Study On Controlling Dissolved And Colloidal Substances By P(DADMAC-AM) Polymers

In papermaking industry, improving the degree of whitewater closure causes high accumulation of dissolved and colloidal substances (DCS) in the system, which often seriously influences the runnability of paper machine, efficiency of chemicals and physical properties of paper. Fixatives, which are polyelectrolytes with high cationic charge density and low molecular weight, can fix DCS onto fibers, bond them into webs and remove them from the system, enabling clean production.Polyamine (PA), polydiallyldimethylammonium chloride (PDADMAC), polyethylene imine (PEI) and highly cationic starch-based fixative (SBF) are generally used fixatives under neutral papermaking conditions. Different fixatives have different advantages, but for a kind of fixative with fixed molecular structure, different charge density and different molecular weight may have great effects on the performances, however, detailed information on such effects are not so clear. Compared with other fixing agents, PDADMAC can have both high charge density and easily controlled high molecular weight (one hundred thousand to one million). cost-effective, and therefore worthy of developing.On the other hand, fixatives with too high charge density or too large molecular weight will cause self-agglomerations of colloidal particles and the agglomerates are easily drained to white water system and causing more severe deposits on the surface of the paper machine. Therefore, this paper planed to synthesize a kind of DCS controlling agent with moderate molecular weight and moderate charge density which performances between coagulant and flocculant. This molecular chain consists of highly charged DADMAC segments which are good to capture colloidal particles and neutral acrylamide (AM) segments which can prevent the colloidal substances from self-agglomerating. The polymer thus designed may achieve preferable effect on controlling DCS.Therefore, in this paper we synthesized a series of PDADMAC with different charge densities and molecular weights through single-factor experiments, where initiator type and dosage, monomer concentration, rising temperature, holding temperature and chelant dosage are investigated thoroughly, and PDADMAC with charge density up to6.5mmol/g was made. The synthesis of P(DADMAC-AM) was carried out with statistical experimental design methodology using a MODDE software, where factors including monomer concentration, initiator amount, chain transfer agent amount, adding speed of AM and chain transfer agent, and holding time are investigated throughly. P(DADMAC-AM)s with charge density from2.97to6.48mmol/g and intrinsic viscosity from80.58to311.33ml/g can be made, and the dosages of chain transfer agent and initiator, and the monomer concentration were found to have large effect on the charge density and molecular weight of the P(DADMAC-AM)s. Then the PDADMACs and P(DADMAC-AM)s with different charge densities and intrinsic viscosities were used to treat an Alkaline Peroxide Bleached Mechanical Pulp(APMP). The effects of PDADMAC and P(DADMAC-AM) on controlling DCS were investigated by measuring the turbidity of white water, pulp drainage speed, weight of suspended fine particles, total dissolved and colloidal substances, cationic demand and chemical oxygen demand (COD) of membrane filtrate. The results showed that PDADMACs and P(DADMAC-AM)s with high charge densities but low molecular weights were inclined to interact with dissolved substances. And those with high charge densities and high molecular weights were apt to interact with colloidal substances, Compared with P(DADMAC-AM), PDADMAC had the priority to interact with the dissolved substances so that more reduction of cationic demand was observed while less colloidal substances were coagulated. P(DADMAC-AM)s had the priority to interact with the colloidal substances, coagulating them more while reducing less cationic demand or dissolved substances.The PDADMACs and P(DADMAC-AM)s synthesized in this study were thought to have good prospect to be used in the industry. The preparation and application of such products laid some foundation for developing more efficient fixatives which functions in colloidal-fixation ways.