FTA (изд-во). Flexography: Principles And Practices. Vol.1-6

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High-density Polyethylene (HDPE). First intro-

duced in 1957, HDPE is the first “linear”

polyethylene. HDPE is characterized by den-

sities above 0.935 g/cc. This higher density

provides stiffness, toughness, good environ-

mental stress-crack resistance (ESCR) and

low-temperature properties. HDPE has

found wide use in many film markets, such

as merchandise bags, cereal/cracker box

liner, extrusions for improved barrier and

grocery sacks.

Linear Low-density Polyethylene (LLDPE). The

next oldest linear film, LLDPE, first appear-

ing widely in 1980, though grades were avail-

able commercially as far back as the 1960s.

Its density is generally 0.916 to 0.940 g/cc).

LLDPE has the same high-density backbone

but is characterized by the side chains as

shown in Figure .

Manufacturers can make up these side

chains from any number of different alpha

olefin copolymers, but commercially the

butene (four-carbon chain), hexene (six-car-

bon chain), and octene (eight-carbon chain)

LLDPE resins are used. Typical comonomer

content in 0.920 g/cc density LLDPE resins, a

workhorse film grade is 8% to 10%.

Properties vary with the type of alpha olefin

used. Overall, properties improve with

increase in short-chain branch length. This

means there is an increase in the tear

strength, tensile strength and impact

strength when going from butene to hexene

to octene.

Metallocene Polyethylene (mLLDPE). mLLDPE

is the latest in the linear polyethylenes to

arrive, with commercial production starting

in the 1990s. These resins derive their name

from the unique nature of the catalyst used

in their manufacture. The catalyst uses vari-

ous metals such as zirconium or other tran-

sition metals to produce a uniform or homo-

geneous resin, since the catalyst has only a

single site for the polymerization to take

place. “Single site” is another name for met-

allocene resins. These resins can range in

density from below 0.860 g/cc to 0.960 g/cc

and above, allowing them to possess the

properties of all the previously mentioned

resins. The main advantage of mLLDPE is

that as the density decreases the melting

point also decreases. This is unlike LLDPE,

where density has minimal or no effect on

melting point. Another advantage is the

lower odor and fewer extractables. These

resins also can incorporate many different

alpha olefins, but those generally available

are the butene, hexene and octene co-

monomers.

Physical Properties

Armed with the multitude of resins avail-

able, film producers multiply the various

possibilities by blending and coextruding all

of the above. In addition, film manufacturers

add color, slip, anti-block, antifog agents,

anti-stats and/or a variety of other items to

modify physical or surface properties. Other

variables include two- to nine-layer coextru-

sion, orientation and mechanical finishes,

such as embossed or matte patterns.

Various polyethylene resins/films do not

have a constant selling price. Each year, one

or more resins are in tight supply and cause

dramatic shifts in relative cost. Factors

affecting film price include, resin, additives,

gauge, surface pattern, production rate, roll

SUBSTRATES 163

Polyethylene families.

HDPE

LDPE

m-LLDPE

LLDPE

164 FLEXOGRAPHY: PRINCIPLES & PRACTICES

configuration and width/diameter of rolls.

Down-gauging has had a major influence

during the last 10 years. In most applica-

tions, down-gauging has been accomplished

by using different formulations incorporat-

ing linear low-density PE (LLDPE) (equal

stiffness and tear at a lower gauge).

Typical yields can be calculated from the

formula:

YIELD



27,690

(IN

/LB) DENSITY CALIPER

(GM/CC)



(MILS)

For example: a 2 mil white pigmented film

with a density of 0.96 has a calculated yield:

YIELD (in

/lb)  27,690  14,422 in

/lb

0.96  2

Typical film manufacturers produce a wide

Table 32

LLDPE LDPE LLDPE LDPE

PROPERTY CAST CAST BLOWN BLWON

Additives (ppm)

Slip 750 750 750 750

Anti-block 4,000 5,000 4,000 5,000

Melt Index 1 2 1 0.3

Density index 0.918 0.925 0.918 0.921

Gauge (mil) 1.0 7.0 2.0 2.0

Haze (%) 4.3 3.5 11 1.7

Gloss 45° (%) 84 87 67 75

Tensile Strength (psi)

6,400 4,700 5,700 3,500

2,200 2,200 5,700 3,400

Elongation (%)

462 457 970 400

550 535 1,080 800

1% Secant modulus (psi)

42,000 47,000 33,000 28,000

44,000 48,000 40,000 31,000

Elmendorf Tear (g)

19 219 430 200

501 112 560 260

Dart impact (g) 93 47 260 230

Machine Direction

Transverse or Cross-machine Direction

PROPERTY LDPE HDPE PP

Density 0.91–0.925 0.941–0.965 906

Tensile Strength (psi) 600–2,300 3,100-5,500 4,300–5,500

Elongation (%) 90–800 20–1,000 200–700

Impact Izod High 5–20 5–2

Resistance to heat (¡F) 180–212 250 225–300

Water Absorption (%) <0.15 <0.01 <0.01–0.03

TYPICAL FILM PROPERTIES

variety of products. Some key traits are clar-

ity (haze/gloss), sealing (initiation tempera-

ture/hot tack), tensile (at yield point/ulti-

mate/machine direction/transverse direc-

tion), stiffness (machine and transverse

secant modulus), barrier (moisture, MVTR/

oxygen, OTR/air) and surface additives.

Polyethylene films do not have the clarity

of a polypropylene-cast film, but have com-

parable prices. LDPE gloss can approach 85%

and 3%–5% haze, versus 90% or higher gloss

and 1.5% haze for polypropylene. Table 32

lists some typical film properties.

General resin types are linear LLDPE,

HDPE, LDPE, mLLDPE, random copolymer

polypropylene (RCPPP), and homopolymer

polypropylene (HPPP). Table 33 shows their

ranking from worst to best for clarity, stiff-

ness and sealing under equal circumstances.

Additives have a major impact on a film’s

physical characteristics and surface proper-

ties. Films should have only those additives

required for a specific application. Common

additives include:

Slip Agents – Erucamide and other fatty amides.

Slip agents lower the coefficient of friction

(COF). A lower COF means the film will have

a higher “slip” film. These additives migrate to

the surface and at a given parts per million

(ppm) additive-loading a 2-mil film will have

twice the amount of slip agent on the surface

than a 1-mil film. Negative aspects include

slip-agent buildup on the surface causing a

“greasy” feel. The slip agent appears as a

waxy powder which can build up on equip-

ment when processing and prevent inks or

adhesives from adhering to the film.

Anti-block – SiO

. Anti-block prevents blocking

(sticking) of adjacent layers of film. Anti-block

lowers COF similar to slip but usually only to

a COF of about 0.4. Low-density ethyl vinyl

acetate (EVAs) and other comonomers

require higher levels of slip and anti-block.

anti-block works by microscopically roughing

the surface so it can make a less smooth sur-

face for printing.

Pigments. The primary pigment used for

white film is titanium dioxide. There is a

whole range of organic and inorganic pig-

ments used to color film. Many of these pig-

ments, when supplied in a concentrate, to

the film producer, may have various disper-

sion aids or lubricants to help disperse the

pigment. These dispersion aids may affect

printability.

Other Additives. These include ultraviolet sta-

bilizers, ultraviolet absorbers, antifogs, anti-

oxidants, processing aids, chill-roll release,

foaming agents, flame retardants, optical

brighteners, delusterants, degradable addi-

tives, clarifiers and antistats.

Printing and Handling

Characteristics

The film surfaces of polyethylene are

notably indifferent to the adhesion of inks or

coatings, and since the first introduction of

polyethylene to the packaging film market,

this has been a hindrance. First the films need

to be “treated” before coating or printing.

An electronic “corona” discharge is the pri-

mary form of treatment. There is also flame

treating and plasma treating. It is commonly

thought that treating “oxidizes” or reorients

the electrons on the surface or forms carbonyl

groups or another chemical change. In “The

Science and Technology of Polymer Films”,

edited by Orville J. Sweeting, the author

SUBSTRATES 165

Table 33

Clarity: Best

HDPE-LLDPE- LDPE-mLLDPE-RCPP-HPPP

Stiffness: Stiffest

mLLDPE-LDPE-LLDPE-RCPP-HPP-HDPE

Sealing:

mLLDPE - LDPE - LLDPE - HDPE - RCPP -HPP

99° 108° C 160° C

RANKING RESIN TYPES

166 FLEXOGRAPHY: PRINCIPLES & PRACTICES

shows that the mechanism was “micropitting

of the surface”. Whatever the mechanism, we

know the surface energy, measured in

dynes/cm, is increased after treatment. This

higher surface energy allows inks and coat-

ings to wet-out the surface.

Common treat levels range from 36 to 42

dynes/cm, depending on the application.

Untreated polyethylene has an inherent treat

level of 31 as compared to polypropylene

with a 29 dyne/cm treat level. The measure-

ment of treat levels is done by a wetting ten-

sion method (ASTM, D2578-67). Other meth-

ods used to measure treat level are: adhe-

sion-ratio test, ink pick-off test and measur-

ing the angle water or other solvents make

with the surface.

Films with additives need more power (or

slower speeds) to reach the same treat levels

as film without additives. An insufficient

treat can cause poor or spotty adhesion, trap-

ping of colors over each other and ghosting.

Over-treating can cause blocking, reduce seal

strength and cause poor printing by the pos-

sible formation of low-molecular-weight

degradation products.

Handling requirements for polyethylene

films can be summarized in two words: Be

Gentle. Marks from handling rolls can cause

films to tear in end-use processing. Storing

film in hot conditions can cause blocking

and sunlight can break down polyethylene.

To get a rough estimate of the tension re-

quired to run polyethylene films, the secant

modulus and the film thickness is required.

The secant modulus is simply the force

required to stretch the film 1%.

Example: Use a typical secant modulus for poly-

ethylene film in the machine direction of 25,000.

With a 1% stretch, the web tension on a 2-mil film

needs to be

25,000  0.002  0.01  0.5 lbs/in of web width

or 0.5 lb/in of web width

For a 40" web, the total web tension would be

40  0.5 = 20 lbs.

Converters print many millions of pounds

of polyethylene each year, but for applica-

tions requiring top-quality print with exact-

ing register standards, polyethylene is usual-

ly not the substrate of choice. In many pack-

aging applications, polyester or oriented

polypropylene is reverse printed and then

laminated with polyethylene to lock in the

print and give excellent protection to the

printed image, in addition to, making the

film heat-sealable. Polyethylene is used in

many printed applications, but often not as

the printed film.

CELLOPHANE

Cellophane is a thin, flexible and transpar-

ent material used for packaging applica-

tions. It is not truly a film in the sense that it

is composed of chemical compounds.

Cellophane is both a bio-degradable and

renewable resource of regenerated cellulose

film, derived from purified wood pulp,

known as “dissolving pulp”.

The use of cellophane followed the growth

of the flexible packaging industry. For 30

years it dominated the industry because it

offered the marketplace a wide variety of

properties that produced a product at rea-

sonable cost. Once the base sheet is pro-

duced, it is coated either with polyvinylidene

chloride copolymer (PVDC) or nitrocellu-

lose, which adds heat sealability, machine-

ability and barrier properties. Uncoated cel-

lophanes are sold mostly for industrial uses,

such as pressure-sensitive tape base, fiber-

glass and rubber mold-release membranes,

and roll-leaf applications. The advent of plas-

tic films, such as polyethylene and polypro-

pylene, eroded the use of cellophane.

Physical Properties

The physical properties of cellophane are

very similar to all types of film and are only

differentiated according to coatings and rein-

forcing structure and thickness. Many film

types, designed for specific applications, are

produced. Regenerated cellulose, the base

sheet for all mono-web cellophanes, varies in

thickness for different basis weights. Other

variations in the base sheet include the

amount and/or type of plasticizer added for

durability, stiffness and dimensional stability.

The most widely used film is plain uncoated,

used for producing pressure-sensitive tapes

and where high-quality printing is desired.

The uncoated film prints readily, with almost

any type of flexographic ink, because it

absorbs liquids.

Each family of cellophane films has similar

properties. Except for the two reinforced

ones (one of which is metallized polyester

and the other a white, opaque polypropylene

core), all of them are clear. The nitrocellulose

films range from 16,000 to 18,000 psi tensile

strength in machine direction and 8,000 to

9,000 psi in transverse direction. Elongation

ranges from 15% to 25% in machine direction

and 30% to 45% in the transverse direction.

Heat-sealable coatings have a wide sealing

range that usually requires temperatures of

200° F to 350° F, depending on machine speed

and pressure. As a rule, these films will run on

any machine that can handle a flexible pack-

aging substrate. The coefficient of friction

ranges from 0.30 to 0.35 (US) to 0.25 to 0.30

(UK). Water-vapor transmission rate (WVTR)

for the moisture-proof films averages 0.05 gm

per 100 in

per 24 hours, while the breathable

“L” types range between 30 to 50 gm/100 in

per 24 hours. Oxygen permeability stays

around 2 cc/100 in

per 24 hours per atm for

most two-sided coated nitrocellulose films.

Table 34 shows the cellophane yields for dif-

ferent gauges.

Printing Characteristics

All cellophanes lend themselves to flexo-

graphic printing and samples are routinely

given to ink manufacturers for testing. They

can make specific suggestions for their use.

Air velocity, exhaust and web temperature

are prime factors to consider when printing

cellophane. Maximum air velocity should be

used and the exhaust should be at least 15%

to 20% greater than the air volume entering

the system.

When web temperature is excessive, there

may be locking in the roll even if the film has

cooled to room temperature before rewind-

ing. Overheating may also cause PVDC-coat-

ed cellophane to stretch during printing and

completely or partially return to its original

shape (snap back). This can cause the repeat

to vary, making it hard for the end user to

properly control the packaging equipment

cutoff. When the film then tries to return to

its original shape, rolls that were wound soft

on the press later become hard. This can

cause ghosting, offsetting and blocking. Web

temperature should be measured as soon as

the film comes out of the overhead oven.

SUBSTRATES 167

Table 34

■ CELLOPHANE

MIL

GAUGE IN

/POUND THICKNESS

116 11,600 1.7

140 14,000 1.4

160 16,000 1.2

180 18,000 1.1

195 19,500 1.0

210 21,000 0.9

220 22,000 0.9

230 23,000 0.8

250 25,000 0.8

■ REINFORCED CELLOPHANE

MIL

GAUGE IN

/POUND THICKNESS

118 11,800 1.6

122 12,200 1.6

123 12,300 1.7

CELLOPHANE YIELDS

SUBSTRATES 169

Appendix A

PROPERTY TEST METHOD

Air Permeability T 251, T 547

Air Resistance T 460, T 536

Bending Stiffness T 535

Bonding Strength T 541

Brightness T 452

Bulking Number T 500

Bursting Strength T 403

Coefficient of Friction T 542

Coefficient of Static Friction T 503

Color T 527

Color Matching T 515

Curl T 520

Degree of Curl/Sizing T 466

Diffuse Opacity T 519

Edge Tearing Resistance T 470

Elongation T 404

Equilibrium Moisture Content T 550

Fiber Analysis T 401

Folding Endurance T 423

Gloss T 653

PROPERTY TEST METHOD

Grammage T 410

Ink Absorbency T 431

Internal Tearing Resistance T 414

Liquid Penetration Resistance T 530

Moisture Content T 208, T 412

Opacity T 425

Optical Properties T 442

pH T 428

Pick Resistance T 459

Smoothness T 538, T 479

Spectral Gloss (20°) T 653

Spectral Gloss T 480

Stiffness/Bending Stiffness T 451, T 489, T 535

Stretch T 495

Surface pH T 529

Surface Strength T 459

Tearing Strength T 496

Tensile Strength T404

Thickness T411, T 551

Wet Strength T 456

TAPPI TEST METHODS: PAPER

170 FLEXOGRAPHY: PRINCIPLES & PRACTICES

PROPERTY TEST METHOD

Abrasion Resistance T 476

Air Permeability T 547

Bending Strength T 495

Bonding Strength T 541

Brightness T 452

Bursting Strength T 807

Colorimetry T 524

Fiber Analysis T 401

Gloss T 653

Grammage T 410

PROPERTY TEST METHOD

Internal Bond Strength T 541

Moisture T 412

Ring Crush T 818

Smoothness T 538

Stiffness T 451, T 489, T 543

Stretch T 495

Tearing Strength T 414, T 496

Tensile Strength T 494

Thickness T411, T 551

Wet Strength T 456

TAPPI TEST METHODS: PAPERBOARD

Reference:

Technical Association of the Pulp and Paper Industry, TAPPI Test Methods, 1997.

PROPERTY TEST METHOD

Bursting Strength T 810

Coefficient of Static Friction

(horizontal-plane method) T 816

Coefficient of Static Friction

(inclined-plane method) T 815

Compressive Strength T 811

Flat-crush Test T 808

Flat-crush Test

(ridged-support method) T 825

Flat-crush of Corrugated Medium T 809

Flexural Stiffness T 820

PROPERTY TEST METHOD

Fluted-crush Test T 824

Ply Separation T 812

Puncture Resistance T 803

Grammage T 545

Folding Endurance T 512

Edgewise Compressive Strength T 811

Compression Test T 802

Impact Resistance T 801

Vibration Test T 817

Water Resistance T 805

TAPPI TEST METHODS: CORRUGATED

anilox roll, 73-80

banded, 79

cell structure, 5, 43-74, 78

ceramic-coated, 74

laser engraving, 74

maintenance, 79-80

mechanical engraving, 73

selection, 77-79

volumetric carrying capacity, 75

central impression press, 67, 101

chambered doctor blade, 72-73, 74

chroma, 22, 53, 54, 65

color management, 50-51

color measurement, 52-53

colorimeter, 56

color matching, 56-59

densitometer, 55-56

L*a*b, 53-55

L*C*h°, 53-55

spectrophotometer, 56

color theory, 51

color matching theory, 56-57

color tolerancing, 54-55

metamerism, 52

corrugated

board construction, 142-143

caliper, 144

container, 146

flute integrity, 143

substrates, 145

warped, 145

washboarding, 144

corrugated board, 129, 138, 141-146

physical properties, 141-143

dyes, 5, 23, 27, 87

films

polyester, 155-158

polyethylene, 162-166

polypropylene, 158-161

polystyrene, 158-161

polyvinyl chloride (PVC), 155

pressure-sensitive, 150

fountain roll, 13, 64, 68-71

hue, 8, 18, 22, 51, 53-54, 56, 57, 65, 105

ink

additives, 32-34

adhesion, 4, 8, 9, 10, 146, 160, 165

assembly, 61-62

catalytic, 40

characteristics, 34-36, 132

climatic effects, 97-99

coatings and adhesives, 7, 8, 10, 11, 12, 14,

24, 41-42, 165

color, 8, 21-22

colorants, 23

color matching, 22

cost as applied (ink value), 112-114

distribution, 103

drying, 6, 10, 11, 14, 24, 31, 32, 34, 35, 38,

39, 40, 41, 135, 144, 160

dyes, 5, 23, 27, 87

electron-beam cured, 41-42

formulation, 37-39

ink metering, 92, 93, 103, 104, 113

pH, 93-95

control, 73

measurement, 94-95

pigments, 23-29

organic, 25

inorganic, 24, 25-27

fluorescent, 27

metallic, 27

pearlescent, 29

thermochromatic, 29

press-side adjustment, 70, 71

proofing, 49, 59-66, 112

pumps, 34, 46, 48, 68-69, 71, 80-81

resins, 29-31

solvent-based, 5, 6, 36, 39, 40, 42, 43

solvents, 31-32

substrates, 3, 5, 6, 9, 11, 12, 13-20, 132,

133-135, 136-140, 144

systems

dispensing, 48, 49, 63-64

ink-blending, 47, 49, 61, 63-64

ink-distribution, 68-74, 103

ink-metering, 9, 34, 35, 37, 67, 68-71

ink pumps, 44, 80-81

proofing, 49, 165

tolerancing, 64-66

thixotropy, 90, 91

UV-cured, 41-42

viscosity

control, 31-32, 34, 40, 58-59, 67, 88

measurement, 91-92

water-based, 37-39

SUBSTRATES 171

Index

172 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ink appearance, 18

inkroom, 47, 48, 49

equipment, 50

safety, 49

procedures, 49-50

inks

catalytic, 40

electron-beam cured, 41-42

process, 9, 10, 104

solvent-based, 5, 6, 36, 39, 40, 42, 43, 148,

154, 157

UV, 41-42, 146, 149

water-based, 5,6, 37-39, 130, 154, 157

ink test

acid/alkalai resistance, 17

block resistance, 14

boiling water resistance, 17

coefficient of friction, 19

color measurement, 18

crinkle adhesion, 14

fade resistance, 19

gloss, 19

heat resistance, 15

ice-water crinkle test, 16

image detail, 19

lamination adhesion, 14

moisture bleed, 16

moisture vapor transmission resistance, 16

odor, 20

oil resistance, 17

opacity/contrast ratio, 19

plasticizer bleed resistance, 18

print density, 18

rub resistance, 15

scratch resistance, 14

soap and detergent resistance, 17

substrate adhesion, 13

tone quality, 19

transfer resistance, 16

in-line press, 67, 81

laminates, 147-151

lightness, 22, 53, 54, 61, 65

paper

acid, 133

alkaline, 133

chemical properties

fiber content, 132

moisture, 132

pH, 133

sizing, 133

coated, 134, 136

finishes

antique, 136

cast coated, 136

coated one side, 136

eggshell, 136

embossed, 136

embossed coated, 136

enamel coated, 136

felt, 136

laid, 136

machined English, 136

matte coated, 136

supercalendared, 136

manufacture, 125-128

properties

basis weight, 129

bulk, 129

burst, 130

caliper, 130

curl, 130

density, 130

dimensional stability, 130

folding endurance, 130

formation, 130

grain direction, 130

internal bond, 131

porosity, 131

stiffness, 131

stretch, 131

tear, 131

tensile energy absorption, 131

tensile strength, 131

roll length, 135, 150

roll quality, 135

storage/handling, 135

surface appearance

brightness, 131

coefficient of friction, 132

color, 132

gloss, 132

opacity, 132

smoothness, 132

uncoated, 136

paperboard, 128-129, 130, 135, 136, 137-138

pigments, 23-29

inorganic, 25-27

fluorescent, 27

metallic, 27

organic, 25

pearlescent, 29

thermochromatic, 29

polyester (PET), 148, 151, 153, 156, 166, 167

area yield factor

physical properties, 156

printing characteristics, 156

polyethylene, 137, 139, 147, 148-149

additives

anti-blocking, 165

pigments, 165

slip agents, 165

physical properties, 163-165

printing characteristics, 165-166

polypropylene, 147, 149, 158-161

oriented (OPP), 158, 166

physical properties, 158-160

printing characteristics, 160-161

polystyrene, 147, 148, 158-161

polyvinyl chloride (PVC), 147-148, 155-156

physical properties, 156

printing characteristics, 156-158

press approval, 65, 107

press characterization, 77, 104-107

presses

chill rollers, 89

corona discharge, 39, 41, 83, 160, 165

dryers, 82, 84-85, 125

ink system requirements, 47, 48, 50

rewind tension, 88

viscometers, 90, 91

pressure-sensitive labels, 149

release liner, 149-150

process color printing, 10, 103-104, 105-107

process inks, 9, 10, 104

reverse-angle doctor blade, 71-72

saturation, 22, 53, 54

Shell Cup, 91

solvency power, 27, 31

solvent balance, 32, 39, 40

spectrophotometer, 18, 19, 22, 48, 53, 56-57,

61, 63, 65, 105, 108

stack press, 67

substrates

cellophane, 160, 166-167

corrugated board, 137-138, 140

envelope paper, 138

facestocks, 147, 150-151

films, 155-167

polyester, 155-158

polyethylene, 162-166

polypropylene, 158-161

polyvinyl chloride (PVC), 155

pressure-sensitive, 150

foils, 138, 150, 152-154

glassine, 139

label stock, 134, 136, 138, 148

metals, 154

multiwall bags, 138

paper and paperboard, 122, 128, 132, 136

pressure-sensitive, 149

release liner, 149-150

tissue, 140

Zahn cup, 91, 103, 113

SUBSTRATES 173