The effect of an inorganic filler on the properties of high-density polyethylene

Plasticheskie Massy, No. 4, 2012, pp. 10 13 The effect of an inorganic filler on the properties of high-density polyethylene M.M. Kuliev and R.S. Ismaiilova Institute of Radiation Problems, Azerbaidzhan Academy of Sciences Selected from International Polymer Science and Technology, 39, No. 8, 2012, reference PM 12/04/10; transl. serial no. 17012 Translated by P. Curtis Summary Low-frequency dielectric spectroscopy was used to study polymer composites with a piezoelectric filler. In a wide frequency range (ν = 25 x 10 6 Hz) and temperature range (273-423 K), the dielectric characteristics (ε and tg δ) and electrical conductivity (σ) of polymer composites with a piezoelectric filler were investigated. The dependences of ε, tg δ, and σ on the concentration (up to 10 vol%) of filler, the frequency of the electric field, and temperature were studied. It was established that the introduction into high-density polyethylene of PKR-3M segnetopiezoelectric filler of the lead zirconate titanate family has a considerable effect on the given characteristics and changes the form of the dependences tg δ(ν), tg δ(t), ε(t), and σ(t). The variance dependence σ(ν) satisfies the condition σ ~ n s and exhibits two linear sections with parameters S 0.8 and S > 1.2 respectively. INTRODUCTION Composite materials based on a polymer matrix and a piezoceramic filler are promising materials for the creation of film sensor elements, transducers, and design elements with bulk-distributed physical properties. Such elements produced by thermal pressing and subsequent polarisation possess anisotropy of properties and increased strength. They can be used in combination with film materials as smart materials for the active damping of vibrations and noise in airplanes and helicopters and in auxiliary hydrophones for identifying underwater objects, for medical ultrasound apparatus, and so on [1]. There is great potential for controlling the properties of polymer composites by varying their electron structure and filler concentration. In turn, the concentration and form of defects largely determine parameters such as the concentration of free and localised charge carriers, the conductivity, etc. The magnitude and stability of these parameters have a considerable influence on the working characteristics of various electronic instruments and devices, and in some cases even determine these characteristics. The production and study of the given materials are of interest from the viewpoint of the miniaturisation of electronic elements and switching to technologies on the molecular level, which enables complex functions of directed charge and energy transfer to be carried out. It is also known [2-8] that the charge states of a polymer composite system, owing to the phase boundaries formed as a result of the interaction of inorganic phase and polymer matrix, have a great influence on their active properties. In this case, an informative method determining the correlation between the structure of the composite and its macroscopic properties, besides thermal activation spectroscopy, is dielectric spectroscopy, which makes it possible to clarify the pattern and details of the mechanisms of charge transfer and localisation [9]. This paper sets out the results of an experimental investigation of the dielectric (ε and tg δ) and electrical (σ) properties of polymer composites based on highdensity polyethylene (HDPE) with a piezoceramic filler in an alternating current. 2013 Smithers Rapra Technology T/33

EXPERIMENTAL The investigation was conducted on composites based on high-density polyethylene of grade 20806-024. As the additive, use was made of piezoceramic PKR-3M of the lead zirconate titanate (PZT) family. Composites containing up to 10 vol% of the indicated additive were obtained from a homogeneous blend of HDPE and filler powders with the aid of a manual heated press at a temperature of 433 K and a pressure of 15 MPa. The crystallisation regime was quenching rapid cooling of the specimen in a water ice mixture. The particle size of PKR-3M amounted to (63 100) x 10-6 m. Specimens comprised films of about (160 ± 10) x 10-6 m thickness and 40 x 10-6 m diameter. In view of the high surface tackiness of the specimens, reliable electrical contact of the electrodes was ensured by using 30 mm diameter superimposed electrodes of 7 µm thick aluminium foil. Use was made of a sandwich -type cell, which consisted of two electrodes of stainless steel of 30 x 10-6 m diameter, the distance between which was determined by the thickness of the specimen. Using an E7-20 wideband immittance gauge at electric field frequencies ranging from 25 to 10 6 Hz and at temperatures ranging from 293 to 423 K, the capacitances and dielectric losses were measured at a measuring voltage amplitude of 1 V. The dielectric permittivity ε was measured according to the standard relationship, with account taken of the capacitance and geometric dimensions of the capacitor structure investigated. In order to avoid errors in calculations of the dielectric permittivity, after the end of measurements, the thickness of the specimens was remeasured. The accuracy of measurement of the dielectric parameters was 1%. The electrical conductivity in the alternating field was determined in a direction perpendicular to the plane of pressing of the specimen by calculation according to the relationship σ = ω C tg δ, where ω = 2pν and C is the capacitance of the investigated specimen). RESULTS AND DISCUSSION Figure 1 gives the dependences ε(ν) and tg δ(ν) measured at a temperature T = 293 K for HDPE and its composites with a filler concentration of up to 10 vol%. The given results indicate that the dielectric permittivity of the specimens at all PKR-3M concentrations in the frequency range ν = 25-10 6 Hz remains practically constant (curves 1 to 4). Such a shape of the dependence ε(ν) indicates that neither the CO groups appearing in the polymer matrix during manufacture and processing nor the domains of filler are able to be oriented by the electric field. Meanwhile, the dependence tg δ(ν) of specimens with a PKR-3M concentration of up to 5 vol% (curves 5 and 6) exhibits two unpronounced maxima at 10 2 and 2 x 10 4 Hz respectively. As can be seen from Figure 1, when up Figure 1. The frequency dependence of the dielectric permittivity (1-4) and dielectric loss tangent (5-8) of HDPE + PKR-3M composites at a measuring voltage amplitude of 1 V. PKR-3M concentration: 0 vol% (1, 5), 5 vol% (2), 8 vol% (3, 7), and 10 vol% (4, 8) to 10 vol% PKR-3M is introduced into HDPE, the value at a frequency of 1 khz and room temperature increases roughly 1.8-fold (from 2.3 to 4.15) by comparison with the dielectric permittivity of pure HDPE. Increase in the filler concentration to 10 vol% (curves 7 and 8) increases tg δ appreciably and changes the shape of its frequency dependence. With increase in frequency to 2 x 10 5 Hz, tg δ decreases monotonically, which indicates the excessive sensitivity of the non-polar polymers to the presence in the material of impurities of polar nature. These impurities increase tg δ of the material and change the shape of the frequency dependence of tg δ. At a frequency of 1 x 10 6 Hz, a small increase in tg δ is found for all specimens investigated. Increase in tg δ in this region with increase in measuring field frequency may be due to polarisation processes associated with the fairly short relaxation time. The maximum observed in the high-frequency region of the dependence tg δ(ν) for specimens with filler concentrations of up to 5 vol% seems to be determined by the relaxation losses characteristic of most dielectrics. Here, structural elements of the polymer system, and also of the low-molecular-weight impurities, can act as relaxers. The use of polymer composites in electronic and radio devices in recent years has required the creation of new materials with a specified level of electrical properties, including in alternating fields. Figure 2 gives the frequency dependence of conductivity on a double logarithmic scale. The figure presents the family of frequency dependences of conductivity σ for different concentrations. It can be T/34 International Polymer Science and Technology, Vol. 40, No. 8, 2013

seen that the dependence σ(ν) is linear in logarithmic coordinates. This indicates the power-law nature of the dependence of conductivity on frequency (σ ~ n s ). All dependences have a common property increase in conductivity with increasing frequency. The dependence σ(ν) reveals two linear sections with parameter S < 1 (first section) and parameter S > 1 (second section). On the first section, for all the specimens investigated, S 0.8, which is typical of the multiplet leap transfer of charge carriers, which, on achievement of a certain frequency n 0, is replaced by a regime of leap transfer by clusters of small size [9]. According to this work, the discovered difference in the frequency boundaries of leap conductivity is due to the different concentration of localised states, which is determined in turn by the difference in concentration of structural defects responsible for their generation. The boundary frequency n 0, starting from which the leap conductivity changes with increase in PKR-3M filler concentration, is shifted towards high frequencies (from 2 x 10 3 to 5 x 10 4 Hz), which may be due to the difference in the nature of localisation of charge carriers. For the second section of the dependence σ(ν) for HDPE and composites with a PKR-3M concentration of 5 vol%, parameter S 1.3, while for composites with a PKR-3M concentration of 10 vol%, S 1.5. As can be seen, in this region, for all specimens, parameter S > 1.2, which according to Deng and Mauritz [10] reflects cooperative interaction during leap diffusion of charge carriers. The temperature dependence of ε at constant frequency (1 khz) for HDPE and its composites with a PKR-3M content of up to 10 vol% is shown in Figure 3. As can be seen from this figure, in the case of pure HDPE and of composites with a PKR-3M content of 8 and 10 vol%, the shape of the dependence ε(t) is characteristic of non-polar polymers: with increase in temperature, ε remains constant or decreases slightly, while in the region of the softening point T s there is a comparatively sharp change (reduction) in ε. In the case of composites with a PKR-3M concentration of 3 and 5 vol%, the nature of the dependence ε(t) changes: in the region of T s, the dielectric permittivity ε, increasing, reaches a maximum and then, decreasing, forms a blurred maximum. The abnormal increase in dielectric permittivity as the temperature of specimens with a PKR-3M concentration of 3 and 5 vol% approaches the softening point T s may be due to an increase in segmental mobility, which causes electrode polarisation. The temperature at which there is a reduction in ε is shifted 12 15 deg towards lower temperatures. The results concerning change in the dielectric loss tangent as a function of the temperature of polymer systems with different degrees of filling are presented in Figure 4. From the figure it can be seen that in all cases the introduction into HDPE of PKR-3M piezoceramic leads to an increase in the dielectric loss values (from 0.14 x Figure 2. Frequency dependences of conductivity for composites based on HDPE + PKR-3M. Concentration of PKR-3M: 0 vol% (1), 5 vol% (2), and 10 vol% (3) Figure 3. Temperature dependence of dielectric permittivity at a frequency of 1 x 10 3 Hz for HDPE + PKR-3M specimens. PKR-3M concentration: 0 vol% (1), 3 vol% (2), 5 vol% (3), 8 vol% (4), and 10 vol% (5) 10 2 to 3.06 x 10 2 ). The temperature dependences of tg δ differ for specimens with different PKR-3M contents. In the spectrum of dielectric losses of composites with a filler content of up to 5 vol%, tg δ decreases with increase in temperature to 333 K, and in the range 333 383 K it remains practically constant and then increases. Increase in the filler content to 10 vol% leads to the following changes in the spectrum of dielectric losses: at the start, the value of tg δ increases, and at 313 K, passing through a maximum, it decreases, and in the range 383-423 K it hardly changes. Such a shape of the temperature dependence of tg δ of composites with different PKR-3M contents indicates a considerable change in intensity 2013 Smithers Rapra Technology T/35

Figure 4. Temperature dependence of the dielectric loss tangent tg δ at a frequency of 1 x 10 3 Hz for HDPE + PKR- 3M specimens. PKR-3M concentration: 0 vol% (1), 3 vol% (2), 5 vol% (3), 8 vol% (4), and 10 vol% (5) Figure 5. Dependences of the logarithm of conductivity on inverse temperature for HDPE + PKR-3M specimens. PKR- 3M concentration: 0 vol% (1), 5 vol% (2), 8 vol% (3), and 10 vol% (4, 5). 5 HDPE + 10 vol% PKR-3M with prior exposure for 180 s to an electric field with E 6.25 x 10 5 V/m of molecular motions in the investigated systems with variation in the filler concentration. The cause of the appearance of high-temperature increase in specimens with a PKR-3M concentration of up to 5 vol% may be due to crystallisation of these composites with increase in the mobility of macromolecules and their segments or to polarisation in the electric field of electric charges in the bulk of the composite. Increase in the filler content causes a decrease in the intensity of molecular motions, and increase in tg δ at high temperatures is not observed. Important information about the mechanism of conductivity for composites with high resistivity, like the systems investigated in the present work, may be given by the temperature dependence of conductivity [11]. The temperature dependences of conductivity in Figure 5, in lg σ 1/T coordinates, have characteristic breaks. From the graphs presented in this figure it follows that there are at least two conduction mechanisms, which can be found in appropriate temperature ranges with different activation energies. For the initial HDPE and a composite with a PKR-3M content of up to 5 vol% (curves 1 and 2), the shape of the dependence σ = f(t) is identical: with increase in temperature, the conductivity of these specimens in an alternating field initially decreases, remains constant, and then, close to the softening point T s, begins to increase. We assume that reduction in conductivity with increasing temperature indicates the electron conduction of these specimens at comparatively low temperatures. Freeing the electrons from energy traps during heating reduces the proportion of the electron component of total conductivity. It must be pointed out that the degree of change in conductivity for HDPE + 5 vol% PKR-3M specimens is considerably greater. Increase in the content of PKR-3M to 10 vol% (curves 3 and 4) leads to an increase in conductivity and changes the form of the dependence σ = f(t). With increase in temperature to 323 K, the conductivity increases negligibly, in the range 323 385 it decreases sharply, and then, with increase in temperature, there is a small increase in σ, and here its maximum value at high temperatures (up to 423 K) remains below the value of σ of the HDPE + 5 vol% PKR-3M composite. Figure 5 also presents the temperature dependence of conductivity in an alternating electric field of an HDPE + 10 vol% PKR-3M composite (curve 5) with prior exposure, at each temperature for 180 s, to a constant electric voltage (100 V) creating in the specimen an electric field of the order of ~6.25 10 5 V/m. It can be seen that in this case the shape of the dependence σ(t) remains as before, but the value of σ in the entire temperature range remains much greater than the value of σ for an HDPE + 10 vol% PKR-3M composite not treated with an electric field. This indicates that the polarisation of the composite material promotes an increase in its conductivity. CONCLUSIONS Thus, it has been established that the introduction into HDPE of up to 10 vol% piezoceramic PKR-3M affects the frequency and temperature dependences of the dielectric characteristics and conductivity of the HDPE + PKR-3M composite system in an alternating electric field. We assume that mutually complementary information on the dielectric, electrical, and relaxation properties of composites will make it possible to discuss on a sounder basis the mechanisms of conduction associated with T/36 International Polymer Science and Technology, Vol. 40, No. 8, 2013

change in the relaxation processes occurring in the composite. The presented experimental results are still insufficient to enable any clear conclusion to be drawn concerning the specific mechanism of conduction of the investigated composites. Work in this direction is continuing. REFERENCES 1. Lusheikin G.A., FTT, 48(6):963 (2006). 2. Chmutin I.A. et al., Vys. Soed., A46:1061 (2004). 3. Young R.H. and Fitzgerald J.J., Chem. Phys., 102(15):6290 (1995). 4. Voilov D.N. et al., Plast. Massy, (3):15 (2008). 5. Avanesyan V.T. and Puchkov M.Yu., Fiz. Tv. Tela, 49(11):2088 (2007). 6. Kuliev M.M. and Ismaiilova R.S., Plast. Massy, (11):10 (2007). 7. Lotonov A.M. et al., Vys. Soed., B48(10):1898 (2006). 8. Kurbanov M.A. et al., Proc. 3rd Int. Conf. Electrical Insulation 2002, St Petersburg, p. 78 (2002). 9. Karulina E.A. and Khanin S.D., Proc. 3rd Int. Conf. Electrical Insulation 2002, St Petersburg, p. 190 (2002). 10. Deng Z.D. and Mauritz K.A., Macromolecules, 25(9):2369 (1992). 11. Gul V.E. and Shenfil L.Z., Electrically Conductive Polymer Composites. Khimiya, Moscow, 240 pp. (1984). 2013 Smithers Rapra Technology T/37