DESC:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a photoactive and self-healable acrylic resin, a synthetic route for preparing the acrylic resin and a coating formulation containing said resin therein. The present invention specifically relates to the use of tannic acid and polyphenylene methylene modified acrylic resin in coating formulations.

BACKGROUND OF THE INVENTION
Solar paint is being considered as an ecofriendly route of alternate energy and development of such types of coating is a challenging area. Traditionally, silicone based solar cells are very effective compared to polymeric photovoltaic material in terms of power conversion efficiency and outdoor stability. However, specially designed conjugated organic polymers are considered as an alternative to silicone based solar cells in view of their strong absorbance of visible light, easy processability as well as application flexibility by wet processing techniques like spin coating, roll to roll coating etc. Several research articles have been published on polymer solar cells focusing on preparation of low band gap materials that can provide stable devices under outdoor exposure. There is a limitation on the performance of polymer solar cells due to their inferior stability. Attempts have been made to improve the stability of solar cells based on different carbonyl bithiophene and methyl ester compounds after thermal elimination of solubilizing ester groups [Martin Helgesen, Suren A. Gevorgyan, F. C. Krebs, and R. A. J. Janssen, Substituted 2,1,3-Benzothiadiazole- and Thiophene-Based Polymers for Solar Cells -Introducing a New Thermocleavable Precursor, Chem. Mater. 21 (2009) 4669-4675]. But thermal cleavage of polymeric material leads to inferior performance. So, there is a need to develop an alternative route to synthesize photovoltaic material having easy processibility and stable under outdoor exposure.
Poly phenylene methylene (PPM), a hydrocarbon polymer having alternating sequence of phenylene and methylene units, is thermally stable, hydrophobic, and fluorescent in nature. However, polyphenylene methylene has limitation in electrical conductivity as phenylene units are separated by an electronically insulating methylene group and form rigid film due to the presence of phenylene moiety. To improve flexibility and adhesion of PPM based coating formulation different rheological additives, plasticizer have been used. However, the major drawbacks of such plasticized coating film are limited lifetime. It is also very difficult to form a flexible copolymerized system of PPM in view of its lack of sufficient reactive sites.
Tannic acid is one of the known biomaterials having polyphenolic structure with abundant terminal phenolic hydroxyl groups. It is reported that tannic acid retarded corrosion of ferrous metal by forming insoluble ferric tannates as a barrier against oxygen diffusion of metal. Most of the reported techniques for incorporation of tannic acid in coating system are physical mixing of tannin or modified tannin in resin matrix which can only be used for temporary protection of metal against corrosion. Moreover, the use of tannic acid in coating formulation has some limitation due to its high-water sensitivity, low reactivity and thermal stability.
Several research articles have been published for development of self-healing coating by adopting dynamic covalent bonding through the reaction of Diels-Alder, retro Diels-Alder, esterification, or disulfide bonds etc. [L. Feng, Z. Yu, Y. Bian, J. Lu, X. Shi, C. Chai, Self-healing behavior of polyurethanes based on dual actions of thermos-reversible Diels-Alder reaction and thermal movement of molecular chains, Polymer 124 (2017) 48-59 and Y. Wei, X. Ma, The self-healing cross-linked polyurethane by Diels-Alder polymerization, Adv. Polym. Technol. 37 (2018) 1987-1993] The self-repairing capability of polymers is attributed to those reversible bonds which can be incorporated in polymer matrix.
Marco F. D’Elia et. al., Poly (phenylene methylene) – Based Coatings for Corrosion Protection: Replacement of Additives by Use of Copolymers, August 2019, Applied Sciences 9 (17) :3551 discloses that copolymer based on polyphenylene methylene units can be used for corrosion protection. The PPM-related copolymer coating contains different fractions of n-octyloxy side chain introduced into the PPM backbone. The poly (phenylene methylene) derivative was synthesized by copolymerization of a mixture of benzyl chloride and variable fractions of 4-octylbenzyl chloride in presence of tin tetrachloride as catalyst.
D'Elia MF, Magni M, Romano T, Trasatti SPM, Niederberger M, Caseri WR, Smart Anticorrosion Coatings Based on Poly (phenylene methylene): An Assessment of the Intrinsic Self-Healing Behavior of the Copolymer. Polymers, 2022, 14(17):3457 discloses corrosion resistant PPM-based coatings (blend and copolymer) were prepared and applied by hot processing on aluminium alloy. Both blend and copolymer of PPM showed effective corrosion protection when the coating thickness is about 50 µm but when the coating thickness was reduced to 30µm, PPM copolymer showed better corrosion resistance than blended PPM. Furthermore, the coatings show intrinsic self – healing ability made by PPM copolymer, contrary to blend PPM.

Xuan Wang, et.al., Bio-based polyphenol tannic acid as universal linker between metal oxide nanoparticles and thermoplastic polyurethane to enhance flame retardancy and mechanical properties, Composites Part B: Engineering, Volume 224, 2021, 109206 discloses that three kind of metal oxide nanoparticles (ZnO, Fe2O3 and Co3O4) modified with tannic acid are dispersed in thermoplastic polyurethane (TPU) matrix to improve the flame retardancy and mechanical properties of TPU composites.
Chen Y, Li S, Liu Z, Wang Z. Anticorrosion Property of Alcohol Amine Modified Phosphoric and Tannic Acid Based Rust Converter and Its Waterborne Polymer-Based Paint for Carbon Steel. Coatings, 2021; 11(9): 1091 discloses alcohol amines were tested to improve the anticorrosion performance of the phosphoric and tannic acid (PTA) based rust converter.
Wang, J., Tan, W., Yang, H. et. al. Towards weathering and corrosion resistant, self-warning and self-healing epoxy coatings with tannic acid loaded nano particles, npj Mater Degrad 7, 39 (2023) discloses incorporating tannic acid (TA) loaded mesoporous silica (MSN-TA) nanoparticles provide weathering resistance, corrosion-warning, and self-healing properties to epoxy resin coatings.
Shicheng Li, et. al. Enhanced corrosion resistance of self-healing waterborne polyurethane coating based on tannic acid modified cerium-montmorillonites composite fillers, Progress in Organic Coatings, Vol 178, 2023, 107454 discloses corrosion resistance of Polyurethane coatings by the action of corrosion inhibitors Ce3+ and tannic acid. The coating also showed self-healing properties in the physical damage was effectively repaired after heat treatment.
Although the existing literature indicates that tannic acid and polyphenylene methylene may impart anti-corrosive, self-healing, fluorescent properties to polymer coatings; however, both polyphenylene methylene and tannic acid have limitations to form flexible coating system due to their high rigidity and low reactivity.
Thus, there is a need to develop a resin system that to provide a polymer system that has potential to perform photo conducting and self-healing properties under solar radiation.

OBJECT OF THE INVENTION
It is an object of the present invention to overcome the drawbacks of the prior art.
It is another object of the present investigation is to provide a method for developing a tannic acid and polyphenylene methylene modified acrylic resin.
It is yet object of the present invention is to provide an acrylic resin modified with tannic acid and polyphenylene methylene.
A further object of the present invention is to provide a coating formulation containing a self-healable and photoactive acrylic resin.
It is also an object of the present invention to provide a coating to perform self – healing and photo conducting under simulation of solar radiation.

SUMMARY OF THE INVENTION
The following disclosure presents a simplified summary of the invention to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.
Accordingly the present invention provides a tannic acid and polyphenylene methylene modified acrylic resin comprising 13 – 18% by weight of xylene; 12 – 16% by weight of 2-Butoxy ethanol; 12 – 18% by weight styrene; 12 – 15% by weight n-butyl acylate; 6 – 10% by weight 2-Ethyl hexyl acrylate; 1 – 4% by weight of Glycidyl methacrylate; 1 – 4% by weight of Tannic acid; 2 – 4% by weight of Di-tertiary butyl peroxide; 22-28% by weight of Benzyl chloride; and 0.2 - 0.5 by weight of Anhydrous aluminium chloride.
According to another aspect of the present invention there is provided a process for preparing the tannic acid and polyphenylene methylene modified acrylic resin of the present invention, said process comprising:
i. synthesizing tannic acid modified acrylic resin by solution polymerization technique;
ii. chemically modifying tannic acid modified acrylic resin with benzyl chloride to form tannic acid-polyphenylene methylene modified acrylic resin via in situ formation of polyphenylene methylene polymer.

According to yet another aspect of the present invention there is provided a polyurethane coating composition exhibiting self-healing and photosensitivity properties under solar radiation, said coating composition comprising of 70 – 80 wt. % of Tannic acid-polyphenyle methylene modified acrylic resin as described herein, 16 – 30 wt. % of Hexa methylene di isocyanate trimer and 5 – 10 wt. % of Di butyl tin di laurate solution (1 wt. % in Xylene).
According to a further aspect of the present invention there is provided a process for preparing the polyurethane coating composition, said process comprising the steps of mixing tannic acid polyphenylene methylene modified acrylic resin along with di butyl tin di laurate solution and hexamethylene diisocyanate trimer in a weight ratio 3 – 4: 1 at room temperature.
According to yet another aspect of the present invention there is provided a kit comprising (i) a base having the tannic acid polyphenylene methylene modified acrylic resin of the present invention along with di butyl tin di laurate solution, and (ii) a hardener having hexamethylene di isocyanate trimer.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The above and other aspects, features and advantages of the embodiments of the present disclosure will be more apparent in the following description taken in conjunction with the accompanying drawings, in which:
Figure 1 illustrates schematic presentation for preparation of tannic acid polyphenylene methylene modified acrylic and polyurethane coating composition comprising the same as disclosed in one of the embodiments.
Figure 2 illustrates FTIR absorption spectra of unmodified tannic acid (a), unmodified acrylic (b), tannic acid polyphenylene methylene modified acrylic resin (c), as disclosed in the embodiments of present invention.
Figure 3 illustrates self-healing performance of tannic acid polyphenylene modified acrylic resin based polyurethane coating panel after different extent of outdoor exposure. (a) after immediate scratch, (b) recovery of scratch after 1 month and (c) recovery of scratch after 2 months.
Figure 4 illustrates the monitoring of self-healing ability of polyurethane coating through surface morphology study by using Scanning Electron Microscope. Typical repair pattern in the exposed scratches at different exposure times, (a) after 1 month and (b) after 2 months of recovery.
Figure 5 illustrates the photosensitivity properties of tannic acid polyphenylene methylene modified acrylic resin measured through spectral studies. (A) UV- vis absorption spectra and (B) photoluminescence spectra of excitation at different wave lengths (a) 315 nm (b) 370 nm and (c) 440 nm
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may not have been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION OF THE PRESENT INVENTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding, but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness.
All terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which various embodiments belong. Further, the meaning of terms or words used in the specification and the claims should not be limited to the literal or commonly employed sense but should be construed in accordance with the spirit of the disclosure to most properly describe the present disclosure.
The terminology used herein is for the purpose of describing various embodiments only and is not intended to be limiting of various embodiments. As used herein, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprise" and/or "comprising" used herein specify the presence of stated features, integers, steps, operations, members, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, members, components, and/or groups thereof.
The present disclosure will now be described more fully with reference to the accompanying drawings, in which various embodiments of the present disclosure are shown.

In one embodiment of the present invention there is provided a unique acrylic resin system having both polyphenylene methylene and tannic acid combined together in a single acrylic resin system. The resin system in accordance with the present invention relates to a novel modified acrylic resin, formed by the copolymerization of different components like styrene, n-butyl acrylate, 2-ethyl hexyl acrylate, glycidyl methacrylate and tannic acid (active ingredient of all these reactive components is 99.9 %)
In a preferred embodiment the novel acrylic resin system comprises 13 – 18% by weight of xylene; 12 – 16% by weight of 2-Butoxy ethanol; 12 – 18% by weight styrene; 12 – 15% by weight n-butyl acylate; 6 – 10% by weight 2-Ethyl hexyl acrylate; 1 – 4% by weight of Glycidyl methacrylate; 1 – 4% by weight of Tannic acid; 2 – 4% by weight of Di-tertiary butyl peroxide; 22-28% by weight of Benzyl chloride; and 0.2 - 0.5 by weight of Anhydrous aluminium chloride. The modified acrylic resin can be used in a polyurethane coating formulation having self-healing and photo conducting properties.
The present inventors have surprisingly found that although polyphenylene methylene is not a conducting material in view of its’ inherent chemical structure; the novel modified acrylic resin when used in a polyurethane coating formulation possesses self-healing and photo conducting properties. Not bound by any theory it is believed that in the hybrid resin some polyphenylene moieties are formed during in situ reaction of modified tannic acid and polyphenylene methylene. In view of this novel hybrid resin structure, delocalization of electrons in phenyl structures as well as p-p stacking interaction for close packing of aromatic rings leads to absorption of solar radiation for electronic transition at lower energy. Such novel hybrid resin-based coating provides self-healing and photo sensitivity properties through the formation of phenolic polyurethane linkages of tannic acid under simulation of external energy.
The FTIR spectra of the novel resin of the present invention shows the presence of sharp characteristic peak at 3425, 3028, 2957, 2930 and 2873 cm -1 corresponding to the stretching vibration of phenolic hydroxyl, olefin C-H, methyl and methylene groups respectively. The peaks at 1727 cm -1 correspond to the ester group (Fig. 2 b, c) and peaks at 1602, 1494, 1453 cm-1 correspond to phenyl ring. The characteristic IR absorption peak for aromatic phenyl and methylene of polyphenylene methylene structure have merged with the corresponding acrylic components (Fig. 2 c). However, the presence of broad absorption peak at 3425 cm -1 for phenyl hydroxyl, implies the modification of tannic acid in tannic acid polyphenylene methylene modified acrylic resin system (Fig. 2 a, c).
In another embodiment of present invention there is provided a process of incorporation of tannic acid and polyphenylene methylene in acrylic resin system by adopting a novel synthetic route. The process comprises the steps of:
i. synthesizing tannic acid modified acrylic resin by solution polymerization technique;
ii. chemical modifying said tannic acid modified acrylic resin with benzyl chloride to form tannic acid-polyphenylene methylene modified acrylic resin via in situ formation of polyphenylene methylene polymer.

The said step (i) synthesis of tannic acid modified acrylic resin by solution polymerization technique comprises: ring opening reaction of epoxy group of Glycidyl methacrylate with phenolic hydroxyl of tannic acid in 2-butoxy ethanol solvent followed by copolymerization with different monomers.
Thus step (i) comprises
(a) heating 2-butoxy ethanol solvent and tannic acid gradually to 110 – 1200C under stirring and holding for 30 – 40 min to form a clear solution,
(b) adding glycidyl methacrylate gradually to the said clear solution of step (a) under stirring at a temperature of 110 – 1200C and holding at this temperature for 1-2 hr. to form a tannic acid – glycidyl methacrylate adduct,
(c) heating the solution of step (b) gradually to 125 – 1300C and adding monomer-initiator mixture comprising styrene, n-butyl acrylate, 2-ethyl hexyl acrylate and di tertiary butyl peroxide for a period of 4-6 hrs.
(d) adding solvent xylene and continuing polymerization at 1300C for another 3-4 hrs. for complete conversion of monomers into polymer.
The step (ii) chemical modification of tannic acid modified acrylic resin with benzyl chloride to form tannic acid-polyphenylene methylene modified acrylic resin via formation of polyphenylene methylene polymer comprises: in situ polymerization of benzyl chloride using anhydrous aluminium chloride as catalyst along with tannic acid modified acrylic resin as precursor.
The step (ii) comprises
(a) mixing tannic acid modified acrylic resin, benzyl chloride and anhydrous aluminium chloride,
(b) purging nitrogen gas into the reaction of step (a),
(c) gradually heating the mixture of step (b) to 1300C and maintaining for 7 – 8 hrs. under stirring in presence of nitrogen gas to complete the reaction.
Purging nitrogen gas enables removal of by product hydrogen chloride gas produced during said reaction in step (c).
In Step (a) 70-80 % of by weight tannic acid modified acrylic resin was mixed with 22-28% by weight of benzyl chloride and 0.2-0.5% by weight of anhydrous aluminium chloride.
It a further embodiment of the present invention there is provided a self-healing, and photo conducting polyurethane coating formulation; comprising the modified acrylic resin of the present invention and diisocyanate adduct (i.e. adduct of hexa methylene diisocyanate trimer, solid content 88 – 92 % by weight and isocyanate content 18 – 22 % by weight).

In a yet another embodiment of present invention there is provided a process for preparing a polyurethane coating composition, said process comprises mixing chemically modifying tannic acid polyphenylene methylene modified acrylic resin and poly isocyanate (i.e. modified acrylic resin: diisocyanate) in a specific ratio at room temperature to form polyurethane coating.
It has been found that the ratio of tannic acid polyphenylene methylene modified acrylic resin to diisocyanate is preferably in the range 3:1 to 4:1.
In a preferred embodiment of the present invention there is provided a polyurethane coating composition having self-healing and photo conducting properties comprising 70-80% by weight Tannic acid-polyphenylene methylene modified acrylic resin; 16-30% by weight Hexa methylene di isocyanate trimer and 5-10% by weight Di butyl tin di laurate solution.
According to a more preferred embodiment, in the polyurethane coating composition of the present invention, 70-80% by weight Tannic acid-polyphenylene methylene modified acrylic resin along with 5-10% by weight Di butyl tin di laurate solution to forms a base and 16-30% by weight Hexa methylene di isocyanate trimer as hardener and the base and the hardener are provided separately.
In use the two components (liquid base and liquid hardener) are mixed and applied on a substrate and allowed to dry till a hard coating film is obtained. The substrate can be any substrate, preferably mild steel panels and aluminium panels.
In a further embodiment of the present invention there is provided a kit comprising (i) a base having the tannic acid polyphenylene methylene modified acrylic resin of the present invention along with di butyl tin di laurate solution, (ii) a hardener having hexamethylene di isocyanate trimer. The base and hardener are mixed in the above described amounts to obtain the polyurethane coating composition of the present invention. The polyurethane coating composition of the present invention exhibits self-healing and photo activity under simulation of solar radiation.
Different embodiments of present invention could be better understood with the help of few examples provided below:

EXAMPLE 1
Tannic acid and polyphenylene methylene modified acrylic resin synthesis, composition and characterization
The modification of tannic acid and its’ incorporation in acrylic resin matrix is carried out through ring opening reaction of epoxy group of glycidyl methacrylate with phenolic hydroxyl of tannic acid in 2-Butoxy ethanol solvent followed by copolymerization with suitable monomers and initiator combination.
All amounts used in the below example is according to Table 1.
In the synthesis process, (a) solvent 2-Butoxy ethanol and tannic acid are charged into a four necked round bottom flask equipped with a mechanical stirrer, a reflux condenser, a thermometer, and a dropping funnel for feeding of the monomer-initiator mixture. The solvent and tannic acid mixture is heated gradually to 110 – 1200C under stirring and maintained for 30 minutes to form clear solution.
(b) Glycidyl methacrylate is added gradually to the clear solution (a) under stirring and maintained for 1 hr. at 110 – 1200C to form tannic acid and glycidyl methacrylate adduct.
(c) The solution (b) is heated gradually to 120 – 1300C and monomer initiator mixture containing styrene, n-butyl acrylate, 2-ethyl hexyl acrylate and initiator di tertiary butyl peroxide is added for 4 hrs. at that temperature.
After complete addition of monomer – initiator mixture, solvent xylene was added, and polymerization is continued at 120 – 1300C for another 4 hrs. for conversion of monomers into polymer.
Incorporation of polyphenylene methylene into tannic acid modified acrylic resin is carried out by in situ polymerization of benzyl chloride using anhydrous aluminium chloride as catalyst along with tannic acid modified acrylic as precursor. In this process, tannic acid modified acrylic resin 74.5 wt. %, benzyl chloride 25 wt.% and anhydrous aluminium chloride 0.5 wt. % are taken in a four necked round bottom flask equipped with a mechanical stirrer, a reflux condenser, a thermometer, and an inlet for purging of nitrogen gas into the reaction flask to remove by product hydrogen chloride gas formed during reaction. The reaction mixture gradually heated to 1300C and maintained for 8 hrs. under stirring in presence of nitrogen to complete the reaction.
Composition of tannic acid – polyphenylene methylene modified acrylic resin is presented in Table 1
Table 1
Resin components Quantity in wt. percentage
Xylene 15.60
2-Butoxy ethanol 14.40
Styrene 14.30
n-Butyl acrylate 13.80
2-Ethyl hexyl acrylate 8.00
Glycidyl methacrylate 2.30
Tannic acid 3.00
Di-tertiary butyl peroxide 3.10
Benzyl chloride 25.00
Anhydrous aluminium chloride 0.50

Fig. 2 FTIR illustrates absorption spectra of unmodified tannic acid (a), unmodified acrylic resin (b), tannic acid – polyphenylene methylene modified acrylic resin (c). In the FTIR spectra, the presence of sharp characteristic peak at 3425, 3028, 2957, 2930 and 2873 cm -1 correspondence to the stretching vibration of phenolic hydroxyl, olefin C-H, methyl and methylene groups respectively. The peaks at 1727 cm -1 correspond to the ester group (Fig. 2 b, c) and peaks at 1602, 1494, 1453 cm-1 correspond to phenyl ring. The characteristic IR absorption peak for aromatic phenyl and methylene of polyphenylene methylene structure have merged with the corresponding acrylic components (Fig. 2 c). However, the presence of broad absorption peak at 3425 cm -1 for phenyl hydroxyl, implies the modification of tannic acid in tannic acid polyphenylene methylene modified acrylic resin system (Fig. 2 a, c).

EXAMPLE 2
Effect of tannic acid modification into resin system on coating properties
To investigate the effect of tannic acid modification in polyurethane coating system, the hybrid resin was made by using tannic acid at different extent in resin formulation. The resin was processed as per Example 1 and polyurethane coating was made by mixing of such modified acrylic and diisocyanate adduct in a particular mixing ratio 3 - 4:1. The composition of unmodified acrylic and tannic acid polyphenylene methylene modified acrylic resin at different tannic acid concentration is presented in Table 2. The process of preparation of the tannic acid polyphenylene methylene modified acrylic resin system and the polyurethane coating composition is schematically shown in Figure 1.

Table 2 Composition of unmodified and modified acrylic at different tannic acid concentration (i.e. base)
Resin component Unmodified acrylic Modified Acrylic I Modified Acrylic II Modified Acrylic III
Xylene 15.60 15.60 15.60 15.60
2-Butoxy ethanol 14.40 14.40 14.40 14.40
Styrene 27.20 16.60 14.30 12.10
n-Butyl acrylate 23.30 13.80 13.80 13.80
2-Ethyl hexyl acrylate 12.80 8.00 8.00 8.00
Glycidyl methacrylate 3.20 2.00 2.30 2.50
Tannic acid --- 1.00 3.00 5.00
Di-tert butyl peroxide 3.50 3.10 3.10 3.10
Benzyl chloride ---- 25.00 25.00 25.00
Aluminium chloride ----- 0.50 0.50 0.50
Total 100.00 100.00 100.00 100.00

Hexa methylene di isocyanate trimer having 90 percent solid and 20 percent isocyanate content was used as hardener.
Polyurethane coating is formed by mixing of the base along with di butyl tin dilaurate and hardener in a ratio i.e., 3 - 4 : 1 by wt. After proper mixing, the coating is applied on to mild steel panels by air-assisted spraying application technique at dry film coating thickness of 70 – 80 microns.
Mechanical properties like pull-off adhesion, flexibility, abrasion resistance, pendulum hardness, cross hatch adhesion, impact resistance of different acrylic polyurethane coating systems was measured as per standard methods. The test results are presented in Table 3.

Table 3 Mechanical properties of different polyurethane coatings
Coating properties Coating using Unmodified acrylic Coating using Modified Acrylic I Coating using Modified Acrylic II Coating using Modified Acrylic III
Pull off adhesion strength in MPa, IS 101 10.40 14.10 20.88 14.92
Flexibility on ¼ inch mandrel, ASTM D522 Pass Pass Pass Fail
Abrasion resistance, weight loss in mg/1000 cycle ASTM 4060 150 120 80 110
Pendulum hardness@ 120, ASTM D 4366 96 238 272 296
Cross hatch adhesion, ASTM D3389 Less than 20% 0 0 Less than 10%
Impact resistance, ASTM D 2794-93 Fail Pass Pass Fail

The performance of coating based on modified acrylic II, is optimum to meet satisfactory mechanical properties. Higher extent of tannic acid modification resulted in inferior mechanical properties in view of increased hardness of coating film.

EXAMPLE 3
Effect of mixing ratio of tannic acid polyphenylene methylene modified acrylic resin and hexamethylene di isocyanate trimer on the performance of polyurethane coating
To study the effect of mixing ratio, polyurethane coating was made by taking tannic acid polyphenylene methylene modified acrylic resin along with Di butyl tin di laurate solution (base) and hexamethylene di isocyanate trimer (hardener) at different ratio. The performance of polyurethane coating made with different amounts of base and hardener in terms of physical properties is presented in Table 4.

Table 4 Physical properties of polyurethane coating at different mixing ratio (base: hardener)
Properties of Polyurethane coating Polyurethane coating at mixing ratio 2 : 1 by wt. Polyurethane coating at mixing ratio 3 : 1 by wt. Polyurethane coating at mixing ratio 4 : 1 by wt. Polyurethane coating at mixing ratio 5 : 1 by wt.
Pot life, hrs 4 8 9 Not dried
Drying of coating film @ 300C, hrs 2 4 5 Not satisfactory
Pendulum hardness@ 120, ASTM D 4366 394 306 282 Could not be measured
Flexibility on ¼ inch mandrel, ASTM D522 Fail Pass Pass Could not be measured

The polyurethane coating composition containing base: hardener mixing ratio 3 – 4: 1 performs satisfactory in terms of different physical properties. The coating composition containing base: hardener in a ratio 3: 1 by wt. was the optimum and met the desired properties.

EXAMPLE 4
Performance evaluation of tannic acid polyphenylene methylene modified acrylic resin based polyurethane coating with respect to self-healing and photosensitivity properties
The polyurethane coating was prepared by mixing of tannic acid polyphenylene methylene modified acrylic resin (prepared in accordance with example 1) along with Di butyl tin di laurate solution as base and hexamethylene diisocyanate trimer as hardener in a ratio of 3: 1 by wt. After proper mixing, the liquid mixture is applied on to mild steel panels and aluminium panels by air-assisted spraying application technique at dry film coating thickness of 70 – 80 microns and allowed to dry overnight to form hard polyurethane coating films. The performance of the polyurethane coated panels is evaluated after maturation of 7 days at room temperature.
Composition of polyurethane coating based on tannic acid – polyphenylene methylene modified acrylic resin and diisocyanate trimer is presented in Table 5.

Table 5 Polyurethane coating composition
Coating components Quantity in wt. percentage
Tannic acid-polyphenylene methylene modified acrylic resin 70 - 80
Di butyl tin di laurate solution (1 wt. % in Xylene) 5 - 10
Hexa methylene di isocyanate trimer 16 - 30

The self-healing capability of coating was monitored after scratching the coating surface in the shape of cross and the scratched panel was exposed under outdoor exposure. Fig. 3, shows the images of tested panel after outdoor exposure which indicates that occurrence of healing of the scratch with outdoor exposure time. This is due to the reformation of polyurethane linkages from reaction of isocyanate and phenolic hydroxyl groups of tannic acid. The surface morphology result reveals the reformation of damaged areas in scratched panel after exposure to sun light as monitored by using scanning electron microscope (Fig. 4).
The photosensitivity properties of tannic acid polyphenylene methylene modified acrylic based polyurethane coatings were performed by measuring UV-visible absorption in 250-750 nm and photoluminescence (PL) in 300 – 700 nm spectral regions. It is evident from UV-visible spectral data, the absorption centered at three different wave lengths (?abs) i.e. 315 nm, 370 nm and 440 nm. (Fig. 5A). Here, the normal UV- absorption band of polyphenylene ring has been shifted from 250 nm to 315 nm. The absorption band at 370 nm is due to p – p* transition, for delocalization of electron over several aromatic rings present in polyurethane coating. Again, the absorption band at 440 nm is due to p – p* transition of ester moieties in the acrylic resin structure. The corresponding PL emission spectrum at different wavelengths as presented in Fig. 5B, indicates the appearance of absorption band at 370 nm. This is attributed to S0 – S1 HOMO-LUMO transition in view of the formation of aromatic p p stacking interaction in the modified acrylic resin based polyurethane coating.
The results of both UV-Vis, and photoluminescence (PL) study reveal the absorption of energy by such modified polyurethane coating is in the visible region. The absorption of energy in the visible region is due to delocalization of electrons through the formation of special chemical structural during reaction of tannic acid modified acrylic and polyphenylene methylene. This has not reported earlier. The absorption of energy in the visible region is the main requirement for harvesting solar energy. Such tannic acid polyphenylene methylene modified acrylic resin based polyurethane coating can have potential for application in the harvesting of solar energy.
,CLAIMS:
1. A tannic acid and polyphenylene methylene modified acrylic resin comprising 13 – 18% by weight of xylene; 12 – 16% by weight of 2-butoxy ethanol; 12 – 18% by weight styrene; 12 – 15% by weight n-butyl acylate; 6 – 10% by weight 2-ethyl hexyl acrylate; 1 – 4% by weight of glycidyl methacrylate; 1 – 4% by weight of tannic acid; 2 – 4% by weight of di-tertiary butyl peroxide; 22-28% by weight of benzyl chloride; and 0.2 - 0.5% by weight of anhydrous aluminium chloride.

2. A process for preparing the tannic acid and polyphenylene methylene modified acrylic resin as claimed in claim 1, said process comprising:
i. synthesizing tannic acid modified acrylic resin by solution polymerization technique.
ii. chemically modifying tannic acid modified acrylic resin with benzyl chloride to form tannic acid-polyphenylene methylene modified acrylic resin via in situ formation of polyphenylene methylene polymer.

3. The process claimed in 2, wherein said step (i) comprises
(a) heating 2-butoxy ethanol solvent and tannic acid gradually to 110 – 1200C under stirring and holding for 30 – 40 min to form a clear solution,
(b) adding glycidyl methacrylate gradually to the said clear solution of step (a) under stirring at a temperature of 110 – 1200C and holding at this temperature for 1-2 hr. to form a tannic acid – glycidyl methacrylate adduct,
(c) heating the solution of step (b) gradually to 125 – 1300C and adding monomer-initiator mixture comprising 12-18 % by weight styrene, 12-15 % by weight n-butyl acrylate, 6-10 % by weight 2-ethyl hexyl acrylate and 2-4 % by weight di tertiary butyl peroxide for a period of 4-6 hrs.
(d) adding13-18% by weight solvent xylene and continuing polymerization at 1300C for 3-4 hrs. for complete conversion of monomers into said tannic acid modified acrylic resin.

4. The process as claimed in claim 2, wherein said step (ii) comprises
(a) mixing 70-80 % by weight of said tannic acid modified acrylic resin, 22-28% by weight of benzyl chloride and 0.2-0.5% by weight of anhydrous aluminium chloride,
(b) purging nitrogen gas into the reaction of step (a),
(c) gradually heating the mixture of step (b) to 1300C and maintaining for 7 – 8 hrs. under stirring in presence of nitrogen gas to complete the reaction.

5. A polyurethane coating composition comprising of 70 – 80 wt. % of Tannic acid-polyphenyle methylene modified acrylic resin as claimed in claim 1, 5 – 10 wt. % of Di butyl tin di laurate solution (1 wt. % in Xylene) and 16 – 30 wt. % of Hexa methylene di isocyanate trimer.

6. A process for preparing the polyurethane coating composition as claimed in claim 5, said process comprising the steps of mixing said tannic acid polyphenylene methylene modified acrylic resin in Di butyl tin di laurate solution with hexamethylene diisocyanate trimer in a weight ratio 3 – 4: 1 at room temperature.

7. A kit comprising (i) a base having the tannic acid polyphenylene methylene modified acrylic resin as claimed in claim 1 along with di butyl tin di laurate solution, and (ii) a hardener having hexamethylene di isocyanate trimer.

8. The polyurethane coating composition as claimed in claim 5, wherein said composition exhibits self-healing and photo activity under simulation of solar radiation.