fi

Transfer and Setting of Water-Based Ink. Part 1 1 : pH, Water Absorbency and Uncoated Paper Structure

J.S. ASPLER, J.H. DE GRACE, M.-C. BELAND, C. MAINE and L. PIQUARD

A series of kraft handsheets of con- trolled pH and water absorbency was pre- pared and printed on a single-sheet flexographic proofpress, with a water-based ink. Ink transfer increased with increasing water absorbency. Ink penetration also in- creased with water absorbency, although the effect is small in the commercial range of ink levels. There is a slight increase in transfer of this alkaline ink to more acidicpapers, but there is no apparent influence of pH on ink setting. A series of newsprint samples was also printed. Within related wood species, print density on newsprint increases with fines content, while set-off decreases with increasing suqlace fines. The correlation be- tween print-through and opacity is much better than was found with oil-based inks on the same newsprints, due to the absence of oil penetration effects.

INTRODUCTION There is considerable interest in the

future of water-based inks. This interest in- cludes the flexographic printing of news- print and other uncoated papers, plus the possible replacement of solvent-based inks in all forms of gravure printing.

Water-based inks are up to 1000 times less viscous than oil-based newsinks and set very rapidly, while oil-based newsinks set slowly. On these grounds alone, one might expect significant differences in both trans- fer and setting properties. We have devel- oped a press, described in Part I of this work

& J.S. Aspler, J.H. De Gr2ce, M.-C. Beland, C. Maine, and L. Piquard Paprican 570 St. John's Blvd. Pointe Claire, Que. H9R 3J9

[ 11, capable of printing single handsheets made under a wider range of conditions and properties than is possible with webs.

We report here the results of quantita- tive ink transfer measurements. We have printed handsheets made from a bleached kraft pulp at controlled pH and controlled water absorbency. Print density, print- through, and set-off measurements have also been made on a well-characterized range of newsprints.

EXPERIMENTAL Press

A simple schematic diagram of the press is shown in Fig. 1. Typically, about 1 mL of ink forms the ink pond between the

engraved inking roller (anilox cylinder) and the first doctor blade. After the ink is distrib- uted on the anilox cylinder, the plate is inked and the sample printed. The press normally runs at 200 rpm (1.6 d s ) .

For set-off, a Kromecote cast-coated paper is used as reference. The nip between the print and the reference paper is closed immediately after printing. Print density, print-through, and set-off are determined from reflectance measurements, as de- scribed in Ref. [2].

Handsheets Handsheets were prepared according

to Technical Section, CPPA Standard C.4 [3]. A bleached spruce kraft pulp was

--NIP 3 n CY LlNDER

DOCTOR BLADE (FIXED) FORMING INK POND

CYLINDER

REVERSE ANGLE DOCTORBLADE

ANiLOX CY LlNDER

Fig. 1. Schematic diagram of flexo proof press. See Ref. [ l ] for details.

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 19 NO. 5 SEPTEMBER 1993 5203

used. Unsized handsheets were prepared in buffer solutions with pH values of 1 to 13. Cold extract pH values of the handsheets were determined according to the TAPPI standard [4]. Nitrogen gas was continuously bubbled through the solutions to eliminate artifacts that may result from the presence of dissolved atmospheric C02.

Sized samples were also prepared with acidic and neutral sizing. Solutions of these sizes were buffered. Rosin-alum size was used at pH 4, and alkenyl succinic an- hydride (ASA) size was used at pH 7 and pH 10.

The Cobb test [5] was used to esti- mate the degree of sizing. A specially made, smaller sample holder 3.6 cm in diameter was used. Since Cobb test values were not measured on all the unsized handsheets, a Cobb value of 150 g/m2 is assumed on the unsized sheets.

1.9-

1.8- E Ln

u) 1.6- C 2 1.5- y 1.4- C - 1.3-

5 lm7- *

c

much wider than would be found in com- mercial papers.

Ink transfer data to sized handsheets are given in Fig. 3, again using a single anilox cylinder (cylinder 360 P, Table I). A Cobb value of 20 g/m2 means complete water repellency. A Cobb value of 150 g/m2 means that the sheet absorbs water virtually instantaneously. Figure 3 shows that ink transfer increases with water absorb- ency. More ink is transferred to the more absorbent samples. However, the range of water absorbencies is wider than would be found in commercial papers, so the influence of water absorbency on ink transfer is still relatively small.

Prints made on the hard-sized kraft handsheets were less uniform than prints on the absorbent samples. Lower uniformity may have resulted either from partial dewet- ting of the ink on these low-energy surfaces or from lower ink transfer. Wetting problems are a known cause of mottle in printing coated board with water-based flexo inks [8,

Table I1 shows the correlations among ink transfer, print properties and handsheet characteristics, combining data from Figs. 2 and 3. For 21 samples, IRI must be greater than 0.433 for the correla- tion to be significant at the 95% confidence level; greater than 0.549 at the 99% confi- dence level; and greater than 0.665 at the 99.9% confidence level.

91.

Newsprint Samples Nineteen newsprints, already charac-

terized in studies with oil-based inks [6] were printed. The sheets were split into four layers on a Beloit sheet splitter. The fines content of the top layer was determined on the Kajaani FS-100 fibre classifier. In this report, fines are defined to be fi- bres I 0.1 mm in length.

Ink A black water-based flexo newsink

from Flint Ink Corp. (formulation KZK075/5, containing 18% carbon black) was used at a viscosity of 0.015 PPS, as measured on a Shell No. 3 cup viscometer. The formulation and viscosity are both typi- cal of current commercial inks. For quanti- tative ink transfer measurements, a similar ink containing CdS as a label (also supplied by Flint) was blended with the first. The blended ink contained 4.88 mg of Cd per gram. The amount of ink transferred to the paper was determined from the Cd content of the ashed prints. For some experiments, the labelled ink was used at full strength (8.59 mg of Cd per gram). The initial pH of the ink was about 8.5.

Application of Excess Ink Flexo ink was applied at low pres-

sure, in large amounts, using the Paprican- Bristow sorption apparatus [7] to a conventional newsprint and to the same newsprint after aging to full water repel- lency (Table 111).

RESULTS AND DISCUSSION Ink Transfer, Setting and Print Properties pH and Wafer Absorbency

Ink transfer data for unsized hand- sheets of different pH are shown in Fig. 2. Prints were made using a single anilox cyl- inder (cylinder engraving 360 P, Table I). At each pH, single ink transfer measurements are shown rather than averages, to show the spread in the data. Ink transfer decreases with increasing pH (R2 = 0.46). More of this alkaline ink transfers to more acidic papers. We note that this effect of pH on ink transfer is not great, even across a pH range which is

Anilox Cylinders Since the only practical way to vary

the ink level in a flexo press is to vary the anilox engraving, eight different engravings were prepared with a range of ink levels (Table I), as described by Aspler et al. [I]. For measurements with a single inking level, the 360 P anilox cylinder was used, as this is most representative of current commerical practice.

2.2r

2.0r

i l

N-Neutral

A N

N

A

AA

N N

N

N I

'I m

m m

m m

I

Y c 1.71

1.2' I I I I I I I I 0 20 40 60 80 100 120 140 160

Water absorbency, Cobb, glm 1.6' I I I I I

I I

3 4 5 6 7 8 9 10

PH Fig. 3. Ink transfer as a function of water absorbency, sized handsheets. A: Acidic samples (pH between 4 and 5). N: Neutral samples (pH between 6.3 and 7.1). 0: alkaline samples (pH greater than 8). Fig. 2. Ink transfer as a function of pH, unsized handsheets.

J204 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 19 NO. 5 SEPTEMBER 1993

Table I1 confirms that the lower ink transfer to more sized and less water absorb- ent samples is significant (R = 0.82). Al- though the nip dwell time is short (-1 ms), there is still sufficient time for the paper surface chemistry to influence ink transfer. Theories dealing with fluid transfer in nips, such as the Hagen-Poiseuille equation, pre- dict increased transfer when wetting is greater (contact angle close to zero). Figure 2 also shows that there may be a need for an additional acid-base interaction term.

There is no correlation between paper pH and print density, set-off, and print- through normalized for the amount of ink (PDIIT, SOIIT, and PTIIT). That is, while pH contributes to ink transfer-albeit to a lesser extent than the absorbency-once the ink is on the paper, pH has no discernable influ- ence on ink setting.

These results may resolve an earlier anomaly [lo]. Newsprint samples had been subjected to accelerated aging, which made the sheets more acidic andless water absorb- ent. Aging reduced the set-off. If we assume that less ink was transferred to the aged, less absorbent paper, the smaller amount of ink on the surface would therefore have led to less set-off, so the acidity of the aged sam- ples may not have been relevant in the lower amount of set-off that was measured.

As seen in Fig. 3, print density in- creases with increasing water absorbency for the handsheets. However, more ink is transferred to the more absorbent paper. If print density is normalized to a print density value per gram of ink, to correct for the different amounts of ink transferred to the different samples, there is a significant de- crease in the normalized print density with increasing water absorbency (R = -0.79). The “printing efficiency” [ll] (effective print density per gram of ink) is lower on highly absorbent paper. That is, although more ink is transferred to an absorbent paper in the nip, there is a second influence of water absorbency, resulting in greater ink penetration.

The range of print densities is still small, despite the broad range of water ab- sorbencies. It is, perhaps, a matter of degree but, as will be shown later, only when 10 or more times the usual amount of ink was applied did water absorbency become a ma- jor factor in ink holdout.

Om7’ r S

W E

EE E w s E I W S

>, 0.77 u) C 0.76 Q) U

C

CI .-

+ 0.75 .-

0.74

S

I I I I I W

0.7 1 I l l I 2 0 22 24 26 28 30 32 34 36 38

% Fines, 0.1 mm or smaller

Fig. 4. Flexo print density as a function of percentage fines in the sheet surface, 0.1 mm and smaller. Samples from reference [6]. E = Eastern North America; W =Western North America; S = Southeastern United States.

In Table 11, print-through appears to increase with increasing water absorbency. This correlation results from greater ink transfer to more absorbent paper. If print- through is normalized to the same ink trans- fer, no correlation with absorbency is found. The apparent insensitivity of the normalized print-through to water absorbency probably results from the fact that the surface component of print-through, PTsr, predomi- nates, and is not influenced by penetration. This point will be expanded upon later.

Ink Holdout-Structural Properties Print Density

For the 19 commercial newsprints, no correlation was found between flexo print density and air leak roughness, as found previously with oil-based ink [6]. Contrary to conventional wisdom, while a good cor- relation between print density and air leak roughness may be found with paper from a single machine, a good correlation is usually not found when dealing with papers from a variety of machines, regions, and furnishes [ 1 I]. Instead, in Fig. 4, flexo print density is plotted as a function of the percentage of fines in the sheet surface. For the three groups of wood species (east, west, and south), the flexo PD covers the same range. However, similar wood species give sepa-

rate groups. Within each group, there is a trend towards increasing flexo print density with increasing fines content on the surface. While these results are scattered, the trends are the same as shown earlier [6] for the print density of oil-based inks printed on the same newsprints.

It is well known, e.g. [12], that in order to obtain similar pulp properties, refining energy follows the order: eastern species (mostly spruce) > western species (hemlockhalsam) > southern pine. The quantity of fines produced during refining to a given set of pulp properties increases in the same order, yielding the observed improve- ment in ink holdout.

Set-off The correlation between set-off and

surface fines content is significant (R2 = 0.43). Not only do surface fines improve the holdout of water-based inks, but they also provide more surface area for the drying and adhesion of the ink and its binder; increasing print density and reducing set-off. We note that the range of set-off values is extremely small (0.023 to 0.033).

Prin t-through Print-through (PT) of oil-based inks

has three components [ 131:

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 19 NO. 5 SEPTEMBER 1993 J205

0.075 r 0.070

0.065

0.060

0.055

0.050

0.045

0.040

0.035

- - - -

-

-

-

-

I

- 0.35

3 0.30- 0.25-

0.20-

2 0.15-

u, 0.05-

e

9.

; 0.10-

0.00

I

B

**. .a?.** B B

.B

I I I I I I I

I-

- = I

II

0.030' I I I I I I 90 91 92 93 94 95 96

Printing opacity, % 1

Fig. 5. Flexo print-through as a function of printing opacity.

=total print through

c 0.07 0.06

0.05

C

0.01

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

ink transfer, g/m

Fig. 6. Components of flexo print-through as a function of ink transfer. Total PT: (W); PTsr (pigment on the surface): (0); PTP (pigment penetration): (A).

hydrophobic newsprint

g 1.0 . .o 0.4 . .. 0 0 O 4-

e 0.2 i I I n D I I I

0.oL 10 20 30 40 50 60 0

Ink transfer, g/m 2

Fig. 7. Fraction of sheet penetrated by ink (Eq. 5). (A): Low ink level; flexo ink (Table I) :(W); oil-based offset ink [6] (a). (B): High ink level, flexo ink; from Tables I and 111. Conventional newsprint (w); Self-sized, hydrophobic newsprint (0).

-The show-through or surface compo- nent, resulting from the initial opacity of the paper (PTsT);

- The pigment penetration component (PTP);

- The opacity loss component, resulting from the penetration of vehicle oil into the sheet (PTv).

For water-based inks, only the first two components are relevant. Larsson and Trollsls [ 131 showed that the P T ~ T and PTp components can be separated. The separa- tion of print-through into its components and the calculation of the penetration depth are shown in the Appendix.

For the 19 newsprints, R2 between the print-through of an oil-based ink and opacity is only 0.53 [6]. A poorer correlation be- tween opacity and oil-based print-through results from the additional interaction be- tween ink vehicle oil and sheet pore struc- ture.

On the other hand, as shown in Fig. 5, for the same 19 newsprints, the correlation between flexo print-through and printing opacity is excellent (R2 = 0.86). Since the water in the flexo ink does not remain in the paper pore structure, it would be expected that opacity would control flexo print- through.

Figure 6, calculated from the print- through data for the single newsprint in Table I, shows that the show-through or sur- face component, PTsT, is virtually constant with increased ink weight on the paper. As ink weight increases, there is little change in the surface (opacity) component of print- through but there is a considerable increase in the pigment penetration. This was also shown by Larsson for oil-based inks. Once a limiting blackness level is reached on the sheet surface, governed by the amount and type of carbon black in the ink, there is no further change in the reflectance of that sur- face layer. However, increased pigment penetration with increased ink weight on the paper will cause PTp to continue to increase.

The fraction of sheet thickness that the ink penetrates is derived in the Appendix and is shown in Figs. 7(a) and (b). In Fig. 7(a), small differences in the calculated fractional penetration between water and oil- based inks in the commercial 2-3 g/m2 re- gion are not meaningful, since the calculations are very sensitive to small errors in the reflectance values. In Fig. 7(b), the penetration of excess ink increases very dra- matically when the newsprint is deliberately made water-repellent. This will be discussed in detail in the next section.

Changes in Newsprint Water Absorbency

Table I11 shows ink transfer, print density, and print-through data for excess flexo ink applied to newsprint, before and after accelerated aging to a lower absorb- ency. At exaggerated ink levels, the conven- tional newsprint shows little increase in print density, but shows a large increase in print- through. The print-through of the non-ab- sorbent newsprint is extremely low at exaggerated ink levels, even though the print density is exceptionally high.

We conclude that excess ink of low viscosity will penetrate a conventional, water absorbent newsprint, since a thick ink layer does not have time to set by evapora- tion. The less absorbent newsprint slows or stops the penetration of excess ink, giving the water time to evaporate, and giving the observed high print density combined with very low print-through. That is, only when very thick ink films are applied is the tradi- tional approach followed, which states that penetration of an aqueous system decreases with decreasing water absorbency. It has been assumed [ 141 that, because of the low viscosity of flexo inks, greater penetration would be a problem. However, with realistic

,

5206 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 19 NO. 5 SEPTEMBER 1993

ink films it is difficult to see water absorb- ency effects.

Conflicting suggestions have been made on whether more [15] or less [16] water absorbency is desirable. Rapid immo- bilization of a thin water film not only ex- plains the lack of a water absorbency effect at normal ink levels, but also the small vis- cosity effect on print-through. It may be that only when large amounts of water-based ink are placed on the paper-for example, in commercial 4-colour printing or in water- based gravure printing-will water absorb- ency be important to ink setting.

IMPLICATIONS FOR WATER-BASED GRAVURE PRINTING

Since water-based gravure inks are similar to flexo inks, this work has implica- tions for future studies of water-based gra- vure inks. Serafano and Rosinski [I71 found that, with water-based gravure inks, dot spreading was greatest on unsized paper. Hmzewicz [ 181 suggested that the setting of alkaline, water-based gravure inks is faster on an acidic paper surface. This contradic- tion between gravure and flexography may result from the thinner flexo ink film. In flexo, rapid evaporation or immobilization of the water is more likely while, in gravure, the thicker ink film would give more time for acid-base interactions between paper and ink.

SUMMARY AND CONCLUSIONS Ink transfer increases with water ab-

sorbency. At constant water absorbency, transfer of an alkaline ink to acidic papers is also greater, although the effect is small. The influence of water absorbency on ink trans- fer is greater than that of pH.

Print density normalized for different ink levels decreases with increasing sheet absorbency, and calculated ink penetration increases with sheet absorbency. Within a commercially realistic range of paper water absorbency, the effect is small. While pH has a small effect on ink transfer, it has no dis- cernable effect on ink setting. Only for the extreme case of applying 10 times (or more) the usual amount of ink to the paper surface does water absorbency have a major impact on ink penetration.

Within related wood species, the print density of newsprint increases with the sur- face fines content. Set-off decreases with increasing surface fines. Both of these re- sults are consistent with studies using non- drying oil-based inks.

The correlation between print-through and opacity is much better than was found with oil-based inks on the same newsprints, due to the absence of an oil penetration coniponent in the print-through.

ACKNOWLEDGEMENTS The authors would like to thank

Sylvie Sauriol and Johanne Lacoursikre for their contributions to the experimental work, and Paprican’s Chemical Analysis Section for numerous Cd analyses.

APPENDIX A Separation of Print-Through Components

Ref. [21: Print-through (Py) is obtained from

(1) R-’

PT 5 log RP, reverse side

where R,’ = the reflectance of the reverse side of the unprinted sheet and RP, reverse side = the reflectance of the reverse side of the print.

The show-through component PTST (pigment on the surface) is defined as:

where Rx = the reflectance of the unprinted sheet over a print. Bristow [ 191 showed that Rx can be calculated from Rp (reflectance of the print), R, (reflectance of the unprinted paper with a black background), and R,‘ :

Therefore, P T ~ T is obtained from Eqs. (2) and (3), and the pigment penetration

component of the print-through, PTp, is given by:

PTp = PT - PTST (4)

Bristow also suggested that the weight fraction of the paper that is pene- trated by the ink, Wp / W, is:

wp/w=

In Eq. (5) , Rpp is the reverse side reflectance from pigment penetration:

R,‘ RPP =

REFERENCES 1. ASPLER, J.s., BELAND, M.-c., BOLUK,

Y and DALPHOND, J.E., 1992 Proc. Tech. Assoc. Graphic Arts, Rochester, NY, p. 207.

2. SCAN P36:77, “Newsprint Evaluation of Test Prints”, Scandinavian Pulp, Paper, and Board Testing Committee, Stockholm (1976).

3. “Forming Handsheets for Physical Testing of Pulp”, Tech. Sect., CPPA Standard C. 4, Montreal.

4. “Hydrogen Ion Concentration (pH) of Paper Extracts (Cold Extraction Method)”, TAPPI PRESS, T 509 om-88, Atlanta, (1988).

5. “Water Absorptiveness of Sized (Non-Bibu- lous) Paper and Paperboard (Cobb Test)”, T 441 om-90, TAPPI PRESS, Atlanta (1990).

6. ASPLER, J.S., DE G R k E , J.H. and DAL- PHOND, J.E.,J. PulpPaperSci. 17(5):J149 (1991); DE G R k E , J.H., J. Pulp PaperSci. 19(5):J208 (1993).

7. LYNE, M.B. and ASPLER, J.S., Tappi 65(12):98 (1982).

8. JENSEN, K.W., Graphic Arts in Finland, 18:14 (1989).

9. BASSEMIR, R.W. and KRISHNAN, R., Proc. 1991 Papercoating Fundamentals Symp., Tech. Sect., CPPA, Montreal, p. 69.

10. ASPLER, J.S., J. Pulp Paper Sei. 14(3): J66 (1988).

11. MANGIN, P.J. and DE G R k E , J.H., Proc. 1984 Intl. Printing and Graphic Arts Conf.,

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 19 NO. 5 SEPTEMBER 1993 J207

12.

13.

14.

15.

16.

17.

18.

19.

TAPPI PRESS, Atlanta, p. 134. LEASK, R.A,. Chapter XI in “Pulp and Pa- per Manufacture, Vol. 2. Mechanical Pulp- ing”, Joint Textbook Commitee of the Paper Industry, TAPPVCPPA (1987). LARSSON, L.O. and TROLLSAS, P.O., in “The Fundamental Properties of Paper Re- lated to its Uses”, Trans. Cambridge Symp., Tech. Div., BPBIF, 600 (1976). IONIDES, G.N. and THORBURN, I., Pulp Paper Can. 92(4):47 (1991). ARLOV, A,, “Newsprint Production and Use: The Future of Newspapers”, PIRA, London (1987). WALLSTROM, E., Advances in Printing Sci. and Technol. 19:199 (1988). SERAFANO, J. and ROSINSKI, J., Ameri- can InkMaker p. 44 (Sept. 1991). HRUZEWICZ, J.N., Roundtable presenta- tion, TAPPI Coating Conf., Orlando, FL (May 1992). BRISTOW, J.A., Adv. Printing Sci. and Technol. 19:137 (1988).

The Print-Through Prop sity of Newsprint r I J.H. DE GRACE

,/

During the investigation ofthe print- ing properties of 19 North American news- prints, it was found that several had similar unprinted opacities, but greatly different print-throughs when printed with non-dry- ing, oil-based newsinks. This was attributed to differences in the propensity of the ne prints to lose opacity in the presence vehicle oils. The newsprints had d fines content. It was foun increased the number of s sheets so that more pores c by the oils. Thus, the new fines contents suffered g opacio.

570 St. John’s Blvd. Pointe Claire, Que. H9R 3J9

INTRODU ION Pri -through is the unwanted ap- ,$: pearanc of a printed At a \ im ge on print the reverse density

ness), print-through epends upon pa- er opacity, as it is modifi d by the ink [ l].

During our investigation o the printing and ruboff properties of co mercial North American newsprints [2] , i ,was observed that many had similar opa’ities prior to printing. However, at constan&rint density, those newsprints had greatly different print- through values when printed with non-dry- ing, oil-based newsinks.

In this article, the large differences in print-through among newsprints with simi- larunprinted opacities are explained in terms of differences in the wood species used to manufacture the papers, and the effect this had on their propensity to lose opacity upon printing.

EXPERIMENTAL Newsprints

Nineteen North American newsprints with a basis weight of 45 g/m2 (0.d.) were tested. The papers were formed on either Fourdrinier, top-former, or twin-wire ma- chines from groundwood and TMP pulps consisting of various wood species. These newsprints have been previously charac- terized in work on their ruboff properties when printed with oil-based offset inks [2], and on their printing properties using a water-based ink [3]. Selected properties are shown in Table I.

Fibre coarseness and the number of fines measuring 0.2 mm or less in whole sheets of the newsprints were determined with the Kajaani FS-200 fibre classifier. The average pore diameter of the newsprints were determined by mercury porosimetry

J208 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 19 NO. 5 SEPTEMBER 1993