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KAJETAN MÜLLER ET AL.

The development of new formula-tions for film applications, includ-ing process optimization and the de-

termination of barrier properties in qual-ity monitoring, has until now been atime-consuming and cost-intensiveprocess. Determining gas permeation, inparticular, has proved a challenge in thisrespect, since these measurements cantake several days and, for high-barriertechnical applications, even several weeks.For this reason, it would be desirable todevelop a method that would allow thebarrier properties of polymer films to bemeasured during the actual productionprocess. This would also dramatically ac-celerate the development of new materi-als and process configurations, since therapid feedback of information on perme-

ation properties would make it possibleto carry out continual iterative develop-ment directly on the production line.

One approach to speeding up perme-ation measurement is to use gases withsmall molecules or atoms – such as heli-um. Permeation processes can also be ac-celerated by increasing the temperature.By these means, measuring times can bereduced by several orders of magnitudeas compared to standard measurementswith oxygen or carbon dioxide.

This article describes a rapid testmethod based on the above principles forspeeding up quality monitoring of filmsand the development of film materials. Toprovide meaningful values, the results ofthe helium permeation measurementsmust be correlated with those of stan-dardized methods. This process is dis-cussed as follows and used to evaluate thepossibilities and limitations of themethod. In conjunction with a direct filmextrusion line, the feasibility of using thismethod for accelerated material and

process optimization and rapid determi-nation of film quality is demonstrated.

Improving Barrier Properties

The permeability of commodity plasticsand also most high-performance plasticsto water vapor, oxygen and other mole-cules has already been intensively studiedand documented [1, 2] but is too high formost technical applications. Figure 1 showsthe permeability values of some typicalpackaging plastics. These values are stan-dardized to a film thickness of 100 µm.Because of the poor barrier properties ofthese plastics, the development of meas-ures to improve barrier performancestarted in the 1960s. Today, there are ba-sically four approaches to improving bar-rier properties in the plastics packagingsector [3]:� Vacuum coating methods:

Barrier layers can be applied onto theplastics using vacuum coating meth-ods. In the case of packaging film, the

Faster ResultsPermeation Measurement of Films. By using helium instead of other gases such

as oxygen and carbon dioxide, permeation measurement of films can be speeded

up considerably. Measurement with helium is therefore a very suitable rapid test

method for predicting gas permeability or monitoring film quality during produc-

tion.

Water vapor permeability [g/m2d at 23 °C, 85 % RH]

a)

104

103

102

101

100

10-1

10-2

10-15

10-16

10-17

10-18

10-19

10-20

10-2

Oxy

gen

perm

eabi

lity

[cm

3(S

TP)/

m2d

bar]

Perm

eatio

n co

effic

ient

[mol

m-1

Pa-1

s-1]

10-1

HDPELDPE

PPPS

PCPLA

PVC-P

PVC-UPET

PANPENPA6

Cellulose

PP

PCTFE

PVDC

EVOH, 38 %

EVOH, 44 %EVOH, 32 %

EVOH, 27 %

COCBOPP

100 101 102 103

Permeation coefficient[mol m-1 Pa-1 s-1]

10-15 10-14 10-13 10-12 10-11

Water vapor permeability [g/m2d at 23 °C, 85 % RH]

b)

103

102

101

100

10-1

10-2

10-16

10-17

10-18

10-19

10-20

10-2

Oxy

gen

perm

eabi

lity

[cm

3(S

TP)/

m2d

bar]

Perm

eatio

n co

effic

ient

[mol

m-1

Pa-1

s-1]

10-1 100 101 102

Permeation coefficient[mol m-1 Pa-1 s-1]

10-15 10-14 10-13 10-12

PCPSUETFE

PI

PVFPA6

PAN

PVOHORMOCERLCP

PVDC

PENPUR, 2C

PETAc

PVDFECTFE

Fig. 1. Permeability and permeation coefficient at 23°C of typical packaging plastics for a) food and b) technical products according to [3] (source: Fraunhofer IVV)

© Kunststoffe

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barrier layer is always covered by addi-tional polymer layers or plastic films.Today, some 15 billion m2 of packag-ing film are produced using thismethod. In most cases, PET or BOPPfilms are coated with aluminum, and asmall proportion with transparent ox-ides (silicon oxide, aluminum oxide)[4].

� Plastic containers with different barri-er layers:In these cases, the primary material isusually PET. The vast majority of thesecontainers with excellent barrier prop-erties have a multi-layer structure. Tofurther improve barrier performance,exfoliated plastic particles are increas-ingly being incorporated in the poly-mer. A smaller proportion of plasticcontainers are coated on the inner sur-face, e.g. with silicon oxides.

� Nanocomposites:The third category has so far been usedfar less than the two previously men-tioned categories. In this case,homoge-nously dispersed nanoparticles arecompounded into the polymer matrixas a barrier layer to inhibit gas perme-ability.

� Multi-layer films:Finally, a fourth class of materials hasbeen developed for packaging techni-

cal articles or sensitive foods, for whichthe very highest barrier requirementsare specified. This class of materials forthe protection of e.g. thin-film batter-ies or optoelectronic devices has a mul-ti-layer structure. Here, inorganic andpolymer layers are deposited alternate-ly on a flexible polymer substrate.

Measurement of PermeationProperties

The oxygen and water vapor permeabil-ity of plastic films are most commonlymeasured using specific carrier gas meth-ods. These test methods are standardized,such as DIN 53380-3 for oxygen perme-ability. The disadvantages of these testmethods are costs and measuring time.For example, to determine the oxygenpermeability of a 100 µm PET film takes

about two days – owing to the relativelylow diffusion coefficient. Measuring timecan be considerably reduced with the rap-id test described here.

Measuring time depends on the timerequired for a steady state to be reachedand therefore on the dimensionless timeor Fourier number F0 [5]:

(1)

This means that the measuring time t ishalved when the diffusion coefficient D isdoubled and quadrupled when the layerthickness l is doubled.

In addition, diffusion D, solubility Sand permeation P are dependent on thetesting temperature T according to theArrhenius equation, where ΔHS is theheat of solution, ED the activation energyand R the universal gas constant [6]:

(2)

Therefore, a gas with a high diffusion co-efficient, such as helium, should be usedfor a rapid test. In addition, the equationshows that the measuring time can be

further reduced by increasing the testingtemperature. The diffusion coefficientsof helium in different plastics are high-er than for oxygen by a factor of 15 upto 500 [2]. This gives the penetrationtimes shown in Table 1. To obtain the ac-tual testing time (including zero pointmeasurement and setting the perme-ation equilibrium), the penetration timemust be multiplied by a factor of about20. This means that the measurement ofhelium permeation through films witha thickness of 100 µm takes only a fewminutes.

Permeation Coefficient of Filled Plastics

Permeation is dependent not only on dis-persion but also on filler content (see equa-tion 3). If a polymer is filled with largely

impermeable particles accounting for avolume fraction ϕ, then the gas moleculescan essentially only diffuse through theremaining volume fraction of the poly-mer (1–ϕ). In this case, the diffusion pathx can be lengthened.

The ratio of the permeation coefficientof a filled specimen P1 to that of the un-filled specimen P0 can be represented asfollows [7]:

(3)

The lengthening of the effective diffusionpath x can be described with the aid of τ. ,where τ. is the ratio of the average lengthof the diffusion path of a gas molecule tothe specimen thickness.

If now the geometry of the particles isconsidered to be the ratio of the length Lto the width w (known as the aspect ra-tio, then the permeation coefficient P1 canbe estimated as follows [7]:

(4)

Rapid Permeation TestDeveloped

At the German Plastics Center (DasKunststoffzentrum SKZ) in Würzburg,Germany, a rapid permeation test (Fig. 2)was developed and introduced in collab-oration with the Fraunhofer Institutefor Process Engineering and Packaging(Fraunhofer IVV) in Freising, Germany.A manometric measuring system fortests as per DIN 53380-2 is coupled witha helium leak detector. With the inte-grated mass spectrometer, very smallhelium leakage rates down to below5·10–12 mbar·l·s–1 can be detected. Directhelium detection has the advantage of in-creasing measuring accuracy by a factorof 104 as compared with manometric he-lium permeation measurement.

Figure 3 shows a schematic representa-tion of the at-line permeation measure-ment. In this diagram, the helium flow isplotted logarithmically against the meas-uring time.

In this test, the top measuring cell isfirst flushed with nitrogen to desorb anydissolved helium gas molecules thatmight still be present in the polymer. Asa rule, this operation takes 1 to 2 min withthe tested film specimens. The requiredflushing time is, however, dependent onthe material, thickness and quality of thetest specimen. As soon as a constant val-

Penetration time [min]

Material O2 CO2 He

LDPE 0.6 0.8 0.04

PVC 23 111 0.10

PET 62 335 0.14

PA11 15 0.08

Table 1. Penetrationtimes in measure-

ment of O2, CO2 andhelium permeation

through some select-ed plastics

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ue is achieved with the nitrogen flushing(zero point measurement), it is possibleto switch over to helium (see Fig. 3, A). Thehelium gas molecules now need a certainamount of time to diffuse through thepolymer to the underside of the speci-men. The measuring time until a steadystate, i.e. constant volumetric flow rate, isestablished, ranges between 3 and 5 minfor the monofilms tested.

The gas molecules now desorb in thelower measuring cell. The resulting heli-um leakage rates can be determined withthe coupled mass spectrometer based onthe change in quantity and concentration.The helium leakage rate when the finalsteady state is reached is used as a refer-ence value for determining the perme-ation value in the rapid test (Fig. 3, B).

So the total measuring time for this gaspermeation test is composed of the timerequired for nitrogen flushing plus thetime for subsequent measurement of he-lium permeation (about 4 to 7 min).

With this rapid test system, the heliumleakage rate Q

.can be determined in

mbar·l/s. However, helium permeabilityis generally quoted in the unitcm3·cm/(cm2·s·bar). With the aid of testarea A and pressure difference Δp, the he-

lium permeability Q can be calculatedfrom the helium leakage rate:

(5)

Application in Direct FilmExtrusion

Figure 4 shows the schematic layout of thedirect extrusion line combined with therapid permeation test system, which is in-tegrated into the production line as an at-line measuring unit for accelerated mate-rial and process development.

The direct extrusion line was set up ac-cording to this layout in the processinglaboratory of the SKZ (Fig. 5). This trial fa-cility consists of a co-rotating twin-screwextruder (Leistritz ZSE 27Maxx, manu-facturer: Leistritz, Nuremberg, Ger-many), a flat-film die (width approx.350 mm) and a roll unit. For taking sam-ples from the running production lineand subsequent at-line permeationmeasurement, a special sampling devicewas developed and integrated into the ex-trusion line. Through the combinationof a film storage device and specimenpunch, it is possible to punch out film

disks from the moving film web duringproduction.

Results

Measuring Time and Reproducibility:The previous estimates of measuring timewere confirmed by the measurements.The time savings are illustrated by themeasuring times for a 100 µm PET film.The total testing time for helium perme-ation measurement, excluding test spec-imen preparation, is about 7 min, whichcomprises about 3 min for zero pointmeasurement and another 4 min for de-termination of the actual test value.

Fig. 2. Layout of therapid permeationmeasurement systemfor at-line use (figure: SKZ)

Measurement of theHe leakage rate

Measuring time

Flushing with N2

SteadyHe flow

He

flow

A

B

Fig. 3. Schematic rep-resentation of perme-ation measurement inthe rapid test method(A: switching from ni-trogen to helium (zeropoint measurement),B: helium leakagerate) (source: SKZ)

© Kunststoffe

Extruder

Feed

Permeationmeasurement

Calender

Accelerated optimization of barrier propertiesthrough at-line measurement of permeation

Sampling device

Fig. 4. Schematic dia-gram of the direct

film extrusion linewith the at-line per-

meation measure-ment system for ac-celerated material

and process develop-ment (figure: SKZ)

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The total time required to measure theoxygen permeation through a similar100 µm PET film according to the rele-vant standard is 48 h (= 2,880 min). Sothe helium rapid test method is 360 timesfaster than the conventional measuringtime.

In Figure 6, the helium permeation co-efficient P is plotted against filler con-tent ϕ for various HDPE/talc compoundsthat were processed directly into filmwith a thickness of 250 µm by the directextrusion process. In addition to the fillercontent, the filler feed position/methodwas varied. The polymer was introducedvia the main feed opening and the talc ei-ther via a side feed opening, the main feedopening or as a 50 % masterbatch.Through repeat measurements, it waspossible to achieve very high repro-ducibility in this measuring method, witha variance of about ±1 %. So test speci-men preparation and the way in whichthe measuring equipment is handled

have only minimal influence on the testvalues.

Influence of Filler Content on Heli-um Permeation: Up to a filler content ofabout 5 %, it may be assumed that themorphological properties of the polymerpredominate, because with all three feedoptions there is a significant increase inthe permeation coefficient (see Fig. 6). Itmay be concluded from this that the filmsin these cases have more amorphous re-gions and smaller spherulites. This phe-nomenon has already also been observedwith polypropylene/talc compounds [8].It is clear that, from a filler content of 5 %,the properties of the fillers ensure an ap-preciably lower permeation coefficient. Inother words, the properties of the filleronly start to predominate from a talc con-tent of 5 %.

The relatively highest permeation co-efficients were obtained when the fillerswere added via the main feed opening(Fig. 6). In this case, reagglomeration (Fig. 7)of the filler particles takes place in the

melting range, which prevents homoge-neous dispersion of the talc in the HDPEbase polymer.

Calculation of the permeation coeffi-cient of an HDPE/talc compound with afiller content of 10 % P1 according to equa-tion 4 gives a value of 3,321 cm3·100 µm/(m2·d·bar). The assignment of a value of10 for the aspect ratio was taken from theliterature [9]. In this example, the differ-ence between the measured and estimat-ed permeation coefficients for helium isabout 2 %. Similarly accurate predictionswere also obtained with other com-pounds of polyamide 6 and nanoclays.From this, it is clear that a good estimateof the permeation coefficient of a filledmonofilm can be made in advance.

Correlation between Helium,Oxygenand Carbon Dioxide Permeationthrough Monofilms: Figure 8 shows thecorrelation between oxygen and carbondioxide permeation through talc-filledHDPE and helium permeation. Oxygenand carbon dioxide permeation were

Fraunhofer-Institut für Verfahrenstech-nik und Verpackung IVVD-85354 FreisingGermanyTEL +49 8161 491-120> www.ivv.fraunhofer.de

Das Kunststoff-Zentrum SKZ D-97076 WürzburgGermanyTEL +49 931 4104-447> www.skz.de

Contacti

Fig. 5. Layout of thedirect extrusion line(figure: SKZ)

Filler content φ

5,500

5,000

4,500

4,000

3,500

3,000

2,500

00

Perm

eatio

n co

effic

ient

P[c

m3×

100

µm/(

m2d

bar)

]

2 4 6 8 10 12 14 16 18 % 22

Side feed openingMain feed openingMasterbatch

Fig. 6. Helium perme-ation coefficient(measured at 40°C) of HDPE/talc com-pounds as a functionof filler content fordifferent feed options(source: SKZ)

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measured with a manometric measuringsystem according to DIN 53380-2. Heli-um permeation was determined using thehelium rapid permeation test already de-scribed.

In a direct comparison of gas perme-ability, permselectivity must be taken in-to account. Permselectivity means in thiscase that the film has different barrierproperties for different gases, with no lin-ear correlation between the properties.There is a very good correlation betweenthe oxygen and helium permeability, withoxygen/helium permselectivity beingabout 0.4 1. This corresponds to the typ-ical value for unfilled HDPE [2]. There isalso a correlation with carbon dioxide butwith a somewhat lower coefficient of de-termination of 0.91 than for oxygen.These results demonstrate the suitabilityof helium permeation measurement as arapid test method for the functional eval-uation of filled plastics.

Correlation between Helium andOxygen Permeation through Compos-ite Films with EVOH Barriers: Threecomposite films with the structure PP/ad-hesion promoter/EVOH/adhesion pro-moter/PP with different EVOH layerthicknesses were produced at the Fraun-hofer Institute for Process Engineeringand Packaging (Fraunhofer IVV) on a

7-layer coextruder manufactured by theDr. Collin company in Ebersberg, Ger-many.EVOH is commonly used as an oxy-gen barrier in packaging materials for sen-sitive foods. The EVOH grade selectedhere contained 38 % ethylene.The EVOHlayer thickness, which is crucial for gaspermeability, averaged 5 µm, 8 µm and10 µm respectively for the three samplefilms produced. The total thickness of thefilms in all three cases was around 50 µm.

Figure 9 shows the correlation betweenthe oxygen and helium permeability ofthese composite films. Just as with thepreviously described monofilms, there isagain a very good correlation between thehelium and oxygen permeability of thecomposite films. Consequently, the heli-um permeation measurement can also beused as a rapid test method in the devel-opment of barrier films with EVOH. As aresult, development cycles and testingtimes in quality control can be signifi-cantly shortened.

Correlation Limits: Besides the advan-tages of the helium rapid test method, itslimitations and special characteristicsmust also be taken into account. For thispurpose, the helium and oxygen perme-ability values and permselectivities of anumber of monofilms are compared inTable 2.

A crucial factor determining the suit-ability of helium permeation measure-ment as a rapid test method for predict-ing e.g. oxygen permeability is permse-lectivity P(He)/P(O2), i. e. whether it ispossible to define a fixed ratio betweenhelium and oxygen permeability. Perms-electivity differs significantly between dif-ferent plastics.Whereas the permselectiv-ity of non-polar polyolefins is 2 or 3, thepermselectivity of polar plastics such as

PET is much higher with a value rangingfrom 40 to 50. It can be concluded, thatthe helium permeation measurementcannot be used as a method for predict-ing oxygen permeability across the wholerange of plastics but always only within aplastics group.

The same also applies to compositefilms. The permselectivity P(He)/P(O2)of composite films with EVOH barrierstypically ranges from 400 to 500. On the

other hand, the permselectivityP(He)/P(O2) of other barrier films, suchas metalized PET films, is between 40 and50, i. e. within the same range as thepermselectivity of unmetalized PET.

Conclusion

The advantage of the helium rapid testover permeation measurement with oth-er gases such as oxygen or carbon diox-ide lies in the significantly shorter meas-uring time. Depending on the materialcombination, the measuring time withmonofilms is only a few minutes andtherefore some two orders of magnitude

faster than with e.g. oxygen permeationmeasurement by standard methods. Itwould therefore seem sensible to use he-lium permeation measurements as a rap-id test method for predicting the perme-ability of mono- and composite films toother gases such as oxygen or carbondioxide.

This is perfectly possible provided thatsome limiting conditions are observed.Only material combinations with approx-

Fig. 7. Scanning electron micrograph of a talcagglomerate (figure: SKZ)

4,000

3,000

2,000

1,000

00

Gas

perm

eabi

lity

[cm

3×

100

µm/(

m2d

bar)

]

Helium permeability[cm3×100 µm/(m2d bar)]

500 1,000 1,500 2,000 2,500 3,000

P (CO2) = 1.29×P (He)R2 = 0.91

P (O2) = 0.41×P (He)R2 = 0.99

Fig. 8. Correlation of the oxygen and carbon dioxide permeability (meas-ured at 23°C) of HDPE/talc compounds with their helium permeability(source: Fraunhofer IVV)

© Kunststoffe

Helium permeability [cm3/(m2d bar)]

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

00

Oxy

gen

perm

eabi

lity

[cm

3/(

m2d

bar)

]

200 400 600 800 1,000 1,200 1,400

Q (O2) = 0.0018 ×Q (He)R2 = 0.98

Fig. 9. Correlation of the oxygen permeability (measured at 23°C) of PP/adhesion promoter/EVOH/adhesion promoter/PP composite films withtheir helium permeability (source: Fraunhofer IVV)

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imately the same permselectivity can becompared. A typical field of applicationwould be the further development of bar-rier systems, in which no fundamentalchanges to the film composition are be-ing made, such as in the further develop-ment of composite films with EVOH bar-riers or with metalized aluminum barri-ers. Within these groups, there is compa-rable permselectivity. On the other hand,direct comparison between e.g. compos-ite films with EVOH barriers and metal-ized composite films is not possible.

The helium rapid test method is alsovery suitable for at-line production mon-itoring of film quality in a product seriesafter defined quality limits have beenspecified. With the rapid test method,possible shortcomings in gas permeabil-ity can be identified at an early stage, soenabling reject rates to be significantly re-duced as compared with using the con-ventional, very protracted test methods.�

This article is dedicated to Prof. Dr.-Ing.Dr.-Ing. E. h. Walter Michaeli on theoccasion of his 65th birthday and hisretirement.

ACKNOWLEDGMENT

Joint Industrial Research and Development (IGF) proj-ect 15968N of the German Industry Association forFood Technology and Packaging (Forschungsvereini-gung IVLV e.V.) was funded through the German Fed-eration of Industrial Research Associations (AiF) un-der the Program to Promote Joint Industrial Researchand Development (IGF) set up by the German FederalMinistry of Economics and Technology (Bundesminis-terium für Wirtschaft und Technologie) on the basis ofa decision by the German Federal Parliament (Bun-destag). We would like to express our thanks for thisfinancial support.

REFERENCES

1 Comyn, J.: Polymer Permeability. Ed., Chapman &Hall, London, 1985

2 Pauly, S.: Permeability and Diffusion Data. In: J.Branddrup, E. H. Immergut (Ed.): Polymer Handbook.Wiley Interscience Publication, New York 1989,pp. 435–449

3 Langowski, H.-C.: Permeation of Gases and Con-densable Substances Through Monolayer and Multi-layer Structures. In: Piringer, O.G.; Baner, A.L.: Plas-tic Packaging, Interactions with Food and Pharma-ceuticals. 2nd edition, Wiley-VCH, Weinheim 2008

4 Smith, A. W.; Copeland, N.: 49th Annual TechnicalConference Proceedings, Society of VacuumCoaters, Albuquerque 2006, pp. 636–641

5 Müller, K.: Oxygen Permeability of Plastic Bottlesfor Oxygen Sensitive Beverages. Brewing Science65 (2007) 3, pp. 74–83

6 Fujita, H.: Diffusion in Polymer-diluent systems.Fortschr. Hochpolym. Forsch. 3 (1961) 1, pp. 1–47

7 Sperling, H.: Introduction to Physical Polymer Sci-ence. Wiley Interscience Publication, New York2006

8 Ferrage, E.: Talc as nucleating agent of polypropy-lene: morphology induced by lamellar particles ad-dition and interface mineral-matrix modelization.Journal of Materials Science 37 (2002) 8,pp. 1,561–1,573

9 Xanthos, M.: Functional Fillers for Plastics. Wiley-VCH, Weinheim 2005

THE AUTHORS

DR.-ING. KAJETAN MÜLLER, born in 1970, is ascientist at the Fraunhofer Institute for Process Engi-neering and Packaging (Fraunhofer-Institut für Ver-fahrenstechnik und Verpackung IVV) in Freising, Ger-many, where he is responsible for cross-disciplinaryactivities in the area of material development, barrierproperties, and content quality.

DIPL.-ING. JULIA BOTOS, born in 1986, is a scien-tist at the German Plastics Center (SüddeutschesKunststoff-Zentrum – SKZ) in Würzburg, Germany,where she works for the Measuring Technology Busi-ness Area in the Plastics Research and DevelopmentDivision on topics including the gas permeability ofplastics.

PROF. DR.-ING. PROF. MARTIN BASTIAN, born in1966, has been institute director of the SKZ inWürzburg since 2006. He is also Professor for Func-tional Polymer Materials at the University ofWürzburg.

DR.-ING. PETER HEIDEMEYER, born in 1959, hasbeen managing director of the Plastics Research andDevelopment Division of the SKZ in Würzburg since2009.

DIPL.-PHYS. THOMAS HOCHREIN, born in 1979, ishead of the Measuring Technology Business Area inthe Plastics Research and Development Division ofthe SKZ in Würzburg.

Test specimen Thickness He O2 P(He)/P(O2)

[µm] P [cm3·100 µm/(m2·d·bar)]

LDPE specimen 1 76 3,610 1,718 2.1

LDPE specimen 2 74 3,678 1,761 2.1

HDPE specimen 1 317 1,686 663 2.5

HDPE specimen 2 181 3,204 1,421 2.3

PS 175 10,745 1,509 7.1

PLA specimen 1 49 6,370 176 36.2

PLA specimen 2 47 6,580 166 39.5

ETFE 205 7,688 638 12.1

PET 12 µm specimen 1 12 629 15 40.9

PET 12 µm specimen 2 12,5 664 16 41.2

PET 100 µm specimen 3 99.5 672 16 41.7

PET 100 µm specimen 4 101 582 12 49.7

PEN 100 µm 97 371 4.5 82.4

Table 2. Comparison between the helium and oxygen permeability values and permselectivities ofdifferent monofilms

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