The color development performance of thermal paper finished products is observed to decline when using mechanical pulp thermal raw paper. This decline is fundamentally attributed to the chemical reaction between lignin in the mechanical pulp and the thermal paper color developer agent under conditions of high temperature and humidity. This reaction results in the destruction of the original color system of thermal paper.

In the market, it has been noted that the color rendering performance of many thermal papers rapidly diminishes or even loses its effectiveness. Interestingly, these problematic thermal papers exhibit normal color rendering performance when initially produced. The degradation of pattern and color occurs gradually over time during circulation or warehousing, as illustrated in Figure 1 depicting the faded color rendering phenomenon of thermal paper.

1. The reaction mechanism of lignin to thermal paper color developer: The typical thermal paper color developers contain solid acid organic matter with a phenol structure. The phenolic hydroxyl group (R-OH), a key thermosensitive color development functional group, undergoes a nucleophilic substitution reaction with the methoxy group (-OCH3) in lignin under specific temperature and humidity conditions. This leads to a gradual decrease in the phenolic hydroxyl group's reactivity.
2. Lignin: Lignin refers to a three-dimensional aromatic polymer compound with a structure connecting phenylpropane units through carbon-carbon and ether bonds. Its main structural units include Guaiacolylpropane, Syringyl Propane, and p-hydroxyphenylpropane, as depicted in Figure 2.
3. The thermal color developer: As a proton donor, the thermosensitive color developer undergoes a color reaction with the colorless dye when the printer head is heated. It converts the lactone ring into a resonance chromophoric group, requiring an appropriate melting point. The degree of reaction with lignin varies among thermal paper color developers with different structures, affecting their color development performance.
4. Methods to solve the problem of decreased color rendering performance:

• Use thermal base paper without mechanical pulp or with a minimal amount of mechanical pulp.
• Opt for a heat-sensitive color developer with high steric hindrance in its chemical structure.
• Increase the thermal paper coating weight to mitigate the impact on color development.
• Avoid high-temperature and high-humidity conditions during storage and logistics for thermal paper made from mechanical pulp base paper.
• Implement isolation measures, such as using mechanical pulp in the core layer and chemical pulp on the front and back sides of the paper, or applying surface gluing with high barrier properties to the thermal base paper.

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