DTO and TOFA are renewable, natural products that the metalworking fluid industry can use globally, whether in their neat form or as derivatives. Many of the common amine salts (such as triethanolamine) are registered on most of the main country databases where metalworking fluids are used. DTO and TOFA are produced directly from pine trees. The Kraft process, which is used to convert pine wood into pulp in paper mills, produces crude tall oil as a byproduct. The crude is then further processed in a biorefinery into DTO, TOFA and tall oil rosin fractions. Of these three, metalworking fluids primarily use DTO and TOFA.

Distilled tall oil is the Swiss Army knife of raw materials for the metalworking fluid industry. DTOs have unique versatility because they are a mixture of TOFA and 5 to 50 percent tall oil rosin. These components enable DTOs to provide multifunctional characteristics to metalworking fluids including reduced foam, improved emulsion stability, better hard-water tolerance than fatty acids, and bioresistance. Each of these characteristics is important for the development of robust metalworking fluids that can provide longer-lasting value to end users.

The Foam Killer

Foam is a problem in metalworking fluid applications because entrained air negatively effects lubricating and cooling properties. It can also lead to cleanup problems in end-user facilities. Rosin acid in DTO is believed to reduce foaming due to its bulky structure, which limits the acids ability to orient and pack tightly at the air-water interface of foam bubbles. The surface tension of the foam bubbles is therefore not lowered as effectively as it would be in a system with more linear chain fatty acids, such as TOFA or a low rosin content DTO. (See Figure 1.)

A direct correlation between a higher DTO rosin content and a lower foam level is demonstrated using the Ross-Miles test, which measures foam generation under low-agitation conditions. The test (ASTM D1173) is done in an emulsifiable oil formulated with 5, 30 and 50 percent rosin acid and then diluted to 10 percent in 100 parts per million water.

As clearly demonstrated with the 50 percent rosin formulation, rosin acid serves two purposes: It helps to minimize the amount of foam that is generated initially and leads to a faster breakdown of the bubbles that do form. Both of these factors are critical for controlling foam in metalworking fluids.

The Emulsifier

DTOs are effective emulsifiers when neutralized with amines or inorganic bases, and the potassium salts of DTOs were one of the first emulsifiers used in emulsifiable oils.

The ability of DTO to emulsify in hard water was shown in a study Ingevity conducted in water hardened to 1,500 ppm. An emulsifiable oil was formulated with DTO containing 28 percent rosin and compared to an oil formulated with the same components but without DTO. The two emulsifiable oils were diluted to a 5 percent concentration in 1,500 ppm water over a 24-hour period.

The emulsions were then evaluated using a Turbiscan instrument, which projects light at a wavelength of 850 nanometers into the sample. The instrument measured the light backscattered after interacting with the oil droplets in the sample at an angle of 135 degrees. Data is expressed as a Turbiscan Stability Index, which is a measure of emulsion destabilization. Higher TSI values indicate decreased emulsion stability.

The emulsifiable oil formulated with DTO exhibited only a very small increase in TSI over the 24-hour test period. In contrast, the TSI for the sample without DTO started to increase significantly after only five hours of testing. The TSI then quadrupled after 24 hours, as the non-DTO emulsion became less stable.

The Microbe Quasher

Microbial contamination is one of the most challenging issues facing the metalworking fluid industry because it impacts fluid performance and durability. At a time when end users are looking to maximize fluid life, microbes are present in the environment to consume the key additives which would accomplish that. The use of DTO in metalworking fluids is one option that can improve bioresistance.

Biocides protect fluids against microbial contamination, but the available selection is declining, leaving metalworking fluid suppliers vulnerable. One logical option is to formulate a fluid with as many components that are resistant to microbes as possible. As part of this process, it is necessary to minimize components that contain linear carbon chains because microbe enzymes can easily degrade these molecules. DTOs can be prepared with tall oil rosins that are cyclic materials-not linear in nature and therefore much harder to degrade. Abietic acid and dehydrobietic acid are two examples of such compounds.

The Adjustable Tool

If distilled tall oil is the utility knife of the metalworking industry, tall oil fatty acid is the screwdriver with interchangeable heads. It is primarily used in metalworking fluids as the basis for emulsifiers and boundary lubricity additives.

TOFA has a distinct advantage as a starting material when compared to saturated fatty acids. Saturated fatty acids, like stearic acid, mainly produce solid derivatives with melting points above room temperature, which makes handling these derivatives more difficult when trying to include and stabilize them in a metalworking fluid. In contrast, TOFA is highly unsaturated, which enables derivatives to remain as liquids so that they can be readily incorporated into metalworking fluid formulations.

TOFA with lower rosin content (less than 5 percent) is useful as an emulsifier. The double bonds and carboxylic acid groups, which are reactive sites, enable the material to be readily used in the preparation of derivatives used as value-added metalworking fluid additives. TOFA is currently being derived into five types of products useful in the metalworking fluids industry.

1. The role of esters is becoming more important as the industry searches for alternatives to medium-chain and long-chain chlorinated paraffins. The esters made from TOFA are used in boundary lubricity additives, such as the monobasic ester isopropyl oleate, and the polyol ester trimethylolpropane trioleate.

2. TOFA can also be used in complex esters. The main emulsifiers derived from TOFA are the nonionic surfactants known as polyethylene glycol monoesters and diesters. The monoesters are prepared by ethoxylation of TOFA, while the diesters are produced through an esterification reaction. A range of PEG esters with varying hydrophilic-lipophilic balance values gives the metalworking fluid industry wide options to find the right emulsifier for use in a specific product.

3. Self-emulsified esters are a multifunctional derivative that act as both boundary lubricity additives and emulsifiers. TOFA can be an integral raw material in developing a self-emulsified ester suitable for specific metalworking fluid types and applications.

4. TOFA has also been the traditional raw material of choice in the preparation of 2:1 amides used as emulsifiers, corrosion inhibitors and boundary lubricity additives in emulsifiable oils and semisynthetic fluids. While diethanolamine was the original amine used with TOFA, diisopropanolamine has been used since the early 1990s.

5. Dimer acids can be prepared by combining two molecules of TOFA. Dimer acids are used by themselves as boundary lubricity additives, mainly in metal-forming operations; however, they can also be used to form oil-soluble and water-soluble ester derivatives.

Pine chemicals allow metalworking fluid producers and formulators to do more with less inside tightening boundaries. As formulators search for more value-added tools in todays challenging business climate, they can consider adding DTO and TOFA to their toolkit.

Monica Ford, Ph.D., is a senior surface scientist at Ingevity in the Industrial Specialties Division. She supports technical efforts for the group, with an emphasis on the lubricants market. Ford graduated from Tuskegee University with a B.S. in Chemical Engineering, and from the University of Florida with a Ph.D. in Chemical Engineering. Contact her at (843) 746-8341 or monica.ford@ingevity.com.