Page 692 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Selected Topics 655

as a bridge between synthetic and natural fibers in that it processes like synthetic fibers but has the

touch, comfort, and moisture management of natural fi bers.

As noted above, new materials must “fight-it-out” with existing materials for existing and devel-
oping areas. This is particularly true in the automotive area. Thus, polyethylene-polyethylene-
copolypropylene panels and bumpers have replaced “rubber” HIPS as automotive bumpers. Other
materials were available with greater toughness and scratch resistance, but they were also more
expensive. Here, classical monomers were employed to develop nonclassical materials.
In the automotive industry, the rule of thumb is that every 10% reduction of vehicle weight gives
about a 5% increase in fuel economy. Weight reduction is only one of the driving forces for the
replacement of metals with polymeric materials. It is worth mentioning other driving forces because
these are also involved in the selection of certain polymers over other polymers. Reduction in cost is
also a factor. Reduction in cost involves a number of factors, including, but not exclusive of, simple
pound per pound cost. Polymers are out performing metals because of the ease with which complex
structures can be made. Some polymers lend themselves to easy fabrication and this represents both
a cost savings and enhances the opportunity for the material to become an automotive material.
Interestingly, the number of parts also is involved in deciding what material to be used. For instance,
if the number of a part to be used is less than about 100,000 annually, then injection-molded plastic
parts are less expensive than ones stamped from steel. Injection-molded parts also provide aesthet-
ics that are hard to obtain from steel and glass. In fact, many of the plastics can be made to resemble
chromes and gold so are used to enhance the look of some automobiles. Polycarbonate tail lenses
are favored over glass for a number of reasons, including ability to make more complex and attrac-
tive shapes. Circular taillights are no longer the only option.
Flexibility of design as well as material flexibility favors polymers. Again, plastics can be made

into almost any shape and when necessary even bent a little to fit them into almost impossible spaces
within the automobile’s interior.
Along with traditional plastics, composites are also becoming more widely employed. GM has

traditionally employed fiber-glass composites for the Corvette’s body panels (now using carbon-

fiber composites for some of these panels), but is now looking to use composites for the body panels
of other automobiles.
Performance is also a consideration and in some instances favors polymers over steel. DuPont
developed a nylon water jacket spacer for the 2006 Toyota Crown and Lexus GS-300. It did not
reduce weight, but did increase the heat-transfer effi ciency between the engine coolant system and
the cylinders lowering the fuel consumption by 1% or an equivalence of more than 50 pounds.
Another indirect way that polymers can assist fuel economy is by allowing alternative fuel uses.
Plastics can “create” free space needed to store bulky batteries encouraging their use as alternative
fuel sources.
Along with the more classical polymers some new ones have been developed for automotive use.
Thus, GE developed Noryl GTX resins that are being used on the fenders of the some cars, including the
Volkswagen New Beetle and Hummer H3. The Noryl GTX resins are alloys of PPO and nylon-66.
The automotive industry is continuing looking for new materials and new uses for more classical
materials.
Many of the so-called new materials are actually “old materials” arranged in different settings.
Following is a brief description of three of these materials. Trgris, developed by Milliken, is a PP
thermoplastic composite. It is made from coextruded PP tape yarn that has a highly drawn core cre-
ating additional strength for the PP yarn. The pulling encourages better alignment of the PP chains.
The tape yarn is then woven into a fabric and layered and finally heated with nonyarn PP, with pres-

sure applied forming rigid sheets that are used for panel applications or premold material. Thus, the
yarn acts as the discontinuous phase with the nonoriented PP used as the continuous phase forming
a PP–PP composite. In some fashion, Trgris is similar to plywood in that various layers are stacked,
generally with the sheets placed at different angles to one another. The entire PP composite is strong
and it is lighter than most composites. It also has good impact resistance since the entire composite is

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