Study On The Characteristics Of Metal Ion Release During Photo-aging Of Typical Plastics And Its Mechanism

Light is considered to be the most important environmental factor in plastic aging,and the accompanying process of plastic photoaging leads to the release of various chemical additives.The current research around the photo-aging characteristics of plastics mainly focuses on the changes of physical and chemical properties,and around the release of endogenous chemical additives during the photo-aging of plastics mainly focuses on organic additives.However,there is a lack of in-depth understanding of the relationship between changes in the molecular structure and physicochemical properties of plastics themselves and changes in the microstructure of plastics during photoaging,and the understanding of the release of intrinsically loaded metal additives in plastics is not comprehensive enough,especially there are few studies on the release characteristics of endogenous additives（metal ions）and their mechanisms in plastic materials of different colors and molecular structures during photoaging.Based on this,this dissertation selects two types of transparent and black colored and widely used typical plastics: hydrocarbon polypropylene（PP）and polyvinyl chloride（PVC）,and aromatic polycarbonate（PC）and polyethylene terephthalate（PET）as research objects.We investigate the aging characteristics of typical plastics under UV irradiation,explore the relationship between the changes of the microstructure of the molecular structure and physicochemical properties of plastics and the photoaging time,analyze the photoaging mechanism,and elute the plastics with different aging degrees to study the characteristics and mechanisms of metal ion release during the photoaging of plastics.The main results of this paper are as follows:（1）The formation of new functional groups（carbonyl and hydroxyl groups）in hydrocarbon polymers after photoaging,but not in aromatic polymers,suggesting that aromatic polymers are more stable to photoaging than hydrocarbon polymers.The main chains of both the original hydrocarbon and aromatic polymers were broken,and with the prolonged photoaging time,more oxygen-containing functional groups were generated in the hydrocarbon polymers,but not in the aromatic polymers,indicating that the aromatic polymers were less susceptible to UV irradiation and more stable in nature.The overall decrease in crystallinity of the major crystalline peaks in both hydrocarbon and aromatic polymers with increasing aging time.The surface morphology of hydrocarbon compounds during photoaging is characterized by holes,cracks,folds and pits,while that of aromatic polymers is characterized by caking,folds and flaking.The relationship between aging degree and aging time of hydrocarbon and aromatic polymer surfaces was modeled,showing that their photo-aging mechanisms are: carbonyl index（CI）and oxygen content（O/C）of both hydrocarbon and aromatic polymers show positive correlation with photo-aging time,and the aging degree follows the pattern of transparent PP> transparent PVC> transparent PVC>black PP> transparent PC>black PC>black PET> transparent PET.The increased oxygen content in hydrocarbon and aromatic polymers suggests that plastics can be photoaged through the absorption of UV light by groups,and there are some differences in the photo-oxidation rates and mechanisms for plastics of different colors and different molecular structures.（2）The analysis of the total concentration of metal ions in virgin plastics by ICP-MS showed that 13 metals（Cr,Mn,Fe,Co,Ni,Cu,Zn,Cd,Sn,Sb,Ba,Hg,and Pb）were detected in almost all virgin hydrocarbons and aromatic polymers.The metal additives in both hydrocarbon and aromatic polymers have a continuous tendency to be released as the photoaging process proceeds,and the release of Fe,Cu,Cd and Pb metals all show a steady increase.The order of metal release rate in polymers of two color types,transparent and black,is: transparent PP > black PVC > black PP > transparent PVC > transparent PC >black PC > black PET > transparent PET,indicating that there are great differences in metal ion release during photo-aging of plastics of different colors and different molecular structures,and the law is similar to the degree of aging of plastics.The order of magnitude of metal ion release in hydrocarbon polymers is roughly Fe> Zn> Cr> Cu> Mn> Pb> Cd,while the order of magnitude of metal ion release in aromatic polymers is roughly Fe> Cr> Zn> Cu>Mn> Pb> Cd,with the exception of Cd,Mn,and Cu,the order roughly satisfies the IrvingWilliams metal complex stability sequence and the distribution of metals in plastics as well as the intrinsic loading content.By fitting nonlinear kinetic equations to the release process of metal ions during photoaging of hydrocarbon and aromatic polymers,it is shown that the release kinetics are more in line with the secondary kinetic model;the mechanism of the whole release process is further found to be determined by both the surface diffusion rate and the intraparticle diffusion rate through the intraparticle diffusion model.