Polyethylene Production from Granules Using High Voltage

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1 Polyethylene Production from Granules Using High Voltage Boris Pavlovich Chesnokov Olga Valerevna Naumova Vladimir Aleksandrovich Strelnikov Fyarid Kinzaevich Abdrazakov Boris Alexandrovich Tronin Al Farabi Kazakh National University, 7 Al Farabi avenue, Almaty , Kazakhstan. Abstract Granules of high-density polyethylene (HDPE) were processed with bremsstrahlung X-rays from an electron accelerator. The ready polyethylene tubes were repeatedly irradiated by electrons using the same accelerator. The structure and thermo-mechanical properties of the material obtained by radiation processing were studied. Potentialities of electron irradiation and deceleration X-ray emission as applied to industrial production of cross-linked polyethylene are discussed. Keywords: High-density polyethylene, electron irradiation, bremsstrahlung X-rays, cross-linking, mechanical properties, melt flow index, internal friction. Introduction This work was accomplished in the frames of a wide program of technological studies and approbation of the methods developed in industrial conditions. This program includes development of the methods for improved characteristics of oxide and impregnated cathodes, luminophorous coatings, application of radiation technology in powder metallurgy for upgrading hard metals and ceramics, production of glass and synthetic crystals [,, 3, 4], and such traditional field for radiation chemistry as polymer radiation processing. Polyethylene is presently one of the most widely used polymers. The history of industrial applications of polyethylene radiation cross-linking covers about half a century. The reviews of technological approaches to radiation processing of polyethylene articles can be found, for example, in the papers [5, 6, 7, 8, 9, 0, ]. The advantages of radiation cross-linking of polyethylene tubes (PEX-c ) are high capacities of the standard technological lines used at the first production stage (tube extrusion) and low costs of feedstock used in the process without sensitizing additives []. The industrial radiation cross-linking methods were criticized for insufficient degree of cross-linking [], difficulties of radiation processing of the original feedstock components and limited application of the extrusion method because of the low fluidity of the material and the possibility of radiation processing application only to the thin-walled articles [3]. However, these disadvantages are not inherent to radiation technology and can be overcome by the proper selection of the type and modes of radiation and thermal processing both of the original materials and ready articles. In this study, a two-stage processing was used for obtaining ready articles from the granulated feedstock. The experiment was planned subject to application of radiation processing in the specific industrial technology for HDPE production. Bremsstrahlung X-ray irradiation from an electron accelerator was used for polyethylene processing in granules in view of the high penetrating ability of the X-rays providing uniform irradiation of the material in bulk. The efficiency of electron beam conversion to X-rays, i. e. the ratio of the X-ray power to the electron beam power, can be approximately evaluated using equation [4]: ku Z () where k is constant (it was experimentally established that k ), U is voltage sated in Volts, Z is the atomic number of the target material. The equation does not strictly hold for high operation voltages U (U = E/e where E is electron energy and e is the electron charge). The maximal production rate of a plant using deceleration X- ray emission can be evaluated as kg k U ( V ) Z W ( kw) q () s D( kgy) In equation (), W (kw) is electron beam power, D (KGy) is absorbed dose needed for the process realization. Equation () shows that the mass of a material processed per unit of time is proportional to the electron energy and the electron beam power. Selection of the radiation source with the appropriate characteristics can provide the given production rate of a radiation facility. Technique Irradiation of the granulated feedstock with bremsstrahlung X- rays. At the first stage of the process, the granulated feedstock for polyethylene production was irradiated with bremsstrahlung X-rays from MeV electrons. 40

2 Irradiation of polyethylene in granules was proposed in a series of works. White (98) developed a technique where granulated polyethylene was processed in a steam atmosphere at heightened temperatures and then irradiated by electrons or gamma rays. To reduce the level of free radicals, the irradiated material was additionally subjected to the steam processing. In the paper [5], a method was reported for polyethylene radiation processing before its conversion to final products although the process conditions were not disclosed. Oxidation of the polymer when irradiated in air, low degree of cross linking, and modification of long chain branching were noted as the most important changes in the properties of irradiated polyethylene. It was reported that irradiation decreased the melt flow index of the polymer by about 50% that resulted in a broader molecular mass distribution and considerable increase in melting temperature. As distinct from the mentioned above studies, radiation processing of polyethylene granules in this work was performed in conditions close to industrial environment. The layout of the irradiation process is shown in Fig.. The granulated polyethylene was irradiated with bremsstrahlung X-rays from MeV electrons using electron accelerator ELU- 4. The dose rate of X-ray irradiation was. 5 Gy/s. The absorbed dose was varied from 0. through 500 kgy. Figure : Schematic extrusion facility. extruder, former, 3 calibrator, 4 cooler, 5-caterpillar pull mechanism, 6 cutter, 7-packaging Results of the Experiments The extrusion facility used in this study is actually a line for HDPE production shown in Fig. 3. Thus, irradiation process was incorporated into the existing technological scheme of polyethylene tube production. Figure 3: HDPE production line Figure : Technological scheme of ground chalcedony irradiation. rotary table, irradiated feedstock, 3 source of bremsstrahlung X-rays (a tungsten target), 4 output window of the electron accelerator Irradiated granules were processed using a standard extrusion facility (Fig. ). The granules were loaded into the receiver tank and transported to the pressing bunker through the vacuum drive connected with extruder (). In the extruder, polymer granules were continuously moved by the auger and converted under heating to the mass suitable for molding. The hot polymer was forced through nozzle () where it was shaped in a form of a tube. Then using driving device (5), it was drawn through calibrator (3) where the required outer diameter was finally formed, cooler (4), cutter (6), and went to packaging (7). Melting and extrusion of irradiated granules lead to profound alterations in the polyethylene molecular structure making it more homogeneous and improve the polymer properties. An important characteristic of polyethylene fluidity and formability is the melt flow index stated in grams of material extruded during 0 minutes in standard conditions. The material mass extruded through the capillary viscometer IIRT- 5 during 0 minutes was determined by a standard technique at the temperature of 90 0 C and pressure of 49Н. The dose dependence of this characteristic (Fig. 4) allows evaluation of the efficiency of HDPE pretreatment for production of practically unlimited variety of materials with smoothly varying values of the melt flow index. It provides an opportunity to flexibly regulate granulated feedstock technological properties coming from the requirements of the specific properties and configurations of ready articles. Fig. 4 shows that, at low absorbed doses, melt flow index of polyethylene produced by radiation technology increases with the dose reaching a maximum at the dose of 0 kgy and then decreases as dose increases. In the existing industrial technology of HDPE production, the optimal value of the melt 4

3 flow index corresponds to the maximum observed in Fig. 4. Increase in the melt flow index at low doses is probably connected with destructive processes and accumulation of free radicals, while cross-linking of macromolecules prevails at higher doses. melt flow index D, kgy Figure 4: Dependence of melt flow index on absorbed dose of bremsstrahlung X-rays Due to fluctuations inherent to the technological process of feedstock production, its properties may vary from batch to batch that leads to the parameter spread in ready articles. Irradiation contributes to leveling these characteristics. Microscopy studies (Fig. 5) have shown that reactive radiation-induced centers initiate formation of polymer chains in the form of thin threads providing strong bonds between macromolecules (Fig. 5b). Owing to the spatial network with a high degree of structural order formed by radiation, irradiated HDPE is characteristic for increased strain resistance and rigidity. The material strength increases due to appearance and interweaving of the thin fibers and stress relaxation in the connecting links localized at small building blocks in the course of crosslinking. In particular, the reactive radiation centers initiate formation of additional inter-and intramolecular bonds in the main and side chains. Irradiation of ready articles by accelerated electrons At the second stage, the ready polyethylene articles (tubes) were irradiated with electrons at the dose rate of kgy/s. To provide uniform surface irradiation, the polyethylene tubes were rotated under the electron beam. The absorbed dose was varied in the range from 30 to 000 kgy. As distinct from X- ray irradiation of the granulated feedstock where the tube structure was formed in the overall volume, the repeated irradiation with accelerated electrons provided additional strengthening of the surface layer. The visual control had shown pronounced changes in the outward appearance of the samples: the electron-irradiated samples had a smoother and brighter surface. The study of the mechanical properties of the irradiated samples included determination of their relative elongation under the given stress. A stress of the plastic flow was measured for the two types of tubes made of HDPE by conventional industrial technology and radiation technology. The values of the load and the relative elongation for tensile stress of the polyethylene samples having the I-shaped crosssection were determined for the longitudinal and transversal directions of the force application (Fig. 6). In the case of the longitudinal disruption, electron irradiation with the dose of 000 kgy results in increase in the ultimate stress by. 5 times and increase in the relative elongation by. 3 times. In the case of the transversal load, the ultimate stress for tensile stress and relative elongation of the sample increase by 8% and 0%, respectively, as a result of radiation processing. a) b) Figure 5: Microstructure of HDPE cleavage facet. Enlargement 300х. a conventional industrial technology, b radiation technology a): F, kgf / cm а, b conventional technology c, d radiation technology a, b longitudinal disruption c, d transversal disruption 4

4 cross-linking of the polymer macromolecules. The internal friction background becomes lower after heating and does not demonstrate a tendency to increase with temperature that could be associated with a viscous flow. In electron-irradiated polyethylene samples, more pronounced and narrower internal friction peaks associated with the transition from a glassy to a high-elastic state were observed (Fig. 8). It indicates to prevalence of free molecular segments having similar sizes in irradiated samples. b): L,, 3 conventional technology, 4 radiation technology, longitudinal disruption 3, 4 transversal disruption Q Figure 6: Dependence of the ultimate stress and relative elongation of HDPE samples at longitudinal and transversal disruption in the method of sample preparation Т, о С Additional information on structural alterations in the irradiated polyethylene was obtained form mechanical spectroscopy of electron-irradiated samples. Internal friction measurements were conducted using a torsional pendulum at the oscillation frequency of about Hz. The unirradiated polyethylene samples were characteristic for availability of internal friction peaks having very broad relaxation spectra, obviously coming from a broad length distribution of the free segments of macromolecules (Fig. 7). Q Т, о С Figure 7: Temperature dependence of internal friction in HDPE made by conventional technology. -first heating, second heating, 3 third heating Preheating the samples made of conventional feedstock up to a temperature close to the melting temperature makes temperature dependence smoother that indicates to a more uniform distribution of the molecular segments. The temperature and the height of the glassy-transition peak retain their small values, i. e. heating doses not cause considerable 3 Figure 8: Temperature dependence of internal friction in electron-irradiated HDPE with the dose of 30 kgy. first heating, second heating, 3 third heating Discussion Each subsequent heating leads to increase in the temperature of the internal friction maximum. A higher rate of internal friction increase after each heating testifies to a viscous flow leading to additional irreversible unfreezing of the molecular segments. This process results in the increase of the internal friction peak heights. On the other hand, formation of the cross-linked structure under electron irradiation leads to increased activation energy needed for excitation of molecular segment oscillations. As a result, internal friction peaks shift to higher temperatures. Changes in internal friction with temperature in electronirradiated polyethylene were accompanied by the changes in the sample dimensions after heating and their recovery after cooling which is characteristic for thermoplastics with a spatial network formed by radiation. Conclusion Application of the described above technology allows production of high quality cross-linked polyethylene in industrial scales using one-stage irradiation of the feedstock in granules with bremsstrahlung X-rays. Additional strengthening and increase in the polyethylene cross-linking degree can be achieved by repeated irradiation of a ready article. In particular, electron irradiation allows obtaining heterogeneous structures with a strengthened surface layer which thickness depends on electron energy [6, 7]. In this study, the thickness of the strengthened layer obtained by polyethylene irradiation with MeV electrons was about. 5 mm. 43

5 In the cases when additional uniform strengthening of the massive ready articles is required, bremsstrahlung X-ray irradiation can be also applied. Sufficient irradiation power and capacity of the facility can be provided by an appropriate selection of electron energy and beam power of the accelerator. [7] V. A. Astapenko. Interaction of Radiation with Atoms, Molecules and Nano-Particles. Moscow: Intellect, 0. [8] References [] B. P. Chesnokov. High Technologies in Electrovacuum Industry. Saratov: Saratov State Agrarian University, 000. [] Y. A. Zaikin, B. A. Aliyev. Radiation effects in high-disperse metal media and their application in powder metallurgy, Radiat. Phys. Chem., vol. 63, no., рр. 7-30, 00. [3] Y. A. Zaikin, B. P. Chesnokov, B. A. Aliyev, V. A. Kiryushatov. Radiation processing of powders for improved fusion structural materials, J. Nucl. Mater., vol. 7/7, рр , 999. [4] Y. Zaikin, S. Korenev. Structural alterations in metal powders under ionizing irradiation, Proceedings of International Conference «Actual problems of energy science, KUBiK, Saratov, pp. 3-7, 00. [5] G. G. A. Böhm, J. O. Tveekrem. The radiation chemistry of elastomers and its industrial applications, Rubber Chemistry and Technology, vol. 55/3, рр , 98. [6] D. W. Clegg, A. A. Collyer. Irradiation Effects on Polymers, Elsevier Appl. Sci., рр. 450, 99. [7] J. G. Drobny. Radiation Technology for Polymers. Boca Raton: CRC Press, 00. [8] M. S. Ivanov. Radiation Chemistry of Polymers. New Concepts of Polymer science. Untrecht: VSP- III, 99. [9] K. Makuuchi, S. Cheng. Polymer Materials and Its Industrial Application. Hoboken: John Wiley and Sons, Inc., 0. [0] Y. Tabata, Y. Ito, S. Tagawa. CRC Handbook on Radiation Chemistry. Boca Raton: CRC Press, 99. [] R. J. Woods, A. K. Pikaev. Applied Radiation Chemistry. Radiation Processing. NY: John Wiley & Sons, Inc., 994. [] Extrusion of metalloplastic tubes. Date Views June 0, 03. www. newchemistry. ru [3] Irradiated polyethylene. Date Views June 0, 03. www. polymerics. ru [4] L. A. Parks. Radiation Crosslinking of Polymers, Sterigenics Advanced Applications, 00 [5] Plastemart. A new method of cross linking of PE improves properties and widens scope, SPE Annual Technical Conference, May 004. [6] J. T. Floyd, W. J. Chappas. Dose depth simulations in standard construction geometries, Radiat. Phys. Chem., vol. 48, рр ,