It doesn't take much observation to conclude that car exteriors have changed dramatically from the days when chromed trim dominated. Today's sleek body styles are complemented by molded body-color trim, rocker panels, cladding, and even grilles. While these applications have traditionally been the domain of lower-cost plastic materials, two factors are changing that picture.

Consumers are demanding better aesthetics, and OEMs want better dimensional stability to correspond with narrower gaps and tighter designs. Thermoplastic polyurethanes (TPUs) are increasingly selected as the answer, according to Jerry Blayne, manager of applications engineering at BFGoodrich Estane TPU Div. (Cleveland), the largest global supplier of TPUs (Estane) and reinforced, alloyed RTPUs (Estaloc).

Urethanes come in several forms-adhesives, paint, foam, casting, sheet, extrusion-and are used in many applications because they have a wide variety of properties. TPUs, the molding compounds, are no exception. Traditionally this material hasn't been a heavy favorite in automotive because of competitive pricing situations-TPU is at the high end of the cost/performance scale compared to competing thermoplastics. However, Blayne explains, "Incoming resin prices are becoming less important now than the total cost or out-the-door finished piece cost for most OEMs and Tier Ones. TPU typically runs faster than competitive materials, and yields are better. These resins also have low viscosity and can fill 6-ft-long parts without difficulty." The end result is they are more cost competitive and are, again, becoming the material of choice for demanding applications.

Design Freedom
Most importantly, TPUs tend to have low, controllable shrinkage rates and, with RTPUs, excellent dimensional stability over time. Of course, dimensional stability is a big issue with large car parts that have to fit exactly. Some RTPU compounds have the same coefficient of linear thermal expansion as sheet metal, which helps maintain the fit on door pieces from Alaska to Phoenix.

Because of low shrinkage rates, TPUs can handle different thickness sections for fit and style purposes. Parts that are impossible to mold can be done in TPU. In the area of body side cladding, which often combines thick and thin sections in one piece, non-TPU parts shrink and expand significantly because of their lower dimensional stability while RTPUs are unaffected. Blayne confirms he is currently working with a supplier on production parts with a nominal 3-mm wall and an adjacent 10-mm wall. "The RTPU parts have no sink marks. One of the areas in which TPU shines is this design freedom. With other materials, you can inject gas to eliminate sinks, but part yields go down and expense goes up," he adds.

Painless Painting
According to Blayne, painted parts represent some of the most exacting molding requirements in the industry because even a minor defect is noticeable on a high-gloss, painted exterior body part. "And there is no texturing allowed, obviously, when you are trying to achieve a Class A surface." The absence of sink marks is a plus for TPU.

In addition, with other materials, additives are required to make the resin accept paint and make it stick. "These are two totally different chemistries that are not compatible, so additives and primers are a necessity," adds Blayne. Not so with TPUs. "Paint is a urethane and adheres to TPU very well. No primers, adhesion promoters, or additives are required," he says. By cutting out the primer step in the production process, TPU's overall per part cost is further reduced.

Also, as a result of the tremendous adhesion between a urethane paint and a TPU substrate, the parts are more resistant to paint chips. Damage resistance is superior as well. Blayne explains, "TPU chemistry allows for rubber-like properties even at very low temperatures. This elastomeric backbone imparts durability that is well suited to damage-prone applications. Overall, these materials excel in abrasion and impact resistance."

Flexibility and Appearance
Automotive applications for TPUs are growing, especially in luxury-class models, according to Blayne. They are used to make rocker panels, cladding, body side trim, and ditch covers.

"The area where the roof joins the side of the vehicle, called the ditch, used to be spot welded, filled with filler, and painted," he says, "a very labor intensive process. Today, entire sides are stamped out of one sheet of metal. Some truck models simply weld the side of the car to the roof. Low-cost, low-aesthetic plastic strips are used to cover many of these ditches."

Because this area of the car appears at eye level when standing next to a vehicle, automakers looking for better aesthetics on this high appearance part turned to TPU. "Our customers would like the ditch cover to look more like the vehicle," adds Blayne. "Urethane is ideal because it is flexible and paints well. Also, it has the ability to conform to the ditch, which varies in width and depth because sheet metal tolerances are not exact."