Adhesion Activation of Aramid Fibers for Industrial Use

Para-aramid is used as reinforcement fiber in many composite materials, including rubbers. Optimal fiber-matrix adhesion is essential in these materials. Since untreated aramid always shows poor adhesion, an adhesion activation process is required. This contribution describes and explains such processes, with focus on those applicable in industry. For rubber applications, adhesion can be achieved by resorcinol-formaldehyde latex (RFL) dipping of the fiber cord. This dipping has to be preceded by a treatment with an epoxy-based finish or epoxy-based pre-dip. Fundamental studies have improved the understanding of the maturation, application, and curing step of the epoxy treatment, including the resulting chemical and physical fiber surface structure. In addition, these studies have shown migration of curatives from rubber to RFL, during curing of the rubber. In this way, the complete aramid-rubber adhesion mechanism has been revealed. The epoxy treatment has also added value in applications of aramid fibers in thermoset matrices, such as epoxy, and in thermoplastic matrices, such as polyamide and polyurethane. This has been proven using several micro and macro-mechanical tests. Special attention is given to possible negative effects of processing oil on the adhesion performance. Some oil turns out to be necessary to reach the maximum adhesion and higher amounts do not directly lead to adhesion problems, both for rubber and for epoxy matrices. This is in line with long-standing industrial experience. Plasma treatment is potentially able to replace epoxy treatment of aramid fibers for rubber applications. However, considerable challenges are encountered in case the activation is carried out on bundles of filaments. On the other hand, plasma activation of cords seems to be feasible as an industrial alternative. There are also important applications where aramid is used as short-cut fiber. For rubber matrices, "dipped chopped fibers" are commonly used, resulting in a higher modulus of the reinforced material. This can be attributed to improved rubber adhesion and improved dispersion of the fibers throughout the rubber. For engineering plastics, the primary objective of adding short-cut fibers is to improve the abrasion resistance. This property does not seem to be affected by adhesion activation.