Application of Infrared Spectroscopy in Determining the Content of Rolling Oil Additives in Aluminum Foil, Sheets, and Strips
Tianjin Port East Technology Co., Ltd. Applied Analysis Department
Abstract： Rolling oil is a crucial auxiliary material in the production process of aluminum sheets, strips, and foils; it directly affects process operation and product quality, and the additive content in rolling oil is a key indicator of its performance. The additives in rolling oil primarily consist of acids, alcohols, and esters, all of which contain oxygen-containing substituents (carboxyl, hydroxyl, and ester groups, respectively). The characteristic absorption peaks of these three components do not interfere with one another and are not affected by the background (base oil). Based on the Lambert-Beer law, calibration curves relating the concentration of each component to its absorbance can be established. By measuring the absorbance of each absorption peak in the sample spectrum, the concentration of each component in the additives can be accurately determined.
Determining the additive content in rolling oils using infrared spectroscopy allows for rapid and accurate results, while completely avoiding the drawbacks of chemical methods, such as significant errors, cumbersome procedures, and high costs (due to the large amount of reagents required).
Keywords： Infrared spectroscopy, aluminum foil/sheets/strips, additives, quantitative analysis
Click to view related products：FTIR-650 Fourier Transform Infrared Spectrometer
· Principle
Taking advantage of the fact that the various components in aluminum foil rolling oil additives have their own characteristic absorption peaks at different wavenumbers in the infrared region (e.g., alcohols have a characteristic absorption at 1052 cm⁻¹, while acids and esters have characteristic absorptions at 1709 cm⁻¹ and 1743 cm⁻¹), respectively. A series of standard substances is diluted with base oil (kerosene-based lubricating oil with C12–C16 carbon chains) in specific proportions to form standard solutions. According to the Lambert-Beer law, as the concentration increases, the concentration of each solute is linearly related to its absorbance. By substituting the absorbance values of the test sample’s characteristic absorption peaks into the linear equation, the concentrations of the sample’s various components can be determined.
· Experimental Conditions
Instruments and Accessories：
FTIR-650 Fourier Transform Infrared Spectrometer;
Liquid testing accessories: Fixed liquid cell (φ=30 mm);
Other：
Dodecyl alcohol (C₁₂H₂₆O), 99% pure;
Hexadecyl alcohol (C₁₆H₃₄O) with a purity of 98%;
Methyl hexadecanoate (C₁₇H₃₄O₂) Purity >97%;
Lauric acid (C₁₂H₂₄O₂) with a purity of 97.5%;
n-Tridecane (C₁₃H₂₈) with a purity of 99%;
· Test Conditions
Resolution: 4 cm⁻¹
Number of scans: 32
Detector: DTGS
· Determination of the optical path length in a liquid cell
Using the interference fringe method, a fixed liquid cell containing no liquid is placed in the optical path and scanned (over a range of 1900 cm⁻¹ to 600 cm⁻¹), yielding interference fringes with maxima and minima. The length of the fixed liquid cell is then calculated using the formula:
· Sample Analysis
1. Carefully inject the sample into the liquid cell using a glass syringe (ensure there are no large or small bubbles in the liquid cell; if there are, repeat the injection). Place the liquid cell in the optical path and perform a scan using the n-tridecane spectrum as the background;
2. Using the tangents of the respective absorption peaks as the baseline, measure the absorbance of the spectrum at the maximum absorption peaks at approximately 1745 cm⁻¹, 1709 cm⁻¹, and 1052 cm⁻¹. Substitute the obtained absorbance values into the calibration curve to determine the concentrations of each component in the sample.
· Construction of a Calibration Curve
1、Preparation of Standard Solutions: Using n-tridecane as the solvent and various additives as the solutes, prepare the following standard solutions based on mass percentage. Substitute these values into the calibration curve to determine the concentrations of each component in the sample.
（A）Standard Solution for Aluminum Foil Rolling Oil:
（B）Standard solution for rolling oil for aluminum sheets and strips:
2、Preparation of Standard Solutions
Using n-tridecane as the reference, measure the absorbance of the two standard solutions and record the absorbance values of each absorption peak (see the sample spectrum below).
2.1Spectral groups corresponding to the working curves of rolling oils for aluminum sheet and strip:
2.1.1Characteristic absorption peak of hexadecanol:
2.1.2Absorption peaks of methyl hexadecanoate and lauric acid:
2.2Spectral groups corresponding to the working curves of aluminum foil rolling oils:
2.2.1Characteristic absorption peak of 12-hydroxyethanol:
2.2.2Characteristic absorption peak of lauric acid:
3、Establishing a Performance Curve
Plot a standard curve with the mass fraction of the standard solution on the x-axis and the absorbance on the y-axis, and determine the equation of the calibration curve and the value of R² (see example below).
4、回收率验证
· Conclusion
The standard curve method for determining the additive content in rolling oils for aluminum foil, sheets, and strips is accurate, rapid, and easy to perform; the calibration curve has an R² value greater than 0.999, and the recovery rate ranges from 99% to 103%, meeting the requirements for quantitative analysis.