Abstract
A polystyrene (PS)/paraffin wax composite coating with excellent hydrophobic property was prepared on stainless steel strainer using a simple, inexpensive immersion drying method. The thickness and bonding strength of the composite coating were measured. Scanning electron microscopy (SEM) was used to characterise the coating surface of the stainless steel strainer. The effects of the paraffin concentration in the PS composite coating and the drying temperature of the coating were studied. The contact angles for oil and water were investigated. The results indicated that the composite coating on stainless steel strainer obtained the best hydrophobic property under paraffin concentration 5 mg mL− 1 and drying temperature 50°C. The water contact angle reached 154 ± 1°, and the coating of the stainless steel strainer also showed a distinct oleophilic characteristic, which could be used for oil–water separation.
Introduction
Hydrophobic surfaces have attracted extensive interest because of their numerous properties, such as corrosion resistance, drag reduction, antiwettability, tribological property, 1 etc. They have been widely used in drag reduction coating on the internal surface of pipeline, 2 self-cleaning windows and oil–water separation devices. Many examples of hydrophobicity also exist in nature.3,4 Up to date, hydrophobic surface can be acquired mainly by two ways: (i) roughen the surface with hydrophobic materials; (ii) modify rough surface with low surface energy compounds. 5 The key is to get suitable structure of the rough surface for researchers. Recently, preparation of micro nano structure on metal surface is mainly through laser processing method, 6 chemical etching, 7 surface coating methods, 8 phase separation, 9 etc. Then, the surface can be modified to improve its hydrophobicity by materials with low surface energy. Using special wettability surface for oil–water separation is a hot issue in current materials science.10,11 Oil–water separation technology is very important in the field of petrochemical, including oil pollution on the sea, 10 wastewater recycling during the oil recovery, separation of the organic solvent, 11 etc. Hence, the aim of our research is to establish a simple, low cost and efficient method to fabricate hydrophobic coatings on the stainless steel strainer surface, which can be used for oil–water separation.
Tu et al. 12 dissolved polystyrene (PS) in tetrahydrofuran solution, sprayed them on glass substrate through a sprayer, superhydrophobic coating with irregular distributed micro nano structures was formed. Jiang et al. 13 used reticular structure in oil–water separation for the first time. They sprayed Teflon (PTFE) emulsion on the surface of stainless steel mesh and finally obtained the hydrophobic and hydrophilic property. Wu et al. 14 used a new method to modify stainless steel mesh. A surface chemical deposition layer was obtained using the ZnO micro nano secondary structures. Then, it is modified with PTFE, and finally, a stable superhydrophobic surface was formed. Wang et al. 15 found that the size of stainless steel mesh has a great influence on oil–water separation. Chen et al. 16 dissolved polyurethane in mixed solution of tetrahydrofuran and N,N-dimethyl formamide for next electrospinning process, polyurethane membrane with bead wire structure was prepared and the water contact angle was increased. It could separate the mixture of oleic acid and water. The oil–water separation using hydrophobic composite coating has been studied by some scholars at home and abroad. However, stainless steel mesh structure is still seldom applied in this aspect, and reports about the use of stainless steel strainer combined with PS/paraffin composite coating are rare in this field.
Paraffin wax has been employed as the main coating additive due to its high yield, low cost and numerous industrial applications.17,18 It is the major constituent of petroleum wax. Paraffin wax is safe, environmentally friendly, non-corrosive and chemically inert. Furthermore, paraffin wax is hydrophobic due to the n paraffins constituents, 19 which makes it widely used in the coating industry.
In the present work, a coating with both hydrophobic and oleophilic properties was prepared using a facile and low cost immersion drying method. The water contact angle on this film can be >150°. The coating was made using a solution containing a mixture of PS and paraffin. The stainless steel strainer structure with the special chemical composition on its surface contributed to these unique properties. It contained no fluorine, which made it a low cost and less polluting method for the separation of oil and water.
Experimental
Materials
Polystyrene (M w : 666 g mol− 1) and paraffin were supplied by Beijing Yanshan Petrochemical Co., Ltd. Epoxy resin was bought from Bluestar Wuxi Petrochemical Co. Ltd. The low molecule weight polyamide resin was the curing agent for epoxy resin. Petroleum ether, acetone, butanone, and xylene were supplied by Sinopharm Chemical Reagent Co. Ltd. All the reagents are analytical reagents.
Preparation of polymer solutions and coating
First, petroleum ether, acetone, epoxy resin and polyamide in accordance with 1:1:1:1 were blended on medium high speed for 30 min until they were blended well. This process was for the preparation of the soluble epoxy resin adhesive. Second, solvents of butanone and xylene in accordance with 1:1 were put into a 250 cc three-necked, round bottomed flask. In the condition of stirring, the PS and paraffin were added into the three-necked, round bottomed flask under mass concentration of 25 and 1.0–8.0 mg mL− 1 respectively. Third, reflux reaction was carried out under 80–100°C for an hour to obtain clear, transparent and colourless polymer solution I. Finally, the soluble epoxy resin adhesive and the polymer solution I with 1:6 were blended on medium high speed ∼30 min until blended well. A supersonic cleaner was used for 10 min at last to obtain the homogeneous polymer solution II.
The clean stainless steel substrates were immersed into the polymer solution II for 1 min using tweezers. Then, the substrates were dried in an oven for 30 min. With the above process, the composite PS coating was successfully prepared on the sample surface. The schematic of preparation process of the polymer solutions and coating was shown in Fig. 1.
Preparation process of polymer solutions and coating
Results and discussion
Thickness of PS composite coating
The thickness of different composite PS coatings on the stainless steel plate substrate was investigated using an ellipsometer of SOPRAGES5 type. Table 1 lists the coating thickness under different concentrations of paraffin addition. The thickness of the PS composite coating under each paraffin concentration was tested on different spots of the substrate, and the coating under all the four concentrations was prepared using the immersion drying process. It can be seen from the Table 1, the composite coating is thickened from ∼17.0 to 18.7 μm. Moreover, the composite coating is homogeneous of the small error value for each paraffin addition concentration.
Thickness of PS coatings under different concentrations of paraffin addition in PS composite coating
Coating bond strength with substrate
An acoustic emission micro scratch tester (WS-2004) was used to investigate the bonding strength between the substrate and the PS composite coatings. The clean diamond indenter with apex angle 120° and the tip size radius 200 ± 5 μm was fixed in the chuck perpendicularly. The substrate samples with PS coatings were fixed on the sample platform horizontally. The test loading rate was 60 N min− 1, and the platform sliding velocity was 1 mm min− 1. Then, with the increase in the test load, when it reached a point, the acoustic emission signal strength elevated sharply, this indicated that the indenter scratched through the coating, and the coating was striped from the substrate surface continuously. This load is just the critical load of adhesion failure between the PS coating and the substrate. The entire tests were carried out under temperature of 20 ± 1°C, relative humidity of 35 ± 2% without any vibration and shock. Each sample was tested five times repeated scratch, and the average critical loads of different samples are shown in Tables 2 and 3.
Bond strength of PS coatings under different concentrations of paraffin addition in PS composite coating with drying temperature 50°C
Bonding strength of PS coatings under different drying temperature with paraffin addition concentration 5 mg mL− 1
Figure 2 shows coating bonding strength under four paraffin addition concentrations, Fig. 2a–d are 1, 3, 5 and 8 mg mL− 1 respectively. From Fig. 2 and Table 2, the scratch curves and critical loads of PS composite coating with different concentrations of paraffin addition under drying temperature of 50°C can be easily found. It can be seen from Fig. 4 that the critical load decreases with the increase in the paraffin concentration from 1 to 8 mg mL− 1. As shown below, when the paraffin concentration reaches 8 mg mL− 1, the bonding strength is too weak for us to apply the coating. It is due to that too much paraffin in the composite PS coating weakens the bonding strength between the coating and substrate surface. Furthermore, the drying temperature influences the bonding strength obviously, which can be seen from Fig. 3 and Table 3. The four images of Fig. 3 are under the same conditions with Table 3, Fig. 3a–d are under the temperature of 50, 60, 70 and 80°C respectively. The critical load is ∼80 N under the drying temperature of 50°C. With the increase in drying temperature from 50 to 80°C, the critical load of adhesion failure decreases observably. It is due to that the paraffin precipitation accumulates from the mixture of composite PS coating when the drying temperature rises.
Scratch curves of PS composite coatings under different concentrations of paraffin additions with drying temperature 50°C
Scratch curves of PS composite coatings under various drying temperatures in paraffin concentration of 5 mg mL− 1
Fourier transform infrared spectroscopy of coating
Polystyrene, paraffin wax and the composite coating were characterised using FT-IT respectively. The composite coating was under the optimal condition of our tests, paraffin concentration of 5 mg mL− 1 and drying temperature of 50°C. The spectra were shown in Fig. 4.
Fourier transform infrared spectroscopy (FT-IR) spectra of PS, paraffin and composite coating
For PS, the absorption peaks at 3082, 3057, 3025 and 3002 cm− 1 are the stretching vibration of = C–H. Peaks at 2919 and 2851 cm− 1 are the stretching vibration of –C–H; 1600, 1580, 1493 and 1453 cm− 1 are rocking vibration of benzene ring; 758 and 698 cm− 1 are outplane C–H rocking vibration in benzene ring. For pure paraffin, the absorption peaks at 2955, 2917 and 2849 cm− 1 are characteristic of the aliphatic C–H stretching vibration. Peaks at 1470 and 1465 cm− 1 are the bending vibration of the –CH2 group; 1375 cm− 1 is the inplane bending vibration of C–H and C–C; 728 and 720 cm− 1 are the rocking vibration of C–H in the paraffin. For PS composite coating, 3081, 3059 and 3025 cm− 1 are the stretching vibration of = C–H. 2954, 2917, and 2849 cm− 1 are the stretching vibration of –C–H; 1600, 1492, 1463 and 1452 cm− 1 are rocking vibration of benzene ring. The peaks at 757, 719 and 697 cm− 1 are rocking vibration of aromatic C–H. Compared with the three spectrums, it can be seen that no other characteristic absorption peak appeared. As seen from above, there is no chemical reaction between PS and paraffin.
Wetting properties of PS composite coating
The contact angle of water and oil was measured using the sessile drop method performed on a contact angle goniometer (JC2000A, China) under an ambient laboratory condition, the temperature and relative humidity were controlled to 20 ± 1°C and 35 ± 2%. Poly alpha olefin (PAO2) was used as representative to investigate the contact angle of oil on PS composite coating surface on the stainless steel plate substrate, and the contact angles of oil were < 5°. It showed oleophilic property.
An ultrapure water droplet (∼20 μL) was applied to the coating surface, it allowed droplet to settle for several seconds, and then photographed them using a CCD camera. Measurements were made on five different spots.
Water contact images are shown in Figs. 5 and 6, and contact angle reached maximum 154° under the best condition. As shown in Fig. 7b, the stainless steel substrate surface is rougher when coated with PS/paraffin coating. The search result can be explained by Cassie superhydrophobic model,20,21 raised to describe the contact angle for liquid droplet on a rough solid surface. The water droplet does not fill the rough coating surface for the micro convex and concave structure, and air still exist in the concave structure. Polystyrene and paraffin are the main hydrophobic constituents that we use to obtain the rough coating surface on stainless steel substrate.
Water contact angles with increase in paraffin concentration in composite PS coating
Water contact angles with increase in drying temperature
Topographies (SEM) of stainless strainer
Figure 5 shows the contact angles of PS coating with different concentrations of paraffin addition under drying temperature 50°C. It can be seen that the contact angle increases and then deceases with the increase in paraffin addition concentration from 1 to 8 mg mL− 1. When the concentration of paraffin addition is 5 mg mL− 1, the contact angle is ∼154°. It shows superhydrophobic property. 22
The water contact angles of different drying temperatures are shown in Fig. 6. The concentration of paraffin addition is 5 mg mL− 1, and the drying temperature from 50 to 95°C. From Fig. 6, it can be seen that the contact angle decreases with the increase in drying temperature from 50 to 95°C. The contact angles are 154 and 148° for drying temperature 50 and 95°C respectively. High drying temperature made more paraffin precipitate on the interface of the coating and substrate. The paraffin content in the coating decreased. Therefore, the contact angle for water also changed.
Considering the influence of the drying temperature and the concentration of the paraffin addition, when the concentration of paraffin addition reached 5 mg mL− 1 under drying temperature of 50°C, the hydrophobic behaviours of composite PS coating were optimal.
A stainless steel strainer was also selected as substrate to investigate the property of the composite PS coating for oil and water separation. Figure 7 shows the SEM topographies of strainer substrate surface. Figure 7a is the clean surface of stainless steel strainer. Figure 7b shows the surface of substrate coated with PS composite film well proportioned after immersing into the polymer solution II and drying processes. Clean surface of the stainless steel is bright, but surface with the PS composite coating is dim. The results could be got from careful comparison of the two photographs a and b. The size of the stainless steel wire is increased for the composite coating. In order to characterise the surface microstructure, higher resolution SEM topographies of the surface are shown in Fig. 7c and d. It can be seen that the composite coating was formed. Substrate surface is roughened with PS coating. In our research, the immersion time was controlled to 1 min to prevent the sharp increase in the coating thickness.
Figure 8a and b shows the schematic diagrams for water and oil on the stainless steel strainer surface respectively. It can be seen from Fig. 8c that the water droplet is approximately a water polo, the stainless steel strainer surface processed by polymer solution is hydrophobic. The coating surface structure shown in Fig. 7d still meets the Cassie superhydrophobic model. However, the oil droplet will immediately spread on the surface and infiltrate into the strainer. The PS composite coating in this study is hydrophobic. In consequence, the water droplet would aggregate on stainless strainer but not infiltrate. The low contact angle for oil shows oleophilic. When oil drops onto the stainless steel strainer surface with composite coating, it would immediately spread on the stainless strainer and infiltrate. The oleophilic property, gravity factor and capillarity of the mesh pore promote the oil infiltration; the schematic diagram is shown in Fig. 9. The results indicate that the PS composite coating can be used for oil–water separation.
Schematic diagrams a,b for water and oil on stainless strainer surface, and c for image of water and oil droplets on surface of strainer
schematic of oil infiltration process on stainless steel strainer with composite coating
A stainless steel plate substrate was used to investigate the optimum condition for hydrophobic property of PS/paraffin composite coating. Then, a stainless steel strainer was used as substrate, and the coating was prepared on it under the same condition for oil–water separation. It can be used in many aspects, especially petrochemical industry. To date, this is the first time the substrate and this coating was combination, and the composite coating can also be employed on other substrates for more applications.
Conclusions
Polystyrene/paraffin composite coating has been prepared by immersing into polymer solution and drying process. The hydrophobic property of coating surface accords with the Cassie superhydrophobic model for the micro convex and concave structure. Paraffin concentration and drying temperature influenced the wetting behaviours and the bonding strength of the coating obviously. The best processing condition of PS composite coating was paraffin concentration of 5 mg mL− 1 and drying temperature of 50°C. Under this condition, the stainless strainer with PS composite coating shows excellent oleophilic and hydrophobic property, and the contact angle for water reached 154° and for oil was < 5°. The strainer with the coating can be used for oil–water separation, and further research is under progress.
Footnotes
Acknowledgements
This research is supported by the National Natural Science Foundation of China (No. 51305459) the Science Foundation of China University of Petroleum, Beijing (No. 2462013YJRC032) and the Tribology Science Fund of State Key Laboratory of Tribology (No. SKLTKF14A08).