Preparation And Properties Of Porous Microfibrillated Cellulose(MFC) Films

Nano-cellulose is a kind of natural polymer material which is biodegradable and renewable.Microfibrillated cellulose(MFC)with high specific surface area and a certain aspect ratio can be used to prepare films with unique pore structure and mechanical properties.This porous MFC films have broad application prospects in the fields of conductive energy storage,separation and adsorption.In this paper,the scraping and coating process of MFC films was explored and optimized firstly.On this basis,the effects of the degree of carboxyethylation pretreatment and mechanical grinding as well as the types of raw materials on the properties of the obtained MFC and MFC films were studied in depth.The optimum initial raw material was determined to be bleached bamboo pulp.Finally,the effects of organic solvents,drying methods and cationic additives on the pore structure and mechanical properties of MFC films were systematically studied.It provides theoretical and technical support for the preparation and application of porous microfibrillated cellulose films.The detailed research contents are as follows:1.The feasibility of preparing MFC films by scraping and coating technology was discussed.The technology includes defoaming,scraping,pre-drying and hot pressing drying.The effects of initial solid content of MFC suspension,coating substrate and film thickness on the properties of MFC films were studied,and the optimum preparation process was determined.The results showed that the appropriate initial solid content of MFC suspension is 1%~2%.Within this concentration range,not only the scraping process can be ensured smoothly,but also the drying process can be accelerated and the film-making efficiency can be improved.The aramid paper was selected as the suitable coating substrate through experiments.In addition,by analyzing the relationship between the thickness of MFC film and its mechanical properties,it is concluded that the suitable thickness range of MFC film is 35~70 ?m.2.MFC was prepared by carboxyethylation pretreatment combinied with mechanical grinding.By changing pretreatment conditions and grinding degree,a series of MFCs with different carboxyl content and grinding degree were obtained.The properties of MFC and the basic physical properties,mechanical properties and pore structure of the MFC films were measured.Meanwhile,the effects of six different raw materials,namely bamboo pulp,hardwood pulp,softwood pulp,chemical-mechanical pulp,cotton short pulp and corn linter cellulose on the MFC and film properties were compared and analyzed.The results showed that when the carboxyl content of the raw material is about 1 mmol/g,the water retention value of MFC is moderate,the slurry will not separate solid from liquid,and has good fluidity in the coating process,which can ensure the smooth process of scraping and coating,and the tensile strength of the obtained MFC film is high.In addition,with the intensification of the grinding degree of raw materials after pretreatment,the degree of fibrillation and porosity of the MFC obtained increased,and the strength of the prepared MFC films increased first and then decreased.In order to take into account the strength and porosity of the films,the optimum grinding conditions are rough grinding for 5 times and fine grinding for 9 times.Under these conditions,MFC prepared from bleached bamboo kraft pulp has good microfibrillation and the obtained film has the best comprehensive performance.3.In order to achieve the controllable regulation of the pore structure of MFC films,the effects of organic solvents,different drying methods and cationic polyelectrolytes on the mechanical properties and pore structure of MFC films were studied in detail.The results showed that when the water in the MFC suspension is partly or completely replaced by ethanol,the porosity and specific surface area of the membrane increase,and the median pore size increases.When organic solvents such as ethanol,isopropanol and n-butanol were used to replace the water in MFC wet film,the pore-forming effect of the dry MFC wet film could be achieved,but the strength of the film would also decrease significantly.The thickness,tightness,transparency,tensile strength and pore structure of MFC films were mainly affected by the drying methods.The MFC films obtained by dring at room temperature or under vacuum thermal pressure had high tensile strength but low porosity.In contrast,the film obtained by freeze-drying has high porosity but low strength.Besides,the addition of cationic polyelectrolytes in MFC suspension can significantly increase the pore structure of the films,and the most suitable addition is a small amount of cationic polyelectrolytes with high charge density and high molecular weight.Adding AKD(alkylketene dimer)to MFC suspension can increase the hydrophobicity and double the porosity of the films.