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Silicone sealant (caulk)

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Misc comments from a kind of boring article I wrote a couple of years ago. 
Chemically Curing Sealants
Chemically curing materials can be either single component or multi-component.  Single component sealants cure by reaction with moisture and oxygen in the atmosphere.  Their depth must be restricted to approximately 1/2 in. because of this dependency on moisture and oxygen.  Multi-component sealants cure by chemical reaction with a curing agent or catalyst which allows for placement depths greater than 1/2 in. Multi-component materials tend to have better movement capabilities than single component materials.  They are also recommended for joints that will be covered with a membrane or coating soon after the sealant is installed.  Once single component materials are covered and don't have access to atmospheric oxygen and moisture, their curing stops.
The base polymer of chemically-curing elastomers is usually polysulfide, polyurethane, or silicone.  Polysulfides were initially the most commonly used sealants but they are being replaced by lower-cost polyurethanes and silicones that have equal or better performance characteristics, including greater movement capabilities and better resistance to UV and ozone.
Polyurethanes are the most commonly used sealants for building joints - they adhere to most construction materials without a primer and are considered "user-friendly" by installers.  Single component polyurethanes are typically used for non-moving column-slab and wall-slab "caulk" joints.  Multicomponent polyurethanes are generally used for expansion joints, particularly those subject to vehicle traffic, because they have good hardness, good tear resistance, excellent recovery and a fairly fast cure.
Almost all sealants, including polysulfides and polyurethanes, are organics which means they are based on carbon-hydrogen chains.  Silicones, however, are non-organics composed of silicon-oxygen chains.   As a result, silicones have somewhat different properties than other sealants.  Silicone sealants typically have higher movement capabilities and deteriorate less with age than other sealants.   They also have good high- and low-temperature resistance and low shrinkage. Many do not have good abrasion resistance, though, and are not recommended for traffic joints unless they can be well protected. In addition, silicones are not suitable for applications where they will be continuously immersed in liquids. The curing agent for some multi-component silicone sealants is an acid - these sealants are not recommended for joint substrates such as marble, copper, portland cement based materials, or galvanized steel.
Gail Kelley
note:  you can usually tell acid-cured sealants - they smell like vinegar