Module 5
Plasticizers and Antidegradants
What these ingredients do and why every compound has them
The supporting cast
Polymer, filler, and cure system get most of the attention, but a compound also contains ingredients whose job is to make it processable and to keep it alive in service. They are used at modest loadings and are easy to overlook, and they are responsible for a disproportionate share of both good outcomes and mysterious failures.
Three groups matter most: plasticizers and oils, which make the compound processable; antidegradants, which protect the finished part; and process aids, which improve behaviour on equipment.
Plasticizers and oils
Petroleum oil is the most common plasticizer in rubber, and it does several things at once.
It lowers viscosity, so the compound mixes, extrudes, and moulds more easily. It softens the cured compound, lowering hardness and modulus. It improves low-temperature flexibility a little. And because it is cheaper than polymer, it reduces cost.
Crucially, oil does not react with the rubber. It sits between the chains, letting them slide past one another more easily. Everything it does follows from that simple physical effect.
Loading is typically 5 to 40 phr, and it usually tracks the filler level. High filler and no oil gives a compound too stiff to process; high oil and low filler gives one too soft to be useful. The two are adjusted together.
The three oil types
Petroleum process oils are classified by which type of hydrocarbon dominates.
| Type | Character | Typical use |
|---|---|---|
| Paraffinic | Straight-chain hydrocarbons. Least polar, most resistant to oxidation, best colour stability. | EPDM, butyl, and peroxide-cured compounds generally, where it interferes least |
| Naphthenic | Ring hydrocarbons without aromatic character. Middle ground on compatibility and cost. | General purpose; the usual default |
| Aromatic | Contains aromatic rings. Most polar, best solvency for diene rubbers, darkest, and staining. | NR, SBR, BR in black compounds. Note that high-PAH aromatic extracts are restricted for tyre applications in the EU and comparable markets, with treated grades (TDAE, MES, RAE) used instead |
The choice is a compatibility decision. An oil that is too different from the polymer will not stay in it — it migrates to the surface and bleeds out. As a rough guide, aromatic oils suit the highly unsaturated diene rubbers, paraffinic oils suit the saturated ones, and naphthenic sits in between and is the safest starting point when you are unsure.
What attacks rubber in service
Rubber degrades by chemical attack, and three agents cause most of it.
| Agent | What it does | Where it shows |
|---|---|---|
| Oxygen | Attacks the polymer chain, causing scission or additional crosslinking depending on the polymer. | Everywhere, accelerated by heat |
| Ozone | Attacks carbon-carbon double bonds at the surface, cutting chains. | Cracks perpendicular to the direction of strain, and only where strain exceeds a threshold — an unstrained surface is attacked chemically but does not crack |
| Heat | Accelerates oxidation and degrades crosslinks. | Hardening or softening depending on the polymer |
| Sunlight and metal ions | Ultraviolet initiates surface oxidation; copper and manganese residues catalyse it. | Surface crazing; unexpectedly fast aging traced to contamination |
Light, particularly ultraviolet, accelerates oxidation, and certain metal ions — copper and manganese especially — catalyse it. This is why copper contamination in a compound is taken seriously out of proportion to the quantity involved.
Antioxidants and antiozonants
These are the protective chemicals, and they work differently enough that they are not interchangeable.
- Antioxidants interrupt the chain reaction of oxidation. They are consumed as they work, so protection is finite.
- Antiozonants migrate to the surface and react with ozone before it can attack the polymer, forming a protective film. They too are consumed, and they must keep migrating to maintain the film.
- Waxes bloom to the surface and form a physical barrier. They protect under static conditions but the film cracks under flexing, so they are not sufficient alone for dynamic parts.
Most compounds use a package rather than a single chemical: an antioxidant, an antiozonant, and often a wax, because each covers a different failure route. Typical total loading is 1 to 4 phr for the chemical antidegradants, with wax counted separately.
Staining — the constraint that shapes the choice
The most effective antioxidants and antiozonants are amine-based, and amines discolour. They darken the compound itself and can stain adjacent light-coloured materials by migration.
For a black compound this is irrelevant, so the best-performing chemicals are used. For a light-coloured or white compound it rules them out, and non-staining phenolic antioxidants are used instead — which are genuinely less effective.
This is a real and unavoidable trade-off rather than a formulating preference. A white sidewall or a light-coloured seal has less aging protection available to it than a black one, and that has to be accounted for in the design rather than assumed away.
Process aids
A small group of ingredients exists purely to improve behaviour on equipment. They do not appear in the finished part's property requirements, which is why they are sometimes left out of a recipe by someone economising, usually to their regret.
| Type | What it does |
|---|---|
| Peptizers | Chemically break polymer chains during mixing, lowering viscosity permanently. Used mainly with natural rubber. |
| Lubricants and release agents | Reduce sticking to mill rolls, mixer surfaces, and moulds. |
| Tackifiers | Increase the ability of uncured compound to stick to itself, needed when building a part from layers. |
| Homogenising agents | Help disperse blends of polymers that do not mix readily. |
| Dispersing aids | Improve filler wetting and shorten mixing cycles. |
Peptizers deserve a distinction from oils. An oil lowers viscosity by getting between chains and can in principle be washed out. A peptizer lowers viscosity by cutting the chains, which is permanent and reduces molecular weight. They achieve a similar processing result by completely different means and with different consequences for final properties.
Check yourself
7 questions. One correct answer each. Keys A–D select, Enter submits.
1.What does process oil actually do in a compound?
2.Which oil type would you choose for a peroxide-cured EPDM compound?
3.Why does oil loading usually track filler loading?
4.What is the difference between how an antioxidant and an antiozonant protect a compound?
5.Why is wax alone insufficient to protect a dynamic part against ozone?
6.Why does a white compound generally have less aging protection available than a black one?
7.What distinguishes a peptizer from a process oil?
Terms from this module
- Process oil
- Petroleum plasticizer that lowers viscosity by separating chains without reacting with them.
- Paraffinic / naphthenic / aromatic
- The three process oil types, from least to most polar; matched to polymer for compatibility.
- Bleeding
- Oil migrating out of a compound to the surface, from incompatibility or overloading.
- Extraction
- Loss of plasticizer into a contacting fluid; can cause a part to shrink and harden rather than swell.
- Antioxidant
- Chemical that interrupts the free-radical chain reaction of oxidation; consumed as it works.
- Antiozonant
- Chemical that migrates to the surface and reacts with ozone before it attacks the polymer.
- PPD
- Paraphenylenediamine, the most common chemical antiozonant class. Staining.
- Staining
- Either the compound darkening itself, or an ingredient migrating out and discolouring what it touches.
- Peptizer
- Additive that permanently lowers viscosity by cutting polymer chains during mixing.
- Tackifier
- Additive that increases the ability of uncured compound to stick to itself.