V M Traders


Harnessing the Power of Electronic media for the Print media


Resistance tests for inks.

Essentially a printing is no more than a means of transferring a coloured pigment onto a substrate. Having got it there quickly and efficiently –not always the easiest of tasks –the ink system must ensure that it remains there under whatever conditions the end user requirements of the print demands.
            Sometimes these demands can be very stringent, and have necessitated a range of tests for resistance properties to be established by ink makers, many of which have applications that are independent of the printing process.


            A basic requirement of any ink is that it adheres to the substrate upon which it is printed. With paper and board, this is really a problem. With non-absorbent substrates such as plastics and foils, it can be. Three very simple test methods have been established which give a very good assessment of ink adhesion- fingernail scratching, scotch tape test and wrinkling. The fingernail test is a very useful technique for foils, films and plastic, including polythene bottles and sheet plastics. The ink film is scratched with the nail and the ease or difficulty with which the ink is removed is a measure of the adhesion or the hardness of the ink film.  

The scotch tape is probably the most widely used, but in many ways, is the least reliable.  A length of tape is pressed down onto the ink film and quickly pulled off. The print is then examined for ink removal. Unfortunately the test is very dependent on how well the tape is pressed onto the ink, what type is used, how long it remains and at what angle and how rapidly it is pulled off. The wrinkle test is commonly used on film. A piece of print is grasped between the thumb about one inch apart. The hands are then put together and moved in a rotary motion so that the face of the print is rubbed against itself for a specified number of rubs. This is a quite a severe test, but more realistically matches the conditions that a printed film will come under during its lifetime.

Rub resistance

The need for good resistance to rubbing, or abrasion, is most significant for carton inks as they are subjected to considerable scuffing. Rub resistance of ink is a difficult property to quantity and should always be assessed by comparison with standard ink of known performance on the board under test. Several mechanical rub tests are available of which the Patra is the most familiar and the most likely to give reproducible results. A large disc of plain board is placed face upwards on the circular platforms and a smaller a printed disc is placed face downward on top of this. A weight is dropped onto the test disc and the machine is run for a specified number of revolutions (usually 50). The large and small discs revolve at the same rate but in such a way that their centers do not coincide, thus causing planetary motion. The print is removed and the degree of ink transfer to the larger plain disc assessed in comparison to a standard. The severity of the test can be varied in two ways; firstly by means of different weights of loading and secondly, by altering the number of revolutions. This test cannot be readily modified to suit lightweight substrates but others have been devised for such cases, for instance heavy duty sacks and dry rub of coloured inks against white on packaging lines.  

Grease / rub resistance

            Grease / rub resistance is important for inks designed for use on snack food packs. A satra rub tester assesses the ink by rotating a weighted 1" diameter felt soaked in a known quantity of a particular grease or oil on a print sample for a given number of revolutions, normally 500. One such test specifies that after 100 rubs, no removal should be noted with a maximum of 50 percent removal after the full 500 if the ink is to be considered satisfactory. 

Blocking and set off

            Blocking is the term used to describe the condition when sheets or reels of print stick together. Set-off describes the transfers of ink from one print onto the back of the print above. Both can be related to the drying of ink in some way. One of the major causes if blocking is failure to remove all solvent from the ink film. It is most common in flexo and gravure printing of plastic films can be a problem in screen printing of sheet plastic, particularly PVC. Set-off is less severe manifestation of slow drying and although the problem can arise in any of the print processes, the term is normally used to describe the poor settings of a litho ink.

Laboratory equipment used to evaluate these two properties may simply be a stack of paper or board left on a bench or a sophisticated hydraulic press needed to simulate the extremely high pressures (5-20 tons per square inch) that may be found at the center of a reel of film whatever the test method, any transfer of ink is noted and, as the result can't be assessed quantitatively, compared against control sample. 

Other physical resistance properties

            The action of light on an ink film can induce and catalyze chemical changes in the pigment molecules which lead to marked colour changes or fading. Stability to light is an important property when the in is to be used for outdoor work or for display in a shop window. Testing for light fastness can be carried out by the traditional method of exposing the print outdoors for several months. This has the advantage of assessing the inks weather ability at the same time and is therefore particularly suitable for testing the inks for road signs for example. A much faster method but which gives no clue to weather ability is to usr an accelerated exposure machine such as an Xenotester where exposure of the print to a xenon discharge lamp can simulate 3 months sunlight in less than one day. On many occasions, it is desirable to be able to heat a pack over an ink film without the ink smearing. The ink film must thus be resistance to heat. The apparatus used consists of a pair of electrically heated jaws with provision for accurate control of temperature, pressure and dwell time. A cruder method uses to domestic iron with base drilled to take the stem of a small dial thermometer.

More and more print is now being subjected to deep-freeze conditions which can place severe demands on ink and varnish films. A satisfactory ink will maintain adhesion and/ or flexibility at temperatures of -30°c to -40°c and can be tested by subjecting the print to a series of deep-freeze/thawing cycles plus wrinkle testing in the presence of water and checking the appearance of the print.

Resistance to chemical influences.

Many inks need to be formulated to resist the product that they are designed to pack and may also be required to withstand other chemical influences. Sometimes a chemically resistance varnish may be used to seal off ink films from their surroundings and, in so doing, ink formulation and colour availability are made that much easier.

            The resistance of inks to chemical influences is dependent on both the pigments used and the resin system. Some resins have poor resistance to specific chemicals (e.g. the poor grease resistance of polyamides) and their use is avoided at the initial formulating stage. Resistant pigments are often more expensive that alternative grades and are only to be used if it is absolutely essential. Thus it is important that any requirement for chemical resistance is specified when the ink is ordered. Chemical resistance tests can, if carefully chosen, indicate the likely product resistance of an ink or varnish but often the only way to obtain absolutely reliable results is to test with the exact product being packed. No two detergents for example are exactly alike and may give different results. The packaging buyer often specifies test methods. The technique varies depend on the product, whether be solid or liquid for example, but essentially all such tests are similar and involve bringing the print into contact with the product under specified conditions. Typical of them all is the soap gel test in which a 25% soap solution is prepared and allow to cool to form to a gel. A portion of print is damped and pressed, face down, on the gel to ensure close contact. After storage, the print is removed and compared for shade and strength with the original print, and the soap gel is examined for any sign of bleed.

            In such a limited space, it has only been possible to outline the test methods used. Most are far more detailed in their application; often modified to test specific properties identified by printers and end user. The ink maker may even design new tests to simulate specific production conditions. It is however, difficult to simulate packaging and transit conditions in the laboratory and therefore, close cooperation between ink maker, printer and end user is essential, ideally, this should be followed from laboratory testing through print and production trials.



Copyright ©  printingindia.com  All rights reserved
Website designed and Hosted by Quartette InfoTech Pvt. Ltd.