Development of anticorrosive two pack polyurethane coatings based on modified fatty amide of Azadirachta indica Juss oil cured at room temperature – a sustainable resource

Abstract

We report the modification of Azadirachta indica Juss oil (renewable source) fatty amide by the piperazine molecule to develop a new polyol. Two pack polyurethane (PU) coatings on mild steel plates were prepared by reacting newly developed polyol with toluene diisocyanate (TDI) at room temperature. Spectral studies of Azadirachta indica Juss oil based fatty amide and piperazine modified fatty amide were carried out using spectroscopic techniques to confirm the modification. The prepared resins were also characterized by end group analysis such as amine and hydroxyl values. Anticorrosive properties of the prepared PU coatings were examined by immersion test in an aqueous salt solution. The thermal stability of coatings was studied by TGA. Other coating properties such as gloss, scratch hardness, adhesion, flexibility, impact resistance and chemical resistance were evaluated using standard methods. It was observed that the presence of nitrogen containing piperazine moiety in the back bone of the PU chain shows better anticorrosive properties compared with the Azadirachta indica Juss oil fatty amide based PU coatings.

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Introduction

The increasing cost of raw materials based on petroleum feedstock and their decreasing availability in nature forced scientists to focus their attention towards the use of renewable resources in the preparation of polymeric resins.^1–4 Renewable resources such as cellulose, carbohydrates, polysaccharides, starch, fatty acids and vegetable oils are natural and agricultural products which have been preferred as raw materials in the preparation of polymers.^5–8 Considering their sustainability and easy availability, vegetable oils and their derivatives have given more wattage as renewable-source-based feedstocks for the polymer sector.^9,10 Plant oils such as soybean (Glycine max), linseed (Linum usitatissimum), tall (liquid rosin), palm (Elaeis), nahar (Mesua ferrea), canola (Brassica), cotton (Gossypium), jatropha (Pongamia pinnata), tung (Vernicia fordii), castor (Ricinus communis) and neem (Azadirachta indica) oils have been used to prepare polymeric resins by appealing to the characteristic properties of the individual oils.^11–18 Vegetable oils have their own fatty acid compositions: the diversity in fatty acid compositions and variation in the degree of unsaturation depend on growing conditions, season and environment present to the particular plant or crop – for example, colder climate increases the level of unsaturated fatty acids in oil.¹⁹ Oils such as castor oil and lesquerella oil have naturally occurring hydroxyl groups on their triglyceride molecules. The structures of ricinoleic and lesquerolic acids of these oils contain hydroxyl groups at C12 and C14 positions, respectively.²⁰ Therefore, these oils are referred to as hydroxyl polyols, which have the ability to react with diisocyanates to develop PU or highly cross-linked polymers. Most of the vegetable oils do not show hydroxyl groups in their structure; as a result, the production of hydroxyl-terminated derivatives of vegetable oils is now an ongoing task for current polymeric researchers. A number of reports for the conversion of vegetable oils into polyols by chemical modification such as ozonolysis,¹⁹ epoxidation,²¹ hydroformylation,²² esterification²³ and amidation^24–27 are found in the literature. During chemical modifications, hydroxyl groups are introduced to the structure of vegetable oils to form the bio-based polyols.²⁸ In the last ten years, extensive work has been done to develop diethanol amides of fatty acids by amidation of various vegetable oils. The prepared diethanol amides are popularly used as non-ionic surfactants and foam boosters for the shampoo, detergent and cosmetics sectors.²⁹ Indeed, coating formulation scientists are now focusing on the use of such fatty amides in the preparation of polyesteramides,^30,31 polyetheramides¹⁷ and useful hydroxyl-terminated polyols for applications in PU coatings. In the case of vegetable-oil-based polymers, polyurethanes have tremendous importance and versatile applications in adhesives, elastomers, foams and surface coatings.

The Azadirachta indica Juss oil is plentiful in the Indian continent and other parts of the world. There is a lot of attraction for its use in agrochemical³² and pharmaceutical³³ applications. Peter et al.³⁴ investigated the use of Azadirachta indica seed extract as a corrosion inhibitor and found that it has high inhibition efficiency against metallic corrosion compared with root and leaf extracts of the plant. Azadirachta indica seed contains 33–38% oil, which possesses five major fatty acids, viz. palmitic, stearic, arachidic, oleic and linoleic acids. Considering the fatty acid composition and unsaturations present in Azadirachta indica Juss oil, it has potential in the making of polymeric resins such as poly(urethane–fatty amide),¹⁸ poly(urethane–esteramide)³⁰ and poly(urethane–ether amide)³⁵ as directed in earlier reports.

Utilization of this oil in the development of different polyols will definitely show environmental benefits, such as a reduction in the requirement of fossil or petroleum resources and the formation of very low greenhouse gas emission. Use of Azadirachta indica Juss oil in the synthesis of polyurethane resin by piperazine-modified fatty amides has not been previously reported or focused on. Piperazine is a cyclic compound containing N atom in its cyclic ring and which has widely been used as a condensing agent in the preparation of polyamides.^36,37 Corrosion of metal surfaces is now the major issue all over the globe; one of the challenges is how to eliminate or reduce the corrosion of metals. The nitrogen atom present in the piperazine molecule contains a lone pair of electrons, which introduces hydrophilicity in the coatings that can enhance adhesion of the coating against the hydrophilic substrates (metals) and can also be used as a corrosion inhibitor for iron and steel.³⁸ One more objective behind this modification was to develop high molecular weight resin in order to get more flexible and soft PU coatings.

The present work attempted to develop renewable-source Azadirachta indica Juss oil based fatty amide and its further modification by piperazine to develop anticorrosive two pack PU coatings. The prepared coatings were cured at room temperature conditions. Coating properties including anticorrosion, thermal resistance, chemical resistance, gloss, impact resistance, adhesion and flexibility were also studied.

Experimental

Azadirachta indica Juss oil was collected from a local supplier and characterized according to its physical properties, such as specific gravity, refractive index, saponification value, hydroxyl value, peroxide value and iodine value. These characterizations were matched with the standards and avoided any further chemical modification. Piperazine (diazacyclohexane) and dibutyltin dilaurate (DBTDL) were from Aldrich Chemicals. Toluene diisocyanate (TDI) was of commercial grade and characterized by determining isocyanate content by ASTM D-2572-97 before use. Cylcohexanone, tetrahydrofuran (THF) and diethanol amine of analytical grade were obtained from SDFCL, India. The remaining chemicals and solvents used for synthesis or analysis were of either synthesis or analytical grades and purchased from Aldrich Chemicals.

Synthesis of AIJFA

Synthesis of Azadirachta indica Juss oil based fatty amide (AIJFA) was carried out by reacting the oil with diethanol amine as per our previous report.¹⁷ The general reaction for synthesis of AIJFA is as given in the Scheme 1.

Scheme 1 Synthesis of AIJFA.

Synthesis of piperazine modified fatty amides (PAIJFA)

The synthesized AIJFA (1.3 mol) was dissolved in 50 mL xylene and transferred to a three necked round-bottom flask that was equipped with a Dean–Stark trap, nitrogen inlet tube, thermometer and magnetic stirrer. Piperazine (1.0 mol) was added in a solution of AIJFA, and the reaction mixture was refluxed at 150 °C until the calculated amount of water was collected in the Dean–Stark trap. The modification reaction was also confirmed by determining the amine value of the mixture at regular intervals. The amine value of the sample (dissolved in a mixture of isopropanol and water), taken at regular intervals, was estimated by titrating it against a 0.5 N HCl solution until the solution became yellow in the presence of bromophenol blue as an indicator. As the amine value went below 10 mg of KOH per g of sample, the reaction was stopped. After completion of the reaction, the product was cooled to room temperature. The trace amount of xylene was evaporated in a rotary vacuum evaporator under reduced pressure to obtain the pure PAIJFA. The general reaction for modification of AIJFA by piperazine is given in Scheme 2.

Scheme 2 Synthesis of PAIJFA.

Preparation of PU coatings

Synthesis of PU was done by reacting the prepared PAIJFA resin at room temperature with TDI in the NCO/OH ratio of 1.1 : 1. The excess NCO group hydrolyzes gradually by reacting with moisture present in the air and giving urea groups, thus increasing the crosslink density and improving the physical properties and chemical resistance of PU. This also rules out the possibility of poor durability of coatings due to the presence of unreacted OH groups. In the actual process, 50% solid content solution of PAIJFA was prepared in a cyclohexanone and THF (80 : 20) mixture with catalyst DBTDL (0.05%) for controlling the rate of reaction. The PAIJFA solution containing the catalyst was mixed with TDI and stirred manually for next 5 min to attain pourable viscosity. Then, by use of a bar applicator, the reaction mixture was coated onto MS steel panels of 4 × 6 inch dimensions, and the prepared coating panels were subjected to cure at room temperature under visual examination. The same procedure was used for preparation of the PU coatings using the previously synthesized AIJFA. Before application of the coatings, all the steel panels used in the study were pretreated by sandpaper, washed with acetone and dried in air. The PU samples prepared from AIJFA and PAIJFA were coded as PU–AIJFA and PU–PAIJFA. The general reaction scheme for the preparation of PU coatings obtained from PAIJFA is given in Scheme 3.

Scheme 3 Preparation of polyurethane coatings.

Characterizations

Analysis of raw materials used in PU coatings. Physical properties such as iodine value, specific gravity and refractive index of AIJFA and PAIJFA were determined by standard experimental methods. Viscosities of AIJFA and PAIJFA were determined on a Brookfield Rheometer (RHEO 2000 version 2.5, Brookfield Engineering Laboratories Inc., USA) in triplicate at 25 °C temperature and 20 rpm speed for 120 seconds. The FTIR spectra of AIJFA and PAIJFA were obtained by a PerkinElmer 2000 FTIR spectrometer in the range of 4000 to 500 cm⁻¹ in KBr pellets. ¹H-NMR measurements of AIJFA and PAIJFA were also performed on a Varian Mercury 300 MHz spectrometer using TMS as an internal standard in the presence of CDCl₃ as a solvent.

Hydroxyl and amine value determination. The hydroxyl and amine values of Azadirachta indica Juss oil, AIJFA and PAIJFA were estimated by acetylating reagent (ASTM D 4274-99) and isopropanol methods (ASTM D 2017-04), respectively.

Polyurethane coatings characterization

Gloss. The importance of gloss measurement is associated with the capacity of a surface to reflect more light. The digital gloss meter (Model BYK Additive & Instruments) was kept on the coated sample in auto-calibration mode at an angle 60°. Precautions were taken during the measurement in such a way that the coated panel sample should not come in contact with oil and the geometry of the gloss meter should remain at 60°.

Flexibility. The film or coatings attached to the metal substrates are elongated when the substrates are bent, which tests the ability of coatings to resist cracking when elongated. Flexibility of the coated samples was tested on a conical mandrel. While performing the test, the mandrel was free to rotate on its axis, and then coated samples were kept in between the rotating axis. The handle of the conical mandrel was lowered in a vertical direction to obtain specific angles.

Pencil hardness. The pencil hardness of the coating was measured by using a pencil hardness tester (Model BYK Additive & Instruments, Germany). In this test, pencils having different (soft and hard) grades were used to move over the surface of the coated panels from a distance of 6 mm at a fixed 45° angle by using a standard holder. Force was applied to the pencil, moving it over the surface of the test samples at a fixed angle. The coated panels were checked for removal of the film. The same procedure was repeated with pencils of higher grades of hardness as long as the film was not scratched or penetrated. The hardest pencil that does not remove the coating denotes pencil hardness of that coating.

Cross-cut adhesion. In this test, a cross-cut adhesion tester consisting of a die with 11 closed sets of parallel blades was passed and pressed on the PU coated panels in two directions at right angle to each other so that a lattice of 100 squares of 1 mm area each was formed. Then adhesion tape was placed at the centre over the lattice present on panels, and within 5 minutes the tape was removed by pulling it steadily for 2 seconds at an angle as close as possible to 60°. Then the samples were examined carefully with the help of a magnifier to determine the percentage of cubes that remained on the coating panels.

Impact and mar resistances. An impact resistance test was carried out to evaluate the load carrying capacity of the coated samples. During this test, the PU coated panels were fitted properly on the sample holder of the impact tester, and a movable weighted indenter (1.818 lb) was lifted from certain heights starting from 5–40 inches and dropped until the film cracked. After falling, weighted indenter peeling, cracking and film detachment from substrate were examined. If the presence was found of any cracks on coatings, the test was considered as failed; otherwise it was considered as passed.

The mar resistance of the prepared PU coating samples was evaluated in the laboratory by a mar resistance tester from BYK Instruments. This resistance is expressed in ‘g,’ in terms of load that failed to spoil the coated film.

Water and chemical resistance. The chemical resistance tests of PU coated panels were performed in water, solvent (xylene), acid (5% HCl) and alkali (5% NaOH) in beakers. Periodic visual inspection was taken for 7 days to find any evidence of softening, deterioration or development of cracks.

Corrosion resistance. The corrosion of metal generally depends upon the type of metal, the presence of electrolytes such as hard water, salt water, battery fluids, etc., and the period of exposure. The anticorrosion properties of prepared PU coatings were evaluated in our laboratory through an immersion test in an aqueous salt solution. Three sample panels, viz. uncoated (steel panel), PU–AIJFA coated and PU–PAIJFA coated panels were immersed in a 3.5% NaCl solution. The test was continued until the deterioration of the coatings due to the corrosion to the surface of the panels was observed. Once blistering was found on one of the sample panels of PU coatings, the panels were removed from the salt solution and examined visually. The visual inspection of surface corrosion of MS panels was recorded using a digital camera (Sony, 16 mega pixels) in the form of images.

Thermal analysis of PU coatings. Thermal analysis of the prepared PU coatings was performed on a thermogravimetric analyzer (TGA 4000, PerkinElmer, USA) in the range of room temperature to 700 °C at a heating rate of 20 °C min⁻¹ in an inert (N₂) atmosphere. Around 3 mg of the samples were taken for the thermal study, and interpretation of results was done by the Pyris software program provided by the manufacturer.

Results and discussion

Characteristic properties of Azadirachta indica Juss oil, AIJFA and PAIJFA

The prepared AIJFA and PAIJFA resins showed solubility in xylene, toluene, chloroform, carbon tetrachloride, ether, acetone, cyclohexanone and THF. The presence of long fatty acid hydrocarbon chains in resins may be associated with the solubility behaviour of resins. Specific gravity, viscosity, refractive index, saponification value, acid value and iodine value of oil, AIJFA and PAIJFA were determined by standard methods, and results are given in the (ESI) Table S1.†

Change in amine value during reaction

The formation of PAIJFA was monitored by determining the amine value of the sample taken from the reaction mixture at various time intervals. The decreasing amine value with reaction time for the preparation of PAIJFA samples is shown in Fig. 1. It shows that at the initial stage of reaction, the rate of decrease of amine value was higher; later on the rate decreased more slowly. This may be due to higher availability of functional groups at the initial stage of condensation reaction and the presence of equilibrium more towards the product's direction at the early stage. In addition, visual inspection of the reaction mixture revealed a change in viscosity due to the increase in length of product formed. The reaction was stopped after 225 min, when the final amine value obtained was 6.75 mg of KOH per gram of sample. From the ratio of difference between initial and final amine values to the initial amine value, the extent of reaction P_av was calculated. It was observed from the value of P_av that almost 97.64% of the reaction was completed. The degree of polymerization and molecular weight of prepared PAIJFA were found to be 6.576 and 2937, respectively.

Fig. 1 Plot of change in amine value versus reaction time.

FTIR and NMR spectrum of PAIJFA

The FTIR spectrum of piperazine modified AIJFA is shown in Fig. S1 (ESI).† The characteristic band of OH groups occurred at 3291 cm⁻¹. At 2855 and 2926 cm⁻¹, symmetrical and asymmetrical stretching vibrations respectively of –CH₂ and –CH₃ were attributed. The peak at 1615 cm⁻¹ was obtained for amide carbonyl, while the absorption band for C–N of amide was observed at 1465 cm⁻¹, and the band at 1358 cm⁻¹ was attributed to C–N of piperazine. The formation of PAIJFA was supported by the presence of characteristic peaks in its FTIR spectrum.

The ¹H-NMR spectrum [Fig. S2 (ESI)†] of piperazine-modified AIJFA showed the peak of terminal –CH₃ at 0.88 ppm. A chemical shift for –CH₂ of long fatty amide chain was seen at 1.25 ppm. Appearance of peaks at 3.57 and 3.59 ppm corresponded to –CH₂– linked with nitrogen of the piperazine molecule. Methylene adjacent to olefin = CH–CH₂– and –OH was found at 1.61 and 3.84 ppm respectively, while –CH₂– attached to amide nitrogen appeared at 3.51 ppm. The peak at 3.89 ppm was attributed to the proton of the hydroxyl group and the protons near to olefinic unsaturation given the peak at 5.37 ppm. The above observations and structural features confirmed the formation of PAIJFA.

Coating properties

The prepared PU coatings on MS panels were cured at room temperature. The outcome of tested coating properties for the prepared coatings included surface dry test, gloss, mar resistance, pencil hardness, impact resistance, flexibility and cross-cut adhesion, which are furnished in Table S2 (ESI).† Surface dry or dry-to-touch of PU–AIJFA and PU–PAIJFA coatings were found at 80 and 65 min, respectively. This indicated that the presence of the piperazine moiety in the PU–PAIJFA coating decreased the drying time of the coatings because its presence introduces a tertiary amine, and it is well known fact that tertiary amines act as catalysts for the isocyanate–hydroxyl reactions.³⁹ Adhesion of both the PU samples with metal substrate was within the range of 95–98%. The gloss of the PU samples was observed as 68 for PU–AIJFA and 82 for PU–PAIJFA coatings. The increased gloss of the PU–PAIJFA coating may be associated with the increase in oil based component, i.e. the amount of fatty acid in final coatings, which increases the capacity of a surface to reflect more light. Impact and mar resistances of the PU–PAIJFA coatings were found higher than that of the PU–AIJFA. The increase in impact and mar resistances of the PU–PAIJFA might have arisen due to the increased amount of soft segment in the PU–PAIJFA coatings compared to the PU–AIJFA coatings. In the case of the pencil hardness test, the film of both PU coating samples were not cut up to the level of a 1H-grade pencil, indicating that both types of coatings had good pencil hardness. Both prepared PU coatings passed the mandrel flexibility test, and not any sort of failure was found.

Water and chemical resistance. The results obtained by chemical resistance tests of the prepared PU coated panels are furnished in the Table S3 (ESI).† It was found that both PU coating samples showed notable water resistance. In the case of acid and solvent resistance tests, the PU–AIJFA sample showed some swelling and blistering to the film while only slight loss in gloss was found in the PU–PAIJFA sample. Swelling, blistering and slight cracks of the PU–AIJFA coatings might be associated with shorter length of soft segment in the case of PU–AIJFA than PU–PAIJFA, which resulted in the brittleness and poor adhesion discussed in the previous section. No significant change in the physical appearance of the PU–PAIJFA coating sample was found after exposing it to water, while minor loss in gloss was found when it was immersed in acid, alkali and solvent. Thus, from the above observations it is clear that the PU–PAIJFA coating exhibited better resistance towards water, acid, alkali and solvent than the PU–AIJFA coating.

Thermo-gravimetric analysis (TGA). TGA curves of the PU–AIJFA and PU–PAIJFA coating samples are shown in three steps of degradation as given in Fig. S3 (ESI).† First and second step degradations for both the samples showed the same temperature range, while the third step was slightly different. The first step degradation was seen in the temperature range of 187–224 °C, and degradation resulted in 9% weight loss, which may be attributed to loss of solvent and moisture present in the coatings during the curing process. Second step degradation occurred in between 223 and 301 °C due to decomposition of the urethane group of the prepared PU coatings, and up to 16% weight loss was found. Finally, third step degradation was obtained in the temperature range of 305–517 °C, with decomposition of around 55% for both the samples. The PU–PAIJFA coatings showed a slightly higher degradation temperature than PU–AIJFA. Third step degradation may be due to the decomposition of hydrocarbon chains of fatty acids contributed by the oil. Therefore, based on the results of thermal analysis, we concluded that the piperazine-modified fatty amide based PU coatings showed comparable and slightly better thermal stability in the third step than unmodified fatty amide based PU coatings.

Weight gain and weight loss study of coatings in water and NaCl solution. The prepared PU coated samples (2 × 2 cm dimension) were subjected to dip study in water and NaCl solutions to evaluate coating stability with the substrate. As indicated in Fig. S4 (ESI),† it was observed that PU–PAIJFA coatings showed a better anticorrosive nature than PU–AIJFA coatings. The weight gain and weight loss were studied by measuring the sample's weight every day, and results are given in the Table S4 (ESI).† From the data we found that panels dipped in water showed an initial weight gain for up to 5 days for the PU–AIJFA sample and 7 days for the PU–PAIJFA sample. The increment in weight gain for the PU–PAIJFA sample may be due to the higher molar mass of PU–AIJFA compared with that of PU–AIJFA. Higher weight loss was found for the PU–AIJFA compared with the PU–PAIJFA in water.

When both prepared sample panels were subjected to dip study in corrosive media (3.5% NaCl), initially weight gain was observed for up to 4 days for the PU–AIJFA sample and 7 days for the PU–PAIJFA sample. Weight losses due to the corrosion in the PU–AIJFA and PU–PAIJFA samples started after 4 and 7 days, respectively. From Fig. S4† it is clear that a higher anticorrosive nature was found in the PU–PAIJFA compared with PU–AIJFA in both water and NaCl solution.

Anticorrosive study by immersion in salt solution. Uncoated steel panels, PU–AIJFA coated and PU–PAIJFA coated steel panels were exposed in the salt solution for 240 h for the study of their anticorrosive performance. The results in the form of images of the anticorrosive test are shown in Fig. 2. It was found that uncoated steel panels showed the highest corrosion compared with the PU–AIJFA coated and PU–PAIJFA coated panels during the testing period. In the case of the coated panels, the PU–PAIJFA coatings showed better anticorrosive resistance than the PU–AIJFA coatings. No severe corrosion was found for the PU–PAIJFA coatings on steel panels, and adhesion of the film to the substrate was not affected much more, even after 240 h. The enhanced anticorrosive properties of the PU–PAIJFA coatings may be due to the existence of the nitrogen atom containing cyclic ring of the piperazine moiety in PU–PAIJFA coatings.³⁸ Better adhesion of the PU–PAIJFA coating to the metal surface may also be the supplementary reason behind the improved anticorrosive performance. The results visibly revealed that corrosion was decreased due to the incorporation of piperazine moiety in the PU coating's network. Therefore, from the salt solution immersion test, it can be concluded that the prepared coatings protected the metal surface and the piperazine-modified fatty amides offered better anticorrosion property than fatty amides based PU coatings.

Fig. 2 Images of anticorrosive resistance test of prepared PU coatings.

Corrosion protection mechanism. Corrosion processes on metallic surface involve conversion of metal (iron) atoms into the ionic state by having oxidation reaction at the anode:^40,41

Fe → Fe²⁺ + 2e⁻

Further reaction can only occur if there is a suitable electron acceptor to combine with the electron released by the iron atom. Water contains dissolved atmospheric oxygen, which readily serves the purpose of electron acceptor and is electrochemically reduced to hydroxyl ions in the cathodic reaction:

O₂ + 2H₂O + 4e⁻ → 4OH⁻

The ferrous ions and hydroxyl ions combine together to produce ferrous hydroxide, which further reacts with more oxygen to form hydrated ferric oxide, the familiar reddish brown rust (corrosion):

Fe²⁺ + 4OH⁻ → Fe(OH)₂

Fe(OH)₂ + O₂ → Fe₂O₃·H₂O

The reaction in the corrosion process can be simply represented as:

PAIJFA-based PU formed uniform and well-adhered coating over the metal substrate, which prohibits the permeation of corrosive media. The protection mechanism shown in Fig. 3 is purely through the barrier action attributed to the hydrophobic oil based resin.⁴² Due to the electro-neutrality principle, electrons released from the iron atom need to be taken up by the oxidizing agent.

Fig. 3 Corrosion protection mechanism.

The amide and carbonyl (polar) groups have a strong affinity with the iron surface, which may be due to the abundance of lone pairs of electrons on the nitrogen/oxygen atom that can interact with the vacant d-orbital of mild steel to form coordination bonds.^43,44 In this way, improvement in adhesion between the piperazine-modified resin and the mild steel substrate can result in a well-adhered coating system that does not allow corrosive media such as O₂ and water to penetrate easily through the coating network, and that protects panels from attack by corrosive species as mentioned in earlier reactions.

Conclusions

The piperazine-modified fatty amide of Azadirachta indica Juss oil was successfully characterized by the determination of physical properties and spectroscopic studies. The PU coatings obtained from piperazine-modified AIJFA with TDI showed excellent improvement in drying property over AIJFA-based PU coatings. The physico-chemical and anticorrosive performance have also been studied and have revealed satisfactory results. In the immersion test for anticorrosive properties, a significant role was played by the piperazine moiety in improving the PU coating properties. This modified resin has good potential for making formulations of surface coating as a binder for paints, considering the equivalence ratio 1.1 : 1 of NCO/OH. The renewable neem oil after modifications was utilized successfully for the synthesis of industrial PU coatings and can be used to substitute petroleum based materials.

Acknowledgements

The authors are thankful to the University Grants Commission, Govt. of India, New Delhi, for its financial assistance.

Notes and references

1. A. Gandini, Macromolecules, 2008, 41, 9491.
2. M. A. R. Meier, J. O. Metzger and U. S. Schubert, Chem. Soc. Rev., 2007, 36, 1788–1802.
3. L. M. Espinosa and M. A. R. Meier, Eur. Polym. J., 2011, 47, 837.
4. J. V. Haveren, E. A. Oostveen, E. Micciche, B. A. J. Noordover, C. E. Koning, R. A. T. M. Benthem, A. E. Frissen and J. G. J. Weijnen, J. Coat. Technol. Res., 2007, 41, 77.
5. D. N. S. Hon, Adhesives from Renewable Resources, ACS Symposium Series 385, ed. R. W. Hemingway and A. H. Conner, American Chemical Society, Washington, DC, 1989, vol. 291–310.
6. L. C. Wadworth and D. Daponte, Cellulose Chemistry and Its Applications, ed. T. P. Nevell and S. H. Zeronian, Ellis Horwood, Chichester, UK, England, 1985, vol. 349–365.
7. A. Anand, R. D. Kulkarni and V. V. Gite, Prog. Org. Coat., 2011, 74, 764.
8. J. C. Ronda, G. Lligadas, M. Galia and V. Cadiz, Eur. J. Lipid Sci. Technol., 2011, 113, 46.
9. V. V. Gite, A. B. Chaudhari, D. G. Hundiwale and R. D. Kulkarni, Pigm. Resin Technol., 2013, 42, 353.
10. F. S. Guner, Y. Yagci and A. T. Erciyes, Prog. Polym. Sci., 2006, 31, 633.
11. M. Ionescu, Z. Petrovic and X. Wan, J. Polym. Environ., 2007, 15, 237.
12. M. Alam, S. M. Ashraf, A. K. Roy and S. Ahamad, J. Polym. Environ., 2010, 18, 208.
13. R. V. Nimbalkar and V. D. Athawale, J. Am. Oil Chem. Soc., 2010, 87, 947.
14. Z. Chen, B. Chisholm, R. Patani, J. Wu, S. Fernando, K. Jogodzinski and D. Webster, J. Coat. Technol. Res., 2010, 7, 603.
15. S. Oprea, J. Mater. Sci., 2010, 45, 1315.
16. S. Narine, J. Yue and X. Kong, J. Am. Oil Chem. Soc., 2007, 84, 173.
17. P. D. Meshram, R. G. Puri, A. L. Patil and V. V. Gite, J. Coat. Technol. Res., 2013, 10, 331.
18. A. B. Chaudhari, A. Anand, S. D. Rajput, R. D. Kulkarni and V. V. Gite, Prog. Org. Coat., 2013, 76, 1779.
19. L. H. Sperling and J. A. Manson, J. Am. Oil Chem. Soc., 1983, 60, 1887.
20. D. L. Compton, J. A. Laszlo and T. A. Isbell, J. Am. Oil Chem. Soc., 2004, 81, 945.
21. Z. S. Petrovic, W. Zhang and I. Javni, Biomarcomolecules, 2005, 6, 713.
22. B. Roy and N. Karak, Polym. Bull., 2012, 68, 2299.
23. A. Guo, D. Demydov, W. Zhang and Z. S. Petrovic, J. Polym. Environ., 2002, 10, 49.
24. S. Pramanik, R. Konwarh, K. Sagar, B. K. Konwar and N. Karak, Prog. Org. Coat., 2013, 76, 689.
25. M. Kashif, F. Zafar and S. Ahmad, J. Appl. Polym. Sci., 2010, 117, 1245.
26. O. Saravari, P. Phapant and V. Pimpan, J. Appl. Polym. Sci., 2005, 96, 1170.
27. S. D. Rajput, P. P. Mahulikar and V. V. Gite, Prog. Org. Coat., 2014, 77, 38.
28. M. A. Mosiewicki, U. Casado, N. E. Marcovich and M. I. Aranguren, Polym. Eng. Sci., 2009, 49, 685.
29. C. S. Lee, T. L. Ooi and C. H. Chuah, Am. J. Appl. Sci., 2009, 6, 72.
30. A. B. Chaudhari, P. D. Tatita, R. K. Hedaoo, R. D. Kulkarni and V. V. Gite, Ind. Eng. Chem. Res., 2013, 52, 10189.
31. A. Zlantanic, C. Lava, W. Zhang and Z. S. Petrovic, J. Polym. Sci., Part B: Polym. Phys., 2004, 42, 809.
32. A. V. Bagle, R. S. Jadhav, V. V. Gite, D. G. Hundiwale and P. P. Mahulikar, Int. J. Polym. Mater. Polym. Biomater., 2013, 62, 421.
33. K. Biswas, I. Chattopadhyay, R. K. Banerjee and U. Bandyopadhyay, Curr. Sci., 2002, 82, 1336.
34. O. C. Peter, E. E. Ebenso and J. E. Udofot, Int. J. Electrochem. Sci., 2010, 5, 978.
35. A. B. Chaudhari, V. V. Gite, S. D. Rajput, P. P. Mahulikar and R. D. Kulkarni, Ind. Crops Prod., 2013, 50, 550.
36. J. A. Mikroyannidis, J. Appl. Polym. Sci., 1992, 46, 1001.
37. P. Nylen and E. Sunderland, Modern Surface Coatings, John Wiley, London, 1965.
38. N. K. Patel and J. Franco, Mater. Corros., 1975, 26, 124.
39. M. Barrere and K. Landfester, Macromolecules, 2003, 36, 5119.
40. H. Ma, S. Chen, Z. Liu and Y. Sun, J. Mol. Struct.: THEOCHEM, 1999, 488, 223.
41. T. Yang, C. W. Peng, Y. L. Lin, C. J. Weng, G. Edgington, A. Mylonakis, T. C. Huang, C. H. Hsu, J. M. Yeh and Y. Wei, J. Mater. Chem., 2012, 22, 15845.
42. S. Pathan and S. Ahmad, J. Mater. Chem. A, 2013, 1, 14227.
43. M. Alam, A. R. Ray, S. M. Ashraf and S. Ahmad, J. Am. Oil Chem. Soc., 2009, 86, 573.
44. M. Alam, S. M. Ashraf and S. Ahmad, J. Polym. Res., 2008, 15, 343.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01880j

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