Acetoxy cure system

Some of the compounding ingredients such as the catalyst may be an irritant or toxic. Also, by-products evolved during the curing of the elastomer may be an irritant (e.g. acetic acid is evolved in the acetoxy-cured system). 

Both the heat-cured and RTV silicones are formed by condensation polymerization and are sometimes referred to as alkoxy- or acetoxy-cured systems. The basic hydroxysilane starting materials are produced by the hydrolysis of mixtures of mono-, di-, and trichlorosilanes as depicted in Figure 3.7. " Condensation with elimination of water produces hydroxyl-terminated alkylpolysiloxanes. These resins can be further polymerized in the presence of alkoxysilanes such as propylorthosilicates to yield the RTV silicones (Figure 3.8). Catalysts such as organotitanates or dibutyltindilaurate may accelerate the curing process. 

Condensation-cured silicones. Both the heat-cured and RTV silicones are formed by condensation polymerization and are sometimes referred to as alkoxy- or acetoxy-cured systems. The basic hydroxysilane... 

RTV Silicone Chemistry. There are two basic cure chemistries used by RTV sihcones the acetoxy-based and the alkoxy-based cure systems. Acetoxy-based RTV sihcones were first commercialized in the early 1960s (422,423). The general chemical reactions of these first-generation products are shown in Figure 8. 

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. 

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... 

Cure mechanism acetoxy, neutral, neutral catalytic (moisture cured systems) ... 

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. 

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. 

The relationship between structure and photoinitiation activity has been examined for polymeric systems bearing side-chain 1-substituted cyclohexyl-phenyl ketone moieties in the UV curing of the HDDA/BA equimolar mixture [19,20]. Indeed, the activity of poly[(l-acryloxycyclohexyl)phenyl ketone] [poly (APK)] and styrene/4-chloromethyl-styrene/l-(4-styrylmethyloxy)cyclohexyl phenyl ketone copolymers (PABOK) has been compared with that of the corresponding low-molecular-weight structural models such as 1-hydroxy-cyclohexyl phenyl ketone (HPK), 1-acetoxy-cyclohexyl phenyl ketone (ACPK) and l-(4-isopropyl-benzyloxy) cyclohexyl phenyl ketone (PIBOK). 

In acetoxy systems acetic acid is produced during cure. This type of system is not ideal for use on alkaline surfaces. However, it has excellent adhesion to glass and glazed ceramics, making it ideal for use in all types of glazing and sanitary applications. Acetic system types should not be used on sensitive surfaces (e.g. concrete, zinc, lead, copper, brass, iron and some coated glasses). 

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