| Electromagnetic waves not only cause interference and damage to electronic instruments and equipment,but also endanger human health.Flexible electromagnetic shielding materials exhibit excellent foldability and adhesion,and have broad application prospects in fields such as radiation protection suits and portable military communication equipments.Building high-strength flexible electromagnetic shielding materials is one of the research focuses in this field.Leather has advantages such as flexibility,good hygiene,and biodegradability.It contains abundant carboxyl and amino groups,which can act as electric dipoles and undergo relaxation,thereby converting electromagnetic waves into heat energy.At the same time,its multi-layer structure will cause multiple diffuse reflections,resulting in electromagnetic wave losses.In addition,the wastewater and sludge containing chromium generated by the chrome tanning method in leather manufacture pose a risk of harm to human health and the ecological environment.Therefore,development of chrome-free tanning is imperative.Designing material modified leather with both chromium free tanning and electromagnetic shielding properties is expected to reduce the modification steps of green chromium free tanning electromagnetic shielding leather and improve the bonding strength between the material and leather.Based on the above background,this study proposed to design and synthesize a polyionic liquid/MOF-LDH/Zr composite material(PMLZ)with both tanning performance and high conductivity.Then,the PMLZ was used to the leather tanning process.By utilizing the-COOH and metal ion defects in the PMLZ composite material and the interaction between-NH2 and-COOH of collagen fibers,multi-point cross-linking was formed to improve the wet heat stability of the leather.By utilizing the π-π conjugated structure of poly imidazole groups and the cavity structure of MOF-LDH,the charge transfer efficiency in the conductive network was increased,endowing the leather with excellent electromagnetic shielding performance.This study provides new ideas and methods for the development of green and flexible electromagnetic shielding materials.The specific research content as following.(1)ZnAl-MOF-LDH,ZnZr-MOF-LDH,and ZnFe-MOF-LDH with both porous cavity and layered structure were synthesized using template guided method.The effects of reaction time,system pH,and nitrate dosage on the morphology of MOF-LDH were investigated.The results showed that when the reaction time was 5 h,pH was 13,and the amount of Al(NO3)3·9H2O was 0.6 g,the morphology of ZnAl-MOF-LDH was the best,with a regular dodecahedral structure and a size of 100-150 nm,obvious protrusions on the surface.When the reaction time was 3 h,pH was 13,and the amount of Zn(NO3)2·6H2O was 0.6 g,the morphology of ZnZr-MOF-LDH was the best,with a small spherical structure and a size of 100-150 nm,obvious protrusions on the surface.When the reaction time was 5 h,pH was 13,and the amount of Zn(NO3)2·6H2O was 1.2 g,the morphology of ZnFe-MOF-LDH was the best,which was a regular octahedral structure with a size of 300-500 nm and obvious protrusions on the surface.Applying it to the tanning process of leather,the effects of different types of MOF-LDH on the tanning,dyeing,fatliquoring,and electromagnetic shielding properties of the leather were investigated.The results showed that the ZnZr-MOF-LDH tanned leather had the best tanning performance,with a slrinkage temperature increase of 11.9℃.The leather tanned by ZnAl-MOF-LDH exhibited the best charge transfer performance,with an equivalent series resistance value as low as 4 Ω.Meanwhile,the leather tanned by ZnAl-MOF-LDH had the best dyeing and fatliquoring performance,with a fatliquor absorption rate of 82%and a dye absorption rate of 75%.(2)Polyionic liquids(PILs)containing carboxyl and imidazole groups were synthesized by using 1-allyl-3-methylimidazole chloride([AMIM]Cl)and methaciylic acid(MAA)as raw materials based on free radical polymerization.The effects of solid content and[AMIM]Cl dosage on the conversion rate and conductivity of PILs were investigated.The results showed that when the solid content was 40%and the amount of[AMIM]Cl was 40%,the conversion rate of PILs could reach 94%,and the conductivity could reach 38.4 ms/cm.The structure of PILs was characterized by fourier transform infrared spectroscopy(FT-IR)and nuclear magnetic resonance hydrogen spectroscopy(1H NMR).The results showed that PILs containing carboxyl and imidazole groups were successfully prepared.The application of PILs in leather tanning process was investigated to inspect the effect of PILs dosage on leather tanning performance and electromagnetic shielding performance.The results showed that when the amount of PILs was 6%,the tanning performance and electromagnetic shielding performance of leather were the best.The shrinkage temperature of the tanned leather could be increased by 16.3℃,and the total electromagnetic shielding efficiency was 9.2 dB.The tear strength of the tanned leather for PILs was 30.1 N/mm,and the tensile strength was 15.1 MPa.(3)By introducing MOF-LDH into the polyionic liquid polymerization system,PILs/MOF-LDH were synthesized through in-situ polymerization.And further compounded it with zirconium sulfate to prepare PILs/MOF-LDH/Zr composite materials(PMLZ).The effect of MOF-LDH dosage on the conversion rate and conductivity of composite materials was investigated.The results showed that when the amount of MOF-LDH was 1.5%,the conversion rate of PMLZ composite material could reach 87%,and the conductivity could reach 96 ms/cm.The structure of PMLZ composite material was characterized by FT-IR and transmission electron microscopy(TEM).The results indicated that carboxyl,imidazole,and metal ions coexist in the PMLZ composite material,and MOF-LDH was uniformly distributed in the PMLZ composite material.It was applied to the leather tanning process and the effects of penetration pH and binding pH on the tanning performance of leather were studied.The results showed that when the penetration pH was 2.0 and the binding pH was 4.0,the leather tanning performance was optimal,with a shrinkage temperature of 96.2℃ and a thickening rate of 65%.The tanning performance,electromagnetic shielding performance,and dyeing and fatliquoring performance of MOF-LDH,PILs,and PMLZ composite materials tanned leather were compared.The results showed that the PMLZ composite material had the best performance,with a shrinkage temperature of 96℃;the total electromagnetic shielding efficiency could reach 14.5 dB,and it was mainly an absorption based electromagnetic shielding,the dye absorption rate could reach 91%,and the fatliquor absorption rate could reach 87%.(4)Polyaniline(PANI)and polypyrrole(PPy)were grown in situ on the surface of PMLZ tanned collagen fibers,respectively,to obtain PANI/PMLZ tanned leather(PA/CF)and PPy/PMLZ tanned leather(Py/CF).The results of scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS)showed that PANI was in the shape of a mulberry,while PPy was in the shape of a small ball,they all encapsulated on the surface of skin collagen fibers.The porosity results indicated that the porosity of PANI grown in situ on the surface of PMLZ composite material tanned collagen fibers(PA/CF)was 55.91%,while the porosity of PPy grown in situ on the surface of PMLZ composite material tanned collagen fibers(Py/CF)was 65.46%.The electromagnetic shielding results showed that Py/CF had the best electromagnetic shielding performance,with a total electromagnetic shielding efficiency of 58.19 dB.After 1000 times twist and turn or 3000 times friction,its total electromagnetic shielding efficiency could still maintain 64%.The results of permeability and water vapor permeability showed that compared to PA/CF,Py/CF had better permeability and water vapor permeability,with a permeability of 40.98 mL/(cm2 h)and a water vapor permeability of 740.68 mg/(10 cm2·24 h).The mechanical performance results indicated that compared to PA/CF,Py/CF had better mechanical properties,with a tensile strength of up to 21.0 MPa.(5)Taking MOF-LDH,PILs,PMLZ composite material,PANI,and PPy as research objects,the interaction among them with skin powder and collagen was investigated to inspect their interaction with skin collagen fibers and their impact on the multi-level structure of skin collagen fibers.A tanning and electromagnetic shielding mechanism model was established.The results showed that the carboxyl groups in the long chain of PMLZ composite material molecules mainly bind to amino groups in collagen in the form of electrovalent bonds,metal defects mainly bind to carboxyl groups in collagen through electrovalent and coordination bonds,the amino groups on PANI and PPy mainly bind to the carboxyl groups in collagen through electrovalent and hydrogen bonds,and PPy forming more hydrogen bonds with collagen than PANI.The results of FT-IR,SEM,and TEM indicated that the introduction of MOF-LDH,PILs,and PMLZ composite materials did not damage the triple-helical structure of collagen and had a good dispersion effect on collagen.PANI and PPy acted on collagen fibers or fiber gaps,and PPy was more distributed on collagen fibers.The results of electromagnetic shielding effectiveness indicated that the dipole of collagen fibers,the multi network cross-linking structure of leather ensured by PMLZ composite material,and the conductivity of PANI and PPy themselves were the determining factors for the electromagnetic shielding performance of chrome free tanned electromagnetic shielding leather. |
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