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McKague, E. L.; Halkias, J. E.; Reynolds, J. D. J. Compos. Mater. 1975, 9 , 2. Michaels, A. S.; Vieth, W. R.; Barrie, J. A. J. Appl. h y s . 1983, 2 4 , 1. Mlkols, W. J.; Seferls, J. C.; Aplcella, A.; Nlcolais, L., Polym. Composites 1982, No. 3 , 118. Mijovlc, J.; Koutsky, J. A. Po/ymer 1979, 20, 1095. MIJovIc, J.; Tsay, L. Po/jmer 1981, 22, 902. Moy, P.; Karasr, F. E. Polym. Eng. Scl. 1980, 2 0 , 315. Morgan, R. J.; Mones, E. T.; Steele, W. J. Polymer 1982, 23, 295. Morgan, R. J.; O'Neal, J. J. Mater. Sci. 1977, 12, 1966. Morgan, R. J.; O'Neal, J.; Miller, D. E. J . Meter. Sci. 1979, 14, 109. Schneider, N. S.; Sprouse, J. F.; Hagnouer, G. L.; Gillham, J. H. Po/ym. Eng. Scl. 1979, 19, 304. Vleth, W. R.; Howell, J. M.; Hoselh, J. H. J . Membr. Sci. 1976, 1 , 177. Vinson, J. D., ASTM-858 1977, 43C. Zimm, E. H.; Lundemberg, J. J . Chem. Whas. 1958, 6 0 , 425. Weitsman, Y. J. Compos. Mater. 1978, 10, 193.
Apicella, A.; Nicolais, L.; Astarlta, G.; Drloil, E. Polymer 1979, 20, 9. Aplcella, A.; Nlcolais, L.; Carfagna, C.; de Notaristefani, C.; Voto, C, Proceedings of the 27th National SAMPE Meeting, San Diego, CA, 1982; "The Effect of the Prepolymer Composition on The Environment Aging of Epoxy Based Resins". Apicella, A.; Nlcolais, L.; Halpin, J. C., Proceedings of the 28th National S A M E Meeting, Anaheim, CA, 1983; "The Role of the Processing Chemorheology on the Environmental Ageing Behavlour of High Performance Epoxy Matrices". Aplcella, A.; Tessierl, R.; de Cataldls, C. J. Memb. Sci. 1983, in press. Barrer, R. M.; Barrle, J. A.; Slater, J. J . Polym. Sci. 1958, 2 7 , 177. Berens, A. R. Angew. Makromol. Chem. 1975, 47, 97. Browning, C. E. Polym. Eng. Sci. 1978, 18, 16. Chi-Hung; Springer, G. S. J . Compos. Mater. 1978, 10, 2. Dusek, K.; Plestie, J.; Lendnicky, F.; Lunak, J. Polymer 1978, 19, 393. Enscore, D. J.; Hopfenberg, H. E.; Stannett, V. T.; Berens, A. R. Po/ymer 1877, 18, 1005. Gillham, J. K. Polym. Eng. Sci. 1979, 19, 676. Illinger, J. R.; Schnider, N. S. Polym. Eng. Sci. 1980, 20, 310. Lewis, A. F.; Doyle, M. J.; Gillham, J. K. Polym. Eng. Scl. 1979, 19, 687. Loos, A. C.; Springer, G. S. J . Compos. Mater. 1979, 13, 17. Kwei. T. K. ; Zupko, H. M. J. Polym. Sci. 1989, A2(7), 876.
Received for review June 2, 1983 Revised manuscript received October 31, 1983 Accepted November 23, 1983
Substrate Effects on the Oxidative Cross-Linking of a Polybutadiene Coating Ray A. Dlckle,' Roscoe 0. Carter, 111, John S. Hammond,+John L. Parsons, and Joseph W. Holubka Research Staff, Ford Motor Company, Dearborn, Michigan 48 12 1
Fourier transform infrared spectroscopy has been employed to compare the oxidative cross-linking of a high vinyl content polybutadiene on various substrates. Spectra were obtained by transmission on salt plates and by grazing angle absorption-reflection from gold, chromium, and cold rolled steel substrates. I n the initial unoxldized state, essentially identical spectra were obtained independent of substrate. After Oxidative cross-linking (accomplished by baking the specimens in air at 180 O C for 30 min) substantial differences in the type and relative amounts of oxidized species were observed. Relative to the essentially bulk specimen observed in transmission, the thin film specimens cured on steel and chromium were more oxidized, while the specimen cured on gold was less oxidized.
Introduction The synthesis and properties of liquid polybutadiene resins and their use as coatings have been discussed (Auckett and Luxton, 1977). Typically these materials cross-link by oxidative processes upon heating in air to form tough, solvent-resistant films. Polybutadiene coatings have been extensively used in model studies of the corrosion of painted steel (e.g., Touhsaent and Leidheiser, 1972; Kendig and Leidheiser, 1976; Leidheiser and Kendig, 1976, 1978; Leidheiser and Wang, 1981; Castle and Watts, 1981; Dickie et al., 1981). XPS studies of the composition of the interfacial surfaces generated as a result of corrosion induced adhesion loss from steel substrates suggest that the coating near the substrate may be more oxidized than is the bulk of the coating film (Dickie et al., 1981). These results, and the results of a subsequent study of the interfacial chemistry of humidity-induced adhesion loss (Holubka et al., 1984) suggest that the nature of the substrate may influence the composition and properties of the organic coating in the interfacial region, and in particular that the normal oxide surface of the cold rolled steel may promote the oxidation of polybutadiene coatings in the interfacial region. To explore this possibility, the autoxidation of thin films (ca. 0.1 pm) of polybutadiene on polished steel has been investigated by infrared spectroscopy by use of a grazing angle absorption-reflection
method. These results have been compared with those of similar studies using gold and chromium substrates and of transmission infrared studies of thicker films (ca. 5 pm) on NaC1. Experimental Section Materials. The polybutadiene coatings used in this study were prepared from Lithene RH, a high vinyl content (49% 1,2-, 22% trans-1,4-, and 29% cis-1,4- addition) low molecular weight (M,= 2500) polybutadiene supplied by Lithium Corporation of America, Bessemer City, NC. For preparation of thin polymer films, a 1% solution of Lithene RH in toluene was used. Immersion of the substrate in this solution, followed by evaporation of solvent at room temperature and (for the cured films) baking in an air circulating oven at 180 "C for 30 min resulted in formation of cross-linked films ca. 0.1 pm thick. For conventional transmission spectra, films ca. 5 pm thick were prepared by drawing down and curing neat polymer on NaCl plates. Gold mirrors were prepared by vacuum deposition of gold on glass microscope slides (metal thickness, ca. 70 nm). Steel mirrors were prepared by metallographic polishing of SAE 1010 cold rolled steel paint test panels purchased from OMI-Parker; 0.3-pm alumina was used for the final polishing stage. The chromium substrates were electrodeposited chromium on an aluminum substrate. Infrared Spectroscopy. Spectra were obtained with a Nicolet 7001 interferometer equipped with a broad band mercury-cadmium telluride detector interfaced to a Digital
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
Table I. Frequency Assignments for Uncured Polybutadiene frequency, cm-' intensitya 3075
m
3028
w, sh
3005
sh
2988
w, sh
2975
sh
2917
vs
2845 1838 1652 1640
S W
sh m
1496
W
1453
m
1438
sh
1419
m
1406
vw,sh
1325-1 350 1294-1320 1238 1080
w w vw, br w, br
995
S
967
S
911
vs
7 27
w, br
698 681
W W
assignment CH, asymmetric stretch in -CH=CH,, 1,2-addition CH stretch of terminal phenyl CH stretch in cis-CH=CH-, 1,4-addition CH stretch in -CH=CH,, 1,2-addition CH, symmetric stretch in -CH=CH,, 1,2-addition -CH,-symmetric stretch plus -CH- stretch -CH, symmetric stretch 995 + 911, 1,2-addition -C=C- stretch, 1,4-addition -C=C- stretch in -C=CH,, 1,2-addition ring mode of terminal phenyl -CH,- deformation, 1,2-addition -CH,- deformation, 1,4-addition -CH,- in-plane deformation; 1,2-addition -CH- in-plane deformation in cis-CH=CH-, 1,4addit ion -CH,- wag -CH- in-plane rock -CH,- twist -CH,- in-plane rock of -CH=CH,, 1,2-addition CH out-of-plane bending in -CH=CH,, 1,2-addition CH out-of-plane bending in trans-CH=CH-, 1,4-addition CH, out of plane bending in -CH=CH, CH out-of-plane bending in cis-CH=CH-; 1,4addition terminal phenyl unknown; 1,a-addition
600
2200
I400
3000
3800
WAVENUMBER (cn?)
Figure 1. Representative transmission spectra of uncured (A) and cured (B)polybutadiene on NaCl.
a s = strong, m = medium; w = weak; v = very; sh = shoulder; br = broad.
Equipment Corporation PDP 11/60 computer. A Perkin-Elmer variable angle external reflectance accessory and a Perkin-Elmer gold wire grid on AgBr polarizer were used; the angle of incidence of the parallel polarized light was 8 4 . 5 O relative to the surface normal. The spectra were the result of 100 co-additions. Resolution was 2 cm-l; boxcar appidization was used in the transform process. All spectra were corrected for carbon dioxide and water by conventional spectral manipulation procedures; a baseline flattening procedure was also employed. The location, shape, and relative intensity of the major spectral features were unaffected by these manipulations. Experiments involving NaC1, steel, and gold substrates were repeated independently by several of the authors. The experiments using chromium as the substrate were performed in duplicate. All spectra obtained were completely consistent with the representative spectra, and with the interpretation, given in this paper. Results and Discussion Transmission spectra of cured and uncured polybutadiene films on NaCl are shown in Figure 1. Reflection spectra of uncured polybutadiene films on gold, chromium,
, 1
600
I
1400
I
/
I
,
I
, I ,
I
O
2200
I
I
I
I 1
3000
I
I
,
,
,
, 3800
WAVENUMBER (Cm"1
Figure 2. Representative reflection spectra of uncured polybutadiene on gold (A), chromium (B), and steel (C).
and steel substrates are shown in Figure 2. The corresponding spectra of cured films are shown in Figure 3. Band aasignments for the uncured films are summarized in Table I. These assignments are based in part on the normal coordinate calculations for syndiotactic 1,2-polybutadiene by Zerbi and Gussoni (1966) and for trans-1,4polybutadiene by Net0 and DiLauro (1967), and in part on the suggested assignments for cis-1,4-polybutadiene given by Cornell and Koenig (1969) and Binder (1963). In addition, several bands suggest the presence of phenyl terminal groups. For the uncured films, the spectra are essentially independent of substrate and sampling techniques. The spectra of the cured samples (Figures l b and 3a-c) all show evidence of extensive oxidation of the polybutadiene. Since these spectra all display absorptions in the same frequency regions it is likely that the types of
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
I 1400 I
600
I
I
,
I
I
I I I
I
,
,
2200
I
,
I
I , ,
3000
, , ,
, ,
3800
WAVENUMBER (en?)
Figure 3. Representative reflection spectra of cured polybutadiene on gold (A), chromium (B),and steel (C).
functional groups formed in the oxidative cure are similar. However, the relative intensities of the various functional groups are very different for films formed on different substrates. Consider first the spectrum of the film cured on an NaCl plate. This film was relatively thick, and hence the transmission spectrum obtained is taken to be representative of the reaction products formed in the bulk of the coating rather than at either interface. In addition to the oxidized species formed, as discussed subsequently, there is evidence for residual unreacted unsaturation. For example, the high-frequency carbon hydrogen stretch attributed to vinyl hydrocarbon (3075 cm-') and the band at 1640 cm-l assigned to the vinyl carbon-carbon stretch are still in evidence. The methylene scissors region is also quite similar to the corresponding region for the uncured sample. The aliphatic ester carbonyl stretch (1730 cm-') and cyclic ester carbonyl stretch (1770 cm-') prominently indicate the formation of oxidized products. At lower frequency, the spectrum of the cured film consists of three broad features which are generally associated with ester and alcohol functionality. The identifiable maxima at 1180 and 1260 cm-' are assigned to the 0-H bend and C-0 stretch for the (principally primary) alcohol. Presence of alcohol functionality is also confirmed by the very broad hydrogen-bonded 0-H stretch between 3300 and 3600 cm-'. The band at 1180 cm-' is attributed to the C-0 stretch of ester groups. The remainder of the spectrum, the three sharp bands between 900 and loo0 cm-' and the broad feature centered at 700 cm-l, are similar to the spectrum of the uncured polybutadiene. The oxidative cross-linking of the polybutadiene coating on gold resulted in a sample with a spectrum (Figure 3a) very similar to that of the bulk film with one striking difference: for the gold substrate there is significantly less intensity at the 1180, 1730, and 1770 cm-l features relative to the rest of the spectrum indicating a much reduced relative abundance of ester functionality in the film. Hence the spectrum of the film formed on the gold substrate seems to be accounted for primarily by alcoholic and vinylic functionality. The spectrum of the film cured on the chromium substrate (Figure 3b) is consistent with a much higher degree of oxygen incorporation than the corresponding spectrum for the gold substrate. The only clear evidence of residual
299
vinyl functionality consists of the 911-cm-' band which has substantially less intensity on the chromium substrate than on gold. The relative mix of alcohol and ester moieties seems to be similar to that observed in the bulk (Le., the f i i cured on NaC1) based on observed relative intensities. The film formed on steel (Figure 3c) has evidently been even further oxidized as the 911-cm-' band has all but vanished from the spectrum. Relative to chromium, the steel substrate seems to favor ester formation: the 1180, 1730, and 1770-cm-' bands are more intense relative to the 1070-cm-' alcohol associated feature. Although it is obvious that there are significant differences in these spectra which do indicate real differences in the products formed during oxidative cure on different substrates, a note of caution must be sounded regarding quantitative interpretation of the results. In the reflection experiment, only one dipole moment is observable; differences in molecular orientation relative to the substrate could be invoked to account for some spectral differences. Even so, it seems certain that the major differences in relative intensities, and to a lesser extent in band shape, between the spectra of what are expected to be essentially amorphous films do indicate substantial differences in the extent of oxidation and type of products formed on the different substrates. Interactions of metals with polymers have been studied by a number of authors; studies related to the present work have been published by Chan and Allara (1974a,b),Allara (1977), and by Cullis and Laver (1978). Chan and Allara studied the mechanism of oxidation at a copper-polyethylene interface; copper carboxylate salts were found to be formed early in the reaction, and were concentrated initially on the metal surface. See also Allara and White (1978) and Allara et al. (1976). Allara (1977), in a rigorous study of the oxidation of poly(1-butene)on gold and copper surfaces, found that the oxidation products formed on gold were essentially the same as those formed in bulk oxidation. The oxidation on copper yielded carboxylate ions as the major species. The copper-polymer interfacial chemistry was found to completely control the degradation of the polymer. The effect of metal oxides on the oxidation of polybutadiene has been studied by Cullis and Laver (1978); carboxy-terminated polybutadiene was oxidized as a coating on a variety of metal oxides. It was found that the oxidation was catalyzed by certain of the oxides; the catalysis was shown to be associated with those oxides for which the width of the forbidden zone between the valence and conduction bands was less than 1.9 eV. For both Fe203 and Cr203,the width of the forbidden zone is 1.9 eV, and both are moderately more active than for example the oxides of nickel, zinc, and tungsten. (It may be noted that the corresponding value for CuO is 1.4 eV.) Although these results are of interest, they may not be directly relevant to the present case, since the functionality and microstructure of the polybutadiene used is probably substantially different. Although the microstructure of the polybutadiene used was not specified, these materials typically have a fairly low amount of 1,2-substitution. The adhesion and corrosion performance of the polybutadiene coatings studied in the present work are likely to be strongly influenced by the chemical changes that occur in the interfacial region of the polymer during cure. The interfacial chemistry of adhesion loss in corrosion, for example, involves the saponification of ester groups formed during the oxidative cure of the polymer (Dickie et al., 1981); the presence of polar functionality probably contributes to the adhesion of these coatings under noncor-
Ind. Eng. Chem. Prod. Res. Dev. 1984,
300
rosive conditions (Holubka et al., 1984). Interfacial chemical effects are not, however, restricted to the polymer. Castle and Watts (1981) and Leidheiser et al. (1982) have shown that changes occur in the oxide film at the interface between the metal and a polybutadiene coating during oxidative curing. Changes in the oxide film have been observed under intact organic coatings upon exposure to corrosive environments (Ritter and Kruger, 1981; Ritter, 1982). It is apparent that the chemistry of the interface, and specifically the interaction of the organic coating with the substrate to which it is applied, must be taken into account in considering mechanisms of the establishment and loss of coating adhesion. Registry No. Polybutadiene (homopolymer), 9003-17-2; steel, 12597-69-2; chromium, 7440-47-3; gold, 7440-57-5.
Literature Cited Allara, D. L. I n "Characterization of Metal and Polymer Surfaces", Lee, L-H.. Ed.; Academic Press: New York, 1977; Vol 2, p 193. Allara, D. L.; White, C. W.; Meek, R. L.; Briggs. T. H. J. Polym. Sci. 1978, 14, 93. Allara, D. L.; White, C. W. I n "Stabilization and Degradation of Polymers", Allara, D. L.; Hawkins, W. L., Ed.; American Chemical Society: Washington, DC, 1978; p 273.
23,300-303
Aukett, P.; Luxton, A. R. J. Oil Col. Chem. Assoc. 1977, 6 0 , 173. Binder, J. L. J. Polym. Sci. 1963, A I , 47. Castle, J. E.; Watts, J. F. I n "Corrosion Control by Organic Coatings", Leidheiser, H., Jr., Ed.; National Assoclation of Corrosion Engineers: Houston, TX, 1981; p 78. Chan, M. G.; Allara, D. L. Polym. Eng. Sci. 1974, 4 , 12. Chan, M. G.; Allara, D. L. J. Co//o/dInterface Sci. 1974, 4 7 , 697. Cornell, S.W.; Koenig, J. L. Macromo/ecu/es 1989, 2 , 540. Cullis, C. P.; Laver, H. S.Eur. Polym. J. 1978, 14, 575. Dickie, R. A.; Hammond, J. S.;Holubka, J. W. Ind. Eng. Chem. Prod. Res. D e v . 1981, 2 0 , 339. Holubka, J. W.; deVries, J. E.; Dickie. R. A. Ind. Ena. Chem. Prod. Res. Dev. 1984, 2 3 , 63. Kendig, M. W.; Leidheiser, H., Jr. J. Electrochem. SOC. 1976, 723, 982. Leldhelser, H., Jr.; Kendlg, M. W. Corroslon NACE 1976, 3 2 , 69. Leidheiser, H., Jr.; Kendig, M. W. Ind. Eng. Chem. Prod. Res. Dev. 1978, 1 ..7 ,. 54 - .. Leldheiser, H., Jr.; Music, S.; Simmons, G. W. Nature (London) 1982, 297, 667. Leldhelser, H., Jr.; Wang, W. J. Coarlngs Techno/. 1981, 5 3 , 77. Neto, D.; DeLauro, C. Eur. Polym. J. 1987, 3 , 645. Ritter, J. J. J. Coatings Techno/. 1082, 54(695), 51. Ritter, J. J.; Kruger, J. I n "Corroslon Control by Organic Coatings", Leidheiser, H., Jr., Ed.; National Association of Corrosion Engineers: Houston, TX, 1981; p 28. Touhsaent, R. E.; Leidheiser, H., Jr. Corrosion NACE 1972, 2 8 , 435. Zerbi, G.; Gussoni, M. Spectrochim. Acta 1988, 22, 21 11.
Received for review October 14, 1983 Accepted January 19, 1984
Urethane TPE Monofilament and Braided Orthodontic Tensioning Devices. Creep and Stress Relaxation Properties 1. L. Sterrett," P. D. Kidd, and C. A. Andseiko ORMCO/S YBRON Corporation, Glendora, California 9 1740
The initial mechanical and long-term loadbearing properties are measured for both monofilament and novel braided orthodontic tensioning devices. The braided device exhibited 11 % higher tensile strength, 90% higher 100-200% modulus, and lower tensile set at failure as compared to the equivalent monofilament device. Braid creep strains ranged from 0.65 to 1.15 vs. monofilament values of 1.03 to 2.50. Six week force retention was also higher for the braid. The effects of prestressing the device are addressed and related to device performance.
Introduction In the current practice of orthodontics, teeth are repositioned to a functional and aesthetically pleasing arch form by forces generated by a wire acting through brackets banded or directly bonded to the teeth. Since the working range of the arch form wire is often limited, orthodontists often use elastomeric tensioning devices to obtain the required movement. Elastic usage is particularly helpful where movement of a tooth along the arch wire is needed, or a tooth must be intruded or extruded to the level of the rest of the arch (Andreasen, 1971; Berman, 1969; Kwapsis, 1972; Reitzik, 1971). A clinically effective force is thought to lie in the range of 200-1000 g, applied over periods of one to several months (Cousins and Harkness, 1970). Basic requirements for the clinical usage of elastomeric tensioning devices are that they be (1)small enough to fit comfortably within the oral cavity, (2) nontoxic and compatible with oral tissues, (3) resistant to the effects of oral fluids and foods, and (4) ideally maintain a clinically useful force over the typical patient recall period of four to six weeks. The devices now used by the profession fall into two broad categories (1)natural latex "rubber bands" and (2) urethane-based devices in the form of monofilament or die-cut chain. The latex devices are very short lived and
must be replaced by the patient daily, thereby calling for a high level of patient cooperation. The urethane devices are generally replaced by the doctor at regular intervals. Several studies are found in the dental literature concerning the stress relaxation properties of urethane devices (Ash, 1978; Bishara and Andreasen, 1970; Brantley et al., 1979; Ware, 1970, 1971). In general, these studies have shown a rapid decline in load under simulated clinical conditions. The results are supported by clinical studies showing a loss of effective force in as little as two days (Andreasen and Bishara, 1970, 1971). A further result of previous studies is the quantitization of the effects prestressing has on the long-term load-bearing properties of orthodontic elastomeric tensioning devices. In general it has been shown (Brantley et al., 1979; Young and Sandrik, 1979) that prestressing orthodontic elastomeric tensioning devices improved their long-term load bearing properties (i.e., improved resistance to stress relaxation) so long as the devices were immediately used after after prestressing. The creep properties of urethane orthodontic devices have not been previously investigated. This study looked at the stress relaxation and creep properties of both a typical urethane-based monofilament tensioning device and an eight-strand braided device of equal diameter. The ultimate and long-term mechanical
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