Impact of Polymer Binders on the Structure of Highly Filled Zirconia Feedstocks

polymers

Article Impact of Polymer Binders on the Structure of Highly Filled Zirconia Feedstocks

Claire Delaroa 1,2, René Fulchiron 1 , Eric Lintingre 2, Zoé Buniazet 2 and Philippe Cassagnau 1,*

1 Ingénierie des Matériaux Polymères, Univ Lyon, Université Lyon 1, CNRS UMR 5223, 15 Boulevard Latarjet, 69622 Villeurbanne CEDEX, France; [email protected] (C.D.); [email protected] (R.F.) 2 Saint-Gobain CREE, Grains et Poudres, 550 Avenue Alphonse Jauﬀret, BP 20224, 84306 Cavaillon, France; [email protected] (E.L.); [email protected] (Z.B.) * Correspondence: [email protected]

Received: 7 September 2020; Accepted: 28 September 2020; Published: 29 September 2020

Abstract: The impact of polypropylene and high-density polyethylene backbone binders on the structure of organic matrix, feedstock, and ceramic parts is investigated in terms of morphology in this paper. The miscibility of wax with polyethylene and polypropylene is investigated in the molten state via a rheological study, revealing wax full miscibility with high-density polyethylene and restricted miscibility with polypropylene. Mercury porosimetry measurements realized after wax extraction allow the characterization of wax dispersion in both neat organic blends and zirconia filled feedstocks. Miscibility differences in the molten state highly impact wax dispersion in backbone polymers after cooling: wax is preferentially located in polyethylene phase, while it is easily segregated from polypropylene phase, leading to the creation of large cracks during solvent debinding. The use of a polyethylene/polypropylene ratio higher than 70/30 hinders wax segregation and favors its homogeneous dispersion in organic binder. As zirconia is added to organic blends containing polyethylene, polypropylene, and wax, the pore size distribution created by wax extraction is shifted towards smaller pores. Above zirconia percolation at 40 vol%, the pore size distribution becomes sharp attesting of wax homogeneous dispersion. As the PP content in the organic binder decreases from 100% to 0%, the pore size distribution is reduced of 30%, leading to higher densification ability. In order to ensure a maximal densification of the final ceramic, polyethylene/polypropylene ratios with a minimum content of 70% of high-density polyethylene should be employed.

Keywords: polymer binder; zirconia feedstocks; ceramic density

1. Introduction Ceramic injection molding (CIM) allows the production of complex ceramic parts via several processing steps presented in Figure1. First, high proportions (40–70 vol%) of ceramic fillers, such as zirconia or alumina, are dispersed under high shear rate in an organic binder to create a blend called feedstock. The organic matrix is usually composed of a low molar mass binder, commonly waxes, providing flow properties enabling injection molding, and one or multiple high molar mass polymers, called backbone binders, which develop feedstock mechanical properties. Backbone binders currently used in CIM process are polyolefins (i.e., polypropylene (PP), low-density polyethylene (LDPE), and high-density polyethylene (HDPE)), polyethylene glycol (PEG), polystyrene (PS), ethylene-vinyl acetate (EVA), or poly(methyl methacrylate) (PMMA) [1,2]. As fine ceramic powders used in CIM industry present high aggregation ability, compatibilizers such as stearic or oleic acid are usually added to control fillers dispersion state [3–7].

Polymers 2020, 12, 2247; doi:10.3390/polym12102247 www.mdpi.com/journal/polymers Polymers 2020, 12, 2247 2 of 17 Polymers 2020, 12, x FOR PEER REVIEW 2 of 18

FigureFigure 1. 1. CeramicCeramic injection injection molding molding processing processing steps. steps.

TheThe produced produced feedstock feedstock is is injected injected in in a a mold mold under under highhigh injectioninjection pressure to to create create compact compact green parts.green Debinding parts. Debinding steps aresteps then are employedthen employed to extract to extract the the organic organic matrix, matrix, whichwhich constitutes 30 30 vol% to 60vol% vol% to 60 of vol% the part.of the This part. processingThis processing step step is critical is critical and and the the selection selection ofof debindingdebinding routes routes has has been extensivelybeen extensively reviewed reviewed in the in literature the literature [8–10 [8–10].]. In order In order to favorto favor shape shape retention, retention, and and toto preventprevent defect creation,defect creation, low molar low mass molar binder mass isbinder first removedis first removed via solvent via solvent extraction. extraction. Subsequently, Subsequently, the remainingthe organicremaining contents organic are thermallycontents are degraded thermally to degraded obtain a to fragile obtain powder a fragile structure powder calledstructure brown called part [11]. brown part [11]. The latter is sintered to create a dense final ceramic. The latter is sintered to create a dense final ceramic. Actually, technical ceramics such as doped zirconia require high processing quality to achieve Actually, technical ceramics such as doped zirconia require high processing quality to achieve full densification and the expected high mechanical properties. However, critical defects and fulldistortions densification can be and grown the at expected each step high of the mechanical CIM process, properties. due to unsuitable However, process critical or formula defects and distortionsparameters can [11–13]. be grown The latter at each is responsible step of the fo CIMr poor process, dimensional due tocontrol unsuitable and low process mechanical or formula parametersperformances,[11– characterized,13]. The latter for example, is responsible by a sign forificant poor decrease dimensional of Weibull control modulus, and resulting low mechanical in performances,poor quality parts. characterized, In fact, every for inhomogeneity example, by a created significant during decrease the process of Weibull cannot be modulus, removed resulting and in poormay quality transform parts. into In a fact,macroscopic every inhomogeneity defect that may induce created critical during failure the of process the final cannot ceramic be [14–16]. removed and may transformDue to complex into a macroscopic processing defectand multicomponent that may induce formulas, critical failurenumerous of the parameters final ceramic might [14 –16]. modifyDue tofeedstock complex behavior processing and structure. and multicomponent The influence of formulas, processing numerous parameters parameters on defect creation might modify has been extensively reviewed in the open literature. Structural evolutions of the sample during feedstock behavior and structure. The influence of processing parameters on defect creation has been debinding and sintering steps have been monitored by porosimetry [17–19], imaging [15,20], and extensivelydimensional reviewed studies [10,21], in the in open order literature. to understand Structural phenomena evolutions involved. of Feedstock the sample rheological during and debinding andmechanical sintering behaviors, steps have which been master monitored its processing by porosimetry ability, have [17– been19], imaginganalyzed [to15 predict,20], and injection dimensional studiesmolding [10 ,behavior21], in order [22]. Those to understand properties phenomenaare strongly influenced involved. by Feedstock the characteristics rheological of the and polymer mechanical behaviors,binder, fillers, which and master additives, its as processing well as their ability, proportions have and been interactions analyzed [23–25]. to predict injection molding behaviorEven [22 ].though Those many properties feedstock are stronglyformulas influencedhave been developed by the characteristics and industrialized, of the polymeronly few binder, fillers,studies and focused additives, on asthe well impact as their of organic proportions matrix and formula interactions on feedstock [23–25 ].structure. Concerning low-molarEven though mass manyselection, feedstock Hsu et formulas al. studied have the been impact developed of wax andcharacteristics industrialized, on LDPE-based only few studies focusedfeedstock on thehomogeneity impact of and organic sintering matrix behavior formula [26]. on Karatas feedstock et al. structure. investigated Concerning injection low-molarability of mass feedstock composed of HDPE and various waxes namely paraffin, carnauba, and bee’s waxes [27]. selection, Hsu et al. studied the impact of wax characteristics on LDPE-based feedstock homogeneity They concluded that the use of paraffin wax offered more appropriate flow properties for the CIM andprocess. sintering behavior [26]. Karatas et al. investigated injection ability of feedstock composed of HDPE andOn the various other waxesside, the namely critical para roleffi ofn, backbo carnauba,ne binder and bee’s has been waxes attested [27]. Theyduring concluded the whole that the useprocess of para byffi rheologicaln wax offered measurem more appropriateents [23,28] and flow shape properties retention for studies the CIM on debinded process. and sintered partsOn the[21,29,30]. other side,However, the critical few rolerelationships of backbone between binder feedstock has been structure attested duringand backbone the whole binder process by rheologicalselection have measurements been really [made.23,28] and shape retention studies on debinded and sintered parts [21,29,30]. However,Setasuwon few relationships et al. employed between two LDPEs feedstock presenting structure different and molar backbone masses binder as backbone selection binder have been reallyand made.observed no clear impact of molar mass on debinding and sintering behaviors of feedstocks [31]. SetasuwonKim et al. et employed al. employed EVA or two PE LDPEs wax as presenting minor binders diff erentin paraffin molar wax masses based as feedstocks backbone [32]. binder and Using mercury porosimetry during wick-debinding, they highlighted a significant modification of observed no clear impact of molar mass on debinding and sintering behaviors of feedstocks [31]. porosity evolution. The latter could be relative to a modification in binder affinity; however, the behaviorKim et of al. neat employed polymer blends EVA orwas PE not wax studied. as minor binders in paraffin wax based feedstocks [32]. Using mercuryWen et al. porosimetry employed EVA, during PP, wick-debinding, LDPE, HDPE, and they their highlighted blends as a significantbackbone binders modification in of porosityfeedstocks evolution. containing The latterparaffin could wax be as relative plasticize to ar modification[33]. Using scanning in binder electron affinity; microscopy however, the and behavior of neatdeviation polymer of density, blends they was concluded not studied. that feedstocks composed of multiple backbone binders, such as WenLDPE/HDPE et al. employed or PP/HDPE, EVA, present PP, LDPE, the more HDPE, homo andgenous their structure blends as after backbone mixing, binders leading into feedstocksthe containinghighest densification paraffin wax ability. as plasticizer They supposed [33]. it Using resulted scanning from synergistic electron microscopyeffects of multi-polymer and deviation of density,blends; they however, concluded the behavior that feedstocks of neat polymer composed binders of multiplewas not studied. backbone binders, such as LDPE/HDPE or PP /HDPE, present the more homogenous structure after mixing, leading to the highest densification ability. They supposed it resulted from synergistic effects of multi-polymer blends; however, the behavior of neat polymer binders was not studied. Polymers 2020, 12, 2247 3 of 17

Moreover, the impact of low-molar mass and backbone binder ratio on injection ability, green microstructure, and sintering ability has also been reviewed. Wen et al. studied PE-paraffin wax based feedstock and concluded that binders containing 70% of wax presented the best mixing behavior, the sharper pore size distribution after thermal debinding, resulting in the highest ceramic density [34]. However, Tseng and Hsu observed the creation of cracking defects during thermal debinding of PVA-paraffin wax based feedstocks when the content of wax reaches 60% of the organic binder [35]. As the wax content increased, the mechanical properties of feedstock were weakened and became insufficient to withstand internal pressure built up by degradation products. By using a powder-bed favoring the evacuation of molten binder, the creation of defect was avoided. Liu and Tseng also showed that pore size distribution obtained in PVA-paraffin wax-based feedstock after thermal debinding was shifted towards smaller pores when the wax content is reduced, due to enhanced fillers dispersion [6]. Since the 1980s, a number of studies have been conducted to understand and improve CIM process at the industrial scale, leading to the development of many feedstock formulas. However, studies related to the impact of backbone polymer selection on parts structure and properties are scarce. Moreover, none of the existing publications investigated the morphology of both neat and filled polymer blends in order to draw conclusions about the importance of the affinity of backbone and low-molar mass binders. In this paper, the impact of PP and HDPE backbone binders on the structure of organic matrix, feedstock, and ceramic parts is investigated in terms of morphology. In order to have a good insight of the structure of such highly loaded systems, wax dispersion in both neat organic blends and zirconia filled feedstock is studied and compared depending on the selected backbone polymers. Final ceramic quality is also reviewed to deeply understand the link between backbone binder selection, organic matrix morphology, feedstock structure, and final product densification ability.

2. Experimental

2.1. Materials Yttria-stabilized Zirconia powder (CY3Z-RA, Saint-Gobain ZirPro, Handan, China) presenting 2 a d50 of 0.3 µm and a specific surface area of 7 m /g was used as filler. Multicomponent organic binders composed of high-density polyethylene (HDPE), polypropylene (PP), and paraffin wax (56 wt%) were used. These binders have been selected based on their injection ability and degradation behaviors. Different weight ratios HDPE/PP between polyethylene and polypropylene were investigated: 0/100, 70/30, 50/50, 30/70, and 100/0. Solid content of the blends varied from 0 vol% to 50 vol%, and fully loaded blends are mentioned as feedstocks. In order to favor fillers dispersion, 0.55 wt% of stearic acid was added, relative to the filler amount. Using contact angle and rheological measurements in previous works, Auscher et al. demonstrated that stearic acid can be chemically bonded to zirconia particles surface, reducing interparticle interactions and improving fillers dispersion state [3,24]. The main characteristics of the organic components are presented in Table1.

Table 1. Characteristics of binder components.

Crystallization Degradation Binder Supplier Density Temperature Temperature

Polyethylene Axalta, Bulle, Swizterland 0.950 115 ◦C 460 ◦C Polypropylene Total, Paris, France 0.905 120 ◦C 430 ◦C Paraﬃn wax Alphawax, Alphen, Neetherlands 0.785 47 ◦C 310 ◦C Sigma Aldrich, Stearic acid 0.845 65 C 250 C Saint-Louis, MO, US ◦ ◦ Polymers 2020, 12, x FOR PEER REVIEW 4 of 18 Polymers 2020, 12, x FOR PEER REVIEW 4 of 18 Table 1. Characteristics of binder components. Table 1. Characteristics of binder components. Crystallization Degradation Binder Supplier Density Crystallization Degradation Binder Supplier Density Temperature Temperature Temperature Temperature Polyethylene Axalta, Bulle, Swizterland 0.950 115°C 460°C PolypropylenePolyethylene Axalta,Total, Bulle, Paris, Swizterland France 0.9500.905 115°C 120°C 460°C 430°C Polypropylene Total,Alphawax, Paris, Alphen, France 0.905 120°C 430°C Paraffin wax Alphawax, Alphen, 0.785 47°C 310°C Paraffin wax Neetherlands 0.785 47°C 310°C Sigma NeetherlandsAldrich, Saint-Louis, Stearic acid Sigma Aldrich, Saint-Louis, 0.845 65°C 250°C PolymersStearic2020 acid, 12, 2247 MO, US 0.845 65°C 250°C 4 of 17 MO, US 2.2. Processing 2.2.2.2. Processing Processing Zirconia, stearic acid, and organic binders were mixed for 15 min at 180 °C in an internal mixer (HAAKEZirconia,Zirconia, Rheomix stearic stearic 600, acid, acid, ThermoFisher, and and organic organic binders binders Waltham, were were MA mi mixedxed, US) for for before 15 15 min min being at at 180 180 injected °C◦C in in anan (Babyplast internal internal mixer mixer 10T, (HAAKEMolteno,(HAAKE Italy)Rheomix Rheomix at 170 600, 600, °C ThermoFisher, ThermoFisher,into dog bone samples,Waltham, presented MA,MA, USA)US) in before beforeFigure being being2. Waxes injected injected were (Babyplast (Babyplastextracted in10T, 10T, an Molteno,isopropanolMolteno, Italy) Italy) bath at at at170 170 70 °C ◦°CC into intofor 24dog dog h andbone bone allowed samples, samples, to presenteddry presented at room in in temperature.Figure Figure 22.. Waxes were extracted in an isopropanolisopropanol bath bath at at 70 70 °C◦C for for 24 24 h h and and allowed allowed to to dry dry at at room room temperature. temperature.

Figure 2. InjectedInjected dog dog bone bone sample. sample. Figure 2. Injected dog bone sample. Systematic weight weight tracking tracking of of the the samples samples before before and after solvent extraction was was realized to ensureensureSystematic a a complete complete weight extraction. extraction. tracking As As ofthe the mass samples mass loss lossof beforeevery of every sampleand sampleafter was solvent at was least atextraction 96 least wt% 96 of wt%was the waxrealized of the weight wax to ensureintroducedweight a introduced complete in the extraction. inblends, the blends, we As concluded the we mass concluded loss that of thatwa everyx wax network sample network was was athighly least highly 96continuous continuouswt% of the in inwax the the weight whole whole introducedsample.sample. Its Its extractionin extraction the blends, created created we an concluded anopen-pores open-pores that networ wa networkx knetwork characteristic characteristic was highly of the of sample thecontinuous sample structure structurein the [20]. whole The [20]. sample.latterThe latter reflects Its reflects extraction binder–binder binder–binder created andan andopen-pores particle–binder particle–binder networ interactionsk interactions characteristic developed developed of the sampleinin each each structureblend blend and and [20]. can can The be be lattercharacterizedcharacterized reflects binder–binderby porosimetry. and particle–binder interactions developed in each blend and can be characterizedInIn thethe casecase by of porosimetry.of blends blends containing containing 50 vol% 50 vol% of Zirconia, of Zirconia, remaining remaining organic componentsorganic components were thermally were thermallydebindedIn the anddebinded case the of samples blends and the werecontaining samples sintered were 50in vol% sintered an oven of Zirconia,in to an produce oven remaining to dense produce ceramics. organic denseThermal componentsceramics. profile Thermal were and thermallyprofileprocedure and debinded details procedure are and respectively detailsthe samples are shownrespectively were insintered Figure shown in3 andan inoven Table Figure to2. produce Those 3 and thermal denseTable ceramics. procedures2. Those Thermalthermal were profileproceduresdeveloped and forwereprocedure this developed particular details for Zirconia arethis respectivelyparticular grade in Zirconia order shown to grade favor in Figure fullin order densification. 3 andto favor Table full 2. densification. Those thermal procedures were developed for this particular Zirconia grade in order to favor full densification.

Figure 3. Temperature history for thermal debinding and sintering steps.

Table 2. Thermal debinding and sintering steps details.

Temperature (◦C) Speed (◦C/h) Holding Time (min) 20 0 0 150 30 0 350 15 60 550 9 60 820 180 0 1450 150 120 Polymers 2020, 12, 2247 5 of 17

2.3. Characterization

2.3.1. Rheological Study Using a rheological model developed by Robert et al. [36] and detailed in the next section, viscosity measurements were used to investigate waxes miscibility in HDPE and PP in molten conditions. Those particular tests were realized on neat polymer/wax blends with increasing wax proportions (Table3). Polymer and wax were mixed for 15 min at 180 ◦C in an internal mixer (HAAKE Rheomix 600, ThermoFisher, Waltham, MA, USA). Samples of 25 mm diameter and 1 mm thick were shaped in a hot press at 180 ◦C. Rheological experiments in dynamic frequency sweep mode with parallel plates geometry were conducted at 180 ◦C using an ARES rheometer (TA Instruments, New Castle, DE, USA). Before applying a 0.8 mm gap and starting the experiment, the specimens were maintained 5 min at 180 ◦C. From 300 to 1 rad/s the strain was set at 10% and increased to 60% at lower frequencies to maintain measurements precision.

Table 3. Components (polymers and wax) concentrations of blends used for miscibility study.

HDPE (wt%) PP (wt%) Wax (wt%) 90 0 10 80 0 20 35 0 65 0 90 10 0 80 20

2.3.2. Scanning Electron Microscopy (SEM) To evaluate the dispersion state of the ﬁllers and porosity creation, scanning electron microscopy observations were realized (Quanta 250 FEG, ThermoFisher, Waltham, MA, USA). After waxes extraction, 1 mm thick samples were fractured in liquid nitrogen, coated with copper, and the secondary electron signal was analyzed.

2.3.3. Mercury Injection Porosimetry After wax extraction, mercury was injected in the porous sample at high pressures with AutoPore IV 9500 porosimeter (Micromeritics, Norcross, GA, USA), allowing the incremental ﬁlling of pores from 100 µm to 6 nm. Pore size distribution was approximated from the injected volume and the increasing mercury injection pressures. In fact, the pressure needed to access a pore corresponds to the pressure needed to access the throat of the pore [37,38]: 4γcosθ D = − P (1) 2 1 where D is the diameter of the pore throat, γ is the mercury surface tension (485.10− N.cm− ), θ is the contact angle between mercury and the sample wall (assumed at 130 ◦C), and P is the measured pressure (Pa). As the pressure is increased to enter small pores, the presence of larger pores inside the sample can be covered. Therefore, mercury porosimetry is a powerful comparative method which provides information about pore size distribution, total pore volume, and skeleton density.

2.3.4. Penetration Tests Penetration tests were realized with an indent on 2 mm thick injected samples (Figure4) using a dynamic mechanical analysis machine (DMA Q800, TA instruments, New Castle, DE, USA). A constant force of 1 N and a temperature rate of 3 ◦C/min were applied on the sample spanning a 50 to 200 ◦C temperature range. By registering indent displacement as a function of the temperature, the evolution of mechanical properties of the sample is measured. Mechanical losses were observed at temperatures corresponding to the melting temperature of the organic components presenting a Polymers 2020, 12, 2247 6 of 17 continuous phase in the sample. Thus, depending on the composition of the multicomponents blend, the number of mechanical losses can vary, giving an insight into phase organization.

Figure 4. Penetration tests set-up.

2.3.5. Density Measurements Quality of Zirconia sintered part was evaluated by density measurements. Those tests track remaining porosity, which is mainly responsible for low mechanical performances of the part. Density tests were realized via hydrostatic measurements on a ME204 Mettler-Toledo scale (Colombus, OH, USA). Samples were weighed in air and in water, and the hydrostatic density of the sample, ρsample, was calculated based on Archimedes’ principle: mair ρsample = ρwater (2) m mwater air − where mair and mwater correspond, respectively, to the sample weight in air and in water, and ρwater is the density of water at the measurement temperature. The measured ceramic densities are compared with 6.08, the maximum theoretical density of the selected Zirconia powder.

3. Results and Discussions

3.1. PP/Wax and PE/Wax Morphologies Due to a large viscosity ratio, low molar wax addition induces a dilution eﬀect of the backbone polymer strongly decreasing its viscosity [39,40]. Robert et al. studied paraﬃn wax miscibility in both PP and HDPE at the molten state via viscoelasticity measurements [36]. They developed a model based on the comparison between polymer/wax viscosity behaviors and generalized Carreau–Yasuda equation in order to conclude on blends homogeneity. In fact, considering an entangled regime of HDPE or PP chains, the zero shear viscosity of the homogeneous blend η0 can be expressed as a function of the backbone polymer viscosity η0P, its content in the blend ϕ, and a factor relative to the free volume correction with temperature (Equation (3)) aϕ. The latter can be expressed as a function of the temperature, the universal gas constant, and activation energies of the blend and the polymer [36].

4 η0 = η0Paϕϕ (3) Polymers 2020, 12, x FOR PEER REVIEW 7 of 18

In fact, considering an entangled regime of HDPE or PP chains, the zero shear viscosity of the homogeneous blend η0 can be expressed as a function of the backbone polymer viscosity η0P, its content in the blend φ, and a factor relative to the free volume correction with temperature

(Equation (3) 𝑎. The latter can be expressed as a function of the temperature, the universal gas constant, and activation energies of the blend and the polymer [36]. 𝜂 =𝜂 𝑎 𝜑 (3) Polymers 2020, 12, 2247 7 of 17 The influence of polymer dilution on relaxation time can be expressed by (Equation (4)) . 𝜏𝜑 =𝜏𝑎𝜑 (4) The inﬂuence of polymer dilution on relaxation time can be expressed by (Equation (4)) where τP represents the relaxation time of the neat polymer. Assuming the applicability of Cox– Merz rule (𝛾 ≡𝜔) the viscosity of polymer/wax blends can be described by the generalized τ(ϕ) = τ a ϕ 1.75 (4) Carreau–Yasuda equation (Equation (5)), where mP andϕ − a are respectively related to the power law slope and the transition between power law and zero-shear domains. where τP represents the relaxation time of the neat polymer. Assuming the applicability of Cox–Merz . rule (γ ω) the viscosity of polymer/wax blends can be described ⁄ by the generalized Carreau–Yasuda(5) ≡ 𝜂𝜔𝜑 =𝜂 𝑎 𝜑1 + 𝜏 𝑎 𝜑. equation (Equation (5)), where manda are respectively related to the power law slope and the transition betweenBy comparing power law andPP/wax zero-shear and HDPE/wax domains. viscosity behaviors to generalized Carreau–Yasuda equation, Robert et al. concluded that PP-based blends underwent phase separation above 5 wt% of a m 4h 1.75 i a wax while HDPE-blends were homogeneousη(ωϕ) = η0Paϕ upϕ 1to+ 30τ Pwt%aϕϕ of wax, in the molten state. They also(5) investigated the effect of crystallization on PP/wax and HDPE/wax morphology with polarized light measurements.By comparing It was PP/wax shown and HDPEthat low/wax molar viscosity mass behaviors component to generalized was rejected Carreau–Yasuda in interspherulitic equation, regionsRobert etduring al. concluded PP crystallization that PP-based [36]. blends DSC measurements underwent phase have separation also been above used 5in wt% the ofliterature wax while to investigateHDPE-blends structure were homogeneous of PE/wax and up PP/wa to 30 wt%x blends of wax, after in crystallization the molten state. [41,42]. They also investigated the eﬀectIn of this crystallization study, HDPE/wax on PP/ waxand PP/wax and HDPE blends/wax pres morphologyenting weight with ratios polarized of 90/10, light 80/20, measurements. and 35/65 wereIt was investigated. shown that Based low molar on the mass work component of Robert et was al., rejected experimental in interspherulitic viscosities were regions measured during and PP . comparedcrystallization to generalized [36]. DSC measurementsCarreau–Yasuda have equation also been by used applying in the factors literature of to1/𝑎 investigate𝜑 and structure 𝑎𝜑 on of thePE/ waxfrequencies and PP /andwax viscosities, blends after respectively crystallization [36] [41,42]. InIn the this case study, of HDPEHDPE/wax/wax andblends, PP/ waxexperimental blends presenting and expected weight viscosities ratios of are 90/ 10,comparable 80/20, and on 35 the/65 wholewere investigated. concentration Based range on (Figure the work 5a). ofWe Robert can conclude et al., experimental that HDPE/wax viscosities blends were form measured a unique and 1.75 4 homogeneouscompared to generalized phase. Therefore, Carreau–Yasuda HDPE and equation wax are byfully applying miscible factors in the of molten1/aϕϕ state.and aϕϕ on the frequenciesIn PP-based and viscosities, blends, a viscosity respectively difference [36]. is observed even at low wax contents (Figure 5b). PP and waxIn the present case ofa low HDPE miscibility/wax blends, in the experimental molten state, and creating expected multiphased viscosities blends. are comparable Those results on are the inwhole good concentration agreement with range the (Figure results5a). mentioned We can conclude in the thatliterature HDPE where/wax blends HDPE form develops a unique higher and miscibilityhomogeneous with phase. waxes Therefore, compared HDPE to PP [36,41,42]. and wax are fully miscible in the molten state.

FigureFigure 5.5. AbsoluteAbsolute complex complex viscosity viscosity of polymer of polymer/waxes/waxes blends blends with varying with waxvarying content: wax ( acontent:) HDPE/ wax(a) HDPE/waxand (b) PP/ wax.and (b The) PP/wax. model isThe from model the workis from of the Robert work et of al. Robert [36]. et al. [36].

In PP-based blends, a viscosity diﬀerence is observed even at low wax contents (Figure5b). PP and wax present a low miscibility in the molten state, creating multiphased blends. Those results are in good agreement with the results mentioned in the literature where HDPE develops higher miscibility with waxes compared to PP [36,41,42]. Miscibility results at the molten state can be compared to morphology information at the solid state observed from SEM observations. HDPE/wax and PP/wax blends containing 56 wt% of wax were studied after selective wax extraction (Figure6). Polymers 2020, 12, x FOR PEER REVIEW 8 of 18

Miscibility results at the molten state can be compared to morphology information at the solid state observed from SEM observations. HDPE/wax and PP/wax blends containing 56 wt% of wax were studied after selective wax extraction (Figure 6). In the case of HDPE/wax blends, the surface is quite homogeneous and reveals uniform porosity surrounded by thin HDPE walls at high magnification. At the solid state, HDPE and wax form two interpenetrating phases. Despite the high miscibility of wax in HDPE in the molten state observed previously, phase separation is observed during crystallization. However, HDPE/wax high affinity favors the creation of thin domains presenting high contact area. PP and wax form multiphased blends in the molten state and this behavior is amplified during cooling. Large cracks have been observed between PP grains at low magnification. The latter arises from wax segregation at PP grain boundaries during PP early crystallization (Table 1). At higher magnification, a thin surface roughness is detected on PP grains surface, attesting of uncompleted phase separation at the solid state. This observation can arise from partial miscibility of wax in PP in Polymersthe molten2020, 12state., 2247 Those results are in good accordance with Robert et al. study in which8 wax of 17 segregation was favored by PP crystallization [36].

Figure 6. SEM observations after waxes (56 wt%) solvent extraction: ( (aa)) and and ( (b)) HDPE/wax, HDPE/wax, and ( c) and (d) PPPP/wax./wax.

In theaddition, case of open-pores HDPE/wax blends,networks the created surface isby quite waxes homogeneous extraction andfrom reveals those uniformHDPE/wax porosity and surroundedPP/wax blends by have thin HDPEbeen characterized walls at high by magnification. mercury porosimetry At the solid (Figure state, 7). HDPE and wax form two interpenetratingHDPE/wax phases.presented Despite a unique the highlarge miscibility pore popula of waxtion incentered HDPE inat the200 moltennm. This state result observed is in previously,accordance phasewith homogeneous separation is observedpore structure during obse crystallization.rved in Figure However, 4. PP-based HDPE blends/wax showed high affi nitytwo favorsdifferent the pore creation populations of thin domainscentered, presenting respectively, high at contact1500 and area. 40 nm. Based on SEM observations, we PP and wax form multiphased blends in the molten state and this behavior is amplified during cooling. Large cracks have been observed between PP grains at low magnification. The latter arises from wax segregation at PP grain boundaries during PP early crystallization (Table1). At higher magnification, a thin surface roughness is detected on PP grains surface, attesting of uncompleted phase separation at the solid state. This observation can arise from partial miscibility of wax in PP in the molten state. Those results are in good accordance with Robert et al. study in which wax segregation was favored by PP crystallization [36]. In addition, open-pores networks created by waxes extraction from those HDPE/wax and PP/wax blends have been characterized by mercury porosimetry (Figure7). Polymers 2020, 12, x FOR PEER REVIEW 9 of 18 can conclude that the large population corresponds to wax segregation at grain boundaries, while the thinnest is related to surface roughness. In both cases, miscibility and structural conclusions are in good agreement. Wax miscibility in selectedPolymers 2020 backbone, 12, 2247 polymer at high temperature highly influence matrix morphology after9 of 17 crystallization.

FigureFigure 7. 7. Pore sizesize distributiondistribution after after waxes waxes (56 (56 wt%) wt%) solvent solvent extraction extraction in HDPE in HDPE/wax/wax and PP and/wax PP/wax blends. blends. HDPE/wax presented a unique large pore population centered at 200 nm. This result is in accordance with homogeneous pore structure observed in Figure4. PP-based blends showed two 3.2. HDPE/PP/wax Blends Morphology different pore populations centered, respectively, at 1500 and 40 nm. Based on SEM observations, we can concludeIn this that part, the blends large populationcomposed of corresponds HDPE, PP, toand wax wax segregation (56 wt%) atwith grain varying boundaries, HDPE/PP while ratios the (e.g.,thinnest 70/30, is related 50/50, to and surface 30/70) roughness. were studied and compared to binary morphology described previously.In both cases, miscibility and structural conclusions are in good agreement. Wax miscibility in selectedHDPE/PP backbone blends polymer have at highbeen temperature widely studied highly influencein the literature matrix morphology and are afterknown crystallization. to form multiphased structures in both molten and solid states [43–45]. After wax extraction, SEM observations3.2. HDPE/PP showed/wax Blends porous Morphology structures composed of two phases corresponding to HDPE and PP with varyingIn this part, proportions blends composed(Figure 8). ofThose HDPE, phases PP, andpresent wax different (56 wt%) pore with characteristics, varying HDPE which/PP ratios are comparable(e.g., 70/30, 50to /that50, and of neat 30/70) PP were and HDPE studied in and presence compared of wax, to binary observed morphology in Figure described4. previously. InHDPE the /casePP blends of 70/30 have HDPE/PP been widely ratio, studied the st inructure the literature is homogeneous and are known and to wax form seems multiphased to be preferentiallystructures in bothintegrated molten in and HDPE solid phase, states due [43 –to45 their]. After high wax affinity. extraction, As HDPE/PP SEM observations ratio decreases showed to 30/70,porous HDPE structures phase composed becomes ofsaturated two phases in wax corresponding and cracks toare HDPE progressively and PP with grown varying in the proportions samples due(Figure to wax8). Those segregation. phases present di fferent pore characteristics, which are comparable to that of neat PP and HDPE in presence of wax, observed in Figure4. In the case of 70/30 HDPE/PP ratio, the structure is homogeneous and wax seems to be preferentially integrated in HDPE phase, due to their high affinity. As HDPE/PP ratio decreases to 30/70, HDPE phase becomes saturated in wax and cracks are progressively grown in the samples due to wax segregation. Pore networks created during wax extraction have been analyzed via mercury porosimetry (Figure9). For the 70 /30 HDPE/PP ratio, we observe a unique pore population similar to that obtained with neat HDPE. This result is consistent with SEM observations, where wax seems mainly absorbed by HDPE phase (Figure8a). For both the 50/50 and 30/70 ratios, two distinct pore populations are observed. The thinnest is centered at 40 nm and its behavior presents a clear dependence on the PP content. This population seems to characterize the volume of wax dispersed in PP grain surface roughness. The second pore population is centered at 350 nm, presenting a characteristic size slightly larger than that obtained with neat HDPE. As the HDPE/PP ratio decreases, HDPE phase becomes saturated in wax leading to an enlargement of the dispersed wax network. This observation is also consistent with SEM observations, where the apparition of small cracks for HDPE content lower than 50% has been pointed out (Figure8b,c). Polymers 2020, 12, 2247 10 of 17

In conclusion, the selection of backbone binder and its interaction with wax has a critical impact on the structure of the organic matrix. To avoid crack development, due to wax segregation in the matrix, a HDPE/PP ratio of 70/30 or higher should be employed. Polymers 2020, 12, x FOR PEER REVIEW 10 of 18

FigureFigure 8. SEM 8. SEM observations observations of of HDPE HDPE/PP/wax/PP/wax morphologymorphology after after wax wax extraction, extraction, HDPE/PP: HDPE /(PP:a) 70/30, (a) 70 /30, (b) 50(/b50,) 50/50, and (andc) 30 (c/)70. 30/70. Polymers 2020, 12, x FOR PEER REVIEW 11 of 18 Pore networks created during wax extraction have been analyzed via mercury porosimetry (Figure 9). For the 70/30 HDPE/PP ratio, we observe a unique pore population similar to that obtained with neat HDPE. This result is consistent with SEM observations, where wax seems mainly absorbed by HDPE phase (Figure 8a). For both the 50/50 and 30/70 ratios, two distinct pore populations are observed. The thinnest is centered at 40 nm and its behavior presents a clear dependence on the PP content. This population seems to characterize the volume of wax dispersed in PP grain surface roughness. The second pore population is centered at 350 nm, presenting a characteristic size slightly larger than that obtained with neat HDPE. As the HDPE/PP ratio decreases, HDPE phase becomes saturated in wax leading to an enlargement of the dispersed wax network. This observation is also consistent with SEM observations, where the apparition of small cracks for HDPE content lower than 50% has been pointed out (Figure 8b,c). In conclusion, the selection of backbone binder and its interaction with wax has a critical impact on the structure of the organic matrix. To avoid crack development, due to wax segregation in the matrix, a HDPE/PP ratio of 70/30 or higher should be employed.

FigureFigure 9. Pore9. Pore size size distribution distribution from wax wax extraction extraction in inHDPE/PP/Wax HDPE/PP/Wax blends. blends.

3.3. Zirconia Addition in HDPE/PP/wax In this part, zirconia is added from 0 to 40 vol% in HDPE/PP/wax (70/30) blends. At such HDPE/PP ratios, previous results highlighted that wax is mainly dispersed in the HDPE phase, in which it is highly miscible. Using the model developed by Shivashakar et al. describing a suspension of monosized spheres, interparticle distance δ can be evaluated as a function of the solid loading ϕ, the maximum packing fraction of the fillers, and their diameter D (Equation (6)) [46]. 1 5 . 𝛿=𝐷 + −1 3. 𝜋. 𝛷 6 (6) Considering random close packed monomodal spheres presenting a maximum packing fraction of 0.64, theoretical interspacing parameter can be calculated as a function of solid loading (Table 4) [47].

Table 4. Interspacing parameter values in function of solid loadings.

Solid Loading (vol%) Interspacing Parameter (nm) 10 330 20 190 30 120 40 70 50 40

SEM observations were employed after wax extraction to identify morphology evolution with increasing solid contents (Figure 10). At low zirconia contents, clusters of zirconia particles with sizes ranging from 300 nm to 600 nm are dispersed in the organic matrix, leading to slight modifications in the matrix morphology (Figure 10a). As the solid loading increases, cluster sizes expand and organic domain sizes are restricted, leading to a decrease of the observed pore size. The latter, created by waxes extraction are easily identified in black on SEM images (Figure 10b,c).

Polymers 2020, 12, 2247 11 of 17

3.3. Zirconia Addition in HDPE/PP/Wax In this part, zirconia is added from 0 to 40 vol% in HDPE/PP/wax (70/30) blends. At such HDPE/PP ratios, previous results highlighted that wax is mainly dispersed in the HDPE phase, in which it is highly miscible. Using the model developed by Shivashakar et al. describing a suspension of monosized spheres, interparticle distance δ can be evaluated as a function of the solid loading φ, the maximum packing fraction of the ﬁllers, and their diameter D (Equation (6)) [46].

0.5 δ = D 1 + 5 1 (6) 3.π.Φ 6 − Considering random close packed monomodal spheres presenting a maximum packing fraction of 0.64, theoretical interspacing parameter can be calculated as a function of solid loading (Table4)[ 47].

Table 4. Interspacing parameter values in function of solid loadings.

Solid Loading (vol%) Interspacing Parameter (nm) 10 330 20 190 30 120 40 70 50 40

SEM observations were employed after wax extraction to identify morphology evolution with increasing solid contents (Figure 10). At low zirconia contents, clusters of zirconia particles with sizes ranging from 300 nm to 600 nm are dispersed in the organic matrix, leading to slight modiﬁcations in the matrix morphologyPolymers 2020, 12 (Figure, x FOR PEER 10 REVIEWa). As the solid loading increases, cluster sizes expand12 of 18 and organic domain sizes are restricted,Above 40 vol% leading of zirconia, to a fillers decrease seem to ofform the a percolating observed phase pore in size.the blends, The leading latter, to created a by waxes extraction are easilyfurther identiﬁeddecrease in organic in black domain on sizes. SEM The images structure (Figure and pores 10 becomeb,c). more homogeneous (Figure 10d) [6].

Figure 10. SEM observations after waxes solvent extraction (HDPE/PP ratio of 70/30), with increasing Figure 10. SEM observations after waxes solvent extraction (HDPE/PP ratio of 70/30), with increasing solid loadings of zirconia particles: (a) 10 vol%, (b) 20 vol%, (c) 30 vol%, and (d) 40 vol%. solid loadings of zirconia particles: (a) 10 vol%, (b) 20 vol%, (c) 30 vol%, and (d) 40 vol%. Wax network evolution has also been characterized by mercury porosimetry (Figure 11). Under 20 vol% of zirconia, the pore size distribution is barely modified by the addition of zirconia, presenting a large population centered at 200 nm. Between 20 vol% and 40 vol% of zirconia, the organic matrix is gradually confined between zirconia clusters, leading to a shift of the main pore population towards smaller pore sizes. In fact, above 20 vol% of zirconia, the theoretical interspacing parameter becomes lower than 200 nm, corresponding to the main pore size in the neat organic matrix. As a result, a secondary pore population is developed around 45 nm. This pore diameter is similar to the characteristic size of wax network dispersed in PP surface roughness. As the solid loading increases, the behavior of wax dispersed in HDPE phase becomes restricted, favoring the dispersion of wax at the surface of PP grains. For solid contents above 40 vol%, interparticle distances are expected to be lower than 100 nm. Particles are closed-packed and their interparticle distances are highly restricted [48]. As a result, the main pore population previously observed disappears and a new sharp pore population centered on 20 nm is created. The phenomenon can be linked to percolation phenomenon observed on SEM results. As the matrix behavior is highly restricted and this leads to smaller and sharper pore size distributions [5,18].

Polymers 2020, 12, 2247 12 of 17

Polymers 2020, 12, x FOR PEER REVIEW 13 of 18 Above 40 vol% of zirconia, ﬁllers seem to form a percolating phase in the blends, leading to a furtherLiu and decrease Tseng instudied organic the domain effect of sizes. solid content The structure on the developed and pores becomepore network more after homogeneous complete binder(Figure removal.10d) [6]. Depending on ceramic powder characteristics and debinding parameters, solid contentsWax between network 45 evolutionvol% and has70 vol% also were been studie characterizedd [5,6,18,49]. by mercuryAs these porosimetrycontents are higher (Figure than 11). fillersUnder percolation 20 vol% of threshold, zirconia, the shar porep pore size populations distribution iswere barely observed, modiﬁed presenting by the addition characteristic of zirconia, size shiftedpresenting towards a large smaller population pores as centered the solid at 200content nm. increased.

FigureFigure 11. 11. PorePore size size distribution distribution after after waxes waxes solvent solvent ex extractiontraction of of HDPE/PP/wax HDPE/PP/wax (70/30) (70/30) blends blends for for differentdiﬀerent zirconia zirconia particle concentrations.

InBetween addition, 20 vol% penetration and 40 vol% tests of were zirconia, used the to organic investigate matrix mechanical is gradually loss confined as a betweenfunction zirconia of the meltingclusters, temperature leading to a shiftof the of thebinders. main poreThose population observations towards allow smaller us to poreidentify sizes. the In continuous fact, above 20phases vol% andof zirconia, understand the theoreticalorganic binder interspacing morphology parameter and its becomes modification lower with than zirconia 200 nm, addition corresponding (Figure to 12). the mainUnder pore size40 vol% in the of neatzirconia, organic a unique matrix. and As critical a result, loss a secondary of mechanical pore performance population is is developed recorded aroundaround 130 45 nm. °C, Thiscorresponding pore diameter to HDPE is similar melting to the temperature. characteristic HDPE size ofphase, wax networkin which dispersed waxes are in mainlyPP surface dispersed, roughness. leads As mechanical the solid loadingperformances increases, of the the blends behavior and of constitutes wax dispersed the matrix, in HDPE while phase PP formsbecomes a dispersed restricted, phase. favoring the dispersion of wax at the surface of PP grains. BeyondFor solid 40 contents vol% of above fillers, 40 a vol%,second interparticle step is observed distances at 160 are expected°C, corresponding to be lower to thanPP melting 100 nm. temperature.Particles are closed-packedZirconia particles and theirare close-packed interparticle distancesleading to are a highlymodificati restrictedon of [48organic]. As amatrix result, structure,the main porein which population HDPE previouslyand PP phases observed become disappears co-continuous. and a new sharp pore population centered on 20 nm is created. The phenomenon can be linked to percolation phenomenon observed on SEM results. As the matrix behavior is highly restricted and this leads to smaller and sharper pore size distributions [5,18]. Liu and Tseng studied the effect of solid content on the developed pore network after complete binder removal. Depending on ceramic powder characteristics and debinding parameters, solid contents between 45 vol% and 70 vol% were studied [5,6,18,49]. As these contents are higher than fillers percolation threshold, sharp pore populations were observed, presenting characteristic size shifted towards smaller pores as the solid content increased. In addition, penetration tests were used to investigate mechanical loss as a function of the melting temperature of the binders. Those observations allow us to identify the continuous phases and understand organic binder morphology and its modification with zirconia addition (Figure 12).

Polymers 2020, 12, x FOR PEER REVIEW 13 of 18

Liu and Tseng studied the effect of solid content on the developed pore network after complete binder removal. Depending on ceramic powder characteristics and debinding parameters, solid contents between 45 vol% and 70 vol% were studied [5,6,18,49]. As these contents are higher than fillers percolation threshold, sharp pore populations were observed, presenting characteristic size shifted towards smaller pores as the solid content increased.

Figure 11. Pore size distribution after waxes solvent extraction of HDPE/PP/wax (70/30) blends for different zirconia particle concentrations.

In addition, penetration tests were used to investigate mechanical loss as a function of the melting temperature of the binders. Those observations allow us to identify the continuous phases and understand organic binder morphology and its modification with zirconia addition (Figure 12). Under 40 vol% of zirconia, a unique and critical loss of mechanical performance is recorded around 130 °C, corresponding to HDPE melting temperature. HDPE phase, in which waxes are mainly dispersed, leads mechanical performances of the blends and constitutes the matrix, while PP forms a dispersed phase. Beyond 40 vol% of fillers, a second step is observed at 160 °C, corresponding to PP melting Polymerstemperature.2020, 12, 2247 Zirconia particles are close-packed leading to a modification of organic matrix13 of 17 structure, in which HDPE and PP phases become co-continuous.

Polymers 2020, 12, x FOR PEER REVIEW 14 of 18

Figure 12. Penetration tests on blends HDPE/PP/wax (70/30) for different Zirconia particle contents.

3.4. Zirconia Addition with Varying HDPE/PP Ratios In this subsection, 20 vol% or 50 vol% Zirconia fillers were added to HDPE/PP/wax blends presenting different backbone binder compositions. Those samples were submitted to mercury intrusion analysis after wax extraction (Figure 13). Those results are compared to behaviors of neat organic matrices with varying HDPE/PP ratios (Figure 10). Without zirconia, two distinct pore populations were observed: the first one was highly influenced by PP content and centered at 40 nm, and the second was shifted from 1500 nm to 200 nm as HDPE/PP ratio is increased from 0/100 to 100/0. Those populations have been related to wax dispersed, respectively, in PP and HDPE phases. At 20 vol% zirconia addition, pore size distributions are also composed of two populations. The thinnest is centered at 40 nm, regardless of HDPE/PP ratio, and is enhanced by PP increasing content. This behavior is similar to the thinnest population observed on neat polymer matrices. Thus, Figure this 12. populationPenetration mainly tests on characterizes blends HDPE the/PP portion/wax (70of /wax30) for dispersed different in Zirconia PP surface particle roughness. contents. The second population is centered at 150 nm for HPDE/PP ratios above 30/70, and is enlarged to 220Under nm 40in the vol% case of of zirconia, neat PP (Figure a unique 13a). and As criticalobserved loss in ofFigure mechanical 9, when the performance matrix is confined is recorded between zirconia clusters, the larger pore size is shifted towards smaller values, leading to smaller around 130 ◦C, corresponding to HDPE melting temperature. HDPE phase, in which waxes are mainly pores than that developed in neat matrices. At 20 vol% of Zirconia, the PP-based blend generates a dispersed, leads mechanical performances of the blends and constitutes the matrix, while PP forms second population 30% larger than that of HDPE based blends. a dispersed phase. When 50 vol% of zirconia is added to create fully loaded feedstocks, pore distributions become monomodalBeyond 40 and vol% sharp of fillers, due to afillers second percolation step is an observedd organic at matrix 160 ◦ C,confinement. corresponding However, to PPas the melting temperature.HDPE/PP Zirconiaratio is increased particles from are close-packed0/100 to 100/0, leadingthe pore topopulation a modification is shifted of from organic 30 nm matrix to 20 structure, nm in which(Figure HDPE 13b). andSimilarly PP phases to blends become containing co-continuous. 20 vol% of zirconia, the PP-based blend generates pores 30% larger than that of the HDPE-based blends. 3.4. ZirconiaIn the Addition literature, with Wen Varying et al. HDPEmeasured/PP pore Ratios size distribution created in feedstock presenting 50 vol%In this solid subsection, content and 20 a vol%backbone or 50binder vol% composed Zirconia of fillers LDPE were(50 vol%) added and toHDPE HDPE (50/ PPvol%)/wax after blends solvent and thermal debinding [34]. They observed the creation of monomodal pore distributions presenting different backbone binder compositions. Those samples were submitted to mercury centered at 40 nm. This characteristic pore size is larger than that obtained in the present publication intrusionsince only analysis solvent after debinding wax extraction was performed. (Figure 13). Those results are compared to behaviors of neat organic matricesIn conclusion, with backbone varying polymer HDPE/ PPselection ratios has (Figure a critical 10 impact). Without on wax zirconia, dispersion, two influencing distinct pore populationsnot only were neat observed:organic matrix the first morphology one was highlybut also influenced filled blends by PPmorphology. content and Even centered in highly at 40 nm, and theconfined second systems was shifted such as from feedstock, 1500 nma modification to 200 nm of as 30% HDPE of the/PP pores ratio network is increased characteristic from 0/100 size to is 100/0. Thosemonitored populations between have HDPE/PP been related ratios toof wax100/0 dispersed, and 0/100. respectively, in PP and HDPE phases.

FigureFigure 13. Pore 13. Pore size size distribution distribution after after waxes waxes solvent solvent extraction extraction for for di differentﬀerent HDPE HDPE/PP/PP ratios:ratios: (a) 2020 vol% and (vol%b) 50 and vol% (b)of 50zirconia vol% of zirconia particles. particles.

At 20Finally, vol% samples zirconia presenting addition, 50 porevol% sizeof zirconia distributions particles arewere also thermally composed debinded oftwo and sintered populations. The thinnestwith adapted is centered thermal at procedures 40 nm, regardless (Figure 3). of HDPESintered/PP density ratio, andwas isme enhancedasured via by hydrostatic PP increasing

Polymers 2020, 12, 2247 14 of 17 content. This behavior is similar to the thinnest population observed on neat polymer matrices. Thus, this population mainly characterizes the portion of wax dispersed in PP surface roughness. The second population is centered at 150 nm for HPDE/PP ratios above 30/70, and is enlarged to 220 nm in the case of neat PP (Figure 13a). As observed in Figure9, when the matrix is confined between zirconia clusters, the larger pore size is shifted towards smaller values, leading to smaller pores than that developed in neat matrices. At 20 vol% of Zirconia, the PP-based blend generates a second population 30% larger than that of HDPE based blends. When 50 vol% of zirconia is added to create fully loaded feedstocks, pore distributions become monomodal and sharp due to fillers percolation and organic matrix confinement. However, as the HDPE/PP ratio is increased from 0/100 to 100/0, the pore population is shifted from 30 nm to 20 nm (Figure 13b). Similarly to blends containing 20 vol% of zirconia, the PP-based blend generates pores 30% larger than that of the HDPE-based blends. In the literature, Wen et al. measured pore size distribution created in feedstock presenting 50 vol% solid content and a backbone binder composed of LDPE (50 vol%) and HDPE (50 vol%) after solvent and thermal debinding [34]. They observed the creation of monomodal pore distributions centered at 40 nm. This characteristic pore size is larger than that obtained in the present publication since only solvent debinding was performed. In conclusion, backbone polymer selection has a critical impact on wax dispersion, influencing not only neat organic matrix morphology but also filled blends morphology. Even in highly confined systems such as feedstock, a modification of 30% of the pores network characteristic size is monitored between HDPE/PP ratios of 100/0 and 0/100. Finally, samples presenting 50 vol% of zirconia particles were thermally debinded and sintered with adapted thermal procedures (Figure3). Sintered density was measured via hydrostatic measurements in order to track remaining porosities which are highly detrimental for final ceramic parts mechanical performances (Table5). The results showed that the highest HDPE contents lead to the highest sintered density, 6.065, corresponding to 99.7% of the maximal theoretical density of zirconia powder.

Table 5. Ceramic densities after sintering process of 50 vol% ﬁlled HDPE/PP/wax blends. Note that the maximal theoretical density of zirconia ceramic is 6.08.

HDPE/PP Ratio Sintered Density 100/0 6.065 70/30 6.065 50/50 6.053 30/70 6.048 0/100 6.040

From previous results, we can assume that a higher wax miscibility in the backbone binder produces more homogeneous feedstocks, presenting thinner and sharper pores network and leading to higher sintering ability of the samples. In fact, pores network created during solvent debinding plays a critical role in both thermal debinding and sintering steps. A thin and homogeneous initial wax network reflects good fillers dispersion and general homogeneity of the whole organic matrix in the sample [6]. It favors the growth of thinner and more homogeneous pore channels after total debinding, which improves the evacuation of degradation products during thermal debinding and sintering ability [50,51], thus leading to higher final density and mechanical properties [5,49]. In the literature, Wen et al. obtained zirconia relative densities of 97.4%, 97.5%, and 98.6% for feedstocks respectively based on HDPE, PP, and HDPE/PP backbone binders [33]. They concluded that multi-polymer-based feedstocks presented higher densification ability. These lower densification abilities observed by Wen et al. can originate from either formula parameters, such as differences in wax/backbone binders affinity or processing parameters, such as the large heating rate of 5 ◦C/min employed during thermal debinding which can easily induce the enlargement of the pore network [6]. Polymers 2020, 12, 2247 15 of 17

4. Conclusions In the present publication, the use of HDPE, PP, and their blends as backbone binders in CIM feedstocks was investigated by studying the morphology of both neat organic matrices and zirconia filled blends, and the final ceramics properties. The miscibility of paraffin wax, employed as plasticizer, with backbone binders was first studied in the molten state. The high miscibility of wax in melted HDPE hinders its segregation during crystallization, favoring the creation of thin and interpenetrating domains. The resulting pore network created after wax extraction is characterized by a unique pore population. On the contrary, wax is only partially miscible in PP in the molten state, resulting in phase segregation at high temperature. During blend crystallization, wax is mainly rejected in PP interspherulitic regions and a small proportion is dispersed in PP grains surface roughness, creating a bimodal pore population shifted toward larger pores after wax extraction. As zirconia powder is added to create CIM feedstock, the pore size distribution resulting from wax extraction is shifted towards smaller pores. The fillers percolation is observed for solid loadings above 40 vol%, highly restricting the organic matrix organization. At such high filling ratios, the size of the pores created during wax extraction becomes sharp attesting to the homogeneity of the blend. As the PP content in backbone binder decreases from 100 wt% to 0 wt%, the characteristic size of the pore distribution is reduced by 30%. Such refinement allows the evacuation of degradation products while limiting diffusion distances, resulting in a high ceramic densification ability. Thus, a minimum content of 70 wt% of HDPE (relative to backbone binders content) is necessary to reach the highest final zirconia density of 6.065. The main results can be summarized as follows.

A high miscibility of plasticizer in backbone binders in the molten state limits phase segregation ◦ during crystallization, thus facilitating the creation a finer and more homogeneous wax network in organic matrices. As solid content increases, interparticle distances are restricted, and the wax network is shifted ◦ towards smaller pores. Above 40 vol%, the percolation of fillers induces major modifications in the organization of organic phases and the wax network becomes monomodal and sharp, attesting to blend homogeneity. As HDPE content in backbone binder increases in 50 vol% loaded feedstocks, a thinner wax ◦ network is promoted, restricting diffusion distances and favoring high ceramic densification ability. Using a minimum HDPE content of 70 wt%, sintered densities of 6.065 are obtained.

In the future, further investigations concerning the eﬀect of organic components structural features such as molar mass on feedstock behavior can be realized. Similar studies can also be realized in presence of other backbone polymers such as PVA, and other common plasticizers (e.g., carnauba or PE waxes).

Author Contributions: Supervision, P.C., R.F.; Validation, E.L. and Z.B.; Writing—original draft, C.D. All authors have read and agreed to the published version of the manuscript. Funding: The authors are grateful to the French National Association for Research and Technology and Saint-Gobain Provence for their financial support (CIFRE convention n_2017/0589). The authors are grateful to Saint-Gobain ZirPro for providing the powders. Conflicts of Interest: The authors declare no conflict of interest.

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