Plasma is mainly generated by gas discharge. Taking water as an example, when the temperature increases, water changes from ice (solid) to liquid water (liquid) and water vapor (gas). As the temperature further increases, the water molecules that make up the water vapor also gain sufficient kinetic energy and begin to separate from each other, a process called dissociation. On this basis, further increasing the temperature will lead to a completely new phenomenon: the outer electrons of atoms will break free from the constraints of the atomic nucleus and become free electrons, and atoms that have lost electrons will become positively charged ions. This process is called ionization. Due to the intensified thermal motion of material molecules, collisions between them can also cause ionization of gas molecules, resulting in a mixture of freely moving and interacting positive ions and electrons, known as plasma.

Principles of Plasma Processing

Plasma treatment utilizes high voltage to break down protective gases such as N2, O2, and Ar2 molecules, generating plasma state nitrogen atoms, oxygen atoms, and argon atoms, and breaking the C-H bonds on the polymer surface to generate plasma state hydrogen atoms, thereby generating free radicals such as - OH and - NH2. Finally, the free radicals recombine with the molecules on the polymer surface, grafting new functional groups onto the polymer surface and obtaining new surface properties. Low temperature plasma treatment can improve the hydrophilicity, hydrophobicity, and adhesion of the material surface.

The mechanism of plasma treatment can be summarized into the following three aspects:

(1) Etching effect of plasma on material surface

Low temperature plasma can etch material surfaces. These etching effects mainly come from the positive ions, active free radicals, and active atoms produced in the system (these active atoms can react with the activated functional groups on the material surface and generate volatile small molecule substances). In addition, etching can not only remove weak boundaries on the surface, but also make it very rough. Often presenting a pitted morphology, which increases the specific surface area of the material. In addition, other neutral atoms and free radicals generated during the reaction process will form a deposition layer on the surface of the material. In addition, etching reduces the surface contact angle of the material, which is beneficial for improving the dispersibility of the material in water.

(2) Plasma promotes the generation of cross-linking on the surface of materials

If the gas used in the discharge process is an inert gas, the high-energy active particles generated during the discharge process can break the old chemical bonds on the material surface, thereby generating new free radicals. If there are no other substances present in the reaction system except for the base material, the newly generated free radicals will re bond with each other, forming a new network cross-linking structure on the surface of the material, which significantly improves the mechanical and surface properties of the material.

(3) Plasma promotes the introduction of functional groups on the surface of materials

If the gas used in the discharge process is a reactive gas or when the substrate material is mixed with organic functional groups, complex chemical reactions will occur on the surface of the plasma activated material. According to the performance expectations of the required materials, characteristic functional group molecules such as - OH, - NH2, and - COOH can be introduced on their surfaces through low-temperature plasma treatment.

The above is a basic introduction to the principle of plasma treatment. Plasma treatment will not cause damage to the material itself, and the surface thickness of the material it can treat is only tens to thousands; And this method has no rigid requirements for the substrate material, a wide range of applications, short surface treatment time, and low cost.