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Interaction with fluids

Cobb test (ISO 535, Tappi T441):

WhatDetermination of the absorbing capacity of the substrate, expressed in g/m², in relation of which the degree of sizing can be established as a percentage.

Sizing has impact on the printability and the degree in which the substrate is writeable. In addition, the Cobb value plays a role when applying glue or the bonding with another substrate. Also some applications need to be watertight (e.g. deepfreeze board)

HowAn area of 100 cm² of substrate is weighed. During a defined time – usually 30, 60, 120 or 300 seconds – a specific volume of distilled water is poured on to the substrate that is clamped between a rubber clad metal surface and a metal ring. After respectively 20, 45, 105 or 285 seconds the water is removed and another 15 s later the excess is blotted with a metal roller of 10 kg. Then the substrate is weighed again. The difference between the weight wet and dry is de weight of the absorbed water.

A = absorptiveness
m1 = dry mass in grams of the test piece
m2 = wet mass in grams of the test piece
F = 10 000/test area in cm²Report the average weight of absorbed water of at least 5 significant values at both front and back and the standard deviation.
NoteThis International Standard specifies a method of determining the water absorptiveness of sized paper and board, including corrugated fibreboard, under standard conditions. It may not be suitable for paper weighing less than 50 g/m2 or embossed paper. It is not suitable for porous papers such as newsprint or unsized papers such as blotting paper or other papers having a relatively high water absorptiveness.
This method is not intended to be used for precise evaluation of the writing properties of paper although it does give a general indication of suitability for use with aqueous inks.

Dynamic water absorption (TAPPI T558):

WhatThe absorbing capacity of the substrate measured in time, following the behaviour of a drop of water and expressed in a contact angle °. This test provides in following the absorption during a period of time (usually 10 to 12 s)

The registration of the absorption in time gives information about ink absorption into the substrate and consequently about the economical use of the ink as well as the ink/water balance on the printing press.

HowWith a FibroDAT Dynamic Absorption Tester a 4µl drop of distilled water is laid upon the substrate by a fully automatic dispenser. An internal camera makes a beforehand determined amount of images per second and software subsequently calculates the contact angle (being the inner angle) of the drop with the substrate. The sharper the angle, the more the drop has penetrated the substrate.
CalculationReporting the average angle of 8 measurements per sample/drop in time and the standard deviation.
NoteNext to the contact angle data on drop base, drop volume and drop height can be provided upon request.

Surface energy measurement derived from water absorptiveness (TAPPI T558):

WhatEach fluid has its own surface tension: the force with which it wants to stay more or less a drop (cohesion) Also a substrate has a surface energy: the force to break the surface tension of the drop (adhesion) Surface energy is expressed in dyne/cm or, officially, in mN/m.

This measurement gives information on the amount of IPA (or alternative additives) that should be added while printing in order to influence the surface tension of the fount. Thus the balance between ink and fount is reached quicker. Using less fount means using less ink. Since offset is an indirect printing method (plate – blanket – substrate – counter pressure) some additional moisture is lost during the process. The substrate however, still needs to be capable of attracting moisture (to prevent scumming) If too much moisture is attracted the printing image will be poor and too little leads to dot gain.

HowAfter having measured the dynamic water absorption, the same test is repeated where the fluid is replaced by DMSO (dimethyl sulfoxide) and the drop placed will be 5 or 6 µl.
CalculationReporting the average angle of 8 measurements per sample/drop in time and the standard deviation.
NoteThe dynamic water absorption indicates a lot about surface energy already: if a drop has a wide base and a sharp contact angle then it indicates high surface energy. Only in case the surface energy needs to be put down in a value, a test with DMSO is necessary.
Both results of distilled water and DMSO are compared and used to calculate the surface energy (software)A simpler alternative to this test can be done by making use of dyne markers or dyne fluid. This is frequently used on manmade substrates. In order to make it possible for ink to adhere, its surface is treated (so called corona treatment) Different printing procedures need different minimal dyne values for the ink to adhere. Offset needs between 35 and 38 mN/m. The dyne fluid is applied to the substrate’s surface and may not bead. The dyne value at which it starts to bead is called the critical dyne value.
This test may be simple; however, a downside is that the dyne fluids or markers have a relatively short shelf-life.

K&N oil absorption

WhatThis test is a simple way to see how quick the mineral oils in ink are absorbed by the substrate. It also reveals other possible effects like cloudy printing image, the so called mottle effect.

To determine the speed with which the oils are absorbed.

HowA cork is dipped in black ink (usually absorption test ink) that subsequently is applied to the substrate. After 30 s this oil is smeared over the surface, creating a ‘comet’s tail.’ After 60 s the density of both the ‘head’ and ‘tail’ of the comet is measured. (See split-screen picture below of two different substrates) Density measurement may be repeated in a pre-determined interval.

Report at least 5 density values of both head and tail and the coefficient of variation.


This test is a quick and dirty variation on the IGT oil absorption test as described in Information Leaflet W24

Making pinholes visible

WhatPinholes are minuscule holes (pores) in the substrate.

Pinholes have a negative impact on strength properties and on the printed image.

HowBy performing the K&N oil absorption test while a blank sheet of copy paper lies underneath, pinholes are visible on the copy paper (the oil penetrates through the holes and leave a mark on the copy paper) See split-screen picture below of two different substrates.

Report by means of pictures, to enable visual assessment.

Vetwerendheid palmkernolie (ISO Grease resistance – permeability test with palm kernel oil (ISO 16532):

WhatResistance of a substrate against penetration of grease, expressed in time of break through and/or show through.

Especially relevant for wrapping and packaging of greasy (food)products like butter, chocolate, cake, etc.

HowThe substrate is creased diagonally. With a metal template the palm kernel oil (colorized red) is applied to the surface. The sample is sandwiched between a SC paper and cellophane, the palm kernel oil towards the cellophane. The sandwich is placed on a clear glass platen. A weight is then placed on this stack.

Grease penetration now is checked with regular intervals by means of a mirror. The grease permeation in time is determined. (See figure below)
Both the folded and unfolded part of the surface are evaluated separately.

CalculationReport the duration in time of at least 2 significant measurement cycles.
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