Influence of the Binder
The influence of the binder (polymer) chemistry is illustrated for a modified silicone construction sealant. In Table 1, an unfilled sealant model formula based on a modified silicone binder (polypropylene oxide backbone) is compared with two sealants that use an acrylic modified silicone binder.
In the formula that is based on an acrylic modified silicone binder, the cracking and chalking resistance could be significantly improved over the conventional modified silicone prepolymer (polypropylene oxide backbone). No cracks were observed even in thin films. When a 50/50 mixture of modified silicone and acrylic modified silicone was used, improved weathering resistance was still observed; cracks and chalking were detected only after extended carbon arc weathering (approximately 2000 h).
Next a range of formulas with increasing amount of acrylic modified silicone was prepared to study the influence of the acrylic modified silicone polymer in more detail. Figure 1 illustrates how the crack formation changes with the acrylic modified silicone content after various exposure times in the carbon arc
eHeat stability: change of Shore A hardness after 2 months at 80°C relative to the value before heat exposure; 1: 0-5 2: 5-33 %; 3: 34-67 % 4: 67-95 %; 5; 95-100
weathermeter. No cracking was observed for an acrylic modified silicone content of 67 % or higher, even after approximately 3000 h of accelerated weathering. Crack formation was significantly reduced even when lower amounts of acrylic modified silicone binder were used: No cracking was observed in case of 50 % acrylic binder and 1350 h of exposure, at 2950 h crack formation was reduced by half. Only when 33 % or less of acrylic modified silicone binder was used, were cracks detected already in the early stages of weathering (800 h). However, already an addition of acrylic modified silicone binder in an amount as low as 17 % resulted in an improvement of the cracking performance compared to the reference sample.
This example illustrates that the binder chemistry is one of the main factors that determine the weathering performance of a sealant.