Corona treatment: a key process to improve the ink adhesion of thickened weak solvent PP synthetic paper

Why does PP synthetic paper need corona treatment?

The main component of PP synthetic paper is polypropylene, which is a non-polar polymer material. Its surface energy is low (usually 30-35 mN/m) and its chemical properties are stable, which makes it difficult for ordinary ink to form a firm adhesion on its surface. If it is printed directly, problems such as ink shedding and blurred patterns may occur, which seriously affect the quality and service life of the printed product.

What is corona treatment?

Corona treatment is a physical process that uses a high-frequency and high-voltage electric field to modify the surface of a material. Its core principle is to generate free radicals and polar groups on the surface of the material through corona discharge, thereby increasing the surface energy and improving its wettability and adhesion.

1. Equipment composition of corona treatment
Corona treatment equipment usually includes the following parts:

High-voltage generator: generates a high-frequency and high-voltage electric field.

Electrode: usually composed of a metal roller and a knife-shaped electrode, with a discharge gap formed between the two.

Treatment roller: used to convey the material to ensure that the material passes through the discharge area evenly.

Control system: adjusts voltage, frequency and processing speed to meet the needs of different materials.

2. Working principle of corona treatment
When PP synthetic paper passes through the corona treatment equipment, the high voltage electric field generates corona discharge between the electrode and the material surface. This discharge phenomenon produces the following effects:

Ionized air: Corona discharge ionizes oxygen and nitrogen in the air to generate active particles such as ozone and nitrogen oxides.
Surface oxidation: These active particles react chemically with the surface of PP materials to introduce polar groups such as hydroxyl and carboxyl groups.
Microscopic roughening: Corona discharge also forms tiny pits and cracks on the surface of the material, increasing the surface roughness.
Through these effects, the surface energy of PP synthetic paper is increased from the original 30-35 mN/m to 40-50 mN/m, significantly improving its wettability and ink adhesion.

Advantages of corona treatment
The application of corona treatment in PP synthetic paper has the following advantages:
High efficiency: Corona treatment is a fast and continuous process suitable for large-scale production. The processing speed can reach hundreds of meters per minute, and the effect is stable.
Environmental protection: Corona treatment does not involve chemical solvents and does not produce harmful substances. It is a green and environmentally friendly surface treatment technology.
Economical: Compared with the use of special inks, corona treatment has a lower cost and a long-lasting effect, which can significantly reduce printing costs.
Wide applicability: Corona treatment is not only suitable for PP synthetic paper, but also can be used for surface modification of various plastic materials such as PE, PET, PVC, etc.

Application of corona treatment in thickened weak solvent PP synthetic paper
Thickened weak solvent PP synthetic paper is often used in high-end printing fields such as outdoor advertising and labels due to its increased thickness and solvent resistance. However, its smooth surface and low surface energy characteristics put forward higher requirements for ink adhesion. The application of corona treatment in this field is mainly reflected in the following aspects:

Improving ink adhesion
Through corona treatment, the polar groups introduced on the surface of PP synthetic paper can form chemical bonds with the resin components in the ink, significantly improving the adhesion of the ink. The ink adhesion of the treated material can reach 4B-5B level (according to ASTM D3359 standard).

Improve printing effect
The surface wettability of PP synthetic paper after corona treatment is better, the ink can be spread evenly, reducing spots and missed printing during the printing process, and improving the clarity and color saturation of the pattern.

Enhance weather resistance
Corona treatment not only improves the adhesion of ink, but also enhances the antioxidant properties of the material surface, making the printed products more resistant to UV and aging in outdoor environments.

Adapt to a variety of printing processes
PP synthetic paper treated with corona is compatible with a variety of printing processes such as digital printing, offset printing, and screen printing to meet the needs of different application scenarios.

Precautions for corona treatment
Although corona treatment has many advantages, the following points should still be noted in actual applications:
The intensity of corona treatment (usually expressed in power density) needs to be adjusted according to the thickness and characteristics of the material. Insufficient treatment may lead to unsatisfactory results, while excessive treatment may damage the surface of the material. The effect of corona treatment will gradually weaken over time, so it is recommended to print as soon as possible after treatment to ensure the best results. Corona treatment equipment needs to be cleaned and maintained regularly, especially the electrode part, to avoid uneven discharge or equipment failure.