Although PC has been used for many years in auto headlamp lenses, its progress in window glazing has been slowed by its its UV sensitivity as well as its tendency to scratch easily, which makes it less weatherable than glass. To improve abrasion resistance of PC auto glazing, major polymer companies have developed various coating technologies.
Heat-cured wet formulations are the traditional coatings applied to small-area PC glazing. But wet coating methods do not always provide adequate scratch resistance in large-area glazing such as windshields. One newer dry coating method that offers sufficient hardness is plasma-enhanced chemical vapor deposition (PECVD) of various organosilicon coatings. In PECVD, an electric field enhances formation of ionized species in the coating chamber, allowing much lower coating deposition temperatures than other methods of chemical vapor deposition. As a result, PECVD allows high rates of deposition without thermal damage to the polycarbonate substrate.
PC glazing must also be protected against UV light, which can turn it yellow. Hard coats of silicon are the most common approach for UV protection. PC auto glazing systems also contain layers with adhesion promoters, as well as decorative and printed layers. The underlying polycarbonate can contain various dyes and additives to improve the aesthetics and performance of the part.
For glazing applications, PC layer must be moulded under low-stress conditions, as moulded-in stresses can degrade optical clarity, cause crack formation and hinder adherence of coatings. Keeping stresses to a minimum becomes a greater challenge in moulding PC windshields, sunroofs and other large-area glazing. Injection moulding techniques, that operate under low pressure have been developed for low-stress molding of large-area PC glazing. These techniques generally operate under low pressure and employ platens that keep width variations in moulded parts to a minimum. Beyond roof modules and windshields, PC glazing has made important inroads in other areas. These include the rear quarter windows, fixed side windows, front quarter windows and rear-door fixed window sections. Polycarbonate's other drawback is its cost vis a vis glass- as polycarbonate production technology is based on oil based feedstocks, it will always be more costly to manufacture than sand-based glass. However reducing the weight of vehicles by the use of polycarbonate for glazing will significantly reduce fuel costs for consumers, especially as more of the car's surface is covered by glazing.
In the near future, automotive glazing is poised to become a technological marvel as advances in design, nanotechnology and materials science will allow car makers to create glazing profiles with never-before-seen capabilities. Until now, the promise of Polycarbonate has been blunted by limits of abrasion-resistance and weatherability. However, plasma technology is on the verge of offering a solution in the form of a glazing layer with a glass-like plasma topcoat with superior abrasion properties and a 10 year weathering life. Nanotechnology, using nanoscale additives to create compounds with entirely new properties, will also play a significant role in tomorrow's emerging technology, for improving the scratch-resistance of topcoats with microparticles of silicon dioxide. Another feature ready for adoption by auto makers is a hydrophobic windshield coating. This safety innovation prevents water drops from splattering when they hit the glass, as the drops remain spherical and simply fall off the vehicle. Another innovation, a quick-reacting suspended particle device (SPD) works through a thin film sandwiched in the interlayer that conducts low-voltage electricity. As the current passes through it, suspended particles either gather or disperse, allowing more or less light to pass, ranging from totally light to completely dark when the car is shut off and the current is cut. Also integrated in the interlayer will be a nearly invisible set of antennas no thicker than a strand of spider's web. The antennas will provide communications links for Telematics devices for Internet, cell phones, navigation, and entertainment.
With its many benefits, PC glazing is poised to emerge from its niche role and make a much larger imprint on the automotive scene. The biggest stimulus for future growth will be the energy saving aspects of the technology as well as the positive styling aspects combined with the labor-saving features of parts integration.