Factors affecting the characteristics of heating devices employing conductive polymers

This study centres on conductive polymers with particular reference to those exhibiting positive temperature coefficient of resistance (PTC) behaviour. Applications of PTC polymers are identified including, with special relevance to this study, automotive applications. The electrical properties and the structure of a particular conductive polymer that has a carbon black conductive filler and exhibits a positive temperature coefficient characteristic, have been measured. Early problems affecting the testing and service performance of these devices are described. Initial concepts of the factors affecting their structure and performance are outlined. The importance of the characteristic known as resistance linearity is described and its important relationship with device performance is noted. The results of a literature survey are detailed. Typical materials and manufacturing processes employed for the type of heater circuit central to this study are described. The results of initial testing and analysis of PTC heater circuits in relation to manufacturing process variables, ageing tests and service failure are summarised. The results of further testing of heater samples and analysis using advanced techniques including Fourier transform IR spectroscopy (FTIR), Calorimetric analysis with scanning microscopy (CASM), Auger electron spectroscopy (AES), Static secondary ion mass spectrometry (SSIMS) and Laser microprobe mass spectrometry (LIMA) are given. The structure of the PTC polymer used in construction of the heater circuit is examined with a high resolution scanning electron microscope (FEGSEM) and the distribution of the carbon filler is determined. Structural changes and changes in the distribution of the carbon resulting from thermal conditioning and other stresses are investigated. A cause of functional failure of heater circuits in service has been identified. Conclusions have been drawn and further work proposed.