Thursday 20 June 2019

Polyphenylene sulfide (PPS) – The Conquering of Electric Car Parts

In this blog post I explain why polyphenylene sulfide (PPS) is conquering more and more key parts in higher voltage electric cars. In my electrification series, I have discussed the requirements of certain parts already in detail as well as which advanced polymers can be used. A major one is PPS.

Currently, a typical internal combustion engine (ICE) has around 700 grams of PPS polymer on board. New numbers form Asia reveal that there will be 3 to 4 kg of PPS in electric vehicles (EVs) and hybrid electric vehicles (HEVs). This is a huge increase. Let’s find out why it is so heavily in use.

Reason number 1: Elevated temperatures during usage over a long lifetime (over 6000 hours)
Applications such as capacitor cases, invertor cores, motor cores and housings have to withstand elevated temperatures during use and need to have excellent heat cycle performance. PPS can fulfill these set of requirements in an economic manner.

Reason number 2: Thermal management systems
Cooling of the battery and the electric motor requires water pumps. The latter need to perform when constant exposed to the water-glycol mixture. The outstanding thermal and chemical performance of PPS makes it a very suitable candidate for any application inside the water pump, e.g. impellers. Cooling of the battery is necessary when the car is charging as well. As such the lifetime expectation of (plastic) parts is also higher.

Reason number 3: Good metal overmolding capabilities
Busbars are usually thick copper lines which need to be overmoulded. Here again, processing of PPS due to good flow properties is an advantage together with its good electrical insulation properties.

Reason number 4: Dimensional stability at ambient conditions
Exterior positioning sensors in electrical vehicles need to be dimensional stable at various ambient temperatures and humidity. This ensures accurate positioning detection. In this context, PPS exhibit negligible water uptake which makes them suitable for external positioning sensors. For instances, Nylons would be less of suitable candidates here due to their inherent higher hydrophilicity.

Reason number 5: Price level suitable for automotive market
With PPS you will get a lot of value for a reasonable price, i.e. continuous use temperature of 200-240°C, UL94 V0 rating, chemically resistance up to 200°C and dimensional stability at ambient conditions. Price is a major advantage in comparison to other high performance polymers.

A short word on linear vs cross-linked PPS:
There are three major routes to obtain PPS. First one is called flash process with curing. The curing step is needed to increase molecular weight [2]. This process results in branched PPS. The second route encompasses the flash process and metal carboxylates and results in linear PPS [3]. The third route is over the quench process and results in linear PPS as well [3]. Leaders in the industrialization of the PPS polymerizations were Chevron Philips and Kureha back in the 1980s. For electric vehicles, linear types of PPS are optimal. These show superior toughness and improved weldline strength. In general, linear PPS does not process as well as cross-linked PPS. However, less moulding flash is generated by linear PPS types.

What are the potential downsides of using PPS for e-mobility applications?
One aspect to consider is the comparative tracking index of PPS which is between 250 and 275 Volts. This is low compared to PPA which can easily reach 600 Volts. Another point is its low thermal conductivity, 0.3 -0.5 W/mK and finally its brittleness.

Besides PPS, what are next best candidate materials?
In the table below, I have listed polymers which can be in competition for applications using PPS: syndiotactic polystyrene (sPS), polybutylene terephthalate (PBT), and polyphthalamide (PPA).

PPS, branched or linear, are in the lead for e-mobility applications, especially for high temperature electronics. Automotive, regardless of ICE or EV, will be the main driver for using PPS. This was a wrap up on PPS used in electrification applications.

I hope you have enjoyed it!
Till next time!
best regards,
Herwig Juster

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[2] Nexant Chemical Systems
[3] Solvay Specialty Polymers – Ryton PPS

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