Cross-Linked Cable Compounds: How Cross-Linking Works and Why It Matters

Cross-linked cable compounds are produced by starting with a thermoplastic or elastomeric base polymer and then introducing chemical bonds between adjacent polymer chains. In an un-cross-linked (thermoplastic) polymer, the chains slide freely past one another when heated, which is why thermoplastics soften and flow above their melting point. Once cross-linked, the inter-chain bonds prevent this flow, turning the material into a thermoset network that can be deformed elastically under stress and recovers its shape, but will not melt or flow even at temperatures well above the base polymer's original melting point. This property transformation is the fundamental reason cross-linked cable compounds are specified for applications requiring sustained high-temperature performance, resistance to short-circuit heating events, and mechanical integrity during and after a fire.

Performance Improvements Delivered by Cross-Linking

The cross-linking of cable insulation and jacket compounds delivers measurable improvements across multiple performance parameters:

• Elevated temperature rating: XLPE (cross-linked polyethylene) achieves a continuous conductor temperature rating of 90 degrees Celsius for standard grades and 150 degrees Celsius for high-temperature grades, compared to 70 to 75 degrees Celsius for un-cross-linked LDPE or HDPE insulations of equivalent thickness. This improvement directly translates to higher current-carrying capacity for the same conductor size.
• Short-circuit temperature withstand: Cross-linked cable compounds maintain mechanical integrity at short-circuit conductor temperatures of 250 degrees Celsius (for XLPE), compared to only 160 degrees Celsius for PVC and 130 degrees Celsius for thermoplastic PE. This prevents insulation deformation and inter-conductor contact during short-circuit events, which could otherwise cause fire.
• Resistance to environmental stress cracking: Un-cross-linked polyethylene can crack when simultaneously under mechanical stress and in contact with certain chemical environments (oils, surfactants, pipe compounds). Cross-linking eliminates this susceptibility by restricting the chain mobility that allows crack initiation and propagation.
• Retention of mechanical properties after ageing: Standard IEC 60811-401 ageing tests place cable specimens in air ovens at elevated temperatures for extended periods (168 hours at 100 to 150 degrees Celsius depending on grade) and measure the retained tensile strength and elongation. Cross-linked cable compounds typically retain over 70% of their original tensile strength and elongation after accelerated ageing tests that cause thermoplastic compounds to fail completely.

LSZH Compound: Formulation, Fire Safety Standards, and Why Halogen-Free Matters

LSZH Compound is formulated by replacing the halogen-containing polymers (most commonly PVC, which contains approximately 57% chlorine by weight) with halogen-free base polymers and adding high loadings of inorganic mineral flame retardants to compensate for the loss of the halogen-based flame suppression mechanism. The base polymers used in LSZH formulations include ethylene vinyl acetate (EVA), ethylene methyl acrylate (EMA), ethylene butyl acrylate (EBA), polyolefin elastomers (POE), and linear low-density polyethylene (LLDPE), often blended together to achieve the target combination of flexibility, mechanical strength, and processability.

The Flame Retardant Mechanism in LSZH Compounds

The primary flame retardant in most LSZH Compound formulations is aluminium trihydroxide (ATH, also called aluminium hydroxide) or magnesium dihydroxide (MDH, also called magnesium hydroxide). Both work through the same endothermic decomposition mechanism: when heated to their decomposition temperature (ATH at 180 to 220 degrees Celsius, MDH at 300 to 340 degrees Celsius), they release chemically bound water vapour. This water release is strongly endothermic, absorbing heat from the flame and reducing the temperature of the burning material. The released water vapour also dilutes the combustible gases and oxygen in the flame zone, further suppressing combustion.

MDH is preferred over ATH when the cable jacket must be processed at temperatures above 200 degrees Celsius (which would cause ATH to begin decomposing prematurely in the extruder). The higher decomposition onset of MDH (300 degrees Celsius) allows extrusion at the higher temperatures required for certain base polymer blends without undesirable water release in the extruder barrel. The trade-off is that MDH costs approximately 1.5 to 2.5 times more than ATH at equivalent flame retardant performance loadings.

Key Standards That LSZH Compounds Must Meet

A compliant LSZH Compound for cable applications must satisfy a series of fire performance tests that together define the LSZH classification:

• IEC 60754-1 (halogen content test): The compound is combusted in a tube furnace and the resulting gases are absorbed in water. The resulting solution must have a pH of greater than 4.3 and a conductivity below 10 microsiemens per millimetre, confirming halogen content below the LSZH threshold.
• IEC 60754-2 (acidity of combustion gases): Measures the acidity index of the gas mixture produced during combustion, with a minimum pH of 4.3 required for LSZH designation, ensuring that the gas mixture is not corrosively acidic to electronic equipment in the vicinity of the fire.
• IEC 61034-2 (smoke density, 3-metre cube test): A cable bundle is burned in a 3-metre cube chamber and the minimum light transmission through the smoke is measured. LSZH-compliant cables must achieve at least 60% light transmission, ensuring that escape routes remain visible during a fire.
• IEC 60332-1 and IEC 60332-3 (flame propagation): Series -1 tests single cable vertical flame propagation; series -3 tests bunched cables in categories A (the most demanding, 7 litres of fuel per metre per minute) through D. The LSZH Compound must achieve the required flame spread category for the installation class specified in the building or infrastructure specification.
• EN 50399 (European CPR classification): The EU Construction Products Regulation requires cables installed in buildings to be classified under EN 50399 and EN 50575 in categories Aca through Fca based on combined fire performance (heat release, smoke production, and flaming droplets). Most LSZH data cable products target Dca, s1, d1, a1 or better classification for use in public buildings and transport infrastructure.

MPO LSZH Optical Cable Compound: Specific Requirements for High-Density Fibre Systems

An MPO LSZH optical cable compound is a specialised formulation developed specifically for the outer jacket and inner buffer tube or sub-unit jacket materials used in MPO (Multi-fiber Push-On) fibre optic trunk cables and high-density fibre distribution cables. MPO cables are the backbone of modern data centre interconnect infrastructure, where 12-fibre, 24-fibre, and 72-fibre MPO trunk cables replace large bundles of individual fibre patch cords, significantly reducing installation time and increasing port density in fibre management panels.

The compound requirements for MPO cable constructions are distinct from those of conventional copper data cables in several important ways. First, the outer jacket of an MPO trunk cable must protect multiple individually jacketed fibre sub-units without imposing micro-bending stress on the optical fibres inside, because micro-bending increases insertion loss and reduces signal integrity across the link. Second, the compound must extrude cleanly to thin wall thicknesses (outer jacket wall thickness of 0.5 to 1.5 mm for compact MPO cables) without voids or surface irregularities that would affect dimensional conformity. Third, the compound must meet the fire safety standards required for the installation environment (typically IEC 60332-3 Category C or Category B, and EN 50399 Dca or Cca class for European installations, or NFPA 262 Steiner Tunnel rating for North American riser or plenum installations).

Comparing MPO LSZH Optical Cable Compound to Standard LSZH Jacket Compounds

While both product types fall under the LSZH classification, the MPO LSZH optical cable compound differs from a general-purpose LSZH Compound for copper cable jacketing in several specific ways:

• Lower filler loading: Copper cable LSZH compounds can accommodate ATH or MDH loadings of 50 to 65% by weight because the copper conductors inside are mechanically robust and are not degraded by the increased compound stiffness that high filler loadings produce. MPO optical cable compounds must limit filler loading to typically 40 to 55% by weight to maintain the flexibility and low modulus needed to protect glass fibres from micro-bending. This lower filler loading requires more sophisticated flame retardant synergist systems (such as char-forming additives, smoke suppressants, or nano-fillers) to compensate and still meet fire test requirements.
• Better surface finish and printability: MPO cable outer jackets must carry clear sequential metre-marking print and cable identification codes that remain legible throughout the cable's service life. The compound surface must be smooth and sufficiently polar to accept UV-cured inkjet or hot-stamp printing without primer, which places additional requirements on the compound's surface energy and the absence of surface-bloom from wax or silicone processing aids at the concentrations used in some copper cable LSZH formulations.
• Tear resistance and field handling: MPO trunk cables are frequently handled, bent, and pulled through tight cable trays and conduits during data centre installation. The MPO LSZH optical cable compound must achieve a minimum tear strength of 10 to 15 N/mm to resist jacket splitting during installation without requiring excessive thickness, which would increase the cable outer diameter and reduce the number of cables that can fit in a cable management pathway.

Frequently Asked Questions About Cable Compound Selection