BACKGROUND OF THE INVENTION

The invention is in the field of food packaging. In particular the present invention is directed to a method for providing an antibiofouling and/or antimicrobial coating layer on a food product contact surface, as well as a food product contact surface obtained using that method.

Many applications exist wherein surfaces come into contact with biological compounds or organisms. This includes many laboratory equipment, food and beverage containers, marine vessels, underwater construction, microfluidics chips, fluidized bed reactors, etc.

In many of these cases biological organisms or compounds may adhere to the surface, thereby contaminating the surface. Cleaning these surfaces may be very difficult and time-consuming.

Current approaches involve protecting surfaces against bio contamination by deposition of a coating. These coatings can be applied using a wet coating deposition technique. Examples can be found in:

- Joh et al, P. Natl. Acad. Sci. USA. 2017, 114, E7054-E7062, “Inkjet-printed point-of-care immunoassay on a nanoscale polymer brush enables subpicomolar detection of analytes in blood.”

- Hucknall et al., Adv. Mater. 2009, 21, 1968-1971, “Simple Fabrication of Antibody Microarrays on Nonfouling Polymer Brushes with Femtomolar Sensitivity for Protein Analytes in Serum and Blood”

- Den g et al., J. Am. Chem. Soc. 2014, 136, 12852-12855, “Poly(ohgoethylene glycol methacrylate) Dip -Coating: Turning Cellulose Paper into a Protein-Repellent Platform for Biosensors”

- Sato et al, Macromolecules, 2018, 51, 10065-10073, “Sol-Gel Preparation of Initiator Layers for Surface-Initiated ATRP: Large-Scale Formation of Polymer Brushes Is Not a Dream” - Liu et al., Biomacromolecules, 2012, 13, 1086-1092, “Ultralow Fouling Polyacrylamide on Gold Surfaces via Surface-Initiated Atom Transfer Radical Polymerization”

- Van Andel et al., Langmuir, 2019, 35, 1181-1191, “Systematic Comparison of Zwitterionic and Non-Zwitterionic Antifouling Polymer Brushes on a Bead-Based Platform”

- Koc et al., Langmuir, 2019, 35, 1552-1562, “Low-Fouling Thin Hydrogel Coatings Made of Photo-Cross-Linked Polyzwitterions”

- Sun et al., Anal. Chem. 2017, 89, 10999-11004, ’’Paper Sensor Coated with a Poly(carboxybetaine)-Multiple DOPA Conjugate via Dip- Coating for Biosensing in Complex Media”

- Cheng et al., Colloids and Surfaces B: Biointerfaces, 2019, 173, 77-84, “Surface functionalization of polytetrafluoroethylene substrate with hybrid processes comprising plasma treatment and chemical reactions”

In the above examples, polymer brushes are produced based on surface-initiated atom-transfer radical polymerization (ATRP) in the following way:

- surface preparation: wet chemical deposition of a component that can initiate a radical chain growth reaction (i.e., an initiator);

- polymer brush formation: wet chemical deposition of a monomer that grows from the initiator sites through radical chain growth.

The thickness of the deposited layer hereby is typically in the range between 60 and 100 nm. The coating techniques described above make use of wet coating’, wherein a surface is subjected to a liquid solution containing the coating material. Such wet coating techniques suffer from a number of drawbacks such as:

- a long drying time;

- large amount of waste resulting in a large stress on the environment;

- homogeneity and conformality of the coating is not always as desired, - the thickness of the coating is not always under control, and may locally be much higher than desired.

A coating technique which has gained momentum in the last few decades is plasma coating. In this process, a precursor which is to form the coating on the surface of a substrate is brought at least partially in a plasma state, and the surface of the substrate is subjected to the plasmized precursor. As a result, strong bonds can be formed between the precursor and the substrate and cross-links can be formed between the coating molecules, thereby resulting in a coating which may be thin, yet very durable, homogeneous and conformal. If the precursor is a polymerizable monomer, polymerization may occur directly onto the surface of the substrate.

Plasma coating techniques can be divided into two categories: vacuum techniques having an operating pressure which is significantly lower than atmospheric pressure, and atmospheric techniques which operate at or near atmospheric pressure, for instance between 400 and 1600 hPa, but preferably very close to atmospheric pressure, e.g. between 950 and 1050 hPa. The present invention relates to an atmospheric plasma technique, which presents a number of advantages over vacuum plasma techniques, such as that no time-consuming depressurizing step is required and that both batch processing, and inline processing, a continuous process in which the multiple objects which are to be treated, are treated sequentially, are easily achievable.

A technique for providing a surface with a protective layer is vacuum plasma coating, examples of which can be found in Materials 2016, 9, 515, and references therein.

Plasma coating can also be performed using an atmospheric pressure plasma, as for instance in Plasma Processes and Polymers 2012, 9, 1176-1183, and in Plasma Processes and Polymers 2015, 12, 2015 1208- 1219. An object of the present invention is to provide a coating layer to a food product contact surfaces in order to alter the surface’s affinity for and/or effect on biological organisms, such as microbes, and biological materials. Another object of the invention is to apply such a coating layer with a high throughput. Another object of the present invention is to improve on the techniques know in the art.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a method for providing an antibiofouling and/or antimicrobial coating layer on a food product contact surface, comprising the steps of: a) ionizing a plasma gas, thereby creating a plasma; b) introducing one or more precursors into said plasma; c) exposing the food product contact surface to said plasma comprising said one or more precursors, thereby forming a coating onto said food product contact surface, wherein the one or more precursors are selected from the group consisting of amines, glycols, fluorocarbons, siloxanes, organosiloxanes, sulfonates, phosphonates, ammonium compounds, such as quaternary ammonium compounds, metal nanoparticles, enzymes, surfactants, peptides, lipopeptides, and combinations thereof.

The present invention provides means to apply an antibiofouling and/or antimicrobial coating layer on a food product contact surface. Using the method, an antibiofouling and/or antimicrobial coating layer can be applied onto food product contact surfaces of a wide range of different shapes and materials. Furthermore, the exact properties of the antibiofouling and/or antimicrobial coating layer can be by optimized, for instance by the selection of the one or more precursors and/or plasma conditions. In accordance with another aspect of the invention, there is also provided a food product contact surface with an antibiofouling and/or antimicrobial coating layer which coating layer is applied using the method as disclosed herein.

The present applicants have found that the abovementioned precursors significantly alter the surface properties with respect to microbes and/or biological compounds. Furthermore, the atmospheric low- temperature plasma coating technique, wherein one or more precursors are introduced into the plasma, and wherein the surface onto which the coating layer is applied is exposed to plasma comprising these one or more precursors, can be applied using a high throughput compared to techniques known in the art, while still allowing for a very smooth, thin coating. The plasma technique also allows treatment of surfaces having a wider range of compositions, shapes, and sizes.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1: at t=t0 the plasma is on. A precursor R-X is added to the plasma gas and the plasma is contacted with the substrate. Hereby the precursor R-X is radicalized, and the substrate is activated.

Figure 2: at t=tl the plasma is on. Radical recombination reactions are taking place on the surface, resulting in a covalent bond between substrate and precursor.

Figure 3: at t=t2, the plasma is on. Film growth and thickness depend on treatment time. Also cross-linking is taking place.

Figure 4: at t=t3 the plasma is off. After the plasma treatment, a functional plasma deposited film remains which is grafted onto the substrate. DETAILED DESCRIPTION OF THE INVENTION

According to the invention there is provided a method for providing an antibiofouling and/or antimicrobial coating layer on a food product contact surface, comprising the steps of: a) ionizing a plasma gas, thereby creating a plasma; b) introducing one or more precursors into said plasma; c) exposing the food product contact surface to said plasma comprising said one or more precursors, thereby forming a coating onto said food product contact surface, wherein the one ore more precursors are selected from the group consisting of amines, glycols, fluorocarbons, siloxanes, organosiloxanes, sulfonates, phosphonates, ammonium compounds, such as quaternary ammonium compounds, metal nanoparticles, enzymes, surfactants, peptides, lipopeptides, and combinations thereof.

As used herein, the term biofouhng refers to the adhesion or accumulation of microbes (such as bacteria, fungi) and/or other biologic material, (such as proteins) on a surface. Analogously, the term antibiofouling refers to preventing or decreasing adhesion or accumulation of microbes and/or other biologic material to a surface.

As used herein, the term antimicrobial refers to decreasing proliferation of microbial material that may be present on a surface.

As used herein, the term aerosol refers to a suspension of fine sohd particles or hquid droplets, in air or in another gas.

Using the method as described herein, an antibiofouhng and/or antimicrobial coating layer can be obtained that is non-specific, meaning that the coating layer is effective against many different kinds of biofouhng and/or microbial contamination.

The antibiofouhng and/or antimicrobial coating layer applied using the method of the invention may provide the food product contact surface with antibiofouling and/or antimicrobial properties in one or both of the following ways:

- decreasing adhesion of proteins and/or microbes, e.g. bacteria and/or fungi, to the food product contact surface, wherein the one or more precursors are selected from the group consisting of amines, glycols, fluorocarbons, siloxanes, organosiloxanes, ammonium compounds, such as quaternary ammonium compounds, and combinations thereof;

- decreasing proliferation of microbial material adherent to the food product contact surface, wherein the one or more precursors are selected from the group consisting of amines, siloxanes, organosiloxanes, ammonium compounds, such as quaternary ammonium compounds, metal nanoparticles, enzymes, surfactants, peptides, lipopeptides, and combinations thereof.

These specific effects for the different precursors are summarized the table below:

In the table above, abbreviations have been used for at least the following chemical compounds: lH,lH,2H,2H-perfLuorodecyl acrylate (PFDA), lH,lH,2H,2H-perfLuorodecyltriethoxysilane (PFDTES), poly(ethylene glycol) methyl ether acrylate (PEGMEA), di(ethylene glycol) ethyl ether acrylate (DEGEA), 2-(tert-butylamino)ethyl methacrylate (BUTAMA), 2-(dimethylamino)ethyl methacrylate (DMAEMA). In addition to providing the coating layer with antibiofouling and/or antimicrobial functionality, the one or more precursors may also provide advantageous structural properties to the coating layer, such as strength, degree of cross-linking, etc. To this end, the one or more precursors preferably comprise one or more of the following: acrylate, methacrylate, vinyl, organosiloxane, and/or saturated compounds. The one or more precursors may comprise antibiofouhng and/or antimicrobial functionality, or structural functionality, or both. Examples of precursors comprising structural functionality, as well as antibiofouling and/or antimicrobial functionality are:

- an organosiloxane with a functional group,

- a polymerizable compound with a functional group wherein preferably said polymerizable compound is an acrylate, a methacrylate or a vinyl, and

- a saturated compound with a functional group, wherein the functional group may be selected from selected from the group consisting of amines, glycol-based groups, fluorinated groups, sulfonates, phosphonates, and ammonium groups. The amine group may be a primary amine group, a secondary amine group, or a tertiary amine group. The ammonium group can for instance be a quaternary ammonium group.

In a preferred embodiment, the one or more precursors comprise lH,lH,2H,2H-perfluorodecyl acrylate (PFDA) and/or 2-(tert- butylamino)ethyl methacrylate (BUTAMA).

As previously discussed, the precursors preferably comprise amine, glycol, fluorocarbon, siloxane, sulfonate, ammonium, phosphonate, quaternary ammonium, metal nanoparticles, enzyme, surfactant, peptide, lipopeptide, chitosan, organic acid, ammonium chloride, a mixture of alcohol and ammonium chloride, a natural antibacterial substance or any combination thereof. Note that the resulting bio-active layer is non-specific. More preferably, the precursor comprises any or any combination of the substances in the table below (chemical structure formulae are shown for easy reference):

The plasma deposition process of the present invention is based on the simultaneous generation of surface radicals (i.e., activation of the difficult-to-treat substrate) and radicalized species in the plasma gas phase, leading to radical recombination reactions of the species to the substrate (i.e., grafting based on covalent bonding). The chemical nature of the precursor can range from classic monomers to saturated molecules, from organic to inorganic molecules, from low molecular weight ( e.g . monomers, oligomers) to high molecular weight (e.g. polymers being dissolved or emulsified).

The plasma chemically activates the precursors and/or the surface. This activation of the precursors and/or the surface may occur by double atomic bonds opening, radical removal and/or ion formation. This allows and/or improves the reactions required to form the coating layer. These reactions may involve:

- reactions between precursors, such as polymerization reactions and cross-linking reactions, and/or - reactions between precursors and the surface, such as covalent bonding reactions. Preferably, the coating layer is covalently grafted to the surface.

The resulting plasma deposited coating layer may have one or more of the following features:

^• Covalently bonded to the surface of the substrate;

^• Antibiofouling and/or antimicrobial, for instance:

(i) adhesion of proteins and microbes, such as bacteria and/or fungi to the surface is decreased, and (ii) proliferation of microbial material adherent to the surface is decreased.

^• Heterogeneous:

Compared to polymeric analogues having a distinct repeat unit, the plasma deposited film may be heterogeneous in nature. This means that besides a polymeric backbone that may provide structure to the coating layer, e.g. a carbon chain, also other elements can be incorporated (originating from the introduced one or more precursors).

^• Cross-linked:

During the growth phase of the plasma deposited coating layer, also radical sites are generated on the surface of the growing film itself. These radical sites are created randomly, leading to the creation of cross links.

^• High molecular weight:

The molecular weight of the fully functional plasma deposited coating layer may be high (comparable to conventional thermosets) compared to layers applied using other techniques, due to the cross-linked nature of the coating layer. In order to achieve such high molecular weight, the presence of O2 is preferably avoided in the treatment area of the plasma process. When there is a significant amount of O2 present (such as > 100 ppm by volume or even > 1 % by volume), radical recombination reactions may be quenched, leading to low molecular weight fragments residing in the plasma deposited layer, having an undesired plasticizing effect on the coating layer. Hence, preferably, the plasma gas comprises 1 % by volume or less of O2, more preferably 0.1 % by volume or less, or most preferably 0.01 % by volume or less of O2.

^• Durable:

Due to the cross-linked nature and/or high molecular weight of the plasma deposited film, the durability of the film is greatly enhanced compared to layers applied using conventional techniques. Overall, it was tested that the time between the plasma deposition process and the apphcation of an adhesive or topcoat can be extended to a period of minimum 6 months.

^• Dry:

After the plasma deposition process, the resulting film does not require any subsequent drying step. A subsequent curing step may also not be necessary, but may lead to improvement of the molecular weight of the film.

In an embodiment, the one ore more precursors are administered in the plasma as a gas, as a liquid or as a solid, preferably as a gas or as a liquid in the form of an aerosol, most preferably as a liquid in the form of an aerosol.

Preferably, the plasma used in the method disclosed herein is a low-energy plasma. A low-energy plasma is defined herein as a plasma of which the power density is high enough to activate the precursors, the substrate, allowing a chemical reaction to take place, but low enough to prevent destruction of the precursors, the substrate. The power density may be in the range of 0.2 - 8 W/dm ³ , more preferably between 0.5 W/dm ³ and 7 W/dm ³ , still more preferably between 0.8 W/dm ³ and 6 W/dm ³ , yet more preferably between 1 W/dm ³ and 5 W/dm ³ , even more preferably between 1.5 W/dm ³ and 4 W/dm ³ , still even more preferably between 2 W/dm ³ and 3 W/dm ³ , such as 2 W/dm ³ , 2.1 W/dm ³ , 2.2 W/dm ³ , 2.3 W/dm ³ , 2.4 W/dm ³ , 2.5 W/dm ³ , 2.6 W/dm ³ , 2.7 W/dm ³ , 2.8 W/dm ³ , 2.9 W/dm ³ , 3 W/dm ³ or any value therebetween, most preferably in the range of 2.4W/dm ³ to 2.6 W/dm ³ .

Preferably, the plasma used in the method disclosed herein is a cold plasma. A cold plasma is defined herein as a plasma of which the temperature is sufficiently low to not melt or otherwise damage the precursor and/or preform that are exposed to said cold plasma. The temperature of the plasma may be 150 °C or lower, preferably 130 °C or lower, more preferably 100 °C or lower, yet more preferably 70 °C or lower, even more preferably 60 °C or lower, yet more preferably 55 °C or lower, still even more preferably 50 °C or lower, even yet more preferably 45 °C or lower. The temperature of the plasma may be as low as room temperature, i.e., the temperature surrounding the plasma. Depending on the location where the coating process is carried out, room temperature may be in the range of 10-40 °C, preferably 15-30 °C, such as 20-25 °C. The temperature of the plasma will generally not be lower than room temperature.

When depositing temperature sensitive coatings it is important to keep the temperature of the plasma steady at the optimal value. Depending on the type of precursor or precursor mixture and/or the pressure, the optimal temperature may be selected. Hence, in an embodiment the temperature of the plasma is selected taking into account the type of precursor, the precursor mixture and/or the plasma pressure.

The plasma of the present invention is preferably an atmospheric plasma which has a pressure around ambient pressure. Such plasma is created and discharged typically at a pressure of between 400 and 1600 hPa, preferably at a pressure between 450 and 1400 hPa, even more preferably at a pressure between 500 and 1300 hPa, yet more preferably between 600 and 1250 hPa, even more preferably between 700 hPa and 1200 hPa, still more preferably between 800 hPa and 1150 hPa, yet more preferably between 900 hPa and 1100 hPa, most preferably about ambient pressure, which is typically about 1013 hPa. Pressure of the plasma can play an important role in the quahty of the deposited layer. Some plasma precursors are sensitive to too low and/or too high plasma pressures compared to the atmospheric pressure, while other precursors provide a better coating at lower or higher plasma pressures. However, low-energy, cold plasma can typically be apphed under reduced pressure of lower than 400 hPa down to vacuum, or increased pressure of more than 1600 hPa, both types requiring a pressure vessel to maintain such low or high pressures. The use of a plasma with pressures in the currently preferred ranges around the ambient pressure reduces costs and difficulties relating to maintaining pressure differences and pressure gradients.

In a preferred embodiment, the plasma is a dielectric barrier discharge plasma (DBD plasma), preferably under atmospheric pressure.

The functionality of the layer may depend on the plasma conditions, e.g. temperature and pressure, in which the layer is deposited. The temperature and/or pressure conditions of the plasma may therefore be selected taking into account the desired functionality of the coating layer.

In a preferred embodiment, the plasma gas is ionized by means of electrodes, wherein more preferably said plasma gas is ionized by said electrodes with a power of at most 10 Watt per cm ² of the electrode surface, more preferably at most 9 W/cm ² , still more preferably at most 8 W/cm ² , even more preferably at most 7.5 W/cm ² . In many embodiments of the present invention, the power applied by the electrodes is minimally 1 W/cm ² , preferably minimally 2 W/cm ² , still more preferably minimally 2.5 W/cm ² . The power is most preferably between 2.5 and 7.5 W/cm ² .

In a preferred embodiment, the plasma gas comprises inert gas for at least 99 % by volume. The use of an inert gas as plasma gas essentially ensures that no reactions take place with the plasma gas and the equipment, between molecules of the plasma gas themselves, even not if temperature is increased. In fact, the lack of reactions also seems to allow to keep the plasma temperature low, e.g. less than 50 °C and preferably around room temperature. The low temperature of the plasma allows treatment of substrates made from a wide range of materials. Furthermore, the use of an inert gas as plasma gas allows for a good control over the formed coating layer and the adhesion properties thereof. Without wishing to be bound by theory, the inventors believe that the lack of reactive gas in the plasma gas ensures that none to very few chemical reactions between the plasma gas and the surface onto which the coating layer is applied, hence the better control over the adhesion properties. If the plasma gas is or mainly comprises N2, it is observed that, under the low power conditions apphed to the plasma in embodiments of the present invention, this results in very little to no nitrogen incorporated in the resulting coating layer. Similarly, the use of noble gases, such as Ar and/or He, as plasma gas also results in very httle to no incorporation of the plasma gas into the coating layer. This is in stark contrast with the use of e.g. O2, NH3 or CH4 as a plasma gas, all of which are deemed reactive gasses, and all of which seem to leave more traces in the coating layer, thereby leading to loss of control over the adhesion properties.

In a preferred embodiment, the one or more precursors are added in a plasma gas afterglow. Hereby plasma gas flows over and between a plasma-inducing system, e.g. a set of electrodes. Downstream of the plasma- inducing system, a plasma gas afterglow is present, which comprises a large number of ionized plasma gas molecules which did not have the time to de ionize. The precursor is preferably introduced in said plasma gas afterglow. As a result, the one or more precursors do not need to be introduced in between e.g. electrodes which are used to ionize the plasma gas, and thus the electrodes may be kept clean for a long duration as the precursor cannot form a layer onto the electrodes.

In a preferred embodiment, said plasma gas comprises inert gas for at least 99 % by volume, i.e., 1 % by volume (vol.%) or less of the plasma gas is a reactive gas. More preferably at least 99.5 vol.%, still more preferably at least 99.8 vol.%, still more preferably at least 99.9 vol.%, even more preferably at least 99.95 vol.%, yet more preferably at least 99.99 vol.% of the plasma gas is an inert gas. This means that the plasma gas preferably comprises 1 vol.% or less O2, more preferably at most 0.5 vol.%, still more preferably at most 0.2 vol.%, yet more preferably at most 0.1 vol.%, still more preferably at most 0.05 vol.%, even more preferably at most 0.01 vol.% of O2. In the atmospheric plasma process of the present invention, this can for instance be achieved by using an overpressure with respect to ambient pressure, e.g. the plasma gas is delivered at a pressure of at least 1013 mbar, preferably at least 1020 mbar, more preferably at least 1030 mbar, even more preferably at least 1040 mbar, still more preferably at least 1050 mbar. Such slight overpressures allow to create an oxygen- poor or even oxygen-free zone in the plasma afterglow.

The atmospheric plasma coating process of the present invention allows both batch processes and inhne processes. Hence, in an embodiment, the surface moves during step c and in another embodiment, the surface is static during step c. In yet another embodiment, the surface moves and remains static during step c according to a predetermined trajectory. This allows to provide e.g. a thicker coating on some portions of the surface and thinner coating on other portions of said surface.

In an embodiment of the invention, the plasma gas flow is between 1 and 1500 standard liter, i.e., a liter of the gas at atmospheric pressure and at room temperature, per minute (“slpm”), more preferably between 50 and 1500 slpm. More preferably the plasma gas flow is between 80 and 1000 slpm. Preferably the plasma gas comprising the precursor is jettisoned from an outlet of a plasma jet nozzle. In a preferred embodiment, the plasma gas flow is determined taking into account a distance between the surface of the substrate and the outlet of a plasma jet nozzle. The larger such distance, the more plasma gas flow is required to ensure that the surface is subjected to a plasma without reactive gases other than the used precursor. In particular, one can ensure that the plasma is essentially free of oxygen coming from e.g. the surrounding air.

In an embodiment of the present invention, the surface undergoes a plasma pre-treatment step prior to being subjected to the plasma comprising the precursor. This is preferably performed in case of extremely inert surfaces, such as glass, silicon wafers, gold, high performance engineering thermoplastics or thermosets, etc. Hereby, the plasma pre treatment preferably activates the surface of the substrate, i.e., it generates surface radicals, and may also preferably a least partially oxidize the surface, leading to an increased surface energy in most cases.

In preferred embodiments, the pre-treatment is performed:

- in an oxygen-rich plasma environment, more preferably using air or CO2 or other oxygen containing species;

- at higher power compared to the power used during the plasma gas ionization step a); and/or

- without the addition of chemical precursors.

In embodiments of the present invention, the coated substrate undergoes an atmospheric plasma post-treatment step. Preferably during this post-treatment step, the molecular weight of the plasma film is increased and/or the thermal stability of the plasma film is increased.

In preferred embodiments, the plasma post-treatment is performed:

- in absence of oxygen, using inert plasma gases, such as N2, Ar, or He (or mixtures thereof);

- at lower plasma power than the power used during the plasma gas ionization step a); and/or

- without the addition of chemical precursors.

In embodiments of the invention, the plasma coating has a thickness between 5 and 600 nm, preferably between 5 and 500 nm, more preferably between 10 and 500 nm, even more preferably between 10 and 300 nm, yet more preferably between 10 and 200 nm, still more preferably between 10 and 80 nm, such as 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm or any value therebetween, most preferably about 20 nm. The plasma coating thickness can be well-controlled by controlling the exposure time of the surface to the plasma and/or the precursors. In a preferred embodiment, the plasma temperature is at most 50 °C, more preferably at most 40 °C, still more preferably at most 30 °C and most preferably around room temperature.

In a preferred embodiment, the plasma temperature is controlled, more preferably by the cooling electrodes used for ionizing the plasma gas. This can be e.g. water-cooled or air-cooled electrodes. Preferably the temperature of the electrodes is measured and/or the temperature of the substrate is measured in order to allow better control the temperature of the plasma gas. Typically this can be achieved by using a temperature control system, e.g. a PID controlling system, which allows to control the cooling of the plasma, e.g. by cooling the electrodes, and by checking how a predetermined desired plasma temperature relates to the measured temperature. Preferably the temperature of the electrodes and of the substrate is measured and the temperature control system ensures that the desired plasma temperature lies between the electrode temperature and the substrate temperature.

The surfaces which can be treated with the present invention may have any type of shape and size, and may be made from a large range of materials, for example:

Polymers:

^• commodities (e.g. polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), ethylene propylene diene monomer (EPDM), polyolefins, etc.)

^• engineering thermoplastics (e.g. polyethylene terephthalate (PET), polybutylene terephthalate (PBT), poly(methyl methacrylate) (PMMA), polycarbonate (PC), poly(ethylene succinate) (PES), polyamides, aramids, acrylonitrile styrene acrylate (ASA), acrylonitrile butadiene styrene (ABS), etc.)

^• fluorinated polymers (e.g. polytetrafLuoroethylene (PTFE), polyvinylidene difluoride (PVDF), fluorinated ethylene propylene (FEP), etc.) ^• biodegradable polymers ( e.g . polylactic acid (PLA), polycaprolactone (PCL), etc.)

^• cross-linked polymers (e.g. epoxy-amines, polyurethanes, silicones, etc.)

^• carbon fibers

^• water-soluble polymers (polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyacrylamides, divinyl ether-maleic anhydride (DIVEMA), polyoxazoline, polyphosphates, polyphosphazenes, etc.)

- Natural materials: rayon or viscose, polysaccharides, chitosan, collagen, proteins, xanthan gum, pectins, dextran, carrageenan, guar gum, hyaluronic acid (HA), leather, etc.

- Metals: gold, silver, iron, brass, lead, iron, copper, tin, stainless steel, aluminum, zinc, etc. (including all possible alloys)

- Ceramics: glass, silicon wafers, metal oxides (e.g. AI ₂ O ₃ , ZnO, etc.), carbides (e.g. SiC, titanium carbide, etc.), nitrides (e.g. S ₁₃ N ₄ , etc.)

As used herein, the term food product contact surface refers to any surface that may be in contact with a solid or liquid food product, including beverages. Food product contact surfaces can for instance be present in production (e.g. membranes), transportation (e.g. pipelines, trucks), storage (bottles, cans, kegs, barrels, casks), serving (e.g. dispensing systems, jugs), and consumption (e.g. cups) of food products, such as beverages.

Products having a food product contact surface to which a coating layer as disclosed herein may be applied include, but are not limited to, the following examples:

- a membrane and/or a filtration system, such as a submersed membrane and/or a submersed filtration system, which may be used in e.g. the production of beer, desalination plants, or water recycling plants. An antibiofouling layer may prevent or limit clogging and efficiency loss as a result of biofouling of a filter and/or membrane, whereas an antimicrobial layer may prevent contamination of the food or beverage due to contamination of the membrane by e.g. bacteria;

- a transportation system for food products such as conveying systems, pipelines, and/or tank trucks, wherein the antibiofouling and/or antimicrobial layer may help e.g. in avoiding cross contamination of the (liquid) food product;

- packaging of food products, wherein the shelf life of the products in the packaging can be drastically increased by an antibiofouling and/or antimicrobial coating layer. Examples of food products that may benefit from packaging with an antibiofouhng and/or antimicrobial coating layer include beverages, such as beer, soft drinks, and dairy products, as well as ketchup, mayonnaise, etc., and the packaging items to which such a layer may be applied include bottles, bottle caps such as crown caps, cans, can lids, kegs, barrels, etc.;

- a dispensing assembly for dispensing beverages and/or parts thereof, such as a keg, barrel, valve, dispensing line, dispensing tap, etc. An antibiofouling and/or antimicrobial layer helps to avoid microbial contamination of the beverage, thereby increasing the shelf hfe of the beverage, also after opening of the beverage container. This helps in reducing waste that would otherwise be caused by having to discard half- empty beverage containers;

- a drain system for collecting spilled food products, such as a drip and/or drain tray for use with a beverage dispensing assembly. An antibiofouling and/or antimicrobial layer in such systems helps to reduce clogging and contamination, thereby reducing maintenance frequency and costs.

In the above examples, as well as in all other food product contact surfaces, it may be advantageous to apply a coating layer with antibiofouling properties, antimicrobial properties, or both. In case both antibiofouling and antimicrobial properties are preferred, a coating layer which comprises both antibiofouhng and antimicrobial properties, may be provided in accordance with the method of the present invention.

In an embodiment of the present invention, a coating layer is apphed using the method of the invention onto a food product contact surface made from a stretchable material. For example, the stretchable material may be a thermoplastic polymer. Such stretchable thermoplastic polymer materials are for instance used for thermoplastic beverage containers. These containers may be produced by making a preform for a container, which preform is subsequently stretched into a container. Other products that may be obtained by stretching of a stretchable material are for instance tubes or films. Containers and/or bag-in-containers that are produced by stretching of a preform are described in Dutch patent apphcation nos. 2024296 and 2024298, filed on 22 November 2019, both of which are incorporated herein by reference.

Other examples of stretchable materials to which a coating layer may be applied using the method according to the invention are metals, such as aluminum. In the production of e.g. beverage cans and/or kegs, one of the steps may involve stretching of a metal surface (such as an aluminum surface), for instance by deep drawing. In embodiments, a coating layer is apphed to a metal food product contact surface using the method according to the invention, which metal food product contact surface is subsequently stretched.

The inventors have found that the antibiofouhng and/or antimicrobial functionality of a coating layer applied using the method of the present invention may be preserved when the stretchable material (e.g. the preform, or the metal food product contact surface) is stretched (e.g. using stretch blow molding, or deep drawing) after application of the coating layer. Preferably, an antibiofouhng and/or antimicrobial coating layer is apphed to a preform for a container using the method of the present invention, after which the preform with the applied coating layer is stretched into the container, e.g. by stretch blow molding. The coating layer applied on the preform using the method according to the present invention preferably comprises crosslinks and is preferably covalently bound to the surface of the preform. These crosslinks and covalent bonds are beheved to be responsible for, or at least to improve, the structural integrity of the coating layer. If the coating layer is applied to a preform which is afterwards stretched in order to produce a container, e.g. by stretch blow molding, the crosslinks and covalent bonds between the coating layer and the preform prevent the coating layer to be damaged, even when the preform is stretched.

Therefore, using the method as described herein, the coating layer stays intact and the antibiofouling and/or antimicrobial functionality is preserved when the preform is stretched, for instance during blow molding, or stretch blow molding. Without wishing to be bound by theory, it is believed that crosslinks in the coating layer and covalent bonds between the coating layer and the preform prevent the coating layer from being damaged, even when the preform is stretched. This makes it possible to apply a coating layer using the method of the present invention to the food product contact surface of a preform for a container or for a bag-in container, which results in an antibiofouling and/or antimicrobial coating layer on the food product (e.g. beverage) contact surface of the container or bag-in-container after the preform is stretched.

The scheme outlined in figures 1 to 4 indicates the different phases during the atmospheric plasma deposition process in an embodiment:

Figure 1: at t=t0 the plasma is on. A precursor R-X is added to the plasma gas and the plasma is contacted with the surface of the substrate. Hereby the precursor R-X is radicalized, and the surface is activated.

Figure 2: at t=tl the plasma is on. Radical recombination reactions are taking place on the surface, resulting in a covalent bond between surface of the substrate and precursor. Figure 3: at t=t2, the plasma is on. Film growth and thickness depend on treatment time. Also cross-linking is taking place.

Figure 4: at t=t3 the plasma is off. After the plasma treatment, a functional plasma deposited film remains which is grafted onto the surface of the substrate.

In Step 1, the plasma is generated (can be based on direct or indirect plasma configurations, using an inert plasma gas such as N2, argon, helium, or any mixtures thereof), instantaneously generating radicalized species in the plasma gas phase. These species can be added to the plasma as a gas (or gas mixture), or a liquid ( e.g . an aerosol, a spray, a liquid mixture, an emulsion, a dispersion, or polymer solution), preferably as a gas or as an aerosol. In the scheme outlined in figs. 1-4, we used the connotation “R-X” to denote the initial precursor, and “R-X · ” the radicalized form of the precursor. “R” being the targeted functionality, and “X” being a part of the molecule being able to be radicalized. For example, “X” can be reactive (such as C=C double bonds, C=0, epoxy, isocyanate, etc), but can also be unreactive (i.e., saturated), in this specific case, the radical will be formed based on hydrogen abstraction.

In addition to the radicalized species in the gas phase, also surface radicals are formed on the surface of the substrate which is also in contact with the plasma. The generation of these surface radicals can be mainly based on hydrogen abstraction or breaking of covalent bonds located at the surface of the substrate.

In Step 2, radical recombination reactions are taking place between the radicalized species and surface radicals. This radical recombination reaction results in a permanent grafting of the precursor to the surface by the formation of a covalent bond. It must be remarked that presence of reactive gasses such as O2 is preferably avoided during this phase.

In Step 3, film growth is taking place by the continuous incorporation of species by radical recombination. It must be remarked that the plasma process is ‘non-specific’, meaning that a specific precursor can be built in on the surface on any location, leading to a heterogeneous conformation of the plasma deposited film on a molecular level.

Furthermore, the film growth can take place in a ‘continuous’ plasma or in a ‘pulsed’ plasma process. This pulsed plasma has a specific plasma off time, where recombination reactions are favored, similar to propagation in conventional polymer synthesis.

In the final phase of the plasma deposition process (Step 4), the plasma is switched off, or similarly the substrate has left the plasma afterglow zone, leading to a fully functional coating layer which is covalently linked to the surface of the substrate.

An apparatus for providing a coating layer onto a surface, in accordance with the present invention, can be found in European patent apphcation No. EP18179354.8, filed on 22 June 2018 and in international patent application No. PCT/EP2019/066647, filed on 24 June 2019, both of which are incorporated herein by reference. Such apparatus and the methods described in these documents can be used to provide the coating layer discussed in the present document.

The coating layer can be provided using an apparatus for depositing a coating via an atmospheric pressure plasma jet, the apparatus comprising:

- a plasma jet generator comprising a jet outlet; and

- a nozzle comprising an adaptor and a shield, preferably a replaceable shield, the shield comprising a jet inlet, a nozzle outlet and a sidewall extending from the jet inlet to the nozzle outlet, wherein the adaptor is preferably configured for detachably attaching the shield onto the plasma jet generator and thereby communicatively coupling the jet outlet and the jet inlet.

Preferably, the shield comprises at the jet inlet a flange attached to the sidewall, and wherein the adaptor comprises a retaining wall comprising an opening with size and shape adapted for retaining the flange. Preferably, the shield is monolithic.

Preferably, the shield comprises an insulating material. More preferably, the shield comprises, and preferably is made of, a polymer material. Preferably, the nozzle outlet of the shield comprises a non-planar edge.

Preferably, the jet outlet comprises an opening, and the jet inlet comprises an opening larger than the opening of the jet outlet.

Preferably, the sidewall comprises a tapering portion. Preferably, the sidewall of the shield comprises at least one precursor inlet.

Preferably, the nozzle comprises a homogenization means, preferably the shield comprising flow disturbance elements.

Preferably, the nozzle is adapted for cooling, preferably the sidewall of the shield comprising a channel for passage of a cooling fluid.

Preferably, the nozzle outlet of the shield comprises an edge, and the apparatus is configured for maintaining said edge at a distance of at least 0.1 mm and at most 5 mm, preferably at least 0.2 mm and at most 2 mm, more preferably at least 0.5 mm and at most 1 mm, of said surface of said substrate.