Publicação
IMPACT OF PRECOAT STRUCTURE ON TOPCOAT PERFORMANCE IN COATED BOARD

Authors: Eli Gaskin1, Janet Preston1, Andrew Findlay1, Peter Heard2, Alexandre Lucato3, Edenil Costa3
1 Imerys Minerals Ltd., Par Moor Centre, Par Moor Rd, Par, Cornwall, UK PL24 2SQ
2 Interface Analysis Centre, H.H. Wills Physics Laboratory, University of Bristol, Bristol, BS8 1TL, UK
3 Imerys do Brasil - Av. Valentina M.F. Borestein, 545 - 08735-270 - Mogi das Cruzes/SP

O PAPEL vol. 81, num. 01, pp. 66 - 73 - JAN 2020



ABSTRACT
Multiple coatings are often employed to give a smooth,
highly opaque and even surface on coated board which is
suitable for high-quality printing. The first coating applied
generally contains coarser pigments which give high bulk
and good coverage of the fibres. The topcoat is often designed
to have finer pigments in order to obtain the required gloss,
optical properties and end functionality (laser marking and
printing). As well as the impact of the pigment choice in the
formulation, the coating application method and the rate
that it dries also have a significant influence on the resulting
topography and pore structure. The binders, thickeners and
additives in the coating formulation will control the rate of
water loss during the coating process and this determines
the runnability of the coating layer on the board machine as
well as its final structure.
This publication is the first in a series of 3 which explore
factors that impact the structure of the precoat and its
subsequent influence on the topcoat. In this work, the
same basepaper, pigment and latex binder systems are
used throughout and the variables studied are the speed
of the coating drying, the weight % solids of the coating
colour applied to the base and finally the impact of adding
thickener to the precoat. Mercury porosimetry and stain
length tests were used to probe the bulk and surface porosity
of the layers respectively and in each case the impact on the
porosity of the precoat alone and the final top coated board
were assessed. A comparison of the bulk porosity of coatings
applied onto porous basepaper and onto non-porous plastic
was also made to determine the impact of dewatering on the
dried layer structure. SIMS sectioning and labelling of the
precoat and topcoat with markers also allowed assessment
of movement of water and starch within the coated board.
The work has proven that the pore structure of the
precoat is very dependent on the substrate to which it is
applied (porous or non-porous), the % solids content and
the amount of thickener added as well as the speed of drying.
These differences then follow through in influencing the
structure of the final topcoat. The work will help provide
levers for paper and board producers to be able to optimise
their whole coating formulation.


Keywords: coated board, precoat, topcoat, porosity,
dewatering


Corresponding author: Janet Preston, Imerys Minerals Par Moor Rd, Par, Cornwall, PL24 2SQ UK Phone: +44-7768427259.
e-mail janet.preston@imerys.com.

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