WOOD ADHESIVE BONDLINES BY NANOINDENTATION
Johannes Konnerth1, Andreas Jäger2, Josef Eberhardsteiner2, Ulrich Müller1,3, Wolfgang Gindl1
1
Institute of Wood Science and Technology, Department of Material Sciences and Process
Engineering, BOKU-University of Natural Resources and Applied Life Sciences, Vienna, Austria
2
Institute for Mechanics of Materials and Structures - Vienna University of Technology, Vienna, Austria
3
Wood Kplus - Competence Center for Wood Composites and Wood Chemistry, Linz, Austria
[email protected]
ABSTRACT
The mechanical properties of both the adhesive and interphase cell walls in wood adhesive bonds were determined by
Nanoindentation.
Therefore seven different polymers used frequently as adhesives and/or matrix polymers in wood, wood composites, and
natural fibre reinforced composites were studied by nanoindentation and compared with results from previous uniaxial tensile
tests. It was shown that the elastic modulus, the hardness, and the elastic-, plastic-, and viscoelastic work of indentation of the
seven different polymers is essentially the same regardless whether the polymers were tested in the form of pure films or insitu, i.e. in an adhesive bond line with spruce wood. An excellent correlation was found between the elastic modulus measured
by tensile tests and the elastic modulus measured by nanoindentation. In spite of the good correlation, the elastic modulus
measured by nanoindentation is significantly higher than the elastic modulus measured by tensile tests.
Furthermore the elastic properties of wood cell walls in the interphase region of four different adhesive bonds were
determined. In comparison with reference cell walls unaffected by adhesive, interphase cell walls from melamine-ureaformaldehyde (MUF) and phenol-resorcinol-formaldehyde (PRF) adhesive bonds showed improved hardness, and in the case
of MUF also an improved elastic modulus. By contrast, cell walls from the interphase region in polyvinyl acetate (PVAc) and
one-component polyurethane (PUR) bonds showed lower elastic modulus and hardness than the reference. Considering the
different cell-wall penetration behaviour of the adhesive polymers studied here, it is concluded that damage and loss of elastic
modulus to surface cells occurring during the machining of wood is recovered in MUF and PRF bond lines, whereas damage
of cell walls persists in PVAc and PUR bond lines.
Introduction
An adhesive bond in wood, and wood- or natural fibre reinforced composites consists of zones (Figure 1) of pure polymer
(adhesive) and pure wood or fibre, a zone, termed interphase, where the polymer and the wood/fibre mix with each other, and
the interface between wood/fibre and polymer.
The mechanical properties of wood and natural fibres as adherents are well known and data is available [1, 2]. The
determination of mechanical properties for adhesives is less easy, but in recent studies also the micro- and macro mechanical
properties of adhesive films and in-situ properties of adhesive polymers in wood bond lines could be determined [3-6]. The
stress strain behaviour in adhesive bonds is significantly influenced by the interphase [7], which may act as important factor
when failure occurs. In the case of wood adhesive bonds the interphase consists of wood cell walls which are often
significantly damaged as a result of surface preparation [8], a phenomenon termed mechanical weak boundary layer [9]. The
second constituent of the interphase is the adhesive which penetrates the adherent surface and can fill microscopic cell
cavities [10-15]. Its properties are depending among others on polymer type and viscosity, and the surface energy. Parallel to
its presence in microscopic cell cavities, polymer was also observed in cell walls [16-20]. Polymer in cell walls can have a
significant effect on the mechanical properties [4, 5, 21]. An accurate knowledge of the mechanical properties of all bond line
components is necessary in order to understand the mechanical behaviour of the composite wood-adhesive interphase and
ultimately stress transfer across the total adhesive bond.
Wood
adherent 1
Interphase
Adhesive
Interphase
50 µm
Wood
adherent 2
Figure 1 Light microscope image of a wood joint with a PRF bond line. Arrowheads indicate selected locations of tested
interphase wood cell walls.
Nanoindentation is a method of hardness testing at very small scale applied to the study of mechanical properties of a variety
of materials [22-24]. It was introduced to the study of cell wall mechanics by Wimmer et al. [25-26].
In the present work we aim to characterise the mechanical properties of typical adhesive/matrix polymers used for adhesive
bonds in wood and wood composites, and as matrix polymers in wood- and natural fibre reinforced composites in-situ, i.e.
directly in the adhesive bond. Results from these measurements will be compared to previously obtained results from
macroscopic tensile tests [6], in order to evaluate the validity of the micromechanical measurements on polymer films.
Furthermore the influence of adhesive penetration into the wood cell wall on their mechanical properties is studied by means of
Nanoindentation on four polymer systems, typically used for wood adhesives.
Materials and methods
1. Adhesive film preparation:
Films were prepared from four different representative wood adhesives: polyvinylacetate (PVAc, PV/H Holzleim Standard,
Henkel Austria GmbH, Vienna), melamine-urea-formaldehyde (MUF, Dynomel L-435 with hardener H469, Dynea Austria
GmbH, Krems, Austria), phenol-resorcinol-formaldehyde (PRF, Aerodux 185 with hardener HR150, Friebe, Mannheim,
Germany), one-component polyurethane (1K PUR, Purbond HB110, Collano AG, Sempach, Switzerland). In addition, the
following matrix polymers used for the production of fibre reinforced composites were tested: epoxy (Epoxidharz L Nr. 236349
with hardener L Nr. 236357, Conrad Electronic, Hirschau, Germany), polyester (Polyester-Laminierharz VIPAL VUP 4782
BEMT with hardener MEKP M300, Gerber GFK-Systeme, Stuttgart, Germany), and polypropylene (PP, PP301460/14 film,
0.5mm, Goodfellow Cambridge Ltd, Huntingdon, England).
MUF, PRF, epoxy, and polyester films were prepared between two polytetrafluoroethylene (PTFE) panels, ensuring a constant
film thickness by means of 0.5 mm thick glass plates placed between the PTFE panels. The films were cured overnight at
ambient temperature and conditioned for one week between screw-clamped wood panels to prevent them from warping. In the
case of MUF and PRF, which contain a considerable amount of water, absorptive paper sheets were placed between polymer
films and wood panels and were changed repeatedly to ensure slow drying without distortion. PVAc and PUR films were
produced by pouring the liquid adhesive onto a PTFE panel, where a film with a homogenous adhesive thickness of about 0.2
mm for PVAc and 1.4 mm for PUR was prepared by means of a spatula. PUR films were repeatedly turned over to ensure
curing on both sides.
Nanoindentation specimens were prepared by cutting strips with a width of 4 x 4 mm from the cast films.
2. Adhesive bonds
In addition to pure polymer films as described above, adhesive bonds with pieces of spruce wood were manufactured using all
seven polymers. Pieces of spruce wood (picea abies) with a length of 100 mm, a width of 100 mm, and a thickness of 15 mm,
were bonded by their radial anatomical planes (the inclination of annual rings was approximately 60 to 90°). For liquid polymer
formulations, the procedure recommended by the manufacturer was followed and curing was done at ambient temperature. PP
films were sandwiched between two pieces of wood in a hot press heated to 230°C for 20 min and then cooled under pressure
for 2 hours. For all specimens, the pressure applied was 0.8 MPa. Post-curing and conditioning was done by storing the
specimens in a standard climate (20°C, 65% relative humidity) for 3 weeks. Pieces with a length of 2 mm, a width of 2 mm,
and a thickness of 0.5 mm were taken directly from the adhesive bond region, dried overnight in an oven at 60°C, and
embedded in Spurr epoxy resin [27] by alternating vacuum-pressure treatment. A smooth surface was cut using a Leica
Ultracut-R microtome equipped with a Diatome Histo diamond knife. Same as pure adhesive films, the embedded and
sectioned bond-line specimens (Figure 1) were glued to metal discs with epoxy resin in order to be clamped magnetically to
the nanoindenter sample stage.
3. Nanoindentation
Nanoindentation was chosen for the characterisation of adhesives and wood cell walls in the interphase of adhesive bonds. NI
was already used in a number of studies on interphases for metal- and glassfiber composites [28-31], for polymers [32] and
wood cell walls [4, 5, 26, 33, 34].
All NI experiments were performed with a Hysitron TriboIndenter system (Hysitron Inc., Minneapolis, USA, www.hysitron.com)
equipped with a three sided pyramid diamond indenter tip (Berkovich type). The samples specified above were clamped
magnetically to the indenter stage.
4. Adhesive characterisation
Two different specimens for each pure polymer film and wood-polymer bond-line, respectively, were examined by performing
24 indents in films and 18 indents in bond-lines. In the case of PVAc, the number of bond-line specimens was increased to
five, due to very high variability of measured properties. Experiments were performed in load-controlled mode using a preforce of 1.5 µN and a three segment load ramp (Figure 2): load application within 3 seconds, hold time 20 seconds, and
unload time 3 seconds. The peak load was varied from 100 µN to 500µN in steps of 50µN and from 500 µN to 1300 µN in
steps of 100µN in order to monitor a possible influence of peak load and unloading rate on the elastic modulus.
A
B
load P
depth h
h2
2
h1
3
1
Wp
Wv
We
depth h
t1
t2
time t
Figure 2 A: schematic load depth graph from nanoindentation with loading (1), holding (2) and unloading (3) segments
(continuous lines). The elastic (We), plastic (Wp), and viscoelastic (Wv) parts of the total work of indentation were calculated by
integrating the respective areas under the load-displacement curve. B: schematic depth-time graph of a nanoindentation
experiment indicating the relative change of indentation depth (related to creep) during the holding segment. The holding
segment (segment 2 on the left) starts at t1 and ends at t2.
5. Cell wall characterisation
Two different specimens for each wood-adhesive bond line were examined by performing 32 to 43 indents in the S2 layer of
early wood reference cell walls (i.e. cell walls unaffected by adhesive) and up to 98 indents in the S2 layer - distant from micro
cracks - of bond line cell walls per adhesive type. Experiments were performed in load-controlled mode using a pre-force of 1.5
µN and again a three segment load ramp (Figure 2): load application within 3 seconds, hold time 20 seconds, and unload time
3 seconds. The peak load was 150 µN for all indents in the experiment resulting in an indent diameter of less than one third
[33] of the cell wall thickness in order to avoid influence from the embedding material. Pre-positioning of the indenter tip was
done by means of an incident light microscope. Imaging of the cell walls was performed on the indenter stage by means of
scanning probe microscopy (SPM) with the indenter tip and a scanning size of 20 µm x 20 µm. Indent positions on the cell
walls were marked on the SPM image and executed from this mode. All indent positions were verified by SPM (Figure 3) after
the indentation procedure. Indents with insufficient distance to cell wall borders were removed from the data set.
Figure 3 Scanning probe microscopy images (20x20µm) of wood cell walls showing position of indents from Nanoindentation.
Left: reference cell walls at a distance > 500µm from the bond line. Right: deformed and broken cell walls in the interphase
region of a bond with adhesive contact.
An incident light microscope image of a PRF bonded specimen as used for nanoindentation is shown in Figure 1. Cell walls
and cell wall fragments from the first row of cells adjoining the central adhesive layer, where an adhesive penetration could be
possible, were the object of nanoindentation measurements. These cell walls were in direct contact with adhesive and showed
varying degrees of damage due to the machining of the bonded wood surfaces. For comparison, cell walls at a distance of
more than 500 µm from the bond line (not visible in Figure 1), where no influence from adhesive penetration was expected,
were tested as a reference.
The load-indentation depth curves recorded during nanoindentation experiments were evaluated according to the Oliver and
Pharr method [35]. From the load-depth graph recorded during the experiment the peak load (Pmax) and the contact area at the
end of the holding segment (A) are determined and the hardness is obtained by dividing Pmax by A. From the initial slope of the
unloading curve the unloading stiffness (S) is determined and the reduced elastic modulus Er is calculated according to
equation 1.
Er =
1
S
π
2
A
(1)
Er is termed the reduced elastic modulus because it takes into account the compliance of the indenter tip according to equation
2.
1 ⎛⎜ 1 − ν m
=
E r ⎜⎝ E m
2
⎞
⎛ 1 −ν i 2
⎟
+ ⎜⎜
⎟
⎠ material ⎝ Ei
⎞
⎟
⎟
⎠ indenter
Since the elastic modulus of the diamond indenter tip is very high (Ei=1140GPa, Poisson’s ratio
(2)
ν i =0.07),
the effect of
indenter compliance on Er is negligibly small in soft materials such as wood and adhesive polymers and no correction was
performed.
Finally, the elastic (We), viscoelastic (Wv) and plastic (Wp) parts of the work of indentation were calculated by integrating the
respective area under the load-displacement graph as shown in Figure 2.
Results and discussion
The elastic modulus of polymer films and adhesive bond lines measured by nanoindentation, respectively, are compared in
Fig. 4a. For both parameters, an essentially good agreement between measurements on films and bond lines is achieved,
indicated by a highly significant coefficient of determination. The only exception from this overall trend is PVAc, which shows
very high variability for both the elastic modulus and the hardness (not displayed) measured in the bond line. While the
coefficient of variation varies between 10% and 30% throughout all measurements, it is 70% for the elastic modulus and 100%
for the hardness of PVAc measured in the bond line. In view of this unusually high variability, additional experiments were
performed with varying unloading rates in order to monitor a potential influence of viscoelastic effects on the measured
parameters, but no statistically significant effect was found. It seems therefore possible that PVAc is less homogeneous in
terms of mechanical properties in an adhesive bond line bonding two pieces of wood, than in a pure polymer film.
10
10
2
2
R = 0.89
R = 0.83
MUF
8
8
PRF
6
Ebond line-NI (GPa)
Ebond line-NI (GPa)
MUF
Polyester
PVAc
4
PRF
6
4
Epoxy
PUR
Polyester
PVAc
Epoxy
PUR
2
2
PP
PP
a)
b)
0
0
0
2
4
6
8
10
0
2
Efilm-NI (GPa)
4
6
8
10
Efilm-macro (GPa)
Figure 4 Comparison of the elastic modulus from nanoindentation in the bond line with both a) the elastic modulus of the
adhesive films determined by nanoindentation and b) the elastic modulus from uniaxial tensile tests [6] (error bars correspond
to standard deviation).
The capability of polymers to absorb elastic, plastic, and viscoelastic deformation work, respectively, is presented in Figure 5.
140
Work [Joule 1E-12]
W elastic
120
W plastic
100
W viscoelastic
80
60
40
20
0
EP
MUF
PEst
PRF
PP
PUR
PVAc
Figure 5 Components of Nanoindentation work of indentation (W) for seven polymers in the bond line.
This evaluation is based on two indents per polymer taken in an adhesive bond with a peak load of 800 µN. It is shown that
MUF is the polymer with the lowest capability to absorb total deformation work, which is the sum of We, Wp, and Wv. This is
perfectly in agreement with the thermosetting behaviour of MUF whose crosslink density may be high compared to that of
epoxy which is another thermosetting polymer. The ductile polymers PP, PUR, and PVAc are capable of absorbing more than
twice the work of MUF. Assuming that the ratio between We and Wp is an indicator for brittleness, MUF with a ratio of 1.04 is
most brittle, whereas PVAc with a ratio of 0.28 is very ductile. On average, We/Wp was 0.73. The capability of a polymer to
absorb deformation energy may be interpreted as an indicator of fracture toughness, which increases with increasing
deformation energy.
Finally, the elastic moduli determined by nanoindentation in the bond line were compared (Fig. 4b) with elastic moduli
determined by tensile tests using a macromechanical extensometer described in more detail by Konnerth et al.[6]. Again a
highly significant correlation, but also a significant offset was observed. Throughout all measurements, the elastic modulus of
the studied polymers was overestimated by nanoindentation. Also Zheng and Ashcroft [36] and Van Landingham et al. [32]
found significantly increased elastic moduli determined by depth sensing indentation compared to uniaxial tests. They
concluded that the difference was caused by different stress conditions, size effects and the chosen unloading rates which are
contributing to viscoelastic effects. The overestimation observed in the case of the tested PUR was very high (>400 %). This
difference is, however, easily explained by the porosity of PUR in macro-sized specimens. Also in the adhesive bond line, a
porosity of about 25 % was observed (determined by incident light microscopy). However, measurements by nanoindentation
were taken only in solid PUR, which explains the considerably higher modulus.
Measurements of wood cell walls were performed as displayed in the scanning probe microscopy images in Figure 3.
Unaffected reference cell walls distant from the bond line and cell walls within the bond line region have been determined. The
small size of indents with respect to the overall cell wall thickness ensures that measured cell wall properties remain
unaffected by the surrounding embedding medium or surrounding adhesive.
The results from nanoindentation measurements are presented in Figure 6. The elastic modulus and the hardness show clear
effects of adhesive on the mechanical properties of cell walls. Statistically homogenous groups were identified by a one-way
Anova evaluation (alpha=0.05). The elastic modulus of interphase cell walls are dependent distinctly on the type of adhesive
applied. The elastic modulus of cell walls contacting PUR of PVAc adhesive is significantly reduced with regard to reference
cell walls situated far away from the bond line. By contrast, the elastic modulus of interphase cell walls in contact with PRF is
equal to the elastic modulus of reference cell walls, and cell walls from the MUF bond line reveal a significantly improved
elastic modulus compared to the reference. The results of hardness measurements by means of nanoindentation (Figure 6b)
are similar, with the exception of interphase cell walls in the PRF bonded specimens, which show a very significant increase of
hardness compared to reference cell walls.
interphase cell walls
0.70
a)
interphase cell walls
reference cell walls
b)
reference cell walls
25
Hardness [GPa]
Reduced Elastic Modulus [GPa]
0.60
20
0.50
0.40
15
0.30
N= 39
32
95
42
98
43
46
N= 39
40
10
32
95
42
98
43
46
40
0.20
PUR
PRF
MUF
PVAc
PUR
PRF
MUF
PV Ac
Figure 6 Reduced elastic modulus a) and Hardness b) from nanoindentation of interphase cell walls with adhesive contact
compared to reference cell walls distant from the bond line (PUR = one-component polyurethane, PRF = phenol-resorcinolformaldehyde, MUF = melamine-urea-formaldehyde, PVAc = polyvinyl acetate, N = number of indents).
Differences in the elastic modulus and hardness of interphase cell walls compared to reference cell wall go well in line with
published data on the penetration of adhesives into the wood cell wall. For PVAc and PUR, respectively, no detectable
penetration of adhesive into the wood cell wall was found [4, 5, 8, 15, 37]. It is therefore assumed that interphase cell walls in
these bond lines are unaffected by adhesive penetration. The observed reduction in mechanical properties of interphase cell
walls in PVAc and PUR bond lines – with respect to cell walls far from the bond line observed by nanoindentation – is most
probably a result of damages occurring during the machining of the wood surface. By contrast, abundant diffusion of phenolformaldehyde and melamine-formaldehyde based adhesives into the cell wall was repeatedly proven [18, 19]. Apparently, the
presence of MUF or PRF adhesive in the cell walls recovers loss of elastic behaviour due to machining and even improves
mechanical properties with regard to cell walls unaffected by adhesive.
Conclusion
It was shown in the present study that the tested polymers perform essentially equal in the shape of pure films and in an
adhesive bond line with spruce wood. A notable exception from this observation is PVAc, which shows very high variability in
the bond line. A highly significant correlation exists between the elastic modulus measured by tensile tests and by
nanoindentation. In general, the elastic modulus measured by nanoindentation is higher than the elastic modulus measured by
tensile tests.
Regarding the cell walls in the interphase of bond lines MUF experience a significant improvement of elastic modulus and
hardness. PRF performs similar to MUF with the exception of the elastic modulus, which is in the same magnitude as
reference cell walls.
Interphase cell walls in PVAc and PUR bond lines have diminished elastic modulus and hardness compared to the reference.
Acknowledgements
W. Gindl and J. Konnerth gratefully acknowledge financial support by the Austrian Science Fund FWF under grant P16837N07.
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