Ammonia vs. thermally modified timber comparison of physical and mechanical properties

Eur. J. Wood Prod. (2012) 70:233 239 DOI 10.1007/s00107-011-0537-z ORIGINALS ORIGINALARBEITEN Ammonia vs. thermally modified timber comparison of physical and mechanical properties Martin Weigl Ulrich Müller Rupert Wimmer Christian Hansmann Received: 22 September 2010 / Published online: 30 March 2011 Springer-Verlag 2011 Abstract Gaseous ammonia treatments as well as thermal modification of timber are both applied to modify the surface colour of wood species. Physical and mechanical properties of solid wood after gaseous ammonia treatments have been so far scarcely investigated. Selected physical and mechanical properties, i.e., colour, equilibrium moisture content, wood density, bending strength, bending stiffness, and impact bending strength were investigated for oak, Norway spruce and Scots pine after gaseous ammonia treatment. Obtained data were compared with thermally modified timber data. While wood colour changes were comparable, most properties of the ammonia treated wood did not alter. There was no significant density change of ammonia treated wood; equilibrium moisture content changed moderately. Mechanical properties also remained within acceptable fluctuations. Results proved that gaseous ammonia treatment is a modification that improves the decorative value but has minimal effect on technologically relevant properties of solid wood. In contrast, property alterations of colour-equivalent thermally modified timber were much more pronounced, with possible limitations to some applications. M. Weigl ( ) U. Müller C. Hansmann Kompetenzzentrum Holz GmbH (Wood K plus), St.-Peter-Str. 25, 4021 Linz, Austria e-mail: m.weigl@kplus-wood.at M. Weigl U. Müller C. Hansmann University of Natural Resources and Life Sciences Vienna (BOKU), Peter Jordan Str. 82, 1190 Vienna, Austria R. Wimmer Institute for Wood Biology and Wood Technology, Georg-August-Universität Göttingen, Büsgenweg 4, 37077 Göttingen, Germany Ammoniak vs. thermisch modifiziertes Holz Vergleich von physikalischen und mechanischen Eigenschaften Zusammenfassung Sowohl Ammoniakbehandlungen (Räuchern) als auch thermische Modifikation werden zur Farbmodifikation von Holzoberflächen eingesetzt. Physikalische und mechanische Eigenschaften von geräuchertem Holz wurden bisweilen selten untersucht. Ausgewählte physikalische und mechanische Kenngrößen, d.h. Farbe, Ausgleichsfeuchte, Holzdichte, Biegefestigkeit, Biegesteifigkeit und Schlagbiegefestigkeit wurden für Eiche, Fichte und Rotkiefer nach gasförmiger Ammoniakbehandlung untersucht. Die gemessenen Daten wurden mit Daten für thermisch modifiziertes Holz verglichen. Während die Holzfarbe vergleichbar war, änderten sich die meisten weiteren Eigenschaften von geräuchertem Holz nicht. Es gab keine signifikante Veränderung der Dichte und die Ausgleichsfeuchte änderte sich lediglich geringfügig. Auch die mechanischen Eigenschaften bewegten sich innerhalb üblicher Variationen. Die Ergebnisse zeigen, dass Räuchern ein Modifikationsverfahren ist, welches den dekorativen Wert des Holzes erhöht, während technologisch relevante Holzeigenschaften nicht beeinträchtigt werden. Im Vergleich dazu zeigt farblich ähnliches thermisch modifiziertes Holz wesentlich stärkere Veränderungen der Holzeigenschaften, welche für manche Anwendungen limitierend wirken können. 1 Introduction Gaseous ammonia treatment and thermal modification have been both applied to modify the apparent colour of different wood species. Ammonia treatment of timber is a wellknown method but has recently gained new interest by industry (Tinkler 1921; Weigl et al. 2007, 2009a; Weigl and

234 Eur. J. Wood Prod. (2012) 70:233 239 Müller 2009). With proper treatment light-coloured wood is darkened and may imitate exclusive timber (Weigl et al. 2009a). Ammonia treated timber (AMT) is therefore frequently applied in interior design such as for furniture or flooring. Oak wood is often used as AMT but there is potential to treat a number of other wood species as well, showing decorative and partly intensive colour changes. With respect to processing time, cross sectional dimensions and ammonia gas concentration, modification might be effective throughout or on superficial wood layers only. Tannins are seen as the wood extractive being responsible for the reaction with ammonia, leading to the desired colour change (Tinkler 1921; Bariska 1969). Colour changes are commonly determined using the L a b colour space system (CIE 1976 colour system). Colour differences are derived from the three coordinates L for brightness, a for saturation in red/green, and b for saturation in yellow/blue (1): Eab = ( L ) 2 + ( a ) 2 + ( b ) 2 (1) According to Sundqvist and Morén (2002), Eab values greater than 2 to 3 can be recognized by the human eye. Colour changes of 38 different wood species following AMT procedure were studied earlier (Weigl et al. 2009a), and Eab values were found to vary between 8 and 42. For high extractive wood species extensive colour changes following the AMT process are reported (Weigl et al. 2007, 2009a). Higher extractive contents and chemical alterations of the wood polymers may arise along with ammonia treatment processes (e.g., Kalnin š et al. 1967;Oniśko and Matejak 1971; Parameswaran and Roffael 1984; Weigl et al. 2009b). Ammonia treatment may also cause changes in cellulose crystallinity, with partial disintegration of hemicelluloses and lignin (e.g. Bariska 1969; Besold and Fengel 1983a; Yatsu et al. 1986). Ammonia treatment not only alters the wood chemical composition, but also wood structural features. Authors have observed changes in lumen sizes, cell deformations, swelling of middle lamella, primary walls, and secondary walls (Bariska 1969, 1975; Berzin š 1972; Berzin š et al. 1970; Parham et al. 1971; Stamm1955). After evaporation of excessive ammonia, temporally disconnected chemical bonds may reorganize and recover in changed sterical positions, leading to a reduction of encapsulated growth stresses. This phenomenon also enables certain rearrangements in the anatomical structure. Weigl et al. (2009b) observed that rays have protruded out of the tangential surface of oak wood after swelling in aqueous ammonia and subsequent drying. With respect to all the chemical alterations and wood structural rearrangements following ammonia treatments, changes in the physical and mechanical wood properties are expected as well. Most of the chemical, physical, and mechanical changes mentioned above were investigated and described for wood soaked in pure or aqueous ammonia (e.g., Bariska 1969, 1975; Berzin š 1972; Berzin š et al. 1970). Modification was partly achieved at moisture contents above fibre saturation and by applying compressive forces for concurrent material densification (e.g., Bariska 1969, 1975; Berzin š 1972; Berzin š et al. 1970; Kalnin š et al. 1967;Oniśko and Matejak 1971). Compared to immersion in liquid ammonia, reduced effects in terms of chemical alterations and structural changes are expected with gaseous ammonia treatments. There is a lack of knowledge with respect to chemical changes as well as physical properties of dry wood treated in gas-phase ammonia, compared to liquid ammonia immersion. Thermal modification was originally developed to achieve better dimensional and biological stability of wood (Hill 2006), with significant changes in colour seen as a side effect (e.g., Patzelt et al. 2002; Hill 2006; Weigl et al. 2009a). Today, thermally modified timber (TMT) processes are widely aiming at this colour-change side effect (Allegretti et al. 2008). Different TMT processes used in industry cause chemical alterations that depend on factors such as temperature, presence of water, duration of treatment, atmosphere, wood species, timber dimensions, or presence of catalysts (Hill 2006). Industrially produced TMT includes a wide range of commercially available species, including spruce, pine, and oak. Similar to AMT the colour changes achieved through the TMT process are characterized by a significant shift towards dark brown, leading to negative L -values (e.g., Patzelt et al. 2002; Hill 2006; Welzbacher et al. 2007; Esteves et al. 2008b; González-Peña and Hale 2009a, 2009b; Weigl et al. 2009a). Another reason for TMT processing is to achieve evenness of colour (Seeling et al. 2007). According to Hill (2006) changes in a prevalent colour hue may be associated with e.g. degradation of tannins, or formation of phenolic breakdown products. Disintegration of holocelluloses with formation of furan, furfural and other breakdown products (Fengel and Wegener 1984) has an impact on the lightness as well. Diffusion processes of lower molecular substances (e.g., fats, waxes, resin acids) towards the surface might lead to changed optical properties. Long-lasting and high-temperature treatments may lead to a severe destruction of wood chemical components. Although more temperature-stable than most extractives or hemicelluloses, changes were also observed for lignin: Hill (2006) has shown lignin alterations, including changed ratios for syringyl over guaiacyl units. Alterations of physical and mechanical TMT-properties due to the modification process are well documented (e.g., Esteves et al. 2008a; Sailer et al. 2000). In this study the following questions are addressed: (1) To what extent will physico-mechanical properties alter due to gaseous ammonia treatment? (2) How do physicomechanical properties of AMT compare to those obtained

Eur. J. Wood Prod. (2012) 70:233 239 235 through TMT? (3) What are the similarities and differences in colour dynamics of the two employed processes? Here, wood samples modified with ammonia were compared with untreated samples, as well as with literature-based TMT data showing similar degree of coloration. 2 Material and methods The species European oak (Quercus sp.), Norway spruce (Picea abies L. [Karst.]), and Scots pine (Pinus sylvestris L.) were chosen for this study. Flawless boards with a length of 750 mm were cut into 23 23 mm 2 studs. Boards were cut lengthwise into halves to prepare paired samples. Working length for the bending tests was 360 mm, and 300 mm for the impact bending test. One of the two cut samples per board was used for the AMT process, while the other one served as a reference (21 to 26 pairs for each testing method). Specimens of all three wood species were exposed to gaseous ammonia up to four weeks. The AMT process was performed at room temperature under atmospheric pressure in a closed high density polyethylene barrel. Samples were placed on plastic grids above the open surface of one litre 30% aqueous ammonia, following Weigl et al. (2009b). The wood mass over ammonia ratio was approximately ten times higher than the ammonia sorption maximum of 0.4 mol/100 g, known for spruce wood (Besold and Fengel 1983b). After evaporation of excessive ammonia for a minimum of 14 days, samples were planed to 20 20 mm 2 studs. Treated samples and references were stored at standard climate (i.e., 20 C, 65% RH) until equilibrium moisture content was reached (i.e., mass change within 24 h of less than 0.1%). Incomplete colour changes were observed with pine in rare cases; in the direct vicinity of epithelial cells that were filled with resin. The following analyses were performed: (1) volumetricgravimetric wood density determination (DIN 52182; n = 47, 47, and 45 for each sample pair of oak, spruce, and pine, respectively), (2) equilibrium moisture content (EMC) by calculation of mass loss after drying at 103 C until steady state (DIN 52183; n = 47, 47, and 45 for each sample pair of oak, spruce, and pine, respectively), (3) tree-point bending via continuous loading in tangential direction until rupture within 90 ± 30 s, to obtain modulus of elasticity (MOE) and modulus of rupture (MOR) (DIN 52186; n = 26, 22, and 21 for each sample pair of oak, spruce, and pine, respectively), and (4) impact bending strength via abrupt tangential loading (ω; DIN 52189; n = 21, 25, and 24 for each sample pair of oak, spruce, and pine, respectively). Shapiro-Wilk statistics were applied to test for normal distribution, followed by two-tailed paired samples t-tests (probability of error = 5%) to evaluate significance of property changes due to AMT processing, compared to the untreated wood. In addition to the collected data so far unpublished L, a, and b data from earlier AMT experiments (Weigl et al. 2009a) were utilized, as well as literature values for TMT to define process settings that lead to colour equivalence of both processes (Patzelt et al. 2002; Welzbacher et al. 2007; Esteves et al. 2008b; González-Peña and Hale 2009a). 3 Results and discussion 3.1 Colour, EMC and wood density The significant shift in colour due to the AMT procedure is most likely due to darkening. L values for spruce, oak, and pine were 17, 22, and 26, respectively. Less changes were observed for yellow saturation ( b = 1, 8, and 10 for spruce, oak, and pine, respectively), and for red-green saturation ( a =+1, 2, and 1 for spruce, oak, and pine, respectively). The corresponding values for the overall colour change ( Eab ) due to the AMT process as published by Weigl et al. (2009a) were 19, 24, and 27 for spruce, oak, and pine, respectively. Table 1 presents different TMT processes resulting in wood colour comparable to the colour shade gained by the AMT process. These TMT process conditions can be seen as relatively moderate as short duration times of only a few hours in combination with relatively low temperatures (165 to 210 C) were used. Significant changes in EMC due to the AMT process were found with all three species, which was consistent with earlier findings (Weigl et al. 2009b). Oak and pine showed significant EMC increases (p <0.001), whereas spruce showed a significant inverse trend (p <0.001). However, the observed magnitude is not of practical relevance in the case of oak (average increase of 0.5% points only), and the same is true for the two softwood species showing even lower differences compared to oak. No significant changes in wood density were found with AMT for any of the investigated species. Hence, EMC and density variations seem to be irrelevant with respect to the possible impact on the mechanical propertiesofamt. 3.2 Mechanical properties Significance levels of the corresponding paired samples t- test concerning changed properties due to the AMT processareshowninfigs.1 to 3. Different changes in MOE and MOR were observed for AMT. While oak was the only species showing a significant reduction in MOE (Fig. 1), MOR (Fig. 2) was significantly reduced for oak and also spruce. Pine did not show any reduction in bending properties. The AMT process did not affect the impact bending strength of oak and spruce (Fig. 3), while pine showed a slight but significant reduction. As a conclusion, there was

236 Eur. J. Wood Prod. (2012) 70:233 239 Table 1 TMT process conditions leading to AMT equivalent colour (based on literature). ( L = shift in brightness, a = shift in red/green saturation, b = shift in yellow/blue saturation; Patzelt et al. 2002; Welzbacher et al. 2007; González-Peña and Hale 2009a; Esteves et al. 2008b) Tab. 1 TMT-Prozessbedingungen die zu AMT-äquivalenter Farbe führen (basierend auf Literatur). ( L = Veränderung der Helligkeit, a = Veränderung der Rot/Grün-Sättigung, b = Veränderung der Gelb/Blau-Sättigung; Patzelt et al. 2002; Welzbacher et al. 2007; González- Peña und Hale 2009a; Esteves et al. 2008b) Source Species Colour parameter comparable to AMT TMT conditions Patzelt et al. (2002) Norway spruce L 90 min, 165 C, 0.8 MPa Welzbacher et al. (2007) Norway spruce L 10 h, 180 C González-Peña and Hale (2009a) Norway spruce L, b 4 h, 210 C González-Peña and Hale (2009a) Scots pine L 2 4 h, 210 C Esteves et al. (2008b) Maritime pine (Pinus pinaster) L, a 2 h, 180 C, mixture of superheated and saturated steam Fig. 1 Box-and-whisker plot for modulus of elasticity (MOE) in bending of untreated references (empty) and ammoniated (grey) oak, spruce and pine samples. Bold line represents the median, the box the interquartile range (i.e., the range for the innermost 50% of all values), the whiskers the outermost measured value within the 1.5-times IQR-distance from the box, and the circles partly occurring outliers. Paired sample t-test was significant (p < 0.001) for oak only Abb. 1 Box- und Whiskers Darstellung des Biege-E-Moduls (MOE) von unbehandelten Referenzen (ungefüllt) und Ammoniak behandelter (grau) Eichen-, Fichten- und Kiefernproben. Die dicke Linie repräsentiert den Median, die Box den Interquartilabstand IQR (d.h. die Schwankungsbreite der innersten 50 % aller Messwerte), die Whiskers den äußersten gemessenen Wert innerhalb des 1.5-fachen IQR von der Box, und die Kreise fallweise auftretende Ausreißer. Der gepaarte t-test war nur für Eiche signifikant (p < 0.001) Fig. 2 Box-and-whisker plot for modulus of rupture (MOR) of untreatedreferences (empty) and ammoniated (grey) oak, spruce and pine samples. The bold line represents the median, the box the interquartile range IQR (i.e., the range for the innermost 50% of all values), the whiskers the outermost measured value within the 1.5-times IQR-distance from the box, and the circles partly occurring outliers. Paired sample t-test was significant (p < 0.001) for oak and spruce Abb. 2 Box- und Whiskers Darstellung der Biegefestigkeit (MOR) von unbehandelten Referenzen (ungefüllt) und Ammoniak behandelter (grau) Eichen-, Fichten- und Kiefernproben. Die dicke Linie repräsentiert den Median, die Box den Interquartilabstand IQR (d.h. die Schwankungsbreite der innersten 50 % aller Messwerte), die Whiskers den äußersten gemessenen Wert innerhalb des 1.5-fachen IQR von der Box, und die Kreise fallweise auftretende Ausreißer. Der gepaarte t-test war für Eiche und Fichte signifikant (p < 0.001) no evidence for a general trend for soft- and hardwoods concerning mechanical properties. Increased bending properties of ammonia plasticized and densified wood were reportedbygrafetal.(1971, 1972), Kalnin š et al. (1967, 1969), and Oniśko and Matejak (1971). During the AMT process the wood was less plasticized compared to procedures applied in the before mentioned studies (e.g., Berzin š et al. 1970). After total evaporation of excessive ammonia the plasticization effect disappears. Significant changes in MOE and MOR were partly observed for oak and spruce (Figs. 1 and 2). However, stiffness and strength losses were in the range of 6 to 15%, which are negligible with respect to common solid wood applications. Davidson and Baumgardt (1970) described temporarily reduced bending properties during gaseous ammonia treatment as well as a post-

Eur. J. Wood Prod. (2012) 70:233 239 237 and intercellular structures are left in a reorganised position. However, as no leaching of temporarily formed disintegration products occurs, recovery of previous mechanical properties appears. 3.3 Comparing AMT and TMT Fig. 3 Box-and-whisker plot for impact bending strength (ω) of untreatedreferences (empty) and ammoniated (grey) oak, spruce and pine samples. The bold line represents the median, the box the interquartile range IQR (i.e. the range for the innermost 50% of all values), the whiskers the outermost measured value within the 1.5-times IQR-distance from the box. Paired sample t-test was significant (p < 0.05) for pine only Abb. 3 Box- und Whiskers Darstellung der Schlagbiegefestigkeit (ω) von unbehandelten Referenzen (ungefüllt) und Ammoniak behandelter (grau) Eichen-, Fichten- und Kiefernproben. Die dicke Linie repräsentiert den Median, die Box den Interquartilabstand IQR (d.h. die Schwankungsbreite der innersten 50 % aller Messwerte), die Whiskers den äußersten gemessenen Wert innerhalb des 1.5-fachen IQR von der Box. Der gepaarte t-test war nur für Kiefer signifikant (p < 0.05) recovery of the mechanical properties. These results agree with own data. Similar relations were also found for impact bending strength. Former studies (e.g., Kalnin š et al. 1967) showed that impact bending strength can be improved by ammonia treatment in combination with densification. Due to this treatment wooden cells will collapse, which raises wood density. Increase in density is usually accompanied with higher strength properties. For AMT (without densification) no or negligible strength losses were observed (Fig. 3). Low but significant reductions were shown only for pine-amt. It is assumed that the found alteration in impact bending strength is of low relevance, especially for indoor applications without bearing load. To summarize, significant reductions in mechanical properties due to the AMT process were observed, however, these changes are technologically negligible and therefore of little relevance. The authors data also indicate that extractives play a more important role in the modification process than the structural wood polymers. During the AMT process ammonia causes a temporary disconnection of van der Waals forces, H-bonds, and covalent bonds within the wooden cell wall. This leads to a reduction of encapsulated growth stresses during the modification process. After evaporation of volatile ammonia, the relative positions of cells AMT equivalent colour can also be achieved under moderate TMT process conditions. These shifts in colour as well as further altered properties are often a side effect of the desired increase in natural durability and dimensional stability in case of TMT. Table 2 shows a comparison of altered AMT and TMT wood properties. When processing TMT aiming at AMT equivalent colour, a decrease of the EMC at standard climate conditions by 1.5 to 2% points can be expected (e.g. Welzbacher et al. 2007; Esteves et al. 2008a). Accordingly, alterations in EMC following the AMT process may vary between +0.5 and 0.1% points, which is less pronounced compared to colour-equivalent TMT. Mass loss due to disintegration of cell wall polymers and reduced volume is typical for TMT. Therefore, impact on timber mass is higher than on volume changes, leading to reduced wood density. Typical mass loss due to conventional TMT processes reaches values up to approximately 14% (e.g. Welzbacher et al. 2007; Esteves et al. 2008a, 2008b). González-Peña and Hale (2009a) found a mass loss of even 28% under severe conditions (245 C, 16 h). A mass loss between 2 and 3% was reported by Welzbacher et al. (2007), Esteves et al. (2008b), and González-Peña and Hale (2009a) for TMT showing a change in colour equivalent to AMT. Hence, alterations concerning mass loss are significantly higher for TMT compared to AMT. Minor improvement of mechanical wood properties at the start of thermal modification is known to origin from increased density due to depolymerisation induced shrinkage and reduced EMC. However, industrial TMT is usually characterized by significantly decreased mechanical properties. For example, the bending properties alter with mass loss and TMT process duration time, following a parabolic function (González-Peña and Hale 2009a). Under TMT process conditions such as in González-Peña and Hale (2009a), leading to a change in colour equivalent to AMT, mechanical properties actually passed their maximum and began to drop. At a mass loss of TMT associated with a shift in colour equivalent to AMT, MOE is reduced by less than 5%, but MOR decreases by approximately 40% (Esteves et al. 2008a). The latter reduction in MOR was found to be aligned with hemicelluloses degradation. Comparable results were also found by Esteves et al. (2007). Alterations in the bending properties following the AMT process are less pronounced, compared to colour equivalent TMT. While reduction in MOE is low for both treatments (AMT: 2 to 8%; TMT: 5%), a remarkable difference was seen for MOR (AMT: +1 to

238 Eur. J. Wood Prod. (2012) 70:233 239 Table 2 Comparison of altered wood properties due to modification for AMT and TMT (referring to literature: Sailer et al. 2000; Welzbacher et al. 2007; Esteves et al. 2008a, 2008b; González-Peña and Hale 2009a) Tab. 2 Vergleich von veränderten Holzeigenschaften aufgrund von AMT- und TMT-Modifikation (bezugnehmend zur Literatur: Sailer et al. 2000; Welzbacher et al. 2007; Esteves et al. 2008a, 2008b; González-Peña und Hale 2009a) AMT (original values) TMT (literature based values) Source for TMT results TMT conditions within the sources EMC +0.5 to 0.1% points 1.5 to 2% points Welzbacher et al. (2007) 10 h, 180 C Esteves et al. (2008a) 2 4 h, 170 200 C Mass loss 0 2to 3% Welzbacher et al. (2007) 10 h, 180 C Esteves et al. (2008b) 2 h, 180 C González-Peña and Hale (2009a) 2 4 h, 210 C MOE 2 to 8% 5% Esteves et al. (2008a) 2 4 h, 170 200 C MOR +1 to 15% 40% Esteves et al. (2008a) 2 4 h, 170 200 C ω +2 to 8% 63% Sailer et al. (2000) 4.5 h, 180 C 15%; TMT: 40%). Impact bending strength decreases even stronger due to thermal modification. This effect has already been seen under moderate TMT process conditions. Boonstra et al. (2007) found a reduction of impact bending strength of 56% for Scots pine, and 79% for Norway spruce after treatment at 165 C and 0.8 to 1 MPa for 30 minutes. Sailer et al. (2000) reported a reduction in impact bending strength for Scots pine by 63%, combined with a mass loss of only 2.5%, following a 4.5 h treatment at 180 C. The biggest differences between the two modification processes were found for impact bending strength (AMT: +2to 8%; TMT: 63%). Those differences are likely due to density changes, which are strong for TMT but negligible for AMT. In addition, colour equivalent TMT processes showed a significant mass loss and a distinct increase in brittleness. Consequently, depolymerisation within AMT is assumed to be lower compared to TMT. Hill (2006) summarized that compared to softwood mechanical properties of hardwood alter more strongly under equal process conditions. Accordingly, it can be assumed that relative differences in mechanical properties between AMT and TMT are more pronounced for oak than for spruce and pine. 4 Conclusion Based on the obtained results it is concluded that gaseous ammonia treatment of solid wood (i.e., the AMT process) is a soft modification treatment with respect to physical and mechanical properties. However, AMT may significantly upgrade the decorative value of the wood. Decorative colour changes cannot only be achieved with oak, but also with other wood species such as spruce or pine. In this respect, AMT is competitive to TMT if the wood is used for interior applications such as floorings or furniture. Acknowledgements This study was supported by the Austrian government and the federal governments of Lower Austria, Upper Austria, and Carinthia supporting the Austrian COMET programme. The authors wish to acknowledge support from Murat Gündüz concerning laboratory work. The authors declare that they have no conflict of interest. References Allegretti O, Cividinir R, Tessadri B (2008) Thermal treatment in saturated vapour pressure for spruce. In: Gard WF, van de Kuilen JWG (eds) Proceedings of conference COST E53, 29 30 October 2008. Delft, pp 125 134 Bariska M (1969) Plastifizierung des Holzes mit Ammoniak in Theorie und Praxis. Holz-Zent.bl 95(84):1309 1311 Bariska M (1975) Collapse phenomena in beechwood during and after NH 3 -impregnation. Wood Sci Technol 9:293 306 Berzin š G, Wagenführ R, Steiger A (1970) Strukturveränderungen des Holzes beim Plastifizierungsvorgang. Holztechnologie 11(4):233 236 Berzin š G (1972) Holzplastifizierung als Weg zur qualitätserhöhenden Werkstoffsubstitution. Holztechnologie 13(2):103 110 Besold G, Fengel D (1983a) Systematische Untersuchungen der Wirkung aggressiver Gase auf Fichtenholz, Teil 2: Veränderungen an den Polysacchariden und am Lignin. Holz Roh- Werkst 41:265 269 Besold G, Fengel D (1983b) Systematische Untersuchungen der Wirkung aggressiver Gase auf Fichtenholz, Teil 3: Sorptionsversuche, Festigkeitsprüfung und Erstellung eines Beurteilungsschemas. Holz Roh- Werkst 41:333 337 Boonstra MJ, Van Acker J, Tjeerdsma BF, Kegel EV (2007) Strength properties of thermally modified softwoods and its relation to polymeric structural wood constituents. Ann For Sci 64(7):679 690 Davidson RW, Baumgardt WG (1970) Plasticizing wood with ammonia a progress report. For Prod J 20(3):19 25 DIN 52182 (1976) Testing of wood; determination of density DIN 52183 (1977) Testing of wood; determination of moisture content DIN 52186 (1978) Testing of wood; bending test DIN 52189 (1981) Testing of wood; determination of impact bending strength Esteves B, Marques AV, Domingos I, Pereira H (2007) Influence of steam heating on the properties of pine (Pinus pineaster) andeucalypt (Eucalyptus globulus) wood. Wood Sci Technol 41:193 207

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