Production of filled paper

DETAILED DESCRIPTION OF THE INVENTION A process according to the invention for making filled paper comprises providing an aqueous feed suspension containing 2.
5 to 20% by weight of filler and cellulosic fibre in a dry weight ratio of 10:1 to 1:50 (preferably 1:1 to 1:50), making an aqueous thinstock suspension by diluting with water an aqueous thickstock suspension consisting of or formed from the feed suspension, adding bentonite or other anionic particulate material to the thinstock or to the thickstock from which the thinstock is formed, subsequently adding polymeric retention aid to the thinstock, draining the thinstock to form a sheet, and drying the sheet, and in this process the filler is coagulated with the fibre in the feed suspension by adding cationic coagulating agent to the feed suspension.
Although it is known to add similar cationic coagulating materials to the filler before addition to the feed suspension or to the thinstock, we obtain significant benefit by adding the coagulant at the stage where the filler is present as a mixture with fibre in a relatively concentrated suspension of the filler and fibre.
It seems that there are three reasons for this.
First, the presence of fibre with the filler means that filler is coagulated in the presence of fibre to form aggregates of filler and fibre that are then trapped in the sheet during the drainage, thereby improving retention.
Second, as a result of adding the coagulant at a time when the suspension is relatively concentrated, the coagulant can more effectively interact with the suspended material to form mixed aggregates of filler and fibre and the effectiveness of the coagulant is not lessened by, for instance, interference due to chemical interaction with impurities in white water or other dilution water utilised for making the thinstock.
Third, the filler is retained preferentially as a result of being present at a high relative concentration, especially if the concentration of fibre fines is low.
The thickstock may consist wholly of the defined aqueous feed suspension, in which event this feed suspension is diluted after the coagulation stage to form the thinstock