Impact of PP Impurities on ABS Tensile Properties: Computational Mechanical Modelling Aspects

Article Impact of PP Impurities on ABS Tensile Properties: Computational Mechanical Modelling Aspects

Charles Signoret 1, Anne-Sophie Caro-Bretelle 2,*, José-Marie Lopez-Cuesta 1, Patrick Ienny 2 and Didier Perrin 1

1 Polymers Composites and Hybrids (PCH), IMT Mines Ales, 30100 Ales, France; [email protected] (C.S.); José[email protected] (J.-M.L.-C.); [email protected] (D.P.) 2 LMGC, IMT Mines Ales, Université Montpellier, CNRS, 30100 Ales, France; [email protected] * Correspondence: [email protected]; Tel.: +33-(0)-6-23-39-62-92

Abstract: Recycling of plastics is hindered by their important variety and strong incompatibility. However, sorting technologies bear costs and meet limits. Very high purities (<2 wt%) are difficult to reach. Yet, such rates may be detrimental to functional properties. In this work, an ABS matrix (major plastic in Waste of Electrical and Electronic Equipments) was filled with 4 wt% of PP to mimic impurities in ABS after recycling. PP-g-MA was introduced in the blend to improve the compatibility. A finite element model was developed from the mechanical behavior of each component. ABS and PP were individually characterized from tensile tests instrumented with photomechanics and their behaviors were modelled through a set of numerical parameters (elasto-visco-plasticity with a Gurson’s criterion behavior). Comparison between the determinist model results and the experimental data (strength, volumetric variation) shows that this type of modelling could be a predictive tool in order Citation: Signoret, C.; Caro- Bretelle, A.-S.; Lopez-Cuesta, J.-M.; to anticipate composite mechanical properties and to understand micromechanisms of deformation Ienny, P.; Perrin, D. Impact of PP (damage, cavitation). The main result is that PP introduced at 4 wt% into ABS does not alter the static Impurities on ABS Tensile Properties: mechanical properties despite polymers incompatibility. The addition of PP-g-MA modifies the local Computational Mechanical properties and possibly conduct to a premature breakage of the polymer blend. Modelling Aspects. Polymers 2021, 13, 1647. https://doi.org/10.3390/ Keywords: polymer recycling; WEEE; predictive modeling; volume change polym13101647

Academic Editors: David Garcia-Sanoguera and 1. Introduction Giulia Fredi Waste of Electrical and Electronic Equipment (WEEE) Plastics (WEEP) are not currently intensively regenerated through mechanical recycling for several different reasons [1,2]. Received: 18 April 2021 Dismantling these devices is a complex subject [3,4]. Moreover, the rapid increase of Accepted: 12 May 2021 Published: 19 May 2021 tonnages [5], the potential toxicity of these products [6], and the presence of valuable materials as precious metals [7] strongly motivate the establishment of proper end-of-life

Publisher’s Note: MDPI stays neutral management scenarios. Strong occurrence of toxic and/or environmentally harmful and with regard to jurisdictional claims in now forbidden (or soon to be) brominated ﬂame retardants within polymeric parts [8] published maps and institutional afﬁl- is also a serious barrier to waste management as it is unconceivable to reemploy these iations. contaminated materials [9]. High purity rates of sorted polymers are probably unattainable, especially as purity and yield are often contradictory notions [10]. However, even a few percent of impurities can be detrimental to mechanical properties [11] as most polymers are incompatible. In the area of 3D printing, recycling plastics has become increasingly important as this technology can produce large quantities of polymer wastes (support Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. structures, parts with defects) [12–14]. This article is an open access article As one of the most present polymers within WEEE [15,16] and owing to its potential distributed under the terms and added value because of its properties, ABS (acrylonitrile-butadiene-styrene) is here consid- conditions of the Creative Commons ered as the material to be valorized. As the most present plastic after styrenics (ABS, HIPS Attribution (CC BY) license (https:// and their blends, ABS/PC, HIPS/PPE ... ) and bringer of strong incompatibility because creativecommons.org/licenses/by/ of its different nature, PP (polypropylene) was chosen for the role of contaminant. Some 4.0/). studies were reported about ABS/PP blends, focusing on PP-rich systems or exploring

Polymers 2021, 13, 1647. https://doi.org/10.3390/polym13101647 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 20

Some studies were reported about ABS/PP blends, focusing on PP-rich systems or exploring the whole range by 10 or 20 wt% steps [17–25]. Apart from strain at break, most of ABS properties are seriously impaired with PP incorporation. The major point is the weak interaction between ABS and PP due to their chemical differences leading to poor me- Polymers 2021, 13, 1647 chanical properties. Several compatibilizers were tried but they improved2 of 19 properties mainly in PP-rich zones, performances still far below pure ABS [19–21,24]. The aim of this study is to improve the comprehension of the micromechanics of de- theformation whole range involved by 10 or during 20 wt% stepsa tensile [17–25 test]. Apart with from PP strainadded at to break, ABS most at a of low ABS weight fraction properties are seriously impaired with PP incorporation. The major point is the weak inter- actionwith betweenor without ABS PP-g-MA. and PP due toA theirpredictive chemical model differences associated leading towith poor tensile mechanical test and optical in- properties.strumentation Several is compatibilizers introduced. From were tried the butidentification they improved of propertiesthe mechanical mainly inbehavior of each PP-richphase, zones, the Gurson-Tvergaard-Needleman performances still far below pure ABS (GTN [19–21) ,model24]. [26–28] has been used to describe the Theelasto-visco-plastic aim of this study is and to improve damageable the comprehension behavior of of ABS the micromechanicsand PP matrices. of Knowing the deformation involved during a tensile test with PP added to ABS at a low weight fraction microstructure, a predictive modeling based on finite element analysis (FEA) was con- with or without PP-g-MA. A predictive model associated with tensile test and optical instrumentationducted to understand is introduced. the Fromlocal the phenomena identification leading of the mechanical to blends behavior breakage. of each phase,The the Gurson-Tvergaard-Needleman paper is structured as follows: (GTN) Section model [26 2 –presents28] has been materials used to describe and experiments; Sec- thetion elasto-visco-plastic 3 highlights modelling; and damageable Section behavior 4 discusses of ABS about and PP results; matrices. and Knowing Section 5 brings a con- theclusion. microstructure, a predictive modeling based on finite element analysis (FEA) was conducted to understand the local phenomena leading to blends breakage. The paper is structured as follows: Section2 presents materials and experiments; Section2. Materials3 highlights and modelling; Methods Section4 discusses about results; and Section5 brings a2.1. conclusion. Materials 2. MaterialsVirgin and unformulated Methods polymer references based on Terluran GP22 (Styrolution) ABS 2.1.and Materials PPH 9020 (PP Homopolymer-Total) were used for this study. With the aim to improving Virgininterfacial unformulated interactions, polymer Bondyram references based1001 onPP-g-MA Terluran provided GP22 (Styrolution) by Polyram ABS Plastic Indus- andtries PPH (Ram-on, 9020 (PP Homopolymer-Total)Israël) was used. were used for this study. With the aim to improving interfacial interactions, Bondyram 1001 PP-g-MA provided by Polyram Plastic Industries (Ram-on,ABS Israël) is biphasic was used. by nature. SAN, a statistical copolymer of styrene and acrylonitrile oftenABS at is 25 biphasic wt% by[29]), nature. is its SAN, majority a statistical phase. copolymer However, of styrene this and amorphous acrylonitrile polymer is very oftenbrittle at 25and wt% thus [29 unsuitable]), is its majority for many phase. applicat However,ions. this amorphousThus, polybutadiene polymer is very (PB) rubber is pre- brittlesent as and a thusnodular unsuitable minority for many phase, applications. as shown Thus,by AFM polybutadiene cartography (PB) on rubber Figure is 1, to improve present as a nodular minority phase, as shown by AFM cartography on Figure1, to its impact properties. For common applications, PB loading rates are reported between 10 improve its impact properties. For common applications, PB loading rates are reported betweenand 30% 10 within and 30% ABS within depending ABS depending on if on flow if flow properti propertieses or or impact impact resistanceresistance is is desired [29]. desiredFor emulsion [29]. For grafting, emulsion grafting,the main the synthesis main synthesis route route for forcommercial commercial ABS, ABS, sizesize distribution is distributionreported between is reported 0.1 between and 1.0 0.1 µm, and but 1.0 mainlyµm, but with mainly a median with a median diameter diameter around 0.3 µm [29] around(Figure 0.3 1).µm PP [29 is] (Figure also biphasic1). PP is also in another biphasic inway another as it way is semi-crystalline, as it is semi-crystalline, meaning that crys- meaning that crystallites, where chains are spatially organized, are dispersed within an amorphoustallites, where phase. chains are spatially organized, are dispersed within an amorphous phase.

FigureFigure 1. 1.Nodular Nodular dispersion dispersion of PB within of PB ABS—AFM within ABS—AFM picture produced picture by combination produced ofby topogra- combination of topog- phyraphy and and modulus modulus cartography cartography owing to AM-FMowing to mode. AM-FM mode.

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ABS and PP were mixed at 4 wt% since previous works from our laboratory [30] highlighted that properties loss began at this rate. PP-g-MA was tried at 3 phr (parts per hundred of resin) to improve interfacial adhesion between phases. ABS pellets were dried at 80 ◦C for at least 16 h before processing. ABS and PP were manually mixed at desired ratio before extrusion within a 900 mm Clextral twin-screw extruder of type BC21 at 250 rpm, 220 ◦C along the screw, with a 5 mm nozzle, at a 6 kg/h feed speed owing to a K-Tron KQx-2 weighing feeder from Coperion. Extrudate was then pelletized owing to an SGS-E50 from CF Scheer & Cie (Stuttgart, Allemagne). For mechanical properties, Type A dogbones specimen corresponding to ISO 3167, was injected using a Krauss-Maffei 180/CX 50 molding injection press at 230 ◦C.

2.2. Methods Uniaxial tensile tests were performed on an MTS criterion model 45 universal testing machine (Eden Prairie, MN, USA) following ISO 527 standard on above-mentioned specimen. Tests were conducted at 1 and 10 mm/min (corresponding respectively to strain rates 10−4/s, 10−3/s) to highlight possible time dependences. Specimens were brought to 1% strain, unloaded, and then brought to break. The software used is TestXpert® (Ars- Laquenexy, France) and allows the recording of time, load, and elongation. During these works, x is deﬁned as the tensile direction, y perpendicular to x and in the planar surface, and z the out of plane direction. The axial nominal stress is given by the following expression (Equation (1)):

F σx = , (1) S0

where F is the recorded load and S0 the initial sample section surface. The optical extensometer involves a high-resolution charge-coupled device (CCD) camera (Redlake Megaplus II (Tuscon, AZ, USA), 1920 × 1080 contiguous and square pixels, coded in 256 grey levels), set in front of the specimen, which records images during the test. The optical axis of the camera remains perpendicular to the in-plane surface of the specimen during the test (Figure2). The images acquisition is commanded by the LabVIEW ® software (National Instruments, Austin, TX, USA) which allows the simultaneous acquisition of the images and the data from the testing machine (such as load and crosshead displacement). According to the test speed used, images are recorded every 0.5 s. The scale factor is ﬁxed to 42 µm per pixel. The in-plane Green Lagrange strains Ex, Ey were deduced from digital image cor- relation (DIC) following a method well described in previous works [31]. A transverse isotropic assumption was considered and validated as in a previous study on SEBS specimen submitted to uniaxial tensile test [32]. The volumetric strain is deﬁned using front-view data only and assuming isotropy:

∆V 2 p = λxλy − 1 where λi = 2Ei + 1, i = x, y, (2) V0

V is the current volume, V0 the original volume, and λx and λy are the in-plane principal stretch ratios in the x and y direction. The part of elastic volumetric strain can be computed (Equation (3)): ∆V σ = x (1 − 2ν), (3) V0 elas E where E and ν are the elastic material parameters (Young modulus and Poisson’s ratio). Polymers 2021, 13, 1647 4 of 19 Polymers 2021, 13, x FOR PEER REVIEW 4 of 20

Figure 2. Testing apparatus. Figure 2. Testing apparatus.

3. ResultsThe in-plane Green Lagrange strains 𝐸,𝐸 were deduced from digital image corre- 3.1.lation Mechanical (DIC) following Characterization a method well described in previous works [31]. A transverse iso- 3.1.1.tropic Mechanical assumption Characterization was considered ofand ABS validated and PP as in a previous study on SEBS specimen submittedABS and to uniaxial PP behave tensile mechanically test [32]. quite differently. The ﬁrst polymer is brittle in comparisonThe volumetric to the second strain one. is defined During using the ﬁrst front-view loading/unloading data only and cycle assuming until 1% isotropy: of strain, the elastic properties (E and∆ ν) of virgin polymers are computed for both imposed speeds =𝜆𝜆 − 1 where 𝜆 = 2𝐸 +1,𝑖 =𝑥,𝑦, (2) (see average values with standard deviation in Table1). Regarding elastic properties, PP

seemsV more is the impacted current volume, by the speed V0 the rate, original especially volume, with and respect 𝜆 and to the𝜆 Poisson’sare the in-plane ratio,with prin- ancipal increase stretch of ratios 10% while in the increasing x and y direction. the speed The from part 1 to of 10 elastic mm/min. volumetric strain can be computed (Equation (3)): Table 1. Elastic properties of virgin polymers (ABS and PP) deduced from instrumented tensile tests ∆𝑉 𝜎 at 1 and 10 mm/min. = (1−2𝜈), (3) 𝑉 𝐸 Loading Speeds (mm/min) 1 1 10 10 where E and ν are the elastic material parameters (Young modulus and Poisson’s ratio). Polymers ABS PP ABS PP E (MPa) 2337 ± 9 1236 ± 25 2349 ± 12 1193 ± 42 3. Results ν 0.36 ± 0.01 0.39 ± 0.01 0.37 ± 0.01 0.43 ± 0.01 3.1. Mechanical Characterization 3.1.1.Moreover, Mechanical the Characterization ABS matrix behaves of ABS as a damageableand PP elastic material. Its global behavior is timeABS independent and PP behave (Figure mechanically3) and its quite volumetric differently. strain The (deduced first polymer from is Equation brittle in (2))com- followsparison an to elastic the second law (deduced one. During from the Table first1 andloading/unloading Equation (3)) until cycle 1.7% until of imposed1% of strain, strain the (Figureelastic4 properties). However, (E PPand polymer ν) of virgin behavior polymers is more are sensitivecomputed to for the both strain imposed speed with speeds regard (see toaverage the stress values values. with Moreover, standard deviation the volumetric in Table strain 1). Regarding deviates from elastic the properties, elastic curve PP seems from 2.3%more of impacted strain and by is the time speed independent. rate, especially For a samewith levelrespect of strain,to the thePoisson’s stress curveratio, with is very an differentincrease forof 10% both while polymers increasing whereas the the speed volumetric from 1 to strain 10 mm/min. is almost similar.

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Table 1. Elastic propertiesTable 1. Elastic of virgin properties polymers of virgin(ABS andpolymers PP) deduced (ABS and from PP) instrumented deduced from tensile instrumented tensile tests at 1 and 10 testsmm/min. at 1 and 10 mm/min.

Loading SpeedsLoading Speeds 1 1 1 1 10 10 10 10 (mm/min) (mm/min) Polymers PolymersABS ABSPP PPABS ABSPP PP E (MPa) E (MPa) 2337 ± 9 2337 1236 ± 9 ± 25 1236 2349 ± 25 ± 12 2349 1193 ± 12 ± 42 1193 ± 42 ν 0.36ν ± 0.01 0.36 0.39 ± 0.01 ± 0.01 0.39 0.37 ± 0.01 ± 0.01 0.37 0.43 ± 0.01 ± 0.01 0.43 ± 0.01

Moreover, the ABSMoreover, matrix the behaves ABS matrix as a damageable behaves as elastica damageable material. elastic Its global material. behav- Its global behavior is time independentior is time (Figure independent 3) and (Figureits volumetric 3) and strainits volumetric (deduced strain from (deduced Equation from (2)) Equation (2)) follows an elasticfollows law an(deduced elastic lawfrom (deduced Table 1 andfrom Equation Table 1 and(3)) untilEquation 1.7% (3))of imposeduntil 1.7% of imposed strain (Figure 4).strain However, (Figure PP 4). polymer However, behavior PP polymer is more behavior sensitive is moreto the sensitive strain speed to the with strain speed with Polymers 2021, 13, 1647regard to the stressregard values. to the Moreover, stress values. the voluMoreover,metric thestrain volu deviatesmetric strainfrom thedeviates elastic from curve the elastic5 curve of 19 from 2.3% of strainfrom and2.3% is of time strain independent. and is time Fo independent.r a same level Fo ofr a strain, same levelthe stress of strain, curve the is stress curve is very different forvery both different polymers for bothwhereas polymers the volumetric whereas the strain volumetric is almost strain similar. is almost similar.

Figure 3. Stress Figurevs.Figure Lagrangian 3. 3.Stress Stress vs.strain vs. LagrangianLagrangian in uniaxial strain strain tens inile in uniaxial testsuniaxial at several tensile tensile testsimposed tests at at several several strainimposed rates imposed for strain strain rates rates for for ABS ABS and PP. andABS PP. and PP.

Figure 4. Volumetric strain vs. Lagrangian strain in uniaxial tensile tests at several imposed strain rates for ABS and PP, focus on strain until % to enhance the deviation point at 2.4% from elastic behavior for PP sample.

3.1.2. Mechanical Characterization of Composites Once PP is added into ABS matrix at 4 wt%, the global shape of the stress/strain curve is similar to virgin ABS (see for example Figure5 for the higher speed test). As expected, the Young’s modulus is lowered by the introduction of a less stiff polymer just like the associated maximum stress. This phenomenon is enhanced by the introduction of PP-g-MA (see Table2). The volumetric strain (Figure6) is almost unchanged as PP is introduced into ABS with or without the presence of PP-gMA. The introduction of PP impurities into ABS does not impact ABS volumetric strain, even if the stress/strain curve is slightly lowered as PP-g-MA is introduced into ABS/PP. Polymers 2021, 13, x FOR PEER REVIEW 6 of 20

3.1.2. Mechanical Characterization of Composites Once PP is added into ABS matrix at 4 wt%, the global shape of the stress/strain curve is similar to virgin ABS (see for example Figure 5 for the higher speed test). As expected, the Young’s modulus is lowered by the introduction of a less stiff polymer just like the associated maximum stress. This phenomenon is enhanced by the introduction of PP-g- MA (see Table 2). The volumetric strain (Figure 6) is almost unchanged as PP is introduced Polymers 2021, 13, 1647 into ABS with or without the presence of PP-gMA. The introduction of PP impurities6 of into 19 ABS does not impact ABS volumetric strain, even if the stress/strain curve is slightly lowered as PP-g-MA is introduced into ABS/PP.

Figure 5. Stress vs. Lagrangian strain in uniaxial tensile tests at 10 mm/min for ABS, PP, and com- Figure 5. Stress vs. Lagrangian strain in uniaxial tensile tests at 10 mm/min for ABS, PP, posites. and composites. Table 2. Elastic and ultimate properties deduced from experiments for ABS, PP, ABS/PP compo- Tablesites at 2. 10Elastic mm/min. and ultimate properties deduced from experiments for ABS, PP, ABS/PP composites at 10 mm/min. ABS PP ABS/PP ABS/PP/PP-g-MA ABS PP ABS/PP ABS/PP/PP-g-MA E (MPa) 2349 2349 ±± 1212 1193 1193 ±± 4242 2329 2329± ±24 24 2212 2212± ±45 45 ν 0.37 ± 0.01 0.43 ± 0.005 0.37 ± 0.04 0.37 ± 0.02 ν 0.37 ± 0.01 0.43 ± 0.005 0.37 ± 0.04 0.37 ± 0.02 Polymers 2021, 13, x FOR PEER REVIEW σmax(MPa) 42 ± 5 31.7 ± 3 41 ± 4 39.4 ± 0.2 7 of 20

εu(−) 0.02 ± 0.01 0.18 ± 0.01 0.02 ± 0.01 0.027 ± 0.01 𝜎(MPa) 42 ± 5 31.7 ± 3 41 ± 4 39.4 ± 0.2

𝜀(−) 0.02 ± 0.01 0.18 ± 0.01 0.02 ± 0.01 0.027 ± 0.01

Figure 6. Volumetric Strain vs. Lagrangian strain in uniaxial tensile tests at 10 mm/min for ABS, Figure 6. Volumetric Strain vs. Lagrangian strain in uniaxial tensile tests at 10 mm/min for ABS, PP, PP, and composites. and composites.

3.2.3.2. MorphologyMorphology CharacterizationCharacterization MorphologiesMorphologies were were monitored monitored owing owing to SEM to (scanningSEM (scanning electron electron microscopy) microscopy) (Quanta FEG(Quanta 200, FEG Thermo-Fisher 200, Thermo-Fisher Scientiﬁc, Scientific, Waltham, Waltham, MA, USA) MA, on cryofracturedUSA) on cryofractured dogbones dog- (or- thogonallybones (orthogonally or 45◦) and or post-mortem 45°) and post-mortem Charpy impact Charpy fragments. impact fragments. To produce To cryofractured produce cry- dogbonesofractured polymer dogbones samples polymer were samples ﬁrst immersed were first into immersed liquid nitrogeninto liquid for nitrogen approximately for approximately 5 min and then taken out and struck with a hammer. AFM (Oxford Asylum Research MFP3D, Santa Barbara, CA, USA) was also performed on samples prepared using a Leica EM UC7 ultracryomicrotome (Leica, Nanterre, France). AM-FM (amplitude modulation-frequency modulation) mode of AFM was applied to enable mapping of local pseudo-moduli, enabling easier differentiation between SAN and PB as their respective moduli are different.

3.2.1. Nodular Dispersion of PP within ABS As expected, polypropylene forms a nodular dispersion within ABS as Figure 7 shows. Cryofractures prove that PP nodules adopt a spherical shape and not an ellipsoidal or cylindrical one. Moreover, PP nodule sizes are reduced with PP-g-MA addition. A possible adhesive interaction is visible in Figure 8 as fibrils are seen between what could be a PP nodule and the matrix. However, this micrograph corresponds to a very specific area of the fracture and was not found elsewhere. Part 2.4.2 below presents micrographs more representative of the post-mortem samples.

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5 min and then taken out and struck with a hammer. AFM (Oxford Asylum Research MFP3D, Santa Barbara, CA, USA) was also performed on samples prepared using a Leica EM UC7 ultracryomicrotome (Leica, Nanterre, France). AM-FM (amplitude modulation- frequency modulation) mode of AFM was applied to enable mapping of local pseudo- moduli, enabling easier differentiation between SAN and PB as their respective moduli are different.

3.2.1. Nodular Dispersion of PP within ABS As expected, polypropylene forms a nodular dispersion within ABS as Figure7 shows. Cryofractures prove that PP nodules adopt a spherical shape and not an ellipsoidal or cylindrical one. Moreover, PP nodule sizes are reduced with PP-g-MA addition. A possible adhesive interaction is visible in Figure8 as ﬁbrils are seen between what could be a PP nodule and the matrix. However, this micrograph corresponds to a very speciﬁc area of Polymers 2021, 13, x FOR PEER REVIEW 8 of 20 the fracture and was not found elsewhere. Part 2.4.2 below presents micrographs more representative of the post-mortem samples.

FigureFigure 7. 7.Nodular Nodular morphology morphology of of PP PP within within ABS ABS at at 4 wt%4 wt% with with and and without without PP-g-MA- PP-g-MA- two two magniﬁ- mag- nifications. cations.

Size distribution of nodules was determined by image analysis (AphelionTM 3.2 (ADCIS)). SEM micrographs were binarized and PP nodules were identified and filtered by surface area (see Figure9). SEM pictures on samples cut with an ultracryomicrotome were preferred as micrographs issued from cryofractures were too uneven to enable image analysis. AFM pictures were also more complex to analyze as both PP and PB were visible. Each identified object is assumed to have a disk shape well described by its diameter. ABS/PP and ABS/PP/PP-g-MA were analyzed owing to this procedure. Around 800 PP

Figure 8. SEM picture of cross-sections of ABS-PP-PP-g-MA samples after tensile test—fibrils linking a potential PP nodule to the matrix.

Size distribution of nodules was determined by image analysis (AphelionTM 3.2 (ADCIS)). SEM micrographs were binarized and PP nodules were identified and filtered by surface area (see Figure 9). SEM pictures on samples cut with an ultracryomicrotome were preferred as micrographs issued from cryofractures were too uneven to enable image analysis. AFM pictures were also more complex to analyze as both PP and PB were

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Polymers 2021, 13, 1647 8 of 19

nodulesFigure 7. were Nodular identiﬁed morphology for both of PP compositions within ABS at with 4 wt% nodules with and diameters without PP-g-MA- from 0.2 totwo 1.7 mag-µm fornifications. ABS/PP blend and from 0.1 to 1.0 µm for ABS/PP/PP-g-MA blends (Figure 10).

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visible. Each identified object is assumed to have a disk shape well described by its diam-

eter. ABS/PP and ABS/PP/PP-g-MA were analyzed owing to this procedure. Around 800 Figure 8. SEM picture of cross-sections of ABS-PP-PP-g-MA samples after tensile test—fibrils link- FigurePP nodules 8. SEM were picture identified of cross-sections for both of ABS-PP-PP-g-MAcompositions with samples nodules after diameters tensile test—ﬁbrils from 0.2 linking to 1.7 ing a potential PP nodule to the matrix. aµm potential for ABS/PP PP nodule blend to the and matrix. from 0.1 to 1.0 µm for ABS/PP/PP-g-MA blends (Figure 10). Size distribution of nodules was determined by image analysis (AphelionTM 3.2 (ADCIS)). SEM micrographs were binarized and PP nodules were identified and filtered by surface area (see Figure 9). SEM pictures on samples cut with an ultracryomicrotome were preferred as micrographs issued from cryofractures were too uneven to enable image analysis. AFM pictures were also more complex to analyze as both PP and PB were

FigureFigure 9.9. BinarizationBinarization of of SEM SEM micrographs micrographs of ofcuts cuts performed performed using using an ultracyromicrotome—ABS an ultracyromicrotome— + PP and ABS + PP + PP-g-MA systems. ABS + PP and ABS + PP + PP-g-MA systems.

3.2.2. Cavitation Phenomenon Figure 11 compares the formation of porosity or cavities on the broken cross section within ABS and PP after tensile tests pursued until failure. These pictures show porosities/cavities areas which seem larger in the case of PP but this polymer came to break at higher strains in comparison with ABS.

Figure 10. Frequencies (a) and cumulated frequencies (b) of diameters sizes distribution for ABS/PP and ABS/PP/PP-g-MA composites.

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visible. Each identified object is assumed to have a disk shape well described by its diameter. ABS/PP and ABS/PP/PP-g-MA were analyzed owing to this procedure. Around 800 PP nodules were identified for both compositions with nodules diameters from 0.2 to 1.7 µm for ABS/PP blend and from 0.1 to 1.0 µm for ABS/PP/PP-g-MA blends (Figure 10).

Polymers 2021, 13, 1647 9 of 19 Figure 9. Binarization of SEM micrographs of cuts performed using an ultracyromicrotome—ABS + PP and ABS + PP + PP-g-MA systems.

Polymers 2021Figure, 13, x FOR10. FrequenciesPEER REVIEW (a) and cumulated frequencies (b) of diameters sizes distribution for 10 of 20 Figure 10. Frequencies (a) and cumulated frequencies (b) of diameters sizes distribution for ABS/PP and ABS/PP/PP-g-MA composites.ABS/PP and ABS/PP/PP-g-MA composites.

Figure 11. Comparison of cavitation phenomena within ABS and PP—SEM pictures on cryofrac- Figure 11. Comparison of cavitation phenomena within ABS and PP—SEM pictures on cryofractured tured dogbones and post-mortem samples from tensile tests at 10 mm/min. dogbones and post-mortem samples from tensile tests at 10 mm/min.

FigureFigure 12 12 shows shows that that the the cavitation cavitation process process related related to to PB PB particles particles is is very very similar similar withinwithin virgin virgin ABS ABS even even with with different different strain strain speeds. speeds. This This is is in in accordance accordance with with the the obser- obser- vationsvations of of Figure Figure4 for4 for which which volumetric volumetric strains strains of of virgin virgin polymers polymers are are insensitive insensitive to to the the imposedimposed speeds. speeds. Figure 13 shows cryofractures and post-mortem samples of SAN, ABS, ABS/PP, and ABS/PP/PP-g-MA. As a fragile material, SAN does not form cavities and fractures are thus very similar from cryofracture to post-mortem. As reported in the literature [33], PB generates cavitation within ABS, enabling an improvement toward fracture as it dissipates energy. Smallest holes on the picture are about 0.2 µm in diameters and biggest ones at 1.4 whereas most of them are around 0.4–0.5 µm. PP presence worsens dramatically the phenomenon as most holes seen here are more around 1.0–1.5 µm. With PP-g-MA the

Figure 12. ABS cavitation independence toward strain speed—SEM pictures of post-mortem samples.

Figure 13 shows cryofractures and post-mortem samples of SAN, ABS, ABS/PP, and ABS/PP/PP-g-MA. As a fragile material, SAN does not form cavities and fractures are thus very similar from cryofracture to post-mortem. As reported in the literature [33], PB generates cavitation within ABS, enabling an improvement toward fracture as it dissipates energy. Smallest holes on the picture are about 0.2 µm in diameters and biggest ones at 1.4 whereas most of them are around 0.4–0.5 µm. PP presence worsens dramatically the phenomenon as most holes seen here are more around 1.0–1.5 µm. With PP-g-MA the hole

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Figure 11. Comparison of cavitation phenomena within ABS and PP—SEM pictures on cryofractured dogbones and post-mortem samples from tensile tests at 10 mm/min. Polymers 2021, 13, 1647 10 of 19 Figure 12 shows that the cavitation process related to PB particles is very similar within virgin ABS even with different strain speeds. This is in accordance with the obser- holevations size of is Figure around 4 0.5–1for whichµm, closervolumetric to pure strains ABS. of This virgin is surely polymers due to are the insensitive morphological to the reﬁnementimposed speeds. described in Figure 10.

Figure 12. ABS cavitation independence toward strain speed—SEM pictures of post-mortem sam- Figure 12. ABS cavitation independence toward strain speed—SEM pictures of post-mortem samples. ples. 3.3. Modelling 3.3.1.Figure Behavior 13 shows cryofractures and post-mortem samples of SAN, ABS, ABS/PP, and ABS/PP/PP-g-MA. As a fragile material, SAN does not form cavities and fractures are thus The literature reports an important number of damage models, able to account for mechanicalvery similar degradation from cryofracture in materials. to post-morte Possibly,m. these As reported models include in the literature new internal [33], variables PB gen- anderates evolution cavitation laws within coupled ABS, to plasticityenabling problemsan improvement [34]. The toward macroscopic fracture measured as it dissipates volume changeenergy. is Smallest assumed holes to be on related the picture to cavitation are abou occurringt 0.2 µm within in diameters the polymers and phasesbiggest by ones void at nucleation1.4 whereas and most growth of them [35]. are The around present 0.4–0.5 work focusesµm. PP on presence a Gurson-type worsens approach dramatically which the is basedphenomenon on mechanics as most of holes porous seen media. here are The more damage around indicator, 1.0–1.5 related µm. With to the PP-g-MA degradation the hole of the material, is the void volume fraction (i.e., the porosity) which progressively downscales the yield surface. In the early eighties, Tvergaard and Needleman [28,36] extended the Gur- son approach, namely GTN model, including material hardening, multiple voids, and void coalescence. This GTN model, available in numerous ﬁnite elements software, has been already used by authors to describe the micromechanisms of deformation of a PP/Bakelite blend [37] for which the PP matrix exhibits porosity following a Gurson-type model. In this damage formulation, the yield criterion uses hydrostatic pressure and porosity: the criterion of plasticity and the plastic potential, depend on a macroscopic stress σ∗ and an effective porosity fraction ft in the following way (Equation (4)):

3J (σ) q I (σ) 2 e + 2 1 e − + 2 2 = 2 2 ftq1cosh 1 q1 ft 0, (4) σ∗ 2σ∗

1 where eσ is the stress tensor J2 = 2 dev(eσ) : dev(eσ), I1 = trace(eσ) and q1, q2 are model parameters. The q1 parameter allows to adjust the inﬂuence of ft on the yield surface and the q2 parameter is related to the pressure effect. Effective stress σ∗ is implicitly evaluated from Equation (4). The plastic ﬂow potential is written as (Equation (5)): −b1 p f = σ∗ − R(p), R = R0 + Q1 1 − e , (5)

where p is the effective plastic strain and R0, Q1, b1 are material parameters. The evolution of viscoplastic strain is written through a Norton’s law:

n . . ∂σ∗ . f eε = (1 − p)pne∗, ne∗ = , p = , (6) ∂eσ K where n, K are material parameters. Polymers 2021, 13, 1647 11 of 19

The total porosity change rate can be split into two components, one attributed to the . . void growth f g and the other one to the nucleation f n: Polymers 2021, 13, x FOR PEER REVIEW 11 of 20 . . . f t = f g + f n, (7)

size is around. 0.5–1 µm,. closer. to pure. ABS. This is surely due to the morphological re- where f = (1 − f )trε and f = Ap, A is a constant. finementg describedt in eFigure 10.n

Figure 13. Cavitation phenomena observed on post-mortem tensile tests fragments through Figure 13. Cavitation phenomena observed on post-mortem tensile tests fragments through SEM— SEM—comparison to cryofractured samples—virgin SAN, virgin ABS, and ABS contaminated comparisonwith 4 wt% PP to cryofracturedand ABS contaminated samples—virgin with 4 wt% SAN, PP virginwith 3 ABS,phr of and PP-g-MA. ABS contaminated with 4 wt% PP and ABS contaminated with 4 wt% PP with 3 phr of PP-g-MA. 3.3. Modelling 3.3.2. Determination of ABS and PP Model Properties 3.3.1. Behavior The behavior of PP and ABS can be well described through this modelling. The The literature reports an important number of damage models, able to account for Young’s modulus E, the Poisson’s ratio ν and the yield stress R0 were determined from stress/strainmechanical degradation data. The GTN in materials. model has Possibly, been implemented these models using include the new Matlab internal software. varia- The bles and evolution laws coupled to plasticity problems [34]. The macroscopic measured volume change is assumed to be related to cavitation occurring within the polymers phases by void nucleation and growth [35]. The present work focuses on a Gurson-type

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porosity evolution parameters q1 and q2 are arbitrarily ﬁxed to 1, adjustments of these values would necessitate some local data (as porosity distribution) [35]. The GTN model is therefore reduced to the native Gurson model. The remaining material parameters were Polymers 2021, 13, x FOR PEER REVIEWidentiﬁed by using a mean square method on stress/strain and volumetric strain/strain13 of 20

Polymers 2021, 13, x FOR PEER REVIEWdata between experiments and numeric data at each timestep of the simulation. Parameters13 of 20 present in Table3 lead to a good accordance between experiments and modelling for both stress (Figure 14) and volumetric strain (Figure 15). The contrast on the K value prevails Table 3. Material model parameters for ABS and PP. forTable the 3. low Material time plasticitymodel parame dependencyters for ABS for and the PP. ABS. Material Properties ABS PP Table 3.MaterialMaterial Properties model parameters for ABS and ABS PP. PP q1(−), q2() 1, 1 1, 1 Materialq1(− Properties), q2() ABS 1, 1 1, PP 1 A() 0.53 0.23 q1(−), q2() 1, 1 1, 1 A()A() 0.530.53 0.23 0.23 n(), K (MPa.s) 2.5, 410 2.4, 51.2 n(), K (MPa.s) 2.5, 410 2.4, 51.2 n(), K (MPa.s) 2.5, 410 2.4, 51.2 R (MPa), Q (MPa), b (−) 27, 2, 210 11, 17, 58.3 𝑅0 (MPa), 𝑄1 (MPa), 𝑏1(−) 27, 2, 210 11, 17, 58.3 𝑅 (MPa), 𝑄 (MPa), 𝑏(−) 27, 2, 210 11, 17, 58.3

Figure 14. Modelling of ABS, stress and volumetric strain vs. Lagrangian strain. Figure 14. Modelling of ABS, stress and volumetric strain vs. Lagrangian strain. Figure 14. Modelling of ABS, stress and volumetric strain vs. Lagrangian strain.

Figure 15. Modelling of PP, stress, and volumetric strain vs. Lagrangian strain. FigureFigure 15. 15.Modelling Modelling of of PP, PP, stress, stress, and and volumetric volumetricstrain strain vs.vs. LagrangianLagrangian strain. 3.3.3. . Prediction of ABS/PP Blend Properties 3.3.3.For . Prediction the sake of simplicity,ABS/PP Blend a plane Properties stress assumption is made for the numerical com- putationsFor the even sake if thisof simplicity, strong hypothesis a plane stress is far assumptionfrom reality. is The made interface for the betweennumerical matrix com- andputations fillers evenis assumed if this strongto be perfecthypothesis (displ isacement far from continuity)reality. The and interface phase between properties matrix are describedand fillers through is assumed the setto beof parametersperfect (displ ofacement Table 3. continuity)This assumption and phase of perfect properties interface are willdescribed be discussed through in thenext set section of parameters as the compatibility of Table 3. ofThis ABS assumption and PP is quiteof perfect questionable. interface will be discussed in next section as the compatibility of ABS and PP is quite questionable.

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3.3.3. Prediction of ABS/PP Blend Properties Polymers 2021, 13, x FOR PEER REVIEW 14 of 20 For the sake of simplicity, a plane stress assumption is made for the numerical com- putations even if this strong hypothesis is far from reality. The interface between matrix and ﬁllers is assumed to be perfect (displacement continuity) and phase properties are We assume that thedescribed properties through of ABS the setand of PP parameters in the blend of Table are3 similar. This assumption to those of of the perfect neat interface will be discussed in next section as the compatibility of ABS and PP is quite questionable. ABS and PP. We assume that the properties of ABS and PP in the blend are similar to those of the neat From the particleABS and size PP. distribution shown in Figure 10, two kinds of microstructures were generated via DIGIMATFrom the particlesoftware size (for distribution both ABS/PP shown and in FigureABS/PP/PP-g-MA 10, two kinds blends). of microstructures Mesh microstructureswere generated based on via triangular DIGIMAT elements software (for and both boundary ABS/PP andconditions ABS/PP/PP-g-MA are pre- blends). sented in Figure 16.Mesh The microstructures RVE (representative based on vo triangularlume element) elements sizes and boundary were chosen conditions to stabi- are presented lize the macroscopicin Figure stress/strain 16. The RVEresponses. (representative As local volume behavior element) is of interest sizes were for chosen this study, to stabilize the the chosen mesh macroscopicrefinement leads stress/strain to stabilize responses. local blend As local response. behavior We is ofassume interest the for struc- this study, the ture symmetry. Achosen prescribed mesh reﬁnementdisplacement leads (co torresponding stabilize local to blend 10% response. of strain) We was assume applied the structure symmetry. A prescribed displacement (corresponding to 10% of strain) was applied on on the top of the sample. The non-linear numerical simulation was carried out using the the top of the sample. The non-linear numerical simulation was carried out using the FE FE software Zebulon,software which Zebulon, was developed which was developed at Mines atParistech Mines Paristech [39]. This [38 ].RVE This model RVE model is is thus thus useful to predictuseful the to mechanical predict the mechanicalproperties propertiesof ABS/PP of composites. ABS/PP composites. The size of The the size PP of the PP nodules is much noduleslower when is much a compatibilizer lower when a compatibilizer is added: this is induces added: this both induces a reduction both a reductionin in the RVE size andthe interparticular RVE size and interparticularnodules distances nodules for distances the ABS/PP/PP-g-MA for the ABS/PP/PP-g-MA blend (see blend (see Figure 16). We canFigure deduce 16). from We can these deduce results from that these interface results that areas interface between areas ABS between and PP ABS are and PP are increased since nodulesincreased are since smaller. nodules are smaller.

FigureFigure 16. Finite 16. Finite element element modelling modelling of ABS of ABS and and PP PP blend: blend: structure structure and boundaryboundary conditions. conditions.

4. Discussion 4. Discussion Comparison betweenComparison numerical between models numerical and experimental models and experimentaldata for ABS/PP data blends for ABS/PP is blends given in Figure 17.is given The insimulated Figure 17 result. The simulateds for both results systems for both(ABS/PP systems and (ABS/PP ABS/PP/PP-g- and ABS/PP/PP- MA) coincide. Asg-MA) a first coincide.result, input As a material ﬁrst result,s parameters input materials lead to parameters good predictive lead to goodmod- predictive modelling as “ABS/PP num” and “ABS/PP exp” are similar until 1.7% of strain. Though elling as “ABS/PP num” and “ABS/PP exp” are similar until 1.7% of strain. Though unicity unicity of the solution to the model is not proven, the good accordance between experimen- of the solution total the results model and is those not predictedproven, the by thegood model accordance assures the between validity experimental of the set of parameters results and those usedpredicted here. Afterby the 1.7% model of strain, assure in thes the experiments, validity of the the blends set of broke parameters quickly. used Obviously, this here. After 1.7% ofis notstrain, the casein the in experiments, the numerical simulationsthe blends broke because quickly. no rupture Obviously, criterion hasthis been is added. not the case in the numerical simulations because no rupture criterion has been added.

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Figure 17. Stress vs. Lagrangian strain for ABS/PP and ABS/PP/PP-g-MA composites, experiments FigureFigure 17. 17. StressStress vs. vs. Lagrangian Lagrangian strain strain for for ABS/PP ABS/PP and and ABS/PP/PP-g ABS/PP/PP-g-MA-MA composites, composites, experiments experiments and modelling. andand modelling. modelling.

AAA first ﬁrstfirst approach approachapproach to toto simulate simulatesimulate polymer polymerpolymer blen blendsblendsds breakdown breakdownbreakdown is is isdevoted devoteddevoted to toto evaluate evaluate evaluate the thethe equivalentequivalentequivalent stress stressstress (Von (Von(Von Mises) Mises)Mises) in inin each eacheach elemen elementelement oft ofof the thethe RVE. RVE.RVE. A AA stress stressstress mappingmapping mapping at atat 1.7% 1.7%1.7% of ofof macroscopicmacroscopicmacroscopic strain strainstrain is is ispresented presentedpresented in in in Figure Figure Figure 18 18 18 for for for both both both blends. blends. blends.

FigureFigureFigure 18. 18.18. VonVon Von Mises Mises Mises stress stress stress cartography cartography cartography at at at1.7% 1.7% 1.7% of of of macroscopic macroscopic macroscopic strain strain strain in in in ABS/PP ABS/PP ABS/PP and andand ABS/PP/PP- ABS/PP/PP-ABS/PP/PP- g-MAg-MAg-MA blends. blends.blends.

ToToTo quantify quantify quantify this thisthis mapping, mapping,mapping, we wewe analyzed analyzedanalyzed the thethe Von VonVon Mises Mises Mises stress stress stress distribution distribution distribution within within within the the the ABS.ABS.ABS. As AsAs local locallocal behavior behaviorbehavior is is isnot notnot element elementelement size sizesize dependent,dependent, dependent, the thethe comparison comparisoncomparison between betweenbetween compati- compati-compati- bilizedbilizedbilized and andand uncompatibilized uncompatibilizeduncompatibilized ABS/PP ABS/PPABS/PP is ispossibl possiblepossible eeven eveneven if ififRVE RVERVE and andand elements elementselements sizes sizessizes are areare not notnot equivalent.equivalent.equivalent. The TheThe Figure FigureFigure 19 19 19 reflects reﬂectsreflects the thethe distribution distributiondistribution of ofof Von VonVon Mises Mises Mises stresses stresses stresses into into into ABS ABS ABS polymer polymerpolymer forforfor several severalseveral macroscopic macroscopic macroscopic strains strains strains (0.3, (0.3, (0.3, 1.5, 1.5, 1.5, 1.9, 1.9, and and 2.1%) 2.1%) forfor for ABS/PPABS/PP ABS/PP and and ABS/PP/PP-g-MA ABS/PP/PP-g-MA composites.composites.composites. The The The ultimate ultimate ultimate stress stress stress of of ABS of ABS ABS polymer polymer polymer is isaround around is around 43 43 MPa, MPa, 43 MPa, beyond beyond beyond which which which the the sam- sam- the plesampleple breaks. breaks. breaks. The The main The main mainconclusion conclusion conclusion is isthat that is redu that reducing reducingcing the the size thesize of size of PP PP of nodules PPnodules nodules by by compatibiliza- compatibiliza- by compatibi- tionlizationtion leads leads leads to to a alocal to local a local increase increase increase of of the the of level the level level of of equivalent equivalent of equivalent stress stress stress by by the bythe diminution the diminution diminution of of the ofthe in- the in- interparticular distance between nodules (see Figure 16). The frequency curves are thus terparticularterparticular distance distance between between nodules nodules (see (see Figure Figure 16). 16). The The frequency frequency curves curves are are thus thus shifted to the right. shiftedshifted to to the the right. right.

Polymers 2021, 13, x FOR PEER REVIEW 16 of 20 Polymers 2021, 13, 1647 15 of 19

Figure 19.FigureFrequencies 19. Frequencies of elements of elements versus equivalent versus equivalent stresses for severalstresses macroscopic for several strainsmacroscopic for ABS strains into ABS/PP for and ABS/PP/PP-g-MAABS into ABS/PP blends. and ABS/PP/PP-g-MA blends.

The number of elementsThe number with of equivalent elements with stresses equivalent higher stresses than higher43 MPa than (corresponding 43 MPa (corresponding to the value of ABS breakage) was computed from these data (see Table4: Percentage to the value of ABS breakage) was computed from these data (see Table 4: Percentage of of elements in ABS). At 1.9% of macroscopic strain both blends show a large number of elements in ABS).broken At 1.9% elements of macroscopic in ABS, 39.3% strain for ABS/PPboth blends and 71.8%show fora large ABS/PP/PP-g-MA. number of bro- Even if the ken elements in ABS,macroscopic 39.3% for simulated ABS/PP Von and Mises 71.8% stresses for ABS/PP/PP-g-MA. are similar for both Even microstructures, if the mac- the local roscopic simulatedstress Von repartition Mises stresses leads toare a similar more premature for both microstructures, breakage in the case the of local ABS/PP/PP-g-MA stress repartition leadsblend. to a more This observation premature can breaka explainge the in resultthe case presented of ABS/PP/PP-g-MA in Figure5, the number blend. of elements This observationthat can haveexplain reached the result the critical present breakageed in Figure value is5, higherthe number in the presenceof elements of PP-g-MA that thus have reached theinducing critical breakage progressively value a reduction is higher of in stress the presence in experiments of PP-g-MA for ABS/PP/PP-g-MA thus induc- blend ing progressivelyin a comparison reduction of with stress ABS/PP. in experiments for ABS/PP/PP-g-MA blend in comparison with ABS/PP. Table 4. Percentage of elements in ABS with an equivalent stress higher than the ABS strength for ABS/PP and ABS/PP/PP-g-MA blends. Table 4. Percentage of elements in ABS with an equivalent stress higher than the ABS strength for ABS/PP and ABS/PP/PP-g-MA blends. % of Elements in ABS with an Equivalent Stress Higher than 43 Mpa % of Elements in ABS with Inan ABS/PP Equivalent Stress Higher In ABS/PP/PP-g-MAthan 43 Mpa εmacro = 0.3%In ABS/PP 0.0In ABS/PP/PP-g-MA 0.0 ε = 1.5% 4.2 5.6 εmacro = 0.3% macro 0.0 0.0 ε = 1.9% 39.3 71.8 εmacro = 1.5% macro 4.2 5.6 ε = 2.1% 80.6 86.9 εmacro = 1.9% macro 39.3 71.8 εmacro = 2.1% 80.6 86.9 To complete this analysis cartography of porosity is given for 1.7% of macroscopic To completestrain this (Figureanalysis 20 cartography). In the PP nodules of porosity the porosity is given never for exceeds1.7% of 1% macroscopic meaning that the PP never reaches the ﬂow stress state. The porosity shows a concentration around the PP strain (Figure 20). In the PP nodules the porosity never exceeds 1% meaning that the PP nodules in a direction perpendicular to the tensile direction. Even if these cartographies never reaches theare flow quite stress similar, state. the distributionThe porosity of shows porosity a aroundconcentration the nodules around seems the different. PP The nodules in a directionFigure perpendicular21 regroups the to number the te ofnsile elements direction. versus Even local if porosities these cartographies into ABS polymer for are quite similar,several the distribution macroscopic of strains porosi (1.5,ty around 1.9, and the 2.1%) nodules for ABS/PP seems and different. ABS/PP/PP-g-MA The Fig- blends. ure 21 regroups theThe number more the of macroscopic elements versus imposed local strain porosities increases, into the ABS more polymer the shift betweenfor sev- ABS/PP eral macroscopicand strains ABS/PP/PP-g-MA (1.5, 1.9, and 2.1% is pronounced.) for ABS/PP The and breakage ABS/PP/PP-g-MA is initiated at blends. the interface The ABS/PP more the macroscopic imposed strain increases, the more the shift between ABS/PP and ABS/PP/PP-g-MA is pronounced. The breakage is initiated at the interface ABS/PP by cavitation; this phenomenon is increased while reducing the size of nodules using a compatibilizing agent for example.

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PolymersPolymers 20212021,, 1313,, xx FORFOR PEERPEER REVIEWREVIEWby cavitation; this phenomenon is increased while reducing the size of nodules using1717 ofof a2020

compatibilizing agent for example.

Figure 20. Porosity cartography at 1.7% of macroscopic strain in ABS/PP and ABS/PP/PP-g-MA FigureFigure 20.20. PorosityPorosity cartographycartography atat 1.7%1.7% ofof macroscomacroscopicpic strainstrain inin ABS/PPABS/PP andand ABS/PP/PP-g-MAABS/PP/PP-g-MA composites. composites.composites.

Figure 21. Frequencies of elements versus effective porosity fraction (see Equation (4)) for several FigureFigure 21. 21.Frequencies Frequencies ofof elementselements versusversus effectiveeffective porosityporosity fraction (see Equation (4)) for several several macroscopicmacroscopic strainsstrains forfor ABSABS intointo ABS/PPABS/PP andand ABS/PP/PP-g-MAABS/PP/PP-g-MA composites.composites. macroscopic strains for ABS into ABS/PP and ABS/PP/PP-g-MA composites. Regarding the volumetric strain, once again the prediction gives satisfactory results RegardingRegarding the the volumetric volumetric strain, strain, once once again again the the prediction prediction gives gives satisfactory satisfactory results results (Figure 22). Nevertheless, a deviation is observed from 1.7% of strain: the model underes- (Figure(Figure 22 22).). Nevertheless, Nevertheless, a a deviation deviation is is observed observed from from 1.7% 1.7% of of strain: strain: the the model model underesti- underes- matestimatestimates the thethe volume volumevolume change. change.change. This ThisThis result resultresult is inisis in accordancein accordanceaccordance with withwith previous previousprevious analysis, analysis,analysis, after afterafter 1.7% 1.7%1.7% of macroscopicofof macroscopicmacroscopic strain strainstrain a large aa large numberlarge numbernumber of ABS ofof elements ABSABS elementselements have exceeded havehave exceededexceeded the ABS stress thethe ABSABS breakage. stressstress Thebreakage.breakage. comparison TheThe comparisoncomparison between experiments betweenbetween experimentexperiment and numericss andand is therefore numericnumeric invalid. isis thereforetherefore Experimentally, invalid.invalid. Exper-Exper- an interfacialimentally,imentally, decohesion anan interfacialinterfacial between decohesiondecohesion PP and betweenbetween ABS occurs PPPP andand thus ABSABS increasing occursoccurs thusthus the increasingincreasing volumetric thethe strain vol-vol- explainingumetricumetric strainstrain the observedexplainingexplaining deviation. thethe observedobserved deviation.deviation.

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FigureFigure 22. 22.Volumetric Volumetric strain strain vs. vs. Lagrangian Lagrangian strain strain for fo ABS/PPr ABS/PP composite, composite, experiments, experiments, and and modelling. modelling. 5. Conclusions 5. ConclusionsPolymer mechanical recycling is viewed nowadays as an efficient way to reduce plasticPolymer pollution mechanical and reduce recycling the environmental is viewed nowadays footprint as an of efficient its production way to fromreduce fossil plas- resources.tic pollution Sorting and isreduce primordial the environmental to achieve economic footprint competitiveness of its production since mostfrom polymers fossil re- aresources. incompatible. Sorting is Residual primordial impurities to achieve are however economic inevitable. competitiveness Understanding since most the potentialpolymers impactare incompatible. of these impurities Residual is impurities thus important are however to evaluate inevitable. recyclability Understanding of end-of-life the plastics. poten- Atial first impact step is of to these work impurities on a model is blend. thus PPimportant was incorporated to evaluate at recyclability 4% in weight of into end-of-life ABS as theyplastics. are two A first major step constituents is to work of on WEEE. a model PP-g-MA blend. was PP addedwas incorporated to these blends at 4% with in the weight aim tointo improve ABS as their they mechanicalare two major performances. constituents of Experiments WEEE. PP-g-MA and predictive was added modelling to these blends were bothwith used the toaim assess to improve first the abilitytheir mechanical of this blend performances. to be reused atExperiments least in static and applications, predictive andmodelling second, were the necessity both used of compatibilization.to assess first the ability of this blend to be reused at least in staticThe applications, main conclusion and second, of this the paper necessity is that of staticcompatibilization. properties of ABS are almost not impairedThe bymain possible conclusion PP contamination of this paper is at that 4 wt% static from properties impact of tests ABS with are almost this impurity not im- rate.paired Hence, by possible this recycled PP contamination material can at be 4 wt% used from as an impact ABS polymer, tests with in this the caseimpurity of static rate. application.Hence, this recycled Contrary material to expectations, can be used the as addition an ABS of polymer, PP-g-MA, in the which case was of static expected applica- to improvetion. Contrary ABS/PP to adhesionexpectations, and tothe reduce addition the sizeof PP-g-MA, of the nodular which phase, was expected induced ato negative improve effectABS/PP on theadhesion ultimate and stress. to reduce The the main size reason of the is nodular that around phase, the induced interfacial a negative area, which effect increaseson the ultimate as the nodules stress. The sizes main decrease, reason the is that level around of stress the triaxiality interfacial in area, ABS iswhich much increases higher. Locally,as the nodules the ABS sizes around decrease, PP nodules the level cavitates of stress much triaxiality more when in ABS nodules is much are higher. even smaller.Locally, Tothe go ABS further around in this PP predictivenodules cavitates analysis, much cohesive more model when zonesnodules should are even be used smaller. to mimic To go variousfurther interfacialin this predictive compatibilities. analysis, Ancohesive alternative model could zones also should be the be use used of to an mimic intermediate various layerinterfacial between compatibilities. phases with graduateAn alternative properties could [ 39also]. The be the situation use of may an intermediate be very different layer forbetween dynamic phases applications with graduate as softer properties inclusions [4 (PP0]. areThe softer situation inclusions may be in very ABS) different can help tofor dissipate energy and accommodate stress. dynamic applications as softer inclusions (PP are softer inclusions in ABS) can help to dissipate energy and accommodate stress. Author Contributions: Conceptualization, C.S., A.-S.C.-B., J.-M.L.-C., P.I. and D.P.; methodology, A.-S.C.-B. and P.I.; software, A.-S.C.-B.; validation; formal analysis, A.-S.C.-B. and P.I.; investiga- Author Contributions: Conceptualization, C.S., A.-S.C.-B., J.-M.L.-C., P.I., D.P.; methodology, A.- tion, C.S.; writing—original draft preparation, A.-S.C.-B.; writing—review and editing, All authors; S.C.-B. and P.I.; software, A.-S.C.-B.; validation; formal analysis, A.-S.C.-B. and P.I.; investigation, project administration, D.P. and P.I. All authors have read and agreed to the published version of C.S.; writing—original draft preparation, A.-S.C.B.; writing—review and editing, All authors; pro- the manuscript. ject administration, D.P. and P.I. All authors have read and agreed to the published version of the Funding:manuscript.This work was supported by BPI France via the FUI 20 (Fonds unique interministériel) grant. Acknowledgments:Funding: This workThe was authors supported would by BPI like France to thank via Benjamin the FUI 20 Gallard, (FondsAlexandre unique interministériel) Cheron and Jean-Claudegrant. Roux for technical support, respectively for polymer processing, mechanical testing and morphology characterization. Suez and Pellenc ST are gratefully acknowledged for partnership in Acknowledgments: The authors would like to thank Benjamin Gallard, Alexandre Cheron and this work. Jean-Claude Roux for technical support, respectively for polymer processing, mechanical testing

Polymers 2021, 13, 1647 18 of 19

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Wang, R.X.Z. Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review. Waste Manag. 2014, 34, 1455–1469. [CrossRef][PubMed] 2. Buekens, A.; Yang, J. Recycling of WEEE plastics: A review. J. Mater. Cycles Waste Manag. 2014, 16, 415–434. [CrossRef] 3. Vanegas, P.; Peeters, J.R.; Cattrysse, D.; Tecchio, P.; Ardente, F.; Mathieux, F.; Dewulf, W.; Duflou, J.R. Ease of disassembly of products to support circular economy strategies. Resour. Conserv. Recycl. 2018, 135, 323–334. [CrossRef] 4. Movilla, N.A.; Zwolinski, P.; Dewulf, J.; Mathieux, F. A method for manual disassembly analysis to support the ecodesign of electronic displays. Resour. Conserv. Recycl. 2016, 114, 42–58. [CrossRef] 5. Robinson, B.H. E-waste: An assessment of global production and environmental impacts. Sci. Total. Environ. 2009, 408, 183–191. [CrossRef] 6. Turner, A. Black plastics: Linear and circular economies, hazardous additives and marine pollution. Environ. Int. 2018, 117, 308–318. [CrossRef] 7. Cucchiella, F.; D’Adamo, I.; Koh, S.L.; Rosa, P. Recycling of WEEEs: An economic assessment of present and future e-waste streams. Renew. Sustain. Energy Rev. 2015, 51, 263–272. [CrossRef] 8. Hennebert, P.; Filella, M. WEEE plastic sorting for bromine essential to enforce EU regulation. Waste Manag. 2018, 71, 390–399. [CrossRef] 9. Guzzonato, A.; Puype, F.; Harrad, S.J. Evidence of bad recycling practices: BFRs in children’s toys and food-contact articles. Environ. Sci. Process. Impacts 2017, 19, 956–963. [CrossRef] 10. Beigbeder, J.; Perrin, D.; Mascaro, J.-F.; Lopez-Cuesta, J.-M. Study of the physico-chemical properties of recycled polymers from waste electrical and electronic equipment (WEEE) sorted by high resolution near infrared devices. Resour. Conserv. Recycl. 2013, 78, 105–114. [CrossRef] 11. Perrin, D.; Mantaux, O.; Ienny, P.; Léger, R.; Dumon, M.; Lopez-Cuesta, J.-M. Influence of impurities on the performances of HIPS recycled from Waste Electric and Electronic Equipment (WEEE). Waste Manag. 2016, 56, 438–445. [CrossRef] 12. Das, A.; Chatham, C.A.; Fallon, J.J.; Zawaski, C.E.; Gilmer, E.L.; Williams, C.B.; Bortner, M.J. Current understanding and challenges in high temperature additive manufacturing of engineering thermoplastic polymers. Addit. Manuf. 2020, 34, 101218. [CrossRef] 13. Goh, G.; Toh, W.; Yap, Y.; Ng, T.; Yeong, W. Additively manufactured continuous carbon fiber-reinforced thermoplastic for topology optimized unmanned aerial vehicle structures. Compos. Part B Eng. 2021, 216, 108840. [CrossRef] 14. Banerjee, S.S.; Burbine, S.; Shivaprakash, N.K.; Mead, J. 3D-printable PP/SEBS thermoplastic elastomeric blends: Preparation and properties. Polymers 2019, 11, 347. [CrossRef][PubMed] 15. Martinho, G.; Pires, A.; Saraiva, L.; Ribeiro, R. Composition of plastics from waste electrical and electronic equipment (WEEE) by direct sampling. Waste Manag. 2012, 32, 1213–1217. [CrossRef][PubMed] 16. Maris, E.; Botané, P.; Wavrer, P.; Froelich, D. Characterizing plastics originating from WEEE: A case study in France. Miner. Eng. 2015, 76, 28–37. [CrossRef] 17. Bonda, S.; Mohanty, S.; Nayak, S.K. Influence of compatibilizer on mechanical, morphological and rheological properties of PP/ABS blends. Iran. Polym. J. 2014, 23, 415–425. [CrossRef] 18. Tostar, S.; Stenvall, E.; Foreman, M.R.S.J.; Boldizar, A. The influence of compatibilizer addition and gamma irradiation on mechanical and rheological properties of a recycled WEEE plastics blend. Recycling 2016, 1, 101–110. [CrossRef] 19. Patel, A.C.; Brahmbhatt, R.B.; Devi, S. Mechanical properties and morphology of PP/ABS blends compatibilized with PP-g-2- HEMA. J. Appl. Polym. Sci. 2001, 88, 72–78. [CrossRef] 20. Patel, A.C.; Brahmbhatt, R.B.; Sarawade, B.D.; Devi, S. Morphological and mechanical properties of PP/ABS blends compatibilized with PP-g-acrylic acid. J. Appl. Polym. Sci. 2001, 81, 1731–1741. [CrossRef] 21. Deng, Y.; Mao, X.; Lin, J.; Chen, Q. Compatibilization of polypropylene/Poly(acrylonitrile-butadiene-styrene) blends by polypropylene-graft-cardanol. J. Appl. Polym. Sci. 2015, 132, 41315. [CrossRef] 22. Luo, Z.; Lu, Q.; Ma, F.; Jiang, Y. The effect of graft copolymers of maleic anhydride and epoxy resin on the mechanical properties and morphology of PP/ABS blends. J. Appl. Polym. Sci. 2014, 131.[CrossRef] 23. IIbrahim, M.A.H.; Hassan, A.; Wahit, M.U.; Hasan, M.; Mokhtar, M. Mechanical properties and morphology of polypropylene/ poly(acrylonitrile–butadiene–styrene) nanocomposites. J. Elastom. Plast. 2017, 49, 209–225. [CrossRef] 24. Kum, C.K.; Sung, Y.-T.; Kim, Y.S.; Lee, H.G.; Kim, W.N.; Lee, H.S.; Yoon, H.G. Effects of compatibilizer on mechanical, morphological, and rheological properties of polypropylene/poly(acrylonitrile-butadiene-styrene) blends. Macromol. Res. 2007, 15, 308–314. [CrossRef] 25. Lee, Y.K.; Lee, J.B.; Park, D.H.; Kim, W.N. Effects of accelerated aging and compatibilizers on the mechanical and morphological properties of polypropylene and poly(acrylonitrile-butadiene-styrene) blends. J. Appl. Polym. Sci. 2013, 127, 1032–1037. [CrossRef] 26. Gurson, A.L. Continuum theory of ductile rupture by void nucleation and growth: Part I—Yield criteria and flow rules for porous ductile media. J. Eng. Mater. Technol. 1977, 99, 2–15. [CrossRef] 27. Tvergaard, V. On localization in ductile materials containing spherical voids. Int. J. Fract. 1982, 18, 237–252. 28. Tvergaard, V.; Needleman, A. Analysis of the cup-cone fracture in a round tensile bar. Acta Met. 1984, 32, 157–169. [CrossRef] Polymers 2021, 13, 1647 19 of 19

29. Moore, J. Acrylonitrile-butadiene-styrene (ABS)—A review. Composites 1973, 4, 118–130. [CrossRef] 30. Signoret, C.; Girard, P.; Guen, A.; Caro-Bretelle, A.-S.; Lopez-Cuesta, J.-M.; Ienny, P.; Perrin, D. Degradation of styrenic plastics during recycling: Accommodation of PP within ABS after WEEE plastics imperfect sorting. Polymers 2021, 13, 1439. [CrossRef] 31. Christmann, A.; Ienny, P.; Quantin, J.; Caro-Bretelle, A.; Lopez-Cuesta, J. Mechanical behaviour at large strain of polycarbonate nanocomposites during uniaxial tensile test. Polymer 2011, 52, 4033–4044. [CrossRef] 32. Caro-Bretelle, A.; Ienny, P.; Leger, R. Constitutive modeling of a SEBS cast-calender: Large strain, compressibility and anisotropic damage induced by the process. Polymer 2013, 54, 4594–4603. [CrossRef] 33. Hall, R.A. Computer modelling of rubber-toughened plastics: Random placement of monosized core-shell particles in a polymer matrix and interparticle distance calculations. J. Mater. Sci. 1991, 26, 5631–5636. [CrossRef] 34. Besson, J. Continuum models of ductile fracture: A review. Int. J. Damage Mech. 2010, 19, 3–52. [CrossRef] 35. Boisot, G.; Laiarinandrasana, L.; Besson, J.; Fond, C.; Hochstetter, G. Experimental investigations and modeling of volume change induced by void growth in polyamide 11. Int. J. Solids Struct. 2011, 48, 2642–2654. [CrossRef] 36. Tvergaard, V. Influence of voids on shear band instabilities under plane strain conditions. Int. J. Fract. 1981, 17, 389–407. [CrossRef] 37. Caro, A.; Bernardeau, F.; Perrin, D.; Leger, R.; Benezet, J.; Ienny, P. Computational modelling of void growth in Phenolic Molding Compounds filled PolyPropylene from optical measurements. Polym. Test. 2018, 71, 209–216. [CrossRef] 38. Besson, J.; Leriche, R.; Foerch, R.; Cailletaud, G. Object-oriented programming applied to the finite element method Part II. application to material behaviors. Rev. Eur. Élém. Finis 1998, 7, 567–588. [CrossRef] 39. Liu, J.L.; Xia, R.; Zhou, X.H. A new look on wetting models: Continuum analysis. Sci. China Phys. Mech. Astron. 2012, 55, 1–9. [CrossRef]