Transparent film adds value to elastomers.

A thin, transparent and flexible polymer film known as parylene has been shown to enhance the performance of robber and elastomer components by protecting surfaces and modifying surface properties. These improvements are made without degrading the functional performance of the part. Objects coated with this polymer film include a diverse range of robber components, ranging from gaskets and seals to rubber keypads and medical catheters.

Parylene is a very thin, transparent polymer film that is applied to substrates in an evacuated deposition chamber by means of gas phase polymerization. There is no liquid phase, and the process does not involve solvents, catalysts or environmentally restricted materials. This film resists chemical attack from organic solvents, inorganic reagents and acids, and exhibits dielectric strength greater than 5,000 volts per mil of coating thickness.

Coating functions

Pinhole-free parylene coating can prevent the transfer of substances into or out of a coated substrate, even in one or two micron layers. 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.

Film elasticity and surface adhesion integrity enable the coating to accommodate elongation of the underlying elastomer without fracture or loss of film-to-substrate bond.

This polymer tends to eliminate surface tack and stickiness without adding stiffness - a useful property for items such as robber keypads used on calculators, cell phones and other devices. In the case of keypads, the coating prevents dirt and oils from penetrating the elastomer surface, protects printed legends and preserves keypad appearance.

The film also adds chemical resistance to robber keypad surfaces, and in some cases it may be possible to substitute a conventional robber keypad for a more costly and higher performance fluoroelastomer by coating the rubber with parylene. The coating allows conventional robber keypad materials to be used in demanding industrial applications, with improved resistance to petrochemical and solvent exposure. The applied film has high dielectric strength, and does not compromise the electrical insulating properties of underlying robber 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 robber seals, plungers or diaphragms. Such substrates 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, including Acute Systemic Toxicity, Intracutaneous Toxicity and Implantation.

Parylene's static and dynamic coefficients of friction, which are in the range of 0.25 to 0.33, allow coated elastomers to approach the dry film lubricity of PTFE (Teflon). For example, tiny lead wires used inside flexible medical catheters must be both electrically insulated and chemically protected. The excellent dry film lubricity of parylene coating allows such surfaces to move easily in fight quarters. The external surfaces of robber catheters also benefit functionally from the coating's lubricity.

The coefficient of friction for parylene C, as measured by ASTM D 1984, is 0.29 for both static and dynamic observations. Figure 1 shows approximate coefficient of friction values for uncoated and parylene coated robber specimens.

[Figure 1 ILLUSTRATION OMITTED]

One of the most important distinguishing features of vacuum polymer deposition compared to conventional liquid coatings is the absence of surface tension, meniscus, bridging or pooling. Parylene is deposited at the molecular level, resulting in a very high degree of conformality - on flat surfaces, over inside and outside comers, and in deep crevices. Film thickness is essentially uniform regardless of substrate geometry.

This coating is applied to many substrates in addition to elastomers, including glass, metal, paper, resin, plastics, ceramic, ferrite and silicon, and even powdered and granular substances. The mechanical dampening and loading effects of parylene are minimal in any application because of coating thinness.

There are three conventional forms of parylene: N, C and D. Each of these has its own mix of properties, with variations in crevice penetration, dielectric constant, dissipation factor, thermal stability and gas permeability values specific to each variant. The raw material for each of these parylene variants, di-para-xylylene dimer, is a white crystalline powder.

Film deposition

Parylene is applied to rubber parts and other substrates by means of gas phase polymerization in a vacuum chamber (figure 3). There is no liquid stage, the parylene coating process does not use solvents or environmentally restricted materials, and no foreign substances are introduced that could degrade coated surfaces. Since parylene is non-liquid, it does not pool, bridge or exhibit meniscus properties during coating.

[Figure 3 ILLUSTRATION OMITTED]

The dimer is first vaporized at approximately 150 [degrees] C. The resulting gas is then heated in a second process to 680 [degrees] C to form the diradical, para-xylylene. This monomeric gas is then introduced to the vacuum chamber at room temperature, where it polymerizes and grows as a conformal film on substrates at room temperature.

During the deposition stage, the active monomeric gas polymerizes spontaneously on substrate surfaces, with no cure-related hydraulic or liquid surface tension forces. Under vacuum chamber conditions, the mean free path of the gas molecules is in the order of 0.1 cm, and free molecular dispersion causes all sides of exposed objects to be uniformly impinged by the gaseous monomer. This results in a high degree of coating conformity, with polymerization occurring in crevices, between closely spaced items and on outer surfaces at the same rate.

The thickness of film buildup is related to the amount of vaporized dimer and dwell time in the vacuum chamber, and can be controlled accurately to [+ or -] 10% of final thickness. Coatings from four microinches to three mils (3,000 microinches) can be applied in a single coating cycle at a typical rate of 0.0002 in./hr.

Effective parylene coating requires that surfaces be absolutely clean, with no residual chemicals, oils, fingerprints, dust or other contaminants, which could interfere with film bonding. Substrate cleanliness can be confirmed prior to coating by immersing parts in an instrumented rinse tank and taking ionograph measurement.

Cleaned substrates must be handled carefully to avoid recontamination before coating. Workers commonly use clean room gloves and handling tools to minimize contact with components during coating preparation.

Rubber and elastomeric components are generally coated in a continuous tumbling process that exposes all surfaces to the monomer. No part fixturing is required with tumble coating, and this process makes efficient use of the parylene raw material.

Since the cure cycle is automatic with parylene and occurs before deposition, substrates are not subjected to cure forces, solvents or elevated temperatures, and no testing is required to confirm that full cure has occurred.

Vacuum-deposited parylene film contributes to enhanced performance of certain rubber substrates in ways that cannot be achieved with alternate materials or processes. Its application is controllable, and provides consistent, repeatable results.

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Bruce Humphrey is plant manager of the Specialty Coating Systems Clear Lake, WI coating facility. For the past 15 years he has worked in conformal coating, with experience spanning industrial, automotive and aerospace applications.