Calendering Response of Calcium Carbonates in Double Coated Woodfree Paper, 2001 Coating Conference Proceedings

Anthony Hiorns--Fine ground calcium carbonate (GCC) is used as the major component in topcoating of woodfree paper. It generates high brightness and has been a cost effective choice. It does not normally generate sufficient paper and print gloss on its own, so a small proportion of clay is usually added to boost these properties. The development of precipitated calcium carbonate (PCC) into a high quality coating pigment now means that there are more options available to the paper maker.

The engineered size distribution of PCC means that they typically have worse rheology than a pigment with a broader size distribution. Although this means that PCC’s may have to be applied at slightly lower colour solids levels, the open and bulky coating structure generated still gives excellent fibre coverage.

This study shows that aragonitic PCC can outperform GCC and generate excellent sheet optics, gloss and smoothness. However, the open coating structure of the PCC coatings can give fast ink setting, effectively locking in any ink film splitting patterns.

New calendering technology using faster speeds and higher roll temperatures allows for a wide range of conditions to be used in order to achieve a given target sheet gloss level. This study reveals that the coatings made from the aragonitic PCC respond well to calendering, generating higher gloss values at less severe calendering conditions compared to GCC.