Note: Descriptions are shown in the official language in which they were submitted.
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Coated base paper and a method for manufacturing coated base paper
Field of the invention
The object of the invention is coated base paper particularly suitable as base
paper
or release paper for labels, and a method for manufacturing coated base paper,
wherein a barrier, i.e., protective layer is applicated to coated paper based
on
cellulose fibres.
Prior Art
Base papers, also known as release paper or backing papers, are used in the
manufacture of label laminates and self adhesive labels, and increasingly,
also as
packaging material for adhesive materials. Most commonly, base paper or
release
paper either is glassine-type transparent super-calendered and surface sized
paper,
or paper usually blade coated with pigments containing kaolin, for example,
fine
paper super-calendered as to be as impermeable as possible. Typical known
prior
art laminate structures of pressure-sensitive labels are shown schematically
in
Figure 1 appended.
In Figure 1, base paper 10 has been applicated with a release coating 20, most
frequently comprising silicone. A pressure-sensitive glue layer 30 has been
applicated to the release coating 20 containing silicone, and a surface paper
40
forming a label has been affixed to the adhesive layer 30.
The silicone layer applicated to the base paper must be smooth and lacking
holes,
as the surface paper of the label then will detach in a smooth manner as
desired,
from the surface of the base paper. To achieve a smooth silicone layer lacking
holes, the base paper cannot have holes, wrinkles, dust or unevenness. The
surface
of the base paper should be sufficiently ~ sealed to prevent the silicone from
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penetrating into the base paper. Neither may the base paper chemically affect
the
catalytic hardening of the silicone. If holes are present in the base paper,
adhesive
from the adhesive layer may penetrate the silicone layer to the base paper,
resulting in fissures and breaks in the web of the litter path and of the
label
laminate during subsequent processing.
In general, the silicone is applicated onto the base paper in a thickness of 1
to 1.25
g/m2 and cured at approximately 140 °C. Silicones being expensive,
efforts are to
applicate the silicone in as thin layers as possible, but on the other hand,
due to the
good detachment characteristics, nonetheless is necessary to manufacture the
silicone layer to have sufficient thickness so as to in advance prevent the
appearance of possible problems and breaks because of holes, porosity, and
surface unevenness often present in fine paper. To be able to decrease the use
of
silicone, the surface of the base paper should be as impermeable and smooth as
possible.
The publication Wochenbl. Papierfabr. 122, No 11, p. 46~ - 472 (1994) presents
a
process, wherein, to reduce silicone consumption, bleached paper having a
grammage of 67 g/m2 was applicated with surface sizing compositions comprising
polyvinyl alcohol and carboxymethylcellulose, or polyvinyl alcohol and starch.
In patent JP 171 600/93 the coating of paper on both sides with polyvinyl
alcohol
is described, followed by treatment with pressurized steam and calendering.
In paper manufacturing, polyvinyl alcohol is commonly used as protective
colloid
in coating pastes. Particularly, polyvinyl alcohol grades having low molecular
weight are suitable for coating pastes comprising kaolin-containing pigment
mixtures and the like. Polyvinyl alcohol is also used as primary or auxiliary
binding agent in coating and surface sizing compositions and as carriers for
excipients such as fluorescent brighteners.
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The drying of polyvinyl alcohol as a separate layer on a web is known to be
difficult and energy consuming, as a leathery surface easily appears on the
polyvinyl alcohol from where the water it contains is difficult to remove,
resulting
in the surface remaining uneven.
Based on the above it can be seen that there exists an obvious need for
impermeable and smooth base paper of greater bulk and lower grammage, that is
based on coated paper manufactured of chemical pulp, and by which one may
achieve material savings, and which base paper needs only a thin layer of
silicone
as release layer when manufacturing label laminates, and for a method for
manufacturing said base paper.
Object of the invention
An object of the invention is coated impermeable base paper, based on chemical
pulp.
A fiuther object of the invention is a method for the manufacturing of coated
impermeable base paper, based on chemical pulp.
In addition, an object of the invention is the use of coated impermeable base
paper,
based on chemical pulp, as base paper and release paper for labels.
The characteristic features of the base paper according to the invention, of
the
method for manufacturing it, and of its use are set forth in the claims.
Summary of the invention
It has been perceived that the problems occurring in base paper according to
prior
art can be avoided or significantly reduced by using the solution according to
the
invention, wherein pigment-coated base paper based on cellulose fibres,
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preferably on chemical pulp, is coated with a barrier layer preferably
comprising
polyvinyl alcohol.
Detailed description of the invention
S
In the solution according to the invention, one or several thin barrier layers
are
applicated onto pigment-coated paper manufactured from cellulose fibres,
preferably primarily, particularly preferably in excess of SO%, from chemical
pulp.
The thickness of the layer is defined through measurement of the layer's
grammage (g/m2) by weighing the base paper and coated paper, or calculating by
squares metres used and the flow of barrier chemical (g/s/m2/s). Preferably,
the
paper is fine paper pigment-coated as known in the art, for whose coating has
been used pigment paste comprising kaolin or carbonate, preferably kaolin
paste,
having a binding agent selected from the group comprising styren-butadiene
latex,
1 S polyvinyl alcohol and other common binding agents used in the art.
Onto the coated paper, manufactured by methods according to prior art, 1 to S
layers, preferably one layer of an aqueous solution comprising a barrier
chemical
is applicated, using a method non-contacting the web known per- se, which
method is . selected from the group comprising a curtain coating method and a
spray method performed by spraying. The barrier chemical is selected from the
group comprising polyvinyl alcohol, acrylate-based barrier latexes of which as
examples may be mentioned the Rebarco~ series of the company Raisio
Chemicals Oy, and their mixtures. Preferably polyvinyl alcohol is used having
a
2S short chain, the viscosity (mPas) and degree of hydrolysis (mol-%) varying
in the
ranges 3 to 98 - 1S to 99. The barrier chemical is applicated as an aqueous
solution having a baxrier-chemical content of 3 to SO weight-%, preferably 3
to 2S
weight-%, and particularly preferably 3 to S weight-%, so that the solution
being
applicated has a viscosity of at most 100 cP, preferably at most SO cP. When
using
acrylate latexes as barrier chemicals, these may also contain fillers,
pigments,
styrene-butadiene latexes and other conventional binding agents. The barrier
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chemical is applicated so as to have an amount of it preferably 0.5 to 3 g/m2
in the
base paper. Preferably, the application of the barrier layer is performed as
on-line
coating.
5 After the application of the barrier layer, drying is carried out using non-
contacting drying techniques, with methods known in the art, such as IR or
fluid
bed drying methods. After drying, optional super calendaring may be performed.
Already as such, the barrier layer according to the invention is impermeable
to an
extent that super calendering is not necessarily needed. In spray application
and
curtain coating methods, optionally pigments, surface-tension regulators,
drying
agents, anti-foaming agents and other excipients known in the art may be added
to
the barrier layer.
It is essential for the invention that a non-contacting method such as spray
or
curtain coating method is used for applicating the barrier layer, by which
methods
an impermeable homogenous surface is provided. As no device having parts
contacting the web is used in these methods, they cause no breaks.
By the method according to the invention, base paper having an exceptionally
impermeable, homogenous and smooth surface lacking holes can be manufactured.
This results in that the amount of silicone used for the release layer can be
reduced
to approximately 0.7 g/ma, as the silicone is not absorbed into nor penetrates
the
base paper. Siliconizing may be performed by any methods known in prior art,
using any silicone or silicone emulsions known in the art from labels and
release
papers.
In addition, the amount of fibre material in the base paper can be reduced, as
also
the amount of binding agent in the coating paste, and the amount of actual
coating
paste can be reduced by up to 20 %, compared with conventional ones. Thus it
is
possible to manufacture base papers of lighter grammage, but being of at least
as
good quality as base papers known in prior art, and having a bulk similar to
or
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even higher than those. Calendering can be made lighter and standard thickness
is
achievable with lighter paper. Using the solution according to the invention,
it is
also possible to achieve significant reductions in the consumption of
material.
The invention will be illustrated in more detail by the following examples;
however, the invention is not intended to be limited to these.
Examples
Example 1
Manufacture of coated base paper, coated with polyvinyl alcohol using a spray
method
Non-calendered paper Simkraft MF Special~ having a grammage of 85 g/m2 and
blade coating of 12.5 g/m2 was coated with a thin layer of polyvinyl alcohol
by
spray coating using a Helicoater device. Sample 1 was a reference sample not
coated. In sample 2 polyvinyl alcohol Mowiol~ having a degree of hydrolysis of
10 - 98, solid content of 5 %, viscosity of 64 cP, and the amount applicated
being
1 g/ma was used. In sample 3 polyvinyl alcohol Mowiol~ having a degree of
hydrolysis of 15 - 99, solid content of 5 %, viscosity of 27 cP, and the
amount
applicated being 1 g/m2 was used, and the same grade was used in sample 4, but
then the solid content was 3 %, viscosity 18 cP, and amount applicated 0.5
g/m2.
Pigment coating of the base paper was carried out as blade coating, followed
by
drying, then barrier layer coating using either spray coating or curtain
coating,
drying, and optional calendaring, which was performed as sheets with a
supercalender of production scale. The results with non-calendered samples are
shown in Table 1 below, and with calendered samples in Table 2 below.
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Table 1: Non-calendered samples
Simla~aft Simkraft Simlffaft Simla~aft
MF MF+ MF+ MF+
reference 1 g/m2 PVA 1 g/mz PVA 0.5 g/m''
Sample Sample 2 Sample 3 PVA
1 Sample 4
BW (g/m2) 90.4 90.2 90.9 90.3
Thickness 112 116 113 112
(gym)
Bulk (cm3/g)1.24 1.29 1.24 1.24
Gloss 75 13 22 23 21
(%)
Ink abs. 42 30 30 33
(%)
NP mk abs.
Oil abs. 2.08 0.76 0.73 0.54
(g/m')
Unger oil
abs.
Table 2: Calendered samples
2 nips 2 nips 2 nips 2 nips 8 nips
SimlffaftSimkraft Simlcraft Simla~aft Simkraft
MF+ MF+ MF+ MF+ MF+
tamb 272 1 g/m2 1 g/m2 0.5 g/m2 tamb 272
PVA PVA PVA
BW (g/mz) 89.4 90.6 91.3 90.5 91.2
Thickness 83 82 81 83 82
(~.m)
Bulk (cm3/g)0.93 0.91 0.89 0.92 0.91
Gloss 75 48 69 68 66 52
(%)
Ink abs. 33 22 21 20 30
(%)
Oil abs. 0.71 0.06 0.19 0.13 1.1
(g/m2)
PVA amount0 0.4 0.4 0.2 0
(g/m2)
Ink-absorption values were significantly lower when using polyvinyl alcohol
compared with the sample not having polyvinyl alcohol, and oil-absorption
values
were exceptionally lower when using polyvinyl alcohol than without it. Even
without calendering, the grade from these tests was usable for siliconizing.
