Performance Analysis of High Viscosity and Low Viscosity Offset Printing Inks (I)

1 Introduction

The ideal offset ink should have the dual characteristics of high viscosity and low viscosity. As we all know, the basic performance of ink is mainly determined by the binder. In the production process, different resin binders are used, and the quality and performance of the ink are also completely different. At present, rosin-modified phenolic resins are mostly used for lithographic offset printing inks. There are many varieties of this type of resin, the main difference is the use of different phenols, such as phenol, bisphenol A, tert-butyl phenol, octyl phenol, nonyl phenol, dodecyl phenol and so on. With the gradual increase of phenolic molecular weight, the performance of the resin will be significantly improved and improved. In this regard, many people have done research. Some researches have been conducted on rheological properties and printing suitability (1), and some have conducted detailed discussions on the constituent raw materials, synthetic processes, and ink applications of rosin-modified phenolic resin binders (2, 3). Various kinds of information in recent years indicate that there are not many domestic manufacturers that have mastered the production technology of high-grade alkylphenol phenolic resins.

However, the earlier rosin phenolic resins, such as phenol, bisphenol A, tert-butylphenol and other modified rosin phenolic resins, have always had a conflict of increasing viscosity and solving solubility problem. It is difficult to ensure high viscosity and low viscosity at the same time when producing inks using such low molecular weight resin binders. Of course, using low molecular weight resins can also increase ink viscosity. For example, increase the viscosity of the resin or increase the amount of resin in the binder. However, both of these conditions are bound to result in high viscosity and insufficient fluidity of the ink. In fact, it is a "high-viscosity, high-viscosity" product; the molecular weight of the resin can also be made lower, but such a resin is difficult to ensure high viscosity. The usual practice is to mix them and get some performance trade-offs. This is often very passive and often overlooks others.

The author starts with the analysis of the influence of the viscosity and fluidity of ordinary low viscosity (low molecular weight) resin binder inks on printing, and focuses on the excellent performance of high viscosity and low viscosity inks.

2 Effect of ink stickiness and fluidity on printing

Inks produced with low molecular weight resin binders, whether they are sticky or fluid, can only be selected within a certain range. Therefore, there is a problem that the stickiness is high or low, the fluidity is too large or too small, and thus the printing is directly affected.

(1) If the viscosity is high and the fluidity is large, the ink appears thin and sticky. When printing, there will be flying ink and pulling paper hair, aAB, referred to as flying ink area.

(2) If the viscosity is high and the fluidity is small, the ink becomes thick and sticky. When printing, there will be more severe paper pulling and poor transfer. bBC, referred to as the paper pull area.

(3) If the viscosity is low and the fluidity is high, the ink appears thin and soft. There will be dirty prints and dot gains when printing. dAD, referred to as dirty area.

(4) If the viscosity is low and the fluidity is too small, the ink will appear soft and weak. When the printing occurs, there will be a situation that the ink piles and the dots are not clean, cCD, or the ink accumulation area.

Among them, the most important is the O point. This is a theoretical pole value that does not exist in practice. However, the diamond ABCD area can be considered as an acceptable product area. The four corner areas a, b, c, and d are the most problematic areas.

3 High Viscosity and Low Viscosity Ink Properties

The above analysis refers to those ordinary low viscosity low molecular weight resin binder inks. At present, most of the high-quality lithographic four-color inks used in the link material, mostly by high-level alkyl phenol (octyl phenol, nonyl phenol, dodecyl phenol, etc.) phenolic resin production. The ink produced with such a resin binder has the characteristics of high viscosity and low viscosity.

The excellent properties of high viscosity and low viscosity inks are closely related to the structure of the resin. Mainly have the following characteristics:

The resin has a large molecular weight, a high softening point and a high viscosity. The high viscosity of the resin is the premise of the high viscosity of the ink, and it is also the guarantee that the ink can achieve low viscosity. Rosin-modified phenolic resins are complex macromolecular compounds obtained by the addition of rosin (polymerized rosin) and a phenolic phenolic resin, followed by esterification with a polyol. The macromolecule high-viscosity rosin modified phenolic resin has high structural characteristics, good gelability and strong cohesion. Therefore, when the ink is produced, even if the viscosity is made lower, it can maintain sufficient high viscosity, good rubber elasticity and cohesion. Essentially speaking, high viscosity and low viscosity are a relative concept and they are two contradictory aspects. The higher the viscosity, the better, and the lower the viscosity, the better. If we do not talk about uniformity, we can use ordinary low-viscosity resins, can also produce very high viscosity inks (open sticky) and very low viscosity inks (open sticky). However, such inks are not easy to use, and they have no use value. The real meaning of the so-called high viscosity and low viscosity should be "to maintain a sufficiently high viscosity in the case of low viscosity." The ink with this dual characteristic can be called good ink.

(to be continued)