Vol. 21 No. 3

Journal of Wuhan Universityof Technology- Mater.Sci. Ed.

Sept. 2006

The Influence of Unidirectional Pressure on Electrical Conductivity of Carbon Black Filled Polyethylene W A N G Ke 1'2 Z H A N G Guo .1'2 Z H A O Zhudi 1'2 P E N G Yi 2 DI Weihua 1'2 D U Chuang 1'2 (l. Key Laboratory of AutomobileMaterials of Education, J'dinUniversity,Changchun 130023; 2. College of Materials Science and Engineering, Jilin University, Changchun 130023) Abstract: High density polyethylene filled with conductive carbon black was prepared by conventional meltmixing method. The effect of unidirectional pressure on the conductivity was studied. Wide angle X- ray diJ~action (WAXD) was used to show the irg'lucnce of pressure on the aggregate structure of the polymer filled with carbon black (CB) fillers. A model on the basis of the formation and destruction of conductive networks was proposed to explain the change in the coudactivity with the application of pressure. Key words: high density polyethylene; carbonblack; electricalconductivity; pressuresensors

1

Introduction

Intelligent materials can detect a change in the environment and respond to it by performing both sensing and actuating functions. The usual stimuli are pressure, temperature, electricity, vibration, etc. The useful responses are changes of conductivity, heating, mechanical damping, or acoustic damping I~l . One of the most commonly used intelligent materials is pressure sensors, which can display a change in electrical conductivity with increasing pressure. Such theories as percolation, conduction network theory, tunnel effect mechanism and electric emission theory are proposedI2'31 . The change in conductivity with pressure depends on the polymer characteristics (crystallinity, molecular weight, and aggregate structure), on structures of carbonaceous fillers (surface area, chemical groups, and DBP value), and on the interaction between the polymer and the filler. In this work, the effect of unidirectional pressure on aggregative structure of polymer matrices and distribution of fillers in the polyethylene filled with CB, which consequently leads to the changes in electrical conductivityI41 , was explored. A model on the basis of the formation and destruction of a conductive network was set up to explain the change in the conductivity with the application of pressure.

2 Experimental High density polyethylene (HDPE) was used as the polymer matrix and the conductive carbon black, whose specification is listed in Table 1, was.used as the filler. (Received:May 17,2005; Accepted:March 25,2006) WANG Ke (SE~): Ph D; E-mail: [email protected] * Corresponding author: ZHANG Guo ( ~ [] ) : Prof. ; E-mail: guozhang@mail. jlu. edu. cn

HDPE was preheated in a Brabender at 140 ~ then CB was mixed with melting HDPE for 5 min. Mixtures were roll-milled for 5 min at the same temperature after 24 h, subsequently, compression-molded, and the samples prepared were cooled down in air to room temperature. Various pressures were applied along the thickness direction of samples on a css-44100 electronic universal testing machine at ambient temperature at a constant deformation rate of 1 ram/rain, followed by cutting them to the same dimensions of ( 1 . 2 x 1.5 x 1.0)cm 3 . Wide angle X-ray diffraction (WAXD) was conducted on a Rigaku Denki Diffractometer ( D ~ - yA ) with a rotating Cu-anode. The electrical resistivity of composites was measured at ambient temperature by a digital multimeter, and then converted to a corresponding conductivity. Table 1 Specificationof the conductive carbon black Particlesize/ Surfacearea nm (m2/g) DBPvalue1

pH value (m3/Kg)

Specific resistivity (~2"em)

36 1 028 6 • 10 -3 7.6 0.26 IDibutylphthalateabsorptionvalue. (ASTMTestMethodD2414-65T)

3

Results and Discussion

Fig. 1 shows the conductivity as a function of the pressure, from which it can be seen that there are three characteristics described as follows in the effect of the pressure on the conductivity. Firstly, the conductivity of CB/HDPE compounds decreases at the low pressure, with the increase of pressure, reaches a minimum value, then increases with the further increase of pressure. Secondly, the more the content of CB, the higher the conductivity. Thirdly, the effect on the conductivity is not very sensitive to the pressure with CB loading level. The effect of pressure on the conductivity is associated with the aggregate structure of polymers. WAXD patterns, as shown in Figs. 2 and 3, were used to illustrate the phenomenon mentioned above. It is obviously noted

Vol.21 No.3

WANGKeet d:The hffluenceof UnidirectionalPressure on Electrical. . . .

that there are similar diffraction characteristics with pressure for 10wt% CB filled and 15wt% CB filled composites. From WAXD patterns it can be seen that with increasing pressure not only the diffraction intensity of the (110) plane decrease, but the diffraction peaks are broadened. However, it is interesting to see that the (200) diffraction intensity remarkably increases at the initial ~ressure, which indicates that the orientation of the (200) )lane in the composites occurs when a 9ressure is 55

applied,i e the (200) plane rotates parallel to the surface of the sample, but with the further increase of pressure, the (200) diffraction intensity decreases accompanied with the broadening of the diffraction peaks, whieh indicates that the orientation is relatively inactive at the initial pressure. The decrease in intensity and broadening of diffraction peaks indicates that the pressure produces crystal fragmentations gradually.

(1 o)

50

(110) (110)

(200)

45 40

MP. 200 MPa

35

~30

2 % CB+HDPE 9 12.5% CB+HDPE 9 10%CB+HDPE

7. 25 20

, ,,

IV-JALU_MPo

10

~ 0 N P a I

I

,

0

Fig. 1

77

1OO 200 300 400 Pressure/NPa The dependence of conductivity on the pressure for eomposites with different CB loading

18

2J0

22

24

26

, 28

___J, 18

20

0Mpa, 22

24 26 28 20t ~ 20/~ Fig.2 X-ray diffraction patterns of lOwt% Fig.3 X-ray diffraction patterns of 15wt% CB filled composite at various CB filled composite at various pressure pressure

Rich CB-containing region

Crystal

Poor CB filled region (a) no pressure

Destruction of conductive networks (b) low pressure

Reestablishing of condutive networks (c) high pressure

Fig.4 Schematicmodel of the change in the aggregage structure of the compositesunder various pressures for CB/HDPE composites We recognize that when it is mixed with melting semicrystalline polymer, CB is entirely and uniformly dispersed in the polymer melt, which is a homogenous phase. However, carbon particles have been excluded from crystallites during the process of eooling-recrystallization of compositesISl , but the polymer is so viscous that carbon particles can not fully penetrate into the amorphous re0on, CB is induced to disperse on the crystal surface, so it seemed to be eoated by a rich CB-containing layer for the crystallites, however, less CB for the amorphous region, as a result, CB conductive channels are preferentially formed along the crystal surface, as depicted in Fig. 4(a). It is considered that two processes having an opposite effect on the electrical conductivity exit at the same time when a pressure is applied: one is that the gap among CB aggregates becomes small due to compression, which induees a high eonduetivity; another, as shown in Fig. 4 (b), is that the conductive pathways are partially inter-

rupted due to uhe orientation of the (200) plane, these two processes would oppose each other, so the electrical conductivity would be determined by the net result of these two processes. At a low pressure, the latter process is the leading factor determining the conductivity, during which the mechanism of electron tunneling is hindered and the uninterrupted length of the conductive paths and the strength of the formed filler networks are lessened, as shown in Fig.4(b). Simultaneously, it is not sensitive to compress for the CB aggregates at a lower pressure, hence, a low conductivity is obtained. However, with increasing pressure, the gaps among the CB aggregates are compressed further, furthermore, the orientation of the (200) plane becomes less active. In addition, as shown in Fig.4(c), the CB layers on the crystal surface are almost connected with each other due to crystal fragmentation, hence, new eonduetive pathways are reestablished, so the electrical conductivity increase gradually. The minimum in conductivity shifts to high pressure

78

Journal of Wuhan Universityof Technology- Mater.Sei. Ed.

: with CB filler concentration, which is associated with the filler loading level. The higher the CB concentration, the more powerful the strength of formed filler channels, therefore, it appears to be more difficult to break the conductive pathways due to the orientation of the (200) plane at the same pressure for the composite with a high filler content than one with a low filler content. In this way, the minimum in conductivity turns to higher pressure with the increase of filler content. It is obvious for the 10wt% CB filled composite that there is a dramatic drop in conductivity with pressure up to 100 MPa, subsequently, a significant increase. The tendency appears to be weak with increasing CB loading. This phenomenon is mainly attributed to CB content. With the increase of CB content, the speed of destruction of conductive networks becomes slow, for they are not easily interrupted in the case of wide range of CB content.

4

Conclusions

CB-filled HDPE composites were prepared by conventional melt-mixing method. The effect of unidirectional pressure on the conductivity of the composite was studied. The conductivity of HDPE/CB compounds decreased at a

Sept. 2006

low pressure, with the increase of pressure, reached a minimum value, then increased with the further increase of pressure. The more the content of CB, the higher the conductivity. The effect on the conductivity was not very sensitive to the pressure in the high content CB-containing composite.

References [ 1] R Strumpler. PolymerCompositeThermistorsfor Temperature and Current Sensors[ J ]. J. Appl. Phys., 1996, 80 ( 11 ) : 6 091-6 096 [2] R D Sherman, L M Middleman, S M Jacobs. ElectronTransport Processes in Conductor Filled Polymers[ J ]. Polym. Eng. Sci. ,1983,23(1) :36-46 [3] E K Sichel, J Z Gittleman, P Sheng. Transport Properties of the Composite Material[J]. Phys. Rev. B., 1978, 18( 10): 5 712-5 716 [4] K P sau, T K chaki, D Khastgir. The Effectof Compressive Strain and Stress on Electrical Conductivityof Conductive Rubber Composites[ J ]. RUBBER. CHEM . TECHNOL., 2000,73(12) :310-324 [5] M Narkis, A Ram, Z Stein. Electrical Properties of Carbon Black Filled Crosslinked Polyethylene[ J ]. Polym. Eng. Sci., 1981,21(16) : 1 049-1 054