Structure And Performance Regulation Of Gelatin-Based Multi-Cross-Linked Biodegradable Film

Plastic film is ubiquitous in industrial production and daily life,and plays an indispensable role in various fields.The wide use of petroleum-based nonbiodegradable plastic film has caused serious threats to the ecological environment and human health.Developing and applying biodegradable plastic film as substitute for petroleum-based non-biodegradable plastic film is an effective way to solve the problems caused by non-renewable petroleum-based plastic film.Biodegradable films prepared from petroleum are always difficult to meet the requirements as biodegradable plastic film substitutes for the petroleum-based non-biodegradable plastic film due to their non-renewable characteristic.Gelatin extracted from leather and food processing waste is biodegradable,biocompatible,and readily available,which makes it a great raw material for biodegradable plastic film with low cost.However,gelatin-based films are often brittle and have poor water resistance,which restricted their application as a substitute for petroleumbased plastic films.In this thesis,different types of bonding are introduced as multi-cross-linking into gelatin matrix to improve the strength and toughness of gelatin-based film.Fabricated gelatin-based films can replace petroleum-based non-biodegradable plastic films in various application scenarios.The effects of different bonds composed multi-cross-links on the structure and properties of the films are studied,and the application performances of the gelatin-based films are optimized and studied according to the corresponding application scenarios.In this thesis,the hydrophobic surface of gelatin film is also constructed to improve the water resistance of gelatin-based film through the repulsion between long chain alkanes of oil and gelatin molecular chain.The detailed research contents including:（1）According to the application requirements of mulch film,gelatin extracted from leather waste is used as raw material.The covalent bonds between gelatin chains and crystalline domains are introduced by adding tetramethyl phosphorous chloride（THPC）and polyvinyl alcohol（PVA）,respectively.The inter and intra hydrogen bonds are also formation between the molecular chains of PVA and gelatin.All these interactions are combined to form a multi-cross-linked gelatinbased film（TCG/PVA）.The covalent cross-linking and crystalline domains introduced by THPC and PVA are confirmed by Fourier transform infrared spectroscopy（FTIR）and X-ray diffraction（XRD）.The cyclic tensile test determines the energy dissipation effect of multi bonds in gelatin matrix.The covalent bonds,crystalline domains,and hydrogen bonds in TCG/PVA act as sacrificial bonds for energy dissipation when the film is subjected to external force,giving TCG/PVA high strength and toughness:the maximum fracture stress（σ_b）and fracture strain（ε_b）are～31.99 MPa and～3.79,respectively.Such mechanical properties conform the requires of Chinese standard GB/T 35795-2017（Biodegradable mulching film for agricultural uses）,and are comparable to the traditional low-density polyethylene film.TCG/PVA also has excellent light transmittance（the maximum transmittance is 92.90%）and biodegradability.The practical application test shows that TCG/PVA can be used as mulch film and gradually biodegrade after crops sprout.Thus,it is suitable for use as mulch film.（2）In order to meet the application requirements of food packaging film,the gelatin extracted from waste hides of food processing is used as raw material,and the THPC（to cross-link gelatin）is replaced by the non-toxic,antibacterial,and antioxidant dialdehyde oligosaccharide（DOC）.The covalent cross-links between the molecular chains of gelatin,the crystalline domains introduced by PVA,and the hydrogen bonds between all the molecular are combined to form a multi-crosslinked gelatin-based film（GPD）.The covalent bonds and crystalline domains in the fabricated GPD are confirmed by FTIR,XRD,and polarizing microscope.The energy dissipation of multi bonds in the gelatin-based film is confirmed by cyclic tensile test.GPD shows high mechanical strength with a highest σ_b of～37.87 MPa.DOC can significantly improve the antioxidant and antibacterial properties of the film,and provide the film with inhibitory ability against the foodbome pathogenic bacteria Escherichia coli（CMCC 44103）and Staphylococcus aureus（ATCC 25923）.The covalent cross-linking between the gelatin molecular chains can effectively inhibit the swelling of GPD and improve the moisture resistance stability.The research shows that the mechanical strength of GPD is better than that of traditional polyethylene,polyvinyl chloride,and commercial biodegradable cellulose-based food packaging film.During stretching,the stretching induced orientation structure can effectively prevent the film from failure under repeated external forces.The practical application evaluation shows that GPD has the same preservative ability as the traditional polyethylene preservative film.After being discarded,it can completely biodegrade in the natural soil after 10 weeks,and is suitable for use as a food packaging film.（3）In order to further improve the moisture resistance stability of the gelatinbased film,the multi-cross-linked gelatin-based film（GC-Zr）containing covalent bonds,metal coordination bonds,and hydrogen bonds is fabricated using dialdehyde carboxymethyl cellulose（DCMC）and Zr⁴⁺.The formation of covalent cross-linking and metal coordination bonds between DCMC and gelatin formed are confirmed using FTIR,XRD,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）,and energy dispersive spectroscopy（EDS）.Meanwhile,these bonds uniformly distribute in the gelatin matrix.Multi-crosslinking endows GC-Zr a maximum σ_b of～32.15 MPa,which is better than traditional polyethylene and PVC packaging film.The mechanical properties of the film can be controlled by adjusting the content of Zr⁴⁺.Due to the stability of covalent bonds and metal coordination bonds in the water environment,GC-Zr has strong moisture resistance stability.GC-Zr-6 samples（with a radius of 3 cm and a thickness of 0.129 mm）can still support 200 g of weight after being placed in the watery environment for 24 h.In addition,GC-Zr has good light transmittance（the maximum transmittance is 91.98%）.Due to the dynamic nature of metal coordination bonds,GC-Zr also has excellent weld ability and aging resistance.After welding,σ_b and E of GC-Zr-6 are 13.89±0.56 MP and 26.46±1.29 MPa,respectively.After placed one year,σ_b and E of GC-Zr-6 are 19.05±0.93 MPa and 60.76±2.67 MPa,respectively.It can completely biodegrade when discarded in natural environment after 7 weeks and is suitable for use as packaging film.（4）Although introducing a variety of bonds into the gelatin matrix can improve the moisture resistance stability of the gelatin-based film,whereas the improvement is limited due to the high hydrophilicity of gelatin.Constructing hydrophobic surface on the gelatin-based film can prevent water molecules from entering into the film substrate effectively,and improve the moisture resistance stability of the film effectively.In this way,a surface water-repellent gelatin-based film（TCG/ODA）was prepared by using the mutual repulsion of octadecylamine（ODA）long-chain alkane and gelatin chains,ODA was further crosslinked with gelatin by THPC to prevent the phase separation and improve the durability of the ODA on the film surface.The results shows that the cross-linking effect of THPC can significantly improve the compatibility of ODA and gelatin,the mechanical strength of the film,and the durability of the ODA hydrophobic layer on the film surface.In addition,there is no phase separation in TCG/ODA with low proportion of ODA,and the TCG/ODA appears the highest σ_b of～26.37 MPa.TCG/ODA has good surface water repellency and can maintain high mechanical strength with the maximum σ_b of～12.76 MPa even after water washing.The mechanical properties of TCG/ODA are adjustable,the mechanical properties of the film can be adjusted by adjusting the ratio of gelatin to ODA.TCG/ODA also has good light transmittance（the maximum transmittance is 90.57%）and can be completely biodegrade in natural soil after being discarded for 8 weeks.（5）Combining the low surface energy water-repellent surface design and the dynamic reversibility of metal coordination bonds,the sulfited fish oil（FOS）and sodium lignosulfonate（LS）that can form metal coordination bonds were introduced into the matrix from gelatin extracted from leather processing waste.The water-repellent and recyclable gelatin-based film(GL/LS/FOS/Fe³⁺)crosslinked by metal coordination bonds was constructed by introducing Fe³⁺,as Fe³⁺can form multi metal coordination bonds between gelatin,LS,and FOS to fix LS and FOS,and increase the cross-linking density of gelatin matrix.FOS migrates to the surface of GL/LS/FOS/Fe³⁺ under the repulsion of gelatin chain.Due to the metal coordination effect of Fe³⁺,FOS and gelatin matrix had good compatibility,and the film structure is uniform without phase separation.Due to the metal coordination effect of Fe³⁺,the water-repellent surface formed by FOS has a longterm effect,and it still exists firmly on the surface of GL/LS/FOS/Fe³⁺ even after water washing.GL/LS/FOS/Fe³⁺ has excellent water resistance and recyclability,and can still maintain high mechanical strength with σ_b of～18.98 MPa after water washing.Due to the recoverability of metal coordination bonds and hydrogen bonds,the mechanical properties and surface water repellency of GL/LS/FOS/Fe³⁺are equivalent to the initial state after three recycling cycles,giving a maximum σ_b of～36.06 MPa.GL/LS/FOS/Fe³⁺also has good welding property,which can be encapsulated by hot pressing.The welding matrix is uniform and no phase separation occurs.After welding,the σ_b and E are～15.92 MPa and～141.81 MPa,respectively.GL/LS/FOS/Fe³⁺can completely block UV light（200-400 nm）.After discarding,it can completely biodegrade in natural soil for 9 weeks,making it suitable for express packaging film.In this thesis,a series of multi-cross-linked gelatin-based films are prepared based on gelatin extracted from leather and food waste,and the structure and properties of different cross-linked gelatin-based films are systematically studied,and the energy dissipation effect of multi-cross-linking in gelatin matrix is also clarified.At the same time,the hydrophobic surface of the gelatin-based film with certain durability is also constructed by the repulsion of oil alkane and gelatin chain combined with the fixation of covalent bond and metal coordination bond,which improves the water resistance of the gelatin-based film and expands the application field of the gelatin-based film.The systematic study of different gelatin-based films in this thesis provides a theoretical and technical foundation for the development and application of gelatin-based biodegradable plastic films for specific applications.