LED lighting technology has the potential to lower energy consumption and reduce maintenance costs on outdoor installations such as area lighting and parking lights, street lights and other “difficult to reach” outdoor locations. When operating in controlled temperatures and weatherproof environments, solid state LED lights are designed to operate for 5 to 10 years. Outdoor LED housings and lighting enclosures are constructed from powder coated aluminum to resist the effects of direct sun and weather for many years. However, if the full payback on the investment in outdoor LED lighting is to be realized, the gaskets required to protect the solid state electronics must function for the 5 to 10 year life of the product.
LED lighting enclosure gaskets, or housing gaskets, are specified to seal exterior aluminum housings. Housing gaskets must seal out rain and moisture for years and function over broad temperature ranges. LED lighting enclosures typically have the International Protection or Ingress Protection rating of IP66 — defined as protected against high pressure jets of water from all directions — with limited ingress permitted. For reference, an IP66 rating is equivalent to a NEMA 4 or 4X rating by the National Electrical Manufacturers Association.
Traditional Gasket Materials
Cost has been a major consideration when designing gaskets for the lighting fixture industry. The materials specified have generally been closed cell EPDM (ethylene propylene di-monomer) and closed cell neoprene sponge. These traditional gasket materials are provided in extruded profiles, die cut gaskets and slit-to-width rolls with or without adhesive backing.
EPDM sponge has been used on incandescent lighting fixtures for many years, and generally provided good value as a gasket material on fixtures where some moisture incursion was not critical to the performance of the lighting fixture.
Closed Cell EPDM (ethylene propylene di-monomer) sponge is molded into buns and skived or split into sheets from .062" through .500" thick. Closed cell EPDM sponge is available in soft, medium and firm densities. Although EPDM is good for UV resistance and aging, the compression set and sealing properties of closed cell EPDM are not considered adequate for long term sealing of enclosures rated for IP66.
Closed Cell Neoprene sponge is molded into buns and skived or split into sheets like EPDM sponge. Closed cell neoprene is frequently blended with other elastomers such as styrene-butadiene rubber (SBR) and EPDM. Blended neoprene sponge is prone to hardening due to aging and may take a permanent compression set, rendering it unreliable for long term outdoor LED lighting installations.
PORON® Microcellular Urethane Foam is cast into continuous rolls from .032" to .500" thick. PORON has excellent compression set resistance. Despite its open cell structure, if deflected approximately 35 to 50%, PORON gaskets may permit an enclosure to pass a NEMA 4 wash down if the edge of the gasket is not directly exposed to pressurized water. However, intermittent moisture contact and freezing may cause PORON gasketing to deteriorate over time if exposed to long term outdoor conditions.
Table 1 shows four gasket materials that function very well in many gasket applications. However, the long-term performance requirements of the outdoor LED lighting industry may disqualify these materials on applications where the long-term compression set performance is critical to the sealing integrity of the enclosure. The outdoor temperature excursions in direct sunlight will continue to degrade long-term sealing performance in these traditional gasket materials.
Silicone Rubber Materials
The major silicone rubber producers, such as Momentive (formerly GE Silicones), Dow, and Wacker Silicones to name a few, have enhanced the performance properties of this versatile elastomer. As a percentage, the material costs of silicones have not increased as much as some of the traditional gasket materials since the recent energy cost increases.
Silicone rubber has the combined properties of resilience, high temperature stability and general inertness that are unavailable in any other elastomer. Silicones are generally unaffected by extended exposure to temperatures from -100°F to 500°F, and are also resistant to aging and degradation from sunlight and ozone. Silicone rubber is a versatile elastomer, as it can be processed into silicone foam, closed cell silicone sponge, and solid silicone rubber from either a traditional gum base or two-part liquid silicone base.