Production of paper, board and cardboard in the presence of copolymers containing N-vinylformamide units

Paper, board and cardboard are produced by draining a paper stock in the presence of a nonhydrolyzed copolymer which contains, as polymerized units, PA0 (a) from 99 to 1 mol % of N-vinylformamide and PA0 (b) from 1 to 99 mol % of one or more water-soluble basic monomers of the formula ##STR1## where R.sup.1 is H, CH.sub.3 or C.sub.2 H.sub.5, R.sup.2, R.sup.3 and R.sup.4 are each H, CH.sub.3, C.sub.2 H.sub.5 or (--CH.sub.2 --CH.sub.2 --O--).sub.n H, R.sup.5 and R.sup.6 are each C.sub.1 -C.sub.10 -alkyl, A is C.sub.1 -C.sub.6 -alkylene, n is from 1 to 6 and Y.sup..crclbar. is an anion, in an amount of from 0.01 to 3.5% by weight, based on dry paper stock.

The present invention relates to a process for the production of paper, 
board and cardboard by draining a paper stock in the presence of 
copolymers containing N-vinylformamide units. 
JP-A-118 406/86 discloses water-soluble polyvinylamines which are prepared 
by polymerizing N-vinylformamide or mixtures of N-vinylformamide with 
other water-soluble monomers, such as acrylamide, N,N-dialkylacrylamides 
or diallyldialkylammonium salts and subsequently hydrolyzing the polymers 
with bases, e.g. ethylamine, diethylamine, ethylenediamine or morpholine. 
The polyvinylamines are used as drainage aids and retention aids in 
papermaking and as flocculants for wastewaters. 
U.S. Pat. No. 4,421,602 discloses polymers which are obtainable by partial 
hydrolysis of polyl-N-vinylformamide with acids or bases. As a result of 
the hydrolysis, these polymers contain vinylamine and N-vinylformamide 
units. They are used, for example in papermaking, as drainage aids, 
flocculants and retention aids. 
EP-A-0 220 603 discloses, inter alia, that N-vinylformamide can be 
subjected to copolymerization together with basic acrylates, such as 
dimethylaminoethyl acrylate, or N-vinylimidazolines, in supercritical 
carbon dioxide. The resulting finely divided copolymers are used in the 
partially hydrolyzed form, in which they contain vinylamine units, for 
example as retention aids and flocculants in papermaking. 
EP-A-0 282 761 discloses a process for the production of paper, board and 
cardboard having high dry strength, in which the dry strength agent used 
is a mixture of cationic polymers, which may also contain, among typical 
monomers, polymerized units of vinylamine, and natural potato starch, the 
potato starch being converted into a water-soluble form by heating in an 
aqueous medium in the presence of a cationic polymer to temperatures above 
the gelatinization temperature of natural potato starch in the absence of 
oxidizing agents, polymerization initiators and alkali. 
It is an object of the present invention to provide papermaking assistants 
which ideally are more effective than the conventional ones and which are 
technically more readily available. 
We have found that this object is achieved, according to the invention, by 
a process for the production of paper, board and cardboard by draining a 
paper stock in the presence of a polymer containing N-vinylformamide 
units, if a nonhydrolyzed copolymer which contains, as polymerized units, 
(a) from 99 to 1 mol % of N-vinylformamide and 
(b) from 1 to 99 mol % of one or more water-soluble basic monomers of the 
formula 
##STR2## 
where R.sup.1 is H, CH.sub.3 or C.sub.2 H.sub.5, R.sup.2, R.sup.3 and 
R.sup.4 are each H, CH.sub.3, C.sub.2 H.sub.5 or (--CH.sub.2 --CH.sub.2 
--O--).sub.n H, R.sup.5 and R.sup.6 are each C.sub.1 -C.sub.10 -alkyl, A 
is C.sub.1 -C.sub.6 -alkylene, n is from 1 to 6 and Y.sup..crclbar. is an 
anion, is used in an amount of from 0.01 to 3.5% by weight, based on dry 
paper stock, as the polymer containing N-vinylformamide units. 
The advantage of the nonhydrolyzed copolymers containing N-vinylformamide 
units over the previously used hydrolyzed copolymers which contain 
vinylamine units after the hydrolysis is that the hydrolysis, which is 
difficult to carry out in many cases, is dispensed with and effective 
papermaking assistants are obtainable by direct copolymerization. 
A suitable monomer (a) of the copolymers is N-vinylformamide. This monomer 
is present in the copolymers in an amount of from 1 to 99, preferably from 
60 to 95, mol %. 
Suitable monomers of group (b) are the compounds of the formula I, of which 
the following compounds may be stated by way of example: 
N-trimethyl-N-(acrylamidoethyl)-ammonium chloride, 
N-trimethyl-N-(methacrylamidoethyl)-ammonium chloride, 
N-trimethyl-N-(acrylamidoethyl)-ammonium methosulfate, 
N-trimethyl-N-(methacrylamidoethyl)-ammonium methosulfate, 
N-ethyldimethyl-N-(methacrylamidomethyl)-ammonium ethosulfate, 
N-ethyldimethyl-N-(acrylamidomethyl)-ammonium ethosulfate, 
N-trimethyl-N-(acrylamidopropyl)-ammonium chloride, 
N-trimethyl-N-(methacrylamidopropyl)-ammonium chloride, 
N-trimethyl-N-(acrylamidopropyl)-ammonium methosulfate, 
N-trimethyl-N-(methacrylamidopropyl)-ammonium methosulfate, 
N-ethyldimethyl-N-(methacrylamidopropyl)-ammonium ethosulfate and 
N-ethyldimethyl-N-(acrylamidopropyl)-ammonium ethosulfate. 
N-Trimethyl-N-(methacrylamidopropyl)-ammonium chloride is preferred. 
Other suitable monomers of group (b) are the compounds of the formula II. 
Examples of compounds of this type are diallyldimethylammonium chloride, 
diallyldimethylammonium bromide, diallyldiethylammonium chloride and 
diallyldiethylammonium bromide. Diallyldimethylammonium chloride is 
preferably used. The anion Y.sup..crclbar. is an acid radical and is 
preferably chloride, bromide, iodide, sulfate, methosulfate or 
ethosulfate. 
Among the monomers of group (b), the compounds of the formula I or II may 
be present in the copolymers either alone or as a mixture with one 
another. It is also possible to use a plurality of compounds of the 
formula I or II in the copolymerization of the monomer (a). The monomers 
of group (b) are present in the copolymers in an amount of from 99 to 1, 
preferably from 40 to 5, mol %. 
The copolymerization of the monomers (a) and (b) is carried out in aqueous 
solution in the presence of polymerization initiators which decompose into 
free radicals under the polymerization conditions. Examples of suitable 
polymerization initiators are hydrogen peroxide, alkali metal and ammonium 
salts of peroxydisulfuric acid, peroxides, hydroperoxides, redox catalysts 
and in particular nonoxidizing initiators, such as azo compounds which 
decompose into free radicals. Water-soluble azo compounds, such as 
2,2'-azobis-(2-amidinopropane) dihydrochloride, 
2,2'-azobis-(N,N'-dimethyleneisobutyramidine) dihydrochloride or 
2,2'-azobis-[2-methyl-N-(2-hydroxyethyl)-propionamide], are preferably 
used. The polymerization initiators are employed in conventional amounts, 
for example in amounts of from 0.01 to 5% by weight, based on the monomers 
to be polymerized. Polymerization can be carried out in a wide temperature 
range, under atmospheric pressure, reduced or superatmospheric pressure, 
in appropriately designed apparatuses. The polymerization is preferably 
effected under atmospheric pressure and at not more than 100.degree. C., 
in particular from 30.degree. to 80.degree. C. The concentration of the 
monomers in the aqueous solution is preferably chosen to give polymer 
solutions whose solids content is from 10 to 90, preferably from 20 to 
70, % by weight. The pH of the reaction mixture is brought to 4-10, 
preferably 5-8. 
Depending on the polymerization conditions, copolymers having different 
molecular weights are obtained. To characterize a copolymer, the K value 
according to H. Fikentscher is stated instead of the molecular weight. The 
K values (measured in 5% strength aqueous sodium chloride solution at 
25.degree. C. and at a polymer concentration of 0.1% by weight) are from 5 
to 350. Copolymers having low molecular weights and correspondingly low K 
values are obtained by the conventional methods, i.e. the use of 
relatively large amounts of peroxide in the copolymerization or the use of 
polymerization regulators or combinations of the two measures stated. 
Polymers having a high K value and high molecular weights are obtained, 
for example, by polymerizing the monomers by reverse suspension 
polymerization or by polymerizing monomers (a) and (b) by the water-in-oil 
polymerization process. In the reverse suspension polymerization process 
and in water-in-oil polymerization, saturated hydrocarbons, for example 
hexane, heptane, cyclohexane or decalin, or aromatic hydrocarbons, such as 
benzene, toluene, xylene or cumene, are used as the oil phase. The ratio 
of oil phase to aqueous phase in reverse suspension polymerization is, for 
example, from 10:1 to 1:10, preferably from 7:1 to 1:1. 
In order to disperse the aqueous monomer solution in an inert hydrophobic 
liquid, a protective colloid is required, the purpose of which is to 
stabilize the suspension of the aqueous monomer solution in the inert 
hydrophobic liquid. The protective colloids furthermore affect the 
particle size of the polymer beads formed by polymerization. 
Examples of suitable protective colloids are the substances described in 
U.S. Pat. No. 2,982,749. The protective colloids which are disclosed in 
German Patent 2,634,486 and are obtainable, for example, by reacting oils 
and/or resins, each of which have allyl hydrogen atoms, with maleic 
anhydride are also suitable. Other suitable protective colloids are 
disclosed in, for example, German Patent 2,710,372 and are obtainable by 
thermal or free radical solution or mass polymerization from 60-99.9% by 
weight of dicyclopentadiene, 0-30% by weight of styrene and 0.1-10% by 
weight of maleic anhydride. 
Other suitable protective colloids are graft polymers which are obtainable 
by grafting polymers (a) of 
a) from 40 to 100% by weight of monovinylaromatic monomers, 
b) from 0 to 60% by weight of monoethylenically unsaturated carboxylic 
acids of 3 to 6 carbon atoms, maleic anhydride and/or itaconic anhydride 
and 
c) from 0 to 20% by weight of other monoethylenically unsaturated monomers, 
with the proviso that the sum of the percentages by weight (a) to (c) is 
always 100 and the polymers (A) have a number average molecular weight of 
from 500 to 20,000 and a hydrogenation iodine number (according to DIN 
53,241) of from 1.3 to 51, with monomer mixtures of 
1) from 70 to 100% by weight of acrylates and/or methacrylates of 
monohydric alcohols of 1 to 20 carbon atoms, 
2) from 0 to 15% by weight of monoethylenically unsaturated carboxylic 
acids of 3 to 6 carbon atoms, maleic anhydride and/or itaconic anhydride, 
3) from 0 to 10% by weight of acrylic monoesters and/or methacrylic 
monoesters of at least dihydric alcohols, 
4) from 0 to 15% by weight of monovinylaromatic monomers and 
5) from 0 to 7.5% by weight of acrylamide and/or methacrylamide, with the 
proviso that the sum of the percentages by weight a) to e) is always 100, 
at not more than 150.degree. C. in an inert hydrophobic diluent in the 
presence of polymerization initiators, the monomers being used in an 
amount of from 97.5 to 50% by weight, based on the mixture of polymer (A) 
and monomers. Protective colloids of this type are described in EP-A-0290 
753. 
When an aliphatic hydrocarbon is used as the inert hydrophobic liquid in 
the reverse suspension polymerization, a mixture of an inorganic 
suspending agent based on modified finely divided minerals and a nonionic 
surfactant has proven very advantageous as the protective colloid. 
The inorganic suspending agents, which have a low hydrophilic/lyophilic 
balance, are the agents usually employed in reverse suspension 
polymerization processes. The mineral component of these substances is, 
for example, bentonite, montmorillonite or kaolin. Finely divided minerals 
are modified by being treated with salts of long-chain amines, for example 
C.sub.8 -C.sub.24 -amines, or quaternary ammonium salts, the amine salts 
or the quaternary ammonium salts being intercalated between the individual 
layers of the finely divided minerals. The quaternized ammonium salts 
which may be used for modification preferably contain 1 or 2 C.sub.10 
-C.sub.22 -alkyl radicals. The other substituents of the ammonium salts 
are C.sub.1 -C.sub.4 -alkyl or hydrogen. The content of free ammonium 
salts of the amine-modified minerals is not more than 2% by weight. Finely 
divided minerals modified with ammonium salts are commercially available. 
The inorganic suspending agents for reverse suspension polymerization 
include silica which has been reacted with organosilicon compounds. A 
suitable organosilicon compound is, for example, trimethylsilyl chloride. 
The purpose of the modification of the inorganic finely divided minerals is 
to improve the wettability of the minerals with the aliphatic hydrocarbon 
used as the outer phase of the reverse suspension polymerization. In the 
case of the natural minerals having a layer-like structure, for example 
bentonite and montmorillonite, the result of modification with amines is 
that the modified minerals swell in the aliphatic hydrocarbon and thus 
disintegrate into very fine particles. The particle size is about 1 .mu.m, 
in general from 0.5 to 5 .mu.m. The silicas reacted with organosilicon 
compounds have a particle size of about 10-40 nm. The modified finely 
divided minerals are wetted both by the aqueous monomer solution and the 
solvent and thus accumulate in the phase interface between the aqueous 
phase and the organic phase. They prevent coagulation on collision of two 
aqueous monomer droplets in the suspension. 
After the end of the copolymerization, some of the water is distilled 
azeotropically so that copolymers having a solids content of from 70 to 
99, preferably from 80 to 95, % by weight are obtained. The copolymers are 
in the form of fine beads having a diameter of from 0.05 to 1 mm. 
In contrast to the prior art, the copolymers described above are used in 
nonhydrolyzed form as an additive to the paper stock in the production of 
paper, board and cardboard. These copolymers contain no vinylamine units. 
They increase the rate of drainage of the paper stock, so that the 
production speed in papermaking can be increased. The copolymers also act 
as retention aids for fibers and fillers and simultaneously as 
flocculants. To achieve the stated effects, the copolymers are added to 
the paper stock in amounts of from 0.01 to about 0.8% by weight, based on 
dry paper stock. Using larger amounts of copolymers imparts dry strength. 
In order to achieve such effects, the polymers are used in amounts of 
about 0.5-3.5% by weight, based on dry paper stock. The use of the stated 
copolymers together with natural potato starch as dry strength agents is 
particularly preferred. Such mixtures have good retention for paper fibers 
in the paper stock. The COD of the white water is considerably reduced by 
means of these mixtures compared with natural starch. The troublesome 
substances present in the water circulations of paper machines have only a 
slight adverse effect on the efficiency of the mixtures of the copolymers 
to be used according to the invention and natural starch. The pH of the 
paper stock suspension may be from 4 to 9, preferably from 6 to 8.5. These 
mixtures of natural starch and cationic polymer which are added to the 
paper stock for imparting dry strength are preferably prepared by heating 
natural potato starch in the presence of the nonhydrolyzed copolymers in 
aqueous solution to temperatures above the gelatinization temperature of 
the natural potato starch, in the absence of oxidizing agents, 
polymerization initiators and alkali. The natural potato starch is 
modified in this manner. 
The gelatinization temperature of the starch is the temperature at which 
the birefringence of the starch particles is lost (cf. Ullmanns 
Enzyklopadie der technischen Chemie, Urban und Schwarzenberg, 
Munich-Berlin, 1965, 16th volume, page 322). 
Modification of the natural potato starch can be carried out in various 
ways. A digested natural potato starch which is in the form of an aqueous 
solution can be reacted with the suitable cationic polymers at from 
15.degree. to 70.degree. C. At even lower temperatures, longer contact 
times are required. If the reaction is carried out at even higher 
temperatures, for example up to 110.degree. C., shorter contact times, 
e.g. from 0.1 to 15 minutes, are required. The simplest method of 
modifying natural potato starch is to heat an aqueous suspension of the 
starch in the presence of the suitable cationic copolymers to above the 
gelatinization temperature of the natural potato starch. For modification, 
the starch is generally heated to 70.degree.-110.degree. C., the reaction 
being carried out in pressure-resistant apparatuses at above 110.degree. 
C. However, it is also possible first to heat an aqueous suspension of 
natural potato starch to 70.degree.-110.degree. C. and to bring the starch 
into solution and then to add the cationic copolymer required for 
modification. Solubilizing of the starch is carried out in the absence of 
oxidizing agents, initiators and alkali, in the course of about 3 minutes 
to 5 hours, preferably from 5 to 30 minutes. Higher temperatures require a 
shorter residence time here. 
From 1 to 20, preferably from 8 to 12, parts by weight of a single suitable 
nonhydrolyzed cationic copolymer or of a mixture of such copolymers are 
used per 100 parts by weight of natural potato starch. As a result of the 
reaction with the cationic copolymers, the natural potato starch is 
converted into a water-soluble form. The viscosity of the aqueous phase of 
the reaction mixture increases. A 3.5% strength by weight aqueous solution 
of the dry strength agent has viscosities of from 50 to 10,000 mPa.s 
(measured according to Brookfield at 20 rpm and 20.degree. C.). 
The copolymers to be used according to the invention can be employed in the 
production of all known paper, cardboard and board grades, for example for 
the production of writing, printing and packaging papers. The papers may 
be produced from a large number of different fiber materials, for example 
from bleached or unbleached sulfite or sulfate pulp, mechanical pulp, 
waste paper, thermomechanical pulp (TMP) and chemothermomechanical pulp 
(CTMP). The basis weight of the papers may be from 30 to 200, preferably 
from 35 to 150, g/m.sup.2, while that of cardboard may be up to 600 
g/m.sup.2. The papers produced using the copolymers, to be used according 
to the invention, as a mixture with natural potato starch have markedly 
improved strength compared with papers obtainable in the presence of the 
same amount of natural potato starch. 
In the Examples which follow, parts and percentages are by weight. The 
viscosities were determined in aqueous solution at a solids concentration 
of 3.5% by weight and at 20.degree. C. in a Brookfield viscometer at 20 
rpm. 
Sheet formation was carried out on a Rapid-Kothen laboratory sheet former. 
The dry breaking length was determined according to DIN 53,112, Sheet 1, 
the Mullen dry bursting pressure according to DIN 53,141, the CMT value 
according to DIN 53,143 and the Brecht-Inset tear propagation strength 
according to DIN 53,115. Testing of the sheets was carried out after 
conditioning for 24 hours at 23.degree. C. and a relative humidity of 50%. 
The K value of the copolymers was determined according to H. Fikentscher, 
Cellulosechemie 13 (1932), 58-64 and 71-74, at 25.degree. C. in 5% 
strength aqueous sodium chloride solution and at a polymer concentration 
of 0.1% by weight; K=k.multidot.10.sup.3. 
The following starting materials were used: 
Copolymer 1 
Copolymer of 90 mol % of N-vinylformamide (VFA) and 10 mol % of 
3-methacrylamidopropyltrimethylammonium chloride (MAPTAC) 
Copolymer 1 was prepared by initially taking 800 g of cyclohexane and 3 g 
of protective colloid described in Example 1 of EP-A-0 290 753 in a 2 1 
flask provided with a stirrer, a thermometer, a gas inlet tube and a 
reflux condenser. The initially taken mixture was heated to 50.degree. C. 
under a nitrogen atmosphere and while stirring at a stirrer speed of 300 
revolutions per minute. As soon as this temperature had been reached, a 
solution of 117 g of N-vinylformamide, 80 g of a 50% strength by weight 
aqueous solution of 3-methacrylamidopropyltrimethylammonium chloride, 0.15 
g of sodium diethylenetriaminepentaacetate, 0.65 g of 
2,2'-azobis-(2-amidinopropane) dihydrochloride and 100 g of water was 
added in the course of 30 minutes. The pH of the aqueous phase was 6.5. 
The reaction mixture was then stirred for 16 hours at 50.degree. C. 
Thereafter, the temperature was increased to 78.degree. C. and 134 g of 
water were distilled off azeotropically with the aid of a water separator. 
The resulting white bead-like solid was filtered off, washed with 200 g of 
cyclohexane and freed from the residual solvent under reduced pressure. 
163 g of a copolymer having a solids content of 96.4% by weight were 
obtained. The K value was 180. 
Copolymers 2 to 5, whose compositions are shown in Table 1, were prepared 
similarly to the abovementioned preparation method. 
TABLE 1 
______________________________________ 
Mol % Mol % Solids 
Copolymer 
VFA.sup.1) 
MAPTAC.sup.2) 
content (%) 
K value 
______________________________________ 
2 80 20 96.1 180 
3 70 30 91.0 203 
4 60 40 94.1 189 
5 50 50 88.0 200 
______________________________________ 
.sup.1) VFA = Nvinylformamide 
.sup.2) MAPTAC = 3methacrylamidopropyltrimethylammonium chloride 
The following polymers were used for comparison: 
Copolymer 6: Homopolymer of N-vinylformamide having a solids content of 
96.6% and a K value of 203, prepared similarly to the method for copolymer 
1 by homopolymerization of N-vinylformamide. 
Copolymer 7: Partially hydrolyzed polymer 6, which was obtained by 
homopolymerization of N-vinylformamide by the preparation method stated 
for copolymer 1, 105 g of a 38% strength hydrochloric acid being added 
before removal of the water and the mixture being stirred for 3 hours at 
50.degree. C. before the water was distilled off azeotropically. The 
degree of hydrolysis was 42%, the K value was 185 and the solids content 
was 93.5%. 
Copolymer 8: This is likewise a hydrolyzed homopolymer of N-vinylformamide 
which was prepared similarly to copolymer 7, except that 211 g of 38% 
strength hydrochloric acid were used in the hydrolysis. The degree of 
hydrolysis was about 90%, the K value was 195 and the solids content was 
90.6%. A degree of hydrolysis of 90% means that 90% of the formamide 
groups originally present in the polymer have been converted into amino 
groups or the corresponding ammonium salt groups.

EXAMPLES 
Wood-containing and kaolin-containing newspaper stock having a consistency 
of 2 g/l, a pH of 6 and an alum content of 0.5% by weight was first 
prepared. This paper stock was used as a model substance for all Examples 
and Comparative Examples. With the aid of a Schopper-Riegler apparatus, 
the freeness (.degree.SR), the drainage time (i.e. the time in which 600 
ml of white water flow out of the apparatus) and the optical transmittance 
of the white water in % were first determined for the paper stock model 
described above. 1 l samples of the paper stock described above together 
with the amounts of copolymers 1 to 8 stated in Table 2 were then tested. 
The results obtained are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
% by weight of added polymer 
Transmittance 
Comparative Copoly- 
Freeness (.degree.SR) 
Drainage time (s) 
(%) 
Example 
Example 
mer 0.01 
0.02 
0.04 
0.01 
0.02 
0.04 
0.01 
0.02 
0.04 
__________________________________________________________________________ 
1 58 93.6 26 
1 1 49 46 42 66.0 
57.9 
51.7 
45 55 63 
2 2 47 43 38 60.6 
51.2 
43.1 
48 62 68 
3 3 44 39 34 54.5 
45.4 
35.2 
54 66 79 
4 4 44 38 35 55.0 
43.1 
37.0 
55 67 75 
5 5 44 41 36 54.6 
47.7 
38.3 
53 63 73 
2 6 56 56 56 89.9 
88.9 
88.3 
28 33 36 
3 7 52 47 36 75.0 
59.3 
38.8 
34 59 66 
4 8 54 54 45 82.0 
81.2 
58.6 
34 35 48 
__________________________________________________________________________ 
To test the paper strength, the strength agents 1 to 5 which are stated 
below and were prepared by heating natural potato starch with the 
copolymers stated in Table 3 were tested. 
TABLE 3 
______________________________________ 
Viscosity of the 
aqueous solution of 
Strength the strength agent 
agent Obtained by reaction with 
[mPa .multidot. s] 
______________________________________ 
1 Copolymer 1 314 
2 Copolymer 3 850 
3 Copolymer 5 858 
4 Copolymer 6 (comparison) 
180 
5 Copolymer 7 (comparison) 
668 
______________________________________ 
Strength agents 1 to 5 described above were each tested in the 
abovementioned paper stock. The amount added was 3.0% by weight, based on 
dry paper stock, in all cases. The test results are shown in Table 4. 
TABLE 4 
______________________________________ 
Strength 
agent No. Dry Dry COD of 
added to CMT bursting breaking 
white 
paper value pressure length water 
stock [N] [kPa] [m] [mg O.sub.2 /l] 
______________________________________ 
Example 
6 1 169 169 3266 128 
7 2 185 173 3457 167 
8 3 184 184 3322 112 
Com- 
para- 
tive 
Example 
5 -- 126 136 2667 162 
6 Natural 145 148 2836 276 
potato 
starch 
7 4 148 149 2971 327 
8 5 200 194 3349 146 
______________________________________ 
Further strength agents were prepared by heating natural potato starch in 
aqueous suspension for 15 minutes at 90.degree.-110.degree. C. in the 
presence of the copolymers stated in Table 5. 
TABLE 5 
______________________________________ 
Viscosity of the 
Obtained by reaction with 
aqueous so- 
copolymer of lution of the 
Strength 
. . . mol % 
. . . mol % of 
of K strength agent 
agent of VFA and DADMAC.sup.1) 
value [mPa .multidot. s] 
______________________________________ 
6 30 70 93 169 
7 50 50 91 180 
8 70 30 94 140 
______________________________________ 
.sup.1) DADMAC = Diallyldimethylammonium chloride 
To test strength agents 6 to 8 with regard to their efficiency, they were 
added to the paper stock described in Example 1 in an amount of 3.0% by 
weight, based on dry paper stock. The results obtained are shown in Table 
6. 
TABLE 6 
______________________________________ 
Strength 
agent No. Dry Dry COD of 
added to CMT bursting 
breaking 
white 
paper value pressure 
length water 
Example 
stock [N] [kPa] [m] [mg O.sub.2 /l] 
______________________________________ 
9 6 182 191 3336 206 
10 7 173 186 3177 251 
11 8 171 178 3331 260 
______________________________________ 
In order to test copolymers 1, 3 and 5 and copolymer 6 (comparison) with 
regard to their efficiency as dry strength agents even in the absence of 
added starch, they were added to the paper stock described in Example 1 in 
an amount of 0.5% by weight, based on dry paper stock. The results 
obtained are shown in Table 7. 
TABLE 7 
______________________________________ 
Co- 
polymer Dry Dry 
No. burst- break- 
COD of 
added to CMT ing ing white 
Comp. paper value pressure 
length 
water 
Ex. Ex. stock [N] [kPa] [m] [mg O.sub.2 /l] 
______________________________________ 
12 1 143 151 2932 162 
13 3 134 145 2794 120 
14 4 132 143 2857 61 
9 6 117 140 2616 153 
______________________________________