Investigation of solvent interaction in epoxy coatings

Investigation of solvent interaction in epoxy coatings

Polymer Testing 19 (2000) 111–114 Short Communication: Material Characterisation Investigation of solvent interaction in epoxy coatings I. Glavcheva...

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Polymer Testing 19 (2000) 111–114

Short Communication: Material Characterisation

Investigation of solvent interaction in epoxy coatings I. Glavcheva,*, K. Petrovaa, I. Devedjievb b

a University of Chemical Technology and Metallurgy, 1156, Sofia, Bulgaria Institute of Polymers, Bulgarian Academy of Sciences, 1113, Sofia, Bulgaria

Received 10 July 1998; accepted 10 September 1998

Abstract The rate of evaporation of solvents from epoxy coatings was investigated. The coatings were obtained from solutions of epoxy olygomers based on Bisphenol A with molecular weights 600 and 2500. It was established that the rate of evaporation depends on the kind of solvents, molecular weights of olygomers and the possibility of formation of H-bonds. The formation of H-bonds was investigated with UV and IR spectroscopies of the solutions and coatings. From the UV spectra were calculated the shift of the peaks ⌬␯. The ratio A⬘ ⫽ A3440/Amax, the shifts ⌬␯v and ⌬␯d and the peak width at half height ⌬1/2 were calculated from IR spectra.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Obtaining epoxy coatings depends on the rates of two processes: the evaporation of the solvents and crosslinking of epoxy groups. The temperature, volatility of solvents, and the possibility of H-bonds formation and the thickness of the coating determine the rate of evaporation. In Flory’s theory of polymer solutions, the interaction between macromolecule and solvent is taken down with ␹o [1]. There are no systematic data in the literature for ␹o for solutions of epoxy olygomers, maybe because the influence of their molecular weight. Increasing molecular weight increases the –OH and oxygen content and the physicochemical interaction with solvents. The possibility of formation of H-bonds is determined by the concentration of polar epoxy and – OH groups and oxygen atoms. The presence of the cyclic rings from bisphenol A suggests com* Corresponding author. Tel.: ⫹ 359-2-6254579; fax: ⫹ 359-2-685488; e-mail: [email protected] 0142-9418/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 8 ) 0 0 0 7 3 - 7

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patibility with aromatic solvents. The formation of H-bonds can possibly be characterized by UV and IR spectroscopies. The shift of the peaks is a measure of H-bonds. In IR spectra, the shift of the peaks of deformation vibrations ⌬␯d is hypsochromic and of valent vibration ⌬␯v is bathochromic. The peak width at half height increases with H-bonds formation. The ratio between the absorbance at 3440 cm−1 and the maximal one in the peak for –OH group is the measure for quantitative characterization of H-bond formation between –NO2, –OH or –O– and H-atoms in macromolecules of nitrocellulose [2]. The aim of this investigation is: to prove the influence of the molecular weight of epoxy olygomers based on bisphenol A on the rate of evaporation of polar or non polar solvents; to relate these data to formation of H-bonds in the solutions and coatings and to find available values from UV and IR spectra for characterization of these bonds.

2. Experimental The molecular weights of Diepox 600 (Lakprom, Bulgaria) and Epicote 1007 (Shell, UK) are 600 and 2500. The amount of polar groups and oxygen atoms were calculated from these values. The molecular weights were measured by the method described in [3]. The solvents toluene, xylene, ethylacetate, ethylglycolmonoethyl ether (EGME) and butanol are grade ‘pure’. The calculated amounts of the solutions of epoxy olygomer with or without addition of curing agent diethylenetriamine (DETA) were put in containers to obtain 0.01 mm thick coatings. The weight of these containers after different periods of time at 20°C were measured with an accuracy of 0.0001 g and the rates for 2, 48 and 500 h were calculated. The UV and IR spectra were obtained with a Perkin Elmer apparatus.

3. Results and discussion The values of the rates of evaporation Ve of different solvents were calculated by the equation: Ve ⫽

Gn ⫺ G0 G0

where Gn is the weight of the sample after n hours stay at 20°C; G0 is the initial weight of the sample. The results are given in Table 1. The temperatures of evaporation of the solvents are: toluene—110°C; xylene—138°C; ethylacetate—77°C; butanol—114°C and EGME—135°C. The –OH content of Diepox 600 is 2.6%; of Epicote 1007 is 6.7%, and the oxygen content is 15.8 and 13%, respectively. The influence of molecular weight of epoxy olygomers, respectively the –OH content, is evident from compositions 1 and 2 in Table 1. Increasing the –OH content leads to decreasing the rate of evaporation. The same effect is obtained by the compositions with DETA—the lower rate of evaporation is due to the higher content of polar groups as well as to the formation of crosslinking. There is no proportional relation between the rates of evaporation and the boiling temperatures of the solvents. For example, the rates of evaporation of polar solvents are smaller than the rates of evaporation of toluene and xylene. The explanation is formation of H-bonds between the olyg-

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Table 1 The rates of evaporation, g m−1 No.

Composition

2h

48 h

500 h

1 2 3 4 5 6 7 8 9 10 11

Diepox 600/toluene Epicote 1007/toluene Epicote 1007/toluene/DETA Diepox 600/xylene Diepox 600/xylene/DETA Diepox 600/ethylacetate Diepox 600/ethylacetate/DETA Diepox 600/butanol Diepox 600/butanol/DETA Diepox 600/EGME Diepox 600/EGME/DETA

24 × 10−2 21 × 10−2 60 × 10−2 5.1 × 10−2 3.5 × 10−5 50 × 10−2 1.8 × 10−7 3.2 × 10−2 7.17 × 10−5 1.8 × 10−2 1.93 × 10−4

9 × 10−2 1.23 × 10−10 1.87 × 10−12 4.5 × 10−2 3.37 × 10−5 4.8 × 10−2 1.74 × 10−4 2.9 × 10−2 7.09 × 10−5 1.5 × 10−2 1.85 × 10−4

6.48 × 10−2 2.31 × 10−14 1.13 × 10−14 1.2 × 10−2 2.96 × 10−5 4.48 × 10−12 1.15 × 10−4 1 × 10−2 6.37 × 10−5 0.2 × 10−2 1.25 × 10−4

omer and the molecules of solvents. A measure of the quantitative evaluation of these bonds are the data from UV and IR spectra. There is a shift ⌬␯ in UV spectra of the solutions of Diepox 600 and Epicote 1007 in ethylacetate at ⫻ 7 dilution. The shift ⌬␯ is bigger for the maximum at 285 nm for a solution of Epicote 1007, but the shift ⌬␯ for the maximum at 279 nm is bigger for the solution of Diepox 600. This is the reason for IR spectroscopy investigation. The results are shown in Table 2. The values of A⬘ ⫽ A3440/Amax are bigger for the olygomer with bigger content of –OH group. Consequently, for epoxy olygomers this relation could be a quantitative characterization for Hbonds formation, as for nitrocellulose [2]. The values of A⬘ are higher for spectra of solutions with polar solvents which confirms the possibility of using this relation. The bigger number of conformations of macromolecules leads to increasing of the peak width at half height ⌬1/2 [4,5]. The formation of H-bonds increases the number of conformations. Actually, the value of ⌬1/2 for a solution of Epicote 1007/ethylacetate is 10 cm−1 bigger than this value Table 2 Data UV and IR spectra No.

1 2 3 4 5 6 a

Composition

Diepox 600/toluene Epicote 1007/toluene Diepox 600/ethylacetate Epicote 1007/ethylacetate Diepox 600/EGME Epicote 1007/EGME

UV

IR

⌬␯285

⌬␯279

⌬⬘ (cm−1)

⌬820 (cm−1)

I⌬␯dIa (cm−1) I⌬␯vIa (cm−1)

– – 0.1

– – 1.5

0.423 0.488 0.63

– – 53

5 8 8

7 9 2

0.7

0.8

0.873

63

10

3

– –

– –

– 0.8

– –

9 11

3 4

I⌬␯dI and I⌬␯vI are the absolute values of the shift of the absorbances at 820 and 1250 cm−1.

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for a solution of Diepox 600. The shift of the absorbances for the C–O–C group at 820 and 1250 cm−1, ⌬␯d and ⌬␯v, confirm the formation of H-bonds: the values are bigger in the case of Epicote 1007. The results thus obtained can be used for evaluation of the interaction between solvents and epoxy olygomers in the process of coatings formation. References [1] Flory PJ. Principles of polymer chemistry. Ithaca, NY: Cornell University Press, 1953. p. 24. [2] Kovalenko VI. Himia I Reakcionnaja Sposobnost Celulozi I ejo Proizvodnih. Moscow: Institute Himiceskoj Fiziki, 1991. p. 198. [3] May CA, Tanaka Y. Epoxy resins. New York: Marcel Dekker, Inc., 1973. p. 680. [4] Kovalenko VI. Zhournal Strukturnoj Himii 1993;34:59. [5] Saunders CW, Taylor LT. Appl Spectrosc 1991;45:604.