Hello and welcome to this post on design properties for plastics engineering. In this post we deep dive into the Comparative Tracking Index (CTI) of aliphatic Polyamides, Polybutylene terephthalate (PBT) as well as Polyphenylene sulfide (PPS) used for electronic components made out of plastic.
In previous posts we discussed the CTI of high performance polymers such as PPS and how we can improve it. The values shown in past posts were estimated according to the standard IEC-60112.
What is the Comparative tracking index (CTI) and why the CTI is important?
In general, when the plastic surface, which is the insulation material, carbonizes due to voltage exposure, a conducting path is formed and tracking occurs. Over time, the surface erodes and a conduction of electricity takes place continuously. The resistance to the occurrence of tracking and erosion is represented by the Comparative tracking index (CTI).
How is the CTI value changing if flame retardant additives and glass-fiber reinforced are added to PA and PBT?
Figure 1 compares the CTI values of Polyamide 6 (PA 6), Polyamide 6.6 (PA 6.6) and Polybutylene terephthalate (PBT) with and without reinforcements, as well as with and without halogen free flame retardants. For Polyamides and PBT, adding reinforcement is not leading to a decline in CTI performance. PBT shows a decline in CTI performance in case flame retardants are added.
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| Figure 1: CTI of PA 6, PA 6.6 and PBT with and without reinforcements, as well as with and without HFFR [1]. |
How is the CTI of PA, PBT, and PPS changing after heat aging and moisture influence?
Figure 2 [2] shows the results of the CTI measurements on untreated and treated PA 6.6, PBT, and Polyphenylene sulfide (PPS) samples (all with glass or glass/mineral reinforcement). For heat aging and moisture influence, the samples were exposed to 85°C at 85% relative humidity for 1,000 hours (in line with the international standards e.g. IEC 60068) . PA 6.6- GF33 wt% (Zytel® 70G33L) and PBT-GF30 wt% (DURANEX® CG7030) reached in the untreated test scenario the maximum achievable value of 600 V. PPS-(GF+MF) 65 wt% (TEDUR® HTR PPS 2465) reached 500 V in the untreated test scenario. After the heat aging and moisture treatment, PA 6.6 and PPS did not show a decline in CTI performance. The advantage of Polyamides is their molecular structure which enables an inherent resistance to tracking and erosion. Achieving a 500 V level with PPS needs for example a special additive modification which we discussed here. PPS has a CTI in the range of 250 V. In hot and humid environments, PBT showed a decline in CTI; however, it can still keep the CTI above 500 V.
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| Figure 2: CTI of PA 6.6, PBT, and PPS before and after heat aging and moisture treatment (85°C/85%RH/1000h) [2]. |
Additional influences on CTI performance - part surface structure
Apart from moisture and temperature influence, part surface influences the CTI performance of your plastic part too (Figure 3). In case of PPS (Tedur HTR), a highly polished surface can increase the CTI from 500 V to 550 V [3]. On the other hand, rough surface structures such as the K29 and K30 (Knauf Industries), decrease the CTI value from 500 to 450 V [3].
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| Figure 3: CTI of PPS and influence of different surface finishes onto the CTI value [3]. |
Conclusions
CTI plays an important role when designing electronic components such as busbars for traction motors and power electronics. Selecting the optimal polymer material which can withstand temperature, humidity, time and mechanical impacts is key in order to make your design compact and safe without having short-circuits in the long run.
Thanks for reading & #findoutaboutplastics
Herwig Juster
Literature:
[1] https://en.kunststoffe.de/a/specialistarticle/against-the-current-2586918
[2] https://www.polyplastics.com/en/product/lines/pbt_pa66/index.html
[3] https://www.plastverarbeiter.de/markt/elektroniktauglich-polyphenylensulfid-compound-mit-hoher-kriechstromfestigkeit.html
