Borealis Borlink XLPE LS4258DC is supplied as a peroxide-containing, crosslinkable low-density polyethylene compound intended for insulation layers in extruded HVDC cable systems. Downstream cable producers validate melt rheology, cure kinetics, and residual-byproduct content on full-scale cable lines because laboratory torque-rheometer data does not capture the thermal history of a triple-head extrusion process. The application routes below are separated by installation environment and converter operating mode; each route imposes different degassing, screen compatibility, and electrical qualification constraints.

Submarine HVDC Interconnector Insulation

Submarine HVDC interconnector cable cores are extruded with LS4258DC as the primary dielectric between a coextruded semiconductive conductor screen and an insulation screen. The compound is typically processed at 100 parts per hundred resin without dilution by unfilled LDPE; reintroduction of clean edge trim is limited to a maximum of 10% by mass because partially crosslinked scrap alters melt viscosity and residual peroxide distribution. A vertical continuous vulcanization line is preferred when insulation wall thickness exceeds 20 mm, as sag in a catenary line creates radial eccentricity at the conductor interface. Melt temperature at the crosshead is held below 130°C to prevent premature decomposition of dicumyl peroxide; the screw is configured with an L/D ratio between 25:1 and 30:1 and a low compression ratio to limit shear heating. Nitrogen pressure in the curing tube is maintained at 1.0 MPa to 1.5 MPa to suppress microvoid formation during peroxide decomposition. The resulting cable core is degassed and then tested according to IEC 62895 for HVDC extruded cable systems, with additional electrical design checks under IEC 60287 for ampacity and thermal ratings. End products include subsea interconnectors operating at ±320 kV or higher, where the insulation wall, conductor cross-section, and screen grades are type-tested as a system rather than as isolated layers.

What Limits the Thermal Processing Window During Cable-Core Extrusion of LS4258DC?

The upper thermal boundary in the extrusion of LS4258DC is fixed by the decomposition behaviour of dicumyl peroxide rather than by melt flow rate alone. Dicumyl peroxide exhibits a half-life of approximately 1 hour at 135°C; at the same time, the compound must be heated sufficiently to reduce melt viscosity for uniform flow through the crosshead and around the conductor. This creates a narrow processing window with barrel zone temperatures between 110°C and 125°C and a maximum allowable melt temperature of 130°C at the die land. Production machines therefore use low-shear single-screw geometry, often with water-cooled feed throats and screen packs retained at 80/120/80 mesh to reduce frictional heating while retaining foreign particles. If the melt temperature exceeds 130°C, local scorch produces gel particles that create insulation protrusions; if the melt temperature falls below 110°C, the insulation can develop high melt pressure at the crosshead and unstable flow marks at the screen interface. The acceptable processing window is also influenced by moisture: pellets exposed to air at relative humidity above 60% should be pre-dried in a desiccant hopper at 60°C for 2 hours before extrusion, because surface moisture becomes a void source during peroxide cure. Amine-bearing additive masterbatches should not be combined with LS4258DC because amine groups accelerate peroxide decomposition and shift the scorch time downward. These limits apply to land cable insulation cores where control of insulation thickness and electrical cleanliness is verified by on-line X-ray diameter measurement and spark testing.

Underground HVDC land links place more severe degassing constraints on the insulating wall than submarine cables because the cable is often installed in duct banks with limited thermal dissipation. The insulation wall must be degassed after the continuous vulcanization line until residual byproducts such as methane, acetophenone, and cumyl alcohol are reduced below cable-specific acceptance limits. For thick-wall land cable sections, degassing chambers are typically operated at 60°C to 70°C for periods that scale with insulation thickness squared; published data for this specific compound under all land-cable wall thickness configurations is limited, so each cable manufacturer validates degassing completeness by headspace analysis or gravimetric residual measurements. The conductor screen and insulation screen are coextruded to avoid air exposure between layers, and the triple-layer structure is tested under IEC 62895 with additional thermomechanical ageing steps where the route includes buried and tunnel sections. End products include underground HVDC links for city-centre infeed and long-distance direct-current transmission corridors, where the cable system must maintain dielectric integrity under cyclic load variations and varying soil thermal resistivity.

When Bidirectional Converter Operation Triggers Space Charge Accumulation

When voltage-source converters operate with fast power-flow reversals, the insulation wall is subjected to repeated DC polarity changes and temperature cycles at the screen interface. LS4258DC is selected for these duty cycles only if the cable manufacturer can demonstrate low space charge accumulation and low apparent DC conductivity after degassing. Measurements of volume resistivity are performed according to ASTM D257 at controlled electric stress and temperature; insulation samples that retain high acetophenone content after degassing can exhibit higher apparent conductivity and should be disqualified. The insulation-to-screen interface is critical because charge injection from the semiconductive screens is a stronger variable than bulk dielectric loss. Coextruded screen materials must be matched to LS4258DC without excessive migration of ionic additives into the insulation; the interface must survive thermomechanical cycling tests under IEC 62895 qualification. End products include bidirectional HVDC links for asynchronous grid interconnection and renewable-energy export corridors, where loading direction changes at intervals below 1 hour under some converter dispatch schedules.

A Triple Head Applies Conductor Screen, Insulation, and Insulation Screen Without Intermediate Cooling

Offshore wind export cable production uses triple-head extrusion in which LS4258DC is delivered to the middle extruder of a triple crosshead while semiconductive compounds are fed to the inner and outer layers. All three layers are combined in the head at melt temperatures below 130°C before entering the high-pressure nitrogen curing tube. The triple-head route avoids extended exposure of the insulation surface to ambient air and reduces particulate contamination between layers. Offshore wind export cable production for high-voltage direct-current systems prioritizes long continuous lengths without radial interface voids; therefore, conductor preheating is adjusted to 80°C to 120°C to equalize thermal contraction during cooling. The insulation wall in submarine export cables is designed for high hydrostatic pressure at water depths up to 1000 m; published data for LS4258DC in dynamic submarine cable applications is limited, and static export cable designs are more common. The finished cable is subjected to partial discharge, dissipation factor, and DC withstand tests according to customer specification and IEC 62895, with end products serving offshore wind farm export circuits and island interconnections.

Power-from-shore HVDC feeders to offshore oil and gas production platforms use LS4258DC where the insulation must tolerate long-term intermittent loading and salt-spray-exposed terminations. The insulation wall is typically thinner than that of submarine interconnectors because the transmitted power is lower, but the cable length can exceed 100 km in remote field developments. Processing of thinner insulation layers allows higher line speed; however, the cable core must still be degassed until residual methane and acetophenone are reduced below acceptance limits to avoid space charge accumulation. The cable is qualified under IEC 62895 with additional tests for fire performance where platform topside sections are installed indoors; hydrocarbon exposure and flame propagation requirements may invoke IEC 60332 or IEC 60754 depending on project specification. End products are high-voltage direct-current feeders connecting shore-based converters to offshore oil and gas installations, where the insulation system replaces traditional gas-insulated or mass-impregnated cable designs in some brownfield retrofits.