Direct thermal labels are not appropriate for any applications where the labels may be exposed to harsh chemicals, intense heat or prolonged exposure to light.
Direct thermal printing uses thermosensitive labels, that are coated with a special dye that can switch between two chemicals forms when exposed to heat, light or pH.
Specifically, direct thermal printers use a heated print-head to directly heat the thermosensitive labels, which react to the heat and darken, creating the desired print.
Because of this, direct thermal labels are not suitable for certain applications. This can include sterilization protocols that require high temperatures, such as steam autoclaves and dry heat ovens.
The coating also reacts with various classes of chemicals, including acids, clearing agents (e.g. xylene) and alcohols. Moreover, direct thermal labels are also fairly light sensitive, and will completely darken if exposed to UV light.