Comparative Study of Water Vapor Permeability of Saran Films I k s p i t e the c o r i s p i c i i o i i ~position M hich Dar.in l i l i n c occupy in a n y rlassification of transparc.nt organic: self-supporting uncoated water vapor barriers. there h a s been little agreement a m o n g investigators cwncerning t h e absolute values of water vapor tran.cmission. T h i s appears t o be d u e chiefly to thr diverne m e t h o d s of a t t a c k employed a n d h a s resulted i n such divergent values : i s t o bring i n t o question t h e reliability of piihlished d a t a . In t h e present paper a c o m p a r a t i \ e study of t h e m e t h o d s a n d d a t a of t h r e e r a t h e r different techniques i s macle: s t a n d ard c u p tests (with moclifiratione) which comprist. a constant pressurc e v d u a t i o n : a variable-pressure technique with new results on t h e modified Ckneral Foods gas transmission cell: and a high-vilcuuni variable-pressore technique. When all t h e factorh t h a t m i g h t influence t h e transmissions are exa m i n e d , two a r e fnuncl t o c o n t r i b u t e most t o t h e a p p a r e n t discrepancies in reparted values: t h e difference i n transfer r a t e d u e to t h e vapor pressure differential a s exeni plified try t h e constant-pressure a n d t h e variable-pressure extremes, a n d t h e t i m e
D
URING recent years, niciny technical data have been published on the gas and water vapor permeability of synthetic
organic thin films having exceptionally high impedance. Despite the latitude of method arid technique involved in the determination of water vapor pt~rnieability,saran films have gencrally t)een rat’ed as the most resistant, transparent, unsupported. rinc.oated water vapor barriers available t o industry. Sotwithstanding the general agreement on the relative position of saran films in the water vapor permeability spectrum, the over-all results of the numerous publications have merely rea,ffirmed the fact without elucidat,ing the wide apparent discrep:iiiries. It is the general purposis of this research to resolve some of th(5 :rpparent discrepancies in pernical~ilityvalues derived for saran films through a more critiwl examination of the methods and techniques employed. Specsifically, a comparative study is madr of data obtained by Iloty, Aiken, and Mark (2) using a high-vacuum variable-pressure technique; by using const,antpres sui^^ General Foods vapor cup technique; and by using a variable-pressure modifird (kneral Foods gas permeability cell ( 6 ) . Furthermore, new data for water vapor transmission at wrious temperatures arid viipor pressures using a technique descrihd by No11 ( 4 ) are reportcad for saran film, in addition to itii iriterpret,ation which is applicable in principle to water vapor permeability data obtained from cup techniques for any film mat,erial. Sol1 employs a standard vacuum desiccator which contains :t largc beaker of drying agent and, above this, a portable rack iu which are located four aluminum water vapor cups. Water vapor transmission is determined by measuring the weight loss of thv \rater-containing film-covered cups. The technique of No11 produced such interesting experimental data that it was felt worth while t.0 include all the results before making comparisons with high-varuum techniques.
f a r t o r iiirolvcvl i n thc. d r t r r n i i n a t i o n s . ‘The lattcr varies from 2 t o mort’ t h a n 22 hours, a n d t h i s variation is extremely hignifimint in view of experimental results. Finally. t h e rrnio\al crf volatile r o n s t i t u c ~ t t sfrorn plastic films u w t a l l y in\ol\ecl in Fariablepwssure techniques c o n t r i h t e k to t h e iipparent discrepancy. I ~ e d u r t i o no f t h e data t o t h e 4ame cwntlitions where possible indicates t h a t piihlishecl rt-sitltr a r e artiiallj r1ost.r topet1it.r t h a n is apparent rnerrl> f r o m transmihsinn \aliirs. ‘I’hr final clifw hivh may be considered fwenrt. is less than 10 hat i d a r t ory where perinrabi l i t y (wnst a n t s in t hr c,itreriiely l o w range I’ = IO * are inrolved. The remnining differenre rniist he a t t r i b u t e d to t h e two vaeuuni drdistinguishing steps in technique: plasticization a n d presscire differential. A graphical analysis of water vapor transfer d a t a obtained from c u p tests indicates t h a t better correlation is m a d e by using “equivalent” transmission (caleulated f r o m AE“/At where IF’ is t h e same, irrespective of time, vapor pressure, temperature, etc.) t h a n by using s h o r t - or long-intrrral transmission d a t a .
*’,
APPAH \’I‘I.S 4 h D PROCEDURE
No essentially new apparatw was employed in the experimental work of this research, but the gas permeability cell introduced by Elder (3) and Shuman ( 6 ) and modified by the author ( 5 )was a p plied t o the determination of water-vapor permeability, after the calcium chloride drying tubes had been removed and a small beaker of deaerated distilled water placed within the bell-jar vacuum system. The experimental work on which the larger part of this report is based involves standard calculations of water vapor transfer rates and employs apparatus found in almost any cdmmercial or academic laboratory. Deep Petri dishes, a large desiccator, an elect1i d l y controlled oven, analytical balance, beeswax-paraffin sealer, and film samples comprise the essential materials required. In order to obtain data on water vapor transfer rates for saran films at several different temperatures and vapor pressures, space limitations made it necessary to employ a somewhat simpler arrangement than the standard control testing W.V T. cabinet. The technique selected wab later found to be previously described by No11 ( 4 ) . Essentially, it consists in placing film specimens sealed to deep cups over a quantity of desiccant in a large desiccator in which is placed a beaker of watei. The cupq are weighed a t regular intervals and the rates of water vapor transfer are calculated from weight gain, area, and time factors according t o standard proceduie. The entire desiccator may be transfeired easily t o an oven for determination at viiriou~temperatures. D 4 l A A l l ) C4LCULATIOYS
The present work, in addition to supplying data on water vapor cup transmission for saraii films, resulted in an evaluation of the following conditions: Reversal of desiccant-water location. One set of tests \rap coilducted using desiccant (sulfuric acid) in the cups and water outr side: the other, using water inside the cups and desiccant outside. Effect of different quantities of desiccant or water. 111 one set of cups, approximately twice as much desiccant was used in one cup as in the other; in another, approximately twice as much water was used in one cup as in the other. 1541
ANALYTICAL CHEMISTRY
1542 Rearing in mind these conditions, it can be seen by superimposing Figures 1 and 2 that the location or quantity of desiccant or water apparently makes no substantial difference in the rate of transfer. For practical purposes, therefore, the remainder of the work was confined to the system wherein the water was inside the vapor cups, in view of possible sulfuric acid attack on the wax-paraffin seal during manipulation.
IO00
1
CALCULATED CURVE FOR 38%. a 4 4 m m . VAPOR PRESSURE
// 1
-6 800
I
IO00
I
I ' I /
900.
I
-J
I
500
*
0
400
v)
5
8
300
v; u)
a a
0
-1
-J
3 500
200
5
e
IO0
U0
- 400 x
0
v)
I
0
2 300 a
Figure 2.
i
s 200 s
168 TIME IN HOURS
336
Cumulative Weight Loss of Saran 517 Film
IO0
0
168
0
336
T I M E IN HOURS
Figure 1.
Cumulative Weight Gain of 0.001-Inch Saran 517 Film
The calculated curves shown in Figures 1 and 2 were included to indicate correlation with two general rules for estimating rates of water vapor transfer (or weight gain or loss):
For every 10" C. rise in tem erature, the water vapor transfer rate is approximately double$ Water vapor transfer rates are directly proportional to the vapor pressure. Incorporating the two geiieralizations into a working equation, one obtains (Tcalcd.
ItlVT&d. ( s gain or loss) = 2
c. -
10
Tobsd.
"'1 x
In Figures 1 and 2, the broken lines were obtained from the data a t 25' C., 24-mm. vapor pressure, applied to Equation 1 as the observed values. The close parallelism of these calculated broken lines with observed values a t 38' C. and 44-mm. vapor pressure is satisfactory proof of the utility of the generalizations within reasonable limits of temperature and vapor pressure. In view of the apparent reliability of the cup tests under these conditions, the remaining graphical data contain only single cumulative curves for each temperature, the single curve representing the average value in each case of two different weights of water in vapor cups. Figure 3 gives the complete data for weight loss through commercial Saran 517 film. The departure from linearity observed in the curves a t temperatures above 25" C. suggested two further experiments involving the cup technique: One investigated the effect of heat treatment on transmission-i.e., the effect of shrinkage with consequent in-
outside cup)
168
0 TIME
Figure 3.
336
I N HOURS
Average Cumulative Weight Loss of Saran 517 Film
crease in film thickness a t elevated temperatures-and the other investigated the effect of plasticizer on the over-all transmission. Figure 4 shows the results of vapor transmission tests a t various temperatures on Saran 517 film which was flash heabtreated. Figure 5 shows the results of vapor transmission tests on unplasticized Saran 517 film.
1543
V O L U M E 2 2 , NO. 12, D E C E M B E R 1 9 5 0
< ‘oniparison ait,h Figure 3 indicates that rapid heat treatment has riot substantially affected the rate of transmission. I n fact, the curvm practically coincide. On the other hand, unplasticixed film shows a markedly lower transmission. This substantiates the known effect of plasticizers, and incidentally shows t,hat rapid heat treatment removes little if any volatile matwial from thc film. At least three portions o f the curves can be distiiiguislirtl for t,hc purpose of vvaluntioii:
A. Long-interval average, as the average rate coniputed from total weight change divided by the total time change, and converted t o standard units.
2 .- 700
LA-
1 1
1000
900
38%.
p2m
‘44mm.
31%. 34mm.
I68 IN HOURS
336
a K
0 TIME
Figure 5 .
Average Cumulative Weight Iuss of Saran ,517 Film, Unplasticized
“E q u iva Ien t ” o r Specified -vTranrmission Level
TIME
Figure 4.
IN
HOURS
4verage Cumulative Weight Saran 517 Film, lieat Treated
Loss
TIME
of
Figure 6.
Equivalent ‘Iransrnission Level
B. Short-iiiterval average, the avc~ragerate computed from the weight change divided by the time change over the initinl 24- to ;3&hour interval. C. “J:~quivalent” transmission average, as the average rate computed from a chosen t o t a l w e i g h t change level, regardless of the time i n t e r i d and of the teniper:tture.
sa,
Table 1. 7‘
Vapor I’resxiire
25 34 38 44
49
Long Interval I r e r a g e ~~
~
~~~~
G.,
ffg
24 40 44 68 88
0.031468 0.03398 0.03524 0.03938 0,021625
0 0498 0.1332 0.1779 0.318.5 0.3520
34
0,03167
34 40
0,03278 0,03494 0,03823 0.02135 0.02214
0.0367 0.0744 0.1680 0.27!1.5 0.4580 0.727
0,04658 0.04833 0.03119 0.03175 0.03351 0.0,?677
0.0223 0.0283 0.0373 0.039.5 0 1190 0.1201
sp.
~
~~. .- Short Interval .iverage G. ,J
100 sq. G./7.07 100 sq. inches: -74 sq. inches anches/24 hours I’ X 108 hour hours I” X 108 A. Saran 517 (0.001 Inch), Commercial
C./7.0: inches; hour
Sf in. C.
Eater \-apor ’Iraiismissioii and Permeability Constant
1 18 1.91 2.02 2 6.5 3..X
0.031458 0 03448 0.03656 0.0210 0.021625
0.0495 0,1525 0.5230 0.3400 0.5520
1.17 2.16 2.52 2.84 3.56
-
Equivalent Transmission Average G./7.07
sq. inches’ hour
G./
100 sq.
inches/24 hours P X IO8
0.031463 0.03425 0.03560 0,03956 0.021625
0.0498 0.1445 0.1908 0.3250 0.5520
1.18 2.05 2.10 2.72 3.56
0,03167 0,03291 0.03625 0.03893 0.02139 0 02208
0.0567 0,2121 0.3034 0.4720 0.7060
1,339 1.652 3.018 3.918 3.938 4,560
0.01661 0.04906 0.03134 0.03208 0,03416
0.0224 0.0308 0.0485 0.0706 0.1412 0.2178
0.528 0.513 0.644 0.912 1.179 1.393
B. Saran 517 (0.001 Inch), Heat Treated 25 31 34 38 44 49
44
68 88
1.33:l 1.576 2.518 3.600
3.880 4 OSIO
0.03167 0.03312 0.03688 0 03875 0 02144 0 02204
0,0367 0.1060 0.2338 0 2071 0 4880 0 6930
1.339 1.772 3.318 3.839 4.080 4.480
0.0988
C. Saran 517 (0.001 Inch), Unplasticized 23
31 34 38 44 49
24 34 40 44
68 88
0.327 0 472 0 336 0 776 1.00.5 1.494
0.0462.5 0.03104 0.03167 0.03208 0.03416 0.03646
0 0216 0.0353 0.0367 0.0706 0.1412 0.2193
0.512 0.589 0.803 0.908 1.179 1.416
0.03641
Method A would be expectcd to give low vnlues; method 13, a t least erratic and probalily high values; ant1 method (‘, medium values, or a t least comparable values in the sense that the film samples have been subjected to the sanie quantity of water vapor passage regardless of the time interval or temperature level. In general, conveiitional procedure ( 7 ) arbitrarily chooses
ANALYTICAL CHEMISTRY
1544 -
'J'able 11. SO.
h r s n Film
Thickness
Permeability Constants at 25" C., 24 M m . Vapor Pressure G./100 sq.
517 commercial 517 heat trested 517 unplasticised Type M T y p e M plwticized T~,X xr 5 1 i rt,eiilxr
Inch 0,001 0.001 0,001
0.OOOR 0.0026
o.nm
0 0011'
Equivalent Long Intervai Transmission -. Steady State . G . / f 0 0 sq. G./100 sq. G./lOO sq. inches/d4 inches/@ inche8/84 hour8 I' X loa hour8 P X 10' hourm I' X I O 8 1.18 o . n m 1.18 0.0498 ,. 0.0887 1.34 0,0567 1.31 ... 0.53 0.0224 0.53 0.0"":( , 0,00002?0.0003 . , .
Short Interval .___ __.___..._ inchea/84 hourm 0.0495 0.0567 0.0216
o.nooi4
o.oonii 0.0024' n.noa}
n.( m i 2
I' X I O r
1.17 1.34 0.51
n.ooa
n.oni
I
.
. .
. . ..,
(1. 0o:i
Figure 7. Water Vapor Transmission and Absolute Pressure Differential of Water Vapor at 25" C. Sarsn T y p e B.1 Film. 0.0005 inch thick
a time intervitl (68 to 72 hours), whereas an equivalent transmission level is reported here, as shown in Figure 6. Table I gives the complete data obtained from a graphical analysis as indicated above.
.
I
...
,.
0,Oi: 0.11
... ..
,
,
,
.
.
n oonnoot ...
, . .
(I
0')31J.1
n.ooo.1 ..
I
0 IIXlk
Source
Table I A , cui, Table I B our) Table IC,' cup hlod. G.F. cell Mod. G.F. cell (2) \lad. G.b' r d i
fourfold, \rhilt, the nmxiiiiiini dit?r-~.cs~ict\ is :r,t~out, oiits t r u ~ i ~ l i ~ c ~ ~ l ~ ~ ~ l i l . H o w v r r , t,he differt7ncw ftj)po:ir to i'ollo\v :t relationship nit ti tinic of tc,st: Cup tests rtquirc. 24 t o '18 hours PVPII for the "short," IiktAn, :trd Mark ( 2 ) rvintcrv:il testing: the tc,chniqiic*of I)o qiiirw 1 to 4 hours; and t , l r inodificd C : c > r i c n l Footls pas prrmcw 1)ility rrll ( 6 ) may b t r e d at : ~ n yiritclrval ( 1 to 2 hours for thts short-intcrv:tl results). In thv s:tmr c ) x I ~ r , the perme:ttiilitF constxiits dccreasc, h y :i fitctor of :Ipproximat.elv 10 I. T h r iniportancr of the tinw f:ic*tc~ri-: tlic~rc~frirc~ :spp:i,rvnt mtwn tliffr~rrrrt 3 , 4 , and 5 inciic:i,te thitt, even trvhniqucs are comp:iwd. I~Ygurc~s for t,hr same test, method, transmission gr;adually decline8 with t,init'. This s:rrnv f:wf is o l ~ , i . v i ill ~ lroutirw or oontml trating of sarin filiw. Ariothrr important condit,ion n1iiN.h influrnocs the t.rausmissiiiri rrsults is thc vapor pressure differc~ntial. I n the cup techniques, the vapor pressure o n one ski(' o f thc. film is substantially zero: hence thtx full effect of transmission is ohst,rvtJd at, all times. With and t,hc variable-pressure technique rmployrd by Doty, Aikt:~~, Mark ( 2 ) and the author ( 6 ) ,on(% side of thr film is momentarily zero, and gradually increases uiit,il the vapor pressure inside thr film equals the vapor prc'ssure outuide the film, as in Figures 7 and 8. It follows, therefore, that the transmission becomes slightly lower with each increment of time. For this lemon alone, water vapor transfer rates and permeability constants derived from varialde-pressure methods should be lower than those derived from constant pressure or cup met'hods. It follows also that variable-pressure techniques which require evacuation of the film samples tend to give absolute or "steady state" values which are more nearly indicative of the true transmission of the basic film material largely devoid of volatile matprials such as plastivizrrs whirh :irv known to influrnre permea-
COBIPARISON ITH VACUUM TECHNIQUES
Foi cwmpar2ttive purposes, the data a t 25" C'. and 24-mm. vapor pressure from the various sources are compiled in Table 11. The apparent discrep:tnc+s in results are not nearly so far apart when the important factor of time is considered. Tests 1 to 3 :we cup methods as previously deAcribed, while test results 4 to 7 mere o b t a i n e d w i t h a p p a r a t u s which necessitates evacuating the film samples. Accordingly, the data for unplasticized film M in Nos. 3 and 4 t o 7 should be most nearly comparable. The permeability constants for values (1t.i ived from short-interval testing ~ I O Wsome degree of correlation. The minimum difference is about
TIME I N HOURS
Figure 8.
Water Vapor Transmission and Absolute Pressure Differential of Water Vapor at 25" C. Saran T ~ p %I e Film. 0.0028 inch thick, unplasticizad
V O L U M E 22, NO. 1 2 , D E C E M B E R 1 9 5 0 bility (1). The values given in Table I1 for Sos. 4,5, and 7 shoN that there is a large difference between initial (short-interval) and steady-state permeability. These data were taken from Figures 7 and 8, which clearly show the transitions observed in long-r:inge tests on the modified General Foods gas permeability cell. Thev show also that the data of Doty, Aiken, and Mark are high (arid closer to vapor cup results) because the latter are taken i n the initial period of the test. Further evidence of the essential steady-state nature of the results for 3 0 s . 4 and 5 of Table I1 is the final permeability constant comparison: P = 0.0003 X lo-* for 0.0005-inch film and P = 0.0004 X lo-* for 0.0028-inch film, an exceptionally good check for films of such widely different thicknesses and treatment. CONCLU ?JON
Widely different test apparatus and technique for determining water vapor permeabilities for synthetic films result in widely different transmission values. In the case of saran films the data from different techniques are riot so divergent as f i s t appears when due consideration is given the over-all conditions, and the time interval involved in making the determination. For practicnl purposes, therefore, it is necessary to know to what condi-
1545 tions the film sanipleb \%ill kw rsposed befort. choosing :i test method that will give applicable results. Keither the evacuating nor the nonevacuating method give. conditions universally encountered in film packaging. If film packaging with desiccant is employed, water vapor rates derived from nonevacuating techniques (such as the cup method) should be employed. If film packaging without desiccant as in food and meat packaging is employed, the evacuating methods or lower rates should be considered. In other words, the specific application to which the film material is put should dictate the test nwthod for rates of water vapor transmission, LITERATURE CITED
(1) Doty, P., J. Chem. Phys., 14,244 (1946). (2) Doty, P. M., Aiken, W. H., and Mark, H., IND.ENQ ( : H E M ANAL.ED.,16, 686 (1944). (3) Elder, L. W., ModernPackaging, 16,69 (1943). (4) Noll, A., Papierfabr. Wochbl. Papierfabr., 5, 151 (1944). (5) Sarge, T.W., ANAL.CHEM.,19,396(1947). (6) Shuman, A. C., IND.ENG.CHEM., ANAL.ED.,16,58(1944). 17) Southwick, C.A., Jr., M o d m i Packaging, 19,No.11, 137 (1 $146). l i a c t i r r i i Srpteinl)ri 12 lq40
Impact Resilience as a Brittleness Test for Polyvinyl Plastics GERARD FRIEDLANDER D r . Rosin Industrial Research Co., Ltd.. Wembley, Middlesex, England
l h e paper deals w-ith Lupke impact resilience of plasticized polyvinyl chlorides. The impact resilience temperature curve passes through a minimum which defines the transition from ordinar) to rubberlike elasticity. The position of this minimum with regard to temperature is characterized by the percentage concentration and type of plasticizer used. .it temperatures lower than the re-
T
HE loss of flexibility of polyvinyl sheets which occurs :it lo\\
dience minimum-i.e., in the absence of rubberlike elasticity-polymer compounds which give rise to high resilience are not capable of large viscous deformation during the short time of impact. In this temperature region impact resilience can be used to classify then1 according to their tendency toward brittleness. Merits of the method are discussed and compared with other hrittleness tests.
temperatures and the consequent brittleness have been a serious drawback in the application of these plastics for articles for out-of-door use, such as handbags. Various test methods have been proposed and tentatively introduced to establish a brittleness standard for these materi;ils. The two best known ones are the A.S.T.R.I. test ( 2 ) , which measures the temperature a t ufhich a plastic test sample ce to be brittle when subjected to a certain impact, and the flex temperature test (S), which measures the temperature at which a certain torsional deflection is attained when a test strip i.; subjected to a constant torsional moment. The A.S.T.M. test suffers from the disadvantage that thv so-called tirittleness temperatures determined by it are far lower than the temperatures the plastic has to withstand in actual practice and therefore cannot give an adequate representation of the behavior of a plasticized polymer under actual working conditions. For instance, many failures due to brittleness have occurred with polyvinyl rhloride handbags at teinpernturcs
temperat'ures of plttsticixed polyviiiyl chloi,ides are appr affected by changes in the rate of deform:tlion. The Clash and Berg method does not measure brittleness but loss of plasticity or increase i n rigidity modulus as a function of temperature. The so-called flex temperature of the Clash and Berg test is defined as the temperature a t which n specimen of standard dimensions is twisted through an ai'c of 200" under a fixed torque of 5.68 X lo6dyne-cm. which is applied for 5 seconds. These specifications lead again to low test temperatures which for H niisturc. having an .LF.T.XI. hrittlentw temperature of - j O " C. are below -20" C. Though the Clash and Berg test is c:rrried out over a relatively short time interval ( 5 seconds), thtj r;:impIe probably undergoes other than purely elastic deformation. hiltrn ~t al. in their paper on creep behavior of plasticized T'iriylite ( 1 ) conclude that in a test lasting only a fraction of a second B trioctyl phosphate plasticized sample of T'inylite may be several times as flexible as R tricresyl phosphate one, though its "5-second stiffness" is the same.
:il~oundo o A further disadv:rntage of t h e tmt i* its dependence on the rat? of' impact. Iienip et nl. ( A ' h : i w reported that brittle
The tendency to brittleness in a plastic is characterized by the :tbsence of rubberlike elasticity aud by a low plasticity-i.c~.. low
c.
THEORY
