THE EFFECT OF THE SURFACE CHARGE OF ELECTRETS ON THE RESISTANCE TO FUNGI OF FLUOROCARBON POLYMER MATERIALS
This paper deals with the study of the resistance to fungi of the nanostructured fluorocarbon films produced on the surface of polymers, depending on the value of the surface charge. Apparently, the resistance to fungi of the film is affected not only by the surface charge but also by other surface characteristics, such as relief and chemical composition. The fluorocarbon films were formed under transient conditions using a two-component fluorocarbon gas mixture (CF4 + C6H12).
Polymeric materials are used in many areas of human activity, such as aerospace, medicine and electronics. Their widespread use is due to their unique physical and chemical properties. One of the main properties that limit the use of polymers is their low resistance to biodegradation, i.e. low resistance to biological degradation . This disadvantage leads to reduction of the equipment lifetime, failures of various systems as well as significant economic losses which account for 2–5% of the GDP of the industrially developed countries .
Filamentous fungi cause the most significant harm of all microorganisms. Destruction occurs when the polymer is directly consumed as food and when the polymer surface interacts with the metabolic products of the fungi .
Biodegradation of polymers is caused by production of enzymes by microorganisms that increases degradation of macromolecules. The main signs of polymer biodegradation are:
tarnishing of the surface;
appearance of mycelium deposits that can be seen visually;
changes in dielectric properties;
loss of mechanical strength;
change in shape;
In order to prevent biodegradation on polymers, it was proposed in works [4–7] to create barrier layers based on nanostructured fluorocarbon films, which have antimicrobial anti-adhesion characteristics, a "peaked" surface topography, and a surface charge that forms the electret states. Electrets mean those areas on a dielectric that remain polarised for long periods of time and, therefore, create electric field in the environment. In order to determine the effect of electretic states on interaction between a surface and microorganisms, it is advisable to investigate surface charges.
The anti-adhesion properties of nanostructured fluorocarbon films are due to the following factors: exposure to fluorine and formation of a specific polymer surface topography where the distance between the roughness peaks is smaller than the microbial cell diameter. To create such relief, the coating has to be formed using a two-component gas mixture containing a film application component (C6H12) and an etching component (CF4). This process must take place under transient conditions (transition from film deposition to etching). Moreover, this barrier layer can serve to produce hydrophobic and superhydrophobic surfaces on polymeric materials .
The purpose of this work is to study the electretic states effect (surface charge value) on the resistance to fungi of polymers modified with fluorocarbon anti-adhesion coatings against fungi and microorganisms.
Polyethylene terephthalate (PET) and polytetrafluoroethylene (PTFE) were chosen as test samples because these polymers are widely used in aviation, electronics, medicine, aerospace, biotechnology, etc.
The barrier layer formation was carried out in two stages in a UVN71-P3 vacuum apparatus equipped with two II-4-0.15 ion sources. At the first stage, the PTFE and PET samples were placed in the chamber and the polymer surface was treated with tetrafluoromethane ions (CF4) for 30 minutes. The first step was necessary to clean the surface, improve adhesion of the fluorocarbon film and, also, produce a preliminary nano-relief. At the second step, a fluorocarbon film was applied from another ion source using a two-component gas mixture of CF4 + С6Н12 at different component ratios.
After formation of the coating, the surface charge was measured using IPEP-1 (electrostatic field meter) according to GOST 25209-82.
Measurements were taken at five points immediately after treatment for 28 days (after 28 days the resistance to fungi tests were completed). Then, the arithmetic mean values of the surface charge were calculated for each material and correlated to the fungus resistance data. The resistance to fungi tests were carried out in accordance with GOST 9.048-89 in compliance with a five-point grading system where zero point corresponds to the condition when no spores and conidia germination is found under the microscope, and five – to the naked eye clearly visible development of fungi covering more than 25% of the test surface. On this basis plots of the surface charge and resistance to fungi as a function of the CF4 content in the plasma gas mixture CF4 + C6H12 were plotted, as well as resistance to fungi plots as a function of the surface charge.
RESULTS AND DISCUSSION
It can be observed in the graph (Fig.2) that the minimum by modulus surface charge of the fluorocarbon film produced on the PET surface corresponds to the best resistance to fungi. As can be seen on the graph, the charge on the original PET surface is higher than that of the one treated for 30 minutes with CF4 ions. The situation is the same with the resistance to fungi (the original resistance to fungi equals two and for treated one equals unity). The resistance to fungi at this site is equal to unity. The surface charge then begins to increase and becomes positive. The highest value of the charge is observed at 40%, at which point the resistance to fungi becomes zero. The resistance to fungi also equals zero at 60%. Thereafter the charge on the surface decreases steadily. From 70% to 100% the resistance to fungi becomes equal to one again.
Figure 3 shows that the minimum modulus surface charge of the fluorocarbon film produced on the PET surface, as well as on the PTFE surface, corresponds to the higher resistance to fungi. A similar situation is also observed with the surface charge and resistance to fungi of the original and treated PET (the charge and resistance to fungi value of the original PET is higher than of the treated one). Between 0% and 10% the surface charge increases rapidly, and then the increase slows down to 40%, where, as with PET, the highest value of charge is observed. From 40 to 70% the surface charge decreases steadily, with a particularly steep fall starting at 60%. At 70% the charge becomes less than the initial charge but more than the treated charge. The resistance to fungi decreases from 0% to 25%, then, between 25% and at 70%, it becomes zero. And from 70 to 100% an increase can be seen in both resistance to fungi and surface charge (at 100% the charge becomes greater than at 60%, but less than at 25%, and the resistance to fungi is equal to unity).
Figure 4 shows that the minimum surface charge value of PET was observed after treatment, when the resistance to fungi equals unity. As the durface charge increases, the following values of CF4 content in the gas mixture were observed respectively: 100, 0 and 70%. The resistance to fungi at these values is also equal to unity. Then, at 60% the resistance to fungi becomes zero. Afterwards the resistance to fungi increases: at 25 and 10% it becomes unity. The initial (untreated) PET has a resistance to fungi of two units. The surface acquires the maximum surface charge at 40%, and the resistance to fungi is zero at this value.
In Figure 5, similar to the previous graph (Fig.4), the minimum charge is obtained for PTFE after treatment. In this case, the resistance to fungi is equal to unity. In this case, the PTFE on which a film with 70% CF4 content in the gas mixture is applied has zero resistance to fungi. Then the original (untreated) sample indictes a sharp increase in the resistance to fungi up to three points. At the next three points, the resistance to fungi begins to decrease by one point relative to the previous point. First it reaches 0%, then 10% and finally 60%, with a strong increase between 0 and 10%. Next, when the resistance to fungi equals one, follows the film produced with 100% CF4. As in the case of PET, the surface acquires the maximum surface charge is at 40%, and the resistance to fungi equals zero.
After treatment with CF4 ions of the surfaces of both polymers, the resistance to fungi increases to one point.
The minimum modulus surface charge of the fluorocarbon film produced on the surfaces of PET and PTFE is observed at 40% CF4 in the CF4 + C6H12 gas mixture and corresponds to the best resistance to fungi (0 points).
The resistance to fungi of fluorocarbon films is probably influenced not only by the surface charge but also by other surface characteristics such as topography and chemical composition.
The study was completed with the financial support of the RFBR, Project No. 20-32-90092.
PEER REVIEW INFO
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Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.