Conformal coatings enhance product surface.

Parylene conformal coatings, chemically known as poly-paraxylylene, were developed more than 30 years ago. The thin-film polymer coating has proven effective in modifying surface characteristics of rubber and elastomer parts as a means of improving performance in one or more categories.

Parylene is applied via a room temperature vapor deposition polymerization process (VDP). The coating is never a liquid, and as it is applied from a vapor, the resultant film is ultra-thin, perfectly conformal and has excellent moisture, chemical and dielectric properties. Parylene is also used to tie down substrate particulates and add lubricity to substrates.

Today, Parylene coatings are used in a number of applications in the medical, automotive, electronics, military and aerospace industries. Parylene can be applied to many substrates, including glass, metal, resin, plastics, ceramic, ferrite and silicon, and even powdered and granular substances.

Coating elastomers

SCS researchers discovered some time ago that the performance of rubber and elastomer components could be measurably enhanced with a thin layer of this flexible, transparent film. Parylene seals and protects elastomer surfaces, and is able to modify surface properties without degrading functional performance. This sealing property is useful for items such as gaskets and seals, rubber keypads, medical catheters and pharmaceutical packaging.

Parylene coating establishes a barrier (figure 1) that can prevent the transfer of substances into or out of the coated substrate, even when the film layer is only one to two microns in thickness. High molecular weight elastomer compounds such as silicone plasticizers are restricted from leaching out by the film barrier, and the coating also prevents external solubilizing chemicals from degrading an elastomer.

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The film resists chemical attack from organic solvents and inorganic reagents, acids or saline solutions. It also contributes electrical insulation with a dielectric strength greater than 5,000 volts per rail of coating thickness.

The chemical structure of Parylene film contributes to its unique performance capabilities in this and other coating applications. It has high molecular weight, a crystalline form and an all-carbon backbone. The absence of polar entities in the essential molecular makeup of the Parylenes makes these polymers quite hydrophobic, stable and resistant to chemical attack.

In contrast, other polymeric coating systems have oxygen, nitrogen or sulfur atom links in their backbone structures, and may require crosslinking to achieve the desired properties. This makes them more vulnerable to solvents and chemical degradation. Some may also contain solvents, fillers, stabilizers, plasticizers or other potentially mobile components that could out-gas, leach or be extracted in use.

Parylene film elasticity and surface adhesion integrity enables the coating to handle substantial elongation of the underlying elastomer without fracturing or breaking the film-to-substrate bond (figure 2).

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Parylene reduces elastomer tack and surface stickiness without adding stiffness to a part. This is a useful property for the rubber keypads used on calculators, cell phones and other devices because the film prevents dirt and oils from penetrating the surface, and it protects printed legends and preserves keypad appearance.

The film also adds chemical protection to rubber keypads, including resistance to petrochemicals and solvents. In some cases, it may be possible to substitute a conventional rubber keypad for a more costly and higher performance fluoroelastomer by coating the rubber with Parylene. The applied film does not compromise the electrical insulating properties of underlying rubber components.

The barrier performance and chemical inertness of Parylene are particularly useful for biomedical objects such as catheters or single-use medical instrumentation items with rubber seals, plungers or diaphragms. Substrates such as these may come in temporary or prolonged contact with human tissues, and could otherwise be compromised. Two of the Parylenes are certified to comply with USP biological testing requirements for Classification VI. Additionally, these Parylenes have been subjected to an array of ISO-10993 biological evaluations, including acute systemic toxicity, intracutaneous reactivity, cytotoxicity, sensitization, hemocompatibility and implantation (2, 12 and 26 weeks).

Parylene can increase the dry film lubricity or slipperiness of elastomeric surfaces to a level approaching that of PTFE (Teflon). Recently introduced SCS Parylene HT[SM] has an excellent coefficient of friction lower than that of PTFE. This is a useful effect for objects such as rubber catheters that must move freely in tight quarters. Parylene improves catheter lubricity, and also isolates the surface from biofluids.

One of the most important distinguishing features of vacuum coating compared to conventional liquid coatings is the absence of surface tension, meniscus, bridging or pooling. This is due to the fact that Parylene is deposited from a gas at the molecular level rather than as a liquid. Vacuum deposition results in a very high degree of coating conformity. Coating thickness is equivalent on all surfaces, including inside and outside comers, and deep crevices (figure 3).

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The mechanical dampening and loading effects of Parylene on elastomers are minimal because of coating thinness and because there is no conventional cure force or phase change.

Rubber and elastomeric components are generally coated in a continuous tumbling process (figure 4) that exposes all surfaces uniformly to the monomer. No part fixturing is required with tumble coating, and this process makes particularly efficient use of the Parylene raw material. Larger parts, and parts that require masking for selective coating, may need to be fixtured in the coating chamber.

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Parylene does not contain any solvents or plasticizers, and no cure forces are needed to obtain the finished polymer film. SCS Parylene coatings are lead-free and RohS compliant.

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 results.