I
W. R. R. PARK1 and JESSE BLOUNT, Jr. Department of Chemistry and Chemical Engineering, Case Institute of Technology, Cleveland, Ohio.
Oxidative Degradation of Epoxy Resin Coatings Dimensional changes can lead to loss of good protective coating properties
IN
THE course of studies on the continuous weight change of cured epoxyphenolic resin paint films upon air aging to at elevated temperatures (150 200' C.), it became evident that slow volatilization was not the only process that was occurring. After a few days' exposure to hot air, such films started to absorb moisture from the air as soon as they were removed from the oven, so that reliable film weights could not be obtained. I t was postulated that the development of this hygroscopic tendency could be traced to the introduction of polar, oxygen-containing groups into the film structure. To clarify this behavior, a quantitative study of the oxidation of epoxy resins has been made under a variety of conditions. i
Experimental
The apparatus used for these oxygen absorption studies was essentially that described by Shelton and others (7, 5 ) (see diagram). I t consists of an aluminum cylinder about 2 feet in diameter and 1 foot deep. The aluminum block is kept at a constant temperature ( ~ 0 . 2 'C.) through the use of a thermostatically controlled heater which is'inserted in a central hole. Around the periphery of this block, eight 2-inch vertical holes are drilled, 2 inches from the outside edge. Into these holes are inserted the glass oxidation tubes, and each tube is in turn connected by capillary tubing and a three-way stopcock to a gas 1
Present address, The Dow Chemical
Go., Midland, Mich.
buret which is held at room temperature. Each buret can be connected to a manifold for simultaneous filling or evacuation of the eight tubes. After the oxidation tubes containing the samples are filled with pure oxygen, each tube is connected solely to its own gas buret, and the pressure therein is kept close to atmospheric by raising or lowering a leveling bulb containing mercury. For comparison purposes, all oxygen absorption values are corrected to 25" C. To eliminate errors due to pressure fluctuations, the pressure of oxygen is always adjusted to 760 mm. of mercury before readings are taken. In addition, an empty tube is used so that blank corrections may be applied to each reading. The results are expressed as cubic centimeters of oxygen absorbed per gram of coating at 25' C. Selection
of
Coating
Formulation.
Preliminary experiments showed that blends of epoxy resins with phenolics gave much greater oxygen absorption values than straight epoxy formulations. Amine-cured epoxy systems also gave higher absorption values. As it was impossible to differentiate between the oxidation that occurred because of the added phenolics or amines and that of the epoxy itself, it was decided to use straight acid-cured epoxy films in this investigation. Phosphoric acid was selected as the curing agent in the coating formulation and was used to the extent of 1 to 3910, based on solids. The films obtained did not have the best combination of coating properties that it is possible to attain by blending with other resins; however, the oxygen absorption
shown by the films used can arise only from oxidation of the epoxy resin. Preparation of Samples. Clean, 24gage, mild steel panels (1 .O X 4.0 inches) were degreased with toluene, dried, weighed, and then dip-coated with the epoxy formulations at uniform speeds of 1 to 7 inches per minute. Generally, all the panels for a single run were dipped simultaneously to eliminate any effect of coating variables on the oxidation rate of the finished films. After each dip the panels were allowed to air dry for definite periods which ranged from 10 to 30 minutes. Each batch of panels was baked for 10 minutes a t 205' C. in a TO V A C . OR O s 1
i
B
Glass oxidation tube, inserted in thermostatically controlled aluminum block, is connected by capillary tubing to gas buret VOL. 49, NO. 11
NOVEMBER 1957
1897
0.
50
150
100 Houri
Figure 1 .
tiours
Effect of air dry time on rate of oxidation
Figure 2.
Effect of final cure on rate of oxidation
Air Dry, Min. Epon 1004 Epon 1007
0
10
30
9
0 (3
0
c)
so
0
C
Epon 1007
100
forced draft oven after each coating operation. Final bakes varied from 30 minutes at 205' C. to 60 minutes at 260" C. The cured panels were again weighed before being placed on hangers in the oxidation tubes each of which contained 0.5 gram of anhydrous powdered calcium oxide. The tubes were closed with a silicone-greased ground-glass stopper, inserted in the heating block, and connected to the gas buret by borosilicate glass capillary tubing. All eight tubes were then evacuated for an hour before the introduction of oxygen. After filling with oxygen, each tube and buret was disconnected from the manifold and readings were taken. At the end of the oxidation period the panels were removed and weighed as soon as they had cooled to room temperature. All oxidations herein reported were carried out a t 180' 2~ 0.2' C.
150
Effect of curing in nitrogen on rate of oxidation 205' C., Min. -
Epon 1007
30
60
@
0
Table 1.
Run
No.
49.6
1
...
... ... 33.5 ... ...
2 3 4
...
5
...
6 a
Epon 10045
Epon 10074
1009"
... ... ... ... ... ... 27.5
...
34.0
34.1 34.1
... 34.0 ...
MEK
... ...
28.0 34.2
Shell Chemical Co. ~~
1 898
260° C., Min. 30 60 0 0
232' C., Min. 60 e 9
30
Epon
260 0
0
C
Hours
Figure 3.
C.
60 Min. ot 232
205
INDUSTRIAL AND ENGINEERING CHEMISTRY
49.6
... ... ... ... ...
... 34.1 ...
Formulation Data
MIBK
... 45.8
Hap01
volatile, %
%
Viscosity, Sec. in No. 4 Ford CUP
19.6
0.8 0.8
50.3 35
1.5 1.7
53 64 64
Butyl Cellosolve . . I
Xon-
Hap04 of Solids,
45.8
19.6
0.6
35
1.7
45.8
19.6
0.6
35
1.7
64
45.0 45.8 51.4
20.8 19.6 20.7
0.6 0.7 0.6
34 35 28
2.2 1.6
25 64 53
37.4
0.4
28
1.7
28
15.2
1.2
35
3.3
42
...
49.4
1.7
O X I D A T I V E D E G R A D A T I O N O F EPOXY R E S I N C O A T I N G S Discussion of Results The effects of such coating variables as dry time, final bake, film thickness, and molecular weight of resin on the oxidation rate of epoxy films were examined. Run 1. Effect of Air Dry Time on Rate of Oxidation. I t might be expected that variations in structure could develop in epoxy films having different air-dry periods and lead to different rates of oxidation. T o test this hypothesis, one set of panels was prepared from a solution of Epon 1004 in methyl ethyl ketone, and a second was coated with a solution of Epon 1007 in methyl isobutyl ketone and butyl Cellosolve (Table I). After each coating operation the panels were allowed to air dry for 0, 10, and 30 minutes before receiving the intermediate bake. Figure 1 shows that the rate of oxidation of these epoxy films is not appreciably affected by different air dry periods, but it does show that Epon 1004 oxidizes more rapidly than Epon 1007. Thus, any structural differences that may arise in these epoxy films because of different air dry periods are apparently eliminated during the intermediate or final bake cycle. Run 2. Effect of Final Bake Schedule on Rate of Oxidation. Panels were coated simultaneously with Epon 1007 (Table I) and then subjected to different final bake schedules-Le., GO minutes a t 205", 232", and 260" C. The films cured at 260" C. were oxidized almost twice as rapidly as those cured a t 205' and 232 " C. (Figure 2). Run 3. Effect of Curing in Inert Atmosphere on Oxidation Rate. For this run, all panels were dip-coated with a solution of Epon 1007 (Table I). All intermediate and final bakes were carried out in a nitrogen atmosphere to eliminate any oxidation which might occur during a baking cycle in air. The final bakes used were 30 *and 60 minutes a t 205", 232", and 260" C. Films baked in nitrogen a t 260" C. for 60 minutes absorbed four times more oxygen than similar films cured in air a t 260" C. (Figure 3)." The difference in oxygen absorption of the air and nitrogen-cured films must be a result of oxygen absorption during the final bake period. Comparison of the cured film weights obtained in runs 2 and 3 tends to support this theory. The data in Table 11 indicate that the weights of cured films for run 3 show a regular decrease with increasing curing temperature. This could mean that either low molecular weight resin is volatilizing or that the nonvolatile resin is undergoing weight loss by thermal degradation, as all panels are the same size and coated simultaneously. Possibly both processes occur together. The cured film weights obtained in run
50
0
Figure
4.
100 Hours
150
Effect of molecular weight on rate of oxidation Min. at 205' C.
Epon 1004a Epon 1007" Epon 1009" Cured in nitrogen.
30
60
0
0
c)
(3
e
e _
Table II. Run
Resin Type
No.
1004
No. of Dips
Dip Speed, In./Min.
_
_
_
_
_
_
_
_
~
~
Preparation of Test Panels Air Dry Time, Min.
Final Bake, Min. a t ' C.a
Cured Av. Film ThickWeight, ness, Grams Milsb
Film Density Before After oxidaoxidation tion
5 0 60-205' 1.0415 4.28 1.207 5 10 60-205 1.0150 4.11 1.208 5 30 60-205 0.9945 4.06 1.209 1007 1 0 60-205 0.5856 2.92 1.197 1 10 60-205 0.5724 2.88 1.193 1 30 60-205 0.5862 2.87 1.192 2 1007 3 15 60-205' 0.7134 3.69 1.157 1.189 3 15 60-232 0.7134 3.64 1.164 1.197 3 15 60-260 0.7248 3.87 1.191 1.215 3 1007 3 10 60-205d 0.5415 2.60 1.203 3 10 60-232 0.5162 2.36 1.194 1.245 3 10 60-250 0.3988 2.95 1.176 1.306 3 10 30-205 0.5305 2.31 1.200 3 10 30-232 0.5073 2.50 1.136 1.226 3 10 30-260 0.4727 2.28 1.178 1.256 4 1004 7 3 10 60-205d 0.5612 2.80 1.276 7 3 10 30-205 0.5506 2.66 1.276 1007 3 3 10 60-205 0.4666 2.86 1.246 3 3 10 30-205 0.5840 2.95 1.236 1009 6 3 10 60-205 0.4859 2.65 1.256 6 3 10 30-205 0.4761 2.33 5a 1007 1 3 10 60-205' 0.0582 0.27 1.115 5b 1007 2 3 10 60-205 0.1166 0.59 1.168 5c 1007 3 3 10 60-205 1.05 1.200 0.2227 5d 1007 5 3 10 60-205 0.5024 2.38 1.214 5e 1007 9 3 10 60-205 1.2561 6.35 1.217 5f 1007 11 3 10 60-205 1.5961 7.80 1.214 6-1 1007 2 3 25 60-210° 0.2255 1.15 1.195 6-2 1007 2 3 25 60-210 0.2269 1.18 1.197 6-3 1007 2 3 25 60-210 0.2194 1.10 1.208 6-4 1007 2 3 25 60-210 0.2182 1.07 1.209 6-5 1007 2 3 25 60-210 0.2288 1.13 1.199 6-6 1007 2 3 25 60-210 0.2292 1.21 1.189 6-7 1007 2 3 25 60-210 0.2340 1.18 1.195 6-8 1007 2 3 25 60-210 0.2339 1.17 1.195 a Intermediate bake, 10 minutes a t 205" C. All film thicknesses were average of 10 determinations on an American Instrument Co. Magne Gauge film thickness meter. ' Cured in air. Cured in nitrogen. 1
5 5 5 6 6 6 6 6 6 3 3 3 3 3 3
%
VOL. 49, NO. 11
0
NOVEMBER 1957
1899
~
~
2, however, do not show this decrease. Thus the amount of oxygen absorbed by these films during the final bake must be enough to more than balance the weight loss by volatilization. The difference in oxygen absorption of
r
films formed from Epon 1009 (Figure 4). The Epon 1007 films had an intermediate oxygen absorption rate. Molecularly speaking, there are two essential differences between these three resins. The basic Epon structure is
1
CH 3
nitrogen- and air-cured films baked a t 205" C. is relatively small compared to that of films baked a t 260' C. (Figures 2 and 3). Thus, at curing temperatures above 205' C., some readily oxidizable structure must be formed in these epoxy films.
shown above, where 12 has values of approximately 5, 10, and 15 for Epon 1004, 1007, and 1009, respectively.
Number of epoxide groups Degree of polymerization
Run 4. Effect of Molecular Size on
Oxidation Rate. Films were prepared for this run from acid-catalyzed formulations of Epon 1004, 1007, and 1009 (Table I). The films were all baked in a nitrogen atmosphere, then oxidized as before. The Epon 1004 films absorbed oxygen more than twice as rapidly as
the ratio of epcxide groups to hydroxyl groups per molecule also varies for these materials. Epon 1004
Epon 1007
Epon 1009
2 -
-
2 -
2 10
2 -
5
Number of epoxide groups Number of hydroxyl groups,molecule
.
The first obvious difference between them is molecular weight. Epon 1009 has a molecular weight 2.5 times greater
c: 0 0
Y)
c! 5(
:
I
c 0
u 0 .u
0
E
5
2 -
5
2
10
I
15
15
The only major chemical difference between these resins, then, is in the relative weight per cent content of epoxy groups. O'Neill and Cole ( 4 ) have shown that the epoxide content of amine-cured epoxy films is always appreciable unless a very large proportion of amine is used. I n these acid-cured films it is improbable that all the epoxide groups react during curing; thus, films of Epon 1004 would be expected to retain the greatest content of epoxide groups. Therefore, it seems likely that unreacted epoxy groups are present in the acidcured films and are acting as focal points for oxidation. Run 5. Effect of Film Thickness on Oxidation Rate. For this study, panels
L Y
a 2!
0"
v
0
c 50
100
I50
Hours
Figure 5.
Effect of film thickness on rate of oxidation No. of Coats
Epon a
1007"
1
2
3
5
9
11
0
0
9
e
a
0
All cured 60 minutes a t 205' C.
Table 111.
Film Data for Run 5
0xggen Film Absorbed Final Bake Cured Film Film Density Weight Loss after at 205' C., Weight, Thickness, after after 150 Hr., Min. Grams Mils" Oxidationb Oxidation, % Cc./Gram 0.27 60 0.0582 1.115 11.7 50 0.59 60 0.1166 1.168 9.4 37 1.05 1.200 8.3 25 60 0.2227 60 0.5024 2.38 1.214 7.0 27 60 1.2561 6.35 1.217 6.2 20 60 1.5691 7.80 1.214 5.7 21 a Average of five readings on the American Instrument Co. Magne Gauge. Determined by a flotation technique using aqueous solutions of sodium nitrate at 22' C.
1 900
than Epon 1004. This in itself should probably not cause differences in oxidation rate. The second, less obvious, difference is in the ratio of epoxide groups to degree of polymerization in these three resins. Stated differently,
INDUSTRIAL AND ENGINEERING CHEMISTRY
having one, two, three, five, nine, and 11 coats of Epon 1007 were prepared (Table I). The thicknesses varied from 0.25 to 8 mils. Films having thicknesses greater than 1 mil have closely similar oxidation rates, while the 0.25-mil film is oxidized twice as fast, and the 0.5-mil film is intermediate in its oxygen absorption rate (Figure 5). Because the oxidation rate is almost the same for identical size films of 1- to 8-mil thickness, the oxidation appears not to be diffusion limited at the temperature of the experiment. At the same rime, there must be an appreciable surface oxidation effect which becomes noticeable or operative only when the ratio of surface-to-volume is large, as films thinner than 1 mil show increased rates of oxidation. The percentage weight loss of these films due to oxidation closely parallels the amount of oxygen that is absorbed (Table 111). The thinnest film loses the greatest proportion (11.7%) of its weight and absorbs the most oxygen per gram, while the thicker films absorb less oxygen and lose a lower proportion of their weight. The probability that volatile
O X I D A T I V E D E G R A D A T I O N OF E P O X Y R E S I N C O A T I N G S oxidation products cannot diffuse so readily out of thick films as out of thin films may explain this observation and may also explain why the density of the oxidized films varies with film thickness. Volatile yet relatively dense oxidation products would remain in the thicker films longer. The high oxidation rate of very thin films may possibly be explained on this basis. The volatile and relatively dense oxidation products that arise in a thin film would diffuse more rapidly into the surrounding atmosphere, leaving a less dense film that would be even more susceptible to oxidation. As a result, thin films might normally be expected to show a greater mass rate of oxidation than thicker films. Run 6. Effect of Oxidation on Permeability of Epoxy Films. To obtain un-
i
supported films which had sufficient smoothness to give reproducible permeability constants, a special type of tinplate panel had to be used. The ordinary 24-gage mild steel test panels were first buffed to give a mirrorlike surface. A thin layer of tin was then electrolytically deposited on this smooth steel surface. In turn, the tinplate was polished until no pits or scratches were visible on the surface under 30-power magnification. These panels were degreased with toluene and dip-coated, as before, at a withdrawal speed of 3 inches per minute. Half of these coated panels were exposed to oxygen a t 180" C., while the other half were aged in lamp-grade nitrogen at the same temperature. After varying periods, the panels were removed from the gas absorption apparatus, and the films were stripped from the surface using the mercury amalgam technique ( 6 ) . The free films so obtained were aged for a minimum of 24 hours a t 23" C. and 50% relative humidity before their permeability constants were aetermined. Preliminary experimentation with films stripped from the specially prepared tinplate panels had shown that the permeability constants could be obtained within a &5 to 10% precision. On the other hand, films stripped from commercial, hot-dipped tinplate panels frequently showed variations cf 200 to 30070 in their permeability constants. The device used for these permeability measurements was essentially that described by Brubaker and Kammermeyer (Z), modified to handle smaller pieces of film. The permeability constant is defined as the number of cubic centimeters of gas (at standard temperature and pressure) that are transmitted through a film 1 cm. thick per second per square centimeter per centimeter of mercury pressure difference across the film.
Permeability constant
P
KOx v x
=
100
273 - x 21 T
d A$
1
x 3x eo
where P = atmospheric pressure, millimeters of mercury V = volume of gas passing through film, cubic centiqeters T = absolute temperature, K. A = area of film, square centimeters t = time, seconds d = film thickness, centimeters Ap = pressure differential across film, centimeters of mercury
100 (Y
c
e
=
8
50
E
8 100 3
4
O'
8
J
12
Micron8
The results obtained are shown in Tables IV and V. For each set of films, whether aged in oxygen or nitrogen, the permeability constants decrease with increasing time of aging at elevated temperature. At first glance, this result might appear to be anomalous. However, for all films studied the rate of oxygen absorption even\ tually decreased with time. The accompanying decrease in permeat ility may be explained as follows. First, the molecular structure may become more greatly cross linked during extended heating. That there is a relationship between the permeability and extent of cross linking in drying oil films has been shown by Harris and Bialecki(3). They found that the permeability toward water vapor of films cast from the
Table IV.
Infrared spectra of films from Run 6 7-1,
Spectrum of 1.5-mil Epon 1007 fllm (6-1)
7-2. Difference spectrum of Films 6-1 and 6-4 after oxidation
7-3. Difference spectrum of Films 6-5 and 6 - 8 after exposure to hot nitrogen
triglyceryl esters of pure fatty acids decreased in this order: oleate > linoleate > linolenate. The extent of cross linking in these films would increase in the same order. The fact that epoxy films aged in nitrogen do not increase in density, yet do decrease in permeability, would seem to indicate that the acid curing mechanism is causing fur-
Permeability of Epoxy Films to Helium Gas
Hours in
Hours in
Oxygen
Nitrogen at 180' C.
Film No.
at 180' C.
6-1 6-2 6-3 6-4 6-5 6-6 6-7 6-8
0 17 72 114
Permeability
x
... ... ...
after
Oxidation
7.2 5.3 4.4 3.7 7.3 6.5 4.1 5.4
...
0 17 72 114
Table V.
10"
Film Density
1.195 1.197 1.208 1.209 1.199 1.189 1.195 1.195
Permeability Data Perme-
p,
Film No. 6-1 6-2 6-3 6-4 6-5 6-6 6-7 6-8
Mm.
v,
T,
A,
t,
Hg
Cc. X lo3
OK.
Sq. Cm.
See.
739 739 739 739 739 741 74 1 741
7.6 5.4 4.7 4.0 7.8 6.5 4.2 5.6
298 298 298 298 298 298 298 298
1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60
600 600 600 600 600 600 600 600
VOL. 49, NO. 1 1
d,
Ap,
Cm. X
Cm.
ability Constant
10s
Hg
X 10"
2.92 3.00 2.80 2.82 2.87 3.08 3.00 2.97
285 285 285 285 285 285 285 285
7.2 5.3 4.4 3.7 7.3 6.5
~
NOVEMBER 1957
4.1
5.4
190 1
ther cross linking in the film on aging. Secondly, the permeability constants of films aged in oxygen were generally lower than those of films aged in nitrogen for the same length of time. Thus, some supplementary mechanism besides more extensive cross linking must be operative in the case of the oxygen-aged films. This effect can probably be traced to the introduction of new chemical groups into an already compact film. The greater density of oxidized over unoxidized films seems to support such a suggestion. Infrared Studies on Epon 1007 Films.
I n the hope of obtaining a more precise conception of the mechanism of oxidation of Epon films, some infrared studies were made on the films used in run 6 with a Perkin-Elmer Model 21 recording spectrophotometer. The sample spectrum of an unoxidized Epon film (6-1) (Spectrum 1) shows that the percentage transmittance is very low over a broad range, because of the thickness of the film. Thus, it is unlikely that chemical changes produced by oxidation can be detected by the single spectrum technique. As a result, small changes in the spectrum of the treated material were studied using compensation techniques. Spectrum 2 was obtained by using matched thickness specimens of films 6-1 and 6-4 (after oxidation). Film 6-1 was placed in the reference beam, while film 6-4 was placed in the sample beam. T h e two sharpest peaks, a t 3.1 and 5.8 microns, show a decrease in hydroxyl group content and an increase in carbonyl group content for the oxidized film as compared to the nonoxidized film. The absorption band a t 8.7 microns is attributed to a carbon-oxygen bond. To ensure that these spectral changes were indeed due to oxidation and not simply to molecular rearrangement caused by prolonged heating, a third spectrum was run. Spectrum 3 is a difference spectrum of film 6-8 (aged 114 hours in nitrogen a t 180" C.) and film 6-5 (no aging at high temperature) and was run the same as spectrum 2. No sharply defined absorption bands are present. Thus, the changes observed in spectrum 2 must definitely be caused by oxidation. A second conclusion may be drawn from spectrum 3. This is that no major chemical alterations occur in Epon 1007 films which are aged in nitrogen a t 180' C. Therefore, the only mechanism that can account for the decrease in permeability constant of films aged in nitrogen has to be one of ever-increasing molecular weight due to slow continuing cross linking of already large molecules. The formation of relatively few new cross-linking'bonds would not show up conclusively on this difference spectrum, yet this same new bond formation could greatly increase the structural complexity of the film.
1 902
Mechanism of Oxidation. AS it is known that hydroxyl groups disappear and that carbonyl groups appear during the oxidation of Epon films, several oxidation mechanisms are possible. The simplest of these would be an oxidation of the alcohol groups to keto groups. Second, the hydroxyls on adjacent molecules may undergo ether formation while other portions of the molecule are being oxidized to carbonyl or possibly carboxyl groups. Third, the Epon films may suffer intramolecular dehydration. This would account for the loss of hydroxyl function. The unsaturation that would result would then be capable of further oxidation to introduce carbonyl groups. Unfortunately, the reported spectra are not sufficiently definitive to allow a choice between these reactions. None of these postulated mechanisms will adequately explain the observed increases in density of Epon films during oxidation. Much more work will have to be done before a clear picture is obtained of the oxidation mechanism. Mechanism of Film Failure. When Epon-containing films are exposed continuously to high temperature service, many of the physical and chemical changes observed in this accelerated study will eventually take place and contribute to film failure, although the conditions here studied will seldom approximate those of actual service. Primarily, the film will increase in density. Changes in density imply dimensional changes or, if no actual alterations in physical size occur, certainly many stresses will develop within the film. Either of these effects will lead to a stressing of the film-substrate adhesion interface and eventual loss of many of the focal points of adhesion. Epon coatings in high temperature service undergo a slow weight loss due to volatilization of resin and loss of oxidation fragments. Thus, the film slowly becomes thinner and eventually unservkea ble. Although Epon films have very low permeability constants toward helium and probably toward other gases, eventually water vapor or other corrosive gases of the environment will permeate the film and start to attack the substrate. Loss of adhesion points through corrosion may follow, and the film will fail. A further factor that may be of considerable magnitude in causing film failure is the continuation of the curing process on aging. As the polymer becomes more highly cross-linked, it also becomes more brittle and less flexible because of the increasingly complex molecular structure. This continued curing is also probably accompanied by slight dimensional changes. Such changes can affect the adhesion of the film considerably, as they will occur after the film has become solid
INDUSTRIAL AND ENGINEERING CHEMISTRY
and has formed its adhesive bonds with the substrate. Finally, the oxidation of the film undoubtedly contributes to failure by introducing new chemical groups that have less chemical resistance than those of the parent structure. Also fragments of molecular structure may be broken off by oxidation. The introduction of new groups and loss of old ones would also be accompanied by dimensional changes and physical stresses which would contribute to film failure through loss of adhesion.
Conclusions The length of air dry time of an Epon coating before baking has little effect upon the rate at which the film will subsequently oxidize. Thus, any molecular orientation effects that result from differing air dry periods must be eliminated during the baking cycles. The temperature and duration of the final bake of an Epon coating can have profound effects on the subsequent oxidizability of the coating. At final bake temperatures above 205' C., some chemical 'change takes place which enhances the oxidation potential of the film. Bake temperatures above 205" C. cause slow volatilization of the coating. The molecular weight or, more particularly, the epoxide-degree of polymerization ratio has definite effects on the oxidation rate of Epon films. Apparently, unreacted epoxy groups present in the cured film represent focal points for oxidation. In the Epon systems studied, the rate of oxidation a t 180" C. was almost independent of film thickness in films having a thickness range of 1 to 8 mils. Thus, the rate of film oxidation is not diffusion limited under these conditions. I n films thinner than 1 mil, however, a greater oxidation rate was observed. The permeability of Epon films toward helium decreases on aging. This is believed to be due to introduction of new chemical groups into the molecular structure as well as to continuation of the curing reaction.
Literature Cited (1) Blum, G. W., Shelton, J. R., Winn, H., IND.ENG.CHEM.43, 464 (1951). ( 2 ) Brubaker, D. W., Kammermeyer, K., Anal. Chem. 25, 424 (1953). (3) Harris, B. L., Bialecki, A., Oflc. Dig.
Federation Paint B Varnish Production Clubs 24, 884 (1952). (4) O'Neill, L. A., Cole, C. P., J. A@. Chem. 6 , 356 (1956). (5) Shelton, J. R., Winn, H., IND.ENG. CHEM.38, 71 (1946). ( 6 ) Talen, H. W., J. Oil & Colour Chemists' Assuc. 34, 455 (1951).
RECEIVED for review October 17, 1956 ACCEPTED April 10, 1957 Division of Paint, Varnish, and Printing Ink Chemistry, 130th Meeting, ACS, Atlantic City, K. J., September 1956.
