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quarter-sawn straight staves are planed interiorly and

exteriorly throughout their thickness to achieve a stave of

cylindrical width. After the wooden material has been air-

and/or kiln-dried to approximately 12% moisture content,

the staves and heads are assembled into steel-hoop-bound

barrels. Assembly operations include: setting up staves;

steaming and winching staves to achieve the belly, bilge,

or circumference arch; heating to make wood pliable and

give one last drying after being steam bent; tapping out for

uniform thickness; trussing to tighten stave joints; crozing

interior grooves in each end of the staves where the heads

will be inserted; heading up by inserting heads in the

croze at each end of the staves; hooping up by driving

riveted-steel hoops onto exterior of staves; boring for

testing, lining, and future ﬁlling; bunging up the bored

hole; and rolling the barrel out to the marketplace.

Dozens of species of timber from all over the world have

been used to make tight (for liquid) and slack (for non-

liquid) cooperage. Hardwood barrels for spirits and wine

include oak timber from Limousin and Nevers in France;

Alastian and Italian oak; and fork-leafed American white

oak, found principally in the slow-growing forest regions

of the United States of Missouri, Indiana, Tennessee,

Kentucky, and Arkansas. Virtually all of the wooden

barrels made in the United States today [1–2 million

(10

)/yr] are 50-gal (189-L)-capacity barrels used by the

bourbon whisky trade. Barrels for bourbon are charred

interiorly about 1/16 in. (1.6 mm) of their 1 in. (25.4 mm)

thickness to bring out the tannin in the wood. Tannin aids

in the coloring and ﬂavoring of spirits and wine.

Wooden barrels have had numerous names, depending

on their size and use. A small sampling of these names

include: pickled-pig’s-feet kit; ﬁsh pail; one-quarter, one-

half, and full-beer ponies; hogshead; salmon tierce; tallow

cask; rum puncheon; and port wine pipe. A list of common

international cask sizes is presented in Table 1 (2).

Just as the wooden barrel replaced the crude basketry

used centuries ago, many other types of container have

replaced the wooden barrel: steel and ﬁber drums, plastic

pails, aluminum and steel cans, ﬁberglass and cement

tanks, and so on; aluminum and stainless steel replaced

wood for beer barrels (3).

To date, no industrial engineer has come up with a

blueprint to replace the strength of a wooden barrel. In

tests involving high stacking, they can perform better

than steel drums.

BIBLIOGRAPHY

F. J. Sweeney, ‘‘Barrels’’ in The Wiley Encyclopedia of Packaging

Technology, 1st edition, John Wiley & Sons, New York, 1986,

pp. 47–48.

Cited Publications

1. Code of Federal Regulations, Title 49, Sect. 178.155–178.161.

2. F. A. Paine, The Packaging Media, Blackie, Glasgow and

London; John Wiley & Sons, New York, 1977.

3. H. M. Broderick, Beer Packaging, Master Brewers Association

of the Americas, Madison, WI, 1982.

BARRIER AND OVERPRINT COATINGS

MICHAEL W. LEONARD

Sun Chemical—Functional

Coating Research, Kent, United

Kingdom

INTRODUCTION

Barrier coatings provide barriers to certain gases, liquids,

radiation, or other substances. These may include oxygen,

water vapor, aroma, light (including ultraviolet), and, in

the case of food packaging applications, products such as

Figure 1. The wooden barrel.

Table 1. Wooden Casks for International Shipments of

Alcoholic Beverages

Beverage Name Gallons

Liters

Wine

Sherry Butt 137.5/140 500

Hogshead 67.5/69 250

Port Pipe 145/147.5 53

Hogshead 72.5/74 265

Vermouth Hogshead 67.5/70 250

Burgundy Hogshead 57.5/60 215

Spirits

Rum Puncheon 137.5/144 520

Hogshead 67.5/72.5 255

Barrel 50 180

Brandy Hogshead 77.5/79 280

Quarter case 39/40 140

Beer Butt 135 500

Hogshead 67.5 245

Barrel 45 165

Kilderkin 22.5 82

Firkin 11 41

Pin 6 20

Gallon (U.S.) and liter sizes are not equivalent.

98 BARRIER AND OVERPRINT COATINGS

oils and fats, and so on. The barrier is usually intended to

prevent both penetration and loss from the package,

which could otherwise cause advanced spoilage of the

packaged product. There is an ongoing industry require-

ment to improve barrier properties in order to extend shelf

life of packaged foods. This section deals only with barrier

coatings, applied to ﬂexible plastic packaging ﬁlms, for

the purpose of minimizing oxygen and water vapor trans-

mission. In describing the various coating technologies,

comparisons will be drawn with alternative methods of

achieving these barriers, in order to provide a balanced

view of this important property in relation to food

packaging.

The ultimate packaging barriers to oxygen and moist-

ure barrier are tin and glass closely followed by aluminum

foils. These materials provide zero gas and vapor trans-

mission and, in the case of glass, also visibility of the

product, which is often an important point of sale market-

ing consideration. While these materials have served the

food packaging industry very well for many years and are

likely to continue doing so, they have certain drawbacks

which leave the door open for new advances in barrier

technologies. Glass and tin, while inﬁnitely recyclable, are

heavy and therefore expensive to transport, resulting in

less environmentally friendly greenhouse gas (CO

) emis-

sion compared to transportation of lightweight plastic

packaging. Plastic packaging laminates including alumi-

num foils, while lighter than tin and glass, render the

packaging difﬁcult (if not impossible) to recycle, resulting

in the need to dispose of it in landﬁll or by incineration,

both of which are negative options in a world where

sustainability is of increasing importance.

The gradual replacement of tin, glass, and more re-

cently aluminum foils by plastic packaging materials has

been evolving for several decades. The improvements to

polymer ﬁlm technologies, coupled with inorganic and

organic surface treatments and coatings, continue to close

the gap with tin and glass in terms of barrier performance,

while ensuring that due attention is paid to the environ-

mental impact.

BACKGROUND

There are limited choices for the packaging technologist

who seeks the ultimate gas barrier in single or even

multilayer plastic package structures. This becomes even

more limited when there is a requirement for product

visibility, reducing the options to structures that are

aluminum foil or (Al) metallized ﬁlm free. The common

polymeric packaging ﬁlms based upon polyethylene ter-

ephthalate (PET), oriented polypropylene (OPP), cellulose

in many coated guises (e.g., MXXT/W, MS, etc.), and

biaxially oriented nylon (BON/OPA) do not have sufﬁcient

oxygen or moisture vapor barrier properties for the many

food packaging speciﬁcations that call for longer shelf

life of foods that spoil if exposed to atmospheres depleted

or rich in these gases. It is usual in many packaging

situations to replace the atmosphere inside a package

with an inert gas substitute. This is known as modiﬁed

atmosphere packaging (MAP) or sometimes protective

atmosphere packaging (PAP). Clearly, it is important

that the substitute gas is retained for as long as possible.

Typical MAP gases include nitrogen, oxygen, and carbon

dioxide which as a rule of thumb permeate through

polymeric ﬁlms in the ratio 1:4:10. Since oxygen is the

most reactive of these three, with respect to food degrada-

tion, it is seen as the most important, while combinations

of nitrogen and carbon dioxide may be selected for reasons

of control of microbial growth, ripening, or cost.

Permeability Data of Several Packaging Films

Low-Density PE

PET

BON

Nitrogen

0.7

0.0108

0.00713

Oxygen

2.2

0.0444

0.0285

Carbon

Dioxide

9.5

0.227

0.066

Ratio

1:3.1:13.5

1:4:21

1:4:9

The manufacturers of such polymeric ﬁlms have over

the past 60 years developed coatings for their products

which, to varying degrees, improve the gas barrier proper-

ties as well as other speciﬁc performance properties such

as heat-seal threshold, product resistance, gloss, and

clarity. Such coatings include organic solvent and aqueous

applied polyvinylidene chloride (PVdC), aqueous or extru-

sion-coated ethylene vinyl alcohol (EVOH), inorganic and

organic sol–gels, and in a few instances aqueous polyvinyl

alcohol (PVOH). Each of these technologies has certain

compromises in performance. Certain polymeric ﬁlms

have an intrinsic barrier to moisture or oxygen, but rarely

both. Cellulose, being a reasonable barrier to oxygen but

poor moisture barrier, is one example.

Alternatives to aqueous or solvent-based coating appli-

cations have been available for some time now. These

essentially inorganic layers are deposited either as vapor

in a vacuum or by various electron-sputtering or plasma-

enhanced chemical vapor deposition (PECVD) processes.

The most common vacuum-deposited barrier coating is

aluminum, but more recent developments include silicon

oxide (SiOx) and aluminum oxide (AlOx) where the

proportion of oxygen is nonstoichiometric. These coatings

have one distinct advantage of being excellent barriers to

both oxygen and water vapor. The oxide coatings are also

completely transparent and most commonly used on PET

and BON. Although improvements to the gas barrier of

OPP are also achieved, this is less common due to the

expensive nature of the process resulting in mediocre

barrier performance. Since the important substrates are

not heat-sealable, they are invariably used in multilayer

structures, which also serve to protect the oxide coatings

which are particularly susceptible to damage through

physical abuse, which may result in a loss of barrier

properties. Some oxide-coated ﬁlms are also in-line coated,

with sol–gel technologies, to enhance both barrier and

abuse resistance properties. Post application of protective

coatings is rare due to the risk of damage to the barrier

before coating.

Very recently, there has been a resurgence in the

interest shown by converters to apply barrier coatings

in-line with the printing process for high-barrier

BARRIER AND OVERPRINT COATINGS 99

packaging laminates. New organic polymer technologies

combined with nano-particulate materials applied from

aqueous or solvent-based systems that may not require

extended drying conditions are at an early stage of

commercialization and demonstrating certain advantages

over existing technologies.

BARRIER COATING TECHNOLOGIES

The gas permeation process for any packaging ﬁlm or

coating is speciﬁc to the chemical structure and morphol-

ogy of that material (1). The transmission of gases across a

barrier usually involves several processes. After colliding

with the barrier surface, the gas molecule must be ad-

sorbed and subsequently dissolved in the voids of the

barrier polymer matrix. Diffusion of the gas molecules

then takes over and is governed by the kinetic energy of

the molecules directly inﬂuenced by temperature. Diffu-

sion of the gas generally proceeds through a barrier layer

from a more to a less concentrated atmosphere or partial

pressure. In all cases, permeation is controlled by the

solution and diffusion steps, according to Henry’s Law of

Solubility and Fick’s Law of Diffusion, respectively. This is

why metals and the previously mentioned metal oxides

exhibit such high barrier properties in exceptionally thin

ﬁlms, to both moisture and oxygen. Crystallinity in or-

ganic polymer barriers plays a less important role,

although in coatings based upon PVdC copolymers, where

the drying temperature is critical to imparting crystal-

linity, variation in oxygen and water vapor transmission

has been reported previously by the author (2). An in-

depth consideration of the theories governing gas trans-

mission are dealt with elsewhere in this publication.

PVDC CO POLYMER COATINGS

PVdC was developed by Ralph Wiley and co-workers at the

Dow Chemical Company during the period 1932–1939 (3).

It was initially commercialized under the trade name of

‘‘Saran’’ in 1939. PVdC is still used today by both packaging

ﬁlm manufacturers and converters. Although it can be

applied from solution, it is far more popular to use the

aqueous dispersion form which enables much higher solids

application at the required viscosities. One reason for the

longevity of this technology is its undoubtedly unique

properties that distinguish it from all other polymer dis-

persions, even today. It is fair to say that PVdC coatings are

still the only organic polymers capable of providing both

moisture and oxygen barrier. The fact that they also

provide heat-sealability, high gloss, transparency, and ﬂex-

ibility are further reasons for its continued use. However,

its use has gradually declined in various regions of the

world as increased importance is placed on environmental

factors. Chlorinated polymers are now excluded in many

packaging structures due to the problems related to incin-

eration and the difﬁculties associated with handling the

toxic by products including dioxin. Furthermore, as higher

barrier requirements have been introduced, partly driven

by the need for longer shelf life, the need for alternative

polymer technologies and inorganic barrier solutions has

resulted in superior barrier technologies, albeit with their

own set of compromises.

POLY VINYL ALCOHOL AND ETHYLENE VINYL

ALCOHOL-BASED COATINGS

Ethylene vinyl alcohol (EVOH) copolymers are hydrolyzed

copolymers of ethylene and vinyl alcohol. Polyvinyl alcohol

(PVOH), made by the hydrolysis of polyvinyl acetate

(PVAc), provides increasing oxygen barrier with the per-

centage conversion of PVAc, but this is accompanied by

increasing water sensitivity. Consequently, the oxygen

barrier of PVOH, while superior to polymers such as

PVdC at low relative humidity, is drastically compromised

under most ambient conditions that packaging must

resist. Occasionally, PVOH coatings are used where the

coating is protected by an efﬁcient moisture vapor barrier

such as that conferred by polyethylene or polypropylene

ﬁlms in multi-ply laminates. However, it is not suited,

even in these structures, for high oxygen barrier speciﬁca-

tions where relative humidity is above 50% or the proces-

sing of the packaged product involves pasteurization or

autoclave steam cooking.

EVOH copolymers became commercially available in

1972 in Japan, although their use was very limited until

some 10 years later. By copolymerizing vinyl alcohol with

ethylene, the hydrophobic properties of the latter improve

the resistance to humidity, while preserving much of the

oxygen barrier. Copolymers with the highest oxygen bar-

rier contain in the region of 27–32% ethylene and provide

an environmentally acceptable alternative to PVdC under

similar conditions of use. Again, the oxygen barrier prop-

erties are still sensitive to increasing levels of humidity,

which limits this barrier in certain (e.g., tropical) speciﬁ-

cations. The solubility of EVOH is troublesome; although

some lower ethylene content copolymers are soluble in

alcohol/water solutions, they have a tendency to gel

upon storage, making them less than convenient to use.

In fact, storage of the solutions at elevated temperatures

is the only way to avoid gelation. Therefore, the use of

EVOH in barrier packaging tends to be more commonly

achieved by extrusion coating which is an expensive

converting exercise.

INORGANIC COATINGS

Aluminum-metallized substrates such as PET, OPP, and

cellulose are manufactured by the vacuum metallization

process. These ﬁlms have been available for around 30

years and provide high levels of barrier to oxygen and

moisture vapor and have undergone an evolutionary

process that has seen improvements to surface stability,

ﬁlm adhesion, and printing ink adhesion. The presence of

aluminum (foil or metallized ﬁlm) in food packaging is

gradually being discriminated as environmentally unac-

ceptable and will ultimately decline. The use of other

vacuum metal oxide deposition processes such as

plasma-enhanced chemical vapor deposition (PECVD) to

100 BARRIER AND OVERPRINT COATINGS

lay down ultrathin layers of aluminium oxide or silicon

oxide is also well-advanced, producing transparent ﬁlms

with equivalent barrier to the more common aluminum

metallization or in some cases even better. The major

disadvantage with these ceramic-type layers is their

brittleness, which makes them prone to damage either

during the converting process or in the ﬁnal laminate

structure during the packaging operation.

NEW BARRIER COATINGS TECHNOLOGY

Over the last ﬁve years, there has been considerable

worldwide investment in nanotechnology. Some of this

investment has been focused on the development of new

barrier coating technology. In an effort to take cost out of

the process whilst achieving the highest possible barrier

properties combined with sustainability (low carbon foot-

print, compostability or recyclability) coatings applied via

the conventional coating processes (gravure, reverse roll,

ﬂexo, blade coat, etc) from water or organic solvent

are ready for commercialization. These coatings exhibit

a balance of properties which makes them suitable for

both ﬁlm manufacturer and converter applied applica-

tions. Their performance in packaging will now be dis-

cussed with reference to the graphs referenced.

Oxygen Barrier (see Figure 1). Compared to other well-

known organic barrier polymers, this new generation of

barrier coatings offers a superior oxygen barrier that is

less susceptible to high levels of relative humidity. Com-

pared to inorganic barriers the oxygen barrier is very

similar, but inorganics still offer slightly better stability to

high relative humidity. Owing to the combination of inert

particle technologies and ‘‘green’’ polymer selection, the

new-generation coatings are generally more sustainable

and provide better abuse resistance as is demonstrated in

Figure 2. Indeed the latest coating technology can be used

to reinforce the performance of aluminium-metallized and

oxide-coated ﬁlms, achieving barriers of less than 0.1 cm

/day as well as improving the ﬂex resistance of these

substrates. In Figure 3 it can be seen that signiﬁcant

improvements to the oxygen barrier of common packaging

substrates is possible without the need for complex appli-

cation/vapor deposition processes, opening up the poten-

tial for conversion of high barrier packaging at various

points along the value chain (e.g., ﬁlm manufacturer or

converter). In many respects these coatings may be seen

as a means to produce cheaper laminate high-barrier

structures (e.g., using barrier-coated OPP) with equiva-

lent barriers to those currently used (e.g., PET).

Compostable and sustainable substrates such as those

derived from cellulose or poly lactic acid (PLA) are being

OTR (cm

/day @23°C & 50% RH)

No Coatin

EVOH PVDC New Tech

100-110 cm

/day

23°C/50% RH

23°C/75% RH

Figure 1. Comparison of new-generation oxygen barrier coating

on PET with other barrier polymers.

Al-PET/PE

PET + New Gen/PE Al-PET+ New Gen/PE

No flexes

20 Gelbo flexes

OTR (cm

/day @23°C & 50% RH)

Figure 2. Oxygen transmission versus ﬂex resistance (in

Gelbo ﬂexes)-new-generation barrier coatings in laminate

structures.

PET OPP OPA

1500cc/m

/day

110cc/m

/day

OTR (cm

/day @23°C & 50% RH)

No Coating

23°C/50% RH

23°C/75% RH

Figure 3. Comparison of new generation oxygen barrier coatings

on common ﬂexible packaging substrates.

BARRIER AND OVERPRINT COATINGS 101

speciﬁed more and more by the large retail outlets. These

substrates do not have the appropriate barrier properties

required for many perishable food types. Improvements

through inorganic and new-generation-type coatings are

presently a high priority. In Figure 4 the improvement to

oxygen barrier on PLA is very signiﬁcant. Cellulose,

however, which has even higher sensitivity to moisture,

shows less improvement.

Moisture Vapor Barrier. Combining oxygen and moist-

ure vapor barrier in organic polymer-based coatings re-

mains a target. Although nano particulate materials are

capable of improving both barrier properties it is still

necessary to apply separate coatings to achieve the high-

est barriers. In Figure 5, improvements to moisture vapor

barrier on PET, PLA, and paper are compared with a

commercial barrier board. With this type of coating tech-

nology, it is possible to confer moisture vapor barrier

performance on substrates such as polylactic acid (PLA)

and cellophane approaching that of polyethylene or OPP.

Present technologies include both water- and solvent-

based products permitting applications from gravure,

ﬂexographic, and lithographic (tower) coaters.

NEAR FUTURE DEVELOPMENTS

The new-generation barrier coating technologies are

set to evolve rapidly in-line with the requirement for a

single coating combining both oxygen and moisture bar-

riers, sustainability and high abuse resistance at low

applied ﬁlm weights. Heat-sealability and surface print

100

120

140

160

180

200

PLA Cellophane

OTR (cm

/day @23°C & 50% RH)

No Coating (23°C/50% RH)

Coating (23°C/50% RH)

No Coating (23°C/75% RH)

Coating (23°C/75% RH)

Figure 4. Indication of oxygen barrier improvements

with new-generation technology on sustainable

substrates.

PET

PLA

Paper

0 20406080

Moisture Vapor Transmission Rate (g/m

/24h)

Coated Board

No Coating; 32°C; 80% RH + New Gen’; 32°C; 80% RH

+ New Gen; 25°C; 50% RHNo Coating; 25°C; 50% RH

Figure 5. New-generation barrier coatings: moisture vapor

transmission on sustainable substrates.

102 BARRIER AND OVERPRINT COATINGS

performance will follow. The use of barrier coatings,

especially oxygen barrier types, will also allow considera-

tion for simplifying or lightweighting complex laminates,

resulting in waste reduction and a lower carbon footprint

than is currently the case (4). Furthermore, these highly

transparent and ﬂexible coatings will allow new packa-

ging design innovations where product visibility is an

advantage. Combining these gas barrier coating technol-

ogies with UV light-curing barrier coatings will result in

clear plastic packaging with the potential to supplant

glass, cans, and conventional laminates in a wider variety

of packaged produce.

BIBLIOGRAPHY

1. D. Zagory, in Encyclopedia of Packaging Technology, 2nd

edition, John Wiley & Sons, New York, 1997.

2. M. W. Leonard, ‘‘A Study of the Effects of Increasing Crystal-

linity in PVdC Coated Polypropylene (Prop c) and Cellulose

(MxxT/s),’’ J. Oil Colour Chem. Assoc. 62(12), 461–469 (1979).

3. D. C. Thompson and A. L. Barney, ‘‘Vinylidene Polymers’’ in

Encyclopedia of Chemical Technology, 2nd edition, Vol. 21,

Interscience, New York, 1970.

4. D. R. Illsley, private communication.

BARRIER POLYMERS

BRADLEY FINNIGAN

Precision Fabrics Group,

Newport News, Virginia

Modern synthetic polymers have been used for over 50

years as barriers to mass transport of liquids and gases.

Major applications of barrier polymers include food and

pharmaceutical packaging. The growth in polymer-based

packaging materials at the expense of metal and glass

containers has seen the importance of barrier polymers

continue to rise. Polymeric packaging offers the advan-

tages of weight reduction, formability into useful and

attractive shapes, reduced breakage, transparency, and

cost savings. Recently, emphases on longer-term storage

and convenience factors such as microwavability have

become important. Research remains strong in industry

and academia to offer improved polymer properties and

fabrication methods and to address environmental issues

including recyclability, source reduction, sustainability,

and degradability.

Traditionally, the deﬁnition of a barrier polymer

has been strongly attached to the oxygen permeability.

Barrier polymers had oxygen permeabilities less than

about 2.0 nmol/m s GPa ( = 1.0 cc(stp) mil/100 in.

day 

atm). (See Table 1 for unit conversions.) This deﬁnition

unnecessarily limits the range of barrier polymers. Some

polymers with higher oxygen permeabilities are useful

barriers for other molecules.

THE PERMEATION PROCESS

A basic understanding of the permeation process can help

clarify the barrier characteristics of polymers. A permeant

molecule moves through a barrier in a multistep process.

First, the molecule collides with the polymer surface.

Then, it must adsorb and dissolve into the polymer

mass. In the polymer, the permeant ‘‘hops’’ or diffuses

randomly as its own thermal kinetic energy keeps it

moving from vacancy to vacancy as the polymer chains

move. The random diffusion yields a net movement from

the side of the barrier polymer that is in contact with a

high concentration or partial pressure of permeant to the

side that is in contact with a low concentration of per-

meant. After crossing the barrier polymer, the permeant

moves to the polymer surface, desorbs, and moves away.

In virtually every case, the permeation is controlled by

the solution and diffusion steps. The diffusion coefﬁcient,

D, is a measure of the speed of molecules moving in the

polymer. The solubility coefﬁcient, S, is an indication of

the number of permeant molecules that are diffusing.

Together, the diffusion coefﬁcient and the solubility coefﬁ-

cient describe the permeability coefﬁcient, commonly

called the permeability, P.

P ¼ D S ð1Þ

A low permeability may result from a low diffusion

coefﬁcient or a low solubility coefﬁcient or both.

Table 1. Permeability Units

with Conversion Factors

Multiply - to obtain k

nmol

m s GPA

cc mil

100 in:

d atm

cc mil

d atm

cc cm

s atm

cc cm

s cm Hg

cc 20 mm

d atm

nmol

m s GPa

1 2 0.129 4.390 10

3.336 10

0.1016

cc mil

100 in:

d atm

0.50 1 6.452 10

2

2.195 10

1.668 10

5.08 10

2

cc mil

d atm

7.75 15.50 1 3.402 10

2.585 10

0.787

cc cm

s atm

2.278 10

11

4.557 10

11

2.939 10

12

1 76.00 2.315 10

12

cc cm

s cm Hg

2.998 10

13

5.996 10

13

3.860 10

14

1.316 10

2

1 3.046 10

14

cc 20 mm

d atm

9.84 19.68 1.27 4.32 10

3.283 10

Throughout the Encyclopedia,cm

(or mL) is used in preference to cc. However, the advantage of using cc here is an obvious visual aid in the complex units

and there are further comments regarding cc versus cm

in the text.

BARRIER POLYMERS 103

The permeability for a given polymer–permeant combi-

nation can be used to describe the steady-state transport.

Equation (2) is Fick’s First Law adapted for packaging:

PADp

ð2Þ

where DM

/Dt is the steady-state rate of permeation of

permeant x through a polymer ﬁlm with area A and

thickness L. P is the permeability, and Dp

is the differ-

ence in pressure of the permeant on the two sides of the

ﬁlm.

Equation (2) shows why reliable tables of permeabil-

ities are important. The packaging engineer has no control

over the Dp

since the conditions of the environment and

the contents are ﬁxed. Mechanical, economic, and contain-

ment requirements limit the allowable ranges of area and

thickness. Only P has a wide range of possibilities.

One caveat must be considered before applying equa-

tion (2). The permeation must be at steady state. With

small molecules such as oxygen, steady state is usually

attained in a few hours or less, depending the polymer and

the thickness. However, with larger molecules in barrier

polymers, especially glassy polymers, the time to reach

steady state can be very long, possibly exceeding the

anticipated storage time. The time to reach steady state,

, can be estimated with equation (3):

ﬃ L

=4D ð3Þ

Equation (2) should only be used when t

is small

compared to the storage time.

Water-vapor transmission is treated differently. The

industry has arrived at a standard condition for reporting

and comparing performance, 37.81C (1001F) and 90% rh

difference. Equation (4) shows how the rate of water-vapor

transmission can be calculated using the value of the

water-vapor transmission rate (WVTR) and the package

geometry:

WVTR A

ð4Þ

When the actual conditions differ from the standard, the

WVTR can be adjusted with great care. Ideally, data have

been reported at the actual conditions; otherwise, adjust-

ments for both the relative humidity difference and tem-

perature must be made. If the polymer is known to be

insensitive to humidity, such as a polyoleﬁn, the humidity

adjustment is merely multiplication by the actual relative

humidity difference on the two sides of the ﬁlm divided by

90% rh. The temperature effect will be discussed later.

UNITS

The units for permeability are complex, and many correct

combinations are used in the literature. Lamentably,

many incorrect combinations are used too. Table 1 con-

tains conversion factors for several common units for the

permeability. Table 2 contains conversion factors for

several common units for WVTR. In these units the

quantity of permeant is a molar unit, typically a cc(stp).

For the permeability of ﬂavors and aromas, a unit using

the mass of the permeant is useful. A modiﬁed unit, the

MZU ( = 10

20

kg m/m

s Pa), can be converted to a

molar unit according to equation (5), where MW is the

molecular weight of the permeant in daltons (g/mol).

P in MZU ð10=MWÞ¼P in nmol=m s  Gpa ð5Þ

PERMEABILITY DATA

Table 3 contains oxygen, nitrogen, and carbon dioxide

permeability data for several polymers at 201C and 75%

rh. Generally, the permeabilities of N

, and CO

are in

the ratio 1:4:14. The polymers are ranked roughly in order

of increasing permeability. This list contains polymers

that meet the traditional criterion for barrier polymers

and several that do not. The range of permeabilities here

is more than four orders of magnitude. Table 4 contains

diffusion coefﬁcients and solubility coefﬁcients for oxygen

and carbon dioxide in many of the same polymers. Note

that these values are useful for comparison with the

ﬂavor, aroma, and solvent permeation that is presented

later in the chapter.

Barrier polymers are typically used as discrete layers,

as coatings, or in blends. Depending on the packaging

application, oxygen barrier polymers are frequently se-

lected from the group consisting of ethylene–vinyl alcohol

copolymers, polyvinyl alcohol, polyvinylidene chloride,

polyacrylonitrile, polyesters, and various types of polya-

mides. Moisture barrier polymers include polyethylene

(particularly high density polyethylene), polypropylene,

polyvinylidene chloride, cyclic oleﬁn copolymers, and poly-

chlorotriﬂuoroethylene. OTR and WVTR data for selected

polymers is shown in Table 5. A comprehensive list of

permeability properties of polymers can be found in

reference 4.

The increased demand for transparent packaging has

seen a number of organic, inorganic and ceramic barrier

coatings introduced as alternatives to metallized coatings

and aluminum foil (3). Examples of commercially avail-

able barrier coating materials include polyacrylic acid,

polyvinylidene chloride, polyvinyl alcohol, nitrocellulose,

epoxy-amine and polyamino ether coatings, and nanocom-

posites thereof. In addition to high-barrier, transparency,

and cost-savings drivers, retort and microwave packaging

requirements contributed to the introduction of thin sili-

con oxide (SiO

) and aluminum oxide (AlO

) coatings.

These coatings are typically applied to bottles and or-

iented polyester ﬁlms via vapor deposition processes.

Table 2. Water Vapor Transmission Rate Units with

Conversion Factors

Multiply - to obtain k

nmol

m s

g mil

100 in:

d

g cm

d

nmol

m s

1 0.253 6.43

g mil

100 in:

d

3.95 1 25.40

g cm

d

0.155 3.94 10

2

104 BARRIER POLYMERS

While SiO

and AlO

coatings are not polymeric, they offer

an excellent combination of oxygen and moisture barrier

properties to polymer-based packaging.

Multilayer barrier structures are commonly used for

both ﬂexible and rigid applications. These structures can

be the result of coextrusion, lamination, or coating. Typi-

cally, one of the layers provides most of the barrier while

other layers provide inexpensive mechanical integrity,

printability, opacity, sealability, formability, adhesion, or

merely a place to locate reground scrap. The total barrier

performance of a multilayer structure can be estimated

with equation (6):

þ ð6Þ

where L

, L

, and L

are the thicknesses of layers and P

, and P

are the respective permeabilities of the layers.

is the total thickness, and P

is the effective perme-

ability of the multilayer structure. The quantity L

the ‘‘permeance.’’ L

and P

may be used in equation (2) to

calculate the expected performance. An example of a

seven-layer barrier ﬁlm is shown in Figure 1.

NANOCOMPOSITE APPROACH TO BARRIER

PERFORMANCE

Polymer nanocomposites containing nanometer-sized

layered silicates (1 nm thick, 100- 500-nm-diameter plate-

lets) have been the focus of considerable research interest

in recent years (5). This is because signiﬁcant improve-

ments in mechanical, thermal, and barrier properties

have been reported, particularly in nylon systems.

Improved barrier performance in nanocomposites can

result from the high-aspect-ratio platelets providing a

tortuous path for the permeant to travel through, or by

the ﬁller inclusions altering the diffusivity of the host

matrix—for example, by decreasing the free volume of the

polymer—or by changing the polymer crystal type/orien-

tation. Improvements in barrier performance upon ﬁller

addition do not always occur.

A number of companies currently manufacture poly-

oleﬁn and nylon nanocomposites for packaging applica-

tions. However, the use of these materials for barrier

packaging has been limited to niche applications. At

present, research is ongoing to overcome the challenges

that are preventing widespread commercialization. These

Table 3. Permeabilities of Selected Polymers

Gas Permeability (nmol/m s GPA)

Polymer Oxygen Nitrogen Carbon Dioxide

Vinylidene chloride copolymers 0.02–0.30 0.005–0.07 0.1–1.5

Ethylene–vinyl alcohol copolymers, dry 0.014–0.095

at 100% rh 2.2–1.1

Nylon-MXD6

0.30

Nitrile barrier polymers 1.8–2.0 6–8

Nylon-6 4–6 20–24

Amorphous nylon (Selar

PA 3426) 5–6

Polyethylene terephthalate 6–8 1.4–1.9 30–50

Polyvinyl chloride 10–40 40–100

High-density polyethylene 200–400 80–120 1200–1400

Polypropylene 300–500 60–100 1000–1400

Low density polyethylene 500–700 200–400 2000–4000

Polystyrene 500–800 80–120 1400–3000

Reference 1; see Table 1 for unit conversion.

Trademark of Mitsubishi Gas Chemical Co.

Trademark of E. I. du Pont de Nemours & Co., Inc.

Table 4. Diffusion and Solubility Coefﬁcients for Oxygen and Carbon Dioxide in Selected Polymers at 231C, Dry

Oxygen Carbon Dioxide

Polymer D,m

/s S, nmol/(m

GPa)

D,m

/s S, nmol/(m

GPa)

Vinylidene chloride copolymer 1.2 10

14

1.01 10

1.3 10

14

3.2 10

Ethylene–vinyl alcohol copolymer

7.2 10

14

2.4 10

Acrylonitrile barrier polymer 1.0 10

13

1.0 10

9.0 10

14

4.4 10

Polyethylene terephthalate 2.7 10

13

2.8 10

6.2 10

14

8.1 10

Polyvinyl chloride 1.2 10

12

1.2 10

8.0 10

13

9.7 10

Polypropylene 2.9 10

12

1.1 10

3.2 10

12

3.4 10

High-density polyethylene 1.6 10

11

7.2 10

1.1 10

11

4.3 10

Low-density polyethylene 4.5 10

11

2.0 10

3.2 10

11

1.2 10

Reference 1.

For unit conversion, see equation (5).

42 mol% ethylene.

BARRIER POLYMERS 105

challenges include processing methods and inexpensive

surface modiﬁcations to ensure excellent particle disper-

sion and particle alignment, which are required in order to

optimize property performance and reproducibility.

FLAVOR/AROMA/SOLVENT BARRIER

In addition to functioning as gas and moisture barriers,

polymers are also utilized in packaging applications that

require ﬂavor, aroma, or solvent barriers. Table 6 contains

data for the transmission of ﬂavor, aroma, and solvent (F/

A/S) molecules in a few polymers. These data are for 251C

at 0% rh and very low activity (or partial pressure) of the

permeant. This list represents only a small fraction of the

virtually limitless combinations of F/A/S compounds and

polymer ﬁlms. Reliable data at low activities, as typically

encountered in foods, are difﬁcult to ﬁnd. However, the

data in Table 6 are consistent with some general rules.

First, polyoleﬁns are not good barriers for F/A/S com-

pounds. Second, vinylidene chloride copolymers and ethy-

lene–vinyl alcohol copolymers are good barriers. A third

rule is not apparent from Table 6. A polymer below its

glass-transition temperature (T

)—that is, glassy poly-

mer—is an excellent barrier for F/A/S compounds. Data

are extremely rare; hence, only a few data are given for an

ethylene–vinyl alcohol copolymer. The problem is that the

diffusion coefﬁcients are so low that the experiments take

too long to do with accuracy. The previous author and

associates were unsuccessful in many attempts to test

polystyrene, PET, and nylons.

In these tables of data, different polymers occupy

the top barrier positions. For oxygen, vinylidene chloride

copolymers and ethylene–vinyl alcohol copolymers are the

best barriers. For water vapor, vinylidene chloride copoly-

mers and the polyoleﬁns are the best barriers. For F/A/S

compounds, vinylidene chloride copolymers remain good.

Some glassy polymers that are not given are the best

barriers. Yet of all these polymers, only a few meet the

traditional deﬁnition of a barrier polymer.

FACTORS AFFECTING PERMEABILITY

The permeability increases with increasing temperature

for all known cases. A plot of logarithm P versus 1/T in

Table 5. Oxygen and Water-Vapor Transmission Rates of Selected Polymers

Polymer OTR, nmol/(m s)

WVTR, nmol/(m s)

Polychlorotriﬂuoroethylene 36

0.004–0.009

AlOx (Toppan GX-P)

0.04 0.006

Vinylidene chloride copolymers 0.02–0.30 0.005–0.05

High-density polyethylene (HDPE) 200–400 0.095

Cyclic oleﬁn copolymers 360 0.02

Polypropylene 300–500 0.16

Amorphous nylon (EMS G21) 7.7 0.23

Low-density polyethylene (LDPE) 500–700 0.35

Ethylene–vinyl alcohol, 44 mol% ethylene 0.12 0.36

Polyethylene terephthalate (PET) 6–8 0.45

Polyvinyl chloride (PVC) 10–40 0.55

Ethylene–vinyl alcohol, 32 mol% ethylene 0.03 0.96

Nylon-6,6 7 0.95

Polyamino ether (ICI’s OxyBLOC) 0.2–1.6 1

Nitrile barrier resins 1.8–2.0 1.5

Polystyrene 500–800 1.8

Nylon-6 4–6 2.7

Polycarbonate 520 2.5

Nylon-12 15.9

Polyacrylic acid (Kureha’s Besela) 0.03

Polyvinyl alcohol 0.12

At 231C and 0% rh unless otherwise noted.

At 381C and 90% rh unless otherwise noted.

Measured at 251C. Reference (4).

Measured at 381C and 100% rh. Reference (4).

Aluminium oxide coated 48ga polyester ﬁlm. Units cm

or gm/(100 in

day atm).

Measured at 231C and 85% rh. Reference (4).

Measured at 401C and 90% rh. Reference (4).

PAA-coated 48ga polyester ﬁlm. Units cm

/(100 in

day atm). Measured at 231C and 80% rh after retort.

Measured at 241C. Reference (4).

LDPE/LLDPE

tie

Nylon 6

EVOH

Nylon 6

tie

m-LLDPE sealant

7-Layer Barrier Food Packaging Structure

Figure 1. Seven-layer barrier ﬁlm used in food packaging.

106 BARRIER POLYMERS

kelvin yields a straight line with a slope proportional to the

activation energy for permeation. Usually the slope is

steeper above T

than below T

. Hence, knowledge of the

permeability at two temperatures allows calculation of the

permeability at a third temperature, provided that T

is not

in the range. For many polymers the oxygen permeability

increases about 9% per 1CaboveT

and about 5% per 1C

below T

. The temperature sensitivity is greater for larger-

permeant molecules. The temperature sensitivity for the

WVTR is theoretically a little more complicated; however, it

is about the same as for the oxygen permeability.

Humidity can affect the permeability of some polymers.

When a polymer equilibrates with a humid environment,

it absorbs water. The water concentration in the polymer

might be very low as in polyoleﬁns or it might be several

weight percent as in ethylene–vinyl alcohol copolymers.

Absorbed water does not affect the permeabilities of some

polymers including vinylidene chloride copolymers, acry-

lonitrile copolymers, and polyoleﬁns. Absorbed water in-

creases the permeabilities in some polymers including

ethylene–vinyl alcohol copolymers and most polyamides.

A few polymers show a slight decrease in the oxygen

permeability with increasing humidity. These include

PET and amorphous nylon. Since humidity is inescapable

in many packaging situations, this effect cannot be over-

looked. The humidity in the environment is often above

50%rh, and the humidity inside a food package can be

nearly 100% rh.

Table 6. Examples of Permeation of Flavor and Aroma Compounds in Selected Polymers at 251C,

Dry

Flavor/Aroma Compound Permeant Formula P, MZU

D,m

/s S, kg/(m

Pa)

Vinylidene Chloride Copolymer

Ethyl hexanoate C

570 8.0 10

18

0.71

Ethyl 2-methylbutyrate C

3.2 1.9 10

17

1.7 10

3

Hexanol C

O 40 5.2 10

17

7.7 10

3

trans-2-Hexenal C

O 240 1.8 10

17

0.14

d-Limonene C

O 32 3.3 10

17

9.7 10

3

3-Octanone C

O 52 1.3 10

18

0.40

Propyl butyrate C

42 4.4 10

18

9.4 10

2

Dipropyl disulﬁde C

270 2.6  10

–18

1.0

Ethylene–Vinyl Alcohol Copolymer

Ethyl hexanoate 0.41 3.2 10

18

1.3 10

3

Ethyl 2-methylbutyrate 0.30 6.7 10

18

4.7 10

4

Hexanol 1.2 2.6 10

17

4.6 10

4

trans-2-Hexenal 110 6.4 10

17

1.8 10

2

d-Limonene 0.5 1.1 10

17

4.5 10

4

3-Octanone 0.2 1.0 10

18

2.0 10

3

Propyl butyrate 1.2 2.7 10

17

4.5 10

4

Low-Density Polyethylene

Ethyl hexanoate 4.1 10

5.2 10

13

7.8 10

2

Ethyl 2-methylbutyrate 4.9 10

2.4 10

13

2.3 10

2

Hexanol 9.7 10

4.6 10

13

2.3 10

2

trans-2-Hexenal 8.1 10

d-Limonene 4.3 10

3-Octanone 6.8 10

5.6 10

13

1.2 10

1

Propyl butyrate 1.5 10

5.0 10

13

3.0 10

2

Dipropyl disulﬁde 6.8 10

7.3 10

14

9.3 10

1

High-Density Polyethylene

d-Limonene 3.5 10

1.7 10

13

2.5 10

1

Menthone C

O 5.2 10

9.1 10

13

4.7 10

1

Methyl salicylate C

1.1 10

8.7 10

14

1.6

Polypropylene

2-Butanone C

O 8.5 10

2.1 10

15

4.0 10

2

Ethyl butyrate C

9.5 10

1.8 10

15

5.3 10

2

Ethyl hexanoate 8.7 10

3.1 10

15

2.8 10

1

d-Limonene 1.6 10

7.4 10

16

2.1 10

1

Values for vinylidene chloride copolymer and ethylene–vinyl alcohol are extrapolated from higher temperatures.

Permeation in the vinylidene chloride copolymer and the polyoleﬁns is not affected by humidity; the permeability and diffusion coefﬁcient in the ethylene–

vinyl alcohol copolymer can be as much as 1000 times greater with high humidity (1).

MZU = (10

20

kg m)/m

s Pa); see equation (5) for unit conversions.

BARRIER POLYMERS 107