TOFA in rubber compounds: cure activator, filler dispersion and processing aid

In rubber compounds, tall oil fatty acid is the fatty acid component that activates the cure system together with zinc oxide; it also supports filler wetting and the processability of the mix. Rubber processing and compound preparation is listed as an application in both the TOFA 180 and TOFA 185 product data. It is a reactive input, distinct from a process oil.

What does a fatty acid do in a rubber compound?

A fatty acid looks like a small line item in a rubber recipe, but it carries three jobs at once. The first is chemical: together with zinc oxide it builds the activator leg of the cure system. The second is physical: it wets the filler surface and helps carbon black and silica disperse through the rubber matrix. The third is process related: it softens mixing viscosity and behaviour on the machine, and eases release from the mould.

All three functions come from the two ends of one molecule. The carboxyl end is polar and reactive; it forms a salt with a metal oxide and anchors to the polar sites on a filler surface. The long hydrocarbon tail is compatible with the non-polar rubber phase. The fatty acid works as the interfacial molecule that ties the two phases together.

Tall oil fatty acid enters this role as an application listed directly in the product data. Rubber processing and compound preparation appears in the application list for both TOFA 180 and TOFA 185, and rubber processing is among the production areas of both products.

Activator chemistry: fatty acid and zinc oxide together

In a sulphur cured system, zinc oxide alone is not enough. Zinc oxide is practically insoluble in the rubber phase and has to be mobilised before it can form the effective complex with the accelerator. The fatty acid does that job: it reacts with the metal oxide and forms, in situ, a zinc soap that can disperse through the rubber. The resulting zinc species combines with the accelerator to give the active form that carries sulphur to the polymer chain.

For the formulator the critical variable here is the free acid content of the fatty acid, because it is the un-esterified free carboxyl groups that take part in activation. That value is defined in the specification as the acid value: min 180 for TOFA 180 and min 185 for TOFA 185 (ASTM D1980). Predictable lot-to-lot activator behaviour depends on keeping that value tight.

The second variable is the fraction that does not react. Unsaponifiables are capped at max 6% in TOFA 180 and max 4% in TOFA 185 (ASTM D1065). That fraction takes no part in activation and behaves like a diluent in the compound. A tighter unsaponifiables limit means more effective fatty acid per unit weight.

Filler wetting and dispersion: the process side contribution

Carbon black and silica enter the mix as agglomerates. Breaking those agglomerates down and distributing them through the rubber phase depends on how quickly the filler surface is wetted, as well as on the shear in the mixer. The polar end of the fatty acid anchors to the filler surface while the hydrocarbon tail orients towards the rubber, and that orientation makes re-agglomeration harder.

The effect is more visible in silica filled compounds, because the silica surface is strongly polar and has little natural affinity for non-polar rubber. The fatty acid is not a coupling agent on its own here and does not replace a silane coupling agent. What it does is ease wetting and flow during mixing.

The liquid form carries a practical advantage as well. TOFA 180 is an amber to dark amber liquid and TOFA 185 a light yellow to amber liquid, and both ship as bulk liquid in ISO tank containers. That means a directly pumpable input with no melting step and no powder handling.

Do not confuse it with process oil: two separate tall oil inputs to rubber

Tall oil derivatives enter the rubber industry in two completely different roles, and treating them as the same line item is a common mistake. A process oil is a largely physical input added at high loading as a plasticiser and filler carrier; in the KORKİMYA portfolio that product is Ecosky-3103 bio-based rubber process oil, and separate articles on this site cover the TDAE comparison, dosage and EUDR there. A fatty acid is a reactive input added at low loading that takes direct part in the cure chemistry.

One value shows the difference most clearly: the acid value. Ecosky-3103 has an acid value of max 15 (ASTM D465 / ASTM D664), meaning it carries no reactive free acid, because its job is not chemical. In TOFA 180 and TOFA 185 the same value is min 180 and min 185. The two are not alternatives to each other: both can sit in the same compound, each doing its own job.

Unsaturation and rosin acid: two critical values in selection

The first property that separates tall oil fatty acid from a saturated fatty acid is unsaturation. The iodine value is min 150 in both TOFA grades (ASTM D5768). Unsaturation means double bonds available for oxidative crosslinking and dimerisation. In resin and binder applications that is an advantage; in a sulphur cured rubber compound it is a variable that can affect cure and scorch behaviour and must be validated compound by compound.

The second property is rosin acid content. The abietic-type acids left after tall oil distillation are measured by ASTM D1240 and are capped at max 3% in TOFA 180 and max 2% in TOFA 185. Rosin acid influences tack and colour behaviour; compounds that are colour sensitive or have a narrow rosin acid tolerance take the grade with the lower limit.

These are the two values to read at incoming goods control before any laboratory trial begins. Both are reported on the per-lot certificate of analysis.

TOFA 180 or TOFA 185?

Rubber processing is in the application list of both grades; the choice is made on colour sensitivity, rosin acid tolerance and effective fatty acid share. The table below compares the two grades as they appear in the product data.

The practical reading is this: in technical rubber compounds with a wide colour and rosin acid tolerance, TOFA 180 is the sufficient and more economical choice. Where a lighter colour, a lower rosin acid content and a higher effective fatty acid share are needed, move to TOFA 185.

Sample, validation and supply

A raw material change in a rubber compound is never made without laboratory validation. The right sequence is to prepare a trial compound with a sample in place of the current fatty acid line item, measure cure and scorch behaviour in your own recipe, and only then move to scaled production.

• CAS number: 61790-12-3 (TOFA grades).
• Packaging: ISO tank container 20-24 MT net for TOFA 180 and 20-22 MT net for TOFA 185; carbon steel or stainless steel tank.
• Storage: 20-40°C, closed tank or container, away from direct sunlight.
• Shelf life: 12 months, extendable to 24 months with quality control testing.
• KKDIK registered; Turkish Safety Data Sheet (SDS/GBF) and a certificate of analysis (CoA) for every lot.