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Permeation of gases through metallized polymer membranes
Journal of Membrane Elsevier
Science, 24 (1985) 297-307 Publishers B.V., Amsterdam - Printed
Science
PERMEATION MEMBRANES
P. MERCEA,
L. MURESAN
Institute of Isotopic (Romania) (Received
July
OF GASES THROUGH
METALLIZED
POLYMER
and V. MECEA
and Molecular
11,1984;
297 in The Netherlands
accepted
Technology in revised
R-3400 form
June
Cluj-Napoca
5, P.O. Box 700
7,1985)
Summary The permeation of He, H,, CO,, Ar, N, and Kr at 50°C through polyethyleneterephthalate, PET, membranes metallized with Pd, Ni and Cu was studied. It was found that metallizing a PET membrane changed its permeability for the gases studied, and that the permeability for H, varied slightly with differing H, pressure. In the range of 0-50°C the temperature dependence of the permeability for He and H, was determined. The results obtained were interpreted by assuming that the permeation of all gases, including H,, through the metal layers of the membranes takes place by diffusion through fine defects which exist in their structure and, moreover, that H, also permeates through the Pd and Ni layers themselves. An important point is that by this method an increase of up to an order of magnitude of the membrane selectivity for H, was obtained.
1. Experimental method 1.1 Preparation of metallized PET membranes The metallized membranes (“sandwich membranes”, SMs) were obtained by vacuum depositing Pd, Ni, or Cu - of at least 99.9% purity - consecutively on each side of a PET membrane, produced by an inland factory under BASF licence. The thickness of the bare PET was measured with a micrometer - with an accuracy of * 2 pm - and found to be 30 of: 2 pm. A PET disk, with a diameter of about 100 mm, was degreased and then degassed for several hours in preliminary vacuum at 80°C. Afterwards the PET membrane was mounted in a circular stainless steel mask and introduced in a Hochvacuum Dresden vacuum deposition system. The deposition of the metal layer was performed, under 5 X 10m6 Torr, by evaporating Pd and Ni from a W basket coated with alumina and Cu from a MO boat. The metal vapour source was made to ensure (as far as possible) a constant vapour density in the solid angle which delimitates the PET membrane. The distance from the vapour source to the center of the plane PET membrane was rc = 25 cm. Hence the deposited metal layer will be thicker at the center, d&, than at the which had an exposed area of edge, d& , of the SM. For our membrane, about 35 cm’, one can easily show that the theoretical ratio d&/d& is about
0376-7388/85/$03.30
0 1985
Elsevier
Science
Publishers
B.V.
1.02 for each layer. The thickness of the layer was measured during the deposition process with a piezoelectric thin film thickness monitor. The quartz crystal of the monitor was located next to the edge of the exposed area. The overall accuracy of the monitor was 2%. Thus one may consider that the metal layer thickness can be determined with an accuracy of 4%. In order to deposit 0.05 pm of Pd, Ni or Cu, the deposition process takes 3, 5 or 0.5 minutes, respectively, with the PET membrane heated up to 80°C. The layers had good polymer adherence and 5 + 0.3, 3 f 0.2 and 0.5 + 0.03 Q-cm electric resistivity for Pd, Ni and Cu, respectively. These values are very large in comparison to those of bulk Pd, Ni and Cu, which are 10.5 X lO-‘j, ‘7 X 10e6 and 1.7 X 10s6 Q-cm respectively [l]. The SMs showed no major defects under 800X optical magnification. In examination of the surface of the layers by scanning electron microscopy (with up to 30,000X magnification) fine cracks, with a width less than 0.1 pm, and defects smaller than 10e4 mm2 were found. These defects probably appear during the cooling of the SM as a result of a difference between the contraction of the PET and of the metal layer. Unfortunately, by choosing different rates of deposition, one could only reduce the number of the defects but not eliminate them. A rough estimate of their density is 103lo4 defects/cm’. Inspecting again the SMs after the experimental runs, one finds that this density increases slightly, probably due to a small pressure distortion which appears during the runs. The uniformity of the obtained layers was determined by measuring, with a Zeiss-made Abbe comparator, the relative light transmittancy, P, of about 30 points from the SM’s surface. In agreement with our assumption that the layers are thicker at the center than at the edge of the SM, the transmittancy was found to be larger at the edge, Fe, than at the center, Id, of the SM. The maximum relative transmittancy ratio re/rd was 1.06, in fair agreement with the theoretical value, 2(d&/d&) = 1.04. Therefore one may consider that a reproductibility better than 10% can be achieved by preparing the SMs by this method. 1.2. Measurement
of permeability
coefficients
Permeability coefficients were determined by means of the steady-state diffusion method [2] . The apparatus, experimental procedures and data analysis were similar to those described in detail in Refs. [3] and [4]. The SM was clamped between the two halves of a diffusion cell which were held together by means of six bolts. The SM was supported on a stainless steel gauze. The cell was connected to the system via metal cones and Pyrex sockets which were sealed with Rhodorsil silicone rubber (RhbnePoulenc). The cell was completely immersed in a thermostat bath controlled to + O.l”C. The high pressure end of the cell was coupled to a lOOO-cm3 reservoir of gas, the pressure of which could be read on a Hg manometer. The low pressure end was joined to a calibrated volume, V = 32 + 0.4 cm3, and a silicone oil vacuometer. The whole system was evacuated and then
299
degassed for 8 hr with the cell maintained at 50°C. The low pressure end was closed off periodically and the rate of the pressure build up measured to check for completeness of degassing and absence of leaks. When the background rate of pressure rise was negligibly small, gas was admitted from the reservoir in the high pressure end, and the pressure rise at the low pressure end was measured with the silicone oil vacuometer. The pressure readings were made with a cathetometer with an accuracy of f 0.05 mm. The pressure, PI(i) in the downstream volume never exceeded 0.5% of the upstream pressure, pz(i); this last did not change detectably during the period of measurements. The ambient temperature around the downstream volume, which was not immersed in the bath, was recorded during each run. The downstream volume, V, was calibrated after each run by gas expansion from a precalibrated volume and pressure measurement with a precision WIKA vacuometer. Corrections were made to take into account the fact that only a part of this volume was at bath temperature. The downstream volume increased slightly up to 5%, when at a constant temperature of T = 5O”C, the upstream pressure was lowered from 70 to 30 cmHg, or when, at a constant upstream pressure of 70 cmHg, the bath temperature was lowered from 50 to 10°C. This is probably the result of a small mebrane distortion which the metal gauze could not remove. All the gases were at least 99.9% pure except COz which was 99% pure. Graphs of pressure rise in the downstream volume against time were plotted for each run. The slope of the linear portion, dp(ij/dt, was calculated, with a mean least squares fit, and used to evaluate the permeation coefficient of the SM for the gas i as follows [4] :
dN)
PsM(i) = dt
VdsM -
273 -
A Ap(i)
T
(1)
Here V is the downstream volume; ClsMis the thickness of the SM, 30.1 f 2 pm; A is the area of the SM, 32.2 + 0.3 cm2; Ap(i) the gas differential pressure (* 0.1 cmHg), Ap(i) = p,(i)-pI(i)=p2(i) because p2(i)Spl(i); and T is the bath temperature. For every reported permeability coefficient at least five runs were made. The permeabilities of the bare PET membranes were determined under identical conditions. The relative deviations of these average permeabilities were less than 5%. Studying a series of five Ni-PET-Ni membranes under identical conditions, the obtained results were within a range of 10%. 2. Phenomenology
of gas permeation through a sandwich membrane
Gas permeation through a nonporous membrane is a complex process which occurs after the application of gas pressure at one interface and results in the following sequence of events: (a) sorption of the gas into the membrane at that interface, (b) diffusion of the gas in and through the membrane
300
and (c) desorption of the gas at the opposite interface. Thus the permeation process is controlled by reactions on the entry and releasing interfaces and gas diffusion through the membrane. When the specimen is thick and the rate of the reactions on the interfaces are rapid relative to the diffusion rate, the permeation process becomes diffusion controlled. On the other hand, when the specimen is thin, the contribution of diffusion through the specimen becomes negligible and permeation becomes sorption controlled [ 51. In our case the metal layers of the SMs behave as thin membranes and the PET layer as a thick membrane. The process of permeation of a gas through an SM is complicated by the fact that the three main events listed above must occur for each layer of the SM, and that the gas also diffuses through defects in the metal layers. Let us try to qualitatively describe this process. Permeation into the metal layer In order to permeate through a metal membrane, a gas must first dissociate into atoms or ions at the upstream interface and then, in this form, dissolve and diffuse in and through the membrane [6]. As a result, the steadystate permeation rate is proportional to dm In our case, at T = 5O”C, and p*(H,) ranging from 30 to 70 cmHg, only H2 dissolves in Pd [7] and to a certain extent in Ni [8]. In our experimental conditions the other gases do not dissolve in Pd, Ni or Cu (including H, in Cu). Thus one may assume that because the permeation process through the thin metal layers is sorption controlled, only Hz can permeate through the Pd and Ni layers themselves, while all other gases studied diffuse in molecular (or monoatomic) form through the defects in the metal layers only (including Hz through Cu). The permeability of Pd and Ni for Hz at T = 50°C and P,(H,) within 30-70 cmHg, is P&H,) = 26 barrer [9] and PNi(H,) = 2.0 X 10m3 barrer [8], where 1 barrer= lo-” X cm3-cm/cm2-set-cmHg [lo]. For Cu, only an estimated value PcU(H2) = 2.0 X lo-’ barrer, extrapolated from higher temperature solubility [ll] and diffusion [ 121 data, may be given. Emerging from the defects of the metal layers, He, Hz, CO?, N2,Ar and Kr dissolve in molecular form in the PET membrane. The diffusion rate through these defects is proportional to Ap(i) and to the total area of the defects. Moreover, in the case of H2 permeation through the Pd and Ni layers, released hydrogen atoms associate spontaneously to form H2 molecules which also dissolve in the PET membrane. Permeation through PET The rate of permeation of a gas through a polymer depends on both the nature of the gas and of the polymer, and its pressure and temperature dependence can be markedly affected by the glass transition of the polymer [ 131. In our case, in the range O-50’%, PET is a glassy polymer [ 141. Therefore the rotational motion of the polymer chains is hindered, and not sufficiently rapid to completely homogenize the penetrant gas molecule’s en-
301
vironment. Thus, this molecule can sit in “holes” of different sizes and with very different intrinsic mobilities. The process of gas solution is necessarily affected by the presence of these “holes” in the polymer matrix, and therefore the permeation behaviour of gases, which depends both on solution and diffusion, will differ considerably above and below the glass transition of PET (Tg = 30°C). The solution of gases in glassy PET can be explained satisfactorily, by means of the dual-sorption model [15] for 2’ > T, (T, being the critical temperature of the penetrant gas). Assuming that the penetrant gas dissolves in the polymer in two thermodynamically distinct populations: (i) ordinary dissolved molecules as at above T,, and (ii) dissolved in the preexisting “holes” of the polymer matrix, and taking into account a partial immobilization of the latter species, a pressure-dependent permeability coefficient can be written [15] : P = k,, &, [l+ FKI(1
+ y)]
(2)
where K = c;I b/k,,, y = bp, k, is the solubility coefficient in the Henry’s law limit, Do = Do = constant is the diffusion coefficient of the ordinary dissolved molecules, F = D,/D,, D, is the diffusion coefficient of the molecules dissolved in the “holes”, cn is the “hole saturation constant”, b is the “hole affinity constant” and p = p2(i) is the upstream pressure. According to eqn. (2), increasing p2(i) at constant T will decrease the permeability of PET. It was found that when pz(He) increases from 0 to 20 atm, the permeability of PET for He, P&He) decreases 6% [ 151. Hence one may accept that Prxr(He) is not pressure dependent when p,(He) increases from 30 to 70 cmHg. Similarly, like He, Hz interacts weakly with PET and also dissolves in small amounts in it [16]. Hence P&HZ) is not pressure dependent in our experimental p,(H,) pressure range. Per-meation through the exit me taf layer After emerging, in molecular form, from the PET membrane, all gases studied which do not dissolve in the exit metal layer of the SM diffuse through its defects and finally enter into the downstream space. Moreover, for the Pd-PET-Pd and Ni-PET-Ni membranes, when the Hz molecules are able to dissociate again at the PET-Pd or PET-Ni interface, they dissolve and therefore diffuse through the Pd or Ni layer. It would be interesting to learn if at this interface the catalytic activity of Pd and Ni in close contact with the polymer is diminished or not. If this is the case, the amount of Hz which dissolves in the Pd or Ni layer will be less, and thus the rate of Hz permeation through the exit layer will also be reduced. In the end, these hydrogen atoms are also released from the SM, they associate spontaneously to form Hz molecules and thereafter enter the downstream space. With this picture in mind let us analyze our experimental data,
302
3. Results and discussion The permeability coefficients, P&i), of the SMs and of bare PET, PPET(i) determined at T = 50°C and Ap(i) z 70 cmHg are given in Table 1. TABLE 1 Permeability coefficients Gas
H* He CO, Ar N, Kr
of bare and metallized PET membranes for simple gases at 50°C
Membranes PET
Pd-PET-Pd
PPET (X 10-2
PSM
barrer)
barrer)
131 194 32.3 20 12.1 8.5
124 76 23 2.1 1.5
1 barrer = lo-”
pSM
ASM
(X 1o-2
f 4 +5 + 1.5 +l f 0.7 f 0.3
Ni-PET-Ni
+4 k3 f 1.2 * 0.3 + 0.1
w
1.33 0.77 4.80 0.66 0.74
)
(x 10-Z barrer)
75 58 20.3 1.5 1.2 0.46
+3 +2 f 1 f 0.1 f 0.1 + 0.1
Cu-PET--h ASM
&iM
(g”“)
(x 10-Z barrer)
0.81 0.60 4.20 0.48 0.57 0.44
56 67 20.1 2.5 1.6
k2 i3 +1 i: 0.2 + 0.1 _
ASM
(g”‘) 0.60 0.69 4.20 0.78 0.70 -
X cm3-cm/cm2-set-cmHg
One can observe from Table 1 that metallizing the PET membranes reduced its permeability for all gases studied. The decrease is dependent both on the nature of the deposited metal and on the penetrant gas. One can also observe that P&I’) for a given gas, i, is about the same for each SM, except for the cases of H, and the Pd-PET-Pd and Ni-PET-Ni membranes, which will be discussed separately. Roughly, P&I’) for a gas which diffuses only through the defects of the metal layers can be given as: hM@)
=
PPET(i)
&
&M
where Waiis the mass of gas i and As, is a constant proportional to the gas conductance of the SM’s metal layer. The values of ASM are given in Table 1. Equation (3) seems to indicate that the metal layers act as a microporous barrier through which the gases exhibit a molecular flow [17] . Indeed, at T = 50°C and p2(i) = 70 cmHg, the mean free path of the gases studied is within the range 0.2-0.06 pm [ 181. Hence, one can consider that the diffusion through the defects, which are less than 0.1 pm large, takes place in a regime of molecular flow [19]. The larger values of ASM for CO* in all SMs and for Hz in Pd-PET-Pd and Ni-PET-Ni are probably the result of the larger solubility of COz in PET [16], and the result of the additional flow of Hz through the Pd and Ni layers themselves; respectively. From Table 1 one also finds that PsM(H2) for the Pd-PET-Pd membrane
303
is nearly equal to that of bare PET, P&Hz), Cu-PET-Cu membranes its value is smaller. follows. Assuming that the SM behaves as a can calculate its theoretical average intrinsic to eqn. (4) [ 201: d SM
while for the Ni-PET-Ni and This fact may be explained as laminated membrane [20], one permeability Ps,(H2) according
dk =;
k=l
P,,(b)
(4)
&d-b)
where Pk(H,) is the permeability for Hz of the &h layer of thickness dk and d SM = c:dk the thickness of the SM. TO evaluate PsM(H,) one must know the value of each dk, and from independent measurements the individual permeabilities, Pk(H,), of each component of the SM. Thus for the Pd-PET-Pd membrane one has: -‘%M P&Hz)
=
drxr
dPd+ PPd(Hz)
dpd
PPET(Hz)
+
bd
(5) (6)
The value obtained from eqn. (5) forTsM(H2), see Table 2, is only a little larger than that of PPET(H2). This result is not unexpected. It was also shown that in a laminated membraneFSM(HZ) satisfies the following relation [20] :
[J’tzW)Imin
GFsM(Hz)
G
max
[&(Hz)l
Therefore the evaluation from eqn. (5) is in agreement with eqn. (6). From eqn. (5) it is also seen that FsM(H2) for the Pd-PET-Pd membrane will be larger than PpxT(H2) with an amount pro_portional to the ratio dpd/dPET. In our case this ratio is small and thus the P.&H,) will be only slightly larger than PPET(H2). In other words, this means that, although the permeability of Pd for H2 is much larger than that of PET, because the Pd layers are very thin their contribution to the increase of the SM’s permeability for H2 is negligible. TABLE 2 Calculated and experimental membranes for H, at 50°C
permeability
coefficients
of Pd-PET-Pd
and Ni-PET-Ni
Membranes PET
Pd-PET-Pd
Ni-PET-Ni
PPET( 5 ) (x 10-z
P.sM(%)
~sM(H,)
PsM(%)
barrer)
(x lo-* barrer)
(X 1o-2 barrer)
(x 1o-7 barrer)
(X lo-* barrer)
131.0 f 4
124.0 f 4
131.4 c 4
75.0 f 3
42.0 + 3
1 barrer = IO-“’ X cm3-cm/cm*-set-cmHg.
%M(&)
304
In the case of the Ni-PET-Ni membrane, because the PNi(H,) is smaller than &,&HZ), according to eqn. (6), F&HZ) calculated with eqn. (4) must be smaller than Pi&HZ). The calculated PsM(HZ) for the above-mentioned membrane and the experimentally found P&H,) given in Table 2 are in agreement with this requirement. However, the experimental value is larger than that calculated. To explain this fact one must consider also the Hz which diffuses through the defects of the Ni layers. On the other hand, because Hz diffuses only through the defects of the Cu layers, the Cu-PET-Cu membrane cannot be discussed in a similar manner. In this case, the decrease in permeability for H2 is the largest. One can see from Fig. 1 that P&H,) increases with AJJ(H~) increasing from about 30 to 70 cmHg, while for the bare PET P&Hz) does not vary with Ap(H,).
-23 NkEi=Ni
* ,,_.,_.o_‘.....
.O.”
,. ,...,... O.‘.“”
4
s
____-o_o-__--o-----m----~“_pET_cu
Fig.
1. Differential
Apq
- -
lcmHg)
gas pressure
dependence
of permeability
of mettillized
PET
for H, at
50°C. I
,
4.9- 02
3.0
Fig. 2. Temperature
I 32
5.4G.2
3.4 1/T x lo3 (K-l]
dependence
3.6
of permeabilities
of metallized
PET
for He and H,.
305
As mentioned earlier, the permeation rate of Hz through the Pd and Ni layers is proportional to (Ap(H*)‘.’ while this rate through the PET membrane and through the defects of the layers it is proportional to Ap(H2). Thus, at a given temperature T = 50°C the rate of pressure build up in the downstream volume, dp(H,)/dt, will be proportional to (Ap(H#, where 0.5 Q n < 1, because, as shown, the effect of the Pd or Ni layer is small. Thus, by evaluating P&H,) from eqn. (1) for the Pd-PET-Pd and NiPET-Ni membranes, for an increasing Ap(H2) = p,(H,), P&H?) will slightly decrease. Hence, in order to explain the results plotted in Fig. 2, it must be that P&H,) increases with Ap(H2) because the total area of the metal layer defects increases with Ap(H,). This supposition may be validated by the similar behaviour of the Cu-PET-Cu membrane where Hz diffuses only through the defects of the Cu layers. The temperature dependence of PsM for He and Hz, in the range of O50°C was also determined. The results and the activation energies, E,, of the permeation process are presented in Fig. 2. It was found that by depositing the metal layers, E, for He and Hz increased slightly for each SM by about the same amount. This is the result of the fact that the permeation rates of H, and He increase more with temperature through a SM than through a bare PET membrane. 4. Conclusions A polymer-based membrane with different permeation behaviour for simple gases has been prepared. It was proved that the thin metal layers deposited on each side of a PET membrane act as gas-selective barriers and determine modifications of the permeability properties of the membranes. Theoretically, all gases which are unable to dissolve in metals cannot permeate through them. The fact that He, CO*, Ar, Nz and Kr still permeate through our SMs shows possibly the level of defects in the metal layers. The differences existing between the values of PSMfor He, COz, Ar or Nz and the corresponding A,, constants, given in Table 1, are probably due to different defect densities in the different metal layers. Thus the Ni-PET-Ni membrane seems to have the highest-quality layers while the Pd-PET-Pd membrane has the worst. Moreover, in the case of Hz permeation through these two membranes, it was proved that Hz also permeates the Pd or Ni layers of the SM themselves. An interesting result of this fact is that Hz permeates faster than He through these membranes, opposite to the case of bare PET where, in agreement with the general behaviour of glassy polymers, He permeates faster than Hz [ 211. At 50” and p*(H,) = 70 cmHg, the permeability of Cu for Hz is too small to produce a similar effect and so He permeates faster than H, through the Cu-PET---&r membrane. Undoubtedly, introducing the notion of defects in the deposited metal layers and postulating simultaneous transport through the layers and through their defects, one can explain virtually any of the results obtained. Unfor-
306
tunately we were not able to prepare metal layers without defects. We also believe that it would be hard to prevent the occurrence of these defects during the preparation and study of the SMs. Therefore, in attempting to formulate a picture of the permeation process of a gas through a SM, one must consider their contribution to the overall process. We feel that an important fact is that, by this method, an increase of more than one order or magnitude of membrane selectivity for H2 is obtainable. Acknowledgements The authors are indebted to Dr. R. Bucur for useful discussions during the preparation of the paper and to Dr. R. Campeanu for helping to draw up the paper.
References
6 7
8 9 10 11
12
13 14
15
C. Kittel, Introduction to Solid State Physics, 4th edn., John Wiley, New York, 1971, p. 283. J. Crank, The Mathematics of Diffusion, Clarendon Press, Oxford, 1956, p. 42. R. Hughes, AWRE Report No. O-17/62, Aldermaston, Berkshire, October 1962. F.P. Evans, R. Gibson,C.G. Hutcheson and P.M.S. Jones, AWRE Report No. O-45/63, Aldermaston, Berkshire, October 1963. Y. Hayashi and A. Tahara, Surface processes in permeation of hydrogen in metals, in: Proceedings of the Third International Congress on Hydrogen and Materials, Paris, 1982, Chap. 3, p. 257. R.M. Barrer, Diffusion in and through Solids, Cambridge University Press, Cambridge, 1951, p. 144. B. Siegel and G.C. Libowitz., The covalent hydrides and hydrides of the group V to VII transition metals, in: W. Mueller, J.B. Blackledge and G.C. Libowitz (Eds.), Metal Hydrides, Academic Press, New York, 1968, p. 545. M.R. Loutham, Jr., J.A. Donovan and G.R. Caskey Jr., Hydrogen diffusion and trapping in Ni, Acta Metall., 23 (1975) 745. G. Bohmholdt and E. Wick@, Diffusion von H und D in Pd und Pd-Legierungen, Z. Phys. Chem. N.F., 56 (1967) 133. V.T. Stannett, W.J. Koros, D.R. Paul, H.K. Lonsdale and R.W. Baker, Recent developments in membrane science and technology, Adv. Polym. Sci., 32 (1979) 69. K.H. Lieser and H. Witte, Solubility of hydrogen in alloys. I. Measuring method and examination of the systems MgCu,-MgZn, and MgNi,-MgZn,, Z. Phys. Chem. (Leipzig), 202 (1954) 321. J. Viilkl and G. Alefeid, Hydrogen diffusion in metals, in: A.S. Nowik and J.J. Burton (Eds.), Diffusion in Solids: Recent Developments, Academic Press, New York, 1975, p. 231. S.A. Stern and H.L. Frisch, The selective permeation of gases through polymers, Ann. Rev. Mater. Sci., 11 (1981) 523. A.S. Michaels and H.J. Bixler, Membrane permeation: Theory and practice, in: E.S. Perry (Ed.), Progress in Separation and Purification, Interscience, New York, 1968, p. 143. W.J. Koros, R.T. Chern, V. Stannett and H.B. Hopfenberg, A model for permeation of mixed gases and vapours in glassy polymers, J. Polym. Sci., Polym. Phys. Ed., 19 (1981) 1513.
307 16 17 18 19 20 21
A.F. Stancell, Diffusion through polymers, in: A.V. Tobolsky and H. Mark (Eds.), Polymer Science and Materials, John Wiley, New York, 1971, p. 247. P.C. Carman, Flow of Gases through Porous Media, Butterworths, London, 1956, p. 62. J.O. Hirschfelder, C.F. Curtis and R.B. Byrd, Molecular Theory of Gases and Liquids, John Wiley, New York, 1954, p. 14. A. Roth, Vacuum Technology, and revised edn., North-Holland, Amsterdam, 1982, p. 64. C.H. Lee, Permeation properties in laminated membranes, Sep. Sci., 9 (1974) 479. , V. Stannett, Simple gases, in: J. Crank and G.S. Park (Eds.), Diffusion in Polymers, Academic Press, New York, 1968, p. 43.
Science, 24 (1985) 297-307 Publishers B.V., Amsterdam - Printed
Science
PERMEATION MEMBRANES
P. MERCEA,
L. MURESAN
Institute of Isotopic (Romania) (Received
July
OF GASES THROUGH
METALLIZED
POLYMER
and V. MECEA
and Molecular
11,1984;
297 in The Netherlands
accepted
Technology in revised
R-3400 form
June
Cluj-Napoca
5, P.O. Box 700
7,1985)
Summary The permeation of He, H,, CO,, Ar, N, and Kr at 50°C through polyethyleneterephthalate, PET, membranes metallized with Pd, Ni and Cu was studied. It was found that metallizing a PET membrane changed its permeability for the gases studied, and that the permeability for H, varied slightly with differing H, pressure. In the range of 0-50°C the temperature dependence of the permeability for He and H, was determined. The results obtained were interpreted by assuming that the permeation of all gases, including H,, through the metal layers of the membranes takes place by diffusion through fine defects which exist in their structure and, moreover, that H, also permeates through the Pd and Ni layers themselves. An important point is that by this method an increase of up to an order of magnitude of the membrane selectivity for H, was obtained.
1. Experimental method 1.1 Preparation of metallized PET membranes The metallized membranes (“sandwich membranes”, SMs) were obtained by vacuum depositing Pd, Ni, or Cu - of at least 99.9% purity - consecutively on each side of a PET membrane, produced by an inland factory under BASF licence. The thickness of the bare PET was measured with a micrometer - with an accuracy of * 2 pm - and found to be 30 of: 2 pm. A PET disk, with a diameter of about 100 mm, was degreased and then degassed for several hours in preliminary vacuum at 80°C. Afterwards the PET membrane was mounted in a circular stainless steel mask and introduced in a Hochvacuum Dresden vacuum deposition system. The deposition of the metal layer was performed, under 5 X 10m6 Torr, by evaporating Pd and Ni from a W basket coated with alumina and Cu from a MO boat. The metal vapour source was made to ensure (as far as possible) a constant vapour density in the solid angle which delimitates the PET membrane. The distance from the vapour source to the center of the plane PET membrane was rc = 25 cm. Hence the deposited metal layer will be thicker at the center, d&, than at the which had an exposed area of edge, d& , of the SM. For our membrane, about 35 cm’, one can easily show that the theoretical ratio d&/d& is about
0376-7388/85/$03.30
0 1985
Elsevier
Science
Publishers
B.V.
1.02 for each layer. The thickness of the layer was measured during the deposition process with a piezoelectric thin film thickness monitor. The quartz crystal of the monitor was located next to the edge of the exposed area. The overall accuracy of the monitor was 2%. Thus one may consider that the metal layer thickness can be determined with an accuracy of 4%. In order to deposit 0.05 pm of Pd, Ni or Cu, the deposition process takes 3, 5 or 0.5 minutes, respectively, with the PET membrane heated up to 80°C. The layers had good polymer adherence and 5 + 0.3, 3 f 0.2 and 0.5 + 0.03 Q-cm electric resistivity for Pd, Ni and Cu, respectively. These values are very large in comparison to those of bulk Pd, Ni and Cu, which are 10.5 X lO-‘j, ‘7 X 10e6 and 1.7 X 10s6 Q-cm respectively [l]. The SMs showed no major defects under 800X optical magnification. In examination of the surface of the layers by scanning electron microscopy (with up to 30,000X magnification) fine cracks, with a width less than 0.1 pm, and defects smaller than 10e4 mm2 were found. These defects probably appear during the cooling of the SM as a result of a difference between the contraction of the PET and of the metal layer. Unfortunately, by choosing different rates of deposition, one could only reduce the number of the defects but not eliminate them. A rough estimate of their density is 103lo4 defects/cm’. Inspecting again the SMs after the experimental runs, one finds that this density increases slightly, probably due to a small pressure distortion which appears during the runs. The uniformity of the obtained layers was determined by measuring, with a Zeiss-made Abbe comparator, the relative light transmittancy, P, of about 30 points from the SM’s surface. In agreement with our assumption that the layers are thicker at the center than at the edge of the SM, the transmittancy was found to be larger at the edge, Fe, than at the center, Id, of the SM. The maximum relative transmittancy ratio re/rd was 1.06, in fair agreement with the theoretical value, 2(d&/d&) = 1.04. Therefore one may consider that a reproductibility better than 10% can be achieved by preparing the SMs by this method. 1.2. Measurement
of permeability
coefficients
Permeability coefficients were determined by means of the steady-state diffusion method [2] . The apparatus, experimental procedures and data analysis were similar to those described in detail in Refs. [3] and [4]. The SM was clamped between the two halves of a diffusion cell which were held together by means of six bolts. The SM was supported on a stainless steel gauze. The cell was connected to the system via metal cones and Pyrex sockets which were sealed with Rhodorsil silicone rubber (RhbnePoulenc). The cell was completely immersed in a thermostat bath controlled to + O.l”C. The high pressure end of the cell was coupled to a lOOO-cm3 reservoir of gas, the pressure of which could be read on a Hg manometer. The low pressure end was joined to a calibrated volume, V = 32 + 0.4 cm3, and a silicone oil vacuometer. The whole system was evacuated and then
299
degassed for 8 hr with the cell maintained at 50°C. The low pressure end was closed off periodically and the rate of the pressure build up measured to check for completeness of degassing and absence of leaks. When the background rate of pressure rise was negligibly small, gas was admitted from the reservoir in the high pressure end, and the pressure rise at the low pressure end was measured with the silicone oil vacuometer. The pressure readings were made with a cathetometer with an accuracy of f 0.05 mm. The pressure, PI(i) in the downstream volume never exceeded 0.5% of the upstream pressure, pz(i); this last did not change detectably during the period of measurements. The ambient temperature around the downstream volume, which was not immersed in the bath, was recorded during each run. The downstream volume, V, was calibrated after each run by gas expansion from a precalibrated volume and pressure measurement with a precision WIKA vacuometer. Corrections were made to take into account the fact that only a part of this volume was at bath temperature. The downstream volume increased slightly up to 5%, when at a constant temperature of T = 5O”C, the upstream pressure was lowered from 70 to 30 cmHg, or when, at a constant upstream pressure of 70 cmHg, the bath temperature was lowered from 50 to 10°C. This is probably the result of a small mebrane distortion which the metal gauze could not remove. All the gases were at least 99.9% pure except COz which was 99% pure. Graphs of pressure rise in the downstream volume against time were plotted for each run. The slope of the linear portion, dp(ij/dt, was calculated, with a mean least squares fit, and used to evaluate the permeation coefficient of the SM for the gas i as follows [4] :
dN)
PsM(i) = dt
VdsM -
273 -
A Ap(i)
T
(1)
Here V is the downstream volume; ClsMis the thickness of the SM, 30.1 f 2 pm; A is the area of the SM, 32.2 + 0.3 cm2; Ap(i) the gas differential pressure (* 0.1 cmHg), Ap(i) = p,(i)-pI(i)=p2(i) because p2(i)Spl(i); and T is the bath temperature. For every reported permeability coefficient at least five runs were made. The permeabilities of the bare PET membranes were determined under identical conditions. The relative deviations of these average permeabilities were less than 5%. Studying a series of five Ni-PET-Ni membranes under identical conditions, the obtained results were within a range of 10%. 2. Phenomenology
of gas permeation through a sandwich membrane
Gas permeation through a nonporous membrane is a complex process which occurs after the application of gas pressure at one interface and results in the following sequence of events: (a) sorption of the gas into the membrane at that interface, (b) diffusion of the gas in and through the membrane
300
and (c) desorption of the gas at the opposite interface. Thus the permeation process is controlled by reactions on the entry and releasing interfaces and gas diffusion through the membrane. When the specimen is thick and the rate of the reactions on the interfaces are rapid relative to the diffusion rate, the permeation process becomes diffusion controlled. On the other hand, when the specimen is thin, the contribution of diffusion through the specimen becomes negligible and permeation becomes sorption controlled [ 51. In our case the metal layers of the SMs behave as thin membranes and the PET layer as a thick membrane. The process of permeation of a gas through an SM is complicated by the fact that the three main events listed above must occur for each layer of the SM, and that the gas also diffuses through defects in the metal layers. Let us try to qualitatively describe this process. Permeation into the metal layer In order to permeate through a metal membrane, a gas must first dissociate into atoms or ions at the upstream interface and then, in this form, dissolve and diffuse in and through the membrane [6]. As a result, the steadystate permeation rate is proportional to dm In our case, at T = 5O”C, and p*(H,) ranging from 30 to 70 cmHg, only H2 dissolves in Pd [7] and to a certain extent in Ni [8]. In our experimental conditions the other gases do not dissolve in Pd, Ni or Cu (including H, in Cu). Thus one may assume that because the permeation process through the thin metal layers is sorption controlled, only Hz can permeate through the Pd and Ni layers themselves, while all other gases studied diffuse in molecular (or monoatomic) form through the defects in the metal layers only (including Hz through Cu). The permeability of Pd and Ni for Hz at T = 50°C and P,(H,) within 30-70 cmHg, is P&H,) = 26 barrer [9] and PNi(H,) = 2.0 X 10m3 barrer [8], where 1 barrer= lo-” X cm3-cm/cm2-set-cmHg [lo]. For Cu, only an estimated value PcU(H2) = 2.0 X lo-’ barrer, extrapolated from higher temperature solubility [ll] and diffusion [ 121 data, may be given. Emerging from the defects of the metal layers, He, Hz, CO?, N2,Ar and Kr dissolve in molecular form in the PET membrane. The diffusion rate through these defects is proportional to Ap(i) and to the total area of the defects. Moreover, in the case of H2 permeation through the Pd and Ni layers, released hydrogen atoms associate spontaneously to form H2 molecules which also dissolve in the PET membrane. Permeation through PET The rate of permeation of a gas through a polymer depends on both the nature of the gas and of the polymer, and its pressure and temperature dependence can be markedly affected by the glass transition of the polymer [ 131. In our case, in the range O-50’%, PET is a glassy polymer [ 141. Therefore the rotational motion of the polymer chains is hindered, and not sufficiently rapid to completely homogenize the penetrant gas molecule’s en-
301
vironment. Thus, this molecule can sit in “holes” of different sizes and with very different intrinsic mobilities. The process of gas solution is necessarily affected by the presence of these “holes” in the polymer matrix, and therefore the permeation behaviour of gases, which depends both on solution and diffusion, will differ considerably above and below the glass transition of PET (Tg = 30°C). The solution of gases in glassy PET can be explained satisfactorily, by means of the dual-sorption model [15] for 2’ > T, (T, being the critical temperature of the penetrant gas). Assuming that the penetrant gas dissolves in the polymer in two thermodynamically distinct populations: (i) ordinary dissolved molecules as at above T,, and (ii) dissolved in the preexisting “holes” of the polymer matrix, and taking into account a partial immobilization of the latter species, a pressure-dependent permeability coefficient can be written [15] : P = k,, &, [l+ FKI(1
+ y)]
(2)
where K = c;I b/k,,, y = bp, k, is the solubility coefficient in the Henry’s law limit, Do = Do = constant is the diffusion coefficient of the ordinary dissolved molecules, F = D,/D,, D, is the diffusion coefficient of the molecules dissolved in the “holes”, cn is the “hole saturation constant”, b is the “hole affinity constant” and p = p2(i) is the upstream pressure. According to eqn. (2), increasing p2(i) at constant T will decrease the permeability of PET. It was found that when pz(He) increases from 0 to 20 atm, the permeability of PET for He, P&He) decreases 6% [ 151. Hence one may accept that Prxr(He) is not pressure dependent when p,(He) increases from 30 to 70 cmHg. Similarly, like He, Hz interacts weakly with PET and also dissolves in small amounts in it [16]. Hence P&HZ) is not pressure dependent in our experimental p,(H,) pressure range. Per-meation through the exit me taf layer After emerging, in molecular form, from the PET membrane, all gases studied which do not dissolve in the exit metal layer of the SM diffuse through its defects and finally enter into the downstream space. Moreover, for the Pd-PET-Pd and Ni-PET-Ni membranes, when the Hz molecules are able to dissociate again at the PET-Pd or PET-Ni interface, they dissolve and therefore diffuse through the Pd or Ni layer. It would be interesting to learn if at this interface the catalytic activity of Pd and Ni in close contact with the polymer is diminished or not. If this is the case, the amount of Hz which dissolves in the Pd or Ni layer will be less, and thus the rate of Hz permeation through the exit layer will also be reduced. In the end, these hydrogen atoms are also released from the SM, they associate spontaneously to form Hz molecules and thereafter enter the downstream space. With this picture in mind let us analyze our experimental data,
302
3. Results and discussion The permeability coefficients, P&i), of the SMs and of bare PET, PPET(i) determined at T = 50°C and Ap(i) z 70 cmHg are given in Table 1. TABLE 1 Permeability coefficients Gas
H* He CO, Ar N, Kr
of bare and metallized PET membranes for simple gases at 50°C
Membranes PET
Pd-PET-Pd
PPET (X 10-2
PSM
barrer)
barrer)
131 194 32.3 20 12.1 8.5
124 76 23 2.1 1.5
1 barrer = lo-”
pSM
ASM
(X 1o-2
f 4 +5 + 1.5 +l f 0.7 f 0.3
Ni-PET-Ni
+4 k3 f 1.2 * 0.3 + 0.1
w
1.33 0.77 4.80 0.66 0.74
)
(x 10-Z barrer)
75 58 20.3 1.5 1.2 0.46
+3 +2 f 1 f 0.1 f 0.1 + 0.1
Cu-PET--h ASM
&iM
(g”“)
(x 10-Z barrer)
0.81 0.60 4.20 0.48 0.57 0.44
56 67 20.1 2.5 1.6
k2 i3 +1 i: 0.2 + 0.1 _
ASM
(g”‘) 0.60 0.69 4.20 0.78 0.70 -
X cm3-cm/cm2-set-cmHg
One can observe from Table 1 that metallizing the PET membranes reduced its permeability for all gases studied. The decrease is dependent both on the nature of the deposited metal and on the penetrant gas. One can also observe that P&I’) for a given gas, i, is about the same for each SM, except for the cases of H, and the Pd-PET-Pd and Ni-PET-Ni membranes, which will be discussed separately. Roughly, P&I’) for a gas which diffuses only through the defects of the metal layers can be given as: hM@)
=
PPET(i)
&
&M
where Waiis the mass of gas i and As, is a constant proportional to the gas conductance of the SM’s metal layer. The values of ASM are given in Table 1. Equation (3) seems to indicate that the metal layers act as a microporous barrier through which the gases exhibit a molecular flow [17] . Indeed, at T = 50°C and p2(i) = 70 cmHg, the mean free path of the gases studied is within the range 0.2-0.06 pm [ 181. Hence, one can consider that the diffusion through the defects, which are less than 0.1 pm large, takes place in a regime of molecular flow [19]. The larger values of ASM for CO* in all SMs and for Hz in Pd-PET-Pd and Ni-PET-Ni are probably the result of the larger solubility of COz in PET [16], and the result of the additional flow of Hz through the Pd and Ni layers themselves; respectively. From Table 1 one also finds that PsM(H2) for the Pd-PET-Pd membrane
303
is nearly equal to that of bare PET, P&Hz), Cu-PET-Cu membranes its value is smaller. follows. Assuming that the SM behaves as a can calculate its theoretical average intrinsic to eqn. (4) [ 201: d SM
while for the Ni-PET-Ni and This fact may be explained as laminated membrane [20], one permeability Ps,(H2) according
dk =;
k=l
P,,(b)
(4)
&d-b)
where Pk(H,) is the permeability for Hz of the &h layer of thickness dk and d SM = c:dk the thickness of the SM. TO evaluate PsM(H,) one must know the value of each dk, and from independent measurements the individual permeabilities, Pk(H,), of each component of the SM. Thus for the Pd-PET-Pd membrane one has: -‘%M P&Hz)
=
drxr
dPd+ PPd(Hz)
dpd
PPET(Hz)
+
bd
(5) (6)
The value obtained from eqn. (5) forTsM(H2), see Table 2, is only a little larger than that of PPET(H2). This result is not unexpected. It was also shown that in a laminated membraneFSM(HZ) satisfies the following relation [20] :
[J’tzW)Imin
GFsM(Hz)
G
max
[&(Hz)l
Therefore the evaluation from eqn. (5) is in agreement with eqn. (6). From eqn. (5) it is also seen that FsM(H2) for the Pd-PET-Pd membrane will be larger than PpxT(H2) with an amount pro_portional to the ratio dpd/dPET. In our case this ratio is small and thus the P.&H,) will be only slightly larger than PPET(H2). In other words, this means that, although the permeability of Pd for H2 is much larger than that of PET, because the Pd layers are very thin their contribution to the increase of the SM’s permeability for H2 is negligible. TABLE 2 Calculated and experimental membranes for H, at 50°C
permeability
coefficients
of Pd-PET-Pd
and Ni-PET-Ni
Membranes PET
Pd-PET-Pd
Ni-PET-Ni
PPET( 5 ) (x 10-z
P.sM(%)
~sM(H,)
PsM(%)
barrer)
(x lo-* barrer)
(X 1o-2 barrer)
(x 1o-7 barrer)
(X lo-* barrer)
131.0 f 4
124.0 f 4
131.4 c 4
75.0 f 3
42.0 + 3
1 barrer = IO-“’ X cm3-cm/cm*-set-cmHg.
%M(&)
304
In the case of the Ni-PET-Ni membrane, because the PNi(H,) is smaller than &,&HZ), according to eqn. (6), F&HZ) calculated with eqn. (4) must be smaller than Pi&HZ). The calculated PsM(HZ) for the above-mentioned membrane and the experimentally found P&H,) given in Table 2 are in agreement with this requirement. However, the experimental value is larger than that calculated. To explain this fact one must consider also the Hz which diffuses through the defects of the Ni layers. On the other hand, because Hz diffuses only through the defects of the Cu layers, the Cu-PET-Cu membrane cannot be discussed in a similar manner. In this case, the decrease in permeability for H2 is the largest. One can see from Fig. 1 that P&H,) increases with AJJ(H~) increasing from about 30 to 70 cmHg, while for the bare PET P&Hz) does not vary with Ap(H,).
-23 NkEi=Ni
* ,,_.,_.o_‘.....
.O.”
,. ,...,... O.‘.“”
4
s
____-o_o-__--o-----m----~“_pET_cu
Fig.
1. Differential
Apq
- -
lcmHg)
gas pressure
dependence
of permeability
of mettillized
PET
for H, at
50°C. I
,
4.9- 02
3.0
Fig. 2. Temperature
I 32
5.4G.2
3.4 1/T x lo3 (K-l]
dependence
3.6
of permeabilities
of metallized
PET
for He and H,.
305
As mentioned earlier, the permeation rate of Hz through the Pd and Ni layers is proportional to (Ap(H*)‘.’ while this rate through the PET membrane and through the defects of the layers it is proportional to Ap(H2). Thus, at a given temperature T = 50°C the rate of pressure build up in the downstream volume, dp(H,)/dt, will be proportional to (Ap(H#, where 0.5 Q n < 1, because, as shown, the effect of the Pd or Ni layer is small. Thus, by evaluating P&H,) from eqn. (1) for the Pd-PET-Pd and NiPET-Ni membranes, for an increasing Ap(H2) = p,(H,), P&H?) will slightly decrease. Hence, in order to explain the results plotted in Fig. 2, it must be that P&H,) increases with Ap(H2) because the total area of the metal layer defects increases with Ap(H,). This supposition may be validated by the similar behaviour of the Cu-PET-Cu membrane where Hz diffuses only through the defects of the Cu layers. The temperature dependence of PsM for He and Hz, in the range of O50°C was also determined. The results and the activation energies, E,, of the permeation process are presented in Fig. 2. It was found that by depositing the metal layers, E, for He and Hz increased slightly for each SM by about the same amount. This is the result of the fact that the permeation rates of H, and He increase more with temperature through a SM than through a bare PET membrane. 4. Conclusions A polymer-based membrane with different permeation behaviour for simple gases has been prepared. It was proved that the thin metal layers deposited on each side of a PET membrane act as gas-selective barriers and determine modifications of the permeability properties of the membranes. Theoretically, all gases which are unable to dissolve in metals cannot permeate through them. The fact that He, CO*, Ar, Nz and Kr still permeate through our SMs shows possibly the level of defects in the metal layers. The differences existing between the values of PSMfor He, COz, Ar or Nz and the corresponding A,, constants, given in Table 1, are probably due to different defect densities in the different metal layers. Thus the Ni-PET-Ni membrane seems to have the highest-quality layers while the Pd-PET-Pd membrane has the worst. Moreover, in the case of Hz permeation through these two membranes, it was proved that Hz also permeates the Pd or Ni layers of the SM themselves. An interesting result of this fact is that Hz permeates faster than He through these membranes, opposite to the case of bare PET where, in agreement with the general behaviour of glassy polymers, He permeates faster than Hz [ 211. At 50” and p*(H,) = 70 cmHg, the permeability of Cu for Hz is too small to produce a similar effect and so He permeates faster than H, through the Cu-PET---&r membrane. Undoubtedly, introducing the notion of defects in the deposited metal layers and postulating simultaneous transport through the layers and through their defects, one can explain virtually any of the results obtained. Unfor-
306
tunately we were not able to prepare metal layers without defects. We also believe that it would be hard to prevent the occurrence of these defects during the preparation and study of the SMs. Therefore, in attempting to formulate a picture of the permeation process of a gas through a SM, one must consider their contribution to the overall process. We feel that an important fact is that, by this method, an increase of more than one order or magnitude of membrane selectivity for H2 is obtainable. Acknowledgements The authors are indebted to Dr. R. Bucur for useful discussions during the preparation of the paper and to Dr. R. Campeanu for helping to draw up the paper.
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