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XYRON™ modified PPE resins boast a number of advantages derived from PPE—including heat resistance, low specific gravity, and flame retardance—that have made them the right choice for many vehicle-mounted batteries and peripheral components. However, resins used for battery applications must not only offer low specific gravity and flame retardance, but must also be resistant to various types of oils—a requirement that demands improvements over existing materials based on PPE, a non-crystalline resin.
In response to this demand, Asahi Kasei has combined our proprietary compatibilizer and flame-retardance technologies to develop a new solution: the XYRON™ TF series of PP-alloy grades. These are non-halogen flame-retardant materials that deliver high-level flame retardance (UL94 V-0, 1.5mm thickness) while simultaneously achieving oil resistance thanks to alloying with PP (polypropylene), a crystalline resin.
XYRON™ TF701 is an oil-resistant flame-retardant PP+PPE alloy grade that exhibits flame retardance on par with that of flame-retardant PS+PPE grades together with good mechanical properties, low specific gravity, and excellent electrical properties.
To characterize oil resistance, we measure critical strain. We mount a test fragment on a bending bar designed to ensure continuously varying strain (Figure 1), coat this fragment with various oils, and measure the minimal strain at which the fragment ruptures (the critical strain).
Table 3 compares the critical strains measured for
XYRON™ TF701 and for an existing flame-retardant PS/PPE resin grade coated by three types of oil: engine oil, brake fluid, and lubricating oil. For all three oils, the critical strain measured for TF701 is significantly higher than that for the existing flame-retardant PS/PPE resin, making TF701 the right choice of material for components that may come into contact with these oils.
To characterize formability and ease of processing for injection molding, we conducted fluidity tests using a spiral-flow mold, and the results are shown below. At a mold temperature of 80°C and a maximum injection pressure of 50 MPa, tests at various cylinder temperatures confirmed that the fluidity of TF701 is superior to that of an existing flame-retardant PS+PPE resin. One likely application for this material will be to fabricate cell spacers for vehicle-mounted batteries; we confirmed the successful use of TF701 to form a thin-walled component mimicking such a spacer (dimensions: 100 mm × 100 mm × 0.4 mm thickness) via standard injection molding. In general, given the limited space available to house vehicle-mounted batteries, the ability to form thinner spacers is advantageous from a space-reduction perspective.
Another key imperative is to minimize warping when forming components, as materials susceptible to significant warping may yield components that cause interference problems in product assembly. The following figure quantifies the extent of warping observed in the formation of a slab of dimensions 150 mm × 150 mm × 2 mm thickness. We see that, in comparison to an existing flame-retardant PP material, TF701 exhibits significantly less warping and is thus superior for component formation.
To characterize resistance to thermal aging (in an 80°C environment), we measured Charpy impact strength retention, and the results are shown in Figure 3. TF701 retains more than 80% of its strength after 3,500 hours, demonstrating the suitability of this material for applications requiring long-term retention of physical properties in high-temperature environments.
We also measured the retention of Charpy impact strength to characterize aging properties in a warm, humid environment (85°C with 85% relative humidity). As shown in Figure 4, here again, TF701 exhibits high retention of physical properties, indicating excellent robustness in warm, humid environments.
PP and other olefin-based resins are known to exhibit poor resistance to copper contamination: in environments involving contact with copper or other metals, these materials suffer oxidation reactions that degrade their physical properties. To characterize resistance to copper contamination, we measured retention of tensile strength for test fragments of materials wrapped in copper foil and aged at 150°C. The results are shown in Figure 5. Whereas the tensile strength of an existing flame-retardant PP material begins to decline rapidly after 500 hours, TF701 retains more than 80% of its tensile strength after 1000 hours. This demonstrates the ability of TF701 to retain stable material properties over long periods of time in environments involving contact with copper or other metals.
The copper-contamination resistance of TF701—in addition to its low specific gravity, tracking resistance, and flame retardance—makes this material an ideal choice for many types of components, such as busbar covers for vehicle-mounted battery systems (components that prevent electric shocks and short circuits for busbars made from copper or other metals).
An insulating component that separates the cells of an EV battery.
Thin walls, light weight, flame resistance, etc. are required.
An insulated protective cover for the conductor through which high voltage and large current of an EV battery flows.
In addition to requiring electrical tracking ability, flame resistance and heat resistance are also required.
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