Silicone sealants chemistry

The role played by the various ingredients in the composition of sealant, and in particular on the durability of adhesion has been discussed recently [77]. Inert plasticizers, such as trimethylsilyl-endblocked-PDMS, are typically added to silicone sealant compositions in order to adjust the rheology of the uncured sealant. They result in a reduction of the modulus and hardness of the cured sealant. Differences in the durability of silicone sealants are found to be due to differences in their cure chemistry, and more specifically to the nature and... 

An important end group in silicone chemistry is the acetoxy group the familiar silicone sealants release acetic acid during moisture cure of these acetoxy-stopped polymers. Acetoxysilanes hydrolyze more readily than alkoxy groups. Acylation of a chlorosilane can he accomplished by the addition of sodium acetate or by reaction with acetic anhydride. Other reactions that permit formation of oiganofunctional sflicones ate shown in Figure 3. 

The chemistry just outlined produces silicone sealants which cure to a fairly tough, resilient rubber. Generally, sealants made by the above route are the higher modulus, strong silicone sealants used in products like silicone glazing sealants, silicone adhesives and silicone bath tub caulk. 

Thus far we have addressed many of the initial properties of silicone sealants and the chemistry that leads to those properties. Many of the long term properties of silicone sealants can be summed up in the general term, weatherabllity. 

All the properties mentioned above are the result of the intrinsic qualities of the silicone or silane components of the sealant and the chemistry of the formulation. These intrinsic qualities cannot be duplicated in organic sealants with or without silicon-containing additives. The chemistry of silicone sealants is, to a large degree, a balance between the chemistry of sand or quartz and the chemistry of hydrocarbon side groups. This unique balance of chemistries has resulted in a well-deserved "top-of-the-line" reputation for silicone sealants. 

Chapter 4 Chemistry of Silicone Sealants and Adhesives, by Dr Andreas Wolf, a scientist at Dow Coming. This is a very important and comprehensive chapter (180 pages), which explains, with many exclusive technical and scientific information, the chemistry of silicones, their technical characteristics, the formulation of silicone sealants, their applications in all industries, and mainly in constmction of course, including a very large bibliography. 

The chemistry of silicone sealants is much the same as that discussed above for silicone pressure-sensitive and mbber-based adhesives, with the exception of the MQ taddfier addition. The basic polymer for silicone sealants is that based upon poly (dimethyl siloxane)diol. This material can be cured by any one of a number of polyfunctional curing agents, all of which are based upon tetrachlorosilane. Thus, such materials as methyl triacetoxysilane or methyl trimethoxysilane can be used in conjunction with a poly(dimethyl siloxane)diol to yield a room... 

A final curing chemistry for silicone sealants is the reaction of a hydride functional silicone with a vinyl-terminated silicone fluid. A platinum catalyst is used for this reaction which is usually blocked by some form of complexing agent. Heat will liberate the platinum, causing the material to cure. This chemistry can be used in a two-part composition. 

The surface of the substrate, the silicone/substrate interface, and the bulk properties of silicones all play significant and influential roles that affect practical adhesion and performance of the silicone. The design of silicone adhesives, sealants, coatings, encapsulants or any products where adhesion property is needed requires the development chemist to have a thorough understanding of both silicone chemistry and adhesion phenomena. 

This chapter first reviews the general structures and properties of silicone polymers. It goes on to describe the crosslinking chemistry and the properties of the crosslinked networks. The promotion of both adhesive and cohesive strength is then discussed. The build up of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The final section of the chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. 

This chapter will deal with the chemistry and applications of epoxies, phenolics, urethanes, and a variety of current vogue high-temperature polymers. Applications in fiber-reinforced plastics will be discussed in the individual sections on resin chemistry where appropriate. Separate sections will deal with adhesives and sealants. Adhesives are most important because, as early history demonstrates, they led the way to the application of resins in aerospace. A section is also included on silicone and polysulfide sealants. Although these materials are elastomers rather than resins, no discussion of aerospace polymers would be complete without some mention. Some major thermosetting polymers have been omitted from this review. Among these are the unsaturated polyesters, melamines, ureas, and the vinyl esters. Although these products do find their way into aerospace applications, the uses are so small that a detailed discussion is not warranted. 

The Chemistry of Silicone Room Temperature Vulcanizing Sealants... 

Silicone adhesives and sealants were introduced approximately 40 years ago and many of the silicones used in the early days are still performing. Products are available in a variety of forms, from pastelike materials to flowable adhesives. Both single- and multicomponent versions are available, with several different cure chemistries. Most of the silicones of commerce are based on polydimethylsiloxane (PDMS) polymers. Other siloxane polymers may be used when resistance to ultrahigh temperature, ultralow temperature, or solvents is required. 

Usually, sealants and adhesive materials for construction applications are evaluated by looking at the engineering side, butnotthe chemistry of the material. As a result, only tests that measure the mechanical properties are used. Most of the studies on the viscoelastic properties use traditional tests such as tensile testing to obtain data, which can be used in complicated mathematical equations to obtain information on the viscoelastic properties of a material. For example, Tock and co-workers studied the viscoelastic properties of stmctural silicone rubber sealants. According to the author, the behavior of silicone mbber materials subjected to uniaxial stress fields carmotbe predicted by classical mechanical theory which is based on linear stress-strain relationship. Nor do theories based on ideal elastomers concepts work well when extensions exceed... 

These versatile polymers can be compounded with other chemicals and fillers into an enormous variety of products that serve in a multitude of applications. This chapter will describe the product types, chemistry, and applications of only one major class of silicone products, the adhesive sealants. In addition, the class of silicone adhesive products will be described and discussed these materials are employed where release characteristics are desired, rather than adhesion. 

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