Elastomeric Silicone Adhesives
The majority of silicone adhesives and sealants use polydimethylsiloxane (PDMS) as the base polymer, which affords these adhesives their special properties, such as excellent ultraviolet (UV) light and ozone resistance, low-temperature resistance, flexibility at temperatures down to —40°C, high-temperature resistance (retention of elasticity up to 100° C), and long-term durability under adverse service conditions.
In most cases, the hydroxy-terminated PDMS (see Fig. 1) is used as polymer base in most silicone adhesives and sealants. Table 1 provides a typical formulation for an elastomeric silicone adhesive.
Condensation cure one-part and two-part room-temperature vulcanizing (RTV) systems are typically formulated from hydroxy-terminated polymers with molecular weights ranging from 15,000 to 150,000. One-part systems are the most widely used in practical applications. These systems are cross-linked with moisture-sensitive multi-functional silanes in a two-stage reaction. In the first stage, after compounding with fillers, the silanol is reacted with an excess of multi-functional silane . The silanol is in essence displaced by the silane. This is depicted in Fig. 2 for an acetoxy-cross-linked system.
FIG. 1—Hydroxy-terminated polydimethyl siloxane (PDMS) polymer .
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Hydroxy-terminated silanol polymer (PDMS): MW 15,000 to 150,000 60-85
Plasticizer: trimethylsilyl-terminated PDMS 10-20
Fumed silica (treated and/or untreated) 5-10
Tin catalyst (e. g., dibutyl tin dilaurate) 0.05-0.1
Adhesion promoter (e. g., aminopropyltriethoxy silane (y-APS) 0.25-2
As seen from Fig. 2, after the first-stage reaction, the silicone has two groups at each end that are susceptible to hydrolysis. The silicone adhesive in the form presented in Fig. 2 is stored and protected from moisture until ready for use. The second stage of the reaction takes place immediately after installation, as the end groups are exposed to moisture and a rapid cross-linking reaction occurs.
The cross-linker system is typically a combination of a reactive tri – or tetra – functional silane and a condensation catalyst. Examples of typical functional groups in cross-linkers are shown in Table 2.
The schematics of the cross-linking reactions of the most commonly used cross-linkers in 1-component moisture-curing silicone adhesives are outlined in Fig. 3 .
The cross-linking reaction is catalyzed by titanates, frequently in combination with tin compounds and other organo-metallic compounds. One-part ace – toxy-cross-linked systems usually rely solely on tin catalysts, e. g., dibutyltin diacetate, dibutyltin dioctanoate, or dibutyltin dilaurate. The tin contents in one-part RTV systems is above 50 ppm with a ratio of 2500:1 for Si-OR to Sn. Typical formulations, however, have up to ten times the above minimum .
The moisture-curing one-part silicone systems gradually polymerize as atmospheric humidity diffuses into the adhesive. In this process, water reacts with the prepolymer molecules, forming cross-linked macro-molecules [2-4].