U.S. patent application number 11/931734 was filed with the patent office on 2008-11-13 for polymer membrane, method for the production and use thereof.
Invention is credited to Jochen Baurmeister, Frauke Jordt, Joachim Kiefer, Werner Kraus, Jurgen Pawlik, Oemer Uensal.
Application Number | 20080280182 11/931734 |
Document ID | / |
Family ID | 7675913 |
Filed Date | 2008-11-13 |
United States Patent
Application |
20080280182 |
Kind Code |
A1 |
Uensal; Oemer ; et
al. |
November 13, 2008 |
POLYMER MEMBRANE, METHOD FOR THE PRODUCTION AND USE THEREOF
Abstract
The present invention relates to an acid-doped polymer membrane
based on polyazoles. The acid-doped polymer membrane can be used in
a variety of applications because of its excellent mechanical
properties and is useful as polymer electrolyte membrane (PEM) in
PEM fuel cells. A doped polymer membrane based on polyazoles is
obtained by a process comprising the steps of: A) casting a film
using a solution of polymers based on polyazoles in a polar,
aprotic organic solvent; B) drying the film formed in step A) until
it is self-supporting; C) treating the film obtained in step B)
with a treatment liquid at a temperature in the range from room
temperature to the boiling point of the treatment liquid; D) drying
and/or dabbing the film treated according to step C) to remove the
treatment liquid from step C); and E) doping the film treated
according to step D) with a doping agent.
Inventors: |
Uensal; Oemer; (Mainz,
DE) ; Kiefer; Joachim; (Losheim am See, DE) ;
Baurmeister; Jochen; (Eppstein, DE) ; Pawlik;
Jurgen; (Frankfurt, DE) ; Kraus; Werner;
(Niedernhausen, DE) ; Jordt; Frauke; (Eppstein,
DE) |
Correspondence
Address: |
HAMMER & ASSOCIATES, P.C.
3125 SPRINGBANK LANE, SUITE G
CHARLOTTE
NC
28226
US
|
Family ID: |
7675913 |
Appl. No.: |
11/931734 |
Filed: |
October 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10468385 |
Jun 21, 2004 |
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PCT/EP02/02216 |
Mar 1, 2002 |
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11931734 |
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Current U.S.
Class: |
429/492 ;
528/423 |
Current CPC
Class: |
C08G 61/12 20130101;
B01D 67/0095 20130101; H01M 8/1048 20130101; B01D 71/62 20130101;
Y02P 70/50 20151101; H01M 8/103 20130101; Y02E 60/50 20130101; C08G
73/18 20130101; B01D 67/0088 20130101; C08G 73/22 20130101; H01M
8/1088 20130101 |
Class at
Publication: |
429/33 ;
528/423 |
International
Class: |
H01M 8/10 20060101
H01M008/10; C08G 73/06 20060101 C08G073/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 1, 2001 |
DE |
101 09 829.4 |
Claims
1. A doped polymer-membrane based on polyazoles, obtained by a
process comprising the steps A) casting a film using a solution of
polymers based on polyazoles in a polar, aprotic organic solvent,
B) drying the film formed in step A) until it is self-supporting to
form a dried film, said dried film having a residual content of
said polar, aprotic organic solvent in a range of 10 to 23%, C)
treating the film obtained in step B) with a treatment liquid at a
temperature in the range from 20.degree. C. to the boiling point of
the treatment liquid, D) drying and/or dabbing the film treated
according to step C) to remove the treatment liquid from step C),
E) doping the film treated according to step D) with a doping
agent, where said doping agent is selected from the group
consisting of: Lewis acids and BrOnsted acids, and where doping is
of a degree from 3 to 15 mol of acid per mol of recurring unit of
polymer.
2. A polymer membrane as claimed in claim 1 having a degree of
doping from 6 to 12 mols of acid per mol of recurring units of said
polymer.
3. A polymer membrane as claimed in claim 1, wherein the polymer
based on polyazoles comprises recurring azole units of the formula
(I) and/or (II) ##STR00003## where Ar are identical or different
and are each a tetravalent aromatic or heteroaromatic group which
may have one or more rings, Ar.sup.1 are identical or different and
are each a divalent aromatic or heteroaromatic group which may have
one or more rings, Ar.sup.2 are identical or different and are each
a trivalent aromatic or heteroaromatic group which may have one or
more rings, X are identical or different and are each oxygen,
sulfur or an amino group which bears a hydrogen atom and a group
having 1-20 carbon atoms, preferably a branched or unbranched alkyl
or alkoxy group, or an aryl group as other radical.
4. A polymer membrane as claimed in claim 3, wherein the polymer
comprising recurring azole units is a copolymer comprising at least
two units of the formula (I) and/or (II) which differ from one
another.
5. A polymer membrane as claimed in claim 4, wherein the polyazole
consists only of units of the formula (I) and/or (II).
6. A polymer membrane as claimed in claim 1, wherein the polyazole
is a polymer comprising recurring benzimidazole units of the
formula (III) ##STR00004## where n is an integer greater than or
equal to 10, preferably greater than or equal to 100.
7. A polymer membrane as claimed in claim 1, wherein doping is
performed for 1 to 96 hours at a temperature ranging from
20.degree. C. to 100.degree. C.
8. A membrane-electrode unit comprising at least one polymer
membrane as claimed in claim 1 and at least one electrode.
9. A polymer electrolyte fuel cell comprising at least one
membrane-electrode unit as claimed in claim 8.
10. A polymer membrane as claimed in claim 1 where said doping
agent is selected from the group consisting of: inorganic Lewis
acids and inorganic BrOnsted acids.
11. A polymer membrane as claimed in claim 7 where said doping
agent is selected from the group consisting of: sulfuric acid and
phosphoric acid.
12. A polymer membrane as claimed in claim 7 where said doping
agent phosphoric acid.
13. A polymer membrane as claimed in claim 12 where said phosphoric
acid has a concentration of 85%.
14. A doped polymer membrane based on polyazoles, obtained by a
process comprising the steps A) casting a film using a solution of
polymers based on polyazoles in a polar, aprotic organic solvent,
B) drying the film formed in step A) until it is self-supporting to
form a dried film, said dried film having a residual content of
said polar, aprotic organic solvent in a range of 10 to 23%, C)
treating the film obtained in step B) with a treatment liquid at a
temperature in the range from 20.degree. C. to the boiling point of
the treatment liquid, where said treatment liquid is selected from
the group consisting of: water, acetone and methanol, D) drying
and/or dabbing the film treated according to step C) to remove the
treatment liquid from step C), E) doping the film treated according
to step D) with a doping agent, where said doping agent is selected
from the group consisting of: sulfuric acid and phosphoric acid,
where said doping is performed for 1 to 96 hours at a temperature
ranging from 20.degree. C. to 100.degree. C. and said doping agent
has a concentration of 85%, and where doping is of a degree from 3
to 15 mol of acid per mol of recurring unit of polymer.
Description
RELATED APPLICATIONS
[0001] This case is a continuation of U.S. patent application Ser.
No. 10/468,385.
BRIEF DESCRIPTION OF THE DRAWINGS
[0002] The foregoing will become more readily apparent by referring
to the following detailed description and the appended drawings in
which:
[0003] FIG. 1 is a graph showing the results of the KF
filtration;
[0004] FIG. 2 is a graph showing proton conductivity;
[0005] FIG. 3 is a graph showing the results of tensile strength on
the polymer membranes.
DETAILED DESCRIPTION OF THE INVENTION
[0006] The present invention relates to an acid-doped polymer
membrane based on polyazoles, a process for producing it and its
use.
[0007] The acid-doped polymer membrane of the invention can be used
in a variety of applications because of its excellent chemical,
thermal and mechanical properties and is particularly useful as
polymer electrolyte membrane (PEM) in PEM fuel cells.
[0008] Acid-doped polyazole membranes for use in PEM fuel cells are
already known. The basic polyazole membranes are doped with
concentrated phosphoric acid or sulfuric acid and act as proton
conductors and separators in polymer electrolyte membrane fuel
cells (PEM fuel cells).
[0009] Due to the excellent properties of the polyazole polymer,
such polymer electrolyte membranes can, when processed to produce a
membrane-electrode unit (MEE), be used in fuel cells at continuous
operating temperatures above 100.degree. C., in particular above
120.degree. C. This high continuous operating temperature allows
the activity of the catalysts based on noble metals present in the
membrane-electrode unit (MEE) to be increased. Particularly when
using reformates from hydrocarbons, significant amounts of carbon
monoxide are present in the reformer gas and these usually have to
be removed by costly gas treatment or gas purification procedures.
The opportunity of increasing the operating temperature enables
significantly higher concentrations of CO impurities to be
tolerated over the long term.
[0010] The use of polymer electrolyte membranes based on polyazole
polymers enables, firstly, part of the costly gas treatment or gas
purification procedures to be omitted and, secondly, the catalyst
loading in the membrane electrode unit to be reduced. Both are
indispensable prerequisites for large-scale use of PEM fuel cells,
since otherwise the costs of a PEM fuel cell system are too
high.
[0011] The acid-doped, polyazole-based polymer membranes known
hitherto display a favorable property profile. However, owing to
the applications sought for PEM fuel cells, in particular in
automobile and stationary applications, they still require overall
improvement. Furthermore, the polymer membranes known hitherto have
a high content of dimethylacetamide (DMAc) which cannot be removed
completely by known drying methods.
[0012] Thus, the polyazole-based polymer membranes known hitherto
still display mechanical properties which are unsatisfactory for
the above application after they have been doped with acid. This
mechanical instability is reflected in a low modulus of elasticity,
a low ultimate tensile strength and a low fracture toughness.
[0013] It is an object of the present invention to provide
acid-doped polymembranes based on polyazoles which, firstly, have
improved mechanical properties and, secondly, have the advantages
of the polymer membrane based on polyazoles and allow an operating
temperature above 100.degree. C. without additional humidification
of the combustion gas.
[0014] We have now found that a specific after-treatment of the
polyazole-based film to be doped with acid surprisingly leads to
doped polymer membranes having improved mechanical properties, with
the good proton conductivity being retained or even improved. In
addition, the after-treatment rids the membrane of residual organic
constituents such as dimethylacetamide (DMAc) which would otherwise
reduce the catalyst activity.
[0015] The present invention provides a doped polymer membrane
based on polyazoles, obtainable by a process comprising the
steps
[0016] A) casting a film using a solution of polymers based on
polyazoles in a polar, aprotic organic solvent,
[0017] B) drying the film formed in step A) until it is
self-supporting,
[0018] C) treating the film obtained in step B) with a treatment
liquid at a temperature in the range from room temperature to the
boiling point of the treatment liquid,
[0019] D) drying and/or dabbing the film treated according to step
C) to remove the treatment liquid from step C),
[0020] E) doping the film treated according to step D) with a
doping agent.
[0021] The preparation of polymer solutions based on polyazoles has
been comprehensively described in the prior art. Thus, EP-A-0816415
describes a method of dissolving polymers based on polyazoles using
N,N-dimethylacetamide as polar, aprotic solvent at temperatures
above 260.degree. C. A substantially more gentle process for
preparing solutions based on polyazoles is disclosed in the German
patent application 10052237.8.
[0022] As polymers based on polyazoles, preference is given to
polymers comprising recurring azole units of the formula (I) and/or
(II)
##STR00001##
[0023] where
[0024] Ar are identical or different and are each a tetravalent
aromatic or heteroaromatic group which may have one or more
rings,
[0025] Ar.sup.1 are identical or different and are each a divalent
aromatic or heteroaromatic group which may have one or more
rings,
[0026] Ar.sup.2 are identical or different and are each a trivalent
aromatic or heteroaromatic group which may have one or more
rings,
[0027] X are identical or different and are each oxygen, sulfur or
an amino group which bears a hydrogen atom and a group having 1-20
carbon atoms, preferably a branched or unbranched alkyl or alkoxy
group, or an aryl group as other radical.
[0028] Preferred aromatic or heteroaromatic groups are derived from
benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane,
diphenyldimethylmethane, bisphenone, diphenyl sulfone, quinoline,
pyridine, bipyridine, anthracene and phenanthrene, all of which may
also be substituted.
[0029] Ar.sup.1 can have any substitution pattern; in the case of
phenylene, for example, Ar.sup.1 can be ortho-, meta- or
para-phenylene. Particularly preferred groups are derived from
benzene and biphenylene, each of which may also be substituted.
[0030] Preferred alkyl groups are short-chain alkyl groups having
from 1 to 4 carbon atoms, e.g. methyl, ethyl, n-propyl or isopropyl
and tert-butyl groups.
[0031] Preferred aromatic groups are phenyl or naphthyl groups. The
alkyl groups and the aromatic groups may be substituted.
[0032] Preferred substituents are halogen atoms such as fluorine,
amino groups or short-chain alkyl groups such as methyl or ethyl
groups.
[0033] If polyazoles comprising recurring units of the formula (I)
are used for the purposes of the present invention, the radicals X
within a recurring unit should be identical.
[0034] The polyazoles used according to the invention can in
principle also have different recurring units which differ, for
example, in their radical X. However, they preferably have only
identical radicals X in a recurring unit.
[0035] In a preferred embodiment of the present invention, the
polymer comprising recurring azole units is a copolymer comprising
at least two units of the formula (I) and/or (II) which differ from
one another.
[0036] In a particularly preferred embodiment of the present
invention, the polymer comprising recurring azole units is a
polyazole comprising only units of the formula (I) and/or (II).
[0037] The number of recurring azole units in the polymer is
preferably greater than or equal to 10. Particularly preferred
polymers comprise at least one 100 recurring azole units. For the
purposes of the present invention, polymers comprising recurring
benzimidazole units are preferably used. An example of an extremely
advantageous polymer comprising recurring benzimidazole units is
represented by the formula (III):
##STR00002##
[0038] where n is an integer greater than or equal to 10,
preferably greater than or equal to 100.
[0039] Casting of a polymer film from a polymer solution according
to step A) is carried out by means of measures known per se from
the prior art.
[0040] Drying of the film in step B) is carried out at temperatures
in the range from room temperature to 300.degree. C. Drying is
carried out under atmospheric pressure or reduced pressure. The
drying time depends on the thickness of the film and is preferably
from 10 seconds to 24 hours. The film which has been dried in step
B) is subsequently self-supporting and can be processed further.
Drying is carried out by means of drying methods customary in the
films industry.
[0041] As a result of the drying procedure carried out in step B),
the polar, aprotic organic solvent is very largely removed. Thus,
the residual content of the polar, aprotic organic solvent is
usually 10-23%.
[0042] A further lowering of the residual solvent content to below
2% by weight can be achieved by increasing the drying temperature
and drying time, but this significantly prolongs the subsequent
doping of the film, for example with phosphoric acid. A residual
solvent content of 5-15% is thus useful for reducing the doping
time.
[0043] The treatment of the film which has been dried in step B)
uses a treatment liquid and is out in the temperature range from
room temperature (20.degree. C.) and the boiling point of the
treatment liquid at atmospheric pressure.
[0044] As treatment liquid for the purposes of the invention and
for the purposes of step C.), use is made of solvents which are
liquid at room temperature [i.e. 20.degree. C.] selected from the
group consisting of alcohols, ketones, alkanes (aliphatic and
cycloaliphatic), ethers (aliphatic and cycloaliphatic), esters,
carboxylic acids, with the abovementioned group members being able
to be halogenated, water, inorganic acids (e.g. H.sub.3PO.sub.4,
H.sub.2SO.sub.4) and mixtures thereof.
[0045] Preference is given to using C1-C10 alcohols, C2-C5 ketones,
C1-C10-alkanes (aliphatic and cycloaliphatic), C2-C6-ethers
(aliphatic and cycloaliphatic), C2-C5 esters, C1-C3 carboxylic
acids, dichloromethane, water, inorganic acids (e.g.
H.sub.3PO.sub.4, H.sub.2SO.sub.4) and mixtures thereof.
[0046] The treatment liquid introduced in step C) can be removed by
means of the drying procedure carried out in step D). The drying
procedure depends on the partial vapor pressure of the treatment
liquid chosen. Drying is usually carried out at atmospheric
pressure and temperatures in the range from 20.degree. C. to
200.degree. C. More gentle drying can also be carried out under
reduced pressure. In place of drying, the membrane can also be
dabbed to free it of excess treatment liquid in step D). The order
is not critical.
[0047] In step E), the doping of the film obtained from step C) or
D) is carried out. For this purpose, the film is wetted with a
doping agent or laid in this. As doping agent for the polymer
membrane of the invention, use is made of acids, preferably all
known Lewis and BrOnsted acids, in particular inorganic Lewis and
BrOnsted acids.
[0048] Apart from these abovementioned acids, the use of polyacids,
in particular isopolyacids and heteropolyacids, and of mixtures of
various acids is also possible. For the purposes of the present
invention, heteropolyacids are inorganic polyacids which have at
least two different central atoms and are in each case partial
mixed anhydrides formed from weak, polybasic oxo acids of a metal
(preferably Cr, Mo, V, W) and a nonmetal (preferably As, I, P, Se,
Si, Te). They include, inter alia, 12-molybdophosphoric acid and
12-tungstophosphoric acid.
[0049] Doping agents which are particularly preferred for the
purposes of the invention are sulfuric acid and phosphoric acid. A
very particularly preferred doping agent is phosphoric acid
(H.sub.3PO.sub.4).
[0050] The polymer membranes of the invention are doped. For the
purposes of the present invention, doped polymer membranes are
polymer membranes which, owing to the presence of doping agents,
display increased proton conductivity compared to the undoped
polymer membranes.
[0051] Processes for preparing doped polymer membranes are known.
In a preferred embodiment of the present invention, they are
obtained by wetting a film of the polymer concerned with
concentrated acid, for example with highly concentrated phosphoric
acid, for a suitable time, preferably 5 minutes-96 hours,
particularly preferably 1-72 hours, at temperatures in the range
from room temperature to 100.degree. C. and atmospheric or
super-atmospheric pressure.
[0052] The conductivity of the polar membrane of the invention can
be influenced via the degree of doping. The conductivity increases
with increasing concentration of doping agent until a maximum value
has been reached. According to the invention, the degree of doping
is reported as mol of acid per mol of recurring units of the
polymer. For the purposes of the present invention, a degree of
doping of from 3 to 15, in particular from 6 to 12, is
preferred.
[0053] The polymer membrane of the invention has improved materials
properties compared to the previously known doped polymer
membranes. In particular, they have very good mechanical properties
and perform better than untreated membranes.
[0054] The polymer membranes of the invention display improved
proton conductivity compared to untreated membranes.
[0055] Possible applications of the doped polymer membranes of the
invention include, inter alia, use in fuel cells, in electrolysis,
in capacitors and in battery systems. Owing to their property
profile, the doped polymer membranes are preferably used in fuel
cells.
[0056] The present invention also relates to a membrane-electrode
unit comprising at least one polymer membrane according to the
invention. For further information on membrane-electrode units,
reference may be made to the specialist literature, in particular
the U.S. Pat. No. 4,191,618, U.S. Pat. No. 4,212,714 and U.S. Pat.
No. 4,333,805. The disclosure in the abovementioned references
[U.S. Pat. No. 4,191,618, U.S. Pat. No. 4,212,714 and U.S. Pat. No.
4,333,805] in respect of the structure and production of
membrane-electrode units is incorporated by reference into the
present description.
[0057] The invention is illustrated below by means of examples and
a comparative example, without the invention being restricted to
these examples.
EXAMPLES
Untreated Film
[0058] The untreated films were laid in 85% strength
H.sub.3PO.sub.4 for 96 hours. Prior to doping with H.sub.3PO.sub.4,
the H.sub.2O content and residual solvent content of the film are
determined by Karl Fischer (KF) titration. The water content of the
film is determined directly as follows by KF titration using a
Mettler-Toledo apparatus. The sample, which is present in a closed
sample vial, is heated to 250.degree. C. and dried at this
temperature. The gas liberated in this way is passed directly into
a closed titration vessel and analyzed by means of a Karl Fischer
[KF] reagent. Apart from the determination of the water content,
the residual solvent content is determined by determining the
weight before and after drying.
[0059] Washing with H.sub.2O and Subsequent Thermal Drying:
[0060] The films were boiled in water for 1 hour. The water bath
was then changed and the films were boiled for a further hour. The
films were subsequently rinsed with fresh water and finally dried
at 160.degree. C. for 3 hours. H.sub.2O content and residual
solvent content were determined on the treated films by KF
titration. The membranes were obtained by doping the films in 85%
strength H.sub.3PO.sub.4 for 96 hours.
[0061] Washing with H.sub.2O:
[0062] The films were boiled in water for 1 hour. The water bath is
then changed and the films are boiled for a further hour. The films
were subsequently dabbed with a cloth and used further in moist
form. H.sub.2O content and residual solvent content of the film
were determined by KF titration. The membranes were doped in 85%
strength H.sub.3PO.sub.4 for 96 hours.
[0063] Washing with Methanol:
[0064] The films were placed in methanol and boiled under reflux
for 2 hours (beginning when the methanol started to boil). The
films were taken out and dried firstly for 1 minute in air minute
in air and then at 100.degree. C. under reduced pressure in a
drying oven for 2 hours. H.sub.2O content and residual organic
solvent content of the film were determined by KF titration. The
membranes were doped in 85% strength H.sub.3PO.sub.4 for 96
hours.
[0065] Washing with Acetone:
[0066] The films were placed in acetone and boiled under reflux for
2 hours (beginning when the acetone started to boil). The films
were then dried firstly for 1 minute in air at RT and subsequently
at 100.degree. C. under reduced pressure in a drying oven for 2
hours. H.sub.2O content and residual solvent content of the film
were determined by KF titration. The membranes were doped in 85%
strength H.sub.3PO.sub.4 for 96 hours.
[0067] FIG. 1 shows the result of the KF titration. The residual
organic solvent is removed completely by washing with water. The
residual organic solvent content is reduced from 16.6% to 3.7 or
2.3% by washing with acetone or with methanol, respectively.
[0068] FIG. 2 shows a proton conductivity which is improved by 10%
even at room temperature and is retained or improved further at
elevated temperature.
[0069] The specific conductivity is measured by means of impedance
spectroscopy in a 4-pole arrangement in the potentiostatic mode
using platinum electrodes (wire, 0.25 mm diameter). The distance
between the current collector electrodes is 2 cm. The spectrum
obtained is evaluated using a simple model consisting of a parallel
arrangement of an ohmic resistance and a capacitor. The specimen
cross section of the membrane doped with phosphoric acid is
measured immediately before mounting of the specimen. To measure
the temperature dependence, the measuring cell is brought to the
desired temperature in an oven and the temperature is regulated via
a Pt-100 temperature sensor positioned in the immediate proximity
of the specimen. After reaching the temperature, the specimen is
maintained at this temperature for 10 minutes before commencement
of the measurement.
[0070] To determine the mechanical properties, uniaxial tensile
tests are carried out on tension bars. A Zwick tester equipped with
a 100 N load cell and a heatable oven is used for this purpose. The
length of specimen between the chucks is 10 cm and the separation
velocity is set at 50 mm/min. The deformation is determined
directly via the distance of travel. The tensile tests on membranes
doped with phosphoric acid are carried out at 100.degree. C. To
calculate the stress automatically, the cross section of each
specimen is determined and entered before commencement of the test.
To determine mean values of modulus of elasticity, tensile
strength, elongation at break and rupture energy (toughness), at
least 5 measurements are carried out on each membrane.
[0071] The results of the tensile tests on the polymer membranes
according to the invention compared to untreated membranes are
shown by way of example in FIG. 3. It can be seen from the figure
that a membrane washed with water has the highest elongation at
break and the highest tensile stress at break.
[0072] An untreated membrane displays an elongation at break of 55%
while a membrane according to the invention has an elongation at
break in the range from 58% to 75%.
[0073] The results of the tensile tests are summarized in Table
1.
TABLE-US-00001 TABLE 1 Results of the tensile tests on membranes
after different washing procedures compared to an untreated
membrane. Error in Error in Error in Tensile tensile Elongation
elongation Rupture rupture Washing E Error in E strength strength
at break at break energy energy Method [MPa] [MPa] [MPa] [MPa] [%]
[%] [kJ/m.sup.2] [kJ/m.sup.2] untreated 4.7 0.7 1.5 0.13 55 5 54 5
washed 5 0.55 1.7 0.25 71 11 74.5 18 with water washed 5.45 0.4
1.55 0.14 64.7 6 63 8.8 with acetone washed 5.3 0.5 1.36 0.22 61.2
13 54 18.6 with methanol
* * * * *