| abstract |
Compositions, devices and processes related to etching of a very thin layer or fine particles of a metal are disclosed for monitoring a variety of parameters, such as time, temperature, time-temperature, thawing, freezing, microwave, humidity, ionizing radiation, sterilization and chemicals. These devices have capabilities of producing a long and sharp induction period of an irreversible visual change. The devices are composed of an indicator comprising a very thin layer of a metal (e.g., polyester film having an extremely thin, e.g., about one hundred Angstroms layer of aluminum) and an activator, e.g., a reactant, such as water, water vapor, an acid, a base, oxidizing agent or their precursors, which is capable of reacting with the said indicator. The indicator retains its opacity and metallic luster, e.g., silvery white, mirror like finish of aluminum layer for a long time. The activator destroys the indicator layer including the naturally formed oxide layer. The indicator layer does not have a matrix/binder. As the device becomes transparent from being highly opaque, any color, message or image printed under it become visible at a pre-determined time, thus making the device self reading. A sealing tape or large label made from an activator and an indicator can be used for sealing boxes and monitoring self-life of perishables inside it. Ink formulations composed of a metal powder and a proper activator can be used for monitoring several sterilization processes, such as sterilization with steam. When water is used as an activator, a thin layer of metals, such as that of aluminum can be used as steam sterilization or humidity indicator. The process can also be used for making patterns, printing plates, electronic circuits and RFIDs (radio frequency identification devices). |