Amines 101 for Metalworking Fluids
Patrick E. Brutto, CMFS | TLT Technical Analysis August 2013
A primer on the basic functions and fluid characteristics of these unique compounds.
Editor’s Note: Following are two condensed articles on metalworking fluids that were published in STLE’s technical e-newsletter, which provides targeted information to those working in specific areas in the fields of tribology and lubrication engineering. The e-newsletters are available on several topics, including MWFs, power generation, synthetics, hydraulics, grease, environmentally friendly fluids, among others. Log on to
www.stle.org for more information and to sign up to receive our e-newsletters.
AMINES AND AMINO ALCOHOLS are widely used in water-dilutable metalworking fluid (MWF) concentrates to neutralize acid-functional components and develop and maintain alkaline pH once diluted by the end-user. These components are also sometimes added tank-side to adjust and buffer the pH of in-use fluids. Commonly used amines such as monoethanolamine (MEA), triethanolamine (TEA), monoisopropanolamine (MIPA) and 2-amino- 2-methyl-1-propanol (AMP) have different advantages in terms of neutralization and buffering efficiencies. This is illustrated in Figures 1 and 2, where comparisons are made among amines and inexpensive caustics. Although caustics are cheap, efficient neutralizers, they have significant performance disadvantages relative to amines. Caustics are poor pH buffers, reacting rapidly with atmospheric carbon dioxide.
Figure 1. Neutralizer Efficiency
Figure 2. pH Buffering Efficiency
Because of differences in amine cost, neutralization and buffering efficiencies, MWFs are often formulated with multiple products. For example, MEA and TEA have been used together for many years to provide a balance of cost-effective neutralization, pH development and buffering. Inspection of Figures 1 and 2 makes it clear why these amines have historically been popular.
In addition to the basic functions described above, amines often impact other fluid characteristics such as corrosion control, foam development, cobalt leaching and microbiological resistance. In fact, for today’s higher-performing, longer-life fluids, these secondary characteristics are often more important than the primary ones. For example, amine choice can significantly influence the resistance of a fluid to microbes, which, in turn, impacts fluid longevity. This is demonstrated in Figure 3 for a synthetic MWF containing 1000 ppm (at dilution) of the registered biocide hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine. The fluid containing MEA/ TEA fails bacterial control (two consecutive weeks above 105 CFU/mL) after four weeks, while partial replacement with OA increases bacterial resistance to 12+ weeks. OA is believed to increase permeability of the bacterial cell wall, enabling the biocide to work more efficiently.
Figure 3. Bacterial Control with Triazine Biocide
Last, but not least, it is important to consider regulatory factors and customer requirements, which may limit the types of amines used. For example, German regulation TRGS 611 prohibits secondary amines from intentionally being added to MWFs used in its jurisdiction. This regulation also limits secondary amine impurities to maximum 0.2 percent in MWF concentrates. The only exception is where a given secondary amine has been specifically exempted by the regulatory authorities. Some multinational users of MWFs have also requested that current or future generations of products not be formulated with secondary amines.
Pat Brutto is a technical service & development specialist for Dow Consumer & Industrial Solutions, a division of The Dow Chemical Co., in Buffalo Grove, Ill. In this role, Pat leads the technical effort to understand performance benefits of DC&IS products and works with customers to solve problems using these chemistries. Pat holds a master’s of science degree in chemistry from DePaul University in Chicago. He is an STLE Certified Metalworking Fluid Specialist,™ and is the current chairman of the STLE Chicago Section. You can reach him at pebrutto@dow.com.