Influence of Non-Reactive Epoxy Binder on the Permeability And

applied sciences

Article Inﬂuence of Non-Reactive Epoxy Binder on the Permeability and Friction Coeﬃcient of Twill-Woven Carbon Fabric in the Liquid Composite Molding Process

Hyeong Min Yoo 1 , Jung Wan Lee 2, Jung Soo Kim 2 and Moon Kwang Um 2,*

1 School of Mechanical Engineering, Korea University of Technology and Education (KOREATECH), 1600, Chungjeol-ro, Byeongcheon-myeon, Dongnam-gu, Cheonan-si 31253, Korea; [email protected] 2 Composites Research Division, Korea Institute of Materials Science (KIMS), 797, Changwon-daero, Seongsan-gu, Changwon-si, Gyeongsangnam-do 51508, Korea; [email protected] (J.W.L.); [email protected] (J.S.K.) * Correspondence: [email protected]; Tel.: +82-55-280-3315; Fax: +82-2-772-1659

Received: 16 May 2020; Accepted: 5 October 2020; Published: 10 October 2020

Abstract: In the liquid composite molding process, a binder is used to fix the preform. In this study, the influence of a non-reactive epoxy binder was investigated. To allow the measurement of permeability, the preform specimen was produced under three preforming conditions: neat fabric preform, binder-treated fabric preform without heat treatment, and binder-treated fabric preform with heat treatment. The in-plane directional permeability, K1 (having maximum flow velocity), and K2 (having minimum flow velocity) of the binder-treated fabric preform decreased approximately 80% compared to the neat fabric preform. The permeability in the out-of-plane direction decreased approximately 80% in the binder-treated fabric preform without heat treatment and about 98% in the binder-treated fabric preform with heat treatment. This decrease occurred because the treated binder on the fiber hindered resin impregnation. The effect of the binder on the friction coefficient of carbon fabric was also investigated. The friction coefficient was high when the binder was on the friction surface and increased 40–200% at 110 ◦C, compared to 25 ◦C.

Keywords: composites; processing; permeability; ﬁbers; injection molding

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 [1–4]. 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 [5–7].

Appl. Sci. 2020, 10, 7039; doi:10.3390/app10207039 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7039 2 of 13

Many studies on the use of binders in the fiber preform process for LCM have been conducted [8–11]. Tanoglu 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]. Estrada 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]. Tanoglu and Seyhan 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]. Brody and Gillespie Jr 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]. Asareh 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 CO2 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 Adams et al. [17], Weitzenböck et al. [18,19], and Fauster et al. [20] using optical radial flow in a porous medium in the in-plane direction. For the out-of-plane directional permeability, Endruweit et al. [21] and Francucci 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]. In this study, using equipment capable of measuring permeability, changes in the permeability of a twill-woven carbon fabric with a non-reactive epoxy binder treatment was investigated in the LCM process. In addition, the changes in the coefficient of friction between fabrics and between fabric and the metallic mold were observed via friction tests. These tests demonstrated another important effect of binder on the friction coefficient, using a parameter for predicting fiber wrinkles in the LCM process. By analyzing both kinds of process parameters, it was possible to determine the characteristic changes in reinforcements in LCM as a function of the binder usage during the preforming process.

2. Experimental

2.1. Materials Twill-balanced woven carbon fabric with T300(3K) was supplied by Toray Corporation. HUNTSMAN’s Araldite LT3366 was used as an epoxy binder, which is a non-reactive powder type, for ﬁxing the ﬁbers through cold stamping after melting at an appropriately high temperature. The epoxy binder has a softening point above 110 ◦C and a glass transition temperature of 75–85 ◦C. To measure the permeability, silicone oil (KF-96-350cs) from ShinEtsu Corporation, which has a viscosity range (340cp at room temperature) similar to that of the epoxy widely used in resin transfer molding (RTM), was used.

2.2. Preforming Process Binder-treated fabric was manufactured using an epoxy binder applied uniformly (5 1 g/m2) to ± one side of Toray’s twill-woven carbon fabric roll. For the preforming process to prepare the preform specimen, the carbon fabrics were laminated 8-ply (Figure1) in the same direction and compressed using a vacuum bag, and, if required, heat treatment was performed for 30 min. at 110 ◦C in a Appl. Sci. 2020, 10, 7039 3 of 13

Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 13 convectionAppl. Sci. 2020 oven., 10, x FOR For PEER investigating REVIEW the effects of the binder, three types of preform specimens were3 of 13 fabricated: neat fabric preform, binder-treated fabric preform without heat treatment (preforming at (preforming at room temperature), and binder-treated fabric preform with heat treatment room temperature), and binder-treated fabric preform with heat treatment (preforming at 110 C). Every (preforming(preforming at at 110 room °C). Everytemperature), case was fabricatedand binder-t withreated the same fabric size (300preform mm ×with 300 mm),heat◦ andtreatment in the case was fabricated with the same size (300 mm 300 mm), and in the measurement of permeability measurement(preforming at of 110 permeability °C). Every the case fiber was volume fabricated frac× withtion ofthe the same preform size (300 was mm determined × 300 mm), by fixingand in the the the fiber volume fraction of the preform was determined by fixing the thickness using the upper and thicknessmeasurement using of the permeability upper and thelower fiber jigs volume before frac thetion test offluid the injectionpreform was(Figure determined 2). The fiber by fixing volume the lower jigs before the test fluid injection (Figure2). The fiber volume fraction was calculated except for fractionthickness was using calculated the upper except and for lower the binderjigs before volume the testfraction fluid because injection it (Figurewas negligibly 2). The fibersmall volume (<3%) the binder volume fraction because it was negligibly small (<3%) compared to the amount of fiber comparedfraction was to thecalculated amount except of fiber for (Table the binder 1). volume fraction because it was negligibly small (<3%) (Tablecompared1). to the amount of fiber (Table 1).

FigureFigure 1.1. PreformingPreforming processprocess forfor thethe carboncarbonfabric fabricpreform preformusing usingvacuum vacuum bagging. bagging. Figure 1. Preforming process for the carbon fabric preform using vacuum bagging.

Figure 2. Schematic of the permeability measurement system. Figure 2. Schematic of the permeability measurement system. Table 1. BinderFigure content 2. Schematic in the fabric of the preform permeability specimen measurement in each ﬁber system. volume fraction.

Binder Fiber Volume Fraction (%) Weight (g) Volume Fraction (%) 50 3.6 1.9 55 3.6 2.1 60 3.6 2.2 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 13

Table 1. Binder content in the fabric preform specimen in each fiber volume fraction.

Binder Fiber volume fraction (%) Weight (g) Volume fraction (%)

50 3.6 1.9

55 3.6 2.1

Appl. Sci. 2020, 10, 7039 60 3.6 2.2 4 of 13

2.3. PermeabilityPermeability MeasurementMeasurement

2.3.1. In-Plane Direction The permeability forfor two-dimensionaltwo-dimensional flow flow was was calculated calculated using using polar polar coordinates coordinates expression expression of ofDarcy’s Darcy’s law law as as follows: follows: Kunsat ∂P v𝑣= ∙ .. (1) (1) µ · ∂r The expressionexpression waswas used used to to obtain obtain the the unsaturated unsaturated permeability permeability by observingby observing the flowthe flow front front over overtime, time, where where the injection the injection pressure pressure was constant. was constant. In the aboveIn the expression, above expression,v is the volume-averaged𝑣 is the volume- ∂P averagedfluid velocity, fluidK unsatvelocity,is the unsaturatedK is the permeability, unsaturated and permeability,µ and represent and μ the and viscosity represent of the fluid the ∂r viscosityand the pressureof the fluid gradient, and the respectively. pressure gradient, Figure respectively.3 shows the Figure change 3 in shows the flow the change front of in the the resin flow frontover time.of the Usingresin over the time. method Using suggested the method by Adams suggested et al. by [ 17Adams] and et assuming al. [17] and that assuming the flow that front the is flowelliptical, front the is permeabilityelliptical, the was permeability obtained through was obtained curve fittingthrough using curve the leastfitting squares using method.the least Withsquares the method.measurement With systemthe measurement shown in Figure system2, the shown test forin eachFigure case 2, wasthe test conducted for each 5 timescase was and conducted the average 5 valuetimes and is shown. the average value is shown.

FigureFigure 3. ObservationObservation of of the the resin resin flow ﬂow front front over time (V. F.: 50%, neat fabric preform).

2.3.2. Out-of-Plane Direction In the general RTM process, the permeability in the out-of-plane direction was calculated using the saturated permeability equation as follows:

Q µ L Ksat = · , (2) A · ∆P

3 where Ksat is the saturated permeability, Q is the flow rate (m /s), A is the cross-sectional area of the porous medium, and L and ∆P represent the length of the porous medium and the pressure difference, respectively. Ksat was calculated by measuring the difference between the inlet pressure and the outlet Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 13

Appl. Sci.In the2020 general, 10, x FOR RTM PEER process, REVIEW the permeability in the out-of-plane direction was calculated using5 of 13 the saturated permeability equation as follows: In the general RTM process, the permeability ∙in the out-of-plane direction was calculated using K ∙ , (2) the saturated permeability equation as follows: ∆ where K is the saturated permeability, 𝑄 is the flow∙ rate (m3/s), A is the cross-sectional area of the K ∙ , (2) porous medium, and L and ∆𝑃 represent the length ∆ of the porous medium and the pressure 3 difference,where K respectively. is the saturated K permeability, was calculated 𝑄 isby the measuring flow rate the (m difference/s), A is the between cross-sectional the inlet area pressure of the andporous the medium,outlet pressure and L after and waiting ∆𝑃 represent for the saturathe lengthtion ofof resin the flow—thatporous medium is, the andflow therate pressure became Appl. Sci. 2020, 10, 7039 5 of 13 constant.difference, With respectively. the measurement K was system calculated shown by measuringin Figure 2, the the difference test for each between case thewas inlet conducted pressure 5 times.and the outlet pressure after waiting for the saturation of resin flow—that is, the flow rate became constant.pressure afterWith waitingthe measurement for the saturation system of shown resin flow—thatin Figure 2, is, the the test flow for rate each became case constant.was conducted With the 5 2.4. Friction Coefficient Test times.measurement system shown in Figure2, the test for each case was conducted 5 times. In this study, the friction coefficient between the fabric and mold surface, as well as the fabric to 2.4. Friction Coefficient Test fabric2.4. Friction friction Coe coefficient,fficient Test was measured as a function of the presence of the binder and the temperatureInIn this this study, study,conditions. the the friction friction The kinetic coefficient coefficient friction between between coefficient the the fabric fabric was and andmeasured mold mold surface, surface,using an as as FP-2260,well well as as the thewhich fabric fabric can to to controlfabricfabric frictionthe movement coecoefficient,fficient, speed was was measuredof measuredspecimen, as a assupplied function a function ofby the the presenceof Thwing-Albert the presence of the binder ofCorporation, the and binder the temperature with and a the20 mm/stemperatureconditions. sliding conditions. Thespeed kinetic and a The friction0.5 kgkinetic weight coe ffifrictioncient (Figure wascoefficient 4). measured Because was the usingmeasured fabric an used FP-2260, using in this an which studyFP-2260, can had which control an epoxy can the bindercontrolmovement onthe one movement speed side ofonly, specimen, speed the test of suppliedcasesspecimen, were by suppliedset the up Thwing-Albert as byshown the Thwing-Albertin Figure Corporation, 5. In Figure Corporation, with 5, a 20cases mm with1–3/s sliding werea 20 intendedmm/sspeed sliding and toa examine 0.5speed kg and weight the a 0.5change (Figure kg weight in4). the Because (Figure coefficient the4). Bec fabric auseof friction used the fabric in between this used study infabrics hadthis study an according epoxy had binderan toepoxy the on conditionsbinderone side on only, oneof the side the binder testonly, cases treatment,the test were cases set and upwere case as shownset 4 wasup as inintended shown Figure 5into. InFigureshow Figure the 5. 5In ,same cases Figure change 1–3 5, werecases between intended1–3 were the fabricintendedto examine and tothe theexamine metallic change mold.the in change the coe ffiincient the ofcoefficient friction betweenof friction fabrics between according fabrics to according the conditions to the of conditionsthe binder of treatment, the binder and treatment, case 4 was and intended case 4 was to show intended the same to show change the betweensame change the fabric between and the the fabricmetallic and mold. the metallic mold.

Figure 4. Equipment for measuring the friction coefficient. Figure 4. Equipment for measuring the friction coeﬃcient.

Figure 4. Equipment for measuring the friction coefficient.

Figure 5. Test cases of the coefficient of friction of carbon fabric. Figure 5. Test cases of the coefficient of friction of carbon fabric. 3. Results and Discussion 3. Results and DiscussionFigure 5. Test cases of the coefficient of friction of carbon fabric. 3.1. Permeability in the in-Plane Direction 3.1.3. Results PermeabilityFigure and6 shows Discussion in the the in-Plane permeability Direction in the in-plane direction for each fiber volume fraction. The tests wereFigure conducted 6 shows on twothe permeability cases with fiber in the volume in-plane fractions direction of 55% for and each 60%. fiber The volume permeability fraction. of The the 3.1. Permeability in the in-Plane Direction testsfiber were in the conducted 90◦ direction on two (having cases awith maximum fiber volume flow velocity) fractions is of K1, 55% and and the 60%. permeability The permeability of the fiber of thein thefiberFigure 0 ◦indirection the6 shows 90° direction (having the permeability a (having minimum a in maximum flowthe in-plane velocity) flow direction isvelocity) represented for is K1,each by and fiber K2. the Involume permeability each measurement,fraction. of Thethe teststhe neatwere fabric conducted preform on and two binder-treated cases with fiber fabric volume preform fractions with a of heat 55% treatment and 60%. during The permeability the preforming of theprocess fiber werein the used 90° direction to investigate (having the a e ffmaximumect of the binderflow velocity) on the in-planeis K1, and permeability. the permeability The average of the value of each test is indicated, and the error bar shows the maximum and minimum of the sample. As shown in Figure6 and Table2, the permeability of the binder-treated fabric preform decreased in every case, regardless of in-plane direction (K1, K2) and the fiber volume fraction. It can be inferred that the treated binder hindered the impregnation of resin in the preform, decreasing the permeability by approximately 80% for K1 and 82% for K2, at a 55% fiber volume fraction, and 80% for K1 and 77% for K2, at a 60% fiber volume fraction. In spite of the consideration of the amount of porosity reduction due to the binder content noted in Table1, the permeability in the in-plane direction of Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 13 fiber in the 0° direction (having a minimum flow velocity) is represented by K2. In each measurement, the neat fabric preform and binder-treated fabric preform with a heat treatment during the preforming process were used to investigate the effect of the binder on the in-plane permeability. The average value of each test is indicated, and the error bar shows the maximum and minimum of the sample. As shown in Figure 6 and Table 2, the permeability of the binder-treated fabric preform decreased in every case, regardless of in-plane direction (K1, K2) and the fiber volume fraction. It can be inferred that the treated binder hindered the impregnation of resin in the preform, decreasing the permeability by approximately 80% for K1 and 82% for K2, at a 55% fiber volume fraction, and 80% for K1 and 77% for K2, at a 60% fiber volume fraction. In spite of the consideration of the amount of porosityAppl. Sci. 2020reduction, 10, 7039 due to the binder content noted in Table 1, the permeability in the in-plane6 of 13 direction of the binder-treated fabric preform significantly decreased, which means that the binder treatment changed the permeability characteristics of the fabric preform itself. In addition, the permeabilitythe binder-treated of the fiber fabric tended preform to decrease significantly as the decreased, fiber volume which fraction means increased that the in binderboth the treatment K1 and K2changed directions, the permeability regardless of characteristics the binder treatment, of the fabric and preform especially itself. for Inneat addition, fabric, thewhich permeability exhibited ofa largerthe fiber decrease tended (K1: to decrease −16%, K2: as the−28%) fiber than volume that of fraction the binder-treated increased in bothfabric. the It K1is assumed and K2 directions, that the permeabilityregardless of of the the binder binder-treated treatment, preform, and especially which had for neatalready fabric, been which reduced exhibited due to a the larger effect decrease of the (K1: 16%, K2: 28%) than that of the binder-treated fabric. It is assumed that the permeability of binder,− was less −affected by the decrease in the fiber volume fraction than that of the neat fabric preform.the binder-treated preform, which had already been reduced due to the effect of the binder, was less affected by the decrease in the fiber volume fraction than that of the neat fabric preform.

Figure 6. Change in the permeability in the in-plane direction according to the binder treatment. Figure 6. Change in the permeability in the in-plane direction according to the binder treatment. Table 2. The permeability of the fabric preform in the in-plane direction. Table 2. The permeability of the fabric preform in the in-plane direction. Permeability–K ,K (m2) Fiber 1 2 2 Volume Neat FabricPermeability – K ,K Binder (m ) (Thermal Treated) Fraction (%) 1 2 Fiber volume K1 K2 K2/K1 K1 K2 K2/K1 11 Neat11 fabric 1 Binder11 (Thermal11 treated) 1 55 7.29 10− 5.97 10− 8.19 10− 1.45 10− 1.06 10− 7.31 10− fraction (%) × × × × × × 60 6.16 10 11 4.30 10 11 6.98 10 1 1.27 10 11 9.93 10 12 7.82 10 1 × − × − × − × − × − × − K1 K2 K2/K1 K1 K2 K2/K1

Figure7 shows the K2 /K1 values according to the fiber volume fraction. The K2/K1 value is a factor that can be obtained by dividing the permeability in the 0◦ direction (having a minimum flow velocity) of K2 into the permeability in the 90◦ direction (having a maximum flow velocity) of K1. As the shape of flow front of the resin is closer to circular, or isotropic, the factor approaches 1. The fabric preform exhibited a K2/K1 ratio of between 0.7 and 0.82. Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 13

7.29 × 5.97 × 8.19 × 1.45 × 1.06 × 7.31 × 55 -11 -11 -1 -11 -11 -1 10 10 10 10 10 10

- 6.16 × 4.30 × 6.98 × 1.27 × 9.93 × 7.82 × 10 60 -11 -11 -1 -11 -12 1 10 10 10 10 10 Figure 7 shows the K2/K1 values according to the fiber volume fraction. The K2/K1 value is a factor that can be obtained by dividing the permeability in the 0° direction (having a minimum flow velocity) of K2 into the permeability in the 90° direction (having a maximum flow velocity) of K1. As

Appl.the shape Sci. 2020 of, 10 flow, 7039 front of the resin is closer to circular, or isotropic, the factor approaches 1. The 7fabric of 13 preform exhibited a K2/K1 ratio of between 0.7 and 0.82.

Neat fabric 1.00 Binder treated 0.95 0.90 0.85 0.80 0.75 0.70

K2 / K1 K2 0.65 0.60 0.55 0.50

0.05 0.00 55 60

Volume fraction (%) FigureFigure 7. 7.The Thevalue valueof ofK2 K2/K1/K1 according according to to the the binder binder treatment. treatment.

3.2.3.2. PermeabilityPermeability inin thethe out-of-Planeout-of-Plane DirectionDirection InIn the the LCM LCM process, process, the the permeability permeability along along the the out-of-plane out-of-plane direction direction ( z(z-axis)-axis) is is also also important important whenwhen the the resin resin is is first first injected injected into into the the fiber fiber surface surface because because of of the the flow flow media media or or the the gap gap between between the the moldmold and and then then impregnated impregnated in in the the vertical vertical direction direction of of the the surface. surface. Consequently,Consequently, the the permeability permeability inin the the out-of-plane out-of-plane directiondirection waswas measured measured usingusing thethe equation equation ininSection Section 2.3.2 2.3.2,, andand thethe results results are are shownshown inin FigureFigure8 8 and and TableTable3 3according according toto thethe threethree types of of fiber fiber volume volume fractions: fractions: 50%, 50%, 55%, 55%, and and60%. 60%. To investigate To investigate the effect the e offfect the of binder the binder treatm treatment,ent, neat fabric neat fabricpreform preform and two and types two of types binder- of binder-treatedtreated fabric fabricpreform preform (without (without heat heat treatment treatment and and with with heat heat treatment treatment during during the preformingpreforming process)process) were were prepared. prepared. As As with with the the tendencytendency of of changechange inin thethe permeabilitypermeability inin thethe in-planein-plane direction,direction, allall threethree casescases showedshowed aa tendencytendency toto decreasedecrease thethe permeability permeability ofof thethe preformpreform inin the the out-of-plane out-of-plane directiondirection when when the the fiber fiber volume volume fraction fraction increased. increa Insed. addition, In addition, the binder-treated the binder-treated preforms exhibitedpreforms aexhibited lower permeability a lower permeability than that of than the that neat of fabric the neat preforms. fabric Whenpreforms. the fiberWhen volume the fiber fraction volume was fraction 50%, 55%,was and50%, 60%, 55%, the and permeability 60%, the permeability of the binder-treated of the binder-treated preform without preform heat without treatment heat exhibited treatment a 78–79%exhibited reduction, a 78–79% and reduction, the binder-treated and the binder-treat preform withed heatpreform treatment with heat exhibited treatment a 97–98% exhibited reduction a 97– compared98% reduction to the permeabilitycompared to ofthe the permeability neat fabric preform. of the neat These fabric results preform. indicate These that theresults binder indicate treatment that hindered the impregnation of resin in the out-of-plane direction. In particular, the permeability of the binder-treated fabric preform with heat treatment during the preforming process decreased more than that of the binder-treated fabric preform without heat treatment because the binder was softened during the heat treatment and permeated between the fibers under the 1 atm pressure applied by a vacuum bag. This phenomenon can greatly affect the reduction in the permeability of the fiber preform. To support this argument, micro observations of the binder in the preform is discussed in Section 3.3 below. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 13 the binder treatment hindered the impregnation of resin in the out-of-plane direction. In particular, the permeability of the binder-treated fabric preform with heat treatment during the preforming process decreased more than that of the binder-treated fabric preform without heat treatment because the binder was softened during the heat treatment and permeated between the fibers under the 1 atm pressure applied by a vacuum bag. This phenomenon can greatly affect the reduction in the permeability of the fiber preform. To support this argument, micro observations of the binder in the preform is discussed in Section 3.3 below.

Appl. Sci. 2020, 10, 7039 8 of 13

Figure Figure8. Change 8. Change in the in permeability the permeability in in the the out-of-plane direction direction according according to the binderto thetreatment. binder treatment. Table 3. Permeability of the fabric preform in the out-of-plane direction. Table 3. Permeability of the fabric preform in the out-of-plane direction. 2 Permeability–K3 (m ) Fiber Volume Fraction Binder (%) Neat Fabric Non-ThermalPermeability Treated - K3 (m Thermal2) Treated 50 1.12 10 12 2.36 10 13 2.38 10 14 × − × − × − 55 9.13 10 13 1.88 10 13 2.09 10 14 Fiber volume × − × − Binder× − 13 13 14 fraction (%) 60 6.87 10− 1.53 10− 1.85 10− Neat × × × 3.3. Microstructure Observationsfabric Non-Thermal Thermal To observe the shape of the binder in the preform, an ECLIPSEtreated LV 150N optical microscopetreated was used. Unlike the smooth surface of the neat fabric preform (Figure9a), the original size of the powder binders lightly attached to the fiber surface was observed on the binder-treated preform without heat 1.12 × treatment50 during the preforming process (Figure9b). In2.36 contrast, × 10 the-13 preform that underwent2.38 × heat 10-14 treatment during the preforming10-12 process exhibited binders pressed in the in-plane direction and that adhered to the fiber in a flat shape (Figure9c). The binder processed with a temperature above the glass transition temperature was flattened while being applied by a vacuum bag of 1 atm, and some of these binders permeated between9.13 × the fibers and had a flat shape in the fiber in-plane direction to 55 1.88 × 10-13 2.09 × 10-14 reduce the out-of-plane directional10-13 space where the fluid can flow. In the two types of binder-treated fabric preform (Figure9b,c), the impregnation of the test fluid was disturbed by the binder occupying space to pass, resulting in a decrease in the permeability of the preform, especially in the heat-treated preform (Figure9c), where the6.87 binders × having a flat shape in the fiber in-plane direction led to a 60 1.53 × 10-13 1.85 × 10-14 significant decrease (>97%) in10 the-13 out-of-plane directional permeability.

Appl. Sci. 2020, 10, 7039 9 of 13

3.4. Friction Coefficient Results The results of the measurement of the kinetic friction coefficient are shown in Figure 10 and Table4. The measurements were taken at room temperature (25 ◦C) and a binder treatment temperature of 110 ◦C. The friction coefficient of the surface with the binder was higher than that of the neat fabric; in the cases that the binder was on the friction surface (Cases 2 and 3), the friction coefficient at 110 ◦C was increased by 40% and 50% compared to the friction coefficient at room temperature (25 ◦C), respectively. Given that there was no change in the friction coefficient as a function of temperature in CaseAppl. Sci. 1, which2020, 10, had x FOR no PEER binder REVIEW on the friction surface, the friction surface of cases 2 and 3 was changed9 of 13 due to the effect of temperature changes in the binder. Because the epoxy binder used in this study has a3.3. glass Microstructure transition temperature Observations of 75–85 ◦C at 110 ◦C, above the glass transition temperature, the binder was inTo a observe viscous state,the shape which of increasedthe binder the in energythe prefor lossm, at an the ECLIPSE friction surfaceLV 150N due optical to viscous microscope dissipation. was Inused. addition, Unlike the smooth adhesion surface force of at the the neat friction fabric surface preform also (Figure increased 9a), the owing original to the size viscous of the powder binder, andbinders consequently lightly attached the friction to the coefiberffi cientsurface of thewas surface observed with on thethe binderbinder-treated increased. preform Similarly, without in case heat 4, whichtreatment shows during the friction the preforming coefficient process between (Figure the binder-treated 9b). In contrast, fabric the and preform the metallic thatmold, underwent the friction heat coetreatmentfficient during at 110 ◦ theC was preforming nearly three process times exhibited higher than binders the frictionpressed coein thefficient in-plane at room direction temperature. and that adhered to the fiber in a flat shape (Figure 9c). The binder processed with a temperature above the Table 4. Friction coefficient of the carbon fabric as a function of temperature. glass transition temperature was flattened while being applied by a vacuum bag of 1 atm, and some of these binders permeated between the fibers andFriction had a Coe flatffi shapecient– µin the fiber in-plane direction to Temperature reduce the out-of-plane directional space whereFabric-Fabric the fluid can flow. In the Fabric-Moldtwo types of binder-treated fabric preform (FigureCondition 9b,c), (◦C) the impregnation of the test fluid was disturbed by the binder occupying Case 1 Case 2 Case 3 Case 4 space to pass, resulting in a decrease in the permeability of the preform, especially in the heat-treated preform (Figure 9c), 25where the binders0.289 having 0.454a flat shape 0.479in the fiber in-plane 0.220 direction led to a significant decrease 110(> 97%) in the out-of-plane0.270 directional 0.633 permeability. 0.716 0.633

Figure 9. Cont. Appl. Sci. 2020, 10, 7039 10 of 13 Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 13

Figure 9. Micro observations of diﬀerent types of preform. Figure 9. Micro observations of different types of preform.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 13

3.4. Friction Coefficient Results The results of the measurement of the kinetic friction coefficient are shown in Figure 10 and Table 4. The measurements were taken at room temperature (25 °C) and a binder treatment temperature of 110 °C. The friction coefficient of the surface with the binder was higher than that of the neat fabric; in the cases that the binder was on the friction surface (Cases 2 and 3), the friction coefficient at 110 °C was increased by 40% and 50% compared to the friction coefficient at room temperature (25 °C), respectively. Given that there was no change in the friction coefficient as a function of temperature in Case 1, which had no binder on the friction surface, the friction surface of cases 2 and 3 was changed due to the effect of temperature changes in the binder. Because the epoxy binder used in this study has a glass transition temperature of 75–85 °C at 110 °C, above the glass transition temperature, the binder was in a viscous state, which increased the energy loss at the friction surface due to viscous dissipation. In addition, the adhesion force at the friction surface also increased owing to the viscous binder, and consequently the friction coefficient of the surface with the binder increased. Similarly, in case 4, which shows the friction coefficient between the binder- treated fabric and the metallic mold, the friction coefficient at 110 °C was nearly three times higher than the friction coefficient at room temperature. Appl. Sci. 2020, 10, 7039 11 of 13

FigureFigure 10. The 10. Thechange change in friction in friction coe coefficientﬃcient of ofthe the carbon carbon fabric fabric as as a afunction function of of temperature.

4. Conclusions Table 4. Friction coefficient of the carbon fabric as a function of temperature. The effect of a non-reactive epoxy binder generally used to fix the preform of the product in the LCM process was investigated. First, the permeabilityFriction of the coefficient preform in the– 𝝁 in-plane and out-of-plane directions were measured respectively using the unsaturated and saturated permeability equation Fabric according to the binder treatment. All the casesFabric of fabric - Fabric preform exhibited the highest permeability in theTemperature 90◦ direction (K1) and the lowest permeability in the 0◦ direction (k2), and both K1 and– K2 Mold decreased bycondition about 80% ( with℃) binder treatment at fiber volume fractions of 55% and 60%. The permeability in the out-of-plane direction also decreased with the binder treatment. In this case, the permeability in Case Case Case the out-of-plane direction of the binder-treated fabric preform (without heat treatmentCase during 4 the 1 2 3 preforming process) decreased by approximately 80% compared to the neat fabric preform, and the binder-treated fabric preform (with heat treatment during the preforming process) decreased by approximately25 98%. The reduction0.289 in permeability0.454 was significant0.479 even considering the 0.220 reduction in the porosity of the preform arising from the presence of the binder. Thus, in agreement with the measurement results of permeability, this reduction was due to the fact that the binder changed the permeability characteristics of the preform. In particular, the binder was flattened and impregnated within the fibers during the preforming process including heat treatment, as was observed through micro-optical observation, and this binder significantly hindered the flow in the out-of-plane direction, resulting in a 98% reduction in the permeability. This decrease in permeability lowered the impregnation rate of the resin, which was closely related to the rate of production in the LCM process, resulting in a higher production cost. Additionally, the change in the kinetic friction coefficient used as a variable in forming simulations for wrinkle prediction in the LCM process was investigated according to the binder treatment. The friction coefficient was high when the binder was on the friction surface, and the binder became viscous at 110 ◦C, resulting in an increase in the friction coefficient by 40–200% compared to room temperature (25 ◦C). Therefore, based on the results of this study, changes in the characteristics of carbon fabric used as a reinforcement for composite materials can be predicted when a binder is used in the LCM process. Appl. Sci. 2020, 10, 7039 12 of 13

Author Contributions: H.M.Y. carried out the experimental design of this study and wrote the original draft. J.W.L. also carried out experimental design of this study. J.S.K. organized experimental data. M.K.U. supervised this study. All authors read and approved the final manuscript. Funding: This work was funded by the Development of a Skin Spar Integrated Wingtip Composite Structure for a Single Aisle Aircraft using High Temperature Resin Infusion Process (20005403), Development of Mass Production Technology for Trunk Lid based on Hybrid Wet Compression Molding (10083584), Civil-Military Technology Cooperation Program (No.15-CA-MA-13-MKE) and the Technology Innovation Program (No.10053841) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). Conflicts of Interest: The authors declare no conflict of interest.

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