TECHNICAL FIELD

The present disclosure relates to a method for preparing a polyimide -coated carbon steel substrate, a method for preparing a polyamic acid intermediate, and the polyamic acid intermediate obtained thereof.

BACKGROUND OF THE DISCLOSURE

Corrosion, a thermodynamically favorable chemical and electrochemical process, is one of the most challenging problems encountered in the marine, aerospace and steel industry. Annually, the corrosion damages cost the United States and Japan alone an approximately US$ 350 billion and ¥4 trillion, respectively (3-4% GDP for both countries). The cost of corrosion will grow further as the demand for steel manufacturing rises in the developed countries in the use of everyday domestic applications to the construction of complex engineering structures for industrial purposes. Therefore, corrosion mitigation is an important subject of increasing interest to the steel industry. Zinc coating is being used in most of the steel plants in the world as it provides continuous barrier protection which does not allow water to penetrate. Nevertheless, zinc coating also gets oxidized over the time due to exposure to moisture and atmospheric pollutants. The products of zinc oxidation have a high surface area and produce blisters that adversely affect the appearance of the covering paint work. In this regard, organic coatings have evolved as an excellent solution to improve corrosion resistance on metal substrate by forming an insulating barrier which prevents the passage of corrosion ions and molecules to the metal surfaces. Organic coating efficiency largely depends on the mechanical properties of the resin system and the adhesion of the coating to the underlying metal base. They are expected to provide a continuous, homogeneous coating that prevents water permeability and physical aging over a long period of time to justify the cost. However, most of the conventional organic polymers show poor adhesion to the carbon steel leading to reduction in coating integrity and corrosion resistance performance. For example, organic coatings based on epoxy resins such as diglycidylether bisphenol A (DGEBA) and its derivatives have been deposited directly on metal substrates as corrosion resistant coatings. This process also involved creating a thin oxide film on the metal surface, which was intended to increase adhesion between the coating and the substrate. Despite the presence of the oxide film, coating adhesion was unsatisfactory due to reactive oxirane (-CH2-CH-O ) groups characteristic of the epoxy, largely disappearing upon curing. As a consequence of the poor adhesion, a reduction in corrosion resistance was observed.

In view of this, there is a clear need for an innovative and cost friendly organic coating having reliable substrate/coatings interface adhesion, and superior corrosion resistance properties over prolonged period of time. Further, the coating should provide necessary flexibility so that there is no coating peeled off during any forming operation of the metal sheet. Interestingly, polyimides (Pls) are considered to be high performance materials in the electronics and aerospace industries due to their excellent mechanical properties, thermo -oxidative stability as well as good adhesion, electric properties and chemical stability, i.e., they provide many of the surface characteristics for which organic coatings are pursued. However, polyimides of this type are semicrystalline and rigid due to the presence of ordered aromatic groups along the polymeric backbone resulting poor flexibility and formability. Forming operation of such kind of polyimide coated metal sheets can have a detrimental effect on coating integrity and corrosion resistance due to crack formation. On the other hand, rigid aromatic groups provide the polyimide necessary stiffness, mechanical properties and corrosion resistance.

Prior studies have explored preparation of polyimide coatings for steel or other metal substrates. US20120225309A1 is directed to a polyetherimide coating on a steel substrate, where said polyetherimide is prepared by polymerization of an aromatic anhydride with two diamines. WO2011154132 discloses a polyetherimide-coated steel substrate, where said polyetherimide is prepared by polymerization of an aromatic anhydride with two aliphatic polyetherdiamines. WO2015028143 is directed to a corrosion protective coating for steel substrates, wherein the corrosion protective coating comprises a water-soluble synthetic polymer such as a polyimide or a polyetherimide and a carbohydrate. W02015151509 discloses a coating film for a steel sheet, wherein the coating film contains a polyimide and a cured silicone. JP7090616 is directed to a high-strength alloyed hot-dip galvanized steel sheet having good low-temperature chipping resistance, perforated corrosion resistance, and spot weldability, wherein the steel sheet comprises an organic film coating having a thickness of 0.01-100 pm on one side of the steel sheet. The organic film is formed of an organic compound such as epoxy resin, polyethylene, polystyrene, polyacetylene, polyester, nylon, and polyimide. US2014150247A1 discloses a steel substrate suitable for forming operations comprising a corrosion protective coating wherein the corrosion protective coating comprises a layer of polyamide -imide (PAI), wherein the PAI layer further comprises a hydroxyl amine. USI0,876,017 discloses a coating layer for metal or polymer surfaces comprising a composite of a fluoropolymer and a thermoplastic polyimide. KR101686695 discloses a composite coating comprising one or more fluoropolymer; one or more epoxy resin; and one or more polyamideimide. CN105838239A discloses a polyimide composite coating prepared by polymerizing an ether and a dianhydride. JP2000033337 discloses a polyimide resin paint coated pleated steel sheet where a thick coating of polyimide is applied by repeating the process of coating and drying twice. The polyimide is prepared by reacting an anhydride and a diamine.

Huttunen-Saarivirta et al. (“Corrosion protection of galvanized steel by polyimide coatings: EIS and SEM investigations”, Progress in Organic Coatings, Volume 72, Issue 3, 2011, pages 269-278) disclose a comparison of two types of polyimide (PI) coatings - poly(4,4'- oxydiphtalic anhydride-co-2,5-bis(4,4'-methylenedianiline)-l,4-benzoquino ne) (AQ) and poly(pyromellitic dianhydride-co-4,4'-oxydianiline) (PM) - synthesized on galvanized steel panels. The results showed that, although both studied PI coatings provided the galvanized steel substrate with corrosion protection during the test period, there were differences in electrochemical behaviour of the coatings.

Qian et al. (“Electrochemical Impedance Spectroscopy Investigation of a Polyimide Coating on Q345 Steel”, Int. J. Electrochem. Sci., 12 (2017) 3063-3071) studied corrosion characteristics of a polyimide coating having different thicknesses prepared by reacting an anhydride and a diamine.

Sezer Hicyilmaz and Celik Bedeloglu (“Applications of polyimide coatings: a review”, SN Appl. Sci. 3, 363 (2021)) discuss properties and applications of polyimide coatings. The article discloses that polyimides are prepared by reacting an anhydride and a diamine.

Malav et al. disclose (“Enhancement of corrosion protection of low nickel austenitic stainless steel by electroactive polyimide-CuO composites coating in chloride environment”, AntiCorrosion Methods and Materials, Vol. 66 No. 6, pp. 774-781) a composite coating comprising polyimide and copper oxide.

The polyimide coatings disclosed in the art still have certain drawbacks such as rigidity due to the presence of ordered aromatic groups resulting in poor flexibility and formability, high cost of monomers, etc. It is necessary to keep a balance between aromatic and aliphatic groups in the polymer backbone in designing a polyimide with superior corrosion resistance as well as flexibility and formability. Further, for the development of polyimide coating on metal substrate, technical challenges to overcome include a high cost of the monomers and selection of right monomer combinations which will give enhanced corrosion resistance but at the same time will provide essential flexibility and formability. The present disclosure attempts to address these challenges and drawbacks associated with polyimide coatings provided in the art.

STATEMENT OF THE DISCLOSURE

The present disclosure provides a method for preparing a polyimide-coated steel substrate, comprising: a) providing an aliphatic diamine and an aromatic diamine in an organic solvent; b) adding a first aromatic dianhydride and a second aromatic dianhydride in the organic solvent to form a reaction mixture; c) stirring the reaction mixture in an inert atmosphere to obtain a polyamic acid intermediate; d) applying the polyamic acid intermediate to a steel substrate; and e) curing the polyamic acid intermediate to obtain the polyimide -coated steel substrate.

The present disclosure further provides a method for preparing a polyamic acid intermediate, comprising: a) providing an aliphatic diamine and an aromatic diamine in an organic solvent; b) adding a first aromatic dianhydride and a second aromatic dianhydride in the organic solvent to form a reaction mixture; and c) stirring the reaction mixture in an inert atmosphere to obtain the polyamic acid intermediate.

The present disclosure provides polyimide-coated steel substrates and polyamic acid intermediates obtained by the methods disclosed herein.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

Figure 1 shows a 'H-NMR spectrum of polyamic acid intermediate of Example 2 (PAA-2) in DMSO-d6 at 298K.

Figure 2 shows a 'H-NMR spectrum of polyamic acid intermediate of Example 4 (PAA-4) in DMSO-d6 at 298K.

Figure 3 shows FTIR spectra of polyamic acid intermediates (PAIs) at 298K.

Figure 4 shows FTIR spectra of polyimides (Pls) at 298K. Figure 5 shows a TGA curve for polyimides.

Figure 6 shows images of polyimide-coated mild steel substrate samples after a SST test for 10 days.

Figure 7 shows a Bode impedance plot of polyimide coated steel samples.

Figure 8 shows an Electrochemical Equivalent Circuits (EECs) used to fit the Bode impedance plot of (panel (a)) PI-1 and PI-3 coated steel (panel (b)) PI-2 and PI -4 coated steel.

DETAILED DESCRIPTION OF THE DISCLOSURE

With respect to the use of any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” or “has” or “having” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Reference throughout this specification to “some embodiments”, “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in some embodiments”, “in one embodiment” or “in an embodiment” in various places throughout this specification may not necessarily all refer to the same embodiment. It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. The term “about” as used herein encompasses variations of +/-10% and more preferably +/- 5%, as such variations are appropriate for practicing the present invention.

Several methods are disclosed in the art for preparing polyimide-coated steel substrates. However, the polyimide coatings provided in the art have certain drawbacks such as poor flexibility and formability, high cost of monomers, etc. The polyimide coatings disclosed in the art are mainly prepared by polymerizing a single dianhydride with one or more diamines. The present disclosure provides a method of designing and preparing a low-cost polyimide coating on metal surfaces having anticorrosion, flexibility and formability properties. The polyimide intermediate which is polyamic acid (PAA) prepared in accordance with the present disclosure comprises a first dianhydride, a second dianhydride, a first diamine and a second diamine. The first and the second dianhydrides contain rigid aromatic groups which contribute to improve stiffness, mechanical property, thermal resistance and corrosion resistance of the polyimide coating on steel surface. The two dianhydrides are copolymerized with the first diamine and the second diamine which is a long -chain aliphatic diamine to bring the amorphous nature in the polyimide coating having enhanced flexibility and formability as well. The polyamic acid (PAA) intermediates in organic solvent are applied to the metal substrate and then cured at elevated temperature to furnish polyimide coating having excellent corrosion resistance and formability properties.

The present disclosure is directed to developing polyimide coating for carbon steel substrates based on two aromatic dianhydrides, a first diamine and a second aliphatic long chain diamine. These precursors provide desirable properties like good adhesion to the metal substrate, corrosion resistance and formability.

A method for preparing a polyimide-coated substrate according to the present disclosure broadly comprises preparing a polyamic acid intermediate (PAI) by employing two aromatic dianhydrides, an aromatic diamine and an aliphatic diamine; applying the PAI to the substrate; and curing the substrate to obtain the polyimide -coated substrate.

In some embodiments, a method for preparing a polyimide-coated steel substrate comprises: a) providing an aliphatic diamine and an aromatic diamine in an organic solvent; b) adding a first aromatic dianhydride and a second aromatic dianhydride in the organic solvent to form a reaction mixture; c) stirring the reaction mixture in an inert atmosphere to obtain a polyamic acid intermediate; d) applying the polyamic acid intermediate to a steel substrate; and e) curing the polyamic acid intermediate to obtain the polyimide-coated steel substrate. The present disclosure also provides a method for preparing a polyamic acid intermediate (PAI) comprising: a) providing an aliphatic diamine and an aromatic diamine in an organic solvent; b) adding a first aromatic dianhydride and a second aromatic dianhydride in the organic solvent to form a reaction mixture; and c) stirring the reaction mixture in an inert atmosphere to obtain the polyamic acid intermediate.

An exemplary schematic showing the preparation of the PAI; applying it to a substrate; and curing the PAI to obtain the polyimide coating according to the present disclosure is shown below:

Aromatic dianhydride 1 Aromatic dianhydride 2 Aromatic diamine Aliphatic diamine

Solvent

170-250 °C i Applied on the steel substrates

Scheme I In some embodiments, the aromatic groups, Ar ¹ , Ar ² , and Ar ³ are benzene, biphenyl, benzophenone, diphenyl ether etc.

In some embodiments, the aliphatic diamine (NH2-R-NH2) is a linear or branched C3-C12 alkyl diamine or a Jeff amine. Accordingly, in some embodiments, the “R” group in the aliphatic diamine is a linear or branched C3-C12 alkyl; a linear or branched C3-C10 alkyl; a linear or branched C3-C8 alkyl; a linear or branched C5-C12 alkyl; a linear or branched C5-C10 alkyl; a linear or branched Cs-Cs alkyl; a linear or branched C6-C12 alkyl; a linear or branched Ce-Cio alkyl; a linear or branched Ce-Cs alkyl; or a linear or branched C8-C12 alkyl. In some embodiments, the “R” group in the aliphatic diamine is a polyether comprising a propylene oxide, ethylene oxide or mixed propylene oxide/ethylene oxide repeating unit. The “R” group of the aliphatic diamine is a long chain group to provide amorphous characteristics, enhanced flexibility and formability to the polyimide coating.

In some embodiments, the linear or branched C3-C12 alkyl diamine is selected from propane-

1.3-diamine; propane- 1,2-diamine; 2-methylpropane-l,2-diamine; 2,2-Dimethyl-l,3- propanediamine; butane -1,4 -diamine; pentane-I,5-diamine; pentane-2,3-diamine; pentane-

1.4-diamine; I,5-diamino-2-methylpentane; hexane- 1,6-diamine; 1,7-diaminoheptane; 1,8- diaminooctane; 1,9-diaminononane; 1,10-diaminodecane; 1,11 -undecanediamine; or 1,12- diaminododecane. In an exemplary embodiment, the aliphatic diamine is hexane-I,6-diamine (also called hexamethylenediamine (HMDA)).

In some embodiments, the aliphatic diamine is a Jeff amine. The term “Jeff amine” as used herein refers to a polyether compound that contains at least one primary amino group attached to a terminus of a polyether backbone, wherein the polyether backbone is based on propylene oxide, ethylene oxide or mixed propylene oxide/ethylene oxide. In some embodiments, the Jeff amine is selected from O,O’-Bis(2-aminopropyl) polypropylene glycol-block-polyethylene glycol-block polypropylene glycol; 4,7,10-trioxa-I, 13-tridecanediamine; polypropylene glycol) bis(2-aminopropyl) ether having a molecular weight 230; polypropylene glycol)bis(2- aminopropyl) ether having a molecular weight 400; l,2-bis(2 -aminoethoxy ethane); 4,9-dioxa- 1,12-dodecanediamine; l,l l-Diamino-3,6,9-trioxaundecane; 2,2'-

(ethylenedioxy)bis(ethylamine); 2, 2-bis(aminoethoxy propane; or l,8-diamino-3,6- dioxaoctane. In some embodiments, the aromatic diamine has a structure of formula (A): NH2-Ar ³ -NH2. In some embodiments, the Ar ³ groups are phenyl, diphenyl ether, triphenyl ether, diphenyl methane etc.

In some embodiments, the aromatic diamine is a monoaromatic diamine or an aromatic polyetherdiamine or an aromatic methyldiamine. In some embodiments, the aromatic diamine is a meta substituted monoaromatic diamine such as m-phenylenediamine (MPA). In some embodiments, the aromatic diamine is selected from an aromatic polyetherdiamine such 4,4’- Oxydianiline; 4,4’-(l,3-Phenylenedioxy) dianiline; 4,4'-(4,4'-isopropylidenediphenyl-l,l'- diyldioxy)dianiline; 4,4'-(l,l'-Biphenyl-4,4'-diyldioxy)dianiline. In some embodiments, the aromatic diamine is selected from an aromatic methyldiamine such as 4,4'- diaminodiphenylmethane or 4, 4'-methylene-bis(2 -methylaniline).

The methods of the present disclosure employ two aromatic dianhydrides for synthesizing the PAI and the polyimide coating. The aromatic dianhydrides employed in the present methods have a structure of formula (B):

Aromatic dianhydride

(B).

In some embodiments, the aromatic group, Ar (in Scheme I - Ar ¹ and Ar ² ), in formula (B) is benzene, biphenyl, benzophenone, diphenyl ether etc. The dianhydrides having formula (B) comprise aromatic groups which improve the corrosion resistance and the mechanical properties of the polyimide coating.

In some embodiments, the first and the second aromatic dianhydride are selected from 3,3’,4,4’-Biphenyltetracarboxylic dianhydride (BPDA), benzophenone-3,3’,4,4’- tetracarboxylic dianhydride (BTDA), pyrometallic dianhydride (PMDA), 4,4 ’-Bisphenol adianhydride (BPADA), 4,4 ’-oxydiphthalic anhydride (ODPA); 4,4’-(hexafluoro- isopropylidene) diphthalic anhydride (FDA); and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (HFDA). In the methods of the present disclosure, the aliphatic diamine and the aromatic diamine are added to an organic solvent in an inert atmosphere followed by addition of the first and the second dianhydrides to form a reaction mixture. The reaction mixture is stirred in the inert atmosphere to obtain a polyamic acid intermediate (PAI). The PAI has a structure of formula (C):

(C).

In some embodiments, the organic solvent is selected from dimethylacetamide (DMAc), N- methylpyrrolidone (NMP), dimethylformamide (DMF), or a combination thereof.

In some embodiments, the reaction mixture comprising the first and the second dianhydride, the aliphatic diamine and the aromatic diamine in the organic solvent is stirred in the inert atmosphere for about 16-20 hours at about 20-30°C to obtain the PAI. In some embodiments, the inert atmosphere is provided by nitrogen, argon, or a combination thereof.

The PAI is applied to the surface of a steel substrate and is cured to convert the PAI to a polyimide. In some embodiments, the PAI is cured at a temperature of about 170-250°C for about 5-15 minutes to obtain the polyimide-coated steel substrate. For example, in some embodiments, the PAI is cured at a temperature of about 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, or 250 °C for about 5, 10, or 15 minutes to obtain a polyimide coating on the steel substrate. In some embodiments, the polyimide has a structure of formula (D):

(D). In exemplary embodiments, polyimides obtained by the present methods have the following structures:

In some embodiments, the polyimide coatings provided by the present methods have a thickness of about 1 to 10 microns, including values and ranges thereof, such as about 1-9, 1- 8, 1-7, 1-5, 1-4, 2-10, 2-8, 2-7, 2-5, 3-10, 3-8, 3-5, 5-10, 5-8, 6-10, 6-9, 7-10, or 8-10 microns. In some embodiments, the polyimide coatings provided by the present methods show a glass transition temperature (Tg) of about 150-250°C, including values and ranges thereof. In some embodiments, the polyimide coatings provided by the present methods show a glass transition temperature (Tg) of about 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245 or 250 °C. In exemplary embodiments, the polyimides show a Tg of about 180, 187, 194 or 220 °C.

In some embodiments, the polyimide coatings provided by the present methods are stable up to a temperature of about 300, 325, 350, 375, 400, 425, 450, 475, or 500 °C. A salt spray test (SST) is one of the tests employed to determine a corrosion resistance of organic coatings. In this test, samples of coated steel substrates are placed in an enclosed chamber at 35° C and exposed to a continuous indirect spray (fogging) of 5% salt solution (pH 6.5 to 7.2). This climate is maintained under constant steady state conditions. The samples are placed at a 30° angle from vertical. After exposure to a salt spray, the samples are observed periodically for blisters, delamination and red rust. In some embodiments, the polyimide coatings provided by the present methods show a corrosion resistance of about 160-250 hours as measured by a salt spray test.

Another test employed to determine the corrosion resistance of a coating is an Electrochemical Impedance Spectroscopy (EIS) test. In some embodiments, the polyimide coatings provided by the present methods show a total resistance to corrosion (Rt) of at least 3.50E+05 ohm as determined by fitting a Bode impedance plot with electrochemical equivalent circuit (EEC). In some embodiments, the polyimide coatings provided by the present methods show a total resistance to corrosion (Rt) of about 3E+05 to about 3E+10, including values and ranges thereof.

The present disclosure also provides polyimide-coated steel substrates and polyamic acid intermediates obtained by the methods disclosed herein.

In some embodiments, the substrates coated according to the methods of the present disclosure include, but are not limited to, a mild steel, stainless steel, galvanized steel, or tin substrate.

The present methods for preparing polyimide-coated substrates and the polyimide-coated substrates obtained therefrom provide several advantages. The present methods employ low- cost monomers to prepare polyimide coatings. The polyimide coatings of the present disclosure show excellent flexibility and formability, adhesion, and corrosion resistance properties. Further, due to the excellent adhesion and corrosion resistance characteristics of the present coatings, pre-treating carbon steel substrates is no longer necessary, and the coatings can be applied directly on bare carbon steel substrates thereby providing further cost reduction.

It is to be understood that the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Similarly, additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon description provided herein.

Descriptions of well-known/conventional methods/steps and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for examples illustrating the above-described embodiments, and in order to illustrate the embodiments of the present disclosure certain aspects have been employed. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein.

EXAMPLES

Example 1: Synthesis of polyimide-1 (PI-1)

7.2 gm (61 mmol) ofhexamethylenediamine (HMDA) and 6.7 gm (61) of m-phenylenediamine (MPDA) were taken in a two necked round bottom flask in the presence of 207 mL of dry DMAc (dimethylacetamide) under nitrogen atmosphere with constant stirring. Then the flask was charged with 18.1 gm (61 mmol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPD A) and 19.7 gm (61 mmol) of benzophenone-3,3',4,4'-tetracarboxylic dianhydride (BTDA). The reaction mixture was then allowed to stir under N2 atmosphere for 18 hours to form a yellow viscous polyamic acid intermediate (PAA-1) liquid with inherent viscosity of 0.47 dL/g. PAA- 1 is then applied directly on the steel substrate and cured in an oven at 250°C for 5 minutes to form a PI-1 polyimide coated steel product. PI-1 has the following structure:

Example 2: Synthesis of polyimide-2 (PI-2) 6.5 gm (55 mmol) of hexamethylenediamine (HMD A) and 11.2 gm (55 mmol) 4,4'- Oxydianiline (ODA) were taken in a two necked round bottom flask in the presence of 207 mL of dry DMAc (dimethylacetamide) under nitrogen atmosphere with constant stirring. Then the flask was charged with 16.3 gm (55 mmol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 17.9 gm (55 mmol) benzophenone-3,3',4,4'-tetracarboxylic dianhydride (BTDA). The reaction mixture was then allowed to stir under N2 atmosphere for 18 hours to form a yellow viscous polyamic acid intermediate (PAA-2) liquid with inherent viscosity of 0.61 dL/g . PAA-2 is then applied directly on the steel substrate and cured in an oven at 250°C for 5 minutes to form a PI-2 polyimide coated steel product. PI-2 has the following structure:

Example 3: Synthesis of polvimide-3 (PI-3)

12.6 gm (54 mmol) of jeffamineD230 (JeffD230) and 5.9 gm (54 mmol) of m- phenylenediamine (MPDA) were taken in a two necked round bottom flask in the presence of 207 mL of dry DMAc (dimethylacetamide) under nitrogen atmosphere with constant stirring. Then the flask was charged with 15.9 gm (54 mmol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 17.5 gm (54 mmol) of benzophenone-3,3',4,4'-tetracarboxylic dianhydride (BTDA). The reaction mixture was then allowed to stir under N2 atmosphere for 18 hours to form a yellow viscous polyamic acid intermediate (PAA-3) liquid with inherent viscosity of 0.30 dL/g. PAA-3 is then applied directly on the steel substrate and cured in an oven at 250°C for 5 minutes to form a PI-3 polyimide coated steel product. PI-3 has the following structure:

Example 4: Synthesis of polyimide-4 (PI-4)

11.4 gm (49 mmol) of jeffamineD230 (JeffD230) and 10 gm (49 mmol) of 4,4'-Oxydianiline (ODA) were taken in a two necked round bottom flask in the presence of 207 mL of dry DMAc (dimethylacetamide) under nitrogen atmosphere with constant stirring. Then the flask was charged with 14.5 gm (49 mmol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 15.9 gm (49 mmol) of benzophenone-3,3',4,4'-tetracarboxylic dianhydride (BTDA). The reaction mixture was then allowed to stir under N2 atmosphere for 18 hours to form a yellow viscous polyamic acid intermediate (PAA -4) liquid with inherent viscosity of 0.55 dL/g. PAA- 4 is then applied directly on the steel substrate and cured in an oven at 250°C for 5 minutes to form a PI-4 polyimide coated steel product. PI-4 has the following structure:

Example 5: Characterization of PAA intermediates and PI Coatings of Examples 1-4

For the characterization of the PAA intermediates, some portion of the reaction mixture was extracted with DCM to precipitate out the polymer and filtered. The residue was washed with water and methanol for 3-4 times to completely remove DMAc followed by drying in vacuum oven. A white powdery mass was obtained which was subjected for NMR and FTIR analysis for structure confirmation. FTIR spectroscopy indicated the reaction between anhydrides and amines and hence formation of PAA intermediate as the peaks correspond to amide appeared in the region of 1620 and 1540 cm" ¹ (Figure 3). In the NMR study, the peaks at 10.5 ppm indicated the -COOH and -CONH functionality in the PAA intermediates (Figures 1 and 2). The PAA intermediates were directly applied on the steel surfaces and then subjected to thermal imidization at 200-250°C to form PI coatings. FTIR spectroscopy confirmed the conversion of PAA to PI by heating as seen by the appearance of the cyclic imide bands in the region of 1770 and 1705 cm" ¹ and the disappearance of amide bands at 1630 and 1540 cm" ¹ (Figure 4). Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) were conducted using a netzsch DSC. The thermal stability of the polymer is evaluated by TGA. Temperature for 5% wt and 10% wt loss are standard measures of the polymer stability. The Pls were found to be stable up to around 450°C (Figure 5). Scans were recorded in the following settings: for TGA and DSC, heating rate was 10°C/min and the temperature range was 30°C to 650°C.

The Salt spray test (ASTMB117 standard) was used to study the corrosion resistance of polyimide-coated metallic specimens. The samples of the size 40x 100x0.8 mm were cleaned, coated, and then placed in a salt spray chamber. Specifically, the coated and cured steel substrates were placed in an enclosed chamber at 35°C and exposed to a continuous indirect spray (fogging) of 5% salt solution (pH 6.5 to 7.2). This climate was maintained under constant steady state conditions. The samples were placed at a 30° angle from vertical. After exposure to a salt spray, the samples were critically observed periodically for blisters, delamination and red rust. The surface appearance of the polyimide-coated samples after the salt spray analysis is shown in Figure 6. PI-1 and PI-3 coatings showed excellent corrosion resistance property. There was no red appearance for the PI-1 and PI-3 coated mild steel sheet after 240 hrs of SST (salt spray test) exposure. However, for the PI -2 and PI-4 coated mild steel sheets showed some rust after 10 days of SST test.

The dry film thicknesses of the polyimide coated steel sheets were in the range of 3 pM to 8 pM. Generally, the thicker coating improved the corrosion resistance property, however, thinner coating gave rise to advantageous formability property. The PAA intermediates were applied on the steel surfaces by dip or roll coating method and then cured using conventional heating. The superior adhesion of the coating and excellent corrosion resistance property can be attributed to the chemical interaction through electron transfer from electron rich hetero atom of the polymer to the metal vacant d orbital. The excellent adhesion and corrosion resistance properties that characterise the polyimide coating of the present invention indicate that it is not necessary to pre-treat carbon steel substrates, instead, the coatings can be applied directly on a bare carbon steel substrate.

Table 1: Various important parameters of polyimides

Electrochemical Impedance Spectroscopy (EIS) tests were performed to understand the corrosion resistance property of polyimide-coated cold rolled cold steel samples. The Bode impedance plot of Figure 7 reveals that the polyimide-coated steel samples have resistive behavior at lower frequency region. It is evident that the coated steel samples have the higher impedance at the lowest frequency which is a clear signature of excellent barrier resistance of the coated steel substrate. The EIS data was fitted with electrochemical equivalent circuit (EEC) to get the values of all electrochemical parameters. The Bode impedance data of PI-1 and PI-3 coated steel sheet was fitted with simple Randles circuit (having one time constant) where the solution resistance (Rsoin) is in series with the parallel combination of constant phase element (CPE _c t) and charge transfer resistance (Ret) (Figure 8(a)). However, PI-2 and PI-4 coated steel sheet was fitted with EEC of two time constants where (CPEctRct) is in parallel with (CPEcRp) which is again in series with Rs (Figure 8(b)). The CPEcRp was incorporated to understand the contribution of the coating in the electrochemical process whereas the CPEctRct represents the corrosion process. Here, Rs, Rp and Ret are the solution resistance, coating resistance and charge transfer resistance respectively. The CPEc and CPEct are the constant phase elements corresponding to the capacitance of coating film and double layer capacitance. The ideal capacitance is replaced by constant phase element to represent the inhomogeneous electrochemical behavior of surface. The CPE is defined by the following equation:

Where, C is the capacitance, co is the angular frequency (c =27if rad s" ¹ ), j = f— and p is the exponent (0<p<l). All the EIS data were fitted by corresponding EECs with the help of Gamry fitting software. The electrochemical parameters of coated steel sheets were obtained after fitting the EIS data with the proposed EECs as shown in Table 1. The total resistance to corrosion (Rt) represent the corrosion resistance property of each samples. Higher value of Rt signifies better corrosion resistance than the lower. PI-1 and PI-3 coated mild steel substrates have higher R _t values than the PI-2 and PI-4 coated substrates. It can be observed that the coated substrates of composition PI-2 and PI-4 are comparable in terms of corrosion resistance since their total resistance to corrosion (R _t ) values are almost comparable.

Table 2: Electrochemical parameters obtained by fitting the Bode impedance plot of different samples with the EECs