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 Two solid objects pressing against each other (but not sliding) will require some threshold of force parallel to the surface of contact in order to overcome static cohesion. Stiction is a threshold, not a continuous force.
In situations where two surfaces with areas below the micrometer range come into close proximity (as in an accelerometer), they may adhere together. At this scale, electrostatic and/or Van der Waals and hydrogen bonding forces become significant. The phenomenon of two such surfaces being adhered together in this manner is called stiction.
Stiction may be related to hydrogen bonding or residual contamination.
Source: Wikipedia
Results are in – Parylene Coating is the best option to prevent stiction in Smart Products’ valves.
As reported in the August SmartTalk, Stiction can occur when inactive valves sit unused for a long period of time due to friction occurring between rubber o-rings sitting against a plastic poppet. If stiction occurs, there is an elevated opening pressure. To keep opening pressures precise, we have spent the last few months testing three different o-ring compounds for a stiction solution.
Test
Smart Products tested the following types of o-rings:
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Buna-N Standard (oiled per procedure)
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Buna-N Internally Lubricated with Nitrile (no oil)
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Buna-N Coated with Parylene (no oil)
Each type of o-ring was inserted into 30 Model #122 valves with an ABS body material and 2 PSI spring. An initial test was conducted to measure the difference in the Steady State Regulated Pressure (SSRP) average compared to the Open Pressure average. After 30 days, we tested all valves again to measure the Open Pressure average increase or decrease.
Results
With only a small difference in opening pressure after sitting for 30 days, Parylene is the clear winner for preventing stiction in our valves.
Chart comparisons of both tests for each type of o-ring treatment are below.
These results will lead us to continue testing Buna-N o-rings coated with Parylene in various sizes and combinations, as well as beginning tests on the other six types of o-rings we offer – EPDM, Viton®, Silicone, Fluorosilicone, Aflas, and Kalrez®.
Disclaimer- Results are from our testing procedures and do not reflect testing in any other environment. Results may vary.
About Parylene Coatings
Parylene is used as a protective polymer coating for a variety of applications including plastic and rubber. There are a variety of Parylene types, including Parylene C (the mono-chloro substituted compound) and Parylene N (the un-substituted compound) that are used as coatings.
A Parylene coating is not simply created and brushed onto our o-rings; it entails a certain process with temperature requirements and specific machinery. At a molecular level, Parylene’s original solid form is converted at about 150°C to dimeric gas. Next, the new Parylene vapor form is further broken down at 650°C to become a thin, clear film coating. This coating is applied at room temperature with specialized vacuum deposition equipment that allows control of the coating rate and thickness. The vapor condenses on all surfaces equally and can pass through holes as small as 1 micrometer, which is particularly beneficial for seeping into the small ridges and gaps naturally found in any type of rubber o-ring. With the entire coating process completed at room temperature, using Parylene alleviates any possible temperature stress on o-rings.
Beyond preventing stiction due to its resistance to friction, a Parylene coating has many beneficial characteristics to ensure it works well with a wide variety of design requirements. For the Medical or Food and Beverage Industries, Parylene is FDA approved with a Class VI bio-compatibility rating. It also has military approval with a MIL-I-46058C, Type XY rating. Parylene is an excellent barrier to oxygen, moisture, chemicals, solvents, acids, carbon dioxide, fungus, bacteria, water vapor, and many other elements. Additionally, Parylene is light-weight, highly reliable, and has excellent adhesion properties.
As always, the more we know about your device’s requirements – including any and all conditions our valves will be exposed to – the better we can create a precise flow-control product unique to your application.
For more information about this study or Parylene coated o-rings, contact a Product & Application Specialist at (800) 338-0404 or sales@smartproducts.com.
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