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Nanocomposites from cellulose and precipitated CaCO3 for the strength enhancement of paper

The alkaline conditions ensuing from the precipitation of CaCO3 by means of Ca(OH)2 and CO2 were to be used in an extruder to produce MFC and simultaneously deposit the PCC obtained on the large specific surface of the fibre wall nanostructures of MFC. The resulting new PCC-cellulose nanocomposites were to be used for the strength enhancement of papers and to lower the raw material costs of papermaking.

MFC-PCC composites were produced, varying the proportions of MFC and CaCO3 contained in them. A very high PCC immobilisation in the range of 43 61 % could be achieved even with highly loaded composites containing 65 80 % CaCO3 and after intense washing. Key criterion for this was the consistency achievable in the extruder, which can be influenced by the lime milk concentration. The higher the consistency in the extruder, the less PCC was precipitated into the aqueous phase. The MFC-PCC composites increased the filler retention in paper; significant gains were found when using short-fibre pulps because of their greater specific surface areas and denser structures.

Whether or not the extruder process leads to the complete conversion of Ca(OH)2 into CaCO3 depending on the reaction speed, length of the extruder and speed of the screws (dwell time distribution). In the ZSK 26 extruder used in the project, full conversion was achieved at a Ca(OH)2 concentration of around 35 % in the stock system. At concentrations > 35 %, the remaining amount of Ca(OH)2 could be fully precipitated into CaCO3 with CO2 afterwards.

The higher the mechanical degradation of fibres in the extruder, the better was the retention during sheet forming and the greater were the strength gains in paper. When substituting only 50 % of the filler quantity by extrusion-precipitated CaCO3 during sheet forming, the tensile strength of handsheets could be increased by 25-30 %. All results – from the extrusion process to papermaking - could be confirmed on pilot scale by trials on the pilot paper machine of PTS.

Another way of producing PCC - a double exchange reaction with carbonate and CaCl2 – could be successfully realised in the extruder as well. Because the aqueous phase of this reaction is much greater than in the precipitation with Ca(OH)2 and CO2, the bonding of PCC is much weaker after the precipitation with CaCl2. A disadvantage of the precipitation with CaCl2 is the formation of salts that must be washed out from the composites afterwards.

The project results are expected to lead to strength gains and, thus, cost savings. This opens up new business opportunities for small and medium-sized enterprises in the fields of plant engineering, machine building and filler production. The findings on strength and volume enhancement are of great importance to manufacturers of graphic, packaging and specialty papers.
Moreover, the project results can be used to derive starting points for further grammage reductions and to save raw material costs.

The research project MF 140082 was funded by the German Federal Ministry of Economic Affairs and Energy BMWi in the programme for the "Promotion of Research, Development and Innovation in disadvantaged areas" based on the decision of the German Parliament and carried out under the umbrella of EuroNorm in Berlin. We would like to express our warm gratitude for this support. We would also like to express our thank to the involved German and Scandinavian companies for providing proper samples as well as for supporting project performance.