48
Ind. Eng. Chem. Res. 1989, 28, 48-51
Camacho,-F.; Diaz, F.; Fernandez, J. “Oxidacibn de n-Parafinas en Fase Liquida. 11. Velocidad de Formaci6n de Hidroper6xido”. Anal. Quim., Ser. A. 1980b, 76(3), 375-9. Cavalieri dOro, P.; Dan6czy, E.; Roffia, P. “On the low temperature oxidation of p-Xylene”. Ozid. Commun. 1980,1(2), 153-62. Chen, Z.; Li, G.; Li, W.; Dai, L.; Niu, Y.; Hu, J.; Gu, J.; Mao, X.; Li, M. “The kinetics of the Oxidation of o-Xylene”. Int. Chem. Eng. 1985, 25(4), 738-46. Dawkins, A. W. “Oxidation of n-paraffins”. Eur. Chem. News. Normal Paraffins Suppl. 1966, 49-58. Eakman, J. M. “Strategy for Estimation of Rate Constants from Isothermal Reaction Data”. Ind. Eng. Chem. Fundam. 1969,8(1), 53-9. Emanuel, N. M. ”Modelling of Oxidation Processes”. Akad. Kiado (Budapest) 1986, 1. Farkas, A.; Passaglia, E. “The Decomposition of Cyclohexyl Hydroperoxide and the Peroxide-catalyzed Polymerization of Styrene”. J . Chem. SOC.1950, 72, 3333-7. Himmelblau, D. M.; Jones, C. R.; Bischoff, K. B. “Determination of Rate Constants for Complex Kinetics Models”. Ind. Eng. Chem. Process Des. Dev. 1967, 6(4), 539-43. Mair, R. D.; Graupner, A. J. “Determination of Organic Peroxides by Iodine Liberation Procedures”. Anal. Chem. 1964, 36(1), 194-204. Marquardt, F. W. “An Algorithm for Least-Squares Estimation of
Nonlinear Parameters”. J . SOC.Ind. Appl. Math. 1963, 11(2), 431-4 1. Prengle, H. W., Jr.; Barona, N. ”Make Petrochemicals by Liquid Phase Oxidation. Part 2: Kinetics, Mass Transfer and Reactor Design”. Hydrocarbon Process. 1970,49(3), 106-18. Rumbea, M. A.; Lanchec, G.; Blouri, Bi “Sur I’Oxydation Thermique des Hydrocarbures d Longues Chaines en Phase Liquide”. Reu. Roum. Chem. 1975,20(8), 1103-14. Saunby, J. B.; Kiff, B. W. “Liquid-Phase Oxidation ...Hydrocarbons to petrochemical”. Hydrocarbon Process. 1976,55, 247-52. Sundaram, K. M.; Froment, G. F. “Modeling of Thermal Cracking Kinetics-I. Thermal Cracking of Ethane, Propane and Their Mixtures”. Chem. Eng. Sci. 1977, 32(2), 601-8. Sundaram, K. M.; Froment, G. F. “Modeling of Thermal Cracking Kinetics. 3. Radical Mechanisms for the Pyrolysis of Simple Paraffins, Olefins and Their Mixtures”. Ind. Eng. Chem. Fundam. 1978, 17(3), 174-82. Tang, Y. P. “On the Estimation of Rate Constants for Complex Kinetic Models“. Ind. Eng. Chem. Fundam. 1971, 10(2), 321-3. Toland, W. G. “Oxidation Liquid Phase”. Ind. Eng. Chem. Unit Proces. Reu. 1960, 52(10), 873-6.
Received for review December 11, 1987 Revised manuscript received September 6 , 1988 Accepted September 14, 1988
MATERIALS AND INTERFACES Infrared Studies of Interfacial Reactions of Dicyandiamide on Zinc Roscoe 0. Carter, 111, Ray A. Dickie,* Joseph W. Holubka, and Nancy E. Lindsay Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121
Infrared reflection-absorption spectroscopy has been used to study the interfacial reactions of a dicyandiamide-cross-linkedepoxy system and of neat dicyandiamide on polished zinc and steel substrates. Changes in frequency and relative intensity of bands in the 2300-190O-cm-‘ region of the infrared spectrum of dicyandiamide-epoxy mixtures and of neat dicyandiamide were observed for thin films heated on zinc. No spectral changes were observed when dicyandiamide was heated on steel or on zinc oxide coated steel. The results suggest that dicyandiamide is reduced by reaction with metallic zinc. The structure of the polymer-metal interface is one of the critical factors determining the adhesion and the corrosion performance of polymeric coatings and adhesives. The importance of chemical bonding across the interface to adhesion and to adhesive bond durability and the nature of interfacial bonding have been extensively discussed and debated. These are relatively short-range effects. Substantially longer range effects, involving chemical changes within at least the first several molecular layers adjacent to the nominal interface, have also been observed and can profoundly influence overall system performance. The degradation of polyethylene on copper is illustrative of polymer oxidation involving relatively long-range interface effects (Chan and Allara, 1974a,b). The oxidation of polybutadiene can be catalyzed by certain metal oxides (Cullis and Laver, 1978), and the curing of polybutadiene on metal surfaces has been demonstrated to result in the formation of an interfacial region of polymer more highly oxidized than the bulk (Dickie et al., 1981, 1984). Substrate-mediated interfacial degradation processes have been observed for vinyl polymers applied to zinc (deVries et al., 1988). 0888-5885/89/2628-0048$01.50/0
The present study is directed toward elucidation of possible surface chemical effects in the curing of epoxydicyandiamide adhesives on steel and galvanized steel substrates. As reported elsewhere, adhesive bonds prepared from electrogalvanized steel and epoxy-dicyandiamide adhesive exhibit substantially better strength retention upon exposure to corrosive environments than do bonds prepared from cold-rolled steel (Holubka et al., 1988). The difference in bond performance suggests that there may be a qualitative difference in the structure of the polymer-metal interfaces. In the present study, the interfacial chemistry of an epoxy-dicyandiamide adhesive formulationand of dicyandiamide itself applied to polished steel and zinc substrates has been investigated by using infrared surface spectroscopy.
Experimental Section Materials. Dicyandiamide and dimethylformamide were obtained from Aldrich Chemical Company. The epoxy resin was Epon 828 (a bisphenol A-epichlorohydrin product from Shell Chemical Company having an epoxide 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 49
A
Y
z u 0
B
v)
4 m
I " " / " " I " " I " " I " " I " "
4000 4000
3000
2000
1000
WAVENUMBERS (CM-1)
Figure 1. FTIR spectrum of dicyandiamide-epoxy film on CRS (A) before and (B)after heating.
equivalent weight of about 190). All materials were used as received. Substrates were cold-rolled steel supplied by Parker Chemical Company in the form of standard paint test panels and heavy zinc foil from Alfa Products. The substrates were polished to a mirror finish, buffed on a polishing cloth with alumina, and dried in a stream of dry air. The substrates were found (by X-ray photoelectron spectroscopy) to be free of alumina. An approximately 15-nm zinc oxide film was produced for use in a control experiment by sputtering 13 nm of zinc onto a polished steel substrate and allowing it to oxidize in air for 18 h. The zinc-dicyandiamide complex used as a model compound in this study was prepared essentially according to a procedure described by Panda et al. (1980). To 1.68 g (20 mmol) of dicyandiamide in 10 mL of dimethylformamide was added 1.36 g (10 mmol) of ZnC1,. After stirring at room temperature for 2 h, the reaction mixture was spread on an IR crystal and heated for 30 s at 180 O C prior to acquiring the infrared spectrum. Spectroscopy. Infrared spectra were recorded on a Mattson Sirius 100 spectrometer with a nitrogen-cooled HgCdTe detector. Reflection-absorption spectra were recorded at an incident angle of 80 deg to the normal by using a Harrick variable-angle reflection accessory. A substrate reference spectrum was recorded prior to coating the substrate in every experiment. Thus, the indication of surface structures, such as oxides, is only present in the spectra if there has been a change in their composition, thickness, or form. Films were formed by casting from dilute dimethylformamide solution. No spectral changes were detected for dimethylformamide alone on either zinc or steel substrates.
Results and Discussion The midinfrared reflection-absorbtion spectra of very thin films of a cross-linking mixture of the epoxy and dicyandiamide and of neat dicyandiamide on polished, oxidized metallic substrates are reproduced in Figures 1-4. The upper spectrum in each figure was obtained before heating, while the lower one was obtained after heating the
3000
2000
1000
WAVENUMBERS (CM-1)
Figure 2. FTIR spectrum of dicyandiamide-epoxy film on zinc (A) before and (B)after heating.
W V
z
2 p: 0
m v)
F.olLi I \
B
u'v!!l
I\
10.005 A
wuu
JUUU
LUUU
LUUU
WAVENUMBERS (CM-1)
Figure 3. FTIR spectrum of dicyandiamide film on zinc (A) before and (B) after heating.
specimen at 170 "C for 1 min (5 min in the case of Figure 4). The spectra of the as-deposited films (top spectra in Figs. 1 and 2) are essentially the sum of epoxy and dicyandiamide spectra and are nearly identical, except for changes in the relative intensities of the quartet of bands in the 1600-cm-l region. These similarities are to be expected, as the films were derived from the same solution and differ only in reflective substrate used. The minor difference in the 1666- and 1647-cm-' pair are attributed to differences in the crystal orientation of dicyandiamide on the two substrates. Heating of the epoxy-dicyandiamide films resulted in a number of spectral changes. Water was eliminated from both films, as evidenced by the downward appearing 0-H stretching band at ca. 3300 cm-l. In both cases, the most
50
Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989
A
10.02 A
0
3000
2000
1000
WAVENUMBERS (CM-1)
Figure 4. FTIR spectrum of dicyandiamide film on zinc oxide coated steel (A) before and (B) after heating.
intense band is reduced by half or more and the dominant features retained are those of the epoxy. The dicyandiamide related features are no longer present in the spectrum obtained from the steel surface. The spectrum obtained from the zinc surface after heating has broad bands at 2160 and 2018 cm-' which appear to have developed from the sharp bands at 2211 and 2169 cm-l present in the spectrum from the surface before heating. These bands do not represent merely a broadening of the original bands but indicate the formation of new molecular entities. The experiment was repeated with neat dicyandiamide applied to the zinc surface (Figure 3). Essentially the same changes were observed along with other spectral changes which had been disguised by the presence of the epoxy in the initial set of spectra. The principal bands observed for dicyandiamide on zinc before and after heating are summarized in Table I; for comparison, frequencies and assignments for bulk dicyandiamide (taken from Jones and Orville-Thomas (1959)) are included. To determine if zinc oxide was a sufficient reagent to produce the changes observed on zinc, the experiment was repeated again with dicyandiamide applied to a thin layer of zinc oxide formed on steel. The initial spectrum was identical with that seen on the zinc surface (Figure 4, top). No evidence could be seen for the products observed on the metallic zinc substrate (Figure 4, bottom). The spectral intensity obtained from the zinc surface was reduced to a third of its initial value in 1 min at 170 "C. A much more gradual reduction in intensity was observed on the zinc oxide/steel surface; even after 5 min of heating, about half the original intensity was retained. The spectral changes observed on zinc, although dramatic, suggest a well-defined reaction rather than a random degradation of the dicyandiamide: there are many common features in the spectra before and after heating, consistent with retention of much of the molecular structure. There is evidence in both cases of N-H stretching modes. The bands at 1255, 930,670, and 580 cm-' are retained. The results obtained in this study are substantially different from those reported in a similar study on anodized aluminum. Brockmann et al. (1986) found that dicyandiamide hydrolyzed to form guanylurea upon
Table I. Infrared Frequencies of Dicyandiamide a n d Zinc Reaction Products bulk thin films on Zn Zn dicyandivibrational heated unheated complex amideb 3434 m 3440 s 3382 m 3380 s 3341 s 3330 s 3332 m 3336 m NH2 asym. stretch 3242 sh 3236 m 3185 s 3176 w, br 3190 m NH2 sym. stretch 3154 s 3156 m 3159 sh 2237 s 2204 sh 2211 s 2208 m N-C=N asym. 2195 s stretch (C=N) 2160 m 2169 s 2165 m 2018 m ? 1666 s 1658 s 1642 s 1638 m NH2 deformation 1639 s 1647 s asym. 1576 s 1551 s 1587 m N-C-N 1568 s stretch asym. 1510 m 1506 m N=C-N stretch (C=N) 1433 w ? 1421 m 1277 sh 1285 w 1254 m N=C-N sym. 1257 m 1255 m 1245 vw stretch (C-N) 1184 m ? 1146 m 1118 m 1098 w 1091 w 1096 w NH2 rock 971 vw 929 m N-C-N sym. 929 w 929 w 931 w stretch (C-N) 813 w 722 vw 724 vw 720 w N=C-N wag 720 w 670 w 671 w 663 w 669 m N=C-N deformation 588 m 554 m 571 w NH2 wag N=C-N twist 508 m 528 m N=C-N rock 493 m 500 m "Abbreviations: w, weak; m, medium; s, strong; v, very; sh, shoulder; br, broad; asym., asymmetric; sym., symmetric. All frequencies in cm-*. Dicyandiamide vibrational frequencies and assignment consistent with Jones and Orville-Thomas (1959).
heating on a chromic acid anodized aluminum surface. The published spectra show the development of a strong carbonyl absorption and a shift of nitrile bands to somewhat higher wavenumbers. Infrared spectra of complexes of dicyandiamide with a number of divalent metals, including zinc, have been reported (Begley et al., 1985; Hubbersley and Falshaw, 1982; Panda et al., 1980), but in each case the frequency shifts observed for the nitrile band have been to higher frequencies. A zinc-dicyandiamide complex has also been prepared in the course of the present study; the principal infrared frequencies are set forth in Table I. Consistent with the cited earlier studies on such complexes, a shift of the nitrile bands to higher frequencies is observed. The formation of a unique product on the zinc surface suggests a surface-mediated reaction involving zinc and species derived from the dicyandiamide. The (negative) results obtained on zinc oxide indicate that metallic zinc is necessary to produce the observed spectral changes. This in turn suggests the reduction of dicyandiamide by zinc metal. The apparent shift of the nitrile bands to lower frequencies is consistent with the formation of multiply bonded structures. The structure of the product formed on the zinc surface has not been established, but it is clearly different from the hydrolysis products and metal
I n d . Eng. Chem. Res. 1989,28, 51-57
ion complexes reported in other studies.
Literature Cited Begley, M. J.; Hubbersley, P.; Moore, C. H. M. Structural and Spectroscopic Properties of Coordinated 1-Cyanoguanidine. Part 2. Bis(1-cyan0guanidine)di-p-aquocopper(I1)Dinitrate Dihydrate and Related Copper (11)Complexes. J. Chem. Res. 1985, (S)378, (M)4001. Brockmann, W.; Hennemann, 0.-D.; Kollek, H.; Matz, C. Adhesion in Bonded Aluminium Joints for Aircraft Construction. Int. J. Adhesion Adhesives 1986,3,115. Chan, M. G.; Allara, D. L. Infrared Reflection Studies of MetalPolymer Interfaces. Polym. Eng. Sci. 1974a,4, 12. Chan, M. G.; Allara, D. L. Infrared Reflection Studies of the Mechanism of Oxidation A t a Copper-Polyethylene Interface. J . Colloid Interface Sci. 1974b,47, 697. Cullis, C. P.; Laver, H. S.The Effect of Metal Oxides on the Oxidation of Polybutadiene. Eur. Polym. J. 1978, 14, 575. deVries, J. E.; Holubka, J. W.; Dickie, R. A. Interfacial Chemistry of Poly(Viny1 Chloride) Adhesive on Cold Rolled and Galvanized Steel. J . Adhesion Sci. Technol. 1988,in press.
51
Dickie, R. A.; Hammond, J. S.; Holubka, J. W. Interfacial Chemistry of the Corrosion of Polybutadiene-Coated Steel. Znd. Eng. Chem. Prod. Res. Deu. 1981,20, 339. Dickie, R. A.; Carter, R. O., 111; Hammond, J. S.; Parsons, J. L.; Holubka, J. W. Substrate Effects on the Oxidative Cross-Linking of a Polybutadiene Coating. Ind. Eng. Chem. Prod. Res. Deu. 1984,23,297. Holubka, J. W.; Chun, W.; Dickie, R. A. Durability of Adhesive Bonds to Zinc-Coated Steels: Effects of Corrosive Environments on Bond Strength. J. Adhesion 1988,in press. Hubbersley, P.; Falshaw, C. P. Structural and Spectroscopic Properties of Coordinated Cyanoguanidine: Di-p-sulphato(tetraaquobis-p-cyanoguanidine dicadmium(I1)) and Related Cadmium(I1) Complexes. J . Chem. Res. 1982, (S)176, (M)1809. Jones, W. J.; Orville-Thomas, W. J. The Infra-Red Spectrum and Structure of Dicyandiamide. Trans. Faraday SOC.1959,55, 193. Panda, P. K.; Mishra, S.B.; Mohapatra, B. K. Complexes of Cobalt(II), Nickel(II), Copper(II), and Zinc(I1) with Dicyandiamide. J. Inorg. Chem. 1980,42, 497.
Received f o r review J u n e 23, 1988 Accepted October 17, 1988
Emulsion Copolymerization of Styrene-Methyl Acrylate and Styrene-Acrylonitrile in Continuous Stirred Tank Reactors. 2. Aqueous-Phase Polymerization and Radical Capture Richard N. Mead and Gary W. Poehlein* School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
A steady-state model including particle growth by aqueous-phase polymerization is developed for emulsion copolymerization in a continuous stirred tank reactor (CSTR). Expressions are derived for the concentrations and average chain length of oligomeric radicals in the aqueous phase. Results include simulation of styrene-methyl acrylate and styrene-acrylonitrile emulsion copolymerization experiments. Radical capture coefficients are determined for the experimental systems. 1. Introduction The ability of relatively water-soluble monomers to undergo polymerization in the aqueous phase has been under investigation for some time. Priest (1952) conducted a series of experiments where vinyl acetate (VAC) was polymerized in aqueous dispersion with potassium persulfate initiator. The polymerizations without emulsifier resulted in the formation of large, narrow size distribution (PSD) particles. The polymerizations with emulsifier produced smaller particles with a broader PSD. Priest (1952) proposed a “homogeneous nucleation” theory where aqueous-phase polymerization without emulsifier resulted in the formation of many small primary particles. The primary particles were assumed to be only semistable and tended to flocculate and form larger particles. Priest (1952) concluded that micellar nucleation and homogeneous nucleation could occur simultaneously when emulsifier micelles were present. Fitch and Tsai (1971) developed a kinetic theory to quantitatively describe the homogeneous nucleation mechanism. A detailed model for homogeneous nucleation was developed by Hansen and Ugelstad (1978, 1979). These authors assumed the aqueous-phase radical could undergo polymerization, capture by a latex particle, termination with another radical in the aqueous phase, or precipitation as a primary particle. The rate of capture of oligomers of length j , Rcj,is Rcj
= kcsjNpRj
of length j by seed latex particles. The concentration of j oligomers in the aqueous phase is Rj. Hansen and Ugelstad (1978) assumed a mean capture coefficient, k,,, could be applied: L - 1 kcSjRj
R,,
=
cRtot
1=1
The total concentration of monomer and oligomer radicals in the aqueous phase, Rtot, is defined by (3) The total rate of capture of radicals was defined as R,: Rc = RcsNpRtot (4) Hansen and Ugelstad (1978) presented a simplified expression for ha derived by assuming irreversible absorption of oligomer radicals and no electrostatic repulsion between oligomer radicals and latex particles: k,, = 4xDwOrp (5) The average diffusivity of the oligomers in the aqueous phase is Dwo. The radius of the monomer-swollen latex particle is rp. Feeney (1986) derived a rate constant for flocculation of primary particles onto latex particles, Bn6
(1)
The term k, is the capture constant for oligomer radicals
The term kB is the Boltzmann constant, and
0888-588518912628-0051$01.50/0 0 1989 American Chemical Society
K
is the
