Dynamic Mechanical Analysis

The adhesives were also subjected to a thermal analysis. The viscoelastic mate­rial properties of the adhesives were determined by means of a dynamic mechanical analysis (DMA). The test involves subjecting the specimen to a sine-wave-type loading over a given temperature range. The specimen likewise exhibits a sine-wave-type response with the same period and a deformation.

FIG. 5—Stress—strain diagram for polyurethanes PUR1 and PUR2 at various temperatures.

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FIG. 6—Stress-strain diagram for acrylates AC3 and AC4 at various temperatures.

The amplitudes of the force and the deformation, as well as the phase shift between force and deformation, are recorded during the test.

These tests involved subjecting each specimen to tensile load (maximum load 7 N) with an excitation frequency of 1 Hz, using a DMA 242 C (Netzsch, Germany) in tension mode. Rectangular bar specimens were used for the study. The material specimens were heated from -60 to + 120°C at a rate of 3 K/min; the evaluation took place in the range between -25 and + 100°C. This involved ascertaining the viscoelastic properties of the material by way of the storage modulus (E’), the loss modulus (£"), and the dissipation factor (tand). The rela­tionship between these properties is given by d = E"IE’. The larger the storage modulus (E’), the greater is the amount of induced mechanical energy that can be recovered from the specimen. The energy irreversibly converted into heat is known as the loss modulus (E”).

The results of the DMA for the adhesives shortlisted are shown in Figs. 7-9. In these diagrams, the storage modulus (E’) is shown as a solid line, the

FIG. 7—DMA graph for epoxy resins EP1 and EP 4 (glass transition temperature Tg is marked).

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FIG. 8—DMA graph for acrylates PUR1 and PUR2 (glass transition temperature Tg is marked).

dissipation factor (tand) as a dotted line. Every adhesive was tested by means of at least two specimens.

The epoxy resins tested (EP1 and EP4) exhibit high storage moduli at low temperatures. In the service temperature range, e. g., room temperature, the adhesives exhibit a very definite glass transition temperature zone in which the storage modulus drops to a very low value within a narrow temperature range. Afterward, the storage modulus remains approximately constant at a low level. Polyurethanes PUR1 and PUR 2 are quite similar in their behavior. In contrast, the Acrylates AC3 and AC4 also have a high storage modulus at a low tempera­ture, but it decreases continuously as the temperature continues to rise. The graph of the dissipation factor forms a plateau with an extended range of the glass transition temperature. The peak is not very distinctive. Furthermore, in the temperature range above + 40° C these adhesives have a higher storage mod­ulus than the epoxy resins and polyurethans tested.

FIG. 9—DMA graph for acrylates AC3 and AC4 (glass transition temperature Tg is marked).

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The results with respect to the glass transition temperature of each adhesive obtained from the DMA are summarized in Table 2. The glass transition temper­ature (Tg), as the mean value of the softening range, is evaluated according to ASTM D 4065 [19] for the peak of the dissipation factor (tand). This reveals that the glass transition temperatures for the acrylates tested have higher values than the epoxy resins and the polyurethanes and are more suitable for the intended application.

Results and Selection of Adhesive

On the basis of the behavior of individual adhesives established over the temper­ature range considered, acrylic adhesive AC4 was selected for the intended ad­hesive joints at the glass frame corners. In contrast to the epoxy resins and the polyurethanes, this adhesive exhibits a less pronounced temperature-dependent behavior and a higher glass transition temperature. Compared with acrylate ad­hesive AC3, acrylate AC4 is characterized by better strength at low temperatures and the ease with which it can be injected into the gap.