Polymer film's thinly veiled design disguises strength

Parylene can be used for coating electronics, such as this Sauer-Danfoss sensor.

If it's possible for a product to be considered a breakthrough technology four decades removed from its first application—and Specialty Coating Systems (SCS) of Indianapolis does—a thinly veiled polymer film known as Parylene coating is on its way to becoming a hot trend for automotive manufacturers interested in providing a reliable measure of protection for components exposed to the harsh environment of today's sophisticated engines.

For years, the company says, designers have explored various options for protecting electronic engine devices, including protective housings, liquid coatings, and encapsulation. Unsealed housings also have been examined and proved to be largely ineffective, allowing contaminants and moisture to reach into and damage critical devices. By comparison, viscous liquid coatings pose mechanical challenges due to differences in the thermal coefficient of expansion, which exist between any coating and the device itself. This condition often results in fractured leads and mechanical damage. Encapsulated parts may also suffer from similar thermal expansion problems, as well as poor heat dissipation.

In its place, the vacuum-deposited polymer film—first discovered some 40 years ago by Union Carbide Corp.—is gaining recognition with component manufacturers, primarily because it possesses good barrier properties in extremely thin layers, SCS says. Proved effective in numerous aviation, aerospace, and medical device applications, it resists chemical attack from organic solvents, inorganic reagents, and acids. The dielectric strength of Parylene film in a layer 25.4 µm (1000 µin) thick is greater than 5000 V.

Defined chemically as poly-para-xylylene, it is a transparent film applied to substrates in an evacuated deposition chamber by means of gas phase polymerization. There is no liquid phase in the process, and no catalysts, solvents, or other environmentally restricted materials are required. The average cured thickness of a conventional liquid conformal coating is generally in the range of 0.005-0.010 in (0.13-0.25 mm). Flat surfaces are often treated at that thickness for hard-to-reach applications such as outside corners, sharp points, and around edges.

Complete protective encapsulation of an object is achieved with a Parylene film thickness of 0.75 µm (30 µin) or less. Because the coating is nonliquid, it does not pool, bridge, or exhibit meniscus properties when applied to surfaces. Film thickness varies little from point to point, whether measured on planar surfaces, in crevices, or on outside corners. In addition to its dielectric and barrier properties per unit thickness, Parylene coating offers extreme chemical inertness and freedom from pinholes, according to SCS. Parylene is easily deposited on such diverse substrates as silicon, glass, metal, paper, resin, plastics, ceramic, and ferrites. Its mechanical damping and loading effects are minimal due to its extremely low mass.

The Parylene raw material, di-para-xylylene dimer, is a white crystalline powder. Dimer is first vaporized at approximately 150°C (300°F) before being molecularly cleaved or pyrolyzed in a second process phase at about 680°C (1250°F). This forms the diradical, para-xylylene, which is introduced into the room-temperature vacuum deposition chamber as a monomeric gas that polymerizes evenly on substrates.

Substrate temperatures remain at a near-ambient level in this gaseous process, and the coating grows as a conformal film on all exposed surfaces. There are no cure-related hydraulic or liquid surface tension forces in the Parylene coating cycle.

Parylene thickness is related to the amount of vaporized dimer and dwell time in the vacuum chamber and can be controlled accurately to ±10% of its final thickness. Film thicknesses from 4.0-3000 µin (0.1-76.0 µm) can be applied in a single operation at a typical rate of 0.200 µin/h (5.08 µm/h).

Parylene coating exists in four variations known as N, C, D, and SCS Nova I-IT. Each of these polymer precursors has a unique molecular form and particular strengths. They are all applied in the same manner, with minor differences in the rate of polymerization. Several benefits exist with each of the variations including:

• High penetration—Because of its molecular activity in the monomer state, Parylene N has the highest penetrating power of the Parylenes and is able to coat relatively deep recesses and blind holes.
• Low permeability—Parylene C possesses a chlorine atom on the benzene ring, giving this variant a combination of electrical and physical properties that include very low permeability to moisture and corrosive gases.
• Thermal stability—Parylene D, with two chlorine atoms on the benzene ring, offers increased temperature performance compared to Parylenes N and C.
• Extended temperature range—SCS Nova I-IT, a proprietary fluorinated Parylene developed by SCS, has advanced properties for demanding applications requiring higher temperature performance and resistance to ultraviolet radiation.

A three-step process is used by SCS to produce Parylene. Click to enlarge

One of the earliest automotive applications of Parylene was as a coating for a pressure sensor used in monitoring the air/fuel mixture in intake manifolds. The hydrocarbon resistance of Parylene film in a very thin layer allowed the delicate electronic sensors to continue to operate accurately.

The material is appropriate for extending racing engine oil pan and valve cover gasket life. Frequent disassembly of racing engine components leads to gasket damage, as does normal thermal cycling.

Parylene has been key to the performance of many spacecraft components, including elements of the Mars Global Surveyor spacecraft camera, the International Space Station vision system, and the ion engine in NASA's Deep Space probe.

- Patrick Ponticel

Side mirror foam from Schefenacker

This foam-filled exterior mirror weighs 20-30% less than a conventional one.

An exterior mirror using foam for the structural housing is a lightweight solution has debuted on a passenger vehicle in Australia.

"Foam supports the plastic surface and unites the two parts together with no fasteners, making it very rigid to withstand vibrations," Rob Gilbert, Regional Managing Director-The Americas for Schefenacker International, said during a technology showcase event at the company's Marysville, MI, exterior mirror manufacturing center.

Without the usual mirror frame's structural elements, the foam version weighs about 20-30% less than a conventional side mirror. "Ultimately, although not a reality yet, we intend to make the product for less money than today's production costs," said Gilbert, adding, "We're already quoting customers in North America, and we anticipate that North America will be the next market introduction."

Second-generation technology will use a foam bracket base, which attaches to the vehicle. A third-generation of the foam-filled frame will replace the molded, painted exterior with a colored pressure-formed film, which is attached and supported by the foam. "So we'll have a foam core, and a colored environmentally resistant exterior film surface," said Gilbert, noting the third-generation product is likely to be seen around MY 2007.

Schefenacker claims about a 30% share of the world automotive interior and exterior mirror market.

- Kami Buchholz

Leather-like coverstock material

Canadian General-Tower's new interior coverstock is designed for use in instrument panel and other applications.

An automotive interior material described as a breakthrough by the company that has exclusive rights to market and make it in North America can be used for seating surfaces as well as door trim, instrument panels, steering wheels, consoles, and shift knobs. A comfortable feel with good moisture-absorption and release properties make the Canadian General-Tower Ltd. polymer the most leather-like nonleather material available, according to the company. It will be offered as both a PVC and thermoplastic polyolefin coverstock.

Canadian General-Tower was granted rights for the product from Tokyo-based Idemitsu Technofine Co. Ltd., which introduced it to the Japanese automakers.

The polymer is the most leather-like of nonleather materials, says Canadian General-Tower.

To make the unique converstock, naturally occurring organic materials are produced in a patented process and combined with special resins. Good moisture absorption and release properties reduce clamminess, providing comfort and functionality close to those of the natural materials.

- Patrick Ponticel