Page 4 - Jabar Catalog Silicone2

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4

Original Values

After 125 Months

Immersion

Volume Resistivity, (ohm-cm)

1x10

16

5.5 X 10

15

Dielectric Constant, 60 Hz

3.0

3.2

Power Factor, 60 Hz

0.0011

Durometer Hardness, Shore A

25-90

Tensile Strength, psi

Up to 1600

Elongation, %

Up to 700

Tear Resistance, lb/in

Up to 250

Compression Set, %

Down to 5

OZONE and CORONA

, as stated, cause most organic rubbers

to break down rapidly, and loose their inherent physical

properties. Silicone on the other hand can withstand high

concentrations of ozone (200ppm) with comparable effects

resembling that of heat aging on silicone. The corona

resistance of silicone approaches that of mica with the added

advantage of silicone’s flexibility.

ELECTRICAL PROPERTIES

of silicone compare favorably with

the best insulating organic materials.

Table 16G

depicts the

typical electrical properties to expect from silicone. Silicone

rubbers fatigue life under continuous stress is high, allowing it

to outperform organic rubbers when subjected to voltage for

a prolonged period of time.

DIELECTRIC FATIGUE and ELECTRICAL STRENGTH

stability are

two more attributes silicone provides where organic rubbers

often fail. At normal temperatures the electrical strength of

silicone rubber is in the same order as the values for organic

elastomers.

Figure 16G

shows that there is very little change

in voltage resistance over the operating temperature range for

which silicones are used. When continuously subjected to

high voltage stresses for prolonged periods of time, other

elastomers will evidence a drop-off in electric strength or

show dielectric fatigue. Values shown in

Figure 16G

, depict

those of silicone rubber. Dielectric constant and power factor

vary in different compounds. The absolute values in

Table16H

will also be influenced by current frequency. For this reason

silicone rubber is not used for applications where constancy in

these properties as a function of frequency are essential.

Volume Resistivity, ohm-cm

1x10

14

- 1x10

16

Electric Strength volts/Mil

400-700

Dielectric Constant 60 Hz

2.95-4.0

Power Factor, 60 Hz

0.001-0.0100

Immersion

Condition

Swell, Percent

Hardness change,

Shore A Scale Points

7 Days at 73.4°F

+1

-1

7 Days at 158°F

+1

-3

7 Days at 212°F

+1

-5

1 Day at 249.8°F

+5

-6

3 Days at 249.8°F

+6

-6

1 Day at 350.6°F

+15

-16

3 Days at 350.6°F

Sample Broke Up

WEATHERING RESISTANCE

is another important concern

when specifying material to be used. Silicone rubber resists

the deteriorating effects of sunlight, ozone, and gasses, which

cause weathering. Inherently water repellent, silicone is not

affected by moist operating conditions. Very dry conditions

and low humidity will not leach, dry out, or affect silicone in

any way.

GOOD FUNGUS RESISTANCE

When rubber is used in any

warm, damp environment, its properties resist attack by mold

or fungus. And, although silicone rubber is not antifungicidal,

its not a nutrient for fungi nor is it adversely affected by

fungus or mold.

RESISTANCE TO WATER

Silicone also resists the deteriorating

effects of the agents found in rainwater: nitrates, sulfates,

chloride, and hydrogenous. Surface water containing

minerals, acids, bases, and salts from the soil normally have

no detrimental effect on silicone. We show in

Table 16F

the

minimal effects long term immersion have on silicone due to

it's resistance to moisture absorption. In fact, testing

performed by the Navy Applied Science Lab, showed that

silicone rubber did not change in appearance or physical

properties after four years of undersea exposure (comparisons

made with control samples shelf-aged at ambient

temperatures).

TABLE 16E SUMMARY OF PHYSICAL

PROPERTIES AT ROOM TEMPERATURE

THE PHYSICAL STRENGTH

of elastomers is measured by

testing the Tensile, Tear, Elongation, and Compression Set. In

general, silicone rubber is strong, resilient, and stretchable at

temperatures where organic rubber fails. When compared

with many popular organic rubbers at room temperature,

silicone is relatively weak. But, as shown previously, silicone

shows far superior stability when subjected to temperature

fluctuations. When specifying an elastomer for a specific

application, one must consider the environments in which the

elastomer is asked to perform, and the expected life of the

unit into which the elastomer is installed. The typical values

one can expect of silicone at room temperature are shown in

Table 16E

.

TABLE 16G SUMMARY OF ELECTRICAL

PROPERTIES-SILICONE RUBBER

Technical Data

TABLE 16F INFLUENCE OF 23°C

WATER IMMERSION

Frequency

Dielectric Constant

Power Factor

60 Hz

2.96

0.0004

10 Hz

2.96

0.0004

10 Hz

2.95

0.0010

10 Hz

2.91

0.0051

TABLE 16H INFLUENCE OF FREQUENCY ON

DIELECTRIC CONSTANT AND POWER FACTOR