Silyl terminated polyether sealants, known as MS sealants, have become the most used type of construction sealant in Japan since their introduction some 20 years ago. In the U.S., MS sealants have been primarily used for industrial application; construction use is still in the developmental stages. The MS sealants have high performance capabilities with many of the same characteristics as a urethane sealant. However, the one-part MS sealants cure much faster than a one-part urethane sealant and find use in the construction industry where property development speed is important.

The structure of an MS polymer consists of a polyether backbone and methyl-dimethoxysilyl terminal functionality. It does not have urethane, urea, or other functional groups that are typical in polyurethane sealants. MS polymer's polyether main chain provides low viscosity, low glass transition temperature, flexibility over a wide temperature range, and low odor.

The advantages of MS construction sealants are numerous. These specific benefits are described below. However, a significant benefit is the fact that the final products are isocyanate-free and solvent-free. MS sealants are based on a polymer, which is silyl-terminated, eliminating the isocyanate content to those involved in the manufacturing process.

The rise in MS sealant popularity has been due to its versatility and well-balanced properties. MS sealants are suitable for a wide range of applications, except for glazing. The initial adhesion of MS sealants onto glass is good, but its exposure to sunlight or UV over a long period of time generates bond strength deterioration. MS sealants, like polyurethane sealants, should not be used at temperatures greater than 85°C.

Table 1 compares the performance of silicone rubber, polyurethane, and MS sealants. Compared with the other two types of sealants, MS sealants have well balanced properties and performance. Some of the other unique properties of MS sealants are:

• Environmental friendliness (solvent free)
• Low temperature gunnability: the viscosity of MS sealants is less dependent on temperature changes
• Storage stability: shelf life is excellent although sealant must be protected from moisture
• Weather resistance and durability: MS sealant shows no cracking, splitting, discoloration or adhesion failure after seven years' testing in Arizona climate
• Stain resistance: MS sealants do not stain as some silicone sealants do because of low molecular weight silicon materials that bleed from the surface of sealed joints
• Paintability: MS sealants provide good paintability unlike silicone sealants
• Adhesion: MS sealants provide adhesion to various substrates including metals, plastics, wood, and ceramics.

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