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

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Treofan is a large multinational OPP manufacturer

with ofﬁces in the United States, England, and Europe;

manufacturing operations are located in Mexico, Ger-

many, Belgium, Italy, and South Africa.

The three largest producers in North America are:

. ExxonMobil, with plants in the United States, Ca-

nada, Belgium, Netherlands, and Italy

. A. E. T. (Applied Extrusion Technologies, Inc.), with

three North American plants

. Amtopp

Other North American manufacturers include: Vifan,

Toray America, 3 M, BIAX Int. (Canada), B.C.F. (Bemis-

Curwood Films), Intertape Polymer, General Electric, and

Steiner Films.

The two largest manufacturers in South and Central

America are VITOPEL (Votorantin) in Brazil and Bioﬁlm

in Colombia and Mexico, with more than 10 other produ-

cers (Treofan, Sigdopack, Polo Films, OppFilm, Bopp

Ecuador, Teleplastic, Agusa, Altopro, and Masterpack).

In Europe, ExxonMobil Plastics Ltd. and Innovia Films

Ltd. are the largest producers, and consolidation of many

of the original manufacturing companies continues in

Europe. Other producers are: Dor Films, Treofan, Vifan,

BOLLORE

, Manuli, Polinas, Bimo, Radici, Derprosa, and

about 12 other, smaller producers spread from Finland to

the old Eastern Block countries to Greece to Spain.

In the Far East there has been a large growth of OPP

capacity over the last 10 years, with large increases in

capacity in China and India. Japanese capacity has de-

creased as older capacity is shut down and replaced with

imported materials. A number of other producers are

located in Taiwan, South Korea, Indonesia, Malaysia,

Turkey, Egypt, India, and Israel.

RAW MATERIALS

The four resin categories used for OPP ﬁlms are: homo-

polymer, copolymer, terpolymer, and modiﬁed resins (see

Polypropylene). Homopolymer resins are made by polymer-

izing propylene monomer with a variety of catalyst. Copo-

lymers incorporate 0.1–15% ethylene as a comonomer with

propylene. The amount of comonomer used depends on the

ﬁlm’s end use. Terpolymer production uses butene along

with the ethylene and propylene feeds. Polypropylene

homopolymer resins are also modiﬁed with terpenes to

give them certain heat sealing properties (see Sealing,

Heat) and to improve moisture barrier and ﬁlm production.

The homopolymer PP, is typically isotactic PP, and ad-

vances in molecular architecture control has permitted

improvements in ﬁlm properties and productivity. Due to

advanced catalyst technology, syndiotactic PP, is now avail-

able and has been used in OPP shrink ﬁlm production (3)

but has not resulted in any broad change in OPP ﬁlm.

MANUFACTURING PROCESS

When a ﬁlm is biaxially oriented, it is mechanically

stretched in the solid state (at a temperature below its

melting point), in perpendicular directions. This aligns the

ﬁlm’s molecules in the transverse machine direction (TD)

and in the machine direction (MD). Once biaxial orienta-

tion has taken place, the ﬁlm exhibits a marked improve-

ment in optical properties: mechanical strength, moisture

barrier, and low temperature durability over cast poly-

propylene ﬁlm (see Film, Non-oriented Polypropylene).

The increase in strength allows OPP ﬁlm to be made as

thin as 0.45 mils (11 mm) and still function as a laminating

substrate for packaging applications (see Laminating;

Multi-layer Flexible Packaging). Films as thin as 0.25

mils (6 mm) are routinely used for capacitors and under

special conditions ﬁlms as thin as 0.04 mil (1 mm) can be

made. Balanced orientation is obtained when the degree of

orientation, as expressed by the ﬁlm’s tensile strength, is

approximately equal in each principal orientation direc-

tion. There are two different methods of orienting OPP

ﬁlm. They are called the ‘‘double bubble tubular’’ and

‘‘tenter frame’’. While both processes were in completion

in the early stages of OPP technology, today the Tenter

frame process dominates world-wide production of OPP

ﬁlms.

Double Bubble Tubular Process

In this process, molten polypropylene resin is extruded

through a circular die (see Extrusion) and is water-

quenched to control the polymer morphology of the tube

and the tube is collapsed by a pair of nip rolls. At this

point, the quenched tube is relatively thick (e.g., 1000 mm

for a 25-mm ﬁnished ﬁlm) and capacity of the bubble line is

generally controlled by quenching rate of the tube. Next

the tube is transported to the reheating section, where it is

reheated by circular radiant heaters to it orientation

temperature which is above the polymer’s T

(see Polymer

Properties) and below the melting temperature. Air is

injected into the tube after the ﬁrst nip and held captive

by a second nip, and the increased air pressure forms an

expanded bubble between the nips (see Figure 1). At the

same time the bubble is expanded, the bubble is pulled by

the second nip at a speed faster than it is fed by the ﬁrst

nip. The size of the bubble’s circumference controls the

amount of transverse (TD) orientation and the speed

differential between the ﬁrst and second nips controls

the degree of machine direction (MD) orientation.

After orientation, the ﬁlm bubble is collapsed at the

second nip and is either wound up as a heat shrinkable

ﬁlm or is reinﬂated and annealed for use as a stabilized,

heat-set packaging ﬁlm (4). Heat stabilization in the

double bubble process is accomplished by one of several

methods. In the ‘‘third bubble’’ approach (5), the oriented

bubble is reinﬂated while the ﬁlm’s temperature is ele-

vated to a point chosen for the required level of stability,

but below the orientation temperature, and then is cooled.

Alternatively, the collapsed bubble is passed over a series

of heated rollers (6), raising the ﬁlm’s temperature as

outlined above, and then quenched on cold rolls. In other

cases the ﬁlm has been held by clips and reheated in a

tenter oven with parallel chains as described below (7).

In all heat stabilization methods, the ﬁlm is mechani-

cally restrained from shrinking back in the machine

478 FILM, ORIENTED POLYPROPYLENE

direction (MD). The amount of restraint in the transverse

direction (TD) varies from no restraint or 1–3% percent

width relaxation, down to essentially no restraint. This

heat treatment relaxes most of the high internal stresses

and tensions built up in the ﬁlm during orientation

and provide a functionally stabilized product. However,

most double bubble ﬁlms will be less dimensionally stable

than a ﬁlm produced on a tenter frame, and in some

instances (e.g., tobacco over wrapping ﬁlms) this can be

an advantage.

In the bubble-orientation process, after heat stabiliza-

tion the sides of the collapsed bubble are slit and the ﬁlm

is wound simultaneously into two mill rolls. During this

winding operation the ﬁlm is frequently corona treated

(see Surface Modiﬁcation). This surface treatment con-

sists of a controlled electron and ion bombardment. The

ﬁlm is slightly oxidized (3–8% oxygen incorporation),

thereby increasing its surface free energy, which produces

a multitude of sites that are chemically attracted to the

polymers used in off-line coatings, printing inks, and

laminating bonding-agents such as adhesives and poly-

oleﬁn extrudate.

Tenter Frame Process

In this process, polypropylene resin is extruded out of a

horizontal ﬂat die, sometimes called a slot die. The

extrudate is quickly cooled by a chilled roll, and a chilled

roll is immersed in a water bath or in a cold water bath.

Today the majority of lines use a chilled roll immersed

in a water bath. The quenching produces a sheet of

controlled morphology approximately 1000 mm thick (for

a 25-mm ﬁlm). This heavy sheet is then reheated in contact

with heated rollers to its orientation temperature (above

the polymer’s T

but below its melting point). After

reheating, it is ﬁrst oriented in the MD. The ﬁrst mechan-

ical stretching is accomplished by drawing the sheet

between a set of two, closely spaced, rollers, with one

roller rotating faster (e.g., the fast roll) than the preceding

roller (the slow roll) (Figure 2). On some lines a number of

faster and slower roll sets are used, stretching and thin-

ning the sheet in two or more sequential steps.

The next step is TD orientation, which is done in a

‘‘tenter frame.’’ The tenter frame has two closely spaced

endless chains with ﬁlm holding clips which diverge

apart to give a wider spacing at the end of the tenter

(see Figure 3). The MD-stretched ﬁlm enters the smaller

open end and is gripped securely by its sides with spring-

loaded mechanical clips (Figure 4). The clips are mounted

on moving chains that run along lubricated tracks on each

side of the frames. As the chains move forward and out-

ward along these tracks, the ﬁlm is stretched in the TD.

Evolutions in clip design have greatly improved the

manufacturing capabilities of orienting equipment and

the subsequent increases in ﬁlm speeds.

To achieve good heat stabilization, the ﬁlm, still se-

curely held by the clips, moves on into a heat-setting

(annealing) oven section. In this section, the clip/chain

systems at each side of the ﬁlm have passed out of the

diverging TD stretching section into a section of the tenter

in which the chains run parallel to each other. Here the

ﬁlm is subjected to elevated temperatures for a few

seconds while it is completely restrained in both direc-

tions. In some instances the TD stability can be improved

by decreasing the width of the ﬁlm, by bringing the chains

S-wrap MD Stretching Stretching Roll to Roll

< V

Figure 2. Schematic diagram of the machine

direction orientation.

Figure 1. Schematic diagram of double bubble process.

FILM, ORIENTED POLYPROPYLENE 479

1–4% closer together during the annealing process. The

ﬁlm is then cooled, released from the clips, has the thicker,

unstretched portion under the clips removed, is generally

corona- or ﬂame-treated, and ﬁnally is wound up into mill

rolls. Most modern OPP Tenter lines wind a roll the full

width of the orienter, but some tenter frame users split the

full width ﬁlm into two roughly equal-sized mill rolls by

slitting in-line (see Slitting and Rewinding) for subse-

quent use in ﬁlm coating or metallization equipment.

Following orientation and stabilization, regardless of

the process used, the freshly produced rolls are usually

stored on racks for several days. During this time, remain-

ing internal tensions are relaxed and a higher degree of

crystallinity develops from secondary crystallization in

the ﬁlm. Additives included in the resin, such as those

needed for slip, are also given time to bloom to the surface

(see Additives, Plastics). After the proper time interval,

the OPP ﬁlm is ready for further processing in an off line

coating and/or a ﬁnished slitting operation.

In some instances an additional MD stretching unit

has been added after the TD oven to induce an additional

MD stretch to the biaxially oriented ﬁlm. This is done

to increase the MD shrinkage of the ﬁlm to make it

suitable for heat-shrinkable labels and may be done inline

on the ﬁlm orienting equipment (8) or out of line during

slitting (9).

In the early days of OPP ﬁlm production when outputs

were relatively low, double bubble lines were 120 in.

(3.05-m) in circumference and tenter frames were 120 in.

(3.05-m) wide. Over the years, larger lines have been

engineered for both processes . At one time, 24 0-in. (6.1-m)

circumference double bubble lines were in operation in

Europe, but due to their low speeds and outputs, the

majority of production capacity installed since the mid-

1980s has been high-speed 6.1-m-to 10.0-m-wide tenter

frame lines. Today the ‘‘standard’’ tenter line is 8.2 m and

produces ﬁlm at greater than 500 m/min. This movement to

a much wider (and faster) line has greatly increased the

OPP ﬁlm producers’ productivity and has also signiﬁcantly

reduced the generation of narrow side rolls in slitting

operations as ﬁlm converting (e.g ., coating, metallizing,

printing, and laminating) equipment, has also increased

in width. Both factors have contributed to OPP ﬁlm’s

reputation of being the most economical of all highly

engineered, high-performance ﬂexible packaging materials.

OPP FILM PROPERTIES

The very low density of polypropylene (0.905 g/cm

) allows

OPP ﬁlm to have the greatest area-coverage yield per unit

of weight of any commercially signiﬁcant thermoplastic or

cellulose-based sheet. Only cavitated opaque ﬁlms can

have a lower density (e.g., 0.6 g/cm

) than clear OPP.

This high area yield and ability to be made into very

thin ﬁlms are important contributions to its overall favor-

able economics, because the ﬁlm is sold by the kilogram

Casting MD Stretching

Preheat TD stretching Annealing

Extruding

Winding

Stretching Oven

Figure 3. Schematic diagram of sequential biaxial ﬁlm orienter.

Film

Closing

Spring

Gripping arm

Gripping foot

Opening Arm

Clip Body

Base attachment point to Chain

Figure 4. Side view of early style ﬁlm stretching clip.

480 FILM, ORIENTED POLYPROPYLENE

but used by the square meter, which give it the lowest cost/

coverage area of comparable thickness ﬁlms.

Thermal Properties

OPP ﬁlms are intermediate in their temperature resis-

tance as compared to other polymer ﬁlms. With a melting

point of 1651C, polypropylene ﬁlms are more temperature-

resistant than polyethylene ﬁlms but less resistant than

polyester ﬁlms.

Normally, a heat-set OPP ﬁlm can withstand brief

contact (0.5 to 1 s) with sealing mechanisms that are

1491C (3001F), or slightly above that, without signiﬁcant

distortion in the seal areas (10). Sealing mechanisms

above this temperature are possible with reduced resi-

dence time (o0.5 s) but do create progressively greater

distortion potential until at about 1541C (typical orienta-

tion temperature) the ﬁlm attempts to return to its

unoriented dimensions, leaving the heated portions of

the seal severely distorted. Because of this thermal

shrinkage at higher temperatures, OPP ﬁlm’s sealing

range has an upper limit of 149–1521C (300–3051F).

When subjected to elevated air temperatures for sev-

eral minutes, heat-set OPP ﬁlm will display a range of

dimensional changes, depending on the temperature to

which they are exposed. Below 751C, OPP ﬁlm shows very

little or no shrinkage at extended heating times, and a

package wrapped in this material maintains its dimen-

sional integrity at normal packaging and distribution

temperature histories. At a temperature of approximately

1051C the ﬁlm will show 1–2% MD shrinkage and perhaps

a TD width change of 0.5% shrinkage to 0.5% growth

within 7 min. However, at 1351C and 7 min, the ﬁlm will

shrink approximately 3–5% in the MD and 2–3% in the

TD. The actual dimensional change values and shrinkage

versus. temperature proﬁle will depend upon the manu-

facturing methods used to produce the ﬁlm. Consequently,

heat-set OPP ﬁlms will show some shrinkage if they are

exposed to very hot environments such as is found in

retort packaging (2501F (1211C) at 15 psi 103 kPa)), and

care should be exercised when choosing OPP for high-

temperature applications.

In some cases OPP ﬁlms are made purposely unstable

so that they may be used for heat shrink ﬁlms. These are

used in two principal markets today, namely, shrink over-

wrap ﬁlms and roll-on shrink on bottle labels. For shrink

overwrap, at 1351C the ﬁlm may shrink 35–50% in both

the MD and TD directions while for shrink labels the ﬁlms

are designed to shrink uniaxially from 8–25% in the MD

with 0% TD shrinkage.

Physical Properties

There are relatively few physical property differences

between ﬁlms made by the two processes (see Table 1).

Both ﬁlms possess excellent moisture-barrier properties

but poor oxygen and aroma barriers. Moisture barrier

increases and decreases proportionately with the thick-

ness of the ﬁlm under consideration. Tensile strength and

elongation of blown ﬁlms tends to be about the same in

both directions (balanced). TD strength is higher than MD

strength in tenter-frame ﬁlms, and MD elongation is

higher than TD elongation (unbalanced). Tenter-frame

ﬁlm has generally superior heat stability over blown ﬁlm

because of more severe annealing conditions that can be

practiced in the tenter-frame operation. Double bubble

OPP ﬁlms have somewhat better impact strength. How-

ever, only in the most demanding and arduous packaging

applications would these relatively small differences be-

come signiﬁcant.

Optical Properties

Standard OPP ﬁlms are naturally high-clarity and high-

gloss ﬁlms offering excellent printing graphics product

visual identiﬁcation. However, many OPP suppliers also

offer clear ﬁlms with a low-gloss, matte surface, to aid in

product graphic appeal and differentiation. Opaque, white

ﬁlms with 20–30% light transmission are also widely

available as are metallized ﬁlms with 1% or less light

transmission and high reﬂectivity for light-sensitive

products.

PRODUCT DEVELOPMENTS AND COEXTRUSION TO

SEVEN LAYERS

The ﬁrst two OPP ﬁlms produced were single-layer homo-

polymer shrink ﬁlm and heat-stabilized ﬁlm that could not

be heat-sealed. The former had limited application be-

cause of its need for hot-wire sealing and tremendous

shrink energy, which originally precluded it from wrap-

ping high proﬁle products. The development of copolymers

allowed the widespread development of propylene shrink

ﬁlms which today have replaced much of the original PVC

shrink ﬁlms and compete strongly with crosslinked poly-

ethylene

shrink ﬁlms.

Additionally, the development of

special polymeric coatings and polypropylene copolymers

and terpolymers during the 1960s, 1970s and 1980s

respectively, has alleviated the lack of sealability problem

in many applications.

The original heat-set OPP ﬁlm gained wide acceptance

as a laminating substrate when combined with heat-

sealing polymer-coated cellophanes (see Cellophane) and

glassine papers (see Glassine). The OPP ﬁlm contributed

strength, moisture barrier, and high surface gloss to the

lamination. However, before acceptance was forthcoming

from the marketplace, OPP ﬁlm producers had to improve

the ﬁlm’s slip characteristics over the relatively high

coefﬁcient of friction (COF) that is natural to the ﬁlm

and improve the low surface wetability. The addition of

internal surface-blooming slip agents reduces the ﬁlm-to-

ﬁlm COF to 0.2–0.4. This COF provides excellent machi-

neability on automatic bag-forming machinery used in the

snack food and candy industries, while surface oxidation

improved wettability. Converters developed the hot-melt

adhesive-based, heat-sealing thermal stripe that allowed

the production of lap back seals on vertical form/ﬁll/seal

machines (see Form/Fill/Seal, Vertical). This technology

widened the use of OPP ﬁlm laminations for snack foods,

candy, and pasta products. Today the use of three-layer

coextruded ﬁlms permits the direct formation of lap seals

without the need for thermal strip technology. Another

FILM, ORIENTED POLYPROPYLENE 481

Table 1. Properties of Oriented Polypropylene

Films, 1 mil (25.4

Typical Film Properties ASTM Test

Blown OPP

Film

Tenter-Frame

OPP Film

Slip-Modiﬁed

Film

Acrylic-Coated

Film

PVDC-Coated

Film

Coextruded

Film

White

Opaque

Film

Metallized

Film

Speciﬁc gravity

D 1505 0.905

0.905

Yield, in.

/(lb mil)

/g m)

30,600

27,300

30,600 48,000

[1714]

[1529]

[1714] [2688]

Haze, %

D 1003

3.0

Light transmission, %

50–60

Tear strength, gf/mil

(N/mm), propagating

D 1922

4–6

(1.5–2.3) (1.5–2.3) (1.5–2.3)

(1.5–2.3)

Service temperature,

1F(

1C), range

70(39)

50(28)

40–110

(22–61)

Heat seal temperature,

1F(1C), range

230–300 25–300 190–300

(110–149) (121–149) (88–149)

Oxygen permeability,

mil/(100

in.

d atm) [cm

mm/

d kPa)];

D1434

771F (23

1C), 0% rh

160

150

1–3

160

4–18

[622]

[583]

[4–12]

[622]

[622] (16–70)

4–6

771F (23

1C), 50% rh

[16–23]

Water vapor

transmission rate

(WVTR) g

mil/100

in.

d) [g 

mm/(m

d)];

0.25

0.30

0.60 0.1–0.3

[0.1]

[0.12]

[0.24] [0.04–0.12]

1001F (38

1C), 90% rh D1894

0.2–0.4

0.25

0.2–0.4

COF, face-to-face back-

to-back

0.4

0.2–0.4

0.25

0.2–0.4

Test conditions, 73

(23 1C), 50% rh

482

application for this non-heat-sealing ﬁlm is the inner liner

of paper bags for products such as cookies and pet foods.

Since this early beginning, the history of OPP product

development of has been the creation of improved barrier

technologies and the development of surfaces with special

attributes to permit the use of the base sheets barrier

properties in packaging and industrial applications. The

primary protection that OPP inherently supplies is that of

a moisture barrier. Additional barriers have been added

during the development of new products such as oxygen,

barriers, aroma (chemical) barriers, and light barriers.

However, it is the moisture barrier property, combined

with the unusually low polymer density, which results in

the maximum possible product coverage with one of the

lowest moisture permeabilities for clear ﬁlms. When these

two features are combined with special surface attributes

such as sealability, printability, laminatability, coatability,

metallizability, and machineability, a high-quality, low-

cost packaging ﬁlm emerged which replaced almost all of

the cellophane then in use in snack food packaging. In

addition to cellophane replacement, OPP has made

further inroads into glassine paper and some foil markets

as well. Today this combination of properties still drives

the expansion of OPP ﬁlms into new packaging applica-

tions and developing markets.

In order to reach this point, several weaknesses of OPP

ﬁlm relative to packaging in general and cellophane in

particular had to be addressed. The inherent limitations

to homopolymer OPP ﬁlm are: no sealability, high COF

surfaces, low surface wettability, static generation, and a

lower use temperature range than cellophane.

In order to overcome these weaknesses in the product,

both ﬁlm and packaging machine changes were required.

The temperature limitations were overcome by improve-

ments in packaging machine design and temperature

control of sealing jaws as well as the use of special acrylic

and PVDC polymer coatings and the use of low-melting

copolymers and terpolymers as ﬁlm sealing surfaces. To-

day the machine evolution is complete and packaging

machines are designed with OPP ﬁlm as a primary

wrap. Static generation occurs at all surfaces and, due to

the high dielectric strength of OPP, must be eliminated at

each point where static interferes with packaging ma-

chine performance. Product formulation approaches have

been hampered due to a lack of Food and Drug Adminis-

tration FDA-approved additives and the relatively high

additive levels that would be required. Surface wettability

is overcome by coating with acrylic polymers; for uncoated

ﬁlms, it is overcome by the surface oxidation of the ﬁlm

with corona, ﬂame, or atmospheric plasma treatment on

the ﬁlm orienter and perhaps at subsequent converting

steps. The development of multilayer coextrusion systems

has also permitted the incorporation of non-propylene

skins to be used, such as EVOH and nylon, which has

further broadened to the range of product developments.

The development of low COF surface modiﬁcations has

relied almost exclusively on ﬁlm formulation develop-

ments and has seen many developments over the years.

The original surface modiﬁcation for COF was a formu-

lated acrylic coating that combined excellent printability,

low COF, and broad sealability. The second development

was the use of fatty acid amides that when added to the

polypropylene, would diffuse to the ﬁlm surface (called slip

bloom) with time and heat treatment after ﬁlm orienta-

tion. While this lowered ﬁlm COF, it is difﬁcult to control

and the additive often interferes with printing ink adhe-

sion, metallization, and lamination bonds. Today this slip

blooming technology has been replaced by the use of

mineral and polymeric particles that are dispersed in

the ﬁlm surface layers as well as the use of silicone oils

for higher temperature applications. This gives a ﬁlm

COF’s of 0.25–0.5. While these COF values appear high

by the original standards, the ﬁlms all exhibit a low but

uniform force to transport the ﬁlm over the forming collar

and through packaging machines. It is this low ﬁlm

transport force which is the critical property for proper

machineability and not the ﬁlm-to-ﬁlm COF.

Today, OPP ﬁlm has found wide acceptance and con-

tinues to grow as new markets are developed making use

of its special product characteristics. Many of the technol-

ogies developed to permit the use of OPP have gone

through several developmental generations as new resin

and additive technologies were invented to overcome the

limitations of the ﬁrst generation ﬁlm modiﬁcations. To-

day, developments in polymer metallocene catalysis con-

tinues, thereby generating new propylene resins such as

high-crystallinity polypropylene and syndiotactic polypro-

pylene. How these new development and resins will im-

pact future growth in OPP packaging and new ﬁlm

modiﬁcation technologies is currently unclear.

SEALABILITY

The quest for functionality through heat-sealing on con-

ventional sealing mechanisms used on most packaging

machines let OPP ﬁlm producers down four avenues of

development. The ﬁrst was the modiﬁed polymer route.

This ﬁlm uses a polypropylene that is modiﬁed with

natural and synthetic terpene resins. The ﬁlm is heat-

sealable, although its sealing range is relatively narrow

(251ForB141C), and it does not have good hot-tack-seal

strengths needed for form/ﬁll/seal packaging. In over-

wrapping applications where the product to be packaged

is contained in a carton or some other self-supporting

structure, this OPP ﬁlm has found good acceptance.

Because this ﬁlm is only partially heat-stabilized, it can

snugly tightened around

the carton or bundle through

the use of a heat tunnel. One version is used today as a

cigarette-packaging material. However, this technology

has largely been supplanted by surface sealability

modiﬁcations.

The second method of imparting sealability to OPP ﬁlm

is by addition of an ofﬂine heat-sealable coating based on

an acrylic polymer (see Acrylics). This coating adds ﬂavor

and aroma barrier, but no moisture or oxygen barrier

properties to the OPP ﬁlm. However, it does provide a

relatively wide sealing range of 801ForB451C and adds

sparkle and good machine slip characteristics. It has a

ﬁlm-to-ﬁlm COF of 0.2–0.3. It has high-volume use in

horizontal form/ﬁll/seal operations (see Form/Fill/Seal,

Horizontal) for items such as baked foods (cookies in

FILM, ORIENTED POLYPROPYLENE 483

particular) and candy. As an overwrap ﬁlm, it is used for

pet foods and various tobacco products such as cigarettes,

cigars, and pipe-tobacco cartons. Its hot-tack and seal-

strength characteristics are such that it is used exten-

sively for VFFS packaging of lightweight products, such as

snack foods, but is not generally proposed for similar

packaging of heavy products. These acrylic coatings are

compatible with many inks (see Inks) and adhesives (see

Adhesives) used by converters and are also heat-sealable

with the PVDC coatings used on cellophane, glassine, and

other OPP ﬁlms (see Vinylidene Chloride Copolymers).

The third method to obtain heat sealability is to coat

the OPP ﬁlm ofﬂine with a PVDC coating, which also

overcomes the gas-barrier deﬁciency. This is required

because oriented polypropylene ﬁlm in its uncoated form

is not considered to be a gas-barrier ﬁlm (see Table 1). Its

oxygen permeability is relatively high at 731F (231C) using

1.00-mil (25.4-mm)- thick ﬁlm (see Table 1). PVCD-coated

ﬁlms fall into four categories. The ﬁrst is readily machine-

able on heat-sealing, automatic packaging equipment. The

sealing range in this case is relatively wide (60–801For

33–451C), and the oxygen-transmission rate is improved

(see Table 1). These ﬁlms are available with one or two

sides coated. The former is a laminating substrate for use

with other OPP ﬁlms and cellulosic materials, to package

snack foods and candies. The two-side-coated product is

used in unsupported form for bags and carton overwraps.

In all of these applications, an aroma barrier (which the

PVdC supplies) is generally desired by the end-user.

The second category of PVDC-coated OPP ﬁlm has a

superior gas barrier and an oxygen-transmission rate of

8–12 cm

/(m

d). However, because its sealing range is no

more than 501F (281C), this ﬁlm is not used for its heat

sealability. Rather it is used as a laminating substrate

with various other materials for the controlled atmo-

sphere packaging of coffee, natural cheese, and processed

meats (see Controlled Atmosphere Packaging).

The third category combines an acrylic coating on one

side and a PVDC coating on the other side. The advantage

cited for this ﬁlm is that it provides a degree of gas and

improved aroma barrier, is lap sealable, machines well,

and costs somewhat less that two-sided, heat-sealable,

PVDC-coated OPP ﬁlms.

The fourth category of PVDC-coated ﬁlms is the combi-

nation of the high-barrier PVDC coating with a broad-

seal-range, coextruded sealant skin. This ﬁlm is used as a

laminating ﬁlm with the high-barrier PVDC buried in the

structure and makes use of the excellent seal properties of

the coextruded skins (see below).

The fourth avenue of development of heat sealability of

OPP ﬁlms is through coextrusion. Although coextruded,

multi-ply, heat-sealing OPP ﬁlms have been available

since 1955; this type of ﬁlm came into large-volume usage

during the late 1970s and early 1980s. This was in large

part due to the development of ethylene propylene copo-

lymers (EP copolymer) and later by ethylene, butylene

propylene terpolymers (EBP terpolymer), resins, and ad-

vances in the technology of coextrusion die design. Coex-

trusions are made using several basic approaches. In one

approach, two or more extruders feed their individual

molten polymer streams to a single manifold extrusion

die (see Coextrusion, Flat; Coextrusion, Tubular). In the

upstream section of the die is a round or rectangular ‘‘feed

block’’ mechanism that brings these polymer ﬂows to-

gether into layers before passing them to the single cavity

(manifold) where they are deformed into a single wide,

thin rectangular sheet as it passes out of the die lips.

Another method is termed ‘‘tandem’’ coextrusion. With

this procedure the ﬁlm’s core material is extruded from its

own extruder and die. This extrudate is then extrusion-

coated (see Extrusion Coating) from one or more extruders

and dies, or it is hot-nip laminated to other compatible,

independently manufactured ﬁlms. In recent years this

‘‘tandem’’ coextrusion has been largely replaced with

multi-manifold coextrusion technology. In this OPP tech-

nology, generally three separate die manifolds are con-

tained together in the same die body. As the polymer ﬂows

toward the die exit, the separate layers are combined

together in layers and then exit the die lips together.

This technology has permitted the combination of resins

that cannot be coextruded by the feed block method or

‘‘tandem’’ methods describe above. More recent develop-

ments have combined the feed block with the multi-

manifold dies to produce ﬁve and more layers in a single

sheet. In all coextrusion cases the multilayered sheets are

moved into the orientation units of the line. Coextrusions

can be made by both the blown bubble and tenter frame

processes.

Polypropylene does not anchor well to many other

polymers. As a result, great care must be taken in the

choice of materials used in OPP ﬁlm coextrusions; other-

wise inter-ply bonds may be broken during orientation or,

later, by the stresses of package forming techniques and

distribution. Other demands on outer ply materials in-

clude hot-slip characteristics that allow easy ﬁlm move-

ment over hot-packaging machine parts and surface-slip

characteristics for good overall trouble-free automatic

high-speed machining. The ply destined to be printed or

laminated must also be corona- or ﬂame-treated.

Today, most coextruded OPP ﬁlms consist of two to ﬁve

layers. The majority of the ﬁlm’s mass is in the core ply

that is usually made with virgin homopolymer and some

regrind. One or both sides of the core may be covered with

a polypropylene-rich copolymer or terpolymer, an ionomer

(see Ionomers), or a vinyl acetate-modiﬁed polyethylene

layer. The choice of polymer depends on the purpose for

which the ﬁnished ﬁlm will be used. Polypropylene-rich

copolymers of propylene and ethylene are used for their

good hot-tack seal strengths and excellent ﬁnished seal

strengths. Terpolymers of propylene, ethylene, and butene

are used to improve the ﬁlm’s breadth of sealing range,

and today’s terpolymers have been developed to give

excellent hot-tack strength as well. Vinyl acetate-modiﬁed

polyethylene plies are used for their anchoring potential

in ofﬂine coating, laminating, and metallizing operations

(see Metallizing). Ionomer resin is used for its wide sealing

range and ability to repel grease and oils. It is somewhat

difﬁcult to handle in the orientation process; but once

oriented and heat stabilized, it provides lower level seal

strengths of about 300 g/in. (116 N/m) which can be used

advantageously in packages that demand an easy-open

feature. Ionomer-skinned ﬁlms are not generally made by

484 FILM, ORIENTED POLYPROPYLENE

the tenter process due to its aggressive adhesion to hot

metal surfaces used in the process and are only produces

on bubble equipment. These ﬁlms are therefore at risk of

disappearing as lower productivity bubble lines are shut

down in favor of higher productivity tenters.

Coextrusion sealing temperature ranges very greatly,

depending on the sealing ply materials. General cate-

gories are listed in Table 2.

Thin coextruded OPP ﬁlms, 0.57–0.70 mil (14–18 mm)

thick, are often laminated to other substrates such as

polymer-coated cellophane, glassine, paper, and other

OPP ﬁlms for snack food, candy, processed cheese, and

pasta-product packaging. In unsupported form they pack-

age lightweight products such as single-serving packages

of crackers used in restaurants.

Medium-weight ﬁlms, 0.800.90 mil (20–30 mm) thick, are

used for carton and tray overwraps by the bakery and

candy industries, as well as for form/ﬁll/seal single wall

pouches by pet food producers. A specially designed med-

ium weight material is used as a cigarette package over-

wrap. Heavy, thick coextruded OPP ﬁlms, over 1.00mil

(25.4 mm) thick, are normally used in unsupported form to

package cookies, candies, snack foods in small bags, and

other products that need the stiffness, strength, excellent

moisture barrier, and rich feel offered by these ﬁlms.

OPAQUE FILMS

White opaque ﬁlm is one of the fastest growing OPP ﬁlm

developments in the United States and Western Europe.

The most widely accepted ﬁlm products of this type are

made by the tenter-frame process. Homopolymer resin is

evenly mixed with a small amount of foreign particulate

matter. In one product, when the thick ﬁlled sheet is

oriented, the polypropylene pulls away from each particle,

creating an air-ﬁlled void (closed cell). After heat stabili-

zation the OPP ﬁlm is similar to a micropore foamed

product. In the second product, the material is produced

as a ﬁlled ﬁlm without voids. The opacity is a direct result

of the amount of particulate material included in the ﬁlm.

In the ﬁlm with air-ﬁlled voids, the imparted opacity

and whiteness is created to a small degree by the encap-

sulated particulate matter. However, the primary opaciﬁ-

cation is brought about by light rays bouncing off the

polypropylene cell walls and the air within each cell. The

refractive index of the air is less than for polypropylene.

This refraction difference results in TAPPI opacity of

about 55%. The light diffusion gives the ﬁlm the visual

effect of pearlescence. Brightness values are calculated at

65–75% by the GE method. The actual gauges of this white

opaque ﬁlm are deceptively thick when their area yield

per unit of weight is considered. A 1.5-mil (38-mm)-thick

cavitated OPP ﬁlm has an area yield of 30,000 in.

/lb

(427 cm

/g), which is about the same yield provided by a

1.0-mil (25.4-mm)- thick transparent OPP ﬁlm.

Coextrusion and out-of-line coating techniques have

greatly expanded the market acceptability of white opa-

que OPP ﬁlm. By using these approaches to ﬁlm manu-

facture, the ﬁlm can be made one- or two-side heat-

sealable. These steps can also increase the ﬁlm’s moisture,

oxygen, and aroma barrier. White opaque OPP ﬁlms are

ﬁnding growing volumes in snack-food packaging, candy-

bar overwraps, beverage-bottle labels, soup wrappers, and

other applications that traditionally have used specialty

paper-based packaging materials. White opaque ﬁlms are

also ﬁnding new applications such as photographic paper

and digital printing substrates.

METALLIZING

The use of OPP ﬁlm as a metallizing substrate is another

area of new market growth (see Metallizing). In this

process, aluminum is vaporized onto one side of a ﬁlm or

paper in a high-vacuum environment. The speed of the

substrate’s passage over the vaporization pots controls the

amount of metal deposited on the ﬁlm. If the metallized

surface is to be used primarily for decorative purposes, a

relatively light coating of approximately 4 O

(1.6 optical

densities, OD) is applied. This gives OPP ﬁlm, originally

in transparent form, a light-transmission rate of about

5%. If the metal coating is used for signiﬁcant improve-

ment of the ﬁnished ﬁlm’s barrier properties, a heavier

disposition of 2 O

(2.2 OD) is employed. This level lowers

light transmission to about 0.6%. Moisture and oxygen

barrier are improved dramatically (see Table 1). In packa-

ging, metallized OPP ﬁlms are widely used in snack-food

laminations and for confectionery wraps.

Today further product design advances have been made

by coextruding non-polypropylene polymer surfaces, that,

when metallized, lower the moisture barrier properties of

the metallized ﬁlm relative to metallized OPP and below

that of metallized polyester (PET) ﬁlms while approaching

the oxygen barrier properties of metallized PET (11).

Further developments have produced developmental sam-

ples of metallized OPP that have moisture and oxygen

barriers signiﬁcantly better than metallized PET and

approaching the properties of foil. This is accomplished

by the incorporation of non-polypropylene layers such as

PETG (12), nylon (13), HDPE (14), and EVOH (15) for the

metallized surface.

LABELS

This is a growth area for OPP ﬁlms with cavitated and

clear ﬁlms gaining wide acceptance in many labeling

applications. The broad label categories where OPP is

making inroads are inmold labels, roll-fed labels,

Table 2. Coextruded OPP Film Sealing Temperature

Ranges

OPP Film

Sealing Temperature

Range

Copolymer group A (0.5–e.0%) 30–501F (16–281C)

Ethylene copolymer group B (Z4%

ethylene)

60–701F (33–391C)

Terpolymers (ethylene and butane) 85–951F (47–531C)

Ionomer 95–1051F (53–581C)

FILM, ORIENTED POLYPROPYLENE 485

pressure-sensitive labels, roll-fed shrink labels, cut and

stack for hot melt, and wet glue. The OPP ﬁlm functions as

a carrier for sophisticated coatings that give many of the

surface properties needed for the label applications.

BIBLIOGRAPHY

W. R. R. Park, ed., Plastics Film Technology, Plastics Applications

Series, Van Nostrand Reinhold Company, New York, 1969.

O. J. Sweeting, ed., The Science and Technology of Polymer Films,

Vol. I, Interscience Publishers, New York, 1968.

O. J. Sweeting, ed., The Science and Technology of Polymer Films,

Vol. II, Interscience Publishers, New York, 1968.

C. J. Benning, Plastic Films for Packaging; Technology, Applica-

tions and Process Economics, Technomic Publishing Co., Lan-

caster, PA, 1983.

T. Kanai, and G. Campbell, eds., Film Processing, Hanser/Gard-

ner Publications, Cincinnati, 1999, pp. 339–343.

K. R. Osborn, and W. A. Jenkins, Plastic Films Technology and

Packaging Applications, Technomic Publishing Company,

Lancaster, PA, 1992.

R. J. Samuels, Structured Polymer Properties: The Identiﬁcation,

Interpretation, and Application of Crystalline Polymer Struc-

ture, Wiley-Interscience, New York, 1974.

M. Bakker, Ed., The Wiley Encyclopedia of Packaging Technology,

Ist edition, John Wiley & Sons, 1986.

Cited Publications

1. M. Gschwandtner, Proceedings of AMI BOPP Film, 2007.

2. W. Pinneger, Proceedings of AMI BOPP Film, 2007.

3. M, Amon, R. G. Peet, and S. J. Pellingra, Jr., U.S. Patent

6,113,996, ‘‘Composition for Uniaxially Heat Shrinkable

Biaxially Oriented polypropylene Film,’’ U.S. Patent and

Trademark Ofﬁce, Washington, DC, 2000.

4. C. J. Benning, Plastic ﬁlms for Packaging; Technology, Appli-

cations and Process Economics, Technomic Publishing Co.,

Lancaster, PA., 1983, p. 39.

5. G. M. Reade, U.S. Patent 3,814,785, ‘‘Heat Stabilization of

Oriented Thermoplastic Films,’’ U.S. Patent and Trademark

Ofﬁce, Washington, DC, 1974.

6. M. Yazawa, U.S. Patent 3,632,733, ‘‘Heat Treating Two-Ply

Biaxially Oriented Films,’’ U.S. Patent and Trademark Ofﬁce,

Washington, DC, 1972.

7. P. J. Barham, J. A. Odell, and F. M. Willmouth, European

Patent Application 0 023776 A1, 1981.

8. L. E. Keller, and M-F. Nothnagle, U.S. Patent 5,691,043,

‘‘Uniaxially Shrinkable Biaxially Oriented Polypropylene

Film and Its Method of Preparation,’’ U.S. Patent and Trade-

mark Ofﬁce, Washington, DC, 1997.

9. W. J. Ristey, G. A. Senich, W. J. Hill, and H. S. Anderson, U.S.

Patent 5,851,610, ‘‘Shrink Films and Articles Including the

Same,’’ U.S. Patent and Trademark Ofﬁce, Washington, DC,

1998.

10. http://www.oppﬁlms.com/Public_Files/OPPFilms/Oriented_

PP_;Films/NorthAmerica/Test_Method_Dimensional_

Stability.pdf

11. Eldridge, M. Mount III, John R. Wagner, ‘‘Aroma, Oxygen and

Moisture Barrier Behavior of Coated and Vacuum Coated

OPP Films for Packaging,’’ J. Plastic Film & Sheeting V17

(July), 2001, pp. 221–237.

12. E. M. Mount, A. J. Benedict, European Patent 444340;

‘‘Metallizable Heat-Sealable, Oriented Polypropylene Film

Has Layer of Co-polyester to Improve Bonding to Metal,’’

1991.

13. R. A. Migliorini, E. M. Mount III, U.S. Patent 5,591,520,

‘‘High Barrier Film’’, U.S. Patent and Trademark Ofﬁce,

Washington, DC, 1997.

14. R. A. Migliorini, E.M. Mount III, U.S. Patent 5194,318,

‘‘Multilayer Film with Metallized Surface,’’ U.S. Patent and

Trademark Ofﬁce, Washington, DC, 1993.

15. R. A. Migliorini, U.S. Patent 5,153,074, ‘‘Metallized Film

Combination,’’ U.S. Patent and Trademark Ofﬁce, Washing-

ton, DC, 1992.

FILM, PERFORATED

NAZIR MIR

Perftech, Inc., Oswego,

Illinois

INTRODUCTION

There is a growing interest to use ﬂexible polymeric ﬁlms

with ‘‘improved’’ gas and water vapor transmission proper-

ties for horticultural packaging and other food and indus-

trial applications. Fresh produce is still respiring, taking in

oxygen and giving out carbon dioxide, and thus its packa-

ging requires polymeric ﬁlms with selective oxygen and

carbon permeability matching the requirements of this

respiring product. The word ‘‘improved’’ in the ﬁrst sentence

above means those ﬁlms having suitable gas and water

vapor transmission properties to match the requirements of

the application. While high-barrier ﬁlms (those with low

transmission rates) are usually used to protect shelf-stable

foods against oxygen and water vapor, those ﬁlms are not

suitable for packaging fresh produce. Films with improved

gas and moisture transmission properties for fresh produce

are generally achieved by one or a combination of the

following: (a) selection of polymeric ﬁlms, (b) addition of

ﬁllers during ﬁlm extrusion, and (c) perforations.

Improved ﬁlms are constantly being developed to sa-

tisfy the needs of the marketplace. Some of those ﬁlms

include high (6–18%) ethylene vinyl acetate content, low-

density polyethylene (Elvax) ﬁlms by DuPont, oriented

polypropylene (OPP) laminates by BP Amoco, styrene

butadiene block copolymer (K-Resin) ﬁlms by Phillips

Chemical, ultra-low-density ethylene octene copolymer

(Attane series) ﬁlms by Dow, and polyoleﬁn plastomer

octene copolymer (Afﬁnity series) ﬁlms by Dow (1).

Plastic ﬁlms can also be incorporated with inert organic

ﬁllers such as CaCO

and SiO

or organic minerals such as

zeolite. Such ﬁlms may then be biaxially oriented to create

small perforations generally in the range of 0.14–1.4 mmin

diameter (2). The gas and vapor transmission properties of

these ﬁlms can be manipulated by adjusting the particle

size of the ﬁller and the degree of stretching.

486 FILM, PERFORATED

Ethylene manipulation is extremely important for stor-

ability of some horticultural crops. Some additives such as

zeolites may also confer ethylene adsorption properties to

the ﬁlm. Ethylene adsorption capacity by zeolite may be

reduced markedly by the presence of water vapor. It has

been shown that the ethylene adsorption capacity of a

zeolite impregnated ﬁlm was negligible at 100% RH

compared to its maximum capacity at 0% RH (3). Because

most horticultural packaging applications are at condi-

tions near water vapor saturation, the use of such ﬁlms

has not yet had any major commercial success.

MANUFACTURING TECHNOLOGY OF PERFORATED

FILMS

Perforated ﬁlms are plastic ﬁlms that have holes drilled

through all the layers of the ﬁlm. Depending on the size

of perforation, the ﬁlm may be called microperforated

or macroperforated ﬁlms. Microperforated ﬁlms have

perforations in the range of 5–300 mm in diameter,

while macroperforated ﬁlms have perforation exceeding

300 mm in diameter. Generally speaking, microperforated

ﬁlms are largely used to increase the gas properties of

packages containing high-respiring produce such as broc-

coli, bananas, and leeks.

Currently, much marketing effort is focused on improv-

ing the presentation and utility of value-added fresh

produce products. As a result, the produce industry is

shifting more and more toward product offerings in trays

as opposed to traditional breathable bags. The most

commonly trays used are either polyester or polypropy-

lene, both of which are quite impermeable to the gas

exchange in the thicknesses used for these applications.

Thus, to account for the respiratory requirement of

the packaged product, the trays need to be lidded with a

highly gas-permeable membrane. This can conveniently

be achieved using microperforation technology while

keeping the sealabilty, machineability, ink adhesion, and

other properties unchanged during the process. Macro-

perforated ﬁlms are largely used to increase the moisture

properties of ﬁlms for packing whole produce (such

as tomato, pepper, banana) and cold and hot bakery

products.

There are several postmanufacturing technologies

available for perforating plastic ﬁlms. These technologies

include (a) mechanical puncturing by cold needle, and (b)

plastic melting at directed spots by hot needles, ﬂame,

low-energy electrical discharge, and high-energy electrical

discharge and lasers. The cold and hot needle technologies

and the ﬂame technology are commonly used for macro-

perforation, while the electrical discharge and lasers

technologies are commonly used for microperforation.

The technologies for macroperforation have been avail-

able in the industry for a relatively long time. Electrical

discharge systems for microperforation of plastic ﬁlms is a

mature technology. Although the perforations on plastic

ﬁlms can be achieved reliably with electrical discharge

systems, most systems have certain ﬁlm thickness limita-

tions beyond which it is almost impossible to achieve

perforation consistently.

Laser technologies for perforation of plastic ﬁlms are

relatively new. Practically speaking, laser systems contain

three key elements: (1) a laser medium that generates the

laser light, (2) a power supply that delivers energy to the

laser medium in the form needed to excite it to emit laser

light, and (3) an optical cavity that concentrates the light

to stimulate the emission of laser radiation. Since the

introduction of laser systems for perforation of plastic

ﬁlms to the marketplace, technological improvements in

all these three components have improved dramatically.

For instance, the systems have evolved from weak 20-W

power sources to 2-kW power sources, from split beam

technology to beam compression and recently to polygon

mirror directing, which ensures that the beam strength

stays constant throughout the laser exposure interval.

This function has remarkably increased the speed and

quality of the perforations on a number of plastic ﬁlms.

Figure 1 illustrates such a system.

Perforation of plastic ﬁlms is commonly achieved by

laser systems. The laser energy generated through

such a system is absorbed by a plastic ﬁlm in directed

spots. The ﬁlm in those spots are heated, melted, and

eventually evaporated, leaving perforations in the ﬁlm.

Although the principle of laser-drilled perforations is

common in all plastic ﬁlms, there is a strong interaction

between the nature of the ﬁlms and their laser absorption

characteristics. Some ﬁlms may need a very small dose of

energy to melt, while for others it may be almost impos-

sible to drill holes under commercial requirements.

TRENDS

A new trend in ﬂexible ﬁlm business is to offer materials

as blends of two or three different polymers, in which each

polymer performs a speciﬁc function such as improving

strength, transparency, or gas transmission to meet spe-

ciﬁc product requirements. Laminated ﬁlm structures are

also becoming common for a number of value-added fresh

produce applications. The structural complexity of these

new ﬁlms imposes a number of challenges and, at times,

imposes restrictions on the successful application of lasers

on the perforation quality and frequency of these ﬁlms.

The perforation quality is often measured by shape, size,

and aspect ratio. Depending on the shape of perforations,

the aspect ratio can be 1 or >1. The circular perforation

has an aspect ratio of 1; and as the shape of the perforation

deviates from the circularity, the aspect ratio begins to

increase. The oblong perforation has an aspect ratio of 1.2

to 1.4. The aspect ratio is deﬁned as the length of the

perforation in machine direction divided by the length of

the perforation in transverse direction. It is either circular

or oblong.

The adhesives in laminated structures can also inter-

fere with the laser energy absorption process and may

dramatically reduce the speed of laser perforation process.

In some instances, the adhesive may serve as a strong sink

for the laser energy, thus making it impossible to drill a

hole through all layers of the laminate. Thus, it is con-

ceivable to think that the development of operating per-

foration procedures for ﬁlms that vary in chemical

FILM, PERFORATED 487