Application Note. An Optimum Solution for the Automatic Control of Cutting Oils

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Harold McGee
5 years ago
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The chemical and physical parameters of the oils must be carefully controlled for

1 An Optimum Solution for the Automatic Control of Cutting Oils Cutting oils are widely used in many mechanical machining processes such as cutting and grinding. The chemical and physical parameters of the oils must be carefully controlled for them to function properly. This article explains how the different parameters can be determined.

2 Cutting and grinding are two classical mechanical machining processes. Both require professional products to optimize the speed and improve the performance of cutting tools. It is extremely important to monitor the variation of oil parameters over time in order to know when to adjust the concentration of constituents in the oil tank or even replace the contents of the tank completely. This eliminates unnecessary costs and maximizes the lifetime of the cutting oil. A cutting oil is a fluid that is used to lubricate the tools that cut or grind the metal. Another important function of the oil is to remove the heat generated by friction during these processes. Cutting oils exist in many formulations optimized for different materials or alloys. The products generally consist of an oilin-water emulsion (3 to 10%). Anionic and cationic surfactants are used as emulsifiers and boron compounds as biostabilizers. In addition, amines are employed as buffers and several other components are also added. Many of the largest producers of cutting oils have their plants in the Piemont region of Italy where a famous Italian automobile manufacturer has its headquarters. The effectiveness of the cutting oil can be controlled and verified by performing different chemical or biological tests. Very often, manufacturers offer these tests as part of their customer support service. The number of the samples analyzed during the year can therefore be very high (8000 to 9000 samples per year). Recently, we visited two of the largest producers of cutting oils to learn about how the oils are analyzed and how the analyses can be automated. Table 1 presents an overview of the principal tests with reference to specific problems and the concentration limits of the parameters. All the parameters except the last two can be monitored by an automatic titrator of the METTLER TOLEDO Excellence series. Table 1: The most important parameters of cutting oils Parameter Function Problem Concentration limit Conductivity ph Alkalinity This represents an index of the total dissolved salts. The initial value and its variation over time are monitored. The ph of new products must be between 8.5 and 9.5. The buffering capabilities of the products and initial concentration of the amines are monitored. If the conductivity is high, the emulsion tends to break and corrosion phenomena occur. CO 2 absorption from the air and the formation of bacteria tend to lower the ph over time. This causes the emulsion to break. The alkalinity tends to decrease over time, reducing the stability of the emulsion. Boron Biostabilizer The boron concentration tends to increase over time due to the evaporation of the water. Salt deposits are then formed. Chlorides Chlorides are impurities that Chlorides promote corrosion phenomena. originate from the raw materials and water used. Surfactants Emulsifiers Surfactants are digested by bacteria. The concentration decreases over time and the risk of the emulsion breaking increases. Total hardness Bacterial counts Corrosion This is derived from the water used. Ca 2+ and Mg 2+ bind with the surfactants and form insoluble salts, which causes the emulsion to break. Bacterial action causes offensive odors, product degradation, and lowers the ph and the surfactant concentration. Corrosion itself is a problem. <5.000 µs/cm 8.5 < ph < 9.5 ~ 500 ppm <150 ppm

Simultaneous measurement of conductivity, ph and alkalinity The most difficult analysis of all those mentioned in Table 1 is the simultaneous measurement of conductivity, ph and alkalinity.

3 Simultaneous measurement of conductivity, ph and alkalinity The most difficult analysis of all those mentioned in Table 1 is the simultaneous measurement of conductivity, ph and alkalinity. Depending on the formulation of the cutting oil, soaps may precipitate during the titration. These tend to stick to the conductivity probe and the ph sensor. A very flexible and effective cleaning routine is therefore required. This problem is optimally solved by the METTLER TOLEDO T90 titrator. The best results were obtained using the following sequence: The sensors and inserts (tip of burette and stirrer) are cleaned with propylene glycol monomethyl ether in the sample beaker that has just been analyzed. Two conditioning beakers are then used. The first beaker contains a mixture of acetone, THF and isopropyl alcohol (one third of each component). The second beaker contains water to regenerate the aqueous film on the ph sensitive glass membrane. All this can be done thanks to the flexible setting possibilities of the T90 titrator and the two Rondo 20 sample changers (Fig. 1). The ingenious construction of the METTLER TOLEDO InLab710 conductivity sensor (Fig. 2) makes cleaning easy and very effective. The titration is performed by using an acid. The consumption at ph 7.00 and 4.00 is calculated for a sample volume of 100 ml. In very critical cases, the measurement of conductivity and the initial ph can be performed independently by using a second tower on the Rondo 20 (Fig. 3). After the analysis, the sample beaker is automatically emptied so that the working environment is not polluted by solvent vapors. Figure 1: The current T90 installation: The Rondo on the right is used to measure the conductivity, ph and alkalinity. The Rondo on the left is for chloride determination. The two peristaltic pumps are used to empty the beakers at the end of the titration.

Boron titration The boron analysis is similar to that used for boric acid in plating baths. In this titration, boric acid forms an ester with mannitol and releases protons.

Chlorides in low concentrations The determination of chloride content is an analysis that is rather sensitive to the sensor problems because the classical silver sensor has a porous ceramic diaphragm.

4 Boron titration The boron analysis is similar to that used for boric acid in plating baths. In this titration, boric acid forms an ester with mannitol and releases protons. The protons are titrated with sodium hydroxide. The boron content can be directly analyzed in the sample used for the alkalinity determination. Chlorides in low concentrations The determination of chloride content is an analysis that is rather sensitive to the sensor problems because the classical silver sensor has a porous ceramic diaphragm. This tends to become clogged with particles especially when used cutting oils are analyzed. METTLER TOLEDO has an excellent solution for this problem namely the DMi148-SC sensor (see Fig. 4), which uses a ph glass membrane as reference. Since the traditional internal Ag/AgCl reference element is not needed, the ceramic diaphragm is no longer required. The only thing to note is that the potential of the titration curve decreases instead of increasing as in the classical argentometric titration. The analysis is performed by adding concentrated nitric acid using a The ph is first adjusted to 8.4 with NaOH. A solution of mannitol is then added which causes the ph to decrease due to the esterification reaction. The sample is again titrated to ph 8.4 and the boron content determined from the consumption of sodium hydroxide in mg/l. burette. This breaks the oil emulsion so that all chlorides are in the aqueous part of the sample and hence can be titrated. Thanks to this addition of acid, the ph of the sample remains stable during the entire titration so the DMi148-SC sensor can be reliably used. Another problem with this analysis is that more than one equivalence point might exist because some formulations contain iodine salts. This problem of the right assignment of chloride calculation and correct equivalence point is solved using the logical conditions of the T90 (see Fig. 5). Since the number of equivalence points cannot be known in advance, the titration is stopped at a predefined potential (0 mv). Figure 2: The InLab 710 conductivity sensor. Figure 4: DMi148-SC with ph glass membrane reference. Figure 3: Rondo 20 with a second tower.

Anionic surfactants and total hardness The Mettler method M377 describes how anionic surfactants can be quantitatively determined in cutting oils by

The total hardness of water is determined classically by titration using the Ca 2+ electrode or the DP5 phototrode in alkaline media.

5 Anionic surfactants and total hardness The Mettler method M377 describes how anionic surfactants can be quantitatively determined in cutting oils by titration with Hyamine1622 using the DS800 TwoPhase sensor. This new sensor allows you to perform a twophase titration according to DIN EN The total hardness of water is determined classically by titration using the Ca 2+ electrode or the DP5 phototrode in alkaline media. Figure 5a: Titration curve of chloride the titration is terminated at 0 mv. Figure 5b: Titration curve of chloride with iodide as impurity (first equivalence point).

Conclusions The system of T90 titrator and Rondo 20 sample changer is ideally suited to perform the

Full automation of the workflow and result calculation provides increased productivity and secure results.

Surfactant Titration Figure 6: Selected applications for surfactant titration, METTLER TOLEDO application

6 Conclusions The system of T90 titrator and Rondo 20 sample changer is ideally suited to perform the titrimetri analyses of cutting oils. Full automation of the workflow and result calculation provides increased productivity and secure results. AGC Surfactant Book 20% Titration Cyan 22 Application Brochure General Titrators Selected Applications for Surfactant Titration Figure 6: Selected applications for surfactant titration, METTLER TOLEDO application brochure No. 22 Mettler-Toledo AG PO Box VI-400, CH-8606 Greifensee Tel Fax Subject to technical changes 06/13 Mettler-Toledo AG For more information

Titration Sensors. Titration Sensors DS800-TwoPhase DS500 DP5 Phototrode. Titration of a large variety of surfactants Fast, accurate and reliable

Titration Sensors. Titration Sensors DS800-TwoPhase DS500 DP5 Phototrode. Titration of a large variety of surfactants Fast, accurate and reliable Titration Sensors Titration Sensors DS800TwoPhase DS500 DP5 Phototrode Titration of a large variety of surfactants Fast, accurate and reliable TwoPhase Surfactant Titration DS800TwoPhase Superior performance