[0001] The present invention relates to a base paper, its use, and a multilayer structure
comprising at least one layer made from said base paper, according to the preambles
of the enclosed claims.
[0002] Various multilayer products are used nowadays for a wide variety of purposes. Common
examples of multilayer products are different laminate structures, such as high-pressure
laminates, continuously pressed laminates, as well as technical laminates and electrotechnical
laminates. Atypical laminate structure may comprise a base or core, possibly a decorative
paper layer(s) arranged on the base or core and finally an overlay layer. The base
or core may be a material board, such as a particle board, fibreboard or the like,
or the base or core may be formed from a number of layers of core paper or base paper
that have been permanently attached together during the manufacture of laminate structure,
i.e. multilayer structure of multilayer product. In case the base or core of the laminate
structure comprises a material board, the laminate structure usually comprises one
or several layers of base paper between the material board and the possible decorative
layer(s) and the overlay layer. Other multilayer products are, for example, evaporative
cooling pads. In practice, multilayered products, such as laminate structures, usually
thus comprise multiple layers of base paper, which usually is an absorbent kraft paper.
[0003] In multilayer products and multilayer structures, the layers may be attached together
by a thermosetting resin which is cured during the manufacture of the multilayer product
or structure. The individual base or core paper layers and the decorative paper layers
are impregnated with the thermosetting resin, the layers are stacked in the desired
order, pressed together and the resin is cured, resulting the final multilayer product
or structure, such as laminate structure. In order to guarantee a proper attachment
of the individual layers with each other and the formation of the laminate structure,
it is important that the used core or base paper absorbs resin effectively and uniformly.
[0004] Most resins used in manufacture of laminate structures are synthetic polymers, such
as melamine resins, formaldehyde resins, phenolic resins, and the like. There is a
rising interest in sustainability in the industry, which has led to reduction of the
use of the synthetic petroleum-based polymer resins, both due to the environmental
reasons as well as the growing market pressure. This means that there is an increased
demand and desire to use resins which are, at least partly, based on natural and/or
renewable resources. One interesting resin possibility to use in multilayered structures
or products would be various thermosetting resins, which are at least partly based
on lignin. Lignin is a natural polymeric substance that is available in large quantities
as a by-product, for example, from pulp industry. These lignin-based resins could
provide a sustainable alternative to petroleum-based polymers, as they are at least
partly based on renewable sources and they can be manufactured by using raw-materials
that otherwise might be considered as waste and incinerated. However, lignin-based
resins have not worked properly with the available base papers, and this have limited
or prevented their use in these applications. Lignin-based resins have not been uniformly
absorbed into the base paper, and a resin poor area has been formed in the middle
of the base paper. As the consequence of the non-uniform resin absorption, the attachment
of the layers during the manufacture of the multi-layered structures and products
has been inadequate, which have resulted failed and/or non-usable structures or products.
[0005] In general, there is a constant need to improve the properties of base papers intended
for use in multilayer structures or products in order to improve the quality of the
produced multilayer structures or products, to ease the production process as well
as to enable wide use of the new, exciting, at least partly lignin-based thermosetting
resins, which are emerging on the market.
[0006] An object of this invention is to minimise or possibly even eliminate the disadvantages
existing in the prior art.
[0007] Another object of the present invention is to provide a base paper, which shows improved
properties in a manufacture of multi-layered structures or products, especially a
good ability to absorb resin uniformly.
[0008] Another object of the present invention is to provide a base paper, which is especially
suitable for use with at least partly lignin-based thermosetting resins in manufacture
of multilayer products.
[0009] In order to achieve,
inter alia, the objects presented above, the invention is characterised by what is presented
in the characterising parts of the enclosed independent claims.
[0010] Some preferable embodiments of the invention will be described in the dependent claims.
[0011] The embodiments mentioned in this text relate, where applicable, to all aspects of
the invention, even if this is not always separately mentioned.
[0012] A typical base paper according to the present invention for producing multilayer
structure or multilayer product, such as laminate structure, comprises cellulosic
fibre material and has
- a bulk density value ≤590 kg/m3,
- a Bendtsen roughness value ≥800 ml/min, and
- a Gurley porosity value ≤15 s/100 ml.
[0013] A typical multilayer structure according to the present invention comprises at least
one layer made from a base paper according to the invention impregnated with a thermosetting
resin comprising lignin.
[0014] Now it has been surprisingly found that an improved absorption of a thermosetting
resin, especially a lignin-based thermosetting resin, into the base paper is obtained
when the base paper has a bulk density value ≤590 kg/m
3, a Bendtsen roughness value ≥800 ml/min, and a Gurley porosity value ≤15 s/100 ml,
sometimes ≤10 s/100 ml or ≤5 s/100 ml. The selected combination of the base paper
properties provides unexpectedly effective absorption of the resin into the base paper.
It was found that the specific combination of the bulk density, Bendtsen roughness
and Gurley porosity values enable uniform and fast absorption of the thermosetting
resins, especially at least partly lignin-based thermosetting resins, throughout the
base paper layer, while the absorbed resin amount is maintained at a proper level.
The present invention solves many of the problems that have this far limited or prevented
the use of lignin-based thermosetting resins in manufacture of multi-layered products,
such as laminates.
[0015] The present invention enables uniform absorption of thermosetting resin, especially
at least partly lignin-based thermosetting resin, throughout the base paper. This
means that a high resin content can be attained even in the middle region of the base
paper, and a uniform or nearly uniform absorption profile for the thermosetting resin
can be achieved throughout the base paper. The base paper has a first large surface
and a second large surface, which are parallel with each other. The distance between
the first and the second surface defines the thickness of the base paper. According
to one embodiment, the base paper may comprise a first edge region, a second edge
region and a middle region extending between the first edge region and the second
edge region. The first edge region extends from the first large surface of the base
paper in the thickness direction towards the middle region of the base paper and the
second edge region extends from the second large surface of the base paper in the
thickness direction towards the middle region of the base paper. The respective edge
region usually comprises 15 % of the thickness of the base paper, and the middle region
usually comprises 70 % of the thickness of the base paper. According to one preferable
embodiment the base paper is impregnated with a thermosetting resin, preferably a
thermosetting resin comprising lignin, i.e. at least partly lignin-based thermosetting
resin. Preferably the concentration of the thermosetting resin in the first and second
edge regions is essentially the same as in the middle region. For example, the difference
in concentration of the thermosetting resin between the first and second edge regions
and the middle region is at most 30 %, preferably at most 20 %, sometimes at most
10 %. Sometimes, the difference in concentration of the thermosetting resin between
the first and second edge regions and the middle region may be less than 15 % or even
less than 5 %.
[0016] The multilayer structure or multilayer product, preferably a laminate structure,
which comprises at least one layer made from base paper according to the present invention,
may comprise at least 10 weight-%, preferably at least 20 weight-%, of thermosetting
resin, especially thermosetting resin comprising lignin, i.e. at least partly lignin-based
resin. The resin content in the multilayer structure or product in percentages is
determined by calculating the weight difference between the impregnated base paper
(structure/product) and unimpregnated base paper (structure/product), whereafter the
weight difference is divided by the weight of the impregnated base paper (structure/product)
and multiplied with 100. All weights are given as dry weights.
[0017] The base paper according to the present invention is especially suitable for impregnation
or absorption with a thermosetting resin comprising lignin, i.e. at least partly lignin-based
resin. The thermosetting resin solution, which is used to impregnate the base paper
may comprise at least 4 weight-%, preferably 8 weight-%, more preferably at least
12 weight%, of lignin, calculated from the total weight of the resin solution.
[0018] In the present context, the term "base paper" denotes a paper made from cellulosic
fibre material and intended for use as a layer in a multilayer structure or product,
such as laminate structure. Typically, the base paper comprises bleached and/or unbleached
cellulosic fibre material, preferably unbleached cellulosic fibre material obtained
by kraft pulping. According to one embodiment the cellulosic fibre material may comprise
virgin cellulosic fibres and/or recycled cellulosic fibres. Sometimes the cellulosic
fibre material may comprise cellulosic fibres obtained by mechanical pulping methods
and/or cellulosic fibres originating from kraft pulping of sawdust. Preferably the
cellulosic fibre material comprises unbleached cellulosic fibres, recycled cellulosic
fibres and/or cellulosic fibres originating from kraft pulping of sawdust. According
to one embodiment, the cellulosic fibre material may comprise 0 - 100 weight-%, preferably
10 - 90 weight-% or 25 - 75 weight-%, of unbleached cellulosic fibres, and/or 0 -
100 weight-%, preferably 10 - 90 weight-% or 25 - 75 weight-%, of recycled cellulosic
fibres.
[0019] The base paper may have a bulk density value ≤590 kg/m
3 or ≤555 kg/m
3, preferably ≤550 kg/m
3, more preferably ≤540 kg/m
3, sometimes even ≤525 kg/m
3. According to one embodiment of the invention the base paper may have a bulk density
value in a range of 425 - 590 kg/m
3 or 440 - 555 kg/m
3, preferably 450 - 550 kg/m
3, more preferably 475 - 540 kg/m
3. It has been observed that the bulk density of the base paper may be significantly
lower compared to conventional base papers, where the bulk values are typically round
600 - 680 kg/m
3. It is assumed that the low bulk value of the base paper, when combined with the
selected porosity values, unexpectedly improves the absorption, i.e. impregnation
of the thermosetting resin, especially at least partly lignin-based thermosetting
resin, into the base paper. Improved resin impregnation enables smooth and effective
operation of the manufacturing process for the multilayer structure or product, as
well as enables the production of stronger and qualitatively more uniform and satisfactory
structures or products. In the present context the bulk values are measured by using
standard ISO 534:2005.
[0020] The base paper may have a Bendtsen roughness value ≥800 ml/min, preferably ≥900 ml/min,
more preferably ≥950 ml/min. According to one embodiment the base paper may have the
Bendtsen roughness value in the range of 800 - 1300 ml/min, preferably 900 - 1200
ml/min, more preferably 950 - 1150 ml/min. Bendtsen roughness indicates the smoothness
of the base paper surface. Conventionally, it has been assumed that the base paper
should have a smooth surface, i.e. relatively low Bendtsen roughness value. However,
now it has been unexpectedly found that increased surface roughness makes the base
paper surface more "open" and facilitates the absorption or impregnation of the thermosetting
resin, especially at least partly lignin-based thermosetting resin. In particular
thermosetting resins having a large molecular size, such as at least partly lignin-based
thermosetting resins, are more easily absorbed when the Bendtsen roughness of the
base paper is within the selected range. In the present context the Bendtsen roughness
values are measured by using standard ISO 8791-2:1990 part 2.
[0021] According to one embodiment of the invention the base paper may have a Gurley porosity
value ≤15 s/100 ml, preferably in the range of 1 - 15 s/100 ml, more preferably 2
- 12.5 s/100 ml, even more preferably 4 - 8 s/100 ml. For example, the base paper
may have a Gurley porosity value in the range of 3 - 9 s/100 ml, preferably 4 - 8
s/100 ml, more preferably 5 - 7 s/100 ml. The base paper may have a Gurley porosity
value even in the range of 7 - 13 s/100 ml, preferably 8 - 12 s/100 ml, more preferably
9 - 11 s/100 ml. According to one preferable embodiment of the base paper may have
a Gurley porosity value ≤5 s/100 ml, preferably ≤4.5 s/100 ml, more preferably ≤4
s/100 ml. The Gurley porosity value may be in the range of 1 - 5 s/100 ml, preferably
1.5-4.5 s/100 ml, more preferably 2 - 4 s/100 ml. In general, the Gurley porosity
value describes the air permeability of the base paper. It was surprisingly found
that when the Gurley porosity value is within the defined values, especially when
the Gurley porosity value is ≤5 s/100 ml, in combination with the defined bulk density
and Bendtsen roughness values, the base paper is able to absorb increased amounts
of thermosetting resin, especially lignin-based thermosetting resin. The increased
ability of the base paper to absorb resin ensures that the resin amount is sufficient
during the pressing step in the manufacture of the multilayer product and the quality
of the produced multilayer products is high. As the defined Gurley porosity for the
base paper enables the absorption of proper amounts of thermosetting resin into the
base paper, the overall efficiency of the process is improved. In the present context
the Gurley porosity values are measured by using standard ISO SCAN-P 19:78.
[0022] According to one embodiment of the invention the base paper may have a moisture content
of 4 - 10 weight-%, preferably 4 - 8 weight-%, sometimes 5 - 8 weight-%. In the present
context the moisture content is measured by using standard ISO 287:1985. It has been
observed that the higher moisture content of the base paper further improves the absorption
of the thermosetting resin into the base paper during the manufacture of multilayer
products. At the same time, it is possible to achieve savings in energy consumption
of the manufacturing process for the base paper as the drying need for the base paper
in the process is reduced.
[0023] According to one embodiment of the present invention the base paper may have a basis
weight in a range of 30 - 350 g/m
2. According to one embodiment the base paper may preferably have a basis weight in
a range of 30 - 120 g/m
2, more preferably 40 - 100 g/m
2. In the present context the basis weight is measured by using standard ISO 536:1995.
The base paper according to this embodiment is thus especially suitable for manufacture
of overlays.
[0024] According to another embodiment of the present invention the base paper may have
a basis weight in a range of 120 - 350 g/m
2, preferably 150 - 300 g/m
2. The base paper according to this embodiment is thus especially suitable for manufacture
of high-pressure laminates (HPL).
[0025] The base paper may preferably have a wet strength value at least 300 N/m, more preferably
at least 350 N/m, even more preferably at least 400 N/m, sometimes even at least 450
N/m. The wet strength value is measured by using test method ISO 3781. Wet strength
is an important property of the base paper, as it allows the effective absorption
or impregnation of the base paper with the thermosetting resin. The base paper may
comprise one or more suitable wet strength resins, such as polyamidoamine epichlorohydrin
resins or glyoxylated polyacrylamide, which can be added to the cellulosic fibre material
during the manufacture of the base paper. According to the one embodiment the base
paper may comprise a wet strength resin in amount of at least 1 kg/ton paper (dry),
preferably at least 2 kg/ton paper (dry), more preferably at least 4 kg/ton paper
(dry).
[0026] The base paper according to the present invention is suitable for producing multilayer
structures or multilayer products, such as laminate structures. The base paper according
to the present invention is especially suitable to be used for making a multilayer
product selected from decorative high-pressure laminates, continuous pressed laminates,
postform laminates, fire retardant laminates, compact laminates, drilling boards,
electrotechnical laminates, and technical laminates. The base paper according to the
present invention is further especially suitable for use as cushion pads used in laminate
impregnation, evaporative cooling pads, as plywood films, or as surface films.
[0027] The base paper according to the present invention can be manufactured as conventional
in the art. By proper selection of process parameters at the press and drying sections
as well as at the calandering, it is possible to produce base paper according to the
present invention. The person skilled in the art is able to determine the required
parameters with a few standard experiments. The present invention is not related to
the manufacture of the base paper as such, but the specific combination of base paper
properties providing unexpected advantages.
[0028] An embodiment of the present invention is described more closely in the following
non-limiting example.
EXAMPLE
[0029] Base paper was made from unbleached chemical pulps. The main raw material of base
paper stock suspension was unbleached sawdust pulp. Another raw material used for
the base paper stock suspension was recycled fibre originating from old corrugated
cartons (OCC). Minimum virgin pulp quantity was 70 %. To increase wet strength properties
of the base paper, wet strength resin was added to furnish. Wet strength resin quantity
was 4 kg/ton to reach the required wet strength.
[0030] The headbox fed a very dilute stock suspension onto the wire. On the wire section,
as much water as possible was removed from the stock suspension fed from the headbox.
Water was removed by means of gravity and suction. After the wire section paper web
was pressed in a nip rolls and press felt, forcing water out of the paper to the felt.
The dryer section comprised steam-heated drying cylinders. The dryer section was followed
by a machine calender which adjusted the thickness and density of the produced base
paper.
[0031] Produced base paper had following properties:
Bulk density 525 kg/m3
Bendtsen roughness average 1143 ml/min
Gurley porosity 3.5 s/100 ml
Grammage 193.3 g/m2
Moisture content 5.9 weight-%
[0032] When impregnated with thermosetting lignin-based resin, it was observed that the
absorption of the resin throughout the base paper was uniform, and no significant
difference in resin content could be observed between the edge regions and the middle
region of the base paper.
[0033] Even if the invention was described with reference to what at present seems to be
the most practical and preferred embodiments, it is appreciated that the invention
shall not be limited to the embodiments described above, but the invention is intended
to cover also different modifications and equivalent technical solutions within the
scope of the enclosed claims.
1. Base paper for producing a multilayer product, such as laminate structure, the base
paper comprising cellulosic fibre material and having
- a bulk density value ≤590 kg/m3,
- a Bendtsen roughness value ≥800 ml/min, and
- a Gurley porosity value ≤15 s/100 ml.
2. Base paper according to claim 1, characterised in that the base paper has the bulk density value ≤555 kg/m3, preferably in a range of 450 - 550 kg/m3, more preferably 475 - 540 kg/m3.
3. Base paper according to claim 1 or 2, characterised in that the base paper has the Bendtsen roughness value in the range of 800 - 1300 ml/min,
preferably 900 - 1200 ml/min, more preferably 950 - 1150 ml/min.
4. Base paper according to claim 1, 2 or 3, characterised in that the base paper has the Gurley porosity value <15 s/100 ml, preferably in the range
of 1 - 15 s/100 ml, more preferably 2 - 12.5 s/100 ml, even more preferably 4 - 8
s/100 ml.
5. Base paper according to claim 1, 2 or 3, characterised in that the base paper has the Gurley porosity value ≤5 s/100 ml, preferably in the range
of 1 - 5 s/100 ml, more preferably 1.5 - 4.5 s/100 ml, even more preferably 2 - 4
s/100 ml.
6. Base paper according to any of preceding claims 1 - 5, characterised in that the base paper has a moisture content of 4 - 10 weight-%, preferably 4 - 8 weight-%.
7. Base paper according to any of preceding claims 1 - 6, characterised in that the base paper has a basis weight in a range of 30 - 350 g/m2.
8. Base paper according to claim 7, characterised in that the base paper has a basis weight in a range of 30 - 120 g/m2, preferably 40 - 100 g/m2.
9. Base paper to according claim 7, characterised in that the base paper has a basis weight in a range of 120 - 350 g/m2, preferably 150 - 300 g/m2.
10. Base paper to any of preceding claims 1 - 9, characterised in that the base paper has a wet strength value at least 300 N/m, preferably at least 350
N/m, more preferably at least 400 N/m.
11. Base paper according to any of preceding claims 1 - 10, characterised in that that the base paper comprises wet strength resin in amount of at least 1 kg/ton paper
(dry), preferably at least 2 kg/ton paper (dry), more preferably at least 4 kg/ton
paper (dry).
12. Base paper according to any of preceding claims 1 - 11, characterised in that the cellulosic fibre material comprises virgin cellulosic fibres and/or recycled
cellulosic fibres.
13. Base paper according to any of the preceding claims 1 - 12, characterised in that the base paper is impregnated with a thermosetting resin, preferably thermosetting
resin comprising lignin.
14. Use of base paper according to any of claims 1 - 13 as cushion pads used in laminate
impregnation; as evaporative cooling pads; as plywood films; or as surface films;
or for making a multilayer product selected from decorative high-pressure laminates,
postform laminates, compact laminates, fire retardant laminates, continuous pressed
laminates, drilling boards, electrotechnical laminates and technical laminates.
15. A multilayer structure, comprising at least one layer made from a base paper according
to any of claims 1 - 13 impregnated with a thermosetting resin comprising lignin,
wherein the multilayer structure preferably comprises at least 10 weight-%, preferably
at least 20 weight-%, of thermosetting resin comprising lignin.