Acetoxy curing

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. 

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. 

Injection molded liquid silicone rubber. Silastic LSR 9280M0. B = One-part RTV acetoxy cure, Dow Corning 732. C = One-part RTV alcohol cure, Dow Corning 737. D = One-part RTV oxime cure, Dow Coming 739. 

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... 

Some have an acetoxy cure mechanism, releasing acetic acid during cure which may cause corrosion problems. Others are described as neutral curing. Most are cured at RT for several days (RTV - Room Temperature Vulcanisation), although higher temperatures may be used to reduce cure times. 

Silicone - One part, acetoxy cure. Silicone - One part Neutral cure. ... 

Acetoxy cure (releases acetic acid). 100% solids. ... 

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. 

Fig. 8. Basic chemistry of acetoxy-based RTV sihcones. The reactions for curing methoxy-based RTV sihcones are the same in that case, the methoxy group (OCH ) replaces acetoxy (OOCCH ), and methanol (CH OH), rather than acetic acid (CH COOH), is formed.

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 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. 

These are silicones in liquid or paste form that can be transformed into cross-linked elastomers at or near room temperature. There are two basic types, one component and two component. The one-component room temperature vulcanizing (RTV) elastomers cure upon exposure to air. A typical material of this type is synthesized from a dihydroxy-terminated siloxane oligomer and methyltriacetoxysilane (Scheme 2). The resulting tetraacetoxy compound will flow as a paste and so can be molded into shape. Hydrolysis of the acetoxy groups takes place over a period of hours. Condensation of the resulting silanols gives a highly cross-linked end product. 

RTV sUoxanes with alkoxy groups in place of acetoxy groups are now being used, and offer the advantage that no acid is formed during the cure. The hydrolysis of alkoxy groups is slower, but can be catalyzed with metal salts. 

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 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). 

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 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). 

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. 

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