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Types and Properties of Moldable Silicone Rubber

TYPES AND PROPERTIES OF MOLDABLE SILICONE RUBBER
Source: Dow Corning, http://www.dowcorning.com/content/rubber/rubberprop/default.asp
The unique properties of silicones enable them to be used for a variety of devices and components – Charles Heide

Notes: -Base polymer, reinforcing filler, and special additives can change tensile strength significantly.
           -Different processing methods and oven cures change the elongation values considerably.

Chemical Structure and Resistance
Sources:Charles Heide, "Silicone Rubber for Medical Applications", Medical Device and Diagnostic Industry Magazine; http://www.mddionline.com/article/silicone-rubber-medical-device-applications (November 1999)

Silicone rubbers are synthetic polymers with an alternating Si-O chain backbone. The molecules will crosslink with the addition of a catalyst.

Peroxide catalyst silicones, used in early silicones and still used today, are known to leave an acid residue in the rubber and outgas peroxide byproducts if not fully cured and cross-linked.

Platinum catalyst silicones have become more popular as they, not only do not leave a peroxide bloom, but they also cure faster and are available in an injectable liquid form. Silicones utilizing platinum cure systems are shipped in a two part kit; typically referred to as Part A and Part B. The two parts are mixed in a predetermined ratio, forming a compound ready to be cured.

Platinum-cured silicones are available as a high-consistency rubber, sometimes referred to as gumstock, as well as liquid silicone rubber or LSR. Silicone resists many chemicals, including water, isopropyl alcohol, some acids, oxidizing chemicals, and ammonia and can meet chemical resistance requirements in elevated temperatures. Liquid silicone can be formulated to enhance its chemical resistance to a given chemical solution. For more information regarding silicone’s resistivity to other chemical compounds please visit: http://www.coleparmer.com/TechInfo/ChemComp.asp

Avoid using concentrated acids, alkaline, and solvents with silicone. Flourosilicone rubbers (FSR) resist solvents and fuels and are generally the best silicones to use in corrosive settings.
All methyl silicones resist ozone.
 
Electrical
Source: Charles Heide, "Silicone Rubber for Medical Applications", Medical Device and Diagnostic Industry Magazine; http://www.mddionline.com/article/silicone-rubber-medical-device-applications (November 1999)

The dielectric insulating property of silicone is the best of any of the available elastomers. For example, a half inch thick piece of silicone has the same dielectric properties as that of 18 inches of air. Another useful benefit is that the dielectric and physical properties are not affected by temperature extremes. For more information regarding the dielectric properties of silicone please visit: http://www.dowcorning.com/content/rubber/rubberprop/elec_dielectric.asp
Examples of electrical engineering applications include:
- Cables and cable terminations
- Corona-resistant insulation tubing
- Keyboards and contact mats
- Conductive profiled seals
- EMI/RFI applications                       

Here is a Silicone Dielectric Insulator designed for Homeland Security to be used in airport imaging equipment for luggage.

Flammability
Typically silicones are flammable; however silicone rubber can be compounded and fabricated to meet many specifications, including:
- UL-94, V-1 or V-0
- UL Code 62
- BMS 159-C
Silicone can be used to improve a product flammability rating.
Silicone can be formulated to change flammability properties.

Fungus Resistance
When rubber is used in any warm, damp environment, its properties must resist attack by mold or fungus. Although silicone rubber is not antifungicidal, it is not a nutrient for fungi nor is it adversely affected by fungus or mold. With test procedures described in 005272B (USAF), several classes of Military Specification MIL-E, silicone rubber was exposed to chaetominum, globum, aspergillus niger, aspergillus terreus, penicillium lutem, and fusarium moniliforne. None of these microorganisms deteriorated the specimens. In another test, silicone rubber samples were buried in 5 inches of warm (28°C) moist soil for 6 weeks with no evidence of microbial attack. Finally, in a third test, samples were sprayed with a mixed spore suspension of fungi and then placed in a tropical test chamber at 27°C, 90/100 percent relative humidity. None were attacked by mildew.

Silicone can be formulated to improve fungus resistance properties.

Food Contact Status and FDA Regulations
FDA Regulation – FDA ZZR-765-E Class 2A&B
Silicone can be rated for food contact but special tests should be run.

Mechanical
Source: Charles Heide, "Silicone Rubber for Medical Applications", Medical Device and Diagnostic Industry Magazine; http://www.mddionline.com/article/silicone-rubber-medical-device-applications (November 1999)
• Very high elongation (up to 1200%)
• Tensile strength (up to 1500 psi)
• Tear strength of silicone is related to the durometer. The higher durometer, the higher tear (typical to 250 ppi)
• Compression Set is as low as 6% at room temperature. The resistance to compression set is maintained at low and elevated temperatures.
The electrical, physical, thermal and chemical resistance properties can be enhanced with special
formulations.
For more information see:
http://www.dowcorning.com/content/rubber/silicone-organics.asp

A. Compression Set Resistance

C. Hardness Range from 0000 shore a to 70 shore D
The 10 to 80 shore A, durometer hardness range offered by silicone rubber gives the designer freedom to select the desired hardness to best perform a specific function. Variations in blending polymer bases, fillers, and additives permit all intermediate hardness values. And, the length of time and the temperature used for oven curing also change the hardness without destroying other physical characteristics.
The normal hardness tolerance is plus or minus 5 (shore A scale). Mold prototypes in different durometer to
see how product performs.

D. Tensile Strength
Typical tensile strength ranges are listed here. (Review the links to each selection guide for more details.)
HCR – High Consistency Silicone Rubber – typical tensile strength range from 4.0 to 12.5 MPa.
FSR – Fluorosilicone Rubber – typical tensile strength from 8.7 to 12.1 MPa.
LSR – Liquid Silicone Rubber – typical tensile strength range from 3.6 to 11.0 MPa.
• Base polymer, reinforcing filler, and special additives, such as fiberglass can change tensile strength significantly.

E. Elongation
Generally refers to "ultimate elongation" or percent increase in original length of a specimen when it breaks.
Typical elongation ranges are listed here. (Review the links to each selection guide for more details.)
HCR – High Consistency Silicone Rubber typical elongation range from 90 to 1120%.
FSR – Fluorosilicone Rubber – typical elongation range from 159 to 699%.
LSR – Liquid Silicone Rubber – typical elongation range from 220 to 900%.
*Different processing methods and oven cures change the values considerably.
The thermal characteristics section shows that the elongation of silicone rubber varies linearly with
temperature, and holds up longer than most other elastomers.

F. Bulk modulus
This property refers to the elastic properties of a material when compressed by external pressures. Silicone rubber offers the advantage that under high-pressure it will deform approximately half that of a comparable organic rubber. In addition, at high temperatures, silicone rubber remains stable, while organic rubbers continue to deform. At low temperatures, silicone rubber will remain stable while many organic rubbers suffer from brittle failure.

G. Flex life
These properties refer to the ability of the material to undergo extreme flexing, but still retain its original shape without any significant loss of its property profile. Silicone rubber can withstand thousands of cycles over a wide range of temperature making this material ideal for keyboard or exhaust hanger and vibration damping applications. Typically natural rubbers require more energy to deform and breakdown when exposed to ozone and UV light, again silicone rubber remains stable over a wide range of temperatures.
Flexometer tests prove the outstanding crack growth resistance of silicone rubber. When subjected to flexometer test punishment – 18,000 cycles an hour until failure – cut-growth samples of a high-performance silicone elastomer lasts up to 500,000 cycles.

H. Tear Strength
Tear Strength is defined as the resistance to growth of a cut or nick when tension is applied to the cut specimen. High-performance silicone rubber resists tear even when nicked and placed under severe twisting stress. Typical tear strength ranges are listed here. (Review the links to each selection guide for more details.)
HCR – High Consistency Silicone Rubber – typical tear strength range from 9 to 55 kN/m.
FSR – Fluorosilicone Rubber – typical tear strength range from 17.5 to 46.4 kN/m.
LSR – Liquid Silicone Rubber – typical tear strength range from 11 to 52 kN/m.

I. Coefficient of Friction
The coefficient of friction of silicone rubber ranges from less than 0.25 to more than 0.75. Steel is about .10. Compared to steel silicone rubber is 2 to 7 times greater. Lower durometers have higher coefficients. The texture can be used to reduce the co-efficient of friction.
Note: Polished surfaces have a higher coefficient of friction. Texture lowers the coefficient of friction.

J. Ozone and Oxidation Resistance
Silicone rubber, when tested for resistance to ozone, shows excellent stability. After both static and dynamic testing for periods of two, four, six, and eight hours, samples had no significant change in durometer hardness, tensile strength, or elongation. Under a magnification of 10, no cracking or checking was visible.

K. Permeability (Gas and Liquid)
Silicone gas permeability is approximately 400 times that of rubber. Gas permeability can be increased by
compounding but not decreased.

L. Bondability

This property refers to the ability to adhere a rubber to another substrate. Parts fabricated from silicone rubber can be easily bonded to metal, glass, ceramics, silicone-glass laminates, or to silicone rubber itself. Best bonding procedures depend on the exact materials and engineering requirements. Typically primers are coated onto the substrate to be adhered to, however this involves additional preparation and coating process. Silicone rubber now offers the advantage of incorporating a bonding agent into the rubber, so that when the fabricated part cures it automatically bonds to the substrate. Silicone rubber can be bonded to any porous or non-porous material; the bond formed is highly durable and will readily withstand flex movement at high and low temperatures. Most organic rubbers cannot match the combination of bond and physical properties that silicone rubber offers. The silicone bond to the substrate is stronger than the rubber itself giving the rubber a tearing bond.

M. Radiation Resistance
Radiation causes changes in the properties of silicone rubber similar to those caused by heat aging. As the total radiation dose is increased, hardness of the rubber increases; tensile strength may increase at first, but later decreases sharply; elongation decreases.
These direct effects of radiation are proportional to the total amount of radiation level – as long as the radiation level is low or moderate. However, with high radiation levels, the heating effects cause additional changes.

N. Temperature Resistance
Temperature extreme stability is silicones most outstanding property. Under normal operating conditions temperatures as high as 600 F and as low as -150 F do not destroy the physical and electrical properties of silicone. At elevated temperatures, the tensile, elongation, and abrasion resistance of silicone is far superior to that of most organic elastomers.
Low temperature flexibility is another advantage silicone has over most organic rubbers. Silicones durometer and modulus show little change at temperatures as low as -100 F.

O. Weather Resistance
Weathering, UV, sunlight and environmental water exposure has less effect on silicone than other materials.

P. Bio-testing
Source: Charles Heide, "Silicone Rubber for Medical Applications", Medical Device and Diagnostic Industry Magazine; http://www.mddionline.com/article/silicone-rubber-medical-device-applications  (November 1999)
Biocompatibility. In extensive testing, silicone rubbers have exhibited superior compatibility with human tissue and body fluids and an extremely low tissue response when implanted, compared with other elastomers. Odorless and tasteless, silicones do not support bacteria growth and will not stain or corrode other materials. They are often formulated to comply with FDA, ISO, and Tripartite biocompatibility guidelines for medical products. Additionally Medical Grade platinum cured silicone meets Class VI medical standards. Silicone oils, namely dimethicone, simethiocone and cyclomethicone are widely used today in cosmetics, antiperspirants in lotions and creams.

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