Polymer additives and compounding

In short

Compounding is the art of mixing a base polymer with additives — plasticisers, stabilisers, fillers, colourants, lubricants, modifiers — into a homogeneous, processable, fit-for-purpose material. A virgin resin is almost never used neat: PVC without a heat stabiliser degrades in the extruder, polypropylene without an antioxidant chain-scissions, and polyethylene without carbon black fails in Indian sunlight within a year. Formulations are written in phr (parts per hundred resin) and converted to batch weights on the shop floor. It is examined in Polymer Materials, Plastics Processing and Polymer Technology papers and in GATE XE-F/Polymer Science, and it is the core competence tested in interviews at Supreme, Finolex, Astral, Prince Pipes, Reliance, SABIC, Clariant, Cabot, Plastiblends, Kabra Extrusiontechnik and every masterbatch and PVC compound house in Daman, Vapi, Ahmedabad and Bhiwandi.

Explained

Why additives, and the phr convention

• Purpose classes: (i) make it processable (plasticisers, lubricants, processing aids), (ii) keep it alive (heat stabilisers, antioxidants, UV stabilisers), (iii) change properties (fillers, impact modifiers, flame retardants, nucleants), (iv) make it saleable (pigments, optical brighteners, antistats, biocides).
• phr: parts by weight per 100 parts of resin. A recipe can total more than 100 (e.g. 176 phr); weight % must be computed from it. The convention comes from rubber compounding and is used across PVC and thermoplastics.
• Three delivery forms: *compound* (fully formulated pellets, ready to mould), *masterbatch* (additive concentrated 20-70 % in a carrier resin, diluted at the machine), *dry blend* (free-flowing powder mix, used for PVC pipe/profile where melt compounding is skipped).
• Cost logic: additives are both a cost and a cost-saver — 20-40 phr of coated CaCO₃ at ₹10-15/kg against resin at ₹90-110/kg cuts the rupee-per-kg price, but raises density, so the honest comparison is cost per litre of product, not per kg.
• Permanence: an additive must not bloom, migrate, extract or volatilise. Low molecular weight = effective but fugitive; polymeric/reactive versions = permanent but costlier.

Plasticisers, lubricants and processing aids

• Mechanism: a plasticiser solvates the polymer, increases free volume and chain mobility, lowers Tg and modulus, raises elongation and low-temperature flexibility. 80-90 % of all plasticiser worldwide goes into PVC.
• Primary plasticisers (compatible, used alone): phthalates — DOP/DEHP, DINP, DIDP; non-phthalates — DOTP (dioctyl terephthalate, now the Indian workhorse), DINCH, citrates (toy/medical grades).
• Secondary plasticisers / extenders: chlorinated paraffin, epoxidised soybean oil (ESBO, also a co-stabiliser) — cheapen the blend but limited compatibility.
• Specialty: adipates (DOA) for cold flexibility, trimellitates (TOTM) for high-temperature cable, polymeric adipate polyesters for migration-resistant gaskets and food-contact uses, phosphate esters for flame retardance.
• Internal plasticisation is copolymerisation (VC-vinyl acetate) — permanent, non-extractable, no migration, but set at the resin stage.
• Regulatory point: DEHP, DBP and BBP are restricted in toys and childcare articles and are avoided in food-contact and medical PVC; Indian toy grades are built on DOTP/DINCH or citrates. Limits are revised, so always check the current BIS/FSSAI text.
• Lubricants: *external* (paraffin wax, PE wax, oxidised PE wax) migrate to the metal wall and reduce friction and plate-out; *internal* (glycerol monostearate, fatty acid esters) reduce melt viscosity; calcium stearate and stearic acid act as both and also as PVC co-stabilisers. Overdosing delays fusion and kills weld-line strength.
• Processing aids: acrylic copolymers (high MW PMMA-type) raise melt strength and promote PVC fusion in foam and profile; fluoropolymer PPA removes melt fracture ("sharkskin") in LLDPE film.

Stabilisers: heat, oxidation and light

• PVC heat stabilisers absorb the HCl split off above ~150 °C (autocatalytic dehydrochlorination → discoloration → charring):
  • Ca/Zn and Ba/Zn soaps — the standard non-toxic systems (pipe, profile, food film, toys).
  • Organotins (octyltin, methyltin mercaptide) — best clarity, used for rigid clear sheet and bottles.
  • Lead salts (tribasic lead sulphate, dibasic lead stearate) — excellent and cheap but toxic; progressively removed from Indian potable-water applications, and the current IS 4985 for uPVC pipes requires a lead-free (unleaded) stabiliser system. Still seen in some wire & cable.
  • Co-stabilisers/synergists: ESBO, β-diketones, hydrotalcite, organic phosphites, polyols, zeolites.
• Antioxidants (polyolefins, styrenics, engineering plastics) break the auto-oxidation cycle R• → ROO• → ROOH:
  • Primary / chain-breaking: hindered phenols and secondary aryl amines — scavenge peroxy radicals (long-term thermal stability).
  • Secondary / preventive: phosphites, phosphonites and thioesters — decompose hydroperoxides (melt-processing stability).
  • A primary + secondary blend is synergistic: the phosphite protects colour during extrusion, the phenol protects service life. Metal deactivators are added where copper contacts the polymer (cable insulation).
• Light/UV stabilisers: HALS (hindered amine light stabilisers — regenerative radical scavengers, the most efficient), UV absorbers (benzotriazoles, benzophenones), quenchers (nickel complexes), and carbon black at about 2-2.5 % — the cheapest and best UV screen, which is why water and gas pipe is black; IS 4984 (PE pipes for water supply) specifies that content band together with a dispersion requirement.
• Why it matters in India: solar radiation and 45 °C ambient in Rajasthan/Gujarat make UV packages non-negotiable for tarpaulin, geomembrane, agri-film and tanks; greenhouse film additionally needs NOR-HALS because sulphur/halogen agrochemicals poison conventional HALS.

Fillers, reinforcements, colourants and specialty additives

• Extender fillers: ground/precipitated CaCO₃ (stearic-acid coated for wetting), talc, kaolin, dolomite, barytes (density/sound damping), fly-ash. They raise modulus, hardness, HDT and thermal conductivity; they lower impact strength, elongation and cost.
• Functional/reinforcing: glass fibre (short 3-4 mm, side-fed to protect length; 10-40 % in PA66, PBT, PP), wollastonite and mica (platy/acicular, high aspect ratio), precipitated silica, carbon fibre, nano-clay, CNT/graphene (conductive at low loading).
• Carbon black grades: furnace blacks for UV/colour; conductive blacks (high structure) for antistatic and semiconductive cable layers — effective above the percolation threshold (~5-10 vol %).
• Coupling agents and compatibilisers: silanes (aminosilane, vinylsilane) for glass, silica and ATH; maleic anhydride-grafted PP/PE for talc, natural fibre and PA/PP blends; titanates/zirconates. They convert a mere filler into a stress-transferring reinforcement.
• Impact modifiers: CPE, MBS, ABS and acrylic core-shell for PVC; EPDM/EPR, POE and SEBS for PP and PA; rubber-toughening works by cavitation and shear-band initiation at the dispersed phase.
• Flame retardants: halogen (brominated) + Sb₂O₃ synergist — gas-phase radical trapping; phosphorus/intumescent (APP + pentaerythritol + melamine) — char formation; ATH and MDH — endothermic water release, need 50-65 wt % of the compound (≈ 100-185 phr), give halogen-free low-smoke (LSZH cable); nitrogen and expandable graphite. PBB and PBDE are restricted to 0.1 % in electrical and electronic equipment under the E-Waste (Management) Rules, 2022, which is why halogen-free FR compounds dominate new designs.
• Blowing agents: chemical exothermic azodicarbonamide (PVC/EVA footwear, foam sheet), endothermic sodium bicarbonate/citric acid, physical pentane/CO₂; with nucleants for fine cell structure.
• Others: antistats (GMS, ethoxylated amines), slip (erucamide/oleamide) and antiblock (natural/synthetic silica) for film, nucleating and clarifying agents (sorbitol-based) for PP, optical brighteners, antifog, biocides, peroxides and silanes for crosslinking (XLPE cable, PEX pipe), chain extenders for recycled PET IV recovery, laser-marking and anti-counterfeit additives.

Compounding equipment and the process

• Batch — internal (Banbury/intermix) mixer: chamber + ram + two rotors; dispersive mixing at high shear; discharged to a two-roll mill or a dump extruder for pelletising. Used for masterbatch, highly filled and rubber-like compounds. Controlled by fill factor (0.6-0.75) and dump temperature.
• Continuous — co-rotating intermeshing twin-screw extruder (TSE): the industry standard. Segmented barrels and modular screws give conveying, melting, mixing (kneading blocks for dispersive, toothed/distributive elements), devolatilisation (vacuum vent) and pressure build-up zones; L/D 32-48; side feeder for glass fibre and high filler loads; gravimetric loss-in-weight feeders for every stream.
• Other machines: counter-rotating TSE (PVC pipe/profile direct extrusion), Buss co-kneader (shear-sensitive, highly filled, cable), FCM/continuous mixer, single-screw with Maddock/pin mixers (dilution only, not dispersion), high-intensity planetary mixer.
• Pelletising: strand die → water bath → strand pelletiser (general purpose); die-face/hot-water-ring or underwater pelletiser (low viscosity, tacky, foaming grades); dicing of sheet; micro-pellets for masterbatch.
• PVC dry blend route (no melting): resin + stabiliser + lubricant + filler + pigment into a high-speed turbo mixer, frictional heating to ~110-120 °C so the porous suspension-PVC grain absorbs the liquid plasticiser and additives, then transfer to a cooling mixer to ~45 °C to prevent lump formation; the free-flowing powder goes straight to a pipe/profile extruder. Saves one heat history on a heat-sensitive polymer.
• Order of addition matters: resin + solids first, liquids after (otherwise balling); stabiliser before pigment; lubricant last so it does not coat the resin and block absorption.
• Mixing physics: distributive mixing spreads the minor phase (needs strain, i.e. many reorientations); dispersive mixing breaks agglomerates (needs shear *stress* above the cohesive strength of the agglomerate). Elongational flow in kneading blocks is more efficient than simple shear. Residence time 20-90 s in a TSE; RTD breadth decides colour change-over time.
• Thermal management: all mechanical energy becomes heat. Specific energy input (SEI) of 0.10-0.35 kWh/kg is typical; too high means degradation, fibre breakage and yellowing, too low means poor dispersion. Pre-dry hygroscopic resins (PA, PET, PC) or vent, else hydrolysis.

Formulation design, testing and quality control

• Design sequence: define end-use requirement and standard → pick resin grade (MFI/K-value) → essential stabiliser package → property modifiers → filler for cost/stiffness → colour and processing aids → trial, test, iterate; verify cost per litre and processing window.
• Dispersion checks: filter/screen pressure value (FPV), optical microscopy of pressed film, dispersion rating charts, ash streaking.
• Composition checks: ash content (filler loading), TGA (filler + volatiles + polymer fractions), FTIR and XRF (element/heavy-metal screening), Soxhlet extraction for plasticiser content.
• Processability: melt flow index/MVR, capillary rheometry, torque rheometry (Brabender) for PVC fusion time and stability time, Congo-red/dehydrochlorination test.
• Performance: tensile, flexural, notched Izod/Charpy, HDT, Vicat, OIT by DSC (antioxidant reserve, used for PE pipe), oven ageing, QUV/xenon accelerated weathering, LOI (limiting oxygen index) and UL 94 for flammability, volume resistivity for antistat/conductive grades, colour ΔE in CIELAB.
• Food, water and toy compliance (India): the Food Safety and Standards (Packaging) Regulations, 2018 require food-contact plastics to conform to the specified Indian Standards, with overall migration testing into food simulants (the BIS method for overall migration of plastic constituents) and a specified list of permitted colourants and pigments for use in contact with foodstuffs, pharmaceuticals and drinking water; IS 10151 covers PVC and its copolymers for safe use in such contact. Toy safety is covered by the IS 9873 series (including migration of heavy elements) under the BIS Toys Quality Control Order.
• Waste and recyclate: under the Plastic Waste Management Rules, 2016 as amended (EPR regime with registration and recycling targets on producers, importers and brand owners, administered by CPCB), recycled plastic is barred from direct food contact, carry-bag thickness and compostable-plastic certification requirements apply, and recyclate must be re-stabilised — a "recycling additive package" (antioxidant top-up, acid scavenger, compatibiliser, chain extender, odour absorber) is now a mainstream compounding product. These Rules are amended frequently; confirm the current amendment before quoting a number.
• Plant safety: carbon-black and silica dust, plasticiser mist, peroxide and azodicarbonamide handling and solvent exposure are regulated as occupational hazards under the OSH&WC Code, 2020 (in force with the four Labour Codes from 21 November 2025, replacing the former Factories Act) — enclosed/vacuum conveying, LEV, dust masks, earthing against dust explosion, and SDS-based training.

Key points

• Compounding - converting resin + additives into a homogeneous, processable material with designed properties.
• phr - parts by weight per 100 parts resin; recipe totals exceed 100, so weight % must be derived.
• Masterbatch - concentrated additive in a carrier, diluted at the machine by the let-down ratio; a compound needs no dilution.
• Plasticiser - raises free volume, lowers Tg and modulus; DOTP/DINCH replacing restricted phthalates.
• Primary vs secondary plasticiser - fully compatible (used alone) vs extender used only in part.
• Internal plasticisation - comonomer built into the chain; permanent, no migration.
• PVC heat stabiliser - scavenges evolved HCl; Ca/Zn and Ba/Zn (non-toxic), organotin (clear), lead (phased out of potable water; IS 4985 pipes are lead-free).
• Primary antioxidant - hindered phenol/amine traps peroxy radicals; secondary - phosphite/thioester decomposes hydroperoxides; used together for synergy.
• HALS - regenerative light stabiliser, most efficient UV protection; carbon black at ~2-2.5 % is the cheapest UV screen (black pipe).
• Extender vs functional filler - cost/stiffness (CaCO₃, talc) vs property-giving reinforcement (glass fibre, wollastonite, nano-clay).
• Coupling agent - silane or MA-grafted polyolefin that bonds filler to matrix and transfers stress.
• Dispersive mixing needs shear stress to break agglomerates; distributive mixing needs strain to spread the phase.
• Fill factor - fraction of the internal mixer's free volume charged, typically 0.6-0.75.
• Specific energy input - net kWh per kg; ~0.10-0.35 kWh/kg in a twin-screw; sets dispersion and degradation risk.
• Flame retardants - halogen + Sb₂O₃ (gas phase), phosphorus/intumescent (char), ATH/MDH at 50-65 wt % ≈ 100-185 phr (endothermic, halogen-free); PBB/PBDE restricted to 0.1 % in EEE by the E-Waste Rules, 2022.
• PVC dry blend - high-speed mixer to ~110-120 °C for absorption, then cooling mixer; avoids one melt history.
• Antagonism exists - sulphur antioxidants deactivate HALS; HALS are poisoned by halogenated FRs and agrochemicals; always test the whole package, not single additives.
• Cost per litre, not per kg, is the true comparison once fillers change density.

Formulas and examples

Example 1 — phr to weight %, density and cost of a flexible PVC cable compound

• wt % PVC = 100/176 × 100 = 56.82 % (others likewise).
• ρ_c = 176 / 133.99 = 1.314 g/cm³.
• Cost = 16,650/176 = ₹94.60/kg, i.e. 94.60 × 1.314 = ₹124.3 per litre.
• Filler volume fraction φ = 7.38/133.99 = 5.5 vol % (only 5.5 % by volume though 11.4 % by weight — the usual trap).
• For a 250 kg batch, PVC charge = 100 × 250/176 = 142.0 kg, DOTP = 71.0 kg, CaCO₃ = 28.4 kg, stabiliser = 4.26 kg, stearic acid = 1.42 kg, TiO₂ = 2.84 kg.
• Raising CaCO₃ to 40 phr: total 196 phr, cost = 16,890/196 = ₹86.17/kg — 9 % cheaper per kg, but ρ rises, so check ₹/litre and the loss of elongation before celebrating.

Example 2 — Tg of the same compound by the Fox equation

• Tg(PVC) = 80 °C = 353 K; Tg(DOTP) ≈ −90 °C = 183 K. Weight fractions on a resin + plasticiser basis: w_PVC = 100/150 = 0.667, w_DOTP = 0.333.
• 1/Tg = 0.667/353 + 0.333/183 = 0.001889 + 0.001821 = 0.003710 → Tg = 269.5 K = −3.6 °C.
• Interpretation: the compound stays flexible at Indian ambient; for a −20 °C cold-flex cable specification, replace part of the DOTP with an adipate (DOA) or raise the loading.

Example 3 — masterbatch let-down and conductive carbon black

• Requirement: 1.2 % TiO₂ in a moulded part using a 60 % TiO₂ white masterbatch. Dosage = 1.2/60 × 100 = 2.0 %, i.e. 2 kg masterbatch + 98 kg natural resin per 100 kg; let-down ratio 1:49. Check: 2 kg × 0.60 = 1.2 kg TiO₂ in 100 kg ✓.
• Conductive grade: 15 wt % carbon black (ρ 1.8) in PE (ρ 0.95). Volumes per 100 g = 15/1.8 = 8.333 and 85/0.95 = 89.47 cm³ → φ = 8.333/97.80 = 8.5 vol % — above the ~5 % percolation threshold, so the compound will be electrically conductive.

Example 4 — internal mixer batch size and output

• Banbury free volume 50 L (50,000 cm³), fill factor 0.70, compound density 1.314 g/cm³.
• m = 50,000 × 0.70 × 1.314 = 45,990 g ≈ 46.0 kg per batch.
• Cycle time 4 min (= 15 batches/h) → output = 46.0 × 15 = 690 kg/h.
• If the fill factor is pushed to 0.85 the ram cannot seat, shear stress on the agglomerates falls and dispersion worsens despite the higher nominal throughput.

Example 5 — specific energy and melt temperature rise in a twin-screw

• TSE runs at 150 kg/h, motor draws 26 kW net of idle losses.
• SEI = 26/150 = 0.173 kWh/kg = 0.173 × 3600 = 624 kJ/kg.
• Adiabatic rise with c_p ≈ 2.0 kJ/kg·K: ΔT = 624/2.0 = 312 K. Feeding at 40 °C would imply ~350 °C — impossible for PP, so the barrel must remove heat, or screw speed/kneading severity must be cut. This calculation is the standard answer to "why is my compound yellowing?"

Where it is used

• PVC pipes, fittings, profiles and cable: Finolex, Supreme, Astral, Prince, Polycab, KEI — dry-blend plants, lead-free Ca/Zn or organotin packages, IS 4985 potable-water compliance, LSZH ATH cable compounds.
• Masterbatch and compound houses: Plastiblends, Poddar Pigments, Clariant, Ampacet, Cabot, Kabra; roles as formulation chemist, colour matcher, TSE process engineer, QC/ASTM-IS testing analyst.
• Automotive and appliance plastics: 20-40 % talc- or glass-filled PP for bumpers and HVAC ducts, impact-modified PA66-GF for under-bonnet parts, FR-ABS/PC for enclosures — supplied to Maruti, Tata Motors, Mahindra, Whirlpool, Voltas tier-1 moulders.
• Agriculture and infrastructure films: multi-layer greenhouse and mulch film, tarpaulin, geomembrane and HDPE pipe with HALS/UV-absorber and carbon-black packages engineered for Indian insolation.
• Packaging and FMCG: slip/antiblock and antifog in BOPP and PE film, nucleated/clarified PP for thin-wall containers, FSSAI-compliant food-contact additive packages.
• Recycling and sustainability: re-stabilisation and compatibilisation of post-consumer PP/PE/PET under the EPR obligations of the Plastic Waste Management Rules, 2016 as amended — a fast-growing employer of compounding engineers.

Interview questions

• Q: What does phr mean and why is it not the same as weight %? A: phr is parts by weight per 100 parts of resin, so the recipe can total more than 100; weight % = phr of the component divided by the sum of all phr, times 100.
• Q: Why does PVC need a heat stabiliser when polyethylene does not? A: PVC starts splitting off HCl above about 150 °C in an autocatalytic reaction that discolours and crosslinks it; the stabiliser scavenges that HCl. PE degrades by oxidation instead, so it needs an antioxidant.
• Q: Difference between a primary and a secondary antioxidant? A: Primary (hindered phenols/amines) scavenge peroxy radicals and give long-term thermal stability; secondary (phosphites, thioesters) decompose hydroperoxides and protect during melt processing. They are used together because they are synergistic.
• Q: Why are water and gas pipes black? A: Carbon black at about 2-2.5 %, well dispersed, is the cheapest and most durable UV screen and also gives the pipe its required weathering life.
• Q: Dispersive versus distributive mixing? A: Dispersive mixing breaks agglomerates and needs high shear *stress*; distributive mixing spreads the phase uniformly and needs total *strain*. Kneading blocks do the first, toothed and reversing elements the second.
• Q: Why is a co-rotating twin-screw preferred over a single-screw for compounding? A: Its modular screws and barrels give independent control of melting, dispersive and distributive mixing, side feeding of fillers and vacuum devolatilisation, with a narrow residence-time distribution and self-wiping, intermeshing screws.
• Q: A customer complains that his filled compound is cheaper per kg but costs more per part. Why? A: Filler raises density, so fewer parts come out of a kilogram; the correct comparison is cost per unit volume, (₹/kg) × density.
• Q: What does a silane coupling agent do? A: Its alkoxy end hydrolyses and bonds to the hydroxyl groups on glass, silica or ATH while its organofunctional end reacts with or entangles in the matrix, so stress is transferred across the interface instead of debonding.
• Q: Why is ATH needed at 50-65 wt % (≈ 100-185 phr) while brominated FR works at 10-15 phr? A: ATH acts physically — it decomposes endothermically and releases water to dilute and cool — so a large mass is needed; brominated FRs act chemically, trapping flame radicals in the gas phase with Sb₂O₃ as synergist.
• Q: What is a let-down ratio and compute it for 1 % of an additive from a 25 % masterbatch. A: The dilution of masterbatch into natural resin; dosage = 1/25 × 100 = 4 %, i.e. a let-down ratio of 1:24.
• Q: Give an example of additive antagonism. A: Sulphur-containing (thioester) antioxidants deactivate HALS, and halogenated flame retardants or acidic agrochemical residues poison them; so a greenhouse-film package uses NOR-HALS and avoids thio synergists.
• Q: Why is PVC pipe made from a dry blend rather than a pellet compound? A: Porous suspension PVC absorbs liquid additives in a high-speed mixer without melting, so the heat-sensitive polymer sees only one heat history in the pipe extruder, saving energy, cost and colour.
• Q: How would you check whether a supplier actually put the specified antioxidant in a PE compound? A: Measure oxidative induction time (OIT) by DSC and confirm by extraction with FTIR/HPLC against the reference formulation.

Common mistakes

• Treating phr as weight % — a 176 phr recipe then gives wrong batch weights and a wrong cost.
• Adding weight fractions as if volumes were weights; filler volume fraction is always far lower than its weight fraction, and property models need the volume fraction.
• Confusing a plasticiser with a lubricant: a plasticiser permanently lowers Tg of the solid; a lubricant reduces friction and melt viscosity during processing only.
• Saying "lead stabilisers are banned in India" without qualification — they are excluded from potable-water uPVC pipe (IS 4985 is unleaded) and from food/toy contact, not blanket-prohibited across every product.
• Dumping liquid plasticiser onto a cold high-speed mixer charge, or adding lubricant first — the blend balls up or absorption stalls.
• Over-filling the internal mixer or over-speeding the twin-screw to raise output: dispersion worsens, glass fibre length collapses and the compound degrades, as the SEI calculation shows.
• Quoting an old withdrawn IS number or a superseded phthalate/FR limit in an interview — name the standard's subject and say the limits are revised if you are not sure of the current edition.
• Testing additives one at a time and ignoring antagonism in the full package, then blaming the resin for field failures.

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