Black masterbatch for cables: which grade for which layer

A cable uses carbon black in up to three places, and each one is specified differently. What to ask for at the jacket, at the insulation and at the screen.

By Zoe Lin, Export Manager, E-LUCK (Guangdong Yijuyuan Plastic Industry Technology Co., Ltd.)

**A cable can use carbon black in three different places, and each one is bought on a different specification. The jacket is bought on carbon content and dispersion. The insulation is bought on what is not in it. The semi-conductive screen is bought on a volume resistivity figure, not on a carbon percentage at all.** Sending one grade to the wrong layer is the single most expensive specification mistake in cable compounding — and it usually only shows up at type testing.

This note walks through the three layers, what each one is actually specified on, and the questions worth asking before you place an order.

The three layers, and what each is bought on

Layer	What it does	What the purchase spec should say
Outer jacket / sheath	Mechanical and UV protection for the whole cable life	Carbon black content and dispersion (see below)
Insulation	Holds voltage without breakdown	Limits on carbon content and foreign particles — plus cleanliness of handling
Semi-conductive screen	Smooths the electrical field at the conductor and at the insulation surface	Volume resistivity limit, taken from the cable standard

Most black masterbatch sold into the cable industry goes into the first row. It is also the row where buyers most often over-specify on carbon content and ignore dispersion entirely.

Jacket: content is necessary, dispersion is what protects

A black polyethylene jacket that will be exposed to sun is normally specified at around 2.5% carbon black in the finished compound, with the industry test methods being direct combustion or thermogravimetric analysis to confirm the content. That figure is a floor for UV protection, not a target to exceed — more carbon black does not buy proportionally more life, and it costs money and dispersion quality.

The number that decides whether the jacket is still flexible in twenty years is dispersion. Two jackets can measure 2.5% carbon black by the same test method and behave completely differently after two summers. The difference is whether the carbon is distributed as fine particles or as agglomerates.

Why it matters physically: UV degradation attacks the polymer where it is not shielded. A large agglomerate is a shielded island with unshielded polymer around it — and that ring of unprotected polymer is where a crack starts. Dispersion is assessed under ISO 18553, and it is worth asking for the result by lot rather than accepting an unqualified "good dispersion" claim.

Practical rule: if a supplier quotes you a carbon black percentage but cannot quote a dispersion figure or a filter pressure value, you have half a specification.

Insulation: the specification is mostly about what is not there

Insulation is not specified on how much carbon black it contains — it is specified on how little contamination it contains. An undispersed agglomerate, a metal particle or a gel in the insulation wall is a point where the electrical field concentrates, and that is the origin of the treeing failures that type tests are designed to catch.

What this means commercially: insulation-grade compounds are a different product from jacket-grade compounds, made on cleaner lines with tighter handling. If a supplier offers you "black masterbatch, works for insulation and jacket", that is a warning sign rather than a saving.

Screen: bought on resistivity, not on carbon percentage

The semi-conductive screen — the layer that evens out the electrical field around a MV conductor — is specified with a volume resistivity limit taken from the cable standard the cable has to meet, together with the base polymer of the insulation system it has to be compatible with.

Two consequences for buyers:

• Do not ask for "a conductive masterbatch with X% carbon". Resistivity is the property under test, and it depends on carbon type, loading and dispersion together. A supplier who quotes only a carbon percentage is not quoting to your test.
• Bring your cable standard to the conversation. The acceptable resistivity window comes from the standard, not from the masterbatch supplier. A supplier who invents their own number without asking for your standard is guessing.

It is worth separating the vocabulary here, because "conductive" is used loosely in the market:

• Antistatic / ESD dissipation — used in plastics for trays, packaging and industrial parts, where the goal is to stop static building up. Specified on surface resistivity, commonly in the ranges set out in ANSI/ESD S541.
• Semi-conductive screen compound — used inside power cable, where the goal is field control. Specified on volume resistivity against a cable standard.
• Conductive carbon black — not the same grade as pigment carbon black. It is a specialty carbon with high structure and surface area, chosen to build a conductive network at a much lower loading than a pigment grade would allow.

These are three different products with three different test methods. A single datasheet cannot honestly cover all three.

What to ask before you place a cable order

For a jacket grade:

1. Carbon black content by lot, with the test method named (IEC 60811-4-1 direct combustion or TGA).
2. A dispersion figure or filter pressure value, not an adjective.
3. MFR matched to your sheathing line speed and jacket polymer.
4. Whether the carrier is a match for your jacket compound — a carrier mismatch is a common source of gel and die lines.

For an insulation grade:

1. Contamination limits, and how they are controlled between lots.
2. Whether the compound is produced on lines dedicated to insulation work.
3. Compatibility with your existing insulation system, not just the colour.

For a screen compound:

1. The volume resistivity limit from your cable standard.
2. Which base polymer the screen is built on, and whether that matches your insulation.
3. How resistivity is measured and reported per lot.

A short note on where we sit

Our plant compounds black masterbatch for the jacket layer as its main cable business — H6045 and CM50 are the two grades most often specified for sheathing work. We also compound conductive and antistatic grades against customer resistivity targets for industrial and ESD applications.

If your enquiry is for insulation or for a semi-conductive screen compound, tell us the specification and the standard. We will tell you plainly whether it is in scope for our plant rather than quote for something we cannot document. That is a shorter conversation, and a cheaper one.

The short version

• Jacket: specified on carbon black content and dispersion. Both numbers, every lot.
• Insulation: specified on contamination limits. Different product, different lines.
• Screen: specified on volume resistivity taken from your cable standard. Not on carbon percentage.
• Conductive carbon black is a different material from pigment carbon black. Do not let the two be quoted interchangeably.
• Ask for test methods by name. An adjective is not a specification.

Common questions

What carbon black content should a cable jacket actually have? For UV-exposed PE jacketing, around 2.5% in the finished compound is the common starting point, confirmed by combustion or thermogravimetric testing. Treat it as a floor for UV protection rather than a target — and pair it with a dispersion requirement, because content alone will not predict service life.

Can you supply a semi-conductive screen compound? Send us the volume resistivity limit from your cable standard and the base polymer of your insulation system, and we will confirm what we can do. If it is outside our scope we will say so instead of quoting a number we cannot back with lot data.

What is the difference between conductive and antistatic? Antistatic (or ESD dissipative) grades are specified on surface resistivity and are aimed at stopping static build-up on parts and packaging. Semi-conductive cable screens are specified on volume resistivity against a cable standard. Same word in the market, different test method entirely.

We are switching jacket supplier. What should we test first? Run your normal sheathing trial, then compare three things against your current compound: dispersion or filter pressure value, MFR, and the surface finish of the extruded jacket. Those three catch most of the problems that show up later as wall defects or screen pressure rise.

Send your RFQ block

Continue reading

• How to read a black masterbatch CoA — the lot-level document that proves what actually shipped.
• How to calculate let-down ratio — why the cheaper bag per kilo often costs more per finished part.
• How to vet a black masterbatch supplier — twelve checks before the first container.

Related pages

• E-LUCK certifications — ISO 9001/14001/45001 and IATF 16949, with certificate references
• Production & plant — the Huizhou floor, lines and quality process
• All 12 black masterbatch grades — data sheets by application series