Synthesis and characterization of new quinolinyl phenol based polybenzoxazine: thermal stability, hydrophobicity and corrosion resistant properties

Abstract A new class of quinolinyl phenol-based benzoxazines (QP-a, QP-cha, QP-2eha) were synthesized by using quinolinyl phenol [4-(6-chloro-4-phenylquinoline-2-yl)phenol] with three structurally distinct amines, namely aniline (a), cyclohexylamine (cha), and 2-ethylhexylamine (2eha) through Mannich condensation reaction using an appropriate solvent and reaction conditions. By using FTIR and 1H-NMR spectroscopy methods, the molecular structures of the benzoxazines QP-a, QP-cha, and QP-2eha were identified. From the differential scanning calorimetry (DSC) analysis, the exothermic peak maximum of benzoxazine monomers viz., QP-a, QP-cha, QP-2eha were observed at 218, 202 and 187 °C respectively. The thermogram obtained from TGA studies the amount of char yield obtained for poly(QP-a), poly(QP-cha) and poly(QP-2eha) were found to be 45, 24, and 32%, respectively. The value of limiting oxygen index (LOI) for polybenzoxazines was calculated using the value of percentage char yield obtained from TGA studies infer that these benzoxazines exhibit good flame-retardant characteristics. Data from water contact angle studies ascertain that these samples possess good hydrophobic properties in the range of 129–138°. Results from corrosion studies infer that the mild steel specimens coated with these benzoxazines were found to be more stable and offer better surface protection against corrosion under the conditions studied.


Introduction
Polybenzoxazines are one of the most important and versatile classes of modified phenolic thermosetting high-performance polymers, [1] which possess good mechanical and thermal properties, low moisture uptake, high carbon residue, exhibit enhanced flame retardant behavior, low surface free energy and excellent dielectric properties. [2][3][4] These properties make them useful as sealants in fabrication of electronic devices, as adhesives and coatings for different industrial and engineering applications. [5][6][7] Polybenzoxazines possess certain deficient behaviors, such as brittleness and high polymerization temperature. To overcome these drawbacks, the development of polybenzoxazine matrices with varying molecular skeletons capable of contributing high performance characteristics is warranted. [8] The improvement of polybenzoxazines with excellent thermo-mechanical [9][10][11] and electrical properties [12,13] is the major challenge to widening their utility toward load bearing and insulation under harsh environmental conditions. Different approaches have been made to enhance their physico-chemical properties; (1) low temperature curing benzoxazine resin [14,15] (2) enhancing thermal stability of polybenzoxazines [16,17] (3) achieving desirable hydrophobicity [18] through the skeletal modification of polybenzoxazine matrices.
In the recent years, the different benzoxazine monomers have been developed through Mannich reaction, using a phenolic compound, formaldehyde, and a primary amine. The benzoxazine monomers showed the outstanding significant molecular design due to the different functional groups in the phenol and amine derivatives, and thus a wide range of reactants with different functional groups, such as the alkyl, alkenyl, aryl, halogen, cyano, aldehyde, maleimide, nitro, furan and carboxylic groups. The different functional groups terminated benzoxazines have been employed for specific high-performance applications, such as aerospace, electronic circuit board, adhesives, matrices, composites, blends and alloys industry. [19][20][21][22] Recently, our research group developed different phenols and amines for the preparation of new types of benzoxazines suitable for specific application. The phenolic precursors synthesized are chalcone based bisphenol, [23] imidazole core bisphenol, [24] and amine precursors developed are pyridine based triamine, [25] imidazole core mono amine, [26] imidazole core diamine, [27] and quinoline core amine. [28] In addition, our research group developed various biobased benzoxazine for wide range of applications. [14,29] Under the corrosion environment, benzoxazine resins can be utilized in the form of coatings to protect mild steel surfaces from corrosion, due to the metal binding ability of N and O atoms or functional groups and also possess good hydrophobic nature. [30,31] Research on corrosionresistant polybenzoxazine coatings has become most vital in order to prevent corrosion-related damages of industrial equipment and to extend their working life. To achieve the effective anticorrosion applications, various benzoxazine coatings have been developed to obtain the required corrosion efficiency. Recently, our research group developed various benzoxazines for corrosion protection on mild steel; cardanol, bisphenol-F, and imidazole core benzoxazines with zirconium phosphate reinforced composite coatings to protect the mild steel surfaces from corrosion [32] . KM Mydeen reported the effect of thymol based polybenzoxazines coatings for corrosion resistant application. [29] R. Sharanya et al. [33] and A. Hariharan et al. [7] developed some cardanol based benzoxazine coatings to protect mild steel surfaces from corrosion.
In the present work, a new class of structurally modified quinolinyl phenol based benzoxazines have been developed with an objective of curing at relatively lower temperature with improved film forming behavior and enhanced hydrophobic properties in order to utilize them for high performance corrosion resistant application. In this context, the 4-(6-chloro-4-phenylquinoline-2-yl)phenol derivatives were synthesized using 2-amino-5-chloro-benzophenone and p-hydroxyacetophenone through Friedlander quinoline synthesis. [34] Then, quinolinyl phenol and paraformaldehyde were separately reacted with aniline, cyclohexylamine and 2-ethylhexylamine to obtain corresponding QP-a, QP-cha, QP-2eha benzoxazines, respectively. Curing and thermal studies were also performed to assess the polymerization behavior and thermal stability of the synthesized benzoxazine derivatives. Protection of mild steel surfaces from corrosion using benzoxazines was checked using an electrochemical workstation. Data from different studies indicates that the benzoxazines developed in the present work can be utilized in the form of adhesives, coatings and matrices where application demands under harsh thermal and moist environments.

Materials
The materials required for the synthesis of benzoxazines were procured from respective chemical agencies, viz., 2-amino-5-chloro benzophenone (Sigma-Aldrich, India), p-hydroxyacetophenone Synthesis of [4-(6-chloro-4-phenylquinoline-2-yl)phenol] 17.01 g (0.073 mol) of 2-amino-5-chlorobenzophenone was mixed with 10 g (0.073 mol) of phydroxyacetophenone and 13.96 g (0.073 mol) of para toluene sulfonic acid were added in portions to a 250 mL double necked round bottomed flask. To this mixture 40 mL of ethanol was added. The content was kept under constant stirring. The temperature was raised to 100 C and maintained for more than 6 h. After the completion of the reaction, the reaction product was allowed to stand at room temperature for 8 h. Furthermore, a yellow precipitate was filtered and washed with ethanol and dried well at 60 C to obtain the title compound (Scheme 1), and its molecular structure was confirmed through 1 H-NMR and MASS spectroscopy ( Figure S1-S2).
Synthesis of quinolinyl phenol/aniline benzoxazine (QP-a), cyclohexylamine benzoxazine (QP-cha), 2-ethylhexylamine benzoxazine (QP-2eha) 1.39 g (0.015 mol) of aniline was mixed with 5 g (0.015 mol) of quinolinyl phenol and 0.90 g (0.030 mol) of paraformaldehyde were added in portion to a 100 mL double necked round bottomed flask. To this mixture, 10 mL of 1,4-dioxane, was added as a solvent. This was kept under constant stirring. Then, the temperature was raised to 110 C and maintained for more than 5 h until the completion of formation of benzoxazine. The resinous crude product obtained was dissolved in 50 mL of ethyl acetate and washed twice with 2 N NaOH for removal of unreacted phenolic compounds. Further organic layer was washed twice with distilled water. Then, the organic phase was dried over an anhydrous Na 2 SO 4 and ethyl acetate mixture, which was removed using the rotary evaporator, and the product was recovered. The synthesized quinolinyl phenol based benzoxazine was labeled as QP-a. Similarly, quinolinyl phenol/cyclohexylamine benzoxazine (QP-cha) and quinolinyl phenol/2-ethylhexylamine benzoxazine (QP-2eha) were also synthesized by adopting the above procedure (Scheme 2).

Ring-opening polymerization of quinolinyl phenol based benzoxazines
The quinolinyl phenol based polybenzoxazines (QP-a, QP-cha, QP-2eha) were prepared through thermal ring opening polymerization. [35] In brief, the respective benzoxazine was separately dissolved in 1,4-dioxane and sonicated to obtain the homogenous product. The resultant product was heated at 80 C for 8 h to remove the solvent. After the removal of solvent, the temperature was raised to 230 C at a heating rate of 20 C/h. The heating was continued for another 3 h at 230 C for the subsequent curing. The polymerization was confirmed by Fourier transform infrared spectroscopy. Upon thermal curing, the chemical bond that exists between oxymethylene and nitrogen gets cleaved followed by the rearrangement with ortho hydrogen present in the neighboring oxazine ring by the reactive methylene group and thus polymerization occurs. The representation for the formation of network structured polybenzoxazine is represented in Scheme 3.

Preparation of polybenzoxazine coated mild steel specimen
The 2 mm thickness of carbon mixed (0.25%) mild steel (MS) plate specimen having the specimen size of 2.5 Â 1.5 cm were first surface polished by emery paper and cleaned thoroughly using ethanol solvent and dried well. Further, those MS plate specimens were placed on the table. The required volume of benzoxazine monomers weighed and dissolved separately in THF solvent and  was drop coated on the mild steel plate and it was dried well at room temperature. Then, the coated specimens were cured in an hot air oven at 200 C for 2 h. The thickness of benzoxazine film coating was found to be about 100 mm.
Characterization techniques FTIR spectra measurements were carried out using Shimadzu IRAffinity-1S FTIR Spectrophotometer using Miracle10 Single Reflectance Attenuated Total Reflection (ATR) Accessory. The ATR measurement area was a circular spot of approximately 1.5 mm diameter prism. The samples were kept directly on ZnSe prism and the spectrum recorded in the wavenumber range 400-4000 cm À1 . NMR spectra were obtained with Bruker (400 MHz) using deuterated chloroform (CDCl 3 ) as a solvent and tetramethylsilane (TMS) as an internal standard. The DSC analysis was carried out on NETZSCH STA 449F3 all monomers using empty aluminum pan as reference with heating rate of 10 C/min in nitrogen atmosphere. Temperature and heat flow scale of the instrument was calibrated under N 2 purge (60 mL min À1 ) at scanning rate of 10 C min À1 . Thermogravimetric analysis (TGA) was obtained using NETZSCH STA 449F3, taking 5 mg of sample under N 2 flow (260 mL min À1 ) and controlling the heating rate at 20 C min À1 . Water contact angle measurements were carried out using a Kyowa goniometer with 5 ll of water as the probe liquid. The benzoxazines coated mild steel plates were tested for their corrosion protection behavior in 3.5% sodium chloride solution. The corrosion experiments were carried out on low carbon mixed (0.25%) mild steel specimen. By the method of spray coating, thickness of 100 mm polymer coated mild steel specimens were used for open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The morphology of the coated materials was analyzed from an FEI QUANTA 200 F high-resolution scanning electron microscope (HRSEM).

Results and discussion
The quinolinyl phenol based benzoxazines (QP-a, QP-cha and QP-2eha) were prepared by the Friedlander quinoline synthesis of quinolinyl phenol with different amino compounds and paraformaldehyde at appropriate reaction conditions as shown in Scheme 2. The quinolinyl phenol benzoxazine monomers were confirmed by FTIR and 1 H-NMR spectral analyses. The curing behavior of benzoxazines was studied by DSC techniques. The thermal stability of the synthesized benzoxazines was studied by TGA technique.

FTIR spectral analyses of monomers
The FTIR spectrum of quinolinyl phenol, QP-a, QP-cha and QP-2eha benzoxazine monomers are shown in Figure 1. The presence of stretching vibration at 3080 cm À1 confirms the phenolic -OH of quinolinyl phenol and the absence of a peak at a similar wavelength for QP-a QP-cha and QP-2eha showed the successful formation of corresponding benzoxazine monomers.
The bands that appeared at around 1108 and 1214 cm À1 represent the asymmetric and symmetric stretching vibrations of the C-O-C bond present in the benzoxazines, respectively. The appearance of a peak at 1221 cm À1 confirms the presence of asymmetric stretching of C-N-C. Similarly, the characteristic bands obtained at 930 and 956 cm À1 which confirm the formation of oxazine ring in the aliphatic and aromatic amine based benzoxazines, respectively. The stretching of C¼C and C¼N were located at 1591 and 1635 cm À1 . The characteristic peak that appeared at around 2900 cm À1 represents the stretching vibration of CH 2 of oxazine ring. The appearance of the band around 670-760 cm À1 is attributed to C-Cl bond present in quinolinyl phenol.

H-NMR spectral analyses
The molecular structure of QP-a, QP-cha and QP-2eha benzoxazines was confirmed by 1 H-NMR spectra. The 1 H-NMR spectra of QP-a, QP-cha and QP-2eha benzoxazines are given in Moreover, the 1 H-NMR shows the multiplet appeared in the range of 0.9 À 3 ppm was corresponds to the aliphatic protons present in cyclohexylamine and 2-ethylhexylamine. The aromatic protons appeared in the range of 6.5À7.5 ppm.

Curing behavior
The curing behavior of quinolinyl phenol based benzoxazines was studied by DSC analysis at a heating rate of 10 C/min. under a nitrogen atmosphere is given in Table 1. The exothermic peak maximum of benzoxazine monomers viz., QP-a, QP-cha and QP-2eha were observed at 218, 202 and 187 C, respectively (Figure 3). Among the synthesized benzoxazines, 2-ethylhexylamine containing benzoxazine (QP-2eha) exhibits the lowest curing temperature (T p ). This may be due to more number of aliphatic methylene units in its structure, which reduces the curing temperature. The appearance of an exothermic peak with regard to the benzoxazines confirms the polymerization proceeds through thermal ring-opening polymerization (Scheme 3).

FTIR spectral analyses of polybenzoxazines
The FTIR spectra of quinolinyl phenol based polybenzoxazines are shown in Figure 4. The absence of peak at 930 cm À1 which corresponds to N-C-O bond indicates the complete cleavage of benzoxazine ring and the formation of three dimensional cross-linked network structured polybenzoxazines [poly(QP-cha) and poly(QP-2eha)]. Similarly, the polymerization of aromatic amine (poly(QP-a)) showed the absence of the peak at 956 cm À1 . The appearance of new band around 3100 cm À1 indicates the completion of the ring-opening polymerization of the oxazine rings. Further, the appearance of a new peak at 1472 cm À1 confirms the formation of a tetra substituted benzene structure.

Thermogravimetric analysis
The thermal stability of the cured poly(QP-a), poly(QP-cha) and poly(QP-2eha) benzoxazines was studied using TGA at a heating rate of 20 C/min in a nitrogen atmosphere and the results obtained are presented in Figure 5. The data obtained from TGA studies are given in Table 2.
There is no weight loss below 272 C which infers the complete removal of volatile molecules like solvents or water. The initial 5% weight loss of cured polybenzoxazines viz. poly(QP-a), poly(QPcha) and poly(QP-2eha) is noticed at above 298, 312 and 272 C respectively. Similarly, the values of maximum degradation temperature (T d ) is observed at 407, 410 and 400 C for poly(QP-a), poly(QP-cha), and poly (QP-2eha) respectively. The maximum degradation was occurred due to the bond cleavage of tertiary amine group. Then, the residual percentage char yield left at 850 C  for polybenzoxazines has been noted. The char yield of poly(QP-a), poly(QP-cha) and poly(QP-2eha) observed are 45, 24, and 32%, respectively.

Flame retardant behavior
The flame retardant behavior of the cured benzoxazines was also ascertained by using the value of residual char yield. The limiting oxygen index (LOI) value was calculated from van Krevelen equation (Equation 1) [36][37][38] and the results obtained are presented in Table 2.
Where h is the percentage char yield of materials remains at 850 C. The char yield values of the polybenzoxazines viz., poly(QP-a), poly(QP-cha) and poly(QP-2eha) were found to be 45, 24 and 32% respectively and the corresponding LOI values calculated are 36, 27 and 30 respectively, indicating their flame retardant behavior. The LOI values of the polymers are to be above the threshold value of 26, in order to possess flame retardancy. [39] All the synthesized polybenzoxazines viz., poly(QP-a), poly(QP-cha) and poly(QP-2eha) possess the value of LOI above the threshold value and infer that these benzoxazines can be considered as a self-extinguishing flame retardant material. The good flame retardant behavior arises due to the presence of three dimensional cross-linked network structure with inherent heterocyclic moiety of benzoxazine.  Water contact angle studies The values of water contact angle and images of poly(QP-a) and poly(QP-cha) and poly(QP-2eha) are presented in Figure 6. The values of water contact angle for poly(QP-a), poly(QP-cha) and poly(QP-2eha) were observed at 129 , 134 and 138 , respectively. Poly(QP-2eha) possesses better hydrophobic character than that of poly(QP-a) and poly(QP-cha) due to the presence of aliphatic chain group. The lower water affinity of the developed polybenzoxazines toward water indicates that the inherent chemical nature associated with intramolecular hydrogen bonding exists between the polybenzoxazine molecules. All the synthesized polybenzoxazine samples possess an excellent hydrophobicity and are comparable with those of existing conventional organic matrices. [40] In addition, the intra-molecular hydrogen bonding exist between the benzoxazine molecules enhanced values of water contact angle which in-turn contributes to lower surface free energy and consequently contributes to hydrophobic behavior. [41] The values of water contact angle suggest that these materials can be used as an effective insulation materials under humid environmental conditions.

Corrosion resistant studies
In the present work, in addition to synthesis and characterization of physico-chemical, thermal behavior and hydrophobic properties their corrosion resistant behavior was also studied in 3.5% NaCl solution in order to predict their utility for the protection of mild steel surfaces. By using a cross-cut test, the adhesive quality of the polybenzoxazine coated MS material was evaluated. After a cross-cut has been applied down to the level of the surface below, it can be used to evaluate how resistant a coating is to detaching from that surface. No square in the grid has come loose; the margins of the cuts are all perfectly smooth. In general surfaces that have coatings that are simple to remove from them are no longer protected from the effects of the environment. In this context, the corrosion protection efficiency of the quinolinyl phenol based polybenzoxazines viz., poly(QP-a), poly(QP-cha) and poly(QP-2eha) were coated on mild steel and immersed in 3.5% sodium chloride solution and studied their corrosion inhibition efficiency. The corrosion studies of the specimens were carried out using open-circuit potential (OCP), electrochemical impedance spectroscopy and potentiodynamic polarization. Quinolinyl phenol based polybenzoxazines were used in the form coatings for corrosion protection applications. [42,43] Quinolinyl based polybenzoxazines were used in the form coatings for corrosion protection applications. It was noticed that the poly(QP-2eha) coating exhibits better resistance toward corrosion on mild steel than the rest of the polybenzoxazines coated on mild steel specimens. This is due to the higher basic nature of 2-ethylhexylamine. The presence of aliphatic chain in poly(QP-2eha) increases its hydrophobic nature, which in turn increases the stability of its coating on MS specimen. Electrochemical impedance spectroscopy was used to assess the corrosion resistance property of the polybenzoxazines coatings on mild steel. Impedance analysis was performed using mild steel plate having 1 cm 2 area. The specimens after coating were immersed in 3.5% NaCl solution for different time periods. The OCP values were measured. It can be understood that OCP values of the coated specimens shifted significantly to the anodic direction when compared with that of the bare mild steel specimen. It can also be seen that OCP values of neat polybenzoxazines coated specimens decreased much slower rate when compared to that of bare mild steel specimen. The OCP shift progressively increasing toward positive values indicated that the high corrosion resistance offered by the coatings. [44] The poly(QP-2eha) coated specimen exhibits more anodic shift of OCP values which indicated that less porous, non-penetrable adherent film formed on the surface which in turn reduced the permeability of the corrosion medium into the film. [7] Nyquist plots derived from the EIS measurements (Figure 7) for three different benzoxazine coatings and uncoated mild steel specimens after immersing in 3.5% NaCl solution for 5 days. For uncoated mild steel specimen exhibits a small capacitive loop indicating that the poor corrosion resistance. The uncoated mild steel specimen has a single capacitive loop. Hence, fitting of EIS data is done using the equivalent circuit model (Figure 8(a,b)). Benzoxazines coated steel specimens exhibited two capacitive loops, hence, fitting of EIS data is done using the equivalent circuit model (Figure 8(b)). Out of the two constants the first one is related to the coated polybenzoxazine films, while the second time constant may be related to the corrosion taking place beneath the coatings, i.e the polymer-steel specimen interface.
The impedance spectra were analyzed using the electrical equivalent circuit represented in Figure 8(a,b), and a perfect fit for experimental data obtained is presented in the Table 3.
The two equivalent circuits, Figure 8(a,b) are used to evaluate the data, R ct is the charge transfer resistance, which are used to measure the resistance of the electron transfer across the metalsolution interface, which is inversely proportional to the corrosion rate of the metal. QY is the double layer capacitance, with constant phase element due to non-ideal behavior of the polymer-NaCl solution interface R 7 is the diffusion resistance of coating and C 6 is the diffusion capacitance of the coating. The calculated values of the corrosion parameters from EIS measurements are presented in Table 3.
The improved corrosion resistant behavior offered by the quinolinyl phenol based benzoxazines may be due to the formation of three dimensional cross-linked network structure in the coatings. The roughness factor value (n) are lower for poly(QP-2eha) matrix based coating also corroborates to the reduction of pores/cavities on the steel surface, however, all the values are close to unity indicating enhanced corrosion resting behavior.
Generally, all the organic coatings are not completely impenetrable for long time, their barrier properties could decrease when immersion time increases because of the water/corrosion medium penetration into the coatings. In the case of bare mild steel specimen, the corrosion medium possesses a direct contact with the metal surface which led to the generation of many electroactive sites and corrosion will take place freely, whereas, polybenzoxazine coatings prevent the diffusion of oxygen and aggressive medium into the polymer matrix due to the complex crosslinking network structure of polymer coatings. From the values of water contact angle, it can also be understood that, all the polybenzoxazine samples are close to super hydrophobic nature, which could effectively reduce the surface wettability of polymer coating, which in turn ultimately reduces the sorption of water molecules on the surface of coatings. [44,45] Figure 9, shows the Tafel plots of poly(QP-a), poly(QP-cha), poly(QP-2eha) coated MS specimens. The corrosion rate (CR, in millimeters per year, mm year À1 ) was calculated using I corr values with the help of the following Equation 2, Where, M is the molecular mass of copper (58.69 g mol À1 ), I corr is the corrosion current density (Acm À2 ), F is the Faradays constant (96500 A s mol À1 ), q is the density of the mild steel specimen (7.85 g cm À3 ) and the number of electrons transferred during corrosion reaction is assumed to be 2. The E corr values of all the benzoxazine samples coated specimens shifted anodically. Maximum anodic shift was observed for poly(QP-2eha) coated mild steel specimen. I corr values the polybenzoxazine coated specimens are also reduced indicating that the corrosion resistance of the coated specimens were improved after coating (Table 4). Further, the poly(QP-2eha) possesses the better corrosion resistance behavior than that of the other two benzoxazines coated MS samples. The significantly enhanced corrosion resistance again reiterates the presence of cross-linkages, which suppressed the anodic corrosion reactions (Figure 9). In general, aliphatic amines would act as a good inhibitor toward corrosion and make stable and non-porous coating over the substrate and it has been evidenced in the corrosion studies. Poly(QP-2eha) coating offers better corrosion resistance when compared with that of similar types of benzoxazines studied and reported earlier. [46] This is due to the higher basic nature of 2-ethylhexylamine. The presence of aliphatic chain in poly(QP-2eha) increases its hydrophobic nature, which in turn increases the stability of its coating on MS specimen. This is also a main reason that poly(QP-2eha) possesses better corrosion resistance when studied in 3.5% NaCl solution.
In addition, the surface morphology of the poly(QP-a), poly(QP-cha) and poly(QP-2eha) coated MS specimens were studied using scanning electron microscopic (SEM) technique ( Figure S3). The SEM images show a smooth, uniform morphology with no substantial aggregation, indicating that the benzoxazine monomers are uniformly dispersed and coated on MS specimens.  Table 3. Calculated values of corrosion parameters of the polybenzoxazines coated and bare mild steel specimens in 3.5% NaCl solution from potentiodynamic polarization studies.

Conclusion
In this new type of quinolinyl phenol (QP) was synthesized using 2-amino-5-chlorobenzophenone and 4-hydroxy acetophenone through Friedlander quinoline synthesis reaction in the presence of p-TSA catalyst and ethanol solvent. The quinolinyl phenol reacted with different amines and paraformaldehyde through Mannich condensation reaction to obtain benzoxazines. Structural characterizations of benzoxazines were performed by FTIR and 1 H-NMR spectroscopic techniques and confirm the successful synthesis of targeted compounds. The curing studies of quinolinyl phenol based benzoxazines namely QP-a, QP-cha QP-2eha were observed at 218, 202 and 187 C, respectively. Ring opening and crosslinking of the oxazine ring were confirmed with FTIR spectra of benzoxazines. All the synthesized polybenzoxazine exhibited good thermal stability and the char yield of 45, 24 and 32%, respectively were observed for poly(QP-a), poly(QP-cha) and poly(QP-2eha) at 850 C. The values of LOI calculated also indicated that all the polybenzoxazines possess self-extinguishing property, which will be suitably exploited for flame retardant applications in future. Poly(QP-2eha) possesses better corrosion inhibition efficiency (99.9%) than that of poly(QP-a) and poly(QP-cha). Data obtained from different studies suggest that these benzoxazine materials can be used as an effective coating materials for different industrial applications under humid environments.