Thermoresponsive Gelcasting: Improved Drying of Gelcast Bodies

Xiaofeng Wang,z Richu Wang,w,z Chaoqun Peng,z Haipu Li,y Bing Liu,z and Zhiyong Wangz

zSchool of Materials Science and Engineering, Central South University, Changsha 410083, China

ySchool of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

An improved ceramic processing method, thermoresponsive gel-casting, has been developed. The method uses a rapid thermo-responsive gel system to solve the problem of inefficient dryingwithin the conventional gelcasting procedure. The gel system ofpoly(N-isopropylacrylamide) gel containing poly-(ethylene ox-ide) graft chains with a freely mobile end was taken as an ex-ample for thermoresponsive gelcasting of alumina. The gellingbehavior of organic aqueous solution was evaluated. The slurries(solids loading, 50 vol%) with the new gelcasting system wereprepared. The wetting ceramic parts were almost completelydried in 70 min at 501C. The microstructure of the green body ishomogeneous. The flexural strength of the sintered ceramics is282737 MPa with a theoretical density of 97.6%.

I. Introduction

GELCASTING is an attractive near-net-shape forming tech-nique for making high-quality complex-shaped ceramic

parts, which was developed by Janney et al.1–4 at the Oak RidgeNational Laboratory to overcome some drawbacks of the in-jection molding. In the typical gelcasting process, slurry madefrom ceramic powder and water-based monomer solution is castin a mold, followed by in situ polymerization to immobilize theparticles. And then the formed part is removed from the mold,dried and sintered in a conventional way. It has been applied tofabricate various ceramics in the past decade.5–8 However, de-spite its obvious advantages, gelcasting is not a very efficientproduction process because it has a time-consuming and tediousdrying step, especially for large-sized parts. To avoid warpageand cracking caused by unrelieved stresses in the body, themoisture should be removed slowly via the process of vapordiffusion through the porous solid, so the temperature and hu-midity of the drying furnace are controlled carefully.2,9,10 There-fore, drying gelcasting parts is the slowest step in the procedure,which almost becomes a bottleneck of the development of gel-casting from laboratory to industry.4,9–11 In order to solve thisproblem, Allied Signal Incorporated11 even designed a real-timecontrol system to control environment for drying ceramic parts.The study of Ghosal et al.10 reported that the drying cycle hadthree stages: constant-rate drying period, falling-rate period, andslow-rate period. The corresponding physical mechanisms werecapillary forces, evaporation and diffusion, respectively. On theother hand, a more efficient drying method, liquid desiccantdrying method, was developed.12 In this method, the wet greenpart immersed in liquid desiccant is dried due to the osmoticdifference between the liquid desiccant and the gelled polymer inthe part.12 However, although the drying method is improved,

the process is still inefficient. Thirty percent solvent of the gelpart withdrawn with an appropriate liquid desiccant required atleast three hours.13

In our experiments, we found that a dense skin layer entrap-ping water in the green gelcasting parts were formed on the sur-faces during drying. The reason is the nature of the pure gel thatforms a skin layer.14,15 Therefore, whether drying green gelcast-ing part with furnace or liquid desiccant, this skin layer leads tovery slow drying rate. Obviously, if the skin layer is not formedor the gel has channels for releasing water throughout the skinlayers, the efficiency of drying will be higher.

Fortunately, a rapid thermoresponsive poly(N-isopropylacryl-amide) (PIPAM) cross-linked gel containing graft chains with afreely mobile end was reported for biomedical technology byYoshida et al.16 and Kaneko et al.15,17 The graft chains are hy-drophilic at lower temperature but hydrophobic at higher temper-ature. Thus, the advantage of this gel system is that it can quicklyrelease water with the graft chains throughout the skin layer uponheating. The stimulus temperature depends on the kind, length(average molecular weight) and amount of the graft chain, and it isalways not higher than 401C.15–17 They reported that there are twoorganics that can be used as the graft chains: semitelechelic poly(N-isopropylacrylamide) (semitelechelic PIPAM)16 and acryloyl-ter-minated methoxy-poly-(ethylene oxide) (PEO).15 This suggests anovel method to improve the gelcasting procedure for solving theproblem of inefficient drying rate, which is forming ceramic partswith the interesting gel systems. In fact, this kind of gelcastingsystem can be specially designed. For example, some hydrophilicgraft chains might be designed and introduced into the conven-tional acrylamide gelcasting system. In general, we describe theimproved gelcasting procedure with thermoresponsive gel contain-ing hydrophilic graft chains as thermoresponsive gelcasting. In thepresent work, the gelcasting system of PIPAM gel containing PEOgraft chains was taken as an example, and alumina ceramic wasfabricated with this gelcasting system.

II. Experimental Procedures

(1) Synthesis of Acryloyl-Terminated Methoxy-PEO

In order to introduce graft chains into PIPAM cross-linked net-work, the acryloyl-terminated methoxy-PEO (PEO macro-monomer) that can copolymerize with N-isopropylacrylamide(IPAM) was synthesized in the beginning.15 PEO methyl ether(a-Hydroxy-o-methoxy-PEO) (10 g, Aladdin Regent Co.,Shanghai, China) of nominal molecular weight 5000 was dis-solved in 100 mL of tetrahydrofuran (THF, Xilong ChemicalCo., Santou, China). Triethylamine (3 mL, Shanghai ChemicalReagent Co., Shanghai, China) as a scavenger for hydrochloricacid and a small amount of tert-butyl catechol (Aladdin RegentCo., Shanghai, China) as a polymerization inhibitor were addedinto this solution. Acryloyl chloride (Aladdin Regent Co.) wasadded dropwise at 01C and the solution was kept at 401C for24 h with vigorous stirring. After the reaction, the solution was

K. Faber—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: wrc@mail.csu.edu.cnManuscript No. 28969. Received November 26, 2010; approved March 5, 2011.

Journal

J. Am. Ceram. Soc., 94 [6] 1679–1682 (2011)

DOI: 10.1111/j.1551-2916.2011.04557.x

r 2011 The American Ceramic Society

1679

separated from precipitated triethylamine hydrochloride by vac-uum filtration. The solution was poured into diethyl ether torecover PEO macromonomer.

(2) Gelcasting Steps

N-Isopropylacrylamide (IPAM; Aladdin Regent Co.) as amonomer, PEO macromonomer as graft chains and N,N0-meth-ylenebis(acrylamide) (MBAM; Shanghai Chemical ReagentCo.) as a cross-linker were dissolved in distilled water as pre-mix solutions(13 wt%). Mass ratio of IPAM, PEO macromono-mer to MBAM was 40:16:1. The alumina powder (MRS-1,Martin, Germany) with an average particle size of 1 mm (vendorspecification) was poured into the premix solutions for prepa-ration of ceramic slurries with a solids loading of 50 vol%. Fordispersion, 4 mg of NH4PAA (A-6114, Toagosei Chemical,Tokyo, Japan) per gram of Al2O3 was used. The pH of mix-tures were controlled to be around 8.5 with ammonia (NH4OH)solutions (Shanghai Chemical Reagent Co.) due to the ioniza-tion of the polyelectrolyte dispersant.18 These powder mixtureswere ball-milled using 10 mm diameter high-purity zirconia ballsin high-density poly(ethylene) bottle of a certain volume for24 h. For reference, a slurry (solids loading, 50 vol%) withoutthe organics was also prepared. Ammonium persulfate (APS,Shanghai Chemical Reagent Co.) as initiator and N,N,N0,N0-tetramethylethylenediamine (TEMED, Shanghai ChemicalReagent Co.) as catalyst were used to initiate the free radicalpolymerization in suspensions. Before casting into molds, theslurry was degassed under vacuum for several minutes toremove trapped air bubbles. Then the molds were kept at151C to make sure that the PEO macromonomer was hydro-philic and dissolved completely in suspension. After consolida-tion and demolding, the green bodies were dried at 501C inan air oven until no further loss of weight was observed,and then the temperature was raised gradually to 1201C for1 h. Both binder burnout and sintering were carried out in air.The firing schedule was 11C/min to 8001C, and subsequently to16801C for 2 h.

(3) Characterization

The rheological behaviors of suspensions with and without or-ganics were conducted with a stress-controlled rheometer(AR2000, TA, New Castle, PA) with a parallel plate (40 mmin diameter). The viscosity measurements were performed insteady-state mode with a steady increment of the shear rate from1 to 1000 s�1. Oscillatory mode was used to study the gellingbehaviors of the aqueous solution with different concentrationsof initiator and catalyst. The frequency was 1 Hz and the trainwas controlled to be 0.4%. All measurements were performed atconstant temperature (151C).

The flexural strength was measured by three-point tests usinga CSS-2205 universal testing machine (with a crosshead speedof 0.5 mm/min) (Changchun Research Institute for TestingMachines, Changchun, China). The dimension of all the testsamples was 30 mm� 4 mm� 3 mm. The bulk densities of sin-tered bodies were determined by the Archimedes method. Themicrostructures of the green body and sintered ceramics wereobserved by scanning electron microscope (SEM, Quanta-200,FEI company, Eindhoven, the Netherlands).

III. Results and Discussion

Similar to most gelcasting systems,2,19 the reaction of IPAM,PEO macromonomer, and MBAM is a typical radical polymer-ization, as shown in Fig. 1. The polymerization of the gel systemwas monitored by changes of storage modulus (G0).20 Figure 2shows the dynamic behavior of the premix solutions withdifferent amounts of initiator and catalyst. As described in theliterature,2,19 the gelation process of radical polymerization iscomposed of an induction period, the period between the addi-tion of the initiator/catalyst and the initiation of gelation, and an

reaction period, the period between the commencement andcompletion of gelation. The G0 within induction period remainssteady, which suggests that the solution is still smoothly flowing.The G0 within reaction period sharply increases, which indicatesthe increase of the viscosity and the formation of elastic gel. Formanipulation in gelcasing process such as mixing and casting,the induction period should be optimized. From Fig. 2, it canalso be seen that the induction period greatly depends on theamounts of initiator and catalyst. When the amount of initiatoris 1.25 mg/mL, the induction period is the shortest, around 26 s.However, it increases with decreasing the amount of initiator,and the induction period is 42500 s when the amount of ini-tiator is 0.25 mg/mL. Besides, the induction time is prolongedfrom 810 to 1020 s when the content of catalyst is decreasedfrom 2 to 4 mL/L (Both of the amounts of initiator are 0.5 mg/mL). Consequently, it is easy to control the gelcasting process bychanging the amounts of initiator and catalyst.

It is very important to attain slurry with low viscosity andsuitable solids loading for gelcasting process. Figure 3 shows theviscosity of slurries (solids loading, 50 vol%), with and withoutthe new gel system, as a function of the shear rate. It can be seenthat both slurries show shear-thinning behavior, but the viscos-ity is enhanced by the addition of the new gel system, especiallyat high shear rate. However, the viscosity at the shear rate of100 s�1 is around 0.157 Pa � s, which is still very low and suitablefor mixing, casting and attaining high green body density.

Fig. 1. Radical copolymerization of IPAM monomer with poly-(ethyl-ene oxide) macromonomer using MBAM as cross-linker.

Fig. 2. Storage modulus (G0) of premix solution with different amountsof initiator and catalyst during gelation at 151C.

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It is a key step to dry the green body in the gelcasting process.The drying curve of the alumina gelcasting part prepared withthe new gelcasting system is shown in Fig. 4. The drying ratewas also calculated using the mass fraction measurements, asshown in Fig. 4(b). It is shown that most of the water is re-moved in 70 min (the mass fraction of water in the as-gellledbody is about 20%), which is very efficient. The green bodycontains the PIPAM cross-linked gel with PEO graft chainswith a freely mobile end, which changes from hydrophilic tohydrophobic upon heating. Besides, the graft chains are pokingthrough the skin layer formed during drying. Therefore, thegreen body can quickly release water due to the rapid dehydra-tion of the graft chains in response to small temperature in-crease. In addition, it can also be seen that the drying processconsists of two stages: fast drying period (stage 1) and slowdrying period (stage 2). This suggests that the physical mech-anism is changed. The physical mechanism in stage 1 is the re-pulsive force between the graft chains and water, while that instage 2 might be the evaporation of water. The total averagelinear shrinkage in the drying step is 5%–6%.

The relative density of dried green bodies is 52.3% (2.08 g/cm3), and their average strength is 1573MPa. The average bulkdensity of sintered ceramic is 3.89 g/cm3 (97.6% of theoreticaldensity), and its average strength is 282737 MPa. These prop-erties are not less than those of alumina ceramics prepared withother gelcasting systems.2,21 The microstructures of the greenbody and sintered ceramics are shown in Fig. 5. Figure 5(a)shows that the powder is well dispersed, and significant draw-backs such as bigger conglomerations and larger pores are not

observed in the green body. Figure 5(b) suggests that the averagegrain size of the sintered body is enlarged, and there are someporosities mainly at the triple points and a few within the grains.Hence, the IPAM–PEO–MBAM system is suitable for gelcast-ing of alumina.

IV. Conclusions

In order to solve the problem of drying in conventional gelcast-ing procedure, a kind of rapid thermoresponsive gel system con-taining the graft chains with a freely mobile end can be used toimprove the gelcasting procedure, which is named as thermore-sponsive gelcasting. The gel system of PIPAM gel containingPEO graft chains was taken as an example. The viscosity of theslurry containing this new system with solids loading of 50 vol%is low, which is suitable for mixing, casting, and attaining highgreen body density. The induction period of polymerization canbe controlled conveniently by adjusting the amounts of initiatorand catalyst. The water of the ceramic parts can be released invery short time upon heating. The microstructure of the greenbody is homogeneous, and no large drawbacks are observed.Both the microstructure and the flexural strength of the sinteredceramic are similar to those of the ceramics prepared by theconventional gelcasting procedure.

Fig. 3. Influence of the new gelcasting system on rheological behaviorof slurries with solids loading of 50 vol%.

Fig. 4. Drying curves of the wetting green body prepared by aluminasuspensions with solids loading of 50 vol% at 501C. The mass fraction ofwater in the as-gellled body is about 20 wt%. The sample diameter andthickness are 5.0 and 3.2 cm, respectively. (a) The dynamic sample massfraction measured during drying, (b). the drying rate calculated using themass fraction measurements.

Fig. 5. Microstructure of the (a) green body and (b) sintered alumina ceramic. The sintered specimen was polished and thermally etched at 16001C for 1 h.

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