Thermal paper is a recording media to be printed with heat from the thermal printer. The image is produced by heat, without printing ink, when the selected area of the thermal paper passes over the thermal print head.
The image is created through the direct transfer of heat (thermal energy) to the thermal paper: thermal paper has a coating, which changes colour to black when exposed to heat. No ink ribbon, toner or ink cartridge is required to record information on the thermal paper. Thermal printing is done with small, compact thermal printers that do not need much maintenance. Thermal paper is widely used in many applications, such as labels, tickets, POS receipts, medical charts, because of its high reliability and convenience.
Thermal paper structure
The thermal paper consists of base paper and a few special coating layers. On the surface side (the recording side), the precoat layer and the thermal layer are coated.
The first layer of the thermal paper is base paper specially developed for thermal paper technology. The base paper is coated by a thermal layer.
The precoat layer improves heat insulation, smoothness, uniformity, and anchoring of the thermal layer.
The purpose of this layer is to produce images with a thermal reaction. In practice, the paper turns black in the heated areas due to this layer with chemicals like dye, developer, and sensitizer.
Topcoated thermal paper structure
A topcoat layer is applied to the thermal layer when high durability of the thermal printing image is needed. Also, the backside layer can be coated to improve both the printability and the reverse side protection against the adhesive.
The topcoat layer protects the thermal layer from water, oil, solvent, humidity, scratches, etc.
The backcoat layer is applied for de-curling, antistatic electricity treatment, and back-barrier against adhesive.
Thermal paper features
The main function of thermal paper is “thermal printing”. This is why, in addition to normal paper properties like grammage, thickness or strength, the thermal printing quality, and image stability are important. Furthermore, pre-printability by conventional printing methods like Flexo or Offset is also required for thermal paper.
General paper properties
- Basic properties: grammage, thickness, moisture
- Strength properties: tensile and tearing strength, elongation
- Surface properties: brightness, opacity, whiteness, smoothness/roughness, surface strength
- Absorption properties: water, oil, printing ink
Thermal paper properties
- Image stability refers to how many years the thermal print will be readable when stored in correct conditions.
- Density is an optical density value measured from thermal printed image.
- Static sensitivity shows at which temperature the thermal image starts to formulate. Low static sensitivity means that the thermal image starts to formulate at high temperatures. High static sensitivity, in turn, means that thermal imaging starts at low temperatures. If thermal paper is used at high-temperature conditions, the static sensitivity is a very significant feature.
- Dynamic sensitivity shows how fast and with what energy level the thermal paper can be printed. Basically, higher dynamic sensitivity equals better quality.