Page 632 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Applications 599

3.2 Adhesives

Dynamic mechanical analysis (DMA) has been used to study the
flow behavior of hot-melt adhesives. [31] Brummer and co-workers [32] used
DMA to study the viscoelastic behavior of adhesives. They found that
dynamic mechanical measurements in adhesives provided insight in the
macromolecular mobility of the polymer or rubber system studied. The
viscoelastic behavior at various temperatures can be correlated with stan-
dard measurements such as adhesive force, shear strength, and tack. The
authors concluded that three-dimensional DMA plots from frequency-
temperature sweeps provide a complete overview of the frequency and
temperature dependence of the adhesive. Foster, et al., [33] characterized the
hot-tack differences in hot-melt adhesives using DMTA.
Although thermoanalytical techniques have been widely used in
the characterization of different types of adhesives, little work has been
published on the use of these to characterize construction adhesives.
Onic and co-workers [34] reported the results of their work on the use
of DMTA to investigate the viscoelastic behavior of wood joints bonded
with cross-linking thermoset adhesives. The resins studied were phenol
formaldehyde (PF), melamine-urea-formaldehyde (MUF), resorcinol-form-
aldehyde (RF), tannin-formadehyde (TF), and tannin-hexamethylenetetra-
mine (TH). The curing of the adhesives was followed by observing the
changes in the storage modulus E´ and tan δ. The authors reported that the
value of the joint E´ increases as the adhesives pass from the liquid to the
rubbery state and then finally to the glassy state. From this, three distinct
zones were observed.
The first zone was observed at very low values of E´ where the
adhesive behaves as a liquid (T ). The second zone starts at E´
gel min
characterized by an increase in the E´ value up to E´ max •E´ min (the elastic
modulus of the joint occurring at the adhesive gel temperature (T ). E´
gel max
occurs at T where the hardened adhesive network is likely to become tighter
f
(vitrification). The third zone was identified as that where the value of the E´
decreases because of physical and chemical changes in the properties (e.g.,
degradation/softening) of the materials in the joint.
Onic, et al., [34] considered that the decrease in modulus of the
bonded joint could not just be attributed to degradation of the adhesive
because all of the adhesives used, with exception of MUF resin, were
thermally stable in the temperature range used. It was rather due to the wood
substrate and delamination of the joint. Therefore, they concluded that the