Influence of Process Parameter Variation on Ceramic Feedstock Flow ...
Influence of process parameter variation on ceramic feedstock flow behaviour T. Hanemanna,b, J. Aronia a
b
Forschungszentrum Karlsruhe, Institut f. Materialforschung III, D-76021 Karlsruhe, Germany Albert-Ludwigs-Universität Freiburg, Institut f. Mikrosystemtechnik (IMTEK), D-79110 Freiburg, Germany
Abstract With respect to feedstock development for different ceramic injection molding techniques the influence of various process parameters during feedstock development was investigated systematically. First the dispersant concentration at the fillers surface was changed in a wide range. The impact on the particle size distribution was measured. Second the size and the geometry of the used stirrers for compounding in an unsaturated polyester resin as polymer matrix were varied. The resulting composite flow properties at a fixed solid load and different temperatures were determined experimentally using a cone and plate rheometer. Increasing dispersant amounts at the alumina surface lead to a change of the particle size distribution and to a significant composite viscosity drop. The use of different stirrers affects directly the composite viscosity as well as the flow behaviour to a certain extent. Keywords: Ceramic feedstock development, dispersants, rheological properties
1. Introduction In microsystem technologies ceramic parts carrying a microstructured surface with details below 100 µm and aspect ratios larger than 1 become more and more important due to their outstanding thermomechanical properties and chemical stability in harsh environments. For a successful commercialization a low cost fabrication has to be established. This means a realization of a process chain covering the single steps 1. Filler conditioning (e.g. surface treatment) 2. Compounding 3. Molding 4. Thermal postprocessing (debinding, sintering) In the last years different variants of injection molding techniques using reactive resins, wax or thermoplastics as binder in the investigated feedstock systems have been developed. These techniques cover the whole replication field starting from rapid prototyping up to mass fabrication [1-5]. For the realisation of dense ceramic parts using polymer binder-ceramic filler-composites a powder load of at least 50 vol% is necessary, which causes a significant increase of the composite's viscosity [6]. In literature a large number of publications deals with the realisation of feedstock systems with large filler loads, mostly related to wax, paraffin or thermoplastic binders applying different kinds of dispersing agents for a reduction of the feedstock viscosity [7,8]. In 2005 Zürcher and Graule gave a comprehensive overview of the influence of the dispersant structure on the flow behaviour of zirconia/organic solvent/dispersions [9]. Detailed rheological investigations of composites consisting of a polymer reactive resin as matrix and surface treated ceramic fillers like alumina and zirconia using different types of organic dispersants have been published in 2006 and 2007 [10-12]. In feedstock development the use of dispersing agents is strictly recommended. The addition of suitable dispersants allows for a viscosity reduction at constant load or, consequently, a load increase at constant viscosity. Furthermore the homogeneity of the feedstock and the greenbody strength, important for demolding, is improved.
Fig. 1. Dispersants attached at particle surface. On a molecular level dispersants, designed for the use in polymer matrix materials, are small amphiphilic molecules or oligomers as well as polymers. These molecules adsorb via van-der-Waals-forces or hydrogen-bridge-linkage on the filler’s surface and reduce the interactive forces between the individual filler particles preventing reagglomeration and supporting wetting by the binder. Figure 1 shows exemplarily the surface attachment of a mono-functional dispersant molecule like a polyethyleneglycolalkylether to ceramic particles, dispersed in a polymer matrix. The addition of fillers to a liquid changes the temperature influence on the composite’s viscosity, which can be described with an Arrhenius-type approach [6] (1), η1 and η2 are the apparent viscosities at the two different temperatures T1 and T2, R the gas constant and ∆Ea is the flow activation energy, which depends mainly on the composition of the investigated system. In general an increasing solid load causes a raise of the flow activation energy which is equivalent with an improved sensitivity to temperature changes. ln
η1( T1 ) ∆Ea 1 1 = ( − ) η2 ( T2 ) R T1 T2
Multi-Material Micro Manufacture S. Dimov and W. Menz (Eds.) © 2008 Cardiff University, Cardiff, UK. Published by Whittles Publishing Ltd. All rights reserved.
(1)
With respect to compounding even a slight temperature increase yields a noticeable viscosity drop, especially at high loads close to the maximum accessible filler load. Low viscous polymer-based reactive resins like polymethylmethacrylate solved in methylmethacrylate or unsaturated polyester solved in styrene can be used as model systems for high viscous polymer melts and for a rapid prototyping of microstructured parts made of plastic, ceramic and metals [3-5]. The apparent viscosity of the pure reactive resin is under ambient conditions below 5 Pa s and enables a rapid composite processing using simple laboratory equipment e.g. for dispersant screening experiments [10-12]. In this work the influence of the used dissolver stirrer size and geometry as well as the dispersant amount on the composite flow behaviour will be discussed. 2. Experimental 2.1. Ceramic powder surface coating The commercial Almatis alumina CT3000 SG was selected as ceramic test material and treated with a dispersant of the polyethyleneglycolalkylether-type (Brij72, see Fig. 2, SigmaAldrich) always doubling the dispersant amount starting with 3.3 g per kg ceramic. The molecule consists of a small hydrophilic moiety, which can adsorb at the alumina surface, and an extended aliphatic tail, which can interact with the nonpolar polymer matrix. To ensure a homogenous surface hydrophobization a large sized rotary evaporator equipped with a 5 l evaporation flask was used (Buechi AG). 1 kg of dry alumina was coated with the dispersant in 1 l of ethanol at 50°C. After two hours the ethanol was removed, the remaining coated alumina was dried at elevated temperature. The resulting particle size distribution was measured using a Beckman Coulter LS230. The relative large batch amount enables a feedstock preparation in bigger volumes. Table 1 lists the different compositions realized by surface coating.
Fig. 2. Molecular structure of Brij72. Table 1 Investigated alumina/Brij72 compositions. Name
Alumina content (g)
Brij content d50-value (g) (µm)
Sample 1 Sample 2 Sample 3 Sample 4 Sample 5
1000 1000 1000 1000 1000
0.0 3.3 6.7 13.3 26.6
Fig. 3. Applied different sized stirrers. Despite the fact, that both stirrers are claimed to be dissolvers stirrers with a donut-like flow profile allowing large shear forces during compounding, the shape and the rotor blades are totally different; the smaller stirrer shows more similarity to a turbine or blade mixer. The latter one generates smaller shear forces and a reduced deagglomeration potential in comparison to the larger dissolver stirrer. This one needs at least a sample volume around 40 ml, the smaller one only a sample volume around 20 ml for successful mixing in a suitable sized glass beaker. An untreated alumina, dispersed in the resin (sample 1), was used as reference material. The low filler content enables the use of a cone and plate rheometer (CVO50, Bohlin) avoiding experimental complications like sticking or gap emptying at larger shear rates. All viscosity measurements were done at 20, 40 and 60°C in the shear rate range between 1 and 200 1/s. The experimental uncertainty of the obtained data is in the range of ± 5%. 3. Results and Discussion 3.1. Impact of surface coating on the particle size distribution Figure 4 shows the influence of the surface coating on the differential particle size distribution. The uncoated alumina (sample 1) possesses two main fractions: a small one between 100 and 400 nm and a large fraction around 2 µm. Increasing dispersant amounts cause a reduction of the fine fraction (sample 3) which disappears at the largest Brij concentration (sample 5) completely. In the latter case only the main fraction remains which is shifted to larger values between 5 and 6 µm. As a consequence the average particle size value d50 increases especially at very large dispersant amounts (see Table 1).
1.9 2.0 2.0 2.3 5.0
2.2 Reactive resin based feedstocks An unsaturated polyester resin (Roth GmbH) with a polymer content around 65 wt% and styrene as reactive thinner was used as polymer binder. Composites containing 50 wt% coated alumina (22.4 vol%) have been prepared using two different sized dissolver stirrers (diameters 42 and 29 mm, see Fig. 3).
Fig. 4. Influence of surface coating on particle size distribution.
3.2. Impact of stirrer size and dispersant concentration on the flow behaviour It has been shown earlier, that the used stirrer type and dispersant concentration have a strong influence on the resulting unsaturated polyester-aluminacomposite viscosity [10]. The stirrer blade geometry affects directly the deagglomeration capability, with respect to particle deagglomeration sharp blade edges as in case of the large dissolver stirrer are favourable in comparison to smooth blades as in turbine stirrers. Increasing dispersant amount can influence the composite viscosity in different manners [9-11]: a) viscosity drop b) viscosity drop with local viscosity minimum for a certain dispersant concentration c) viscosity increase Figure 5 shows for the samples 1 and 5 the flow curves at 20, 40, and 60°C using the 42 mm dissolver stirrer for dispersion. As expected with increasing temperature the viscosity drops significantly. Sample 5 shows at all investigated temperatures a reduced viscosity, especially at larger shear rates. In a rough approximation all flow curves show Newtonian flow behaviour. Table 2 lists for all investigated composites using the large stirrer for compounding the viscosities at a shear rate of 100 1/s. Increasing dispersant amounts cause a pronounced viscosity drop at all investigated temperatures (percentage reduction @20°C: 41%; @40°C: 32%; @60°C: 35%).
Fig. 6. Flow curves of the samples 1 & 5, using the 29 mm stirrer for compounding. Table 3 summarizes for all mixtures fabricated by the 29 mm stirrer the viscosity data. Increasing dispersant amounts lead again to a significant viscosity reduction (percentage reduction @20°C: 42%; @40°C: 31%; @60°C: 27%). The stirrer size and the shape cause two basic differences in the resulting flow curves and viscosity values: Firstly, almost all viscosity values listed in Table 3 are smaller than the ones presented in Table 2, especially at the lowest measuring temperature. The differences vanish at larger temperatures. Secondly the mixture (sample 5) prepared with the 29 mm stirrer show a pseudoplastic flow at 40°C and 60°C, whilst the same sample prepared with the 42 mm stirrer show a Newtonian flow. Table 3 Composite viscosity (small stirrer). Sample
Composite viscosity(@100 1/s) @20°C
Fig. 5. Flow curves of the samples 1 & 5, using the 42 mm stirrer for compounding. Table 2 Composite viscosity (large stirrer). Sample
Composite viscosity(@100 1/s) @20°C
@40°C
@60°C
Sample 1 Sample 2 Sample 3 Sample 4
6.89 5.08 4.63 4.97
1.43 1.16 1.11 1.14
0.57 0.50 0.48 0.45
Sample 5
4.07
0.97
0.37
In case of the composites using the 29 mm stirrer the flow curves are different (Fig. 6). Whilst the addition of uncoated alumina to the unsaturated polyester matrix (sample 1) yield a Newtonian flow in the investigated shear rate range the surface coating introduces a slight pseudoplastic flow at 40°C and 60°C. As in the previous mentioned composites increasing temperatures result in a viscosity reduction.
@40°C
@60°C
Sample 1
6.12
1.31
0.52
Sample 2 Sample 3 Sample 4 Sample 5
4.70 4.46 4.66 3.58
1.14 1.06 1.10 0.91
0.52 0.42 0.43 0.38
Hence for screening purposes the small stirrer may be applied exploiting the small sample volume necessary despite the slight differences in the resulting viscosity data. These differences are expected to be significantly larger in case of highly filled composites or feedstocks suitable for the different variants of micro powder injection molding. 3.3. Impact on the flow activation energy As shown earlier using micro- and nano-sized ceramics [11,13] surface coating and the resulting surface hydrophobization has a strong influence on the flow activation energy, which is a reliable measure for the viscosities sensitivity to temperature changes. In general increasing solid filler content (uncoated filler) cause a significant flow activation energy increase especially close to the critical filler load [14,15]. A surface hydrophobization cause a better coupling of the filler to the polymer chains, the sensitivity to temperature changes drops, especially in case of nanosized fillers with their large specific surface areas.
References
Fig. 7. Relative flow activation energy change. At a small filler load, as investigated here, only a small impact on the flow activation energy can be expected as published earlier [11,13-15]. Figure 7 shows the flow activation energy change relative to the pure polyester resin. The addition of untreated alumina to the resin cause a slight flow activation energy increase independent of the used stirrer type. Within the experimental error the values are almost identical. Increasing Brij72 concentrations cause in all cases a reduction of the flow activation energy, but the numerical values scatter, especially the ones for sample 2 and 5. In both cases the use of the small stirrer results in a small reduction of flow activation energy change relative to the pure resin. An unequivocal trend cannot be observed, which is due to the relative low solid load of 22.4 vol% in the composite far away from the critical filler load. The scattering of the numerical value is within the range observed in earlier investigations using microsized fillers [11,13-15]. 4. Conclusion The influences of the dispersant concentration and the dispersing conditions on the composite viscosity have been investigated systematically. Increasing dispersant amounts cause a shift to larger average particle sizes. A reduction of the dissolver stirrer size and especially the blade geometry cause changes in resulting flow properties, which can be enhanced in case of larger solid loads in the composite. At constant solid load increasing dispersant amounts cause a pronounced viscosity reduction at all measured temperatures. As in earlier investigations a clear impact on the flow activation energy cannot be observed. With respect to feedstock development the Brij72 concentration should be as large as possible to ensure a low viscosity value. As a drawback very large dispersant amounts can reduce the greenbody strength due to a reduced coupling between the coated filler and the polymer matrix resulting in molding defects as well as a reduced sinter density. Hence in all cases a compromise between the different criteria has to be found experimentally for each investigated binder-filler-system individually. Acknowledgements The authors gratefully acknowledge the financial support by the European Commission within the 4MNetwork of Excellence and the Deutsche Forschungsgemeinschaft DFG (SFB 499). TH thanks R. Schillinger for sample processing at IMTEK.
[1] Ruprecht R, Finnah, G, Piotter, V. Microinjection Molding – Principles and Challenges. In: Löhe D and Haußelt J (Eds) Microengineering of Metals and Ceramics, Wiley-VCH, 253-287, Weinheim, 2005. [2] Bauer, W, Haußelt, J, Merz, L, Müller, M, Örlygsson, G, Rath. Micro Ceramic Injection Molding. In: Löhe D and Haußelt J (Eds) Microengineering of Metals and Ceramics, Wiley-VCH, 325-356, Weinheim, 2005. [3] Hanemann T, Honnef K, Hausselt J. Rapid prototyping of microstructured ceramic and metal parts st using reaction molding techniques. Proc. 1 Intern. Conf. on Multi-Material-Micro-Manufacture (4M), 29.06.-01.07.2005, Karlsruhe, FRG. [4] Hanemann T, Honnef K, Hausselt, J. Process chain development for the rapid prototyping of microstructured polymer, ceramic and metal parts: Composite flow behaviour optimization, replication via reaction molding and thermal postprocessing. Intern. J. of Adv. Manuf. Techn. 33 (2007) 167-175. [5] Hanemann T, Bauer W, Knitter R, Woias P. Rapid prototyping and rapid tooling techniques for the manufacturing of silicon, polymer, metal and ceramic microdevices. In: Leondes CT (Ed) MEMS/NEMS Handbook: Techniques and Applications. Springer Publisher, 187-255, Berlin 2006, Vol. 3. [6] German RG. Powder Injection Molding. Princeton: Metal Powder Industries Federation, 1990. [7] Xie Z-P, Luo J-S, Wang X, Li J-B, Huang Y. The effect of organic vehicle on the injection molding of ultra-fine zirconia powders. Materials and Design 26 (2005) 79-82. [8] Trunec M, Dobsak P, Cihlar J. Effect of powder treatment on injection moulded zirconia ceramics. J. Europ. Ceram. Soc. 20 (2000) 859-866. [9] Zuercher S, Graule T. Influence of dispersant structure on the rheological properties of highlyconcentrated zirconia dispersions. J. Europ. Ceram. Soc. 25 (2005) 863-873. [10] Hanemann T. Influence of dispersants on the flow behaviour of unsaturated polyester-aluminacomposites. Composites A 37 (2006) 735-741. [11] Hanemann T. Viscosity change of unsaturated polyester-alumina-composites using polyethylene glycol alkyl ether based dispersants. Composites A 37 (2006) 2155-2163. [12] Hanemann T, Heldele R, Haußelt, J. Structureproperty relationship of dispersants used in ceramic rd feedstock development, Proc. 4M 2007 – 3 Intern. Conference on Multi-Material-Micro-Manufacture (4M), 03.-05.10.2007, Borovets, Bulgarien, 73-76. [13] Hanemann T, Heldele R, Haußelt J. Particle size dependent viscosity of polymer-silica-composites, nd Proc. 4M 2006 - 2 Intern. Conference on MultiMaterial-Micro-Manufacture (4M), 20.-22.09.2006, Grenoble, Frankreich, 191-194. [14] Hanemann T. Influence of particle properties on the viscosity of polymer-alumina-composites, Ceramics International, 2008, online available, doi: 10.1016/j.ceramint.2007.08.007. [15] Hanemann T, Honnef K. Process chain development for the realization of zirconia microparts using composite reaction molding, Ceramics International, 2008, online available, doi: 10.1016/j.ceramint.2007.10.005.
b
Forschungszentrum Karlsruhe, Institut f. Materialforschung III, D-76021 Karlsruhe, Germany Albert-Ludwigs-Universität Freiburg, Institut f. Mikrosystemtechnik (IMTEK), D-79110 Freiburg, Germany
Abstract With respect to feedstock development for different ceramic injection molding techniques the influence of various process parameters during feedstock development was investigated systematically. First the dispersant concentration at the fillers surface was changed in a wide range. The impact on the particle size distribution was measured. Second the size and the geometry of the used stirrers for compounding in an unsaturated polyester resin as polymer matrix were varied. The resulting composite flow properties at a fixed solid load and different temperatures were determined experimentally using a cone and plate rheometer. Increasing dispersant amounts at the alumina surface lead to a change of the particle size distribution and to a significant composite viscosity drop. The use of different stirrers affects directly the composite viscosity as well as the flow behaviour to a certain extent. Keywords: Ceramic feedstock development, dispersants, rheological properties
1. Introduction In microsystem technologies ceramic parts carrying a microstructured surface with details below 100 µm and aspect ratios larger than 1 become more and more important due to their outstanding thermomechanical properties and chemical stability in harsh environments. For a successful commercialization a low cost fabrication has to be established. This means a realization of a process chain covering the single steps 1. Filler conditioning (e.g. surface treatment) 2. Compounding 3. Molding 4. Thermal postprocessing (debinding, sintering) In the last years different variants of injection molding techniques using reactive resins, wax or thermoplastics as binder in the investigated feedstock systems have been developed. These techniques cover the whole replication field starting from rapid prototyping up to mass fabrication [1-5]. For the realisation of dense ceramic parts using polymer binder-ceramic filler-composites a powder load of at least 50 vol% is necessary, which causes a significant increase of the composite's viscosity [6]. In literature a large number of publications deals with the realisation of feedstock systems with large filler loads, mostly related to wax, paraffin or thermoplastic binders applying different kinds of dispersing agents for a reduction of the feedstock viscosity [7,8]. In 2005 Zürcher and Graule gave a comprehensive overview of the influence of the dispersant structure on the flow behaviour of zirconia/organic solvent/dispersions [9]. Detailed rheological investigations of composites consisting of a polymer reactive resin as matrix and surface treated ceramic fillers like alumina and zirconia using different types of organic dispersants have been published in 2006 and 2007 [10-12]. In feedstock development the use of dispersing agents is strictly recommended. The addition of suitable dispersants allows for a viscosity reduction at constant load or, consequently, a load increase at constant viscosity. Furthermore the homogeneity of the feedstock and the greenbody strength, important for demolding, is improved.
Fig. 1. Dispersants attached at particle surface. On a molecular level dispersants, designed for the use in polymer matrix materials, are small amphiphilic molecules or oligomers as well as polymers. These molecules adsorb via van-der-Waals-forces or hydrogen-bridge-linkage on the filler’s surface and reduce the interactive forces between the individual filler particles preventing reagglomeration and supporting wetting by the binder. Figure 1 shows exemplarily the surface attachment of a mono-functional dispersant molecule like a polyethyleneglycolalkylether to ceramic particles, dispersed in a polymer matrix. The addition of fillers to a liquid changes the temperature influence on the composite’s viscosity, which can be described with an Arrhenius-type approach [6] (1), η1 and η2 are the apparent viscosities at the two different temperatures T1 and T2, R the gas constant and ∆Ea is the flow activation energy, which depends mainly on the composition of the investigated system. In general an increasing solid load causes a raise of the flow activation energy which is equivalent with an improved sensitivity to temperature changes. ln
η1( T1 ) ∆Ea 1 1 = ( − ) η2 ( T2 ) R T1 T2
Multi-Material Micro Manufacture S. Dimov and W. Menz (Eds.) © 2008 Cardiff University, Cardiff, UK. Published by Whittles Publishing Ltd. All rights reserved.
(1)
With respect to compounding even a slight temperature increase yields a noticeable viscosity drop, especially at high loads close to the maximum accessible filler load. Low viscous polymer-based reactive resins like polymethylmethacrylate solved in methylmethacrylate or unsaturated polyester solved in styrene can be used as model systems for high viscous polymer melts and for a rapid prototyping of microstructured parts made of plastic, ceramic and metals [3-5]. The apparent viscosity of the pure reactive resin is under ambient conditions below 5 Pa s and enables a rapid composite processing using simple laboratory equipment e.g. for dispersant screening experiments [10-12]. In this work the influence of the used dissolver stirrer size and geometry as well as the dispersant amount on the composite flow behaviour will be discussed. 2. Experimental 2.1. Ceramic powder surface coating The commercial Almatis alumina CT3000 SG was selected as ceramic test material and treated with a dispersant of the polyethyleneglycolalkylether-type (Brij72, see Fig. 2, SigmaAldrich) always doubling the dispersant amount starting with 3.3 g per kg ceramic. The molecule consists of a small hydrophilic moiety, which can adsorb at the alumina surface, and an extended aliphatic tail, which can interact with the nonpolar polymer matrix. To ensure a homogenous surface hydrophobization a large sized rotary evaporator equipped with a 5 l evaporation flask was used (Buechi AG). 1 kg of dry alumina was coated with the dispersant in 1 l of ethanol at 50°C. After two hours the ethanol was removed, the remaining coated alumina was dried at elevated temperature. The resulting particle size distribution was measured using a Beckman Coulter LS230. The relative large batch amount enables a feedstock preparation in bigger volumes. Table 1 lists the different compositions realized by surface coating.
Fig. 2. Molecular structure of Brij72. Table 1 Investigated alumina/Brij72 compositions. Name
Alumina content (g)
Brij content d50-value (g) (µm)
Sample 1 Sample 2 Sample 3 Sample 4 Sample 5
1000 1000 1000 1000 1000
0.0 3.3 6.7 13.3 26.6
Fig. 3. Applied different sized stirrers. Despite the fact, that both stirrers are claimed to be dissolvers stirrers with a donut-like flow profile allowing large shear forces during compounding, the shape and the rotor blades are totally different; the smaller stirrer shows more similarity to a turbine or blade mixer. The latter one generates smaller shear forces and a reduced deagglomeration potential in comparison to the larger dissolver stirrer. This one needs at least a sample volume around 40 ml, the smaller one only a sample volume around 20 ml for successful mixing in a suitable sized glass beaker. An untreated alumina, dispersed in the resin (sample 1), was used as reference material. The low filler content enables the use of a cone and plate rheometer (CVO50, Bohlin) avoiding experimental complications like sticking or gap emptying at larger shear rates. All viscosity measurements were done at 20, 40 and 60°C in the shear rate range between 1 and 200 1/s. The experimental uncertainty of the obtained data is in the range of ± 5%. 3. Results and Discussion 3.1. Impact of surface coating on the particle size distribution Figure 4 shows the influence of the surface coating on the differential particle size distribution. The uncoated alumina (sample 1) possesses two main fractions: a small one between 100 and 400 nm and a large fraction around 2 µm. Increasing dispersant amounts cause a reduction of the fine fraction (sample 3) which disappears at the largest Brij concentration (sample 5) completely. In the latter case only the main fraction remains which is shifted to larger values between 5 and 6 µm. As a consequence the average particle size value d50 increases especially at very large dispersant amounts (see Table 1).
1.9 2.0 2.0 2.3 5.0
2.2 Reactive resin based feedstocks An unsaturated polyester resin (Roth GmbH) with a polymer content around 65 wt% and styrene as reactive thinner was used as polymer binder. Composites containing 50 wt% coated alumina (22.4 vol%) have been prepared using two different sized dissolver stirrers (diameters 42 and 29 mm, see Fig. 3).
Fig. 4. Influence of surface coating on particle size distribution.
3.2. Impact of stirrer size and dispersant concentration on the flow behaviour It has been shown earlier, that the used stirrer type and dispersant concentration have a strong influence on the resulting unsaturated polyester-aluminacomposite viscosity [10]. The stirrer blade geometry affects directly the deagglomeration capability, with respect to particle deagglomeration sharp blade edges as in case of the large dissolver stirrer are favourable in comparison to smooth blades as in turbine stirrers. Increasing dispersant amount can influence the composite viscosity in different manners [9-11]: a) viscosity drop b) viscosity drop with local viscosity minimum for a certain dispersant concentration c) viscosity increase Figure 5 shows for the samples 1 and 5 the flow curves at 20, 40, and 60°C using the 42 mm dissolver stirrer for dispersion. As expected with increasing temperature the viscosity drops significantly. Sample 5 shows at all investigated temperatures a reduced viscosity, especially at larger shear rates. In a rough approximation all flow curves show Newtonian flow behaviour. Table 2 lists for all investigated composites using the large stirrer for compounding the viscosities at a shear rate of 100 1/s. Increasing dispersant amounts cause a pronounced viscosity drop at all investigated temperatures (percentage reduction @20°C: 41%; @40°C: 32%; @60°C: 35%).
Fig. 6. Flow curves of the samples 1 & 5, using the 29 mm stirrer for compounding. Table 3 summarizes for all mixtures fabricated by the 29 mm stirrer the viscosity data. Increasing dispersant amounts lead again to a significant viscosity reduction (percentage reduction @20°C: 42%; @40°C: 31%; @60°C: 27%). The stirrer size and the shape cause two basic differences in the resulting flow curves and viscosity values: Firstly, almost all viscosity values listed in Table 3 are smaller than the ones presented in Table 2, especially at the lowest measuring temperature. The differences vanish at larger temperatures. Secondly the mixture (sample 5) prepared with the 29 mm stirrer show a pseudoplastic flow at 40°C and 60°C, whilst the same sample prepared with the 42 mm stirrer show a Newtonian flow. Table 3 Composite viscosity (small stirrer). Sample
Composite viscosity(@100 1/s) @20°C
Fig. 5. Flow curves of the samples 1 & 5, using the 42 mm stirrer for compounding. Table 2 Composite viscosity (large stirrer). Sample
Composite viscosity(@100 1/s) @20°C
@40°C
@60°C
Sample 1 Sample 2 Sample 3 Sample 4
6.89 5.08 4.63 4.97
1.43 1.16 1.11 1.14
0.57 0.50 0.48 0.45
Sample 5
4.07
0.97
0.37
In case of the composites using the 29 mm stirrer the flow curves are different (Fig. 6). Whilst the addition of uncoated alumina to the unsaturated polyester matrix (sample 1) yield a Newtonian flow in the investigated shear rate range the surface coating introduces a slight pseudoplastic flow at 40°C and 60°C. As in the previous mentioned composites increasing temperatures result in a viscosity reduction.
@40°C
@60°C
Sample 1
6.12
1.31
0.52
Sample 2 Sample 3 Sample 4 Sample 5
4.70 4.46 4.66 3.58
1.14 1.06 1.10 0.91
0.52 0.42 0.43 0.38
Hence for screening purposes the small stirrer may be applied exploiting the small sample volume necessary despite the slight differences in the resulting viscosity data. These differences are expected to be significantly larger in case of highly filled composites or feedstocks suitable for the different variants of micro powder injection molding. 3.3. Impact on the flow activation energy As shown earlier using micro- and nano-sized ceramics [11,13] surface coating and the resulting surface hydrophobization has a strong influence on the flow activation energy, which is a reliable measure for the viscosities sensitivity to temperature changes. In general increasing solid filler content (uncoated filler) cause a significant flow activation energy increase especially close to the critical filler load [14,15]. A surface hydrophobization cause a better coupling of the filler to the polymer chains, the sensitivity to temperature changes drops, especially in case of nanosized fillers with their large specific surface areas.
References
Fig. 7. Relative flow activation energy change. At a small filler load, as investigated here, only a small impact on the flow activation energy can be expected as published earlier [11,13-15]. Figure 7 shows the flow activation energy change relative to the pure polyester resin. The addition of untreated alumina to the resin cause a slight flow activation energy increase independent of the used stirrer type. Within the experimental error the values are almost identical. Increasing Brij72 concentrations cause in all cases a reduction of the flow activation energy, but the numerical values scatter, especially the ones for sample 2 and 5. In both cases the use of the small stirrer results in a small reduction of flow activation energy change relative to the pure resin. An unequivocal trend cannot be observed, which is due to the relative low solid load of 22.4 vol% in the composite far away from the critical filler load. The scattering of the numerical value is within the range observed in earlier investigations using microsized fillers [11,13-15]. 4. Conclusion The influences of the dispersant concentration and the dispersing conditions on the composite viscosity have been investigated systematically. Increasing dispersant amounts cause a shift to larger average particle sizes. A reduction of the dissolver stirrer size and especially the blade geometry cause changes in resulting flow properties, which can be enhanced in case of larger solid loads in the composite. At constant solid load increasing dispersant amounts cause a pronounced viscosity reduction at all measured temperatures. As in earlier investigations a clear impact on the flow activation energy cannot be observed. With respect to feedstock development the Brij72 concentration should be as large as possible to ensure a low viscosity value. As a drawback very large dispersant amounts can reduce the greenbody strength due to a reduced coupling between the coated filler and the polymer matrix resulting in molding defects as well as a reduced sinter density. Hence in all cases a compromise between the different criteria has to be found experimentally for each investigated binder-filler-system individually. Acknowledgements The authors gratefully acknowledge the financial support by the European Commission within the 4MNetwork of Excellence and the Deutsche Forschungsgemeinschaft DFG (SFB 499). TH thanks R. Schillinger for sample processing at IMTEK.
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