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Today's automotive design engineer must take into account a mind-boggling array of considerations when planning the features and layout of an automobile. In addition to vehicle performance and efficiency, passenger comfort and safety continue to be leading concerns in automotive design. And while seat belts and airbags may get the big news headlines, there are many other areas where the right design, and the right material, can make a big difference in making passengers comfortable, keeping them safe, and/or extending the life and performance of the vehicle.
Many of these details are not readily apparent as you sit in the driver's seat, but nevertheless play a key role in the overall comfort, safety, and quality of the vehicle. One of these unsung heroes of automotive design is thermoplastic polyurethane (TPU), a versatile material that plays an invisible role in a number of technologically advanced components for cars of all makes and models. JPS Elastomerics Corp., a global supplier of Stevens Urethane film, sheet, and tubing, has seen a marked increase in the number of automotive applications for TPU, and anticipates further growth as designers respond to consumer demands for comfort, quality, and safety.
Thermoplastic Polyurethane (TPU)
Thermoplastic polyurethane compounds possess a combination of properties unavailable in any other thermoplastic material, and provide design engineers with unparalleled flexibility. Polyurethane combines the best properties of rubber and plastic, without the weaknesses inherent in plasticized vinyl films. Because they contain no plasticizers, TPUs are not subject to the brittleness and other problems caused by plasticizer migration. Urethane is easy to work with, and is readily modified to suit a particular application through the addition of colors, stabilizers, lubricants, and other additives.
Noted for its high performance and general overall toughness, urethane's unique characteristics make it an extremely versatile material that outperforms many other thermoplastics. Low-temperature flexibility, high tensile strength, abrasion and chemical resistance, elasticity and elongation, and a wide range of durometers (hardnesses) are all important performance features available with TPU.
The number of different ways in which urethane can be fabricated further enhances the material's design versatility. TPU can be heat or RF sealed, vacuum formed, adhesive bonded, and laminated to any number of substrates. Film and sheet can be fabricated with urethane tubing to provide the benefits of urethane to an entire product system, such as in linked bladder applications found in lumbar support systems.
Typical uses for TPU include bladder systems for inflatablelumbar supports and moisture and air barriers for roof liners and door cavities. A thin film laminated to car seat fabric allows it to be vacuum formed prior to foaming the cushion. It also prevents the cushioning foam from penetrating the seat fabric and acts as an adhesive between foam and fabric.
With violent attacks such as car-jackings and drive-by shootings, break-ins and vandalism on the rise, particularly in metropolitan areas, consumer concerns about security are growing quickly. In addition, today's luxury cars are full of electronic communication devices and gadgets, which offer tempting targets to thieves and vandals.
In response, many automotive manufacturers are offering specialized window glazing systems as optional extras on some top models which are virtually impenetrable to forced entry attempts, and will even stop small caliber bullets. Although long available through specialized armored car converters, until recently these security windows have been unfamiliar to the consumer at large.
Security glazing is made from laminated layers of glass and plastic, usually a polycarbonate. The adhesive in these laminations is critical to ensure a strong, durable bond, with no delamination once it is installed. Visibility is also an issue, since the windows have to be as clear as regular glass.
Aliphatic-grade thermoplastic polyurethane film is the only adhesive which is both compatible with polycarbonate and which will maintain its physical properties over a wide range of temperature extremes. Aliphatic TPU film allows the polycarbonate sheet to expand and contract over temperature changes within the confines of the glass layers, which do not change dimensions over temperature. The strength of the TPU polymer also enhances the impact resistance of the finished laminate, and keeps the glass attached to the polycarbonate after impact. Aliphatic TPU offers both ultra high optical clarity, so vision is not distorted, and long term light stability, which eliminates problems with yellowing from UV rays.
A popular feature available in many luxury car models is an inflatable lumbar support built into the seat. Typically a bladder and air tubing system made from TPU, this feature allows the passenger to custom set the amount of support provided. Performance requirements demand extremely high burst strength to protect against over inflation; durability to withstand frequent changes in pressure and inflation; flexibility over a long working life; and reliable performance under temperature extremes, since a car's interior temperature can range from -20űF (-29ˇC) in winter to 200űF (93ˇC) in summer. TPU offers all of these attributes, and its tensile strength permits the use of thin films which provide added comfortable support for the driver and passenger, where other plastics would need a thicker layer for similar performance. TPU films are also specified for their ease of weldability and consistency of production.
Noise, both inside and outside the car, is yet another area automotive designers focus on to improve the driving experience. Reduced road noise, engine noise and vibration, and air noise means a quieter, more enjoyable ride. TPU films contribute to noise reduction in several key applications.
For example, polyurethane foam, an excellent sound insulation material, is used to fill cavities in doors, instrumentation panels, under seats, and in other areas. However, it is prone to attracting dirt, water, oil, road salt and grease, which impairs its insulating ability. To eliminate this problem, the foam is encapsulated with a thin layer of TPU film, which protects the foam and helps retain its sound deadening properties. TPU film is the only material currently available that is tough enough and durable enough for such an application. Its strength and durability allow a very thin film to be used, less than .001-inch (.025 mm), which protects the foam while still permitting sound waves to pass through and be absorbed by the foam.
In addition to concerns about interior noise, new regulations mandated by Europe's unified market call for reduced exterior noise pollution by cars, trucks and buses, caused chiefly by the vehicles' engine. The same type of polyurethane foam encapsulated by TPU film used for interior applications, is also being used for insulation elements inside the engine bay. Harsh under-the-hood operating conditions require a material with resistance to high heat, oils, and chemicals. TPU is the best choice for these applications, and new products in development will offer increased performance under these conditions.
The automotive industry also uses TPU film as a barrier to air and water. For example, a thin layer of TPU film adhered to the inside of a door cavity will prevent water penetration of the interior door panel. With more and more electronic components such as window and lock mechanisms located in car doors, this type of moisture protection is essential. TPU films are also used in roofing liners, where they act as both an adhesive and an air barrier. They ensure that interior fabrics bond well to the roof liner, and at the same time, by preventing air from passing through, keep the fabric clean. TPU films are used in a number of other applications as a heat seal film for laminating fabric to foam, metal or other plastic substrates.
Adhesion is critical in specialized break-in proof window glazing systems, typically made from laminated layers of glass and polycarbonate. Aliphatic TPU is the only adhesive that is both compatible with polycarbonate and that will maintain its physical properties through temperatrue extremes. The plastic can then expand and contract withing the glass layers, resulting in no change in dimension over these temperatures. Ultra high optical clarity means vision is not distorted and long-term light stability eliminates yellowing from UV rays.
In addition, TPU films have long been used in the manufacture of car seating. A very thin film laminated to the back of the seat fabric allows the fabric to be vacuum formed into a mold prior to foaming the cushion. The film then serves two distinct yet important purposes: it prevents cushioning foam from penetrating into the seat fabric, and it acts as an adhesive between the foam and the fabric.
Paint chip protection is another application where TPU film serves as a protective barrier. Here, a very thin layer of film is applied over the paint in areas sensitive to chipping by stone and other road debris.
The use of TPU film and sheet is increasingly important in automotive design. As advances in TPU technology further expand its capabilities, new developments such as TPU alloys, flame retardant and breathable materials will open up even more applications. Faced with increasingly demanding design criteria, automotive design engineers will benefit from knowing how TPU can help solve their most challenging performance issues.
JPS Elastomerics Corp. designs and manufactures polyurethane film, sheet, and tubing, sold under the Stevens brand name. Stevens film and sheet can be manufactured in thicknesses from .001" to .125" and in widths from 5 inches. Tensile strength up to 10,000 psi assures reliability and durability over a long working life, and urethane film and sheet can elongate significantly and still return to its original dimension with no loss of "memory." Stevens Urethane is available in hardnesses from 75 to 95 Shore A, and in a range of standard and custom colors. Various opacities and surface textures are also available.
Based in Holyoke, Mass., JPS Elastomerics manufactures a full range of high-performance urethanes for use in a wide variety of "can't fail" applications for the consumer, medical, commercial, and industrial markets. JPS Elastomerics is the only company in the world that offers urethane in blown film, extruded sheet, tubing, cord, and profile form.
For additional information, or a free brochure, write, call, or fax JPS Elastomerics Corporation, Nine Sullivan Road, Holyoke, MA 01040. Phone: (413) 533-8100. FAX: (413) 552-1195. Internet: www.jpselastomerics.com.
In Europe, contact Polymar, Donksesteenweg 13, B-2930 Brasschaat, Belgium. Phone: (03) 652-04-48. FAX: (03) 652-11-09.
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