From the examples above one can see that with Pcc the coating colour often also contains some clay or talc. An effect of using these platy pigments is that they improve the water retention of the coating colour. If no water is retained, it will drain into the paper instantly during application, causing a hard cake of pigment to build up on the metering device. For example, if the coating is applied with a blade, poor water retention causes hard form that catch under the blade and leave particles to visible scratches on the coating surface. It is generally known that water retention is poorer with GCC than with clay and, in some cases, even poorer with PCC. The theory behind this is illustrated in Figure 62. In the case of kaolin, water has to go a longer way through the tortuous path created by the packing of platy As the the water particle size distribution becomes narrower and the shape NOT platy, has a shorter penetration path and the "tortuosity factor" is reduced. This may not be the only reason, but perhaps the most important. The other side of the coin is the blistering tendency. A coating colour with an open structure/poor water retention has been found to significantly reduce the blistering tendency of paper, which is important today as the operating speeds of heatset offset presses are rising
It is known that water retention can be improved by adding some CMC or synthetic thickener to the coating colour. Using starch as a binder also improves water retention
5.5 Coating applications of various grades of PCC
For high glossing double- or triple-coated papers, fine-particle-size PCC has been found to be the best choice. Fine particle size in this application means an APS of 0.3-0.4 um. In this case, it does not matter whether the base stock is woodfree (contains only chemical pulp) or wood-containing (contains at least 10 mechanical pulp). If plastic pigment is in use, it can be replaced with PCC, which makes the econ gloss of the coating colour very attractive. If high gloss is not need the high stiff can be exploited with lighter calendering, which means higher bulk, higher stiffness and better opacity and brightness than with other pigments
For precoating, coarser coating pigments are necessary. Pccs with an APs of 0.6-0.8 um recommended to open the of the layer. This signifcantly improves opacity by increasing the number of efficient light scattering voids With this open structure, water retention can be a concern and it may be necessary to mpensate by adding some long chain-length CMC starch synthetic to the coating colour