Liquid Silicone Rubber – Tailoring Properties with Chemistry

Liquid Silicone Rubbers are compounds formed from a fundamental silicone oxide base. The compound structure is given below:

Figure 1. Schematic of the structure of silicone rubber.

Modifying Properties of Liquid Silicone Rubbers

By altering certain side groups [R] (in this case CCH3) of the compound, the material may be modified to meet specifically defined functions or requirements. Hence, there can be several LSR materials, each with different characteristics though some features will remain the same for all liquid silicone rubbers. LSRs have an extreme temperature range performance. In their pure state they will be stable from 50°C to 180°C. By adding stabilizers the material can remain stable to temperatures above 250°C and by use of special compounds a low temperature flexibility of -90°C. LSRs are normally transparent but may be easily colored by use of non-hazardous pigments. LSRs have superior chemical and oil resistance, are very stable, highly resistant to oxidation, light, UV and X-rays.

Grades of LSSR

There are many different grades of LSR available. Some examples include electrically conductive, medical grade, FDA compliant, self-adhesive, self lubricating and combinations of these.

Electrically conductive grades are normally carbon filled to achieve the required conductivity. Medical grade materials are prepared under thorough testing and regulations to offer superior quality. There are various kinds of self-adhesive LSRs. In general, LSRs have exclusive compatibility with thermoplastics but many formulations are available so that they can suit a variety of applications. The most general formulations allow direct strong bonds onto most thermoplastic surfaces and metals including aluminum, magnesium and steel. When overmolding onto thermoplastics, due to the high curing temperatures, glass-filled temperature resistant thermoplastics such as polymides, PBTTs and PCs are preferred.

Unique Properties of Liquid Silicone Rubbers

The unique properties of liquid silicone rubbers ensure that they are truly special when compared to other materials in the market. Some of the unique properties of liquid silicone rubber are listed below:

  • Liquid silicone rubbers do not have any stabilizers, plasticizers, or promoters. Their exclusive flex properties are due to their distinct chemistry. As a crosslinked material it can withstand very high temperatures with no heat ageing and will retain its mechanical properties and resilience.
  • The material is cured through a method that contains non-hazardous substances therefore, even in an under cured part, there is no threat of hazardous chemicals being released.
  • LSRs are translucent, odorless and tasteless.
  • They are considered hypoallergenic and will prevent bacteria or mold growth due to their inorganic molecular backbone.
  • Their superior heat range and weather resistance along with its low ageing features make it a very unique material. LSR parts will not become brittle in time.

Mechanical Properties of Silicone Rubbers

Silicone Rubbers have exclusive mechanical properties. The following prioperties are described in more detail in the following sections:

  • Superior elongation and tear resistance
  • Low compression set and stress relaxation

Superior Elongation and Tear Resistance

Silicone rubbers are cured during processing to create a strong intermolecular crosslinked bond. This bond will offer extreme elongation of the material with no permanent deformation and a superior tear resistance, providing a highly resilience material.

Low Compression Set and Stress Relaxation

LSR is a high resilience material both in elongation and compression. The compression of the material will create an equal opposing force that will not alter with time making it an ideal material for gaskets and seals. LSR materials are crosslinked which will prevent any creep or stress relaxation problems.

Variable Alternatives for Material Properties

Standard ranges for properties are provided in the following table as a reference. Ranges shown are approximate and depend on the individual materials but these values are the most commonly seen in the market today.

Specific Gravity Durometer (Shore A) Tensile Strength (psi) Tear Strength (ppi) Elongation (%)
1.07-1.14 5-70 400-1520 50-590 150-980

Conclusion

Advanced molding technology, high precision molds and cold runner systems result in flash-free parts. Simtec’s expertise in materials and technology ensures that their processes are optimized resulting in lowest possible cycle times and excellent product quality. On comparing Simtec’s LSR injection molding techniques with other methods used for making rubbers, LSR high precision injection molding is unique especially for its volume, purity, and quality. The purity of LSR purity is definitely higher than any thermoplastic elastomer, polyurethane or natural rubber. There is no ash content on cured parts and parts may be made totally and perfectly transparent. Unlike peroxide based vulcanizations there is perfect batch-to-batch consistency. This is made possible because of the simplicity of the materials to be mixed and the use of modern automated mixing systems. High-cavitation molds provide high volume of parts in shorter periods of time.

About Simtec

SIMTEC is a research and technology driven company. Since their establishment in 2002 in Madison, Wisconsin, USA, they have been continuously developing Extraordinary Solutions™ for leading industries worldwide with services ranging from prototyping to serial production of high precision Liquid Silicone Rubber (LSR), overmolded and Two Shot (LSR/Thermoplastics) components.

This information has been sourced, reviewed and adapted from materials provided by Simtec.

For more information on this source, please visit Simtec.

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