Presented at the United States - Japan Cooperative Research in Natural Resources 2005
Mechanical and electrical properties of thermoplastic starch composites using carbon black as a conductive filler Victoria L. Finkenstadt and J.L. Willett National Center for Agricultural Utilization Research, United States Department of Agriculture, 1815 North University Street, Peoria, Illinois 61604 USA
[email protected] Abstract Thermoplastic starch was blended with carbon black to form an electroactive polymer composite. Thermoplastic starch is naturally insulative, and the electroactive polymer composite takes advantage of the conductive pathways formed through the percolation of conductive particles through the polymer matrix. The polymer is filled up to 40% w/w with carbon black. Tensile strength and modulus increase and the elongation decreases with increasing carbon black concentration. The conductance increases 10-fold at 20% w/w carbon black content. Above the percolation threshold, the mechanical properties of the carbon-thermoplastic starch composite overwhelm the conductive pathways. Introduction Electroactive polymer composites (EPCs) were developed using intrinsically insulative thermoplastic starch (TPS) as the polymer matrix and carbon black (CB) as a conductive filler. Starch is a biodegradable, renewable resource. While starch is packaged into granules in its native state, the properties of starch materials are exhibited when the granular structure is broken down by mechanical and thermal means. Starch is composed of a mixture of linear and branched polysaccharides. Amylose is a linear polymer of (1,4) linked anhydroglucose units (AGU). Amylopectin is a highly branched polysaccharide composed of linear (1,4) AGU chains with branch points having (1,6) linkages between amylose chains. Starch obtains its plastic properties upon the gelatinization and destructuring of starch granules with enough water and thermomechanical energy. Starch can be gelatinized by benchtop heating of a water dispersion or cold gelatinized with NaOH, but true thermoplastic starch (TPS) is easily obtainable through extrusion. Carbon black has been used as a conductive filler in several polymer systems [1,2,3,4,5]. Highly filled composites form a continuous pathway from particle to particle through which electrical current flows. Around the critical concentration or percolation threshold, a small increase in the concentration of the carbon black increases the conductivity by several orders of magnitude. The percolation threshold of electroactive polymer composites usually range between 10 and 30% w/w [6]. EPCs have commercial applications in sensors (chemi-resistors) for organic vapor detection [4]. When EPCs are exposed to different analytes and they diffuse into the polymer matrix, the conductance of the material decreases significantly as the polymer swells because of the percolation mechanism of the filler. We report on the preliminary material development of an EPC composed of thermoplastic starch and carbon black. Materials and Methods Starch. Commercially available corn starch (CPC International) was used. Ambient moisture was approximately 9% weight basis. Carbon black, with particle sizes between 10-100 nm, was obtained from Fisher Scientific. The starch and starch-carbon black mixture was blended with water using a commercially available mixer and refrigerated overnight to distributed moisture uniformly through the starch. Extrusion. Batches were extruded using a Brabender single-screw extruder with four temperature zones (profile: 80°-90°-110°C and a die temperature of 100°C). A 3:1 high shear mixing zone screw was employed. Ribbons (100 mm wide and approximately 0.5 mm thick) were extruded using a hangar-type die. Conductance. Electrical resistance (R, ohms) was measured by a Keithley constant current source and an ElectroTech Systems four-point probe at room temperature and humidity according to ASTM D-257. A direct current technique was used. Conductance is equal to 1/R, was corrected for volume, and is reported as Siemens per centimeter (S/cm). This test method involves a direct-current procedure for volume resistance/resistivity and
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conforms to the standards established by ESD S11.11 and ASTM D257 [7]. Resistance is directly measured using a defined electrode configuration (concentric ring electrode), specific test voltage (0.75 V) and controlled environmental conditions. Samples were stored and measured in 50% RH and standard room temperature (23°C). For clarity on plots, the log of conductance is reported. Mechanical Properties. Tensile properties of CB-TPS composites were evaluated using an Instron Model 1122 mechanical property testing machine. The thickness of the individual ASTM D638 Type D dogbones was measured before testing. The gauge length was 40 mm, and the strain rate was 50 mm/min. Mechanical properties included tensile strength, modulus and elongation. Tensile strength is the maximum stress a sample can sustain without fracture. Modulus describes the stiffness of the material and is determined from the slope of the line tangent to the stress-strain curve. All samples were conditioned at 50% RH and 23°C for at least 48 hours before testing. Individual thicknesses were accounted for in the calculations. Results and Discussion The samples are composed of thermoplastic starch with a conductance of 10-8.5 S/cm and carbon black with a conductivity of 10-2 S/cm. Starch-based materials are moisture sensitive, and the amount of moisture in the system will affect the conductive pathways within the system as well as the quality of physical properties of TPS. While moisture contributes to the overall bulk conductance of TPS, it does not at or above MCs of about 20% [7]. Therefore, the moisture content was kept constant at 20% w/w. As carbon black was added to thermoplastic starch, the composite became more brittle and strong than the control sample of TPS (Figure 1-3). As expected when filling a plastic with rigid particles, TPS becomes less plastic. Looking at the tensile properties, one can clearly see a significant increases in tensile strength (Figure 1) and stiffness (Figure 2) and a concurrent loss of elongation (Figure 3) at 20% w/w carbon black. Higher levels of carbon black resulted in high melt viscosity during processing (data not shown) and deterioration of the mechanical properties of the CB-TPS composites. Addition of carbon black to thermoplastic starch caused the conductance of the composite to increase by a factor of 10 at a filler content of 20% w/w carbon black (Figure 4). While the conductance level is comparable to that achieved with other electroactive polymer composites, the range of conductivity is not as good. Polypropylene filled with carbon black had a percolation threshold of 20%, but the increase in conductance was at least 5 orders of magnitude [3]. This large increase is needed for the composite material to be used as a vapor chemi-resistor sensor with suitable sensitivity. Two possible explanations for the smaller range of conductance is that TPS does not have good adhesion to the carbon black or the carbon black particles are not forming a cohesive pathway. In other words, the system may be too dispersed or homogeneous and the pathways may not be well-defined. Adhesion and cohesion has been shown to lead to lower percolation thresholds (i.e. lower loading of conducting material giving higher conductivity) and improved mechanical properties in composites [6]. One should also note that the initial conductivity of TPS is relatively high compared to polypropylene (10-11 S/cm). One reason for this is the water content. Water is necessary to process the composite, but the product could be dried after production which would lower the initial conductance. Indeed, this may be necessary to impart adequate sensitivity of the chemi-resistor to gas/liquid analytes. The product may have to be protected from moisture which may reduce its usefulness in different environments. Future work is planned to assess different types of starch-graft-copolymers blended with carbon black for EPCs. Conclusion Thermoplastic starch was blended with carbon black to form an electroactive polymer composite. TPS is naturally insulative, and the EPCs take advantage of the conductive pathways formed through the percolation of conductive particles through the polymer matrix. The highest conductance achieved was 10-7 S/cm at 20% w/w. Other researchers blended carbon black with polypropylene and achieved a percolation threshold of 6% w/w and a conductance of 10-5 S/cm [3]. Clearly, further development is needed in order to minimize the percolation threshold and maximize the conductivity range of starch based composites using carbon black for chemiresistors in gas or liquid phase sensors with appropriate sensitivity. Acknowledgements The authors would like to thank Mr. Richard Haig for his contributions to this work.
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Figure 1. Tensile strength versus carbon black content in thermoplastic starch. 1.2
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Figure 3. Elongation versus carbon black amount in thermoplastic starch.
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Figure 4. Conductance of carbon black filled thermoplastic starch composites. References 1. Jean-François Feller IL. Mechanical and rheological properties of poly(ethylene-co-ethyl acrylate) as a function of carbon black content. Polymer Testing. 2003; 203:317-324. 2. Tang H, Chen X, Luo Y. Electrical and dynamic mechanical behavior of carbon black filled polymer composites. European Polymer Journal. 1996; 32:963-966. 3. Kozlowski M, Frackowiak S. Chemical sensors based on polymer composites. Sensors and Actuators B. 2005; 109:141-145. 4. Hu JW, Chen SG, Zhang MQ, Li MW, Rong MZ. Low carbon black filled polyurethane composite as candidate for wide spectrum gas-sensing element. Materials Letters. 2004; 58:3606-3609. 5. Feller JF, Chauvelon P, Linossier I, Glouannec P. Characterization of electrical and thermal properties of extruded tapes of thermoplastic conductive polymer composites (CPC). Polymer Testing. 2003; 22:831-837. 6. Wessling B. Dispersion as the key to processing conductive polymers. Handbook of Conductive Polymers. ed Skothiem, TA et al. 2005; 467-530. 7. Finkenstadt VL, Willett JL. A direct-current resistance technique for determining moisture content in native starches and starch-based plasticized materials. Carbohydrate Polymers. 2004; 55:149-154.
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