Selecting Lubricants with Minimal Impact on Flammability in Cable Compound Formulations

Selecting Lubricants with Minimal Impact on Flammability in Cable Compound Formulations

Introduction

The selection of lubricants in cable compound formulations requires careful consideration of their impact on flame retardancy. An optimal lubricant should provide excellent processing aid without compromising the material's fire resistance. This article outlines recommendations based on chemical structure, thermal stability, and synergistic effects with flame retardant systems, drawing from industry practices and research data.

1. Recommended Lubricant Types and Mechanisms

1.1. Silicone-based Lubricants (Silicone Powder/Oil)

• Key Advantages: The Si-O bond energy in silicones (452 kJ/mol) is significantly higher than C-C bonds (348 kJ/mol). At high temperatures, they form a dense silica protective layer that inhibits flame propagation. For instance, adding 0.5-3% of Javachem® GT series (Zhejiang Jiahua) to halogen-free flame-retardant polyolefin cable compounds can increase the Oxygen Index (OI) to over 37%, reduce die buildup, and increase line speed by 20%.

• Application: Suitable for EVA/PE-based cable compounds, especially in highly filled systems (>60% filler). Their hydrophobic nature reduces moisture absorption and improves weatherability.

• Typical Grades: Dow Corning DC-3200, Shin-Etsu KF-96, Zhejiang Jiahua GT-300.

1.2. Metallic Soaps (Calcium/Zinc Stearate)

• Flame Retardancy Mechanism: Calcium stearate decomposes at 200-250°C, generating CaO and CO₂. CaO can react with Aluminum Trihydroxide (ATH) to form calcium aluminate, enhancing char layer density. Studies show that 2-3% calcium stearate can reduce the Peak Heat Release Rate (PHRR) by 15% and improve filler dispersion.

• Process Compatibility: Shows significant synergy with phosphorus-nitrogen flame retardants (e.g., MPP). Can replace part of traditional lubricants in halogen-free formulations without affecting UL94 V-0 rating when used at 1-2%.

• Note: Excessive use may cause blooming; recommended to use in combination with internal lubricants (e.g., Pentaerythritol stearate).

1.3. Oxidized Polyethylene Wax (OPE Wax)

• Characteristics: The carbonyl content (1.5-3%) improves compatibility with polar flame retardants like Magnesium Hydroxide (MDH). The oxidized layer formed at high temperatures can suppress combustion. Tests show cable compounds with 1.5% OPE wax maintain an OI of 32%, 5 points higher than those with standard PE wax.

• Application Advice: Prefer high melting point grades (Drop Point: 105-115°C) with molecular weights between 8000-15000, suitable for extrusion processes at 180-220°C.

• Typical Grades: Honeywell A-C 629, Clariant Licowax OP.

1.4. Polytetrafluoroethylene (PTFE) Micropowder

• Flame Retardancy Features: PTFE has a high decomposition temperature (~500°C), producing only trace amounts of CO₂ and HF upon combustion. The formed char layer prevents melt dripping. Adding 0.5-1% PTFE micropowder to flame-retardant PP can reduce melt dripping incidence from 70% to below 10%.

• Specific Value: Suitable for low-smoke cables (e.g., rail transit), where its very low coefficient of friction (0.05-0.1) reduces interfacial friction heat during high-speed extrusion.

• Typical Grades: DuPont Teflon® MP100, Daikin Polyflon® L-15.

2. Lubricant Types Requiring Caution

2.1. Fatty Acids (Stearic Acid/Oleic Acid)

• Risk Analysis: Stearic acid (C18H36O2) has a high heat of combustion (42 MJ/kg, ~10% higher than PE). Its decomposition produces long-chain hydrocarbons that can promote flame spread. Adding over 0.5% may cause UL94 rating to drop from V-0 to V-2.

• Alternatives: Fully replace with calcium stearate or use low molecular weight hydroxystearic acid (e.g., 12-hydroxystearic acid), which has 18% lower heat of combustion.

2.2. Standard Amides (EBS)

• Limitations: EBS decomposes above 300°C, generating ammonia and nitrile gases, which may interfere with the char-forming mechanism of phosphorus-based flame retardants. Experiments show 1% EBS can increase vertical burning time by 2-3 seconds.

• Improvement Direction: Use silane-modified EBS (e.g., Clariant Licowax EBS-S), where released siloxanes during combustion can partially counteract the negative effects of amide decomposition.

2.3. Paraffin Waxes (Liquid Paraffin/Microcrystalline Wax)

• Combustion Risks: Volatile components of paraffin tend to migrate to the surface, forming a flammable layer. In OI tests, adding 2% paraffin can decrease the OI value by 3-5 points.

• Alternatives: Use high melting point (>90°C) Fischer-Tropsch waxes, which have a narrow molecular weight distribution, better thermal stability than paraffin, and higher char residue upon combustion.

3. Selection Strategy and Process Optimization

3.1. Synergistic Design with Flame Retardants

• Phosphorus-Silicon Synergy: When silicone lubricants are combined with aluminum phosphinate, siloxanes can promote the surface enrichment of phosphorus-based flame retardants, forming a "Si-P-char" composite protective layer, increasing OI to over 35%.

• Metallic Soap-Hydroxide Synergy: At a mass ratio of 1:10 (calcium stearate:ATH), the formed calcium aluminate enhances char strength, increasing residue at 800°C from 22% to 28%.

3.2. Processing Parameter Matching

• Temperature Control: The optimal processing temperature for silicone lubricants is 180-200°C; avoid exceeding 220°C to prevent Si-O bond breakage. Add metallic soaps later in the mixing cycle (130-150°C) to prevent premature decomposition.

• Dispersion Process: For highly filled systems, use twin-screw extruders with high shear (screw speed 300-400 rpm) for uniform dispersion of lubricants and flame retardants. Pre-mixing silicone powder with ATH and adding in two steps can increase tensile strength by 12%.

3.3. Certification and Testing Validation

• Basic Tests: Oxygen Index (GB/T 2406.2) ≥32%; Vertical Burning (UL94) V-0; Smoke Density (GB/T 8323.2) Dm(4min) ≤75.

• Long-term Performance: After thermal aging (120°C×168h), the change in tensile strength should be ≤±10%, and the change in elongation at break should be ≤±15%.

• Environmental Compliance: Prefer lubricants compliant with RoHS and REACH. For medical cables, comply with standards like USP Class VI.

4. Typical Formulation Examples

4.1. Halogen-Free Flame-Retardant Polyolefin Cable Compound

• Formulation (parts by weight): EVA (VA 18%) 100, Magnesium Hydroxide 120, Silicone Powder 2, Calcium Stearate 1.5, Antioxidant 1010 0.5, Light Stabilizer 770 0.3.

• Properties: OI 37%, Tensile Strength 11 MPa, Elongation at Break 160%, Heat Shrinkage (120°C×24h) 0.8%.

4.2. High Flame-Retardant PVC Cable Compound

• Formulation (parts by weight): PVC 100, Antimony Trioxide 5, Phosphate Ester Flame Retardant 20, Calcium Stearate 1.2, OPE Wax 1.0, Epoxidized Soybean Oil 5.

• Properties: UL94 V-0, OI 34%, Surface Resistivity >10^14 Ω·cm. Suitable for industrial control cables.

5. Risk Control and Industry Trends

• Batch Stability: Perform Thermogravimetric Analysis (TGA) on incoming lubricant batches to ensure initial decomposition temperature >250°C and volatiles ≤0.5%.

• Alternative Validation: Use a "stepwise replacement method" for substituting imported lubricants: start with 30% domestic product, gradually increase to 100% after performance verification. For example, Yanshan Petrochemical's silicone powder has successfully replaced Dow Corning DC-3200 in photovoltaic cables.

• Sustainability: Bio-based lubricants (e.g., castor oil-based amides) have ~40% lower carbon emissions than traditional ones, and the CO₂ released during combustion can be absorbed by plants, aligning with regulations like the EU's CBAM.

Conclusion

Silicone-based lubricants, metallic soaps, oxidized polyethylene wax, and PTFE micropowders are ideal choices for cable compounds that balance lubrication and flame retardancy. Practical application requires optimization based on the specific flame retardant system, processing conditions, and performance requirements, validated through small-scale trials for compatibility and burning performance.