Absorption is the speed with which ink penetrates into paper after printing. For the offset process this property is very important. If the ink is absorbed too slowly or too fast, it may be the cause of problems. An absorption, which is too slow, may result in smearing because the ink does not dry fast enough. An absorption, which is too fast, may result in a reduction of the dry ink properties because too much binding agent has penetrated into the paper and mainly the pigments remain at the surface. Because of this, for instance, the abrasion resistance or gloss may be reduced. The varnish ability is influenced by the absorption as well.
The test is important when using oxidative drying offset inks and varnishes. The absorption tests are usually carried out as comparative tests. The chosen absorption times may differ per type of paper.
Principle
A strip of the paper to be tested is printed with an IGT printability tester under standard conditions with a set off ink. After certain times the strip is brought into contact with a blank strip of a standard set off paper. Part of the ink, which is still present at the surface of the printed strip, will smear on the set off strip. The more ink is being absorbed into the printed strip, the less ink will smear on the set off strip. The density of the smeared ink will be a value for the absorption. Since smearing mainly concerns the ink, which is at the top of the surface of the paper, the test strip is printed with a hard (aluminium) printing form.
The complete range of recommended set off times is about 0.1, 3, 6, 10, 15, 30, 60 and 120 s. The short interval times < 10 s can be tested with the IGT Global Standard Tester 2; all other times with the IGT Global Standard Testers 2, 3 and 3H. For paper with little absorption, longer times may be of importance. If the penetration of an ink has to be tested, a standard paper will be printed with the ink to be tested. Further, the test is carried out as described in this information leaflet.
Method of operation
• It is recommended to execute the test in the standard atmosphere; to most standards it is 23.0 ± 1.0 °C (73.4 ± 1.8 °F) and 50 ± 2% rh.
• For the operation of the AIC2-5T2000, Global Standard Tester, High Speed Inking Unit 4 and ink pipette follow the instructions of the manuals, IGT information leaflet W100 and the displays accurately.
• Handle the samples carefully.
Assessment
1. After > 4 hours of making the print, measure the contrast density of the set off strips.
2. Calculate the average of the measured densities per set off time per type of paper.
3. If desired calculate spreading and/or standard deviation. Sometimes it may be useful to note the highest and lowest values as well.
4. If desired make a table or graph.
