| 摘要 |
Security device 10 has layers 12,15 of a copolymer which switches from opaque to transparent at a specified activation temperature. These layers comprise a hard segment and a crystallisable soft segment or at least one soft segment which is crosslinked and contains a crystallisable soft segment. The device may include demetallised indicia 14 in metal layer 13. The layers 12,15 are opaque at temperatures below the activation temperature (preferably 35-37 {C) and switch to become transparent when warmed above it, revealing the indicia 14. The polymers may be oligomers. Adhesive layers 16 bond the device as a thread 17 on to a substrate from which a secure document 11 is made. Alternatively, the device may overlay an optically variable feature (30, figure 13) which becomes visible through the polymeric film 12 above the activation temperature. |
