Oil-repellent and antifog coatings based on poly(vinyl alcohol)/hydrolyzed poly (styrene-co-maleic anhydride)/fluorocarbon surfactant

Highlights

• High transparency for poly(vinyl alcohol)/hydrolyzed poly (styrene-co-maleic anhydride)/fluorocarbon surfactant coatings.

• A large space between FC molecules provides enough hydrophilicity on PVA/H-PSMA surface, and thus achieves excellent antifog performance.

• The mobile fluorinated alkyl chains on the surface can be reorganized to slide oil droplets easily at low TA, providing oil repellency.

Abstract

In order to improve the oil-repellent and antifouling ability of the antifog coatings, a poly(vinyl alcohol)/hydrolyzed poly (styrene-co-maleic anhydride)/fluorocarbon surfactant (PVA/H-PSMA/FC) was prepared by incorporating a fluorocarbon surfactant into PVA/H-PSMA via hydrogen bonds. The microstructure, antifog and oil-repellent properties were investigated in our study, and the results indicate that the strong interaction between the PVA/H-PSMA and the FC provides a good fixation for the FC inside or on the surface of the coatings. The water contact angle (WCA) ranges from 59° to 69° and the oil contact angle (OCA) ranges from 35° to 50°, which means that the PVA/H-PSMA/FC coatings are hydrophilic and oleophilic. The results also demonstrate that the prepared PVA/H-PSMA/FC coating exhibits excellent antifog properties and high transparency. Moreover, the oil droplets may slip off the coating surface without any residue and the glide angle is below 12°. A low content of FC loaded creates a large space between FC molecules, thus providing enough hydrophilicity on the PVA/H-PSMA surface and achieving excellent antifog performance. Meanwhile, the mobile fluorinated alkyl chains on the surface can be reorganized and act as “slides” to easily move oil droplets at low TA, providing oil repellency. The excellent oil-repellency makes the transparent PVA/H-PSMA/FC antifog coatings have potential application prospects in the field of surface protection of optical equipment and solar panel.

Introduction

Due to a mass condensation of water vapor on the surface of material, atomization causes severe light scattering and reduces the transparency of the material. Therefore, antifog coatings are essential for lenses such as optical devices, glasses, windshields, solar panels [[1], [2], [3], [4]]. Some physical operation methods have been put into use, such as adjusting the surface temperature of the materials, controlling the relative humidity of the environment and the speed of the airflow to suppress the appearance of fog [2]. However, these methods require complex equipment and high-cost components.

With the advent of antifog materials, most of the above deficiencies have been solved. The antifog materials originally reported are primarily hydrophilic or superhydrophilic [5,6]. These materials allow water droplets to flow and diffuse quickly, ensuring light transmission and reducing light scattering [7,8]. Some inorganic materials [[9], [10], [11], [12], [13], [14]], such as TiO₂, SiO₂ or graphene oxide (GO), can be used to prepare superhydrophilic coatings. Xiong et al. [9] prepared super-smooth, transparent and superhydrophilic TiO₂ coating on glass substrates by improved dipping and coating methods with good antifog performance. Chen et al. [12] fabricated a superhydrophilic and anti-reflection silica nanoparticles antifog coating by vapor deposition. Hu et al. [14] spin-coated GO onto a glass substrate to obtain a superhydrophilic and transparent antifog coating. However, these superhydrophilic surfaces are easily damaged by scratching and cutting. Polymers having polar groups including hydroxyl (OH), carboxyl (COOH) and ester group (COOR) [[15], [16], [17]] can be combined with other suitable polymers to fabricate hydrophilic antifog coatings and the supramolecular assembly among the polymer chain provides good self-healing ability to reduce the impact of scratches, cuts and other damages on thin coatings. Zhang et al. [15] constructed an inherently antifog coating with good self-healing ability via hydrogen bonding between poly(vinyl alcohol) (PVA) and polyacrylic acid. Wang et al. [16] presented a new healable antifog coating via LBL assembly of poly(ethylenimine) (PEI) and a blend of hyaluronic acid and poly (acrylic acid) (HA-PAA). In the meantime, our team [17] prepared a low-cost and high-performance antifog coatings via a mixed solution of PVA and hydrolyzed poly (styrene-co-maleic anhydride) (H-PSMA). PVA/H-PSMA antifog coatings have self-healing ability, which is attributed to self-assembly among the hydroxyl groups of the PVA chains or between the hydroxyl group of PVA and the carboxylic group of the H-PSMA through a large number of hydrogen bonds. Once the coatings are damaged by external forces, such as scratching or cutting, it can quickly repair itself by forming hydrogen bonds in a certain humidity environment.

However, in practical applications, hydrophilic antifog coatings are susceptible to contamination by oleophilic reagents [18]. Oily substances are adsorbed on the surface of the coatings, causing the coatings to lose its antifog performance [19,20]. In order to solve this problem, Guo et al. [21] developed multifunctional coatings with antifog, self-cleaning and antibacterial properties by grafting poly (N-vinylpyrrolidone-co-maleic anhydride) onto glass slides. The hydrophilic brushes had no affinity for the hydrophobic organic fluids and provided oil repellency. Compared with the tedious molecular structure design, fluoropolymers, or fluorine-containing small molecules have been widely incorporated into polymer coatings, and because of its low surface energy and simple processing, it can provide oil-repellent and antifouling properties [[22], [23], [24], [25]]. Sun et al. [23] demonstrated a novel poly (tetrafluoride-r-vinylpyrrolidone) (F-VP)/polyvinylidene difluoride (PVDF) membrane with excellent superhydrophilicity, superoleophobicity, ultralow oil-adhesion performance. The antifouling of the F-VP modification layer endowed the F-VP/PVDF membrane with superoleophobic property to various oils. Howater et al. [24] prepared hydrophilic-oleophobic coatings from fluorosurfactant-Macromers, and the surface of the coatings showed the ability to “self-clean” oil-based foulants. Wang et al. [25] investigated the antifog properties of perfluoropolyethers polymers with different terminal groups and found that perfluoropolyethers with hydroxyl terminations showed both oleophobic and hydrophobic properties on silica substrates. Their unique simultaneous oleophobicity/hydrophilicity reduces the airborne hydrocarbon contamination and improves the long-term antifog performance. Xu et al. [18] fabricated hyaluronic acid (HA)/branched poly(ethylenimine) (bPEI) coatings via layer-by-layer assembly and immersed the coatings in the aqueous solutions of perfluorooctanesulfonic acid potassium salt to obtained excellent antifog and oil repellency ability. However, a strong bonding between fluorocarbon chains and polymer coating is required for long-term oil-repellent stability.

In our previous work, PVA/H-PSMA coatings were prepared with excellent antifog and self-healing ability [17]. However, those coatings are unable to sustain intrinsically high transparency and antifog performance once their surfaces were contaminated with oleophilic materials such as paraffine and alkane. In this study, FC was covalently incorporated into the PVA/H-PSMA coatings to improve anti-fouling performance while maintaining excellent antifog behavior of the PVA/H-PSMA coatings. The antifog behavior and oil-repellency of the PVA/H-PSMA/FC coatings were investigated. The fluorinated moieties were immobilized inside or on the surface of the coatings via strong interactions between the FC and PVA/PSMA blends. On the one hand, the large space among FC molecules provided hydrophilicity to obtain excellent antifog ability. On the other hand, the reorganization of the mobile fluorinated alkyl chains on the surface of the coatings provided excellent oil repellency.