kinetic response of thermosetting adhesive systems to heat

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).