Grafting epoxy resins - European Coatings

Quelle/Publication: European Coatings Journal 12/2004 Ausgabe/Issue: 32 Seite/Page:

Grafting epoxy resins Randhir Parmar, Kalpesh Patel, Niranjan Patel, Jayant Parmar Waterborne industrial coatings are a key area of interest for researchers because of concern about environmental pollution, industrial safety and stringent legislations. Epoxy resins of higher molecular weight were grafted with acrylic monomers, neutralized and reacted with N-butoxymethacrylamide to generate curing site on its backbone and give water compatibility. Sets of self curing coating compositions were prepared and their physico-chemical characteristics such as particle size and viscosity were measured. The performance properties of the cured films reveal their potential applications as high performance coating binders for waterborne coatings. Due to ecological reasons, solvent-based industrial coatings are increasingly being replaced by waterborne products. This trend has influenced the development of novel curing agents. The majority of the coating properties in thermosetting binders are altered by the degree of curing and the type of curing agents. Thus tremendous efforts have been made by the researchers to develop newer curing technologies with better ease of handling and use, low toxicity and low pollution of the environment, better storage stability and with improved film performance. Owing to their versatility due to latitude of selecting curing agents [1], epoxies are the best choice for further modifications. In the present work self curing epoxy-acrylic graft copolymers are prepared by using epoxy resin ("EEW-4000"). The grafting of styrene (St), methacrylicacid (MA), ethylacrylate (EA) and N-butoxymethacrylamide (NBMA) is carried out by using free radical initiator. The neutralization of this graft-copolymer using amine, result into water dispersibility. Various aspects such as preparation, optimization of major variables in compositions and characterization of their film performance properties necessary for a high performance coating binder based on waterborne coatings are studied. Materials used The epoxy resin ("EEW-4000"), was procured from M/s. Synpol, Ahmedabad (India). The acrylic monomers, MA, EA and St were supplied by M/s Saurashtra Paints Ltd., Ahmedabad (India), and butyl cellosolve used in the present study was obtained from M/s. Marigold Paints Pvt. Ltd., V.V.Nagar (India). The solvent methylethyl ketone (MEK) and monomer NBMA were supplied by M/s. Himalaya Resins Pvt. Ltd. Baroda (India). The benzoyl peroxide (BPO) and dimethyl amino ethanol (DAE) were of AR (Analytical Reagent) grade, Synthesis of self-curing EDs (SDs) Synthesis of SDs were carried out by reacting epoxy resin with various vinyl monomers, St, EA, MA and NBMA using free radical initiator at 90°C. Several SDs were prepared as per compositions (Table 1). All the SDs prepared were either bluish hazy or opaque in colour. The coating compositions (Table 2) were diluted to an applicable viscosity with water and applied on tin panels with a brush. The panels were put in a stove at 175°C for 15 min and after this the coatings were examined for performance characteristics [2, 3]. Evaluation of physical properties of the SDs The characterization of the SDs were carried out as

described in standard literature [2, 3] (Table 3). The SDs were also characterized by their IR-spectra using a Nicholet - FTIR spectrophotometer (Figure 1). The molecular weights of the SDs were measured by gel permeation chromatography (GPC). Evaluation of performance characteristics (film) The characterization of the cured films for various mechanical properties such as adhesion, flexibility, impact resistance and scratch hardness was carried out (Table 4). The films were also characterized [4] for their chemical resistance, solvent and hot water resistance, as well as salt water immersion resistance (Table 5). These films were also characterized as described earlier by IR-spectra [5] as well as by TGA (hermogravimetric Analysis) and DSC ( Differential Scanning Calorimetry) for thermal properties. Representative IR-spectra of the films are shown (Figure 1) and representative thermograms of DSC and TGA are also shown (Figure 2). Very good compatibility with water From Table 3 it can be seen that, except for SDs-6, 7, 14, & 15, the compositions gave very good compatibilities with water. This could be due to the solvating effect of acrylamide moiety over the other comonomers which can have the direct effect on their compatibility, solubility and stability of the resulting dispersions. Best temperature for curing: 300°C From the IR spectra of cured films (Figure 1) the characteristic peak of the -NH group of NBMA is observed to be weakened (1510 cm-1), which clearly indicates its participation in the curing reaction. At the same time since NBMA undergoes grafting onto the epoxy backbone, the new -CH2-groups (Reaction Scheme 1) are observed to increase in intensity of -CH bending at 2952 cm-1. The DSC thermograms of the uncured resins (Figure 2) indicate curing at 380°C with a very high exothermic heat of 105.3 J/g. At the same time the TGA of the cured composition (Figure 2) indicates decomposition in two steps. The resin is quite stable up to 300°C and then starts degradation (1st step up to 460°C) having highly cross-linked system (79.5 % weight loss). Thus the best temperature for curing is around 3000C. Viscosity studies From the results of viscosity (Table 3) it is clear that the compositions with even low NBMA content (10 %) show higher viscosity which is due to clustering of the molecules whereas those with higher content of NBMA (50%) show higher viscosity due to increased affinity for water molecules. In the present study majority of compositions based on NBMA show quite satisfactory shelf life. Good flexibility and weather resistance The results (Table 4) support and agree well with the literature stating that, compared to melamine formaldehyde based curing agent containing triazine structure and subsequent densely crosslinked structure of self condensed MF-resin moiety, the substituted acrylamide based coatings in the present study show good flexibility and weather resistance if designed properly [9]. Scratch resistance decreases with more than 40 % NBMA From the results in Table 4, it is very clear that the optimum

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level of NBMA for the impact strength can be 20-30% based on monomers. Compositions with higher NBMA were found to have increased scratch hardness due to higher CLD (Crosslinked Denisity), but in the case of compositions with more than 40% NBMA (SDs 5 & 10), a decrease in the value of scratch hardness is observed. This may be due to the self condensation of alkoxy methyl groups as the number of hydroxyl and epoxy groups may not be sufficient to react with the acrylamide moiety. It is reported that the self condensation reaction leads to the localized stresses as imperfections falls within the coating film [10]. Table 5 indicates that, in deciding chemical resistance, a higher CLD content enhances solvent resistance, hot water resistance and salt water resistance. The best: 20% of the NBMA, with epoxy:acrylic ratio of 70:30 Preparation of self curing waterborne dispersions from epoxy resins by grafting with a substituted acrylamide (NBMA) along with other co-monomers is a relatively novel approach. Thus the ideal self curing composition comprised around 20% of the NBMA, with the epoxy:acrylic ratio of 70:30. No need for any external curing agent The compositions imparted excellent mechanical properties and solvent, chemical and water resistance. These coating systems become a single component systems, which do not require any external curing agent and of course giving either equal or better performance compared to conventional melamine curing agent based systems. Thus these systems can be successfully exploited for high performance industrial waterborne coating applications. Acknowledgement The authors express thanks to Dr. R. M. Patel, Prof. and Head, Dept. of Chemistry, S.P.Uni. for providing the necessary facilities for this research work. Thanks are also due to Dr. B. G. Patel, Director, ISTAR and Dr. M. M. Patel, Hon.Coordinator ISTAR, C.V.M. Institution, V.V.Nagar for providing necessary facilities.

- Several self-curing coating compositions have been made by grafting epoxy resins onto acrylic monomers such as N-butoxymethacrylamide (NBMA). - Most of the compositions gave very good compatibilities with water. - In the IR spectra of cured films the characteristic peak for the -NH group of NBMA is observed to be weakened, which indicates its participation in the curing reaction. - Even compositions with low NBMA content (10 %) show higher viscosity. - The optimum level of NBMA for the impact strength can be 20-30% based on monomers. - In the case of compositions with more than 40% NBMA, a decrease in the value of scratch hardness is observed. - In deciding chemical resistance, a higher CLD content enhances solvent resistance, hot water resistance and salt water resistance. The authors: -> Dr.J.S.Parmar is a professor of polymer chemistry with more than 25 years of research experience. He has completed several research projects sponsored by CSIR and UGC, New Delhi, India. His research interests are in ion-exchange resins, polymeric UV-absorbers, interpenetrating polymer networks, vinylester coatings and UV-curable coatings. -> Dr.R.J.Parmar is a senior lecturer at the Dept. of Ind. Chem., Paints and Varnishes Section, ISTAR and has good experience of industrial and academic research in the area of waterborne and industrial high performance coatings. -> Kalpesh Patel has an M.Sc. in Ind. Polym. Chem. from S.P. Uni. and has experience in a Alkyd resin plant. Presently he is a lecturer at the Dept. of Ind. Chem., Paints and Varnishes section, ISTAR, V. V.Nagar. -> Dr. N. V. Patel is a research fellow at the Department of Chemistry, S. P. Uni.. He has good research experience.

REFERENCES [1] P. K. T. Oldring "Waterborne & Solvent based Epoxies and their end use Applications," Vol. II, John Wiley & Sons and Sita Tech. Ltd, London, U. K., (1997), p. 188-313. [2] IS : 197 - 1969, Methods of sampling and test for varnishes and lacquers (Ist revision), Indian Standard Institute, New Delhi, (1970). [3] Brookfield Dial Viscometer, Operating instruction manual no. m/85 - 150 - I - 495. [4] F. Konstandt, "Organic Coatings : Properties and Evaluation", Chemical Pub. Co., New York, (1985). [5] C. R. Hegedus, F. R. Pepe, J. B. Dickenson and F. H. Walker, J. Coat. Tech., 74, (2002) 927, p. 31. [6] J. T. K.Woo, R. M. Marcinko, US Patent 5093392 (1991). [7] A. Wegmann, A. G. Ciba-Geigy, J. Coat. Tech., 65, (1993) 827, p. 27. [8] I. Varghese, Paintindia, L, (2000), 8, p. 94. [9] Z. W. Wicks, F. N. Jones, P. S. Pappas, "Organic coatings : Science and Technology" Vol. I, John Wiley & Sons Inc. Chechester, (1992), p. 102. [10] V. E. Basin, Prog. Org. Coat, 12, (1984), p. 213. [11] K. Hotta, K. Tomihara, N. Kosugi, Mitsubishi Rayon Co. Ltd. Japan, Jpn. Kokai Tokyo Koho Jp. 09,255,911 (1997), C.A. 127 pp. 308469 b, (1997). Results at a glance



Figure 1: IR Spectra of (a) Uncured SD; (b) Cured film of SD.



Reaction scheme 1: SEAGC: Self curing epoxyacrylic graft copolymer; SEAGD: Self curing epoxyacrylic graft copolymer dispersion.

Figure 2: Representative thermograms of SD composition for DSC and TGA .



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