Natterer, J.

Stamm, B.

Wegener, G.

Windeisen, E.

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DOI 10.1007/s00107-005-0002-yK U R Z O R I G I N A L I A · B R I E F O R I G I N A L SHolz als Roh- und Werkstoff (2005) 63: 388–389B. Stamm · E. Windeisen · J. Natterer · G. WegenerThermal behaviour of polysaccharides in wood during friction weldingPublished online: 16 August 2005© Springer-Verlag 20051 IntroductionThis paper deals with the joining of wooden pieces withoutany additional material, using the technology of friction wel-ding. Sutthoff et al. (1996) made the first attempts for the joi-ning of wood by means of friction welding in Germany, re-sulting in a first patent. Since the year 2000, the Chair ofTimber Construction (IBOIS) at the Swiss Federal Institute ofTechnology in Lausanne is studying and developing this me-thod for wood connection (Gliniorz and Natterer 2000, Gli-niorz et al. 2001) in collaboration with the Wood ResearchMunich. The times necessary to complete a bond are shor-ter than one minute. No preparation of the welded surfaces isrequired.Chemical analysis was carried out to determine the thermalreactions taking place during the welding process. This articledeals with the thermal decomposition and the behaviour of thepolysaccharides of wood in a temperature range from ambienttemperature to about 450 ◦C, which is representative for the wel-ding process.At the beginning of the welding process, Coulomb-frictioncauses the interface to heat up. After a few seconds (3–10 s)at a temperature of about 320–350 ◦C the surfaces start to de-compose in consequence of the increased temperature. Thewood adjoining to the rubbed surfaces starts to soften, for-ming a viscous film. At about 420–450 ◦C a maximum tem-perature is reached and a steady state is attained. A finalcooling down after the termination of the frictional move-ment leads to solidification of the interfacial film forming theconnection.B. Stamm () · J. NattererIBOIS, Chair of Timber Construction, Swiss Federal Institute of TechnologyLausanne, 1015 Lausanne, SwitzerlandE-mail: bernhard.stamm@epfl.chE. Windeisen · G. WegenerWood Research Munich, Technical University of Munich, 80797 Munich,Germany2 Material and methods2.1 SamplesThe chemical analyses were carried out with Norway spruce(Picea abies) samples taken at different stages of thermaldecomposition:• Native wood of Norway spruce as reference• Start of thermal decomposition at about 320–350 ◦C (DS1(Decomposition Stage 1))• Transient phase at about 350–380 ◦C (DS2)• Steady-state phase at about 420–450 ◦C (DS3)All samples were ground by a ball mill.2.2 Methods2.2.1 ExtractionThe extraction of the samples was carried out at room tempe-rature with acetone as solvent with all the test samples. Thistreatment shows the way in which the solubility of wood com-pounds increases as a consequence of thermal attack due to thefrictional heat generation.2.2.2 Analysis of polysaccharides after hydrolysis by means ofion-exchange chromatographyThe hydrolysis of the extracted samples with trifluoroace-tic acid according to Fengel and Wegener (1979), leads toa splitting of the macromolecules into their monomer com-ponents. The hydrolysis was carried in a thermo-block at110 ◦C with different concentrations of TFA. The dried hy-drolysates were solubilised in a defined volume of distilledwater. The samples prepared in the manner described, wereanalysed in respect of the composition of polysaccharides bymeans of their monomer compounds by means of ion-exchangechromatography.3893 Results and discussionThe amounts of extract content of the different samples obtainedby extraction with acetone are shown in Table 1.Sample Acetone Extract [%]Reference 2.3DS1 2.7DS2 4.5DS3 15.8Table 1 Amounts of acetone ex-tractsTabelle 1 Mengen der Acetonex-trakteThe amount of acetone extract increases with an increase oftemperature. The highest increase occurs between DS2 at about350–380 ◦C and DS3 at about 420–450 ◦C. The big differencebetween the two amounts of extract is due to a partial splitting ofthe polymeric compounds by the thermal influence. The most si-gnificant decomposition takes place between 350 ◦C and 450 ◦C.This is indicated by a higher solubility of the joint material inacetone in comparison to native wood.The proportions of the different saccharides were found bydifferent kinds of hydrolysis carried out. Not any kind of hy-drolysis is ideal for every polysaccharide. The glycosidic bondsof xylan for instance are more susceptible to acidic attack thanthose of other polyoses. Thus, an amount of xylan is alreadydecomposed to smaller units than monomers and cannot be re-traced, while other polyoses are perfectly split into their mono-meric compounds. An exact analysis requires optimised hydro-lysis conditions, adapted to the different polymeric compounds.As the proportions of hexuronic acid and acetyl groups havenot been determined during the analyses carried out, the totalamounts of polyoses in the reference samples are not as high asgiven by the relevant literature.The results of the sugar analyses are presented in Fig. 1.The percentage of polyoses decreases steadily with increasingtemperature. The decomposed wood, which is finally serving asinterconnecting medium, contains 4.5% of polyoses compared to25.9% in the reference sample. Thereby, the thermally less sta-ble xylan is more affected than mannan. The strongest decompo-sition of polyoses takes place in the temperature range between320 ◦C and 380 ◦C. The very short duration of the process (10to 20 seconds) moves the thermal decomposition of wood com-pounds to higher temperatures in comparison to bibliographicaldata concerning the thermal degradation of wood (Fengel andWegener 1984, Wienhaus 1999). Nevertheless, the order in whichthe different polymeric compounds of wood become decompo-sed stays the same. Due to their structural characteristics, cellu-lose and lignin behave thermally more stable than polyoses. Theamount of cellulose stays nearly the same and rises somewhat dueto the thermal decomposition of the other compounds.Microscopical examinations (Stamm et al. 2005) of thecontact zone showed that the interfacial material resemblesa dense amorphous mass, containing fragments of wood cells.Fig. 1 Average values from repeat determinations of the monomericsugar-units converted into anhydro sugars for different stages of thermaldecomposition1 2 , literature values from Fengel and Wegener (1984)Abb. 1 Mittelwerte der monomeren Zuckereinheiten aus Doppelbestimmun-gen, umgerechnet in Anhydrozucker zu verschiedenen Stufen des thermi-schen Abbaus1 2, Literaturangaben nach Fengel and Wegener (1984)These wood cells, still showing the cell wall structure, areless decomposed than the amorphous material in which theyare embedded. Thus, the polyoses detected at the interfa-cial layer are assumed to be mainly part of these cell wallfragments.Further papers will treat the behaviour of wood with regardto friction welding in a more extensive manner.ReferencesFengel D, Wegener G (1979) Hydrolysis of polysaccharides with trifluoroa-cetic acid and its application to rapid wood and pulp analysis. In: BrownJr RD, Jurassek L (eds) Hydrolysis of cellulose; mechanisms of enzy-matic and acid catalysis. Adv in Chem Series, no 181. Am Chem Soc,Washington, pp 145–158Fengel D, Wegener G (1984) Wood – Chemistry, Ultrastructure, Reactions.Walter de Gruyter, Berlin New YorkGliniorz KU, Natterer J (2000) Holzschweißen – Innovative Verbindungs-technologien im Holzbau. Tagungsunterlagen, Symposium der LignaPlus/Weltmesse für die Forst- und Holzwirtschaft in Hannover, pp 9–18Gliniorz KU, Mohr S, Natterer J, Navi P (eds) (2001) Wood Welding. In:Proceedings of the First International Conference of the European Societyfor Wood Mechanics, Lausanne, Switzerland, pp 571–574Stamm B, Natterer J, Navi P (2005) Joining wood by friction welding. HolzRoh Werkstoff (in press)Sutthoff B, Franz U, Hentschel H, Schaaf A (1996) Verfahren zum reib-schweißartigen Fügen und Verbinden von Holz. Patentschrift DE 196 20273 C2, Deutsches Patent- und MarkenamtWienhaus O (1999) Modifizierung des Holzes durch eine milde Pyrolyse– abgeleitet aus den allgemeinen Prinzipien der Thermolyse des Holzes.Wiss Z U Dresden 48:17–22

Thermal behaviour of polysaccharides in wood during friction welding

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Thermal behaviour of polysaccharides in wood during friction welding

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