Thermoplastic Reaction Injection Pultrusion for Continuous Glass Fiber-Reinforced Polyamide-6 Composites

materials

Article Thermoplastic Reaction Injection Pultrusion for Continuous Glass Fiber-Reinforced Polyamide-6 Composites

Ke Chen , Mingyin Jia, Hua Sun and Ping Xue *

Beijing University of Chemical Technology, Institute of Plastic Machinery and Engineering, Beijing 100029, China; [email protected] (K.C.); [email protected] (M.J.); [email protected] (H.S.) * Correspondence: [email protected]; Tel.: +86-10-6443-6016

Received: 14 January 2019; Accepted: 30 January 2019; Published: 2 February 2019

Abstract: In this paper, glass fiber-reinforced polyamide-6 (PA-6) composites with up to 70 wt% fiber contents were successfully manufactured using a pultrusion process, utilizing the anionic polymerization of caprolactam (a monomer of PA-6). A novel thermoplastic reaction injection pultrusion test line was developed with a specifically designed injection chamber to achieve complete impregnation of fiber bundles and high speed pultrusion. Process parameters like temperature of injection chamber, temperature of pultrusion die, and pultrusion speed were studied and optimized. The effects of die temperature on the crystallinity, melting point, and mechanical properties of the pultruded composites were also evaluated. The pultruded composites exhibited the highest flexural strength and flexural modulus, reaching 1061 MPa and 38,384 MPa, respectively. Then, effects of fiber contents on the density, heat distortion temperature, and mechanical properties of the composites were analyzed. The scanning electron microscope analysis showed the great interfacial adhesion between fibers and matrix at 180 ◦C, which greatly improved the mechanical properties of the composites. The thermoplastic reaction injection pultrusion in this paper provided an alternative for the preparation of thermoplastic composites with high fiber content.

Keywords: glass ﬁber; thermoplastic composites; pultrusion; thermal analysis; mechanical properties

1. Introduction In recent years, environmental degradation and the energy crisis have made people aware of the importance of using recyclable materials. Fiber-reinforced thermoplastic composites have attracted more and more attention in various fields, particularly in the automotive industry, due to their manufacturing flexibility and recyclability [1,2]. Long fiber-reinforced thermoplastic composites have been applied to seatback frames and battery storage for hybrid vehicles [3]. As for structural applications requiring improved physical properties, continuous fiber reinforcements are necessary. Thermoplastic composites with continuous fiber have much higher mechanical performance than composites with short or long fiber reinforcements. Pultrusion is one of the most cost-effective and energy-efficient methods of manufacturing continuous fiber-reinforced composite profiles. Over the past 70 years, thermosetting resins have dominated the pultrusion industry owing to their rapid curing and effective impregnation with low viscosity. However, thermosetting resins have several disadvantages. They are brittle, sensitive to impact, and cannot be recycled. In addition, volatile compounds would be released during the pultrusion process of many thermosetting resins, which is contrary to environmental protection policies. Thermoplastic polymers completely avoid the above shortcomings and offer improved impact strength, damage tolerance, toughness, and reparability [4]. Nevertheless, a relative high viscosity

Materials 2019, 12, 463; doi:10.3390/ma12030463 www.mdpi.com/journal/materials Materials 2019, 12, 463 2 of 15

(100–10,000 Pa·s) of molten thermoplastics [5] limits the impregnation ability, resulting in the poor quality of the pultruded products. Several impregnation techniques for thermoplastic pultrusion have been developed by researchers. One way was the direct melting with high-fluidity polymers, such as high fluidity polyamide-66 (PA-66) [6]. The other way was to put the polymer and fibers in intimate contact prior to the final molding step. Pre-impregnated materials, such as prepreg tapes, commingled fibers, and powder coated towpregs had been well-developed for pultrusion processes [7–10]. However, there was still a large gap in achieving good impregnation with high fiber content between thermoplastic polymers and thermosetting resins. In order to solve this problem, reactive processing of thermoplastic composites with anionic ring-opening polymerization of polyamide-6 (PA-6) has been developed by researchers [11,12]. The impregnation of high content fibers can be easily achieved through caprolactam (monomer of PA-6) with extremely low viscosity (5 mPa·s) [11]. It provided an alternative to the manufacture of thermoplastic composites. Gong and Yang prepared all-polyamide composites using resin transfer molding (RTM) in which PA-6 matrix was formed by in-mold anionic polymerization of caprolactam in the presence with PA-66 plain cloth [13]. By comparing the mechanical properties of all-polyamide composites prepared at different temperatures, an optimal mold temperature of 180 ◦C was found. Yan et al. produced continuous glass fiber-reinforced anionic PA-6 composites through vacuum assisted resin infusion (VARI) [14]. The tensile strength and flexural strength of plain-woven fabric composites reached 434 MPa and 407 MPa, respectively. The anionic ring-opening polymerization of PA-6 can be completed in tens of seconds to 60 min, depending on the different activator–initiator combinations [15]. This provided the possibility for its application in pultrusion. Luisier et al. developed a pilot reactive injection pultrusion line in the base of anionic polymerization of polyamide-12 (PA-12) [16]. Epple et al. successfully prepared thermoplastic composites with anionic PA-6 using a pultrusion process, but the optimization of process condition, properties, and microstructure of pultruded composites were not reported in detail [17]. In this study, a novel thermoplastic reaction injection pultrusion test line was developed to manufacture continuous glass fiber-reinforced PA-6 composites. Using the specifically designed injection chamber, thermoplastic composites with fiber contents up to 70 wt% can be successfully prepared. The effects of temperature of injection chamber on the wet-out of fibers was investigated. Thermal properties and mechanical properties of the composites were also studied under different heating zone temperatures and different fiber contents. Besides, scanning electron microscopy was applied to analyze the microstructure of composite materials.

2. Experimental

2.1. Materials Commercial grade caprolactam (CPL, melting point was 69 ◦C) was purchased from BASF Co, Heilbronn, Germany. The sodium hydroxide (≥96% by mass, “NaOH”) was kindly provided by Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. Difunctional hexamethylene-1, 6-dicarbamoylcaprolactam (2 mol/kg concentration in caprolactam, ‘C20’) was obtained from Brüggemann Chemical, Heilbronn, Germany, which was used as the activator. The initiator sodium caprolactamate (2 mol/kg concentration in caprolactam, “C10”) was self-prepared via the reaction of caprolactam and NaOH under a vacuum environment. The chemical structures of these materials are presented in Figure1. C20 concentration of 1.0 mol% and C10 concentration of 2.0 mol% in caprolactam were chosen (C20 is a difunctional activator). Based on the preliminary study [18], these concentrations were intended to shorten the polymerization time while ensuring the molecular weight of PA-6 matrix. Materials 2019, 12, 463 3 of 15 Materials 2018,, 12,, xx FORFOR PEERPEER REVIEWREVIEW 3 of 15

FigureFigure 1.1. StructuresStructures ofof materialsmaterials used used in in this this study: study: (a ()a caprolactam,) caprolactam, (b ()b sodium) sodium caprolactamate, caprolactamate, and and (c) hexamethylene-1,6-dicarbamoylcaprolactam.(c) hexamethylene-1,6-dicarbamoylcaprolactam.

TheThe glass glass fiber fiber reinforcements reinforcements were were provided provided by by CPICCPIC Co.,Co., Ltd, Ltd, Chongqing, Chongqing, China. China. These These unidirectionalunidirectional E-glassE-glass rovings (ECT 4301R) were 2400 tex andand treatedtreated withwith aa silanesilane couplingcoupling agentagent suitablesuitable forfor PA-6.PA-6. TheThe initiatorinitiator C10C10 waswas particularly sensitivesensitive toto moisturemoisture inin thethe atmosphere andand could not remain reactivereactive for long long pe periodsriods of of time. time. Therefore, Therefore, it had it had to be to prepared be prepared before before use useto ensure to ensure reactivity. reactivity. The Theschematic schematic of the of thepreparation preparation of the of the initiator initiator C10 C10 is isshown shown in in Fig Figureure 2.2. A A three three-neck-neck flaskflask containingcontaining caprolactamcaprolactam andand aa certaincertain proportionproportion ofof NaOHNaOH waswas placedplaced inin thethe oiloil bath,bath, whichwhich waswas maintainedmaintained at ◦ 135135 °C.C. The The mixture mixture was was heated heated to to boiling boiling in ain vacuum a vacuum environment environment for 10–20 for 10 min–20 minutesuntil no particles until no wereparticle presents were in present the three-neck in the three flask,-neck and flask, then and the then initiator the initiator C10 was C10 prepared. was prepared. The prepared The prepared liquid initiatorliquid initiator was poured was poured into a 500 into mL a beaker500 mL and beaker cooled and to cooled solidify to for solidify later use. for Nitrogen later use. gas Nitrogen was blown gas intowas theblown beaker into to the isolate beaker the to air isolate before the being air before covered being with covered a glass with lid. a glass lid.

Figure 2.2. The schematic of the preparation ofof thethe initiatorinitiator C10.C10.

2.2. Development of Thermoplastic Reaction Injection Pultrusion Test Line 2.2. Development of Thermoplastic Reaction Injection Pultrusion Test Line 2.2.1. Pultrusion Test Line 2.2.1..2.1. PultrusionPultrusion Test Line The thermoplastic reaction injection pultrusion test line comprises a guiding device for glass The thermoplastic reaction injection pultrusion test line comprises a guiding device for glass ﬁbers, a ﬁber preheating and arranging unit, an injection unit, a pultrusion die, and a puller, as shown fibers, a fiber preheating and arranging unit, an injection unit, a pultrusion die, and a puller, as shown in Figure3. in Figure 3.

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Figure 3. The developed thermoplastic reaction injection pultrusion test line.

The glass fibers were preheated and arranged prior to entering the pultrusion die to avoid inhibition of polymerization by moisture on the cold fibers. The fibers were heated by the hot air at 150 °C and heat conduction from the metal tension rollers, which is illustrated in Figure 4(a). Two heating techniques worked together to ensure the removal of moisture from the surface of the glass fibers. Tension rollers and guide plates were utilized to make the fiber bundles tightly aligned. Figure 3. TheThe developed developed thermoplastic thermoplastic reaction reaction injection pultrusion test line. The pultrusion die consisted of three parts: an injection chamber, a heating zone, and a cooling zone. In this study, rectangular profiles with a cross-section of 20 mm × 4 mm were pultruded. A The glass fibersfibers were preheated and arranged prior to entering the pultrusion die to avoid trumpet-shaped injection chamber with a downward angle of 8° was designed. The upper part of the inhibition of polymerization by moisture on the cold fibers. The fibers were heated by the hot air at inhibitionpultrusion of polymerization die is shown in byFig uremoisture 4(b). This on shapethe cold of the fibers injection. The chamber fibers were can effectively heated by reduce the hot the air at 150 ◦C and heat conduction from the metal tension rollers, which is illustrated in Figure4a. Two heating 150 °Cbackflow and heat of resin conduction and contri frombute theto the metal impregnation. tension rollers, At the samewhich time, is illustratedthe inclination in Figof 8°ure ensured 4(a). Two heatingtechniquesthat techniques the worked traction worked together resistance together to was ensure notto ensure the that removal large. the removal It of was moisture proved of moisture from to be the from feasible surface the in surface of the the following of glass the fibers.glass fibers.Tensionexperiments. Tension rollers androllers The guide total and length platesguide of wereplates the pultrusion utilized were utilized to was make determined to the make fiber the to bundles befiber 1.0 m.bundles tightly aligned.tightly aligned. The pultrusion die consisted of three parts: an injection chamber, a heating zone, and a cooling zone. In this study, rectangular profiles with a cross-section of 20 mm × 4 mm were pultruded. A trumpet-shaped injection chamber with a downward angle of 8° was designed. The upper part of the pultrusion die is shown in Figure 4(b). This shape of the injection chamber can effectively reduce the backflow of resin and contribute to the impregnation. At the same time, the inclination of 8° ensured that the traction resistance was not that large. It was proved to be feasible in the following experiments. The total length of the pultrusion was determined to be 1.0 m.

FigureFigure 4. Components 4. Components of of the the pultrusion pultrusion line:line: (a)) f ﬁbersibers preheating preheating and and arranging arranging unit, unit, and and(b) the (b ) the upperupper part part of the of the pultrusion pultrusion die. die.

The pultrusion die consisted of three parts: an injection chamber, a heating zone, and a cooling zone. In this study, rectangular proﬁles with a cross-section of 20 mm × 4 mm were pultruded. A trumpet-shaped injection chamber with a downward angle of 8◦ was designed. The upper part of the pultrusion die is shown in Figure4b. This shape of the injection chamber can effectively reduce the backﬂow of resin and contribute to the impregnation. At the same time, the inclination of 8◦ ensured that the traction resistance was not that large. It was proved to be feasible in the following experiments. The totalFigure length 4. Components of the pultrusion of the pultrusion was determined line: (a) f toibers be 1.0preheating m. and arranging unit, and (b) the upper part of the pultrusion die.

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Materials 2018, 12, x FOR PEER REVIEW 5 of 15 2.2.2. Mixing and Injecting of Reaction Materials The2.2.2. injection Mixing and device Injecting designed of Reaction for the Materials anionic polymerization of PA-6 was different from traditional resin injectionThe injection molding device (RIM) designed device. Speciﬁc for the anionicequipment polymerization was required of PAto remove-6 was different moisture from under a high vacuumtraditional and resin high injection temperature molding and (RIM) to protect device. the Specific molten equipment reaction materials was required from air.to remove The device had tomoisture include under the followinga high vacuum features: and high (a) totemperature vacuum theand twoto protect kinds the of molten liquid reaction materials materials to dropfrom themoisture air. The device content had below to include 300 ppm,the following (b) to storefeatures: the (a) molten to vacuum caprolactam the two abovekinds of the liquid melting temperaturereaction materials (69 ◦C) andto drop protect the moisture it from content moisture below and 300 oxygen, ppm, (b) (c) to tostore mix the the molten two caprolactam components in above the melting temperature (69 °C) and protect it from moisture and oxygen, (c) to mix the two an accurate ratio, and (d) to inject the both reaction materials into the pultrusion die under certain components in an accurate ratio, and (d) to inject the both reaction materials into the pultrusion die pressure. These features are achieved by the device shown in Figure5. under certain pressure. These features are achieved by the device shown in Figure 5.

FigureFigure 5. The 5. The specially specially designed designed resin resin injection injection moldingmolding (RIM) (RIM) device device:: (a)( aoverview) overview of RIM of RIM device device,, and (band) schematic (b) schematic diagram diagram of theof the device. device.

TwoTwo kinds kinds of liquid of liquid reaction reaction materials materials were used: used: the the caprolactam caprolactam and andthe initiator the initiator C10 were C10 in were tank A while the caprolactam and the activator C20 were in tank B. When the device was activated, in tank A while the caprolactam and the activator C20 were in tank B. When the device was the two reaction materials in the tanks were heated to 135 °C in a vacuum environment. The activated, the two reaction materials in the tanks were heated to 135 ◦C in a vacuum environment. temperatures of reaction materials could be detected by the thermocouples set on the inner wall of The temperaturesthe tanks. After of degassing reaction materialsfor 20 minutes, could nitrogen be detected was introduced by the thermocouples into the tanksset and on the the reaction inner wall of thematerials tanks. After were cooled degassing to an for appropriate 20 min, nitrogen injection temperature was introduced by stirring. into the Then tanks the andtwo kinds the reaction of materialsreaction were materials cooled were to an fed appropriate to the static mixer injection by the temperature pneumatic piston by stirring. pumps according Then the to twoa certain kinds of reactionratio materials (1:1 in this were study) fed. The to the pressure static-limiting mixer by valves thepneumatic were used to piston adjust pumps the pressure according of the topiston a certain ratiopumps (1:1 in thisto meet study). the flow The requirements. pressure-limiting The injection valves werepressure used was to set adjust to 0.2 the–0.5 pressure bar to ensure of the the piston pumpsimpregnation to meet the based ﬂow on requirements. pultrusion speed Thes. All injection the pipes, pressure tanks, and was the set static to mix 0.2–0.5er were bar placed to ensure in a the impregnationhot air oven based at 100 on °C pultrusion to avoid solidification speeds. All of the caprolactam pipes, tanks, in advance. and the The static injection mixer gun were was placed also in a heated with an electric heater to maintain the temperature at 100 °C. hot air oven at 100 ◦C to avoid solidiﬁcation of caprolactam in advance. The injection gun was also ◦ heated2.3. with Characterizations an electric heater to maintain the temperature at 100 C.

2.3. Characterizations2.3.1. Differential Scanning Calorimetry

2.3.1. DifferentialDifferential Scanning scanning Calorimetry calorimetry (DSC) (Mettler Toledo TGA/DSC1, Zurich, Switzerland) analysis was performed to determine the degree of crystallinity (Xc) and melting point (Tm) of the PA- Differential6 matrix of the scanning pultruded calorimetry composites. (DSC) The (Mettlermelting enthalpy Toledo TGA/DSC1, (Hm) of PA-6 Zurich,matrix was Switzerland) tested under analysis a was performednitrogen atmosphere, to determine over a thetemperature degree ofrange crystallinity from 25 °C to (X 250c) and°C with melting a heating point rate ( ofTm 10) of°C/min. the PA-6 matrixThe of PA the-6 pultrudedmatrix content composites. (mresin) was Thedetermined melting using enthalpy thermogravimetric (Hm) of PA-6 analysis. matrix Then, was the tested melting under a ◦ ◦ ◦ nitrogenenthalpy atmosphere, per unit of over mass a temperature(∆Hm = Hm/mresin range) was fromobtained. 25 XCc towas 250 calculatedC with according a heating to rate the following of 10 C/min. equation: The PA-6 matrix content (mresin) was determined using thermogravimetric analysis. Then, the melting enthalpy per unit of mass (∆Hm = Hm/mresin) was obtained. Xc was calculated according to the following equation:

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∆Hm Xc = · 100% (1) ∆H100 where ∆H100 is the melting enthalpy of fully crystalline PA-6: ∆H100 = 190 J/g [19].

2.3.2. Thermogravimetric Analysis Thermogravimetric analysis (TGA) tests of the composites were carried out under a nitrogen ﬂow of 50 mL/min on a TGA instrument (Mettler Toledo TGA/DSC1, Zurich, Switzerland). The temperature range was from 25 ◦C to 600 ◦C and the heating rate was 10 ◦C/min.

2.3.3. Heat Deflection Temperature The heat deflection temperature (HDT) of the composites was measured using a Thermal Deformation Vicat Softening Point Temperature Tester (KXRW-300CL-3, Taiding test, Chengde, China), following ISO 75-2-2013 [20]. The size of the rectangular specimen was 80 mm × 10 mm × 4 mm. The applied flexural stress was 1.80 MPa and the heating rate was 2 ◦C/min. HDT was measured at a standard deflection of 0.34 mm.

2.3.4. Mechanical Properties The mechanical properties of composites were measured by using a universal testing machine (KXWW-20C, Taiding test, Chengde, China). Flexural strength and ﬂexural modulus were tested according to ASTM D-790 [21]. The size of the rectangular specimen was 80 mm × 10 mm × 4 mm and the span was 64 mm. The short beam shear tests were conducted in accordance with ASTM D-2344 [22] and the size of the rectangular specimen was 24 mm × 8.0 mm × 4.0 mm. A three-point bending jig equipped with a 3 mm diameter supports and a 6 mm diameter loading nose was adjusted to a span of 16 mm. The interlaminar shear strength (ILSS) can be calculated according to the following equation:

F ILSS = 0.75 · m (2) w · t in which w and t are the width and thickness of the test specimen. At least ﬁve specimens per set of conditions were tested. The densities of the samples were also measured according to ASTM D-792 [23].

2.3.5. Scanning Electron Morphology Scanning electron morphology (SEM) study was carried out using an electron microscope (S-4700, Hitachi, Tokyo, Japan). Cross-sections of the samples were observed after polishing with water abrasive papers. The morphology of the fracture surface was observed after fracturing the sample in the liquid nitrogen.

3. Results and Discussion

3.1. Process Feasibility of Thermoplastic Reaction Injection Pultrusion Different from traditional thermoplastic pultrusion with a melting process, for reaction injection pultrusion, fiber impregnation can be easily achieved by the reaction materials with an ultra-low viscosity. Anionic polymerization occurred in the pultrusion die to manufacture composite materials. This technique was more similar to pultrusion with thermosetting resins, but it required more critical conditions. The anionic polymerization of caprolactam was particularly sensitive to moisture in the atmosphere. When the relative humidity was higher than 40% in the air, the polymerization was difficult to proceed smoothly. Therefore, an impregnation technique with open resin bath was not ideal. In this study, an injection chamber directly connected to the heating zone was adopted to isolate Materials 2019, 12, 463 7 of 15 the air. It avoided the problem of insufficient adhesion of the reaction materials to the fibers when using the open resin bath. The reaction materials injected into the impregnation chamber would move along with the fibers, so a long pot life of reaction materials was also unnecessary. Because commercial caprolactam was used in this study, a strict vacuum dehydration process must be implemented to ensure that the water content of the reaction materials dropped below 300 ppm. The reaction materials in both tanks were heated to boil under 0.01 MPa for 20 min. Before the injection process, the reaction materials should be cooled to 100 ◦C by stirring to avoid polymerization during the mixing phase. Another characteristic that must be followed was the high reactivity of reaction materials. Researchers have demonstrated that the combination of initiator C10 and activator C20 provided a very short induction time for polymerization, and the reaction time can be controlled within 2 min [11,24]. In this study, higher contents of activator and initiator were used to accelerate the reaction. Besides, the addition of fibers resulted in higher thermal conductivity of composites than that of neat polymer, resulting in a faster reaction. A pultrusion speed up to 80 cm/min can be achieved.

3.2. Effect of Temperature of Injection Chamber on the Wet-Out of Fibers When the reaction materials were injected into the injection chamber, the impregnation of the unidirectional glass fiber reinforcements was also carried out simultaneously. The injection chamber needed to be maintained at a suitable temperature to prevent solidification of the caprolactam and to ensure a low viscosity of the reaction materials to impregnate the fibers. Although the reaction temperature range recorded in the literature was 130–170 ◦C[15], this did not mean that the reaction was not underway below this temperature range. The reaction also took place, but the reaction rate was relatively slow, which can lead to the increase of viscosity of the reaction materials. Both temperature (T) and monomer conversion (α) had effects on the viscosity of reaction materials (µ). This can be described using the following equation [25]:

3525 µ(T, α(T, t)) = 2.7 × 10−7 · exp( + 17.5 · α) (3) T In general, the viscosity should be less than 1 Pa·s to allow the resin to penetrate the fiber reinforcement thoroughly [16]. Therefore, the temperature of the injection chamber was very important and had a great influence on the rheological behavior of the reaction materials, which determined the wet-out of fibers. Surfaces of pultruded composites with 70 wt% glass fibers at various temperatures of the injection chamber are present in Figure6. It can be seen that some voids appeared on the surfaces of the composites. The higher the temperature, the more serious the phenomenon of voids on the surface of the samples. The surfaces of samples with injection chamber temperatures of 110 ◦C and 120 ◦C were unacceptable for qualified pultruded products. The voids on the surface would have a negative effect on the final mechanical properties of the pultruded products. Materials 2018, 12, x FOR PEER REVIEW 8 of 15

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FigureFigure 6. Surfaces 6. Surfaces of pultrudedof pultruded composites composites withwith 70 wt% wt% glass glass fibers ﬁbersfibers at atvarious various temperatures temperatures of the of the injectioninjection chamber. chamber.

This was caused by the following effects. Because the injection chamber was directly connected This was caused by the following effects. Because the injection chamber was directly connected with the heating zone, which had a relative high temperature (150–180 °C),◦ the temperature at the with the heating zone, which had a relativerelative highhigh temperaturetemperature (150–180(150–180 °C),C), the temperature at the interfaceinterface between between the injection the injection chamber chamber and heating and heating zone was zone actually was actually higher higher than the than set temperature the set interfacetemperature between due the to theinjection heat conduction. chamber The and reaction heating materials zone on was the surface actually of the higher compos thanites thein set due to the heat conduction. The reaction materials on the surface of the composites in contact with the temperaturecontact withdue theto the heating heat moldconduction. was reacted The aheadreaction of materials time, resulting on the in surface a sharp of rise the in compos viscosity.ites in heatingcontactAdequate moldwith the waspenetration heating reacted of moldahead fiber bundles was of time, reacted on resulting surfaces ahead inof of acomposites sharp time, rise resulting could in viscosity. not in be a achieved sharp Adequate rise by reaction in penetration viscosity. ofAdequate fibermaterials bundles penetration with on higher surfaces of viscosity. fiber of bundles composites This phenomenon on surfaces could notwas of becompositesparticularly achieved noticeable could by reaction not on be the materials achieved sample surfaces withby reaction higher ◦ ◦ viscosity.materialsof 110 Thiswith °C and phenomenonhigher 120 °C.viscosity. Samples was This particularlyat 90 phenomenon °C and noticeable100 °C was had particularly onflat thesurfaces sample noticeablebecause surfaces the on reaction of the 110 sample materialsC and surfaces 120 C. ◦ ◦ Samplesof 110were °C at notand 90 heated 120C and °C. to 100theSamples temperatureC had at flat90 °Crequired surfaces and 100 for because the°C sharphad the flat rise reaction surfaces in viscosity. materials because werethe reaction not heated materials to the temperaturewere not SEMheated required micrographs to the for temperature theof the sharp polished riserequired incomposites viscosity. for the with sharp the injectionrise in viscosity. chamber at 90 °C and 100 °C are shown in Figure 7. In spite of scratches caused by polishing, the pictures confirmed◦ excellent ◦ SEM micrographs of of the the polished polished composites composites with with the the injection injection chamber chamber at 90 at 90°C andC and 100 100°C areC impregnation. The void content of the composite samples was nearly zero. Therefore, it was a good areshown shown in Fig in Figureure 7. 7 In. In spite spite of of scratches scratches caused caused by by polishing, polishing, the pictures pictures confirmed confirmed excellentexcellent choice to set the temperature of the injection chamber at 90–100 °C. For the following sections, the impregnation. The The void content of of the composite sample sampless was nearly zero. Therefore, it waswas a good temperature of the impregnation chamber was set at 100 °C in order◦ to get a faster reaction speed. choicechoice to set the temperature of the injectioninjection chamber at 90–10090–100 °C.C. For the followingfollowing sections,sections, the ◦ temperature of thethe impregnationimpregnation chamberchamber waswas setset atat 100100 °CC inin orderorder toto getget aa fasterfaster reactionreaction speed.speed.

Figure 7. SEM micrographs of the polished composites with injection chamber at 90 °C (a) and 100 °C (b).

FigureFigure 7. SEM7. SEM micrographs micrographs of of the the polished polished composites composites with with injection injection chamber chamber at at 90 90◦C( °Ca )(a and) and 100 100◦C( °Cb ). (b).

Materials 2018, 12, x FOR PEER REVIEW 9 of 15 Materials 2019, 12, 463 9 of 15 3.3. Effect of Heating Zone Temperature on the Thermal Properties of PA-6 Matrix of the Pultruded Composites 3.3. Effect of Heating Zone Temperature on the Thermal Properties of PA-6 Matrix of the Pultruded Composites In general, the molding die for pultrusion included a high temperature zone for the resin reactionIn general, and a low the moldingtemperature die forzone pultrusion for cooling included and shaping. a high temperature In this study, zone the for temperature the resin reaction of the coolingand a low zone temperature was set to be zone only for 20 cooling °C lower and than shaping. that of Inthe this heatin study,g zone. the temperatureThis was because of the the cooling PA-6 ◦ matrixzone was of setthe to composites be only 20 wasC lower a semi than-crystalline that of the polymer heating and zone. a Thisslow was cooling because rate the can PA-6 improve matrix the of crystallinity,the composites which was ahad semi-crystalline a great influence polymer on the and mechanical a slow cooling properties rate can of improvethe polymer. the crystallinity, Due to the whichaddition had of afiber greats, the influence heat resistance on the mechanical of composites properties was greatly of the improved polymer. and Due the to feasibility the addition of the of coolingfibers, the zone heat with resistance a relative of compositeshigh temperature was greatly was ensured. improved The and specific the feasibility process of temperatures the cooling zoneand withpultrusion a relative speed high are temperature shown in Table was ensured. 1. High die The temperature specific process can temperaturesaccelerate the and react pultrusionion, thus achievingspeed are showna high pultrusion in Table1. Highspeed die. temperature can accelerate the reaction, thus achieving a high pultrusion speed. Table 1. The specific process parameters of the thermoplastic reaction injection pultrusion. Table 1. The specific process parameters of the thermoplastic reaction injection pultrusion. Temperature (°C) ◦ Pultrusion speed Injection Temperature ( C) Pultrusion Speed Heating zone Cooling zone (cm/min) Injectionchamber Chamber Heating Zone Cooling Zone (cm/min) 150150 130 13030 30 160 140 40 100 160 140 40 100 170 150 60 170 150 60 180 160 80 180 160 80

In order to determine the crystallinity of PA PA-6-6 matrix in thethe composites,composites, TGA tests should be carried out to determine the proportion of the matrix. matrix. Fig Figureure 88 is is a a typical typical TGA TGA diagram diagram ofof the the ◦ composites pultruded with a heating zone temperature of 180 °C.C. It can be seen from the figure figure that ◦ the PA-6PA-6 matrixmatrix decomposed completely at 500 °C,C, leaving only glass fibers.fibers. The weight percent of glass fibers fibers was 73.23%. There may also be residue residuess in the composites, which were not involved in ◦ the reaction. According According to to the the literature literature [2 [266], the residual caprolactam caprolactam was was volatilized volatilized below below 200 200 °C.C. Except for that part, the proportion of matrix PA-6PA-6 cancan bebe determined.determined.

FigureFigure 8. 8.A A typical typical TGA TGA diagram diagram of theof compositesthe composites pultruded pultruded with with heating heating zone temperaturezone temperature of 180 of◦C. 180 °C. Since the reaction was carried out below the melting point of PA-6, polymerization and crystallizationSince the occurred reaction wassimultaneously. carried out The below temperature the melting of the point pultrusion of PA-6, die polymerization had a great effect and crystallizationon the crystallinity occurred and meltingsimultaneously. point of PA-6.The temperature of the pultrusion die had a great effect on the crystallinityFigure9 shows and that melting the degree point ofof crystallinity PA-6. of the PA-6 matrix in composites gradually decreases as the heating zone temperature increased from 150 to 180 ◦C. The reason was that the higher the

Materials 2018, 12, x FOR PEER REVIEW 10 of 15 Materials 2019, 12, 463 10 of 15 Figure 9 shows that the degree of crystallinity of the PA-6 matrix in composites gradually decreases as the heating zone temperature increased from 150 to 180 °C. The reason was that the higherpolymerization the polymerizat temperatureion temperature was, the stronger was, the the stronger thermal motionthe thermal of the motion polymer of chain the polymer was, resulting chain was,in a lowerresulting equilibrium in a lower degree equilibrium of crystallinity, degree of crystallinity, which meant which that moremeant time that wasmore needed time was to achieveneeded toa crystallization achieve a crystallization equilibrium equilibrium [12]. With [12]. the increasing With the increasing die temperature, die temperatur the increasee, the of increase molecular of molecularweight of theweight PA-6 of matrix the PA caused-6 matrix by thecaused branching by the reactionbranching was reaction another was reason another [14]. reason The increase [14]. The in increasemolecular in weightmolecular enhanced weight theenhanced entanglement the entanglement between polymerbetween chains,polymer which chains, limited which thelimited internal the internalrotation rotation of polymer of polymer chains, thuschains, affecting thus affecting the orderly the order accumulationly accumulation of molecular of molecular chains, chains and had, and a hanegatived a negative impact impact on the on crystallization the crystallization behavior behavior of the of PA-6 the matrix.PA-6 matrix. The presence The presence of fibers of fibers also made also madethe crystallinity the crystallinity of PA-6 of matrix PA-6 in matrix composites in composites higher than higher that of than the thatpure of PA-6 the that pure was PA reported-6 that was [12]. reportedDue to the [12]. heterogeneous Due to the nucleation, heterogeneous fibers nucleation, may act as nucleatingfibers may agentsact as in nucleating the composites. agents What in the is compositemore, in thes. What presence is more, of fiberglass, in the presence the total of fiberglass heat release, the of total the heat resin release system of wasthe resin lower, system and more was lowerpolymers, and had more the polymers tendency ha ofd crystallization. the tendency of crystallization.

Figure 9. 9. DegreeDegree of ofcrystallinity crystallinity and andmelting melting point ofpoint PA-6 of matrix PA-6 for matrix various for heating various zone heating temperature zone temperaturemeasured by measured DSC. by DSC.

◦ The melting melting point point of ofthe the PA-6 PA matrix-6 matrix decreased decrease by mored by than more 3 C than with 3 the °C increase with the of temperature increase of ◦ temperaturefrom 150 to 180fromC. 150 This to 180 may °C. have This been may due have to thebeen transformation due to the transformation of crystal structure of crystal caused structure by a branching reaction [27]. The α-structure crystal (T = 220 ◦C) and γ-structure crystal (T = 214 ◦C) caused by a branching reaction [27]. The α-structurem crystal (Tm = 220 °C) and γ-structurem crystal (Tm =appeared 214 °C) appeared in the anionic in the PA-6 anionic simultaneously. PA-6 simultaneously. The branching The reactionbranching led reaction to an increase led to an in theincrease content in theof γ content-structure of γ crystals,-structure thereby crystals, the thereby melting the point melting was reduced. point was reduced. 3.4. Effect of Heating Zone Temperature on the Mechanical Properties of the Pultruded Composites 3.4. Effect of Heating Zone Temperature on the Mechanical Properties of the Pultruded Composites Four composite samples with 70 wt% glass fibers depending on the heating zone temperatures Four composite samples with 70 wt% glass fibers depending on the heating zone temperatures were tested to investigate the flexural properties and interlaminar shear strength (ILSS). From Figure 10, were tested to investigate the flexural properties and interlaminar shear strength (ILSS). From Figure we can see that in the heating zone temperature of 150 ◦C, the flexural strength and the flexural 10, we can see that in the heating zone temperature of 150 °C, the flexural strength and the flexural modulus reached the maximum value of 1061 MPa and 38,384 MPa, respectively. As the heating modulus reached the maximum value of 1061 MPa and 38384 MPa, respectively. As the heating zone zone temperature rose from 150 ◦C to 170 ◦C, the flexural properties of the composites decreased temperature rose from 150 °C to 170 °C, the flexural properties of the composites decreased gradually, gradually, which was consistent with the decreasing trend of crystallinity of the PA-6 matrix in Figure9. which was consistent with the decreasing trend of crystallinity of the PA-6 matrix in Figure 9. When When the heating zone temperature was further increased to 180 ◦C, the flexural properties of the the heating zone temperature was further increased to 180 °C, the flexural properties of the composites were improved. This variation tendency can be explained using the following reasons. composites were improved. This variation tendency can be explained using the following reasons. In In the heating zone temperature range of 150 ◦C to 170 ◦C, as with most semi-crystalline polymers, the heating zone temperature range of 150 °C to 170 °C, as with most semi-crystalline polymers, the the mechanical properties of the PA-6 matrix of the composites mainly depended on the degree of mechanical properties of the PA-6 matrix of the composites mainly depended on the degree of crystallinity. Therefore, the decrease of crystallinity would reduce the mechanical properties of PA-6 crystallinity. Therefore, the decrease of crystallinity would reduce the mechanical properties of PA-6

MaterialsMaterials 20182019,, 1212,, x 463 FOR PEER REVIEW 1111 of of 15 15 Materials 2018, 12, x FOR PEER REVIEW 11 of 15 matrix, resulting in a decrease in the flexural properties of the composites. As the temperature reachedmatrix, resulting resulting180 °C, the in in a interfacial adecrease decrease inbonding the in the flexural flexuralbetween properties propertiesfibers ofand the ofPA composites. the-6 matrix composites. was As the greatly As temperature the improved temperature reached and ◦ playedreached180 C, a the 180major interfacial °C ,role the ininterfacial bondingthe flexural bonding between properties between fibers of and the fibers PA-6 composites and matrix PA-,6 waswhich matrix greatly can was be improved greatly demonstrated improved and played by andthe a followingplayedmajor role a major analysis in the role flexural of in ILSS the properties offlexural the composites. properties of the composites, of the composites which can, which be demonstrated can be demonstrated by the following by the followinganalysis of analysis ILSS of of the ILSS composites. of the composites.

Figure 10. Flexural properties of the pultruded composites depending on the heating zone temperature.Figure 10. 10.Flexural Flexural properties properties of the of pultruded the pultruded composites composites depending depending on the heating on the zone heating temperature. zone temperature. FigFigureure 1111 showsshows the the t testest results results of of the the ILSS ILSS of of specimens specimens with with 70 70 wt% wt% glass glass fibers ﬁbers at at various various ◦ ◦ heatingheatingFig zoneure 11 temperatures.temperatures. shows the test WhenWhen results the the temperature oftemperature the ILSS wasof was specimens in in the the range range with of 150of70 150wt%C to°C glass 170 to 170 C,fibers there°C, atthere was various littlewas littleheatingdifference difference zone between temperatures. between the ILSS the When of ILSS the ofthepultruded the temperature pultruded composites. was composites. in However,the range However, ILSSof 150 of ILSS°C the to composites of170 the °C, composites there reached was ◦ ◦ reachedlittle71.5 MPa difference 71.5 at heatingMPa between at zoneheating temperature the zone ILSS temperature of of the 180 pultrudedC, of which 180 °Ccomposites. was, which 13.7% was higher However, 13.7% than higher ILSS that than at of 170 the that C. composites at This 170 was°C. Thisreachedbecause was 71.5 thebecause de-blockingMPa atthe heating de- ofblocking the zone activator temperature of the C20 activator occurred of 180 C20 °C to ,occurred which a larger was extendto 13.7%a larger at higher heating extend than zone at thatheating temperature at 170 zone °C. temperatureThisof 180 was◦C because and of more 180 the °C free deand isocyanate-blocking more free of groups isocyanate the activator were groups generated, C20 wereoccurred which generated, to increased a larger which extend the increased chance at heating ofthe the chance bondzone oftemperatureformation the bond of formation of aminosilane 180 °C ofand aminosilane urea more links free on isocyanateurea the links glass ongroups surface the glass were [12,27 surface generated,]. The [12,2 interfacial which7]. The bondingincreased interfacial between the bonding chance the betweenofﬁbers the andbond the PA-6 formationfibers matrix and ofPA was aminosilane-6 greatly matrix enhanced.was urea greatly links enhanced. on the glass surface [12,27]. The interfacial bonding between the fibers and PA-6 matrix was greatly enhanced.

FigureFigure 11. 11. TheThe interlaminar interlaminar shear shear strength strength of the of thepultruded pultruded composites composites depending depending on heati onng heating zone temperature.Figurezone temperature. 11. The interlaminar shear strength of the pultruded composites depending on heating zone temperature.

Materials 2019, 12, 463 12 of 15

It can be concluded that it was the best choice to set 180 ◦C as the temperature of the heating zone. Not only the pultruded composites exhibited relatively excellent mechanical properties, but a high pultrusion speed could also be achieved.

3.5. Effect of Fiber Content on the Properties of the Pultruded Composites Table2 illustrates the properties of the pultruded continuous glass fiber-reinforced PA-6 composites with different fiber contents. The density and HDT of pultruded composites increased with increasing fiber content due to the higher specific gravity and thermal stability of the glass fiber. The composite material reached an HDT much higher than that of pure PA-6, reaching about 200 ◦C. With the increase of fiber content, the flexural strength, flexural modulus, and ILSS of composites also increased. This was because, as the reinforcement, the glass fiber played a dominant role in the mechanical properties of composites.

Table 2. The properties of pultruded composites with different ﬁber contents.

Fiber Roving Flexural Flexural Density HDT ILSS Contents Numbers Strength Modulus (kg/m3) (◦C) (MPa) (wt%) (2400 tex) (MPa) (MPa) 50 26 1.56 × 103 197.8 731.64 24,526.76 51.14 60 34 1.72 × 103 201.6 834.36 33,664.22 58.85 70 42 1.84 × 103 204.5 865.82 36,991.41 63.11

3.6. Scanning Electron Microscopy Analysis of the Pultruded Composites The microstructures of pultruded composites were observed using scanning electron microscopy. The cross sections of the pultruded composites with different fiber contents after polishing are shown in Figure 12a–c. With the increase of fiber content, the fibers became denser and the fibers dispersed more and more evenly. Except for the cross-sections in Figure 12a, there were no indications of specific regions where the fibers bundles form for cross-sections with fiber contents of 60 wt% and 70 wt%. However, micron-sized voids were found in the cross section of 50 wt% products, which may be due to the high resin contents and resin shrinkage. There was almost no void in the cross-sections of the pultruded composites with fiber contents of 60 wt% and 70 wt%. Figure 12d shows the fracture surfaces of the composites with 70 wt% glass fibers prepared at a heating zone temperature of 180 ◦C. It can be seen from the figure that there was a good interfacial bond between the fiber and the matrix under the action of the coupling agent. Materials 2019, 12, 463 13 of 15 Materials 2018, 12, x FOR PEER REVIEW 13 of 15

Figure 12. SEM micrographs of the pultruded composites: ( a)) cross sections with 50 wt% glass fibers, fibers, ((b)) crosscross sectionssections withwith 6060 wt% glass fibers,fibers, ((c)) cross sectionssections with 70 wt% glass fibers,fibers, and (d) fracture surfaces with 70 wt% glass fibers,fibers, allall atat aa heatingheating zonezone temperaturetemperature ofof 180180 ◦°C.C.

4. Conclusions Continuous glassglass fiber-reinforcedfiber-reinforced PA-6PA-6 compositescomposites werewere successfullysuccessfully pultruded via the anionic ring-openingring-opening polymerization polymerization of of caprolactam. caprolactam. As As the the injection injection chamber chamber temperature temperature was was set at set 90 at– ◦ 90–100100 °C, goodC, good fiber fiber impregnation impregnation and anda smooth a smooth surface surface of pultruded of pultruded composites composites were were achieved. achieved. The Thetemperature temperature of the of heating the heating zone zonehad significant had significant effects effects on both on boththe thermal the thermal properties properties of the of PA the-6 PA-6matrix matrix and mechanical and mechanical properties properties of composites. of composites. When the When heating the zone heating temperature zone temperature was 150 °C, was the ◦ 150crystallinityC, the of crystallinity the PA-6 matrix of the was PA-6 the matrix highest was (43.3% the) highestand flexural (43.3%) strength and flexuraland flexural strength modulus and flexuralof composites modulus reached of composites the maximum reached of 1061 the MPa maximum and 38384 of 1061MPa, MPa respectively. and 38,384 However, MPa, respectively. the highest ◦ However,interlaminar the shear highest strength interlaminar (ILSS) of shear composites strength occurred (ILSS) of at composites 180 °C, reaching occurred 71.5 at 180MPa,C, due reaching to the 71.5interfacial MPa, duebonding to the between interfacial the bonding fibers and between PA-6 matrix the fibers was and greatly PA-6 improved matrix was by greatly de-blocking improved of the by de-blockingactivator C20 of. theMechanical activator C20.properties, Mechanical heat properties,deflection temperatures,heat deflection and temperatures, densities andof pultruded densities ofcomposites pultruded increased composites with increased increasing with glass increasing fiber content. glass Micron fiber content.-sized voids Micron-sized were found voids in cross were- foundsections in of cross-sections composites with of composites 50 wt% fibers, with but 50 wt%not in fibers, composites but not with in composites higher fiber with content highers. Great fiber contents.interfacial Great bonding interfacial between bonding the fibers between and the the fibers matrix and was the matrixalso observed was also usingobserved scanning using scanning electron electronmicroscopy. microscopy.

Author Contributions: Conceptualization, K.C.; Data curation, H.S.; Funding acquisition, M.J.; Investigation, Author Contributions: Conceptualization, K.C.; Data curation, H.S.; Funding acquisition, M.J.; Investigation, H.S.; Project administration, P.X.; Resources, P.X.; Writing—review and editing, K.C. H.S.; Project administration, P.X.; Resources, P.X.; Writing—review and editing, K.C. Funding: This research received no external funding. Funding: This research received no external funding Acknowledgments: The authors would like to acknowledge the support of the Fundamental Research Funds forAcknowledgement the Central Universitiess: The authors (JD1610). would The like authors to acknowledge would also the like support to appreciate of the Fundamental Mrs. Jianing GengResearch (School Funds of Internationalfor the Central Education, Universities Beijing (JD1610 University). The authors of Chemical would Technology) also like to for appreciate her help Mrs. in modifying Jianing Geng the language (School of the manuscript. International Education, Beijing University of Chemical Technology) for her help in modifying the language of Conﬂictsthe manuscript. of Interest: The authors declare no conﬂict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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References

1. Matabola, K.P.; De Vries, A.R.; Moolman, F.S.; Luyt, A.S. Single polymer composites: A review. J. Mater. Sci. 2009, 44, 6213. [CrossRef] 2. Stewart, R. Thermoplastic composites-recyclable and fast to process. Reinf. Plast. 2011, 55, 22–28. [CrossRef] 3. Kim, B.J.; Cha, S.H.; Park, Y.B. Ultra-high-speed processing of nanomaterial-reinforced woven carbon fiber/polyamide 6 composites using reactive thermoplastic resin transfer molding. Compos. Part B. Eng. 2018, 143, 36–46. [CrossRef] 4. Bechtold, G.; Wiedmer, S.; Friedrich, K. Pultrusion of thermoplastic composites-New developments and modelling studies. J. Thermoplast. Compos. 2002, 15, 443–465. [CrossRef] 5. Van Rijswijk, K.; Teuwen, J.J.E.; Bersee, H.E.N.; Beukers, A. Textile fiber-reinforced anionic polyamide-6 composites. Part I: The vacuum infusion process. Compos. Part A Appl. Sci. Manuf. 2009, 40, 1–10. [CrossRef] 6. Babeau, A.; Comas-Cardona, S.; Binetruy, C.; Orange, G. Modeling of heat transfer and unsaturated flow in woven fiber reinforcements during direct injection-pultrusion process of thermoplastic composites. Compos. Part A Appl. Sci. Manuf. 2015, 77, 310–318. [CrossRef] 7. Novo, P.J.; Silva, J.F.; Nunes, J.P.; Marques, A.T. Pultrusion of fibre reinforced thermoplastic pre-impregnated materials. Compos. Part B. Eng. 2016, 89, 328–339. [CrossRef] 8. Silva, R.F.; Silva, J.F.; Nunes, J.P.; Bernardo, C.A. New powder coating equipment to produce continuous fibre thermoplastic matrix towpregs. Mater. Sci. Forum 2008, 587, 246–250. [CrossRef] 9. Angelov, I.; Wiedmer, S.; Evstatiev, M.; Friedrich, K. Pultrusion of a flax/polypropylene yarn. Compos. Part A Appl. Sci. Manuf. 2007, 38, 1431–1438. [CrossRef] 10. Linganiso, L.Z.; Bezerra, R.; Bhat, S.; John, M. Pultrusion of flax/poly (lactic acid) commingled yarns and nonwoven fabrics. J. Thermoplast. Compos. 2014, 27, 1553–1572. [CrossRef] 11. Van Rijswijk, K.; Bersee, H.E.N.; Jager, W.F.; Picken, S.J. Optimisation of anionic polyamide-6 for vacuum infusion of thermoplastic composites: Choice of activator and initiator. Compos. Part A Appl. Sci. Manuf. 2006, 37, 949–956. [CrossRef] 12. Van Rijswijk, K.; Bersee, H.E.N.; Beukers, A.; Picken, S.J. Optimisation of anionic polyamide-6 for vacuum infusion of thermoplastic composites: Influence of polymerisation temperature on matrix properties. Polym. Test. 2006, 25, 392–404. [CrossRef] 13. Gong, Y.; Yang, G. All-polyamide composites prepared by resin transfer molding. J. Mater. Sci. 2010, 45, 5237–5243. [CrossRef] 14. Yan, C.; Li, H.; Zhang, X.; Zhu, Y.; Fan, X.; Yu, L. Preparation and properties of continuous glass fiber reinforced anionic polyamide-6 thermoplastic composites. Mater. Des. 2013, 46, 688–695. [CrossRef] 15. Van Rijswijk, K.; Bersee, H.E.N. Reactive processing of textile fiber-reinforced thermoplastic composites-An overview. Compos. Part A Appl. Sci. Manuf. 2007, 38, 666–681. [CrossRef] 16. Luisier, A.; Bourban, P.E.; Månson, J.A.E. Reaction injection pultrusion of PA12 composites: Process and modelling. Compos. Part A Appl. Sci. Manuf. 2003, 34, 583–595. [CrossRef] 17. Epple, S.; Bonten, C. Production of continuous fiber thermoplastic composites by in-situ pultrusion. AIP Conf. Proc. 2014, 1593, 454–457. 18. Chen, K.; Jia, M.Y.; Sun, H.; Xue, P. Optimization of initiator and activator for reactive thermoplastic pultrusion. J. Polym. Res. 2019, 26, 40. [CrossRef] 19. Cartledge, H.C.Y.; Baillie, C.A. Studies of microstructural and mechanical properties of nylon/glass composite Part I The effect of thermal processing on crystallinity, transcrystallinity and crystal phases. J. Mater. Sci. 1999, 34, 5099–5111. [CrossRef] 20. ISO 75-2-2013 Plastics-Determination of Temperature of Deflection under Load-Part 2: Plastics and Ebonite; ISO: Geneva, Switzerland, 2013. 21. ASTM D-790 Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials; ASTM: West Conshohocken, PA, USA, 2007. 22. ASTM D-2344 Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates; ASTM: West Conshohocken, PA, USA, 2016. 23. ASTM D-792 Standard Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement; ASTM: West Conshohocken, PA, USA, 2013. Materials 2019, 12, 463 15 of 15

24. Khodabakhshi, K.; Gilbert, M.; Fathi, S.; Dickens, P. Anionic polymerisation of caprolactam at the small-scale via DSC investigations. J. Therm. Anal. Calorim. 2014, 115, 383–391. [CrossRef] 25. García-Manrique, J.A.; Hoto, R.; Gascón, L.; Andrés, J. A numerical simulation of woven/anionic polyamide 6 composite part manufacturing using structural reactive injection moulding process. J. Thermoplast. Compos. 2016, 29, 219–233. [CrossRef] 26. Cho, B.G.; McCarthy, S.P.; Fanucci, J.P.; Nolet, S.C. Fiber reinforced nylon-6 composites produced by the reaction injection pultrusion process. Polym. Compos. 1996, 17, 673–681. [CrossRef] 27. Ricco, L.; Russo, S.; Oreﬁce, G.; Riva, F. Anionic poly (ε-caprolactam): Relationships among conditions of synthesis, chain regularity, reticular order, and polymorphism. Macromolecules 1999, 32, 7726–7731. [CrossRef]

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