Silicone adhesives: condensation cure

The two-part sihcone adhesive/sealants do not require moisture to cure and produce a superior deep-section cure. Two types are available addition-cure and condensation-cure. Addition of curing produces no by-products, can be heat-accelerated, produces negligible shrinkage, and provides the best high-temperature resistance of all silicone adhesives. Condensation-cure silicones are not easily inhibited and can be used on a greater variety of materials. ... 

Further discussion can be found in Silicone adhesives condensation cure. 

Information on particular types of adhesive may be found in articles cited above and in the following entries Hot melt adhesives. Selection of adhesives. Silicone adhesives addition cure. Silicone adhesives condensation cure. Toughened adhesives. 

Related articles are Silicone adhesives condensation cure and Silicone adhesives addition cure. 

Part of the wide variety in Silicone structures results from different curing mechanisms. Silicone adhesives may cure by addition or condensation reactions the former is considered here, the latter in Silicone adhesives condensation cure. 

Silicone adhesives condensation cure B PARBHOO Cross-linking by step reactions... 

Silicone adhesives addition cure B PARBHOO Cross-linking by addition reactions Silicone adhesives condensation cure B PARBHOO Cross-linking by step reactions Silicones structures B PARBHOO Chemical structure... 

Silicones are widely used in industry, for example, as adhesives, sealants and release agents see Silicone properties and Silicone adhesion. Silicone adhesives may cure by condensation or addition reactions. The former is considered here and the latter in Silicone adhesives addition cure (see also Step polymerization). 

Newer silicone adhesives having solids levels up to 97% are also commercially available [109]. Instead of using silanol condensation reactions, they rely on addition chemistry between vinyl functional silicone oligomers and silicon hydride terminated silicones. This addition reaction is typically facilitated with platinum derived catalysts. This hydrosilation process can be run at reduced oven temperatures, but the finished products typically do not yield the same balance of properties as seen for condensation cure materials. 

Scheme 5. Common moisture RTV condensation cure systems for silicone adhesives and sealants. R is typically methyl (CH3-) or ethyl (CH3CH2-) group.

There are many applications for silicone adhesives, sealants, or coatings where the condensation curing systems are not suitable. This is because they are relatively slow to cure, they require moisture to cure that can itself be in some cases uncontrollable, and they evolve by-products that cause shrinkage. Adhesives needed in automotive, electronics, microelectronics, micro electromechanical systems, avionic, and other hi-tech applications are usually confined to vei7 small volumes, which can make access to moisture difficult. Also, their proximity to very sensitive mechanical or electronic components requires a system that does not evolve reactive chemicals. 

To prevent the mold from adhering to the model, it is necessary to seal porous or, in the case of condensation-curing rubber, moisture-absorbing surfaces as well as surfaces consisting of silicate materials such as glass or porcelain prior to applying the silicone rubber. Suitable coating materials are polyvinyl alcohol, methyl cellulose (wallpaper adhesive), soft soap or Vaseline. 

Atomic spectroscopy (including atomic absorption spectrometry, atomic emission spectrometry, and atomic fluorescence spectrometry) is of use across the span of reactive adhesive technologies. For example, the cure of anaerobic adhesives on non-reactive surfaces is usually assisted by the use of an active metal-based primer. Similarly, the cross-linking of silicone adhesives is promoted by the use of organometallic salts of cobalt, tin, iron, lead, and platinum. In the case of polyurethane adhesives, the key condensation reactions are catalyzed by tin salts (e.g., dibutyl tin dilaurate and stannous octoate). 

Condensation reaction chemistry of silicones can be divided in two sub-groups. One sub-group is the moisture condensation cure system where, as the name implies, moisture in the air is utilized for hydrolysis reactions that lead to cross-linking of the polymer chains. The cross-linking reaction proceeds from the surface into the bulk and generally takes place at room temperature. The second sub-group is based on direct condensation reactions between polymers with different functional groups (cf. Scheme 1). These latter cure systems offer adhesives with fast rate of deep section cure at both room and elevated temperature. 

The formulation of a typical silicone adhesive based on condensation cure may contain up to 10 components as described in Table 1. 

Table 1. Typical components of a silicone adhesive/sealant based on condensation cure system...

Some silicone adhesives and sealants require condensation catalysts to accelerate curing. These are usually organotin and organotitanimn compounds such as dibutyl tin dilamate, dibutyl tin diacetate, dibutyl tin dioctoate, or tetrabutyl titanate. 

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