Starch-based PHCH
The invention relates to a process for preparing self-binding pigment particle suspensions, to a self-binding pigment particle suspension as well as to a paper product comprising self-binding pigment particles and to the use of the self-binding pigment particle suspension in paper applications, such as in paper coating or as filler material.
1. A self-binding pigment particle suspension obtained by a process comprising the following steps of:
a) providing an aqueous pigment material suspension comprising pigment material, wherein the pigment material comprises calcium carbonate,
b) providing a starch having a net negative charge and consisting of at least one anionic starch,
c) mixing the starch of step b) with the aqueous pigment material suspension of step a), wherein the starch is added to the aqueous pigment material suspension in an amount from 0.5 to 20 wt.-%, based on the total dry weight of the pigment material in the suspension, and
d) grinding the aqueous pigment material and starch of step c) during and/or after step c) at a temperature from 20° C. to 40° C., wherein grinding step d) is carried out to reduce the particle size of the pigment material of step a) and to obtain a fraction of self-binding pigment particles in which greater than 20 wt.-%, based on the total weight of the pigment particles have a particle size of less than 2 μm, as measured with a Mastersizer 2000,
thereby obtaining the self-binding pigment particle suspension, wherein the self-binding pigment particle suspension comprises self-binding pigment particles at least partially coated with the starch and the amount of free starch in the self-binding pigment particle suspension is less than 50 wt.-% based on the total amount of starch added in step c) and the pigment material in the self-binding pigment particle suspension has a surface charge density after step d) in the range of +2.5 μEq/g and −10 μEq/g.
2. The self-binding pigment particle suspension according to claim 1 , wherein the pigment material of step a) comprises calcium carbonate and one or more of-dolomite, a calcium associated with magnesium, clay, kaolin, titanium dioxide, talc, aluminium hydroxide, mica, synthetic fibers, or natural fiber.
3. The self-binding pigment particle suspension according to claim 1 , wherein the pigment material is a ground natural calcium carbonate, a precipitated calcium carbonate, a modified calcium carbonate, or any mixture thereof.
4. The self-binding pigment particle suspension according to claim 1 , wherein the pigment material is a ground natural calcium carbonate.
5. The self-binding pigment particle suspension according to claim 1 , wherein the at least one-anionic starch of step b) is an anionic starch comprising anionic groups selected from the group consisting of carboxyl groups, carboxymethyl groups, carboxymethyl hydroxypropyl groups, carboxymethyl hydroxyethyl groups, phosphate groups, sulfonate groups, and any mixture thereof.
6. The self-binding pigment particle suspension according to claim 1 , wherein the at least one-anionic starch of step b) is an anionic starch having a degree of carboxylation in the range of 0.001 to 0.08.
7. The self-binding pigment particle suspension according to claim 1 , wherein in step c) the starch is added to the aqueous pigment material suspension in an amount from 1 to 20 wt.-%, based on the total weight of the dry pigment material in the aqueous pigment material suspension.
8. The self-binding pigment particle suspension according to claim 1 , wherein the solids content in step c) is adjusted such that it is at least 1 wt.-%, based on the total weight of the aqueous pigment material suspension.
9. The self-binding pigment particle suspension according to claim 1 , wherein grinding step d) is carried out during step c).
10. The self-binding pigment particle suspension according to claim 1 , wherein grinding step d) is carried out until the fraction of self-binding pigment particles having a particle size of less than 1 μm is greater than 10 wt.-%, and/or until the fraction of self-binding pigment particles having a particle size of less than 2 μm is greater than 20 wt.-%, based on the total weight of the pigment particles.
11. The self-binding pigment particle suspension according to claim 1 , wherein grinding step d) is carried out until the fraction of self-binding pigment particles having a particle size of less than 1 μm is greater than 70 wt.-%, and/or until the fraction of self-binding pigment particles having a particle size of less than 2 μm is greater than 80 wt.-%, based on the total weight of the pigment particles.
12. The self-binding pigment particle suspension according to claim 1 , wherein the pigment material in the obtained self-binding pigment particle suspension has a surface charge density in the range of +2 μEq/g and −8 μEq/g.
13. The self-binding pigment particle suspension according to claim 1 , wherein the pigment material in the obtained self-binding pigment particle suspension has a surface charge density in the range of +0.5 μEq/g and −6 μEq/g.
14. The self-binding pigment particle suspension according to claim 1 , wherein the obtained self-binding pigment particle suspension has a Brookfield viscosity in the range of 1 to 3500 mPas.
15. The self-binding pigment particle suspension according to claim 1 , wherein grinding step d) is carried out such that the amount of free starch in the obtained self-binding pigment particle suspension is less than 45 wt.-%, based on the total amount of starch added in step c).
16. The self-binding pigment particle suspension according to claim 1 further comprising performing a concentration step e) before step d), such that the solids content in the self-binding pigment particle suspension obtained after step d) is at least 45 wt.-% based on the total weight of the self-binding pigment particle suspension, and the self-binding pigment particle suspension comprises a fraction of self-binding pigment particles in which greater than 20 wt.-% based on the total weight of the pigment particles have a particle size of less than 2 μm as measured with a Mastersizer 2000, and self-binding pigment particles at least partially coated with the starch, and wherein the amount of free starch in the self-binding pigment particle suspension is less than 50 wt.-% based on the total amount of starch added in step c) and the pigment material in the self-binding pigment particle suspension has a surface charge density in the range of +2.5 μEq/g and −10 μEq/g.
17. The self-binding pigment particle suspension according to claim 1 further comprising performing a concentration step e) after step d), such that the solids content is the self-binding pigment particle suspension after step e) is at least 45 wt.-% based on the total weight of the self-binding pigment particle suspension, and the self-binding pigment particle suspension comprises a fraction of self-binding pigment particles in which greater than 20 wt.-% based on the total weight of the pigment particles have a particle size of less than 2 μm as measured with a Mastersizer 2000, and self-biding pigment particles at least partially coated with the starch, and wherein the amount of free starch in the self-binding pigment particle suspension is less than 50 wt.-% based on the total amount of starch added in step c) and the pigment material in the self-binding pigment particle suspension has a surface charge density in the range of +2.5 μEq/g and −10 μEq/g.
18. The self-binding pigment particle suspension according to claim 1 , wherein before or during or after step c) and/or step d) a dispersing agent is added.
19. The self-binding pigment particle suspension according to claim 1 , wherein the starch of step b) is in a form of a starch solution or starch suspension having a starch concentration from 1 wt.-% to 50 wt.-%, based on the total weight of the starch solution or starch suspension, and having a temperature of <40° C.
20. The self-binding pigment particle suspension according to claim 19 , wherein the starch of step b) is in form of a starch solution or starch suspension having a starch concentration from 15 wt.-% to 45 wt.-%, based on the total weight of the starch solution or starch suspension.