Application Research Of Copper And Silver In VMD Developing Latent Fingerprints On Plastic

Vacuum metal deposition(VMD)utilizing sequential Au and Zn depositions has been an effective technique to develop latent fingerprint on non-porous and semi-porous surfaces.However,conventional Au/Zn VMD has not been widely used in practice due to its high cost,cumbersome operation and the “empty prints” on some plastic.In order to explore a more costeffective alternative metal,simplify the process and further improve the identifiable quality of fingerprints,this thesis used copper and silver instead of gold,and selected 5-6 types of common plastic as objects to carry out the following research:The experiment optimized the condition by exploring the locations of fingerprint-carrying substrates and the amount of coating metal.It turned out that the best effects were found in the position between the evaporation boat and the middle part of VMD560.The optimal amount of different coating metals varied greatly,while the optimal amounts of the same coating metal on general plastics were relatively stable.However,it was worth noting that due to the special surface structure of PP woven bags,the optimal amounts of coating metal were greatly reduced.Then the effects of Cu,Ag,Cu/Zn,Ag/Zn,Au/Zn deposition were compared to determine the best coating metal on different kinds of plastic.The results showed that copper and silver could be used as effective supplements to gold.Within the 50-day fingerprint remaining time,Cu/Zn VMD was more effective than Au/Zn VMD on HDPE and PP woven bags;Ag VMD was more effective than Au/Zn VMD on LDPE,PET,PVC and black PE.Based on the best coating metal,the effects of VMD and “502” fuming were compared and the sequence of combining the two methods was investigated.The results showed that VMD behaved generally better than “502” fuming,and the longer remaining time,the more obvious the advantage of VMD;when “502” fuming used as the first technique with VMD,there was an obvious improvement of the quality of fingerprints on the PET,PVC and black PE,but there was little benefit and would be gradually deleterious to fingerprint enhancement on HDPE,LDPE and PP woven bags.When “502” fuming was used as a post-treatment after VMD,it would significantly reduce the quality of fingerprints on HDPE,LDPE,PET and PVC,but it was very effective at enhancing fingerprints on the black PE and PP woven bags.Considering the effect and operation process,the optimal fingerprints development solution could be achieved: using Cu/Zn VMD and Ag VMD on HDPE and LDPE respectively;using“502” fuming +Ag VMD on PET,PVC and black PE,and using Cu/Zn VMD+ “502” fuming on PP woven bags.Finally,the feasibility of the best alternative metal deposition to develop fingerprints on plastic exposed to water was explored.Compared with conventional Au/Zn VMD,it was found that the alternative metal on most plastics still has absolute advantages.However,the performances of Au/Zn VMD on PET and PVC were special,which showed a significant improvement with the increase of immersion time.When fingerprints were immersed for more than 2 days,the effect of Au/Zn VMD was even better than Ag VMD.Therefore,it is recommended to use Ag VMD for the PET and PVC immersed within 1 day and use Au/Zn VMD for the two types of plastics immersed for more than 2 days.The effects of optimized VMD and SPR were compared.The results showed that VMD performed better,and the longer remaining and soaking time,the more obvious the advantage of VMD.