POSITIVE TEMPERATURE-COEFFICIENT EFFECTS IN THE ELECTRICAL-RESISTIVITY OF CARBON BLACK-FILLED CHLOROPRENE RUBBER

Positive temperature coefficient (PTC) effects have been studied in carbon black-filled chloroprene rubber samples with a high degree of crystallinity (Bayprene 320). For unvulcanized chloroprene rubber the PTC effect is pronounced at around 50-degrees-C where the crystalline structure melts. Immediately above this temperature there is only a slight decrease in the resistivity with temperature, followed by a resumed increase. We found that temperature cycling of the unvulcanized samples led to poor reproductibility in the PTC characteristics when the samples were loaded with carbon black close to the percolation threshold. The reproducibility in the PTC characteristics was improved by increasing the carbon black content to 30 wt%. Alternative ways to improve PTC characteristics are to combine different types of carbon black or vulcanize the chloroprene rubber. Also, the adding of chalk (CaCO3) to the compound was investigated; it increased the resistivity of the compound but reduced the PTC effect. A strong PTC effect is achieved for 25 wt% carbon black in vulcanized chloroprene rubber in the temperature interval between -35-degrees-C and + 120-degrees-C, while a limited PTC effect is achieved in samples loaded with 30 wt% carbon black.