Overcoming Processing Bottlenecks in LSZH Cables: Thermal Stability, Interface Compatibility & Process Optimization

Overcoming Processing Bottlenecks in LSZH Cables: Thermal Stability, Interface Compatibility & Process Optimization

Why Processing Excellence Defines LSZH Cable Quality

Low Smoke Zero Halogen (LSZH) cables are critical in transportation, power, construction, and communication applications where fire safety, low smoke emission, and halogen-free performance are non-negotiable. However, even the most advanced halogen-free flame retardant compound will underperform if extrusion processes are unstable or if thermal stress causes delamination or mechanical weakening in the final product.

In this article, we dissect key bottlenecks in LSZH cable processing—particularly during extrusion, co-extrusion, and multi-layer assembly—and outline how manufacturers can optimize formulations and machinery to ensure consistent, high-quality output.

I. Thermal Decomposition & Micro-Bubbles During Extrusion

1. Problem: Processing Instability of Highly-Filled LSZH Cable Compounds

During screw extrusion, especially when the melt temperature exceeds 200 °C, highly filled LSZH cable compounds face a significant challenge:
premature thermal decomposition of flame retardant fillers such as aluminum trihydrate (ATH) and magnesium hydroxide (MDH).

These hydrated fillers begin to release water vapor when subjected to shear-induced overheating inside the screw barrel. As the water vapor escapes, it forms micro-bubbles within the polymer melt, leading to:

• Reduced melt density and compactness

• Increased void content

• Poor fusion between layers

• Surface defects or blowholes in finished cables

Mechanical test data shows that these micro-defects can reduce the final tensile strength of the LSZH cable sheath by more than 30%. This not only compromises performance but can also result in failed QC during pull tests or bending simulations.

The problem is intensified under the following extrusion conditions:

• High screw compression ratio (e.g., >1:3) → Excess shear stress and frictional heating

• Excessively long residence time → Prolonged exposure to elevated temperature

• Unbalanced die cooling or improper melt flow → Localized hot zones and vapor build-up

2. Process Optimization: Engineering Solutions for Bubble-Free LSZH Compounds

To mitigate thermal decomposition and achieve stable, high-quality extrusion of low smoke halogen free cable compounds, the following process adjustments are highly recommended:

✅ Use Low Compression Ratio Screws (≤ 1:2)

• Why: Reduces shear stress and overheating

• Effect: Protects hydrated flame retardants from premature breakdown

• Result: Fewer bubbles, more consistent melt

✅ Adopt Semi-Squeezing Die Structures

• Why: Provides a smoother pressure transition between extruder and die

• Effect: Prevents sudden melt expansion and bubble entrapment

• Result: Denser extrudate with improved mechanical performance

✅ Control Draw-Down Ratio Close to 1:1

• Why: Ensures uniform material thickness and minimal tension stress

• Effect: Avoids orientation defects and internal strain

• Result: Better layer bonding and dimension stability

✅ Maintain Melt Temperature Below 190 °C

• Why: Keeps processing temperature safely below ATH/MDH decomposition thresholds

• Effect: Reduces vaporization, moisture release, and void formation

• Result: Smooth, defect-free sheath with strong physical properties

II. Multi-Layer Structure & Interface Compatibility

1. Problem: Delamination and Structural Failure in Multi-Layer LSZH Cables

To meet strict B1-level flame retardant classifications under China GB/T 31247 and EU EN 50575, modern LSZH cables often require a multi-layer construction, typically composed of:

• Ceramicized fire protection layer – forms a rigid barrier under flame exposure

• Flame retardant binding tape – holds the structure during combustion

• Halogen-free outer jacket – provides mechanical protection and insulation (often polyethylene-based)

While this design enhances flame resistance, it introduces a new challenge: thermal expansion mismatch.

Each layer—particularly ceramicized silicone rubber and polyethylene sheaths—has a different coefficient of thermal expansion (CTE). Under fire conditions or rapid temperature cycling, this difference can trigger:

• Interface peeling

• Blistering and air gaps

• Delamination under load

In real-world tests, the bond strength between a ceramicized silicone layer and PE sheath drops by over 50% at 150 °C. Once interfacial integrity is lost, the cable fails to maintain shape and continuity during fire tests, risking catastrophic flame spread or loss of function.

2. Direction of Improvement: Advanced Interface Engineering

To improve compatibility and structural integrity in multi-layer LSZH cable compounds, manufacturers are turning to gradient layer technologies and interface engineering techniques, including:

✅ Add Compatibility Modifiers (e.g., PE-g-MAH)

• What it does: Maleic anhydride-grafted polyethylene (PE-g-MAH) chemically bridges between polar (ceramic/silicone) and non-polar (PE) materials

• Result: Enhances interlayer adhesion by up to 2× at elevated temperatures

• Application: Blend into the ceramicized or adhesive layer at 3–8% ratio

✅ Use Tie Layers or Co-Extruded Blends

• What it does: Introduces a compatibility transition layer between materials with different expansion rates

• Result: Minimizes thermal stress and delamination risk

• Application: Co-extrude or laminate a soft, elastomer-modified PE or EVA copolymer as a buffer layer

✅ Optimize Interface Wetting and Surface Energy

• What it does: Improves the physical bonding through better flow, contact, and fusion during co-extrusion

• Result: Reduces internal voids and blister formation

• Application: Adjust melt temperature, surface pre-treatment, or use adhesion promoters

These improvements ensure that multi-layer LSZH cable structures maintain mechanical bonding and integrity during fire exposure, enabling them to pass critical tests such as:

• Flame spread (EN 60332-3)

• No dripping (IEC 60332-1-2)

• Structural retention (GB/T 31247 section 6)

3. Summary Table: LSZH Processing Bottlenecks & Solutions

✅ YINSU FRP-950X: Reducing Thermal Risk While Enhancing Compatibility

To help solve extrusion and multi-layer issues, YINSU Flame Retardant Company offers FRP-950X, a coated red phosphorus flame retardant designed for advanced LSZH formulations.

FRP-950X Features:

• Thermal stability: Won' t decompose or foam prematurely

• Low addition (3–5%): Reduces filler-induced stress

• Excellent dispersion: Compatible with EVA, PE, PP, and blends

• Improved char layer: Enhances multi-layer integrity under fire

By replacing or reducing ATH/MDH loading with FRP-950X, cable manufacturers can lower processing temperatures, avoid micro-bubble defects, and achieve better inter-layer bonding in multi-layer LSZH constructions.

Interested in improving your LSZH cable production? Contact YINSU or visit the FRP-950X product page.

III. Conclusion

While LSZH cables provide clear fire safety advantages, their production involves complex material and process challenges. From thermal decomposition during extrusion to interface failures in multi-layer structures, optimizing each step is crucial to ensure consistent performance and regulatory compliance.

With advanced formulation tools—such as FRP-950X and compatibility enhancers—manufacturers can overcome these bottlenecks, delivering low smoke halogen free cables that are safe, flexible, and reliable.