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Acrylic-melamine modified DGEBA-epoxy coatings and their anticorrosive behavior
Progress in Organic Coatings 50 (2004) 47–54
Acrylic-melamine modified DGEBA-epoxy coatings and their anticorrosive behavior Eram Sharmin, L. Imo, S.M. Ashraf, Sharif Ahmad∗ Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi 110025, India Received 22 August 2003; accepted 1 October 2003
Abstract Acrylic acid modified melamine resin (AM) was synthesized and used as a curing agent, in different compositions (10–40 wt.%), for DGEBA-epoxy resin (EAM). EAM resins were applied on mild steel strips of standard size to study their coating properties. The structures of AM and EAM were established by IR and 1 H NMR spectroscopies. The physico-mechanical and anticorrosive behavior of these coatings was evaluated by standard methods. The solubility of AM was determined in various organic solvents at room temperature. TGA and DSC techniques were used to analyze the thermal properties and curing aspects of the resin. The sample with 30 wt.% loading of AM (EAM-30), among all EAM resins gave better physico-mechanical and corrosion resistance performance under various corrosive media. The acrylic-melamine modified DGEBA-epoxy coatings were compared with the reported DGEBA-polyamide coating system. It was found that EAM-30 exhibited better anticorrosive properties than the reported DGEBA-polyamide coating system. © 2003 Elsevier B.V. All rights reserved. Keywords: Acrylic melamine; DGEBA; Anticorrosion; Solubility
1. Introduction The need to develop high performance coatings material with superior thermal, mechanical and anticorrosive properties to suit the stringent environmental conditions is pressing. Epoxy resins are the premier candidates to this end. They posses an exceptional combination of properties like good toughness, adhesion and chemical resistance and are versatile in applications [1]. The selection of a suitable curing agent plays an important role to determine the properties and life of epoxy resins. Recently, the modifications of epoxy resins with boron [2], phosphorus [3–5], naphthalene [6], silicone [7], calcium containing methacrylate resin [8], benzyl methacrylate and styrene monomer [9], metal acetyl acetonate [10], polyethersulphones [11], isocyanurate-oxazolidone [12], polyurethanes [13], acrylics [14], melamine and melamine phenol-formaldehyde resin have been reported [15,16]. Melamine resins hold potential to be used as curing agent for epoxies and have been used as melamine formaldehyde, sulphonated melamine formaldehyde, butylated melamine formaldehyde and hexamethoxymethyl melamine [17,18,23]. They impart better hardness, water, ∗ Corresponding author. E-mail address: sharifahmad [email protected] (S. Ahmad).
0300-9440/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2003.10.003
alkali and solvent resistance as well as adequate thermal stability and electronic insulation to the resin provided by the s-triazine ring [17,19]. The curing temperature of melamine formaldehyde cured alkyds and polyesteramides has been observed around 150 ◦ C while with epoxies it is as high as 180–200 ◦ C [16,20,21]. A major disadvantage with melamine resin is its poor solubility in water (0.32 g per 100 cm3 at 20 ◦ C and 5 g per 100 cm3 at 100 ◦ C), in dimethyl sulphoxide (6 g per 100 cm3 at 30 ◦ C), in glycols and glycerine (4–11 g per 100 g) [19]. The above studies motivated us to pursue the modification of melamine with acrylic acid. The approach aimed to explore the possibility to enhance the solubility of melamine in various organic solvents as well as to utilize the synthesized acrylic melamine as a curing agent for DGEBA-epoxy. Literature survey reveals that no such modification of melamine with acrylic acid has been reported so far [17,18]. The films of acrylic-melamine modified epoxy resins were applied on mild steel strips to study their anticorrosive behavior. It was found that the above modification of melamine improved the solubility of melamine. And, curing of DGEBA-epoxy with former further improved the flexibility, gloss retention properties as typical of acrylic resin [24] and reduction in baking temperature of the modified epoxy resin coatings. We report in this paper the synthesis of acrylic modified melamine (AM), acrylic-melamine cured DGEBA-epoxy
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coatings (EAM) and their spectroscopic, physico-mechanical characterization as well as corrosion resistance performance. The performance of EAM coatings was also compared with those of DGEBA-polyamide (DGEBA-PA) coatings cured at room temperature [22].
nitrogen atmosphere at a heating rate of 10 ◦ C/min. The solubility of AM was checked in various organic solvents at room temperature. Iodine value (ASTM D 555-61), saponification value, refractive index and density were determined by standard laboratory methods. 2.5. Preparation and testing of coatings
2. Experimental
Melamine was reacted with acrylic acid in 1:3 molar ratio in DMSO in a 250 ml three neck round bottomed flask equipped with a stirrer and thermometer. The temperature was maintained at 160 ◦ C throughout the reaction. The reaction was monitored by acid value and thin layer chromatography at regular intervals. The excess of solvent was removed from the synthesized product in a rotary vacuum evaporator. It was further washed with butanone.
Coatings were applied by brush (40 wt.% of the resin in solvent) on commercially available mild steel strips (30 mm × 10 mm × 1 mm) for chemical resistance test and (70 mm × 25 mm × 1 mm) strips for determination of specular gloss at 45◦ by gloss meter (model RSPT-20; digital instrument, Santa Barbara, CA), scratch hardness (BS 3900), bending test (ASTM-D3281-84) and impact resistance (IS: 101 part 5/sec-31988). The coated samples were baked at 75–78 ◦ C for 35–45 min (Table 3a). Coating thickness was measured by elcometer as 75 ± 5 m (Model 345, Elcometer Instruments, Manchester). Corrosion tests were performed in water, acid (5 wt.% HCl), alkali (5 wt.% NaOH), NaCl (3.5 wt.%) and xylene by placing them in 3 in. diameter porcelain dishes and dipping the coated samples in aforementioned media. Periodic examination was conducted till the coatings showed evidence of softening or deterioration (Table 3a). The solvent resistance tests were performed by repeated rubbing of the coated samples with the solvents after every 2 min of interval (10 times double rub, Table 3b).
2.3. Synthesis of acrylic-melamine modified DGEBA-epoxy (EAM)
3. Results and discussion
2.1. Materials Epoxy resin (diglycidyl ether of bisphenol A, DGEBA) used was of epoxy equivalent 180 (Ciba Speciality Chemicals, India). Melamine and acrylic acid (Merck, India) were of analytical grade. Tetrabutyl ammonium hydroxide (s.d.fiNE Chem, India) was used as catalyst while dimethylsulphoxide (DMSO) (Merck, India) as solvent. 2.2. Synthesis of acrylic melamine (AM)
DGEBA-epoxy along with required amount of AM was placed in a three neck round bottomed flask equipped with a thermometer and stirrer. The temperature of the reaction was maintained at 130 ± 5 ◦ C in the presence of a few drops of 60% aqueous solution of tetra butyl ammonium hydroxide (TBAH). The reaction was monitored by thin layer chromatography and epoxy equivalent at regular intervals. After the completion of the reaction, the solvent was removed in a rotary vacuum evaporator to the extent that there is no loss of flow property. 2.4. Test methods AM and EAM were characterized by FTIR and 1 H NMR spectroscopies and by thermal analysis techniques (TGA and DSC). FTIR spectra were taken on Perkin Elmer 1750 FTIR spectrophotometer (Perkin Elmer Cetus Instruments, Norwalk, CT) using a NaCl cell. 1 H NMR spectra were recorded on JEOL 300 MHz FX-1000 Spectrometer (JEOL, Peabody, MA) using dimethyl sulphoxide (DMSO) as solvent and tetra methyl silane (TMS) as an internal standard. TGA (TA 2000) was carried out in nitrogen atmosphere to study the thermal stability of the polymer. The curing behavior of EAM was studied by DSC (Dupont 910) in
Scheme 1 shows the synthesis of AM. Scheme 2 depicts the curing reaction of epoxy with AM. Figs. 1 and 2 represent the IR spectra of AM and EAM. Figs. 3 and 4 represent the 1 H NMR spectra of the same. Comparison of IR and 1 H NMR spectra of AM and EAM reveals the following structural characteristics (Table 1a and b). 3.1. Spectral analysis 3.1.1. Comparison of IR spectra of AM and EAM Multiple bands in the range of 3300–3200 cm−1 in case of AM appear as typical of –NH– stretching bands of secondary amide due to hydrogen bonding. –CO– stretching band of amide linkage at 1650 cm−1 is observed in AM while in EAM this band is at 1609 cm−1 due to –CO– of tertiary amide. The –NH– bending band at 1560 cm−1 is present in AM, it is conspicuous by its absence in EAM. Both the spectra reveal –CN– stretching absorption at 1470 cm−1 due to melamine moiety. Sharp bands in EAM at 1183.2 and 1121.4 cm−1 correspond to ether linkages. EAM spectra also features a prominent band at 1249.4 cm−1 related to the asymmetrical –C–O–C– stretching of aryl alkyl ether of DGEBA-epoxy, while the symmetrical stretching band at about 1070 cm−1 .
E. Sharmin et al. / Progress in Organic Coatings 50 (2004) 47–54
49
Scheme 1.
3.1.2. Comparison of 1 H NMR spectra of AM and EAM The details of 1 H NMR spectra of AM and EAM are given in Table 1b. 1 H NMR spectra for AM give peaks at δ = 8.1–8.2 ppm for –NH– amide proton and at δ = 5.9 and 6.2 ppm for CH2 =CH– and CH2 =CH, respectively. In case of EAM, the peak for –NH– disappears while those at δ = 5.9 and 6.2 ppm for CH2 =CH and CH2 =CH confirm the incorporation of AM in DGEBA-epoxy as given in Table 1b. The polymerization at unsaturation of acrylic acid cannot be ruled out. This is supported by the appearance of peaks at δ = 1.10 and 1.25 ppm for –CH2 –CH– and –CH2 –CH–, respectively, in either spectra.
3.2. Thermal analysis The TGA and DSC thermogram of resin composition EAM-30 are given in Figs. 5 and 6. The DSC thermogram shows a small endothermic peak at about 145 ◦ C. The TGA thermogram at this temperature shows a perceptible weight loss. We therefore correlate this peak to the evaporation of the trapped solvent. We further noticed another exothermic peak centered at 245 ◦ C followed by another peak at about 275 ◦ C in the DSC thermogram. Subsequently, two more decomposition events are observed at about 340 and 420 ◦ C. Beyond this a fifth decomposition event is noticed in the thermogram at 545 ◦ C. The TGA thermogram shows onset
Scheme 2.
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Fig. 1. FTIR spectra of AM.
Fig. 2. FTIR spectra of EAM.
E. Sharmin et al. / Progress in Organic Coatings 50 (2004) 47–54
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Fig. 3. 1 H NMR spectra of AM.
of decomposition at 145 ◦ C, the decomposition continues up to 420 ◦ C apparently as a single event. However, as observed earlier, the DSC thermogram shows at least four exothermic events in this range. It appears that the TGA fails to resolve these decomposition events. In fact the structure of the resin shows a number of groups which decompose independent of each other giving rise to these decomposition events. The TGA distinctly shows another decomposition event after 450 ◦ C and continuing up to 620 ◦ C involves 20% weight loss. However this event in DSC thermogram is highly pro-
nounced and shows that a very large enthalpy is involved in this exothermic decomposition event. TGA thermogram shows a 50% weight loss at 410 ◦ C. The DSC thermogram depicts an exotherm in this range. A closer examination of TGA and DSC thermograms reveals that the resin can be cured safely in the temperature range of 130–160 ◦ C. Also the samples show a considerable thermal resistance up to approximately 220 ◦ C which implies a safer application of these coatings up to the given temperature.
Fig. 4. 1 H NMR spectra of EAM.
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Table 1 IR and 1 H NMR values of AM and EAMa Functional group
AM (cm−1 )
EAM (cm−1 )
(a) IR values –NH– –C=O –O–CH2 –CH–O– –CH2 –O–Ph– –CN– –NH– bending
3300–3200 1650 – – 1470 1560
– 1609 1183.2, 1121.4 1249.4 1470 –
(b) 1 H NMR values –NH– –CH=CH2 –CH=CH2
8.1–8.2 6.2 5.9
– 6.2 5.9 Fig. 6. DSC thermogram of EAM-30.
3.7
–O–CH2 –CH–O– –CH2 –O–Ph– –Ph (protons) –CH3 –CH2 –CH– –CH2 –CH– a
Where Ph is
– – – – 1.10 1.25
3.8 3.5–3.4 3.9 7.02–6.6 0.97 1.10 1.25
.
3.3. Physico-chemical characterization Table 2 provides information about iodine value, refractive index, saponification value and the density of the
Table 2 The physico-chemical characterization of AM and EAM Resin code
Iodine value
Saponification Refractive value index
Specific gravity at 25 ◦ C (g/cm−3 )
EAM EAM-10a EAM-20a EAM-30a EAM-40a
29.0 12.5 12.8 13.2 13.4
16.8 11.5 12.10 12.56 12.90
1.552 1.555 1.558 1.560 1.561
1.492 1.498 1.504 1.515 1.517
a Last digit in the resin code indicates percent loading of acrylic melamine on DGEBA-epoxy.
samples. Iodine value is found to decrease from AM up to 10 wt.% addition of AM into DGEBA-epoxy due to the presence of lower amount of unsaturation in EAM-10. From 10 wt.% loading of AM into epoxy (EAM-10) to 40 wt.% loading of AM into epoxy (EAM-40), iodine value increases though marginally. The increase in iodine value supports the increase in amount of loading of AM into DGEBA-epoxy that corresponds to increase in unsaturation.
Fig. 5. TGA thermogram of EAM-30.
E. Sharmin et al. / Progress in Organic Coatings 50 (2004) 47–54
The marginal increase in iodine value indicates the increase in molar mass of EAM along with polymerization at unsaturation of acrylic group. Saponification value also follows the same trend. Increase in density and refractive indices of the resins further confirm the increase in molar mass with increase in percent loading of AM from EAM-10 to EAM-40. AM was subjected to the solubility tests in solvents like dimethyl sulphoxide, ethanol, methanol, butanol, water, xylene, toluene and butanone. In contrast to melamine, acrylic melamine was completely soluble in DMSO. AM was insoluble in xylene, benzene and butanone; in methanol, ethanol and butanol the solubility varied from 20 to 40%; in water the solubility was 7–8%. 3.4. Coating properties Table 3a provides the values of physico-mechanical and corrosion resistant properties of DGEBA-AM (baked) and DGEBA-PA (room temperature cured) systems. From EAM-10 to EAM-40, it is noticeable that the drying time firstly decreases up to EAM-20 and then becomes constant from EAM-30 to EAM-40. It shows that the optimum extent of cross-linking of coatings is achieved at 30 wt.% loading of AM (EAM-30). Scratch hardness increases up to EAM-30 followed by a decrease at EAM-40. Up to
53
EAM-30, the number of melamine groups, aromatic moieties and cross-linking through oxirane oxygen as well as via polymerization through unsaturation of acrylic group provide sufficient hardness and cross-link density to the resin which leads to an increase in the scratch hardness. Beyond 30 wt.% loading, the hard nature of melamine and the increase in cross-link density produce strain in the films that lowers down the scratch hardness and introduces brittleness. Compared to the DGEBA-polyamide (DGEBA-PA) system, the impact resistance of all compositions of EAM is much higher. The increased number of polar groups in EAM-based coating material imparts very good adhesion of the resin to the metal substrate as compared to the DGEBA-PA system. All EAM compositions exhibit high gloss comparable to DGEBA-PA system due to the presence of acrylic moiety in the resin [24]. It is further noted that with the increased addition of AM in DGEBA the increase in gloss is observed due to increased acrylic moiety. It is interesting to note that only for compositions EAM-20 and EAM-30 the coatings pass the 1/8 in. conical mandrel bend test presumably due to greater adhesion with the substrate because of the presence of an appropriate number of polar groups and also due to higher polyacrylic moiety. For the same reason, these systems show better flexibility with respect to the DGEBA-PA system.
Table 3 Coating properties of AM and EAM cured DGEBA-epoxy and DGEBA-PA systems and solvent action test Resin code
(a) Coating properties Baking timeb (min) Scratch hardness (kg) Impact test (lb/in.) Gloss at 45◦ Bending (1/8 in.) 5 wt.% HCl (168 h) 3.5 wt. NaCl (168 h) 5 wt.% NaOH (168 h) Xylene (45 days) Water (45 days) (b) Solvent action test Xylene Acetone Butanone MIBKc Butanol Chloroform a
EAM-10a
EAM-20a
EAM-30a
EAM-40a
DGEBA-PA(22)
48 2.20 200 50 Fail Loss in gloss and weight Loss in gloss and weight Loss in gloss
45 2.50 200 55 Pass Loss in gloss and weight Loss in gloss and weight Loss in gloss
35 3.50 250 60 Pass Loss in gloss
35 3.20 250 60 Fail Loss in gloss
– >5.0 120 59 Fail Film partially removed
Loss in gloss
Loss in gloss
Film partially removed
Unaffected Loss in gloss and weight
Loss in gloss and weight Unaffected Loss in gloss and weight
Film partially removed
Unaffected Loss in gloss and weight
Loss in gloss and weight Unaffected Loss in gloss and weight
– Film slightly affected but remains intact
Unaffected Unaffected Unaffected Loss in gloss and weight Loss in gloss
Unaffected Unaffected Unaffected Unaffected
Unaffected Unaffected Unaffected Unaffected
Unaffected Unaffected Unaffected Unaffected
– – – –
Loss in gloss Unaffected
Loss in gloss and weight Unaffected
–
Loss in gloss and weight
Loss in gloss and weight Unaffected
Last digit in resin code indicates wt.% loading of AM into epoxy resin. Coatings were baked at 75–78 ◦ C. c Methyl isobutyl ketone. b
–
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In contrast to the DGEBA-PA system, all EAM compositions show a remarkable alkali resistance attained by the presence of melamine, amide and ether linkages, collectively. These systems also show good resistance to xylene and fair acid resistance attributed to the presence of ether linkages. The rub tests performed on the coated samples to test the effect of the action of solvents exhibit no visual change in xylene. Contrarily, a minor deterioration is observed with other solvents and a perceptible change is noticed in case of butanol (Table 3b).
4. Conclusion The synthesis of acrylic modified melamine overcomes the solubility problem of melamine fairly. Thus, it offers an opportunity to utilize the properties of melamine by a novel route in the field of coatings. The introduction of acrylic group further provides adequate flexibility and gloss to the resin. TGA thermogram indicates better thermal properties of EAM-30. A comparative study of DGEBA-PA system with acrylic melamine epoxy system indicates better physico-mechanical and chemical resistance properties of the latter. Such a combination of melamine, amide, acrylic and ether linkages imparts to the coatings an outstanding performance under the above described corrosive media. EAM thus stands as a good coating material where high performance is needed in terms of physico-mechanical properties as well as chemical resistance, in particular against alkalis.
Acknowledgements Authors are thankful to Prof. Shakeel Ahmad, Head, Department of chemistry, for the facilities he provided in
carrying out this work. They are also acknowledging the help of Ms. Ufana Riaz, in the preparation of this manuscript. References [1] H. Lee, K. Neville, Hand Book of Epoxy Resins, McGraw-Hill, New York, 1967. [2] J. Gao, Y. Liu, J. Appl. Polym. Sci. 76 (2000) 1054. [3] P. Jain, V. Choudhary, I.K. Verma, J. Macromol. Sci.-Polym. Rev. C 42 (2) (2002) 139. [4] T.S. Wang, J.F. Yeh, M.D. Shau, J. Appl. Polym. Sci. 59 (1996) 215. [5] C.S. Wang, J.Y. Shieh, J. Appl. Polym. Sci. 73 (1999) 353. [6] C.S. Wang, M.C. Lee, J. Appl. Polym. Sci. 73 (1999) 1611. [7] K.S. Ananda, N.T.S.N. Sankara, Prog. Org. Coat. 45 (2002) 323. [8] K.E. Abraham, A. Francis, P. Ramesh, R. Joseph, J. Appl. Polym. Sci. 82 (2001) 2342. [9] K. Mimura, H. Ito, H. Fujioka, Polymer 42 (2001) 9223. [10] Z. Zhang, C.P. Wong, J. Appl. Polym. Sci. 86 (2002) 1572. [11] K. Mimura, H. Ito, H. Fujioka, Polymer 41 (2000) 4451. [12] K.S. Chian, S. Yi, J. Appl. Polym. Sci. 82 (2001) 879. [13] H. Hariani, S. Fellahi, M. Baker, J. Appl. Polym. Sci. 70 (1998) 2603. [14] D.N. Ajinkya, K. Agarwal, I.G. Vaidya, M.A. Sheroy, S.P. Potnis, J. Polym. Mater. 8 (1991) 39. [15] J. Lubczak, J. Appl. Polym. Sci. 58 (1995) 559. [16] J.Y. Shieh, C.S. Wang, Polymer 42 (2001) 7617. [17] P. Nylan, E. Sunderland, Modern Surface Coatings, Wiley, London, 1965, pp. 193–195. [18] H. Yunchao, Z. Fanser, Y. Hu, L. ChunYing, W. Zhaoqiang, L. Weining, Y. Shukai, J. Appl. Polym. Sci. 56 (1995) 1523. [19] Z. Wripsza, M. Kucharspi, J. Lubczak, J. Appl. Polym. Sci. 67 (1998) 1039. [20] T.K. Roy, V.M. Mannari, D.A. Rawal, J. Sci. Ind. Res. 56 (1997) 1661. [21] V.M. Mannari, R.P. Patel, J. Sci. Ind. Res. 57 (1998) 76. [22] S. Ahmad, S.M. Ashraf, A. Hasnat, Paint India (2002) 47. [23] A.V. Patsis (Ed.), Proceedings of the 13th International Conference on Advances in Organic Coatings Science and Technology, vol. 11, 6–11 July 1987, Technomic Publishing Co. Inc., 1987, p. 112. [24] S. Paul, Surface Coatings, Science and Technology, 2nd ed., Wiley, New York, 1996, p. 313.
Acrylic-melamine modified DGEBA-epoxy coatings and their anticorrosive behavior Eram Sharmin, L. Imo, S.M. Ashraf, Sharif Ahmad∗ Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi 110025, India Received 22 August 2003; accepted 1 October 2003
Abstract Acrylic acid modified melamine resin (AM) was synthesized and used as a curing agent, in different compositions (10–40 wt.%), for DGEBA-epoxy resin (EAM). EAM resins were applied on mild steel strips of standard size to study their coating properties. The structures of AM and EAM were established by IR and 1 H NMR spectroscopies. The physico-mechanical and anticorrosive behavior of these coatings was evaluated by standard methods. The solubility of AM was determined in various organic solvents at room temperature. TGA and DSC techniques were used to analyze the thermal properties and curing aspects of the resin. The sample with 30 wt.% loading of AM (EAM-30), among all EAM resins gave better physico-mechanical and corrosion resistance performance under various corrosive media. The acrylic-melamine modified DGEBA-epoxy coatings were compared with the reported DGEBA-polyamide coating system. It was found that EAM-30 exhibited better anticorrosive properties than the reported DGEBA-polyamide coating system. © 2003 Elsevier B.V. All rights reserved. Keywords: Acrylic melamine; DGEBA; Anticorrosion; Solubility
1. Introduction The need to develop high performance coatings material with superior thermal, mechanical and anticorrosive properties to suit the stringent environmental conditions is pressing. Epoxy resins are the premier candidates to this end. They posses an exceptional combination of properties like good toughness, adhesion and chemical resistance and are versatile in applications [1]. The selection of a suitable curing agent plays an important role to determine the properties and life of epoxy resins. Recently, the modifications of epoxy resins with boron [2], phosphorus [3–5], naphthalene [6], silicone [7], calcium containing methacrylate resin [8], benzyl methacrylate and styrene monomer [9], metal acetyl acetonate [10], polyethersulphones [11], isocyanurate-oxazolidone [12], polyurethanes [13], acrylics [14], melamine and melamine phenol-formaldehyde resin have been reported [15,16]. Melamine resins hold potential to be used as curing agent for epoxies and have been used as melamine formaldehyde, sulphonated melamine formaldehyde, butylated melamine formaldehyde and hexamethoxymethyl melamine [17,18,23]. They impart better hardness, water, ∗ Corresponding author. E-mail address: sharifahmad [email protected] (S. Ahmad).
0300-9440/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2003.10.003
alkali and solvent resistance as well as adequate thermal stability and electronic insulation to the resin provided by the s-triazine ring [17,19]. The curing temperature of melamine formaldehyde cured alkyds and polyesteramides has been observed around 150 ◦ C while with epoxies it is as high as 180–200 ◦ C [16,20,21]. A major disadvantage with melamine resin is its poor solubility in water (0.32 g per 100 cm3 at 20 ◦ C and 5 g per 100 cm3 at 100 ◦ C), in dimethyl sulphoxide (6 g per 100 cm3 at 30 ◦ C), in glycols and glycerine (4–11 g per 100 g) [19]. The above studies motivated us to pursue the modification of melamine with acrylic acid. The approach aimed to explore the possibility to enhance the solubility of melamine in various organic solvents as well as to utilize the synthesized acrylic melamine as a curing agent for DGEBA-epoxy. Literature survey reveals that no such modification of melamine with acrylic acid has been reported so far [17,18]. The films of acrylic-melamine modified epoxy resins were applied on mild steel strips to study their anticorrosive behavior. It was found that the above modification of melamine improved the solubility of melamine. And, curing of DGEBA-epoxy with former further improved the flexibility, gloss retention properties as typical of acrylic resin [24] and reduction in baking temperature of the modified epoxy resin coatings. We report in this paper the synthesis of acrylic modified melamine (AM), acrylic-melamine cured DGEBA-epoxy
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E. Sharmin et al. / Progress in Organic Coatings 50 (2004) 47–54
coatings (EAM) and their spectroscopic, physico-mechanical characterization as well as corrosion resistance performance. The performance of EAM coatings was also compared with those of DGEBA-polyamide (DGEBA-PA) coatings cured at room temperature [22].
nitrogen atmosphere at a heating rate of 10 ◦ C/min. The solubility of AM was checked in various organic solvents at room temperature. Iodine value (ASTM D 555-61), saponification value, refractive index and density were determined by standard laboratory methods. 2.5. Preparation and testing of coatings
2. Experimental
Melamine was reacted with acrylic acid in 1:3 molar ratio in DMSO in a 250 ml three neck round bottomed flask equipped with a stirrer and thermometer. The temperature was maintained at 160 ◦ C throughout the reaction. The reaction was monitored by acid value and thin layer chromatography at regular intervals. The excess of solvent was removed from the synthesized product in a rotary vacuum evaporator. It was further washed with butanone.
Coatings were applied by brush (40 wt.% of the resin in solvent) on commercially available mild steel strips (30 mm × 10 mm × 1 mm) for chemical resistance test and (70 mm × 25 mm × 1 mm) strips for determination of specular gloss at 45◦ by gloss meter (model RSPT-20; digital instrument, Santa Barbara, CA), scratch hardness (BS 3900), bending test (ASTM-D3281-84) and impact resistance (IS: 101 part 5/sec-31988). The coated samples were baked at 75–78 ◦ C for 35–45 min (Table 3a). Coating thickness was measured by elcometer as 75 ± 5 m (Model 345, Elcometer Instruments, Manchester). Corrosion tests were performed in water, acid (5 wt.% HCl), alkali (5 wt.% NaOH), NaCl (3.5 wt.%) and xylene by placing them in 3 in. diameter porcelain dishes and dipping the coated samples in aforementioned media. Periodic examination was conducted till the coatings showed evidence of softening or deterioration (Table 3a). The solvent resistance tests were performed by repeated rubbing of the coated samples with the solvents after every 2 min of interval (10 times double rub, Table 3b).
2.3. Synthesis of acrylic-melamine modified DGEBA-epoxy (EAM)
3. Results and discussion
2.1. Materials Epoxy resin (diglycidyl ether of bisphenol A, DGEBA) used was of epoxy equivalent 180 (Ciba Speciality Chemicals, India). Melamine and acrylic acid (Merck, India) were of analytical grade. Tetrabutyl ammonium hydroxide (s.d.fiNE Chem, India) was used as catalyst while dimethylsulphoxide (DMSO) (Merck, India) as solvent. 2.2. Synthesis of acrylic melamine (AM)
DGEBA-epoxy along with required amount of AM was placed in a three neck round bottomed flask equipped with a thermometer and stirrer. The temperature of the reaction was maintained at 130 ± 5 ◦ C in the presence of a few drops of 60% aqueous solution of tetra butyl ammonium hydroxide (TBAH). The reaction was monitored by thin layer chromatography and epoxy equivalent at regular intervals. After the completion of the reaction, the solvent was removed in a rotary vacuum evaporator to the extent that there is no loss of flow property. 2.4. Test methods AM and EAM were characterized by FTIR and 1 H NMR spectroscopies and by thermal analysis techniques (TGA and DSC). FTIR spectra were taken on Perkin Elmer 1750 FTIR spectrophotometer (Perkin Elmer Cetus Instruments, Norwalk, CT) using a NaCl cell. 1 H NMR spectra were recorded on JEOL 300 MHz FX-1000 Spectrometer (JEOL, Peabody, MA) using dimethyl sulphoxide (DMSO) as solvent and tetra methyl silane (TMS) as an internal standard. TGA (TA 2000) was carried out in nitrogen atmosphere to study the thermal stability of the polymer. The curing behavior of EAM was studied by DSC (Dupont 910) in
Scheme 1 shows the synthesis of AM. Scheme 2 depicts the curing reaction of epoxy with AM. Figs. 1 and 2 represent the IR spectra of AM and EAM. Figs. 3 and 4 represent the 1 H NMR spectra of the same. Comparison of IR and 1 H NMR spectra of AM and EAM reveals the following structural characteristics (Table 1a and b). 3.1. Spectral analysis 3.1.1. Comparison of IR spectra of AM and EAM Multiple bands in the range of 3300–3200 cm−1 in case of AM appear as typical of –NH– stretching bands of secondary amide due to hydrogen bonding. –CO– stretching band of amide linkage at 1650 cm−1 is observed in AM while in EAM this band is at 1609 cm−1 due to –CO– of tertiary amide. The –NH– bending band at 1560 cm−1 is present in AM, it is conspicuous by its absence in EAM. Both the spectra reveal –CN– stretching absorption at 1470 cm−1 due to melamine moiety. Sharp bands in EAM at 1183.2 and 1121.4 cm−1 correspond to ether linkages. EAM spectra also features a prominent band at 1249.4 cm−1 related to the asymmetrical –C–O–C– stretching of aryl alkyl ether of DGEBA-epoxy, while the symmetrical stretching band at about 1070 cm−1 .
E. Sharmin et al. / Progress in Organic Coatings 50 (2004) 47–54
49
Scheme 1.
3.1.2. Comparison of 1 H NMR spectra of AM and EAM The details of 1 H NMR spectra of AM and EAM are given in Table 1b. 1 H NMR spectra for AM give peaks at δ = 8.1–8.2 ppm for –NH– amide proton and at δ = 5.9 and 6.2 ppm for CH2 =CH– and CH2 =CH, respectively. In case of EAM, the peak for –NH– disappears while those at δ = 5.9 and 6.2 ppm for CH2 =CH and CH2 =CH confirm the incorporation of AM in DGEBA-epoxy as given in Table 1b. The polymerization at unsaturation of acrylic acid cannot be ruled out. This is supported by the appearance of peaks at δ = 1.10 and 1.25 ppm for –CH2 –CH– and –CH2 –CH–, respectively, in either spectra.
3.2. Thermal analysis The TGA and DSC thermogram of resin composition EAM-30 are given in Figs. 5 and 6. The DSC thermogram shows a small endothermic peak at about 145 ◦ C. The TGA thermogram at this temperature shows a perceptible weight loss. We therefore correlate this peak to the evaporation of the trapped solvent. We further noticed another exothermic peak centered at 245 ◦ C followed by another peak at about 275 ◦ C in the DSC thermogram. Subsequently, two more decomposition events are observed at about 340 and 420 ◦ C. Beyond this a fifth decomposition event is noticed in the thermogram at 545 ◦ C. The TGA thermogram shows onset
Scheme 2.
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E. Sharmin et al. / Progress in Organic Coatings 50 (2004) 47–54
Fig. 1. FTIR spectra of AM.
Fig. 2. FTIR spectra of EAM.
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Fig. 3. 1 H NMR spectra of AM.
of decomposition at 145 ◦ C, the decomposition continues up to 420 ◦ C apparently as a single event. However, as observed earlier, the DSC thermogram shows at least four exothermic events in this range. It appears that the TGA fails to resolve these decomposition events. In fact the structure of the resin shows a number of groups which decompose independent of each other giving rise to these decomposition events. The TGA distinctly shows another decomposition event after 450 ◦ C and continuing up to 620 ◦ C involves 20% weight loss. However this event in DSC thermogram is highly pro-
nounced and shows that a very large enthalpy is involved in this exothermic decomposition event. TGA thermogram shows a 50% weight loss at 410 ◦ C. The DSC thermogram depicts an exotherm in this range. A closer examination of TGA and DSC thermograms reveals that the resin can be cured safely in the temperature range of 130–160 ◦ C. Also the samples show a considerable thermal resistance up to approximately 220 ◦ C which implies a safer application of these coatings up to the given temperature.
Fig. 4. 1 H NMR spectra of EAM.
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E. Sharmin et al. / Progress in Organic Coatings 50 (2004) 47–54
Table 1 IR and 1 H NMR values of AM and EAMa Functional group
AM (cm−1 )
EAM (cm−1 )
(a) IR values –NH– –C=O –O–CH2 –CH–O– –CH2 –O–Ph– –CN– –NH– bending
3300–3200 1650 – – 1470 1560
– 1609 1183.2, 1121.4 1249.4 1470 –
(b) 1 H NMR values –NH– –CH=CH2 –CH=CH2
8.1–8.2 6.2 5.9
– 6.2 5.9 Fig. 6. DSC thermogram of EAM-30.
3.7
–O–CH2 –CH–O– –CH2 –O–Ph– –Ph (protons) –CH3 –CH2 –CH– –CH2 –CH– a
Where Ph is
– – – – 1.10 1.25
3.8 3.5–3.4 3.9 7.02–6.6 0.97 1.10 1.25
.
3.3. Physico-chemical characterization Table 2 provides information about iodine value, refractive index, saponification value and the density of the
Table 2 The physico-chemical characterization of AM and EAM Resin code
Iodine value
Saponification Refractive value index
Specific gravity at 25 ◦ C (g/cm−3 )
EAM EAM-10a EAM-20a EAM-30a EAM-40a
29.0 12.5 12.8 13.2 13.4
16.8 11.5 12.10 12.56 12.90
1.552 1.555 1.558 1.560 1.561
1.492 1.498 1.504 1.515 1.517
a Last digit in the resin code indicates percent loading of acrylic melamine on DGEBA-epoxy.
samples. Iodine value is found to decrease from AM up to 10 wt.% addition of AM into DGEBA-epoxy due to the presence of lower amount of unsaturation in EAM-10. From 10 wt.% loading of AM into epoxy (EAM-10) to 40 wt.% loading of AM into epoxy (EAM-40), iodine value increases though marginally. The increase in iodine value supports the increase in amount of loading of AM into DGEBA-epoxy that corresponds to increase in unsaturation.
Fig. 5. TGA thermogram of EAM-30.
E. Sharmin et al. / Progress in Organic Coatings 50 (2004) 47–54
The marginal increase in iodine value indicates the increase in molar mass of EAM along with polymerization at unsaturation of acrylic group. Saponification value also follows the same trend. Increase in density and refractive indices of the resins further confirm the increase in molar mass with increase in percent loading of AM from EAM-10 to EAM-40. AM was subjected to the solubility tests in solvents like dimethyl sulphoxide, ethanol, methanol, butanol, water, xylene, toluene and butanone. In contrast to melamine, acrylic melamine was completely soluble in DMSO. AM was insoluble in xylene, benzene and butanone; in methanol, ethanol and butanol the solubility varied from 20 to 40%; in water the solubility was 7–8%. 3.4. Coating properties Table 3a provides the values of physico-mechanical and corrosion resistant properties of DGEBA-AM (baked) and DGEBA-PA (room temperature cured) systems. From EAM-10 to EAM-40, it is noticeable that the drying time firstly decreases up to EAM-20 and then becomes constant from EAM-30 to EAM-40. It shows that the optimum extent of cross-linking of coatings is achieved at 30 wt.% loading of AM (EAM-30). Scratch hardness increases up to EAM-30 followed by a decrease at EAM-40. Up to
53
EAM-30, the number of melamine groups, aromatic moieties and cross-linking through oxirane oxygen as well as via polymerization through unsaturation of acrylic group provide sufficient hardness and cross-link density to the resin which leads to an increase in the scratch hardness. Beyond 30 wt.% loading, the hard nature of melamine and the increase in cross-link density produce strain in the films that lowers down the scratch hardness and introduces brittleness. Compared to the DGEBA-polyamide (DGEBA-PA) system, the impact resistance of all compositions of EAM is much higher. The increased number of polar groups in EAM-based coating material imparts very good adhesion of the resin to the metal substrate as compared to the DGEBA-PA system. All EAM compositions exhibit high gloss comparable to DGEBA-PA system due to the presence of acrylic moiety in the resin [24]. It is further noted that with the increased addition of AM in DGEBA the increase in gloss is observed due to increased acrylic moiety. It is interesting to note that only for compositions EAM-20 and EAM-30 the coatings pass the 1/8 in. conical mandrel bend test presumably due to greater adhesion with the substrate because of the presence of an appropriate number of polar groups and also due to higher polyacrylic moiety. For the same reason, these systems show better flexibility with respect to the DGEBA-PA system.
Table 3 Coating properties of AM and EAM cured DGEBA-epoxy and DGEBA-PA systems and solvent action test Resin code
(a) Coating properties Baking timeb (min) Scratch hardness (kg) Impact test (lb/in.) Gloss at 45◦ Bending (1/8 in.) 5 wt.% HCl (168 h) 3.5 wt. NaCl (168 h) 5 wt.% NaOH (168 h) Xylene (45 days) Water (45 days) (b) Solvent action test Xylene Acetone Butanone MIBKc Butanol Chloroform a
EAM-10a
EAM-20a
EAM-30a
EAM-40a
DGEBA-PA(22)
48 2.20 200 50 Fail Loss in gloss and weight Loss in gloss and weight Loss in gloss
45 2.50 200 55 Pass Loss in gloss and weight Loss in gloss and weight Loss in gloss
35 3.50 250 60 Pass Loss in gloss
35 3.20 250 60 Fail Loss in gloss
– >5.0 120 59 Fail Film partially removed
Loss in gloss
Loss in gloss
Film partially removed
Unaffected Loss in gloss and weight
Loss in gloss and weight Unaffected Loss in gloss and weight
Film partially removed
Unaffected Loss in gloss and weight
Loss in gloss and weight Unaffected Loss in gloss and weight
– Film slightly affected but remains intact
Unaffected Unaffected Unaffected Loss in gloss and weight Loss in gloss
Unaffected Unaffected Unaffected Unaffected
Unaffected Unaffected Unaffected Unaffected
Unaffected Unaffected Unaffected Unaffected
– – – –
Loss in gloss Unaffected
Loss in gloss and weight Unaffected
–
Loss in gloss and weight
Loss in gloss and weight Unaffected
Last digit in resin code indicates wt.% loading of AM into epoxy resin. Coatings were baked at 75–78 ◦ C. c Methyl isobutyl ketone. b
–
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In contrast to the DGEBA-PA system, all EAM compositions show a remarkable alkali resistance attained by the presence of melamine, amide and ether linkages, collectively. These systems also show good resistance to xylene and fair acid resistance attributed to the presence of ether linkages. The rub tests performed on the coated samples to test the effect of the action of solvents exhibit no visual change in xylene. Contrarily, a minor deterioration is observed with other solvents and a perceptible change is noticed in case of butanol (Table 3b).
4. Conclusion The synthesis of acrylic modified melamine overcomes the solubility problem of melamine fairly. Thus, it offers an opportunity to utilize the properties of melamine by a novel route in the field of coatings. The introduction of acrylic group further provides adequate flexibility and gloss to the resin. TGA thermogram indicates better thermal properties of EAM-30. A comparative study of DGEBA-PA system with acrylic melamine epoxy system indicates better physico-mechanical and chemical resistance properties of the latter. Such a combination of melamine, amide, acrylic and ether linkages imparts to the coatings an outstanding performance under the above described corrosive media. EAM thus stands as a good coating material where high performance is needed in terms of physico-mechanical properties as well as chemical resistance, in particular against alkalis.
Acknowledgements Authors are thankful to Prof. Shakeel Ahmad, Head, Department of chemistry, for the facilities he provided in
carrying out this work. They are also acknowledging the help of Ms. Ufana Riaz, in the preparation of this manuscript. References [1] H. Lee, K. Neville, Hand Book of Epoxy Resins, McGraw-Hill, New York, 1967. [2] J. Gao, Y. Liu, J. Appl. Polym. Sci. 76 (2000) 1054. [3] P. Jain, V. Choudhary, I.K. Verma, J. Macromol. Sci.-Polym. Rev. C 42 (2) (2002) 139. [4] T.S. Wang, J.F. Yeh, M.D. Shau, J. Appl. Polym. Sci. 59 (1996) 215. [5] C.S. Wang, J.Y. Shieh, J. Appl. Polym. Sci. 73 (1999) 353. [6] C.S. Wang, M.C. Lee, J. Appl. Polym. Sci. 73 (1999) 1611. [7] K.S. Ananda, N.T.S.N. Sankara, Prog. Org. Coat. 45 (2002) 323. [8] K.E. Abraham, A. Francis, P. Ramesh, R. Joseph, J. Appl. Polym. Sci. 82 (2001) 2342. [9] K. Mimura, H. Ito, H. Fujioka, Polymer 42 (2001) 9223. [10] Z. Zhang, C.P. Wong, J. Appl. Polym. Sci. 86 (2002) 1572. [11] K. Mimura, H. Ito, H. Fujioka, Polymer 41 (2000) 4451. [12] K.S. Chian, S. Yi, J. Appl. Polym. Sci. 82 (2001) 879. [13] H. Hariani, S. Fellahi, M. Baker, J. Appl. Polym. Sci. 70 (1998) 2603. [14] D.N. Ajinkya, K. Agarwal, I.G. Vaidya, M.A. Sheroy, S.P. Potnis, J. Polym. Mater. 8 (1991) 39. [15] J. Lubczak, J. Appl. Polym. Sci. 58 (1995) 559. [16] J.Y. Shieh, C.S. Wang, Polymer 42 (2001) 7617. [17] P. Nylan, E. Sunderland, Modern Surface Coatings, Wiley, London, 1965, pp. 193–195. [18] H. Yunchao, Z. Fanser, Y. Hu, L. ChunYing, W. Zhaoqiang, L. Weining, Y. Shukai, J. Appl. Polym. Sci. 56 (1995) 1523. [19] Z. Wripsza, M. Kucharspi, J. Lubczak, J. Appl. Polym. Sci. 67 (1998) 1039. [20] T.K. Roy, V.M. Mannari, D.A. Rawal, J. Sci. Ind. Res. 56 (1997) 1661. [21] V.M. Mannari, R.P. Patel, J. Sci. Ind. Res. 57 (1998) 76. [22] S. Ahmad, S.M. Ashraf, A. Hasnat, Paint India (2002) 47. [23] A.V. Patsis (Ed.), Proceedings of the 13th International Conference on Advances in Organic Coatings Science and Technology, vol. 11, 6–11 July 1987, Technomic Publishing Co. Inc., 1987, p. 112. [24] S. Paul, Surface Coatings, Science and Technology, 2nd ed., Wiley, New York, 1996, p. 313.