Although transparent liquid silicone rubber grades exist, moldable silicones represent a more advanced material for LED lamps and luminary applications.
A silicone elastomer is a good fit for electrical insulation applications because of their high dielectric strength. In addition to their resistance to high temperature, UV, and ozone, they are hydrophobic, so they do not promote surface insulation failures.
The basic materials and the process employed to fabricate individual or discrete electronic components have been known, and in use, since the late 1950s. At that time, silicon had become the substrate material of choice because of its adequate electron properties, availability, and most importantly, the ease of formation of its high quality native oxide, SiO2.
In the electronics field, the use of a silicone elastomer for a wide variety of applications has increased due to the fact that the industry can count on the extreme diversity of silicone polymer chemistry and architectures for meeting technical challenges. For example, the Light Emitting Diodes (LED) applications components market has the challenge of increasing the light flux and thermal loads.
Newer designs may not be compatible with the common materials used by lighting manufactures. Until recently, silicone was only used for the interior of electronic devices, but now, silicone rubber is used for consumer and industrial lamps and luminaries, signage, street and architectural lighting, entertainment lighting, health-care and automotive headlamps, and lights for car interiors. A silicone elastomer that is used for electronic applications has the combination of an inorganic chain similar to silicates, which is associated with high surface energy. They incorporate side methyl groups that are conversely very organic and thus exhibit low surface energy.
The Si-O bonds of silicone are strongly polarized and, without protection, should lead to strong intermolecular interactions. Methyl groups weakly interact with each other so they effectively shield the main molecular chain. This is further facilitated by the absence of side groups at the oxygen atom, which results in the high flexibility of the siloxane chain. This open molecular structure also explains the lower barrier rotation around the Si-O bond. In more practical terms, this means silicones’ polydi-methylsiloxane chain can easily adopt many shapes to improve processing and molding.
These low intermolecular interactions lead to low glass transition temperature (-127°C), high flexibility, good vibration absorption characteristics, and high impact resistance. Additionally, due to high electrical resistivity, resistance to environmental degradation, and to electrical aging, as well as their hydrophobicity, a silicone elastomer can be used in cable end terminations and connections made at the end of underground high voltage cables; they can also be used as insulators for power lines.
Insulation occurs without chemical bonding between the termination and the cable. It takes advantage of the silicone terminations ability to exclude any entrapped air (in this case, particularly in areas of high electrical field and around the edges at the cable end). The high gas permeability of silicones allows any trapped air to diffuse, leaving an air-free joint.
Such silicone rubber cable end terminations are produced by rubber injection molding using a silicone high consistency rubber (HCR) or by liquid injection molding using a two-part liquid silicone rubber (LSR).