Acetoxy sealant

With these tools, it has been possible to develop a whole range of silicone sealants with cure kinetics designed for many different requirements (Fig. 11). The acetoxy sealants are fast curing, and thus well adapted to glazing applications. The oxime sealants are better suited for adhesion on plastics. For applications where a longer tooling time is required, such as expansion seals or perimeter joints, Alkoxy sealants have been developed with a longer skin-over time. 

Adhesion characteristics depend on the chemistry of the specific sealant and the substrate to be bonded. For example, acid-liberating (acetoxy) sealants bond quite well to glass and to many aluminums, but should not be used against porous cementatlous substances because, with time, a thin powdery film of... 

Acetoxy sealants are widely being used in a large number of applications in the construction industry. Where they are incompatible with specific substrates, they can be replaced by neutral cure systems. The alkoxy and oxime cure systems are available in either transparent (silica-filled composition only), or non-transparent/solid colour (silica-and/or chalk-filled composition containing appropriate pigment) versions. Their chemical neutrality enables these systems to offer a wide range of adhesive-bonding capabilities to... 

These materials are stable in storage for many months or years either as one-package or as two-package systems. In general, many two-package systems tend to have longer shelf age capabilities because the chemically reactive components can be separated until the time of use. However, some one-component products, such as the acetoxy sealants, also have excellent shelf stability and remain useful for up to three or four years after manufacture. 

The moisture-reactive cross-linkers used in sihcones are of the form R Si(OR )4 where n = 0 or 1 and R may be any organic group, such as methyl, ethyl, or vinyl. R also varies acetoxy, alkoxy, oxime, and propenoxy are among the most typical. Common cross-linkers are hsted in Table 1, and a typical formulation for a one-part sihcone sealant is given in Table 2. 

Condensation reactions are used for cross-linking at ambient temperatures. The acetoxy-functional condensation system is widely used in fluorosilicone sealants. The cross-linking occurs after exposure to atmospheric moisture.62 The limitation of this system is that it is effective for only relatively thin layers. Moreover, it often requires up to 14 days to cure and the acetic by-product may corrode certain substances. 

Another well-known type is the RTV ( room-temperature vulcanizing ) silicone sealants, in which adsorbed water causes hydrolysis of acetoxy groups on silicon atoms within the chemical structure, creating sites for cross-linking and the consequent hardening of the sealant to a rubbery state. 

Another example is the polymer formed with an acetoxy-terminated prepolymer and water as a curing agent. However, water is not directly incorporated but moisture is absorbed from air. The reaction is completed in 24 hr, and these polymers are used as adhesives and sealants. 

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. 

X2 Construction sealants. Silicone sealants have proven to be very useful in a number of applications related to concrete construction (Figure 6.5). Systems based on neutral or alcoholic cure are suitable. Acetoxy-cured sealants are not suitable because of the evolution of acetic acid by-product, which reacts with calcium-bearing surfaces such as concrete and mortar, resulting in uncertain adhesion (Spells and Klosowki, 1981). 

Mid-to-high-modulus acetoxy-cured sealants are the most common. In certain applications, neutral cure silicones are also employed. Whatever the curing system, a sanitary sealant should be equipped with a fungicide. The sanitary field includes bathrooms (Figure 6.7), kitchens, refrigerators or cold storage rooms. 

This correctly implies that all the acetoxy functionality present on the crosslinker will be released during the cure as acetic acid. The acetic acid corrodes copper and some other metals, and also reacts with concrete forming a layer of Ca(0Ac)2, which interferes with the stability of the bond between the sealant and the calclferous surface. Acetic acid also has a pungent odor which some find offensive. Thus, sealants containing this most common crosslinker are not desirable for all applications. 

Because reaction (b) is chemically neutral there is no corrosion hazard affecting electronic equipment. However, the most common family of one-component RTV elastomers employs the acetoxy reaction shown in (c) which liberates acetic acid with its characteristic vinegary smell when curing. This type of elastomer is widely used for general industrial and household sealants but should be avoided in favour of the neutral variety for most electronic applications because acetic acid residues can promote corrosion and undesired electrical conductivity. 

In the global market, the acetoxy, methoxy, and oxime curing systems are the commercially predominant curing chemistries for one-component sealants. The tetrafunctional ethylor-thosilicate is used in two-component formulations. 

One-Component Silicones. The liquid silicone prepolymers used in one-component sealants are usually methyl-substituted polysiloxanes having more than two blocked, hydrolyzable end groups. These groups may be acetoxy, ketoxime, alkylamino, or benzamido. When the composition is extruded into a joint... 

The acetic acid hy-product can be corrosive, particnlarly to copper terminals on printed circnit boards and other electrical items. For this reason other RTV silicone sealants have been developed with oxime- or alkoxy-based by-products. These can be slightly slower to cure than the acetoxy-based products but often the adhesion to plastics is considerably improved. 

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