Note: Descriptions are shown in the official language in which they were submitted.
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1 26927-36
LAMINATED BOARD MATERIAL
TECHNICAL SCOPE
The invention relates to a laminated board material which is
very stiff and lends itself well to increasing and which does not
impart unwanted smell or taste to liquid foods, being a laminate
of at least three layers. More specifically the invention relates
to a laminated paper board to be used for containers for liquids,
especially so called Gable Top containers. Those containers which
are marketed as TETRA RE ~ and PURE PA ~ are examples of Gable
Top containers.
BACKGROVND ART
Paper board containers for liquids are presently manufactured
mainly from bleached sulphate pulp. This material is very suitable
for containers but is expensive, due to the great expense of raw
material which is required. Containers for other products than
liquids are often made from composite materials, the stiffness of
which has been increased by laminating, corrugating, or otherwise,
and in some instances these principles have been employed also in
the production of containers for liquids. These types of materials
have not been used for Gable Top containers, however, since they
do not have the necessary qualities therefor. Laminated board
materials are also known, e.g. for folding board, where a central
layer of groundwood pulp and/or thermo-mechanical pulp (TMP) is
covered by surface layers of bleached chemical pulp. These materi-
als cannot either be accepted for use with liquid foods, however,
especially not for milk, mineral water and the like.
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la 26927-36
DISCLOSURE OF THE INV:E~NTION
The purpose of the invention is to offer a laminated board
material which is as good a material for containers as is homogene-
ous fully bleached board but which makes more efficient use of the
wood fibre. In particular the invention aims at providing a lamin-
ated board material for so called Gable Top containers, which
demand a material
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with good creasability, flat surfaces and no taste or smell.
These and other objectives may be achieved by providing the
material with at least one centre layer of a cellulosic material
of large bulk, i.e. of low density, which has also good
creasability and foldability, and on each side of the centre
layer/s surface layers of cellulosic material of considerably
greater material strength than the centre layer/s. The primary
purpose of the centre layer/s is to keep the surface layers apart,
providing the laminated board with such properties as are desirable
in particular for Gable Top containers.
Accordingly, in one aspect, the invention provides a
laminated paper board which is stiff and creasable
and which does not impart undesirable smell or taste to liquid
foods, consisting of at least three layers, characterized by
having
at least one central layer of cellulosic material of high
bulk, and a density of between 500 and 700 kg/m3 comprising a
mixture consisting mainly of chemically modified thermo-mechanical
pulp (CTMP) and bleached long fibre sulphate pulp,
and on eachSide of said at least one central layer surface
layers of cellulosic material of higher strength and ductility
than the at least one central layer and a density of between 640
and 750 kg/m ,
the primary purpose of the at least one central layer being to
keep the surface layers apart.
The centre layer/s is/are made from a mixture mainly of
chemically modified thermo-mechanical pulp, so called CTMP, and
bleached sulphate pulp. The surface layers should be made from
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bleached sulphate pulp, preferably from sulphate pulp with long
fibres. Long fibre sulphate pulp signifies pulp from soft wood
with a mean fibre length of at least 2 mm.
The density of the centre layer/s should be between 550
and 700 kg/m3, preferably between 575 and 675 kg/m3, while that of
the surface layers should be between 64n and 750 kg/m3, preferably
between 660 and 720 kg/m3. Further, the centre layer/s should
contain between 30 and 70, preferably between 40 and 60, % per
weight CTMP, the rest mainly being bleached long fibre sulphate
pulp. In order to obtain the best possible product properties,
for example to keep the board from splitting, the pulp of the
centre layer/s should consist at least in part of pulp which has
been further refined compared to the sulphate pulp of the surface
layers, which part will function as reinforcement for the pulp of
the board. More specifically, it is suitable that this
reinforcing fraction constitute 5 - 20 % of the total mass of the
centre layer/s and be refined to a drainage resistance of at
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least 60 SR, preferably to 70 - 80 SR, while the sulphate pulp
of the surface layers should be refined to between 20 and 30 SR,
preferably 22 - 28 SR.
The different types of pulp constituting the different
layers also contain glue, starch, and retention agents of
types and quantities
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as are well known in the art The total grammage of the material may
vary between 200 and 400 g/m .
Further aspects and characteristics of the invention will be apparent
from the patent claims and from the following description of a pre-
ferred embodiment.
DESCRIPTION OF A PREFERRED EMBODIMENT
The laminated paper board according to the preferred embodiments was
10 manufactured with grammages of 255 and 310 g/m . In both cases one of
the surface layers (the exterior layer) had a grammage of 70 g/m and
the other (the interior layer) a grammage of 50 g/m . The surface
layers consisted of bleached long fibre sulphate pulp refined to
24 SR (desired value) and with a density of approximately 690 kg/m
(mean value). The centre layer consisted of about 45 % CTMP and 55 %
bleached long fibre sulphate pulp, the raw material of which was a
mixture of wood from Nordic conifers (Pinus silvestris and Picea
abies). The centre layer contained about 10 % (of the total mass of
the centre layer) reinforcing pulp, refined to 75 SR (desired value).
The density of the centre layer was about 625 kg/m (mean value). The
mean value of the drainage resistance of the centre layer was 30SR
(desired value). The raw material for the CTMP was wood from Picea
abies.
