Study on the wettability of the stainless steel pieces prepared by different methods

With the increase in air quality problems derived from the emission of cooking oil fumes, the oil fume purifier was widely installed in household kitchens to eliminate the oil fume. However, because of the great adhesion of the oil, the elimination efficiency of the oil fume purifier goes down and its cleaning difficulty goes up. To solve the problem, four reagents were chosen to fabricate the amphiphobic surface by immersion and chemical vapor deposition. The results showed that the surfaces modified by alcohol reagent, organic fluorine group resin reagent, and heptafluorodecyl trimethoxysilane were only hydrophobic, while perfluorodecanoic acid-modified surface was amphiphobic and its contact angles for water and peanut oil were 138.3° and 125.5° respectively. Therefore, perfluorodecanoic acid could be used to prevent sticking and recycle the condensate of cooking oil fumes on the stainless steel surface of the oil fume purifier.


Introduction
In China, catering sales in 2018 were about 84.79 billion dollars, which has increased by 7.5% compared with 2017 about 78.86 billion dollars [1].Meanwhile, the air quality problems derived from the emission of cooking oil fumes (COFs) have also sharply increased.COFs refer to a gas-liquid-solid three-phase mixture, mainly including water vapor, oil mist, solid particulate matter, volatile organic compounds, polycyclic aromatic hydrocarbons, heavy metals, and carbonyl compounds [2].Studies have shown that COFs may cause irreversible damage to human health, such as mutagenicity, genetic toxicity, and carcinogenicity effects [3][4].
To prevent and control air pollution and protect people's health, the Chinese national standard "Emission Standard for Oil Fume Pollutants in the Catering Industry GB18483-2001" was issued in 2001 and revised in 2019, which stipulated that the maximum emission concentration of COFs should not be greater than 1 mg/m 3 .Therefore, the oil fume purifier was widely installed in household kitchens in China.However, when the mixture of COFs went away from the heating device, they would collide violently with the air, and their temperature decreased rapidly to 60℃-80℃, forming the condensate and sticking to the surface of the oil fume purifier and the oil fume exhaust pipe at last.Due to the great adhesion of the oil, the oil fume purifier and the oil fume exhaust pipe would be blocked after several months of utilization, thus reducing the elimination efficiency of the oil fume, and increasing their cleaning difficulty.
Solid surfaces with special wettability, such as hydrophobic surfaces, oleophobic surfaces, and amphiphobic surfaces with contact angles greater than 90° for water, and oil, and both are widely studied in fields ranging from biology, chemistry, and physics to material science.These surfaces could be applied practically in many fields, such as the self-cleaning of automobile glass and oil fume purifiers.In general, these methods of constructing special wettability were electrospinning, hydrothermal processing, chemical vapor deposition, and sol-gel processing.However, there were few reports on the preparation of amphiphobic surface, which was applied to the self-cleaning of oil fume purifier.In addition, waste cooking oil had great potential as a biofuel feedstock [5].Therefore, to prevent sticking and recycle fully the condensate of COFs, some methods were chosen to fabricate the amphiphilic surface, and their feasibilities were estimated in this study.

Materials
The collected stainless-steel pieces (length× width× height: 3.5 cm×3.5 cm×0.1 cm) were purchased from the local processing plant, which was widely used as the raw material of the oil fume purifier and the oil fume exhaust pipe.They were washed ultrasonically with anhydrous ethanol for 5 min firstly, then were washed with distilled water several times to remove anhydrous ethanol, and then dried in an oven at 80℃ for 12 h.At last, a part of them was stored in a desiccator for further use.

Methods
The stainless-steel pieces were immersed into AR or OF for 5 min and then were dried in an oven, and stored in a desiccator for further use.Furthermore, the stainless-steel pieces were washed with HCl solution (0.5 mol/L) for 30 min to remove surface oxides and were washed with acetone for 30 min to remove surface organic matter.They were immersed in the ethanol solution of PA (0.02 mol/L and 0.005 mol/L) for an appropriate time at room temperature, and then took out the sample from the solution, rinsed several times with ethanol and deionized water, and dried at room temperature.
Another method was that HT was coated on the surface of the stainless-steel pieces by chemical vapor deposition, and the specific steps were as follows: The stainless-steel pieces were ultrasonic washed for 10 min with petroleum ether, anhydrous ethanol, and deionized water successively, and then dried with high-pressure nitrogen.The dried stainless-steel pieces were placed in HCl solution (1 mol/L) for 10 min to remove the oxide film from the surface, and then repeatedly cleaned with deionized water and dried with high-pressure nitrogen.The pre-treated stainless-steel pieces were placed in FeCl 3 solution (1 mol/L) for etching for 30 min, and then repeatedly cleaned with deionized water and dried with nitrogen.The etched stainless-steel strainer was calcined in a Muffle furnace at 400℃ for 2 h.Finally, the stainless-steel pieces were placed in a PTFE box containing 45 μL of HT and modified by chemical vapor deposition for 1 h-2 h.
Contact angle (CA) was measured by the instrument JC2000D1 (POWEREACH, China).CA was conducted by using the drops of water and oil at room temperature, in which the droplets of deionized water and oil were dropped carefully onto the surface, and the average value was obtained by three measurements at different positions of the sample.

Results and discussion
In this study, the stainless-steel pieces were immersed in AR for 5 s, 10 s, 15 s, 30 s, 60 s, 120 s, 180 s, 240 s, and 300 s, and then were dried by the irradiation of ultraviolet curing machine (with lamp power of 1 KW, wavelength of 365 nm-366 nm, and power density of 76 W/cm) for 0.3 s, and were cooled at room temperature finally.Deionized water (W), peanut oil (P), and rapeseed oil (R) were used to investigate the CA of the treated stainless steel pieces.It can be seen from Figure 1 that the CA of W, P, and R does not increase with the prolonged immersion time, which indicates that the chemical reaction between AR and the stainless steel pieces takes place quickly, and their CA reaches about 101.2°, 59.3°, and 57.8° respectively.To compare with the CA of the untreated stainless steel pieces, their CAs are elevated at about 49.7°, 27.8°, and 23.9° respectively.However, the AR-modified surface was only hydrophobic but not oleophobic, which could not meet the requirement of an amphiphobic surface.The reason might be that the main component of AR was alcoholic, which affected the effect of hydrophobic and oleophobic on the stainless steel pieces.
The same process was applied to OF, and the result showed that the CA of W, P, and R on the surface of the stainless steel pieces was similar to the contact angle of AR, which was about 95.0°, 54.0°, and 55.0° respectively, and elevated about 43.5°, 22.5°, and 19.7° respectively (Figure 2), but it did not reach the requirement of the amphiphobic surface.The phenomenon of lower contact angle might be caused by the organic fluorine resin of OF.
Figure 3 shows the contact angle of HT modified surface.When the modified time was 1 h, the CAs of W, P, and R were 113.8°, 78.6°, and 81.2° respectively.When the modified time was 1.5 h, the CAs of W, P, and R were 117.6°, 92.8°, and 85.5° respectively.Moreover, when the modified time was 2 h, the CAs of W, P, and R were 126.4°, 94.2°, and 87.9° respectively.The result indicated that the modified time had an important impact on the hydrophobicity and oleophobicity of the stainless steel pieces, and 2 h was the best modified time.By comparing the CA of the untreated stainless steel pieces, the CAs elevated 74.9°, 62.7°, and 52.6° respectively.The reason was that the long chain molecule of HT contained the groups of -CF 2 (with a surface tension of 18 mN/m) and -CF 3 (with a surface tension of 6.7 mN/m), and their surface tension was low and they could effectively reduce the free energy on the surface of stainless steel pieces.In this study, the CA of W on the HT surface reached the standard of hydrophobic, but the CAs of P and R were on the edge of oleophobic, which did not exhibit excellent oleophobicity.However, some researchers found that the CA of soybean oil on the HT surface of the stainless steel filter was increased from 35° to 140.5° [6].The difference might be that the contact state between the liquid drop and the strainer surface was the composite contact state of liquid-solid and liquid-gas.However, the contact state between the liquid drop and the piece surface was only the composite contact state of liquid-solid in this study.It showed that the immersed time was the critical factor, which affected the CAs of water and oil (Figure 4).When the ethanol solution of PA was 0.005 mol/L, the water CA could reach as high as 138.0° (FW), and the oil CA could also reach as high as 124.8° (FP).To investigate the effect of the concentration of the ethanol solution of PA, 0.02 mol/L of PA ethanol solution was chosen.The result indicated that when the ethanol solution of PA was 0.02 mol/L, the water CA could reach as high as 138.3° (TW), and the oil CA could also reach as high as 125.5° (TP), and the CA had almost no difference between 0.005 mol/L and 0.02 mol/L.Compared with the untreated surfaces, the CAs for water and oil were increased to about 86.5° and 93.3° (Figure 5).According to the CA standard of hydrophobicity or oleophobicity, the surface of PA was hydrophobic and oleophobic.Some researchers found that the conductive surface could be superhydrophobic.When the electrolyte was an ethanol solution of PA (0.02 mol/L) and the applied volt dc was 10 V, the CAs for water and salad oil on the copper substrate were 161.3° and 142.8°, respectively [7].However, the CAs for water and peanut oil on the surface of the stainless steel piece were only 138.3° and 125.5° in this paper, which was lower than the CAs for water and salad oil on the copper plate surface because the substrate and their conductive were different.

Figure 1 .
Figure 1.The contact angle of AR.Figure 2. The contact angle of OF modified surface modified surface.

Figure 2 .
Figure 1.The contact angle of AR.Figure 2. The contact angle of OF modified surface modified surface.

Figure 3 .
Figure 3.The contact angle of HT modified surface.

Figure 4 .
Figure 4.The contact angle of the PA-modified surface.

Figure 5 .
Figure 5.The contact angle of untreated and modified PA. (a: The water CA of untreated surface; b: The water CA of 0.02 mol/L of PA modified surface; c: The water CA of 0.005 mol/L of PA modified surface; d: The oil CA of untreated surface; e: The oil CA of 0.02 mol/L of PA modified surface; f: The oil CA of 0.005 mol/L of PA modified surface.)