Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 578-582
578
Kinetics of Gelation in Model Polycondensates DlmRrls S. Argyropoulos' and Henry I. Bolker Pulp and Paper Research Institute of Canada, Pointe Claire, Quebec, Canada HQR 3J9,and Department of Chemistry, McGill Universlty, Montreal, Quebec, Canada H3A 2A7
The kinetics of network formation in model polycondensations of 1,3,5-benzenetriacetic acid (BTA) with decamethylene glycol (DMG) and hexadecane glycol (HDG) were determined. By the application of a simple kinetic model the overall apparent energies of activation of the BTA/DMG and BTAIHDG systems were found to be 59.6 and 58.7 kJ mol-', respectively. The kinetic results revealed that the reaction is affected by diffusion beyond the gel point. The diffusion effect was found to be less significant in the BTAIHDG networks.
Introduction Relatively little research appears to have been done on the course of polymerizations beyond the gel point because investigations have been hindered by the insolubility in any solvent of the resulting network polymers. Any investigations of the kinetics of network formation beyond the gel point have required indirect methods. Thus, in experiments on epoxy resins and thermoset polyester compositions, the extent of conversion of monomer to polymer has been followed by observing the change in physical properties such as refractive index (Dannenberg, 1959), electrical resistivity (Aukward et al., 1958; Miller, 1966), viscosity (Kakurai and Noguchi, 1962), infrared spectrum analysis (Dannenberg, 1963), and, most importantly, heat capacity measurements (Prime, 1970; Prime and Sacher, 1972, 1973; Horie, 1970; Kamal et al., 1973). There is only one account of a kinetic study of model random polycondensates formed from 1,3,5-benzentriacetic acid (BTA) and decamethylene glycol (DMG) (RossMurphy, 1975). It was undertaken in order to examine the suitability of the cascade theory (Gordon and Scantlebury, 1967a; Gordon and Temple, 1972) and to determine the degree of cyclization only up to the gel point. This paper represents an attempt to determine kinetic parameters and thus elucidate the process of network formation beyond the gel point. It should also fill a gap in the literature, which has little information on rate-controlling processes in rigid polymeric media. Theoretical Considerations In polymerization reactions diffusional processes come into play under several different circumstances: (1) in reactions between free radicals; (2) in polymerizations on the solid surface of a catalyst (Le., Ziegler-Natta processes); and (3) in polymerizations in which the reaction has proceeded to high conversions, where the decreased mobility of the reactants makes diffusional processes rate-determining. The formation of polymer networks beyond the gel point probably parallels the last circumstance, where physical rather than chemical factors are important and substant i d y every collision between reactive functions is effective in producing chemical reaction. The formation of a polymer network may be further complicated by a considerable rise in the glass transition temperature, Tg, of the reacting system. If the Tgapproaches or exceeds the reaction temperature, then segmental motions are slowed *Author to whom correspondence should be addressed at the
Pulp and Paper Research Institute.
down, and the chemical reaction is controlled by these motions rather than by chemical factors. Such processes must be taken into account when reaction kinetics beyond the gel point are examined. There is also evidence that the rate coefficients of diffusion-controlled reactions are inversely proportional to the viscosity of the reaction medium (Allen and Patrick, 1974). No model exists, however, that treats the region beyond the gel point as a diffusion-controlled process. The rate of a chemical reaction (dp/dt) can be described by eq 1 (Piloyan et al., 1966), where A. is a constant, f ( p ) dp/dt = Aofb) exp(-EA/RT)
(1)
is a function of the extent of reaction (and is often taken as f(p) = (1- p)", where n is the order of reaction; but in more general cases f ( p ) = p"(1- p)", where m and n are constants), and EA is energy of activation. Acitelli et al. (1971) fitted thermal analytical data for the cure of epoxy resins to eq 2, where k is the overall rate dp/dt = k(1 - p)"
(2)
constant and n is the overall reaction order (Prime, 1970; Prime and Sacher, 1972,1973; Acitelli et al., 1971; Shiraldi et al., 1981). In their conclusions Acitelli et al. (1971) indicated that such reactions are diffusion controlled, because the overall energy of activation was similar to literature values given for the self-diffusivities of some polymers. The order of reaction, n, as defined by eq 2, was found to vary (with temperature) from 0.5 to 1.1 (Acitelli et al., 1971). Since it is therefore difficult to know what significance to attach to energies of activation derived from overall rate constants k , Schiraldi et al. (1981) referred to n as a "phenomenological" order of reaction. More recently the cross-linking of various epoxy resin systems has been described by another kinetic expression (eq 31, where p dp/dt = ( k , + k2pm)(1 - p)"
(3)
is the fractional degree of reaction, k , and k2 are temperature-dependent kinetic rate constants, and m and n are kinetic exponents (Horie, 1970; Kamal et al., 1973; Sourour and Kamal, 1976; Ryan and Dutta, 1979). This model suggesta that the curing process is governed by more than a single rate constant. Whenever the model has been used, the kinetic parameters have been obtained by least-squares fitting of thermal data (DSC) to it or by combining the variables in order to produce a linear relationship from which the kinetic parameters can be determined. The latter model was further modified by Barton (1980) in order to account for the diffusion control
0196-4321I86/1225-Q578$01.50/0 0 1986 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 579 Scheme I
6 COOH I
HOOCCH~‘
CH~COOH
+
Polyester Networks
H OfCH&OH
/ BTA’DMG +
H20
\BTAiHDG
x = 10, Decamethylene Glycol, DMG
x = 16, Hexadecane Glycol, HDG 1,3,5-Benzenetriacetic Acid (BTA)
of the overall cross-linking reaction. Equation 2 is a useful expression for it may lead to fundamental qualitative conclusions on the nature of the Cross-linking process. Such conclusions are of great interest in the overall perspective of this work. Therefore the application of this expression is discussed in this paper.
Results and Discussion The model random polycondensates produced from the esterification of 1,3,5-benzenetriacetic acid (BTA) with decamethylene glycol (DMG) and hexadecane glycol (HDG), respectively, were easily followed kinetically by monitoring the amount of water evolved during polymerization. A representation of the actual chemistry operating is shown in Scheme I. The overall kinetic results at various temperatures are shown in Figures 1and 2. For the polymerization of a trifunctional monomer with a difunctional monomer &e., RAf + BB, where f = 3) in stoichiometric equivalence, Flory’s theory predicts that the critical point should appear at p = 0.707 (Flory, 1953). It is apparent from Figures 1and 2 that a retardation in the rate of reaction takes place after the gel point (above 70% conversion), obviously because of network formation. Accurate critical point determination was beyond the scope of this work, but in both reactions it appeared within the range predicted by Flory’s theory. Since the region beyond the gel point was of primary concern, it was examined more closely by taking more conversion readings than were taken in the pregel region. Another noteworthy feature of Figures 1 and 2 is that the rates of BTA/HDG polymerization are faster than those of the polymerization of the BTA/DMG system, in the region beyond the gel point. The difference between the two polymerizations lies in the number of methylene groups between cross-links (10 in BTA/DMG and 16 in BTA/HDG). This observation suggests that the parameters determining reactivity beyond the gel point include polymer flexibility and its effect on segmental mobility. The presence of an extra six methylene groups between cross-links has a plasticizing action in the polymer, thus somewhat lowering its glass transition temperature (Shen and Eisenberg, 1970) and hence enhancing segmental mobility at the elevated temperatures of the polymerization reaction. This interpretation is essentially based on the free-volume concept; the extra methylene groups increase mobility because they add “excess free volume” between cross-links (Bueche, 1962). Theoretical expressions correlating the glass transition temperature with the degree of cross-linking have been derived by Fox and Loshaek (1955) and DiMarzio and Gibbs (1963). Their treatments, however, refer to processes of cross-linking preformed chains. There are no published relationships for networks formed de novo from polyfunctional and difunctional monomers. In any case, Fox and Loshaek’s treatment arrives at an expression that predicts a linear relationship between the glass transition temperature of the network and the number of cross-links per unit mass. Thus an increase in the molecular weight between cross-links is expected to decrease glass transition
A 14OoC 155OC 0 18OOC
I
J
50
100
150
200
250
350
300
Polymerization time, min
Figure 1. Overall kinetic results from the polymerization of BTA/DMG.
A 140°C e 155’C 0 l800C
I
J
50
100
150
200
250
300
Polymerization time, min
Figure 2. Overall kinetic results from the polymerization of BTA/HDG.
temperature and explains the enhanced mobility observed in the BTA/HDG networks. It therefore became of interest to attempt to evaluate the magnitude of the effect of increasing molecular weight between cross-links by calculating the “phenomenological” rate constants and Yapparent”energies of activation of both network types.
Application of dp/dt = k(1 - P ) ~ The kinetic data from both reactions was fitted to eq 2 with the results given in Tables I and 11. Two sets of calculations were made, one treating the overall polymerization process (Table I) and one that looks only at the region beyond the gel point (Table 11). Plots of log (dpldt) vs. log (1- p ) at the various temperatures of polymerization gave the “phenomenological”single rate constants, k , and overall reaction orders, n. The rate constants, in turn,
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
Table I. Summary of Kinetic Parameters Derived for the Overall Processes of BTA/DMG and BTA/HDG Polymerization by ADDlYinb? Eauation 2 BTA/DMG BTA/HDG reaction reaction temp, "C 1O4k: s correln coeff order, n av n temp, "C 104kbs correln coeff order, n av n 140 6.30 0.953 2.2 140 4.16 0.999 1.2 155 9.50 0.973 1.8 1.9 155 12.30 0.977 1.9 1.6 180 29.70 0.979 1.8 180 20.70 0.977 1.8 "Temperature dependence of k: k = 1.9 X 58.7/R11. EA in kJ mol-'.
lo4 exp(-59.6/Rn. EA in kJ mol-'.
bTemperaturedependence of k : k = 1.28 x lo4 exp(-
Table 11. Summary of Kinetic Parameters Derived for the Beyond the Gel Point Processes of BTA/DMG and BTA/HDG Polvmerization bv ADDlYing Eauation 2 BTA/DMG BTA/HDG reaction reaction temu, "C 104k,"s correln coeff order, n av n temp, "C 104kbs correln coeff order, n av n 140 2.63 0.987 140 0.49 0.962 1.0 1.0 0.996 1.6 1.3 0.996 1.3 1.1 155 6.30 155 1.95 180 13.00 0.970 1.4 0.963 1.2 180 5.05 "Temperature dependence of k : k = 4.4 59.5/RT). EA in kJ mol-'. -4
-10
X
lo6 exp(-86.1/RT). EA in kJ mol-'. bTemperaturedependence of k : k = 9.4
,
I
1
I
I
2.2
2.3 T-1
I O
I
2.4
.10w
Figure 3. Arrhenius plots of rate constants as derived from the model dpldt = k(1- p)" for the two different stages of polymerization for the overall process and for that beyond the gel point (lines drawn according to linear regression analysis).
were treated by the logarithmic form of the Arrhenius equation, In k = In A - EA/RT. The rate constants of both reactions increase with increasing polymerization temperature, as expected from elementary kinetic considerations. The Arrhenius plots of both overall polymerization processes (Table I; Figure 3) resulted in almost equal apparent activation energies and preexponential factors (within the range of experimental error). The value of the apparent energy of activation, 59.6 kJ mol-', of the overall process of BTA/DMG gelation compares well with the 55.2 kJ mol-' reported by Peniche-Covas (1973) for the same reaction. This latter value was derived from accurate measurements of esterification rates obtained by following the steam pressures in a sealed reaction vessel as a function of time in the 80-90 "C temperature range (Peniche-Covas, 1973; Love, 1968); the mathematical model used to derive the kinetic parameters was based on the theory of branching processes developed by Gordon (Harris, 1963). (Details of the model have been given by Gordon and Scantlebury (1967b)J Love (1968), using similar techniques, reported an overall reaction constant, k , for the same reaction at 170 "C as 9.1 X s-l. Thus the values of EA and k in the literature are supported by those determined in this laboratory by applying the relatively simple eq 2. Fitting only the kinetic data from beyond the gel point region into eq 2 gave the results of Table 11. The fact that the apparent energy of activation of the BTA/DMG re-
X
lo3 exp(-
action beyond the gel point is higher than that of the overall process suggests that diffusional factors determine the kinetics in this region. This argument does not apply, however, to the BTA/HDG polymerization, which showed very similar apparent energies of activation for both the overall reaction and the reaction beyond the gel point (Tables I and 11). Therefore, in the cross-linking of the BTA/HDG networks beyond the gel point, diffusion is less critical. These observations further indicate that the somewhat lower overall energy of activation of the BTA/DMG system that was reported by Peniche-Covas (1973) (as compared to ours) may be due to the low temperature range (80-90 "C) of their polymerization: it is highly probable that they obtained only limited conversion. Our studies showed that in this temperature range the reaction ceases at p = 0.84, probably because the Tgof the network becomes higher than the polymerization temperature. Conducting the process at elevated temperatures (140-180 "C)increases mobility and hence encourages diffusion, thus permitting the sampling of kinetic information to almost complete conversion. It is very likely that as conversion increases, diffusion progressively becomes the dominant factor and, in turn, increases the value of EA. The "phenomenological" overall reaction orders as determined by the model under examination show that the overall process tends to second-order kinetics (Table I). The reaction order beyond the gel point, however, tends to first-order kinetics (Table 11). Kinetic studies on the BTA/DMG system, at various dilutions with inert 1,3,5tris(diphenylmethylcarboxymethy1)benzene in the melt state, have shown that the initial stages of the polycondensation follow third-order kinetics (Ross-Murphy, 1975). As the formation of intramolecular cyclic links increases, the reaction progressively falls below third order, because self-catalyzed cyclization obeys second-order rate laws (Gordon and Scantlebury, 1967b). James and Guth (1947) suggested that after the gel point gel-gel reactions should be considered intermolecular and not intramolecular. Consequently the rate of cyclization should decrease after the gel point, and the reaction order should then again increase to a third-order rate process. A third-order rate law accords with the original work of Flory (1939), who showed that linear polyesterification follows third-order kinetics, in close agreement with earlier work on simple esterifications (Rolfe and Hinshelwood, 1934). Similar
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 581
".'I// 0.3
0 Monodisperse chain tunctionallty 2.8 0 D )) )) 8.0
/ 2 4 6 8 10 12 14 16 18 Polymerization time, h
Figure 4. Overall kinetic results for the cross-linking of monodisperse polystyrene carboxylated a t two different functionalization levels with decamethylene glycol. Table 111. Kinetic Parameters for the Cross-Linking of PS-COOH with DMG at Two Different Degrees of Cross-Linking at 130 "C monodisperse overall overall chain reaction reaction functionality rate, s-' order, n 2.8 4.9 x 10-4 1.6 8.0 5.0 X 0.9
kinetics are not to be expected, however, in reactions where diffusion operates. The work of Ross-Murphy (1975) shows a considerable lowering in the reaction order beyond the gel point. This deviation has been rationalized by Gordon and Parker (1971) and is believed to indicate that "the reaction rate is decreased and becomes controlled by the rate at which partner functionalities can diffuse together''. In the present research, kinetic studies were also made of the random cross-linking of preformed chains at 130 "C. Monodisperse polystyrene (A& = 2100, A&, = 1900) appropriately functionalized with -COOH groups (Argyropoulos and Bolker, 1986) at two different levels was cross-linked with stoichiometric amounts of DMG by melt polycondensation, giving the kinetic results shown in Figure 4. The presence of 8 cross-linking sites per primary chain led to an early gel point and lowering of the overall reaction rate relative to the material possessing 2.8 cross-linking sites per primary chain. The application of eq 2 to these results gave the rate parameters in Table 111 which show that an increase in the cross-link density substantially lowers the overall reaction rate. Furthemiore, a lowering of the overall reaction order with increasing cross-link density is observed. At high cross-link densities the kinetics of network formation tend to n = 1. It is worth noting here that the overall rate of reaction of the monodisperse polymer possessing a functionality equal to 2.8 is close to that of the reaction of BTA/HDG at 140 "C (Table I). This partly justifies, in a kinetic sense, Berry's (1980) theoretically proven statement that "...for f = 4 is the same theoretically whether the functional groups are distributed along a chain or distributed around a central point''. Edwards (1975) found that network formation of telechelic brominated polybutadiene well beyond the gel point followed first-order kinetics. He pointed out that "this is what should be expected for a diffusion controlled network building step". The result itself is quite surprising when one realizes that diffusion operates even in systems that have M , values of 40 X lo3 and 5 X lo3 in the initial and final stages of polymerization, respectively (M, = molecular
weight between cross-links). Such large M, values are expected to yield networks with enhanced functional group accessibility. The glass transition temperatures of Edwards's networks were in the -53 to -104 "C temperature range, which is well below the temperature of the kinetic experiments (25 "C). Even then, diffusion was found to operate beyond the gel point. The delay of the gel point relative to that predicted by the Flory-Stockmayer theory in networks formed by addition polymerization has been extensively studied (Walling, 1945; Minnema and Staverman, 1958; Loshaeck and Fox, 1953; Gordon and Roe, 1956; Shultz, 1958; Price et al., 1958). Walling (1945) attempted to attribute the effect to the diffusion control of the reactions, but Gordon and Roe (1956) produced convincing evidence to the contrary. In a detailed investigation Gordon and Roe attributed the delaying gelation effect to cyclization and chain transfer operating before the gel point. Their data, however, explicitly admits that diffusion operates beyond the gel point. It is not at all surprising, therefore, that the present results on the BTA/DMG and BTA/HDG networks, beyond the gel point, tend to indicate diffusion-controlled kinetics.
Summary The kinetics of network formation show rate retardation with increasing conversion beyond the gel point. The application of eq 2 to the kinetic results gave an apparent energy of activation for the BTA/DMG reaction equal to 59.6 kJ mol-', which compares well with the literature value of 55.2 kJ mol-' (Peniche-Covas, 1973). Diffusion is less critical in the cross-linking of BTA/HDG than in the BTA/DMG reaction. An increase in the molecular weight between cross-links reduces the domination of diffusion beyond the gel point, but never eliminates it. The rate of diffusion is a factor not taken into account in the theoretical derivation of gelation equations. The assumption of random bond formation within the growing network is thus violated, which may seriously affect the applicability of such equations beyond the gel point. Experimental Section Networks were formed by the polycondensation of 1,3,5-benzenetriacetic acid (BTA) with the diols 1,lOdecamethylene glycol (DMG) and 1,16-hexadecane glycol (HDG) and by the polycondensation of monodisperse carboxylated polystyrene (synthesized according to Argyropoulos and Bolker (1986)) with 1,lO-decamethylene glycol. The conversion parameter, p , was monitored by weighing the polymerization tube at various time intervals; the reduction in weight, due to H 2 0 losses, was related to the extent of reaction by eq 4, where Wo is the original P = (WO- W,)/(WO - WlC€l) (4) weight of the reaction mixture, W , is the weight of the mixture after time t, and W,, is the theoretical weight at reaction completion. To simplify the calculations, the ratio of trifunctional component to difunctional component was always maintained as 2:3; i.e., exact stoichiometry between acid groups and hydroxyls was kept. Both components were dried to constant weight in a vacuum oven prior to use. All equipment used in the polymerizations was kept in an oven at 110 "C, and then, before beginning the reaction, the equipment was cooled in a desiccator, thus minimizing errors occurring from water condensing on its surface. The two predried components were accurately weighed in a polymerization tube. A small magnetic stirrer was also placed in the mixture to facilitate mixing during
582
Ind. Eng. Chem. Prod. Res. Dev. 1986, 25,582-585
the reaction. The tube was sealed with a rubber septum, and dry nitrogen was passed in at a moderate rate to exclude oxygen. The needles used for nitrogen inlet and outlet were removed, and the tube was immersed in an oil bath preset at 170 "C (or 180 O C for collecting data at this temperature), heated by a hot plate with a magnetic stirring motor. Within 5 min the low-melting-diol component had melted, and the mixture was partly homogenized. The temperature was rapidly lowered to the appropriate temperature when needed by adding cold oil into the heating bath. At selected time intervals the tube was withdrawn and wiped clean, and while it was still hot, its septum was removed, and the tube was placed in a vacuum desiccator (at mmHg) over KOH to reach room temperature and constant weight. This ensured complete removal of water at each data point. Finally, the tube's surface was dried and degreased with acetone. The difference in weight was used to calculate the extent of reaction at the particular time. Subsequent repetition of this procedure with the same sample produced additional points for the conversion vs. time plots. The reproducibility of this method was f0.02 in the determination of
P* Registry No. BTA, 4435-67-0; HDG, 7735-42-4; DMG, 11247-0; (BTA)(DMG)(copolymer), 34606-50-3; (BTA)(HDG)(copolymer), 104241-62-5.
Literature Cited Acitelli, M. A.; Prime, R. B.; Sacher, 0. Polymer 1071, 72,335. Allen, P. E. M.; Patrick, C. R. Kineflcs and Mechanisms of Polymerization Reactions; Ellls Horwood: Chichester, 1974; Chapter 2, pp 116-117. Argyropoulos, D. S.; Bolker, H. I.Polym. Prepr. ( A m . Chem. Soc., Div. Powm. Chem.) 1086, 27, 457. Aukward, J. A.; Warfield, W.;Petree, M. C. J. folym. S d . 1058, 27, 199. Barton, J. M. Polymer 1980, 21,603. Berry, R . M. Ph.D. Thesis, McGill University, 1980, Chapter 5, p 222.
Bueche, F. Physical fropertes of Polymers; Interscience: New York, 1962. Dannenberg, H. S E J. 1050, 75, 875. Dannenberg, H. SPE Trans. 1063, 3 , 78. DiMarzio, E. A.; Gibbs, J. H. J. Polym. Sei., Polym. Chem. Ed. 1983, 7 , 1417. Edwards, D. C. Rubber Chem. Technol. 1075, 48(2), 202. Flory, P. J. J. Am. Chem. SOC. 1030, 61,3334. Flory, P. J. Principles of Polymer Chemistry; Cornell University: Ithaca, NY, 1953: Chapter I X . Fox, T. G.; Loshaek, S. J. Polym. Sci. 1055, 40, 371. Gordon, M.; Roe, J. J . Polym. Sci. 1056, 21, 27, 75. Gordon, M.; Scantlebury, G. R. J. Chem. SOC. 19678, C-I. Gordon, M.; Scantlebury, G. R. J. Chem. SOC. B 1067b, 1. Gordon, M.; Parker, T. W. Proc.-R. SOC.Edinburgh, Sect. A : Math. Phys. S d . 1071, A69, 181. Gordon, M.; Temple, W.B. Makromoi. Chem. 1072. 263, 160. Harris, T. E. The Theory of Branching Processes : Sprlnger-Veriag: West Berlin, 1963. Horie, K. J. folym. Sci. folym. Chem. Ed. 1070, 8 , 1357. James, H. M.; Guth, E . J. Chem. fhys. 1947, 75, 669. Kakurai, T.; Noguchi, T. Kobunshi Kagaku 1962, 19, 547. Kamal, M. R.; Sourour, S.: Ryan, M. Annu. Tech. Conf.-Soc. fiast. Eng. 1073, 19, 187. Loshaek, S.; Fox, T. G. J. Am. Chem. SOC.1053, 75, 3544. Love, J. A. Ph.D. Thesis, Strathclyde University, 1968. Miller, 8. J. Appl. Polym. Sci. 1066. 10, 217. Minnema, L.; Staverman, A. J. J. Polym. Sci. 1058, 29, 281. Peniche-Covas, A. L. Ph.D.Thesis, University of Essex, 1973. Ryabchikov, I.D.; Novikova, 0. S. Nature (London) 1066, Piloyan, G. 0.; 212, 1229. Price, F. P.;Glbbs, J. H.; Zimm. B. H. J. Phys. Chem. 1058, 62,972. Prime, R. B. Anal. Caiorim. 1970, 2. 201. Prime, R. 6.: Sacher, E. Polymer 1072, 13,455. Prime, R . B.; Sacher, E. folym. Eng. Sci. 1073, 13, 365. Rolfe, C. N.; Hinshelwood, G. N. Trans. Faraday SOC. 1934, 3 0 , 935. Ross-Murphy, S.B. J. folym. Sci., Polym. Symp. 1975, No. 53,11. Ryan, M. E.; Dutta, A. Po/ymer 1070, 20,203. Schiraldi, A.; Wagner, V.: Samanni, G.; Rossi, P. J. Therm. Anal. 1081, 21, 299. Shen, M. C.; Eisenberg, A. Rubber Chem. Techno/. 1970, 43(1), 95. Shultr, A. R. J. Am. Chem. SOC.1058, 8 0 , 1854. Sourour, S.; Kamal, M. R. Thermochim. Acta 1076, 14, 41. Walling, C. J . Am. Chem. SOC. 1045, 67,441.
Receiued for review December 2, 1985 Accepted April 14, 1986
Nickel Powders into Sintered Structures for the Alkaline Battery: Porosity Studies Victor A. Tracey" Inco Alloy Products Ltd., Birmingham 8 1 6 OAJ, Unitecl Kingdom
INCO nickel powder type 255 is used for the production of porous struclures for the alkaline battery. Typical porosity levels obtained by processing the powder using the optimum conditions of the slurry sinter route are about 81 YO. The increase in porosity possible by varying sintering temperature, addition of spacing agents, and use of loose sintering has been explored. The increase in porosity (up to about 90%) has been related to strength and pore structure changes.
Introduction The porous nickel used for the manufacture of electrodes of alkaline batteriea is prepared from irregular INCO nickel powder types 255 and 287 (Inco, 1981a,b) through processes involving sintering. Two processes are employed, loose sintering and slurry sintering, and the latter process has been favored by many manufacturers because of its
* Address correspondence to International Nickel Inc., Saddle Brook, N J 07662.
continuous nature (Falk and Salkind, 1969). In the manufacture of the sinter, time, temperature, powder characteristics, and atmospheres are important, and an earlier paper looked at these parameters with the objective of optimizing the process (Tracey, 1982). The paper showed that sintering at 950-1000 "C gave the best strength on the basis of economic considerations. Under the conditions specified, INCO nickel powder type 255 would produce a sintered nickel porosity of about 81% (Table I).
0 196-432118611225-0582$01.50/0 0 1986 American Chemical Soclety
