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Effect of high-temperature drying on spruce and larch Shape stability, mechanical properties and mould growth

Frühwald, Eva LU (2007)
Abstract (Swedish)
Popular Abstract in Swedish

Formstabilitet, dvs. förmågan för träet att behålla sin form, är en av de viktigaste parametrarna när trä används som byggnadsmaterial eller för andra ändamål. I allmänhet ändras träets dimensioner och form när dess fuktkvot ändras. Detta är dock inte önskvärt i konstruktioner, varför det behövs metoder som förbättrar formstabiliteten.



I denna avhandling har inverkan av högtemperaturtorkning på den omedelbara formen, men också den mer långsiktiga formstabiliteten under användningstiden studerats för svensk gran samt några lärkarter. Fokus har lagts på skevhet, då det är denna deformation som oftast leder till kassering av nordiskt virke.



Små prover har... (More)
Popular Abstract in Swedish

Formstabilitet, dvs. förmågan för träet att behålla sin form, är en av de viktigaste parametrarna när trä används som byggnadsmaterial eller för andra ändamål. I allmänhet ändras träets dimensioner och form när dess fuktkvot ändras. Detta är dock inte önskvärt i konstruktioner, varför det behövs metoder som förbättrar formstabiliteten.



I denna avhandling har inverkan av högtemperaturtorkning på den omedelbara formen, men också den mer långsiktiga formstabiliteten under användningstiden studerats för svensk gran samt några lärkarter. Fokus har lagts på skevhet, då det är denna deformation som oftast leder till kassering av nordiskt virke.



Små prover har torkats vid hög temperatur (120?C och 170?C) och jämförts med lågtemperaturtorkning (80?C). Alla torkprogram, oavsett temperatur, inkluderade även basning och konditionering vid hög temperatur och luftfuktighet (98?C, 98 % relativ luftfuktighet). Effekten av inspänning (pålastning) under torkning har studerats genom att hälften av brädorna har spänts in under torkprocessen. Efter avslutad torkning fuktväxlades samtliga prover för att undersöka om den reduktion av deformationer som har åstadkommits med hjälp av inspänning är permanent. Inverkan av torkningstemperaturen på formstabiliteten hos gran har studerats och även jämförts med en industriell värmebehandlingsprocess.



Resultaten visade att högtemperaturtorkningen är ett alternativ för att torka nordiska träslag som gran och lärk. Deformationerna, speciellt skevheten, kunde effektivt minskas genom inspänning under torkningen. Formstabiliteten var permanent under varierande fuktförhållanden. Torkningstemperaturen hade dock ingen effekt på formstabiliteten. Snarlika resultat har erhållits för formstabilitet efter torkning vid 80, 120 och 170?C. Att lågtemperaturtorkat virke (80?C) hade bra formstabilitet har troligtvis åstadkommits mestadels under basning och konditionering, som utgjorde korta värmebehandlingar med uppmjukning och krypning som följd.



Brädor med stor skevhet har värmebehandlats vid 95?C för att studera möjligheten att reducera skevhet. Värmebehandlingen minskade skevheten effektivt, dock ökade skevheten något i de efterföljande fuktväxlingarna. Dessa resultat bekräftar tidigare studier i det att extremt skeva brädor kan rätas ut med hjälp av värmebehandling, då termomekanisk krypning hjälper till att få brädorna att anta den önskade formen.



Högtemperaturtorkning inverkar både på träets kemiska och fysikaliska egenskaper. Sorptionsbeteende, styvhet (statisk och dynamisk elasticitetsmodul i böjning), böjhållfasthet och slagseghet har undersökts i denna studie. Det kunde visas att styvheten är mer eller mindre oberoende av torkningstemperaturen medan böjhållfastheten minskar med ökad torkningstemperatur. Slagsegheten visade inget tydligt beroende av torkningstemperatur.



Inverkan av torkningstemperatur eller värmebehandling på mottagligheten för mögelpåväxt har också studerats. Allt artificiellt torkat virke hade snarlik mögelpåväxt, medan klimatisering av prover vid 20?C resulterade i mindre och värmebehandling i mycket mindre mögelpåväxt. Andra faktorer som påverkade mögelpåväxten var närvaro av kärn- och splintved och vilken yta som studerats, den ursprungliga torkningsytan eller en yta som erhållits genom sågning efter torkningsprocessen. (Less)
Abstract
Shape stability, the ability of the wood to retain its shape, is one of the most important parameters when using timber as a building material and for other applications. Generally, timber undergoes changes in dimensions and shape when its moisture content is changed. However, this can often not be tolerated in constructions and thus, methods to improve the shape stability have to be found.



In this thesis, the influence of high-temperature drying on the immediate shape stability after drying but also on the long-term behaviour under in-service conditions was studied for Norway spruce and different larch species. Special focus was on twist, as this is the predominant distortion type leading to downgrading of Scandinavian... (More)
Shape stability, the ability of the wood to retain its shape, is one of the most important parameters when using timber as a building material and for other applications. Generally, timber undergoes changes in dimensions and shape when its moisture content is changed. However, this can often not be tolerated in constructions and thus, methods to improve the shape stability have to be found.



In this thesis, the influence of high-temperature drying on the immediate shape stability after drying but also on the long-term behaviour under in-service conditions was studied for Norway spruce and different larch species. Special focus was on twist, as this is the predominant distortion type leading to downgrading of Scandinavian timber species.



Small-scale specimens were dried at high temperatures (120?C and 170?C) and compared to conventional drying (80?C). All drying schedules, independent of temperature, included pre-steaming and steaming phases at saturated conditions (98?C, 98 % relative humidity). The effect of restrained drying or top-loading was investigated by restraining half of the boards during the drying process. After completed drying, all specimens were moisture-cycled to investigate the permanency of shape stability and distortion reduction attained during restrained drying. The influence of drying temperatures on shape stability of spruce was also compared to commercial heat-treatment.



The results showed that high-temperature drying is an alternative for Scandinavian timber species such as Norway spruce and larch. Distortions, especially twist, were successfully reduced by restrained drying and the obtained shape stability was permanent, even in changing humidity conditions. Drying temperature did not influence shape stability, but similar results were found for shape stability for drying at temperatures levels 80, 120 and 170?C. The good shape stability for low-temperature drying (80?C) is most probably achieved by the pre-steaming and steaming phases, which constituted a short high-temperature treatment with corresponding softening and creep behaviour.



Excessively twisted boards were heat-treated at 95?C to investigate the possibility to reduce twist. Twist could be effectively reduced by heat-treatment, however, twist increased slightly in subsequent moisture cycling. These results confirm earlier studies in that excessively twisted boards can be straightened by heat-treatment, as mechano-thermal creep helps to attain the desired shape of the board.



High-temperature drying influences both the chemical and physical properties of timber. In this study, sorption behaviour, stiffness (static and dynamic modulus of elasticity in bending), bending strength and toughness were investigated. It could be shown that stiffness is more or less unaffected by drying temperature, whereas bending stress decreases with increasing drying temperature. The toughness tests did not show a clear tendency of strength loss with increasing temperature.



Finally, the effect of different drying temperatures and heat-treatment on the susceptibility for mould growth was studied. It could be stated that all kiln drying temperatures resulted in about equal mould growth levels, whereas air-drying had lower and heat-treatment substantially lower mould levels. Other factors influencing the amount of mould growth were presence of sapwood or heartwood and the type of surface ? original drying surface or re-sawn surface. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Dr Salin, Jarl-Gunnar, SP, Trätek, Stockholm
organization
publishing date
type
Thesis
publication status
published
subject
keywords
twist, shape stability, Norway spruce, mould susceptibility, larch, high-temperature drying, distortion, Technological sciences, Teknik, Material technology, Materiallära, Building construction, materialteknik, heat-treatment, Byggnadsteknik, KSTRWood
pages
200 pages
publisher
Division of Structural Engineering, Lund Institute of Technology, Lund University
defense location
Lecture hall A, V-building John Ericssons väg 1 Lund University Faculty of Engineering
defense date
2007-06-01 13:15
external identifiers
  • other:ISRN:LUTVDG/TVBK-1034/07-SE
ISSN
0349-4969
language
English
LU publication?
yes
id
eea20ffb-433a-4718-9596-36a24742564c (old id 548680)
date added to LUP
2007-09-07 11:33:52
date last changed
2016-09-19 08:44:52
@phdthesis{eea20ffb-433a-4718-9596-36a24742564c,
  abstract     = {Shape stability, the ability of the wood to retain its shape, is one of the most important parameters when using timber as a building material and for other applications. Generally, timber undergoes changes in dimensions and shape when its moisture content is changed. However, this can often not be tolerated in constructions and thus, methods to improve the shape stability have to be found.<br/><br>
<br/><br>
In this thesis, the influence of high-temperature drying on the immediate shape stability after drying but also on the long-term behaviour under in-service conditions was studied for Norway spruce and different larch species. Special focus was on twist, as this is the predominant distortion type leading to downgrading of Scandinavian timber species.<br/><br>
<br/><br>
Small-scale specimens were dried at high temperatures (120?C and 170?C) and compared to conventional drying (80?C). All drying schedules, independent of temperature, included pre-steaming and steaming phases at saturated conditions (98?C, 98 % relative humidity). The effect of restrained drying or top-loading was investigated by restraining half of the boards during the drying process. After completed drying, all specimens were moisture-cycled to investigate the permanency of shape stability and distortion reduction attained during restrained drying. The influence of drying temperatures on shape stability of spruce was also compared to commercial heat-treatment.<br/><br>
<br/><br>
The results showed that high-temperature drying is an alternative for Scandinavian timber species such as Norway spruce and larch. Distortions, especially twist, were successfully reduced by restrained drying and the obtained shape stability was permanent, even in changing humidity conditions. Drying temperature did not influence shape stability, but similar results were found for shape stability for drying at temperatures levels 80, 120 and 170?C. The good shape stability for low-temperature drying (80?C) is most probably achieved by the pre-steaming and steaming phases, which constituted a short high-temperature treatment with corresponding softening and creep behaviour.<br/><br>
<br/><br>
Excessively twisted boards were heat-treated at 95?C to investigate the possibility to reduce twist. Twist could be effectively reduced by heat-treatment, however, twist increased slightly in subsequent moisture cycling. These results confirm earlier studies in that excessively twisted boards can be straightened by heat-treatment, as mechano-thermal creep helps to attain the desired shape of the board.<br/><br>
<br/><br>
High-temperature drying influences both the chemical and physical properties of timber. In this study, sorption behaviour, stiffness (static and dynamic modulus of elasticity in bending), bending strength and toughness were investigated. It could be shown that stiffness is more or less unaffected by drying temperature, whereas bending stress decreases with increasing drying temperature. The toughness tests did not show a clear tendency of strength loss with increasing temperature.<br/><br>
<br/><br>
Finally, the effect of different drying temperatures and heat-treatment on the susceptibility for mould growth was studied. It could be stated that all kiln drying temperatures resulted in about equal mould growth levels, whereas air-drying had lower and heat-treatment substantially lower mould levels. Other factors influencing the amount of mould growth were presence of sapwood or heartwood and the type of surface ? original drying surface or re-sawn surface.},
  author       = {Frühwald, Eva},
  issn         = {0349-4969},
  keyword      = {twist,shape stability,Norway spruce,mould susceptibility,larch,high-temperature drying,distortion,Technological sciences,Teknik,Material technology,Materiallära,Building construction,materialteknik,heat-treatment,Byggnadsteknik,KSTRWood},
  language     = {eng},
  pages        = {200},
  publisher    = {Division of Structural Engineering, Lund Institute of Technology, Lund University},
  school       = {Lund University},
  title        = {Effect of high-temperature drying on spruce and larch Shape stability, mechanical properties and mould growth},
  year         = {2007},
}