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Effect of Extrusion on the Electrical, Mechanical and Rheological Properties of an Ethylene Butylacrylate/Carbon Black/Graphite Nanoplatelets Nanocomposite by Elena Álvarez Díez Diploma work No. 134/2014

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Effect of Extrusion on the Electrical, Mechanical and Rheological Properties of an Ethylene Butylacrylate/Carbon Black/Graphite Nanoplatelets Nanocomposite by Elena Álvarez Díez Diploma work No. 134/2014 at Department of Materials and Manufacturing Technology CHALMERS UNIVERSITY OF TECHNOLOGY Gothenburg, Sweden Diploma work as an Erasmus exchange student 134/2014 Performed at: Department of Materials and Manufacturing Technology Chalmers University of Technology, SE Gothenburg Examiner: Professor Mikael Rigdahl Department of Materials and Manufacturing Technology Chalmers University of Technology, SE Gothenburg Supervisor: Doctor Ruth Ariño Mariné Department of Materials and Manufacturing Technology Chalmers University of Technology, SE Gothenburg Effect of Extrusion on the Electrical, Mechanical and Rheological Properties of an Ethylene Butylacrylate/Carbon Black/Graphite Nanoplatelets Nanocomposite Elena Alvarez Díez Elena Álvarez Díez, 2014 Diploma work no 134/2014 Department of Materials and Manufacturing Technology Chalmers University of Technology SE Gothenburg Sweden CHALMERS Reproservice Gothenburg, Sweden 2014 Effect of Extrusion on the Electrical, Mechanical and Rheological Properties of an Ethylene Butylacrylate/Carbon Black/Graphite Nanoplatelets Nanocomposite Elena Álvarez Díez Department of Materials and Manufacturing Technology Chalmers University of Technology Abstract The possibility of introducing graphite nanoplatelets (GNP) in a semiconductive polymeric composite* filled with carbon black (CB) through the extrusion mixing was studied, the application being the semiconductive screens in high voltage direct current extruded cables. An appropriate extrusion processing aiming for adequate semiconductive properties of the extruded material was searched for. The possible detrimental effect by adding GNP and CB on the mechanical and rheological properties of the material was also studied. The influence of different processing variables was examined through the application of two different extrusion temperatures (160 C and 180 C), two rotation screw speeds (50 rpm and 100 rpm) and two types of screw (barrier screw and conventional screw). Materials with 3.5, 5 and 12 volumen-% of hybrid filler content were analyzed, the hybrid filler consisting of 80 weight-% GNP and 20 weight-% CB. Direct current measurements were performed in order to determine the electrical conductivity. Scanning electron microscopy and X-ray diffraction were used for characterizing the morphology and the structure of the fillers and composites. Uniaxial tensile tests and viscosity measurements with a capillary viscometer were conducted in the study of the mechanical and rheological properties, respectively. The results showed a beneficial effect on the electrical properties with the introduction of GNP. The reason could be a synergistic effect between the two fillers due to the geometries of the fillers. The best results regarding the electrical properties were obtained with the conventional screw, showing lower percolation thresholds than those observed with the barrier screw. A possible de-agglomeration and low breakage of the GNP obtained with the conventional screw whereas high breakage of GNP with the barrier screw could be the reasons for the observation. No significant differences in the electrical properties were observed when changing the extrusion temperatures and screw speeds. In addition, an acceptable melt viscosity and stiffness was achieved at the filler contents studied. *Semiconductive polymeric composite: composite that uses a polymer as matrix (in this work ethyl butyl-acrylate) and conductive particles such as carbon black as filler. Key words: semiconductive polymeric composite, hybrid conductive filler, graphite nanoplatelets, carbon black, extrusion mixing, barrier screw, conventional screw, synergistic effect Contents NOMENCLATURE... I 1. INTRODUCTION BACKGROUND MATERIALS Ethylene butyl acrylate copolymer (EBA) Carbon black (CB) Graphite nanoplatelets (GNP) NANOCOMPOSITE BEHAVIOR THEORIES FOR CONDUCTIVITY Percolation theory Synergistic effect Rheological properties Mechanical properties PROCESSING TECHNIQUES Fundamentals of compounding Internal mixer Extrusion mixing Compounding variables Capillary rheometer CHARACTERISATION TECHNIQUES Electrical properties EXPERIMENTAL RAW MATERIALS PERCOLATION CURVE DETERMINATION METHODS Compounding of masterbatches Extrusion experiments Verification of filler content (TGA) Electrical measurements (two-point measuring technique) Morphological characterization of composite material and fillers (SEM and Optical Microscopy) Structural analysis of GNP (XRD) Uniaxial tensile tests Rheological measurements (Capillary Viscometer) RESULTS AND DISCUSION CONCLUSIONS... 45 Appendix A References... 51 Acknowledgements First I would like to express my gratitude to my supervisor Ruth Ariño for her excellent guidance throughout this thesis and for having shared all her knowledge and time with me. I would also like to thank my examiner Mikael Rigdahl for having offered me the opportunity to carry out this thesis and for his time and precious supervision. Second, the department of Materials and Manufacturing Technology are thanked for giving me the opportunity to work with them and for the treatment received. Thanks to Borealis AB and Swerea Group for providing the materials and sharing their equipment for the development of this thesis. Thanks to all my friends, colleagues and staff at the department of Materials and Manufacturing Technology for the friendly atmosphere and time we shared. Particularly I thank Raquel de Oro Calderón for always having the time to help me, Sujith Guru for being a magnificent opponent in the defense of this thesis and Mahesh V Sudaram for his time, knowledge and advice. Finally, I would like to thank my family for the support received during my whole life, the education I have received and my brother César Luis Álvarez for always advising me and helping with his knowledge. I Nomenclature Symbols c P V D L E σb Percolation threshold Pressure Velocity Diameter Length Young s Modulus Ultimate Strength [Vol% ] [Pa] [m/s] [m] [m] [Pa] [Pa] εb Final Elongation [%] σ τa Conductivity Apparent shear stress [S/m] [Pa] µa Apparent viscosity [Pa s] γa Apparent shear rate [s -1 ] Acronyms CB CNT EBA EG GIC GNP HVAC HVDC SEM TGA Vol % Wt % XRD Carbon Black Carbon Nanotubes Ethyl Butyl Acrylate Expanded Graphite Graphite Intercalation Compound Graphite Nanoplatelets High Voltage Alternating Current High Voltage Direct Current Scanning Electron Microscope Thermogravimetric Analysis Volume percentage Weight percentage X-ray Diffraction II Introduction High voltage direct current cables (HVDC cables) have aroused great interest in the recent years. The continuous growth in energy consumption has pushed the electricity grid into a new challenge. Nowadays, large amounts of energy need to be carried for large distances in a globalized world. Because of their capacitive and inductive limitations, high voltage alternating current cables (HVAC cables) are not suitable for the present transmission grid. HVDC are widely used since they do not present any capacitive or inductive effects and they also provide advantages when linking asynchronous systems preventing cascading failures. Despite of their different functioning, both types of cables have a similar structure. Figure 1 shows a typical structure of an extruded HVDC cable. [1] Conductor (copper or aluminum) Conductor screen (semiconductive polymer) Insulation (insulating polymer for HVDC cables) Insulation screen (semiconductive polymer) Metallic screen (aluminium, lead or copper sheath) Other layers Figure 1. Structure of an extruded HVDC cable In HVDC cables, two semiconductive* screen layers are required. The inner one is called conductor screen or conductor shield and the outer one insulation screen or insulation shield. Coextrusion is the manufacturing process used for cables with voltages up to 300kV and powers up to 1000MW. In this process, conductor shield, insulation, insulation screen and metallic screen are extruded around the conductor. [1] The main function of the conductor shield is to provide a smooth, continuous and void-free interface between the conductor and the insulation as well as contribute to a homogenous radial electric field. Gaps of air, together with voltage differences, can produce electrical discharges (also called corona discharges) that may deteriorate the insulation. In addition, it is important to provide a homogeneous radial electric field in the insulation to avoid stresses that can damage the insulation. An analogous function applies to the insulation screen, in this case working between the insulation and the metallic screen layers. The presence of these two layers reduces premature failures and increases the lifetime. [1] Therefore the materials used for these applications require on one hand some conductivity (to avoid high voltage differences at the interfaces) and on the other hand compatibility with the insulation (to be attached closely to the insulation). The cable standards demand a volume resistivity for the conductor shield below cm at 90 C and 130 C for continuous and emergency operating conditions respectively. In the case of the insulator shield the limiting requirement is cm at 90 C for 1 continuous operation temperature and 110 C as emergency conditions [2]. Composites with ethylene copolymers as matrix material and carbon black (CB) as conductive filler are the materials most widely used. In order to achieve the required conductivity, the amount of filler often needs to be between 25 and 40 weight percent (wt %). This high loading is a drawback for the manufacturing process due to the viscosity increase as well as for the mechanical properties, decreasing the flexibility [3]. Furthermore, a decrease in the amount of CB used would be preferred for material cost savings. In previous studies, a synergistic effect on the electrical conductivity by replacing part of the CB by graphite nanoplatelets (GNP) was observed, with the best results for the hybrid systems consisting of 80 wt % GNP and 20 wt % CB [4] (see appendix A, Figure 35). The measured conductivities overcome those of the composites filled with CB or GNP separately. It is believed that by modifying different parameters in the extrusion process, GNP could be further exfoliated or de-agglomerated (as has been observed with layered silicate platelets when extruded in a polymeric matrix, Figure 2) [5] resulting in a higher electrical conductivity. Besides the GNP exfoliation/de-agglomeration, a suitable orientation and dispersion of the filler is also desired and particle fragmentation should be avoided to some extent. In this work, the effect of extrusion mixing on the electrical and mechanical properties was studied. Morphological and rheological studies were also performed. Figure 2. Effect of extrusion in filled polymer with layered silicate platelets. Aim of this work The aim of this work is to study the influence of extrusion on a polymer nanocomposite with ethylene butylacrylate as the matrix and carbon black and graphite nanoplatelets as the conducting fillers, with particular regard to the electrical conductivity of the material and, to a lesser extent, its mechanical properties and viscosity. The study will include case studies of different processing temperatures, screw speeds and types of screw. Special attention will be paid to the analysis of a possible exfoliation or de-agglomeration of the graphite nanoplatelets on account of the recent interest in industrial processes to obtain single layers of graphite, i.e. graphene. 2 *In this thesis, semiconductive material is defined as a material with conductivity between that of the conductor and that of the insulator of high voltage cables. It is not referred to N-type or P-type nonlinear resistive materials. 3 1. Background This chapter introduces the materials used in this work and provides an understanding of the theories that describe the performance of filled polymers with regard to electrical, mechanical and rheological properties. The processing techniques used and the effect of the different processing techniques and parameters on the material are also presented. An overview of the influential parameters is given for this reason. Finally, the basics for some characterization techniques used are given at the end of the chapter Materials Ethylene butyl acrylate copolymer (EBA) In this work, an ethylene butyl acrylate copolymer (EBA), also called ethylene n-butyl acrylate (EnBA) or poly (ethylene co-butyl acrylate) was used as a matrix material. This copolymer is composed of randomly arranged ethylene and butyl acrylate monomer units. [6] Figure 3 shows its structure. Figure 3 Polymerization and structure of ethyl butyl acrylate EBA belongs to the copolymer group of acrylates, which together with the acetates are the common polymers used for semiconductive screens. Acrylates have better thermal stability than acetates, which in this case is of a great importance since not only higher temperatures in the extrusion process increase the production rate but the polymer must resist the heating from the cross-linking of the insulation ( C).[3] EBA is polymerized by the same high-pressure process as low-density polyethylene (LDPE) [7]. Although it is a thermoplastic it can be crosslinked by peroxide addition and applying heat and pressure. With this, the physical and heat resistant properties are improved without impairing other essential properties. [8] In cable manufacturing, crosslinking takes place after the cable leaves the co-extruder head [3]. The amount of acrylate monomers as well as the presence of fillers affect the crystallinity of the copolymer [9], [10]. The acrylic acid units reduce the crystallinity [9] whereas fillers may impede the crystallization process of EBA [10]. The crystalline regions constitute very compact structures and therefore, the presence of fillers in these regions is inhibited, creating regions with high amounts of fillers. In the case of conductive fillers, as in this work, this can contribute to a higher conductivity of the material [10]. In copolymers, the melting and glass transition temperatures depend on the weight fractions of monomers [11]. EBA has a melting point between 95 and 115 C, increasing as the butyl acrylate content decreases [12]. Other characteristics of EBA copolymers are good flexibility and additive compatibility what makes it suitable for highly filled compounds [13]. In addition, EBA exhibits a certain polarity, caused by the 4 butylester side groups [14] that increases the conductivity of the polymer [12], [15] and can improve the affinity to the fillers as has been observed with expanded graphite (EG) [16] Carbon black (CB) Carbon black was chosen as one of the components of the hybrid system. It is electrically conducting and widely used in polymer matrix composites for different applications [17]. It exists in the form of aciniform aggregates (the smallest dispersible unit) formed of spheroidal particles of elemental carbon [17], [18]. The size of the particles varies between 15 and 75 nm whereas for the aggregates it varies from 50 to 400 nm. Furthermore aggregates tend to collapse in agglomerates with a size between 100 and 1000 nm [19]. Figure 4 shows agglomerates of the carbon black used in this work. The particles microstructure however is not well understood. Paracrystalline domains where planes of graphite lay in a turbostratic* stacking have been found [20]. For the graphitic structures, graphite plates with an interplane distance deviating from the expected have been observed by X-Ray diffraction [17]. In the center of the particles a lack of order in the structure is suspected [17]. Figure 4 Carbon black agglomerates ENSACO 260G Carbon black is obtained from the partial combustion or thermal decomposition of hydrocarbons [21] usually through the furnace process (from petrochemical or carbochemical origin) or the acetylene process (from acetylene gas) [17]. The conductivity of carbon black varies between 10-1 and 10 2 ( cm) -1. Carbon black grades with high specific volumes, high structures and absence of impurities or chemical groups are preferred for conducting filled polymers. In addition, because porosity in carbon black increases the specific volume, higher porosity affects positively the conductivity of the material. High structured carbon, i.e. with larger number of particles per aggregates and branched sides, gives higher conductivity when added to a polymer although the specific volume is decreased [17]. This apparent contradiction is attributed to small particles, as in the case of high structured carbon blacks, that tend to aggregate and form agglomerates which can enhance the electrical properties. 5 Among the main drawbacks of carbon black, health concerns must be considered with caution [21]. The International Agency for Research on Cancer (IARC) has classified it in the Group 2B, carcinogens. Other drawbacks, especially when added in large amounts to a polymer matrix, are the increase of viscosity and the detrimental effect on mechanical properties such as flexibility [18]. *Turbostratic stacking of graphene sheets: random stacking that can be observed in many carbon materials in contrast to the regular stacking of the graphite structure [22] Graphite nanoplatelets (GNP) GNP is a type of carbon-based nanoparticle in the form of platelets. These platelets are stacks of graphene sheets building up aggregates 2-15 nm thick with diameters between the sub-micron range and 50 µm. [23] As a result, the GNP particles have high aspect ratios and large surface areas. At the atomic level, graphene layers are one atom thick layer of carbon atoms, each of them sp2 hybridized and forming three covalent bonds with the adjacent carbon atoms. The atoms form a lattice of hexagons in each graphene sheet and the sheets are bonded to each other by weak van der Waals forces. Consequently, highly anisotropic properties are obtained [24]. The high aspect ratio and large surface area of GNPs promote the formation of an electrically conductive network at lower filler contents than in the case of conventional carbon black (CB) [25]. GNP also enhances the barrier properties, thermal conductivity, the surface of the matrix polymers and mechanical properties such as the stiffness or the strength [23]. GNP can be obtained by intercalation and delamination of natural graphite flakes. Atoms, ions or molecules known as intercalants can be intercalated between the parallel graphene layers giving as a result the graphite intercalation compound (GIC). The intercalants are based on strong oxidizing acids, such as sulfuric and nitric acids and metal chlorides. Vapor transport or thermal method, chemical oxidation and electrochemical methods are the typical techniques used for the intercalation. Once the intercalants are introduced between the graphene sheets, high temperatures are applied (typically 1050 C for 30s) expanding the GIC along the thickness direction by thermal shock and decomposition of the intercalants. The graphite layers are separated, the resulting material being called expanded graphite (EG). This is a porous worm rod material with weak bonds in its structure. The graphite nanoplatelets (GNP) are obtained through ultrasonication of the EG in a solvent, first breaking down the bonds of the EG flakes and after delamination giving smaller particles as a result. [24] In some investigations EG has been used as the filler for conducting polymer applications. The electrical conductivity of EG-filled polymers is two orders of magnitude higher than obtained with GNP showing at the same time poorer mechanical properties. However, composites with GNP have lower percolation thresholds due t
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