Audi first, and now Daimler – after the job cuts in Ingolstadt, Germany, the Swabian company is now following suit. Around 15,000 are now to be lost, following a significant drop in profits last year. This is due to heavy investment in technical development and electric vehicle production, and to liabilities resulting from the diesel scandal. Daimler is currently paying the price for missing out on alternative drive technologies, and cheating on its environmental emissions.

So some of the omens suggest that 2020 could be another difficult year for the automotive groups. However, that does not necessarily mean bad news for the plastics sector. I can see two points that seem positive. First, the automakers’ weakness is principally a company crisis and not a demand crisis. This is clear from the rising sales figures. In 2019, VW sold more vehicles than ever, and remains the world’s biggest auto manufacturer. Daimler and BMW also increased their sales figures in the past year. There are also the first indications that the market for electric cars is picking up speed. VW, for example, sold 140,000 hybrid and all-electric cars last year – an increase of 80 %. And Tesla, too, with 367,500 units in 2019, increased its electric car sales by 50 % year on year. The planned Gigafactory in Brandenburg, Germany, also shows that the Americans are confident about sales growth in Europe.

This bodes well for the plastics industry. Second, the industry is already very well prepared to meet the changing demands made on electric cars. Not only the materials manufacturers, with heat-conducting, hydrolysis-resistant and electrically insulating materials, but also plastics processors. How far the industry has already come is shown in our large Automotive Extra from page 7, featuring among other things an interview with the Nobel Prize winner Dr Akira Yoshino (p. 14.) So from the point of view of the plastics industry, bring on electromobility!

Florian Streifinger [Florian.Streifinger@hanser.de]

Editorial Streifinger

Flame Retardancy, Heat and Tracking Resistance of Electric Vehicles

The transformation from the combustion to the electric engine entails several imponderabilities for the plastics industry. In part, electric drives have completely different material requirements which are becoming specifically evident with the increasing use of appropriate vehicles. The following article presents an overview of the most important changes.

The transformation from the combustion to the electric engine entails several imponderabilities for the plastics industry. In part, electric drives have completely different material requirements which are becoming specifically evident with the increasing use of appropriate vehicles.

With the move towards electromobility, the automotive industry is facing a radical change. In the transition period, manufacturers will strongly diversify the powertrain. By optimizing internal combustion engines (ICEs), switching to alternative fuels and introducing hybrid cars, they are reducing their overall fleet emissions. At the same time, they are building competences in electric engines expand their offers of fully electric vehicles (EVs). The fuel cell vehicles developed by some manufacturers are also driven by electric engines.

Electrification requires various new systems and components. This is the case regardless of whether the electric engine is powered by electricity from a large battery module by a fuel cell in conjunction with a smaller battery. Some of these additional components include high-voltage chargers, lithium-ion batteries, fuel cells, powerful electric engines, inverters for direct-to-alternating (DC-AC) transformation and DC-DC converters (Fig. 1).

The electric powertrain (see Box p. 13) typically operates at high voltage with currents of several hundred amperes (A). Therefore, the safety and reliability of the drive system have become more essential than ever. Engineering plastics used for electrical insulation or structural housing components will see sharp increases in requirements for thermal aging resistance, flame retardancy and insulation properties. Flame retardants as well as dielectric strength and comparative tracking index (CTI) are clearly gaining in importance. Ignoring these requirements would result in significant safety risks and considerable reduce the reliability of vehicles. For these reasons, material suppliers are collaborating with tiers and OEMs to develop materials that will meet the requirements and boost the performance of EVs.

Flame retardants for use in plastics are a good example of the change in requirements. In the automotive environment as such they are not desirable, as they mechanic properties and the flowability of materials, while also resulting in higher weight and cost. However, the need for flame retardancy is expected to greatly increase with all the components in high-voltage (HV) powertrains, from the charging plug to the electric engine. On the one hand, this is due to proactive initiatives taken by OEMs or tiers. On the other hand, it can be assumed that supervisory authorities will also impose stricter regulations.

Flame protection in transportation is all about increasing the escape time in case of an emergency. Hence, it is no surprise that in airplanes, where escape is complicated, even seats and carpets need to pass very high flame retardancy specifications. Clearly, passengers can escape much readily from a car. Currently, passing the required flame tests is therefore rather easy for cars. At present, there are only limited applications and manufacturers that require the use of V0 certified plastics according to UL standard 94 in vehicle on-board electrical insulations.

However, the situation is very likely to change fundamentally with the transition to high-voltage electrification and unattended battery charging. High currents and high charging loads increase the fire hazard. Moreover, the HV system is associated with general risks such as potential short circuits or sparks. There have already been multiple reports of serious fires caused by on-board lithium-ion batteries even in parked and switched-off cars. EVs without dedicated flame protection are a safety risk. Higher requirements must therefore be expected in terms of flame retardancy (UL94 V0) and glow wire ignition temperature (GWIT).

In addition, lightweighting continues to be of great importance for automotive designers, albeit for fundamentally different considerations with regard to the different drive systems. In vehicles with combustion engines, every gram of weight reduction directly contributes to lower emissions, hence lower potential penalties as per the latest CO2 targets in the EU. For battery electric drives, designers need to compensate the extra weight of the high-capacity batteries, which can quickly add up to around 800 kg for a car with a good range. More and improved lightweighting structures are also needed for preventing a driving experience compromised by the inertia of a heavyweight vehicle. This will increase the use of plastics in applications hitherto dominated by metals, such as for parts requiring enhanced stiffness at high temperatures between 150 °C and 180 °C or for housings of electronic control devices. In order to meet the requirements of electromobility, materials supplier DSM, Heerlen, The Netherlands, has developed several engineering plastics targeted specifically at HV systems of hybrid and fully electric vehicles (Fig. 2).

These include aliphatic polyamide (PA) 6, 46, 66, 666 and 410, aromatic PA 4T (polyphthalamide, PPA), PPS, PBT, PET and thermoplastic copolyether ester (TCP) grades. Among all these polymers, PA6, PA66, PA 4T, PBT and PPS are very well suited to the needs of the electric powertrain.

Quite often, halides such as copper-iodide (CuI) based heat stabilizers are used polymers to achieve good thermal resistance. This keeps the mechanical properties of the structural or insulating plastic on a high level even at long term exposure to elevated temperatures. Heat and humidity, however, can cause some hydrolysis in the plastic, during which iodide (I2) is released, which in turn can lead to electrical corrosion of metal contacts or pins. This will not only affect metal parts in direct contact with the heat stabilized plastic, but also others located only in physical proximity to the plastic, e. g. enclosures with critical electronic components inside (Fig. 3).

Given the ionic nature of I2, a constant bias voltage will accelerate the migration of these ions and thus accelerate the effect of electrical corrosion significantly. This can reduce the conductivity of components, such as busbars, leading to increased heat generation up to component failure. Moreover, it can also change the current/voltage characteristics and subsequently the switching profile of components such as integrated circuits (ICs) or transistors.

As a long-standing material supplier to the electronics and automotive industry, DSM has developed various different engineering plastics custom-tailored to critical electrical applications (Table 1).

Free of metals and inorganic heat stabilizers such as CuI, these materials ensure high reliability for sensitive automotive components such as sensors, connectors, ADAS or electronic enclosures. Furthermore, in close cooperation with Stuttgart, Germany, based automotive supplier Bosch, DSM has developed a dedicated test facility for measuring the impact of electrical corrosion for different application cases both with and without direct metal contact.

To further minimize the problem of electrical corrosion caused by additives, DSM has introduced ForTii Ace as a high temperature PPA with the highest Tg (glass transition temperature) of 160 °C amongst all polyamides. In applications requiring a continuous use temperature up to 150 °C and above, the high Tg eliminates the need for heat stabilizers in the compound without impairing the mechanical properties of the polyamide.

The efforts of automotive manufacturers to increase the driving range of EVs are also affecting the plastics used. So far, the acceptance and sales of EVs are still rather low, above all due to high purchase prices, an insufficient charging infrastructure and the aforementioned, unsatisfactory range. EVs will reach cost parity with combustion engine cars only if battery pack costs drop to USD 100 per kWh. We can expect that growing volume demand and advances in battery performance will keep the costs shrinking. Advances in this direction were confirmed last year by Volkswagen AG during the IAA International Motor Show in Frankfurt, Germany. Along with the launch of their new ID.3 EV, the company also announced that given their huge battery deal with Asian suppliers, they have reached a cost level of USD 100. However, it remained unclear if this referred to the cost of the complete battery module or only the cells. At the same time, with increased governmental focus on reducing municipal fine dust pollution, we can also expect the charging infrastructure to be expanded.

One of the key focal points for automotive designers, therefore, is to increase the range of EVs. This can be done in four different ways:

<ul><li>By raising the battery capacity through either more cells or by newer battery technologies,</li><li>by using higher-efficiency DC-AC inverters and DC-DC converters,</li><li>by using higher-efficiency AC motors with further reduced component count,</li><li>by boosting the battery power through higher DC voltages.</li></ul>

Mild hybrid cars have a 48 V low power battery. They cannot drive fully electrically, as their electric motor only assists the combustions engine. In contrast, most fully hybrid cars and commercially available battery electric vehicles run at voltages between 200 V and 400 V, with a trend towards further increasing the battery voltage up to 800 V. Likewise, the charging infrastructure in Europe is also being expanded to accommodate such high voltages. Ionity GmbH, Munich, Germany, a joint venture of BMW, Daimler, Ford, VW, Audi and Porsche, is currently developing a pan-European network of ultra-fast high-voltage chargers (for up to 800 V), with the new Porsche Taycan being one of the first vehicles in the market to utilize this HV technology.

In addition, various industry players are actively working to move to even higher voltages in the range of 1000 V and above. A high-voltage charging system of 1000 V can reduce the battery charging time for a range of 1600 km to about 1 hour, which means the driver can fill up power for 400 km in just 15 minutes. Such charging times are considered acceptable for consumers and a big relief when traveling longer distances.

To achieve an overall higher electrical power at minimized power losses, the increase of voltage is preferred as opposed to increasing the current, since electric losses in conduction (P) scale with the square of current (I²) (Fig. 4).

Therefore, the electric powertrain requires high-voltage components and systems capable of delivering sufficient power to drive the electric engine while also minimizing the battery charging time. With high voltages, engineers need to take extra care in the design of parameters such as dielectric strength, creep distance and tracking resistance.

A key requirement for increasing the safety and reliability of the electrical systems is to avoid tracking between contact pins. Such tracking can occur if the surface of an insulation material gets carbonized and hence increasingly conductive. The root cause for such carbonization is surface contamination through ions, which are included in moisture, dust or other particles that get deposited over time on the plastic surface between two contact pins.

In principle, designers have three options to prevent this from happening:

<ul><li>Increase the creep distance between the two contact pins (the actual surface distance of the plastic) by introducing a rib or recess.</li><li>Avoid contamination of the plastic insulator surface from particles or moisture by encapsulating the entire system.</li><li>Use an insulation plastic with an intrinsically higher CTI.</li></ul>

Creep distance is the actual surface distance between the contacts (Fig. 5). If the direct distance or pitch between the contacts remains the same, the creep distance must be increased to avoid tracking between these pins at higher voltage. When designers are forced to squeeze additional electronic functionality such as glycol cooling into the often confined available space, it will hardly be possible to increase the creep distance, especially not without increasing the pitch between the contact pins. Encapsulating such connectors and plugs to avoid external contamination is not always feasible, either. In such cases, the designer can use insulating plastics with a higher CTI value (Fig. 6). In this way, the risk of tracking can significantly be reduced.

DSM offers halogen-free flame retardant plastics that deliver the required electrical performance, and is the first and only company worldwide that has globally launched compounds for high-voltage applications with a certified CTI rating of >700 V (for Akulon PA 66) and >850 V (for ForTii PPA). These compounds are free of metals, halogens, red phosphorous and halide-based heat stabilizers, so that potential electrical corrosion of any metal used in the assembly of the components can be avoided. They are also available in orange colored grades.

To ensure highest safety and reliability – not only during technically critical application testing on component level, but also during the entire lifetime specified by OEMs – DSM has carefully studied the insulating properties of plastics at room temperature as wesll as after long-term aging at high temperature. The data provides a good reference to engineers on the actual lifetime performance of the electric powertrain at actual working temperatures.

Fundamentally, there are three electric breakdown mechanisms for a plastic resin as an insulation material:

Intrinsic or material-specific breakdown, which depends on the presence of free electrons capable of migration. When an electric field is applied, these electrons are accelerated and tend to escape from the trap of the molecular structure. When the field strength reaches a certain limit, they are then released from the molecules. Accelerated by the electric field, the electrons collide with each other, which eventually leads to an avalanche effect resulting in electric breakdown.

Thermal breakdown, which depends on the Tg of the plastic compound and on the polarity of the molecular structure, and it is the main failure mechanism for a plastic compound. When the temperature rises above Tg, this has two effects: First, the molecules of the polymer start to become movable, which makes them “rub” each other and creates energy loss (dielectric loss). As a consequence, the temperature of the resin will increase further. Second, the volumetric resistance of the polymer will decrease. This weakens the molecular trapping of electrons, which will tend to cross the from the valence into the conduction band. In a high-voltage field, this will result in a higher amount of current passing through the polymer, which can increase the temperature of the polymer dramatically.

Figure 7 shows the correlation of the electric breakdown strength decrement vs. the temperature increment of plastic compounds before aging. Characteristically, the dielectric strength of many polymers decreases at higher temperatures.

Discharging breakdown, which can occur when there is a flaw in a plastic part, caused by voids, contamination or impurity. Normally, these flaws are consistent or show a medium or lower dielectric strength. In a high-voltage field, such an inhomogeneous structure will lead partial discharge, which can result in breakdown.

Figure 8 illustrates the electric breakdown strength of different polymers after heat aging.

Electrical properties, such as CTI, electric breakdown and volumetric resistance are important parameters when selecting the right insulation material for high-voltage components.

Figure 9 provides an overview of potential material choices for electrical applications within the powertrain. For materials used in the high voltage path DSM recommends a lower limit of 10 kV/mm of electric breakdown strength with a related volumetric resistance of 108 Ωcm at the respective operative temperature of the application.

Due to their high Tg and aromatic content, halogen-free ForTii PPA compounds offer a sufficient electrical safety margin even at elevated continuous use temperatures of 160 °C. Xytron PPS, because of its linear and symmetrical molecular structure in combination with high aromatic density, shows much lower polarity than most other polymers. This makes it an ideal material for components designed to work in the harshest conditions and stand up to peak operating temperatures over 200 °C. For these reasons it is often utilized in environments where applications can be exposed to severe chemical, oil or other aggressive agents.

Fig. 4. In order to increase the battery performance, it is usually more advisable to increase the voltage rather  than the current

Fig.5. The creep distance is often deliberately increased to prevent the flow of current between two contacts

Fig. 6. Comparison of different polymer classes by tracking resistance. Colors indicate respective mechanical strength

Fig. 7. Electric breakdown strength before aging at elevated temperatures. Most polymers show a decline in breakdown strength at rising temperatures

Fig. 8. Electric breakdown strength after aging at elevated temperatures. The curves of the materials are similar before and after aging. As a result of aging, the dielectric strength decreases as expected 

Fig. 9. DSM high-voltage material portfolio for high operating temperatures. Different materials are recommended depending on the temperature range

Functional diagram of the powertrain in a battery electric vehicle. Electrification adds further components (source: DSM)

In electric vehicles, the change in engine technology also entails new requirements for the plastic materials used

Table 1. DSM portfolio of compounds without critical halide salts or red phosphorous, which could lead to electro corrosion

Fig. 1. Key components of the high voltage drive train system of a Chevrolet Bolt

Fig. 2. PA is a very suitable material also for EV applications. For instance, PA6 and PA66 are used in HV connectors, HV PA 4T busbars and contactors

Fig. 3. Terminal corrosion caused by outgassed heat stabilizers or migration of other ionic impurities in the plastic

DSM - Hauptartikel

A standard electric powertrain system consists of a power distribution unit (PDU), an on-board charger (OBC), an inverter, a converter and an e-motor.

Inverter: Sometimes also referred to as an intelligent power control module, the inverter transforms the direct current (DC) supplied from the battery to three-phase alternating current (AC) to drive the e-motor. Components found inside the inverter can include insulated gate bipolar transistors (IGBT), thick metal busbars to carry high currents, three-phase terminal blocks, capacitors and inductors.

PDU: The power distribution unit centrally distributes power to various units, working as a dispatch station in the EV system. Busbars and contactors are typical key components inside the PDU.

Convertor: The converter changes the voltage from the higher battery levels of in between 400V and 800 V to the lower 12 V required by other power consuming units, such as the infotainment system in the car, without changing its DC character. Field effect transistors, capacitors, inductors and busbars are typical components assembled in the converter.

OBC: An on-board charging device is the interface between an external charging plug and the PDU. In contrast to the inverter, it transforms AC to DC voltage. It works under the slower charging mode and is required if charging is done on the AC net, such as at common domestic wall plugs. Given the typically much lower power range of the grid between 3.3 kW and 7.9 kW, charging is slower compared to the high-voltage DC fast chargers.

Dip.-Eng. Yu Bin has been Advanced Engineering Manager Electrical Vehicles at DSM Engineering Plastics in Shanghai, China, since 2014.

Dr. Tamim P. Sidiki is Marketing Director Automotive & Consumer Electronics at DSM Engineering Plastics. He joined the Dutch company in 2007 and focuses on advances in the electronic and automotive applications.

Dr. David Zhu has been working as scientist for DSM in Shanghai since 2013. His fields of activity include aerials, solar cells, 5G and the electric powertrain of vehicles.

Read the German version of the article in our magazine Kunststoffe or at <a href="http://www.kunststoffe.de">www.kunststoffe.de</a>

Elektromotoren

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Automotive

Elektronik

Elektromobilität

Mobilität

Kunststoffe

Photonik

Optik

Polyamide

Kunststoff Werkstoffe

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Elektronische Bauelemente

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Technische Kunststoffe

Hochleistungskunststoffe

“Plastics Play a Key Role in  Lithium-Ion Batteries”

Interview with One of this Year’s Winners of the Nobel Prize in Chemistry,  Dr Akira Yoshino

Lithium-ion batteries are the basis of most mobile devices. Auto manufacturers also rely on them in their electric vehicles. The importance they have now acquired for our society is clearly shown by the award of the Nobel Prize for Chemistry 2019 to Dr Akira Yoshino, Dr Michael S. Whittingham, and Dr John B. Goodenough for their contributions to the development of lithium-ion batteries. Kunststoffe interviewed Dr Yoshino about the key role plastics play in these batteries and how the range problems of electric vehicles are being solved.

Interview with One of this Year’s Winners of the Nobel Prize in Chemistry, Dr Akira Yoshino

Electromobility is on the advance. Practically all auto manufacturers are investing in both electric vehicles and the charging infrastructure. The brainchild at the heart of electric cars but at the same time their greatest problem child is the lithium-ion battery. It is still too heavy and its capacity too small, so limiting the driving range of these vehicles. Kunststoffe interviewed the “father of the lithium-ion battery”, Japanese chemist Dr Akira Yoshino, about the current difficulties and future of his creation.

Kunststoffe: What role do plastics play in lithium-ion batteries?

Dr Akira Yoshino: A key role – particularly in relation to lightweight design. Take pouch cells, for example. Their outer casing generally consists of an aluminum-plastic laminated film. Before this development was introduced in 1995, batteries used metallic cylinders. The new cell type made batteries lighter and slimmer. In the automotive sector, too, a decision will have to be made in future as to whether the batteries use metallic cylinders or pouch cells.

Kunststoffe: What improvements in lithium-ion batteries are you working on at present?

Yoshino: Lithium-ion batteries are already a mature technology for small electronic devices. In the automotive sector, on the other hand, I still see very great potential for improvement. I am particularly keen to research the fundamentals of lithium ions again. The movements of lithium ions differ significantly – depending on whether they are in a liquid or solid electrolyte. When it comes to optimizing the batteries, it will play a major role as to how we can increase the speed of the lithium ions. Further development of the electrolytes – liquid and solid – is the key here and is already forging ahead.

Kunststoffe: You once said you hoped that your invention would change the whole world in the automotive sector. What changes do you hope and expect to see?

Yoshino: The abbreviation CASE, for Connected, Autonomous, Shared und Electric, perfectly describes the direction in which the world is heading. Autonomous vehicles will be connected worldwide and shared with other users. I believe that a new mobile society is emerging. Not only with the increasing popularity of electric vehicles but also with the aid of other technologies such as AI, IoT and 5G.

Kunststoffe: But there are still some problems with electromobility, for example the limited driving range of the vehicles. The battery is often blamed for this. How can this problem be solved?

Yoshino: I firmly believe in a “shared automotive society” from 2025. Private vehicles will gradually be replaced by fully autonomous, smart vehicles moving through the streets with the best possible efficiency and always recharging in good time. Limited range will then no longer be an issue.

Kunststoffe: You are called the “father of the lithium-ion battery”. This year you were awarded the Nobel Prize in Chemistry. Which means more to you?

Yoshino: Both are an incredible honor for me and make me very happy. The two accolades are also directly connected with each other. After all, the development of the lithium-ion battery played no small part in my being awarded the Nobel Prize in Chemistry. (smiles)

The interview was conducted by Florian Streifinger, Editor

Yoshino - Hauptartikel

For nearly 40 years now, the Japanese chemist, Dr Akira Yoshino, has been working on lithium-ion batteries. In 1981 he started development of rechargeable batteries, initially with polyacetylene as the anode material. Two years later, he had developed a functioning battery. In 1985, there followed the prototype of the lithium-ion battery in its present form. Since then, Dr Yoshino had been working continually on the further development of these batteries, improving, for example, their safety and charging methods. For his invention, he was awarded the 2019 Nobel Prize in Chemistry. Since gaining his Master’s degree in 1972 at the University of Kyoto, Japan, Dr Yoshino has been employed by the Japanese company Asahi Kasei. He is also currently President of the Japanese research consortium for batteries, Libtec. He completed his doctorate at the University of Osaka, Japan, in 2005. Since 2017, he has been a Professor at Meijo University in Nagoya, Japan.

“With fully autonomous,  smart vehicles, limited driving range will no longer be an issue“

Batterien

Energiespeicherung

Underbody Protection for Batteries in Electric Vehicles

Batteries in electric vehicles must be specially protected to counter the risk of fires. Battery trays made from aluminum frequently offer insufficient protection. The automotive supplier ElringKlinger has therefore developed underbody protection manufactured from glass fiber-reinforced PP.

Batteries in electric vehicles must be specially protected to counter the risk of fires. Battery trays made from aluminum frequently offer insufficient protection.

Until now, battery trays for electric vehicles have been produced mainly from aluminum. Recently, however, a number of vehicles have caught fire after objects lying in the road were flung up at high speed against the battery trays. These components were designed to resist low-speed impact stresses and could not withstand this ballistic impact. As a result, the objects pierced the trays and damaged the batteries, which then began to burn.

ElringKlinger AG, Dettingen an der Erms, Germany, therefore developed a ballistic-impact-resistant battery tray system. It had to weigh no more than an aluminum component and incur no significant extra costs through the use of expensive materials such as aramid fibers or titanium. In the interest of sustainability, the aim was to achieve the greatest possible recyclability of the materials used and the finished battery tray.

The battery trays in electric vehicles are primarily underbody systems designed to protect the batteries from impact stresses arising from the road. These are simulated in the development process by a bollard test with defined energy input. From the available installation space, mounting system, and selected material, the possible component thickness, weight, and costs can be calculated.

The behavior of different fiber composites under impact stress was tested by ElringKlinger using the puncture impact test as per standard ISO 6603–2. The matrix systems tested were polypropylene (PP) and an epoxy resin, while the reinforcing materials used were glass and carbon fibers. The tests showed that in a direct comparison, glass fiber-reinforced PP recorded the highest maximum forces and energy absorption (Fig. 1).

The following material properties and mechanisms are responsible for this:

<ul><li>Ductility of PP in contrast to brittle epoxy resin,</li><li>high elongation at break of glass fibers (5 %) in comparison with carbon fibers (0.5 %),</li><li>high energy transfer through fiber pullout due to limited adhesion of the polar glass fiber surface to the non-polar PP.</li></ul>

As the results show, laminate structures based on unidirectional (UD) tapes achieve the highest test values. The UD tapes developed in-house with 80 wt. % and 60 vol. % glass fiber content not only have improved maximum force and energy absorption but also significantly increased stiffness. Currently available UD tapes with 70 wt. % glass fiber content have a tensile modulus of approximately 34 GPa. Through the increase in fiber content, the value could be increased to 47 GPa. This is a step in the right direction for creating a system with an installation space similar to that of the aluminum tray. But it is still not enough, since the elastic modulus of aluminum (70 GPa) is considerably higher. So installation space equivalence can only be achieved by increasing the structural stiffness of the battery tray.

The technical solution lies in a sandwich structure known as ElroSafe. This uses UD tapes for the outer layers, because of their lower density, and a direct long fiber thermoplastic (DLFT) as the core material. In this way, through higher wall thickness, the required structural stiffness can be obtained for the same areal weight. Comparative calculations and validation tests show that with a 5 mm-thick sandwich comprising 1.5 mm outer layers made from UD tapes and a 2 mm core made from a DLFT with 30 wt. % glass fiber content, an installation space equivalent to that for a 3 mm-thick aluminum sheet (comparative material: EN AW-5754 H22) is obtained. Installation space equivalence here means the same component thickness plus deformation under defined load. Testing was carried out using application-appropriate geometry with a free bending length of 254 mm. In the testing instrument, a standard bollard with a diameter of 180 mm and weighing 100 kg was dropped from a height of 85 mm onto the 340 x 290 mm test panel (Title figure). This corresponds to an energy input of 84 J. Deflection over time was measured on the underside of the test panel.

Table 1 compares test panels with different glass fiber contents in the UD tapes of the outer layers and a 3 mm-thick aluminum sheet.

These results, taken together with curves shown in Figure 2 (left aluminum, right ElroSafe), lead to the following conclusions:

<ul><li>Only with a UD tape having a glass fiber content of 80 wt. % can installation space equivalence with aluminum be obtained.</li><li>The ElroSafe sandwich transfers the input energy fully elastically. This is possible due to the high tensile strength of the tape (up to 1000 MPa) and the high elongation at break of the composite (>2.5 %).</li><li>The aluminum sheet deforms plastically in the form of a permanent bulge of 12.2 mm (Fig. 3) due to its low yield strength of about 100 MPa.</li></ul>

It is assumed that damage events of this type and intensity rarely or never happen in a vehicle’s life. Nevertheless, when driving off paved roads or descending from a curb, such damage events may possibly occur. A battery tray produced from ElroSafe effectively prevents damage to the battery in these cases. Even after repeating the bollard test ten times on the same sandwich panel with an energy input of 84 J, no permanent damage can be seen. Following these tests, the drop height was gradually raised in a type of increasing-load test. The first visible sign of damage, a ductile failure on the compression side, occurred at a drop height of 785 mm. At 735 mm, which corresponds to an energy input of 721 J, there was still no damage. ElroSafe can therefore withstand very high loads without damage but requires additional deformation space for this.

Following the battery fires occurring as a result of running over objects in the road, ballistic impact tests were conducted on behalf of a German auto manufacturer. In these tests, 5 kg steel projectiles were fired onto various materials at 110 km/h and an impact angle of 15°. Only the ElroSafe sandwich, in the described overall thickness of 5 mm, withstood the test without penetration. There was only slight linear abrasion of the outer glass fibers on the side hit by the projectile. In this case, too, the input energy was fully converted into elastic deformation.

In accidents between a gasoline or diesel vehicle and a battery-powered vehicle, spilt fuel can ignite under the battery vehicle. It is therefore important to ensure that batteries do not catch fire before the passengers of the battery-powered vehicle have been rescued. Standard ECE180 defines a suitable test for this. A trough of burning gasoline with a flame temperature of 700 to 800 °C is slid under the component being tested or a significant section of it. It remains there for 130 s. During flame exposure, the component must not lose its structural integrity. In addition, the temperature on the inner side of the component must not rise so high as to ignite the battery.

The fire test rig can be seen in Figure 4. On an ElroSafe sheet measuring 695 mm x 695 mm, aluminum profiles were fitted all round, simulating the construction of a current battery tray. The tray was sealed on top with fire-resistant gypsum plasterboard. On the outside of the profiles, two thermocouples were laterally mounted and another was passed through the plasterboard to the inner side of the ElroSafe sheet.

Figure 5 shows the temperature curves measured on the profiles (yellow and green). These curves represent the combined temperature of the flames and the aluminum profiles. The red curve shows the temperature curve on the inner side of the ElroSafe sheet. Because of the low thermal conductivity of the thermoplastic fiber composite, a temperature of only 100 °C was reached there.

The high fiber content leads to extremely dense packing of the unidirectional glass fibers, which prevents oxygen reaching the deeper layers. For this reason, only a very thin matrix film burns on the side exposed to flame. The weight loss of the sheet was only 14 g.

The low increase in temperature on the inner side of the ElroSafe sheet shows the good thermal insulating effect of ElroSafe. Since these batteries only work optimally within a certain temperature range, their temperature must be controlled. When outdoor temperatures are low, a by no means negligible amount of energy is required for this. The energy demand is higher the more energy is lost to the environment as a result of thermal conduction. In this respect, a battery tray made from ElroSafe achieves a substantial improvement over aluminum with its very high thermal conductivity and so also extends the vehicle driving range.

The initiation and progression of a sound event arising as a consequence of an external stimulus such as a stone strike are characterized by measuring the acoustic impedance. Here, the pulse height, i. e. the sound volume, and the decay behavior of the pulse over time are measured. Even in this regard, a battery tray made from ElroSafe has considerable advantages over its aluminum counterpart. The sound pulse is lower and decays faster. This is particularly important with high-end vehicles.

The tests during the development process clearly show that ballistic-impact-resistant battery trays can be produced from ElroSafe. Besides the crucial safety aspect, such trays have very good suitability for everyday use because small impact stresses do not cause lasting damage. The trays also offer good protection from fires under the vehicle and are very good thermal and acoustic insulators. On account of these technical advantages and the low costs, a number of development projects with this material have already been started. Serial production is expected to follow in 2022 or 2023.

ElroSafe can also be used in lightweight design applications with thermoplastic fiber composites, which until now have been held back by high system costs. Possible applications include rear seat backs, bulkheads, and rear bumper beams. These components require additional functionalization in the form of ribs or mounting elements, which can be readily pressed from the DLFT core or produced in a subsequent injection molding process in the compression mold.

The starting materials for ElroSafe consist solely of PP, glass fibers, and commonly used additives such as carbon black for coloration, stabilizers, and adhesion promoters. All waste products along the production chain, such as edge trim from tape manufacture, but also reject parts, can be comminuted and used in the production of new components instead of an injection molding masterbatch. The regrind is blended with virgin PP on the injection molding machine to produce PP-GF30 or PP-GF40. This ensures a sustainable material loop, which has been validated by two studies carried out at Rosenheim University of Applied Sciences (TH Rosenheim), Germany <a href="http://[1]">[1]</a>.

Fig. 1. UD tapes made from PP with a high glass fiber content had much higher values than the other materials tested

Fig. 2. Deflection under load (180 mm diameter, 100 kg, 84 J): in contrast to aluminum, the PP sandwich structure transfers the energy fully elastically

Fig. 3. After three repetitions of the bollard test, the aluminum sheet had significantly deformed by 12. 2 mm, whereas the ElroSafe sheet was undamaged

Fig. 4. The material must withstand temperatures of up to 800 °C in the fire test. This ensures that the battery does not catch fire 

Fig. 5. Temperature curves in the fire test: the temperatures measured on the aluminum frame (green and yellow curves) are initially far higher than those measured on the ElroSafe sheet (red curve)

To be suitable for the underbody protection of electric vehicles, a material must first pass a bollard test

Table 1. Results of the bollard test on UD tapes with different glass fiber contents and an aluminum sheet

ElringKlinger - Hauptartikel

Harri Dittmar works for ElringKlinger as a consultant in Global Business Development Lightweight Solutions; <a href="mailto:harri.dittmar@elringklinger.com">harri.dittmar@elringklinger.com</a>

The results presented are not an individual effort and at this point the author would like to extend sincere thanks to the whole team involved: Martin Eberhardt (Head  of Development Compression Molding), Jaroslav Antoschenko and Markus Höfler (Development Compression Molding), Robert Witzgall (Predevelopment Lightweight Solutions), and Julian Kleih and Sebastian Hemmerle (Component Calculation).

Polypropylen-Platten

Halbzeuge und Zwischenprodukte

Kunststoff-Halbzeuge

Glasfasern / -matten

Verstärkungsstoffe

Aluminium

Metall Werkstoffe

NE-Metalle

Polypropylen

Large-Area Curved Plastic Panels with Integrated Displays

Harmoniously integrating displays into the lines of a vehicle cockpit has always been a real challenge. Continental now uses a combination of film insert molding, in-mold decoration and injection-compression molding to produce large, three-dimensionally formed plastic panels.

The display increasingly sets the tone of a vehicle cockpit. Most current car models already have two full-color, graphic-capable displays. The instrument cluster usually consists of a large display (full digital cluster, FDC). In addition, there is usually a second display in the center console, which presents a wide range of content that is not directly relevant to driving, such as navigation, infotainment and operation. It often has a touch function. For years, the trend has been toward increasing the number of displays, as well as their diagonals and resolution.

Precisely harmonizing the technical properties of such side-by-side displays (white point, color temperature, color coordinates and black homogeneity) represents one major challenge. Another is to integrate the display surfaces harmoniously into the lines of a cockpit. Glass-based displays turn out to be somewhat intractable in this respect. The curvature that can be achieved is in the range from about 800 to 3000 mm radius. Flexible displays such as AMOLED (active matrix OLED) are significantly more promising. However, AMOLED displays must have a protective layer to make them suitable for the comparatively harsh use in vehicles with touch operation.

To solve this challenge, Continental has developed a continuous 3D plastic panel (Title figure). It protects the two 12.3 inch AMOLED displays that it covers against mechanical damage. In addition, the partially printed panel can be merged seamlessly into the surrounding surface, so that the entire curved surface acts like a single continuous instrument. To produce this special interior trim, multiple special production processes were combined with one another for the first time. So-called high-pressure forming (HPF) is used, e. g., for forming the two-dimensionally printed film to the three-dimensional contour of the final part. This is subsequently inserted into a mold and back-molded. This process is called film insert molding (FIM). With in-mold decoration (IMD), the hard coating is applied to the part while it is still in the injection mold in a roll-to-roll process. In combination with injection-compression process, this ensures very low stress conditions in the part. The technical challenge in developing this production process resulted from the dimensions of the panel: the displayed pattern measures around 200 mm x 700 mm. These dimensions cannot be processed by either IMD or FIM processes with >100 mm forming height. At the time of their commissioning, some of the machines used were unique in the world.

The 3D instrument panel is constructed of three functional layers. The reverse side is formed by a printed film (FIM) with 375 µm thickness, the front side is a 15 µm-thick hard layer, which is applied from a transfer film (IMD). The core of the part consists of 2.1 mm-thick polycarbonate (PC), which is injected between the FIM film and the IMD film at the front. It subsequently remains in the mold under homogeneous pressure and temperatures of initially 300 °C during the cooling phase. Despite the size of 200 mm x 700 mm, the roughly 2.5 mm thick end product only weighs 350 g.

This integration would not be possible without extensive technical experience with all the manufacturing technologies used. With the combination of all the steps, together with the three-dimensional contour of the panel, it is actually a completely novel process. Continental benefited here from its many decades of experience in printing and forming films, for example for the dials of traditional tachometers. The automotive supplier’s experience in injection molding optical and highly transparent components, such as mirrors for head-up displays and cover glasses was extremely important. The transfer application of thin films, too, has already been a large-series process for many years, for example for providing transparent indicator points with a luminous surface or producing high-quality piano black surfaces on plastic panels.

Besides the manufacturing technologies, another major challenge was the choice of suitable raw materials. The automotive sector actually makes very severe demands on the materials. They include heat resistance, for example to direct sunlight or the heat emitted from the displays, dimensional stability, scratch resistance despite numerous abrasion strokes and optical quality that must be comparable to high-quality materials such as mineral glass. High impact strength is also necessary in order to prevent splintering of the cover glass, even in the event of head impact in the worst case. In addition, it must also be possible to integrate touch operation. As with every lifetime component in the vehicle, a fatigue strength of over 10 years, or 300,000 km is expected – and has also been exhaustively tested. Only specially selected materials therefore come into consideration.

Figure 1 shows an overview of the two strands of the manufacturing process. The upper strand generates the printed reverse side of the panel by FIM; the lower strand feeds the transfer film with the hard coating. Where the two strands meet, the two film layers are backmolded by injection-compression.

At the beginning of the upper strand there is a large web of PC film (grade: Makrofol HF, supplier: Covestro AG, Leverkusen, Germany). The Makrofol HF is ideal for the purpose due to its high thermal resistance and transparency, as well excellent formability and elasticity over a wide temperature range. This 375 µm-thick film is first printed with functional and decorative motifs, as well as colored features, using a process combining digital and screen printing. Digital printing is used for creating challenging color gradients and graphic patterns. Even photorealistic prints can be implemented by these means.

In an HPF machine, heating tiles heat the finished printed 2D film webs up to its glass transition temperature of 150 °C. Then the heated web is positioned over the 3D mold and the forming mold closes. Air pressure at about 60 bar applies the film to the 3D contour of the mold (Fig. 2).

After removal from the machine, the formed film is cured under UV light. In the prototype production, the 3D blank is then trimmed to the part contour all round with a milling machine, and the required bores are drilled. This process produces the reverse side of the subsequent instrument panel. The HPF machine, from the manufacturer Niebling GmbH, Penzberg, Germany, allows genuine 3D forming of large-size parts with drawing depths of ≤150 mm at web sizes of 1150 mm x 650 mm. The forming tool was specially designed for this process.

On the lower strand of the production line in Figure 1, the transfer is prepared with the hard coating. This IMD film is fed to the injection-molding machine as roll material. With this manufacturing step, too, it was necessary to cross existing technical boundaries due to the three-dimensional and large-size design of the display panel.

Before backmolding, the printed 3D film is inserted into the complex injection mold. The two mold halves together weigh over 16 t. In addition, due to the complexity of the mold, it has a certain built-in intelligence in the form of a programmable logic controller (PLC), which, among other things, supports operation by smartphone via WLAN. The PLC is responsible for monitoring and coordinating the over 60 actuators and sensors that are located in the mold. It independently handles interplay with the injection-molding machine and two other secondary peripheries. During insertion of the FIM, the IMD protection film (type: Bright View; supplier Leonhard Kurz Stiftung GmbH & Co. KG, Fürth, Germany) is positioned in the mold by a special film feeder. To fix the film in the mold, a twelve-part clamping frame concept is used. This fixes the film in the mold using a precisely defined sequence of velocity and position control (Fig. 3). The transfer film is now laid wrinkle-free in the mold under vacuum.

When the two films have been positioned, the mold halves are closed and the films are backmolded with PC (grade: Makrolon Ai, supplier: Covestro) at up to 300 °C. Due to the high temperatures and pressures, the printed FIM and the transparent IMD protective coating become an integral part of the panel. To ensure that the subsequent panel has a uniformly high optical quality, the plastic is injected into the cavity while it is still slightly open, and it is then only closed fully after complete injection. This ensures a low filling pressure and a homogeneous pressure distribution during the cooling phase. Because of the resulting opening forces, the hydraulics of the machine must generate clamping forces of up to 1500 t. As the machine opens, the IMD transfer film is released from the front side of the 3D panel, while the hard layer (protective coating) remains on the PC.

The PC, with its glassy transparency, has high impact strength even at low temperatures and the good dimensional stability gives the 3D instrument panel its strength. After cooling, the IMD protective coating on the final panels is hardened under UV light. The sprues are subsequently removed and the panels are now ready for the optical dry bonding of the two 12.3-inch AMOLED displays.

Plastic panels allow additional 3D structures to be generated on the panel front side, which guide the driver’s fingers during operation of the center console (Fig. 4).

This allows digital operating elements, such as slide controllers for volume control to be replicated. These tactile orientation aids make good hand-eye coordination less critical. The driver can reduce the time he takes his eyes off the road, and maintains a better overview of the traffic situation. In addition, the 3D instrument panel can give the driver tactile feedback. For example, he feels it in his fingertips as soon as he has successfully performed a touch input. The visual check is therefore briefer or entirely unnecessary, which further increases safety. The instrument panel, which is 3D formed for design reasons, thus also supports ergonomic requirements on an operating interface in the vehicle.

The manufacturing processes, comprising FIM-3D forming, roll-to-roll IMD coating and injection-compression, allow large-size instrument panels with high optical quality to be produced. Besides their low weight, they possess many key advantages. First, the combined process allows the integration of curved display surfaces, which principally shifts the center control better into the arc of reach of the driver’s right hand. This makes the functions integrated in the center console display more ergonomic. The possibility of additionally generating three-dimensional structures on the surface of the panel makes operation during driving even safer and more intuitive. And finally, the process permits the seamless integration of individual AMOLED displays without visible transitions between the screen and panel surface. The result is a harmonious cockpit with attractive flowing curves. The innovative manufacturing technology with purpose-adapted machines has proven its effectiveness in the manufacture of prototype panels. The innovative process is therefore suitable for future series deployment.

Fig. 3. The twelve-part tensioning frame for positioning the IMD film ensures precision fixing of the film before backmolding

Two displays are seamlessly integrated into the continuously 3D-formed instrument panel

Fig. 1. The plastic panel is manufactured in two separate processes, which are then merged together

Fig. 2. The FIM machine for 3D heat forming of large-size polycarbonate arcs operates at 60 bar