SILICONE RUBBER

Silicone rubbers are a group of synthetic elastomers noted for their (1) resilience over a very wide temperature range, (2) outstanding resistance to ozone and weathering, and (3) excellent electrical properties.

Composition

The basic silicone elastomer is a dimethyl poly-siloxane. It consists of long chains of alternating silicon and oxygen atoms, with two methyl (-CH3) side chains attached to each silicon atom. By replacing a part of these methyl groups with other side chains, polymers with various desirable properties can be obtained. For example, where flexibility at temperatures lower than -57°C is desired, a polymer with about 10% of the methyl side chains replaced by phenyl groups (-C6H5) will provide compounds with brittle points below -101 °C. Side-chain modification can also be used to produce elastomers with lower compression set, increased resistance to fuels, oils, or solvents, or to permit vulcanization at room temperature.

Curing, or vulcanization, is the process of introducing cross-links at intervals between the long chains of the polymer. Silicone rubbers are usually cross-linked by free radical-generating curing agents, such as benzoylperoxide, which are activated by heat, or the cross-linking can be accomplished by high-energy radiation beams. Room temperature vulcanized compounds are cross-linked by the condensation reaction resulting from the action of metal-organic salts, such as zinc or tin octoates. Pure polymers upon cross-linking change from viscous liquids into elastic gels with very low tensile strength. To attain satisfactory tensile strength, reinforcing agents are necessary. Synthetic and natural silicas and metallic oxides are commonly used for this purpose. In addition to the vulcanizing agents and reinforcing fillers described, other additives may be incorporated into silicone compounds to pigment the stock, to improve processing, or to reduce the compression set of certain types of silicone gum.


Types

Silicone-rubber compounds can be conveniently grouped into several major types according to characteristic properties. Typical properties of several types are shown in Table S.5. According to one such classification system, types are (1) general purpose, (2) extremely low temperature, (3) extremely high temperature, (4) low compression set, (5) high strength, (6) fluid resistant, (7) electrical, and (8) room temperature vulcanizing rubbers.

1. General-purpose compounds are available in Shore A hardnesses from 30 to 90, tensile strengths of 4.8 to 8.24 MPa, and ultimate elongations of 100 to 500%. Their service temperature range extends from -55 to 260°C, and they have good resistance to heat and oils, along with good electrical properties. Many of these compounds contain semireinforcing or extending fillers to lower their cost.

2. Extremely low temperature compounds have brittle points near -118°C and are quite flexible at -84 to -90°C. Their physical properties are usually about the same as those of the general-purpose stocks, with some reduction in oil resistance.

3. Extremely high temperature compounds are considered serviceable for over 70 h at 343°C and will withstand brief exposures at higher temperatures; for example, 4 to 5 hr at 371°C and 10 to 15 min at 399°C. In comparison, general-purpose compounds are limited to about 260°C for continuous service and 316°C for intermittent service.

4. Low compression set compounds provide typical values of 10 to 20% compression set after 22 h at 149°C. They have improved resistance to petroleum oils and various hydraulic fluids, and are particularly suitable for use in O-rings and gaskets.

5. High-strength compounds, in Shore A hardnesses of 25 to 70, provide tensile strengths from 0.82 to over 136 MPa and elongations from 400 to 700%, with tear strengths from 26,790 to 58,045 g/cm. Compounds of this class may operate over a service temperature range from -90 to 316°C.

6. Excellent resistance to a wide range of fuels, lubricants, and hydraulic fluids is offered by compounds based on a silicone polymer with 50% of its side methyl groups substituted by trifluoropropyl groups. Physical properties are similar to properties of other types of silicone compounds. However, service temperature range is somewhat limited. Its low-temperature properties are about the same as those of the dimethyl polymer, with a brittle point around -68°C and an upper service temperature of around 260°C.

7. In general, silicone-rubber compounds have excellent electrical properties, which, along with their resistance to high temperatures, make them suitable for many electrical applications. With proper compounding, dielectric constant can be easily varied from about 2.7 to 5.0 or higher, while the power factor can be varied from 0.0005 up to 0.1 or higher. The volume resistivity of a typical silicone compound will be in the range 1014 to 1016 Q-cm and its dielectric strength will be about 450 to 550 V/mil thickness (measured on a slab 2.0 mm). Resistance to corona is excellent and water absorption is low. In most cases, excellent electrical properties are retained over a wide temperature and frequency range. Compounds can also be prepared with very low resistivity, as low as about 10 Q-cm, for special applications. Insulated tapes for cable-wrapping applications can be prepared from electrical-grade compounds with a partial cure or from a completely cured self-adhering sili-cone compound.

8. Room-temperature vulcanizing sili-cone rubbers are available to provide most of the performance characteristics of silicone rubbers in compounds that cure at room temperature.

In addition to having excellent heat resistance, silicone rubbers retain their properties to a much greater extent at high temperatures than do most organic rubbers. For example, a sili-cone compound with a tensile strength at temperature of 1.36 MPa will have a tensile strength at 316°C of 0.48 MPa or over. Most organic rubbers, although their initial properties are much higher, will be virtually useless at 260°C (except for the fluoroelastomers). In applications where a silicone rubber part operates in low oxygen atmosphere, such as sealing on high-altitude aircraft, its heat resistance will be still further improved.

In using silicone rubber at high temperatures, care must be taken to prevent reversion or depolymerization, which may occur where a part is required to operate in an enclosed environment. Here again, where this problem cannot be eliminated by the design engineer, the sili-cone-rubber fabricator can produce compounds with relatively high resistance to reversion.

Fabrication and Uses

In general, silicone rubbers may be handled on standard rubber-processing equipment. Their fabrication differs from that of organic rubbers chiefly in that uncured silicone compounds are softer and tackier and have much lower green strength. Also, an oven postcure in a circulating air oven is often required after vulcanization to obtain optimum properties. Silicone rubbers can be extruded, molded, calendered, sponged, and foamed. Since compounding of the rubber stock determines to a great extent the processing characteristics of the material, the fabricator should be consulted before the material is specified, to determine whether compromises are necessary to obtain the best combination of physical properties and the most desirable shape. Silicone-rubber compounds can also be applied to fabrics by calender coating, knife spreading, or solvent dispersion techniques.

For certain applications, very soft or low durometer materials are required. Suitable for such applications are low durometer solid sili-cone rubbers, closed or open cell expanded sil-icone rubbers (designated here as sponge and foam, respectively), and fibrous silicone rubber.

TABLE S.5

Properties of Some Typical Silicone Rubbers

General

Extreme Low

Extreme High

Low Compression

High

Fluorosilicone (General

Purpose

Temp.

Temp.

Set

Strength

Purpose)

RTVa

Hardness (Shore A)

50 ± 5

25 ± 5

50 ± 5

60 ± 5

50 ± 5

60 ± 5

65 ± 5

Tensile strength, psi

1000

1000

1200

900

2000

950

750

Elongation, %

400

600

300

130

600

225

110

Tear strength, lb/in.

80

120

150

60

300

75

40

Compression set (22 h at 300°F), %

20

20

15

10

30

15

13b

Max service temp., °F

Continuous

500

500

550

500

500

500

500

Intermittent

600

600

700

600

600

550

600

Low temp. flex., °F

-65

-130

-130

-65

-130

-65

-65

Volume swell

7

10

+9


+5

+10

+1

+3

(ASTM No. 1 Oil;

70 h at 300°F), %

a Room temperature vulcanizing silicone rubber. b After additional postcure 24 h at 480°F.

Silicone-rubber sponge is available in molded sheets, extrusions, and simple molded shapes. As in the case of solid silicone rubber, improved resistance to fluids or abrasion can be obtained by bonding molded or extruded sponge to a fabric or plastic cover. Silicone foam rubber can be fabricated in heavy cross sections and complex shapes, and can be foamed and vulcanized either at ambient or elevated temperature. It is suitable for use where an extremely soft, low-density silicone material is required. Like sponge, it can be bonded to fabrics and plastics.

For some applications a solid, low durometer material has advantages over sponge or foam. For example, it should probably be specified for gaskets or seals where the low compression set of foam, the compress-ion-deflection characteristics of sponge, and the higher tensile and tear strength of solid silicone rubbers must be combined.

The last highly compressible material, fibrous silicone rubber, consists of hollow rubber fibers sprayed in a random manner and bonded into a low-density porous mat. Its properties include excellent compression set combined with good tear and tensile strength, and very high porosity. As manufactured at present, it is serviceable from -55°C to over 260°C, and is available in mats 3.2 mm thick and 228 mm wide.

Silicone rubbers are most widely used in the aircraft, electrical, and automotive industries, although their unique properties have created many other applications. Specific examples would include seals for aircraft canopies or access doors, insulation for wire and cable, dielectric encapsulation of electronic equipment, and gaskets or O-rings for use in aircraft or automobile engines. An example of another field in which they are useful is the manufacture of stoppers for pharmaceutical vials, since silicone rubbers are tasteless, odorless, and nontoxic.

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