Parylene conformal coatings boost elastomer seal performance in alternative fuels.

Materials suppliers have initiated evaluation programs that help predict performance of elastomeric materials in alternate fuels such as methanol and alcohol fuel blends. For example, fuel and solvent resistant silicones exhibit good performance over a broad temperature range in several automotive applications. A patented development, applying a parylene conformal coating to a wide range of elastomer seals and gaskets promises to enhance the performance of a fairly wide range of elastomeric materials, specifically silicones. Extensive testing in a variety of aggressive alternative fuels reveals that parylene coatings improve property retention such as hardness, tensile strength, elongation and volume swell.

The mid 1970s fuel supply crisis initiated serious interest in the development of alternate fuels. In the late 1980s, gasohol, predicted by a few experts to be the automotive fuel of the future, had a brief successful run. That is, until forward thinking elastomer seals suppliers cautioned the industry on the possibility of detrimental effects on elastomer parts in engines; especially on carburetor cups, grommets and fuel system diaphragms.

During the past decade, elastomeric material suppliers, seal designers and manufacturers have developed formulations that will allow them to respond quickly, if and when the government mandates automobile engines capable of running on alternate fuels. However, the "hazardous aggressive fuel syndrome" continues to cause concern regarding the performance of elastomers in gasolinemethanol mixtures.

Acadia currently is producing and testing a variety of seals for critical automotive applications. The basic elastomeric materials, selected through extensive research and development, are the optimum materials for the specific sealing needs. For several seal designs, an added performance edge is provided by a patented post coating operation (U.S. patent 5,075, 174); the application of a parylene conformal coating that promises to enhance and extend performance life of the seal. Under test are fuel system seals, o-rings, hoses and diaphragms. Basic parylene research was performed, coating typical laboratory dumbbells, platens and slabs, as well as fuel system parts made of fluorocarbon, HNBR, NBR and silicone. The bottom line: Parylene conformal coatings on elastomer seals improve volume swell performance, especially for silicone, while helping to either improve or retain other key physical properties.

Basics of parylenes

Parylene, the generic I.D. for a unique polymer series, will uniformly protect most component configurations including sharp edges, points, flat surfaces, crevices, exposed internal surfaces, etc. The Union Carbide development was first used to coat electronic and computer components, such as high-density disk drive components, printed circuit boards, connectors, etc. In these applications, parylene helps exclude minute dust, fibers, smoke particles and other microscopic contamination.

Parylene polymers are vacuum-deposited at room temperature which allows the polymer to form with equal, time-controlled densities on all types of surfaces. The penetrating nature of vacuum-deposition produces inert, nonreactive, pinhole free coating conformality that provides environmental barrier protection, chemical resistance and mechanical and dielectric strength. Coating layers may be as thin as 0.1 microns, or as thick as several mils.

The basic member of the series (figure 1 ), parylene N provides high dielectric strength and a dielectric constant that does not vary with frequency. A second series member, parylene C, provides a useful combination of electrical and physical properties, plus very low permeability to moisture and corrosive gases. A third member, parylene D, improves thermal stability while maintaining physical and electrical properties at higher temperature.

Deposition process

Parylene polymers are vapor phase deposited at around 0. 1 tort, which produces a truly conformal nature of coating. All sides and surfaces of an object being coated are uniformly impinged by the gaseous monomer. Parylene C is norrnally deposited at about 0.2 microns per minute; parylene N is somewhat slower.

It is a three step process. First, the solid dimer is vaporized at 150[degrees]C. Next, the quantitative cleavage (pyrolysis) of the dimer at the two methylene-methylene bonds at about 680[degrees]C, yields the stable monomeric diradical, p-xylene. Last, the monomer enters the room temperature deposition chamber where it simultaneously adsorbs and polymerizes on the substrate elastomer. Rate of deposition is directly proportional to the square of the monomer concentration, and inversely proportional to the absolute temperature.

Preliminary solvent resistance research

Basic research on the effects of a wide variety of organic solvents on parylene N, C and D was conducted by Nova Tran. A brief summary of this research follows.

Six classes of organic solvents were examined: Alcohol (isopropyl), ketones (acetone and 2,4-pentanedione), aliphatic hydrocarbon (iso-octane), aromatic hydrocarbon (xylene), chlorinated olefin (trichloroethylene), chlorinated aromatic (chlorobenzene and O-dichlorobenzene), heterocyclic base (pyridene), and fluorinated solvent (trichlorotrifluoroethane).

The solvents had a minor swelling effect on the parylenes with 3% maximum increase in film thickness. The swelling was completely reversible after the solvents were removed by vacuum drying.

Inorganic reagents examined include: Deionized water; 10% solutions of sodium hydroxide and ammonium hydroxide; non-oxidizing acids, hydrochloric and sulfuric, in concentrated and 10% solutions; and oxidizing acids, nitric and chromic, concentrated and 10% solutions. The diluted inorganic reagents had little effect on the parylenes.

The acids at 10% concentrations had virtually no effect at room temperature and, except for chromic, no effect at 75[degrees]C. Concentrated acids at room temperature (23[degrees]C) had little effect. Under severe conditions, 75[degrees]C for 90 minutes, all acids had a measurable effect ranging from 0.7% swelling with hydrochloric to 8.2% with chromic.

In addition, nitric acid under these same severe conditions caused severe degradation. Both concentrated nitric and sulfuric acids caused some discoloration. A complete report on this basic research is available from Nova Tran.

Performance in alternate fuels

Acadia has received a utility patent for the application of parylene on elastomeric material seals and gaskets. Elastomers covered in the application include: silicone, nitrile, natural rubbers, fluorocarbon, styrene butadiene, ethylene-propylene, polyurethane, neoprene, polyacrylate, ethyl acrylates, fluorosilicone, highly saturated nitrile and carboxylated nitrile. In automotive applications, parylene coating of silicone shows the most promise.

Test fuel series

A series of test fuels, specifically formulated for accelerated testing of elastomeric materials, has been developed by the SAE. The test fuels that eliminate possible variations in commercially available gasolines and gasoline/methanol mixtures allow direct comparison and performance evaluation of materials.

The basic fuel recipes shown in table 1 are for elastomeric and plastic material testing only, and not to be used in engine development. Some components in gasoline, chloride ions, formic acid and water, may decompose through auto-oxidation to form aggressive substances that may attack polymers. Copper ions and hydroperoxide are added to the test fuel to determine the effect of auto-oxidized fuels on polymers.

Lab testing

Candidate engine and fuel system seals for parylene coating include nitrile grommets in carburetors, silicone o-rings, diaphragms and cups, among many others. Acadia's programs were designed to investigate the effect several aggressive fuels have on parylene coated fluorocarbon, HNBR, nitrile and silicone. Properties of most interest are volume swell, tensile change, elongation change and hardness change.

Continuing studies provide data on the stability of coated elastomers in a range of fuels, over specific time periods. The use of different fuels in this study is significant, because the parylene coated elastomer selected for a specific fuel application will be required to provide acceptable low volume swell and good physical properties in a wide range of methanol/fuel blends. Also, the coated elastomer part may encounter other fuel types during the operating life of the alternate fuel engine.

Standard ASTM test procedures being used to measure physical properties include ASTM D-2240 for durometer hardness and ASTM D-412 for tensile strength and elongation.

Test setups must include a Reflux Condensor to prevent boiling-off of methanol or gasoline to assure accurate test results. Exact fuel recipes must be maintained. Any proportional changes in the methanol to gasoline mix will have a detrimental effect on test results.

The Acadia tests require the soaking of various parylene coated elastomeric materials in a selected number of reference fuels. For example, volume swell platens and dumbbells are evaluated both before and after parylene coating. All samples are soaked in various fuels for 168 hours at 158[degrees]F. Then the standard ASTM test for volume swell, and changes in tensile, elongation and hardness properties provide the before and after comparative data. Table 2 provides a means of comparing the data.

In this specific test program, the degree of improvement with fluorocarbons, HNBR and NBR appears to be minimal. However, there is significant improvement with silicone.

Conclusion

Test data provided in table 2 allow the following conclusions. Silicone and HNBR test samples show volume swell inversely proportional to percent of methanol concentration, while swell is proportional to percent of methanol concentration on the FKM test samples.

With FKM there is a moderate increase in elongation as the percent of methanol increases. With silicone, HNBR and NBR moderate to severe reduction in elongation is evident, with NBR having an inverse relationship to the percent of methanol concentration.

HNBR retains a functional level of tensile strength with some protection offered by the parylene coating. The tests or hardness revealed that NBR exhibited fair to good hardness retention with moderate protection by the parylene coating. HNBR exhibited good hardness retention and protection by the parylene coating.

The degree of property retention with fluorocarbons, HNBR and NBR appears to be a minor factor when parylene coatings are applied. However, there are significant benefits, particularly in the area of volume swell, when the coatings are applied to silicone.

Parylene coatings appear to have an applications niche in automotive fuel systems, particularly on otings, stationary seals and on fuel level indication components. They provide the additional benefit of lowering the surface energy attraction of rubber and thermoplastic components. A most promising application area is applying parylene coating to all the components of an automobile's fuel level indicator system, the entire cartridge, floats, etc.