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Acrylic co-polymer emulsion binders for green machining of ceramics
European Polymer Journal 36 (2000) 1503±1510
Acrylic co-polymer emulsion binders for green machining of ceramics D.B. Rohini Kumar a, M. Rami Reddy b, V.N. Mulay b, N. Krishnamurti a,* a
Organic Coatings and Polymers, Indian Institute of Chemical Technology, Hyderabad 500 007 (A.P), India b Material Science, Indian Institute of Chemical Technology, Hyderabad 500 007 (A.P), India Received 12 October 1998; received in revised form 22 June 1999; accepted 12 August 1999
Abstract Manufacturing of complex shaped ceramic materials is becoming very complicated due to the diculties involved in the machining of ceramics. Therefore green machining of pressed forms is gaining more attention in the ceramic industry. In this communication acrylic-based co-polymer emulsions were prepared and used as binders in the ceramic materials such as Lanthanum chromite or strontium chromite. These were then pelletized. The green density and green strength of these pellets were determined. The acrylic co-polymers were characterized by 1H NMR spectroscopy to ensure the absence of unreacted monomer. Glass transition temperature (Tg) of all the co-polymers was calculated theoretically and their eect on the green strength of the ceramic-binder composites has been studied. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Ceramic machining has been widely used in advanced ceramic manufacturing. Machining processes can be used to make high precision, complex-shaped ceramic parts inaccessible through normal ceramic forming methods. Despite their excellent strength retention at high temperature, their oxidation resistance and their chemical stability, ceramics for high temperature structural applications continue to be popular only on a limited basis because of their brittle nature. A simple and inexpensive way to overcome this problem is to introduce weak interlayer that de¯ects cracks. This renders the
* Corresponding author. Fax: +91-40-7173387. E-mail address: [email protected] or [email protected] (N. Krishnamurti).
material notch insensitive and results in an apparent fracture toughness. Ceramic machining can be divided into two categories: ®red and green. Grinding and cutting wheels are generally used to machine ®red ceramics. Thus the abrasives used to produce these wheels must be harder than the ceramics being machined. Typical abrasives used are diamond or cubic boron nitride (CBN). The high cost of these wheels and slow feed rates associated with machining hard and brittle materials limit the mass production of ceramic parts by ®red process. Whereas green machining of pressed preforms is presently used for forming small and symmetrical parts such as spark plug insulators [1]. However, machining is limited to grinding or cutting and cannot withstand milling, drilling and lathing. The dry press process of forming clay products is not, as the name implies, one in which dry clay is used. Rather, it is a process in which the clay or mix used relatively dries (<3% free moisture) as compared
0014-3057/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 1 9 9 - 8
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D.B. Rohini Kumar et al. / European Polymer Journal 36 (2000) 1503±1510
to other common forming processes. Dry pressing of technical ceramics is a fundamental method of producing high quality ceramic components. The goal of dry pressing technical ceramics is to obtain products having uniform size and green density, consistent part-topart green density and defect free compact [2]. The binder is the most important additive for spray-dried ceramics. The binders are of two types (1) inorganic and (2) organic. Amongst these organic binders are in vast use. Nowadays the use of polymeric binders has increased even in the manufacture of Silicon carbide sheets [3]. The major issue in green machining is that the selection of a binder system that can provide sucient green strength. The binder is most important in dry pressed ceramics [4,5]. Binders have strong in¯uence on the ceramic granule properties such as bulk density, ¯ow rate and compaction behavior. A good binder for ceramic application should provide high green strength at a low usage level. Generally green density should decrease with binder addition. But the high performance binder has a less negative impact on green density [6]. The most used binders for dry pressed ceramics are PVA and PEG. PVA produces high green strength, whereas PEG produces high green density. An adequate Tg and polymer backbone structure minimizes the negative impart of the binder on green density. The glass transition temperature (Tg) of the polymer binder is one of the most important parameters controlling binder performance during dry pressing [7,8]. The Tg eects the green density and green strength of dry pressed ceramic parts. Keeping this in view polyacrylic binder emulsions based on methylacrylate, methylmethacrylate, 2-ethylhexylacrylate and styrene were prepared. These emulsions are ready to use dispersions having 0.05±0.5 mm particles in water.
2. Experimental
2.1. Materials Methylmethacrylate and 2-ethylhexylacrylate (BDH, Bombay), styrene (Fluka, Switzerland) were washed with 0.5% sodium hydroxide and distilled at atmospheric or reduced pressure to remove the polymerization inhibitors completely which otherwise eect the polymerization process to a great extent. Initiators potassium persulphate and sodium metabisulphite (Loba Chemie) were used as such without any further puri®cation. Emulsi®ers both anionic and nonionic types were of HICO, Bombay and are used as received.
2.2. Emulsion co-polymerisation Distilled water (100 ml) and surfactants (7.5 g) were taken in a three-neck round bottom ¯ask ®tted with overhead stirrer, condenser and a dropping funnel. The ¯ask was ¯ushed with nitrogen and this inert atmosphere was maintained throughout the experiment. The contents of the ¯ask were heated to 70±758C. The monomer mixture, as shown in Table 1, was added through dropping funnel by a delayed process over a period of 1±1.5 h. Initiators (0.05 g) of each were dissolved in distilled water (10 ml) separately and were added through the top of the condenser before the addition of the monomer mixture. After completion of the addition of the monomer mixture, the polymerization was continued for further 30 min. The emulsion was then cooled to 308C and stored in glass bottles. In all the experiments the percentage solids was maintained at 35% by weight. The viscosities of polymer emulsions were determined by using Brook®eld Viscometer (Spindle No. 1, 50 rpm). The viscosities (m.Pa s) of the emulsions are given in Table 1. 2.3. Nuclear magnetic resonance spectroscopy 1
H NMR technique was used essentially to identify the acrylic co-polymers. The spectra were recorded using Gemini 200 MHz coupled with computer attachments. This computer records the integration value of each peak. From these values, mol% of each p(monomer) present in co-polymer was estimated according to the procedure already described in the literature [9,10]. The emulsions were precipitated using acetic acid and separated polymer powder was ®ltered. This powder was washed several times with water to remove acetic acid till the washings are neutral to pH. Samples were then dried and were used in the form of solutions in CDCl3 at ambient temperature. Tetramethyl silane (TMS) was used as an internal standard. The nonappearance of peaks between 4±5 ppm (0C1CH2) indicate the absence of free monomer in the co-polymer. All the combinations of acrylic monomers contain ester group, which is indicated by the presence of a peak at 3.5±3.7 ppm. The 1H NMR spectra of co-polymers MA±MMA and 2-EHA±MMA are shown in Figs. 1 and 2, respectively. The identi®cation of each peak is recorded on the spectrum, which is self-explanatory and hence not discussed here again in detail. 2.4. Preparation of pellets Lanthanum chromite or Strontium chromite were mixed with 2%, 3.5% and 5% (by weight) acrylic co-polymer emulsion binder and the mixture was
D.B. Rohini Kumar et al. / European Polymer Journal 36 (2000) 1503±1510
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Table 1 Compositions of dierent acrylic monomers used in emulsion polymerisation Co-polymer
Comonomers ratio (by weight)
Comonomersa
Viscosity (m.Pa s)
Tg (K) calculated
1 2 3 4 5 6 7 8 9 10
60:40 40:60 90:10 65:35 30:68 66:33 60:40 50:50 55:43:2 30:68:2
S:MA S:MMA MA:MMA MA:MMA MA:MMA EHA:MMA EHA:MMA EHA:MMA EHA:MMA:MAA EHA:MMA:MAA
75 75 65 69 70 80 80 80 100 100
333.30 377.30 284.60 305.71 342.46 236.24 243.40 248.20 250.26 295.74
a
S, styrene; MMA, methylmethacrylate; MA, methylacrylate; 2-EHA, 2-ethyl hexylacryate; MAA, methacrylic acid.
tumbled in a stainless steel container on a roller mill for 20 min to ensure a uniform mixed product. Samples of acrylic-lanthanum chromite or strontium chromite were pressed in a 12.7 mm diameter cylindrical uniaxial die, mounted in a laboratory press. Samples were pressed at 34.5 MPa (5000 psi) for 30 s.
Samples were then dried at 608 C prior to their green strength measurement. 2.5. Green strength Axial and radial green strengths were determined at
Fig. 1. 1H NMR spectrum of co-polymer of (MA±MMA).
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loading rate 1 mm/min by means of a Mikrotech tensometer. Loading devices were padded on both sides with 0.5 mm thick cardboard pieces to minimize sample friction. Compression strength was calculated using the following equation a 2p=PDL where a is the maximum compression strength, p is the applied load at fracture, D is the diameter of the sample block and L is the length of the sample block [11,12].
3. Results and discussions 3.1. Nuclear magnetic resonance (NMR) 1 H NMR technique has been adopted to determine quantitatively the p(monomer) units present in various acrylic co-polymer systems. In the following Table 2, schematic structures of p(monomer) unit, its structure and their characteristic protons to estimate the mol%
of the p(monomer) units present in the co-polymer systems are given. In the present study MA±MMA co-polymers could not be estimated quantitatively as there are no characteristic peaks which are dierent from each other. In all the other co-polymers, the p(monomer) units in the co-polymers were estimated and the results are given in Table 3. 3.2. Acrylic binders Normally PVA and PEG are used as binders. PVA is highly eective and versatile formulating tool because of its solution stability, its compatibility with many common formulation ingredients and its wide range of grades availability [13]. But both these suer from their hydrophilic nature. However, acrylic binders seem to be better option to PVA and PEG due to their hydrophobic nature. These acrylic co-polymer binders can be made by solution, suspension and emulsion polymerization techniques. In the emulsion polymerization technique, the viscosity of the emulsions is found to be
Fig. 2. 1H NMR spectrum of co-polymer of (2-EHA and MMA).
D.B. Rohini Kumar et al. / European Polymer Journal 36 (2000) 1503±1510
quite independent of polymer molecular weight. Emulsion viscosity is much lower than that of solutions of similar concentrations and the strength of emulsion binders can be adjusted independent of its viscosity. The solution polymerized acrylic co-polymer in toluene with 35% solid content has been found to be in the range of 500 to 1000 m.Pa s. Therefore, when the solution polymerized acrylic co-polymers were mixed with the ceramic materials and when pressed into the form of pellets and on storage due to the evaporation of solTable 2 Schematic structure of the p(monomer) units and their characteristic
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vents, the bulk density of the ceramic material decreased due to the formation of voids in the shaped material. The bulk density (green density) of such systems with 2% polymer solution loading was found to be in the range of 1.25 to 1.50 g/ml. Due to this reason, the axial strength of 2% loaded pellets was found to be in the region of 10±15 MPa. The radial tensile strength was very much lower and the pellets broke into pieces during the loading of the pellets in the tensometer. The solution polymerized acrylic co-
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polymers exhibited many such disadvantages like decreased bulk density and decreased green strength. Another disadvantage is that the solvents cause the ®re hazards and pollution problems. The suspension polymerized acrylic binders cannot be easily mixed uniformly with the ceramic material due to their non-wetting characteristic nature and also the varied particle sizes of the acrylic co-polymers. Therefore, we have adopted emulsion polymerized acrylic binders, as these polymers produced a wide range of properties to meet the requirements of dierent ceramic forming methods. These emulsions have polymer particle size in the range of 0.05±0.5 mm. Usually the viscosity of an emulsion is much lower than that of polymer solutions of equivalent concentrations (Table 1). This low viscosity allows the preparation of ceramic slurries at high loading values. When the ceramic slurries are dried, the acrylic copolymers form a thin coating over the ceramic particle and thus impart hydrophobicity. This is the result of coalescence of the minute resin particles suspended in aqueous medium as the water evaporates. This results in better process control for ceramic manufacture. Another advantage of polyacrylic emulsion binders is that these co-polymers can be tailor made to produce a wide range of properties to meet the requirements of dierent ceramic forming materials. The mechanical properties of granules are controlled by the glass transition temperature (Tg K) of binder phase and the ambient temperature during compaction. The densi®cation takes place in ceramic machining by three predominant mechanisms: (1) rearrangement of granule; (2) densi®cation by elimination of intergranule porosity. This is accomplished by granules plastically deforming and fractioning. The type of fracture depends on the moisture content of the binder. At low humidities the fracture is clear and brittle. At high humidities there is considerable stretching of the binder due to its high elongation properties. The fracture is now more plastic in nature; and (3)
intragranular porosity decreases and densi®cation occurs primarily due to particle sliding and rearrangement into a denser packing con®guration. At higher pressures particle fragmentation may become important [8]. Tg plays major role in selecting polymer binder. The advantage of selecting acrylic monomers is that one can get the desired product with desired Tg of the copolymer. The Tgs of co-polymers prepared in this present study are given in Table 1. These Tgs were calculated theoretically by using the following equation [14]. 1=Tg w1 =Tg1 w2 =Tg2 Where w1 and w2 are the weight fractions of components 1 and 2, Tg1 and Tg2 are the glass transition temperatures of individual homopolymers. The eect of Tg of a co-polymer binder on the green strength of the ceramics has been discussed later in this communication.
3.3. Green density Green density is one of the most important parameters in forming the ceramic articles. For a given ceramic powder, higher green density indicates better packing and results in lesser shrinkage during ®ring. For all the acrylic binders studied, the green density decreased linearly with the increase in the amount of binder. The volume occupied by the binder phase caused separation of ceramic particles, leading to the decrease in observed density. Optimum loading of binder in ceramic materials is necessary for obtaining the highest density and most uniform compact [15]. Among the formulations, co-polymers No. 5 and No. 7 gave high green density at 2% binder loading. The co-polymer No. 7 gave high green density at all percentages of loading (Table 4). This is due to the plastisizing eect of EHA.
Table 3 The quantitative estimation of p(monomer) units (mol%) in acrylic co-polymers Serial number 1 2 3 4 5 6 7 8 9 10
S:MA S:MMA MA:MMA MA:MMA MA:MMA EHA:MMA EHA:MMA EHA:MMA EHA:MMA:MAA EHA:MMA:MAA
Co-polymers
Co-polymers
As estimated by NMR (mol%)
60:40 40:60 90:10 65:35 30:70 66:34 60:40 50:50 55:43:2 30:68:2
55.4:44.6 39.1:60.9 91.3:8.7 68.4:31.6 33.3:66.7 51.4:48.6 44.9:55.1 35.2:64.8 39.8:57.2:3.0 18.8:78.5:2.7
53:47 37:63 ± ± ± 48:52 42:58 33:67 36:62.5:1.5 18:80:2
D.B. Rohini Kumar et al. / European Polymer Journal 36 (2000) 1503±1510 Table 4 Green density of ceramic materials (Strontium Chromite) with dierent acrylic monomer compositions Co-polymers (serial number)
1 2 3 4 5 6 7 8 9 10
Green density (g/ml) 2%
3%
5%
2.50 2.48 2.67 2.54 2.84 2.58 2.85 2.55 2.63 2.55
2.48 2.45 2.54 2.51 2.48 2.56 2.67 2.50 2.60 2.51
2.20 2.40 2.45 2.42 2.48 2.56 2.60 2.46 2.60 2.46
3.4. Green strength The green strength of the ceramic materials increased linearly with the increasing amounts of binder incorporated into the ceramic base. It is observed that MA±MMA (30:70) (co-polymer 5) and EHA± MMA±MAA (55:43:2) (co-polymer No. 9) co-polymers gave highest green strength to the ceramic material (Table 5). The advantage of these two copolymers is that, even at low levels of their loading as binders in ceramic materials, high green strength of the composite material is obtained. Thereby the machining properties of these composites are much superior. It has also been observed that as the percentage of MMA increased in the co-polymers (Nos. 3, 4, 5 of Table 1) there is a gradual increase in the green strength of the ceramic material. This is due to the in-
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¯uence of glass transition temperature (Tg) of the acrylic binder systems. The Tg of MA±MMA co-polymer systems (284±342 K) increased with the increase in the percentage of MMA. A similar observation (Tg 236±295 K) was also noticed in another set of co-polymers containing EHA and MMA monomers (co-polymers 6±10). These observations seems to be in agreement with the previous studies [6,16] and EHA has more plasticizing eect on the product. MMA is playing major role in controling Tg of the polymer. In the binder systems containing EHA±MMA copolymers as the percentage of EHA increased, the ¯exibility of the co-polymer also increased considerably. The ceramic materials have exhibited an excessive spring back property. This is due to the deformation allowed stress relief during the pressing operation (Table 5). In case of co-polymer No. 3 the theoretical Tg is 284.60 K. The green strengths were determined at 298 K, which is above its Tg and the plasticizing eect of rubbery co-polymer predominated and the green strength was less compared to the other two. In copolymer No. 1 though the Tg is 333.30 K which is less compared to co-polymer No. 2, the green strength is less because of the crystallinity of polystyrene.
4. Conclusions It is concluded from this study that the properties of binder play a major role in ceramics that is ultimately very helpful in improving green strengths of the ceramic machining. The emulsion polymerized acrylic binders are best suited for ceramic processing. It has been found that the acrylic co-polymers of MMA with MA (70:30) and with EHA, MMA and
Table 5 Green strength of ceramic materials with dierent acrylic co-polymer compositions at dierent percentages Green strength (MPa) Co-polymers (serial numbers from Table 1)
1 2 3 4 5 6 7 8 9 10
Axial
Radial
2%
3.5%
5%
2%
3.5%
5%
22 32 33 37 56 32 29 30 47 23
20 26 25 66 68 19 27 33 53 31
23 24 16 19 35 15 17 32 25 33
0.70 0.77 1.88 1.60 0.52 0.70 0.85 0.81 1.91 1.19
1.91 0.83 7.73 1.74 0.66 0.71 1.05 1.73 2.56 1.74
± 0.61 2.19 1.24 0.59 1.24 1.45 1.32 2.31 2.06
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MAA (55:43:2) are good binders for the dry pressing of the ceramic materials. These types of polymers can be tailor made to suit to the end application. Glass transition temperature also in¯uences the binder behavior in the ceramic processing.
References [1] Sheppard LM. The challenges of ceramic machining continue. Am Ceram Soc Bull 1992;71(11):590±610. [2] Lewis Jr WA. Ceram Eng Sci Proc 1996;17(1):137±43. [3] Vandeperre L, Beist Vanders O, Bouyer F, Foissy A. Am Ceram Soc Bull 1978;77(1):53±8. [4] Lukasiewiez SJ, Reed JS. Am Ceram Soc Bull 1978;52(9):798. [5] Reed JS, Runk RB. `Dry pressing' in ceramic processes. In: Wang FFY, editor. Treatise on material science and technology, vol. 9. New York: Academic Press, 1976. p. 77±98. [6] Wu XK, Whitman DW, Kaufel WL, Finch WC, Cumbers DI. Am Ceram Soc Bull 1997;76(1):49±52.
[7] Nies CW, Messing GL. J Am Ceramic Soc 1984;67(4):301. [8] Dimilia RA, Reed JS. Am Ceram Soc Bull 1983;62(4):484. [9] Yeagle ML. In: Craver CD, editor. Polymer characterisation interdisciplinary approaches. New York: Plenum Press, 1971. p. 201. [10] Vijayalakshmi V, Vani JNR, Krishnamurti N. Polym Paint Col J 1991;181(4290):506. [11] Rudnick A, Hunter AR, Holden FC. An analysis of the diametrical compression test. Mater Res Stand 1963;3(4):283±9. [12] Negre F, Sanchez E, Garcia J, Gines F, Carty WM. Am Ceram Soc Bull 1998;77(1):63±8. [13] Bassner SL, Klingenberg EH. Am Ceram Soc Bull 1998;77(6):71±5. [14] Roe RJ. In: Mark HF, Bikales NM, Overberger CG, Menges G, editors. Encyclopedia of polymer science and engineering. New York: John Wiley & Sons, 1987. p. 539. [15] Lannutti JJ, Deis TA, Kong CM, Phillips DH. Am Ceram Soc Bull 1997;76(1):53. [16] Wu XK, Whitman DW, Kaufel WL, Finch WC, Cumbers DI. Am Ceram Soc Bull 1995;74(5):61.
Acrylic co-polymer emulsion binders for green machining of ceramics D.B. Rohini Kumar a, M. Rami Reddy b, V.N. Mulay b, N. Krishnamurti a,* a
Organic Coatings and Polymers, Indian Institute of Chemical Technology, Hyderabad 500 007 (A.P), India b Material Science, Indian Institute of Chemical Technology, Hyderabad 500 007 (A.P), India Received 12 October 1998; received in revised form 22 June 1999; accepted 12 August 1999
Abstract Manufacturing of complex shaped ceramic materials is becoming very complicated due to the diculties involved in the machining of ceramics. Therefore green machining of pressed forms is gaining more attention in the ceramic industry. In this communication acrylic-based co-polymer emulsions were prepared and used as binders in the ceramic materials such as Lanthanum chromite or strontium chromite. These were then pelletized. The green density and green strength of these pellets were determined. The acrylic co-polymers were characterized by 1H NMR spectroscopy to ensure the absence of unreacted monomer. Glass transition temperature (Tg) of all the co-polymers was calculated theoretically and their eect on the green strength of the ceramic-binder composites has been studied. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Ceramic machining has been widely used in advanced ceramic manufacturing. Machining processes can be used to make high precision, complex-shaped ceramic parts inaccessible through normal ceramic forming methods. Despite their excellent strength retention at high temperature, their oxidation resistance and their chemical stability, ceramics for high temperature structural applications continue to be popular only on a limited basis because of their brittle nature. A simple and inexpensive way to overcome this problem is to introduce weak interlayer that de¯ects cracks. This renders the
* Corresponding author. Fax: +91-40-7173387. E-mail address: [email protected] or [email protected] (N. Krishnamurti).
material notch insensitive and results in an apparent fracture toughness. Ceramic machining can be divided into two categories: ®red and green. Grinding and cutting wheels are generally used to machine ®red ceramics. Thus the abrasives used to produce these wheels must be harder than the ceramics being machined. Typical abrasives used are diamond or cubic boron nitride (CBN). The high cost of these wheels and slow feed rates associated with machining hard and brittle materials limit the mass production of ceramic parts by ®red process. Whereas green machining of pressed preforms is presently used for forming small and symmetrical parts such as spark plug insulators [1]. However, machining is limited to grinding or cutting and cannot withstand milling, drilling and lathing. The dry press process of forming clay products is not, as the name implies, one in which dry clay is used. Rather, it is a process in which the clay or mix used relatively dries (<3% free moisture) as compared
0014-3057/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 1 9 9 - 8
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D.B. Rohini Kumar et al. / European Polymer Journal 36 (2000) 1503±1510
to other common forming processes. Dry pressing of technical ceramics is a fundamental method of producing high quality ceramic components. The goal of dry pressing technical ceramics is to obtain products having uniform size and green density, consistent part-topart green density and defect free compact [2]. The binder is the most important additive for spray-dried ceramics. The binders are of two types (1) inorganic and (2) organic. Amongst these organic binders are in vast use. Nowadays the use of polymeric binders has increased even in the manufacture of Silicon carbide sheets [3]. The major issue in green machining is that the selection of a binder system that can provide sucient green strength. The binder is most important in dry pressed ceramics [4,5]. Binders have strong in¯uence on the ceramic granule properties such as bulk density, ¯ow rate and compaction behavior. A good binder for ceramic application should provide high green strength at a low usage level. Generally green density should decrease with binder addition. But the high performance binder has a less negative impact on green density [6]. The most used binders for dry pressed ceramics are PVA and PEG. PVA produces high green strength, whereas PEG produces high green density. An adequate Tg and polymer backbone structure minimizes the negative impart of the binder on green density. The glass transition temperature (Tg) of the polymer binder is one of the most important parameters controlling binder performance during dry pressing [7,8]. The Tg eects the green density and green strength of dry pressed ceramic parts. Keeping this in view polyacrylic binder emulsions based on methylacrylate, methylmethacrylate, 2-ethylhexylacrylate and styrene were prepared. These emulsions are ready to use dispersions having 0.05±0.5 mm particles in water.
2. Experimental
2.1. Materials Methylmethacrylate and 2-ethylhexylacrylate (BDH, Bombay), styrene (Fluka, Switzerland) were washed with 0.5% sodium hydroxide and distilled at atmospheric or reduced pressure to remove the polymerization inhibitors completely which otherwise eect the polymerization process to a great extent. Initiators potassium persulphate and sodium metabisulphite (Loba Chemie) were used as such without any further puri®cation. Emulsi®ers both anionic and nonionic types were of HICO, Bombay and are used as received.
2.2. Emulsion co-polymerisation Distilled water (100 ml) and surfactants (7.5 g) were taken in a three-neck round bottom ¯ask ®tted with overhead stirrer, condenser and a dropping funnel. The ¯ask was ¯ushed with nitrogen and this inert atmosphere was maintained throughout the experiment. The contents of the ¯ask were heated to 70±758C. The monomer mixture, as shown in Table 1, was added through dropping funnel by a delayed process over a period of 1±1.5 h. Initiators (0.05 g) of each were dissolved in distilled water (10 ml) separately and were added through the top of the condenser before the addition of the monomer mixture. After completion of the addition of the monomer mixture, the polymerization was continued for further 30 min. The emulsion was then cooled to 308C and stored in glass bottles. In all the experiments the percentage solids was maintained at 35% by weight. The viscosities of polymer emulsions were determined by using Brook®eld Viscometer (Spindle No. 1, 50 rpm). The viscosities (m.Pa s) of the emulsions are given in Table 1. 2.3. Nuclear magnetic resonance spectroscopy 1
H NMR technique was used essentially to identify the acrylic co-polymers. The spectra were recorded using Gemini 200 MHz coupled with computer attachments. This computer records the integration value of each peak. From these values, mol% of each p(monomer) present in co-polymer was estimated according to the procedure already described in the literature [9,10]. The emulsions were precipitated using acetic acid and separated polymer powder was ®ltered. This powder was washed several times with water to remove acetic acid till the washings are neutral to pH. Samples were then dried and were used in the form of solutions in CDCl3 at ambient temperature. Tetramethyl silane (TMS) was used as an internal standard. The nonappearance of peaks between 4±5 ppm (0C1CH2) indicate the absence of free monomer in the co-polymer. All the combinations of acrylic monomers contain ester group, which is indicated by the presence of a peak at 3.5±3.7 ppm. The 1H NMR spectra of co-polymers MA±MMA and 2-EHA±MMA are shown in Figs. 1 and 2, respectively. The identi®cation of each peak is recorded on the spectrum, which is self-explanatory and hence not discussed here again in detail. 2.4. Preparation of pellets Lanthanum chromite or Strontium chromite were mixed with 2%, 3.5% and 5% (by weight) acrylic co-polymer emulsion binder and the mixture was
D.B. Rohini Kumar et al. / European Polymer Journal 36 (2000) 1503±1510
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Table 1 Compositions of dierent acrylic monomers used in emulsion polymerisation Co-polymer
Comonomers ratio (by weight)
Comonomersa
Viscosity (m.Pa s)
Tg (K) calculated
1 2 3 4 5 6 7 8 9 10
60:40 40:60 90:10 65:35 30:68 66:33 60:40 50:50 55:43:2 30:68:2
S:MA S:MMA MA:MMA MA:MMA MA:MMA EHA:MMA EHA:MMA EHA:MMA EHA:MMA:MAA EHA:MMA:MAA
75 75 65 69 70 80 80 80 100 100
333.30 377.30 284.60 305.71 342.46 236.24 243.40 248.20 250.26 295.74
a
S, styrene; MMA, methylmethacrylate; MA, methylacrylate; 2-EHA, 2-ethyl hexylacryate; MAA, methacrylic acid.
tumbled in a stainless steel container on a roller mill for 20 min to ensure a uniform mixed product. Samples of acrylic-lanthanum chromite or strontium chromite were pressed in a 12.7 mm diameter cylindrical uniaxial die, mounted in a laboratory press. Samples were pressed at 34.5 MPa (5000 psi) for 30 s.
Samples were then dried at 608 C prior to their green strength measurement. 2.5. Green strength Axial and radial green strengths were determined at
Fig. 1. 1H NMR spectrum of co-polymer of (MA±MMA).
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D.B. Rohini Kumar et al. / European Polymer Journal 36 (2000) 1503±1510
loading rate 1 mm/min by means of a Mikrotech tensometer. Loading devices were padded on both sides with 0.5 mm thick cardboard pieces to minimize sample friction. Compression strength was calculated using the following equation a 2p=PDL where a is the maximum compression strength, p is the applied load at fracture, D is the diameter of the sample block and L is the length of the sample block [11,12].
3. Results and discussions 3.1. Nuclear magnetic resonance (NMR) 1 H NMR technique has been adopted to determine quantitatively the p(monomer) units present in various acrylic co-polymer systems. In the following Table 2, schematic structures of p(monomer) unit, its structure and their characteristic protons to estimate the mol%
of the p(monomer) units present in the co-polymer systems are given. In the present study MA±MMA co-polymers could not be estimated quantitatively as there are no characteristic peaks which are dierent from each other. In all the other co-polymers, the p(monomer) units in the co-polymers were estimated and the results are given in Table 3. 3.2. Acrylic binders Normally PVA and PEG are used as binders. PVA is highly eective and versatile formulating tool because of its solution stability, its compatibility with many common formulation ingredients and its wide range of grades availability [13]. But both these suer from their hydrophilic nature. However, acrylic binders seem to be better option to PVA and PEG due to their hydrophobic nature. These acrylic co-polymer binders can be made by solution, suspension and emulsion polymerization techniques. In the emulsion polymerization technique, the viscosity of the emulsions is found to be
Fig. 2. 1H NMR spectrum of co-polymer of (2-EHA and MMA).
D.B. Rohini Kumar et al. / European Polymer Journal 36 (2000) 1503±1510
quite independent of polymer molecular weight. Emulsion viscosity is much lower than that of solutions of similar concentrations and the strength of emulsion binders can be adjusted independent of its viscosity. The solution polymerized acrylic co-polymer in toluene with 35% solid content has been found to be in the range of 500 to 1000 m.Pa s. Therefore, when the solution polymerized acrylic co-polymers were mixed with the ceramic materials and when pressed into the form of pellets and on storage due to the evaporation of solTable 2 Schematic structure of the p(monomer) units and their characteristic
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vents, the bulk density of the ceramic material decreased due to the formation of voids in the shaped material. The bulk density (green density) of such systems with 2% polymer solution loading was found to be in the range of 1.25 to 1.50 g/ml. Due to this reason, the axial strength of 2% loaded pellets was found to be in the region of 10±15 MPa. The radial tensile strength was very much lower and the pellets broke into pieces during the loading of the pellets in the tensometer. The solution polymerized acrylic co-
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polymers exhibited many such disadvantages like decreased bulk density and decreased green strength. Another disadvantage is that the solvents cause the ®re hazards and pollution problems. The suspension polymerized acrylic binders cannot be easily mixed uniformly with the ceramic material due to their non-wetting characteristic nature and also the varied particle sizes of the acrylic co-polymers. Therefore, we have adopted emulsion polymerized acrylic binders, as these polymers produced a wide range of properties to meet the requirements of dierent ceramic forming methods. These emulsions have polymer particle size in the range of 0.05±0.5 mm. Usually the viscosity of an emulsion is much lower than that of polymer solutions of equivalent concentrations (Table 1). This low viscosity allows the preparation of ceramic slurries at high loading values. When the ceramic slurries are dried, the acrylic copolymers form a thin coating over the ceramic particle and thus impart hydrophobicity. This is the result of coalescence of the minute resin particles suspended in aqueous medium as the water evaporates. This results in better process control for ceramic manufacture. Another advantage of polyacrylic emulsion binders is that these co-polymers can be tailor made to produce a wide range of properties to meet the requirements of dierent ceramic forming materials. The mechanical properties of granules are controlled by the glass transition temperature (Tg K) of binder phase and the ambient temperature during compaction. The densi®cation takes place in ceramic machining by three predominant mechanisms: (1) rearrangement of granule; (2) densi®cation by elimination of intergranule porosity. This is accomplished by granules plastically deforming and fractioning. The type of fracture depends on the moisture content of the binder. At low humidities the fracture is clear and brittle. At high humidities there is considerable stretching of the binder due to its high elongation properties. The fracture is now more plastic in nature; and (3)
intragranular porosity decreases and densi®cation occurs primarily due to particle sliding and rearrangement into a denser packing con®guration. At higher pressures particle fragmentation may become important [8]. Tg plays major role in selecting polymer binder. The advantage of selecting acrylic monomers is that one can get the desired product with desired Tg of the copolymer. The Tgs of co-polymers prepared in this present study are given in Table 1. These Tgs were calculated theoretically by using the following equation [14]. 1=Tg w1 =Tg1 w2 =Tg2 Where w1 and w2 are the weight fractions of components 1 and 2, Tg1 and Tg2 are the glass transition temperatures of individual homopolymers. The eect of Tg of a co-polymer binder on the green strength of the ceramics has been discussed later in this communication.
3.3. Green density Green density is one of the most important parameters in forming the ceramic articles. For a given ceramic powder, higher green density indicates better packing and results in lesser shrinkage during ®ring. For all the acrylic binders studied, the green density decreased linearly with the increase in the amount of binder. The volume occupied by the binder phase caused separation of ceramic particles, leading to the decrease in observed density. Optimum loading of binder in ceramic materials is necessary for obtaining the highest density and most uniform compact [15]. Among the formulations, co-polymers No. 5 and No. 7 gave high green density at 2% binder loading. The co-polymer No. 7 gave high green density at all percentages of loading (Table 4). This is due to the plastisizing eect of EHA.
Table 3 The quantitative estimation of p(monomer) units (mol%) in acrylic co-polymers Serial number 1 2 3 4 5 6 7 8 9 10
S:MA S:MMA MA:MMA MA:MMA MA:MMA EHA:MMA EHA:MMA EHA:MMA EHA:MMA:MAA EHA:MMA:MAA
Co-polymers
Co-polymers
As estimated by NMR (mol%)
60:40 40:60 90:10 65:35 30:70 66:34 60:40 50:50 55:43:2 30:68:2
55.4:44.6 39.1:60.9 91.3:8.7 68.4:31.6 33.3:66.7 51.4:48.6 44.9:55.1 35.2:64.8 39.8:57.2:3.0 18.8:78.5:2.7
53:47 37:63 ± ± ± 48:52 42:58 33:67 36:62.5:1.5 18:80:2
D.B. Rohini Kumar et al. / European Polymer Journal 36 (2000) 1503±1510 Table 4 Green density of ceramic materials (Strontium Chromite) with dierent acrylic monomer compositions Co-polymers (serial number)
1 2 3 4 5 6 7 8 9 10
Green density (g/ml) 2%
3%
5%
2.50 2.48 2.67 2.54 2.84 2.58 2.85 2.55 2.63 2.55
2.48 2.45 2.54 2.51 2.48 2.56 2.67 2.50 2.60 2.51
2.20 2.40 2.45 2.42 2.48 2.56 2.60 2.46 2.60 2.46
3.4. Green strength The green strength of the ceramic materials increased linearly with the increasing amounts of binder incorporated into the ceramic base. It is observed that MA±MMA (30:70) (co-polymer 5) and EHA± MMA±MAA (55:43:2) (co-polymer No. 9) co-polymers gave highest green strength to the ceramic material (Table 5). The advantage of these two copolymers is that, even at low levels of their loading as binders in ceramic materials, high green strength of the composite material is obtained. Thereby the machining properties of these composites are much superior. It has also been observed that as the percentage of MMA increased in the co-polymers (Nos. 3, 4, 5 of Table 1) there is a gradual increase in the green strength of the ceramic material. This is due to the in-
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¯uence of glass transition temperature (Tg) of the acrylic binder systems. The Tg of MA±MMA co-polymer systems (284±342 K) increased with the increase in the percentage of MMA. A similar observation (Tg 236±295 K) was also noticed in another set of co-polymers containing EHA and MMA monomers (co-polymers 6±10). These observations seems to be in agreement with the previous studies [6,16] and EHA has more plasticizing eect on the product. MMA is playing major role in controling Tg of the polymer. In the binder systems containing EHA±MMA copolymers as the percentage of EHA increased, the ¯exibility of the co-polymer also increased considerably. The ceramic materials have exhibited an excessive spring back property. This is due to the deformation allowed stress relief during the pressing operation (Table 5). In case of co-polymer No. 3 the theoretical Tg is 284.60 K. The green strengths were determined at 298 K, which is above its Tg and the plasticizing eect of rubbery co-polymer predominated and the green strength was less compared to the other two. In copolymer No. 1 though the Tg is 333.30 K which is less compared to co-polymer No. 2, the green strength is less because of the crystallinity of polystyrene.
4. Conclusions It is concluded from this study that the properties of binder play a major role in ceramics that is ultimately very helpful in improving green strengths of the ceramic machining. The emulsion polymerized acrylic binders are best suited for ceramic processing. It has been found that the acrylic co-polymers of MMA with MA (70:30) and with EHA, MMA and
Table 5 Green strength of ceramic materials with dierent acrylic co-polymer compositions at dierent percentages Green strength (MPa) Co-polymers (serial numbers from Table 1)
1 2 3 4 5 6 7 8 9 10
Axial
Radial
2%
3.5%
5%
2%
3.5%
5%
22 32 33 37 56 32 29 30 47 23
20 26 25 66 68 19 27 33 53 31
23 24 16 19 35 15 17 32 25 33
0.70 0.77 1.88 1.60 0.52 0.70 0.85 0.81 1.91 1.19
1.91 0.83 7.73 1.74 0.66 0.71 1.05 1.73 2.56 1.74
± 0.61 2.19 1.24 0.59 1.24 1.45 1.32 2.31 2.06
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MAA (55:43:2) are good binders for the dry pressing of the ceramic materials. These types of polymers can be tailor made to suit to the end application. Glass transition temperature also in¯uences the binder behavior in the ceramic processing.
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[7] Nies CW, Messing GL. J Am Ceramic Soc 1984;67(4):301. [8] Dimilia RA, Reed JS. Am Ceram Soc Bull 1983;62(4):484. [9] Yeagle ML. In: Craver CD, editor. Polymer characterisation interdisciplinary approaches. New York: Plenum Press, 1971. p. 201. [10] Vijayalakshmi V, Vani JNR, Krishnamurti N. Polym Paint Col J 1991;181(4290):506. [11] Rudnick A, Hunter AR, Holden FC. An analysis of the diametrical compression test. Mater Res Stand 1963;3(4):283±9. [12] Negre F, Sanchez E, Garcia J, Gines F, Carty WM. Am Ceram Soc Bull 1998;77(1):63±8. [13] Bassner SL, Klingenberg EH. Am Ceram Soc Bull 1998;77(6):71±5. [14] Roe RJ. In: Mark HF, Bikales NM, Overberger CG, Menges G, editors. Encyclopedia of polymer science and engineering. New York: John Wiley & Sons, 1987. p. 539. [15] Lannutti JJ, Deis TA, Kong CM, Phillips DH. Am Ceram Soc Bull 1997;76(1):53. [16] Wu XK, Whitman DW, Kaufel WL, Finch WC, Cumbers DI. Am Ceram Soc Bull 1995;74(5):61.