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

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and other ﬁtments on separate converting equipment than

on in-line form/ﬁll/seal equipment. Preformed pouches, to

date, have tended to be more reliable in distribution

performance than in-line-made form/ﬁll/seal pouches. Be-

cause of this reliability factor, most liquid-containing

standup ﬂexible packages to date have used preformed

pouches. Since many different ﬁll/seal machines begin-

ning with preformed pouches are commercially available,

preformed pouches have commonly been used.

STANDUP FLEXIBLE-POUCH MACHINERY

In general, there are two types of machines for producing

standup pouches: preformed pouchmaking machines and

horizontal form/ﬁll/seal machines.

Preformed pouch machines are used by converters to

make standup pouches that are supplied to a packaging

company that will ﬁll the pouch through the open top.

These machines utilize single-wound laminated (nonex-

tensible) rollstock. They make pouches in either (a) two

lanes of production, for pouches smaller than approxi-

mately 7 in.  11 in, or (b) one lane of production, for

pouches larger than 7 in.  11 in. The same pouch

machines can add zipper reclosures and easy-open fea-

tures, or can be used to make conventional three-side-seal

pouches. Some pouch machines use a separate bottom

gusset web and can then easily manufacture in two lanes

(‘‘2-up’’ or two pouches at one time). Other machines use

one web and plow the bottom gusset into the center-folded

material, but are limited to one lane of production.

The advantages of using the preformed pouch are its

superior seal integrity and the ease of adding a reclosure

feature, such as the reclosable pour spout. Preformed

pouch equipment is used more often for packaging liquids

and for small-market-niche products.

Packagers utilize form/ﬁll/seal machinery to fabricate

and ﬁll the standup pouch. Again, single-wound lami-

nated sheeting is used on the machine to form the pouch

and then ﬁll and seal it in-line. This is known as a one-step

operation, while the preformed pouches must be shipped

from the converter to the packager for ﬁlling and sealing

in a second operation.

The form/ﬁll/seal (one-step) operation offers economies

to the packager. Even though the speeds of the pouch-

forming operation are typically much slower than those

using preformed pouches, the fact that all operations are

done in-line saves on storage and labor costs. Standup

pouch form/ﬁll/seal machinery is more typically used for

the packaging of large-volume, dry granular products

such as nuts and snacks (see Form/ﬁll, seal horizontal

and vertical).

USE OF STANDUP FLEXIBLE POUCHES WORLDWIDE

The worldwide use of ﬂexible standup pouches, exclusive

of Capri Sun, was about 700–900 million in 1993, with the

largest fraction being in Japan, with the grand total in the

3–3.5 billion range. There was limited but steady growth

during the mid- to late 1990. During recent years, 2005 to

2007, Standup pouches have been growing at a steady

rate.

The introduction of the ﬂat bottom standup pouches

began in late 2005. The ﬂat-bottom pouch can be made as

a relacement for rigid and semirigid packages. The year

2007 has seen the format grow in human and pet food

applications. This format is expected to grow steadily into

the future.

The following is a discussion of the use of standup

ﬂexible pouches throughout the world, broken down by

area.

North America

Package material source reduction was a major factor

behind the development of product packaging in standup

ﬂexible pouches in the United States and Canada.

Although there is really no practical way to recycle the

standup ﬂexible pouch itself, the initial package reduces

material use and results in less material going to the

landﬁll. Standup ﬂexible pouches offer the potential for up

to 80% package mass reduction for liquid products such as

household laundry products, window cleaners, chemicals,

detergents, fabric softeners, lotions and creams, and

shampoos. Packagers with environmental concerns began

to investigate and test standup ﬂexible pouches that had

been commercialized in Germany in the mid-1980s. Using

structures of polyester ﬁlm and sometimes nylon ﬁlm plus

LLDPE and a variety of European packaging equipment,

packages were introduced as environmentally ‘‘friendlier.’’

A popular application was as a reﬁll for high-density

polyethylene (HDPE) bottles.

The ﬁrst use of a large-size standup ﬂexible pouch in

the United States was for evaporated milk. It was retorted

to achieve ambient temperature shelf stability.

Standup ﬂexible pouches appear to have effected a

more successful entry into the Canadian market for liquid

laundry products under the umbrella of being environ-

mentally more ‘‘friendly.’’ Some of these laundry products

are packaged in the United States. Other products being

marketed in standup ﬂexible pouches in Canada include

hair shampoo/conditioner, motor oil, and liquid household

cleaner.

Europe

Among the products being packaged in standup ﬂexible

pouches in Europe are liquid detergents, fabric softeners,

shampoos, hand creams, windshield-washer ﬂuids, and

latex paint in p5-L sizes. Standup ﬂexible pouches re-

present a rather large usage for liquid packaging in

Europe.

Most

European standup ﬂexible

pouches for liquids are

premade and appear to be three-piece, using 12-mm

(0.00048-in.) PET ﬁlm/0.004- to 0.006-in. LLDPE for the

two major faces and the softer, stronger nylon ﬁlm/LLDPE

or PET/LLDPE for the bottom panel. Thus, the ﬂexible

structures for standup ﬂexible pouches are not especially

unique or difﬁcult to fabricate.

1158 STANDUP FLEXIBLE POUCHES

Latin America

In Latin America, standup ﬂexible pouches have been well-

received. One reason is the apparent low cost of ﬂexible

materials versus their semirigid plastic package counter-

parts. The pouches have become established in a number of

food markets, including mayonnaise, mustard, ketchup,

salad dressings, and hot-ﬁlled jams and jellies in the ﬂuid

category, as well as for dry (powdered) gelatin desserts,

syrups, toppings, rice, and analogous ﬂowable dry pro-

ducts. In the nonfood category, several companies pack

liquid fabric softeners and detergents in standup ﬂexible

pouches. Capri Sun is also present in the Latin American

market. The number of standup ﬂexible pouches in Latin

America is estimated to be in the 200-million-unit range.

Japan

Almost all are preformed pouches fabricated from polye-

ster ﬁlm/aluminum foil/cast polypropylene ﬁlm, with some

in glass-coated polyesters as the barrier, packaged on

several different types of equipment. A few of the alumi-

num foil laminated standup ﬂexible retort pouches are

distributed in the United States.

In Japan, several supply companies developed both

equipment and materials that allowed for a relatively

healthy market niche for liquid food products. The concept

was so well engineered that it could be applied for retort

pouches of low-acid ﬂuid foods and beverages such as milk.

India

Prominent uses include dry rice, motor oil, and related

automotive ﬂuids. Environmental factors once again play

a role in the choice of pouch packaging. Motor oil repre-

sents a signiﬁcant environmental solid-waste problem,

since the conventional extrusion blow-molded HDPE bot-

tles are not recyclable. Large numbers of semirigid bottles

become part of the municipal solid-waste stream. The

incorporation of motor oil into ﬂexible packages helps

reduce this package mass.

ECONOMIC ISSUES

Historically, the standup ﬂexible pouches have not offered

lower packaging costs, although this has now begun to

change. The actual costs of the standup ﬂexible pouches

with liquid dispensing spouts have been more than double

the cost of extrusion blow-molded plastic bottles with

labels and closures. Another added cost is importing

from Europe, Israel, and even Japan. Thus the expected

cost savings initially did not materialize, and, in fact, they

were negative or at best even. In late 1993, however, a

signiﬁcant change in the costing situation emerged as

domestic sources for both preformed and form/ﬁll/seal

standup ﬂexible pouches began to develop. In the United

States, the costs have come down considerably and are

now competitive with the costs for more traditional packa-

ging, such as an extrusion blow-molded HDPE bottle. On

the horizon, however, is the ability of plastic bottle blowers

to reduce the wall thicknesses and hence the package

mass and the economics even more—and, of course, their

increasing use of postconsumer scrap plastic for nonfood/

cosmetic contents.

Yet another major problem is that all automatic

standup pouch ﬂexible-packaging equipment is much

slower than bottle-packaging equipment, some down to

25–30 one-liter units per minute. Maximum output speed

for any standup pouch ﬂexible packager is in the range of

120 per minute. Most bottling lines for liquid household

products begin at 100 and go up to 400 bottles per minute,

with some even higher.

Some pouch equipment manufacturers are developing

higher-speed machines, with speeds of X400 per minute.

This new equipment will generate greater attention for

standup ﬂexible pouches for liquid packaging.

Standup ﬂexible pouches have almost zero vertical

compression strength and thus must be totally protected

by higher-cost and higher-package-mass secondary and

tertiary packaging in distribution. Furthermore, standup

ﬂexible pouches are much more difﬁcult to handle than

their bag-in-box equivalents.

Another key to the use of standup ﬂexible pouches is a

lack of equipment to handle and unitize the pouches into

secondary and tertiary packages. Much of the North

American output is actually hand-packed into corrugated

ﬁberboard cases. On the other hand, equipment for utiliz-

ing ﬂexible pouches is commercially available.

The standup ﬂexible liquid-content pouch market is

relatively small, representing only about 1% of the total of

all ﬂexible pouches.

BIBLIOGRAPHY

General References

A. L. Brody, Rubbright-Brody, Inc., ‘‘The Source Reduction Pros

and Cons of Liquid Stand-up Pouches,’’ paper presented at

Green Packaging ‘94, Washington, DC.

A. L. Brody, Rubbright-Brody, Inc., Worldwide Technology, Mar-

kets and Prospects for Stand-up Flexible Pouches, Packaging

Strategies, Inc., 1994.

‘‘Cleaner Reﬁlls Spout a Cap,’’ Packag. Digest (Sept. 1992).

Liquid Pouch Packaging vs. Rigid Blow Molded Containers:

Opportunity or Threat and Environmental Implications

1991–1996, Mastio & Company Pouch Study, 1992.

W. J. Noone, ‘‘Stand-up Flexible Pouches: Has Their Time Come?’’

Packag. Print. Converting (March 1995).

STAPLES

J. R. CHARTERS

Paslode Corporation

INTRODUCTION

In packaging, staples are used to assembly wooden pallets,

skids, and boxes and crates (see Boxes, Wood). In

STAPLES 1159

fastening wood to wood, the relatively heavy shank of a

nail provides greater shear strength, but staples provide

greater holding power because their two legs provide more

wood-to-metal contact surface area. Staples are used as a

replacement for nails in many applications including

furniture, housing, fencing, etc, but the following discus-

sion focuses on packaging applications only.

The staple has three basic parts: the crown, two legs,

and two points (see Figure 1). In selecting the proper

staple, four factors must be considered: wire size (gauge),

crown width, leg length, and type of point.

WIRE SIZE

Heavy-duty staples are designated by the gauge (ga) of the

wire prior to manufacturing the staples.

16 ga ¼ 0:062 in: ¼ 1:587 mm

15 ga ¼ 0:072 in: ¼ 1:829 mm

14 ga ¼ 0:080 in: ¼ 2:032 mm

In packaging applications, 16-gauge is the most com-

mon. Heavier-gauge staples are used if a particularly

strong combination of holding power and shear strength

is required, as in the assembling of some heavy-duty skids.

Lightwire staples are designated by the width of the wire

after manufacturing; for example, ‘‘30’’ is equivalent to

0.030 in. (0.762 mm) ﬁnished width; ‘‘50’’ is equivalent to

0.050 in. (1.270 mm) ﬁnished width. A ‘‘50’’ staple is hea-

vier and more power is required to drive it.

CROWN WIDTH

The widths most common in packaging are shown in

Figure 2.

The staple used most frequently for packaging applica-

tions is the heavy-duty 1/2 -in. (1.3-cm) crown. The wide-

crown 15/16-in. (2.4-cm) heavy-duty staple is used to

prevent pull-through on attachment of porous materi-

als—for example, fastening a corrugated box to a wooden

skid. The wide-crown staple is also used over steel strap-

ping to secure it in place. Lightwire staples are used to

attach identiﬁcation tags to the outside of wooden boxes.

Plastic liners are attached to boxes with lightwire staples

as well. These are just a few of the numerous applications

for heavy-duty and lightwire staples in packaging.

LEGS

A staple has two legs of equal length. The staple must be

long enough to penetrate the receiving member with

sufﬁcient length to adequately hold two pieces of material

together. The stress of the application must be considered

in order to determine the minimum allowable penetration

of the staple in order to do the job.

POINTS

Staple points are cut at different angles to meet various

application requirements (see Figure 3). A divergent point

causes the staple legs to divert in opposite directions when

it is driven into the wood. This type of point is advanta-

geous when clinching is required. Clinching is the process

of bending the staple points over the opposite side of the

work piece. The staple legs penetrate the wood members

then strike a steel plate diverting the points back into the

wood. The chisel point, which is designed to cut through

the wood without diverging, is the most common in the

industry.

Figure 1. Three basic parts of the staple.

Figure 2. Common crown widths.

Figure 3. Staple points.

1160 STAPLES

BIBLIOGRAPHY

J. R. Charters, ‘‘Staples’’ in M. Bakker, ed., The Wiley Encyclope-

dia of Packaging Technology, 1st edition, John Wiley & Sons,

New York, 1986, pp. 615–616; 2nd ed., R. J. Brody and K. S.

Marsh, eds., 1997, pp. 856–857

General References

Products 1, Paslode Company, Lincolnshire, IL, 1979.

Paslode Products, Paslode Company, Lincolnshire, IL, 1983.

STATIC CONTROL

H. SEAN FREMON

Cole Static Control, Inc., Akron,

Ohio

Since its inception in the packaging industry approxi-

mately 80 years ago, static control has become an integral

part of packaging processing. As machinery speeds have

increased and materials have changed from paper to

plastics and combination laminates and coatings, the

need for static control has rapidly expanded.

As a result, the last 10 years have seen the greatest

increase in the use and development of static control

products to enhance manufacturing processes.

Static is the excess or deﬁcit of electrons on a material.

A regular nonpolar atom has a nucleus with a positive

charge and negatively charged electrons orbiting it. When

one atom comes in close proximity to another, electrons

from the orbit draw or tear away electrons, leaving one

atom with an excess of electrons and the other with a

deﬁcit. This process, called ionization, leaves the atom

with the excess carrying a negative charge, while the one

with the deﬁcit carries a positive charge.

CAUSES OF STATIC

Static in packaging manufacturing is caused by friction,

separation, heat change and improper grounding. Packa-

ging and material handling equipment often incorporate a

combination of these to accomplish its function.

Friction

Static is most commonly caused by friction, the rubbing of

two materials during packaging processing. When a plas-

tic substrate in web form rubs over a steel roller, or when a

pouch is shaped around a forming collar on a vertical form

ﬁll seal machine, for example, the plastic picks up the

negative charge while the steel roller carries the positive

charge. Because metal is such a good conductor, however,

the positive charge is bled off. The charge on the plastic

is not.

Table 1 shows the likely relative polarity a material will

exhibit if it is rubbed with another. The material higher on

the chart will hold a positive charge, while those in the

lower positions will hold the negative charge.

Separation

When material separation occurs, such as the unwinding

of packaging material on a form–ﬁll–seal machine, static

charges are produced at the tangent of the unwind. This

can result in very high charges that attract dust and

contaminants and can cause operator shock hazards. Any-

one who has handled Scotch tape may recognize this

phenomena. As a piece of tape is pulled off its roll and

the tape wraps around itself, it becomes difﬁcult to handle

because static has occurred from the separation.

Heat Change

The change in heat during the thermoforming, injection

and blow molding of packaging products can cause static

charges. As plastic is melted to shape and removed from

its appropriate tooling, it undergoes a cooling process that

can add more static to existing products. As a part cools,

its charge increases and can draw existing dust from the

plant atmosphere. In extreme situations, the dust can

draw to the product surface and adhere to the hot part,

making it nearly impossible to remove later. If the parts

undergo additional manual operations, such as bottle

printing or container sealing, operators are susceptible

to shock (see also Thermoforming).

Improper Grounding

One of the common misconceptions in static control is that

if a machine is grounded, no static can be present. How-

ever, despite grounding efforts from the material to

Table 1. Triboelectric Series

Air Positive charge

Human skin

Rabbit fur

Glass

Human hair

Nylon

Wool

Silk

Aluminum

Paper

Cotton

Steel

Wood

Hard rubber

Nickel, copper

Brass, silver

Gold, platinum

Acetate ﬁber (rayon)

Polyester

Cling ﬁlm

Polyethylene

PVC

Silicon

Teﬂon Negative charge

STATIC CONTROL 1161

ground, there can be blocked passages in the form of

greased bearings, poor metal-to-metal connections and

charges too high to be properly grounded. Because many

packaging products are insulators, partial static reduction

can be attained, but full grounding is impossible.

Results of Untreated Static Electricity

The development of static-related problems are often

inevitable in the manufacturing environment and can

lead to a variety of different problems.

Production Problems and Slow Downs

A static charge generates an electrical ﬁeld that acts like a

magnet, repelling similar charges and attracting opposite

or neutral charges. This accounts for the attraction be-

tween charged materials and machine frames or rollers

causing machine jams. In many cases, the machine op-

erator will be forced to run at slow speeds to avoid the

problems caused by static.

Dust Attraction

Many products, such as plastic molding materials and

ﬁlm, develop high static charges on their surfaces. This

highly charged surface will attract airborne dust, some-

times from more than three feet away, and hold it tightly

to the surface. This can lead to high scrap rates. Subse-

quent operations (such as molding, printing, or laminat-

ing) and their end products can be seriously affected by

such difﬁcult-to-remove contamination.

Shocks to Personnel

When handling highly charged material, people receive

unpleasant shocks either directly from material or indir-

ectly. This can become a safety issue if the operator, after

being shocked, recoils into other moving machinery in the

plant.

Fires and Explosions

Static is usually found on nonconductive materials where

high resistivity prevents the movement of the charge.

There are at least two situations where the static charge

an move quickly and be dangerous in a combustible

atmosphere. The ﬁrst is where a grounded object intensi-

ﬁes the static ﬁeld until it overcomes the dielectric

strength of the air and allows current to ﬂow in the form

of a spark. The second case is where the charge is on a

ﬂoating conductor such as an isolated metal plate. Here

the charge is very mobile and will ﬂash to a proximity

ground at the ﬁrst opportunity.

Damage and Interference in Electronic Components

Strong static ﬁelds and static discharges can cause inter-

ference with electronic equipment. This can cause metal

detectors to malfunction, ink-jet printer boards to burn

out, and weighing equipment to give false readings.

TYPES OF STATIC CONTROL DEVICES

Active/Electrical Ionizers

Active electrical ionizing equipment, one of the most

effective ways of eliminating static charges, is seen in

nearly every large packaging plant that processes plastic

or paper at high speeds. These units create ionization by

forcing high voltage to ground and breaking down air

molecules in between into low levels of ozone. This makes

available millions of free positively and negatively

charged ions and effectively ‘‘feeds’’ away the static charge

by ‘‘matching up’’ ions of opposite charges. Active ionizing

equipment comes in many shapes and sizes, including

alternating current (ac) bars or rods, blowers, nozzles, and

air curtains, steady-state direct current (dc), and pulsed

dc units.

Electrical ionizers are driven by power supplies, which

transform 110 V or 220 V into voltage ranging from 4500 V

to 14,000 V while lowering amperages to a safe level. This

equipment is normally custom manufactured and retroﬁt

to the machine it serves. If this equipment is not employed

properly, it is useless in packaging applications.

Many bar- or rod-type units are used in web and sheet-

based applications, while air-driven units are used for

static control on asymmetrical parts such as closures, jars,

or thermoformed packages. Most electrical ionizers cannot

be used in hazardous areas.

Static Bars. Static bars are most often used in web-

based applications in the packaging industry. This equip-

ment is necessary on many form–ﬁll–seal, overwrapping,

tamper evident sealing, and stretch wrapping machines.

Bars are normally placed within close proximity to the

web and do not require any air to operate. Most bar-type

ionizers must be within three in. of the substrate as it

passes beneath the bar(s).

Air Ionizers. Air ionizers, driven by small fans, com-

pressed air or large blowers for large parts, are used for

static control on asymmetrical products. In conveying,

unscrambling, ﬁlling, denesting, and sorting operations,

air ionizers may be necessary to prevent parts from

sticking together, prevent operator shock, or eliminate

dust attraction.

Nozzles, Guns, and Pinpoint Ionizers. Nozzles are used

in packaging to clean packaging products and to eliminate

static and dust. An example of this is during composite can

cleaning. Ionizing guns are used in more manual environ-

ments where cycle times afford the luxury of cleaning each

unit. Pinpoint ionizers are used in powder ﬁlling, for

example, when a small area must be ionized with a small

piece of equipment. Powder ﬁlling operations often use

pinpoint ionizers to discharge materials.

Steady-State DC.

Steady-state DC generators allow the

production

of a desired

polarity, positive or negative or

both, which makes them ideal for high-speed webs where

the polarity of the material is known. This allows control

1162 STATIC CONTROL

of static with higher voltage where conventional lower-

voltage AC static control is less effective.

Pulsed DC. Pulsed DC controllers allow adjustment of

positive and negative ions and control of the frequency of

ions, in hertz, that the unit produces. This type of equip-

ment is used in clean room and benchtop applications

where the use of air is not allowed or where large areas

need to be ionized. A ‘‘cloud’’ of ions is produced, making it

ideal for hand packaging of medical devices, for example.

Passive

Passive static control incorporates grounding and conduc-

tive materials. Gold and silver are excellent conductors

but cost-prohibitive. Carbon ﬁber, stainless steel, and

phosphor bronze are more commonly used passive static

control devices in the packaging industry. However, tinsel

remains most commonly seen because of its low cost and

availability. It is recognized by its similarity to Christmas

garland. Tinsel is usually loosely tied to the frame of a

machine’s ground. Its effectiveness is considered mixed,

but if it reduces static to an acceptable level, it does away

with the need for more expensive electrical ionizers.

Nuclear Ionizers

Nuclear ionizers, radioactive ionizing devices, incorporate

polonium-210, an alpha-emitting isotope. When the alpha

particles collide with an air molecule, it creates thousands

of positive and negative ions without the use of electricity.

This makes nuclear ionizers ideal for explosion-proof

environments where electricity cannot be used and the

possibility of an electrically related spark can exist. Coat-

ing and chemical packaging are examples of plants where

nuclear ionizers must be used for static control. Most

nuclear ionizing units have an active life of approximately

one year, after which they must be returned and reﬁlled.

The Nuclear Regulatory Commission requires yearly

registration of these devices, which must be continually

leased instead of owned. The cost of leasing nuclear static

control devices versus buying electrical ones is much

higher and can make them cost-prohibitive. If a nuclear

unit is damaged, the site can be susceptible to strict clean-

up requirements by the NRC.

Static Generators

Static generators are used to temporarily bond materials

together, generally in web handling processes. Static

generators work the same way as static eliminators,

except that a ground is not present locally. It is placed

on the opposite side of the materials passing between

forcing two substrates to accept the ionization, thereby

pinning them. Static generators operate on DC, allowing

polarity control in packaging operations. Examples of

static generator use in the packaging industry include

air bubble removal, overwrapping, laminating, in-mold

decorating, and heatshrink packaging.

Static control devices, when used properly, can make a

great deal of difference in production efﬁciency and overall

costs. It is important that you meet face-to-face with your

static eliminator supplier or know your application well

when you are specifying or purchasing to assure the

equipment is necessary and correct for the application

intended. When ionizers results are minimal, it is usually

because equipment is improperly placed or the wrong

equipment is being used.

In the packaging industry, static ‘‘problems’’ are rela-

tive. In one plant, such as a plastic bag converting plant,

charges of 10,000 V may be barely noticed. In another

plant, such as a medical device packager, charges as low as

1000 V may be attracting airborne dust and foreign mat-

ter. It is important to know what actions can be taken to

remove existing problems relating to static and to what

extent they will enhance the packaging manufacturing

and material handling operations.

STRUCTURE/PROPERTY RELATIONSHIPS OF

PACKAGING MATERIALS

KIT L. YAM

Department of Food Science,

Rutgers University, New

Brunswick, New Jersey

The properties of materials are closely related to their

structures. This article provides an introductory under-

standing of the structure/property relationships of packa-

ging materials. Four levels of atomic and molecular

structures are discussed: (a) chemical constituents, what

atoms are composed in the material; (b) chemical bonding,

what forces hold the atoms together to form a molecule; (c)

intermolecular forces, what forces attract molecules to-

gether to form a material; and (d) spatial arrangements,

how molecules are arranged in three-dimensional space.

Although each level has distinct characteristics, the four

levels are closely related, with each level affecting the

sequential levels in determining the observed material

properties.

In a large sense, the ﬁrst two levels of structures—

chemical constituents and chemical bonding—are respon-

sible for the chemical properties of the materials. Chemical

properties determine the sensitivity of a material to che-

mical changes, which involve breaking chemical bonds to

transform part or all of the original material into new

substances. Examples of chemical changes important to

packaging include oxidation, corrosion of metal containers,

and incineration of packaging wastes. However, the last

two levels of structures—intermolecular forces and spatial

arrangement—are responsible for the physical properties

of materials. Physical properties determine the physical

behavior of a material under conditions, such as heat,

pressure, and concentration gradient, which do not involve

breaking chemical bonds and forming new substances.

Examples of physical properties important to packaging

include gas permeation through package walls, migration

of volatile compounds from package to food, and shock and

vibration during distribution.

STRUCTURE/PROPERTY RELATIONSHIPS OF PACKAGING MATERIALS 1163

CHEMICAL CONSTITUENTS

The ﬁrst level of structure considers the chemical consti-

tuents, which are the atoms composed in the packaging

material. These atoms determine what type of chemical

bonds can be formed and what chemical properties can be

expected. In general, consumer packages are constructed

of four basic types of materials (plastics, paper, glass, and

metals) with chemical constituents shown in Table 1. It is

important to note that plastics and paper are carbon-

based organic materials, whereas glass and metal are

inorganic materials, because this categorization has pro-

found implications on their properties, sources of raw

material, manufacturing technologies, and economic va-

lue. Like most organic materials, plastics and paper are

inherently lighter, weaker, and more susceptible to che-

mical reactions than inorganic materials such as glass

and metals.

Although most packages are made of single materials,

some packages are made of two or more different materi-

als to provide the necessary functionalities for certain

applications. For example, aseptic cartons for milk and

juices are made of a laminate consisting of 70% paper, 6%

aluminum (Al), and 24% polyethylene (PE). The typical

structure of the laminate is PE/paper/PE/Al/PE, in which

the paper layer provides strength and stiffness, the thin

aluminum foil layer provides gas and light barrier; the

polyethylene layers provide inner and outer protection,

heat sealing, and binding between paper and aluminum.

CHEMICAL BONDING

The second level of structure is chemical bonding that

determines how atoms are bound together to form a

molecule. A chemical bond is formed when the outermost

electrons from two atoms interact with each other to

achieve a lower and more stable energy state. These

interactions involve sometimes the transferring of elec-

trons to form ionic bonds and sometimes the sharing of

electrons to form covalent bonds or metallic bonds, de-

pending on the electronegativities of the atoms (i.e., their

abilities to attract electrons). Ionic, covalent, and metallic

bonds are known as primary bonds, which are character-

ized by strong bond strengths as shown in Table 2.

Some properties of materials generally associated with

the different types of chemical bonding are shown in

Table 3.

Ionic Bonding. An ionic bond is formed when two atoms

come together to transfer one or more electrons between

each other. For example, a sodium atom will transfer an

electron to a chloride atom to yield a Na

ion and a Cl



ion:

Na atom Cl atom

ion

−

ion

The electrostatic attraction binding the oppositely

charged ions together is known as the ionic bond.

Formation of an ionic bond occurs between two atoms of

greatly different electronegativities. Typically, one atom is

a metal (such as Li, Na, Al, or Fe) because of its tendency

to donate electrons, and the other atom is a nonmetal

(such as O, F, S, or Cl) because of its tendency to accept

electrons. Ionic materials are strong, hard, brittle, and

resistant to aggressive chemicals, and they also have a

high melting point.

Compared with covalent and metallic bonds, ionic

bonds are found less commonly in packaging materials.

Nevertheless, ionic compounds are found in protective

layers for packaging applications, such as alumina

(Al

) on foil and tin oxide (SnO

) on tin plate. Ionic

bonds are also found in ionomers, which are copolymers

that contain both ionic and nonionic repeat units. Shown

below is the chemical structure of Surlyn (DuPont,

Wilmington, DE), which is an ionomer polymer (see

the Inomer article) that contains a small portion of

Table 1. Chemical constituents, chemical bonds, and intermolecular forces in packaging materials

Material Commonly found atoms Chemical bond Intermolecular forces

Plastics Major: C, H C–C backbone, ionic van der Waals forces

Minor: O, N, Cl, etc.

Paper C, H, O Covalent van der Waals forces

Glass Major: Si, O Covalent, ionic Ionic solid

Minor: Na, K, Ca, Al, etc.

Metals Major: Fe, Al Metallic Coulombic forces

Minor: Cr, Sn, etc.

Table 2. Typical bond strengths of chemical bonding and

intermolecular forces

Interaction type

Bond energy

(kJ mol

1

)

Chemical bonding Covalent bonds 200–800

Metallic bonds 100–400

Ionic bonds 40–500

Intermolecular forces Hydrogen bonds 4–40

Dipole–dipole forces 0.15–15

Ion–dipole 5–60

Dispersion forces 0.4–4

1164 STRUCTURE/PROPERTY RELATIONSHIPS OF PACKAGING MATERIALS

methacrylic acid partially substituted with Na in its

carboxyl group:

−

The ionic groups in an ionomer may interact each other

to form a cluster with ionic bonds as shown in Figure 1.

This type of bonding may be considered partly as intra-

molecular bonding and partly as intermolecular interac-

tion. The ionomer is used as sealant between a plastic

layer and a foil in packaging applications where excellent

heat sealability and hot tack are required.

Metallic Bonds. Metallic bonds are found in aluminum

foil, metal containers, and other metal-containing packa-

ging materials. Atoms in metal typically have low electro-

negativities and are willing to share electrons freely, not

only with neighbor atoms but also with all other atoms in

the lattice. For example, Figure 2 shows the formation of

metallic bonding, when aluminum atoms lose their outer

electrons, become positively charged Al

ions, and those

electrons in turn form a ‘‘sea of electrons’’ to surround the

regularly spaced Al

ions. It is important to note that the

electrons in metallic bonds are ‘‘delocalized’’ or free, which

means that they are nondirectional and free to move in all

directions in the lattice, unlike the electrons in covalent

bonds that are bound to speciﬁc atoms and locations.

Metallic bonds are largely responsible for the strength,

malleability, and ductility of metals. Malleability is the

ability to be hammered into various shapes; and ductility

is the ability to be drawn out into thin wires, whereas the

shape of the material is changed without cracking or

breaking in both cases. Therefore, a metallic material

such as an aluminum sheet may be rolled into a thin

aluminum foil, or an aluminum disk may be hammered

into cups used for the popular soda cans.

To explain why metallic bonded materials are malle-

able and ductile, Figure 3 shows several layers of metal

ions held closely together.

Although strong, metallic bonds are not bound to

speciﬁc metal ions. Hence, forces from hammering or

stretching can cause layers of metal ions to slide over

each another. As the metal ions move, they also attract the

delocalized electrons to their new locations, whereas the

sea of electrons is not broken and still holds the metal ions

together.

Metallic bonds are also responsible for high electrical

and thermal conductivities of metals. The high electrical

conductivity may be explained by the ability of the delo-

calized electrons to ﬂow freely in the lattice of positive

metal ions when a voltage is applied. The high thermal

conductivity may be explained by the ability of metal ions

to absorb heat energy by vibrating faster. Because the

metal ions are closely packed in the lattice, the faster

vibrating ions can collide with their neighboring ions to

transfer the heat energy.

Covalent Bond Model. Covalent bonds are the most

abundant chemical bond found in plastics, paper, and

glass. A covalent bond is formed when two atoms come

together to share their outermost electrons. Typically,

these atoms have high and similar electronegativities,

which means that they have strong afﬁnity to electrons

and are willing to share them with their partner atoms.

These atoms are from nonmetal elements (such as C, O, N,

and Cl) located on the right side of the periodic table. As

an example, shown below are two oxygen atoms sharing

Table 3. Properties associated with chemical bonding

Property Ionic bond materials Covalent bond materials Metallic bond materials

Melting and

boiling points

High melting and boiling points

because of strong attractions

between atoms (ions)

Relatively low thermal

transition temperatures

caused by weak attractive

forces between molecules

High melting and boiling points

caused by attraction of metal ions

with the delocalized electrons

Electrical and

thermal

conductivity

No electric conductivity and poor

thermal conductivity because of the

absence of mobile charged particles

The electron sharing makes

the electrons tightly

bound to atoms, thus no

charge available for

conductivity

High conductivity because of easy

transmission of vibration energy

through the ‘‘electron cloud’’;

metallic luster coming from free

electrons

Mechanical

properties

(hardness and

brittleness)

Generally hard because the ions are

strongly bound to the lattice; also

often brittle because of strong

repulsion between equal charges at

close proximity at the distortion

Generally soft because the

intermolecular bonds are

weak and easily displaced

Dense, hard, and strong with tightly

packed atoms in the lattice;

malleable and ductile as the

distortion is possible without

disrupting the interatomic bond

Ionomer

molecules

Figure 1. Ionic interactions in ionomer molecules.

STRUCTURE/PROPERTY RELATIONSHIPS OF PACKAGING MATERIALS 1165

two pairs of outermost electron to form an oxygen mole-

cule joined by a double covalent bond:

Two ox

en atoms One ox

en molecule

Although small molecules such as O

and H

O are

important to products such as food, the molecules in

packaging materials are much larger and more complex.

For example, polyethylene (PE), which is the most fre-

quently used packaging polymer shown below, is a mole-

cule consisting of thousands of C–C and C–H covalent

bonds:

For a polymer molecule, the degree of polymerization

determines the number of covalent bonds and thus mole-

cular weight, which affects the property and performance

of the material. Although C–C and C–H bonds are the

most common, CQC, CQO, O–H, and other covalent

bonds are also frequently found in plastics and paper.

The most common covalent bond in glass is Si–O.

An important attribute of a covalent bond is bond

polarity, which is determined by the difference in electro-

negativities between the atoms joined by the bond. When

the difference is zero or negligible, the electrons are

shared equally between atoms, and a nonpolar covalent

bond such as C–C is formed. When the difference is small

but signiﬁcant, the electrons are no longer shared equally

between atoms, and a polar covalent bond such as CQOis

formed. When the difference is large, the electrons are no

longer shared but are transferred from one atom to

another, and an ionic bond is formed.

Bond polarity can greatly affect the polarity of the

molecule and thus the properties of the material. If all

its bonds are nonpolar, the molecule is also nonpolar. If

some of its bonds are polar, the molecule could be polar or

nonpolar, depending on the symmetry of the molecule.

This concept is illustrated by considering the covalent

bonds in two common packaging polymers, polyvinyl

chloride (PVC) and PE:

PVC (

olar) PE (non

olar)

Because C–H and C–Cl are both polar bonds, PVC is a

polar molecule. But PE is a nonpolar molecule, because its

C-H bonds are arranged symmetrically in such a manner

that the dipoles cancel out, which results in no net dipole.

The more polar the molecule, the stronger are the inter-

molecular forces, and the stronger intermolecular forces

accounts for the reason why PVC has higher mechanical

and barrier properties than PE.

Covalent bonds may be broken when packaging mate-

rials such as plastics and paper are exposed to severe

conditions, such as aggressive chemicals, high tempera-

ture, and high shear. High temperature, for example, is

encountered during heating of food packages in the micro-

wave oven (especially when a heating device known as

susceptor is used in the package) or in the conventional

oven. High temperature and high shear are frequently

encountered during the processing of polymers such as

extruding polymer resins to form plastic ﬁlms or contain-

ers. A major concern associated with the breakage of

chemical bonds is possible formation of undesirable oligo-

mer compounds, which can migrate from package to food.

Sea of electrons

Positive

metal ions

Figure 2. Formation of metallic bonding.

ers of ions after slidin

ers of ions before slidin

Force

Figure 3. Sliding between layers of metal ions.

1166 STRUCTURE/PROPERTY RELATIONSHIPS OF PACKAGING MATERIALS

Covalent bonds are typically stronger than metallic

and ionic bonds. The amount of energy required to break

a covalent bond between two linked atoms is known as

bond dissociation energy. Some covalent bond energies

relating to packaging materials are shown in Table 4.

It needs to be mentioned that a molecule may have

more than one type of chemical bond. For example,

ionomer is a packaging polymer that consists of mostly

of covalent bonds and a small but signiﬁcant number of

ionic bonds (Na

or Zn

ions) to its side chains. Its

covalent-ionic nature provides ionomer with unique prop-

erties (heat seal strength, abrasion resistance, and melt

elasticity) not found in polymers with only covalent bonds.

INTERMOLECULAR FORCES

The third level of structure considers the intermolecular

forces that attract molecules together. Although atoms are

bond together tightly by strong covalent bonds to form a

molecule, a group of molecules are held together loosely by

weak intermolecular forces. Typically covalent bonds are

10–100 times stronger than intermolecular forces. Inter-

molecular forces can be cohesive between like molecules in

surface tension or adhesive between unlike molecules.

Plastics and paper are composed of polymer molecules,

and their physical properties are greatly inﬂuenced by the

intermolecular forces between these molecules. When a

PE ﬁlm is punctured or melted, what are broken are only

the intermolecular forces that hold the PE molecules,

whereas the much stronger covalent bonds holding the C

and H atoms remain unaffected. The weak intermolecular

forces are largely responsible for the low strength and low

melting point of these materials.

However, glass and metal are not composed of a collec-

tion of molecules. A glass or metal material is composed of

a single giant molecule, in which all the atoms are bound

together by primary bonds to form a network. Glass is a

network of Si-O bonds extending throughout the lattice,

and metal is a network of metallic ions held in the sea of

electrons. Hence, cutting a metal container involves

breaking metallic bonds, and shattering a glass bottle

involves breaking covalent bonds. Metal and glass are

inherently stronger and stiffer than plastics and paper,

because metallic bonds and covalent bonds are much

stronger than intermolecular forces.

Intermolecular forces, which are often called van der

Waals forces, correspond to a combination of different

ways of attraction among molecules according to their

dipole moment (dipole–dipole, dipole-induced dipole, and

dispersion forces). Dispersion forces are the weakest

intermolecular force (one hundredth-one thousandth the

strength of a covalent bond), hydrogen bonds are

the strongest intermolecular force (about one-tenth the

strength of a covalent bond): dispersion forcesodipole-

dipole interactionsohydrogen bonds.

Dipole–Dipole Forces. Dipole–dipole forces are intermo-

lecular forces between polar molecules: There are attrac-

tive forces between the positive end of one polar molecule

and the negative end of another polar molecule. They have

a signiﬁcant effect only when the molecules involved are

close together (touching or almost touching). Dipole–

dipole forces have strengths that range from 0.15 to

15 kJ mol

1

, being much weaker than ionic or covalent

bonds.

Consider the chemical structures of two polar mole-

cules, PVC and ethylene vinyl alcohol copolymer (EVOH):

PVC EVOH

For PVC, each C–H bond is polar because carbon is

more electronegative than hydrogen, and the C–Cl bond is

polar because chlorine is more electronegative than either

carbon or hydrogen. Each hydrogen atom will take on a

partial positive charge, and the chlorine atom will take on

a partial negative charge resulting in a net dipole because

the dipoles will not cancel out because of the difference in

electronegativities of carbon, hydrogen, and chlorine. For

EVOH, vinyl alcohol part is polar because the OH group is

more electronegative than hydrogen. When these molecu-

lar chains are in close proximity, dipole–dipole attractions

are formed.

Hydrogen Bonding. This is a special type of dipole–

dipole interaction. It occurs when a hydrogen atom is

covalently bonded to a small highly electronegative atom

such as nitrogen or oxygen. The result is a dipolar

molecule. The hydrogen atom has a partial positive charge

and can interact with another highly electronegative atom

in an adjacent molecule (again N, O, or F). This results in a

stabilizing interaction that binds the two molecules to-

gether. Hydrogen bonds vary in their bond strength from

about 4 to 40 kJ mol

1

being much weaker than typical

covalent bonds, but are stronger than dipole–dipole or

dispersion forces.

Hydrogen bonding is important for some paper and

plastic packaging materials. For example, the high oxygen

barrier of EVOH is largely caused by the formation of

hydrogen bonding between the EVOH molecular chains.

Table 4. The strength of some selected covalent chemical

bonds at 251C

Bond

Bond energy

(kJ mol

1

) Bond

Bond energy

(kJ mol

1

)

C–Cl 338 O–H 463

C–F 565 C–C 348

O–O 146 CQC 612

Si–O 466 C–N 305

C–H 412 C–O 360

N–H 388 CQO 743

STRUCTURE/PROPERTY RELATIONSHIPS OF PACKAGING MATERIALS 1167