Another one-part silicone used for bonding, sealing, and coating is MasterSil 415, which features high transparency and is especially useful in optical and display applications. This easy-to-use paste can be applied to vertical surfaces without sagging or running, and it cures to a flexible, rubbery solid when exposed to moisture in the air. Setup time is 8-12 minutes at 75F. MasterSil 415 can be used over a temperature range of -185F to 400F and is useful where low-temperature reliability is of critical importance to the application. This formula also withstands vibration, shock, impact, exposure to moisture, and other weather conditions and is FDA-compliant.
Two-part specialty silicones
Two-part silicones, also called addition curing systems, employ a combination of a silicone polymer plus a catalyst to initiate the cure. Although these two-part systems are not quite as easy to use as one-part silicones, they offer several advantages compared to their simpler counterparts, especially in production environments. Because a catalyst is required to initiate the cure -- rather than ambient moisture -- two-part silicones have no problems with section thickness. Therefore, they are well suited for applications such as potting, encapsulation, and large castings. Another advantage is they release no reaction byproducts, so they can be used in closed environments. Because their cure can be accelerated with heat, some systems can be treated with a fast, partial heat cure to enable a quick setup of the adhesive, after which the work piece is allowed to finish curing at room temperature off the production line.
Due to their flexibility, two-part silicones feature stress-relieving capabilities over a wide temperature range and provide resistance to vibration, shock, corrosion, and heat. They offer strong adhesion to glass, plastics, metals, rubbers, and other substrates. Within the two-part silicone adhesives family, specific product formulations feature resistance to humidity and temperature extremes, superior flexibility, electrical insulation, and thermal and electrical conductivity. Certain two-part systems are approved for use in specific industries, such as those certified to meet USP Class VI for biocompatibility.
Despite these advantages, it's important to note that certain substances and substrates may inhibit the curing of these two-part systems. Users must be aware of materials in the application and cure area that might cause this inhibition. For example, it is very important to avoid contamination from other silicones that are cured using metallic salts, chlorinated rubber compounds, PVC plasticizers, amines, sulfur-containing materials, and butyl elastomers.
Robert Michaels is VP Technical Sales for Master Bond Inc.