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Antistatic behavior of PAN-based low-temperature carbonaceous fibers
Journal of Electrostatics 71 (2013) 1036e1040
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Antistatic behavior of PAN-based low-temperature carbonaceous fibers Shan Zhang a, b, Chengguo Wang b, *, Hua Yuan b, Bingming Zhang b, Xue Lin b, Zhitao Lin b a b
Key Laboratory for LiquideSolid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China Carbon Fiber Engineering Research Center, Faculty of Materials Science, Shandong University, Jinan 250061, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2013 Received in revised form 6 October 2013 Accepted 8 October 2013 Available online 15 October 2013
A kind of antistatic coatings which were applied to nonconductive surfaces were prepared with polytetrafluoroethylene (PTFE) paint as matrix, polyacrylonitrile (PAN)-based low-temperature carbonaceous fibers as conductive filler. The influences of carbonaceous fiber content, carbonization temperature, size and testing voltage on the surface resistivity of the coating were investigated. The surface resistivity could be well controlled in the static dissipative range (106e109 U) by changing the content, size and carbonization temperature of carbonaceous fibers. The present study could be useful for the application of chopped carbonaceous fibers in antistatic materials. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Carbonaceous fibers Surface resistivity Antistatic Composite coating
1. Introduction Most polymers are typical insulators with high surface resistivity, tending to acquire strong electrostatic charge buildup by polymers in frictional contact with other materials which can result in very large static voltages that may lead to dangerous discharge spark [1]. The occurrence of these electrostatic charges is related to the surface resistivity of the material. The surface resistivity required to effectively dissipate these charges and prevent charge accumulating is usually rather low, 106e109 U [2,3]. Too high surface resistivity prevents control of charge build-up and limits dissipation of static charges. Too low surface resistivity will result in fast electrostatic discharges or arcing from the plastic part. In some application, e.g. packaging for electronics, this may result in damage to electronic components by way of polarization, heating, or shorting out by contacting with the conductive materials. The electrostatic problem can be solved by incorporation of antistatic agents [4,5] or conductive fillers into the polymer which reduces the surface resistivity. The classical antistatic agents are general “soap like” molecules with a hydrophobic and a hydrophilic part which migrate to the surface and, by attracting a layer of water, reduce the surface resistivity [6]. However, these materials have some important drawbacks: they give no volume conduction
* Corresponding author. Tel./fax: þ86 531 88395322. E-mail address: [email protected] (C. Wang). 0304-3886/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elstat.2013.10.007
beneath the surface, and they are generally easily removed and washed out, consequently limiting their effect over time [7]. Permanency is an essential property in many application and the conductive fillers can be a choice. Mixed to the polymer matrix, such conductive fillers form a percolating, conductive network inside the polymer matrix. Common conductive fillers involve carbon series (carbon black [8,9], graphite [2,10], carbon fiber [11e13], carbon nanotube [14e16]), metal powder series [17], metallic oxide series (nano ATO [18], nano ZAO [19]) and hybrid conductive filler [20,21]. However, the applications of carbonaceous fibers in antistatic polymers have been scarcely reported. In this work, the PANbased low-temperature carbonaceous fibers (700 Ce900 C) were prepared on a self-designed production line. The influences of carbonaceous fiber content, carbonization temperature, size and testing voltage on the surface resistivity of composite coatings were investigated. The present study could be useful for the application of chopped carbonaceous fibers in antistatic materials. 2. Experimental 2.1. Raw materials The matrix used in this work was polytetrafluoroethylene (PTFE) paint and its physical properties are shown in Table 1. The PAN-based low-temperature carbonaceous fibers were prepared on a self-designed production line as shown in Fig. 1. The line is composed of two thermal stabilization furnaces with five
S. Zhang et al. / Journal of Electrostatics 71 (2013) 1036e1040 Table 2 Some properties of carbonaceous fibers.
Table 1 Physical properties of PTFE paint. Viscosity (mPa S)
Fineness (mm)
Solid content (%)
Tensile strength (MPa)
600
40
45
20
separate temperature zones respectively, a low-temperature carbonization furnace with three zones and seven sets of stretching equipments. The temperature in 10 furnace zones were set stepwise as 190 C, 200 C, 210 C, 220 C, 230 C, 240 C, 250 C, 260 C, 270 C and 260 C respectively. Subsequently, the oxidized fibers were subjected to low-temperature carbonization in pure nitrogen atmosphere (purity ¼ 99.9999%). The temperature of the first two zones were designated in sequence as 350 C and 500 C, and the temperature of the last zone was set as 700 C, 750 C, 800 C, 850 C and 900 C respectively to obtain different carbonaceous fibers. Properties of carbonaceous fibers are shown in Table 2 and the length is variable according to experiment requirement.
2.2. Samples preparation Chopped carbonaceous fibers were uniformly dispersed in PTFE paint by mechanical stirring. The carbonaceous fibers/PTFE paint mixture was sprayed onto glass fiber/paint composite plate. Average thickness of the composite coating was about 100 mm, and the composite coating was fully cured at room temperature for 48 h. The optical microscope image (magnification of 20 times) of the cured coating is shown in Fig. 2.
2.3. Testing Surface resistance of the composite coating was measured by 1508 type insulation resistance tester (Fluke, USA). The surface resistivity of the composite coating rs could be calculated by Eq. (1).
rs ¼ Rs b=l
1037
(1)
where Rs is the surface resistance of the composite coating; b and l are the electrode width and the electrode spacing respectively (see Fig. 3). Double-sided conductive copper tapes are used as the electrodes. In the test, the conductive tapes adhere to the surface of the composite coating. The testing voltage is 500 V when the surface resistance is below 5.5 108 U, otherwise 1000 V. When the effect of testing voltage on
Fig. 1. Scheme of carbonaceous fibers production line (1)e(10) thermal stabilization furnace zones; IeVII stretching rollers; ①e③ low-temperature carbonization furnace zones.
Temperature ( C)
Carbon content (wt.%)
Electrical resistivity (U cm)
Tensile strength (GPa)
Young’s modulus (GPa)
Elongation at break (%)
700 750 800 850 900
69.9 71.48 73 74.03 74.8
299 13.3 3.79 0.378 8.27 10
1.20 1.43 1.63 1.81 2.02
47.76 70.27 76.57 92.54 99.13
2.27 2.07 2.03 2.01 1.95
2
the surface resistivity is investigated in Section 3.4, it is variable in the range of 50e1000 V according to experiment requirement. 3. Results and discussion 3.1. Effect of fiber content The variation of surface resistivity as a function of fiber content is shown in Fig. 4 for carbonaceous fiber of 900 C and 4 mm length. It can be seen that surface resistivity is very high at low fiber loading (<0.3 wt.%), which is more or less equal to the resistivity of pure paint, exceeding the measurement range of the tester. When fiber content increases from 0.3 to 0.5 wt.%, the surface resistivity of coating falls sharply from 1011 to 106 U. This denotes the conversion of the coating performance from poor antistatic to good antistatic behavior and there exists a critical value (percolation threshold Fc) of fiber content between 0.3 and 0.5 wt.%. When carbonaceous fiber content exceeds the critical value, the coating has good antistatic performance. When fiber content exceeds 0.5 wt.%, the surface resistivity tends to decrease more and more slowly with increasing fiber content. Corresponding to the fiber content from 0.5 to 0.7 wt.%, the surface resistivity value of the coating falls only from 1.25 106 to 2.6 105 U. It’s worth mentioning that the antistatic performance is not improved when fiber content exceeds 0.7 wt.%. In addition, excessive addition of fibers will not only cause the waste of fibers but also increase the difficulty of dispersion for chopped fibers in matrix. As we all know, the coatings would demonstrate inferior performances if chopped fibers were poorly dispersed. Fig. 4 also illustrates the fiber distribution and interconnections in the matrix for different fiber content. Point (1) corresponds to low fiber content, the fiber contacts are rare and there is no longrange continuous conductive channel in the matrix; point (2) is
Fig. 2. Optical microscope image of the cured coating (magnification of 20 times).
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S. Zhang et al. / Journal of Electrostatics 71 (2013) 1036e1040
Fig. 3. Schematic illustration of the surface resistivity of the composite coating test.
The surface resistivity values versus fiber content with fibers of different carbonization temperature curves are plotted in Fig. 5. It
can be seen that all surface resistivity versus fiber content curves for carbonaceous fibers/PTFE paint composite coating have typical features of percolation phenomena. According to the percolation theory [11,23,24], the surface resistivity remains high until about 0.3e0.5 wt.% of chopped carbonaceous fiber has been added, then decreases rapidly and finally levels off at about 0.7 wt.% addition of fiber. From Fig. 5, it is obviously demonstrated that when fibers content reaches to Fs, the surface resistivity of coatings drops from 1010 to 105 U for carbonization temperature of 700 Ce900 C. The reason is primarily due to the difference in the electrical resistivity of carbonaceous fibers. The electrical resistivity of fiber decreases obviously with increasing carbonization temperature and the data is shown in Table 2. The electrical resistivity of conductivity filler is one of the key factors that predominates the resistivity of the composite [20]. Lower electrical resistivity of carbonaceous fiber results in lower surface resistivity of the coating. In ESD (electrostatic discharge)-protected environments the optimal surface resistivity is in the range of 106e109 U, and the optimal carbonization temperature of fibers is 750 Ce850 C. Therefore, it is much easier to add carbonaceous fibers to achieve the optimal surface resistivity by changing the content and carbonization temperature of fibers. From Fig. 5 it is clearly visible that the carbonization temperature of fibers has no significant effect on the values of Fc and Fs. They are about 0.4 and 0.7, respectively. It can be concluded that it is not necessary to exceed the value Fs for the fiber content and that the use of different carbonization temperature of fibers allows the required value of surface resistivity to be obtained first.
Fig. 4. Effect of fiber content on surface resistivity of the coating with 4 mm fiber length; carbonization temperature of fiber is 900 C.
Fig. 5. Effect of fiber carbonization temperature on surface resistivity with 4 mm fiber length.
representative of the value near the percolation threshold and the continuous conductive network is beginning to form. Beyond this value-point (3) the surface resistivity of coating decreases slightly with the increase of fiber content, and further fiber addition point (4) will not produce significant variations to the surface resistivity [12,22]. This process may be regarded as a growth process of percolation group which is interconnected fiber clusters. The growth rate of percolation group is very rapid near the percolation threshold Fc. At this stage, a small increase of fiber content will markedly makes percolation ratio increase, thus decreasing rapidly surface resistivity of the coating. The percolation ratio is the ratio of the amount of fibers that involved in the continuous conductive network and total fibers. When the fiber content exceeds point (3), the growth rate of percolation group becomes slow, so the surface resistivity of the coating tends to decrease more and more slowly with increasing fiber content. When fiber content exceeds point (4) (used Fs as follows), the percolation ratio approaches 100%. That is, all individual chopped fibers are involved in the continuous conductive network. At this moment, the increase of fiber content no longer has a significant impact on the growth rate of percolation group and the surface resistivity becomes leveling off.
3.2. Effect of fiber carbonization temperature
S. Zhang et al. / Journal of Electrostatics 71 (2013) 1036e1040
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Table 4 Effect of testing voltage on surface resistivity. Testing voltage (V)
Surface resistivity (U)
50 100 250 500 1000
>55 52.6 37.9 27.3 12.1
through the tunnel barrier from a single fiber hopping to another with the aid of the tunneling effect. Sometimes the phenomenon of partial discharge occurs on the coating surface under 1000 V voltage, which is because the fibereresin interface (tunnel barrier) is breakdown by electronics. The antistatic mechanisms of the short carbonaceous fiber based antistatic coating mainly depend on the conductive path theory, tunneling effect and field emission theory. Fig. 6. Effect of fiber length on the surface resistivity of the coating, the carbonization temperature of fiber is 750 C.
3.3. Effect of fiber length The effect of fiber length on surface resistivity is shown in Fig. 6. As expected, the values of Fc and Fs decrease when fiber length increases. From Table 3, the values of Fc and Fs are 1.0% and 1.6% respectively in the case of 1 mm, while Fc and Fs are 0.3% and 0.6% respectively as the fiber length is 8 mm. The longer fibers are more likely to be interconnected to form continuous conductive path. From the graph it is clearly visible, for 8 mm and 4 mm fibers, the surface resistivity drops drastically in the zone of Fc Fs; on the contrary for the fibers 2 mm and 1 mm long a relatively gradual decrease in surface resistivity has been found. The growth rate of percolation group of long fibers is faster than that of short fibers in the zone of Fc Fs. When carbonaceous fibers content exceeds Fs, the surface resistivity of the coating is substantially maintained at about 107 U for the fibers of all the length. That is, the influence of fiber length on the surface resistivity of composite coating drops gradually with the raise of fiber content. The result again shows that the electrical resistivity of carbonaceous fibers is a key factor that predominates the surface resistivity of the composite coating.
4. Conclusions (1) The composite coating performance changes from poor antistatic to good antistatic behavior near the percolation threshold Fc and the addition of more fiber content, above Fs value, does not produce any significant decrease in the surface resistivity of the coating. (2) When fibers content reaches to Fs, the surface resistivity of the coating using the 700 Ce900 C carbonaceous fibers as conductive filler is adjustable between 1010 and 105 U. In ESD-protected environments the optimal carbonization temperature of fibers is 750 Ce850 C and the surface resistivity is in the range of 106e109 U. (3) Carbonization temperature of fibers has little effect on the values of Fc and Fs, while the fiber length has a greater impact. The values of Fc and Fs are lower in the case of long fibers than that in the case of short fibers. The growth rate of percolation group of long fibers is faster than that of short fibers in the zone of Fc Fs. (4) The surface resistivity values of the coatings decrease with increasing testing voltage. The antistatic mechanisms of the short carbonaceous fiber based antistatic coating mainly depend on the conductive path theory, tunneling effect and field emission theory. Acknowledgments
3.4. Effect of testing voltage The effect of testing voltage on surface resistivity was analyzed for carbonization temperature of fibers equal to 750 C, length of 4 mm and 0.8 wt.% of fiber content. The experimental data is shown in Table 4. The surface resistivity values of the coatings decrease with increasing testing voltage. Each conductive filler has a thin layer of adsorbed polymer [25]. There is the tunnel barrier between any two adjacent monofilaments. According to the field emission theory, the height and width of the tunnel barrier are reduced by a strong external electric field. When the width of the tunnel barrier closes to the wavelength of the electron, electron can penetrate
Table 3 The values of Fc and Fs with different fiber lengths. Carbonaceous fiber length/mm
Fc/%
Fs%
1 2 4 8
1.0 0.6 0.4 0.3
1.6 1.2 0.7 0.6
This work has been supported by National Basic Research Program of China (973 Program) (2010CB631102, 2011CB605601). References [1] J.A. Johnson, M.J. Barbato, S.R. Hopkins, M.J. O’malley, Dispersion and film properties of carbon nanofiber pigmented conductive coatings, Prog. Org. Coat. 47 (3e4) (2003) 198e206. [2] S.S. Azim, A. Satheesh, K.K. Ramu, S. Ramu, G. Venkatachari, Studies on graphite based conductive paint coatings, Prog. Org. Coat. 55 (1) (2006) 1e4. [3] M. Narkis, G. Lidor, A. Vaxman, L. Zuri, New injection moldable electrostatic dissipative (ESD) composites based on very low carbon black loadings, J. Electrostat. 47 (4) (1999) 201e214. [4] F.X. Zhang, M.P. Srinivasan, Crosslinked polyimide polythiophene composite with reduced surface resistivities, Thin Solid Films 479 (1/2) (2005) 95e102. [5] M.E.L. Wouters, D.P. Wolfs, M.C. van der Linde, J.H.P. Hovens, A.H.A. Tinnemans, Transparent UV curable antistatic hybrid coatings on polycarbonate prepared by the solegel methods, Prog. Org. Coat. 51 (4) (2004) 312e319. [6] J. Ward, R. Simmons, P. Chatham, Internal anti-static agents for engineering plastics, Annu. Tech. Conf. Soc. Plast. Eng. 56 (2) (1998) 1782e1786. [7] C. Li, T. Liang, W. Lu, C. Tang, X. Hu, M. Cao, J. Liang, Improving the antistatic ability of polypropylene fibers by inner antistatic agent filled with carbon nanotubes, Compos. Sci. Technol. 64 (13e14) (2004) 2089e2096.
1040
S. Zhang et al. / Journal of Electrostatics 71 (2013) 1036e1040
[8] I. Novak, I. Krupa, I. Janigova, Hybrid electro-conductive composites with improved toughness, filled by carbon black, Carbon 43 (4) (2005) 841e848. [9] S. Rastegara, Z. Ranjbar, DC and AC electrical conductivity of electro-deposited carbon-blackeepoxy composite films, Prog. Org. Coat. 63 (1) (2008) 1e4. [10] J. Jin, S. Leesirisan, M. Song, Electrical conductivity of ion-doped graphite/ polyethersulphone composites, Compos. Sci. Technol. 70 (10) (2010) 1544e 1549. [11] J. Vilcakova, P. Saha, O. Quadrat, Electrical conductivity of carbon fibers/ polyester resin composites in the percolation threshold region, Eur. Polym. J. 38 (12) (2002) 2343e2347. [12] M. Chiarello, R. Zinno, Electrical conductivity of self-monitoring CFRC, Cement Concrete Comp. 27 (4) (2005) 463e469. [13] J. Xu, W. Yao, R. Wang, Nonlinear conduction in carbon fiber reinforce cement mortar, Cem. Concr. Compos. 33 (3) (2011) 444e448. [14] I. O’Connor, S. De, J.N. Coleman, Y.K. Gun’Ko, Development of transparent, conducting composites by surface infiltration of nanotubes into commercial polymer films, Carbon 47 (8) (2009) 1983e1988. [15] P. Xue, K.H. Park, X.M. Tao, W. Chen, X.Y. Cheng, Electrically conductive yarns based on PVA/carbon nanotubes, Compos. Struct. 78 (2) (2007) 271e277. [16] C. Li, E.T. Thostenson, T.W. Chou, Effect of nanotube waviness on the electrical conductivity of carbon nanotube-based composites, Compos. Sci. Technol. 68 (6) (2008) 1445e1452.
[17] I. Krupa, G. Mikova, I. Novak, I. Janigova, Z. Nogellova, F. Lednicky, J. Prokes, Electrically conductive composites of polyethylene filled with polyamide particles coated with silver, Eur. Polym. J. 43 (6) (2007) 2401e2413. [18] D. Zhang, Z. Deng, J. Zhang, L. Chen, Microstructure and electrical properties of antimony-doped tin oxide thin film deposited by solegel process, Mater. Chem. Phys. 98 (2e3) (2006) 353e357. [19] W. Lin, R. Ma, W. Shao, B. Liu, Structural, electrical and optical properties of Gd doped and undoped ZnO: Al (ZAO) thin films prepared by RF magnetron sputtering, Appl. Surf. Sci. 253 (11) (2007) 5179e5183. [20] S. Wu, L. Mo, Z. Shui, Z. Chen, Investigation of the conductivity of asphalt concrete containing conductive fillers, Carbon 43 (7) (2005) 1358e1363. [21] M. Drubetski, A. Siegmann, M. Narkis, Electrical properties of hybrid carbon black/carbon fiber polypropylene composites, J. Mater. Sci. 42 (1) (2007) 1e8. [22] B. Chen, K. Wu, W. Yao, Conductivity of carbon fiber reinforced cement-based composites, Cem. Concr. Compos. 26 (4) (2004) 291e297. [23] F. Bueche, Electrical resistivity of conducting particles in an insulating matrix, J. Appl. Phys. 43 (8) (1972) 2279e2286. [24] X. Liang, L. Ling, C. Lu, L. Liu, Resistivity of carbon fibers/ABS resin composites, Mater. Lett. 43 (3) (2000) 144e147. [25] F. Lux, Review models proposed to explain the electrical conductivity of mixtures made of conductive and insulating materials, J. Mater. Sci. 28 (1993) 285e301.
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Antistatic behavior of PAN-based low-temperature carbonaceous fibers Shan Zhang a, b, Chengguo Wang b, *, Hua Yuan b, Bingming Zhang b, Xue Lin b, Zhitao Lin b a b
Key Laboratory for LiquideSolid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China Carbon Fiber Engineering Research Center, Faculty of Materials Science, Shandong University, Jinan 250061, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2013 Received in revised form 6 October 2013 Accepted 8 October 2013 Available online 15 October 2013
A kind of antistatic coatings which were applied to nonconductive surfaces were prepared with polytetrafluoroethylene (PTFE) paint as matrix, polyacrylonitrile (PAN)-based low-temperature carbonaceous fibers as conductive filler. The influences of carbonaceous fiber content, carbonization temperature, size and testing voltage on the surface resistivity of the coating were investigated. The surface resistivity could be well controlled in the static dissipative range (106e109 U) by changing the content, size and carbonization temperature of carbonaceous fibers. The present study could be useful for the application of chopped carbonaceous fibers in antistatic materials. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Carbonaceous fibers Surface resistivity Antistatic Composite coating
1. Introduction Most polymers are typical insulators with high surface resistivity, tending to acquire strong electrostatic charge buildup by polymers in frictional contact with other materials which can result in very large static voltages that may lead to dangerous discharge spark [1]. The occurrence of these electrostatic charges is related to the surface resistivity of the material. The surface resistivity required to effectively dissipate these charges and prevent charge accumulating is usually rather low, 106e109 U [2,3]. Too high surface resistivity prevents control of charge build-up and limits dissipation of static charges. Too low surface resistivity will result in fast electrostatic discharges or arcing from the plastic part. In some application, e.g. packaging for electronics, this may result in damage to electronic components by way of polarization, heating, or shorting out by contacting with the conductive materials. The electrostatic problem can be solved by incorporation of antistatic agents [4,5] or conductive fillers into the polymer which reduces the surface resistivity. The classical antistatic agents are general “soap like” molecules with a hydrophobic and a hydrophilic part which migrate to the surface and, by attracting a layer of water, reduce the surface resistivity [6]. However, these materials have some important drawbacks: they give no volume conduction
* Corresponding author. Tel./fax: þ86 531 88395322. E-mail address: [email protected] (C. Wang). 0304-3886/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elstat.2013.10.007
beneath the surface, and they are generally easily removed and washed out, consequently limiting their effect over time [7]. Permanency is an essential property in many application and the conductive fillers can be a choice. Mixed to the polymer matrix, such conductive fillers form a percolating, conductive network inside the polymer matrix. Common conductive fillers involve carbon series (carbon black [8,9], graphite [2,10], carbon fiber [11e13], carbon nanotube [14e16]), metal powder series [17], metallic oxide series (nano ATO [18], nano ZAO [19]) and hybrid conductive filler [20,21]. However, the applications of carbonaceous fibers in antistatic polymers have been scarcely reported. In this work, the PANbased low-temperature carbonaceous fibers (700 Ce900 C) were prepared on a self-designed production line. The influences of carbonaceous fiber content, carbonization temperature, size and testing voltage on the surface resistivity of composite coatings were investigated. The present study could be useful for the application of chopped carbonaceous fibers in antistatic materials. 2. Experimental 2.1. Raw materials The matrix used in this work was polytetrafluoroethylene (PTFE) paint and its physical properties are shown in Table 1. The PAN-based low-temperature carbonaceous fibers were prepared on a self-designed production line as shown in Fig. 1. The line is composed of two thermal stabilization furnaces with five
S. Zhang et al. / Journal of Electrostatics 71 (2013) 1036e1040 Table 2 Some properties of carbonaceous fibers.
Table 1 Physical properties of PTFE paint. Viscosity (mPa S)
Fineness (mm)
Solid content (%)
Tensile strength (MPa)
600
40
45
20
separate temperature zones respectively, a low-temperature carbonization furnace with three zones and seven sets of stretching equipments. The temperature in 10 furnace zones were set stepwise as 190 C, 200 C, 210 C, 220 C, 230 C, 240 C, 250 C, 260 C, 270 C and 260 C respectively. Subsequently, the oxidized fibers were subjected to low-temperature carbonization in pure nitrogen atmosphere (purity ¼ 99.9999%). The temperature of the first two zones were designated in sequence as 350 C and 500 C, and the temperature of the last zone was set as 700 C, 750 C, 800 C, 850 C and 900 C respectively to obtain different carbonaceous fibers. Properties of carbonaceous fibers are shown in Table 2 and the length is variable according to experiment requirement.
2.2. Samples preparation Chopped carbonaceous fibers were uniformly dispersed in PTFE paint by mechanical stirring. The carbonaceous fibers/PTFE paint mixture was sprayed onto glass fiber/paint composite plate. Average thickness of the composite coating was about 100 mm, and the composite coating was fully cured at room temperature for 48 h. The optical microscope image (magnification of 20 times) of the cured coating is shown in Fig. 2.
2.3. Testing Surface resistance of the composite coating was measured by 1508 type insulation resistance tester (Fluke, USA). The surface resistivity of the composite coating rs could be calculated by Eq. (1).
rs ¼ Rs b=l
1037
(1)
where Rs is the surface resistance of the composite coating; b and l are the electrode width and the electrode spacing respectively (see Fig. 3). Double-sided conductive copper tapes are used as the electrodes. In the test, the conductive tapes adhere to the surface of the composite coating. The testing voltage is 500 V when the surface resistance is below 5.5 108 U, otherwise 1000 V. When the effect of testing voltage on
Fig. 1. Scheme of carbonaceous fibers production line (1)e(10) thermal stabilization furnace zones; IeVII stretching rollers; ①e③ low-temperature carbonization furnace zones.
Temperature ( C)
Carbon content (wt.%)
Electrical resistivity (U cm)
Tensile strength (GPa)
Young’s modulus (GPa)
Elongation at break (%)
700 750 800 850 900
69.9 71.48 73 74.03 74.8
299 13.3 3.79 0.378 8.27 10
1.20 1.43 1.63 1.81 2.02
47.76 70.27 76.57 92.54 99.13
2.27 2.07 2.03 2.01 1.95
2
the surface resistivity is investigated in Section 3.4, it is variable in the range of 50e1000 V according to experiment requirement. 3. Results and discussion 3.1. Effect of fiber content The variation of surface resistivity as a function of fiber content is shown in Fig. 4 for carbonaceous fiber of 900 C and 4 mm length. It can be seen that surface resistivity is very high at low fiber loading (<0.3 wt.%), which is more or less equal to the resistivity of pure paint, exceeding the measurement range of the tester. When fiber content increases from 0.3 to 0.5 wt.%, the surface resistivity of coating falls sharply from 1011 to 106 U. This denotes the conversion of the coating performance from poor antistatic to good antistatic behavior and there exists a critical value (percolation threshold Fc) of fiber content between 0.3 and 0.5 wt.%. When carbonaceous fiber content exceeds the critical value, the coating has good antistatic performance. When fiber content exceeds 0.5 wt.%, the surface resistivity tends to decrease more and more slowly with increasing fiber content. Corresponding to the fiber content from 0.5 to 0.7 wt.%, the surface resistivity value of the coating falls only from 1.25 106 to 2.6 105 U. It’s worth mentioning that the antistatic performance is not improved when fiber content exceeds 0.7 wt.%. In addition, excessive addition of fibers will not only cause the waste of fibers but also increase the difficulty of dispersion for chopped fibers in matrix. As we all know, the coatings would demonstrate inferior performances if chopped fibers were poorly dispersed. Fig. 4 also illustrates the fiber distribution and interconnections in the matrix for different fiber content. Point (1) corresponds to low fiber content, the fiber contacts are rare and there is no longrange continuous conductive channel in the matrix; point (2) is
Fig. 2. Optical microscope image of the cured coating (magnification of 20 times).
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S. Zhang et al. / Journal of Electrostatics 71 (2013) 1036e1040
Fig. 3. Schematic illustration of the surface resistivity of the composite coating test.
The surface resistivity values versus fiber content with fibers of different carbonization temperature curves are plotted in Fig. 5. It
can be seen that all surface resistivity versus fiber content curves for carbonaceous fibers/PTFE paint composite coating have typical features of percolation phenomena. According to the percolation theory [11,23,24], the surface resistivity remains high until about 0.3e0.5 wt.% of chopped carbonaceous fiber has been added, then decreases rapidly and finally levels off at about 0.7 wt.% addition of fiber. From Fig. 5, it is obviously demonstrated that when fibers content reaches to Fs, the surface resistivity of coatings drops from 1010 to 105 U for carbonization temperature of 700 Ce900 C. The reason is primarily due to the difference in the electrical resistivity of carbonaceous fibers. The electrical resistivity of fiber decreases obviously with increasing carbonization temperature and the data is shown in Table 2. The electrical resistivity of conductivity filler is one of the key factors that predominates the resistivity of the composite [20]. Lower electrical resistivity of carbonaceous fiber results in lower surface resistivity of the coating. In ESD (electrostatic discharge)-protected environments the optimal surface resistivity is in the range of 106e109 U, and the optimal carbonization temperature of fibers is 750 Ce850 C. Therefore, it is much easier to add carbonaceous fibers to achieve the optimal surface resistivity by changing the content and carbonization temperature of fibers. From Fig. 5 it is clearly visible that the carbonization temperature of fibers has no significant effect on the values of Fc and Fs. They are about 0.4 and 0.7, respectively. It can be concluded that it is not necessary to exceed the value Fs for the fiber content and that the use of different carbonization temperature of fibers allows the required value of surface resistivity to be obtained first.
Fig. 4. Effect of fiber content on surface resistivity of the coating with 4 mm fiber length; carbonization temperature of fiber is 900 C.
Fig. 5. Effect of fiber carbonization temperature on surface resistivity with 4 mm fiber length.
representative of the value near the percolation threshold and the continuous conductive network is beginning to form. Beyond this value-point (3) the surface resistivity of coating decreases slightly with the increase of fiber content, and further fiber addition point (4) will not produce significant variations to the surface resistivity [12,22]. This process may be regarded as a growth process of percolation group which is interconnected fiber clusters. The growth rate of percolation group is very rapid near the percolation threshold Fc. At this stage, a small increase of fiber content will markedly makes percolation ratio increase, thus decreasing rapidly surface resistivity of the coating. The percolation ratio is the ratio of the amount of fibers that involved in the continuous conductive network and total fibers. When the fiber content exceeds point (3), the growth rate of percolation group becomes slow, so the surface resistivity of the coating tends to decrease more and more slowly with increasing fiber content. When fiber content exceeds point (4) (used Fs as follows), the percolation ratio approaches 100%. That is, all individual chopped fibers are involved in the continuous conductive network. At this moment, the increase of fiber content no longer has a significant impact on the growth rate of percolation group and the surface resistivity becomes leveling off.
3.2. Effect of fiber carbonization temperature
S. Zhang et al. / Journal of Electrostatics 71 (2013) 1036e1040
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Table 4 Effect of testing voltage on surface resistivity. Testing voltage (V)
Surface resistivity (U)
50 100 250 500 1000
>55 52.6 37.9 27.3 12.1
through the tunnel barrier from a single fiber hopping to another with the aid of the tunneling effect. Sometimes the phenomenon of partial discharge occurs on the coating surface under 1000 V voltage, which is because the fibereresin interface (tunnel barrier) is breakdown by electronics. The antistatic mechanisms of the short carbonaceous fiber based antistatic coating mainly depend on the conductive path theory, tunneling effect and field emission theory. Fig. 6. Effect of fiber length on the surface resistivity of the coating, the carbonization temperature of fiber is 750 C.
3.3. Effect of fiber length The effect of fiber length on surface resistivity is shown in Fig. 6. As expected, the values of Fc and Fs decrease when fiber length increases. From Table 3, the values of Fc and Fs are 1.0% and 1.6% respectively in the case of 1 mm, while Fc and Fs are 0.3% and 0.6% respectively as the fiber length is 8 mm. The longer fibers are more likely to be interconnected to form continuous conductive path. From the graph it is clearly visible, for 8 mm and 4 mm fibers, the surface resistivity drops drastically in the zone of Fc Fs; on the contrary for the fibers 2 mm and 1 mm long a relatively gradual decrease in surface resistivity has been found. The growth rate of percolation group of long fibers is faster than that of short fibers in the zone of Fc Fs. When carbonaceous fibers content exceeds Fs, the surface resistivity of the coating is substantially maintained at about 107 U for the fibers of all the length. That is, the influence of fiber length on the surface resistivity of composite coating drops gradually with the raise of fiber content. The result again shows that the electrical resistivity of carbonaceous fibers is a key factor that predominates the surface resistivity of the composite coating.
4. Conclusions (1) The composite coating performance changes from poor antistatic to good antistatic behavior near the percolation threshold Fc and the addition of more fiber content, above Fs value, does not produce any significant decrease in the surface resistivity of the coating. (2) When fibers content reaches to Fs, the surface resistivity of the coating using the 700 Ce900 C carbonaceous fibers as conductive filler is adjustable between 1010 and 105 U. In ESD-protected environments the optimal carbonization temperature of fibers is 750 Ce850 C and the surface resistivity is in the range of 106e109 U. (3) Carbonization temperature of fibers has little effect on the values of Fc and Fs, while the fiber length has a greater impact. The values of Fc and Fs are lower in the case of long fibers than that in the case of short fibers. The growth rate of percolation group of long fibers is faster than that of short fibers in the zone of Fc Fs. (4) The surface resistivity values of the coatings decrease with increasing testing voltage. The antistatic mechanisms of the short carbonaceous fiber based antistatic coating mainly depend on the conductive path theory, tunneling effect and field emission theory. Acknowledgments
3.4. Effect of testing voltage The effect of testing voltage on surface resistivity was analyzed for carbonization temperature of fibers equal to 750 C, length of 4 mm and 0.8 wt.% of fiber content. The experimental data is shown in Table 4. The surface resistivity values of the coatings decrease with increasing testing voltage. Each conductive filler has a thin layer of adsorbed polymer [25]. There is the tunnel barrier between any two adjacent monofilaments. According to the field emission theory, the height and width of the tunnel barrier are reduced by a strong external electric field. When the width of the tunnel barrier closes to the wavelength of the electron, electron can penetrate
Table 3 The values of Fc and Fs with different fiber lengths. Carbonaceous fiber length/mm
Fc/%
Fs%
1 2 4 8
1.0 0.6 0.4 0.3
1.6 1.2 0.7 0.6
This work has been supported by National Basic Research Program of China (973 Program) (2010CB631102, 2011CB605601). References [1] J.A. Johnson, M.J. Barbato, S.R. Hopkins, M.J. O’malley, Dispersion and film properties of carbon nanofiber pigmented conductive coatings, Prog. Org. Coat. 47 (3e4) (2003) 198e206. [2] S.S. Azim, A. Satheesh, K.K. Ramu, S. Ramu, G. Venkatachari, Studies on graphite based conductive paint coatings, Prog. Org. Coat. 55 (1) (2006) 1e4. [3] M. Narkis, G. Lidor, A. Vaxman, L. Zuri, New injection moldable electrostatic dissipative (ESD) composites based on very low carbon black loadings, J. Electrostat. 47 (4) (1999) 201e214. [4] F.X. Zhang, M.P. Srinivasan, Crosslinked polyimide polythiophene composite with reduced surface resistivities, Thin Solid Films 479 (1/2) (2005) 95e102. [5] M.E.L. Wouters, D.P. Wolfs, M.C. van der Linde, J.H.P. Hovens, A.H.A. Tinnemans, Transparent UV curable antistatic hybrid coatings on polycarbonate prepared by the solegel methods, Prog. Org. Coat. 51 (4) (2004) 312e319. [6] J. Ward, R. Simmons, P. Chatham, Internal anti-static agents for engineering plastics, Annu. Tech. Conf. Soc. Plast. Eng. 56 (2) (1998) 1782e1786. [7] C. Li, T. Liang, W. Lu, C. Tang, X. Hu, M. Cao, J. Liang, Improving the antistatic ability of polypropylene fibers by inner antistatic agent filled with carbon nanotubes, Compos. Sci. Technol. 64 (13e14) (2004) 2089e2096.
1040
S. Zhang et al. / Journal of Electrostatics 71 (2013) 1036e1040
[8] I. Novak, I. Krupa, I. Janigova, Hybrid electro-conductive composites with improved toughness, filled by carbon black, Carbon 43 (4) (2005) 841e848. [9] S. Rastegara, Z. Ranjbar, DC and AC electrical conductivity of electro-deposited carbon-blackeepoxy composite films, Prog. Org. Coat. 63 (1) (2008) 1e4. [10] J. Jin, S. Leesirisan, M. Song, Electrical conductivity of ion-doped graphite/ polyethersulphone composites, Compos. Sci. Technol. 70 (10) (2010) 1544e 1549. [11] J. Vilcakova, P. Saha, O. Quadrat, Electrical conductivity of carbon fibers/ polyester resin composites in the percolation threshold region, Eur. Polym. J. 38 (12) (2002) 2343e2347. [12] M. Chiarello, R. Zinno, Electrical conductivity of self-monitoring CFRC, Cement Concrete Comp. 27 (4) (2005) 463e469. [13] J. Xu, W. Yao, R. Wang, Nonlinear conduction in carbon fiber reinforce cement mortar, Cem. Concr. Compos. 33 (3) (2011) 444e448. [14] I. O’Connor, S. De, J.N. Coleman, Y.K. Gun’Ko, Development of transparent, conducting composites by surface infiltration of nanotubes into commercial polymer films, Carbon 47 (8) (2009) 1983e1988. [15] P. Xue, K.H. Park, X.M. Tao, W. Chen, X.Y. Cheng, Electrically conductive yarns based on PVA/carbon nanotubes, Compos. Struct. 78 (2) (2007) 271e277. [16] C. Li, E.T. Thostenson, T.W. Chou, Effect of nanotube waviness on the electrical conductivity of carbon nanotube-based composites, Compos. Sci. Technol. 68 (6) (2008) 1445e1452.
[17] I. Krupa, G. Mikova, I. Novak, I. Janigova, Z. Nogellova, F. Lednicky, J. Prokes, Electrically conductive composites of polyethylene filled with polyamide particles coated with silver, Eur. Polym. J. 43 (6) (2007) 2401e2413. [18] D. Zhang, Z. Deng, J. Zhang, L. Chen, Microstructure and electrical properties of antimony-doped tin oxide thin film deposited by solegel process, Mater. Chem. Phys. 98 (2e3) (2006) 353e357. [19] W. Lin, R. Ma, W. Shao, B. Liu, Structural, electrical and optical properties of Gd doped and undoped ZnO: Al (ZAO) thin films prepared by RF magnetron sputtering, Appl. Surf. Sci. 253 (11) (2007) 5179e5183. [20] S. Wu, L. Mo, Z. Shui, Z. Chen, Investigation of the conductivity of asphalt concrete containing conductive fillers, Carbon 43 (7) (2005) 1358e1363. [21] M. Drubetski, A. Siegmann, M. Narkis, Electrical properties of hybrid carbon black/carbon fiber polypropylene composites, J. Mater. Sci. 42 (1) (2007) 1e8. [22] B. Chen, K. Wu, W. Yao, Conductivity of carbon fiber reinforced cement-based composites, Cem. Concr. Compos. 26 (4) (2004) 291e297. [23] F. Bueche, Electrical resistivity of conducting particles in an insulating matrix, J. Appl. Phys. 43 (8) (1972) 2279e2286. [24] X. Liang, L. Ling, C. Lu, L. Liu, Resistivity of carbon fibers/ABS resin composites, Mater. Lett. 43 (3) (2000) 144e147. [25] F. Lux, Review models proposed to explain the electrical conductivity of mixtures made of conductive and insulating materials, J. Mater. Sci. 28 (1993) 285e301.