PTC/NTC Behaviors of Nanostructured Carbon Black-filled HDPE ...

Carbon Science

Vol. 2, No. 3&4 December 2001 pp. 159-164

PTC/NTC Behaviors of Nanostructured Carbon Black-filled HDPE Polymer Composites Soo-Jin ParkH , Min-Kang Seo and Jae-Rock Lee Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-600, Korea H e-mail: [email protected] (Received November 8, 2001; accepted December 9, 2001)

Abstract In this study, the effects of carbon black (CB) content and anodic oxidation treatment with AgNO3 on positive temperature coefficient (PTC) behavior of CB/HDPE nanocomposites were investigated. Also, the addition of elastomer as a toughing agent was studied. The 20~50 wt% of CB, 0~5 wt% of elastomer, and 1 wt% of AgNO3-filled HDPE nanocomposites were prepared using the internal mixer in 60 rpm at 160oC and the compression-molded at 180oC for 10 min. As a result, the room temperature resistivity and PTC intensity of the composites were dependent, to a large extent, on the content of CB, addition of elastomer, and surface chemical properties that were controlled in the relative arrangements of the carbon black aggregates in a polymeric matrix. Moreover, the composites with relatively low room temperature resistivity and suitable PTC intensity could be achieved by treatment of AgNO3. Consequently, it was noted that PTC effect was due to the deagglomeration or the breakage of the conductive networks caused by thermal expansion or crystalline melting of the polymeric matrix. Keywords : Carbon black (CB), Positive temperature coefficient (PTC), Nanocomposites, Elastomer, Room temperature resistivity

1. Introduction Polymer materials can become semiconductive materials when doped with conducting particles, resulting in extrinsic conducting system, which are of considerable importance in the electrical and electronic industries. The electrical resistivity of carbon black (CB) loaded polymer composites shows an anomalous increase at temperature around the melting point of the matrix. This effect is commonly referred to as positive temperature coefficient (PTC) of resistivity [1-3]. Important industrial applications of PTC materials include overcurrent protectors and self-regulation heaters [4]. Such composites used as heating elements possess several advantages such as self-adjusting heat output by environment temperature and simple fabrication. It is well known, however, that non-crosslinked carbon black-filled semicrystalline polymer composites exhibit a very sharp decrease in resistivity, when the temperature is above the melting point of semicrystalline polymers. This phenomenon is referred to as the negative temperature coefficient (NTC) effect [5-9]. It has been suggested that the NTC effect of CB-filled semicrystalline polymer composites be caused by the formation of conductive chains resulting from the relaxation of polymer structure and agglomeration of CB particles. This is due to the low viscosity of the polymer at elevated temperatures, leading to a significant increment in the mobility of the CB particles in the composites [10].

In general, non-crosslinked CB-filled semicrystalline polymer composites cannot be used as thermistors in over-temperature and over-current protections due to their NTC effects and poor reproducibility in thermal cycling. In addition, they have poor resistance for fracture, as well. In order to overcome these disadvantages, researchers have proposed and developed many methods to eliminate the NTC effect and to improve the PTC intensity as well as toughness of CB-filled polymer composites. Among these methods, the first approach used is to crosslink the semicrystalline polymer matrix by a crosslinking agent, such as peroxide [3, 11] or silane [12]. For instance, Narkis [10] successfully used peroxide to crosslink CB-filled high-density polyethylene composites without sacrificing the PTC intensity. In addition to the use of a crosslinking agent, Gamma and electron beam radiations have been used to crosslink CB-filled semicrystalline polymer composites [13-15]. It was also reported that third filler could be used to stabilize the polymer matrix and eliminate the NTC effect of CB-filled semicrystalline polymer composites including PTC intensity [16]. Many factors, such as structure of carbon black, polymeric matrix, and processing parameters, affect the conductivity of CB-filled semicrystalline polymer composites at room temperature, as well as, the PTC/NTC behaviors [17, 18]. Although, the effects of the carbon black structural parameters (such as particle size and aggregation) on the conductivity of CB-filled semicrystalline polymer composites are

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generally understood, the effects on PTC behavior of the composites, including carbon black-polymer interfacial properties have not been systematically studied. The objective of this study, thus, is to elucidate the PTC and NTC effects of CB-filled semicrystalline polymer composites when CB is selectively localized in the dispersed phase. And, we also investigate the effect of third element, i.e., AgNO3 as a magnetic powder, on PTC behaviors of CBfilled semicrystalline polymer (HDPE)/elastomer composites.

2. Experimental 2.1. Materials and Sample Preparation The polymer matrix used was high-density polyethylene (HDPE, density 0.967 g/cm3, number average molecular weight 30000, melt index 0.35 g/10 min supplied from Honam Petrochem. Co. of Korea) and the carbon black (CB, 40B1 supplied from Korea Carbon Black Co. of Korea) was used with addition to 20~50 wt%. The elastomer as a toughing agent used was styrene butadiene rubber (SBR, 1502 supplied from Kumho Petrochem. Co. of Korea). In addition, metal powders, i.e., silver nitrate (AgNO3, F.W.=169.89 supplied from Kojima Chem. Co. of Japan) was used to treat a carbon black using anodic oxidation method. Table 1 lists the characteristics of the CB used. The nanostructured CB/ HDPE mixtures treated with AgNO3 were mixed in an internal mixer at 60 rpm for 5 min, as already reported in previous paper [4]. They were taken out of the mixer, and then compression-molded at 180oC into 1 mm thickness using the hot-press. The samples were cut into pieces to measure electrical resistivity in the thickness direction of the CB-filled polymer composites. 2.2. Resistivity In order to measure the volume resistivity of the CB-filled polymer composites at room temperature and progressively elevated temperatures, we connected the heating electrode of insulating resistance tester with a programmed temperature controller. The samples were cut into 1 × 1 cm pieces, and the electrical resistivity was measured in the thickness direction of the CB-filled polymer composites at a heating rate of 2oC/min using a digital multimeter. Copper paste was used Table 1. Characteristics for carbon black used in this study Characteristics

CB 40B1 2

Specific surface area (m /g) DBP absorption (cc/100g) pH Average particle size (nm) Ash (%) Volatile contents (%)

149 129 7.9 22 0.006 1.28

to ensure good contact of the sample surface with the electrodes of the conduction tester. 2.3. Dynamic Scan by DSC A Perkin Elmer DSC-6 was used to measure the thermal properties of the sample. Dynamic DSC scans of all samples were carried out over a range of temperature from 30 to 250oC at a heating rate of 10oC/min under the condition of N2. The sample weight was about 12 mg. 2.4. PTC Intensity The PTC properties of CB-filled polymer composites are estimated by the measurement of PTC intensity. Which is defined as the ratio of the maximum resistivity (ρmax) to the resistivity at room temperature (ρRT), as shown by eq. (1) [19]. PTC intensity = ρmax / ρRT

(1)

2.5. Impact strength and Morphology A Tinius Olsel Model 66 Izod Impact Tester was used for the measurement of impact strength of the composites. The morphology of the composites was investigated by using an optical microscope operating at the transmission mode. Thin sections of the composites of 1 µm in thickness were prepared by a cyromicrotome at -100oC.

3. Results and Discussion To begin with, CB-filled HDPE conductive composites are studied for the correlation of carbon black content and resistivity. Fig. 1(a) shows the room temperature resistivity (ρRT) of CB-filled HDPE composites as a function of CB content. With increasing the CB content, the ρRT is decreased. Fig. 1 (b) shows the melting temperature resistivity (ρmax) of the composites according to the temperature. The ρmax is also decreased with increasing the content of CB. However, in case of the ρmax in Fig. 1(b), one sharp resistivity curve is observed at the melting temperature region of the HDPE with increasing the temperature, which is a slightly higher than the melting point of neat HDPE. We refer to this phenomenon as the PTC behavior. However, the temperature further increase, the composites do not show PTC effect but rather a strong NTC behavior. This result can be accounted for by the formation of conductive chains, resulting from the relaxation of polymer structure and agglomeration of CB particles [20]. And, it has been also suggested that the NTC effect of a CB-filled single semicrystalline polymer composites is caused by the formation of a flocculated structure when the viscosity of the polymer is sufficiently low at elevated temperature [11, 21]. To reveal the PTC mechanism of the composites showing similar PTC behavior, a differential scanning calorimeter (DSC) was used to study the melting behaviors of the 20 and

PTC/NTC Behaviors of Polymer Composites

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Fig. 2. DSC scans for CB-filled HDPE composites.

Fig. 1. Resistivity for CB-filled HDPE composites as a function of CB content. (a) room temperature resistivity and (b) melting temperature resistivity

30 wt% CB-filled HDPE composites. Fig. 2 shows the DSC scans of the composites obtained at a heating rate of 10oC/ min. A comparison of Figs. 1 and 2 exhibits that the melting of the neat HDPE crystallite and the sharp increase in the resistivity of the composites occurs in a similar temperature range. Hence, the PTC effect observed at 148oC and 150oC is attributed to the volume expansion as a result of the melting of neat HDPE crystallite with different content of CB particles. Fig. 3(a) and (b) shows the optical micrographs of the 20 and 30 wt% CB-filled HDPE composites. As depicted in these micrographs, two-phase structure is observed. The right areas are identified as HDPE particles having high viscosity so that CB particles cannot go inside the HDPE particles, while the dark areas forming a continuous phase are identified as a mixture of HDPE and CB particles. The interfaces between HDPE matrix and CB particles are much darker, indicating that a much higher concentration of CB is

Fig. 3. Optical micrographs of CB-filled HDPE composites. (a) CB : 20 wt% and (b) CB : 30 wt%

located at the interface. As shown in Fig. 3, the segregation of CB at interfaces can clearly be seen at this low concentration of CB. When the interfacial regions between HDPE matrix and CB particles are saturated with CB particles, the excess CB particles are forced to be dispersed in the matrix.

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of CB is needed to ensure that the CB interparticle distances are small to allow electron tunneling between particles and the conductivity of interfacial region shows more effective conductivity than that of HDPE matrix only [22]. Consequently, it is thought that the interfacial region controls the

Fig. 4. PTC intensity for CB-filled HDPE composites as a function of CB content.

Generally, it is known that the lower CB content composites show a higher room temperature resistivity and a lower PTC intensity, while the higher CB content composites exhibit a lower room temperature resistivity and a higher PTC intensity [21]. Fig. 4 shows the PTC intensity of the composites as a function of CB content. As expected, the CB content can significantly influence the PTC intensity of the composites and though the 20 wt% CB-filled composites have the highest value of ρmax, the ρRT is relatively much increased compared to other composites, resulting in decreasing the PTC intensity. Therefore, the 30 wt% CBfilled HDPE composites exhibit the highest PTC intensity. This result can be explained by the fact that CB particles are distributed adequately at the interface between HDPE and CB before it is dispersed in the HDPE matrix. And, the CB concentration at the interface is much higher than that of the HDPE matrix, as shown in Fig. 3. That is, a proper amount

Fig. 5. Impact strength for CB-filled HDPE/SBR composites as a function of SBR content.

Fig. 6. Resistivity and PTC intensity for 30 wt% CB-filled HDPE/SBR composites as a function of SBR content. (a) room temperature resistivity, (b) melting temperature resistivity, and (c) PTC intensity

PTC/NTC Behaviors of Polymer Composites

resistivity of the composites predominantly, resulting in influencing the PTC intensity. It is well known that the elastomer has been used to increase the toughness of the polymer composites [23] and the impact strength of elastomer-toughened polymer composites depends on the morphology of the composites, the characteristics of the rubber, and the nature of the interface between these phases [24, 25]. Fig. 5 shows the Izod impact strength of 30 wt% CB-filled HDPE/elastomer composites as a function of elastomer content. With increasing the content of elastomer, the impact strength goes through an increase and then declines a little as a level of maleation of the SBR-type rubber increase. This maleation is possibly suitable for carbon blacks to be incorporated between hydrocarbon elastomer and HDPE matrix, resulting in improving the toughness of the resulting composites due to the increase of interfacial bond [26, 27]. Fig. 6 shows the PTC intensity of 30 wt% CB-filled HDPE/elastomer composites. The difference of a tendency for resistivity between room temperature and elevating temperature is happened. This is probably due to the intrinsic characteristic of HDPE matrix. With increasing the content of elastomer, PTC intensity of the composites is decreased. Especially, in case of 5 wt% SBR, though the maximum resistivity at crystalline melting region is increased, the resistivity at room temperature is much higher than any other composites, resulting in decreasing the PTC intensity. Therefore, it is drawback that the composites containing elastomer show the decrement of PTC intensity largely, though it makes an important role to increase the impact strength of the composites. This is probably due to the increment of resistivity at room temperature, resulting from the interruption of conductive paths of the composites [28]. To overcome this phenomenon and eliminate the NTC effect of the composites, thus, we treat the CB particles with AgNO3 as magnetic powders using anodic oxidation method. Fig. 7 shows the PTC intensity of 30 wt% CB-filled HDPE/SBR composites, treated with AgNO3. In case of the CB-filled HDPE/SBR composites treated by anodic oxidation, the PTC intensity is improved considerably compared to the composites with SBR only. It seems that the thermomechanical properties of the composites and particularly, the temperature dependent expansion of the matrix plays an important roles in the PTC behavior according to the nature of the elements, i.e., CB and AgNO3 [29-31]. Therefore, the anodic oxidation treatment with AgNO3 leads to a decrease of the resistivity at room temperature, while improvement of the resistivity at crystalline melting region, resulting in increasing the PTC intensity of the composites. This is probably attributed to the strong interactions between carbon blacks and polymer chains. That is, the surface functional groups of CB, such as quinones, carboxyl, phenol, and lactones may react with polymer free radicals during mastication [32], fixing the polymer molecules to their surface. The

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Fig. 7. Resistivity and PTC intensity for 30 wt% CB-filled HDPE/SBR composites using an anodic oxidation treatment with AgNO3. (a) room temperature resistivity, (b) melting temperature resistivity, and (c) PTC intensity

strong polymer-filler interactions can partly prevent the flocculation of carbon black aggregates during temperature increase. Therefore, the composites have a higher PTC intensity and a weaker NTC effect. However, the more important

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point seems to get fracturation, i.e., nano particle size of CB, and a suitable localization of the conductive fillers [33]. This is why biphasis system can be good candidates to force the structuration of the conductive polymer composites.

[6] [7] [8] [9] [10]

4. Conclusion

[11]

In this work, we are to investigate the effects of carbon black (CB) content, elastomer, and anodic oxidation treatment with AgNO3 on positive temperature coefficient (PTC) behavior of nanosized carbon black-filled HDPE composites. As a result, the PTC intensity of conductive polymer composites depends strongly on the CB content. At 30 wt% CB contents, the PTC intensity is relatively large at 360, owing to the distribution of CB particles at the interface of the HDPE matrix. However, as the CB content increases, the CB concentration at the interface remains relatively constant, because the particle size of HDPE does not change during the melt-mixing process due to their extremely high viscosity. Therefore, the conductivity paths become more important in controlling the resistivity of the composites. Although, the PTC intensity of the composites containing elastomer is lowered, they show an improvement of impact strength, resulting from improving the toughness of the composites. However, anodic oxidation treatment with AgNO3 leads to an increment of PTC intensity in CB-filled HDPE/ elastomer composites. This is probably due to the strong interactions between carbon blacks and polymer chains, resulting from the anodic oxidation treatment of CB particles with AgNO3. Consequently, it is found that the anodic oxidation treatment with AgNO3 is one of relevant methods to compensate for loss of PTC intensity in CB-filled HDPE polymer composites containing elastomeric materials.

[12]

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