Properties of the High-Strength Structural Silicone Film Adhesive (TSSA)
Cure Chemistry and Behavior
The structural silicone film adhesive is a one-part material that features a heat – activated addition-cure mechanism, as shown in Fig. 1.
The addition-cure mechanism causes no cure by-products (and no odor) to be evolved during the reaction. The film adhesive is cured at temperatures of 120-130°C for a period of 20-30 min between the substrates while applying a pressure, typically 0.15-1.3 MPa, in order to provide efficient wet-out on the substrate. The optimum cure conditions are achieved in an autoclave, such as those used in the production of laminated glass. Alternative production methods, such as prepressing (prior to heat cure) or vacuum-bagging (during heat cure) are currently being investigated.
The onset of cure occurs rather rapidly; however, the heat exposure time of 20-30 min is needed to achieve a uniform activation temperature and complete, homogeneous cure within the glass/glass or glass/metal assembly. The cure behavior of the film adhesive was characterized in a rotorless cure meter (oscillating die rheometer) under isothermal conditions at a temperature of 130°C (no preheat) and constant maximum strain (arc 3°) and constant oscillation frequency (100 cycles/min) for a total running time of 30min . The rheometry trace is shown in Fig. 2. After approximately 5 min at 130°C, the torque rapidly increases within about 2 min by a factor of 8 and after a total cure time of about 7-8 min, the torque value reaches about 80% of its final value. The steep increase in torque is an indication that the chemical reactions associated with the cross linking of the silicone film adhesive have begun.
The rheometer gives two important pieces of information: First is the maximum torque (Tf), which is a measure of the final set rigidity. Second is a value called T90, the amount of time it takes for the torque to reach 90% of the maximum value. T90 is correlated to the material’s setting time. At 130°C, T90 is reached after approximately 15 min of cure and Tf after about 30 min.
RCH CH2+ H SiR’3 —► RCH CH2 SiR’3
FIG. 1—Silicone addition cure reaction yielding no cure by-products.
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FIG. 2—Rheometry trace for silicone film adhesive cured at 130°C.