Yam, Kit L. (ed.). The Wiley encyclopedia of packaging technology

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MOLECULAR WEIGHT OF PACKAGING

POLYMERS

KIT L. YAM

Department of Food Science,

Rutgers University, New

Brunswick, New Jersey

An important characteristic of packaging polymers is their

molecular sizes relative to the much smaller organic or

inorganic molecules. The molecular weight of a packaging

polymer, a measure of its molecular chain length, can

signiﬁcantly affect the physical properties of the polymer.

As molecular weight increases, tensile and impact

strengths increase sharply before leveling off, while melt

viscosity increases slowly and then sharply (Figure 1).

When selecting molecular weight for a speciﬁc applica-

tion, the strength and melt viscosity should be compro-

mised. The molecular weight should be high enough to

acquire adequate strength, but low enough to avoid high

melt viscosity, which makes the polymer difﬁcult to pro-

cess, such as ﬂowing into a mold or through an extruder

die. Typically, the practical molecular weight range for

packaging polymers is between 50,000 and 200,000 as

mentioned above.

Unlike small molecules such as oxygen or water, a

polymer does not have a single molecular weight. This is

because polymer molecules of different sizes are formed

during the polymerization reaction. The relative fraction

of different sizes may be characterized using a plot of

frequency versus molecular weight, known as molecular

weight distribution (MWD), as shown in Figure 2.

Two average molecular weights are often obtained from

the MWD. The number-average molecular weight

deﬁned as

ð6:1Þ

where N

and M

are the number and molecular weight of

polymer molecule i, respectively. The weight-average mo-

lecular weight

Mw is deﬁned as

ð6:2Þ

where w

is weight of polymer molecule i.

The polydispersity index (PDI) is deﬁned as

PDI ¼

ð6:3Þ

which is a measure of the spread of the molecular weight

distribution. The value of PDI is always Z1. PDI = 1

means that all the polymer molecules have the same

molecular weight and thus

¼ M

However, PDI = 1 is

difﬁcult if not impossible to achieve; most commercial

packaging polymers have PDI greater than 2. A higher

PDI is associated with a broader MWD (Figure 2(b)),

meaning more variation in molecular weights among the

polymer molecules.

A broader MWD tends to decrease the tensile and

impact strengths of a polymer but increase its ease of

processability, due to the smaller molecules in the dis-

tribution acting as a plasticizer or processing aid.

Chain Entanglement

A unique characteristic associated with large molecular

weight of packaging polymers is chain entanglement, the

ability of polymer chains to become entangled with one

another. The degree of chain entanglement increases with

length of the polymer chain or molecular weight of the

Tensile

strengt

Molecular wei

Impact

strength

Melt

viscosity

Physical properties

Figure 1. Effects of molecular weight on polymer properties.

Weight-average, M

Number-average, M

Molecular weight

(a) (b)

Number of polymer

Molecular weight

Number of polymer

MWD

Broad

MWD

Narrow

MWD

Figure 2. Molecular weight distribution.

798 MOLECULAR WEIGHT OF PACKAGING POLYMERS

polymer. Polymers consisting of short chains are weak and

brittle but become strong and ductile above some critical

length. A minimum of 1000 to 2000 repeating units, for

example, is necessary for polyethylene to acquire the

strength adequate for packaging foods.

Chain entanglement is important for packaging poly-

mers. For example, during thermoforming or blow mold-

ing of a thermoplastic, the polymer is in molten state with

its molecular chains resembling cooked spaghetti (or, more

vividly, wriggling long worms) tangled up on a plate.

Chain entanglement holds the polymer chains together

but also allows them to slide pass one other, thereby

providing both the necessary strength and the pliability

for the polymer to be molded into a ﬁlm, bottle, or other

shapes. After cooling, sliding between polymer chains is

more difﬁcult and chain entanglement makes the polymer

strong and resilient.

Summation of Intermolecular Forces

The physical properties of a material are greatly inﬂu-

enced by the forces (such as dipole–dipole forces, hydrogen

bonding, and dispersion forces) between its molecules (see

Structure/Property Relationships of Packaging Materi-

als). Although intermolecular forces exist between all

molecules, their inﬂuences are more pronounced for poly-

mers. Summation of intermolecular forces refers to the

total force between neighboring molecules—this total

force is particularly signiﬁcant for packaging polymer

molecules due to their long molecular chains.

The effects of summation of intermolecular forces may

be observed in the physical states of large and small

molecules: for example, polyethylene of solid versus

ethane of gas. Except for the difference in size, the

chemical structures are the same and weak nonpolar

intermolecular forces operate on both compounds.

Although weak nonpolar forces are operating in both

compounds, the summation of intermolecular forces is

much greater for the long polyethylene chains of solid

than for ethane gas. Yet polyethylene molecules are held

more strongly together due to greater summation of

intermolecular forces.

Summation of intermolecular forces and chain entan-

glement can signiﬁcantly inﬂuence the mechanical, ther-

mal, rheological, and other properties of thermoplastics.

MULTILAYER FLEXIBLE PACKAGING

THOMAS J. DUNN

Printpak Inc., Atlanta,

Georgia

Multilayer ﬂexible packaging offers packaged-goods man-

ufacturers the opportunity to match the functional needs

of their products precisely to the materials used to pack-

age them. The same materials used as complete, sealed

ﬂexible packages are also suitable for use as lidding on

rigid containers, ﬁlm covers on carded and blister packs,

and wrapping materials. These functional needs, broadly

deﬁned, are (a) package appearance, (b) package barrier,

and (c) product containment.

Multilayer ﬂexible packaging consists of combinations

of metal, plastic, or cellulosic substrates (foils, ﬁlms, and

paper), individually ranging in thickness from 0.0001 in.

(0.1 mil or 2.5 mm) to about 0.005 in. (5 mils or 125 mm).

The aggregate thickness of the combination is deﬁned

by convention to be less than 0.01 in. (10 mils 250 mm).

(Thicker composites are typically considered ‘‘sheet’’ ma-

terials.) This material is fashioned into various container

formats (pouch, bag, wrapper, etc.) that hold a product.

The manufacture of multilayer ﬂexible packaging is

known as converting. In producing material for a particu-

lar application, speciﬁc text and related graphic informa-

tion (e.g., pictorials and use suggestions or bar codes)

are typically printed onto one of the substrates. Several

processes—coating, laminating, and coextrusion—then

combine this printed layer with others to provide the

overall material functionality.

Packaged-goods manufacturers usually buy the compo-

site directly in roll form. They use various types of

integrated packaging machinery to form the material

into a pouch or bag-like container, ﬁll it with a premea-

sured product amoount, and ﬁnally seal it. Alternately, the

converter will fabricate bags or pouches from the compo-

site material. The packaged-goods manufacturer then ﬁlls

and seals these containers using a variety of manual or

automated means.

PACKAGE APPEARANCE

Consumers and industrial users require various visual

cues and information on the packages of products they

use. Multilayer ﬂexible packages must communicate to

these users speciﬁc information about the product within:

product identiﬁcation (including for example, name, in-

gredients and speciﬁc type), product quantiﬁcation (e.g.,

count, volume, or net weight), manufacturer’s identity and

address, and warnings about any hazards presented by

the product. In addition, manufacturers will usually pro-

vide product-use information (with text, icons, and/or

pictorials) on the package. If the product is branded, the

package must effectively communicate trademarks, logos,

and brand colors. The appearance of a multilayer ﬂexible

package may also be used to communicate a variety of

marketing claims for the product/package (e.g., new

formulations, new packaging types/formats nutritional

positioning, and compliance with certain religious require-

ments), promotional efforts of the manufacturer intended

to stimulate sales, or linkages to other products. Multi-

layer ﬂexible packaging provides abundant options to ﬁll

these needs.

Materials Options

Multilayer ﬂexible packaging ranges in opacity from

totally opaque to completely transparent. In the opaque

form, it serves to deliver light-sensitive photographic and

MULTILAYER FLEXIBLE PACKAGING 799

X-ray ﬁlms to users. When transparent, it can merchan-

dise a variety of consumer products, allowing the would-be

buyer to directly view them before purchase. Aluminum

foil historically served as the most effective material for

blocking light. With mirrorlike spectral reﬂecting proper-

ties, composite materials with foil have come to connote

product quality and freshness (1). Metallized plastic ﬁlms

with vacuum-deposited aluminum coatings have been

effective in imitating this shelf appearance, but they

typically do not provide all the light barrier or other

preservative qualities of foil.

A variety of plastic ﬁlms combine to make transparent

multilayer ﬂexible materials. Cellophane, the earliest

transparent ﬁlm, is a cellulosic product; however, as a

plain, uncoated material it had severe limitations (2). This

material was ﬁrst used as an unsupported (single-layer)

packaging material. In fact, much of the product–process

development in multilayer ﬂexible packaging over the

past 50 years derives from the need to overcome various

use limitations of cellophane (e.g., brittleness from cold-

temperature or low-humidity storage). Modern transpar-

ent ﬁlms include polyethylene, polypropylene, polyesters,

and nylons, among others. These ﬁlms can also be pig-

mented with white, black, or various hues to complement

colors printed on the package.

Printing Options

Multilayer ﬂexible-packaging materials are printed using

primarily rotogravure and ﬂexographic processes. Unlike

traditional lithographic printing process, both ﬂexible

material processes lend themselves to the effectively limit-

less choice of package size available with ﬂexible packa-

ging. These high-quality, high-speed methods provide a

full range of textural, machine-readable, and pictorial

information. Some materials are printed on their outside

surface (surface printing). In such cases, a protective

coating is often applied over the ink to protect it from

scufﬁng, chemical destruction, or other abuse. In contrast,

the more common practice for multilayer ﬂexible materi-

als is to print on the inner surface of an outer, transparent

layer of the multilayer ﬂexible material (reverse printing).

The surface is then laminated to the other layers. The

outside layer itself then serves to protect the ink from

abuse. It also usually presents a very glossy appearance to

the package as it is displayed (see also Printing).

PACKAGE BARRIER

Multilayer ﬂexible-packaging materials provide documen-

ted levels of barrier to environmental factors: light, moist-

ure vapor, and oxygen (Table 1). These factors can interact

with and impair packaged products for which multilayer

ﬂexible ﬁlms provide protection. The materials are gen-

erally considered impervious to microorganisms, although

packaging shelf-stable processed foods in them requires

conﬁrmation of this barrier function down to the level of a

submicrometer void (3).

Oxidation of oils fats and vitamins in foods and other

products can be retarded by barrier multilayer ﬂexible

materials if the oxygen-rich atmosphere within the pack-

age is replaced with an inert gas such as nitrogen (gas

ﬂush) or evacuated and then sealed under vacuum. Inter-

action with water vapor can be more dynamic. Dry food

(moisture content o3%) and hygroscopic granular pro-

ducts need protection from moisture migrating from a

humid exterior into the package. Moist products, includ-

ing aqueous solutions or suspensions, need moisture-

barrier materials that will prevent the loss of product

moisture into a low-humidity exterior. Recent advances in

controlled and modiﬁed atmosphere packaging systems

utilize the barrier of ﬂexible multilayer packaging materi-

als to nitrogen, carbon dioxide, and ﬂavors/aromas as well.

These barrier functions and standard methods for quanti-

fying them are not generally established in the industry.

Extended Shelf Life

Flexible multilayer packaging is successfully used for

retort processing of various food formulations. These

‘‘retort pouches’’ are capable of preserving their contents

for 48 months. The primary barrier properties are pro-

vided by a layer of aluminum foil or, in a few cases, by a

transparent layer of saran (polyvinylidene chloride,

‘‘PVDC’’) ﬁlm or aluminum-oxide/silicon-oxide-coated

plastic ﬁlms.

Industry-standard practices are established for the safe

and reliable production of these packaged products (4).

Shelf-life extension is also achieved in practice by

aseptic packaging of sterile products. In this process, a

sterilizing technique, such as a steam or hydrogen peroxide

bath, or exposure to an electron beam, is used to clean the

product-contact surfaces of the ﬂexible multilayer material

Table 1. Typical Transmission by Selected Flexible Materials

Transmission Rate

Gauge (mil) Material Optics: Haze or Optical Density Moisture vapor Oxygen

1 LDPE 6% (haze) 0.8 W100

1 HDPE 30% (haze) 0.35 W100

0.7 Oriented polypropylene 1.6% (haze) 0.49 90

0.5 Oriented polyester 2.5% (haze) 2.8 6

0.5 Oriented nylon 3.0% (haze) 24 4

0.7 Metallized-oriented polypropylene 2.3 (OD) 0.02 8

0.5 Metallized-oriented polyester 3.0 (OD) 0.05 0.08

Values as supplied by various commercial ﬁlm suppliers. Moisture-vapor values: g/(100 in.

 24 h) at 1001F and 90% RH. Oxygen values: ml/ (100 in.

 24 h)

at 1001F and 90% RH.

800 MULTILAYER FLEXIBLE PACKAGING

in a sterile environment. Then, while still in a sterile

environment, a commercially sterile product is packaged

in this material and hermetically sealed.

The barrier properties of ﬂexible multilayer packaging

materials are typically not as dependable as that of rigid

packages (e.g., metal cans and glass bottles) traditionally

used for extended shelf-life packaging. This is the result of

(a) imperfections in the thin layer of materials involved,

(b) measurable transmission of oxygen through fabricated

multilayer polymeric containers, and (c) voids in heat-

sealed (i.e., welded) areas of the fabricated containers.

Ongoing research is directed toward developing reliable,

nondestructive testing of 100% of ﬂexible pouches contain-

ing long-shelf-life processed foods (3) (see also Shelf Life).

Material Barrier Properties

The barrier presented by a plastic ﬁlm to migrating

gaseous molecules is a complex property of that ﬁlm (see

also Barrier Polymers). It is considered to be a mass-

transport phenomenon in which the gas ﬁrst dissolves into

the polymer; then, it diffuses through small voids in the

polymer in response to the net gas concentration differ-

ences on either side of the polymer; and ﬁnally, desorbs

and leaves the polymer on the side of lower concentration

(5). In practice, the industry uses an equilibrium (iso-

static) technique in which a constant concentration gra-

dient for the permeating gas is maintained across the ﬁlm.

In this steady-state condition, transmission of the per-

meant is expressed as a mass (or standard volume) of the

permeant per unit area in a 24-h period. Ambient tem-

perature and ﬁlm thickness are also reported (6). Table 1

provides a summary of barrier performance for commonly

used ﬁlms.

The barrier to migrating gaseous molecules presented

by aluminum foil is not characterized by this plastic ﬁlm

model. Foil, free from small voids as in polymers, is a

perfect barrier to migrating molecules. With the inevitable

pinholes in foils (representing o0.001% of the surface

area), however, the transport mechanism is one of ﬂow

through oriﬁces (7). At the point of ﬂow, the polymeric

model applies, if the foil is laminated to a plastic ﬁlm.

Barriers to Organic Volatiles

Food and nonfood products often contain volatile compo-

nents critical to the utility and purpose of that product.

Inside a sealed ﬂexible multilayer package, these compo-

nents can volatize into the headspace of the container. In

this mobile form, they can dissolve and diffuse through

a polymer ﬁlm, much like environmental gases. If the

solubility and transmission rates are of sufﬁcient magni-

tude, product shelf life can be signiﬁcantly decreased.

Similarly, volatiles from the enviroment external to the

package or from packaging materials themselves can

desorb into the package headspace and contaminate pro-

ducts inside. While a variety of proprietary studies have

been reported, attempts to develop standardized methods

and measures for typical volatiles have gained the support

of industry participants only very recently.

PRODUCT CONTAINMENT

This function of multilayer ﬂexible packaging is the most

complex, and perhaps the most critical. If the material

fails to contain its product properly, its intended appear-

ance and barrier performance are quickly compromised.

Containment relies on the physical strength of the com-

posite material to deliver a product securely to its in-

tended market. It also depends on the ability to ‘‘seal’’

packages by using heat and/or pressure to weld opposing

surfaces of a two-dimensional leaf of material into the

three-dimensional container (e.g., pouch, bag).

Physical Strength

Standard physical measures of material strength are used

to characterize ﬁlms, foils, and papers. Table 2 summarizes

typical properties of interest. The speciﬁc product to be

packaged dictates the level of strength (as measured by

any particular physical property) that a composite ﬂexible

material must have. For example, dried soup mixes and

ground coffee contain small, sharp, granular particles that

can readily puncture or rub through a material unless

‘‘puncture resistance’’ is adequate. The actual environmen-

tal contexts within which punctures take place are com-

plex and dynamic—so much so that traditional strength of

materials measurements taken alone are not sufﬁcient

predictors of success in a given application. The pouch of

dried soup mix, for example, is typically sold as a multi-

pack inside a paperboard carton. Through distribution and

storage, any pouch face in direct contact with the paper-

board will experience abrasive forces greater than ones

rubbing against other pouches. To satisfy the need to

specify materials without the time and expense of dynamic

simulation or actual ﬁeld testing, a variety of specialized

tests with associated testing apparatus have been devel-

oped to discriminate among the relevant performance

levels of various materials (Table 3). Many of these have

been adapted from the paper industry where analogous

Table 2. Materials Properties of Interest

Property Reference

Ultimate tensile ASTM D882

Tensile at elongation ASTM D882

Elongation at break ASTM D882

Secant modulus ASTM D882

Density ASTM D1505

Table 3. Specialized Materials Properties

Property Reference

Tear strength TAPPI T414

Dart drop impact ASTM D4272

Puncture ASTM F1306

Burst TAPPI T403

Flex durability ASTM F392

Abrasion resistance ASTM F735

Bond strength ASTM F904

MULTILAYER FLEXIBLE PACKAGING 801

needs for distinguishing among materials in nonpackaging

uses have long been recognized.

The growing interest in packaging relatively large

volumes of liquids (i.e., W200 mL) demands a new level

of strength in multilayer ﬂexible-packaging materials.

Linear low-density polyethylene (a copolymer of ethylene

and longer-chain a oleﬁns, typically octene) is the material

of choice for such applications. Its tensile strength and

elasticity can withstand the surges of hydraulic pressure

as liquids in pouches are compressed during shipping and

handling. Laminating this ﬁlm to oriented-polyester ﬁlm

with its high tensile strength provides a reinforced com-

posite material with the ability to contain several liters of

liquid. This ﬁlm laminated to oriented-nylon ﬁlm for its

high puncture strength might be the choice if the package

must withstand small diameter puncture abuse.

Heat-Seal Strength

Obviously, the container made of a multilayer ﬂexible

material can be no stronger than the seals that weld it

together. Seals are typically made by applying pressure

with heated surfaces to opposing faces of the composite

materials, melting the thermoplastic material of the con-

tacting surfaces. The two surfaces then fuse together as

the thermoplastics cool.

Cohesive coatings are the major exception to heat

sealing. Such coatings are similar to pressure-sensitive

ones in that they will adhere when they are applied with

pressure to another surface. Unlike pressure sensitives,

cohesives adhere only to surfaces coated with similar

coatings. In practice, cohesive coatings are applied in a

perimeter pattern on the inside surface of a packaging

ﬁlm. The cohesive property allows this pattern-coated ﬁlm

to be unwound from roll form on a packaging machine.

The package is formed around a product using only

pressure to adhere the opposing cohesive surfaces. Heat-

sensitive products, such as chocolate bars, can be wrapped

in this manner without melting. Packaging line speeds

can also be increased as the sealing pressure can be

transmitted to the interface essentially instantly (8).

Heat sealing, in contrast, requires time to raise the inter-

face temperature to the melting point because the heated

surfaces can contact only the opposite side of the surface to

be sealed.

The process of making heat seals relies on the basic

thermoplastic properties of the polymers on the inside of a

multilayer ﬂexible package (the ‘‘seal’’ layer) (9). In theory,

this welding process involves a dynamic ﬂuid material

moving under pressure. However, heat-sealing practice

for the most part ignores the rheological properties of the

molten polymer and uses a static set of temperature,

pressure, and time variables to control the heat-sealing

process on packaging machinery. A matched pair of seal-

ing ‘‘jaws’’ is maintained at constant temperature and

brought together with the packaging material between

them. Assuming that the packaging machine’s pressure

settings are correct, the thickness of the material and the

minimum distance between the jaws dictate the pressure

at the sealing interface. The ‘‘dwell’’ time during which

maximum pressure is maintained is dictated by the line

speed of the machinery. The process assumes that the

outer layers of the multilayer ﬂexible-packaging material

are themselves sufﬁciently heat-resistant to avoid melt-

ing. Paper and cellophane can easily withstand sealing

temperatures, typically 250–3501F. Heat-set oriented-

plastic ﬁlms of nylon, polyester, or polypropylene are the

usual thermoplastic options. Pressure effects at the inter-

face of seal layers can be enhanced by using serrated or

ridge surfaces on the seal jaws that mate with comple-

mentary patterns in the closed position. (In theory, such

shapes can be used to impart shear forces to the molten

polymers. This would cause stronger welds more quickly.)

Various preheating techniques (e.g., infrared lamps or

heated guide surfaces) have been developed to speed up

the process of melting the interface layers. New grades of

‘‘metallocene polyethylene’’’ (linear low-density polyethy-

lene, polymerized using catalyst systems made of metal-

locene materials) have densities lower than traditional

grades. As a result of this combination of density and

chemical composition, such products have strength and

the ability to seal faster and/or at lower temperatures.

(See Sealing, Heat)

While the integrity of the package itself depends on

reliable seals, consumer access to the packaged products

can require that the seals be broached with relatively little

effort. Such ‘‘easy-open’’ seals have been accomplished by a

variety of means (Figure 1). One major technique (‘‘seals’’)

involves using a thin layer of low-melt-point polymer as

the seal layer. This is welded to itself in the sealing process.

As the consumer pulls on the seal to open the pouch, the

thin layer breaks away from the other layer(s) of the

multilayer material. This allows access to the interior of

the package past a layer that was previously buried within

the composite material. Alternatively, the seal layer can be

contaminated with an immiscible material that is blended

into the polymer that will actually melt to form the weld.

This can be either an inorganic material or an immiscible

polymer with a higher melt point. When the seal is made,

the welded area is decreased by the amount of interface

area comprised of the ‘‘contaminate.’’ This is designed to be

large enough to allow access to the package interior by

Clean-peel

Outer Layer

Barrier layer

Sealant Layer

Barrier layer

Outer layer

Delamination

Outer Layer

Barrier layer

Sealant Layer

Barrier layer

Outer layer

Figure 1. Easy-open seal types.

802 MULTILAYER FLEXIBLE PACKAGING

using a speciﬁed force to separate the original seal inter-

face (‘‘clean-peel seals’’).

Reclosing ﬂexible packages remains an industry chal-

lenge. Dry products (e.g., cereals, snacks, cookies, crack-

ers, and pharmaceutical products) can quickly lose their

crispness in an opened package. And moist products, such

as shredded cheese, can dry out, particularly in refriger-

ated air. Pressure-sensitive tapes and interlocking plastic

ridges (plastic ‘‘zippers’’) are used in several applications

at present. Widespread adoption of recloseable features

for ﬂexible packages awaits a reliable, low-cost technique.

Alternatives to separate pieces of pressure-sensitive tapes

have been developed that actually incorporate pressure

sensitive layers into the multilayer structure (10). Re-

closeable zippers for packages have been enhanced with

rigid ‘‘sliders’’ that are easily gripped and reliably press

the zipper ‘‘ridges’’ together into the closed position (11).

MANUFACTURING MULTILAYER FLEXIBLE PACKAGING

The converting operations for manufacturing multilayer

ﬂexible packaging are printing, laminating, and ﬁnishing

processes. Such operations are designed and equipped to

convert a variety of raw materials into many kinds of

customized products. Even within a speciﬁc category of

packaged product, the actual multilayer ﬂexible packa-

ging speciﬁed by one customer may differ dramatically

from that purchased by another. Differences in product

formulations, packaging machinery, distribution systems,

and marketing approaches combine to require different

levels of appearance, barrier, and containment. These

functional differences, in turn, dictate differences in spe-

ciﬁed raw materials.

Printing

Flexible packaging printing involves both ﬂexographic

(images on rubber plates that stamp ink onto a substrate)

and rotogravure (images etched into metal cylinders that

release ink to a substrate from small engraved cells)

printing processes. The choice between processes is now

primarily one of customer preference. Historically, con-

verters specializing in foil and opaque packaging were

rotogravure printers. Until the 1980s, this process was the

only one capable of reproducing quality continuous tone

(pictorial) graphics. Flexographic printers were more

common among cellophane and plastic-ﬁlm converters,

where transparent packaging required only basic text

printing. Flexographic continuous-tone printing has be-

come much more acceptable for high-quality graphics over

the past 10 years. With consolidation of converters within

the industry and internal diversiﬁcation by the remaining

suppliers, this distinction of capabilities along opaque and

transparent substrate lines has virtually disappeared.

Both printing processes use liquid inks comprised of

colored pigments, a polymeric ‘‘vehicle’’ (binder), and a

volatile solvent. The solvent (an organic liquid or water) is

evaporated from the printed substrates as the vehicle

solidiﬁes and adheres to the printed substrate. Each of

these three ink components must be selected to meet

various functional and environmental requirements (12).

When the printed surface is to be the outside of the

outer layer of the multilayer ﬂexible package, a protective

layer, or overprint varnish, is often applied over the ink to

prevent scufﬁng and to provide a high-gloss effect. This

coating must also be resistant to hot sealing jaws that will

contact it as heat seals are made. In contrast, when the

printed surface is on the inside of the outer layer, the ink

must be formulated to adhere to the material that will be

used to bond it to the next layer. It must also have an

internal strength sufﬁcient to support the overall strength

requirements of the entire multilayer material. These

properties must be maintained at the temperatures that

the ink will experience during heat sealing and any

thermal process of the product in the package.

Adhesive Laminating

The majority of adhesive laminations in multilayer ﬂex-

ible packaging are manufactured using the dry-bond

process. In this technique, a liquid adhesive is applied to

one substrate. Any one of a number of web-coating meth-

ods (e.g., direct, offset, or reverse gravure; wirewound

rods; air knife) is used to meter and apply the adhesive

onto the substrate. The adhesive coating is then dried

with hot air. This dried surface can be adhered to a second

substrate using heat and pressure at a nip point. The

newest generation of ﬂexible packaging adhesives involve

2 or more liquids comprised of reactive prepolymeric

chemicals. When mixed, these so-called 100% solids ad-

hesives undergo chemical reactions that essentially bind

up all of the applied coating materials into a solid layer of

cross-linked adhesive. No volatile diluents need be evapo-

rated from the coatings to achieve the solid layer.

The adhesive formulations themselves represent a

reactive chemistry (typically urethanes or acrylics) that

is chosen to withstand the processing and distribution

environment of the ﬁlled product. This can be high

temperatures of retort processing (2801F), migrating vo-

latile organics that can redissolve adhesive solids, or other

packaging components that can interact to discolor com-

ponents of the lamination. Because the adhesives are

reactive chemicals that are expected to polymerize and/

or crosslink when coated, government food-additive reg-

ulations control the presence of any unreacted residuals in

the composite materials (13). (See also Adhesives.)

Extrusion Lamination

Extrusion lamination involves using a thin (as low as 0.3

mil or 9 mm) layer of plastic (typically low-density poly-

ethylene (LDPE) or a copolymer of LDPE) to bond together

two layers of ﬁlm, paper, and/or foil. The method has the

advantage

over adhesive laminations

of adding substan-

tial thickness to a multilayer lamination as well as con-

tributing to the overall strength of the material. It grew

out of the process of coating paperboard with low-density

polyethylene (an alternative to wax coatings for such

packages as milk cartons) (14). At present, the technique

accounts for the majority of printed multilayer ﬂexible

packaging.

MULTILAYER FLEXIBLE PACKAGING 803

The process enjoys signiﬁcant cost advantages over the

alternative of producing separate thin layers of polyethy-

lene and using dry bond adhesives to adhere the separate

layers together. It has no environmental emissions and

permits the processing of thinner layers of plastic than

can be handled otherwise. The intrinsic adhesive afﬁnity

of low-density polyethylene (LDPE) toward other sub-

strates is limited. However, a variety of LDPE copolymers,

adhesion primers, and processing aids have been devel-

oped to broaden the applicability of the method (15).

Coextrusion

The cost advantages of extrusion laminations are exploited

in the extreme in the coextrusion process. This method

entirely eliminates the use of any separately manufac-

tured substrates. It simultaneously extrudes several layers

of molten plastic into a single multilayer material. Each

plastic maintains its identity as a separate layer in the

ﬁlm and can contribute various functions accordingly. For

example, a coextruded ﬁlm of nylon, ethylene vinyl alcohol,

and ionomer provides, respectively, heat resistance, oxygen

barrier, and low-temperature sealing for the material used

to vacuum-package processed meats. Of course, a pure

coextruded ﬁlm precludes the use of (a) reverse-printed

substrates and (b) paper and foil entirely. However, hybrid

processes in which a multilayer extruded curtain is used to

‘‘coextrusion-laminate’’ or ‘‘coextrusion-coat’’ traditional

substrates have been developed.

Coextrusion has proven to be a powerful option for

achieving a mix of functional features in ultra-thin-base

ﬁlms. In effect, base-ﬁlm suppliers can mix and match

materials in a manner analogous to a converter’s lamina-

tion processes. For example, a current standard seal layer

for snack food packaging is a coextruded oriented-polypro-

pylene ﬁlm comprised of a B0.05 mil (1-mm) bonding layer

for metal adhesion, a core layer of B0.6 mil (15-mm) of

homopolymer polypropylene, and a seal layer of B.01 mil

(2 mm) of propylene–ethylene copolymer. This ﬁlm is va-

cuum-metallized and extrusion-laminated to a reverse-

printed 0.7 mil (18 mm) oriented-polypropylene ﬁlm to pro-

vide a strong, easy-open package with excellent light and

moisture barrier and sufﬁcient oxygen barrier to maintain

an internal nitrogen-ﬂush headspace for about 3 months.

Finishing

The converter will print and laminate multilayer ﬂexible-

packaging materials in typical widths of 40–60 in. (1–

1.7 m). These are usually wider than the rolls that a

packaging machine can form, ﬁll, and seal. The converter

then slits the wide rolls into several narrower lanes

suitable for the customers’ machines. This product is

then sold in roll form to the customer in very compact

form. A typical 0.003-in. (75-mm)-thick multilayer mate-

rial wound on a core 3 in. (75 mm) in diameter can be

delivered in 18-in. (450-mm)-diameter rolls containing

about 7000 packages 12 in. (300 mm) long.

An alternative to multilayer ﬂexible packaging in rolls

is the premade bag or pouch form. For this product, the

front and back of the pouch are printed side-by-side. This

single package is folded longitudinally and heat-sealed

inside-to-inside along the fold and two of the other three

sides. These ﬂat pouches are sent to the customer, who ﬁlls

them with product and seals shut or otherwise closes the

fourth side.

CURRENT TRENDS IN MULTILAYER FLEXIBLE

PACKAGING

Innovation has been a reliable characteristic of ﬂexible

packaging over past decades (16), and signiﬁcant devel-

opment efforts continue to introduce new ﬂexible ﬁlms

and composites that improve package functionality in

essentially all of the areas discussed here. Recently, two

emergiing issues, packaging sustainability and active/

smart packaging, have emerged to further drive industry

inovation.

Appearance

New technical processes for achieving appropriate quality

for the graphical images reproduced on ﬂexible packaging

materials are being actively developed in the ﬁrst decade

of the 21st century. Increasingly, graphics origination,

review, and appproval efforts are accomplished using

Internet-based exchange of digital ﬁles. Such ﬁles are in

turn used to generate digital printing media (ﬂexo plates

or cylinders). Techniques exist to actually drive digital

print engines to print ﬂexible packaging materials, but are

not widely used at this writing because of ink costs and

limited throughput.

Advanced digital controls on printing presses permit

the adoption of ‘‘expanded color gamut’’ printing. This

technology tracks the color value and printed density of

six or seven standard colors (compared to the four tradi-

tional ‘‘process colors’’: cyan, magenta, yellow, and black)

in order to provide speciﬁc colors and color effects with

regularity and consistency between production lots.

Criminal efforts to market counterfeit branded goods

have

stimulated the development

of various appearance

techniques to authenticate the source of packaged goods

in the market place. These include overt (visible to the

consumer) methods as holographic reproductions. Covert

means (such as ﬂuorescent inks) support scrutiny of

wholesale or retail stocks by trained agents of producers

or buyers of the packaged goods.

Barrier

Transparent alternatives to opaque foil and metallized

multilayer barrier materials increasingly ﬁnd their way

into commercial packages. The most common of these are

ceramic or metallic oxides (for example, silicon and alumi-

num oxides) that are deposited on plastic ﬁlms in vacuum

coating operations comparable to those used to produce

widely used metallized barrier materials. The oxide vapors

are produced with evaporative methods from solid forms of

the compounds or with reactive methods in which precur-

sors of the oxides are introduced into a chamber and

plasma or chemical reaction energy used to form the

oxides. Other transparent coating techniques have been

developed to combine the oxygen barrier properties of

804 MULTILAYER FLEXIBLE PACKAGING

polyvinyl alcohol with the moisture barrier properties of a

metallic-compound-containing material (17).

Nanotechnology in ﬂexible barrier materials is most

evident in attempts to disperse nanometer-thick platelets

of naturally occurring clay minerals in polymers. The

objective is to arrange the platelets so as to form a

‘‘tortuous path’’ for gaseous molecules that would perme-

ate through a polymer layer. The technology is now best

practiced with polymers, such as nylon and ethylene vinyl

alcohol, that have regions of relativlely strong electro-

static polarity distributed throughout them. Nonpolar

molecules, especially polyoleﬁns, are unable to break

apart the natural clay particles to achieve the needed

distribution of platelets in the polymer. Research into the

use of surfactants (with their distinct polar and nonpolar

ends) has shown promise in effectively dispersing plate-

lets into polyoleﬁns.

Containment

Ironically, ﬂexible packages seem to be trying to become

more rigid, at least stiff enough to stand up. The Doyen

standup pouch concept has been used for hot-ﬁll juice

containers for over three decades (18). European and

Japanese consumers are exposed to many other commer-

cial examples at present. In the United States, many

domestic market introductions have succeeded. The for-

mat is now a mainstay for consumer products in the

States. In the 2007 Flexible Packaging Association (FPA)

Achievement awards and Innovation showcase, 13 Doyen-

style pouches were recognized (19). Further evolution

includes pouches shaped to suggest their prior rigid

package formats, such as (a) bottles or jars (three such

examples were included in the 2007 FPA awards) and (b)

pouches fabricated with ﬂat bottoms perpendicular to

distinct side panels (six such examples included in the

2007 FPA awards).

Ease of opening for ﬂexible packaging has emerged as a

signiﬁcant challenge to growth for the industry (20). An

aging population with decreasing manual dexterity has

come to demand easy-opening ﬂexible packaging and can

recognize the differences, as ‘‘Consumer Reports’’ found

with three different brands of ready-to-eat cereal with

ﬂexible coextruded ﬁlm liners in folding cartons (21).

Packaging Sustainability

By nature, multilayer ﬂexible packaging contains many

materials intrinsically bonded together. This fact miti-

gates against recycling the component materials, as has

been possible for many monomaterial packaging systems

(e.g., HDPE merchandise bags and milk bottles, PET

beverage containers, aluminum cans, and glass bottles;

note that some progress has been made in separating at

least some components of multilayer materials for eco-

nomic recovery) (22). Rather than minimizing packaging

residuals after use by recycling, the ﬂexible-packaging

industry has been urging consumer and regulatory con-

sideration of the advantages of using less packaging

material for a product as it is put into commerce (23).

Flexible-material packaging systems offer signiﬁcant re-

duction of both waste volumes and weights when

compared to traditional systems. A few of these are

summarized in Table 4.

Other strengths regarding sustainable industrial de-

velopment practices exhibited by ﬂexible packaging come

from the ability to use ‘‘biopolymers,’’ polymers from

carbon sources grown in contemporary farming, forestry,

or aquaculture systems, rather than from fossil fuels.

Modiﬁed starch has been extruded into ﬁlm and fabricated

into bags. Lactic acid has been extracted from corn and

then polymerized into polylactic acid. Numerous ﬂexible

barrier packages have incorporated this material into

commercial packages at this writing.

Active/Smart Flexible Packages

‘‘Active packaging’’ (packaging that changes conditions

and/or environment in the package to extend shelf life,

Table 4. Flexible-Packaging Source Reduction

Packaging System Reduction (%)

Baseline Reduced Weight Volume

Coffee can Brick pack 70 55

Detergent bottle Concentrate pouch 85 84

Diaper carton Film bag 85 86

Soup can Soup pouch 93 97

Glass bottle Flexible pouch 96 82

Glass bottle Drink box 90 70

Source: Flexible Packaging Association.

Table 5. Active and Smart Packaging Types

Package Type Example Source Integral/Separate

Active

Microwave susceptors Graphic

Packaging

Integral

Purge absorbers Maxwell Chase Separate

Oxygen absorbers Mitsubishi

Ageless

Separate

Oxygen indicators Toppan Separate

Oxygen scavenging ﬁlm Cryovac Integral

Carbon dioxide

controller

Guinness Widget Separate

Antibody immobilizer Toxin Alert, Inc. Separate

Temperature reliant

permeation

Landec

Intelimer

Separate

Volatiles: absorption/

release/transmission

CSP Technologies Integral

Odor controllers Cell Resin

Technologies,

Integral

Antimicrobials Englehard Integral

Smart

Antitheft devices/tags Sensormatic Separate

Temperature indicators Tempilabel Separate

Time-temperature

integrators

3M Separate

Quality/spoilage

indicators

Cox Technologies,

Inc.

Separate

Ripeness indicators Ripesense

Limited

Separate

RFID Alien Technology Separate

MULTILAYER FLEXIBLE PACKAGING 805

enhance sensory properties, and/or improve food safety)

and ‘‘smart packaging’’ (packaging that senses change

and/or changes package properties) technologies have

long been recognized in the packaging industry as possi-

ble, but for a variety of commercial and legal issues have

not been widely adopted in the U.S. consumer market. The

European Union, in contrast, has actively encouraged

commercialization of the technologies. The European

‘‘FAIR’’ Project CT98-4170 sought ‘‘[t]o enable the applica-

tion of active and smart concepts throughout Europe and

to establish and implement these concepts in the current

relevant regulations for food packaging in Europe.’’ In

their view, ‘‘Active packaging y will likely emerge as the

preservation technology of the twenty-ﬁrst century’’ (24).

Cooksey reported on commercial applications of active

and smart packaging technologies in 2006 (25) A summary

of these examples is given in Table 5. The table also

indicates whether the technology is ‘‘integral’’ to the

packaging material or a ‘‘separate’’ label, sachet, or other

device incorporated into the package.

As is apparent here, much of the commercial and

proposed application of active/smart packaging technology

intends the use of separate and additional components in

or on the package (along with the product itself) rather

than a packaging material that inherently provided the

functionality itself. Such merged material performance is

the subject of much current research (26).

BIBLIOGRAPHY

1. Aluminum Association, Aluminum Foil, 2nd edition, Wa-

shington, DC, 1981.

2. W. A. Jenkins, and J. P. Harrington, Packaging Foods with

Plastics, Technomic Publishing, Lancaster, PA, 1991, p. 34.

3. B. Blakistone, ‘‘New Developments in Plastic Packaging Seal

Integrity Testing: One Key to the Future of High Speed

Plastic Packaging’’ in Proceedings of the 1994 IOPP Packa-

ging Technology Conference, Institute of Packaging Profes-

sionals, Herndon, VA, 1994.

4. ASTM, F1168, Standard Guide for Use in the Establishment

of Thermal Processes for Foods Packaged in Flexible Contain-

ers, American Society for Testing and Materials, Philadel-

phia, 2000.

5. Michigan State University, School of Packaging, ‘‘Product

Storage Stability Based on the Permeability of the Package

Storage System,’’ Course Notes, July 10–13, 1984, East Lan-

sing, MI, 1984.

6. ASTM,

D3985, Standard Test Method for Oxygen Gas Trans-

mission Rate through Plastic Film and Sheeting Using a

Coulometric Senso and

F1249, Standard Test Method for

Water Vapor Transmission Rate through Plastic Film and

Using a Modulated Infrared Sensor, American Society for

Testing and Materials, Philadelphia, 2005.

7. British Aluminium Foil Rollers Association, Barrier Proper-

ties of Aluminum Foil: A Handbook, London, 1973.

8. R. J. Ginsberg, ‘‘Cohesive Coatings Offer Food Field Cost

Advantages, Higher Speeds,’’ Packag. Devel. Syst. July/Au-

gust, 23–35 (1979).

9. T. Dunn, ‘‘Use of Differential Scanning Calorimetry in Devel-

oping and Apply Films for Flexible Packaging’’ in M. L.

Troedel, ed., Current Technologies in Flexible Packaging,

ASTM STP912, American Society for Testing and Materials,

Philadelphia, 1984.

10. L. S. Timm and T. E. Bublitz, U.S. Patent 5,993,962, ‘‘Reseal-

able packaging system,’’ November 30, 1999; and S. J. Strauss

et al., U.S. Patent 5,089,320; ‘‘Resealable packaging mate-

rial,’’ February 18, 1992.

11. D. Van Erden, U.S, Patent 6,112,374; ‘‘Zipper for slider

package,’’ September 5, 2000; and T. J. May, U.S. Patent

6,491,432, ‘‘Resealable closure mechanism having a slider

device and methods,’’ December 10, 2002.

12. T. Dunn, ‘‘Flexible Packaging and Environmental Control in

the 80’s and 90’s,’’ in Proceedings of Envirocon ‘89, The

Packaging Group Inc., Princeton, NJ, 1989.

13. U.S. Food and Drug Administration, ‘‘Indirect Food Additives:

Adhesive Coatings and Components,’’ Code of Federal Reg-

ulations, Title 21, Part 175.

14. T. Bezigian, ‘‘Extrusion Coating and Laminating—The

Growth of an Industry,’’ Converting Mag. (Part I, p. 48ff.,

January 1992; Part II, p. 30ff., February 1992).

15. R. Isbister, ‘‘Chemical Priming for Extrusion Coating,’’ J.

TAPPI 71(5), 101–104 (1988).

16. T. Dunn, ‘‘Flexible Packaging 1994: Issues, Trends, and Life

Cycle Analysis,’’ in Proceedings of the TAPPI International

Packaging Symposium, Chicago, July 1994.

17. H. Ohba, et al., U.S. Patent # 6,605,344;‘‘Gas-barrier ﬁlms’’;

August 12, 2003; 16 pp.

18. A. Brody, ‘‘The Source Reduction Pros and Cons of Liquid

Stand-up Pouches’’ in Proceedings Green Packaging ‘94 Con-

ference,’’ Packaging Strategies, Washington, DC, June, 1994.

19. Anonymous, ‘‘Honoring Innovation: 2007 Flexible Packaging

Achievement Awards and Innovation Showcase’’; Flexible

Packaging Association, Linthicum, MD, March 2007, 36pp.

20. Anonymous, ‘‘The Oyster Awards: Consumer Reports’s Hard-

to-Open-Packaging Hall of Shame Welcomes New Inductees,’’

Consumer Reports, March 2007.

21. Anonymous, ‘‘The Oyster Awards: Consumer Reports’s picks

for America’s hardest-to-open packages,’’ Consumer Reports,

March 2006.

22. E. Klein, ‘‘The Greening of Coated Paperboard’’ in Proceed-

ings of the TAPPI Polymers, Laminations, & Coatings Con-

ference, Nashville, TN, August 1994.

23. T. Dunn, ‘‘Source Reduction Needs Deﬁning to Have Max-

imum Impact,’’ Packag. Technol. Eng. 3(4), 44–47 (1994).

24. L. Vermeiren, et al., 1999; ‘‘Developments in the Active

Packaging of Foods’’ in Trends in Food Science & Technology,

Vol. 10, Elsevier Science Ltd., Amsterdam, pp. 77–86.

25. K. Cooksey, ‘‘General Overview of Active and Intelligent

Packaging,’’ in National Plastics Exposition-Educational

Conference, Chicago, IL, June 21, 2006.

26. A. L. Brody, et al., Active Packaging for Food Applications,

CRC Press, Boca Raton, FL, 2001, 236 pp.

806 MULTILAYER FLEXIBLE PACKAGING

NANOCOMPOSITE PACKAGING MATERIALS

M. D. SANCHEZ-GARCIA

J. M. LAGARON

Novel Materials and

Nanotechnology Lab., IATA,

Valencia, Spain

NANOCOMPOSITES CURRENT INTEREST AND

APPLICATIONS IN FOOD PACKAGING

Nanotechnology is by deﬁnition the creation and use of

structures with at least one dimension in the nanometer-

length scale that creates novel properties and phenomena

otherwise not displayed by either isolated molecules or

bulk materials. Since Toyota researchers in the late 1980s

found that mechanical, thermal, and barrier properties of

nylon-clay composite materials improved to a signiﬁcant

extent by reinforcement with less than 5% of clay (1),

extensive research work has been performed in the study

of nanocomposites for packaging application. The term

‘‘nanocomposite’’ refers to composite materials that con-

tain typically low additions of some kind of nanostruc-

tured materials. Most nanocomposites being considered in

the packaging sector are based in low additions, typically 1

to 7 wt.-%, of modiﬁed nanoclays.

Nanoscale structures display a high surface-to-volume

ratio, which becomes ideal for applications that involve

composite materials, chemical reactions, drug delivery,

controlled and immediate release of substances in active

an functional packaging technologies, and energy storage,

for instance, in intelligent packaging (2, 3).

Until recently, the most interesting packaging technol-

ogy based on blending to generate barrier properties was

the so-called oxygen scavengers. This technology is known

to lead to relatively low levels of oxygen in contact with

the food because it traps permeated oxygen from both the

headspace and the outside. However, in carbonated bev-

erages for instance, a barrier to carbon dioxide is also a

requirement. As most commodity plastic packaging mate-

rials, e.g., polyethylene terephthalate (PET) and its main

sustainable counterpart the PLA, are not sufﬁcient bar-

rier to these gases, multilayer structures had to be devised

in which one layer (made of Ethylene vinyl alcohol copo-

lymer (EVOH), MXD6, polyethylene naphthalene (PEN),

and nanocomposites of PA6) needs be high barrier to

carbon dioxide and to oxygen. An alternative, technology

is the use of oxygen scavenger but these only reduce O

and not CO

, at the packaging headspace. The nanobio-

composites technology can overcome this in monolayer

solutions because barrier properties are usually not only

to oxygen but also to other gases and low-molecular-

weight components such as water vapor and food aroma

components (4).

In general, the rationale behind the interest of the

nanocomposites is that with low additions of the

nanoﬁller, many relevant properties can be enhanced

without a signiﬁcant drop in interesting properties of

the matrix such as transparency or toughness. As stated

above, one property that has attracted more interest in the

food-packaging area, because of its direct implication in

food quality and safety issues, is the possibility of increas-

ing the barrier properties to gases and vapors of plastics

and bioplastics. Thus, by simple application of models

such as the one of Nielsen, and for the case of laminar

structures with high aspect ratios (L/2W of 180), the model

predicts permeability reductions of the matrix of up to

about one order of magnitude for fractional volume (j)

layered clay additions of ca. 0.05 (see Figure 1).

In this technology, it is reckoned that the presence of

exfoliated layers of clay in the polymer results in enhance-

ment of the so-called tortuosity (or detour) factor. The

equation in Figure 1 shows a simple formalism to model

permeability of systems comprising impermeable plates of

a layered ﬁller oriented perpendicular to the permeant

transport direction and evenly dispersed across the poly-

mer matrix. The use of other models such as the one of

Cussler predicts even higher permeability reductions but

most experimental data with barrier improvements below

10-fold ﬁt better the above Nielsen model (5).

Numerous studies in the literature report that experi-

mental improvements in barrier properties to water vapor,

i.e., about 60% permeability reduction in polyvinyl alcohol

(PVOH) (6) and even larger in gluten at 11% relative

humidity (RH) (7), and to oxygen [above 75% in EVOH (8–

10) and up to 64% in polylactic acid (PLA) (11, 12)], can be

achieved via polymer nanocomposites. It is therefore a

proven fact that the integration of ceramic nanoparticles

with tailor made modiﬁcations in plastics matrices does

help increase the barrier to oxygen and vapors and reduce

the water sensitivity of biopolymer matrices.

There are several technologies to achieve nanodisper-

sion, namely, in situ polymerization, dispersion in solu-

tion, and dispersion by melt blending. The latter route is

perhaps the most interesting methodology from an applied

view point because it can be used more widely to convert

most plastics into ﬁnal articles (13–15).

Nanoclays are currently being considered as barrier

layers in multi-layer packaging to improve barrier

matrix

nano

clay

1− φ

clay

d ′

Figure 1. Schematics showing the tortuosity effect imposed by

nanolayered clays to the transport of low-molecular-weight com-

ponents through a ﬁlm and a typical model equation that takes

this effect into account in the permeability coefﬁcient.

807