Plasma is the "fourth state" of matter that exists in nature, apart from solid, liquid, and gas. It is a collection of various particles such as atoms, molecules, electrons, ions, and free radicals. The entire system has an equal number of positive and negative charges, making it electrically neutral as a whole. Aurora, lightning, and the starry sky in nature are all plasma phenomena. Plasma is essentially an ionized gaseous substance, with its constituent electrons, ions, etc. possessing a certain amount of energy. When they interact with the surface of a material, they transfer their own energy to the atoms and molecules on the surface, causing complex chemical and physical changes that alter the wettability, adhesion, surface energy, and other properties of the material surface.

Principles of Plasma Surface Treatment

There are many theoretical explanations for the principle of plasma on material surfaces, such as surface molecular chain degradation theory, oxidation theory, hydrogen bonding theory, cross-linking theory, ozone oxidation theory, and surface dielectric theory. However, the reaction mechanism of plasma on material surfaces can be summarized into three steps:

(1) A small number of free electrons in the air are accelerated in a high-voltage electric field to obtain higher kinetic energy, and they will inevitably collide with other molecules in space during motion. The impacted molecules simultaneously receive some energy and become excited molecules with activity.

(2) Excited state molecules are unstable and decompose into free radicals, consuming absorbed energy; It may also dissociate into ions or retain their energy and remain in a metastable state.

(3) When free radicals or ions react on the surface of materials, they can form a dense cross-linked layer; Plasma undergoes polymerization reaction with existing gases or monomers, depositing on the surface of materials to form coatings with designable properties; Plasma reacts with surface free radicals or ions to form a modified layer.

Plasma surface treatment effect

The interaction process between plasma and material surface is very complex, and different types of plasma reaction processes are different. They are related to the particles involved in the reaction, plasma state parameters, surface materials, material surface states, and the surface acting as electrodes, walls, or samples in the reactor, all of which will affect the interaction process between plasma and surface. There are roughly four types of effects of plasma on material surfaces: surface cleaning; Surface etching; Surface cross-linking and formation of surfaces with new chemical structures.

The process of action may introduce specific functional groups on the surface, resulting in surface etching, the formation of cross-linked structural layers, or the generation of surface free radicals. These actions are generally not singular, and often one action is dominant while several actions coexist.

1) Surface cleaning

Plasma cleaning, also known as dry cleaning, is the process of exciting process gases such as oxygen and argon into ionic states under the excitation of a radio frequency power source, and then reacting chemically and physically with impurities on the surface of the workpiece to be cleaned, generating volatile products and achieving the purpose of cleaning.

2) Surface etching

Plasma surface treatment can cause etching on materials for two reasons: firstly, sputtering erosion caused by the impact of active particles such as electrons and ions in the plasma on the material surface (mainly inert gases), and secondly, chemical erosion of the material surface by chemical active species in the plasma (reactive gases).

3) Crosslinking reaction

The free radical parts on the surface of the processed material will also react with each other, forming a dense cross-linked layer on the material surface, which can strengthen the surface layer, improve the material's wear resistance, adhesion, and chemical resistance.

4) Forming a new chemical structure

If the discharge gas is a reactive gas such as N2, 02, C2H4, etc., complex chemical reactions will occur on the activated material surface. Therefore, the new chemical structure is closely related to the discharge gas, and selecting the appropriate discharge gas is a key step in material surface modification.

Low temperature plasma technology (hereinafter referred to as plasma) is commonly used to treat the surface of polymer materials by utilizing gases such as Ar, O2, N2, and air to generate excited states of atoms, molecules, free radicals, and ions through low-pressure glow discharge, which act on the material surface. The main effects of plasma on material surfaces include clearing weak boundary layers, surface etching, surface cross-linking, and introducing specific functional groups on the material surface. During the modification process, several effects may occur simultaneously and compete with each other, causing changes in the mechanical properties and surface energy of the material or obtaining a more hydrophilic surface.

The process of action may introduce specific functional groups on the surface, resulting in surface etching, the formation of cross-linked structural layers, or the generation of surface free radicals. These actions are generally not singular, and often one action is dominant while several actions coexist.