Thermally cured silicones

Several types of primary coating materials have been employed commercially. The earliest materials used in large scale production were thermally-cured silicones (14). They are notable... 

Deviees designed to transmit information by means of photons now incorporate polymeric materials with the appropriate structures. Passive applications include the eoating of optieal fibers by UV eurable epoxy aerylates, thermal curing silicones, or heat-shrinkable polyethylene, to protect them from mechanical wear. Polymers have also been used in the manufaeture of wave guides. Suitable polymers can now be synthesized having aetive nonhnear optical properties, which depend on the eleetronie exeitation of the 7t-electron system. 

Alternatively, thermally cured silicone elastomers are often used, notably in Japan. They provide excellent microbending protection but the low rate of curing (about 1.5 m s ) is an economic draw-back. 

Hydrosilation silicones or addition cure systems utilize a hydride functional crosslinker with a vinyl functional base polymer and a noble metal catalyst. While the cure can be initiated with UV [48,49], thermal cure versions dominate the commercial market [23,50]. In thermal cure systems, inhibitors are necessary for processing and anchorage additives are common. 

Epoxy cured silicones were developed to be photo initiated rather than thermally cured [54]. The chain length of these materials ranges to 200 monomer repeat units, but the majority component of most formulations is significantly shorter. The structure of a typical base polymer is shown in Fig. 4. The chain can be terminal and/or pendant functional, with degree and type of epoxy function-... 

Thermal cure system. The thermal cure system is based on a hydrosilylation addition reaction between vinyl-functionalized and silicon-hydrido functionalized polysiloxanes [32,33,35], Unsaturated organic groups react with a Si-H functionality in the presence of a platinum-based catalyst (Scheme 10). 

Cure systems based on hydrosilylation can be formulated as one and two-part silicone products, that can be either flowing or non-flowing. These formulations provide fast thermal cure rates, they are resistant to humid and other harsh environments, and they have good dielectric properties. These formulations can be self-priming or alternatively the substrate may require priming before application of the silicone. 

PDMS based siloxane polymers wet and spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with adhesive, giving an interfacial phase that is continuous and without voids. The gas permeability of the silicone will allow any gases trapped at the interface to be displaced. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. It must be noted that suitable liquids reaching the adhesive-substrate interface would immediately interfere with these intermolecular interactions and displace the adhesive from the surface. For example, a study that involved curing a one-part alkoxy terminated silicone adhesive against a wafer of alumina, has shown that water will theoretically displace the cured silicone from the surface of the wafer if physisorption was the sole interaction between the surfaces [38]. Moreover, all these low energy bonds would be thermally sensitive and reversible. 

On the other hand, silicones are used in organic composites. The application of alloys composed of the fully cured silicone rubber particles in a continuous thermoplastic phase, called thermoplastic silicone vulcanizate, which offers high-performance materials, has been reported.506,507 Functionalized POSS was not only co-polymerized, but also blended with poly(methylmethacrylate) to study the effect of silsesquioxanes on the thermal stability of thermoplastics.508... 

DuPont polyimide, Pyer-ML RC5057 was used for comparison. Pyer-ML varnish was diluted to I.IPa s (11 poise) using N-methyl-2-pyrrolidone and spin coated on silicon wafers and thermally cured. 

Jansen and co-workers [86] have evaluated temperature-controlled outgassing processes of plastics and rubbers using both off-line and on-line TD-GC-FTIR-MS. Decomposition of polyesterurethanes by means of TG-Tenax off-line sampling followed by TD-GC-FTIR-MS revealed C02, H20, tetrahydrofurane, cyclopentanone, dicarbonic acid, aliphatic diols and esters [86]. The same authors have also described the detection of polychlorinated biphenyls (PCB) in 2,4-dichlorobenzoylperoxide cured silicone rubbers after outgassing products of a rubber silicone part obtained after desorption for 10 minutes at 200 °C in the thermal desorption cold-trap and subsequent analysis by means of TD-GC-MS. Using a mass range of 290-294 Da the MS can be used as a selective detector for these substances. 

The state-of-the-art silicone systems used in label stock application are normally solventless and thermal curing. Base polymers for these systems are vinyl-functionalized polydimethylsiloxanes having viscosities of around 200 - 600 mPa.s. Cross-linkers normally are hydride-functionalized polydimethylsiloxanes with a viscosity of around 25 mPa.s. These two components are cross-linked by a platinum catalyst, which can be the Karstedt catalyst. Additionally an inhibitor is added to the silicone mixture to prevent curing before it is applied on the substrate. These inhibitors ate... 

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