4676
Ind. Eng. Chem. Res. 2010, 49, 4676–4681
Improved Prediction of Offset Ink Setting Rates Based on Experimental Data and Filtration Equations Hanna Koivula,*,†,‡ Douglas Bousfield,‡ and Martti Toivakka† Center for Functional Materials, Laboratory of Paper Coating and ConVerting, Åbo Akademi UniVersity, Åbo/Turku, Finland, and Paper Surface Science Program, Department of Chemical and Biological Engineering, UniVersity of Maine, Orono, Maine 04469
The setting rates of inks on paper influence the printing operation and the quality of the final product. Although much work has been reported on the effect of the coating structure on setting rates, much less work has been done understanding the ink parameters, especially the influence of emulsification. A series of four papers were tested with three inks. The inks were characterized in terms of printing properties, rheology, and their filtration behavior. The filtration rate is used to obtain a permeability value of the ink filtercake. The value of the maximum tack was determined to be more dependent on the smoothness of the paper tested than on the type of ink. The dynamics and magnitude of the tack test were dependent on the ink viscosity, but viscosity alone does not explain the results. The measured ink filtercake permeabilities are different between the ink types by a factor of 2. The setting rate is found to be primarily a function of the viscosity of the oil used in the ink, rather than a function of the pigment and resin system. Introduction The absorption of fluids into coated paper is important for the end-use application in printing processes; the absorption of ink oils determines ink setting rates, which link back to print quality issues and production limits. The role of the coating pore structure to the ink setting has been studied by many groups over the last 15 years.1–10 However, the role of the ink components and ink emulsification on absorption and setting has not been well-described in the literature. A few different hypotheses have been proposed to describe the absorption of oils from the ink. The multiphase theory suggested by Donigian11 describes ink oil absorption, under the hypothesis that the resin phase and oil phase of the ink have differing rates of absorption for the oils to penetrate to the coating structure. The gradual solidification of the ink, as suggested by Gane et al.,12,13 states that ink oils absorb into the coating structure, gradually solidifying and drying the ink, without any clear layering of components. The filter cake theory suggested by Xiang and Bousfield14 suggests that there is a filter cake of pigment particles and hard resin accumulating at the interface between the ink and the coating. It is understandable that the fountain solution and water added in the printing press changes the viscosity of printing inks. Ercan15 showed that both high and low shear rate viscosities have some correlation to the filamentation of the ink in the printing nip. She found that, especially, the increased elastic nature of the ink, including the interaction of dissolved soft resins when present, compared to the purely viscous behavior of the diluent oil (e.g., mineral oil) resulted in smaller filaments. This was already earlier hypothesized by Xiang.16 Xiang’s later work17 did show differences in the initial stages of ink film leveling between emulsified and pure inks. Both Xiang16 and Fro¨berg18 have studied the tack of emulsified inks. Both authors * To whom correspondence should be addressed. Tel.: +358 2 2154305. E-mail:
[email protected]. † Center for Functional Materials, Laboratory of Paper Coating and Converting, Åbo Akademi University. ‡ Paper Surface Science Program, Department of Chemical and Biological Engineering, University of Maine.
found that the initial rate of tack rise seems to be unaffected by the fount addition. Emulsification had a greater influence on the maximum tack and the rate of tack decrease. This paper concentrates on the influence of ink oil components and ink emulsification on ink setting rates. Theoretical predictions based on the filter cake theory are compared to experimental results. Results from the filtration experiments and rheological tests are fitted into the equations describing ink setting. The difference in setting rates between two inks and an emulsified ink on fast and slow setting papers is described. Materials and Methods Two inks were used in this study. The first is a heat-set magenta ink (ACP series, Sun Chemicals) consisting of ∼40% ink oils (mostly mineral oil), 23% hard resin, and 11% pigment. The remaining portion is a combination of waxes and miscellaneous additives. The second ink was a sheet-fed cyan ink (QK series, Hostmann Steinberg). This ink has generally fairly similar composition to heat-set ink, except for the main difference, with respect to the oil mixture used, because sheet-fed formulations have a portion of vegetable oil(s) and, in addition, possibly some soft (alkyl) resins. Fountain solution concentrate (Majesta Varn 635, Day International), was mixed with distilled water in a ratio of 4 oz/gal, which was equivalent to 31.2 mL/L, to create the fountain solution for emulsification of the inks. The heatset ink was emulsified together with 30% (by weight) fountain solution. This ink/fountain solution combination was emulsified for ∼5 min with a high-speed mixer, using a high-shear mixer head. The presence of a vortex was taken as an indication of good mixing. The emulsification in laboratory scale must always be considered very carefully. In a process situation the constant supply of fountain solution and high speed of the printing machine are aiding the emulsification process. However, in a laboratory environment, the time scales are much longer. Although a known amount of water was emulsified into the ink, during the process of distributing ink into a thin layer and printing it on the paper, water will evaporate, to some extent. Tests were conducted in a humidity- and temperature-controlled room at 50% relative humidity (RH) and 23 °C.
10.1021/ie9014028 2010 American Chemical Society Published on Web 04/20/2010
Ind. Eng. Chem. Res., Vol. 49, No. 10, 2010
4677
Figure 2. Illustration of the ÅAGWR setup for the ink filtration experiment. Figure 1. Matrix of the papers and their abbreviations used for laboratory print testing of tack, gloss, and print density. Table 1. Data Describing the Ink Set Times, Coating Gloss, and Surface Roughness of the Matrix of Coated Papers Used in This Work Paper Glossy Finish standard
fast
standard
fast
a
110 min 72.6 0.68 µm
20 min 74.6 0.7 µm
140 min 45.5 1.57 µm
30 min 48.1 1.58 µm
printer IST paper gloss (75°) roughness PPS10 (soft) a
Matte Finish
parameter
IST ) ink set time of standard density black print.
A series of commercial papers were obtained from SAPPI North America. The papers were described by their optical and ink setting behavior, as shown in Figure 1. The basic properties relating to the surface appearance and ink setting are described in Table 1. The fast ink setting paper’s coating pore structure and/or chemical composition have been modified to be fast ink setting and drying; other than modifications made to the coating to achieve those goals, the papers are indistinguishable from their standard grade line contemporaries. Tack tests were performed using a Microtack tester, as described by Xiang.19 The probe consists of a rubbery material of contact size with the ink surface 2 mm × 2 mm. The tack test was performed directly after printing with a laboratory print tester (KRK Universal Printability Tester, MPT 8000, Kumagai Riki Kogyo Co., Ltd., Japan). A volume of 0.3 cm3 ink was distributed on the rollers for 60 s. Ink was distributed on the printing roll for 30 s. The printing roll was a smooth aluminum roll 4.0 cm wide and 6.4 cm in diameter. The printing force was set to 1000 N, and the printing speed was 1.0 m/s. The amount of transferred ink was measured by weighing the inked printing roll before and after the printing. The difference in the weights gives the gravimetric amount of ink transferred to the paper. Assuming that 1 g/m2 of ink corresponds to an ink layer thickness of 1 µm, the transferred ink layer thickness was ∼2-3 µm, depending on the paper-ink combination used. The final print gloss was measured using a MicroTri-Gloss (BYK Gardner USA), using a 60° geometry. Print densities were measured by QUIKDens 100 Portable Reflection Densitometer (X-Rite, GretagMacbeth) with paper white as the reference. Viscosities of the inks and ink mixtures were measured with a controlled stress rheometer (Bohlin CVO, Malvern Instruments, Ltd.), using a CP 4°/40 mm cone-and-plate geometry. Temperature was held constant at 25 °C during the measurements. An Åbo Akademi Gravimetric Water Retention tester (ÅAGWR) (Kaltec Scientific, Inc.) was used for the ink filtration experiment. Mineral oil (Product SUS 60, Sun Chemicals) was used as a reference and to dilute the inks for filtration experiments. A reference ink jet printing paper (Brochure, HP), with good absorptive properties was used for the filtration tests to compare the inks. The filtration pressure was 103 kPa. The filter used was an Isopore membrane filter (Millipore) with a pore size of 1.2 µm. Filtration times were varied between 2
Figure 3. Print density data for ink mixtures printed on an array of papers.
min and 20 min. Figure 2 shows a schematic illustration of the setup. Ink was placed in the chamber on top of the membrane filter and multiple sheets of a porous paper, which were to absorb the ink oils. The reference paper was chosen because of its known small and narrow pore size, having pore sizes in the 30-nm range and 1-µm range, promoting fast absorption of ink oils. Several papers were added to have a sufficiently large absorption capacity. Offset inks have a thick and pasty consistency. Therefore, the inks needed to be diluted to be filtered. Mineral oil was chosen as a diluent, because it is a common component of offset inks and has one of the lowest viscosities among the ink components. Dilution ratio of ink and mineral oil was 50%:50% (by weight). Papers were weighed before and after filtration to determine the amount ink that was filtered gravimetrically. According to Gros et al.,20 the filtration resistance coefficient that is obtained should not be dependent on the dilution level. This method, which was first described by Gros et al.,20 may be a useful way to characterize and understand the setting rate of a particular ink. The filtration and absorption calculation equations are explained in detail in the Supporting Information section, as Appendix 1. Results and Discussion Optical data, in the form of print density and print gloss (60° geometry), were measured for the different inks printed on the commercial papers. Figure 3 shows the print density data. The inks are identified in the graphs by abbreviations: heat-set ink (HS), emulsified heat-set ink (HS E), and sheet-fed ink (SF). The papers are described as stated in Figure 1. The print densities using the same ink amount on rollers is smaller for the emulsified ink than the nonemulsified one. This is an expected effect of replacing part of the ink with water. The effect of adding water will reduce the transferred amount of ink. The high print density of the sheet-fed ink may come from the higher pigment loading in that ink. The print gloss values of the printed samples are given in Figure 4. Although the emulsified ink had a lower viscosity, which should lead to high gloss, because of increased filament
4678
Ind. Eng. Chem. Res., Vol. 49, No. 10, 2010
Figure 4. Print gloss data for the coated papers using same amount of ink on the rolls. Figure 6. Maximum tack values with standard deviations for the four different coated papers.
Figure 5. Microtack tester curves for the pure inks on a fast-setting matte paper.
leveling, the emulsified inks had print gloss values that were very similar to those of the pure inks. Since the ink film is thinner, because of added water and subsequent changes in the ink transfer, the effects of the surface roughness provides a larger contribution. Therefore, the surface roughness might work against the positive effect of the lower viscosity improving the printed gloss. The sheet-fed ink had the highest print gloss. As expected, the high-gloss papers gave high print gloss. Figure 5 shows the tack graphs of the inks used in this study on a fast-setting matte paper measured with the tack tester. The sheet-fed ink has clearly the highest tack, and it is the slowest of the three, likely due to the highest linseed oil content. The heat-set ink has lower tack and the emulsified heat-set ink has the lowest tack values. This result agrees with the work of Xiang and Bousfield for the emulsified ink.16 The maximum tack values presented in Figure 6 show both the tack values and the standard deviations of the measurements. The standard deviations are quite large, as often is the case in printing. However, it seems that the deviations of heat-set inks on the standard papers are smaller than those of the faster-setting papers. The trends of the maximum tack and optical and structural surface properties of the paper follow the trends given in the literature (see, for example, ref 21). Matte papers result in tack values that are lower than those of smooth glossy papers. Fast-setting coatings have slightly lower tack values than slowsetting coatings. The time needed to reach a low tack value, defined as
