Several New Types of Thermal Paper

1. Two-Color Thermal Paper

Two-color thermal paper refers to paper that produces two different colors after thermal development. It consists of two heat-sensitive layers. The upper layer is a low-melting-point heat-sensitive layer, and the lower layer is a high-melting-point heat-sensitive layer. When heated, the upper layer develops first, producing one color. As the temperature rises, the high-melting-point heat-sensitive layer melts, producing the second color. This second color is a mixture of the colors produced by the high-melting-point and low-melting-point heat-sensitive layers. For example, if the upper layer develops red and the lower layer develops green alone, then the second color is black. While this produces two colors, it doesn't produce vibrant colors. To address this, a two-color thermal paper with a clearing layer has been developed. The lower layer is a high-melting-point heat-sensitive layer, and a clearing layer is sandwiched between them. The melting point of the substance in the clearing layer is close to that of the lower layer. When this substance melts and mixes with the upper layer, it weakens or even eliminates the color of the developed image in the upper layer, making the developed color of the lower layer more vibrant. This results in a noticeable color difference between the two-color images.

2.PTM Paper 

Thermal recording systems have many advantages, such as small size, low price, and fewer supporting equipment requirements. However, ordinary thermal paper also has several disadvantages, such as easy surface scratching, image blurring by oil, detergents, etc., and high surface gloss making image interpretation difficult. To overcome these shortcomings, PTM paper (Plain Thermax) was developed. This type of thermal paper possesses the characteristics of ordinary paper. PTM paper consists of three layers: a heat-sensitive layer, a bottom layer, and a paper base. The heat-sensitive layer, or top layer, has scratch and stain resistance. A large amount of organic dyes are added to this layer to minimize the visibility of scratches. Additionally, some chemicals that promote ink curing also reduce the chance of heat head scratches. The special adhesive used in the top layer improves water resistance, and there is no sticky feeling when touched with wet hands. To make images and text easy to read and interpret, the surface is treated with a matte finish. The bottom layer affects the optical density and resolution of the paper. To achieve high thermal sensitivity and good image resolution, an oil-based dye absorbent is added to the bottom layer, forming a porous space with a heat-insulating effect, thus preventing energy loss from the heat head. Paper base: To obtain the characteristics of ordinary paper, a stiff paper base is necessary, with a thickness of approximately 70-90 μm. The thermal sensitivity and image stability of PTM paper are significantly improved compared to ordinary thermal paper. For thermal paper, the size of the dye particles in the thermal layer has a significant impact on thermal performance. The smaller the dye particle size, the better the thermal performance; for example, reducing the average particle diameter from 1.5 μm to 0.8 μm can save 30% of heat head energy. PTM paper utilizes this technology, thus achieving good image quality on low-power fax machines.

Ordinary thermal paper development achieves its effect by reacting leuco dyes with phenolic compounds upon heating to generate chromogenic dyes. However, because this reaction is reversible, image quality often deteriorates when affected by plasticizers or oils. Therefore, PTM paper incorporates compounds with epoxy functional groups to improve image stability.

In this system, the leuco dye in the heat-sensitive layer cannot react directly with the epoxy compound; it can only react with the product of the reaction between the leuco dye and the developer—the developing dye—and this reaction is irreversible. Therefore, image attenuation is suppressed, and image stability is enhanced.

3. Color Thermal Paper

Color thermal paper is printing paper that produces all colors through the action of heat. They encapsulate the color-developing agent in microcapsules, which are then coated onto the surface of the paper to create color thermal paper. Once heat is applied to the surface of the color thermal paper, the capsules rupture, producing yellow, magenta, and deep red colors depending on the heat applied.

4. Transfer Thermal Paper

Transfer thermal paper is an overlapping of two sheets of paper. A special ink is coated on the back of the base paper (such as capacitor paper or polyester film with a thickness of approximately 6-10 μm), and then a sheet of white paper (with an extremely smooth surface and high absorbency) is placed on top. When the heat source contacts the paper surface, it causes the ink to melt and transfer onto the white paper. This thermal recording method allows for the direct use of plain paper in fax machines without the need for thermal paper.