Study On Controlling Pore Structure Of Nanofibrillated Cellulose Based Foam Materials

Development of biodegradable,resource-rich and recyclable foam materials has recently gained a strong interest due to the increasing environmental concern and depletion of petroleum.Recently,nanocellulose have been widely used as reinforcement materials in polymeric nanocomposites due to the environmental concerns.Nanocellulose based foam has been considered as one of the most promising biodegradable foams.Researching the effect of preparation process on the porous structure and mechanical properties of foams,which has significance for the development of nanocellulose-based foam material with different structure and application performance.In this work,nanofibrillated cellulose(NFC)based foam materials were prepared by using freeze-drying method,with a focus on controlling the pore structure of NFC based foam materials.The physical and chemical properties of industrialized NFC were characterized.Then,the pore structure of NFC based foam material was controlled by suspension concentration and freezing temperature.Results showed that the pore size of NFC foams decreased with increasing the suspension concentration from 1wt% to 10wt%,but the large pore size and dense structure can be found in the network of NFC foams.The density and pore size of the NFC foam can be improved by frozen with liquid nitrogen,which help to increase the mechanical property.Five types of alcohols were selected for controlling the size,shape and distribution of the pore in NFC foams.Results showed that the pore size and lamellar structure can be well controlled by addition of ethanol,isopropanol and n-butanol into NFC suspension.The use of methanol can decrease the pore size but the unevenly distribution can be observed.Among the alcohols used in this work,addition of tert-butanol resulted in the minimum pore size with 8.65?m,but the pore shape can not be controlled effectively.Compared to the NFC foams without addition of alcohols,the density and shrinkage rate of all foams became smaller after alcohols addition,and the porosity increased after alcohols addition,but the thermal conductivity and thermal stability were not affected.Based on the pore size,distribution and shape,ethanol was the best candidate for controlling the pore structure of NFC foams.Based on the above results,the optimization of the pore structure and mechanical properties of NFC foam by ethanol addition was conducted in terms of concentration,freezing temperature and suspension concentration.Results showed that the circle pore can be obtained by addition of ethanol,the pore distribution of NFC foams became uniform with increasing the ethanol concentration up to 10wt%,but the pore size was increased.Decreasing freezing temperature and increasing suspension concentration resulted in regular layered pores structure,and some columnar structure between the layers can be observed.When the freezing temperature of NFC suspension was lower than the freezing point of ethanol,or the suspension concentration increased up to 10wt%,the layered structure of NFC foams would disappeared,and the pore size was decreased.The optimal process conditions were as follows: ethanol concentration 5wt%,suspension concentration 3wt%,freezing temperature-55?.Under this conditions,the mean pore area,mean pore wall thickness and pore density were 3231.44?m2,2.46?m and 4.69×106/cm3,respectively.Finally,the effects of amylopectin content,freezing temperature and solid content(amylopectin and NFC)on the pore structure and mechanical properties of amylopectin/NFC composite foams were investigated.With increasing amylopectin content,the pore distribution of composite foams became uniform,and the pore size was increased.The pore shape gradually turned into circle,and the pore wall of the foams were generated with the addition of amylopectin;while their pore size became smaller than contrast sample and the pores were distributed unevenly when amylopectin content exceeded to 70%.The increase of solid content led to unevenly pore distribution,irregular pore shape,decreased pore size.In aspect of temperature effect,the pores were distributed more uniformly,but the pore size was decreased when the composite foams were frozen by liquid nitrogen.The mechanical properties of composite foams were reinforced by adding gelatinized amylopectin into NFC suspension.The optimal process conditions were as follows: amylopectin content,solid content and freezing temperature were 30%,3wt% and-55?,respectively.The mean pore area,mean pore wall thickness and pore density of composite foams were 195.99?m2,1.04?m and 4.68 ×108/cm3,respectively.The Young's modulus,yield strength and energy adsorption were 713.14 KPa,92.28 KPa and 13.24 kJ/m3,respectively.Biodegradable NFC-based composite foams with low density,high porosity,and high mechanical strength were successfully prepared.Its pore structure and mechanical properties can be controlled efficiently by controlling the suspension concentration and freezing temperature,as well as the addition of alcohols and amylopectin.The preparation process is environmental friendly and the product is biodegradable,the results indicating excellent potential to use NFC-based composite foams as cushion packaging material and structural insulation material.