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DGEBA epoxy blends for coating applications: Preparation, characterization, ageing studies and coating properties
Progress in Organic Coatings 67 (2010) 170–179
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Linseed amide diol/DGEBA epoxy blends for coating applications: Preparation, characterization, ageing studies and coating properties Eram Sharmin a , M.S. Alam b , Renjish K. Philip b , Sharif Ahmad a,∗ a b
Materials Research Lab., Dept. of Chemistry, Jamia Millia Islamia, New Delhi 110025, India Dept. of Chemistry, Jamia Hamdard University, New Delhi, India
a r t i c l e
i n f o
Article history: Received 28 April 2009 Received in revised form 25 August 2009 Accepted 17 September 2009 Keywords: Ageing Blends Thermogravimetric analysis FTIR Coatings
a b s t r a c t Linseed amide diol [HELA] was used as modifier for conventionally available epoxy resin [DGEBA] by blending in different ratio. Blends [DGEBA/HELA] were subjected to spectral, physico-chemical, ageing and antibacterial studies. Interesting features of the blends were complete miscibility of HELA with DGEBA, principally due to hydrogen bonding and chemical reaction between the two constituents, and their moderate antibacterial activity against S. aureus. DGEBA/HELA blends were further treated with triethylenetetramine [TETA-A] [DGEBA/HELA/A] as curing agent to evaluate their performance as corrosion protective coating materials. DGEBA/HELA/A coatings showed good physico-mechanical and chemical resistance behavior, in particular against alkaline media. Thermal analysis of DGEBA/HELA/A revealed their single to multi-step degradation behavior with safe usage upto 220 ◦ C. Our investigations confirm the dual role of HELA as environment-friendly, reactive-modifier and mild curing agent for epoxy resins. Besides, DGEBA/HELA/A may find potential applications as “solvent free”, ambient temperature cured antibacterial coating materials. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Epoxy resins comprise an important class of thermosetting materials, with unique combination of properties, that can be tailor made depending upon the type of curing agent and modifier employed [1–3]. Their cured networks exhibit high modulus, low creep, good adhesion, physico-mechanical and corrosion/chemical resistance properties as well as good performance at elevated temperatures for various end use applications such as adhesives, lubricants, industrial tooling, reinforced plastics, paints and coatings. The major drawback of epoxy resins is that on curing, they generally contract and develop internal stresses, which deteriorates their resistance against the attack of moisture and chemicals, lowers their fracture energy, hydrophobicity and impact strength [2–6]. Over the last few years, modifications of epoxies have been accomplished with polystyrene, polymethylmethacrylate, PEEK, PES, PEI, polyurethanes [4,7–10], liquid rubber, siloxane based polymers, hydroxyl terminated polybutadiene, hydroxyl functionalized hyperbranched polymer, polyesters, liquid elastomers, inorganic particles or nanofillers either by blending or by chemical reaction between the virgin epoxy and the modifier [11–20]. Blending with
∗ Corresponding author. Tel.: +91 9891290510; fax: +91 011 26827508. E-mail addresses: eram [email protected] (E. Sharmin), sharifahmad [email protected] (S. Ahmad). 0300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2009.09.012
thermoplastics leads to enormous increase in viscosity of the resin, which causes problems especially during paint formulation. The major disadvantage with rubber and siloxane modifiers is that of miscibility, phase separation and bleeding of the modifier, respectively [2,15–28]. Several vegetable oil based polymers have been used as reactive modifiers and diluents of epoxy composites and nanocomposites [17,18]. Research investigations have been carried out by Kobayashi and co-workers [29], Miyagawa et al. [30], Erhan and co-workers [31], Chandrashekhara et al. [32], Ray and Bousmina [18], Uyama et al. [33] and Jin and Park [34] on synthesis, fabrication, mechanical, thermo-physical and thermo-mechanical properties of DGEBA [Diglycidyl ether of Bisphenol A]/oil epoxy. These systems show good miscibility, improved fracture toughness and impact properties compared to the virgin epoxy resin. Blends of Mesua ferrea L. seed oil polyurethanes/DGEBA epoxy have shown good performance as surface coatings and paints according to separate reports by Karak and co-workers [35,36]. Fatty amide diols are obtained through the base-catalysed aminolysis of oils such as linseed, karanj, castor, soybean and others, as monomers with functionalities viz., hydroxyl, double bonds, active methylenes, amides and long alkyl chain of parent oil [37]. They find extensive applications as one of the starting materials of several polymeric resins i.e., polyesteramide, polyetheramide, polyurethaneamide for their use in coatings and adhesives. They exhibit the characteristics typical of an epoxy modifier such as low molecular weight, ease of processing, suitable functionality, misci-
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bility or solubility with epoxies, inherent fluidity to afford curing in a “solvent free” environment (to limit the harmful effects of VOCs and to avoid disadvantages of imperfections e.g., intramolecular loop and void formation during cross-linking reactions, in solution) at ambient temperature, and most importantly, renewability/sustainability [1,2,27,28]. However, literature survey reveals that they have not yet been used for the same [35,36]. In this work, we have attempted for the first time, to develop DGEBA epoxy coating materials by blending, in different formulations by weight (DGEBA/HELA) in the absence of any organic solvent, with amide diol-N,N -bis (2-hydroxyl ethyl) linseed amide [H, MW: 367] obtained from linseed oil. DGEBA/HELA blends were characterized by spectral, physico-chemical, antibacterial and ageing studies using standard methods. Blends were further treated with triethylenetetramine [TETA = A] curing agent [DGEBA/HELA/A] to prepare their coatings; the latter were analyzed by physico-mechanical, thermal (TGA, DTG, DSC), X-ray and chemical resistance tests. Our investigative studies revealed that HELA can be used both as sustainable resource based antibacterial, environment friendly, reactive modifier and mild curing agent for epoxy resins. We further observed that such DGEBA/HELA combinations may serve as potential candidates in the field of coatings and paints.
2. Experimental 2.1. Materials Seeds of L. usitatissimum or linseed (obtained from local market) were air-dried, ground to a powdered form and were further subjected to oil extraction in Soxhlet apparatus. Petroleum ether (b.p. 60–80 ◦ C) was used as solvent in the extraction procedure. DGEBA epoxy [E] resin (LY 556; EE = 280) and triethylenetetramine [TETA] were procured from Ciba Specialty Chemicals, Pvt. Ltd. (India). Sodium methoxide, sodium chloride, diethylether (Merck, India) and diethanolamine (s.d. Fine Chem., India) were used as received.
2.2. Synthesis of N,N -bis(2-hydroxyethyl) linseed amide (HELA) The synthesis of HELA is reported elsewhere [37].
2.3. Preparation of DGEBA/HELA blends DGEBA epoxy and HELA were placed in vacuum oven at 70 ◦ C, prior to blend preparation, to remove any trapped air or moisture. Epoxy and HELA were mixed in predetermined ratio (90/10, 80/20, 70/30, 60/40) by weight to obtain their blends (I, II, III and IV), respectively (Table 1). Each of these samples was mixed by continuous agitation over magnetic stirrer for 15 min. Blends were left at ambient temperature for 24 h, under observation. No visual phase separation and gelation were observed indicating proper compatibilization between the two constituents.
2.4. Characterization FTIR spectra of blends were taken on PerkinElmer 1750 FTIR spectrometer (PerkinElmer Cetus Instrument, Norwalk, CT, USA) using NaCl Cell. 1 H NMR spectra (E/H-I) were recorded on JEOL GSX 300 MHz FX-1000 spectrometer using deuterated chloroform as solvent and tetra methyl silane (TMS) as an internal standard. Iodine value (IV), Saponification value (SV), Epoxide equivalent (EE), Refractive index (RI), Density (D), Inherent viscosity (InV) of LO, HELA, E and Epoxy/HELA blends were determined by standard laboratory methods. Antibacterial activity of these systems was evaluated by agar diffusion method [38]. All samples were dissolved in dimethylsulphoxide (DMSO) and were tested against E. coli and S. aureus using standard drug Amekasin (30 g/disc). DGEBA/HELA/A were applied on commercially available mild steel strips of standard sizes, 70 mm × 25 mm × 1 mm and 30 mm × 10 mm × 1 mm, for the determination of gloss (at 45◦ by gloss meter, model RSPT-20; Digital Instruments, Santa Barbara, CA, USA), physico-mechanical (scratch hardness-BS 3900, impact resistance-IS: 101 part 5/s-3,1988, bend test-ASTM-D328184) and chemical/corrosion resistance tests (in water, acid, alkali) performance of coated panels, respectively. Heat treatment of aforementioned coated panels was carried out in vacuum oven in the temperature range of 160–280 ◦ C and the effect of high temperature on color, gloss, adhesion and weight of coatings was also recorded. Thermal analysis of the cured networks was carried out by TGA 51 (TA Instrument, USA), DTG and DSC 10 (TA Instrument) in nitrogen atmosphere and their X-ray diffractograms were recorded on X-ray diffractometer model Phillips PW 3710 using Copper K␣ radiations. 3. Results and discussion Blends of HELA and epoxy [DGEBA/HELA] were prepared in different compositions by weight under mechanical agitation in the absence of any solvent (Table 1) taking advantage of inherent fluidity of HELA. Blends (I, II, III and IV) existed as clear, homogenous and transparent to translucent solutions (from I to IV) at the working temperature (28–30 ◦ C) upto 15 days. During blend formation, epoxy and HELA (in uncured form) were found to be completely miscible with each other, which determines the systems’ ease of processability up to the aforementioned time period and may be attributed to the strong intermolecular interactions (probably in the form of hydrogen bonding) between the two (uncured) components through their epoxide or hydroxyl groups [39]. Such good miscibility between these two (uncured) components eventually has positive effect on the entanglements formed between their cured networks (after curing with TETA). However, this phenomenon may also be regarded as a consequence of chemical reaction between the two components as established by their
Table 1 Composition of DGEBA/HELA [E/H] blends. Sample code
DGEBA
HELA
E H I II III IV
100 – 90 80 70 60
– 100 10 20 30 40
171
Fig. 1. Plot of InV. vs. weight fraction of E blends.
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Fig. 2. Curing reactions of epoxy.
ageing studies using (i) IR (Fig. 5), (ii) 1 H NMR spectra (Fig. 6) and (iii) EE results (Table 3). Blends I, II, III and IV turned to insoluble, hard solid mass after a period of 45, 40, 25 and 20 days, respectively. Thus, it can be envisaged that these blends have the pot life in the range of 15–45 days depending upon the amount of HELA; the more the amount of HELA, the lower the pot life of blends. 3.1. Physico-chemical characterization of DGEBA/HELA blends Plots of EE, RI, D and InV of blends were made as a function of weight fraction of epoxy. Among all the systems, EE value
was found to be the lowest for E (280) indicating the presence of comparatively higher number of oxirane rings. As the amount of HELA increases or DGEBA decreases from E to IV, EE values increase due to corresponding decrease in the amount of epoxy or oxirane moieties (Table 2). RI value of each blend system was found to have intermediate values between the two of its pristine components (H = 1.4878; E = 1.560) (Fig. 7). However, as the proportion of epoxy in blend systems increases, RI values also show an increasing trend (approaching value closer to that of E) since RI of epoxy is higher than that of HELA.
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(Fig. 4). In DGEBA epoxy, absorption band for secondary hydroxyls (appears much reduced in intensity as compared to that of HELA) occurs at 3469 cm−1 . In case of freshly prepared blends (I, II, III and IV), IR spectra (Fig. 4) reveal that the most prominent variation occurs in the absorption bands of –OH. As the amount of HELA increases (from I to IV), the –OH bands increase in depth due to increase in the number of hydroxyl moieties. At the same time, these absorption bands shift towards lower frequency values (from 3469 in pure epoxy to 3405 in IV, which has the highest amount of HELA), as also evident from Fig. 4, probably due to hydrogen bonding between –OHs of HELA. In DGEBA/HELA-III and IV, bands for C–H str. (unsaturation of fatty amide) become evident; those for –CH2 sym. str. and –CH2 asym. str. also increase in intensity. No variation is observed in absorption band values of –CN, –C O (amide) and epoxy indicating that interactions largely develop between primary hydroxyls of HELA and not between aforementioned groups in freshly prepared blends.
Fig. 3. Plot of density vs. weight fraction of E in blends.
The density values for pure E and HELA were found to be 1.1882 and 1.0029, respectively. The actual density value of each blend system was found to be in between that of its constituents. The theoretical densities of blends were calculated by the volume additive principle, which states that = WE E + WH H , where is the density of blend [40]. WE , WH and E , H are the weight fractions of the constituents and density values of epoxy and HELA, respectively. The variation of of blends with composition is shown in Fig. 3. As the proportion of epoxy increases in blends, the density values increase correspondingly. The experimental and calculated density values of blends were found to be in between those of their constituents just as RI values and not much variation was found between experimental and calculated density values indicating complete miscibilization or proper compatibilization between components. InV of HELA was obtained as 0.6306 and that of epoxy was found to be 0.4514. The InV values of blends were observed in between the values of the two components and closer in value to that component which had higher InV value and was higher in amount (Fig. 1). 3.2. Spectral (IR) analysis of blends The absorption bands for functional groups of epoxy and HELA as observed in their IR spectra are provided below: • Pure Epoxy (IR, cm−1 ): 1291–1185 (C–O–C, aryl alkyl ether, asym. str.), 1075 (aryl alkyl ether, sym. str.), 2966 (–CH3 of quaternary carbon), 3056, 1582, 787, 750 (aromatic rings str.), 2930 (–CH2 sym. str.), 2874 (–CH2 , asym. str.), 972–915 (oxirane ring), 1383 ((CH3 )2 C), 3469 (–OH) (as also observed in literature) [6,41]. • HELA (IR, cm−1 ): 3390 (–OH), 2925 (–CH2 sym. str.), 2853 (–CH2 , asym. str.), 1465 (–CN str.), 1622 (carbonyl), 3009 ( C–H str.), 1635 (–C C–) (as also observed in literature) [37]. In HELA, electrostatic interactions are likely to develop within the molecule in the form of hydrogen bonding, evident from lower absorption band value of its primary hydroxyls (3390 cm−1 ) in IR Table 2 EE of freshly prepared (F)/aged (*A)-I, II, III and IV. System
F
*A10
*A20
*A25
I II III IV
295.74 350.72 429.82 499.0
308.82 368.84 448.09 552.5
332.57 505.55 912.50 1022.0
438.0 762.4 – –
EE of E = 280, F: freshly prepared, *A10: aged for 10 days, *A20: aged for 20 days, *A25: aged for 25 days.
3.3. Ageing behavior of DGEBA/HELA (IR, 1 H NMR, EE) IR: In aged samples (*A-I, II, III and IV), a contrary trend in IR spectra is observed as compared to their freshly prepared counterparts (I, II, III and IV), i.e., as the content of HELA is increased from *A-I to IV, –OH band intensity (depth) decreases and the absorption band value (–OH) shows shifting from 3438, 3417, 3408, 3405 in I, II, III and IV to 3392, 3446, 3446 and 3448 in *A-I, II, III, IV, respectively (Figs. 4 and 5). The absorption bands for oxirane rings (973–830 cm−1 ) and unsaturation (3009 ( C–H str.), 1635 (–C C–)) diminish and eventually disappear in *A-III and *A-IV, attributed to the chemical reaction of epoxy with –OH of HELA (Fig. 2); consequently both get consumed in the process. No variation is observed in absorption band values of –CN, –C O (amide) indicating that chemical reactions largely occur between primary hydroxyls of HELA and oxirane of epoxy and none of the aforementioned groups. In *A-III and *A-IV, only absorption bands for phenyl ether and substituted aromatics (1732, 1288, 1122, 1073, 786 cm−1 ) as well as those of secondary hydroxyls (3446, 3448 cm−1 ) of epoxy are observed as un-crosslinked, soluble portion, amenable to analyses. It is anticipated that the diminished bands at 3446 and 3448 cm−1 may exclusively be assigned to those of secondary hydroxyls of epoxy in *A-III and IV since they are closer in value to hydroxyls of pure epoxy (3465 cm−1 ) (Fig. 4). Fig. 2 shows chemical reaction of epoxy with HELA by polyaddition reaction with the likely formation of two products -primary and secondary nascent hydroxyls, as also corroborated by literature reports [42–44]. However, it was not possible to identify the exact nature (primary or secondary) of hydroxyls in the final product because we understand that the formed hydroxyls become incorporated part of the cured epoxy network and were not amenable to analyses [45]. In IR spectra of all *A-I, II, III and IV samples, band for unsaturation is visible as a small peg that eventually disappears in *A-III and *A-IV; this clearly indicates that the unsaturation in the pendant alkyl chain of HELA also participates in curing reaction. 1 H NMR: Spectrum of *A-II (after 10 days of preparation of blend) (Fig. 6) also shows chemical reaction of epoxy with HELA. Here, occurrence of characteristic peaks of both cured and uncured epoxy networks and those of HELA (unsaturation, hydroxyls, methylene and methyl) are clearly visible. The spectrum clearly shows the remarkable changes at the functional groups (hydroxyls, epoxies) and curing reaction sites when compared with the spectrum of pure epoxy and HELA [6,37]. EE (Table 2): Blends were monitored for their EE values after 10, 20 and 25 days of ageing. A remarkable increment in EE values of *A-I, II, III and IV was observed after ageing. It was also observed
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Fig. 4. IR spectra of I,II,II, IV.
that *A-IV with highest content of HELA showed highest EE value after 20 days and beyond this time period it existed as a hard solid mass. *A-III showed similar trend after 25 days. The higher EE value in these systems can be attributed to lower content of oxirane ring after ageing (due to their consumption by curing/chemical reaction with –OH of HELA as also evidenced by IR studies). The chemical reactions can preferentially be termed as curing reactions of epoxy where HELA behaves as a curing agent. However, since the curing reactions proceed at a mild pace only on ageing for a considerable period (within 20–45 days), HELA can also be termed as a mild curing agent for epoxies. 3.3.1. Probable curing reactions of DGEBA/HELA/A (Fig. 2) As DGEBA/HELA (I, II, III and IV) involved longer curing times (20–45 days), TETA was added to accelerate curing phenomenon [42]. It was found that the presence of amine speeds up curing reaction as evident from their curing times (90–120 min and about 24 h,
respectively as compared to 15–45 days) at ambient temperature and also corroborated by literature reports [39,44,46]. It is not possible to ascertain the preferential sequence of curing (chemical) reactions. However, on the basis of high reactivity of amines to oxirane rings it can be speculated that at the initial stages, the curing reaction occurs between terminal primary amines and epoxy groups of DGEBA [2,39,44,46] by polyaddition reaction in presence of proton donor (–OHs of HELA), which leads to the opening of the epoxy ring by hydrogen bonding with the oxygen atom (of epoxy) in transition state. The resulting secondary amines, along with those of TETA, further participate in epoxy curing reaction in similar fashion. The –OHs formed during epoxy-amine (both primary and secondary) curing reactions and those of HELA undergo etherification of epoxy groups in presence of (formed) tertiary amine [44,46]. The possible occurrence of auto-oxidation at active methylene sites in HELA also cannot be ruled out as typical of several oil-based polymers.
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Fig. 5. IR spectra of *A- I/II/III/IV.
Fig. 6.
1
H NMR spectrum of II.
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Fig. 10. Plot of gloss (at 45◦ ) vs. weight fraction of E in cured networks.
Fig. 7. Plot of RI vs. weight fraction of E in blends.
Fig. 8. Plot of scratch hardness vs. weight fraction of E in cured networks.
Thus, the overall curing reactions in DGEBA/HELA/A involve “single-pot, multi-step, zero-solvent, ambient-temperature” curing strategy. 3.4. Physico-mechanical and chemical resistance tests (Figs. 8 and 9) 3.4.1. Physico-mechanical analyses In plain (DGEBA/TETA) E/A and I, II, III, IV/A coated panels, Scratch Hardness values were obtained in the range of 5.0–6.0 kg. The plain E/A and system I/A failed 250 lb/in. Impact Resistance test. In II/A, at this load, coating material was slightly chipped off the substrate. In III/A and IV/A, coated panels successfully passed the test under 250 lb load. Aromatic epoxies are brittle in nature (as discussed in Section 1). The incorporation of HELA overcomes the brittle nature of epoxy and their higher cross-linking further renders toughness to the system. This may be attributed to the flexibility conferred by HELA as a typical inherent characteristic of oil based resins attributed to the long aliphatic hydrocarbon chains of oil. Here, flexibility is conferred by long and pendant aliphatic fatty amide chains of HELA, which have actually been incorporated into the systems by chemical reaction (as discussed in previous section). Coated panels of pure E/A failed 1/4 in. conical mandrel bend test. System I/A passed 1/4 in. bend on conical mandrel. Systems III/A and IV/A passed 1/8 in. bend on conical mandrel while in sys-
Fig. 9. Plot of impact resistance vs. weight fraction of E in cured networks.
tem II/A, coating material was slightly chipped off the coated panel. This indicates that as the content of HELA increases in blended systems, its flexible nature overcomes the drawback of brittleness of DGEBA epoxy. Scratch hardness, impact resistance performance and bending ability of coatings mainly depend on the crosslink density of the polymeric resin, adhesion of the coatings to the substrate as well as the flexibility of polymeric chains, respectively. The curing scheme (Fig. 2) apprises about the presence of cross-linking sites (viz. epoxies and –OH), ether, aromatic moieties of DGEBA as well as long aliphatic chains of fatty amide, which collectively introduce sufficient crosslink density, adhesion, toughness and improve the physico-mechanical performance of the resins in cured form (coatings). However, a slight deterioration in scratch hardness is observed on increasing soft segments of HELA in rigid epoxy matrix [17]. The schemes also suggest that here both TETA and HELA act as curing agents. Also, with increase in the amount of HELA (along with the inclusion of TETA), drying time of coatings decreases, gloss as well as IRt and bend test values show gradual improvement. These facts further confirm the role of HELA, both as a curing agent and modifier, which had a profound influence on physico-mechanical characteristics and chemical resistance performance of coatings. From plain E/A, I/A-IV/A, gloss values increase from 39 to 61 with increase in the amount of HELA as a typical characteristic of fatty amide based coating material (Fig. 10). 3.4.2. Chemical resistance All coated panels were subjected to exposure in various chemical media to study their corrosion resistance behavior viz. changes in their weight, adhesion and overall appearance. In 5 wt.% HCl, systems I/A and II/A showed loss in adhesion after 20 h of immersion; coating material however, remained unaffected. In HCl, systems III/A and IV/A also showed similar results. In 5 wt.% NaOH, coated panel of sample II/A was found to be unaffected even after dipping in aforementioned media for 72 h. No change in gloss and weight of coated panels was observed. In samples III/A and IV/A, slight loss in gloss and adhesion was observed, respectively, while in sample I/A, corrosion was visible (in the form of loss in weight and adhesion of the coated panel). This trend is attributed to the presence of ether linkages formed by curing of epoxy resin and amide moieties of HELA which are resistant to alkaline hydrolysis. As the content of HELA is increased, from sample I-IV/A, the performance in alkaline media also improved. These results are extraordinarily remarkable in contrast to conventional resins (polyesteramide, polyetheramide, polyurethaneamide) obtained from HELA owing to the excellent combination of functionalities e.g., amide and ether junctions, which show good alkali resistance. In 3.5 wt.% NaCl, loss in weight of coated panels became evident after 20 h due to dissolution of coating material in case of I-IV/A coated panels. In sample IV/A, slight loss in adhesion was observed after 5 h of immersion.
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177
Fig. 11. TGA thermograms of EA, I/A, II/A, III/A.
In xylene, swelling occurred in panels coated with samples III/A and IV/A after 70 h, while in I/A and II/A, no change was observed. This could be attributed to higher amount of HELA in these samples, which shows good solubility in xylene. In water, sample I/A and II/A coated panels were found to be unaffected after 70 h while coated panels of samples III/A and IV/A showed slight change in weight due to dissolution because of higher content of HELA, which is water soluble in nature. As discussed earlier, good miscibility between the two (uncured) components has a positive effect on the entanglements formed between their cured networks (after curing with TETA), which eventually have a cumulative effect on the overall physicomechanical and chemical resistance performance of systems I-IV/A.
3.5. Effect of temperature on color, gloss, adhesion and weight of coatings All the systems (I-IV/A) were subjected to heating in air in an oven from 100 ◦ C to 280 ◦ C for the time and at temperature intervals of ten minutes and 20 ◦ C, respectively. No deterioration in gloss, color and weight of coated panels was observed up to 150 ◦ C. However, on heating beyond 150 ◦ C, color of the coated panels turned brownish (translucent from initial water white transparent). A slight deterioration in the gloss of coated panels (from their initial values) was also visible. Between 170 and 180 ◦ C, all the systems turned dark brown; system IV/A that had the highest amount of HELA, showed loss in adhesion due to thermoplastic nature of HELA, which cannot withstand high temperature and in an uncombined form starts melting above 60 ◦ C. However, here, in combination with E and TETA, probably due to cross-linking by TETA, thermoplasticity became evident only at temperature as high as 170–180 ◦ C. System III/A showed slight loss in adhesion on further heating beyond 180–200 ◦ C. The percent loss in gloss and
weight of coated panels in III/A and IV/A was found as 21–24% and 0.20–0.27%, respectively. In I/A and II/A, adhesion was found to be unaffected by further heating the coated panels up to 280 ◦ C; however, percent deterioration in their gloss and weight was found as 30–34% and 2–2.5%, respectively. All the systems turned black when heated up to 280–300 ◦ C with evident loss in weight of coated panels indicating the onset of degradation of polymers or lysis of bonds.
3.6. Thermal stability of cured DGEBA/HELA/A networks TGA, DTG and DSC thermograms of DGEBA/HELA/A systems are provided in Figs. 11 and 12. The shapes of DSC thermograms are almost identical. While TGA and DTG thermograms of II/A and III/A show identical degradation pattern, those of E-A and I/A show pronounced variation. All the thermograms show the onset of degradation beyond 220 ◦ C. DTG thermogram of E-A shows multiple but muffed degradation stages which are evident in TGA thermogram corresponding to 10 wt.% (200–300 ◦ C), 40 wt.% (300–375 ◦ C), 20 wt.% (375–425 ◦ C) and 30 wt.% (475–660 ◦ C) loss. I/A distinctively reveals three degradation stages; the first two are closely spaced in a temperature span of 335–425 ◦ C, the third one occurs at a higher temperature. DTG thermograms of II/A and III/A show almost single step thermal degradation incurring about 85 wt.% loss. Thus, in other words, the higher the content of HELA, the lesser is the number of degradation steps. It can be anticipated that in E-A and I/A, there are a number of separately existing entities (cured and uncured oxirane rings, –OH, –NH2 , double bonds) that disintegrate independently under the influence of high temperature as evident from multiple degradation steps in DTG. While in II/A and III/A due to higher amount of HELA, it is envisioned that both the systems comprise a cross-linked, homogenous network
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E. Sharmin et al. / Progress in Organic Coatings 67 (2010) 170–179 Table 3 Antibacterial activity of blends. Antibacterial activity
H
I
II
III
IV
E. coli S. aureus
− +
− +
− +
− ++
− ++
Inactive +: mildly active (10–12 mm), ++: moderately active (13–20 mm), +++: highly active (21–26 mm).
3.8. Antibacterial behavior of blends (Table 3)
Fig. 12. DSC thermograms of I/A, II/A and III/A.
of cured oxirane rings, –OH, –NH2 and double bonds (IR spectra Figs. 4 and 5 and Reaction scheme Fig. 2) that results in their single-step thermal degradation behavior as (not separate but one) single entity favored by chain entanglements. E-A, I/A, II/A and III/A show 25 wt.% loss at 340 ◦ C, 330 ◦ C, 332 ◦ C, 323 ◦ C, 50 wt.% loss at 375 ◦ C, 365 ◦ C, 368 ◦ C and 365 ◦ C and 75 wt.% loss at 425 ◦ C, 410 ◦ C, 393 ◦ C, 382 ◦ C, respectively. Decrease in decomposition temperatures can be attributed to increase in soft segments of HELA in rigid epoxy matrix, slightly lowering the cross-linking density [17]. DSC thermograms show endotherms centered at 300–350 ◦ C while TGA thermograms clearly show decomposition in this temperature range signifying some configurational changes in the polymeric resin prior to its decomposition. DTG shows maximum weight loss rate beyond this temperature range. Derivative curves in this study also reveal that the temperatures corresponding to the maximum weight loss rate first increase marginally from E-A (345 ◦ C) to I/A (348 ◦ C), sharply to II/A (366 ◦ C) and then decrease with increased amount of HELA from II/A (366 ◦ C) to III/A (353 ◦ C) while the rate of weight loss at these temperatures first decreases from E-A to I/A and then increases from I/A to III/A. The exact reason for this behavior cannot be deduced; however, we understand that the thermoplastic nature of HELA may be responsible for this behavior. Interestingly, DTG of pristine E/A shows an additional, relatively short degradation step, revealing a very small percent of weight loss at about 225 ◦ C, (which is not observed in any other DTG thermogram) followed by a major degradation process between 345 and 366 ◦ C predominant in all the systems. The former is attributed to the breaking of unreacted oxirane or amine molecules; the latter is associated with the thermal degradation of cured epoxy network eventually followed by the decomposition of aromatic moieties and aliphatic chains of fatty amide under the influence of high temperature [39].
All the systems were found to be inactive and mildly to moderately active against E. coli and S. aureus, respectively. The long pendant hydrocarbon chains of HELA are abode to amide, hydroxyl, double bonds and methylene groups while epoxy has oxirane rings, hydroxyl moieties, aromatic rings and ethereal linkages. The probable mode of action of these polymeric resins involves bacterial protein denaturation, damage of lipid complexes in cell membranes or dehydration of bacterial cells [37,38,47]. E. coli (Gram-negative) and S. aureus (Gram-positive) show varying response against these systems. The presence of outer membrane containing lipopolysaccharide in the former, protects the bacteria from several antibiotics, dyes and detergents that normally damage the inner membrane or cell wall peptidoglycan. Consequently, HELA and DGEBA/HELA/A systems show mild to moderate antibacterial activity against S. aureus and are inactive against E. coli. It is interesting to note that while pure HELA exhibits mild activity against S. aureus, system IV/A with maximum amount of HELA shows better (moderately active) antibacterial activity. 4. Conclusion Blends and their coatings were prepared at ambient temperature without the use of any organic solvent. DGEBA/HELA showed mild to moderate antibacterial activity against S. aureus, which gradually improved with higher content of HELA. Ageing studies of blends confirm that HELA can be employed as a mild curing agent and reactive modifier for epoxies operative at ambient temperature by zero solvent approach. Physico-mechanical characteristics such as IRt, BT and gloss improved with increase in the amount of HELA. The approach provides an alternate pathway to cut off the use of commercially available epoxy resins by partial substitution of fatty amide monomers derived from seed oils—a sustainable resource. It also provides a novel opportunity to develop “solvent free” coating materials and adhesives due to inherent fluidity of HELA—a typical characteristic of seed oils and their derivatives that facilitates its use as solvent, reactant and curing agent, simultaneously. However, further improvement for mild curing behavior of HELA is required, which is in progress in our laboratory. Acknowledgement Dr. Eram Sharmin is thankful to CSIR, New Delhi, India, for Research Associateship against Grant No. 9/466(0102)2K8-EMR-I. References
3.7. XRD of cured DGEBA/HELA/A networks DGEBA/HELA/A systems reveal insignificant broadening of peaks from I/A to III/A indicative of their amorphous nature, which increases with increase in HELA. It is interesting to note that from III/A to IV/A, no such change (peak broadening) is observed, which reveals that at former loading, HELA completely envelops epoxy. The presence of amorphous peaks also informs that HELA has no effect on crystalline nature of polymers.
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Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Linseed amide diol/DGEBA epoxy blends for coating applications: Preparation, characterization, ageing studies and coating properties Eram Sharmin a , M.S. Alam b , Renjish K. Philip b , Sharif Ahmad a,∗ a b
Materials Research Lab., Dept. of Chemistry, Jamia Millia Islamia, New Delhi 110025, India Dept. of Chemistry, Jamia Hamdard University, New Delhi, India
a r t i c l e
i n f o
Article history: Received 28 April 2009 Received in revised form 25 August 2009 Accepted 17 September 2009 Keywords: Ageing Blends Thermogravimetric analysis FTIR Coatings
a b s t r a c t Linseed amide diol [HELA] was used as modifier for conventionally available epoxy resin [DGEBA] by blending in different ratio. Blends [DGEBA/HELA] were subjected to spectral, physico-chemical, ageing and antibacterial studies. Interesting features of the blends were complete miscibility of HELA with DGEBA, principally due to hydrogen bonding and chemical reaction between the two constituents, and their moderate antibacterial activity against S. aureus. DGEBA/HELA blends were further treated with triethylenetetramine [TETA-A] [DGEBA/HELA/A] as curing agent to evaluate their performance as corrosion protective coating materials. DGEBA/HELA/A coatings showed good physico-mechanical and chemical resistance behavior, in particular against alkaline media. Thermal analysis of DGEBA/HELA/A revealed their single to multi-step degradation behavior with safe usage upto 220 ◦ C. Our investigations confirm the dual role of HELA as environment-friendly, reactive-modifier and mild curing agent for epoxy resins. Besides, DGEBA/HELA/A may find potential applications as “solvent free”, ambient temperature cured antibacterial coating materials. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Epoxy resins comprise an important class of thermosetting materials, with unique combination of properties, that can be tailor made depending upon the type of curing agent and modifier employed [1–3]. Their cured networks exhibit high modulus, low creep, good adhesion, physico-mechanical and corrosion/chemical resistance properties as well as good performance at elevated temperatures for various end use applications such as adhesives, lubricants, industrial tooling, reinforced plastics, paints and coatings. The major drawback of epoxy resins is that on curing, they generally contract and develop internal stresses, which deteriorates their resistance against the attack of moisture and chemicals, lowers their fracture energy, hydrophobicity and impact strength [2–6]. Over the last few years, modifications of epoxies have been accomplished with polystyrene, polymethylmethacrylate, PEEK, PES, PEI, polyurethanes [4,7–10], liquid rubber, siloxane based polymers, hydroxyl terminated polybutadiene, hydroxyl functionalized hyperbranched polymer, polyesters, liquid elastomers, inorganic particles or nanofillers either by blending or by chemical reaction between the virgin epoxy and the modifier [11–20]. Blending with
∗ Corresponding author. Tel.: +91 9891290510; fax: +91 011 26827508. E-mail addresses: eram [email protected] (E. Sharmin), sharifahmad [email protected] (S. Ahmad). 0300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2009.09.012
thermoplastics leads to enormous increase in viscosity of the resin, which causes problems especially during paint formulation. The major disadvantage with rubber and siloxane modifiers is that of miscibility, phase separation and bleeding of the modifier, respectively [2,15–28]. Several vegetable oil based polymers have been used as reactive modifiers and diluents of epoxy composites and nanocomposites [17,18]. Research investigations have been carried out by Kobayashi and co-workers [29], Miyagawa et al. [30], Erhan and co-workers [31], Chandrashekhara et al. [32], Ray and Bousmina [18], Uyama et al. [33] and Jin and Park [34] on synthesis, fabrication, mechanical, thermo-physical and thermo-mechanical properties of DGEBA [Diglycidyl ether of Bisphenol A]/oil epoxy. These systems show good miscibility, improved fracture toughness and impact properties compared to the virgin epoxy resin. Blends of Mesua ferrea L. seed oil polyurethanes/DGEBA epoxy have shown good performance as surface coatings and paints according to separate reports by Karak and co-workers [35,36]. Fatty amide diols are obtained through the base-catalysed aminolysis of oils such as linseed, karanj, castor, soybean and others, as monomers with functionalities viz., hydroxyl, double bonds, active methylenes, amides and long alkyl chain of parent oil [37]. They find extensive applications as one of the starting materials of several polymeric resins i.e., polyesteramide, polyetheramide, polyurethaneamide for their use in coatings and adhesives. They exhibit the characteristics typical of an epoxy modifier such as low molecular weight, ease of processing, suitable functionality, misci-
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bility or solubility with epoxies, inherent fluidity to afford curing in a “solvent free” environment (to limit the harmful effects of VOCs and to avoid disadvantages of imperfections e.g., intramolecular loop and void formation during cross-linking reactions, in solution) at ambient temperature, and most importantly, renewability/sustainability [1,2,27,28]. However, literature survey reveals that they have not yet been used for the same [35,36]. In this work, we have attempted for the first time, to develop DGEBA epoxy coating materials by blending, in different formulations by weight (DGEBA/HELA) in the absence of any organic solvent, with amide diol-N,N -bis (2-hydroxyl ethyl) linseed amide [H, MW: 367] obtained from linseed oil. DGEBA/HELA blends were characterized by spectral, physico-chemical, antibacterial and ageing studies using standard methods. Blends were further treated with triethylenetetramine [TETA = A] curing agent [DGEBA/HELA/A] to prepare their coatings; the latter were analyzed by physico-mechanical, thermal (TGA, DTG, DSC), X-ray and chemical resistance tests. Our investigative studies revealed that HELA can be used both as sustainable resource based antibacterial, environment friendly, reactive modifier and mild curing agent for epoxy resins. We further observed that such DGEBA/HELA combinations may serve as potential candidates in the field of coatings and paints.
2. Experimental 2.1. Materials Seeds of L. usitatissimum or linseed (obtained from local market) were air-dried, ground to a powdered form and were further subjected to oil extraction in Soxhlet apparatus. Petroleum ether (b.p. 60–80 ◦ C) was used as solvent in the extraction procedure. DGEBA epoxy [E] resin (LY 556; EE = 280) and triethylenetetramine [TETA] were procured from Ciba Specialty Chemicals, Pvt. Ltd. (India). Sodium methoxide, sodium chloride, diethylether (Merck, India) and diethanolamine (s.d. Fine Chem., India) were used as received.
2.2. Synthesis of N,N -bis(2-hydroxyethyl) linseed amide (HELA) The synthesis of HELA is reported elsewhere [37].
2.3. Preparation of DGEBA/HELA blends DGEBA epoxy and HELA were placed in vacuum oven at 70 ◦ C, prior to blend preparation, to remove any trapped air or moisture. Epoxy and HELA were mixed in predetermined ratio (90/10, 80/20, 70/30, 60/40) by weight to obtain their blends (I, II, III and IV), respectively (Table 1). Each of these samples was mixed by continuous agitation over magnetic stirrer for 15 min. Blends were left at ambient temperature for 24 h, under observation. No visual phase separation and gelation were observed indicating proper compatibilization between the two constituents.
2.4. Characterization FTIR spectra of blends were taken on PerkinElmer 1750 FTIR spectrometer (PerkinElmer Cetus Instrument, Norwalk, CT, USA) using NaCl Cell. 1 H NMR spectra (E/H-I) were recorded on JEOL GSX 300 MHz FX-1000 spectrometer using deuterated chloroform as solvent and tetra methyl silane (TMS) as an internal standard. Iodine value (IV), Saponification value (SV), Epoxide equivalent (EE), Refractive index (RI), Density (D), Inherent viscosity (InV) of LO, HELA, E and Epoxy/HELA blends were determined by standard laboratory methods. Antibacterial activity of these systems was evaluated by agar diffusion method [38]. All samples were dissolved in dimethylsulphoxide (DMSO) and were tested against E. coli and S. aureus using standard drug Amekasin (30 g/disc). DGEBA/HELA/A were applied on commercially available mild steel strips of standard sizes, 70 mm × 25 mm × 1 mm and 30 mm × 10 mm × 1 mm, for the determination of gloss (at 45◦ by gloss meter, model RSPT-20; Digital Instruments, Santa Barbara, CA, USA), physico-mechanical (scratch hardness-BS 3900, impact resistance-IS: 101 part 5/s-3,1988, bend test-ASTM-D328184) and chemical/corrosion resistance tests (in water, acid, alkali) performance of coated panels, respectively. Heat treatment of aforementioned coated panels was carried out in vacuum oven in the temperature range of 160–280 ◦ C and the effect of high temperature on color, gloss, adhesion and weight of coatings was also recorded. Thermal analysis of the cured networks was carried out by TGA 51 (TA Instrument, USA), DTG and DSC 10 (TA Instrument) in nitrogen atmosphere and their X-ray diffractograms were recorded on X-ray diffractometer model Phillips PW 3710 using Copper K␣ radiations. 3. Results and discussion Blends of HELA and epoxy [DGEBA/HELA] were prepared in different compositions by weight under mechanical agitation in the absence of any solvent (Table 1) taking advantage of inherent fluidity of HELA. Blends (I, II, III and IV) existed as clear, homogenous and transparent to translucent solutions (from I to IV) at the working temperature (28–30 ◦ C) upto 15 days. During blend formation, epoxy and HELA (in uncured form) were found to be completely miscible with each other, which determines the systems’ ease of processability up to the aforementioned time period and may be attributed to the strong intermolecular interactions (probably in the form of hydrogen bonding) between the two (uncured) components through their epoxide or hydroxyl groups [39]. Such good miscibility between these two (uncured) components eventually has positive effect on the entanglements formed between their cured networks (after curing with TETA). However, this phenomenon may also be regarded as a consequence of chemical reaction between the two components as established by their
Table 1 Composition of DGEBA/HELA [E/H] blends. Sample code
DGEBA
HELA
E H I II III IV
100 – 90 80 70 60
– 100 10 20 30 40
171
Fig. 1. Plot of InV. vs. weight fraction of E blends.
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Fig. 2. Curing reactions of epoxy.
ageing studies using (i) IR (Fig. 5), (ii) 1 H NMR spectra (Fig. 6) and (iii) EE results (Table 3). Blends I, II, III and IV turned to insoluble, hard solid mass after a period of 45, 40, 25 and 20 days, respectively. Thus, it can be envisaged that these blends have the pot life in the range of 15–45 days depending upon the amount of HELA; the more the amount of HELA, the lower the pot life of blends. 3.1. Physico-chemical characterization of DGEBA/HELA blends Plots of EE, RI, D and InV of blends were made as a function of weight fraction of epoxy. Among all the systems, EE value
was found to be the lowest for E (280) indicating the presence of comparatively higher number of oxirane rings. As the amount of HELA increases or DGEBA decreases from E to IV, EE values increase due to corresponding decrease in the amount of epoxy or oxirane moieties (Table 2). RI value of each blend system was found to have intermediate values between the two of its pristine components (H = 1.4878; E = 1.560) (Fig. 7). However, as the proportion of epoxy in blend systems increases, RI values also show an increasing trend (approaching value closer to that of E) since RI of epoxy is higher than that of HELA.
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(Fig. 4). In DGEBA epoxy, absorption band for secondary hydroxyls (appears much reduced in intensity as compared to that of HELA) occurs at 3469 cm−1 . In case of freshly prepared blends (I, II, III and IV), IR spectra (Fig. 4) reveal that the most prominent variation occurs in the absorption bands of –OH. As the amount of HELA increases (from I to IV), the –OH bands increase in depth due to increase in the number of hydroxyl moieties. At the same time, these absorption bands shift towards lower frequency values (from 3469 in pure epoxy to 3405 in IV, which has the highest amount of HELA), as also evident from Fig. 4, probably due to hydrogen bonding between –OHs of HELA. In DGEBA/HELA-III and IV, bands for C–H str. (unsaturation of fatty amide) become evident; those for –CH2 sym. str. and –CH2 asym. str. also increase in intensity. No variation is observed in absorption band values of –CN, –C O (amide) and epoxy indicating that interactions largely develop between primary hydroxyls of HELA and not between aforementioned groups in freshly prepared blends.
Fig. 3. Plot of density vs. weight fraction of E in blends.
The density values for pure E and HELA were found to be 1.1882 and 1.0029, respectively. The actual density value of each blend system was found to be in between that of its constituents. The theoretical densities of blends were calculated by the volume additive principle, which states that = WE E + WH H , where is the density of blend [40]. WE , WH and E , H are the weight fractions of the constituents and density values of epoxy and HELA, respectively. The variation of of blends with composition is shown in Fig. 3. As the proportion of epoxy increases in blends, the density values increase correspondingly. The experimental and calculated density values of blends were found to be in between those of their constituents just as RI values and not much variation was found between experimental and calculated density values indicating complete miscibilization or proper compatibilization between components. InV of HELA was obtained as 0.6306 and that of epoxy was found to be 0.4514. The InV values of blends were observed in between the values of the two components and closer in value to that component which had higher InV value and was higher in amount (Fig. 1). 3.2. Spectral (IR) analysis of blends The absorption bands for functional groups of epoxy and HELA as observed in their IR spectra are provided below: • Pure Epoxy (IR, cm−1 ): 1291–1185 (C–O–C, aryl alkyl ether, asym. str.), 1075 (aryl alkyl ether, sym. str.), 2966 (–CH3 of quaternary carbon), 3056, 1582, 787, 750 (aromatic rings str.), 2930 (–CH2 sym. str.), 2874 (–CH2 , asym. str.), 972–915 (oxirane ring), 1383 ((CH3 )2 C), 3469 (–OH) (as also observed in literature) [6,41]. • HELA (IR, cm−1 ): 3390 (–OH), 2925 (–CH2 sym. str.), 2853 (–CH2 , asym. str.), 1465 (–CN str.), 1622 (carbonyl), 3009 ( C–H str.), 1635 (–C C–) (as also observed in literature) [37]. In HELA, electrostatic interactions are likely to develop within the molecule in the form of hydrogen bonding, evident from lower absorption band value of its primary hydroxyls (3390 cm−1 ) in IR Table 2 EE of freshly prepared (F)/aged (*A)-I, II, III and IV. System
F
*A10
*A20
*A25
I II III IV
295.74 350.72 429.82 499.0
308.82 368.84 448.09 552.5
332.57 505.55 912.50 1022.0
438.0 762.4 – –
EE of E = 280, F: freshly prepared, *A10: aged for 10 days, *A20: aged for 20 days, *A25: aged for 25 days.
3.3. Ageing behavior of DGEBA/HELA (IR, 1 H NMR, EE) IR: In aged samples (*A-I, II, III and IV), a contrary trend in IR spectra is observed as compared to their freshly prepared counterparts (I, II, III and IV), i.e., as the content of HELA is increased from *A-I to IV, –OH band intensity (depth) decreases and the absorption band value (–OH) shows shifting from 3438, 3417, 3408, 3405 in I, II, III and IV to 3392, 3446, 3446 and 3448 in *A-I, II, III, IV, respectively (Figs. 4 and 5). The absorption bands for oxirane rings (973–830 cm−1 ) and unsaturation (3009 ( C–H str.), 1635 (–C C–)) diminish and eventually disappear in *A-III and *A-IV, attributed to the chemical reaction of epoxy with –OH of HELA (Fig. 2); consequently both get consumed in the process. No variation is observed in absorption band values of –CN, –C O (amide) indicating that chemical reactions largely occur between primary hydroxyls of HELA and oxirane of epoxy and none of the aforementioned groups. In *A-III and *A-IV, only absorption bands for phenyl ether and substituted aromatics (1732, 1288, 1122, 1073, 786 cm−1 ) as well as those of secondary hydroxyls (3446, 3448 cm−1 ) of epoxy are observed as un-crosslinked, soluble portion, amenable to analyses. It is anticipated that the diminished bands at 3446 and 3448 cm−1 may exclusively be assigned to those of secondary hydroxyls of epoxy in *A-III and IV since they are closer in value to hydroxyls of pure epoxy (3465 cm−1 ) (Fig. 4). Fig. 2 shows chemical reaction of epoxy with HELA by polyaddition reaction with the likely formation of two products -primary and secondary nascent hydroxyls, as also corroborated by literature reports [42–44]. However, it was not possible to identify the exact nature (primary or secondary) of hydroxyls in the final product because we understand that the formed hydroxyls become incorporated part of the cured epoxy network and were not amenable to analyses [45]. In IR spectra of all *A-I, II, III and IV samples, band for unsaturation is visible as a small peg that eventually disappears in *A-III and *A-IV; this clearly indicates that the unsaturation in the pendant alkyl chain of HELA also participates in curing reaction. 1 H NMR: Spectrum of *A-II (after 10 days of preparation of blend) (Fig. 6) also shows chemical reaction of epoxy with HELA. Here, occurrence of characteristic peaks of both cured and uncured epoxy networks and those of HELA (unsaturation, hydroxyls, methylene and methyl) are clearly visible. The spectrum clearly shows the remarkable changes at the functional groups (hydroxyls, epoxies) and curing reaction sites when compared with the spectrum of pure epoxy and HELA [6,37]. EE (Table 2): Blends were monitored for their EE values after 10, 20 and 25 days of ageing. A remarkable increment in EE values of *A-I, II, III and IV was observed after ageing. It was also observed
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Fig. 4. IR spectra of I,II,II, IV.
that *A-IV with highest content of HELA showed highest EE value after 20 days and beyond this time period it existed as a hard solid mass. *A-III showed similar trend after 25 days. The higher EE value in these systems can be attributed to lower content of oxirane ring after ageing (due to their consumption by curing/chemical reaction with –OH of HELA as also evidenced by IR studies). The chemical reactions can preferentially be termed as curing reactions of epoxy where HELA behaves as a curing agent. However, since the curing reactions proceed at a mild pace only on ageing for a considerable period (within 20–45 days), HELA can also be termed as a mild curing agent for epoxies. 3.3.1. Probable curing reactions of DGEBA/HELA/A (Fig. 2) As DGEBA/HELA (I, II, III and IV) involved longer curing times (20–45 days), TETA was added to accelerate curing phenomenon [42]. It was found that the presence of amine speeds up curing reaction as evident from their curing times (90–120 min and about 24 h,
respectively as compared to 15–45 days) at ambient temperature and also corroborated by literature reports [39,44,46]. It is not possible to ascertain the preferential sequence of curing (chemical) reactions. However, on the basis of high reactivity of amines to oxirane rings it can be speculated that at the initial stages, the curing reaction occurs between terminal primary amines and epoxy groups of DGEBA [2,39,44,46] by polyaddition reaction in presence of proton donor (–OHs of HELA), which leads to the opening of the epoxy ring by hydrogen bonding with the oxygen atom (of epoxy) in transition state. The resulting secondary amines, along with those of TETA, further participate in epoxy curing reaction in similar fashion. The –OHs formed during epoxy-amine (both primary and secondary) curing reactions and those of HELA undergo etherification of epoxy groups in presence of (formed) tertiary amine [44,46]. The possible occurrence of auto-oxidation at active methylene sites in HELA also cannot be ruled out as typical of several oil-based polymers.
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Fig. 5. IR spectra of *A- I/II/III/IV.
Fig. 6.
1
H NMR spectrum of II.
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Fig. 10. Plot of gloss (at 45◦ ) vs. weight fraction of E in cured networks.
Fig. 7. Plot of RI vs. weight fraction of E in blends.
Fig. 8. Plot of scratch hardness vs. weight fraction of E in cured networks.
Thus, the overall curing reactions in DGEBA/HELA/A involve “single-pot, multi-step, zero-solvent, ambient-temperature” curing strategy. 3.4. Physico-mechanical and chemical resistance tests (Figs. 8 and 9) 3.4.1. Physico-mechanical analyses In plain (DGEBA/TETA) E/A and I, II, III, IV/A coated panels, Scratch Hardness values were obtained in the range of 5.0–6.0 kg. The plain E/A and system I/A failed 250 lb/in. Impact Resistance test. In II/A, at this load, coating material was slightly chipped off the substrate. In III/A and IV/A, coated panels successfully passed the test under 250 lb load. Aromatic epoxies are brittle in nature (as discussed in Section 1). The incorporation of HELA overcomes the brittle nature of epoxy and their higher cross-linking further renders toughness to the system. This may be attributed to the flexibility conferred by HELA as a typical inherent characteristic of oil based resins attributed to the long aliphatic hydrocarbon chains of oil. Here, flexibility is conferred by long and pendant aliphatic fatty amide chains of HELA, which have actually been incorporated into the systems by chemical reaction (as discussed in previous section). Coated panels of pure E/A failed 1/4 in. conical mandrel bend test. System I/A passed 1/4 in. bend on conical mandrel. Systems III/A and IV/A passed 1/8 in. bend on conical mandrel while in sys-
Fig. 9. Plot of impact resistance vs. weight fraction of E in cured networks.
tem II/A, coating material was slightly chipped off the coated panel. This indicates that as the content of HELA increases in blended systems, its flexible nature overcomes the drawback of brittleness of DGEBA epoxy. Scratch hardness, impact resistance performance and bending ability of coatings mainly depend on the crosslink density of the polymeric resin, adhesion of the coatings to the substrate as well as the flexibility of polymeric chains, respectively. The curing scheme (Fig. 2) apprises about the presence of cross-linking sites (viz. epoxies and –OH), ether, aromatic moieties of DGEBA as well as long aliphatic chains of fatty amide, which collectively introduce sufficient crosslink density, adhesion, toughness and improve the physico-mechanical performance of the resins in cured form (coatings). However, a slight deterioration in scratch hardness is observed on increasing soft segments of HELA in rigid epoxy matrix [17]. The schemes also suggest that here both TETA and HELA act as curing agents. Also, with increase in the amount of HELA (along with the inclusion of TETA), drying time of coatings decreases, gloss as well as IRt and bend test values show gradual improvement. These facts further confirm the role of HELA, both as a curing agent and modifier, which had a profound influence on physico-mechanical characteristics and chemical resistance performance of coatings. From plain E/A, I/A-IV/A, gloss values increase from 39 to 61 with increase in the amount of HELA as a typical characteristic of fatty amide based coating material (Fig. 10). 3.4.2. Chemical resistance All coated panels were subjected to exposure in various chemical media to study their corrosion resistance behavior viz. changes in their weight, adhesion and overall appearance. In 5 wt.% HCl, systems I/A and II/A showed loss in adhesion after 20 h of immersion; coating material however, remained unaffected. In HCl, systems III/A and IV/A also showed similar results. In 5 wt.% NaOH, coated panel of sample II/A was found to be unaffected even after dipping in aforementioned media for 72 h. No change in gloss and weight of coated panels was observed. In samples III/A and IV/A, slight loss in gloss and adhesion was observed, respectively, while in sample I/A, corrosion was visible (in the form of loss in weight and adhesion of the coated panel). This trend is attributed to the presence of ether linkages formed by curing of epoxy resin and amide moieties of HELA which are resistant to alkaline hydrolysis. As the content of HELA is increased, from sample I-IV/A, the performance in alkaline media also improved. These results are extraordinarily remarkable in contrast to conventional resins (polyesteramide, polyetheramide, polyurethaneamide) obtained from HELA owing to the excellent combination of functionalities e.g., amide and ether junctions, which show good alkali resistance. In 3.5 wt.% NaCl, loss in weight of coated panels became evident after 20 h due to dissolution of coating material in case of I-IV/A coated panels. In sample IV/A, slight loss in adhesion was observed after 5 h of immersion.
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Fig. 11. TGA thermograms of EA, I/A, II/A, III/A.
In xylene, swelling occurred in panels coated with samples III/A and IV/A after 70 h, while in I/A and II/A, no change was observed. This could be attributed to higher amount of HELA in these samples, which shows good solubility in xylene. In water, sample I/A and II/A coated panels were found to be unaffected after 70 h while coated panels of samples III/A and IV/A showed slight change in weight due to dissolution because of higher content of HELA, which is water soluble in nature. As discussed earlier, good miscibility between the two (uncured) components has a positive effect on the entanglements formed between their cured networks (after curing with TETA), which eventually have a cumulative effect on the overall physicomechanical and chemical resistance performance of systems I-IV/A.
3.5. Effect of temperature on color, gloss, adhesion and weight of coatings All the systems (I-IV/A) were subjected to heating in air in an oven from 100 ◦ C to 280 ◦ C for the time and at temperature intervals of ten minutes and 20 ◦ C, respectively. No deterioration in gloss, color and weight of coated panels was observed up to 150 ◦ C. However, on heating beyond 150 ◦ C, color of the coated panels turned brownish (translucent from initial water white transparent). A slight deterioration in the gloss of coated panels (from their initial values) was also visible. Between 170 and 180 ◦ C, all the systems turned dark brown; system IV/A that had the highest amount of HELA, showed loss in adhesion due to thermoplastic nature of HELA, which cannot withstand high temperature and in an uncombined form starts melting above 60 ◦ C. However, here, in combination with E and TETA, probably due to cross-linking by TETA, thermoplasticity became evident only at temperature as high as 170–180 ◦ C. System III/A showed slight loss in adhesion on further heating beyond 180–200 ◦ C. The percent loss in gloss and
weight of coated panels in III/A and IV/A was found as 21–24% and 0.20–0.27%, respectively. In I/A and II/A, adhesion was found to be unaffected by further heating the coated panels up to 280 ◦ C; however, percent deterioration in their gloss and weight was found as 30–34% and 2–2.5%, respectively. All the systems turned black when heated up to 280–300 ◦ C with evident loss in weight of coated panels indicating the onset of degradation of polymers or lysis of bonds.
3.6. Thermal stability of cured DGEBA/HELA/A networks TGA, DTG and DSC thermograms of DGEBA/HELA/A systems are provided in Figs. 11 and 12. The shapes of DSC thermograms are almost identical. While TGA and DTG thermograms of II/A and III/A show identical degradation pattern, those of E-A and I/A show pronounced variation. All the thermograms show the onset of degradation beyond 220 ◦ C. DTG thermogram of E-A shows multiple but muffed degradation stages which are evident in TGA thermogram corresponding to 10 wt.% (200–300 ◦ C), 40 wt.% (300–375 ◦ C), 20 wt.% (375–425 ◦ C) and 30 wt.% (475–660 ◦ C) loss. I/A distinctively reveals three degradation stages; the first two are closely spaced in a temperature span of 335–425 ◦ C, the third one occurs at a higher temperature. DTG thermograms of II/A and III/A show almost single step thermal degradation incurring about 85 wt.% loss. Thus, in other words, the higher the content of HELA, the lesser is the number of degradation steps. It can be anticipated that in E-A and I/A, there are a number of separately existing entities (cured and uncured oxirane rings, –OH, –NH2 , double bonds) that disintegrate independently under the influence of high temperature as evident from multiple degradation steps in DTG. While in II/A and III/A due to higher amount of HELA, it is envisioned that both the systems comprise a cross-linked, homogenous network
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E. Sharmin et al. / Progress in Organic Coatings 67 (2010) 170–179 Table 3 Antibacterial activity of blends. Antibacterial activity
H
I
II
III
IV
E. coli S. aureus
− +
− +
− +
− ++
− ++
Inactive +: mildly active (10–12 mm), ++: moderately active (13–20 mm), +++: highly active (21–26 mm).
3.8. Antibacterial behavior of blends (Table 3)
Fig. 12. DSC thermograms of I/A, II/A and III/A.
of cured oxirane rings, –OH, –NH2 and double bonds (IR spectra Figs. 4 and 5 and Reaction scheme Fig. 2) that results in their single-step thermal degradation behavior as (not separate but one) single entity favored by chain entanglements. E-A, I/A, II/A and III/A show 25 wt.% loss at 340 ◦ C, 330 ◦ C, 332 ◦ C, 323 ◦ C, 50 wt.% loss at 375 ◦ C, 365 ◦ C, 368 ◦ C and 365 ◦ C and 75 wt.% loss at 425 ◦ C, 410 ◦ C, 393 ◦ C, 382 ◦ C, respectively. Decrease in decomposition temperatures can be attributed to increase in soft segments of HELA in rigid epoxy matrix, slightly lowering the cross-linking density [17]. DSC thermograms show endotherms centered at 300–350 ◦ C while TGA thermograms clearly show decomposition in this temperature range signifying some configurational changes in the polymeric resin prior to its decomposition. DTG shows maximum weight loss rate beyond this temperature range. Derivative curves in this study also reveal that the temperatures corresponding to the maximum weight loss rate first increase marginally from E-A (345 ◦ C) to I/A (348 ◦ C), sharply to II/A (366 ◦ C) and then decrease with increased amount of HELA from II/A (366 ◦ C) to III/A (353 ◦ C) while the rate of weight loss at these temperatures first decreases from E-A to I/A and then increases from I/A to III/A. The exact reason for this behavior cannot be deduced; however, we understand that the thermoplastic nature of HELA may be responsible for this behavior. Interestingly, DTG of pristine E/A shows an additional, relatively short degradation step, revealing a very small percent of weight loss at about 225 ◦ C, (which is not observed in any other DTG thermogram) followed by a major degradation process between 345 and 366 ◦ C predominant in all the systems. The former is attributed to the breaking of unreacted oxirane or amine molecules; the latter is associated with the thermal degradation of cured epoxy network eventually followed by the decomposition of aromatic moieties and aliphatic chains of fatty amide under the influence of high temperature [39].
All the systems were found to be inactive and mildly to moderately active against E. coli and S. aureus, respectively. The long pendant hydrocarbon chains of HELA are abode to amide, hydroxyl, double bonds and methylene groups while epoxy has oxirane rings, hydroxyl moieties, aromatic rings and ethereal linkages. The probable mode of action of these polymeric resins involves bacterial protein denaturation, damage of lipid complexes in cell membranes or dehydration of bacterial cells [37,38,47]. E. coli (Gram-negative) and S. aureus (Gram-positive) show varying response against these systems. The presence of outer membrane containing lipopolysaccharide in the former, protects the bacteria from several antibiotics, dyes and detergents that normally damage the inner membrane or cell wall peptidoglycan. Consequently, HELA and DGEBA/HELA/A systems show mild to moderate antibacterial activity against S. aureus and are inactive against E. coli. It is interesting to note that while pure HELA exhibits mild activity against S. aureus, system IV/A with maximum amount of HELA shows better (moderately active) antibacterial activity. 4. Conclusion Blends and their coatings were prepared at ambient temperature without the use of any organic solvent. DGEBA/HELA showed mild to moderate antibacterial activity against S. aureus, which gradually improved with higher content of HELA. Ageing studies of blends confirm that HELA can be employed as a mild curing agent and reactive modifier for epoxies operative at ambient temperature by zero solvent approach. Physico-mechanical characteristics such as IRt, BT and gloss improved with increase in the amount of HELA. The approach provides an alternate pathway to cut off the use of commercially available epoxy resins by partial substitution of fatty amide monomers derived from seed oils—a sustainable resource. It also provides a novel opportunity to develop “solvent free” coating materials and adhesives due to inherent fluidity of HELA—a typical characteristic of seed oils and their derivatives that facilitates its use as solvent, reactant and curing agent, simultaneously. However, further improvement for mild curing behavior of HELA is required, which is in progress in our laboratory. Acknowledgement Dr. Eram Sharmin is thankful to CSIR, New Delhi, India, for Research Associateship against Grant No. 9/466(0102)2K8-EMR-I. References
3.7. XRD of cured DGEBA/HELA/A networks DGEBA/HELA/A systems reveal insignificant broadening of peaks from I/A to III/A indicative of their amorphous nature, which increases with increase in HELA. It is interesting to note that from III/A to IV/A, no such change (peak broadening) is observed, which reveals that at former loading, HELA completely envelops epoxy. The presence of amorphous peaks also informs that HELA has no effect on crystalline nature of polymers.
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