Process for producing a high solids pigment suspension comprising carboxymethylcellulose-based dispersant

A process for preparing an aqueous suspension comprising the steps of

• a) providing a mineral pigment material,
• b) providing a depolymerized carboxymethylcellulose having a degree of carboxylation in the range from 0.2 to 2.2, a molecular weight in the range from 5000 to 40000 g/mol, and a polydispersity index in the range from 2 to 10,
• wherein the depolymerized carboxymethyl cellulose is prepared by depolymerizing a high molecular weight carboxymethylcellulose in a process comprising the following steps:
• i) providing a high molecular weight carboxymethylcellulose having a molecular weight of more than 40000 g/mol and a degree of carboxylation in the range from 0.2 to 2.0,
• ii) providing a peroxide selected from hydrogen peroxide and/or an alkali metal salt thereof,
• iii) providing a catalyst,
• iv) mixing at least one part of the high molecular weight carboxymethylcellulose of step i) and/or at least one part of the peroxide of step ii) and/or at least one part of the catalyst of step iii) and water in any order at a reaction temperature from 50 to 85° C., and
• v) adding the remaining part of the high molecular weight carboxymethyl cellulose and/or the remaining part of the peroxide and/or the remaining part of the catalyst in one or more steps to the mixture obtained in step iv) until the mixture of step v) contains from 10 to 60 wt.-% depolymerized carboxymethylcellulose, based on the total weight of the mixture of step v), and until the mixture of step v) at the same time has a Brookfield viscosity between 30 and 10000 mPa·s at 20° C.,
• wherein during step v) the Brookfield viscosity of the mixture is maintained between 200 and 1500 mPa·s, measured at the reaction temperature, and
• c) mixing the depolymerized carboxymethylcellulose of step b), the mineral pigment material of step a) and water to form an aqueous suspension,
• wherein the mineral pigment material is added in an amount from 10 to 80 wt.-%, based on the total weight of the suspension, and
• wherein the depolymerized carboxymethylcellulose is added in an amount from 0.05 to 5.0 wt.-%, based on the total weight of the mineral pigment material in the suspension, and such that the Brookfield viscosity of the aqueous suspension is between 40 and 2000 mPa·s at 20° C.