Characterization of Other Technological Properties by DMA
In addition to mechanical material exploration, the DMA technique also can be employed in further performance-related investigations. For instance, an investigation of the rheology of the wet (uncured) sealant can be utilized in characterizing the sealant’s behavior during installation or handling processes.
The comparative illustration in Fig. 13 shows a higher zero-shear viscosity for Sealant A (pasty, bitumen-like) compared to Sealant B (syrup-like) indicating an easier handling with lower shear loading for the latter. To achieve flow, a certain yield limit has to be exceeded for Sealant A, which simultaneously can be an indicator for the deformation resistance (stability) during processing. To extrude Sealant A from a conventional cartridge requires a force of at least 62 N. Shear loading above a shear rate of 6 x 100, corresponding to a rotational speed above 5 rpm for Sealant A, and a shear rate of 4 x 100 corresponding to a rotational speed above 3 rpm for Sealant B, respectively (see Fig. 13), comprises the danger of flow interruption or air infiltration.
An exploration of the technological material characteristics is not complete without an evaluation of the curing processes, especially the exploration of curing under special environmental conditions. Figure 14 highlights the potential to explore such curing behavior by DMA material analysis for Sealants A and B under exploitation of the minimized mechanical loading options by our modern test equipment. At climatic conditions of approximately + 30°C and 40% RH, the reactive Sealant B (two part) attained after 24 h asymptotically an equilibrium stiffness state but exhibited via a secondary stiffness increase a
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10 2-10 3-10 4 10 5-10 6-10 7-Ю 8-10 1/3 10
Dynamic viscosity Sealant В Comp. A 2
■j Shear stress
Additional test results
yield limit 62 N max. bearable t < 2100 Pa rotational speed < 5 rpm for homogenization recommended steady flow
max. bearable т < 1000 Pa rotational speed < 3 rpm for
FIG. 13—Rheology of Sealants A and B (after mixing of parts A and B) at critical processing temperature (+10° C).
consecutive reaction (cross-linking), which was even not finished after 48 h at the end of the test. Apart from the outer boundary area of the plate-plate-test geometry, Sealant A (one part) shows, even after more than 14 days, no completion of the cross-linking, although test geometry mirrors the geometrical conditions of a real sealant joint geometry.
According to the DMA tests, also the thermal curing of Sealant C was not completely finished within the recommended curing duration. Considerable secondary stiffness increase indicates consecutive cross-linking (not shown as figure here). In summary, the time-sweep mode used in this part of the investigation could prove its suitability for evaluating curing kinetics of SSG sealants and provide additional opportunities for further investigation, even under superimposed mechanical loading if so desired.