Process and coating composition for coating a paper web
View Patent ↗The invention relates to a process for producing a coated paper web, According to the process, a paper web is formed from a fibrous raw material in a paper machine, the paper web is coated with a pigment-containing coating mix, and the coated paper web is calendered. According to the invention, the paper web is coated with a coating composition that contains as the mineral pigment mainly a mixture of gypsum and calcium carbonate, the amount of gypsum being at minimum 10% by weight of the total amount of gypsum and calcium carbonate. With mixtures of calcium carbonate and gypsum, such as mixtures of PCC and gypsum and mixtures of PCC, ground carbonate and gypsum, there are obtained simultaneously a sufficient gloss and smoothness as well as a brightness and opacity better than with PCC-kaolin pastes.
1. A process for producing a coated paper web, according to which process
a paper web having a grammage of 30 to 250 g/m 2 is formed from a fibrous raw material in a paper machine,
the paper web is coated with a pigment-containing coating composition, and
the coated paper web is calendered at a linear pressure of at least 200 kN/m,
characterized in that the coating composition contains as a mineral pigment mainly a mixture of gypsum and calcium carbonate, the amount of gypsum being at minimum 10% by weight of the total amount of gypsum and calcium carbonate to form a coating layer having a grammage of 5 -30 g/m 2 on at least one of the surfaces of the paper web.
2. The process according to claim 1 , characterized in that the coating composition contains
ground or precipitated
10-90 parts by weight and
calcium carbonate
gypsum
90-10 parts by weight
pigment in total
100 parts by weight
and
binder
1-20% by weight of the pigment
thickener
0.1-10% by weight of the pigment.
3. The process according to claim 1 , characterized in that the coating composition contains a plastics pigment in an amount of 1-20% by weight of the mineral pigment.
4. The process according to claim 1 , characterized in that the coating composition contains another mineral pigment at maximum 10%.
5. The process according to claim 1 , characterized in that gypsum constitutes at minimum 10% of the weight of the coating pigment.
6. The process according to claim 1 , characterized in that the coating is carried out by jet application.
7. The process according to claim 1 , characterized in that a coating layer having a grammage of 5-30 g/m 2 is formed on both of the surfaces of the paper web.
8. The method according to claim 1 , characterized in that the coated paper is calendered in an online soft-calender.
9. The process according to claim 8 , characterized in that the calender rolls are not substantially heated.
10. The process according to claim 9 , characterized in that a calendered paper web having a gloss below 50% is produced.
11. The process according to claim 9 , characterized in that the paper web is calendered at a linear pressure of 200-350 kN/m.
12. The process according to claim 1 , characterized in that the calendering temperature is 120-170° C.
13. The process according to claim 12 , characterized in that a calendered paper web having a gloss above 50% is produced.
14. The process according to claim 13 , characterized in that the paper web is calendered in an online calender having at least two nips formed between a soft roll and a hard roll.
15. The process according to claim 13 , characterized in that the paper web is calendered at a linear pressure of 250-450 kN/m.
16. The process according to claim 1 , characterized in that said paper web comprises a fibrous material, at least a part of which is made up of a chemimechanical pulp of a species of the Populus family.
17. The process according to claim 1 , characterized in that the coating composition is used as a single-coat mixture or a pre-coat mixture.
18. Electrophotography paper, characterized in that it has been produced by the process according to claim 1 .
19. The paper according to claim 18 , characterized in that it comprises a coated offset paper.
20. The process according to claim 1 , characterized in that gypsum constitutes at minimum 30% of the weight of the coating pigment.
21. The process according to claim 1 , characterized in that gypsum constitutes at minimum 50% of the weight of the coating pigment.
22. The process according to claim 1 , characterized in that gypsum constitutes at minimum 60% of the weight of the coating pigment.