Fillers suitable for thermal paper coating slurry:Calcium carbonate

Calcium carbonate has now become an independent category in various fields. It comes in many varieties with complex crystal forms, and its abundant sources, wide range of applications, and low price have led to continuous development. It can be broadly classified into two types: natural and precipitated (light). Natural calcium carbonate has a tightly packed crystal structure and hard particles. While it is inexpensive and has good flowability in coatings, it does not meet the requirements for oil absorption. Therefore, various precipitated calcium carbonates can be used as fillers for thermal paper production and thermal paper coating slurry.

Precipitated calcium carbonate can be produced in different crystal forms depending on its source or processing method, including hexagonal, orthorhombic, needle-like, spindle-shaped, or amorphous forms. Its general characteristics are porous and fine structure. Currently, most domestically produced light calcium carbonate is spindle-shaped, while some foreign companies are developing needle-shaped forms with high oil absorption. Various organic agents can be used to treat its surface to improve its properties such as dispersibility and thixotropy; activated calcium carbonate treated with stearic acid has a large surface area and good oil absorption. Some domestically produced micro- or ultra-fine calcium carbonate has a particle size of less than 4µm (over 85%) and an oil absorption of 80-120ml/100g, all of which can be used in thermal paper.

The test results show that various types of untreated light calcium carbonate contribute significantly to the whiteness of the base color, especially the color density, and maintain stability. However, their oil penetration values are all relatively high, resulting in adhesion to the pen tip. Fine calcium carbonate with activated surface treatment exhibits better anti-sticking properties, but it is difficult to disperse. Therefore, more superior varieties of calcium carbonate as a heat-sensitive filler need to be developed,so that better quality of thermal paper coating chemicals and thermal paper can be produced.