Fillers Suitable for Thermal Paper--Clay and Talc

As mentioned above, there are many fillers that can be used for thermal paper. Based on their source characteristics or chemical composition, they can be classified into several categories, we intoduce Clay and Talc as per below.

Clay and Talc are natural inorganic compounds belonging to the silicate category. They have wide sources, are easy to process, and have low cost. Possessing the general performance required of fillers for coated paper, they are widely used in coating printing papers. They can also be selected as fillers for thermal paper.

The main chemical composition of clay is Al₂O₃ 2SiO₂ 2‌H₂O. Its composition and properties vary depending on the origin. Although general kaolin has advantages in whiteness, hiding power, and the fluidity of the coating composition, it is not a good filler for thermal paper. The clay used as filler for thermal paper is generally surface-treated bentonite, montmorillonite, kaolin, or calcined kaolin. Due to certain physical and chemical treatments, their crystal lattice structure contains voids and they have high oil absorption. When used as a filler in thermal paper, they can overcome the sticking phenomenon to a certain extent. Generally, the oil absorption value of this type of treated clay is above 50–80 ml/100g, which can adequately meet this requirement.

The main component of talc is magnesium silicate, i.e., a crystalline complex of MgO SiO₂‌  . It has a certain whiteness and is fine and smooth, which can improve the smoothness of the paper. Although it is difficult to disperse due to its hydrophobic surface and has not been widely used in coated paper, its applications have been expanding recently due to the gradual development of dispersants and wetting agents. In thermal paper, its smoothness is also beneficial for paper feeding during recording and is favorable for color development. Therefore, it can be used alone or mixed with other fillers.

To achieve a better anti-sticking effect with clay and talc, one can increase the amount of binder used or sacrifice some image density to achieve the desired goal.

Here is a set of thermal paper formulation experiments using them as fillers. The basic formula is as follows:

Solution A

Black thermal dye 1 part

PVA 10% aqueous solution 0.5 parts

Water 40 parts

Solution B

 BPA 1 part

PVA 10% aqueous solution 0.5 parts

Water 40 parts

Solution C:

25% filler dispersion (dispersant can be added appropriately)

After dispersing Solutions A, B, and C to an average particle size of approximately 1–2 μm, mix them in the ratio A:B:C = 1:3:4. Then add 1.5 parts of 10% PVA, stir to mix evenly, and coat onto 60 g/m² quality paper with a coating weight of 7–8 g/m². Measure the color development density, background color, oil permeability, and observe the sticking condition. The results are shown in Table 1:

Table 1

Table 2

In another comparative test for sticking condition, the results are shown in Table 2.

The testing conditions for the above data are as follows:

· Color Development Density: Cut the thermal paper into 10 mm wide strips. Apply heat and pressure for a fixed time on a heat-developing block at 150°C to allow color development. Measure the reflection density using a Macbeth RD918 reflection densitometer.

· Background Color: Measure using the same densitometer.

· Oil Permeability: A relative data point indicating the oil absorption of the pigment. Under conditions where other factors such as coating particle size and coating weight are relatively equal, the oil permeability value is measured using an IGT tester. A smaller value indicates better ink permeability and better oil absorption.

· Sticking Condition: After the thermal paper contacts the heating element for color development, wipe the heating element firmly with smooth white paper. Observe the black residue on the paper. No black residue on the paper: ◎; Slight: ○; Moderate: △; Significant: ×.

The above data indicate that although thermal paper made with general clay and talc exhibits good color development performance and background color, their anti-sticking performance is average. In contrast, calcined clay and treated clay show good anti-sticking properties.