The Chemistry of Life: The Plastic in Cars
The Chemistry of Life: The Plastic in Cars
Even if cars soon start running entirely on electricity or hydrogen, they'll still need 100 gallons or more of oil to make their plastic parts, such as seats, dashboards, bumpers, and engine components. And some day that plastic may be recycled back into fuel.
Cars of old were mostly steel, but the use of lightweight alternatives has dramatically increased in the last couple of decades. Whereas almost no plastic could be found on a car from the 1950s, today's automobiles have more than 260 pounds (120 kilograms) of plastic on board, according to the Transportation Energy Data Book.
"It is expected that high oil prices and strict CO2 standards will accelerate the growth [in plastic use]," says Aafko Schanssema from PlasticsEurope, a plastic industry group based in Belgium.
Plastics improve fuel economy by reducing weight, but they also require petroleum as a raw ingredient.
"Plastics are in fact solidified oil," Schanssema explained.
Although different plastics have different recipes, it takes roughly 0.4 gallons of crude oil to make 1 pound of plastic. Globally, around 8 percent of the oil that comes out of the ground is used to make plastic.
The average car is a mix of materials: glass windows, rubber tires, lead batteries, copper wires, as well as traces of zinc, magnesium, tin, platinum and cobalt.
However, steel is still the single most important material in cars. It is strong, durable and malleable. On the flip side, though, it is relatively heavy. For this reason, car manufacturers have been trimming down on its use.
For domestic cars, the percentage of weight in steel and iron has dropped from 75 percent in 1977 to 63 percent in 2004, according to the Department of Energy's Transportation Energy Data Book.
Some of the steel has been replaced by lightweight aluminum, whose percentage has grown from 2.6 percent in 1977 to 8.6 percent in 2004. Plastic has seen a similar rise in prominence, going from 4.6 to 7.6 percent over the same 27-year period. (In Europe, the average car currently has closer to 11 percent plastic, Schanssema said.)
A 2005 PlasticsEurope study showed that every pound of plastic in a car replaces roughly 1.5 pounds of traditional materials.
Based on this weight reduction, the same study calculated that plastics provide a fuel savings of about 3.8 percent. However, cars haven't improved their gas mileage by that much.
"On the whole, U.S.-made cars have increased in total weight, so that whatever effect can be ascribed to plastics has been more than offset," said Michael Renner, a senior researcher for Worldwatch.
Renner thinks an emphasis on particular parts misses the bigger picture: the total size and power of new vehicles have been going in the "wrong direction" for many years. He does agree, however, that "the continued development of lightweight materials will still be critical."
Even if plastics can mitigate some of the fuel use, they are not exactly loved by environmentalists.
"The production of plastics is of course highly energy-intensive and polluting," Renner said. But he added that the same is true for steel and aluminum production.
One concern is that plastic recycling is not as fully developed as metal recycling of vehicle parts. Composite plastics are especially hard to separate and thus make available for re-use.
However, to Schanssema's thinking, this would not justify making cars from heavier metal parts,
"When looking at environmental impact from a life-cycle approach, it has been found that about 95 percent of the environmental impact of a car is during the so-called 'use phase,'" he said. "End-of-life contributes only marginally to the impact."
Besides reducing weight, plastics help to streamline the shape of vehicles, improve the performance of tires and increase the safety of windshields and fuel tanks.
Still, there are ideas for making plastics more sustainable. One way might be to use bio-degradable plastics, or ones that come from renewable resources, such as corn or sugarcane.
Another option is to recover the energy from discarded plastic parts. The company Plas2fuel, based in Washington state, can make a gallon of oil from melting down 8 pounds of plastic. In March, this process was used by Oregon-based Agri-Plas to turn plastic waste into 8,200 gallons of oil.
Ironically, then, the plastic in electric cars and fuel cell vehicles might one day be recycled into oil that could be burned in gasoline-powered cars.
We have customers ranging from manufacturing immaturity to international corporations that have as long a history in plastic parts as Rosti does. That said, we do have customers that surprise us and have been able to teach us about a particular design or material application. We value these interactions, as nothing is ever black and white. There are always compromises and trade-offs to be made. Conversely, we also have customers that believe they have “been there and done it all,” so extra care must be taken to prepare supporting evidence for our proposals. This would include simulation data, past product data, theoretical calculations, prototype tooling and other information.
Processors and end users who use nylon have become very familiar with the effects that water absorption has on that material. In applications where high loads are generated, such as in snapfit assemblies, nylon that is still close to its dry-as-molded state may exhibit brittle failure, and we have learned that this failure mode can be mitigated by conditioning the parts to bring them up to their equilibrium moisture content. This frequently solves problems with the assembly process.
The moisture conditioning process takes many forms. Some simply pour a prescribed amount of water into molded parts contained in a moisture-proof package such as a polybag. Others prefer placing saturated paper towels into the package with the nylon parts and allowing the water to migrate out of the paper and into the nylon. Some go as far as boiling the parts. This not only increases the moisture uptake rate, but also ensures that the moisture is absorbed more uniformly throughout the wall of the part.
While rapid moisture conditioning is a legitimate method for improving the impact resistance of nylon products, there should be concerns with using it indiscriminately. A nylon product may be temporarily brittle while it comes to equilibrium with the atmosphere. But it may also be brittle because the material has been degraded during the molding process. In such situations, the brittle condition is not simply a temporary symptom of low moisture content, but rather is a permanent condition brought about by reduced molecular weight.
The problem is that this shortcoming can be covered up by pumping large amounts of moisture into the polymer. Under such conditions, the polymer becomes sufficiently flexible so that it no longer appears to be brittle. But a moisturizing process that is performed rapidly often introduces more moisture into the polymer than it can retain in the long term. If this happens, then when the excess moisture comes back out of the polymer, the brittle condition can return, usually after the part has gone into the application.
Failure in the field
This occurred in an application involving a critical part in a consumer product. Parts produced in an unfilled nylon 6/6 were received from the molder that appeared to be more brittle than usual. The explanation was that the parts had been molded just a few days earlier. It was winter in a northern state where the indoor humidity was very low and therefore moisture uptake was slow.
The corrective action was to moisture condition the parts. However, this was done very aggressively, and the final moisture content of the conditioned parts was 3.2%. The parts worked initially, going through the assembly and testing process without any obvious problems. However, once in the field the parts began to fail. When the product was brought back in for evaluation, the moisture content of the product had declined to 1.5-1.6%.
Field experience has shown that this is a consistent value that is obtained for parts that have been allowed to come to a true equilibrium with ambient surroundings. It will be higher in extremely hot, humid environments or in situations where the part is immersed in water or used in close proximity to water, but in most cases a part molded in unfilled nylon 6/6 can only hold about 1.5% water by weight.
This experience contradicts a lot of the data published by material suppliers showing the conditioned moisture content at 2.5%. But much of this early work was performed using accelerated techniques that had a tendency to introduce more moisture into the polymer than it could hold in the long term. Field experience shows that values of 1.5% for an unfilled material are much closer to the norm.
It is also important to emphasize that this value is by weight of polymer. If a material contains 33% glass fiber, then one-third of the polymer has been replaced by an inorganic material that is not hygroscopic, and therefore the amount of water that this compound can hold will be proportionally lower.
One of the biggest drivers of change across any industry is the cost of production. If there are lower cost alternatives that provide the same or better results, naturally a company should pursue those. When it comes to metal-to- plastic conversion, perhaps the biggest advantage of plastic parts is their ability to potentially provide an overall cost savings of 25-50% over metal.
Also, using plastic often streamlines the number of secondary operations commonly associated with metal parts and reduces the number of assembly steps required. With plastic, OEMs have the ability to combine multiple components into a single molded part design, as opposed to making numerous individual components out of metal and welding or fastening them together. OEMs can even create complex plastic parts with tight tolerances that require no secondary machining using scientific molding processes. Having fewer production steps and less assembly time can provide a significant cost savings, as well as provide more design flexibility.
In addition to streamlining assembly, injection molding gives manufacturers the time-saving advantage of having the color and surface finish ready to go right out of the mold, instead of tacking on time-consuming steps afterwards, as is the case with metal parts. Also, the injection molding process typically has faster cycle times (more parts made per machine hour) than metal components, all while producing repeatable, durable parts.
Plastic parts are typically 50% lighter than their metal counterparts, and provide more production quantity — that is, you get more more parts per pound with plastic versus metal.