EP0282761B1 - Process for producing paper and board having a high dry strength - Google Patents

Abstract

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 (a) diallyldimethylammonium chloride, (b) N-vinylamine or (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.

Description

To increase the dry strength of paper, it is e.g. from Ullmann's Encyclopedia of Technical Chemistry, 4th edition, publisher. Chemie, Weinheim - New York, 1979 volume 17, page 581 known to use aqueous slurries of native starches, which are converted into a water-soluble form by heating, as a mass additive in the manufacture of paper. However, the retention of the starches dissolved in water on the paper fibers in the paper stock is low. An improvement in the retention of natural products on cellulose fibers in the manufacture of paper is known, for example, from US Pat. No. 3,734,820. It describes graft copolymers which are obtained by grafting dextran, a naturally occurring polymer with a molecular weight of 20,000 to 50 million, with cationic monomers, e.g. Diallyldimethylammonium chloride, mixtures of diallyldimethylammonium chloride and acrylmid 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.

A process for cationizing starch is known from US Pat. No. 4,097,427, in which the starch is boiled in an alkaline medium in the presence of water-soluble quaternary ammonium polymers and an oxidizing agent. Quaternary ammonium polymers include quaternized diallyldialkylamine polymers or quaternized polyethyleneimines. The oxidizing agent used is, for example, ammonium persulfate, hydrogen peroxide, sodium hypochlorite, ozone or tert-butyl hydroperoxide. The modified cationic starches which can be prepared in this way are added to the paper stock as dry strength agents in the production of paper. However, the wastewater is polluted by a very high COD value.

From US-A-4,146,515 a process for the production of cationic starch is known which is used for surface sizing and coating of paper and paper products. According to this method, an aqueous slurry of oxidized starch is digested together with a cationic polymer in a continuous cooker. Suitable cationic polymers are condensates of epichlorohydrin and dimethylamine, polymers of diallyldimethylammonium chloride, quaternized reaction products of ethylene chloride and ammonia, quaternized polyethyleneimine and quaternized polyepichlorohydrin.

The invention has for its object to achieve an improvement in the dry strength of paper when using starch compared to the known methods. In particular, the substantivity of the starch is increased when it is drawn onto the fibers in the paper stock, thereby reducing the COD load in the wastewater.

• a) Diallyldimethylammoniumchlorid,
• b) N-Vinylamin oder
• c) N-Vinylimidazolinen der Formel
  

in der
R¹ = H, C₁- bis C₁₈-Alkyl,

R⁵, R⁶ = H, C₁- bis C₄-Alkyl, Cl,
R² = H, C₁- bis C₁₈-Alkyl,

R³, R⁴ = H, C₁- bis C₄-Alkyl, und
X⁻ ein Säurerest bedeutet,
einpolymerisiert enthalten und die einen K-Wert von mindestens 30 haben, und nativer Kartoffelstärke einsetzt, die durch Erhitzen in wäßrigem Medium auf Temperaturen oberhalb der Verkleisterungstemperatur der nativen Kartoffelstärke in Abwesenheit von Oxidationsmitteln, Polymerisationsinitiatoren und Alkali in eine wasserlösliche Form überführt wird, wobei die native Kartoffelstärke entweder in Gegenwart der kationischen Polymerisate erhitzt oder als bereits aufgeschlossene Kartoffelstärke mit den kationischen Polymerisaten bei Temperaturen im Bereich von 15 bis 70°C zur Reaktion gebracht wird.The object is achieved according to the invention with a process for the production of paper, cardboard and cardboard with high dry strength by adding a dry strength agent from a mixture of cationic polymers and starch to the paper stock and dewatering the paper stock with sheet formation when used as a dry strength agent a mixture of cationic polymers, which are units of

• a) diallyldimethylammonium chloride,
• b) N-vinylamine or
• c) N-vinylimidazolines of the formula
  

in the
R¹ = H, C₁ to C₁₈ alkyl,

R⁵, R⁶ = H, C₁ to C₄ alkyl, Cl,
R² = H, C₁ to C₁₈ alkyl,

R³, R⁴ = H, C₁ to C₄ alkyl, and
X⁻ means an acid residue,
contain in copolymerized form and have a K value of at least 30, and use native potato starch, which is converted into a water-soluble form by heating in an aqueous medium to temperatures above the gelatinization temperature of the native potato starch in the absence of oxidizing agents, polymerization initiators and alkali, the native Potato starch is either heated in the presence of the cationic polymers or is reacted as an already digested potato starch with the cationic polymers at temperatures in the range from 15 to 70 ° C.

The mixtures to be used as dry strength agents according to the invention have good retention towards paper fibers in the paper stock. The COD value in the white water is significantly reduced with the mixtures according to the invention compared to the native starch. The interfering substances contained in the water circuits of paper machines affect the effectiveness of the dry strength agents to be used according to the invention only slightly. The pH of the paper stock suspension can be in the range from 4 to 9, preferably 6 to 8.5.

As has been established with the help of a number of experiments, the task is only solved if native potato starch is used as the starch. In contrast to the known processes for starch modification mentioned above, the modified starch to be used according to the invention is produced in the absence of oxidizing agents, polymerization initiators and also in the absence of alkali. The modification of the native potato starch is preferably achieved by heating it in aqueous slurry with the cationic polymers in question to a temperature above the gelatinization temperature of the native potato starch. The gelatinization temperature of the starch is the temperature at which the birefringence of the starch grains is lost, cf. Ullman's Encyclopedia of Technical Chemistry, Urban and Schwarzenberg, Munich-Berlin, 1965, 16th volume, page 322.

However, the modification of the native potato starch can generally be done in various ways. An already digested native potato starch, which is in the form of an aqueous solution, can be reacted with the cationic polymers in question to temperatures in the range from 15 to 70 ° C. Longer contact times are required at even lower temperatures. If the reaction is carried out at even higher temperatures, e.g. up to 110 ° C, shorter contact times are required, e.g. 0.1 to 15 min., The simplest way of modifying the native potato starch is to heat an aqueous slurry of the starch in the presence of the cationic polymers in question to a temperature above the gelatinization temperature of the native potato starch. In general, the starch for modification is heated to temperatures in the range from 70 to 110 ° C., the reaction being carried out in pressure-tight apparatus at temperatures above 110 ° C. However, one can also proceed by first heating an aqueous slurry of native potato starch to a temperature in the range from 70 to 110 ° C. and bringing the starch into solution and then adding the cationic polymer required for modification. The starch is always solubilized in the absence of oxidizing agents, initiators and alkali in about 3 minutes to 5 hours, preferably 5 to 30 minutes. Higher temperatures here require a shorter dwell time. 1 to 20, preferably 8 to 12 parts by weight of a single polymer or a mixture of the cationic polymers in question are used per 100 parts by weight of native potato starch. The native potato starch is converted into a water-soluble form by heating or reacting with the cationic polymers. The viscosity of the aqueous phase of the reaction mixture increases. A 3.5% by weight aqueous solution of the mixture to be used as dry strength agent has viscosities in the range from 50 to 10,000 mPas (measured according to Brookfield at 20 rpm and 20 ° C.).

(A) Polymers of diallyldimethylammonium chloride are suitable for the preparation of the dry strength agents to be used according to the invention. Polymers of this type are known. Under Polymers of diallyldimethylammonium chloride should primarily be understood to mean the homopolymers and the copolymers with acrylamide and / or methacrylamide. The copolymerization can be carried out in any monomer ratio. The K value of the homopolymers and copolymers of diallyldimethylammonium chloride is at least 30, preferably 95 to 180.

• a) 95 bis 10 mol% N-Vinylformamid und
• b) 5 bis 90 mol% Vinylacetat oder Vinylpropionat

einpolymerisiert enthalten, wobei die Formylgruppen des Copolymerisats zu 70 bis 100 mol% unter Bildung von N-Vinylamin-Einheiten in den Copolymerisaten und die Acetyl- und Propionylgruppen zu 70 bis 100 mol% unter Bildung von Vinylalkohol-Einheiten abgespalten werden. Der K-Wert der hydrolysierten Homo-und Copolymerisate von N-Vinylformamid beträgt vorzugsweise 70 bis 170. Die zu dieser Gruppe gehörenden Polymerisate sind beispielsweise bekannt aus der US-A-4 421 602, US-A-4 444 667 und der DE-A-35 34 273.Cationic polymers of group (b), which contain units of N-vinylamine copolymerized as characteristic monomers, can be obtained by hydrolyzing homopolymers of N-vinylformamide, the formyl groups of the homopolymers of N-vinylformamide being split off to 70 to 100 mol% and N Polymers containing copolymerized vinylamine units are formed. If 100 mol% of the formyl groups are split off from the homopolymers of N-vinylformamide, the resulting polymers can also be referred to as poly-N-vinylamines. This group of polymers also includes hydrolyzed copolymers which

• a) 95 to 10 mol% of N-vinylformamide and
• b) 5 to 90 mol% of vinyl acetate or vinyl propionate

in copolymerized form, the formyl groups of the copolymer being split off to form 70 to 100 mol% to form N-vinylamine units in the copolymers and the acetyl and propionyl groups to be split off to form 70 to 100 mol% with the formation of vinyl alcohol units. The K value of the hydrolyzed homo- and copolymers of N-vinylformamide is preferably from 70 to 170. The polymers belonging to this group are known, for example, from US Pat. Nos. 4,421,602, 4,444,667 and DE- A-35 34 273.

in der
R¹ = H, C₁- bis C₁₈-Alkyl,

R⁵, R⁶ = H, C₁- bis C₄-Alkyl, Cl,
R² = H, C₁- bis C₁₈-Alkyl,

R³, R⁴ = H, C₁- bis C₄-Alkyl, und
X⁻ ein Säurerest bedeutet,
gegebenenfalls zusammen mit Acrylamid und/oder Methacrylamid, in wäßrigem Medium bei pH-Werten von 0 bis 8, vorzugsweise von 1,0 bis 6,8 in Gegenwart von Polymerisationsinitiatoren, die in Radikale zerfallen, polymerisiert.Cationic polymers of group c) are homopolymers and copolymers of optionally substituted N-vinylimidazolines. These are also known substances. They can be prepared, for example, by the process of DE-B-1 182 826 in that compounds of the formula

in the
R¹ = H, C₁ to C₁₈ alkyl,

R⁵, R⁶ = H, C₁ to C₄ alkyl, Cl,
R² = H, C₁ to C₁₈ alkyl,

R³, R⁴ = H, C₁ to C₄ alkyl, and
X⁻ means an acid residue,
optionally polymerized together with acrylamide and / or methacrylamide, in an aqueous medium at pH values from 0 to 8, preferably from 1.0 to 6.8, in the presence of polymerization initiators which break down into radicals.

in der

R¹ =

H, CH₃, C₂H₅, n- und i-C₃H₇, C₆H₅ und

X⁻ =

ein Säurerest ist.

X⁻ steht vorzugsweise für Cl⁻, Br⁻, So₄²⁻, CH₃O-SO₃H⁻, C₂H₅-O-SO₃H⁻, R-COO⁻ und R² = H, C₁- bis C₄-Alkyl und Aryl.1-Vinyl-2-imidazoline salts of the formula II are preferably used in the polymerization

in the

R¹ =

H, CH₃, C₂H₅, n- and i-C₃H₇, C₆H₅ and

X⁻ =

is an acid residue.

X⁻ preferably represents Cl⁻, Br⁻, So₄²⁻, CH₃O-SO₃H⁻, C₂H₅-O-SO₃H⁻, R-COO⁻ and R² = H, C₁- to C₄-alkyl and aryl.

The substituent X⁻ in the formulas I and II can in principle be any acid residue of an inorganic and an organic acid. The monomers of formula I are obtained by using the free base, i.e. 1-vinyl-2-imidazoline, neutralized with the equivalent amount of an acid. The vinylimidazolines can also be neutralized, for example, with trichloroacetic acid, benzenesulfonic acid or toluenesulfonic acid. In addition to salts of 1-vinyl-2-imidazolines, quaternized 1-vinyl-2-imidazolines can also be used. They are prepared by reacting 1-vinyl-2-imidazolines, which may optionally be substituted in the 2-, 4- and 5-position, with known quaternizing agents. Suitable quaternizing agents are, for example, C₁ to C₁₈ alkyl chlorides or bromides, benzyl chloride, benzyl bromide, epichlorohydrin, dimethyl sulfate and diethyl sulfate. Epichlorohydrin, benzyl chloride, dimethyl sulfate and methyl chloride are preferably used as quaternizing agents.

To prepare the water-soluble homopolymers, the compounds of the formula I or II are preferably polymerized in an aqueous medium. The copolymers are obtained by using the monomers of the compound of the formulas I and II with acrylamide and / or methacrylamide polymerized. In the case of the preparation of copolymers, the monomer mixture used in the polymerization contains at least 1% by weight of a monomer of the formula I or II, preferably 10 to 40% by weight. Copolymers of 60 to 85% by weight of acrylamide and / or methacrylamide and 15 to 50% by weight of N-vinylimidazoline or N-vinyl-2-methylimidazoline are particularly suitable for modifying native potato starch.

The copolymers can also by copolymerizing other monomers, such as styrene, vinyl acetate, vinyl propionate, N-vinylformamide, C₁- to C₄-alkyl vinyl ether, N-vinylpyridine, N-vinylpyrrolidone, N-vinylimidazole, acrylic acid esters, methacrylic acid esters, ethylenically unsaturated C₃- to C₅ -Carboxylic acids, sodium vinyl sulfonate, acrylonitrile, methacrylonitrile, vinyl chloride and vinylidene chloride can be modified in amounts up to 25% by weight. In addition to 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 reverse suspension polymerization method, in which bead-like polymers are obtained. The polymerization is initiated with the aid of conventional polymerization initiators or by the action of high-energy radiation. Suitable polymerization initiators are, for example, hydrogen peroxide, inorganic and organic peroxides, and hydroperoxides and azo compounds. Mixtures of polymerization initiators can be used as well as so-called redox polymerization initiators, e.g. Mixtures of sodium sulfide, ammonium persulfate and sodium bromate or mixtures of potassium peroxydisulfate and iron (II) salts. The polymerization is carried out at temperatures in the range from 0 to 100 ° C., preferably 15 to 80 ° C. It is of course also possible to polymerize at temperatures above 100 ° C., but it is then necessary to carry out the polymerization under pressure. For example, temperatures up to 150 ° C are possible. The reaction time depends on the temperature. The higher the temperature during the polymerization, the shorter the time required for the polymerization.

Since the compounds of the formula I are relatively expensive, copolymers of compounds of the formula I with acrylamide or methacrylamide are preferably used as cationic polymers of group (c) for economic reasons. These copolymers then contain the compounds of the formula I only in effective amounts, ie in an amount of 1 to 40% by weight. For the preparation of the dry strength agents to be used according to the invention, preference is given to using copolymers of acrylamides with compounds of the formula I in which R¹ = methyl, R², R³, R⁴ = H and X = is an acid residue, preferably chloride or sulfate.

• a) 70 bis 96,5 Gew.% Acrylamid und/oder Methacrylamid,
• b) 2 bis 20 Gew.% N-Vinylimidazolin oder N-Vinyl-2-methylimidazolin und
• c) 1,5 bis 10 Gew.% N-Vinylimidazol

mit der Maßgabe einpolymerisiert enthalten, daß die Summe der Angaben a) bis c) in Gew.% immer 100 beträgt und die einen K-Wert von 80 bis 150. Diese Copolymerisate werden durch radikalische Copolymerisation der Monomeren a), b) und c) nach dem oben beschriebenen Polymerisationsverfahren hergestellt. Zur Herstellung der erfindungsgemäß als Trockenverfestigungsmittel zu verwendenden Mischungen geht man wäßrigen Aufschlämmungen nativer Kartoffelstärke aus, die pro 100 Gew.-Teile Wasser 0,1 bis 10 Gew.-Teile nativer Kartoffelstärke enthalten. Wie bereits oben angegeben, werden mit anderen Stärkesorten die Vorteile der Erfindung nicht erreicht. Die erfindungsgemäß zu verwendenden Reaktionsgemische aus den oben beschriebenen Polymerisaten und nativen Kartoffelstärke wird dem Papierstoff in einer Menge von 0,5 bis 3,5, vorzugsweise 1,2 bis 2,5 Gew.%, bezogen auf trockenen Papierstoff, zugesetzt. Der pH-Wert der Mischung beträgt 2,0 bis 9,0, vorzugsweise 2,5 bis 8,0. Die Lösung des Trockenverfestigungsmittels in Wasser hat bei einer Feststoffkonzentration von 3,5 Gew.% eine Viskosität von 50 bis 10.000, vorzugsweise 80 bis 4.000 mPas, gemessen in einem Brookfield-Viskosimeter bei 20 Upm und einer Temperatur von 20°C.For the modification of native potato starch, copolymers are also suitable

• a) 70 to 96.5% by weight of acrylamide and / or methacrylamide,
• b) 2 to 20% by weight of N-vinylimidazoline or N-vinyl-2-methylimidazoline and
• c) 1.5 to 10% by weight of N-vinylimidazole

in copolymerized form with the proviso that the sum of the data a) to c) in% by weight is always 100 and that a K value of 80 to 150. These copolymers are obtained by radical copolymerization of the monomers a), b) and c) prepared by the polymerization process described above. To prepare the mixtures to be used according to the invention as dry strength agents, aqueous slurries of native potato starch are used which contain 0.1 to 10 parts by weight of native potato starch per 100 parts by weight of water. As already stated above, the advantages of the invention are not achieved with other types of starch. The reaction mixtures of the polymers and native potato starch to be used according to the invention are added to the paper stock in an amount of 0.5 to 3.5, preferably 1.2 to 2.5% by weight, based on dry paper stock. The pH of the mixture is 2.0 to 9.0, preferably 2.5 to 8.0. The solution of the dry strength agent in water at a solids concentration of 3.5% by weight has a viscosity of 50 to 10,000, preferably 80 to 4,000 mPas, measured in a Brookfield viscometer at 20 rpm and a temperature of 20 ° C.

The dry strength agents to be used according to the invention can be used in the production of all known paper, cardboard and cardboard qualities, for example writing paper, printing paper and packaging paper. The papers can be made from a variety of different types of fiber materials, for example from sulfite or sulfate pulp in the bleached or unbleached state, wood pulp, waste paper, thermomechanical material (TMP) and chemothermomechanical material (CTMP). The pH of the stock suspension is between 4.0 and 10, preferably between 6.0 and 8.5. The dry strength agents can be used both in the production of base paper for papers with low basis weight (LWC papers) and for cardboard. The basis weight of the papers is between 30 and 200, preferably 35 and 150 g / m², while it can be up to 600 g / m² for cardboard. The paper products produced according to the invention have a noticeably improved strength compared to such papers which were produced in the presence of an equal amount of native potato starch, which strength can be quantified, for example, on the basis of the tear length, the burst pressure, the CMT value and the tear resistance.

The parts given in the examples are parts by weight, the percentages relate to the weight. The viscosities of the solidifiers were determined in an aqueous solution at a solids concentration of 3.5% by weight and a temperature of 20 ° C. in a Brookfield viscometer at 20 rpm.

The sheets were made in a Rapid-Köthen laboratory sheet former. The dry tear length was determined according to DIN 53 112, sheet 1, the dry burst pressure according to Mullen, DIN 53 141, the CMT value according to DIN 53 143 and the tear propagation resistance according to Brecht-Inset according to DIN 53 115.

The leaves were tested after 24-hour air conditioning at a temperature of 23 ° C and a relative humidity of 50%.

The K value of the polymers was determined according to H. Fikentscher, Cellulosechemie, 13, 58-64 and 71-74 (1932) at a temperature of 25 ° C in 5% aqueous saline solutions and a polymer concentration of 0.5% by weight ; where K = k · 10³.

The following ingredients were used:

Polymer 1

Homopolymer of diallyldimethylammonium chloride with a K value of 95

Polymer 2

Homopolymer of diallyldimethylammonium chloride with a K value of 110

Polymer 3

Homopolymer of diallyldimethylammonium chloride with 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 with a K value of 119

Polymer 5

Copolymer of 25 mol% N-vinyl-2-methylimidazoline and 75 mol% acrylamide with a K value of 117.

Polymer 6

Homopolymer of N-vinylformamide, from which 99% of the formyl groups have been split off, with a K value of 83.

Polymer 7

Homopolymer of N-vinylformamide, from which 83% of the formyl groups have been split off, with 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 split off, with a K value of 75.

Polymer 9 (comparison)

Copolymer of 30% by weight dimethylaminoethyl acrylate methochloride and 70% by weight acrylamide with a K value of 205

Solidifier 1

A 3% slurry of native potato starch (gelatinization temperature 90 ° C.) in water is mixed with such an amount of polymer 1 that the resulting mixture contains 10% polymer 1, based on the native potato starch used. The mixture is then heated with stirring to a temperature in the range from 90 to 95 ° C. for 15 minutes and, after cooling to a temperature in the range from 10 to 40 ° C. according to the invention, is used as a dry strength agent for paper by placing it in a stock suspension before Sheet formation admits (viscosity: 656 mPa · s).

Solidifier 2

As described above under hardener 1, a dry strength agent for paper is prepared by reacting a 3% aqueous slurry of native potato starch with polymer 2 instead of polymer 1 used there (viscosity: 870 mPa · s).

Solidifier 3

As described above under hardener 1, a dry strength agent for paper is produced by using polymer 3 instead of polymer 1 described there (viscosity: 950 mPa · s).

Solidifier 4

As described above under hardener 1, a dry hardening agent is prepared by using polymer 4 instead of the polymer used there (viscosity: 398 mPa · s).

Solidifier 5

A 3% aqueous slurry of native potato starch (gelatinization temperature 90 ° C) is heated with stirring for 15 minutes to a temperature in the range from 90 to 95 ° C, the starch dissolving. After the starch solution has cooled to a temperature of 70 ° C., a 5% aqueous solution of polymer 2 is added, so that the amount of the polymer, based on the native potato starch, is 10%. The mixture is then stirred for a further 10 minutes at a temperature of 70 ° C. and then cooled to room temperature. A dry strength agent for paper is obtained (viscosity: 784 mPa · s).

Solidifier 6

As described for the preparation of hardener 1, a dry hardening agent is produced by using polymer 5 instead of the polymer used there (viscosity: 250 mPa · s).

Solidifier 7

As described for the preparation of hardener 1, a dry hardening agent is produced by using polymer 6 instead of the polymer used there (viscosity: 150 mPa · s).

Solidifier 8

As described for the preparation of hardener 1, a dry hardening agent is produced by using polymer 7 instead of the polymer used there (viscosity: 206 mPa · s).

Solidifier 9

As described for the preparation of hardener 1, a dry hardening agent is produced by using polymer 8 instead of the polymer used there (viscosity: 86 mPa · s).

Solidifier 10

For comparison, a dry strength agent for paper is produced according to the specification given under hardener 1, but polymer 9 is used instead of the polymer used there (viscosity: 766 mPa · s).

Hardener 11 (comparison)

For comparison, a dry strength agent for paper is made according to the method described in US Pat. No. 4,097,427 in Example 7 using Polymer 3 in an amount of 6.6% based on starch, 5% sodium hydroxide based on starch and ammonium persulfate produced as an oxidizing agent (viscosity: 30 mPa · s).

Solidifier 12

As described above under hardener 1, a dry strength agent for paper is prepared by using polymer 3 instead of polymer 1 described there, in such an amount that the resulting mixture is now only 6.6% polymer instead of 10% 3, based on starch, contains (viscosity: 985 mPa · s).

Solidifier 13 (comparison)

As described for the preparation of hardener 6, a dry hardening agent is produced by using native corn starch instead of the native potato starch used there (viscosity: 290 mPa · s).

Hardener 14 (comparison)

As described for the production of hardener 6, a dry hardening agent is produced by using native wheat starch instead of the native potato starch used there (viscosity: 220 mPa · s).

example 1

Sheets with a weight of 120 g / m² are produced in a Rapid Köthen sheet former. The paper stock consists of 80% mixed waste paper and 20% bleached beech sulfite pulp, which is ground to 50 ° SR (Schopper-Riegler) and to which the solidifier 1 described above is added in an amount such that the solids content of solidifier 1, based on dry Paper stock, 2.2%. The pH of the stock suspension is adjusted to 7.6. The leaves made from this fabric model are air-conditioned and then the CMT value, the dry burst pressure and the dry tear length are measured using the methods specified above. The results are shown in Table 1.

Examples 2 to 9

Example 1 is repeated in each case with the exception that the hardeners given in Table 1 are used instead of the hardeners 1 used in Example 1. The results thus obtained are shown in Table 1.

Comparative Example 1

Example 1 is repeated without adding a dry strength agent, ie a fabric made from 80% mixed waste paper and 20% bleached beech sulfite pulp, which is ground to 50 ° SR is dewatered in a Rapid-Köthen sheet former, whereby sheets with a basis weight of 120 g / m² are obtained. The results of the strength test on the sheets thus obtained are given in Tables 1 and 2.

Comparative Example 2

Comparative example 1 is repeated with the exception that 2% native potato starch, based on dry fiber material, is added to the paper stock. The strength values of the paper sheets thus obtained are given in Table 1.

Comparative Example 3

Example 1 is repeated with the exception that the strengthening agent described therein is replaced by the same amount of strengthening agent 10. The strength values of sheets thus obtained are given in Table 1.

Comparative Example 4

Example 1 is repeated with the exception that the dry strength agent indicated therein is replaced by the same amount of the strength agent 11. The strength values of paper sheets produced in this way are given in Table 2.

Example 10

Example 1 is repeated with the exception that the strengthening agent described therein is replaced by the same amount of strengthening agent 12. The strength values of sheets thus obtained are given in Table 2.

Example 11

Example 1 is repeated with the exceptions that the strengthening agent described therein is replaced by the same amount of strengthening agent 12 and that instead of the paper stock consisting of 80% mixed waste paper and 20% bleached beech sulfite pulp, a paper stock made of 100% unbleached coniferous wood sulfate is used for sheet formation , which is ground to 30 ° SR (Schopper-Riegler), and the sheets formed from it have a basis weight of 100 g / m². The strength values of these sheets are given in Table 3.

Comparative Example 5

Example 1 is repeated with the exceptions that the strengthening agent described therein is replaced by the same amount of strengthening agent 11 and that instead of the paper stock consisting of 80% mixed waste paper and 20% bleached beech sulfite pulp, a paper stock made of 100% unbleached coniferous wood sulfate is used , which is ground to 30 ° SR (Schopper-Riegler), and the sheets formed from it have a basis weight of 100 g / m². The strength values of these sheets are given in Table 3.

Comparative Example 6

Comparative example 1 is repeated with the exception that instead of the paper stock consisting of 80% mixed waste paper and 20% bleached beech sulfite pulp, a paper stock made of 100% unbleached coniferous wood sulfate, which is ground to 30 ° SR (Schopper-Riegler), is used for sheet formation and forms sheets with a basis weight of 100 g / m². The results of the increase in strength on the sheets thus obtained are given in Table 3.

Example 12

Paper with a basis weight of 120 g / m 2 and a width of 68 cm is produced on a test paper machine at a speed of the paper machine of 50 m / min. 80% mixed waste paper and 20% bleached sulfite pulp with a freeness of 50 ° SR are used as paper stock. Before the sheet is formed, hardener 1 is added in an amount of 2.2%, based on dry paper stock. The white water has a pH of 7.6. The strength values of the paper thus produced are given in Table 4.

Example 13

Example 12 is repeated with the exception that the same amount of hardener 3 is used. The strength values of the paper so produced are given in Table 4.

Example 14

Example 12 is repeated with the exception that instead of the dry strength agent used there, the hardener 4 is used. The strength values of the paper thus obtained are given in Table 4.

Example 15

Example 12 is repeated with the exception that the solidifying agent 6 is used instead of the dry hardening agent used there. The strength values of the paper thus obtained are given in Table 4.

Comparative Example 7

On the test paper machine described in Example 12, paper with a basis weight of 120 g / m 2 is produced from a paper stock which consists of 80% mixed waste paper and 20% bleached beech sulfite pulp with a freeness of 50 ° SR. The speed of the paper machine is set to 50 m / min, the pH value of the white water is 7.6. The difference to example 12 is that no dry strength agent is used. The strength values of the paper thus obtained are given in Table 4.

Comparative Example 8

Comparative Example 7 is repeated with the exception that 2% native potato starch, based on dry fiber, is additionally added to the paper stock described there before dewatering. The strength values of the paper thus obtained are given in Table 4.

Comparative Example 9

Comparative example 7 is repeated with the exception that 2% native corn starch, based on dry fiber, is additionally added to the paper stock described there before dewatering. The strength values of the paper thus obtained are given in Table 4.

Comparative Example 10

Comparative Example 7 is repeated with the exception that 2% native wheat starch, based on dry fiber, is additionally added to the paper stock described there before dewatering. The strength values of the paper thus obtained are given in Table 4.

Comparative Example 11

Example 12 is repeated with the exception that the same amount of hardener 13 is used instead of hardener 1. The strength values of the paper thus obtained are given in Table 4.

Comparative Example 12

Example 12 is repeated with the exception that the same amount of hardener 14 is used instead of hardener 1. The strength values of the paper thus obtained are given in Table 4. Table 4 example Solidified No. used CMT value [N] Dry burst pressure [kPa] Dry tear length [m] COD value in white water [mg / l] 12 1 139 163 3381 139 13 3rd 177 151 3151 130 14 4th 130 147 3278 146 15 6 202 161 3488 134 7 - 109 129 2425 129 8th native potato starch 110 118 2823 320 9 native cornstarch 112 105 2672 287 10th native wheat starch 119 117 2652 256 11 13 122 115 2732 185 12 14 117 121 2767 172

Example 16

On the test paper machine described in Example 12, an LWC paper is produced from the following fabric model: 40% bleached wood pulp, 30% bleached softwood sulfite pulp and 30% bleached birch sulfate pulp with a grinding degree of 35 ° SR. Based on dry fibrous material, 20% china clay and 0.3% of a commercially available cationic polyacrylamide with a K value of 120 are added in the form of a 7% aqueous solution. In addition, 0.5% alum is added so that the water running off the sieve has a pH of 6. Before the dewatering on the paper machine sieve, hardener 1 is added in an amount of 2.2%, based on dry pulp. At a production speed of the paper machine of 60 m / min, paper with a basis weight of 50 g / m 2 is obtained, the strength values of which are given in Table 5.

Example 17

Example 16 is repeated with the exception that the same amount of hardener 2 is used instead of the hardener used there. The dry strength values of the paper thus obtained are given in Table 5.

Example 18

Example 16 is repeated with the exception that instead of the hardener stated there, hardener 4 is now used. An LWC paper is obtained, the dry strength values of which are given in Table 5.

Comparative Example 13

Example 16 is repeated, except that LWC paper is made in the absence of a dry strength agent. The strength values of the paper thus obtained are given in Table 5.

Comparative Example 14

Example 16 is repeated with the exception that instead of the hardening agent 1 used there, 2% native potato starch, based on dry fiber material, is now used. The strength values of the LWC paper thus obtained are given in Table 5.

Claims (7)

1. A process for producing paper and paperboard of high dry strength by adding a dry strength enhancer comprising a mixture of a cationic polymer and starch to the paper stock and dewatering the paper stock with sheet formation, which comprises using as the dry strength enhancer a mixture of a cationic polymer which contains as characteristic monomers copolyerized units of

   a) diallyldimethylammonium chloride,

   b) N-vinylamine or

   c) an N-vinylimidazoline of the formula

   

   where
   R¹ is H, C₁-C₁₈-alkyl or

   

   
   R⁵ and R⁶ are each H, C₁-C₄-alkyl or Cl,
   R² is H, C₁-C₁₈-alkyl,

   

   
   R³ and R⁴ are each H or C₁-C₄-alkyl and
   X⁻ is an acid radical,
   and which 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 agent, polymerization initiator or alkali, the natural potato starch either being heated in the pressure of the cationic polymer or being reacted as a previously digested potato starch with the cationic polymer at from 15 to 70° C.
2. A process as claimed in claim 1, wherein from 1 to 20 parts by weight of a cationic polymer or a mixture thereof are used per 100 parts by weight of natural potato starch.
3. A process as claimed in either of claims 1 and 2, wherein the dry strength enhancer used is a mixture obtainable by heating natural potato starch in the presence of a homopolymer of diallyldimethylammonium chloride having a K value of from 60 to 180.
4. A process as claimed in either of claims 1 and 2, wherein the dry strength enhancer used is a mixture obtainable by heating natural potato starch in the presence of a hydrolyzed homopolymer of N-vinylformamide in which from 70 to 100 mol % of the formyl groups on the polymer have been eliminated to form N-vinylamine units, the hydrolyzed polymer having a K value from 75 to 170.
5. A process as claimed in either of claims 1 and 2, wherein the dry strength enhancer used is a mixture obtainable by heating natural potato starch in the presence of a hydrolyzed copolymer which contains

   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 on the polymer having been eliminated to form N-vinylamine units and from 70 to 100 mol % of the acetyl and propionyl groups having been eliminated to form vinyl alcohol units, and the hydrolyzed polymer having a K value from 70 to 170.
6. A process as claimed in either of claims 1 and 2, wherein the dry strength enhancer used is a mixture obtainable by heating natural potato starch in the presence of a homopolymer of a substituted or unsubstitued N-vinylimidazoline or of a copolymer thereof with acrylamide and/or methacrylamide having a K value from 80 to 220.
7. A process as claimed in either of claims 1 and 2, wherein the dry strength enhancer used is a mixture obtainable by heating natural potato starch in the presence of a copolymer which contains

   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 and has a K value from 80 to 220.