Paper surfaces in lithography/offset printing paper... · FPIRC, Paper Surfaces, Offset printing...

Paper surfaces in lithography/offset printing

Göran Ström

Innventia

Offset lithography

FPIRC, Paper Surfaces, Offset printing

Offset Paper

Ink transport

rollers

Ink

Offset

cylinder

Roller with

print form

Water


Content

Introduction - ink transfer - ink splitting - filament pattern - ink levelling. Paper properties, i.e. coating pore structure Ink composition Absorption of ink oils – Ink setting - importance of coating properties - importance of ink properties Print quality, impact of coating structure


Ink transfer, splitting and levelling Ink splitting in nip, image size: 5 x 4 mm


Tentative illustration of the process

2 mm

1. Formation of filament 2. Ink setting (<15 min) 2. Ink levelling (<15 min) 3. Ink drying (>hours )

0.2 s after printing 30 s after printing


0.1 s 2.3 s

6.0 s 19.8 s

Ink levelling


0

0,1

0,2

0,3

0,4

0 50 100 150Pore diameter (nm)

Po

re v

olu

me

(cm

3/m

2)2 µm Top view

SEM image

20 µm

50µm

Pore size distribution


Amount of wet ink (black) on various papers after full scale sheet-fed offset printing

0

0,5

1

1,5

2

2,5

3

0 1 2 3 4 5

Number of Full Tone Layers

Ink A

mo

un

t, g

/m2

.

Matte Silk 1Silk 2 Gloss


Cross section of an ink film (400 %) after full scale printing

2 µm2 µm


Fibre

Ink layer

1 µm


Ink film on coated fine paper 400% print

silk

matt


Ink film thickness distribution 400 % print

Source: Liisa Granat, 2002, diploma work

0

2

4

6

8

10

12

0 1 2Ink Film Thickness, µm

Fre

quency

, %

. Gloss

Silk

Matt

0

2

4

6

8

10

12

0 1 2Ink Film Thickness, µm

Fre

quency

, %

. Gloss

Silk

Matt


Printing ink composition Example: sheet-fed offset

Main components

Pigment 10-20%

Binder (hard resin alkyd resin)

20-40%

Mineral or vegetable oil (carrier phase)

30-50%

Additives: 0-10% Fillers, Antioxidants, Siccatives, Waxes, Rheology modifiers


Ink oil absorption

Ink

Coating

Base paper


Ink drying

Oil absorption by the coating,

leaving binder and pigment on top of the paper. Starts promptly after ink transfer.

This is the physical drying. And for most inks also by…….

Polymerization of the alkyd resin.

Starts several hours after ink transfer.

This is the chemical drying.

Ink setting is the initial

stage of physical drying. Ink setting takes roughly 1 to 20 minutes. The print is touch dry after ink setting but not completely dry. We divide ink setting into: - Initial ink setting rate and - Ink setting time.

The ink dries through:


Drying stages of an offset ink film

Touch-dry. After ink setting.

Semi-dry. After the physical drying is completed.

Fully dry. After the oxidative drying is completed.


Composition in ink film during physical drying

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16 18 20 22

Time, min

Rem

ain

ing

Oil

in Ink

Film

, %

. .

M-E

TG

0

20

40

60

80

100

120

0 2,5 min 20 min 8 h

Ink-paper contact time

Ink layerc

om

po

sit

ion

(w

t.-%

)

Other

Ink oil

Binder

Pigment

0

20

40

60

80

100

120

0 2,5 min 20 min 8 h

Ink-paper contact time

Ink layerc

om

po

sit

ion

(w

t.-%

)

Other

Ink oil

Binder

Pigment

Vegetable-oil ink with a mixture of M-E=Mono-ester and TG=Triglyceride


Why do we have to care about ink setting

Too fast ink setting

• Reduced print gloss

• Ink piling

• Back-trap mottling

• Delamination (board)

Too slow ink setting

• Slow drying

• Set-off

• Smearing

• Blocking


Methods to measure ink setting

100 µm

Force measurement

Tack development of ink after printing (ISIT,

Paper and Ink Stability Tester, Splitting Force Meter, Deltack)

Optical colour density Measuring set-off density after printing


Measurement of ink setting using ISIT Ink Surface Interaction Tester

1

2


0

1

2

3

4

5

6

7

8

9

0 50 100 150

Time, s

Ta

ck F

orc

e,

N

Quick setting

Slow setting

Force-time curve gives setting properties

M Karathanasis, STFI

Ink

Measurement of ”tack force”

Paper

F

Pull-off

wheel

Ink

Ink setting (Ink Surface Interaction Tester)


Ink amount influence the curve

0

1

2

3

4

5

6

7

8

9

0 100 200 300 400 500 600

Time [s]

Ta

ck

Fo

rce

[N

]

2.0 g/m2

1.4 g/m2


How to evaluate ISIT data Same paper but different inks

0

1

2

3

4

5

6

7

8

9

0 500 1000

Time [s]

Ta

ck

Fo

rce

[N

]

NovaGloss FG 966 TopLith


0

1

2

3

4

5

6

7

8

0 50 100 150

Time, s

Tack

Forc

e, N

.

How to evaluate ISIT data

Slope: dF/dt [N/s]

Time to max tack [s]

Time to 5N [s]

Ink setting:

Ink setting rate is quantified by the initial slope.

Ink setting time (Touch-dry) is quantified by the time at a certain tack force on the decay side of the curve e.g. 5N.


Ink setting - driving forces

r

osc2yΔP

(Laplaces equation) (Hagen-Poisuelle equation)

8lη

ΔPr

dt

dl 2

Coating pore structure -Coating porosity -Pore size -Surface pore density Coating composition i.e latex type and content Ink composition -Surface tension -Viscosity

Driving force: Capillary pressure Setting resistance: Vicous drag


Ink/latex interaction Ink setting on Mylar film and Mylar film coated with latex

0

1

2

3

4

5

6

7

8

9

0 200 400 600

Time [s]

Ta

ck

Fo

rce

[N

]

Mylar film

Mylar film + latex


Paper surface properties that influences ink setting

Ink setting is determined by the:

Pore structure of the coating and

Diffusion of oil into the latex binder.

Question: How important is the pore structure? How important is the latex? or What is the relative importance of the pore structure and the latex? for Initial ink setting rate and ink setting time.


Coating composition in vol.-% when the latex content was increased (carbonate coating)

Porous region:

Pigment volume is almost constant. Air is exchanged for latex when latex content is increased.

Non-porous region. Pigment is exchanged for latex

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90 100 110

Latex Content, pph

Vo

lum

e %

Air

Pigment

Latex

Non-porousPorous


A close-up of the porous region

Observation: Increasing latex content decreases porosity and pore size.

0

10

20

30

40

50

60

70

6 8 10 12 14 16 18 20 22 24 26

Latex Content, pph

Vo

lum

e,

% AirPigmentLatex

0

10

20

30

40

50

60

70

80

6 8 101214161820222426

Latex Content, pph

Po

re R

ad

ius,

nm


Examples of ISIT curves

The initial slope gives the initial ink setting rate. Setting time is taken at 5, 4 and 3 N on the decay part of the curve

R2 = 0,98

R2 = 0,96

3

4

5

6

7

8

0 2 4 6 8 10 12

Time, s

Tack F

orc

e, N

.

v-ink

m-ink

0

2

4

6

8

10

0 50 100 150 200

Time, s

Tack F

orc

e, N

.

v-inkm-ink


Comparison between set-off test for touch dry ink film and a certain time on the decay side of the ISIT curve

Conclusion: Time to touch dry correlates well with time to

reach 5N on the decay side of the ISIT curve

m-ink

0

100

200

300

400

500

600

0 5 10 15 20 25

Latex Content, pph

Settin

g T

ime, s

Time to 3 N

Time to 4 N

Time to 5 N

Touch dry

v-ink

0

100

200

300

400

500

600

0 5 10 15 20 25

Latex Content, pphS

ettin

g T

ime

, s

Tim e to 3 N

Time to 4 N

Time to 5 N

Touch dry


0,0

0,1

0,2

0,3

0,4

0 5 10 15 20 25 30

Porosity, %

Initia

l S

lope,N

/s

m-ink

v-ink

Influence of latex content and porosity on initial ink setting rate

Conclusion: Voids have a very strong impact on initial ink setting

rate, while latex content has virtually no impact.

0

0,0

0,1

0,2

0,3

0,4

0 10 20 30 40 50 60 70 80 90 10

0Latex Content, vol.-%

Initi

al S

lope, N

/s

m-ink p

v-ink p

m-ink n-p

v-ink n-p


Conclusion: Both voids and latex have an impact on ink setting

time, but voids have a stronger impact.

Influence of latex content and porosity on ink setting time

0

500

1000

1500

2000

0 10 20 30 40 50 60 70 80 90 10

0Latex Content, vol.-%

Tim

e to 5

N,s

m-ink p v-ink n-p m-ink n-p v-ink n-p

0

0

200

400

600

800

1000

0 5 10 15 20 25 30

Porosity, %

Tim

e to

5 N

,s

m-ink v-ink


It is well established that ink setting rate increased with - increased porosity - decreased pore size

But first a few slides on the structure

Alright, now we know a little bit about latex verses voids, but what about porosity and pore size?


Effect of blending two GCC on porosity and pore size

10

15

20

25

30

35

40

0 20 40 60 80 100

HC 90, pph .

Poro

sity,

%

.

30

40

50

60

70

Pore

radiu

s,

nm

.

Porosity

Pore Radius

ZetaCarb and HC90 11 parts DL 920


Broad size distribution Narrow size distribution gives higher porosity and better coverage

Pigment packing Particle size distribution and effect on porosity


Mercury intrusion technique for pore structure characterization: coatings laid on plastic films. SB(13)

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0 5 10 15 20 25

Latex Amount, pph

Pore

Radiu

s, µ

m

Ngcc75 Bgcc90 Bgcc98

10

15

20

25

30

35

40

0 5 10 15 20 25

Latex Amount, pph

Poro

sity

, %

Ngcc75

Bgcc90

Bgcc98

Higher latex amount gives lower porosity

Narrow particle size distribution gives higher porosity

Finer pigments gives smaller pores


Both pore radius and porosity are changed

(Increased amount of latex)

Ink setting vs. pore structure

Source: Karathanasis, 2005 unpublished

R2 = 0,97

0

50

100

150

200

250

300

350

400

10 15 20 25 30

Porosity, %

Ink

Sett

ing T

ime, s

.

Constant porosity: 26-27 % (Different pigment blends)

R2 = 0,99

0

50

100

150

200

250

300

350

400

30 40 50 60 70

Pore Radius, nm

Ink

Settin

g T

ime, s

.


Effect of ink properties: Ink setting rate increases with the ratio Surface tension/viscosity

r

osc2yΔP

8lη

ΔPr

dt

dl 2


Summary: Effect of pore size and ink properties on setting

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0 1 2 3 4 5

Ink S

ett

ing

(S

lop

e),

N/s

.

Surface tension/viscosity, m/s

C60 & SB (+22)

C90 & SB (+22)

Sanna Rousu et al


Ink setting Fast separation of ink oils to the coating (pigment and binder stays on the surface) Measured with different techniques Takes 1-20 minutes, 4-8 preferably About half of the oil has left the ink film The material can be handled

Ink setting depends on Surface pore structure: Setting rate increases with decreased pore size,

increased porosity and increased pore density.

Latex binder in the coating. Diffusion of oil into the latex

Ink oil viscosity and surface tension (/)

Summary


Chemical drying

An oxygen induced polymerisation of drying oils. Also referred to as auto oxidation. Three steps: 1 Formation of hydroperoxides. 2 Decomposition of hydroperoxides into free radicals. 3 Polymerisation.


The reaction rate depends on: • Access to oxygen. • Type of oil, number of double bonds. • Catalytic driers (sicatives) are soaps of cobalt (Co)and manganese (Mn). • Antioxidants (e.g. BHT and HQ).

OH

OH

OH

C(CH3)3(CH3)3C

CH3

Butylated Hydroxy Toluene ( BHT )

Hydroquinone (HQ)


Equipment for characterizing ink drying

Ink drying 400 % Uncoated paper Pressure 5 kg Elrepho / visual


Ink drying curves from a rub-off test after full scale printing

0,0

10,0

20,0

30,0

40,0

50,0

60,0

70,0

80,0

0 10 20 30 40 50 60

Drying time, h

Ru

b-o

ff x

10

00

, %

Gloss

Silk

Full scale printing, 400 %

Chemical

drying

Physical

drying

Chemical drying starts long after printing.


40°C 6°C 23°C

0

10

20

30

40

50

60

70

80

90

0 100 200 300 400 500 600

Drying Time, h

Ru

b-o

ff (

x1000)

Matte coated

Ink drying is strongly improved by increased temperature

Source: Skogbergs, Diploma work, 2001 Ström et al: Tappi Coating Fundamentals 2003

Ink drying - summary

Physical drying: Absorption of ink oil

by the substrate. Ink setting is the initial

state of this process.

Depends on:

• Ink properties: viscosity and surface tension

• Substrate properties: structure, binder type

and content.

Chemical drying: Polymerization of

drying oils (alkyds and triglycerides)

Depends on:

• Ink chemical composition: type of binder,

siccatives, antioxidants.

• Access to oxygen and temperature


Main components

Content (%)

Pigment 10-20

Binder (hard resin, alkyd resin, triglycerides)

20-40

Carrier (mineral and/or vegetable oil, triglycerides)

30-50


Print quality

Print gloss – strongly influenced by the

topography of the ink layer, which is

controlled by coating topography and ink

setting rate.

Variation in print gloss

Print density

Variation in print density – Print mottle


40

50

60

70

80

90

100

0 1 2 3 4

Surface roughness (PPS), µm

Prin

t g

loss,

%

Matt

Silk

Gloss

The surface topography

is important

40

50

60

70

80

90

100

0 20 40 60 80 100

Paper gloss, %

Print

glo

ss,

%

Matt

Silk

Gloss

Higher paper gloss normally

gives higher print gloss - but


Micro roughness depends on pigment (size, shape, aggregation)

Macro roughness depends on base paper structure

• The ink gradually fills up the

micro structure and gloss increases • Micro roughness can be completely covered but not macro roughness

Paper roughness. Micro and macro roughness


Macro- roughness

Unprinted paper

Micro- roughness

Printed (4x100%)

Ink fills up

micro

roughness

But not

macro

roughness


Print gloss as a function of ink amount after offset printing on coated paper

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3

Ink Amount, g/m2

Paper

and P

rint

Glo

ss,

%

Gloss Silk 1

Silk 2 Matt

Print gloss on commercial papers, increases with coating smoothness and ink amount

Macro roughness increase in the order: Gloss<Silk<Matt


Cross sections of prints with 3 g ink/m2

Smooth substrate

Micro rough substrate

Paper surface structure has impact on ink surface structure


Effect of ink amount and micro roughness on print gloss for macro smooth coatings

0

20

40

60

80

100

0 1 2 3 4 5

Ink Amount, g/m2

Pri

nt G

loss, %

Base

Fine

Coarse

Mineral-oil based ink for sheet-fed offset

Ström, Karathanasis, 2007 TAPPI Coating and Graphic Arts Conference

Smooth substrate (base)

Micro rough substrate (coarse)


Print gloss is also influenced by ink setting rate.

1. Formation of filament 2. Ink setting (<15 min)

2 mm

0.2 s after printing 30 s after printing


30

35

40

45

50

55

60

65

70

75

80

0 50 100 150

Time [s]

Pri

nt G

loss [%

]

Quick setting

Slow setting

Karathanasis, STFI 2001

Print gloss increases with time after print

nip due to ink levelling

Slow setting Low porosity

Fast setting High porosity

100 µm

An ink layer (1.2 g/m2) 1 week after printing


Slower setting (higher setting time) gives higher print gloss due to improved ink leveling

Uncalendered papers

35

40

45

50

55

100 150 200 250 300

Ink Setting Time, s

Pri

nt G

loss, %

1 gsm

2 gsm

Calendered papers

70

75

80

85

100 150 200 250 300 350 400

Ink Setting Time, s

Pri

nt G

loss, %

1 gsm2 gsm

Ström et al. Coating Fundamentals Conference, 2010


Papers are not uniform


1 µm

Bgcc90

10 pph SB(13)


1 µm


Print Gloss Variation

20

21

22

23

24

25

26

0 10 20 30 40 50

Clay Content, pph

Pri

nt G

loss V

ari

atio

n, %

Uncal Cal

Macro and micro roughness

Before calendering

After calendering Smooth and

compact region

5 µm

5 µm5 µm



Print density is higher on a smooth coating

0,0

0,5

1,0

1,5

2,0

2,5

0 1 2 3Ink Amount, g/m2

Dry

Pri

nt D

en

sity

3 uncal 2 uncal

6 uncal 2 cal

3 cal 6 cal

1 cal 1 uncal

Different pilot coated paper

with carbonate, clay and

SB latex. Lab printed



Print mottle – a serious print quality defect

Several causes. The most common are:

Back trap (BTM):

Ink is lifted off from the

print in one nip and

transferred to the paper in

a subsequent nip

Water interference (WIM):

To much water on the

paper surface which

results in in refusal.


Back trap mottle is due to non-uniformity in ink setting (oil absorption).

Non-uniformity in oil absorption is due to non-uniformity in coating layer composition, e.g. non-uniformity in coating structure.

Non-uniformity in coating structure can be due variation in coating thickness but can also be closed area induced by calendering

Back trap mottle


Print mottle vs. coating thickness variation

Source: Kolseth, Sävborg, 2004


BTM vs. non-uniformity in closed area

Source: Chinga, Helle, 2003


Water interference mottle in offset

Important factors.

Poor interaction between paper and fount.

Slow and uneven absorption of fount by the coating which

in turn is due to low and uneven porosity.

High fountain feed and polar paper surface.

Appearance - White spots are formed due to poor ink transfer (ink refusal).

- Raster dots become non-uniform.


GCC and 9 pph latex, uncalendered

High print impression: 0.25 mm

Example with half-tone print Dot uniformity is important for print quality Dot size 130x130 µm

Low print mottle, 1.15% High print mottle, 2.8%

GCC and 13 pph latex, uncalendered

Low print impression: 0.10 mm


White spots are due to ink refusal and/or ink adhesion failure (ink-lift-off)

Ink refusal means that ink is not transferred to the paper surface.

Ink refusal can be characterized by lab printing on a dry and pre-wetted surface. Ink refusal = Litho density/Dry density

Ink-lift-off: Ink is transferred to the paper but due to low adhesion to the coating, the ink is lifted off in subsequent print unit.

Raster dots with white spots

Ink

water

paper

Ink

water

paper


Characterization of dot non-uniformity

White

spots

Black

spots

Perimeter increase


R2 = 0,94

0

0,5

1

1,5

2

2,5

3

0 20 40 60 80 100

Print M

ottle

, C

oV

, %

Perimeter Increase, %

Good correlation to spot area and perimeter increase!

Mean value of 5 sub-areas

Ström and Madstedt in Advances in Printing and Media Technology vol. XXXVI, 2009

C5 halftone

R2 = 0,96

0

0,5

1

1,5

2

2,5

3

20 25 30 35 40 45

Area of Spots, area-%

Pri

nt

Mott

le,

CoV

% W+B SpotsVery high Not OK Low

Paper surfaces in lithography/offset printing paper... · FPIRC, Paper Surfaces, Offset printing...

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Transcript of Paper surfaces in lithography/offset printing paper... · FPIRC, Paper Surfaces, Offset printing...