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Studies on the setting chemistry of glass-ionomer cements
Clinical Materials
7 (19919 289-293
Studies on the Setting Chemistry of Glass-ionomer Cements Eleanor A. Wasson & John W. Nicholson Materials Technology Group, Middlesex TWI 1 IDLY, UK
Laboratory
of the Government
Chemist,
Queens
Road,
Teddington,
Abstract: The Glass-ionomer cement is formed by the reaction of a polymeric acid and a basic glass. Conventional setting theory assumes that the process consists of the formation of a calcium/aluminium polyacrylate matrix, and makes no mention of the role of silica. Three different monomeric acids have been used to form weak cements, yet their calcium and aluminium salts are known to be watersoluble. This, together with the previous finding that silica is leached out of the glass during acid attack, suggests that the formation of a hydrated silicate is important in the setting reaction. The significance of such a silicate network within Glass-ionomer cements is discussed.
INTRODUCTION
material surrounded by a sheath of ion-depleted silica gel. Barry et aL7 concluded from this that such a structure existed in the clinical Gl,ass-ionomer. These studies have led to thie current ideas that the setting of Glass-ionomer cements takes place as follows :I initial mixing causes the acid to attack the glass, liberating first calcium ions and later aluminium ions. These latter ions are assumed to be complexed in some way, for example with fluoride ions or with water.5 This ion-leaching is assumed to leave behind an ion-depleted layer of silica gel around the otherwise unreac ted glass particles. When sufficient neutralisation has taken place the material sets and continues to undergo slow neutralisation with time, gradually binding in further water molecules and improving in strength as it ages. The set material is assumed to be very similar to the zinc poly(carboxylate) cement, consisting of a metal poly(acrylate) matrix with unreacted inorganic powder embedded in it. Recently the authors have carried out a study of the ions released by ionomer glasses when washed with dilute acetic acid solutions.8 They concluded that, contrary to the established theory, there is no sequential release of calcium and aluminium ions. Both ions are released at the earliest stages of reaction. The authors reconciled this, with results of
Glass-ionomer cements are widely used in clinical dentistry. ’ They are formed by reaction of a watersoluble p’olymeric acid such as poly(acrylic acid) with a basic aluminosilicate glass. Their setting chemistry and structure have been extensively studied, making them among the best characterised materials currently used in dentistry. Soon after their invention Wilson and his coworkers established tha.t their setting involved the formation of calcium and aluminium poly(acrylate)s2,3 and classified them as acid-base reaction cements.4 A number of experimental techniques have been used to study the setting process in these cements. They include infrared spectroscopy,2 Fourier Transform infrared spectroscopy,5 13C NMR spectroscopy’ and electron probe microanalysis7 Many of these studies have used model cements that depart in important respects from the formulations used clinically, requiring care in the interpretation of the results. The electron probe studies, for example, used cements prepared from large particle size glasses.7 The resulting material was shown to be a composite consisting of a metal poly(acrylate) matrix with unreacted glass powder embedded in it. The glass particles consisted of a core of unattacked 289
o
1991 Crown
Copyright
21.2
infrared spectroscopic studies which showed that aluminium poly(acrylate) is formed distinctly later than calcium poly(acrylate), by considering the probable nature of the aluminium species released. This species is likely to be a highly condensed oxygenated ion containing a number of aluminium atoms’,’ whose gradual release of essentially discrete aluminium ions leads to the slow formation of the aluminium poly(acrylate) component. These acid washing studies’ have also shown that a number of species are leached out of ionomer glasses in addition to calcium and aluminium. These included sodium, silicon and phosphorus, the latter presumably as silica and phosphate. This implies that the initial acid attack is not mere ion exchange, but wholesale dissolution of a proportion of the glass. This, in turn, implies that such species are present throughout the matrix ofa typical Glassionomer cement and not merely in a small core around the glass particles. If silicon occurs in the matrix of the cement, it may have a structural role. Unfortunately this cannot be studied directly because the matrix is not crystalline. It was therefore decided to investigate the possibility of silicon having a matrix-forming role using an indirect approach The approach chosen was to attempt to prepare cements from acetic acid and from the mineral acids, HCl and HNG,. These acids were selected because they are monomeric and monofunctional. This eliminated the possi ility of an insoluble crosslinked structure holding the cement together. The mineral acids had the additional advantage that the chemistry of the possible calcium an aluminium salts is straightforward. Insolubility of the cement could not be conferred by salt formation since all of the possible salts are water-soluble, Cement formation would therefore indicate a setting mechanism other than simple neutralisation. METHODS
Table I. Post-firing composition
of glasses used in &is study, -“--.-_-----_. Composition(by muss)
Code
MPA A1,0, (35 %I); sio, (28 %); CaO (26 %); Na,O (I 1 %) A&O, (14.2%); SD, (24.9%); CaF, (12+3%);
G338
AlF, (46 %); APO, (24.2 %); P&i&F, --~__
acetic acid cements cements.
and
3-5: 1 for
(19.2 %)
ineral
acid
plied to the crystal of spectra acquired at intervals
over
S AN
ISCIJ
s to for remelts with ~on~~~t~at~ Atte acid not result in the formation of a cement with either of the glasses use
AND MATERIALS
The acids used in this study were : acetic acid, glacial (AR grade, ex. BDH ; 99.8 % assay) ; HCl (AR grade, ex. Fisons; 35-38 % HCl; SG l-18); and HN0, (AR grade, ex. BDH; 69X-705 % ; SG 1.42)~ All of the acids were diluted to give 40 % by volume (v/v) of their original concentration. The two glasses used were MP4 (ex. Pilkingtons) and G338 (ex. LGC; < 45 pm); the post-firing compositions of these glasses are given in Table 1. Cements were mixed at a powder to liquid ratio (p/l) of 3 : 1 for
mineral essentially
acid while G338 water-stabk
forms
cements.
Prior to setting, the cement after immersion in water. Hn
slightly
soft but
Studies
on the setting chemistry
the resulting solution was measured and, in nearly all cases, was found to be between 5 and 6 10 min after mixing. This shows that the acids all undergo rapid neutralisation, although this does not result in the formation of stable cements. The exception to this is in the use of acetic acid. In this case the pH rose from 2.5 (for water plus acid) to a little over 4 after 5 mm: the pH of the water used in this study was 5.5. Comparison with the pH of aluminium acetate (4.5) and calcium acetate (7.5) suggests that neutralisation is less rapid in these cements than with the stronger mineral acids. Determination of the pH during setting clearly shows the initial rapid neutralisation in all of the systems (see Table 2). The apparent discontinuity in the chang,e in pH during the setting of the glassionomer is due to the use of two methods to obtain a full setting profile. The table also shows that in no case does the cement reach full neutralisation within the 2 h pH was studied. This was not expected in the case of the simple acid cements but was expected for Class-ionlomers Twhichare nearer full set in this time. The change in pH observed for the nitric acid and acetic acid cements are very similar, both showing initially rapid neutralisation. The results of infra-red studies on the acetic acid based cement paste is shown in Figs 1 and 2. Comparison of these spectra with those for aluminium and calcium acetates (Figs 3 and 4) show that aluminium acetate is identifiable 70 min after mixing; there is no evidence of calcium acetate however. The final spectrum taken after 69.5 h Table 2. Change in pH during the setting of glass-ionomer experimentma cements
of Glass-ionomer
80
60
7. T
40
20
2500
Fig.
1. IR
2000
spectrum
1750 cm-'
for
'1500
acetic acid mixing.
cement
1250
70 min
1000
1
after
and
4
Gkrss-ionomer
Acetic acid
Nitric acid
1 2 3
2.032 2.179 2.267
2.370 2.605 2.637
0.219
4 5
2.654 2.668
8
2.332 2.385 2.483
10 11 15” 20 30 45 60 90
2.545 2.784 2.974 3.369 3.644 3.780 4.044
3.185
.158 ,280 ,183 .232 ,379 1.045 -
4.266
3.298
1.647
120
291
shows a residual acid peak at 1700 cm-l and a broad band at 1050 cm-l, the rema:ining peaks at 1596, 1467 and 1421 cm-l are a good match for aluminium acetate. These simple experiments lead to some important conclusions concerning the chemistry of the cementforming reactions. It is clear, for example, that neutralisation does not, in itself”,confer insolubility on the mixture. However, the complex chemistry of aluminium carboxylates makes it difficult to be certain that the stability of the acetic acid cement is not conferred by the formation of a ‘basic ’ aluminium acetate ; in these cements slow development of water-stability sugglests that this is not
Change in PH Time (min)
cements
2.729 2.782 2.831 2.930 3.010
a pH values for glass-ionomer from 15-120 min are calculated using method 2 of Kent and Wilson.‘o
8I l-
0.010 %T
0.691 0.830
-
60I-
/-
Fig. 2. IR spectrum
for acetic acid cement 169.5h after mixing.
292
Eleanor A. Wasson, John W. ~~~~~~s~~~ ,
ilities of possible process’”
neutralisation
resulting from
salts -
Saalt
63.7 279 49.9
‘WNQ:,), CaCP, .6H,O AlCl,
%T
..__-.--
~~l~~~~it~in Hz0 (g cm-“) ~~~~~~~t~~~ I”C] ---.____I--_____. 266 0
95-
go-
the
-____
25 n is ; _/
-
85 c
hydrated calcium,
80
I
f
t
2000
I 1500 cm-’
Fig. 3. IR spectrum
t
silicate strut a~~rni~~~rn or
orus
an such
an
I
1000
of calcium
acetate.
ortant for the ins& give Ento the setting rea ionomer cements. The
resistance1 and the increase in unbound water with time.l”
I
I
2000
Fig. 4. IR spectrum
3
I
1
:
1000
1500 cm-1
of aluminium
the equation
ent of true water ratio of bound to
acetate
s that there IS a e ~o~ec~~es are the case. For the mineral acid plus MP4 cements there are no such problems, as all the simple salts likely to be formed during the neutralisation process are water-soluble : values for solubility in water are given in Table 3. These results suggest that an alternative reaction to neutralisation has occurred which confers waterstability on the cements. Such a reaction must be initiated by the acid-base process but be distinct from it. Consideration of the species known to be released from the glass points to the formation of an
~x~erirn~~t~~ly the should give a straight 1 e
Studies on the setting chemistry of Glass-ionomer cements
of the graph is 245 and 1, = 5 x 103. However, when Hill et al.,15 applied these concepts to the glassionomer cement, using the weight average molar masses of various samples of poly(acrylic acid), they found that the graph of log G, versus log M, gave a slalpe of less than 1 and close to 0.5. They also found that there was no upper limit to the range of linearity of this plot. Despite these findings they concluded that the reptation model was of some value in the understanding of the mechanical behaviour of these materials, and suggested that the reason for the deviation from theory was that the ionic crosslinks held to be present in these materials would prevent tlhe chain pull-out mechanism from operating as efficiently as it might otherwise. Of greater significance than the fact that the mechanical properties of Glass-ionomers depart from theory is the fact that they also differ from those of zinc poly(carboxylate) cements.13 These latter cements are composed of a structure that is very similar to the one that has been assumed to exist in Glass-ionomers. In the case of the zinc poly(carboxylate) cement there is no reaction comparable with the formation of the hydrated silicate structure suggested for Glass-ionomers. For zinc poly(carboxylate) cements the slope of the plot of G, versus log M was found to be approximately 1; in fact these cements behaved in many respects like thermoplastic polymer composites. The results for zinc poly(carboxylate)s can be used to assess th’eeffect of ionic interactions between poly(acrylic a&d) chains compared with the predictions of the chain pull-out model of failure. The increase in magnitude of interactive forces between the molecules in such a system reduces the dependency of toughness on polymer molar mass, reducing the slope of the log G, versus log M plot to a value of 1. The implication of the finding that the slope of :such a plot for Glass-ionomer cements is about 0.5 is tlhat there is an additional factor operating in these cements which reduces still further the dependency of toughness on polymer molar mass. This could well be the difficulty of pulling polymer chains through a substantial network of :hydrated silicate. CONCLUSION It has bleen shown that stable cements can be
293
formed from ‘ionomer ’ glasses y aqueous solutions of acetic acid or mineral acids, although the setting reaction in these cases is slo,wer than, and distinct from, the initial neutralisation process. This has been taken as further evidence that cement formation occurs partly via the formation of a hydrated silicate structure in addition to simple salt formation. The existence of such a silicate structure in Glass-ionomer cements may explain the deviation of the mechanical properties both from those predicted by the reptation ideas of de Gennes and from those of the zinc poly(carboxylate) cements. REFERENCES 1 Wilson, A. D. & McLean, J. W., Glastilanomer Cement. Quintessence Publishing Co., Chicago, 1988. 2. Crisp, S. & Wilson, A. D., Reactions in Glass-Ionomer cements I. An infrared spectroscopic study. J. ,Dent. Res., 53 (1974) 1408~13. 3. Crisp, S., Pringuer, M. A., Wardleworth, D. & Wilson, A. D., Reactions in Glass-Ionomer cements II. An infrared spectroscopic study. J. Dent. Res., 53 (1974) 141419. 4. Wilson, A. D., The chemistry of dental cements. Chem. Sot. Revs, 7 (1978) 265-96. 5. Nicholson, J. W., Brookman, P. J., Lacy, 0. M. & Wilson, A. D., Fourier transform infrared spectroscopic study of the role of tartaric acid in Glass-Ionomier dental cements. J. Dent. Res., 67 (1988) 1451-4. 6. Prosser, H. J., Richards, C. P. Br Wilson, A. D., NMR spectroscopy of dental materials, II. The role of tartaric acid in Glass-Ionomer cement. J. Biomed. Muter. Res., 16 (1982) 43145. 7. Barry, T. I., Clinton, D. J. & Wilson, A. D., The structure of a Glass-Ionomer cement and its relationship to the setting. 1. Dent. Res., 58 (1979) 1072-9. E. A. & Nicholson, J. W., A study of the 8. Wasson, relationship between setting chemistry and properties of modified glass-poly(alkenoate) cements. Brit. Rely. J., 23 (1990) 179-83. 9. Walters, D. N. & Henty, M. S., Raman ;spectra of aqueous solutions of hydrolysed aluminium (III) salts. J. Chenz. Sot. Dalton Transactions, (1977) 243-S. 10. Kent, B. E. & Wilson, A. D., Dental Silicate cements: VIII. Acid-base aspects. J. Dent. Res., ‘48 (1969) 412-18. 11. Handbook of Physics and Chemistry, ed. R. C. Weast, CRC Press, Florida, 1988-1990. 12. Wilson, A. D. & Crisp, S., Ionomer cements. Brit. Poly. J., 7 (1975) 279-96. 13. Hill, R. G. & Labok, S. A., The inliuence of polyacrylic acid molecular weight on the fracture of zinc polycarboxylate cements. J. Mat. Sci., 26 4’1991) 67-74. 14. de Gennes, P., Scaling Concepts in Polymer Physics. Cornell University Press, Ithaca, 1979. 15. Hill, R. G., Wilson, A. D. & Warrens, C. P., The influence of polyacrylic acid molecular weighf on the fracture toughness of glass-ionomer cements. J. Mat. Sci., 24 (1989) 363-71.
7 (19919 289-293
Studies on the Setting Chemistry of Glass-ionomer Cements Eleanor A. Wasson & John W. Nicholson Materials Technology Group, Middlesex TWI 1 IDLY, UK
Laboratory
of the Government
Chemist,
Queens
Road,
Teddington,
Abstract: The Glass-ionomer cement is formed by the reaction of a polymeric acid and a basic glass. Conventional setting theory assumes that the process consists of the formation of a calcium/aluminium polyacrylate matrix, and makes no mention of the role of silica. Three different monomeric acids have been used to form weak cements, yet their calcium and aluminium salts are known to be watersoluble. This, together with the previous finding that silica is leached out of the glass during acid attack, suggests that the formation of a hydrated silicate is important in the setting reaction. The significance of such a silicate network within Glass-ionomer cements is discussed.
INTRODUCTION
material surrounded by a sheath of ion-depleted silica gel. Barry et aL7 concluded from this that such a structure existed in the clinical Gl,ass-ionomer. These studies have led to thie current ideas that the setting of Glass-ionomer cements takes place as follows :I initial mixing causes the acid to attack the glass, liberating first calcium ions and later aluminium ions. These latter ions are assumed to be complexed in some way, for example with fluoride ions or with water.5 This ion-leaching is assumed to leave behind an ion-depleted layer of silica gel around the otherwise unreac ted glass particles. When sufficient neutralisation has taken place the material sets and continues to undergo slow neutralisation with time, gradually binding in further water molecules and improving in strength as it ages. The set material is assumed to be very similar to the zinc poly(carboxylate) cement, consisting of a metal poly(acrylate) matrix with unreacted inorganic powder embedded in it. Recently the authors have carried out a study of the ions released by ionomer glasses when washed with dilute acetic acid solutions.8 They concluded that, contrary to the established theory, there is no sequential release of calcium and aluminium ions. Both ions are released at the earliest stages of reaction. The authors reconciled this, with results of
Glass-ionomer cements are widely used in clinical dentistry. ’ They are formed by reaction of a watersoluble p’olymeric acid such as poly(acrylic acid) with a basic aluminosilicate glass. Their setting chemistry and structure have been extensively studied, making them among the best characterised materials currently used in dentistry. Soon after their invention Wilson and his coworkers established tha.t their setting involved the formation of calcium and aluminium poly(acrylate)s2,3 and classified them as acid-base reaction cements.4 A number of experimental techniques have been used to study the setting process in these cements. They include infrared spectroscopy,2 Fourier Transform infrared spectroscopy,5 13C NMR spectroscopy’ and electron probe microanalysis7 Many of these studies have used model cements that depart in important respects from the formulations used clinically, requiring care in the interpretation of the results. The electron probe studies, for example, used cements prepared from large particle size glasses.7 The resulting material was shown to be a composite consisting of a metal poly(acrylate) matrix with unreacted glass powder embedded in it. The glass particles consisted of a core of unattacked 289
o
1991 Crown
Copyright
21.2
infrared spectroscopic studies which showed that aluminium poly(acrylate) is formed distinctly later than calcium poly(acrylate), by considering the probable nature of the aluminium species released. This species is likely to be a highly condensed oxygenated ion containing a number of aluminium atoms’,’ whose gradual release of essentially discrete aluminium ions leads to the slow formation of the aluminium poly(acrylate) component. These acid washing studies’ have also shown that a number of species are leached out of ionomer glasses in addition to calcium and aluminium. These included sodium, silicon and phosphorus, the latter presumably as silica and phosphate. This implies that the initial acid attack is not mere ion exchange, but wholesale dissolution of a proportion of the glass. This, in turn, implies that such species are present throughout the matrix ofa typical Glassionomer cement and not merely in a small core around the glass particles. If silicon occurs in the matrix of the cement, it may have a structural role. Unfortunately this cannot be studied directly because the matrix is not crystalline. It was therefore decided to investigate the possibility of silicon having a matrix-forming role using an indirect approach The approach chosen was to attempt to prepare cements from acetic acid and from the mineral acids, HCl and HNG,. These acids were selected because they are monomeric and monofunctional. This eliminated the possi ility of an insoluble crosslinked structure holding the cement together. The mineral acids had the additional advantage that the chemistry of the possible calcium an aluminium salts is straightforward. Insolubility of the cement could not be conferred by salt formation since all of the possible salts are water-soluble, Cement formation would therefore indicate a setting mechanism other than simple neutralisation. METHODS
Table I. Post-firing composition
of glasses used in &is study, -“--.-_-----_. Composition(by muss)
Code
MPA A1,0, (35 %I); sio, (28 %); CaO (26 %); Na,O (I 1 %) A&O, (14.2%); SD, (24.9%); CaF, (12+3%);
G338
AlF, (46 %); APO, (24.2 %); P&i&F, --~__
acetic acid cements cements.
and
3-5: 1 for
(19.2 %)
ineral
acid
plied to the crystal of spectra acquired at intervals
over
S AN
ISCIJ
s to for remelts with ~on~~~t~at~ Atte acid not result in the formation of a cement with either of the glasses use
AND MATERIALS
The acids used in this study were : acetic acid, glacial (AR grade, ex. BDH ; 99.8 % assay) ; HCl (AR grade, ex. Fisons; 35-38 % HCl; SG l-18); and HN0, (AR grade, ex. BDH; 69X-705 % ; SG 1.42)~ All of the acids were diluted to give 40 % by volume (v/v) of their original concentration. The two glasses used were MP4 (ex. Pilkingtons) and G338 (ex. LGC; < 45 pm); the post-firing compositions of these glasses are given in Table 1. Cements were mixed at a powder to liquid ratio (p/l) of 3 : 1 for
mineral essentially
acid while G338 water-stabk
forms
cements.
Prior to setting, the cement after immersion in water. Hn
slightly
soft but
Studies
on the setting chemistry
the resulting solution was measured and, in nearly all cases, was found to be between 5 and 6 10 min after mixing. This shows that the acids all undergo rapid neutralisation, although this does not result in the formation of stable cements. The exception to this is in the use of acetic acid. In this case the pH rose from 2.5 (for water plus acid) to a little over 4 after 5 mm: the pH of the water used in this study was 5.5. Comparison with the pH of aluminium acetate (4.5) and calcium acetate (7.5) suggests that neutralisation is less rapid in these cements than with the stronger mineral acids. Determination of the pH during setting clearly shows the initial rapid neutralisation in all of the systems (see Table 2). The apparent discontinuity in the chang,e in pH during the setting of the glassionomer is due to the use of two methods to obtain a full setting profile. The table also shows that in no case does the cement reach full neutralisation within the 2 h pH was studied. This was not expected in the case of the simple acid cements but was expected for Class-ionlomers Twhichare nearer full set in this time. The change in pH observed for the nitric acid and acetic acid cements are very similar, both showing initially rapid neutralisation. The results of infra-red studies on the acetic acid based cement paste is shown in Figs 1 and 2. Comparison of these spectra with those for aluminium and calcium acetates (Figs 3 and 4) show that aluminium acetate is identifiable 70 min after mixing; there is no evidence of calcium acetate however. The final spectrum taken after 69.5 h Table 2. Change in pH during the setting of glass-ionomer experimentma cements
of Glass-ionomer
80
60
7. T
40
20
2500
Fig.
1. IR
2000
spectrum
1750 cm-'
for
'1500
acetic acid mixing.
cement
1250
70 min
1000
1
after
and
4
Gkrss-ionomer
Acetic acid
Nitric acid
1 2 3
2.032 2.179 2.267
2.370 2.605 2.637
0.219
4 5
2.654 2.668
8
2.332 2.385 2.483
10 11 15” 20 30 45 60 90
2.545 2.784 2.974 3.369 3.644 3.780 4.044
3.185
.158 ,280 ,183 .232 ,379 1.045 -
4.266
3.298
1.647
120
291
shows a residual acid peak at 1700 cm-l and a broad band at 1050 cm-l, the rema:ining peaks at 1596, 1467 and 1421 cm-l are a good match for aluminium acetate. These simple experiments lead to some important conclusions concerning the chemistry of the cementforming reactions. It is clear, for example, that neutralisation does not, in itself”,confer insolubility on the mixture. However, the complex chemistry of aluminium carboxylates makes it difficult to be certain that the stability of the acetic acid cement is not conferred by the formation of a ‘basic ’ aluminium acetate ; in these cements slow development of water-stability sugglests that this is not
Change in PH Time (min)
cements
2.729 2.782 2.831 2.930 3.010
a pH values for glass-ionomer from 15-120 min are calculated using method 2 of Kent and Wilson.‘o
8I l-
0.010 %T
0.691 0.830
-
60I-
/-
Fig. 2. IR spectrum
for acetic acid cement 169.5h after mixing.
292
Eleanor A. Wasson, John W. ~~~~~~s~~~ ,
ilities of possible process’”
neutralisation
resulting from
salts -
Saalt
63.7 279 49.9
‘WNQ:,), CaCP, .6H,O AlCl,
%T
..__-.--
~~l~~~~it~in Hz0 (g cm-“) ~~~~~~~t~~~ I”C] ---.____I--_____. 266 0
95-
go-
the
-____
25 n is ; _/
-
85 c
hydrated calcium,
80
I
f
t
2000
I 1500 cm-’
Fig. 3. IR spectrum
t
silicate strut a~~rni~~~rn or
orus
an such
an
I
1000
of calcium
acetate.
ortant for the ins& give Ento the setting rea ionomer cements. The
resistance1 and the increase in unbound water with time.l”
I
I
2000
Fig. 4. IR spectrum
3
I
1
:
1000
1500 cm-1
of aluminium
the equation
ent of true water ratio of bound to
acetate
s that there IS a e ~o~ec~~es are the case. For the mineral acid plus MP4 cements there are no such problems, as all the simple salts likely to be formed during the neutralisation process are water-soluble : values for solubility in water are given in Table 3. These results suggest that an alternative reaction to neutralisation has occurred which confers waterstability on the cements. Such a reaction must be initiated by the acid-base process but be distinct from it. Consideration of the species known to be released from the glass points to the formation of an
~x~erirn~~t~~ly the should give a straight 1 e
Studies on the setting chemistry of Glass-ionomer cements
of the graph is 245 and 1, = 5 x 103. However, when Hill et al.,15 applied these concepts to the glassionomer cement, using the weight average molar masses of various samples of poly(acrylic acid), they found that the graph of log G, versus log M, gave a slalpe of less than 1 and close to 0.5. They also found that there was no upper limit to the range of linearity of this plot. Despite these findings they concluded that the reptation model was of some value in the understanding of the mechanical behaviour of these materials, and suggested that the reason for the deviation from theory was that the ionic crosslinks held to be present in these materials would prevent tlhe chain pull-out mechanism from operating as efficiently as it might otherwise. Of greater significance than the fact that the mechanical properties of Glass-ionomers depart from theory is the fact that they also differ from those of zinc poly(carboxylate) cements.13 These latter cements are composed of a structure that is very similar to the one that has been assumed to exist in Glass-ionomers. In the case of the zinc poly(carboxylate) cement there is no reaction comparable with the formation of the hydrated silicate structure suggested for Glass-ionomers. For zinc poly(carboxylate) cements the slope of the plot of G, versus log M was found to be approximately 1; in fact these cements behaved in many respects like thermoplastic polymer composites. The results for zinc poly(carboxylate)s can be used to assess th’eeffect of ionic interactions between poly(acrylic a&d) chains compared with the predictions of the chain pull-out model of failure. The increase in magnitude of interactive forces between the molecules in such a system reduces the dependency of toughness on polymer molar mass, reducing the slope of the log G, versus log M plot to a value of 1. The implication of the finding that the slope of :such a plot for Glass-ionomer cements is about 0.5 is tlhat there is an additional factor operating in these cements which reduces still further the dependency of toughness on polymer molar mass. This could well be the difficulty of pulling polymer chains through a substantial network of :hydrated silicate. CONCLUSION It has bleen shown that stable cements can be
293
formed from ‘ionomer ’ glasses y aqueous solutions of acetic acid or mineral acids, although the setting reaction in these cases is slo,wer than, and distinct from, the initial neutralisation process. This has been taken as further evidence that cement formation occurs partly via the formation of a hydrated silicate structure in addition to simple salt formation. The existence of such a silicate structure in Glass-ionomer cements may explain the deviation of the mechanical properties both from those predicted by the reptation ideas of de Gennes and from those of the zinc poly(carboxylate) cements. REFERENCES 1 Wilson, A. D. & McLean, J. W., Glastilanomer Cement. Quintessence Publishing Co., Chicago, 1988. 2. Crisp, S. & Wilson, A. D., Reactions in Glass-Ionomer cements I. An infrared spectroscopic study. J. ,Dent. Res., 53 (1974) 1408~13. 3. Crisp, S., Pringuer, M. A., Wardleworth, D. & Wilson, A. D., Reactions in Glass-Ionomer cements II. An infrared spectroscopic study. J. Dent. Res., 53 (1974) 141419. 4. Wilson, A. D., The chemistry of dental cements. Chem. Sot. Revs, 7 (1978) 265-96. 5. Nicholson, J. W., Brookman, P. J., Lacy, 0. M. & Wilson, A. D., Fourier transform infrared spectroscopic study of the role of tartaric acid in Glass-Ionomier dental cements. J. Dent. Res., 67 (1988) 1451-4. 6. Prosser, H. J., Richards, C. P. Br Wilson, A. D., NMR spectroscopy of dental materials, II. The role of tartaric acid in Glass-Ionomer cement. J. Biomed. Muter. Res., 16 (1982) 43145. 7. Barry, T. I., Clinton, D. J. & Wilson, A. D., The structure of a Glass-Ionomer cement and its relationship to the setting. 1. Dent. Res., 58 (1979) 1072-9. E. A. & Nicholson, J. W., A study of the 8. Wasson, relationship between setting chemistry and properties of modified glass-poly(alkenoate) cements. Brit. Rely. J., 23 (1990) 179-83. 9. Walters, D. N. & Henty, M. S., Raman ;spectra of aqueous solutions of hydrolysed aluminium (III) salts. J. Chenz. Sot. Dalton Transactions, (1977) 243-S. 10. Kent, B. E. & Wilson, A. D., Dental Silicate cements: VIII. Acid-base aspects. J. Dent. Res., ‘48 (1969) 412-18. 11. Handbook of Physics and Chemistry, ed. R. C. Weast, CRC Press, Florida, 1988-1990. 12. Wilson, A. D. & Crisp, S., Ionomer cements. Brit. Poly. J., 7 (1975) 279-96. 13. Hill, R. G. & Labok, S. A., The inliuence of polyacrylic acid molecular weight on the fracture of zinc polycarboxylate cements. J. Mat. Sci., 26 4’1991) 67-74. 14. de Gennes, P., Scaling Concepts in Polymer Physics. Cornell University Press, Ithaca, 1979. 15. Hill, R. G., Wilson, A. D. & Warrens, C. P., The influence of polyacrylic acid molecular weighf on the fracture toughness of glass-ionomer cements. J. Mat. Sci., 24 (1989) 363-71.