Low Temperature Adhesive Bonding for Structural Wood Materials · 2020. 7. 5. · Low Temperature...
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a b c d Low Temperature Adhesive Bonding for Structural Wood Materials Anita Tran a,b , Melanie Mayr a , Johannes Konnerth a , Wolfgang Gindl-Altmutter a a BOKU - Institute of Wood Technology and Renewable Materials Konrad Lorenz-Straße 24, 3430 Tulln an der Donau, Austria b [email protected] nanoindentation position interphase cell reference cell on hardener application side adhesive embedding epoxy lumen filling INTRODUCTION Strong and reliable bonds are mandatory for load-bearing timber products, therefore manufacturing processes are strictly regulated. Substrate temperature specification for adhesive application is at minimum 18 °C for structural finger joints 1 . This requires in-line pre-heating prior to adhesive bonding, when lamellas are stored outside in winter. RESEARCH QUESTION Can adhesive bonds with identical bond performance also be achieved with significantly lower substrate temperature by simply providing more time for adhesive cure? METHODOLOGY Scarf joint samples from spruce imitating finger joint angles of 7.5° were adhesively bonded in three temperature test series (A - C). The melamine-urea-formaldehyde (MUF) adhesive was applied on separate sides. The samples were cut lengthwise for different testing methods. The following parameters were examined to further understand the influence of substrate temperature on the adhesive bond: • tensile shear strength parallel to the grain via tensile testing • adhesive penetration via microscopic analysis • cell wall penetration and micro- mechanics via of nanoindentation. pressing temperature A 0 °C B 0 °C C 20 °C 1. equilibrate to temperature 2. adhesive application hardener MUF resin 3. pressing (12 h) postcuring temperature A 0 °C B 20 °C C 20 °C 4. postcuring (12 h) 5. conditioning of scarf joints for two weeks at standard climate: 20 °C / 60 % r.h. 6. cutting cured samples with 5 mm (a,d) and 13 mm width (b, c) for testing 7. testing methods a. microscopic analysis b. tensile shear strength (untreated) c. tensile shear strength after 24 h water immersion (pretreated) d. nanoindentation Microscopic analysis Fig.3. Microscopic image of bond line (dottet line) with highlighted nanoindentation positions 2 The adhesive penetrated on both application sides with no depency on temperature. The pure hardener was only visibleon its application side and clearly distinguisable from the MUF resin. Fig. 3 also depicts preselected positions for nanoindentation in interphase and reference cells on both application sides. Fig. 1. Overview scheme showing manufacture of scarf joints and subsequent testing methods 2 Nanoindentation Nanoindentation visualized the penetration of the adhesive into the cell wall with increased hardness and reduced modulus of elasticity from unfilled cells to adhesive filled cells independent of temperature. Fig.4. Reduced elastic modulus of cell walls 2 Fig.5. Topography images after nanoindentation: reference cell (a) and cell filled with adhesive and hardener (b) 2 RESULTS Tensile testing Resulting tensile shear strength varied marginally for different temperatures. Even after exposure to water, the adhesive bond remained durable. Fig. 2. Tensile shear strength and corresponding wood failure percentage in dependence of test temperature 2 b hardener application side resin application side MUF resin MUF resin embedding resin embedding resin hardener hardener 50 μm 50 μm ACKNOWLEDGEMENT The research project is part of the Austrian Biorefinery Center Tulln (ABCT) funded by BOKU - University of Natural Resources and Life Sciences, Vienna and the county of Lower Austria. References 1 EN 15497, Structural finger jointed solid timber - Performance requirements and minimum production requirements, 2014. 2 Tran et al., Adhesive strength and micromechanics of wood bonded at low temperature, submitted to Int J Adhes Adhes. CONCLUSION The substrate temperature at the time of adhesive application has least influence on the macro- and microscopic level and therefore permits bonding at low temperatures providing enough time for cure at sufficient temperature is allowed. This applies to the examined specimen geometry and is possibly transferable to finger joints. interphase cell reference cell on resin application side lumen cell wall adhesive hardener 3 µm
Transcript of Low Temperature Adhesive Bonding for Structural Wood Materials · 2020. 7. 5. · Low Temperature...
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Low Temperature Adhesive Bonding for Structural Wood MaterialsAnita Trana,b, Melanie Mayra, Johannes Konnertha, Wolfgang Gindl-AltmutteraaBOKU - Institute of Wood Technology and Renewable MaterialsKonrad Lorenz-Straße 24, 3430 Tulln an der Donau, [email protected]
bond line
hard
ener
app
licat
ion
side
resi
n ap
plic
atio
n si
de
nanoindentation position
interphase cellreference cell onhardener application sideinterphase cell
reference cell onresin application side
adhesive
embedding epoxy
lumen filling
MUF resinMUF resin
embedding resinembedding resin
hardenerhardener
50 µm50 µm
bond line
hard
ener
app
licat
ion
side
resi
n ap
plic
atio
n si
de
nanoindentation position
interphase cellreference cell onhardener application sideinterphase cell
reference cell onresin application side
adhesive
embedding epoxy
lumen filling
MUF resinMUF resin
embedding resinembedding resin
hardenerhardener
50 µm50 µm
INTRODUCTIONStrong and reliable bonds are mandatory for load-bearing timber products, therefore manufacturing processes are strictly regulated.Substrate temperature specification for adhesive application is at minimum 18 °C for structural finger joints1. This requires in-line pre-heating prior to adhesive bonding, when lamellas are stored outside in winter.
RESEARCH QUESTION
Can adhesive bonds with identical bond performance also be achieved with significantly lower substrate temperature by simply providing more time for adhesive cure?
METHODOLOGYScarf joint samples from spruce imitating finger joint angles of 7.5° were adhesively bonded in three temperature test series (A - C). The melamine-urea-formaldehyde (MUF) adhesive was applied on separate sides. The samples were cut lengthwise for different testing methods. The following parameters were examined to further understand the influence of substrate temperature on the adhesive bond: • tensile shear strength parallel to the grain via tensile testing• adhesive penetration via microscopic analysis• cell wall penetration and micro- mechanics via of nanoindentation.
pressing temperature A 0 °C B 0 °C C 20 °C
1. equilibrate to temperature
2. adhesive application
hardener
MUF resin
3. pressing (12 h)
postcuring temperature A 0 °C B 20 °C C 20 °C
4. postcuring (12 h)
5. conditioning of scarf joints for two weeks at standard climate:
20 °C / 60 % r.h.
6. cutting cured samples with 5 mm (a,d) and 13 mm width (b, c) for testing
7. testing methods a. microscopic analysis b. tensile shear strength (untreated) c. tensile shear strength after 24 h water immersion (pretreated) d. nanoindentation
Microscopic analysis
Fig.3. Microscopic image of bond line (dottet line) with highlighted nanoindentation positions2
The adhesive penetrated on both application sides with no depency on temperature. The pure hardener was only visibleon its application side and clearly distinguisable from the MUF resin. Fig. 3 also depicts preselected positions for nanoindentation in interphase and reference cells on both application sides.
Fig. 1. Overview scheme showing manufacture of scarf joints and subsequent testing methods2
NanoindentationNanoindentation visualized the penetration of the adhesive into the cell wall with increased hardness and reduced modulus of elasticity from unfilled cells to adhesive filled cells independent of temperature.
Fig.4. Reduced elastic modulus of cell walls2
Fig.5. Topography images after nanoindentation: reference cell (a) and cell filled with adhesive and hardener (b)2
RESULTSTensile testingResulting tensile shear strength varied marginally for different temperatures. Even after exposure to water, the adhesive bond remained durable.
Fig. 2. Tensile shear strength and corresponding wood failure percentage in dependence of test temperature2
bond line
hard
ener
app
licat
ion
side
resi
n ap
plic
atio
n si
de
nanoindentation position
interphase cellreference cell onhardener application sideinterphase cell
reference cell onresin application side
adhesive
embedding epoxy
lumen filling
MUF resinMUF resin
embedding resinembedding resin
hardenerhardener
50 µm50 µm
ACKNOWLEDGEMENTThe research project is part of the Austrian Biorefinery Center Tulln (ABCT) funded by BOKU - University of Natural Resources and Life Sciences, Vienna and the county of Lower Austria.
References1 EN 15497, Structural finger jointed solid timber - Performance requirements and minimum production requirements, 2014.2 Tran et al., Adhesive strength and micromechanics of wood bonded at low temperature, submitted to Int J Adhes Adhes.
CONCLUSIONThe substrate temperature at the time of adhesive application has least influence on the macro- and microscopic level and therefore permits bonding at low temperatures providing enough time for cure at sufficient temperature is allowed. This applies to the examined specimen geometry and is possibly transferable to finger joints.
bond line
hard
ener
app
licat
ion
side
resi
n ap
plic
atio
n si
de
nanoindentation position
interphase cellreference cell onhardener application sideinterphase cell
reference cell onresin application side
adhesive
embedding epoxy
lumen filling
MUF resinMUF resin
embedding resinembedding resin
hardenerhardener
50 µm50 µm
lumen
cell wall
adhesive
hardener3 µm
