INDUSTRIAL AND ENGINEERING CHEMISTRY
788
Vol. 38, No. 8
(GALLON5 OF 50LUTION ?fR POUND U f OVEN-DRY WOOD)
Figure 21.
Solution-Rood Ratio Curlr
4 sharp break occurs at 1.4 galluna of sulution per pound of wood
when ufiing
R
O.7.jv0 sodium hydroxide treating solution o n n n ~ ~ g l a a
fir separators. 0
2
4
50L U TION
elertiical Ieyistancr, hair picliiiiiiidii> battciy pciiut tiiaii( C , ai111 m t h w lov strength properties. Of four procedures tt.-tetl. tht one using 5.0yosodium hydroxide plus 5 Ock sodium sulficlt~1 I i i o cedure G, Table 11) produced separators Tyith fair elcctrical resistance, good preliminary battery performance, but -trtq$L properties somewhat lower than those of Douglas fir qeparators. The separators appeared t o have heen cut from rather poor grade stock, which may account for the low strength propertie. Selected stock for the separators would perhap.. result in improveil ieparator strength. CONCLUSIONS
Four chemical treatments of practically equal efhc.it.tic> were developed for separators from Douglas fir. They produced treated separators having electrical resistance and perfor nianccproperties comparable with those of separators from Port Orford white cedar, but their strength properties were slightly inferior. 2. One of the chemical treatments which was found suitable for Douglas fir and Port Orford white cedar also rendered scparators from noble fir and Alaska yellow cedar suitable for storapt, batteries. 3. Two chemical treatments M vru developrd for sepuratoi. from redwood. They produced acceptable elrctrical and battery performance properties, hiit qtrength propcrtieq wniewhaf 1111
8 / D /Z l4 16 18 20 CONG€N T 8 A 7 / O N (P€ R C E N T)
6
Figure 22. Relati\e Efficiencj of Extracting Solutioiis on Matched Redwood Separators Treated 11 Hour4 at 97-100" C. t~l10I
t o tllOse O f separitt(Jr3 IlIaIiC
troiii
the other four
\\ooi1*
rested 4. Out rhemical treatment n a s develuped for bald ckprea-tapdiIatorQ It produced treated separatorq having fair t o goou
t4rctrical rwstance and battery performance propertiw, hut Ytrength piopc'rties lower than those of separators from thc, other ltoods employed The comparatively low strength of the bald cypress might have been due t o second grade stock from \vliii*lthe s e p ra t ors M erp c u t 4CKhOW L h D G M h Y T
'The authorz, in behalf of the Foreit ProduLts Laborator?,
H I ~ I
to thanh the Office of Production Research and Development ot
the War Production Board for financial aid in this cooperattvc research. Sincere appreciation is extendrd to 0 IT7. Brov,n oi the Unireraity of Indiana for helpful wggestions, and to E. T Foote, R . P. Hammond, .J. F Harper, G. C. Appel, Rilliarti Thompson, John Schaefer, and associates of Globe-Union, Iiic for valuable advice and for conducting the commercial te>t>on the treated separators. Valuable assistance was furnishrd by the Evans Products Company, the Scparator Manufartiirinu Companr, a n d the Standaril I3attc.n Separator Compani
Temperature Dependence of Water Vapor Permeability .
iibuur i n the film tiy jurlipirig PAUL RI. DOTY, W H. AlI'iEIV, A N D H . 3lAKh: into hole3 in its immrtiiatt. tion of the water vapor Polytechnic Institute, Brooklyn, iV. Y . neighborhood; these holw ;in. permeability of thin, self*upporting films made of constantly forming :tiid (liborganic polymers is often large erivugh t u be of considerable 2ppearing as a result of the random motion of segmrnts i)i' tht long chain molecules. Because of the concentration graditLnt o! practical importance. A st'udy of t,his temperature dependence promises t o lead t o a better understanding of the fundamental the water, the net effect of this nearly random movenirrit i. >I drift of the water molecules toward the dry side of thrs film. nature of water vapor diffusion through organic polymers. Thir: article contains a number of direct measurements of the influence The number of molecules transported across a giverl ari>;i o f temperature on water vapor permeation for several types of within the film during unit time will necessarily be propoltiomwl to the concentration of water molecules and hence to thy soluself-supporting films, and attempts to contribute to a more debility of water in the film. T h e process of permeation is thu* tailed interpretation of the processes of diffusion and permeation. dependent on both diffusion and solubility. Since hoth diffusiori .is a result of Barrer's work (I) and the measurements preand solubility are temperature dependent. permeation will nlso t w . viously presented ( 6 ) ,the process of permeation of water through a n organic high polymer film is considered t o be roughly as folin general, temperature dependent. The following. measurements were cxrricd out in iiri ap1i:ir:i t 11lows: Water molecules dissolve in the film on the side exposed and by a technique previously dclscribed (6). Thry pc*rnlit : I I I to the vapor, migrate by activated diffusion through the film, evaluat,ion of three constants, only two of w-hirh :ire ind(~pt~titli.r~f : and evaporate from the other side. I n general, flow occurs through preformed capillaries only as the result of a mechanical p = permeability constant,, cubic pc.iitimeters of v a ~ ~ u(:11r injury or imperfection. An individual water molecule moves standard temperature and pressure) passing per ~ ~ r o n thronuli tl
T
EMPERATL'KE varia-
August,
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1946
4 glass cell is described for the measurement of the permeability and diffusion constants of vapors in film materials. The temperature dependence of the permeabilit? constant has been measured for ten homogeneous polymeric films. In all cases there is an exponential dependence of the constant on absolute temperature. In the films studied the permeability constant decreases and the energy of activation for permeation increases as the amount of crystallinity in the polymer increases. The effect of crystallization of natural rubber on permeability constant is show-n. In one case the temperature dependence of both permeability constant and diffuaioti constant was measured. This permitted evaluation of the energy of activation for permeation and diffusion, and also calculation of the heat of solution. This procedure verified the resolution of permeability into diffiiiioii and solution for water kapor.
i i i t a i i i t ) i d i i c I 3q. mi. in area and 1 mni. thick when the pra+ w r e difference is 1 em. of mercury I ) = diffusion constant, cubic centimeters of vapor (at S.T.P.) pas5ing per second through 1 em. cube of the material when thew 15 a unit concentration gradient across the cube ,c' = Yolubility coefficient] cubic centimeters of vapor dissolved i n 1 ( . i . . of thc polymer a t a pressure of 1 cm. mercury
a
I n t t i r - e units the c7onqtants are related by
P
=
IODS
(1)
'Thu- t h r permeability constant may be considered as the product of t a i ) factor.: n r a t e constant, D . and an equilibrium constant,
s
789
TABLE I. D w m r w r o s Film
1
KO.
Conipositioii Polrstvrene PolystGrene Polyvinyl chloride-acetate Polyvinyl chloride-acetate PolyvinLl chloride Polyethylene Polyethylene Rubber hydrochloride Polyvinylidene chloride
IIV
Fr1-m
Remark? Criorian ted Oriented Vinylite V Y N W , viis[ { r t , i t < . mercially roducedi Vinylite cast !,lai,uratory) Geon 101 rpsiii. cas1 (laboratory) Calendered Cast Pliofilm %ran (exptl. filni)
VYK'W.
diows the operation of the cell. T h c wll i b evacuated ~1 itti . T ( J ~ P cock d open. When a measurement is begun, stopcovh .4 1closed and *topcock B is opened so that mater vapoi tsnteia t a known piessure into the lomeer part of the ccll. l'ht bhole unit may be placed i n a thermostat and, by u~iiiy saturated salt solution such as sodium acid t u t r a t e in tht. n ' i t t s r vessel, the relative humidity may be held prartically con-tarit n-hile the temperature is varied. Measurements have heen madl from -10" to 80" C. The cell itself is made from btnndarcj Pyrex pipe parts (two 3-1 inch reducers, clamp, and ruhbri gaskets). -4wire screen placed in the upper part of thv ~ 1 prevents bulging of the film under higher vapor pressures. Pressure us. time plots serve to represent the experimental tiai if In most cases these curves emerge from the abscissa with d vt'rj small slope which continues over a period of a few minutr. t o few hours, depending on temperature and on the naturc. A J N thickness of the film. Then the curves gradually bend upn ird and continue as a long straight line. This straight line r:iu br extrapolated to the time axis so as to cut i t at a n intercrpl I, From the slope of the straight line [Ap (mm.)/At ( i p r . ) 1 t h t permeability constant P can be calculated by
WEASUREMENT OF COXSTANTS P, D , AND S
\I(vihuremcJnts of' P , D,and S were carried out as follows: 'I'he tilm \vas mounted in a cell which was then evacuated t o about 0.1 mirrun. Water vapor a t a knoim pressure was admitted to OIIC aid(, o f the film. The pressure increase in the closed volumt' on the dry side was measured as a function of time by means o f w M:~c*Lrodgage whose capillary was siirroundeti with hot I n t h e first part of this x o r k a metal cell was w e d for mounting the film. A g1a.q:: diffusion cell was recently developed n-hich is n i o i ' i ~i ~ o r r 1 p w c . tan11 riqiiilc.: lrsa timv lor outgassing. Figiirc 1
where V = volume into which gas t'ipwiidt: alter p i s s i t i g t,hrough film, cc.; a = area of film, sq. em.: 1 = thickness, nim. p = vapor pressure of water a t one side of film, mm. Hg. Fick's partial differential diffusion equation for these bouiiciur~ conditions has been solved (I), and a simple relation has h u l i l fqund between diffusion conetant, and intercept I,. ThiF wlation is
D = 12/6L. '/'tit,
TO MACLEOO
11
11
(
x
silribility coeficient S is then obtaiiied from Equat>ioii1 TEXIPEKATURE DEPENDEKCE OF P
Perriictat)ility constants were rnt.asured, by the procetiurt. J U S I described, a t different temperatures for films of several polynwrr (Table I). T h e results are listed in Tahle 11. The film> inIaluded in this investigation were studied at several presaurrh ot water vapor (at 92c; relative humidity) and in most (-art+ at. different thicknesses. The permeability eonstants w w indrpendent of these variablrs. I n Figure 2 most, of these data arc plotted as logarithm of prmeability ronstant against reciprora I of ahdolute temperature. The straight lines obtained in t h w e plots show t h a t temperature deprndenw of the permc>ability const;int may heexprrssed by 1' = p,le-"/K'
(41
where Po = pernieability constant at abaulute zero E = energy of activation for permeation R = gas constant, T = the ahsolute 1 i . n i p c w i t r i r i . The energies of avtivatioii, B, may be obtained from the slopre o f t,hr lines in Figure 2: thpy > I I V listid in Table 111.
1
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
790
3.1
3.0
3.2
3.3
3.4
3.7
3.6
3.5
!x 10’ T
Vol. 38, No. 8
film has a somewhat higher permeability. Pure polyvinyl chloride possesses about the same energy of activation as the vinyl chloride-acetate copolymer but a considerably lower permeability constant. This point will be discussed in the next section. The tn.0 samples of polyethylene have the same E-value but differ again in permeability constant. It is usually observed that calendered films have higher permeability constants than cast Elms of the same composition. This is presumably due to the greater number of inhomogeneities and thin spots in the calendered product, so that the effective thickness is less than that measured. Rubber hydrochloride and polyvinylidene chloride show low pei,meaLility constants and high energies of activation. Thus, for the materials studied there is a good correlation between the value of P and the energy of activation, E. \Ye may conclude, therefore, that, except a t high temperatures the permeability constant is generally lower, the greater the energy bf activation. This generalization may fail if the solubility of Fater vapor or the heat of solution deviates greatly from the magnitudes usually encountered. The data appear t o show that the basis of this general kxhavior is closely connected Kith the amount of crystallinity in the polymeric materials. Polystyrene, polyvinyl chloride, and its acetate copolymer are noncrystalline (9). Polyethylene is about three quarters crystalline a t room temperature (IO). Rubber hydrochloride (3) and polyvinylidene chloride (8) are very highly crys-
Figure 2. Temperature Dependence of Permeabilit? Constants of Polymeric Films (Table I)
Examination of these data shows that, for the materinls studied, the effect of temperature on the permeability constant’ ianges from no effect a t all for polystyrene t o a doubling of P per 5 ” C. increase for polyvinylidene chloride. ‘This result is important in the practical application of the various films. It indicates that routine test data a t one temperature cannot, in grmral, be used t o predict trnnsmission a t another temperature, unlcss the temperature dependence is known for the matfrinl iint1t.r ronsideration. These data seem t o confirm the existence of activated diffusion during the transmission of water vapor through polymeric materials. This may be compared with similar bphavior found by Barrer (I), Dewar (4), and Edwards and Pickering ( 7 ) , nh(i measured the permeability of various rubbers t o permanent g a w . In these experiments the energy of actiration was in the neighborhood of 8000 caloiies. Polystyrene exhibited the same behavior of temperature independence in both the oriented and urioriented state. Vinylite shows a small temperature dependence. The commercinlly made
TEUPERATCRES Polystyrene
3
1’01s styrene
3
Polyvin]-l chloride-acetate copolymer
-1
Polyvinyl chloride-acetate copolymer
5
Polyvinyl chloride
6
Polyethylene
7
Polyethylene
8
Rubber hydrochloride
9
Polyvinylidene chloride
T.ZBI,E111.
3.4
3.5
3.6
+X
Figure 3.
io3
3.7
3.8
3.9
Temperature Dependence of Permeabilit! Constants of Natural Rubber
25 32 38 45 25 32 38
1
Film No 1, 2 3
“3.3
T, C.
318terial
Film S o
4 5
: b 9
25 32 38
0
IO 25 38 0 10 25 30 35 45 55 0 25 32 38 15 20 22 40 GO 80 20 25 30 38 43.5 47.5 52 25 32 38
Px
108
89.5
80.0 87.0 82.0 83.5 84.0 83.3 32.5 38.2 -13 8 19.9 22 2 28.8 32.4 8.6, 10.9 11,5,12 2 11.6, 12.3 14.9 15.5 18 7 20.3 0.73 6.10 8.60 11.05 2.45 3.20 3.60 7.91 22.2 50.0 1.02 1.21 2.08 4.10 4.35 5.15 8.7 0.20 0.52 0.82
E S E R G I E B O F .$CTIV%TIOX FOR P E R V E A T I O X
Material Polystyrene Polyvinyl chloride-acetate Polyvinyl chloride-acetate Polyvinyl chloride Polyethylene (calendered) Polyethylene (caat) Ruhber hydrochloride Polyvinylidene chloride
E. cal /mole
n
Log P6 -6.08 -3.51 -4.80 -5.10 -1.34 +0.13 i-1.63 4-4.20
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1946
functional variation with temperature. that >$ =
0.5 c l
0
'0
- 0.5 -1.0
3.C
I
!
3.1
3.2
1
f Figure 1.
3.3 x lo3
I
~
3.4
3.5
3.6
3.7
'TerriperaLure Dependence of P, D, antl S for Pol?\in?l Chloride
talline. It' the solubility ih const:iiit, crystallinity will give rise to a higher energy of activation and a smaller permeability constant. This seems t o be a reasonable conclusion regarding the, effect of crystallinity o n permeability, for in a crystalline substance tlie ordered spacing of the atoms and molrcul(- presents the most effective arrangemt~ntpossible i n fnrn~iiig(at constant density) a n obstacle t o transmission. Ilortwyer, in the crystalline st:ite t h e atoms are held in their po-itions by forces that are stronger on the average than tlii,,-e existing i n the amorphous st:ite The density of the polymers also increases appi~oximntely i n the same order as d o t h e activation energies. This effect of crystallinity should be especially noticeable il' measurements are made on a film material whicli cryAtallized in the temperature range being studied. T o test this hypothesi*: :I filni oi uitiiral rubber n-as inveatigatcd. 'Hie rewltq art%s h o ~ iii r Figure 3. From 25 to 0" C. (1r.it pottion I Jgraph1, ~ tlie perrnt'abilit,y constant decrwses in a n exponential manner with the reciprocal of absolute temperature. At about 0" ('., lion-ever. there is a break in t h e plot, followd by apotlier itrnight-line portion d o v a to - 10" C. T h e priiliai)le experimental e i m r t1oe.q not pormit determination of t l i t - pr tinuity. It is importnnt t o iii)tc' t1i:rt the temperature r:inge n.hcsrtL l u h l ) c ~ ~ ~ transition in the noncry-t:illinc~region. th(, enc1rgy ( i f ai.tiv:rtioii C ir :?I (crilia\il:ited from the T I ~01'Jt h~e liiic>l transition point i n tliv is about 30,000 caloiic= ! ~krnonitratestlint cry5tiillinity decre:i*e. the permeability con.*t:int by increasing the energy of ac.tivation. Since the thickness i ) i the rubber film untlcr the conditions of t h r experimeiit wxs not :iccurately knoim, the absolute valuez i i f t h e pcrme:ibility f'ull>(:ititnppearing in Fipui,e 3 may tip i n error by as much :L- 2 0 5 .
791 Hence i t would require (6)
$q0r - A H ~ H T
\\\-hered,is a constant and AH the heat of solution. I n order to determine the validity of this analysis i n water-polymer systems, a series of careful experiments were performe8:l on a film of poly vinyl chloride. Ten measurements of thrx permeability aiitl di j.iusion constants ere carried out at different temperature-. The results of these measurements are shown i n Figure 4, whr.rc* log ( P x 108), log ( D x los), and log S are plotted against rtri1)rocal ab>olute temperature. Such a plot yields straight liiic:.~ for all three quantities, 10 that for this system E , ED. and A H , :ippcaring in Equations 4, 5 , and 6, may be evaluated from tht. slopes of the lines in Figure 4: E = 2tiO0, E D = 10,000, and AH = -7400 calories per mole. The value of the energy o t activation, E , vihich chnractc.rizw the temperature dependcnct. I I t permeability, k the algchraic, sum of heat of :jolution and ciic1.gy of activation for diffusion. I t f o l l o ~ sthat E is not easily prc.. dictable. For example, measurements of the temperature d v pendence of P and D in the case of polystyrene show that E Dantl - AH have finite and identical values; con:equently the valiic of zero for E results. I t is even possible thrrt a negative v ~ l i i e of E may exist in some systems. The theoretical significance (Ji the measurements presented here together with those obtainid n-hen plasticizers are added is discussed in $1 forthcoming (*ommunication ( 5 ) . SUIl\lARY
The teinperature depciidence of tlic, ptJriiteitliility of polymer films t o water vapor has been deterrnined, ivith showing a wide range of kic,havior in thi.5 respect. The rrsolution of pernieahilit y into protluct of diffusion am1 solution has been ,--polyvinyl chloride. Proot' is given verified for this system he energy of act..vation for permeae dependence of permeability. The t,rature ck~pendencevaries so much froii: ~ i n ematerial to another that the perforntancc of a given pack:iging film rminot be predicted n-ith certainty a t one t,emperature on the basis of nieasureniente a t another temperature. If su(-Ii pretlictionr arc desired. measurements of pelmeability must bt, made inr a t 1e:tst t r o temperatures. Finally, it is clear t,htit ignorance of the temperature dependence of permeability can b(, -ufficient reason for lack of agreement amrlng various testing iiic.tliiiil* ~ l i i c l id o not require thc same temperature.
-
ACKXOR LEDGIIENT
.. 1he autliors n-ish to express their apprec.i:ition
to tile Sationti1 Itesearch Counril Committee on Quarternlaster Prohlemx and t,o the Research and Development 13r:inch of the Officc: ni the Quarsponsoruhip mo-t ( ~ fthc work termaster Genernl, under lvhc reported here was carried out : i t i f 1 with tvlio?t~ ~ i ~ t ~ n i i t~h~ci ~o ,~ results are published.
a
LITERATURE CITED
Bailer, le. SI., "Diffusion i n and t l i t ~ ~ i i gSuiid..". l~ Seu Y l , i h . SIacmillan Co., 1941. Bekkedahl, S . ,aiid TI-ootl, L . .I.,.J, C'htm. P h i l a . . 9, 19:3 (, I M I : 10,403 (1942). Burin. C. IT., a i d Gntiiei , I;. T.,, J . C'ho?~. ,b~oc.,1942, 434 Dewar, J.,Proc. R o y . I j i a t . Gi. Bril.. 21, 81:; (1914-1lib Doty, P.JI.,J . Chem. Phus., 14, 244 (1046).
Doty, P. A I , , Aiken, TT. H.. and Mark. H ASAL.ED..16.686 11944).
'TEMPERATURE DEPESDE3-CE OF D AND S
Diffusion constants in liquid systems are known t o be exponullti31 functions of temperature. Barrer (I) showed t h a t this is true for diffusion of some permanent gases through rubber. The diffusionconstant may be represented by , I J =
Uoe-Eo/Rr
Isu. Ksc,. C ' I I E M . ,
(1942).
JIeyer, K. H., "High Polytiieric S u h s t a i i w - " . SPT\ I - o r h . Interscience Publisliet.>.1942. Raine, E. C., Richards, 11, B.. tin(] Ryriei , H . . T),ntis.Farc~day
(5)
where D, is a constant and EDis the energy of activation for diffusion. If both P and D behave as Equations 4 and 5 predict, i t is nccessnry t h a t t h r solubility coefficient exhibit thr same
,
Soc., 41, 56 (1946). I'HESENTED
before the Division of Paint, Varnish, a r d Plastics Ciluniiatri A V E R I C h S CHEJ~ICA S OLC I E r Y i n Nra York,
a t the 108th Meeting of r h e
s.1'.
i
