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Shear Fracture Characterisation of Green Glued Polyurethane Wood Adhesive Bonds at Various Moisture and Gluing Conditions
Linnaeus University, Faculty of Science and Engineering, School of Engineering. (Byggteknik)
Lund University.
2012 (English)In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 7, no 2, p. 93-100Article in journal (Refereed) Published
Abstract [en]

The shear fracture properties of green-glued one-component polyurethane (PUR) wood adhesive bonds subjected to kiln drying were investigated. The local shear strength and fracture energy of the wood adhesive bonds were determined from experimentally recorded complete shear stress versus deformation curves of the bond line. A stable test set-up and small specimens that were anti-symmetrically loaded were used in order to get a uniform and pure state of shear stress. Different moisture contents (MCs) and pressing times were investigated. The fracture properties of conventionally dry-glued wood adhesive bonds and of solid wood were used as reference. The results show that the fracture energy of green-glued bonds with PUR adhesive is dependent on the MC of wood and on the pressing time. The same fracture energy and strength can be obtained by green gluing as by dry gluing, but there seems to exist a maximum MC of sapwood, in the range between 78% and 160%, and a minimum pressing time, in the range between 3 h and 48 h, for which it can be achieved. Both dry- and green-glued polyurethane adhesive bonds were more ductile than solid wood.

Place, publisher, year, edition, pages
2012. Vol. 7, no 2, p. 93-100
Keywords [en]
green gluing, fracture energy, shear strength, wood, adhesive bonds, stress versus deformation
National Category
Materials Engineering
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
URN: urn:nbn:se:lnu:diva-18596DOI: 10.1080/17480272.2012.662997Scopus ID: 2-s2.0-84861640741OAI: oai:DiVA.org:lnu-18596DiVA, id: diva2:525524
Available from: 2012-05-08 Created: 2012-05-08 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Characterisation of green-glued wood adhesive bonds
Open this publication in new window or tab >>Characterisation of green-glued wood adhesive bonds
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The gluing of unseasoned wood, called green gluing, is a relatively new sawmill process, implying a radically changed order of material flow in the production of value-added wood-based products. It facilitates the enhancement of raw material recovery and value yield by integrating defect elimination and gluing already before kiln drying. The present study evaluates green glued adhesive bonds in flatwise glued beams and finger joints. The main part of this work deals with green gluing using a moisture curing polyurethane adhesive (PUR). Standardised test methods and specially designed, small scale, specimens were used for the determination of the strength, fracture energy and the ductility of both dry- and green glued bonds in tension and in shear. Using the small scale specimens it was possible to capture the complete stress versus deformation curves, including also their unloading part. An optical system for deformation measurement was used for the analysis of bond behaviour. The influence of moisture content during curing and temperature after curing on the adhesive chemical composition and on the mechanical properties was investigated. Furthermore, the moisture transport through the adhesive bond during curing was tested. Finally, microscopy studies were performed for analysis of bond morphology and fracture. The results show that two significant factors influence the shear strength of green glued bonds: wood density and adhesive spread rate. Bonds which fulfil the requirements according to EN 386 could be obtained within a wide range of process parameters. The small specimen tests showed that green glued PUR bonds can reach the same strength and fracture energy, both in shear and in tension, as dry glued bonds with the same adhesive amount. The local material properties of the bonds could be determined, thanks to the failure in the tests taking place within the adhesive bond itself and not in the wood. Following process factors were shown to cause lower bond strength: a) a low adhesive spread rate, b) high pressure and c) short pressing time in combination with low wood density and high moisture content. Moreover, the heat treatment of the cured PUR adhesive during drying influenced the chemical composition of the adhesive, providing for higher strength, stiffness and Tg of the adhesive, caused by an increased amount of highly ordered bidentate urea.

Place, publisher, year, edition, pages
Växjö, Kalmar: Linnaeus University Press, 2012. p. 67
Series
Linnaeus University Dissertations ; 85/2012
Keywords
green gluing, finger jointing, durability, shear strength, wood failure percentage, fracture energy, tensile strength, PUR adhesive
National Category
Materials Engineering
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-18606 (URN)978-91-86983-57-4 (ISBN)
Public defence
2012-06-01, Myrdal, Hus K, Växjö, 09:00 (English)
Opponent
Supervisors
Available from: 2012-05-09 Created: 2012-05-08 Last updated: 2012-06-15Bibliographically approved

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Sterley, Magdalena

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