Application Of Wax Emulsion In Thermal Paper Coating

The application of wax emulsion in thermal paper coating can indeed significantly improve the performance of printing paper and labels, especially in high-frequency use scenarios such as supermarkets. The following is a systematic analysis of its application and key technologies:

1. The mechanism of action of wax emulsion in thermal coating

Lubrication protection: wax emulsion forms a uniform lubricating film on the surface of the thermal layer, reducing the friction coefficient between the print head and the paper (usually by 30%-50%), and significantly extending the life of the print head (such as from 50,000 prints of traditional materials to 100,000 prints).

Thermal conduction optimization: The thermal conductivity of wax can adjust the response speed of the thermal layer to heat and avoid uneven color display caused by local overheating. For example, wax with a melting point of 60-80℃ can balance color display efficiency and print head protection.
Coating compatibility: By selecting polarity-matched wax (such as modified polyethylene wax), ensure good bonding with the developer (such as bisphenol A) and filler (such as silicon dioxide) in the thermal layer to avoid coating peeling.

2. Key performance indicators and material selection
Wax type:
Synthetic wax (such as polyethylene wax): high melting point (100-140℃), high hardness, suitable for high-speed printing scenarios.
Natural wax (such as palm wax): environmentally friendly but low melting point (45-60℃), need to be compounded for use.
Particle size control: The emulsion particle size needs to be <1μm to ensure the flatness of the coating (surface roughness Ra<0.3μm) and avoid print head jamming.
Solid content: usually 20-40%, taking into account coating efficiency and drying energy consumption.

3. Formula design and process points

Compound system: wax emulsion is often used in conjunction with the following additives:

Defoamer (such as silicone, added at 0.1%-0.5%): prevent coating bubbles.

Dispersant (such as polyacrylate): maintain the stability of wax particle suspension.

Antistatic agent (such as quaternary ammonium salt): reduce paper adhesion.

Coating process:

Quantitative coating: coating amount 3-8g/m², excessive amount will lead to decreased color sensitivity.

Drying conditions: 80-120℃ hot air drying to ensure uniform film formation of wax film.

4. Performance verification and challenges

Abrasion resistance test: verified by Taber abrasion test (CS-10 wheel, 500g load), high-quality wax coating can withstand 1000 cycles without obvious wear.
Thermal response test: It is necessary to ensure that the wax layer does not affect the color development temperature (usually the temperature difference between the color development starting temperature and the wax melting point is required to be ≥20℃).

Long-term stability: After the accelerated aging test (40℃/75%RH, 30 days), the coating has no cracking or stickiness.

5. Environmental and cost considerations

Environmental protection trend: Solvent-free wax emulsion (VOC<50ppm) gradually replaces traditional solvent-based products.

Cost analysis: Wax emulsion accounts for about 15-25% of the total coating cost. By optimizing the solid content and drying process, the cost can be reduced by 10%-20%.

6. Typical application cases

Supermarket price tags: Thermal paper with 5% polyethylene wax emulsion is used, the print head life is increased to 150,000 times, and the color contrast reaches 0.8 (DIN5033 standard).
Logistics label: The composite coating (bottom developer + top wax layer) remains stable in an environment of -20℃ to 60℃.

Conclusion and suggestions

The application of wax emulsion needs to balance lubricity, thermal response and cost. Suggestions:

Preferentially use synthetic wax emulsions with narrow particle size distribution (such as Tianshi new material series).

Use an online coating detection system (such as infrared water analyzer) to monitor the uniformity of the coating in real time.

For high-speed printing scenarios, develop a nano wax emulsion composite system with a high melting point (>120℃).