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Reactions of zinc oxide with molten diacids and their anhydrides
European Polymer Journal Vol. 10, pp. 267 to 272. Pergamon Press. Printed in Great Britain.
REACTIONS OF ZINC OXIDE WITH MOLTEN DIACIDS AND THEIR ANHYDRIDES M. NARrdS, I. RON, A. SIEGMANNand L. K^CIR Plastics Institute, Center for Industrial Research, P.O. Box 31 l, Haifa, Israel (Received 14 July 1973) Abstract--The reactions of some organic diacids and their anhydrides with powdered ZnO were studied. The products are salts of approximately 1: 1 mole composition. Their thermal stability is rather high, in comparison with their organic components. They are insoluble and usually cannot be recrystallized. It is suggested that anhydrides react with ZnO via their parent diacids, resulting initially in a half salt, as an intermediate product, followed by the formation of the normal salt.
INTRODUCTION Ionomers are polymers in which cross-linking is achieved by ionic bridges between pendant ionized carboxyl groups. A peculiar property of ionomers is their dual character; they are "cross-linked" in the solid state, and accordingly possess improved mechanical properties over their parent acid copolymer [1], In the molten state, however, they are thermoplastic and are processed as such. This type of polymer is exemplified by Dupont Surlyn A resins which have wide commercial applications. A summary of the chemistry and properties of ionomers is given in the literature [2, 3]. The idea of linear polycondensed salts of diacids is noted in the early work of a French group [4]. These authors calculated a "degree of polymerization" for Mg 2+, Cd 2+ and Pb 2+ sebacates from the non-stoichiometric metal content of salts precipitated from water. Economy et al. [5] have recently published their work on reactions leading to metal salts of dibasic acids under the title "'Halatopolymers", underlining their polymerlike behaviour in the melt. This behaviour is demonstrated mainly by their high viscosity which is influenced by the interchanging network of dianions with metal cations. The salts were prepared either by double decomposition of divalent metal acetates with the diacids, as shown, M(OAc)2 + HOOC--R--COOH [M2"-OOC--R--COO-]~ + 2AcOH
(1)
or precipitation from aqueous solutions of the sodiumsalts of diacids and metal dichlorides as follows: MCI2 + Na+-OOC--R--COO-Na* ~
yields a low crystalline solid, whereas the second method yields a highly crystalline salt. The products of both methods are similar above their fusion temperature. After fusion and subsequent cooling, similar amorphous solids are obtained. DTA and DSC curves of the so-called "amorphous products" exhibit melting peaks which indicate that they contain microcrystallites and are therefore not truly amorphous. The salts could not be crystallized by slow cooling from the melt or by annealing. The viscosities of several calcium sebacates melts were measured and the time average molecular weight at 386 ° was estimated as 2000, corresponding to approximately 10 repeat units. In the molten state the chains are not polymers in the classic sense, since they dissociate and associate frequently and have no definite "'end-groups". The authors conclude that "metal dicarboxylates can exist either as salts or amorphous polymerlike structures, depending on the method of preparation" [5]. They term the change from salt to polymer as "halatopolymeric transformation". Systems of polymers or copolymers containing carboxylic groups bridged by metal ions were studied by Otocka [6], Satas [7] and Nielsen [8]. We have initially studied the reaction of an alternating styrene/maleic anhydride copolymer with ZnO; for a better understanding of this system, we have experimented with simple model systems, which are reported in this paper. By using a free diacid, the effect of the backbone-polymer on the reaction is eliminated and the acid group concentration is increased. The reason for using anhydrides was to avoid the evolution of water which would interfere in a moulding operation. The anhydride/ZnO reaction was found to be more complex than the acid/ZnO reaction, and most probably proceeds via an acid intermediate.
EXPERIMENTAL Macro-scale preparations were usually performed with X-Ray powder pictures show that the first method 0-14).5 mole quantities. The acid or anhydride was melted 267 [M -'+ -OOC--R--COO-]~ + 2NaCI.
(2)
268
M. NARKIS,1. Roy,, A. SIEGMANNand L. K^cm
in a glass flask and powdered ZnO added with stirring. The increasing viscosity of the medium did not usually allow the addition of a full equivalent ofZnO. Soxhlet apparatus was used to extract the excess organic component. The non-extractable dry solids were later analysed by TGA and DTA. Microscale reactions were accomplished directly in the TGA pan. For a microscale preparation, the components were mixed by co-grinding weighed portions of the solids and up to 30 mg of the mixture were put in a TGA balance pan and heated to a constant temperature above the m.p. of the organic substance. The molten acid or anhydride, on reacting with the oxide, became ionically bound and lost its volatility while the free acid or anhydride was evaporated. The product could be further heated to decomposition, or taken out for DTA, DSC and solubility tests. Both methods of isolation (extraction and volatilization) make use of the fact that the excess acid or anhydride remains non-bonded and removable by physical means. Excess ZnO, if present, cannot be removed by these methods. The DuPont 990 Console with TGA, DTA and DSC modules was used for the thermal analyses. Chemical analysis is based on calcination in the TGA apparatus. Air calcination was used since it yields ZnO, while calcination in nitrogen caused partial reduction of the oxide.
RESULTS AND DISCUSSION Table l summarizes the results of the bulk preparations. Assuming that the molar ratio of the reacted oxide to acid in the product is 1 : 1, it was found by analysis that the product contained excess oxide (after
extraction of the non-bonded acid). This was attributed to insufficient contact between the reactants. Table 2 summarizes the results of the microscale preparations and shows that compositions closer to 1 : 1 mole ratio can be obtained. In this series, pre-mixing of the dry powders and an excess of the organic component (later to be evaporated) provide better contact. In general, since Z n O does not dissolve in the organic phase, concentration effects cannot be expected and contact between the phases is the determining factor. Anhydrides generally yield lower ratios of organic/ oxide for reasons to be discussed later. In the case of an acid, a half-salt as an intermediate product may be expected. The idea of the formation of a half-salt is based on an early "melting point" (i.e. endothermic peak) which was observed between the melting point of the free acid and the decomposition (and/or melting) temperature of the final product. If this early peak represented the melting of the final product, it would be difficult to understand the endothermic form of peaks generally obtained at higher temperatures. The melting point of the half-salt may be reasonably expected to be lower than that of the normal salt. The early peak which is assigned to the half-salt does not always appear. The DTA curve of Fig. I shows that this half-salt disappears on prolonged isothermal heating (see curve B, without peaks) yielding the normal double salt.
Table I. Composition and thermal properties of diacid zinc salts prepared in bulk
Run no. 1
Organic compound Succinic acid Succinic anhydride Succinic anhydride prepared by compression moulding Zinc succinate prepared in solution
Composition organic component (wt %) (mole %)
DSC or DTA endothermic peaks Temperature Initial Maximum
54.2 50.1
48.1 45.1
430 --
450 492
42.3
37-4
--
--
55-7
50.5
--
500
2
Phthalic anhydride
59.5
44.7
3
Sebacic acid
48'8
283
Zinc sebacate prepared in solution
--
--
4
Adipic acid
53"9
42"5
No data
5
Zinc fumarate prepared in solution
44.3
39-8
360 small 385 wide
* Reversible.
No melting 430 = decomposition 300 320*
340 330
TGA Temperature of decomposition Initial Maximum
400 420
420
450
500 "
410
450
300 330
330 400
400 ° decomposition No data 370
400
Reactions of zinc oxide
269
Table 2. Composition and thermal properties of diacid zinc salts prepared from mixed powders (microscale) Composition Run no. 6
7
Organic compound
Temperature of preparation
Succinic acid Succinie anhyd. Phthalic anhyd. Phthalic acid*
8
9
10
Sebacic acid
200 200 120-150
52.1 53.4 54-2
46,5 48,1 49.0
180 220 240 150--180
0 54-0 55-5 60
39.2 40.6 45
250
Maleic anhyd. Maleic acid*
68
4-8-8 57-2
110
180
Furnaric acid
Organic component (wt ~) (mole ~)
285
0
48
3-7-4 52.4
--
DSC or DTA endothermic peaks Temperature Initial Maximum 410 440 w
No melting 430 (exo). --
450 270 500 --
TGA Temperature of decomposition Initial Maximum 400 440 -m
450 500 ---
380 320
430 430
330
400
-260 250 270 360 405 (exothermie)
360
380
360
400
390 400 (exothermic)
330
390
--
280 430
--400 (exo).
335 350 --
* Prepared in situ by addition of water to anhydrides.
I
I
I
I
Tem~ra*ure
Fig. 1. DTA thermograms.(A) scanning of reacting fresh mixture of ZnO/succinic anhydride (B) scanning of zinc suceinate, preparation at 200 ° (Heating rate of 20°/rain).
The melting of the normal salts (ca. 400 °) is usually accompanied by decomposition. In the case of zinc sebacate, however, it was possible, by cooling, to recrystallize the material from the melt. A detailed description of the various reactions and characterization of their products follows. Succinic acid (m.p. 190 °) reacts easily to give a product of approximately 1:1 m o l e ratio, when heated slowly to a constant temperature above the melting point of the acid. This product is stable up to 400 ° as measured by T G A (Table 2). Freshly prepared succinic anhydride/ZnO mixtures give very low yields under similar experimental conditions. It was found that the
yield could be increased simply by letting the mixture stand at r o o m temperature before heating. After a few days the maximum reaction was obtained (49 mole per cent organic). These yields were independent of the amount of excess organic component and the compositions given refer to the solid residue. Similar results were obtained for phthalic and maleic anhydrides. This behaviour suggests that the anhydride is first opened to the acid by the residual water in the anhydride powder and that small amounts of water are sufficient for the reaction to proceed. We have found that succinic anhydride containing water behaves as free acid. On carrying out the reaction between Z n O and succinic anhydride in a DTA capillary, the following endothermic peaks (Fig. 1), were obtained: melting of the anhydride (ca. 80 °, not shown), melting of the acid, a wide peak at 2700 attributed to the half salt, and a final melting of the product starting at about 440 ° with maximum at about 500 °. The 440 ° peak corresponds approximately to the beginning of decomposition found in the T G A curve. For comparison, crystalline zinc succinate was prepared by precipitation from water and found to contain 50-5 mole per cent organic component. When this crystalline salt was analysed by DSC, a melting peak with a maximum at 500 ° was observed. When a mixture of Z n O and anhydride was allowed to react for several hours at about 200 ° and with occasional addition of water, the product showed a DSC maximum at 492 °. In both high melting materials, the endothermic
270
M . N A R K I S , |. R O N , A. SIEGMANN a n d L. K A C I R
2boo
/
o
o
J'
~wK
!i =
'
~0
-i ........
\v
ba
E w
,,
I
I
I Temperatu re,
I
I
eC
2O0
250
I 400
300
Temperot ure,
"C
Fig. 2. DSC thermograms (A) scanning of zinc phthalate, preparation at 140°,-(B) scanning of zinc phthalate which was kept for additii~nal 3 hr at the reaction temperature (heating rate of 20°/rain).
Fig. 3. DSC thermograms (A) scanning of zinc sebacate, preparation at 250 °, (B) scanning of fresh zinc sebacate, preparation in solution, (C) scanning of zinc sebacate which has gone through cycle (B) (heating and cooling rates of 10°/min).
sharp peaks were followed by sharp exothermic peaks indicating decomposition of the melts. Attempts at recrystallization of the salt from the melt by slow cooling gave a product which (on DSC) showed a wide exothermic band, from 500 to 420 °, indicating some recrystallization. Phthalic anhydride (m.p. 130"8°) exhibits more complex behaviour. Its dry mixture with ZnO, when kept at 180 ~ (i.e. above its m.p. but below the m.p. of the acid) leaves ZnO with no bound organic matter. When a little water is added, the anhydride reacts at 150-180 ° to give a product of 45'5 mole per cent organic, stable to 380 °, which is practically the 1:1 salt. Good yields are obtained from the commercial anhydride by quick heating to 220-240 °. The anhydride may contain some free acid (m.p. 206 °) and therefore a reaction via the acid can occur. The acid reacts with the oxide, liberating water which starts a "chain reaction" of anhydride opening. This water transfer has to be faster than evaporation of the water. In a separate experiment where water was added, the formation of acid could be followed by DSC. The peak area of the anhydride decreases while that of the acid increases. Figure 2a shows that the products from reaction between ZnO and anhydride at 130-150 ° contain anhydride, free acid and half salt (melting peaks at 135, 220, 290 °, respectively). The melting range of the zinc phthalate is around about 440 °. After reaction with additional water at 130-150 ° (this run is not shown in Fig. 2), the anhydride peak disappears while the acid, the half salt and the melting of the zinc phthalate peaks are still strong. Curve B in Fig. 2 shows the DSC of the same mixture after three additional hours of reaction at 130-150 °. A very weak peak of the acid is still observed, and two endothermic peaks at 440 and 575 ° are obtained. In summary, phthalic anhydride most probably reacts via the free acid. Terephthalic acid [5-1 gives very stable salts of Sn, Mg, Ca and Ba via their acetates, decomposing without melting at 400, 495, 525 and
590 °, respectively. These relative stabilities might reflect increasing ionic character of the salts. Sebacic acid (m.p. 134 °) was reacted with ZnO at 250 ° to give a product of 48 mole per cent organic (Table 2) and stable up to 330 ° (5 per cent loss by TGA). The crystalline zinc-succinate salt was prepared by reacting aqueous solutions of ZnCI2 and sodiumsebacate. The DSC of this salt (Fig. 3a) shows further crystallization above 250 °, a sharp melting peak at 330 ° and decomposition above 360 °. The TGA of this salt shows initial weight loss at 300 ° similar to data obtained for the product prepared at 250 ° and to literature data [5"1. By heating a fresh sample of the crystalline solution product in the DSC pan up to 350 ° (to avoid any significant decomposition) and then cooling at a rate of 10°/rain, recrystaIlization takes place as shown by the 230 ° exothermic peak in Fig. 3b. Further temperature cycling (Fig. 3c) of the same sample results in a lower and broader melting peak which may indicate partial decomposition. The i.r. spectrum of the crystalline salt (Fig. 4) is much sharper than that of the fusion product, and contains three extra bands at 1200, 1270 and 1340 cm -~. These extra three bands may be due to lattice vibrations. The spectrum of the melt reaction product contains one additional band at 1720 cm -t (which does not appear in the salt), probably due to free acidic groups. It seems that after repeated fusion, the melt reaction and the solution products become practically identical. Maleic anhydride (m.p. 56 °) reacts via the free acid. The dry anhydride mixed with oxide gives a product of 3-7 mole per cent organic only (Table 2). The acid, prepared in situ by adding equivalent amount of water to the anhydride while holding it below 100 °, was further reacted with ZnO at 180 ° to give a product practically 1: 1 mole ratio (52.4 mole per cent organic). Additional experiments have shown that the reaction proceeds at any temperature above the
Reactions of zinc oxide melting point of the acid (130 °) and that the nonbonded acid can be evaporated below 200 °. The product is stable up to 360 ° (TGA) where it starts to lose weight, and reaches a maximum evaporation rate at 380 °. The DSC shows a small broadened peak at 260 °, followed by a moderate exothermic trend. This is attributed to melting, followed by slow disproportionation of the half-salt At higher temperatures, a strong exothermic decomposition peak starts at about 360 ° and reaches its maximum at 405 °, with no melting peak preceding it. Thus, zinc maleate behaves differently from the chemically similar zinc succihate, which first melts sharply and then decomposes. It seems that the double bond in the zinc maleate is the reason for the lower stability of this salt Fumaric acid (m.p. 286 °) is interesting for comparison with the isomeric maleic aci& Here the half-salt is expected as a necessary intermediate since two metal atoms must be involved. Zn fumarate could not be prepared in bulk by fusion probably because the liquid vapour pressure at its high trrp. is too high. In the thermobalance, a iaroduct was obtained by rapid heating of the ZnO/acid mixture to 300 ° and immediate cooling to 285 ° and subsequently slow heating to 330 °. Slow weight loss was observed at 330 °, and a very exothermic decomposition at 390-400 °. The DSC curve of the same product similarly shows exothermic decomposition only in this range. Zinc fumarate, pre•pared from the acid and ZnO in aqueous solution (Table I), exhibits a weak DSC endothermic peak at 360 °, and a stronger endothermic peak at 385 °, indicating two chemical species, the first probably the halfsalt. At 390-40) ° it decomposes very exothermally. The DSC results suggest that the slow weight loss at 330 ° mentioned above may very well be the solid-state formation of the normal salt at the expense of'the half salt, accompanied by the liberation of free acid. This system seems to be suitable for ~tudying the kinetics of the acid--~ half-salt---, normal salt transformation. A reaction to form the salt which does not proceed via the acid is doubtful. Such a hypothetical reaction would proceed as follows: ~0
R/C~o ~,C/ %
4- Zn0
~
/C ~ .
~0 R/C~0 ~C /
271 R/cOOH 4- H20
~
~COOH
ZnO . . . . .
~
half-salt . . . . . ~
(6)
%0 ~/COOH R~.COOH +
normal $011 in chain 1~lructure
(7) The half-salt can be described by either of the following suggested structures: HOOCRCOZnOCRCOOH I) II o
o
(8)
o
II
/COZnOH
R~CO
H
II o
Based on DSC data shown before, it is suggested that the half-salt of step (7) is formed. Economy et al. [5] have described as polymerlike both the melts obtained by fusion of the crystalline metal dicarboxylates, as well as the solids obtained from these melts by cooling. The melt would consist of random length chains, linked by ionic bonds, which continuously undergo changes. Tobolsky has described as very labile salt linkages which may be involved in interchange reactions, where breaking and reforming of bonds occur at equal rates. Since at the freezing temperature the energy of activation for a change in conformation is rather high, upon freezing the .so,called amorphous solid, similar in some respects to polymeric glasses, is formed. The situation here is different from that prevailing in organic acids, which recrystallize by slow cooling of their melts; then in monocarboxylic acids this happens even below the melting temperature. Diacid salts of monovalent cations can also be recrystallized, but in the case of divalent cations the process of recrystallization from the melt is expected to be highly hindered. O'Connor and Maslen [10] reported on copper succinate that "'the structure consists of infinite chains of
O-
WovelenQth, ~m 70 80 90 [ ( I
60 I
lO0
IO I
~2 I
14 t
- -
R~C~o_O 0 - Zn 2+
I 8
(4)
~
B
c o
c//° OR/ Zn z+ \C%o - o-
COO- + Zn + -OOC R/ R R -OOC~ \CO0-+Zn+ - o o c /
E = o
(5) The more reasonable explanation, in conformity with the experimental results, is the formation of the acid by reaction with water, Eqn. (6) followed by generation of the half-sah of Zn and subsequent formation of the double salt
I
1800
I
IE~)O
I
1400
I
1200
[
~000
I
I
800 700
Wovenumber, cm-'
Fig. 4. Infra-red spectra of zinc sebacate. (A) Preparation from solution, (B) preparation in melt
272
M. NARKIS,1. RON, A. SIEGMANNand L. KACIR
covalently linked binuclear units..." The unit cell is comprised of four half molecules and two metal atoms and each unit cell is covalently bonded by four bonds to its neighbouring unit cells. Such crystalline salts are usually not described as polymers, however they also fall in the category of "Halatopolymers" as defined by Economy et al. [5] who have included in this category the amorphous versions only. Thus the crystalline salt and the amorphous solid actually differ in their degree of crystallinity and not in their composition (apart from chemical contamination) and are both "polymerlike". It is suggested that the halatopolymeric transformation is simply a disordering process, which occurs during melting, and is usually irreversible in the case of diacid bivalent metal salts.
REFERENCES 1. T. C. Ward and A. V. Tobolosky, J. appl. Polynt Sci. 11, 2403 (1967). 2. R. H. Kinsey, Appl. Poly~ Syrup. 11, 77 (1969). 3. DuPont Data Sheets on Surlyn A. 4. C. Paquot, R. Perron and C. Vassilieres, Bull. Soc. chim. Ft., 5, 315 (1959). 5. J. Economy, J. H. Mason and L. C. Wohrer, J. Polynt Sci. AI, 8, 2231 (1970). 6. E. P. Otocka and T. K. Kwei, Macromolecules I, 244 (1968). 7. D. Satas and R. Mihalik, J. appl. Polynt Sci. 12, 2371 (1968). 8. W. E. Fitzgerald and L. E. Nielsen, Proc. R. Soc. A, 282, 237 (1964). 9. A. V. Tobolsky, Properties and Structure of Polymers. Wiley, New York (1960). I0. B. H. O'Connor and E. N. Maslen, Acta crystallogr. 20, 824 (1966).
R~sumt----On a 6tudi6 les rtactions de quelques diacides organiques et de leurs anhydrides avec le ZnO poudreux. Les produits obtenus sont des sels de composition molaire approximativement I : 1. Leur stabilit6 thermique est assez grande, par comparaison h celle de leurs constituants organiques. Iis sont insolubles et ne peuvent pas couramment ~tre recristallis~s. Ceci sugg/~re que la rtaction des anhydrides avec ZnO passe par l'intermtdiaire de leurs analogues diaeides, ce qui donne lieu tout d'abord hun h,~misel, puis h la formation de sel normal, Sonumario---Si 6 studiata la reazione di alcuni diacidi organici e delle loro anidridi con ZnO in polvere. Le sostanze ricavate sono sali con composizione molare di circa 1 : 1. A raffronto dei loro componenti organici, la loro stabilit~ termica 6 piuttosto elevata. Tali sail sono insolubili e normalmente non ricristallizzano. Si avanza ripotesi che le anidridi reagiscano con ZnO tramite i loro rispettivi acidi da cui provengono, formando dapprima come prodotto intermedio un semisale e quindi il sale normale. Zasammenhssung--Untersucht wurde die Reaktion verschiedener organischer Disdiuren und ihrer Anhydride mit pulverisiertem ZnO. Als Reaktionsprodukte werden Salze mit der angen~iherten Zusammensetzung 1:1 erhalten. Verglichen mit ihren .organischen Bestandteilen ist deren thermische Best~ndigkeit betr~ichflich. Sic sind unltslich und idtnnen ffir gewthnlich nicht kristallisiert werden. Man mul3 annehmen, dab die Anhydride mit ZnO fiber die korrespondierenden Dis~uren reagieren, wobei zun~ichst das Halb-Salz entsteht, dem die Bildung des normalen Salzes folgt.
REACTIONS OF ZINC OXIDE WITH MOLTEN DIACIDS AND THEIR ANHYDRIDES M. NARrdS, I. RON, A. SIEGMANNand L. K^CIR Plastics Institute, Center for Industrial Research, P.O. Box 31 l, Haifa, Israel (Received 14 July 1973) Abstract--The reactions of some organic diacids and their anhydrides with powdered ZnO were studied. The products are salts of approximately 1: 1 mole composition. Their thermal stability is rather high, in comparison with their organic components. They are insoluble and usually cannot be recrystallized. It is suggested that anhydrides react with ZnO via their parent diacids, resulting initially in a half salt, as an intermediate product, followed by the formation of the normal salt.
INTRODUCTION Ionomers are polymers in which cross-linking is achieved by ionic bridges between pendant ionized carboxyl groups. A peculiar property of ionomers is their dual character; they are "cross-linked" in the solid state, and accordingly possess improved mechanical properties over their parent acid copolymer [1], In the molten state, however, they are thermoplastic and are processed as such. This type of polymer is exemplified by Dupont Surlyn A resins which have wide commercial applications. A summary of the chemistry and properties of ionomers is given in the literature [2, 3]. The idea of linear polycondensed salts of diacids is noted in the early work of a French group [4]. These authors calculated a "degree of polymerization" for Mg 2+, Cd 2+ and Pb 2+ sebacates from the non-stoichiometric metal content of salts precipitated from water. Economy et al. [5] have recently published their work on reactions leading to metal salts of dibasic acids under the title "'Halatopolymers", underlining their polymerlike behaviour in the melt. This behaviour is demonstrated mainly by their high viscosity which is influenced by the interchanging network of dianions with metal cations. The salts were prepared either by double decomposition of divalent metal acetates with the diacids, as shown, M(OAc)2 + HOOC--R--COOH [M2"-OOC--R--COO-]~ + 2AcOH
(1)
or precipitation from aqueous solutions of the sodiumsalts of diacids and metal dichlorides as follows: MCI2 + Na+-OOC--R--COO-Na* ~
yields a low crystalline solid, whereas the second method yields a highly crystalline salt. The products of both methods are similar above their fusion temperature. After fusion and subsequent cooling, similar amorphous solids are obtained. DTA and DSC curves of the so-called "amorphous products" exhibit melting peaks which indicate that they contain microcrystallites and are therefore not truly amorphous. The salts could not be crystallized by slow cooling from the melt or by annealing. The viscosities of several calcium sebacates melts were measured and the time average molecular weight at 386 ° was estimated as 2000, corresponding to approximately 10 repeat units. In the molten state the chains are not polymers in the classic sense, since they dissociate and associate frequently and have no definite "'end-groups". The authors conclude that "metal dicarboxylates can exist either as salts or amorphous polymerlike structures, depending on the method of preparation" [5]. They term the change from salt to polymer as "halatopolymeric transformation". Systems of polymers or copolymers containing carboxylic groups bridged by metal ions were studied by Otocka [6], Satas [7] and Nielsen [8]. We have initially studied the reaction of an alternating styrene/maleic anhydride copolymer with ZnO; for a better understanding of this system, we have experimented with simple model systems, which are reported in this paper. By using a free diacid, the effect of the backbone-polymer on the reaction is eliminated and the acid group concentration is increased. The reason for using anhydrides was to avoid the evolution of water which would interfere in a moulding operation. The anhydride/ZnO reaction was found to be more complex than the acid/ZnO reaction, and most probably proceeds via an acid intermediate.
EXPERIMENTAL Macro-scale preparations were usually performed with X-Ray powder pictures show that the first method 0-14).5 mole quantities. The acid or anhydride was melted 267 [M -'+ -OOC--R--COO-]~ + 2NaCI.
(2)
268
M. NARKIS,1. Roy,, A. SIEGMANNand L. K^cm
in a glass flask and powdered ZnO added with stirring. The increasing viscosity of the medium did not usually allow the addition of a full equivalent ofZnO. Soxhlet apparatus was used to extract the excess organic component. The non-extractable dry solids were later analysed by TGA and DTA. Microscale reactions were accomplished directly in the TGA pan. For a microscale preparation, the components were mixed by co-grinding weighed portions of the solids and up to 30 mg of the mixture were put in a TGA balance pan and heated to a constant temperature above the m.p. of the organic substance. The molten acid or anhydride, on reacting with the oxide, became ionically bound and lost its volatility while the free acid or anhydride was evaporated. The product could be further heated to decomposition, or taken out for DTA, DSC and solubility tests. Both methods of isolation (extraction and volatilization) make use of the fact that the excess acid or anhydride remains non-bonded and removable by physical means. Excess ZnO, if present, cannot be removed by these methods. The DuPont 990 Console with TGA, DTA and DSC modules was used for the thermal analyses. Chemical analysis is based on calcination in the TGA apparatus. Air calcination was used since it yields ZnO, while calcination in nitrogen caused partial reduction of the oxide.
RESULTS AND DISCUSSION Table l summarizes the results of the bulk preparations. Assuming that the molar ratio of the reacted oxide to acid in the product is 1 : 1, it was found by analysis that the product contained excess oxide (after
extraction of the non-bonded acid). This was attributed to insufficient contact between the reactants. Table 2 summarizes the results of the microscale preparations and shows that compositions closer to 1 : 1 mole ratio can be obtained. In this series, pre-mixing of the dry powders and an excess of the organic component (later to be evaporated) provide better contact. In general, since Z n O does not dissolve in the organic phase, concentration effects cannot be expected and contact between the phases is the determining factor. Anhydrides generally yield lower ratios of organic/ oxide for reasons to be discussed later. In the case of an acid, a half-salt as an intermediate product may be expected. The idea of the formation of a half-salt is based on an early "melting point" (i.e. endothermic peak) which was observed between the melting point of the free acid and the decomposition (and/or melting) temperature of the final product. If this early peak represented the melting of the final product, it would be difficult to understand the endothermic form of peaks generally obtained at higher temperatures. The melting point of the half-salt may be reasonably expected to be lower than that of the normal salt. The early peak which is assigned to the half-salt does not always appear. The DTA curve of Fig. I shows that this half-salt disappears on prolonged isothermal heating (see curve B, without peaks) yielding the normal double salt.
Table I. Composition and thermal properties of diacid zinc salts prepared in bulk
Run no. 1
Organic compound Succinic acid Succinic anhydride Succinic anhydride prepared by compression moulding Zinc succinate prepared in solution
Composition organic component (wt %) (mole %)
DSC or DTA endothermic peaks Temperature Initial Maximum
54.2 50.1
48.1 45.1
430 --
450 492
42.3
37-4
--
--
55-7
50.5
--
500
2
Phthalic anhydride
59.5
44.7
3
Sebacic acid
48'8
283
Zinc sebacate prepared in solution
--
--
4
Adipic acid
53"9
42"5
No data
5
Zinc fumarate prepared in solution
44.3
39-8
360 small 385 wide
* Reversible.
No melting 430 = decomposition 300 320*
340 330
TGA Temperature of decomposition Initial Maximum
400 420
420
450
500 "
410
450
300 330
330 400
400 ° decomposition No data 370
400
Reactions of zinc oxide
269
Table 2. Composition and thermal properties of diacid zinc salts prepared from mixed powders (microscale) Composition Run no. 6
7
Organic compound
Temperature of preparation
Succinic acid Succinie anhyd. Phthalic anhyd. Phthalic acid*
8
9
10
Sebacic acid
200 200 120-150
52.1 53.4 54-2
46,5 48,1 49.0
180 220 240 150--180
0 54-0 55-5 60
39.2 40.6 45
250
Maleic anhyd. Maleic acid*
68
4-8-8 57-2
110
180
Furnaric acid
Organic component (wt ~) (mole ~)
285
0
48
3-7-4 52.4
--
DSC or DTA endothermic peaks Temperature Initial Maximum 410 440 w
No melting 430 (exo). --
450 270 500 --
TGA Temperature of decomposition Initial Maximum 400 440 -m
450 500 ---
380 320
430 430
330
400
-260 250 270 360 405 (exothermie)
360
380
360
400
390 400 (exothermic)
330
390
--
280 430
--400 (exo).
335 350 --
* Prepared in situ by addition of water to anhydrides.
I
I
I
I
Tem~ra*ure
Fig. 1. DTA thermograms.(A) scanning of reacting fresh mixture of ZnO/succinic anhydride (B) scanning of zinc suceinate, preparation at 200 ° (Heating rate of 20°/rain).
The melting of the normal salts (ca. 400 °) is usually accompanied by decomposition. In the case of zinc sebacate, however, it was possible, by cooling, to recrystallize the material from the melt. A detailed description of the various reactions and characterization of their products follows. Succinic acid (m.p. 190 °) reacts easily to give a product of approximately 1:1 m o l e ratio, when heated slowly to a constant temperature above the melting point of the acid. This product is stable up to 400 ° as measured by T G A (Table 2). Freshly prepared succinic anhydride/ZnO mixtures give very low yields under similar experimental conditions. It was found that the
yield could be increased simply by letting the mixture stand at r o o m temperature before heating. After a few days the maximum reaction was obtained (49 mole per cent organic). These yields were independent of the amount of excess organic component and the compositions given refer to the solid residue. Similar results were obtained for phthalic and maleic anhydrides. This behaviour suggests that the anhydride is first opened to the acid by the residual water in the anhydride powder and that small amounts of water are sufficient for the reaction to proceed. We have found that succinic anhydride containing water behaves as free acid. On carrying out the reaction between Z n O and succinic anhydride in a DTA capillary, the following endothermic peaks (Fig. 1), were obtained: melting of the anhydride (ca. 80 °, not shown), melting of the acid, a wide peak at 2700 attributed to the half salt, and a final melting of the product starting at about 440 ° with maximum at about 500 °. The 440 ° peak corresponds approximately to the beginning of decomposition found in the T G A curve. For comparison, crystalline zinc succinate was prepared by precipitation from water and found to contain 50-5 mole per cent organic component. When this crystalline salt was analysed by DSC, a melting peak with a maximum at 500 ° was observed. When a mixture of Z n O and anhydride was allowed to react for several hours at about 200 ° and with occasional addition of water, the product showed a DSC maximum at 492 °. In both high melting materials, the endothermic
270
M . N A R K I S , |. R O N , A. SIEGMANN a n d L. K A C I R
2boo
/
o
o
J'
~wK
!i =
'
~0
-i ........
\v
ba
E w
,,
I
I
I Temperatu re,
I
I
eC
2O0
250
I 400
300
Temperot ure,
"C
Fig. 2. DSC thermograms (A) scanning of zinc phthalate, preparation at 140°,-(B) scanning of zinc phthalate which was kept for additii~nal 3 hr at the reaction temperature (heating rate of 20°/rain).
Fig. 3. DSC thermograms (A) scanning of zinc sebacate, preparation at 250 °, (B) scanning of fresh zinc sebacate, preparation in solution, (C) scanning of zinc sebacate which has gone through cycle (B) (heating and cooling rates of 10°/min).
sharp peaks were followed by sharp exothermic peaks indicating decomposition of the melts. Attempts at recrystallization of the salt from the melt by slow cooling gave a product which (on DSC) showed a wide exothermic band, from 500 to 420 °, indicating some recrystallization. Phthalic anhydride (m.p. 130"8°) exhibits more complex behaviour. Its dry mixture with ZnO, when kept at 180 ~ (i.e. above its m.p. but below the m.p. of the acid) leaves ZnO with no bound organic matter. When a little water is added, the anhydride reacts at 150-180 ° to give a product of 45'5 mole per cent organic, stable to 380 °, which is practically the 1:1 salt. Good yields are obtained from the commercial anhydride by quick heating to 220-240 °. The anhydride may contain some free acid (m.p. 206 °) and therefore a reaction via the acid can occur. The acid reacts with the oxide, liberating water which starts a "chain reaction" of anhydride opening. This water transfer has to be faster than evaporation of the water. In a separate experiment where water was added, the formation of acid could be followed by DSC. The peak area of the anhydride decreases while that of the acid increases. Figure 2a shows that the products from reaction between ZnO and anhydride at 130-150 ° contain anhydride, free acid and half salt (melting peaks at 135, 220, 290 °, respectively). The melting range of the zinc phthalate is around about 440 °. After reaction with additional water at 130-150 ° (this run is not shown in Fig. 2), the anhydride peak disappears while the acid, the half salt and the melting of the zinc phthalate peaks are still strong. Curve B in Fig. 2 shows the DSC of the same mixture after three additional hours of reaction at 130-150 °. A very weak peak of the acid is still observed, and two endothermic peaks at 440 and 575 ° are obtained. In summary, phthalic anhydride most probably reacts via the free acid. Terephthalic acid [5-1 gives very stable salts of Sn, Mg, Ca and Ba via their acetates, decomposing without melting at 400, 495, 525 and
590 °, respectively. These relative stabilities might reflect increasing ionic character of the salts. Sebacic acid (m.p. 134 °) was reacted with ZnO at 250 ° to give a product of 48 mole per cent organic (Table 2) and stable up to 330 ° (5 per cent loss by TGA). The crystalline zinc-succinate salt was prepared by reacting aqueous solutions of ZnCI2 and sodiumsebacate. The DSC of this salt (Fig. 3a) shows further crystallization above 250 °, a sharp melting peak at 330 ° and decomposition above 360 °. The TGA of this salt shows initial weight loss at 300 ° similar to data obtained for the product prepared at 250 ° and to literature data [5"1. By heating a fresh sample of the crystalline solution product in the DSC pan up to 350 ° (to avoid any significant decomposition) and then cooling at a rate of 10°/rain, recrystaIlization takes place as shown by the 230 ° exothermic peak in Fig. 3b. Further temperature cycling (Fig. 3c) of the same sample results in a lower and broader melting peak which may indicate partial decomposition. The i.r. spectrum of the crystalline salt (Fig. 4) is much sharper than that of the fusion product, and contains three extra bands at 1200, 1270 and 1340 cm -~. These extra three bands may be due to lattice vibrations. The spectrum of the melt reaction product contains one additional band at 1720 cm -t (which does not appear in the salt), probably due to free acidic groups. It seems that after repeated fusion, the melt reaction and the solution products become practically identical. Maleic anhydride (m.p. 56 °) reacts via the free acid. The dry anhydride mixed with oxide gives a product of 3-7 mole per cent organic only (Table 2). The acid, prepared in situ by adding equivalent amount of water to the anhydride while holding it below 100 °, was further reacted with ZnO at 180 ° to give a product practically 1: 1 mole ratio (52.4 mole per cent organic). Additional experiments have shown that the reaction proceeds at any temperature above the
Reactions of zinc oxide melting point of the acid (130 °) and that the nonbonded acid can be evaporated below 200 °. The product is stable up to 360 ° (TGA) where it starts to lose weight, and reaches a maximum evaporation rate at 380 °. The DSC shows a small broadened peak at 260 °, followed by a moderate exothermic trend. This is attributed to melting, followed by slow disproportionation of the half-salt At higher temperatures, a strong exothermic decomposition peak starts at about 360 ° and reaches its maximum at 405 °, with no melting peak preceding it. Thus, zinc maleate behaves differently from the chemically similar zinc succihate, which first melts sharply and then decomposes. It seems that the double bond in the zinc maleate is the reason for the lower stability of this salt Fumaric acid (m.p. 286 °) is interesting for comparison with the isomeric maleic aci& Here the half-salt is expected as a necessary intermediate since two metal atoms must be involved. Zn fumarate could not be prepared in bulk by fusion probably because the liquid vapour pressure at its high trrp. is too high. In the thermobalance, a iaroduct was obtained by rapid heating of the ZnO/acid mixture to 300 ° and immediate cooling to 285 ° and subsequently slow heating to 330 °. Slow weight loss was observed at 330 °, and a very exothermic decomposition at 390-400 °. The DSC curve of the same product similarly shows exothermic decomposition only in this range. Zinc fumarate, pre•pared from the acid and ZnO in aqueous solution (Table I), exhibits a weak DSC endothermic peak at 360 °, and a stronger endothermic peak at 385 °, indicating two chemical species, the first probably the halfsalt. At 390-40) ° it decomposes very exothermally. The DSC results suggest that the slow weight loss at 330 ° mentioned above may very well be the solid-state formation of the normal salt at the expense of'the half salt, accompanied by the liberation of free acid. This system seems to be suitable for ~tudying the kinetics of the acid--~ half-salt---, normal salt transformation. A reaction to form the salt which does not proceed via the acid is doubtful. Such a hypothetical reaction would proceed as follows: ~0
R/C~o ~,C/ %
4- Zn0
~
/C ~ .
~0 R/C~0 ~C /
271 R/cOOH 4- H20
~
~COOH
ZnO . . . . .
~
half-salt . . . . . ~
(6)
%0 ~/COOH R~.COOH +
normal $011 in chain 1~lructure
(7) The half-salt can be described by either of the following suggested structures: HOOCRCOZnOCRCOOH I) II o
o
(8)
o
II
/COZnOH
R~CO
H
II o
Based on DSC data shown before, it is suggested that the half-salt of step (7) is formed. Economy et al. [5] have described as polymerlike both the melts obtained by fusion of the crystalline metal dicarboxylates, as well as the solids obtained from these melts by cooling. The melt would consist of random length chains, linked by ionic bonds, which continuously undergo changes. Tobolsky has described as very labile salt linkages which may be involved in interchange reactions, where breaking and reforming of bonds occur at equal rates. Since at the freezing temperature the energy of activation for a change in conformation is rather high, upon freezing the .so,called amorphous solid, similar in some respects to polymeric glasses, is formed. The situation here is different from that prevailing in organic acids, which recrystallize by slow cooling of their melts; then in monocarboxylic acids this happens even below the melting temperature. Diacid salts of monovalent cations can also be recrystallized, but in the case of divalent cations the process of recrystallization from the melt is expected to be highly hindered. O'Connor and Maslen [10] reported on copper succinate that "'the structure consists of infinite chains of
O-
WovelenQth, ~m 70 80 90 [ ( I
60 I
lO0
IO I
~2 I
14 t
- -
R~C~o_O 0 - Zn 2+
I 8
(4)
~
B
c o
c//° OR/ Zn z+ \C%o - o-
COO- + Zn + -OOC R/ R R -OOC~ \CO0-+Zn+ - o o c /
E = o
(5) The more reasonable explanation, in conformity with the experimental results, is the formation of the acid by reaction with water, Eqn. (6) followed by generation of the half-sah of Zn and subsequent formation of the double salt
I
1800
I
IE~)O
I
1400
I
1200
[
~000
I
I
800 700
Wovenumber, cm-'
Fig. 4. Infra-red spectra of zinc sebacate. (A) Preparation from solution, (B) preparation in melt
272
M. NARKIS,1. RON, A. SIEGMANNand L. KACIR
covalently linked binuclear units..." The unit cell is comprised of four half molecules and two metal atoms and each unit cell is covalently bonded by four bonds to its neighbouring unit cells. Such crystalline salts are usually not described as polymers, however they also fall in the category of "Halatopolymers" as defined by Economy et al. [5] who have included in this category the amorphous versions only. Thus the crystalline salt and the amorphous solid actually differ in their degree of crystallinity and not in their composition (apart from chemical contamination) and are both "polymerlike". It is suggested that the halatopolymeric transformation is simply a disordering process, which occurs during melting, and is usually irreversible in the case of diacid bivalent metal salts.
REFERENCES 1. T. C. Ward and A. V. Tobolosky, J. appl. Polynt Sci. 11, 2403 (1967). 2. R. H. Kinsey, Appl. Poly~ Syrup. 11, 77 (1969). 3. DuPont Data Sheets on Surlyn A. 4. C. Paquot, R. Perron and C. Vassilieres, Bull. Soc. chim. Ft., 5, 315 (1959). 5. J. Economy, J. H. Mason and L. C. Wohrer, J. Polynt Sci. AI, 8, 2231 (1970). 6. E. P. Otocka and T. K. Kwei, Macromolecules I, 244 (1968). 7. D. Satas and R. Mihalik, J. appl. Polynt Sci. 12, 2371 (1968). 8. W. E. Fitzgerald and L. E. Nielsen, Proc. R. Soc. A, 282, 237 (1964). 9. A. V. Tobolsky, Properties and Structure of Polymers. Wiley, New York (1960). I0. B. H. O'Connor and E. N. Maslen, Acta crystallogr. 20, 824 (1966).
R~sumt----On a 6tudi6 les rtactions de quelques diacides organiques et de leurs anhydrides avec le ZnO poudreux. Les produits obtenus sont des sels de composition molaire approximativement I : 1. Leur stabilit6 thermique est assez grande, par comparaison h celle de leurs constituants organiques. Iis sont insolubles et ne peuvent pas couramment ~tre recristallis~s. Ceci sugg/~re que la rtaction des anhydrides avec ZnO passe par l'intermtdiaire de leurs analogues diaeides, ce qui donne lieu tout d'abord hun h,~misel, puis h la formation de sel normal, Sonumario---Si 6 studiata la reazione di alcuni diacidi organici e delle loro anidridi con ZnO in polvere. Le sostanze ricavate sono sali con composizione molare di circa 1 : 1. A raffronto dei loro componenti organici, la loro stabilit~ termica 6 piuttosto elevata. Tali sail sono insolubili e normalmente non ricristallizzano. Si avanza ripotesi che le anidridi reagiscano con ZnO tramite i loro rispettivi acidi da cui provengono, formando dapprima come prodotto intermedio un semisale e quindi il sale normale. Zasammenhssung--Untersucht wurde die Reaktion verschiedener organischer Disdiuren und ihrer Anhydride mit pulverisiertem ZnO. Als Reaktionsprodukte werden Salze mit der angen~iherten Zusammensetzung 1:1 erhalten. Verglichen mit ihren .organischen Bestandteilen ist deren thermische Best~ndigkeit betr~ichflich. Sic sind unltslich und idtnnen ffir gewthnlich nicht kristallisiert werden. Man mul3 annehmen, dab die Anhydride mit ZnO fiber die korrespondierenden Dis~uren reagieren, wobei zun~ichst das Halb-Salz entsteht, dem die Bildung des normalen Salzes folgt.