The insulation of state-of-the-art extruded highvoltage direct-current (HVDC) power cables is composed of cross-linked low-density polyethylene. Driven by the search for sustainable energy solutions, concepts that improve the ability to withstand high electrical fields and, ultimately, the power transmission efficiency are in high demand. The performance of a HVDC insulation material is limited by its residual electrical conductivity. Here, we demonstrate that the addition of small amounts of high-density polyethylene (HDPE) to a low-density polyethylene (LDPE) resin results in a drastic reduction in DC conductivity. An HDPE content as low as 1 wt % is found to introduce a small population of thicker crystalline lamellae, which are finely distributed throughout the material. The change in nanostructure correlates with a decrease in DC conductivity by approximately 1 order of magnitude to about 10 −15 S m −1 at high electric fields of 30 and 40 kV mm −1 and elevated temperature of 70 °C. This work opens up an alternative design concept for the insulation of HVDC power cables.
The most common type of extruded
power cable insulation is based
on cross-linked polyethylene (XLPE), which cannot be recycled as a
thermoplastic material. Hence, thermoplastic insulation materials
currently receive considerable attention because they would allow
recycling through re-melting. In particular blends of polyethylene
(PE) and polypropylene (PP) would be a compelling alternative to XLPE,
provided that the poor compatibility of the two polymers can be overcome.
Here, we establish an alternative approach that exploits the in situ
formation of a PE–PP-type copolymer through reactive compounding.
Ternary blends of an ethylene-glycidyl methacrylate copolymer, a maleic
anhydride-grafted polypropylene, and up to 70 wt % low-density polyethylene
(LDPE) are compounded at 170 °C. Covalent bonds form through
reaction between epoxy and carboxyl groups, leading to a PE–PP-type
copolymer that shows good compatibility with LDPE. The in situ generated
PE–PP copolymer arrests creep above the melting temperature
of LDPE, mediated by a continuous network that is held together by
PP crystallites. Recyclability is confirmed by reprocessing at 170
°C. Furthermore, the here investigated formulations feature a
low direct-current electrical conductivity of ∼4 × 10–14 S m–1 at 70 °C and 30 kV
mm–1, on a par with values measured for LDPE and
XLPE. Evidently, in situ formation of a PE–PP-type copolymer
through reactive compounding is a promising approach that may enable
the design of thermoplastic insulation materials for power cables.
The mechanical performance of natural fiber reinforced polymers is often limited owing to a weak fiber‐matrix interface. In contrast, melamine‐formaldehyde (MF) resins are well known to have a strong adhesion to most cellulose containing materials. In this Paper, nonwoven flax fiber mat reinforced and particulate filled MF composites processed by compression molding are studied and compared to a similar MF composite reinforced with glass fibers. Using flax instead of glass fibers has a somewhat negative effect on tensile performance. However, the difference is relatively small, and if density and material cost are taken into account, flax fibers become competitive. Tensile damage is quantified from the stiffness reduction during cyclic straining. Compared to glass fibers, flax fibers generate a material with a considerably lower damage rate. From scanning electron microscopy (SEM), it is found that microcracking takes place mainly in the fiber cell walls and not at the fiber‐matrix interface. This suggests that the fiber‐matrix adhesion is high. The materials are also compared using dynamic mechanical thermal analysis (DMTA) and water absorption measurements.
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