At the moment of printing a quantity of ink or varnish is absorbed by the surface of the paper. This amount is determined by the absorption of liquid in the surface recesses (roughness) and the absorption into the paper pores at the surface. With the in this leaflet described test method the sum of the two phenomena is determined: the oil absorption or varnishability. The reciprocal value of this is called print penetration. It is quite safe to assume that, when various batches of paper of the same type are supplied by the same manufacturer and the oil absorption test renders the same result, both the roughness and the absorption have remained the same. It is most unlikely that, for instance, the effect of the roughness has increased in exactly the same ratio as that of the absorption has decreased.
Due to its simplicity the test method is very suitable to be used in quality control systems. The test is suitable to evaluate three paper properties:
• Varnish ability: In the case of varnishability low oil absorption and thus a long stain is essential. For that reason the varnishability is expressed directly as the stain length in mm.
• Print penetration: A large stain indicates a low roughness/absorption of the paper. It seems to be logical to use the reciprocal value of the stain length multiplied by 1,000 as an identifier of this property (i.e. 1,000/stain length in mm). This is then called the print penetration.
• Felt and wire side: see W60 The method has been standardized in the Dutch standard NEN 1836.
Principle
Between the printing disc and a paper strip on the sector of an IGT-printability tester a drop of oil with a volume of 5.8 ± 0.3 mg is spread to a stain (see figure 2). The length of the stain is measured. The stain length is increasing when the roughness and/or absorption of the paper is decreasing.
Method of operation
• It is recommended to execute the test in the standard atmosphere; to most standards it is 23.0 ± 1.0 °C (73.4 ± 1.8 °F) and 50 ± 2% rh.
• For the operation of the AIC2- 5T2000 and Global Standard Tester follow the instructions of the manuals, IGT information leaflet W100 and the displays accurately.
• Handle the samples carefully.
Assessment
1. Measure the length of the stain to the nearest mm. Thus the stain length or varnishability is determined.
2. If required determine the print penetration with the formula: Printing penetration = 1000: stain length in mm
3. Repeat point 1 and 2 for each test strip.
4. Calculate the average and if required the spreading and/or standard deviation. In some cases it may be useful to mention the highest and lowest values as well.
