Compatible With Silicone, Butyl, Polyurethane, and Polysulfide Sealants UV Stabilized CRL Thermoplastic Rubber (TPR) Setting Blocks are compatible with almost anything, meaning you can use them in designs calling for the use of modern structural glazing sealants. They are formed of a 90 durometer shore hardness thermoplastic rubber that performs as well as traditional neoprene setting blocks. They are available in the most popular sizes of neoprene blocks. They have excellent chemical resistance, and do not mar or stain surfaces. Job specific testing prior to start up is recommended with each silicone sealant manufacturer.