Selected properties of gas phase ammonia treated

Holz Roh Werkst DOI 10.1007/s00107-008-0301-1

ORIGINALS · ORIGINALARBEITEN

Selected properties of gas phase ammonia treated wood Martin Weigl · Johannes P¨ockl · Michael Grabner

© Springer-Verlag 2009

Abstract Changes of physical wood properties after ammonia gas phase treatment were tested. In order to cover the wide variability of the investigated parameter, nineteen different wood species were investigated. While density is more or less not affected, equilibrium moisture content at standard climate increases significantly for almost all species. Changes of differential shrinkage and swelling in radial and tangential direction are specific to species. While some species show increased swelling or shrinkage, for other species these values decline. In most cases, ammoniation leads to increased dimension stability in radial direction but to a reduction in tangential direction. So, anisotropy of swelling and shrinkage in general increases due to ammoniation. These results show that changed physical properties of such wood have to be considered concerning conditioning and dimensioning products therefrom.

neunzehn verschiedenen Holzarten durchgef¨uhrt. W¨ahrend die Holzdichte durch die Behandlung weitestgehend nicht beeinflusst wird, zeigt sich f¨ur die Holzausgleichsfeuchte bei Normklima bei fast allen Holzarten ein signifikanter Anstieg. Ver¨anderungen des differentiellen radialen und tangentialen Schwind- bzw. Quellmaßes sind stark von den Holzarten abh¨angig. W¨ahrend einige Holzarten einen Anstieg des Schwind- bzw. Quellmaßes zeigen, weisen andere Holzarten eine Abnahme auf. In den meisten F¨allen f¨uhrt das R¨auchern zu einer Zunahme der Dimensionsstabilit¨at in radialer Richtung, aber zu einer Abnahme in tangentialer Richtung. Somit nimmt die Anisotropie des Schwindbzw. Quellmaßes durch R¨auchern zu. Die Ergebnisse zeigen, dass eine Ber¨ucksichtigung dieser Ver¨anderungen bei der Klimatisierung des Holzes und bei der Dimensionierung von daraus erzeugten Werkstoffen erforderlich ist.

Ausgew¨ahlte Eigenschaften von ger¨auchertem Holz

1 Introduction

Zusammenfassung Es wurden physikalische Ver¨anderungen von Holz nach Behandlung mit Ammoniak in der Gasphase (R¨auchern) untersucht. Um die hohe Variabilit¨at dieser Parameter abzudecken, wurden die Untersuchungen an

Ammonia treatment as a method to change the colour of wood is known by carpenters as a surface modification (Tinkler 1921) and by industry as a process of modifying whole wood pieces. Oak is probably the most commonly ammoniated species. Weigl et al. (2007, 2009a,b) showed the effect of ammoniation on the colour of wood. For example, high colour changes could be found for Black locust (Robinia pseudacacia L.), wild cherry (Prunus avium L.) or oak (Quercus sp.). Such colour modification is based on significant changes in the chemical composition of wood. A wide range of reactions between ammonia and the wood cell wall substances and extractives are possible. However, gaseous treatments aiming at colour modification is much less described in literature than soaking wood with pure ammonia or aqueous solutions and high pressure treatments

M. Weigl (u) · J. P¨ockl Competence Center for Wood Composites and Wood Chemistry, Competence Center Wood GmbH, St. Peter Str. 25, 4021 Linz, Austria e-mail: [email protected] M. Grabner BOKU – University of Natural Resources and Applied Life Sciences Vienna, Peter Jordan Str. 82, 1190 Vienna, Austria

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aiming at wood plasticization. A brief review shall illustrate possible effects of ammoniation on wood even if they might appear to a reduced extend in case of the above mentioned colour modification procedures. A conversion from cellulose I to cellulose III due to high pressure treatment of wood with ammonia was for example reported by Huang et al. (2006), Lewin and Roldan (1971), Parham et al. (1972) or Yatsu et al. (1986). However, Barry et al. (1936) could not find any differences in cellulose structure of ramie fibre after gaseous ammoniation. Parameswaran and Roffael (1984) showed that no conversion from cellulose I to III occurs in the case of aqueous ammonia treatment, but there is an increase in crystallinity. Celluloses degree of polymerisation was found to increase for dry wood and to decrease at elevated wood moisture contents due to gaseous ammonia treatment (Besold and Fengel 1983b). (Bariska 1969) reported that hemicelluloses and low molecular weight lignin might get completely dissolved by ammonia. Besold and Fengel (1983b) found a reduction of about 0.9% lignin and in combination with high wood moisture content (around fibre saturation) an increase in water soluble sugars for spruce due to gaseous ammoniation. Unbound components such as low molecular sugars, pectin, acids, alcohols, tannic acids and even inorganic salts can get dissolved due to soaking in an aqueous ammonia solution (Oni´sko and Matejak 1971). According to Kalnin’ˇs et al. (1967), soaking wood with ammonia increases the amount of extractable substances about seven-fold. Similar results were also found by Besold and Fengel (1983a) for gaseous ammonia treatment, especially at increased wood moisture contents. Parameswaran and Roffael (1984) found indications of deacetylation of wood due to aqueous ammonia treatment. Increased, but still not alkaline pH values and also increased nitrogen concentrations in high pressure ammonia gas treated wood could be found by Amburgey and Johnson (1979) and by Besold and Fengel (1983a), as well as for aqueous treatments by Parameswaran and Roffael (1984). According to Besold and Fengel (1983c), ammonia sorption by spruce wood is in the range of 5.9 to 7.7% by mass, but just 20 to 30% of the absorbed ammonia is chemically bound. This study deals with the effect of ammonia gas phase treatment on solid wood properties. The aim of this work was to investigate to which extent the usability of ammoniated wood is influenced by changed physical properties. The following is hypothesised: 1. Gaseous ammoniation leads to a changed equilibrium moisture content due to remaining ammonium groups in the chemical wood structure, activation of polar groups on the hemicelluloses and the conversion of cellulose to a more amorphous structure. 2. Gaseous ammoniation leads to density changes due to the overall chemical changes such as disintegration of struc-

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tural polymers as well as due to changed structures after temporary plasticization and recovery of disconnected bonds in changed positions. 3. Gaseous ammoniation leads to changed shrinkage and swelling due to the overall chemical modification such as disintegration of structural polymers, activation of functional groups of hemicelluloses, conversion of cellulose into a more amorphous structure and remaining ammonium groups.

2 Material and methods Nineteen different wood species known for significant colour change due to ammoniation were selected according to earlier findings (Weigl et al. 2007): Norway spruce (Picea abies L. Karst), silver fir (Abies alba Mill.), European ash (Fraxinus excelsior L.), Black locust (Robinia pseudacacia L.), ailanthus (Ailanthus altissima (Mill.) Swingle), elm (Ulmus sp.), oak (Quercus sp.), maple (Acer sp.), silver birch (Betula pendula Roth.), European beech (Fagus sylvatica L.) and its coloured heartwood, poplar (Poplar sp.), European hornbeam (Carpinus betulus L.), Black alder (Alnus glutinosa Gaertn.), lime (Tilia sp.), American tulip tree (Liriodendron tulipifera L.), plum (Prunus domestica L.), wild cherry (Prunus avium L.), common walnut (Juglans regia L.). Three boards from different individual trees excluding sapwood were selected per species. From each individual board, a set of two paired samples with dimensions of 60 mm in axial and each 20 mm in radial and tangential direction were prepared. The clear samples were anatomically perfectly oriented. The total number of samples was 114 (19 × 3 × 2). One sample per pair was fumigated while the other one was kept as reference. Fumigation took place in a closed system in presence of air by arranging samples in an elevated position above the surface of an overspill of 30% aqueous ammonia solution for 14 days. After fumigation, samples were ventilated until no more odour of ammonia gas could be detected. All samples, fumigated once and references, were stored at standard climate (20 ◦ C, 65%RH) for four months. Mass and dimensions in all three anatomical directions were determined before and after kiln drying at 103 ◦ C. Furthermore, the same samples were soaked with water at room temperature following several vacuum (5 kPa) and atmospheric pressure stages (100 kPa) within a time period of 10 days. After equilibration, radial and tangential dimensions of the soaked samples were examined again. Based on the measured parameters, changes due to ammoniation of equilibrium moisture content (EMC) at standard climate, oven dry density, radial and tangential shrinkage and swelling were determined. Obtained values for shrink-

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age and swelling were transformed to differential ones (with β = differential shrinkage and α = differential swelling) in order to display the linear dimensional change with increasing respectively decreasing wood moisture content (i.e. percentage swelling/shrinkage per percent wood moisture content change [%/%]). The transformation was based on division of the raw data by the corresponding wood moisture content change (i.e. the change from the determined EMC at standard climate towards 0% in case of shrinkage and the change from 0% towards 30% as an approximation for the fibre saturation point in case of swelling). According to Rasch and Guiard (2004), a t-test is robust enough against non-normality. Hence it was used for comparison of the data for differential swelling and shrinkage concerning effects of the previous ammoniation. Correlation analysis concerning differential swelling and shrinkage was performed using the Sperman’s rho rank correlation procedure due to partial deviations from a normal distribution.

3 Results and discussion The estimated properties are shown in Figs. 1 to 6. Species are always arranged according to increasing values for the reference samples. Data are given as grouped box plots. Bold lines represent the median, boxes the interquartile distance and whiskers the outermost value within the 1.5-times interquartile distance. Grey boxes represent ammoniated

Fig. 1 Equilibrium moisture content (EMC) at standard climate of nineteen different ammoniated and reference wood species Abb. 1 Holzausgleichsfeuchte (EMC) bei Normklima von neunzehn verschiedenen ger¨aucherten und unbehandelten Holzarten

Fig. 2 Kiln dry density of nineteen different ammoniated and reference wood species Abb. 2 Darrdichte von neunzehn verschiedenen ger¨aucherten und unbehandelten Holzarten

samples, white ones the references. In some cases, distribution of data within the species varies remarkably, which could partly be dedicated to the small number of samples within each species. However, results give a general impres-

Fig. 3 Differential radial shrinkage (βr ) of nineteen different ammoniated and reference wood species Abb. 3 Differentielles radiales Schwindmaß (βr ) von neunzehn verschiedenen ger¨aucherten und unbehandelten Holzarten

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Fig. 4 Differential tangential shrinkage (βt ) of nineteen different ammoniated and reference wood species Abb. 4 Differentielles tangentiales Schwindmaß (βt ) von neunzehn verschiedenen ger¨aucherten und unbehandelten Holzarten

Fig. 6 Differential tangential swelling (αt ) of nineteen different ammoniated and reference wood species Abb. 6 Differentielles tangentiales Quellmaß (αt ) von neunzehn verschiedenen ger¨aucherten und unbehandelten Holzarten

sion of changed wood properties due to gaseous ammoniation. Finally, due to the high number of species, common changes should be representative for wood in general.

3.1 Equilibrium moisture content

Fig. 5 Differential radial swelling (αr ) of nineteen different ammoniated and reference wood species Abb. 5 Differentielles radiales Quellmaß (αr ) von neunzehn verschiedenen ger¨aucherten und unbehandelten Holzarten

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Commonly an EMC of about 11.8% at standard climate is expected according to the Loughborough diagram (i.e. an approximation of equilibrium wood moisture content according to the environmental conditions: temperature, relative air humidity and vapour pressure, Stamm and Loughborough (1935), Kollmann (1982)) although it has just been established for Sitka spruce (Picea sitchensis (Bong.) Carr.). It is commonly known that other species behave differently according to EMC. Excepting spruce and fir, the only coniferous species, all other species show increased equilibrium moisture content at standard climate after ammoniation (Fig. 1) with lime and hornbeam showing the strongest and beech heartwood the smallest effect. For lime, walnut, alder and hornbeam, this effect is most distinct. However, discrepancy between expected EMC (according to the Loughborough diagram) and determined EMC gets reduced due to gaseous ammonia treatment. This increase in EMC is also in accordance with Bariska et al. (1970) who found higher water sorption of ammonia treated Yellow birch (Betula alleghaniensis Britton) on a micro scale, which was also partly attributed to the conversion of native cellulose to the type III crystal form. Furthermore, Bariska et al. (1969) discussed that more wood substance is involved in sorption of ammonia than for water, probably due to remaining hydrophilic groups after ammoniation (Amburgey and Johnson 1979). Bariska and Popper (1975) reported about an in-

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crease of hygroscopicity and fibre saturation point for beech, birch and cotton due to ammonia sorption. Our results are also in accordance with Oni´sko and Matejak (1971) who found higher EMCs for oak, beech and pine wood soaked in ammonia than for soaked in water at a relative humidity above 90% due to the formation of an expanded capillary system. Finally, also Parameswaran and Roffael (1984) reported a similar effect for beech and oak wood due to boiling in aqueous ammonia solution respectively water. According to Bariska (1975), an explanation for increased EMC, but also for changed swelling and shrinkage properties of wood due to contact with ammonia is that: • Ammonia leads to a temporary loss of the dimensional stabilisation function of lignin due to additional polar groups • The portion of water active functional groups belonging to hemicelluloses increases due to the partly disconnection from lignin, leading to a higher affinity to water and increased swelling • Van der Waals and dipole bonds (including H-bonds) between structural macromolecules get rearranged in changed positions after evaporation of temporary bound ammonia Especially these changes in polarity seam to be an accurate explanation for increased EMC as increased polarity leads to an increased affinity to water. 3.2 Density Estimated kiln dry density varies between 0.3 and 0.7 g/cm3 (Fig. 2). Kiln dry density shows almost no change due to ammoniation. Just lime and oak show significantly decreased respectively increased values. The observed density changes are within naturally occurring variations. Obviously, the above mentioned changes in the chemical composition of wood do not influence overall wood density. Furthermore no significant differences between oven dry density and density at standard climate (without illustration) can be found. These results are also in accordance with Bariska (1975) who observed an overall density increase while soaking beech wood in anhydrous ammonia and a full recovery of initial density after evaporation. The fact that density is not influenced by gaseous ammonia treatment actually indicates that mechanical properties also remain in their naturally range which was also shown in Weigl et al. (2009a). 3.3 Differential shrinkage and swelling Shrinkage from wet to kiln dry conditions followed by swelling in water towards fibre saturation, showed results varying strongly between species, between swelling and

shrinkage and between radial and tangential considerations (Figs. 3 to 6). Partly no changes appear due to ammoniation, but mostly either significantly increased or decreased values can be found. A paired sample t-test (df = 55) proves that significant differences due to ammoniation occur in case of αr , αt and βr (sig. level = 0.010, 0.027 respectively 0.036), but not for βt (sig. level = 0.749). Most commonly, ammoniation leads to decreased radial and increased tangential shrinkage and swelling. Reduced radial swelling of beech soaked with anhydrous ammonia was also reported by Bariska (1975). For untreated wood within the density range of 0.3 to 0.7 g/cm3 , Kollmann (1982) gave values between 2.22 and 1.66 for the relation εα = αt /αr (with εα = anisotropic swelling coefficient, αt and αr = swelling coefficients in tangential and radial direction). For εβ = βt /βr (with εβ = anisotropic shrinkage coefficient, βt and βr = shrinkage coefficients in tangential and radial direction) values of between 3.68 and 1.41 were reported there. Average values estimated for treated samples and references are within these ranges but show a remarkable increase due to ammoniation: εα = 1.97 (reference) and 2.10 (ammoniated), εβ = 1.58 (reference) and 1.84 (ammoniated). Average changes of ε-values due to ammoniation (ammoniated minus reference) are shown in Fig. 7. Positive values indicate an increase of anisotropy due to ammoniation, negative ones a decrease. More pronounced changes due to ammoniation can be found for εβ . A similar change in swelling anisotropy of beech wood soaked in anhydrous ammonia was also reported by Bariska (1975). Radial and tangential trends of shrinkage within tree rings and even within cell walls were for example described by Trendelenburg and Mayer-Wegelin 1955 or Wardrop 1964. Changes of anisotropy in general must be dedicated to differences in radial and tangential distribution and content of functional groups (e.g. Gierlinger and Schwanninger 2007, Wimmer 1994), being potential reaction partners of ammonia. Values presented here for εα and εβ of ammoniated samples and references are on the same level as in Oni´sko and Matejak (1971) who gave values for oak and beech swelling in water or ammonia and shrinking from the soaked stage. Parham et al. (1972) even reported for Loblolly pine that wood shrinks more in the radial than in tangential direction due to swelling with and drying from ammonia, which leads to a decrease of εβ values. Such a decrease, for example, can also be found for spruce (Fig. 7), however a conversion of anisotropy (i.e. stronger swelling or shrinkage in radial than in tangential direction) can not be found for any sample. Differences between the here presented and the discussed literature data must be attributed to the different treating conditions. Soaking wood with ammonia obviously behaves different than a gaseous modification such as presented here. Some of the discussed chemical changes such as the conversion from cellulose I to cellulose III are described for

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Holz Roh Werkst Fig. 7 Changes of anisotropic swelling (εα ) and shrinkage (εβ ) coefficients due to ammoniation of nineteen different wood species Abb. 7 Ver¨anderung des anisotropen Quell- (εα ) und Schwindmaßes (εβ ) durch R¨auchern von neunzehn verschiedenen Holzarten

more extreme reaction conditions (e.g. increased temperature and/or pressure, treatment in a liquid stage). However, a gaseous treatment as an industrial process is preferable due to easier processing (especially concerning technical drying and evaporation of unbound ammonia). Results of overall species correlation analysis are shown in Tables 1 and 2. For example, highly significant relations can be found between differential swelling of ammoniated samples and references (Table 1; r = 0.589** in radial and r = 0.660** in tangential direction). Furthermore there is a highly significant relation between differential radial and tangential swelling of the references (r = 0.427**), but none for the ammoniated samples (Table 1). No explanation can be given for the highly significant correlation between differential swelling in radial direction of references and differential swelling in tangential direction of ammoniated samples (Table 1). However, among all significant correlations, the latter relationship is the weakest and might be spurious. A highly significant relation can Table 1 Rank correlation coefficients (rSperman’s rho ) for the associations between and within differential radial and tangential swelling (α) of ammoniated samples and references; highly significant values are indicated (**) Tabelle 1 Rangkorrelationenskoeffizienten (rSperman’s rho ) f¨ur die Assoziationen zwischen und innerhalb des differenziellen radialen und tangentialen Quellmaßes (α) ger¨aucherter Proben und Referenzen; hoch signifikante Werte sind markiert (**)

ammoniated radial tangential reference

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radial tangential

ammoniated radial tangential 1.000 0.188 0.188 1.000

reference radial tangential 0.589** 0.217 0.382** 0.660**

0.589** 0.217

1.000 0.427**

0.382** 0.660**

0.427** 1.000

Table 2 Rank correlation coefficients (rSperman’s rho ) for the associations between and within radial and tangential differential shrinkage (β) of ammoniated samples and references; highly significant values are indicated (**) Tabelle 2 Rangkorrelationenskoeffizienten (rSperman’s rho ) f¨ur die Assoziationen zwischen und innerhalb des differenziellen radialen und tangentialen Schwindmaßes (β) ger¨aucherter Proben und Referenzen; hoch signifikante Werte sind markiert (**)

ammoniated radial tangential reference radial tangential

ammoniated radial tangential 1.000 0.014 0.014 1.000 0.210 0.146 0.110 0.436**

reference radial tangential 0.210 0.110 0.146 0.436** 1.000 0.087 0.087 1.000

be found between differential tangential shrinkage of ammoniated samples and references (r = 0.436**), but none for radial direction (Table 2). Furthermore, no significance can be found comparing differential shrinkage values in radial and tangential direction (Table 2). In general, these results further support the above described results concerning the increase of swelling and shrinkage anisotropy due to ammoniation. Anisotropic property changes might be due to changed linkage between wooden cells. For example, if plane oak wood gets submersed in an aqueous ammonia solution and dried afterwards, rays protrude of the tangential surface. Also the anisotropic chemical composition of wood (e.g. Gierlinger and Schwanninger 2007, Wimmer 1994) and the various possible reactions of ammonia with functional groups found in the structural wood components (e.g. changed cellulose crystal type or partly disintegration of lignin and hemicelluloses) might be accurate explanations that need to be proved in further investigations.

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4 Conclusion Ammoniation leads to changed, mostly increased affinity of wood to water. So the first hypotheses could be approved. Changed physical properties such as anisotropic shrinkage and swelling of solid wood have to be considered while conditioning the wood and dimensioning products therefrom. Accretion of wooden products due to ammonia induced colour change and colour stabilization can therefore in many cases be further increased by a significant dimensional stabilization if anatomical directions are respected. So also the third hypotheses could be approved. However, no indication for changed wood density due to ammoniation could be found which rejects the second hypotheses. In general, further explanations of observed effects desire detailed studies on the changed chemical composition of gas phase ammonia treated wood. Acknowledgement This study was supported by the Austrian government and the federal governments of Lower Austria, Upper Austria and Carinthia. The authors like to thank for the very constructive reviewer comments on the earlier versions of the manuscripts.

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Besold G, Fengel D (1983c) Systematische Untersuchungen der Wirkung aggressiver Gase auf Fichtenholz, Teil 3: Sorptionsversuche, Festigkeitspr¨ufung und Erstellung eines Beurteilungsschemas. Holz Roh- Werkst 41:333–337 Gierlinger N, Schwanninger M (2007) The potential of Raman microscopy. Spectroscopy 21:69–89 Huang WY, Zhao M, Zhu HZ, Zhou X (2006) Structure of ramie treated by liquid ammonia. J Donghua Univ (English Edition) 23(1):103–107 Kalnin’ˇs AJ, Darzin’ˇs TA, Jukna AD, Berzin’ˇs GV (1967) Physikalisch-mechanische Eigenschaften mit Ammoniak chemisch plastifizierten Holzes. Holztechnologie 8(1):23–28 Kollmann F (1982) Technologie des Holzes und der Holzwerkstoffe. Springer, Berlin Heidelberg New York Lewin M, Roldan LG (1971) Effect of liquid anhydrous ammonia in the structure and morphology of cotton cellulose. J Polym Sci C Polym Sympos 36:213–229 Oni´sko W, Matejak M (1971) Einfluß 25%iger Ammoniakl¨osung auf die physikalischen und mechanischen Eigenschaften des Holzes. Holztechnologie 12(1):45–54 Parameswaran N, Roffael E (1984) Kenntnisstand und Untersuchungsergebnisse zur Wirkung von Ammoniak auf Holzsp¨ane. Holz Roh- Werkst 42:327–333 Parham RA, Davidson RW, de Zeeuw CH (1972) Radial-Tangential Shrinkage of Ammonia-Treated Loblolly Pine Wood. Wood Sci 4(3):129–136 Rasch D, Guiard V (2004) The robustness of parametric statistical methods. Psychol Sci 46(2):175–208 Stamm AJ, Loughborough WK (1935) Thermodynamics of the swelling of wood. J Phys Chem 39(1):121–132 Tinkler CK (1921) “Fumed” oak and natural brown oak. Biochem J 15(4):477–486 Trendelenburg R, Mayer-Wegelin H (1955) Das Holz als Rohstoff, 2. Auflage. Carl Hanser, M¨unchen Wardrop AB (1964) The structure and formation of the cell wall in xylem. In: Zimmermann MH (ed) The formation of wood in forest trees. Academic Press, New York London, pp 87–134 Weigl M, P¨ockl J, M¨uller U, Pretzl H, Grabner M (2007) UVresistance of ammonia treated wood. In: Hill CAS, Jones D, Militz H, Ormondroyd GA (eds) 3rd European Conference on Wood Modification. 15th–16th October 2007, Cardiff, UK Weigl M, G¨und¨uz M, M¨uller U (2009a) On the mechanical stability of ammonia treated wood. In: Hill CAS, Militz H (eds) 4th European Conference on Wood Modification. 27th–29th April 2009, Stockholm, Sweden Weigl M, Kandelbauer A, Hansmann C, P¨ockl J, M¨uller U, Grabner M (2009b) ,,Application of natural dyes in wood“. In: Bechtold T, Mussak RAM (eds) Handbook of Natural Colorants. John Wiley & Sons, Inc., New York (in press) Wimmer R (1994) Structural, chemical and mechanical trends within coniferous trees. In: Spiecker H, Kahle HP (eds) Modelling of tree-ring development – cell structure and environment workshop Proceedings. 5th–9th September 1994, Freiburg Yatsu LA, Calamari TA, Benerito RR (1986) Conversion of Cellulose I to Stable Cellulose III. Text Res J 56(7):419–424

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