Mechanism and Application of Chitosan Papermaking Aids

With the development of the national economy and the improvement of people’s living standards, the demand for paper and paper products has changed significantly. The supply of specialty paper, processed paper and high-grade paper is in short supply, and the country spends a lot of foreign exchange to import such products. In order to change this situation, one of the effective solutions is to develop and develop corresponding papermaking chemicals. The currently widely used papermaking wet-end additives mainly consist of synthetic polymers and natural polymers. Although the former has many advantages, such as low cost and unique performance, it is basically a petroleum downstream product, and the existing oil resources are short. A decade or so will be exhausted in a matter of a hundred years, and most of these synthetic polymers cannot be biodegraded, and some are even toxic, adversely affecting biology and ecology. Obviously, the scientific concept of development has a direct impact on the development of new types of paper additives. Natural polymers are increasingly favored by people due to their renewable, non-toxic, biocompatible, environment-friendly, abundant sources, and excellent properties, and are gradually replacing synthetic polymers.

The natural polymers commonly used in the industry mainly include starch and its modified products, cellulose and its modified products, guar gum and its modified products, proteins and modified products, but there are generally large amounts and the effects are owed. Good and other shortcomings, such as starch as an internal additive on the paper to maintain poor performance, prone to deposition. Chitosan, which has a molecular structure similar to that of cellulose and is easily modified, has long been concerned by insightful people in paper chemical research.

In recent years, domestic and foreign research on the development and utilization of chitin and its derivatives in the paper industry has been very active; among them, Japan has the most extensive research, including paper sizing, reinforcement, retention and drainage, finishing, and papermaking wastewater treatment. As well as the use of chitosan as a main component to make specialty papers, it has applied for a large number of patents and has produced many mature industrial products. In recent years, some institutions in China have also engaged in this research work, mainly focusing on enhancing, assisting and filtering, and special papers, and the development momentum is good.

In order to make the reader understand the recent research and development direction in this area, this paper summarizes the work of the predecessors and, combined with the results of the author’s recent research, makes various applications and mechanisms of chitosan and its derivatives in the paper industry. A review.

1 Chitosan and its derivatives in the paper industry

Chitosan and its derivatives can strongly interact with cellulose and is an excellent papermaking agent. A large number of relevant research reports and patent documents published so far have almost involved various processes in the paper industry.

1.1 Surface sizing agent or auxiliary sizing agent

The quality of paper made from grass fibers is generally poor, the strength of chitosan is high, the film formation is good, and the interaction with cellulose is large. Therefore, the surface sizing agent of chitosan as a straw pulp fiber paper has more practical significance. Can greatly improve paper performance. 0.1~1g/m2 chitosan is coated on the paper surface to improve the paper surface strength, flexibility and printing performance. N-alkyl chitosan fibers were added to the sulphate pulp, and the resulting paper had high tearing and folding resistance. Compared with rosin gum, when chitosan is used as surface sizing agent, it has higher dry and wet strength, bursting resistance, tearing degree, printing performance, water resistance and electrical insulation properties. For newsprint, printing paper, map paper, and (cyanoethyl chitosan)=1% surface treatment, the crack length, breakage resistance, folding resistance, etc. can be improved without affecting the gloss, and it is suitable for Paper made from various pulps can be used as a surface sizing agent for semi-chemical pulp, sulfite pulp, kraft pulp, cotton fiber, flax fiber paper.

Chitosan is blended with starch or polyvinyl alcohol, and it can also be used as a surface compounding sizing agent with a mass ratio of 1:9 to 9:1 and a paper surface sizing agent with a 10% ratio. And the length of the fracture can be increased by 0.75 to 1 times; chitosan/oxidized starch blending, sizing the surface of double-sided offset paper, coated paper, drying paper, and electrostatic copy paper can significantly improve the mechanical properties and printability of the paper. Dyeing vividness.

Chitosan and its derivatives can also be used to synergize the sizing agent in the slurry. When sizing with alkyldienone (AKD), add w[chitosan (acetic acid or hydrochloric acid solution)] = 0.1% to 0.4% to the slurry, compared with the epichlorohydrin polyamide resin (PAE). Can effectively improve the AKD retention, filler retention and sizing. This is due to the fact that the chitosan cations are adsorbed to the anionic sites of the positive AKD latex when added to the slurry, allowing the AKD milk particles to carry a positive charge that can be adsorbed with negatively charged fibers, thereby increasing the retention of AKD. .

When sizing with w(AKD)=0.05% or w[alkenylphthalic anhydride (ASA)]=0.01%, add (chitosan)=0.5%, w(carboxymethylcellulose)=1% or w When (carboxyethyl cellulose)=1%, the retention of rubber can be increased, and the degree of sizing can reach 38s.

1.2 Paper Enhancers

The ideal reinforcing agent should usually have certain conditions: (1) Linear water-soluble polymer, high relative molecular mass, strong film-forming ability, sufficient adhesive strength to fiber; (2) good compatibility with cellulose Sex, does not destroy the hydrogen bonds between cellulose molecules; (3) functional groups can fully contact the paper fibers, and can be firmly bonded through ionic bonds or hydrogen bonds; (4) non-toxic, biodegradable natural products. Chitosan satisfies the above conditions and is therefore an ideal paper enhancer.

The mechanism of action of chitosan as a paper enhancer has a large amount of literature. Currently, it is generally considered that it can improve the mechanical strength of paper from three aspects, ie dry strength, wet strength and initial wet strength.

1.2.1 dry strength agent

When chitosan is added to the slurry, it is first adsorbed on the surface of the negatively charged cellulose and fills in between the fibers when the paper is formed. This filling action will increase the binding area between the fibers and the numerous groups on the molecule. The corresponding chemical bonds (hydrogen bonds, ionic bonds) are formed with the groups on the surface of the fiber. In the drying process, the evaporation of moisture provides more opportunities for the intermolecular chemical interaction between chitosan and cellulose. More hydrogen bonds are formed to increase the strength of the paper.

W (chitosan) = 0.5% and w (cationic starch) = 0.05% and w (epichlorohydrin polyamide) = 0.01% compounded, for hardwood pulp enhancer, split into paper up to 4. 0 km, Allan et al. showed that the strength of the paper used to produce printing paper is inversely proportional to the size of the chitosan particles, which is proportional to the degree of deacetylation; and for the pulps with high or low beating degree, the tensile strength is significantly increased. The dry reinforcement effect is better than some common additives such as polyacrylamide and polyethyleneimine. Spraying w(chitosan)=0.5%~2% onto the formed wet paper sheet, the strength and burst resistance were improved, and the tearing factor was slightly decreased. The experiment also found that the additive method has an effect on the strength of the paper sheet. Spraying the chitosan on the paper sheet by spraying method has the best effect (less amount, economy, and convenience); the dissolved chitosan is precipitated with alkali solution. On the fiber, the strength of the paper sheet made by papermaking was the second highest; the paper sheets made by dissolving the dissolved chitosan on the surface of the fiber had the greatest increase in strength. The most used were sodium tripolyphosphate, lactic acid, and shell polymer. With sugar-treated vegetable fiber paper, the paper's water resistance is greatly improved.

Copolymers of chitosan and other reagents tend to have better composite reinforcement effects. Slagel et al. copolymerized acrylamide, 2-acrylamido-2-methylpropyl sulfonic acid and chitosan, and added it to the slurry instead. For chitosan, under the same conditions, the burst strength and tensile strength of the handsheets were increased by 54.4% and 22,1%, respectively, compared to blank sheets, while the pure chitosan only increased by 12.1% and 18.5. %. Graft copolymerization of chitosan and cationic starch can effectively increase the physical strength of paper and promote the retention of filler. The optimum dosage is w (copolymer) = 1.0%; compared with blank paper, the fracture length is increased by 77.8 %, burst resistance increased by 44.7%, its enhancement effect is better than cationic starch and chitosan.

Laleg et al. showed that at pH = 5, w (chitosan) = 2% increased the crack length by 8.4%, tensile energy absorption by 24.5%, tear index by 7.7%, and piercing index by 23%. When pH = 10, the above parameters were increased by 68%, 234%, 53%, and 106%, respectively.

1.2.2 Wet Strength Agents

Chitosan can also increase wet strength. As the pulp is bleached and oxidized, the cellulose and hemicellulose molecules contain certain aldehyde groups. These aldehyde groups can interact with the amino groups on chitosan during the general papermaking process to produce a Shiff base. Increased wet strength of paper. In addition, when the chitosan is deposited on the surface of the slurry under alkaline conditions, the wet strength can be greatly improved, such as when the humidity is 60%, the pH is 10, and the amount of chitosan is 2 g/100 g of fiber, the wetness is Increased strength by more than 4 times. At pH 5, the wet strength can also increase more than 3 times. A mixed solution of chitosan salt (0.1 g/100 g of fiber) and epoxy polyamide resin (0.2 g/100 g of fiber) was added to the bleached chemical pulp, and the resulting paper had high dry and wet strength.

1.2.3 Initial wet strength agent

Allan et al. first discovered that the strength of wet webs was significantly increased when chitosan was sprayed on paper sheets that had never been dried; Laleg et al. also found that w (chitosan) = 1 % was added to a grindstone with 40% solids content. On the ground paper pulp wet paper web, when pH = 5, 7.5, and 9, the tear length of the wet paper web increased by 50%, 60%, and 100%, respectively. This is unlike many enhancers, which usually do not increase wet paper strength, and some even reduce this parameter. Therefore, chitosan can not only improve the physical and mechanical properties of paper, but also improve the operating efficiency of paper machine equipment, especially suitable for alkaline papermaking system.

1.3 Retention aids

Both the quaternized chitosan and the chitosan in the acidic solution are polycationic high molecular compounds that can be almost completely absorbed in the pulp fibers, fine fibers, soluble hemicellulose, and by charge neutralization. On the surface of colloidal substances such as lipophilic extracts, polyelectrolyte complexes are formed between soluble carbohydrates such as β- and γ-cellulose; electrostatic neutralization, bridging, inlaying, and hydrogen bonding exist with inorganic fillers. It acts and flocculates, thereby increasing the retention of fine fibers, fillers, and various additives in the slurry.

When Seika studied the retention of polyoxyethylene in deinking news pulp, it was found that chitosan was more effective and less used than conventional phenolic resins. Allen et al. found that compared with chitosan, linear polyoxyethylene (PEO), cationic polyacrylamide (PAM), branched polyvinylamine, colloidal SiO2, and branched polyphenol resin, the retention and drainage performance was improved. The phenolic resin combined with PEO has the best retention and drainage aid properties, followed by chitosan. Allen et al. studied retention and closed water cycle systems for thermomechanical pulp (TMP) newsprint production and found that compared to anionic PAMs and nonionic PEOs, chitosan is disadvantageous to closed water circulation systems, mainly due to the system charge caused by closed loops. accumulation. If you use chitosan derivatives of amphoteric electrolyte, you can solve this problem. Jason et al. also found that the retention of TMP by chitosan was also affected by the retention procedure.

Chitosan can also cause mechanical pulp flocculation, increase the freeness of the pulp, improve the retention rate, reduce the concentration of white water, and improve the water filtration performance of papermaking. At pH = 7.5, the use of w (chitosan) = 1% can increase the Canadian Standard Freeness of Milled Ground Slurry (SGW) from 80 mL to 227 mL, and the dynamic flow meter retention rate from 75% to 87%. The authors recently discovered that chitosan quaternary ammonium salts are more apt to flocculate small fibers and more easily complex with soluble and colloidal carbohydrates. These phenomena all lead to the retention of pulp and the improvement of drainage performance, such as the decrease of beating degree by nearly 30%. The retention rate of fillers is also greatly increased, and its optimal flocculating concentration is only 5% to 10% of cationic starch.

1.4 Chitosan for Papermaking Wastewater Treatment
Chitosan is used for black liquor treatment. Compared with HE (Hexamethlene diamine epichlorohycdrin), polyethyleneimine (PEI), PAM and other chemical reagents, chitosan has the best effect, TOC removal rate reaches 70%, black liquor color removal rate Up to 90%, significantly higher than HE, PEI, PAM, isolated wood