Bigger and brighter. Advances in automotive glazing are improving the comfort, safety, and efficiency of modern vehicles. Now, new layers of invisible technology are creating windows that can adapt their appearance and behavior, offering privacy, information, and even entertainment as required.
Jonathan Ward
A modern vehicle contains around 5 square meters of glass, that’s almost twice as much glazing as 30 years ago. The reasons are clear: in addition to their greater size, today’s cars are more likely to use large, steeply raked windshields and glass sunroof designs. Big windows are popular with designers and customers alike, but they create engineering challenges. And the industry’s rapid transition to electric power makes those challenges more acute. “With an electric vehicle, efficiency is all-important,” says Dr. Marcus Frank, Head of Application and R&D Glass at Bühler Leybold Optics. “You want to use every watt-hour in the battery for extra range, not spend it on auxiliary functions.” The most energy-intensive auxiliary function in a passenger car is the equipment needed to keep the passengers at a comfortable temperature. Large expanses of glass make cabin temperature control tricky. In cold weather, glass windows and roof panels conduct and radiate heat from the interior of the vehicle. Heating comes for free in an internal combustion car, thanks to large quantities of waste heat generated by the engine. With a highly efficient electric motor, cabin heating must draw precious extra power from the battery.
At low temperatures, when battery performance is already impaired, heating loads can cut the range of an electric vehicle by 30 percent. In hot climates, the situation is no better. Solar energy passing through the windows can quickly heat the cabin to uncomfortable or even dangerous temperatures. Modern vehicles deal with that heat by using air conditioning systems that distribute pre-cooled air into the cabin. That’s another energy-intensive job: When it is working hard, a 3-kilowatt air conditioning unit can reduce the fuel efficiency of a conventional vehicle by 20 percent.
One way to reduce the energy required for cabin heating and cooling is to make smarter glass. Coatings and films with carefully engineered optical characteristics allow the creation of glazing that filters incoming and outgoing radiation at specific frequencies.
The filters can perform different functions: reflecting heat radiated by occupants back into the vehicle, for example, or minimizing the solar energy entering the cabin by blocking the transmission of light at visible and non-visible wavelengths. Infrared reflective coatings involve ingenious nano-scale engineering. They are usually made as a stack of alternating layers including silver and metal oxides. “The individual layers in these coating systems have thicknesses in a range between 2 nanometers (nm) up to 70 nm; the thickness of the silver layers is around 15 nm,” says Dr. Frank. “But a full stack might include up to 20 individual layers.”
The stacking doesn’t stop there. To improve its strength and preserve the delicate solar control coatings, advanced automotive glazing uses laminated construction, with two or more layers of glass sandwiched together with robust plastic interlayers.
Putting the coated side of the glass inside the sandwich protects it from impacts, abrasion, and corrosion. Laminated glass and solar control coatings are widely used in architectural glazing to make buildings more comfortable and energy efficient. Vehicle glazing uses the same basic technologies, but automotive applications involve some extra challenges, says Dr. Frank: “The automotive industry has strict standards relating to the optical properties of glass, especially in windshields and front windows. They are there to ensure the driver always has a clear view of the environment around them.” And then there’s weight. Extra mass requires extra energy to move it, and that is a particular problem for electric vehicles. To compensate for heavy batteries, carmakers are looking for ways to reduce weight elsewhere in the vehicle. One way to do that is with thinner glass. “Architectural glazing might use 4 millimeters (mm) to 6 mm glass sheets,” says Dr. Frank. “In automotive applications we see sheets as thin as 1 mm.”
Applying precise, complex coating stacks to large pieces of thin, flexible glass requires sophisticated manufacturing technology. That’s an area where Bühler has developed considerable expertise. Its specialist vacuum coating machines used in the automotive sector include numerous features designed for safe handling and effective quality control. They include closely spaced rollers to support the glass as it travels through the machine, and advanced process control. “After every stage in the process, we use in-line spectrometers to check the optical properties of the glass,” explains Dr. Frank. “Our machines use that data to continually adjust their operating parameters. If you ensure every layer in the stack is within specification, you know that the finished glass will perform as expected.”
In the most advanced vehicles, glazing is evolving even further. Automotive suppliers are now incorporating active materials into their multi-layer glass sandwiches, creating products that can alter their appearance and behavior on demand. Polymer dispersed liquid crystal (PDLC) films, for example, allow the tint of a window or roof panel to change under electronic control. That allows occupants to dim their windows for privacy or comfort reasons. Applied selectively, the technology could replace the ubiquitous sun visor with panels that adapt instantly and automatically to shield the driver’s eyes from low sunlight.
Incorporating organic light-emitting diode display technologies into windshields will enable a new generation of head-up displays that allow drivers to see useful information without taking their eyes off the road. Looking even further ahead, autonomous vehicles could create bold new opportunities for advanced automotive glazing. The windshield of the future might transform into a widescreen monitor, allowing car occupants to work, watch videos, or play games on the move.
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