Heavy-Duty Headlamp Applications Need Better Coatings to Battle Hazing
TMC Working With Industry Groups to Improve Specification Options
During the past decade, there has been an increase in the use of custom-designed headlamps in heavy-duty vehicles. Since the designs are equipped with replaceable bulbs, many of the headlamps are often installed for the full life of the vehicle, barring any damage that may occur. Unfortunately, many fleets are seeing evidence of hazed headlamps in the second year of ownership with significant haze and yellowing occurring in the third year. The haze and yellowing of these headlamps frequently diffuses beam pattern of the lamp and reduces visibility. Anyone who owns a late model car likely has experienced a similar situation, but why is this?
Headlamps, fog lamps and driving lamps were for many years made of glass lenses. The light sources were incandescent or halogen bulbs, which had relatively short lives. When these sealed beam devices failed, an entirely new lamp assembly was installed to replace the failed device. Hazing and yellowing didn’t figure into the picture because the lamps didn’t last that long.
Eventually, glass sealed beam headlamps were replaced with plastic-sealed beam lamp assemblies. These plastic assemblies offered many benefits, such as reduced weight and greater impact resistance, but the entire assembly still needed to be replaced when the lamp failed so hazing and yellowing not a problem.
In 1985, a new forward lighting technology emerged called “aerodynamic headlamps” and the National Highway Traffic Safety Administration (NHTSA) developed new specifications for these devices. Aerodynamic headlamps offered great advantages such as integrated aerodynamic styling, replaceable bulbs and longer service life.
Unfortunately, material and coating specifications for aerodynamic headlamps were originally driven by the automotive industry, which did not consider the requirements of the commercial vehicle market. By 2000, aerodynamic headlamps began appearing on commercial vehicles. New designs looked great but the coatings used were not robust enough to endure commercial vehicle applications.
Now that light emitting diode (LED) light sources are being used for headlamps, lamps are lasting even longer. But the performance of the coatings has to match the durability of the light source to take full advantage of the new designs.
Most headlamps today are made of polycarbonate resin and are coated with a single-component, UV Curable coating. However, a two-Component, thermally cured silicone hard coating is a better option. This coating process consists of a primer applied to the polycarbonate lens, then baked, followed by a silicone topcoat layer and baked again. This coating is applied using a more complex process and has many superior performance benefits when compared to single component coatings, such as:
- abrasion and mar resistance,
- improved weatherability,
- extended service life,
- excellent resistance to micro-cracking,
- excellent ultraviolet radiation protection,
- improved thermal, solvent and chemical resistance.
Fleets that operate vehicles in typical on-highway applications should consider spec’ing the two-component, thermally cured silicone hard coat system. This will help ensure their vehicles’ headlamps will last the typical service life expected by the vehicle’s first owner. Because the appearance of both of these headlamp coating systems is clear, it is important that the purchaser obtains a certification from the vehicle manufacturer that selected hard coat of choice has been applied to the headlamps.
TMC has developed a recommended practice — RP 171, “High-Performance Coatings For Forward Lighting On Commercial Trucks” — with spec’ing recommendations for fleets wanting a more robust coating system. There are no SAE standards or recommended practices for coating system performance for either automotive or the heavy-duty industries — only the elements detailed in the Federal Motor Vehicle Safety Standards No. 108 standards. However, TMC is working with the Society of Automotive Engineers (SAE) to encourage manufacturers to offer these premium coating options. SAE’s Lighting Task Force is launching an Improved Coating Durability Task Force at the suggestion of TMC’s S.1 Electrical Study Group, so progress is hopefully being made in this direction.
Better coating options should translate into increased service life, improved visibility and additional safety. Take a look at your fleet’s headlamp lenses. Do they meet your expectations?