Rheological properties and printability of water-washable inks (below)

(Rheology And Printability of Water Washable Waterless Ink)

Continuous flow ordinary waterless inks—The rheological curves of the four printing inks (yellow, pink, black, and black) used for ordinary horizontalless printing are shown in Figure 1. By observing the viscosity curve (n vs. gamma), we can conclude that these four inks (except perhaps black ink) exhibit Newtonian fluid properties at 0.01-10S-1, followed by large shear rates (>10 S Partial viscosity of -1) decreased. In addition, the shear rate between 10-1000 S-1 is also reduced due to the difference in the resin-based binder in the offset ink or the unwinding of the polymer chain. The black ink's curve dropped after 10 S-1, perhaps because of the poor wetting of the pigment in the resin.

The stress curve is almost linear throughout the test range. Each curve follows the Herschel-Bulkley model δ=δy+Kγη, where δy refers to the deformation stress (minimum stress required to observe the flow), K is the viscosity coefficient, and η is the proportionality factor. The value of δy is listed in Table 1. All δy values ​​were measured at 0 (×10-12) Pa. However, when the shear rate is small, there is no resistance to flow. This feature is extremely important in the transfer of ink to paper, and the initial coating process. The pigment is evenly distributed in the resin, without friction, without resistance, and fully wetted by the resin. In the entire process, only a small amount of stress is generated in the ink.




Fig.1 The rheological curves of traditional anhydrous ink (K,C,M,Y)

Table 1: Stress Yield Values ​​in the Herschel-Bulkley Model Four-Color Ink δy (×10-12) Pa (for normal inks) δy (Pa) (for Class A inks)
K 17500 131
C 5.9 448
M 0.11 513
Y 5.2 785

Low-gloss, water-based, water-washable water-free inks (Type A) - Flow curves for low-gloss washable inks A for four printing primaries are shown in Figure 2. Compare the viscosity curve with the shear rate in the range of 0.01-10 S-1. From Fig.1, there is no Newtonian stagnation in ordinary printing ink. Assume that the stagnation phenomenon occurs because the selected resin does not wet the pigments. Therefore, it is necessary to find new resin-type binders to improve the infiltration of the pigments, thereby improving the smoothness and gloss of the ink on the prints. In both figures, the shear bands with high shear rates (>10S-1) are very narrow.

In the stress-shear rate curve, Herschel-Bulkley's model δ = δy + Kγη yields a large yield stress when the shear rate is low (from 131 Pa of black ink to 785 Pa of magenta ink). In print, these yield stresses must be overcome in order to obtain a high degree of smoothness. This stress not only affects the gloss of the A ink, but also affects the flowability of the ink on the ink fountain roller during the printing process. In fact, magenta inks with high yield stress have poor fluidity in ink fountains.




Figure 2. Flow curve of low gloss washable ink A




Figure 3. Color flow curves of anhydrous A and normal inks

A comparison of the flow curves of the 1:1 ratio of A ink to normal ink of the same color (magenta) in FIG. 3 reveals that the shear rate is lower (
High-gloss, water-based, water-washable water-free ink (Type B) - Since it is sometimes necessary to print high-gloss prints with Type A ink, a second washable paste ink (Type B) is produced . The flowability is compared with ordinary waterless ink below. Figure 4 is the viscosity and stress curve of the ink. It can be seen from the figure that the viscosity of the magenta B-type ink and common ink is the same at 0.01S-1, and there is a very small Newtonian stagnation. There is a deviation from the point of stagnation, and the deviation of type B magenta ink is getting larger and larger. This deviation is caused by the nature of the resin type binder. From the SR curve, it can be seen that the yield stress of the two is very low and is actually the same. When the shear rate is not high, the yield stress gradually decreases and the gloss increases gradually. Formulations Suitable resinous binders improve pigment wettability and dispersion stability.




Figure 4. Viscosity and Stress Curve of the Ink

Viscoelastic properties In the ink-chain measurement, the elastic modulus is defined as G′=(τ0/γ0)cosδ, the viscous modulus is defined as G′′=(τ0/γ0)sinδ, and tanδ=G′′/G′. Tan δ is a measure of the elasticity and viscosity of an offset ink and is related to the ink string frequency. The high stringing ink frequency corresponds to the case of grinding pigments during the printing process, and the low stringing ink frequency corresponds to the smoothing of the ink and the flow within the container.

The magenta ink and A-ink inks show the inking curve as shown in FIG. 5 . The elastic moduli G' and tan δ are represented by different frequencies. When the frequency is low (



Figure 5. Vibration curves for magenta traditional inks and A inks




Figure 6 is a graph of the vibration of a Type B magenta ink (ie, a high gloss, washable magenta ink) and an ordinary ink.

The graph shows that the G' curve is consistent over the entire frequency range. The tan δ value of the two inks is approximately 2 (more than 1.5 in the entire frequency range) in the range of 10-100 Hz, which indicates that the rheological properties and the highlight characteristics of the two inks are consistent. Figure VI. Vibration Curve of Type B Magenta Ink and General Ink

Friction resistance The normal type and B type of anhydrous ink are printed at the same density using Prufbau's printing equipment. This print was subjected to a rub resistance test on the Sutherland Rub Tester. The weight was 4 lbs and 50 cycles of friction. The results are shown in Figure 7. The above two figures are comparisons of the tests performed for normal ink (left) and water-washable anhydrous ink (right) at 5 minutes after printing. Both prints for each ink used for comparison represent different products. As can be seen from the figure, the prints printed with water-washable water-free ink have much higher abrasion resistance than the prints printed with ordinary water-free inks. The following two figures compare the abrasion resistance of the normal (left) and water-washable anhydrous inks after 24 hours of printing. As in the above figure, the two prints for each ink used for comparison represent different products. Similarly, printed matter printed with washable inks performs better than ordinary inks. In addition, the printability of the prints printed with the water-washable ink after 5 minutes (upper right) was better than that of the prints printed with the normal ink at the print 24 hours after the print (bottom right).





Figure 7. Comparison of rub resistance between normal inks and water-washable inks. Conclusion Two types of water-based, water-washable, VOC-free inks have been on the lithographic stage. This will improve the print-on-demand printing of colorless editions from 5000 to 10,000 copies, and it will quickly develop with the digital revolution.

The parameters that reflect the new ink viscoelasticity have been obtained. An important aspect of printability - high gloss, is related to the following properties: 1) at low shear rates (
The ratio of viscous modulus (G") to elastic modulus (G') of high-gloss inks is similar to that of ordinary paste inks.When the vibration frequency is not high, the smoothness and gloss of specialty inks can be observed. At frequencies, more elastic behavior can be observed.Gloss changes are a complex process, and some important properties such as stickiness are not within the scope of current work.Compatible with ordinary waterless inks, washable inks It also shows great advantages in terms of abrasion resistance, and its abrasion resistance measured 5 minutes after printing is better than that of normal waterless ink measured on the day after printing.

The use of polymer chemistry makes the formulation of offset inks more flexible and diverse. It determines the viscoelastic and performance characteristics of the ink.