Ultra high molecular weight polyethylene fibers (UHMWPE) are composed of non-polar methylene molecular chains and possess excellent strength, quantity, chemical stability, corrosion resistance, and impact resistance. They are important materials in the field of high-performance fibers. However, due to its smooth surface, lack of polar functional groups, and low surface energy, the interfacial bonding force between fibers and matrix materials is weak. Without surface modification or functionalization treatment, ultra-high molecular weight polyethylene fibers are difficult to achieve efficient bonding with the matrix, and thus cannot exert their excellent performance. Plasma treatment, as an efficient surface modification technique, can introduce a large number of polar functional groups (such as hydroxyl, carboxyl, etc.) on the surface of fibers, significantly improving surface polarity and adhesion performance.

Plasma treatment technology

Plasma treatment is a comprehensive modification technology composed of neutral particles and high-energy charged particles to form a plasma. The modification depth of the material by plasma treatment is only about 100nm, which does not cause significant damage to the material structure and is beneficial for high-performance fibers to maintain their excellent mechanical properties.

The essence of plasma modified fiber interface technology is to use high-energy active particles generated by plasma treatment equipment to break the C-C and C-H chemical bonds on the fiber surface, promote the molecular chains on the fiber surface to obtain free radicals, and then transform into functional groups with certain activity. In order to effectively achieve fiber surface modification, the particle energy generated by plasma should be higher than the chemical bond energy in UHMWPE fibers. From the energy and chemical bond energy of each particle, it can be seen that plasma is sufficient to break the chemical bonds of fibers (Table 1-1). In addition, plasma also has an etching effect on the surface of fibers, increasing the roughness of the fiber surface and improving the mechanical interlocking performance of the fiber surface.

Table 1-1 Plasma Particle Energy and UHMWPE Fiber Chemical Bond Energy

The general plasma atmosphere consists of oxygen, argon, helium, nitrogen, or a combination of several gases in a certain proportion. When oxygen is used as the generating medium, the new functional groups generated on the surface of the material are more active. The mechanism of oxygen plasma treatment on UHMWPE fibers is that high-energy particles containing oxygen break the C-C and C-H chemical bonds on the surface of UHMWPE fibers, and regenerate oxygen-containing functional groups such as hydroxyl, carboxyl, and carbonyl groups on the fiber surface (Figure 1.1).

Figure 1.1 Oxygen plasma treatment of UHMWPE fibers

The effect of plasma treatment on the surface hydrophilicity of ultra-high molecular weight polyethylene fibers

Contact angle testing is an important means of evaluating the surface hydrophilicity of materials, as shown in Table 1-2. The surface contact angle of UHMWPE fibers without plasma treatment is relatively high, with a contact angle of 113 °, and the surface exhibits strong hydrophobicity. This is because the surface of unmodified UHMWPE fibers is mainly composed of non-polar C-C and C-H bonds, lacking hydrophilic groups, and the surface is smooth without microscopic roughness, resulting in lower surface energy and difficulty in forming effective wetting with water molecules. After plasma treatment, UHMWPE fibers showed significant and rapid infiltration, with a contact angle of 0. This significant change is mainly due to two factors: on the one hand, plasma introduces a large number of polar groups (such as - OH, C=O, and O-C=O) on the fiber surface, significantly improving the chemical activity of the fiber surface; On the other hand, plasma etching has formed significant micro roughness structures. The rough surface amplifies the hydrophilic properties through the Wenzel effect, resulting in excellent wetting performance of the fiber surface.

Table 1-2 Contact angles of UHMWPE fibers with different plasma treatment powers

Plasma treatment not only has significant advantages in improving material properties, but also has become an important means of modern surface modification technology due to its environmental friendliness, continuous operability, and suitability for large-scale production. Plasma treatment technology, with its advantages of environmental friendliness, easy operation, high efficiency, and suitability for large-scale production, has opened up new development space for the application of UHMWPE fibers in high-performance composite materials.