It is found that the mechanical pulp thermal raw paper will cause its the finished product of thermal paper color development performance decline. The fundamental reason is that the lignin in the mechanical pulp reacts chemically with the thermal paper color developer agent under high temperature and high humidity conditions. Thus destroying the original color system of thermal paper.

On the market, it has been discovered that the color rendering performance of many thermal papers declines rapidly or even loses color rendering performance. However, the color rendering performance of these problematic thermal paper is normal when they were first produced. It is a period of time after the circulation or warehousing, the pattern and color are gradually disappeared over time:

Figure 1 The color rendering phenomenon faded of thermal paper

1. The reaction mechanism of lignin to thermal paper color developer

Typical thermal paper color developers are usually solid acid organic matter containing a phenol structure. Its key thermosensitive color development functional group - phenolic hydroxyl group (R-OH) and one of the main functional groups in lignin - methoxy group (- OCH3) can undergo a nucleophilic substitution reaction with the phenolic hydroxyl group R under certain temperature and humidity conditions, causing the phenolic hydroxyl group R-OH to gradually decrease and then lose reactivity.

2. The lignin

Lignin mainly refers to a three-dimensional aromatic polymer compound with a three-dimensional structure in which the units of the phenylpropane structure are connected through carbon-carbon bonds and ether bonds. The main structural units are: a.Guaiacolylpropane; b. Syringyl Propane; c p-hydroxyphenylpropane, as shown in the figure:

Figure 2 Structural unit of lignin

The functional groups in its molecule mainly include: methoxy group (-OCH3), hydroxyl group (-OH), carbonyl group (-CO), and ether oxygen atom. Its structural unit is composed of an aromatic benzene ring and an aliphatic side chain, which are also connected with various functional groups, such as the methoxy group on the benzene ring, the reactive phenolic and alcoholic hydroxyl groups, and carbonyl groups. .

3. The thermal color developer

As a proton donor, the thermosensitive color developer melts with the colorless dye when the printer head is heated to perform a color reaction. Its function is to react with the color former (CVL , ODB-2 ) to convert the lactone ring into a resonance chromophoric group. It requires an appropriate melting point. It has low solubility in water and good compatibility with color dye chemicals.

The degree of reaction between thermal paper color developers of different structures and lignin is different. When the active group R-OH in the developer has a branched chain, the steric hindrance effect increases the interaction between the methoxy group and the hydroxyl group on the phenol. The reaction energy level (i.e., the electrophilic substitution positioning effect of the benzene ring), therefore can show a certain ability to withstand lignin (i.e., the color development performance is not or less affected by mechanical pulp).

Figure 3 Structure and physical properties of various thermal color developers

BPA, BPS, BPB and other color developers have low aging retention rates of white paper, while BPC, BPOAA, D-8, BPZ, BPAP and other color developers have high aging retention rates. The reason is that the proton-releasing groups in BPA, BPS, BPB and other chromogens with symmetrical chemical structures are easily destroyed by the active groups in the mechanical slurry. However, BPC, BPOAA and other chromogenic agents have branched chains (protecting groups) in the ortho position of the proton-releasing group. The steric hindrance effect of chromogens such as D-8, BPZ, BPAP reduces the activity of proton-releasing groups, which to a certain extent alleviates the damage speed of mechanical pulp active groups to the proton-releasing function of chromogens.

4. Methods to solve the problem of decreased color rendering performance of thermal paper caused by mechanical pulp

① Use thermal base paper that does not contain mechanical pulp, or base paper that only contains a very small amount of mechanical pulp.

② Use a heat-sensitive color developer with high steric hindrance in its chemical structure.

③ Appropriately increase the amount of thermal paper coating weight to reduce the impact of color development.

④ Experience shows that high temperature and high humidity can easily promote the reaction between lignin and color developer. Therefore, thermal paper made of mechanical pulp base paper should be to used avoid the high temperature and high humidity situations as much as possible during storage and logistics.

⑤ Take certain isolation measures to reduce the contact between mechanical pulp and thermal paper coating. For example, mechanical pulp is made into the core layer of the paper, and the front and back sides of paper are made of chemical pulp; or thermal base paper is surface-glued with high barrier properties, etc.

Qingdao JIERUIXIN Machinery and Technology Co., Ltd. can produce three-proof thermal paper (waterproof, oil-proof and alcohol-proof), with a service life of more than 5 years and longer image retention time.