Fillers suitable thermal paper:Metal Oxides or Hydroxides

Although using clay or special calcium carbonate with high oil absorption can somewhat improve the sticking problem, it is not sufficient and also brings disadvantages such as reduced color development sensitivity and lower image density. Therefore, it is recommended to use magnesium oxide, aluminum oxide, magnesium hydroxide, or aluminum hydroxide with an oil absorption of 80 ml/100 g or more, which can yield satisfactory results. These compounds should be used in fine powder form to achieve higher oil absorption. Of course, crystal morphology also affects the oil absorption properties of the particles, but when the crystal morphology is the same, particle fineness is an important factor influencing oil absorption. Representative examples of such compounds include:

(1) Fine particulate anhydrous aluminum oxide

Oil absorption: 80–250 ml/100 g

(2) Fine particulate magnesium oxide

Oil absorption: 80–100 ml/100 g

(3) Aluminum hydroxide powder with an average particle size of 0.1–1 μm

Oil absorption: 80–100 ml/100 g

(4) White magnesium hydroxide powder with an average particle size of 1–10 μm

Oil absorption: 80–100 ml/100 g

These oxides or hydroxides, especially magnesium hydroxide, are generally not used as fillers for coated paper. However, as fillers for thermal paper, they are completely devoid of electron-accepting properties (i.e., solid acid properties) and therefore do not participate in the color development reaction, resulting in no background fog. Moreover, they provide good anti-sticking effects, and the desired results can be achieved with only a small amount added.