Silicone HTV

Sealants are often based on butyl rubber, polyisopropylene, polyurethane, or silicone rubber. Reactive sealants may be vulcanized at room temperature (RTV) or at high temperature (HTV). Silicone sealants are chemically-hardening, single-component systems in which cross-linking of the substrate is initiated by moisture from the air. This separates the reaction products. After rapid formation of a skin on the surface, cross-linking continues inside the mixture until it is fully hardened. 

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]...

FIGURE 42.4 Otto Bock SiOCX socket system (Otto Bock Healthcare, GmbH, Duderstadt, Germany). The socket consists of an inner made of HTV silicone that connects to the outer composite socket using a mechanical screw interface. 

Because of their high viscosity, solid rubbers are normally compounded in trough kneaders, on internal mixers or on the roll. Cables, hoses and profiles are produced using an extruder. Molded articles can be produced with HTV silicones in compression, transfer or injection molding processes (see Fig. 7). 

The recently developed self-adhesive HTV silicone rubbers are addition curing, one component materials which remain processable for several weeks at room temperature. Self-adhesive liquid silicone rubbers are always delivered as two components. This is due to the fact that once components A and B are mixed the processing time is approximately 3-10 days at room temperature. 

Self-adhesive HTV silicone rubber is formulated in a similar way to self-adhesive LR. The ready-to-use formulation contains all adhesion promoters. However, other than LR, which is two component and addition curing only, self-adhesive HTV can be purchased both, as peroxide and addition curing. Because of fundamental differences in curing speed between self-adhesive peroxide cured HTV and addition cured HTV the applications differ quite substantially. 

Furthermore, HTV silicone rubbers exhibit better mechanical properties than LR. As a consequence applications might differ for the self-adhesive versions of these two family of products, apart from much better adhesion to steel in case of self-adhesive HCR. 

Table 11.5 gives a general overview of the properties of HTV silicones and LR. It is clear that HTV will never be 100% replaced by LR. So far the mechanical properties and heat resistance of HTV rubbers are more favonrable than those of LR. However, this summary should emphasise the characteristics of each family of materials and give an idea of how to select the most snitable material for an application. 

Table 11.5 Summary of properties of LR versus HTV silicone rubber ...

This section will summarise the processing techniques applied in the manufacture of composite parts from LR and HTV silicone rubber. 

Silicones are readily available in liquid form (LR or LSR), room temperature vulcanising (RTV) and high temperature vulcanising (HTV). 

Itoh et al. [141] patented a blend of polyurethane and silicone gum with vinyl groups. The crosslinking reaction of the silicone component was achieved using a radical generator. Chorvath et al. [142] also patented a system composed of polyamide or polyester as the thermoplastic and vinyl-containing polysiloxane cured by a peroxide or non-peroxidic initiator such as 3,4-dimethyl-3,4-diphenylhexane (HTV). Moreover, they added different compatibilizers such as amino-terminated or amino-grafted polysiloxane, epoxy- or isocyanato-functionalized polysiloxane. 

Silicones are well known for their versatility, which makes them ideally suitable for a variety of applications. The fluids can be used as solvents, as foam-control systems, or as release agents (20% of the total volume). High-molecular-weight silicones are mainly used in mbber applications such as High Temperature Vulcanisable (HTV) and Room Temperature Vulcanisable (RTV) (43%), resins (4%), or specialties (15%). Other applications for silicones are masonry protection (8%), textiles (7%), and paper coatings (3%). Silicones can be uniquely tailored for each application area by substitution by reactive groups, allowing them to be cured by different mechanisms. 

Non-bleeding softener or pot life regulator in silicone elastomers and gels, production of vibration-absorbing rubber (less crosslinked than regular HTV rubber)... 

Hydrophilic and hydrophobic fumed silicas are mainly used to improve the mechanical strength of RTV-, HTV-or liquid silicone mbber. Fumed silicas with higher surface area usually lead to higher transparency of the silicone rubber which is important for special applications... 

Silicone rubber (HTV) containing 0.07% of vinyl groups, produced by Polish Silicones Ltd. (Poland), reinforced by Aerosil 200 fumed silica... 

High temperature vulcanising, solid silicone rubber (HTV),... 

The intention of this chapter is to outline the advantages of using HTV and LR silicone rubbers. These materials are most commonly used by the silicone rubber industry. It would however be wrong to think that silicone rubbers and their bonding were limited to just these two groups of materials and this chapter will also consider other silicone specialities. 

HTV and liquid silicone rubber LR, are used in moulding at elevated temperatures (press, transfer or injection moulding). 

As for silicone rubbers other than LR, most of the above mentioned points apply, too. Silicone HTV rubbers can be delivered in a wide variety of preforms which allows the processor to optimise the production process. Such preforms range from blocks and strips to coils and even pellets, use of the latter allowing a more or less continuous operation. 

Typical applications for silicone HTV and LR rubber composite parts are ... 

Most recently self-adhesive HTVs have been developed. However, they require special release agents or a specially structured mould suface. They are mainly intended for bonding silicone rubber to steel. 

In this case self-adhesive HTV is used as a substitute to the primer. As it is chemically different from primers, which are not silicone rubbers, it is considered in this section. 

A very popular technique using HTV is the combination of moulded silicone parts together with extruded HTV profiles, tubing or mouldings. Examples of such composites are catheters used as medical devices, electrical cables, long rod insulators and composite insulators. 

Such applications are widely used across all fields which use silicone rubbers. The technique is quite simple. In the first step a tubing or a moulding is cured to an incomplete degree of curing. This is achieved by curing at a lower temperature or too short a curing time for complete vulcanisation. These parts are then inserted into another silicone cavity (HTV or LR) and coated with the silicone rubber. It is advisable to make silicone-silicone composites from silicone rubbers with only one curing system. In other words, a peroxide... 

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