Additives, Compounding and Formulations: Ingredients, Carbon Black, Mixing and Vulcanisation
2. Stabilisers: PVC heat stabilisers, antioxidants and UV stabilisers
PVC heat stabilisers. Above about 100 °C PVC loses HCl from labile sites (allylic and tertiary chlorine), leaving conjugated polyene sequences that absorb visible light, so the polymer yellows, browns and finally blackens. The released HCl catalyses further loss (autocatalysis), and the loss is faster in the presence of zinc or iron chloride. A stabiliser works by (i) absorbing HCl, (ii) replacing the labile chlorine by a stable group, (iii) reacting with the polyenes or with oxygen, or (iv) chelating the harmful metal chlorides. Main families: lead salts (tribasic lead sulphate: cheap, excellent long-term heat and electrical stability, opaque, toxic, being phased out), calcium-zinc soaps (non-toxic; the zinc soap replaces labile Cl but forms ZnCl₂, which itself accelerates breakdown, so calcium soap is added to convert ZnCl₂ back to zinc soap), organotin mercaptides (clear rigid PVC) and epoxidised soybean oil, a co-stabiliser that scavenges HCl through its epoxide rings.
Antioxidants. Oxidation is a radical chain: initiation gives R•, then R• + O₂ → ROO• and ROO• + RH → ROOH + R•, and ROOH splits into more radicals when heated. Primary antioxidants (hindered phenols, aromatic amines) donate a hydrogen atom to ROO•, and the radical they become is too stable to continue the chain: they are chain breakers. Secondary antioxidants (phosphites, thioesters) do not touch radicals; they decompose ROOH into harmless alcohols, so they are hydroperoxide decomposers. The two used together are synergistic. In rubbers the antidegradants are mainly aromatic amines (for example the p-phenylenediamine types), which also protect against ozone cracking by migrating to the surface (the bloom), and wax is added to give a protective surface film.
UV stabilisers. Sunlight breaks bonds and starts the same oxidation chain. UV absorbers (benzophenones, benzotriazoles) absorb the radiation and release it as harmless heat; they need thickness to work, so they protect thick parts better than thin films. HALS (hindered amine light stabilisers) do not absorb UV: they trap radicals and are regenerated in a cycle, so they protect thin films and fibres for long times. Screeners such as carbon black and titanium dioxide reflect or absorb the light at the surface.
3. Plasticisers, lubricants, processing aids, impact modifiers, flame retardants and antistats
A plasticiser is a low-volatility, compatible liquid (the usual one for PVC is DOP, also called DEHP, di-2-ethylhexyl phthalate) that sits between chains, increases free volume and screens the dipolar attraction between chlorine and hydrogen sites. It lowers Tg and the modulus and raises elongation: rigid PVC (Tg about 80 °C) becomes flexible at 30 to 60 phr because Tg falls below room temperature. Plasticisers can migrate, evaporate or be extracted; a secondary plasticiser or extender (epoxidised oil, chlorinated paraffin) is cheaper but less compatible. In rubbers, process oils play the same part as plasticisers and extenders, paraffinic oils for non-staining and aromatic for compatibility with SBR and BR.
| Additive | How it works | Typical example |
|---|---|---|
| Internal lubricant | Compatible, lowers melt viscosity and internal friction | Fatty alcohols, esters |
| External lubricant | Poorly compatible, migrates to the metal surface, delays fusion, aids release | Paraffin and PE wax |
| Processing aid | Promotes fusion and melt strength of PVC | High-molar-mass acrylic copolymer |
| Impact modifier | Rubbery particles start crazing and shear yielding that absorb energy | MBS, acrylic, CPE; EPDM in PP |
| Flame retardant | Gas-phase radical trapping, endothermic water release, or char formation | Brominated compound with Sb₂O₃; Al(OH)₃; phosphorus |
| Antistatic agent | Migrates and attracts moisture to conduct charge, or conductive filler | Ethoxylated amines, glycerol esters; carbon black |
Two numbers are worth remembering. Aluminium hydroxide decomposes endothermically near 200 °C, 2Al(OH)₃ → Al₂O₃ + 3H₂O, releasing 54/156 = 34.6 % of its mass as water, which cools and dilutes the flame, but about 50 to 65 % loading is needed and so it cannot be used in polymers processed much above 200 °C; magnesium hydroxide starts near 330 °C. A halogen flame retardant works with antimony trioxide as a synergist: the pair forms antimony halide species that trap flame radicals in the gas phase.
4. Rubber compounding ingredients
A rubber compound carries ingredients of seven kinds. Base elastomer(s); crosslinkers (sulphur for unsaturated rubbers; organic peroxides such as dicumyl peroxide, which form C–C crosslinks and work in saturated rubbers such as EPDM and silicone and in polyethylene; metal oxides ZnO with MgO for polychloroprene); activators (zinc oxide with stearic acid, which makes soluble zinc soap and so lets the accelerator work); accelerators; fillers (reinforcing carbon black or silica, semi-reinforcing and inert fillers such as clay or calcium carbonate); protective agents and processing aids (oils, peptisers that cut chain length during mastication, tackifiers); and retarders (a pre-vulcanisation inhibitor lengthens the safe scorch time).
The order of addition follows the sensitivity of each ingredient. The rubber is first masticated to soften it, fillers, oil and the zinc oxide and stearic acid are added next, and the curatives (sulphur and accelerator) go last, at the lowest temperature, usually below about 100 °C, so that no crosslinking (scorch) starts in the mixer.
5. Carbon black: structure, reinforcement and grades
Furnace carbon black is made by partial combustion and cracking of aromatic oil in a hot furnace. The growing carbon spheres, the primary particles of roughly 10 to 100 nm, fuse into branched, grape-like aggregates, the smallest unit that survives mixing. Three properties matter. Particle size (equivalently surface area, measured by nitrogen adsorption or iodine number): finer black gives more rubber-filler contact, so higher tensile strength, tear and abrasion resistance, but more heat build-up and higher viscosity. Structure, the degree of branching of the aggregate, measured by oil absorption (DBP) number: high structure traps rubber in its voids (occluded rubber), raising the effective filler volume, so modulus and hardness and viscosity rise and die swell falls. Surface activity controls how much rubber is held as bound rubber, which resists solvent extraction.
| Grade | Name | Character and use |
|---|---|---|
| N110, N220 | SAF, ISAF | Very fine, high abrasion resistance: tyre treads |
| N330 | HAF | General-purpose reinforcing grade: treads, mechanical goods |
| N550 | FEF | Medium size, high structure: good extrusion, carcass |
| N660, N774 | GPF, SRF | Coarser, low heat build-up: inner liners, sidewalls, mouldings |
| N990 | MT (thermal) | Largest particles, low structure, very high loading, little reinforcement |
In the N-number, N stands for normal cure rate and the first digit encodes the particle-size class: N1 is the finest and N7, N9 are progressively coarser, so a smaller digit means a smaller particle and more reinforcement. The usual loading is 30 to 60 phr in tyre compounds; in plastics 2 to 3 % carbon black is used as a UV screener and as a conductive filler. Silica with a silane coupling agent is the main non-black reinforcing filler, favoured for low rolling resistance.
6. Polymer mixing equipment
Mixing has two aims. Dispersive mixing breaks agglomerates of filler down to aggregates: it needs a high stress, above the cohesive strength of the agglomerate, so it happens in the high-shear zones. Distributive mixing spreads the ingredients uniformly in space: it needs large total strain, folding and repeated cutting, not high stress. Every machine does both, in different ratios.
| Machine | What it does | Typical use |
|---|---|---|
| Two-roll mill | Two horizontal counter-rotating rolls at different speeds (friction ratio of about 1.1 to 1.25) shear the stock in the adjustable nip; open, cooled rolls, batch, operator cutting | Mastication, adding curatives, sheeting out, laboratory and small batches |
| Internal mixer (Banbury type) | Closed chamber with two rotors, a ram pressing the charge down; high shear at the rotor tip clearance; batch of a few minutes; temperature and fill factor controlled | Large-scale rubber compounding, filler dispersion; the dump goes to a mill or extruder |
| Single-screw extruder | Feed, compression and metering zones convey, melt and pump; poor mixing unless a mixing section is added | Shaping, pelletising, profiles |
| Twin-screw extruder | Intermeshing, self-wiping screws, positive conveying, modular elements; continuous, strong dispersive and distributive mixing, vents for devolatilising | Compounding and masterbatch of plastics, reactive extrusion |
7. Vulcanisation: sulphur systems, accelerators and activators
Vulcanisation converts the plastic, tacky rubber into an elastic network by joining chains at a few sites. Sulphur alone is slow (hours at 140 °C) and needs a high loading; with an accelerator and the activator pair ZnO and stearic acid, the cure takes minutes and is efficient. The accelerator and the zinc soap form an active sulphurating complex that reacts with the allylic positions of the unsaturated chain and gives crosslinks of three kinds: mono-sulphidic (C–S–C), di-sulphidic and poly-sulphidic (Sₓ, x ≥ 3). Crosslinking raises modulus, hardness and resilience, lowers hysteresis, elongation and permanent set, and makes the rubber insoluble: it only swells.
| System | Sulphur | Accelerator | Crosslinks and properties |
|---|---|---|---|
| Conventional | 2.0 to 3.5 | 0.4 to 1.2 | Mostly poly-sulphidic: high strength and fatigue life, poor heat ageing and reversion |
| Semi-EV | 1.0 to 1.7 | 1.2 to 2.4 | Mixed: a compromise of fatigue and ageing |
| EV (efficient) | 0.4 to 0.8 | 3 to 6 | Mostly mono- and di-sulphidic: good heat ageing, low compression set and reversion, lower strength and fatigue |
The shift from conventional to EV is a shift to a high accelerator to sulphur ratio: little sulphur is available per crosslink site, so crosslinks stay short. Short, strong C–S–C bonds resist heat; long, weak Sₓ bonds break and re-form under strain, which gives fatigue life but makes them break down in heat (reversion: a fall in modulus on over-cure). Accelerator classes in rising order of speed are guanidines (such as DPG, a secondary accelerator), thiazoles (MBT, MBTS), sulphenamides (CBS, TBBS: fast with delayed onset, so a long safe scorch time), thiurams (TMTD, which can also donate sulphur) and dithiocarbamates (ultra-fast).
8. The cure curve and cure kinetics
A curemeter records torque against time at a fixed temperature, and the torque is proportional to the shear modulus, hence to crosslink density. The curve gives: ML, minimum torque (compound viscosity); ts2, the scorch time, the time for the torque to rise 2 units above ML; MH, maximum torque; t90, the optimum cure time, the time to reach ML + 0.9 (MH − ML); and the cure rate index CRI = 100/(t90 − ts2) in min⁻¹. After MH natural rubber may show reversion, a falling torque, while some synthetic rubbers show a rising, marching torque. The torque rise ΔM = MH − ML is a relative measure of crosslink density.
Kinetics. The state of cure is α = (M − ML)/(MH − ML), from 0 to 1. After the induction period, to a good approximation the cure is first order: dα/dt = k (1 − α), so ln (1 − α) = − k (t − ts) and a plot of ln (1 − α) against t is a straight line of slope − k. At t90, α = 0.9, so k = ln 10/(t90 − ts) = 2.303/(t90 − ts). The temperature dependence is Arrhenius, k = A exp(−Ea/RT), so between two temperatures (in kelvin) t₂ = t₁ exp[(Ea/R)(1/T₂ − 1/T₁)], with R = 8.314 J mol⁻¹ K⁻¹. The shop-floor form is the Q10 rule: the cure time halves for every 10 °C rise, t₂ = t₁/2^[(T₂ − T₁)/10], which corresponds to an activation energy near 100 kJ/mol at about 150 °C.
Worked example. A compound has t90 = 24 min at 140 °C. By the Q10 rule at 160 °C, which is two steps of 10 °C, t90 = 24/2² = 6 min. With Arrhenius and Ea = 90 kJ/mol, the time of 18 min at 150 °C (423.15 K) becomes, at 170 °C (443.15 K), t = 18 exp[−(90000/8.314)(1/423.15 − 1/443.15)] = 18 exp(−1.1546) = 5.67 min, because (1/423.15 − 1/443.15) = 1.0666 × 10⁻⁴ K⁻¹, times 10825 K gives 1.1546, and e^−1.1546 = 0.3152, so 18 × 0.3152 = 5.67 min. For thick articles, the interior lags the surface: the cure is judged by the equivalent cure ∫ 2^[(T − Tref)/10] dt, and the state at the coldest centre sets the press time, while the surface may over-cure.
Key takeaways
- Compounding makes a polymer processable, gives it the property target, protects it in service and adds special functions; rubber recipes are in phr (parts per hundred rubber), so the ingredients sum to more than 100 and weight per cent is phr divided by the total.
- PVC degrades by autocatalytic loss of HCl, so its stabilisers (lead, Ca/Zn, organotin, epoxidised oil) absorb HCl or replace labile chlorine; primary antioxidants break the radical chain, secondary ones decompose hydroperoxides, UV absorbers need thickness while HALS trap radicals and do not.
- Plasticisers lower Tg and modulus; internal lubricants are compatible and cut melt viscosity, external ones migrate to metal and delay fusion; impact modifiers are rubbery particles; Al(OH)₃ releases 34.6 % water near 200 °C and Mg(OH)₂ near 330 °C; antistats migrate and bind moisture or are conductive fillers.
- Carbon black: finer particles raise abrasion resistance, tensile strength and hysteresis; higher structure (DBP number) raises modulus and viscosity and cuts die swell; N-number first digit small means fine; mixing is dispersive (high stress) and distributive (large strain), done in mills, internal mixers and twin-screw extruders.
- Sulphur vulcanisation needs double bonds and ZnO with stearic acid as activator; conventional systems (high S, low accelerator) are poly-sulphidic and fatigue-resistant but reversion-prone, EV systems (low S, high accelerator) are mono- and di-sulphidic and heat-stable; curatives are added last and cold to avoid scorch.
- Cure curve: ML, ts2, MH, t90 = time to ML + 0.9(MH − ML), CRI = 100/(t90 − ts2). First-order cure gives ln(1 − α) = −k(t − ts), k = 2.303/(t90 − ts); the Arrhenius shift is t₂ = t₁ exp[(Ea/R)(1/T₂ − 1/T₁)], and the Q10 rule halves the time per 10 °C.
Practice questions (16)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
During the heat processing of PVC, which species released by the polymer speeds up its own further degradation?
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Answer: A — Hydrogen chloride, which catalyses further dehydrochlorination
PVC loses HCl from labile chlorine sites and the HCl catalyses further loss, so a stabiliser must absorb HCl or replace the labile chlorine. Water hydrolysis is not the route: the degradation proceeds without water or oxygen, and the other gases listed are not the autocatalyst.A hindered phenol is added to polypropylene as a primary antioxidant. How does it protect the polymer?
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Answer: D — It donates a hydrogen atom to a peroxy radical and so breaks the oxidation chain
Primary antioxidants are chain breakers: they give an H atom to ROO• and become a stable radical. Decomposing hydroperoxides without touching radicals is the role of secondary antioxidants such as phosphites, and that is the tempting wrong choice. UV absorption is the job of UV absorbers.A calcium-zinc stabiliser is used in flexible PVC, with the calcium soap in large excess over the zinc soap. What does the calcium soap do?
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Answer: B — It turns the ZnCl₂ back into zinc soap, as ZnCl₂ speeds degradation
Zinc soap replaces labile chlorine but forms ZnCl₂, a strong catalyst of further dehydrochlorination (zinc burning). Calcium soap exchanges with ZnCl₂ to regenerate zinc soap and form harmless CaCl₂. It is not a plasticiser or UV absorber, and PVC stabilisation does not involve crosslinking.Two compounds differ only in the carbon black: one uses the very fine N220 grade and the other the coarse N774 grade, both at 50 phr. Which statement is correct?
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Answer: C — The N220 compound has higher abrasion resistance and tensile strength but more heat build-up
Finer black gives a larger rubber-filler interface and so higher abrasion resistance, tensile strength and tear, at the price of more hysteresis (heat build-up) and a higher viscosity. The coarse grade is the other way round, so the first and second claims are reversed, and particle size matters even at equal loading.At constant loading and particle size, a carbon black of higher structure (a higher DBP oil absorption number) is used in a rubber compound. Which set of changes is expected?
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Answer: A — Higher modulus and viscosity and lower die swell, as more rubber is occluded in aggregates
A branched aggregate traps rubber in its voids, raising the effective filler volume, so modulus, hardness and uncured viscosity rise and die swell falls. Structure is a separate property from particle size and acts in the uncured state too, so the claim of no change fails.An NR engine mount must keep its modulus after long service at 100 °C, with low compression set, and fatigue life is secondary. Which vulcanisation system suits it best?
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Answer: C — EV: low sulphur (about 0.5 phr) with high accelerator, giving mostly mono- and di-sulphidic crosslinks
Short mono- and di-sulphidic crosslinks are thermally stable, resist reversion and give low compression set; this is what the EV system makes. The conventional system makes weaker poly-sulphidic links that break down in heat, which is the tempting choice only if fatigue life were the priority. Unvulcanised rubber flows and sets permanently.A rubber recipe is: rubber 100, carbon black 45, process oil 5, zinc oxide 5, stearic acid 2, accelerator 1, sulphur 2 (all in phr). What is the carbon black content in weight per cent of the whole compound, to one decimal place?
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Answer: D — 28.1 %
Total = 100 + 45 + 5 + 5 + 2 + 1 + 2 = 160 phr, so carbon black = 45/160 = 28.1 %. Reading 45 phr as 45 % ignores the other ingredients, and 30.4 % (45/148) wrongly leaves out the oil, zinc oxide and stearic acid from the total.A polyethylene film must resist sunlight for years, but the film is only 40 µm thick. Which UV stabiliser is the better choice, and why?
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Answer: B — HALS, which trap radicals and regenerate, so they work independent of thickness
A UV absorber needs path length (thickness) to remove enough radiation, so a 40 µm film is poorly protected by it. HALS act by trapping radicals in a regenerative cycle and do not depend on thickness, which is why they are the standard choice for films and fibres. Calcium stearate and lubricants are not UV stabilisers.A polyamide is melt-compounded at 250 °C and needs a halogen-free flame-retardant filler at high loading. Aluminium hydroxide begins to decompose, releasing water, at about 200 °C, and magnesium hydroxide at about 330 °C. Which choice is sound?
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Answer: D — Magnesium hydroxide, because aluminium hydroxide would release its water and foam the melt during compounding
At 250 °C aluminium hydroxide would already dehydrate in the extruder, wasting its fire-retardant action and creating voids. Magnesium hydroxide is stable up to its onset near 330 °C, so it survives compounding. A higher water yield is irrelevant if the water is lost early, and antimony trioxide is a synergist for halogens, not a water-releasing filler.Which of the following statements about additives for PVC and other plastics are correct?
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Answer: A — A plasticiser such as DOP lowers the glass transition temperature and the modulus of PVC; C — An external lubricant migrates to the hot metal surface and delays the fusion of PVC
A plasticiser lowers Tg and modulus and raises elongation, so the first claim holds and the last is its reverse. External lubricants are poorly compatible and migrate to metal surfaces, delaying fusion and aiding release, so that claim holds. The internal lubricant is the compatible one, which lowers melt viscosity; assigning migration to it mixes up the two classes.Which statements about carbon black in rubber are correct?
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Answer: A — A higher DBP oil absorption number indicates higher structure; B — Bound rubber is the rubber held on the black surface that resists solvent extraction
The DBP number measures the voids of the aggregates, hence structure, and bound rubber is by definition the portion that cannot be extracted. Higher structure does the opposite of the third claim: it raises viscosity and cuts die swell. In the N-system the smaller digit means the finer black, so the last claim is reversed.Which statements about polymer mixing equipment are correct?
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Answer: A — Dispersive mixing needs a stress above the cohesive strength of the filler agglomerates; B — A two-roll mill works by running its two rolls at different speeds, the ratio being called the friction ratio
Breaking agglomerates needs high local stress, and the mill shears the stock in the nip because the rolls run at different speeds. An internal mixer processes closed batches of a few minutes, not a steady stream. Twin-screw extruders are routinely vented for devolatilisation, so the last claim is false.A rubber compound has an optimum cure time t90 = 24 min at 140 °C. Use the Q10 rule (the cure time halves for every 10 °C rise in temperature) to find t90 at 160 °C, in minutes.
Numerical answer — type the value.
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Answer: 6
The rise is 20 °C, which is two steps of 10 °C, so t90 = 24/2² = 24/4 = 6 min. Halving only once (12 min) is the common slip.The optimum cure time of a compound is t90 = 18 min at 150 °C. Assume Arrhenius behaviour of the cure rate, with t90 inversely proportional to the rate constant, so t₂ = t₁ exp[(Ea/R)(1/T₂ − 1/T₁)], with Ea = 90 kJ/mol, R = 8.314 J mol⁻¹ K⁻¹ and T = °C + 273.15. Find t90 at 170 °C in minutes, to two decimal places.
Numerical answer — type the value.
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Answer: 5.67
T₁ = 423.15 K and T₂ = 443.15 K. Ea/R = 90000/8.314 = 10825 K. 1/T₂ − 1/T₁ = −1.0666 × 10⁻⁴ K⁻¹, so the exponent is −1.1546 and t₂ = 18 × e^−1.1546 = 18 × 0.3152 = 5.67 min. A positive exponent (a longer time at a higher temperature) is the sign slip to avoid.A cure curve at 160 °C gives a scorch time ts2 = 3.5 min and an optimum cure time t90 = 12.5 min. Using CRI = 100/(t90 − ts2), with the times in minutes, find the cure rate index in min⁻¹, to two decimal places.
Numerical answer — type the value.
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Answer: 11.11
t90 − ts2 = 12.5 − 3.5 = 9.0 min, so CRI = 100/9.0 = 11.11 min⁻¹. Using t90 alone (100/12.5 = 8) forgets that the induction time must be subtracted.For a curemeter run, ML = 1.0 dN·m and MH = 19.0 dN·m. The torque at 6.0 min is 10.5 dN·m. Take the state of cure α = (M − ML)/(MH − ML), assume first-order cure dα/dt = k(1 − α) with negligible induction time (so α = 0 at t = 0), and find k in min⁻¹, to three decimal places.
Numerical answer — type the value.
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Answer: 0.125
α = (10.5 − 1.0)/(19.0 − 1.0) = 9.5/18 = 0.5278, so 1 − α = 0.4722. For first order, ln(1 − α) = −kt, so k = −ln(0.4722)/6.0 = 0.7503/6.0 = 0.125 min⁻¹. Taking α as 10.5/19 (forgetting ML) gives a different, wrong value.