Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/290460
42 March - April 2014 | www.machinerylubrication.com where metal surfaces grind together under pressure, lubricity is a key concern. But if the application involves only hydrodynamic lubrication where there's no metal-to-metal contact, lubricity is less important. Esters are great for high-temperature hydrody- namic applications because they can survive in extreme environments where no other lubricant can. Manufacturing, Chemical Stability and Application Suitability To this point, the role that chemical structure plays in ester properties has been discussed. However, a second factor is equally important: the manufacturing process and the residuals it can leave behind. Residual Acid Value Ester manufacturing always starts with an acid and an alcohol, both of which may be volatile. It is impossible to achieve 100-percent conversion in any chemical reaction, so there is always some residual carboxylic acid or alcohol in the final product. If this is not properly controlled, it can alter the initial properties of the ester and can also cause the lubricant's proper- ties to change during storage and use. Carbox ylic acids are the primar y concern because they can accelerate hydrolytic breakdown of the lubricant. This is espe- cially problematic in metalworking fluids where water is a main component. Figure 3 shows the effect of residual acid on hydrolytic stability. This is an accelerated hydrolysis test that holds an ester and water in a sealed tube at 125 degrees C. Ester 1 (blue) has an acid number of 0.03 milligrams of potassium hydroxide per gram (mg KOH/g) and shows almost no degradation over the duration of the test. Ester 2 (purple) starts with an acid number of 1, while Ester 3 (red) begins with an acid number of 3. Esters 2 and 3 are highly degraded by the end of the test. Catalyst Residue Esters are usually made with a catalyst to accelerate the synthesis, but ester catalysts also accelerate the degradation of ester in the presence of water. Therefore, it is essential to remove or deactivate the ester catalyst at the end of the manufacturing process to ensure that the ester will maintain its quality during storage, formulation and use. Moreover, mineral acids and certain active metals must be avoided because they can break down any type of ester. Most ester lubricants are not recommended for applications in which they will come in contact with strong acids and bases. Figure 4 shows the effect of mineral acids and metals on hydrolytic stability. The three samples all started with virtually no acid present. One sample (purple) was treated with a mineral acid, while metal fines were added to another (red). As shown on the left, the strong mineral acid completely hydrolyzed the sample within 24 hours. Metal fines were not as fast but had the same effect. The untreated sample (blue) retained its integrity. In conclusion, it is a good idea to consider the expertise and experience of your ester supplier. Esters can be designed and manufactured to work in almost any environment, but this means the selection process is critical. Work with someone who knows the science and technology of esters and is willing to take the time to understand your requirements. This is the only way to ensure you are getting the right product for your lubrication needs. sYNthetICs Figure 3. Residual acid left after manufacturing shortens ester life dramatically. Figure 4. Residual catalyst significantly reduces hydrolytic stability.