In Simple Terms
Researchers found that adding a specially modified rubber to a heat-resistant plastic can make thin insulation films much less likely to crack when bent. The blend also kept an electrical-insulation advantage seen with rubber, and survived about four times as many folds as the comparison materials. It still needs long-term testing before anyone can know how it will perform in electric cars.
Why Motor Insulation Needs to Bend
Insulation in a modern electric drive system has to cope with heat, repeated mechanical bending and fast electrical pulses—all at once. In an electric car motor, rapid switching in the inverter causes the voltage to fluctuate repeatedly. Those pulses can trigger tiny electrical discharges in small gaps inside the insulation. Over time, the resulting damage can weaken the insulation or cause it to fail.
That is why simply showing that a material blocks current in a basic test is not enough. One important measure is partial-discharge inception voltage, or PDIV: the voltage at which small, localized discharges first appear. A low electrical permittivity is also desirable, along with the ability to withstand operating temperatures. But making a film that combines these qualities with resistance to repeated folding is no easy task.
The team of Takenaka, Kobuyama and Ogizawa chose polyphenylene sulfide, or PPS, as the base material. This plastic is known for its thermal stability and low electrical permittivity, and offers a practical balance of performance and cost. But its rigid molecular structure also makes it relatively brittle. Fine cracks can form during manufacturing or after repeated bending.
Adding a flexible rubber might seem like a straightforward fix, but the two materials need to work well together. PPS and a fluoroelastomer called FEPM do not naturally blend easily. They can separate into large regions with weak connections between them. In that case, the rubber does not become a uniform part of the material; instead, the boundaries between the two phases may become weak spots that limit the benefit of its flexibility.
Changing the Boundary Between Plastic and Rubber
Rather than simply mixing the two materials, the researchers tested whether they could improve how well they fit together at the point of contact. They compared unmodified FEPM with a similar rubber whose chain ends had been modified by adding iodine atoms. They called the modified version mFEPM. The idea was that the modification might enable a chemical reaction during heat processing, strengthening the connection between the rubber and PPS.
The hypothesis rests on the fact that carbon-iodine bonds can break apart when heated above 270 degrees Celsius. This could produce reactive radicals that might, in principle, react with the aromatic rings in PPS and perhaps form linking chains called PPS-g-mFEPM at the boundary between the materials. The study does not present this reaction pathway as established fact, however. The radicals could take part in other reactions, and the researchers did not prove that the formation of linking material was the only explanation for their observations.
The blends were prepared in a 30-millimeter twin-screw extruder running at 200 revolutions per minute. PPS and one of the two types of rubber were fed through separate hoppers, at a weight ratio of 70 parts PPS to 30 parts rubber. The mixture passed through four temperature zones, with settings ranging from 270 to 290 degrees Celsius.
After the material was made into pellets, it was processed again to produce uniform films 150 micrometers thick. For this step, the team used a single-screw extruder fitted with a 250-millimeter-wide coat-hanger die, operating at temperatures between 300 and 330 degrees Celsius. These steps matter because the researchers did more than mix ingredients in a lab: they made thin films resembling the form of insulation that might be used in motor components.
From Large Droplets to a Finer Distribution
Images of the materials’ internal structure showed a clear difference between the two blends. In the sample containing unmodified FEPM, the rubber appeared in relatively large, coarse regions scattered through the PPS like islands in a sea, with weak bonding at the boundaries. In the blend containing mFEPM, the rubber regions were smaller and more uniform. Their number-average diameter was about 0.38 micrometers.
This change matters because the size of the rubber regions and how well they connect to the plastic matrix can affect how stress travels through the material. If the regions are large or their boundaries are weak, forces may concentrate in particular spots rather than spreading more evenly. A finer structure could help prevent the rubber regions from merging during processing and might reduce stress at the boundary. The researchers say the results are consistent with the possibility that links formed between the two materials, but this remains a possible explanation, not definitive proof of the reaction mechanism.
The team also examined how PPS crystallized as it cooled. Thermal measurements suggested that the fine boundaries around mFEPM regions may provide sites where crystallization can begin, a process known as heterogeneous nucleation. By contrast, the unmodified rubber appeared to inhibit crystal growth, leaving much of the matrix in an amorphous state after cooling. The study did not establish why this effect occurred.
In the reactive blend, the measured crystallization rate—calculated from the inverse of the time needed to reach half of the final crystallinity—was higher than in the nonreactive blend. Polarized-light microscope images also showed no clearly defined, large spherical crystals. The researchers interpreted this as consistent with the formation of smaller crystals rather than distinctive large structures. These details may matter because the arrangement and size of crystals can affect a material’s stiffness, although the study does not identify any one structural feature as the cause of the mechanical improvement.
Fold Testing Reveals the Biggest Difference
The clearest difference appeared in repeated-fold testing. The blend containing mFEPM withstood about four times as many folding cycles as either pure PPS or the blend containing unmodified rubber. This suggests that adding a flexible rubber alone is not enough to guarantee better performance. How the rubber is distributed through the material and how well it bonds to the matrix may be just as important as its presence.
The team also tested annealing, in which a material is heated under controlled conditions after manufacturing. The samples were annealed at 200 degrees Celsius for one hour. After this treatment, the reactive blend retained its resistance to folding, while the performance of pure PPS and the nonreactive blend fell sharply. The researchers suggested that crystallization-related embrittlement might explain the decline, perhaps because large spherical crystals grew in the samples that became less resistant. But this does not prove that crystal size alone caused the drop, nor does it rule out contributions from bonding between the phases or other structural factors.
The comparison points to a possible practical benefit. Insulating films used in motor windings or in slot liners that hold the windings may need to bend during shaping and installation. A material that retains its ability to withstand folding after exposure to heat could be better suited to that stage of manufacturing. Even so, a fold test cannot reproduce every force and condition experienced by an insulating component inside a motor operating for years.
An Electrical Improvement That Crystallinity Alone Cannot Explain
The electrical improvement was not limited to the reactive blend. Both the blend containing unmodified FEPM and the blend containing mFEPM had higher partial-discharge inception voltages than pure PPS at the temperatures tested. This included measurements at 150 degrees Celsius, even though PDIV fell at that temperature for every sample.
The researchers proposed two possible explanations for the higher PDIV in blends containing the fluoroelastomer. Fluorine atoms may help trap some electrons, while the low electrical permittivity of FEPM regions may help raise the voltage needed to initiate a discharge. These are proposed explanations, however, not proof that a single mechanism has been definitively identified.
After annealing, the degree of crystallinity increased in all three materials, but their PDIV values changed only slightly. This suggests that increased crystallinity was not the main factor behind the blends’ advantage in the discharge-initiation tests. The distinction is important: mechanical and electrical properties may respond to different features of a material’s structure. An improvement in one property cannot automatically be assumed to explain an improvement in another.
What the Results Do Not Prove Yet
The findings give researchers a reason to take the blend seriously, but they do not establish how long insulation made from it would last in an electric car. The study measured partial-discharge inception voltage; it did not directly test how much damage discharges cause over time, long-term electrical life, or the time to insulation breakdown. Nor is folding resistance alone enough to predict performance under every kind of vibration, pressure and heat that a motor may experience.
The experiments also do not pinpoint which specific change produced the large increase in folding endurance. Better bonding between PPS and the rubber may have played a part. Smaller rubber regions, a different crystallization rate or changes in crystal arrangement may also have contributed. Several factors could be working together. Finding out how much each one matters will require experiments that separate them, rather than treating the blend as a package of changes.
The results do not show that the same method will work with other engineering plastics, either. Chemical compatibility, crystallization behavior and processing temperatures vary from one material to another. Other electrical applications—or uses in chemically harsh environments—would need to be tested independently before the findings could be applied to them. For now, the work points to a promising way to develop a particular insulating material, not a ready-made recipe for every industry.
The study was published in Polymer Journal in 2026 and was conducted by Takenaka, Kobuyama and Ogizawa. Its central idea is that controlling the boundaries between components in a polymer blend may make it possible to combine properties that are difficult to achieve in a single material: greater flexibility while retaining an electrical-insulation advantage associated with adding a fluoroelastomer.
The Bottom Line
Reactive blending of PPS with an iodine-terminated fluoroelastomer offers a possible way to reduce brittleness in insulating films. The resulting material had a finer internal structure and withstood about four times as many folds as the comparison samples. Both types of fluoroelastomer also raised PDIV compared with pure PPS, including at 150 degrees Celsius, although values fell for all materials at that temperature. The next steps are to test mechanical and electrical durability over the long term and determine which structural features drive the improvement before the blend can be considered a proven choice for electric-motor insulation.