the peculiarity, after a certain time of use, lose their grip with the polymer with which they were impregnated, which resulted in swelling of this wood.

The object of the present invention is to provide a improvement in wood impregnation techniques by hardenable products, and in particular by monomers, in particular with a view to imparting to them dimensional stability characteristics when are subsequently exposed to a humid atmosphere.

The subject of the present invention is therefore a wood treatment process of the type in which arranges the wood to be treated in an enclosure in which creates a vacuum, where we fill said enclosure of a curable product, in particular a monomer, so as to impregnate the wood by making penetrate the product into its fiber space, which then causes the hardening of said product, by polymerization, characterized by what we do precede the impregnation operation of the wood from a cross-linking step, i.e. a stage during which we heat and maintain the wood at a temperature between 220 ° C and 300 ° C, and in that the impregnation is carried out immediately after the cross-linking step, so as to use the remaining heat of the wood being cooled to thermally polymerize the monomer.

Interestingly said product is a monomer which is hardened: by polymerization, in particular by the action of radiation or thermally.

We know that a crosslinking operation. consists to subject the wood to a heat treatment in controlled conditions to cause reactions thermocondensation at the lignocellulosic structure wood. A cross-linking operation is usually performed in a neutral atmosphere or reducing on previously dried wood, in subjecting it to a temperature between 220 ° C and 280 ° C for a time long enough to that the entire mass of the treated wood reaches the processing temperature, and without exceeding this treatment. We can see that, under these conditions, this duration depends on the nature and thickness of the wood.

The Applicant has found that this operation of retification possessed: three important effects, to know how to slightly increase the porosity of the wood, give it a hydrophobic character, and improve its wettability with respect to the monomers by modifying the surface tension of the wood. These three effects are used in particular according to this invention to promote the impregnation of wood by the monomer.

The tests carried out by the applicant, and which will be described in detail below, thus making it possible to demonstrate that by preceding, according to the invention, the impregnation of the product, and in particular of a monomer, by a cross-linking operation, not only obtained a higher impregnation than that conforming to the prior art, but that it was not no longer necessary to obtain such an impregnation of pressurize the enclosure containing the wood to treat and impregnation product.

The Applicant has also established that the process according to the invention made it possible to carry out a impregnation, with monomers, of varieties of wood which until now were considered almost non impregnated by known methods, such as notably spruce, oak, chestnut, beech.

According to the invention, some of the agents of swelling, such as especially methanol, which are used according to the prior art prior to the impregnation operation, are not more necessary, which represents an economy, a share in terms of the cost of the product itself, and on the other hand in terms of the cost of implementation of the process.

The polymerization of the monomer can, of course, be performed using multiple techniques and in particular among these by the action of rays gamma, by convection heating and by heating by microwave.

Various examples of implementation will be described below. work of the invention with reference to the attached drawing on which :

Figure 1 is a graph that represents the variation in temperature to which a wood to be treated as a function of time during a cross-linking operation.

Figure 2 is a schematic view of a reactor allowing to carry out an impregnation operation of a piece of wood by a monomer.

Figure 3 is a graph that represents the variation of the impregnation rate as a function of time, a sample of wood with a monomer, respectively in the case of a natural wood and a retified wood.

Figure 4 is a graph that represents the variation of the volume swelling as a function of time, wood samples in the presence of an atmosphere wet.

Although the Applicant has established that the present invention is applicable to many wood species, including beech, charm, ash and poplar, we will describe as an example the process according to the invention in the case of a application to charm.

We therefore propose to treat according to the invention samples made up of charming pieces. According to the invention, the first part of the treatment involves subjecting these pieces of wood cross-linking treatment. To do these are introduced into an enclosure of processing where they are subject to variations in programmed temperatures. We have thus represented on the figure 1 the temperature variation T (in ° C) in function of time t (in min), to which we have focused the enclosure containing the charming rooms to be treated and dotted the temperature of the wood.

Such a treatment process includes three essential steps, namely a drying step A, preferably a glass transition step B, and a proper retification step C.

The first drying step A breaks down itself in two phases, a first phase A1 during which gradually raises the temperature of the treatment chamber containing the charm to be treated with a temperature rise rate of approximately 5 ° C / min, from room temperature to a temperature T1, close to 100 ° C., followed by a phase A2 during which the enclosure 1 temperature at a threshold value T1 until the end of drying.

During the second stage B, which is optional, but which, when carried out, greatly improves the efficiency of the process, we raise gradually the temperature of the enclosure with a rate of temperature rise close to previous, from temperature T1 to a temperature Tg close to the transition temperature vitreous of the species of wood considered, namely the charm in this case. We maintain the temperature Tg at this plateau value for the time required so that the entire mass of treated wood reaches the glass transition temperature Tg. will notice that extending the duration of this bearing does not result in any harmful consequence in concerning the respect of the mechanical qualities of the processed product.

During the third stage C we raise gradually, during a phase C1, the enclosure temperature with rising speed in temperature close to the previous one, the temperature glass transition Tg at the temperature of retification Tr and maintained for one second phase C2 the oven temperature at this plateau value, until most hemicelluloses are broken down.

We know that one of the difficulties of this stage specific is that maintaining temperature must be carried out for a time long enough for the hemicellulose to be broken down in almost all but that it is imperative not to exceed this time under penalty of start destroying lignin and cellulose, which would then translate into a drop in mechanical characteristics of treated wood.

This first part made it possible to prepare the almost perfect material in the second part, know the impregnation, which will follow, especially in opening the pores of the wood, giving it a hydrophobic character and increasing its wettability vis-à-vis the monomer.

We then implement this second part of the treatment, which itself comprises two stages, to know an impregnation step with a product curable and in particular a polymerizable monomer, and a step of hardening, or polymerization, of the wood impregnated product.

To implement the first step we uses a device like the one shown on Figure 2, and which is well known from the prior state of technique. This device consists essentially an enclosure 1, or reactor, which has a suction line 3 connected to a pump unladen, not shown in the drawing, a pipe 5 injection of the monomer into enclosure 1, and a pipe 7 for the evacuation thereof.

The wood to be treated 2 is introduced into the enclosure 1 by a door 8, then there is a vacuum in it by sucking in the air it contains through line 3 of so as to empty as much as possible the "cells" wood. We then inject the product we want impregnate the wood in enclosure 1. We leave in the state for a period of about thirty minutes, depending on the viscosity of the product used, then the reactor 1 is emptied via line 7.

The Applicant has established that the processing of retification improved the impregnation of the wood, without that, as in the previous state of the technical, it is necessary to put under pressure the product contained in reactor 1.

The curve of FIG. 3 represents the variation in the charm impregnation rate (in%) in function of the passage time t in reactor 1, for natural charm (curve a) and charm that has undergone previously a crosslinking with a monomer which is GMA (curve b), that is to say methacrylate glycidyle. We note that, in both cases, glicidyle impregnation stabilizes after a duration of about thirty minutes, and that the rate permeating the charm which, in its natural state, was 50%, increased to 55% when it was, prior to impregnation, subject to retification. The plaintiff obtained results from same order with wood species and monomers different, especially for beech, ash, poplar and various conifers.

Then the hardening step of the product. If this is a monomer we can submit impregnated wood, depending on the nature of the monomer used, either by the action of gamma radiation or the action of heat in order to cause its polymerization. Heating can be performed in particular by conventional means of convection or again by microwave.

Regarding polymerization by route thermal, the Applicant has found that wood retified exhibited better behavior than wood natural. Indeed, because of the treatment he has undergone, the retified wood no longer emits gas or vapor water like natural wood, emission which is the cause poor adhesion of the polymer

We can also carry out the impregnation immediately after the cross-linking step, so that benefit from the remaining heat of the wood being cooling, to ensure, thermally the polymerization of the monomer which allows both optimize the process and achieve a gain of energy.

The Applicant has carried out tests in order to compare the dimensional stability of natural wood impregnated with that of the same wood treated following the invention, when they were placed in an atmosphere wet.

These tests were carried out in an enclosure in which kinetics have been carried out humidity recovery. The enclosure had a 75% humidity and a temperature of 25 ° C, depending on standard NF 51-264. We measured the swelling in time function of wood samples made up of the species by the charm previously treated, respectively in the natural state (curve a), in the state impregnated according to the prior art (curve b), and in the impregnated state after crosslinking (curve c). The results were recorded on the figure 4.

It can be seen in this figure that the impregnation has not dimensionally stabilized the wood in the state natural. Indeed there is very little difference between the natural wood (ΔV close to 7%) and natural wood impregnated (ΔV around 6.5%).

We note that the wood which, prior to its impregnation, underwent cross-linking treatment has a very reduced ΔV swelling since it is around 3%.

The Applicant has of course also treated charm samples, on the one hand in the natural state and on the other hand in the crosslinked state, using monomers of various natures, namely HEMA (methacrylate of hydroxyethyl), HEA (2-hydroxyethyl acrylate) and AGE-AM (mixture of 70% allyl glycidile ether and 30% maleic anhydride) and GMA and then, after treatment, subjected these various samples to a swelling test according to the aforementioned standard NF 51-264 and measured the swelling of these samples. The results are given in Table I below, in which they are expressed as a percentage of ASE units, that is to say as a percentage of the conventional dimensional stabilization coefficient. TYPE OF WOOD MONOMER ASE (in% of the conventional dimensional stabilization coefficient) Natural impregnated HEMA 44.1 "" HEA 35.4 "" AGE-AM 42.1 "" GMA 26.6 Impregnated crosslinked HEMA 49.1 "" HEA 64.3 "" AGE-AM 59.9 "" GMA 77

It can be seen that the different monomers used are more or less effective in stabilizing dimensional in a humid atmosphere of treated wood, but that in all cases this stabilization is higher than that obtained according to the previous state of technique, that is to say by impregnation without prior retification.

In addition to dimensional stability, this invention also increases the hardness of the treated wood.

Claims (5)

1. Method of treating wood, in which the wood to be treated is placed in an enclosure in which a vacuum is created, wherein said enclosure is filled with a curable product in order to impregnate the wood by causing the product to penetrate the space between the fibres thereof, wherein said product is then cured by polymerisation, characterised in that the operation of impregnating the wood is preceded by a seasoning step, i.e. a step during which the wood is heated to and maintained at a temperature ranging between 280°C and 300°C and in that impregnation is effected immediately after the seasoning step in order to make use of the residual heat in the wood during cooling to produce polymerisation of the monomer by thermal means.
2. Method as claimed in claim 1, characterised in that said product is a monomer which is cured by polymerisation.
3. Method as claimed in claim 2, characterised in that the monomer belongs to the group of monomers consisting of GMA, HEMA, HEA, AGE-AM.
4. Method as claimed in one of the preceding claims, characterised in that the seasoning process incorporates a step of vitreous transition.
5. Method as claimed in one of the preceding claims, characterised in that the wood to be treated is impregnated with said product in a treatment enclosure in which the internal pressure is close to atmospheric pressure.

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