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Thermal Properties
The coefficients of thermal expansion of the parylenes are similar to epoxies: approximately 35 ppm/degrees C vs 27 to 30 ppm/ºC for most epoxy
molding compounds.
Based on extrapolation of test data, Parylene C is expected to survive continuous exposure to air at 100ºC for ten years (100,000 hr.). In
oxygen free atmospheres, it is expected to survive the period at 220ºC.
The table below illustrates parylene thermal characteristics as compared with epoxies, silicones and urethanes.
Cryogenic
In general, Parylene is capable
of withstanding exposure to cryogenic temperatures. Steel panels coated with Parylene C and chilled in liquid nitrogen at -160ºC have withstood impacts of more than 100
inch-lb. in a modified Gardiner falling ball impact test. This compares with values of approximately 250 in-lb. at room temperature.
Unsupported films of Parylene C
0.002" thick can be flexed 180º six times at -165ºC before failure occurs. Comparable films of polyethylene and ³ Teflon ³ fail at three and one flex respectively.
Neither electrical nor physical properties are affected by temperature cycling from -271ºC to room temperature.
Vacuum Stability
Vacuum tests conducted by the Jet Propulsion Laboratory showed a total weight loss of 0.12% for Parylene C at 121ºF and 10-6 torr. Collectible volatile, condensable material values were less that 0.01% (the sensitivity limits of the test).
Thermal Endurance of the Parylenes in Air
The thermal endurance in air of Parylenes N, C and D has been measured using an induction-time-to-initial
weight-loss method. Based on an Arrhenius extrapolation of these data, the temperature for 100,000 hours' endurance is 106ºC in the case of Parylenes N and C, and 134ºC in the
case of Parylene D.
In inert atmospheres the temperature for 100,000 hours endurance is greater than 200ºC for all three Parylenes.
All plastics undergo
degradation at rates which increase with temperature; the higher the use temperature the shorter the time a plastic will perform a desired function.
From the viewpoint
of design a frequent concern is how long a structure will function under a given set of thermal conditions. Design criteria may specify minimum lifetimes at specific
temperatures. Only aging test on the total structure will answer completely whether a lifetime criterion is met. However, since the properties of a structure's components will
usually combine to give the properties of the total structure, it is generally possible to estimate the limitations on structure lifetime imposed by a specific component.
Thermal Properties of Parylene C vs. Urethane, Epoxy & Silicone
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PROPERTIES
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METHOD OR CONDITIONS
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PARYLENE C
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EPOXIES
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SILICONE
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URETHANES
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T (melting), ºC
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Taken from secant modulus - temperature curve
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280
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cured
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cured
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~170
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T (glass transition), ºC
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Taken from secant modulus - temperature curve
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80-100
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120
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-130
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-10
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T5 (where modulus = 105), ºC
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Taken from secant modulus - temperature curve
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125
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110
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-125
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-30
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T4 (where modulus = 104), ºC
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Taken from secant modulus - temperature curve
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240
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120
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-80
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0
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Linear Coef. of Expansion (10-5/ºC)
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ASTM D696 - 44 (61)
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3.5
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4.5-6.5
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25-30
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10-20
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Specific Heat @ 20ºC
cal/g/ºC
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0.17
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0.25
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0.42
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