To achieve specific application properties, it is sometimes necessary to employ specific LSR technologies.
All silicone elastomers consist of crosslinked polydimethyl siloxane (PDMS) molecules, fillers, and additives. Here, we are going to explain the classifications of Liquid Silicone Rubber (LSR) and the difference between the grades.
First, we must remember that Liquid Silicone Rubbers undergo addition curing at elevated temperatures. The main applications for this material family are silicone moldings, produced by injection molding, and textile coatings.
The curing mechanism is the addition of modified room temperature vulcanization with mixing ratios, typically 1:1.
Standard Liquid Silicone Rubber evolved from market needs over 25 years ago. They are called general purpose Liquid Silicone Rubbers. The existing standard LSR has been modified over time.
The base chemistry of standard silicones has not changed, however the curing process is now faster, and this allows for a decreased production cycle time and a lower cost per unit. One of the advantages of the fast curing process is lower molding temperatures. The typical applications for these materials are in the automotive, healthcare, and domestic appliance industries. The hardness range of these materials is between 10 and 80 Shore A.
Specifically Purposed Silicones
From standard silicones, other silicones with specific purposes have been generated. Some examples of these silicones are briefly described below.
- High tear resistant LSR was developed to provide maximized mechanical strength for baby bottle applications. They provide a good alternative for latex and other organic rubbers.
- No post cure LSR is a formulation with a good compression set right after molding. Standard LSR needs post cure to achieve the desired compression set.
- Heat stabilized LSR has the requirements necessary to operate in ignition systems. Normally, they are colored with a metal oxide in the form of pigment paste. The typical color is black and they do not need to be post cured. Typical applications are in automotive industry.
- Coolant resistant LSRs must have the minutest changes in properties over time as possible. Normally, they are the natural replacements of EPDM rubber, because the coolant resistance has a lower change in the compression set in hot air, and it has other important parameters such as hardness, tensile strength, elongation at break, and weight change. A typical application is gaskets for radiators, wherein these LSRs come into contact with coolants (monoethylene glycol) in water at temperatures above 100°C and the hot air environment on the outer side of the radiator.
- Self lubricating LSRs are also referred to as oil bleeding. In appearance, they are similar to standard silicones. The difference is at some point they form an oily film on the surface of the parts. This film is helpful during assembly, and it improves the hydrophobic behavior of the seal. The oil content range is between 2% and 7%, and the speed and amount of oil bleeding corresponds to the contents.
- Oil resistant LSRs are formulated to increase the mechanical, physical, and chemical behavior of the material when they are in contact with oil, because, by nature, the resistance of silicone and organic rubbers is low.
- Self-adhesive LSRs are tough enough to stick anywhere desired, yet are easy enough to remove when needed. The advantage of these silicones is the simplicity of combining plastic with silicone rubber without using a primer.
- Electrically conductive LSRs are made possible by adding substantial amounts of carbon black. Additionally, the viscosity of these LSRs increases almost 8 times, in comparison with a standard LSR.
- Flame retardant LSRs are used in consumer electronics, especially in high voltage areas.
- Extra low viscosity LSRs are used for manufacturing insulators. They can be processed at lower pressures, but the curing times are longer.