Permeability Properties of Plastics and Elastomers

344 Pages·2012·15.311 MB·English

by  Laurence W. Mckeen (Auth.)

To mydaughter LindseyMcKeen Polizzotti, who continuesto amaze me
PLASTICS DESIGN LIBRARY(PDL)
PDL HANDBOOK SERIES
SeriesEditor:Sina Ebnesajjad,PhD
President, FluoroConsultantsGroup, LLC
ChaddsFord,PA,USA
www.FluoroConsultants.com
ThePDLHandbookSeriesisaimedatawiderangeofengineersandotherprofessionalsworkingintheplasticsindustry,
and related sectors using plastics and adhesives.
PDLisaseriesofdatabooks,referenceworksandpracticalguidescoveringplasticsengineering,applications,processing,
and manufacturing, andapplied aspects of polymer science, elastomers and adhesives.
Recenttitles inthe series
Sastri, Plastics inMedical Devices
ISBN:9780815520276
McKeen, Fatigue andTribological PropertiesofPlastics andElastomers,Second Edition
ISBN:9780080964508
Wagner, Multilayer Flexible Packaging
ISBN:9780815520214
Chandrasekaran,RubberSeals for Fluid and HydraulicSystems
ISBN:9780815520757
Tolinski, Additivesfor Polyolefins
ISBN:9780815520511
McKeen, TheEffectofCreep and Other TimeRelated Factorson Plasticsand Elastomers, Second Edition
ISBN:9780815515852
Ebnesajjad, Handbookof AdhesivesandSurfacePreparation
ISBN:9781437744613
Grot,Fluorinated Ionomers,SecondEdition
ISBN:9781437744576
To submit anewbook proposal for the series, please contact
Sina Ebnesajjad,SeriesEditor
[email protected]
or
Matthew Deans, Senior Publisher
[email protected]
To mydaughter LindseyMcKeen Polizzotti, who continuesto amaze me
PLASTICS DESIGN LIBRARY(PDL)
PDL HANDBOOK SERIES
SeriesEditor:Sina Ebnesajjad,PhD
President, FluoroConsultantsGroup, LLC
ChaddsFord,PA,USA
www.FluoroConsultants.com
ThePDLHandbookSeriesisaimedatawiderangeofengineersandotherprofessionalsworkingintheplasticsindustry,
and related sectors using plastics and adhesives.
PDLisaseriesofdatabooks,referenceworksandpracticalguidescoveringplasticsengineering,applications,processing,
and manufacturing, andapplied aspects of polymer science, elastomers and adhesives.
Recenttitles inthe series
Sastri, Plastics inMedical Devices
ISBN:9780815520276
McKeen, Fatigue andTribological PropertiesofPlastics andElastomers,Second Edition
ISBN:9780080964508
Wagner, Multilayer Flexible Packaging
ISBN:9780815520214
Chandrasekaran,RubberSeals for Fluid and HydraulicSystems
ISBN:9780815520757
Tolinski, Additivesfor Polyolefins
ISBN:9780815520511
McKeen, TheEffectofCreep and Other TimeRelated Factorson Plasticsand Elastomers, Second Edition
ISBN:9780815515852
Ebnesajjad, Handbookof AdhesivesandSurfacePreparation
ISBN:9781437744613
Grot,Fluorinated Ionomers,SecondEdition
ISBN:9781437744576
To submit anewbook proposal for the series, please contact
Sina Ebnesajjad,SeriesEditor
[email protected]
or
Matthew Deans, Senior Publisher
[email protected]
PERMEABILITY PROPERTIES OF
PLASTICS AND ELASTOMERS
Third Edition
Laurence W. McKeen
Amsterdam(cid:1)Boston(cid:1)Heidelberg(cid:1)London(cid:1)NewYork(cid:1)Oxford
Paris(cid:1)SanDiego(cid:1)SanFrancisco(cid:1)Singapore(cid:1)Sydney(cid:1)Tokyo
WilliamAndrewisanimprintofElsevier
William Andrewisanimprintof Elsevier
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Firstedition 1995
Second edition2003
Third edition 2012
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Preface
Thisbookhasisanextensiveupdateandextension The data in each chapter are generally organized
to the second edition by the same title. The second with chemistry, a manufacturer and trade name list,
editionwaspublishedin2002,andalothaschanged an applications and uses list followed by the data.
in the field since then. There are new plastic mate- Tabular data are first, followed by graphical data.
rials. There has been an expanded interest in green The tabular data from the second edition has been
materials, those made from renewable resources or verified and reformatted to take up much less space,
those that decompose relatively quickly in the envi- whereas this new edition does not have many more
ronment. There has been a turnover in ownership of pages,thereisfarmoreinformationcontained.Alist
the plastic producing companies. There has been of conversion factors for gas permeation and vapor
a lot of consolidation, which of course means dis- transmission measures is also included. There is an
continued products. This update is much more appendix that covers the important application of
extensive than the usual “next edition.” gloves. There is also an appendix of standard fuels
Ithasbeenreorganizedfromapolymerchemistry used in testing plastic for auto fuel systems appli-
point of view. Plastics of mostly similar polymer cations,animportantuseastheindustrymovesaway
types are grouped into 10 data chapters. A brief from metal to plastics to saveweight.
explanationofthechemistryofthepolymersusedin Numerous references are included. Some data
the plastics is discussed at the start of each plastic from the earlier edition have been removed or
section. replaced with updated data. Removed data include
An extensive introduction has been added as four discontinued products. Product names and manu-
chapters. The initial chapter focuses on permeation, facturers have been updated.
what it is, how it occurs, is measured and data are I am especially appreciative of the confidence,
presented. The second chapter covers polymer support, and patience of my friend Sina Ebnesajjad.
chemistryandplasticscompositionandhowitrelates He was also the primary proofreader of the manu-
to permeation. The third chapter focuses on script.Iwouldnothavebeengiventheopportunityto
productionoffilms,containers,andmembranes.The do this work had it not been for the support of
fourthchapterfocusesontheusesofbarrierfilmsand Matthew Deans, Senior Publisher at Elsevier. I also
membranes. Membranes were largely ignored in the wish to acknowledge Bill Bennett, Marketing
previous editions, but the function of membranes is Manager, Americas Region of Ansell Limited and
critically affected by their permeation properties. Joe Yachanin of Force 12 Design Ltd for their work
Chapters5through14areadatabankthatservesas on the permeation of gloves index. My family has
anevaluationofpermeationperformanceofplastics. been particularly supportive through the long hours
Each of these chapters starts with a brief outline of of writing and research from my home office.
the chemistry of the polymer in that section. There
are hundreds of uniform graphs and tables for more Laurence McKeen
than60genericfamiliesofplasticsthatarecontained 2011
in these chapters.
xi
1 Introduction to Permeation of
Plastics and Elastomers
This book is about the passage of liquids, vapors, membranes. The fourth chapter focuses on the uses
orgasesthroughplasticorpolymericmaterials,such of barrier films, containers, and membranes.
asfilms,membranes,andcontainers.Thepassageof Chapters 5 to 14 are a databank that serves as an
small molecules through solid materials is called evaluation of permeation performance of plastics.
permeation. Permeability properties are important Each of these chapters starts with a brief outline of
in many applications and are important in everyday the chemistry of the polymer in that section. There
life. The small molecule that passes through the are hundreds of uniform graphs and tables for more
solidiscalledthepermeant.Thepermeabilityofthe than60genericfamiliesofplasticsthatarecontained
packaging materials (wrapping films, containers in these chapters. The data in each chapter are
seals, closures, etc.) needs to be matched with the generally organized with chemistry, a manufacturer
sensitivityofthepackagedcontentstothatpermeant andtradenamelist,andanapplicationsanduseslist
and the specified shelf life. Some packages must followed by the data.
have nearly hermetic seals, while others can
(and sometimes must) be selectively permeable.
Knowledge about the exact permeation rates is 1.1 History
thereforeessential.Therearemanyexamplesforthe
importance of permeation properties, some are as Itappearsthatthefirstscientificmentionofwater
follows: permeation was made by a physicist, Abbe´ Jean-
Antoine Nollet (1700e1770).1 Nollet sealed wine
(cid:1) Theairpressureintiresshoulddecreaseasslowly containerswithapig’sbladderandstoredthemunder
as possible, so it is good to know which gas water. After a while, he noticed that the bladder
permeates slowest through the rubber wall and at bulged outward. Because of his scientific curiosity,
what rate. he did the experiment the other way round: he filled
(cid:1) Packaged meat needs to retain its moisturewithin the container with water and stored it in wine. The
result was a shrinking inward of the bladder. This
the package to prevent it from drying out, but it
was an evidence of water permeation through the
needs to keep oxygen out too, which slows down
pig’s bladder from an area of high concentration to
spoilage.
an area of lower concentration.
(cid:1) Thewatervaporpermeationofinsulatingmaterial
The first study of gas permeation through a poly-
isimportanttoprotectunderlyingfromcorrosion. mer was conducted by Thomas Graham in 1826.2
(cid:1) To meet legal regulations, e.g., California Air Graham observed a loss in volume of a wet pig
Resource Board (CARB) for low-emission vehi- bladder inflated with carbon dioxide. In 1831, John
cles, it is essential to use barrier materials for Kearsley Mitchell, professor of medicine and phys-
fuel hoses and tanks. iology at the Philadelphia Medical Institute,
observedthatballoonsshrunkatdifferentrateswhen
Thisfirstchapterfocusesonpermeation,whatitis, they were filled with different gases.
how it occurs, how it is measured, and how its data In 1856, Henry Gaspard Philibert Darcy (born
are presented. The second chapter covers polymer 1803) formulated his homogeneous linear law of
chemistry and plastics composition and how watertransportthroughaporousmedium(essentially
composition affects permeation. The third chapter sand and gravel beds). Darcy’s law is an important
concentrates on production of films, containers, and basic relationship in hydrogeology. Examples of its
PermeabilityPropertiesofPlasticsandElastomers.DOI:10.1016/B978-1-4377-3469-0.10001-3
Copyright(cid:1)2012ElsevierInc.Allrightsreserved. 1
2 PERMEABILITY PROPERTIES OF PLASTICS AND ELASTOMERS
application include the flow of water through an 3) Prolonged exposure to elevated temperature
aquifer and the flow of crude oil, water, and gas affected the retention capacity of the
throughtherockinpetroleumreservoirs.Darcy’slaw membrane.
isarelationshipbetweentheinstantaneousdischarge
4) Differences in the permeability could be
rate through a porous medium, the viscosity of the
exploited for application in gas separations.
fluid, and the pressure drop over a given distance.
5) Variation in membrane thickness altered the
Darcy’s law in equation form is given in Eqn (1).
permeation rate but not the separation charac-
teristics of the polymer.
(cid:3)kAðP (cid:3)P Þ
Q ¼ b a (1)
mL
1.2 Transport of Gases and
where (refer Fig. 1.1) Vapors through Solid Materials
Q¼totaldischargerate(unitsofvolumepertime,
such as m3/s) There are two ways for small molecules to pass
k ¼ permeability of the medium (units of area, through a solid material. One is by passing through
such as m2) a small hole or leak. The second is for the small
A ¼ cross-sectional area to flow (units of area, molecule to work its way through the solid between
such as m2) the small spaces between the molecules (in the case
(P (cid:3)P )¼pressuredrop[unitsofpressure,such of polymers or plastics) or atoms in the crystal
b a
structure of inorganic solids or metals.
as Pa (Pascal)]
m ¼ dynamic viscosity (units such as Pas)
L ¼ the length, where the pressure drop is taking
1.2.1 Effusion
place (units such as m).
The permeability k from Darcy’s law should not Passage of a gas or a liquid may occur through
be confused with the permeability coefficient P, adefectsuchasapinholeinafilm.Althoughtheend
describedlaterin thischapter,usedwhendescribing result might be the same, the passage of the gas or
permeation through plastic materials. liquid, this is not permeation. This type of transport
A theory for gas permeation through polymeric through a film is more appropriately called effusion.
materials was not developed until 1866, when Effusion is governed by Graham’s law, also known
Thomas Graham proposed the solution-diffusion as Graham’s law of effusion. It was formulated by
process, where he postulated that the permeation Scottish physical chemist Thomas Graham. Graham
process involved the dissolution of penetrant, fol- found through experimentation that the rate of effu-
lowed by transmission of the dissolved species sion of a gas is inversely proportional to the square
through the membrane. The other important obser- root of the mass of its particles as given in Eqn (2):
vations Graham made at the time were as follows: rffiffiffiffiffiffiffi
Rate M
1 ¼ 2 (2)
1) Permeation was independent of pressure. Rate M
2 1
2) Increase in temperature leads to decrease in
penetrant solubility, but made the membrane where
more permeable. Rate is the rate of effusion for gas 1 (volume or
1
number of moles per unit time)
Rate is the rate of effusion for gas 2
2
M is the molar mass of gas 1
1
M is the molar mass of gas 2.
2
When one plots the time, it takes 25mL of gas to
betransportedthroughapinholeintovacuumagainst
themolecularmassofthegas,alinearrelationshipis
obtained as shown in Fig. 1.2.
A complete theoretical explanation of Graham’s
Figure 1.1 Parameter descriptions for Darcy’s law. lawwasprovidedyearslaterbythekinetictheoryof
1: INTRODUCTION TO PERMEATION OF PLASTICS AND ELASTOMERS 3
Table1.1 KineticDiameters ofVarious Permeants3
Molecule Diameter(nm)
Helium (He) 0.26
Hydrogen (H ) 0.289
2
Nitric oxide (NO) 0.317
Carbon dioxide (CO ) 0.33
2
Argon (Ar) 0.34
Oxygen (O ) 0.346
2
Nitrogen (N ) 0.364
2
Carbon monoxide(CO) 0.376
Methane (CH ) 0.38
4
Ethylene (C H ) 0.39
2 4
Xenon(Xe) 0.396
Propane (C H ) 0.43
Figure 1.2 The time required for 25mL samples of 3 8
different gases to diffuse through a pinhole into n-Butane (C H ) 0.43
4 10
a vacuum.
Difluorodichloromethane 0.44
(CF Cl )
2 2
gases. Graham’s law is most accurate for molecular
Propene (C H ) 0.45
3 6
effusion,whichinvolvesthemovementofonegasat
Tetrafluoromethane (CF ) 0.47
a time through a hole. It is only approximate for 4
i-Butane (C H ) 0.50
diffusion of one gas into another. 4 10
An example where effusion would be appropriate
isgivenbelow:oxygenandnitrogenmoleculeswere
spherical. Table 1.1 lists the kinetic diameters of
passing through a relatively large passage way
various permeants.
through the tire wall, such as a leak. Graham’s law
for“effusion”appliesonlyiftheexitpinholethrough
which the molecules pass is relatively large 1.2.2 Solution-Diffusion and
comparedwiththesizeofthemoleculesanddoesnot
Pore-Flow Models
obstruct or constrain one molecule from passing
through relative to the other molecule. Arelationshipthatgoverns“permeation”is based
If there are no pinholes in the material then for onFick’s(First)lawofdiffusionandHenry’slawof
a molecule to get through that material, it must find solubilities.Fick’slawsofdiffusionwerederivedby
its way through a tangle of molecular chains. Since AdolfFickin1855.Hislawsdescribeddiffusionand
all matter are in continuous motionwhen it is above can be used to solve for the diffusion coefficient, D.
absolutezero,asmallmoleculemaybeabletowork Henry’s law is one of the gas laws, formulated by
itswaythroughthatentanglementofpolymerchains William Henry in 1803. It states that: At a constant
astheybendformingapocketinthematerialthrough temperature, the amount of a given gas dissolved in
which a small molecule can fit. Those little pockets a given type and volume of liquid is directly
may continuously close and form again allowing proportional to the partial pressure of that gas in
transport of a small molecule through the material. equilibrium with that liquid. These laws take into
Onewouldexpectthatsmallermoleculeswouldhave account the relative sizes of the molecules and their
an easier time diffusing through that molecular sizes compared to the very small passage way
“jungle.” dimensions in the solid material (such as a rubber)
Thekineticdiameterisareflectionofthesmallest through which the molecules “permeate.” It is often
effective dimension of a given molecule. Another mistakenly assumedthat “molecularsize” correlates
way to characterize kinetic diameter is to determine directlywith“molecularweight.”Oxygendoeshave
the diameter of a molecule, assuming it to be a greater molecular weight (32) than nitrogen
4 PERMEABILITY PROPERTIES OF PLASTICS AND ELASTOMERS
(28), but the oxygen molecule is actually smaller in assumption is that the pressure is uniform through
size. the membrane and equal to the upstream pressure.
Combining Fick’s and Henry’s laws yields the For the pore-flow model, the pressure is assumed to
overall equation governing permeation of small dropuniformlyacrossthemembrane.Thisdifference
molecules, such as gases, into material such as affects the mathematics of modeling the membrane
rubbers and other plastics. processesofdialysis,gasseparation,reverseosmosis,
The solution-diffusion and pore-flow models are and pervaporation.
commonly used to understand and predict perme- Although the difference is small, gas permeation
ationthroughfilms. Boththemodels aresimilar and through a nonporous dense polymer membrane is
suggest permeant transfer through polymer films or usuallydescribedusingthesolution-diffusionmodel,
membranes progresses through five consecutive the basis of which is described by the fairly simple
steps as follows (refer Fig. 1.3): Eqn (3).
P ¼ DS (3)
1. Permeant diffusion to the polymer film from
the upstream atmosphere. where
P ¼ permeability
2. Adsorption of the permeant by the polymer
D ¼ diffusivity
film at the interface with the upstream
S ¼ solubility.
atmosphere.
The units of permeability are (amount of gas(cid:4)
3. Diffusion of the permeant inside and through thickness of membrane)/(area of membrane(cid:4)
the polymer film. The diffusion step is the time(cid:4)pressure).
slowest and becomes the rate-determining Thestandardunitsusedinthisbookare(cm3mm)/
step in gas permeation. (m2dayatm)forgasvolumesor(gmm)/(m2dayatm)
4. Desorption of the permeant at the interface of for gas mass. When gas volumes are used, they are
the downstream side of the film. usually for gas at standard temperature and pressure
5. Diffusionofthepermeantawayfromthepoly- (STP)conditions.ThecurrentversionofIUPAC’sSTP
(cid:5) (cid:5)
mer film into the downstream atmosphere. is a temperature of 0 C (273.15K, 32 F) and an
absolute pressure of 100kPa [14.504pounds per
The primary difference between the solution- squareinch(psi),0.986atm],whileNIST’sversionis
(cid:5) (cid:5)
diffusion model and the pore-flow model is the a temperature of 20 C (293.15K, 68 F) and an
assumption of the pressure differential through absolutepressureof101.325kPa(14.696psi,1atm).
themembrane.4Forthesolution-diffusionmodel,the International Standard Metric Conditions for natural
gas and similar fluids is 288.15K and 101.325kPa.
AppendixAcontainsalargelistofotherunitsandtheir
conversionfactorstothestandardusedinthisbook.
The units of diffusivity are (area of membrane)/
time.
The units of solubility are (amount of gas)/
(volume of polymer(cid:4)pressure).
All three parameters can be experimentally
determined from a time-lag experiment, assuming
that a membrane is totally free of permeant and the
diffusion coefficient is assumed to be constant. The
time-lag technique was originally conceived by
Daynesin1920.5Thebasicexperimentmeasuresthe
amount of vapor that crosses through a film versus
time shown in Fig. 1.4 (further details are given in
Section 1.5). Before the actual experiment starts,
valve A is closed and a vacuum is drawn through
Figure 1.3 Solution-diffusion model of permeation openvalvesBandC.ValveBisclosedandvalveAis
through polymer films or membranes. openedatthebeginningoftheexperiment(t¼0)and
1: INTRODUCTION TO PERMEATION OF PLASTICS AND ELASTOMERS 5
l2
D ¼ (4)
6L
where l is the film thickness and L is the time lag
determined from the plot.
In this particular example, the film thickness, l,
was 0.0177cm and the time lag was 6264s. This
results in the diffusivity calculated to be 8.62(cid:4)
10(cid:3)9cm2/s. Note that this formula for time lag can
apply when the diffusion coefficient is constant and
not a function of concentration. Concentration may
affect the diffusion coefficient if the film swells.
Inthisexample,theDppressuredifferentialwas1.
Theslopeofthelinethroughthelinearportionofthe
plotisthepermeability,P,whichis1.4(cid:4)10(cid:3)9gcm/
cm2satm (which reduces to g/cmsatm).
By rearranging Eqn (5), the solubility can be
calculated:
P 1:4(cid:4)10(cid:3)9 ðg(cid:4)cm=cm2(cid:4)s(cid:4)atmÞ
S ¼ ¼
D 8:62(cid:4)10(cid:3)9 ðcm3=sÞ
Figure 1.4 Schematic representation of the setup ¼ 0:16ðg=cm3(cid:4)atmÞ
for gathering the data for the time-lag measurement
plot. (5)
Solubility can also be determined directly using
the amount of gas passing through the membrane is a microbalance to determine the weight gain before
measured. and after exposure of a given amount of plastic/
Oneplotstheamountofvaporthatcrossesthrough polymer to a permeant at a given pressure.7
a film of l thickness, a area, and Dp pressure differ-
ential versus time as shown in Fig. 1.5.
1.2.2.1 Non-Fickian Diffusion
Astraightline isdrawn throughthelinearportion
oftheplotasshown.Thex-axisinterceptisthetime Fick’s first law of diffusion works when the
lag, L. The time lag is used to determine the diffu- diffusing penetrant is nonswelling. When the pene-
sivity D using thefollowingequation (see Ref. 6 for trant swells, the plastic structural changes are intro-
derivation). duced (the dimensions are changed) and that may
introduce local deformation and stress. In some
cases, the stresses are so large that cracks (often
calledcrazing)mayform.Thestressesgeneratedmay
affect diffusion and solubility, which in turn affect
permeation per Eqn (3). The mathematics of non-
Fickian transport is dependent on the model chosen
andismathematicallycomplex.8e10Furthermore,the
temperature, dimensions, and material history may
have an effect on transport.
1.2.2.2 Dependence of Permeability,
Diffusion, and Solubility Pressure
The solubility coefficient depends on pressure.
Figure 1.5 Typical time-dependent permeation data Pressure is especially important when using
(time-lag plot) for a single-layer film. membranes for separations. It also depends on gas