Effect of Carbon Black Grade on the Microwave Heating of High-

Transcription

1 Effect of Carbon Black Grade on the Microwave Heating of High- Density Polyethylene J. Clarke 1, J. F. Harper 1, D. M. Price 1*, J. Zhang 1,2 1. Institute of Polymer Technology and Materials Engineering, Loughborough University, Leicestershire LE11 3TU United Kingdom. 2. Department of Polymer Science & Engineering, Sichuan University, Chengdu , China ABSTRACT: Blends of high-density polyethylene (HDPE) with different grades of carbon black were prepared by mixing the two materials in a rheometer. Measurements of the samples temperature rise when exposed to microwave energy were conducted. There appeared to be a threshold below which the material did not respond to microwave irradiation which was below the percolation threshold for electrical conductivity. Above this lower limit, the resulting temperature rise was found to be proportional to the amount of carbon black added and its surface area. Mechanical testing of the samples showed that the blends became more brittle with increasing carbon black content and that this was also related to the surface area of the carbon black. Key words: High density polyethylene (HDPE), carbon black, microwave heating, mechanical properties INTRODUCTION Microwave energy is an important means of heating materials. Its uses range from high temperature sintering of ceramics and chemical synthesis to domestic cooking of foodstuffs. Although microwaves have been employed for polymer processing, their applications are limited to those polymers (usually thermosets) which readily absorb energy and are thus self-heating under microwave irradiation. Non-polar polymers (such as polyolefins) are largely transparent to microwaves and indeed are employed to package microwavable meals. In order to enhance the absorption of microwaves by thermoplastics, fillers such as metal powder, zinc oxide and carbon black have been incorporated. 1-5 These additives interact with microwaves and the energy is then transferred to the matrix. Our interests lie in developing filled thermoplastic polymer systems that can be thermoformed using microwave energy. In a previous publication we examined the microwave response of carbon black filled high density polyethylene. 1 The temperature rise during microwave irradiation was found to be proportional to the amount of carbon black added once a certain threshold (ca. 5 % weight) had been reached. The mechanical properties of the * Correspondence to: D. M. Price (d.m.price@lboro.ac.uk) Submitted to: Journal of Applied Polymer Science 1

2 blend were also studied. Addition of carbon black to HDPE reduced the elongation to break and made the material more brittle. Brosseau et al. have previously studied the microwave absorption of polymers at low filler content. 7 and Kynak et al. have examined the microwave absorption of carbon black and carbon fibre filled polypropylene in the 8-12 GHz region. 8 The latter workers also examined the mechanical properties of the composites. They found that the stiffness and brittleness of the materials increased with additive loading. In this work we have extended the initial study to a wider range of carbon blacks so as to determine the optimum properties required to achieve satisfactory microwave absorption without compromising the mechanical properties of the product. EXPERIMENTAL Materials High density polyethylene (HDPE) from Borealis molecular weight g mol -1 and melt flow index 0.3 g min -1 (190 C, 2.16 kg) was used as the matrix. A series of carbon blacks were obtained from the Columbian Chemicals Company (table 1). Carbon blacks can be obtained with different properties depending on the manufacturing conditions employed. This can be described on two levels; firstly the size of the near-spheroidal ultimate particles present in the carbon black (defined by the specific surface area) and secondly by the degree of aggregation of these particles (called structure - measured by the amount of dibutyl phthalate that can be absorbed by 100 g of material up to a point when a specified viscosity is reached). 9 Table 1. Carbon black used in this study Sample preparation Code surface area (m 2 g -1 ) Structure (cc/100 g) MT N C N N N N A Haake PolyLab Rheomix was used to mix the HDPE with different amounts of each carbon black. The mixing temperature was 180 C and rotational speed was set to 40 rpm. Mixing was continued for 10 minutes after addition of the materials during which time the motor torque and mix temperature were recorded. Each blend was 2 Submitted to: Journal of Applied Polymer Science

3 then made into sheets 1.5 mm thick by compression moulding at 180 C. Microwave heating A 800W microwave oven (Daewoo KOR-68CD7) was used for heating experiments and operated at full power for 15 s. Specimens (10 mm x 50 mm x 1.5 mm) were oriented vertically in the centre of the oven turntable using a block of ceramic foam as a support. The initial and final temperatures of the sample were recorded using a thermal imaging camera (FLIR Systems Thermovision A40). Thermal Analysis Differential scanning calorimetry (DSC) was carried out using a TA Instruments Q1000 DSC fitted with an auto-sampler and mechanical cooler. Specimens (15-20 mg) were encapsulated in hermetically sealed aluminium pans, heated from 0 to 200 C at 10 C/min, cooled back to 0 C at the same rate and then reheated to 200 C also at 10 C/min. Each blend was run in triplicate, an average of measurements of the area under the melting peak of the second heating experiment being used to determine the crystallinity of the blend from the relationship: Crystallinity = H fblend /( H fhdpe (1-X CARBON )) Where H fhdpe is the heat of fusion of pure HDPE, H fblend is the heat of fusion of the blend and X CARBON is the weight fraction of carbon present. Tensile Testing A Lloyd Tensometer, Model L10000, was used for tensile testing at room temperature. The crosshead speed was 50mm/min. Dumbell specimens were prepared according to BS903, except their thickness was 1.5 mm. RESULTS & DISCUSSION Data from the preparation of the blends is shown in Figure 1. The average torque for the mixing rotors for the last minute of processing (i.e. once steady-state had been reached) is plotted against the structure of the carbon black used for different loadings of additive. As the amount of carbon black is increased, the viscosity of the blend rises and this effect is more pronounced for higher structure carbon blacks. 10 The surface area of the carbon did not affect the viscosity of the blends. This suggests that there is little interaction between the polymer and the surface of the filler and that it is the swept volume of the carbon black particles (the higher structure particles appearing larger) that influences the viscosity. Plots of the temperature rise on microwave heating against filler loading are shown in Figure 2. Above a certain threshold, the temperature rise was found to be Submitted to: Journal of Applied Polymer Science 3

4 directly proportional to carbon black content. The slope and onset of these linear trends were calculated and used to define the heating efficiency (in C/%) and threshold (%) for each carbon black (table 2). Table 2. Heating efficiency and threshold for carbon black filled HDPE Carbon black heating efficiency threshold (%) ( C/%) MT N C N N N N Figure 3 shows a plot of the heating efficiency of the carbon blacks plotted against its surface area. There is a very good correlation between these two parameters. The threshold for heating shows no clear relationship between the surface area and/or structure although it is apparent that the onset of heating is below the percolation threshold for conductivity in this class of material which is usually reported as lying between 10 and 20%. 11 According to Modine et al. the percolation threshold varies with the shape and degree of agglomeration of the carbon black: it occurs at higher loadings for carbon particles with low surface-to-volume ratio and low structure. 12 Yu et al. discuss the influence of matrix viscosity on the percolation behaviour carbon black in polyolefins which can lead to dispersion problems in low viscosity melts. 13 It is apparent from the shape of the heating curves in Figure 2 and the low onset of heating that the microwave response does not correlate with the usual S-shaped response seen for electrical conductivity. Our findings agree with those of Dishovsky and Grigorova who found that there was a relationship between the electrical conductivity of carbon black filled HDPE below the percolation threshold and ultra-high frequency electromagnetic wave absorption in three frequency regions , and GHz. 14 They also report that the highest attenuation was in the region 2-3 GHz with a discontinuity at the percolation threshold. The relative degree of crystallinity and the onset temperature of crystallisation showed no clear trend between carbon black level and type and its influence on the crystallinity and rate of crystallisation of the blend. Carbon black has low nucleation ability in HDPE with the melting and crystallisation temperatures remaining almost unchanged. 15 Carbon remains dispersed in the interspherulitic regions of the HDPE, which would have implications for the electrical conductivity of the blend but not apparently for is heatability by microwaves. 16,17 The tensile properties of the blends are shown in Figures 4 and 5. The elongations at yield and elongation at break vs. carbon black content for the smallest 4 Submitted to: Journal of Applied Polymer Science

5 and largest surface area carbon blacks are shown in Figure 4. The elongation at yield is little changed by the incorporation of carbon black but the material becomes less ductile as the level of filler is increased. This effect increases with the surface area of the carbon black used as shown in Figure 5 for samples with 10% carbon black content. There was no correlation of mechanical properties with the structure of the carbon black used. The modulus and yield stress were also increased by the addition of carbon black although these could not be related with the surface area or structure of the additive. CONCLUSIONS The addition of carbon black to HDPE allows the polymer to be heated by microwave energy. The temperature rise is proportional to the amount of carbon black added (above a certain threshold) and the surface area of the carbon black. Carbon black addition affects the melt viscosity according to its structure but has negligible effect on the crystallisation of the HDPE. Incorporation of carbon black into HDPE makes the material stiffer and more brittle. Again, the reduction in ductility increases with the surface area of the carbon used, therefore a compromise must be made between microwave processability and mechanical performance. Acknowledgements The authors would like to thank Columbian Chemicals Company for their donation of carbon black to the project. This work was funded under the auspices of the European Commission Project: Safehose. (Number FP ) References 1. French Pat , Japanese Pat , Wu, C. Y.; PhD thesis Ohio State University, Ippen, J. Rubber Chem. Technol. 1971, 44, Meredith, R. J. J Elastom Plast 1976, 8, Clarke, J.; Harper, J.; Price, D.; Zhang, J. Plast Rubber Compos, submitted. 7. Brosseau, C.; Quéffélec, P.; Talbot, P. J Appl Phys 2001, 89, Kaynak, A.; Polat, A.; Yilmazer, U. Mater Res Bull 1996, 31, Donnet, J-B.; Bansal R. C.; Wang, M-J. Carbon Black Science and Technology; Marcel Dekker; New York, Van Bellingen, C.; Probst, N.; Grivei, E. Polym Polym Compos 2002, 10, Wang, Y-J; Pan, Y., Zhang, X-W.; Tan, K. J. Appl Polym Sci 2005, 98, Modine, F. A.; Duggal, A. R.; Robinson, D. N.; Churntski, E. L.; Bartowiak, M.; Mahan, G. D.; Levinson, L. M. J Mater Res 1996, 11, Yu, J.; Zhang, L. Q.; Rogunov, M.; Summers, J.; Hiltner, A.; Baer, E. J Appl. Submitted to: Journal of Applied Polymer Science 5

6 Polym. Sci. 2005, 98, Dishovsky, N.; Grigorova, M. Mater. Res. Bull. 2000, 35, Chodak, I.; Krupa, I. J Mater. Sci. Lett. 1999, 18, Lao, Y.; Wang, G.; Zhang, B.; Zhang, Z. Eur Polym. J 1998, 34, Hopkins, A. R.; Reynolds, J. R. Macromolecules 2000, 33, Submitted to: Journal of Applied Polymer Science

7 Figures: Figure 1 Steady-state mixing torque vs. carbon black structure for HDPE blends containing 5, 10, 20 and 30% carbon black. Figure 2 15s. Temperature rise for specimens exposed to 800 W microwave power for Submitted to: Journal of Applied Polymer Science 7

8 Figure 3 Plot of heating efficiency vs. carbon black surface area. Figure 4 and N115. Plot of elongation at yield and at break vs. carbon black content for MT 8 Submitted to: Journal of Applied Polymer Science

9 Figure 5 Plot of elongation at yield and at break vs. carbon black surface area (10 % w/w carbon black content). Submitted to: Journal of Applied Polymer Science 9