Silicone oil
Composition, characteristics and applications

Silicones comprise a set of synthetic polymers made up of an inorganic frame consisting of a series of bonds between Silicon and Oxygen atoms (-Si-0- Si) and variously branched side functional groups of an organic nature, typical of carbon-carbon (C-C) bonds.

The remarkable versatility of this polymer in industrial contexts has stimulated the developer to compose numerous variants which, for simplicity's sake, are grouped into three main categories:

• Lubricating oils
• Resins
• Elastomers

The best-known silicone oils can be divided into six main categories:

• Dimethylsilicones > The most widely-used silicone oils in industrial lubrication due to their effective price-performance ratio. They possess a high viscosity index and low surface tension. Ideal for protection and friction reduction of polymeric materials such as rubbers and plastics.
  
  
• Flurosilicones > Good lubricating power and chemical resistance to the action of solvents, extending the life of rolling bearings. They are suitable for high-temperature applications and have excellent mechanical load-bearing capacity.
  
  
• Methylphenylsilicones > Characterised by excellent resistance to low and high temperatures, being able to withstand radiation very well and possess high oxidation stability.
  
  
• Methyl-alkyl-silicones > Able to develop an effective lubricating film under conditions of dynamic friction. Compatible with organic materials. Can be used in coating and industrial painting applications.
  
  
• Chlorophenyl methyl silicones > Ideal for extending lubrication intervals at extreme temperatures. The presence of chlorine due to high reactivity with metals ensures effective lubrication under conditions of boundary friction in metal-to-metal couplings. They have a very low freezing point.
  
  
• Organosilicones > Grouped for simplicity into a single category comprising all frames (Si-O-Si), variously combined with organic polymers, the branching of which is chosen to enhance specific properties.

Silicone fluids are classified not only by their chemical composition but also on the basis of their average molecular weight or the dynamic viscosity value measured at a reference temperature of 25° C.

Silicone oils are available on the market with standard viscosity values and tolerances usually between +/-10%.

The silicone oils most commonly used in industrial lubrication belong to the dimethyl silicone family and are the best value for money, being available in a very broad variety of viscosities with remarkable properties that make them the preferred choice where harsh operating conditions are foreseen.

The main characteristics of silicone oils can be summarised as follows:

• Wide operating temperature range from -60° up to 200° C of continuous thermal exposure
• Need for high thermo-viscous stability > reduced viscosity variation upon temperature change
• Mixed mechanical couplings > rubber-metal and metal-plastic
• Good flame resistance and self-extinguishing
• High thermo-oxidation stability due to strong Si-O-Si bonding
• Low toxicity 
• Reduced content of Volatile Organic Compounds (VOCs) for higher viscosity grades
• Excellent compatibility with amorphous plastics and rubbers > NBR, EPDM, FKM
• Unrivalled resistance to wash-off
• Hydrolysis stability
• High dielectric strength

The dynamic viscosity value of a silicone oil is not always constant but can vary in relation to temperature as well as the dynamic stress to which it is subjected. This design parameter should be taken into account if the oil is responsible for the proper functioning of shock absorbers, soft-closing kinematics, dampers or viscous coupling units.

Silicone oils are either used directly for lubrication or as the formulation basis for silicone greases, which differ in their adhesive and cohesive capacity at friction points.

Silicone greases are preferred to silicone oils due to their superior ability to withstand centrifugation and mechanical loading.