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Conductive coatings and composites from latex-based dispersions
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Colloids and Surfaces A: Physicochem. Eng. Aspects 311 (2007) 48–54
Conductive coatings and composites from latex-based dispersions Lorraine F. Francis a,∗ , Jaime C. Grunlan b , Jiakuan Sun c , W.W. Gerberich a a
Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, MN 55455, United States b Department of Mechanical Engineering, Texas A; M University College Station, TX 77843-3123, United States c Rohm and Haas (China) Holding Co. Ltd., China Research and Development Center 1077, Zhang Heng Road, Zhangjiang Hi-tech Park, Shanghai, 201203, China Available online 24 August 2007
Abstract Electrically conductive coatings and composites are prepared from aqueous dispersions of conducting particles and polymer latex particles. Relatively small amounts of conductive particles are needed to develop electrical conductivity, because the particulate nature of the latex leads to a segregated network that lowers the percolation threshold. Several nanosized conductive fillers have been studied: carbon black, antimony-doped tin oxide, indium tin oxide and carbon nanotubes. The latex chosen for most studies was either a poly(vinyl acetate-co-acrylic) polydisperse latex, a poly(vinyl acetate) polydisperse latex, or monodisperse poly(vinyl acetate) latex. This paper reviews the effect of particle size, aggregation and aspect ratio on the microstructure and properties of conductive composites and coatings. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite; Latex; Conductivity; Coating
1. Introduction Composites containing conductive particles in a nonconductive polymer matrix have been used for many years for applications such as electromagnetic shields, thermally sensitive resistors, and sensors [1]. In recent years, the development of flexible electronic devices and large area “macroelectronics” [2] has led to a demand for coatings with tailored electrical properties and functions. In this paper, a common base for liquid applied coatings – a latex dispersion – is combined with electrically conductive nanoparticles to create functional materials and coatings for a variety of applications. To impart electrical conductivity to a composite, the quantity of conductive particles must be high enough for the particles to form an interconnected network [3,4]. The formation of the network is commonly described as a percolation phenomenon in which the conductivity (σ) follows a power law and jumps by orders of magnitude when the volume fraction of conductive particles (V) surpasses a critical percolation threshold (Vc ): σ = σ0 (V − Vc )s
(1)
In Eq. (1), σ 0 is a proportionality constant, and s is the critical conductivity exponent (∼1.5–2.0 for a 3D random system). ∗
Corresponding author. Tel.: +1 612 625 0559; fax: +1 612 626 7246. E-mail address: [email protected] (L.F. Francis).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.08.026
The formation of an interconnected conductive particle network depends both on the morphology of the conductive particles and the characteristics of the matrix and resulting composite microstructure. A variety of theories and models have been proposed to account for the conductivity of composites and the effect microstructure on conductivity [5 and references therein]. For random addition of conductive particles to a continuous nonconductive matrix, the critical conductive particle content needed to establish the network is roughly 16 vol% [3,4]. Lowering the percolation threshold requires altering the conductive particles, the matrix or both. For example, particles with asymmetric morphologies create percolating networks at lower contents than do spherical particles [5]. The microstructure of the matrix can force percolation to occur at a lower value by segregating the conductive particles to a restricted volume within the microstructure. For example, conductive particles that segregate to one phase in a polymer blend form a percolating network selectively in that phase and therefore at a lower overall particle content [6 and references therein]. The magnitude of the conductivity past the threshold strongly depends on the properties of the conductive particles themselves and the connections between the particles [7]. In this paper, latex particles are used to construct a segregated network of conducting particles, as shown schematically in Fig. 1. Conductive particles are mixed with nonconductive latex particles in an aqueous dispersion. On drying, latex undergoes a sequence of microstructure changes [8 and references
L.F. Francis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 311 (2007) 48–54
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Fig. 1. Schematic of microstructure development of a latex-based composite containing conductive particles. Adapted from [17].
therein]. Conductive particles are trapped as the latex forms the polymer matrix. In these systems, several materials and processing variables influence composite microstructure and properties. The focus here is on the effects of the characteristics of the particles. Examples are taken from the authors’ recent papers [9–15] and Ph.D. theses [16,17].
2. Experimental 2.1. Materials The characteristics of the conductive particles and latex dispersions used in the research are given in Tables 1 and 2. The following abbreviations will be used throughout the paper: carbon black (CB), antimony doped tin oxide (ATO), indium tin oxide (ITO) and single walled carbon nanotubes (SWNT), poly(vinyl acetate) (PVAc), poly(vinyl acetate co-acrylic) (PVAc-co-acrylic), and poly(methyl methacrylate co-butyl acrylate) (MMA-BA).
2.2. Preparation of composites and coatings The general procedure for composite preparation has two steps. First, an aqueous dispersion containing conductive particles and latex is prepared. Second, the dispersion is converted into a bulk composite by drying at room temperature (CB1/latex) or a coating by application with a wire wound rod onto a substrate followed by drying at 50 ◦ C (CB2/latex, ATO/latex, ITO/latex). Coating thickness is increased by repeating the deposition and drying. Details of the preparation procedures are given elsewhere for the CB/latex [9–12], ATO/latex and ITO/latex [14,15], and SWNT/latex [13]. 2.3. Characterization Composite microstructures and properties were characterized by several techniques. Scanning electron microscopy was used to characterize the cross-sectional and surface microstructures. Direct current electrical conductivity was measured using a commercial 4-point probe instruments (Veeco FPP-5000 or
Table 1 Characteristics of conductive particles Particle chemistry
Abbreviation
Manufacturer (product name)
Characteristics
Carbon black
CB 1
Dry powder, 20 nm diameter spherical primary particles in open clusters
Carbon black
CB2
Columbian Chem. (Conductex 976 Ultra) Columbian Chem. (Raven 1170)
Antimony-tin oxide Indium–tin oxide Single-walled carbon nanotube
ATO ITO SWNT
Nanophase Technologies (Nanotek® ) Nanophase Technologies (Nanotek® ) Carbon Nanotechnologies
Dry powder, 20 nm diameter spherical primary particles in closed clusters, σ∼ = 1 S/cm* Dry powder, 15 nm (ave) diameter spherical primary particles, σ ∼ = 0.l S/cm* Dry powder, 20 nm (ave) diameter faceted primary particles, σ ∼ = 0.01 S/cm* Dry powder, 1–2 nm diameter and 100+ nm long, σ ∼ = 510 S/cm**
*dry particle coating. **from Ramasubramaniam et al. [29]; measured on a polymer-free coating of SWNT’s. Table 2 Characteristics of latex dispersions Latex chemistry
Abbreviation
Manufacturer (product name)
Characteristics
Poly(vinyl acetate)
PVAc
Air Products (Vinac 21)
Poly(vinyl acetate co-acrylic)
PVAc-co-acrylic
Air Products (Flexbond 325)
Poly(vinyl acetate) Poly(methyl methacylate co-butyl acrylate)
PVAc-M MMA-BA
H.B. Fuller (PD 202) Custom [12]
Aqueous dispersion; Tg = 35 ◦ C, MFFT* = 15 ◦ C; polydisperse (vol. ave size, Dv = 2.6 m, num. ave. size, Dn = 108 nm) Aqueous dispersion; Tg = 19 ◦ C, MFFT* = 10 ◦ C; polydisperse (Dv = 333 nm and Dn = 60 nm) Aqueous dispersion; Tg = 34 ◦ C, monodisperse (Dv = Dn = 116 nm) Aqueous dispersion; Tg = 19 ◦ C, monodisperse (Dv = 1.087 m, Dn = 0.929 m)
*minimum film formation temperature.
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Loresta AP [MCP-T4000]) or a home built apparatus consisting of a Keithley 220 current source, a K&S 4-point probe, and a Keithley 6517A electrometer. 3. Results and discussion 3.1. Effect of latex particle size As shown in Fig. 1, conductive particles are forced to the spaces between the compacting latex particles during drying. As a consequence, the microstructure and properties of the composites are affected by the relative sizes of the latex particles and conductive particles (or clusters). Because of the availability of commercial latex and synthetic methods to make latex of various sizes, particle size effects are most easily explored by varying the size of the latex particles and keeping the conductive particle size constant. In the CB/latex system, the effect of latex particle size was probed using a custom monodisperse latex [12] and a commercial monodisperse latex [16]. Fig. 2 compares the conductivity of composites prepared with CB1 and two different sizes of latex. Composite preparation involved high shear mixing of dispersion and drying at room temperature. The data fit the percolation power law (Eq. (1)) and the percolation thresholds for both composites are below that expected for a random 3D microstructure. Conductive particles are forced to occupy the spaces between the larger latex particles and therefore prevented from adopting a random spatial distribution. This segregation effect is stronger for larger size latex particles; the conductive particles adopt a less random configuration. This result is consistent with theoretical models of size effects, which take into account the effect of size on packing of particles (i.e., the arrangement of smaller, spherical conductive particles in connected layers around larger, spherical insulating particles) [5,18]. Fitting the data to the models quantitatively, however, is not possible because the CB particles are clusters, not monodisperse spheres. In addition, the formation of aggregates of either CB or latex influences packing, microstructure, and therefore percolation [16].
Fig. 2. Effect of latex on the conductivity in CB1/latex composites. Lines represent fits to percolation power law. Adapted from [16].
Fig. 3. Effect of ATO content on the conductivity of ATO/PVAc-co-acrylic and ATO/P VAc coatings. Lines are fits to percolation power law; fits apply above the percolation threshold. From [17]. With kind permission from Springer Science and Business Media.
In studying effects of particle size, variations in glass transition temperature (Tg ) and film formation behavior of the latex should be considered. In a systematic study [16] with monodisperse latex of nearly identical size and varying Tg , the percolation threshold of CB/latex coatings was found to increase as the latex Tg decreased for a given drying temperature. The coalescence of the latex, which is enhanced at lower Tg , disrupts connections between the conductive particles. In the present comparison, the larger latex has the lower Tg and hence will coalesce to a greater degree. Hence, the size effect appears have more influence over the percolation threshold as compared with the Tg effect. The effect of latex size was also probed in the ATO/latex system [17]. Fig. 3 shows the effect of ATO content on the conductivity of ATO/PVAc and ATO/PVAc-co-acrylic coatings. Coatings were prepared from stable dispersions and using drying temperatures above the glass transition temperatures of the latex particles. The percolation threshold drops from around 0.06 for the coatings prepared with PVAc-co-acrylic (smaller latex particles on average) to around 0.03 for the coatings prepared with PVAc (larger latex particles on average). Both ATO/latex systems demonstrate reduction of percolation threshold relative to the expected result for a random composite (0.16). Figs. 4 and 5 reveal the effect of latex particle size on segregation in the microstructure. Both coating microstructures display the segregated network of the smaller ATO particles packed between the larger latex particles. The images show networks fully established with an ATO loading of 0.15 in the final microstructure. SEM images of ATO/PVAc-co-acylic coatings with an ATO volume fraction 0.05 contain disconnected ATO-rich regions [14]. By contrast the image of the PVAc-based coating with an ATO volume fraction of 0.03 reveals connectivity, consistent with the conductivity results. Since both latexes in this comparison are polydisperse, particle size effects are less clearly evaluated; however, the large difference in the average size appears to provide the same effect
L.F. Francis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 311 (2007) 48–54
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Fig. 4. SEM image (backscattered electron, BE, imaging mode) of crosssection of ATO/PVAc-co-acrylic coating with ATO volume fraction of 0.15. In BE mode, ATO-rich phase is brighter than polymer-rich phase. From [17].
as in the monodisperse case. That is, PVAc latex particles are larger on average and therefore provide greater spatial segregation, less randomness and lower percolation threshold. During the segregation, the latex particles on the small end of the size distribution are also expected to be forced to the spaces between the larger latex particles, and hence the ATO-rich phase, at least in the case of the PVAc-co-acylic latex, contains some polymer [14]. This polymer likely influences the magnitude of the conductivity. 3.2. Effect of aggregation Aggregation can occur at several stages during the processing of composites. First, the conductive particles themselves may be created as aggregates during their synthesis. The manufacture of CB particles, for example, creates clusters composed of small primary particles linked together into open or closed structures [19]. Second, aggregation can occur in the dispersion if the repulsive interactions between particles are not strong enough to overcome van der Waal attraction. Both latex particles and conductive particles are susceptible to aggregation in the suspension. Lastly, aggregates may be generated during drying as particles approach one another and the composition of the remaining aqueous medium changes. The effect of the addition of a dispersant [Disperbyk® , an alkanolammonium salt of poly(carboxylic acid)] on the microstructure and properties of CB1/PVAc composites was studied [10]. CB1 particles are open clusters of 20 nm primary
Fig. 5. SEM images (BE mode) of cross-section of ATO/PVAc coating with ATO volume fraction of (a) 0.15 and (b) 0.03. From [17].
particles. These clusters can further aggregate in aqueous dispersions and on drying. Results from sedimentation tests with CB1 dispersed in water or in latex dispersion demonstrated that the addition of dispersant increased the stability of dispersions and hence cut down on aggregation in the dispersion. CB1 dispersed with PVAc latex was less prone to sedimentation compared with CB1 dispersed in water, indicating that the latex either hindered settling of the CB aggregates or that the aggregation process was affected by the latex or perhaps free poly(vinyl alcohol),
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Fig. 7. Effect of dispersant on the conductivity of CB/PVAc composites. From [10].
Fig. 6. SEM micrographs of cross-sections of CB1/PVAc latex composites containing 8 vol% CB: (a) with 1 wt% dispersant and (b) with 5 wt% dispersant.
which is used to stabilize the PVAc particles. The effect of dispersant on microstructure of CB-PVAc microstructure is shown in Fig. 6. Composites prepared with more dispersant tended to contain smaller aggregates though there was some variability in final microstructures. Fig. 7 shows the effect of dispersant on the conductivity of CB/PVac composites. The percolation threshold was increased by the addition of the dispersant. By forming connections between conductive particles, aggregation brings the system closer to having the interconnected network needed for conduction in the final composite. That is, fewer connections need to form between conductive particles as the final microstructure develops during drying. By preventing aggregation, the dispersant stopped the formation of connections between conductive
particles. Fig. 7 shows a positive effect of aggregation—a lower percolation threshold. The negative consequence of aggregation for CB/PVAc composites was a reduction in the break strength in composites prepared with greater than 10 vol% CB. The role of particle–particle interactions and aggregation on the microstructure and properties on ATO/PVAc-co-acrylic and ITO/PVAc-co-acrylic coatings was also studied [15]. ATO and ITO particles used in this study were delivered in the dry state. Aggregates in the dry powder do not completely break up in aqueous dispersions; aggregates on the order of 50–100 nm have been observed in cryogenic SEM studies of stable aqueous dispersions [20]. In these systems, particle–particle interactions are easily changed by altering the dispersion pH. For ATO/PVAc-co-acrylic, dispersions are stable at pH of 3 or more due to the generation of negative surface charges on both ATO and latex particles. Aggregation was induced by lowering the pH, which decreases repulsive interactions. Microstructure studies showed large ATO-rich clusters formed in the coating. Consistent with the CB1/PVAc result, aggregation lowered the percolation threshold from 0.06 volume fraction ATO for the stable suspensions to 0.03 volume fraction ATO for unstable suspensions prepared at pH 1.5. The negative consequence of aggregation in this system was a decrease in the optical transparency of the coatings. Fig. 8 compares the conductivity of coatings prepared with different fillers and PVAc-co-acrylic latex as the polymer matrix. The percolation threshold of the ITO/latex coatings is higher than for CB2/latex or ATO/latex. As explored in detail elsewhere [15], this difference is due at least in part to the tendency for ITO to be attracted to the surface of the latex, which restricts its ability to form a connected network. Comparing the fillers, the conductivity past the percolation threshold correlates well with the conductivities of the dry, polymer-free particle coatings themselves (See Table 1). Hence, the composite conductivity past the percolation threshold depends on the conductivity of the individual particles and the particle–particle contacts. In this aspect, the nanosize of the fillers creates a challenge: the number of particle–particles contacts, which are frequently sites for of increased resistance, increases as the particle size drops [7].
L.F. Francis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 311 (2007) 48–54
Fig. 8. Effect of filler content and type on conductivity of PVAc-co-acrylic coatings. Coating thicknesses were 15 m, 4 m, and 4 m for CB2/latex (pH 4), ATO/latex (pH 3) and ITO/latex (pH 3) CB2/latex, ATO/latex and ITO/latex, respectively. From [17].
3.3. Effect of filler aspect ratio The conductive fillers used in the preceding studies (CB, ATO, ITO) have little or no aspect ratio. When these relatively spherical fillers are replaced with single-walled carbon nanotubes (SWNTs) the percolation threshold of the latexbased composite is dramatically reduced due to the combination of conductive particle segregation and high aspect ratio. Traditional, non-segregated carbon nanotube-filled composites typically have percolation thresholds of 0.2 vol% or more [21–24], which is much lower than that of carbon black-based composites. The high aspect ratio of the nanotubes [25] reduces the number of contacts required to create a conductive path and reduces the percolation threshold. Only a few researchers have taken the additional step of creating a formal segregated network to achieve even greater percolation threshold reductions [13,26,27]. Fig. 9 shows electrical conductivity as a function of filler concentration for the polydisperse PVAc filled with carbon black or SWNT. Segregated nanotube networks created with polymer latex produce percolation thresholds as low as
53
Fig. 9. Effect carbon filler content and type on conductivity of PVAc coatings. Data from [13] was plotted assuming a density of 1.8 g/cm3 for raw SWNT (27 wt% iron impurity) and 1.17 g/cm3 for PVAc.
0.02 vol% [13,26,27]. Changing from carbon black to nanotubes produces an order of magnitude reduction in the percolation threshold, from 2.5 to 0.02 vol%. Fig. 10 shows cross-sectional images of the poly(vinyl acetate) films containing 1.3 and 2 vol% carbon nanotubes. These images provide visual evidence of the segregated network concept, in which the solid polymer particles have effectively squeezed the smaller nanotubes into a networked structure. 4. Summary The electrical conductivity of composites prepared from aqueous dispersions of conductive nanoparticles and latex depends as much on the composite microstructure as it does on the properties of the individual components. Several microstructure tailoring mechanisms for decreasing the quantity of conductive particles needed to establish a network (i.e., the percolation threshold) have been identified. First, the segregation of conductive particles to the interstitial space between the latex
Fig. 10. Freeze-fractured cross-sections of PVAc latex films containing (a) 1.3 and (b) 2 vol% SWNT. From [28]. Copyright Wiley-VCH Verlag GmbH; Co. KGaA. Reproduced with permission.
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particles is enhanced and the percolation threshold drops as the size disparity between the small conductive particles and the larger latex particles increases. Second, aggregation of conductive particles in the dispersion also lowers the percolation threshold. Third, high aspect ratio conductive particles create an interconnected network at much lower volume fractions than more spherical particles. With low percolation thresholds, other properties, such as optical transparency (e.g., in case of ATOand ITO-based composites) and mechanical strength can be improved relative to highly loaded composites, and the cost associated with incorporating higher priced nanoparticles into composites is reduced. Enhancing the electrical conductivity beyond the percolation threshold, however, remains a challenge. The results summarized in this paper show a correlation between the conductivity of the conductive particles, as measured in a polymer-free particulate coating, and the composite conductivity. Hence, future improvements in composite conductivity require nanoparticles with higher conductivity and perhaps more importantly, particle–particle contacts engineered to have lower resistance. Acknowledgements The authors are grateful to the industrial sponsors of the Coating Process Fundamentals group for supporting this research. JCG thanks Eastman Kodak and the graduate school of the University of Minnesota for fellowships, and JS thanks the Industrial Partnership in Interfacial and Materials Engineering (IPRIME) for a fellowship. JCG thanks Carbon Nanotechnologies for supplying carbon nanotubes and Avery Dennison for funding the SWNT-latex research. References [1] L. Rupprecht, Conductive Polymers and Plastics in Industrial Applications, Society of Plastics Engineers, & Knovel, Norwich, NY, 1999. [2] R.H. Reuss, B.R. Chalamala, A. Moussessian, M.G. Kane, A. Kumar, D.C. Zhang, J.A. Rogers, M. Hatalis, D. Temple, G. Moddel, B.J. Eliasson, M.J. Estes, J. Kunze, E.S. Handy, E.S. Harmon, D.B. Salzman, J.M. Woodall, M.A. Alam, Y.J. Murthy, S.C. Jacobsen, M. Olivier, D. Markus, P.M. Campbell, E. Snow, Macroelectronics: perspectives on technology and applications, Proc. IEEE 93 (2005) 1239–1256. [3] S. Kirkpatrick, Percolation and conduction, Rev. Mod. Phys. 45 (1973) 574–588. [4] R. Zallen, The Physics of Amorphous Solids, Wiley, New York, 1983. [5] F. Lux, Models proposed to explain the electrical conductivity of mixtures made of conductive and insulating particles, J. Mater. Sci. 28 (1993) 285–301. [6] J. Huang, Carbon black filled conducting polymers and polymer blends, Adv. Polym. Technol. 21 (2002) 299–313. [7] G.R. Ruschau, S. Yoshikawa, R.E. Newnham, Resistivities of conductive composites, J. Appl. Phys. 72 (1992) 953–959. [8] J.L. Keddie, Film formation of latex, Mater. Sci. Eng. Rep. 21 (1997) 101–170.
[9] J.C. Grunlan, W.W. Gerberich, L.F. Francis, Lowering the percolation threshold of conductive composites using particulate polymer microstructure, J. Appl. Polym. Sci. 80 (2001) 692–705. [10] J.C. Grunlan, F.L. Bloom, W.W. Gerberich, L.F. Francis, Effect of dispersing aid on electrical and mechanical behavior of carbon black-filled latex, J. Mater. Sci. Lett. 20 (2001) 1523–1526. [11] J.C. Grunlan, W.W. Gerberich, L.F. Francis, Electrical and mechanical behavior of carbon black-filled poly(vinyl acetate) latex-based composites, Polym. Eng. Sci. 41 (2001) 1947–1962. [12] J.C. Grunlan, Y. Ma, M.A. Grunlan, W.W. Gerberich, L.F. Francis, Monodisperse latex with variable glass transition temperature and particle size for use as matrix starting material for conductive polymer composites, Polymer 42 (2001) 6913–6921. [13] J.C. Grunlan, A.R. Mehrabi, M.V. Bannon, J.L. Bahr, Water-based singlewalled nanotube-filled polymer composite with an exceptionally low percolation threshold, Adv. Mater. 16 (2004) 150–153. [14] J. Sun, W.W. Gerberich, L.F. Francis, Electrical and optical properties of ceramic-polymer nanocomposite coatings, J. Polym. Sci. B: Polym. Phys. 41 (2003) 1744–1761. [15] J. Sun, B.V. Velamakanni, W.W. Gerberich, L.F. Francis, Aqueous latex/ceramic nanoparticle dispersions: colloidal stability and coating properties, J. Colloid Interface Sci. 280 (2004) 387–399. [16] J.C. Grunlan, Carbon Black-Filled Polymer Composites: Property Optimization with Segregated Microstructures. Ph.D. Dissertation, University of Minnesota, 2001. [17] J. Sun. Transparent, Conductive Coatings from Latex-Based Dispersions. Ph.D. Dissertation, University of Minnesota, 2004. [18] A. Malliaris, D.T. Turner, Influence of particle size on the electrical resistivity of compacted mixtures of polymeric and metallic powders, J. Appl. Phys. 42 (1971) 614–618. [19] J. Donnet, R.B. Bansal, J.-M. Wang, Carbon Black: Science and Technology, 2nd ed., Dekker, New York, 1993. [20] H. Luo, L.E. Scriven, L.F. Francis, Cryo-SEM studies of latex/ceramic nanoparticle coating microstructure development, J. Colloid Interface Sci. (2007) doi:10.1016/j.jcis.2007.07.047. [21] J.M. Benoit, B. Corraze, S. Lefrant, W.J. Blau, P. Bernier, O. Chauvet, Transport properties of PMMA-carbon nanotubes composites, Synth. Met. 121 (2001) 1215–1216. [22] A. Dufresne, M. Paillet, J.L. Putaux, R. Canet, F. Carmona, P. Delhaes, S. Cui, Processing and characterization of carbon nanotube/poly(styrene-cobutyl acrylate) nanocomposites, J. Mater. Sci. 37 (2002) 3915–3923. [23] E. Kymakis, I. Alexandou, G.A. Amaratunga, Single-walled carbon nanotube-polymer composites: electrical, optical and structural investigation, Synth. Met. 127 (2002) 59–62. [24] B.J. Landi, R.P. Raffaelle, M.J. Heben, J.L. Alleman, W. VanDerveer, T. Gennett, Single wall carbon nanotube-Nafion composite actuators, Nano Lett. 2 (2002) 1329–1332. [25] P.M. Ajayan, Nanotubes from carbon, Chem. Rev. 99 (1999) 1787–1799. [26] A. Mierczynska, J. Friedrich, H.E. Maneck, G. Boiteux, J.K. Jeszka, Segregated network polymer/carbon nanotubes composites, Central Eur. J. Chem. 2 (2004) 363–370. [27] O. Regev, P.N.B. El Kati, J. Loos, C.E. Koning, Preparation of conductive nanotube-polymer composites using latex technology, Adv. Mater. 16 (2004) 248–251. [28] J.C. Grunlan, Y.S. Kim, S. Ziaee, X. Wei, B. Abdel-Magid, K. Tao, Thermal and mechanical behavior of carbon nanotube filled latex, Macromol. Mater. Eng. 291 (2006) 1035–1043. [29] R. Ramasubramaniam, J. Chen, H. Liu, Homogeneous carbon nanotube/polymer composites for electrical applications, Appl. Phys. Lett. 83 (2003) 2928–2930.
Colloids and Surfaces A: Physicochem. Eng. Aspects 311 (2007) 48–54
Conductive coatings and composites from latex-based dispersions Lorraine F. Francis a,∗ , Jaime C. Grunlan b , Jiakuan Sun c , W.W. Gerberich a a
Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, MN 55455, United States b Department of Mechanical Engineering, Texas A; M University College Station, TX 77843-3123, United States c Rohm and Haas (China) Holding Co. Ltd., China Research and Development Center 1077, Zhang Heng Road, Zhangjiang Hi-tech Park, Shanghai, 201203, China Available online 24 August 2007
Abstract Electrically conductive coatings and composites are prepared from aqueous dispersions of conducting particles and polymer latex particles. Relatively small amounts of conductive particles are needed to develop electrical conductivity, because the particulate nature of the latex leads to a segregated network that lowers the percolation threshold. Several nanosized conductive fillers have been studied: carbon black, antimony-doped tin oxide, indium tin oxide and carbon nanotubes. The latex chosen for most studies was either a poly(vinyl acetate-co-acrylic) polydisperse latex, a poly(vinyl acetate) polydisperse latex, or monodisperse poly(vinyl acetate) latex. This paper reviews the effect of particle size, aggregation and aspect ratio on the microstructure and properties of conductive composites and coatings. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite; Latex; Conductivity; Coating
1. Introduction Composites containing conductive particles in a nonconductive polymer matrix have been used for many years for applications such as electromagnetic shields, thermally sensitive resistors, and sensors [1]. In recent years, the development of flexible electronic devices and large area “macroelectronics” [2] has led to a demand for coatings with tailored electrical properties and functions. In this paper, a common base for liquid applied coatings – a latex dispersion – is combined with electrically conductive nanoparticles to create functional materials and coatings for a variety of applications. To impart electrical conductivity to a composite, the quantity of conductive particles must be high enough for the particles to form an interconnected network [3,4]. The formation of the network is commonly described as a percolation phenomenon in which the conductivity (σ) follows a power law and jumps by orders of magnitude when the volume fraction of conductive particles (V) surpasses a critical percolation threshold (Vc ): σ = σ0 (V − Vc )s
(1)
In Eq. (1), σ 0 is a proportionality constant, and s is the critical conductivity exponent (∼1.5–2.0 for a 3D random system). ∗
Corresponding author. Tel.: +1 612 625 0559; fax: +1 612 626 7246. E-mail address: [email protected] (L.F. Francis).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.08.026
The formation of an interconnected conductive particle network depends both on the morphology of the conductive particles and the characteristics of the matrix and resulting composite microstructure. A variety of theories and models have been proposed to account for the conductivity of composites and the effect microstructure on conductivity [5 and references therein]. For random addition of conductive particles to a continuous nonconductive matrix, the critical conductive particle content needed to establish the network is roughly 16 vol% [3,4]. Lowering the percolation threshold requires altering the conductive particles, the matrix or both. For example, particles with asymmetric morphologies create percolating networks at lower contents than do spherical particles [5]. The microstructure of the matrix can force percolation to occur at a lower value by segregating the conductive particles to a restricted volume within the microstructure. For example, conductive particles that segregate to one phase in a polymer blend form a percolating network selectively in that phase and therefore at a lower overall particle content [6 and references therein]. The magnitude of the conductivity past the threshold strongly depends on the properties of the conductive particles themselves and the connections between the particles [7]. In this paper, latex particles are used to construct a segregated network of conducting particles, as shown schematically in Fig. 1. Conductive particles are mixed with nonconductive latex particles in an aqueous dispersion. On drying, latex undergoes a sequence of microstructure changes [8 and references
L.F. Francis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 311 (2007) 48–54
49
Fig. 1. Schematic of microstructure development of a latex-based composite containing conductive particles. Adapted from [17].
therein]. Conductive particles are trapped as the latex forms the polymer matrix. In these systems, several materials and processing variables influence composite microstructure and properties. The focus here is on the effects of the characteristics of the particles. Examples are taken from the authors’ recent papers [9–15] and Ph.D. theses [16,17].
2. Experimental 2.1. Materials The characteristics of the conductive particles and latex dispersions used in the research are given in Tables 1 and 2. The following abbreviations will be used throughout the paper: carbon black (CB), antimony doped tin oxide (ATO), indium tin oxide (ITO) and single walled carbon nanotubes (SWNT), poly(vinyl acetate) (PVAc), poly(vinyl acetate co-acrylic) (PVAc-co-acrylic), and poly(methyl methacrylate co-butyl acrylate) (MMA-BA).
2.2. Preparation of composites and coatings The general procedure for composite preparation has two steps. First, an aqueous dispersion containing conductive particles and latex is prepared. Second, the dispersion is converted into a bulk composite by drying at room temperature (CB1/latex) or a coating by application with a wire wound rod onto a substrate followed by drying at 50 ◦ C (CB2/latex, ATO/latex, ITO/latex). Coating thickness is increased by repeating the deposition and drying. Details of the preparation procedures are given elsewhere for the CB/latex [9–12], ATO/latex and ITO/latex [14,15], and SWNT/latex [13]. 2.3. Characterization Composite microstructures and properties were characterized by several techniques. Scanning electron microscopy was used to characterize the cross-sectional and surface microstructures. Direct current electrical conductivity was measured using a commercial 4-point probe instruments (Veeco FPP-5000 or
Table 1 Characteristics of conductive particles Particle chemistry
Abbreviation
Manufacturer (product name)
Characteristics
Carbon black
CB 1
Dry powder, 20 nm diameter spherical primary particles in open clusters
Carbon black
CB2
Columbian Chem. (Conductex 976 Ultra) Columbian Chem. (Raven 1170)
Antimony-tin oxide Indium–tin oxide Single-walled carbon nanotube
ATO ITO SWNT
Nanophase Technologies (Nanotek® ) Nanophase Technologies (Nanotek® ) Carbon Nanotechnologies
Dry powder, 20 nm diameter spherical primary particles in closed clusters, σ∼ = 1 S/cm* Dry powder, 15 nm (ave) diameter spherical primary particles, σ ∼ = 0.l S/cm* Dry powder, 20 nm (ave) diameter faceted primary particles, σ ∼ = 0.01 S/cm* Dry powder, 1–2 nm diameter and 100+ nm long, σ ∼ = 510 S/cm**
*dry particle coating. **from Ramasubramaniam et al. [29]; measured on a polymer-free coating of SWNT’s. Table 2 Characteristics of latex dispersions Latex chemistry
Abbreviation
Manufacturer (product name)
Characteristics
Poly(vinyl acetate)
PVAc
Air Products (Vinac 21)
Poly(vinyl acetate co-acrylic)
PVAc-co-acrylic
Air Products (Flexbond 325)
Poly(vinyl acetate) Poly(methyl methacylate co-butyl acrylate)
PVAc-M MMA-BA
H.B. Fuller (PD 202) Custom [12]
Aqueous dispersion; Tg = 35 ◦ C, MFFT* = 15 ◦ C; polydisperse (vol. ave size, Dv = 2.6 m, num. ave. size, Dn = 108 nm) Aqueous dispersion; Tg = 19 ◦ C, MFFT* = 10 ◦ C; polydisperse (Dv = 333 nm and Dn = 60 nm) Aqueous dispersion; Tg = 34 ◦ C, monodisperse (Dv = Dn = 116 nm) Aqueous dispersion; Tg = 19 ◦ C, monodisperse (Dv = 1.087 m, Dn = 0.929 m)
*minimum film formation temperature.
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Loresta AP [MCP-T4000]) or a home built apparatus consisting of a Keithley 220 current source, a K&S 4-point probe, and a Keithley 6517A electrometer. 3. Results and discussion 3.1. Effect of latex particle size As shown in Fig. 1, conductive particles are forced to the spaces between the compacting latex particles during drying. As a consequence, the microstructure and properties of the composites are affected by the relative sizes of the latex particles and conductive particles (or clusters). Because of the availability of commercial latex and synthetic methods to make latex of various sizes, particle size effects are most easily explored by varying the size of the latex particles and keeping the conductive particle size constant. In the CB/latex system, the effect of latex particle size was probed using a custom monodisperse latex [12] and a commercial monodisperse latex [16]. Fig. 2 compares the conductivity of composites prepared with CB1 and two different sizes of latex. Composite preparation involved high shear mixing of dispersion and drying at room temperature. The data fit the percolation power law (Eq. (1)) and the percolation thresholds for both composites are below that expected for a random 3D microstructure. Conductive particles are forced to occupy the spaces between the larger latex particles and therefore prevented from adopting a random spatial distribution. This segregation effect is stronger for larger size latex particles; the conductive particles adopt a less random configuration. This result is consistent with theoretical models of size effects, which take into account the effect of size on packing of particles (i.e., the arrangement of smaller, spherical conductive particles in connected layers around larger, spherical insulating particles) [5,18]. Fitting the data to the models quantitatively, however, is not possible because the CB particles are clusters, not monodisperse spheres. In addition, the formation of aggregates of either CB or latex influences packing, microstructure, and therefore percolation [16].
Fig. 2. Effect of latex on the conductivity in CB1/latex composites. Lines represent fits to percolation power law. Adapted from [16].
Fig. 3. Effect of ATO content on the conductivity of ATO/PVAc-co-acrylic and ATO/P VAc coatings. Lines are fits to percolation power law; fits apply above the percolation threshold. From [17]. With kind permission from Springer Science and Business Media.
In studying effects of particle size, variations in glass transition temperature (Tg ) and film formation behavior of the latex should be considered. In a systematic study [16] with monodisperse latex of nearly identical size and varying Tg , the percolation threshold of CB/latex coatings was found to increase as the latex Tg decreased for a given drying temperature. The coalescence of the latex, which is enhanced at lower Tg , disrupts connections between the conductive particles. In the present comparison, the larger latex has the lower Tg and hence will coalesce to a greater degree. Hence, the size effect appears have more influence over the percolation threshold as compared with the Tg effect. The effect of latex size was also probed in the ATO/latex system [17]. Fig. 3 shows the effect of ATO content on the conductivity of ATO/PVAc and ATO/PVAc-co-acrylic coatings. Coatings were prepared from stable dispersions and using drying temperatures above the glass transition temperatures of the latex particles. The percolation threshold drops from around 0.06 for the coatings prepared with PVAc-co-acrylic (smaller latex particles on average) to around 0.03 for the coatings prepared with PVAc (larger latex particles on average). Both ATO/latex systems demonstrate reduction of percolation threshold relative to the expected result for a random composite (0.16). Figs. 4 and 5 reveal the effect of latex particle size on segregation in the microstructure. Both coating microstructures display the segregated network of the smaller ATO particles packed between the larger latex particles. The images show networks fully established with an ATO loading of 0.15 in the final microstructure. SEM images of ATO/PVAc-co-acylic coatings with an ATO volume fraction 0.05 contain disconnected ATO-rich regions [14]. By contrast the image of the PVAc-based coating with an ATO volume fraction of 0.03 reveals connectivity, consistent with the conductivity results. Since both latexes in this comparison are polydisperse, particle size effects are less clearly evaluated; however, the large difference in the average size appears to provide the same effect
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Fig. 4. SEM image (backscattered electron, BE, imaging mode) of crosssection of ATO/PVAc-co-acrylic coating with ATO volume fraction of 0.15. In BE mode, ATO-rich phase is brighter than polymer-rich phase. From [17].
as in the monodisperse case. That is, PVAc latex particles are larger on average and therefore provide greater spatial segregation, less randomness and lower percolation threshold. During the segregation, the latex particles on the small end of the size distribution are also expected to be forced to the spaces between the larger latex particles, and hence the ATO-rich phase, at least in the case of the PVAc-co-acylic latex, contains some polymer [14]. This polymer likely influences the magnitude of the conductivity. 3.2. Effect of aggregation Aggregation can occur at several stages during the processing of composites. First, the conductive particles themselves may be created as aggregates during their synthesis. The manufacture of CB particles, for example, creates clusters composed of small primary particles linked together into open or closed structures [19]. Second, aggregation can occur in the dispersion if the repulsive interactions between particles are not strong enough to overcome van der Waal attraction. Both latex particles and conductive particles are susceptible to aggregation in the suspension. Lastly, aggregates may be generated during drying as particles approach one another and the composition of the remaining aqueous medium changes. The effect of the addition of a dispersant [Disperbyk® , an alkanolammonium salt of poly(carboxylic acid)] on the microstructure and properties of CB1/PVAc composites was studied [10]. CB1 particles are open clusters of 20 nm primary
Fig. 5. SEM images (BE mode) of cross-section of ATO/PVAc coating with ATO volume fraction of (a) 0.15 and (b) 0.03. From [17].
particles. These clusters can further aggregate in aqueous dispersions and on drying. Results from sedimentation tests with CB1 dispersed in water or in latex dispersion demonstrated that the addition of dispersant increased the stability of dispersions and hence cut down on aggregation in the dispersion. CB1 dispersed with PVAc latex was less prone to sedimentation compared with CB1 dispersed in water, indicating that the latex either hindered settling of the CB aggregates or that the aggregation process was affected by the latex or perhaps free poly(vinyl alcohol),
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Fig. 7. Effect of dispersant on the conductivity of CB/PVAc composites. From [10].
Fig. 6. SEM micrographs of cross-sections of CB1/PVAc latex composites containing 8 vol% CB: (a) with 1 wt% dispersant and (b) with 5 wt% dispersant.
which is used to stabilize the PVAc particles. The effect of dispersant on microstructure of CB-PVAc microstructure is shown in Fig. 6. Composites prepared with more dispersant tended to contain smaller aggregates though there was some variability in final microstructures. Fig. 7 shows the effect of dispersant on the conductivity of CB/PVac composites. The percolation threshold was increased by the addition of the dispersant. By forming connections between conductive particles, aggregation brings the system closer to having the interconnected network needed for conduction in the final composite. That is, fewer connections need to form between conductive particles as the final microstructure develops during drying. By preventing aggregation, the dispersant stopped the formation of connections between conductive
particles. Fig. 7 shows a positive effect of aggregation—a lower percolation threshold. The negative consequence of aggregation for CB/PVAc composites was a reduction in the break strength in composites prepared with greater than 10 vol% CB. The role of particle–particle interactions and aggregation on the microstructure and properties on ATO/PVAc-co-acrylic and ITO/PVAc-co-acrylic coatings was also studied [15]. ATO and ITO particles used in this study were delivered in the dry state. Aggregates in the dry powder do not completely break up in aqueous dispersions; aggregates on the order of 50–100 nm have been observed in cryogenic SEM studies of stable aqueous dispersions [20]. In these systems, particle–particle interactions are easily changed by altering the dispersion pH. For ATO/PVAc-co-acrylic, dispersions are stable at pH of 3 or more due to the generation of negative surface charges on both ATO and latex particles. Aggregation was induced by lowering the pH, which decreases repulsive interactions. Microstructure studies showed large ATO-rich clusters formed in the coating. Consistent with the CB1/PVAc result, aggregation lowered the percolation threshold from 0.06 volume fraction ATO for the stable suspensions to 0.03 volume fraction ATO for unstable suspensions prepared at pH 1.5. The negative consequence of aggregation in this system was a decrease in the optical transparency of the coatings. Fig. 8 compares the conductivity of coatings prepared with different fillers and PVAc-co-acrylic latex as the polymer matrix. The percolation threshold of the ITO/latex coatings is higher than for CB2/latex or ATO/latex. As explored in detail elsewhere [15], this difference is due at least in part to the tendency for ITO to be attracted to the surface of the latex, which restricts its ability to form a connected network. Comparing the fillers, the conductivity past the percolation threshold correlates well with the conductivities of the dry, polymer-free particle coatings themselves (See Table 1). Hence, the composite conductivity past the percolation threshold depends on the conductivity of the individual particles and the particle–particle contacts. In this aspect, the nanosize of the fillers creates a challenge: the number of particle–particles contacts, which are frequently sites for of increased resistance, increases as the particle size drops [7].
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Fig. 8. Effect of filler content and type on conductivity of PVAc-co-acrylic coatings. Coating thicknesses were 15 m, 4 m, and 4 m for CB2/latex (pH 4), ATO/latex (pH 3) and ITO/latex (pH 3) CB2/latex, ATO/latex and ITO/latex, respectively. From [17].
3.3. Effect of filler aspect ratio The conductive fillers used in the preceding studies (CB, ATO, ITO) have little or no aspect ratio. When these relatively spherical fillers are replaced with single-walled carbon nanotubes (SWNTs) the percolation threshold of the latexbased composite is dramatically reduced due to the combination of conductive particle segregation and high aspect ratio. Traditional, non-segregated carbon nanotube-filled composites typically have percolation thresholds of 0.2 vol% or more [21–24], which is much lower than that of carbon black-based composites. The high aspect ratio of the nanotubes [25] reduces the number of contacts required to create a conductive path and reduces the percolation threshold. Only a few researchers have taken the additional step of creating a formal segregated network to achieve even greater percolation threshold reductions [13,26,27]. Fig. 9 shows electrical conductivity as a function of filler concentration for the polydisperse PVAc filled with carbon black or SWNT. Segregated nanotube networks created with polymer latex produce percolation thresholds as low as
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Fig. 9. Effect carbon filler content and type on conductivity of PVAc coatings. Data from [13] was plotted assuming a density of 1.8 g/cm3 for raw SWNT (27 wt% iron impurity) and 1.17 g/cm3 for PVAc.
0.02 vol% [13,26,27]. Changing from carbon black to nanotubes produces an order of magnitude reduction in the percolation threshold, from 2.5 to 0.02 vol%. Fig. 10 shows cross-sectional images of the poly(vinyl acetate) films containing 1.3 and 2 vol% carbon nanotubes. These images provide visual evidence of the segregated network concept, in which the solid polymer particles have effectively squeezed the smaller nanotubes into a networked structure. 4. Summary The electrical conductivity of composites prepared from aqueous dispersions of conductive nanoparticles and latex depends as much on the composite microstructure as it does on the properties of the individual components. Several microstructure tailoring mechanisms for decreasing the quantity of conductive particles needed to establish a network (i.e., the percolation threshold) have been identified. First, the segregation of conductive particles to the interstitial space between the latex
Fig. 10. Freeze-fractured cross-sections of PVAc latex films containing (a) 1.3 and (b) 2 vol% SWNT. From [28]. Copyright Wiley-VCH Verlag GmbH; Co. KGaA. Reproduced with permission.
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particles is enhanced and the percolation threshold drops as the size disparity between the small conductive particles and the larger latex particles increases. Second, aggregation of conductive particles in the dispersion also lowers the percolation threshold. Third, high aspect ratio conductive particles create an interconnected network at much lower volume fractions than more spherical particles. With low percolation thresholds, other properties, such as optical transparency (e.g., in case of ATOand ITO-based composites) and mechanical strength can be improved relative to highly loaded composites, and the cost associated with incorporating higher priced nanoparticles into composites is reduced. Enhancing the electrical conductivity beyond the percolation threshold, however, remains a challenge. The results summarized in this paper show a correlation between the conductivity of the conductive particles, as measured in a polymer-free particulate coating, and the composite conductivity. Hence, future improvements in composite conductivity require nanoparticles with higher conductivity and perhaps more importantly, particle–particle contacts engineered to have lower resistance. Acknowledgements The authors are grateful to the industrial sponsors of the Coating Process Fundamentals group for supporting this research. JCG thanks Eastman Kodak and the graduate school of the University of Minnesota for fellowships, and JS thanks the Industrial Partnership in Interfacial and Materials Engineering (IPRIME) for a fellowship. JCG thanks Carbon Nanotechnologies for supplying carbon nanotubes and Avery Dennison for funding the SWNT-latex research. References [1] L. Rupprecht, Conductive Polymers and Plastics in Industrial Applications, Society of Plastics Engineers, & Knovel, Norwich, NY, 1999. [2] R.H. Reuss, B.R. Chalamala, A. Moussessian, M.G. Kane, A. Kumar, D.C. Zhang, J.A. Rogers, M. Hatalis, D. Temple, G. Moddel, B.J. Eliasson, M.J. Estes, J. Kunze, E.S. Handy, E.S. Harmon, D.B. Salzman, J.M. Woodall, M.A. Alam, Y.J. Murthy, S.C. Jacobsen, M. Olivier, D. Markus, P.M. Campbell, E. Snow, Macroelectronics: perspectives on technology and applications, Proc. IEEE 93 (2005) 1239–1256. [3] S. Kirkpatrick, Percolation and conduction, Rev. Mod. Phys. 45 (1973) 574–588. [4] R. Zallen, The Physics of Amorphous Solids, Wiley, New York, 1983. [5] F. Lux, Models proposed to explain the electrical conductivity of mixtures made of conductive and insulating particles, J. Mater. Sci. 28 (1993) 285–301. [6] J. Huang, Carbon black filled conducting polymers and polymer blends, Adv. Polym. Technol. 21 (2002) 299–313. [7] G.R. Ruschau, S. Yoshikawa, R.E. Newnham, Resistivities of conductive composites, J. Appl. Phys. 72 (1992) 953–959. [8] J.L. Keddie, Film formation of latex, Mater. Sci. Eng. Rep. 21 (1997) 101–170.
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