Mechanical Investigation of Epoxy Based High-Strength Carbon Fiber Braided Composites Modied with Silane Coupling Agents

Braiding technique is one of the most cost-effective and versatile methods to manufacture braided preforms for producing textile reinforced composites which have been utilized in a number of applications such as aerospace and automotive sectors. Carbon fiber is one of the most common reinforcing fibers having high strength and modulus used in high-performance composites. In this study, epoxy resin was modified with 3 – aminopropyltriethoxysilane (APTES) and 3 – aminopropylmethyldimethoxysilane (APMDMS) in order to enhance interfacial adhesion between matrix and carbon fiber. Composites were produced by vacuum-assisted resin infusion method (VARIM) using braided carbon fabrics and epoxy resin which was treated with silane at different concentrations (from 0.0% to 1.0%). Braided fabrics were manufactured from a high-strength standard modulus type of carbon fiber and using a radial braiding machine. According to the mechanical results, the ideal (optimum) concentration of APTES and APMDMS for the matrix modification has been around 0.5 wt% of the epoxy system. Also, the mechanical properties of APTES-treated epoxy composites are slightly higher than those of APMDMS-treated epoxy composites at the same concentration. When it is compared to silane untreated composite, 0.5 wt% of APTES/APMDMS silane treated epoxy/carbon braided composites have led to an increase of 7.71/6.16 and 7.65/6.05 % in tensile and flexural strength while the corresponding increase has resulted in 17.48/13.51 and 16.63/13.33 % in terms of tensile and flexural modulus, respectively. Impact testing results indicate that


INTRODUCTION
Textile composites are very attractive because of their superior mechanical properties. So, they have been widely used in aeronautical, automotive, petroleum, maritime, defense, and sports industries among others [1,2]. Carbon fiber composites are ideal for applications where strength, stiffness, lower weight, and superior fatigue properties have critical pre-requisites [3,4]. Braiding, in general, is a textile manufacturing process in which three or more yarns are interlaced (in the longitudinal direction) to obtain a stronger or thicker product. Braids may be linear, rounded, plane-shell, or solid structures (1-D, 2-D, or 3-D fabrics) with constant or variable cross-section [5]. Two-dimensional (2D) biaxial and three-dimensional (3D) braided fabric structures are frequently used in the aerospace, automotive, and medical industries as structural elements [6,7]. Braiding is recognizable from the other conventional textile processes such as weaving and knitting, in the textile method approaches to produce yarns into a fabric, and in the peculiarity fiber architectures stemming from those methods [8].
A weak fiber-matrix interfacial adhesion can affect adversely the mechanical performance properties of the textile reinforced composites because of the surface incompatibility between hydrophilic carbon fiber and non-polar resin system [5]. So, the relationship between matrix system and carbon fiber is very important to the mechanical properties of carbon fiber composites. As a consequence, a proper interphase process is required to attain enhancements within the mechanical properties of the composites.
It was proposed by many researchers that poor interfacial adhesion ends up with matrix crack, deflection, and fiber pull-out, which were the cornerstone mechanisms for brittle-matrix fiberreinforced composite structures [9,10]. This brittle fracture mechanism is one of the disadvantages of carbon fiber composites especially epoxy-based ones [11]. Epoxy resins are a very common type of matrix system that can be curable by some reaction of oxirane groups.
There is a wide range of organo-functional silane coupling agents that can work with epoxy resins effectively. However, it needs to be certain generalizations that can be made for selecting the ideal silane for the desired structure [12][13][14].
Silane coupling agent has double functional groups which can form a bridge between the epoxy resin and carbon fiber to enhance the synergy. The use of silane coupling agents to modify the matrix system has many advantages. One of them is that alkoxysilane groups may react with fiber surface and they have a vast number of functional groups that can be adjusted depending on the matrix to be employed. Thus, a silane coupling agent can be used to modify the resin as a chemical method that promotes the interfacial adhesion between matrix and fiber leading to improved mechanical properties of composites [9,10,15,16].
Many studies are related to silanization effects on mechanical properties of woven glass, carbon, or hybrid fabric composites. However, there aren't many studies on braided composites modified with silane coupling agents or a very small amount of work is done to evaluate it. So, in this present study, braided carbon composites were manufactured with high-strength carbon fibers and modified with silane coupling agents which were 3-aminopropyltriethoxysilane (APTES) and 3-aminopropylmethyldimethoxysilane (APMDMS). Mechanical testing (tensile, flexural, and impact) of these composites was carried and the effects of silane coupling treatment on the mechanical properties of the epoxy/carbon braided composites were discussed.
The chemical composition of silane coupling agents and epoxy/carbon braided composites were analyzed by Fourier transform infrared spectroscopy (FTIR). The fracture surfaces of these composites obtained from impact analysis were studied with the help of Scanning Microscope Analysis (SEM).

Production of the Reinforcement System
The braided carbon fabrics used in this study were produced with Herzog RF/144-100 machine at ITA (Institut für Textiltechnik) RWTH Aachen University. T700-24K standard modulus carbon fiber supplied by Toray Company, Japan was used in the production of fabrics (Tab. 1). In order to manufacture a carbon braided fabric structure, there must be two sets of carbon fiber that are intertwined. The final structure is aligned at some angle to the longitudinal axis of the braided fabric. Two sets of carbon fiber bobbins are identical but have counter direction at any area along the braid. Carbon fiber bobbin from one set does not intersect with any others in the same set but intersects every carbon fiber bobbin in the other set. Each set of carbon fibers in the braiding system are moved at the same speed. In order to produce regular braided pattern fabric, all carbon fiber red bobbins within one set are moved in the clockwise direction while the yellow bobbins in the other set are moved in the counter-clockwise direction (Fig. 1). b a To produce the intertwining of the two sets of carbon fiber bobbins, there has to be a motion where some of the bobbins in each set have proceeded toward the center of the tubular mandrel and the rest toward the outside synchronically. Carbon fiber is moving in and out in the radial direction and it is also moving along the circumference of the tubular mandrel. The angular velocity of the braiding carriers is specified as ω. The braiding mandrel is taken forward by a pulling (take-up) mechanism. The take-up speed of the mandrel is specified as ν. In the case of the tubular mandrel, the relationship between the take-up speed of the mandrel (ν), the rotational speed (ω) of the bobbin carriers, and the mandrel radius (R) is written by Equation 1 [17][18][19].
Eq. 1. [19] 2D braiding structures can be open or closed meshed architectures which are characterized by coverage factor that means the measure of the surface area enclosed by strands [20]. Since the number and width of fiber (such as 1K, 3K, 6K, 12K, 24K) cannot be changed easily, the coverage factor is obtained by changing the braiding angle corresponding to the change of mandrel diameter [21]. Equation 2 is given for tubular mandrels. The prepared samples for this particular case were braided on a mandrel by the braiding robot.
Each ply was braided once, cut, and stacked on top of another one. Even though the adjustment is meant to be 45° in orientation, the human error may vary between ±5° (Fig. 2). Other properties of the braided carbon fabric are shown in Table 2.

Resin System
In the matrix system used in the study, MGS L 160 epoxy resin and MGS H 160 hardener produced by HEXION was preferred. This epoxy resin is suitable for manufacturing products according to the vacuum infusion method. In the study, the epoxy/hardener mixing ratio was chosen to be 100:25 by weight, as recommended by the manufacturer (HEXION). Table 3 below shows the properties of the resin, while Table 4 shows the properties of the hardener at room temperatures.

Silane Coupling Agents
In this study, 3-aminopropyltriethoxysilane (APTES) and 3aminopropylmethyldimethoxysilane (APMDMS) produced by Gelest company were used as the coupling agents in the epoxy resin system with 95% concentration and colorless liquid. Both silane chemical properties are given in Table 5 below.

Modification of the Matrix System
In this study, silane coupling agents were separately mixed with the epoxy resin system in order to attain matrix modification. For the modification of the matrix system, the epoxy matrix was modified by silane coupling agents. The percentage of the silane concentration started from 0.0 wt% to 1.0 wt% by increasing 0.25 % for this study. Specimens were prepared for the mechanical testing (tensile, impact, and flexural) in order to understand the optimum concentration percentage of silane coupling agents (APTES and APMDMS). They were separately added to the epoxy matrix and mixed for 5 min at least during the composite production process. After that, the hardener was incorporated into the mixture, which was stirred for approximately 10 min.

Production of the Composite Laminate Materials
Carbon braided composite laminates were produced by the vacuum-assisted resin infusion method (VARIM). The glass mold was cleaned with propanol and the inside of the glass surface covered with releasing agent for easier separation of the composite structure from the mold.
There are some production points that need to be taken into account for the vacuum infusion technique.
Where ply m is the weight of fibers of one-ply, fiber d is the density of the fiber, f v is the predetermined volume fraction of the fiber which is approximately 0.5 [24] for the vacuum infusion process, ply V is the volume of one-ply, and ply A is the area of the ply.   Table 6 below.

Fourier Transform Infrared Spectroscopy (FTIR)
FTIR spectra analysis was carried out using Perkin Elmer -120 in order to define the spectrum of the silane coupling agent 3aminopropyltriethoxysilane (APTES), unmodified and modified epoxy/carbon braided composite samples.

SEM Analysis
Scanning electron microscopy (SEM) was executed out using an LEO 1450 VP (LEO Electron Microscopy Ltd, Cambridge, UK). These images were obtained by using fractured surfaces resulting from the Charpy impact test.

Tensile Strength Properties of Braided Composites
Epoxy-based carbon fiber braided samples were subjected to tensile tests. In this study, the reported data is shown with the confidence interval represented by error bars. Figure   which are 47.08 GPa and 46.56 GPa, respectively. When the silane-treated composite is compared to the untreated composite, the value of tensile strength of 0.5 wt% APTES and APMDMS is improved up to 7.71% and 6.16%, respectively. When it comes to the value of tensile modulus for comparison between untreated and silane-treated composites, the value of 0.5 wt% APTES and APMDMS is gone up to 7.65% and 6.05%, respectively. Considering values obtained from the tensile analysis; tensile strength, modulus, and strain values increase gradually with the incrementation of silane concentration until it reaches the optimum value which is 0.5 wt% for both coupling agents in this study. However, after they come to the optimum value, they decrease with the increase of silane concentration for both of the coupling agents. It is thought that the most important factors affecting the tensile strength in textile-reinforced composite structures are single fiber strength and fiber volume fraction [28,29]. For this reason, a slight improvement in the tensile strength properties of carbon braided composite materials has been achieved in the modification process with silane which can be interpreted by the enhancement of adhesive features between carbon fiber and epoxy matrix system [30]. It can be also deducted that the increase and subsequent decrease are related to the saturation level of the silane which depends on the fact that the excess silane coupling agent can generate a weak boundary structure after optimum silane agent concentration [31].

Flexural Strength Properties of Braided Composites
The   The flexural properties of epoxy/carbon braided composite are affected by the fiber volume fractions, which is determined by the production process, and adhesion between fiber-matrix component. Proper silane coupling process might lead to improved adhesion which results in better flexural performance at braided composite [16,32,33].

Impact Strength Properties of Braided Composites
Charpy impact test was implemented on unnotched samples and Figure 9 illustrates the related test results. According to the results, the optimum concentration of both silane coupling agents is 0.5 wt% as like tensile and flexural properties of the braided composites. ) fiber/matrix system [36][37][38]. The effect mechanism of the fiber material amount on the impact resistance of textile reinforced composites is more sophisticated than tensile and flexural strengths properties due to the impact resistance of energy distribution during stress. The impact energy distribution is very important for matrix cracking, delamination, fiber/matrix debonding, fiber pullout, and fiber rupture modes [39].

FTIR -ATR Analysis
FT -IR spectra were attained by utilizing the Attenuated Total Reflectance (ATR) module.
FTIR-ATR was used for surface functional group analysis of 3-aminopropyltriethoxysilane (APTES) amino-silane coupling agent and its FT-IR spectra are seen in Figure 10.

Microstructure Properties of Braided Composites
Scanning electron microscope images were captured for silane-untreated (Fig. 12a), 0.50 wt% APTES-treated (Fig. 12b), and 0.50 wt% APMDMS-treated (Fig. 12c) to observe the microstructure of the epoxy-based braided composites. Based on the results of the Charpy impact and the obtained SEM images from the same analysis, the damage types seen in the samples were determined as fiber breakage, fiber peel, matrix pore, and matrix crack. In addition to these damages, matrix residues are seen on the fracture surfaces. With the usage of silane, less fiber pulls out and more fiber breakage is detected because of the improvement in interfacial strength according to the SEM images. The fracture surfaces of the epoxy braided carbon composites exhibit various outputs stemming from modification of fiber/matrix with silane coupling agent as represented in Figure 12. When the surface of fracture for silane untreated epoxy composite is examined, it can be observed that less amount of epoxy resin is spotted to adhere onto the fiber surfaces which seems smoother and cleaner comparison with the other two silane treated epoxy braided carbon composites. It can be said that the adhesion between the epoxy resin and carbon fiber is weaker than the others (Fig. 12a).
After modification of the matrix with 0.50 wt% APTES silane, SEM images show that a greater amount of epoxy resin is detected at the fiber surfaces in comparison to silane untreated braided carbon composite and this situation also applies to 0.50 wt% APMDMS-treated samples ( Fig.   12b and Fig. 12c). That is to say, decent modification of carbon fiber with silane coupling agents increases the interfacial adhesion that leads to improving the mechanical performance of the braided carbon composites. Moreover, these observations can be proved by the results of the tensile strength, flexural strength, and impact resistance analysis.

CONCLUSIONS
Two types of silane coupling agents (APTES and APMDMS) were selected to treat epoxy /carbon braided fabrics. APTES and APMDMS were utilized in order to enhance the interfacial bond between the matrix system (epoxy resin) and reinforcing fiber system (carbon fiber). The In this present study, the mechanical properties of epoxy/carbon braided composites decreased above 0.5 wt% silane concentration. This is related to the fact that the excess amount of silane, which causes a lubrication effect, constitutes the weak boundary layer after optimum (0.5 wt%) silane coupling agent concentration. So, all the mechanical properties (tensile, flexural, and impact) of the composites have shown a decrease at higher silane concentrations.
When all mechanical test results are examined, APTES-treated epoxy/carbon braided composites in all concentrations have resulted slightly better than APMDMS-treated epoxy/braided carbon composites. It is thought that the reason for this situation can be explained by the less branched network and more closed/packed structure for APTES that results in slightly better mechanical performance on braided composited compared to APMDMS [22,40].
According to the results of tensile analysis, the tensile strength of the epoxy/carbon braided composite could be improved by both silane coupling agents treatment. However, when it is compared to untreated braided composite, this improvement was limited with 7.71 % and 6.16% in tensile strength value for optimum concentration which is 0.5 wt% both APTES and 0.5 wt% APMDMS, respectively. In terms of tensile modulus, analogous improvement wasn't able to pass 7.65 % and 6.05 % 0.5 wt% APTES and 0.5 wt% APMDMS, respectively.
When it comes to the flexural properties of these epoxy/carbon braided composites, the enhancement has resulted in 17.48 % and 13.51% for the optimum concentration of both APTES and APMDMS in flexural strength value, and the analogous enhancement of flexural modulus has occurred with 16.63% and 13.33 %, respectively. Specimens with APMDMStreated tend to have slightly lower flexural strength and modulus compared to APTES-treated ones.
The effect of the silane coupling process on Charpy impact strength is slightly enhanced by 6.87 % and 4.31 % for the optimum concentration of both APTES and APMDMS, respectively.
The reason for the slight increase can be also linked to that the improvement in interfacial adhesion modified by silane coupling agents has a slight effect on the impact strength of the braided carbon composites. It is thought the effect of interfacial adhesion on impact strength is negligible level.
In summary, the proper silanization process has good compatibility with the epoxy matrix system and so this situation increased the interfacial adhesion between braided carbon fiber and epoxy matrix that results in better mechanical performance on composite structures.

Availability of data and materials
The data that support the findings of this study are available on request from the corresponding author [OE].