Flame retardant masterbatch for cable and wire plastics is one of the most technically demanding products in the entire compounding industry, because it must simultaneously satisfy a fire performance specification written by a safety authority, an electrical specification written by a utility or equipment maker, a mechanical specification written by an installer, and a processing specification written by the cable extruder’s production manager. These four requirements pull in opposite directions. Raising flame retardant loading improves the fire rating and destroys elongation. Choosing a halogenated system delivers excellent flame performance at low loading but generates dense, corrosive smoke. Choosing a halogen-free mineral system solves the smoke problem but forces filler loadings above 60 percent, which slashes throughput, accelerates screw wear and challenges the extruder’s torque envelope.
This guide unpacks the full decision space. It covers the polymer matrices used in cable construction, the chemistry of gas-phase and condensed-phase flame retardancy, the four major retardant systems and their performance envelopes, the testing standards that define acceptance in Europe, North America and China, the architecture of a well-designed cable masterbatch, and — critically — how to compound these formulations on a co-rotating twin-screw extruder without dehydrating the filler, degrading the polymer or scrapping the screws in six months. Kerke, a Wanplas factory specialising in parallel co-rotating twin-screw compounding extruders, lists cable compounding among its core application industries and has supplied KTE-series lines for both halogenated and halogen-free cable compound production across more than 70 countries.
Whether you are a masterbatch producer designing a new halogen-free concentrate, a cable manufacturer evaluating a switch from PVC to LSZH, or an equipment buyer sizing a compounding line for 65 percent aluminium trihydroxide, the sections below provide the loading levels, temperature windows, screw configuration principles and test criteria you need in 2026.
Why Flame Retardant Masterbatch Defines Cable Safety
Cables are the most widely distributed combustible material in any building, and they are installed precisely where fire spreads fastest: in vertical risers, cable trays, ceiling voids and service shafts that behave like chimneys. A single vertical riser can carry hundreds of kilograms of polymer per floor, and once ignited, unmitigated cable insulation propagates flame upward at a rate that outpaces most suppression systems. This is why cable flammability is regulated more prescriptively than almost any other plastics application, and why flame retardant masterbatch is not an optional performance enhancement but a compliance requirement.
The consequences of getting it wrong go beyond flame spread. In enclosed spaces, the majority of fire fatalities result from smoke inhalation and toxic gas rather than from burns. Halogenated cable insulation releases hydrogen chloride or hydrogen bromide when it burns; these gases are acutely toxic, they obscure visibility, and they form corrosive acids on contact with moisture that destroy electronic equipment and structural steel long after the flame is out. A data centre, a hospital operating theatre, an underground metro station or a naval vessel cannot tolerate that outcome, which is why low smoke zero halogen construction has become mandatory in these environments across most of the world.
What the masterbatch actually delivers
A cable flame retardant masterbatch is a concentrated, pre-dispersed, pelletised package that carries the active retardant, its synergists, dispersing aids, coupling agents, lubricants, antioxidants and often a colour system, all on a carrier resin compatible with the cable compound. The reasons for using a masterbatch rather than direct powder addition are compelling in this application specifically:
- Dust and exposure control. Antimony trioxide is a respirable powder with occupational exposure limits; brominated retardants are fine powders; aluminium trihydroxide at sub-micron particle size is highly dusty. Pelletising eliminates the airborne hazard on the cable extrusion floor.
- Dispersion quality determines fire performance. A poorly dispersed retardant leaves polymer-rich domains that ignite and sustain flame. Two compounds with identical formulation can differ by a full UL 94 rating class purely on dispersion. Pre-dispersing on a twin-screw compounder rather than in the cable line’s single-screw extruder is the only reliable route.
- Moisture control. Metal hydroxides and many phosphorus compounds are hygroscopic. A dried, sealed, pelletised masterbatch delivers controlled moisture; loose powder in a humid cable plant does not, and residual moisture produces voids that cause dielectric breakdown.
- Accurate metering. Fire ratings depend on hitting the loading target within a narrow band. Gravimetric feeding of a pellet is far more accurate than volumetric feeding of a low-bulk-density powder.
Two distinct product architectures
The industry uses two very different masterbatch architectures for cable, and confusing them causes procurement errors. The first is an additive concentrate, typically 40 to 70 percent active on a compatible carrier, let down at 10 to 30 percent into a base polymer at the cable plant. This suits brominated and phosphorus systems where the required final loading is modest. The second is a fully formulated compound, supplied ready to extrude at 100 percent, which is the normal route for halogen-free jacketing because the required filler loading of 60 percent or more cannot be reached by letting down a concentrate into virgin polymer without exceeding practical concentrate loadings. In practice, LSZH cable compound is a compounded product rather than a masterbatch in the strict sense, and it is produced on the same class of co-rotating twin-screw equipment.
Cable Polymer Matrices: PVC, PE, XLPE, EVA, TPU and TPE
The base polymer determines which flame retardant chemistries are viable, what processing temperature window is available, and how much of the fire performance comes free from the polymer itself. Selecting the retardant before selecting the matrix is a common and costly sequencing error.
Polyvinyl chloride
PVC remains the highest-volume cable insulation and jacketing polymer worldwide because it is intrinsically flame retardant. Its chlorine content of approximately 57 percent in the unplasticised resin gives an inherent limiting oxygen index in the mid-40s for rigid PVC, dropping to roughly 22 to 28 percent for typical flexible cable compounds after plasticiser addition. Plasticiser is the enemy of fire performance: every part of phthalate or adipate plasticiser adds combustible fuel and dilutes the chlorine. Cable-grade flexible PVC therefore relies on antimony trioxide, zinc borate, molybdenum compounds and ATH to restore the rating and, increasingly, to suppress smoke.
PVC’s advantages are low relative cost, excellent electrical properties, good oil and chemical resistance, easy processing at 165 to 185 °C, and simple colouring. Its disadvantages are decisive in critical installations: dense black smoke, hydrogen chloride evolution, poor low-temperature flexibility without heavy plasticisation, and plasticiser migration over service life. Processing temperature must stay below approximately 205 °C to avoid dehydrochlorination, which limits which flame retardants can be used.
Polyethylene and cross-linked polyethylene
Polyethylene has superb dielectric properties — low permittivity, very low dissipation factor — which makes it the material of choice for medium and high voltage insulation and for data cable. It is also one of the most flammable common polymers, with a limiting oxygen index of about 17.4 percent, meaning it burns readily in ordinary air and drips flaming molten polymer. Every gram of fire performance must therefore be added.
Cross-linked polyethylene, produced by peroxide, silane or irradiation cross-linking, raises the continuous operating temperature from around 70 °C to 90 °C and eliminates melt dripping, which by itself improves behaviour in the vertical flame test. Cross-linking complicates flame retardant selection because peroxide systems are sensitive to acidic and radical-scavenging additives, and silane systems require moisture curing that hygroscopic mineral fillers disturb.
Ethylene vinyl acetate and polyolefin elastomers
EVA is the workhorse base for halogen-free flame retardant cable compounds, and the reason is filler acceptance. The polar acetate groups wet mineral surfaces far better than non-polar polyethylene, allowing 60 to 65 percent aluminium trihydroxide to be incorporated while retaining usable elongation. Vinyl acetate content in cable grades typically ranges from 18 to 33 percent; higher VA gives better filler acceptance and flexibility but lower mechanical strength and worse heat resistance.
Metallocene polyolefin elastomers based on ethylene-octene copolymers are frequently blended with EVA to improve low-temperature flexibility and reduce the acetic acid odour that EVA generates at elevated processing temperature. Maleic anhydride grafted polyethylene or grafted EVA at 3 to 8 percent of the polymer phase acts as a coupling agent, chemically bridging the mineral filler to the polymer and recovering a substantial fraction of the tensile and elongation lost to high loading.
Thermoplastic polyurethane and thermoplastic elastomers
TPU is specified for cable jackets requiring exceptional abrasion resistance, oil resistance, and flexibility over a wide temperature range — mining cables, robotic drag chains, offshore umbilicals, military field cables. Polyester-based TPU offers better oil and abrasion resistance; polyether-based TPU offers better hydrolysis resistance and low-temperature flexibility. TPU processes at 190 to 215 °C and is extremely moisture sensitive, requiring drying to below 0.02 percent before extrusion, which strongly constrains flame retardant selection: hydrated mineral fillers are problematic, and phosphorus-based systems with low moisture uptake are preferred.
Thermoplastic elastomers based on styrenic block copolymers or thermoplastic vulcanisates are used where rubber-like feel and flexibility are required without vulcanisation. They accept flame retardants reasonably well but have limited temperature resistance and require careful oil-extension balancing, since the extender oil is highly combustible.
| Matrix | Baseline LOI | Processing Window | Max Continuous Temp | Compatible FR Systems | Typical Cable Use |
|---|---|---|---|---|---|
| Flexible PVC | 22–28 % | 165–185 °C | 70–105 °C by grade | Sb2O3, zinc borate, ATH, molybdate | Building wire, flexible cords, control cable |
| LDPE / HDPE | 17.4 % | 160–200 °C | 70 °C | ATH, MDH, brominated + Sb2O3 | Data cable insulation, low voltage jacket |
| XLPE | 17–19 % | 120–140 °C pre-cure | 90 °C, 250 °C short circuit | ATH, MDH; peroxide-compatible grades only | MV/HV insulation, solar cable |
| EVA (18–33 % VA) | 19–21 % | 150–180 °C | 90 °C | ATH at 60–65 %, MDH, silicone synergist | LSZH jacket and bedding |
| POE elastomer | 17–18 % | 160–190 °C | 90 °C | ATH, MDH with coupling agent | Flexible LSZH, low-temperature cable |
| TPU (polyester) | 20–22 % | 190–215 °C | 90–125 °C | Phosphorus esters, melamine cyanurate, MDH | Mining, robotic, offshore jacket |
| TPE-S / TPV | 17–19 % | 170–200 °C | 85–110 °C | MDH, IFR, brominated in non-LSZH grades | Appliance cords, soft-touch jackets |
| Silicone rubber | 26–30 % | Cure at 150–200 °C | 180–200 °C | Platinum-cure with ceramifying fillers | Fire-survival and circuit-integrity cable |
The Combustion Cycle and How Flame Retardants Interrupt It
Every flame retardant works by breaking one of the four legs of the fire tetrahedron — fuel, oxygen, heat and the free-radical chain reaction — and understanding which leg a given chemistry attacks explains its loading requirement, its side effects and its interactions. Polymer combustion is a self-sustaining loop: heat pyrolyses the solid polymer into volatile fuel fragments, those fragments mix with oxygen and burn in the gas phase, and the heat released feeds back to pyrolyse more polymer.
Gas-phase inhibition
Gas-phase retardants work by scavenging the high-energy hydrogen and hydroxyl radicals that propagate combustion. Halogenated compounds are the archetype. When a brominated retardant decomposes, it releases hydrogen bromide, which reacts with hydroxyl and hydrogen radicals to form water and much less reactive bromine radicals. The chain-branching reaction slows, the flame cools, and combustion becomes unsustainable. Because radical scavenging is catalytic in effect, gas-phase systems achieve high performance at relatively low loading — typically 12 to 20 percent total including synergist — which is why halogenated chemistry survives despite its drawbacks. Phosphorus can also act in the gas phase through PO radical formation, particularly with volatile phosphorus esters.
Condensed-phase action: char formation
Condensed-phase retardants change what happens in the solid polymer. Phosphorus compounds decompose to phosphoric and polyphosphoric acid, which dehydrate the polymer’s carbon backbone into a carbonaceous char rather than volatile fuel. The char forms a physical barrier that insulates the underlying polymer from heat and blocks the diffusion of volatiles outward and oxygen inward. Char formation works best in oxygen-containing polymers such as polyesters, polyurethanes, cellulosics and EVA; it works poorly in pure hydrocarbons like polyethylene, which have no oxygen to facilitate the dehydration mechanism.
Endothermic cooling and fuel dilution
Metal hydroxides operate through a third mechanism entirely: physical heat absorption and dilution. Aluminium trihydroxide decomposes at roughly 180 to 200 °C, absorbing approximately 1.05 kJ per gram and releasing about 34.6 percent of its mass as water vapour. Magnesium dihydroxide decomposes at 300 to 340 °C, absorbing approximately 1.24 kJ per gram and releasing about 31 percent water. The endothermic decomposition draws heat out of the polymer, keeping it below pyrolysis temperature. The released steam dilutes the combustible gases and displaces oxygen in the flame zone. The residual metal oxide forms a porous ceramic layer that provides some barrier protection.
The catch is that these are stoichiometric, not catalytic, mechanisms. Every joule absorbed requires a proportional mass of filler, which is why halogen-free mineral systems need 60 percent loading where a brominated system needs 15 percent. This single fact drives the entire equipment and processing challenge of LSZH compounding.
Intumescence: the engineered char
Intumescent flame retardant systems combine three functional components to produce a swollen, multicellular char that is far more protective than a simple char layer. An acid source, usually ammonium polyphosphate, decomposes to release phosphoric acid. A carbon source, or charring agent, such as pentaerythritol, dipentaerythritol or a triazine-based polymer, is esterified and then dehydrated by that acid to form carbon. A blowing agent, typically melamine or the nitrogen already present in ammonium polyphosphate, releases ammonia and nitrogen that expand the forming char into a foam. The result can be a char layer many times the original coating thickness with very low thermal conductivity.
| Mechanism | Representative Chemistry | Where It Acts | Typical Loading for V-0 | Main Drawback |
|---|---|---|---|---|
| Radical scavenging | Brominated aromatics plus antimony trioxide | Gas phase, flame zone | 12–20 % combined | Dense smoke, corrosive acid gas |
| Char promotion | Phosphate esters, red phosphorus, DOPO derivatives | Condensed phase | 8–20 % depending on matrix | Weak in pure polyolefins, colour issues with red P |
| Endothermic cooling and dilution | ATH, MDH, huntite-hydromagnesite | Condensed and gas phase | 55–70 % | Very high loading, mechanical property loss |
| Intumescence | Ammonium polyphosphate plus pentaerythritol plus melamine | Condensed phase, expanding barrier | 20–30 % | Water sensitivity, migration, limited to lower temperatures |
| Ceramification | Silicone with mica, wollastonite, glass frit | Condensed phase, ceramic shell | Formulation specific | Premium cost, specialised processing |
| Nanocomposite barrier | Organoclay, layered double hydroxide, carbon nanotube | Condensed phase, char reinforcement | 2–6 % as synergist only | Cannot achieve rating alone; needs exfoliation |
Halogenated Systems: Brominated Retardants and Antimony Synergy
The brominated flame retardant plus antimony trioxide combination remains the most efficient flame retardant system available on a performance-per-unit-loading basis, and it continues to dominate applications where smoke and acid gas are not restricted. Understanding the synergy is essential because the two components are useless in the ratio most people assume.
How the bromine-antimony synergy works
Antimony trioxide is not itself a flame retardant. Used alone in a polyolefin it does essentially nothing. Its function is to react with the hydrogen bromide released by the brominated retardant to form antimony tribromide and antimony oxyhalides. These species are volatile at flame temperatures, so they carry antimony into the gas phase, where antimony halide provides a much more efficient radical-scavenging pathway than hydrogen bromide alone. The result is a strongly super-additive effect: a system containing both performs far better than the sum of its parts.
The optimum bromine-to-antimony weight ratio in most polyolefin and styrenic systems is close to 3 to 1, and departures in either direction waste material. Excess antimony beyond the stoichiometric requirement adds cost, adds opacity, adds density, and can actually reduce performance by consuming bromine that would otherwise be available. Insufficient antimony leaves the bromine system underpowered.
The main brominated chemistries in cable
Decabromodiphenyl ethane has largely replaced decabromodiphenyl oxide in new formulations following regulatory pressure on the older material; it offers approximately 82 percent bromine content, excellent thermal stability well above 300 °C, and low cost per unit of bromine delivered. Ethylene bis-tetrabromophthalimide offers similar performance with better ultraviolet stability, which matters for outdoor cable jackets. Brominated polystyrene and brominated epoxy oligomers are polymeric, which reduces migration and blooming and improves thermal stability, at higher relative cost. Tetrabromobisphenol A derivatives are used in some cable applications, but the parent compound is more commonly a reactive intermediate in epoxy systems.
Chlorinated retardants such as chlorinated paraffin and Dechlorane Plus still appear in specific cable compounds. Chlorinated paraffin doubles as a secondary plasticiser in PVC, which makes it economically attractive, though short-chain grades face regulatory restriction in many jurisdictions.
Trade-offs and mitigation
The two structural disadvantages of halogenated systems are smoke and acid gas. Bromine and chlorine promote incomplete combustion, producing large volumes of dense black soot that reduces visibility dramatically. Hydrogen halide evolution is measured by IEC 60754 and typically exceeds the halogen-free thresholds by an order of magnitude or more. Where a specification permits halogens but demands reduced smoke, formulators use zinc borate, zinc stannate, zinc hydroxystannate, calcium carbonate, magnesium hydroxide as a partial co-filler, and molybdenum compounds such as ammonium octamolybdate as smoke suppressants. Zinc stannate in particular is a well-established partial replacement for antimony trioxide, allowing antimony loading to be cut while maintaining rating and improving smoke behaviour.
| Component | Active Content | Thermal Stability | Typical Loading in Compound | Notes for Cable Use |
|---|---|---|---|---|
| Decabromodiphenyl ethane | ~82 % Br | Above 340 °C | 10–16 % | Workhorse for PE and PP jackets; requires good dispersion |
| Ethylene bis-tetrabromophthalimide | ~67 % Br | Above 400 °C | 12–20 % | Superior UV stability for outdoor and solar cable |
| Brominated polystyrene | ~68 % Br | Above 340 °C | 14–22 % | Polymeric, non-blooming; higher relative cost |
| Antimony trioxide | Synergist | Stable | 3–6 % | Target Br:Sb near 3:1; occupational exposure controls apply |
| Zinc borate | Synergist / smoke suppressant | Above 290 °C | 2–8 % | Partial Sb2O3 replacement; anti-drip and afterglow suppression |
| Zinc hydroxystannate | Synergist / smoke suppressant | Above 180 °C | 1–4 % | Strong smoke reduction; enables reduced antimony |
| Ammonium octamolybdate | Smoke suppressant | Above 250 °C | 1–5 % | Effective in PVC; promotes char, reduces smoke density |
| Chlorinated paraffin | ~70 % Cl in high grades | Limited, dehydrochlorinates | 5–15 % in PVC | Dual plasticiser function; short-chain grades restricted |
Metal Hydroxides: ATH and MDH for Halogen-Free Jacketing
Aluminium trihydroxide and magnesium dihydroxide together account for the large majority of halogen-free flame retardant volume in the cable industry, and the choice between them is decided almost entirely by processing temperature. Everything else about their behaviour — the endothermic mechanism, the very high loading requirement, the mechanical property penalty, the abrasive wear on compounding equipment — is broadly similar.
The temperature ceiling that governs the choice
Aluminium trihydroxide begins to lose water at approximately 180 to 200 °C. That decomposition is precisely what makes it a flame retardant, but during compounding and extrusion it is a defect mechanism: released steam creates voids, porosity, surface blisters and, in severe cases, foaming of the extrudate. As a practical rule, ATH-filled compounds must be processed with melt temperature held below approximately 190 to 195 °C at every point in the extruder, including local shear peaks that do not appear on the barrel thermocouples. This restricts ATH to polyethylene, EVA, POE and other low-melting polyolefins.
Magnesium dihydroxide is stable to approximately 300 to 340 °C, which opens the door to polypropylene, polyamide and higher-temperature processing. Its endothermic capacity per unit mass is actually slightly higher than ATH. The trade-offs are cost, which is Medium to High compared with ATH’s Low, and a stronger tendency toward alkalinity and moisture pick-up, which affects compound electrical properties if not controlled.
Particle size, surface treatment and coupling
The performance of a metal hydroxide compound depends as much on particle engineering as on loading level. Median particle size in cable-grade ATH typically ranges from 0.8 to 2.5 micrometres; finer grades give better mechanical properties and smoother surfaces but sharply increase viscosity and specific surface area, which raises binder demand and reduces throughput. Coarser grades process more easily but produce rougher surfaces and lower elongation.
Surface treatment is essential above roughly 50 percent loading. Three approaches dominate:
- Fatty acid coating, usually stearic acid or stearate, applied at 0.5 to 2 percent on the filler. This is the cheapest option; it reduces viscosity substantially and improves dispersion but does not create a chemical bond to the polymer, so mechanical property recovery is limited.
- Silane treatment, typically vinyl or amino functional silanes, applied at 0.5 to 1.5 percent. Vinylsilane can graft to the polymer during peroxide cross-linking, producing a genuine chemical bridge that dramatically improves tensile strength, elongation and hot-set performance, and also improves water resistance and electrical properties after humid ageing.
- Polymeric coupling via maleic anhydride grafted polyolefin added to the compound rather than to the filler, at 3 to 8 percent of the polymer phase. The anhydride reacts with surface hydroxyl groups. This is often the most cost-effective route to elongation recovery in EVA-based LSZH.
Synergists that reduce required loading
Because every percentage point of filler loading costs mechanical performance and throughput, formulators work hard to reduce it. Several synergists deliver meaningful reductions:
- Silicone-based synergists, either high-molecular-weight silicone gum or silicone-functional powders, promote formation of a silica-rich ceramic surface layer during burning and can allow 3 to 8 percentage points of filler reduction at equal rating, while simultaneously improving processability and lowering the jacket’s coefficient of friction for easier conduit pulling.
- Zinc borate at 2 to 5 percent forms a glassy layer that reinforces the ceramic residue, suppresses afterglow and reduces smoke.
- Organoclay and layered double hydroxide nanofillers at 2 to 5 percent reinforce the char barrier and reduce peak heat release rate, though they cannot achieve a rating on their own and require good exfoliation, which demands high distributive mixing capability.
- Red phosphorus or ammonium polyphosphate at low levels can be combined with reduced metal hydroxide in a hybrid system, though red phosphorus imposes colour limitations and requires encapsulation to prevent phosphine generation.
- Huntite-hydromagnesite natural mineral blends decompose in two stages across a wider temperature range and can partially substitute for ATH while also contributing platelet reinforcement.
| Property | Aluminium Trihydroxide (ATH) | Magnesium Dihydroxide (MDH) | Huntite-Hydromagnesite |
|---|---|---|---|
| Decomposition onset | 180–200 °C | 300–340 °C | Two stages, ~220 °C and ~400 °C |
| Water released | ~34.6 % of mass | ~31 % of mass | ~17 % water plus CO2 |
| Endothermic capacity | ~1.05 kJ/g | ~1.24 kJ/g | Lower, spread over range |
| Max processing temperature | ~190 °C melt | ~280 °C melt | ~210 °C melt |
| Typical loading for V-0 in EVA/PE | 60–65 % | 58–65 % | Partial replacement only |
| Relative raw material cost | Low | Medium–High | Low–Medium |
| Smoke suppression | Excellent | Excellent | Good |
| Abrasiveness to equipment | Moderate to high | Moderate | Moderate to high |
| Effect on elongation at 60 % load | Falls to roughly 120–200 % with coupling | Similar, slightly better with silane | Platelet shape aids stiffness, reduces elongation |
| Suitable matrices | PE, EVA, POE, flexible PVC | PP, PA, TPU, high-temp polyolefin | PE, EVA blends |
Intumescent Phosphorus-Nitrogen Systems for Cable Compounds
Intumescent flame retardant systems offer the halogen-free performance of mineral fillers at roughly one-third to one-half the loading, which makes them extremely attractive wherever mechanical properties or specific gravity matter. The reason they have not displaced metal hydroxides in general cable jacketing comes down to water sensitivity, migration and the difficulty of building an effective char in a pure hydrocarbon matrix.
Composition and the three-component principle
A classical intumescent formulation contains an acid source, a carbon source and a blowing agent, and modern commercial products often combine two or three of these functions in a single molecule. Ammonium polyphosphate is the dominant acid source; crystal form II grades have much better thermal stability and lower water solubility than form I and are the standard choice for polyolefin cable compounds. Microencapsulated grades coated with melamine formaldehyde resin, silane or thermoset shells further reduce water extraction and improve compatibility.
The carbon source in older formulations was pentaerythritol or dipentaerythritol, but these small polyols migrate and are water-soluble. Modern systems use charring agents based on triazine polymers, piperazine pyrophosphate derivatives, or hyperbranched polyol structures with much higher molecular weight and far lower migration. Nitrogen blowing action comes from melamine, melamine phosphate, melamine polyphosphate or the ammonium content of the polyphosphate itself.
Performance envelope
A well-designed intumescent system in polypropylene or polyethylene reaches UL 94 V-0 at 1.6 mm thickness at approximately 22 to 28 percent total loading, compared with 60 percent for ATH. Limiting oxygen index typically rises from 17 to 18 percent in the base polymer to 28 to 34 percent. Because loading is far lower, tensile strength and elongation are much better preserved, specific gravity stays near 1.05 to 1.15 rather than 1.5 to 1.6, and compounding throughput is dramatically higher. For cables where weight matters — aerospace, automotive, portable equipment — this is decisive.
The limitations that constrain adoption
Water sensitivity is the principal weakness. Ammonium polyphosphate and its associated nitrogen compounds are hydrophilic, and prolonged exposure to humidity or water immersion leaches them out, degrading both flame performance and electrical insulation resistance. Cable jackets buried in ducts, exposed to condensation or subjected to the water immersion tests specified in many cable standards can fail. Microencapsulation, silane surface treatment and combination with a modest metal hydroxide loading all improve the situation, and the hybrid intumescent-plus-mineral approach is increasingly common.
Processing temperature is the second limitation. Ammonium polyphosphate begins to decompose in the 275 to 300 °C region, and interaction with the polymer at high shear can trigger early decomposition and foaming. In practice intumescent cable compounds process comfortably in polyolefins but require careful control in engineering polymers.
Colour and appearance form a third constraint. Intumescent packages are white to off-white and can be pigmented, but red phosphorus based hybrids impose a dark red-brown base colour that limits the achievable palette — a real issue in cable, where colour coding is a functional safety requirement governed by installation standards.
LSZH Compounds: Smoke Density, Acid Gas and Toxicity Control
Low smoke zero halogen construction is defined not by what it contains but by what it does not emit, and the specification is enforced through three independent test families: flame propagation, smoke density and halogen acid gas evolution. A compound must pass all three; excelling at one does not compensate for failing another.
Smoke density measurement and targets
Smoke obscuration for cables is most commonly assessed by the three-metre cube test described in IEC 61034, in which a defined bundle of cable specimens is burned in an enclosed chamber over a spirit fire and light transmittance is measured across the chamber. The typical acceptance criterion for LSZH cable is a minimum light transmittance of 60 percent, and premium specifications for metro, tunnel and data centre installations demand higher. Halogenated PVC cable, by comparison, commonly reduces transmittance to a small fraction of that value.
Smoke density can also be measured on material samples using the NBS smoke chamber approach in ASTM E662 or ISO 5659, reporting specific optical density under flaming and non-flaming conditions. These give better formulation discrimination during development, while the three-metre cube test is the compliance test.
Halogen acid gas and corrosivity
IEC 60754-1 determines the amount of halogen acid gas evolved, expressed as hydrogen chloride equivalent, with a common zero-halogen threshold of not more than 0.5 percent by mass. IEC 60754-2 measures the pH and conductivity of the aqueous solution formed from combustion gases, with typical requirements of pH not less than 4.3 and conductivity not more than 10 microsiemens per millimetre. These two tests together establish that combustion products will neither poison occupants with acid gas nor corrode equipment and reinforcement.
Meeting them is straightforward if the formulation is genuinely halogen-free, but there are traps. Halogenated processing aids, certain pigments, halogen-containing antioxidants and cross-contamination from shared compounding equipment can push a nominally halogen-free compound over the threshold. Producers running both halogenated and halogen-free grades on the same twin-screw line must implement rigorous purge and cleaning protocols, and many operate dedicated lines for LSZH production.
Toxicity indices
Beyond acid gas, some specifications — particularly rail, marine and defence — impose toxicity index requirements assessing carbon monoxide, carbon dioxide, hydrogen cyanide, nitrogen oxides, sulphur dioxide and other species. Nitrogen-containing flame retardants such as melamine derivatives can generate hydrogen cyanide and ammonia under some conditions, so an intumescent phosphorus-nitrogen system that easily passes acid gas testing may still face scrutiny under a toxicity index requirement. Metal hydroxide systems generally perform best on toxicity because their decomposition products are water and inert oxide.
The mechanical property recovery problem
The defining engineering challenge of LSZH is that fire performance and mechanical performance are inversely coupled through filler loading. A useful cable jacket needs tensile strength typically above 10 MPa and elongation at break above 125 to 150 percent depending on the governing standard, plus adequate tear strength, low-temperature bend performance and resistance to hot deformation. At 62 percent ATH in unmodified EVA, elongation can collapse below 100 percent. The recovery toolkit is well established: silane coupling on the filler, maleic anhydride grafted polyolefin in the polymer phase, careful selection of EVA vinyl acetate content, blending with POE elastomer for low-temperature flexibility, optimised particle size distribution using a bimodal filler blend, and silicone synergists that permit loading reduction. Applied together, these routinely restore elongation into the 150 to 250 percent range at full flame performance.
| Performance Attribute | Flexible PVC Cable Compound | Brominated Polyolefin Compound | LSZH ATH-Filled EVA Compound | Intumescent Polyolefin Compound |
|---|---|---|---|---|
| Flame retardant loading | Inherent Cl plus 3–8 % additive | 12–20 % | 60–65 % | 22–28 % |
| Limiting oxygen index | 26–32 % | 26–30 % | 32–42 % | 28–34 % |
| Smoke density | Very high | High | Very low | Low |
| Halogen acid gas | Substantial HCl | Substantial HBr | None | None |
| Tensile strength | 12–20 MPa | 12–20 MPa | 9–14 MPa | 14–22 MPa |
| Elongation at break | 180–350 % | 300–600 % | 130–250 % | 250–500 % |
| Specific gravity | 1.35–1.55 | 1.15–1.35 | 1.45–1.60 | 1.05–1.18 |
| Water absorption sensitivity | Low | Low | Moderate | High without encapsulation |
| Extrusion line speed | High | High | Reduced, viscosity limited | Moderate to high |
| Relative compound cost | Low | Medium | Medium–High | High |
Testing and Certification: LOI, UL 94, IEC and GB Standards
Cable flame retardancy is validated through a hierarchy of tests ranging from small-scale material screening to full-scale cable burn chambers, and it is essential to know which level a given result belongs to. A material that achieves UL 94 V-0 on a moulded bar has not demonstrated that a finished cable will pass IEC 60332-3, because the bunched cable test involves far more fuel, a far larger ignition source and a chimney geometry that promotes propagation.
Screening-level material tests
Limiting oxygen index, described in ASTM D2863 and ISO 4589, measures the minimum oxygen concentration in an oxygen-nitrogen mixture that will just sustain candle-like burning of a vertically mounted specimen. Because ambient air contains about 21 percent oxygen, a material with LOI above roughly 26 percent is generally self-extinguishing in normal atmosphere, and cable specifications commonly call for 28 to 32 percent or higher. LOI is the fastest and most discriminating development tool: a formulator can rank twenty candidate formulations in a day.
UL 94 is the most widely cited flammability classification. The vertical burning test on bar specimens produces V-0, V-1 and V-2 ratings based on afterflame time, total afterflame across ten flame applications, and whether flaming drips ignite cotton beneath the specimen. V-0 requires individual afterflame not exceeding 10 seconds, total not exceeding 50 seconds for the set, and no flaming drips. For thin materials the equivalent is the VTM classification. Crucially for cable, UL 94 also includes VW-1, a vertical wire flame test applied to the finished wire rather than a moulded bar, and VW-1 is the rating most often specified in North American appliance and internal wiring applications.
Cable-level flame propagation tests
IEC 60332-1 subjects a single vertical insulated conductor or cable to a one-kilowatt premixed flame for a defined period and requires that charring not extend beyond a specified distance from the burner. IEC 60332-2 applies a smaller flame to small conductors. IEC 60332-3 is the severe bunched cable test conducted in a three-metre-high chamber with a ribbon burner delivering around 20 kilowatts plus forced airflow, classifying cable into categories that reflect the volume of non-metallic material per metre — category A F-R and A representing the highest fuel loads, down through B, C and D for progressively smaller bundles.
In the European Union, the Construction Products Regulation introduced the Euroclass system for cables through EN 50575 and the EN 50399 test method, classifying cables from Aca through Fca with additional ratings for smoke production, acidity and flaming droplets. Achieving the higher Euroclasses requires low heat release and low flame spread measured in a large-scale rig, and this has been a major driver of LSZH adoption across Europe.
Circuit integrity and fire survival
Some cables must continue to function during a fire — emergency lighting, fire alarm circuits, smoke extraction fans, lift control. Circuit integrity is assessed by tests such as IEC 60331, where the cable is subjected to flame while energised and must maintain circuit continuity for a defined period, typically 30, 90 or 120 minutes. These cables use mica tape barriers, ceramifying silicone rubber or ceramifiable polyolefin compounds that convert to a self-supporting ceramic shell rather than simply resisting ignition. Compounding ceramifying formulations is a specialised discipline requiring careful dispersion of mica, wollastonite and low-melting glass frit at high loading.
Chinese national standards
China’s GB standard system mirrors the IEC framework closely for cable flammability. GB/T 18380 corresponds to the IEC 60332 series for flame propagation on single and bunched cables. GB/T 17651 corresponds to IEC 61034 for smoke density in the three-metre cube. GB/T 17650 corresponds to IEC 60754 for halogen acid gas. GB/T 19216 corresponds to IEC 60331 for circuit integrity. GB/T 2406 covers oxygen index determination. Exporters serving both markets generally find that a formulation validated to the IEC set will meet the corresponding GB/T requirements, but certification must still be obtained separately and the CCC compulsory certification scheme applies to many cable categories sold in China.
| Standard | Scope | Specimen Level | Typical Pass Criterion | Chinese Equivalent |
|---|---|---|---|---|
| ASTM D2863 / ISO 4589 | Limiting oxygen index | Moulded material bar | LOI above 26–32 % as specified | GB/T 2406 |
| UL 94 V-0 | Vertical burning classification | Moulded bar at stated thickness | Afterflame ≤10 s each, ≤50 s total, no flaming drips | GB/T 2408 equivalent method |
| UL 94 VW-1 | Vertical wire flame test | Finished insulated wire | Self-extinguish within 60 s, flag and cotton not ignited | Referenced in GB cable specs |
| IEC 60332-1 | Single vertical cable flame spread | Single finished cable | Char within defined distance of burner | GB/T 18380.11-13 |
| IEC 60332-3 | Bunched vertical cable flame spread | Cable bundle in 3 m chamber | Char height below 2.5 m; category A F-R, A, B, C, D | GB/T 18380.31-36 |
| IEC 61034 | Smoke density, three-metre cube | Cable bundle | Light transmittance not less than 60 % | GB/T 17651 |
| IEC 60754-1 | Halogen acid gas quantity | Compound sample | HCl equivalent not more than 0.5 % by mass | GB/T 17650.1 |
| IEC 60754-2 | pH and conductivity of gases | Compound sample | pH not below 4.3, conductivity not above 10 µS/mm | GB/T 17650.2 |
| IEC 60331 | Circuit integrity under fire | Energised finished cable | Continuity maintained 30, 90 or 120 minutes | GB/T 19216 |
| EN 50399 / EN 50575 | Euroclass reaction to fire | Cable bundle, large-scale rig | Class Aca to Fca plus s, a, d sub-ratings | Not directly equivalent |
| RoHS and REACH | Restricted substances | Compound composition | Pb, Cd, Hg, Cr(VI), PBB, PBDE and phthalate limits | GB/T 26572 and related |
Masterbatch Formulation Architecture and Let-Down Design
A cable flame retardant masterbatch is a system, not a single ingredient, and a robust formulation typically contains six or seven functional groups whose interactions must all be validated. Designing the architecture properly is what separates a concentrate that performs at the customer’s line from one that looks correct on paper and fails in production.
The functional building blocks
- Primary flame retardant at the largest fraction: brominated aromatic, metal hydroxide, ammonium polyphosphate system or phosphate ester.
- Synergist to multiply the primary’s effectiveness: antimony trioxide for bromine, zinc borate or silicone for metal hydroxide, melamine derivative for phosphorus.
- Coupling agent to bond filler to polymer: vinyl or amino silane on the filler surface, or maleic anhydride grafted polyolefin in the polymer phase at 3 to 8 percent.
- Dispersing aid and lubricant: polyethylene wax, oxidised polyethylene wax, calcium or zinc stearate, or ethylene bis-stearamide, typically 0.5 to 2.0 phr, to reduce viscosity and protect the compounding line from abrasive wear. The lubricant must be chosen so that it does not measurably lower the limiting oxygen index — a hydrocarbon wax is additional fuel.
- Antioxidant package: a hindered phenolic primary antioxidant with a phosphite or thioester secondary, sized for both compounding heat history and long-term thermal ageing at the cable’s rated temperature. Metal deactivators are essential in compounds in direct contact with copper conductors, because copper ions catalyse polyolefin oxidation.
- Anti-drip agent where required: PTFE micropowder at 0.2 to 0.5 percent forms a fibrillated network that prevents flaming drips, which is often the difference between V-2 and V-0.
- Colour system and UV stabiliser: carbon black for outdoor jackets, or a pigment system matched to the cable colour code, plus hindered amine light stabilisers for exposed cable. Note that hindered amine stabilisers can be deactivated by acidic species from halogenated retardants, so acid-resistant grades are required.
Concentration and let-down arithmetic
The achievable masterbatch concentration is bounded by what the carrier can hold while remaining pelletisable and dispersible. For brominated systems, 60 to 75 percent total actives on a polyolefin carrier is routine. For metal hydroxides, concentrates above about 75 percent become extremely difficult to pelletise and to disperse at the customer’s single-screw extruder, and since the target final loading is 60 to 65 percent, the let-down ratio would need to be around 85 percent — at which point producing a fully formulated compound is simply better engineering.
The practical rule is straightforward: if the required final active loading exceeds roughly 25 to 30 percent, supply a fully formulated compound rather than a concentrate. Below that threshold, a concentrate offers real logistical and inventory advantages. This is precisely why the cable industry buys brominated flame retardant concentrates but buys LSZH as compound.
| Product Format | Active Concentration | Let-Down Ratio | Final Active in Compound | Typical Target Rating |
|---|---|---|---|---|
| Brominated concentrate for PE jacket | 65–72 % (Br FR plus Sb2O3) | 20–28 % | 14–19 % | UL 94 V-0, IEC 60332-1 |
| Brominated concentrate for PP | 60–70 % | 22–30 % | 15–20 % | UL 94 V-0 at 1.6 mm |
| Intumescent concentrate for polyolefin | 55–65 % APP system | 38–48 % | 22–28 % | UL 94 V-0, halogen-free |
| LSZH fully formulated compound | 60–65 % ATH or MDH | Used at 100 % | 60–65 % | IEC 60332-3, 61034, 60754 |
| Smoke suppressant concentrate for PVC | 50–60 % molybdate and zinc borate | 6–12 % | 3–7 % | Reduced smoke PVC |
| Silicone synergist concentrate | 40–50 % silicone gum | 2–5 % | 1–2.5 % | Filler reduction, lower jacket COF |
| Anti-drip concentrate | 10–20 % PTFE micropowder | 1.5–3 % | 0.2–0.5 % | V-2 to V-0 upgrade |
Interaction traps to validate
Several well-documented antagonisms deserve explicit checking during development. Hindered amine light stabilisers are basic and can be deactivated by acidic halogenated species and by acidic phosphorus compounds, so acid-tolerant grades must be selected. Antimony trioxide interacts with certain pigments and can shift colour. Ammonium polyphosphate is incompatible with basic fillers such as calcium carbonate, which neutralise the acid source and destroy intumescence. Metal hydroxides are alkaline and can accelerate hydrolysis of polyester-based TPU. Peroxide cross-linking systems are inhibited by radical scavengers, which includes many antioxidants and some flame retardants, so a compound designed for thermoplastic use will not simply transfer to a peroxide-cured XLPE application. Every one of these has caused expensive field failures, and every one is inexpensive to catch in a laboratory-scale trial.
Twin-Screw Compounding of Highly Filled Cable Masterbatch
Compounding a 60 to 65 percent mineral-filled halogen-free cable compound is among the most demanding duties a twin-screw extruder performs, and machine specification errors made at purchase cannot be corrected afterwards. Three constraints define the problem: the filler volume fraction is so high that feeding becomes the throughput bottleneck, the melt temperature ceiling imposed by ATH decomposition is far below normal polyolefin practice, and the filler is abrasive enough to consume conventional nitrided screws rapidly.
Feeding architecture: the real bottleneck
At 62 percent ATH the mineral is roughly 40 percent by volume, and untreated ATH powder has bulk density in the 300 to 600 kg/m³ range depending on grade, with a great deal of entrained air. Attempting to feed all of that through the main port together with the polymer is impossible: the extruder becomes feed-limited long before it becomes torque-limited, and entrained air backs up out of the feed throat, carrying dust and destabilising the feed.
The established solution on Kerke KTE-series lines is split side feeding. Polymer, coupling agent and additives enter the main hopper through loss-in-weight feeders. The mineral filler is divided between two twin-screw side feeders, the first positioned after the initial melting zone at approximately 14 to 18 L/D, the second at approximately 26 to 30 L/D. Splitting the filler roughly 60:40 or 50:50 between the two positions avoids overwhelming the melt at any single point, keeps local viscosity and shear heating under control, and permits considerably higher total output than single-point addition.
Each side feeder should be fitted with an atmospheric vent immediately upstream so that entrained air escapes rather than being forced into the melt. For very fine ATH grades below about 1.5 micrometres median size, a crammer feeder or a compacting side feeder is needed to overcome the poor flow and high air content. Loss-in-weight gravimetric feeding is mandatory rather than volumetric, because flame rating depends on hitting the loading target consistently and volumetric feeders drift as bulk density changes with humidity and fill level.
Screw configuration principles
The screw design objective is thorough wetting and distribution of the mineral at the lowest possible shear energy input. This differs from a glass-fibre compound, where the goal is fibre length preservation, and from a pigment concentrate, where the goal is agglomerate rupture. Specific recommendations:
- Total length of 44 to 52 L/D. The extra length compared with a general-purpose 40 L/D machine is needed to accommodate two side feed positions, two vacuum vents and adequate distributive mixing between them without raising shear intensity.
- Melt the polymer completely before the first filler addition. A short kneading section using 45-degree forward blocks at 10 to 14 L/D achieves this. Adding filler to unmelted polymer produces agglomerates that never disperse.
- Use moderate-intensity mixing after each filler addition. Combinations of narrow forward kneading discs and toothed or slotted distributive elements wet the filler surface without generating the shear peaks that push local melt temperature past the ATH decomposition threshold. Avoid long blocks of 90-degree neutral or reverse elements, which create high-shear hot spots.
- Include melt seals only where functionally necessary, ahead of vacuum vents, and keep them short. Every reverse element is a local temperature spike.
- Provide generous free volume in the conveying elements between mixing zones. Deep-flight, high-free-volume elements are essential for handling the large volume of low-bulk-density filler.
- Two vacuum vents, one at approximately 32 L/D and one near 42 L/D, operating at 0.05 to 0.09 MPa absolute, remove moisture carried in by the hygroscopic filler and any volatiles from coupling agent reaction. Residual moisture in a cable compound is unacceptable because it produces voids that reduce dielectric strength.
- Keep screw speed moderate, typically 250 to 400 rpm, and accept that this compound will not run at the 600 rpm used for lightly filled polyolefins. Tip speed drives shear heating, and the melt temperature ceiling is the binding constraint.
Thermal control: the defining discipline
For ATH-filled compounds, every barrel zone from the first filler addition onward should be set to deliver melt temperature in the 165 to 185 °C band, with an absolute ceiling around 190 °C. Barrel setpoints are typically 20 to 30 °C below the desired melt temperature in the high-filler zones, because viscous dissipation, not conduction, is the dominant heat source. Effective barrel cooling with adequate water flow and correctly sized cooling channels is not optional. Measure melt temperature with an immersion probe at the die adapter, not with a flush-mounted sensor, which reads barrel wall temperature rather than melt temperature and can under-report by 10 to 20 °C.
For MDH-filled compounds the ceiling rises to roughly 250 to 270 °C, which relieves the constraint considerably and is one of the practical reasons to accept MDH’s higher relative cost when the matrix requires higher processing temperature.
| Process Parameter | LSZH 62 % ATH in EVA/POE | Brominated FR Concentrate on PE | Intumescent APP System on PP | FR PVC Cable Compound |
|---|---|---|---|---|
| Recommended L/D | 44–52 | 40–44 | 40–48 | 32–40 |
| Feed throat | Water-cooled 25–40 °C | Water-cooled 30–45 °C | Water-cooled 30–45 °C | Water-cooled 30–40 °C |
| Melting zone barrel temp | 150–170 °C | 170–190 °C | 190–210 °C | 150–170 °C |
| Mixing zone barrel temp | 145–165 °C | 165–185 °C | 185–205 °C | 155–172 °C |
| Die zone temp | 150–168 °C | 170–188 °C | 190–205 °C | 160–175 °C |
| Target melt temperature | 165–185 °C, ceiling 190 °C | 185–205 °C | 205–225 °C | 170–190 °C, ceiling 205 °C |
| Screw speed | 250–400 rpm | 300–500 rpm | 300–450 rpm | 150–300 rpm |
| Specific energy input | 0.19–0.26 kWh/kg | 0.16–0.22 kWh/kg | 0.18–0.24 kWh/kg | 0.13–0.19 kWh/kg |
| Vacuum level | 0.05–0.09 MPa abs, two vents | 0.06–0.09 MPa abs, one vent | 0.05–0.08 MPa abs, two vents | 0.07–0.09 MPa abs |
| Filler feeding | Split two side feeders | Single side feeder | Single or split side feeder | Pre-blended dry blend |
| Pelletising method | Water-ring or underwater die face | Water-cooled strand | Water-cooled strand or die face | Hot die face, air cooled |
| Barrel and screw metallurgy | Bimetallic liner, wear-resistant elements | Bimetallic or nitrided | Bimetallic, corrosion resistant | Corrosion-resistant overlay |
Wear management and machine specification
Aluminium trihydroxide has Mohs hardness around 2.5 to 3.5, which sounds mild, but at 40 percent volume fraction and with sub-micron particles carried at high pressure across flight lands, the cumulative abrasion is severe. Kerke specifies bimetallic barrel liners with hard alloy overlays for cable compounding duty, together with high-wear-resistance screw elements in the kneading zones where contact pressure concentrates. Because KTE-series machines use segmented barrels and splined screw shafts with individually replaceable elements, a worn mixing section can be renewed without replacing the entire screw or barrel — a significant advantage when running abrasive LSZH formulations continuously.
Torque capability also matters. A highly filled compound at reduced melt temperature is viscous, and the machine must deliver the specified output without hitting the torque limit. High-torque gearbox designs with elevated specific torque per cubic diameter allow the same screw diameter to process more filled material, which is why cable compounders generally specify high-torque configurations rather than standard ones.
Downstream and pelletising
Highly filled LSZH melt has poor melt strength, so water-cooled strand pelletising is often unreliable — strands sag and break. Water-ring die-face cutting or underwater granulation gives far better stability, produces a uniform lens-shaped or spherical pellet that feeds well in the cable extruder, and avoids the extended strand path where breakage occurs. Kerke’s cutting system range covers water-cooled strand, air-cooled strand, air-cooled die face, water-ring die face, eccentric water-mist and full underwater granulation, allowing the pelletising route to be matched to the compound rather than the reverse. Downstream, thorough dewatering and drying are essential because the hygroscopic filler will otherwise carry moisture into the cable line.
For very high filler loadings or heat-sensitive combinations, the KTE-SE double-stage mother-baby system offers a valuable alternative: the first-stage co-rotating twin screw performs mixing and devolatilisation at low pressure, and a second-stage single screw builds the die pressure needed for pelletising at a lower melt temperature. This decoupling is often the cleanest way to keep an ATH compound below its decomposition threshold while still generating adequate head pressure.
Selection Guide by Cable Type and Installation Environment
The right flame retardant system follows from the installation environment and the governing specification, not from a general preference for one chemistry over another. The following guidance maps common cable categories to appropriate systems.
Building wire and general low-voltage installation
For conventional building wire installed in conduit in residential and light commercial buildings, flexible PVC with an antimony-zinc borate package remains the dominant and appropriate solution. It meets IEC 60332-1 comfortably, costs less than alternatives, and the fire risk profile of dispersed conduit installation is different from that of a concentrated riser. Where local building regulations have adopted higher Euroclass requirements, or where the installation is a public assembly building, LSZH becomes mandatory.
Data centres, telecommunications and plenum spaces
These environments combine very high cable density, forced air circulation that distributes smoke rapidly, and extremely valuable electronic equipment that corrosive gases destroy. LSZH is effectively mandatory, and specifications typically require IEC 60332-3 category B or better, IEC 61034 light transmittance above 60 percent, IEC 60754 halogen-free confirmation, and increasingly a Euroclass Cca or B2ca rating in Europe. In North America, plenum-rated cable historically used fluoropolymer insulation for its intrinsic low smoke and flame characteristics, which is a different but equally valid solution path.
Rail, metro and tunnel
Underground transport imposes the most severe combination of requirements: long confined spaces with limited escape routes, high passenger density, and cables that must continue operating during an evacuation. Expect LSZH as a baseline, circuit integrity requirements to IEC 60331 for safety circuits, toxicity index limits, and mechanical requirements for vibration and oil resistance. Ceramifying silicone or mica-taped constructions handle the fire-survival circuits, while ATH-filled EVA compounds handle general jacketing.
Marine, offshore and defence
Salt-fog corrosion resistance, mud and hydrocarbon exposure, and strict smoke and toxicity limits characterise this segment. Halogen-free is standard. TPU jackets with phosphorus-based flame retardants serve where abrasion and oil resistance dominate; ATH or MDH-filled polyolefin serves general jacketing. Note that magnesium dihydroxide is often preferred over ATH in these applications because of better water resistance and higher processing temperature latitude.
Photovoltaic and outdoor installation
Solar cable faces continuous ultraviolet exposure, wide temperature cycling, and a 25-year design life. Cross-linked polyolefin with carbon black, an effective hindered amine light stabiliser package and a UV-stable flame retardant such as ethylene bis-tetrabromophthalimide or a halogen-free mineral system is typical. Ozone and weathering resistance testing supplements the flammability requirements.
Automotive and electric vehicle
High-voltage EV cabling has driven rapid growth in specialised flame retardant compounds. Requirements include halogen-free construction, resistance to battery coolant and automotive fluids, operation to 125 °C or above for some circuits, thin-wall extrusion for weight and packaging reasons, and very strict volatile organic compound and fogging limits for interior applications. Cross-linked halogen-free polyolefin and specialised TPU compounds dominate, and the thin-wall requirement makes intumescent systems attractive because their lower loading preserves the flexibility needed for tight routing.
Troubleshooting Flame Retardant Cable Compound Production
Problems in flame retardant cable compounding cluster into predictable patterns, and the majority trace back to one of four root causes: melt temperature exceeding the filler’s decomposition threshold, moisture, inadequate dispersion, or additive antagonism. The table below maps symptoms to causes and remedies.
| Symptom | Probable Cause | Diagnostic Check | Corrective Action |
|---|---|---|---|
| Foaming or porous strands | ATH dehydration from excessive melt temperature | Immersion probe melt temperature; TGA of pellets | Lower barrel setpoints, reduce screw speed, soften kneading blocks, improve barrel cooling |
| Voids and pinholes in extruded insulation | Residual moisture from hygroscopic filler | Karl Fischer moisture on pellets; check vacuum integrity | Restore full vacuum on both vents, pre-dry filler, seal packaging, add drying before cable line |
| Fails UL 94 V-0 despite correct formulation | Poor dispersion leaving polymer-rich domains | Microtome section under optical microscope; ash content uniformity across samples | Add distributive elements after each side feed, verify filler feed accuracy, check for feeder surging |
| Flaming drips causing V-2 instead of V-0 | No anti-drip agent or insufficient melt strength | Observe drip behaviour during test | Add PTFE micropowder at 0.2–0.5 %, consider cross-linking or higher molecular weight base resin |
| Elongation below specification | Poor filler-polymer adhesion at high loading | SEM of tensile fracture surface for filler pull-out | Add silane treatment or maleic anhydride grafted polyolefin, raise EVA vinyl acetate content, blend POE |
| Rough or grainy jacket surface | Coarse filler fraction or agglomerates | Screen pack pressure rise test; sieve residue on filler | Switch to finer or narrower particle size distribution, add screen changer, strengthen dispersive mixing |
| Compound discolouration and yellowing | Thermal degradation, antioxidant depletion, or copper catalysis | Oven ageing test; check antioxidant level by extraction | Increase antioxidant, add metal deactivator, reduce residence time and melt temperature |
| Halogen test failure in nominally LSZH compound | Cross-contamination or halogenated minor ingredient | Ion chromatography on combustion products; audit every raw material | Dedicated LSZH line or rigorous purge protocol, replace halogenated processing aid or pigment |
| Intumescent compound fails after water immersion | Ammonium polyphosphate leaching | Weight loss and LOI after 7-day immersion | Switch to microencapsulated APP, add silane treatment, hybridise with metal hydroxide |
| Torque limit reached below target output | Viscosity too high at the required low melt temperature | Compare torque percentage against output curve | Optimise lubricant package, use fatty-acid-coated filler, specify high-torque gearbox, consider double-stage system |
| Feed throat surging and dust blowback | Air entrainment from low-bulk-density filler | Observe side feeder vent; check feeder mass flow stability | Add atmospheric vent upstream of side feeder, use compacting or crammer side feeder, split filler addition |
| Rapid screw and barrel wear | Abrasive filler with inadequate metallurgy or lubrication | Measure radial clearance at intervals; monitor output drift at fixed settings | Specify bimetallic liners and wear-resistant elements, increase lubricant, reduce shear intensity |
| Insulation resistance drops after humid ageing | Untreated hydrophilic filler surface, water uptake | Insulation resistance before and after humidity conditioning | Use silane-treated filler, improve coupling, verify complete devolatilisation |
Frequently Asked Questions
Can I simply add a flame retardant masterbatch to an existing cable compound to upgrade its rating?
Rarely without reformulation. Adding a concentrate dilutes everything else in the compound, including plasticiser, antioxidant, colour and any existing filler, so mechanical and ageing properties shift. It also changes viscosity and may require extrusion parameter changes. The correct approach is to redesign the full formulation with the flame retardant included from the start, then validate mechanical properties, electrical properties, ageing and flammability as a set. Treating flame retardancy as a bolt-on almost always produces a compound that passes the burn test and fails something else.
Why does my compound pass UL 94 V-0 but fail IEC 60332-3?
Because they measure fundamentally different things. UL 94 V-0 is a small-scale material test on a moulded bar with a small ignition source. IEC 60332-3 burns a bundle of finished cables in a three-metre chamber with a roughly 20 kilowatt ribbon burner and forced airflow, with far more fuel and a chimney effect that drives propagation. Passing the material test is necessary but not sufficient. Cable-level performance also depends on cable construction: bedding materials, filler yarns, tape layers, conductor size and the total volume of non-metallic material per metre all contribute.
How much does high filler loading reduce compounding throughput?
Substantially, though the exact figure depends on the machine. A 62 percent ATH-filled LSZH compound typically runs at a fraction of the output that the same twin-screw extruder would achieve on an unfilled polyolefin, because the machine becomes limited by feeding capacity for the low-bulk-density filler and by the melt temperature ceiling that caps allowable shear energy input. Split side feeding, high-torque gearboxes, generous free volume in the screw design and an optimised lubricant package are the four levers that recover the most throughput.
Is halogen-free always safer than halogenated?
Halogen-free is clearly better on smoke obscuration, acid gas corrosivity and equipment survival, which is why it is mandated in confined public spaces. But halogenated systems can achieve higher flame retardancy at lower loading, and a halogen-free compound that has been pushed to marginal filler loading in order to preserve mechanical properties may actually be closer to its performance limit. The right framing is to specify the fire scenario and the required test results, then select the chemistry that meets them with adequate margin. In open outdoor installations where smoke dispersal is not a concern, halogenated systems remain technically appropriate.
What causes elongation to collapse in LSZH compounds and how is it fixed?
At 60 to 65 percent mineral loading the polymer becomes a minority phase and the filler particles act as stress concentrators with weak interfacial adhesion, so cracks initiate at particle surfaces at low strain. The fixes work by strengthening that interface and by softening the polymer phase: silane or fatty acid surface treatment on the filler, maleic anhydride grafted polyolefin as a polymeric coupling agent, higher vinyl acetate content EVA for better wetting, POE elastomer blending for chain mobility, and bimodal particle size distribution to improve packing efficiency. Applied together these routinely lift elongation from below 100 percent to 150 to 250 percent.
Can flame retardant cable compounds be recycled?
Halogen-free mineral-filled compounds are the most recyclable of the flame retardant families, because the filler is thermally stable and chemically inert; production scrap can generally be reground and reintroduced at 10 to 20 percent with modest property loss, provided the material has not been thermally degraded. Cross-linked compounds cannot be remelted and are limited to use as ground filler. Halogenated compounds face regulatory restrictions on recycling in some jurisdictions because of brominated substance controls. Within the Wanplas group, Polyretec supplies washing and recycling systems for cable and wire plastic recovery, while Kerke supplies the twin-screw pelletising lines used to re-compound the recovered material.
What antioxidant system should a flame retardant cable compound use?
A combination of a hindered phenolic primary antioxidant with a phosphite or thioester secondary is standard, sized for both the compounding heat history and the long-term thermal ageing requirement at the cable’s rated temperature. Compounds in direct contact with copper conductors must include a metal deactivator, because copper ions catalyse hydroperoxide decomposition and dramatically accelerate polyolefin oxidation. Be aware that acidic species from halogenated flame retardants and from acidic phosphorus compounds can deactivate basic hindered amine light stabilisers, so acid-tolerant grades are required in those systems.
How do I decide between a fully formulated compound and a concentrate?
Use the final active loading as the deciding criterion. If the required loading is below roughly 25 to 30 percent, a concentrate is practical and offers inventory and logistics advantages, letting the cable maker adjust colour and base resin locally. If the required loading exceeds that, as it always does with metal hydroxide systems, supply a fully formulated compound. Attempting to reach 62 percent ATH by letting down a 78 percent concentrate leaves almost no room for the base resin and produces poor dispersion in the cable extruder’s single-screw machine.
What is the role of silicone in halogen-free cable compounds?
Silicone plays three roles simultaneously. As a flame retardant synergist, it migrates to the surface during burning and forms a silica-rich ceramic layer that reinforces the mineral residue, allowing filler loading to be reduced by several percentage points at equal rating. As a processing aid, high-molecular-weight silicone gum reduces melt viscosity and die pressure in a highly filled compound, recovering throughput. As a surface modifier, it lowers the jacket’s coefficient of friction, which reduces the pulling tension needed to install long cable runs through conduit. Its relative cost is High, but the combined benefit across three functions frequently justifies it.
Which twin-screw extruder size should I specify for LSZH cable compound production?
Size the machine on the required output at the reduced throughput that a 60-plus percent filled compound imposes, not on the machine’s rating for unfilled polyolefin, and specify a high-torque configuration with adequate L/D for two side feed positions and two vacuum vents. In practice most commercial LSZH lines use screw diameters in the 65 to 95 mm range for mid-volume production and larger for high-volume dedicated plants. Kerke’s practice is to validate the formulation on a laboratory-scale KTE twin-screw extruder to establish the specific energy input, melt temperature profile and torque demand, then scale the production machine from measured data rather than from generic rules.
Conclusion
Flame retardant masterbatch for cable and wire plastics is a systems engineering problem in which chemistry, mechanics, electrical performance and processing all constrain one another. The polymer matrix sets the processing temperature ceiling and therefore decides whether aluminium trihydroxide or magnesium dihydroxide is viable. The installation environment sets the smoke and acid gas requirements and therefore decides whether halogenated chemistry is permissible at all. The required loading follows from the mechanism — catalytic radical scavenging at 12 to 20 percent, intumescent char formation at 22 to 28 percent, endothermic mineral cooling at 60 to 65 percent — and that loading in turn determines whether the product is delivered as a concentrate or as a fully formulated compound, and how hard the compounding line will have to work.
The practical discipline is consistent across all four systems. Validate against the actual governing test, not a proxy: UL 94 and limiting oxygen index for screening, IEC 60332, IEC 61034 and IEC 60754 for compliance, IEC 60331 where circuit integrity is required, and the corresponding GB/T methods for the Chinese market. Design the formulation as a whole, including coupling agent, lubricant, antioxidant, metal deactivator and anti-drip agent, and test for antagonism before scale-up. On the production side, respect the melt temperature ceiling absolutely, split the filler feed, provide two vacuum vents, use distributive rather than aggressive dispersive mixing, and specify wear-resistant metallurgy from the outset rather than after the first screw set is consumed.
Kerke, a Wanplas factory with more than twelve years of specialisation in parallel co-rotating twin-screw compounding, lists cable compounding among its core application industries and supplies KTE-series lines configured specifically for this duty — high-torque gearboxes, 44 to 52 L/D barrel sets, twin-screw side feeders and crammer feeders for low-bulk-density mineral filler, loss-in-weight gravimetric feeding, multi-stage vacuum devolatilisation, bimetallic barrel liners with replaceable segments, and a full range of pelletising systems from water-cooled strand through water-ring die face to underwater granulation. Laboratory KTE twin-screw machines support formulation development and scale-up validation, and the KTE-SE double-stage system handles the most heat-sensitive highly filled formulations. Backed by the Wanplas brand’s shared engineering standards, open-factory policy, annual free spare-parts allowance and service coverage across more than 70 countries, Kerke supports cable compounders from first formulation trial through commissioning and long-term production optimisation. If you are planning a flame retardant cable compound programme in 2026, define the governing standards first, run a laboratory-scale trial to measure specific energy and melt temperature, and specify the compounding line around the filler loading you actually need to achieve.
