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SUBSTRATES 153

sible is used. This is the reason for the trend

in recent years toward machines that can

handle wider substrates and larger reels.

METALLIZED FILM

In the metallizing process, reel diameter is

limited and turret rewinds and unwinds can’t

be incorporated in the vacuum chamber. For

these reasons, it is more economical on a per

unit area basis to metallize thin films. The

usual cost for metallized 48-gauge polyester

for flexible packaging is under $0.055/msi,

where msi stands for one thousand square

inches.

Physical Properties

The aluminum layer on metallized film is

so thin, usually 20 microns, that the mechan-

ical properties of the film aren’t changed. In

other words, metallized packaging film, such

as OPP, is just as flexible as the unmetallized

type.

Metallizing a plastic film is a very econom-

ical way to enhance its barrier properties,

and the amount of metal used can vary. The

amount usually is measured and controlled

by monitoring the film’s opacity in optical

density units. The barrier property depends

on the metal’s thickness or optical density.

At a typical packaging density of 2, oxygen

permeability through 48-gauge polyester is

improved by a factor of 100 by metallizing.

On coextruded OPP, the improvement

would ordinarily be a factor of 80 to 3.

Similarly, improvements by a factor of 100

can be achieved in moisture permeability on

48-gauge polyester. For light-barrier proper-

ties, both visible and UV transmission is

reduced to less than 1%.

Printing and Handling

Characteristics

Much of the metallized film used in print-

ing is a laminate, with the metal layer sand-

wiched in the middle. In this case, the actual

printing occurs on the unmetallized surface

and the inks to be used should be the right

ones for that substrate (OPP, polyester, etc.).

In cases where printing on metallized surface

is required, standard inks normally can be

used, although some metallized substrates

(paper and some grades of OPP) can have a

surface “poisoning” effect that will harm ink

adhesion, unless printing is done shortly

after metallizing. Usually the metallized sub-

strates are primed to provide a stable surface

with good printing characteristics.

METALLIZED PAPER

Since it was introduced commercially in

North America, metallized paper has made

significant advances in food and beverage

labeling and packaging. It is being used to

label nearly every type of glass and metal

container and is starting to find application

as a packaging material. Prime label applica-

tions include liquor, wine, wine coolers,

beer, paint cans, soap wrappers and person-

al care products.

Following rapid growth in the 1980s, the

metallized paper industry began a period of

consolidation. The primary markets continue

to be giftwrap, glue-applied labels, pressure-

sensitive laminators and bags. Market size in

1995 was estimated at 85–90 million pounds.

Physical Properties

Packaging engineers like metallized paper

because of its printing, finishing and applica-

tion advantages over foil laminates. The

paper quality determines the labeling advan-

tages. Manufacturing metallized paper is

complicated, from the substrate selection to

final remoisturing. These papers provide the

top quality appearance of foil with the pro-

duction efficiency of plain paper. Physical

properties are summarized in Table 23.

Printers especially like the fact that metal-

lized paper lies flat during printing and

resists mechanical and humidity-induced

154 FLEXOGRAPHY: PRINCIPLES & PRACTICES

curl. Foil laminates often curl and jam press

and production lines with misfeeds and

flagged labels. Metallized paper’s tendency

to lie flat also boosts press speed and die

cutting efficiency to similar levels as those of

plain paper. The same holds true at high-

speed labeling lines.

Printing Characteristics

Most metallized papers have a print coat,

which make them compatible with both sol-

vent- and water-based inks. Plain-paper inks

need little adjustment to print metallized

papers; they even strip and dry as easily as

with plain paper. Because of the advantages

outlined above, metallized paper can be

introduced into the pressroom without the

learning curve that usually accompanies an

unfamiliar substrate.

CLEAR METAL

The metallizing industry has made signifi-

cant strides in producing high-barrier,

“clear” films. Both SIO

and ALO

films have

been finding new market applications. SIO

films have been used in several high-end

medical applications.

Table 23

BASIS WEIGHT CALIPER TENSILE STIFFNESS MULLEN

END USE #/REAM

g/m

MIL MD

PAL

Giftwrap 35 57 2.1 2 29 66 40 12

General label 43 70 2.7 27 40 74 41 14

General label 58 94 3.2 32 60 150 22 22

High Gloss 56 91 3.4 34 0 150 23 23

Ream of 500 sheets, 24" x 36"

Machine direction.

Cross-machine direction.

PHYSICAL PROPERTIES OF METALLIZED PAPER

SUBSTRATES 155

Films

ilms represent a large and diverse

class of substrates used in the

flexographic packaging industry.

These clear, plastic substrates fall

into four classes: polyvinyl chlo-

ride, polyester, polypropylene

and polyethylene.

POLYVINYL CHLORIDE (PVC)

PVC is a unique and popular packaging

material because of its ability to accept and

respond to a range of additives. This film is

commonly referred to as vinyl. It can be

blown, cast, or extruded. Thicknesses of

sheets range from 0.0004" to greater than

0.004". The films are inherently odorless,

tasteless, chemical resistant and waterproof.

In the early years of use they were seen pri-

marily as cheaper alternatives to textiles.

Today, vinyl films are versatile and cost

effective for uses including wall covering,

blister packaging, tapes and labels, water-

beds and floppy-disk jackets.

The ingredients used to manufacture vinyl

depend on the intended application. PVC

resins are the major component of the films

and are made by polymerizing vinyl chloride

monomer using suspension, emulsion or bulk

polymerization. Plasticizers are the major

additive and impart flexibility. Food packag-

ing requires the use of an FDA-approved plas-

ticizer. The next ingredients are heat stabiliz-

ers whose functions are to prevent discol-

oration during processing. Further additives

are lubricants and esters of multifunctional

alcohols, which impart antifog and antistatic

properties. Additives or fillers,like talc or clay,

as well as amides, are added for slip and anti-

block properties. Pigments are added for

color.

Physical Properties

For printing, especially labels and decals,

most vinyl films are made by calendering.

This process is best suited for high-volume

production and requires excellent surface

quality and uniform thickness control. It can

produce films up to 84" in width. For thick-

nesses below 0.002" casting is used. Casting

works best for small volume specialty appli-

cations requiring excellent clarity, low strain

and uniform strength in both directions.

Although vinyl films have a broad flexibility

range, the typical physical properties are

tensile strength of 3,400 to 5,000 psi, elonga-

tion of 50% to 200%, and Sheffield smooth-

ness of less than 10 on the face side.

Printing and Handling

Characteristics

The ability to print with both solvent- and

water-based inks without surface treatment

has figured prominently in both meat and

poultry packaging. A recent development has

been the imprinting of a safe-handling label

mandated by the USDA, on the film itself.

This ensures 100% compliance at the store

level on all packages of raw meat.

Although not always necessary, a primer

coat may be used in some applications.

Corona treating of vinyl films to improve sur-

face tension is available but not widely used.

POLYESTER

The unique mechanical, thermal and chem-

ical traits of bi-oriented polyester (polyethyl-

156 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ene terephthalate or PET) film is making it

more and more the substrate of choice in

many flexographic applications. When a sub-

strate must be tear-resistant, stable in heat

and humidity, retain sheet flatness and clari-

ty, and have a good moisture and or oxygen

barrier, polyester film is the right choice,

whether the printed result is a throw-away

package or a long-life graphic.

Once a product with generic types, PET

film today has many forms designed for spe-

cific end uses. These forms may feature a par-

ticular surface chemistry, roughness, clarity

or slip. Also, there are special variations, such

as matte, heat sealable, thermoformable,

shrinkable, low shrink and barrier-coated.

The primary uses for polyester film include:

photography (X-ray, aerial, phototool), mag-

netic recording (computer, audio, instrumen-

tation, video) and reprographics (duplicating

microfilm, engineering, layout), but packag-

ing and printing uses are the fastest-growing.

Most photographic PET base and some mag-

netic and reprographic base is produced by

plants belonging to the coating firm. Virtually

all other uses are supplied by industrial film

producers. (Table 24).

Standard area yield factors for polyester

film are shown in Table 25. Thicknesses

above and below this range are common in

some end uses, especially electrical and mag-

netic recording. Thicknesses listed are the

ones commonly used in flexographic printing.

Roll widths of 60" to 70" are common in

many end uses and roll diameters are very

often 24" to 28", with weights of 1,200 lbs. to

2,000 lbs. With flexo applications, smaller

rolls are more common, with 6" diameter

cores virtually the standard, though 10" is

often supplied for other uses.

Physical Properties

Depending on the requirements, PET film

can be manufactured with a variety of physi-

cal properties, as shown in Table 26. Special

films such as formable or heat-sealable may

have different properties, and the manufac-

turer can supply data on these. Unless other-

wise indicated, all values in Table 27 are at

73° F (23° C) and 50% humidity.

Printing Characteristics

Polyester film’s chemical stability comes

from its basic polymer, polyethylene tereph-

Table 24

APPLICATION % OF CONSUMPTION

Photographic 26.7

Magnetic Recording 15.2

Reprographics 13.4

Packaging, including metallized 11.5

Printing/labels/release coating 3.8

Electrical 3.2

Glazing/specialty vacuum coating 3.2

Transfer printing/roll leaf 2.7

Pressure-sensitive Tape 2.5

Building Products 2.5

General laminates/stationery 1.9

Miscellaneous 13.4

TOTAL 100

END USES OF PET FILM IN U.S.

ESTIMATED

Table 25

AREA YIELD

GAUGE/MICRONS IN

/LB M

48/12 41,250 58.7

75/19 26,400 37.6

92/23 21,500 30.6

142/36 13,900 19.8

200/50 9,900 14.1

300/75 6,600 9.4

400/100 4,950 7.0

500/125 3,960 5.6

700/175 2,830 4.0

AREA YIELD FACTORS FOR

POLYESTER FILMS

SUBSTRATES 157

thalate (PET), a polymerized ester normally

formed in a condensation reaction of ethyl-

ene with terephthalic acid or dimethyl

terephthalate. The natural chemical inert-

ness of this polymer is further enhanced by

two-way stretching (bi-orientation) and high-

temperature crystallization. Orientation in-

creases tensile strength, flexibility, tear

strength and pinhole resistance, while crys-

tallization boosts thermal stability and barri-

er properties.

Chemical inertness, one of PET film’s major

strengths, makes it hard to coat and print

because of solvent and water resistance.

Manufacturers have attacked this problem by

changing the surface chemistry without hin-

dering the thermal and mechanical character-

istics. While these changes do not make PET

film as easy to print on as some other sub-

strates, they do give the printer a wider choice

in both inks and processing conditions.

The earliest of these surface modifications

was corona treatment to allow easier wet-

out. Next came resin treatments that worked

well with solvent-ink systems but resisted

water-based inks. Very recently, polymer

treatments were developed to allow the use

of water- and alcohol- ink systems. These sur-

face modified films are shown in Table 28.

While PET film’s physical traits make it

ideal for flexo printing, some precautions are

in order, especially if press operators are used

to running paper or more extensible films,

such as vinyl or polyethylene. For example,

the film has a residual shrink tendency that

increases alarmingly as the temperature rises.

Needless to say, this can affect such process-

ing parameters as neckdown and tension. In

addition, rising temperature greatly reduces

tensile strength. This can cause the film to

stretch under press tension, affecting register

and promoting wrinkles and creases. PET

film is quite stable at processing tempera-

tures up to 180° F. But once into the 180° F to

Table 26

PHYSICAL PROPERTY NORNAL RANGE

Tensile Strength, at Break 20,000 psi maximum

Elongation at Break 150% maximum

Shrinkage

Low Shrink less than 1%*

Standard 1-5 Maximum*

Shrinkable Over 30%**

WATER VAPOR AND

OXYGEN PERMEABILITY THICKNESS (GAUGE)

Maximum Uncreased,

Uncoated 48.0 75.0 92.0 142.0

Water-vapor Transmission

(g/100 in

/24 hours) 3.5 3.0 2.5 2.0

Oxygen Transmission

(cc/100 in

/24 hours) 10.0 8.0 5.0 3.0

* 150° C, air unrestrained, 30 mins.

** 100° C, water, 1 min.

PHYSICAL PROPERTIES

OF PET FILM

Table 27

GROUP DYNES COMMENT

Plain Film 40-43 Polyester Inks. Primer/Top coats required.

Resin Treated 43-45 Solvent-based Inks. Primer/top coats not required:

treatment does not fade with time.

Corona Treated 50-54 Water/Alcohol inks. Mild solvents;treatment fades with time.

Polymer Treated 58-63 Water-based inks. Primer/top coat not required;

treatment does not fade with time.

SURFACE-MODIFIED FILMS

158 FLEXOGRAPHY: PRINCIPLES & PRACTICES

220° F range, the film goes into an expansion-

to-shrinkage transition that can cause unpre-

dictable web-handling problems.

POLYPROPYLENE

This section will focus on oriented

polypropylene or OPP film. The volume of

non-oriented polypropylene film used is

about one fourth that of OPP film. It replaces

low-density polyethylene (LDPE) film in

applications for which its better clarity, stiff-

ness and barrier properties justify the extra

cost. Non-oriented polypropylene film,

sometimes called cast-polypropylene film,

has physical properties and printing charac-

teristics similar to those of LDPE film. The

inks and printing practices used with cast-

polypropylene film are the same as those for

LDPE (see section on LDPE for details.)

Estimated world usage of oriented poly-

propylene (OPP) for 1997 was approximate-

ly 4 billion pounds, with 20% of it being con-

sumed in North America, 31% in Western

Europe and 38% in the Asian-Pacific Region.

The largest portion of the 800-million pound

North American market is snack packaging

(25%). Other major areas include baked

goods (15%), cookies (10%), labels (10%),

confectionery (5%), other assorted foods

(15%) and nonfoods (20%) (Figure

Polypropylene film changed little between

1960 and 1980. But in recent years. many

new products have appeared and OPP film

has expanded into a large family of materi-

als, some of which call for special printing

considerations.

Recent additions to this family include a

number of composite films in which OPP is

the core, with thin, functional layers of vari-

ous polymer resins coextruded or applied by

coating. Opaque films, which have a foam-

like structure, and metallized films are two

relatively new additions that have achieved

substantial volume. For a view of polypropy-

lene film beyond what this section covers,

the reader is referred to The Encyclopedia of

Polymer Science and Engineering, Volume 3.

The yield of 1 mil (0.001") OPP film is

30,600 square inches per pound, compared

with 19,500 square inches per pound with

cellophane. PP's specific gravity of 0.91 g/cc,

the lowest among plastic films, accounts for

a yield significantly higher than that of any

other clear, oriented plastic film. PP resin’s

high yield and relatively low cost per pound

make it the most cost-effective film of its

kind for flexible packaging. Depending on

thickness and structure, it ranges from $0.05

to $0.10 per 1,000 square inches.

OPP film is available in thicknesses from 40

to 400 gauge (0.0004" to 0.004"). Essentially,

all printed OPP film ranges in thickness from

0.00045" to 0.0012" (0.45 to 1.2 mil). OPP film

is sold by the pound, and comes on 3" or 6"

inside diameter cores. Widths depend on the

type of film. Several types are available from

tenter lines that yield widths of about 200".

Physical Properties

Most of today’s OPP film is clear, biaxially

oriented, slightly formulated homopolymer

not significantly different from when it was

first introduced in 1960.

Polypropylene’s physical characteristics

come from the catalyst and reaction condi-

tions used to produce it. They determine

Polypropylene usage in

the United States.

Baked Goods

15%

10%

Labels

10%

Confectionery

Other

Assorted Foods

15%

Nonfoods

20%

Snack Packaging

25%

whether the polypropylene chains will be

highly symmetrical and, therefore, highly

crystalline and easily oriented. An asymmet-

rical chain will crystallize only slightly and

will not be capable of high orientation or

great strength.

Orientation, OPP film’s distinguishing trait,

means that its long, chain-like molecules have

been aligned in the machine and transverse

directions. In a linear orientation, the aligned

molecular chains form highly symmetrical

matrices known as crystalline spherulites, fit-

ting together in a multilayer, repeating config-

uration of juxtaposed chains to create great

cohesion. This keeps the ranks closed and

resists the penetration of solvents and certain

vapors, including water vapor, giving OPP

film its excellent barrier properties.

Orientation also accounts for other good

characteristics. The high directional-tensile

strength and modulus of a crystalline-orient-

ed film, compared with unoriented film, is

analogous to that of a woven fabric com-

pared to the same mass of fibers in a random

pile. In addition, orientation provides better

clarity and low-temperature flexibility.

Orientation may be equal in both planar

directions, giving equal tensile properties, as

SUBSTRATES 159

Table 28

MECHANICAL/OPTICAL UNBALANCED TENTER PROCESS BALANCE (TUBULAR PROCESS)

PROPERTIES OPP FILM BOPP FILM

Haze, % 2.0 1.0

Gloss, % 85.0 80.0

Tensile Strength, psi (kg/cm

)

22,000 (1,500) 30,000 (2,100)

43,000 (3,000) 30,000 (2,100)

Elongation at Break, %

165 85

50 85

Tensile Modulus, psi (kg/cm

)

280,000 (20,000) 380,000 (27,000)

490,000 (34,000) 380,000 (27,000)

Elmendorf Tear, g/mil 4–64–6

Coefficient of Friction

Film to film 0.25 0.25

General Properties

Water Absorption (%) <0.005

Low-Temperature Usefulness (°C) –60

Chemical Properties

WVTR for 1 mil (g/100 in

, g/m

)

Grease Resistance Excellent

Oil Resistance Excellent

Machine Direction

Transverse or Cross-machine Direction

Slip-modified Film

TYPICAL PHYSICAL PROPERTIES OF OPP FILM

160 FLEXOGRAPHY: PRINCIPLES & PRACTICES

is typical for tubular-process OPP film, usu-

ally known as balanced (BOPP) film. It

might be unbalanced and relatively weaker

in the machine direction but stronger in the

transverse direction, as is usually the case

with tenter-produced OPP film.

Table 28 shows the typical tensile proper-

ties of OPP film. Of special interest when it

comes to printing is the tensile modulus. The

machine-direction modulus is a direct mea-

sure of a film’s resistance to elongation, a

significant trait in continuous web printing.

Printing Characteristics

Any discussion of printing of OPP film

with flexography has to include both bulk

composition, physical properties and sur-

face characteristics. The surface character-

istics are important because they determine

whether or not a particular ink will wet-out

and adhere, while bulk film properties mat-

ter because they affect print quality from the

aspect of web handling.

Polupropylene is made by polymerizing the

unsaturated hydrocarbon gas propylene. The

result is polypropylene, a saturated hydro-

carbon structure of the class of polymers

called polyolefins. Like other saturated

hydrocarbon substances, polypropylene has

very low polarity and very low reactivity. Its

surface-wetting tension (sometimes called

surface energy) is low, 29 dynes per centime-

ter. This inertness means that wetting and ink

adhesion will not occur unless the surface

energy is increased. Usually, this is done by

corona, high-voltage discharge treatment

and, to a lesser extent, by flame treatment.

The energy intensity and technique used in

surface treatment are critical for successfully

printing OPP film. For general purpose print-

ing, a surface treatment equal to 2.5 to 3 watt

minutes of corona discharge per square foot

of film is required. Film manufacturers will do

this to boost the surface energy from 29 to

45+ dynes per centimeter on freshly treated

film. This treatment will fade down to about

the 40 dynes per centimeter level. Converters

can increase the dyne level on already-treated

OPP by corona treating in-line, but the effect

is only temporary. If done improperly, con-

verters (and suppliers) can also cause back-

side treatment, which can be disastrous for

applications which require a non-treated sur-

face such as cold-seal release applications.

OPP films can contain migratory slip- and

anti-blocking agents. These tend to bloom to

the surface, mask the surface treatment and

give misleading, low-wetting tension read-

ings. Solvent inks usually get through these

contaminants easily, while water-based inks

will not, without the addition of some cut-

ting solvent (5% alcohol). Ink adhesion is

typically a function of the film’s surface

chemistry beneath any migratory additives

which bloom to the surface.

Variations in surface composition include

coextruded or coated layers of ethylene-pro-

pylene copolymer, acrylic polymers and alu-

minum. In printing characteristics, polyolefin

copolymers are similar to polypropylene, but

they are usually more receptive to corona

treatment than homopolymer polypropylene.

Acrylic surfaces have wetting tension

higher than that of the polyoefins and show

an advantage in ink adhesion (but a disad-

vantage in retaining ink solvents, requiring

extra care in drying).

Metallized surfaces adhere well when

clean. But they are so reactive that the sur-

face may be contaminated by contact with

the other side of the film, particularly if it

contains any organic substance of low mole-

cular weight. For consistently good print-

ability, the metal surface should be treated in

line, using, for example, a bare-roll corona

treater.

Just as important a consideration in printing

OPP film is resistance to machine-direction

elongation. After the surface energy deficien-

cy of OPP was remedied, the tendency of OPP

film to stretch in the machine direction was

the next major obstacle. Converters found

that while they could use tensions of a few

pounds per inch of web width when printing

cellophane, they had to learn to control web

tensions to as low as 0.25 to 0.5 pounds per

inch and still maintain good web spreading

and flatness when printing OPP film.

As temperature goes up, OPP film’s resis-

tance to elongation goes down, just as with

other thermoplastic materials. At tempera-

tures of about 140° F (typically reached in

converting) a 1 mil thick OPP film under 0.5

lb. per inch tension would stretch about 0.6%,

the maximum allowable elongation for good

registration.

The tensile modulus (not the tensile

strength) reflects the film’s resistance to

elongation. The tensile modulus has been

defined as the value of the load required to

stretch a 1" wide by 1" thick piece of materi-

al to 100% elongation. The tensile modulus is

the initial slope of the load vs. elongation

curve and is measured in units of pounds per

square inch (psi). For a film, converting a

380,000 psi modulus (such as for the

machine direction of BOPP film) to the ten-

sion load that would apply to a 1 mil thick-

ness, yields a value of 380 lbs. for 100% elon-

gation per 1-inch width film (as shown in

Table 29). Note that tenter-process film

stretches more easily in the machine direc-

tion and would require only a 280 lb. load for

the 100% elongation, 2.8 lb. for 1% elonga-

tion. This value would occur at the initial

part of the curve of the stress-strain rela-

tionship and can be relied on as physically

meaningful. Increasing the temperature or

tension, or reducing the film thickness,

would mean higher elongation for a given

tension load.

Comparison of various films’ machine

direction modulus values, as in Table 29,

indicates the relative tendencies of OPP film

and other flexible packaging films to stretch

in the machine direction. This comparison

should bring home the importance of con-

trolling tension in printing them. Table 30

shows common off roll weights.

SUBSTRATES 161

Table 29

LOAD, IN POUNDS, TO STRETCH A 1" WIDE,

TENSILE MODULUS

, PSI 1 MIL THICK FILM TO ELONGATION OF

FILM TYPE MACHINE DIRECTION 100% 1% 0.63%

LDPE 50,000 50 0.50 0.32

PP (cast) 95,000 95 0.95 0.60

OPP (tenter process) 280,000 280 2.80 1.76

BOPP (Balanced tubularProcess) 80,000 380 3.80 2.39

Cellophane 620,000 620 6.20 3.91

Polyester 500,000 500 5.00 3.15

Defined as load required to stretch a 1" wide by 1" thick piece of material to 100% elongation.

TENSILE MODULUS VALUES/RESISTANCE TO ELONGATION

Table 30

CORE DIAMETER ROLL WEIGHT IN

INSIDE OUTSIDE DIAMETER LBS. PER INCH

(IN) (IN) OF WIDTH

3.0 3.75 11.0 2.7

3.0 3.75 15.0 5.5

6.0 6.75 12.5 2.8

6.0 6.75 16.0 5.4

6.0 6.75 19.0 8.1

6.0 6.75 21.0 10.0

OPP FILM WEIGHT/INCH

OF ROLL WIDTH

162 FLEXOGRAPHY: PRINCIPLES & PRACTICES

POLYETHYLENE

Polyethylene (PE) is the most common

film used in the United States (Figure

) .

The first PE resin was made in Great Britain,

but during the 1940s it appeared in the

United States, with Union Carbide being the

first manufacturer. PE’s applications are too

numerous to list but range from tape and dry

cleaner bags to exotic multilayer boil-in-the-

bag laminations and coextrusions.

New resins and in turn, new films, are

appearing on the market quickly and con-

stantly. Ideas generated by resin producers,

film producers, and film users and convert-

ers have expanded the roster of available

films. New resins, such as those using the

new catalyst technology metallocene, have

greatly expanded the universe of polyethyl-

ene films.

A short review of polyethylene resins

should be helpful in understanding the many

different film types available today. Table 31

shows three main families of polyethylene.

These can be further broken down as follows:

Low-density Polyethylene (LDPE). The first

polyethylene commercially manufactured

was low-density polyethylene (LDPE).

LDPE is also referred to as high-pressure

polyethylene, or branched polyethylene.

These names are derived from the type of

reactor and the appearance of the mole-

cules. LDPE is made in a high-pressure reac-

tor (pressures upwards of 40,000 psi) and

has an ethylene backbone with many

branches as shown in Figure . LDPE can

be copoylmerized with at least four com-

mercially available comonomers:

• vinyl acetate (EVA);

• methyl acetate (EMA or EMAC

);

• acrylic or methacrylic acid comono-

mers (EAA or EMAA); and

• ionomer or ionically crosslinked PE

(Surlyn

Polyethylene usage in

the United States.

Average

Annual

Growth Rate

1985 1990 1995

U.S. PE Film Consumption (million lbs.)

10,000

8,000

6,000

4,000

2,000

HDPE

LDPE

LLDPE

HDPE

LDPE

LLDPE

HDPE

LDPE

LLDPE

4.8%

2.8%

7.8%

Table 31

LDPE

LLDPE

HDPE

Melt index (g/10min) 0.2–70 0.2–50 0.01–80

Density (g/cc) 0.91–0.935 0.916–0.94 0.940–0.965

Short chain branching 10–30 10–-30 <10

Short chain branching length C1–C4 C2C4 or C6 C2 or C4

Long chain branching(no./molecule) 30 0 0

Crystalline melting point (° C) 180 122 130

LDPE (low density PE): Best clarity; highest tear; lowest stiffness

LLDPE (linear low density PE): Higher stiffness, greater puncture resistance; improves down gauging potential.

HDPE (high density PE): Highest stiffness, low impact and tear, highest tensile, best barrier.

THREE MAIN FAMILIES OF PE FILMS

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