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Characterisation of green-glued wood adhesive bonds
Linnaeus University, Faculty of Science and Engineering, School of Engineering. (Byggteknik)
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. , 67 p.
Series
Linnaeus University Dissertations, 85/2012
Keyword [en]
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: urn:nbn:se:lnu:diva-18606ISBN: 978-91-86983-57-4 (print)OAI: oai:DiVA.org:lnu-18606DiVA: diva2:525573
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
List of papers
1. Edge and face gluing of green timber using a one-component polyurethane adhesive
Open this publication in new window or tab >>Edge and face gluing of green timber using a one-component polyurethane adhesive
2004 (English)In: European journal of wood and wood products, ISSN 0018-3768, Vol. 62, no 6, 479-482 p.Article in journal (Refereed) Published
Keyword
gluing, polyurethane adhesive, shear strength
National Category
Engineering and Technology
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-18591 (URN)10.1007/s00107-004-0517-7 (DOI)
Available from: 2012-05-08 Created: 2012-05-08 Last updated: 2012-05-31Bibliographically approved
2. Finger jointing of green Scots pine
Open this publication in new window or tab >>Finger jointing of green Scots pine
2005 (English)In: Green gluing of wood- process – products – market : International conference / workshop, Borås, April 7-8, 2005 / [ed] Björn Källander, Borås, 2005, 55-65 p.Conference paper, Published paper (Refereed)
Abstract [en]

Finger jointing of green timber enables the wood industry to produce value-added products by using a new efficient process. Green finger jointing has gradually accelerated in New Zealand, USAand Canada, but is still relatively unknown in Europe. This study evaluates three green adhesive systems used with green Scots Pine timber: 1) a modified Phenol Resorcinol Formaldehyde (PRF) known as Greenweld®, 2) a one-component polyurethane adhesive and 3) a soy protein and PRF (soy/PRF) adhesive. Green adhesives have been compared with a conventional exterior PRF-Reference adhesive used with kiln-dried timber. The evaluation of mechanical performance was carried out with 4-point bending, according to EN 408. The warp of dried finger-jointed timber was measured in the form of bow, spring and twist and microscopy studies of glue lines were performed. The moisture durability of the adhesives was tested according to ASTM-D 4688-95. The tensile strength, WFP (Wood Failure Percentage) and failure modes were determined. Results showed that bending strength did not differ either between green adhesive systems and PRF-Reference dry system, or among green adhesives themselves. The warp of dried timber was on the same level for green glued finger joints and for conventional finger joints. The durability test showed that the PUR adhesive used in the green process had highest dry tensile strength compared to the other tested adhesives. The wet tensile strength after accelerated aging of all four adhesives did not differ significantly. The dry WFP without any treatment was high for all three adhesives used for green gluing and did not differ from the PRF-Reference. However, the wet WFP after cyclic boiling treatment of the green adhesive systems turned out to be lower than the wet WFP of the PRF-Reference adhesive. The results of the failure mode also illustrate that the green adhesives glue line of the wet specimens failed more often in the interface between wood and the adhesive than those with the PRF-Reference adhesive. Additionally, microscopy studies of the glue lines revealed that the penetration of polyurethane adhesive was deepest, Greenweld’s penetration was similar to PRF-Reference and sapwood glue lines of green adhesives had more cavities than heartwood glue lines.

Place, publisher, year, edition, pages
Borås: , 2005
Keyword
finger jointing, green gluing, durability
National Category
Engineering and Technology
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-18593 (URN)91-85533-31-9 (ISBN)
Conference
International conference / workshop Green gluing of wood- process – products – market. Borås, April 7-8 2005COST Action E34 Bonding of Wood in cooperation with SP Swedish National Testing and Research Institute
Available from: 2012-05-08 Created: 2012-05-08 Last updated: 2012-09-14Bibliographically approved
3. Fracture characterisation of green glued-polyurethane adhesive bonds in Mode I
Open this publication in new window or tab >>Fracture characterisation of green glued-polyurethane adhesive bonds in Mode I
2013 (English)In: Materials and Structures, ISSN 1359-5997, E-ISSN 1871-6873, Vol. 46, no 3, 421-434 p.Article in journal (Refereed) Published
Abstract [en]

Unseasoned (green) spruce timber side boards of size 25 × 120 × 600 mm were flatwise-glued with a one-component PUR adhesive. Glued pairs of boards were then kiln-dried to 12 % moisture content. A special small-scale specimen for testing the fracture properties of the adhesive bond in Mode I was developed in order to evaluate the adhesive bond properties. The complete force versus deformation curve, including both the ascending and the descending parts, could be obtained with these small-scale specimens, enabling the strength and fracture energy of the bond line to be calculated. In addition, the fractured specimens were examined by scanning electron microscope. Results show that both the tensile strength and the fracture energy of the green glued PUR adhesive bonds were equal to those of the dry glued bonds. The methodology developed and used in the present study gives new possibilities for analysis of the mechanical behaviour of wood adhesive bonds, and particularly of their brittleness and its correlation with the type of fracture path. This is in sharp contrast to the use of standardised test methods (e.g. EN 302, ASTM D905) with specimens having relatively large glued areas. Using such types of specimens, it is not possible to obtain the complete force versus deformation response of the bond. In addition, when using such test methods, failure takes place in the wood or in the fibres near the bond, thus making it impossible to obtain detailed information about the bond line characteristics.

Keyword
Wood adhesive, Bond line, Fracture, Ductility, Green gluing, Polyurethane
National Category
Materials Engineering
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-18594 (URN)10.1617/s11527-012-9911-5 (DOI)000314889000007 ()2-s2.0-84893644705 (Scopus ID)
Available from: 2012-05-08 Created: 2012-05-08 Last updated: 2017-12-07Bibliographically approved
4. Shear Fracture Characterisation of Green Glued Polyurethane Wood Adhesive Bonds at Various Moisture and Gluing Conditions
Open this publication in new window or tab >>Shear Fracture Characterisation of Green Glued Polyurethane Wood Adhesive Bonds at Various Moisture and Gluing Conditions
2012 (English)In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 7, no 2, 93-100 p.Article 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.

Keyword
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:nbn:se:lnu:diva-18596 (URN)10.1080/17480272.2012.662997 (DOI)2-s2.0-84861640741 (Scopus ID)
Available from: 2012-05-08 Created: 2012-05-08 Last updated: 2017-12-07Bibliographically approved
5. Influence of cure conditions on the properties of a one-component moisture-cured polyurethane adhesive in the context of green gluing of wood
Open this publication in new window or tab >>Influence of cure conditions on the properties of a one-component moisture-cured polyurethane adhesive in the context of green gluing of wood
2012 (English)In: Journal of Applied Polymer Science, ISSN 0021-8995, E-ISSN 1097-4628, Vol. 126, no S1, E296-E303 p.Article in journal (Refereed) Published
Abstract [en]

A commercial one-component moisture-cured polyurethane-urea wood adhesive was investigated under different curing environments to simulate parameters during green gluing, that is, gluing of freshly sawn and undried timber. This process is an eco-efficient and waste eliminating process in which only the finished wood product properties have been tested; however, not the adhesive itself. Therefore, the effect of moisture and postcuring heat treatment on the adhesive properties such as cure, chemical, and physical characteristics, and adhesion to wood were studied. It was determined by rheometry that the water content was proportional to the time to gel point, with moisture content of 2.65.6 wt % water, resulting in a higher initial storage modulus of the adhesive. Additionally, it was found that the strength of the wet glued bonds was significantly higher after the heat treatment, corresponding to the increase in ordered bidentate groups (Fourier transform infrared spectroscopy), higher storage modulus (rheometry), and higher Tg (dynamic mechanical thermal analysis).

Keyword
moisture curing adhesives, polyurethane adhesive, wood adhesive, green gluing, wet gluing
National Category
Materials Engineering
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-18598 (URN)10.1002/app.36895 (DOI)2-s2.0-84864281628 (Scopus ID)
Available from: 2012-05-08 Created: 2012-05-08 Last updated: 2017-12-07Bibliographically approved
6. Flat wise green gluing of Norway spruce for structural application
Open this publication in new window or tab >>Flat wise green gluing of Norway spruce for structural application
2009 (English)In: International Conference on Wood Adhesives 2009 / [ed] Charles Frihart, Madison: Forest Products Society, 2009, 325-334 p.Conference paper, Published paper (Refereed)
Abstract [en]

Unseasoned (green) Spruce timber planks with dimensions25 x 150 x 5400 mm3 were flat-wise glued with a one-componentPUR adhesive, forming laminated beams with 150 x 300 mm2 crosssection.After curing, each beam was divided in two halves and subsequentlydried. The final cross-section of the beams was thus 50 x 300mm2. The evaluation of the beams included bending stiffness andbending strength in 4-point bending and the mechanical properties ofthe adhesive bonds. The adhesive bonds were tested according toEuropean standards for glued-laminated timber (EN 392, EN 391) andaccording to ASTM D 905, but also with a special small-scale specimenfor testing the fracture properties of the adhesive bond in Mode I.The complete force vs. deformation curve, including both the ascendingand the descending parts could be obtained. The deformationswere measured with a contact-free technique, based on two camerasand white light. The equipment made it possible to register the strainin the bond line and in the adjacent wood with a high spatial resolution(0,2 mm). The strength of the bond line and the fracture energywas calculated. Results show that both the stiffness and the strength ofthe beams can comply with the requirements for glued-laminated timberclass L40. The adhesive bonds fulfill the requirements of glulam instandard EN 386. The tensile strength and fracture energy measuredwith the small specimens of green-glued bond lines is on the samelevel as of conventionally glued bond lines.

Place, publisher, year, edition, pages
Madison: Forest Products Society, 2009
Keyword
green gluing, tensite strength, fracture energy
National Category
Materials Engineering
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-18600 (URN)9781892529572 (ISBN)9781617389863 (ISBN)
Conference
International Conference on Wood Adhesives, Lake Tahoe, Nevada, 28-30 sept.,2009
Available from: 2012-05-08 Created: 2012-05-08 Last updated: 2015-09-28Bibliographically approved
7. Finger jointing of freshly sawn Norway Spruce side boards: a comparative study of fracture properties of joints glued with phenol-resorcinol and one-component polyurethane adhesive
Open this publication in new window or tab >>Finger jointing of freshly sawn Norway Spruce side boards: a comparative study of fracture properties of joints glued with phenol-resorcinol and one-component polyurethane adhesive
2014 (English)In: Materials and Joints in Timber Structures: Recent Developments of Technology, Springer, 2014, 325-339 p.Conference paper, Published paper (Other academic)
Abstract [en]

Finger jointing of unseasoned Norway Spruce was studied with respect to tensile strength, adhesive penetration and durability. Finger joints were manufactured with 1) unseasoned wood and one component polyurethane (PUR) adhesive, 2) dried wood and PUR adhesive and 3) dried wood and phenol resorcinol formaldehyde (PRF) adhesive. Two levels of wood density were used. The tensile strength of the finger joints was determined and the deformations within the joint were studied with an optical measurement system (ARAMIS). The penetration of the adhesive was studied with x-ray microtomography. The durability of the joints was determined according to the standard ASTM D 4688. The results show that the tensile strength and the durability of green glued finger joints are on the same level as that of dry glued PUR joints. The penetration of the PUR adhesive is high in the unseasoned wood and cavities within the bonds seem to be smaller than in dry glued PUR joints. The tensile strength of the finger joints is dependent on density, independent on the adhesive system used. The strength of the green glued PUR adhesive bonds in finger joints measured with small scale specimens did not differ from the strength of the dry glued PUR bonds.

Place, publisher, year, edition, pages
Springer, 2014
Series
RILEM Bookseries, ISSN 2211-0844 ; 9
Keyword
finger joint, bond line, fracture, green gluing, digital image correlation, x-ray tomography
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-18603 (URN)10.1007/978-94-007-7811-5_30 (DOI)2-s2.0-84885449084 (Scopus ID)978-94-007-7810-8 (ISBN)
Conference
RILEM International Symposium on Materials and Joints in Timber Structures, Stuttgart, GERMANY, OCT 08-10, 2013
Available from: 2012-05-08 Created: 2012-05-08 Last updated: 2015-09-28Bibliographically approved

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