High-temperature creep resistant ternary blends based on polyethylene and polypropylene for thermoplastic power cable insulation

The impact of a small amount of polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) on the thermomechanical and electrical properties of blends comprising low-density polyethylene (LDPE) and isotactic polypropylene (PP) is investigated. SEBS is found to assemble at the PP:LDPE interface as well as within isolated PP domains. The addition of 10\ua0wt% SEBS significantly increases the storage modulus between the melting temperatures of the two polyolefins, 110 and 160\ub0C, and results in improved resistance to creep during both tensile deformation as well as compression. Furthermore, the ternary blends display a very low direct-current (DC) conductivity as low as 3.4 7 10 \ua0S m at 70\ub0C and 30 kV mm , which is considerably lower than values measured for neat LDPE. The here presented type of ternary blend shows potential as an insulation material for high-voltage direct current power cables

Highly insulating thermoplastic nanocomposites based on a polyolefin ternary blend for high-voltage direct current power cables

Octyl-silane-coated Al2O3 nanoparticles are found to be a promising conductivity-reducing additive for thermoplastic ternary blends comprising low-density polyethylene (LDPE), isotactic polypropylene and a styrenic copolymer. The ternary blend nanocomposites were prepared by compounding the blend components together with an LDPE-based masterbatch that contained the nanoparticles. The nanoparticles did not affect the superior stiffness of the ternary blends, compared to neat LDPE, between the melting temperatures of the two polyolefins. As a result, ternary blend nanocomposites comprising 38 wt% polypropylene displayed a storage modulus of more than 10 MPa up to at least 150 degrees C, independent of the chosen processing conditions. Moreover, the ternary blend nanocomposites featured a low direct-current electrical conductivity of about 3 x 10(-15) S m(-1) at 70 degrees C and an electric field of 30 kV mm(-1), which could only be achieved through the presence of both polypropylene and Al2O3 nanoparticles. This synergistic conductivity-reducing effect may facilitate the design of more resistive thermoplastic insulation materials for high-voltage direct current (HVDC) power cables

Nanocomposites and polyethylene blends: two potentially synergistic strategies for HVDC insulation materials with ultra-low electrical conductivity

Among the various requirements that high voltage direct current (HVDC) insulation materials need to satisfy, sufficiently low electrical conductivity is one of the most important. The leading commercial HVDC insulation material is currently an exceptionally clean cross-linked low-density polyethylene (XLPE). Previous studies have reported that the DC-conductivity of low-density polyethylene (LDPE) can be markedly reduced either by including a fraction of high-density polyethylene (HDPE) or by adding a small amount of a well dispersed, semiconducting nanofiller such as Al2O3 coated with a silane. This study demonstrates that by combining these two strategies a synergistic effect can be achieved, resulting in an insulation material with an ultra-low electrical conductivity. The addition of both HDPE and C8–Al2O3 nanoparticles to LDPE resulted in ultra-insulating nanocomposites with a conductivity around 500 times lower than of the neat LDPE at an electric field of 32 kV/mm and 60–90 \ub0C. The new nanocomposite is thus a promising material regarding the electrical conductivity and it can be further optimized since the polyethylene blend and the nanoparticles can be improved independently

Highly insulating thermoplastic blends comprising a styrenic copolymer for direct-current power cable insulation

The impact of the composition of blends comprising low-density polyethylene (LDPE), isotactic polypropylene (PP) and a styrenic copolymer additive on the thermomechanical properties as well as the direct-current (DC) electrical and thermal conductivity is investigated. The presence of 5 weight percent (wt%) of the styrenic copolymer strongly reduces the amount of PP that is needed to enhance the storage modulus above the melting temperature of LDPE from 40 to 24 wt%. At the same time, the copolymer improves the consistency of the thermomechanical properties of the resulting ternary blends. While both the DC electrical and thermal conductivity strongly decrease with PP content, the addition of the styrenic copolymer appears to have little influence on either property. Evidently, PP in combination with small amounts of a styrenic copolymer not only allows to reinforce LDPE at elevated temperatures but also functions as an electrical conductivity-reducing additive, which makes such thermoplastic ternary formulations possible candidates for the insulation of high-voltage power cables

Repurposing Poly(3-hexylthiophene) as a Conductivity-Reducing Additive for Polyethylene-Based High-Voltage Insulation

Poly(3-hexylthiophene) (P3HT) is found to be a highly effective conductivity-reducing additive for low-density polyethylene (LDPE), which introduces a new application area to the field of conjugated polymers. Additives that reduce the direct-current (DC) electrical conductivity of an insulation material at high electric fields have gained a lot of research interest because they may facilitate the design of more efficient high-voltage direct-current power cables. An ultralow concentration of regio-regular P3HT of 0.0005 wt% is found to reduce the DC conductivity of LDPE threefold, which translates into the highest efficiency reported for any conductivity-reducing additive to date. The here-established approach, i.e., the use of a conjugated polymer as a mere additive, may boost demand in absolute terms beyond the quantities needed for thin-film electronics, which would turn organic semiconductors from a niche product into commodity chemicals

Mechanical Analysis of Melamine-Formaldehyde Composites

Planar random fibre composite materials based on melamine-formaldehyde (MF), one of the hardest and stiffest isotropic polymers that is known, have been developed recently on an industrial scale. In this thesis the mechanical behaviour of these materials is systematically investigated for the first time. Based on microstructural observations, a model predicting stiffness of these ternary materials with various resin/filler/fibre compositions is developed. The results are in agreement with experimental data. In addition, for practical purposes, merit indices for optimising the composition towards minimised weight and material cost are introduced, thus extending the use of the developed model. The effect of cure temperature and amount of catalyst on the rheokinetical behaviour of the MF resin is investigated using a technique called Torsional Substrate Analysis (TSA), developed for this purpose. TSA measurements revealed several transitions. Rheokinetical data are used to construct Time-Temperature-Transformation (TTT) cure diagrams. High Pressure Differential Scanning Calorimetry (HPDSC) measurements are used to estimate the fractional conversion. Tg of the resin is discussed. Damage mechanisms in MF composites with inorganic and organic constituents are studied by scanning electron microscopic observations directly under tensile load (in-situ SEM). Debonding of inorganic filler particles and glass fibres is observed on straining. Critical cracks mainly originate at debonded fibres and at fibres located close to the sample surface. On the other hand, debonding of cellulose is not observed. Damage initiation and development are evaluated by conducting cyclic tensile tests with a systematically increasing maximum strain. The unloading modulus is used to define a damage parameter. The resin/filler/fibre composition and the architecture of glass fibre reinforcement are found to have a strong influence on damage behaviour and performance of MF composites. The mechanical behaviour of several flax fibre reinforced MF composites is studied and compared to that of a glass fibre reinforced grade. Although replacing glass with flax fibres has a somewhat negative effect on tensile performance the difference is small and is compensated for by the lower density and cost, making flax fibre composites a competitive material. Compared to glass, flax fibres constitute a material with a considerably lower damage rate. The work can serve as a foundation for developing MF composites towards various applications and requirements

Mechanical Analysis of Melamine-Formaldehyde Composites

Planar random fibre composite materials based on melamine-formaldehyde (MF), one of the hardest and stiffest isotropic polymers that is known, have been developed recently on an industrial scale. In this thesis the mechanical behaviour of these materials is systematically investigated for the first time.<p /> Based on microstructural observations, a model predicting stiffness of these ternary materials with various resin/filler/fibre compositions is developed. The results are in agreement with experimental data. In addition, for practical purposes, merit indices for optimising the composition towards minimised weight and material cost are introduced, thus extending the use of the developed model.<p /> The effect of cure temperature and amount of catalyst on the rheokinetical behaviour of the MF resin is investigated using a technique called Torsional Substrate Analysis (TSA), developed for this purpose. TSA measurements revealed several transitions. Rheokinetical data are used to construct Time-Temperature-Transformation (TTT) cure diagrams. High Pressure Differential Scanning Calorimetry (HPDSC) measurements are used to estimate the fractional conversion. T_g of the resin is discussed.<p /> Damage mechanisms in MF composites with inorganic and organic constituents are studied by scanning electron microscopic observations directly under tensile load (in-situ SEM). Debonding of inorganic filler particles and glass fibres is observed on straining. Critical cracks mainly originate at debonded fibres and at fibres located close to the sample surface. On the other hand, debonding of cellulose is not observed. Damage initiation and development are evaluated by conducting cyclic tensile tests with a systematically increasing maximum strain. The unloading modulus is used to define a damage parameter. The resin/filler/fibre composition and the architecture of glass fibre reinforcement are found to have a strong influence on damage behaviour and performance of MF composites.<p /> The mechanical behaviour of several flax fibre reinforced MF composites is studied and compared to that of a glass fibre reinforced grade. Although replacing glass with flax fibres has a somewhat negative effect on tensile performance the difference is small and is compensated for by the lower density and cost, making flax fibre composites a competitive material. Compared to glass, flax fibres constitute a material with a considerably lower damage rate.<p /> The work can serve as a foundation for developing MF composites towards various applications and requirements