## 1 Introduction  
As the demand for composite materials with high strength and light weight increases, composite manufacturing processes are being investigated to reduce production costs. Among these processes, liquid composite molding (LCM), including resin transfer molding (RTM) or structural reaction injection molding (S-RIM), is advantageous for manufacturing complex-shaped products. These inexpensive processes can be used without an autoclave process [1; 2; 3; 4]. In LCM, the fiber is preformed in the mold prior to injecting resin. The fiber preform process using a binder is recommended, because fiber layers should be bonded to each other to fix the desired shape. There are many kinds of binders. Depending on the materials used, they can be classified into thermoset and thermoplastic binders and may be powder or liquid-type binders. As the amount of binder used in LCM increases, it can easily fix a preform of a complex shape owing to the high adhesion between the fibers. However, there are several associated issues preventing the large amount of binder use, including changes in the shear properties and the friction coefficient of the fiber preform, as well as the poor permeability. Consequently, optimization studies attempting to identify an efficient preform process by analyzing the effects of binders are needed [5; 6; 7]. Many studies on the use of binders in the fiber preform process for LCM have been conducted [8; 9; 10; 11]. [PERSON] et al. investigated the effect of a thermoplastic polyester binder on the glass transition temperature and mechanical properties of S2-glass-fibre reinforced epoxy composites through a T-peel test, a double cantilever beam (DGB) test, and a short beam shear (SBS) test [8]. [PERSON] et al. examined the influence of binder materials, known as tackifiers, on the characteristics of fiberglass preforms according to the different concentrations of the tackifiers [9]. [PERSON] and [PERSON] manufactured electrical-glass/polyester composite panels using a preform tailored to various concentrations of the thermoplastic binder and analyzed their compressive mechanical behavior and failure modes [10]. [PERSON] and [PERSON] studied the characteristics of two types of preform binder--reactive thermosets and non-reactive thermoplastic--in glass/vinyl ester composites. In their study, the interplay adhesion of woven glass plies and inter-laminar shear strength of woven-glass-reinforced vinylester composites was achieved depending on the type, amount, and processing of the binder [11]. [PERSON] et al. investigated the change in the ultimate strength of 800 Tex HTS-coated carbon fibers through double lap shear under different activation temperatures and activation techniques, including an electric oven, hot plate, microwave, and CO\({}_{2}\) laser [12]. There have been many studies on the effect of binders on the mechanical properties of processed composite materials, and some have shown that binders could affect the permeability of preform in LCM [6; 9; 13; 14]. Recent studies have focused on the change in permeability depending on the chemical state of the binder to minimize the binder effect during LCM [15; 16]. Among the process parameters, the permeability of the fiber preform is an important factor in LCM that determines the total process time, thereby affecting the cost of production. The measuring method of the permeability of fiber preform was introduced by [PERSON] et al. [17], [PERSON] et al. [18; 19], and [PERSON] et al. [20] using optical radial flow in a porous medium in the in-plane direction. For the out-of-plane directional permeability, [PERSON] et al. [21] and [PERSON] et al. [22] used a method applying the saturated permeability. In addition, in the measurement of permeability, it is necessary to test with the same fluid, because the results may change depending on the type of test fluid used [23].