Metrohm has developed a method, based on Karl Fischer titration and utilising the 874 oven sample processor in conjunction with the 851 coulometer, for determining the water level in printing inks.
According to the company, printing inks are complicated mixtures of chemical compounds.
The composition varies by printing process, by whether printing is sheet fed or web fed and by substrate.
Recent developments in printing presses (sheet-fed and web-fed machines in terms of high-speed production and inking/dampening units) and drier equipment (nitrogen blanketing and cold lamps) have led to an increase in the number of applications in the graphic arts industry: boxes for cosmetics, food, tobacco, spirits, business forms, direct mail, lottery tickets and credit cards, for example.
The composition varies by the solvent base, be it oil based or water based.
It varies by drying mechanism and by whether the drying is by primarily chemical or physical processes.
Inks are divided into liquid and paste inks.
Gravure, flexo, inkjet, electrographic and some electrophotographic inks (or toners) are liquid inks, while lithographic and letterpress inks are paste inks.
The distinction between liquid and paste inks can be an arbitrary one, since it is based on apparent viscosity.
Sheet-fed inks dry by oxidation of the vehicles or varnishes carrying the pigment in the ink.
These varnishes, made from linseed oil or Tung oil, will oxidise slowly at room temperature.
To assist in drying, all inks have some drying catalysts added in the form of an inorganic salt, cobalt, manganese and lead (rarely).
When ink is applied to a non-porous surface or where accelerated drying is required, an extra boost of these porous catalysts can be applied by adding cobalt driers to the ink.
The accelerated drying also takes place on the rollers, cobalt and in the ink duct so inks with added driers must be processed quickly or they set in the ink train.
On non-porous surfaces, getting enough oxygen to the ink may be a problem, so driers (calcium perborate) can be porous added to release oxygen into the ink.
The more ink, the slower the drying, and some colours dry slower than others.
Ink drying will be retarded by cold temperature, low pH (acid) and high relative humidity.
High-humidity levels of water will emulsify in the ink and will slow ink drying.
Keeping water to a minimum and using alcohol to reduce the film thickness is important on non-porous surfaces.
This water level needs to be determined both quickly and accurately, preferably at the point of manufacture but also when the inks are applied to different substrates.
Metrohm's new method for this is said to be both fast and accurate.
