Good for 1,000 volts in electric cars - TPU cable sheathing for new vehicle drives
Before the backdrop of ever-stricter emission limits, all car manufacturers and suppliers are working feverishly on new technologies like the electric drive.
- (1888PressRelease) July 08, 2010 - However, it is precisely these innovative electric and hybrid vehicles that place high demands on cable harnesses and lines, all of which are hardly comparable to the on-board power supply systems of today. In fact, due to the high voltages of as much as 1,000 volts and correspondingly high currents, these cables have to be able to withstand temperatures that are much higher than those encountered by their predecessors.
In response to this, BASF Polyurethanes has added the new variant Elastollan® 785 A 10 HPM to its product line of thermoplastic polyurethanes (TPU) for cable sheathing. HPM stands for high performance material.
The Lemfoerde researchers had to comply with the more stringent requirements for automotive cables of electric and hybrid drives, so they developed a new TPU production method and also turned to proven high-quality materials when it came to selecting the basic chemicals and stabilizers. Thus, they succeeded in achieving good polymer structure in combination with a very effective resistance to external influences.
Keeping pace with the requirements
For many years now, products belonging to Elastollan’s 11 series have been doing a good job as sheathing material for cables and connectors in vehicles, including lines for the ABS and ESP systems. Some of the types known in the market are 1185 A 10, 1185 A 10 M and 1195 A 10. These flexible materials are mechanically very strong, vibration- and friction-proof as well as chemical-resistant, and they also ensure a high level of functional reliability. These properties have enabled the materials to prove their worth in the areas near the axles, in the wheel wells and also in the engine compartment as well as in the spaces close to the transmission. Design engineers also like to manufacture battery lines from TPU which, if required, can be supplied in a halogen-free, flame-resistant variant.
With the new Elastollan 785 A 10 HPM, this performance spectrum that has been optimized for sheathing automotive cables, is now also available for the high temperatures found in electric and hybrid cars. Aside from its temperature resistance, which surpasses that of products of the 11 series, the new TPU also stands out for its much lower compression set, and yet for the rest, it exhibits similar mechanical properties.
Automobile manufacturers raise the bar
In keeping pace with the trend towards electric and hybrid cars, but also in view of the ever-higher temperatures found in classic vehicles with gasoline or diesel engines, German carmakers have agreed to apply the temperature class D of the German standard LV 112. This standard stipulates that the cables have to pass the usual winding test, even after having been exposed to 3,000 hours of hot-air ageing at 150 degrees Celsius [302 degrees Fahrenheit]. According to the temperature class C of ISO 6722, which had been required up to now, depending on where it is installed, it was sufficient for the cable sheathing to withstand hot-air ageing at 125 degrees Celsius [257 degrees Fahrenheit] over the course of 3,000 hours.
When compared to ISO 6722, LV 112 – which is the standard primarily being used now – is also more stringent in terms of resistance to hydrolysis at high temperatures. Whereas ISO 6722 accepts 1,000 hours at 85 degrees Celsius [185 degrees Fahrenheit] and 85 percent relative humidity, new LV 112 requires a test duration of 3,000 hours here as well.
The other prerequisites for cable sheathing are largely identical in these two standards, which are the norm for the materials available today. This includes resistance to cold, media resistance and inhibition of fungal growth as well as compatibility with other materials normally found in cable harnesses such as adhesive tape, shrink tubes and connectors.
The new Elastollan 785 A 10 HPM easily meets the requirements of temperature class D as well as the more strict specifications of LV 112 in terms of hydrolysis resistance. Alternative plastics that display a comparable performance in this respect are either considerably more expensive because they come from even higher temperature classes, or else they are more laborious, and thus more costly, to process. The latter especially holds true for cross-linked materials that additionally have the drawbacks that they cannot be recycled and give rise to water-tightness problems when water splashes onto connectors or nozzles.
Surpassing the standard stipulations by far
During long-term tests in hot air at a temperature of 150 degrees Celsius [302 degrees Fahrenheit], Elastollan 785 A 10 HPM largely retains its high strength, even after 3,000 hours. Even though the elongation at break drops to 200 percent after this time period, it is still well above the value required by LV 112. An elongation at break of about 50 percent is sufficient to pass the cable test prescribed here.
After the ageing procedure over 240 hours at 175 degrees Celsius [347 degrees Fahrenheit], which is likewise required by LV 112, and even after 6 hours at 200 degrees Celsius [392 degrees Fahrenheit], the elongation at break of Elastollan 785 A 10 HPM, at a value of 610 percent and 530 percent respectively, falls well within the safe range. In comparison, the tensile strength drops much more markedly at higher temperatures. This, however, is not all that relevant for cable applications since the strength of the cable is determined by the conductor materials and the sheathing.
When the material is stored in hot air at 85 degrees Celsius [185 degrees Fahrenheit] at a relative humidity of 85 percent, its elongation at break over a time period of 3,000 hours actually rises from 690 to 840 percent. The residual strength after this long-term load is still a respectable 20 megapascal.
New TPU being used by leading cable manufacturers
Customer tests with the new product are currently been conducted at leading automotive cable manufacturers and, in many cases, it is about to be specified and approved for high-temperature cable applications.
For cable manufacturers, the behavior in the winding test is the decisive factor. In this test, after the finished cable has undergone hot-air ageing or storage in hot, humid air, it is wound around a mandrel whose diameter is approximately the same as that of the cable. The material is considered to have passed the test if the cable does not exhibit any cracks, deformations, adhesions or excessive color changes. The data gathered with S2 standard test rods at the BASF Polyurethanes laboratory has been consistently and thoroughly confirmed in actual practice tests.
Following the winding test, the cable manufacturer also conducts electric tests. Here, the specific contact resistance of the insulation in one-percent salt water still has to be at least 109 Ω · m. Finally, a voltage of 1000 volts (or 5,000 volts in the case of larger cross sections) is applied for one minute. During this time no disruptive discharge may occur. Elastollan 785 A 10 HPM also easily passes these tests, even after 3,000 hours of hot-air ageing at 150 degrees Celsius [302 degrees Fahrenheit] or 3,000 hours at 85 degrees Celsius [185 degrees Fahrenheit] and 85 percent relative humidity.
The future of TPU cable
Aside from the “soft” Elastollan 785 A 10 HPM, whose Shore A hardness is 85, BASF Polyurethanes also offers the considerably harder Elastollan 745 D 15 HPM (Shore D hardness of 53). In contrast to the softer product, which is recommended for thicker walls of the kind found in sheathed cables or battery lines, Elastollan 754 D 15 HPM is suitable for very thin core insulation. At the present time, this product reaches the resistance level required by ISO 6722 in humid heat over 1,000 hours, but does not yet comply with the 3,000 hours stipulated in the more stringent standard LV 112. Researchers at the laboratory are working on remedying this as soon as possible.
Another focal point of the research are flame-resistant formulations – if required also halogen-free – of the HPM family. Color masterbatches and metal deactivator masterbatches that allow the materials to be specifically tailored for the application in question are already available.
By the way, electric cars are not only an important market for the new HPM family but also hold a very promising future for TPU cables made with the existing 11 series. The flame-resistant products of this series will serve as sheathing for the charging cables, thus playing a major role in ensuring electric mobility.
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