Bioresin-based superhydrophobic coatings with reduced bacterial adhesion

Abstract

Hypothesis

Certain biobased polymers or natural compounds can be effectively used in superhydrophobic coating formulations to reduce environmental impact of fluorinated compounds and related bioaccumulation and toxicity problems. Many environmental concerns have thus far been raised in relation to toxicity of solvents and C8 fluorine chemicals. Elimination of these important elements from non-wettable coating formulations can jeopardize non-wetting performance significantly. However, intelligent and innovative approaches that introduce ecofriendly resins and compounds in superhydrophobic coating formulations without significantly altering self-cleaning superhydrophobicity are possible and being reported.

Experiments

Superhydrophobic coatings based on a biomass-derived bioresin polyfurfuryl alcohol (PFA) were prepared. The coatings were made by blending PFA resin with a C6 perfluorinated acrylic copolymer PFAC in solution and subsequent spray coating. Silica nanoparticles were also added in order to repel some common oils. Coating morphology, chemical and thermal properties, biocompatibility and bacterial adhesion properties were studied in detail.

Findings

Coatings having 50 wt% bioresin revealed equal water-repellency performance comapred to 100% PFAC-based coatings. Healthy cell growth was maintained on the coatings with no cell toxicity using human cell line, HeLa cells. Superhydrophobic coatings demonstrated very low bacterial adhesion to E. coli, S. aureus and Ps. aeruginosa indicating promising biofouling resistance. The coatings did not require any post thermal annealing. This would cause significant energy savings for large-scale adaptation.

Introduction

Fabrication of non-wetting surfaces and coatings by utilizing non-toxic and preferably sustainable materials and methods has been gaining momentum in recent years [1], [2], [3], [4]. This also includes superhydrophobic treatment of textiles for self-cleaning fabrics [5], [6], [7], [8], [9]. Traditionally, non-wetting coatings have been made with long-chained, perfluorinated compounds (PFCs) that impart very good water and oil repellency to surfaces. However, the U.S. Environmental Protection Agency (EPA) has banned longer-chained PFCs having eight or more fluorinated carbons on the polymer backbone (i.e., C8 chemistry), which largely break down into toxic products such as perflurooctanoic acid (PFOA) or perfluorooctanesulfonic acid (PFOS) [10], [11]. Although much safer, some uncertainties also exist for shorter chains known as C6 chemistry. These ever-increasing environmental concerns on such compounds have already driven the need to reduce and eventually eliminate these compounds from the superhydrophobic formulations. To this end, several non-fluorinated hydrophobic macromolecules and resins have been increasingly investigated such as natural or synthetic waxes [12], [13], [14], [15], silicone fluids and resins [16], [17], [18], [19] and certain rubbery polymers [20], [21], [22].

It is well known that smooth coatings or surfaces of perfluorinated macromolecules or polymers like polytetrafluoroethylene (PTFE, Teflon) are inherently hydrophobic with static water contact angles between 110° and 120°. With proper surface texturing, these surfaces display what is known as the lotus effect (droplet roll over), exhibiting static water contact angles higher than 150° [23], [24], [25]. A surface texture having micron and submicron/nano scales (dual-length scale) can allow trapping of microscopic air pockets under the liquid droplets [26], [27]. In fact, the relationship between superhydrophobic states and surface texture is a somewhat intricate. This statement is not complete. Surface features required for optimum superhydrophobicity are surfaces that display high contact angles and low contact angle hysteresis and low sliding angles. They fall into various wetting states: Wenzel, Cassie air-trapping, Cassie impregnating (with a single level of hierarchy of roughness), Lotus-like (with a double level of hierarchy of roughness-micro/nano), and the rose petal-like (sticky superhydrophobic) wetting state. It is highly probable that Cassie air-trapping and Lotus-like wetting states are suitable for anti-fouling purposes as water drops roll off the surface leaving no others behind. In contrast, Wenzel, Cassie impregnating, and rose petal-like wetting states may not meet the anti-fouling requirements, because water drops remain entrapped into the surface features, after the tilting of the surface [28], [29], [30].

Another important parameter about these surfaces is the formation of low contact angle hysteresis (<10˚), which is the difference between the advancing and the receding contact angles. Low (preferably < 5˚) and stable contact angle hysteresis means that the droplets can freely roll off on the surface and associated phenomenon is known as self-cleaning superhydrophobicity. Similarly, droplets can easily slide away or roll-off from certain non-wetting surfaces (i.e., Cassie air-trapping and Lotus-like wetting) once they are tilted. The tilt angle at which the droplets roll or slide off is known as sliding or roll-off angle. The smaller it is, the less sticky the superhydrophobic surface will be. Minimization of the solid-liquid interfacial area is achieved with very low contact angle hysteresis [31], [32]. Similarly, the right combination of fluorine chemistry and surface texturing would also repel many low surface tension liquids (i.e., 31.00–39.00 mN m⁻¹) including oils and solvents [33], [34], [35]. However, eliminating fluorine component from the formulations generally increases the surface energy and makes it more difficult to repel such liquids [36]. Hence, it is expected that new methods and formulations would first aim at reducing fluorinated elements in the formulations, maintaining the low surface energy aspect but without being the main component or matrix of the coatings [15], [37], [38], [39], [40].

Among nature-synthesized materials, waxes show very hydrophobic character but they suffer from low thermal stability, poor substrate adhesion and low resistance to mechanical abrasion. However, certain approaches such as modified wax emulsions [41], [42], [14], wax-polymer blends [15], wax/nanoparticle-in-polymer composite coatings [43] have been demonstrated and used successfully in superhydrophobic applications such as water-oil separation. A recent approach, for instance, used lycopodium spore powder (a hydrophobic flowerless plant) along with cellulose and a waterborne polyolefin matrix dispersion to create ecofriendly superhydrophobic coatings with low contact angle hysteresis [2]. Surfaces of cellulosic particles were also transformed into superhydrophobic by coupling their –OH groups with reactive silane macromolecules by chemical vapor deposition and subsequent polymerization [44]. In this work, we utilize polyfurfuryl alcohol (PFA) bioresin in combination with an acrylic copolymer having C6 side chains (conforms to the EPA 2010/2015 perfluorooctanoic acid (PFOA) stewardship program) having no toxic PFOAs byproducts upon breakdown in the environment [45].

Polyfurfuryl alcohol is synthesized from pentosan-rich biomass, such as bagasse (a wasteproduct from sugar cane production) [46] and has been used as matrix in the manufacturing of various composite materials including compounding with other biodegradable thermoplastics [47] or thermosets [48]. Owing to high compatibilty of PFA with many organic and inorganic materials, it can be used in a broad range of applications such as corrosion-resistant coatings [49], [50], wood adhesives [51], etc. Polyfurfuryl alcohol can be thermally crosslinked and can be used as a robust binder for separation membranes [52], lithium batteries [53], nanostuctured carbon materials [54]. We found that the bioresin, PFA, and the acrylic copolymer were compatible with one another when blended at any ratio with no macroscopic phase separation. We fabricated coatings with various degrees of hydrophobicity by spraying polymer blends and hydrophobic silica nanoparticle dispersions in acetone on aluminum and glass surfaces. The superhydrophobic nanocomposite coating made up of 1:1 PFA: polymer blend containing silica nanoparticles was found to display exteremly minimal bacterial adhesion against E. coli and S. aureus. The coatings were also employed in vitro cytotoxicity test using HeLa cells.