201
Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 261-262
coatings employed do not show sensitivity to humidityinduced adhesion loss, the corrosion-inducedadhesion-loss process must be explained in terms of the unique features of the corrosion process itself. The enormous difference in rate of adhesion loss between the resins used in this study must also be explained. Overall, it is suggested that, for the epoxy ester coating, adhesion loss starts at the scribe with an initial attack on the organic coating by cathodically produced hydroxide at or near the organic coating/phosphate interface. As the interfacial region is disrupted, additional surface area becomes available for the cathodic reaction. Upon continued exposure to hydroxide, both the organic coating and the phosphate conversion layer are degraded in the interfacial region. The conversion coating degradation appears to involve at least partial dissolution of phosphate. A similar process is likely involved in the case of the epoxy amine coating; due to the greater resistance of the organic resin to degradation by hydroxide, the disruption of the interface is probably due principally to dissolution of the conversion coating. With
sufficiently long exposure times, virtually all the conversion coating is dissolved. Literature Cited Alien, C. C.; Curtis, M. T.; Hooper, A. J.; Tucker, P. M. J . Chem. SOC., Danon Trans. 1974, 1526. Chuang, T. J.; Brundle, C. R.; Wandelt, K. Thin Solid Films 1978, 5 3 , 19. Dickie, R. A,; Smlth, A. 0.CHEh4ECH 1980, 10, 31. Hammond, J. S.;Holubka, J. W.; deVries, J. E.; Dickie, R. A. Corros. Sci. 1981.,~ 27. 239. Hammond, J. S.;Holubka, J. W.; Dickie, R. A. J . Coat. Technol. 1979 5716551. 45. Hoiubka, J:'W.; Hammond, J. S.;devries, J. E.; Dickie, R. A. J . Coat. Techno/. 1980, 52(670), 63. Leidheiser, H. Ind. Eng. Chem. Prod. Res. D e v . 1978, 17, 54. Leidheiser, H. Croat. Chem. Acta. 1980, 53(2), 197. Leidheiser, H.; Kendig, M. W. Corrosion NACE 1976, 3 2 , 69. Mayne, J. E. 0. In "Corrosion", 2nd ed.; Shreir, L. L., Ed.; Newnes-Butterworths: London, 1976; Voi. 2, p 1524. McIntyre, N. S.; Zetaruk, D. G. Anal. Chem. 1977, 4 9 , 1521. Smith, A. G.; Dickle, R. A. Ind. Eng. Chem. Prod. Res. D e v . 1978, 17, 42. Wagner, C. D. Anal. Chem. 1977, 99, 1282. Wiggle, R. R.; Smith, A. G.; Petrocelli, J. V. J . faint Technol. 1988, 4 0 , 174.
.~
Received for review June 1, 1982 Accepted November 1, 1982
Cure Behavior of an Epoxy Resin-Dicyandiamide System Accelerated by Monuron Bernard R. LaLlberte,' Joseph Bornstein, and Robert E. Sacher US.Army Materials and Mechanics Research Center, Powmer Research Division, Waterfown, Massachusetts 02 172
The synergistic effect of dicyandiamide and Monuron which takes place in the hardening of epoxy resins was investigated by differential scanning calorimetric analysis conducted under dynamic conditions. Based on the data obtained by this technique, it was found that the accelerated cure behavior may be illustrated by the examination of three distinct systems: epoxy resin -t- dicyandiamide (Dicy), resin Dicy/Monuron, and resin Monuron. The activation energies of these mixtures were found to be 39, 22, and 19 kcal/mol, respectively. The byproduct of the resin Monuron reaction is dimethylamine. I t was found that dimethylamine is able to enhance the reactivity of dicyandiamide dramatically.
+
+
+
Introduction The hardening of dicyandiamide (Dicy)-containing epoxy resins is a high-temperature process where effective cure is realized at approximately 200 "C. In contrast, with the addition of trisubstituted urea accelerators such as N'- (4-chlorophenyl)-N,N-dimethylurea (Monuron) these systems can be cured at temperatures somewhat lower than 130 "C. This report is a review of some of our observations on the accelerated cure mechanism of Dicy-containing epoxy resins as realized by differential scanning calorimetry. Discussion and Results Various types of amines such as dimethylamine are known to react with epoxy resins. This amine commanded the attention of earlier workers (Son and Weber, 1973). They speculated that the acceleration of the Dicy cure of epoxy systems was the result of a reaction between Dicy and Monuron. The reaction was conducted in a solvent and proceeded with considerable reluctancy at about 120 "C. Our interest in the cure mechanism was heightened when it was observed (LaLiberte and Bergquist, 1978) that dimethylamine was readily formed by a N'-(3,4-dichlorophenyl)-NJ-dimethylurea (Diuron)/resin mixture. The speculation of a low-temperature cyclocondensation re-
action occurring between the aryl urea and the epoxy functionality affording 2-oxazolidones was supported by the literature (Iwakwa and Izawa, 1964) and this prompted a mass spectrometric (MS) study (LaLiberte, 1979; LaLiberte et al., 1981). The MS analysis of the reaction of Diuron and Monuron with the model compound, p-tertbutylphenyl glycidyl ether, yielded molecular ions ascribable to the formation of 2-oxazolidone derivatives of the type
I
CI
*
roi
-CHCH2N--C=O
$A
This article not subject to U S . Copyright. Published 1983 by the American Chemical Society
CI CI
R
t HN HN,
/CH3
\ CH3
262
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
where R is equal to either H or C1. The following experiments were conducted which established the especially favorable effect of dimethylamine in lowering the cure temperature of Dicy-containing resins. Two milliliters of liquid dimethylamine (bp 7 "C) in a chilled syringe was rapidly introduced and stirred into an intimate mixture of Shell's EPON 828, diglycidyl ether of bisphenol-A (23.13 g) and Dicy (1.48 g). The resulting three-component system contained in a large test tube was placed in an oil bath held at 116 "C. Solidification of this system was realized in about 10 min whereas, under dimethylamine mixture identical conditions, a resin required approximately 30 min to harden. The EPON 828 + Dicy control showed no signs of solidification with prolonged heating a t 116 "C. The earlier workers (Son and Weber, 1973) stated that the analysis by DSC of a resin + Monuron mixture conducted at a heating rate of 10 "C/min indicated only a slight exotherm at 160 "C. First of all, the exothermic reaction temperature is a function of the heating rate, and secondly, the area of the exotherm is not necessarily an indication of the significance of a chemical reaction. We found that the normalized area of the exotherms became progressively smaller as the heating rate was increased when experimentation was conducted in open aluminum pans. These values were reproducible. Failure to achieve a constant area in all likelihood may be attributed to the rapidity with which dimethylamine escapes from its environment. Further investigation indicated that analysis conducted at an especially slow heating rate of 0.5 "C/min afforded results that were in agreement with areas recorded isothermally. Thus, the calculation of the extent of reaction did not present a serious problem. For the present, we merely wish to say that Monuron undergoes considerable reaction with EPON 828 at 90 "C in 2.5 h. The relationship between the exothermic reaction temperature and the heating rate from a series of DSC runs is expressed as (Carpenter, 1977)
Table I.
3M's
Epoxy Formulation
wt 76 of SP-250 resin
wt % o f
wt %of
Dicy
Monuron
88.60 88.60 88.60
7.51 7.51 0.00
0.00 3.79 3.79
+
log rate = A / T
+B
where rate = heating rate ("C/min) assigned to a sample, T = reaction temperature (K) of the sample, A = constant, related to activation energy (E,), and B = constant, related to the Arrhenuis frequency factor. The dynamic E, values reported in this manuscript were calculated by multiplying the slope ( A ) of the above equation by -2.303R where R is the gas constant. The SP-250 resin manufactured by the 3M Company of St. Paul, MN, is predominantly composed of two epoxides, one of which has an epoxy cresol Novalac structure, ECN 1273, and the other material is EPON 828. The cure behavior of this complex system was investigated (LaLiberte and Sacher, in press) in this laboratory. The effects of Monuron on the hardening process was illustrated by the examination of three mixtures shown in Table I.
4 --+-+-'
si
2.8
2 I
2.6 2.2
2.3
2.q
2.7
2.5
I W K
Figure 1. T h e Arrhenuis plots of Table I.
The above three mixtures adhered to the linear Arrhenuis expression where the log rates vs. the 1000/K values were plotted as depicted in Figure 1. The dynamic E, of the SP-250 + Dicy mixture was found to be 38.51 f 2.02 kcal/mol. In contrast, the three-component system underwent reaction with greater facility because the E, did indeed decrease considerably. The value was calculated to be 21.84 f 0.76 kcal/mol. Significantly, this value is not that much different from that of resin + Monuron mixture which is associated with an E, of only 19.06 f 0.79 kcalfmol. Consequently, the reaction between the trisubstituted urea and the epoxide is an important factor that has to be taken into consideration in providing an explanation of the lowering of the cure temperature of dicyandiamide-containing resins. Registry No. SP 250,64156-32-7; A r a l d i t e ECN 173, 3737068-6; E p o n 828, 25068-38-6; monuron, 150-68-5;dicyandiamide, 461-58-5.
Literature Cited Carpenter, J. F. "Quallty Control of Structural Nonmetallic", Contract No. N00019-764-0136, Prepared for Naval Air Systems Command, Washlngton DC, Oct 14, 1976. Iwakawa, Y.; Izawa, S. J . Org. Chem. 1964, 29, 379-82. LaLiberte, B. R. AMMRC TN 79-1, Jan 1979. LaLlberte, 8. R.; Bergquist, P. R. AMMRC TN 78-5, June 1978. LaLlberte, B. R.; Sacher, R. E. "The Dicyandiamkle Cure of SP-250 Epoxy Resin Accelerated by Monuron", AMMRC TR, in press. LaLiberte, B. R.; Sacher, R. E.; Bornstein, J. AMMRC TR 81-30, June 1981. Son, P.; Weber, C. D. J . Appl. Po&" Sci. 1973, 77, 1305-13.
Received for review September 23, 1982 Accepted December 13, 1982
