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