CA1302021C

CA1302021C - Making paper, board and cardboard of high dry strength

Info

Publication number: CA1302021C
Application number: CA000572015A
Authority: CA
Inventors: Andreas Stange; Hans-Juergen Degen; Werner Auhorn; Volkmar Weberndoerfer; Michael Kroener; Heinrich Hartmann
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1987-07-25
Filing date: 1988-07-14
Publication date: 1992-06-02
Anticipated expiration: 2009-06-02
Also published as: NO883274D0; NZ225536A; KR890002493A; FI883504A0; DK407888A; DE3724646A1; EP0301372A1; NO172402C; DE3864938D1; FI92083C; US4940514A; JP2596593B2; FI92083B; EP0301372B1; FI883504A; NO172402B; DK407888D0; ATE67538T1; NO883274L; DK170826B1

Abstract

O.Z. 0050/39344 Abstract of the Disclosure: Paper, board, and cardboard having high dry strength are made by adding to the paper stock a dry strength agent which is obtainable by mixing enzymatically digested starch having a viscosity of from 20 to 2,000 mPa.s (measured in 7.5% strength aqueous sol-ution at 45°C) and a cationic polymer which contains, as typical copolymerized monomers, a) diallyldimethylammonium chloride, b) N-vinylamine or c) an unsubstituted or substituted N-vinylimidazoline, the K value of the cationic polymer in each case being not less than 30, and draining the paper stock with sheet formation.

Description

~L3~
- 1 - o.Z. 0050/39344 Making paper, board and cardboard of high dry strength To increase the dry strength of paper it is known to add aqueous suspensions of natural starches which are converted into a water-soluble form by heating to the pulp during papermaking. However, the retention of the starches dissolved in water by the paper fibers in the paper stock is poor. An improvement of the re-tention of natural products by cellulose fibers during paper~aking is disclosed in, for example, U.S. Patent 4,734,820, which describes graft ~opoly~ers which are prepared by grafting dextran, a naturally occurring polymer having a molecular weight of from ZO,OOO to 50 million, ~ith cationic monomers, eg. diallyldimethyl-ammonium chloride, ~ixtures of diallyldimethylammon;um chloride and acrylamide or mixtures of acrylamide and basic methacrylates, such as dimethylaminoethyl meth-acrylate. The graft polymerization is preferably car-ried out in the presence of a redox catalyst.
U.S. Patent 4,097,427 discloses a process for the cationization of starch, in which the digestion o~ starch is carried out in an alkaline 0edium in the presenc~ of water-soluble quaternary ammonium po(ymers and an oxidiz-ing agent. Quaternary ammonium polymers include quater-nized diallyldialkylamine polymers or quaternized poly-ethylene;mines. The oxidizing agents used are, for example, ammoniu~ persulfate, hydrogen peroxide, sodium hypochlorite, o~one or tert~butyl hydropero~ide. The modifi~d cationic starches ~hich can be prepared in this manner are added as dry strength agents to the paper stock during papermaking. However, the wastewater has a very high COD value~
It is an object of the present invention to achieve an ;mprovement in the dry strength of paper using starch, in comparison with the known processes. In par-ticular, ;t is intended to increase the substantivity of the starch during adsorption onto the fibers in the paper '~

~3~2~

2 0 ~ Z n 0 0 5 0 / 3 9 3 6~ 4 stock, and hence to reduce the COD in the wastewater.
~ e have found that this object is achieved, according to the invention, by a process for making paper, board and cardboard of high dry strength by adding a dry strength agent to the paper stock and draining the paper stock with sheet formation, if the dry strength agent used is an aqueous solution of 3 mixture of an enzymatically digested starch having a viscosity of from 20 to 2,000 mPa.s (measured in 7~5% s~rength aqueous sol-ution at 45C) and a cationic polymer which contains, ascopolymerized characteristic monomers, a) diallyldimethylammonium chloride, b) N-vinylamine or c) an N-vinylimidazoline of the formula R4HC~N--I--R 1 X
CH=CH2 ( I ) ~s where R1 is H, C1-C1æ-alkyl or ~ , RS and R6 R~
are each H, C1-C4-alkyl or Cl, R2 is H, C1-C1g-alkyl~

-CH2 ~ or -CH2-\H/C~2 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 (determined according to H.
Fikentscher in 5% strength by weiyht aqueous sodium chlor-ide solution at 25C and at a polymer concentration of 0.5% by weight).
The mi~tures to be used according to the inven-tion as dry strength agents have good retention with respect to paper fibers in the paper stock. The COD
value in the backwater is substantially reduced by the - 3 - o.Z~ 0050/39344 mi%tures to be used according to the invention, in co~-parison with a natural starch or an enzymatically di-gested starch. The troublesome substances present in the circulations of paper machines have only a slight adverse S effect on the effectiveness of the dry strength agents to be used according to ~he invention. The pH of the stock suspensions may be from 4 to 9, preferably from 6 to 8.5.
En~ymatically digested starches are an important component of the ~ixtures. All natural starches are suitable for the preparation of the mixtures, for example natural potato starch, wheat starch, corn starch, rice starch and ~apioca starch. The starches are digested with the aid of enzymes, for exa~ple a-amylase fro~
Aspergillus oryzae or from ~acillus lichemiformis or amyloglucosidase fro~ Aspergillus niger, by known methods in which an aqueous suspension of natural starch or of a mixture of a plurality of natural starches in water is first prepared. The sus~ension is prepared using from 0.1 to 60 parts by weight of starch per 100 parts by weight of water. From 0.0001 to 1 part by ~eight, per 100 parts by weight of the suspension, of an enzyme cus-tomarily used for the digestion of natural starches is then added to these starch suspensions. The aqueous sus-pensions of starch and enzyme are heated to about 100C
with thorough mixing. The enzymatic digestion of the starch takes place in the temperature range up to about 90C. The degree of digestion of the natural starch de-pends on the rate of heating of the reaction mixture, the residence time at a certain fairly high te~perature and the amoun~ of enzyme used. The progress of the digestion of the natural starch can readily be determined by taking samples of the mixture and measuring the viscosity of the samples. As soon as the desired degree of digestion of the starch has been reached, the enzy~e is deactivated.
^~5 Deactivation is most easily effected by heat;ng the re-action mixture to above 90C~ for e~ample 92-98C~ At these temperatures, the enzymes lose their activity~ so :~3~'2~

- 4 - O.Z. 0050l393~4 that the enzymatic digestion then ceases. The resulting aqueous solution of the enzymatically digested starch is then cooled, for example to 70C, if necessary diLuted with water and then mixed with the cat;onic polymers, the dry s~rength agent for papermaking being obtained. The concentration of the enzymatically digested starch in the aqueous solution which is then mixed w;th the cation;c polymer is from 40 to 0~5% by weight. The enzymatic digestion is continued until the resulting aqueous sol-utions of enzy~atically digested starch have a viscosity - of from 20 to 2,000, preferably from 25 to 1,50û, ~Pa.s (measured in 7.5% streng~h aqueous solution at 45C).
The aqueous solution of the enzymatically digested starch is then combined with the cat;onic poly-mers described above. This is most easily done by mix-ing the aqueous solution o~ the said starch with the suitable cationic polymers in the form of an aqueous solution directly after the enzymatic digestion. The enzymatically digested starch can be mixed with the cationic poLymers at from 15 to 170C; at above 100C, the reac~ion is carried out in a pressure-tight appara tUSo The two components are preferably mixed at from 40 to 1~0C in the course of from 1 to 60 minutes. Mix-ing of the enzymatically digested starch and the cationic polymers is always carried out in the absence of oxidiz-ing ayents, initiators and alkalis. AlL that is des;red is thorough homogeneous mixing. From 1 to 20, preferably fro~ S to 15, parts by ~eight of one or more cationic polymers are used per 100 parts by weigh~ of an enzymatic-alLy digested starch or of a ~ix~ure of such starches.For example, a 25Z strength by weigh~ aclueous solution of the mixture consisting of enzy~tically digested starch and cationic polymer and to be used as a dry strength agent has a viscosity of from 10 to 10,000 mPa.s (measured by the ~rookfield method at 20 rp~ and 80C).
Exa~pLes of suitable cat;onic poLymers of group a) are poly~ers of diallyldimethylam~onium chloride.

~L3~

- 5 - o.z. 0050/39344 Polymers of this type are known.
Polymers of diallyldimethyLammonium chLoride are prioarily the homopolymers and the copolymers with acryl-a~ide and/or ~ethacrylamide. The copolymerization can be carried out using any monomer ratio. The K value of the homopolymers and copolymers of diallyldimethylammonium chLoride is no~ less than 30~ preferably from 95 to 180.
Cationic polymers of group (b) which contain units of N-vinylamine as typical polymerized monomers are obtainable by hydrolyzing homopolymers of N-vinylform-amide, from 70 to 100 mol % of the formyl groups of the homopoly~ers of N-vinylformamide being eliminated and polymers containing polymerized N-vinylamine units being formed. If 100 mol ~ of the formyl groups are eliminated from the homopolymers of N-vinylformamide, the resulting polymers may also be regarded as poly-N-vinylamines.
This group of polymers includes hydrolyzed copolymers of bl) from 95 to 10 mol % of N-vinylformamide and b2) from 5 to 90 mol % of vinyl acetate or vinyl propio-nate, the sum of the data in mol ~ always being 1aO, and from 70 to 100 mol X of the formyl groups of the cop~lymer having been eliminated with formation of N-vinylarine units ;n the copolymers, and from 70 to 100 mol % of the acetyl and propionyl groups having been eliminated with for~ation of vinyl alcohol units. The K value of the hydrolyzed homopolymers and copolymers of N-vinylform-aoide is preferably from 70 to 170. The polymers belong-ing to this group are disclosed in, for example, U.S.
Patent 4,421,602, US 4,444,667 and German Laid-Open Application DOS 3,534v273.
Suitable cat;onic poLymers of group c~ are homo-polymers and copolymers of unsubstituted or substituted H-vinylimidazolines. These are also kno~n substances.
They can be prepared, for exa~pLe, by the process of Gernan Published Application DAS 1,182,826, by polymeriz-ing a compound of the formula ~3~

- 6 - O . Z . 0050/ ~i9344 R3HC--N_R2 R 4 H C`N--C--R ~ X
CH=CH2 R5 ( I ) ~here R1 is H, C1-C18-alkyl or R6 ' R5 and R6 are each H, C1-C4-alkyl or Cl, R2 jS H, C1-C1g-alkyl~

--CHz_~ --CH2--CH--c~2 3 4 or \ / , R and R are each H or C1-C4-alkyl and X is an acid radical, with or with-out acrylamide and/or methacrylamide, ;n an aqueous medium at a pH of from 0 to 8, preferably from 1.0 to 6.8, in the presence of a polymer;zat;on ;nitiator wh;ch decomposes ;nto free rad;cals.
v;nyl-2-;midazol;ne salts of the formula II

H2f N--R2 ~
L~l2C`I--c--ll ~ ( 11 ) where R1 ;s H, CH3, C2Hs, n-c3H7~ ;-C3~7 or C6~5 and X
is an acid radical, are preferably used in the poly-merization. X is preferably Cl , ar , S04Z-, CH30-S03H , C2Hs-0-S03H or R-C00 and R2 ;5 H, C1-C4-alkyl or aryl.
The substituent X ;n the formulae I and II can in princ;p(e be any acid radical of an ;norganic or of an organic ac;d~ THe monomers of the formula I are obta;ned by neutral;zing the free base, ;e. a 1-vinyl-2-imidazo-line, with the equivalent amount of an acid. The vinyl-imidazolines can also be neutral;zed, for example, w;th tr;chloroacet;c acid, benzenesul~onic acid or toluene-sulfonic acid. In addition to sal~s of 1-vinyl-2-;m;dazolines, quatern;zed 1-v;nyl-2-imidazolines are also ~3~2~

- 7 - o.7. 0050/39344 suitab~e. They are prepared by reacting 1-vinyl-2-imidazolines, which may be substituted in the 2-, 4- and 5-position, with known quaternizing agents. Examples of suitable quaternizing agents are C1-C1g-alkyl chlorides or bromides, benzyl chloride, benzyl bromide, epichloro-hydrin, dimethyl sulfate and diethyl sulfate. Preferably used quaternizing agents are epichlorohydrin, benzyl chloride, dimethyl sulfate and methyl chloride.
For the preparation of the water-soluble homopoly-mers, the compounds of the formula I or II are preferablypolymerized in an aqueous medium. The copolymers are obtained by polymerizing the monomeric comoounds of the formulae I and II with acrylamide and/or methacrylamide.
For the preparation of copolymers, the 00nomer mixture used in the poly~erization contains not less than 1, pref-erably from 10 to 40, % by weight of a monomer of the for-mula I or II. Copo~ymers wh;ch contain from 60 to 85%
by weight of acrylam;de and/or methacrylamide and fror 15 to 40~ by weight of N-vinylimidazoline or N-vinyl-2-methylimidazoline as copolymerized units are particularlysuitable for the modification of enzymatically digested starch.
The copolymers may be further modified by incor-porating other monomers, such as styrene, vinyl acetats, vinyl propionate, N-vinylformamide, C1-C4-alkyl vinyl ethers, N-vinylpyridine, N-vinylpyrrolidone, N-vinyl-imidazo~e, acrylates, methacrylates, ethylenically un-saturated C3-Cs-carboxylic acids, sodium vinylsulfonate, acrylonitrile, methacrylonitrile, vinyl chloride and vinylidene chloride, in amounts of up to 25X by weight, as copolymerized units. In addition to the polymeriza-tion in aqueous solution, it is also possible, for exam-ple, to prepare the homopolymers and copolymers in a water-in-oil emulsionu The monomers can also be polymer-ized by the process of in!erse suspension polymerization,;n which bead polymers are obtained. The polymerization is initiated with the aid of conventional polymerization - 8 - o.z. ~050/3~344 initiators or by the action of high energy radiation.
E~amples of suitable polymerization initiators are hydrogen peroxide, inorganic and organic peroxides, and hydroperoxides and azo compounds. Mixtures of polymer-S ization initiators as well as redox polymerizationinitiators can be used, for example mixtures of sodium sulfite, ammonium persulfate and sodiurn bromate, or mix-tures of potassium peroxydisulfate and iron(II) salts~
The polymeri~ation is carried out at from O to 100C, preferably from 15 to 80C. It is of course also pos-sible to carry out the polymerization at above 100C, but in this case it is necessary to effect the polymer-ization under superatmospheric pressure. Temperatures of, for example, up to 150C are possible. The reac-tion time depends on the temperature. The higher thetemperature at which the polymerization is carried out, the shorter is the time required for the poly0erization.
Since the compounds of the formula I are rela-tively expensive, copolymers of compounds of the for-mula I with acryla~ide or methacryla~ide are preferablyused as cationic polymers of group (c), for economic reasons. These copolymers contain the compounds of the formula I as copoly~erized units only in effective amounts, ;e. in an amount of from 1 to 40~ by weight.
Copolymers of acrylam;de with compounds of the formula I
~here R1 is methylr R2, R3 and R4 are each H and X is an acid radical, preferably chloride or sulfate, are prefer-ably employed for the preparation of the dry strength agents to be used according to the invention.
Other substances which are sui~able for modify-ing enzymatically digested starches are copolymers of c1~ from 70 to 96.5~ by weight of acrylamide and/or meth-acryla~ide, c2) from 2 to 20% by weight of N-vinylimidazoline or N-vinyL-2-methylimidazoline and c3) from 1.5 to 10~ by ~eight of N-vinylimidazole, having a K ~alue of from 80 to 150, and the sum of the ~3~

- 9 - o.~. 0050/39344 percentages by weight always being 100. These copolymers are prepared by free radical copolymerization of monomers c1), c2) and c3) by the polymeriza~ion method described above.
The mixtures to be used according to the inven-tion and consisting of the cationic polymers described above and enzymatically digested starch are added to the paper stock in an amount of from 0.5 to 5.0, preferably from 1.5 to 3.5, % by weight~ based on dry stock. The pH
of the mixture is from 2.0 to 9.0, preferably from 2.5 to 8Ø The solution of the dry strength agent in ~ater has, at a sol;ds content of 7.5% by weight, a viscosity of from 20 to 10,000, preferably from 30 to 4~000r mPa.s, measured in a Brookfield viscometer at 20 rpm and at ~5 45C.
The dry strength agents to be used according to the invention can be employed for making all known D aper, cardboard and board grades, for example writing, printing and packag;ng papers. Papers can be made from a w;de Z0 range of fiber materials, for example fro~ sulfite or sulfate pulp in the bleached or unbleached state, groundwood, ~aste paper, thermomechanical pulp (TMP) and chemothermomechanical pulp (CTMP). The pH of the stock suspension is from 4.0 to 1û~ preferably trom 6.0 to 8.5.
The dry strength agents can be used both for making raw paper for papers having a low basis weight (L~C papers) and for cardboard. The basis weight of the papers is fro~ 30 to 200, preferably from 35 to 150, g/m2, while that of cardboard can be up to 600 9/~2~ Compared ~ith papers made in the presence of the same amount of natural potato starch, the paper products produced according to the invention have markedly improved strength, which can be quantified, for example, with reference to the tear length, the bursting pressure, the CMT value and the tear strength.
In the Examples, parts and percentayes are by weight. rhe viscosities of the strength agents were - 10 - O.Z~ 0050/39344 determined in aqueous solution at a solids content of 7.5%
by ueight at 45C in a ~rookfield viscometer at 20 rpm;
the viscosities of the enzymatically diges~ed starches were determined ;n wat~r at a concentration of 7.5% by weight and at 45C~ likewise in a Prookfield viscometer at 20 rpm.
The sheets were made in a Rapid-Kothen laboratory sheet for~er. The dry tear length was determined accord-ing to DIN 53,112, page 1, the Mullen dry bursting pres-sure according to DIN 53,141, the CMT value according to DIN 53,143 and the ~recht-lnset tear strength according to DIN 53,115.
The sheets were each tested after conditioning for 24 hours at Z3C and a relative humidity of 50~.
The COD value was determined using COD Tester A
from Grove Analysentechnik GmbH.
The K value of the polymers was determined according to H. fikentscher, Cellulosechemie, 13 (t932), 58-64 and 71-74, at 25C in 5~ strength aqueous sodium chloride solutions and at a polymer concentration of 0.5%
by weight; K = k . 103.
The following starting materials were used:
Polymer 1 Homopolymer of diallyldimethylammonium chloride, having a K value of 95.
Polymer Z
Homopolymer of diallyldimethylammonium chloride, having a K value of 110.
Polymer 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 S
Copolymer of 25 mol % of N-vinyl-2-methylimidazoline ~3~2~

- 11 - O~Z. 0050/39344 and 75 mol % of acrylanide, having a K value of 117.
Polymer 6 Ho~opolymer of N-vinylformamide from which 99% of the formyl groups have been eliminated, having a K value of ~3.
Polymer 7 Homopolymer of N-vinylformamide from which 83% of the formyl groups have been eliminated, having a K value of 168.
Polymer 8 Copolymer of 40% by ~eight of N vinylfor~amide and 60~ by ~eight of vinyl acetate, from which 100~ of the formyl groups and 98~ of the acetyl groups have been eli~inated, having a K value of 75.
Strength agent 1 An enzyme (~-amylase from Aspergillus oryzae~ is added to a 25% strength suspension of natural potato starch in water in an amount such that the result;ng mixture contains 0.01%, based on natural potato starch used, of en~yme. This mixture is heated to 90-95C in the course of 15 minutes, while stirring, and is then cooled to 70C. The viscosity of the enzymatically digested natural potato starch is 24 mPa.s, measured at 45C ;n 7.5~ strength aqueous solution.
An aqueous solution of polymer 1 ;s added to the aqueous solution of the enzymatic potato starch, cooled to 70C, in an amount such that the resulting m;xture contains 10X, based on enzymatically digested potato starch used, of polymer 1~ The mixture is then stirred for a further 1U ~inutes at 70C and is used according to the invention as a dry strength agent by adding it to a stock suspension prior to sheet formation. The viscos-ity of the mixture is 82 mPa.s~
Strength agent 2 As described above under strength agent 1~ a dry strength agent for paper is prepared by ~ixing a 25 strength aqueous solution of enzymatically digested 13r~2~2'1 - 12 - O.Z. OOS0/39344 potato starch (viscosity ot a 7.5~ strength aqueous solution at 45C = 24 mPa~s) with the polymer 2 described aboveO A dry strength agent which has a viscosity of 108 mPa.s is obtained.
S Strength agent 3 As described above under strength 3gent 1, a dry strength agent for paper is prepared from the enzymati-cally digested starch stated there and polymer 3. The strength agent has a viscosity of 122 mPa.s.
Strength agent 4 As described above under strength agent 1, a dry strength agent is prepared from the en~ymatically digested potato starch and polymer 4~ The viscosity of the strength agent is 61 mPa.s.
Strength agent S
As described for the preparation of strength agent 1, a dry strength agent is prepared by mixing the enzymatically digested potato starch with polymer S. A
dry strength agent which has a viscosity of 36 mPa.s is Z0 obtained.
Strength agent 6 As described for the preparation of strength agent 1, a strength agen~ is prepared by mixing the enzymat;cally digest~d potato starch ~ith polymer 6. The strength agent has a viscosity of 28 mPa.s.
Strength agent 7 As described for the preparation of strength agent 1, the enzymatically digested potato starch is ~i~ed with polymer 7. This gives a dry strength agent having a viscosity of 31 mPa.s.
Strength agent 8 As described for the preparation of strength agent 1, the enzy~atically digested potato starch is mi~ed with polymer 8. A dry strength agent having a viscosity of 25 mPa.s is obtained.
Strength agent 9 As described above under strengeh agent 1, ~3~

- 13 - o.Z. 0050/39344 natural potato starch is digested with one fourth of the a00unt of ~-amylase (enzyme) stated above, an aqueous starch solution having a viscos;ty (measured at 45C in 7.5~ strength aqueous solution) of 190 mPa.s resulting.
The aqueous solution of the digested starch is then mi~ed at 45C with polymer 5 and used in the form of the aqueous solution of the mixture as a dry strenclth agent for paper.
The viscosity is 210 mPa.s Strength agent 10 1û As described for the preparation of strength agent 1, natural potato starch is digested ~ith only one tenth of the amount of enzyme stated there. The viscos-ity of the enzymatically digested potato starch is 443 (measured in 7.5% strength aqueous solution at 45C).
Instead of the polymer 1 used there, the same amount of polymer 5 is then added to the solut;on of the enzy0ati-cally digested potato starch~ the said solution having been cooled to 45C. A dry strength agent for paper, which has a viscosity of 476 mPa.s, is obtained.
Strength agent 11 (comparison) This is the enzymatically digested potato starch which is described above under strength agent 1 and which has a viscosity of 24 mPa.s ~measured at 45C in 7.5X
strength aqueous solution)~

Sheets having a basis weight of 120 9/m2 are produced in a Rapid-Kothen sheet former. The paper stock consists of 80Z of mixed waste paper and 20~ of bleached beech sulfite pulp which has been beaten to 50SR
{Schopper-Riegler3 and to which the strength agent 1 des-cribed above has been added in an a~ount such that the solids content of strength agent 1 is 3.3%, based on dry paper stock. The pH of the stock suspension is brought to 7.5. The sheets made from this model stock are con-ditioned, after which the CMT value, the dry burstingpressure and the dry tear length are measured by the methods stated above. The results are shown in Table 1.

~3~j 2~

- 14 - O.Z. OOSO/39344 Example 1 is repeated in each case with the ex-ception that the strength agent stated in Table 1 is used instead of the strength agent 1 used in E~a~ple 1. The results thus obtained are shown in Table 1.

Example 1 is repeated without adding a dry strength agent, ie. a stock consisting of 80% of mixed waste paper and 20% of bleached beech sulfite pulp beaten to 50SR is drained in a Rapid-Kothen sheet former, sheets having a basis ~eight of 120 g~m2 being ob-tained. The results of the strength test on the result-ing sheets are shown in Tables 1 and 2.

Comparative Example 1 is repeated~ exce~t that 3%, based on dry fiber, of natural potato starch are added to the paper stock. The strengths of the resulting paper sheets are shown in Table 1.

Comparative Example 2 is repeated, except that the natural potato starch is replaced by the same amount of strength agent 11. ~he strengths of the resulting sheets are shown in Table 1.

~3~J2~

- 15 - o.z. 0050/39344 TA~LE 1 Exa~ple Strength agent CMT value Dry ~ry tear no. added to bursting length paper stock pressure ~N~ tkPa] [m]

Co~parative Exa~ple 2 Natural 121 129 2732 potato starch Paper having a basis weight of 120 9/m2 and a ~idth of 68 cm is made on a test paper machine at a speed of 50 ~tmin. The paper stock used consists of 80% of mixed waste paper and 20% of bleached sulfite pulp having a freeness of 56SR. Prior to sheet for~ation, 3.3%~
based on dry paper stock, of strength agent 9 are added to the paper stock. The backwater has a pH of 7.3. The strengths of the resulting paper are shown in Table 2.

Example 11 is repeated, except that the same a~ount of strength agent 10 is used. The strengths of the resulting paper are shown in Table 2.

13~2`~2~L

- 16 - 0.~. OOSO/39344 On the test paper machine described in Exa~pl~
11, paper having a basis weight of 120 9/m2 is 0ade from a paper stock which consists of 80% of mixed waste S paper and 20X of bleached beech sulfite pulp hauing a freeness of 56SR. The speed of the paper machine is set at SO m/min, and the pH of the backwater is 7.3.
The difference compared with Example 11 is that no dry strength agent is used. The strengths of the resulting paper are shown in Table 2.

Comparative Example 4 is repeated, except that 3~, based on dry fiber, of natural potato starch are furthermore added to the paper stock described there, prior to drainage. The strengths of the resulting paper are shown in Table 2.

Comparative Example 4 is repeated, except that 3X~ based on dry fiber, of strength agent 11 are further-~ore added to the paper stock described there, prior todrainage. The strengths of the resulting paper are sho~n in Table 2.

Example Strength CMT Dry Dry tear COD value agent no. value bursting length of back-used pressure water [N] [kPa] [m] ~mg/l]
. .

12 10 150 172 3~21 203 Comparative Exa~ples 5 Natural 110 131 3149 386 potato starch

Claims

1. A process for making paper, board and cardboard having high dry strength, which comprises adding an aqueous solution of a mixture of enzymaticaLlY digested starch having a viscosity of from 20 to 2,000 mPa.s (measured in 7.5% strength aqueous solution at 45°C) and a cationic polymer which contains, as coPolymerized characteristic monomers, a) diallyldimethylammonium chloride, b) N-vinylamine or c) an N-vinylimidazoline of the formula (I) where R1 is H, C1-C18-alkyl or , R5 and R6 are each H, C1-C4-alkyl or C1, R2 is H, C1-C18-alkyl, or 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, (determined according to H. Fikentscher in 5% strength aqueous sodium chloride solution at 25°C and at a poly-mer concentration of 0.5% by weight), from 1 to 20 parts by weight of one or more cationic polymers being usecl per 100 parts by weight of enzymatically digested starch, as a dry strength agent to the paper stock and draining the paper stock with sheet formation.

2. A process as claimed in claim 1, wherein the cationic polymer used is a homopolymer of diallyldimethyl-ammonium chloride, having a K value of from 60 to 180.

3. A process as claimed in claim 1, wherein the cationic polymer used is a hydrolyzed homopolymer of N-vinylformamide, from 70 to 100 mol % of the formyl groups of the polymer having been eliminated with - 18 - O.Z. 0050/39344 formation of N-vinylamine units and the hydrolyzed polymer having a K value of from 75 to 170.

4. A process as claimed in claim 1 wherein the cationic polymer used is a hydrolyzed copolymer of b1) from 95 to 10 mol % of N-vinylform3mide and b2) from 5 to 90 mol % of vinyl acetate or vinyl Propio-nate from 70 to 100 mol % of the formyl groups of the polymer having been elimindted with formation of N-vinylamine units and from 70 to 100 mol % of the acetyl and propionyl groups having been eliminated with formation of vinyl alcohol units and the hydrol-yzed copolymer having a K value of from 70 to 170.

5. A process as claimed in claim 1 wherein the cationic polymer used is a homopolymer of an unsubstituted or substituted N-vinylimidazoline or a copolymer there-of with acrylamide and/or methacrylamide having a K value of from 80 to 220.

6. A process as claimed in claim 1 wherein the cationic polymer used is a copolymer of c1) from 70 to 96.5% by weight of acrylamide and/or meth-acrylamide c2) from 2 to 10% by weight of a N-vinyli0idazoline or N-vinyl-2-methylimidazoline and c3) from 1.5 to 10% by weight of N-vinylimidazole having a K value of from 80 to 220.