Varying Mixing Ratios

Compared to one-component adhesives, two-component adhesives provide additional complexity in their application, which is also expected to have an impact on durability. Although the mixing ratio is clearly defined by the adhe­sive manufacturers as one key parameter in application, the robustness of the adhesive with respect to the mixing ratio deviations is of interest for confidence in, and reliability of, bonded structures. In this section, experimental results are presented with a focus on pre-defined mixing ratio variations of a two – component silicone adhesive which has been widely used for structural sealant glazing for decades. The nominal mixing ratio by volume is defined as 10:1 [2] while the intended variation of this ratio spans from 7:1 up to 11:1, as shown in Table 1.

In order to judge the fitness of the adhesive in a first step, tensile tests and shear tests were performed and post-processed with respect to strength and stiff­ness properties. All tests were performed at room temperature and at displace­ment rates of 50 mm/min for the tensile tests and 5 mm/min for the shear tests; Fig. 1 shows the load curves obtained by tensile tests of all tested dog-bone type specimens. The nominal size of the specimens was a 25 mm gauge length and a 6 mm x 4 mm cross section area. In order to provide more insight, Fig. 2 is reduced to averaged curves for each mixing ratio. Slight differences in stiffness are visible with the mixing ratio of 7:1 featuring the highest stiffness, and with the mixing ratio of 11:1 featuring the lowest stiffness. A similar trend is also visi­ble for shear tests performed by quadruple ETAG H-type specimens; see Figs. 3 and 4. The nominal dimensions of the adhesive applied in the test specimens are

specimens which are loaded in parallel). The conclusion of our findings is that the mixing ratio cannot be considered a primary parameter in terms of its influ­ence on the elastic behavior of the two-component silicone adhesive.

Regarding strength characteristics, Fig. 5 presents the maximum loads for the tensile tests, while Fig. 6 displays the load levels for the shear tests. Interest­ingly, the shear test results show a superior mechanical behavior for the nomi­nal mixing ratio, while the tensile tests do not. In general, the variations of the strength levels is quite small demonstrating a high robustness of the silicone ad­hesive with respect to mixing ratio errors from a mechanical property point of view for the investigated test conditions. Nevertheless, for the final conclusions, the database of ten tensile tests and four shear test specimens is considered to be quite small.

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—- 7:1

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— 8.5:1

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11:1

Tensile

Strain [%] —

50

100

150

200

FIG. 2—Averaged tensile test results for varying mixing ratios.

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250

FIG. 3—Shear test results for varying mixing ratios.

These results might be interpreted as an indication that the durability of the investigated two-component silicone adhesive is probably quite insensitive to small variations in the mixing ratio in view of its mechanical properties. Please note that this statement is related to mechanical aspects only; the change of chemical charac­teristics due to varying mixing ratios are not considered within these investigations.