Critical evaluations of liquid absorption

testing methods for package printing

Li Yang and Sofia Thorman

li.yang@innventia.com

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Topography + absorption print quality

Appropriate and robust measurements

predictable print quality

How to measure the absorption properties ?

– Total absorption

Cobb60 ?

Bristow wheel ?

IGT penetration tester ?

…

– Absorption non-uniformity

Stain technique

Background

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Measurement methods

Two relatively new methods have been

evaluated

– ASA – Automatic Scanning Absorptometer

– ACT – Automatic Cobb Tester

Both methods provide time-resolved absorbency

measurements

– To examine short or long time absorption

characteristics

– Capillary-driven absorption (no external pressure

applied)

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Paper boards

Six pilot coated paper boards

– Different top-coatings on the same pre-coated base

– Similar surface roughness

– But broad absorbency range

Latex

– Latex A: vinyl acetate acrylate (VAA)

– Latex B: styrene butyl acrylate (SBA)

Pigments

– HC90=Hydrocarb® 90

– SCHG=Setacarb® HG

– Clay=Capim NP delaminated clay

– CCN75 = Covercarb® 75, narrow PSD

No. Board

notation

Coating compositions Roughness,

std. dev ,

height [µm]

Contact

angle,

H2O

1 12.5 Latex

A

12.5 pph VAA Latex A,

60 pph HC90 + 40 pph SCHG.

0.86 71.2

2 15 Latex A 15 pph VAA Latex A, Pigments as 1. 0.88 75.2

3 20 Latex A 20 pph VAA Latex A, Pigments as 1. 0.76 80.8

4 15 Latex B 15 pph SBA latex B, Pigments as 1. 0.89 84.4

5 40 Clay 15 pph VAA latex.

Pigments; 60 pph HC90 + 40 clay

0.83 70.3

6 N75 GCC 15 pph VAA latex. 100 pph CCN75 0.87 73.4

- Pre-coating 100 pph Hydrocarb® 60 and 13 pph

of the VAA latex.

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Measurements

Testing liquids

ASA

– 12% (volume) of the condensed blue liquid,

methylene blue, diluted in deionized water.

– 8.6 wt-% n-propanol in order to reduce the

surface tension similar to flexographic inks.

(38 mj/m2)

ACT

– water

Surface roughness

OptiTopo

– height standard deviation

– wavelength interval 0.06-1 mm

PPS

Bendtsen

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Full-scale printing with an in-line flexographic press at Tetra Pak, Lund, Sweden.

Print density of a solid cyan area was measured on five consecutive signatures.

– Spectro-densitometer, SpectroDens (TECHKON GmbH, Königstein, Germany),

calibrated against the board white and with the following settings: D50 illumination, 2º observer, polarization

filter and density filter ISO E.

Print trial

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Results -- ASA

Boards of the same pigments but

different amounts of latex A

– Absorption rate decreases with increasing

latex

Different types of latexes A vs. B leads

to different liquid absorption

– Type B shows a wetting delay

Wetting delay

y = 12,1x + 4,2

y = 9,7x + 4,4

y = 8,3x + 2,7

0

5

10

15

0,0 0,2 0,4 0,6 0,8 1,0

Wate

r A

bso

rpti

on

[m

l/m

²]

Time½ [s½]

12.5 Latex A

15 Latex A

20 Latex A

0

5

10

15

0,0 0,2 0,4 0,6 0,8 1,0

Wate

r A

bso

rpti

on

[m

l/m

²]

Time½ [s½]

15 Latex A

15 Latex B

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Results -- ASA

The porosity of the coatings affect the

liquid absorption

– Board (N75) having the most porous

surface (GCC of narrow PSD)

– Board with 40% clay is less porous

– Board having GCC of broad PSD has the

lowest porosity.

– The boards have similar contact angle

y = 9,7x + 4,4

y = 10,9x + 4,7

y = 13,1x + 5,2

0

2

4

6

8

10

12

14

16

18

0,0 0,2 0,4 0,6 0,8 1,0

15 Latex A

40 Clay

N75 GCC

Wate

r A

bso

rpti

on

[m

l/m

²]

Time [√s]

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Results -- ACT

The measurements show similar long-

term trends as those from ASA

Less reliable in short-term behaviour

– More latex, higher absorption

– Incorrect measurement at t0

– In contradictory to the ASA observations

0

10

20

30

40

50

60

70

0,0 2,0 4,0 6,0 8,0

Wate

r A

bso

rpti

on

[m

l/m

²]

Time½ [s½]

12.5 Latex A

15 Latex A

20 Latex A

0

2

4

6

8

10

12

14

0,0 0,2 0,4 0,6 0,8 1,0

Wate

r A

bso

rpti

on

[m

l/m

²]

Time½ [s½]

12.5 Latex A

15 Latex A

20 Latex A

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Results -- ACT

The measurements show similar long-

term trends as those from ASA

– For short term, it is less obvious with less

information

No wetting delay observed

0

10

20

30

40

50

60

70

0,0 2,0 4,0 6,0 8,0

Wate

r A

bso

rpti

on

[m

l/m

²]

Time½ [s½]

15 Latex A

15 Latex B

0

2

4

6

8

10

12

14

0,0 0,2 0,4 0,6 0,8 1,0

Wate

r A

bso

rpti

on

[m

l/m

²]

Time½ [s½]

15 Latex A

15 Latex B

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At t=0, the liquid only fills the surface

roughness.

The intercept with the y-axis (t=0) is

proportional to surface roughness

Surface roughness vs. absorption at t0

0

5

10

15

0,0 0,5 1,0

Wate

r A

bso

rpti

on

[m

l/m

²]

Time½ [s½]

15 Latex A 15 Latex A

R² = 0,01

R² = 0,73

-2,0

0,0

2,0

4,0

6,0

0,7 0,8 0,9

Inte

rcep

t y-a

xis

[m

l/m

²]

Surf. rough. Std. Dev [µm], Optitopo (0.06-1mm)

ACT ASA

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Absorption rates

– Both methods reveal similar long-term trends

The absorption rates of ASA measurements are generally

proportional to those of ACT

– the board 15 Latex B is an exception

Their short-term observations differs significantly

– t< 1.0 s

– ASA’s measurements is probably more reliable

makes more sense as it reveal surface topography

Comparison between ASA and ACT measurements

0

5

10

15

12.5 15 20 15 40 N75

Latex A Latex B Clay GCC

Ab

so

rpti

on

ra

te [

ml/

m²s

½]

ASA ACT

0

5

10

15

20

25

0.0 1.0 2.0 3.0 4.0

Wa

ter

Ab

so

rpti

on

[m

l/m

²]

Time½ [s½]

ASA

ACT

15 pph Latex B

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There are a clear correlation between the

absorption rates of the boards and the

respective print densities.

Print density vs. absorption rates

R² = 0.38 R² = 0.71

1.3

1.4

1.5

1.6

0 5 10 15

Pri

nt

den

sit

y

Absorption rate [ml/m²s½]

ACT ASA

15 LatexB

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Being capable of predicting print quality from unprinted surfaces (paper/board) will help the

producers to

– supply substrates with properties demanded by converters and end-users.

– develop products of preferred printing surfaces.

– increase production efficiency and effectiveness by reliable in-house quality control.

Industrial applications

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KRK KUMAGAI RIKI KOGYO Co., Ltd. and FIBRO system AB for supporting with

measurements and allowing us to use their equipment.

BillerudKorsnäs, Stora Enso and Tetra Pak for financial support.

The Research Institute of Sweden (RISE) and the Knowledge Foundation via VIPP Industrial

Research College – Values Created in Fiber Based Processes and Products – at Karlstad

University, Sweden, are also acknowledged.

Acknowledgements