kinetic response of thermosetting adhesive systems to heat

KINETIC RESPONSE OF THERMOSETTING ADHESIVE SYSTEMS TO
HEAT: PHYSICO-CHEMICAL VERSUS MECHANICAL RESPONSES
Christian Heinemann 1 , Ralph Lehnen 2 and Philip E. Humphrey 3
1 Christian Heinemann, University of Hamburg, Center of Wood Science and Technology,
Germany;
heinemann@holz.uni-hamburg.de
2 Ralph Lehnen , Federal Research Centre for Forestry and Forest Products, Institute for Wood
Chemistry and Chemical Technology of Wood, Hamburg, Germany;
r.lehnen@holz.uni-hamburg.de
3 Philip E. Humphrey, Oregon State University, Department of Wood Science and Engineering,
Corvallis, Oregon 97331, USA;
philip.humphrey@orst.edu
Keywords: adhesives, strength development, cure, DSC, ABES, bonding kinetics, testing,
wood-based composites
ABSTRACT
The kinetic response of four thermosetting adhesive systems (UF and UF-m, each with
two molar ratios) to heat has been explored. In preliminary studies, gelation time and Differential
Scanning Calorimetry (DSC) measurements were made and activation energy values derived for
the latter using Kissinger’s method. The results are compared with isothermal bond strength
development data collected using the ABES (Automated Bonding Evaluation System) approach.
In this, miniature bonds are formed under highly controlled conditions at a variety of preselected
isothermal temperatures and pressing times and immediately thereafter destructively
tested in shear mode. Reactivity indices have been determined by analyzing regressed isothermal
bond strength development rates from the ABES tests and these values are compared with
gelation and DSC values. In addition, the ABES technique has been modified to enable the
adhesion kinetics of miniature resinated fiber discs to be explored in shear mode, and
preliminary results are presented.
INTRODUCTION
Our present knowledge about the mechanical transformation processes of thermosetting
adhesives as they cure and how they affect the formation and final properties of wood-based
composites is incomplete. Before composite panels may leave the hot presses used in their
manufacture, the spatial distribution of inter-fiber bond strength values must exceed certain
critical profiles; this is in order to avoid immediate catastrophic failure or the creation of
products with deficient performance attributes. The attainment of such inter-fiber bond strength
profiles depends on complex spatial interactions among thermodynamic and micro-mechanical
mechanisms during pressing. The magnitude of localised stresses transferred to adhesive bonds
when presses open depends upon the residual viscoelastic properties of the hygro-thermally
softened micro-structure in concert with internal gas pressure gradients principally due to
compressed air and water vapor (Thoemen and Humphrey, 2001).

Over the past two decades or so various studies have been conducted with a view to
inferring adhesive cure during composite hot pressing processes. Methods of physico-chemical
analysis (such as DSC, DTA and DMA) have been used to explore the responsiveness of
adhesive systems to heat. Derived activation energy and tan-delta values (δ) have been used to
gauge the reactivity of adhesives at different stages of cure (Mizumachi, 1973; Geimer et al.,
1990). Similarly, mechanical tests have been successfully used to explore isothermal bond
strength accumulation by forming and testing miniature lap-shear bonds under a range of highly
controlled temperature and time combinations (Humphrey and Ren, 1989; Humphrey and
Zavala, 1989; Humphrey, 1991; Wang et al., 1995; Humphrey, 1999; Heinemann 2002).
Differential Scanning Calorimetry (DSC) and two mechanical means of exploring the kinetics of
thermosetting adhesive cure (based on the ABES approach), and how the results pertain to biocomposite
production, are presented and compared here.
EXPERIMENTAL METHODS AND RESULTS
Specimen preparation and properties
Four commercially available urea formaldehyde (UF) and melamine-fortified UF (UF-m)
adhesive systems with differing molar ratios of urea to formaldehyde (F:U) and melamine
content (Table 1) were tested. After the addition of 1% hardener (Ammonium nitrate - NH4NO3),
the solids content was adjusted to 60% by adding distilled water. Differences in viscosity among
the four formulations were minor.
Table 1 – Properties of thermosetting adhesives used as test
specimens in this study.
adhesive
system
molar ratio
F:U
melamine
content
gelation
time [s]
UF#2 High - 68
UF#3 Low - 65
UFm#4 High High 145
UFm#5 Low Low 97
Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry was employed as one means of characterising the
responsiveness of the adhesives to heat. Adhesive sample heating-rates between 1.0 and
4.0°K/min were selected in order to maximize detection of the exothermic reaction peaks, rather
than using rates that correspond to any specific heating cycle that occurs in panels during
pressing (which are almost always faster, non-uniform, and vary with position).
A typical set of curves for the tested adhesives is presented as Figure 1. A distinct single
peak is evident for the UF samples (adhesive systems UF#2 and UF#3), whereas the melaminemodified
specimens (systems UF-m#4 and UF-m#5) show a slight shoulder before reaching their
maxima. Data for the UF-m samples suggest that higher temperatures are necessary to reach the
exothermic peak, and this indicates a slower polymerization reaction that is probably due to an
increased buffer capacity of the melamine. Additionally, a reduced peak of the exothermic
reaction is evident, based on the properties of the aromatic ring structure of the melamine.

Exothermic Heat [0,5 W/g]
UF#2
UF#3
UFm#4
UFm#5
30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190
Temperature [°C]
Lab: METTLER
Figure 1 - DSC curves obtained with a 2°K/min heating rate for samples weighing 10 mg.
The relationship between temperature and the rate at which a first-order reaction proceeds
and temperature is commonly described using the Arrhenius Equation (Eqn. 1):
ln k = ln A – EA/RT (1)
where k is the rate coefficient (kJ/mol), A is the pre-exponential factor, R is the universal
gas constant (8,314*10 -3 kJ*mol -1 *K -1 ) and T is the absolute temperature (°K).
In the present DSC experiments, activation energy values have been extracted by
analyzing temperature peaks measured for runs conducted using three different heating rates of
1°K/min, 2°K/min and 4°K/min following the method of Kissinger (Kissinger 1957).
Automated Bonding Evaluation System (ABES)
The ABES technique may be used to characterize the mechanical response (bond strength
in this case) of adhesive systems to a wide range of parameters; the influence of temperature was
of concern in the present investigation. The test arrangement is shown conceptually as Figure 2
and the bond forming and testing zone of the ABES instrument as Figure 3.
Figure 2 - A schematic representation of the ABES approach.

Figure 3, A close-up of the bond forming and testing zone of ABES (the cooling head is retracted and
was not used in the reported research)
Adhesive was metered onto the end-most 5 mm of pairs of Maple veneer strips measuring
0.7 mm thick, 20 mm wide and 117 mm in length (conditioned to 8 % moisture content).
Adherend pairs where mounted in ABES with an overlapping area of 100 mm² and pressed
together at 1.2 N/mm². Bonds were pressed at temperatures of 95°C, 105°C, 115°C and 125°C.
The wood was especially thin in order to hasten the transfer of heat from the precisely
temperature-controlled pressing blocks to the bonding interface. In this way, near-isothermal
adhesive conditions (±1°C) were attained within 12 seconds following press closure. Measured
bond strength accumulation for all pressing times in excess of 12 seconds could therefore be
attributed to the target temperature. After each pre-selected pressing time, bond strength was
tested almost instantaneously in shear mode (ABES is digitally controlled and pneumatically
driven). All bonds were therefore tested while still at the forming temperature. The effect of
testing temperature on the strength of variously cured bonds (where the cooling head is activated
immediately following press-opening and before bond pulling) is the subject of a separate study
(Kim and Humphrey, 2002; Kim, 2002). Further details of the ABES testing approach, including
issues of shear stress-correction, heat transfer, cooling, and bond analysis may be found in a
publication (Humphrey, 1999).
Temperature effects on bond strength development: A typical family of isothermal bond
strength development curves is shown in Figure 4a (for formulation #3.) It is convenient here to
sub-divide the curves into three portions. During the first section no significant bond strength
occurs (although tack may be quantified when the high-sensitivity mode of ABES is activated.)
Once bonding begins, bond strengths rise at a near-constant rate, before, in the third stage, the
increase of bond strength decreases and the curves level off until they reach their maximum
values. The initial delay in the onset of bond strength development may be caused by a short
period of increasing temperature combined with a loss of energy due to evaporation of water
within the glue line. The adhesive may also still be in the fully liquid state (and even sustain a
temporary decrease in viscosity upon initial temperature rise.) The second and near-linear stage
is likely directly attributable to the polymerization reaction of the adhesive systems by chainextension
and cross-linking processes.

It is the regressed gradient of this dominant portion of the data that will be used here to derive
reactivity indices and activation energy values for the adhesive systems. The decreased-rate stage
may be due both to a slowing of the polymerization reaction of the adhesive combined with the
onset of cohesive failure of the adherend on a micro-scale. The adherends begin to fail by fiber
pull-out and therefore ultimate strength values of the fiber-adhesive interface itself may be
masked. Taking the slope of the linear section of each bond strength curve from Figure 4a
(excluding the initial delay and the leveling-off section at advanced pressing times) the regressed
isothermal bond strength rate can be plotted versus temperature (Figure 4b).
Shear Strength [N/mm²]
5,0
4,0
3,0
2,0
1,0
125°C
115°C
105°C
95°C
0,0
0 60 120 180
Pressing Time [s]
240 300 360
0
90 100 110
Pressing Temperature [°C]
120 130
a) b)
Figure 4. a) A representative family of bond strength development curves for four temperature levels (for
adhesive encoded UF#3), and b) Regressed isothermal bond strength development rate (for the middle
linear stage) versus temperature for all four adhesive systems.
Isothermal Shear Strength
Development [kPa/s]
240
180
120
60
UF #2 UF #3 UFm #4 UFm #5
Repetition of the above analysis for all four adhesive systems enables differences in their
responsiveness to temperature to be explored. These differences are evident in Fig 4b. The higher
formaldehyde content appears to correspond with faster bonding rates, while increasing amounts
of melamine reduces the rates. For each adhesive system a linear correlation can be derived by
plotting the natural logarithm of the regressed isothermal bond strength development rate against
the reciprocal of absolute temperature (Figure 5).
ln k [kPa/s]
5,5
5,0
4,5
4,0
3,5
3,0
UF #2 UF #3 UFm #4 UFm #5
2,5
2,50 2,55 2,60 2,65 2,70 2,75
1 / absolute Temperatur [°K*10 -3 ]
Figure 5. ABES-derived Arrhenius plots (natural logarithm of regressed bond strength
rate vs. the reciprocal of absolute temperature) for each of the four adhesive
systems.

The slope of each regressed line reflects the adhesives' reactivity; the steeper the slope the
higher the adhesive’s reactivity. Humphrey and Ren (1989) coined the expression 'reactivity
index' (Ri) to describe the bonding kinetics of adhesive systems:
Ri = -T * ln A (2)
where T is the absolute Temperature (°K) and A is the rate of bond strength development
(kPa*s -1 ).
Linearity of the plot suggests that the bond strength development rate can be described by
a first order chemical reaction. This analytical method is usually applied to chemical reactions
but might be transferred to evaluate mechanical behaviour of thermosetting adhesives. In order to
obtain activation energy values for the tested adhesive systems the reactivity index can be
multiplied by the universal gas constant (R). Derived reactivity indices and activation energy
values are presented in Table 2.
Table 2. Reactivity index and activation energy values of for the four
systems investigated.
Adhesive System Reactivity Index
Activation Energy
[kJ/mol]
ABES ABES DSC
UF#2 8,70 72,3 95,5
UF#3 7,68 63,9 85,0
UFm#4 8,19 68.1 92,8
UFm#5 7,41 61,6 84,0
The activation energies of the tested adhesive systems from Table 2 determined with
ABES and DSC might be compared by comparing relative ratios. Knowing that the adhesive
system UF#2 shows the highest activation energy values, these values are set to equal a ratio of
'1' for ABES and DSC separately. In Figure 6 the ratios of adhesive systems UF#3, UFm#4 and
UFm#5 show good conformity when related to UF#2. Overall, activation energy values from the
DSC method show slightly lower differences than do those determined with ABES. However,
the determination of key ratios with DSC and ABES might be a promising tool to compare
kinetic response of different adhesive systems to heat. Indeed, the results suggest that the ABES
approach may prove more sensitive to small differences in adhesive formulation than DSC.
Further, these limited results suggest that gelation time may be an unreliable predictor of
subsequent bonding performance (after gelation).

Activation Energy Index
1,00
0,95
0,90
0,85
ABES
DSC
Gelation time
(2,09) (1,46)
UF#2 UF#3 UFm#4
UFm#5
Adhesive System
Figure 6. Ratios of activation energy of the adhesive systems derived from DSC, ABES
and gelation time methods on a basis of reactivities for adhesive #2.
Composites testing system (ComTes)
Design and testing procedure:- ABES has been modified in order to enable the combined
effects of adhesive and fiber network characteristics on bonding of wood-based composite
materials to be explored. Resinated (but uncured) miniature fiber discs (17.5 mm in diameter and
9-mm in thickness) can be pressed under controlled conditions of temperature and tested in shear
mode perpendicular to the forming direction. For this purpose, after forming, two samples are
pressed and tested simultaneously. Aspects of the system are shown as Figure. 7. Two circular
temperature-controlled pressing heads (�) act uniaxially on the outer surfaces of the samples,
while and one angular block (�) in the center with inserted cartridge heaters are used to transfer
heat and load onto the samples surfaces. To impede passive spreading of the samples, the fiber
discs are peripherally constrained by PTFE (Teflon) cylinders (�) that are pneumatically driven
back prior to testing. Small aluminum discs are glued simultaneously onto the discs’ surfaces
with cyanoacrylate adhesive to transfer shear forces from the shear bar (�) into the samples.
Usually, the samples fail in the core layer of the 3 mm thick samples because of the slightly
delayed temperature increase in this plane.
1
1
2
fiber discs
Figure 7 - Overview (left) and detail (right) of the Composites Testing System.
4
4
3

Temperature effects on internal mat characteristics during testing
In order to resolve the role of the adhesive in affecting the shear strength of compressed
fiber samples, a number of fiber samples were compressed in the absence of adhesive, and the
results compared with corresponding samples pressed with adhesive. Clearly, the interplay of
mechanisms which influence the structure and properties of compressed fiber networks is highly
complex; it is beyond the scope of the present discussion and will be the subject of future
publications. Such mechanisms are, however, assuredly influenced by fiber type and
juxtaposition, transient moisture and temperature conditions, and the magnitude and duration of
applied compressive stress.
Figure 8 shows representative shear resistance values of non-resinated miniature fiber
discs. Pressing temperature appears to affect measured values. This is no doubt attributable to the
interplay of mechanisms listed above. Shear resistance does appear to decrease with increasing
pressing temperature, while almost no influence can be detected with increasing pressing time.
Under pressing conditions of low temperature and high specimen density, a decreasing trend of
shear resistance is, however, evident; this might be attributable to thermal softening of cell wall
material.
Shear Strength [N/mm²]
1,2
0,8
0,4
0,0
125°C; 600kg/m³
125°C; 500kg/m³
100°C; 400kg/m³
125°C; 400kg/m³
150°C; 400kg/m³
0 20 40 60
Pressing Time [s]
80 100 120
Figure 8. Shear Strength of non-resinated miniature fiber discs pressed at different
temperatures and densities.
Temperature Effects On Shear Strength Development Of Resinated Samples
The shear strength development of resinated miniature fiber discs pressed at 100°C,
125°C and 150°C are presented in Figure 9. These initial results show a strong correlation
between measured shear strength and pressing time (hollow symbols on Fig. 9). Solid symbols in
Fig. 9 are data generated by offsetting the non-resinated shear strength values from the
corresponding resinated values. As one may expect, inferred adhesive shear strength values
increase with increasing pressing temperature and pressing time.

Shear Strength [N/mm²]
2,0
1,5
1,0
0,5
150°C 125°C 100°C
0,0
0 50 100 150 200 250
Pressing Time [s]
Figure 9 - Shear strength development of resinated fiber discs at 400 kg/m³ (hollow dots
are measured data while filled ones are the inferred adhesive contribution to shear
strength)
CONCLUSIONS
Two methods, one physico-chemical (DSC) and one mechanical (ABES), have been
presented to characterize adhesive systems (UF and UF-m) with different molar ratios and
different melamine content. The kinetic response of thermosetting adhesives to heat has been
described in terms of reactivity indices and activation energy values (following Kissinger’s
method for the former). With the ABES approach, thermosetting adhesives have been cured at
different temperatures (95°C-125°C) and subsequently destructively tested in shear mode.
Veneer strips are used as wooden adherends. The reactivity index of the adhesive systems have
been determined by plotting the natural logarithm of regressed isothermal bonding rates against
the reciprocal of absolute temperature. Taking activation energy indices of all tested adhesives a
satisfying conformity can be seen when comparing ABES and DSC.
In the second part of the study, a method has been summarized which may be used to
determine bond strength development of fibrous composite material. Miniature MDF-like fiber
discs have been formed and subsequently tested in shear mode. Initial results are encouraging:
resinated wood fibrous material pressed under controlled conditions of temperature, load and
density have been presented. Internal mat characteristics have to be addressed when analyzing
absolute strength values. Compared to ABES, which is a reliable method to determine strength of
partially cured bonds, the modified ABES approach makes it possible to explore the micromechanical
behavior of resinated wood fiber mats.

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