Synergy of CMC and modified chitosan on strength properties of cellulosic fiber network

Abstract

Previously, the application of modified chitosan (MCN) and carboxymethyl cellulose (CMC) in a dual polymer system showed promising results in increasing the strength of fiber networks. This work evaluates the influence of CMC molecular weight (MW) and of CMC/MCN ratio on the characteristics of formed CMC/MCN complexes in polymer solutions and on the properties of cellulosic fiber networks. The results showed that the application of MCN and the high MW CMC (HCMC) (1:1 charge ratio) enhanced the tensile, burst, and tear indices, but reduced the light scattering coefficient of the fiber network more significantly than that of MCN and the low MW CMC (LCMC). Increasing the ratio of HCMC/MCN marginally affected the tensile index, the burst index, and the light scattering coefficient of the fiber network, even though the ratio significantly affected the characteristics of the CMC/MCN complexes in polymer solutions. The tear index and brightness were improved by increasing the HCMC/MCN ratio. These results were explained based on the retention of HCMC/MCN complexes and the layer-by-layer assembly of HCMC and MCN on the fiber surface.

Introduction

The application of polymers has been a successful method to enhance the strength properties of cellulosic fiber networks (Fatehi & Xiao, 2008, Fatehi et al., 2009a, Lofton et al., 2005, Reis et al., 2003). Chitosan has been applied in a variety of areas due to its abundance and comparative cost-effectiveness. It was introduced in the paper industry as a dry strength additive and an antimicrobial agent (Chi et al., 2007, Holappa et al., 2006, Lertsutthiwong et al., 2002). However, its application in papermaking is limited due to its poor water solubility, particularly under neutral or alkaline environments (Lim & Hudson, 2004). To address this difficulty, chitosan has been modified (Lim & Hudson, 2004, Lu et al., 2004), and the modified chitosan has been employed as a retention aid in papermaking (Li, Du, & Xu, 2004).

Carboxymethyl cellulose (CMC) is an anionic polymer that has been introduced in papermaking as a strength agent (Jokinen et al., 2006, Watanabe et al., 2004). Since the surface of fibers is also negatively charged, CMC has a limited adsorption on the fiber surface (Jokinen et al., 2006, Watanabe et al., 2004). Modifying CMC to improve its adsorption on fibers has been studied in previous research (Fras-Zemljic et al., 2006, Fujimoto and Petri, 2001, Wang and Somasundaran, 2005). The adsorption of modified CMC on fibers was enhanced under the acidic condition (Watanabe et al., 2004).

The application of modified chitosan (MCN), which is a highly charged cationic polymer, along with CMC showed promising results in improving the strength of the fiber network (Fatehi, Qian, Kititerakun, Rirksomboon, & Xiao, 2009b). In this system, MCN is introduced in the fiber suspension above its maximum adsorption. By introducing CMC to the suspension, some CMC can be adsorbed on top of MCN that already adsorbed on fibers, which may follow the layer-by-layer assembly concept, while the others can form complexes with unadsorbed MCN (Fatehi et al., 2009b, Lofton et al., 2005). The formation of CMC/MCN complexes and their retention characteristics were investigated in our previous research (Fatehi et al., 2009b). We reported that this system improved the paper strength more effectively than the layer-by-layer assembly of CMC and MCN (Fatehi et al., 2009b). The size of polymers influences the size of complexes in solutions (Reis et al., 2003) and the thickness of the polymer layer coated on the fiber surface, which can impact the final properties of fiber networks (Gärdlund et al., 2005, Gärdlund et al., 2003a, Gärdlund et al., 2003b, Lofton et al., 2005). The first objective of the present study was to investigate the influence of the MW of CMC on the efficiency of CMC/MCN in improving the strength of the fiber network.

It was reported that the tensile strength of the fiber network was more significantly improved by adding the complexes of poly amideamine (PAA) and CMC preformed in one-to-one charge ratio than by adding the complexes preformed in other ratios (Gärdlund et al., 2003a). However, if the MCN and CMC are added separately to the fiber suspension, the ratio of such polymers will affect the formation and retention of the formed complexes, as well as the layer-by-layer assembly of the polymers on fibers. The second objective of the current study was to assess how changes in the ratio of CMC/MCN would affect the strength of the fiber network.

In this paper: (1) the complexes formed between MCN and two different MW CMCs were further characterized in water; (2) MCN and CMC were added subsequently to the fiber suspension at different dosages based on the equal charge ratio, and the corresponding properties of fiber networks were systematically analyzed; (3) MCN was added at 2 mg/g on fibers and different dosages of high MW CMC (HCMC) were applied into the fiber suspension and the properties of fiber networks were assessed. The results of this work are directly related to the application of strength additives in dual polymer systems to improve the properties of fiber networks.