Production of paper and paperboard of high dry strength

Paper and paperboard of high dry strength are produced by adding a dry strength enhancer to the paper stock and dewatering the paper stock with sheet formation by using as the dry strength enhancer a mixture of a cationic polymer which contains as characteristic monomers copolymerized units of PA1 (a) diallyldimethylammonium chloride, PA1 (b) N-vinylamine or PA1 (c) a substituted or unsubstituted N-vinylimidazoline and has a K value of not less than 30, and natural potato starch which is converted into a water-soluble form by heating in an aqueous medium at above the gelatinization temperature of natural potato starch in the absence of any oxidizing agents or alkali.

To increase the dry strength of paper, it is known to use aqueous slurries 
of natural starches made water-soluble by heating as pulp additives in 
papermaking. However, the retention of the water-dissolved starches on the 
fibers in the paper stock is low. An improvement in the retention of 
natural products on cellulose fibers in papermaking is known for example 
from U.S. Pat. No. 3,734,820. Said patent described graft copolymers 
prepared by grafting dextran, a naturally occurring polymer having a 
molecular weight from 20,000 to 50 million, with cationic monomers, for 
example diallyldimethylammonium chloride, mixtures of 
diallyldimethylammonium chloride and acrylamide, or mixtures of acrylamide 
and basic methacrylates, such as dimethylaminoethyl methacrylate. The 
graft polymerization is preferably carried out in the presence of a redox 
catalyst. 
U.S. Pat. No. 4,097,427 discloses a process for cationizing starch by 
boiling the starch in an alkaline medium in the presence of water-soluble 
quaternary ammonium polymers and an oxidizing agent. Possible quaternary 
ammonium polymers also include, inter alia, quaternized 
diallyldialkylamine polymers or quaternized polyethylene imines. The 
oxidizing agent used is for example ammonium persulfate, hydrogen 
peroxide, sodium hypochlorite, ozone or tert-butyl hydroperoxide. The 
modified cationic starches preparable in this manner are added as 
dry-strength enhancers to the paper stock in papermaking. However, they 
create a very high COD value in the waste water. 
It is an object of the present invention to obtain an improvement in the 
dry strength of paper, compared with existing processes, by using starch. 
More particularly, the substantivity of the starch during exhaustion onto 
the fibers in the paper stock shall be increased and as a result the COD 
level in the waste water reduced. 
We have found that this object is achieved according to the invention with 
a process for producing paper and paperboard of high dry strength by 
adding a dry-strength enhancer to the paper stock and dewatering the paper 
stock with sheet formation, by using as the dry-strength enhancer a 
mixture of a cationic polymer which contains as characteristic monomers 
copolymerized units of 
(a) diallyldimethylammonium chloride, 
(b) N-vinylamine or 
(c) an N-vinylimidazoline of the formula 
##STR1## 
where R.sup.1 is H, C.sub.1 -C.sub.18 -alkyl or 
##STR2## 
R.sup.5 and R.sup.6 are each H, C.sub.1 -C.sub.4 -alkyl or Cl, R.sup.2 is 
H, C.sub.1 -C.sub.18 -alkyl, 
##STR3## 
R.sup.3 and R.sup.4 are each H or C.sub.1 -C.sub.4 -alkyl and X.sup.- is 
an acid radical, 
and which has a K value of not less than 30, and natural potato starch 
converted into a water-soluble form by heating in an aqueous medium at 
above the gelatinization temperature of natural potato starch in the 
absence of any oxidizing agent, polymerization initiator or alkali. 
The mixtures to be used according to the invention as dry-strength 
enhancers are well retained by the fibers in the paper stock. The COD 
value in the backwater is substantially reduced using the mixtures 
according to the invention compared with natural starch. The interfering 
substances present in the circulating water systems of paper machines 
impair the effectiveness of the dry-strength enhancers to be used 
according to the invention only to a small extent. The pH of the paper 
stock suspension can be within the range from 4 to 9, preferably from 6 to 
8.5. 
As was found in a series of experiments, the stated object is only achieved 
when the starch used is natural potato starch. Unlike the abovementioned 
existing starch modification processes, the preparation of the modified 
starch to be used according to the invention is carried out in the absence 
of oxidizing agents and polymerization initiators and even in the absence 
of alkali. 
The modification of natural potato starch is preferably achieved by heating 
said starch in aqueous suspension together with one or more of the 
cationic polymers which come into consideration at above the 
gelatinization temperature of the starch, the gelatinization temperature 
of a starch being that temperature at which the birefringence of the 
starch grains disappears (cf. Ullmann's Enzyklopadie der technischen 
Chemie, Urban und Schwarzenberg, Munich/Berlin, 1965, volume 16, page 322. 
However, in general the modification of natural potato starch can be 
carried out in various ways. A previously digested natural potato starch 
which is present in the form of an aqueous solution can be made to react 
with one or more of the cationic polymers which come into consideration at 
from 15.degree. to 70.degree. C. Still lower temperatures require longer 
contact times. If the reaction is carried out at still higher 
temperatures, for example up to 110.degree. C., shorter contact times, for 
example from 0.1 to 15 minutes, are required. The simplest method of 
modifying natural potato starch comprises heating an aqueous slurry of the 
starch in the presence of one or more of the cationic polymers which come 
into consideration at above the gelatinization temperature of natural 
potato starch. In general, the temperatures involved in modifying the 
starch range from 70.degree. to 110.degree. C., in the case of 
temperatures above 100.degree. C. the reaction being carried out in 
pressure-tight apparatus. However, it is also possible first to heat an 
aqueous slurry of natural potato starch at from 70.degree. to 110.degree. 
C. to solubilize the starch and then to add the cationic polymer required 
for effecting modification. This solubilizing of the starch always takes 
place in the absence of any oxidizing agents, initiators and alkali in the 
course of from about 3 minutes to 5 hours, preferably from 5 minutes to 30 
minutes. High temperatures here require a shorter residence time. The 
amounts used per 100 parts by weight of natural potato starch range from 1 
to 20, preferably 8 to 12, parts by weight of a single polymer or of a 
mixture of the cationic polymers which come into consideration. The 
heating and/or reacting with the cationic polymers has the effect of 
converting the natural potato starch into a water-soluble form. This 
water-solubilization is accompanied by an increase in the viscosity of the 
aqueous phase of the reaction mixture. A 3.5% strength by weight aqueous 
solution of the mixture to be used as dry-strength enhancer has a 
viscosity within the range from 50 to 10,000 mPas (measured by Brookfield 
at 20 rpm and 20.degree. C.). 
To prepare the dry-strength enhancers to be used according to the invention 
there come into consideration (a) polymers of diallyldimethylammonium 
chloride. Polymers of this type are known. For the purposes of the present 
invention, polymers of diallyldimethylammonium chloride are first and 
foremost the homopolymers and the copolymers with acrylamide and/or 
methacrylamide. The copolymerization may be carried out using any desired 
monomer ratio. The K value of the homopolymers and copolymers of 
diallyldimethylammonium chloride is not less than 30, preferably from 95 
to 180. 
Cationic polymers of group (b), which contain as characteristic monomers 
copolymerized units of N-vinylamine, are obtainable by hydrolyzing 
homopolymers of N-vinylformamide to detach from 70 to 100 mol % of the 
formyl groups in the homopolymers of N-vinylformamide to give polymers 
containing N-vinylamine as copolymerized units. As soon as 100 mol % of 
the formyl groups have been eliminated from the homopolymers of 
N-vinylformamide, the resulting polymers may also be referred to as 
poly-N-vinylamines. This group of polymers also includes hydrolyzed 
copolymers which contain 
(a) from 95 to 10 mol % of N-vinylformamide and 
(b) from 5 to 90 mol % of vinyl acetate or vinyl propionate 
as copolymerized units, from 70 to 100 mol % of the formyl groups in the 
copolymer being eliminated to form N-vinylamine units in the copolymer, 
and from 70 to 100 mol % of the acetyl and propionyl groups being 
eliminated to form vinyl alcohol units. The K value of the hydrolyzed 
homopolymers and copolymers of N-vinylformamide is preferably from 70 to 
170. The polymers belonging to this group are known for example from U.S. 
Pat. No. 4,421,602, U.S. Pat. No. 4,444,667 (incorporated by reference) 
and German Laid-Open Application DOS No. 3,534,273. 
Suitable cationic polymers of group (c) comprise homopolymers and 
copolymers of substituted or unsubstituted N-vinylimidazolines. The 
substances in question here are again known substances. They can be 
prepared as described in German Published Application DAS No. 1,182,826 by 
polymerizing compounds of the formula 
##STR4## 
where R.sup.1 is H, C.sub.1 -C.sub.18 -alkyl or 
##STR5## 
R.sup.5 and R.sup.6 are each H, C.sub.1 -C.sub.4 -alkyl or Cl, R.sup.2 is 
H, C.sub.1 -C.sub.18 -alkyl, 
##STR6## 
R.sup.3 and R.sup.4 are each H or C.sub.1 -C.sub.4 -alkyl, and X.sup.- is 
an acid radical, 
with or without acrylamide and/or methacrylamide, in an aqueous medium at a 
pH from 0 to 8, preferably from 1.0 to 6.8, in the presence of 
polymerization initiators which decompose to free radicals. 
Preference is given to using in the polymerization 1-vinyl-2-imidazoline 
salts of the formula II 
##STR7## 
where R.sup.1 is H, CH.sub.3, C.sub.2 H.sub.5, n- or i-C.sub.3 H.sub.7 or 
C.sub.6 H.sub.5 and X.sup.- is an acid radical. X.sup.- is preferably 
Cl.sup.-, Br.sup.-, SO.sub.4.sup.2-, CH.sub.3 O--SO.sub.3 H.sup.-, C.sub.2 
H.sub.5 --O--SO.sub.3 H.sup.- or R--COO.sup.- and R.sup.2 is H, C.sub.1 
-C.sub.4 -alkyl or aryl. 
The substituent X.sup.- in the formulae I and II may in principle be any 
desired acid radical of an inorganic or organic acid. The monomers of the 
formula I are obtained by neutralizing the free base, ie. 
1-vinyl-2-imidazolines, with an equivalent amount of an acid. The 
vinylimidazolines can also be neutralized for example with trichloroacetic 
acid, benzenesulfonic acid or toluenesulfonic acid. Aside from salts of 
1-vinyl-2-imidazolines it is also possible to use quaternized 
1-vinyl-2-imidazolines. They are prepared by reacting 
1-vinyl-2-imidazolines, which may be substituted in the 2-, 4- and 
5-positions, with known quaternizing agents. Suitable quaternizing agents 
are for example C.sub.1 -C.sub.18 -alkyl chlorides or bromides, benzyl 
chloride, benzyl bromide, epichlorohydrin, dimethyl sulfate and diethyl 
sulfate. The quaternizing agent used is preferably epichlorohydrin, benzyl 
chloride, dimethyl sulfate or methyl chloride. 
To prepare the water-soluble homopolymers, the compounds of the formula I 
or II are preferably polymerized is an aqueous medium. The copolymers are 
obtained by polymerizing the monomers of the compounds of the formulae I 
and II with acrylamide and/or methacrylamide. The monomer mixture used in 
the polymerization contains, if copolymers are to be prepared, not less 
than 1% by weight of a monomer of the formula I or II, preferably from 10 
to 40% by weight. Particularly suitable for modifying natural potato 
starch are copolymers of from 60 to 85% by weight of acrylamide and/or 
methacrylamide and 15 to 40% by weight of N-vinylimidazoline or 
N-vinyl-2-methylimidazoline. 
The copolymers can additionally be modified by inclusion, as copolymerized 
units, of other monomers, such as styrene, vinyl acetate, vinyl 
propionate, N-vinylformamide, C.sub.1 -C.sub.4 -alkyl vinyl ethers, 
N-vinylpyridine, N-vinylpyrrolidone, N-vinylimidazole, acrylic esters, 
methacrylic esters, ethylenically unsaturated C.sub.3 -C.sub.5 -carboxylic 
acids, sodium vinyl sulfonate, acrylonitrile, methacrylonitrile, vinyl 
chloride or vinylidene chloride, in amounts of up to 25% by weight. Aside 
from the polymerization in aqueous solution it is possible for example to 
prepare the homopolymers and copolymers in a water-in-oil emulsion. The 
monomers can also be polymerized by the method of reverse suspension 
polymerization, which produces polymers in bead form. The polymerization 
is initiated with the aid of customary polymerization initiators or by the 
action of high-energy radiation. Suitable polymerization initiators are 
for example hydrogen peroxide, inorganic and organic peroxides, and also 
hydroperoxides and azo compounds. It is possible to use not only mixtures 
of polymerization initiators but also redox polymerization initiators, for 
example mixtures of sodium sulfide, ammonium persulfate and sodium bromate 
or mixtures of potassium peroxodisulfate and iron (II) salts. The 
polymerization is carried out at from 0.degree. to 100.degree. C., 
preferably at from 15.degree. to 80.degree. C. It is of course also 
possible to polymerize at above 100.degree. C., although then it is 
necessary to carry out the polymerization under superatmospheric pressure. 
The temperature can be for example as high as 150.degree. C. The reaction 
time depends on the temperature. The higher the temperature, the shorter 
the time required for the polymerization. 
Since the compounds of the formula I are relatively costly, it is 
preferable, for economic reasons, to use as cationic polymers of group (c) 
copolymers of compounds of the formula I with acrylamide or 
methacrylamide. These copolymers then contain the compounds of the formula 
I only in active amounts, ie. in an amount from 1 to 40% by weight. In the 
preparation of the dry-strength enhancers to be used according to the 
invention preference is given to using copolymers of acrylamide with 
compounds of the formula I where R.sup.1 is methyl, R.sup.2, R.sup.3 and 
R.sup.4 are each H, and X is an acid radical, preferably chloride or 
sulfate. 
To modify natural potato starch it is also possible to employ copolymers 
which contain 
(a) from 70 to 96.5% by weight of acrylamide and/or methacrylamide, 
(b) from 2 to 20% by weight of N-vinylimidazoline or 
N-vinyl-2-methylimidazoline and 
(c) from 1.5 to 10% by weight of N-vinylimidazole as copolymerized units 
with the proviso that the sum of (a) to (c) in percent by weight always 
adds up to 100, and which have a K value from 80 to 150. These copolymers 
are prepared by free radical copolymerization of monomers (a), (b) and (c) 
by the polymerization process described above. To prepare the mixtures to 
be used according to the invention as dry-strength enhancers, the starting 
point is an aqueous slurry of natural potato starch which, per 100 parts 
by weight of water, contains from 0.1 to 10 parts by weight of natural 
potato starch. As stated above, the advantages of the invention are not 
realized with any other type of starch. The reaction mixtures of the 
polymers described above and natural potato starch which are to be used 
according to the invention are added to the paper stock in an amount from 
0.5 to 3.5, preferably from 1.2 to 2.5, % by weight, based on dry paper 
stock. The pH of the mixture ranges from 2.0 to 9.0, preferably from 2.5 
to 8.0. The solution of the dry-strength enhancer in water in a solids 
concentration of 3.5% by weight has a viscosity from 50 to 10,000, 
preferably from 80 to 4,000, mPas, measured in a Brookfield viscometer as 
20 rpm and 20.degree. C. 
The dry-strength enhancers to be used according to the invention can be 
used in the production of all known types of paper and paperboard, for 
example writing, printing and packaging papers. The various types and 
grades of paper can be produced from a wide variety of fiber materials, 
for example from sulfite or sulfate pulp in the bleached or unbleached 
state, groundwood, wastepaper, thermomechanical pulp (TMP) or 
chemothermomechanical pulp (CTMP). The pH of the stock suspension is 
within the range from 4.0 to 10, preferably from 6.0 to 8.5. The 
dry-strength enhancers can be used not only in the production of base 
paper for paper varieties of low basis weight (LWC papers) but also for 
paperboard. The basis weight for paper ranges from 30 to 200, preferably 
from 35 to 150, g/m.sup.2, while for paperboard it can be up to 600 
g/m.sup.2. The paper products produced according to the invention have a 
substantially improved strength compared with paper produced in the 
presence of the same amount of natural potato starch, as can be 
quantitatively expressed for example in terms of their breaking length, 
the burst pressure, the CMT value and the tear propagation resistance. 
In the Examples, the parts and percentages are by weight. The viscosities 
of the strength enhancers 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. 
The sheets were produced in a Rapid-Kothen laboratory sheet former. The dry 
breaking length was determined in accordance with German Standard 
Specification DIN No. 53,112 Sheet 1, the dry burst pressure by the Mullen 
method (German Standard Specification DIN No. 53,141), the CMT value in 
accordance with German Standard Specification DIN No. 53,143 and the tear 
propagation resistance by the Brecht-Inset method in accordance with 
German Standard Specification DIN No. 53,115. 
The sheets were each tested after 24 hours conditioning at a temperature of 
23.degree. C. and a relative humidity of 50%. 
The K value of the polymers was determined by the method of H. Fikentscher, 
Cellulosechemie, 13 (1932), 58-64 and 71-74, at 25.degree. C. in 5% 
strength aqueous sodium chloride solution at a polymer concentration of 
0.5% by weight, K being=k.times.10.sup.3. 
The following substances were used: 
Polymer 1 
Homopolymer of diallyldimethylammonium chloride having a K value of 95. 
Polymer 2 
Homopolymer of diallyldimethylammonium chloride having a K value of 110. 
Polymer 3 
Homopolymer of diallyldimethylammonium chloride having a K value of 125. 
Polymer 4 
Copolymer of 90% by weight of acrylamide, 8% by weight of 
N-vinyl-2-methylimidazoline and 2% by weight of N-vinylimidazole, having a 
K value of 119. 
Polymer 5 
Copolymer of 25 mol % of N-vinyl-2-methylimidazoline and 75 mol % of 
acrylamide, having a K value of 117. 
Polymer 6 
Homopolymer of N-vinylformamide from which 99% of the formyl groups have 
been eliminated and which has a K value of 83. 
Polymer 7 
Homopolymer of N-vinylformamide from which 83% of the formyl group have 
been eliminated and which has a K value of 168. 
Polymer 8 
Copolymer of 40% by weight of N-vinylformamide and 60% by weight of vinyl 
acetate, from which 100% of the formyl groups and 98% of the acetyl groups 
have been eliminated and which has a K value of 75. 
Polymer 9 (comparison) 
Copolymer of 30% by weight of dimethylaminoethyl acrylate methochloride and 
70% by weight of acrylamide, which has a K value of 205. 
Strength enhancer 1 
To a 3% strength slurry of natural potato starch (gelatinization 
temperature 90.degree. C.) in water is added to a sufficient amount of 
polymer 1 for the resulting mixture to contain 10% of polymer 1, based on 
the starting amount of natural potato starch. The mixture is then heated 
with stirring at from 90.degree. to 95.degree. C. for 15 minutes and, 
after cooling down to within the range from 10.degree. to 40.degree. C., 
used in accordance with the invention as a dry-strength enhancer for paper 
by being added to a stock suspension prior to sheet formation (viscosity: 
656 mPa.s). 
Strength enhancer 2 
The procedure for preparing a dry-strength enhancer for paper described 
above for strength enhancer 1 is repeated, except that here a 3% strength 
aqueous slurry of natural potato starch is reacted not with polymer 1 used 
there but with polymer 2 (viscosity: 870 mPa.s). 
Strength enhancer 3 
The procedure for preparing a dry-strength enhancer for paper described 
above for strength enhancer 1 is repeated, except that here the polymer 1 
described there is replaced by polymer 3 (viscosity: 950 mPa.s). 
Strength enhancer 4 
The procedure for preparing a dry-strength enhancer described above for 
strength enhancer 1 is repeated, except that the polymer used there is 
replaced by polymer 4 (viscosity: 398 mPa.s). 
Strength enhancer 5 
A 3% strength aqueous slurry of natural potato starch (gelatinization 
temperature 90.degree. C.) is heated with stirring at from 90.degree. to 
95.degree. C. for 15 minutes, during which the starch becomes solubilized. 
After the starch solution has been cooled down to 70.degree. C., a 5% 
strength aqueous solution of polymer 2 is added in such an amount that the 
amount of polymer based on the starting amount of natural potato starch, 
is 10%. The mixture is then stirred at 70.degree. C. for a further 10 
minutes and thereafter cooled down to room temperature. A dry-strength 
enhancer for paper is obtained (viscosity: 784 mPa.s). 
Strength enhancer 6 
The procedure for preparing a dry-strength enhancer described in the 
preparation of strength enhancer 1 is repeated, except that here the 
polymer used there is replaced by polymer 5 (viscosity: 250 mPa.s). 
Strength enhancer 7 
The procedure for preparing a dry-strength enhancer described in the 
preparation of strength enhancer 1 is repeated, except that here the 
polymer used there is replaced by polymer 6 (viscosity: 150 mPa.s). 
Strength enhancer 8 
The procedure for preparing a dry-strength enhancer described in the 
preparation of strength enhancer 1 is repeated, except that here the 
polymer used there is replaced by polymer 7 (viscosity: 206 mPa.s). 
Strength enhancer 9 
The procedure for preparing a dry-strength enhancer described in the 
preparation of strength enhancer 1 is repeated, except that here the 
polymer used there is replaced by polymer 8 (viscosity: 86 mPa.s). 
Strength enhancer 10 
For comparison, a dry-strength enhancer for paper is prepared by the 
procedure described for strength enhancer 1, except that the polymer used 
there is replaced by polymer 9 (viscosity: 766 mPa.s). 
Strength enhancer 11 (comparison) 
For comparison, a dry-strength enhancer for paper is prepared by the method 
described in Example 7 of U.S. Pat. No. 4,097,427 using polymer 3 in an 
amount of 6.6%, based on starch, 5% of sodium hydroxide, based on starch, 
and ammonium persulfate as oxidizing enhancer (viscosity: 30 mPa.s). 
Strength enhancer 12 
A dry-strength enhancer for paper is prepared as described above for 
strength enhancer 1, except that here the polymer 1 described there is 
replaced by polymer 3, which is used in such an amount that the resulting 
mixture, instead of 10%, here contains only 6.6% of polymer 3, based on 
starch (viscosity: 985 mPa.s). 
Strength enhancer 13 (comparison) 
A dry-strength enhancer is prepared as described in the preparation of 
strength enhancer 6, except that here the natural potato starch used there 
is replaced by natural corn starch (viscosity: 290 mPa.s). 
Strength enhancer 14 (comparison) 
A dry-strength enhancer is prepared as described in the preparation of 
strength enhancer 6, except that here the natural potato starch used there 
is replaced by natural wheat starch (viscosity: 220 mPa.s).

EXAMPLE 1 
A Rapid-Kothen sheet former was used to produce sheets having a basis 
weight of 120 g/m.sup.2. The paper stock comprises 80% of mixed wastepaper 
and 20% of bleached beech sulfite pulp which has been beaten to a freeness 
of 50.degree. SR (Schopper-Riegler) and to which the strength enhancer 1 
described above is added in such an amount that the solids content in 
terms of strength enhancer 1 is 2.2%, based on dry paper stock. The pH of 
the stock suspension is adjusted to 7.6. The sheets produced from this 
model stock are conditioned and thereafter measured in respect of the CMT 
value, the dry burst strength and the dry breaking length by the methods 
specified above. The results are reported in Table 1. 
EXAMPLES 2 to 9 
Example 1 is repeated each time, except that the strength enhancer 1 used 
in Example 1 is replaced by one of the strength enhancers indicated in 
Table 1. The results thus obtained are reported in Table 1. 
COMATIVE EXAMPLE 1 
Example 1 is repeated without addition of a dry-strength enhancer; that is, 
a stock comprising 80% of mixed wastepaper and 20% of bleached beech 
sulfite pulp beaten to a freeness of 50.degree. SR is dewatered in a 
Rapid-Kothen sheet former to produce sheets having a basis weight of 120 
g/m.sup.2. The results of the strength tests on the sheets thus obtained 
are reported in Tables 1 and 2. 
COMATIVE EXAMPLE 2 
Comparative Example 1 is repeated, except that the paper stock is treated 
with 2% of natural potato starch, based on dry fiber substance. The 
strength values of the sheets of paper thus obtained are reported in Table 
1. 
COMATIVE EXAMPLE 3 
Example 1 is repeated, except that the strength enhancer described therein 
is replaced by the same amount of strength enhancer 10. The strength 
values of sheets thus obtained are reported in Table 1. 
COMATIVE EXAMPLE 4 
Example 1 is repeated, except that the dry-strength enhancer specified 
therein is replaced by the same amount of strength enhancer 11. The 
strength values of sheets of paper prepared in this way are reported in 
Table 2. 
EXAMPLE 10 
Example 1 is repeated, except that the strength enhancer described therein 
is replaced by the same amount of strength enhancer 12. The strength 
values of sheets thus obtained are reported in Table 2. 
TABLE 1 
______________________________________ 
Number of 
strength Dry 
enhancer Dry burst breaking 
added to 
CMT value pressure length 
paper stock 
[N] [kPa] [m] 
______________________________________ 
Example 
1 1 171 160 3263 
2 2 164 165 3314 
3 3 162 167 3379 
4 4 161 159 3152 
5 5 172 160 3180 
6 6 168 165 3328 
7 7 161 173 3037 
8 8 165 166 3071 
9 9 159 167 3167 
Comparative 
Example 
1 -- 126 128 2531 
2 natural 125 140 2840 
potato 
starch 
3 10 147 149 2907 
______________________________________ 
TABLE 2 
______________________________________ 
Number of strength Dry burst 
enhancer added to 
CMT value pressure 
paper stock [N] [kPa] 
______________________________________ 
Example 
10 12 151 158 
Comparative 
Example 
1 -- 123 131 
4 11 137 139 
______________________________________ 
EXAMPLE 11 
Example 1 is repeated, except that the strength enhancer described therein 
is replaced by the same amount of strength enhancer 12 and that instead of 
the paper stock consisting of 80% of mixed wastepaper and 20% of bleached 
beech sulfite pulp, a paper stock which consists 100% of unbleached 
softwood sulfate and which has been beaten to a freeness of 30.degree. SR 
(Schopper-Riegler) is used for sheet formation, and the sheets formed 
therefrom have a basis weight of 100 g/m.sup.2. The strength values of 
these sheets are reported in Table 3. 
COMATIVE EXAMPLE 5 
Example 1 is repeated, except that the strength enhancer described therein 
is replaced by the same amount of strength enhancer 11 and that instead of 
using the paper stock consisting 80% of mixed wastepaper and 20% of 
bleached beech sulfite pulp a paper stock which consists of 100% of 
unbleached softwood sulfate and which has been beaten to a freeness of 
30.degree. SR (Schopper-Riegler) is used for sheet formation and the 
sheets formed therefrom have a basis weight of 100 g/m.sup.2. The strength 
values of these sheets are reported in Table 3. 
COMATIVE EXAMPLE 6 
Comparative Example 1 is repeated, except that instead of using the paper 
stock consisting 80% of mixed wastepaper and 20% of bleached beech sulfite 
pulp a paper stock which consists of 100% of unbleached softwood sulfate 
and which has been beaten to a freeness of 30.degree. SR 
(Schopper-Riegler) is used for sheet formation and the sheets formed 
therefrom have a basis weight of 100 g/m.sup.2. The results of the 
increase in strength measured on the sheets thus obtained are reported in 
Table 3. 
TABLE 3 
______________________________________ 
Number of strength 
Dry burst Dry breaking 
enhancer added to 
pressure length 
paper stock [kPa] [m] 
______________________________________ 
Example 
11 12 623 8637 
Comparative 
Example 
5 11 576 8203 
6 -- 504 7535 
______________________________________ 
EXAMPLE 12 
Using an experimental paper machine, paper having a basis weight of 120 
g/m.sup.2 and a width of 68 cm is produced at a paper machine speed of 50 
m/min. The paper stock used comprises 80% mixed wastepaper and 20% 
bleached sulfite pulp having a freeness of 50.degree. SR. Prior to sheet 
formation, the paper stock is treated with strength enhancer 1 in an 
amount of 2.2%, based on dry paper stock. The backwater has a pH of 7.6. 
The strength values of the paper thus produced are reported in Table 4. 
EXAMPLE 13 
Example 12 is repeated, except that the same amount of strength enhancer 3 
is used. The strength values of the paper thus produced are reported in 
Table 4. 
EXAMPLE 14 
Example 12 is repeated, except that the dry-strength enhancer used there is 
replaced by strength enhancer 4. The strength values of the paper thus 
obtained are reported in Table 4. 
EXAMPLE 15 
Example 12 is repeated, except that the dry-strength enhancer used there is 
replaced by strength enhancer 6. The strength values of the paper thus 
obtained are reported in Table 4. 
COMATIVE EXAMPLE 7 
The experimental paper machine described in Example 12 is used to produce 
paper having a basis weight of 120 g/m.sup.2 from a paper stock which is 
80% mixed wastepaper and 20% bleached beech sulfite pulp of freeness 
50.degree. SR. The paper machine speed is set to 50 m/min and the pH of 
the backwater is 7.6. The difference from Example 12 is that no 
dry-strength enhancer is used. The strength values of the paper thus 
obtained are reported in Table 4. 
COMATIVE EXAMPLE 8 
Comparative Example 7 is repeated, except that, before dewatering, the 
paper stock described therein is additionally treated with 2% of natural 
potato starch, based on dry fiber substance. The strength values of the 
paper thus obtained are reported in Table 4. 
COMATIVE EXAMPLE 9 
Comparative Example 7 is repeated, except that, before dewatering, the 
paper stock described therein is additionally treated with 2% of natural 
corn starch, based on dry fiber substance. The strength values of the 
paper thus obtained are reported in Table 4. 
COMATIVE EXAMPLE 10 
Comparative Example 7 is repeated, except that, before dewatering, the 
paper stock described therein is additionally treated with 2% of natural 
wheat starch, based on dry fiber substance. The strength values of the 
paper thus obtained are reported in Table 4. 
COMATIVE EXAMPLE 11 
Example 12 is repeated, except that strength enhancer 1 is replaced by the 
same amount of strength enhancer 13. The strength values of the paper thus 
obtained are reported in Table 4. 
COMATIVE EXAMPLE 12 
Example 12 is repeated, except that strength enhancer 1 is replaced by the 
same amount of strength enhancer 14. The strength values of the paper thus 
obtained are reported in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Number of Dry burst 
Dry COD value 
strength 
CMT value 
pressure 
breaking 
in backwater 
enhancer used 
[N] [kPa] length [m] 
[mg/l] 
__________________________________________________________________________ 
Example 
12 1 139 163 3381 139 
13 3 177 151 3151 130 
14 4 130 147 3278 146 
15 6 202 161 3488 134 
Comparative 
Example 
7 -- 109 129 2425 129 
8 natural 110 118 2823 320 
potato 
starch 
9 natural 112 105 2672 287 
corn 
starch 
10 natural 119 117 2652 256 
wheat 
starch 
11 13 122 115 2732 185 
12 14 117 121 2767 172 
__________________________________________________________________________ 
EXAMPLE 16 
The experimental paper machine described in Example 12 is used to produce 
an LWC paper from the following model stock: 40% of bleached groundwood, 
30% of bleached softwood sulfite pulp and 30% of bleached birch sulfate 
pulp of freeness 35.degree. SR. Based on dry fiber substance, an 
additional 20% of china clay and 0.3% of a commercial cationic 
polyacrylamide having a K value of 120 and in the form of a 7% strength 
aqueous solution. Additionally, 0.5% of alum is added, so that the 
drainage water has a pH of 6. Before dewatering on the paper machine wire, 
the paper stock is treated with strength enhancer 1 in an amount of 2.2%, 
based on dry fiber substance. A production speed on the paper machine of 
60 m/min produces paper having a basis weight of 50 g/m.sup.2, the 
strength values of which are reported in Table 5. 
EXAMPLE 17 
Example 16 is repeated, except that the strength enhancer used therein is 
replaced by the same amount of strength enhancer 2. The dry strength 
values of the paper thus obtained are reported in Table 5. 
EXAMPLE 18 
Example 16 is repeated, except that the strength enhancer specified therein 
is replaced by strength enhancer 4, affording an LWC paper whose dry 
strength values are reported in Table 5. 
COMATIVE EXAMPLE 13 
Example 16 is repeated, except that an LWC paper is produced in the absence 
of any dry-strength enhancer. The strength values of the paper thus 
obtained are reported in Table 5. 
COMATIVE EXAMPLE 14 
Example 16 is repeated, except that here strength enhancer 1 used there is 
replaced by 2% of natural potato starch, based on dry fiber substance. The 
strength values of the LWC paper thus obtained are reported in Table 5. 
TABLE 5 
______________________________________ 
Number of Dry Tear 
strength 
Dry burst breaking propagation 
enhancer 
pressure length resistance 
used [kPa] [m] [mJ/m] 
______________________________________ 
Example 
16 1 52 2913 417 
17 2 51 2781 409 
18 4 54 2943 423 
Comparative 
Example 
13 -- 39 2270 338 
14 natural 46 2558 398 
potato 
starch 
______________________________________