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Characteristics of Automotive Glass
The worlds glazing manufacturers are constantly striving to deliver products that provide the best possible technological solutions to meet automobile manufacturers’ and end-users’ needs from a comfort, safety and security perspective, whilst at the same time helping the industry to meet its climate commitments.
Safety and Security
Glass plays an important role in a car’s ability to offer safety and security to its occupants. The automobile industry is ever increasing its use of laminated glass, which is literally a “sandwich” of two glass sheets with a thin but resilient plastic layer between them. This plastic layer is polyvinyl butyral or PVB. It can be made in two variations, standard and acoustic. Laminated glass can now be found in sidelites and in panoramic roofs on intermediate priced automobiles as well as luxury offerings. The layers are bonded to each other under controlled heat and pressure by using an autoclave.
If the glass suffers an impact, the glass layers may fragment but, depending on the force of the impact, the plastic interlayer will hold. This type of construction is mandated for windscreens and is becoming increasing popular in sidelites for sound attenuation. This safety feature enables the vehicle to absorb some of the energy of the impact and helps to prevent the ejection of the driver and passengers and the penetration of objects in case of an accident. Moreover, after an impact the resulting glass fragments tend to remain attached to the plastic interlayer, reducing the hazards of sharp projectiles during an accident. And the fact that windows made of laminated glass will tend to remain in their frame greatly improves the performance of the side airbags, providing the necessary support during inflation.
The #1, 2, etc. are surface designations. #1 is the outside or class ‘A’ surface, #2 is the surface adhering to the PVB, and same with #3, #4 is the surface inside the cockpit/cabin.
A different kind of passenger safety factor is provided by UV (ultraviolet) filter glass, which blocks harmful UV rays to protect skin and eyes (as well as the materials of the car’s interior). UV filter glass blocks >99% of harmful UV rays from entering the vehicle, keeping the driver and passengers’ safe and greatly extending the life of upholstery and fabrics. UV filter technology can be incorporated into laminated glass for optimal safety and security.
Visibility and Visual Comfort
Good visibility conditions are critical to driving safety, and visual comfort (for example light reflections and glare) is part of visibility. The driver’s field of vision is influenced by the light the windscreen allows into the cabin. Indeed, bright sunlight causes glare, rain smears the windscreen, condensation clouds the view and ice buildup blocks out almost everything. Today’s vehicle glazing technologies can deal with all of these. Windshield installation angle plays a significant part in glare, specifically veiling glare, which is the reflection of objects on your instrument panel.
This is mitigated to great extent by having the instrument panel (IP) top cover, one color; Black. Black absorbs, and does not reflect, it also enables the automaker is carry only one part number in their system, and reduces the complexity of inventory control. This also eliminates matching the various vehicle interiors.
Reflections on the inside of the windscreen can disturb the visual comfort and concentration of the driver. This is compounded by lighter IP top covers that may still exist. Anti-reflective glass, which is treated with a special coating, can reduce these reflections by up to 40% at a reflective angle of 60 degrees, and also helps reduce glare. Reduces exterior and interior visible light reflectance to less than 2%
- Transmits more than 90% visible light
- Superior safety, security and acoustic performance
- Blocks over 99% of UV transmittance
- Durable pyrolytic (hard coat) surface, bendable and temperable
In the field of Nanotechnology, hydrophobic coated glass is often used in automotive and applications. Hydrophobic glass is coated with a polymeric layer that repels water, causing rain or other water to run off the glass without leaving droplets and also greatly improving the effectiveness of windshield wipers in completely clearing water from the windscreen. This leads to significant increases in visibility under wet conditions due to less surface contact with the glass. The early claims from the suppliers were that it would eliminate the use for windshield wipers. This would never be proven successful.
Hydrophilic glass is characterized by a contact angle of ≥105°, whereas Hydrophobic is ≤90°
Electrically heated glass was first developed in World War II to prevent aircraft windscreens from frosting over and obscuring visibility. Since then, this technology has been become almost universally used in aviation. It has even reached supermarket freezers, where glass doors must remain clear for customers to be able to see the merchandise. In order to electrically heat glass, a microscopic Tin (II) Oxide coating is applied to a pane of ordinary float glass. This coating is perfectly transparent and conducts electricity. An electrical current is supplied by two busbars located on opposite sides of the glass. The electrical resistance of the Tin Oxide coating produces heat energy. This heat radiates from the glass in the form of infrared energy. The busbars are typically connected to a power control unit that regulates the flow of electricity and thus the temperature of the glass, much the same as your backlite defogger/defroster system.
Electrically heated glass maintains a steady and consistent temperature across the entire surface, and heat radiates off the glass in only one direction: toward the object or area to be heated. In addition, this technology imparts beauty and elegance to glass. The glass can also be coloured or etched with designs in order to complement the appearance of a room or appliance. At one time the Corvette looked at having the grid pattern created to look like the Chevy ‘Bow Tie” symbol.
Electrically heated glass is a laminated glass, incorporating almost invisible electrically-conductive wires. It comprises two or more sheets of glass interlaid with one or more films of polyvinyl butyral (PVB). This assembly combines comfort with safety, whilst preventing condensation. Electrically heated glass is suitable for any circumstance where there is high moisture content in the air and where the difference between the internal and external temperature may lead to condensation risk. In heatable glass, invisible wires or an invisible metallic coating, in each case embedded into the interlayer of the laminated windscreen, respond to an electrical charge by raising the temperature of the glass, in cold weather speeding the removal of any condensation or ice and then of course preventing any further condensation or ice buildup. The potential failure mode to embedded wires is moisture intrusion and delamination. Depending on how quickly the elements heat up, there is always a risk of thermal shock which could lead to windshield a failure mode.
Today’s increased use of glass in automobiles means that, without the right technologies, more solar energy will penetrate the interior of the vehicle, causing high temperatures. For example, when the outside temperature is 80.6°F/27°C, without anti-heat technologies, a car interior can heat up to 136.4°F/58°C in few minutes. Combating this solely by way of air-conditioning alone would be an environmentally unfriendly approach, leading as it would to higher fuel consumption and CO2 emissions.
In recent years, the industry has developed solar control glazing. This type of glass is specially engineered to reduce vehicle cabin by up to 44.6°F/7°C or 46.4°F/8°C temperature, and reduce heat penetration in vehicles exposed to solar radiation by over 25%. In this way, a significant amount of energy is saved from powering air-conditioning units while passengers’ comfort is safeguarded.
Solar control glazing
Solar control glazing has a coating that allows it to reduce the transmission of solar heat from the external environment while still permitting the optimal light transmission essential to ideal driving conditions. This type of glazing can reduce by over 25% the heat penetration in vehicles exposed to solar radiation, thus reducing the need for the use of fuel-hungry cooling systems. Dark-tinted glazing and switchable glazing also have solar control properties which can reduce the amount of solar heat entering the vehicle’s cabin.
The last decade has seen significant improvements in the design and application of solar control glazings in vehicles. The major drivers are passenger comfort, minimising the degrading effects of the sun’s radiation on interior trims and fabrics, and improving fuel consumption by lowering the load on the vehicle’s air-conditioning unit. Additionally, vehicle styling is impacted by the choice of solar control glazing, from dark tints in the rear of the vehicle, to the differentiated colour of infrared reflective windscreens.
Solar radiation is partly reflected, partly transmitted and partly absorbed by a glazing, the degree of each depending on the glazing fitted. Most tinted glasses can selectively absorb the sun’s energy, whilst glazings with specially designed coatings can be used to reflect solar radiation.
Absorbing Solar Control
Green optimised solar absorbing glasses are providing significant improvement to occupant comfort. The products, designed for the European and North American markets respectively, reduce the heat entering through a vehicle’s glazed area by roughly 20% when compared to a car equipped with standard tinted glass.
Due to vehicle safety legislation, requiring adequate light to provide clear driver vision, use of these dark tinted glazings is restricted to rear passenger compartments and to roof glazings.
The privacy glazing segment, originally developed for the North American market, now finds global applications. This grey glass has a low light transmission and an even lower transmitted energy value. It reduces the transmitted heat to the interior of the vehicle by approximately 65% in comparison to an optimised green glass used in the front of the vehicle. It also prevents more than 95% of ultraviolet radiation from entering the passenger compartment. This glass is increasingly the choice for SUVs and MPVs, in addition to being suitable for roof glazing applications.
One particular manufacture makes a green/blue portfolio of glasses that blends with the green front door glass to allow both a design choice and an improvement to solar loading. The glass has a light transmission in the range 26-45 % over a 3-5 mm glass thickness range. In a vehicle equipped with this technology the heat entering the rear of the vehicle is reduced by 45% when compared to that at the front of the car.
Reflective Solar Control
Many glass manufactures possess the technology and capability to deliver a coated windscreen product that reflects more than 30% of the sun’s energy (more than five times that of a standard glass). This particularly benefits the new generation of vehicles that are commonly designed with larger glass areas. In addition to coated glass product, automotive glazing also reflects the sun’s heat. Some products are constructed from an infrared reflecting film, laminated between two pieces of glass. The product can be used in all vehicle apertures.
Acoustic laminated glass technologies have been developed to combat levels of noise penetration in vehicles. Noise mainly comes from the sound of the car’s engine, air and other environmental factors (e.g. other cars, roadworks, etc.). When using specific acoustic laminated glass, an improvement of some 3-5dB can be achieved, which is quite noticeable to the ear. Lower noise levels mean that the driver will become less tired, and make communication inside the vehicles more comfortable, thus contributing to overall safety.
Today’s automotive engineers are on an unending quest to make vehicle interiors quieter and more comfortable. In the auto industry, this translates to reductions in noise, vibration and harshness, or NVH.
Tests show noise is transmitted through the vehicle’s glazing and comes from several sources:
• Air-pressure fluctuations against the windshield at highway speeds
• Air impinging on the edges of the vehicle’s body panels
• Other vehicles and environmental noise
• Road noise and vibration; this is also transmitted through the body structure.
One way to reduce noise is to increase the thickness of the glass. This provides marginal noise reduction and adds weight to the vehicle. This decision will also impact the run channel and overall packaging space if the intended use is a door. The complete packaging envelope may change. A new power unit may be needed to accommodate the added weight. A windscreen is better addressed by an asymmetrical application with an acoustic graded PVB.
More recently, PVB suppliers have developed vinyl with improved acoustic-damping qualities. These new interlayers are particularly efficient in the 1,500 Hz to 5,000 Hz frequency range.
The decibel reduction typically is 2-3 dB overall. Although not large, it results in a quieter vehicle interior that allows people to converse without shouting.
Acoustic vinyl also enables vehicle designers to decrease the thickness of the windshield without negatively affecting noise levels. This is particularly important as automotive engineers try to shave weight off vehicles to make them more fuel efficient. Acoustic vinyl windshields meet all of the same federal safety specs as regular vinyl.
Acoustic vinyl is also suitable for use in sidelites to further improve interior noise levels. Other benefits include:
• Security or theft deterrence. It takes longer to break through a laminated glass window, therefore reducing “smash and grab” theft.
• Improved health. Laminated glass allows for lower ultraviolet light transmittance, especially beneficial to those with melanoma.
• Interior protection. Lower UV transmittance helps preserve vehicle interiors and slow color fading.
• Vehicle weight reduction. The vinyl interlayer’s lower density allows vehicle designers to maintain the same overall window thickness while reducing the weight of the window compared to monolithic tempered glass.
The main downside to the use of acoustic vinyl is its cost. However, the difference in cost between acoustic vinyl and regular PVB will likely diminish as volumes increase. There are 18 vehicle makes and 40-plus models sold in the United States with acoustic windshields. The types of vehicles sporting acoustic windshields stretch across all sizes and price ranges; their use is not limited to luxury vehicles. In fact, the 2008 Ford Focus compact car will have an acoustic windshield as standard equipment. These windshields also are available standard on Ford Escape, Mariner, Expedition, Navigator and Lincoln MKX vehicles.
Acoustic glazing consists of two or more sheets of glass, bonded together with one or more acoustic interlayers. The interlayers act as a noise-dampening core, weakening the sound as it travels through the glass. Acoustic laminated glass also benefits from all the safety and security properties of standard laminated glass. Moreover, acoustic windscreens reduce the need for heavier glazing, which allows car manufacturers to reduce vehicle weight.
*This will not affect commercial trucks where the need is more in mitigating the damage from stone impingement than noise. This is achieved by using an asymmetrical windshield.
Reducing the noise level of a vehicle cabin is an important factor for passenger comfort. It is also important for safety concerns because noise is believed to reduce driver efficiency over time. Sound transmission loss through a glazing system can be induced by mass, stiffness, and damping. Sound Transmission Loss (STL) represents the amount of sound, in decibels (dB), that is isolated by a material or partition in a particular octave or 1/3 octave frequency band. In reality, there is a limit for increasing the mass, or the thickness of glass, and also for controlling stiffness due to the limit of glass design. To increase damping effectively and obtain good sound attenuation, laminated glass is used with a viscoelastic plastic interlayer that changes vibration energy into heat energy.
The human ear can hear sounds in the frequency range from 20 Hz to 20,000 Hz, and it is especially sensitive to sounds in the range of 1,000 Hz to 4,000 Hz. In case of a 4mm thick single glass plate, the frequency range for its coincidence dip is around 3,000 Hz. Wind noise comes from air blowing across the car body and is transmitted through glass parts, which then increase the interior noise level. This noise mainly includes high frequency sounds around 1,000 Hz to 5,000 Hz. Multi-layer acoustic PVB interlayer shows the highest STL value over monolayer acoustic PVB and normal PVB interlayer, especially in the coincidence dip range of 1,000 to 4,000 Hz. Therefore, high frequency sounds like wind noise from outside is effectively blocked by a multi-layer acoustic PVB interlayer. Special acoustic PVB interlayer, acoustic film has been adopted by several automobile manufacturers not only for high-end luxury vehicles but also for mid-class passenger cars. It achieves a quieter interior. Because of these acoustic properties, the manufactures can reduce the weight of the vehicle by using thinner (normally acoustically inferior) glass, which in turn results in better fuel efficiency.
Style and Taste
A car is, for many people, a reflection of their desired image or style, expressing an element of who they are and what they enjoy. And the car’s glass is an important visual component of this. Some people prefer the privacy of darkly tinted glass (or the perceived “VIP image” that goes with it), others like more light and openness. Many of these selections for the average driver are predicated upon cost.
Visual light and aerodynamic design requires glazed surfaces of increasingly complex geometries, which can be a real manufacturing challenge to ensure durability and perfect vision. Manufacturers are therefore continuously developing advanced glass shaping techniques which allow for the bending and shaping of the glass to offer car makers the freedom to achieve their design and style concepts whilst keeping a watchful eye on ever increasing tooling costs.
Colorants and colours
Some of the most-used colorants and the colours they produce are listed below:
Iron – Green, brown, blue
Manganese – Purple
Chromium – Green, yellow, pink
Vanadium – Green, blue, grey
Copper – blue, green, red
Cobalt – blue, green, pink
Nickel – yellow, purple
Titanium – purple, brown
Cerium – yellow
Selenium – pink, red
Gold – Red
Sulphide – yellow
Carbon & Sulphur – amber, brown
Dark tinted glass keeps the interior cool by absorbing solar energy and at the same time offers privacy for passengers in the back of the car (dark tinted glass is not permitted for the driver and forward-passenger windows, or of course for the windscreen). Dark tinted glass is available in several colours and each colour in several shades from lighter to very dark. The values shown are percentages of light transmission.
Switchable glazing, which responds to varying electrical conditions with correspondingly different colour properties, allows for the creation of a “customized” interior ambience. Via a simple dial, a driver or passengers can control the amount of light entering the vehicle – blocking anywhere from 4% to 40%. Heat from sunlight can be reduced by anywhere from 2% to 20%. One can select a lighter tint on cloudy days, and then darken it when the sun reappears, or simply change the lighting to suit one’s mood or create an appearance appropriate for the occasion.
Glass in photovoltaic applications
Photovoltaics – how it works
Conventional photovoltaic cells are electronic components based on silicon, a semiconductor material. It absorbs photons (constituents of solar rays) converting their energy into electrical particles (electrons). These charges are collected on the front and rear of the cell. The tension this creates generates an electrical current. Each cell only produces a limited quantity of electricity (≈ 0.5 volt). The cells are assembled in modules and panels, which are themselves connected in series.
Photovoltaic technologies are used to convert solar energy directly into electricity. There are many different technologies available to suit various requirements, from domestic systems to utility scale. Photovoltaic panels come in various shapes and colours offering flexibility for design integration. Unfortunately in the automotive industry, the return on investment (ROI) has only been around 10%, not sustainable for production.
The most common photovoltaic technology is based on crystalline silicon solar cells. In this application glass acts as a protective outer layer, while transmitting the solar light to the photovoltaic cells interconnected underneath.
Other photovoltaic technologies include thin film where solar cells are deposited as a sequence of thin films on glass. In these technologies, transparent conductive coated glass can be used as the front glass upon which the films are grown. The conductive coating not only allows light through to the photoactive films, but also conducts the generated electricity out of the modules.
More books and blogs from Lyn R. Zbinden: Glass Engineering: Design Solutions for Automotive Applications: http://books.sae.org/r-433/,