Fibers and Polymers 2014, Vol.15, No.1, 1-7 1 The Application of Gas Plasma Technologies in Surface Modification of Aramid Fiber Youyi Sun 1,2 , Qing Liang 1 , Huijun Chi 1 , Yongji Zhang 1 , Yi Shi 3 , Daining Fang 2 * , and Faxing Li 2 1 Research Center for Engineering Technology of Polymeric Composites of Shanxi Province, North University of China, Taiyuan 030051, P. R. China 2 College of Engineering, Peking University, Beijing 100871, P. R. China 3 Colleges of Materials and Science, Guilin University of Technology, Guilin 541004, P. R. China (Received March 28, 2012; Revised May 7, 2013; Accepted May 11, 2013) Abstract: Gas plasma technologies have been utilized to improve the surface properties of fibers in many applications from textiles to fiber-reinforced composites since the 1960s. This review discusses the feasibility and characteristics of gas plasma technologies applied to aramid fiber. The influence of various plasma treatments on the chemical and mechanical properties of aramid fibers as well as fiber-reinforced composites is described. The moisture regain is emphasized to achieve good bonding between aramid fibers and polymer matrix and to enhance the surface modification of aramid fiber and mechanical properties of the composites. More sophisticated technologies such as plasma-initiated graft polymerization are also discussed to highlight very recent developments. Keywords: Aramid fiber, Surface modification, Plasma, Mechanical properties Introduction Aramid fibers (AF) have frequently been utilized for composite material fabrication because of their high thermal stability in addition to their high modulus, high strength, vibration damping, and resistance to chemicals [1]. However, the adhesion between AF and most polymer matrices is poor, resulting from high crystallization and smooth surface of the fiber [2]. Therefore, it is necessary to modify the surface of AF and the common methods of surface modification on AF are chemical methods [3-6], such as chemical etching, chemical surface grafting, and polymerization modification. Previous studies have showed that the surface roughness, interfacial shear strength (IFSS), and other properties were all remarkably improved after chemical treatments. Despite these advantages, chemical treatments may erode AF, and decrease its mechanical properties [5,6]. In addition, chemical treatments require large amount of water and chemicals, resulting in the environmentally unfriendly. They prevented the large-scale produce of the AF with surface modification. In recent years, plasma treatment which is a physical method for surface modification has been largely developed [7,8]. Depending on the interactions of atomic oxygen, OH radicals, UV photons and ions with polymers surface, plasma treatment not only induces physical and changes in polymers surface, but also keeps furthest the original bulk qualities of AF, such as its mechanical properties. Moreover, it is a dry process and therefore environmentally friendly. So, some works have reported the effect of treatment conditions (eg. time, discharge power, moisture regain, and so on) on the surface properties of AF and mechanical properties of its composites [9-32]. Despite all the studies done so far in this field, the application of gas plasma at the room temperature on the surface modification of AF is a relative new physical process. So, most of the works done so far was devoted to the development and optimization of the effect rather than a deeper understanding of the mechanism of the physical process. The scope of this paper is to discuss some mechanistic aspects of the physical process between gas plasma and AF. It may be helpful for further discussions and the development of surface modification of AF based on gas plasma at the room temperature. Surface Treatment of AF Based on Gas Plasma Technologies The Formation of Plasma The experiment set-up in gas plasma technologies can be obtained from market, eg Atomflo-R, Surfx Company, USA as shown in Scheme 1(A) [11-15]. The experiment set-up can also be designed and prepared by our group as shown in Scheme 1(B) [16-19]. A single barrier covered is used between two metal plate-electrodes. The upper electrode covered with alumina or other materials are connected to a power supply which provides a high AC voltage continuously, the lower electrode is grounded [33,34]. A uniform filamentary discharge comes into the electrodes when proper voltage is applied. And then the gas plasma is obtained around the electrodes in the presence of gas (ca. O 2 , N 2 , NH 3 , Ar, and so on). For example, the action of oxygen plasma constituents, namely, atomic oxygen [O( 1 S)], metastable (singlet 1 ∆ a ) excited molecular oxygen, and charged particles such as O 2+ , O 2- , O + , O - and e - [35]. *Corresponding author: syyi@pku.edu.cn DOI 10.1007/s12221-014-0001-x