Cause Analysis of Surface Bumps and Crystalline Specks in Low-Smoke Halogen-Free Cable Compounds and Compatibilizer Selection Recommendations

During the extrusion processing of low-smoke halogen-free flame-retardant polyolefin cable compounds, the frequent occurrence of bumps, crystalline specks, and discolored spots on the sheath surface is a well‑known quality defect in the industry, characterized by long troubleshooting cycles and high difficulty in pinpointing the root cause. It is not uncommon for finished cables to pass all electrical and mechanical performance tests, yet entire batches are rejected solely due to scattered raised particles or discolored spots on the surface.

Process engineers typically focus troubleshooting efforts on the extrusion equipment—repeatedly adjusting zone temperatures and screw speeds, dismantling dies, and changing screen packs multiple times. Short‑term improvements may be observed, but the defects reappear after several hours of continuous production. When all adjustable parameters on the equipment side have been exhausted and the problem persists, it is reasonable to extend the investigation from the processing stage to the raw material end. Residual gel particles in the compatibilizer are one of the most easily underestimated triggers for such intermittent surface defects.

I. Source of Gel Particles: Side Reactions in Compatibilizer Grafting Processes

Compatibilizers are mostly produced via melt free‑radical grafting, using maleic anhydride (MAH) as the grafting monomer, which reacts with polyolefin molecular chains under the action of an initiator. During this process, if the initiator dosage, screw shear intensity, and reaction temperature profile are not properly matched, the polyolefin chains may undergo not only the desired grafting reaction but also side reactions—crosslinking between molecular chains—generating infusible and insoluble fine gel particles.

These gel particles are encapsulated within the compatibilizer pellets, typically ranging in size from tens to several hundred micrometers, and are difficult to detect by routine factory quality control methods:

• Melt flow index (MFI) characterizes the overall fluidity of the material and cannot distinguish between linear resin and crosslinked components.

• Graft ratio determination by chemical titration or infrared spectroscopy similarly does not reflect gel content.

This gives rise to a typical "passes testing but fails in processing" hidden defect—all specifications on the quality report are met, yet the problem only emerges during actual extrusion. Literature references on cable compounds indicate that pre‑crosslinking reactions increase the molecular weight and viscosity of polyethylene, and in severe cases can cause an uneven insulation surface. The formation mechanism of gels is analogous; both originate from excessive crosslinking of molecular chains.

II. Mechanism by Which Gel Particles Induce Surface Defects

When a compatibilizer containing gel particles enters the cable extrusion process, these gels do not melt and plasticize uniformly like the polyolefin matrix under heating. Instead, they remain as solid microparticles suspended in the melt, eventually being carried onto the core surface, forming visible raised bumps and crystalline specks.

The random behavior of gels further complicates the issue: some gel particles are trapped by the screen pack, while others deform under high shear and may even squeeze through the screen mesh openings, passing through the filtration system along with melt pressure fluctuations. After passing through, the gels are distributed extremely unevenly in the melt, often resulting in a smooth surface on the front section of the same cable but defects appearing on the rear section, leading to inconsistent quality within the same batch. This erratic occurrence makes on‑site troubleshooting extremely difficult, continuously consuming technical, production, and quality control resources.

It is worth noting that moisture introduced during storage and processing of the cable compound can also induce pre‑crosslinking reactions, deteriorating extrusion processability. Such moisture‑induced surface issues can appear similar in appearance to defects caused by gels carried by the compatibilizer, and should be differentiated during troubleshooting.

III. Source Prevention through Compatibilizer Selection

The approach to solving such problems should shift from "post‑production interception" to "source prevention"—the key lies not in using finer mesh screen packs, but in whether the compatibilizer itself possesses low gel content and high purity. In low‑smoke halogen‑free flame‑retardant cable formulations, the compatibilizer is typically added at 5–15 parts by weight, and its quality directly determines filler dispersion effectiveness and the apparent quality of the finished product.

Taking Shanghai Jiuju Polymer Materials Co., Ltd.'s JY‑C31 cable‑dedicated compatibilizer as an example, this product incorporates the following technical features addressing gel issues at both the molecular design and process control levels.

1. Gel suppression: controlling crosslinking side reactions from the reaction stage
   JY‑C31 employs a source‑controlled reaction process. By optimizing the initiator ratio, precisely setting the grafting reaction temperature profile, and matching a custom screw configuration, it suppresses excessive crosslinking of molecular chains while maintaining an effective graft ratio, keeping the final gel content at a low level. Unlike approaches that rely on post‑production screening to passively remove gels, JY‑C31 reduces gel generation from the reaction stage itself, ensuring that each pellet can be fully plasticized and uniformly dispersed during processing.

2. Matrix purity: metallocene polyethylene reduces exudates
   JY‑C31 uses metallocene‑catalyzed polyethylene as the base resin. Metallocene polyethylene has a narrow molecular weight distribution and low oligomer residue, effectively avoiding the exudation issues caused by low‑molecular‑weight fractions found in traditional compatibilizers. Under high‑temperature extrusion conditions, these low‑molecular‑weight components tend to migrate from the melt to the die wall, continuously accumulating to form die deposits (carbon build‑up). After sufficient accumulation, these deposits may peel off in flakes and re‑enter the melt, creating new black specks or bumps. The high purity of the metallocene matrix significantly reduces the risk of small‑molecule exudation and die build‑up, contributing to stable surface quality during long‑term continuous extrusion.

3. Wide processing window adaptable to different production line conditions
   JY‑C31 offers a broad melt processing window with good tolerance to processing temperature fluctuations. Considering that many cable compound manufacturers use older extrusion equipment with limited temperature control precision, JY‑C31 does not rapidly produce perceptible changes in cable appearance when zone temperatures shift slightly, reducing adjustment difficulty and scrap losses caused by process variations.

It should be noted that raised bumps and crystalline specks on the sheath surface are not merely cosmetic flaws. These tiny particles create stress concentration points within the sheath, which can induce micro‑crack initiation and gradual propagation under long‑term thermal cycling and repeated bending conditions, potentially posing risks to the long‑term operational reliability of the cable.

IV. Summary

The recurring issues of bumps and crystalline specks on the surface of low‑smoke halogen‑free cable compounds have complex causes, and troubleshooting must address both equipment and raw material factors. Gel particles in the compatibilizer are a frequently underestimated contributor, and we recommend including them in the troubleshooting scope.

Shanghai Jiuju Polymer Materials Co., Ltd. has been dedicated to the R&D and production of polymer compatibilizers for over two decades, with products widely used in cable compounds, modified plastics, and other fields. The JY‑C31 cable‑dedicated compatibilizer is designed specifically for low‑smoke halogen‑free flame‑retardant systems, with specialized technical expertise in gel control and surface quality optimization.

If you are facing technical challenges related to surface defects in cable compounds or compatibilizer selection, please feel free to contact us for technical data and free samples. We also offer formulation optimization advice and on‑machine testing support.