How Parylene Improves Elastomers and O-Rings

Posted by Scott Curtis on Friday, August 12, 2016

What is Parylene?
Parylene (poly paraxylylene) is a generic name for polymers that belong to a unique chemical family and are used as conformal coatings in protective applications. The coating process exposes product to the gas-phase monomer at low pressure. Through vacuum deposition, parylene condenses on the object’s surface in a polycrystalline fashion, providing a coating that is truly conformal and pinhole free.

What is an Elastomer?
Elastomer is a big fancy word, and all it means is “rubber”. Some polymers which are elastomers include polyisoprene or natural rubber, polybutadiene, polyisobutylene, and polyurethanes. What makes elastomers special is the fact that they bounce. But just saying “they bounce” is kind of vague. Let’s be more specific. What makes elastomers special is that they can be stretched to many times their original length, and can bounce back into their original shape without permanent deformation.

It is a polymer that shows elastic properties. Since the terms rubber-like and elastomeric mean almost the same thing, the terms “rubber” and “elastomer” are often used interchangeably today to describe silicone elastomers.

What is an O-Ring?
An O-ring is a torus, or doughnut-shaped ring, generally molded from an elastomer. O-rings are used primarily for sealing. They are also used as light-duty, mechanical drive belts.

In designing an O-ring seal, it is best to determine the O-ring compound first, as the selected compound may have significant influence on gland design parameters. Essentially, the application determines the rubber compound; the primary factor being the fluid to be sealed. The elastomer however, must also resist extrusion when exposed to the maximum anticipated system pressure and be capable of maintaining good physical properties through the full temperature range expected. In dynamic applications, the selected material must also have the toughness and abrasion resistance so important in reciprocating and rotary seals.


Why use Parylene to coat Elastomers and O-rings?
Parylene’s thin, transparent and flexible characteristics enhance the performance of rubber and elastomer components by protecting surfaces and modifying surface properties. These improvements are made without degrading the functional performance of the part. Parylene yields a truly conformal coated part with a consistent thickness on both flat surfaces and around the internal dimensions of openings and features as small as 0.01mm. Pinhole-free Parylene can prevent the transfer of substances into or out of a coated substrate. In particular, high molecular weight compounds such as silicone plasticizers are retained beneath the film barrier, and the coating also prevents solubilizing chemicals from intruding into an elastomer.

Objects coated with Parylene film also include a diverse range of rubber and elastomer components, from gaskets and seals to keypads and medical catheters. During the vacuum deposition coating process, the Parylene monomer is able to penetrate the surface of rubbers and plastics, providing outstanding adhesion. A 2 micron coating imparts exceptional dry lubricity as well as resistance to surface wear. For example, silicone rubber keypads are coated to improve the feel of the surface by removing the elastomer tack while protecting the keyboard against dirt and oils, and shielding printed legends. O-rings are coated to reduce friction in syringes.

Vacuum-deposited parylene film can improve the surface properties of rubber substrates in ways that cannot be achieved with other materials or processes. Its application is controllable, and the process provides consistent, repeatable cost effective results. Because parylene is deposited as a gas at the molecular level, it yields a conformal coated part with like thickness on the flat surface area, over corners, and in, on and around the internal dimensions of holes. No matter the size or shape of the elastomer application, parylene film conforms to the surface features. With parylene film’s surface adhesion and elasticity, the coating can handle substantial elongation of the elastomer part without any fracturing or breaking of the surface adhesion of the film from the substrate bond. Rubber and silicone parts are usually coated in a horizontal chamber or tumbler. This exposes all surfaces to the parylene monomer. This proves to be cost effective, while offering superior quality. Because curing occurs before the film is deposited, the parts are not exposed to high temperatures and the integrity of the part is maintained.

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