Curing silicones

The chemical bonding theory of adhesion applied to silicones involves the formation of covalent bonds across an interface. This mechanism strongly depends on both the reactivity of the selected silicone cure system and the presence of reactive groups on the surface of the substrate. Some of the reactive groups that can be present in a silicone system have been discussed in Section 3.1. The silicone adhesive can be formulated so that there is an excess of these reactive groups, which can react with the substrate to form covalent bonds. It is also possible to enhance chemical bonding through the use of adhesion promoters or chemical modification of the substrate surface. 

Effect of Room-Temperature-Vulcanized Silicone Cure in Device Packaging... 

Figure 1. RTV silicone cure study (A) Permittivity measurement, (B) Loss factor measurement with time at ambient temperature.

A second class of silicones cures by addition polymerization initiated by a catalyst Polymerization occurs by a free-radical mechanism involving a vinyl, allyl, or other unsaturated group of a silicone monomer (Figure 3.9). Homopolymers can be formed... 

Cure time testing tends to be somewhat subjective, but again there are methods available, such as ASTM C-679, Tack-Free Time of Elastomeric Sealants. Tack-free time is the curing time required for the product to develop a skin that is not damaged when subjected to application and removal of a plastic film. It is important in all cases to determine the cure characteristics of the product in actual working conditions. Since most silicones cure by reaction with moisture in the air, the sensitivity of cure time to humidity should be determined. Surface cure rates can usually be tailored to meet application requirements. 

Fig. 1 Process for preparing S-IPN films (a) solution is drop cast onto treated glass (b) solvent is allowed to evaporate and the RTV silicone cures at room temperature (c) film is peeled off of the glass substrate (d) film is stretched biaxially by 100 x 100% (e) HTV silicone is cured at 180°C for 30 min (f) film is relaxed and some prestrain is preserved. Reproduced with permission fi om [34]...

Polymer Alkenyl functionalized PDMS Backbone of silicone cured network... 

An unprimed silicone adhesive implies that it is free of any adhesion promoter. The substrate on the other hand, may still need to be activated or primed. Adhesion relies mainly on chemical and/or mechanical mechanisms (see Mechanical theory of adhesion and Primary bonding at the interface). The chemical adhesion depends on both the reactivity of the selected silicone cure system and on the natural presence of reactive groups on the surface of the substrates. 

Primers for adhesive bonding chemically functionalize the substrate surface to provide pathways for chemical bonding with a selected silicone cure system. The increase of chemical bond improves adhesion durability. The main disadvantage of priming the substrate is the addition of an extra step in the whole process of adhesive application. The primers are usually Silane adhesion promoters, a reactive alkoxy silane molecule, oligomer, or a mixture of two or more different silanes. 

Property ASTM polyester Epoxy Silicone (50-50) (50-50) diamine cure) sultide Silicone cure) tisol Prepolymer... 

RTV (room temperature vulcanizing) silicones cure in about 24 h. They can be graded according to their room temperature viscosities which range from as low as 1500 cP (general-purpose soft) up to... 

Fig. 13. Room temperature vulcanized (RTV) silicone cure mechanism...

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