Surface coating on aluminum substrate with polymeric guanidine derivative to protect jet fuel tanks from microbial contamination

Highlights

• The antimicrobial coating and the coating technology were environmental-friendly and could be applied on a large scale.

• The developed antimicrobial coating exhibited desirable biocidal efficacy without detectable PHMG leaching.

• The antimicrobial aluminum sheet could quickly kill the mixed microorganisms and exhibited desirable UV resistance.

• The antimicrobial aluminum sheet showed the potential to protect jet fuel tanks from microbial contamination.

Abstract

Antimicrobial coatings can act as an effective barrier to microbial growth and colonization in jet fuel tanks, which can effectively avoid the safety problems and economic losses. In this study, we initially synthesized an antimicrobial and water-insoluble complex, poly (hexamethylene guanidine) hydrochloride-sodium stearate (PHMG-SS), and its minimum inhibitory concentrations (MICs) were measured as 0.25 g/L, 0.25 g/L, 0.25 g/Land 0.5 g/L against Escherichia coli, Staphylococcus aureus, Bacillus subtilis and Yarrowia lipolytica, respectively; then, PHMG-SS was facilely coated onto the surface of aluminum substrate by the aid of a green linker, polyvinyl butyral (PVB). The PHMG-SS-coated aluminum exhibited a powerful microbicidal activity against the above four microorganisms, and provided a greater than 99.999% (5 log) reduction in viable counts of experimental microorganisms within 30 min through contact. In addition, the antimicrobial performance could still be observed after 7 d of ultraviolet radiation or soaking in jet fuel. Furthermore, the antimicrobial mechanism was investigated and concluded as damaging the microbicidal morphologies and suppressing the activity of respiratory chain dehydrogenase. Finally, PHMG-SS-coated aluminum was evaluated as interior surface of jet fuel tanks to protect jet fuel, and the experimental results showed that PHMG-SS-coated aluminum achieved a greater than 99.9999% (6 log) reduction in viable counts of a mixed suspension of Escherichia coli, Staphylococcus aureus, Bacillus subtilis and Yarrowia lipolytica within 30 min. The outputs from this work represent a highly versatile approach to build an antimicrobial barrier to protect jet fuel from microbial contamination, and such approach is also worthy of wide application to treat other surfaces.

Introduction

Jet fuel, mainly composed of C₈-C₁₆ hydrocarbons which are volatile and exhibit high saturated vapor pressure and hygroscopicity, is the crucial source to ensure the normal work of all the systems of aircraft and flying safety [1,2]. When free-water is present in tanks, the jet fuel-associated water environment is suitable for microorganism proliferation, particularity within biofilm communities of tank surfaces. Organic compounds which partition from the fuel to aqueous phase would serve as nutrients and facilitate jet fuel biodeterioration [[3], [4], [5]]. Furthermore, the moisture in the jet fuel provides a suitable environment for the growth of microorganisms, making it more susceptible to microbial contamination than gasoline [6]. Therefore, the microbial control of jet fuel is highly important and worthy of extensive study.

A great deal of publicity has been given in recent years on the problems caused by microbial attack on jet fuel, especially that microbial contamination of jet fuel has been widely aware in the aviation industry, and International Air Transport Association (IATA) originally published “Guidance Material on Microbiological Contamination in Aircraft Fuel Tanks” in 2002 (currently in its 5th edition) which provided a detailed overview of the damage potentially caused by jet fuel-contaminating microorganisms and provided recommendations for minimizing biodeterioration risk in aircraft fuel systems [5,7,8]. In fact, microbial contamination of jet fuel has been reported as early as 1960s [9]. Jet fuel can pick up microbially contamination from each stage of the fuel transport infrastructure from refinery tank to airfield hydrant, and microorganisms can enter the inner fuel tank as contaminants due to the entry of airborne, waterborne microorganisms, or occasional errors in operational procedures, thus leading to the contamination of inner surface and jet fuel [10,11]. Studies have shown that diverse microorganisms, including various bacteria, yeasts and moulds, can all flourish in jet fuel. Of the organisms identified, Hormoconis resinae has been particularly prevalent in jet fuel tanks, which can utilize the hydrocarbons of jet fuel and results in the production of organic acids which are corrosive to metals [12,13]. Besides, the yeast of Yarrowia lipolytica (Y. lipolytica) and the bacterium of Bacillus subtilis (B. subtilis), have previously been isolated from jet fuel [8]. Y. lipolytica is well known for its ability to degrade hydrocarbon fuels by a specific pathway involving alkane monooxygenases, fatty-alcohol oxidases as well as dehydrogenases fatty-acyl-CoA synthetases, hence it can undoubtedly degrade jet fuel [14]. These microorganisms live in the water-bearing areas of jet fuel storage and transportation equipment [15]. Under normal circumstances, it is impossible to remove water thoroughly, and water accumulation can contribute to the biofilm accumulation on interior surface of jet fuel tanks. Consequently, the residual water provides habitats in which microbial communities can thrive, and then microorganisms can soon aggregate. Moreover, they are adept at attaching to surfaces to form wet slimes, and fouling on the interior surface to induce the biofilm accumulation. Finally, the fuel filters will be plugged by the microbial aggregates or biofilms [16]. Microbial growth and contamination in jet fuel storage tanks can cause metal corrosion, fuel filter block, and increased maintenance costs associated with these problems [7,8]. Moreover, once the jet fuel is contaminated by microorganism, the quality of fuel will be reduced [5]. In addition, the cleaning of jet fuel tank is a cumbersome and arduous work, which will require a huge cost and also pose a threat to the cleaners [17]. Therefore, it is of great importance to take effective measures to render the interior surface of jet fuel tanks with potent antimicrobial property and control the microbial contamination.

Surface coating is the most desirable and economic approach to obtain the antimicrobial properties owing to the simple operation and easy scalable production [[18], [19], [20]]. However, the existing common problem of surface coating is the persistence. The antimicrobial components will gradually leach out from the coated surface, thus resulting in a decrease in the durability and effectiveness of antimicrobial property and also possible danger to human health and environmental safety, especially when the coatings contain small molecules and inorganic particles [[21], [22], [23], [24]]. Therefore, polymeric coatings with strong antimicrobial activity and sustainability without leaching should be highly developed but remains a critical challenge. Based on the long-term exploration of antimicrobial materials in our group [[25], [26], [27], [28]], as well as considering the coating process and cost, poly (hexamethylene guanidine) (PHMG) was incorporated into the coating strategy, which is an ideal antimicrobial polymer due to its broad-spectrum and excellent antimicrobial properties, lower toxicity to human beings and environment [29,30]. Although PHMG has been extensively applied in various fields, and acted as a powerful agent to inactivate the microorganisms, the water-soluble characteristic easily causes PHMG leaching out from the coating systems. Therefore, modifying PHMG into water-insoluble derivatives would be a practicable method to avoid the attenuation of antimicrobial performance.

In the present work, we aim to develop an antimicrobial aluminum sheet with water-insoluble PHMG-SS as biocide through a facile surface modification method. PHMG-SS was synthesized via strong electrostatic interaction and embedded in a coating applied on aluminum sheets. The performances of the modified aluminum sheets were evaluated in terms of biocidal efficiency against microorganisms identified as contaminants in jet fuel tanks, the leaching of PHMG-SS, and the coating stability. Some aspects of the biocide action mechanism were presented and discussed (Scheme 1).