EXIT
By Knut Möller
Design
embossing has proven to be a preferred marketing tool
for tissue products. Since the market introduction of
this novel technique enormous progress has been made
with respect to machine design, process optimisation
and, last but not least, with the formulation of coloured
laminating glues.
How can we produce the coloured laminated
glues needed for design embossing? Should we add colour
to the glue or should we make the ink sticky like glue?
The objective
is to design coloured glues having strong colour intensity,
maximum bleeding resistance and good ply bonding. In
addition, they should have the same penetration behaviour
and runnability as non-pigmented laminating glues.
The easiest way seems just to mix existing glues
with commercially available pigment dispersions. Pigment
dispersions do not contain any binder system. Therefore
it is not astonishing that mixtures of pure pigments
with laminating glues show no bleeding resistance at
all.
This is caused
by the water-soluble glue polymers present. During drying
they will cover the pigment particles and forman interlayer
between the pigments and the substrate. In contact with
water the polymers will resolve and the pigment particles
will be re-dispersed, which can easily been observed
on these products by standard bleeding test. It should
be noted that even inferior bleeding results are achieved
if glues are mixed with soluble dyes instead of pigment
dispersions. Another drawback of pigment dispersions
comes from the dispersing agents used for pigment stabilisation.
If added to the laminating glue, they will change markedly
the polarity of the liquid phase and thus the solubility
of the glue polymers. As aforesaid, a change in solubility
will change the viscosity and the penetration behaviour
of the glue. This explains the frequent occurrence of
increased deposits with the addition of pigment dispersions
to laminating glues.
Another
possibility is to combine standard printing inks with
laminating glues. Inks contain film-forming binder systems
providing excellent fastness of the printout after drying.
Unfortunately, at high pigment levels (high
colour intensity) these mixtures show only marginally
improved bleeding resistance compared to pure pigments.
On the other hand, sufficient bleeding resistance can
only be achieved at low pigment content (pastel shades).
This is because the filmformation of the ink binders
is disturbed when water-soluble glue polymers are present.
The binder is unable to build up a sufficient resistance
against water (Figure 1).
In many cases there is an incompatibility between theglue
polymers and the binder used for printing inks. This
may lead to unexpected effects. Incompatibility can cause
an agglomeration of the ink binder and the pigment particles.
Coarse agglomerates of pigments show much lower intensity
than finely dispersed pigment particles. As a consequence,
the intensity of the coloured glue may drop even if further
ink is added. During regular application the incompatibility
may result in cloudiness of the design or even in a sudden
loss of intensity of the coloured glue without any apparent
reason.
The addition of printing inks to laminating glues
may alsoinfluence the solubility of the glue polymers.
Though widely used, the mixing of printing inks and laminating
glues is not more than a sub optimal procedure to formulate
coloured glues.
To find the optimal solution we have to
use specially-developed ink concentrates for the colouration
of laminating glues. They must provide special binder
system for superior bleeding resistance; high pigmentation
for high intensities; full compatibility with laminating
glues; no deterioration ofglue properties; and easy handling.
A
special binder system is needed to ensure a fast andcomplete
film formation in the presence of the water-soluble glue
polymers. Only if the pigment particles are fixed immediately
to the substrate fibres can bleeding after rewetting
be excluded (Figure 2).
In practice, with coloured laminating glues that have
superior bleeding resistance, the pigments do not at
all penetrate through the plies. They will be found on
the surface of the ply to which they have been transferred
by the anilox roll. Nevertheless, the colouration can
be seen from the surface of a multiply product – with
slightly reduced intensity. This is due to the show through
of the plies condensed by the top-to-top embossing process.
To achieve the high intensities required for most designs
and taking into account the weakening of the tissue plies,
the ink concentrates have to be pigmented much higher
thancommon flexographic printing inks.
The ink concentrate has to be stable, compatible and
completely miscible with the laminating glue. This is
to ensure a fine and stable distribution of the pigment
particles during mixing, resulting in an optimal intensity
of the coloured glue.
The formulation of the ink concentrate has to be adjustedto
the chosen laminating glue. It must not change the polarity
of the solution to ensure unchanged penetration properties
and adhesive strength of the glue polymers.
If ink concentrates are mixed on-site with the correspondinglaminating
glues, unproblematic handling is required. This includes
sufficient storage stability, no sedimentation during
storage, appropriate viscosity, and easy mixing with
laminating glues using common stirrers.
For the sake of flexibility, many converters prefer
to mix their coloured laminating glues on-site by means
of a dispensing and mixing unit. During design changes,
this allows to switch very quickly from non-pigmented
glue to coloured laminating glues as well as between
different colours. It would be advantageous to purchase
both laminating glue and ink concentrate from one source
to ensure complete compatibility. But in Europe, most
suppliers manufacture and offer solely either glues or
inks. Normally, ink suppliers are obliged to adjust their
systems to commercially available laminating glues observing
the requirements discussed above.This implies a circumstantial
communication between the tissue converter and all suppliers
involved.
In contrast, buying from a single source the
converter will profit from experience from both sides
in one company rather than dealing with two companies
with their own objectives. Concerning development, implementation
and service it is important to work with people who understand
both, glues and inks, to achieve the maximum performance
and cost in use benefits.
For extended production runs
with one single design, tissue converters prefer to use
ready-for-use coloured glues. Again, there are only few
companies in Europe offering ready-for-use coloured laminating
glues completely developed in-house. The mutual benefit
of ready-for-use systems is obvious. Sincethe supplier
is responsible to deliver coloured glues of the agreed
quality, the customer can count on always constant colour
shade, colour intensity and identical glue performance
guaranteed by the supplier. On-site mixing errors are
excluded. Furthermore, the supplier is able to continuously
optimise both, the ink and glue system, to formulate
the best available coloured glue.
In general, ready-for-use
coloured laminating glues arealso available as concentrates
to save shipping costs. They simply have to be diluted
with water on-site.
Colour embossing is more and more
combined with printingto create novel designs. In most
cases, not flexographic printing, but the so-called dot
embossing technique is used. With this technique a printing
ink is applied during an embossing process creating a
coloured pattern on the tissue product prior to design
embossing.
The correct choice of colour shades is a prerequisite
for an appealing and clear design. Which printing colours
can becombined, for example, with a given green shade
of coloured glue? If you look at an example using the
L*a*b* colour space,all colours close to the chosen colour
will give brilliant mixed colours. The larger the distance
between the colours within the colour space, the more
the mixed colours will appear lacklustre. The combination
of complementary inks will result in grey.
It may serve
as a general rule not to combine complementary inks if
clear mixed colours are desired also in the overlapping
areas of the design.
Conclusion
The main requirements of coloured laminating glues fordesign
embossing are: adhesive strength; water-sensitive ply
bonding; colour shade; colour intensity; bleeding resistance
of pigments; low soiling tendency; unproblematic runnability
and handling; and pleasing combination with printing
colours. These properties can be achieved best by combining
common laminating glues with special ink concentrates.
Crucial factors for the formulation of an optimal system
are: choice of the right pigment binder; compatibility
of pigment and glue system; and careful adjustment of
penetration properties. Though systems from different
suppliers are more or lessaccepted in the market place,
the entire system should preferably originate from
one source to ensure maximum technical and cost performance.
Coloured
laminating glues for design embossing are no‘sticky
inks’. But it is very helpful in creating superior
products not only to consider the glue properties but
also the peculiarities of pigmented inks. TW
Mr Möller
is export manager with Kapp-Chemie Gmbh inGermany. This
article is extracted from his presentation at Tissue
World Nice.
