Help Me Choose
The selection of passenger windows in a vehicle is a critical decision point that can have a significant impact on vehicle aesthetics, passenger comfort, ridership, operating costs, and safety.
When making a selection, several factors should be considered:
Material type refers to the glazing material used in windows. AROW Global offers four different materials: laminated glass, tempered glass, acrylic, and polycarbonate.
Laminated safety glass is produced by permanently bonding together two custom-cut pieces of annealed (a term referring to the controlled, stress-relieving, cooling process that occurs during glass manufacture) glass with a layer of polyvinyl butyral, or pvb (a resin prepared from a reaction of polyvinyl alcohol and butyraldehyde, manufactured in sheets of various colors and constituent materials) in an autoclave under heat and pressure. When broken, the adherence of the glass to the pvb prevents the broken glass pieces from falling out of the window opening and exposing passengers and technicians to sharp edges.
Laminated glass has fairly low impact and scratch resistance compared to other glass alternatives, however through variations to the PVB selected, many different colors, tints and solar properties are available.Laminated glass is typically more expensive than tempered glass, but less expensive than acrylic and polycarbonate options.
Additional laminated glass characteristics:
- Edges are typically hidden (gasket, sealant or framing material ) to minimize exposure of the glass/pvb interface to the elements
- Laminated glass breaks quite easily relative to other glazing options
- Laminated glass cannot be used for flush glass or frameless type windows which have exposed
- Perimeter edges or frames that do not support the perimeter edges
- Laminated glass can be curved in multiple dimensions
Heat Tempered (Toughened) Glass
In the process of annealing glass (typically conducted after the float glass production), glass is heated to a temperature over 1,000 degrees F and allowed to cool slowly and evenly which results in glass that is slightly stronger than un-annealed float glass. For tempered glass destined for motor vehicle glazing systems, annealed glass is cut-to-size, and subsequently placed in a tempering furnace where it is heated to a temperature high enough to permit a reconfiguration of the crystal structure on a molecular level. The heated annealed glass is cooled quickly under forced air which causes the outside surface of the glass to shrink and compress upon itself, and at the same time causes stretching in the core of the glass where it cooled more slowly.
Alternatively, molten salts can be used to chemically temper glass. Fully tempered glass can bend slightly further than annealed glass before breaking, and has a much higher strength (4-5 times greater). In addition, upon fracture, fully tempered glass “shatters” into small, granular pieces which are less prone to causing lacerations when handled.
Additional tempered glass characteristics:
- Edges can be rounded and polished to create an attractive aesthetic
- The edges of tempered glass are substantially more susceptible to fracture than the surface
- Glass cannot be cut or formed after tempering (will result in shatter)
- Tempered glass can be curved in multiple dimensions with specialized tooling and in a single dimension without specialized tooling
Acrylic or Polymethyl methacrylate (PMMA) is a thermoplastic polymer that is known commonly by various brand names like Plexiglass, Almacoat and Lucite, Acrylic has an impact resistance 5-8 times greater than annealed glass and a density of approximately 50% less. Acrylic has a low scratch resistance and is therefore typically specified with an additional hard coat to provide additional scratch protection. Acrylic is less brittle than glass, but more brittle than polycarbonate and breaks in a pattern similar to that of annealed glass. Acrylic is typically the second most expensive glazing option.
Polycarbonate is a thermoplastic polymer that is easily molded and thermoformed and known commonly by various manufacturer brand names like Lexan, Makrolon and Tuffak. Polycarbonate has an impact resistance 250 times greater than glass and a density of approximately 50% less. Polycarbonate has a very low scratch resistance and is therefore typically specified with an additional hard coat to provide additional scratch protection. Polycarbonate is typically the most expensive glazing option available.
Costs for glazing depend on many factors, and the more “features” (high solar performance, exotic colors, atypical thickness, non-standard shapes or edge treatments) desired, the higher the cost.
Nominal Thickness: 3/16″ (5mm) and 1/4″ (6mm) are typical although up to 1/2″ (12mm) may be considered for applications where high strength/impact resistance is required and increased weight is not a problem.
Material impact resistance is measured by the ability of the material to withstand impacts from various forces and is typically governed by the following ANSI/SAE Z26.1 tests:
- 5.6 Impact, Test 6 (Ball Drop, 3.05m [10 ft.])
- 5.7 Fracture, Test 7
- 5.8 Impact, Test 8 (Shot Bag)
- 5.9 Impact, Test 9 (Dart Drop, 9.14m [30 ft.])
- 5.10 Impact, Test 10 (Dart Drop, Table 2 Heights)
- 5.11 Impact, Test 11 (Dart Drop, 3.05m [10 ft.])
- 5.12 Impact, Test 12 (Ball Drop, 9.14m [30 ft.])
- 5.13 Impact, Test 13 (Ball Drop, Table 2 Heights)
Generally speaking, impact resistance increases for like materials with material thickness. There are various security films than improve impact resistance to varying degrees as well.
Material impact resistance is measured by the ability of the material to withstand abrasion over time and is typically governed by the following ANSI/SAE Z26.1 tests:
- 5.17 Abrasion Resistance, Test 17 (Plastics)
- 5.18 Abrasion Resistance, Test 18 (Safety Glazing Material)
Glass abrasion resistance can be enhanced with special coatings like Guardian Diamond Guard.
With continuous upward pressure on energy markets and fuel costs, there is significant interest in managing vehicle weight. As such, consideration is often given to the weight density of each glazing option.
Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays. It is named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet. Although ultraviolet radiation is invisible to the human eye, overexposure can cause sunburn and some forms of skin cancer. As such, it is desirable to minimize UV transmittance through windows.
In very basic terms, for tempered glass, UV energy transmittance decreases as the % light transmittance decreases. There are certain colors that exhibit lower UV energy transmittance relative to % light transmittance.
Laminated glazing systems eliminate virtually all UV light, (typical % UV transmittance is cited as <1%). Additional options to reduced UV transmittance for tempered glass include options similar to those for reducing solar energy transmittance and include: glass with enhanced properties via manufacture or applied coating, as well as solar control window films.
Solar Energy Transmittance
The basic principle of heat transfer is that heat will always migrate through the glazing to the cooler side. So in the summer heat, the direction is from the exterior to the (typically air conditioned) interior and during the winter from the (heated) interior to the exterior. The amount of energy invested in managing the vehicle environment is greatly affected by glazing selection. Heat is transferred through the glazing by four methods:
Solar Energy Transmittance is defined as the percentage of solar energy permitted to pass through the glazing and has a significant effect on all of these modes of heat transfer. Therefore as the total solar energy transmittance decreases, the net heat transfer decreases.
In very basic terms, solar energy transmittance decreases as the % light transmittance decreases. There are certain colors that exhibit lower solar energy transmittance relative to % light transmittance.
It is often desirable to select a glazing configuration that displays high light transmittance, but does not exhibit the corresponding high level of solar energy transmittance. For this reason, there are various options available to improve the solar performance of the glazing system (often referred to as SMG, or “Solar Management Glazing”), such as exotic PVB options in laminated glass, glass with enhanced properties via manufacture or applied coating, as well as solar control window films, all of which reduce the solar energy transmittance while maintaining high light transmittance.
Glazing systems with advanced solar management properties are often selected with consideration to the following:
- Reduction of heat transfer through the glass may improve passenger comfort and therefore increase ridership
- Improved visibility through the glass (from interior and exterior) without the corresponding sacrifice of increased solar heat load
- Reduced demand on cooling system during summer months
- Time required to drop temperature in the vehicle to target temperature may be significantly reduced during summer months
- Glazing systems with high-performance solar management characteristics are often more expensive
Common colors include un-tinted (ie. clear), various shades of gray, green, and azure (bluish-green); colors for laminated glass can be customized in accordance with pvb offerings, however costs increase substantially for non-typical color selections. Light transmittance (%LT) selection is typically made with consideration to the following:
- Fleet make up; fleet operators often make a selection that matches their existing vehicle fleet
- Desired aesthetics; it is often purported that darker tints convey an improved aesthetic, especially when used with the flush glass style of window
- Desired solar performance; azure colored glass displays the best solar performance (ie. higher light transmittance with relatively lower solar energy transmittance due to the combination of green and blue tints and the corresponding effect on the path of the light during transmission) than the other colors; otherwise lower solar energy transmittance necessitates correspondingly lower light transmittance.
- Different glass OEMs (Guardian, PPG, and Pilkington are prevalent in North America) have products that are nominally the same (color and %LT) however a side-by-side comparison may reveal subtle color differences; it is for this reason that it is good practice to avoid comingling glass from different OEMs on the same vehicle.
Light Transmittance or Luminous transmittance is defined as the percentage of visible light permitted to pass through the glazing and often referred to as %LT. As %LT increases, more light passes through the glass, thus leading to a less encumbered view through the glass. The %LT tops out at approximately 89% for (nominally) clear glass and commonly be as low as 9% for 6mm tempered glass. %LT selection is typically made with consideration to the following:
- ANSI Standards mandate that glazing systems required for driving visibility have a %LT of equal to or greater than 70%
- There is no prescribed minimum %LT for side windows, however the most common configurations are gray 44% LT (tempered or laminated glass) and gray 28% LT (laminated glass)
- Glass manufacturers and laminators cite a tolerance of +/- 1-2% for the glass %LT; while a 5% difference is perceptible in controlled conditions, it is not typically detectable by the naked eye in practice
- Fleet make up; fleet operators often make a selection that matches their existing vehicle fleet
- Desired aesthetics; it is often purported that darker tints convey an improved (prestigious) aesthetic, especially when used with the flush glass style of window
- Desired solar performance; darker tints (typically) allow less solar energy to pass through; exotic configurations using high-performance PVB in laminated glass may permit enhanced solar performance (i.e. less solar energy transmittance) while maintaining high light transmittance
- Higher light transmittance permits better visibility into the vehicle from the outside; this is often desirable especially in the event of an emergency and higher security situations
Color/ %LT Representation
Clear (depicted relative to true (i.e. 100%LT) clear on left)
Gray – Tempered (depicted relative to 89% clear on left)
Gray – Laminated (depicted relative to 89% clear on left)
Green – Tempered & Laminated (depicted relative to 89% clear on left)
Azure – Tempered (depicted relative to 89% clear on left)
Azure – Laminated (depicted relative to 89% clear on left)