NO153460B - PROCEDURE FOR MAKING PAPER WITH HIGH DRY STRENGTH AND LOW WATER STRENGTH. - Google Patents

Description

This invention relates to a method for production

of paper with high dry strength and low wet strength by treating the paper's surface with water-soluble salts of copolymers based on ethylenically unsaturated carboxylic acids.

The improvement of paper's strength properties, e.g. wear length, burst index and napping resistance, can be achieved by adding auxiliaries to the fiber suspension before paper web formation, or it can be achieved by impregnating the formed paper with suitable products by surface application, e.g. in the glue press. When adding pulp, starch and starch derivatives are used

ter, as well as synthetic products, mostly based on polyacrylamide. For surface application, almost exclusively starch and starch derivatives are used for these purposes.

It is known that wet strength resins based on, for example, urea-formaldehyde or melamine-formaldehyde, which are used for surface application to increase the paper's wet strength, also lead to an improvement in the paper's dry strength; however, such products are not used in practice as agents to increase dry strength,

because of. the great difficulties in the re-dissolution of the wet-solid prepared paper, in particular the waste obtained during the paper manufacture.

From US patent no. 2 999 038 it is known to produce pa-

pir with high wet strength by applying 2-8 percent by weight, calculated on the weight of the dry paper, of an aqueous solution of ammonium salts of high molecular weight polymers, which as characteristic monomers contain acrylic acid, methacrylic acid or itaconic acid, on the surface of the paper.

According to the invention, for the initially described method, compounds are proposed which, when using

use as an impregnating agent for paper leads to an increase in the dry strength of the paper, while the wet strength is not significantly affected

equal degree. —^

The invention thus relates to a method for producing

ling of paper with high dry strength and low wet strength by treating the paper's surface with water-soluble salts of copolymers of

a) 90 - 30 percent by weight acrylic acid and/or methacrylic acid,

b) 10-70% by weight of acrylonitrile, methacrylonitrile, acrylamide

and/or methacrylamide, as well as optionally

c) up to 30% by weight of an acrylic acid or methacrylic acid ester,

characterized in that water-soluble alkali metal and/or alkaline earth metal salts are used as salts of the copolymers, which in a 2 percent aqueous solution at a temperature of 20°C have a viscosity of 5 - 100 mPas (Brookfield viscometer, 20 revolutions per minute) .

The copolymers are produced according to known methods by polymerization of the monomers, cf. US patent 2 819 189 and US patent 2 999 038. The monomer mixture is polymerised continuously or discontinuously by means of radical polymerization initiators, preferably in water. When the alkali or alkaline earth metal salts of acrylic acid or methacrylic acid are 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 a similar way to a precipitation polymerization. The copolymers thus obtained can be neutralized directly with alkali or alkaline earth metal hydroxides.

During the polymerization, however, it is also possible to polymerize in aqueous solution acrylic acid or methacrylic acid that has been neutralized to a degree of 10 - 40% with ammonium, alkali or alkaline earth metal ions, and then neutralize the resulting aqueous polymer solution completely with alkali or alkaline earth metal salts. A production method for the copolymers in which acrylic acid or methacrylic acid that is partially neutralized with ammonium ions is used is known from German (BRD) publication 20 04 676.

However, it is also possible to isolate the copolymers obtained by precipitation polymerization in water, dry them and mix them with one or more dry, powdery alkali metal hydroxides or alkaline earth metal hydroxides or oxides. These powdery mixtures can then be dissolved in water without difficulty, whereby a clear solution is obtained.

The copolymers of the monomers specified under a), b) and possibly c) can also be produced by the inverse suspension polymerization method known from BRD patent no. 1 081 228, as well as by the inverse emulsion polymerization method according to BRD patent no. .1,089,173.

To the extent that alkali or alkaline earth metal salts of acrylic acid or methacrylic acid are not used during the polymerization, the copolymers are neutralized in conjunction with the polymerization with alkali metal hydroxides or alkaline earth metal hydroxides or oxides. Of particular importance are the sodium and potassium as well as the calcium and magnesium salts of the above-mentioned copolymers. The alkali metal salts used according to the invention, however, do not include the ammonium salts, i.e. of the copolymers. The copolymers preferably contain 90-60% by weight of acrylic acid and/or methacrylic acid, preferably 10-60% by weight of acrylonitrile, methacrylonitrile, acrylamide and/or methacrylamide, and optionally up to 30% by weight of acrylic acid ester or methacrylic acid ester. The esters are preferably derived from primary alcohols with 1-4 carbon atoms.

Calcium and/or magnesium salts are preferably used, or mixtures of alkaline earth and alkali metal salts of copolymers of acrylic acid and acrylamide, acrylic acid and acrylonitrile, acrylic acid, methacrylic acid and acrylamide, and copolymers of acrylic acid, acrylamide and methacrylamide. At least 30% of the copolymer's carboxyl groups are neutralized with alkaline earth metal ions. The degree of neutralization is generally 70 - 100%.

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

The water-soluble alkali and alkaline earth metal salts of the copolymers under consideration are applied to the paper in the form of a 1-10 percent aqueous solution. The paper can be impregnated with the salt solution, e.g. in a glue press, or a solution of the copolymer salt can be sprayed onto the paper. The uptake of the preparation solution depends on the absorbent capacity of the paper used. Calculated on solids, it is sufficient to achieve a satisfactory increase in the paper's dry strength if the paper is impregnated with 1 - 4 percent by weight of the described alkali or io-alkali metal salts of copolymers

tene.

The increase in the firmness or strength of the paper is achieved directly after the drying of the paper under normal conditions, e.g. temperatures of 80 - 110°C. Aging of the impregnated paper is not required. Also of particular advantage is the fact that the alkali and alkaline earth metal salts of the copolymers used according to the invention can also be used together with the starches often used in practice. One can then use mixtures of starch solutions and copolymer solutions containing 2-10 weight percent, preferably 2-6 weight percent starch and 1-3 weight percent of the alkali and alkaline earth metal salts of copolymers used according to the invention.

All known paper qualities, e.g. writing paper, printing paper and wrapping paper can be impregnated with the products used according to the invention. The paper can be made from a number of different fiber materials such as sulphite or sulphate cellulose (bleached or unbleached), abrasive pulp or recycled paper. The use of the alkaline earth metal salts of copolymers of acrylic acid and/or methacrylic acid and the comonomers specified under b) as a surface application agent for paper leads to an unexpected increase in the paper's dry strength, without the paper's wet strength being significantly increased. With regard to the dry strength properties of paper that are improved to a significant extent, it concerns, for example, the wear length, burst strength, napping resistance, resistance to further tearing and the CMT value.

The following examples will further illustrate the invention. The parts given in the examples are parts by weight, and the percentages are on a weight basis. The indicated viscosity values were measured at a temperature of 20°C in a Brookfield viscometer at 20 revolutions/minute. The dry wear length was determined according to DIN 53 112, Blatt 1 and the wet wear length according to DIN 53 112, Blatt 2. The napping resistance of the paper was determined according to the Dennison wax method.

Example 1

A copolymer of 65% acrylic acid and 35% acrylonitrile (produced by polymerization of acrylic acid that was partially neutralized with ammonia, in aqueous solution with potassium peroxide-disulphate as catalyst at a pH value of 3.5) is converted into the calcium salt (pH value = 7.0) by reaction with calcium hydroxide. A wood-free, unglued offset paper produced in a paper machine with a grinding degree 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. In table 1, the viscosity of the preparation solution, the amount of the pure copolymer salt applied to the paper, calculated on the weight of the paper, and some of the properties of the obtained paper are compiled.

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 for the impregnation of paper, which is described in example 1. The results are compiled in table 1.

Comparative example lb

For comparison purposes, the paper described in example 1 was simply impregnated with water and then dried at 100°C.

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 value 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 compiled 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 value of 3.5, for impregnation of it in e.g. 1 described paper. The paper was dried at the same temperature as in example 2. The results are given 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°C.

Example 3

A copolymer produced by the copolymerization of 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 solution has a pH value of

5.5. The solution is applied to a wood-free, alum-free" unglued offset paper with a grinding degree of 25°SR and 10% ash (kaolin). The impregnated paper is dried at a temperature of 100°C. Further information regarding the preparation solution and the properties of the impregnated paper will can be seen from table 3.

I w v ~ w w Comparison example 3a

The copolymer described in example 3 is used in unneutralized form as a 2% aqueous solution at a pH value of 2.6 as an impregnating agent for the offset paper specified in example 3. In table 3, the properties of the obtained paper are indicated together with the paper properties that were measured after treating the paper with water and drying (comparison example 2b).

Example 4

The calcium salt of a copolymer of 80% acrylic acid,

10% acrylamide and 10% methacrylamide are used in a 3% aqueous solution (pH value of 9.0) as a surface impregnating agent for the paper described in example 3. The results are compiled in a table

Comparative example 4a

The unneutralized copolymer according to example 4 is applied as a 3% aqueous solution to the paper described in example 3. The results are given in table 4 for comparison with a paper (comparative example 4b) which was only impregnated with water and dry}

Example 5

A 1% aqueous solution of the calcium salt of a copolymer of 65% acrylic acid and 35% acrylonitrile, which dissolved solution contains 4% of a practically common oxidatively degraded starch, is applied to the paper described in example 3. The results are compiled 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 dissolved solution contains 4% of a practically common oxidatively degraded starch, is used as a preparation solution for the impregnation of paper, which is described in example 3. The results are compiled in table 5.

They are compared with an experiment in which the paper described in example 3 was only impregnated with water and then dried.

The examples and comparative examples show that the copolymer salts used according to the invention, compared to the corresponding ammonium salts and polyacids, lead to a further increase in the paper's dry strength without at the same time leading to an unwanted increase in the paper's wet strength.

Claims (3)

1. Process for producing paper with high dry strength and low wet strength by treating the paper's surface with water-soluble salts of copolymers of a) 90 - 30 weight percent acrylic acid and/or methacrylic acid, b) 10-70 weight percent acrylonitrile, methacrylonitrile, acrylamide and /or methacrylamide, as well as optionally c) up to 30 percent by weight of an acrylic acid or methacrylic acid ester, characterized in that water-soluble alkali metal and/or alkaline earth metal salts are used as salts of the copolymers, which in a 2 percent aqueous solution at a temperature of 20°C have a viscosity of 5 - 100 mPas (Brookfield viscometer, 20 revolutions per minute) . 2. Process according to claim 1, characterized in that water-soluble calcium and/or magnesium salts of copolymers of a) 90-30% by weight acrylic acid and/or methacrylic acid, b) 10-70% by weight acrylonitrile, methacrylonitrile, acrylamide and/or methacrylamide, as well as optionally c) up to 30% by weight of an acrylic acid or methacrylic acid ester, which in a 2% aqueous solution at a temperature of 20°C has a viscosity of 5-100 mPas (Brookfield viscometer, 20 revolutions per minute). 3. Method according to claim 1, characterized in that mixtures of alkali and alkaline earth metal salts of the copolymers are used.