High-voltage tracking resistance in flame-retardant engineering thermoplastics

© pixabay, HansBraxmeier


Our mobility is undergoing a transformation: across the world, electrification is the key to climate-friendly mobility by land, by sea and by air. In automotive technology in particular, this transformation is now more apparent than ever. It ranges from countless interior control elements with aerodynamic add-on parts and various sensors on the exterior to the powertrain of electric vehicles and the charging infrastructure (e.g. components for plugs, sockets, stations, charging boxes, etc.). The properties of engineering thermoplastics such as polyamides, polycarbonates, polyesters and their blends mean they are frequently used in these parts. Depending on the requirements profile, the thermoplastics are reinforced or unreinforced and appropriate additives are employed.

They must have good processing properties and also meet stringent requirements in terms of mechanical properties, dimensional stability, flame retardancy, tracking resistance, aging resistance and weathering resistance. Against the backdrop of increasing charging voltages and charging currents, high-voltage tracking resistance is becoming an increasingly important topic.

In this context, the test for high-voltage tracking resistance according to DIN EN 60587 (Electrical insulating materials used under severe ambient conditions) confirms the extent to which the insulating capacity of a surface changes at high voltages (≥ 1 kV) outdoors when influenced by moisture and contaminants. If the time-to-track under the applied voltage is greater than 60 min, the material is deemed to have passed the test. The applied test voltage is gradually increased (in increments of 0.5 kV) and the test is repeated. The test result is the maximum test voltage, also referred to as the IPT value (Inclined Plane Tracking), for which the time-to-track for five specimens is > 60 min.

This test is very time-consuming and labor-intensive and is thus not well suited for use, for example, in the development of new compounds as a simple and rapid method for evaluating high-voltage tracking resistance. It is also not possible to draw conclusions from other investigations and tests, such as the standardized CTI (Comparative Tracking Index) test for voltages up to 600 V, as the substances behave differently under the various test setups.

Therefore, the aim of this joint research project is to develop and impart comprehensive knowledge on the modes of action of flame-retardant engineering thermoplastics in view of their high-voltage tracking resistance. The first priority is to determine the various factors that influence their behavior in an IPT test. These factors must be evaluated and weighted in order to derive the relevant influencing variables for classifying the materials according to the IPT test. In the process, differences and similarities between the CTI and IPT test methods will be defined and compared. In addition, modes of action and basic mechanistic principles will be studied using suitable model compounds. Based on the findings, structure-property relationships and ideally methods for predicting material behavior will be derived. Once derived, these should enable the project participants to shape their own material and product development processes with more efficiency and precision. The project should also serve as an interdisciplinary platform for players across the entire value chain to develop targeted solutions for the technical problems arising in relation to flame retardancy and (high-voltage) tracking resistance.

Research focus

The first step of the project will be to collate, compare and evaluate the main influencing factors arising from the state of the art in view of the dependencies of CTI and PTI test results. Material- and condition-specific factors will be systematically examined, such as the type of polymer (polyamides, polyesters and polycarbonates) and the presence of flame retardants, additives, fillers and functional and reinforcing materials, taking into account factors such as the applied voltage, the water content or the type and amount of contaminants, etc.

In addition, the current information on modes of action and mechanisms (e.g. concerning crust formation, changes in surface topography, pyrolysis, etc.) of the CTI and IPT test methods will be collated; lastly, the reported correlations with physical and chemical tests (e.g. TGA, laser ablation, etc.) will be described. Based on this, similarities and differences between the CTI and PIT test methods will be defined. Together with the project participants, the influencing factors will be measured and model systems will be selected, taking individual requirement profiles into account. Subsequently, these model systems will be evaluated systematically and in greater detail in terms of their behavior in the CTI and IPT tests as it relates to their composition as well as manufacturing and test conditions. Furthermore, the fire behavior will be evaluated using the UL-94 Vertical Flame Test. As part of the project, several compounding and injection molding campaigns (max. total of 60 variations) will be carried out in order to further reduce the large number of influencing parameters and to gain a better understanding of the underlying dependencies. The experiments will be selected and planned in detail in close collaboration with the participants, taking the relevant input into account.

After each campaign, selected material analysis tests will be performed in order to better understand the underlying modes of action and dependencies with regard to tracking and erosion. Various thermal (e.g. TGA, DSC), microscopic (e.g. SEM, Raman microscopy, white light interferometer), spectroscopic and chromatographic methods (e.g. GPC, pyrolysis GC/MS) are used here. Based on these findings, structure-property relationships will be derived with the aim of gaining a better understanding of dependencies in CTI and IPT testing. In addition, these findings should help with assessing or predicting the behavior of similar materials in the test setups mentioned.