Note: Descriptions are shown in the official language in which they were submitted.
CA 02625075 2008-04-08
1
REDUCING THE WATER AND WATER VAPOUR ABSORBENCE AND
ENHANCING THE DIMENSIONAL STABILITY OF PAPER AND PAPER
PRODUCTS AND USE OF COATED PAPER PRODUCTS
Description
The invention relates to methods for reducing the absorption of water and
water vapor
and for increasing the dimensional stability of paper and paper products by
treatment
with an aqueous solution and/or dispersion of at least one reactive material
which re-
acts with itself and/or with the cellulose fibers with crosslinking, and
heating of the
treated materials to a temperature at which drying and crosslinking takes
place, and
the use of coated paper products and/or of the coated cellulose fibers which
can be
produced therefrom by defibrating as an additive to thermoplastics and as an
additive
to heat-curable plastics.
From the publication "Treatment of timber with water soluble dimethylol resins
to im-
prove the dimensional stability and durability", which appeared in Wood
Science and
Technology 1993, pages 347-355, it is known that the shrinkage and swelling
proper-
ties of wood and the resistance to fungi and insects can be improved by
treating wood
with an impregnating agent which consists of an aqueous solution of dimethylol
dihy-
droxyethyleneurea (DMDHEU or 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-
2-
one) and a catalyst. Catalysts used are metal salts, citric acid and amine
salts, indi-
vidually or in combination. The DMDHEU is used in the aqueous solution in
concentra-
tions of from 5% to 20%. The added amount of catalyst is 20% based on the
DMDHEU. The impregnation is effected under reduced pressure. At elevated tem-
perature, a reaction of the DMDHEU with itself and with the wood takes place.
This
reaction takes place for one hour in a drying oven at temperatures of 80 C or
100 C.
The wood samples thus treated exhibit an improvement in the shrinkage and
swelling
properties of up to 75%, and do so at DMDHEU concentrations of 20%. In this
way,
wood bodies having dimensions of 20 mm x 20 mm x 10 mm were investigated. The
method described can be used only in the case of small dimensions of the wood
bod-
ies, because these tend to crack in the case of larger dimensions.
EP-B 0 891 244 discloses the impregnation of wood bodies comprising solid wood
with
a biodegradable polymer, a natural resin and/or a fatty acid ester - if
appropriate with
application of reduced pressure and/or pressure. The impregnation takes place
at ele-
vated temperatures. The pores in the wood are at least largely filled, and a
molding
which comprises both wood and biodegradable polymer forms. The polymer does
not
react with the wood. With this treatment, the characteristic properties of
wood, the bio-
degradability and the mechanical properties are not lost. The thermoplasticity
can be
increased. Depending on the proportion of polymer introduced, there is an
increase in
the surface hardness by the incorporation of the polymer into the wood matrix,
so that
naturally soft woods are also suitable for high-quality floors.
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SE-C 500 039 describes a method for hardening wood with densification, in
which un-
treated wood is impregnated with various aminoplast monomers based on melamine
and formaldehyde by means of suitable vacuum pressure impregnation, then
dried,
and cured in a press for densification at elevated temperature. Agents
mentioned are,
inter alia, DMDHEU, dimethylolurea, dimethoxymethylurea,
dimethylolethyleneurea,
dimethylolpropyleneurea and dimethoxymethyluron. This method has the disadvan-
tage that the natural wood structure is lost as a result of the densification,
and the for-
maldehyde emission of the finished wood body is relatively high, depending on
the
crosslinking agent used.
WO 04/033171 discloses a method for the production of a wood body having high
sur-
face hardness and low formaldehyde emission, an untreated wood body being
impreg-
nated with an aqueous solution of
A) an impregnating agent consisting of a 1,3-bis(hydroxymethyl)-4,5-
dihydroxyimidazolidin-2-one modified with a Cl-5-alcohol, a polyol or mixtures
thereof, and
B) a catalyst frorn the group consisting of the ammonium or metal salts,
organic or
inorganic acids and mixtures thereof,
dried, and then cured at elevated temperature.
According to the method disclosed in WO 04/033170 the durability, dimensional
stabil-
ity and surface hardness of a wood body is improved by impregnating a wood
body
with a 1 to 50% strength by weight aqueous solution of an impregnating agent
compris-
ing a substance of group A and/or at least one substance of group B and at
least one
substance of group C as a catalyst, and causing the impregnating agent
subsequently
to react with itself and with the wood under humid conditions for avoiding
drying. Suit-
able impregnating agents are, for example, dimethyloldihydroxyethyleneurea
(DMDHEU), urea-glyoxal adducts and urea-formaldehyde adducts. Suitable
catalysts
are, for example, magnesium chloride, zinc chloride, ammonium chloride or
acids, such
as formic acid, maleic acid, hydrochloric acid or sulfuric acid.
WO 2004/025019 discloses a method and an apparatus for exchanging a liquid
present
in fibers with another liquid. The procedure adopted here is to press out
fiber cake to
such an extent that a considerable amount of the liquid which is present in
the fibers is
transferred into the space between the fibers, to meter the other liquid,
which is to re-
place the first liquid, into the compressed fiber cake during the compression
step so
that the first liquid is removed from the space between the fibers, and then
to let down
the pressure on the fiber cake under further action of the other liquid which
is to replace
the first liquid, further replacement liquid being absorbed. Liquid cleaners,
chemical
PF 57237 CA 02625075 2008-04-08
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treatment agents, liquid acids or bases, bleaches, delignification agents,
catalysts,
complexing agents, fluorescence indicators, metal ions, cationic or anionic
poiymers,
colorants and inorganic substances being mentioned as replacement liquid. With
the
aid of the known method, it is possible, for example, at least partly to
remove lignin
constituents from cellulose fibers and thus to prepare a pulp having a better
and more
uniform quality.
Cellulose fibers and paper products produced therefrom, such as paper, board
and
cardboard, readily absorb water and also water vapor from the air. As a
result, how-
ever, the dimensional stability and the mechanical stability of the cellulose
fibers and of
the paper products are reduced to an undesired extent. In order to reduce the
water
absorption of paper products, a wet strength agent can be added, to the paper
stock,
for example during production of said products. Known wet strength agents are,
for
example, urea-formaidehyde resins, which increase not only the wet strength
but also
the dry strength of the paper (cf. EP-A 0 123 196 and US 3,275,605), melamine-
formaldehyde resins (DE-B 10 90 078) and other commercially available
products, for
example epichlorohydrin-crosslinked condensates of polyamidoamines, such as
the
Luresin brands (BASF Aktiengesellschaft).
It is the object of the invention to provide a method for reducing the
absorption of water
and water vapor and for increasing the dimensional stability of paper and
paper prod-
ucts, such as board and cardboard.
The object of the invention is achieved, according to the invention, by a
method for
reducing the absorption of water and water vapor and for increasing the
dimensional
stability of paper and paper products by treatment with an aqueous solution
and/or dis-
persion of at least one reactive material which reacts with itself and/or
cellulose fibers
with crosslinking, and heating of the treated materials to a temperature at
which drying
and crosslinking takes place, if cellulose fibers or a paper product obtained
therefrom
by drainage on a wire are or is first compressed, the compressed paper product
is then
brought into contact with an aqueous solution and/or dispersion of the
reactive mate-
rial, the compression is eliminated with further action of the aqueous
solution and/or
dispersion and the paper product is dried and crosslinked. The crosslinking of
the re-
active materials takes place, for example, at temperatures above 30 C, for
example in
the temperature range of from 35 to 200 C. The method can be carried out
continu-
ously and also batchwise.
A preferred procedure is one in which cellulose fibers which comprise at least
50% by
weight of virgin fibers or a paper product obtained therefrom by drainage on a
wire,
having a water content of in each case at least 0.7 g of water by g of dry
cellulose fi-
bers, are or is first compressed under a pressure of at least 2.1 MPa, the
compressed
paper product is then brought into contact with an aqueous solution and/or
dispersion
PF 57237
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of the reactive material, the compression is eliminated with further action of
the aque-
ous solution and/or dispersion, and the paper product is dried and is heated
to a tem-
perature in the range of from 70 to 200 C for crosslinking.
For example, the aqueous solution and/or dispersion comprises, as reactive
material,
at least one heat-curable binder from the group consisting of the urea-
formaldehyde
adducts, urea-glyoxal adducts, melamine-formaldehyde adducts, phenol-
formaldehyde
adducts, one- and two-component systems based on epoxy resins, polyurethanes
or
isocyanates, polyacrylates, polymethacrylates, styrene-(meth)acrylate
copolymer dis-
persions and/or styrene-butadiene-(meth)acrylic acid copolymer dispersions. In
some
cases, the use of mixtures of at least two reactive materials is of interest,
for example
mixtures of inelamine/urea-formaldehyde condensates. The reactive materials
may be
present as aqueous solution or as aqueous dispersion. Here, transitions
between solu-
tion and dispersion are possible. If dispersions are used for example, the
mean parti-
cle diameter of the polymer particles dispersed in water is less than 1 pm,
preferably
less than 500 nm and generally in the range of from 10 to 100 nm.
The aqueous solution and/or dispersion thus comprises, for example, a group of
a re-
active, crossiinkabie materiai which may consist of
(i) at least one reactive substance which forms a polymer,
(ii) if appropriate, at least one Cl_5-alcohol, at least one polyol or
mixtures thereof
and
(iii) at least one catalyst.
Examples of (i) a reactive substance which forms a polymer are urea-glyoxal
adducts
and derivatives thereof, e.g. 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-
2-one
(referred to below as "DMDHEU"). In the impregnation, it can be used either
alone or
together with (ii) at least one Cl_5-alcohol, a polyol or mixtures thereof. If
1,3-
bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-one is used together with an
alcohol
and/or polyol as the impregnating agent, correspondingly modified 1,3-
bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-ones (referred to below as
"mDMDHEU") form. Such compounds are disclosed, for example, in US 4,396,391
and WO 98/29393. These are reaction products of 1,3-bis(hydroxymethyl)-4,5-
dihydroxyimidazolidin-2-one with at least one Cl_5-alcohol, at least one
polyol or mix-
tures thereof.
The compounds of group (ii) include CI_5-alcohols, for example methanol,
ethanol, n-
propanol, isopropanol, n-butanol, isobutanol, tert-butanol and n-pentanol,
preferably
methanol, and polyols, such as ethylene glycol, diethylene glycol, 1,2- and
1,3-
propylene glycol, 1,2-, 1,3- and 1,4-butylene glycol, glycerol,
trimethylolpropane and
polyalkylene glycols, such as polyethylene glycol, polypropylene glycol, block
copoly-
PF 57237 CA 02625075 2008-04-08
mers of ethylene glycol and propylene glycol. Polyethylene glycols of the
formula
HO(CH2CH2O),H, where n is from 3 to 20, and diethylene glycol are preferred.
In order to prepare modified 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-
2-one
5 (mDMDHEU), DMDHEU and the monohydric alcohol and/or the polyol are mixed,
the
monohydric alcohol and/or the polyol being used in an amount of from 0.1 to
2.0 mol
equivalents each, based on DMDHEU. The mixture of DMDHEU, monohydric alcohol
and/or polyol is reacted, for example, at temperatures of from 20 to 70 C and
a pH of
from 1 to 2.5, the pH being adjusted to 4 to 8 after the reaction.
(i) a reactive substance which forms a polymer is to be understood as meaning
both
urea-formaldehyde adducts and urea-glyoxal adducts and in each case
derivatives
thereof. The following compounds may be mentioned by way of example: dimethy-
lolurea, bis(methoxymethyl)urea, tetramethylolacetylenediurea,
methylolmethylurea
and 1,3-dimethyl-4,5-dihydroxyimidazolidin-2-one, 1,3-
bis(hydroxymethyl)imidazolidin-
2-one or mixtures thereof. These compounds of group (i) can, if appropriate,
also be
used in the presence of (ii) at least one C,_s-alcohol, at least one polyol or
mixtures
thereof as the impregnating agent. Suitable alcohols and polyols have already
been
mentioned above. ivielhanoi, diethylene glycol and mixtures thereof are
preferred.
The aqueous solution of the impregnating agent comprises the reactive
compounds of
group (i) and the compounds of group (ii), for example, in a concentration of
from 1 to
70% by weight, preferably from 10 to 60% by weight and in particular from 20
to 60%
by weight. The impregnating agent preferably comprises 1,3-bis(hydroxymethyl)-
4,5-
dihydroxyimidazolidin-2-one (DMDHEU) as a compound of group (i).
In addition to (i) and, if appropriate, (ii), the impregnating agent always
comprises a
catalyst (iii). Suitable catalysts (iii) are, for example, metal salts from
the group consist-
ing of metal halides, metal sulfates, metal nitrates, metal
tetrafluoroborates, metal
phosphates or mixtures thereof. individual examples of (iii) are magnesium
chloride,
magnesium sulfate, zinc chloride, lithium chloride, lithium bromide, boron
trifluoride,
aluminum chloride, aluminum sulfate, zinc nitrate and sodium
tetrafluoroborate. Said
compounds can be used, either alone or as a mixture, as a catalyst.
Further suitable catalysts (iii) are ammonium salts, such as ammonium
chloride, am-
monium sulfate, ammonium oxalate, diammonium phosphate or mixtures thereof. In
addition, organic and/or inorganic acids may be used as a catalyst. Examples
of these
are maleic acid, formic acid, acetic acid, propionic acid, citric acid,
tartaric acid, oxalic
acid, p-toluenesulfonic acid, hydrochloric acid, sulfuric acid, boric acid or
mixtures
thereof.
PF 57237 CA 02625075 2008-04-08
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Preferably used compounds of group (iii) are magnesium chloride, zinc
chloride, mag-
nesium sulfate, aluminum sulfate or mixtures of these compounds. Magnesium
chlo-
ride is particularly preferred.
The catalyst (iii) is present, for example, in a concentration of from 0.1 to
10% by
weight, preferably from 0.2 to 8% by weight, particularly preferably from 0.3
to 5% by
weight, based on the components (i)-(iii) of the reactive material.
Of the products which are described above and comprise formaldehyde in the
form of
condensed units, in particular low-formaldehyde condensates are used. In the
present
context, low-formaldehyde is to be understood as meaning that the reactive
materials
comprise no substantial amounts of free formaldehyde and that no substantial
amounts
of formaldehyde are released even during drying or curing of the cellulose
fibers or
paper products treated therewith. In general, such reactive materials comprise
< 100
ppm of formaldehyde.
Further reactive materials which react with themselves and/or cellulose fibers
with
crosslinking are formaldehyde-free, heat-curable binders. Such binders are
described,
for exai-iiple, in the following publications, which are hereby incorporated
by reference
as disclosure content of the present invention, namely US 4 076 917, EP-A 0
445 578,
EP-A 0 583 086, EP-A 0 651 088, WO 97/31036, page 4, line 12, to page 12, line
14,
WO 97/31059, page 2, line 22, to page 12, line 5, WO-A-97/31060, page 3, line
8, to
page 12, line 36, DE-A-199 49 591, page 3, line 5, to page 7, line 38, WO
01/27163,
page 5, line 34, page 22, line 2, and the radiation-curable binders disclosed
in DE-A
199 17965.
In addition to the binders which are described in the abovementioned
publications,
suitable heat-curable binders are all curable binders which are described in
the litera-
ture, for example, for strengthening nonwovens and/or are used for this
purpose in pra-
ctice, such as heat-curable resins based on phenol and formaldehyde, the
abovemen-
tioned melamine-formaldehyde and urea-formaidehyde resins, urea-glyoxal resins
and
in particular formaldehyde-free one- and two-component systems based on epoxy
res-
ins or polyurethanes, polyacrylates, polymethacrylates, polyvinyl acetates,
styrene
acrylate copolymer dispersions, styrene-methacrylate copolymer dispersions,
styrene-
butadiene-(meth)acrylic acid copolymer dispersions and mixtures of said
dispersions
with a mixture of a polycarboxylic acid and a polyhydric alcohol as
crosslinking compo-
nent.
Examples of preferred heat-curable binders are mixtures of
(a) a polymer which is obtainable by free radical polymerization and which com-
prises, incorporated in the form polymerized units, from 5 to 100% by weight
of
PF 57237 CA 02625075 2008-04-08
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an ethylenically unsaturated carboxylic anhydride or of an ethylenically
unsatu-
rated dicarboxylic acid whose carboxyl groups can form an anhydride group, and
(b) at least one alkanolamine, which comprises at least two hydroxyl groups in
the
molecule and/or at least one polyhydric alcohol.
Specific examples of such mixtures are aqueous solutions and/or dispersions of
a co-
polymer of 80% by weight of acrylic acid and 20% by weight of maleic acid,
comprising
from about 40 to 60% by weight of solids and having a molar mass M, of from 15
000
to 900 000, in combination with triethanolamine or aqueous solutions of a
copolymer of
55% by weight of acrylic acid and 45% by weight of maleic acid in combination
with
triethanolamine. These binders can comprise, if appropriate, an esterification
catalyst
and/or a compound comprising bound phosphorus, such as hypophosphorous acid,
as
a reaction accelerator.
The copolymer (a) described above may be composed, for example, of
- from 50 to 99.5% by weight of at least one ethylenically unsaturated mono-
or
dicarboxylic acid,
- from 0.5 to 50% by weight of at least one ethylenically unsaturated compound
from the group consisting of the esters of ethylenically unsaturated
monocarbox-
ylic acids and the monoesters and diesters of ethyfenicalfy unsaturated
dicarbox-
ylic acids with an amine having at least one hydroxyl group and
- up to 20% by weight of another monomer.
Heat-curable, aqueous compositions which comprise at least one copolymer (a)
and at
least one alkanolamine or higher-functional 0-hydroxyalkylamine and/or at
least one
polyhydric alcohol can, if appropriate, additionally comprise at least one
surfactant.
Further heat-curable binders are based on aqueous mixtures of
polycarboxylic acids, such as polyacrylic acid, polymethacrylic acid,
copolymers
of acrylic acid and maleic acid, copolymers of methacrylic acid and maleic
acid,
copolymers of ethylene and maleic acid, styrene and maleic acid, or copolymers
of acrylic acid or methacrylic acid and esters of acrylic or methacrylic acid
with
preferably monohydric alcohols comprising 1 to 24 carbon atoms, the polycar-
boxylic acids exhibit a K value of from 50 to 100 (measured with the
polycarbox-
ylic acids in unneutralized form according to H. Fikentscher in dimethylforma-
mide at 25 C and a polymer concentration of 0.1 % by weight) and
- polyhydric alcohols, such as trimethylolpropane, glycerol, 2-
hydroxymethylbutane-1,4-diol and polyvinyl alcohol, and/or polyfunctional
amines
and/or alkanolamines.
PF 57237 CA 02625075 2008-04-08
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Polycarboxylic acids, polyhydric alcohols, alkanolamines and polyfunctional
amines are
preferably used in amounts such that the number of acid functions is
equivalent to the
total number of alcoholic hydroxyl and amine functions, cf. EP-A 0 445 578. In
addi-
tion, crosslinkable materials which consist of an aqueous solution of a
polycarboxylic
acid (homo- or copolymer), preferably having a molar mass MN, of 10 000 or
less, and a
polyol, such as triethanolamine, and in which the ratio of the number of
equivalents of
hydroxyl groups to the number of equivalents of carboxyl groups is in the
range of from
0.4:1 to 1.0:1 are suitable, cf. EP-A 0 990 727.
In the method according to the invention, binders which are sold under the
trade name
Acrodur by BASF Aktiengesellschaft are particularly advantageously used as
reactive
materials. An example of this is an aqueous styrene-acrylate polymer
dispersion which
is modified with a polycarboxylic acid and a polyhydric alcohol as
crosslinking compo-
nent. It crosslinks at a temperature of as low as 130 C. However, in order to
achieve
high production speeds, the crosslinking is preferably carried out at
temperatures of
from 180 to 200 C. A further formaldehyde-free binder is commercially
available, for
example, as a colorless to slightly yellowish, clear, aqueous solution of a
modified
polycarboxylic acid with a polyhydric alcohol as crosslinking component. It
crosslinks,
for example, at drying temperatures of from about 160 to 180 C.
Formaldehyde-free reactive materials which comprise at least one
polycarboxylic acid
and at least one polyhydric alcohol and/or alkanolamine or polyfunctional
amine are
particularly preferred. Compositions which comprise these reactive agents can,
if ap-
propriate, comprise even further formaldehyde-free polymers, e.g.
polyacrylates, which
are sold under the trade name Acronal by BASF Aktiengesellschaft. The aqueous
solutions and/or dispersions of a reactive material which are used for the
impregnation
comprise the reactive material, for example in an amount of from 1 to 70% by
weight,
preferably from 10 to 60% by weight and generally from 30 to 50% by weight.
In the context of the invention, paper products are to be understood as
meaning, for
example, paper itself and board and cardboard. For the method according to the
in-
vention, it is possible to start from cellulose fibers of all types, both from
natural and
from recovered fibers, in particular from fibers from waste paper, which,
however, are
used only as a mixture with virgin fibers. Virgin fibers are to be understood
as meaning
cellulose fibers which have not yet been processed to give a paper product or
which
have not yet been dried. In fiber mixtures comprising virgin fibers and fibers
from
waste paper, the amount of virgin fibers is, for example, at least 50% by
weight, pref-
erably at least 70% by weight. In the particularly preferred process variant,
a pulp
which comprises 100% of virgin fibers is used as a starting material Suitable
fibers for
the production of the pulps are all qualities customary for this purpose, e.g.
mechanical
pulp, bleached and unbleached chemical pulp and paper stocks from all annual
plants.
Mechanical pulp includes, for example, groundwood, thermomechanical pulp
(TMP),
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chemothermomechanical pulp (CTMP), pressure groundwood, semi-chemical pulp,
high-yield pulp and refiner mechanical pulp (RMP). For example, sulfate,
sulfite and
soda pulps are suitable as chemical pulp. Unbleached chemical pulp, which is
also
referred to as unbleached kraft pulp, is preferably used. Suitable annual
plants for the
production of paper stocks are, for example, rice, wheat, sugarcane and kenaf.
In contrast to dried cellulose fibers, virgin fibers have a high porosity, cf.
W. Gindl, F.
Zargar-Yaghubi and R. Wimmer, Bioresource Technology 87, 325-330 (2003). If a
collection of moist cellulose fibers is considered, the water is found both
between the
individual cellulose fibers and in the interior of the cellulose fibers. An
aqueous slurry
of cellulose fibers is pressed during drainage on the wire of a paper machine
to such
an extent that the sheets formed therefrom comprise from 0.7 to 1.0 g of water
per g of
dry cellulose fibers, cf. G.V. Laivins and A.M. Scallan, TAPPI Proceedings,
Engineering
Conference, Book 2, 741-747 (1993). After pressing, for example with the aid
of a size
press, water is present in the spaces between the fibers and in the interior
of the cellu-
lose fibers. As long as the fibers have a sufficient porosity, water can also
be removed
from the interior of the fibers by pressing a fiber structure. In the method
according to
the invention, the paper product drained on a wire is subjected to a pressure
such that
water is forced out of the interior of the cellulose fibers. This pressure is
at least 2.1
MPa and may be, for example up to 50 MPa. Preferably, it is in the range of
from 2.5
to 10 MPa. As a result of the action of a pressure of at least 2.1 MPa on the
moist fiber
product comprising a predominant proportion of virgin fibers, the water
content of the
paper product is reduced to values below 0.7 g of water per g of dry fibers.
It is, for
example, from 0.3 to 0.5 g of water per g of dry cellulose fibers and is
generally in the
range of from 0.3 to 0.4 g of water per g of dry cellulose fibers.
The action of a pressure on the fiber structure and the treatment of the fiber
structure
with an aqueous solution of a reactive material can be effected continuously
or batch-
wise. A continuous procedure is disclosed in WO 2004/025019, page 5, line 3,
to page
8, line 8, mentioned in connection with the prior art. As explained in more
detail there,
a fiber cake on a wire or a belt is passed through a nip formed by two
compression rolls
and is compressed therein. As a result, a part of the water which is present
in the cel-
lulose fibers is forced out of the interior of the cellulose fibers into the
spaces between
the fibers of the compressed fiber cake and partly out of the fiber cake. With
the aid of
a compressible belt which revolves over a roll which, together with the other
roll, forms
the nip for the compression of the cellulose fiber cake, the compressed
cellulose fiber
structure is brought into contact under pressure with an aqueous solution of
the reac-
tive material. As a result, the water which originates from the interior of
the cellulose
fibers and is present in the intermediate spaces between the fibers is
replaced by the
aqueous solution of the reactive material. After leaving the roll nip, the
compressed
cellulose fiber structure is passed through an interstice which is filled with
an aqueous
solution of a reactive material. Relaxation of the compressed cellulose fibers
begins.
PF 57237 CA 02625075 2008-04-08
Similarly to a compressed sponge, which is released, cellulose fibers absorb
aqueous
solution of the reactive material. The solution penetrates not only into the
intermediate
spaces of the paper product but also into the interior of the cellulose
fibers. In this way,
not only coating of the individual cellulose fibers of the paper product with
a reactive
5 material but also at least partial coating of the interior of the fibers is
achieved. After
the treatment with an aqueous solution of a reactive material, the paper
product is dried
and is heated to a temperature of, for example, from 70 to 200 C for
crosslinking the
reactive material.
10 In a batchwise embodiment of the method according to the invention, for
example, it is
possible to adopt a procedure in which first a paper machine wire having a
mesh size
of, for example from 80 to 150 pm and then a sheet which is produced from a
predomi-
nant portion of virgin fibers and has a basis weight of, for exaniple, from 50
to 500
g/m2, in general from 75 to 250 g/mZ, and a water content of, for example,
from 50 to
80% by weight are placed in a press equipped with a perforated tray.
Thereafter, a
papermaker's felt which is impregnated with an aqueous solution of at least
one reac-
tive material and then a sheet of plastic, e.g. polymethyl methacrylate,
polystyrene or
polypropylene, are placed in succession on the paper sheet. A pressure of at
least 2.1
MPa is then exerted on the layers of said materials which are present in the
press with
the aid of a piston inserted into the press. Water which originates from the
cellulose
fiber structure and the interior of the cellulose fibers, together with excess
aqueous
solution of the reactive material is forced out of the perforated tray. The
duration of the
action of the pressure when the method according to the invention is carried
out
batchwise is, for example, from 0.1 to 120 seconds, preferably from 0.5 to 20
seconds.
In the continuous procedure, the duration of the pressure is, for example,
from 0.01 to
20 seconds, preferably from 0.02 to 1 second. After the end of the
compression, the
sheet absorbs further aqueous solution of the reactive material on relaxation.
It is then
removed from the press, and dried and heated to a temperature of, for example,
from
70 to 200 C, preferably from 120 to 170 C, for crosslinking the reactive
material.
While a polymer application of < 5 g/mz, in general from 1 to 3 g/mz, is
usually achieved
in the case of application of an aqueous polymer solution with the aid of a
size press,
the polymer application in the method according to the invention is, for
example, > 5
g/m2, e.g. from 5.5 to 8 g/mz. In comparison with known application methods,
paper
and paper products which have a reduced absorption of water and water vapor
and a
higher dimensional stability are therefore obtained by the method according to
the in-
vention.
Suspensions of cellulose fibers can be produced from the paper products
obtained by
the method according to the invention, for example by disintegration of the
paper or of
the paper products in water, from which suspensions in turn it is possible to
obtain, by
removal of water, coated cellulose fibers which comprise the coating material
at least
PF 57237 CA 02625075 2008-04-08
11
partly in the interior. These cellulose fibers may be present, for example, in
the form of
a powder.
Both writing and printing papers and packaging papers, corrugated board,
wallpapers,
cardboard, laminates of, for example, a composite of board or paper and at
least one
film or sheet of a thermoplastic, and construction elements can be produced by
the
method according to the invention. Of particular interest are mixtures of (i)
the paper
products obtainable by the method according to the invention and/or the coated
cellu-
lose fibers which can be produced by defibrating and (ii) thermoplastics or
heat-curable
plastics. Moldings of any desired design can be produced from such mixtures.
The invention therefore furthermore relates to the use of the coated papers or
paper
products obtainable by the method according to the invention and/or the coated
cellu-
lose fibers which can be produced therefrom by defibrating as an additive to
thermo-
plastics and as an additive to heat-curable plastics.
Such mixtures comprise, for example, from 0.1 to 90% by weight, preferably
from 1 to
70% by weight and in general from 2 to 50% by weight of at least one component
(i).
The composite materials are prepared, for example, by mixing at least one of
the
coated materials with at least one thermoplastic or one heat-curable material.
The mix-
ing can be effected, for example, in an extruder, for example at least one
product
coated according to the invention and a thermoplastic being heated to a
temperature
which is in the respective softening range of the thermoplastic or higher and
the mix-
ture being extruded.
Suitable thermoplastics are, for example, polyolefins, such as polyethylenes,
which are
obtainable by the high-pressure or low-pressure polymerization process,
pofypropyl-
ene, polybut-2-ene or polybut-1-ene, polyisobutylene, polystyrene, polyamides,
such as
polycaprolactam or condensates of hexamethylenediamine and adipic acid,
polyesters,
such as polyethylene terephthalate, polymethyl methacrylate, polycarbonate and
poly-
vinyl chloride.
Examples of heat-curable plastics are all reactive materials which have
already been
described above for the coating of paper and paper products, e.g. urea-
formaldehyde
resins, melamine-formaldehyde resins, one- and two-component systems based on
epoxy resins, polyurethane or isocyanates, crosslinkable polyacrylates and
crosslink-
able polymethacrylates.
The mixtures of the components (i) and (ii) are suitable for the production of
moldings,
in particular for the production of construction elements, such as composites
for the
insulation of walls, as a water vapor barrier, in the form of sheets for the
cladding of
facades or in the interior for the production of doors and claddings, as
material for the
PF 57237 CA 02625075 2008-04-08
12
production of pieces of furniture which are used outdoors and inside, as
housings for
electrical appliances, such as vacuum cleaners, kitchen machines, televisions,
radios,
stereo units and computers, as material for automotive parts, for example
interior door
trims, dashboards and shelves for seats, as material for flower boxes,
flowerpots, wa-
tering cans, plant tubs, walls and supporting parts for summer houses and for
toys and
as packaging material.
Unless otherwise evident from the context, the stated percentages in the
examples are
percentages by weight.
Examples
Determination of the water absorption
Paper samples having the dimensions 4 cm x 4 cm were weighed, then stored for
30
minutes in distilled water at a temperature of 20 C, then removed, dried with
an absor-
bent cloth and weighed. The weight increase is calculated in %.
Determination of the dimensional stability
Paper samples having the dimensions 4 cm x 4 cm were stored for one week over
sil-
ica gel at a temperature of 20 C in a desiccator. They were then weighed. In
addition,
the paper thickness (Di) was determined. The samples were then stored over
water
for one week in a desiccator so that the paper was saturated with water vapor.
The
samples were then weighed and the thickness (D2) of the samples was
determined.
The dimensional stability was determined as follows:
Dimensional stability = Dz-Di = 100 [%]
Di
In the formula, D, is the thickness of the dry paper and D2 is the thickness
of the moist
paper.
Determination of the moisture absorption
Paper samples having the dimensions 4 cm x 4 cm were stored for one week over
sil-
ica gel at a temperature of 20 C in a desiccator. Thereafter, they were
weighed (Wi)
and stored for one week over water in a desiccator so that the paper was
saturated
with water vapor. The samples were then weighed (W2). The moisture absorption
was
determined as follows:
IVz - bVI
Moisture absorption = = IV100 [%]
i
PF 57237 CA 02625075 2008-04-08
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Determination of the stiffness
The stiffness was determined by the beam method according to DIN 53 121. For
this
purpose, samples having a size of 100 x 25.4 mm were cut out of the papers to
be
tested, clamped, and measured under the following conditions: measuring length
I
100 mm, sample width b = 25.4 mm, bending angle a=20 . It was ensured that a
force
Fmax of at least 15 mN was reached at maximum deflection. Measurement was ef-
fected 5 times per sample.
Example 1
A paper stock which consisted of a mixture of 70% of bleached pine sulfate
pulp and
30% of birch sulfate pulp and had a freeness of 35 SR (Schopper-Riegler) was
drained
in a Rapid Kothen sheet former. The sheets had a basis weight of 80 g/mZ. They
were
pressed in each case between filter papers to a water content of 50% and
impregnated
by first placing a sheet in a press whose base consisted of a paper machine
wire hav-
ing a mesh size of 100 pm, then placing in succession a papermaker's felt
which was
impregnated with a 50% strength aqueous solution of Kaurit0 210 (urea-
formaldehyde
resin) on the sheet and then covering with a sheet of polymethyl methacrylate.
A pres-
sure of 2.1 MPa was then exerted on the content of the press for period of 5
seconds.
Water which originated from the paper and aqueous solution of the coating
material
from the papermaker's felt were forced from the bottom of the press.
Thereafter, the
pressure was canceled and the sheet impregnated in this manner was removed
from
the press. The sheet was dried for 4 hours at a temperature of 130 C. Under
these
conditions, the resin which was in the fibers and had been deposited thereon
crosslinked. Stiffness, dimensional stability and water absorption of the
sheet thus
obtained were then tested. The results are shown in the table.
Examples 2-6
Example 1 was repeated with the only exception that in each case an aqueous
solution
and/or dispersion of the heat-curable binders shown in table 1 was used
instead of the
aqueous solution of Kaurit0 210. The results thus obtained are shown in table
2.
PF 57237 CA 02625075 2008-04-08
14
Comparative example 1
Example 1 was repeated with the only exception that the papermaker's felt was
now
impregnated with distilled water instead of the aqueous solution of Kaurit
210. The
results are shown in table 2.
Table 1
Example no. Amount applied [g] Self-crosslinking material
1 8.0 Heat-curable urea-formaldehyde resin (Kaurit
210), 50% strength aqueous solution
2 7.5 Heat-curable melamine-formaidehyde resin
(Kauramin 787), 60% strength aqueous solution
3 8.0 Mixture of polycarboxylic acid and polyfunctional
amine (Acrodur~ 910L), 35% strength aqueous
solution
4 8.2 Mixture of polycarboxylic acid and polyfunctional
amine (Acrodur,5 DS 3515), 35% strength aque-
ous dispersion
5 7.9 Heat-curable urea-formaldehyde resin (Fixapret'O
ECO), 70% strength aqueous solution
Comparative
example no.
1 Impregnation with water
Table 2
Decrease in
Water absorp- Moisture ab- dimensional
Example no. Stiffness [mN] stability after
tion [%] sorption [%]
moisture ab-
sorption [%]
1 1700 100 15 3
2 2200 50 9 5
3 2000 270 20 6
4 3521 250 20 7
5 3000 110 15 4
Comparative
example no.
1 254 475 22 8
