WO2022061410A1

WO2022061410A1 - Coating process

Info

Publication number: WO2022061410A1
Application number: PCT/AU2021/051114
Authority: WO
Inventors: Simon John KING
Original assignee: Nanokote Pty Ltd
Priority date: 2020-09-24
Filing date: 2021-09-23
Publication date: 2022-03-31

Abstract

A process for producing a gas barrier coating on a metal, broadly comprising the steps of applying a gas barrier coating solution to the metal to form a gas barrier coating on the metal, wherein the gas barrier coating is predominantly silicon nitride; and applying a top coat solution to the gas barrier coating.

Description

COATING PROCESS

Technical Field

[0001] The invention relates to a coating process for metal, particularly for producing a gas barrier on a metal surface. The process is particularly suitable for the manufacture of coated metals, such as stainless steel and aluminium, for use in the manufacture of consumer goods.

Background of Invention

[0002] Metal surfaces, such as stainless steel, aluminium or zinc, used in the fabrication of consumer goods are usually coated by prefabrication processes such as as spray coating, slot die coating, roller coating, dip coating, flood coating and coil coating. A spray coating process is generally defined as a process in which a coating material is sprayed onto a metal surface. A slot die coating process is generally defined as a process in which a coating material is applied to a metal surface as a solution, slurry or extruded thin film. A roller coating process is generally defined as a process in which a coating material is applied to a metal surface using rollers. A coil coating process is generally defined as a process in which a coating material is applied on a rolled metal strip in a continuous process. These prefabrication processes may include cleaning, chemical pre-treatment of the metal surface and single or multiple coating treatments on either one-side or two-side which are subsequently cured.

[0003] The coating material maybe be a liquid coating or a coating powder. The coating material is often an organic material and available coatings include polyesters, polyurethanes, polyvinylidene fluorides (PVDF), epoxies, primers, backing coats and laminate films. For each product, the coating may be built up in multiple layers, including primer coatings and topcoats. The coated metal strip can then be fabricated into the desired form.

[0004] At elevated temperatures, all metals react with hot gases. The most common high-temperature gaseous mixture is air, of which oxygen is the most reactive component. For example, stainless steel has a thin layer of chromium oxide on the surface that provides important corrosion protection to the base surface. When exposed to high heat the chromium oxidises and discolours in the form of heat tint. This oxidation process not only renders the stainless steel aesthetically unattractive it also leaves the surface exposed to corrosion. This limits the use of metals such as stainless steel in high temperature applications.

[0005] Applying coatings to metal surfaces to address this issue is possible. These gas barrier coatings often come with long curing times and in most cases require high temperature. For example, coatings such as TEOS (tetraethyl orthosilicate), TMOS (tetramethyl orthosilicate) and MTES (methyltriethoxysilane) mixtures have been used in the past to provide protection to stainless steel. These coatings however are limited in that they require high temperature conversion which is time consuming and expensive on large scale. Other gas barrier coatings including zirconium oxide-based materials have also been used but these require chemical vapor deposition (CVD) application techniques that are both slow and expensive.

[0006] In addition to being time-consuming and expensive, the resulting coating from these previously described systems is very brittle and not flexible. This means that these coating systems are not suitable for prefabrication coating processes as fabrication cannot occur post coating.

[0007] Inorganic polysilazanes have been used to create gas barrier technology. These materials have been created to produce thin silicon dioxide barriers at a variety of temperatures including down to room temperature. These barriers are sufficiently flexible to allow for post-coating fabrication, meaning that they can be used in prefabrication coating processes such as spray coating and slot die coating processes. Nevertheless, these coatings take time to convert to silicon dioxide. This means that the length of time between the application of coats is impractically long for the coating process to be completed in a single run on any coating line. For this reason, these coatings have not been widely applied in prefabrication coating processes to allow for large areas of stainless steel to be coated.

[0008] There is therefore an ongoing need for improved coating systems and processes, which at least partially addresses one or more of the above-mentioned shortcomings or provides a useful alternative. [0009] In addition to being suitable for prefabrication coating, gas barrier coatings for metal surfaces should also maintain other useful properties to widen their utility. These properties include corrosion resistance, scratch resistance, easy cleaning, transparency, food contact safety and cost efficiency.

[0010] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of Invention

[0011] In a first aspect the invention provides a process for producing a gas barrier coating on a metal, comprising the steps of: applying a applying a gas barrier coating solution to the metal to form a gas barrier coating on the metal, wherein the gas barrier coating solution comprising a polysilazane or a mixture of polysilazanes and at least one solvent; curing the coating on the metal at a coating curing temperature and for a coating curing time to produce a gas barrier coating; wherein the gas barrier coating is predominantly silicon nitride; applying a top coat solution to the gas barrier coating, wherein the top coat solution comprises a polysilazane or a mixture of polysilazanes, and at least one solvent; and curing the top coat solution on the metal at a top coat curing temperature and for a top coat curing time to form a top coat..

[0012] The coatings of the present invention provide excellent gas barrier properties. Additionally, the coatings of the present invention are flexible and therefore allow for post-coating fabrication. This is advantageous as they can be used in prefabrication processes such as slot die coating or spray coating. Furthermore, the coatings of the present invention provide excellent corrosion and scratch resistance. They also provide excellent easy clean properties, as they provide surfaces that are both hydrophobic and oleophobic, and they are food contact safe. This makes them desirable for use in the consumer goods and appliance industry, particularly for use in ovens, cooktops and other heating appliances. [0013] The present invention also allows for mass, fast processing. In one embodiment, the process of the present invention is a single run coating process where the coating and top coat can be applied and cured in less than 60 seconds each to produce a durable coating with excellent gas barrier and high temperature, easy clean properties. This is a significant reduction in curing times and temperatures compared with conventional processes.

[0014] The process of the present invention allows access to remarkably high speed and high-volume prefabrication processes such as spray coating and slot die coating. These coating systems can be applied in relatively inexpensive processing lines with short oven sections, which have a small footprint.

[0015] Hence, the present invention allows large areas of metal to be coated at a price point that allows for these metals to be used in high temperature applications. This is especially attractive in the appliance industry, particularly where the coated steel is intended for use in high temperature applications such as in consumer goods. The invention can be applied to a large variety of metals such as of stainless steel, aluminium, zinc and other metals. The process of the present invention allows rapid access to gas barrier coatings in both a cost- and time- efficient manner.

[0016] Further aspects of the invention appear below in the detailed description of the invention.

Brief Description of Drawings

[0017] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[0018] Figure 1 shows an FT-IR spectrum of the coating of the present invention shortly after application and immediately prior to the application of the top coat, as discussed in Example 1.3.

[0019] Figure 2 shows an FT-IR spectrum of the top coating of the present invention shortly after application, as discussed in Example 1 .3. [0020] Figure 3 is a photograph showing the performance of a coating in accordance with the presence invention under pyrolytic self-clean conditions, as discussed in Example 1.6.

Detailed Description

[0021] The present invention relates to a process for producing a gas barrier coating on a metal. The invention also relates to a coating system for use in this process.

[0022] In one embodiment, the process of the present invention comprises the steps of: applying a gas barrier coating solution to the metal to form a gas barrier coating on the metal, wherein the gas barrier coating solution comprising a polysilazane or a mixture of polysilazanes and at least one solvent; curing the coating on the metal at a coating curing temperature and for a coating curing time to produce a gas barrier coating; wherein the gas barrier coating is predominantly silicon nitride; applying a top coat solution to the gas barrier coating, wherein the top coat solution comprises a polysilazane or a mixture of polysilazanes, and at least one solvent; and curing the top coat solution on the metal at a top coat curing temperature and for a top coat curing time to form a top coat.

[0023] The process of the present invention produces a gas barrier coating on a metal. A gas barrier coating protects the metal surfaces from reaction with gases, especially at high temperature. The most common high-temperature gaseous mixture is air, of which oxygen is the most reactive component. High temperature oxidation can cause discolouration as well as reducing the durability and corrosion resistance of the metal. An effective gas barrier coating will delay or prevent oxidation at high temperatures. In some embodiments of the present invention, a metal surface treated with the gas barrier coating of the present invention shows little to no change when heated at 350 ^SC PMT for 24 hours, preferably for 72 hours, more preferably for 140 hours. In some embodiments, a stainless steel surface treated with the gas barrier coating of the present invention shows little to no change when heated at 350 ^SC PMT for 24 hours, preferably for 72 hours, more preferably for 140 hours. In some embodiments of the present invention, a metal surface treated with the gas barrier coating of the present invention shows little to no change when heated at 500 ^SC PMT for 6 hours, preferably for 12 hours, more preferably for 24 hours. In some embodiments, a stainless steel surface treated with the gas barrier coating of the present invention shows little to no change when heated at 500 ^SC PMT for 6 hours, preferably for 12 hours, more preferably for 24 hours.

[0024] Polysilazanes are polymers of silicon and nitrogen with the general formula [R¹R²Si-NR³]_n where R¹-R³ can be hydrogen atoms or organic substituents. If all substituents R are H atoms, then the polymer is commonly referred to as an inorganic polysilazane or perhydropolysilazane (PHPS). If one or more of R¹, R² and R³ is a hydrocarbon then it is known as an organopolysilazane.

[0025] Polysilazanes suitable for use in the present invention have the general formula [R¹R²Si-NR³]_n where R¹, R² and R³ are identical or different and independently of one another represent hydrogen or an optionally substituted alkyl, aryl, vinyl or (trialkoxysilyl)alkyl group and n is a whole number. Polysilazanes suitable for use in the present invention preferably have a numerical average molecular weight of 150 to 250 000 g/mol, in a suitable solvent.

[0026] Heating polysilazanes results in crosslinking to form higher molecular weight polymers. In general, liquid materials will be converted to solids as the temperature increases. At 400-700 °C, the organic groups decompose with the evolution of small hydrocarbon molecules, ammonia and hydrogen. Between 700 and 1200 °C a three-dimensional amorphous network develops containing Si, C and N (referred to as “SiCN ceramics"). A further temperature increase can result in crystallization of the amorphous material and the formation of ceramic materials such as silicon dioxide, silicon nitride, silicon carbide and carbon. These processes can be accelerated by including catalysts in the coating system, allowing curing of a coating to take place at lower temperatures, although this can affect the quality of the resulting coating properties.

[0027] Furthermore, in conventional polysilazane coating systems, the coating takes significant time to cure, even in the presence of a catalyst; usually at least 60 minutes, or even longer to convert to silicon dioxide prior to the application of a top coat. The base coating generally also needs moisture in the form of high humidity to complete the conversion process. The present invention has surprisingly found that a silicon dioxide base coating is not required to produce the desired performance results. The invention shows that a high performance gas barrier coating can be achieved without full conversion of the base coating to silicon dioxide before application of a top coat.

[0028] The gas barrier coating of the present invention is a ceramic material. Particularly, the gas barrier coating is predominantly silicon nitride. In some preferred embodiments of the invention, the gas barrier coating will be at least 50% silicon nitride, preferably at least 60% silicon nitride, more preferably at least 70% silicon nitride, even more preferably at least 80% silicon nitride, most preferably at least 90% silicon nitride.

[0029] The process of the present invention is suitable for coating metal surfaces. Metals suitable for use in the present invention include stainless steel, galvanised steel, aluminium, aluminium alloys, zinc, zinc/iron, zinc-based alloys, chromium alloys, titanium, tinplate, brass and copper.

[0030] In a preferred embodiment, the process of the present invention is a prefabrication coating process selected from the group consisting of spray coating, slot die coating, roller coating and coil coating. In a more preferred embodiment, the process of the present invention is a spray coating process. In another more preferred embodiment, the process of the present invention is a slot die coating process. A person skilled in the art will recognise, however, that the coating system of the present invention can also be applied using other suitable techniques such as flooding, dipping, spraying, slot die, roller coating, coil coating, doctor blade and gravure coating techniques.

[0031] The skilled person will appreciate that the at least one solvent in the gas barrier coating solution and the top coat solution can be any solvent suitable for use with polysilazanes. Solvents preferable for use with polysilazanes are, in particular, organic solvents without water and without reactive groups (such as hydroxyl groups or amine groups) which might react with polysilazane. Examples of suitable solvents are as follows: an aliphatic hydrocarbon, an alicyclic hydrocarbon and an aromatic hydrocarbon such as pentane, hexane, cyclohexane, toluene, xylene, and turpentine; a halogenated hydrocarbon solvent such as methylene chloride and trichloroethane; an ester such as ethyl acetate and butyl acetate; a ketone such as acetone and methyl ethyl ketone; an aliphatic ether such as dibutyl ether; an alicyclic ether such as dioxane and tetrahydrofuran; and alkylene glycol dialkyl ether and polyalkylene glycol dialkyl ethers (such as diglyme). Preferably the solvent is independently selected from the group consisting of aliphatic or aromatic hydrocarbons, halogenated hydrocarbons, pentane, hexane, cyclohexane, toluene, xylene, turpentine; methylene chloride, trichloroethane, ethyl acetate, butyl acetate, acetone, methyl ethyl ketone, tetrahydrofuran, dibutyl ether, dioxane , alkylene glycol dialkyl ether, polyalkylene glycol dialkyl ether, diglyme, and mixtures thereof. Most preferably, the solvent is dibutyl ether or n-butyl acetate. Preferably, the fraction of polysilazane in the solvent in the top coat solution and the coating solution is 1% to 80% by weight polysilazane, preferably 5% to 50% by weight, more preferably 10% to 40% by weight.

[0032] The skilled person will appreciate that the gas barrier coating solution may comprises further components such as adhesion agents, crosslinking agents, wetting agents, viscosity modifiers, smoothing agents and combinations thereof. Examples of such components are 3-aminopropyltriethoxysilane (APTES) and polyether siloxane copolymers such as Tego™ Glide.

[0033] The skilled person will appreciate that the at least one catalyst in the gas barrier coating solution and the top coat solution can be any solvent suitable for catalysing the reaction of polysilazanes. Preferably, the at least one catalyst in the coating solution and the top coat solution is independently selected from the group consisting of ammonia, methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, n-propylamine, isopropylamine, di-n-propylamine, diisopropylamine, tri-n-propylamine, n-butylamine, isobutylamine, di-n-butylamine, diisobutylamine, tri-n-butylamine, n-pentylamine, di-n-pentylamine, tri-n-pentylamine, dicyclohexylamine, aniline, 2,4-dimethylpyridine, 4,4-trimethylenebis(1 - methylpiperidine), 1 ,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1 ,5-diazabicyclo[4.3. 0]non-5-ene (DBN), 1 ,4-diazabicyclo[2.2.2]octane (DABCO), N,N-dimethylpiperazine, cis-2,6-dimethylpiperazine, trans-2,5-dimethylpiperazine, 4,4- methylenebis(cyclohexylamine), stearylamine, 1 ,3-di(4-piperidyl)propane, N,N- dimethylpropanolamine, N,N-dimethylhexanolamine, N,N-dimethyloctanolamine, N,N- diethylethanolamine, 1 -piperidineethanol, 4-piperidinol, palladium, palladium acetate, palladium acetylacetonate, palladium propionate, nickel, nickel acetylacetonate, silver powder, silver acetylacetonate, platinum, platinum acetylacetonate, ruthenium, ruthenium acetylacetonate, ruthenium carbonyls, gold, copper, copper acetylacetonate, aluminium acetylacetonate, and mixtures thereof. More preferably, the at least one catalyst in the coating solution and the top coat solution is independently selected from the group consisting of is palladium propionate, 1 ,8- diazabicyclo[5.4.0]undec-7-ene (DBU), 1 ,5-diazabicyclo[4.3. 0]non-5-ene (DBN), aluminium acetylacetonate, 4, 4-trimethylenebis(1 -methylpiperidine), and mixtures thereof. Preferably, the catalyst in the top coat solution and the coating solution is present in an amount of 0.001 % to 10%, preferably 0.001 % to 6%, more preferably 0.001 % to 3%, based on the weight of the polysilazane.

[0034] The skilled person will appreciate that the coating curing time and the coating curing temperature are tuneable properties that assist in providing a gas barrier coating with excellent gas barrier properties and that does not crack or delaminate when the coated surface is placed under heat. In some embodiments of the present invention, the gas barrier coating curing time is 10-180 seconds at 260 ^SC PMT (Peak Metal Temperature). Preferably, the gas barrier coating curing time is around 60 seconds at 260 ^SC PMT. In certain aspects of the invention, the gas barrier coating curing temperature is 200-450 ^SC PMT. Preferably, the gas barrier coating curing temperature is 220-290 ^SC PMT.

[0035] In some embodiments of the present invention, the top coat curing time is 10-180 seconds, at 260 ^SC PMT. Preferably, the top coat curing time is around 60 seconds, at 260 ^SC PMT. In certain aspects of the invention, the top coat curing temperature is 100-450 ^SC PMT. Preferably, the top coat curing temperature is around 260 ^SC PMT.

[0036] The skilled person will appreciate that the composition of the coating will change during the curing process, as the polysilazanes react with one another. The skilled person will also appreciate that the composition of the coating may further change over time after curing as the coating reacts with the atmosphere. Reaction with air and humidity can lead to an increase in silicon dioxide content. Without being bound by theory, it is speculated that application of a top coat can assist to slow or halt this post-curing reaction. It is speculated that there may be some diffusion through the top coat after application which results in further post-curing reaction of the base coat. In some embodiments of the present invention, the top coat is applied within less than 30 minutes of curing the coating, preferably less than 10 minutes, preferably less than 5 minutes, preferably less than 3 minutes, more preferably less than 120 seconds.

[0037] The skilled person will also appreciate that the composition of the top coat may further change over time after curing as the top coat reacts with the atmosphere. Reaction with air and humidity can lead to an increase in silicon dioxide content. While a thermal cure process achieves the first step cure, the ambient conditions over the following days may complete the chemical cure of the top coat.

[0038] In a preferred embodiment, the coating of the present invention after curing has a dry film thickness of 50-2000 nm, preferably 200-600 nm, more preferably around 250 - 500 nm. It will be appreciated that the thickness of the film depends on the roughness of the surface and, in general, the flatter the surface then the less base coat required.

[0039] The skilled person will appreciate that above a thickness of about 400 nm a red colour (the low end of the visible light spectrum) can be observed in the coating which, while not affecting the efficiency of the coating, is undesirable from an aesthetic viewpoint. Interference at coating thicknesses from 400 - 700nm can generate all the colours of visible light spectrum, which can result in a rainbow effect. These discolourations can be at least somewhat addressed by application of a top coat. The skilled person will also appreciate that while the base coat can be applied in its own right and achieve performance, the addition of a top coat further improves performance from an easy clean and pyrolytic cleaning performance point of view. The additional thickness delivered by the top coat along assists with these properties. Increasing the thickness of the base coat in order to achieve similar properties is possible but becomes too expensive. The skilled person will appreciate that, in one aspect, it is desirable to minimize the base coat thickness while achieving the desired performance from a colour change point of view. The top coat can then assist in providing the additional protection and other properties. [0040] In a preferred embodiment of the present invention, the top coat after curing has a dry film thickness of 0.1 -50 pm, preferably around 1 pm, even more preferably between around 0.3 and 0.5 pm. This still allows for performance and flexibility while addressing any issues with discolouration in the coating.

[0041] In some embodiments, the process of the present invention comprises application of a base coat comprising an inorganic polysilazane at 250 - 260 ² C (PMT) for 20-60 seconds followed by an almost immediate top coat of an organic polysilazane at 250 - 260 ² C (PMT) for 40-60 seconds. In a further embodiment, the process of the present invention comprises application of a base coat comprising an inorganic polysilazane at 250 - 260 ² C (PMT) for 60 seconds followed by an almost immediate top coat of an organic polysilazane at 250 - 260 ² C (PMT) for 60 seconds.

[0042] As used herein, peak metal temperature (PMT) refers to the maximum temperature achieved by the metal substrate during the drying/curing process.

[0043] Where the terms “comprise”, “comprises” and “comprising” are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[0044] The term "and/or" as used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example "A and/or B" is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein.

Examples

[0045] The invention will now be further explained and illustrated by reference to the following non-limiting examples.

[0046] Materials and Methods

[0047] Polysilazanes were supplied by Merck, Germany. Commercial Durazane RC 1500 and Durazane SC 1500 both consist of 3-aminopropyltriethoxysilane substituted polymethyl-(hydro)/polymethylsilazane. Commercial Durazane 2400, 2600 and 2800 all consist of perhydropolysilazane (PHPS) in solution (20 % w/w in dibutyl ether) with various catalysts, and Durazane 2200 consists of perhydropolysilazane in solution (20 % w/w in dibutyl ether) with no catalyst. Dry solvents were used from commercial suppliers. [0048] FT-IR analyses have been recorded on a Perkin Elmer Spectrum 100 instrument, exploiting the attenuated total reflectance (UATR) geometry in the wavenumber range 5500-650 cm^-1 (4851 data points, 4 cm^-1 resolution, force applied I N 88) using a diamond/ZnSe crystal combination as an internal reflective element. [0049] Example 1.1: Coating Preparations

[0050] Table 1 shows a number of example coating preparations.

Table 1 - Example base coat preparations

PHPS refers to perhydropolysilazane. TMDHA refers to N,N,N',N'-tetramethyl-1 ,6- diaminohexane.

[0051 ] Example 1.2: Method of Top Coat Preparation

[0052] 76.09 g of n-butyl acetate was weighed out into a vessel to which 20 g of 3- aminopropyltriethoxysilane substituted polymethyl-(hydro)/polymethylsilazane

(Durazane RC 1500) was slowly added while stirring. After 5 minutes, 3.5 g of 3- aminopropyltriethoxysilane (APTES) was added followed by 0.009 g of 2,3,4,6,7,8,9,10-Octahydropyrimidol[1 ,2-a]azepine (DBU) and 0.2 g of Tego™ Glide 410. Stirring was continued for a further 5 minutes. The resulting liquid was poured into an air tight vessel that had been evacuated of air by nitrogen or argon blanketing.

[0053] Using a similar process to Example 1 .2, a number of top coat examples were prepared, and these are shown in Table 2.

Table 2 - Example top coating preparations

Solvent is n-butyl acetate. DBU refers to 1 ,8-diazabicyclo[5.4.0]undec-7-ene, also known as 2,3,4,6,7,8,9,10-octahydropyrimidol[1 ,2-a]azepine. DBN refers to 1 ,5-Diazabicyclo [4.3.0]non- 5-ene. APTES refers to 3-aminopropyltriethoxysilane. [0054] Example 1.3: Method of Application

[0055] A stainless steel sheet (#4 finish) was prepared by cleaning with isopropyl alcohol. The sheet was fully allowed to dry. Using a Lab Doctor blade applicator at 2 micron WFT, base coating (Example 1 .1 .1 ) was applied to the sheet in the direction of the grain. The sample sheet was left for 30 - 45 seconds before being placed in a fan forced oven at 270 ^SC for 60 seconds. Peak Metal Temperature (PMT) reached 260 ^SC during this time. The sample was removed from the furnace and quenched. The sample coating was clear and flat at this point making it ideal for the top coat application. The dry film thickness was approximately 328 nm (confirmed using light reflectance). FTIR at this point showed about 0.5 - 2.5% Si-H along with no significant Si-0 peaks which indicated the coating was not fully cured and more than 90% Si-N (see Fig. 1 ). To this coated sample, the top coat (Example 1 .2) was applied by drawdown bar. The sample was then placed immediately in a fan forced oven for 60 seconds at 270 ^SC. The sample was removed from the furnace and quenched. The resulting film was clear and flat. As shown in Figure 2, FTIR at this point shows no Si-H which indicates full cure of the top coat. It was noted that the top coat will still continue to cure over the coming weeks to reach full chemical cure with a larger Si-0 proportion than that shown in the FTIR.

[0056] The FTIR spectrum of the base coating is shown in Figure 1 . The peaks at 820 cm^-1 is indicative of Si-N bonds, indicating that most of the coating has not yet converted to silicon dioxide. The peaks at 944 cm^-1 and 1040 cm^-1 may be indicative of Si-0 bonds, suggesting that a small amount of silicon dioxide is present after the curing process. The peak at 2184 cm’¹ is indicative of Si-H bonds, suggesting that the film is not fully cured. It also indicates that the film does not need to be fully cured prior to application of top coat.

[0057] Without wishing to be bound by theory, it is posited that the conversion of the base coating to silicon dioxide is slowed or halted by the top coat. The inorganic polysilazane of the base coating requires moisture and oxygen to convert from silicon nitride to silicon dioxide. It is theorised that either the conversion still takes place under the top coat in a very short period of time or that a dense silicon nitride coating provides for excellent barrier properties. It is speculated that there may be some diffusion through the top coat after application which results in further post-curing reaction of the base coat.

[0058] Example 1.4

[0059] A 20% w/w solution of inorganic polysilazane in dibutyl ether without catalyst (perhydropolysilazane, Durazane 2200, Example 1 .1 .4) was applied to a piece of 2B stainless steel and cured at 260 ^SC (PMT) for 60 seconds. This was then immediately top coated (Example 1 .2) and cured for 60 seconds at 260 ^SC (PMT).

[0060] This sample was then placed in a furnace at 350 ^SC. No colour change was detected in the first 10 minutes. By comparison, uncoated stainless was shown to change colour at 350 ^SC in the first 5 minutes. This result illustrates that the coating has effective gas barrier properties. This short test is particularly relevant for both the Durazane 2200 base and any of the catalysed versions as 10 minutes is not enough time for silicon dioxide conversion. Further, upon leaving the sample in the furnace for a further 18 hours also results in no colour change.

[0061] Other coating systems were prepared as follows: This result provides further evidence that the gas barrier of the present invention is predominantly dense Si-N or silicon nitride. The gas barrier is shown to be effective before silicon dioxide had sufficient time to form. Durazane 2200 (a pure inorganic polysilazane without any catalyst) generally requires a temperature of 500 °C and higher along with significant time (>1 h) to convert to silicon dioxide (as per supplier datasheets).

[0062] Other coating systems were prepared and tested in accordance with the procedure of Example 1 .4 as follows:

Table 3 - Results of Furnace Testing on Various Coating Systems on Stainless Steel

[0063] Example 1.5

[0064] A 20% w/w solution of inorganic polysilazane in dibutyl ether with Pd propionate catalyst (perhydropolysilazane, Durazane 2400, Example 1.1.1 ) was applied to a piece of 2B stainless steel and cured at 260 °C (PMT) for 60 seconds. This was then immediately top coated (Example 1 .2) and cured for 60 seconds at 260 °C (PMT).

[0065] A butane blow torch was then immediately applied to the coated surface. No colour change of the stainless-steel surface was detected. By comparison, uncoated stainless was shown to change colour under the butane blow torch within 30 seconds. This result illustrates that the coating has effective gas barrier properties.

[0066] Other coating systems were prepared and tested in accordance with the procedure of Example 1 .5, trialling the various base coats of Example 1 . All observations were consistent with Example 1 .5.

[0067] Example 2 - Testing

[0068] The coating system of Example 1 .3 was subject to a number of experiments to test the gas barrier properties, corrosion resistance properties, scratch resistance properties, easy cleaning properties, transparency and food contact safety properties. The results of the experiments are shown in Tables 4-6. Table 4 - Results of Various Tests of Coating System on Stainless Steel

Table 5 - Results of Coating System on Stainless Steel in Chemical Resistance

Tests

Table 6 - Results of Various Tests of Coating System on Stainless Steel

[0069] As can be seen from these results, the coating system of Example 1 .3 has good to excellent gas barrier properties, corrosion resistance properties, scratch resistance properties and easy cleaning properties even after heat, chemical and mechanical stress. The coating system of Example 1 .3 is also shown to be food contact safe.

[0070] Example 3 - Pyrolytic cleaning tests

[0071] The coating system of Example 1 .3 was subject to a pyrolytic cleaning test, as follows.

[0072] Ingredients: 165g ready-made Traditional Bisto gravy, 80g Crisco

[0073] Method:

• Added Crisco & contents of gravy sachet to a heat proof bowl.

• Covered & heated in microwave at low power for 60s intervals until Crisco was entirely melted.

• Used a small whisk to mix thoroughly until thick paste formed.

• Applied soil to sample evenly using a pastry brush.

• Baked at 250 °C for 2 hours (including preheat). Allowed sample to cool.

• Operated pyrolytic self-clean cycle (450 ^SC for 4 hours including heat up time - nominally 3.5 hours @ 450 ^SC).

[0074] The results, shown in Figure 3, illustrate that after the pyrolytic clean cycle, the coated surface is pristine and the clean was successful as no food staining or residue is evidenced. The coated side of the sample shows no colour change, cracking, flaking or other damage to the stainless steel. In contrast, the uncoated side is heavily discoloured and stained with food residue ash and left uncleanable.

***

[0075] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Claims

23 CLAIMS

1 . A process for producing a gas barrier coating on a metal, comprising the steps of: applying a gas barrier coating solution to the metal to form a gas barrier coating on the metal, wherein the gas barrier coating solution comprising a polysilazane or a mixture of polysilazanes and at least one solvent; curing the coating on the metal at a coating curing temperature and for a coating curing time to produce a gas barrier coating; wherein the gas barrier coating is predominantly silicon nitride; applying a top coat solution to the gas barrier coating, wherein the top coat solution comprises a polysilazane or a mixture of polysilazanes, and at least one solvent; and curing the top coat solution on the metal at a top coat curing temperature and for a top coat curing time to form a top coat.

2. The process of claim 1 , wherein the gas barrier coating after curing has a dry film thickness of around 50-2000 nm.

3. The process of claim 2, wherein the gas barrier coating after curing has a dry film thickness of around 200-600 nm.

4. The process of claim 2, wherein the gas barrier coating after curing has a dry film thickness of around 250-500 nm.

5. The process of any one of claims 1 to 4, wherein the top coat after curing has a dry film thickness of around 0.1 -50 pm.

6. The process of claim 5, wherein the top coat after curing has a dry film thickness of around 1 pm.

7. The process of claim 6, wherein the top coat after curing has a dry film thickness of between around 0.3 and 0.5 pm.

8. The process of any one of claims 1 to 7, wherein the top coat is applied within less than 30 minutes of curing the gas barrier coating.

9. The process of claim 8, wherein the top coat is applied within less than 5 minutes of curing the gas barrier coating.

10. The process of claim 8, wherein the top coat is applied within less than 120 seconds of curing the gas barrier coating.

11 . The process of any one of claims 1 to 10, wherein the gas barrier coating curing time is less than 15 minutes, at 260 ^SC PMT (Peak Metal Temperature).

12. The process of claim 11 , wherein the gas barrier coating curing time is between 10-180 seconds, at 260 ^SC PMT.

13. The process of claim 11 , wherein the gas barrier coating curing time is around 60 seconds, at 260 ^SC PMT.

14. The process of any one of claims 1 to 13, wherein the top coat curing time is less than 15 minutes, at 260 ^SC PMT.

15. The process of claim 14, wherein the top coat curing time is between 10-180 seconds, at 260 ^SC PMT.

16. The process of claim 14, wherein the top coat curing time is around 60 seconds, at 260 ^SC PMT.

17. The process of any one of claims 1 to 16, wherein the gas barrier coating curing temperature is 200-450 ^SC PMT.

18. The process of claim 17, wherein the gas barrier coating curing temperature is around 220-290 ^SC PMT.

19. The process of any one of claims 1 to 18, wherein the top coat curing temperature is 100-450 ^SC PMT.

20. The process of claim 19, wherein the top coat curing temperature is around 260 ^SC PMT.

21 . The process of any one of claims 1 to 20, wherein the at least one solvent in the coating solution and/or the top coat solution is independently selected from the group consisting of aliphatic or aromatic hydrocarbons, halogenated hydrocarbons, pentane, hexane, cyclohexane, toluene, xylene, turpentine; methylene chloride, trichloroethane, ethyl acetate, butyl acetate, acetone, methyl ethyl ketone, tetrahydrofuran, dibutyl ether, dioxane , alkylene glycol dialkyl ether, polyalkylene glycol dialkyl ether, diglyme, parachlorobenzotrifluoride, and mixtures thereof; and preferably is dibutyl ether or n- butyl acetate.

22. The process of claim 21 , wherein the at least one solvent in the coating solution and/or the top coat solution is dibutyl ether or n-butyl acetate.

23. The process of any one of claims 1 to 22, wherein the gas barrier coating solution and/or the top coat solution further comprise at least one catalyst.

24. The process of any one of claims 1 to 23, wherein the at least one catalyst in the gas barrier coating solution and/or the top coat solution is independently selected from the group consisting of ammonia, methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, n-propylamine, isopropylamine, di-n- propylamine, diisopropylamine, tri-n-propylamine, n-butylamine, isobutylamine, di-n- butylamine, diisobutylamine, tri-n-butylamine, n-pentylamine, di-n-pentylamine, tri-n- pentylamine, dicyclohexylamine, aniline, 2,4-dimethylpyridine, 4,4-trimethylenebis(1 - methylpiperidine), 1 ,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1 ,5-diazabicyclo[4.3. 0]non-5-ene (DBN), 1 ,4-diazabicyclo[2.2.2]octane (DABCO), N,N-dimethylpiperazine, cis-2,6-dimethylpiperazine, trans-2,5-dimethylpiperazine, 4,4- methylenebis(cyclohexylamine), stearylamine, 1 ,3-di(4-piperidyl)propane, N,N- 26 dimethylpropanolamine, N,N-dimethylhexanolamine, N,N-dimethyloctanolamine, N,N- diethylethanolamine, 1 -piperidineethanol, 4-piperidinol, palladium, palladium acetate, palladium acetylacetonate, palladium propionate, nickel, nickel acetylacetonate, silver powder, silver acetylacetonate, platinum, platinum acetylacetonate, ruthenium, ruthenium acetylacetonate, ruthenium carbonyls, gold, copper, copper acetylacetonate, aluminium acetylacetonate, and mixtures thereof.

25. The process of claim 24, wherein the at least one catalyst in the gas barrier coating solution and/or the top coat solution is independently selected from the group consisting of palladium propionate, 1 ,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1 ,5- diazabicyclo[4.3. 0]non-5-ene (DBN), aluminium acetylacetonate, 4,4- trimethylenebis(1 -methylpiperidine) and mixtures thereof.

26. The process of any one of claims 1 to 25, wherein the fraction of polysilazane in the solvent in the top coat solution and/or the gas barrier coating solution is 1 % to 80% by weight polysilazane.

27. The process of claim 26, wherein the fraction of polysilazane in the solvent in the top coat solution and/or the gas barrier coating solution is 5% to 50% by weight polysilazane.

28. The process of claim 26, wherein the fraction of polysilazane in the solvent in the top coat solution and the gas barrier coating solution is 10% to 40% by weight polysilazane.

29. The process of any one of claims 1 to 28, wherein the catalyst in the top coat solution and/or the gas barrier coating solution is present in an amount of 0.001 % to 10%, based on the weight of the polysilazane.

30. The process of claim 29, wherein the catalyst in the top coat solution and/or the gas barrier coating solution is present in an amount of 0.001 % to 6%, based on the weight of the polysilazane. 27

31 . The process of claim 29, wherein the catalyst in the top coat solution and/or the gas barrier coating solution is present in an amount of 0.001 % to 3%, based on the weight of the polysilazane.

32. The process of any one of claims 1 to 31 , wherein a metal surface treated with the gas barrier coating of the present invention shows little to no change when heated at 350 ^SC PMT for 24 hours.

33. The process of claim 32, wherein a metal surface treated with the gas barrier coating of the present invention shows little to no change when heated at 350 ^SC PMT for 72 hours.

34. The process of claim 33, wherein a metal surface treated with the gas barrier coating of the present invention shows little to no change when heated at 350 ^SC PMT for 140 hours.

35. The process of any one of claims 1 to 34, wherein a metal surface treated with the gas barrier coating of the present invention shows little to no change when heated at 500 ^SC PMT for 6 hours, preferably for 12 hours, more preferably for 24 hours.