Article pubs.acs.org/IECR
Transport of Immiscible Vapor Pressure Contrasting Liquids in Multiple Nip Impressions: Application to Heatset Offset Printing of Paper Carl-Mikael Tåg*,† and Patrick A. C. Gane‡,§ †
Center for Functional Materials, Department of Physical Chemistry, Åbo Akademi University, FI-20500 Åbo, Finland School of Chemical Technology, Department of Forest Products Technology, Aalto University, P.O. Box 16300, 00076 Aalto, Finland § Omya International AG, CH-4665 Oftringen, Switzerland ‡
ABSTRACT: A multistation heatset offset printing press was studied here to exemplify the transport of immiscible liquids into a porous substrate under dynamic rolling nip conditions. In offset printing, oil-based ink and a water-based fountain solution are used to define the image and nonimage areas according to the respective surface energy of the printing plate. The fountain solution and ink inevitably form an emulsion after the press has run sufficiently to reach equilibrium. It is this emulsion that is transported via a rubber blanket to the paper and forms the printed image. The emulsification degree depends on the ink properties and the printing process at the equilibrium ink−fountain solution balance, which, in turn, supports stable continuous running of the press in order to achieve a good print quality. Focus is given to the transfer of fountain solution to both nonimage areas of paper as well as to the inked areas carried in the emulsion. In order to monitor the amount transferred, each of two tracing elements, lithium and cesium in this case, was added separately to the fountain solution circulation. Printed paper samples as well as ink samples were taken from different positions in the press at various time intervals to follow the evolution of the process. The amount of lithium and cesium tracers was analyzed using time-of-flight secondary-ion mass spectrometry and inductively coupled plasma mass spectrometry, respectively. It was found that nonimage areas take up a constant amount of liquid per printing unit following the first unit, with the uptake being limited by the coated paper properties, while the tracer concentration in the printed image areas increased as a function of time because of progressive evaporation of the fountain solution in the system over time.
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INTRODUCTION Offset printing is nowadays still the most widely used printing technique for high-volume and high-quality printed products. The dynamic of multiple impressions is of universal interest in many production scenarios where the behavior of immiscible components applied to a porous medium is important. In offset printing, the inks are applied wet-on-wet on the paper substrate from separate printing units. The offset process is based on the mutual repulsion of oil and water. A fountain solution is transferred from the nonimage area of the printing plate via a rubber blanket to the paper through applied nip pressure and by film splitting following surface wetting in the printing nips. However, the printed areas also contain a fountain solution, partly as a liquid layer applied under or on top of the ink from ink stations on a multicolor press, where no prior or subsequent image has been applied, and partly in an emulsified form because of equilibrium uptake at the printing plate and printing blanket−paper interface. Depending on the properties of the printing ink, around 30 wt % of a fountain solution is taken up by the ink.1 The fountain solution is distributed evenly throughout the ink in small droplets.2 The adhesion properties between the liquid and the rollers, plates, and blankets, as well as liquid cohesion, determine the splitting mechanism in liquid transfer. This transfer of the ink and water throughout the roller train is one of the most important factors in the emulsification process. © 2013 American Chemical Society
The ink−water balance is strongly affected by the surrounding changes in the temperature and humidity. Even higher press speeds increase shear forces affecting the balance. Too much water creates an effect known as snowflaking, an effect where voids are left in the image, resulting in a snowlike appearance and unclear images on the substrate.3 The amount of fountain solution transferred to the respective areas, namely, to the nonimage and image areas, has been questioned for several decades. The common belief even today is that more fountain solution is carried with the ink to the paper, resulting in higher moisture content in printed areas compared to nonimage areas.4−6 However, the transferred amount is difficult to determine despite several attempts. Trollsås and Larsson found, using a gravimetric method, that the moisture uptake in newsprint is 0.3−0.5 g m−2 per side per printing unit.7 Hansen measured the uptake with a microwavebased method and found that the uptake in newsprint is 0.4− 0.7 g m−2.8 The results reported for a single nip coldset offset press vary from 0.20 to 0.68 g m−2.8,9 A lot of heat is generated on presses. The only cooling on standard machines is by evaporation of the fountain solution. Uncontrolled temperature Received: Revised: Accepted: Published: 15602
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and humidity in the press room will affect the evaporation rates. Systems relying on gravimetrical methods are neither rapid nor accurate enough to avoid evaporation, and determination of the weight difference before and after drying (as traditional determination of the paper moisture content) for a printed sample also is affected by the mass loss of ink solvents.10 In addition, the fountain solution is lost in the delivery system and in roller trains through spillage and evaporation.11 Already in the 1950s, liquid transfer to paper was measured by adding a known amount of water-soluble dye to the fountain solution and later determining the amount of dye extracted from a known area of the printed paper.11,12 A detectable substance was also added to the fountain solution, and subsequent atomic absorption spectrometry gave results suggesting that the fountain solution content in image areas is more than 10-fold compared to that in nonimage areas.13 Reinius et al. labeled the fountain solution with cesium chloride (CsCl; 25 wt %) and showed the difference in the penetration depth between different coatings, with platy kaolin giving the least fountain solution penetration and greatest surface spreading, and narrow-particle-size-distribution blocky ground calcium carbonate showing the greatest penetration.14 Also Preston et al. have used a tracing element for detection of the water distribution in coatings.15 They used a water solution with 25 wt % CsCl and utilized high-resolution secondary-ion mass spectrometry to map the Cs+ ions in the paper cross sections. They concluded that the pigment surface area is of crucial importance regarding penetration of the fountain solution. The strength of the coatings decreased as the fountain solution penetrated deeper into the coating structure and the base paper, e.g., in the case of narrow-particle-size-distribution calcium carbonate. Lipponen et al. used lithium chloride (LiCl) as a tracer of starch penetration in uncoated fine paper.16 Sodhi et al. investigated the distribution of ink and coating constituents in two coating formulations.17 The argument that ink carries most of the fountain solution has been postulated, but quantitative information on the amount of solution carried by the ink and paper has not been firmly reported so far because of the confusion generated as the tracer concentration can be observed to increase in the ink over time. The study of fountain solution transfer to paper undertaken here accounts in more detail for the equilibrium dynamics of the press, partly via the time-developing levels of tracer in the nonimage areas on the printing plate and partly with the emulsified ink. The aim of the work reported here is to investigate the transfer ratio to the specific print image and nonimage areas as well as how it develops with time, an aspect overlooked or poorly described by other workers in the field. Distribution of the fountain solution transferred is illustrated using a lithium tracer and time-of-flight secondary-ion mass spectroscopy (ToF-SIMS) and the amount transferred quantified from the response of a cesium tracer detected with inductively coupled plasma mass spectrometry (ICP-MS).
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Figure 1. Schematic of the heatset offset printing machine. Note that the black ink (K) unit was left open throughout the trial. Thus, the cyan unit is here referred to as being the first printing unit. zoom-in of one printing unit. Note that the rollers in the unit are simplified and the actual number in a typical unit is higher depending on the machine design. High-tack inks (Premoking 2000; Flint Group, s-Gravenzande, The Netherlands) were applied to target print densities (C = 1.45). The black unit was disengaged, and the printing sequence was thus C−M− Y−fountain solution (FS). Hence, hereafter, the cyan unit is referred to as the first printing unit. The fountain solution used in the trial contained 3 wt % concentrate (Redufix-AF; Hostmann-Steinberg, Vaanta, Finland) and 4 wt % isopropyl alcohol (IPA). The fountain solution had a pH of 5.1 and a conductivity of 1150 μS cm−1, respectively, and these parameters were stable during the trial. The same fountain solution was used in both the printing trial and the laboratory tests performed. The fountain solution temperature was kept constant at 10 °C. The lithium (LiCl) and cesium (CsCl) salts added to the fountain solution were supplied by Scharlau Chemie, Barcelona, Spain. The studied paper substrate was a calcium carbonate double-coated wood-free paper with a grammage of 80 g m−2. For creating the images, an oval dot raster was used, with the raster angles being 15° for C, 45° for M, 0° for Y. The raster pattern was amplitude-modulated (the dot size varies depending on the object density) and linearized, i.e., no compensation, for example, for dot gain in the ink−fountain solution press configuration studied. The ink coverage was controlled by the tonal values; i.e., when printing with an amplitude-modulated raster, the varying dot sizes are placed in a fixed grid (equal spacing). Two different tracing elements were tested in order to locate the fountain solution distribution and its absolute amount in the paper. Choosing an appropriate tracer depending on the characteristic property studied is important for correct interpretation of the data. Initially, the distribution and penetration depth of LiCl was traced from prepared cross sections. Lithium had been chosen as the tracing element because of its higher sensitivity in ToF-SIMS compared to cesium, for example, and, in addition, molecules like C10H13, C9H9O, C6H13O3, C4H13OSi2, and C3H9O2Si2 display peaks that disturb (overlap) the cesium peak in the mass spectra. These peaks are related to silicone oil and polystyrene. Analysis showed, however, the presence of lithium in the ink(s) and fountain solutions in the studied press configuration, and also in the reference materials. This was not a drawback with respect to determining the penetration depth of the fountain solution because ink penetration is generally less deep.18,19 However, for quantitative analysis of the fountain solution transferred in the printing press, cesium was chosen as the tracer instead, having checked that it was absent from the starting reference materials. The tracing salt was CsCl and was mixed into the fountain solution only. The CsCl solution added to the fountain solution was itself made to a 10 wt % concentration before addition. ToF-SIMS for Li+ Detection. Mapping of the elemental and molecular species present on the surface and in the bulk, in order to determine penetration of the fountain solution into the paper coating
MATERIALS AND METHODS
Printing Trials and Tracer Materials. The printing trials were carried out at the erstwhile Forest Pilot Center Oy, Raisio, Finland. The printing trials were performed on a Heidelberg Web-8 heatset web offset printing machine with five-over-five printing units, i.e., with the application potential for five colors, each simultaneously applied to both sides of the paper. Kodak Gold (Dipd Gold, positive) printing plates were used. The press-room conditions were held constant, maintained at 50% relative humidity and temperature T = 23 °C. In Figure 1, a schematic of the printing machine is presented as well as a 15603
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Figure 2. ToF-SIMS cross-sectional images showing the Li+ ion distribution in green taken from nonimage (left) and cyan-printed (right) samples. The Ca2+ ion is marked with red. The scale bar is 10 μm. structure, was studied with the ToF-SIMS technique. The ToF-SIMS instrument used was a PHI Trift II spectrometer. The sample cross sections were coated with platinum before imaging. The homogeneity and distribution of Li+ ions was acquired from a 100 × 100 μm2 area. The samples were scanned from five spots. It should be highlighted that the instrument is not able to differentiate the state of the Li+ ion; i.e., both the free Li+ cations and the salt bound ions are detected the same way. The distribution of lithium was followed by imaging using its isotope lithium-7. Note that the ToF-SIMS technique is highly surface-sensitive with an analyzing depth of less than 2 nm. The detection limit is in the parts per million to parts per billion range. The lateral resolution is 1 μm. ICP-MS for Cs+ Detection. For the sample preparation, 0.5 g of each paper sample was weighed and dissolved with 5 cm3 of 65% nitric acid (HNO3), 1 cm3 of 30% hydrogen peroxide (H2O2), and 0.5 cm3 of 40% hydrofluoric acid (HF) and diluted to 100 cm3 in order to dissolve all components. The mixture was mixed well to produce a clear and colorless solution. The mixture was further diluted 100 times because of the corrosive effect of HF on the ICP-MS glass nebulizer. At this dilution, the samples had no more than 0.2% total dissolved solids in order to achieve the best instrument performance and stability. A total of seven repetitive measurements were carried out for each sample. The Cs+ content used to determine the fountain solution transfer in the samples was analyzed with a Perkin-Elmer Sciex Elan 6100 DRC Plus inductively coupled plasma mass spectrometer. The sample is typically introduced into the ICP as an aerosol, formed by passing the predissolved solid sample into a nebulizer. Once the sample aerosol is introduced into the ICP torch, it is completely desolvated and the elements in the aerosol are converted first into gaseous atoms and then ionized toward the end of the plasma region. In the plasma, the elements are atomized, ionized, and then separated by their charge-to-mass ratio (z/m) in the mass spectrometer. The studied cesium isotope was cesium-133.
drawback is that the amount of fountain solution in the respective areas cannot be absolutely determined because of the fact that the ratio of Li+ ions in ink versus fountain solution is not known and thus cannot be normalized. However, as concluded from the observations, under the same conditions for both areas, in terms of printing, it is obvious that the Li+ ion intensity is located deeper in the structure for the nonimage area compared to the printed inked area. Quantitative Analysis of Cs+ Tracer Transfer via a Fountain Solution with ICP-MS. As previously mentioned, it was found that the printing ink and the studied paper both contained lithium. Hence, the tracer for quantitative analysis was changed to cesium. Table 1 summarizes support for this tracer choice in that the concentration of the Cs+ ion in the reference material samples was found to be negligible.
RESULTS Depth-of-Penetration Profiling with ToF-SIMS Analysis of Li+. Cross sections of the variously printed papers were analyzed with ToF-SIMS. Figure 2 shows the Li+ ion distribution in the cross section for both nonimage and cyanprinted paper. First, the Li+ ion distribution is more surfaceconcentrated in the printed area compared to the nonimage area. The ink forms a partially sealing “barrier” layer on the paper, and thus the fountain solution cannot penetrate into the structure as much as in the case of the neighboring more “open”/surface-permeable nonimage area. Hence, it can be concluded that the fountain solution subsequently applied to the image areas is retained preferentially on top of the ink. The
Transfer of a Cs+ Trace Element in a Fountain Solution. Figure 3 presents transfer of the Cs+ ion from the fountain solution supply tray via the printing plate and rubber blanket to the paper nonimage areas. The Cs+ content was determined after the first printing unit (nonimage areas in the cyan unit) as well as after the fourth printing unit (FS) as a function of machine rotations, i.e., printed copies. It can be seen that a certain amount of the fountain solution is transferred to the paper in the nonimage areas, and the Cs+ concentration at equilibrium remains constant with time when one single unit is observed. Obviously, the Cs+ ion content is higher after the fourth printing unit because the fountain solution is delivered from four printing units.
Table 1. Cs+ and Li+ Concentrations in the Studied Reference Paper, Ink, and Fountain Solution, Determined with ToF-SIMSa Cs+ ion references (without ion doping) paper cyan ink fountain solution
Li+ ion
amount/ mg kg−1
standard deviation (±)/mg kg−1
amount/ mg kg−1
standard deviation (±)/mg kg−1
0.000 0.030 0.003
0.000 0.010 ≪0.001
6.050 5.570 1.011
0.500 0.306 0.025
a
Note that these samples were analyzed as they are, i.e., without any additional ion doping.
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Figure 3. Cs+ ion content in nonimage areas of paper as a function of printed copies: first unit (nonimage, cyan unit, C) and fourth unit (nonimage, FS unit).
Figure 4. Cs+ ion content in the ink tray as a function of printed copies.
Figure 5. Cs+ ion content in cyan image areas of paper as a function of printed copies. Cyan areas after the first and fourth units were studied.
As mentioned in the Introduction, ink takes up fountain solution during printing, establishing finally an equilibrium ink−water balance and thus attaining a stable process, with this being borne in mind as we progress to the next step in the analysis.
An ink sample was taken from the ink tray (cyan unit) as a function of machine rotations. Figure 4 displays a strong increase in the Cs+ ion content as a function of increasing machine rotations. For clarification, the figure represents the movement of the tracing element from the fountain solution 15605
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Figure 6. Cs+ ion content for nonimage and different tone value printed areas presented as the logarithm of the number of printed copies.
Figure 7. Cs+ ion movement along the printing process: analysis of the tape samples from back-trapped first down cyan ink.
according to the single color image in a multicolor offset printing process. An increase in the Cs+ ion content after the fourth unit (FS) is also seen, by way of comparison to the first unit, as a function of printed copies in Figure 5. The results also show that a stronger increase in the Cs+ ion content is seen after the first printing unit, which reflects the action of the ink in effectively sealing the surface at the raster points of the image. The results give indications of a steep initial increase in the tracer amount as a function of machine rotations and printed copies; i.e., the data over the large number of copies (>∼1000) do not extrapolate linearly back to the press start-up. The very first printed sheets were, therefore, further analyzed so as to provide data at the onset of this concentration growth. Figure 6 presents a comparison of the Cs+ ion contents for both nonimage and cyan-printed areas, where samples were taken after the first printing unit and running the machine with all subsequent units open. In order to be able to extrapolate linearly back to the press start-up, the amount of printed copies is presented on a logarithmic scale. It is here worthwhile to clarify that initially the machine is turned on, activating fountain solution circulation, at which the fountain solution and ink are not in contact with each other. As the rollers in the printing
supply to the printing plate where the ink and the fountain solution first come into contact and get mixed. The emulsified mixture is subsequently transported via the rubber blanket to the paper. However, what is not taken up by the paper, after an assumed roughly 50:50 ink film split, will be transported further back once again to the printing plate. The movement of the tracer in the system is evident as it works its way back all the way to the ink tray, as is seen in the strong increase in the Cs+ ion content in the sample taken from the ink tray. At this point, we note that the increase in the tracer concentration in the sampled ink is exponential, suggesting already a concentrating mechanism beyond that of simple linear feedback. Next, the tracer content in the cyan-printed area on the paper was analyzed. Figure 5 shows the increase in the Cs+ ion content in the printed image area as a function of printed copies. It should be highlighted that when the samples were taken after the first printing unit, all subsequent units were open; i.e., there was no transfer of any liquids to the paper after the first cyan unit. In the case of the samples taken after the fourth printing unit (FS), the intermediate units were then closed, such that three subsequent layers of the fountain solution were further applied on the cyan-only ink layer, 15606
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units are brought into contact, the first sheet is printed. The machine is striving for ink−fountain solution equilibrium, and an overload of fountain solution to the paper seems to occur, as is clearly seen for the nonimage area. After this, the system finds equilibrium and a stable regime is observed. However, an increase in measurable Cs+ is again seen in the cyan-printed areas on the paper, starting with a strong initial increase. The Cs+ ion intensity was also studied for different tone values, namely, for 25%, 50%, and 100%. Obviously, a lower tone value, giving lower ink coverage, also results in more fountain solution around the printed dots, i.e., in the space between them. The results in Figure 6 show that the Cs+ ion content is the highest in the fulltone (100%) printed areas, decreasing via the 50% areas to 25%. Interestingly, the shape of the evolution is also different for the different print surface areas. In all cases, the evolution is asymptotic, with clearly different rates. The 100% cyan area exhibits a strong initial increase, after which it is increased toward an asymptote outside the range of the running time as a function of the number of printed copies, but as the cyan tone value is decreased, the evolution levels out much faster as a function of printed copies within the run time of the experiment. The trend is even more pronounced for the 25% raster area, which behaves similarly to the nonimage areas, again related to the different dot sizes and thus the space between the raster dots containing a fountain solution only. The general form of the detected ion content over time follows: [Cs+(t )] = [Cs+lim t →∞](1 − e−t / λ)
carried out from different positions within one printing unit as well as from different positions along the process. The fountain solution applied as a liquid layer on a permeable open area is free to be absorbed, induced by the nip pressure. However, in the case of a printed surface, the fountain solution applied on top of the ink is mainly retained in the uppermost surface layers because of the barrier effect of the ink, as was also stated in previous studies.18 In offset printing, the external pressure has an important role to play in the penetration of low-viscosity liquids, such as the fountain solution, but the exception is for the case of high-viscosity offset inks, where penetration in the nip is minimal.19 A quantitative comparison of the tracer (Cs+ ion) amount between the image and nonimage areas was made as a function of printed copies and printing units. The Cs+ ion content was seen to be stable in the nonimage areas within one unit and increased as a function of units passed. Because observations in Figures 3 and 6 show that there is little to no concentration of the tracer over time because of the level line for [Cs+] in the nonimage area, it can be concluded that the Cs+ level is a direct measure of the fountain solution level transferred to paper. This indicates that the fountain solution replenishes itself so fast that there is no concentrating effect. The increase in the Cs+ ion intensity was, however, not linear along the print units because the surface structure of the printed substrate cannot take up an equal amount of fountain solution at subsequent units compared with the more permeable/“open” structure in unit 1 prior to further application. As the paper passes the printing units, the coated paper surface structure roughness will progressively be filled and fountain solution transfer in the subsequent units has previously been concluded to occur according to the film-splitting mechanism and pressure permeation in the printing nip.20 The [Cs+] evolution in inked areas again showed an increasing trend as a function of printed sheets. The increase in the tracer content was found to approach an asymptote, which indicates a concentrating mechanism of the tracer over time. The logarithmic trend was more obvious in samples taken from the ink tray and roller surfaces, compared to those from the emulsified ink applied on paper. The suggested concentrating mechanism, occurring near and from the surface of the ink and induced by the flow properties, is evaporation, with the ink train rollers enabling more rapid evaporation of the fountain solution and, hence, the tracer concentration until local humidity becomes saturated. The ink picks up the fountain solution in order to maintain equilibrium for process runnability, which subsequently evaporates and thus provides a concentrating mechanism for the tracer, which is building up in the ink as a function of time. In addition, it was found that the tracer mobility is not constrained in the circulation but rather moved within one unit and along the process via a feedback mechanism in the back-trapped ink. The proposed concentrating mechanisms, namely, evaporation and film split transfer, require that the starting saturation liquid amount in ink first be determined independently. This was found using a hydroscope test device (Testprint BV, Heerenveen, The Netherlands) by adding dropwise the CsCllabeled fountain solution into the repeatedly film-split pure nonemulsified ink rotating on the rollers. The ink and fountain solution were the same as the ones used in the printing trials. The fountain solution was added until the ink became saturated, which was visually observed as the point at which droplets were first observed floating on the ink in the roller nip.
(1)
where λ describes the asymptotic approach rate constant. The presented results have shown that the tracer is able to migrate around in the process and even within one printing unit. As the paper moves along from unit to unit, a further transport can be expected. Figure 7 confirms the assumption that, because the offset process is printed wet-on-wet, the emulsified ink applied on the paper in one unit is further transported and partly also taken up by the subsequent units in the ink back-trapping ratio mechanism, i.e., the balance between the previously printed ink and the current contacting print station blanket. Tape samples were taken from the rubber blanket from two different units at three separate times, i.e., development of [Cs+] in the trapped ink from the film split to the blanket. The results suggest that, in the beginning, half of the tracer is transported further to the next unit. The tracer evolution is, however, not as strong in the following unit as in the first one, but, nonetheless, the asymptotic trend is again observed. In order not to confuse the transfer with that of fresh application, it needs to be highlighted that the printing plate at this stage of the study was removed from the second unit (magenta, M, in this test; see Figure 1), avoiding any fountain solution transfer disturbing analysis because the aim was only to pick up the transported ink emulsion and follow its paper and press distribution tendency.
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DISCUSSION Tracing agents were used to investigate the liquid movement in a heatset offset printing process. The tracing elements Li+ and Cs+ were added in the fountain solution only. The distribution of Li+ within the paper structure was analyzed from paper cross sections with ToF-SIMS, and quantitative analysis of Cs+ was performed with ICP-MS. Quantitative determination was 15607
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Figure 8. Ink−fountain solution emulsification degree. The abbreviation A.U. denotes NIR absorbance in arbitrary units. The depicted moisture level is linearly proportional to the baseline-corrected water absorption band area between 1890 and 2000 nm, and it is assumed also to be proportional to the true moisture level.
that evaporation of the fountain solution indeed takes place during processing in what is effectively a multiroller assembly, including transfer rollers, printing plate, rubber blanket, and paper. Thus, the amount of tracer circulating in the system cannot be directly correlated to the amount of fountain solution in liquid form delivered to the printing substrate. The problem in analyzing the data is that there is a printplate prewetting stage before the first contact with the paper. The prewetting stage consists of a couple of revolutions, and during that time, the ink and fountain solution achieve an equilibrium plate balance following an overshoot of the fountain solution concentration. After prewetting, the rollers come into contact with the paper. Hence, the problem in analyzing the data is the situation at start-up of the machine, which is needed to force the ink to balance rapidly with the fountain solution before the actual printing starts. The ink is effectively sampled on the plate by the delivery system during the prewetting stage and is thereafter transferred via the rubber blanket to the paper on the first contact once the printing nip is subsequently closed. What is not known is how much evaporation has already occurred during this prewetting. To be able to solve this, a normalization for the increased Cs+ ion amount at the start was made to account for the unknown amount applied to the ink by the forced initial wetting of the ink. The fountain solution has probably all evaporated at this initial loading in the ink, leaving an overdose of [Cs+] as it comes into contact with the paper, and so follows the trend observed in Figure 6. The assumption made here is that the start-up adopts excess fountain solution on the printing plate, as indicated in Figure 6. Thus, the ink will pick up its maximum emulsion load at this point to reach equilibrium as quickly as possible. The aim is to find out the amount of Cs+ ion theoretically possible in the ink at saturation and to compare this with the observed result from spectroscopic analysis of the press samples. For this, the sample conditions for the analyses are summarized in Table 2: Before the trial on the press, the CsCl tracer was added to the fountain solution to a component mass concentration level of 10 wt %. The cesium atomic weight = 132.9 and chlorine atomic weight = 35.45, such that at the given ratio 1:1 M the
At this condition, the fountain solution addition was stopped and evaporation of the emulsified fountain solution was followed as it took place on the continually running hydroscope. A probe based on near-IR (NIR) diffusereflectance spectroscopy was placed on the roller to monitor addition of the fountain solution to the ink. The measurement was carried out in trans-reflectance mode. Ink samples were taken before start-up and after the measurement. As seen in Figure 8, a steep increase in the moisture level of the ink is observed at t = 180 s to the saturation point at t = 220 s. Thereafter, the fountain solution was allowed to evaporate from the ink. At the end of the measurement, the ink, at equilibrium with the environment, contains an amount of moisture equal to that at the start, before addition of the fountain solution; however, the tracing element is drastically concentrated within the ink. The tracer accumulation, as seen in the print on paper, was different depending on the different tone values created from the different raster patterns (Figure 6). A lower tone value has more fountain solution in the noninked areas around the printed dots. Thus, also the volume of emulsified ink per unit area is smaller, resulting in a slower buildup based on the concentration of tracer in the ink. The buildup in lower tone values also leveled out rapidly as a function of time, generating a saturated condition. The saturated condition was also seen to develop rapidly in the midtone values (50%) but was significantly delayed in the fulltone region to beyond the run time tested, which has more volume capacity to store the tracer. As mentioned in the Introduction, previous studies have proposed that most of the fountain solution transferred to the paper surface is carried by the ink on press. The reason for this conclusion originates from the fact that the evaporation effect has been overlooked, and the concentrating mechanism of tracer in ink has thus not been followed from the press start-up, leading to a likely misinterpretation. Previous conclusions, therefore, have been related only to the assumption that the increase in the tracer content results from the fountain solution gradually being incorporated into the ink to form the emulsion, ignoring the increase resulting from the water phase evaporating from the ink in press, leaving the tracer within the ink concentrating as a function of time. This study shows 15608
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48.3 parts of the fountain solution (saturation level) per 100 parts of the ink, i.e., 0.483 g of fountain solution per gram of ink. We now take this hydroscope saturated condition and calculate how much tracer we would expect to find on the press at the first instance that the ink is saturated with the fountain solution. The mass of the fountain solution within unit mass of ink on the press is now calculated by considering the increased mass proportion (mtracer‑containing FS/mink) provided by the tracer concentration in the fountain solution
Table 2. Measurement Conditions and Differences between Instruments method
condition and procedures
Hydroscope
1. calculation of the total salt tracer concentration (CsCl) 2. Hydroscope effectively equivalent to the fulltone application (100% color) 3. analysis based on the fountain solution emulsified per unit weight of ink when saturated 4. Hydroscope, therefore, determining the tracer content at saturation, which is assumed to be equivalent to the start-up prewetting 1. only the ion concentration [Cs+] determined irrespective of unknown liquid amounts 2. raster fraction chosen for equivalence to the Hydroscope (100% fulltone) 3. analysis based on the weight of tracer per unit weight of printed paper, i.e., ink and paper 4. ICP-MS measuring the tracer at print start-up
ICP-MS
mtracer‐containing FS m ink =
mFS without tracer ([CsCl]M w,CsCl + [H 2O]M w,H2O)/M w,H2O m ink (2)
= 0.483[0.1(132.9 + 35.45) + 0.9(2 + 16)]/(2 + 16) = 0.89 g of tracer-containing fountain solution per g of ink = 890 g kg−1, where Mw refers to the respective molecular weight and [ ] to the component concentration. The methodology determines how much CsCl is present, and so it is necessary to calculate the mass of Cs+ ion contained per kilogram of ink from the value of the fountain solution containing the tracer. The CsCl concentration in the fountain solution is 10 wt %, i.e., 0.1 × 890 = 89 g kg−1 CsCl tracer in the ink. The mass of Cs+ ions per kilogram of ink is therefore accounting for 80% of this salt mass, i.e., 0.8 × 89 ≈ 71 g kg−1 Cs ions in the ink under the condition of saturation right at start-up. By extrapolation of the spectroscopic results from the press, we are able to deduce that there are 17.0 mg kg−1 tracer (Cs ion) in the 100% fulltone printed paper at start-up (Figure 10). This is the Cs+ concentration per unit weight of printed paper that would come from the initial full loading and which later gets left as residual Cs+ in the ink. The printed paper piece dissolved for spectroscopic analysis had the size 10 × 8 cm2, giving an analyzed sample weight of 0.64 g determined from the basis weight of the paper. The assumed ink layer thickness on paper is 1 μm. Note, however, that this is a fulltone area derived from a rastered image, so the ink thickness is a melding of the printed dots (differences between measurements highlighted in Table 2). The mass of Cs+ on the actual sheet sample at start-up is thus 17.0 mg kg−1
Cs+ ion accounts for roughly [132.9/(132.9 + 35.45) × 100] = 80% of the contained salt weight. To determine the potential saturation level for the fountain solution in ink, the fountain solution (without tracer) was added dropwise to ink in the hydroscope until saturation was observed visually. Applied amount of ink on roller = 12 g. According to Figure 9, the amount of fountain solution added
Figure 9. Fountain solution addition (without tracer) to ink with the hydroscope device. A total of 5.8 cm3 (5.8 g) of fountain solution in 12 g of ink at saturation gives 48.3 pph fountain solution in ink.
to ink at saturation is 5.8 cm3; i.e., assuming the density of the fountain solution equals that of water, the ink was able to carry
Figure 10. Extrapolation to the initial fountain solution loading. 15609
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× (0.64 g of paper × 0.001) kg = 0.01088 mg. Because all of this is concentrated in the ink layer, we require knowledge of the weight of the ink on the paper. The ink density is assumed to be 1 100 kg m−3, i.e., ink density × ink volume (=size of the ICP-MS paper piece × ink layer thickness) gives the ink mass, which is 1100 kg m−3 × (10 × 8/10000 m2 × 0.000001 m) ≈ 0.0000088 kg = 8.8 mg (ink mass on the sample piece of paper). So, the amount of Cs+ ions in the ink layer according to the ICP-MS technique is 0.01088 mg/0.0000088 kg = 1236.36 mg kg−1 = 1.24 g kg−1. The corresponding value for the nonimage area is 3.5 mg kg−1. This is a direct measure of the fountain solution level when it was concluded that there was, in practice, no concentrating mechanism of the tracer in the nonimage area observed (extrapolation in Figure 11, neglecting the initial dosage at start-up).
a discrete phase held by the surface energy within the ink and not subject to hydraulic pressure removal or by any surface energy differential between ink, water, and paper contact surface, at least on the time scale in the nip. Nonimage Area versus Image Area Fountain Solution Consumption. We know the saturated level of the fountain solution that is able to be carried by the ink from the hydroscope data, which is 0.483 g of fountain solution per gram of ink. We have extrapolated the log t rate (from ICP-MS data) of the tracer increase in the 100% tone region back to start-up, which equals 1.24 g kg−1. This linear function of logt tells us that it is a concentrating mechanism. The amount at start-up needs now to be reduced according to the ratio of print dot area to the region between the dots (raster correction). This is done by saying that the total tracer is made up of (“what is in ink 1.24 g kg−1 × dot area in measured region” + what is in the space between the dots (=nonimage value) × area of space between the dots”)/area of 100% tone image. Note that this is valid for raster patterns, and in the case of fulltone areas, the value in this example is 1.24 g kg−1. This concentration does not exceed the saturation value because 1.24 g kg−1 < 71 g kg−1, namely, the mass of Cs+ ions per kilogram of ink under the condition of saturation right at start-up. A last reality check is to obtain the fountain solution amount in the ink by subtracting the fountain solution delivery rate in the nonimage area from the total fountain delivery rate amount used on the press. The ratio of the fountain solution being delivered to the rate of new ink being consumed = 1.3244, as derived from Figure 11. Hence, the fountain solution delivery rate: 3.87/1.3244 = 2.92 g s−1. Comparably, the ink delivery rate is given by ink density × ink layer thickness = 1 100 kg m−3 × 0.000001 m = 1.1 g−2 at a running speed of 5.6 m s−1. Therefore, the time taken to pass 1.1 g of ink through the press is equal to the time taken to pass 1 m2 through the press. In 1 s, an area of 0.63 m (sheet width) × 5.6 m = 3.528 m2 passes through. Thus, the time taken to pass 1 m2 through = 1/3.528 = 0.284 s, i.e., 1.1 g of ink pass through every 0.284 s ≥ 1.1/0.284 = 3.87 g s−1. We then deduce the rate of paper mass through the machine at a printing rate of 32000 copies h−1 as 0.0252 kg (the mass of paper printed per copy) × 32000 copies h−1 = 806.4 kg h−1 = 806.4/60/60 kg s−1 and is the mass of paper running through the machine press per second = 0.224 kg s−1 = 224 g s−1. Finally, from the print layout, it can be determined that 30% (in the cyan ink station only) consists of the cyan-printed area (ink dot coverage area). This means that 70% is free fountain solution in the nonimage area. Thus, the fountain solution delivery rate on paper at the printing station = nonimage rate (2.92 g s−1 × 0.7) + image rate = (2.92 g s−1 × 0.3), where the second term gives 0.876 g s−1 as the fountain solution delivery rate in the real inked areas. Although the values above are, of course, rate values, because the fountain solution delivery rate is known and the rate of fountain consumption at the nonimage area and the space between the dots is also known in principle, but the balance is roughly consistent with the derived amounts by tracer determination. Future Perspectives. The general trend in today’s fountain solutions is toward lower or zero isopropyl alcohol (IPA) content, which is compensated for by mainly glycol-based nonionic surfactants. IPA in a fountain solution is expected partly to evaporate when applied to the surface and partly to penetrate with the liquid phase into the substrate. The surface
Figure 11. Rate of increase of the tracer concentration.
Figure 11 shows that the rate of increase of the measured tracer with respect to the logarithm of time, logt, is directly proportional to the tone level. It shows the ratio of the fountain solution being delivered to the rate of new ink being consumed. This demonstrates that the tracer increase is primarily occurring in the ink and very little concentration is occurring within the fountain solution itself, being at a very low level and so only a small perturbation on the value that could be expected for a 0% tone level, i.e., 1 − tone level. Furthermore, the greater the amount of ink being transferred per unit area onto the paper (tone level), the greater the amount of ink per unit area on the plate and blanket and, hence, the greater the evaporation area exposed. That this is log−linearly proportional supports that evaporation is indeed the primary mechanism by which the tracer is being concentrated in the ink. Because the print density (tone level) is a logarithmic function of the ink pigment amount, this accounts for the observed log−linear relationship. Finally, that the rate of increase of the tracer in the fountain solution is so low supports the long-held assumption that the fountain solution is replenishing that being taken up by the paper and that very little is being returned on the blanket back to the plate; i.e., fountain solution absorption dominates over evaporation on the plate and blanket. This supports also the well-known cleansing action of the fountain solution. Further information can also be deduced from Figure 11. We have shown that the ink carries a certain amount of fountain solution. Because the rate of tracer increase is proportional to the exposed area only, without any undue offset at zero tone, and is being returned on the blanket, we can deduce that the fountain solution is not migrating from the inner bulk of the ink to the paper in the nip, such that the fountain solution remains 15610
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active agent again, as the name indicates, will remain on the surface or penetrate into the bulk of the substrate and will thus not evaporate. Hence, it can be assumed that the different solutions will exhibit differences not only in ink−fountain solution behavior but also in paper surface properties generated by the buildup of surfactant. This accumulation will change the ink−fountain solution balance and affect the subsequent wetting behavior. Despite the fact that the surfactant concentration in partly or totally IPA-free fountain solutions is relatively low, its role on the surface properties should not be underestimated, especially during longer production runs with respect to phenomena such as vanishing dots and blanket and plate buildup.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Paul Ek from Åbo Akademi University is gratefully thanked for assisting with the ICP-MS measurements. REFERENCES
(1) Aspler, J. Ink−Water−Paper Interactions in Printing: an Updated Review. Proceedings of the TAPPI Advanced Coating Fundamentals Symposium, Turku, Finland, Feb 8−10, 2006; TAPPI Press: Peachtree Corners, GA, 2006; p 117. (2) Koivula, H.; Bousfield, D.; Toivakka, M. Use of Confocal Laser Scanning Microscopy and Computer Model to Understand Ink Cavitation and Filamentation. Proceedings of the TAPPI Coating and Graphic Arts Conference, Dallas, TX, 2008; TAPPI Press: Peachtree Corners, GA, 2008; pp 1−17. (3) Kipphan, H. Handbook of Print Media; Springer: Heidelberg, Germany, 2001. (4) Lindquist, U.; Karttunen, S.; Virtanen, J. New Models for Offset Lithography. Adv. Print. Sci. Technol. 1981, 16, 67. (5) MacPhee, J. Further Insight into the Lithographic Process with Special Emphasis on where the Water Goes. TAGA J. 1985, 269. (6) Lim, P. Y. W.; Daniels, C. J.; Sandholzer, R. E. Determination of the Fountain Solution Picked up by the Paper and Ink in Offset Printing. Tappi International Printing and Arts Conference, Minneapolis, MN, 1996; TAPPI Press: Peachtree Corners, GA, 1996; p 83. (7) Trollsås , P. E.; Larsson, L. O. Vattenupptagning och Dimensionsförändringar hos Tidningspapper vid Tryckning i Offsetförfarande, Tidningspappersbrukens forsknings-laboratorium, rapport nr. 4:6, Stockholm, Sweden, 1987; p 1. (8) Hansen, Å. Water Absorption and Dimensional Changes of Newsprint during Offset Printing. In Advances in printing science and technology. Proceedings of the 23rd Research Conference of IARIGAI, Paris, France, Sept 17−20, 1995; John Wiley and Sons: New York, 1997. (9) Trollsås, P. E. Water Uptake in Newsprint during Offset Printing. Tappi J. 1995, 78, 155. (10) Beckman, N. J. The Wet Resistance of Coated Offset Papers. Tappi Journal; Technical Association of Pulp and Paper Industry: Atlanta, GA, 1962; Vol. 45, p 855. (11) Beckman, N. J. A Bench Test for Predicting the Effect of the Fountain Solution on Pick and Curl of Offset Papers. TAGA Proceedings of the 11th annual meeting, Rochester, NY, 1959. (12) Pritchard, E. J.; Akers, D. Water Pick-up on an Offset-Litho Press. Printing Laboratory Report No. 41; The Printing and Packaging & Allied Trade Research Association: Leatherhead, U.K., 1961; p 13. (13) Lim, P. Y. W. Method for Measuring the Amount of Fountain Solution in Offset Lithography Printing. U.S. Patent 5,826,507, 1998. (14) Reinius, H.; Pajari, H.; Tahkola, K.; Mikkilä, J.; Pohler, T.; Nieminen, S.; Hermansson, E.; Schulze, U. Knowledge of the Interactions between Fountain Solution and Coating Provides Ways to Improve Printed Paper Quality. Profess. Papermak. 2006, 1, 44. (15) Preston, J. S.; Husband, J. C.; Norouzi, N.; Blair, D.; Heard, P. J. The Measurement and Analysis of the Distribution of Fountain Solution in Kaolin and Calcium Carbonate Containing Coatings. Proceedings of the TAPPI Coating Fundamentals, Montréal, June, 2008; TAPPI Press: Peachtree Corners, GA, 2008; p 1. (16) Lipponen, J.; Lappalainen, T.; Astola, J.; Grön, J. Novel Method for Quantitative Starch Penetration Analysis through Iodine Staining and Image Analysis of Cross-sections of Uncoated Fine Paper. Nord. Pulp Pap. Res. J. 2004, 19, 300. (17) Sodhi, R. N. S.; Sun, L.; Sain, M.; Farnood, R. J. Analysis of Ink/ Coating Penetration on Paper Surfaces by Time-of-Flight Secondary
CONCLUSIONS
Transportation of the fountain solution in the heatset offset printing process as “pure” liquid and as an emulsion with ink has been studied at different time intervals and process positions. This system constitutes two immiscible liquids having very different vapor pressures, insomuch as one component, namely, the fountain solution, undergoes preferential evaporation at room temperature and pressure. The location within the paper structure and in the printing units was monitored by mass spectral analysis of Li+ and Cs+ ion tracing elements in relation to penetration into the substrate and quantitative distribution throughout the process, respectively. The nonimage area on the coated paper showed a constant tracer value over time, t, because all of the fountain solution delivered in the nonimage area is seen to be consumed. The tracing element was found to be surface-concentrated in the paper coating under process conditions, especially in printed image areas, and the liquid fountain solution was found to (i) migrate within one printing unit and (ii) transfer along the process, exhibiting a concentrating mechanism of the tracer amount in the ink, increasing toward an asymptote over time. The conclusion is that the fountain solution in the ink is concentrating the tracer probably because of evaporation and so has a linear trend with logt. The concentrating mechanism of the tracer was shown independently on a hydroscope to be due to evaporation of the water phase from the ink emulsion as it maintains an equilibrium emulsion concentration of the fountain solution. For the ink, the fountain solution is trapped in the ink during the printing but is exposed to evaporation in the split and supply balance on the plate, with increasing tracer concentration following the gradient of the logt plot. It can thus be concluded that the fountain solution in the ink emulsion becomes trapped and is not expelled under the nip pressure. However, in the nonimage areas, evaporation of the fountain solution is only in the desired drying areas of the press, and the fountain solution replenishes itself so fast that there is no concentrating effect. It was determined that, for this specific case, the amount of fountain solution carried by the printing ink to the paper was less than that transferred to the nonimage areas. The results imply that previous conclusions based on tracking tracer compounds dissolved in the fountain solution have been misleading because of the likely oversight of ignoring evaporation before application to the paper and in subsequent recycling through the press. 15611
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Ion Mass Spectrometry (ToF-SIMS) in Conjunction with Principal Component Analysis (PCA). J. Adhes. 2008, 84, 277. (18) Rousu, S.; Gane, P. A. C.; Spielmann, D. C.; Eklund, D. Separation of Offset Ink Components during Absorption into Pigment Coating Structure. Nord. Pulp Pap. Res J. 2000, 15, 527. (19) Oittinen, P. Fundamental Rheological Properties and Tack of Printing Inks and their Influence on Ink Behaviour in a Printing Nip. Ph.D. Dissertation, Helsinki University of Technology, Helsinki, Finland, 1976. (20) Tåg, C. -M.; Toiviainen, M.; Juuti, M.; Ridgway, C. J; Gane, P. A. C. Online Detection of Moisture in Heatset Printing: the Role of Substrate Structure during Liquid Transfer. Ind. Eng. Chem. Res. 2011, 50, 4446.
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