Effects of Expandable Graphite at Moderate and Heavy Loadings on the Thermal and Electrical Conductivity of Amorphous Polystyrene and Semicrystalline High-Density Polyethylene

Abstract

In this work, we prepared and investigated two series of polymer composites, wherein the matrix was either an amorphous polystyrene (PS) or a semicrystalline high-density polyethylene (HDPE) filled with expandable graphite (EGr) at relatively high loadings within the range 5–55 wt %. For the investigation we employed a thermogravimetric analysis and differential scanning calorimetry to assess the thermal transitions and evaluate the various polymer fractions (crystalline (CF), mobile (MAF) and rigid amorphous (RAF)) in addition to broadband dielectric spectroscopy and a laser flash analysis to evaluate the EGr effects on electrical conductivity, σ, and thermal conductivity, λ, respectively. In PS, EGr was found to impose an increase of the glass transition temperature and a systematic decrease of the corresponding heat capacity change. The latter was rationalized in terms of the formation of an interfacial RAF. No glass transition was recorded for HDPE whereas the fillers increased the CF moderately. As expected, σ increased with the filler loading for both matrices, up to 10−3–10−2 S/cm, resulting in a conductive percolation threshold for electrons at > 8 wt % EGr. Simultaneously, the λ of PS and HDPE were strongly increased, from 0.13 and 0.38 W·K–1·m–1 up to 0.55 and ~2 W·K–1·m–1, respectively. λ demonstrated an almost linear EGr loading dependence whereas the semicrystalline composites exhibited a systematically higher λ.

Aryloxyphosphazene-Modified and Graphite-Filled Epoxy Compositions with Reduced Flammability and Electrically Conductive Properties

“…Specific volume electrical resistance was lowered by five orders of magnitude compared to the previously obtained compositions containing graphite [47]. The low value of specific volume electrical resistance of filled compositions and, consequently, high electrical conductivity are due to screening-oriented electron transfer through thermally expanded graphite due to hopping and tunneling mechanisms [48]. In the case of unfilled samples, the cured compositions are dielectrics.…”

Optimization of Thermal Conductivity and Tensile Properties of High-Density Polyethylene by Addition of Expanded Graphite and Boron Nitride

“…This value showcases a significant 137% increase when compared to the study conducted by Soboliček et al, even when employing only half the quantity of EG. A slightly higher percentage of EG (55 wt.%) in the HDPE matrix resulted in an increase in the thermal conductivity of composites (1.97 W/mK), as evidenced in a study by Klonos et al [13], which can be compared with HDPE/EG15/BN15 regardless of the size of BN particles. Research on composites with BN as an additive show similar thermal properties to EG at the same percentage.…”

“…EG treated with poly(vinyl-alcohol) shows an increase of thermal conductivity in the polymer, even at low percentages of additives, which was researched by Yin et al [12]. According to Panagiotis et al, the polystyrene (PS) matrix, compared to the HDPE matrix with EG additive, results in a multiple-fold increase in thermal conductivity [13]. A similar procedure and parameters were applied (temperature of kneader chamber of 185 • C, at 60 rpm/min, and for 10 min) for mixing HDPE/EG/carbon nano tubes (CNT) composite, with EG up to 20 wt.% and CNT up to 3 wt.%.…”

Structure–property relationships in renewable composites of poly(lactic acid) reinforced by low amounts of micro- and nano-kraft-lignin

“…The most significant alternations therein involve the significant roles of the rubbery vs. the glassy state (ionic conductivity) and that of the amorphous vs. the semicrystalline state of the polymer (heat transport). 26,89–92 In this connection, the properties of the initial PLA and that of lignin were found to have an indirect effect, namely, on the T g and the semicrystalline morphology. Please note that the various effects recorded on the ions or heat transfer demonstrate the potential to tune them via processing and the nanocomposite composition.…”