Preparation And Performance Research Of Anti-tarnish Packaging Films For Silver Products

Silver is widely used in various fields due to its excellent performance and strong decorative properties.Among them,the largest amount is used in the industrial field,especially in the electronics and electrical industries.Silver products,made of pure silver（or silver alloy）directly or electroplated pure silver（or silver alloy）on other substrates,are easy to interact with corrosive substances in the air（such as H₂S,etc.）and tarnish（corrosion）leading to changes in their performance or appearance,which affects their use.Therefore,it is very important to use suitable packaging materials to protect it during transportation and storage.At the same time,the package must have certain mechanical properties because the package is damaged easily by industrial silver products for its irregular shapes and protrusions when the package is subjected to vibration or shock during transportation.This paper uses active protective packaging as a means to prevent or slow down the tarnish of silver products.Various anti-tarnish packaging films with different copper powder contents and particle sizes were firstly prepared,and their properties,discoloration effect and protective mechanism were studied,finally an anti-tarnish packaging film for silver products with better anti-tarnish properties and mechanical properties was prepared by modifying with linear Low-density polyethylene（LLDPE）.The packaging film is not only suitable for the protection of industrial silver products but also for other types of silver products.Firstly,based on the existing H₂S corrosion test method,the test method of low concentration H₂S（3～4 ppm）was determined,and the high concentration of H₂S（13～14 ppm）test formula was determined considering the influence of reaction time,reactant concentration,and reaction temperature on the H₂S concentration,following by first dissolving 120 g Na₂S·9H₂O with 300 m L H₂O and 28 g KH₂PO₄ with 90 m L H₂O in 40℃,and then pour into the H₂S test device modified from a 240 mm vacuum desiccator and seal after mixing them evenly to prepare H₂S in 40℃.Secondly,the copper/low-density polyethylene（Cu/LDPE）packaging film was prepared by extrusion casting method,which the copper powder was modified with titanate coupling agent,while the the best formula,8 wt%copper powder with average particle size D50 1.381μm and 92 wt%LDPE,of Cu/LDPE packaging film with relatively better anti-tarnish performance and mechanical properties was finally determined by studying the influence of different copper powder contents and copper powders particle sizes on the thermal properties,barrier properties,mechanical properties,optical properties,anti-tarnish performance of the packaging films.The discoloration effect of Cu/LDPE packaging film was studied by means of chromatic aberration test,laser microscope and Raman spectroscopy,it shows that the Cu/LDPE packaging film prepared by copper powder with average particle size D50 1.381μm has a more obvious discoloration effect,and is more suitable as a"detector"of H₂S gas and an"indicator"of different H₂S concentrations or the end of film life.The protective mechanism of the packaging film was studied by energy dispersive spectroscopy（EDS）,X-ray diffractometer（XRD）,Raman spectroscopy,and X-ray photoelectron spectroscopy（XPS）,indicating Cu₂O and Cu S produced after reacting with corrosive substances such as H₂S,O₂,water vapor,thereby preventing these corrosive substances from continuing to enter the package and causing silver to tarnish.Finally,the result with the best modification effect was 20wt%LLDPE（118W）after using LLDPE with different melt indexes to modify the mechanical properties of the LDPE packaging film.Based on this result,the copper/linear low-density polyethylene/low-density polyethylene（Cu/LLDPE/LDPE）packaging film with good anti-tarnish performance and mechanical properties was prepared by using 20 wt%LLDPE（118W）to modify continually.The anti-tarnish packaging film formula and parameters were determined through various properties studies:8 wt%Cu with average particle size D50 1.381μm,18.4 wt%LLDPE（118W）,73.6 wt%LDPE.After 3 ppm H₂S 10d corrosion test and 14.5±0.5 ppm H₂S 10 d corrosion test,the chromatic aberrations of silver flake packaged by G2 are 0.78 and 2.13respectively,compared with 36.82 and 45.34 of silver flake packaged by G0,the chromatic aberrations values have been reduced by 98%and 95%;the tensile strength and tear strength of G2 are 29.7 Mpa and 75.8 k N/m respectively,which are increased by 18.8%and 3.4%compared with the pure LDPE packaging film G0（25 Mpa and 73.3 k N/m）.