Crosslinked polyethylene cable insulation is one of those materials nobody notices until it fails. The difference between a 20-year cable and a 40-year cable is often a fraction of a percent of peroxide, dispersed evenly, decomposed at the right temperature, and degassed properly afterwards.
This guide is about the practical side of dicumyl peroxide (DCP, CAS 80-43-3) in XLPE insulation and PEX-a pipe: how the compound is built, what the continuous vulcanisation line actually does, why degassing matters, and where the process goes wrong.
Why peroxide crosslinking for cable
Medium and high-voltage cable insulation has to hold its shape and dielectric strength at conductor temperatures of 90 deg C continuous and 250 deg C on short circuit. Thermoplastic PE cannot – it softens and deforms. Crosslinking locks the chains into a network, and the network type decides the temperature rating.
| Crosslinking route | Network | Typical application |
|---|---|---|
| Peroxide (DCP) | Direct C-C bonds between chains | MV/HV cable insulation, PEX-a pipe |
| Silane (VTMS) | Si-O-Si bridges | LV cable, domestic pipe (PEX-b) |
| Irradiation | C-C bonds, formed by electron beam | Thin-wall wire, heat-shrink tubing |
Peroxide crosslinking dominates MV/HV because it delivers high crosslink density in thick sections without the thickness limit that constrains electron-beam curing.
Building the compound
| Component | Function | Practical notes |
|---|---|---|
| LDPE / LLDPE base | Insulation polymer | Clean, low-gel resin; dielectric grade |
| DCP | Radical source for crosslinking | Usually well under 2% of the compound; dispersion is critical |
| Antioxidant | Long-term thermal stability | Competes with DCP for radicals – balance the package |
| Copper deactivator | Protects against copper-catalysed oxidation | Essential in insulation in contact with a copper conductor |
| Tree-retardant additives | Water-tree resistance | For wet-environment cables (TR-XLPE) |
| Processing aids | Extrusion quality | Must not react with the peroxide |
Two rules dominate formulation. First, antioxidant and peroxide compete: the antioxidant scavenges the radicals you need for crosslinking, so raising one forces you to revisit the other. Second, dispersion is everything – a DCP agglomerate is a scorch point, a void and a dielectric defect in one.
What the CV line does
Continuous vulcanisation is a long, pressurised tube. The extruded cable enters, is heated to decomposition temperature of the peroxide, held long enough for the crosslink to develop, then cooled under pressure so the insulation does not expand and tear.
| Stage | What happens | Failure mode if wrong |
|---|---|---|
| Extrusion | Compound melted and shaped onto the conductor | Scorch in the extruder if the melt runs too hot |
| Pressurised heating | DCP decomposes; radicals form; C-C bonds develop | Under-cure – low gel content, poor hot-set |
| Cooling under pressure | Network fixed; volatile fragments stay dissolved | Decompression voids if pressure is released too early |
| Degassing | Acetophenone and other volatiles diffuse out | Odour, bubbles on subsequent heating, dielectric loss |
Degassing: the step that gets skipped
The decomposition of DCP leaves acetophenone and related fragments dissolved in the insulation. If they stay there, they cause a characteristic smell, can form voids when the cable heats up in service, and degrade dielectric performance. Degassing – holding the cable in a warm chamber for a period to let the volatiles diffuse out – is a standard step, not an optional one.
The practical consequence for a compounder: the cycle time of the whole line includes degassing, and a change in peroxide dose changes the degassing requirement.
Hot-set and gel content: how cure is verified
| Test | What it measures | Interpretation |
|---|---|---|
| Gel content (hot xylene extraction) | Fraction of polymer that does not dissolve | Direct measure of crosslink density |
| Hot-set (elongation under load at elevated temperature) | Dimensional stability above the melting point | Closest to the service condition |
| Mechanical tests after ageing | Tensile and elongation retention | Crosslinking is only useful if the compound survives |
| Dielectric loss / tan delta | Volatiles and contamination | Catches under-degassed or contaminated insulation |
Where it goes wrong
| Symptom | Likely cause | Response |
|---|---|---|
| Scorch / gel specks in the extruder | Melt temperature too high, or DCP agglomerates | Lower melt temperature; improve dispersion; check the peroxide dose |
| Low gel content after curing | Under-cure, DCP loss during storage, or antioxidant overdose | Check the peroxide active oxygen on arrival; verify cure temperature and residence time; rebalance the antioxidant |
| Voids after decompression | Pressure released before the insulation is cool | Extend the cooling section |
| Smell and bubbles in service | Insufficient degassing | Extend degassing time or raise chamber temperature |
| Poor hot-set despite high gel content | Uneven crosslink distribution | Check dispersion and mixing, not the dose |
PEX-a pipe: the same chemistry, different constraints
Hot-water pipe uses the same peroxide crosslinking but without the conductor, so the constraints shift: wall thickness, dimensional stability and surface finish dominate. Extrusion is followed by crosslinking in a pressurised salt bath or infrared oven, and the pipe must remain round and smooth. The peroxide must be fully dispersed, because a scorch point in a pipe wall becomes a stress concentration under pressure cycling.
Frequently asked questions
How much DCP is in a cable compound? Well under 2% in most insulation compounds; the exact figure is set by target gel content and validated on the line.
Can I replace DCP with a silane system? For LV cable and domestic pipe, often yes – and it is simpler. For MV/HV insulation the crosslink density and temperature performance of peroxide curing are still the benchmark.
Why does the active oxygen specification matter here? Because the dose is calculated from the peroxide content. A diluted or partly decomposed lot under-cures silently, and you only find out at the gel-content test.
Does DCP need special shipping? Yes – it is an explosive precursor in China and an organic peroxide for transport, so export requires a licence and end-use documentation. Plan lead time accordingly.
Contact PolyBlueChem for DCP samples, COA and export documentation support.