Note: Descriptions are shown in the official language in which they were submitted.
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PREPREG WHICH IS PRODUCED BY IMPREGNATING A BASE PAPER WITH
A COMBINATION OF RESIN SOLUTION AND POLYMER DISPERSION
FIELD
The present disclosure relates to prepregs, to a process for their production
and
to decorative covering materials obtainable therefrom.
BACKGROUND
Decorative covering materials, also called decorative papers or decorative
sheets, are preferably used for covering surfaces in furniture manufacture and
in
interior decoration, especially for laminated floors. These decorative sheets
are
paper webs which have been impregnated with synthetic resin and optionally
surface-treated thereafter; they are decoratively printed, or unprinted, and
are in
a state that can no longer be reactivated under pressure and heat. Depending
on
the type of impregnation process, a distinction is made between thermosetting
decorative sheets with a through-impregnated core (offline process) and
decorative sheets based on prepregs (online process) to which a finish coating
can then be applied. This produces a decorative sheet with a finished surface.
Inside the paper machine, immediately after sheet forming and drying,
decorative
sheets based on prepregs are impregnated with a resin mixture to a greater or
lesser depth, depending on the mass per unit area of the base paper. After
drying
and smoothing, usually also inside the paper machine, the paper or sheet must
be crosslinked and hence acquire interlaminar strength so that no fibre
separation occurs within the decorative sheet during the subsequent processing
steps of finish-coating and lamination onto appropriate substrates. Moreover,
for
further processing, the prepregs are required to have a high covering power
and
surface quality, a good printability in rotogravure, a good varnishability
(high
varnish resistance), a good flatness and the ability to be glued to chipboard
and
MDF (medium density fibreboard).
The thermally crosslinkable impregnating resins conventionally used for the
production of prepregs are water-thinnable, formaldehyde-containing resin
solutions based on urea-formaldehyde, melamine-formaldehyde or phenol-
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formaldehyde resins. Because use of these resin solutions on their own results
in
relatively brittle products, they are generally used in combination with water-
thinnable, thermally crosslinkable polymer dispersions based on acrylic acid,
acrylic acid ester, styrene, butadiene, vinyl acetate or acrylonitrile.
The resin uptake in the case of prepregs can be between 5 and 35 wt.%, the
resin solution component generally penetrating deeper into the base paper than
the polymer dispersion component. The polymer dispersions concentrate to a
greater or lesser extent on the top side and bottom side of the paper and
ensure
that the papers have a good smoothability and hence printability, as well as
surface imperviousness and hence varnishability (varnish resistance). The
resin
solutions, on the other hand, are intended to ensure the good interlaminar
strength and flatness of the prepreg by crosslinking of the fibre-resin
composite
during further processing.
The penetration depth of polymer dispersions can be increased by a variety of
additives, e.g. water-soluble polymers based on polyvinyl alcohols, gelatin,
starches, cellulose derivatives, alginates and mixtures thereof (EP 0 739 435
B1),
maleic acid copolymers (EP 0 223 922 Al) or a (meth)acrylarnide/(meth)acrylic
acid copolymer (DE 197 28 250 Al and DE 197 58 479 Al).
In some cases, when producing the prepregs using the above-mentioned
additives, the use of conventional formaldehyde-containing resin solutions is
dispensed with altogether. However, these now formaldehyde-free prepregs have
a limited interlaminar strength, flatness and printability.
These disadvantages are avoided by using a thermosetting, formaldehyde-free
resin solution which does not crosslink extensively until the prepreg is heat-
treated for a period of about 40 seconds at about 132 C according to DE 101 34
302 Cl (maximum degree of crosslinking of 85%). The resin solution used for
this
purpose, whose preparation and possible uses, inter alia for the impregnation
of
papers for decorative applications, are described in DE 197 35 959 Al. The
disadvantage is that the internal strengths of such a prepreg according to DE
101
34 302 C2, which is completely uncrosslinked until it is hot-pressed on the
substrate material (e.g. chipboard) as the last further processing step, are
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currently no better, or even poorer, than those of known formaldehyde-free or
formaldehyde-containing prepregs. In particular, until it is hot-pressed, it
has a
poorer surface imperviousness and hence poorer varnishability (lower varnish
resistance), as well as a poorer flatness during lamination (gluing), than
formaldehyde-free and formaldehyde-containing prepregs known hitherto.
SUMMARY
An aspect of the invention is therefore to provide partially crosslinked
prepregs
which already extensively avoid said disadvantages by the time they reach the
lo end of the paper machine, which, as far as possible, are distinguished
by a high
surface quality, a good printability and a higher interlaminar strength than
that of
known formaldehyde-free or formaldehyde-containing prepregs, a good
varnishability (high varnish resistance), a good flatness and the ability to
be glued
with conventional adhesives in the subsequent processing stages, and whose
properties are thus almost equivalent to those of offline-impregnated papers.
In one aspect, there is provided a prepreg obtainable by impregnating a base
paper with a thermally crosslinkable impregnating combination containing a
formaldehyde-free resin solution and a polymer dispersion, wherein a mixing
ratio
of resin solution to polymer dispersion in the impregnating combination is
45:55
to 10:90 based on solids, residual moisture content is 2 to 3 wt.% and the
prepreg has a degree of crosslinking of at least 10% and at most 85% after
drying.
In some embodiments, the resin solution contains: a) a polymer obtained by
free-
radical polymerization and containing less than 5 wt.% of an a,13-
ethylenically
unsaturated monocarboxylic or dicarboxylic acid incorporated during
polymerization, b) a polymer obtained by free-radical polymerization and
containing more than 15 wt.% of an a43-ethylenically unsaturated
monocarboxylic
or dicarboxylic acid incorporated during polymerization, and c) an
alkanolamine
having at least two hydroxyl groups.
In some embodiments, the polymer dispersion is based on acrylic acid, acrylic
acid ester, styrene, butadiene, vinyl acetate or acrylonitrile.
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In some embodiments, the mixing ratio of resin solution to polymer dispersion
in
the impregnating combination is 35:65 to 30:70.
In some embodiments, the proportion of impregnating components in the paper is
between 5 and 35 wt.%, based on the weight per unit area of the base paper.
In some embodiments, drying takes place up to the residual moisture content of
2
to 3 wt.%, the degree of crosslinking being between 50% and 85% after drying.
io In some embodiments, the degree of crosslinking is between 70% and 85%
after
drying.
In some embodiments, the prepreg is a decorative prepreg or decorative
covering material.
In another aspect, there is provided a process for the production of a
prepreg,
comprising the following steps: a) impregnation of a base paper with a
thermally
crosslinkable impregnating combination containing a formaldehyde-free resin
solution and a polymer dispersion, a mixing ratio of resin solution to polymer
dispersion in the impregnating combination being 45:55 to 10:90 based on
solids,
and b) drying of the impregnated base paper so that the impregnating
combination has a degree of crosslinking of at least 10% and at most 85% and a
residual moisture content of 2 to 3 wt.%.
In some embodiments, the resin solution contains: a) a polymer obtained by
free-
radical polymerization and containing less than 5 wt.% of an a,p-ethylenically
unsaturated monocarboxylic or dicarboxylic acid incorporated during
polymerization, b) a polymer obtained by free-radical polymerization and
containing more than 15 wt.% of an a43-ethylenically unsaturated
monocarboxylic
or dicarboxylic acid incorporated during polymerization, and c) an
alkanolamine
having at least two hydroxyl groups.
In some embodiments, drying is carried out at 100 C or above.
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In some embodiments, on impregnation, the mixing ratio of resin solution to
polymer dispersion in the impregnating combination is 35:65 to 30:70.
In some embodiments, on impregnation, proportion of impregnating components
in the paper is between 5 and 35 wt.%, based on the weight per unit area of
the
base paper.
In some embodiments, drying takes place in a paper machine.
DETAILED DESCRIPTION
The prepreg according to the invention can be obtained by impregnating a base
paper with an impregnating combination containing a thermosetting,
formaldehyde-free resin solution and a thermally crosslinkable polymer
dispersion. The impregnated base paper is dried and smoothed, the smoothing
expediently taking place inside the paper machine used to produce the prepreg.
The degree of crosslinking of the prepreg is at least 10% and at most 85%. The
impregnated base paper has a residual moisture content of 2 to 3 wt.%.
The degree of crosslinking is determined analogously to the test method
described in DE 101 34 302 Cl, wherein an area of 100 cm2 of prepreg is
stamped out, weighed, kept in water at a temperature of 60 C for 15 min,
rinsed,
dried in the oven at 130 C and weighed again. The soluble proportion of the
polymer(s) can be determined from the weight difference and the known
application rate of the impregnating resin or impregnating combination (e.g.
12
g/m2 dry, for a 60 g/m2 prepreg). The degree of crosslinking in % is then 100
¨
soluble proportion in %.
The invention also provides a process for the production of thermally
partially
crosslinked, formaldehyde-free prepregs inside the paper machine and to their
use as decorative covering materials. Prepregs are understood according to the
invention as meaning papers partially impregnated with formaldehyde-free resin
solutions in combination with polymer dispersions. The proportion of
impregnating components is 5 to 35 wt.%, preferably 10 to 30 wt.%, based on
the
base paper weight.
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The residual moisture content of the prepreg is between 1.5 and 3 wt.%.
Residual moisture contents of more than 3.5 wt.% can cause the prepreg to
stick
to machine parts, for example the smoothing rolls of the paper machine.
Formaldehyde-free resin solutions suitable for impregnation are the polymer
compositions described in DE 197 35 959 Al and containing
A) at least one polymer obtainable by free-radical polymerization and
containing less than 5 wt.% of an a,p-ethylenically unsaturated
monocarboxylic or dicarboxylic acid incorporated during
polymerization,
B) at least one polymer obtainable by free-radical polymerization and
containing more than 15 wt.% of an ot,13-ethylenically unsaturated
monocarboxylic or dicarboxylic acid incorporated during
polymerization, and
C) at least one alkanolamine having at least two hydroxyl groups.
In one particularly preferred embodiment, the base paper can be impregnated
using a resin that contains afore-mentioned components B) and C), but not
component A). Such resins are disclosed in DE 197 35 959 Al.
The water-thinnable impregnating mixture according to the invention,
consisting
of above-mentioned formaldehyde-free resin solutions A) + B) + C) or B) + C)
and conventionally used polymer dispersions, is applied to at least one side
of
the paper web, then dried to a residual moisture content of 2 to 3 wt.% and
then
smoothed. The impregnated base paper is thereby partially crosslinked, the
degree of crosslinking being between at least 10% and max. 85%, depending on
the mixing ratios of the impregnating combination. In the subsequent
processing
steps, such as printing, varnishing and lamination/gluing, this only partially
crosslinked prepreg is sufficiently flexible. It is particularly advantageous
for the
quality of the prepreg if the residual moisture content is already adjusted in
the
paper machine (online). In line with the general usage of the term, a paper
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machine is understood as meaning the paper production unit from the screening
of the fibre suspension to the rolling-up of the base paper.
The mixing ratio of formaldehyde-free resin solution (I) to polymer dispersion
(II)
can be 49:51 to 10:90 (solid:solid), preferably 45:55 to 20:80. Particularly
preferred mixing ratios are from 35:65 to 30:70 because prepregs produced with
impregnating combinations having these mixing ratios have a particularly good
printability and adequate flexibility.
The concentration of the impregnating mixture according to the invention,
which
contains further additives if necessary, can be up to 50 wt.%. The
impregnating
combination according to the invention can be applied to the paper web online
in
an amount of between 5 and 35 wt.%, preferably of between 10 and 30 wt.%,
based on the weight per unit area of the base paper. The application devices
employed can be any units conventionally used inside the paper machine,
preferably roller application devices such as a size press or film press.
Surprisingly, it has been found that the prepregs according to the invention
have
better properties than already known formaldehyde-containing and
formaldehyde-free prepregs in respect of printability and surface strength
(tesa
strength), even immediately on leaving the paper machine. At the same time,
however, they retain a high surface imperviousness (high lacquer resistance)
and
dimensional stability/flatness which formaldehyde-free, uncrosslinked prepregs
according to DE 101 34 302 Cl, i.e. impregnated with the above-mentioned
formaldehyde-free resin solutions according to DE 197 35 959 Al, do not
possess on leaving the paper machine. In the case of the last-mentioned
prepreg, partial crosslinking only takes place at the end of the entire
further
processing chain, i.e. during hot pressing.
These outstanding properties of the prepreg according to the invention are
achieved by combining resin solutions and polymer dispersions in a particular
mixing ratio for which an optimum is established between the penetrating and
film-forming properties of the individual components on impregnation of the
base
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paper, and which has already extensively reached its final state inside the
paper
machine, presumably due to synergistic effects and the influence of
temperature.
The prepreg according to the invention can therefore be used in a
significantly
more universal manner in the further processing steps and is not subject to
any
technological limitations in respect of different process parameters such as
speed, temperature and pressure. In particular, the prepregs according to the
invention can be produced so as to possess a high flexibility, dimensional
stability
and good barrier properties against water and solvents and can be used in
particular as a base material for high-quality special papers such as printing
and
advertising substrates, decorative wall materials, abrasive papers, adhesive
tapes or floor coverings.
It has been possible to achieve particularly good grades of prepreg when its
degree of crosslinking is more than 50% and preferably 70% or more.
The base papers to be impregnated according to the invention are those
described inter alia in patents EP 0 223 922 Al, EP 0 870 872 Al and DE 101 34
302 Al. Likewise, the corresponding impregnation, drying and smoothing
processes are described in detail in said documents and in the journal
"Wochenblatt fur Papierfabrikation" no. 6 (2000), pp 350 to 355, and are known
state of the art.
The Examples which follow illustrate the invention. Unless stated otherwise,
all
amounts are based on the solid or solids content.
EXAMPLES
Example 1 (Comparative)
A base paper consisting of 20% of kraft pulp and 80% of eucalyptus pulp was
produced according to the invention. The freeness value was 30 SR (Schopper-
Riegler). The additives used were 25% of titanium dioxide and 4% of
formaldehyde-free wet strength additive, based on absolutely dry pulp. This
base
paper., without internal sizing and with a weight per unit area of 48 g/m2,
was
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impregnated on both sides in a laboratory size press with a formaldehyde-free
resin solution of modified polyacrylic acid and a polyhydric alcohol
(AcrodurTM
950 L from BASF), having a solids content of approx. 30%, and then dried at
different temperatures for approx. 18 seconds to a residual moisture content
of
approx. 2%.
The application rate after drying was 10 g/m2.
The degrees of crosslinking of the prepregs dried at different temperatures
are
io collated in Table 1. A partial crosslinking of the prepregs to about 30%
then takes
place under drying conditions that are conventionally used in the paper
machine
(up to 110 C, drying time approx. 14 seconds at 300 m/min).
Example 2 (Prepreq according to the invention)
The base paper from Example 1 was impregnated by means of a laboratory size
press with the impregnating mixture according to the invention, consisting of
a
formaldehyde-free resin solution (Acrodur 950 L) and a styrene/acrylate
dispersion (AcronalTM 305 D from BASF) in a ratio of 35:65 (solid:solid), and
dried
for approx. 18 seconds at temperatures of about 110 C to a residual moisture
content of approx. 2%.
The application rate was about 12 g/m2. The degree of crosslinking of the
prepreg according to the invention was approx. 70%. Accordingly, the addition
of
polymer dispersion to the resin solution significantly increases the degree of
crosslinking.
Example 3 (Comparative)
The base paper from Example 1 was impregnated by means of a laboratory size
press with an impregnating mixture conventionally used for prepregs,
consisting
of formaldehyde-free styrene/acrylate dispersion (Acronal TM 305) and
formaldehyde-containing resin solution (Urecoll TS) in a ratio of 70:30 (A)
and
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90:10 (B) (based on solids), and then dried analogously to Example 2. The
application rate was approx. 13 g/m2 in each case.
The degree of crosslinking of the conventional prepreg was 76% (A) or 84% (B),
i.e. significantly higher than that of prepregs according to the invention
containing
formaldehyde-free resin solutions according to Examples 1 and 2.
Example 4
The prepregs produced according to Examples 1 to 3 were smoothed under
io constant conditions in a laboratory calender. The paper properties
achieved are
collated in Table 2.
In comparison with conventionally used formaldehyde-containing or low-
formaldehyde prepregs (Comparative Examples 3 A and 3 B), the formaldehyde-
free prepreg according to the invention (Example 2) has a significantly better
interlaminar strength and printability. The lacquer resistance remains at a
high
quality level, while the flatness deteriorates somewhat.
In contrast to the formaldehyde-free prepreg according to Example 1, the
prepreg
zo according to the invention (Example 2) is distinguished by an improved
surface
imperviousness and hence a significantly higher lacquer resistance, lower
colour
bleeding (pinhole test) and better printability, as well as a greater
flatness.
The significantly better properties of the thermally partially crosslinked
prepreg
according to the invention are due to the combination of resin solution and
polymer dispersion in a specific range of mixing ratios, and the consequently
higher degrees of crosslinking than with the resin solution alone.
35
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Table 1
Degree of crosslinking of prepregs after different heat treatments (contact
drying)
Drying temperature, C
95 100 105 110
Degree of crosslinking, % 011.3 18.6 30.5
Impregnation with formaldehyde-free resin solution
Acrodur 350 L, 30%
Application rate approx. 10 g/m2
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Table 2
Comparison of the properties of different prepregs, smoothed
Comparisons
Prepreg according to
the invention
Paper property Example 3 (A) Example 3 (B)
Example 1 Example 2
Weight per unit area, g/m2 approx. 60 approx. 60
approx. 60 approx. 60
Application rate, gim 2 approx. 13 approx. 13
approx. 10 approx. 12
_
o
Degree of crosslinking, % approx. 76 approx. 84
approx. 30 approx. 70
0
(110 C, <20 seconds)
1..)
_
01
Water absorption, girn2 17.0 20.0
37.8 23.0
co
ko
Bendtsen porosity, ml/min 37.1 36.0
250 14.5 0.
1..)
Bekk smoothness, s 296 310
285 320 "
0
.
1-,
Flatness, s 14 5
6 10
1
0
Interlaminar strength / tesa test 3 5
1 1 co
1
0
Lacquer resistance / gloss, % 84.6 82.1
21.0 81.8 0.
Pinhole test 1 1
4 2
Printability 3 3
4 1
(aqueous rotogravure)
.
Formaldehyde emission, approx. 0.7 approx. 0.35
<0.2 <0.2
. mg HCO/m2-h (low-formaldehyde)
(formaldehyde-free) (formaldehyde-free)
,
_
Score 1 = very good
Score 5 = very poor
