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Permeation of gases through poly (ethylene terephthalate) membranes metallized with palladium
Journal of Membrane Science, 35 (1988) 291-300 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
291
PERMEATION OF GASES THROUGH POLY (ETHYLENE TEREPHTHALATE) MEMBRANES METALLIZED WITH PALLADIUM
P. MERCEA, L. MURESAN, V. MECEA, D. SILIPAS Znstitute of Isotopic and Molecular Technology, P.O. Box 700, R-3400 Cluj-Napoca 5 (Romania) and I. URSU National Centre of Physics, P.O. Box MS, Mkgurele-Bucharest
(Romania)
(Received March 13,1987; accepted in revised form July 7,1987)
Summary The permeation of He, Hz, COP,Ar and Ns at 50’ C through poly(ethylene terephthalate) (PET) membranes metallized with Pd layers ranging from 125 to 1000 A in thickness was studied. It was found that the Pd layers act as gas barriers which reduce the rate of gas transfer through the polymer with up to more than one order of magnitude. The fact that the Pd layers failed to be perfect gas barriers was attributed to the presence of defects in the structure of the Pd layers and to additional gas diffusion through preferential diffusion paths along grain boundaries. A particular case is represented by the permeation of H,. Since H, is able to dissolve into the Pd layers it permeates not only through the defects of the layers but also through the layers themselves. The experimental results show that catalytic activity of Pd in dissociating Hz molecules determines the permeation rate of H, through the metallized PET membranes.
Introduction Metallized polymer membranes are already widely used in space technology (for thermal blanketing) [ 11, as superinsulating materials (in winter clothing and emergency blankets), for winding to form capacitors, for decorative purposes [ 2 ] and in the packaging field (where they tend to replace aluminium foil which has a higher energy content). The majority of commercial metallized polymer membranes consist of aluminium, zinc or gold deposited on polyesters, polypropylene or polyimide as the base. Usually the ratio of coating thickness to membrane thickness is around 1: 1000. This very thin coating has a strong influence not only on the electrical and optical properties but also on the gas permeability of the polymer membrane. In Ref. [ 31 it was found that
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biaxially oriented polyester membranes can have their gas-barrier performance improved by a factor of several hundreds by metallization. We reported an increase, of up to more than one order of magnitude, of the selectivity for H2 of PET membranes metallized on each side with a 500 A thick Ag, Al, Cu, Ni or Pd layer [ 4,5]. Vacuum deposition of metals on polymer membranes, at all but cryogenic temperatures, gives rise to a conventional polycrystalline structure with the grain sizes being of the order of the metal thickness. At early stages the deposited metal layer consists of small clusters or nuclei, a few nanometers in diameter, which, as further metal vapour condenses, grow and eventually coalesce. The coalescence of the nuclei to form a continuous layer occurs at a thickness which depends on the mobility of the depositing atoms, the vacuum quality and the rate of deposition [ 61. Theoretically, gases which are unable to dissolve in metals cannot permeate through them. Therefore a coherent metal layer deposited on the surface of a polymer should endow the polymer with perfect gas-barrier properties. Nevertheless there is experimental evidence that this is not the case [ 3-5,7]. It follows that, for gases which are unable to dissolve in the metal layer, the actual gas-barrier properties of the metallized polymer membrane (MPM) must be governed by defects which are inevitably produced in the metal layer during the preparation and/or testing of the MPMs. There were two main types of defects found in the metal layers obtained by vacuum deposition: pinholes and cracks. As stated in Ref. [ 31 the major cause for the pinhole defects is the presence of dust particles on the polymer membrane during metallization which, subsequently, become dislodged and leave behind an unmetallized shadow. On the other hand, cracks appear probably as a result of the difference between the contraction coefficients of the polymer and of the metal layer. A theory of gas permeation through PMP was given in Ref. [ 81. It was shown that, provided the pinholes in the metal layer are circular and small compared with the thickness of the polymer, d, at steady state the total flux of gas per square centimeter, Q, can be expressed to a good approximation as: Q=DD8(1+1.183L)(c2--c,)/d
(1)
provided 8 4 1 and A> 0.3, where D is the diffusion coefficient, A= d/r, with r, the radius of the pinhole, 8 = nr; n is the fraction per square centimeter of the polymer surface that is not metallized with n the density of pinholes per square centimeter, and ci and c2 are the concentrations of the dissolved gas just below the downstream and upstream surfaces of the polymer, respectively. From eqn. ( 1) results that, in the limit of large 1, Q becomes larger than the flux through a bare polymer membrane of thickness d and surface 8. Qualitatively this result is understandable: the thicker the membrane compared with the size of the pinhole, the more the diffusing gas spreads out in the polymer membrane. On
the other hand, knowing that in similar steady-state conditions the total flux per square centimeter, Q,,, through a bare polymer membrane is given by [ 91:
Qo=D(c:!--1)/d
(2)
it follows that the ratio Q/Q0 is a function only of the geometrical parameters such as membrane thickness, pinhole diameter and pinhole density. From eqn. (1) it also follows that, provided all defects of the metal layer could be eliminated, which would lead to 8 = 0, the permeating flux Q would vanish. Based on the results discussed above as well as on the main statements of the phenomenological model of the gas transport through MPMs, discussed in Ref. [ 41, one can state that: (a) All gases permeate through the metal layers of the MPM in undissociated atomic or molecular form, by diffusing through the defects of the layers. The gas-barrier properties of the metal layers are determined by 0++, the total area of defects, regardless of the shape of the defects. Moreover, there is some evidence that the gas-barrier properties of the metal layers are enhanced when the gases diffuse through the defects in a molecular flow regime [ 41. (b ) In order to permeate through the metal layer a gas must first dissociate into atoms or ions and then, in this form, dissolve and diffuse through the metal layer. (c ) All gases permeate through the polymer substrate of an MPM in an undissociated atomic or molecular form. The rate of permeation depends, as in the case of a bare polymer, both on the nature of the gas and of the polymer [ lo]. Moreover, the permeation rate is dependent on the thickness of the polymer membrane, the density and the size of the defects of the metal layer. Thus, from the point of view of gas permeation, by metallizing a polymer membrane, one can aim for two effects: (1) a gas-barrier effect, in order to decrease the overall rate of gas transfer through the polymer, and (2) a gas-selective effect, in order to increase the permeation selectivity for one gas which is able to avoid the former effect by dissolving and diffusing through the metal layer itself. Because He, COZ, Ar and N, do not dissolve into Pd at 50’ C and a pressure of about 70 cmHg, and H, does, we had the possibility to investigate both effects by studying the permeation of these gases through PET membranes metallized with Pd. We also established the influence on these effects of the thickness of the deposited Pd layer and of its orientation, whether facing the upstream compartment (Case I) or the downstream compartment (Case II) of the apparatus. Moreover, we tried to determine the effect of “poisoning” the Pd layers with air pollutants or H,S on the gas-barrier and selectivity effects. These processes are of interest especially when the MPMs are used for H2 separations.
Fig. 1. Size distribution of pinholes for a Pd( 500 A) -PET membrane.
Experimental
The experimental procedures for obtaining MPMs by vacuum deposition, for measuring the thickness of the deposited layers and for controling their uniformity were described in Ref. [ 41. Pd, of at least 99.95% purity, was deposited on a 30 -t 2 pm thick PET membrane (produced by a Romanian factory). The thicknesses of the deposited Pd layers, dk, were 125 + 15,250 _+20, 500 5 25,750 ? 35 and 1,000 2 50 A. The defect density, pd, was estimated from a series of scanning electron micrographs, and found to be pd= 1.5X 10’ defects/mm’ for &= 500 A. The size distribution of the pinholes for the same MPM is given in Fig. 1. It was found that, generally, for increasing &, the defect density decreases slightly but the size distribution remains practically unchanged. These results are in agreement with those reported in Ref. [ 31. In order to prevent contamination of the Pd layers, the MPMs were introduced into the diffusion cell of the apparatus immediately after vacuum deposition. The permeability coefficients of the MPMs for a given gas i, PIMPM ( i) , and those of bare PET, PPET( i) , were determined by means of the steady-state diffusion method [ 111. All gases were at least 99.9% pure, except COZ which was 99% pure. The experimental set-up, procedures and data analysis were similar to those described in Ref. [ 41, For every reported PMPM(i) value at least five determinations were made. The maximum relative deviation of PMVIPM(i) from its average value was within 15% for MPMs with 125 and 250 A thick Pd layers, and never exceeded 10% for the other MPMs. Results and discussion
Permeation of He, CO,, Ar and N2 through PET membranes metallized with Pd The PMMM(i) values were determined, both for Case I and II, for He, Cog, Ar and N, at a temperature of 50 + 0.1 ‘C and a pressure gradient Ap ( i) = 70 cmHg. The results are given in Table 1.
295
TABLE 1 Permeability constants, at 50 ’ C, for poly ( ethylene terephthalate)
Permeability, PMPM(cbarrer”)
Membrane
Case I
Case II
membranes metallized with Pd
H2
He
COZ
Ar
Nz
PET Pd-PET-Pdb
131 124
194 76
32 23
20.0 2.1
12.0 1.5
Pd(125 &-PET Pd (250 A) -PET Pd (500 A) -PET pd(750 Q-PET pd(iOOO Q-PET
130 129 127 129 130
123 90 80 75 54
29
28 24 17 14
4.0 2.3 2.6 2.4 2.2
3.2 2.0 1.6 1.5 1.2
PET-Pd(125 A) PET-Pd( 500 A, PET-Pd ( 750 A) PET-Pd(1000 A,
100
128 86 74 61
30 24 16 15
4.2 3.0 2.3 2.2
3.3 1.7 1.5 1.4
70 58 48
“1cbarrer = lo-* barrer = lo-‘* cm3-cm/cm*-set-cmHg. bRef. [4].
From Table 1 one can see that, as expected, the Pd layers deposited on the PET membranes reduce the permeability for all gases, in both Case I and Case II. An exception is Hz, which will be discussed separately. In order to compare the results given in Table 1 with the predictions of eqn. (1) PMPM ( i) must be expressed as: P&PM(i)
=PPET(i)
f
Sj(l+l*lSLj)
(3)
j=l
where
represents the total area of the pinholes, L=d/roj and j spans the size distribution of the pinholes (see Fig. 1) . For the MPM with 500 A thick Pd layers we found that 8+ is about 7.5 x lo3 pm2/mm2, meaning that about 0.75% of the surface of this MPM is left unmetallized and Cy!l 0j(l+ l-1811,) is about 7x 10-2. Hence, according to eqn. (3)) regardless of the nature of gas i, PGPM (i) of this MPM must be about 14 times smaller than for the bare PET membrane. The fact that the PMPM(i) values given in Table 1 are larger than those calculated with eqn. (3 ) might be due to an additional gas flow through
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the metal layer of the MPM. There are at least two sources which may contribute to this additional gas flow. First, there is a gas flow through the cracks in the Pd layers, not taken into account in the evaluation of 19+,which results in &> CJpz1 np Second, as suggested in Ref. [ 31, there may be additional gas diffusion through preferential diffusion paths, perhaps along the grain boundaries of the metal layer. On the other hand, the results given in Table 1 show that the values of PMPM(i) depend not only on the thickness of the polymer membrane and on the size and distribution of the defects of the metal layer, but also on the nature of the permeant gas. Responsible for this might be the fact that through defects smaller than the mean free path of the gas molecules, flow takes place in a selective molecular regime. Moreover, through these defects additional surface flow may occur [ 121. This last process in particular is expected to be responsible for the larger value of PMPM(i) for the more condensable gas CO,. From Table 1 can also be seen that the values of PMMPM( i) for He, CO*, Ar and N, are about the same for both Case I and Case II. This means that the gas-barrier effect is about the same regardless of the orientation of the Pd layer (facing the upstream or the downstream compartment of the experimental setUP).
The fact that PMPM( i) for He, COz, Ar and N, decreases with increasing Pd layer thickness is mainly the result of the decrease of Pd with increasing &. Moreover, the thickness of the deposited metal layer will influence the permeation through the MPM in those cases where the defects act essentially as channels. When those channels are comparatively long, the rate of permeation may be determined to a certain extent by the flow of the gas in the channels. This would result in a somewhat lower value of c2. In our case because ]&lmax= 0.1 ,um and because according to Fig. 1 the most frequently occurring pinholes have diameters of about 2 to 6 pm, the contribution of the last effect can be neglected in a first approximation. The discussion above is partially valid for the permeation of H2 through an MPM as well, namely for that part of the permeant H2 flow which occurs by diffusion through the defects in the Pd layers. The remaining part of the H, flux is formed by permeation through the Pd layers themselves. What determines the magnitude of this flux and how it influences PMPM(H,) will be analysed in the next section. Permeation
of H2 through PET membranes
metallized with Pd
From Table 1 one can see that the PMPM (H,) values corresponding to Case I differ appreciably from those corresponding to Case II. Let us try to explain this result. In Case I, regardless of the size of dk, the PMPM (H,) values have approximately the same value, which is only slightly smaller than that of PPET (H,) (Table 1) . At 50” C, Pd is more permeable to H, than PET, PPd= 26 barrer
297
[ 31. Therefore, when one considers that an MPM behaves as a laminated membrane [ 141, one would expect that its permeability for H2 would be slightly higher than that of the bare PET membrane. The opposite tendency, as observed in Table 1, may be caused by a reduction of the H, flow through the Pd layers themselves. This may be the result of a partial inhibition (poisoning) of the catalytic activity of Pd, and/or the result of boundary effects at the Pd-PET interface. The reduction of the catalytic activity of Pd may be caused by hydrocarbon vapours released into the upstream volume by the vacuum pumps and sealing greases of the experimental set-up. It was found in Ref. [ 151 that even a hydrocarbon vapour partial pressure as low as low5 Torr may reduce the catalytic activity of Pd. On the other hand, for Case 1, it is interesting to notice from Table 1 that, regardless of the magnitude of dk, H, permeates faster than He through the MPMs, in contrast to the case of bare PET where, in agreement with the general behaviour of glassy polymers, He permeates faster than Hz [ 161. In Case II the PMPM( H, ) values are considerably smaller than PPET ( H2 ) , regardless of the size of dk. To explain this one must assume that the catalytic activity of Pd in close contact with PET is largely inhibited and that therefore the flow of H, through the Pd layers themselves is considerably diminished. For Case II PMPM( H,) is smaller than the corresponding PMPM( He), in contrast to the situation for Case I. In Ref. [ 41 it was found that the Hz permeability of a Pd-PET-Pd membrane is slightly smaller than that of bare PET. According to the discussion above this may result from the permeation of Hz through the Pd layer facing the downstream compartment being smaller than that through the Pd layer facing the upstream compartment. In order to check the possibility that, due to the deposition of the metal layer, the properties of PET at the metal-polymer interface were altered (by the annealing of PET for example) the following test was done. After determining the PMMPM ( i) values for a Pd( 500 A) -PET membrane the Pd layer was chemically removed and subsequently the permeabilities, P&-(i) , for this membrane were determined. It was found that the PbET ( i) values were, in the limit of the experimental error, equal to those of the undeposited PET membrane, PPET (i) , thus indicating that at the metal-polymer interface no modification of the permeation behaviour of PET has occurred. To find out to what extent the inhibition (“poisoning”) of the catalytic activity of Pd for dissociating H2 molecules influences the permeation rate of H2 through the MPMs, two additional experiments were carried out. First, the values of PMPM( i) were determined for a series of MPMs which were exposed to air for at least three months. The results obtained are given in Table 2. From this table one can notice that for both Case I and Case II PMPM(H,) is smaller than the corresponding PMVIPM ( HZ) given in Table 1. This may be ascribed to the partial poisoning of the Pd layers by pollutants con-
298
TABLE 2 Permeability constants, at 5O”C, for poly(ethylene terephthalate) membranes metallized with Pd, which were exposed to air and the poisoning effect of H$, respectively Membrane
Permeability, PMPM(cbarrer”) He
CO,
Ar
107 97 88 84 71 63 49
120 92 79 73 58 85 60
28 26 24 16 14 23 15
5.1 2.5 2.4 2.2 2.0 2.7 2.0
3.4 1.8 1.6 1.4 1.1 1.7 1.3
55 58
67 68
15 16
2.4 2.4
1.3 1.4
a. Exposed to air for at least three months Case I
Case II
Pd(125 &-PET Pd (250 A, -PET Pd (500 A) -PET Pd(750 &-PET Pd(lOOO &-PET PET-Pd (500 A, PET-Pd (750 ii)
b. Poisoned with H,S Case I Case II “1 cbarrer= lo-’
Pd(750 .&)-PET PET-Pd (750 A, barrer= lo-”
cm3-cm/cm’-set-cmHg.
tained in air. In Case II this poisoning effect has only a limited impact on PMPM( H2 ) , which decreases less than in Case I, because, as previously stated, in Case II the Pd layers themselves are involved only to a limited extent in the transport of H, through the MPM. In another experiment the Pd layers were deliberately poisoned by introducing, for at least 15 minutes, H,S in the upstream or downstream compartment of the diffusion cell. It is known [ 171 that upon contact with H2S the catalytic activity of Pd is inhibited almost completely within a short period of time. Therefore one may assume that both in Case I and Case II all penetrant gases, including HP, diffuse almost exclusively through the defects in the Pd layers. The values of PMPM( ;) for two PET membranes metallized with a 750 A thick Pd layer which were subsequently poisoned with H2S are given in Table 2. As expected, in Case I, P MPM(H, ) is smaller than PPET (H, ) , smaller than the corresponding P MPM( Hz) given in Table 1, and moreover even smaller than PMPM ( He). Thus, even in Case I, the hierarchy of the values of P,,,(i) is similar to that of a bare PET membrane, where PpET (He) > PPET (H,) [ 161. This result is significant because until now in all situations studied in Case I it was found that PMPM (Hz) > PMPM ( He). Conclusions
The results given in Tables 1 and 2, as well as those previously reported in Refs. [ 41 and [ 51, show that, as expected, the gas-barrier effect of an MPM
is mainly determined by the level of defects of the deposited metal layers. The fact that the gas-barrier effect depends not only on geometrical parameters such as membrane thickness, pinhole size and distribution seems to be the result of a more complex process in which surface diffusion occurs in parallel with selective molecular flow through the smaller defects of the metal layer. Moreover, an additional flux of gas diffusing through preferential diffusion paths along grain boundaries may contribute to the overall rate of gas transfer through an MPM. In the range of 125 to 1000 A the thickness of the deposited metal layer, dk, influences the gas-barrier effect to the extent that a variation of the defect density Pd corresponds to a variation of dk. The gas-barrier effect of an MPM bears little, if any, relationship with the orientation of the metal layer (whether facing the upstream or the downstream compartment). This result may be of practical value because it allows one to deposit the metal layer in such a way as to be protected from any mechanical or chemical destruction. An advantageous solution related to this is to manufacture the MPM as a multilayer membrane in which the metal layer is located between two polymer layers. On the other hand, the gas-selective effect of an MPM essentially depends on the ability of a given gas to dissolve in, and diffuse through, the metal layer itself. From the cases studied in Refs. [ 41 and [ 51 and in this paper, only Hz has the ability to dissolve in Pd, and to a smaller extent in Ni. The results given in Table 2 show that the gas-selective effect is strongly diminished or even cancelled when the surface of the Pd layer is chemically “poisoned” by air pollutants or H,S. When trying to protect the Pd layer, for example by means of a multilayer structure or by a protective laquer, one faces another process which diminishes or even cancels the Hz-selective effect. This process is the reduction of the catalytic activity of Pd, in close contact with the polymer, in dissociating H2 molecules. Therefore, in order to obtain and preserve a Hz-selective effect the deposited Pd layer must face the upstream volume, and its contact with poisoning gases or vapours must be avoided. Therefore one can conclude that the key to a high MPM performance, in both effects envisaged, lies in the quality of the deposited metal layers. The gas-barier and selective effects reported in this paper can certainly be improved by specific treatment of the polymer before the vacuum deposition process, a more rigorous control of the deposition process itself, and post-treatment of the obtained MPMs. Acknowledgements
The authors are indebted to I. Chicinas for helping with the scanning electron micrographs, to Dr. Zs. Gulacsi for useful discussions during the prepa-
300
ration of the paper and to Dr. Rodica Candea for help with preparation of the paper.
References 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15
16 17
H. Stoever, Space technology utilisation - The role of ESA and national organisations, ESA Bull., 48 (1986) 18. B.N. Chapman and J.C. Anderson, Science and Technology of Surface Coating, Academic Press, New York, NY, 1974, p. 45. E.H.H. Jamieson and A.H. Windle, Structure and oxygen-barrier properties of metallized polymer film, J. Mater. Sci., 18 (1983) 64. P. Mercea, L. Muregan and V. Mecca, Permeation of gases through metallized polymer membranes, J. Membrane Sci., 24 (1985) 297. P. Mercea, The selective transport of gasesd through polymer membranes, Ph.D. Dissertation, University of Bucharest, Bucharest, 1986. J. Fridrich and K. Kohout, The influence of thickness and condensation rate on the resistivity of evaporated aluminium films, Thin Solid Films, 7 (1971) R49. D.H.W. Carstens and E.P. Ehart, Permeability of deuterium and helium in poly(viny1 alcohoi), J. Appl. Polym. Sci., 29 (1984) 261. W. Prins and J.J. Hermans, Theory of permeation through metal coated polymer films, J. Phys. Chem., 63 (5) (1959) 716. R.M. Barer, Diffusion in and through Solids, Cambridge University Press, Cambridge, 1951, p. 5. V. Stannett, Simple gases, in: J. Crank and G.S. Park (Eds.) , Diffusion in Polymers, Academic Press, New York, NY, 1968, p. 43. J. Crank, The Mathematics of Diffusion, Clarendon Press, Oxford, 1956, p. 62. S.-T. Hwang, Surface diffusion parallel with Knudsen flow, Sep. Sci., 11(l) (1976) 17. G. Bohmholdt and E. Wieke, Diffusion von H und D in Pd und Pd-Legierungen, Z. Phys. Chem. N.F., 56 (1967) 133. C.H. Lee, Permeation properties in laminated membranes, Sep. Sci., 9 (1974) 479. J.F. Lynch and T.B. Flanagan, An investigation of the dynamic equilibrium between chemisorbed and adsorbed hydrogen in the palladium/hydrogen system, J. Phys. Chem., 77 (1973) 2628. S.-T. Hwang, H. Choi and K. Kammermeyer, Gaseous transfer coefficients in membranes, Sep. Sci. 9 (1974) 461. F.A. Lewis, The Palladium Hydrogen System, Academic Press, London, 1967, p. 23.
291
PERMEATION OF GASES THROUGH POLY (ETHYLENE TEREPHTHALATE) MEMBRANES METALLIZED WITH PALLADIUM
P. MERCEA, L. MURESAN, V. MECEA, D. SILIPAS Znstitute of Isotopic and Molecular Technology, P.O. Box 700, R-3400 Cluj-Napoca 5 (Romania) and I. URSU National Centre of Physics, P.O. Box MS, Mkgurele-Bucharest
(Romania)
(Received March 13,1987; accepted in revised form July 7,1987)
Summary The permeation of He, Hz, COP,Ar and Ns at 50’ C through poly(ethylene terephthalate) (PET) membranes metallized with Pd layers ranging from 125 to 1000 A in thickness was studied. It was found that the Pd layers act as gas barriers which reduce the rate of gas transfer through the polymer with up to more than one order of magnitude. The fact that the Pd layers failed to be perfect gas barriers was attributed to the presence of defects in the structure of the Pd layers and to additional gas diffusion through preferential diffusion paths along grain boundaries. A particular case is represented by the permeation of H,. Since H, is able to dissolve into the Pd layers it permeates not only through the defects of the layers but also through the layers themselves. The experimental results show that catalytic activity of Pd in dissociating Hz molecules determines the permeation rate of H, through the metallized PET membranes.
Introduction Metallized polymer membranes are already widely used in space technology (for thermal blanketing) [ 11, as superinsulating materials (in winter clothing and emergency blankets), for winding to form capacitors, for decorative purposes [ 2 ] and in the packaging field (where they tend to replace aluminium foil which has a higher energy content). The majority of commercial metallized polymer membranes consist of aluminium, zinc or gold deposited on polyesters, polypropylene or polyimide as the base. Usually the ratio of coating thickness to membrane thickness is around 1: 1000. This very thin coating has a strong influence not only on the electrical and optical properties but also on the gas permeability of the polymer membrane. In Ref. [ 31 it was found that
0376-7388/88/$03.50
0 1988 Elsevier Science Publishers B.V.
292
biaxially oriented polyester membranes can have their gas-barrier performance improved by a factor of several hundreds by metallization. We reported an increase, of up to more than one order of magnitude, of the selectivity for H2 of PET membranes metallized on each side with a 500 A thick Ag, Al, Cu, Ni or Pd layer [ 4,5]. Vacuum deposition of metals on polymer membranes, at all but cryogenic temperatures, gives rise to a conventional polycrystalline structure with the grain sizes being of the order of the metal thickness. At early stages the deposited metal layer consists of small clusters or nuclei, a few nanometers in diameter, which, as further metal vapour condenses, grow and eventually coalesce. The coalescence of the nuclei to form a continuous layer occurs at a thickness which depends on the mobility of the depositing atoms, the vacuum quality and the rate of deposition [ 61. Theoretically, gases which are unable to dissolve in metals cannot permeate through them. Therefore a coherent metal layer deposited on the surface of a polymer should endow the polymer with perfect gas-barrier properties. Nevertheless there is experimental evidence that this is not the case [ 3-5,7]. It follows that, for gases which are unable to dissolve in the metal layer, the actual gas-barrier properties of the metallized polymer membrane (MPM) must be governed by defects which are inevitably produced in the metal layer during the preparation and/or testing of the MPMs. There were two main types of defects found in the metal layers obtained by vacuum deposition: pinholes and cracks. As stated in Ref. [ 31 the major cause for the pinhole defects is the presence of dust particles on the polymer membrane during metallization which, subsequently, become dislodged and leave behind an unmetallized shadow. On the other hand, cracks appear probably as a result of the difference between the contraction coefficients of the polymer and of the metal layer. A theory of gas permeation through PMP was given in Ref. [ 81. It was shown that, provided the pinholes in the metal layer are circular and small compared with the thickness of the polymer, d, at steady state the total flux of gas per square centimeter, Q, can be expressed to a good approximation as: Q=DD8(1+1.183L)(c2--c,)/d
(1)
provided 8 4 1 and A> 0.3, where D is the diffusion coefficient, A= d/r, with r, the radius of the pinhole, 8 = nr; n is the fraction per square centimeter of the polymer surface that is not metallized with n the density of pinholes per square centimeter, and ci and c2 are the concentrations of the dissolved gas just below the downstream and upstream surfaces of the polymer, respectively. From eqn. ( 1) results that, in the limit of large 1, Q becomes larger than the flux through a bare polymer membrane of thickness d and surface 8. Qualitatively this result is understandable: the thicker the membrane compared with the size of the pinhole, the more the diffusing gas spreads out in the polymer membrane. On
the other hand, knowing that in similar steady-state conditions the total flux per square centimeter, Q,,, through a bare polymer membrane is given by [ 91:
Qo=D(c:!--1)/d
(2)
it follows that the ratio Q/Q0 is a function only of the geometrical parameters such as membrane thickness, pinhole diameter and pinhole density. From eqn. (1) it also follows that, provided all defects of the metal layer could be eliminated, which would lead to 8 = 0, the permeating flux Q would vanish. Based on the results discussed above as well as on the main statements of the phenomenological model of the gas transport through MPMs, discussed in Ref. [ 41, one can state that: (a) All gases permeate through the metal layers of the MPM in undissociated atomic or molecular form, by diffusing through the defects of the layers. The gas-barrier properties of the metal layers are determined by 0++, the total area of defects, regardless of the shape of the defects. Moreover, there is some evidence that the gas-barrier properties of the metal layers are enhanced when the gases diffuse through the defects in a molecular flow regime [ 41. (b ) In order to permeate through the metal layer a gas must first dissociate into atoms or ions and then, in this form, dissolve and diffuse through the metal layer. (c ) All gases permeate through the polymer substrate of an MPM in an undissociated atomic or molecular form. The rate of permeation depends, as in the case of a bare polymer, both on the nature of the gas and of the polymer [ lo]. Moreover, the permeation rate is dependent on the thickness of the polymer membrane, the density and the size of the defects of the metal layer. Thus, from the point of view of gas permeation, by metallizing a polymer membrane, one can aim for two effects: (1) a gas-barrier effect, in order to decrease the overall rate of gas transfer through the polymer, and (2) a gas-selective effect, in order to increase the permeation selectivity for one gas which is able to avoid the former effect by dissolving and diffusing through the metal layer itself. Because He, COZ, Ar and N, do not dissolve into Pd at 50’ C and a pressure of about 70 cmHg, and H, does, we had the possibility to investigate both effects by studying the permeation of these gases through PET membranes metallized with Pd. We also established the influence on these effects of the thickness of the deposited Pd layer and of its orientation, whether facing the upstream compartment (Case I) or the downstream compartment (Case II) of the apparatus. Moreover, we tried to determine the effect of “poisoning” the Pd layers with air pollutants or H,S on the gas-barrier and selectivity effects. These processes are of interest especially when the MPMs are used for H2 separations.
Fig. 1. Size distribution of pinholes for a Pd( 500 A) -PET membrane.
Experimental
The experimental procedures for obtaining MPMs by vacuum deposition, for measuring the thickness of the deposited layers and for controling their uniformity were described in Ref. [ 41. Pd, of at least 99.95% purity, was deposited on a 30 -t 2 pm thick PET membrane (produced by a Romanian factory). The thicknesses of the deposited Pd layers, dk, were 125 + 15,250 _+20, 500 5 25,750 ? 35 and 1,000 2 50 A. The defect density, pd, was estimated from a series of scanning electron micrographs, and found to be pd= 1.5X 10’ defects/mm’ for &= 500 A. The size distribution of the pinholes for the same MPM is given in Fig. 1. It was found that, generally, for increasing &, the defect density decreases slightly but the size distribution remains practically unchanged. These results are in agreement with those reported in Ref. [ 31. In order to prevent contamination of the Pd layers, the MPMs were introduced into the diffusion cell of the apparatus immediately after vacuum deposition. The permeability coefficients of the MPMs for a given gas i, PIMPM ( i) , and those of bare PET, PPET( i) , were determined by means of the steady-state diffusion method [ 111. All gases were at least 99.9% pure, except COZ which was 99% pure. The experimental set-up, procedures and data analysis were similar to those described in Ref. [ 41, For every reported PMPM(i) value at least five determinations were made. The maximum relative deviation of PMVIPM(i) from its average value was within 15% for MPMs with 125 and 250 A thick Pd layers, and never exceeded 10% for the other MPMs. Results and discussion
Permeation of He, CO,, Ar and N2 through PET membranes metallized with Pd The PMMM(i) values were determined, both for Case I and II, for He, Cog, Ar and N, at a temperature of 50 + 0.1 ‘C and a pressure gradient Ap ( i) = 70 cmHg. The results are given in Table 1.
295
TABLE 1 Permeability constants, at 50 ’ C, for poly ( ethylene terephthalate)
Permeability, PMPM(cbarrer”)
Membrane
Case I
Case II
membranes metallized with Pd
H2
He
COZ
Ar
Nz
PET Pd-PET-Pdb
131 124
194 76
32 23
20.0 2.1
12.0 1.5
Pd(125 &-PET Pd (250 A) -PET Pd (500 A) -PET pd(750 Q-PET pd(iOOO Q-PET
130 129 127 129 130
123 90 80 75 54
29
28 24 17 14
4.0 2.3 2.6 2.4 2.2
3.2 2.0 1.6 1.5 1.2
PET-Pd(125 A) PET-Pd( 500 A, PET-Pd ( 750 A) PET-Pd(1000 A,
100
128 86 74 61
30 24 16 15
4.2 3.0 2.3 2.2
3.3 1.7 1.5 1.4
70 58 48
“1cbarrer = lo-* barrer = lo-‘* cm3-cm/cm*-set-cmHg. bRef. [4].
From Table 1 one can see that, as expected, the Pd layers deposited on the PET membranes reduce the permeability for all gases, in both Case I and Case II. An exception is Hz, which will be discussed separately. In order to compare the results given in Table 1 with the predictions of eqn. (1) PMPM ( i) must be expressed as: P&PM(i)
=PPET(i)
f
Sj(l+l*lSLj)
(3)
j=l
where
represents the total area of the pinholes, L=d/roj and j spans the size distribution of the pinholes (see Fig. 1) . For the MPM with 500 A thick Pd layers we found that 8+ is about 7.5 x lo3 pm2/mm2, meaning that about 0.75% of the surface of this MPM is left unmetallized and Cy!l 0j(l+ l-1811,) is about 7x 10-2. Hence, according to eqn. (3)) regardless of the nature of gas i, PGPM (i) of this MPM must be about 14 times smaller than for the bare PET membrane. The fact that the PMPM(i) values given in Table 1 are larger than those calculated with eqn. (3 ) might be due to an additional gas flow through
296
the metal layer of the MPM. There are at least two sources which may contribute to this additional gas flow. First, there is a gas flow through the cracks in the Pd layers, not taken into account in the evaluation of 19+,which results in &> CJpz1 np Second, as suggested in Ref. [ 31, there may be additional gas diffusion through preferential diffusion paths, perhaps along the grain boundaries of the metal layer. On the other hand, the results given in Table 1 show that the values of PMPM(i) depend not only on the thickness of the polymer membrane and on the size and distribution of the defects of the metal layer, but also on the nature of the permeant gas. Responsible for this might be the fact that through defects smaller than the mean free path of the gas molecules, flow takes place in a selective molecular regime. Moreover, through these defects additional surface flow may occur [ 121. This last process in particular is expected to be responsible for the larger value of PMPM(i) for the more condensable gas CO,. From Table 1 can also be seen that the values of PMMPM( i) for He, CO*, Ar and N, are about the same for both Case I and Case II. This means that the gas-barrier effect is about the same regardless of the orientation of the Pd layer (facing the upstream or the downstream compartment of the experimental setUP).
The fact that PMPM( i) for He, COz, Ar and N, decreases with increasing Pd layer thickness is mainly the result of the decrease of Pd with increasing &. Moreover, the thickness of the deposited metal layer will influence the permeation through the MPM in those cases where the defects act essentially as channels. When those channels are comparatively long, the rate of permeation may be determined to a certain extent by the flow of the gas in the channels. This would result in a somewhat lower value of c2. In our case because ]&lmax= 0.1 ,um and because according to Fig. 1 the most frequently occurring pinholes have diameters of about 2 to 6 pm, the contribution of the last effect can be neglected in a first approximation. The discussion above is partially valid for the permeation of H2 through an MPM as well, namely for that part of the permeant H2 flow which occurs by diffusion through the defects in the Pd layers. The remaining part of the H, flux is formed by permeation through the Pd layers themselves. What determines the magnitude of this flux and how it influences PMPM(H,) will be analysed in the next section. Permeation
of H2 through PET membranes
metallized with Pd
From Table 1 one can see that the PMPM (H,) values corresponding to Case I differ appreciably from those corresponding to Case II. Let us try to explain this result. In Case I, regardless of the size of dk, the PMPM (H,) values have approximately the same value, which is only slightly smaller than that of PPET (H,) (Table 1) . At 50” C, Pd is more permeable to H, than PET, PPd= 26 barrer
297
[ 31. Therefore, when one considers that an MPM behaves as a laminated membrane [ 141, one would expect that its permeability for H2 would be slightly higher than that of the bare PET membrane. The opposite tendency, as observed in Table 1, may be caused by a reduction of the H, flow through the Pd layers themselves. This may be the result of a partial inhibition (poisoning) of the catalytic activity of Pd, and/or the result of boundary effects at the Pd-PET interface. The reduction of the catalytic activity of Pd may be caused by hydrocarbon vapours released into the upstream volume by the vacuum pumps and sealing greases of the experimental set-up. It was found in Ref. [ 151 that even a hydrocarbon vapour partial pressure as low as low5 Torr may reduce the catalytic activity of Pd. On the other hand, for Case 1, it is interesting to notice from Table 1 that, regardless of the magnitude of dk, H, permeates faster than He through the MPMs, in contrast to the case of bare PET where, in agreement with the general behaviour of glassy polymers, He permeates faster than Hz [ 161. In Case II the PMPM( H, ) values are considerably smaller than PPET ( H2 ) , regardless of the size of dk. To explain this one must assume that the catalytic activity of Pd in close contact with PET is largely inhibited and that therefore the flow of H, through the Pd layers themselves is considerably diminished. For Case II PMPM( H,) is smaller than the corresponding PMPM( He), in contrast to the situation for Case I. In Ref. [ 41 it was found that the Hz permeability of a Pd-PET-Pd membrane is slightly smaller than that of bare PET. According to the discussion above this may result from the permeation of Hz through the Pd layer facing the downstream compartment being smaller than that through the Pd layer facing the upstream compartment. In order to check the possibility that, due to the deposition of the metal layer, the properties of PET at the metal-polymer interface were altered (by the annealing of PET for example) the following test was done. After determining the PMMPM ( i) values for a Pd( 500 A) -PET membrane the Pd layer was chemically removed and subsequently the permeabilities, P&-(i) , for this membrane were determined. It was found that the PbET ( i) values were, in the limit of the experimental error, equal to those of the undeposited PET membrane, PPET (i) , thus indicating that at the metal-polymer interface no modification of the permeation behaviour of PET has occurred. To find out to what extent the inhibition (“poisoning”) of the catalytic activity of Pd for dissociating H2 molecules influences the permeation rate of H2 through the MPMs, two additional experiments were carried out. First, the values of PMPM( i) were determined for a series of MPMs which were exposed to air for at least three months. The results obtained are given in Table 2. From this table one can notice that for both Case I and Case II PMPM(H,) is smaller than the corresponding PMVIPM ( HZ) given in Table 1. This may be ascribed to the partial poisoning of the Pd layers by pollutants con-
298
TABLE 2 Permeability constants, at 5O”C, for poly(ethylene terephthalate) membranes metallized with Pd, which were exposed to air and the poisoning effect of H$, respectively Membrane
Permeability, PMPM(cbarrer”) He
CO,
Ar
107 97 88 84 71 63 49
120 92 79 73 58 85 60
28 26 24 16 14 23 15
5.1 2.5 2.4 2.2 2.0 2.7 2.0
3.4 1.8 1.6 1.4 1.1 1.7 1.3
55 58
67 68
15 16
2.4 2.4
1.3 1.4
a. Exposed to air for at least three months Case I
Case II
Pd(125 &-PET Pd (250 A, -PET Pd (500 A) -PET Pd(750 &-PET Pd(lOOO &-PET PET-Pd (500 A, PET-Pd (750 ii)
b. Poisoned with H,S Case I Case II “1 cbarrer= lo-’
Pd(750 .&)-PET PET-Pd (750 A, barrer= lo-”
cm3-cm/cm’-set-cmHg.
tained in air. In Case II this poisoning effect has only a limited impact on PMPM( H2 ) , which decreases less than in Case I, because, as previously stated, in Case II the Pd layers themselves are involved only to a limited extent in the transport of H, through the MPM. In another experiment the Pd layers were deliberately poisoned by introducing, for at least 15 minutes, H,S in the upstream or downstream compartment of the diffusion cell. It is known [ 171 that upon contact with H2S the catalytic activity of Pd is inhibited almost completely within a short period of time. Therefore one may assume that both in Case I and Case II all penetrant gases, including HP, diffuse almost exclusively through the defects in the Pd layers. The values of PMPM( ;) for two PET membranes metallized with a 750 A thick Pd layer which were subsequently poisoned with H2S are given in Table 2. As expected, in Case I, P MPM(H, ) is smaller than PPET (H, ) , smaller than the corresponding P MPM( Hz) given in Table 1, and moreover even smaller than PMPM ( He). Thus, even in Case I, the hierarchy of the values of P,,,(i) is similar to that of a bare PET membrane, where PpET (He) > PPET (H,) [ 161. This result is significant because until now in all situations studied in Case I it was found that PMPM (Hz) > PMPM ( He). Conclusions
The results given in Tables 1 and 2, as well as those previously reported in Refs. [ 41 and [ 51, show that, as expected, the gas-barrier effect of an MPM
is mainly determined by the level of defects of the deposited metal layers. The fact that the gas-barrier effect depends not only on geometrical parameters such as membrane thickness, pinhole size and distribution seems to be the result of a more complex process in which surface diffusion occurs in parallel with selective molecular flow through the smaller defects of the metal layer. Moreover, an additional flux of gas diffusing through preferential diffusion paths along grain boundaries may contribute to the overall rate of gas transfer through an MPM. In the range of 125 to 1000 A the thickness of the deposited metal layer, dk, influences the gas-barrier effect to the extent that a variation of the defect density Pd corresponds to a variation of dk. The gas-barrier effect of an MPM bears little, if any, relationship with the orientation of the metal layer (whether facing the upstream or the downstream compartment). This result may be of practical value because it allows one to deposit the metal layer in such a way as to be protected from any mechanical or chemical destruction. An advantageous solution related to this is to manufacture the MPM as a multilayer membrane in which the metal layer is located between two polymer layers. On the other hand, the gas-selective effect of an MPM essentially depends on the ability of a given gas to dissolve in, and diffuse through, the metal layer itself. From the cases studied in Refs. [ 41 and [ 51 and in this paper, only Hz has the ability to dissolve in Pd, and to a smaller extent in Ni. The results given in Table 2 show that the gas-selective effect is strongly diminished or even cancelled when the surface of the Pd layer is chemically “poisoned” by air pollutants or H,S. When trying to protect the Pd layer, for example by means of a multilayer structure or by a protective laquer, one faces another process which diminishes or even cancels the Hz-selective effect. This process is the reduction of the catalytic activity of Pd, in close contact with the polymer, in dissociating H2 molecules. Therefore, in order to obtain and preserve a Hz-selective effect the deposited Pd layer must face the upstream volume, and its contact with poisoning gases or vapours must be avoided. Therefore one can conclude that the key to a high MPM performance, in both effects envisaged, lies in the quality of the deposited metal layers. The gas-barier and selective effects reported in this paper can certainly be improved by specific treatment of the polymer before the vacuum deposition process, a more rigorous control of the deposition process itself, and post-treatment of the obtained MPMs. Acknowledgements
The authors are indebted to I. Chicinas for helping with the scanning electron micrographs, to Dr. Zs. Gulacsi for useful discussions during the prepa-
300
ration of the paper and to Dr. Rodica Candea for help with preparation of the paper.
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