The Role of Fillers in Thermal Paper

When discussing fillers, they are used as a main component in coated paper. Their functions are generally: to fill the unevenness of the paper surface to improve smoothness for better printability, to increase the paper's whiteness, opacity, and gloss, and to improve the paper's appearance. These effects are primarily achieved by adding natural pigments such as clay and talc, as well as synthetic pigments like precipitated calcium carbonate, titanium dioxide, satin white, and aluminum hydroxide.

In thermal paper, fillers not only need to fulfill the requirements mentioned above for coated paper but must also adapt to the specific properties required for thermal recording.

To enhance the color development sensitivity and recording speed of thermal recording paper, low-melting-point substances are added to the thermal coating formulation to lower the thermal melting point of the dye and developer. These low-melting-point substances are generally heat-fusible materials such as fatty acid amides or various waxes, also known as sensitizers. The addition of these low-melting-point materials can lead to various adverse effects when the color-forming layer comes into contact with the thermal head during recording.

To explain this issue, let's first look at the thermal head and the recording process: A thermal recording head is an assembly of heating elements, generally available in thick-film and thin-film types. They consist of a wear-resistant thin layer coated onto a heat-resistant material like Ta₂N or RuO₂. During recording, electrical pulses corresponding to the signal generate Joule heat, which is transferred to the corresponding area of the thermal paper in close contact with the recording head to produce an image. The temperature can reach 300°C, with a surface temperature of 80–100°C. To ensure good thermal conductivity from the head to the paper, considerable pressure must be applied between the paper and the head using a platen roller. The head undergoes instantaneous heating and cooling while continuously applying pressure to feed the paper forward for recording. If the performance of the heat-sensitive material is poor, several situations can occur:

(1) The thermal fusible substances in the heat-sensitive layer can transfer onto the thermal head. The head becomes contaminated by adhering to these molten materials, a phenomenon known as "head sticking" or "residue buildup." This affects the thermal conductivity of the recording head, resulting in insufficient temperature, which impacts color development sensitivity and reduces the color density of the recording layer.

(2) This thermal melt forms an adherent layer on the thermal head. The accumulation of this layer can also lead to poor print clarity and degraded image quality.

(3) The molten material accumulated on the thermal head can cause adhesion between the head and the surface of the recording layer during subsequent thermal recording. This can prevent the recording paper from advancing smoothly during continuous recording, causing operational obstacles such as pitch misalignment, and even shortened characters or distorted images.

(4) The organic and inorganic powder particles contained in the heat-sensitive layer of thermal recording paper can cause varying degrees of physical or electrochemical wear to the print head during the recording process.

For these reasons, thermal paper experts worldwide have been trying various methods to change the above situation, i.e., to overcome the thermal head issues. According to various reports and our own research experience, fillers play a significant role in overcoming the aforementioned problems. Therefore, it can be said that fillers in thermal paper are not only for improving paper whiteness, increasing opacity, and enhancing smoothness but also, and more importantly, for ensuring good adaptability to the thermal head and overcoming the issues of adhesion and sticking mentioned above.