Preparation Of Rape Stalk Hybrid Nanocellulose And Its Application In Fresh-keeping Packaging Film

The rapid development of the commodity economy has led to an increase in packaging output,and the resulting plastic packaging waste has plagued the world with white pollution.In addition,China is the largest agricultural producer,generates a large amount of agricultural and livestock wastes every year,which has also caused tremendous pressure on the rural environment.As the main component of plant fiber,cellulose has the characteristics of renewable,degradable,high strength,high modulus,etc.It is an ideal raw material for the preparation of new environmentally friendly packaging materials.However,the hydrophilicity and poor weather resistance of cellulosic materials limit its application and development.In this study,cellulose nanocrystals（CNC）and cellulose nanofibers（CNF）were extracted from rapeseed straw,and then the hybrid nanocellulose/ethyl cellulose composite film（D-CF/EC composite film）was prepared by using ethyl cellulose（EC）as matrix and modified CNC and CNF as reinforcement phase.Furthermore,based on the D-CF/EC composite film,a silver-loaded hybrid nanocellulose/ethyl cellulose antibacterial composite film（Ag-D-CF/EC antibacterial composite film）and a pH color response intelligent packaging film were prepared successively.The comprehensive performance and mechanism of the composite film were systematically studied,and the specific research contents are as follows:（1）The composition and structure of rape stalk were detected and analyzed,and the cellulose of rape stalk was extracted.The results show that the crystalline structure of rape stalk cellulose is the strongest cellulose type I,and the content is as high as 50.86%,which is much higher than that of rice,wheat and other crop stalks.Furthermore,CNC and CNF were prepared by acid hydrolysis method and TEMPO oxidation-high pressure homogenization method respectively,and their structure and performance were characterized and analyzed.The prepared CNC and CNF both have a large aspect ratio.The CNC is rod-shaped,with a length of 100-500 nm,and a crystallinity of 87.64%.CNF is filamentous,the length is 100-2000nm,and the crystallinity is 68.92%.The thermal stability of CNC and CNF is slightly lower than that of cellulose,the T_onset is around 220°C,and the aqueous dispersions of both have higher stability.（2）The prepared CNC and CNF were mixed together,and the hybrid nanocellulose film was prepared by the vacuum filtration method,and its microstructure and comprehensive performance were characterized and analyzed.The results show that,compared with a single nanocellulose film,the CNC and CNF in the hybrid nanocellulose film are interlaced and complementary,the interwoven network structure is smaller and more balanced,and the film surface has a higher flatness.When the content of CNC and CNF is 1:1,the flatness of the film is the best,and the tensile strength can reach up to 109.72 MPa,which is 39.45%larger than that of a single CNF film.The increase of CNC content in the hybrid nanocellulose film helps to improve the crystallinity,thermal stability and light transmittance.The crystallinity of the pure CNC film is as high as93.05%,and the light transmittance is above 90%.（3）Hybrid nanocellulose film has high tensile strength,but poor toughness and water resistance limit its application range.With the help of dodecenyl succinic anhydride（DDSA）,the surface of CNC and CNF was hydrophobically modified to prepare D-CNC and D-CNF.Then,the hybrid nanocellulose/ethyl cellulose composite film（D-CF/EC composite film）was prepared by using EC as matrix,D-CNC and D-CNF as reinforcement phases,and epoxy soybean oil（ESO）as plasticizer.The structure and comprehensive performance of modified nanocellulose and D-CF/EC composite film were characterized and analyzed.The results show that the hydrophobic long carbon chain of DDSA replaced part of the hydroxyl groups on the surface of nanocellulose,so that D-CNC and D-CNF could be evenly dispersed in the composite film,and the light transmittance of the film could reach more than 85%.ESO has a good toughening effect on EC film,but will greatly reduce its mechanical and thermal properties.Hybrid nanocellulose has obvious enhancement effect.When the addition amount is 3%,the tensile strength of the composite film reaches the maximum value of 39.86 MPa,which is 48.23%higher than that of the composite film with only plasticizer.At the same time,the thermal performance has also picked up.The T_g is around 88℃,and the heat seal strength can reach 4.82 N/15mm.In addition,the addition of nanocellulose can accelerate the degradation rate of the composite film,and its degradation rate under ultraviolet light is much higher than that of soil burying.（4）Based on the prepared full degradation D-CF/EC composite film,the antibacterial function is given to achieve the effect of preservation.Silver nanoparticles（Ag NPs）have broad-spectrum bactericidal properties.Silver-loaded nanocellulose（Ag-D-CNC and Ag-D-CNF）were prepared by using green reducing agent glucose,which were then added into EC as strengthening and antibacterial components to construct a silver-loaded hybrid nanocellulose/ethyl cellulose antibacterial composite film（Ag-D-CF/EC antibacterial composite film）.The structure and antibacterial preservation performance of the silver-loaded nanocellulose and antibacterial composite film were characterized and analyzed,and the antibacterial composite film was applied to the packaging of cherry tomatoes.The results show that nanocellulose as a carrier and dispersant can make the particle size of Ag NPs smaller（about 10-50 nm）and dispersion is more uniform.Ag-D-CNC and Ag-D-CNF could cause Staphylococcus aureus and Escherichia coli to rupture and die.The prepared antibacterial composite film has obvious antibacterial effect on the above two kinds of bacteria,and has stronger antibacterial effect on Escherichia coli.The antibacterial composite film has a good preservation effect on cherry tomatoes.When refrigerated at 5℃,the preservation period can be extended by about 4 days.（5）On the basis of the prepared Ag-D-CF/EC antibacterial composite film,the natural pH indicator anthocyanin extracted from grape skin was coated on the surface to construct an intelligent packaging film with both pH color response and antibacterial functions.The indication and mechanism of anthocyanins to different pH acid-base solutions were analyzed.The structure of the smart packaging film and its responsiveness to acid-base solutions and volatile ammonia were characterized,and it was applied to the freshness detection of fresh meat.The results show that anthocyanins have four different chemical structures of Flavyium cation,quinoid base,methanol pseudobase,and chalcone in different acid-base environments,resulting in different colors.Intelligent packaging film has obvious indications for acid-base solutions of different pH.It turns red when it meets acid,and turns blue-green or yellow-green when it meets alkali.At the same time,it is also sensitive to volatile ammonia,and it will obviously turn blue-green within 2 minutes.Practical applications show that the intelligent packaging film has a relatively obvious freshness indicator effect on fish and pork.With the increase of the pH value of refrigerated fish and pork,the color of the film gradually changes from purple-red to blue,and finally to yellow-green.The results of this paper provide theoretical basis and practical guidance for the comprehensive utilization of agricultural and livestock waste and the development of new green intelligent packaging films,which will help reduce the generation of plastic packaging waste originally and promote the resource utilization of agricultural and livestock waste.