Synthesis Of N-methylated Chitosan Derivatives And Their Application In Papermaking

Chitosan can be used as the additive in papermaking industry due to its better biodegradability,compatibility,antibacterial and film-forming properties.However,chitosan only can be dissolved in special solvents or acidic solution because of the poor solubility.With the papermaking process changing from acid to alkaline,it is necessary to study a green and high-efficiency method to modify chitosan for better applications.N,N,N-trimethyl chitosan(TMC)is a kind of chitosan quaternary ammonium salt after introducing three methyl groups to the amino group of chitosan backbone.Hydration ability can be increased with more positive charge after quaternization,thus the solubility of TMC greatly improves.At present,the preparation of TMC is carried out using methyl iodide or dimethyl sulfate as methylation reagent under high temperature and strong alkali conditions.The two reagents are toxic and have poor selectivity to amino group,which greatly limit the research and application of TMC.Therefore,a green and non-toxic methylation reagent(dimethyl carbonate)was used to prepare TMC instead of methyl iodide or dimethyl sulfate in this paper.N,N-dimethyl chitosan(DMC)with a degree of substitution of 47.7% was firstly prepared by the methylation reaction of chitosan and formaldehyde in formic acid solution,and then chitosan quaternary ammonium salt(TMC)was obtained by the quaternization of DMC and dimethyl carbonate using an ionic liquid([Bmim]Cl)as a solvent and catalyst.The results of 1H NMR,13 C NMR and FTIR showed that TMC with the quaternization degree of 8.8% was successfully prepared.Compared with the preparation method using dimethyl sulfate,dimethyl carbonate had a better selectivity to amino group,there was no O-methylation by-product formation.The XRD,TG and DSC analyses showed that the crystallinity of the product decreased with the increase of N-methylation degree,and the thermal stability of DMC slightly decreased,TMC had better thermal stability.By discussing the effect on the quaternization degree and the yield of the product,the optimum preparation process of TMC can be obtained,the concentration of DMC in [Bmim]Cl was 5% and the dosage of dimethyl carbonate was 21.2 mol/mol GLU(Glucosidic unit),the reaction time was reached at 130 ? for 3.5 h.The effects on paper sheet and pulp properties under different processing conditions were investigated by adding self-made N-methylated chitosan(DMC and TMC)into needle-leaf bleached kraft pulp(NBKP).N-methylated chitosan as the strengthening agent,retention and filter aid can effectively improve the dry strength and the wet strength of paper sheet.In addition,the retention effect on the pulp and the filler and the filter performance are greatly improved.The effect of TMC was better than that of DMC and chitosan.At the same time,the factors such as the dosage of additives,pulp consistence and pH value of pulp had significant influence on the effect of N-methylated chitosan in pulp.When the dosage of 0.1%,pulp consistence of 0.38%,pH value of 7,pulp and paper properties can get a better performance.At this point,the tensile index of TMC-added paper increased by 13.1% comparing to that of chitoan-added paper,the tearing index increased by 6.6%,the burst index increased by 13.0%,the wet tensile index increased by 14.1%,the retention rate of pulp and filler increased by 9.7% and 3.2% respectively,and the dynamic filter water increased by 11.6%.The effect mechanism of N-methylated chitosan was investigated by testing the charge density of N-methylated chitosan and the light scattering coefficient of the paper.The results showed that the charge density of TMC was higher than that of DMC and chitosan and the light scattering coefficient of TMC paper was lower under the same conditions.The higher charge density of TMC had the higher adsorption of negatively charged components in the pulp,thus the relative binding area of paper increased and then the wet and dry strength are greatly improved.Moreover,fibers can be gathered and retained in pulp due to the adsorption of TMC and fibers,therefore contributing to improve the retention of fiber and filler and the filtration performance of pulp.In addition,N-methylated chitosan was coated on the surface of the paper and the physical properties and printability of the coated paper were investigated.With the increase of coating weight,the dry and wet strength and smoothness of paper coated with DMC and TMC were improved,the air permeability was decreased,and the water vapor barrier and oil resistance of coated paper were significantly improved.When the coating weight was 1.0 g/m~2,DMC coated paper was more effective for blocking the passage of water vapor than TMC and chitosan coated paper.TMC coated paper had better grease resistance because TMC with the higher positive charge can adsorb with grease to avoid the penetration of grease.In addition,DMC and TMC coated paper had better printability than chitosan.When the dosage was 1.0 g/m~2,TMC coated paper had higher surface strength than DMC and chitosan,and the ink absorbency of DMC and TMC coated paper was better than that of chitosan coated paper.Permeability was more moderate,less prone to print through.The SEM and contact angle test showed that DMC and TMC were more likely to form uniform film on the surface of the paper with less coating weight,and the water contact angle of the paper was increased.As a result,the performance of the coated paper was greatly improved.Finally,the amphoteric chitosan derivative was prepared by oxidizing TMC with TEMPO/NaBr/NaClO system.The product,N,N,N-trimethyl-6-carboxy chitosan(TMCC),has been demonstrated by the FTIR and 1H NMR analyses.With the increase of the degree of oxidation,the thermal stability of TMCC decreased and the particle size of the product became smaller,and TMCC had better solubility in the larger pH range than TMC and chitosan.When the concentration is less than 2 mg/m L,TMCC can be dissolved in the pH range of 1-11.The effect of NaClO dosage on the oxidation degree was the most important factor.With the increase dosage of NaClO,the carboxyl content of the product increased rapidly.However,excessive NaClO would accelerate the degradation of the TMC main chain and decrease the yield of the product.When the dosage of NaClO was 2.7 mol/mol GLU,the degree of carboxylation and the yield of the product can achieve a better level.Because both positively charged quaternary ammonium groups and negatively charged carboxyl groups were onto the TMCC backbone,TMCC with the amphoteric structure could absorb substances with different charge in black liquor of wheat straw alkaline pulping.When the dosage was 20mg/L(to the volume of wasterwater),the COD removal rate and the transmittance can reach 65.5% and 85%,respectively,and it is better than that of using TMC or CPAM under the same conditions.