Tight seals for the long haul

Fred Seidel
Applications Engineer
High Performance Foams Div.

Cynthia Kiss
Rogers Corp.
Rogers, Conn.

Obviously, tiny water molecules that penetrate exterior automotive joints will wreak havoc on sensitive components for GPS antennas, cell phones, and communication modules. Windshield-wiper motors, backup sensors, and fluid reservoirs also must be sealed tightly from water. Proper seal and gasket selection, however, involves more than looking at how a material resists water. Tests reveal that the vast majority of foam-gasketing materials — EPDM, neoprene, EVA, polyethylene, and various blends — seal well initially, but often collapse over time and under compressive loads.

Exterior water-sealing foams are gaining importance as exterior electronics become more complex. In windshield-wiper motors, for example, gaskets seal the cover and housing to protect electrical contacts inside. If gasketing materials take a permanent set over time, they lose the ability to press against the cover and flange and make a tight seal. Motor housings often need additional watertight seals to plug access holes. Roof-mounted hardware such as roof racks and antennas also need weathertight seals around mounting or access holes. Watershield gaskets in doors are another example. These gaskets must deflect water from the dry side of the door to weep or drain holes located at the door base on its wet side.

A few rating factors help gauge which material will best give a long-lasting exterior seal:

Closed-cell content: Seal and gasket materials typically contain a blend of both open and closed cells. Closed-cell content is often listed as low, medium, or high. Generally, foams rated high in closed-cell content have fewer open pathways through their cell structure and will seal best against water. Additionally, these foams typically better withstand the stresses of heat and pressure than those with ratings of medium or low.

There is no set industry standard, however, for determining a material's closed-cell content. Designers often have differing views on how well a material will seal, especially for those designated as highly open celled. However, properly designed joints can compress both open and closed-celled materials creating effective watertight seals. The best material for particular designs depends on other design factors.

Foam density (pcf): Low-density materials are an attractive option when weight is at a premium. But decreasing density means there is less gasket material and water can more easily permeate through the gasket.

Water absorption: Typically a waterabsorption rating of low is good and high is bad in sealing materials. Manufacturers often supply water-absorption data based on uncompressed materials. ASTM D-570-98 Standard Test Method Water Absorption of Plastics covers the determination of the relative rate of absorption of water by plastics when immersed.

Resistance to standing water at 30% compression: How well a material supports a column of standing water at 30% compression helps determine gasket and seal success. A common test is the U-Channel test (GM6086M Flexible Expanded Rubber Materials Standard) that places a die-cut material in the shape of a U between two clear, plastic plates. The material is compressed and the channel filled with water. It must resist leakage for 1 hr before and after heat cycling.

Resistance to water leaking at 2 psi at 30% compression: This trial may be a good litmus test of how a foam functions as a seal in a pressured environment (such as a transmission).

Air permeability/Gurley porosity: This test determines the airflow passing through a foam from top to bottom. Ratings include sealed or breathable.

Compression-set resistance at 70°C: Materials rated excellent in this test demonstrate that they do not collapse over time and under compressive loads. A material's ability to resist compression set is especially important for gaskets that will see repeated compression. Materials that rate poor demonstrate compromised sealing, despite providing a good seal initially.

Compressive stress relaxation test: This test determines the amount of push-back force a gasket places on an enclosure under pressure. Under continued compression, all polymers relax over time. But the longer they hold their elasticity, the more effective their seal will be. The results of stress relaxation testing shows that a typical Poron cellular urethane formulation, for example, continues to resist compression longer and at higher temperatures compared to the other elastomers. While costly and timeconsuming, these studies are a powerful means of understanding foam cushion durability and ultimate product longevity.

Processibility: Designers must factor in pros and cons for each material investigated including how easy it is to work with and die cut. Higher-grade materials tend to die cut cleanly. There is a minimum of frayed edges left behind. A crisp and well-defined part lends itself to intricate and detailed designs thanks to easy stripping or removal of excess material. Material comparisons should also examine the possible use of pressure-sensitive adhesives. Bond strength between adhesives and gasket material is often key to ensuring a long-lasting seal.

Cost: It's not surprising that OEMs often decide first what price they are willing to pay when designing-in gaskets and seals. But pricing may vary considerably among different seal and gasketing materials based on how well they stand up to environmental abuse. Using economics as the key driver may lead to serious performance pitfalls. Less-expensive materials may provide an immediate economic benefit and a short-term performance solution, but may ultimately cause longterm problems if gaskets leak and bring costly vehicle recalls.

OEMs typically conduct initial evaluations to gauge how seal materials perform under select environmental conditions. Once the material passes initial testing and is spec'd-in, OEMs then run long-term durability trials including hose-directed water tests, rain-driven water tests, and spraying. One of the keys to the success in these trials is seal compression. Compression may need numerous tweaks to get a watertight seal.

MATERIAL-PERFORMANCE COMPARISON

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MAKE CONTACT:

Rogers Corp.
(860) 779-5597
rogerscorporation.com