Positive and negative temperature coefficient effects of an alternating copolymer of tetrafluoroethylene–ethylene containing carbon black-filled HDPE particles

doi:10.1016/S0032-3861(00)00095-1

Polymer

Volume 41, Issue 19, September 2000, Pages 7279-7282

https://doi.org/10.1016/S0032-3861(00)00095-1 Get rights and content

Abstract

A conductive polymer composite was prepared by melt-mixing of an immiscible semicrystalline polymer blend of an alternating copolymer of tetrafluoroethylene–ethylene (ETFE), high density polyethylene (HDPE), and carbon black (CB). The optical microscopy and time-of-flight secondary mass spectrometry results indicated that the CB particles were selectively localized in the HDPE phase. In addition, it was found that the CB-filled HDPE particles formed a dispersed phase in the ETFE matrix. A double-positive temperature coefficient (PTC) effect was observed in the composite, caused by the large thermal expansion due to the consecutive melting of HDPE and ETFE crystallites. The negative temperature coefficient (NTC) that was observed in this system could not have been caused by the formation of flocculated structures because the size of the CB-filled HDPE particles is significantly large, so that their mobility is extremely limited even at high temperatures. This conclusion was confirmed by observing the morphology of the composite at temperature ranging from 25 to 250°C. These results suggest that new mechanisms need to be uncovered to explain the NTC effect of conductive polymer composites.

Introduction

After the positive temperature coefficient (PTC) effect was first observed in a carbon black (CB)-filled low density polyethylene composite by Frydman [1] in 1945, many researchers have studied the PTC effect due to the potential applications of PTC materials in industry [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. In general, it has been well accepted that the strong PTC effect of CB-filled semicrystalline composites is caused by an increase in the average interparticle or aggregate distance of CB due to the large thermal expansion as a result of the melting of the polymer crystallites [7].

It should be noted, however, that most of the earlier studies were focused on the composites comprising CB and a single semicrystalline polymer [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Few studies have been conducted on the PTC and negative temperature coefficient (NTC) effects of CB-filled immiscible polymer blends. Our previous results of CB-filled PVDF/HDPE composites have shown that the morphology can significantly influence the PTC and NTC behaviors of the composites [14]. It was found that when the PVDF and CB-filled high density polyethylene (HDPE) form the dispersed and continuous phases, respectively, the PTC and NTC behaviors are similar to those of a CB-filled neat HDPE composite containing an equivalent amount of CB in the HDPE phase. However, when the PVDF and CB-filled HDPE form a co-continuous phase, the PTC effect is not significantly disturbed but the NTC effect is delayed to a higher temperature.

Uncrosslinked CB-filled semicrystalline polymer composites exhibit a sharp decrease in resistivity when temperature is above the T_m of the polymers. This phenomenon is referred to as the NTC effect. It has been suggested that the NTC effect of a CB-filled single semicrystalline polymer composite is caused by the formation of a flocculated structure when the viscosity of the polymer is sufficiently low at elevated temperatures [11], [12], [13]. However, the NTC effect of CB-filled polymer blends has not been elucidated in detail. The objective of this paper is to elucidate the PTC and NTC effects of a CB-filled immiscible semicrystalline polymer blend when CB is selectively localized in the dispersed phase, which is the semicrystalline polymer that has the lower melting point.

Section snippets

Experimental

The high melting point semicrystalline polymer used was an alternating copolymer of tetrafluoroethylene–ethylene (ETFE) from Du Pont. The low melting point polymer was a HDPE from Philips Petroleum International with an MFI of 0.35 g/10 min. The CB used was N660 from Columbian Chemicals.

The CB-filled ETFE/HDPE composite was prepared by melt-mixing the materials in a Haake mixer at 280°C and 30 rpm for 15 min. The compound obtained was further compressed into a 2-mm sheet by a hot press at 280°C and

Results and discussion

Fig. 1 depicts the log resistivity of the 10 wt% N660-filled ETFE/HDPE composite and a 47.6 wt% N660-filled HDPE composite as a function of temperature. Two sharp resistivity jumps are observed for the ETFE/HDPE composite. This phenomenon is markedly different from that of any CB-filled neat semicrystalline polymer composite which exhibits only one resistivity jump when it is heated. As temperature increases, the first resistivity jump of the ETFE/HDPE composite is observed at about 140°C, which

Conclusions

Our results indicated that for the 10 wt% N660-filled ETFE/HDPE composite, CB particles are only selectively localized in the HDPE phase, and that the CB-filled HDPE forms a dispersed phase in the ETFE matrix. A double-PTC effect was observed and the cause was identified as the large thermal expansion owing to the consecutive melting of HDPE and then ETFE crystallites. The NTC effect that we observed in this system could not have been caused by the formation of flocculated structures because the

Acknowledgements

This work was supported by Industry & Technology Development Council under a Grant No. AF/155/99 and the Hong Kong Government Research Grant Council under the grant No. HKUST 582/95P.

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Polymer CommunicationPositive and negative temperature coefficient effects of an alternating copolymer of tetrafluoroethylene–ethylene containing carbon black-filled HDPE particles

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Jpn J Appl Phys

Polym Engng Sci

J Appl Polym Sci

Polym Engng Sci

Rubbber Chem Technol

Polymer Communication
Positive and negative temperature coefficient effects of an alternating copolymer of tetrafluoroethylene–ethylene containing carbon black-filled HDPE particles