Ink receptivity on paper — characterization of paper materials

Colloids and Surfaces A: Physicochemical and Engineering Aspects 168 (2000) 251 – 259 www.elsevier.nl/locate/colsurfa

Ink receptivity on paper — characterization of paper materials Bohuslava Havlı´nova´ a,*, L’udmila Hornˇa´kova´ b, Vlasta Brezova´ a, Zuzana Lipta´kova´ a, Juraj Kindernay a, Viera Jancˇovicˇova´ a a

Faculty of Chemical Technology, Slo6ak Technical Uni6ersity, Radlinske´ho 9, SK-812 37 Bratisla6a, Slo6ak Republic b Pulp and Paper Research Institute, Lamacˇska´ cesta 3, SK-815 20 Bratisla6a, Slo6ak Republic Received 12 October 1999; accepted 15 February 2000

Abstract The rheological behaviour of three black offset inks was tested using viscometer with the cone and plate geometry. The shear stress versus shear rate measurements confirmed that the investigated offset inks represent similar viscoelastic, pseudoplastic fluids, forming thixotropic structure. Mileage and ink transfer curves were measured onto the paper materials with different properties. The analysis of experimental results using Tollenaar – Ernst and modified Walker–Fetsko equations established the substantial role of the paper characteristics such as smoothness, brightness and penetration in the process of ink film formation on paper substrates during printing. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Offset inks; Rheology; Paper characterization; Ink transfer; Paper – ink interaction

1. Introduction The rate, quality and economics of printing process is tightly linked to the characteristics of printing technology and materials used. The information on the interaction between ink and paper, as well as the laboratory investigations of typography materials properties, provides a better understanding of the printing procedure [1,2]. Since the behaviour of ink at various stages of printing operation can be estimated using rheological mea* Corresponding author. E-mail address: [email protected] (B. Havlı´nova´)

surements, the study of ink rheology was usually performed in attempt to predict the ink performance on the press [3–7]. The formation of ink film with the desired optical density on paper substrates is consequentially dependent on the ink receptivity by paper, which is determined by structure and optical properties of paper surface. The measurements of ink mileage and transfer curves on paper are necessary for the adequate evaluation of interaction between ink and paper [8–10]. The main aim of our study was to obtain the rheological characteristics of different offset printing inks, and to find the relation between ink

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transfer parameters and properties of paper materials. 2. Experimental

2.1. Materials The investigations were performed using following offset printing inks: RAPIDA SCHWARZ 7000 49 (Michael Huber, Germany), black ink for the standard offset sheet printing; UNILASER HMP 95 (Coates Lorilleaux, France), new generation ecological black ink for sheet and rotary printing; IGT black offset ink recommended for print tester (IGT, Netherlands). The uncoated and coated paper substrates were used in our study: offset carton (OC), wood-free uncoated (SCP Ruzˇomberok, Slovak Republic); offset paper (OP), wood-free uncoated (SCP Ruzˇomberok, Slovak Republic); SUPRAPRINT (SP), uncoated (SCP Ruzˇomberok, Slovak Republic); SUPRAART 2/M (SA 2/M), coated (SCP Ruzˇomberok, Slovak Republic); SUPRAART 2/ LH (SA 2/LH), coated (SCP Ruzˇomberok, Slovak Republic) and LUMIART (LA) (Finland). The deionized water was used for the preparation of the ink emulsions.

The transfer of the offset inks onto the paper substrates was determined using IGT AIC 2 print tester (IGT, Netherlands). The optical density of the prepared ink films was measured after 24 h applying reflection densitometer X-Rite 408 (USA). The rheological behaviour of inks and inks emulsions was investigated by means of Viscotester VT500 (Haake Mess-Technik, Germany) with the cone and plate geometry. A 28 mm, 0.5° cone was applied to the study of the flow behaviour. The offset inks were continuously stirred 20 min at 300 rpm before measurements. The ink–water emulsions (10 and 20% of water) were prepared by the addition of calculated amount of water, and the mixtures were stirred 20 min at 300 rpm before measurements. The standard volume of sample (0.130 ml) was placed symmetrically to the centre of plate. The shear stress was measured during the controlled applied shear rate increasing from 0 s − 1 up to 240 s − 1 in 4 min. The thixotropy of ink samples was monitored increasing the shear rate from 0 s − 1 up to 240 s − 1 in 2 min, and immediately the decrease from 240 s − 1 down to 0 s − 1 in 2 min was applied. The measurements were repeated three times with new sampling at the constant temperature of 25°C.

2.2. Apparatus 3. Results and discussion The characteristic properties of the paper substrates were measured according to the STN and STN ISO standards using the following instruments: automatically-operated micrometer, automatic analytical balance (Sartorius, precision of 0.001 g), device for the water absorbency determination according to Cobb60, universal apparatus INSTRON 1011 (England), instrument for the smoothness evaluation according to Beek (Bu¨chelVan der Korput, Netherlands), and filter photometer ELREPHOMAT DFC-5 used in paper brightness measurements. pH values of paper substrates were determined on WTW pH meter using a combined glass electrode at 25°C. In accordance with the STN ISO 187 standard the paper samples were air-conditioned before measurements, which were carried out under the same conditions.

3.1. Characterization of paper substrates Tables 1 and 2 summarize the evaluated mean values (calculated from ten measurements) of properties for paper materials used in the study. The results obtained were explored as the basis for discussion about the efficiency of ink transfer onto the paper substrates.

3.2. Rheological beha6iour of printing inks The shear stress (t) versus shear rate (D) flow curves measured for three different offset inks are depicted in Fig. 1. Nearly comparable behaviour characterized for viscoelastic fluids was observed for all ink systems under study. The inks were creeping out of the measuring gap during rheolog-

B. Ha6lı´no6a´ et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 168 (2000) 251–259

ical measurements, and the shear fracture was observed. The analogous phenomenon was studied previously for various printing inks by Chou [5]. Additionally, the thixotropy effect was measured for the original inks and their emulsions (Fig. 2a). The hysteresis loop area decrease with the increasing water content in the ink emulsions was observed (Fig. 2b). Considering the viscoelastic properties of the inks, the analysis of the shear stress versus shear rate dependencies is limited to shear rate range of DD =Dmax − Dmin (Fig. 1). The experimental data (shear rate in the range from 34 s − 1 up to 93 s − 1) were fitted by the Ostwald, Casson and Cross flow equations [11,12], using least square analysis (programme Scientist, MicroMath). We test all the regression models on the base of the statistical parameters of the fittings. The results of the numerical evaluations along with the R 2 of the fittings for three original offset inks are summarized in Table 3. The high values of R 2 (R 2 ] 0.997) confirmed that we achieved good agreement of the experimental and calculated data for all three flow models (Table 3). The parameters calculated reflected the similar pseudoplastic flow behaviour (b B1) for the investigated inks. The highest values of parameters A, h and h0 were evaluated for UNILASER ink. The small values of Cross parameter a imply a relatively minor shear dependent contribution to the structural breakdown in the ink samples [12].

3.3. Ink transfer onto the paper substrates The formation of the ink film onto the paper substrates by printing process is substantially infl-

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uenced by paper-ink interactions. Consequently, the observed optical density of the ink films, mileage curve, incorporates simultaneously the properties of ink and paper substrates [9].The dependence of optical density, OD, on the ink concentration on the paper, y, may be described by the Tollenaar–Ernst equation [13]: OD= OD (1−exp(−m y))

(1)

where OD is saturation optical density, m is a parameter dependent on the ink pigment distribution and onto the paper characteristics [9,13,14]. Fig. 3 shows the density curves measured for IGT black ink transfer on both sides of OC, SP and SA 2/M papers along with the results of calculations obtained by non-linear least squares analysis using Tollenaar–Ernst equation (Eq. (1)). IGT offset ink was chosen as the standard ink for optical density measurements. The differences in properties of top side and underside of paper materials (Table 2) are reflected in the mileage curve profile (Fig. 3). The comparison of the density dependence versus ink concentration on paper for LA paper using IGT and RAPIDA inks is illustrated in Fig. 4. Here it is clearly demonstrated that the application of the same ink concentration resulted in the higher values of optical density for RAPIDA ink. Therefore, the saturation density is determined predominantly by ink properties, as was stated before by Chou [9]. The measurements with UNILASER ink were also performed, however, the results were strongly disturbed by very short ink drying time limit during the experiments using IGT AIC 2 tester.

Table 1 The mean values of thickness, grammage, specific weight and surface pH for the paper materials used in the study along with the standard specification used for the evaluation Property

Paper material OC

Thickness (mm) Grammage (g m−2) Specific weight (g cm−3) Surface pH (25°C)

249 200 0.81 8.5

OP 176 138 0.79 8.8

Standard SP 146 118 0.81 9.1

SA 2/M 152 136 0.90 9.2

SA 2/LH 82 98 1.19 9.3

LA 96 128 1.34 8.3

STN STN STN STN

ISO 534 ISO 536 ISO 534 50 0374

254

Property

Paper material OC

Smoothness (s)

Standard OP

aa

b

25.2

24.1

SP b

a 30.

21.1 94.2

Penetration (mm

)

Surface strength of paper (mm s−1) a

a, top side; b, underside.

23.1

3320

3590

a

b

15.5

14.4

28.7

28.6

STN ISO 535

93.

93.6

93.6

91.6

91.7

88.8

89.0

STN ISO 2470

19. 13. 13. 19. 1 2 8 2 2590 4200 4580 3130

9.4

9.4

7.1

7.2

6.7

6.5

22. 3

95.

19.

1596

STN ISO 5627

3 93.

5

1626

9

3 95.

6

1234

b

21.1

7

1284

a

25.3

93.9

21

b

18.

23.

32.

a

61.9

4 −1

b

LA

66.2

20.5

11. 7

4 Brightness (%)

a

SA 2/LH

27.

1 Water absorbency by surface (g m−2)

SA 2/M

4

1710

1310

2106

1700

1940

1830

PN 50 0320 STN 50 0368

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Table 2 The mean values of smoothness, water absorbency by surface, brightness, penetration and surface paper strength determined for the paper materials used in the study along with the standard specification used for the evaluation

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predominantly by the paper characteristics and the pigment concentration. The paper smoothness significantly influences the formation of the ink film on paper surface and the process of saturation density achievement. According to our results the exponential dependence for m parameter on paper smoothness was evaluated (Fig. 5a), and consequently, the highest density slopes from the mileage curves were measured for the coated papers SA 2/LH and LA. The monitored optical density of the ink films is related also to the paper brightness. The increase in the paper brightness linearly lowered m parameter, as is depicted in Fig. 5(b). The penetration of paper materials characterizes the pores and capillaries in the papers and plays a dominate role in the procedure of ink film

Fig. 1. The shear stress versus shear rate dependencies measured for the original black offset inks (t =25°C): ( ) RAPIDA 7000 49; () UNILASER HMP 95; ( ) IGT.

All experimental mileage curves were successfully fitted by Tollenaar – Ernst equation (R 2 ] 0.998), and the parameters OD and m were evaluated. Parameter m represents the rate at which the saturation density of ink film on paper substrate is achieved, and its value is determined

Fig. 2. (a) The thixotropic flow curve of original UNILASER HMP 95 offset ink (t =25°C). (b) The dependence of the thixotropy hysteresis loop area on the water content in the ink emulsions: ( ) RAPIDA 7000 49; () UNILASER HMP 95.

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Table 3 The parameters of Ostwald, Casson and Cross models along with the R 2 values of the fitting calculated from the flow curves of the original offset inks (shear rate from 34 s−1 up to 93 s−1) Ostwald model Offset ink RAPIDA UNILASER IGT

A (Pa s)

R2

b

220 240 226

0.57 0.59 0.51

t0 (Pa)

h (Pa s)

0.997 0.998 0.999

Casson model Offset ink RAPIDA UNILASER IGT

440 450 430

12 15 8

R2 0.997 0.998 0.998

Cross model Offset ink RAPIDA UNILASER IGT

h0 (Pa s) 67 91 61

h (Pa s) 1 1 1

a (s2/3)

R2

0.05 0.07 0.07

0.998 0.998 0.997

Fig. 3. Ink mileage curves measured using IGT black offset ink onto the paper substrates: ( ) OC top side; ( ) OC underside; ( ) SP top side; () SP underside; () SA 2/M top side; () SA 2/M underside. The illustrated simulated mileage curves were calculated using Tollenaar – Ernst equation (Eq. (1)).

formation on paper surface. In our study we observed the consequential influence of paper penetration on the saturation density, as well as on m parameter (Fig. 6a and b). The low penetration of coated papers (Table 2, Fig. 6a) is reflected in the high values of m parameter. The highest values of saturation density were obtained for coated SA 2/M paper, characterized by high values of smoothness, brightness and low penetration. The analogous results were previously obtained in the paper testing study by Gebrtova´ et al. [15]. The process of ink transfer from the plate to paper substrates can be described by S-shaped ink transfer curve, as is shown in Fig. 7. Walker–Fetsko equation can be applied for the mathematical declaration of the transfer curve [16]: y=AW – F[bW – F BW – F +fW – F(x − bW – F BW – F)] (2) AW – F =1− exp( − kW – F x)

(3)

Fig. 4. Ink mileage curves measured using IGT and RAPIDA 7000 49 inks on LA paper: RAPIDA 7000 49 on top side ( ) and underside ( ); IGT on top side ( ) and underside (). The illustrated simulated mileage curves were calculated using Tollenaar – Ernst equation (Eq. (1)).

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ink concentration on plate, as proposed by Zang equation [17]: fW – F = fn + (0.5− fn )exp(− 2fn x)

(5)

where fn is the limiting fW – F value for high ink concentrations on plate [17]. Applying this modified Walker–Fetsko equation for the experimental transfer curves fitting we obtained the excellent agreement of the experimental and calculated data (Fig. 7, Table 4). We found that the values of parameters kW – F and bW – F correlated with paper penetration (Fig. 8a and b). If the values of paper penetration are low, then the printing smoothness parameter kW – F is high. On the other hand, coated papers with small penetration are characterized by very low immobilization capacity, bW – F.

Fig. 5. The dependence of m parameter evaluated from Tollenaar – Ernst equation (Eq. (1)) (using IGT ink on the different paper materials) on (a) paper smoothness, (b) paper brightness.

BW – F =1− exp( − x/bW – F)

(4)

where x is the ink concentration on plate before transfer, y is the ink concentration transferred to the paper, kW – F is a constant related to the printing smoothness of paper [9], bW – F is the immobilization capacity of paper under given printing conditions and fW – F is the splitting coefficient (fraction of the free ink transferred to the paper [9]), AW – F is the contact factor and BW – F is the immobilization factor. The experimental transfer curves were obtained using IGT black ink, and were fitted by the Walker–Fetsko equation using the non-linear least squares analysis. However, the values of evaluated constants kW – F, bW – F and fW – F were unreal. Consequently, we incorporated to the evaluation the dependence of fW – F parameter on

Fig. 6. The dependence of m parameter (a) and saturation density (b) evaluated from Tollenaar – Ernst equation (Eq. (1)) on penetration of paper materials (using IGT ink on the different paper materials).

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Fig. 7. The ink transfer curves obtained by IGT ink transfer on OC paper ( , top side; , underside) along with the calculated transfer curves evaluated using modified Walker – Fetsko equation (Eqs. (2) and (5)).

4. Conclusions The investigated offset inks represent viscoelastic, pseudoplastic fluids, forming thixotropic structure. The rheological behaviour of these inks

Fig. 8. The dependence of parameters kW – F (a) and bW – F (b) evaluted from modified Walker – Fetsko equation (Eqs. (2) and (5)) on the penetration of paper materials (using IGT ink on the different paper materials).

Table 4 The parameters of modified Walker–Fetsko equation (Eqs. (2) and (5)) along with the R 2 values of the fitting evaluated from the experimental transfer curves measured utilizing IGT black ink on the different paper materials Paper material OC OP SP SA 2/M SA 2/LH LA

a

aa b a b a b a b a b a b

a, top side; b, underside.

kW–F (m2 g−1)

bW–F (g m−2)

fn

R2

0.24 0.24 0.25 0.25 0.33 0.49 1.4 1.82 1.64 1.81 2.34 2.51

7.3 11.4 4.8 4.4 1.78 1.08 0.002 0.003 0.39 0.39 0.26 0.24

0.023 0.040 0.36 0.37 0.44 0.44 0.52 0.52 0.46 0.46 0.49 0.48

0.999 0.999 0.999 0.999 0.998 0.999 0.999 0.999 0.999 0.999 0.999 0.999

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is similar and the experimental shear stress versus shear rate curves can be described using Ostwald, Casson or Cross flow models. The mileage curves measured using IGT offset ink on the different paper materials were analyzed using Tollenaar – Ernst equation (Eq. (1)), and both saturation optical density and m parameter were calculated. The paper properties such as smoothness, brightness, penetration determine the values of m parameter, as well as influence the saturation optical density. The process of ink transfer from the plate to paper was described using modified Walker–Fetsko equation (Eqs. (2) and (5)). The results obtained confirmed again the dominant role of paper penetration in the ink transfer process.

Acknowledgements We thank the Slovak Grant Agency (Project VEGA 1/6156/99) for financial support and Erich Nova´k is gratefully acknowledged for helpful discussions.

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