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.
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 Â³ Teflon Â³ fail at three and one flex respectively.
Neither electrical nor physical properties are affected by temperature cycling from -271ºC to room temperature.
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
|PROPERTIES||METHOD OR CONDITION||PARYLENE C||EPOXIES||SILICONE||URETHANES|
|T (melting), ºC||Taken from secant moduleus -temperature curve||280||cured||cured||~170|
|T (glass transition), ºC||Taken from secant moduleus -temperature curve||80-100||120||-130||-10|
|T5 (where modules = 105)ºC||Taken from secant moduleus -temperature curve||125||110||-125||>-30|
|T4(where modules = 104)ºC||Taken from secant moduleus -temperature curve||240||120||-80||0|
|Linear Coef. of Expansion (10"5/ºC)||ASTM D696 - 44(61)||3.5||>4.5-6.5||25-30||10-20|
|Specific Heat @ 20ºC cal/g/ºC||0.17||0.25||0.42|