Tall oil is a viscous, dark brown liquid that is a co-product of the wood pulp industry. The name originated as an anglicization of the Swedish word “tallolja,” which means “pine oil”. It is recovered from the chemical process used to convert wood into paper fiber. Tall oil is a complex mixture of organic compounds that serves as a valuable renewable and bio-based resource.
Origin and Initial Extraction
The creation of tall oil is connected to the Kraft pulping process, also known as sulfate pulping, which is the dominant method for producing chemical wood pulp. This process breaks down wood chips, primarily from softwoods like pine, using heat and a highly alkaline solution of sodium hydroxide and sodium sulfide. The high temperature and alkalinity cause the naturally occurring wood resins and fats to saponify, converting the esters and carboxylic acids into soluble sodium soaps.
These soaps, rich in fatty and resin acids, are not dissolved in the final cooking liquid, called black liquor, and begin to float to the surface. The pulp mill collects this floating material through a skimming process, resulting in soap skimmings. The collected soap skimmings contain a significant amount of entrained black liquor, which must be removed.
The raw soap is then transferred to an acidulator where it is heated and treated with sulfuric acid. This acidification step breaks down the sodium soaps, separating the organic acids from the sodium to yield Crude Tall Oil (CTO). CTO is a sticky, dark-colored liquid mixture that represents an economic stream for the pulp mill, with a yield typically ranging from 30 to 50 kilograms per ton of pulp produced.
Primary Chemical Components
Crude Tall Oil (CTO) is a mixture of three primary chemical classes, with the exact ratio depending heavily on the species of wood and the geographical source. The first component is Rosin Acids, which typically make up 30% to 50% of the total mass. These are cyclic organic acids, such as abietic acid and pimaric acid, which are responsible for the natural tackiness and resinous properties of pine wood.
The second main group is Tall Oil Fatty Acids (TOFA), generally between 35% and 55% of the total. This fraction is composed of various saturated and unsaturated fatty acids. Prominent unsaturated examples include oleic acid and linoleic acid, while saturated ones include palmitic acid. The high concentration of long-chain unsaturated fatty acids gives tall oil its oily consistency and chemical reactivity.
The Unsaponifiables are neutral substances that do not react with the alkaline cooking liquor. This fraction usually accounts for 5% to 15% of the CTO and includes various sterols, high-molecular-weight alcohols, and hydrocarbon derivatives. The ratio between the fatty acids and rosin acids is a quality measure, influencing the subsequent refining process and the final market value of the CTO.
Refining and Fractionation
Crude Tall Oil is subjected to a refining process called fractionation to separate its valuable components. The standard method for this separation is high-vacuum distillation, which takes advantage of the different boiling points of the compounds. The CTO is heated under reduced pressure, allowing the compounds to vaporize and be collected at distinct temperature ranges without thermal degradation.
This distillation separates the crude mixture into three commercially distinct intermediate products:
- Tall Oil Fatty Acids (TOFA), the lightest fraction, is a purified stream of fatty acid components.
- Distilled Tall Oil (DTO) is a blend of both fatty acids and rosin acids.
- Tall Oil Rosin (TOR) consists of the concentrated resin acids.
The heaviest, non-volatile residue remaining after the distillation is called tall oil pitch. This pitch contains high-molecular-weight polymers and unsaponifiable material. The fractionation process is controlled to maximize the yield of the more valuable TOFA and TOR fractions.
Diverse Industrial Applications
The refined products derived from tall oil serve as versatile bio-based feedstocks across manufacturing sectors. Tall Oil Rosin (TOR) is used for its natural tackifying properties, making it a component in adhesives, especially in pressure-sensitive tapes and labels. It is also used in the production of printing inks, where it improves the binding and drying characteristics, and as a sizing agent in paper manufacturing.
Tall Oil Fatty Acids (TOFA) are used to produce soaps, detergents, and emulsifiers due to their surfactant properties. They are used for manufacturing specialized lubricants and greases, particularly in metalworking fluids, and are converted into alkyd resins for protective coatings like paints and varnishes. Furthermore, TOFA is chemically modified to create dimer acids, which are building blocks for producing polyamides and epoxy resins.
The raw Crude Tall Oil and certain fractions are utilized in the energy sector. CTO serves as a sustainable feedstock for creating advanced biofuels, including renewable diesel and sustainable aviation fuel, as it does not compete with food crops. Additionally, the heavier tall oil pitch is often used as a binder in cement, an emulsifier for asphalt, or as a fuel source within the pulp mill itself.