The Calenderability of Coating Grit Lines
Rick Crabtree
Chemical Research Associate
Mead Central Research
Chillicothe, Ohio 45601
Gary Fugitt
Senior Research Specialist
Mead Central Research
Chillicothe, Ohio 45601
ABSTRACT
The objective of the study was to determine characteristics of grit lines (coater scratches, blade lines, etc.) that could be identified which would allow prediction of which grit lines may be rendered undetectable after supercalendering.Samples of thirty two grit lines which varied in severity were collected for the analysis.The severity of the grit lines was determined before and after calendering via profilometry, and visually with the aid of low angle and transmitted light.
The calenderability of grit lines did not correlate with grit line width, depth, or combinations of width and depth. Calenderability appeared to depend on the amount of coating which remained in the valleys of the grit lines (as determined by transmitted light) and the steepness of the valley walls.The calenderability of grit lines was enhanced with increasing calendering temperature or pressure. The combination of increased calendering temperature and pressure may have had additive effects on the calenderability of grit lines.Experimental
The samples used for the evaluation were produced on a 47 g/m2 ground wood
containing rawstock. The rawstock was coated (ClS) on a pilot coater at 915 mpm using a
rigid blade set-up with a .38 mm, 40° blade.
The coat weight was approximately 11 g/m2.
The grit lines were naturally occurring
scratches assumed to be caused by solid
material trapped beneath the blade. The
coating formulation consisted of fine clay
(42-46 parts), fine ground calcium carbonate
(14-18 parts), anatase titanium dioxide (2-5 parts), plastic pigment (5-7 parts), and a
starch/latex binder system (9 to 11 parts),
Thirty two samples containing grit lines varying
in severity were collected for the analysis.
Each grit line was rated on a scale of 1 to 10
with the aid of transmitted light, a rating of 1
being the lightest grit line and 10 being the
most obvious or severe. A Beloit Wheeler laboratory hand calender was used for calendering the samples. The calendering conditions included pressures of 144 and 206
kN/m at temperatures of 66°C and 82° C.The samples were sent through the nip three times.
Grit line dimensions were measured using profilometry both before and after calendering.
The profilometer was a Mahr-Perthing Perthometer PRK with an S8P processor and a Focodyn laser optic, non-contact measuring
head. The measurement has a vertical
resolution of .1 micron and individual measurement points were spaced 5 microns
apart on center.The measurement area is a 2 micron diameter circular area.
Discussion
Calenderability, as defined in the context of this evaluation, is the determination of which
coating grit lines could be removed or made undetectable by supercalendering. The
objective of the evaluation was to determine if
1996 Coating Conference 135