Gloss Dry-Back from Wet Trap Printing with Aqueous Coatings
By Theresa Ruehle, H. Tom Mills, Sunil Jayasuriya, Paul Gloor, and Stephen A. Fischer
Editor’s Note: “Gloss Dry-Back from Wet Trap Printing with Aqueous Coatings” is an abridged version of the first-place recipient of the National Printing Ink Research Institute’s (NPIRI) Lecture Series at NPIRI’s 46th Annual Technical Conference at Marco Island, FL from Oct. 2-4, 2002. Readers can request the full report by e-mailing Dawn.Schaab@jwp.com.
Ink dry-back is a term used to describe the decrease in optical density over time after printing that usually results in a loss in gloss and a shift in print color. Ink dry-back is typically observed in the sheetfed offset printing process.
Gloss dry-back is often observed after wet trap printing with aqueous coatings over wet litho ink on a sheet-fed offset printing press. The reduction in gloss is significant over the first 30 minutes, after printing and coating, until finally reaching its final value hours after printing. Typically, more than a 20-point gloss loss can be observed in 24 hours.
Many variables can affect gloss dry-back, such as the paper type, ink penetration into paper, ink set time, coating composition, and printing conditions, i.e., print speed and dry conditions. This paper offers a hypothesis and supporting evidence to minimize gloss dry-back in wet trap printing with an aqueous coating, where the ink must display a high solid (gel-like) microstructure to achieve a faster cessation of flow to slow its penetration into paper along with a coating having a flexible polymeric structure.
A good measure of the ink microstructure is the complex viscosity (η*) calculated from an oscillatory flow experiment. This hypothesis was explored in the laboratory and on a commercial Heidelberg press using various litho inks and waterborne acrylic coatings.
High gloss is a key property that provides visual appeal for a variety of print and coating applications. Gloss is influenced by the surface characteristics of the substrate and the physical properties of the coating, which influence the formation of a smooth and continuous film.
Efforts to achieve high gloss over wet litho ink are diminished by dry-back. Gloss dry-back is often observed after wet trap printing with aqueous coatings over wet litho ink on an offset printing press. The reduction in gloss can drop 20 points before finally reaching equilibrium 24 hours after printing. The gloss dry-back effect is not easily reproduced on a laboratory scale. The ink is typically applied using a Little Joe proofing press and the coating applied either with a hand proof roller or a draw down rod.
This paper examines the effect of commercial litho inks and aqueous coatings on the gloss dry-back effect during wet trap printing. A commercial press study identified variables that affected gloss dry-back. A laboratory procedure was developed to duplicate the gloss dry-back effect observed on a commercial printing press.
Data gathered in the lab was used to develop a hypothesis about the cause of gloss dry-back. The hypothesis was tested on a commercial Heidelberg press. The paper used in the laboratory and on press was SBS 12-point gage paperboard.
Laboratory Dry-Back Test Method: A test method was developed to observe gloss dry-back in the laboratory using a Quickpeek color proofing kit (Thwing-Albert). The instrument controls the amount of ink applied to the surface of the substrate, SBS 12-point paperboard. A metal measuring bar contains two holes of different size, one small and one large, for ink delivery from the steel slab to the hand roller that is applied to the surface of the paper. Ink films can vary from light to heavy depending on the hole size and volume of ink used. Ink densities targeted were between 1.31 and 1.36, which is characteristic of a heavy ink film.
After printing, the ink was coated using a 300Q Flexo hand proof roller. Dry coating film thickness was measured at 2.0 microns (about 0.4 lb./msf) by a scanning electron microscope on a freeze-fracture cross-sectional view of a coated paperboard. The time between inking and coating was strictly controlled. The freshly coated print was immediately dried in a forced air oven for 5 seconds at 43°C. A gloss meter was used to collect 60° gloss readings, at several positions on the print, over time.
Figure 1 illustrates the gloss dry-back of three colors using the laboratory method and an aqueous acrylic coating. The time between printing and coating was very critical for reproducing gloss dry-back in the laboratory, which is illustrated in Figure 2. Figure 3 shows that the time between inking and coating is critical and validates the laboratory method. Initial gloss readings on the press were taken immediately after coating.
Viscoelastic Measurements: When materials have flow properties with both solid and liquid characteristics, their rheology is described as viscoelastic. Viscoelasticity is time-dependent and typical of all polymeric materials, including inks and coatings. Oscillatory flow measurements define the elastic modulus as G’ = (τ/γ) cosδ and the viscous modulus as G” = (τ/γ) sinδ, where τ is the sinusoidal stress and γ is shear. The complex viscosity, which is related to the viscosity of a viscoelastic fluid, is calculated as η* = [(G’/ω)2 + (G”/ω)2]1/2, where ω = frequency. Oscillatory flow experiments were run on a Rheometrics SR-5000 at a gap width of 0.5mm.
Coating Compositions: Three aqueous acrylic coatings were used in the laboratory and one was selected for press trial evaluations. These coatings were all designed for the coater of sheet-fed lithography printing. The coating compositions were typical for the industry, formulated using low molecular weight acrylic resins, film forming and non-film forming emulsion polymers, wax dispersion, defoamer and wetting agents.
Ink Compositions: The litho inks are commercially available and designed for use in the sheet-fed printing process. All inks are described as quick setting and drying. Figure 4 illustrates the complex viscosity (η*)-frequency profiles of the yellow inks from an oscillatory flow experiment.
Ink Gloss Dry-Back: Using the laboratory method for measuring gloss dry-back, several commercial yellow inks were studied. The gloss dry-back results are shown in Figure 5. The yellow inks displayed different rates of gloss loss as a function of time.
Postulating that the gloss loss is related to stress on the dried coating resulting in cracks and other surface structural defects from the ink undergoing flow, there must be differences between the inks. It should be noted that ink could flow both laterally on the paper surface and vertically inside the paper matrix.
A possible explanation for the results could be the set times of the inks, but they are all described as quick setting. Ink setting can be adjusted to about two-minutes or up to 30 minutes depending on the ink formulation, ink thickness, substrate and temperature. A quick-setting ink typically sets within two minutes. Postulating that ink displaying less gloss dry-back is caused by its tendency to laterally flow or penetrate into the paper more slowly before setting, one must consider the ink’s gel structure and flow characteristics after printing. Viscosity at very low shear rates is considered a good measure of the strength of inter-particle interactions and hence the degree of ordering in dispersions, like inks.
For most systems, particularly polymers, low shear η* (complex viscosity related to the viscosity of a viscoelastic fluid calculated from an oscillatory flow experiment) provides a convenient measure of low shear viscosity as stated in the Cox-Merz empirical rule.1Figure 6 illustrates that gloss dry-back is a function of the ink complex viscosity (η*) derived from extrapolating the η* data (Figure 4), obtained from oscillatory flow experiments, at an extremely low frequency that would mimic shear forces after printing. Similar correlations are observed with viscosity (η), and G” (viscous modulus).
The observed results can be rationalized on the basis of the temporal order of cessation of flow for the ink and the coating upon application. If the ink completes its flow before the coating (the ink has set, but the coating has not dried), then the coating will undergo uninterrupted drying resulting in no dry-back.
This situation is observed in dry-trap printing. On the other hand, a dried coating will continue to suffer stresses from ink flow causing surface defects resulting in a loss in gloss over time.
A more solid (gel-like) microstructure in an ink helps achieve faster cessation of flow in two ways. First, faster relaxation times associated with stronger inter-particle interactions cause faster structure recovery upon disturbances during application. Second, higher yield stresses existing in a solid-like microstructure retards penetration inside the paper matrix.
Depending on the polymer structure and the coating components (ratio of hard resin, hard emulsion, and soft emulsion), flexibility (as measured by glass transition temperature, Tg) can be varied to some degree. We postulate that a flexible coating will tend to be more resilient as the ink flows laterally and penetrates into the paper, decreasing the surface deformities resulting in less gloss dry-back.
To test this hypothesis, a more flexible acrylic hybrid structure, experimental resin NMF based on an acrylic-polyether resin,2 was formulated into a coating and tested. The results are also graphed in Figure 6 showing much less gloss dry-back than the other acrylic coatings, HG and CMF. The laboratory study concluded that inks displaying high complex viscosity (η*) at low frequency combined with a more flexible coating results in less gloss dry-back.
Commercial Press Results
Heidelberg Press Data: Two yellow inks displaying different complex viscosity (η*) at low frequency and the CMF coating were selected for press trials. The tests were performed on a Heidelberg Speedmaster 102 at 8000 s/h using SBS 12-point paperboard. Coating weight was applied at about 0.6 pounds/msf (msf = thousand square feet). The ink was always in the sixth (last) station on the press. Calculated time from printing to coating was 2.4 seconds.3
The data from the press trial is summarized in Figure 7 showing that higher final gloss was obtained, with the ink exhibiting a higher complex viscosity (η*), 24 hours after printing and coating. Very little or no gloss reduction is observed when coating was applied over non-inked areas.
Gloss dry-back is strongly influenced by the roughness of the coating surface caused by surface deformation during wet trap printing from the flow of the ink and aqueous coating. The process is dynamic, and gloss dry-back equilibrates in less than 24 hours after printing and coating.The effect can be rationalized on the temporal order of cessation of flow for the ink and the coating upon application. A more solid (gel-like) microstructure in an ink helps achieve faster cessation of flow.
A good measure of the strength of inter-particle interactions is the complex viscosity (η*) at low shear rates. Laboratory data and commercial press data show that inks displaying a higher complex viscosity (η*) at very low frequency result in less gloss dry-back.
This conclusion might be obvious to one skilled in the art of lithographic printing; however, the effect can now be quantified through the understanding of the ink rheology. Also, coatings with a flexible polymer structure can contribute to less gloss dry-back.
This information can be used to select the right combination of ink and coating that result in less gloss dry-back during wet trap printing.
1. Cox, W.P., Merz, E.H., J. PolymerScience, Vol. 28, 619, 1958.
2. Anderson, J.L, Tokas, E., US 6,194,510, February 27, 2001.
3. Communication with Heidelberg AG, 2000.