Borealis HDPE VL4580 is a black, UV-stabilised high-density polyethylene compound supplied for extrusion of cable sheathing and microduct profiles. The grade is produced with a bimodal molecular weight distribution in which a high molar mass fraction and a lower molar mass fraction are combined during polymerisation. Typical lot-release data include a density of 0.958 g/cm³ when measured by ISO 1183-1, a melt flow index of 0.7 g/10 min at 190 °C/2.16 kg in accordance with ISO 1133-1, and a tensile yield stress of 23 MPa determined by ISO 527-2. The compound contains approximately 2.5 wt% carbon black as determined by ASTM D1603-14, which provides UV stabilisation and outdoor resistance. It is supplied as a sheathing compound rather than a pipe resin, and it is not formulated for low-smoke zero-halogen or flame-retardant cable constructions. Typical application targets include outer sheaths for loose-tube fibre optic cables, copper pair telecommunications cables, and high-density polyethylene microduct tubes installed by air blowing.

The bimodal architecture of VL4580 distinguishes it from unimodal HDPE jacket grades of similar melt flow index. In a unimodal HDPE, increasing molecular weight improves slow crack resistance but raises melt viscosity and screw torque. The bimodal design allows the high molar mass fraction to raise the tie-molecule density between crystal lamellae while the low molar mass fraction acts as an internal processing aid. The result is a shear-thinning melt that can be processed at lower head pressure than a unimodal resin of equivalent density and melt flow index. This distinction is most visible in environmental stress crack resistance and in the stability of tube dimensions during high-speed sheathing.

How Does VL4580 Address Slow Crack Growth in Buried Cable Ducts?

Slow crack growth in buried high-density polyethylene is governed by the ability of the amorphous tie-chain network to resist fibril formation ahead of a craze zone. The notched bent-strip test under ASTM D1693-15 evaluates this failure mode by exposing specimens to a 10% solution of Igepal CO-630 at 50 °C. VL4580 typically exhibits an F₅₀ greater than 1000 h under these conditions, whereas a conventional unimodal HDPE cable jacket material with a comparable melt flow index may fail between 50 h and 200 h. The bimodal molar mass distribution is the principal reason for this difference because the high molecular weight species increases the density of stress-transmitting tie molecules in the interlamellar regions. In cable duct applications, residual curvature from coiling and the presence of alkaline soil water create a mechanically aggressive environment. The high ESCR value is therefore relevant not only to fracture mechanics but also to long-term duct integrity after repeated thermal expansion and contraction.

The following table summarises class-typical values for VL4580 against a reference unimodal HDPE cable sheathing compound. Values are representative engineering comparisons and should be verified against the supplier’s current lot-release documentation.

On single-screw extrusion lines for cable jackets, melt temperature control is more significant than barrel set points. VL4580 is suitable for screws with L/D ratios from 24:1 to 30:1 and compression ratios of 3:1 to 4:1. A die-entry melt temperature of 200 °C to 230 °C is recommended to balance thermal homogeneity and stabiliser consumption. Melt temperatures above 240 °C can shorten the oxidative induction time measured at 200 °C by ISO 11357-6. Feed-throat temperature should remain below 50 °C to avoid pellet bridging. If the material has been stored at relative humidity above 60%, pre-drying at 70 °C for 2 h is required to prevent surface defects. Screen-pack pressure drop should not exceed 350 bar during normal production because excessive pressure can increase melt temperature and degrade the stabiliser package. Melt pumps may be used to reduce pressure pulsation, provided the inlet pressure to the pump is monitored to avoid starvation.

Maximum melt residence time at 230 °C should be kept below 20 min because prolonged exposure to residual oxygen can consume the phenolic antioxidant. When the line is interrupted, the barrel temperature should be reduced to 160 °C or the screw should be kept rolling at low speed to avoid stagnant melt zones. On restart, the first 5 kg of extrudate is typically discarded until the melt is free of discoloration and the jacket surface is uninterrupted. These operating limits derive from the oxidative stability of the stabiliser package and are not a replacement for line-specific validation.

Regrind addition from start-up and trimming scrap may be used up to 20 wt% when the material is clean, dry, and free of copper fines or optical fibre glass. The melt flow index of the regrind should be checked at 190 °C/2.16 kg according to ISO 1133-1; a shift of more than 20% from the virgin value indicates thermal or shear damage. Mixing with LLDPE or EVA reduces modulus and increases flexibility, and such blends should be validated against the complete cable specification before use. Incoming-lot testing should include melt flow index, density, and oxidative induction time because these parameters are not guaranteed by a single melt flow measurement. A melt flow index variation of ±0.1 g/10 min at 190 °C/2.16 kg is generally acceptable for jacket lines, but lines running at high take-off speed may show diameter instability when the value drifts to the upper or lower limit. Carbon black dispersion is a separate release criterion: a poorly dispersed batch can pass carbon black content testing while still generating surface defects and reduced weathering life.

Mechanical Property Retention After Thermal Ageing and UV Exposure

The stabilisation system combines dispersed carbon black with phenolic and phosphite antioxidants. Carbon black content is not a sufficient predictor of weathering performance; dispersion quality controls micro-agglomerate formation that can initiate surface cracks. Dispersion is assessed by ISO 18553 or a pressure-rise filtration test in the melt. Virgin oxidative induction time at 200 °C measured by ISO 11357-6 is typically above 30 min. After thermal ageing, tensile retention is measured by ISO 527-2 and may be reported against the cable product standard or IEC 60811-501. Accelerated weathering is conducted using ISO 4892-2 method A with moisture and xenon-arc exposure. The product is intended for black service; non-black UV-stabilised variants are not commonly available because carbon black is the primary UV stabiliser. Continuous conductor temperature is limited by the thermoplastic nature of HDPE; applications above 75 °C conductor temperature should use a crosslinked polyethylene jacket rather than VL4580.

At sub-ambient installation temperatures, the flexural modulus of 950 MPa determined by ISO 178 provides higher crush resistance than LLDPE sheathing compounds. The ductile-to-brittle transition is suppressed by the high molar mass fraction; brittleness temperature by ASTM D746 is typically below -70 °C. Stress whitening can still occur when the cable is bent beyond the minimum bend radius at temperatures below -20 °C, which indicates localised yielding of the outer sheath. For direct-buried cables, compression loads from stones and freeze-thaw cycles are resisted by the hoop stiffness of the sheath, and the wall thickness should be calculated from the maximum expected soil load rather than standard jacket thickness alone.

When a Cable Sheathing Compound Is Selected for High-Speed Fibre Optic Microduct Extrusion

In high-speed microduct extrusion, melt strength and draw-down stability define the practical operating window. VL4580 has higher melt strength than an LLDPE of similar melt flow index, which stabilises tube dimensions at take-off speeds above 100 m/min. The same property restricts attenuation to thin skins; the minimum skin thickness for smooth surface and complete carbon black coverage is approximately 0.15 mm. For coextrusion with foamed or solid polyolefin core layers, die land length should be set to at least 10 times the annular gap to reduce melt fracture. Shrinkback of the finished microduct is measured after conditioning at elevated temperature according to IEC 60811-501 or the relevant cable product specification, and the HDPE sheath must be allowed to relax before cutting to length. The higher modulus of VL4580 improves crush resistance but increases the minimum bend radius at installation temperatures below 5 °C.

In comparison to peroxide-crosslinked polyethylene jackets, VL4580 has lower hot deformation resistance and a lower continuous conductor temperature limit. It is not a substitute for XLPE in cables rated above 90 °C conductor temperature. However, it does not require peroxide dosing or a continuous vulcanisation line, and clean regrind can be reintroduced into the same extrusion process. Heat deformation is not evaluated by hot-set testing because the material is not crosslinked; Vicat softening temperature according to ISO 306/A50 is a more relevant indication of short-term heat resistance. The specific Vicat value for VL4580 is typically 127 °C. The compound also differs from LLDPE jacketing materials by its lower elongation at break and higher modulus, which makes it more suitable for applications where dimensional stability under compressive load is required rather than flexible reeling.

Chemical resistance of VL4580 follows the behaviour of the polyethylene matrix. The material withstands dilute aqueous acids, alkalis, and saline solutions at ambient temperature, which supports direct burial in corrosive soils. Prolonged contact with concentrated oxidising acids, free halogens, and aromatic hydrocarbons is not recommended; published data for this specific grade in continuous contact with strong solvents is limited, and compatibility testing should follow ISO 175 by measuring mass change and tensile retention after immersion. The stabiliser package is also sensitive to amine-based additives that can reduce oxidative stability; such additives should not be blended without repeating the oxidation induction time measurement. The product is a cable sheathing compound and should not be assigned a hydrostatic pipe design basis under ISO 9080 unless the complete pipe-grade certification is available. No potable-water pressure certification should be implied from the density or ESCR values.

Compatibility with cable filling compounds is evaluated by immersion testing at 70 °C for 7 days using ISO 175. Published data for this specific grade with every commercial filling gel is limited; qualification should use the actual filling compound and measure tensile retention and mass change. Failures in copper cable jackets are often caused by environmental stress cracking at the interface between the sheath and a filling compound that contains surface-active components, rather than by bulk chemical dissolution. The ESCR performance of VL4580 reduces this failure likelihood only when the sheath is properly cooled and not stretched during the filling operation.