Page 622 - Handbook of Thermal Analysis of Construction Materials

590 Chapter 14 - Sealants and Adhesives

Dynamic mechanical analysis (DMA) can be considered one of the
most suitable analytical techniques to study the viscoelastic properties of
polymeric materials such as adhesives and sealants. According to Rogers, et
al., [21] it is more meaningful to correlate the performance test with the sealant
viscoelastic properties to explain their behavior. In a proposed test protocol
for selecting concrete pavement joint sealants, the authors recommended
DMA as a technique to initially evaluate the curing of sealants and to study
the effect of mechanical fatigue by measuring, in both cases, the glass
transition. In another study [22] on polyurethane sealants for the same
application, the authors concluded that the viscoelastic properties of the
sealants, as determined by DMA, were able to explain sealant behavior
observed in adhesion-in-peel and shear fatigue tests and that the correlation
was conclusive for a variety of test conditions (e.g., high and low tempera-
ture, thermal cycling, and water and chemical exposure).
Malik [23] used dynamic mechanical analysis to study the effect of
accelerated weathering on the storage and loss moduli of commercially
available polyurethane (PU) and silicone construction sealants. Three
multicomponent PU and one-component silicone sealants were used in the
study. The sealants were exposed to 8 hours of UV exposure at 65°C
alternating with 4 hours of condensation at 50°C for 600 and 1000 hours.
The moduli G´, G´´, tan δ, and the dynamic viscosity (η) were measured in
the temperature range of 25° to 125°C. The temperature was increased to
125°C, held for 2 hours and cooled to 25°C.
The study showed that silicone sealants exhibit very high dynamic
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moduli (10 dyn/cm ) compared to PU (10 dyn/cm ). Moreover, silicone
is stiffer than PU sealants below -30°C (Fig. 5a–b). Malik reported that
silicone and PU sealants behave similarly only above this temperature and
that the modulus of the silicone sealant falls in the acceptable window.
According to the author, the storage modulus for a good construction
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sealant should not exceed 10 dyn/cm at -40°C. Therefore, he indicated
that silicone sealants may not be suitable for temperatures below -30°C
because they are too rigid.
It was reported that a comparison of the DMA data for two of the
polyurethane samples (PU-A and PU-C) showed remarkable differences in
the viscoelastic parameters of the two samples. The curves for G´, G´´, tan
δ and η for PU-C (Fig. 5c) show almost a plateau whereas the same curves
for PU-A display a transition between 50° and 70°C.
From his work, Malik concluded that contrary to the conventional
view by sealant manufacturers and end use, silicone sealants are a lot stiffer
than most of the PU sealants at temperatures below -35°C. Also, some PU