## 3 Results  
### Degreasing Efficiency over Time  
Increasing the number of moles of hydrophilic EO in the SE nonionic surfactants increases the HLB number. The effects of changing the number of moles of EO on the degreasing efficiency of the steel specimens were examined for a constant surfactant content and temperature. Using the variation of samples before and after degreasing, the degreasing efficiency was calculated using Equation (3) as mentioned in Section 2.2. Figure 1 shows the degreasing efficiency over time for each type of SE surfactant in the degreasing solution at temperatures of 30 \({}^{\circ}\)C and 40 \({}^{\circ}\)C. Apparently, the degreasing efficiency increased along with the extension of duration. At 30 \({}^{\circ}\)C, with a degreasing time of 2 min, the degreasing efficiency of SE-9 and -7 were 97.9% and 91.6%, respectively, which are higher than those of SE-12 and SE-20 (90.6% and 87.9%, respectively), while SE-3 had the lowest degreasing efficiency of 73.3%. This trend in degreasing efficiency was observed for all measured times (1-5 min). At 40 \({}^{\circ}\)C and a degreasing time of 2 min, the degreasing efficiency of SE-9 and -7 were 99.1% and 98.3%, respectively, which is higher than the efficiencies of SE-12 and -20 (97.5% and 90.3%, respectively), while the degreasing efficiency of SE-3 was the lowest at 85.0%. Therefore, the degreasing efficiency decreased in the order of SE-9, SE-7, SE-12, SE-20, and SE-3, at both 30 \({}^{\circ}\)C and 40 \({}^{\circ}\)C.  
Figure 2 shows the degreasing efficiency over time for each type of SE surfactant at degreasing solution temperatures of 50 \({}^{\circ}\)C and 60 \({}^{\circ}\)C. Overall, when the temperature increased over 50 \({}^{\circ}\)C, the degreasing efficiency significantly increased for all surfactants and reached maximum after 2 min. At 50 \({}^{\circ}\)C and a degreasing time of 1 min, the degreasing efficiency of SE-9 was 97.1%, which was higher than those of SE-12 and SE-20 (95.9% and  
Figure 1: Degreasing efficiency over time with the various degreasing solutions at 30 \({}^{\circ}\)C (**left**) and 40 \({}^{\circ}\)C (**right**). 93.3%, respectively); the degreasing efficiency of SE-7 was 90.3%, which was lower than that of SE-12 and SE-20, unlike at lower temperatures, while the degreasing efficiency of SE-3 was still the lowest at 78.0%. This order was maintained for all measured degreasing times (1-5 min). At 60 \({}^{\circ}\)C and a degreasing time of 1 min, the degreasing efficiency of SE-12 and SE-20 were 98.4% and 97.2%, respectively, which are higher than the efficiencies of SE-9 and SE-7 (95.9% and 94.4%, respectively), while the degreasing efficiency of SE-3 was the lowest at 87.7%. This order was maintained for all degreasing times. The reason for the differences in the degreasing efficiency between the degreasing solution temperatures was related to the CP. A nonionic surfactant becomes soluble when hydration occurs at the oxygen position of ether groups bonded to the EO chain, which is a hydrophilic group. The number of ether bonds increases as the length of the EO chain increases; therefore, the solubility in water improves as the degree of hydration increases. Precipitation occurs at the Krafft point related to the hydrophobic part of a surfactant molecule, and at the cloud point system which separates into a concentrated and dilute surfactant phase. The temperature at which emulsification occurs is referred to CP, where the CP of nonionic surfactants is higher because the percentage of the hydrophilic part of the CP is higher [18; 19; 20; 21]. The degreasing efficiency decreases at temperatures above the CP as the solubility of the surfactants in water is reduced, while it is optimal when temperatures approach the CP. Therefore, temperatures of 50 \({}^{\circ}\)C and 60 \({}^{\circ}\)C yielded higher degreasing efficiency than temperatures of 30 \({}^{\circ}\)C and 40 \({}^{\circ}\)C, as these temperatures were closer to the CP of most surfactants, except SE-3.