CH637714A5 - METHOD FOR PRODUCING PAPER WITH HIGH DRY RESISTANCE AND LOW WET STRENGTH. - Google Patents

Description

The invention relates to a process for producing paper with high dry strength and low wet strength by treating the surface of the paper with water-soluble salts of polymers based on ethylenically unsaturated carboxylic acids.

Improving the strength properties of paper, e.g. Breaking load, bursting pressure and resistance to picking can be achieved both by adding auxiliaries to the fiber suspension before sheet formation and by impregnating the paper formed with suitable products by surface application, e.g. in the size press. Starch and starch derivatives as well as synthetic products, mostly based on polyacrylamide, are used in the mass addition. Starch and starch derivatives are used almost exclusively for this purpose in the surface application.

It is known that wet strength resins based on e.g. Urea-formaldehyde or melamine-formaldehyde, which are used in the surface application for the wet strengthening of paper, also lead to an increase in the dry strength of paper; However, such products are not used in practice because of the great difficulty in redissolving the wet-strength paper, in particular the rejects incurred in the paper mills.

From US-PS 2999038 it is known to produce paper with high wet strength by applying 2 to 8% by weight, based on the weight of the dry paper, of an aqueous solution of ammonium salts of high molecular weight polymers, acrylic acid, to the surface of the paper , Methacrylic acid or itaconic acid as characteristic monomers.

The object of the invention is to propose compounds for the method described in the introduction which, when used as an impregnating agent for paper, lead to an increase in the dry strength of the paper, but do not influence the wet strength, or influence it only insignificantly.

The object is achieved according to the invention in that, as salts of polymers, water-soluble alkali and / or alkaline earth salts of copolymers of a) 90 to 30% by weight of acrylic acid and / or methacrylic acid,

b) 10 to 70% by weight of acrylonitrile, methacrylonitrile, acrylamide and / or methacrylamide and optionally c) up to 30% by weight of an acrylic or methacrylic acid ester,

which have a viscosity of 5 to 100 mPa-s (Brookfield viscometer, 20 U / min) in 2% aqueous solution at a temperature of 20 ° C.

The copolymers can be prepared by known processes by polymerizing the monomers, cf. e.g. US-PS 2819189 and US-PS 2999038. The monomer mixtures are polymerized continuously or batchwise with the aid of radical polymerization initiators, preferably in water. If the alkali metal or alkaline earth metal salts of acrylic acid or methacrylic acid are already used in the polymerization of the monomers, the copolymer salt solutions to be used according to the invention are obtained directly. Acrylic acid and / or methacrylic acid can be polymerized with the corresponding amides or nitriles in water in the manner of a precipitation polymerization. The resulting copolymers can directly with alkali or. Alkaline earth metal hydroxides are neutralized.

However, it is also possible to polymerize in the polymerization to 10 to 40% neutralized with ammonium, alkali or alkaline earth ions acrylic acid or methacrylic acid in aqueous solution and then to completely neutralize the aqueous polymer solution obtained with alkali or alkaline earth metal salts. A preparation process for the copolymers, in which acrylic acid or methacrylic acid partially neutralized with ammonium ions is used, is known from DT-AS 2004676.

However, the copolymers obtained in the precipitation polymerization in water can be isolated, dried and mixed with one or more dry, powdered alkali metal hydroxides or alkaline earth metal hydroxides or oxides. These powdery mixtures can then be easily dissolved in water to give a clear solution.

The copolymers from the monomers of components a), b) and optionally c) can also be prepared by the reverse suspension polymerization process known from DE-PS 1081228 and by the inverse emulsion polymerization process according to DE-PS 1089173.

If the alkali metal or alkaline earth metal salts of acrylic acid or methacrylic acid are not already used in the polymerization, the copolymers are neutralized after the polymerization, e.g. with alkali hydroxides or alkaline earth hydroxides or alkaline earth oxides. The sodium and potassium as well as the calcium and magnesium salts of the copolymers mentioned above are of particular importance. However, the alkali salts used according to the invention do not include the ammonium salts of the copolymers. The copolymers contain 90 to 30, preferably 90 to 60% by weight of acrylic acid and / or methacrylic acid and 10 to 70, preferably 10 to 60% by weight of acrylonitrile, methacrylonitrile, acrylamide and / or methacrylamide and optionally up to 30% by weight. % of an acrylic acid ester or a methacrylic acid reester. The esters are preferably derived from primary alcohols having 1 to 4 carbon atoms.

Calcium and magnesium salts and mixtures of alkaline earth and alkali salts of copolymers of acrylic acid and acrylamide, acrylic acid are preferably used

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637 714

and acrylonitrile, acrylic acid, methacrylic acid and acrylamide, and copolymers of acrylic acid, acrylamide and methacrylamide. The carboxyl groups of the copolymers are generally at least 30% neutralized with alkaline earth metal ions. The degree of neutralization is generally 70 to 100%.

A 2% aqueous solution of the alkali or alkaline earth salts to be used according to the invention has a viscosity (Brookfield 20 / min) of 5 to 100, preferably 10 to 30 mPas at a temperature of 20 ° C. The pH of the copolymer salt solution is generally in the range between 4.0 and 10.0.

The water-soluble alkali and alkaline earth salts of the copolymers in question can be applied to the paper in the form of a 1 to 10% strength aqueous solution. The paper can be impregnated with the alkali or alkaline earth salt solution of the copolymers, e.g. in a size press, or spray a solution of the copolymer salt onto the paper. The absorption of preparation solution depends on the absorbency of the paper used. In relation to the solids, to achieve a good increase in the dry strength of the paper, it is generally sufficient to impregnate the paper with 1 to 4% by weight of the alkali metal or alkaline earth metal salts of the copolymers described.

The increase in the strength of the paper is immediately after drying the paper under usual conditions, e.g. Temperatures of 80 to 110 ° C, obtained. It is not necessary to age the impregnated paper. Another particular advantage is the fact that the alkali metal and alkaline earth metal salts of the copolymers to be used in accordance with the invention can also be used together with the starches which are frequently used in practice. You can e.g. Use mixtures of starch solutions and copolymer solutions which contain 2 to 10% by weight, preferably 2 to 6% by weight of starch and 1 to 3% by weight of the alkali and alkaline earth metal salts of copolymers to be used according to the invention.

All known paper grades, e.g. Writing, printing and packaging papers are impregnated with the products to be used according to the invention. The paper can be made from a variety of fiber materials, such as sulfite or sulfate pulp (bleached or unbleached), wood pulp or waste paper. The use of the alkaline earth metal salts of copolymers of acrylic acid and / or methacrylic acid and the comonomers mentioned under b) as a surface application agent for paper leads to an unexpected increase in the dry strength of the paper without significantly increasing the wet strength of the paper. The dry strength properties of the paper, which are noticeably improved, are e.g. the tear length, burst pressure, pick resistance, tear resistance and the CMT value.

The invention is illustrated by the following examples. The parts given in the examples are parts by weight, the percentages relate to the weight. The viscosities stated were measured at a temperature of 20 ° C. in a Brookfield viscometer at 20 rpm. The dry tear length was determined in accordance with DIN 53112, sheet 1, and the wet tear length in accordance with DIN 53112, sheet 2. The pick resistance of the paper was determined according to the Dennison wax test.

example 1

A copolymer of 65% acrylic acid and 35% acrylonitrile (produced by polymerizing acrylic acid partially neutralized with ammonia in aqueous solution with potassium peroxidisulfate as a catalyst at a pH of 3.5) is converted into the calcium salt (pH Value = 7.0). A wood-free, unsized offset paper with a

Degree of grinding of 25 ° SR, 14% ash (kaolin) and 1% alum is impregnated with a 2% aqueous solution of the above-mentioned calcium salt of the copolymer of acrylic acid and acrylonitrile and then dried at a temperature of 100 ° C. Table 1 shows the viscosity of the preparation solution, the amount of pure copolymer salt applied to the paper, based on the weight of the paper, and some properties of the paper obtained.

Comparative example la

The ammonium salt of the copolymer of 65% acrylic acid and 35% acrylonitrile described in Example 1 is adjusted to pH 7.0 with ammonia and used to impregnate paper, which is described in Example 1. The results are summarized in Table 1.

Comparative Example Ib

For comparison, the paper described in Example 1 is only impregnated with water and then dried at 100 ° C.

Table 1

Example 1 Comparative Example

(Calcium salt) la lb

(Ammonium (water) sodium salt)

Viscosity of the 2%

8th

25th

5

watery

Preparation solution

(mPa-s)

Product intake (%)

1.85

1.92

-

(Solid based on

dry paper

Dry tear length (m)

3700

3500

2400

Dennison

14

11

3rd

Wet tear length (m)

150

665

0

Example 2

The ammonium salt of the copolymer of 65% acrylic acid and 35% acrylonitrile described in Example 1 is neutralized with sodium hydroxide. The pH of the aqueous, 2% solution of the alkali salt is 7.5. The paper described in Example 1 was impregnated with this solution and dried at a temperature of 100 ° C. The results are summarized in Table 2.

Comparative Example 2a

A copolymer of 65% acrylic acid and 35% acrylonitrile is used without neutralization in the form of a 2% aqueous solution, which has a pH of 3.5, for impregnating paper, which is described in Example 1. The paper was dried at the same temperature as in Example 2. The results are shown in Table 2. For comparison, Table 2 also shows the values obtained when the paper described in Example 2 is impregnated with water and dried at 100.degree.

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Table 2

Example 2 Comparative Example

(Sodium salt) 2a 2b

(Polyacid) (water)

Viscosity of the 2%

30th

20th

5

watery

Preparation solution

(mPa-s)

Product intake (%)

1.95

1.90

-

(Solid based on

dry paper)

Dry tear length (m)

4200

4100

2400

Dennison

14

14

3rd

Wet tear length (m)

290

780

0

Example 3

A copolymer obtained by copolymerizing 80% acrylic acid and 20% acrylamide in aqueous solution using potassium peroxydisulfate as a catalyst.

is neutralized with magnesium hydroxide. A 2% aqueous solution of the magnesium salt of the copolymer is prepared, which has a pH of 5.5. This solution is applied to a wood-free, alum-free, unsized offset paper with a freeness of 25 ° SR and 10% ash (kaolin). The impregnated paper is dried at a temperature of 100 ° C. Further information on the preparation solution and the properties of the impregnated paper can be found in Table 3.

Comparative Example 3a

The copolymer described in Example 3 is used in non-neutralized form as a 2% aqueous solution at a pH of 2.6 as an impregnating agent for the offset paper specified in Example 3. Table 3 shows the properties of the paper obtained together with those properties of the paper which were measured after treating the paper with water and drying (comparative example 2b).

Table 3

Example 3 Comparative Example

(Magnesium salt) 3a 3b

(Polyacid) (water)

Viscosity of the 2%

11

9

5

watery

Preparation solution

(mPa-s)

Product intake (%)

1.82

1.82

-

(Solid based on

dry paper)

Dry tear length (m)

4700

4400

2900

Dennison

18th

16

6

Wet tear length (m)

0

250

0

Example 4

The calcium salt of a copolymer of 80% acrylic acid, 10% Acryk.nid and 10% methacrylamide is used in 3% aqueous solution (pH value of 9.0) as a surface impregnating agent for the paper described in Example 3. The results are summarized in Table 4.

Comparative Example 4a

The unneutralized copolymer according to example 4

is applied as a 3% aqueous preparation solution to the paper described in Example 3. The results are shown in Table 4 in comparison to a paper (Comparative Example 4b) which was only impregnated with water and dried.

Table 4

Example 4 Comparative Example

(Calcium salt) 4a 4b

(Polyacid) (water)

Viscosity of the 2%

26

48

5

watery

Preparation solution

(mPa-s)

Product intake (%)

2.96

2.88

-

(Solid based on

dry paper)

Dry tear length (m)

5100

4800

2900

Dennison

16

14

6

Wet tear length (m)

100

400

0

Example 5

A 1% aqueous solution of the calcium salt of a copolymer of 65% acrylic acid and 35% acrylonitrile, which contains 4% dissolved in an oxidatively degraded starch which is customary in practice, is applied to the paper described in Example 3. The results are summarized in Table 5.

Comparative Example 5a

A 1% aqueous solution of the ammonium salt of a copolymer of 65% acrylic acid and 35% acrylonitrile, which contains 4% dissolved in an oxidatively degraded starch which is customary in practice, is used as a preparation solution for impregnating paper, which is described in Example 3. The results are summarized in Table 5. They are compared to an experiment in which the paper described in Example 3 was only impregnated with water and then dried.

Table 5

Example 5 Comparative Example (Calcium Salt 5a 5b

+ Starch) (ammonium (water) salt +

Strength)

Viscosity of the 2%

10th

18th

5

watery

Preparation solution

(mPa-s),

Product intake (%)

4.58

4.77

-

(Solid based on

dry paper)

Dry tear length (m)

4800

4600

2900

Dennison

16

14

6

Wet tear length (m)

0

250

0

The examples and comparative examples show that the copolymer salts to be used according to the invention lead to a further increase in the dry strength of the paper compared to the corresponding ammonium salts or the polyacids, without undesirably increasing the wet strength of the paper.

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Claims (3)

637 714 1. Process for the production of paper with high dry strength and low wet strength by treating the surface of the paper with water-soluble salts of polymers based on ethylenically unsaturated carboxylic acids, characterized in that water-soluble alkali and / or alkaline earth salts of copoly- merisates from a) 90 to 30% by weight of acrylic acid and / or methacrylic acid, b) 10 to 70% by weight of acrylonitrile, methacrylonitrile, acrylic amide and / or methacrylamide and optionally c) up to 30% by weight of an acrylic or methacrylic acid reester, which have a viscosity of 5 to 100 mPa-s in 2% aqueous solution at a temperature of 20 ° C. 2. The method according to claim 1, characterized in that water-soluble calcium and / or magnesium salts of copolymers of a) 90 to 30 wt .-% acrylic acid and / or methacrylic acid, b) 10 to 70% by weight of acrylonitrile, methacrylonitrile, acrylic amide and / or methacrylamide and optionally c) up to 30% by weight of an acrylic or methacrylic acid reester, which have a viscosity of 5 to 100 mPa-s in 2% aqueous solution at a temperature of 20 ° C. 2nd PATENT CLAIMS 3. The method according to claim 1, characterized in that mixtures of alkali and alkaline earth metal salts of the copolymers are used.