Description:FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[see section 10 and rule13]

“A COMPOSITION, AN ARTICLE, METHODS OF PREPARATION AND APPLICATION THEREOF”

Name and address of the applicants:
TATA STEEL LIMITED
Jamshedpur, 831001, Jharkhand, India

Nationality: Indian

The following specification particularly describes the invention and the manner in which it is to be performed.

TECHNICAL FIELD
The present disclosure relates to field of material sciences. The present disclosure particularly relates to a composition which is significantly cost effective and possess improved coating properties, such as corrosion resistance, impact resistance and formability. The disclosure further relates to an article and methods of preparing the composition and the article.

BACKGROUND OF THE DISCLOSURE
Metal bodies such as rebars, wires, strips or sheets are subjected to protective surface treatments to prolong the longevity and performance of the rebar, wire, strip or sheet. This operation is of particular importance in the automotive industry and white good industries where the metal sheet should exhibit very high corrosion resistance.

Zinc coating have been used in the past since they provide continuous impervious metallic barrier that does not allow moisture to contact the metal and provides sacrificial corrosion protection to the steel substrate underneath. Without direct moisture contact corrosion should not occur. However, due to sacrificial nature, zinc coatings gradually degrade over time due to exposure to water and atmospheric pollutants in open air applications. Barrier life is also proportional to coating thickness and thicker coatings increase the costs of the coating metal substrates with zinc.

Zinc coatings are also oxidised preferentially when bare metal is exposed to moisture if the metal is scratched. In the immediate presence of zinc, iron in a metal is not oxidized until all of the zinc has been sacrificed. However, the products of zinc oxidation have a high surface area and produce blisters and adversely affect the appearance of the covering paint work.

In view of the above, organic coatings have been manufactured to improve corrosion resistance when coating metal substrates. However, it was noted that thicker coatings increase corrosion resistance, thus increasing the cost involved in coating metallic substrates.

Further, organic coatings, such as epoxy, acrylic, urethane and silane based organic coatings were widely used as coating matrix for steel anticorrosive coatings. These coatings are economical, however, such coatings could not provide chemical resistance, high temperature and oxidation resistance.

It is therefore evident that improved and high performance coating composition like engineering polymer (polyimide/polyetherimide) based coating is needed that provides for improved coating properties when applied on metallic substrates and which is cost effective than those previously known.

STATEMENT OF THE DISCLOSURE
Accordingly, the present disclosure describes a coating composition which is cost effective with respect to different kinds of high performance engineering polymers (e.g. PEEK, polyimides) and possess improved coating properties. The composition employs combination of components which are significantly economical, and their combination provides for improved coating properties, such as corrosion resistance, impact resistance and formability.

The coating composition of the present disclosure comprises-
i. mixture of dianhydrides comprising at least two dianhydrides selected from a group comprising 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA), Pyromellitic dianhydride (PMDA), 4,4' Oxydiphthalic Anhydride (ODA), 4,4'-(4,4'-Isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA), 4,4'-(hexafluoro-isopropylidene) diphthalic anhydride (FDA) and 3,3',4,4'-Benzophenonetetracarboxylic dianhydride (BTDA); and
ii. mixture of diamines comprising at least two diamines selected from a group comprising Poly (propylene glycol) bis (2-aminopropyl ether), Metaphynelene diamine (MPDA), Hexamethylenediamine (HMDA), 4,7,10- trioxa-1,13-tridecanediamine (TTDA), p-phenylene diamine (PDA), Ethylene diamine (EDA), and 4,4’ oxydianiline (ODA).

The present disclosure further describes a method for preparing the composition described above, said method comprising-
i. mixing the mixture of dianhydrides and solvent to obtain a blend;
ii. mixing the diamines and solvent to obtain a blend; and
iii. adding the blend of step ii) to the blend of step i), followed by mixing to obtain the composition.

The present disclosure further describes an article comprising polyimide coating according to the composition as described above. The polyimide coating formed from the composition described above demonstrates improved coating properties, such as corrosion resistance, impact resistance and formability.

The present disclosure further describes a method of producing the article comprising the polyimide coating, said method comprises- applying the composition described above on surface of a substrate, followed by curing to obtain the article comprising polyimide coating.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
In order that the present disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, where:

Figure 1 depicts repeat unit structure of polyamic acid formed in the compositions [a) composition-1; b) Composition-2, c) Composition-3; and d) composition-4] of the present disclosure.

Figure 2 illustrates scanning electron microscope (SEM) images (cross sectional view) of the polyimide coatings (coatings 1, 2, 3 and 5) on a substrate obtained from the compositions [a) composition-1; b) Composition-2, c) Composition-3; and d) composition-4] of the present disclosure.

Figure 3 illustrates Fourier-transform infrared spectroscopy (FTIRs) plot of polyamic acid formed in the compositions of the present disclosure.

Figure 4 illustrates Fourier-transform infrared spectroscopy (FTIRs) plot of polyimide coatings obtained from the compositions of the present disclosure.

Figure 5 illustrates Proton nuclear magnetic resonance spectroscopy (1H-NMR) plot of the polyamic acid formed in the compositions of the present disclosure.
Figure 6 illustrates Thermogravimetric analysis plot of polyimide coatings (coatings 1, 2, 3 and 4) obtained from Composition-1, Composition-2, Composition-3 and Composition-4, respectively.

Figure 7a illustrates image of steel substrate comprising polyimide coating (for e.g., coating-2) post impact testing at 9.8 joule impact energy.

Figure 7b illustrates image of intact polyimide coating (for e.g., coating-2) on steel substrate post 0T bend test.

Figure 8 illustrates image of articles comprising polyimide coatings obtained from Composition-1, Composition-2, Composition-3 and Composition-4, respectively post salt spray test (SST).

Figure 9 illustrates Bode Impedance plot of polyimide coatings (coatings- 1, 2, 4 and 4) obtained from Composition-1, Composition-2, Composition-3 and Composition-4, respectively.

Figure 10 illustrates representative diagram of electrical equivalent circuits (EECs) used to fit polyimide coatings (coatings- 1, 2, 3 and 4) obtained from Composition-1, Composition-2, Composition-3 and Composition-4, respectively.

DETAILED DESCRIPTION OF THE DISCLOSURE
Unless otherwise defined, all terms used in the disclosure, including technical and scientific terms, have meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. By means of further guidance, term definitions are included for better understanding of the present disclosure.

As used herein, the singular forms ‘a’, ‘an’ and ‘the’ include both singular and plural referents unless the context clearly dictates otherwise.

The term ‘comprising’, ‘comprises’ or ‘comprised of’ as used herein are synonymous with ‘including’, ‘includes’, ‘containing’ or ‘contains’ and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps.

The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective ranges, as well as the recited endpoints.

The term ‘about’ as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of ±10% or less, preferably ±5% or less, more preferably ±1% or less and still more preferably ±0.1% or less of and from the specified value, insofar such variations are appropriate to perform the present disclosure. It is to be understood that the value to which the modifier ‘about’ refers is itself also specifically, and preferably disclosed.

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 ‘Formability’ as used herein refers to ability of the polyimide coatings formed from the composition of the present disclosure to undergo plastic deformation without being damaged.

The term ‘corrosion resistance’ or ‘salt spray resistance’ as used herein refers to ability of the polyimide coatings formed from the composition to prevent environmental deterioration by chemical or electro-chemical reaction of an article comprising the coating.
The term ‘impact resistance’ as used herein refers to ability of the polyimide coatings formed from the composition to withstand sudden or intense force or shock. In other words, impact resistance means resistance of the polyimide coating to impact, measured as the energy.
The present disclosure relates to a coating composition that is significantly cost effective and possess improved coating properties, such as corrosion resistance, impact resistance and formability.

The inventors of the present disclosure have particularly designed specific mixture of dianhydrides and specific mixture of diamines, which are combined together to provide a coating composition which is significantly cost effective and possess improved coating properties, such as corrosion resistance, impact resistance and formability.

In some embodiments of the present disclosure, the coating composition comprises-
i. mixture of dianhydrides comprising at least two dianhydrides selected from a group comprising 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA), Pyromellitic dianhydride (PMDA), 4,4' Oxydiphthalic Anhydride (ODA), 4,4'-(4,4'-Isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA), 4,4'-(hexafluoro-isopropylidene) diphthalic anhydride (FDA) and 3,3',4,4'-Benzophenonetetracarboxylic dianhydride (BTDA); and
ii. mixture of diamines comprising at least two diamines selected from a group comprising Poly (propylene glycol) bis (2-aminopropyl ether), Metaphynelene diamine (MPDA), Hexamethylenediamine (HMDA), 4,7,10- trioxa-1,13-tridecanediamine (TTDA), p-phenylene diamine (PDA), Ethylene diamine (EDA), and 4,4’ oxydianiline (ODA).

In some embodiments of the present disclosure, the mixture of dianhydrides is mixture of 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA) and Pyromellitic dianhydride (PMDA).

In some embodiments of the present disclosure, the dianhydride is present at a concentration ranging from about 55 mmol to 70 mmol, including all the values in the range, for instance, 56 mmol, 57 mmol, 58 mmol, 59 mmol and so on and so forth. In an embodiment, dianhydrides in the mixture are at same proportion, that is 1:1 equivalent.

In some embodiments of the present disclosure, the mixture of diamines is selected from a group comprising mixture of Hexamethylenediamine (HMDA) and Metaphynelene diamine (MPDA); mixture of Hexamethylenediamine (HMDA) and 4,4’ oxydianiline (ODA); mixture of or Poly (propylene glycol) bis (2-aminopropyl ether) and Metaphynelene diamine (MPDA); and mixture of Poly (propylene glycol) bis (2-aminopropyl ether) and 4,4’ oxydianiline (ODA).

In some embodiments of the present disclosure, the diamine is present at a concentration ranging from about 55 mmol to 70 mmol, including all the values in the range, for instance, 56 mmol, 57 mmol, 58 mmol, 59 mmol and so on and so forth. In an embodiment, diamines in the mixture are at same proportion, that is 1:1 equivalent.

In some embodiments of the present disclosure, the coating composition comprises-
i. mixture of 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA) and Pyromellitic dianhydride (PMDA); and
ii. mixture of diamines comprising at least two diamines selected from group comprising Poly (propylene glycol) bis (2-aminopropyl ether), Metaphynelene diamine (MPDA), Hexamethylenediamine (HMDA), 4,7,10- trioxa-1,13-tridecanediamine (TTDA), p-phenylene diamine (PDA), Ethylene diamine (EDA), and 4,4’ oxydianiline (ODA).

In some embodiments of the present disclosure, the coating composition comprises-
i. mixture of 50 mmol to 70 mmol of 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA) and 50 mmol to 70 mmol of Pyromellitic dianhydride (PMDA); and
ii. mixture of 50 mmol to 70 mmol of Metaphynelene diamine (MPDA) and 50 mmol to 70 mmol of Hexamethylenediamine (HMDA).

In some embodiments of the present disclosure, the coating composition comprises-
i. mixture of 50 mmol to 70 mmol of 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA) and 50 mmol to 70 mmol of Pyromellitic dianhydride (PMDA); and
ii. mixture of 50 mmol to 70 mmol of Hexamethylenediamine (HMDA) and 50 mmol to 70 mmol of 4,4’ oxydianiline (ODA).

In some embodiments of the present disclosure, the coating composition comprises-
i. mixture of 50 mmol to 70 mmol of 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA) and 50 mmol to 70 mmol of Pyromellitic dianhydride (PMDA); and
ii. mixture of 50 mmol to 70 mmol of Poly (propylene glycol) bis (2-aminopropyl ether) and 50 mmol to 70 mmol of Metaphynelene diamine (MPDA).

In some embodiments of the present disclosure, the coating composition comprises-
i. mixture of 50 mmol to 70 mmol of 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA) and 50 mmol to 70 mmol of Pyromellitic dianhydride (PMDA); and
iii. mixture of 50 mmol to 70 mmol to Poly (propylene glycol) bis (2-aminopropyl ether) and 50 mmol to 70 mmol of 4,4’ oxydianiline (ODA).

In some embodiments of the present disclosure, the coating composition comprises solvent selected from a group comprising N, N-dimethylacetamide (DMAc), 1-methyl-2-pyrolidone (NMP), Dimethyl formamide (DMF), chloroform, ethanol and any combination thereof.

In some embodiments of the present disclosure, the solvent in the composition is present at a concentration ranging from about 70 %v/v to 90 %v/v, including all the values in the range, for instance, 71 %v/v, 72 %v/v, 73 %v/v, 74 %v/v and so on and so forth.

In some embodiments of the present disclosure, the coating composition has solid content ranging from about 10 wt.% to 30 wt.%, including all the values in the range, for instance, 11 wt.%, 12 wt.%, 13 wt.%, 14 wt.% and so on and so forth.

The inventors have identified that mixture of 3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA) and Pyromellitic dianhydride (PMDA) in combination with mixture of diamines comprising at least two diamines selected from a group comprising Poly (propylene glycol) bis (2-aminopropyl ether), Metaphynelene diamine (MPDA), Hexamethylenediamine (HMDA), 4,7,10- trioxa-1,13-tridecanediamine (TTDA), p-phenylene diamine (PDA), Ethylene diamine (EDA), and 4,4’ oxydianiline (ODA), provides for coating composition having improved coating properties, such as corrosion resistance, impact resistance and formability. In the coating composition, the mixture of 3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA) and Pyromellitic dianhydride (PMDA) aids in imparting improved coating properties and making the coating composition cost effective. The mixture of the dianhydrides provides rigidity and the mixture of diamines provides flexibility to the composition. Thus, the coating composition is both rigid and flexible, thus offering improved coating properties. Figure 1 describes polyamic acid formed in the compositions of the present disclosure. Further, Figure 2 describes FTIR plot of polyamic acid formed in the compositions of the present disclosure.

The present disclosure further describes method of preparing the composition described above.

In some embodiments of the present disclosure, the method of preparing the composition comprises-
i. mixing the mixture of dianhydrides and solvent to obtain a blend;
ii. mixing the mixture of diamines and solvent to obtain a blend; and
iii. adding the blend of diamines to the blend of the dianhydrides, followed by mixing to obtain the composition.

In some embodiments of the present disclosure, the solvent employed for mixing with the mixture of dianhydrides is selected from a group comprising N, N-dimethylacetamide (DMAc), 1-methyl-2-pyrolidone (NMP), Dimethyl formamide (DMF), ethanol, chloroform and any combination thereof.

In some embodiments of the present disclosure, the solvent employed for mixing with the mixture of diamines is selected from a group comprising N, N-dimethylacetamide (DMAc), 1-methyl-2-pyrolidone (NMP), Dimethyl formamide (DMF), ethanol, chloroform and any combination thereof.

In some embodiments of the present disclosure, the solvent employed for mixing with the mixture of dianhydrides and mixture of diamines respectively, can be same or different.

In some embodiments of the present disclosure, the method of preparing the composition is carried out in inert atmosphere, in presence of gas selected from a group comprising nitrogen, argon and a combination thereof.

In some embodiments of the present disclosure, in the method, mixing of the mixture of dianhydrides and the solvent is carried out for a duration ranging from about 15 minutes to 30 minutes, including all the values in the range, for instance, 16 minutes, 17 minutes, 18 minutes, 19 minutes and so on and so forth. In an embodiment, the mixing is carried out at a temperature ranging from about 20 °C to 40 °C, including all the values in the range, for instance, 21 °C, 22°C, 23 °C, 24°C and so on and so forth.

In some embodiments of the present disclosure, in the method, mixing of the mixture of diamines and the solvent is carried out for a duration ranging from about 15 minutes to 30 minutes, including all the values in the range, for instance, 16 minutes, 17 minutes, 18 minutes, 19 minutes and so on and so forth. In an embodiment, the mixing is carried out at a temperature ranging from about 20 °C to 40 °C, including all the values in the range, for instance, 21 °C, 22°C, 23 °C, 24°C and so on and so forth.

In some embodiments of the present disclosure, in the method, the blend of diamines is added dropwise to the blend of dianhydrides, followed by mixing for a duration ranging from about 15 minutes to 30 minutes, including all the values in the range, for instance, 16 minutes, 17 minutes, 18 minutes, 19 minutes and so on and so forth. In an embodiment, the mixing is carried out at a temperature ranging from about 20 °C to 40 °C, including all the values in the range, for instance, 21 °C, 22°C, 23 °C, 24°C and so on and so forth.

In some embodiments of the present disclosure, in the method, upon mixing the blend of diamines with the blend of dianhydrides, the reaction is allowed to occur for a duration ranging from about 16 hours to 24 hours, including all the values in the range, for instance, 16.1 hours, 16.2 hours, 16. 3 hours, 16.4 hours and so on and so forth. In an embodiment, the reaction is allowed to occur with constant stirring under inert atmosphere having gas selected from a group comprising nitrogen, argon and a combination thereof.

The method of preparing the coating composition of the present disclosure is simple and cost effective. The inventors have designed the method to achieve coating composition possessing improved coating properties, such as corrosion resistance, impact resistance and formability.

The present disclosure further describes an article comprising polyimide coating.

In some embodiments of the present disclosure, the article comprises the polyimide coating formed from the composition described above.

In some embodiments of the present disclosure, the polyimide coating has thickness ranging from about 15 microns to 35 microns, including all the values in the range, for instance, 15.1 microns, 15.2 microns, 15.3 microns, 15.4 microns and so on and so forth.

In some embodiments of the present disclosure, article comprising the polyimide coating is resistant to corrosion. The polyimide coating prevents corrosion, thus protecting the article from corrosion.

In some embodiments of the present disclosure, the article comprising the polyimide coating is resistant to corrosion for a duration ranging from about 20 hours to 140 hours, including all the values in the range, for instance, 21 hours, 22 hours, 23 hours, 24 hours and so on and so forth, according to salt spray test. In an embodiment, the article comprising the polyimide coating is resistant to corrosion for a duration ranging from about 80 hours to 140 hours, including all the values in the range, for instance, 81 hours, 82 hours, 83 hours, 84 hours, according to salt spray test. The salt spray test is an accelerated corrosion test that produces corrosive attack to coated samples in order to evaluate the coating for its resistive capacity to corrosion.

In some embodiments of the present disclosure, the article comprising the polyimide coating exhibits total resistance to corrosion in the range of about 2.5E+05 ? cm2 to 2.30E+11 ? cm2.

In some embodiments of the present disclosure, the article comprising the polyimide coating is resistant to impact having energy ranging from about 8 joules to 10 joules, including all the values in the range, for instance, 8.1 joules, 8.2 joules, 8.3 joules, 8.4 joules and so on and so forth. Figure 7a of the present disclosure describes intact polyimide coating (for e.g., coating-2) under impact testing upon applying about 9.8 joule impact energy.

In some embodiments of the present disclosure, the polyimide coating on the article is intact according to 0T bend test. The bend test is a qualitative test used to evaluate both ductility and soundness of the coating. Figure 7b of the present disclosure describes intact polyimide coating (for e.g., coating-2) on steel substrate according to 0T bend test. From the figure 7b, it is clear that the polyimide coating can withstand impact without forming any crack or peel off.

The improved impact resistance of the coating, according to impact testing and intactness of the coating according 0T bend test, establishes that the polyimide coating in the article has improved flexibility or formability.

In some embodiments of the present disclosure, the article is selected from a group comprising carbon steel, hot rolled steel sheet, hot rolled steel tube, galvanized iron and galvannealed steel sheet.

The present disclosure further describes a method of producing the article comprising the polyimide coating.

In some embodiments of the present disclosure, the method of producing the article comprises applying the composition as described above on a substrate, followed by curing to obtain the article comprising polyimide coating.

In some embodiments of the present disclosure, the composition is applied onto the substrate by technique selected from a group comprising spray coating, dip coating, roll coating, wiping method, and any combination thereof.

In some embodiments of the present disclosure, the curing is carried out at a temperature ranging from about 205 °C to 250 °C, including all the values in the range, for instance, 206 °C, 207 °C, 208 °C, 209 °C and so on and so forth. In an embodiment, the curing is carried out for a duration ranging from about 3 minutes to 5 minutes, including all the values in the range, for instance, 3.1 minutes, 3.2 minutes, 3.3 minutes, 3.4 minutes, 3.5 minutes.

In some embodiments of the present disclosure, the substrate is selected from a group comprising carbon steel, hot rolled steel sheet, hot rolled steel tube, galvanized iron and galvannealed steel sheet.

The coating composition, the article and the methods of preparing thereof described in the present disclosure, provides for the following advantages-
- The coating composition provides for improved coating properties, such as corrosion resistance, formability and impact resistance.
- Mixture of dianhydride comprising at least two anhydrides selected from a group comprising 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA), Pyromellitic dianhydride (PMDA), 4,4' Oxydiphthalic Anhydride (ODA), 4,4'-(4,4'-Isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA), 4,4'-(hexafluoro-isopropylidene) diphthalic anhydride (FDA) and 3,3',4,4'-Benzophenonetetracarboxylic dianhydride (BTDA), provides rigidity to the composition and aids in reducing the cost of the coating composition significantly. Particularly the mixture of 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA), Pyromellitic dianhydride (PMDA) plays significant role in providing rigidity to the composition and aiding in reducing the cost of the coating composition significantly.
- The mixture of diamine comprising at least two diamines selected from a group comprising Poly (propylene glycol) bis (2-aminopropyl ether), Metaphynelene diamine (MPDA), Hexamethylenediamine (HMDA), 4,7,10- trioxa-1,13-tridecanediamine (TTDA), p-phenylene diamine (PDA), Ethylene diamine (EDA), and 4,4’ oxydianiline (ODA), provides flexibility to the coating composition.
- Combination of the mixture of the dianhydrides and the mixture of the diamines in the coating composition provides both rigidity and flexibility to the polyimide coating formed from the composition.
- The polyimide coating in the article formed from the composition of the present disclosure, demonstrates improved impact resistance and formability
- The article comprising the polyimide coating demonstrates improved resistance to corrosion.
- The method of preparing the coating composition and the method of preparing the article comprising the coating is simple and cost effective.

It is to be understood that the foregoing description is illustrative not a limitation. While considerable emphasis has been placed herein on 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 may be practiced and to further enable those of skill in the art to practice the embodiments. Accordingly, following examples should not be construed as limiting the scope of the embodiments herein.

EXAMPLES
Example 1: Preparation of the coating composition
a. Preparation of mixture of dianhydrides
Three neck R.B flask was degassed twice with nitrogen and then filled with about 103.5 g of DMAc (half of the total solvent to be used). About 20.7 g (70 mmol) of BPDA and about 15.3 g (70 mmol) of PMDA was added and mixed to obtain slurry type mixture of dianhydrides in DMAc.

b. Preparation of mixture of diamines
Two necked round bottom flask was degassed twice with nitrogen and then filled with about 103.5 g of DMAc (remaining half of the total solvent). About 8.2 g (70 mmol) of HMDA and about 7.6 g (70 mmol) of MPDA was added to the solvent and mixed to obtain mixture of diamines (solution).

c. Preparation of the composition
The mixture of diamines (solution) was added dropwise to the mixture of dianhydrides in DMAc (solution), followed by mixing and allowed the reaction to occur for a duration of about 18 hours at room temperature (in the range of 20 °C to 40 °C), under constant stirring under nitrogen atmosphere. Initially the reaction mixture appeared like white emulsion which was gradually converted to transparent solution as the reaction progressed due to generation of heat during the reaction. After that the reaction mixture was converted to viscous light yellow solution, which is polymaic acid solution in DMAc, i.e., the coating composition (referred to as Composition-1). The solid content of the composition was 25 % wt/wt.

Example 2: Preparation of the coating composition
a. Preparation of mixture of dianhydrides
Three neck R.B flask was degassed twice with nitrogen and then filled with about 103.5 g of DMAc (half of the total solvent to be used). About 18.3 g (62 mmol) of BPDA and about 13.6 g (62 mmol) of PMDA was added and mixed to obtain slurry type mixture of dianhydrides in DMAc.

b. Preparation of mixture of diamines
Two necked round bottom flask was degassed twice with nitrogen and then filled with about 103.5 g of DMAc (remaining half of the total solvent). About 7.3 g (62 mmol) of HMDA and about 12.6 g (62 mmol) of ODA was added to the solvent and mixed to obtain mixture of diamines (solution).

c. Preparation of the composition
The mixture of diamines (solution) was added dropwise to the mixture of dianhydrides in DMAc (solution), followed by mixing and allowed the reaction to occur for a duration of about 18 hours at room temperature (in the range of 20 °C to 40 °C), under constant stirring under nitrogen atmosphere. Initially the reaction mixture appeared like white emulsion which was gradually converted to transparent solution as the reaction progressed due to generation of heat during the reaction. After that the reaction mixture was converted to viscous light yellow solution, which is polymaic acid solution in DMAc, i.e., the coating composition (referred to as Composition-2). The solid content of the composition was 25 % wt/wt.

Example 3: Preparation of the coating composition
a. Preparation of mixture of dianhydrides
Three neck R.B flask was degassed twice with nitrogen and then filled with about 103.5 g of DMAc (half of the total solvent to be used). About 17.9 g (60 mmol) of BPDA and about 13.3 g (60 mmol) of PMDA was added and mixed to obtain slurry type mixture of dianhydrides in DMAc.

b. Preparation of mixture of diamines
Two necked round bottom flask was degassed twice with nitrogen and then filled with about 103.5 g of DMAc (remaining half of the total solvent). About 14 g (60 mmol) of Poly (propylene glycol) bis (2-aminopropyl ether) and about 6.6 g (60 mmol) of MPDA was added to the solvent and mixed to obtain mixture of diamines (solution).

c. Preparation of the composition
The mixture of diamines (solution) was added dropwise to the mixture of dianhydrides in DMAc (solution), followed by mixing and allowed the reaction to occur for a duration of about 18 hours at room temperature (in the range of 20 °C to 40 °C), under constant stirring under nitrogen atmosphere. Initially the reaction mixture appeared like white emulsion which was gradually converted to transparent solution as the reaction progressed due to generation of heat during the reaction. After that the reaction mixture was converted to viscous light yellow solution, which is polymaic acid solution in DMAc, i.e., the coating composition (referred to as Composition-3). The solid content of the composition was 25 % wt/wt.

Example 4: Preparation of the coating composition
a. Preparation of mixture of dianhydrides
Three neck R.B flask was degassed twice with nitrogen and then filled with about 103.5 g of DMAc (half of the total solvent to be used). About 16.2 g (55 mmol) of BPDA and about 11.9 g (55 mmol) of PMDA was added and mixed to obtain slurry type mixture of dianhydrides in DMAc.

b. Preparation of mixture of diamines
Two necked round bottom flask was degassed twice with nitrogen and then filled with about 103.5 g of DMAc (remaining half of the total solvent). About 12.7 g (55 mmol) of Poly (propylene glycol) bis (2-aminopropyl ether) and about 11 g (55 mmol) of ODA was added to the solvent and mixed to obtain mixture of diamines (solution).

c. Preparation of the composition
The mixture of diamines (solution) was added dropwise to the mixture of dianhydrides in DMAc (solution), followed by mixing and allowed the reaction to occur for a duration of about 18 hours at room temperature (in the range of 20 °C to 40 °C), under constant stirring under nitrogen atmosphere. Initially the reaction mixture appeared like white emulsion which was gradually converted to transparent solution as the reaction progressed due to generation of heat during the reaction. After that the reaction mixture was converted to viscous light yellow solution, which is polymaic acid solution in DMAc, i.e., the coating composition (referred to as Composition-4). The solid content of the composition was 25 % wt/wt.

Figure 1 illustrates the repeat unit structure of the polyamic acid of the compositions obtained in Examples 1 to 4, respectively.

Example 5: Method of preparing the article comprising the polyimide coating.
The composition obtained from Examples 1 to 4 above were coated on CRCA steel surface, respectively by dip coating and cured under hot air-drying oven at 250 °C for about 5 minutes to obtain polyimide coating.
The coating obtained from the composition-1 is referred to as coating-1; the coating obtained from the composition-2 is referred to as coating-2; the coating obtained from the composition-3 is referred to as coating-3; and the coating obtained from the composition-4 is referred to as coating-4.

Example 6: Characterization of the coating in the article
a. Scanning Electron Microscope (SEM) analysis
SEM analysis of the articles comprising the polyimide coating obtained from Example 5 was performed to measure the coating thickness and assess the coating uniformity and cracks.
Figure 2 describes cross sectional SEM images of the polyimide coatings (Coating-1, Coating-2, Coating-3 and Coating-4).
Thickness of Coating-1, Coating-2, Coating-3 and Coating-4 were found to be 17 microns, 35 microns, 20 microns and 15 microns, respectively.
It was noted that all the coatings (coating 1 to 4) were uniform in the article.
SEM images illustrate that Coatings 1, 2 and 4 were free of cracks. However, cracks were noted in Coating-3.

b. Fourier Transformed Infrared Spectroscopy (FTIR) and Nuclear magnetic resonance (1H NMR) spectroscopy analysis:
The compositions obtained in Examples 1 to 4 were precipitated in Dichloromethane (DCM) followed by drying under vacuum to obtain the polyamic acid in solid form. The solid polyamic acids from different compositions (compositions 1 to 4) were characterized by FTIR and NMR spectroscopy to confirm the structure of polymer. The FTIR curve for all the polyamic acid is illustrated in the plot under Figure 3. The peak at 1546 cm-1 represents the N-H bonding of amide whereas the peak at 1606 cm-1 represents the C=O stretching of amide bond formed in the polyamic acid. These two peaks are the signature of amic acid formation due to reaction between di-anhydrides and di-amines.

Figure 4 represents FTIR curve of all the polyimide coating (after curing of the composition) over steel surface. It was noted that peaks at 1770 and 1702 cm-1 was for (C=O), peak at 1361 cm-1 was for (C–N) and peak at 725 cm-1 was for imide ring. The coatings (coating 1 to 4) from the compositions (compositions 1 to 4) showed formation of imide ring and that represented successful formation of polyimide coating.

Figure 5 represents 1H-NMR plot of the polyamic acid. The singlet peak at 10.50 ppm and the broad hump from 10.59 ppm to 10.65 ppm indicate the presence of -COOH and -CONH functional groups in polyamic acid intermediate respectively.

c. Thermogravimetric analysis
Thermogravimetric analysis of the coatings (coating- 1, 2, 3 and 4) was performed in nitrogen atmosphere in the temperature ranging from about 40 °C to 800 °C with a heating rate of 10 °C per minute. Figure 6 illustrates thermogravimetric analysis plot of the coatings.
It was noted that the coatings were thermally stable.
The order of stability of the coatings were noted as- coating-2 > coating-1 > coating -4 > coating-3. The highest thermal stability of coating 2 was attributed to the presence of more bulky aromatic group in the main backbone chain of the polymer.

d. Formability and impact resistance analysis
0T bending and impact test was carried out to understand flexibility or formability of the coatings. Figure 7a depicts image of polyimide coating (for e.g., coating 2) withstanding impact of energy of about 9.8 joule. And Figure 7b depicts image of polyimide coating (for e.g., coating 2) withstanding the bending according to 0T bending test.

e. Salt Spray Test (SST)
Salt spray test was performed as per ASTM B117 standard. This test was performed under aqueous sodium chloride solution fog. The articles obtained prepared according to Example 5 comprising the coating was subjected to salt spray test. Articles having size of about 10 x 3 cm-2 was kept in salt spray test chamber. Formation of red rust and coating blister was observed over time.
Figure 8 describes images of articles comprising the polyimide coating post salt spray test.
It was observed that the polyimide coating in the article was resistive to corrosion for a duration ranging from 24 hours to 120 hours according to salt spray test.
The order of improved corrosion resistance showcased by the coatings according to salt spray test is - coating -2> coating-1 = coating-4 > coating-3.

f. Electrochemical Impedance Spectroscopy (EIS) test:
The EIS test was performed to assess the mechanism of corrosion of all the polyimide coated steel. This test was performed in three electrode flat cell where saturated calomel electrode (SCE) acts as reference electrode, Platinum mesh as counter electrode (CE) and the polyimide coated steel sample as working electrode (WE) with exposure area of 1 cm2. All the tests were performed in 3.5 wt.% aqueous sodium chloride solution. Open circuit potential (OCP) of all the coated samples were first carried out for about 15 minutes to get potential fluctuation less than 6 mV/seconds for last few minutes which is followed by EIS tests. 10 mV sinusoidal potential perturbation was provided for all EIS tests and the impedance data was recorded by ‘Gamry’ software. All the tests were done in ‘Gamry’ potentiostat. Each EIS data was recorded in the frequency range of 10-2 to 105 with 10 points per decade.

Figure 9 describes bode impedance plot showing capacitive behaviour of the coatings at higher frequency region. From the plot it can be noted that coating 2 showed capacitive behaviour in the frequency range of 10-2 to 105 Hz. The coatings 1, 3 and 4 showed capacitive behaviour in the frequency region of 103 to 105 Hz, followed by resistive behaviour in the region of 10-2 to 103 Hz.

Electrical equivalent circuit (EEC) has been used to fit the Bode impedance data to obtain different electrochemical parameters of coatings (shown in Table 1). Figure 10 represents EEC used to fit all the coating where (Ccor Rcor) is in parallel with (CcRpo) which is again in series with the solution resistance (Rsoln). Here, Ccor is the constant phase element representing the electrolyte double layer on steel surface where the charge transfer reactions are going on. Rcor is the resistance of charge transfer reaction. (Rpo) is coating pore resistance and (Cc) is constant phase element related to the coating. Figure 10 provides illustrative diagram of EECs used to fit the coatings (coatings- 1, 2, 3 and 4).
Sample Rsoln (? cm2) Rpo
(? cm2) Cc (F)
m Rcor
(? cm2) Rt=(Rpo+ Rcor)
(? cm2) Ccor (F) n Goodness of fit
Coating-1 2 3.66E+05 2.39 0.98 7.79E+04 4.43E+05 4.56E-05 0.58 9.80E-04
Coating-2 0.29 3.21E+05 1.54 0.95 2.30E+11 2.30E+11 1.26e-10 1 5.54E-03
Coating-3 0.5 1.57E+05 3.55E-10 1 9.39E+04 2.5E+05 6.56E-06 0.81 5.56E-03
Coating-4 0.6 1.01E+05 3.9 0.89 2.72E+05 3.73E+05 4.6E-05 0.57 7.74E-04
Table 1: Electrochemical parameters obtained by fitting the Bode impedance plot of different polyimide coating (coatings- 1, 2, 3 and 4) samples with the EECs.

Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based on the description provided herein. The embodiments herein provide various features and advantageous details thereof in the description. Descriptions of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments herein.

The foregoing description of the specific embodiments fully reveals the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments in this disclosure have been described in terms of preferred embodiments, 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.

As regards the embodiments characterized in this specification, it is intended that each embodiment be read independently as well as in combination with another embodiment. For example, in case of an embodiment 1 reciting 3 alternatives A, B and C, an embodiment 2 reciting 3 alternatives D, E and F and an embodiment 3 reciting 3 alternatives G, H and I, it is to be understood that the specification unambiguously discloses embodiments corresponding to combinations A, D, G; A, D, H; A, D, I; A, E, G; A, E, H; A, E, I; A, F, G; A, F, H; A, F, I; B, D, G; B, D, H; B, D, I; B, E, G; B, E, H; B, E, I; B, F, G; B, F, H; B, F, I; C, D, G; C, D, H; C, D, I; C, E, G; C, E, H; C, E, I; C, F, G; C, F, H; C, F, I, unless specifically mentioned otherwise.

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. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.
, Claims:WE CLAIM:
1. A coating composition comprising-
i. mixture of dianhydrides comprising at least two dianhydrides selected from a group comprising 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA), Pyromellitic dianhydride (PMDA), 4,4' Oxydiphthalic Anhydride (ODA), 4,4'-(4,4'-Isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA), 4,4'-(hexafluoro-isopropylidene) diphthalic anhydride (FDA) and 3,3',4,4'-Benzophenonetetracarboxylic dianhydride (BTDA); and
ii. mixture of diamines comprising at least two diamines selected from a group comprising Poly (propylene glycol) bis (2-aminopropyl ether), Metaphynelene diamine (MPDA), Hexamethylenediamine (HMDA) and 4,7,10- trioxa-1,13-tridecanediamine (TTDA), p-phenylene diamine (PDA), Ethylene diamine (EDA), and 4,4’ oxydianiline (ODA).

2. The composition as claimed in claim 1, wherein the mixture of dianhydrides is mixture of 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA) and Pyromellitic dianhydride (PMDA).

3. The composition as claimed in claim 1, wherein the mixture of diamines is selected from a group comprising mixture of Hexamethylenediamine (HMDA) and Metaphynelene diamine (MPDA); mixture of Hexamethylenediamine (HMDA) and 4,4’ oxydianiline (ODA); mixture of Poly (propylene glycol) bis (2-aminopropyl ether) and Metaphynelene diamine (MPDA); and mixture of Poly (propylene glycol) bis (2-aminopropyl ether) and 4,4’ oxydianiline (ODA)

4. The composition as claimed in claim 1 or 2, wherein in the dianhydride is present at a concentration ranging from about 55 mmol to 70 mmol.

5. The composition as claimed in claim 1 or 3, wherein the diamine is present at a concentration ranging from about 55mmol to 70mmol.

6. The composition as claimed in claim 1, wherein the composition further comprises solvent selected from a group comprising N, N-dimethylacetamide (DMAc), 1-methyl-2-pyrolidone (NMP), Dimethyl formamide (DMF), chloroform, ethanol and any combination thereof.

7. The composition as claimed in claim 6, wherein the solvent is present at a concentration ranging from about 70 % v/v to 90 %v/v.

8. The composition as claimed in claim 1, wherein solid content in the composition is ranging from about 10 wt% to 30 wt%.

9. A method for preparing the composition as claimed in claim 1, said method comprises-

i. mixing the mixture of dianhydride and solvent to obtain a blend;
ii. mixing the mixture of diamine and solvent to obtain a blend; and
iii. adding the blend of step ii) to the blend of step i), followed by mixing to obtain the composition.

10. The method as claimed in claim 9, wherein the solvent in step i) and step ii) are selected from a group comprising N, N-dimethylacetamide (DMAc), 1-methyl-2-pyrolidone (NMP), Dimethyl formamide (DMF), ethanol, chloroform and any combination thereof.

11. The method as claimed in claim 9, wherein the method is carried out in inert atmosphere in presence of gas selected from a group comprising nitrogen, argon and a combination thereof.

12. The method as claimed in claim 9, wherein the mixing in the step i) and step ii) is carried out for a duration ranging from about 15 minutes to 30 minutes, at a temperature ranging from about ranging from about 20 °C to 40 °C.

13. The method as claimed in claim 9, wherein the mixing in the step iii) is carried out for a duration of few minutes ranging from about 15 minutes to 30 minutes, at a temperature ranging from about 20 ? to 40 ?, followed by allowing reaction to occur for a duration ranging from about 16 hours to 24 hours.

14. An article comprising polyimide coating according to the composition as claimed in claim 1.

15. The article as claimed in claim 14, wherein the polyimide coating has thickness ranging from about 15 microns to 35 microns.

16. The article as claimed in claim 14, wherein the article is resistant to impact having energy ranging from about 8 joules to 9.8joules.

17. The article as claimed in claim 14, wherein the article is resistant to corrosion for a duration ranging from about 20 hours to 140 hours according to salt spray test.

18. The article as claimed in claim 14, wherein the article exhibits total resistance to corrosion in range of about 2.5E+05 to 2.30E+11 ? cm2.

19. The article as claimed in claim 14, wherein the polyimide coating in the article is intact according to 0T bend test.

20. The article as claimed in claim 14, wherein the article is selected from a group comprising carbon steel, hot rolled steel sheet, hot rolled steel tube, galvanized iron and galvannealed steel sheet.

21. A method of producing the article as claimed in claim 14, said method comprises applying the composition as claimed in 1 on a substrate, followed by curing to obtain the article comprising polyimide coating.

22. The method as claimed in claim 21, wherein the composition is applied onto the substrate by technique selected from a group comprising spray coating, dip coating, roll coating, wiping method and any combination thereof.

23. The method as claimed in claim 21, wherein the curing is carried out at a temperature ranging from about 205 °C to 250°C, for a duration ranging from about 3 minutes to 5minutes.

24. The method as claimed in claim 21, wherein the substrate is selected from a group comprising carbon steel, hot rolled steel sheet, hot rolled steel tube, galvanized iron and galvannealed steel sheet.