Artigos TÚcnicos | Artigo TÚcnico | 28.08.2017

O PAPEL vol. 78, num. 8, pp. 94 - 104 AUG 2017


Authors: Janet Preston1, Martti Toivakka3, Andrew Findlay1, Rui Sergio Tsukuda2, Evandro Oliveira2, Edenil Costa2
1 Imerys Minerals Ltd., Par Moor Centre, Par Moor Rd, Par, Cornwall, UK PL253EH
2 Imerys do Brasil - Av. Valentina M.F. Borestein, 545 - 08735-270 - Mogi das Cruzes/SP
3 ┼bo Akademi, Laboratory of Paper Coating and Converting, Finland

The reduction of binder concentration in coatings, within
the constraints of acceptable final paper quality, is a constant
focus for cost effective production of paper and board coatings.
As binder levels are reduced, issues of strength and printability
become more and more critical, due to the increasing probability
of creating a heterogeneous binder distribution within the coating
structure. Obtaining the maximum strength for a given amount of
binder added is a goal for the cost-conscious paper maker.
In this presentation, the complexity of strength measurement of
a coating is outlined as well as the factors which impact the IGT
dry pick test. The dry pick test is shown to have a good correlation
with a commercial press trial; where coating pick on the printing
blanket was assessed after a controlled print run; and is more
relevant than a laboratory wet pick test using pre-damping water.
Pigment properties such as particle size distribution, surface
area and shape factor of the minerals in the coating layer have
a profound impact on the resultant strength and these are
discussed. The impact of how the latex is added to the slurry is
also determined. The broad particle size distribution GCC was
separated into two fractions - fine and coarse. The effects of adding
the latex binder to either the fine or the coarse fraction separately,
or in sequence, was explored, and compared to a situation where
the latex is added to the combined blend. It was found that mixing
the latex first with the finer pigment fraction and then blending in
the coarse fraction does not significantly influence the strength of
the coatings or indeed the other properties. It appears that mixing
ensured uniform binder distribution herein.
The process of applying a coating colour onto a porous paper
substrate results in some loss of the aqueous phase into the
substrate. Along with the water, some binder, latex or starch may
be lost, depleting the coating layer and reducing strength. This
might be at least partly offset by enrichment of binder at the
coating-base interface. In this study, we have also attempted to
relate the effects of binder depletion on the type and location of
failure observed in the dry pick test. We present evidence that
there is a depletion of latex binder from the coating surface during
the coating application stage, and a subsequent enrichment of the
binder at the coating-basepaper interface. This effect is greatest
with an absorbent basepaper and a coating pigment which has
poor water retention characteristics.
A total formulation approach, incorporating thickeners and water
retention aids is required to obtain an optimised coating solution.
Keywords: Coating strength, coating color dewatering,
consolidation, IGT dry pick test

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