HTV (High Temperature Vulcanization) Silicone Rubber

Overview of the Silicone Rubber

Silicone rubbers' special features as organosiloxanes polymer are unique in that they carry both inorganic and organic properties in terms of molecular structure unlike ordinary organic rubbers. In other words, due to the inorganic properties pertaining to Si-O as the main chain in terms of the molecular structure, they are superior to ordinary organic rubbers in heat resistance, chemical stability, electrical insulating property, abrasion resistance, weather ability and ozone resistance among others. As high polymer of long chains, polydimethylsiloxane comprising silicone rubbers creates a spiral structure and provides less inter molecular interactions, resulting in abundance of elasticity and superior compression set, and enhanced cold resistance. The branching organic methyl carries special organic properties such as superior reaction, solubility and processability, along with surface character including waterproof and contact resistance. Having armed with these properties, silicone has been widely used to replace petrochemical products in all industries including aerospace, munitions industry, automobile, fine chemicals, construction, electric and electronics, food processing, mechanical engineering, medical and pharmaceutical, cosmetics, home appliances, paper film, solar batteries, and semi conductor. Recently, the scope of silicone application has been expanding at a greater speed.

General Properties

1. High bonding energy

Silicone rubber has siloxane bond (Si-O) of molecular structure as the main chains.
While carbon bond, C-C, carries 84.9 Kcal/mol, siloxane bond carries 106.0Kcal/mol in greater capacity and stability. As a result, silicone rubbers are superior in heat resistance, electric insulation, and chemical stability compared to ordinary organic rubbers. Siloxane bond's energetic stability is secured due to sharp difference between Si and O in terms of electronegativity making Si-O to be closest to ionic bond.

2. Low intermolecular force with spiral structure

With its coil shaped spiral structure and low intermolecular force, silicone (dimethylpolysiloxane) is highly elastic and compressible. In addition, since methyl group located in the outside of the coil structure is free to rotate on its own, silicone rubber has outstanding water repellency and demoldibility.

Heat Resistance

Heat resistance of silicone rubber is one of its most excellent properties and provides the basis for its creation. Silicone rubber is far better than organic rubbers in terms of heat resistance. At 150 , almost no alterations of properties take place that it may be used semi permanently. Furthermore, silicone rubber withstands use for over 10,000 consecutive hours even at 200 and, if used for a shorter term, it may also be used at 300 as well.
Boasting this excellent heat resistance, silicone rubbers are widely used to manufacture rubber components and parts used in high-temperature places.

Cold Resistance

Cold resistance of silicone rubber is the finest among organic rubbers. It provides a critical reason behind the creation of silicone rubbers. Natural and ordinary rubbers demonstrate significant changes in formation depending on temperatures. They become soft at high temperatures and hard at low temperatures so that they may not be available any more. While other organic rubbers may only be used up to -20°C or -30°C , silicone rubber maintains elasticity between -55°C and -70°C Some of its products even withstand temperatures as extremely low as under -100°C

Weather ability

Silicone rubber has superb ozone resistance. Due to corona-discharged ozone, other organic rubbers soften at a higher speed, but silicone rubber is rarely affected. Furthermore, long-term exposures to UV rays, winds, or rain do not substantially change its physical properties.

Electric Properties

Silicone rubbers are used for insulation materials at high temperature with its superior insulation properties. It is particularly known for broader range in temperature and volume resistance between 10¹⁴Ohm cm and 10¹⁶Ohm cm. Silicone rubber experiences lowest change in performance in wet condition and are best fit for use as insulation materials. By adding special conductive fillers, conductive silicone may also be manufactured. In particular, silicone rubber is strongly resistant against corona discharge or arc among others, while being widely used for insulation purposes in high voltage environments.

Electric Conductivity

Conductive silicone rubber is a compound comprising conductive materials such as carbon black, silver and copper. Depending on the type of silicone rubber, they range in resistance level from a few Ohm cm to 10³Ohm cm. One of the properties is that its electric properties are not much affected by variance in temperatures. No rubber materials yet are found to match the electric properties of silicone rubber over 200°C Conductive ones are also used for keyboard interfaces, antistatic parts, and shield materials for high voltage cables.

Radiation Resistance

Relative to other organic rubbers, ordinary (dimethyl) silicone rubber has no special performance in terms of anti radiation. However, methyl phenyl silicone rubber adopting phenyl group in polymer molecules does have radiation resistance to be used for cables at nuclear power plants and connectors.

Steam Resistance

Silicone rubber absorbs only 1% of moisture even after experiencing long exposure to water without being affected in mechanical strength or electric properties. Generally, silicone rubber does not deteriorate even after having contact with steam under atmospheric pressure. In high pressure steam over 150 , siloxane polymer is cut off and rubber properties decline. Such a property may be improved by the composition of silicone rubber, selection of curing agent, and the post curing. Other modified products are also available with improved steam and boiling water resistance.

Flame Redundancy

Silicone rubber does not easily burn when in contact with a flame, but would burn out consistently once ignited. However, by adding a small amount of flame retardant, it may become flame resistant and self-extinguishing. Flame resistant silicone rubbers presently in use would scarcely produce toxic gas during combustion since they do not contain organic halogen compounds discovered in organic polymers.

Oil Resistance

Silicone rubber is inferior to ordinary organic rubber in oil resistance at room temperature. However, for automobiles or aircrafts that require high temperature resistance, it demonstrates higher performance. Even when in contact with automobile oil, silicone rubber does not inflate significantly by reason of swelling. It swells in non polar organic compounds such as benzene, toluene, and gasoline. But its materials do not disintegrate or dissolve unlike ordinary organic rubbers. If solvent is removed, it would be restored to the original conditions.