Use of PCC mineral filler in CMP pulp reinforced with cationic starch-nanosilica layering

Abstract
Background and objectives:
Addition of mineral fillers to the paper pulp results in improvement of some important paper properties. But the paper mechanical properties would decrease due to the impossibility of attachment between filler particle and cellulosic fibers and inhibition of fiber-fiber bonds. One of substantial approaches to overcome such restriction is to modify cellulosic fiber surface to improve their bonding-ability with other paper network fragments. Thus filler can be added to the reinforced fibers network. Recent researches show that nanotechnology layer-by-layer method is a good way for modifying the cellulose fibers characteristics and improving its quality. In this technique, cellulosic fibers and counter-ionic particles are placed in an interaction media. During the interaction, ionic particles are absorbed by fibers surface mainly via electrostatic absorption. Therefore the fibers network strength would improve considerably. In current research, CMP fibers was firstly treated with three layer of cationic starch - anionic nanosilica - cationic starch. Then PCC filler was added to the treated pulp in different levels. The extent of strengths improvement with layering treatment and the strength loss due to the PCC addition was evaluated in order to calculate the level of filler application in reinforced CMP pulp.
Materials and methods:
The layering treatment of CMP fibers was successively conducted using cationic starch polymer and anionic nanosilica. For three subsequent layers constitution, 500 ml pulp suspension with 0.53 consistency was mixed for 10 minutes with solutions of cationic starch (1.5 %) - anionic nanosilica (0.05 %) - cationic starch (1.5 %) applying laboratory mixer. Then PCC filler was added to the 3-layer treated CMP pulp in three levels of 5, 10, and 15 %. Handsheets with 60±3 g/m2 basis weight was prepared from untreated, treated, and treated pulp containing PCC filler; and their physical properties (thickness, apparent density, bulk) and mechanical characteristics (tensile index, burst index, tear index) have been compared. Also, SEM micrographs were prepared from the above samples to evaluate the variation of fiber surface structure and paper network.
Results:
The results indicated that by treatment of CMP fibers with layer-by-layer method, there occurs no meaningful variation in paper physical properties. Whereas this system had considerable effect in paper strengths improvement. Evaluation of strength properties showed that by 3-layer treatment of CMP fibers,
Tensile index of paper improved from 28.54 N.m/g (in untreated fibers) to 39.18 N.m/g, burst index from 1.36 kPa.m2/g to 2.68 kPa.m2/g, and tear index from 6.11 mN.m2/g to 11.12 mN.m2/g. Also, in prepared SEM micrographs a different appearance in 3-layer treated fibers surface compared to the untreated fibers; indicated an increase in gelatinization of treated fiber surface due to an increase in cationic starch absorption, results to improve the bonded surface between fibers and increase in paper strengths. By adding 10 % of PCC filler to this pulp, tensile index, burst index and tear index was measured 26.40 N.m/g, 1.79 kPa.m2/g, and 8.07 mN.m2/g respectively. Furthermore, SEM micrographs confirmed PCC filler retaining and its relative uniform distribution in the treated CMP fibers network.
Conclusions:
Analysis of the results showed that by reinforcing CMP pulp with layer-by-layer method used in current research, paper strength characteristics increases considerably without significant increase in physical properties. From statistical viewpoint, with increasing of the strength in the treated CMP pulp, 10 % of PCC filler can be added to it.