| 摘要 |
A wrapping sheet material (2b) comprises superincumbent layers of heat-activatable adhesive (3b), field responsive matter (20), and a cover film (10b). The film (10b) comprises 30 micron thick PVDC. The field responsive layer (20) comprises electrically conductive matter in the form of vacuum deposited aluminium of about 5 Angstrom unit thickness. The heat-activatable adhesive layer (3b) comprises a hot seal glue of about 15 Angstrom unit thickness. The term "field responsive matter" applies to matter which, when subjected to a suitable field of radio-frequency, electromagnetic or microwave radiation, will become heated to a sufficient level to cause the adhesive (3b) to melt and then set or cure. The sheet (3b) is used in the apparatus (1b), wherein heat-activation takes place by the use of radio-frequency high voltage heating means (25, 26). The sheet (2b) is folded and then passed through a roller nip (6). The means (25, 26) cause induction heating of the contiguous edges of the field responsive layer (20), due to the presence of a radio-frequency field (29). The heated layer (20) transmits heat to activate the glue layer (3b) whereby, when pressure is applied by co-operating rollers (4, 5) the longitudinal edges of the sheet (2b) are joined together to provide a good, substantially tamper-proof seal. <IMAGE> |
