Plastics have recently claimed several firsts in their persistent push to supplant steel—the first plastic-reinforced bridges and the first PET (polyethylene terephthalate) material robust enough to withstand the blistering heat of an e-coat curing oven. Indeed, in steel-dominated industries such as construction and automotive, plastics are muscling their way in because of their hard-to-beat combination of properties such as lightweight strength.

Two new plastic bridges—one in the U.S. and the other in the U.K.—represent the first of their kind in the world, at least say the designers of these structures.

In Vermont, construction on a 44-meter bridge reinforced with plastic instead of steel began in August 2002. Forgoing a steel rebar, the bridge instead depends on two layers of rebar made out of glass fiber-reinforced polymer—which is suitable for a state that sees a lot of snow, ice and salt because it does not corrode.

The new rebar, which is inside the bridge's concrete deck, is substantially lighter than steel rebar. While steel rebar weighs in at a pound and a half per foot, the new rebar tips the scale at less than half a pound. The polymer's only drawback is its higher cost, which will pile an additional $50,000 to the bridge's $1.4 million cost—an extra expense covered by a federal grant.

Elsewhere, in Europe, the first highway bridge made from a revolutionary plastic material recently opened in grand fashion—with a 34-ton Sherman tank barreling across the structure to underscore its strength and durability.

The bridge in Shrivenham, Oxfordshire, is made from a new type of composite material named ASSET, Advanced Structural Systems for Tomorrow's Infrastructure, composed of glass and carbon fiber-reinforced polymer. The material is strong and lightweight, making it a viable alternative to reinforced concrete or steel decks.

Meanwhile in the automotive industry, a custom-made PET material recently accomplished an unprecedented feat—it faced the searing heat of the curing oven stage in electrocoating, a vehicle painting method, and emerged from it without damage.

Electrocoating or e-coat is a painting process that utilizes an electrical current to deposit paint particles quickly and evenly over large surfaces, handling the size and complexity of an auto body. Developed in 1961, the painting method has streamlined production, completely priming a vehicle in roughly two minutes. The curing oven phase of this painting method, however, has stumped most plastics.

With temperatures hovering between 350 and 375°F for a 30-minute time span, the curing oven stage has been deadly for most plastics—causing them to warp, sag or become distorted. As a result, plastic parts have to be put together after e-coat, instead of on the assembly line, making production more costly and time-consuming.

No longer. Ford, molder Venture Industries and resin supplier DuPont have developed a PET material that can take the heat.

To handle blazing temperatures, the material's polymeric mix had to be tinkered with to exhibit certain properties—maximum crystallinity and superior heat resistance. In addition, being isotropic—uniformly elastic in all directions—was a must so mineral and glass fillers were thrown in. "We were especially sensitive to making sure the material has enough isotropic behavior to resist warp due to differential shrinkage," says Ken Nelson, a senior technical consultant for DuPont.

DuPont made a special grade of its Rynite PET material for Ford's GOR, grill-opening reinforcement, which was previously made of sheet molding compound, SMC. Part design was also crucial to the material's success. Project members added ribs when an area of the GOR bent more than it should. This addition helped the material cope with shrinkage stresses.

While other thermoplastic materials have also been e-coat survivors, this custom-made PET has the distinction of being the biggest e-coat success story ever. "Other parts were not as large and were unrestrained to allow for thermal expansion," says Nelson. "Here there is almost total restraint of the part, which has to absorb the stresses and return to within specified tolerances."

The piece marks the first time that PET has been used in such a big part—13 lbs., 65 in. long—with adequately low expansion. It does not contain any metal whatsoever, and this one-material part is very simple, requiring one mold, no stamping tools, no trimming, no drilling and no milling. It also surpasses the SMC version in durability.

The all-thermoplastic GOR hit the commercial market in March 2002 and can be found on the 2003 Lincoln Navigator and Ford Expedition.

And plastics are not stopping there. Rod Lowman, president of the American Plastics Council, recently unveiled a 20-year strategy, developed by the APC Automotive Group, aimed at making the cars and trucks of the future "plastic-intensive."

Imagine a future, says Lowman, with paintless cars sporting glossy plastic body panels with molded-in hues and "smart" plastics that undo the damage caused by rock chips, door dings and minor collisions. It's not farfetched either considering that 35 years ago, plastics accounted for only 2% of the total weight of a typical North American vehicle, but now represent 8-10%.

In other words, in 2020, we may very well be driving plastic cars and zooming over plastic bridges.

Sources: Plastic's Possible Future
Gary S. Vasilash, Editor-In-Chief
Automotive Design & Production, Oct. 2002
http://www.autofieldguide.com/columns/gary/1002mat.html

Plastics Industry Reveals 20-Year Vision
Drew Winter
WardsAuto.com, Aug. 8, 2002
http://www.industryclick.com

It's a First! PET Survives E-Coat Curing
Michelle Maniscalco
http://www.immnet.com/articlelibrary/archive/getOneArticle.php3?getArtID=2034

UK: Army Tank Tests Europe's First Plastic Highway Bridge
International Construction, Nov. 4, 2002
http://intlconstruction.com

USA: Bridge Innovation Underway
International Construction, Aug. 22, 2002
http://intlconstruction.com