EP0191276A1 - Method for preparing wood to impregnate it - Google Patents

Abstract

The heartwood parts (2) are perforated by making a plurality of perforations (1) of small diameter, for example between 1 and 5 mm, in particular along a path inclined relative to the longitudinal plane of the part. The method prepares the wood for its impregnation at heart. It applies in particular to wood for constructing industrial cooling towers. <IMAGE>

Description

The invention relates to the protective treatment of wood, in particular for industrial cooling towers (also called atmospheric refrigerants).

The invention relates more particularly to a process for preparing wood for its impregnation to the core.

The process of the invention is prior to the impregnation of the wood and is independent of the technique and of the impregnation product which will be used.

Three wood impregnation processes are known: the Rueping process, the Bethell process and the Lowry process. The official document "Timber Preservation in New Zealand: Specifications" summarizes them. The NEN 3255 standard also mentions the Rueping process. These three processes have been universally known for a long time, even if official standards do not always cite them by their proper names.

For many woods used industrially the penetration is negligible because it takes place in the direction of the length (of the order of one to several meters) of the pieces of wood, essentially along the axis of the trunks of trees. However, it is highly desirable to be able to treat the wood in depth, throughout the mass, if possible, in order to ensure its durability over time, for example 15 years, at least 10 years, or even 30 years or more if the pieces of wood are used, for example, in the construction of atmospheric refrigerants for power plants, and this in an environment conducive to its degradation, both chemical and bacteriological. Indeed, inside atmospheric refrigerants, wood is subjected to the action of water to be cooled, often charged with various chemical substances, to that of atmospheric air, carrying innumerable biological germs (algae, mushroom gnons, (fungi), ...), and to strong variations in temperature and humidity, especially between the stops and the steps of the atmospheric refrigerant.

The pieces of wood used in industrial constructions, and in particular in atmospheric refrigerants, are obviously treated by impregnation before assembly, all the various cuts and perforations, in particular for the passage of bolts, having however been carried out before the impregnation treatment. The manipulation of pieces of wood, their assembly, their assembly in industrial constructions constitute numerous occasions to injure the wood; if the impregnation is very shallow, the slightest injury is a primer to the subsequent attack of the wood by aggressive biological and chemical agents which easily reach the non-impregnated part. The slightest defect in a thin impregnation layer also makes it locally porous and therefore conducive to the penetration of destructive agents. A thin layer is also easily pierced mechanically, both by the accidental action of man and by the action of animals and various parasites. It is therefore important to seek to improve the penetration of impregnation products.

The only way currently known to increase the penetration of impregnation products is to incise (notch). wood. This technique is cited in 1.6 of standard C1-82 of the AMERICAN WOOD-PRESERVER'S ASSOCIATION.

This technique is described in the paragraph "Incising" on page 18-13 of the book "Wood handbook: Wood as an Engineering Material" from Forest Products Laboratory, Forest Service, US Department of Agriculture (Library of Congress Catalog Card n ° 73 -600335) and illustrated in Figure 18-2. This incision technique is also described in section 7.1 of the Cooling Tower Institute (CTI) standard "Pressure Preservative Treatment of Lumber _for industrial water-cooling towers" (CTI Bulletin WMS-112).

Although this technique of mechanical preparation of wood by incision, with a view to its impregnation, incontestably increases the relative volume of wood which is impregnated, it is unsatisfactory. Indeed, on the one hand it only slightly increases the impregnated volume and, on the other hand, it frankly strengthens the wood. There is a trade-off between increasing the impregnated volume and the deteriorating mechanical properties of the wood; this compromise generally limits the penetration of incisions to 10 to 20 mm (see Wood handbook); however for cooling towers, where the wood is particularly stressed both mechanically and by the environment, the atmosphere, to which it is subjected, the Cooling Tower Institute recommends not to exceed 0.4 inch deep (see CTI Bulletin WMS- 112), or about ten mm. If for hard-to-impregnate woods, the penetration depth has thus been multiplied by 5 or even 10, this remains however small; it is very far from a penetration to the core of the impregnation products, this penetration being always very low in the direction perpendicular to the fibers ("grains" in English). As for the allocation of mechanical resistance, it is greater than it appears at first glance according to the arrangement of the notches, however parallel to the trunk of the tree, that is to say to the fibers; in fact, the fibers are not strictly coaxial with the trunk (theoretically vertical for a straight upright tree); they have inclinations relative to the axis of the trunk, or more exactly by relation to the direction of movement of the logs ("log" in English) in the incising machine, on the order of 10% on average, random, on either side of the axis; the length of the notches will therefore affect a quantity of fibers which is all the greater as the notches are, on the one hand, longer and on the other hand, closer to one another in the direction of the length. However, these notches are relatively long (0.4 inch or 10 mm typically) and the longitudinal pitch is relatively small (0.4 inch or 10 mm between two successive notches offset tangentially by 0.2 inch, or 5.0 mm; 3 inches or 75 mm, between two aligned notches). The reduction in mechanical properties will be greater the smaller the diameter of the log, and will be particularly sensitive in bending for example, because it is essentially the outer layers which provide resistance to bending. In the calculation of resistance of wooden constructions, particularly atmospheric refrigerants, it will be necessary to admit significant reductions in the maximum admissible tensions, of the order of 20 to 25%.

To overcome the shortcomings and disadvantages of the known method cited above, it is proposed to perforate the logs to the heart, possibly right through, of a multitude of perforations of small diameters, in judicious directions and according to a pattern (pattern ) appropriate.

The diameters of the perforations are preferably 2 to 3 mm; they may however be between 1 and 5 mm depending on the case (nature of the wood, length of the perforation, nature of the subsequent impregnation, method of perforation). The holes are always drilled perpendicular to the fibers; but in any plane perpendicular to the fibers, the perforations are oriented either perpendicular to the ring rings, or tangentially to the ring rings, or at any angle with respect to the ring rings. Preferably, the perforations are substantially perpendicular to the rings; this is the case for round pieces whose axis is substantially that of the tree (logs, posts, piles).

For parts with a substantially square cross-section offset from the axis of the shaft, preferably perforated perpendicular to the faces which give the highest angles (closest to 90 °) between the perforation and the annular rings.

On the other hand, for parts of rectangular section with a large width / height ratio, the parts are preferably perforated on the faces which give the shortest perforation lengths. ''

Optionally, these square and rectangular section pieces can be perforated on faces perpendicular to each other. The pieces of any section (triangular, semi-cylindrical, etc.) are preferably perforated on their flat faces and are perforated perpendicular to these faces. The perforations on one side are offset from each other both in the direction of the fibers and in the perpendicular direction. During the impregnation, the impregnation product will fill the holes resulting from the perforations and will diffuse in the wood especially along the fibers, from a few cm to a few tens of cm as indicated above; in the other directions the penetration will be weaker.

Consequently, the spacing of the holes is different in the direction of the fibers and in the direction perpendicular to the fibers. It is preferably larger in the direction of the fibers than in the perpendicular direction fiber laire.

When the perforations are made, it can be done by any known process, in particular by drilling using drill bits or forests. But it can also be done in a new way by hollow needles, the central channel of the needle ensuring the evacuation of the wood.

The central recess of the needles seems essential; in fact, a solid needle would crush the wood over its entire length and create tensions which could crack the wood and create breaking points. Such hollow needles have a length at least equal to that of the hole to be made, for example 35 mm, and have a diameter, for example, outside of 3 mm, and inside of 1.8 mm, the thickness of the walls of the needle being 0.6 mm; these needles would be, for example, special hardened tool steel; the perforation would be by depression, either continuous or jerky; a correct draft of the tip of the tool ensures a good cut, avoiding any crushing of the wood, and ensures the reflux of the chips by the center of the needle.

The essential advantages of the hollow needle compared to the forest are that it does not require rotating equipment, which is a source of danger and is more expensive, and that it does not heat the wood locally.

The avant-garde technique of laser drilling is also possible for making perforations.

A description of the invention is now given below according to the attached drawings in which:

Figure 1 is a sectional view of a piece of wood showing the penetration of an impregnated product gnation in the absence of incisions or perforations, (prior art);

Figure 2 shows in elevation a piece of perforated wood according to the invention and Figures 3 to 6 show in cross section various modes of perforation.

In FIG. 1, which illustrates the prior art, the shaded part shows how the impregnation product penetrates negligently, in the direction of the length of the fibers, essentially along the axis of the tree trunks, in the absence of incisions.

In Figure 2, we see the perforations 1 made in the minimum thickness of the piece of wood 2 along an oblique path relative to the longitudinal plane of the piece (piece of rectangular or any section). The spacing of the holes is different in the direction of the fibers ("d" in Figure 2) and in the perpendicular direction ("e" in Figure 2); in the direction perpendicular to the fibers, the spacing is relatively small, of the order of magnitude of 2 to 15 mm (2 mm is a limit slightly less than the 0.2 inch in FIG. 1 of the standard CTI-WMS 112) ; in the direction of the fibers, the spacing is preferably greater (certainly not smaller) so as not to force the piece of wood, the easy penetration of impregnation products in this direction allowing it; this spacing is of the order of magnitude from 4 to 40 mm.

The new wood preparation process therefore ensures good penetration to the core; as for the deterioration of the mechanical properties, it is significantly lower than for the incision process; the derating factors of the maximum admissible tensions are of the order of 6 to 15%, depending on the arrangement of the perforations and the nature of the constraints.

In FIG. 3, the perforations 1, executed from a single face, pass through the part; on the other hand, in FIG. 4, relating to a thicker part, the perforations 1 are executed on two opposite faces and are blind, stopping shortly before the center 3 of the part; they are aligned there opposite one another; alternatively, they can be offset.

In FIG. 5 relating to a log or a post, the perforations 1 are radial, of equal length, stop substantially before the center 3, and are preferably arranged in a helix around the periphery of the part.

As a variant, it is possible to have series of perforations of different lengths, of which at least one 4 arrives practically at the center 3, the other perforations 5 ending so as to leave enough material between their ends to avoid reducing the mechanical properties too much. while ensuring good impregnation (Figure 6).

Claims (20)

1. A method of preparing wood before impregnation, characterized in that the logs or pieces of wood are pierced through by practicing a multitude of small diameter perforations in the directions and according to an appropriate pattern. 2. Method according to claim 1 characterized in that the perforations have a diameter between 1 and 5 mm. 3. Method according to claim 2, characterized in that the perforations have a diameter of 2 to 3 mm. 4. Method according to claims 1 to 3, characterized in that the perforations are drilled. perpendicular to the fibers. 5. Method according to claim 4, characterized in that the perforations are oriented as desired according to one of the provisions below: - perpendicular to the annular rings; - tangentially to the annular rings; - at any angle relative to the annular rings. 6. Method according to claim 4 applied to round parts whose axis is substantially that of the tree, characterized in that the perforations are made in substance perpendicular to the annular rings. 7. Method according to claim 4 applied to parts of substantially square section offset from the axis of the shaft, characterized in that the perforations are made perpendicular to the faces which give the highest angles, the closest to 90 ° between the perforations and the annular rings 8. Method according to claim 4 applied pieces of rectangular section with a large width / height ratio, characterized in that the perforations are made on the faces which give the shortest perforation lengths. 9. Method according to claims 7 and 8, characterized in that the perforations are formed on faces perpendicular to each other. 10. Method according to claim 4 applied to parts of any section, characterized in that the perforations are formed on the flat faces and perpendicular to these faces. 11. Method according to claims 4-10, characterized in that the perforations are offset from one another both in the direction of the fibers and in the perpendicular direction. 12. Method according to claim 11, characterized in that the spacing of the perforations is different in the direction of the fibers (spacing d) and in the direction perpendicular to the fibers (spacing e). 13. Method according to claim 12, characterized in that the spacing e is from 2 to 15 mm and the spacing d is from 4 to 40 mm. 14. Method according to claim 4, characterized in that the perforations are carried out from a single face and pass through the part. 15. The method of claim 4, characterized in that the perforations are performed on two opposite faces and are blind, stopping shortly before the center of the room. 16. The method of claim 15, characterized in that the perforations of two opposite faces are optional - aligned with each other - offset from each other 17. Method according to claim 15, applied to a log or a post, characterized in that the perforations are radial, of equal length and stop substantially before the center. 18. The method of claim 15 applies to a log or to a post, characterized in that the perforations are radial, of different lengths, at least one arriving practically in the center, the other perforations ending so as to leave between their ends enough material to avoid harming the mechanical properties of the part while allowing its good impregnation. 19. Method according to claims 1 to 18, characterized in that all known methods are used for the perforations such as drilling using drill bits or forests 20. Method according to claims 1 to 18, characterized in that one implements for the perforations of the hollow needles.

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