TOFA in lubricant ester production: polyol esterification, base stock and boundary lubrication

In lubricant ester production, tall oil fatty acid is esterified with polyols into an oil-soluble ester structure. That ester is then used as a base stock or as a lubricity additive. Lubricant ester formulations are listed directly as an application in the TOFA 185 product data.

Two separate routes into a lubricant: soap or ester?

Tall oil fatty acid enters the lubricant world through two different chemistries, and confusing them is a source of formulation errors. The first route is saponification: the fatty acid is neutralised with an alkali or an amine, a water-dispersible soap forms, and that soap builds the emulsifier and lubricity leg of water-miscible metalworking fluids. The second route is esterification: the fatty acid is reacted with an alcohol and the product is soluble in oil rather than water.

This article covers the second route. The saponification, emulsion and water-miscible cutting fluid side is the subject of a separate article on this site and is not repeated here. The ester route is the domain of oil-based hydraulic fluids, compressor oils, greases, chain oils and neat machining oils.

How does esterification with polyols work?

Most lubricant esters are made by esterifying a multifunctional alcohol (a polyol) with several fatty acid molecules. The carboxyl group of the fatty acid and the hydroxyl group of the alcohol form the ester bond while releasing water; removing that water drives the reaction forward. The resulting molecule is a branched structure with the polyol at the centre and long hydrocarbon tails around it.

The value of that structure in a lubricant comes from three places: the polar ester bond supports adhesion to the metal surface, the long hydrocarbon tails form a separating film under load, and the branched geometry sets the flow behaviour. On the fatty acid side, the variables the formulator controls are chain length, unsaturation and free acid content.

TOFA offers a measurable advantage here: it enters the process as a free fatty acid. The acid value of TOFA 185 is min 185 (ASTM D1980) and esterification stoichiometry can be calculated directly from it. The unsaponifiables limit of max 4% (ASTM D1065) caps the fraction that does not react and behaves like a diluent in the finished ester. Unsaturation is defined by a min 150 iodine value (ASTM D5768).

Base stock or additive? Two different use levels

A fatty acid derived ester can enter a lubricant at two different levels, and the purchasing consequences of the two are different.

• As a base stock: the ester forms the main liquid phase of the lubricant or is blended with a mineral base oil. Volumes are high and raw material consistency feeds straight through to the product specification.
• As a lubricity additive: the ester is added to a mineral or synthetic base oil to improve friction and wear behaviour. Volumes are low, but the expectation on surface activity and purity is higher.
• As an intermediate: the ester is a semi-finished chemical sold on to grease and specialty lubricant producers.

All three start from the same place: the colour, acid value and impurity profile of the fatty acid that feeds the ester. The final colour and odour of a lubricant ester reflect the refining level of the raw material that went into it.

Boundary lubrication: why is a fatty acid so effective?

Three lubrication regimes are defined. In the hydrodynamic regime the surfaces are fully separated by a fluid film and viscosity governs. In the mixed regime the film has thinned and asperities begin to touch. In the boundary regime the fluid film can no longer carry the load and contact is governed by molecular layers adsorbed on the surface. Heavy load, low speed and high temperature make that regime unavoidable.

What matters in the boundary regime is not viscosity but the ability of molecules to adhere to the metal surface. The polar end of a fatty acid and of a fatty acid ester does exactly that: it orients to the surface, the hydrocarbon tails line up outwards and a layer forms that reduces metal-to-metal contact. That is why fatty acid derivatives sit among the classic lubricity additives.

Bio-based lubricant positioning

Tall oil is a by-product of kraft pulp production, meaning it is a forest-derived raw material that does not compete with the food chain. That is a direct positioning advantage for lubricant programmes with a bio-based content or renewable raw material target. KORKİMYA supplies these products KKDIK registered, with a Turkish Safety Data Sheet (SDS/GBF) and a per-lot certificate of analysis (CoA).

Being bio-based is not a performance claim on its own; the oxidation stability, low-temperature behaviour and hydrolytic resistance of the finished lubricant are set by the formulation and the additive package. The right approach is to validate the raw material choice with a sample in your own ester process.

Which feedstock for the ester route?

Two tall oil feedstocks come up in ester production. TOFA 185 is a tightly specified fatty acid. DTO Premium is a distilled tall oil grade that carries rosin acid and is listed in the product data with an esterification reactions for lubricant esters application. The difference between them is whether rosin acid functionality enters the ester.

Incoming goods control and supply

For an ester producer the critical values are the four that determine reaction yield and finished product colour: acid value, iodine value, rosin acid content and unsaponifiables. All of them appear on the per-lot certificate of analysis.

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