Synthetic Metals 145 (2004) 245–252 Electrically and thermally conductive polyethylene/graphite composites and their mechanical properties I. Krupa, I. Novák ∗ , I. Chodák Polymer Institute, Slovak Academy of Sciences, Dúbravská Cesta 9, 842 36 Bratislava, Slovak Republic Received 1 December 2003; received in revised form 28 April 2004; accepted 18 May 2004 Available online 19 July 2004 Abstract Electrical conductivity, thermal conductivity and ultimate mechanical properties, namely elongation at break and stress at break of low and high density polyethylene/graphite composites were investigated in this paper. Percolation concentration at about 11 vol.% of the filler for both filled polyethylene’s was found. The degree of crystallinity of polyethy- lene matrix did not have an significant influence neither the percolation concentration nor electrical conductivity of composites. DSC measurement indicated that filler has no significant influence on the change of degree of crystallinity of polyethylene matrices. The thermal conductivity of filled HDPE is higher then thermal conductivity of filled LDPE in the whole concentration range due to higher degree of crystallinity of high density polyethylene. The nonlinear behavior was observed for the dependency stress at break versus filler content. After initial decrease in stress at break, a reinforcing effect was observed. The reinforcing effect was more pronounced for HDPE matrix. © 2004 Elsevier B.V. All rights reserved. Keywords: Polyethylene/graphite composites; Thermal conductivity; Electrical conductivity; Ultimate mechanical properties 1. Introduction Graphite is naturally abundant as well as synthetically prepared inorganic material, which has significant influence on the electrical and thermal conductivity of filled materi- als, their barrier properties as well as mechanical properties. It is frequently used especially for an improvement of elec- trical conductivity, antistatic properties as well as thermal conductivity of plastics. Polymer/graphite composites have long been used in structural, aerospace and sporting goods applications [1–3]. Electroconductive polymeric composites are often used as heating elements, temperature-dependent resistors and sen- sors, self-limiting electrical heaters and switching devices, antistatic materials for electromagnetic interference shield- ing of electronic devices, etc. [4]. The importance of thermal conductivity in polymer com- posites is associated with the need for appreciable levels of thermal conductance in circuit boards, heat exchangers, ap- pliances, and machinery [5]. The of information on thermal conductivity of materials is also necessary for modeling of ∗ Corresponding author. Tel.: +421-2-5477-3308; fax: +421-2-5477-5923. E-mail address: upolnovi@savba.sk (I. Nov´ ak). optimum conditions during processing of materials, as well as for a analysis of transport of heat in materials during practical applications. The presence of graphite particles has also significant in- fluence on the mechanical behavior of filled polymers [6–8]. Physical behavior of filled LDPE and HDPE, filled with synthetic graphite particles, namely electrical conductivity, thermal conductivity and ultimate properties was investi- gated in this paper. 2. Experimental Two grades of polyethylene were used as the ma- trix, namely low density polyethylene Bralen RA 2–19 (MFI = 1.7 g/10 min, density = 0.916 g/cm 3 , particle size <50 m) from Slovnaft, Slovakia, high density polyethy- lene Stamylan HD 9089 V (MFI = 8 g/10 min, density = 0.963 g/cm 3 ) (DSM, The Netherlands), while graphite (EG-10, synthetic graphite, SGL Carbon, UK) was used as the filler. Granulometric parameters of the graphite EG-10 are summarized in Table 1. The blends were prepared by mixing both components in a 50 ml mixing chamber of Brabender Plasticorder PLE 331 at 170 ◦ C (LDPE matrix) or 200 ◦ C (HDPE matrix) for 10 min at the mixing speed 35 rpm. 0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.05.007