Novel Poly(Caprolactone) Epoxy Blends By Additive Manufacturing

Materials 2020, 13, 8193 of 17of PCL and the glass transition temperature of the epoxy matrix have been used to fix the shapeduring the recovery cycle [38,43]. In a recent paper by Karger-Kocsis et al., the thermally inducedhealing through thermoplastic PCL dissolved in 12.5, 25, 37.5, and 50 wt %, respectively, in epoxysystems differing for their glass transition temperature (Tg ), was investigated [38]. It was found thatthe transition of PCL from disperse to continuous phase depends not only on the PCL amount, butalso on the epoxy type and on the healing temperature.Because of its peculiar properties, epoxy resin is the main thermosetting material used as matrixin composites, but it is quite brittle and has low fracture toughness properties. The most commonsolution for toughening epoxy resin is the blending with rubber elastomeric materials [44]. However,the obvious consequence is a reduction of the mechanical and thermal properties needed for high-endapplication such as aerospace engineering. In order to toughen and retain or even increase thethermo-mechanical properties of epoxy resin, different polymers with high molecular weight and glasstransition temperatures (Tg ), like polysulfone (PSF), poly(ether sulfone) (PES), and Poly (ether imide)(PEI) were considered [3]. In these works, it has been shown that, in order to maximize the tougheningeffect, high performance and high molecular weight engineering thermoplastics must be used in contentexceeding 20 wt %. In this way, co-continuous and phase-inverted morphologies are developed [45].However, the increase in viscosity can lead to some problems in the processing steps. The tougheningeffect upon the addition of a soft rubbery phase is given by the introduction of several differentmechanisms that are able to absorb energy during the fracture propagation depending on shape, size,volume fraction, surface modification, and type of the filler used. It is possible to identify in rubberreinforced materials a large number of different toughening mechanisms that can also be combined.Among the different mechanisms to toughen epoxy matrices with rigid thermoplastic particles, themost important are particle bridging, crack pinning, crack path deflection, particle-yielding-inducedshear banding, particle yielding, and microcracking [46].In the present work, a tridimesionally interconnected structure of PCL produced through filamentfused fabrication (FFF) was utilized to toughen an epoxy system. FFF is a technology which allows theproduction of complex three-dimensional objects starting from a computer-aided design computeraided design (CAD) model. The model, in order to be recognized by the printer, is first converted intodigital format and then, using a specific software, is sliced into layers [47–49]. Then, the model is readyfor starting the printing process [50]. In this process, thermoplastic filaments are mechanically fed froma spool into an extrusion head. The extruder is heated above the characteristic critical temperature(glass transition temperature for amorphous polymers and melting temperature for semicrystallinepolymers), and the softened or molten polymers are extruded through a nozzle in a x-y predefinedpath on a heated bed, and then solidify by cooling. After the deposition of a single layer, the heatedbed moves down and the next layer is deposited, and the process continues until the final 3D objectis completely produced [51,52]. The main advantages correlated to this process are its versatility, itslow cost, the possibility to achieve complex geometries and to use a wide variety of polymers withmultimaterial filaments with different functionalities in a single step [50]. The most used polymersfor FFF technology are acrylonitrile–butadiene–styrene (ABS), poly(lactic acid) (PLA), polyamides(especially Nylon 12), polycarbonate (PC), and thermoplastic polyurethane (TPU) [53]. In a recentwork of Karger-Kocsis et al., fused deposition modeling (FDM) was used to create 2D layered structuremade of PCL rods that were patterned with unidirectional carbon fibers, and the resulting materialswere then infiltrated with an amine-curable epoxy resin [54]. In this case, self-healing mechanismcould be triggered by heat treatment above the melting temperature of the PCL phase. On the otherhand, in literature, only an attempt can be found on the reinforcement of a 3D printed thermoplasticstructure with a high strength resin [55], but the investigation of the toughening effect provided by thethermoplastic phase was not considered.The aim of the present work was the development of novel thermoplastic (i.e., PCL)/thermosetting(i.e., epoxy) blends through FFF, in which the final morphology of the blend can be controlled andnot solely determined by the thermodynamics of mixing of the two phases. The thermoplastic phase

Materials 2020, 13, 8194 of 17Materials 2020,13, 819designed, a priori, with a computer-aided-design software and then produced4 byof 17geometryhas beenFFF. In a second step, the PCL scaffold has been impregnated with a liquid epoxy resin. After curingnovel system have been carefully analyzed and correlated with its microstructural features. In orderof the epoxy matrix, the mechanical properties, the toughening, and the self-healing features of thisto perform a direct comparison, also a conventional melt-mixed poly(caprolactone)/epoxy blend hasnovel system have been carefully analyzed and correlated with its microstructural features. In order tobeen produced and characterized with the same experimental techniques.perform a direct comparison, also a conventional melt-mixed poly(caprolactone)/epoxy blend has beenproduced and characterized with the same experimental techniques.2. Materials and Methods2. Materials and Methods2.1. Materials2.1. MaterialsIn order to build the 3D printed structure, a PCL99 Naturel poly(caprolactone) filament (density 1.145g/cmMn the47,500w 84,500g/mol),Naturelhaving a poly(caprolactone)diameter of 1.75 mm,was purchasedIn orderto3,build3D g/mol,printedMstructure,a PCL99filament(density EC157 (density 1.16 baseElantech 1.145 g/cm , Mn 47,500 g/mol, Mw 84,500 g/mol), having a diameter of 1.75 mm, was purchased EC157 (densityviscosityat 25 C (Haarlem, 800 mPa·s)and the hardenerElantechW 342 (density0.96 g/cm, viscosityfrom3D4MAKERSNetherlands).The epoxybase Elantech 31.16g/cm3 ,at C 800 WSrl25 C at7025mPa·s)werekindlyandprovidedby ElantasEuropeAll3materialsviscositymPa·s)the hardenerElantech342(Collecchio,(density Italy).0.96 g/cm, viscositywereat 25usedC as70received.mPa·s) were kindly provided by Elantas Europe Srl (Collecchio, Italy). All materials wereused as received.2.2. Preparation of the Samples2.2. Preparation of the Samples2.2.1. 3D Printed Poly(caprolactone)-Epoxy Blends2.2.1. 3D Printed Poly(caprolactone)-Epoxy BlendsAll the PCL structures were designed with the software Solidworks , provided by Dassault , provided by DassaultAllthe SEPCLstructures were e),withandtheaccordingtoSolidworksthe model representedin Figure 1a,b.SystemesSE (Velizy-Villacoublay,France),modeland accordingthe3)modelrepresentedin Figure1a,b.The nominaldimensions of the designed(80 10 to4 mmwere selectedaccordingto ISO1793 ) were selected according to ISO 179-2Thenominaldimensionsofthedesignedmodel(80 10 4mm2 standard [56]. The dimensions of the cross section were also valid for the determination of thestandardThe dimensionscrossD5045-14section werealso validdeterminationof ard[57] for(2 theW/B 4), where WandfractureB are thetoughnessaccordingtoASTMD5045-14standard[57](2 W/B 4),whereWandBaretheand towidth and thickness of the specimen, respectively. The proposed model was designedwidthin orderthicknessthe specimen,respectively. Theproposed modelin theorderto allowa completeallow a bothdesignedphases cturesthe PCL structures were put in the cavity of a silicone mold for the impregnation with the epoxywereputitinwasthe possiblecavity of toa siliconemoldshrinkagefor the impregnationwith theofepoxyresin, it wasTherefore,possible totheresin,see a slightin the mid-sectionthe structures.seesolutiona slight wasshrinkagein the ofmid-sectionof the structures.Therefore,the tosolutionwassomethe designofthe designfour transversalsupport partsin etricalstiffness. The main geometrical characteristics of the PCL structure, determined with the Solidworks characteristicsthe following:PCL structure,themmSolidworksthe following:total3 (35.0 vol software,software, areofthetotaldeterminedvolume with1119.5% of PCLarerespectto the external33volume 1119.5vol3),%volumeof PCLinrespectto theexternaldimensions80 10 4 mm), volumedimensions80mm 10 (35.04 mmthe spanlength(accordingto ASTMD5045standard[57]) 3 (32.6 vol % respect to thein 521.3the spanlength(accordingtoASTMD5045standard[57]) 521.3mm33mm (32.6 vol % respect to the dimensions 40 10 4 mm ), amount of thermoplastic PCL in32 (20.0% ofdimensions40 10 4 mmof thermoplasticPCL incontentthe midwithcrossrespectsection to 8themmcross-sectionthe mid crosssection 8), amountmm2 (20.0%of PCL surface2PCLsurface contentwith2 ofrespectto the cross-sectiondimensions 10 4 mm of the designed model).dimensions10 4 mmthe esentationPCLmodel.Figure1. nof ofthethePCLmodel. sDassaultSystemes,MA,USA) odewascompiledand exported in stereolithography format (STL). Through the software Slic3r, the G-code was , nozzle temperaturewiththe followingprintingparameters:percentage100%,infill angle 45infillcompiledwith age100%,angle 45 , nozzletemperature 220 C, bed temperature 40 C, deposition rate 15 mm/s. The 3D printer used toproduce the samples was a Sharebot Next Generation desktop 3D printer, provided by Sharebot Srl(Nibionno, Italy), equipped with a nozzle diameter of 0.35 mm. The liquid epoxy base Elantech

Materials 2020, 13, 8195 of 17 220 C, bed temperature 40 C, deposition rate 15 mm/s. The 3D printer used to produce thesamples was a Sharebot Next Generation desktop 3D printer, provided by Sharebot Srl (Nibionno, Italy),Materials 2020, 13, 8195 of 17equipped with a nozzle diameter of 0.35 mm. The liquid epoxy base Elantech EC157 was degassed for20 minroom temperature,andthentemperature,the hardenerandElantechwas addedandmanually W342EC157wasatdegassedfor 20 min atroomthen cast intoadded and manually mixed for 2 min. The mixture was degassed for 5 min at room subsequently cast into the 3D printed PCL structure, that was previously placed inside a siliconewasthenthe curingprocedureat roomfor 24 ath, roomfollowedby the postcuringmold.Thesubjectedresultingtoblendwas thensubjectedto thetemperaturecuring proceduretemperaturefor 24 h, P),neatPCLfilament(i.e.,PCL(FIL))followed by the postcuring at 60 C for 15 h. Therefore, in this work neat epoxy samples (EP), neatandcorresponding3D printed(i.e., PCL(3D))were consideredthePCL(3D))preparationofPCL thefilament(i.e., PCL(FIL))and structurethe corresponding3D ontentwaskeptconsidered for the preparation of 3D printed poly(caprolactone)/epoxy blends (i.e., EP-PCL(3D)), inconstant35 vol%. In wasFigure2 arereportedrepresentativeimagesofreportedthe preparedPCL(3D), EP,andwhich theatPCLcontentkeptconstantat 35vol %. In Figure2 arerepresentativeimagesEP-PCL(3D)samples.of the prepared PCL(3D), EP, and EP-PCL(3D) samples.FigureFigure2. Representativeimagesimagesof the preparedPCL(3D),EP, andsamples.2. Representativeof the preparedPCL(3D),EP,EP-PCL(3D)and EP-PCL(3D)samples.2.2.2.Melt-Mixed Poly(caprolactone)-EpoxyPoly(caprolactone)-Epoxy Blends2.2.2. Melt-MixedBlendsAepoxyresinwaswaspreparedwithwitha PCLa contentequal to20.0 wt%. TheA blendblendofofPCLPCLandandepoxyresinpreparedPCL contentequalto 20.0wt selected%. noftheproducedEP-PCL(3D)blends.selected amount of PCL was the same present in the mid cross section of the produced rrespondingfilaments,andthentheywereaddedtoblends. Small PCL pieces were prepared by cutting the corresponding filaments, and then they were theepoxybaseepoxyElantechin a steelcontainerconnectedto aconnectedwater circuit.temperatureofaddedto thebase EC157ElantechEC157in a steelcontainerto a Thewatercircuit. The C and PCL was melt-mixed with the liquid epoxy base by mechanical stirringthewaterwassetto85temperature of the water was set to 85 C and PCL was melt-mixed with the liquid epoxy base byat750 rpm for90 mina mechanicalmixture mixer.was thenmindegassedat roommechanicalstirringat with750 rpmfor 90 minmixer.with aThemechanicalThedegassedmixture forwas30then and the hardener Elantech W342 was addedtemperature.The temperatureraised at 70for 30 min at d at 70 C and the hardener Elantech andmanuallymixed2 min. mixedThe mixtureswereimmediatelycastedinto a siliconeW342was addedand formanuallyfor 2 min.Thethenmixtureswere thenimmediatelycastedmolds,into aanddegassedAfter performingtheperformingsame curingreportedfor the3D printedsiliconemolds,again.and degassedagain. Aftertheproceduresame curingprocedurereportedfor thePCL3D3 specimens were3epoxyblends,80 10 nted PCL epoxy blends, 80 10 4 mm specimens were obtained. The acronym EP-PCL wasthesesamples.utilizedto design these samples.2.3.Experimental TechniquesTechniques2.3. Experimental2.3.1. Characterization of the Constituents2.3.1. Characterization of the ConstituentsThe elastic modulus of the blend constituents (i.e., epoxy and PCL 3D printed structure) wasThe elastic modulus of the blend constituents (i.e., epoxy and PCL 3D printed structure) wasevaluated with quasi-static tensile tests performed using an Instron 5969 testing machine (Instron;evaluated with quasi-static tensile tests performed using an Instron 5969 testing machine (Instron;Norwood, MA, USA) equipped with a load cell of 50 kN. According to ISO 527 standard [58] 1BANorwood, MA, USA) equipped with a load cell of 50 kN. According to ISO 527 standard [58] 1BAsamples were produced by 3D printing for PCL and by casting for EP. The crosshead speed was setsamples were produced by 3D printing for PCL and by casting for EP. The crosshead speed was setequal to 0.25 mm/min and the strain was measured with an extensometer Instron 2620 (Instron;equal to 0.25 mm/min and the strain was measured with an extensometer Instron 2620 (Instron;Norwood, MA, USA), with a gauge length of 12.5 mm. The maximum deformation reached in allNorwood, MA, USA), with a gauge length of 12.5 mm. The maximum deformation reached in all thethe tests was limited to 1% and the elastic modulus was evaluated with the method of the secanttests was limited to 1% and the elastic modulus was evaluated with the method of the secant lineline between deformation levels of 0.05% and 0.25%. Failure properties of the constituents werebetween deformation levels of 0.05% and 0.25%. Failure properties of the constituents were measuredwith the same machine and the crosshead speed was set equal to 100 mm/min and the strain valueswere obtained by simply referring to the crossbar displacement. Instead, for EP specimens, thecrosshead speed was set equal to 1 mm/min. The strain at break was calculated normalizing theelongation at break with the initial distance between the grips, set at 58 mm. Five specimens were

Materials 2020, 13, 8196 of 17measured with the same machine and the crosshead speed was set equal to 100 mm/min and the strainvalues were obtained by simply referring to the crossbar displacement. Instead, for EP specimens,the crosshead speed was set equal to 1 mm/min. The strain at break was calculated normalizing theelongation at break with the initial distance between the grips, set at 58 mm. Five specimens weretested for both PCL(3D) and EP samples.2.3.2. Evaluation of the Flexural and Fracture BehaviorThree-point bending flexural tests were performed according to American Society for Testingand Materials (ASTM) D790 standard [59], with an Instron 5969 test machine (Instron; Norwood, MA,USA) with a load cell of 50 kN. The nominal dimensions of the tested EP, EP-PCL(3D), and EP-PCLspecimens were 80 10 4 mm3 . The span length was set to 65 mm and the crosshead speed was fixedat 1.68 mm/min. At least five specimens were tested for each sample. In this way, the tangent modulusof elasticity (EB ), the flexural stress at break (σfB ) and the flexural strain at break (εfB ) were determined.For as concerns the evaluation of fracture behavior under quasi-static conditions, single edgenotched bending (SENB) specimens with nominal dimensions 80 10 4 mm3 were tested, for EP,EP-PCL(3D), and EP-PCL blends, in three-point bending mode by using a universal Instron 5969testing machine, equipped with a load cell of 50 kN. The measurements were carried out at roomtemperature using a span length of 40 mm and a crosshead speed of 10 mm/min, according to the ASTMD5045 standard. Before testing, the samples were sharply notched with a razor blade (notch radiusof 0.01 mm) and the notch depth (a) was one half of the sample width. In this way, it was possibleto determine the critical stress intensity factor (KIC ) of the prepared samples, with the expressionsreported in Equations (1) and (2)PQKQ f (x)(1)B W 1/2 21 1.99 x(1 x)(2.15 3.93x 2.7xf (x) 6x 2(2)3(1 2x)(1 x) /2where KQ is the tentative value for KIC , PQ is the tentative value for the load and it could be equalor lower to Pmax , that is the maximum load sustained by the samples, B and W are respectively thethickness and the width of the samples, and f(x) is a calibration factor, with x a/W. Five specimenswere tested for EP, EP-PCL(3D), and EP-PCL blends, and the linearity and plasticization validitycriteria required by the ASTM D5045 were carefully checked.For EP, EP-PCL(3D), and EP-PCL blends, a new set of SENB specimens, with the same dimensionsreported for fracture tests under quasi-static conditions, was tested under impact conditions. Themeasurements were performed using a Charpy impact machine provided by CEAST, and theload–displacement curves were recorded using a tup extensometer in the hammer. A mass ofthe hammer equal to 2.5 kg, a starting angle of 32 , an impact speed of 1 m/s and a span length of 40mm were utilized to test the samples. The procedure for the determination of the fracture toughnesswas set according to [60], which was a test protocol used for the compilation of the ISO 17281 standard.In this way, KIC and specific energy absorbed under impact conditions (Uspec ), computed as the totalimpact energy divided by the resisting cross section of the material, were determined. Even in thiscase, five specimens were tested for each sample. Three-point bending configurations was selected toevaluate the toughness of the studied materials under quasi-static conditions, because it resembled theCharpy impact test to study the fracture toughness at higher speed.2.3.3. Evaluation of the Healing BehaviorEP, EP-PCL(3D), and EP-PCL blends samples, tested and fractured both in quasi-static modeand under impact conditions, were then healed by holding the two broken halves under pressure (7MPa) in an oven at T 80 C for 30 min, by using a home-made screw device. The above-mentionedhealing temperatures and time were selected according to the parameters reported in literature [38].

Materials 2020, 13, 8197 of 17The healed samples were re-tested following the same procedure reported for the evaluation of thefracture behavior in quasi-static and impact mode, and the healing efficiency (ηhealing ) was determinedaccording to Equation (3)KIC, healedηhealing (3)KIC, virginwhere KIc,healed is the fracture toughness after healing, and KIc,virgin is the fracture toughnessbefore healing.2.3.4. Microstructrural CharacterizationThe top view and the fracture surfaces of EP-PCL(3D) blend specimens, tested under impactconditions, were observed, before and after healing, through a Zeiss Axiophot optical microscope,equipped with a Leica DC300 digital camera. Microstructural observations of the fracture surfacesof EP-PCL blend were performed by a Zeiss Supra 40 high resolution Scanning Electron Microscope(SEM) operating at an accelerating voltage of 2.5 kV, after that a platinum palladium conductive coatingwas applied on the samples.3. Results and

phase separate during the curing process. Examples of epoxy blends which are heterogeneous are those with polyamides, polyvinylidene fluoride, and polybutylene terephthalate [11]. During curing, the molecular weight of the epoxy resin increases and this can lead to a phase separation between the thermoplastic phase and the epoxy matrix.