Sustainable Additive Manufacturing: Mechanical Response of Polyamide 12 Over Multiple Recycling Processes

materials

Article Sustainable Additive Manufacturing: Mechanical Response of Polyamide 12 over Multiple Recycling Processes

Nectarios Vidakis 1, Markos Petousis 1,* , Lazaros Tzounis 2, Athena Maniadi 3 , Emmanouil Velidakis 1, Nikolaos Mountakis 1 and John D. Kechagias 4

1 Mechanical Engineering Department, Hellenic Mediterranean University, 71410 Heraklion, Crete, Greece; [email protected] (N.V.); [email protected] (E.V.); [email protected] (N.M.) 2 Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece; [email protected] 3 Department of Materials Science and Technology, University of Crete, 70013 Heraklion, Crete, Greece; [email protected] 4 General Department, University of Thessaly, 41500 Larissa, Greece; [email protected] * Correspondence: [email protected]; Tel.: +30-2810-37-9227

Abstract: Plastic waste reduction and recycling through circular use has been critical nowadays, since there is an increasing demand for the production of plastic components based on different polymeric matrices in various applications. The most commonly used recycling procedure, especially for thermoplastic materials, is based on thermomechanical process protocols that could significantly alter the polymers’ macromolecular structure and physicochemical properties. The study at hand focuses on recycling of polyamide 12 (PA12) filament, through extrusion melting over multiple recycling courses, giving insight for its effect on the mechanical and thermal properties of Fused Filament Fabrication (FFF) manufactured specimens throughout the recycling courses. Three-dimensional (3D) FFF printed specimens were produced from virgin as well as recycled PA12 filament, while they have been experimentally tested further for their tensile, flexural, impact and micro-hardness mechanical Citation: Vidakis, N.; Petousis, M.; properties. A thorough thermal and morphological analysis was also performed on all the 3D printed Tzounis, L.; Maniadi, A.; Velidakis, E.; samples. The results of this study demonstrate that PA12 can be successfully recycled for a certain Mountakis, N.; Kechagias, J.D. Sustainable Additive Manufacturing: number of courses and could be utilized in 3D printing, while exhibiting improved mechanical Mechanical Response of Polyamide properties when compared to virgin material for a certain number of recycling repetitions. From this 12 over Multiple Recycling Processes. work, it can be deduced that PA12 can be a viable option for circular use and 3D printing, offering an Materials 2021, 14, 466. https:// overall positive impact on recycling, while realizing 3D printed components using recycled filaments doi.org/10.3390/ma14020466 with enhanced mechanical and thermal stability.

Received: 27 December 2020 Keywords: additive manufacturing (AM); three-dimensional (3D) printing; recycling; polyamide 12 Accepted: 15 January 2021 (PA12); tensile test; ﬂexural test; Charpy’s impact test; Vicker’s micro-hardness; scanning electron Published: 19 January 2021 microscopy (SEM)

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional afﬁl- 1. Introduction iations. Polymers are materials widely used by various industries nowadays, due to their relatively low-cost as well as unique intrinsic properties, such as ease of processability and light weight character, resulting into products with high mechanical, electrical and thermal properties [1–3]. As the demand for plastic products grows rapidly, the global Copyright: © 2021 by the authors. manufacturing of plastics is increasing at an approximately annual rate of 8.2% [4]. To date, Licensee MDPI, Basel, Switzerland. more than 3 billion tons of plastics has been produced globally [5], and the increasing use of This article is an open access article plastic products leads to a larger quantity of plastics ending up as waste in the environment. distributed under the terms and conditions of the Creative Commons Several strategies exist on how waste plastic should be used, such as recycling, reuse and Attribution (CC BY) license (https:// resource and energy recovery [6]. Amongst all, the one that is considered to be the most creativecommons.org/licenses/by/ cost efﬁcient and effective and deemed to be consistent enough for being implemented by 4.0/). the plastics’ industry is the mechanical recycling [7–9].

Materials 2021, 14, 466. https://doi.org/10.3390/ma14020466 https://www.mdpi.com/journal/materials Materials 2021, 14, 466 2 of 15

Fused Filament Fabrication (FFF) is the most commonly used 3D printing AM method to date, especially for thermoplastic in nature polymeric materials, mainly due to characteristics, such as rapid processing, simplicity and cost-efficiency [10,11]. Research is available but limited regarding the recycling of thermoplastics by repetitive melt extrusion cycles and consecutive 3D FFF printing, while basically studies exist only for the most commonly used thermoplastics in FFF such as acrylonitrile butadiene styrene (ABS) [12] and polypropylene (PP) [13]. Typical materials as filaments used in Fused Filament Fabrication nowadays are engineering thermoplastic polymers, such as polylactic acid (PLA) [14] and acrylonitrile butadiene styrene (ABS) [15–20], with extensive literature available for their mechanical, electrical and thermal properties. Literature is not yet enriched enough with data on the mechanical and thermal properties of semi-crystalline materials such as polyamide 6/66/12 (PA 6/66/12) used for 3D printing via FFF, with only a few reports available on the tensile strength and impact strength [21–23] as well as on the flexural strength [24]. This is mainly due to the difficulties of the material’s processing in FFF 3D printing, i.e., (i) distortion and warping of these materials, and (ii) upon their crystallization, they tend to have volume reduction associated with the formation of ordered, more densely packed regions during crystallization, when compared to amorphous polymers, such as ABS [25–27]. Consequently, crystallization, as a mechanism, is believed to drastically decrease molecular chain mobility and thus can prevent interlayer fusion to establish sufficiently strong adhesion between the 3D printed layers. Regarding continuous recycling, research exists for the mechanical and thermal properties of recycled ABS [12], while literature in not yet available on the continuous recycling of semi-crystalline thermoplastic polymeric materials such as polyamides for 3D printing applications. Moreover, the occurring mechanisms upon recycling based on melt processing methods, i.e., change of crystallinity, scission of polymeric macromolecular chains, etc., which can significantly affect the mechanical properties of the 3D printed recycled materials and their thermal stability have not been yet fully elaborated. Polyamide (PA) contains the amide repeat sequence linkage in the polymeric backbone, as well as H-bonds that are formed between the adjacent polymeric chains. Different types of polyamides exist, based on the macromolecular chains’ architectures and the main chemistry of the backbone, e.g., PA66, PA6,12, PA12, etc. As such, polyamides are typically tough, semi-crystalline polymers, with low glass transition temperature (Tg). Moreover, polyamides are known to be extensively used in automotive industry to manufacture automobile parts, as well as in textile industry and fabrics, due to their superior specific mechanical properties such as impact strength and their ease of processability [28]. Literature is available on the recycling of different types of polyamides and polyamide- based composites consisting of recycled polyamide [29–34]. However, no research is available on the recycling of virgin PA12 for a number of recycling courses and its effect on the mechanical and thermal properties of 3D printed specimens. Furthermore, from an economical point of view, besides the environmental necessity to recycle this type of polymer, polyamide (polyamide group of Plastics) holds the 8% of the global recycling market share, translating to 5.8 billion dollars in 2020 (Figure1), creating thus a strong interest for recycling. The current research work seeks initially to promote the production of recycled, sustainable PA12 filaments for circular use and to identify the underlying mechanisms that directly affect the mechanical and thermal stability of the recycled polymers. Moreover, this study aims to enrich literature with data upon the mechanical and thermal properties of 3D printed PA12 over multiple recycling courses, proving that the continuous recycling process for a certain number of repetitions can increase the mechanical properties of the recycled polyamide, when compared to virgin material. This research work focuses on the behavior of virgin and recycled PA12 material throughout six (6) recycling courses, with specimens manufactured via FFF, 3D printing, to investigate the thermomechanical process impact on the mechanical strength of the produced specimens as well as the structural and Materials 2021, 14, x FOR PEER REVIEW 3 of 15

behavior of virgin and recycled PA12 material throughout six (6) recycling courses, with specimens manufactured via FFF, 3D printing, to investigate the thermomechanical process impact on the mechanical strength of the produced specimens as well as the structural and morphological changes, resulting from the alteration of crystallinity and thermal properties. Mechanical testing (tension, flexion, impact, micro-hardness) was employed to evaluate the effects of degradation processes in the mechanical properties of the specimens made with virgin and recycled filament. It was found that there is an overall increase in the mechanical properties throughout the recycling courses until the 5th recycling course, both in tension and in flexion. For the corresponding thermal properties, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed, while Scanning Electron Microscopy (SEM) was conducted for the evaluation of their morphological characteristics in all recycling

Materials 2021, 14, 466 courses studied. DSC analysis showed a slight decrease in the material’s crystallinity3 after of 15 the 4th recycling course. Morphological analysis indicated that after the 4th recycling course, 3D printed specimens had degraded interlayering fusion, cause of the minor inability of the material to flow due to possible increased of crystals’ size affecting the melt morphologicalrheological properties changes, of resulting the polymer. from theWith alteration the data ofderived crystallinity from this and study, thermal it was properties. shown Mechanicalthat there is testing an economical (tension, and ﬂexion, a manufacturin impact, micro-hardness)g benefit in recycling was employed at least to up evaluate to five thetimes effects of PA12 of degradation and especially processes a commercially in the mechanical available propertiesPA12 material of the used specimens for FFF madeAddi- withtive Manufacturing. virgin and recycled ﬁlament.

FigureFigure 1.1. 20202020 recycledrecycled plasticsplastics marketmarket andand parametersparameters affectingaffecting plasticsplastics circularcircular economyeconomy (market(market volumevolume datadata source:source: Technavio.Technavio. GlobalGlobal RecycledRecycled PlasticsPlasticsMarket Market 2020–2022 2020-2022 (IRTNTR21968).(IRTNTR21968).

2. MaterialsIt was found and Methods that there is an overall increase in the mechanical properties throughout the2.1. recyclingMaterials courses until the 5th recycling course, both in tension and in flexion. For the corresponding thermal properties, thermogravimetric analysis (TGA) and differential scanningA polyamide calorimetry from (DSC) Arkema were (Colombes, performed, France) while Scanningwas used Electronand particularly Microscopy PA12 (SEM) resin wasAESNO conducted TL grade. for Arkema’s the evaluation PA12 ofis theirsold under morphological Rilsamid characteristicsPA12 AESNO TL in allbrand recycling name. coursesThe fundamental studied. DSC properties analysis of showed this PA12, a slight as they decrease are shown in the in material’s the vendor’s crystallinity correspond- after 2 theing 4thTechnical recycling Sheet, course. include Morphological a density of 1.01 analysis g/cm indicated(according that to ISO after 1183), the 4thMelt recycling Volume- 3 course,Flow Rate 3D (MVR) printed of specimens8.0 cm /10 min had (according degraded interlayeringto ISO 1133) at fusion, 235 °C/5.0 cause kg, of Vicat the Soften- minor inabilitying Temperature of the material at 142 °C to (according flow due to to possible ISO 306/B50) increased and Melting of crystals’ Temperature size affecting at 180 the °C melt(according rheological to ISO properties 11357-3). ofAccording the polymer. to the With vendor, the data in this derived PA12 from grade, this some study, additives it was shownhave been that added there isas anfor economicalheat stabilizing, and alubrication, manufacturing and UV benefit stabilization. in recycling at least up to five times of PA12 and especially a commercially available PA12 material used for FFF Additive Manufacturing.

2. Materials and Methods 2.1. Materials A polyamide from Arkema (Colombes, France) was used and particularly PA12 resin AESNO TL grade. Arkema’s PA12 is sold under Rilsamid PA12 AESNO TL brand name. The fundamental properties of this PA12, as they are shown in the vendor’s corresponding Technical Sheet, include a density of 1.01 g/cm2 (according to ISO 1183), Melt Volume-Flow Rate (MVR) of 8.0 cm3/10 min (according to ISO 1133) at 235 ◦C/5.0 kg, Vicat Softening Temperature at 142 ◦C (according to ISO 306/B50) and Melting Temperature at 180 ◦C (according to ISO 11357-3). According to the vendor, in this PA12 grade, some additives have been added as for heat stabilizing, lubrication, and UV stabilization.

2.2. Methods 2.2.1. Recycling Process In this study, a procedure of repeated extrusion processes was followed in order to simulate the degradation mechanism of consequent recycling process [35]. The material selected for this work, PA12, was purchased from Arkema (Colombes, France), in the form of fine granules. After a drying procedure at 80 °C for 24 h, granules were extruded through a single screw extruder (3D Evo Composer 450, 3D Evo B.V., Utrecht, The Neth- erlands), in order to produce 1.75 mm diameter filament appropriate for FFF 3D printing. The produced filament was then dried at the same conditions, and part of it was used to produce 3D printed specimens, while the rest of the filament was shredded back into pellets in order to be recycled. The above procedural steps were repeated 6 times, otherwise defined, and denoted in this work as 6 corresponding recycling courses. In each recycle course, filament quality control was performed under a real-time monitoring system working with optical sensors (extruder’s built-in system). A deviation of 0.07 mm on average was measured in each recycle course’s produced filament. Quality control was also conducted under optical and dimensional measures in each manufactured specimen. Fig- ure 2 summarizes all processes followed for the purposes of this study. Materials 2021, 14, 466 4 of 15 The shredder machine that was utilized in order to reshape the filament into pellets was a Shreddit (3D Evo B.V., Utrecht, The Netherlands). Through all recycling courses, the extrusion parameters in the 3D Evo Composer 450 (3D Evo B.V., Utrecht, The Nether- lands)a single were screw kept extruder constant. (3D More Evo Composerspecifically, 450, heat 3D zone Evo 1 B.V., (closer Utrecht, to extruder’s The Netherlands), nozzle) was in setorder to to210 produce °C, heat 1.75 zone mm 2 diameterwas set to filament 220 °C, appropriateheat zone 3 forwas FFF set 3D to printing.220 °C and The finally produced heat zonefilament 4 (closer was thento extruder’s dried at thehopper) same was conditions, set to 185 and °C. partExtruder’s of it was screw used rotational to produce speed 3D wasprinted set to specimens, 8.5 rpm, while the restcooling of the fans filament after the was nozzle shredded were back set to into 50% pellets speed. in order to be recycled.In this study, The above Fused procedural Filament Fabrication steps were was repeated selected 6 times,to manufacture otherwise specimens defined, and for eachdenoted recycling in this course. work as Specimens 6 corresponding were fabricat recyclinged courses.using a Craftbot In each recycle Plus (Craftbot course, filamentltd, Bu- dapest,quality controlHungary) was Fused performed Filament under Fabrication a real-time technology monitoring 3D system Printer, working with the with all-metal optical 3Dsensors printing (extruder’s head (hot built-in end) in system). its setup. A deviationThe 3D printing of 0.07 mmparameters on average are summarized was measured in Figurein each 3. recycle It has to course’s be mentioned produced that filament. a masking Quality tape (3M control 101+) was was also used conducted over the alumi- under numoptical build-plate and dimensional (heatbed) measures in order into eachreduce manufactured the wrapping specimen. effect on Figurethe PA12.2 summarizes Finally, no coolingall processes (3D printer followed nozzle’s for the fans purposes were set of to this 0%) study. was used.

Figure 2. Flow chart with the methodology followed for the PA12 recycling in the current study.

The shredder machine that was utilized in order to reshape the filament into pellets was a Shreddit (3D Evo B.V., Utrecht, The Netherlands). Through all recycling courses, the extrusion parameters in the 3D Evo Composer 450 (3D Evo B.V., Utrecht, The Netherlands) were kept constant. More specifically, heat zone 1 (closer to extruder’s nozzle) was set to 210 ◦C, heat zone 2 was set to 220 ◦C, heat zone 3 was set to 220 ◦C and finally heat zone 4 (closer to extruder’s hopper) was set to 185 ◦C. Extruder’s screw rotational speed was set to 8.5 rpm, while the cooling fans after the nozzle were set to 50% speed. In this study, Fused Filament Fabrication was selected to manufacture specimens for each recycling course. Specimens were fabricated using a Craftbot Plus (Craftbot ltd, Budapest, Hungary) Fused Filament Fabrication technology 3D Printer, with the all-metal 3D printing head (hot end) in its setup. The 3D printing parameters are summarized in Figure3. It has to be mentioned that a masking tape (3M 101+) was used over the aluminum build-plate (heatbed) in order to reduce the wrapping effect on the PA12. Finally, no cooling (3D printer nozzle’s fans were set to 0%) was used. Materials 2021, 14, x FOR PEER REVIEW 5 of 15

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Figure 2. Flow chart with the methodology followed for the PA12 recycling in the current study.

Figure 3. ParametersParameters employed in the study for the co constructionnstruction of the FFF 3D printed specimens.

2.2.2.2.2.2. Tensile Tensile Specimens’ Fabrication and Testing TensileTensile experiments werewere carried carried out out under unde ther the ASTM ASTM D638-02a D638-02a international international standard. standard.According According to the to standard, the standard, a type a type V specimen V specimen of 3.2 of mm 3.2 thicknessmm thickness was chosen,was chosen, and aand total a totalof six of (6) sixspecimens (6) specimens were were manufactured manufactured and and tested tested for for each each recycling recycling course. course. Tensile test’stest’s device device used used for for this this purpose purpose was was an an Imada Imada MX2 MX2 (Imada (Imada inc., inc., Northbrook, Northbrook, IL, IL, USA) USA) tension/flexiontension/ﬂexion test test apparatus apparatus in in tensile tensile mode mode set set up, up, using using standardized standardized grips grips (Figure (Figure 44a).a ). ElongationElongation speed forfor thethe tensile tensile test test was was set set to to 10 10 mm/min mm/min as peras per the the standard standard speciﬁcations specifica- ◦ tionsand the and experimental the experimental lab temperature lab temperature when when tests weretests were conducted conducted was 21 wasC. 21 °C.

2.2.3. Flexion Specimens’ Fabrication and Testing Flexural tests were conducted according to the ASTM D790-10 international standard. A total of six (6) specimens for each recycling course were fabricated with a thickness of 3.2 mm and tested in the same device referred above in three-point bending test setup. Imada’s chuck speed was set to 10 mm/min, and the flexural setup used is presented in Figure 5a. The experimental lab temperature when tests were conducted was 21 °C.

2.2.4. Impact Specimens’ Fabrication and Testing Impact tests were conducted according to the ASTM D6110-04 international standard. Specimens were built with dimensions of 80 mm length, 8 mm width and 10 mm thickness. A total of six (6) specimens for each recycling course were tested in a Terco MT 220 (Terco AB, Huddinge, Sweden) Charpy’s impact apparatus. Release height of the apparatus hammer was the same for all the experiments. The experimental lab temperature when tests were conducted was 21 °C.

2.2.5. Micro-hardness Measurements Microhardness measurements were conducted according to the ASTM E384-17 inter-

national standard. The specimens’ surface was fully polished before each set of measure- Figure 4. Tensile tests: (a) devicements experimental according to setup the standard’s and specimens requiremen after the test,ts. An (b )Innova stress strain Test graphs300 Vickers from the(Innovatest no 2 specimen of each recycle course, (c) comparison of the tensile strength average values along with the calculated deviation Europe BV, Maastricht, The Netherlands) was used. Applied force was set to 100 gF and from each recycle course studied,a duration and (ofd) comparison10 s was selected of the tensilefor indentation. modulus of Imprints elasticity were average measured values along under with six the (6) dif-

calculated deviation from eachferent recycle specimens course studied. for each one of the six (6) recycle courses.

2.2.3. Flexion Specimens’ Fabrication and Testing 2.2.6. Thermal Analysis Flexural tests were conducted according to the ASTM D790-10 international standard. A totalThermogravimetric of six (6) specimens analysis for each (TGA) recycling under course oxygen were was fabricated performed with to aobtain thickness infor- of mation3.2 mm about and testedthe critical in the degradation same device temperature referred above of the in PA12 three-point virgin material bending selected test setup. for thisImada’s research chuck to speed identify was the set appropriate to 10 mm/min, extrusion and the and ﬂexural 3D printing setup used temperature. is presented The in measurementsFigure5a. The experimentalwere carried labout temperature with a Perkin when Elmer tests Diamond were conducted TG/TDA was (PerkinElmer, 21 ◦C.

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Figure 5. Flexural tests: ( (aa)) d deviceevice experimental experimental setup setup and and specimens specimens after after the the test, test, (b (b) )stress stress stra strainin graphs graphs from from the the no no 2 specimen of each recycle course, (c) comparison of the flexural strength average values along with the calculated deviation 2 specimen of each recycle course, (c) comparison of the ﬂexural strength average values along with the calculated deviation from each recycle course studied, and (d) comparison of the flexural modulus of elasticity average values along with the from each recycle course studied, and (d) comparison of the ﬂexural modulus of elasticity average values along with the calculated deviation from each recycle course studied. calculated deviation from each recycle course studied.

3.3.2.2.4. Impact Impact Results Specimens’ Fabrication and Testing Charpy’sImpact tests notched were conductedImpact Test according results are to theshown ASTM in Figure D6110-04 6a. internationalSpecifically, in standard. Figure 6aSpecimens, the mean were calculated built with impact dimensions strength of 80of mmeach length, recycling 8 mm course width for and PA12 10mm is presented, thickness. whileA total Figure of six (6)6b specimenspresents the for Charpy’s each recycling test setup course and were broken tested specimens in a Terco after MT the 220 experi‐ (Terco ment.AB, Huddinge, Sweden) Charpy’s impact apparatus. Release height of the apparatus hammer was the same for all the experiments. The experimental lab temperature when tests were conducted was 21 ◦C.

2.2.5. Micro-Hardness Measurements Microhardness measurements were conducted according to the ASTM E384-17 international standard. The specimens’ surface was fully polished before each set of measurements according to the standard’s requirements. An Innova Test 300 Vickers (Innovatest Europe BV, Maastricht, The Netherlands) was used. Applied force was set to 100 gF and a duration of 10 s was selected for indentation. Imprints were measured under six (6) different specimens for each one of the six (6) recycle courses.

2.2.6. Thermal Analysis Figure 6. Impact tests: (a) comparisonThermogravimetric of the impact strength analysis average (TGA) values under along with oxygen the calculated was performed deviation to from obtain each infor- recycle course studied and (mationb) device about experimental the critical setup degradation and specimens temperature after the test. of the PA12 virgin material selected for this research to identify the appropriate extrusion and 3D printing temperature. The measurementsRecycling werecourses carried comparative out with graphs a Perkin (a) Elmer calculated Diamond average TG/TDA values (PerkinElmer, and deviation Inc., of ◦ theWaltham, impactMassachusetts, strength for all Unitedthe recycling States courses of America) studied with and a heating(b) Charpy’s cycle impact from 32 testC ex‐ to ◦ ◦ perimental550 C with setup a heating alongside step of with 10 theC/min. broken specimens. Differential scanning calorimetry (DSC) was also performed to obtain information 3.4.about Micro the- effecthardness of recycling,Results on the melting point (Tm) and the shift in the degree of crystallinity of the samples. The measurements were taken via a Perkin Elmer Diamond DSC Figure 7a below presents the mean micro-hardness Vickers (HV) values calculated (PerkinElmer, Inc., Waltham, Massachusetts, United States of America) with a temperature for each recycling course of PA 12, while the experimental setup is depicted in Figure 7b. cycle of 50 ◦C to 300 ◦C with a heating step of 10 ◦C/min and then cooling down to 50 ◦C.

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2.2.7. Morphological Characterization Scanning electron microscopy (SEM) characterization was performed for the morphological characterization of specimens’ internal/external structure and interlayer fusion via fractured surfaces’ microstructural analysis and 3D printed samples side surface morphol- ogy, respectively. The SEM analysis was carried out using a JEOL JSM 6362LV (Jeol Ltd., Peabody, MA, USA) electron microscope in high-vacuum mode at an accelerating voltage of 5 kV.

3. Results 3.1. Tension Results In Figure4 below, the tensile testing setup utilized in this work is depicted ( Figure4a ), alongside with the fractured specimens after the tensile test experiments (Figure4a), the comparative graphs of the tensile stress versus strain (Figure4b), the average tensile strength and the deviation in each recycling step (Figure4c) and the average tensile modulus of elasticity and deviation (Figure4d) for all six (6) recycling courses.

3.2. Flexion Results

In Figure5 below, the flexural testing setup utilized in this work is presented (Figure5a ), Figure 5. Flexural tests: (a) device experimental setup and specimens after the test, (b) stress strain graphs from the no 2 specimen of each recycle course,alongside (c) comparison with the of specimens the flexural after strength the average experiment values (Figure along with5a), thethe calculated comparative deviation graphs of from each recycle course studied,the flexural and (d) stresscomparison versus of strainthe flexural (Figure modulus5b), the of averageelasticity flexuralaverage values strength along and with the the devia- calculated deviation from eachtion recycle in each course recycling studied step. (Figure5c) and the average flexural modulus of elasticity and deviation (Figure5d) for all six (6) recycling courses. 3.3. Impact Results 3.3. Impact Results Charpy’s notched Impact Test results are shown in Figure 6a. Specifically, in Figure 6a, theCharpy’s mean calculated notched Impact impact Test strength results of are each shown recycling in Figure course6a. Specifically, for PA12 is in presented, Figure6a, the mean calculated impact strength of each recycling course for PA12 is presented, while while Figure 6b presents the Charpy’s test setup and broken specimens after the experi‐ Figure6b presents the Charpy’s test setup and broken specimens after the experiment. ment.

Figure 6. ImpactImpact tests: tests: ( (aa)) c comparisonomparison of of the the impact impact strength strength average average values values along along with with the the calculated calculated deviation deviation from from each recycle course studied and (b) device experimental setup and specimens after the test. recycle course studied and (b) device experimental setup and specimens after the test.

RecyclingRecycling courses courses comparative comparative graphs graphs (a) (a) calculated calculated average average values values and and devi deviationation of theofthe impact impact strength strength for forall the all therecycling recycling courses courses studied studied and and (b) Charpy’s (b) Charpy’s impact impact test testex‐ perimentalexperimental setup setup alongside alongside with with the the broken broken specimens. specimens.

3.4.3.4. Micro Micro-Hardness-hardness R Resultsesults FigureFigure 77aa belowbelow presents presents the the mean mean micro-hardness micro-hardness Vickers Vickers (HV) (HV) values values calculated calcula forted foreach each recycling recycling course course of PAof PA 12, while12, while the the experimental experimental setup setup is depicted is depicted in Figure in Figure7b. 7b.

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Figure 7. Micro-hardness Vickers tests: (a) comparison of the Vickers micro-hardness average values along with the calculated deviation from each recycle course studied and (b) device experimental setup and specimens after the test.

3.5. Thermal Analysis Results Regarding the thermal analysis, the TGA results of pure PA12 in direct comparison with PA12 of the 6th recycling course are presented in Figure 8a, while a zoom in the degradation area is depicted in Figure 8b. DSC analysis was employed for the qualitative determination of the degree of crystallinity, as well as the melting temperature (Tm) and glass transition temperature (Tg) of the investigated materials. The degree of crystallization was calculated by the following equation [36]:

Figure 7. 7. MMicro-hardnessicro-hardness Vickers Vickers tests: tests: (a) ( ac)omparison comparison of ofthe the VickersΔΗ Vickersm micro micro-hardness-hardness average average values values along along with with the cal- the culated deviation from each recycle courseXc( studied%crystallinity and (b)) device= experimental∗ 100% setup and specimens after the test.(1) (1) calculated deviation from each recycle course studied and (b) deviceΔHo experimental setup and specimens after the test.

3.5.whereas3.5. Thermal Thermal ΔH A Analysismnalysis is the Rmelting Resultsesults enthalpy (the area under the melting curve), and ΔΗο is a theoretical value of the melting enthalpy of 100% crystalline PA12. The value ΔH0 = 230 RegardingRegarding the the thermal thermal analysis, analysis, the the TGA TGA results results of of pure pure PA12 PA12 in direct comparison J/g was used in a degree of crystallinity calculation. DSC results are shown in Figure 9a with PA12 PA12 of the 6 6thth recycling course are presented in Figure 88a,a, whilewhile aa zoomzoom inin thethe and Table 1, while a magnification of a specific temperature window of the graph is de- degradationdegradation area area is depicted in Figure 88b.b. picted in Figure 9b. DSC analysis was employed for the qualitative determination of the degree of crystallinity, as well as the melting temperature (Tm) and glass transition temperature (Tg) of the investigated materials. The degree of crystallization was calculated by the following equation [36]: ΔΗm Xc(%crystallinity) = ∗ 100% (1) (1) ΔHo

whereas ΔHm is the melting enthalpy (the area under the melting curve), and ΔΗο is a theoretical value of the melting enthalpy of 100% crystalline PA12. The value ΔH0 = 230 J/g was used in a degree of crystallinity calculation. DSC results are shown in Figure 9a and Table 1, while a magnification of a specific temperature window of the graph is depicted in Figure 9b.

Figure 8 8.. (a) TGA graphs for for pure pure PA12 PA12 1st1st recycling course versus 6 6thth recycling course; ( b) magnified magniﬁed TGA temperature window for the material’s thermal degradation region.region.

DSC analysis was employed for the qualitative determination of the degree of crystallinity, as well as the melting temperature (Tm) and glass transition temperature (Tg) of the investigated materials. The degree of crystallization was calculated by the following equation [36]: ∆Hm Xc(%crystallinity) = ∗ 100% (1) ∆Ho

whereas ∆Hm is the melting enthalpy (the area under the melting curve), and ∆Ho is a theoretical value of the melting enthalpy of 100% crystalline PA12. The value ∆H o = 230 J/g Figure 8. (a) TGA graphs forwas pure used PA12 in 1st a degreerecycling of course crystallinity versus 6 calculation.th recycling course; DSCresults (b) magnified are shown TGA intemperatureFigure9a and Table1, while a magniﬁcation of a speciﬁc temperature window of the graph is depicted in window for the material’s thermal degradation region. Figure9b.

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Figure 9. (a) DSC 2nd2nd heat/coolheat/cool cycle cycle curves curves for for PA12 PA12 1st,1 1st,st, 3 3rd,rd, and 6th6 th recycling courses and ( b) magniﬁedmagnified temperature window of the DSC curves. window of the DSC curves.

Table 1. Basic thermal characteristics of the PA12 samples as obtained and calculated from DSC results. TableTable 1. 1.BasicBasic thermal thermal characteristics characteristics of of the the PA12 PA12 samples asas obtainedobtained andandcalculated calculated from from DSC DSC results. results.

◦ Tc (°C) Crystallization tem- ◦ SampleSample T ( C) CrystallizationTc (°C) Crystallization Temperature Tem- T Tm m((° (°C)C)C) Melting Melting Temperature temperature Tempera- X Xc (%)(%) Crystallinity Crystallinity Degree degree Sample c m Xc (%)c Crystallinity Degree perature ture PA12 1st cyclest 147 175 36.5 PA12 1st cycle 147 175 36.5 PA12 3rd cyclerd 144 174 36.1 PA12 3rd cycle 144 174 36.1 PA12 6th cycleth 145 175 30.6 PA12 6th cycle 145 175 30.6

3.6.3.6. Morphological Morphological CharacterizationC Characterizationharacterization ResultsR Resultsesults InIn the the following Figure Figure 10,10, the SEM images of the tensile specimens’specimens’ side surface is depicteddepicted in in order to identify any defects in the layering of the specimens as well as to examineexamine the layering interfusion of the specimens after each recycling course.

Figure 10. Specimens’ side surface SEM images: (a) 1st recycling course, (b) 2nd recycling course, (c) 3rd recycling course, Figure 10. Specimens’ side surface SEM images: ( (aa)) 1st 1st recycling recycling course, course, ( (bb)) 2nd 2nd recycling recycling course, course, ( (cc)) 3rd 3rd recycling recycling course, course, (d) 4th4th recycling course,course, (e) 5th5th recycling course and (f) 6th6th recycling course. (d) 4th recycling course, (e) 5th recycling course and (f) 6th recycling course.

In FigureFigure 11, the fracturedfractured areas of the tenstensileile specimens, one of each recycling course are depicted, in order to study and elaborate the fracture mechanics as well as to find any possible correlations with the mechanical propertyproperty results.

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In Figure 11, the fractured areas of the tensile specimens, one of each recycling course Materials 2021, 14, x FOR PEER REVIEWare depicted, in order to study and elaborate the fracture mechanics as well as to ﬁnd10 of any 15

possible correlations with the mechanical property results.

Figure 11. TensileTensile s specimens’pecimens’ fracture fracture su surfacerface SEM SEM images: images: (a) (1sta) 1strecycling recycling course, course, (b) 2nd (b) 2ndrecycling recycling course, course, (c) 3rd (c )recy- 3rd cling course, (d) 4th recycling course, (e) 5th recycling course and (f) 6th recycling course. recycling course, (d) 4th recycling course, (e) 5th recycling course and (f) 6th recycling course.

4. Discussion 4.1. Mechanical Properties Regarding the effect of continuous recyclingrecycling processes on the mechanical properties of PA12, it waswas provedproved inin thisthis studystudy thatthat thethe mechanicalmechanical propertiesproperties overall increase untiluntil the 4th recycling course regarding the tensile strength, while there is anan overalloverall increaseincrease until the 3rd recycling course in flexuralflexural strength.strength. Moreover, the impact strength resultsresults follow a similar trend as the overall increase ceasesceases atat thethe 3rd3rd recyclingrecycling course.course. Regarding the tensile strength results, as statedstated above, there is a notable increase in the tensile strengthstrength valuesvalues until until the the 5th 5th recycling recycling course, course, with with a peaka peak value value of 39.1of 39.1 MPa MPa on onthe the 4th 4th recycling recycling course. course. This This result result proves prov therees there is a total is a 17.7%total 17.7% improvement improvement in the tensilein the tensilestrength, strength, when compared when compared to virgin to PA12virgin material. PA12 material. The same The trend same is trend visible is onvisible the tensileon the tensilemodulus modulus of elasticity of elasticity resultsthat results clearly that show clearly a peak show value a peak of 176.6value MPa of 176.6 on the MPa 4th on recycling the 4th recyclingcourse. It course. has to beIt notedhas to thatbe noted the tensile that the modulus tensile ofmodulus elasticity of experimentallyelasticity experimentally obtained valuesobtained and values sample’s and sample’s behavior behavior could be could more preciselybe more precisely attributed attributed to (i) the to “filamentous” (i) the “filamentous”type of printing type of in printing a layer-by-layer in a layer-by-layer manner ofmanner the 3D of printed the 3D bulkprinted objects, bulk asobjects, well as (ii) some plausible defects/voids between the sample’s layers, due to the nature of 3D well as (ii) some plausible defects/voids between the sample’s layers, due to the nature of printing, that could act as stress accumulation points. From these micro-cracks and while 3D printing, that could act as stress accumulation points. From these micro-cracks and the material is being stressed at an external quasi static tensile field, it can be envisaged while the material is being stressed at an external quasi static tensile field, it can be envis- that some cracks could initiate and propagate in the longitudinal direction that the tensile aged that some cracks could initiate and propagate in the longitudinal direction that the test stress field is applied, resulting into a “plasticization”-like effect and yielding further tensile test stress field is applied, resulting into a “plasticization”-like effect and yielding to generally lower tensile modulus of elasticity values or, in other words, a knock-down further to generally lower tensile modulus of elasticity values or, in other words, a knock- effect on the material’s modulus. Specifically, similar behavior has been also observed for down effect on the material’s modulus. Specifically, similar behavior has been also ob- ABS, PLA, HDPE and PP as reported in our previous recent study [37], corroborating the served for ABS, PLA, HDPE and PP as reported in our previous recent study [37], corrob- behavior that was also experimentally determined for PA12 tensile modulus of elasticity orating the behavior that was also experimentally determined for PA12 tensile modulus values in the study at hand. of elasticityFinally, values tensile in stress the study versus at strainhand. graphs, indicate that specimens of the 4th cycle behaveFinally, in a “stiffer”tensile stress manner, versus when strain compared graphs, to indicate the specimens that specimens of the rest of ofthe the 4th cycles. cycle behave in a “stiffer” manner, when compared to the specimens of the rest of the cycles. There is no similar work in literature yet available to directly correlate the above findings. Similar studies regarding laser sintering with PA12 indicate that PA12 shows improved tensile strength after recycling [38]. It was also noted that in the Selective Laser Sintering

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Materials 2021, 14, x FOR PEER REVIEW 11 of 15 There is no similar work in literature yet available to directly correlate the above findings. Similar studies regarding laser sintering with PA12 indicate that PA12 shows improved tensile strength after recycling [38]. It was also noted that in the Selective Laser Sintering ((SLS)SLS) processes with PA materials, the thermal stress of the unused materialmaterial is strong enough to cause aging [ 39]. Flexural properties show a similar behavior. Flexural strength (and flexural flexural modulus of elasticity) starts t too decrease after the 2 2ndnd recycling step and significantly significantly decrease after the 4th 4th repetition. repetition. More More specifically, specifically, the peak value in the flexure flexure strength is visible in the 22ndnd recycling course in this case with a value of 44.1 MPa. This indicates an increase in the flexural flexural strength of 15.4%, when compared to thethe virginvirgin material.material. Flexural modulus of elasticity follows the same trend with a peak value of 882.6 MPa in the 2nd2nd recyclingrecycling course. In In a similar way, fromfrom FigureFigure5 5bb,, stress/strain stress/strain curves’curves’ slope,slope, itit isis derivedderived thatthat thethe 22ndnd recycling course specimens shown a “stiffer” behavior. Regarding the impact strength results, it was shown that there is an increase in the 2 impactimpact strength until thethe 4th4th recyclingrecycling course.course. AA peakpeak valuevalue ofof 61.561.5 kJ/mkJ/m2 is present at the 33rdrd recycling coursecourse andand marksmarks a a 47.3% 47.3% increase increase in in the the impact impact strength, strength, when when compared compared to tothe the 1st 1 recyclingst recycling course course [24 [].24]. Finally, regarding the micro-hardnessmicro-hardness Vickers results, it was shown that a peak of 28.5 HV value was present present at the the 4 4thth recycling recycling course, course, stating stating a a slight slight improvement improvement of 4%, 4%, when compared to the 1st1st recycling course. The overall micro micro-hardness-hardness findingsfindings indicate that there there is not a clear improvement or decrease in the micro-hardnessmicro-hardness of the polyamide 12 specimens. This can be due to the the layering layering structure of the FFF specimens. No literature isis yet yet available available for the micro micro-hardness-hardness on the recycled PA12. Regarding the overall mechanical properties results ( (FigureFigure 1122), data data from from other other stud- stud- iesies [ 36,39–41] suggest that chain chain-scission-scission and chain branching may occur simultaneously from the start of the reprocessing due to chain scission caused by the shear forces intro- from the start of the reprocessing due to chain scission caused by the shear forces intro- duced in the extrusion system. These competing mechanisms alongside with a possible duced in the extrusion system. These competing mechanisms alongside with a possible degree of crosslink might be the cause of the shift in the mechanical properties. degree of crosslink might be the cause of the shift in the mechanical properties.

FigureFigure 12 12.. VirginVirgin and and recycled recycled in in the the 6 6 recycle courses studied in this work PA12 mechanical properties summary.summary.

4.2. Thermal Analysis Analysis Figure 88 depicts the overall thermal thermal stability stability and and critical critical degradation degradation temperature temperature of PA12of PA12 of 1 ofst 1stand and 6th recycling 6th recycling courses, courses, showing showing a slight a slightdegradation degradation of the polymer. of the polymer. More- ovMoreover,er, the TGA the TGAgraph graph shows shows the bandwidth the bandwidth and the and maximum the maximum operating/processing operating/processing tem- peraturetemperature that thatthe material the material can be can processed be processed by means by means of extrusion of extrusion melting melting and 3D and print- 3D ing.printing. TGA TGAdata dataindicates indicates that a that possible a possible knock knock-down-down effect effect might might be present be present due dueto the to aforemthe aforementionedentioned degradation, degradation, affecting affecting the thermal the thermal stability stability of the of thepolymer polymer and and thus thus resulting in specimens with inferior mechanical properties after multiple recycling courses. This is in agreement with the tensile and flexure strength results of the Sections 3.1 and 3.2 indicating that mechanical properties are indeed deteriorating after the 4th recycling

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resulting in specimens with inferior mechanical properties after multiple recycling courses. This is in agreement with the tensile and flexure strength results of the Sections 3.1 and 3.2 indicating that mechanical properties are indeed deteriorating after the 4th recycling course. Finally, the TGA curves show that the working/process temperature utilized in this work for all PA12 samples, both for recycling and 3D printing processes, are below the critical temperature of 401 ◦C, where the material in all cases starts rapidly to degrade with an abrupt weight loss. DSC results in Figure9 and Table1 indicate a slight decrease in the degree of crystallinity with the recycling course, while there is not any noticeable shift in the corresponding melting (Tm) and crystallization (Tc) temperatures. The glass transition temperature (Tg) for all samples was determined by the melting endotherm curves (Figure9b), while it has been found in the temperature range of 50–55 ◦C. As it can be seen from Figure9b , all samples exhibited similar crystallization and melting behavior, revealing thus that the crystallinity of the samples has not been significantly altered and/or affected by the multiple consecutive recycling and 3D printing processing cycles, until the 6th recycling course. Literature suggests that decrease in crystallinity generally leads to the reduction of the effect that material molecules can pack and form crystal regions. The changes in crystallinity that are evident from the recycling procedure, more profound on the 6th recycling course, suggest that there might be a simultaneous occurrence of chain branching or polymer cross-linking along with chain scission, when using extrusion systems in the recycling process [36,42]. Furthermore, changes in the degree of crystallinity may result in changes of mechanical properties, i.e., creep, modulus, and hardness, presented in continuation. It should be mentioned that (i) some variation in the polymer chain lengths (chain scission) due to the consecutive recycling courses, affecting also a possible high quality of crystallite formation and the resulting polymer’s degree of crystallinity, accompanied with some plausible crosslinking of the polymer chains in the final 5th and 6th recycling courses are most likely the cause of the decrease in mechanical properties, as well as the inability to process further the material as it could no longer flow through the extrusion systems after the 6th recycling course. Another interesting finding to mention from the DSC analysis shown with a dashed circular line in Figure9b; specifically, in the melting endotherm curves for all samples, is an observed peak at ~155 ◦C. This is most likely attributed to cold recrystallization behavior of the samples, known to occur during the heating cycle and the endotherm curves before the melting endotherm peak. This phenomenon is explained by the fact that glassy fraction/part of the polymeric material (amorphous polymer chains) crystallizes with the thermal energy provided to the sample during the heating cycle. It was also evident that in the 6th recycling course there was found a slight shift of about +5 ◦C, while being slightly increased from the 1st to the 6th recycling course. The fact that the cold recrystallization is more prominent for the material with the increased recycled courses is explained by the fact that due to recycling/extrusion consecutive cycles, a higher fraction of short chains could be found. At the same time, above the Tg that the material/polymeric chains get some mobility, the polymeric chains that are in amorphous (glassy) state could more easily crystallize exhibiting thus a more intense cold recrystallization behavior for the 6th recycling course sample.

4.3. Morphological Characterization As it can be seen from Figure 10, the specimens from recycling courses 1st to 4th exhibit uniform layer thickness and deposition, without any observed voids and/or discontinuities in the interlayers, most likely attributed to the optimum 3D printing parameters used in the current study, as well as the slight plausible degradation of the PA12 polymeric chains by the consecutive recycling cycles that can affect further the homogeneity of the polymer melt viscosity (affecting thus the 3D printing FFF process and the ﬁnal quality of the 3D printed object). Materials 2021, 14, 466 13 of 15

The 5th and 6th recycling courses, on the other hand, show discontinuities in the layering and gaps from poor interfusion layering. These structural faults directly affect the mechanical properties of the 5th and the 6th recycling courses as the tensile/ﬂexural and impact results showed. Moreover, as it can be seen from the tensile specimens’ fracture area in Figure 11, specimens of the 6th recycling course had poor inter-layering fusion that led to the partial detachment/separation of the 2 outer (shell) layers of the printed parts; thus, tensile strength results for the 6th cycle indicated poor stress loading capacity. Regarding the fractography of the samples, it was evident that the fracture mode changes after each recycling course. More speciﬁcally, it was shown that the failure mode of the specimens is initially more ductile, and throughout the recycling courses it becomes more brittle by each recycling step. Specimens from the 1st recycling course (Figure 11a) failed in a “ductile” manner than the samples of the 5th (Figure 10e) and 6th (Figure 10f) recycling courses. This brittle fracture mechanism observed by the SEM fractography for the 3D printed PA12 samples after the 3rd and 6th recycling courses (Figure 10c,f, respectively) is in agreement with the experimental stress–strain curves acquired in this work that demonstrate the stiffness and ductility of the different PA12 materials after each course of recycling. All the above could be more precisely attributed to the plausible chain scission and chain branching occurring phenomena during the material’s recycling and reprocessing steps, as have been well-documented by the corresponding DSC, TGA analyses in this study.

5. Conclusions This research studies the effect of the thermomechanical treatment during the recycling process on the mechanical and thermal properties of PA12 polymer, over a certain number of recycling courses. In total, six (6) recycling courses were conducted, and tensile, flexural and impact properties along with micro-hardness were determined on FFF specimens. The following were derived: 1. The findings of this study prove that the overall mechanical behavior of the recycled PA12 polymer is generally improved over the recycling steps, for a certain number of repetitions, making PA12 a suitable polymer to be used in circular use. 2. It became evident that the recycling steps alter the mechanical properties of PA12 polymer, resulting in an average 15% increase in all mechanical properties studied herein, between the second and 3rd recycling course, while the polymer seems to be rapidly degrading after the fifth recycling course. 3. The crystallinity of PA12 polymer decreased slightly with the increase of the extrusion’s cycles, and the cross-linking and branching predominated over chain scission during multiple extrusions, thus the increase in the mechanical properties until the 3rd cycle (no reduction in crystallinity yet) and the decrease in the mechanical properties from 5th to 6th recycling course. 4. The materials’ flow was jeopardized after the 5th recycling course, making the filament hard to 3D print due to flow interruption. A possible low-grade crosslinking might be present affecting the flow rate of the recycled material.

Author Contributions: Conceptualization, N.V. and A.M.; methodology, N.M., E.V. and A.M.; soft- ware, E.V., A.M. and N.M.; validation, A.M., N.V. and M.P.; formal analysis, A.M., J.D.K. and E.V.; investigation, M.P., N.V. and J.D.K.; resources, N.V., M.P., J.D.K. and L.T.; data curation, A.M., E.V., N.M. and L.T.; writing—original draft preparation, A.M. and E.V.; writing—review and editing, A.M, N.V., M.P. and L.T.; visualization, N.V. and A.M.; supervision, N.V.; project administration, N.V. and M.P.; funding acquisition, N.V. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Materials 2021, 14, 466 14 of 15

Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: Authors would like to thank Ioan Valentin Tudose and Mirela Suchea from the Hellenic Mediterranean University for the SEM images. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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