What is tack?
Tack is a measure of the forces required to split a single film into two. In case of an ink, such film splitting is influenced by rheological and adhesive ink properties and the internal cohesion of the ink.
Tack depends heavily on the ink formulation. Inks must be formulated to optimize the quality of printing and minimize undesired effects in the printing process, such as substrate rupture or coating pick-up, web contamination, dot gain, poor setting, loss of gloss, loss of colour density etc. that can be caused by too high or too low tack.
When tack is too high it is hard to separate the paper from the printing blanket or when it is too low the ink does not transfer properly down the inking roller chain. A major influence on ink tack has the vehicle system used for the formulation, so that good tack properties of vehicles are crucial for good performance of litho inks on the press.
Tack is a critical issue in the lithographic process and tack is relevant to all stages of distribution or transfer in the printing process. Therefore, accurate tack measurements are essential for a good performance of an ink on the press.
How to measure ink tack?
Conventional instruments being developed to measure tack all rely on the same principle of measuring the force exerted on a measuring roller that is positioned on the ink film of a driven roller. This method suffers various disadvantages due to the fact that the result of both compression and splitting forces are measured.
In addition to this, different manufacturers of tackmeters have established their own arbitrary scales, so that it is difficult to compare the results of tack measurements of an instrument from “manufacturer A” with the ones from a “manufacturer B”. Even comparability of measurement results of instruments from a single manufacturer is difficult, mainly due to the fact that different roller surfaces produce different interactions of the cohesive, rheological and adhesive components that contribute to tack. The RealTack test method is a
breakthrough in tack measuring technology by elimination of errors caused by roller surface conditions.
Traditional method of tack measurement
Instruments for tack measurements from different manufacturers currently on the market are all using the same principle of operation:
A defined weight of ink is placed on a three roller system. The roller system consists of a middle, metallic driving roller, an ink distribution roller and a measuring roller for tack determination. These two outer rollers are covered with an elastomer layer. After speed adjustment and temperature stabilization the axial force on the measurement roller is determined. This axial force is used as indication of tack. The higher the axial force, the higher the determined tack number.
The principle disadvantage of this method is that there is no separation between the forces needed for film formation and film splitting as both forces are combined in the measurement. Film formation is mainly influenced by the inner friction of the ink. Film separation is mainly influenced by the inner cohesion of the ink. Because film formation and film splitting are based on different physical effects, there is no general prediction for any correlation between these two forces.
The second phenomenon, which can mask the tack measuring result, is the influence of the elastomer layer on measurement roller. Its dynamic mechanical properties have different levels of effect depending on the test conditions such as temperature, surface speed and pressure. The dynamic mechanical properties of the elastomer layer are not constant during use of the instrument but are influenced by material aging, solvent contamination, mechanical stress on surface, etc. This may explain the large variations in tack measuring results from
instruments of the same design.
The RealTack method – setting a standard in accuracy of tack measurement
The new method overcomes the principle problems of the traditional method. A newly developed, small, high speed force sensor is integrated into the surface of the driving roller. This sensor forms a common hard plane with the roller surface. The force on this sensor is scanned continuously versus rotation angle in real time.
With this unique approach it is not only possible to separate the measurement of the forces involved in creation and splitting but also the detailed force progress versus rotation angle. Also the different viscoelastic properties of elastomer roller can not influence the tack measurement anymore...
As a result of this new approach to tack measurement a lot of new information is now available from a single test method:
- Length of film splitting process
- Peak force and integral force of film splitting process (TACK, inner cohesion of ink)
- Form and statistical variations of splitting curve (ink length, misting and flying tendency)
- Active length of roller gap
- Length of film formation process
- Maximal force and integral force of film formation process (indicates inner friction of ink)
- tack force measurements in dependence on line force, speed and film thickness without principle influences caused by the instrument
Measurement on roller surface eliminates systematic errors caused by differences in viscoelastic properties of elastomer roller. In addition to this the inner friction of the film doesn’t affect the tack result anymore. These advantages are leading to new levels of measuring accuracy and reliability.
Real time data acquisition provides rotation angle dependent force measurements which can be evaluated as force function versus angle graphically. Extensive tack characterization is possible by systematical curve analysis providing a wealth of information instead of a single tack number.
The miniature sensor devices in the roller surface enable new insights in ink morphology and may be used for instance to characterize nanostructered formulations.
The innovation driven technology of the RealTack is setting a standard in accurate tack measurement. It is a new benchmark for R & D and QC in the printing ink industry.
For further information feel free to download the instrument specification and/or contact Novomatics.
