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Assessment of the Need for a Coagulant (poly DADMAC or PAC) with Tapioca or Potato Starch in Mechanical Papers Filled with PCC

Abstract: Precipitated calcium carbonate (PCC) is used to produce mechanical grade papers to reduce the production cost and to improve paper properties. However, adding PCC reduces the strength of pap…

Abstract: Precipitated calcium carbonate (PCC) is used to produce mechanical grade papers to reduce the production cost and to improve paper properties. However, adding PCC reduces the strength of paper. Papermakers compensate for this loss by adding strength additives like starch. It is also customary to add coagulants in order to fix the dissolved and colloidal substances (DCS) which reduce the effectiveness of cationic starch. These coagulants are commonly known as anionic trash collectors (ATC). A common coagulant is poly DADMAC. Recent work in our laboratory showed that tapioca starch has a strong affinity for TMP fibers. It was also conjectured in that study that an ATC might not be necessary to add in the pulp suspension when using tapioca starch. On the other hand it was suggested that potato starch would be more effective in the presence of a proper ATC. However, these suggestions were not proven experimentally. In this work the performance of commercially available poly DADMAC and poly aluminum chloride (PAC) as anionic trash collectors (ATC) in the presence of tapioca or potato starch in a thermo-mechanical pulp (TMP) suspension loaded with PCC was evaluated through charge measurements, first pass retention, drainage and aggregation with photometric dispersion analysis (PDA).

Thermo-mechanical pulp (TMP) was obtained from a Catalyst Paper mill at 3.5-4% consistency and stored at 4C without any preservatives. The filler is a commercially available, negatively charged, acid-tolerant precipitated calcium carbonate (PCC). It has a particle size of 1.5 m and a brightness of 98 % PCC and was supplied as slurry at 33% solids content. The tapioca starch used in this work had a degree of substitution (DS) of 0.1 and viscosity of 570 5 cP. The potato starch had a DS of 0.1 and a viscosity of 550 5 cP. The two starches were received in powder form and prepared as suspensions at solids content of 2 %. Subsequently, they were cooked in a microwave for 10 min to a final temperature of about 95 C. Stirring continued for 30 min and starch was kept at a temperature of 50 C.

It was found that when using potato starch it is necessary to use an ATC to enhance adsorption on the fibres. On the other hand, the addition of tapioca starch (8 kg/t of pulp) in a peroxide-bleached thermo-mechanical pulp (TMP) suspension loaded with PCC does not necessarily require the use of poly DADMAC or PAC to deal with the anionic trash. A split addition of tapioca starch (2 kg/t to PCC and 6 kg/t to TMP) which was found beneficial in a mill to combat linting problems is optimal since it does not necessarily require a coagulant. Also, from a flocculation point of view tapioca starch does not necessarily need a coagulant whereas the effectiveness of potato starch increases with poly DADMAC. Finally, it was also found that tapioca starch which is added as a strength additive also enhances fines and filler retention in a high anionic trash environment.

Paper presented at the PAPTAC 94th Annual Meeting in Montreal, February 5-7, 2008.

Keywords: STARCH, PCC, MECHANICAL PULP, COAGU-LANT, RETENTION

Full manuscript available at www.paptac.ca.