Tuesday, 30 April 2019

High Performance Polymers in Electrification: A Must-Have Or A Nice-To-Have (Part 1: Introduction and Battery Systems)

High Performance Polymers in Electrification: A Must-Have Or A Nice-To-Have (www.findoutaboutplastics.com)

In automotive industry, electrification gains traction and design engineers all over are working to increase the energy density of batteries, create higher voltage traction motors and increase allover system voltage. In this three part series, we will discuss the requirements of different system components, i.e. batteries, traction motor, power electronics and autonomous driving in terms of polymers. Let’s get started with batteries.

- Plastic parts in electric vehicles (EV’s)

In 2019, we see all major OEMs and Tier-1 supplier continue their efforts toward electrification and new mobility solutions. Currently, development progress of internal combustion engines is at 1% per year [1], whereas electrification developments progress at a rate of 7 %. Industry experts predict that the connected mobility can grow according metcalfe's law with a rate of 70% (taking Facebook user rates as a baseline). The overall plastic weight per car will not change significantly with EV’s. However, there will be a slight increase in weight in total. There are currently 10 000 parts made out of plastic in an average car and these use ca. 39 different polymers. Out of the 39, 6 are used the most, i.e. polypropylene, polyurethane, polyamides, polyethylenes, acryle-butadien-sytrenes, and polyvinylchloride [2].

Check out here my infographic on “Plastics in Automotive”

Also in electrification, light weighting together with fuel economy will continue to be a megatrend. The rule of thumb says that for every 10% of weight reduction, fuel economy improves by ca. 6-8%. This additionally drives the consumption of plastics in automotive. The Chinese car market shows a lot of potential for light weighting since most cars run on old technology platforms. Furthermore, this is the largest car market today.

- What changes with EV’s?
EV’s have no longer the need for fuel tanks, AdBlue tanks, pumps and fuel connections. Here, we will see a drop in engineering polymers, together with elastomers since fuel lines and gaskets are not in demand. On the commodity side, high density polyethylene (HDPE) will also lose out since it is used for making tanks. Under the hood, combustion engine covers will not be needed either, however new engine covers for traction motor will rise.

- Which challenges are there concerning material selection?
In the past, material selection for internal combustion cars was easier since lots of experience as well as know-how were already built-up. This included the handling of specifications. For under-the-bonnet applications for example, aliphatic Nylons such as PA 6, PA 6.6., and aromatic Nylons such as polyphtalamide (PPA) are widely used. Furthermore, polyphenylene sulfide (PPS) is used where the highest heat and chemical requirements over the lifetime of the car are needed. PBT, PP, PU and POM/Acetals round up the polymers used in several exterior and interior applications.

Today, in hybrid and full EV’s, material selection tends to be much more differentiated, since applications need to fulfill specific requirements. The one-fits-all approach is no longer working in a similar sense. Standards from other industries such as electronics influence now material selection in automotive. As a result, a “wedding” between e.g. consumer electronics and automotive standards may take place.

Looking into the high performance material portfolio, semi-crystalline polymers fulfill stringent electronic requirements such as high CTI (>600), intrinsic flame retardancy, dielectric strength, creep, tracking resistance, EMI shielding, and high (140°C) Relative Temperature Index (RTI).

Lithium ion batteries – the heart of the EV
For coating the separator, fluoropolymers such as PVDF in aqueous dispersions can be used. Commodity polymers such as ultra-high-molecular-weight polyethylene (UHMWPE) can be used as separator substrate material. PVDF plays a role in making electrode binders (anode and cathode) too. In cell manufacturing, high adhesion between electrodes and separator is needed to obtain good laminates. Most common cell systems are pouch and jelly roll systems.

Battery pack and module – holding all together
Going outside the cell to the finished battery pack, several challenges need to be overcome. In general, a battery pack has between 10 to 16 modules and each module can have between 10 to 12 pouch cells.

For battery packs, three things need to be balanced:
• energy density (range),
• power (rapid charging),
• safety (non-flammable).

Every battery is connected over the pack’s housing with an electrical control unit, which protects the cells from overloading. The housing must ensure that all batteries stay in place and impact or vibration do not compromise performance. Figure 1 summarizes the potential plastic materials for cells, module, and pack.
Figure 1:  Overview of polymeric materials for battery technologies in the electrical powertrain.
Cells are connected to one another over busbars. Stability must be ensured. Busbars are usually long copper parts (up to 30 cm) overmoulded with PPS to obtain low dimensional changes. Flame-retardant plastics become increasingly important for packaging of the cells into modules. Modules have endplates and separator plates. Polymers such as PPS are a perfect candidate to be used as endplate and separator material in such environments since it ensures a best-in class thermal and chemical stability (RTI up to 220°C), inherently flame retardancy, and high dimensional stability. Selecting a linear PPS grade is beneficial since it has lower flashing during moulding. This minimizes post-processing steps after moulding and the investment of a de-flashing station. In addition, polycarbonate and their blends offer long-term capability and low-temperature impact strength which are needed for the battery modules as well.

Generally, all different assembles can benefit from thermal conductive materials which support the removal of the heat generated by the cells. Different polymer grades can be improved in this context by means of additives.

Thermal management – keep it cool
Next to the classic thermal management systems which use water-glycol, new ways of cooling are on the rise. This is driven by water-glycol reaching its performance borders with higher voltage system (>500 V). A new way is the so called “direct liquid cooling” which uses a dielectric cooling fluid and the cooling component is directly immersed in this fluid. This can be realized with fluorinated fluids such as perfluoropolyether (PFPE). These kinds of fluids have good thermal conductivity and combine a low electrical conductivity with low viscosity. Furthermore, excellent chemical resistance is given through the fluoro properties.

Heating systems for interior and battery
In contrast to traditional combustion cars, EV’S dissipate less heat to enable sufficient heating of the passenger cabin. Therefore, additional heaters need to be installed. Most of such heaters are based on the Positive Temperature Coefficient (PTC) effect. The ceramic based PTC elements can be lined up next to each other holding together via a support frame.

Here, a stringent requirement is not having ions in your polymer formulation. This is important to prevent corrosion of overmoulded metal parts and connectors. Galvanic corrosion can reduce the electric performance down to failure. Therefore, electrical friendly, halogen free stabilized semi-aromatic Nylons or PPS might be suitable here.

During operation, the battery pack temperature should be kept at 60°C. Heaters are used to keep temperatures on a certain temperature to ensure efficient charging. Heaters can operate in the high-voltage range (500V) with an output of 7 kW. High performance polymers will ensure their proper function over lifetime.

Charging Systems – high voltage with high safety
Charging the batteries of your electric vehicle in a reasonable time requires high voltage charging systems. Temperatures should be kept between 0°C to 45°C. Additionally, interconnection systems provide the power for the electric engine. Electrical properties such as dielectric strength, volume resistivity, creep, tracking resistance need to be carefully considered by design engineers.

Color coding helps safely handle parts in the event of an accident. Above 60 V, a system is considered to be high voltage and then orange color coding of connectors and cables is required.

For covering all these needs, polymeric materials need to fulfill Comparative Tracking Index (CTI) values of over 600, dielectric strength in the range of 30 kV to 35 kV, and Relative Temperature Index (RTI) of 140°C.

Aliphatic Nylons such as PA6, PA66, PA 46 fulfill this stringent criteria. Exception are certain temperature levels. Semi-aromatic Nylons such as PPA (6T/6I; 6T/6.6; 6T/6I/6.6) fulfill the temperature and electrical requirements. High mechanical strength combined with reflow soldering durability and laser welding properties make them a good choice. Important is to select types which are halogen free and free of red phosphorous which in turn allows Nylons to reach high CTI values. Electric corrosion of assembly bins is prevented by not using ionic heat stabilizers. Most Nylons have an HB flame rating and V0 rating is possible, however it might involve inorganic heat stabilizers.

With this I will close the first part of this high performance polymers series for EV.

In the second part we will look into the traction motor.

Thank you for reading!

Till next time!

Herwig Juster

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[1] http://www.professoren.tum.de/lienkamp-markus/ and https://www.youtube.com/watch?v=5moHQFbEsDU&t=2238s
[2] A. Patil: An overview of Polymeric Materials for Automotive Applications, Materials Today: Proceedings 4 (2017) 3807-3815

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