Preparation And Properties Of Keratin/cellulose Nanocrystalline Composite Biofilms

With the progress of society,petroleum-based plastics have been widely used in people’s daily life,but the environmental pollution caused by their non-degradability is becoming more and more serious.It is an inevitable trend using bioplastics to replace petroleum-based plastics,among which the bioplastics derived from renewable resources are considered to be the most promising type.Chicken feathers are one kind of the waste materials produced by animal husbandry,which is produced in a large amount every year.The existing usual treatment methods of chicken feathers are landfill,incineration or composting,while efficient utilization method of chicken feathers is still lacking.The keratin content in chicken feathers accounts for more than 90%.It is of great potential and significance to extract keratin and prepare biofilm to replace petroleum-based polymer plastics being used in some occasions.Due to the brittleness and insufficient mechanical properties of pure keratin film,the application is limited,it is necessary to adopt some technical means to enhance its mechanical properties to meet the application requirements.Physical blending,chemical crosslinking,surface modification,plasticizing and other film strengthening methods have achieved certain progresses,but there is still a lot of research and optimization potential in the selection of reinforcing materials and modification methods.Cellulose is one of the most abundant polysaccharides in nature,widely distributed in wood fiber.Cellulose nanocrystals(CNC)are a form of cellulose.Its rigid structure and easy modification make cellulose nanocrystals become a potential film reinforcing material.The composite of cellulose nanociystals and keratin might effectively overcome some of the defects of keratin film.Meanwhile,the easily-achieved modification of cellulose nanocrystals also provides more possibilities for improving and expanding the function of biofilms.In this thesis,keratin and cellulose nanocrystals were extracted from chicken feathers and filter paper respectively.After the cellulose nanocrystals were modified with NaIO4,they were blended with keratin and glycerin.The biofilms with good tensile strength and adhesion properties were prepared by heating to induce the reaction between the aldehyde group of cellulose nanocystals and the amino group of keratin.Then,cellulose nanocrystals were modified by transesterification grafting,and then blended with keratin and citric acid to prepare film with excellent tensile properties and antibacterial properties.This thesis could provide method guidance and technical support for using natural biological materials to prepare functional packaging film.The first part is the extraction,modification and characterization of keratin and cellulose nanocrystals.Keratin was extracted from chicken feather with urea as dilatant and cysteine as reducing agent.The extraction rate functional groups,particle size and Zeta potential of the keratin were determined.The results showed that the reduction method selectively destroyed the disulphide bond in the keratin structure without destroying the peptide bond,so that the extracted keratin had a larger particle size and certain negative charge.Cellulose nanocrystals were extracted from filter paper by sulfuric acid hydrolysis method,and were modified with aldehyde group by sodium periodate.XPS analysis proved that the aldehyde groups were introduced onto cellulose nanocrystals surface,which provided necessary functional groups for subsequent crosslinking reaction.The changes in the structure and properties of cellulose nanocrystals before and after modification were also investigated.It was found that the cellulose nanocrystals were partially hydrolyzed dining the oxidation of sodium periodate,which further extended the polymer structure and enhanced the interaction between the polymers,leading to the increase of particle size and the decrease of crystallinity.The second part is the preparation of keratin/cellulose nanocrystals chemical crosslinking composite biofilm and the test of tensile adhesion.The cross-linked films were prepared by solvent construction method through the Schiff base reaction between the amino group of keratin and the aldehyde group of modified cellulose nanocrystals.Cellulose nanocrystals as physical reinforcement phase and chemical crosslinking phase improve the mechanical properties of composite films from two aspects.The effects of cellulose nanocrystals oxidation degree,material ratio and heat treatment time on the mechanical properties of crosslinked film were investigated.It was found that the increase of oxidation degree of cellulose nanocrystals and heat treatment time could both enhance the degree of Schiff base reaction and thus improve the mechanical properties of crosslinked film.At the same time,the adhesive properties of the films are tested,the film has good adhesion on a variety of substrate surfaces.Based on the chemical structure the adhesion mechanism was analyzed,it is believed that the adhesion properties of electrostatic interaction polarity interaction and hydrogen bond interaction all play important role.The crosslinked composite film could be dispersed or dissolved in water,so when it’s applied as disposable packaging or wrapping materials,post-processing and biodegradation could be facilitated,thereby can greatly reduce the harm to the environment.The third part is the preparation and performance of keratin/cellulose nanocrystals composite biofilms with excellent tensile and antibacterial properties.Firstly,the ternary keratin/cellulose nanocrystals/citric acid composite biofilms were prepared by using citric acid as a plasticizing and antibacterial bi-functional additive.The microstructure,thermal and mechanical properties of the films were characterized.It was found that the addition of citric acid changed the surface flatness of the films,improved the thermal stability of the films,and increased the elongation at break and shape recovery rate of the films.The interaction between the components of the complex system was also investigated.It was found that citric acid could change the secondary structure of keratin and increase the content of α-helix structure,thus significantly improving the tensile property of the film.Cellulose nanocrystals played an important role in maintaining the uniform dispersion of keratin macromolecules in the complex system and promoting the formation of hydrogen bond network structure of macromolecules through hydrogen bond interaction.Therefore,the prepared film is uniform and stable.Then part of citric acid was grafted onto cellulose nanocrystals by citric acid esterification method,and then the composite biofilm was prepared by ternary blending.It was found that the modified pretreatment method could further improve the tensile property and thermal stability of the film,and the molecular size of cellulose nanocrystals increased after esterification.Citric acid molecules act as crosslinking molecules to connect multiple cellulose nanocrystals,which not only promotes the formation of macromolecular network structure,but also enhances the slip ability of the macromolecular network,and significantly improves the tensile properties of the film.The generation of ester group is helpful to reduce the decomposition rate and amount of the film.The results of bacteriostatic zone test showed that the films prepared by the two strategies had certain bacteriostatic effect,and had potential application in food packaging preservation,antibacterial packaging and other related fields.In this thesis,the natural renewable biological wastes were used as raw materials to prepare functional biofilms.The application performances of the films such as tensile strength,elongation at break,water vapor transmittance,adhesion and antibacterial properties,were modulated by physical and chemical methods,which might provide method guidance and data support for the application of natural biomaterials in the field of functional packaging films.