Oxygen Absorption by Cast Plastic Films J. W. WE STWATER lYniversity of Delaware, .Vetoark, Del.
To
determine the oxygen absorption ability of different plastic materials, cast films of 98 commercial plastics .rc(ere exposed to dry oxygen at room temperature. A variety of solvents was used for the casting medium, and film thicknesses were varied from 0.65 to 1.94 mil. Glass was used as the material coated. Weight determinations were made a t wgular intervals. For a thickness of 0.5 mil and a time of 10 days, some films gained up to 69" i n weight; a few resins lost weight. The effects of time, solvent, pressure, and film thickness are illustrated. The data suggest that plastic films contain both oriented and unoriented molecules, with the ratio depending on the polymer type, film ,thickness, and choice of solvent. Highly oriented films and poorly oriented films are not reactive toward oxygen to the same degree. Observations of secondary interest, concerning solvent retention, aging, and effect of light, are also included.
T
HE widespread use of plastic films as wrapping foils, paints, lacquers, adhesives, photographic film, and in countless other applications, has led to an interest in the absorption of oxygen by these films. The role of oxygen in influencing polymerization as well as the degradation of plastic films has been indicated by past observers. The subject of film stability has seemed t o be of sufficient importance to warrant a study of the action of oxygen on the great number of commercially available plastics. For this reason, representative plastic materials were obtained covering the major types of polymers and the present investigat,ion was carried out. The use of plastic f i h s by man predates written history. Well preserved wall paintings by prehistoric man (50,000 B.C.) are in existence. The early Egyptians (1000 B.C.) used protective and ornamental coatings of high durability probably made by cooking together natural oils and natural resins. At this time, the Orientals were using japans, lacquers containing resinous insect exudations. During the Renaissance, the use of drying oils became widespread, and skilled craftsmen built, up remarkable coatings on'wood by applying numerous coats of linseed oil with laborious rubbing between coats. By the end of the last century the manufacture of polymer films had become a large scale industry. The utilization of these materials depends on their ability to form continuous, inert, coherent sheets. Their electrical properties are sometimes of primary importance as are their color, gloss, etc. I n use, the overwhelming majority of plastic films are in almost constant contact with air. It is well known that certain film formers, t,he drying oils and resins, react with oxygen in some manner during the phenomenon of drying. It is also known that raw rubber and vulcanized sheets of rubber are capable of reacting with oxygen t o a considerable extent. In addition, the literature indicates that oxygen catalyzes certain polymerization and condensation reactions. However, no data have been presented to indicate whether the majorit,y of plastic films are sensitive to oxygen. The purpose of this study was to determine
the comparative degree of sensitivity t o oxygen of many of the plastics in use today, as measured by ability to absorb oxygen. I n addition, it is hoped that a study of the process of oxygen absorption may give indications as to the mechanism of film formation. APPARATUS AND PROCEDURE
All plastic films used in this study were cast from solution onto glass plates. Figure 1 shows two photographs of the sharp edge mechanism used to control film thickness. This apparatus consisted of two hollon., concentric, brass cylinders threaded one inside the other. Only the inner cylinder \>-asin actual contact with the glass surface; the outer cylinder had a sharp lover edge which served as the doctor blade. A segment of the bottom rim of the inner cylinder was removed t o allow escape of resin solution as the apparatus was moved along the horizontal suiface to be coated. The inside diameter of the inner cylinder was 1 inch. The rig was set vertically on a glass sheet and the outer cylinder rotated to obtain the desired clearance above the sheet. A few drops of polymer soIution were placed within the inner cylinder; the rig was then moved across the glass plate leaving behind a thin film of solution. Normally the films were about 1 inch wide and 5 inches long. Afterflow was usually slight because the solutions were rather viscous and because solvent evaporation from a thin film is initially quite rapid. The freshly cast films were allowed to air-dry'for about 15 hours and viere then baked for 30 minutes a t 112' * 2 " C. Pieliminary tests showed that this air drying was sufficient to prevent bubble formation during baking. The heat treatment was sufficient to cure all the thermosetting resins used, and it served to remove most of the residual solvent. I t is recognized that the drying scheme employed may be satisfactory for some films but less than ideal for others. After the baking period. the glass
weighed in air, and returned to the oxygen exposure. Light was excluded from all films, except one set discussed later, during the oxygen exposure. Inasmuch as the films remained on glass plates during the entire exposure, the absorption of oxygen occurred a t one surface only for each film. Filni areas were measured with a planimeter (accuracy 1.0.03 square inch), the weights by an analytical balance (*0.2 mg.), and the thicknesses were computed by use of these measurements and the specific gravity of the polymers as given by the suppliers. I n a fern cases specific gravity measurements were carried out in this laboratory. The measured weight changes for the fil~lisvaried from a negligible value (0.2 mg. or less) to ns high as 4.9 mg. per film. The film weights were usually 50 to 60 mg., and the cxposed areas were usually 5 t o 6 square inches Figure 1. Apparatus for Casting Films of Desired Thickness
1494
INDUSTRIAL AND ENGINEERING CHEMISTRY
1948
August
20
0
40
60
80
100
120
140
160
100 200
T i m e , Hours
Figure 2 Four films were prepared alike for each test and were tested simultaneously. The data presented are the averages for the four determinations. DISCUSSION OF RESULTS
Table I shows t h a t on11 COMPARISON OF VARIOUS PLASTICS. 4 of 98 plastic substances tested absorbed more than 3% of their weight in oxygen in 10 days. Two were alkyds, one was a rosin ester, and the other a n oxidizing resin of petroleum origin. These four are used as lacquer or varnish resins and are known to be sensitive t o oxygen. They are seldom used pure, but a r e ordinarily incorporated with other resins or oils to obtain the desired coating characteristics. Thirty-six resins exhibited weight gains smaller than 3% in 10 days. Most ureas, phenolics, alkyds, coumarone-indenes, and rosin esters appear in this class. It is likely t h a t these have unreacted constituents which tend t o oxidize. The effects could be due t o reactions of extraneous side chains or of impurities. It is noted that a few of these resin types also gave negligible weight changes. Thirty-nine resins gave negligible weight changes. These represent the polyacrylates, polystyrenes, polyvinyls, cellulose derivatives, pure and heat treated natural resins, plus a few phenolics and alkyds. It is particularly noticeable that most straight-chain thermoplastics fall in this group which indicates that in general no points of attack for oxygen exist on such molecules.
0
40
80
120
160
200
Time,Hours
Figure 3
240
200
1495
Twenty-two resins lost weight when exposed to oxygen. This class is composed principally of three cellulose derivatives, two rosin derivatives, two coumarone-indenes, three alkyds, seven phenolics, one urea resin, and one melamine resin. Such losses are probably due to slow release of solvents, plasticizers, or unreacted constituents, even though the films were at apparent equilibrium before exposure t o oxygen. On the basis of a 50-mg. samRle, a weight change of 1% which corresponds t o 0.5 * 0.2 mg. gives a n uncertainty of 40%. This indicates a possibility t h a t some of the slight weight changes are nonsignificant, even though four films were used t o obtain each value listed. EFFECTOF TIME. The great effect of time on the amount of oxygen absorbed by a modified alkyd i s shown i n Figures 2 and 3. It would have been desirable to determine the complete time curves for numerous other resins also, but because of the scope of the investigation, this was impracticable. Dyal V15038 was chosen for the study because a preliminary investigation indicated t h a t this resin is capable of absorbing large quantities of oxygen. This resin is an alkyd made from a nonphthalic, polybasic acid and is modified with linseed oil to the extent of about 85 parts oil to 100 parts resin. The rate of oxygen absorption was constant for 60 or 70 hours and then gradually decreased toward zero. By the end of 7 or 8 days the films had nearly reached equilibrium. An analysis of the curves shows t h a t they do not follow the laws of diffusion; therefore the process by which these films add oxygen is not one of simple absorption. An initial induction period was not encountered for the particular tests shown. However, the phenomenon was met with on occasion during preliminary observations. It was found that the behavior of the films during the first 10 or 20 hours was sometimes erratic. Some observers (9, 27, 32) have reported induction periods during the air drying of various oils and resins whereas other workers (29) show curves with practically no induction period. It is likely that the purity as well as the past history of a resin determine what its initial behavior will be. Traces of impurities which could act as antioxidants would be expected t o have considerable influence. The effect of film thickness for a EFFECT OF FILM THICKNESS. modified alkyd is shown i n Figures 2, 3,4,and 5. Figures 4 and 5 were construFted using values from the smoothed curves in Figures 2 and 3. For the thinner films (about 1 mil or less) the amount of oxygen absorbed at any time is nearly proportional to the thickness. For thicker films, the amount absorbed was considerably greater. per each additional increment of thickness. Therefore, i t was concluded t h a t the &ucture of a
Film Thcckness. M ~ l s
Figure 4
1496 fiiin is dependent o n it,s thickness; also, the thickness effect does not follow diffusion laws-that is, the dependence of oxygen absorbed on thicknesv is not, e s p r e s s i b l e by integrated forms of Fick's law. Skin effect is frequent'ly given as one outstanding difference between mat films and films prepared by pressure or other mea,ns. Tlie xhnormal behavior of the cast films i n Figure 4 during rhe first 6 hours may be attributed t o this factor. S o matter what thickness the films had, the weight gain in 6 hours was apparenfly constant at about 0.1 mg. per square inch. The skin, supposedly a tough, preoxidized outer portion or t h e films, r e t a r d e d t h e passage of oxygen. However, once penetrated (about, 1 day), the underlying plastic t rehaved i n a normal manner. Hut since each film area was roughly 5 square inches the actual amount absorbed at the 6-hour level is uncert,ain by about 4070,as previously explained. I t was desirable for purposes of comparison that all plastics classified in Table I be of the Same thickncss. All bhe films were cast using a blade cleasance and solution concentration calculLtt.ed t,o give a constant thiclrn of 0.5 mil. However, vxr t,ions from 0.5 mil did occur, aud t.he st,andard deviation of ail the thicknesses compared t,o the 0.5 mil value is 0.17 rnii. % . time-consuming trial and error technique would be required t o obtain b trol. EFFECTOF S O L V ~ N'l'he T. solvent, used as the cast8ing medium has a pronounced effect on the resistance of plastics to oxygen. I n some cases polar solvents resulted in less weight gains than did nonpolar solvents; in other case8 the reverse was true. An example of each is: For a drying acid mpdified alkyd cast from benzol (Rezyl869), the weight change was not measurable but when trichloroethylene was used the weight gain was appreciable; for a modified condensate
INDUSTRIAL AND ENGINEERING CHEMISTRY
T\ einht
h1ethJ-1ester of rosin Oxidizing resin of petroleum origin Ilrying-acid modified alkyd Oil-modified alkyd
Puiyatyrene Polyvinyl acetiit? Polyisohntene l'ulyisoliiitene P-couniaroiie-iiida~ir
P-conmarone-indene Processed roumaronrindene Processed coumnronrindene c(Jii1nnrowPhenol indene Polyterpene Glycerol ester of roaiu
(:RIII
Vol. 40, No 8
More than 37, by Weielir
Ahalyn
14
c
0.42
4 .2
0 ,3 0
Velsicol AF-3
27
H
0.46
3.9
0.31)
Rezyl 869 Dyal V-I .50:38
2
RL
0.79 0.50 0.58 0.63 0.48 0.49 0 , .50
5 , r,
0.77 0 39 0.56 0.64 0.42 0.38
n
24
Lualin KO.4000 Gelva V-7 Vistanex polybiitrrir (medium) Vistanex polyhiitrrw 6 Cumar W- I / . ('umar F X
7
B
4.8C 6.0 6.3 .i, 4
4.7 6.1 '
0.49e Coumarone-indelri, Coumarone-indent, Natural robin Recent dammar Recent . . ~ .dammar Recent copal Fossil copal (run at, 650° F.) Pine tree resin Heat treated rosin Polymerized wood rosin
'I'rede N a m e Lucite HG-1 Aoryloid .\-I 0 Styron A-1.5 Vinylite AY.44 G e l m V-60
Snpplier 10
For 111 'ai 1 .5/95 k
2:3
Solvent 1,
2T
22 9 6 23
k iir
'I'hic k rit:s*I Xils 0.91 0.51 0.77 0.57 0.54
I
+ 2c'
0.60 0.58
H
0.38 0.53
Alrar 7 / 7 0 Luniarith C H . Sz
23
PAP-331-30 Nitrocellulose K h I/*" Uitrocellulose RS125-175" Nitrocellulose RS 1000" Ethocel, 20 ~p Paradene 2 R-5 Nevindene Water M hite Batavia Damar A/E standard Batu E. India bold scraped Manila WY soft pale
26
14
0.43 0.43
14
0 , 2:;
14 9
0.2u 0.66
Congo 33 Vinsol Hyrex Poly-Pale resin
8
tkr
+ 31
0.78 0.78 0.41
17 17 18
0.35
3
3
a
14 18
14
+ ac
0.78 0.53
+ 21 CC+ 2~ + in
0.54 0.46 0.43 0.58
?€I
3
ZH
c
I
August
1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
1491
I J pe of surface, type of polgmer, and the solvent used, (10-day exposure, 97% 0s.1 a t m . , 74' to 77' P.) but in any case the thickness Thickness, of the oriented layer has an Trade Name Supplier" Solvent6 Mils Type upper limit and cannot be Weight Change Negligible increased indefinitely by in0.63 c Zitro 18 Zinc resinate 0.39 C 18 Whito Rosin + catalyat creasing the total film thick0.42 C 12 X H eater gum Rosin-glycerine ester 0.63 C 12 Resin 340 Congo ester ness. This limit may be Rosin-pentaerythritol quite different for every solB 0.64 14 Pentalyn M ester Unsaturated petroleuin vent used. The film portion 0.32 c Naftolen RC 28 residue 0.64 I 16 Chlorinated diphenyl Aroclor 5460 [arthest from the coated sur0.53 C 2 Rezyl 12 Nondrying alkyd face will be of a brush-heap, Alkyd modified with na0.48 2 C Teglac 2152 tural resin acids unoriented structure and can Xiodified maleic lacquer 0.51 C 12 Resin 4 resin be increased indefinitely bjRosin modified maleic increasing the total thickness. I 1.04 14 Lewis0124 resin Alkyd of maleic anhyFigure 7 then shows that 0.50 C Paraplex AL-111 22 dride, glycerol, rosin C 0.49 Resin 220 11 Turpene phenolic films cast from solvent S 0.69 l I € + 1x Aroohem 520 25 Modified condensate would give almost a straight,Rosin modified phenol 22 c 0.72 hmberol M-88 formaldehyde line graph of oxygen absorbed Phenol formaldehyde in (: 0.48 2 Phenao 615-N against film thickness, as ester gum Drying-acid modified each film is composed almost 0 50 2 c: Hezyl86R alkyd solely of unoriented molecules and is therefore nearly honioWeight 1205s W t Gain, Calod. _______ geneous. However, the films Thickness, bfq.1 from solvent Y would give a Type Trade Xame Suppliero Solvent b Mils Wt, % s q . in. straight line for only very Cellulose acetate Lumarith IS, Hs 8 1H 31 0.46 -0.7 -0.07 Cellulose acetate Cellulose acetate small or very large film thjckA-13 26 1IT + 31 0.42 -0.8 -0.07 Cellulose acetate-bunesses, i n which cases t,he tvrate EAB-160-40 26 I 0.66 -0.6 -0.07 films would be nearly all Pure rubber hydrocaru 0.63 -0.6 -0.06 bon Marbon B 15 oriented or nearly all UIIC 0.33 -1.0 -0.09 Chlorinated rubber Parlon 125 cp. 14 I 0.68 -0.4 -0.05 Coumarone-indene R-15 medium resin 17 oriented, relatively speaking. 0.40 -0.8 -0.06 P-coumarone-indene Cumar V 5 C For the in-between region 0.48 -0.4 -0.03 Lime-hardened rosin Helix 18 C Hydrogenated rosin where both oriented and un0.45 -0.6 -0.05 ester Hydroguni P-190 21 C oriented molewles are in Chlorinated naphtha- 0 09 6 C 0.32 -1 3 Halowax 1012 lene comparable amounts, the abTerpinene, maleio an14 11% + 8C 0 50 --I 4 -0.14 Petrex A6HT hvdride a1kvd sorption against thickness TeFninene. maleic an1 H + 6C 0.43 -1.3 14 -0.11 Petrex A5HT h$dride B.11.-.-1 ~ Y U curves could not be straight,. Alkyd modified with This theory was first, ex1 -0.4 0.70 -0.05 2 natural resin acids Teglac Z-152 Pure phenol formalpressed in 1937 by Young 0.40 -1.0 11 C -0.08 Resin 202 dehyde 0.30 14H + 1B -0.4 7 -0.03 CatFbond 701 Phenol formaldehyde and his co-workers ($6,36), 0.55 C -0.8 4 -0.08 Reapoid BR-298i Pure phenolic but has been modified some0.74 C -0.5 -0.07 12 Concentrated uhenolic Resin 153 Rosin modified phenol what for presentation here. formaldehyde Beckaoite 1116 21 C 0.57 -0.7 -0.08 4 C 0.66 -0.7 -0.08 Resin modified phenolic Resinoid BR-2963 If the polarity of solvents Phenol formaldehyde is of importance, as the rein ester gum Phenac 615-N 2 I 0.76 -0.5 -0.07 Melamine reein Melmac 566-9 2 2 0 C + 5K + 3E + 2E' 0.50 -1.4 -0.13 sults indicate, the choice of Urea, melamine formaldehyde resin Catavar UM-I 18 7 1H 3N 0.61 -1.4 -0.17 benzene and acetone is a fortunate one. The dipole a As shown in Table 11. Q Symbols used for solvents: A = acetone; B = benzene; C = benzol (Industrial). 1) = butyl abetate. E: = moment of acetone is over 40 but 1 alcohol. F = capryl alcohol; G = cellosolve acetate; H = eth 1 alcohol; I A ethylene dichloridk; J = metKyl alcohdl. K = mineral spirits; L = toluene; M = trichloroethyfme; N = water; P = xylol (industrial). times as great as t8hatof benC Values werk interpolated from graphs because no tests were made for 0.6 mil thickness. zene ( 1 1 ) . EFFECT O F PRESSURE, h n effect of pressure on the oxpgen absorption of the particular alkyd previously described is shown in Figure 8. T h e first (Arochem 520) cast from benzene, a measurable weight gain sections Of the curves were given in Figure 2. The films occurred, but when a 1 t o 1 mixture of benzene and ethanol was were first allowed t o reach equilibrium with oxygen at a presused the weight change was not detectable. Additional example! sure of 1 atmosphere; this required about 6 weeks, and then the may be found in Table I. pressure was reduced t o 0.1 atmosphere. Figure 8 shows that Figure 6 is an excellent illustration of the effect of two solvents oxygen began t o escape from the Polymer as Soon as the Pressure was released and t h a t the films continued to lose weight foi o,l Dyal (oil-modified alkyd). the thickness range 17 days at which time the pressure was returned t o 1 atmosphere During these 17 days the thickest films (1.49 mils) lost nearly investigated, the films cast from acetone were much more tive to oxveen than those cast from benzene. The films from 25010 of the oxygen Dreviously absorbed. Other tests. not rePGrted here, d&o&rated that when the Pressure was reacetone gave curves which are nearly straight, whereas those from turned to its original value of 1 atmosphere the films gained benzene are decidedly curved in the upper range. weight to return t o their previous equilibrium values. Some of the effects of solvent and film thickness can be ParThis pronounced sensitivity to pressure is indicative of tjle tially explained by assuming each film t o be partly oriented oxygen being held by forces of absorption or adsorption; it is not likely t h a t oxygen held by primary bonds would be bound 80 and partly unoriented as shown in Figure 7. Assuming that the loosely. Nevertheless, it is almost, certain that some of the oxygeii forces betw,een a polymer and a coated surface (in this case glass) does combine chemically with polymers. Rubber, for example, will cause orientation of the pOlyn1er molecules closest t o the releases carbon dioxide, water, and other products as it absorbb coated surface, the amount of orientation may depend on the and similar gases are released during the air oxygen (18,31, M),
TABLE I. OXYGENARsoRrTIoN
BY V A R r o c s
PLASTICS FIms (Continued)
+
+
v-15038 I
I
1498
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 8
Absorption j o f O x y q e n by Oil Modified Alkyd
2
(Sherx
n
-Williami
Dyal V
- I50
I A t m , 7 8 - 8 3 ' F , 97%
F 2 0
2 a
_ I c 3
; I c
a 0 0
5 0
04
0 Oo
04
oa
12
08
16
20
Film Thickness, Mils
12
16 20 Film Thickness, Mds
Figure 6
Figure 5
drying of drying oils and resins (9, 22). So it is concluded here that some of the oxygen held by polymer films is held by chemical bonding and some is held by secondary forces. AGING. One resin, an oxidizing type of petroleum origin (Velsicol AF-3), was selected to demonstrate the fact that aging
Cast from Solvent X
mil for each set,. One set vias exposed to dry oxygen in darkness and one set to room air next to the pane of a closed window. In order to imitate the nat'ural circulation of air contacting the samples in the light, the oxygen atmosphere of the samples in the da.rk was flushed and renewed on four occasions during the 10-day exposure. The following weight changes occurred in 10 days: Films in dry oxygen in darkness lost' 0.07 mg. per square inch; films in room air in light lost 0.18 mg. per square inch. The films exposed t o light lost about three times as much weight as did the dark films in spite of the fact that. the dark films were in an atmosphere having an oxygen concentration nearly five times that used for the light exposures. It is not known whether the observed weight losses were due to splitt,ing off of oxidation products, plasticizer evaporation, residual solvent evaporation, or to some other cause, but it is obvious that light greatly accelerated the process.
SOLVENT RETENTIDS.The ret,ention of solventmsby macromolecular substances is characteristic behavior. It may be impossible t o remove all solvent complet'ely from a resinous material without destroying the material. Figure 10 illust,rates a n attempt to drive off solvents from various air-dried plastic films by use of heat. The tests were performed as folioTYs: A powdered resin sample was spread thinly on a glass plate and was oven-dried a t 85" to 90" C. until t h e weight appeared constant. from one 30-minute period t o the next. Figure 7
will cause a decrease in capacity to absorb oxygen. A quantity of the resin was stored, as a solution in benzene, for 5 months, and t'hen cast films of the aged resin were compared with films of the same resin which was prepared and tested when first received. The results are given i n Figure 9. The fresh resin absorbed six or seven times as much oxygen as did t,he aged resin in 10 days of measurement. Presumably the test demonstrates t h a t resins which are able to oxidize or t o polymerize further xyill do so during storage and thereby decrease gradually in capacity for oxygen absorption. No special precaution was used t o exclude air from the benzene used. The prepared solution was kept in a corked bottle during storage. EFFECT OF LIGHT,Because light is known to affect many reactions, all the test,s previously discussed were conducted with the careful exclusion of light a t all times except during the periods of weighing. However, it was felt that a rough estimate should be made of the magnitude of effects which light could produce if not excluded. Consequently tlvo sets, each of four cellulose acetate samples (Tennessee Eastman Type A-13, cast from 1 : 3 mixture by weight of ethanol and ethylene dichloride) were prepared with an average thickness of 0.42
-
EFFECT Absorption o f
OF
(Sherwin -Wiliiomr
'0
200
400
PRESSURE Oil Modified Alkyd
O x y g e n by
600
D i o l V - 15038 1
1038 0 200 Time, Hours
Figure 8
400
600
-
August 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE 11. SOURCEOF RESINSTESTED
EFFECT OF A G i N G Absorplion of O x y g e n by A n Onidizinq Resin o f Petroleum Oriqin (Vilsicol
1499
Symbols Used in Table I
AF-31
1
iAim, 74-77"F, 9 7 % 0 2 , IODays
2 F r e s h Resin
3 4 5
6
7 8
e
10 11 12
13
14
15 16 17 18 19
20 21 22
Film T h c k n e s s , Mils
Figure 9
This required approximately 4 hours, and the final weight was taken to be the true resin weight.' ,4 solvent was applied from an eye dropper, and the plate was agitated until solution appeared complete. The film was allowed to dry for 15 hours in air and then the heating procedure was begun. Films prepared in this manner are not likely to be of uniform thickness, but this scheme seemed t o be the best for obtaining films of known resin content. Duplicate tests were run for each solvent-resin combination, and each point in Figure 10 is the average of readings for three films. After a heating period of 0.5 hour a t 85" t o 90" C., the amount of retained solvent was sometimes as high as several per cent by weight. Additional heating caused a slow release of solvent. But if additional heating beyond the half-hour period was omitted the subsequent solvent release was negligible in 10 days, as indicated by other tests. This is important for i t means that the weight changes recorded in Table I are probably due t o oxygen and not due to solvent, although it is recognized t h a t the t b solute film weights and thicknesses may be in error by several per cent because of retained solvent. Figure 10 shows that solvent retention by a resin is not necessarily determined by the volatility of the solvent. The difference seems t o be more closely related t o the polarity of the resin and solvents. For example, Figure 10 demonstrates that e thy1 cellulose, a nonpolar polymer, has such a strong affinity for toluene (also nonpolar) that even after 20 hours of heating, the amount of retained toluene is over 2% by weight. On the other o hand, butyl acetate and ethylene dichloride, both polar solvents, were not retained by ethyl c cellulose in this manner. For a polar plastic, w > phenol formaldehyde modified with rosin, polar solvents were retained in greatest amount and a nonpolar solvent was not retained to a comparable degree. Some resin-solvent combinations lost more weight than possible if only solvent were removed. I n those cases slow decomposition of the resin, or escape of volatile ingredients, must be responsible. However, these effects presumably occurred, no matter what solvent was chosen, so the relative position of weight against time curves in Figure 10 were assumed t o be correct.
23 24 25 26 27 28
Name of Supplier
Address
Advance Solvents and Chemical Corp. American Cyanamid and Chemh oal Cor@ American Gum Importers Laboratories, Inc. Bakelite Corp. The B p e t t Division, Allied Chemical and Dye Corp. Carbide and Carbon Chemicals Corp. Catalin Corp. Celanese Celluloid Corp. Dow Chemical Co. E . I. du Pont de Nemours and Co., Inc. Durez Plastics and Chemicals, 1,nc. France, Campbell and Darling, Inc. Glyco Products Co., Inc. Hercules Powder Co., Inc. Marbon Corp. Monsanto Chemical Co. The Neville Co. Newport Industries, Inc. Pennsylvania Industrial Chemical Corp. Plaskon Co., Inc. Reichhold Chemicals, Inc. The Resinous Products and Chemical Co. Shawinigan Products Corp. The Sherwin-Williams Co. Stroook and Wittenberg Corp. Tennessee Eastman CorD. Velsicol Corp. Wilmington Chemical Corp
New York, N. Y. New York, N. Y. Brooklyn, N. Y. New York, N. Y.
New York, N. Y. New York, N. Y. New York, h-.Y. New York, N. Y. Midland, Mich. Wilmington, Del. N. Tonawanda, N. Y. Iienilworth, N. J. Brooklyn, N. Y. Wilmington, Gary, Ind. Del.
St. Louis, Mo. Pittsburgh, Pa. Pensacola, Fla. Clairton, Pa. Toledo, Ohio Detroit, Mich. Philadelphia, Pa. New York, N.Y . Philadelphia, Pa. New York, N. Y. Kingsport, Tenn. Chicago, Ill. New York, N. Y.
CONCLUSIONS
From a study of the data presented, i t is concluded t h a t : 1. Many types of plastic films are capable of absorbing oxygen, some as much as 6% by weight; other types are completely inert t o oxygen. 2. The rate of oxygen absorption and the oxygen content a t equilibrium are dependent not only on the plastic used but also on'the solvent, the film thickness, the oxygen pressure, and the age of the plastic material. 3. Oxygen may be held in a polymer by two types of forcesprimary bonds and secondary means, such as polar forces. 4. Plastic films contain both oriented and unoriented molecules; the relative amounts of each are influenced by type of polymer, solvent used, and film thickness. 5. Solvent retention by plastic films may be considerable. In particular, polar polymers tend t o retain polar solvents, and nonpolar polymers retain nonpolar solvents.
T RFTFNTION Films Cast from
Solvent &
Heated a i 85-90% Vapor Pressures At
2
------- - --
88' C
ETHER-3500 mm BENZENE --920 ETHYLENE Cl -850 TOLUENE-2- - 3 6 0 b?e-10uKETONE---300 w r Y L ACETATE -200 BUTYL ALCOHOL-250 XYLENE---I60
---
3
0 4 Time o f Heaiinq. Hours
Figure 10
8
12 16 20
Hq
INDUSTRIAL AND ENGINEERING CHEMISTRY
1500
ACKNOU’I,EI)(J~~EN‘T
.Icilinovl-letigine~it~ is ciur G . E. Laiidt for suggesting t,he prol,ltiin and for his valuable sdvic~,concerning t,he invest~igatioii, and to C. R. Lynam for his hclp iii construct,iiig t h r casting apparat,us. Appreria,t,ioii is estended also t,o the twvcnt,y-eight lastic ni:triufactur~c~r.s(soe T:hIe I T) who supplitd thv I ~ A cd. BIB LIOGKA PHY
(1) .kUei’, L.. I S D .
k:NG. (>H$>>i,,30, 466-72
(2) Bass, S.L., and Kauppi, T ( 3 ) Bender, H. L., Ibid.,pp. 11 (4) Bodenstein, Max. Sitrbe,.
(1988). id., 29, 678-86 (1937).
Akad. W k . , 1931, 73 -88. (5) Bradley, T. F., IWD.ENG.CHEM.,29, 440-5; 579--84(1937):
30,689-92; 1087-8 (1938). (6) Bradley, T. F.,Krogn, E . L., and Johnaton, W.H., I b i d . , 29, 1270-6 (1937). (7) Bradley. T. F., and P i a m , H. L.’., Ibid.,32, 694--7 (1940). (8: Rurk, R. E., Thompson, H. E., Weith, 1.J., and Williams, I., “Polyineriaation,” gp. 210-1, New York. Reinhold Pub. Corp., 1937. (9) Burns, R. M., aiid Schuh, A . JC., “Prot,ecti\-e (:oatings for Metals,” pp. 276-8. 282-8, 316-7, 325, New York, Reinhold . Pub. Coip., 1939. ,” pi). 613-5, Yen. York, Mo(IO) Clark, G. L., “Applied X-K Giaw-Hill Book Go., 1940. Ill) Debye, P., “Polar Molerules,” p. 50, Sew trork. Cheniical Catalog Co., 1929. (12) Delmonte, J., “Plastics i i i Iiiigiueering,” p . 51 1 , Cleveland, Peiiton Pub. Co., 1939. (13) Gee, G., “The Phenomena of Polyuieriw,tiorr aiid Condensation,” pp. 187-95, London, Gurney aiid Jackson, 1935. (14) H-arkins, M’.D., and Jura, G . , .J. Am. Ch,em. Soc.. 6 6 , 919--27 11944).
Matter,” chap. 8, 41. (11, 1937.
Yol. 40, No. 8 I,oii(loii,
Caiiibridge Univ. P ~ P , ? ,
(16) Jones, G. G., ;tiid Mile,, 1;. 11.. J . Soc. Cht7n. I n d . . 52, 251T
(1933). (17) Kauppi, 7’. A , , a,nd Haah. S. L., 1 ~ 1 ) I.:vG. . CHEM.,30, 74-9 ( 1938). (1s) Keiiip, A. ti., Bishop, IT, S.,: i l l t i I,;rssellc, P. .i., l t ~ i c l . ,23,
1444-9 (1931). (19) Kieiile, R. € I . , aiid -\da,iii>, I-. \.., I h L , 21, 1279-82 (1929). (20) Kienle, R. I-I.,and E’ergu. l b i d . , pp, 349-52. (21) Krumbhaar, W.,“The C of Synthetic Surface (:oariugs,” pp. 62-5, 128-9, ork,Neinliold Pub. Corp., 1 9 3 7 . (22) Long, J. S.,Rheineck, A . I.:. xiid Hall, C . L., IND.ENG.( > H E M . , 25, 1086 --91(1933). ( 2 3 ) Mark, H., “Physical ~ h e i n i a L r yof LIigli Polymeric Sg’stten,b,’’ pp. 254-5, New York, Int~erscieiicePublishers, 1940. (24) Aleyer, K. H., “Natural and Synthetic High Polymers,” pp, 200-1, 281, 608, 612--18, 620, New York. Interscience €%I)liaheus, 1942. (25) Miller, K. P., DuPon[ Jlluga2me, 32, No. 5, 8 (1938). (26) Aforrell, I
