The performance of adhesive-hardwood bonds can often be sensitive to
humidity and temperature variation. Therefore, it is frequently challenging to achieve
standard requirements for structural applications. To gain a better understanding of the
wood-adhesive bond, the properties of the individual constituents as well as the local
interface of European beech (Fagus sylvatica L.) wood
cell walls in contact with structural adhesives were analyzed by means of
nanoindentation. These results are compared to classical lap-shear strength. As
adhesives two different one-component polyurethane adhesives (1C PUR) and a phenol
resorcinol formaldehyde adhesive (PRF) were used. In one case, the beech wood was
additionally pre-treated with an adhesion-promoting agent (primer) prior to bonding with
1C PUR. Beech wood joints were analyzed subsequent to several treatments, namely
standard climate, after wet storage and in re-dried conditions. In addition, the
influence of the primer on the hydroxyl accessibility of beech wood was investigated
with dynamic vapor sorption (DVS). The lap-shear strength revealed good performance in
dry and re-dried conditions for all adhesives on beech. Both polyurethane adhesives
obtained deficits when tested in wet conditions. The use of a primer significantly
improved the PUR performance in wet condition. DVS experiment demonstrated a decrease in
hydroxyl group accessibility when using a high primer concentration. As novelty,
nanoindentation was used for the first time to characterize the local
wood–adhesive-interface properties in wet conditions. Nanoindentation showed that all
tested 1C PUR perform quite similar in room climate, while PRF achieves considerable
higher values for reduced E-modulus and hardness. Wet storage led to a considerable
reduction in mechanical properties for all adhesives, while the highest relative change
was observed for PRF. After re-drying, the adhesives re-gained a large part of their
original mechanical properties in room climate. No distinct effect of the primer on the
local micromechanical properties could be detected with nanoindentation in terms of
specific work of indentation.
Notes
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1 Introduction
The on-going trend towards mixed forests in Europe and a growing stock of
hardwood challenges wood industry and science for an increasing material usage of
hardwood. Until now, most of the hardwood is used for thermal energy production.
Adhesive bonding can be one way to enable the use of hardwood for structural
applications, thus using hardwood in a more profitable, competitive and sustainable way.
However, some hardwood species still show difficulties in meeting requirements for
structural standard testing methods, such as delamination resistance according to EN
302-2 (Konnerth et al. 2016). While higher
strength of hardwood balances positive in wood-engineered products, the response to
humidity reduces the competitiveness and potential of some hardwood species (e.g., beech
wood).
One-component polyurethane adhesives (1C PUR) are being successfully used
for structural applications using spruce as substrate. However, PUR is associated with
comparably poor performance on some alternative wood species and some hardwoods,
especially when tested for humid or very dry environments. In order to overcome these
issues, adhesion deficits were addressed with the combination of different
adhesion-promoting agents (primer) as reported in various studies (Ohnesorge et al.
2010; Amen-Chen and Gabriel 2015; Kläusler et al. 2014a, b). Richter
(1999) described the general
characteristics of primers by a polar part that enables strong intermolecular
interactions, a hydrophobic spacer grid and a part that enhances the wetting with the
adhesive. The application of primers can enhance the mechanical performance of 1C PUR
bonds on hardwood in order to allow for complying with standard requirements (Kläusler
et al. 2014a, b; Clerc et al. 2018). It was recently shown that a primer is capable of penetrating
wood cells to a certain extent (Casdorff et al. 2018). However, the function of the primers at the local interface is
not fully understood yet and demands further research. Similar to other references
(e.g., Frihart 2012), in this context, the
term “interphase” refers to the region within an adhesive bond where the adhesive
penetrates the pores of the wood substrate. Within this interphase multiple local
“interfaces” are present. The latter is defined as the direct (local) boundary between
the wood cell wall and the adhesive.
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Different approaches have been conducted to evaluate the performance of
adhesives on wood (Amman et al. 2013;
Konnerth et al. 2006; Kläusler et al.
2014a, b). Testing of single and pure adhesive films might not be able to
display the real conditions in a bond line, since the surrounding wood affects the
curing, mechanical relaxation and morphology as found by Ren and Frazier (2012). Therefore, investigations on
wood–polymer-interactions are preferably conducted in a real joint.
Next to standardized tests, nanoindentation (NI) has shown to be a
reliable technique that allows for investigating essential parameters relevant for
adhesive bonding. The usage of nanoindentation to determine the properties of wood cell
walls was introduced by Wimmer et al. (1997). Various studies on wood cell walls, adhesives and their
interactions at the interphase followed and contributed to a better understanding of the
joint performance (Amman et al. 2016; Zhang
et al. 2015; Jakes et al. 2008; Konnerth et al. 2006; Rindler et al. 2018; Obersriebnig et al. 2013). NI has also been used in high humidity environment as shown by
Jakes et al. (2015), but not yet applied to
water-stored glue lines of hardwood in combination with a primer.
However, indentation values reported for studies on wood cell walls have
to be interpreted carefully due to the three-dimensional stress state in combination
with the anisotropic nature of wood, as well as the importance of proper sample
preparation (Konnerth et al. 2009). NI is
capable of analyzing the properties of the individual components present in the
interphase region of wooden bonds (e.g., Zhang et al. 2015; Konnerth et al. 2006) as well as the performance of the local interface at the
micro-scale level (Obersriebnig et al. 2013). Studies in this field mainly addressed the interphase region of
wood-adhesive-bonds in dry conditions or the influence of moisture on polymer films
(Konnerth et al. 2010). Mechanical
properties of adhesives are typically available in dry conditions as summarized by
Stöckel et al. (2013). Literature using
different climatic conditions is less frequently accessed (Rindler et al. 2018; Stöckel et al. 2013). Wood properties and their dependence on moisture have been well
described at the macroscopic level (Niemz and Sonderegger 2017).
Little information is available on micromechanical properties including
the influence of moisture and the performance of the interface at the local level,
possibly due to a lack in available methodology. One possible approach to test interface
performance was proposed by Obersriebnig et al. (2013). Knowledge of moisture-dependent mechanical properties of single
constituents present in hardwood bonds could therefore help to better understand the
behavior of the joint and possible influence of a primer. Next to the
wood–adhesive-interactions, the influence of primer on the surface hydroxyl
accessibility is of high interest. The available hydroxyl groups are assumed to play a
crucial role in the physiochemical interactions in the wood bonding process (Frihart
2012). Dynamic vapor sorption analysis
has shown to be useful for the determination of accessible hydroxyl groups of wood with
deuterium (Sepall and Mason 1961; Thybring
et al. 2017) and could be useful to
describe the effect of primer application.
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In the present study, the aim was a better understanding of the mechanisms
contributing to moisture resistance of beech wood adhesive joints. Therefore, the
following was investigated:
Tensile shear strength and wood failure of beech wood bonds using
different 1C PUR adhesives, partly in combination with a primer, and a PRF
adhesive in dry, wet and re-dried conditions.
Influence of adhesion-promoting agent (primer) and lack of
extractives (hot water and hexane) on hydroxyl groups accessibility of beech
wood by dynamic vapor sorption (DVS) analysis.
Mechanical properties of individual regions (adhesive, wood
cells) of beech wood bonds by nanoindentation in dry, wet and re-dried
conditions.
Performance of the local interface between cell wall and adhesive
in dry, wet and re-dried conditions.
2 Experimental
2.1 Wood
European beech wood (Fagus sylvatica
L.) with an average density of 700 ± 34 kg/m3 from one lot
without any type of irregularities such as knots, heartwood or discoloration was
selected and cut to lamellas of 10 mm thickness. The lamellas were stored in standard
climate (20 °C/65% relative humidity) until a moisture content of approximately 12%
was reached. After conditioning, the material was planed with fresh knives down to 5
mm, cut to size and bonded according to EN 302-1 for single lap-joints within 30 min.
Wood intended for nanoindentation and dynamic vapor sorption (DVS) experiments was
used from one single board and from the same annual ring.
2.2 Adhesives and wetting promoting agent
Two commercial one-component polyurethane (1C PUR) adhesives were
tested in this study and compared with a commercial phenol resorcinol formaldehyde
(PRF) adhesive. The 1C PUR adhesives mainly differ by their reactivity (open time).
1C PUR B is recommended to use with a wetting promoting agent (primer) and the other
system can be used without primer when bonding alternative wood species, for example
beech or larch. The primer was used with 1C PUR B to create variant 1C PUR C. The use
of primer is further described in the literature (Amen-Chen and Gabriel 2015; Richter 1999).
PRF has proofed to reliably bond wood for structural and outdoor
applications (Dunky and Niemz 2002). The
selection of adhesives and some of their processing parameters are listed in Table
1.
Table 1
Selected properties of adhesives and their processing
parameters
Adhesive
1C PUR (A)
1C PUR (B)
PRF
Viscosity @ 25 °C (mPas)
20,000–30,000
24,000
400–1500
Open time (min)
< 60
70
120
Application (g/m2) one
side
160
160
450
Closed assembly time (min)
0
0
30
Pressure (MPa)
0.8
0.8
0.8
Press time (h)
10
10
10
2.3 Longitudinal tensile shear strength and wood failure on beech wood
The climatized and freshly planed lamellas were cleaned by compressed
air prior to bonding. For the variants using a primer, the liquid primer was diluted
in deionized water to a 10%-solution for variant (C). To ensure homogenous primer
distribution, the lamellas were transported with a conveyor belt and a constant feed
speed through a self-made spray application device. The defined amount of 20
g/m2 was afterwards controlled by a scale without
giving the solution time for evaporation. The used spread rate and concentration have
recently been determined to be ideal for hardwood bonding (Clerc et al. 2018). After adhesive application the bonded
lamellas were subsequently stacked into an apparatus to ensure precise pressure
distribution and pressed in a hydraulic press (Lindenberg, Altendorf, Switzerland)
for 10 h at 0.8 MPa at ambient temperature for all adhesive systems. After pressing,
the bonded lamellas were stored in standard climate for three weeks to ensure
complete curing and sample conditioning. Subsequent to specimen treatment described
in Table 2, lap joint specimens were tested
in tensile shear mode according to EN 302-1 using a universal testing machine (Zwick
30 KN, Ulm, Germany). Specimens were tested in load-controlled mode at 2 kN/min. For
each variant and treatment, 15 specimens were tested and compared with solid beech
wood references using the same specimen geometry, but without an adhesive bond line.
Wood failure percentage (WFP) was determined visually in 10%-steps.
Table 2
Treatment of tensile shear strength samples according to EN
302-1
Treatment
Definition
A1
Testing in standard climate 20 °C/65% relative
humidity
A2
4 days immersed in cold water (20 ± 5 °C), testing of
specimen in wet condition
A5
Boiling in hot water for 6 h, then 2 h cold water storage
(20 ± 5 °C), condition in standard climate until original mass is
reached, testing in dry conditions
2.4 Dynamic vapor sorption analysis
For the gravimetric determination of hydroxyl group accessibility, the
dynamic vapor sorption equipment (DVS-ET1, Surface Measurement Systems, London, UK)
was used. The samples were prepared from one beech wood panel within the same annual
ring. Approximately 10 g of early wood was separated with a razor blade and further
cut into very thin sections. Any chemical modification to the wood cell wall is
usually more pronounced in early wood than late wood. Therefore, early wood was
chosen for this experiment.
Samples were dipped into solutions with distilled water and primer in
the following concentrations 0.1%, 1% and 10% (based on weight). To determine the
influence of wood extractives on the hydroxyl group accessibility, further samples
were extracted (Automatic Solvent Extraction 200, Dionex, Reinach, Switzerland) with
hot-water (at 100 °C, 1 h) and hexane (at 60°, 1 h). For each variant, three
replicates were tested. Specimens were dried at 0% RH and 60 °C for 6 h while purging
with dry nitrogen gas to remove the wood’s bound water. A 1 h stabilization time at
25 °C (deuteration condition) followed. Afterwards, samples were conditioned with
D2O (Liquid D2O 99.9 atom% D, Sigma
Aldrich, Buchs, Switzerland) for 10 h at 95% RH. Specimen weight was determined
before and after conditioning and hydroxyl group accessibility was calculated from
the corresponding difference. The number of accessible OH groups was quantified
according to Väisänen et al. (2018) as
follows:
A is the accessible OH group content in dry mass of the sample (mol
g− 1). mi is the dry mass of the
sample before exposing it to D2O vapor (g).
mf is the dry mass of the sample after the
D2O exposure (g). MD is the molar
mass of deuterium (2.014 g mol− 1).
MH is the molar mass of hydrogen (1.008 g
mol− 1).
2.5 Nanoindentation
Samples for the nanoindentation experiments were obtained from one
beech wood lamella of 10 mm thickness. It was stored in standard climate (20 °C, 65%
RH) and small parts of around 25 × 25 mm were cut out with a chisel. Each time, two
counterparts were used to create an adhesively bonded assembly. Prior to bonding, a
fresh surface at its radial anatomical plane was created with the help of a rotary
microtome (Leica RM2155, Wetzlar, Germany) in order to keep cell wall damage at a
minimum level. The primer was applied to variant 1C PUR C with a spray bottle and its
application weight was controlled with a scale before bonding with 1C PUR.
Afterwards, the adhesive was applied with a spatula. For PRF, a closed assembly time
of 30 min was used. Small screw clamps were used to pressurize both counterparts for
12 h and stored without clamps in standard climate for three weeks to allow for
sufficient post-curing and conditioning of the sample. Samples for nanoindentation
were cut out with a razor blade with a size of 5 mm length, 2 mm thickness and 2 mm
width. The samples were then bonded by a two-component epoxy resin (UHU Plus
Sofortfest, Bolton, Switzerland) to a metal disc with 12 mm in diameter to fit into
the ultra-microtome sample holder. Further microtoming with diamond blades (Ultratrim
and Histo, Diatome, Nidau, Switzerland) using an ultra-microtome (Ultracut-R, Leica,
Vienna, Austria) ensured flat surface. To control surface quality and for
pre-selecting proper indentation points, incident light microscopy (Axioplan 2,
Zeiss, Jena, Germany) was used. To allow for testing multiple specimens
simultaneously, samples were bonded to flat metal plates. Three samples at a time
were surrounded by a polymer ring to enable storing the samples in water during the
later indentation experiments. The prepared samples were clamped magnetically onto
the indenter stage. All nanoindentation experiments were performed with a Hysitron
TriboIndenter (Hysitron Inc., Minneapolis, USA) equipped with an extremely sharp
cone-shaped tip with a total opening angle of 60°. The scanning probe microscopy mode
of the indenter was used to control the precise positioning (Fig. 1) of the indents. As recommended by Obersriebnig et
al. (2013), indents were performed in a
displacement-controlled mode with a maximum indentation depth of 850 nm. Load was
applied in a three-segment load ramp with a load increase for 3 s, peak load holding
for 20 and 3 s of unloading.
×
Measurements were taken to analyze the individual components present in
an interphase of a bond, namely the bulk adhesive, the wood cell wall (S2) and at the
direct interface between the adhesive and the wood cell wall lumen (S3), as well as
between the adhesive and the wood cell wall (S2) (Fig. 1).
For each climate condition and adhesive variant, eight wood cells were
tested with eight indents. For bulk material properties, the obtained results were
reduced E-modulus (Er) and hardness (H), evaluated according
to the method by Oliver and Phaar (1992). The specific work of indentation (Wd)
spent during each indent at the interface between adhesive and wood cell wall was
determined by integrating the total area under the load-displacement curve as
proposed by Obersriebnig et al. (2012).
Statistical analysis of the results of nanoindentation and lap-joints
on beech wood was conducted with a single factor variance analysis (ANOVA, 5%
confidence interval) with a post hoc least significant difference to allow comparison
between the mean values of each adhesive variant.
3 Results and discussion
3.1 Tensile shear strength and wood failure on beech wood
The results of tensile shear strength and wood failure percentage (WFP)
are shown in Fig. 2. After conditioning in
standard climate, all adhesive systems were able to meet the standard requirement of
10 MPa according to EN 302-1. PRF even surpasses solid wood in tensile shear strength
and had the highest wood failure percentage (90%) of all tested adhesives. While the
PRF joints performed significantly higher than the 1C PUR adhesives joints, no
significant difference was observed within the 1C PUR variants. All 1C PUR adhesive
bonds were characterized by a similar wood failure percentage in standard climate
conditions of around 30–40%.
×
The storage in water and subsequently testing in wet state (A2) showed
a considerable reduction in tensile shear strength and wood failure percentage for
lap-joints, including the solid wood reference, in comparison to the performance in
dry climate (A1). However, the PRF maintained its high WFP. Characteristic for all 1C
PUR variants was the absence of the wood failure for all cases. While the 1C PUR C
with primer application demonstrated a similar tensile shear strength to PRF, the
other PUR adhesive assemblies performed below 50% of the solid wood value reference.
1C PUR B was significantly lower in tensile shear strength than 1C PUR A. Despite no
significant difference in strength, the type of failure between 1C PUR C and PRF
differed considerably. The application of a primer led to a significant increase in
tensile shear strength compared to the same adhesive applied without any adhesion
promoter. Considering the standard requirements for A2 conditions, 1C PUR A and B
were not able to reach the 6 MPa threshold value.
For treatment A5, specimens were re-conditioned to their original mass
in standard climate, after boiling in water and cold-water storage. All adhesives
joints were able to reach similar or even better values compared to their standard
climate reference, which was in accordance with another study (Kläusler et al.
2014a, b). Yet, significant differences appeared between all variants. The
variant with applied primer (1C PUR C) obtained a significantly higher tensile shear
strength than the variant 1C PUR B without primer and nearly the same as the variant
1C PUR A without primer, but this time with higher WFP than for the PRF.
As a main result from the macroscopic test it can be concluded that the
investigated two commercial 1C PUR systems meet standard requirements when tested in
dry ambient, while they lack in performance when tested in wet conditions. The
well-established PRF adhesive joints were able to meet all standard requirements. The
application of the primer (1C PUR C) sufficiently improved the bonding performance
for surpassing the standard requirements in wet conditions (A2). However, the primer
application did not increase the wood failure percentage for treatment A2.
3.2 Gravimetrically determined hydroxyl group accessibility to
D2O vapor
The results of the DVS experiments are depicted in Fig. 3. The reference obtained an average value of around
7.5 mmol/g accessible hydroxyl groups for early wood of beech.
×
No difference was observed between the reference and the samples
immersed in 0.1% primer solution. The samples treated with 1% primer concentration
obtained higher scattering and two out of three values with a lower amount of
accessible hydroxyl groups, but no statistically significant trend could be
determined. The variant exposed to 10% primer concentration showed a substantial
decrease in hydroxyl group accessibility down to approximately two thirds of the
reference accessible hydroxyl groups.
It is proposed that the primer may deposit on the wood polymer hydroxyl
groups and could therefore block the access to deuteration in high concentrations of
10%. However, this concentration exceeds industrial primer application. Own studies
with samples that have been sprayed with industrial application devices using a
common spread rate and concentration showed a similar trend for samples but a
considerable smaller influence on the hydroxyl accessibility (results not
shown).
The extraction treatments applied to the beech wood did not show a
difference in hydroxyl group accessibility. The hot water extraction aimed to mainly
dissolve polar components such as tannins, organic salts and carbohydrates.
Extraction with hexane focused on dissolving of mainly non-polar extractives such as
fats, waxes and phenols (Sixta 2006).
The hypothesis of the study was that some extractives can reduce the hydroxyl group
accessibility by creating a surficial chemical weak boundary layer of water-soluble
extractives, and when the extractives were dissolved, the amount of accessible
hydroxyl groups would be expected to increase. While the hot-water extraction showed
consistently high hydroxyl group accessibility, a slightly higher scattering was
observed for hexane-extracted wood, while no significant difference to the reference
could be detected.
3.3 Nanoindentation
Optical focusing, proper positioning and subsequent tip approach with
the nanoindentation device was not possible with the samples being covered in water.
Therefore, after full sample immersion in water for 48 h, the water level was lowered
below the sample surface 120 min prior to the first measurement for the condition
“wet storage”. The ongoing shrinking of the swollen sample required permanent focus
adjustment for each measurement. The results of the nanoindentation (NI) experiments
on bulk materials (adhesive, wood cell walls) are summarized in Fig. 4. In general, the PRF adhesive revealed an approximate
three times higher reduced (red.) E-modulus and an approximate four times higher
hardness than the polyurethane adhesives in dry conditions, which is in accordance
with other studies (Amman et al. 2013;
Stoeckel et al. 2013) performed on
similar substrates. Comparing all 1C PUR variants in room climate, no significant
difference in mechanical properties was found. During storage in water, the red.
E-modulus and hardness of all adhesive systems dropped considerably. Noticeable is
the high relative change of PRF in comparison to all 1C PUR variants in terms of red.
E-modulus and hardness. The polyurethane adhesives were reduced in red. E-modulus to
around 70% and PRF to 10% of its initial values at room climate. The hardness of
polyurethane adhesives was reduced to 30–40% and that of PRF to 30% of its original
value.
×
No significant influence on bulk adhesive properties was observed when
a primer was used. In general, the investigations on the bond line and the results
for the bulk adhesive were comparable with earlier studies on moisture influence
tested on cured adhesive polymer films (Konnerth et al. 2010). While the differences in dry and wet
conditions for red. E-modulus and hardness were quite similar, the 1C PUR C had
significantly lower values for red. E-modulus and hardness in re-dried conditions for
the bulk adhesive as well as the cell wall. After testing in wet conditions, the
samples were dried for two days in room climate. All bulk adhesives were able to
re-gain a considerable part of their original mechanical properties in room climate.
Indentations in the wood cell showed that the red. E-modulus at room climate was
somewhat higher for the wood cells next to PRF compared to wood cell walls in contact
with the group of 1C PUR. However, this effect is superimposed by a high degree of
scattering resulting from the natural variability of the wood substrate. Higher
mechanical values for cell walls in contact with in-situ polymerizing adhesives
(Frihart 2012), such as the PRF used, can
generally be expected by the penetration of low-molecular weight substances from the
liquid PRF into the wood cell prior to curing. As a consequence, stiffening of the
wood cell walls is frequently observed (e.g.,Gindl et al. 2004; Konnerth et al. 2006). However, this effect was not visible for
the hardness of wood cells near PRF measured in other studies (Obersriebnig et al.
2013). Between the 1C PUR variants,
wood cells of 1C PUR A and 1C PUR C did not show any significant difference, while 1C
PUR B was significantly lower. Due to the incapability of penetrating the cell walls,
this difference may have its origin rather in the variability of the wood structure
than by the influence of the adhesive.
After water storage, variant 1C PUR A showed significantly higher red.
E-modulus and hardness compared to the remaining variants, which did not show
significant differences, including the PRF variant. After drying for two days, the
red. E-modulus differed significantly for all variants. However, for hardness, only
1C PUR C showed a significantly lower hardness. As moisture is a main bias for
mechanical properties of polymers, differences in drying rate of the individual
adhesives and assemblies may be assumed. It remains unclear whether the mechanical
properties are able to re-gain their original values for the case of longer storage
times. Macroscopic properties observed using the lap-joints described above might be
an indicator that properties lost during wet storage may be recuperated.
The results of the specific work of indentation at the direct interface
between adhesive and wood, separated into the different contact regions between
adhesive and the wood cell wall S2 and S3, are depicted in Fig. 5.
×
The specific work of indentation presented in Fig. 5 consists mainly of the work consumed for deforming
the wood cell wall and the adhesive. Only a comparably small amount of around 10–20%
can be attributed to real adhesion as expected by Obersriebnig et al. (2012). However, a visible crack was observed by
the authors, which exceeded the size of the indentation tip towards a partial
delamination of both surfaces.
Due to methodological restrictions, differences in adhesion can only be
observed when mechanical properties of the individual constituents are comparable. As
a consequence, only differences in specific work of indentation between different
contact regions (S2 vs. S3) of one adhesive assembly and condition state may be
interpreted as adhesion differences. In contrast, the considerable differences
between the specific work of indentation of different adhesives, as visible for PRF
and the 1C PUR versions, may not be interpreted as differences in adhesion, but have
their origin mainly from differences in mechanical properties of the constituent
phases (cell wall, adhesive) in their corresponding state.
Considering these restrictions for interpreting the specific work of
indentation, PRF was found to adhere similar to S2 and S3 cell wall areas in dry
state, as well as after water storage. Only after re-drying, the adhesion towards S3
cell wall areas may be considered to be lower. For 1C PUR A, specific work of
indentation at the interfaces between adhesive and both cell wall areas S2 and S3 was
found to be similar in all three conditions. Since the similar 1C PUR adhesive was
used to produce variant B and the primered variant C, a careful comparison can be
drawn. 1C PUR B and the primered variant of the same adhesive 1C PUR C showed higher
specific work of indentation at the adhesive/S3 interface in room climate, while no
difference could be found for the other climatic conditions.
Before performing the present work, higher adhesion between PUR
adhesive and cell wall areas was hypothesized when using a primer, especially in wet
state as a high amount of adhesion failure (lack of wood failure) is frequently
observed for PUR-wood bonds (Fig. 2, A2
condition). Comparing the specific work of indentation at the interface of 1C PUR B
and the primered variant of the same adhesive 1C PUR C, in wet state an insignificant
specific work of indentation is visible for 1C PUR C. In re-dry state, specific work
of indentation of 1C PUR C is significantly below the value of 1C PUR B. As
considerable differences in bulk mechanical properties of the two PUR assemblies are
evident for the same conditions, deriving information about adhesion differences is
not possible.
3.4 Overall discussion
Lap-shear joints of beech wood demonstrated once again (Konnerth et al.
2016; Clerc et al. 2018) that the used PRF adhesive is capable of
meeting standard requirements for all conditions. However, a formaldehyde free and
colorless alternative for safely bonding hardwood is frequently desired.
The used 1C PUR systems showed good performance in dry and re-dried
conditions, but a primer was needed to surpass standards requirements in wet
conditions on beech wood. Despite improving strength, the application of the primer
did not lead to an improvement in wood failure percentage. Contrary, another study
(Lüdtke et al. 2015) showed that WFP can
be increased by using a primer. This difference may be explained by possible
differences in the primer application, or by the small processing window of the
primer (Clerc et al. 2018).
The investigated hydroxyl group accessibility of European beech wood
was in accordance with findings by Tarmian et al. (2017). They further showed only minor differences between the
hydroxyl group accessibility of European beech, spruce and pine wood. In contrast,
Teleman et al. (2002) found a lower
amount of accessible hydroxyl groups for hardwoods, for example beech wood, due to
the beech’s lower amount of hydroxyl groups of its hemicelluloses. However, this
possible disadvantage of beech wood may only be a subordinated factor to explain the
more challenging bonding of hardwood in comparison to softwood. Far more important
for hardwood bonding could be their higher density as well as higher swelling and
shrinking coefficients (Niemz and Sonderegger 2017). These properties result in higher stresses in the bond
region as a result of changing moisture conditions.
The removal of polar and nonpolar extractives did not show any
considerable influence on the hydroxyl group accessibility of beech wood. Its low
extractive content of around 2% based on the dry wood mass (Jiang et al. 2014) could be the reason for not revealing a
possible influence. Hence, this approach could have more impact on wood species with
considerably higher extractive contents such as larch or pine.
The primer is expected to enhance wetting of 1C PUR on hardwood as well
as to increase the adhesion. With the methods used, an improvement of the adhesion at
the interface due to primer application could not be found. As the mechanical
properties of the bulk wood cell wall remained at a lower level after the two days of
re-drying, a possible influence of the primer on the water absorption and/or release
rate in the interphase area could be assumed. Studies by Väisänen et al.
(2018) demonstrated that there is a
connection between the equilibrium moisture content and the accessibility of hydroxyl
groups, while another study revealed only poor correlation (Rautkari et al.
2013). In this regard, the deep
penetration of the primer in the wood cell walls, as shown by Casdorff et al.
(2018), might be favorable. However,
this finding could not be validated in the present study, as the amount of primer
necessary to decrease the hydroxyl accessibility noticeably exceeds industrial
application rates by far. Therefore, a reduction in hydroxyl group accessibility
under optimal industrial primer application cannot be proven.
As a novelty, the nanoindentation experiments revealed that
water-stored wood-adhesive composites can also be tested in wet conditions. However,
analyzing the specific work of indentation did not show any influence of the applied
primer on the adhesion in different climate conditions. The proportionally higher
reduction in red. E-modulus and hardness of PRF can be attributed to a softening of
the polymers as a result of water uptake. Wimmer et al. (2013) revealed that PRF adhesive can take up to
18% moisture, while 1C PUR only gained 3.5%, which was explained by the process of
polycondensation and the production of methylol phenol derivates. The involved
hydroxyl groups may take up two water molecules (Bentz and Neville 1949). Furthermore, the hydromechanical performance
of PRF was considered to be similar to wood (Musznyski et al. 2002). In combination with cell wall impregnation
and the reduction in local swelling and shrinking in the interphase, PRF is capable
of creating a moisture-resistant composite with high mechanical strength even in wet
conditions.
Kläusler et al. (2013)
showed that the tensile strength of 1C PUR polymer films was reduced by 19–30% when
ambient moisture was increased from standard climate to a relative humidity of 95%.
In addition, its E-modulus was reduced in these conditions between 31–56%. In
comparison, PRF did not show a decline in tensile strength with increasing moisture
content, but the E-modulus was significantly reduced to 50%. For polyurethane, it can
be expected that water uptake is also leading to structural changes such as free
volume variations, relaxation effects and changes in visco-elastic behavior (Smith et
al. 2004).
4 Conclusion
The novel and challenging approach to characterize wood-adhesive
interfaces of bonds in wet conditions by nanoindentation extends possible applications
for nanoindentation and was expected to provide new insights into the mechanisms how a
primer is affecting PUR bonds. In contrast to the authors' assumptions, no measurable
effect of the primer on the local adhesion between adhesive and cell wall by specific
work of indentation could be observed.
While macroscopic mechanical performance of PRF adhesive bonds are on a
high level, the storage in water showed a dramatic reduction in mechanical properties of
the PRF adhesive itself. This effect was much less pronounced for the polyurethane
adhesives. No direct influence of the primer on the local micro-mechanical properties of
the bonding line could be found in dry and wet conditions. Only after re-drying, the
mechanical properties of the wood cell walls pre-treated with the primer remained longer
on a lower level, while other adhesives re-gained their original values already. It was
further shown that the primer application can reduce the hydroxyl accessibility of beech
wood, when applying high spread rates. The mechanism of the primer responsible for
improving 1C PUR adhesive bonds is still not fully understood and requires further
research.
Acknowledgements
The authors thank the Swiss Innovation agency Innosuisse for the financial
support.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states there is no conflict of
interest.
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