Page 633 - Handbook of Thermal Analysis of Construction Materials

600 Chapter 14 - Sealants and Adhesives

DMTA results can only be interpreted as the behavior of the total joint
rather than the adhesive glueline alone.
Dynamic mechanical thermal analysis (DMTA) has also been used
by Brinson, et al., [35] to determine the suitability of the technique for
evaluating damage in the adhesive bond from the viscoelastic properties of
bonded beams and for evaluating the effects of various environmental
conditions and various surface treatments. The authors considered that if
the bond becomes damaged (either adhesive and/or interphase) due to
excessive load, fatigue, moisture, or corrosion, it would seem likely that
dissipation mechanisms or loss modulus and tan δ would change. There-
fore, they used DMTA to measure the viscoelastic properties of beams with
simulated flaws and beams taken from lap specimens, which had been
exposed to humidity and/or corrosion for extended periods.
The DMTA data for the simulated flaw obtained at the two
frequencies used (1.0 and 10 Hz) showed a substantial change in storage and
loss moduli (E´ and E´´) with the inclusion of simulated flaws from no flaw,
0%, to a flow the length of the beam, 100%. Also, the glass transition as
given by E´´ appears to decrease with increasing simulated flaw length. The
authors also reported that the significant changes were observed with
increasing exposure to humidity. Specimens exposed for over a month had
higher storage moduli, E´, than some of those exposed for shorter periods
of time. Furthermore, exposure to corrosive conditions showed to be more
complicated than for humidity. However, initial changes in the dynamic
mechanical properties are essentially the same as in humidity exposure.
From the results of the work, it was concluded that it does appear
that DMTA studies on bonded beams may allow the determination of
progressive damage due to fatigue, moisture, corrosion, or, perhaps, to
other environmental parameters. But it may be necessary to build special
DMTA equipment more sensitive to small changes in damping behavior.
Differential scanning calorimetry (DSC), DMA and TG were used
by Tabaddor and co-workers [36] to investigate the cure kinetics and the
development of mechanical properties of a commercial thermoplastic/
thermoset adhesive, which is part of a reinforced tape system for industrial
applications. From the results, the authors concluded that thermal studies
indicate that the adhesive was composed of a thermoplastic elastomeric
copolymer of acrylonitrile and butadiene phase and a phenolic thermo-
setting resin phase. From the DSC phase transition studies, they were able
to determine the composition of the blend. The kinetics of conversion of
the thermosetting can be monitored by TG. Dynamic mechanical analy-
sis measurements and time-temperature superposition can be utilized to