Aramid fiber is a high-performance polymer material renowned for its outstanding strength, modulus and heat resistance. Thanks to these superior properties, it is widely applied in aerospace, automotive industry, bulletproof and puncture-resistant protection and other fields.

The excellent strength and toughness of aramid fiber stem from its highly ordered chain structure formed by aromatic ring linkages between polyamide chains. For this reason, aramid fiber is commonly used as a reinforcing phase for rubber and epoxy resin, and has been proven highly effective in enhancing the overall performance of composite materials.

When combined with resin matrix, aramid fiber suffers from limited adhesion to polymer substrates due to its inherent surface characteristics. Therefore, surface modification is essential to strengthen the bonding strength between aramid fiber and the matrix.

Plasma Modification Treatment

Plasma is generated by applying an electric field to gas. It is a mixture of neutral atoms, molecules, free radicals, excited species, ions and electrons. Macroscopically, the total charge of electrons is approximately equal to that of positive ions, making plasma quasi-neutral. As a special state of mixed gas, it exhibits collective effects (see Figure 1).

Figure 1 Schematic Diagram of Plasma Components

Low-temperature plasma surface treatment is an advanced technology that induces reactions on material surfaces via plasma. On one hand, it can introduce specific functional groups onto material surfaces and cause surface etching, thus forming cross-linked structures or free radical layers. On the other hand, the gas temperature remains nearly at room temperature during treatment, and the whole process consumes relatively low energy, which effectively cuts down operational costs in practical applications.

This technology causes negligible damage to the bulk material. It can remarkably optimize various surface properties while keeping the intrinsic properties of the base material intact and stable.

Mechanism of Enhanced Adhesion of Aramid Fiber via Plasma Modification

1. Physical Modification

During plasma treatment, large quantities of active particles such as ions, excited molecules and free radicals in plasma bombard the aramid fiber surface and produce an etching effect. This roughens the fiber surface and creates numerous micro-pits, which increases the specific surface area of the fiber.

The original fiber surface is smooth. After 3 minutes of plasma treatment, multiple indentations appear and the surface becomes significantly rougher, which facilitates the bonding between fiber and matrix. Figure 2(b) to Figure 2(e) show the micromorphology of aramid fiber before and after plasma treatment.

Figure 2 Micromorphology of Aramid Fiber Before and After Plasma Treatment

2. Chemical Modification

Activation: The bond energy on the aramid fiber surface is about 3~8 eV per bond, while the energy of plasma particles ranges from 0 to 20 eV per bond. When plasma acts on the fiber surface, it breaks the chemical bonds on the surface. The free radicals in plasma then form a network cross-linked structure with the broken bonds, greatly improving the surface activity.

Grafting: During plasma treatment, carbon, nitrogen and oxygen molecules in the ambient atmosphere are excited and react on the fiber surface, introducing abundant functional groups such as hydroxyl, amino and carboxyl groups. Based on polar compatibility, these functional groups interact and bond with polar groups in polyurethane, thereby greatly increasing the overall bonding strength.