Methods of passive impregnation are partly aimed at enhancing a timber's natural capacity to withstand insects and fungi, as well as partly reducing access to nutrients and humidity.
Self-impregnation is a well-known traditional method used in most cultures. The usual procedure is to lop off the top of a pine or similar tree and remove a few stripes of bark from the bottom to the top. Three or four of the highest branches are left to 'lift' the resin. After a few years, the whole trunk is filled with resin and the timber will gain a quality similar to heartwood.
Cleaning out the cell content makes the timber less attractive to most insects and fungus. This can be achieved by storing it under water for a time. In salt water the results will be even better due to the salt absorbed, since this has an antiseptic effect.
In the past, it was common practice in Scandinavia to boil wooden shakes before they were used for roofing. Boiling is a very effective way of washing out the content of the cells.
Heat treatment. Burning the outer layer of wood is a traditional way of increasing the durability of any part of the wooden piles that were going to be placed underground. The carbon coating that forms lacks nutrients and is almost impenetrable to insects and fungus. The heating also enriches phenolic resins and aldehydes in the outermost part of the pile. These are natural biocides. The greatest impact occurs with pine, which is rich in resin, while burning spruce and deciduous trees is not so effective. During burning the timber can easily split, and it is easy for fungus to get access through the splits, so burning must be carefully controlled, preferably by using a blow lamp. The depth of the burning should be 1-3 mm, after which the surface is brushed with a bronze brush. This process takes a long time. Julius Caesar described the technique in his book De Bello Gallico in connection with setting up fortifications in the Roman Empire. This method has also been used for centuries in Portuguese and British timber warships, as it not only increases resistance to rot but also makes the surface less water-absorbing.
A modern version of heat treatment is done by heating the timber, usually pine or spruce, in a closed chamber at 200 to 250 °C for
24 hours. The process is quite energy intensive, but in addition to increasing the timber's resistance to fungi, it gives the wood a golden brown patina which has made it an attractive competitor to tropical hardwoods. This method seems to give the timber the same resistance as heartwood (Edlund, 2004).
Chemical modification is a process that normally uses acetylation through a process whereby the timber reacts with acetic anhydride -which comes from acetic acid (see page 177). This effectively reduces the ability of wood to absorb water. The result is a highly stable timber that is no longer digestible. The resultant resistance against fungi is greater than for heat treated timber and about the same as for timber impregnated with copper (Edlund, 2004).
In a second type of chemical modification, furfuryl alcohols produced from biomass waste such as sugar canes, corn cobs and sunflower are used. The furfury alcohols are driven by pressure into the cell walls of the wood and heated with steam (80-140 °C) for 6 to 8 hours to achieve polymerization. The impact strength of the wood is thereby strongly reduced, but at the same time a higher stiffness is achieved, and the treated wood is no longer attractive to fungi (Figure 19.3).
In a third type of chemical modification, the cells in the wood are filled up to 90% with synthetic monomers. After heating and gamma radiation a plastic polymer is formed that is virtually inaccessible for fungus and insects.
Seen from the environmental point of view there is no doubt that acetylation and furfurylation are preferable to this last method. As with normal timber these treatments involve few environmental doubts. The chemicals are based on renewable resources and the timber will be suitable for energy recovery. If modified with synthetic polymers the waste will have to be treated like other plastics.
Saturation with linseed oil is done by pressure treating the timber with linseed oil, in amounts of about 90 litres per m3 of timber. The cell pores are then filled and thus become inaccessible to insects and fungus, as in heartwood. Linseed oil treatment is also water repelling. The resistance against fungal attack is considered equal to treatment with copper based fungicides (Edlund, 2004).
pH-regulating substances can be used both as a preventive measure against fungal attack and for remedial treatment. Mould will not grow if the pH level is higher than 6.0. Treatment with pH-regulating substances is also effective against insect attacks. Exceptions are the fungus Merulius lacrymans and the longhorn house beetle, which are not affected. The pH-regulating substances available are primarily alkalis such as clay, cement, lime, waterglass and lye. Treatment with lye also brings the resins and tar to the surface of the wood in the same way as burning. The pH-regulating substances are not poisonous in themselves, so they do not cause problems in the indoor climate of the building or for the surrounding environment.
Waterglass (seepage90) isveryalkaline. In addition, itformsacoatsohardthatinsects cannot penetrate it to lay their eggs. It is, however, susceptible to leaching when exposed to rain, and can therefore only be used indoors or on protected parts of the building. Waterglass needs a rough surface; it does not bind well to a planed surface. It is dissolved in boiling water and applied to the wood with a brush. It can also be applied to straw materials, using a solution of one part waterglass to two parts water. Waterglass is often used as a flame retardant and it is very open to water vapour.
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