Designing GLASS for Style – Polycarbonates

                                                        Polycarbonates

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Glass is not the designer’s only option to glaze an opening. Lightweight materials may find exciting opportunities in the automotive industry as a means of increasing fuel efficiency. With 75% of fuel consumption relating directly to vehicle weight, potential weight reductions that result in improved price-performance ratio promote use of lightweight materials. One of the key plastics used in automotive sector is polycarbonate (PC). PC has dominated the market for vehicle headlamp covers for 15 years, and now it challenges glass in windows. Plastics are lightweight materials ideal to improve fuel efficiency and design flexibility without compromising on performance or safety. The automotive industry can expect an impressive 6-8% improvement in fuel usage with only 10% reduction in vehicle weight. This translates into a reduction of around 20 kg of CO2 per kilogram of weight reduction over the vehicle’s lifetime. With newer advances in plastics materials, the weight of these parts is bound to reduce. Lighter vehicles facilitate easier braking, reduced collision impact overall vehicle performance and superior driving experience.

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What is polycarbonate?

Polycarbonates are a particular group of thermoplastics. They are easily worked, molded, and thermoformed; as such, these plastics are very widely used in modern manufacturing. They are called polycarbonates because they are polymers having functional groups linked together by carbonate groups in a long molecular chain.

Polycarbonate is a very durable material, and can be laminated to make bullet-resistant “glass”. This polymer is highly transparent to visible light and has better light transmission characteristics than many kinds of glass. The primary advantages of polycarbonates in automotive glazing are lowered weight and associated CO2 emissions reductions along with greater styling freedom and simpler functional integration. The wide choice of styling options with polycarbonates is testing conventional assumptions of automotive window design and creating whole new opportunities for advanced vehicle styling.

Polycarbonate is becoming more common in industry. It is often used to create protective features, for example in banks as well as vandal-proof windows and lighting lenses for many buildings. Other products made from polycarbonate include sunglass/eyeglass lenses, compact discs, DVDs, and automotive headlamp lenses.

PC auto glazing permits parts integration previously not possible. Features such as colored glazing for tinted sunroofs and windows aft of the “B” pillar are now possible along with gaps in the window, sharp corners, smooth corner radii or complex three dimensional shapes. These features are undesirable, unmoldable or cost-prohibitive with laminated or tempered glass. PC auto glazing permits parts integration previously not possible. It is possible to create a roof-module frame and optical panel in one piece.

fisker-karma-atlantic-7

Fiskar Karma Atlantic 7

Molding heating/defrosting elements and fractal antennas into a PC window panel is also possible using in-mold films with preprinted circuitry. This is a versatile material to use, but advances in this application have been slowed by UV sensitivity of the material and its tendency to scratch easily, which makes it less weatherable than glass. If your production run is niche and well-funded, and you are looking to distinguish yourself from your competition, by all means, look at it. This is one material that is position specific. Legally it cannot be used everywhere, it is very expensive once it has been made fully coated for the durability cycle demanded of commercial use, usually 10 years.

The designer must now ask him or herself “What is my total cost versus what can be gained in mass reduction and design freedom?” The main drawback to a PC is weathering, resistance to scratch and scuffing. By the time you apply a hard coat, any potential savings derived from weight is now lost due increased piece price cost which can be as much as 4:1. There are infinite number ways that Design (Styling/Industrial Design) and engineering can and should work together upfront in the process. This can be used in the fixed vent window of, let’s say a Ferrari or framed sidelites for the XL1.

fixed vent

It saves weight, becomes a unique feature, like carbon fibre, but is unrealistic in normal production due to cost vs. value. Value still must be derived.

VW-XL1-door-open

The Volkswagen XL1 uses polycarbonate windows to help save weight, just one of many distinguishing features

The Volkswagen hybrid concept, XL1 plug-in is one such vehicle that makes use of the innovative automotive lightweight solutions mentioned. The Volkswagen XL1 becomes the first vehicle to feature advanced plasma coating technology on two-component (2K) injection molded PC windows. Using polycarbonate resin, which is an amorphous engineering thermoplastic, characterized by outstanding mechanical, optical, electrical and thermal properties, Volkswagen reportedly estimates a reduction in weight in previous all glass applications at 33%, while delivering a high-quality optical appearance and scratch-resistant surface.

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The ability to create an aggressive curvature of the upper portion of the sidelite adds to form and function. Since the upper lite is polycarbonate and stationary there are no additional abrasion issues due to traveling between a door moldings or packaging issues with getting the glass in a confined opening like the door inner and outer envelope. The occupant will generally not depend on that area for general viewing.

VW_front_side_lo_resLooking at the XL1’s fixed and roll-down side windows pictured above, notice that the sidelite is two separate glass lites. The smaller of the two lites is glass and dynamic. The larger stationary lite is framed polycarbonate on its periphery to provide its structural integrity. This is because a polycarbonate side window will billow like a sail once the vehicle is mobile if not supported. The frame around the window adds rigidity, but also adds weight.

The requirements for plastic glazing are met by depositing plasma coating on the parts after molding. This glass-like plasma coating, applied over a hard coat, delivers enhanced weathering, durability, and abrasion performance and enables compliance with global requirements and regulations for plastic glazing applications.

wet_coat

Wet coating

Federal regulations prohibit the use of polycarbonate in an AS1 position (windshield). Federal regulations further state, for these positions must demonstrate the ability to meet ≥ 70% Light transmission (LT) for AS1 (Windshield) and AS2 (Sidelites) applications. In 2005, the U.S. Dept. of Transportation’s National Highway Traffic Safety Administration (NHTSA) confirmed that polycarbonate may be used in vehicle areas specified for Item 2 (AS2, AS3) glazing (safety glazing material for use anywhere in a motor vehicle except windshields), provided that the product satisfies the existing performance standards for Item 2 glazing. Questions persist regarding retention of the necessary transparency over the useful lifetime of the vehicle. A test has been developed for coated polycarbonate automotive glazing to measure their resistance to windshield wipers. This test enables simulation of the actual stress exerted on the wiped glazing far more realistically, accurately, and with a higher reproducible quality than is possible with the Taber abrasion test stipulated in the regulations. Initial tests have shown that, when dirty panes with a polysiloxane coating are wiped under primarily wet conditions, they exhibit virtually no clouding that is visible to the naked eye – even after 30,000 double-wiper cycles.

 

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Wiper performance is critical in the design, engineering, and testing acceptance of a windshield for a saleable vehicle.

PC glazing technology continues to enable the development of advanced, lightweight window designs for the automotive industry. Polycarbonate glazing may enable weight reductions up to 50% versus comparable glass materials, depending on the complexity and design of the window part, but as you saw with the XL1 33% will probably be the realistic range.

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Toyota’s two-panel Prius V moonroof made of polycarbonate plastic.

In conclusion; Polycarbonate is sneaking into the western auto industry as glass replacements in non-strategic areas. Hyundai has used it on concept cars and Bugatti on a new lightweight targa top for the Veyron (saving 13 pounds).

Bugatti Targa top

Bugatti Targa top

 The most prominent mainstream users of polycarbonate right now are Mercedes (which puts it into fixed Smart side windows, as well as those roof panels) and Toyota, which is introducing it on the V, the new station wagon version of the Prius.

GM Volt

General Motors also introduced it in the concept Chevy Volt roof panel.

New legislations in Western countries have been introduced to limit pollution caused by vehicles. European legislations such as EC 715/2007 (Euro 5, 6 standards) and U.S. regulations such as EPA 2010 are applying stricter rules to reduce CO2 emissions from new cars. The United States government announced a plan to increase fuel efficiency by more than 5% pa starting in 2012. These requirements necessitate lighter vehicles and improved fuel efficiency. European regulation currently permits polycarbonate (PC) use in all automotive glazing applications except the windshield. In the United States, the US Department of Transportation’s National Highway Traffic Safety Administration (NHTSA) has approved the use of PC for all non-windshield glazing applications provided that it meets all existing auto glazing specifications for laminated glass as prescribed in the Federal Motor Vehicle Safety Standard (FMVSS) No. 205. This includes < 2% delta haze in the all-important Taber 1000 abrasion test. However, uncertainty over long term weathering and compliance with the ANSI regulation has held back adoption of PC glazing in the US.

 

Technical data on polycarbonates

Polycarbonate has:

Density = 1.20 g/cm3

Use range from -100°C to +135°C

Melting point around 250°C

Refractive index equal to 1.585 ± 0.001

Light transmission index equal to 90% ± 1%

Poor weathering in an ultraviolet (UV) light environment

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