ISSN 00125008, Doklady Chemistry, 2015, Vol. 462, Part 2, pp. 156–159. © Pleiades Publishing, Ltd., 2015. Original Russian Text © L.B. Boinovich, V.M. Bouznik, P.N. Grakovich, V.I. Gryaznov, A.S. Pashinin, G.Yu. Yurkov, 2015, published in Doklady Akademii Nauk, 2015, Vol. 462, No. 4, pp. 431–434.
CHEMICAL TECHNOLOGY
Creation and Modification of Superhydrophobic Materials Based on Fibrous Polytetrafluoroethylene Corresponding Member of the RAS L. B. Boinovicha, Academician V. M. Bouznikb, c, P. N. Grakovichd,
V. I. Gryaznovc, A. S. Pashinina, and G. Yu. Yurkovb, e Received December 9, 2014
DOI: 10.1134/S0012500815060014
Materials having superhydrophobic properties attract attention of researchers not only for explora tion of an interesting phenomenon but also for possi ble practical applications (surface selfcleaning, miti gation of icing, decrease in the hydrodynamic resistance) [1]. As regards superhydrophobicity, fluoropolymers [2], in particular, fibrous nonwoven materials are promis ing; these materials can be used to form surfaces with desired roughness, which is important for superhydro phobic behavior to appear [1]. A specific feature of fluoropolymers, first of all, the basic one, polytetrafluoroethylene (PTFE), is the technological difficulty of producing fibers. The known electro and aerodynamic spinning methods [3–5] based on the use of polymer solutions are inap plicable to PTFE as it is insoluble. Meanwhile, it is most attractive due to high chemical and thermal sta bility, low friction coefficient, and unique dielectric properties [6]. The only method for the production of PTFE fibers is laser treatment (ablation) of the block polymer [7]. It appears pertinent to study nonwoven PTFE materials to elucidate the influence of surface mor phology on wetting and to develop modification meth
a
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia b AllRussian Scientific Research Institute of Aviation Materials, ul. Radio 17, Moscow, 105005 Russia c National Research Tomsk State University, Tomsk, 634050 Russia d Belyi Institute of Mechanics of Metal–Polymer Systems, Belarus National Academy of Sciences, Gomel, Belarus e Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii pr. 49, Moscow, 119991 Russia email:
[email protected]
ods in order to achieve superhydrophobicity and con trol over the wetting behavior. The nonwoven PTFE materials are manufactured as cotton wool or felt, the latter being more convenient for practical applica tions. EXPERIMENTAL The material production process and equipment were reported earlier [7]. The process conditions used to manufacture the felt samples result in deformed fibers with considerably changed morphology (Fig. 1a), unlike cotton wool samples. Rectangleshaped specimens were studied. They were characterized by considerably different millime ter size roughness of the upper and lower sides caused by the specific features of the production process. Wet ting was measured using digital processing of the video image of the sitting drop; the experimental setup was described in [7, 8]. The initial contact angles were measured at 3–5 different points of the surface of each sample side; for each point, the average angle was determined based on 10 successive images of the drop. The scatter of angles measured at different sites of the coating could amount to several degrees, which reflected the spatial inhomogeneity of the surface roughness. Dust and watersoluble contaminations were removed by preliminary washing of samples with water for 5 min using a Grad 1335 ultrasonic bath. RESULTS AND DISCUSSION The initial contact angles of the samples are simi lar, 143° ± 1° and 144° ± 2°, and the rolloff angles are 35° ± 6° and 13° ± 4°; the rougher side has a smaller rolloff angle corresponding to smaller wetting hyster esis. The equilibrium angles obtained in various areas of the same surface differ by up to 10°. It follows from the obtained values of angles that the material is char
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10 µm (b)
(a)
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10 µm
Fig. 1. SEM images of the surface of nonwoven felt material F4: (a) initial sample; (b) sample obtained by thermal pressing at 120°C and 100 kg/cm2.
A study of the evolution of the contact angle upon longterm contact of a water drop with the surface demonstrated that the angles on both sides markedly decrease and equilibrium values ranging from 6° to 20° are established after 4–8 h of the contact. This behav ior is related to inhomogeneous contamination of the sample. The decrease in the contact angle is a result of for mation of a wetting/adsorption film near the drop perimeter, the film thickness being determined by the composition of the liquid. The presence of watersol uble or hydratable impurities results in film thickening and, hence, in a smaller contact angle [9]. To verify this hypothesis, the samples were additionally washed with distilled water for 10 days with daily replacement of wash water and then airdried. After that, the time variation of the contact angles substantially decreased (3°–5°), indicating a decrease in the chemically inho mogeneous distribution of impurities on the sample surface (Fig. 2). Thus, the hypothesis of the influence of material surface impurities on the hydrophobicity in the contact with aqueous media is in line with the data for additionally washed samples. Of considerable significance for hydrophobic material service is stability against salt, acid, and alkali solutions; therefore, evolution of wetting of the mate rial upon contact with a 0.5 M solution of sodium chloride, 0.1 M aqueous solution of sulfuric acid, and 0.05 M aqueous solution of potassium hydroxide were investigated. The slight decrease in the contact angle after 6–8 h of the contact (from 144 ° to 142° for NaCl solution; from 144° to 139° for H2SO4; and from 145° to 143° for KOH) attest to high chemical stability of the material after additional washing. DOKLADY CHEMISTRY
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The chemical stability of the material on contact with these corrosive liquids was additionally analyzed by measuring the surface tension at the test liquid drop–air interface. The surface tension values obtained immediately after deposition of a solution drop differ little for all cases and are 72 mN/m. The surface tension of the drops decreases with contact time for all solutions, indicating that during contact with aqueous media, fragments of the material having surfactant properties are desorbed. These may be monomers and oligomers present in the PTFE fibers. The decrease in the surface tension is more pro nounced for acid and alkali solutions (by 7 and 5 units, respectively, while for the salt solution, the decrease was only by 3). The presence of a mixed homogeneous/heteroge neous wetting regime precludes creation of useful superhydrophobic functions; therefore, attempts were made to modify the surface of the felt sample for estab lishing the superhydrophobic wetting regime. The felt sample was impregnated with a dispersion of silica nano particles in a solution of 1(3((2,2,3,3,4,4,5,5,6,6,7,7
Contact angle, deg
acterized by mixed homogeneous/heterogeneous wet ting regime on both surfaces [1].
147.5
142.5
137.5
0
4
8
12
16
20 Time, h
Fig. 2. Evolution of the contact angles formed by distilled water drops on the initial sample after additional washing. The data obtained for a rough surface are designated by rhombuses and those for the smoother surface are shown by squares.
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dodecafluoroheptyl)oxy)propyl)N,N,Ntrimethylsi lanetriamine in decane [10]. The multimodal texture of the material surface was provided by aggregation of SiO2 nanoparticles both in the bulk dispersion and in the wetting film during the solvent evaporation [11]. The material with such coating had initial wetting angles of 158.7° ± 3.5° and rolloff angles of 6.8° ± 7.5°, while the surface tension of a water drop was 72.3 ± 0.4 mN/m. The contact angle and the surface tension decreased slightly (to within experimental error) during 20 h of the experiment. Thus, the modified nonwoven material sample can be considered as promising for service in the open atmosphere with increased humidity and contamina tion. The dust and soot particles specially deposited on the coating surface are easily removed by drops that fall on the surface or by a water jet (selfcleaning effect). The time dependences for water and aqueous sodium chloride solution demonstrate a slight (within the experimental error) decrease in the contact angle and a moderate decrease in the surface tension. The water wetting parameters of the considered materials, their time variation, and selfcleaning unambiguously attest to the superhydrophobicity of the PTFE felt sur face after modification. Since the surface roughness determines to a large extent the type of material wetting [12], it is pertinent to develop procedures that would control the surface morphology of the felt sample. An example is thermal pressing, which makes it possible to vary the surface morphology and the material porosity. For treatment, a sample of the nonwoven material was sandwiched between polyamide films, then placed between pol ished metal sheets, and mounted into a manual hydraulic press where it was treated with temperature and pressure. Upon thermal pressing with low pressure and temperature, the material retained the fibrous structure of the untreated sample (Fig. 1a); however, as the pressure and temperature increased, neighbor ing fibers fused together at the contact points. At tem perature above 120°C and a pressure of 100 kg/cm2, the sample turned into a solid film (Fig. 1b) with a sub stantially different roughness; its contact angle was 107° ± 4°, which is a typical value for polished block PTFE samples. Hence, the contact angle can be sub stantially changed by thermal pressing. The used equipment makes it possible to form sites with different surface roughness over large areas; this may give rise to the contact angle gradient. Samples with considerable variation of surface morphology were produced. The contact angles measured succes sively at different points spaced by 5 mm toward the most rough area were 109° ± 5°, 124° ± 4°, 130° ± 4°, 141° ± 5°, and 150° ± 3°. Thus, thermal pressing of fibrous materials can be used to produce materials
with considerable variation of surface wetting ranging from hydrophobic to superhydrophobic states. Fur thermore, it is possible to manufacture samples with substantial contact angle gradient on the surface. Our investigations demonstrated that PTFE felt material obtained by laser ablation has a surface mor phology providing high hydrophobicity. Mixed (heter ogeneous/homogeneous) wetting regime was estab lished. The decrease in the contact angle upon long term contact of the drop with the surface was found to be caused by surface contamination by external impu rities, which can be eliminated rather easily. Studies of sample wetting with salt, alkali, and acid solutions demonstrated high chemical stability of the non woven materials in question, which is important for service in corrosive environments. A method for surface modification of nonwoven samples on treatment with a dispersion of silica nano particles in an organic solvent to achieve the superhy drophobic state was proposed (the initial contact angle was 158.7° ± 3.5° and the rolloff angle was 6.8° ± 7.5°). Thermal pressing modification of felt samples pro vided considerable and controlled variation of the sur face morphology with the contact angle being changed by 40°; moreover, samples with a contact angle gradi ent were prepared. The studied PTFE felt materials appear to be promising for practical use, especially in view of the proposed modification methods. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (project no. 13–03–12168_ofim). REFERENCES 1. Boinovich, L.B. and Emel’yanenko, A.M., Usp. Khim., 2008, vol. 77, no. 7, pp. 619–638. 2. Buznik, V.M., Aviats. Mater. Tekhnol., 2013, no. 1, pp. 29–34. 3. Filatov, Yu.N., Elektroformovanie voloknistykh materia lov (EFVprotsess) (Electroformation of Fibrous Mate rials (EFF Process)), Moscow: GNTs RF NIFKhI im. L.Ya. Karpova, 1997. 4. Medeiros, E.S., Glenn, G.M., Klamczynski, A.P., Orts, W.J., and Mattoso, L.H.C., J. Appl. Polym. Sci., 2009, vol. 113, pp. 2322–2330. 5. Zhang, L.F., Kopperstad, P., West, M., Hedin, N., and Fong, H., J. Appl. Polym. Sci., 2009, vol. 114, pp. 3479– 3486. DOKLADY CHEMISTRY
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CREATION AND MODIFICATION OF SUPERHYDROPHOBIC MATERIALS 6. Loginov, B.A., Udivitel’nyi mir ftorpolimerov (Amazing World of Fluoropolymers), Kirov: Dom pechati, Vyatka, 2009. 7. Grakovich, P.N., Ivanov, L.F., Kalinin, L.A., Ryab chenko, I.L., Tolstopyatov, E.M., and Krasovskii, A.M., Ross. Khim. Zh., 2008, vol. 52, no. 3, pp. 97–105. 8. Emel’yanenko, A.M. and Boinovich, L.B., Inorg. Mater., 2011, vol. 47, no. 15, pp. 1667–1675. 9. Emel’yanenko, A.M. and Boinovich, L.B., Prib. Tekh. Eksp., 2002, no. 1, pp. 52–57.
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10. Pashinin, A.S., Emel’yanenko, A.M., and Boino vich, L.B., Fizikokhim. Poverkhn. Zashch. Mater., 2010, vol. 46, no. 6, pp. 734–739. 11. Boinovich, L.B., Emelyanenko, A.M., and Pashi nin, A.S., ACS Appl. Mater. Interfaces, 2010, vol. 2, no. 6, pp. 1754–1758. 12. Boinovich, L.B. and Emelyanenko, A.M., Langmuir, 2009, vol. 25, no. 5, pp. 2907–2912.
Translated by Z. Svitanko
