Development of Commercial Wood Preservatives ... - ACS Publications

Chapter 22

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Processes and Properties of Thermally Modified Wood Manufactured in Europe Holger Militz Institute of Wood Biology and Wood Technology, Georg-August-University Goettingen, Buesgenweg 4, 37077 Goettingen, Germany

Several processes to thermally treat wood have been commercialised in Europe in the past decade. Due to the high temperatures with most processes using 180 - 220 °C, the chemical structure of the wood components are greatly changed. Heat treated lumber has altered biological and physical properties. The wood is more resistant against basidiomycetes and soft rot fungi, and has a lower equilibrium moisture content and fibre saturation point. Consequently, the dimensional stability is improved. Because of the increased brittleness of the wood, some strength properties are greatly decreased. Due to the enhanced durability, dimensional stability, and good appearance, thermally-treated wood is currently used in Europe in many indoor and outdoor applications.

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© 2008 American Chemical Society In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Introduction Research efforts have long examined processes to chemically modify wood. Because of the availability of tropical timbers with high natural quality and cheap and effective wood preservatives, however, only a few wood modification processes were commercialized in the past. This has changed in the last few years, with increased interest to find alternatives for tropical timbers and preservative-treated wood leading to several new wood treatments that have recently been commercialized in Europe. Acetylation with acetic anhydride, furfurylation with furfural alcohol, or treating wood with modifying resins are examples of processes that have been, or will shortly be, commercialized in Europe (7). Most of these new treatments are non-biocidal alternatives to conventional wood treatment with biocidal preservatives, and provide wood with improved dimensional stability and a pleasant appearance for interior or exterior use. It has long been known that wood properties can be altered when wood is heated at elevated temperatures. However, only recently has this knowledge led to the development of commercial processes. Today, many production units with varying production capacities exist in Finland, Sweden, Germany, France, Switzerland, Austria, The Netherlands and possibly other East European countries. This article will give an overview of the existing technology and the material properties of thermally treated wood.

Treatment Processes Initial attempts to use the scientific knowledge of Stamm et al. (2) and Burmester (5) to develop a commercial heat-treatment process for mid-European wood species were made by Giebeler in Germany (4). For more than 20 years knife handles were produced in a small scale production plant. The original goal, introduction of a large scale process for exterior wood, was not reached because of a lack of interest from the wood industry in the 1980's. Only about 10 years later was the idea of thermally treating wood taken up by several research groups and industry in Europe. More or less independently from each other, several processes were developed and taken from the laboratory to commercial production. A l l of these processes have in common a thermal treatment at elevated temperatures (160 - 240 °C) than that normally used to dry lumber (50 - 120 °C). The main differences between the various processes are the process conditions and treatment technology. To produce wood with good decay resistance and physical properties, the temperature, wood moisture content and minimizing oxygen during the elevated temperatures are the key paramaters. The main characteristics of some of the processes are (5):

In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Plato-Process ( P L A T O B V , the Netherlands) The PLATO-process (6, 7) involves three treatment steps and combines the hydrothermolysis step with a dry curing step. The process time depends on the wood species, thickness, shape of wood, etc., and uses a thermolysis step (1-2 hours at 160 - 190 °C ) followed by an intermediate drying step (3-5 days) and a final curing step (8-12 hours at 170 - 190 °C). In some cases a conditioning step (2-3 days) is needed. Depending on wood species and thickness of the material, these times can be shorter. The heating medium can be steam or heated air (8).

Retification Process ( N O W New Option Wood, France) This is a one-step process that starts with relatively dry wood (approx. 12 % M C ) . The material is heated under oxygen-poor conditions (less than 2% oxygen) to 200 - 240 °C. A nitrogen atmosphere is used to minimize the oxygen present. The total duration of the process depends on the wood dimensions and wood species, and is approx. 9-12 hours. There are different production sites in France (9). The usual energy source is electricity.

O H T Process (oil-heat treatment, Menz Holz, Germany) The main characteristic of the O H T process is the use of linseed oil as a drying medium and to improve heat flow into the timber. At the same time, the oxygen level in the vessel is low due to the oil. Fresh or pre-dried timber can be used in this process. The process is performed at 180 to 220°C for 2-4 hours in a closed vacuum-pressure process vessel. Additional time involves heating up and cooling down, with this time dependent on the wood dimension. Typical process duration for a whole treatment cycle (including heating up and cooling down) for logs with a cross section of 100 mm χ 100 mm and length of 4 meters is 18 hours (10).

Thermo Wood Process (Stora, Finnforest, Finland) A n industrial scale wood heat treatment process, under the trade name of ThermoWood was developed in Finland. Today the process is licensed to members of the Finnish ThermoWood Association. The ThermoWood process consists of three steps. In a first step, the wood is dried in a high temperature kiln. The temperature is increased steadily to 130 °C, during which time high temperature drying takes place. In a second step, the temperature is raised to 185 - 230°C. The temperature is held for 2-3 hours, depending on the end-use


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application. The third step is a cooling and conditioning step. This final stage lowers the temperature using a water spray system, and at a temperature of 80 90°C re-moisturising and conditioning takes place to bring the wood moisture content to 4-6 %. The temperature inside the wood is used to regulate the temperature rise in the kiln. The wood employed can be freshly sawn or kiln dried (77).

Stellac Treatment (Finland) The Stellac treatment is similar to the ThermoWood treatment described above, and is run under atmospheric conditions. The process takes place in an air tight stainless steel kiln. During the first step, the temperature is raised up to 100 °C, followed by a conditioning phase. The actual heat treatment takes place at temperatures up to 250 °C for several hours. The total process duration is about 24 hours.

Further Processes An Austrian joint-venture (Muhlbôck/ Mitteramskogler) produces thermally modified wood for mainly interior and exterior water resistant products. The technology uses kiln drying chambers where the wood is thermally modified at approx. 160 - 220 °C. New processes were recently introduced in Switzerland (Balz) and by a Russian-German cooperation (Barkett).

Products and Production of Heat Treated Wood Depending on the wood species and the production process, several biological and technological properties of the wood are changed by the treatment. The colour of the wood turns brownish, which is used by some companies to give local wood the appearance of expensive tropical wood. Because no chemicals are used, heat treated wood can be used in both exterior (with increased resistance against wood degrading organisms) and interior applications. At the present time thermally treated wood is used in many applications, including windows, claddings, play ground equipment, sauna interiors, bath rooms, parquet flooring, decking, etc. Official data on the real production of heat treated wood and the production capacity is not available. Militz (72) estimated the production in 2001 at approx. 165,000 m . The capacity could easily be increased, because the equipment to thermally treat the wood is relatively simple and has a low capital cost. 3


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Chemical and Anatomical Changes Chemical changes to the wood structural polymers caused by the high treatment temperatures, leads to altered wood properties, such as biological resistance against wood degrading organisms, altered physical/strength properties, darker colour, etc. Intensive research (13-22) has shown that many different chemical transformations occur during the thermal treatment. Kotilainen (75) studied changes in the chemical composition of different softwood and hardwood species that were heated to 150 - 260 °C for several hours under steam, air or nitrogen atmospheres. He found that the reaction conditions influenced the extent and type of chemical changes in the wood components. Hardwood species tended to decompose more than softwood species. The main volatile compounds, beside water, were acids (formic acid, and acetic acid) liberated from the hemicelluloses. Sivonen et al. (25) used Electron Spin Resonance (ESR) and C P - M A S C - N M R to show increased cellulose crystalinity and the presence of stable free radicals. Relatively mild PLATO-treated wood was investigated with solid phase CPMAS C - N M R and Fourier transform infrared spectroscopy (FTIR) to understand at the molecular level the reasons for the enhanced properties improvements (14- 16). Acetic acid is liberated from the hemicelluloses, which leads to further acid-catalyzed carbohydrate hydrolysis causing a reduction in the degree of polymerisation of the carbohydrates. Acid catalysed dehydration and other reactions result in formation of formaldehyde, furfural and other aldehydes, as well as some lignin cleavage at the Cot and 04 interunit bonds. Some aldehyde groups may be produced from the lignin Cy carbon. Lignin autocondensation through the cleaved, positively charged benzylic Cot may lead to formation of some methylene bridges in this first phase. A n increase in the number of free reactive sites on the aromatic ring of some lignin units also occurs in the early phase, and continues into the next phase. In the second treatment step, autocondensation of lignin is believed to continue with formation of methylene bridges that connect adjacent aromatic rings. The aromatic nuclei sites are formed by demethoxylation and then react with the cleaved, positively charged benzylic Cot. Reactions occur with some of the aldehyde groups that were formed in the first step phase with lignin aromatic nuclei sites to form additional methylene bridges. This leads to an increase in cross-linking with consequent improvement in a dimensional stability and decreased hygroscopicity of wood. Esterification reactions were found to occur under dry conditions at elevated temperature in the curing step, indicated by an increase in the ester carbonyl peak at 1740 cm" in the FTIR spectrum. The formed esters were apparently mainly linked to the lignin complex, based on the observation that the newly formed carbonyl groups were present in heat-treated wood but absent in the isolated holocellulose. Finally, Weiland and Guyonnet (24) performed DRIFT spectroscopy on thermally modified Pinus pinaster and Fagus sylvatica 13

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377 and reported the formation of new ether linkages, along with the previouslyreported acid hydrolysis. The importance of hemicellulose degradation in combination with lignin changes on the swelling behaviour of thermally treated pine (Pinus pinaster) and beech (Fagus sylvatica) was demonstrated by Repellin and Guyonnet (18). Furthermore, Wikberg and Maunu (19) studied the chemical changes caused at temperatures between 160 and 195 °C in spruce (Picea abies), birch (Betula pendula), aspen (Populus tremula) and oak (Quercus robur) by C C P M A S N M R spectroscopy. Spectra revealed the degradation of amorphous cellulose and hemicelluloses, which increased the relative amount of crystalline cellulose. Furthermore they reported changes in the lignin structure by cleavage of the β-Ο4 bonds. In softwood lignin, a decrease in methoxyl groups was measured leading to a more condensed lignin structure. An increase in cellulose crystallinity due to heat treatment was reported by studies of Bhuyian et al. (25) and Bhuyian and Hirai (77), based on X-ray diffractometry. This may be important for further process optimisation, in that the process conditions (oven drying versus high-moisture conditions) likely affects the final crystallinity. Specifically, when heating under dry conditions the crystallinity change was observed to be much less than with moist process conditions. Nuopponen et al. (21) used FT-IR and U V Resonance Raman spectroscopy to analyse thermally modified scots pine (Pinus sylvestris). Analysis of acetone extracts of the modified wood showed that that the acetone solubility of the lignin increased with process temperatures above 180 °C. Increased levels of free phenolic hydroxyl groups were found in the lignin, probably due to the cleavage of β-Ο-aryl ether linkages. The amount of extractable lignin increased with increasing temperature, while the resin content in the extract decreased. Sander and Koch (22), based on UV-spectroscopy studies, reported an increase in the 280 nm maximum in the S2 layer of sprucewood (Picea abies) caused by hydrothermal treatment, and concluded that the change in the lignin chromophoric behaviour could be due to hydrolysis reactions of carbohydrates. Wood treated by the P L A T O treatment was evaluated by light and scanning electron microscopy (26). This study suggested that, depending on the process and wood species, damage to the ray tissue and the vessels will occur. With optimised processes, however, no major tissue changes were seen.


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Properties In the last decade many publications have studied the material properties of heat treated timber. Overview articles are given by Rapp (5), Militz (72), Ewert and Scheiding (27). In general, as was earlier shown by Stamm et al. (2), Burmester (3) and Giebeler (4\ the durability, sorption, shrinkage and swelling,



Figure 1. Two stage heat treated Scots pine: radial section of tracheids (A), bordered pit with opened pit chamber(B) radial section, crossing field (C). No changes of the tissue are visible (26)

Figure 1. The typical fracture of a heat treated Norway spruce specimen after a bending test (A). Microscopical photo of a heat treated (B) and non-treated (C) Radiata pine, fracture surface after bending test (26)


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and strength properties are changed by a heat treatment. The level of change depends on the wood species and process conditions, in which the temperature, the duration of treatment, the wood moisture content and the absence of oxygen are the critical process factors.

Figure 3. Heat treated Scots pine (cross section). Radial cracks in the earlywood occur due to harsh conditions (26)

Resistance Against Fungi and Insects Many authors have shown that the durability of wood against decay fungi can be improved considerably by a thermal treatment of wood (8, 10, 11, 14,2732). The efficacy depends on the wood species and treatment conditions. The durability of non-durable softwood species, like Norway spruce (Picea ahies) Scots Pine (Pinus sylvestris) and Maritima Pine (Pinus maritime) sapwood, can be increased, with the durability obtained dependent on the treatment temperature and process duration (10, 11, 33). Tjeerdsma et al. (8) studied the influence of wood moisture during the hydrothermal step of PLATO-treated wood. The resistance against all of the examined fungi, especially soft rot fungi, improved considerably. Decay resistance was found to be dependent on the applied process conditions. In the research of Tjeerdsma et al. (8\ the treatment effectiveness was improved by employing a hydrothermal step prior to the dry heat-treatment step. The process conditions in the curing step appear to have the largest effect on the resistance against soft and brown rot decay. White rot decay was less dependent on the curing conditions and more affected by the hydrothermolysis step. The enhanced resistance to brown and soft rot degradation was partly assigned to the reduced hygroscopicity of the wood. y


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381 For the OHT-process, Rapp and Sailer (70) and Sailer et al. (35) studied the resistance of heat treated wood to Coniophora puteana with different oil loadings. With increasing temperatures in the range of 180 - 220 °C, the resistance of heat-treated spruce and pine to the brown rot fungus C puteana improved considerably. Mass loss of less than 2% was found in pine sapwood treated in oil at 200°C. With spruce, a decisive increase in resistance was only obtained at 220°C. Matsuoka et al. (36) treated Sugi (Cryptomeria japonica) in liquid paraffin at temperatures between 90 and 150 °C, but did not find much durability against brown rot fiingi or termites, suggesting that temperatures greater than 150 °C are necessary. Militz and Krause (37) and Ewert and Scheiding (27) tested the resistance of beechwood (Fagus sylvatica) and Pine sapwood (Pinus sylvestris) from several commercially available heat treatment processes against soft rot fungi and basiodiomycetes. The durability of both wood species (natural durability class 5) was considerably improved; however, the durability varied. Depending on the process and test fiingi, some wood species were very resistant (class 1-2), while others showed only a slight improvement (class 3-4). Also, resistance against soft rot fiingi was lower than the resistance against most basiodiomycetes. As shown earlier by Tjeerdsma et al. (30, 33), a treatment temperature of at least 180-200 °C is needed to improve the durability to class 1-2. Ewert and Scheiding (27) tested resistance against blue stain fiingi using E N 152-1 method with Aureobasidium pullulan and Sclerophoma pithyophila, and reported no difference between treated wood and controls in the colonisation at the surface of the samples. However, penetration of the hyphae into the wood was only seen with non treated (control) pine, whereas the heat treated wood was only superficially colonised. Welzbacher and Rapp (57) took material from several commercial treatment batches of pine sapwood (Pinus sylvestris) and spruce wood (Picea abies). For most of the material, an improvement to durability class 3 was obtained. Mayes and Oksanen (77) and Viitanen et al. (38) also reported a higher resistance of heat treated timber (Thermowood, Finnforest) against several fimgi. Furthermore, an improved durability of several bamboo species against basidiomycetes was demonstrated by Leithoff and Peek (39). Research performed at the University of Kuopio (Finland) and at the French institute C T B A showed a higher resistance of thermally treated wood against longhorn beetles, Annobium punctatum and Lyctus brunneus. However, preliminary trials with termites showed no improved resistance (77).

Sorption and Dimensional Stability Because of the chemical alteration of the wood cell wall structural polymers, the sorption behaviour of the thermally treated wood is altered (40). Tjeerdsma et al. (8) measured the hygroscopicity of Plato-treated wood. The strong impact



382 of the treatment on reducing hygroscopicity with softwood and hardwood was illustrated by the reduced sorption curves of the treated versus control wood. Reduced hygroscopicity was most pronounced at higher relative humidities (R.H. > 70 %). The hysteresis effect between sorption and desorption was found to be unchanged by the heat treatment of wood. From corresponding research, it is known that the hygroscopicity of heat-treated wood can vary considerably with varying process time and temperature in the second treatment step (8). Popper et al. (41) investigated the influence of temperatures between 100 and 200 °C on sorption properties and swelling properties of several wood species (Pinus radiata, Pseudotsuga menziesii, Laurelia sempervirens, Castanea sativa, Quercus robur). They noticed that even low temperatures resulted in a lower equilibrium moisture content, with the effect greater with increasing temperature. The sorption analysis, according to the Hailwood-Horrobin model, suggested that changes in the void volume and cross linking of the holocellulose could be responsible for this effect. By a heat treatment of Fagus orientalis at temperatures above 180 °C, Yildiz (42) reduced tangential swelling and the rate of water absorption. Goroyias and Hale (43) treated wood strands for OSB production and reported on the effects of temperature and treatment time on mechanical properties, dimensional stability and water absorption. High temperature treatments resulted in significant reductions in thickness swelling of wood strands but reduced M O R and M O E by up to 20%. In a joint research programme with the German window industry, the physical properties of several commercial heat treatment processes were compared (37). A l l evaluated processes lowered the equilibrium moisture content in the range of relative humidities examined, as well as the volumetric swelling. At higher humidity, the volumetric swelling was reduced to approx. 5060 % of its original values (Fig. 6). Liquid water uptake usually is affected much less by a heat treatment (Fig. 7) than bound water absorption. As shown by microscopical analysis, the high increase in free water uptake of heat-treated material may have been caused by blue stain fungi that had colonised the wood before the heat treatment.

Mechanical Properties The changes in the cell wall chemistry (changes in the hemicellulose and lignin structures, cellulose depolymerisation and increased crystallinity, etc.) affect the mechanical properties of heat treated wood. (8, 37, 44- 49). For PLATO-treated wood, the bending strength measurements of several wood species, non-treated versus heat-treated, showed an average strength loss of 5 to 18 % for heat-treated planks (40 mm χ 150 mm χ 2200 mm). Earlier


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Figure 6. Maximal volumetric swelling of untreated Scots pine (ut) (Pinus sylvestris) and beech (Fagus sylvatica) and heat treated Scots pine from three commercially available processes (heat a-c)


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Figure 7. Capillary water uptake in tangential direction of untreated Scots pine (ut) (Pinus sylvestris) and beech (Fagus sylvatica) and heat treated Scots pine from three commercially available processes (heat a-c). Microscopical studies revealed that some samples from process a showed blue stain attack (code(a)b) wheras others werefreeof attack (code (a)n).

studies on this subject showed in general a strength loss to approximately 50% or more (50). Strength results often are based on small wood samples free of defects and planks treated under mild conditions. During the process, high tension can occur in the wood as it is exposed to high temperatures and rapid evaporation of water. Some o f the wood species were found difficult to treat and showed a number of defects, mainly cracks, i f not treated carefully. Several softwood species are known to have a high resistance against liquid impregnation. These wood species were found to be difficult to heat treat and showed a relatively high strength loss. Altogether, the final strength was dependent the process conditions and affected predominantly by the process temperature in combination with wood species (26). The M O E and M O R of OHT-treated wood (JO) was determined in a three point bending test with medium force applied on 150x10x10 mm samples parallel to the grain. There was no reduction in the M O E of coniferous wood with either heat treatment process. The M O R of wood that was oil-heat-treated at 220°C was reduced to about 70% of the value of untreated controls. Bengtsson et al. (57) tested the strength of heat treated beams (45 χ 145 mm) of spruce (Picea abies) and pine (Pinus sylvestris) from higher temperature ranges (200 - 220 °C) and found reduced bending strengths of up to 50% but only minor M O E changes. Schmid et al. (46) compared several mechanical properties of material from three commercial heat treatment processes (Table 1). The compression strength remained relatively unchanged and the Brinell hardness was slightly 3



385 reduced. Because of the increased brittleness, the impact bending strength decreased considerably. This was reported earlier by other authors (10, 11, 52, 53), who showed that the impact bending strength is the strength property most reduced by all heat treatment processes. Under non optimum process conditions, the impact bending strength can be reduced to about 50 % of its original values. Kubojima et al. (48) examined the influence of temperature and oxygen with sitka spruce (Picea sitchensis) on static bending strength and impact bending. They found that at lower temperatures the static Young's modulus increased, but it decreased with longer treatment times and higher treatment temperatures. The decrease was less in nitrogen than an oxygen rich atmosphere.

Table 1. Bending strength ( M O R ) , Modulus of elasticity ( M O E ) , Compression strength [all in N/mm ], impact bending strength [kJ/m ] and Brinell hardness of untreated and heat treated Scots pine (Pinus sylvestris) from 3 commercially available processes (heat a-c) 2

MOR MOE Compression strength Impact bending Brinell hardness

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Heat (b)

Heat (c)

Scots pine 80 10589 37

71 9944 47

91 13144 57

93 11729 54

56 15

17 14

21 17

19 13

Colour and Odor Due to the high temperatures employed, all heat-treated wood species show a characteristic brownish colour (comparable to the natural colour of Thuja plicata, Western Red Cedar). The colour is affected by the treatment temperature and the duration of the processes. The higher the temperature and the longer the duration the darker the colour. After treatment, the wood has a characteristic caramellish smell, likely due to to furfural formation. Measurements (11, 13) also showed that monoterpenes emission from treated pine is considerably reduced, but heat-treated wood does emit acetic acid.

Paintability and Coating Performance The resistance of heat treated timber against weathering (UV-light, wetting) is not greatly changed compared to untreated wood, making a surface treatment with oils or paints necessary. No changes in paintability of heat treated wood


386 with water borne acrylic or solvent borne alkyds were found (54). Due to its UV-degradability, opaque systems are recommended over priming oils and stains (77). After several years of outside exposure, heat-treated timber performs considerably better than non-treated wood. This is likely due to its higher dimensional stability, resulting in less flaking and cracking (77, 30, 54, 55).


Gluability Several authors reported on the gluability of heat-treated wood (77, 30, 55). The glued wood was evaluated, following different German and European standards, for strength and moisture performance (46). Heat-treated timber can be glued with many industrial adhesives (polyvinyl alcohol and other polyvinylic glues, polyurethane, isocyanate, and resorcinol-phenolic glues). Due to the lower shear strength and tension strength perpendicular to grain with heat-treated lumber, a higher wood failure is found. Furthermore, the hydrophobic wood surface causes a slower penetration of the solvents from the glue to the surrounding wood, which makes it necessary to modify the gluing process.

References 1.

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387 13. Kotilainen, R.: Chemical changes in wood during heating at 150 - 260 °C. PhD thesis, Jyväskylä Universitiy, Research Report 80, Finland. 2000. 14. Tjeerdsma, B . F.; Boonstra, M.; Pizzi, Α.; Tekely, P.; Militz, H . Holz als Roh- und Werkstoff 1998, 56, 149-153. 15. Tjeerdsma, B . F.; Boonstra, M.; Militz, H . : Chemical changes in hydro thermal treated wood; FTIR analysis of combined hydro thermal and dry heat-treated wood. Submitted for publication. 2005. 16. Tjeerdsma, B . F.; Militz, H . Holz als Roh- und Werkstoff 2005, 63, 102111. 17. Bhuiyan,M.T. R.; Hirai, N. J. Wood Sci. 2005, 51, 42-47. 18. Repellin, V . ; Guyonnet, R. Holzforschung 2005, 59, 28-34. 19. Wikberg, H . ; Maunu, S. L . Carbohydrate Polymers 2004, 58, 461-466. 20. Mitsui, K . ; Murata, Α.; Tolvaj, L . Holz als Roh- und Werkstoff 2004, 62, 164-168. 21. Nuopponen, M.; Vuorinen, T.; Jämsä, S.; Viitaniemi, P. J. Wood Chem. Tech. 2004, 24, 13-26. 22. Sander, C.; Koch, G . Holzforschung 2001, 55, 193-198. 23. Sivonen, H . ; Maunu, S. L . ; Sundholm, F.; Jämsä, S.; Viitaniemi, P. Holzforschung 2002, 56, 648-654. 24. Weiland, J. J.; Guyonnet, R. Holz als Roh- und Werkstoff 2003, 61, 216220. 25. Bhuiyan,M.T. R.; Hirai, N.; Sobue, N. J. Wood Sci. 2000, 46, 431-436. 26. Boonstra, M. J.; Sander, C.; Tjeerdsma, B . ; van Acker, J.; Stevens, M.; Militz, H . : The effect of a two-stage heat treatment on the anatomical structure of wood. Submitted to: Wood Research, 2005. 27. Ewert, M.; Scheiding, W. Holztechnologie 2005, 46, 22-29. 28. Buro, A . Holz als Roh- und Werkstoff 1954, 12, 297-304. 29. Scheffer, T. C.; Eslyn, W. E. Forest Products J.l 1961, 11, 485-490. 30. Tjeerdsma, B . F.; Stevens, M.; Militz, H . ; van Acker, J. Holzforschung und -verwertung 2002, 54, 94-99. 31. Welzbacher, C. R.; Rapp, A . O. Intern. Res. Group on Wood Pres. Doc. no. IRG/WP 02-40229 2002. 32. Kamdem, D . P.; Pizzi, Α.; Jermannaud, A . Holz als Roh- und Werkstoff 2002, 60, 1-6. 33. Tjeerdsma, B . F.; Stevens, M.; Militz, H . Intern. Res. Group on Wood Pres. Doc. no. IRG/WP 00-40160 2000. 34. D I N E N 113:1996 DIN Taschenbuch 132 Holzschutz Beuth Verlag GmbH -Berlin - Wien - Zurich, 1998, ISBN 3-410-14278-9. 35. Sailer, M.; Rapp, A . O.; Leithoff, H . Intern. Res. Group on Wood Pres. Doc. no. IRG/WP 00-40162 2000. 36. Matsuoka, Y.; Ohmura, W.; Fujiwara, S., Kanagawa, Y. Intern. Res. Group on Wood Pres. Doc. no. IRG/WP 02-40221 2002. 37. Militz, H . ; Krause, Α.: Modified wood for window and cladding products. Proceedings of the COST conference Florence. 2004. In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


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