Printing Properties : Behavior towards Liquids
The printing properties of paper result from complex interactions between print¬ing ink, printing process, and paper. Practice-oriented printability tests must be performed to evaluate these properties. Test printers are also suitable for this pur¬pose. Instruments for offset printing, gravure printing, and flexographic printing can be used with standard printing inks under laboratory conditions to test the dry pick resistance or the wet pick resistance of papers. Missing dots, mottling and ink penetration properties can also be tested. Conversely, the behavior of various print¬ing inks towards standard papers can also be evaluated.
Using an image analyzer for testing printed image and printing process has increased during the last decade. This is due to the improved capability of the analyzers and because the importance of the quality of the printed image has increased. The image analyzer and its use in paper testing has already been men¬tioned earlier in connection with paper tests. The image analyzer is used to meas¬ure the uniformity of paper (print mottle, number of missing dots, and properties of dot), surface properties of paper (fiber rising, picking, and contact angle meas¬urement), and many other properties of paper, i. e., width of cracking at the fold.
Furthermore, defined proof copies produced with the test printers can be used to test full ink coverage, color density, color gloss, shade, abrasion resistance, stack¬ing ability, and contact yellowing.
More reliable results obviously come from tests that closely simulate the actual printing process. That is why tests developed for full-scale printing machines give the best predictions of the actual printability properties of a paper
The behavior of liquids towards paper is characterized by the processes of wetting and penetration. In both cases, the characteristic physical property is the surface tension. This value can be measured directly and tensiometrically in the case of liquids and indirectly, via the contact angle of test liquid droplets, in the case of solids such as paper. A liquid wets the surface of paper only if its surface tension is lower than that of the paper. The same holds for the wetting of the capillary walls upon penetration of liquids into the capillaries of the paper.
If the wetting and the penetrating capacity of liquids are to be changed, the surface tension of the paper must also be changed. This is achieved, for instance, by sizing the paper, a process which must fulfill the requirements regarding printability with aqueous inks. According to DIN 53 126 (2001), paper is considered to be printable if a standard ink line drawn with an adjusted drawing pen has neither run nor penetrated into the paper after 24 h.
The water absorption WA (Cobb) (ISO 535, 1991) refers to the amount of water that is absorbed by a certain area of paper on one-sided contact for a specified exposure time. The time of exposure to water must be chosen such that a sufficient amount of water enters into the fiber matrix, but does not penetrate to the opposite side of the sample.
In the determination of grease permeability (DIN 53 116), red-colored palm kernel oil is used as the testing agent. The passage of fat through the sample under specified conditions is then evaluated.