Final Report Summary - AMICOAT (Development of new antimicrobial nanostructured durable coatings for fuel tanks)
Executive Summary:
AMICOAT APPROACH SUMMARY:
Microbial growth in aircraft fuel tanks is one of the main sources of contamination of aircraft fuel, plugging of flow of fluids systems and, particularly microbially influenced corrosion of fuel tanks.
Microorganisms from aircraft fuel need water to growth and this water come form the intrinsically fuel moisture content and additionally water from atmospheric moisture condenses in the wall of fuel tanks. Among different microorganism Hormoconis resinae (also known as the “kerosene fungus”) and Pseudomonas aeruginosa are the most prevalent.
The current main methods to control the microorganisms’ growth in aircraft fuel are based on maintenance, periodical removal of water from fuel tanks, and treatment, addition of additives as fuel preservatives.
These methods are not enough to solve the problem and there is an urgent need to develop better methods to control the microbial contamination.
In that sense the AMICOAT project proposes to develop new antimicrobial coatings to apply onto fuel tank walls. The antimicrobial coatings will confer antibioadhesion properties to the fuel tanks walls and they will allow the controlled release of biocides from the fuel tank walls to the water/fuel interphase.
The aim of AMICOAT project is to develop two different coating systems, a polymeric nanocomposite antimicrobial coating and a silica sol-gel antimicrobial coating. The main characteristics of the two new coating systems will be: i) good adhesion with epoxy primer due to the grafting strategy ii) durable antimicrobial properties due to the encapsulation of biocides, that will be incorporated in the two both polymeric and inorganic-organic coating systems.
Project Context and Objectives:
The aim of AMICOAT project can be disclosed in the achievement of three main targets:
• The development of new durable antimicrobial coating which practically eliminate the problems caused by the presence of fungi in the fuel tanks.
• The coating formulation incorporating biocides for long-lasting antimicrobial action; coating formulation including sol-gel technology.
• The development of new protocols to test the efficiency of antimicrobial coatings in biofilm elimination.
More specifically, the objectives of AMICOAT are:
• To define the requirements to prevent and control contamination in fuel tanks.
• To define the target microorganisms and tests to measure the efficiency of microbial growth inhibitors activities in fuel tanks.
• To select the materials of aircraft fuel tank, the surface treatments to be applied on them and the components of coatings for the two systems to be developed in the project (nanocomposites and sol-gel coatings).
• To do a screening of antimicrobials based on active inorganic compounds, organic molecules and nanoparticles, including metal and metal oxides with different inhibition mechanisms.
• To address especial interest in potential nano-safety and nano-toxicity issues.
• To analyze minimum inhibition concentration for target fuel microorganisms in order to define the most appropriate inhibitors with the most favourable inhibition mechanisms.
• To evaluate the controlled release of selected antimicrobials in polymeric, inorganic-organic and/or micro and/or nanoencapsulated systems in order to define the most suitable methods to control biological activity and durability of biocide effects.
• To determine if the formulated coatings do present antimicrobial properties and which is the mode of action of the antimicrobials tested.
• To formulate and apply antimicrobial durable nanocomposite-based coatings for fuel tanks by using suitable polymeric resins and micro and/or nanoencapsulated antimicrobial systems with the better efficiency of fungi growth inhibition activity.
• To formulate and apply antimicrobial durable sol-gel coatings for fuel tanks by using suitable inorganic-organic precursors and biocide components with the most efficient growth inhibition activity in active molecular and/or nanoparticulated form.
• To analyse the growth inhibition for specific microorganisms present in aircraft fuel according to microbiological analysis and studies of formation, adhesion and elimination of biofilms onto the new developed antifouling coating surfaces.
• To analyze surface morphology, surface chemistry and surface tension of antimicrobial coatings developed in the project.
• To assess the performance for the new coatings in order to validate them for their application
The antimicrobial nanostructured coating developed in AMICOAT project represent a promising alternative for eliminating fungi growth in the fuel tank, considering that some of the current solution like regular removal of water essential for fungi growth, or periodic washing out of tanks, have limited effect and complicate the maintenance process.
The solution that AMICOAT project proposes would practically eliminate the problems caused by the presence of fungi in the fuel tanks, which in practical terms would be translated into a faster, more efficient and low cost maintenance process. The project follows the 2050 European’s Commission Flight path Vision for Aviation, which states that providing the best associated services in aeronautics and air transport are part of the general objective of enhancing European competitiveness in world air transport markets. Integrating technology at all levels including for providing a quality and affordable maintenance process, it is part of the strategy for safeguarding Europe’s place in the global aeronautic market.
Project Results:
S/T RESULTS AND FOREGROUND:
The AMICOAT project has achieved the main Scientific & Technological target objectives established at its beginning. As main results, based on the test and validation activities carried out at LEITAT facilities, we can state that the antimicrobial nanostructured coatings developed in AMICOAT project represent a promising alternative for eliminating fungi growth in the fuel tank.
This fact represents an important progress, considering that some of the current solution like regular removal of water essential for fungi growth, or periodic washing out of tanks, have limited effect and complicate the maintenance process.
In accordance to the S&T objectives of the project, the main results are detailed in the following paragraphs:
1. Definition of the target microorganisms and tests to measure the efficiency of microbial growth inhibitors activities in fuel tanks; and selection of materials of aircraft fuel tank, the surface treatments to be applied on them and the components of coatings for the two coating systems to be developed.
The microorganism selection was done following the recommendations of the ASTM E 1259 “Standard method for evaluation of antimicrobials in distillate fuels” which specifies the following as the most representative species of pollution problems in fuel tanks:
• Pseudomonas aeruginosa ATCC 33988
• Candida tropicalis ATCC 20495
• Hormoconis resinae ATCC 48138
Additionally, MicrobMonitor2 ® kit was selected as the most appropriate for determining the inhibition of microbial growth in fuel tanks by the innovative solutions researched in AMICOAT project. Moreover, although there are not standard protocols for evaluating the ability to inhibit the formation of biofilms in tank fuel, since it is a very critical process for preventing microbial contamination, an innovative model of biofilms formation into representative surfaces was developed for evaluating the effectiveness of AMICOAT solutions for preventing microbial contamination in fuel tanks.
A study of the aircraft fuel tank materials has been done. Taking into account the results of this study 2024 aluminium alloy was selected as a standard material for coating deposition.
Two standard coating systems were defined. Both of them were based on two coatings: 1st) Epoxy primer; 2nd) Polyurethane or Epoxy top coat. A selection of coating components including commercially available epoxy and polyurethane binders with excellent chemical resistance, pigments and solvents was also done. Moreover, brush and spray were established as techniques for coating application onto aluminium substrate.
2. Analysis of microbial growth inhibition through the screening of antimicrobials with different inhibition mechanisms, the minimum inhibitory concentration for target fuel microorganisms and the evaluation of the controlled release of selected antimicrobials in the coating systems to be developed:
Mechanisms of action of biocides:
According to this study and different guidelines, a pre-selection of biocides to test was done. The pre-selection is shown in Table 1: (in attachment)
(See attached file for complete table)
Minimum inhibitory concentration:
The biocides pre-selected: zinc pyrithione (ZP), silver nitrate, hexadecyltrimehylammonium bromide (HDTAB), 3-iodo-2-proponyl-N-butylcarbamate, 2-methyl-4-isothiazolin-3-one (MIT), carvacol and acriflavine were tested according the standard M07- A8 “Dilution methods to determine the antimicrobial susceptibility of bacteria that grow aerobically” against the previously selected microorganisms (Pseudomonas aeruginosa, Candida tropicalis and Hormoconis resinae). The results of the minimum inhibitory concentration for each biocide/microorganism are shown in graph(See attached file for Figure 1. Minimum inhibitory concentration of biocides for different microorganims.)
Based on the results obtained, and comparing the inhibitory activity against the 3 microorganisms showed by the different biocides tested, 3-Iodo-2propynyl-N-Butylcarbamate, HDTAB and carvacrol were discarded. Therefore, due to the high antimicrobial activity against P. aeruginosa, C. tropicalis and H. resinae and also their molecular compatibility with the technologies proposed in the project development, the following biocides were selected for the following studies:
- Zinc pyrithione.
- Silver nitrate.
- 2-methyl-4-isothiazolium-3-one (MIT).
Biocides antimicrobial activity in fuel:
The most active biocides previously selected were evaluated against the three microorganisms in fuel samples according to the adapted standard 1259 “Control standards test method for evaluation of antimicrobials in liquids fuels below 390ºC”. The results obtained showed that the three selected biocides for development of new nanostructured durable antimicrobial coatings for fuel tanks were effective for reducing microbiological contamination. The biocides, at the concentrations analyzed, were able to reduce in case of Pseudomonas aeruginosa at least 7 logarithms, for yeast 6 logarithms and for fungi 4 logarithms the microbial growth compared with the control where non biocide is added.
On the other hand, microbial contamination was, as expected, detected in the aqueous phase while the fuel phase was free of microbial contamination after 5 days of incubation.
Biofilm development:
Biofilm models of P. aeruginosa and H. resinae onto epoxy coated aluminium plates were also developed for evaluating the ability of the selected biocides to inhibit the formation of that biofilms. According to the general scheme of coatings for aircraft fuel tanks, epoxy coatings were selected as a model for the biofilm development. Different protocols were tested until reached the optimal conditions for biofilm obtaining.
The results showed that silver nitrate and zinc pyrithione were able to avoid Pseudomonas aeruginosa and Hormoconis resinae biofilm formation. On the contrary, MIT although was effective against H. resinae, it showed reduced ability to inhibit the Pseudomonas’s biofilm formation.(see attachment for Figure 2. Biofilms on epoxy coated aluminium surfaces with biocides in free form)
Initial antimicrobial coatings development:
For the evaluation of the controlled release of antimicrobials, antimicrobial coatings based on polymeric and sol-gel systems were developed. The antimicrobial coatings were produced by dispersing the biocide agent into the coating. All formulations were developed on a laboratory scale, using a high speed disperser.
The procedure to prepare the polymeric formulations can be summarized in a three-step process:
a) Resin dilution: Polyfunctional resin is diluted in a suitable solvent as to achieve a suitable viscosity for spray application (aprox. 120 cPs).
b) Biocide wet-out /dispersion: A dispersant/wetting agent is added to the mixture (in an amount of about 10%wt. of the biocide content) and dispersed homogeneously. Then, the biocide is slowly added to the formulation and stirred until properly dispersed.
c) Hardener addition: Finally, a crosslinking agent is added to the former preparation, according to the resin-hardener mixing ratios recommended by the supplier.
Sol-gel coatings were based on tetraethoxysilane (TEOS) and (3-Glycidyloxypropyl) trimethoxysilane (GPTMS) as silica precursors. GPTMS was added to provide flexibility and avoid the formation of cracks in the coating. The epoxide-ring of GPTMS can be opened and as a consequence also react with the hydroxyl groups of the epoxy primer. The effect of different GPTMS/TEOS molar ratio was studied in order to achieve a free-cracking coating surface.
We started studying the hydrolysis and condensation reactions by ATR-IR spectroscopy of a TEOS and GPTMS sol, separately. Sols have been prepared at 13 wt% and 50 wt% (solid content) in water using HNO3 as catalyst. The solubility of biocides (AgNO3, 2- methyl-4-isothiazolin-3-one and zinc pyrithione) in the water-based sol-gel was also tested. Silver nitrate and 2- methyl-4-isothiazolin-3-one were soluble in water-based sol while zinc pyrithione was only soluble in DMSO solvent.
Microcapsules as controlled release systems:
In parallel, a selection of the microencapsulation technologies including the shell material was done according to the current scientific bibliography.(in attachment is the Table 2. Materials and microencapsulation technology selection for microcapsules development)
The microencapsulation procedure for each shell material was developed using zinc pyrithione as a core material. Zinc pyrithione microcapsules of PMMA were obtained following an experimental procedure based on the solvent evaporation microencapsulation technique. The best results showed a microencapsulation efficiency of 86 % and a microencapsulation yield of 84 %. The ZnP/PMMA microcapsules were dispersed in fuel for 3 days and no damage was observed by optical microscopy. (in attached file is the Figure 3. ZnP/PMMA microcapsules)
However, PMMA microcapsules did not show enough solvent resistance once they were introduce in coating formulation. On the other hand, melamine-formaldehyde microcapsules of a model core (oil) showed good solvent resistance and microencapsulation of water-soluble biocides (MIT and AgNO3) in MF have been tried but not positive results were obtained. As alternative, halloysite clays were proposed to immobilize water-soluble biocides. Positive results were obtained when AgNO3 was used.
Biocide release studies:
Prior to the design of the biocide release study, solubility analysis of the selected biocides in fuel and fuel/water solutions, chemical analysis of fuel JP8 (the selected fuel), and establishment of the analytical protocol to quantify the biocide release were performed.
Biocide release studies from different developed coatings were investigated in order to determine the retention of the biocide in the coating over extended-periods of time. Determination of biocide leakage rate for each kind of coating (Epoxy-PUR-SolGel) gave us an idea of the durability of the developed coating. If there is an uncontrolled biocide release, a slow release technology is needed to ensure the long-lasting effect. Selection of a proper release system allows keeping the concentration of the active agent above the minimum inhibition concentration (MIC) while decreasing initial release.In order to design the experimental setup for the release studies, previous measurements of water content in JP8 fuel and solubility studies of the selected biocides in fuel and water were performed. JP8 water content was determined to be around 0,004 %. The evaluation of the solubility of the target compounds in JP8 was analyzed by ICP-MS (AgNO3, ZnP) and UHPLC-MS (MIT). While ZnP was not soluble in fuel or water; MIT and AgNO3 were soluble in water but not in fuel. Considering the previous results, the release studies were performed for MIT and AgNO3 in an aqueous media. After 5 days of immersion, ICP-MS analytical results indicated that under these experimental conditions no AgNO3 release was detected for polymeric-based coatings. Regarding the polymeric based-coatings incorporating MIT, only a small percentage of biocide was released from the coatings (0.9 % for Epoxy coatings and 4.6 % for PUR based coatings) when the temperature raised up to 50ºC. A large amount of MIT subsisted into the antimicrobial coating matrix during the time of the release studies.(in attached file Figure 4. Release of MIT from epoxy (left), PUR (middle) and sol-gel (right) coatings).
MIT total release from sol-gel coatings after 5 days in water at room temperature was estimated around 1.8-2.3 %. The diffusion of MIT from sol-gel coating was easier than the diffusion from epoxy and PUR-based coating. When sol-gel was the coating matrix, silver release could be detected by ICP-MS whereas this was not possible with polyurethane and epoxy-based coatings. In this case, silver nitrate release from sol-gel coating was very low (up to 3 ppm).
Due to the slow biocide release from inorganic and organic coatings, no solutions to delay the biocide release were needed. At that point, it was decided not to incorporate microcapsules or nanoparticles in coating development, as their inclusion would only have delayed even more the biocide release.
Microbiological analysis results:
The different antimicrobial coatings were analysed microbiologically by applying the diffusion test method. With this method the activity of the different coatings was evaluated against one of the most respresentative fungus in aircraft fuel tanks: Hormoconis resinae ATCC 20495.
Aluminium coated plates incorporating a 3%wt of biocidal agent were observed at two different times, after 3 days and 7 days of incubation with the fungus. After three incubation days, all the coatings containing zinc pyrithione showed antimicrobial activity and the biocide diffused from the coating to the agar plate generating an inhibition zone. The other coatings containing Silver nitrate and MIT did not show any inhibition zone, which means that either the coating has not antimicrobial activity or the antimicrobial did not diffuse to the agar plate.(in attachment Figure 5. Results of microbiological analyses for the different antimicrobial coating systems with biocides)
According to the antimicrobial character, zinc pyrithione seemed to be the best choice. Above all coating materials, sol-gel and PUR exhibited a faster release of biocide resulting in better antimicrobial results. In conclusion, sol-gel and polyurethane formulations containing zinc pyrithione were chosen for further optimization.
3. Formulation and functional optimization of antimicrobial coatings:
The antimicrobial coating systems developed were based on two subsequent layers: A beige coloured epoxy primer and a polymeric or sol-gel topcoat incorporating zinc pyrithione as active antimicrobial substance.
Different polyurethane based formulations were prepared using different additive systems for a better dispersion and stability of zinc pyrithione. In parallel, sol-gel formulations were obtained in different experimental conditions: aqueous solution, organic solvent and anhydrous conditions. Anhydrous sol-gel process with MEK as solvent for coating showed the best performance results (good wettability and appearance).
Different sol-gel precursors (TEOS/GPTMS), combined in different weight ratio, different amounts of biocide in the final sol-gel coating and the dispersion of the biocide in the silica sol were studied.
From all of them, best formulations (based on characterization results) were finally applied on epoxy coated aluminium plates by spray technique for their characterization incorporating different amounts of ZnP: a 3%wt and a 5%wt in dry coatings.(in attached file is the Figure 6. Sol gel systems (left) and conventional coating systems (right) on aluminium substrates incorporating a different amounts of active substance).
Real samples of the aforementioned coatings (corresponding to Deliverable 3.1) were shown during the dissemination event that took place in Leitat Technological Center on 2014-05-22.
4. Analysis of the growth inhibition of the new developed antimicrobial surfaces, superficial and performance characterization of antimicrobial coatings:
The characterization of the optimum antimicrobial sol-gel and polyurethane coatings was performed, including: the analysis of antimicrobial growth inhibition and biofilm formation; the surface properties determination and the performance assessment of the new antimicrobial coatings developed.
Evaluation of microbial growth inhibition:
The antimicrobial and antibiofilm abilitiy against Hormoconis resinae ATCC 20495 of the new antimicrobial coatings was evaluated using two different methods:
- Diffusion test method
The diffusion method is a rapid method that consists in evaluating the antimicrobial behaviour by direct contact between agar plates with the target microorganism and the coatings tested. With this method it is possible to determine if the coatings have the ability to avoid spores’ germination and consequently, to inhibit-the vegetative growth.(in attached file is the Figure 7. Results of microbiologically analyses for polyurethane based coatings (left) and sol-gel coatings (right).)
- Biofilm prevention
This test allows evaluating the antibiofilm properties of the developed coatings by assessing the reduction in the spore attachment to the surfaces when compared to the negative control.(in attachment Table 3. Biofilm development in the sol-gel coatings)(in attached file is the Table 4. Biofilm development in polyurethanic formulations)
All these results together demonstrate that the developed formulations have high ability to avoid the spores germination and, consequently, inhibit the vegetative growth of H. resinae. Additionaly, these formulations also present a slight ability to prevent the attachment of spores into the surfaces.
Surface properties characterization:
Epoxy-Al plates coated with sol-gel and polyurethane based formulations were characterized by SEM.(attached file contains Figure 8. SEM Images of 3% ZnP, 5% ZnP and biocide-free sol-gel coated Al plates and Figure 9. SEM Images of 3% ZnP, 5% ZnP and biocide-free polyurethane coated Al plates).
The ink test was used to measure the surface tension of the developed coatings. The surface tension for polyurethane based coatings was around 26-28mN/m, while the sol-gel coatings presented a surface tension around 22-24mN/m.
Coating thickness of optimum antimicrobial sol-gel and polyurethane coatings have been evaluated by 3D Optical Confocal Profilometry. Results indicated that thickness were in the range of 219-319 µm for sol-gel coatings and in the range of 280-432 µm for polyurethane coatings. These thicknesses depend on the properties of the paint itself (i.e. rheology) and on the application technology (i.e. HVLP spray) and procedure. AFM analysis for the epoxy aluminium plates coated with optimum sol-gel and polyurethane formulations containing 0%, 3% and 5% of ZnP (in the final coating) were also carried out in order to observe surface topography and nanoroughness for the optimum antimicrobial samples.
Coating performance characterization:
The performance of the developed coatings was assessed according to the standard tests used to evaluate the fuel tank coatings. Tests carried out to assess the coatings performance were:
- Crosshatch adhesion test
- Pencil hardness test
- Resistance immersion test
Next figure summarizes the results after performing each of these tests.(in attached file Figure 10. Coating performance evaluation for sol-gel and PUR systems)
Excellent adhesion properties were achieved for all of the coatings. As expected, hardness in polyurethane-based coatings was lower than in sol-gel coatings and the biocide incorporation in the formulations seemed to increase the scratch resistance when compared with those formulations with no biocide.
Regarding the results obtained in the immersion tests, no visible changes were observed in any of the immersed coated plates after 45 days, indicating an excellent fuel resistance.
CONCLUSIONS:
Main conclusions after AMICOAT project are listed below:
• Hybrid silica sol-gel and PUR antimicrobial top coatings have been developed, incorporating zinc pyrithione as active agent.
• Different protocolos and methodologies have been adapted for the evaluation of the antimicrobial activity in different conditions.
• The antimicrobial functionality has been optimized:
• The incorporation of ZnP in a 3% by weight in dry PUR and sol-gel coatings is enough to inhibit Hormoconis resinae growth, showing an slight antibiofilm capacity.
• Performance characterization tests have been carried out.
(Note:Complete File with Figures and Tables can be found in attachment S T Results Foreground for this section)
Potential Impact:
Strategic Impact of AMICOAT:
The antimicrobial nanostructured coating that the AMICOAT project developed, is promising alternative for eliminating funghi growth in the fuel tank, considering that some of the current solution like regular removal of water essential for fungi growth, or periodic washing out of tanks, have limited effect and complicate the maintenance process. The solution that AMICOAT project developed practically eliminate the problems caused by the presence of fungi in the fuel tanks, which in practical terms would be translated into a faster, more efficient and low cost maintenance process. The project follows the 2050 European’s Commission Flight path Vision for Aviation, which states that providing the best associated services in aeronautics and air transport are part of the general objective of enhancing European competitiveness in world air transport markets. Integrating technology - at all levels including for providing a quality and affordable maintenance process, it is part of the strategy for safeguarding Europe’s place in the global aeronautic market. New development is expected to identify the suitable coating technology to create the right synergies with the new developed inhibitors to be offered to the aeronautics. Main steps are to consider some chemistry coatings technologies, carrying out screening tests to verify suitability of the considered technologies, select their components and evaluate their synergies with the active molecules and nanoparticles synthesized taking in account stability, compatibility and effectiveness. Coatings property specification should comply AMS-C-27725A standards. In the design of the AMICOAT project’s solutions, the guiding principle was to contribute in raising the energy efficiency and the plane safety, knowing that these two objectives are at the top of the agenda of the European Aeronautical Strategies formulated by ACARE. Safety and environmental protection are growing concerns in the context of an expected rise of the aeronautical transport from 9,4 million commercial flights in 2011, to 25 million in 2050. Even with the advent of the high speed rail, the air transport remains the only viable direct way of connecting Europe’s regions. Even for shorter distances, in some geographical areas, aviation sometimes offers the only viable direct way of connecting Europe’s regions. The effect of aircraft emissions on the environment is complex and not fully understood. In absolute terms, it is clear that aviation will have a significant effect on climate change, greater than that suggested by the industry’s CO2 emissions alone. For the mitigation of the negative environmental effects, the target for 2050 is to employ technologies and procedures for reducing a 75% the CO2 emissions per passenger kilometre, and a 90% of the nitrogen oxide (NOx) emissions. The AMICOAT project incorporates a wide range of solutions that have effects both on the plane performance and safe. The solutions proposed are efficient and fast implementing, shortening the time to market for added value improvements that would contribute rising the competitiveness of the European aeronautical sector.
Dissemination of AMICOAT Project:
• A public webpage for AMICOAT project has been established.
• AMICOAT project updates have been published in EASN newsletter.
• AMICOAT public workshop in May 2014 has been organised to attract interesting stakeholders
Exploitation of AMICOAT:
• Patent consideration by Project coordinator is underway to exploit the solution developed in AMICOAT.
List of Websites:
Website:
http://ipo.leitat.org/amicoat/
Coordinator:
Leitat Technological Center
Contact Person:
Mr.Ahmad Bilal
(abilal@leitat.org)
C/ de la Innovació, 2 , 08225 Terrassa (Barcelona)
Tel. (+34) 93 788 23 00 • Fax (+34) 93 789 19 06
AMICOAT APPROACH SUMMARY:
Microbial growth in aircraft fuel tanks is one of the main sources of contamination of aircraft fuel, plugging of flow of fluids systems and, particularly microbially influenced corrosion of fuel tanks.
Microorganisms from aircraft fuel need water to growth and this water come form the intrinsically fuel moisture content and additionally water from atmospheric moisture condenses in the wall of fuel tanks. Among different microorganism Hormoconis resinae (also known as the “kerosene fungus”) and Pseudomonas aeruginosa are the most prevalent.
The current main methods to control the microorganisms’ growth in aircraft fuel are based on maintenance, periodical removal of water from fuel tanks, and treatment, addition of additives as fuel preservatives.
These methods are not enough to solve the problem and there is an urgent need to develop better methods to control the microbial contamination.
In that sense the AMICOAT project proposes to develop new antimicrobial coatings to apply onto fuel tank walls. The antimicrobial coatings will confer antibioadhesion properties to the fuel tanks walls and they will allow the controlled release of biocides from the fuel tank walls to the water/fuel interphase.
The aim of AMICOAT project is to develop two different coating systems, a polymeric nanocomposite antimicrobial coating and a silica sol-gel antimicrobial coating. The main characteristics of the two new coating systems will be: i) good adhesion with epoxy primer due to the grafting strategy ii) durable antimicrobial properties due to the encapsulation of biocides, that will be incorporated in the two both polymeric and inorganic-organic coating systems.
Project Context and Objectives:
The aim of AMICOAT project can be disclosed in the achievement of three main targets:
• The development of new durable antimicrobial coating which practically eliminate the problems caused by the presence of fungi in the fuel tanks.
• The coating formulation incorporating biocides for long-lasting antimicrobial action; coating formulation including sol-gel technology.
• The development of new protocols to test the efficiency of antimicrobial coatings in biofilm elimination.
More specifically, the objectives of AMICOAT are:
• To define the requirements to prevent and control contamination in fuel tanks.
• To define the target microorganisms and tests to measure the efficiency of microbial growth inhibitors activities in fuel tanks.
• To select the materials of aircraft fuel tank, the surface treatments to be applied on them and the components of coatings for the two systems to be developed in the project (nanocomposites and sol-gel coatings).
• To do a screening of antimicrobials based on active inorganic compounds, organic molecules and nanoparticles, including metal and metal oxides with different inhibition mechanisms.
• To address especial interest in potential nano-safety and nano-toxicity issues.
• To analyze minimum inhibition concentration for target fuel microorganisms in order to define the most appropriate inhibitors with the most favourable inhibition mechanisms.
• To evaluate the controlled release of selected antimicrobials in polymeric, inorganic-organic and/or micro and/or nanoencapsulated systems in order to define the most suitable methods to control biological activity and durability of biocide effects.
• To determine if the formulated coatings do present antimicrobial properties and which is the mode of action of the antimicrobials tested.
• To formulate and apply antimicrobial durable nanocomposite-based coatings for fuel tanks by using suitable polymeric resins and micro and/or nanoencapsulated antimicrobial systems with the better efficiency of fungi growth inhibition activity.
• To formulate and apply antimicrobial durable sol-gel coatings for fuel tanks by using suitable inorganic-organic precursors and biocide components with the most efficient growth inhibition activity in active molecular and/or nanoparticulated form.
• To analyse the growth inhibition for specific microorganisms present in aircraft fuel according to microbiological analysis and studies of formation, adhesion and elimination of biofilms onto the new developed antifouling coating surfaces.
• To analyze surface morphology, surface chemistry and surface tension of antimicrobial coatings developed in the project.
• To assess the performance for the new coatings in order to validate them for their application
The antimicrobial nanostructured coating developed in AMICOAT project represent a promising alternative for eliminating fungi growth in the fuel tank, considering that some of the current solution like regular removal of water essential for fungi growth, or periodic washing out of tanks, have limited effect and complicate the maintenance process.
The solution that AMICOAT project proposes would practically eliminate the problems caused by the presence of fungi in the fuel tanks, which in practical terms would be translated into a faster, more efficient and low cost maintenance process. The project follows the 2050 European’s Commission Flight path Vision for Aviation, which states that providing the best associated services in aeronautics and air transport are part of the general objective of enhancing European competitiveness in world air transport markets. Integrating technology at all levels including for providing a quality and affordable maintenance process, it is part of the strategy for safeguarding Europe’s place in the global aeronautic market.
Project Results:
S/T RESULTS AND FOREGROUND:
The AMICOAT project has achieved the main Scientific & Technological target objectives established at its beginning. As main results, based on the test and validation activities carried out at LEITAT facilities, we can state that the antimicrobial nanostructured coatings developed in AMICOAT project represent a promising alternative for eliminating fungi growth in the fuel tank.
This fact represents an important progress, considering that some of the current solution like regular removal of water essential for fungi growth, or periodic washing out of tanks, have limited effect and complicate the maintenance process.
In accordance to the S&T objectives of the project, the main results are detailed in the following paragraphs:
1. Definition of the target microorganisms and tests to measure the efficiency of microbial growth inhibitors activities in fuel tanks; and selection of materials of aircraft fuel tank, the surface treatments to be applied on them and the components of coatings for the two coating systems to be developed.
The microorganism selection was done following the recommendations of the ASTM E 1259 “Standard method for evaluation of antimicrobials in distillate fuels” which specifies the following as the most representative species of pollution problems in fuel tanks:
• Pseudomonas aeruginosa ATCC 33988
• Candida tropicalis ATCC 20495
• Hormoconis resinae ATCC 48138
Additionally, MicrobMonitor2 ® kit was selected as the most appropriate for determining the inhibition of microbial growth in fuel tanks by the innovative solutions researched in AMICOAT project. Moreover, although there are not standard protocols for evaluating the ability to inhibit the formation of biofilms in tank fuel, since it is a very critical process for preventing microbial contamination, an innovative model of biofilms formation into representative surfaces was developed for evaluating the effectiveness of AMICOAT solutions for preventing microbial contamination in fuel tanks.
A study of the aircraft fuel tank materials has been done. Taking into account the results of this study 2024 aluminium alloy was selected as a standard material for coating deposition.
Two standard coating systems were defined. Both of them were based on two coatings: 1st) Epoxy primer; 2nd) Polyurethane or Epoxy top coat. A selection of coating components including commercially available epoxy and polyurethane binders with excellent chemical resistance, pigments and solvents was also done. Moreover, brush and spray were established as techniques for coating application onto aluminium substrate.
2. Analysis of microbial growth inhibition through the screening of antimicrobials with different inhibition mechanisms, the minimum inhibitory concentration for target fuel microorganisms and the evaluation of the controlled release of selected antimicrobials in the coating systems to be developed:
Mechanisms of action of biocides:
According to this study and different guidelines, a pre-selection of biocides to test was done. The pre-selection is shown in Table 1: (in attachment)
(See attached file for complete table)
Minimum inhibitory concentration:
The biocides pre-selected: zinc pyrithione (ZP), silver nitrate, hexadecyltrimehylammonium bromide (HDTAB), 3-iodo-2-proponyl-N-butylcarbamate, 2-methyl-4-isothiazolin-3-one (MIT), carvacol and acriflavine were tested according the standard M07- A8 “Dilution methods to determine the antimicrobial susceptibility of bacteria that grow aerobically” against the previously selected microorganisms (Pseudomonas aeruginosa, Candida tropicalis and Hormoconis resinae). The results of the minimum inhibitory concentration for each biocide/microorganism are shown in graph(See attached file for Figure 1. Minimum inhibitory concentration of biocides for different microorganims.)
Based on the results obtained, and comparing the inhibitory activity against the 3 microorganisms showed by the different biocides tested, 3-Iodo-2propynyl-N-Butylcarbamate, HDTAB and carvacrol were discarded. Therefore, due to the high antimicrobial activity against P. aeruginosa, C. tropicalis and H. resinae and also their molecular compatibility with the technologies proposed in the project development, the following biocides were selected for the following studies:
- Zinc pyrithione.
- Silver nitrate.
- 2-methyl-4-isothiazolium-3-one (MIT).
Biocides antimicrobial activity in fuel:
The most active biocides previously selected were evaluated against the three microorganisms in fuel samples according to the adapted standard 1259 “Control standards test method for evaluation of antimicrobials in liquids fuels below 390ºC”. The results obtained showed that the three selected biocides for development of new nanostructured durable antimicrobial coatings for fuel tanks were effective for reducing microbiological contamination. The biocides, at the concentrations analyzed, were able to reduce in case of Pseudomonas aeruginosa at least 7 logarithms, for yeast 6 logarithms and for fungi 4 logarithms the microbial growth compared with the control where non biocide is added.
On the other hand, microbial contamination was, as expected, detected in the aqueous phase while the fuel phase was free of microbial contamination after 5 days of incubation.
Biofilm development:
Biofilm models of P. aeruginosa and H. resinae onto epoxy coated aluminium plates were also developed for evaluating the ability of the selected biocides to inhibit the formation of that biofilms. According to the general scheme of coatings for aircraft fuel tanks, epoxy coatings were selected as a model for the biofilm development. Different protocols were tested until reached the optimal conditions for biofilm obtaining.
The results showed that silver nitrate and zinc pyrithione were able to avoid Pseudomonas aeruginosa and Hormoconis resinae biofilm formation. On the contrary, MIT although was effective against H. resinae, it showed reduced ability to inhibit the Pseudomonas’s biofilm formation.(see attachment for Figure 2. Biofilms on epoxy coated aluminium surfaces with biocides in free form)
Initial antimicrobial coatings development:
For the evaluation of the controlled release of antimicrobials, antimicrobial coatings based on polymeric and sol-gel systems were developed. The antimicrobial coatings were produced by dispersing the biocide agent into the coating. All formulations were developed on a laboratory scale, using a high speed disperser.
The procedure to prepare the polymeric formulations can be summarized in a three-step process:
a) Resin dilution: Polyfunctional resin is diluted in a suitable solvent as to achieve a suitable viscosity for spray application (aprox. 120 cPs).
b) Biocide wet-out /dispersion: A dispersant/wetting agent is added to the mixture (in an amount of about 10%wt. of the biocide content) and dispersed homogeneously. Then, the biocide is slowly added to the formulation and stirred until properly dispersed.
c) Hardener addition: Finally, a crosslinking agent is added to the former preparation, according to the resin-hardener mixing ratios recommended by the supplier.
Sol-gel coatings were based on tetraethoxysilane (TEOS) and (3-Glycidyloxypropyl) trimethoxysilane (GPTMS) as silica precursors. GPTMS was added to provide flexibility and avoid the formation of cracks in the coating. The epoxide-ring of GPTMS can be opened and as a consequence also react with the hydroxyl groups of the epoxy primer. The effect of different GPTMS/TEOS molar ratio was studied in order to achieve a free-cracking coating surface.
We started studying the hydrolysis and condensation reactions by ATR-IR spectroscopy of a TEOS and GPTMS sol, separately. Sols have been prepared at 13 wt% and 50 wt% (solid content) in water using HNO3 as catalyst. The solubility of biocides (AgNO3, 2- methyl-4-isothiazolin-3-one and zinc pyrithione) in the water-based sol-gel was also tested. Silver nitrate and 2- methyl-4-isothiazolin-3-one were soluble in water-based sol while zinc pyrithione was only soluble in DMSO solvent.
Microcapsules as controlled release systems:
In parallel, a selection of the microencapsulation technologies including the shell material was done according to the current scientific bibliography.(in attachment is the Table 2. Materials and microencapsulation technology selection for microcapsules development)
The microencapsulation procedure for each shell material was developed using zinc pyrithione as a core material. Zinc pyrithione microcapsules of PMMA were obtained following an experimental procedure based on the solvent evaporation microencapsulation technique. The best results showed a microencapsulation efficiency of 86 % and a microencapsulation yield of 84 %. The ZnP/PMMA microcapsules were dispersed in fuel for 3 days and no damage was observed by optical microscopy. (in attached file is the Figure 3. ZnP/PMMA microcapsules)
However, PMMA microcapsules did not show enough solvent resistance once they were introduce in coating formulation. On the other hand, melamine-formaldehyde microcapsules of a model core (oil) showed good solvent resistance and microencapsulation of water-soluble biocides (MIT and AgNO3) in MF have been tried but not positive results were obtained. As alternative, halloysite clays were proposed to immobilize water-soluble biocides. Positive results were obtained when AgNO3 was used.
Biocide release studies:
Prior to the design of the biocide release study, solubility analysis of the selected biocides in fuel and fuel/water solutions, chemical analysis of fuel JP8 (the selected fuel), and establishment of the analytical protocol to quantify the biocide release were performed.
Biocide release studies from different developed coatings were investigated in order to determine the retention of the biocide in the coating over extended-periods of time. Determination of biocide leakage rate for each kind of coating (Epoxy-PUR-SolGel) gave us an idea of the durability of the developed coating. If there is an uncontrolled biocide release, a slow release technology is needed to ensure the long-lasting effect. Selection of a proper release system allows keeping the concentration of the active agent above the minimum inhibition concentration (MIC) while decreasing initial release.In order to design the experimental setup for the release studies, previous measurements of water content in JP8 fuel and solubility studies of the selected biocides in fuel and water were performed. JP8 water content was determined to be around 0,004 %. The evaluation of the solubility of the target compounds in JP8 was analyzed by ICP-MS (AgNO3, ZnP) and UHPLC-MS (MIT). While ZnP was not soluble in fuel or water; MIT and AgNO3 were soluble in water but not in fuel. Considering the previous results, the release studies were performed for MIT and AgNO3 in an aqueous media. After 5 days of immersion, ICP-MS analytical results indicated that under these experimental conditions no AgNO3 release was detected for polymeric-based coatings. Regarding the polymeric based-coatings incorporating MIT, only a small percentage of biocide was released from the coatings (0.9 % for Epoxy coatings and 4.6 % for PUR based coatings) when the temperature raised up to 50ºC. A large amount of MIT subsisted into the antimicrobial coating matrix during the time of the release studies.(in attached file Figure 4. Release of MIT from epoxy (left), PUR (middle) and sol-gel (right) coatings).
MIT total release from sol-gel coatings after 5 days in water at room temperature was estimated around 1.8-2.3 %. The diffusion of MIT from sol-gel coating was easier than the diffusion from epoxy and PUR-based coating. When sol-gel was the coating matrix, silver release could be detected by ICP-MS whereas this was not possible with polyurethane and epoxy-based coatings. In this case, silver nitrate release from sol-gel coating was very low (up to 3 ppm).
Due to the slow biocide release from inorganic and organic coatings, no solutions to delay the biocide release were needed. At that point, it was decided not to incorporate microcapsules or nanoparticles in coating development, as their inclusion would only have delayed even more the biocide release.
Microbiological analysis results:
The different antimicrobial coatings were analysed microbiologically by applying the diffusion test method. With this method the activity of the different coatings was evaluated against one of the most respresentative fungus in aircraft fuel tanks: Hormoconis resinae ATCC 20495.
Aluminium coated plates incorporating a 3%wt of biocidal agent were observed at two different times, after 3 days and 7 days of incubation with the fungus. After three incubation days, all the coatings containing zinc pyrithione showed antimicrobial activity and the biocide diffused from the coating to the agar plate generating an inhibition zone. The other coatings containing Silver nitrate and MIT did not show any inhibition zone, which means that either the coating has not antimicrobial activity or the antimicrobial did not diffuse to the agar plate.(in attachment Figure 5. Results of microbiological analyses for the different antimicrobial coating systems with biocides)
According to the antimicrobial character, zinc pyrithione seemed to be the best choice. Above all coating materials, sol-gel and PUR exhibited a faster release of biocide resulting in better antimicrobial results. In conclusion, sol-gel and polyurethane formulations containing zinc pyrithione were chosen for further optimization.
3. Formulation and functional optimization of antimicrobial coatings:
The antimicrobial coating systems developed were based on two subsequent layers: A beige coloured epoxy primer and a polymeric or sol-gel topcoat incorporating zinc pyrithione as active antimicrobial substance.
Different polyurethane based formulations were prepared using different additive systems for a better dispersion and stability of zinc pyrithione. In parallel, sol-gel formulations were obtained in different experimental conditions: aqueous solution, organic solvent and anhydrous conditions. Anhydrous sol-gel process with MEK as solvent for coating showed the best performance results (good wettability and appearance).
Different sol-gel precursors (TEOS/GPTMS), combined in different weight ratio, different amounts of biocide in the final sol-gel coating and the dispersion of the biocide in the silica sol were studied.
From all of them, best formulations (based on characterization results) were finally applied on epoxy coated aluminium plates by spray technique for their characterization incorporating different amounts of ZnP: a 3%wt and a 5%wt in dry coatings.(in attached file is the Figure 6. Sol gel systems (left) and conventional coating systems (right) on aluminium substrates incorporating a different amounts of active substance).
Real samples of the aforementioned coatings (corresponding to Deliverable 3.1) were shown during the dissemination event that took place in Leitat Technological Center on 2014-05-22.
4. Analysis of the growth inhibition of the new developed antimicrobial surfaces, superficial and performance characterization of antimicrobial coatings:
The characterization of the optimum antimicrobial sol-gel and polyurethane coatings was performed, including: the analysis of antimicrobial growth inhibition and biofilm formation; the surface properties determination and the performance assessment of the new antimicrobial coatings developed.
Evaluation of microbial growth inhibition:
The antimicrobial and antibiofilm abilitiy against Hormoconis resinae ATCC 20495 of the new antimicrobial coatings was evaluated using two different methods:
- Diffusion test method
The diffusion method is a rapid method that consists in evaluating the antimicrobial behaviour by direct contact between agar plates with the target microorganism and the coatings tested. With this method it is possible to determine if the coatings have the ability to avoid spores’ germination and consequently, to inhibit-the vegetative growth.(in attached file is the Figure 7. Results of microbiologically analyses for polyurethane based coatings (left) and sol-gel coatings (right).)
- Biofilm prevention
This test allows evaluating the antibiofilm properties of the developed coatings by assessing the reduction in the spore attachment to the surfaces when compared to the negative control.(in attachment Table 3. Biofilm development in the sol-gel coatings)(in attached file is the Table 4. Biofilm development in polyurethanic formulations)
All these results together demonstrate that the developed formulations have high ability to avoid the spores germination and, consequently, inhibit the vegetative growth of H. resinae. Additionaly, these formulations also present a slight ability to prevent the attachment of spores into the surfaces.
Surface properties characterization:
Epoxy-Al plates coated with sol-gel and polyurethane based formulations were characterized by SEM.(attached file contains Figure 8. SEM Images of 3% ZnP, 5% ZnP and biocide-free sol-gel coated Al plates and Figure 9. SEM Images of 3% ZnP, 5% ZnP and biocide-free polyurethane coated Al plates).
The ink test was used to measure the surface tension of the developed coatings. The surface tension for polyurethane based coatings was around 26-28mN/m, while the sol-gel coatings presented a surface tension around 22-24mN/m.
Coating thickness of optimum antimicrobial sol-gel and polyurethane coatings have been evaluated by 3D Optical Confocal Profilometry. Results indicated that thickness were in the range of 219-319 µm for sol-gel coatings and in the range of 280-432 µm for polyurethane coatings. These thicknesses depend on the properties of the paint itself (i.e. rheology) and on the application technology (i.e. HVLP spray) and procedure. AFM analysis for the epoxy aluminium plates coated with optimum sol-gel and polyurethane formulations containing 0%, 3% and 5% of ZnP (in the final coating) were also carried out in order to observe surface topography and nanoroughness for the optimum antimicrobial samples.
Coating performance characterization:
The performance of the developed coatings was assessed according to the standard tests used to evaluate the fuel tank coatings. Tests carried out to assess the coatings performance were:
- Crosshatch adhesion test
- Pencil hardness test
- Resistance immersion test
Next figure summarizes the results after performing each of these tests.(in attached file Figure 10. Coating performance evaluation for sol-gel and PUR systems)
Excellent adhesion properties were achieved for all of the coatings. As expected, hardness in polyurethane-based coatings was lower than in sol-gel coatings and the biocide incorporation in the formulations seemed to increase the scratch resistance when compared with those formulations with no biocide.
Regarding the results obtained in the immersion tests, no visible changes were observed in any of the immersed coated plates after 45 days, indicating an excellent fuel resistance.
CONCLUSIONS:
Main conclusions after AMICOAT project are listed below:
• Hybrid silica sol-gel and PUR antimicrobial top coatings have been developed, incorporating zinc pyrithione as active agent.
• Different protocolos and methodologies have been adapted for the evaluation of the antimicrobial activity in different conditions.
• The antimicrobial functionality has been optimized:
• The incorporation of ZnP in a 3% by weight in dry PUR and sol-gel coatings is enough to inhibit Hormoconis resinae growth, showing an slight antibiofilm capacity.
• Performance characterization tests have been carried out.
(Note:Complete File with Figures and Tables can be found in attachment S T Results Foreground for this section)
Potential Impact:
Strategic Impact of AMICOAT:
The antimicrobial nanostructured coating that the AMICOAT project developed, is promising alternative for eliminating funghi growth in the fuel tank, considering that some of the current solution like regular removal of water essential for fungi growth, or periodic washing out of tanks, have limited effect and complicate the maintenance process. The solution that AMICOAT project developed practically eliminate the problems caused by the presence of fungi in the fuel tanks, which in practical terms would be translated into a faster, more efficient and low cost maintenance process. The project follows the 2050 European’s Commission Flight path Vision for Aviation, which states that providing the best associated services in aeronautics and air transport are part of the general objective of enhancing European competitiveness in world air transport markets. Integrating technology - at all levels including for providing a quality and affordable maintenance process, it is part of the strategy for safeguarding Europe’s place in the global aeronautic market. New development is expected to identify the suitable coating technology to create the right synergies with the new developed inhibitors to be offered to the aeronautics. Main steps are to consider some chemistry coatings technologies, carrying out screening tests to verify suitability of the considered technologies, select their components and evaluate their synergies with the active molecules and nanoparticles synthesized taking in account stability, compatibility and effectiveness. Coatings property specification should comply AMS-C-27725A standards. In the design of the AMICOAT project’s solutions, the guiding principle was to contribute in raising the energy efficiency and the plane safety, knowing that these two objectives are at the top of the agenda of the European Aeronautical Strategies formulated by ACARE. Safety and environmental protection are growing concerns in the context of an expected rise of the aeronautical transport from 9,4 million commercial flights in 2011, to 25 million in 2050. Even with the advent of the high speed rail, the air transport remains the only viable direct way of connecting Europe’s regions. Even for shorter distances, in some geographical areas, aviation sometimes offers the only viable direct way of connecting Europe’s regions. The effect of aircraft emissions on the environment is complex and not fully understood. In absolute terms, it is clear that aviation will have a significant effect on climate change, greater than that suggested by the industry’s CO2 emissions alone. For the mitigation of the negative environmental effects, the target for 2050 is to employ technologies and procedures for reducing a 75% the CO2 emissions per passenger kilometre, and a 90% of the nitrogen oxide (NOx) emissions. The AMICOAT project incorporates a wide range of solutions that have effects both on the plane performance and safe. The solutions proposed are efficient and fast implementing, shortening the time to market for added value improvements that would contribute rising the competitiveness of the European aeronautical sector.
Dissemination of AMICOAT Project:
• A public webpage for AMICOAT project has been established.
• AMICOAT project updates have been published in EASN newsletter.
• AMICOAT public workshop in May 2014 has been organised to attract interesting stakeholders
Exploitation of AMICOAT:
• Patent consideration by Project coordinator is underway to exploit the solution developed in AMICOAT.
List of Websites:
Website:
http://ipo.leitat.org/amicoat/
Coordinator:
Leitat Technological Center
Contact Person:
Mr.Ahmad Bilal
(abilal@leitat.org)
C/ de la Innovació, 2 , 08225 Terrassa (Barcelona)
Tel. (+34) 93 788 23 00 • Fax (+34) 93 789 19 06
