Persistent surface pitting and white spots on cables? A high-grafting compatibilizer improves filler dispersion.

In the production of low-smoke zero-halogen (LSZH) cable compounds, surface quality issues are among the most vexing challenges for formulation engineers. Pitting, white spots, and surface bumps recur repeatedly—not only impairing product appearance but also potentially causing localized defects in the insulation layer that may lead to breakdown hazards. Many manufacturers repeatedly adjust extrusion process parameters and switch filler batches, yet the problem persists stubbornly. The real cause may lie in a long‑overlooked factor: insufficient grafting degree of the compatibilizer, resulting in substandard dispersion of inorganic fillers in the polymer matrix.

1. Recurrent pitting and white spots – process adjustments treat symptoms, not root causes

To achieve high flame‑retardant grades such as B1, LSZH cable compounds typically require the addition of 50–70 wt% inorganic flame‑retardant fillers, such as aluminum hydroxide (ATH) and magnesium hydroxide (MDH), into the polyolefin matrix. These filler particles have polar surfaces, while the polyolefin matrix (PE, EVA, etc.) is non‑polar—the two are inherently incompatible.

The role of the compatibilizer is to act as a “molecular bridge” between the filler and the matrix: its maleic anhydride polar groups anchor onto the filler surface, while its non‑polar polyolefin backbone entangles and co‑dissolves with the base resin, thereby uniformly dispersing the inorganic particles in the organic phase. When the grafting degree is insufficient, compatibilizer molecules cannot effectively coat every filler particle. The uncoated particle surfaces remain highly polar, and the cohesive forces between particles far exceed their affinity for the matrix. A large number of particles agglomerate into micron‑ or even millimeter‑sized aggregates, which cannot be broken apart by shear forces during extrusion, eventually manifesting as pitting and white spots on the cable surface.

2. From pitting/white spots to insulation breakdown – the chain reaction of filler agglomeration

When ATH/MDH loading exceeds 55%, the total surface area of the fillers increases dramatically, and the number of anchoring sites required per unit volume rises exponentially. The functional groups provided by conventional compatibilizers (with grafting degrees of 0.6%–0.8%) are grossly inadequate. Filler particles that are not effectively coated begin to agglomerate during the melt compounding stage, are pushed toward the die under the high temperature and pressure of the extruder, and form raised pits and recessed white spots on the cable surface. These surface defects not only compromise appearance but may also become stress concentration points and electric field distortion sites, potentially leading to insulation breakdown during long‑term service.

3. How to solve the dispersion problem at the source?

To fundamentally resolve the dispersion issue, the key is to select a compatibilizer with a sufficiently high grafting degree, ensuring that every filler particle is effectively coated even in highly filled systems.

JP‑M13 cable compound compatibilizer from Shanghai Jiuju Polymer Materials Co., Ltd. is a POE‑based maleic anhydride grafted compatibilizer specially developed for highly filled LSZH cable compound systems. JP‑M13 employs a proprietary grafting process that consistently maintains a high grafting degree—significantly higher than conventional products—thereby greatly increasing the number of anchoring sites provided per unit mass of compatibilizer. In systems filled with >55% ATH/MDH, the high density of maleic anhydride groups achieves nearly complete coverage of the filler surface, effectively shielding inter‑particle cohesion and enabling uniform, finely dispersed filler distribution throughout the polymer melt.

The POE elastomer backbone itself offers excellent low‑temperature toughening properties (glass transition temperature approx. –50°C), imparting both good flexibility and crack resistance while maintaining rigidity. This makes it particularly suitable for demanding applications such as thin‑wall cables and fine wires, where surface quality and insulation reliability are critical.

4. Selection and validation recommendations for formulation engineers

For formulation engineers, the following key points are worth attention during the selection and validation stage:

• Request the grafting degree test report from the supplier—verify whether the value reaches a high‑grafting level, as this is the most direct basis for judging the compatibilizer’s dispersion capability.

• Observe filler dispersion via scanning electron microscopy (SEM) —focus on whether obvious agglomerates exist and whether the dispersed particle size is within a reasonable range.

• Conduct power‑frequency withstand voltage tests (3500 V/5 min) —this is the most direct means of verifying insulation integrity, while simultaneously inspecting the extruded cable surface for pitting and white spots.

• Track the yield rate—compare the scrap rate differences when using different compatibilizers; high‑dispersion compatibilizers are typically associated with lower processing energy consumption and more stable extrusion pressure fluctuations.

JP‑M13 is specifically developed for highly filled LSZH cable compound systems. Its high grafting degree ensures uniform and fine filler dispersion, while the POE elastomer backbone provides excellent low‑temperature toughening properties, making it an ideal choice for achieving superior cable surface smoothness and reliable insulation performance.

If you are also troubled by persistent pitting and white spots on cable surfaces, feel free to contact us. We have been stably serving multiple leading cable manufacturers and can provide grafting degree test reports and free samples, with direct technical support from our engineers.