Wood preservation process

Chemical wood preservation processes serve to protect wood preventively against wood-destroying or wood-discolouring organisms , or to combat infestation by these organisms . The aim of modern wood protection is to ensure a high, uniform penetration depth of the wood preservative using a process that is economical and environmentally friendly. The most commonly used methods are boiler pressure impregnation (KDI) and boiler vacuum pressure impregnation (KVD), but several types of wood ( Douglas fir , larch and spruce ) are only partially suitable for these treatments. By perforation , some success has been achieved, but these have to be tailored to the specific application of the wood.

history

Probably the first attempts to protect wood consisted of rubbing or painting the surface of the wood with substances that were expected to have a corresponding protective effect. The most effective wood preservatives and processes were determined through tests . With the Industrial Revolution , there were more options for wood preservation and the need for treated wood increased. This triggered a sharp increase in new processes and techniques in the early 19th century, with the greatest innovations between 1830 and 1840. In that decade, Bethell, Boucherie, Burnett and Kyan made history in wood protection with numerous inventions. Since then, innumerable innovations, inventions, and process improvements have been developed.

Impregnability

Classification of impregnability

In EN 350-2 the treatability of wood is defined and classified into four impregnability (good, moderate, severe and very difficult to be soaked) made. The information relates primarily to the sapwood and only in some cases to the entire cross-section. In principle, the core cannot be impregnated. For some types of wood (e.g. spruce) technical pretreatments are specified for successful impregnation (laser perforation, drilling perforation or mechanical needling).

• Class 1: easy to soak (pine ( sapwood area), beech)
• Class 2: moderately drinkable
• Class 3: difficult to drink (Douglas fir)
• Class 4: very difficult to water (spruce, larch)

It should be noted that there can also be a high degree of variability within individual species.

Influence of the microstructure of the wood

In addition to the chemical properties of the impregnating agent and the process design of the wood protection method, the microstructural properties of the wood have a major influence on the impregnability.

The transport of liquids in the wood, which is responsible for impregnability, can take place in three ways: through the cell walls , through the pits and via continuous connecting paths . One of the most important factors affecting the impregnability of wood is the anatomical direction of penetration. Due to the elongated shape of the transport cells, wood can be soaked in a lengthwise direction many times better than across it.

In the industrial impregnation of softwood sawn timber , the longitudinal penetration is negligible compared to the radial penetration due to the sawn timber dimensions. Radial penetration by wood rays and the resin channels they contain is the main route through which liquids penetrate into the sapwood of long specimens.

The liquid transport in the hardwood takes place mainly in the longitudinally oriented vessels consisting of numerous vessel members, even with the usual sawn timber dimensions . In contrast to a possible verticalization , the vessel perforations generally do not lead to a restriction of the flow.

When wood is impregnated, a large part of the liquid in question penetrates into the wood via the pits. Their construction is therefore of crucial importance for the impregnability of the wood.

Pressureless processes

There are numerous pressureless wood preservation processes that involve different steps. The most commonly used methods include brushing, spraying, dipping, or cold and hot baths. There are also a number of less well-known or outdated methods of application such as charring, diffusion processes and juice displacement.

Painting and spraying

Painting is an old and widely used application method. It is mainly used in the carpentry sector. Thanks to further technological developments, it is also possible to apply wood preservatives by spraying. A priority in the formulation of modern chemical wood preservatives is the development of active ingredients which, despite their low toxicity, guarantee an interruption of the development cycle of the wood-damaging larvae. Here, the depth of penetration of the funds is of particular importance for the effect of the funds. Oil- based, glycol-based agents and some water-soluble and water- emulsifiable wood preservatives can be applied. A major difficulty when using water-based borates is the premature crystallization of the active ingredient in the area close to the surface. When painting and spraying with water-based or glycol-based agents, adequate pre-watering is therefore particularly important for dry woods. Since the eggs of animal wood pests are deposited in cracks and crevices, their careful handling is of particular importance. When using timber with high moisture content, cracks and crevices that form after the wood protection treatment remain untreated due to the drying process. In the event of subsequent treatment in the existing building, particular care must be taken to treat the cracks and crevices thoroughly.

Diving

This process consists of immersing all of the wood in a wood preservative bath for a period of a few minutes to several hours. Edge protection can thus be achieved, i.e. H. a penetration depth of the order of millimeters. In addition, there is also what is known as “brief dipping”, in which the wood only remains in the wood preservative solution for seconds to minutes. The depths of penetration achieved with short dives are of the same order of magnitude as with brushing; H. only surface protection without a defined penetration depth can be achieved. The immersion process requires larger amounts of funds and a system designed for this purpose, so they are not suitable for smaller jobs. The most common area of ​​application is the treatment of wooden components such as formwork, timber, windows and doors. Color pigments are also often used when diving.

Trough impregnation

The wood is immersed in the wood preservative for hours, which enables even and deep penetration. Trough impregnation is mainly used for protective salt impregnation.

The German Society for Wood Research (DGfH) has described the procedure for such self-monitoring in a leaflet “Procedure for treating wood with wood preservatives, Part 2, non-printing process”.

Areas of application

Use class 1, 2 and 3

target

Edge protection, a few mm penetration in the edge area. The following is required: storage of the wood for at least one to several days for wood moisture levels: up to 20% dry and semi-dry up to 30%, in special cases: moist up to max. 50%

Protective agent

(practically only) soluble in water

Aftercare

non-fixing (GK 1.2 without test rating W): Positioning afterwards always protected from rain, fixing (GK 3 with test rating W): certain fixation positioning, temporarily (at least 7 days) protected from rain

Kyanize

The Kyanisierung was in 1823 as the oldest consciously applied chemical wood preservation method of John Kyan invented an Englishman. In this process, patented in 1832, dried wood, e.g. B. sleepers or telegraph poles, immersed in a 0.67% mercury (II) chloride solution. This process, which is now meaningless, is only effective on the surface of the wood.

Trough suction process

This is a long term process. The logs are immersed in the wood preservative for several days to weeks. Because of this type of treatment, it is sometimes referred to as the emplacement procedure . This procedure was introduced by Kyan in the 19th century . The penetration depths and quantities absorbed depend on the type of wood (impregnability), moisture content, the type of protective agent and the duration of impregnation. With increasing soaking time, protective agent absorption and penetration slow down. In the case of dried wood, a second drying process must be carried out after the treatment. Posts are only placed in the trough with the endangered end. However, they must be impregnated at least 30 cm above the level of the earth. The woods that are submerged in the troughs must be secured against floating and stacked in such a way that the wood preservative can penetrate to all surfaces.

The penetration depths achieved are between 5 and 10 mm, but can also be up to 30 mm for pine and beech . Because of the lower penetration depth compared to printing processes, a slightly higher concentration should be selected. The concentration of the water-soluble solution must be constantly monitored by spindles and, if necessary, corrected by adding salt. The minimum concentration is 5% and 10% for fresh wood. This process is mainly used with freshly juiced wood. However, if oils or solvent-based preparations are used, the wood must be at least semi-dry. Despite its former popularity in Europe, this procedure is only used minimally today.

Hot / cold trough impregnation

Invented and patented by CA Seeley, this process consists of repeated dipping of wood in cold and hot baths of wood preservatives. During the hot bath, the air inside the wood expands. After switching to the cold bath (or switching to the cooled wood preservative), a certain vacuum effect occurs, and this leads to increased absorption. A small amount of wood preservative is also absorbed during the hot bath, but the majority is absorbed during the cold bath. The process is repeated several times, thereby saving time compared to other trough-soaking processes. Each bath lasts 4 to 8 hours, in some cases longer. The wood protection temperature in the hot bath is 60 to 110 ° C and in the cold one 30 to 40 ° C. Both water-soluble preparations and oil-based agents can be used in this process. An average penetration depth of 30 to 50 mm is achieved.

Osmosis process

This process was developed in Germany and consists of the diffusion principle. A wood preservative in pasty form is applied to the freshly juiced wood. Then the wood is stacked tightly and well covered to avoid any loss of moisture. The stacks are then left to stand for 30 to 90 days while the water-soluble wood preservative diffuses into the wood. In the United States and Canada, the osmosis process is used to treat fence posts as well as post-treatment on standing posts and telephone poles. However, this method is not often used because of the great effort involved.

Juice displacement process

In this process, as the name suggests, the soaking liquid is introduced into the wood by displacing the cell sap. It follows that with this type of drying the wood to be impregnated is still standing or freshly felled. Several attempts have been made over the years to find a workable method of implementation. The original Boucherie process also consisted of this principle (see Boucherie process). Attempts have also been made to use this principle to make trees or other plants resistant to fungi, insects and parasites (mistletoe), with individual successes in maize .

Water is released into the air through perspiration in leaves or needles. This creates a negative pressure inside the branch and later in the tree trunk. Water enriched with minerals comes from the roots as replenishment through the sapwood. The course of applied wood preservative will mainly take place in the sapwood and in the vertical direction.

This process was popular in the GDR to discolor the wood of trees. Holes were drilled in the trunk, filled with dye and sealed. The resulting wood was streaked with paint. The penetration of the solution is the most complex of all the variations in the process. Levi S. Gardner tried to solve this with a cut around the tree. The outer cut was then sealed and the internal voids filled with dye.

H. Renner's (1929) method consisted of drilling a hole through the entire tree trunk and then cutting through the splint with a double-edged saw. This process was repeated with a hole 30 cm above at right angles to it. Carl Schmittutz (1934) combined the osmosis process with the sap displacement process by peeling the tree trunk up to a height of 1 m and then coating it with paste-like wood preservative. The injured areas were then covered with a protective paper and the agent diffused into the splint.

Other solutions have been developed by the US Bureau of Entomology and Plant Quarantine. Their suggestions were to cut the tree down and prop it on another, then put the trunk in a tub of wood preservative. In general, the advantages of this method are treatment of the entire sapwood area with little use of energy. However, this process is rarely used in industrial wood protection.

Printing process

With pressure impregnation, the impregnating agent is pressed into the wood under high pressure (9-10 bar) for several hours. The aim is to ensure that the protection agent is distributed as uniformly and as deeply as possible in the impregnable part of the wood (sapwood), although this varies in larger areas depending on the impregnability of the wood species. In general, pressure impregnations achieve a more even distribution than pressureless methods. The depth of penetration depends on the type of wood. This process can be adapted to machine treatment of larger quantities of wood. Pressure impregnation is not suitable for treating spruce and fir, but only to a limited extent with larch and Douglas fir.

One method of pressure impregnation is pressure impregnation , where the wood is treated in cigar-shaped kettles. These include the full impregnation process, economy potion process and alternating pressure process. The Boucherie process, as a purely hydrostatic pressure process, works without a pressure vessel. Which method is chosen depends on the type of wood, wood moisture at the time of protective treatment, wood preservative used, penetration depth and the planned use of the wood. Before the actual treatment with wood preservatives, the air is sometimes drawn out of the wood with a vacuum of a maximum of 40 mbar absolute pressure; this vacuum phase usually lasts 30-40 minutes. Then the impregnation cylinder is flooded with the impregnating agent and put under pressure, sometimes with an increase in temperature. A final vacuum (usually 10–15 minutes) after the cylinders have been emptied is usually carried out in order to recover excess fluid.

Full soaking process

This procedure was developed by William Burnett (for water-soluble agents) and by John Bethell (for oil-soluble preservatives) in 1838. The aim of this process is to achieve the highest possible absorption of the wood preservative. Dry to semi-dry (“ready-to-drink”) woods are suitable for impregnation, i. H. with wood moisture below the fiber saturation point. Above all, water-soluble wood preservatives are used. Rarely used, e.g. B. in railway sleepers , find coal tar and impregnation oils and solvent-based preparations. The desired wood preservative density is regulated by the concentration of the solution. The main feature is a vacuum before the protective agent is charged.

Saving drink process

This process is suitable for easily impregnable types of wood. A large part of the wood preservative that has been introduced is sucked off again, leaving a protective layer within the cells. Mainly oil-based agents are used, but it is also possible to apply this method to water-soluble protective agents. Thresholds, bars, posts and construction timber are treated in this way. There are two basic methods, Rueping and Lowry, with several sub-variations.

Simple Rüping procedure

Patented by Max Rüping from Germany in 1902, the main feature of this process is the air pressure at the beginning of the process. Use with the wood types pine and oak.

Double Rüping procedure

Two simple Rüping procedures are strung together. This process is used for beech wood.

Lowry process

Developed by CB Lowry in 1906, this process is similar to the Rueping process, but without prior vacuum and without air pressure.

Alternating pressure method

In contrast to other "static" processes, the alternating pressure process is a "dynamic" process. Vacuum and pressure phases alternate in a short time, with the vacuum and pressure transitions taking place in a fraction of a second. During the vacuum phases, a small amount of liquid is sucked out of the wood, in the pressure phase protective liquid is then injected. For these processes special machinery with a fully automatic control must be available.

The same amounts of protective agent are to be used as for the pressure impregnation. In the past, attempts were often made to reverse the closure of the spruce spots using this method , without success. A maximum penetration depth of 10 mm is achieved with spruce (without perforation).

Boucherie process

Developed in 1838 by Dr. Boucherie from France. This procedure initially consisted of adding a sack or container of wood preservative to the end of a felled tree. The wood preservative is introduced into the sap flow through transpiration (see sap displacement).

In the modified “Boucherie process”, the trunks are placed next to each other on a wooden frame in such a way that the end of the trunk is slightly higher than the end of the plait. The horizontally stored trunk ends of the freshly felled, decapitated and delaminated tree trunks are then connected to a high-standing storage container for the impregnation solution and the solution is pressed out of the container into the trunks under its hydrostatic pressure.

The tree trunks can be sealed with plates, caps or stubble. To ensure that the solution spreads, star-shaped holes are drilled with a central hole that is connected to the main line. The connection of the individual trunks with the liquid supply lines is designed in such a way that it can be interrupted at each trunk without disturbing the impregnation process for the other trunks. The container stands on a frame about 10-15 m high so that the liquid is pressed into the logs under its own hydrostatic pressure.

As a rule, the trunks to be impregnated by this method are felled in the months of April to October, after which the impregnation must be started as soon as possible - within one day at the latest. With this method you are bound to the frost-free season. Based on the experience available, watering should generally begin no later than 8-14 days after the trees have been felled.

The average uptake in pine trees is about 5.5 kg when using a 1% soaking solution and about 8 kg of crystallized copper sulfate per cubic meter when using a 1.5% solution (values ​​date from the 1970s). The use of oil-soluble agents is not advisable in this process. This method is used to treat poles as well as larger trees, and for the past decade it has been widely used to protect bamboo in states such as Costa Rica , Bangladesh , India, and the US state of Hawaii .

High pressure juice displacement process

Developed in the Philippines. This method uses a pump attached to a diesel engine (instead of the elevated tank) to generate the necessary pressure. It can be used on grasses and bamboo.

Perforation

First patented by Kolossvary, Halteberger and Berdenich from Austria in 1911 and 1912 (US pat. 1,012,207 and 1,018,624), improved by OPM Goss, DW Edwards and JH Mansfield u. a. The perforation of the wood increases the absorption of the wood preservative. Perforation (also perforation, from the Latin foramen = hole) is the general term used to describe the perforation or the perforation of a flat object. An example of the use of perforation in another area is the so-called “convenience food” pack. The shelf life of these products is essentially determined by the air exchange and moisture preservation. Laser perforation technologies offer the possibility of making targeted micro-holes in the packaging so that air circulation is guaranteed, but moisture is preserved.

The problem with chemical pressure impregnation is that the most important central European timber, spruce, is very difficult to impregnate. The shallow penetration depths of usually just a few millimeters are generally not sufficient for effective protection of wooden components. In order to improve the impregnability of spruce wood, perforation has been used since the fifties of the last century. This method consists in making slots or holes in the wood surface in a relatively narrow grid, that is, "perforating" the wood to a desired depth. In North America, where this process is widely used, the slots are pressed in with special machines. The wood is passed between rotating rollers that are equipped with teeth or knife-like tips. Starting from the punctures, the penetration distance of the protective agent is on average about 20 mm in the direction of the fibers and about 2 mm across it. This is used to derive the hole or slot grid required for a complete impregnation of the perforated zone.

In North America, where mostly small wood cross-sections are used, perforation depths of 4 to 6 mm have become established. However, this is hardly sufficient for large wood cross-sections, as are common in Europe. In order to permanently protect exposed wooden components, penetration depths of around 10 to 12 mm are required. The punctures are quite clearly visible to the naked eye, which is seen as a certain aesthetic flaw in Europe rather than America. The slender teeth of a newer Canadian machine allow narrower grooves to be achieved. It should also be pointed out that the slits partially close again during the impregnation in aqueous salt solution due to the swelling of the wood. With laser beams or water jets, even finer holes can be made technically very elegant. You can also perforate by pressing in pin cushions. In contrast to slot perforation, this method is expensive. The costs for the various perforation methods are approximately € 0.50 per m² for double-sided slot perforation; with laser perforation to € 3.60 / m²; and for needle perforation to € 1.00 (values ​​are from 1997).

The strength losses caused by the slots are small, they are even negligible with cross-sectional thicknesses of the timber over approx. 80 mm. It is advantageous that the many slits distribute the superficial shrinkage stresses when the wood is dried and thus counteract the formation of individual large and deep drying cracks.

The mechanical slot perforation is used as a technically simple and robust process in industrial applications not only in North America but also in Europe. Current developments aim to improve the material for the slotted disks in order to keep the surface changes of the perforated products as low as possible through slimmer tooth shapes. The needle perforation improves the impregnation quality while at the same time minimizing the aesthetic impairment of the wooden surfaces and the strength. However, there are still procedural problems with continuous feed: Up to now, practical use has been made more difficult because the needles that break off lead to long downtimes.

The laser-optical perforation avoids the disadvantages of the mechanical incising process through contact-free work. Despite the overall positive processing properties, practical application has so far failed due to the high costs. Future changes in the field of laser technology can lead to improvements here. Although the needle perforation has advantages with regard to the lower fiber and wood surface disruption, the slot perforation has a superior position due to its economic efficiency and technical feasibility and will be used for a wide range of applications in the future, for example for rough-sawn cladding, noise barriers, etc.

literature

• BA Richardson: Wood Preservation. 2nd Edition. Chapman & Hall, 1993, ISBN 0-419-17490-7 . (English)
• Franz Kollmann: Technology of wood and wood-based materials: Volume 2, 2nd edition. Springer Verlag, 1955, ISBN 978-3-642-52948-1 .

Individual evidence

1. ↑ M. Rosenthal, E. Bäucker, CT Bues: Wood structure and impregnability. About the influence of the microstructure of wood on the penetration behavior of liquids. 2011.
2. ↑ DIN 68800-3: 2012-02, Section 3 “Terms”.
3. ↑ Michael Stahr (Ed.): Building renovation: Recognizing and remedying structural damage. 5th, enlarged and updated edition. Vieweg + Teubner Verlag , 2011, ISBN 978-3-8348-1406-7 , p. 222.
4. ↑ DIN 68800-3: 2012-02, Section 3 “Terms”.