Reactions in Glass lonomer Cements: V. Effect of Incorporating Tartaric Acid in the Cement Liquid STEPHEN CRISP and ALAN D. WILSON Laboratory of the Government Chemist, Cornwall House, Stamford Street, London SEI 9NQ, England A description is given of the effect on the ASPA cement reaction of tartaric acid incorporated in the cement liquid. Tartaric acid acts as an accelerator that aids in the extraction of ions from the aluminosilicate glass and facilitates their binding to the polyanion chains. Postgelation hardening is significantly increased. Working time is unaffected possibly because cations are initially present as complexes. In previous articles,1-3 an account has been given of the reactions taking place in the basic prototype glass ionomer cement, ASPA I. In the reaction, the calcium fluoroaluminosilicate powder is partly decomposed by the poly (acrylic acid) (PAA) solution to silica gel and the cations liberated are bound to poly (acrylate) forming a hard saltlike gel. Barry, Clinton, and Wilson4 have elucidated the microstructure of this cement with the use of an electron microscope combined with a nondispersive X-ray analyzer. The matrix was shown to be a mixture of aluminum and calcium poly (acrylates), with the former predominating, in which were embedded particles consisting of a core of unreacted glass sheathed by a silica gel. Kent, Lewis, and Wilson5 have reported the physical and mechanical properties of ASPA I cement. Subsequently, Wilson, Crisp, and Ferner6 described improvements in the rheological characteristics of the cement system which were imparted by the incorporation of low molecular weight chelating agents in the (PAA) solutions. Tartaric acid was found to be the most effective and this variant of the glass-ionomer cement, which exhibits improved working qualities, is termed ASPA II cement. Received for publication September 22, 1975. Accepted for publication April 13, 1976.

Rheological studies using an oscillating rheometer shlowed that the postgelation rate of hardening was accelerated. The mechanism of this effect was discussed and ascribed to the formation of metal ion binuclear complexes. This present article reports on studies on the setting reaction in the ASPA II cement and compares results with those obtained for ASPA I cement, using methods described previously.1-3 The initial stage of the reaction, the acid decomposition of the calcium fluoroaluminosilicate powder, was followed in a model system by chemically determining the rate of extraction of ions from a suspension of the powder in a dilute polyacid-chelating acid solution. Subsequent stages of the reaction were studied during a 94-hour period, using two complementary methods. The use of the attenuated total reflectance (ATR) technique of infrared spectroscopy made it possible to monitor the transformation of COOH groups to COOgroups and the formation of silica gel, and a chemical method was used to follow the associated variations in concentration of soluble ionic species present in the setting cement. By collating the results of these various studies, a comprehensive account of the cement-forming reactions was constructed. Materials and Methods The cement powder, an ion-leachable glass designated G200, was prepared by fusing a mixture of silica and alumina in a flux of Ca, Al, and Na fluorides and Al phosphate. The melt was shock cooled and the opal glass formed was ground so it could pass through a sieve with a mesh opening of 45 micrometers (,um) 1 (Table 1). The PAA solution was prepared by the 1023

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J Dent Res November-December 1976

CRISP AND WILSON TABLE 1 COMPOSITION OF THE GLASS G200 Glass, Fusion Mixture

Wt

Component

175 100 207 30 60

SiO2

A1203 CaF2

Na3AIF,f A1PO4 A1F3

32

aqueous polymerization of acrylic acid, using ammonium persulfate as the initiator and propan-2-ol as the chain transfer agent. The final product had a weight average molecular weight of 23,000, as determined by ultracentrifugal sedimentation, and was concentrated to 50% m/m by vacuum distillationl.7 (liquid I for ASPA I cement). Analytical grade reagent D-tartaric acid was added to the liquid (5% m/m) and dissolved by rotating the solution. The final composition of the liquid was 47.5% m/m PAA, and 5% m/m tartaric acid (liquid II for ASPA II cement.

A deuterated sample of cementing liquid was prepared by drying the aqueous solution at 37 C until a glass was formed that was then dissolved in sufficient deuterium oxide (D20) to produce an approximately 25% solution. This solution was dried and the residue redissolved in D20. This process was repeated twice. The final dried product was powdered in a ball mill. The deuterated powdered polymer was mixed in the required proportion with the glass powder and the cement formed by the addition of the requisite amount of D20 to this mixture. All of the experiments were done at 23 C.5 In all of the wet-chemical studies, the proportion of cement components used was 3.0 gm powder to 1.0 ml liquid or its equivalent in diluted solutions. In infrared spectroscopic studies, a lower proportion of powder to liquid, 1.5/gm/ml, was used to moderate the reaction. The techniques of study and analytical procedures have been fully described elsewhere'-3 and are only outlined here. The initial decomposition of the glass powder was studied in a model system. Weighed amounts of powder were shaken with measured volumes of 1:100 diluted

iII -S

.-

Minutes

FIG 1.-Progressive extraction of ions from glass powder G200 by diluted liquid II. Results expressed, for each ion, as percentage fraction of amount originally present in glass. ASPA II cement: solid circle, Na; solid square, Ca; solid triangle, Al; solid inverted triangle, F; solid diamond, P20r.

FIG 2.-Progressive extraction of ions from glass powder G200 by diluted liquid I (broken lines) and diluted liquid II (continuous line). Results expressed as milligrams of ion per gram of glass powder. ASPA I cement: open square, Ca; open triangle, Al; open inverted triangle, F; open circle, Na. ASPA II cement: solid square, Ca; solid triangle, Al; solid inverted triangle, F; solid circle, Na; solid diamond, P205.

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TARTARIC ACID AND CEMENT LIQUID

liquid I and liquid II solutions for various periods of time. The suspensions were centrifuged and the concentration of Al, Ca, Na, F, and phosphate determined following the methods described previously.1 No suitable methods were available for the determination of PAA and tartaric acid. The reaction in the actual cement pastes was followed by crushing cement specimens of various ages, extracting the soluble ionic species with water, and determining the extracted ion by the same chemical methods as for the acid extraction. The method has been detailed in a previous report.1 Infrared spectroscopic studies were done on cements and deuterated cements. Details have been given previously.2

Results

MTODEL SYSTEM.-The decomposition of the aluminosilicate glass powder within the first ten minutes of the reaction is represented graphically as the progressive extraction of ionic species from the glass into the dilute acid of the model system (Figs 1, 2). In Figure 1 results are shown as a fraction of the amount of the species originally present in the glass, whereas in Figure 2 they are shown as milligrams extracted per gram of cement. Results recorded for a longer period are given in Table 2. These have little

ExTRAcTION

OF

significance, except for the sodium ions, because of interfering precipitations. In general, ions are extracted more rapidly from the glass by liquid II than by liquid I solution. SOLUBLE ION CONCENTRATION OF CEMENT PASTES.-The extraction of ions from the glass powder and their precipitation within the cement paste are shown by the soluble ion/ time curves seen in Figures 4 and 5. The maximums present in the Ca2+/time and Al3+/time curves for ASPA I cement are not shown for ASPA II cement where a steady reduction in the concentration of these ions is recorded. Otherwise, ASPA I and ASPA II cement behaved in a similar fashion with a steady increase in soluble sodium content and broad maximums in the F-/time and P905/time curves. INFRARED SPECTRUMS.-The ATR spectrums of setting ASPA I and ASPA II cement pastes in D20 are shown in Figure 3. The use of D20 clarifies the spectrums by elimination of the H20 band at about 1,630 cm-'. The general features in both sets of spectrums are similar. There is a decrease in the intensity of the asymmetric stretching mode at 1,700 cm-1 resulting from the COOH group and bands appear at about 1,540 and 1,600 cm-1 because of the asymmetric stretching modes of the Ca and Al COO- salts, re-

TABLE 2 SPECIES FROM THE GLASS BY DILUTED LIQUiDS (1:100)

IN MODEL SYSTEM Time (min) I

10

10'

10'

10'

2.7 3.1

4.1 5.0

6.7 7.3

6.9 9.1

9.5 11.9

4.6 7.2

5.7

7.9

8.5

11.5

12.6

13.1

9.7 13.3

1.2 0.6

1.5 1.8

3.1 1.6

3.4 3.2

5.9 5.7

Mg Ca/gm Liquid I Liquid II M, Al/gm

12.3 10.2

15.0 15.3

25.3 18.5

26.3 25.5

15.5 33.6

Liquid I Liquid II

5.0 6.5

8.7 13.3

15.0 20.1

18.0 30.3

16.7 39.5

Solution

Mg P205/gm Liquid 1 Liquid II Mg F/gm Liquid I Liquid II Mg Na/gm Liquid I Liquid II

1025

Note: Liquid I and II correspond to the ASPA I and 11 cements, respectively. Downloaded from jdr.sagepub.com at UNIVERSITE DE MONTREAL on June 9, 2015 For personal use only. No other uses without permission.

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J Dent Res November-December 1976

CRISP AND WILSON 7

9

8

I

I

12 10Il 15 pm

~~~~

3 min

.lh _._ --- 3h -

,rJ'

-24h KBr d

isk

IS . I

,I

1800

1400

1600

7

6

1000

1000

1200

9

8

cm-

15 rIn

12

10

6mmn 1h -

6h

\

/

-.......... KOrdlak

PI,/

f\~~~~~~~~~~~~~~~~~ .....

1800

1600

1400

1200

FIG 3.-Top, ATR infrared spectrums of ASPA infrared spectrums of deuterated ASPA II cement.

spectively, together with a symmetric stretching band at about 1,400 cm-1. The broad band, maximum about 1,080 cm-', arises from the formation of silica gel and increases in intensity with time. Both cements retain some residual COOH groups after 24 hours, an observation confirmed by KBr disk transmission spectrums.

B000 C.-

1000 II

cement. Bottom, ATR

It is difficult to compare the rates of reaction of the two cement variants. The intensity of the bands is a function of the contact of the sample to the ATR crystal and not only varies from sample to sample but changes with a set of measurements on the same sample, thus rendering precise quantification impossible.

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TARTARIC ACID AND CEMENT LIQUID

Vol 55 No. 6

1027

1-

6-

5E

E 40

E

3-

2-

10-

-.*--

.,

,2 'O-

-- -

-,O"

.--

1-

'..

j --

~~ ~-G_

10

1000

100

Si 02 5P205

-2 o

P205 10000

Minutes

FIG 4.-Soluble nonmetallic ions present in cement. Broken lines refer II cement. ASPA I cement: open circle with line, SiO,; open circle, Na; open inverted triangle, F; open diamond, P205. ASPA II cement: same as for ASPA I cement only solid geometric figures. to ASPA I cement and continuous lines to ASPA

Discussion GENERAL.-A broad outline of the progress of the reaction is given by the changes recorded in the ATR spectrums (Fig 3). The general features are a progressive conversion of acid COOH groups (asymmetric stretching mode 1,700 cm-') to salt COO- groups (asymmetric stretching modes at 1,540 and 1,600 cm-1) as metal salts are formed, accompanied by an increase in the amount of silica gel formed (broad band at 1,080 cm-') Both variants of the cement show some free COOH groups present after 24 hours. These results are consistent with an acid decomposition of the aluminosilicate glass to silica gel that liberates cations that form PAA salts. Infrared spectroscopy is insufficiently precise to compare the reaction rates of the two ASPA cement variants, and it is impractical to record changes occurring in the first few minutes after mixing that are of particular significance to working and hardening characterization. A more detailed account is .

given by extraction experiments. DECOMPOSITION OF THE POWDER.-For

both ASPA I and ASPA II cements there is an initial rapid extraction of ions (Figs 1, 2) that, as the ATR results show (Fig 3), is accompanied by the conversion of some COOH groups to COO- groups and the formation of silica gel. After the first minute

the extraction rate decreases sharply. The particularly rapid extraction of Ca ions in both ASPA cement variants illustrates the importance of its role in initial set and shows that tartaric acid which only forms weak complexes with Ca (pKj = 2.17) has little effect on the extraction rate. Al is extracted more extensively from ASPA IL than from ASPA I cement. This difference is most likely associated with the role of F which will form complexes with Al of the type A1F2+ and AIF2+. In the presence of tartaric acid, complexes will also form between this acid and aluminum, thus increasing the extraction of this metal. The complexes involved have similar stability constants: AIF2+, pKj = 6.08; AIF2+, pK2 = 5.02; and Al/tartaric acid, pK, = 6.35.8 The extraction of F is greatly increased in the ASPA LI cement. This is probably due to increased Al extraction accompanied by the formation of complex fluorides, and to the effect of tartaric acid on the glass which is a stronger acid than PAA. GELATION AND HARDENING.-There are marked differences between the soluble ion/ time curves for ASPA I and ASPA II cements (Figs 4, 5). The clearly discernible maximums present in the Ca ion/time and Al ion/time curves for ASPA I cement are absent in corresponding curves for ASPA II

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J Dent Res November-December 1976

CRISP AND WILSON

--

A

1. square, Ca; open riangl,A1;pencicle}N.ASPAIIcem °

I

.

Na

for3ASPA I cement only solid geometric figures.

2

Al

-

~

~

0 01

a

~ ~~~

t~~~oio

turn

teas

FIG 5.-Soluble metal ions present in cement. Broken lines refer to ASPA I cement and continuous lines to ASPA II cement. ASPA I cement: open square, Ca; open triangle, Al; open circle, Na. ASPA II cement: same as for ASPA I cement only solid geometric figures. cement; however, these probably occur in the period before observations are possible using the present techniques. A point of particular interest is that Ca ions become more rapidly bound in an insoluble form in ASPA II cement. This difference is most noticeable when cements are 5 minutes old. The soluble Ca content of ASPA II cement is not only considerably lower than in ASPA I cement but is decreasing much more rapidly. These results account for the rapid hardening characteristics of this form of the glass-ionomer cement. Thus, although tartaric acid does not assist the extraction of Ca ions from the powder, it does apparently accelerate the binding of these ions into the gel-matrix. This may possibly be attributed to the formation of binuclear complexes proposed previously.6 In both the ASPA I and ASPA IL cement systems, Ca ions are much more rapidly bound in an insoluble form than Al ions, and initial gelation and setting of these cements is to be attributed mainly to Ca ions. The data from ATR spectrums show that the COO-(Ca) groups predominate over the COO-(Al) groups in the first hour or so of the setting reaction.

The differences between the soluble Al ion content of ASPA I and ASPA II cements are not marked. However, it must be remembered that results from the powder decomposition experiments indicate that more Al is liberated in the ASPA II cement system, so it follows that more Al ions are present in insoluble form either as part of the matrix or as an insoluble complex. Since Na does not form insoluble salts, the soluble ion content can be regarded as an indicator of the extent of the reaction. It is noteworthy that after two minutes the concentration of soluble Na in the ASPA II cement is twice that in the ASPA I cement, indicating that the aluminosilicate powder is being decomposed that much more rapidly. (The conclusion is supported by results for F and phosphate.) However, since ultimately the two Na ion/time curves meet, the total proportion of powder consumed in the reaction is the same for both cements. The F ion/time curves, which show broad maximums, do not correspond to the pattern of either the Ca ion/time curves or the Al ion/time curves. The binding of the F ion into the matrix takes place slowly. The content of soluble F is higher for ASPA II cement which is consistent with the more

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Vol 55 No. 6

TARTARIC ACID AND CEMENT LIQUID

rapid decomposition of the powder. The behavior of phosphate ions is similar to F. The amount of soluble silica is similar for both cements which indicates that the extent of the reaction is approximately the same in both variants. ROLE OF COORDINATE ANIONS.-It is appropriate in considering the role of tartaric acid to comment on the coordination of the metal ions, especially A13+, in the cementing reaction. The first stage of the reaction is the decomposition of the glass surface to release A13+, Ca2+, Na+, and F- ions. Cations will be transported in the aqueous phase surrounded by a cluster of coordinated ligands: H20, OH-, and F-. Na+ and Ca2+ form simple ions with hydration spheres. In aqueous solution, Al adopts a coordination number of 6 and in consequence of its high electrostatic potential is surrounded by a strongly bound coordination sphere of hydroxyl ions and water molecules. A13+ exists as [AI(H,O)6]3+ in strongly acid conditions, but at a higher pH, it hydrolyzes to [Al (H20) 5OH]2+ or possibly such various binuclear species as [ (H20) 5Al (OH) ]24+ [A17 (OH) 17]4+, and [Al13 (OH) 34]5+ which involve hydroxyl bridging.9,10 In the glass ionomer cement system, F ions are present that can form a series of complexes with Al ranging from A1F2+ to AlF63-. Cationic complexes will form in acid solutions. As the F ion is about the same size as the OH- ion, it may substitute OH- ions in the previously described hydroxyl complexes Ca can also form complexes with F- ions, but these are less stable than the Al complexes. As the polyacid ionizes and negatively chiarged carboxylate groups appear along the chain, the configuration changes from a coil bound by hydrogen bonding to an extended linear form. The polyelectrolyte phase gels as the polyanions form salts with metal cations. Several forms of salt are possible, and these have been discussed by Crisp, Prosser, and Wilson,"1 but the most important from a structural viewpoint is the crosslinked salt that bridges two polyanions formed with Ca and Al ions. Crisp, Lewis, and Wilson'2 in discussing zinc polycarboxylate cements have pointed out that there must be ligands, other than poly (acrylate) groups, attached to the metal ion to satisfy sixfold coordination requirements. These may be neutral ligands such as water mole-

H

H

,~

2-1

"Id0

I

CH2 CH-C

CH2

o-

M

~

C- CH

0

C H2CIH-C/

1029

CH2 H

H

H

H

\ M'/

CHG

M

CH2

0

H H

H

H

~~~A-

I CH2

0

CH-C

M

GH2

0 H

CH2 3

c--CH

CH2 H

FIG 6.-Postulated forms of salts in gel matrix where A can be F or OH.

cules that do not affect the bonding situation, or anionic ligands such as OH- or F- ions tthat do. Anionic ligands reduce cross-linking ability as is seen from the inspection of Figure 6. Such cross-link breakers must be present;otherwise, in view of kinetic and steric constraints, the ionomer matrix would be continuously cross-linked and could possibly possess immense strength. The binding of Ca ions to the polyanion takes place more rapidly than with Al ions. Several factors account for this difference. A13+ has a lower mobility than Ca2+ because it is more heavily hydrated and possibly forms binuclear species. A13+ is extracted more slowly than Ca2+ from the glass. To fully react with the polyanion, A13+ requires three carboxylate groups and, although not impossible, such steric configurations are not very probable. It is more likely that it reacts with two carboxylate groups, with a OH- or F- as the

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1030

CRISP AND WILSON

remaining anionic ligand. Another factor is the stability of F complexes formed by Ca2+ and A13+ ions. The metal F bonds may have to be ruptured before salt formation can occur with the polyanion. This depends on the distribution of complexes which is a function of pH. Bond breaking will be slower with A13+ since its F complexes are considerably more stable than those of Ca2+. Indeed, previously Crisp and Wilson13 found that good workability in this system could only be obtained if the glass powder was prepared so as to readily release F- ions, an observation that indicated that the formation of F complexes was necessary to prevent the premature binding of the cations. The effect of organic anions, tartrate in this instance, must be similar and dependent on their ability to form complexes with metal ions. Tartaric acid acts as both an acid and a complexing agent and has the ability in 50% aqueous solution to form weak, hydrolytically unstable cements with alumino-silicate powders. In the glassionomer cement system it plays a dual role in the reaction scheme. First, by virtue of the acidity and complex-forming ability already noted, tartaric acid aids the extraction of glass ions, which accounts for the increase in ion extraction observed when it is present. Second, by forming complexes with metal ions it withholds them, for a short time, from the polyanion. By the time the polymer has assumed a linear configuration, metal ions will be available to cross-link from the glass, either from the F complexes, as in the original system, or, in addition, from the tartrate complexes. The increased availability of cations and the fact that tartaric acid prevents reaction with the polyanion in this initial period gives the cement a more definite set. The reaction is more rapid in the first few minutes and this results in a marked improvement in early hardness. Infrared spectroscopic studies indicate that the relative rates of binding of Ca2+ and A13+ are unaffected by the presence of tartaric acid, an observation that is explained by the fact that the F and tartrate complexes of each of these anions are of similar stability. It is probably of significance that tartaric acid may form complexes with two cations and so bridge them. This bridged structure may be represented thus:

J Dent Res November-December 1976

0 I) __

H I

C

0

/

I

CH

f

CH

/ C II 0

H

It would seem, therefore, that the metal complexes may act not only as a reaction intermediate but also as a flexible bridging unit between polymeric chains. Conclusions The chemistry of the setting reaction of ASPA II cement is essentially the same as for ASPA I cement: the calcium fluoroaluminosilicate glass is partly decomposed by acid attack to silica gel, whereas liberated Al and Ca ions are bound to the poly (acrylate) chains causing the cement paste to gel and set hard. The prime difference between the two variants is in the rate of reaction. Tartaric acid, incorporated into PAA solutions, acts as an accelerator facilitating the extraction of ions from the glass powder. Thus, there is a higher concentration of cations available to react with the polyanion with a consequent increase in the hardening rate. Working time is not affected and it must be presumed that complex formation prevents premature binding of cations to the poly (acrylate) chain. The authors thank Dr. H. J. Prosser for infrared spectroscopic analysis and the Government Chemist, Dr. H. Egan, for permission to continue this paper. Crown Copyright. Reproduced by permission of Her Britannic Majesty's Stationery Office.

References 1. CRISP, S., and WILSON, A.D.: Reactions in Glass lonomer Cements: I. Decomposition of the Powder, J Dent Res 53: 1408-1413, 1974. 2. CRISP, S.; PRINGUER, M.A.; WARDLEWORTH, D.;

and WILSON, A.D.: Reactions in Glass lonomer Cements: II. An Infrared Spectroscopic Study, J Dent Res 53: 1414-1419, 1974. 3. CRISP, S., and WILSON, A.D.: Reactions in Glass Ionomer Cements: III. The Precipitation Reaction, J Dent Res 53: 1420-1424, 1974. 4. BARRY, T.I.; CLINTON, D.J.; and WILSON,

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Vol 55 No. 6

TARTARIC ACID AND CEMENT LIQUID

A.D.: Unpublished report. 5. KENT, B.E.; LEWIS, B.G.; and WILSON, A.D.: The Properties of a Glass lonomer Cement, Br Dent J 135: 322-326, 1973. 6. WILSON, A.D.; CRISP, S.; and FERNER, A.J.: Reactions in Glass Ionomer Cenments: IV. The Effect of Chelating Co-Monomers on the Setting Behavior, J Dent Res 55: 489-495, 1976. 7. WILSON, A.D., and KENT, B.E.: Surgical Cement, Br Patent no. 1 316 129, 1973. 8. Stability Constants of Metal-Ion Complexes, Special Publication No. 17, London: The Chemical Society, 1964. 9. AKITT, J.W.; GREENWVOOD, N.N.; and LESTER, G.D.: Aluminium -27 Nuclear Magnetic Resonance Studies of Acidic Solutions of

1031

Aluminium Salts, J Chem Soc A 803-807, 1969. 10. AVESTON, J.: Hydrolysis of the Aluminium Ion: Ultracentrifugal and Acidity Measurements, J Chem Soc 4438-4443, 1965. 11. CRISP, S.; PROSSER, H.J.; and WILSON, A.D.: An Infra-Red Spectroscopic Study of Cement Formation Between Metal Oxides and Aqueous Solutions of Poly(Acrylic Acid), J Mater Sci 11: 36-48, 1976. 12. CRISP, S.; LEWIS, B.G.; and WILSON, A.D.: Zinc Polycarboxylate Cements: A Chemical Study of Erosin and Its Relationship to Molecular Structure, J Dent Res 55: 299-308, 1976. 13. CRISP, S., and WILSON, A.D.: Unpublished data.

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Reactions in glass ionomer cements: V. Effect of incorporating tartaric acid in the cement liquid.

Reactions in Glass lonomer Cements: V. Effect of Incorporating Tartaric Acid in the Cement Liquid STEPHEN CRISP and ALAN D. WILSON Laboratory of the G...
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