Cars have to lose weight if we want to be able to buy them with a good conscience in the future. If not, there will be a fuel price and CO2 overkill. One of the reasons for this is that cars consume more energy than necessary because they are too heavy. A promising method for cutting down vehicle weight is to use fiber-reinforced plastics, with which chemists and engineers at LANXESS help making components lighter and lighter.
Heavy but not sluggish
Although automobiles have become steadily safer and more comfortable in recent decades, they have also gained a lot of weight in the process. “Drivers put much higher demands on their vehicles today than was the case even ten years ago,” says Julian Haspel, a project manager for global application development at the High Performance Materials business unit of the Leverkusen-based specialty chemicals company LANXESS. “Certain types of equipment such as power windows and air conditioning are now considered indispensable.”
Major automakers also have a lot of ideas in the pipeline to make driving easier. Many of these ideas require new sensors or additional small motors. Safety systems are making great progress as well, and features such as multiple airbags and crash-resistant occupant cells are now defining a high standard of automotive safety that nobody seriously wants to do without.
“In 2012, the latest generation of Volkswagen’s best-selling Golf model weighed 450 kilograms more than its angular ancestor of the 1970s,” says Haspel, who is an expert in lightweight engineering. Even though the trend toward more weight was already reversed in the current model series, the Golf VII has a curb weight of at least 1.2 tons, depending on its equipment. “However, you also have to take into account the fact that cars have become bigger in recent years,” adds Haspel. Despite their weight, today’s cars are by no means sluggish, because engine developers have done a good job of boosting the automobiles’ performance over the past few years.
Emissions have to decline
However, the automotive industry’s solution of simply sticking a more powerful engine underneath the hood will become more difficult in the future. “This is mainly due to climate change,” explains Haspel. “After all, engines turn fuel into the greenhouse gas carbon dioxide — it’s a law of nature that can’t be changed.” Although today’s vehicles consume remarkably little fuel compared to earlier generations of cars, the European Commission has nevertheless set a strict new limit, which stipulates that, beginning in 2020, each manufacturer’s vehicles will only be allowed to emit an average of 95 grams of CO2 per kilometer. In 2008 the limit was still 165 grams. The new target can only be achieved through further reductions in fuel consumption, since every liter of fuel that is burned emits 2.33 kg (gasoline) or 2.66 kg (diesel) of CO2.
Good reasons for being overweight
Automakers are employing a variety of measures to meet the strict emissions limits. One of them is to simply put their cars on a “diet.” “Even a weight reduction of 100 kilograms can cut fuel consumption by 0.35 to 0.5 liters per 100 kilometers,” says Haspel. “Depending on the engine and the type of fuel used, this reduction will cause carbon dioxide emissions to decline by 8.8 to 12.5 grams per kilometer. And the range of electric cars is increased if they are made lighter. That’s because the battery doesn’t get depleted as rapidly if the motor has to move less mass. As you can see, cutting weight has lots of advantages!”
But don’t the increased number of equipment features and stronger vehicle bodies automatically add to the car’s weight, we ask. “Not necessarily,” replies Haspel, who points out that the trick here is to use materials that are extremely strong but light as well. For example, the metals industry has for some time now offered special high-strength steels that can absorb more force than was previously the case. Moreover, the light metal magnesium and special aluminum alloys can replace comparatively heavy traditional types of steel in crucial areas. Haspel and his colleagues have created new materials that can turn heavy automobiles into agile and extremely speedy cars. These state-of-the-art vehicles weigh less than their predecessors and can therefore improve the CO2 balance.
A part of the solution for a long time
Automobile developers have cut large amounts of metal from their construction plans in recent years because ingenious engineers have managed to replace the metal with plastics. The list of components that are now made of polyamide and similar products is quite impressive. It includes dashboard supports, air intakes, armrests, engine covers, connecting rods, mountings, and seat components.
That’s why it’s extremely difficult to find substitutes for the parts that have not yet been replaced. Although steel is heavy, it is also strong, so manufacturers have traditionally used it for automobiles. Steel is also good at protecting vehicle occupants during accidents; that’s why structural body parts, doors, sidewalls, fenders, hoods, and other components are still mostly made of sheet metal. As a result, polymer materials have to be about as strong as their metallic counterparts if they are to compete with steel, aluminum, and magnesium in their last remaining domains. However, the new materials should not be too expensive either.
Fibers for strength
One of the most promising approaches to solving this problem is to use not plastic but materials that are much stronger. Examples include fibers made of glass or carbon. If these fibers are embedded into plastic, it is as if the rather soft polymer material were given a kind of skeleton. If force is applied to a fiber-reinforced component, the fibers act like bones and carry a large part of the load. As a result, the component can be subjected to far more stress before it breaks or tears. “Several factors have to be taken into account here as well,” says Haspel. “For example, the fibers are most effective if they are as long as possible.” This is because long fibers are better at transmitting forces through the material in which they are embedded.
The “wet tablecloth” principle
Until a few years ago, it was still considered state-of-the-art to use many short fibers. For example, glass fibers were cut very short so that they could be mixed well with polyamide. This process made it possible to create components the like of which had never been thought of before. For example, there is an upper-range sedan that has a spare tire well made of Durethan, a LANXESS polyamide with a glass-fiber content of 60 percent. The plastic component is glued into the vehicle’s aluminum body and is so strong that it even helps to reinforce the car’s rear section; this is crucial for good handling.
Developers are now also experimenting with “long” glass fibers that are several millimeters in length. Although such fibers can absorb a greater amount of force, it is not easy to mix them with plastic without cutting them up in the process.
A mesh in a plastic casing
“That’s why we at LANXESS are also employing a third approach,” explains Haspel. “We are using virtually endless glass fibers to create textiles such as those found in jeans. We then impregnate this material with our plastic in order to create fiber-reinforced composite sheets.” If these Tepex fiber composites are heated, they can be turned into almost any shape, just like wet tablecloths. And because the fibers retain their full length, they can absorb a considerable amount of force.
“What’s great about this research is that the semifinished Tepex materials can be very easily combined with other types of plastic,” explains Haspel. The new Tepex technique thus joins the “hybrid” method as another means of using plastic to build lightweight cars. Although LANXESS engineers have been using the hybrid method for a long time now to combine metals and plastic, Tepex plates have the advantage of being lighter than the aluminum profiles of “classic” hybrid components.
LANXESS labs are currently creating ultra-strong support members whose extreme firmness is the result of a Tepex core. Stabilizing ribs, threaded sleeves, and clips are shaped by means of injection molding. As a result, the parts can be mounted quickly and easily. This also reduces assembly costs. In addition, component prices are similar to those of aluminum and magnesium parts.
Half the weight
The main benefit of hybrid components (especially those using Tepex) is that they are extremely light. Some fiber-reinforced materials are now about as strong as metals. Depending on the component and the task, substituting these materials for steel can reduce weight by up to 50 percent.
“Our Tepex composites can, for example, reduce the thickness of the sidewalls of airbag casings to between 0.5 millimeters and 1 millimeter without affecting the component’s strength,” explains Haspel. By contrast, conventional injection-molded plastic parts still need walls that are between three and four millimeters thick. The weight of the LANXESS solution is correspondingly lower. Using a brake pedal as an example, the experts from LANXESS have demonstrated how the new technique can reduce weight. While the all-steel version of the brake pedal weighs 794 grams, the plastic-metal composite version reduces weight by a respectable amount to 526 grams. But even this weight reduction is far exceeded by the Tepex hybrid solution, which only weighs 355 grams. “The plastic-metal hybrid technique makes front ends up to 40 percent lighter than those made exclusively of metal,” says Haspel. “The weight of aluminum front ends can be reduced further by using Tepex inserts.”
It is therefore quite possible that plastic is about to invade the last domain that is currently still dominated by metals — the vehicle body. Together, these two types of material can lead to many improvements.