Low-voltage cable insulation used to be cured in a continuous vulcanisation line: peroxide, high pressure nitrogen, a long tube, a lot of capital. Silane crosslinking replaced that with a warm water bath, and the monomer that made it possible is vinyltrimethoxysilane (VTMS, CAS 2768-02-7).
This guide is for cable and pipe compounders: how the silane route works, how it compares with peroxide and irradiation curing, what the compound and masterbatch design look like, where the process goes wrong, and how to test whether the product is actually crosslinked.
Why silane crosslinking exists
Polyethylene melts. For a cable insulation or a hot-water pipe that is a problem, so the polymer is crosslinked – chains joined into a network so the material no longer flows at service temperature. Three industrial routes do this:
| Route | How | Capital | Typical use |
|---|---|---|---|
| Peroxide (PEX-A) | Dicumyl peroxide plus high temperature and pressure | High – CV line, nitrogen, long tube | MV/HV cable, high-performance pipe |
| Irradiation (PEX-C) | Electron beam after extrusion | High – accelerator | Thin-wall wire, heat-shrink tubing |
| Silane (PEX-B) | VTMS grafting plus moisture cure | Low – grafting line plus water bath | LV cable, hot-water pipe, tubing |
The silane route wins on capital and on line speed for small and medium sections. It loses on crosslink density and on temperature rating, which is why it dominates LV cable and domestic pipe but not MV cable.
The chemistry in three steps
- Grafting. VTMS is reacted with polyethylene in the presence of a peroxide. The vinyl group adds to polymer radicals, so each grafted site carries three methoxy groups.
- Shaping. The grafted compound is extruded into its final form – insulation on a conductor, or a pipe.
- Moisture cure. Water diffuses in, hydrolyses the methoxy groups to silanols, and a condensation catalyst joins neighbouring silanols into siloxane bridges between chains.
Because the crosslink is formed after shaping, the material stays thermoplastic throughout the extrusion step – no scorch, no premature cure, no scorch risk from a hot die.
Two-step (Sioplas) versus one-step (Monosil)
Two-step
| Component | Content | Made where |
|---|---|---|
| Graft masterbatch (A) | PE with grafted VTMS, plus antioxidant | Dedicated grafting extruder |
| Catalyst masterbatch (B) | PE carrier plus organotin condensation catalyst | Separate compounding step |
| Let-down ratio | Typically a few percent of each, set by target silane content | On the cable/pipe line |
The advantage is separation: the catalyst never meets the silane before the melt, so nothing cures in the bag. Masterbatches ship and store well, which is why most LV cable producers buy them rather than handling silane on site.
One-step
Silane, peroxide and catalyst are metered separately into a purpose-built extruder with a long mixing section. Simpler material logistics, tighter process control demanded. The failure mode is a metering error: too much peroxide gives gel and die build-up, too little gives low graft content and a compound that never reaches the required gel fraction.
Curing: the step that is easy to get wrong
| Cure condition | Typical time | Notes |
|---|---|---|
| Hot water bath, 80-95 deg C | Hours, depending on wall thickness | The standard in-line or off-line cure |
| Low-pressure steam | Hours | Faster heat transfer, more equipment |
| Ambient storage | Days to weeks | Acceptable for thin sections; slow and uncontrolled for thick ones |
| Humid environment | Variable | Not a substitute for a defined cure schedule |
Cure is diffusion-limited: water has to reach the centre of the insulation. A 1 mm wall cures in hours; a 4 mm wall needs substantially longer and benefits from higher temperature. This is why the same compound can pass a gel-fraction test on thin wire and fail on a thicker section.
Compound design
| Component | Function | Practical note |
|---|---|---|
| Base PE (LDPE/LLDPE blend) | Carrier polymer | Melt index and density set the extrusion behaviour |
| VTMS | Graft monomer | Graft level set by the masterbatch ratio |
| Peroxide (e.g. DCP type) | Radical initiator for grafting | Dose tuned to give graft without crosslinking in the extruder |
| Antioxidant | Processing and long-term stability | Phenolic plus phosphite; must not consume the peroxide |
| Condensation catalyst | Silanol condensation | Kept in a separate masterbatch |
| Cu stabiliser / metal deactivator | Protects against copper-catalysed oxidation | Essential in insulation in contact with copper conductor |
| Fillers / flame retardants | Property modification | Acidic or moisture-bearing fillers interfere with cure and must be dried and often silane-treated |
Where it goes wrong
| Symptom | Likely cause | Response |
|---|---|---|
| Low gel fraction after cure | Insufficient graft, catalyst underdosed, cure too short or too cool | Check graft level by FTIR or gel test on the graft masterbatch; verify catalyst dosing; extend cure |
| Gel or die build-up during extrusion | Peroxide overdose, or moisture in the feed | Reduce peroxide; dry the compound and silane; check drum seals |
| Surface defects and voids | Methanol released by hydrolysis during extrusion, trapped in the melt | Vent the extruder; reduce moisture in feed; lower melt temperature |
| Slow cure, tacky surface | Catalyst masterbatch not dispersed, or silane partly hydrolysed before grafting | Check dispersion; check silane water content on arrival |
| Poor ageing performance | Antioxidant and copper stabiliser package inadequate for the service temperature | Review the stabiliser package, not the silane |
How to verify crosslinking
- Gel fraction (hot xylene extraction). The standard measure – the percentage of polymer that does not dissolve tells you how much network formed.
- Hot set / elongation under load at elevated temperature. Directly relevant to the service condition.
- FTIR for graft content on the masterbatch, before extrusion, so you know whether the raw material is right rather than discovering it after cure.
- Mechanical tests after ageing – tensile and elongation retention, since crosslinking is only useful if the compound survives its service life.
Frequently asked questions
Can silane-crosslinked PE replace peroxide-cured insulation in MV cable? Generally no. The silane route gives a lower crosslink density and is used for LV cable and pipe, not medium-voltage insulation.
How much VTMS ends up in the compound? The graft level is typically well under 2% of the compound, set by the masterbatch let-down ratio rather than by direct silane addition.
Why does moisture matter so much in the extrusion step? Water hydrolyses the methoxy groups prematurely, releasing methanol that causes voids, and reducing the silane available for crosslinking later.
What silane quality do I need? 99.0% min assay, colour (Pt/Co) max 30, and a low water content – a hazy or dark drum will graft poorly.
Contact PolyBlueChem for VTMS samples, COA and compound design support.