Physical and Chemical Drying in Sheet-fed Offset Printing on Coated Paper

摘要

Ink drying of sheet-fed offset prints on coated papers involves depletion of ink oil from the ink film (physical drying) and polymerisation of ink binder (chemical drying or oxidative drying). New techniques have been developed and used to study these phenomena. Depletion of ink oil was studied by chemical analysis, while polymerisation of model ink vehicles was characterised by ATR-FTIR (Attenuated Total Reflectance Fourier Transform Infrared Spectrosco-py). Ink drying after full-scale printing was followed by scuffing the ink film against an unprinted paper at high pressure and measuring the rub-off smear on the recipient paper. It was shown that the initial oil depletion, i. e. ink setting, was fast and very much enhanced by decreasing the pore size of the coating. Roughly, half of the original amount of ink oil had been absorbed after a few minutes. The time very much depended on the ink-setting rate. Oil depletion continued for many hours, although at a low rate. All or almost all the binder stayed in the ink film on top of the paper. The ink film c ontained high amounts of binder and may be regarded as a polymer matrix filled with pigment. Model studies showed that the polymerisation of the vehicle was very much determined by the driers and antioxidants. Drying after full-scale printing was highly determined by temperature. Type of paper had a significant influence on physical drying and less influence on chemical drying. Access to air strongly influenced the time for the chemical drying to be completed. Pile drying was six to ten times slower than drying with full access to air.