Study On Preparation Of Microcrystalline Cellulose By Permeation-assisted Pretreatment And Particle Size Control Technology

Microcrystalline cellulose(MCC)is an important cellulose-based functional material,which is widely used in medicine,food,cosmetics,leather,3D printing and other industrial fields.The preparation of conventional MCC usually uses inorganic acid to hydrolyze cellulose fibers directly,destroying the relatively loose amorphous zone in the aggregation structure and retaining the crystalline zone,so as to obtain rod-like MCC products with high crystallinity and a certain length.However,the crystalline and amorphous regions in the aggregated structure of cellulose are arranged alternately and wrapped each other,and the fine fibers in the biological structure of plant fibers are closely intertwined.These two structural characteristics will lead to result that the acidic solution has low accessibility and poor selectivity to the amorphous region of cellulose,and slow acid penetration,acid hydrolysis average rate.Eventually MCC also show large size and poor homogeneity.In order to solve the impact of these problems,this thesis is to pretreat softwood pulp chemically or mechanically before acid hydrolysis,which is to improve the permeability efficiency of acid solution and the reactivity of cellulose,and to realize the control of MCC particle size.The specific research contents of this thesis are as follows:First of all,NaOH solution was used to pretreat cellulose fibers,and the research explored the effect of alkali pretreatment on the structure of cellulose fibers and the quality of MCC.The results showed that with the increase of NaOH concentration,the water retention value of cellulose fiber increased,the crystallinity increased first and then decreased,the crystallinity of MCC did not change much,and crystal structure of cellulose transformed when NaOH concentration was more than 9%,and some cellulose I transformed into cellulose II.The average particle size of MCC decreased first and then increased.With the decrease of alkali pretreatment temperature,the water retention value of cellulose fibers increased,and the crystallinity decreased,the crystallinity of MCC had little effect.After alkali treatment at 10 °C,crystal structure transformed and some cellulose I would be transformed into cellulose II.The average particle size of MCC decreased first and then increased.The reaction time and solid-liquid ratio had little effect on the structure of cellulose fibers and the quality of MCC.The optimum conditions of alkali pretreatment were as follows: 9% NaOH,25 °C of reaction temperature,60 min of reaction time and 1:20 of solid-liquid ratio.The average particle size of MCC after alkali pretreatment decreased from 52.116 ?m to 41.831 ?m,the uniformity of particle size increased,and the crystallinity of MCC remained basically unchanged.The crystal structure of cellulose remained unchanged after pretreatment,and it was still natural cellulose I.Then,two different mechanical methods,PFI refining and Valley beating,were used to pretreat cellulose fibers.The effects of different mechanical methods on the structure of cellulose fibers and the quality of MCC were explored,and the differences between the two methods were analyzed.The results showed that with the increase of beating degree of two mechanical methods,the water retention value of cellulose fiber increased,the crystallinity of cellulose decreased,and the length of cellulose fiber shortened.The average particle size of PFI-MCC decreases first and then increases,but the crystallinity of PFI-MCC does not change much.The average particle size of Valley-MCC decreases gradually and its crystallinity decreases slightly,but it also meets the requirements of high crystallinity of MCC.By analyzing the difference between the two beating methods,it was found that PFI refining had stronger effect on the fibrillation of fibers,Valley beating had stronger cutting effect on fibers,so the length of fibers decreased greatly.Valley-MCC has better homogeneity and more concentrated size distribution than PFI-MCC.Finally,the reaserach found that Valley beating is more suitable for preparing MCC by comprehensive comparison.Comprehensive preparation process of Valley beating and acid hydrolysis was optimized by response surface methodology.The effects of hydrochloric acid concentration,reaction temperature,reaction time and beating degree on the quality of MCC were investigated.The results showed that the order of significant effects of various factors on the crystallinity of MCC was reaction temperature > acid concentration > reaction time > beating degree.The optimum acid hydrolysis process were 2.5 mol/L acid concentration,80 °C of reaction temperature,45 minutes of reaction time and 25 °SR of beating degree.On the premise that the size and crystallinity of MCC are basically the same,compared with the untreated acid hydrolysis process,the Valley beating acid hydrolysis process reduces the amount of acid solution by 16.67%,the reaction time by 35.71%,and the uniformity of MCC particles is better.In summary,Valley beating is a suitable pretreatment method for preparing MCC,which improves the penetration efficiency of acid and the accessibility of acid to cellulose,and realizes the control of MCC particle size.Besides,acid hydrolysis of Valley beating reduces the amount of acid,shortens the reaction time,and reduces the production cost.