The role of pore density in the setting of offset printing ink on coated paper.

Application: Knowing which factors govern ink setting rate is a valuable tool in designing coated papers that have improved press runnability and ink gloss. This interpretation of coating porosity should help paper mils work with suppliers to engineer desired qualities in coating structures.

Printed pages that have a positive "snap," where printed areas have greater gloss than the unprinted paper, are highly appealing. Coating pigment can be a critical factor in print quality. Very fine pigments give high paper gloss, often resulting in poor snap. Ultrafine pigments coatings also give a rapid tack buildup, leading to press problems such as back trapping and piling. The presence of very fine pores may promote fast ink setting, which may mean there is not enough time for the ink film to level after printing, reducing smoothness.

For this study, we determined the mean pore diameter and pore volume of pilot coated papers prepared using commercial pigments under controlled conditions. Coatings with ultrafine pigments (kaolin and calcium carbonate) had more pores in a given area of paper than coatings with engineered or platey grades of clay.

We found that ink sets more slowly as the average pore size decreases, if the coatings have the same number of pores. When we plotted the data conventionally, we found that as pores become finer, ink setting rate becomes faster, in agreement with previous studies. This effect is a direct consequence of the increasing number of pores present as their size becomes finer.

Our results indicate that if ink setting rate is normalized for a constant number of pores per unit area of coating, larger pores will give taster ink setting. However, if compared at equal pore volume, coatings having more numerous smaller pores will set ink faster, as normally observed. In practice, ultrafine pigments, which give very fine pores, set ink very fast because the large number of pores has a greater influence than their size. This principle has general applicability to most clay and calcium carbonate coatings used in gloss papers.

Preston, Elton, Legrix, Nutbeem, and Husband are with Paper Research Group, European Paper Group, Imerys Minerals Ltd., Par Moor Road, St Austell, Cornwall PL24 2SQ, United Kingdom. E-mail Husband at [e-mail] or Preston at [e-mail].