Characterization of Surface Topography of Coated Paper, 1999 Coating Fundamentals Symposium Proceedings

Kari Juvonen, R. Hannele Järvinen and Olli Suontausta

The Finnish Pulp and Paper Research Institute

Besides high level of smoothness, several other aspects of paper surface topography are important for high print quality. These include the scale of roughness in the xy-direction and the uniformity of surface topography. Standard air leakage roughness measurements employ a relatively large xy-scale and give only the average level of roughness.

Laser profilometry is a noncontact method for measuring coated and uncoated paper topography on a relatively small scale. A laser beam 1 µm in diameter is focused on the paper surface and the z-directional position of the surface is determined from the focal length of the focusing system. The

sample is moved on an xy-table so that either two-dimensional areas or one-dimensional line scans can be measured. The areas measured typically range from 2*2 mm² to 50*50 mm². The maximum scanning length for line measurements is 190 mm.

The method of analysis involves dividing the roughness variation into separate wavelength bands and calculating the RMS value (intensity) of roughness variation as a function of wavelength (scale). The overall effect of calendering is readily seen using this procedure. Roughness at all wavelengths is affected by calendering, but the most pronounced changes take place at wavelengths typical of fiber bundles or small flocks (wavelengths from 50 µm to 1 mm).

A more sophisticated way to view the effect of calendering is to analyze the local variations in roughness. At short roughness wavelengths, in particular, we have detected large local variations, i.e. the intensity of roughness variation changes from point to point. One reason for this may be that the calender roll surface does not make uniform contact with all the features of the surface (e.g. valleys, steep slopes and hills).

In the heatset offset printing of LWC paper, the filling effect of the printing ink reduces roughness at wavelengths below 25 µm. At the same time, the fountain water and ink-fountain water emulsion cause fiber roughening, which can be detected at wavelengths over 100 µm. Fiber roughening after printing is more pronounced on ink surfaces than on water surfaces. It is assumed that the printing ink acts like a glue by preventing the roughened paper from regaining its original smoothness during drying.