Properties Required for Fillers Used in Thermal Paper

Fillers in thermal paper must not only possess the properties typical of pigments used in coated paper but also meet specific requirements.

As pigments for general coated paper, they should have the following characteristics:

(1) Good whiteness and a certain degree of opacity (hiding power).

(2) An appropriate particle size distribution range.

(3) Easy dispersibility in water.

(4) Good chemical stability.

(5) Good compatibility with other components in the coating.

(6) Low consumption of binders.

(7) Low abrasiveness.

(8) Low cost.

When selecting pigments for thermal paper, in addition to the above requirements, the following factors must also be considered:

(1) To achieve high recording clarity and resolution, as well as to provide good thermal conductivity for high-speed recording, the recording layer must have a smooth surface with fine coating particles. Therefore, the filler is required to have a high degree of fineness, generally with a particle size in the range of 0.2 -5μm or an average particle size of 1 -2μm  being preferable.

(2) The thermal head relies on the adsorptivity of the added filler; thus, the selected filler must have high oil absorption, with an oil absorption value of 50 ml/100 g or higher.

(3) Since the colorless dye in thermal paper tends to develop color when exposed to acidic or electron-withdrawing substances, the filler may also react with the dye if it contains active groups, leading to undesirable background coloration. Therefore, it is essential to select a filler with low surface activity that does not easily cause background color formation.

Based on these requirements, there are many suitable types of fillers, each with its own advantages and disadvantages. Examples include various types of clay, calcium carbonate, magnesium oxide, aluminum oxide, magnesium hydroxide, aluminum hydroxide, and silicates.