Preparation And Application Of Highly-Branched Polyacrylamide And Its Derivative Aaplied As Paper Strengthening Agents

In recent years, the demand for ideal paper strengthening agent has increasingly grown because of the following trends in pulp and paper industry:the papermaking system changed from acid to neutral or alkaline, further closed water circuit, elevated paper-machine speed, and the increased use of fillers, high-yield pulps and recycled fibers. Consequently, novel papermaking additives clearly need to be developed to meet the demand for more cost-effective strengthening agents.Two series of paper strengthening agents were developed in this paper to meet the different requirements for the paper strength. The two series were as follows:branched amphoteric polyacrylamides (AmPAM) and cross-linked glyoxalated polyacrylamide (GPAM). Then the products were characterized by FT-IR, NMR and GPC, and the strengthening effects of the agents were especially studied. In order to provide the theoretical foundation for practical production and application, the feasible technical conditions were presented through pilot scale trails.To meet the demand for enhancing dry strength of paper, the branched AmPAM was synthesized. According to the technology of free radical polymerization, a series of AmPAM with different molecular weights were prepared as water solution by using acrylamide (AM),2-methacryloxyethyl trimethyl ammonium chloride (DMC), and acrylic acid (AA) as main materials, N,N-methylene bis-acrylamide (MBA) and sodium methylallyl sulfonate (SMAS) as molecular structure modification additive, ammonium persulfate and sodium bisulfite as redox initiation system. The handsheets strengthening effect of AmPAMs were compared with several commercial strengthening agents. Results showed that AmPAM demonstrated excellent performance and had a good strengthening effect under the following polymerization conditions:the dosage of AM91.94mol%, reaction temperature50℃, the charge of initiator0.18wt%, reaction pH value5.5, the dosage of cross-linking monomer MBA0.03mol%, the dosage of cationic monomer DMC5mol%, the dosage of anionic monimer AA3.0mol%, the dosage of chain transfer agent SMAS0.03mol%, the consistency of reactant15%. Under the conditions, the cationic charge density was moderate (0.56～0.61mmol·g-1), the molecular weight was around16×104, the pH value was5-6and the quality was stable. The mechanical strength of paper was remarkably enhanced, the tensile and tear indices of the handsheets increased by27.1%and8.8%, respectively, compared with those of handsheets without addition of AmPAM. The increased degree of tensile and tear indices of paper added with AmPAM were also both much greater than those of paper added with commercial CPAM and YG-20.To simultaneously increase the dry strength and the wet strength of paper sheets, a novel paper strengthening agent, cross-linked cationic glyoxalated polyacrylamide (GPAM) of cationic polyacrylamide (CPAM) and glyoxal was produced. In this study, the reaction conditions of the cross-linked CPAM and the modified conditions of CPAM with glyoxal were investigated, respectively. The optimized conditions of the synthesis of CPAM were: pH5, temperature of50℃, AM dosage of55wt%, DMC dosage5.5wt%, N-methylolacrylamide (NMA) dosage8.0wt%,2-mercaptoethanol dosage1.0wt%, and initiator dosage0.5wt%. The optimal modified conditions were:glyoxal dosage30wt%, temperature of60℃, reaction time140min, and pH7.0～8.0. Under the optimized conditions, the GPAM obtained has excellent properties:the cationic charge density was1.2～1.4mmol·g-1, the molecular weight was around2.6×10, the pH value was3-4and the quality was stable. When GPAM was used as a strengthening agent, the mechanical strength of the paper was remarkably enhanced, the dry and wet tensile indices of the handsheets increased by24.6%and381%, respectively, compared with those of handsheets without addition of GPAM. The strengthening effect of the GPAMs was much better than that of other commercial strengthening agent, such as CPAM, YG-20and PAE.The molecular structure and weight of AmPAM and GPAM were characterized by FT-IR, NMR and GPC, respectively. The results indicated that AmPAM included AM, DMC and AA organic functional groups and had a branched structure. The data of GPAM demonstrated that GPAM contained AM, DMC, NMA and glyoxal functional groups, had a relatively narrow molecular distribution and, was obtained by synthesizing in two stages.The pilot scale experiments were conducted according to the synthesis process found in the laboratory. The properties of products accorded with the demands of paper mills, and they had good strengthening effect which was as good as that of products obtained in the laboratory. Through pilot trails, the feasible technical conditions of AmPAM and GPAM were presented to meet the industrialization.