COATING TECHNOLOGY FOR PLASTIC CONTAINERS

Description

[0001] The invention lies in the field of plastic coating by means of plasma-enhanced chemical vapor deposition (PECVD).

[0002] In industry, plastic containers, such as bottles or canisters, are used to store a wide variety of liquids. The properties of the plastic affect the material's suitability for storing the respective medium, as well as the container's manufacturing costs.

[0003] The suitability of the material depends in particular on the permeation properties of the plastic towards various gases (e.g. oxygen from the ambient air), solvents, aromatics, dyes or other substances, as well as the resistance of the plastic to the medium to be stored (e.g. acidic or alkaline media).

[0004] For the safe storage of a medium, such as an alkaline solution, it must be protected from penetrating substances from the environment or from loss of substances. The contents of the container must not escape into the environment. However, depending on the properties of the container material, gases and other substances can diffuse through the container wall (permeation). The barrier performance of the container material describes how well the container, or the stored medium, is protected against permeation.

[0005] Furthermore, the medium itself should not leak from the container or damage it. The container material must withstand the stored medium for extended periods and ensure adequate storage safety. Particularly when storing chemicals, many of which are classified as hazardous materials, high demands are placed on the resistance of the container materials.

[0006] In principle, the production of containers from plastic, e.g., by extrusion blow molding, is a popular approach in industry because it allows for cost-effective container manufacturing and plastic containers are easy to store. However, current technology does not offer a cost-effective plastic that simultaneously provides sufficient barrier performance and resistance to aggressive, especially alkaline or acidic, media.

[0007] In current technology, plastics or special composite plastics, which are expensive to manufacture or difficult to transport or store, are therefore often not used for storing aggressive media.

[0008] Containers often manufactured by co-extrusion blow molding are used, their walls consisting of several layers of different plastics. Frequently, a mixture of high-density polyethylene (HDPE), polyamide (PA), and / or ethylene-vinyl alcohol copolymer (EVOH) is employed. HDPE is highly resistant to acids, alkalis, oils, and fats, but has high permeability to certain substances, which is why EVOH and / or PA are used. These composite plastic packaging solutions are not environmentally friendly, as only mono-material solutions can be recycled sustainably.

[0009] Besides the use of non-recyclable multi-material plastic solutions, the only other options for barrier treatment of plastics are fluorination or coating using plasma technology. In fluorination, the containers are placed in a vacuum chamber and exposed to a fluorine gas mixture in the absence of the atmosphere. Due to its high reactivity, fluorine partially replaces hydrogen atoms on the material surface. However, this only creates an adsorption barrier, which can only prevent the migration of media such as solvents. Fluorine is also toxic and highly corrosive, making this technology unattractive and expensive. Furthermore, there is a growing body of research attributing negative environmental and health impacts to fluorinated plastics.

[0010] Plasma polymer coatings, on the other hand, are not only inexpensive, but their production also requires only minimal energy and material input. Using PECVD, fluorine-free, silicon-based barrier coatings can be produced that act as passive barriers, preventing the migration of various substances. These coatings are therefore suitable, for example, as solvent barriers, barriers against the migration of any contaminants from recycled plastics into a product, or as gas barriers against oxygen, hydrogen, or carbon dioxide, opening up a wide range of potential applications.

[0011] DE 36 32 748 A1 describes a process for the PEVCD coating of hollow bodies under low pressure, which is now widely used in industry. First, the entire vacuum chamber, together with the bottle to be coated, is evacuated to the required working pressure. Then, the process gases are introduced into the interior of the bottle and excited to plasma using microwaves. The plasma ignites inside the bottle, and a barrier coating can be applied.

[0012] Compared to competing permeation reduction methods, plasma-polymer coatings are not only cost-effective but also environmentally friendly, as their production requires minimal energy and material input and eliminates the need for solvents or other harmful chemicals. This is particularly true for low-pressure plasma processes due to their closed reaction chamber and minimal process gas usage.

[0013] Silicon oxide SiO x barrier coatings known from the prior art are extremely susceptible to hydrolysis, so that they decompose even upon contact with contents with slightly elevated pH values ​​> 7.

[0014] From DE 10 2006 048 658 A1 a multilayer composite coating is known which consists of a SiO x barrier layer and a silicon-organic passivation layer.

[0015] One disadvantage of these multilayer composite coatings, whose layers are built up one after the other (often in separate process steps), is that, due to the layer growth mechanisms typical for PECVD processes, defects and pores in an underlying layer propagate as flaws in a subsequent coating, so that such layer systems always exhibit an open porosity.

[0016] Such PECVD processes for depositing a barrier layer against gas permeation and a protective passivation layer on plastic containers (so-called composite coatings) are mainly used in the beverage industry today.

[0017] For certain contents, e.g. aggressive alkaline chemicals, which require high mechanical stability of the containers, high barrier performance against gases and solvents, and high resistance to acidic and alkaline media, the known composite coatings are not suitable or at least require optimization.

[0018] The growth processes of the materially different layers of the composite layer create pores on the layer surface. These pores, among other things, cause gaps in the passivation layer, through which the underlying barrier layer is exposed to the aggressive medium. Pores can form particularly through so-called island growth during layer deposition. Island growth prevents homogeneous layer formation.

[0019] Layer growth in PECVD is a complex process largely determined by the interplay of kinetic and thermodynamic forces. At the start of the coating process, layer-forming particles are accelerated towards the substrate surface. Upon impact, they are reflected back from the surface if their kinetic energy is too low. If their kinetic energy is sufficiently high, they are adsorbed as adatoms. These adatoms then diffuse across the substrate surface until they either desorb or condense with other adatoms, forming nuclei and settling into their final positions within the layer. The surface energy of the substrate to be coated significantly influences the mobility of the adatoms and thus the nature of the layer growth. Similar surface energies of the layer material and the substrate material promote homogeneous layer growth.

[0020] Island growth occurs primarily when the substrate has a lower surface energy than the forming layer, resulting in a weaker interaction between the substrate and the adatom than the interaction between the particles. In this case, the layer material preferentially grows in three-dimensional islands. Since atom condensation on the substrate is not initially thermodynamically favored in this growth mode, closed layers only form as the layer grows and islands merge.

[0021] Furthermore, depending on the plasma process and reactor type, a different number of collisions occur in the plasma before the reactive particles reach the substrate. With few collisions, only a few reactions take place in the gas phase, so the deposition rate decreases. If the particles or radicals undergo many collisions or reactions before reaching the substrate, bulk polymerization can occur, in which oligomers are already formed in the gas phase. The larger the particles, the lower the probability of surface diffusion, so that island growth occurs more frequently and the layers are later closed, as the islands must first grow together.

[0022] Island growth thus leads to a granular surface structure and, consequently, to nanoscopic pores at the grain boundaries of the merging islands. When several grain boundaries meet, pores form. The lateral extent of the pores is on the order of several nanometers. Therefore, these layer defects are significantly larger than, for example, molecular oxygen or ions in an alkaline solution and thus represent weak points in the layer system.

[0023] If such pores are located in an underlying SiO x layer (barrier layer), increased nucleation occurs at these pores, leading to the formation of agglomerates, which results in the continuation of these layer defects into a possible subsequent protective layer (passivation layer).

[0024] Experiments have shown that a large number of nanoscopic pores always form in SiO x layers deposited on plastic or silicon-organic adhesion promoter layers, which can be detected using electrochemical methods such as cyclic voltammetry or electrochemical impedance spectroscopy.

[0025] This can be explained, among other things, by the different surface energies and chemical compositions of the materials, as well as increased bulk polymerization. Measurements of existing pores after the further application of a silicon-organic passivation layer to the SiO₂ x layers showed that the number of nanoscopic pores hardly decreased as a result of the additional coating.

[0026] When using composite coatings known from the prior art, the system fails due to pore or defect-driven exposure to chemically aggressive media. These media can penetrate open pores and reach the hydrolysis-prone barrier layer beneath the passivation layer, causing it to decompose. This leads to undermining of the protective layer and failure of the entire system. Therefore, this coating system is unsuitable for the industrial storage of chemicals.

[0027] One approach to solving these problems with composite layers is the use of so-called gradient layers, in which the chemical composition changes gradually along the layer thickness between organic (passivation or adhesion promotion) and oxide (migration barrier). In contrast to composite layers, which consist of a few (micro)layers with sharply distinguishable chemical compositions, gradient layers exhibit a continuous gradient in chemical composition along the layer thickness.

[0028] Gradient layers are known from other areas of technology, e.g. as a scratch-resistant layer on plastic spectacle lenses.

[0029] From EP 0 718 418 A1 it is known that the plasma pulsing parameter can be used to generate a PECVD gradient layer while keeping all other parameters constant. However, this parameter is only suitable for depositing, for example, scratch-resistant layers, for which no significant differences in chemical composition are required, so that the same gas composition can be used throughout the entire layer deposition process.

[0030] DE 42 38 279 A1 discloses a further coating process for producing a scratch-resistant layer.

[0031] From EP 2 630 273 B1, a method for plasma treatment of workpieces is known in which the plasma is ignited with pulsed microwave energy, whereby the ratio between the duration of the on-phases and the off-phases is varied. The quotient of the on-time and the off-time is initially increased and then decreased again during the coating process in order to first reduce and then subsequently increase the carbon content in a barrier layer.

[0032] German patent DE 197 32 217 C2 discloses a plasma coating process for a photovoltaic semiconductor device with a gradient layer exhibiting gradually changing layer properties through variation of the plasma parameters and gas supply. The gradient layer comprises a diffusion barrier layer, an elastic polymer protective layer zone, and a scratch-resistant surface layer. Deposition takes place in a microwave plasma. However, this process is not suitable for producing the required layer properties on a plastic substrate for plastic containers used in aggressive media such as chemicals. In particular, it lacks guidance on how and using which process parameters the plasma power can be controlled to generate the desired gradient layer.

[0033] From DE 101 39 305 A1 a PICVD process in a pulsed microwave plasma for pharmaceutical packaging is known, which, however, does not include a sufficient passivation layer.

[0034] WO 01 / 94448 A2 discloses a coating for beverage bottles with an improved barrier against gas permeation, low wall thickness, and good recyclability. A coating with a SiOxCyHz base layer containing organic components and a barrier layer applied thereon is proposed. One embodiment is a continuum layer structure with variable composition, ranging from SiOxCyHz on the substrate side to SiOx on the outer surface of the coating. Such coatings do not exhibit sufficient resistance to chemically aggressive media (e.g., sodium hydroxide).

[0035] US patent 2009 / 148633 A1 discloses a plasma-CVD coating on a plastic bottle consisting of an adhesive layer with a higher carbon concentration, a barrier intermediate layer with a lower carbon concentration, and a surface protective layer with a higher carbon concentration. This technique can be used to create coatings for beverage bottles that withstand the slightly alkaline effects of mineral water (pH 7.3) but fail in more corrosive environments.

[0036] The known methods are insufficient for producing a pore-free gradient layer that can represent a wide spectrum of chemical composition from highly oxidized (SiO x ) for barrier to organosilicon (SiOCH ) for corrosion protection.

[0037] In particular, simply changing the pulse ratios is not sufficient to achieve the desired range between barrier and passivation effects in a continuous gradient layer while simultaneously ensuring homogeneous, pore-free layer growth.

[0038] With regard to the solution known from EP 2 630 273 B1, the present invention makes it possible to deposit an advantageous gradient layer in which the pulse parameters, in particular the ratio of on-time to off-time, are not necessarily increased first and then decreased. Rather, research has shown that the targeted control of the energy density in the plasma as a function of the gas composition has a significant influence on the coating properties. For example, it is possible to control the energy density in the plasma, with at least an initially constant ratio between the on-time and off-time of the pulses, by adjusting the energy input, the absolute duration of the pulses, and / or the gas composition.

[0039] Furthermore, gradient coatings known from the prior art, e.g., scratch-resistant coatings, are unsuitable due to their mechanical properties. Scratch-resistant coatings with a thickness of over one micrometer, often over two or three micrometers, tend to crack or break at stressed points under mechanical stress, thus impairing their chemical protective effect.

[0040] Further development of the gradient layer deposition methods developed in the 1990s was limited by the long response times of commercially available mass flow controllers, which were approximately 0.5 seconds. The formation of pore-free gradient layers with a gradient resolution of a few nanometers was not possible with the technology available at that time.

[0041] The object of the present invention is therefore to demonstrate an improved coating technology for plastic containers.

[0042] It has been shown that special process conditions and layer properties are necessary to form a chemically resistant coating with good barrier performance.

[0043] In order to achieve a sufficiently high chemical resistance for the storage of chemicals, high carbon content organosilicon coatings must be produced, preferably by retention of methyl groups from the precursor.

[0044] Plasma-polymerized layers deposited from organosilicon precursors (for example, silicon dioxide barrier layers) consist of silicon, carbon, hydrogen, oxygen, and / or nitrogen. The typical instability of such layers in, for example, alkaline media is due to the high electronegativity difference between oxygen / nitrogen and silicon, because ions in solution primarily break bonds with large partial charge shifts. The more inorganic or oxidized the layers are, the more positive partial charges are present on carbon and especially silicon atoms. Since the electronegativity difference between silicon and oxygen is greater than between silicon and carbon, silicon possesses the largest positive partial charge, which makes these bonds, and thus especially the frequently used SiOₓ barrier layers, particularly vulnerable.

[0045] Research has shown that, within a narrow process window of plasma energy density, gas composition, process pressure, and temperature, it is possible to retain the methyl groups contained in the precursor, e.g., hexamethyldisiloxane (HMDSO) molecule, within the plasma, thus preventing bulk polymerization and incorporating them into the layers. Considering the structural formula and bond energies of the HMDSO molecule, the PECVD of HMDSO in the plasma initially involves the elimination of the methyl groups and subsequently the hydrogen. Normally, these groups fragment in the plasma, react with other particles to form volatile components, and leave the reaction space without participating in layer formation.

[0046] By retaining the methyl groups, increased layer stability can be achieved through the mechanism of steric shielding. Steric shielding refers to the spatial isolation of a weak bond by adjacent, space-filling substituents. In organic chemistry, a substituent is defined as a group of atoms (organyl residue or other residue) that replaces a hydrogen atom in a molecule (in this case, the methyl groups).

[0047] The use of oxygen as a reactive gas for the deposition of a chemically resistant passivation layer must therefore be limited or even avoided. However, the addition of a large amount of oxygen is essential for the production of oxide SiO₂ x layers (barrier layer). Therefore, generating both functional layers solely by controlling the energy input (e.g., via the pulse parameters of the plasma excitation) during the process is insufficient.

[0048] The invention relates to both the coating according to claim 10 and the method for producing the coating according to claim 1.

[0049] A first aspect of the invention is to use a gradient layer deposited by PECVD for coating a plastic container for aggressive media (e.g., alkalis). This gradient layer has at least one oxide-dominant barrier zone and one organic-dominant passivation zone. At the nanoscale, the gradient layer consists of a multitude of nanolayers with a resolution of a few nanometers (down to 1 nanometer), the material composition of which changes gradually from nanolayer to nanolayer. Along the layer thickness, the imprinting of the material composition changes gradually from organic to oxide or vice versa. Within a nanolayer, the material composition of the layer is essentially homogeneous. The material composition of the individual nanolayers is determined by the process parameters of the PECVD process.

[0050] The barrier zone is oxide-based and offers high barrier performance to prevent gas permeation through the container wall. Preferably, the barrier zone has a high concentration of Si-O-Si compounds.

[0051] The passivation zone is organically composed and offers high resistance to aggressive media, particularly alkaline media. The passivation zone is preferably arranged on a barrier zone and protects the barrier zone from environmental influences, especially aggressive media within the container. Preferably, the passivation zone has a high concentration of organic compounds, preferably CH₃ and / or CH₂ compounds, and a low concentration of oxide compounds, particularly low concentrations of Si-OH and Si-O-Si compounds.

[0052] Depending on the type of plastic substrate to which the coating is to be applied, it is advantageous to deposit an organically influenced adhesion zone between the plastic substrate and the oxide barrier zone.

[0053] Polyolefins are typically coated with an adhesive layer. Certain plastics, e.g., PET The detention zone can be dispensed with.

[0054] In a first embodiment, the gradient layer comprises at least one oxide barrier zone and one organic passivation zone (2-zone layer). In In another embodiment, the gradient layer comprises an organic adhesion zone, an oxide barrier zone and an organic passivation zone (3-zone layer).

[0055] The different zones gradually merge into one another. They form a continuous gradient layer with smooth transitions between them. The zones preferably have an oxide or organic concentration, located either in the center or at the edge of the zone, depending on whether it is a boundary zone of the gradient layer or a zone within the gradient layer. In In certain embodiments, the coating can comprise several gradient layers. The gradient layer can also comprise several zones of the same type (passivation zone, barrier zone, or adhesion zone).

[0056] A particular advantage of gradient layers over composite layers is that the layers can be formed to be especially homogeneous, i.e., pore-free and with low residual stresses. During the deposition of the individual nanolayers, chemically similar nanolayers always meet, thus preventing island growth.

[0057] The gradually transitioning zones can be distinguished from one another in various ways. Preferably, the zones are distinguished based on the proportion of organic and oxide compounds. The gradient layer is composed of a large number (approximately 80–1000) of nanolayers, each a few nanometers thick. If oxide compounds predominate in a nanolayer, this nanolayer can be classified as part of the barrier zone. If organic compounds predominate in a nanolayer, it can be classified as part of the passivation or adhesion zone.

[0058] Another aspect of the invention is to vary the concentration of the reactive gas used, preferably oxygen, during the deposition of the gradient layer in order to achieve a sufficiently strong gradient between the oxide barrier zone and the organic passivation zone or adhesion zone.

[0059] For the deposition of the gradient layer in the PECVD process, a process gas mixture is introduced into a previously evacuated reaction chamber in which the polymer substrate is located. The process gas mixture comprises one or more silicon-containing precursors, preferably hexamethyldisiloxane (HMDSO) and / or hexamethyldisilazane (HMDSN). The process gas mixture also includes, at least temporarily, one or more reactive gases, preferably oxygen. In certain embodiments, further auxiliary gases, e.g., noble gases, can be added.

[0060] The composition, i.e., the mixing ratio, of the process gas mixture is dynamically adjustable, preferably by means of mass flow controllers. In particular, the reactive gas mass flow, the precursor mass flow, and the ratio of the two mass flows to each other are controllable. In In certain embodiments, several precursors and / or reactive gases can be used. In In these cases, the relative proportions of the individual precursor mass flows are also controllable. Preferably, the ratio between reactive gas mass flow and precursor mass flow is controlled.

[0061] The reactive gas fraction C can be described in particular by the following formula: C = RGF PGF C: Reactive gas content RGF: Reactive gas mass flow PGF: Precursor mass flow

[0062] The precursor mass flow can result from the sum of the mass flows of several precursors.

[0063] An increase in the reactive gas admixture, particularly the oxygen admixture, to the process—i.e., an increase in the reactive gas fraction C—leads to a higher degree of fragmentation of silicon-containing monomers and greater oxidation of the deposited layers. The oxidation of the layers, in turn, correlates with the barrier effect, or hydrolysis stability, of the layers.

[0064] By controlling the proportion of reactive gas, preferably the oxygen proportion, the material composition and thus the function of the deposited nanolayers can be controlled across a broader spectrum. In this way, a high barrier effect and strong passivation can be achieved while maintaining a homogeneous, i.e., pore-free, layer structure.

[0065] Another aspect of the invention is to selectively control the excitation energy during the deposition of the gradient layer with respect to the process gases involved in the reaction. In particular, the excitation energy is controlled mass-related depending on the composition of the process gas mixture. The targeted adjustment of the mass-related excitation energy prevents bulk polymerization, which also prevents pore formation.

[0066] In plasma-enhanced chemical vapor deposition (PECVD), the process gas mixture is excited to a plasma by an energy source. A magnetron is preferably used as the energy source. Excitation is preferably carried out using microwaves. Preferably, the plasma is excited in pulsed mode. Pulsed excitation prevents excessive heating of the plastic substrate by the plasma.

[0067] Plasma excitation supplies energy to the reaction of the process gases. The mass-related excitation energy describes the energy supplied to the reaction per particle mass. By precisely adjusting the energy supplied per particle mass, a particularly homogeneous layer deposition can be achieved.

[0068] It has been shown that completely pore-free layers can be deposited if the excitation energy supplied to the reaction is controlled depending on the process gas mixture. Preferably, the mass-related excitation energy in the respective zones of the gradient layer is set within a specific process window depending on the respective reactive gas fraction.

[0069] To calculate the mass-related excitation energy, it is advantageous to correct the mass flow of the process gases involved in the reaction using a correction factor to account for the reactivity of the reactive gas. The correction factor for oxygen is K = 0.6 and for nitrogen K = 0.5.

[0070] The mass-related excitation energy, also called energy density, can be described with the following formula: E = Pp Fc = P × ton ton + toff PGF + K × RGF Jkg − 1 E: mass-related excitation energy (energy density) [J / kg] Pp: mean pulse power [J / s] Fc: corrected process gas flow [kg / s] P: power [J / s] ton: pulse on time [s] toff: pulse off time [s] PGF: precursor mass flow [kg / s] RGF: reactive gas mass flow [kg / s]

[0071] In the preferred embodiment, a relatively high mass-related excitation energy is supplied for the deposition of the barrier zone at a relatively high reactive gas content, preferably with 25 ≤ C ≤ 250, particularly preferably with 50 ≤ C ≤ 100, preferably 400 kJ / kg ≤ E ≤ 2700 kJ / kg, particularly preferably 1800 kJ / kg ≤ E ≤ 2200 kJ / kg.

[0072] For the deposition of the passivation zone, a relatively low to medium mass-related excitation energy is supplied at a relatively low reactive gas content, preferably 0 ≤ C ≤ 20, particularly preferably 0 ≤ C ≤ 10, preferably 30 kJ / kg ≤ E ≤ 1700 kJ / kg, particularly preferably 500 kJ / kg ≤ E ≤ 800 kJ / kg.

[0073] To deposit the optional adhesion zone, a relatively low mass-related excitation energy is supplied at a relatively low to medium reactive gas content, preferably 0 ≤ C ≤ 50, particularly preferably 0 ≤ C ≤ 30, preferably 20 kJ / kg ≤ E ≤ 200 kJ / kg, particularly preferably 30 kJ / kg ≤ E ≤ 140 kJ / kg.

[0074] The interval limits mentioned here are disclosed both in combination and individually and contribute to the technical effect of the invention. The lower and upper limits of each interval are also expressly disclosed separately.

[0075] Another aspect of the invention is to make the gradient layer, in particular the individual zones, sufficiently thin. The total thickness of the coating is advantageously less than 1000 nanometers. Preferably, the coating is up to 500 nanometers thick.

[0076] A thin coating exhibits lower layer stress. This lower layer stress reduces the risk of cracks forming or the coating breaking under mechanical stress. In contrast to thicker gradient layers used in scratch-resistant coatings (e.g., for eyeglass lenses), the requirements for mechanical strength are higher for plastic containers. Thin-walled plastic containers are generally soft. During storage or transport, the containers can deform (e.g., due to impacts or stacking). Such deformations must not lead to coating failure. It has been shown that thin coatings with a thickness of less than 1000 nanometers, preferably less than 500 nanometers, are particularly well-suited for plastic containers, especially for storing aggressive media.

[0077] The zones formed in the gradient layer also preferably have a thickness optimized for their function and / or the layer formation process.

[0078] The thickness of the optional adhesion zone is preferably a maximum of 10 nanometers. The adhesion zone is preferably thinner than the barrier zone or the passivation zone. Such a thin adhesion zone is sufficient to protect the polymer substrate from the oxidatively degrading effects of the oxygen-rich plasma used to deposit the barrier zone. With increasing thickness of the adhesion zone, increased porosity may occur during the deposition of the subsequent barrier zone. A thin adhesion zone with a maximum thickness of 10 nanometers is therefore advantageous for homogeneous layer formation. Such a thin adhesion zone provides the most homogeneous possible base for the deposition of the subsequent barrier zone.

[0079] The barrier zone preferably has a maximum thickness of 100 nanometers. This thin barrier zone is sufficient to form an adequate migration barrier. Simultaneously, homogeneous layer growth is possible. Preferably, the barrier zone is thinner than the passivation zone. A thinner barrier zone is advantageous for crack-free layer growth.

[0080] Highly oxidized layers grow with intrinsic layer stresses. In contrast, the organically influenced passivation layer grows almost free of layer stresses and can therefore be thicker.

[0081] The passivation zone has a preferred thickness of up to 800 nanometers. Since the passivation zone must provide sufficient protection for the barrier zone against the aggressive media in the container, it can be thicker than the barrier zone.

[0082] The coating can be composed of one or more gradient layers. The gradient layer structure described above can be repeated in several contiguous gradient layers to enhance the protective effect. A zone located at the edge of the coating typically has its organic or oxide concentration at the outer end, i.e., the end facing the medium or substrate. A zone located within a gradient layer typically has its oxide or organic concentration in the center.

[0083] In contrast to the consecutive application of different layer types in a composite layer, the coating material in a gradient layer always encounters a chemically similar surface, thus promoting layer growth and preventing the formation of pores on merging islands.

[0084] Adjusting the gas flows, i.e., the composition of the process gas mixture, allows for the setting of the required chemical composition of barrier and passivation zones. Furthermore, by precisely controlling the energy input per molecule depending on the process gas mixture, bulk polymerization is prevented to stop large oligomers from participating in layer formation and creating pores.

[0085] Using the method according to the invention, it is thus possible to produce layers that have a high barrier performance against gases and other permeants and at the same time high stability against aggressive media or other environmental influences, since the upper layers are chemically stable and at the same time pore-free.

[0086] For polyethylene containers coated with gradient layers, barrier improvements of 50 to 2000 times, based on the oxygen permeation rate, can be achieved, depending on the surface quality, volume, and weight of the containers. Furthermore, the gradient layers remain stable even after prolonged storage of highly alkaline NaOH solution or similar aggressive chemicals, and the barrier is maintained. This allows them to be used to make packaging made from conventional packaging plastics such as PE or PET suitable for storing sensitive chemicals.

[0087] Instead of silicon, other suitable semimetals or metals are also conceivable for use in the invention. The invention is not limited to silicon-based starting materials or coatings. Alternatively or additionally to the preferred precursors HMDSO and HMDSN, other (semi-)metallic monomers can also be used. Furthermore, alternatively or additionally to oxygen, other reactive gases, e.g., nitrogen, can be used. The disclosure also includes embodiments in which, instead of the preferred process gases, the alternatives mentioned herein or known to those skilled in the art for PECVD are used.

[0088] Further advantageous embodiments are described below with reference to the drawings.

[0089] The invention is illustrated in the drawings in an exemplary and schematic manner. They show: Figure 1: Reactor setup; Figure 2: Gradient layer setup (3-zone layer); Figure 3: Multi-gradient coating setup; Figure 4: Favorable profile of process parameters C and E during gradient layer deposition; Figure 5: FT-IR spectra at the centroids of an adhesion zone (EH) and a barrier zone (WB); Figure 6: FT-IR spectra at the centroid of a passivation zone.

[0090] Figure 1 Figure 1 shows a schematic diagram of a reactor (300). The reactor (300) comprises a process chamber (301). The substrate (100) is arranged in the process chamber (301). A reaction chamber (302) is formed in or on the plastic substrate (100). The reaction chamber (302) can be evacuated using a pump (310). In the embodiment shown, the reaction chamber is formed inside the container (100) in order to coat the inside of the plastic container (100), e.g., a canister.

[0091] A process gas mixture (PGM) is fed into the reaction chamber (302) via pipes. The reactor includes a gas supply (320) and one or more gas flow controllers (321), preferably mass flow controllers. The process gas mixture (PGM) consists of one or more process gases (PG1, PG2, RG) whose mixing ratios can be controlled.

[0092] In the preferred embodiment, two precursors are used. The first precursor (PG1) is preferably HMDSN. The second precursor (PG2) is preferably HDMSO.

[0093] One or more precursors (PG1, PG2) can be used, in particular for a specific zone of the gradient layer. It is especially advantageous to use (predominantly) HMDSO as a first precursor for the deposition of the adhesion zone (201) and / or the passivation zone (203). Alternatively or additionally, it is particularly advantageous to use (predominantly) HMDSN as a second precursor for the deposition of the barrier zone (202).

[0094] Particularly in the transition between two zones of the gradient layer, two or more precursors can be mixed, at least temporarily.

[0095] The precursor mass flow rate is preferably based on the sum of the mass flows of the several precursors. For chemically similar precursors (e.g., HMDSN and HMDSO), the mass flows of the individual precursors in the process gas mixture can be combined to determine the mass-related excitation energy. In some embodiments, it may be advantageous to weight the individual gas mass flows by a factor.

[0096] Oxygen is preferably used as the reactive gas (RG). Alternatively or additionally, nitrogen can be used as a (further) reactive gas (RG).

[0097] In the reaction chamber (302), the process gas mixture (PGM) is excited by means of an energy source (350). Preferably, a magnetron is used as the energy source (350). The plasma is preferably excited by pulsed microwaves (351). Alternatively or additionally, ICP plasmas can also be used.

[0098] The coating (200) is deposited onto the plastic container (100) from the process gas mixture excited to plasma (352).

[0099] The process chamber (301) can comprise one or more reaction chambers (302). The reaction chamber (302) can occupy the entire process chamber or only a part of it. In particular, the reaction chamber (302) can be formed inside the plastic container (100). Preferably, the container is sealed. Alternatively or additionally, another reaction chamber can be formed, e.g., on the outside of the container.

[0100] The reactor (300) can also include several process chambers (301) with one or more reaction spaces (302).

[0101] Depending on the layer structure, the size of the substrate (100), and the process parameters, the coating (200) is preferably deposited over a period of approximately 5 to 60 seconds. Longer process times may occur in particularly large containers. During the deposition process, at least one gradient layer is deposited. The mixing ratio of the process gas mixture (PGM) is varied during the process. In particular, the mass flow rate of the precursors (PG1, PG2) and / or the reactive gas (RG) is changed. This variation of the process gases influences the material composition of the coating (200).

[0102] An advantageous progression of the process parameters is in Figure 4 depicted.

[0103] Preferably, the reactive gas content (C) and / or the mass-related excitation energy (E) between the adhesion zone (201) and the barrier zone (202) is gradually increased in the layer build-up direction (x).

[0104] According to the invention, the mass-related excitation energy (E) between the barrier zone (202) and the passivation zone (203) is gradually reduced in the layer build-up direction (x). The variation of the reactive gas content (C) and the mass-related excitation energy (E) is preferably uniform, and in particular stepless.

[0105] Particularly advantageously, the excitation energy (E) is controlled depending on the composition of the process gas mixture (PGM), especially depending on the precursor gas mass flow and / or the reactive gas mass flow.

[0106] Several options are available to a person skilled in the art for controlling the mass-related excitation energy. In particular, the average pulse power can be adjusted. Advantageously, the power of the energy source and / or the pulse parameters are controlled. It is especially advantageous to calculate a setpoint for the mass-related excitation energy (E) and to control or regulate it with one or more manipulated variables, e.g., a selection or combination of (nominal) power input, pulse on-time, pulse off-time, or pulse rate.

[0107] Figure 2 Figure 1 shows a schematic structure of a coating (200) with a gradient layer (200') as a 3-zone layer. The coating (200) is deposited onto the plastic substrate (100). The deposition, i.e., the build-up in nanolayers (L), of the coating takes place in the layer build-up direction (x).

[0108] The thickness of a suitable coating (d) is preferably in the range of 80–1000 nanometers. The plastic substrate (100) is several orders of magnitude thicker. Typical wall thicknesses for plastic containers are in the millimeter range. For clarity, the plastic substrate is therefore shown as a fractional representation.

[0109] With a 3-zone layer ( Fig. 2 First, an organically influenced adhesion zone (201) is deposited on the substrate (100). Subsequently, an oxide-influenced barrier zone (202) is deposited on the adhesion zone (201), with the zones transitioning smoothly into one another. Finally, an organically influenced passivation zone (203) is deposited on the barrier zone (202).

[0110] In a 2-zone layer, the barrier zone (202) can also be deposited directly on the substrate.

[0111] The gradient layer (200') comprises at least one barrier zone (202) and one passivation zone (203). The passivation zone (203) is located between the surrounding surface (211) of the coating (200) and the barrier zone (202). The passivation zone (203) protects the barrier zone (202). The passivation zone is resistant to the influence of aggressive media (M).

[0112] The coating (200) forms a migration barrier, i.e., also a diffusion barrier, for ambient gases (A) that can diffuse through the plastic substrate (100). The formation of one or more barrier zones (202) interrupts permeation pathways. The oxide barrier zone (202) acts as a migration barrier.

[0113] The gradient layer (200') is built up during the deposition process in a multitude of nanolayers (L). The material composition of the individual nanolayers (L) changes gradually in the layer build-up direction (x).

[0114] The gradient (G) schematically shows the gradual progression of the material composition of the gradient layer.

[0115] The individual zones, i.e., the adhesion zone (201), the barrier zone (202), and the passivation zone (203), can be described based on their material composition and the proportion of specific bond types. The respective center of gravity of a zone can be considered representative of the gradually changing composition within that zone. The respective center of gravity (S1, S2, S3) of a zone can be viewed as a specific nanolayer (L) at an extremum of a particular property profile, e.g., the gradient (G), along the layer stacking direction (x).

[0116] The chemical structure of the gradient layers is defined by the extrema (centroids), in particular local or global maxima or minima, at which the maximum layer properties for the respective layer type are established, such that the process is subsequently geared towards achieving the next extremum. The chemical structure at the extrema (S1, S2, S3) is described in more detail below. Table 1 lists the preferred elemental compositions of the layers at the extrema, which were determined by X-ray photoelectron spectroscopy (XPS).

[0117] To further characterize the chemical structure of the different zones, especially that of the centers of activity within each zone, Fourier-transform infrared spectroscopy (FT-IR) is used. Table 2 lists the vibration modes important for evaluating the FTIR measurements. Table 2: Vibration mode and FT-IR peak position A b sym CH3 1376 cm -1< B s sym CH3 1257 cm -1< Si-O-Si 440 cm -1< C 1010 - 1035 cm -1< 1070 cm -1< D s sym Si-OH 930 cm -1< E r Si-CH 3 , b Si(CH3) 3 839 cm -1<

[0118] In Table 2, the prefixes denote: s = stretching; b = bending; r = rocking; as well as the subscript: sym = symmetrical.

[0119] Figures 5 and 6 These are exemplary FT-IR spectra of the extrema of the respective zones. The parameter W denotes the wavenumber with the unit cm⁻¹, and the dimensionless parameter X denotes the extinction.

[0120] Figure 5 shows FT-IR spectra at a focus of an HMDSO-based bonding agent zone (EH) and at a focus of an HMDSN-based barrier zone (WB).

[0121] The coating at the extremum of the barrier zone (WB), see focus S2 in Fig. 2, exhibits a strongly pronounced Si-O-Si network structure (440 cm⁻¹ and 1070 cm⁻¹). The broadband peak between 900 and 1300 cm⁻¹ can be mainly attributed to three antisymmetric Si-O stretching vibrations in Si-O-Si compounds. The peak around 1135 cm⁻¹ can be attributed to a Si-O-Si cage structure with bond angles of approximately 150°. Network structures with bond angles around 144° are attributed to the peak around 1070 cm⁻¹. If the structures have bond angles smaller than 144°, suboxides are assumed to be present in the networks (1010–1030 cm⁻¹). The peak at 839 cm -1< is characteristic of stretching vibrations and rocking vibrations of methyl groups and shows that at a center of gravity of the adhesion promoter zone (EH) a higher retention of these functional groups from the monomer is achieved compared to the barrier zone (WB), giving this zone an organic character.

[0122] The intensity and precise peak position of the Si-O-Si network peak provide information about the degree of crosslinking of the layers. A higher peak maximum position indicates a higher degree of Si-O-Si crosslinking. Such a peak shift is clearly visible when comparing the two spectra. The barrier zone (WB) exhibits a highly crosslinked coating with a pronounced Si-O-Si network structure, in which little C and CH are bound.

[0123] Figure 6 This shows an exemplary FT-IR spectrum of a centroid of a passivation zone. The figure depicts the FT-IR spectrum of an HMDSO-based passivation zone (EP).

[0124] The spectrum of the passivation zone in Figure 6 similar to the spectrum of the detachable zone (DZ) in Figure 5Three particularly pronounced peaks are observed. The peak in the range of approximately 700–900 cm⁻¹ is composed of the superposition of a series of individual peaks. The peak at a wavenumber of 1070 cm⁻¹ is also attributable to stretching vibrations of the Si-O-Si network. The peak at 1257 cm⁻¹ is characteristic of symmetrical stretching vibrations of methyl groups.

[0125] The chemical structure of the zones is of paramount importance, especially with regard to the barrier performance of the barrier zone and the hydrolysis stability of the passivation zone.

[0126] To characterize the chemical structure of the zones, the structure parameter V is therefore introduced, which represents the ratio of the areas (Gaussian curves) under the peaks measured by FT-IR, each of which can be clearly assigned to CH 3 - or Si-O groups (The assignment of the peaks can be found in Table 2): V = A + B + E C + D V:Structural parameters [-] A:Extinction X [-] at W = 1376 cm -1< B:Extinction cm -1< D: Extinction X [-] at W = 930 cm -1<

[0127] The barrier performance of the barrier zone is determined primarily by the degree of cross-linking and oxidation of the layers with respect to their chemical structure. For the extremum of the barrier zone (WB), an advantageous range of V < 0.2 and a particularly advantageous range of V < 0.05 were determined.

[0128] The chemical stability of the passivation zone is explained by considering the electronegativity of the elements involved in the layer material, which allows for the evaluation of local partial charge shifts in the bonds. The greater the difference in electronegativity between the bonding partners, the more strongly the bonding electrons are localized to the bonding partner with the higher electronegativity. This results in an electron boost for that element.

[0129] Among the bonds present in the layers, the oxygen in Si-O experiences the greatest electron boost. As a result, oxygen carries negative partial charges (δ -< ) and silicon carries positive partial charges (δ +< ). Silicon thus experiences an electron deficiency. Negatively charged ions in solution therefore attack the silicon bonded to oxygen due to its positive partial charges. Si-CH3 bonds, in turn, have small partial charges. The layer is thus attacked nucleophilically by the ion at Si-O, for example. This results in the formation of Si-OH bonds and the cleavage of the Si-O-Si bond. Within the Si-O network, more and more gaps are created through the introduction of anionic end groups. The CH3 groups sterically shield weak bonds, which can significantly slow down or even completely halt the reaction. The higher the ratio of CH3 groups to Si-O, the slower the degradation of the layer can proceed.The larger the V, the more durable the coating. For the extremum of the passivation zone, at least one structural parameter V > 0.20 and particularly advantageously a V > 0.35 is achieved in order to provide the necessary protective performance.

[0130] To deposit an advantageous adhesion promoter layer, a structural parameter V of 0.1 < V < 0.3 is achieved at the extremum of the adhesion promoter zone (EH).

[0131] The structural parameter V can be used to determine the oxide or organic centers and / or the functional zones of a gradient layer. Figure 2 For example, a schematic diagram of the structural parameter V(x) is shown along the layer construction direction x. The extrema (S1, S2, S3) of the structural parameter V(x) can be used to determine the centroids of the gradient layer and the individual functional zones of a gradient layer.

[0132] Depending on the layer architecture, the respective oxide or organic concentration of a zone can be located at the edge or in the center of the zone. The transitions between the zones are gradual.

[0133] The zones are preferably defined by their material composition. The material composition of the individual nanolayers determines the function of the layer zones.

[0134] To delineate the zones (201, 202, 203), e.g., to determine the thicknesses (d1, d2, d3), various analytical methods can be used. For example, the material composition of the individual nanolayers can be determined using the SIMS method (secondary ion mass spectrometry) or, furthermore, the nanoSIMS method. By bombarding the surface with ions, the layers can be ablated and analyzed at nanometer resolution.

[0135] To define the thickness of the zones, the structural parameter V or the elemental composition of the nanolayers along the layer structure direction can be determined. If the proportion of organic compounds predominates, the position of the passivation or adhesion zone can be assigned. If oxide compounds predominate, the position of the barrier zone can be assigned.

[0136] The material composition of two adjacent nanolayers (L) is always similar. This material similarity of the individual nanolayers (L) promotes homogeneous layer growth.

[0137] The advantage of the gradual build-up of the gradient layer zones is that during deposition, each nanolayer (L) finds a chemically similar base. The chemical properties of the nanolayers change only gradually. This prevents island growth.

[0138] Figure 3Figure 1 shows an exemplary structure of a coating (200) with several gradient layers (200', 200", 200‴). The structure of the gradient layers or the individual zones can be repeated in the layer build-up direction. Preferably, the zones between two successive gradient layers (200', 200") also transition gradually into one another.

[0139] The multilayer structure of the gradient layers can be particularly advantageous for enhancing the migration barrier and / or chemical resistance, i.e., the protection of the barrier zone. Alternatively or additionally, the thinness of the individual zones can still be maintained, thereby promoting homogeneous layer growth and the mechanical properties (e.g., low layer stresses) of the coating.

[0140] InIn an embodiment not specifically shown, the coating (200) can also comprise a gradient layer (200‴) with only one barrier zone (202) and one passivation zone (203). In particular, for PET The adhesion zone (201) can be omitted for substrates.

[0141] In a 2-zone layer (200‴), the barrier zone (202) is preferably arranged directly on the plastic substrate (100) or on an intermediate gradient layer (200', 200"). In a 2-zone layer, the barrier zone (202) can, in particular, have an oxide center located at the edge of the zone.

[0142] Figure 4 shows an advantageous profile of the reactive gas fraction (C) or the mass-related excitation energy (E) during the deposition of a gradient layer (200') with an adhesion zone (201), a barrier zone (202) and a passivation zone (203).

[0143] The process variables C and E can each be controlled or regulated within a specific process window (W).

[0144] The adhesion zone (201) is deposited with a low to medium proportion of reactive gas (C). In In an advantageous embodiment, at least a small amount of oxygen is added at the beginning of the process. The resulting oxygen-rich plasma activates the surface of the plastic substrate (210). In this way, prior surface activation of the substrate (100) can be omitted, thereby shortening the process times. The surface activation improves the adhesion of the coating (200) to the substrate surface.

[0145] The adhesion zone (201) protects sensitive plastic substrates (100), especially polyolefins, from the oxide barrier zone (202). In this respect, the adhesion zone (201) has a protective effect for the plastic substrate.

[0146] By gradually varying the reactive gas content (C) and / or the mass-related excitation energy (E), the gradient (G) of the material composition of the deposited layer is controlled, thus promoting homogeneous and pore-free layer growth.

[0147] In During certain phases of the deposition process, the proportion of (at least one) reactive gas (RG) in the process gas mixture can range from 0 to 100%. For the deposition of a highly oxide region, it may be advantageous to use a very high proportion of reactive gas (C), preferably 50 ≤ C ≤ 100. Alternatively or additionally, for an organic region of the gradient layer, it may be advantageous to use a low proportion of reactive gas (C) or even no reactive gas at all.

[0148] InIn a particularly advantageous embodiment, the excitation energy (E) of the plasma is controlled as a function of the current process gas composition. It is particularly advantageous to maintain the energy introduced per particle mass within a specific range and to adapt this range to the changing reactive gas fraction.

[0149] By varying the proportion of reactive gases, a wide range of material compositions can be achieved, particularly between the oxide and organic components of the gradient layer. This allows for especially strong barrier and passivation effects, providing sufficient migration barrier and chemical resistance for containers holding aggressive media.

[0150] By adjusting the mass-related excitation energy (E), the excitation, and thus the reaction, can be kept within an optimal range for homogeneous layer growth (despite highly variable gas composition). This, in particular, prevents bulk polymerization. As a result, pore-free layer deposition is possible.

[0151] The embodiments described, illustrated, or claimed here can also be combined with one another in their individual features. Features can be replaced or supplemented with individual features of the other embodiments. Reference symbol list 100 plastic substrate plastic substrate 200 coating coating 200' gradient layer coating layer 201 Contact zone contact zone 202 Barrier zone barrier zone 203 Passivation zone passivation zone 210 Substrate side, substrate surface substrate face 211 surrounding area environment face S1 organic focus organic focal point S2 oxide center oxidic focal point S3 organic focus organic focal point 300 reactor reactor 301 Trial Chamber process chamber 302 reaction chamber reaction chamber 310 pump pump 320 Gas supply Gas supply 321 Gas flow regulator gas flow controller 350 Energy source, magnetron plasma source, magnetron 351 Microwave ovens microwave 352 plasma plasma PGM process gas mixture process Gas mixture PG Precursor, polysiloxane precursor, polysiloxane PG1 (First) precursor, HMDSN (first) precursor PG2 (Second) precursor, HMDSO (second) precursor RG Reactive gas, oxygen reactive gas, oxygen A ambient gases ambient gases C Reactive gas content reactive gas content E Excitation energy, mass-related excitation energy, mass-related FC corrected process gas flow corrected process gas flow G gradient gradient K Correction factor correction factor L nanolayer nano layer M medium medium P Power input power supply r resolution resolution RGF Reactive gas mass flow reactive gas mass flow PGF Precursor mass flow precursor gas mass flow clay Pulse activation time pulse on-time toff Pulse shutdown time pulse off-time W Process window process window x Layer structure direction layer buildup direction

Claims

1. Process for the production of a coating (200) for producing a chemically resistant migration barrier on a plastic substrate (100), in particular a plastic container, by means of plasma-enhanced chemical vapor deposition (PECVD) wherein a process gas mixture (PGM) is fed into a reaction space (302) and is excited by means of an energy source (350) with an excitation energy (E) to form a plasma (352), the process gas mixture (PGM) having an adjustable composition of one or more precursors (PG) and / or one or more reactive gases (RG), wherein a gradient coat (200') with an adhesion zone (201), a barrier zone (202) and a passivation zone (203) or a gradient coat (200") with a barrier zone (202) and a passivation zone (203) is deposited on the substrate (100), characterized in that the deposition of the gradient coat (200', 200") is controlled by varying the reactive gas content (C) in the process gas mixture (PGM), wherein the excitation energy (E) is controlled on a mass-related basis as a function of the precursor gas mass flow and the reactive gas mass flow, the mass-related excitation energy (E) being gradually reduced for the deposition of an organically characterized passivation zone (203) on an oxidically characterized barrier zone (202).

2. Process according to claim 1, wherein one or more pulse parameters of the energy source (350), in particular the quotient of the pulse on-time and the pulse off-time of the excitation pulses, are kept constant at least temporarily during the deposition of the gradient coat, in particular during the deposition of the adhesion zone (201) and / or the barrier zone (202).

3. Process according to claim 1 or 2, wherein the reactive gas content (C) and / or the mass-related excitation energy (E) is gradually increased between the adhesion zone (201) and the barrier zone (202) along the layer build-up direction (x).

4. Process according to one of the preceding claims, wherein the reactive gas content (C) is gradually reduced between the barrier zone (202) and the passivation zone (203) along the layer build-up direction (x).

5. Process according to one of the preceding claims, wherein the plasma is excited by pulsed microwaves (351).

6. Process according to one of the preceding claims, wherein the power supply (P) and / or the pulse parameters, in particular the pulse on-time (ton) and / or the pulse off-time (toff), the pulse rate or the pulse ratio, of the plasma excitation is controlled in order to vary the excitation energy (E).

7. Process according to one of the preceding claims, wherein - during the deposition of the barrier zone (202) ▪ the reactive gas content (C) is 25 ≤ C ≤ 250, preferably 50 ≤ C ≤ 100, and / or ▪ the mass-related excitation energy (E) is 400 kJ / kg ≤ E ≤ 2700 kJ / kg, preferably 1800 kJ / kg ≤ E ≤ 2200 kJ / kg. - during the deposition of the passivation zone (203) ▪ the reactive gas content (C) is 0 ≤ C ≤ 20, preferably 0 ≤ C ≤ 10, and / or ▪ the mass-related excitation energy (E) is 30 kJ / kg ≤ E ≤ 1700 kJ / kg, preferably 500 kJ / kg ≤ E ≤ 800 kJ / kg. - during deposition of the adhesion zone (201) ▪ the reactive gas content (C) is 0 ≤ C ≤ 50, preferably 0 ≤ C ≤ 30, and / or ▪ the mass-related excitation energy (E) is 20 kJ / kg ≤ E ≤ 200 kJ / kg, preferably 30 kJ / kg ≤ E ≤ 140 kJ / kg.

8. Process according to one of the preceding claims, wherein the process gas mixture (PGM) comprises one or more silicon-containing precursors (PG, PG1, PG2), in particular organosilicon compounds, siloxanes, silazanes or silanes, preferably hexamethyldisiloxane (HMDSO) and / or hexamethyldisilazane (HMDSN), and / or the reactive gas (RG) comprises oxygen.

9. Process according to one of the preceding claims, wherein hexamethyldisilazane (HMDSN) is used as precursor (PG1) for depositing the barrier zone (202) and hexamethyldisiloxane (HMDSO) is used as precursor (PG2) for depositing the adhesion zone (201) and / or the passivation zone (203).

10. Coating on a plastic substrate (100), in particular for a plastic container, for producing a chemically resistant migration barrier, wherein the coating (200) comprises one or more gradient coats (200', 200", 200‴), wherein the at least one gradient coat contains organosilicon compounds (SiOxCyHz) which have been deposited from one or more silicon-containing precursors, in particular siloxanes, silazanes or silanes, wherein at least one gradient coat comprises at least one oxidically characterized barrier zone (202) and an organically characterized passivation zone (203), wherein the chemical configuration of the gradient coat changes gradually starting from the substrate face of the coating (210) along the layer build-up direction (x) and changes from at least one barrier zone (202) to a passivation zone (203), wherein the chemical structure of the gradient coat is determined by a structural parameter V which can be measured using Fourier-transform-infrared- spectroscopy (FT-IR): V = A + B + E C + D A: Extinction at W = 1376 cm-1 B: Extinction at W = 1257 cm-1 E: Extinction at W = 839 cm-1 C: Extinction at W = 440 cm-1, 1010 cm-1 to 1035 cm-1, 1070 cm-1 D: Extinction at W = 930 cm-1, characterized in that the structural parameter V at its maximum (S3) in the passivation zone (203) is V > 0.2.

11. Coating according to claim 10, wherein a gradient coat (200') additionally comprises at least one organically characterized adhesion zone (201) between a barrier zone (202) and the substrate face (210).

12. Coating according to claim 10 or 11, wherein the structural parameter V at the extremum (S2) of the barrier zone (201) is smaller than at the extremum (S3) of the passivation zone (203).

13. Coating according to one of the preceding claims, wherein the structural parameter V at the extremum (S2) of the barrier zone (201) is V < 0.2, preferably V < 0.05.

14. Coating according to one of the preceding claims, wherein the structural parameter V at the extremum (S3) of the passivation zone (203) is V > 0.35.

15. Coating according to claim 11, wherein the structural parameter V at the extremum (S1) of the adhesion zone (201) is 0.1 < V < 0.3.

16. Plastic container with a coating according to one of the preceding claims.