Application of Infrared Spectroscopy in Determining the Structural Families and Composition of Mineral Insulating Oils and Lubricants
Tianjin Port East Technology Co., Ltd. Applied Analysis Department
Abstract: Mineral oil refers to a liquid insulating material refined from petroleum. The main components of petroleum are alkanes, naphthenes, and aromatics, which possess excellent electrical properties and aging stability. By appropriately controlling the content of each component according to specific applications, various insulating oils—such as transformer oil and switchgear oil—as well as various lubricating oils can be produced. For example, aromatic hydrocarbons can effectively adsorb gases and are used in the production of cable oil or capacitor oil, while naphthenic hydrocarbons can lower the oil’s pour point and are used in the production of low-temperature lubricating oils.
According to D L/T 929-2005, “Infrared Spectroscopic Determination of Structural Group Composition in Mineral Insulating Oils and Lubricants,” infrared spectrometers can be used to accurately and rapidly perform qualitative and quantitative analyses of the structural group composition in oils.
Keywords： Infrared Spectroscopy Mineral Insulating Oil, Lubricating Oil Structural Group Composition
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· Principle
Based on the characteristic absorption peaks of aromatic carbon at 1610 cm⁻¹ and alkanes at 720 cm⁻¹ in the infrared spectrum of mineral insulating oils and lubricants, the absorbance values are substituted into empirical formulas to calculate the content of aromatic carbon (CA%), the aliphatic carbon (CP%), and the cycloalkane (CN%) content in the structural groups of the insulating oil and lubricating oil.
· Experimental Conditions
Instruments and Accessories:
FTIR-650 Fourier Transform Infrared Spectrometer;
Hydroxyl Test Accessories; Hydroxyl Test Holder;
Test conditions:
Resolution: 4 cm⁻¹
Number of scans: 32
Detector: DTGS
Other:
Carbon tetrachloride (CCl₄): Analytical grade (A.R.)
Determination of the optical path length in a liquid cell:
Using the interference fringe method, a empty fixed liquid cell is placed in the optical path and scanned (over a range of 1900 cm⁻¹ to 600 cm⁻¹), yielding interference fringes containing maxima and minima. The path length of the fixed liquid cell is then calculated using the formula:
· Sample Analysis
1. Carefully inject the oil sample into the liquid cell using a glass syringe (make sure there are no large or small air bubbles in the liquid cell; if there are, repeat the injection). Place the liquid cell in the optical path and perform the scan;
2. Establish the baseline for the scanned spectrum as follows, and record the absorbance of the sample at 1610 cm⁻¹ and 720 cm⁻¹;
3. Substitute the absorbance obtained in step 2 into the empirical formula to calculate the percentages of aromatic carbon (C A %), aliphatic carbon (C P %), and cycloalkane carbon (C N %).
4. The table below lists the results of aromatic carbon content tests for several samples:
· Conclusion
By using infrared spectroscopy to determine the structural group composition of mineral insulating oils and lubricants, and combining this with the empirical formulae in DL/T 929-2005, the structural group composition of these oils can be determined conveniently, quickly, and accurately.