Polyester fiber (also known as PET) is a condensed polymer of terephthalic acid (PTA) and ethylene glycol (EG). Through the esterification and condensation reaction of PTA and EG, a polymer compound called polyethylene terephthalate (PET) is produced. Its molecular structure formula is shown in Figure 1-1.

Figure 1-1 Schematic diagram of polyester molecular structure formula

The molecular structure of polyester is mainly composed of a rigid benzene ring and a flexible fatty hydrocarbon group, where the ester group forms a rigid conjugated system with the benzene ring, restricting the free rotation of the flexible fatty hydrocarbon group. Therefore, polyester fibers exhibit high melting points, high rigidity, and high strength, and are widely used in various fields. The symmetrical structure of polyester molecules enables various functional groups to be arranged neatly on both sides of the molecule, resulting in a high degree of geometric regularity and good crystallization tendency of the molecular structure. Because its molecular structure does not contain any other polar groups, only one hydroxyl group at each end, polyester fibers have high crystallinity and strong surface inertness, making it difficult to form a highly adhesive interfacial layer with the resin matrix, which limits their composite with other materials.

Due to the regular molecular structure, high crystallinity, smooth fiber surface, and poor wettability with resin, polyester is difficult to mechanically interlock with the resin matrix. In addition, there are only hydroxyl groups at both ends of the molecular chain in macromolecules, and no other polar groups, resulting in strong surface inertness. Therefore, surface modification is necessary. Plasma surface treatment can improve its surface properties.

Plasma surface treatment

Plasma is a non condensed state system produced by ionizing gas under specific conditions, which can be understood as an excited gas. It contains stripped electrons, ions with opposite charges, and excited atoms, and still exhibits neutrality as a whole.

Plasma surface modification material technology is one of the earliest techniques for preparing materials using plasma. It is a plasma treatment of the shallowest surface of the material. Due to the higher energy of plasma compared to chemical bonds, it can cause etching, cross-linking, chemical modification, and other effects on the surface of the material.

Low temperature plasma treatment of polyester fibers is a relatively environmentally friendly technology. Its principle is that low-temperature plasma discharge generates high-energy particle flow. On the one hand, high-energy particles can change the roughness of the fiber surface, increase its surface energy and hydrophilicity, thereby improving the fiber's wetting performance; On the other hand, high-energy particles can initiate grafting reactions or chemical bond breaks on the surface of fibers, introducing new active groups and improving adhesion.

The advantages of using plasma to treat polyester fiber materials mainly include: (1) the treatment belongs to a dry process, which does not require the treatment of waste liquid and gas, saves energy, and has low pollution, which is conducive to environmental protection; (2) The treatment only involves the surface layer (tens to thousands of angstroms) and does not affect the inherent properties of the substrate; (3) Efficient, with short processing time (seconds to minutes). As an environmentally friendly process, plasma treatment is becoming increasingly popular in changing the surface properties of polymers.