A high performance acrylic sealant and a high performance polyurethane sealant, both conforming to the ASTM C920-05 Class 25 specification, were chosen for this evaluation.
The polyurethane sealant was specified by the moisture-proofing contactor as part of a commercial restoration project. A two part polyurethane was chosen due to the low humidity in El Paso and the extended cure times required for one part polyurethanes under these conditions. The specific product selected was not the contractor’s first choice-product selection was dictated by availability at the local distributor. However, the product selected is commonly available and widely used. The contractor has had extensive experience with this product
TABLE 1—High performance acrylic sealant formulation.
aAcrylic latex from The Dow Chemical Company, Midland, MI 48674.
and has found that it crazes more than other commercially available polyurethanes upon weathering. However, in his experience, this crazing has not led to complaints or call backs.
A laboratory prepared acrylic sealant, based on a commercially available binder, was chosen for comparison. A laboratory prepared sealant (Table 1) was used instead of a commercially available sealant so that the authors could control the formulation and understand the relationship between formulation ingredients and exterior performance. A plasticizer free formulation was chosen to minimize sealant dirt pickup. Since contractor application preferences were unknown at the time that the sealant was formulated, no attempt was made to optimize the viscosity, toolability, or open time of the formulated material that was sent to El Paso.