A method of curing a layer deposited on a substrate and a substrate made using the method.

Abstract

The present invention relates to a method of curing a layer deposited on a substrate, the layer comprising a coating formulation containing hydrolysable Si-X groups wherein X is selected from a halogen, C1-6 alkoxy, hydrogen, -NH-Si or -NH2. The coating formulation is dissolved in a solvent when applied to the substrate. The method includes drying the layer at a temperature not greater than 20 °C above the boiling point of the solvent and subjecting the layer to a curing vapour. The present invention also relates to a substrate made using this method.

Description

[0001] A method of curing a layer deposited on a substrate and a substrate made using the method.

[0002] Technical Field

[0003] The present invention relates to a method for curing of layers comprising coating formulations containing hydrolysable Si-X groups. X is selected from a halogen, C-i- Ce alkoxy, hydrogen, -NH-Si and -NH2, and a substrate comprising a coating made using the method.

[0004] Background Art

[0005] Compounds containing hydrolysable Si-X groups are commonly used in coating formulations, either as the main component or additives. Curing of such coatings involves the hydrolysis of the Si-X group to form silanols, followed by condensation reaction between the silanol groups to form Si-O-Si bond. Since they do not require or involve toxic curing agents or groups - either as part of the molecule or separate additives - they are considered environmentally friendly. Furthermore, the Si-O-Si bond is highly stable to UV and other environmental factors, as well as to a large variety of chemicals.

[0006] The Si atoms of the Si-X or other Si atoms in the molecules can also be bonded to organic groups, such as methyl, ethyl, phenyl or vinyl (CH=CH2). These Si-organic groups, being highly inert, confer good hydrophobicity and even oleophobicity to the coating. This is the reason for use of such compounds as anti-soiling coatings or modification of surfaces to confer anti-soiling behaviour. Examples of compounds containing hydrolysable Si-X group include (poly)alkoxysilanes, (poly)halosilanes, and (poly)silazanes.

[0007] Quite often coatings containing hydrolysable Si-X groups are left to cure in ambient atmosphere, that is their curing is effected by atmospheric moisture. Such a curing method is simple and environmentally friendly, since it does not utilise toxic curing agent, such as tin based compounds. However, such curing is usually top down, that is the top surface of the coating cures first or faster than the interior of the coating. The consequences of this are:

[0008] 1 . Curing will generally slow down with time, since the cured top surface can act to slow down penetration of moisture to the interior. This is especially the case when Si atoms are bonded to organic groups, e.g., methyl and vinyl in polysilazane. These Si-alkyl groups (e.g., Si-CHs) or Si-aryl (Si-phenyl groups) are hydrophobic and inert and therefore can impede moisture ingress. In fact, in organic chemistry, they are only second to carbon-fluorine bonds (e.g., CF2 or CF3) in inertness and hydrophobicity. This is the reason polydimethylsiloxane (PDMS) is widely used as a water repellent surface.

[0009] 2. Depending on weather conditions, curing may stall, leading to an uncured interior and a cured top surface and hence varying properties across the width of the coating.

[0010] Among compounds with hydrolysable Si-X groupswhich are used in coating formulations, organic polysilazane (OPSZ) is popular and widely used for coating vehicles in order to impact dirt repellence and protection of car paint from UV. OPSZ coatings are also applied to solar PV glass, windows, eyewears, touchscreen, etc as anti-soiling or easy to clean surfaces.

[0011] The commonly recommended curing method for OPSZ is to keep and cure in ambient environment, with curing (up to 95%) taking as long as 7 days. The inventors have noted that OPSZ coatings, whether commercially available or formulated in house, showed very limited or poor curing even after heating at 200°C for 1-2 hours. Therefore, the use of such high temperatures, which may not be practicable for some substrates, for example plastics, does not necessarily lead to satisfactory curing. Even after raising the relative humidity to more than 50%, the inventors found out that curing, as evidenced by the disappearance of NH (or expressed as Si-NH or Si- NH-Si) FT-IR peak at about 900 cm-1, is still poor or limited at elevated temperature. Therefore, a need for a cost effective and timely process which leads to (near) complete curing of polysilazane and other compounds containing hydrolysable Si-X groups remains. The patent application WO2023 / 282768 A1 teaches an improved curing method for polysilazane which involves the following process: a) drying in an oven maintained at 80°C for 10 mins, followed; b) curing in a high humidity atmosphere containing H2O2 vapour. The authors noted that for coating formulation which does not contain the cross-linking catalyst, tetrabutylammonium fluoride (TBAF), curing - hydrolysis of Si- NH to form silanols and subsequent condensation to yield Si-O-Si - is limited, just like a TBAF-containing coating left to cure in ambient conditions.

[0012] Tetrabutylammonium chloride (TBAC) is also known to enhance curing of polysilazane. TBAC and TBAF belong to a group called quaternary ammonium salts (QAS). QAS are easily hydrated, and this may be the reason they are able to enhance the curing of polysilazanes.

[0013] A polysilazane coating method is also described in WO 2022 / 002844 A1 . In that method, polysilazanes, catalysts (TBAF), and reactive nanomaterials and / or reactive molecules are introduced into a coating composition vessel and the solution is mixed for a predetermined time period. Then, within a further predetermined time after the mixing step, the mixed solution is applied to a substrate, and a coating layer is formed thereon and subsequently cured.

[0014] It is the objective of the current invention to improve the curing method of coatings containing hydrolysable Si-X groups. A secondary objective of this patent is to enhance the curing of polysilazane coating formulations, which does not contain QAS, such as TBAF.

[0015] Summary of invention

[0016] The present invention provides a method of curing a layer deposited on a substrate, the layer comprising a coating formulation containing hydrolysable Si-X groups wherein X is selected from a halogen, C1-C6 alkoxy, hydrogen, -NH-Si or -NH2, and wherein the coating formulation is dissolved in a solvent, the method comprising the steps of: a) heating the layer at a temperature not greater than 20 °C above the boiling point of the solvent and b) subjecting the layer to a curing vapour.

[0017] Further, the present invention provides a substrate made using the inventive method.

[0018] Detailed description of the invention

[0019] The present invention provides an improved and rapid method of curing a layer deposited on a substrate, wherein the layer comprises a coating formulation containing hydrolysable Si-X groups, wherein X is selected from halogens, alkoxy groups, hydrogen, NH-Si and NH2. Examples of compounds containing hydrolysable Si-X groups include (poly)alkoxysilanes, (poly)halosilanes, and (poly)silazane. Their hydrolysis and condensation reaction can be summarised as follows:

[0020] Si-X + H2O = Si-OH + HX . (1 )

[0021] Si-OH + Si-OH = Si-O-Si + H2O . (2) where X may be a halogen, preferably chlorine or bromine (e.g. chlorine in dimethyldichlorosilane), alkoxy group - e.g. ethoxy in (3 -aminopropyl)triethoxysilane APTES) and methoxy in methyltrimethoxysilane - hydrogen e.g. in polysilazane and NH-Si or NH2, e.g. in polysilazane. The hydrolysis of Si-NH-Si in polysilazane has a different stoichiometry and leads to formation of two silanol molecules and ammonia:

[0022] Si-NH-Si + H2O = Si-OH + Si-NH2. (3)

[0023] Si-NH2+ H2O = Si-OH + NH3 . (4)

[0024] The combined equation is therefore:

[0025] Si-NH-Si + 2H2O = 2Si-OH + NH3 . (5)

[0026] X in equations 3 and 4 are therefore NH-Si and NH2, respectively.

[0027] The coating formulation comprising hydrolysable Si-X groups wherein X is selected from a halogen, C-i-Ce alkoxy group, hydrogen, NH-Si or NH2, may be dissolved in one or more solvents. The solvent may be a polar aprotic solvent and / or a non-polar solvent. For Si-X, when X is alkoxy, polar solvents such as ethanol, acetone may be used. The solvent may be an anhydrous solvent. In some embodiments, all components are completely dissolved or suspended as a stable suspension before their introduction into the coating composition vessel. In other embodiments, one or more liquid components is introduced neat, that is without the presence of a solvent, into the coating composition vessel, whereas all other components are completely dissolved or suspended as a stable suspension before their introduction into the coating composition vessel. In some embodiments, all components are dissolved or suspended in the same solvent. In other embodiments, different solvents are used for two or more of the components. In some embodiments, a solvent is chosen from the list comprising, but not limited to, dimethyl sulfoxide (DMSO), n-butyl acetate, tertbutyl acetate, tetrahydrofuran (THF), dimethylformamide (DMF), 2-methyl- tetrahydrofuran (MTHF), dibutyl ether (DBE), xylene. In preferred embodiments, coating compositions comprising substantial amounts of PHPS comprise only nonpolar solvents such as dibutyl ether and xylene.

[0028] The substrate may be glass, ceramic, metal, polymer (plastic), wood or a composite. The substrate may be rigid or flexible.

[0029] When the coating formulation is deposited as a layer on a substrate, the layer is heated at a temperature not greater than 20 °C above the boiling point of the solvent. Preferably, the temperature is not greater than 10 °C above the boiling point of the solvent. Alternatively, the temperature may be equal to the boiling point. In embodiments of the invention, the layers may be heated at temperatures 10 to 20°C lower than the boiling point of the solvent. In one embodiment, wherein butyl acetate is used as the solvent, the temperature may be 30 to 60 °C lower than the boiling point of the solvent.

[0030] As previously stated, curing in a moisturized atmosphere is a top-down process. This means that curing starts at, or is faster at, the top layer which is more exposed to moisture. A natural consequence of this is that curing can get stalled due to the formation of a cured and impervious layer. This cured top layer, after reaching a sufficient thickness, will allow for a dry-to-touch state and good surface properties (e.g., high water contact angle) even without depth curing. However, depth curing is important to ensure a product that will be durable in demanding applications. The heating step may promote more uniform drying through the thickness of the layer. Overdrying or non-uniform drying means a more dry top surface compared to an underlayer which might be wetter.

[0031] The temperature and the duration of the heating step may preferably be controlled so that the coating layer does not become completely dried.

[0032] Drying means densifying the coating, which involves closer packing of the polysilazane molecules. This results in more severe steric hindrance and greater obstruction of the curing vapor. Closer packing of polysilazane molecules may lead to the quicker formation of an inert top segment during the drying and / or early stages of the curing vapor treatment, which then prevents the ingress of the curing vapor to the hydrolysable polar Si-X groups below.

[0033] The drying may be performed within a period of not more than 20 to 30 minutes. The drying may be performed during a period of between 10 seconds to 30 minutes, preferably between 20 seconds to 20 minutes, preferably between 10 seconds to 10 minutes, preferably between 20 seconds to 5 minutes, preferably between 20 seconds to 2 minutes, or preferably between 30 seconds to 1 .5 minutes.

[0034] The curing vapour comprise compounds known in the art to hydrolyse or promote the hydrolysis of Si-X groups. The curing vapour may comprise water, H2O2, NH3, or organic / inorganic acids or a combination of two or more compounds.

[0035] The curing compound may exist originally as a liquid and therefore will have to be transformed to the vapour phase. Generation of the vapour may be accomplished by any suitable process, for example using heat.

[0036] Generation of vapour may be accomplished using any other known process known in the art, such as processes used in generating water vapour from water - for example for the purpose of humidification. Examples include ultrasonic and impeller humidifiers. A preferred embodiment is generation of the curing vapour using an ultrasonic humidifier. An ultrasonic humidifier uses ultrasonic frequency to atomize (vapourize) liquid into fine mist and may be referred to in this patent document as ultrasonic vapourizer.

[0037] An impeller vapour generator uses a disc (impeller) rotating at high speed to break liquid into fine droplets which can float in air and may be referred to in this patent document as impeller vapourizer.

[0038] The curing vapour may comprise a mixture of water and H2O2. The curing vapour may comprise from 1 to 30 wt% H2O2 in water, preferably from 5 to 25 wt% H2O2 in water. Preferably, the concentration of H2O2 in water is 10 wt% or above.

[0039] The curing vapour may comprise a mixture of ammonia and water. The curing vapour may comprise from 0.25 to 30 wt% NH3 in water, preferably from 2 to 20 wt% NH3 in water.

[0040] The aforementioned percentages (of H2O2 and NH3) in water refer to the compositions of the liquid from which the curing vapour is generated. It is assumed that the generated vapour will have the same concentrations as the liquid.

[0041] After the coating layer is subjected to the curing vapour, it may be heated to complete or further enhance the curing process. Preferably the temperature does not exceed 200 °C for 60 minutes or less.

[0042] In the curing experiments described below, the coating formulations contain polysilazanes. The curing method can also be performed on coating formulations containing hydrolysable Si-X groups wherein X is selected from a halogen or an alkoxy group. EXPERIMENTS

[0043] General note for the experiments:

[0044] • Except otherwise indicated, the curing vapour is 20 wt% H2O2 in water.

[0045] • For the sake of simplicity, % H2O2 vapour means a vapour generated from an aqueous solution containing same percentage of H2O2. That is, the concentration of H2O2 in the liquid and vapour phases are considered to be the same.

[0046] • The H2O2 vapour is generated from an ultrasonic humidifier and redirected or uniformly distributed by flowing air - from the oven fan. The vapour produced by the humidifier is soft and easily swayed so the redirecting and distributing air / gas is essential for uniform treatment of samples.

[0047] • Curing vapour is any vapour capable of hydrolysing the polar groups in polysilazane to form silanols (Si-OH).

[0048] • Curing is rated from 6.75 (practically no curing) to 1 (excellent curing): 1 -1.125 (excellent curing, EC); 1.25-1.5 (very very good curing, VVGC); 1.75-2 (very good curing, VGC); 2.25-2.75 (good curing, GC); 3-3.5, (fair curing, FC); 3.75 (borderline curing, BC); 4-4.5 (poor curing, PC); 4.75-5.5 (very poor curing, VPC) and 5.75-6.75 (extremely poor curing, EPC).

[0049] • Curing rating is based on FT-IR measurement.

[0050] • D1500RC is an organic polysilazane with (3-aminopropyl)triethoxysilane (APTES) to ensure rapid curing. Structure is believed to be:

[0051] (a, b and c are proprietary information)

[0052] • 7640 Permanent Profi Protect (PP) is a commercial formulation of organic polysilazane from Creative Chemical Manufacturers, CCM. According to CCM, it contains APTES. n-butyl acetate is the main solvent and accounts for 35% - <55% of the composition while organic polysilazane accounts for 15% - <35% and APTES accounts for 5% - <10%.

[0053] • Boiling point of n-butyl acetate is 126 °C.

[0054] • TBAF is a solution of tetrabutylammonium fluoride in THF (0.5 wt%). TBAF acts as cross-linking catalyst to prevent or minimize fragmentation.

[0055] • TG4 is a solution of TEGO® Glide 410, (from Evonik) in THF (5.5 wt%). TG4 is a polyether modified siloxane and acts as a levelling agent.

[0056] • Boiling point of THF is 66 °C.

[0057] • B17 is a solution of a dumbbell POSS (10 wt% in THF). The structure is shown below:

[0058] • A85 is a solution of an open cage POSS (trisilanol isobutyl POSS, sourced from HybridPlastics) - 10 wt% in THF. The structure is shown below:

[0059] R = isobutyl

[0060] Coating formulations:

[0061] • B 17-66:

[0062] A: 2000 pl D1500RC + 1000 pl TG4 B: 2000 pl B17 POSS + 660 pl TBAF + 250 pl THF

[0063] • B 17-80

[0064] A: 1182 pl D1500RC + 590 pl TG4

[0065] B: 1420 pl B17 POSS + 400 pl TBAF + 2318 pl THF

[0066] • A85-8

[0067] A: 1182 pl D1500RC + 590 pl TG4

[0068] B: 1182 pl A85 + 400 pl TBAF + 2550 pl THF

[0069] • PP-1 :

[0070] 3940 pl PP + 3940 pl n-butyl acetate

[0071] • D1500-R2:

[0072] 2000 pl D1500RC + 3910 pl THF

[0073] • D1500-R9:

[0074] 2000 pl D1500 RC + 9820 pl n-butyl acetate jeriment 1

[0075] • Coating formulation (B17-66) was prepared by combining two solutions:

[0076] A: 2000 pl D1500 RC + 1000 pl TG4

[0077] B: 2000 pl B17 POSS + 660 pl TBAF + 250 pl THF

[0078] The two solutions were mixed by vortexing and allowed to stand for 5 minutes before ultrasonic spray coating.

[0079] • The formulation was applied to glass slide substrates with the ultrasonic spray coater using the following parameters: flow rate of 0.8 ml / min, velocity 25 mm / s, line spacing 10 mm, air pressure 0.45 bar, height 50 mm and run power 36%.

[0080] • After drying, the coatings were cured by subjecting them to H2O2 vapour in an oven maintained at 70 °C for 10 minutes, and thereafter heated in an oven at 120 °C for 1 hour.

[0081] • The drying conditions and curing levels attained are indicated in Table 1 and indicate that drying at 55 °C led to better curing than 70 °C which was better than 100 °C for the same duration of drying.

[0082] • The cured coatings have an average thickness of 8.5 pm. When drying was done for 30 minutes at 55 °C, the curing level was about the same or similar to drying at 100 °C for 10 minutes. This indicates that even at lower drying temperature, a short drying duration is favourable for enhanced curing.

[0083] Table 1

[0084] Experiment 2

[0085] Coating formulation (A85-8) was prepared by combining two solutions:

[0086] A: 1182 pl D1500 RC + 590 pl TG4

[0087] B: 1182 pl A85 POSS + 400 pl TBAF + 2250 pl THF

[0088] The two solutions were mixed by vortexing and allowed to stand for 5 minutes before ultrasonic spray coating.

[0089] The formulation was applied to glass slide substrates with the ultrasonic spray coater using the following parameters: flow rate of 0.8 ml / min, velocity 25 mm / s, line spacing 10 mm, air pressure 0.45 bar, height 50 mm and run power 36%.

[0090] After drying, the coatings were cured by subjecting them to a high humidity atmosphere containing H2O2 vapour at 70 °C for 10 minutes, and thereafter heated in an oven at 120 °C for 1 hr.

[0091] The drying conditions and curing levels attained are indicated in table 2 and indicate that drying at 55 °C for 2 minutes led to better curing than same temperature for 10 minutes which was in turn better than 100 °C for 10 minutes.

[0092] The cured coatings have an average thickness of 3.7 pm. Table 2

[0093] Experiment 3

[0094] • Two solutions were prepared:

[0095] A: 200 pl D1500 RC + 100 pl TG4 + 700 pl THF

[0096] B: 100 pl A85 POSS + 32 pl TBAF + 868 pl THF

[0097] • Coating formulation was applied to glass slide substrates with the ultrasonic spray coater thus: o Part B was sprayed first: flow rate 0.4 ml / min, velocity 25 mm / s, line spacing 10 mm, air pressure 0.45 bar, height 50 mm and run power 36%. o Part A was sprayed 4-5 minutes later with same parameters.

[0098] • After drying, the coatings were cured by subjecting them to a high humidity atmosphere containing H2O2 vapour at 70 °C for 10 minutes, and thereafter heated in an oven at 120 °C for 1 hour.

[0099] • Average thickness of cured coatings was 2.3-2.5 pm.

[0100] • Samples were dried at 70 °C for 30, 60 and 80 seconds. The curing level achieved was similar at 1 .25 to 1 .5. This indicates that a quick exposure to warm air is sufficient to dry the coating for the curing process.

[0101] Experiment 4

[0102] • Coating formulation (PP-1 ): 3940 pl PP + 3940 butyl acetate

[0103] • The formulation was applied to glass slide substrates with the ultrasonic spray coater thus: flow rate 0.8 ml / min, velocity 35 mm / s, line spacing 7 mm, air pressure 0.45 bar, height 50 mm and run power 36 %.

[0104] • After drying, the coatings were cured by subjecting them to a high humidity atmosphere containing H2O2 vapour at 70 °C for 10 minutes, and thereafter heated in an oven at 120 °C for 1 hour. • Average thickness of cured coatings was 4-4.5 pm.

[0105] • The drying conditions and curing levels attained are indicated in table 3 and indicate that drying at 60 °C for 5 minutes is slightly better than 115 °C for 5 minutes and both of them significantly better than 160 °C for 5 minutes.

[0106] Sample 2 which was dried at 70 °C for 1 .5 minutes gave a better curing than 60 °C for 5 minutes, which indicates that the even with high boiling point solvents, low temperature drying in short duration of less than 5 minutes is sufficient to prepare the coating for curing.

[0107] Table 3

[0108] • Coating formulation (PP-1): 3940 pl PP + 3940 butyl acetate

[0109] • The formulation was applied to glass slide substrates with the ultrasonic spray coater thus: flow rate 0.8 ml / min, velocity 35 mm / s, line spacing 7 mm, air pressure 0.45 bar, height 50 mm and run power 36 %.

[0110] • After drying, the coatings were cured by subjecting them to a high humidity atmosphere containing H2O2 vapour at 70 °C for 10 minutes, and thereafter heated in an oven at 120 °C for 1 hr. The concentration of liquid H2O2 in the ultrasonic humidifier was varied from 20 to 2.5 wt%.

[0111] • Average thickness of cured coatings was about 4.5 pm.

[0112] • The drying and curing conditions and curing levels attained are indicated in Table 4. The results indicate that at as low as 5% H2O2 concentration, curing level attained varied from fair to good. Table 4 jeriment 6

[0113] • Two coating formulations were prepared: o B17-80 (Parts A and B were mixed by vortexing for 5 minutes)

[0114] A: 1182 pl D1500 RC + 590 pl TG4

[0115] B: 1420 pl B17 POSS + 400 pl TBAF + 2318 pl THF o D1500-R2

[0116] 2000 pl D1500RC + 3910 pl THF

[0117] • The formulations were applied to glass slide substrates with the ultrasonic spray coater thus; flow rate 0.3 ml / min, line spacing 7 mm, air pressure 0.45 bar, height 50 mm and run power 36 %. Velocities for B 17-80 and D1500-R2 were 50 and 40 mm, respectively. B17-80 was applied in two layers, i.e. sprayed twice, while D1500-R2 was a single layer.

[0118] • After drying at 70 °C for 1 .5 min, the coatings were cured by subjecting them to a high humidity atmosphere containing H2O2 vapour at 70 °C for 1.5 minutes. Thereafter the coatings were heated for specific duration.

[0119] • Average thickness of cured coatings was about 3 pm.

[0120] • The post- H2O2 treatment conditions are listed in table 5 and indicates that:

[0121] 1 ) Heating for 10, 30 and 60 minutes at 120 °C does not provide significant changes in curing level for B17-80, while for D1500-R2, 30 and 60 minutes heating at same temperature showed slightly better curing than 10 minutes heating. However, the later gave a curing level of good to very good. This shows that a shorter post- H2O2 heating period of 10 mins or less or lower temperature heating is sufficient to achieve good curing.

[0122] 2) Without post- H2O2 heating, B17-80 curing level is fair to good, while for D1500-R2, curing level varies from poor to extremely poor. The presence of TBAF, a quaternary ammonium salt (QAS) in B17-80 improved curing, as previously disclosed in WO2023 / 282768 A1 , while its absence in D1500-R2 lead to limited curing without the additional post-H2O2 exposure heating.

[0123] Table 5

[0124] As can be seen from the experiments above, the curing method according to the invention shows that excellent curing of a coating formulation deposited as a layer on a substrate can be obtained at low temperature and within short curing period.

Claims

Claims1 . A method of curing a layer deposited on a substrate, the layer comprising a coating formulation containing hydrolysable Si-X groups wherein X is selected from a halogen, C1-6 alkoxy, hydrogen, -NH-Si or -NH2, and wherein the coating formulation is dissolved in a solvent, the method comprising the steps of: a) drying the layer at a temperature not greater than 20 °C above the boiling point of the solvent, and b) subjecting the layer to a curing vapour.

2. The method of claim 1 , wherein steps a) and b) are performed simultaneously.3 The method of claim 1 or 2, wherein the curing vapour comprises one or more of water, H2O2, NH3, organic and inorganic acids.

4. The method of any of the preceding claims, wherein the curing vapour contains from 1 to 30 wt% H2O2 in water.

5. The method of any one of the preceding claims, wherein drying is performed at temperature is not greater than 10 °C above the boiling point of the solvent.

6. The method of any one of the preceding claims, wherein step a) is performed in a period not more than 20 to 30 minutes.

7. The method of any one of the preceding claims, wherein step a) is performed during a period of between 10 seconds to 30 minutes, preferably between 30 seconds to 5 minutes.

8. The method of any one of the preceding claims wherein after the layer is subjected to the curing vapour it is heated to a temperature at or above the boiling point of the solvent.

9. The method of claim 8, wherein the temperature is below 200 °C for 60 minutes or less.

10. The method of any of the preceding claims, wherein X is selected from chlorine, bromine, methoxy, ethoxy, hydrogen, NH-Si or NH2. 11 . The method of any one of the preceding claims wherein the solvent is selected from tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF), n-butyl acetate, tert-butyl acetate or dibutyl ether (DBE).

12. A substrate made using the method of any one of the preceding claims.

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