CRACKING AND ROUGHENING OF COATED PAPER SURFACES

Roughening is a well-known phenomenon with both uncoated and coated pa pers; it takes place when the paper comes into contact with water or o ther agents during coating or printing, Roughening is caused by an exp ansion of the fibre network, which results in fibre rise followed by a decrease in gloss. Such a reduction in gloss is highly undesirable fo r a coated and/or printed surface, since the required smooth surface d oes not fully develop. This study focuses on surface roughening, with particular reference to the cracking of coated surfaces when carboxyme thylcellulose (CMC) of different molar masses was added to the coating colour systems in varying amounts along with SPS clay or ground CaCO3 (90% < 2 The ESEM technique, which allows reproducible wetting and dr ying of a specific area, was used in the present study. ESEM was carri ed out as a real-time study of water contact with both the coated surf ace and the base paper. The ESEM study revealed that coating colours b ased on CaCO3 pigments cause a higher degree of both surface roughenin g and cracking than coating colours in which clay was used as the pigm ent. The distortion of the surface is due to contact with water during the wetting procedure, due to rising of the long fibre fraction, This effect seems to be related either to extremely low adsorption of CMC onto the pigment particles or to differences in particle shape. The sh ape of the pigment particle determines the contact area between partic les in a dried coating layer. The recovery of the cracks is also relat ed to the quantity and molecular mass of the water-soluble thickener f or ground CaCO3 coating colours. Clay-based coating colours are genera lly more resistant towards surface roughening caused by fibre expansio n or fibre rising, and no significant cracking occurs in the surface.