Note: Descriptions are shown in the official language in which they were submitted.
~lZ6594
Process for the manufacture of paper having a hiyh dry
strength and a low wet strength
The present invention relates to a process for the
manufacture of paper having a high dry strength and a low wet
strength by treating the surface of the paper with a water-
soluble salt of a polymer based on one or more ethylenically
unsaturated carboxylic acids and drying the paper.
The strength characteristics of paper, e.g. the breaking
load, bursting pressure and pick resistance can be improved,
however, by adding auxillaries to the suspension of fibrous
material before forming the sheet, or by impregnating the
resulting paper by surface-coating with suitable products, for
example in a size press. For addition to the pulp, starch and
starch derivatives, and also synthetic products, mostly based
on polyacrylamide, are employed. For surface-coating, starch
and starch derivatives are used almost exclusively.
It is known that wet-strength resins based, for example,
on urea-formaldehyde or melamine-formaldehyde and employed to
improve the wet strength of paper by surface-coating, also
increase the dry strength of paper; such products are however
not used industrially as means of improving the dry strength
because of the great difficulty in re-disintegrating the wet-
strength paper, especially the waste material from paper
factories.
U.S. Patent 2,999,038 discloses that paper having a
high wet strength can be manufactured by treating the surface
of the paper with from 2 to 8% by weight, based on the weight
of dry paper, of an aqueous solution of an ammoniurn salt of
a high molecular weight polymer which contains acrylic acid,
methacrylic acid or itaconic acid as characteristic monorners.
The present invention seeks to provide compounds
which, when used as paper impregnating agents, increase the
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dry strength of the paper with only an insignificant effect,
if any, on the wet strength.
According to the present invention paper having a
high dry strength and a low wet strength may be provided by
treating the surface of paper with an aqueous solution of a
water-soluble alkali metal and/or alkaline earth metal salt of
a copolymer of
a) from 90 to 30% by weight of acrylic acid and/or
methacrylic acid and
b) from 10 to 70~ by weight of acrylonitriler
methacrylonitrile, acrylamide and/or methacrylamide, with or
without
c) up to 30% by weight of an ester of acryllc acid
and/or an ester of methacrylic acid,
which copolymer has a viscosity of from 5 to 100 mPas (measured
in a Brookfield viscometer at 20 revolutions per minute) in 2%
by weight strength aqueous solution at 20C, and drying paper.
The copolymer may be manufactured by polymerizing the
monomers using conventional methods, of U.S. Patents 2,819,189
and 2,999,038. In these methods, the monomer mixture is
polymerized continuously or batchwise, preferably in water,
uslng free radical initiators.
I~ the polymerization of the monomers is carried out using
the alkali metal or alkaline earth metal salts o~ acrylic
acid or methacrylic acid, the copolymer salt solutions to be
used according to the invention are obtained directly. Acrylic
acid and/or methacrylic acid can also be polymerized with the
corresponding amides or nitriles in water by a precipitation
polymerization method. The copolymer thereby obtained can be
dlrectly neutralized with an alkali metal and/or alkaline
earth metal hydroxide.
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~Z6594
However, it is also possible to carry out the poly-
merization in a~ueous solution with acrylic acid or methacrylic
acid which has been neutralized to the extent of from 10 to ~0
with ammonium, alkali metal or alkaline earth metal ions, and
then to neutralize the resulting aqueous polymer solution
completely with an alkali metal or alkaline earth metal hy-
droxide. The process for the manufacture of copolymers in
which acrylic acid or methacrylic acid partially neutralized
with ammonium ions is employed is disclosed in German Published
Application DAS 2,004,676.
However, it is also possible to isolate the copolymer
obtained by precipitation polymerization in water, dry it and
mix it with one or more dry, pulverulent alkali metal hydroxides
or alkaline earth metal hydroxidesor oxides. This pulverulent
mixture can then readily be dissolved in water, giving a clear
solution.
The copolymer obtained from the monomers listed under
a) and b), with or without c), can also be manufactured by the
reverse suspension polymerization disclosed in German Patent
1,081,228 and by the inverse emulsion polymerization process
disclosed in German Patent 1,089,173.
If the polymerization has not already been carried out
~ith an alkali metal or alkaline earth metal salt of acrylic
acid or methacrylic acid, the copolymer is neutralized, after
polymerization, with an alkali metal hydroxide or alkaline earth
metal hydroxide or alkaline earth metal oxide. The sodium,
pota~sium, calcium and magnesium salts of the above copolymers
are of particular importance. However, the alkali metal salts
according to the invention do not include the ammonium salts
of the copolymers. The copolymers contain from 90 to 30,
preferably from 90 to 60, ~ by weight of acrylic acid and/or
methacrylic aci~ and from 10 to 70, preferably from 10 to ~0 90
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l~Z6S94
by weight of acrylonitrile, methacrylonitrile, acrylamide or
methacrylamide, with or without up to 30~ by weight of an ester
of acrylic acid and/or an ester of methacrylic acid. The
ester is preferably derived from a primary alcohol of 1 to 4
carbon atoms.
Preferably, calcium and magnesium salts, and mixtures
of alkaline earth metal salts and alkali metal salts, of
copolymers of acrylic acid and acrylamide, of acrylic acid and
acrylonitrile, of acrylic acid, methacrylic acid and acrylamide,
and of acrylic acid, acrylamide and methacrylamide, are used.
At least 30~ of the carboxyl groups of the copolymer are
preferably neutralized with alkaline earth metal ions when
mixture~ are u~ed. In general, the degree of neutralization
i~ from 70 to 100%.
A 2% by weight strength aqueous solution of the alkali
metal and/or alkaline earth metal salts to be used according to
the invention has a viscosity tmeasured on a Brookfield
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viscometer at 20 revolutions per minute) of from 5 to lO0,
preferably from lO to 30, mPas at 20C. The pH of the co-
polymer salt solution is generally found to be from 4.0 to 10Ø
The water-soluble alkali metal salts and alkaline
earth metal salts of the copolymers in question are suitably
applied to paper in the form of their aqueous solution of from
1 to 10% by weight strength~ The paper can, for example, be
impregnated with the alkali metal or alkaline earth metal salt
solution of the copolymer in a size press, or a soiution of
the copolymer salt can be sprayed onto the paper. The amount
of treatment solution taken up depends on the absorbency of the
paper employed. To achieve a good increase in the dry strenyth
of the paper it suffices to impregnate the latter wîth from
preferably 1 to 4~ by weight - based on paper solids, of the
above alkali metal or alkaline earth metal salt of the copolymer.
The increase in strength of the paper results direct-
ly after drying the paper, which may be carried out under con-
ventional conditions, e.g. at from 80 to 110C. Aging of the
impregnated paper is not necessary. ~t is a particular advan-
tage that the alkali metal and/or alkaline earth metal salts
of copolymers, to be used according to the invention, can also
be employed together with the starches commonly used in indus- t
trial practice. In such cases it is possible to use a mixed
aqueous solution of starch and copolymer, which contains from
2 to 10~ by weight, preferably from 2 to 6% by weight, of
starch and from 1 to 3~ by weight of the alkali metal and/or
i
B
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594
alkaline earth metal salt of the copolymer, to be employed
according t~ the invention
i
B 5a-
~Z6S94
All conventional grades of paper, e.g. wrlting paper,
print stock and packaging paper, can be impregnated with the
copolymer salt to be used according to the invention. The
papers may have been manufactured from a plurality of fibrous
materials, e.g. bleached or unbleached sulfite or sulfate pulp,
mechanical pulp or waste paper. The use of an alkaline earth
metal salt of a copolymer of acrylic acid and/or methacrylic
acid and the comonomer(s) mentioned under b) as a surface
coating for paper leads to an unexpected increase in the dry
strength of the paper without a significant increase in the
wet strength. Examples of the dry strength characteristics
which are substantially improved are the tear length, the
bursting pressure, the pick resistance, the tear resistance
and the CMT value. The Examples which follow illustrate
embodiments of the invention. In these Examples, parts and
percentages are by weight. The viscosities were measured at
20C in a Brookfield viscometer at 20 revolutions per minute.
The dry tear length was determined as described in DIN 53,112,
page 1 and the wet tear length as described on page 2 of the
same Standard. The pick resistance of the paper was deter-
mined by the Dennison wax test.
EXAMPLE 1
A copolymer of 65% of acrylic acid and 35% of
acrylonitrile (manufactured by copolymerizing acrylic acid,
partially neutralized with ammonia, with acrylonitrile in
aqueous solution with potassium peroxydisulfate as the
cataly~t, at pH 3.5)
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o.z. 32 J 792
. is converted to the calcium salt (pH 7.0) by reaction with
calcium hydroxide. A wood-free,non-sized offset paper~
obtained from a pulp with a freeness of 25SR, and contain-
ing 14% of ash (kaolin) and 1% of alum is produced on a
papermaking machine, and is impregnated with a 2% strength
aqu~ous solution of the above calcium salt of the copolymer
of acrylic acid ard acrylonitrile, and is then dried at
100C. Table 1 lists the viscosity of the treatment solu-
tion, the amount of pure copolymer salt applied to the
paper~ based on the weight o~ paper, and some properties of
the paper obtained.
COMPARATIVE EXAMPLE la
The ammonium salt of the copolymer of 65% of acrylic
acid and 35% of acrylonitrile, described in Example 1, is
brought to pH 7.0 with ammonia and used to impregnate the
paper described in Example 1. The results are listed in
Table 1.
COMPARATIVE EXAMPLE lb
For comparison, the paper described in Example 1 is
lmpregnated with water only, and then dried at 100C.
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TABLE 1
Example l Comparative Example
(Calcium la -lb
salt) Ammonium Water
salt
Viscosity of the 2~
strength aqueous treat-
ment solution ~mPa.s] 8 25 5
Product pick-up (%~
(solids, based on dry
paper) 1.85 1.92
Dry tear length (m) 3,700 3,500 2,400
Dennison value 14 ll 3
Wet tear length (m) 150 665 0
_ . _
EXAMPLE 2
The ammonium salt of the copolymer of 65% of acrylic
acid and 35% of acrylonitrile, described in Example l, is
neutralized with sodium hydroxide. The pH of the aqueous
2% strength solution of the alkali metal salt is 7.5. The
paper described in Example l is impregnated with this solution
and dried at 100C. The results are listed in Table 2.
COMPARATIVE EXAMPLE 2a -.
A copolymer of 65% of acrylic acid and 35% of
acrylonitrile is used without neutralization, in the form
of a 2% strength aqueous solution having a pH of 3.5, for
impregnating the paper described in Example 1. The paper
i5 dried at the same temperature as in Example 2. The results
are given in Table 2. For comparison, Table 2 also gives the
values obtained if the paper described in Example l is
impregnated with water and dried at 100C.
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O.Z. 32,792
TABLE 2
Example 2 Comparative Example
(Sodium 2a 2b
__ salt) (Polyacid) (~ater~
Viscosity of the 2%
strength aqueous treat-
ment solution [mPa.s]30 20 5
Product pick-up (%)
(solids, based on dry
paper) . 1.95 1.90
Dry tear length (m)4,200 4,100 2,400
~ea-aison value 14 14 3
Wet tear length (m) 290 780 0
,
EXAMPLE 3
A copolymer obtained by copolymerizing 80% of
acrylic acid and 2~/o of acrylamide in aqueous solution,
using potassium peroxydisulfate as the catalyst, is neutral-
ized with ~agnesium hydroxide. A 2% strength aqueous
solution of the magnesium salt of the copolymer is pre-
-pared; its pH is 5.5. This solution is applied to a
~ood-free, alum-free, non-sized offset paper produced from
~ pulp having a freeness of 25SR~ and containing 10% of
ash (kaoIin). The impregnated paper is dried at 100C.
Further details of the treatment solution, and of the pro-
perties of the impregnated paper, are to be found in Table
3.
COMPARATI~E EXAMPLE 3a
e copolymer described in Example 3 is used in a
non-neutralized form, as a 2% strength aqueous solution of
pH 2.6, as an impregnating agent for the offset paper speci-
fied in Example 3~ Table 3 shows the properties of the
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o.z. ~2~792
paper obtained together with those properties of the paper
which were measured after treating the paper with water and
drying it (Comparative Example 3b).
TABLE 3
Example 3 Comparatlve Example
(Magnesium 3a 3b
salt) (Polyacid) (~ater)
Viscosity of the ~/0
strength aqueous treat-
ment solution [mPa.s] 11 9 5
Prodl1ct pick-up (%)
(solids, based on dry
paper) 1.82 1.82
Dry tear length (m)4,700 4,400 2~ 900
Dennison value 18 16 6
Wet tear length (m) o 250 0
.
EXAMPLE 4
The oalcium salt of a copolymer of 8~o Of ac~Jlic
acid, l~/o O~ acrylamide and 10% of methacrylamide is used7
as a 3% strength aqueous solution of pH g.0, as a surface
impregnant for the paper described in Example 3~ The
results are summarized in Table 4.
COMPARATIVE EX~PLE 4a
The non-neutralized copolymer of Example 4 is
applied, as a ~% strength aqueous treatment solution, to the
paper described in Example 3. The results are shown in
Table 4, in comparison with a paper (Comparative Example 4b)
which was impregnated with water only, and ~hen dried.
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TABLE 4
Example 4 Comparative Example
(Calcium 4a 4b
salt) (Polyacid) (Water)
Viscosity of the ~/0
strenth aqueous treat-
ment solution [~Pa.s] 26 48 5
Product pick-up (%)
(solids, based on dry
paper) 2.962.88
D~y tear length (m) 5,1004,800 2,900
Vennison value - 15 14 6
We~-tear length (m) 100 400
.
EXAMPLE 5
A 1~ strength aqueous solution of the calcium salt
of a copolymer o~ 650/~ of acrylic acid and 35yO of acrylo-
ni~rile, with 4% of an oxidatively degraded starch,
con~entionally used in the paper industry, dissolved in the
solution, is applied to the paper described in Example 3.
The results are summarized in Table 5.
COMPARATIVE EXAMPLE 5a
. A 1% strength aqueous solution o~ the ammonium salt
o~ ~ copolymer of 65% of acrylic acid and 3~% o~ ac~ylo-
nitrile, with 4% of an oxidatively degraded starch,
co~ventionally used in the paper industry, dissolved in the
solution, is used as a treatment solution for impregnating
the paper described in Example 3. The results are
summarized in Table 5. They are compared with an experi-
-~: ment in which the paper described in Example 3 is only
impregnated with water and then dried.
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O.z. 32,732
TA3LE 5
Example 5 Comparative Example
(Calcium 5a 5b
salt + (Ammonium (Water)
; - starch) salt +
starch)
_ .
Viscosity of the ~,~
strength aqueous treat-
ment solution ~Pa.s~ 10 18 5
Product pick-up (%)
(solids, based on dry
paper) - 4.584~77
Dry -tear length (m) 4,8004,600 2,900
Dennison value 16 14 6
Wet tear length (m) o 250 0
The Examples and Comparative Examples show that the
copolymer salts to be used according to the invention lead,
compared to the correspondir.g ammonium salts or polyacids, to
a ~urther increase in the dry strength of the paper, without
producing an undesirable increase in the wet strength of
the paper.
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