Study on the interactions between dissolved and colloidal substances under the treatment of fixing agents in papermaking

摘要

Fixing agents, highly cationic and low molecular weighted polyelectrolytes, are widely used in papermaking industry to attach the dissolved and colloidal substances (DCS) to wood fibers and remove them from papermaking systems. However, the interaction between the fixing agents and DCS is still not understood well. In this study, oxidized starch, rosin acid, and their mixture were used as models for the dissolved substances (DS), colloidal substances (CS), and dissolved and colloidal substances (DCS) in papermaking furnishes, respectively. Three polyamines fixing agents (PA) with different characteristics were used for treating the simulated solutions. The effect of PAs on the stability of the systems was compared via analyzing the residual concentrations of the model compounds after solid-liquid separation, and the interactions between the dissolved substances and the colloidal substances were inferred. The results showed that the effectiveness of the fixing agents in removing the oxidized starch was poor while their effectiveness in removing the rosin acid was good in the singular systems. However, in the mixed DCS system, the removal of the oxidized starch was better, while that of the rosin acid was worse, than that in the singular DS or CS system. The results indicate that when a fixing agent was added into the mixed DCS system, the DS was first changed into DS-polyelectrolyte complexes (PEC), then the PEC was attached to the CS particle surfaces which improved the stability of the latter, i.e. the removal of CS became harder. On the other hand, once the CS stability was disturbed and its agglomeration took place, more DS could be removed compared to the singular DS system. The accumulation and deposition of dissolved and colloidal substances (DCS) in papermaking system have been a troubling problem for years. One popular choice for many paper mills to tackle this problem is to use the so-called fixing agents which have low molecular weights and high cationic charge densities, to fix the an ionic DCS onto fibers, and remove them from papermaking system as a part of paper product. There are many types of fixing agents. Those typical ones used in neutral pH and moderately alkaline condition include polyamines (PAs), polydimethyldiallylammonium chlorides (PDADMACs), polyvinylamines (PVams), polyethyleneimines (PEIs), etc., the efficacis of different fixing agents on differnt pulp systems vary widely. In order to select the optimal fixing agent for a given pulp system, it is necessary to build a proper evaluation system and investigate the mechanisms of fixing agents with different characteristics. Traditional methods used to evaluate the efficacy of fixing agents focus on investigating the variation in wet-end chemistry chatacteristics of pulp during the treatment of fixing agents, which include the measurments of pulp properties such as drainage speed and zeta-potential, the measurements of pulp filtrates properties such as cationic demand, turbidity, chemical oxygen demand (or total organic carbon) of the filtrates. In addition to this, the deposition tendency test of DCS remained in filtrate are included as well. Ususally, two mechanisms are applied to explain the behaviour of different fixing agents on DCS, which are charge neutralisation and patch model mechanism . Charge neutralisation means negative charges of DCS being completely neutralized by positive charges of fixing agent in stoichiometric ratio, while patch model is here referred to as cationic fixing agent adsorbing onto surfaces of anionic CS particle, the adsorbed patch of the latter can serve as an anchoring point for another anionic particle or fibers which are also negatively charged, thus the aggregation occured when negative charge of DCS were partially neutralized. Based on the fundamental mechanisms, the interaction between fixing agent and components in pulp can be concluded as a) neutralization of negative charges on fibers, or coagulation of fibers