Silicones moisture-curing

Silicones. Commercially avaHable sHicone sealants are typicaHy one of three curing types moisture-reactive (curing) sealants, moisture-releasing (latex) sealants, and addition-curing sealants. Of these three types, moisture-curing sHicones make up the vast majority of sHicone sealants sold. 

Moisture-Curing Silicones. The formulation of moisture-curing sHicones includes a sHicone polymer, filler, a moisture-reactive cross-linker, and sometimes a catalyst. The most common sHicone polymer used in sealant formulations is an alternating sHicon—oxygen backbone with methyl groups attached to the sHicon such as the sHicone polymer (1). 

Scheme 9. Crosslinking reaction of a two-part moisture cure silicone adhesive system.

A WBL can also be formed within the silicone phase but near the surface and caused by insufficiently crosslinked adhesive. This may result from an interference of the cure chemistry by species on the surface of substrate. An example where incompatibility between the substrate and the cure system can exist is the moisture cure condensation system. Acetic acid is released during the cure, and for substrates like concrete, the acid may form water-soluble salts at the interface. These salts create a weak boundary layer that will induce failure on exposure to rain. The CDT of polyolefins illustrates the direct effect of surface pretreatment and subsequent formation of a WBL by degradation of the polymer surface [72,73]. 

Silicon oxychloride, in fiber optic fabrication, 11 138 Silicon-oxygen (Si-O) backbone, in moisture-curing silicones, 22 32... 

Most moisture-curing silicones have good general adhesion to a variety of substrates. However, adhesion can be markedly improved with different combinations of silanes. The more common silane adhesion promoters are categorized as amine functional, eg, 3-aminopropyltrimethoxysilane [13822-56-5]y 3-aminopropyltriethoxysilane [919-30-2] y or IV-(2-aminoethyl)-3-aminopropyltrimethoxysilane [1760-24-3] as epoxy functional, eg, 3-glycidoxypropyltrimethoxysilane [2530-83-8] (2) and 2-3(3,4-epoxycyclohexyl)ethyltrimethoxysilane [3388-04-3] (3) as mercapto functional, eg, 3-mercaptopropyltrimethoxysilane [4420-74-0] or as methacrylate functional, eg, 3-methacryloxypropyltrimethoxysilane [2530-85-0] (4). 

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. 

As already mentioned, one-component silicone sealants cure by moisture pick-up from the atmosphere to form permanently elastic rubber. There are different types of curing systems for one-part silicone sealants (Table 6.5). However, the most commonly available systems are the acetoxy and neutral curing systems. 

In the rubber industry, moisture absorbed on the surface of silicate, impacts the rate and extent of cure and results in sponge-like textures. In moisture cured systems such as polyurethanes, polysulfides and silicones, moisture causes a premature reduction in shelf-life. In extrusion and injection molding the moisture absorbed on fillers contributes to various defects and a strict regime must be followed regarding the drying time and the conditions prior to processing. Lacing, a less well known phenomenon, is caused by the absorption of moisture on the surface of titanium dioxide. ... 

Moisture-curing adhesives are formulated with resins that react with atmospheric moisture to form a cured polymer. Most of these resins are one-part cyanoacrylates, silicones, or polyurethanes. The key advantage, of course, is their ability to cure without the use of heat or other external form of energy. However, in many cases, several days of ambient exposure are required to achieve full properties since the rate... 

Silicone adhesives were introduced commercially in 1944 [5, p. 213]. In 1960 the silicone sealants were introduced to the construction industry [11, p. 86]. Silicones are useful at both high and low temperatures and are available today as solvent-based moisture-curing adhesives, one-part moisture-curing adhesives and sealants, two-part curing adhesives and sealants, and pressure-sensitive adhesives. 

Silicones are available in one- and multicomponent forms. The one-component types are commercially the most important and will be the focus of most of this discussion. These products, which generally cure by reaction with atmospheric moisture, are called RTV (room-temperature vulcanizing) sealants or adhesives. The surface cure rate of these products is a function of the cure system, but the rate of cure in depth depends on the ability to transmit water vapor through the mass of sealant. Silicones are highly permeable to moisture vapor, and generally the one-component types cure at a rate of about 0.3 cm/day. Due to this high vapor permeability, the one-component silicones typically cure faster than do their nonsilicone counterparts. 

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