## 1 Introduction  
Rayon flock is a synthetic leather having a flexible backing and multiple upstanding flock fibers adhered to the backing. It has a layer of rayon flocking hair held by adhesive on a rayon cloth. It is usually used as upholstery for sofas and automobiles. Owing to the hydrophilic nature of rayon (regenerated cellulosic fiber), it can be easily stained. This problem can be alleviated by improving surface hydrophobicity. Many researchers have explored ways of improving the flock surface hydrophobicity. [PERSON] _et al._ prepared a transparent, superhydrophobic, silica-based surface film by introducing a nano-scale rough surface [1]. [PERSON] _et al._ confirmed that a superhydrophobic surface can be achieved by aligned carbon nanotubes coated with a thin zinc oxide film [2]. [PERSON] and [PERSON] enhanced the surface hydrophobicity of nylon flock fabric by grafting with poly(acrylic acid) and 1_H_-1_H_-perfluorocotyl-amine [3]. [PERSON] _et al._ fabricated a hydrophobic coating by CF\({}_{4}\) plasma treatment [4].  
Among the different methods, plasma treatment of a polymer surface is an effective technique because of its unique ability to form a thin hydrophobic film on surfaces [5]; this technique can be integrated into continuous production processes [6] also, which means a single-step method for surface modification. Plasma treatment of polymer surface is a well-established technique [7; 8; 9]. Atmospheric pressure plasma overcomes the disadvantages of low-pressure plasma--its integration into an in-line production process is cumbersome and expensive [10]. Traditional plasma sources include transferred arcs, corona discharge, and dielectric barrier discharge (DBD) [10]. Corona and DBD produced non-equilibrium plasmas, with gas temperature between 50 \({}^{\circ}\)C and 400 \({}^{\circ}\)C, which may limit their usage in materials processing [10]. On the other hand, atmospheric pressure plasma offers the advantages of a temperature range from 20 \({}^{\circ}\)C to 200 \({}^{\circ}\)C with a high concentration of reactive species from 10 ppm to 100 ppm [10]. Atmospheric pressure plasma jet is an effective way to create a plasma zone with its movable torch [11]. A number of researchers have studied the possible applications of the plasma torch [11; 12]. It could be a single-step method for surface hydrophobization. Because of its stability and low toxicity, organosilane is one of the monomers for introducing a hydrophobic surface [12; 13; 14; 15]. In previous studies [16; 17; 18; 19], tetramethylsilane (TMS) has been applied to modify a synthetic leather surface under the influence of atmospheric pressure plasma treatment. The hydrophobicity of a TMS-plasma-treated synthetic leather surface, which is a flat surface, was improved significantly. However, TMS plasma treatment for synthetic leather with a flock surface has seldom been reported. Therefore, in this study, the surface of rayon flock synthetic leather is modified by means of atmospheric pressure plasma using a plasma jet with TMS and the chemical and morphological changes on the surface are evaluated by instrumental methods. In addition, the statistical method SPSS is used for finding the correlation between the treatment parameters.