Dimensionally stable paper and production method thereof

1. A method of producing a paper having good dimensional stability, comprising the steps of:

I. producing of a water-based impregnation mixture comprising at least

a sugar alcohol having a general formula HOCH 2 [CH(OH)] n CH 2 OH wherein n=2, 3, 4 or 5,

a urea,

a polyalkylene glycol and/or glycerin, and

a) a latex dispersed in the mixture, wherein the impregnation mixture has a pH of more than 7, or

b) a dry strength agent, wherein the impregnation mixture has a pH of at least 6 and at most 7,

II. lowering of the pH of the impregnation mixture to at most 5.5 by addition of a coagulant releasing aluminum ions, and

III. raising the pH at least to the values indicated in step I) for alternative a) or b), respectively, and

IV. impregnating a raw paper with the impregnation mixture of step III).

2. The method according to claim 1 , wherein the impregnation mixture before step II) includes a dry weight ratio

I. of the latex to

a. the total of sugar alcohols is 5:95 to 15:85, and/or

b. the urea is 15:85 to 30:70, or

II. of the dry strength agent to

a. the total of sugar alcohols is 5:95 to 20:80, and/or

b. the urea is 12:88 to 21:79.

3. The method according to claim 1 , wherein the latex is selected from the group consisting of polyacrylonitrile, polyacrylates, polyvinylacetates, styrol-acetate-copolymers, styrol-acrylate-copolymers, styrol-butadiene-copolymers, and a mixtures of two or more of these substances.

4. The method according to claim 1 , wherein the dry strength agent comprising a polymer made with one or more of the monomers of acrylic and acrylamide or a terpolymer having hydroxyl groups and being made using the monomers of acrylic acid, acrylamide and (hydroxyethyl)methacrylate.

5. The method according to claim 1 , wherein the raw paper is impregnated by an application of solids of at least 10 wt.-% relative to the raw paper dry weight.

6. The method according to claim 1 , wherein the impregnation mixture in further comprises a wetting agent, selected from terpenic wetting agents and di(2-ethylhexyl) sulfosuccinate salts.

7. The method according to claim 1 , wherein the ratio of sugar alcohol or, if more than one sugar alcohol is present in the impregnation mixture, the dry weight ratio of sugar alcohols to urea is 80:20 to 20:80.

8. The method according to claim 1 , wherein the raw paper is produced from a fiber mixture comprising one or more of the following fibers: TMP (thermomechanical pulp), CTMP (chemi-thermomechanical pulp), recycled fibers and cellulose, wherein the dry weight ratio of cellulose to TMP is 10:90 to 100:0,

and wherein the content of synthetic fibers is at most 5 wt.-% of the raw paper equilibrium dry weight.

9. The method according to claim 1 , wherein the polyalkylene glycol is polyethylene glycol of a molecular weight of 300 to 800 g/mol,

and wherein the amount of polyethylene glycol is 4 to 8 wt.-% based on the total of C5- and C6-sugar alcohols and urea dry weights.

10. A method for producing a paper having good dimensional stability, comprising the steps of:

I. producing a water based impregnation mixture comprising:

a sugar alcohol having a general formula HOCH 2 [CH(OH)] n CH 2 OH wherein n=2, 3, 4 or 5;

a urea; and

a polyalkylene glycol and/or glycerin;

II. impregnating a raw paper with the impregnation mixture;

III. producing of a water-based impregnation mixture comprising at least:

a sugar alcohol having a general formula HOCH 2 [CH(OH)] n CH 2 OH wherein n=2, 3, 4 or 5;

a urea; and

 a polyalkylene glycol and/or glycerin; and

 a) a latex dispersed in the mixture, wherein the impregnation mixture has a pH of more than 7; or

 b) a dry strength agent, wherein the impregnation mixture has a pH of at least 6 and at most 7;

IV. lowering of the pH of the impregnation mixture of step III) to at most 5.5 by addition of a coagulant releasing aluminum ions; and

V. raising the pH of the impregnation mixture of step IV) at least to the values indicated in step III) for alternative a) or b), respectively, and

VI. impregnating a raw paper with the impregnation mixture of step V).

11. The method according to claim 10 , wherein immediately before step III) the dry weight ratio

I. of the latex to

a. the total of sugar alcohols is 5:95 to 15:85, and/or

b. the urea is 15:85 to 30:70, or

II. of the dry strength agent to

a. the total of sugar alcohols is 5:95 to 20:80, and/or

b. the urea is 12:88 to 21:79.

12. The method according to claim 10 , wherein the latex is selected from the group consisting of polyacrylonitrile, polyacrylates, polyvinylacetates, styrol-acetate-copolymers, styrol-acrylate-copolymers, styrol-butadiene-copolymers, and a mixtures of two or more of these substances.

13. The method according to claim 10 , wherein the dry strength agent comprising a polymer made with one or more of the monomers of acrylic and acrylamide or a terpolymer having hydroxyl groups and being made using the monomers of acrylic acid, acrylamide and (hydroxyethyl)methacrylate.

14. The method according to claim 10 , wherein the raw paper is impregnated by an application of solids of at least 10 wt.-% relative to the raw paper dry weight.

15. The method according to claim 10 , wherein the impregnation mixture in further comprises a wetting agent, selected from terpenic wetting agents and di(2-ethylhexyl) sulfosuccinate salts.

16. The method according to claim 10 , wherein the ratio of sugar alcohol or, if more than one sugar alcohol is present in the impregnation mixture, the dry weight ratio of sugar alcohols to urea is 80:20 to 20:80.

17. The method according to claim 10 , wherein the raw paper is produced from a fiber mixture comprising one or more of the following fibers: TMP (thermomechanical pulp), CTMP (chemi-thermomechanical pulp), recycled fibers and cellulose, wherein the dry weight ratio of cellulose to TMP is 10:90 to 100:0,

and wherein the content of synthetic fibers is at most 5 wt.-% of the raw paper equilibrium dry weight.

18. The method according to claim 10 , wherein the polyalkylene glycol is polyethylene glycol of a molecular weight of 300 to 800 g/mol,

and wherein the amount of polyethylene glycol is 4 to 8 wt.-% based on the total of C5- and C6-sugar alcohols and urea dry weights.