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Influence of minute metal ions on the idle time of acrylamide polymerization in gelcasting of ceramics
November 2002
Materials Letters 56 (2002) 990 – 994 www.elsevier.com/locate/matlet
Inf luence of minute metal ions on the idle time of acrylamide polymerization in gelcasting of ceramics Lei Zhao*, Jin-long Yang, Li-guo Ma, Yong Huang Department of Materials Science and Engineering, The State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China Received 29 October 2001; received in revised form 8 March 2002; accepted 12 March 2002
Abstract Usually, some minute metal ions, such as Fe2 + , Cu2 + , Al3 + , Ca2 + , exist in ceramic powders as impurities. These minute metal ions have large influences on the idle time of acrylamide polymerization in gelcasting. Different ions and their concentrations have different effects on the gel system. In this work, by introducing these ions into the gel system, the influence mechanisms and their principles have been investigated and generalized, which can be an important guide for the control of gelcasting processing. D 2002 Published by Elsevier Science B.V. Keywords: Gelcasting; Free-radical polymerization; Minute metal ions; Idle time; Ceramics
1. Introduction Gelcasting, an important colloidal forming for the shaping of ceramic green body, was first developed in the Metals and Ceramics Division—Ceramic Processing Group at Oak Ridge National Laboratory (ORNL), USA [1– 3]. It can fabricate ceramic bodies by means of in situ polymerization of organic monomers which results in a macromolecular network to hold the ceramic powders together. Gelcasting is a near net shape-forming process, which has many advantages [4]. Compared to slip casting, gelcasting can produce much more homogeneous green bodies with uniform density over the part. Compared to injection moulding, gelcasting uses
*
Corresponding author.
only small quantities of organic binder. This makes a critical binder removal step not needed before sintering. Ceramic bodies with good properties can be acquired by gelcasting. Thus, this forming process has been applied in all kinds of ceramics [5– 7]. And, now, gelcasting has extended to industrial application [8]. The main steps of gelcasting are as follows. First, monomer, crosslinker, medium (water or organism) are mixed together to prepare a premix solution. Second, the premix solution, initiator, catalyst, and ceramics powders are thoroughly mixed into a homogeneous suspension with high solid volume loading and low viscosity. Next, the suspension is cast into a non-pore mold, where the monomer polymerizes to form a three-dimensional structure. Thus, the slurry solidifies in situ. At last, after demolding and drying, a uniform green body is acquired.
0167-577X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 6 6 0 - 2
L. Zhao et al. / Materials Letters 56 (2002) 990–994
At present, the aqueous system of acrylamide gelcasting has been investigated widely. How to control the procedure effectively has aroused considerably worldwide attention. In the aqueous gelcasting system, initiator and catalyst are added into the premix solution. The polymerization of acrylamide is a free-radical reaction. Such reaction has three stages. The first is chain-initiation stage during which free radicals come into being and become more and more. The second is chain-growth stage. When the quantity of free radicals is enough, the polymerization begins and keeps on going along gradually. Free radicals disappear and the polymerization stops during the third stage called chain-end stopping stage. Idle time is defined as the time between the addition of the initiator and catalyst and the commencement of polymerization [1]. Once the polymerization begins, the viscosity of the slurry will increase so fast that the slurry cannot flow at all soon. Thus, the slurry must be cast into a mold before polymerization. Idle time should be considered as a key to improve the controllable ability of the slurry. Since the polymerization is exothermic, a temperature change of the system can be observed after the polymerization begins. The idle time can be measured by means of observing temperature change of the slurry. The factors which influence the idle time of acrylamide polymerization have been studied in some literatures [9,10]. Free-radical polymerization of acrylamide may be initiated by using compounds that easily break into radicals under polymerization conditions. Such radiation polymerization is a combined effect of physical factors and material initiators. The more the dose of initiator and catalyst is, the shorter the idle time is. When temperature increases, the idle time becomes short too. The investigation in this work shows that some minute metal ions in the solution can also influence the idle time. However, this factor has been always overlooked by many researchers. In general, minute metal ions, such as Fe2 +, Cu2 +, Al3 +, Ca2 +, exist in ceramic powders as impurities. Furthermore, the species and quantities of such impurity ions will change because of different preparation processes of the same ceramic powder. The influences of these ions, therefore, should be considered. In this work, such influences were investigated by introducing the specific ion into the solution. The influence mechanisms can provide an important guide for controlling of gelcasting processing.
991
2. Experimental 2.1. Materials Solvent is deionized water with conductivity of 1.02 AScm 1. Monomer is monofunctional acrylamide (AM) produced by Mitsui Toatsu Chemical, Japan. Crosslinker is difunctional N,NV-methylenebisacrylamide (MBAM) produced by Hongxing Biological and Chemical Factory of Beijing, China. Such above reagents are mixed into the premix solution in certain rations. The amount of all the organic monomers is about 5 wt.% in the premix solution. Catalyst is N,N,NV,NV-tetramethylenediamine (TEMED) produced by Xingfu Fine Chemical Institute, China. Initiator is ammonium persulfate, (NH4)2S2O8 produced by the Third Reagent Works of Beijing, China. Minute metal ions were introduced into the gel system by means of adding of the following chemical reagents: CuSO 45H2O, FeSO 47H2O, Al 2(SO 4)3 9H2O, CaCl2, which are all in reagent grade and produced by Yili Fine Chemical of Beijing, China. 2.2. Procedure Under 20 jC, specific ion was introduced into the premix solution, which is 50 ml, in a certain concentration first. Then, the constant initiator and catalyst were added. Thus, the gel system was prepared, in which a thermometer is placed to measure the temperature change while the polymerization begins. Hence, the idle time was achieved. For a specific ion, different idle time can be achieved by changing its concentration with keeping the other conditions not to change, which results to an ion concentration– idle time curve, according to which the influence mechanism can be generalized.
3. Results and discussion [11] 3.1. Fe2+ ion The influence of Fe2 + on the idle time of acrylamide polymerization is shown in Fig. 1. While the concentration of iron(II) increases, the idle time becomes short gradually. When the concentration of
992
L. Zhao et al. / Materials Letters 56 (2002) 990–994
increasing of iron(II), iron(II) can form a redox system with the initiator (NH4)2S2O8: 2þ 3þ ! SO2 S2 O2 8 þ Fe 4 þ SO4 þ Fe
Fig. 1. Influence of Fe2 + on the idle time of acrylamide polymerization.
iron(II) is between 0.4 and 0.5 mmol/l, the descending rate of the idle time becomes slow. After that, the idle time shortens fast again. Such behavior shows there are different mechanisms during different stages. When the concentration of iron(II) is smaller, iron(II) exists in the solution as a hydro-ion. Hydrogen ion can engender from the hydro-layer of iron(II). As a result, the acidity of solution will augment. The reaction can be expressed as the following Eq. (1): þ þ FeðH2 OÞ2þ 6 ! FeðH2 OÞ5 OH þ H
ð1Þ
The rate coefficient of initiation of (NH4)2S2O8 can be expressed as: Kd ¼ K1 þ K2 ðHþ Þ
ð2Þ
where K1 is the rate coefficient of initiation reaction without acid, and K2(H + ) is the rate coefficient of initiation reaction under acid. From Eq. (2), it can be achieved that the more H + there is in the solution, the faster the free radicals engender. Thus, the idle time becomes shorter step by step with the increasing of the concentration of iron(II). However, while the concentration of iron(II) is greater than 0.1 mmol/l, a complex reaction will beget between Fe2 + and the catalyst (TEMED) [12], which will result that the ability of TEMED to activate free radicals decreases. However, the negative effect of such reaction is not more than the positive effect of H + ; thus, the result is that the rate of free-radical generation still increases but with a slower acceleration than before. This makes the idle time keep up becoming shorter. Following the more
ð3Þ
The influence rule of the redox system is shown in Fig. 2. The initiation rate of the redox system is much higher than that of TEMED – (NH4)2S2O8. Thus, the idle time becomes short faster when the concentration of iron(II) is above 0.4 mmol/l. However, the redox system can be influenced easily by oxygen in air. Further more, the polarity and pH of medium also have great influence on the reaction. All of these will make the reaction is difficult to control. On the other hand, an electron transfer reaction can beget between iron (III) and free radicals, which results in the reduction of free radicals. The effect can be shown by the following Eq. (4): fCH2 C ˙ HR þ Fe3þ ! fCH ¼ CHR þ Fe2þ þ Hþ
ð4Þ
Thus, if the concentration of iron(II) is too large, iron(III) is overfull at last. The above result will happen, which makes the radical quantity decrease and the idle time become longer. This effect has been monitored during the experiment. (It is not shown in the Fig. 1 for the concentration of iron(II) is too much at that time.)
Fig. 2. Influence of the redox of Fe2 + – (NH4)2S2O8 on the idle time of acrylamide polymerization.
L. Zhao et al. / Materials Letters 56 (2002) 990–994
993
3.2. Cu2+ ion From Fig. 3, we can get the following result. The free-radical generation is accelerated under very low concentration of copper(II). A deceleration, however, is observed after the copper(II) concentration is over 0.22 mmol/l. The mechanisms can be achieved as follows. When copper(II) is little, it can coordinate with the monomer. During the coordination an electron transfers to copper(II) from monomer. As a result, copper(II) is reduced into copper(I), which can be oxidized by the initiator (NH4)2S2O8. Thus, a highspeed redox system forms as shown in Eq. (5). þ 2 2þ S2 O2 8 þ Cu ! SO4 þ SO4 þ Cu
ð5Þ
Hence, copper(II) can be taken as an effective catalyst at that time. However, when an overdose of copper(II) is administered, the above reaction will not be major. Here, copper(II) will mostly react with the catalyst (TEMED). The activation of TEMED will decline greatly. Furthermore, copper(II) has the same effect on free radicals as iron(III) does. Thus, the concentration of copper(II) is greater than a certain value, 0.22 mmol/l, as shown in Fig. 3. The synthesis of above reactions makes the rate of free-radical generation descend and the idle time become longer.
Fig. 4. Influence of Al3 + on the idle time of acrylamide polymerization.
ation effect is so small that it is not easy to observe when the concentration of Al3 + is under 0.1 mol/l. The result that the mechanism of Al3 + is not related to its concentration can be achieved since the curve in Fig. 4 is so smooth. The compound of Al3 + and water has such a reaction expressed as the following Eq. (6): ½ðH2 OÞ5 AlðOH2 Þ 3þ ! ½ðH2 OÞ5 AlðOHÞ 2þ þ Hþ
3.3. Al3+ ion Fig. 4 shows that Al3 + has an acceleration effect on the free-radical generation. However, the acceler-
ð6Þ From Eqs. (2) and (6), the result that the freeradical generation accelerates due to the increase of H + . However, it needs a large concentration of Al3 + as shown in Fig. 4. Thus, if the concentration of Al3 + in the solution is too little, the acceleration effect is not distinctive. 3.4. Ca2+ ion
Fig. 3. Influence of Cu2 + on the idle time of acrylamide polymerization.
The rate of free-radical generation declines with adding of Ca2 + , but the regulation is not distinctive. Fig. 5 shows the curve of the idle time vs. the concentration of Ca2 + . The result, which is consistent with Niu’s [13], attributes to the steric hindrance effect of Ca2 + reducing the activation of TEMED. The changes in conformation of macromolecules and the sizes of macromolecular coils after the introduction of quantities of Ca2 + lead to a decrease of the
994
L. Zhao et al. / Materials Letters 56 (2002) 990–994
polymerization must be considered, the disadvantages of which should be removed. On the other hand, a new way to control the polymerization may be found according to such influence mechanisms.
Acknowledgements This research work was supported by the National Science Foundation of China (Grant No. 59872018) and ‘‘973 Program’’ of the People’s Republic of China (Grant No.G2000067203). The authors are grateful for the grants. Fig. 5. Influence of Ca2 + on the idle time of acrylamide polymerization.
rate of interaction of the growing macroradical with the monomer. Thus, the idle time becomes long.
4. Conclusions Different metal ions adding to the polymerization system of gelcasting have different effects on the idle time. The different mechanisms have been found as the followings. (1) The effects of iron(II) and copper(II) are very complicated. The mechanisms vary with the changes of their concentrations. Both of them can form a redox system with the gel system under some certain concentration, which accelerates the free-radical generation. However, if their concentrations are too great, on the contrary, a restraint effect will occur. This results from a radical consumption of iron(III) or copper(II) and the complex reaction between iron(II) or copper(II) and TEMED. (2) Al3 + has an acceleration effect on the rate of free-radical generation. However, when the concentration of Al3 + is too little, the effect is not distinctive. Such effect attributes to the hydration of Al3 +. (3) Ca2 + has a restraint effect on the polymerization system for a steric hindrance effect although the effect is much slightly. Since it is ineluctable that minute metal ions exist as impurities in the ceramic powders of gelcasting, the influences of such ions on the idle time of the
References [1] A.C. Young, O.O. Omatete, M.A. Janney, et al., Gel-casting of alumina, J. Am. Ceram. Soc. 74 (3) (1991) 612 – 618. [2] O.O. Omatete, M.A. Janney, R.A. Streklow, Gelcasting—a new ceramic forming process, Am. Ceram. Soc. Bull. 70 (10) (1991) 1641 – 1649. [3] O.O. Omatete, M.A. Janney, C.A. Walls, et al., Development of low-toxicity gelcasting systems, J. Am. Ceram. Soc. 81 (3) (1998) 581 – 591. [4] R. Gilissen, J.P. Erauw, A. Smolders, Gelcasting, a near net shape technique, Mater. Des. 21 (2000) 251 – 257. [5] J. Yang, Z. Xie, Q. Tang, Study on rheological behavior and gel casting of a-Al2O3 suspension, J. Chin. Ceram. Soc. 26 (1) (1998) 41 – 46. [6] J. Sun, L. Gao, J. Guo, Study on rheological properties and gelcasting with Y-TZP/a – Al2O3, J. Chin. Ceram. Soc. 26 (5) (1998) 662 – 667. [7] J.P. Maria, J.O. Kiggans, T.N. Tiegs, et al., Gelcasting of sintered reaction bonded silicon nitride for improved mechanical properties, Ceram. Eng. Sci. Proc. (1995) 1071 – 1075 (Am. Ceram. Soc.). [8] O.O. Omatete, M.A. Janney, S. Nunn, Gelcasting: from laboratory development towards industrial production, J. Eur. Ceram. Soc. 17 (1997) 407 – 413. [9] Y. Huang, L. Ma, Q. Tang, et al., Surface oxidation to improve water-based gelcasting of silicon nitride, J. Mater. Sci. 35 (14) (2000) 3519 – 3524. [10] B. Waesche, G. Steinborn, Influence of slip viscosity on the mechanical properties of high purity alumina made by gelcasting, Key Eng. Mater. 132 – 136 (1997) 374 – 377. [11] Z. Pan, Free-Radical Polymerization, Industry Publishing, Beijing, 1983. [12] S. Zhu, Y. Zhou, B. Shen, Short Tutorial of Complex Chemistry, Science and Technology Publishing, Tianjin, 1990. [13] T. Niu, Influence of ion Ca(II) on the polymerization of acrylamide, Jiangxi Chem. Ind. 3 (1997) 7 – 9.
Materials Letters 56 (2002) 990 – 994 www.elsevier.com/locate/matlet
Inf luence of minute metal ions on the idle time of acrylamide polymerization in gelcasting of ceramics Lei Zhao*, Jin-long Yang, Li-guo Ma, Yong Huang Department of Materials Science and Engineering, The State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China Received 29 October 2001; received in revised form 8 March 2002; accepted 12 March 2002
Abstract Usually, some minute metal ions, such as Fe2 + , Cu2 + , Al3 + , Ca2 + , exist in ceramic powders as impurities. These minute metal ions have large influences on the idle time of acrylamide polymerization in gelcasting. Different ions and their concentrations have different effects on the gel system. In this work, by introducing these ions into the gel system, the influence mechanisms and their principles have been investigated and generalized, which can be an important guide for the control of gelcasting processing. D 2002 Published by Elsevier Science B.V. Keywords: Gelcasting; Free-radical polymerization; Minute metal ions; Idle time; Ceramics
1. Introduction Gelcasting, an important colloidal forming for the shaping of ceramic green body, was first developed in the Metals and Ceramics Division—Ceramic Processing Group at Oak Ridge National Laboratory (ORNL), USA [1– 3]. It can fabricate ceramic bodies by means of in situ polymerization of organic monomers which results in a macromolecular network to hold the ceramic powders together. Gelcasting is a near net shape-forming process, which has many advantages [4]. Compared to slip casting, gelcasting can produce much more homogeneous green bodies with uniform density over the part. Compared to injection moulding, gelcasting uses
*
Corresponding author.
only small quantities of organic binder. This makes a critical binder removal step not needed before sintering. Ceramic bodies with good properties can be acquired by gelcasting. Thus, this forming process has been applied in all kinds of ceramics [5– 7]. And, now, gelcasting has extended to industrial application [8]. The main steps of gelcasting are as follows. First, monomer, crosslinker, medium (water or organism) are mixed together to prepare a premix solution. Second, the premix solution, initiator, catalyst, and ceramics powders are thoroughly mixed into a homogeneous suspension with high solid volume loading and low viscosity. Next, the suspension is cast into a non-pore mold, where the monomer polymerizes to form a three-dimensional structure. Thus, the slurry solidifies in situ. At last, after demolding and drying, a uniform green body is acquired.
0167-577X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 6 6 0 - 2
L. Zhao et al. / Materials Letters 56 (2002) 990–994
At present, the aqueous system of acrylamide gelcasting has been investigated widely. How to control the procedure effectively has aroused considerably worldwide attention. In the aqueous gelcasting system, initiator and catalyst are added into the premix solution. The polymerization of acrylamide is a free-radical reaction. Such reaction has three stages. The first is chain-initiation stage during which free radicals come into being and become more and more. The second is chain-growth stage. When the quantity of free radicals is enough, the polymerization begins and keeps on going along gradually. Free radicals disappear and the polymerization stops during the third stage called chain-end stopping stage. Idle time is defined as the time between the addition of the initiator and catalyst and the commencement of polymerization [1]. Once the polymerization begins, the viscosity of the slurry will increase so fast that the slurry cannot flow at all soon. Thus, the slurry must be cast into a mold before polymerization. Idle time should be considered as a key to improve the controllable ability of the slurry. Since the polymerization is exothermic, a temperature change of the system can be observed after the polymerization begins. The idle time can be measured by means of observing temperature change of the slurry. The factors which influence the idle time of acrylamide polymerization have been studied in some literatures [9,10]. Free-radical polymerization of acrylamide may be initiated by using compounds that easily break into radicals under polymerization conditions. Such radiation polymerization is a combined effect of physical factors and material initiators. The more the dose of initiator and catalyst is, the shorter the idle time is. When temperature increases, the idle time becomes short too. The investigation in this work shows that some minute metal ions in the solution can also influence the idle time. However, this factor has been always overlooked by many researchers. In general, minute metal ions, such as Fe2 +, Cu2 +, Al3 +, Ca2 +, exist in ceramic powders as impurities. Furthermore, the species and quantities of such impurity ions will change because of different preparation processes of the same ceramic powder. The influences of these ions, therefore, should be considered. In this work, such influences were investigated by introducing the specific ion into the solution. The influence mechanisms can provide an important guide for controlling of gelcasting processing.
991
2. Experimental 2.1. Materials Solvent is deionized water with conductivity of 1.02 AScm 1. Monomer is monofunctional acrylamide (AM) produced by Mitsui Toatsu Chemical, Japan. Crosslinker is difunctional N,NV-methylenebisacrylamide (MBAM) produced by Hongxing Biological and Chemical Factory of Beijing, China. Such above reagents are mixed into the premix solution in certain rations. The amount of all the organic monomers is about 5 wt.% in the premix solution. Catalyst is N,N,NV,NV-tetramethylenediamine (TEMED) produced by Xingfu Fine Chemical Institute, China. Initiator is ammonium persulfate, (NH4)2S2O8 produced by the Third Reagent Works of Beijing, China. Minute metal ions were introduced into the gel system by means of adding of the following chemical reagents: CuSO 45H2O, FeSO 47H2O, Al 2(SO 4)3 9H2O, CaCl2, which are all in reagent grade and produced by Yili Fine Chemical of Beijing, China. 2.2. Procedure Under 20 jC, specific ion was introduced into the premix solution, which is 50 ml, in a certain concentration first. Then, the constant initiator and catalyst were added. Thus, the gel system was prepared, in which a thermometer is placed to measure the temperature change while the polymerization begins. Hence, the idle time was achieved. For a specific ion, different idle time can be achieved by changing its concentration with keeping the other conditions not to change, which results to an ion concentration– idle time curve, according to which the influence mechanism can be generalized.
3. Results and discussion [11] 3.1. Fe2+ ion The influence of Fe2 + on the idle time of acrylamide polymerization is shown in Fig. 1. While the concentration of iron(II) increases, the idle time becomes short gradually. When the concentration of
992
L. Zhao et al. / Materials Letters 56 (2002) 990–994
increasing of iron(II), iron(II) can form a redox system with the initiator (NH4)2S2O8: 2þ 3þ ! SO2 S2 O2 8 þ Fe 4 þ SO4 þ Fe
Fig. 1. Influence of Fe2 + on the idle time of acrylamide polymerization.
iron(II) is between 0.4 and 0.5 mmol/l, the descending rate of the idle time becomes slow. After that, the idle time shortens fast again. Such behavior shows there are different mechanisms during different stages. When the concentration of iron(II) is smaller, iron(II) exists in the solution as a hydro-ion. Hydrogen ion can engender from the hydro-layer of iron(II). As a result, the acidity of solution will augment. The reaction can be expressed as the following Eq. (1): þ þ FeðH2 OÞ2þ 6 ! FeðH2 OÞ5 OH þ H
ð1Þ
The rate coefficient of initiation of (NH4)2S2O8 can be expressed as: Kd ¼ K1 þ K2 ðHþ Þ
ð2Þ
where K1 is the rate coefficient of initiation reaction without acid, and K2(H + ) is the rate coefficient of initiation reaction under acid. From Eq. (2), it can be achieved that the more H + there is in the solution, the faster the free radicals engender. Thus, the idle time becomes shorter step by step with the increasing of the concentration of iron(II). However, while the concentration of iron(II) is greater than 0.1 mmol/l, a complex reaction will beget between Fe2 + and the catalyst (TEMED) [12], which will result that the ability of TEMED to activate free radicals decreases. However, the negative effect of such reaction is not more than the positive effect of H + ; thus, the result is that the rate of free-radical generation still increases but with a slower acceleration than before. This makes the idle time keep up becoming shorter. Following the more
ð3Þ
The influence rule of the redox system is shown in Fig. 2. The initiation rate of the redox system is much higher than that of TEMED – (NH4)2S2O8. Thus, the idle time becomes short faster when the concentration of iron(II) is above 0.4 mmol/l. However, the redox system can be influenced easily by oxygen in air. Further more, the polarity and pH of medium also have great influence on the reaction. All of these will make the reaction is difficult to control. On the other hand, an electron transfer reaction can beget between iron (III) and free radicals, which results in the reduction of free radicals. The effect can be shown by the following Eq. (4): fCH2 C ˙ HR þ Fe3þ ! fCH ¼ CHR þ Fe2þ þ Hþ
ð4Þ
Thus, if the concentration of iron(II) is too large, iron(III) is overfull at last. The above result will happen, which makes the radical quantity decrease and the idle time become longer. This effect has been monitored during the experiment. (It is not shown in the Fig. 1 for the concentration of iron(II) is too much at that time.)
Fig. 2. Influence of the redox of Fe2 + – (NH4)2S2O8 on the idle time of acrylamide polymerization.
L. Zhao et al. / Materials Letters 56 (2002) 990–994
993
3.2. Cu2+ ion From Fig. 3, we can get the following result. The free-radical generation is accelerated under very low concentration of copper(II). A deceleration, however, is observed after the copper(II) concentration is over 0.22 mmol/l. The mechanisms can be achieved as follows. When copper(II) is little, it can coordinate with the monomer. During the coordination an electron transfers to copper(II) from monomer. As a result, copper(II) is reduced into copper(I), which can be oxidized by the initiator (NH4)2S2O8. Thus, a highspeed redox system forms as shown in Eq. (5). þ 2 2þ S2 O2 8 þ Cu ! SO4 þ SO4 þ Cu
ð5Þ
Hence, copper(II) can be taken as an effective catalyst at that time. However, when an overdose of copper(II) is administered, the above reaction will not be major. Here, copper(II) will mostly react with the catalyst (TEMED). The activation of TEMED will decline greatly. Furthermore, copper(II) has the same effect on free radicals as iron(III) does. Thus, the concentration of copper(II) is greater than a certain value, 0.22 mmol/l, as shown in Fig. 3. The synthesis of above reactions makes the rate of free-radical generation descend and the idle time become longer.
Fig. 4. Influence of Al3 + on the idle time of acrylamide polymerization.
ation effect is so small that it is not easy to observe when the concentration of Al3 + is under 0.1 mol/l. The result that the mechanism of Al3 + is not related to its concentration can be achieved since the curve in Fig. 4 is so smooth. The compound of Al3 + and water has such a reaction expressed as the following Eq. (6): ½ðH2 OÞ5 AlðOH2 Þ 3þ ! ½ðH2 OÞ5 AlðOHÞ 2þ þ Hþ
3.3. Al3+ ion Fig. 4 shows that Al3 + has an acceleration effect on the free-radical generation. However, the acceler-
ð6Þ From Eqs. (2) and (6), the result that the freeradical generation accelerates due to the increase of H + . However, it needs a large concentration of Al3 + as shown in Fig. 4. Thus, if the concentration of Al3 + in the solution is too little, the acceleration effect is not distinctive. 3.4. Ca2+ ion
Fig. 3. Influence of Cu2 + on the idle time of acrylamide polymerization.
The rate of free-radical generation declines with adding of Ca2 + , but the regulation is not distinctive. Fig. 5 shows the curve of the idle time vs. the concentration of Ca2 + . The result, which is consistent with Niu’s [13], attributes to the steric hindrance effect of Ca2 + reducing the activation of TEMED. The changes in conformation of macromolecules and the sizes of macromolecular coils after the introduction of quantities of Ca2 + lead to a decrease of the
994
L. Zhao et al. / Materials Letters 56 (2002) 990–994
polymerization must be considered, the disadvantages of which should be removed. On the other hand, a new way to control the polymerization may be found according to such influence mechanisms.
Acknowledgements This research work was supported by the National Science Foundation of China (Grant No. 59872018) and ‘‘973 Program’’ of the People’s Republic of China (Grant No.G2000067203). The authors are grateful for the grants. Fig. 5. Influence of Ca2 + on the idle time of acrylamide polymerization.
rate of interaction of the growing macroradical with the monomer. Thus, the idle time becomes long.
4. Conclusions Different metal ions adding to the polymerization system of gelcasting have different effects on the idle time. The different mechanisms have been found as the followings. (1) The effects of iron(II) and copper(II) are very complicated. The mechanisms vary with the changes of their concentrations. Both of them can form a redox system with the gel system under some certain concentration, which accelerates the free-radical generation. However, if their concentrations are too great, on the contrary, a restraint effect will occur. This results from a radical consumption of iron(III) or copper(II) and the complex reaction between iron(II) or copper(II) and TEMED. (2) Al3 + has an acceleration effect on the rate of free-radical generation. However, when the concentration of Al3 + is too little, the effect is not distinctive. Such effect attributes to the hydration of Al3 +. (3) Ca2 + has a restraint effect on the polymerization system for a steric hindrance effect although the effect is much slightly. Since it is ineluctable that minute metal ions exist as impurities in the ceramic powders of gelcasting, the influences of such ions on the idle time of the
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