Material selection for the thermal management system in an electric vehicle
An automotive thermal management system (TMS) is, in general terms, a system that manages the temperature of automotive units, such as engines. If a component is to operate under optimum conditions, it has to be kept within a corresponding temperature range. A TMS is designed to optimize a component's operating conditions by controlling its ambient temperature in order to extend its service life and reduce energy consumption.
Vehicle electrification is gaining momentum, particularly in the form of hybrid carsvehicles and fully-ebattery electric carsvehicles. These two technologies have brought completely different challenges to thermal management systems. Reasonable selection of engineering plastic materials in TMS designs can enable automobile makers to stay ahead of the competition in terms of design cycles, versatility of components, and total costs. This article will first shed some light on the exacting requirements for materials chosen in TMS manufacturing for EVs, and then analyze several ideas for selecting materials.
The ultimate challenge brought about by automotive electrification for TMS materials are two-pronged: exposure temperature and exposure time.
Let's start by looking at fully battery-driven carsvehicles. The actual operating temperature of the TMS in a fully-ebattery electric vehicle is lower than that of a fuel vehicle. But what poses a real challenge to the TMS is doubled runtime. The temperature of a fully-ebattery electric vehicle’s batteries cannot be below freezing. A TMS that performs at high latitudes still needs to operate to warm the batteries in the winter when the vehicle stops. This requires double the maximum time of exposure to water-resistant glycol coolant. The figure for fuel vehicles needs to be 1,000 to 3,000 hours, while the figure for fully-ebattery electric vehicles is required to reach 6,000 to 10,000 hours. After so many hours of thermal aging, the performance of many materials tends to drop dramatically.
Now, a closer look at hybrid carsvehicles. A hybrid car is equipped with both a gasoline engine and electric-driven oil and a gas system. Its engine, newly fitted electric-drive motor, and powerelectronic control system are combined into onea hybrid engine. Its compact layout and insufficient space for higher local temperature arising from poor heat dissipation mean that the TMS mighthas to work under higher pressures and at higher temperatures. OEMs are paying close attention to aging resistance data at 135C and even 150C. The performance of engineering plastics after heat aging under these temperatures is severely challenged.
Are various engineering plastics able to meet these challenges? Let us now study the relationship between the exposure temperature and exposure time of a variety of current engineering plastics. The following graph shows how the exposure temperature of some mainstream engineering plastics used in thermal management systems changes with exposure time: polyphenylene sulfide (PPS), polyphthalamide (PPA), long chain polyamides (LCPA), and polyamides 66(PA66).
We tried to analyze the performance of these materials from two angles. The purple line shows that the number of materials that are able to resist heat reduces as the exposure temperature increases; when the temperature exceeds 130C, the stability of PPS and PPA becomes apparent. The yellow line shows that the longer the exposure time is, the less material becomes suitable for selection. The material with the lowest mechanical degradation after being kept at high temperatures for long periods of times is PPS, followed by PPA, LCPA, and PA66.
Why do these materials differ so vastly in terms of their aging resistance performance? To put it simply, it depends on the hydrolysis resistance of the resins that make up the materials. A TMS is based entirely on the coolant, which is a mixture of water and glycol. At high temperatures, water can cause severe damage to many materials as a result of hydrolysis. Therefore, the stronger a material’s resistance to hydrolysis, the better. PPA, LCPA, and PA66 belong to the polyamide family. The resistance of the polyamide amino bond to hydrolysis is insufficient. As a member of the polyamide family, PPA has the highest hydrolysis resistance, and it can be further improved upon by modification. PPS is different from nylon in its intrinsic molecular structure. Its molecular structure, which consists of thioether bonds and benzene rings, is simple but stable, enabling it to withstand concentrated sulfuric acid. Consequently, it is highly resistant to hydrolysis. Long-term performance under complex working conditions is crucial for electric vehicles. Therefore, PPS and PPA are recommended in the design of key TMS components.
Since PPS has such good properties, why is it still susceptible to physical aging? The engineering plastics used in a TMS are made up of resins and glass fibers. Another of these materials weaknesses is that the bonding interface between the glass fiber and the resin is prone to cracking under the influence of water. The bonding interface determines the difference in the anti-aging performance of varying PPS materials. DSM technology can make the direct bonding of glass fiber and PPS very strong, and thereby significantly slow the aging of PPS.
Changes in the bonding interface between glass fibers and PPS resin before and after 135C thermal aging with water-glycol fluid – viewed under an atomic force microscope.
Although the materials chosen for the test were all PPS reinforced with 40 percent glass fiber, the differences in the bonding interfaces after aging were obvious. We used atomic force microscopy to visualize the differences at the micro-level. The white areas in each photo represent glass fibers, the yellow areas are PPS resin, and the black areas are the width of the bonding interface. Before aging, the bonding interfaces of the two materials showed virtually no black areas, indicating that the interface is both smooth and seamless. Over time, the black area became increasingly greater in width at the bonding interface of the competition (the second line), which means that a deep crack has occurred on the bonding interface. However, the bonding interface of the DSM G4080HR only showed a thin line after 3000 hours of aging at 135C, indicating that the bonding of the resin and glass fibers was still very strong. Due to growing aging time or challenging driving conditions, microscopic cracks on the surface and inside of parts will extend until the part eventually cracks or even breaks. The resultant leaks are familiar to everybody.
Driven by advances in bonding interface technology, DSM has launched G4080HR, a Xytron PPS product range for commercial use in the automotive industry. This innovation provides long-term super hydrolysis resistance, helping DSM's clients meet the challenges brought by the TMS revolution. The figure below shows the tensile strength and elongation at break (EAB) of G4080HR and its competition. After 3000 hours of aging in water-glycol fluid at 135C, the tensile strength of G4080HR decreased by only 21%, while the competition decreased by 61%. The result is a tensile strength 114% higher than the competition. The EAB of G4080HR went down by only 29%, while the competition fell by 49%. The outcome is an actual EAB that is 63% higher than the competition after aging.
Weld lines inevitably appear on TMS parts in injection molding. They are regarded as the weakest links of the overall structure. It can also be said that the strength of the weld lines determines the strength of the entire part and the required thickness of the product. With its bonding interface technology, DSM has greatly improved the tensile strength and elongation at break of weld lines in G4080HR. The following figure shows a comparison of the aging resistance of different samples with specially prepared weld lines: after 1000hrs of aging at 135C, the measured weld line tensile strength was maintained at 75MPa, 85% higher than the competition, and the weld line EAB was still 0.6%, 50% higher than the competition. This data suggests that the mechanical properties of the G4080HR at the weld line can also withstand rigorous testing.
In order to improve the long-term reliability of cooling systems for a new generation of electric vehicles, the possibility of upgrading coolant has also been considered. The aging resistance of materials towards different coolants is the third factor that affects material selection. There is no question that PPS products, especially the Xytron G4080HR, can minimize the need to alter design because of coolant changes, since they are resistant to strong corrosives like sulfuric acid.
The Xytron G4080HR product range has facilitated more flexible designing of lighter TMS components with thinner walls. The long-term performance of the final component can be more easily predicted due to its superior aging resistance and weld line strength retention. More importantly, the walls of TMS components can also be made thinner to cut costs. DSM has accumulated a wealth of experimental data on the Xytron G4080HR and other common products through tests conducted in various mediums at different temperatures and periods of time, including authoritative data from third-party Automotive test lab certification bodies in Germany. This data is instrumental in helping customers effectively predict the long-term performance and service life of components.
Electrification of vehicles boosts TMS market growth. The Xytron G4080HR offers to Automotive OEMs and TMS Tiers Recently, OEMs and suppliers of TMS components are facing a complex competitive environment and increasingly demanding technical requirements. The testing of new models for anti-aging performance in various working conditions is both time-consuming and labor-intensive. The adoption of a more reliable and stable solution when selecting materials can, makinge it easier for car manufacturers to reduce design-related risks and accelerate product development when upgrading their vehicle models. The Xytron G4080HR offers better long-term performance and is able to survive the most demanding conditions. It opens up the possibility of optimizing product design, as well as reducing both costs and weight. As a consequence, it allowssupports OEMs to apply reliable TMS components to a wide range of vehicle models.