Austr;ilim Dental luurnal. February. I977

31

Volume 22. No. I

The clinical development of the glass-ionomer cements. 1. Formulations and properties* Clitiiccil

J. W. McLean, M.D.S., L.D.S. R.C.S. of the Go~~ertrtiietir C/iettii,vt, Lotidoti

C m i s i i l ~ ~ i tLuhorotory i~,

AND

A. D. Wilson, D.Sc., C.Chem., F.R.I.C. Seriior I'riiicipcil Scietitific Officer, Lobortilory of t h e Goveriinietrt Clrenii.st, Lotidon

AHslKACT-71'he formulations of the glass-iononier cements their properties and clinical development a r e described. (Reccivrd f o r ptrhlicfrliotl M n y .

Introduction I he formulation of the glass-ionomer cements hy Wilson and his co-workers'-4 antl their development for clinical use by McLean and Wilson",'; had in objective the combining of the best properties in the silicate cements, composite resins and polycarboxylnte cements. Silicate cement has the good properties of low thermal expansion. high abrasion resistance when not attacked by acids, and the ability to afford some caries protection by the liberation of fluoride ions. T h e good features of the composite resins of Bowen' include excellent aesthetics. resistance to acid attack and higher flexural strength than the phosphate bonded cements. T h e polycarboxylate cements of Smiths-'" I.

*

From a lecture, 21st Australian Dental Congress. Adelaide, February, 1976.

Wilson. A. D. and Kent, B. E.-The glass-ionomer cement, a new translucent dental filling material. J . Appl. Chem. Biotech.. 21: 313. 1971. 2 Wilson. A. 13.. and Kent, B. E.-A new trnnslucent cement for dentistry. The glass-ionomer cement. Hrit. D. J.. 132:4, 133-135 (Feb. 1 5 ) 1972. a Wilson, A . 13.. and Kent. B. E.-Surgical cement. Brit. Pat. No. I 316 129. 1973. 4 Crisp. S. Ferner. A. J.. Lewis, B. G.. and Wilson. A. D. -Properties of iniproved glass-ionomer cement formulations. J . Dent. 3:3, 125-130 ( M a y ) 1975. :McLean. J . W., and 'Wilson. A. I1.-Fissure sealing and filling with an adhesive glass-ionomer cement. Brit. D. J . . 136:7. 269-276 (Apl. 2) 1974. 0 McLean. J . W.. and Wilson, A. D.-Die klinische entwicklung von Glas-lonomer-Zementen. Schweiz. Mschr. Zahnheilk. 84: 697-708 (July) 1974. 7 Uowen. K. L.-Dental filling material comprising vinyl silane treated fused silica and :I binder consisting of the reaction product of bisphenol and glycidyl acrylate. U S . Pat. No. 3 066 122, 1962. 5 Smith. D. C.-A new dental cement. Brit. 13. J.. 125:9. 381-384 (Nov. 5 ) 1968. 1' Smith. 13, C.-Improvements relating to surgical cement. Brit. Pat. No. I 139 430, 1969. 'USmith. 11. C.-A review of the zinc polycarboxylate cements. J . Canada. D. A,. 37: 22-29 (Jan.) 1971.

1976)

are noted for their hytlrophilic properties. good adhesion to tooth structure and apparent blandness. Clearly, t o combine all these properties in one material is unlikely but many of these objectives have been achieved in developments of the glassionomer cement system. T h e glass-ionomer cement has strength characteristics similar to those o f the silicate cenientI1, but is more resistant t o acid attack. I t is also bland, like t h e polycarboxylate cements, but with the oddeci advantage of translucency. T h e problem of achieving and maintaining adhesion to tooth material has been the occasion of much research. While resins can now be bonded to enamel by use of acid-etching techniques, attachment is by tags o f resin mechanically interlocked in etched-fissures. There is n o question of adhesion by molecular forces and for this reason bonding to dentine presents much greater problems. Acid etching does not create ii sufficiently porous surface antl the mechanical retention of resins is minimal: nor would such pre-treatment be clinically desirable. Glass-iononier cements and polycarboxylate cements a r e the only tlentnl materials that bond t o enamel and dentine because they adhere to substrates by means of polar and ionic attractions - physicocheniical adhesion. This paper, which is the first of ii series. reports o n further progress made in t h e formulation of Kent, B. E., Lewis, B. G.. and Wilson. A. D.-The properties of a glass-ionomer cement. Brit. 13. J., 135:7. 322-326 (Oct. 2) 1973.

32

Australian Dental Journal, February, 1977

these new cements designed for a number of specific clinical applications. Glass-ionomer cements

Glass-ionomer (ASPA) cements a r e based o n the hardening reaction between ion-leachable glasses and aqueous solutions or homo- and copolymers of acrylic acid. T h e setting reaction has been elucidated in it number of studiesI2-l4. On mixing t h e two components, hydrated protons from the liquid penetrate the surface layers of the powder p;trticles. Cations (mainly A W and GI?+) a r e displaced and the aluminosilicate network is degraded t o a hydrated siliceous gel. Cations, either simple or a s fluoride complexes, migrate into the aqueous phase of the cement paste where metallic salt bridges are formed between the long chains of charged polycarboxylate ions, cross-IinkFng them and causing the aqueous phase to gel and the cement to set (Fig. I ) .

Fig. I .-Schematic

representation of the cement-forming reaction.

Calcium ions are more rapidly hound to the polyacrylate chains than aluminiuni ions and a r e chiefly responsible for its initial set''.' 4 . Later. :iluminium salt bridges form and the cement hortlens; the high ionic potential of the trivalent aluminium ion ensures a much stronger crosslinking than is possible with divalent ions alone. T h e slower reaction of aluminium is attributed to Ihe more stringent steric requirements imposed by ;I trivalent ion o n polyanion chain configuration. T h e glass-iononier cement has 21 double setting reaction that confers interesting rheological properties on the hardening paste. Initially. the cement sets to a condition which enables i t t o be carved like ;in anialgnm (calcium ion-exchange), later it sets rock hard (aluminium ion-exchange).

I2

I::

Crisp, S.. and Wilson, A . I>.-Relrclions in gl;rss-iononirr cements: I Decomposition of the powder. J . I>. Res.. 53:6, 1408-1413 (Nov.-Dee.) 1974. Crisp, S., Pringuer. M. A.. Wardleworth, D.. and Wilson. A . I).-Reactions in glass-ionomer cements: I I . An

infrared spectroscooic study. J . D. Res.. 5 3 : h 1414-1411) ( N o v , - D e c . ) 1974.Crisp, S.. and Wilson, A . I>.-Reactions in glass-iononier cements: 111. The precipitation reaction. J . D. Kes.. 53:fi. 1420-1424 (Nov.-Dec.) 1974.

Glass-iononier types

-ionomel cements used ;it present are in the form of ii calcium oluniino silicate glass powder and poly (acrylic acid) based liquid (ASPA). Modifications in working time, setting time, manipulative properties, opacity and strength may be made by using different glass and polyacitl and by the addition of low molecular weight cheloting agentsl(;. Other niodifications can he ni;tde by varying the size o f the powder particles and the molecular weight, concentration and viscosity of the liquidl7. Wilson and Kent in 1973:' reported a number of formulations and o n e glass, designated (3200. was found which when combined with an aqueous solution o f polyacrylic acid (50 per c e n t ) yielded cements with desirable properties for restoration, linings ;ind lutings. T h e original cement was labelled ASPA I . Compared with the dental silicate cement it maintained its surface integrity against attack by weak acids, but i t was it slow hardening cement, and found to be unstritable for general clinical use. A more rapidly hardening cement, ASPA II, was developed which proved more successful under clinical conditions. T h e ASPA II cement was developed :IS the resuli of ;I systematic study of the effect o n setting properties of chelating agents dissolved in the liquid. It was found that addition of minor ;imounts of tartaric acid had dramatic effects. While the working time of the cement was unaffected, its post set hardening rate was considerably enhanced. This desirable result wits attributed t o two causes: ( i ) the enhancement of the extraction of cations froni the glass. which niitst accelerate hardening, and (ii) the formation of niet;il-tartr:tte complexes which prevented premature cation-binding t o polyanion chains which otherwise would shorten working time. ASPA I I was found to have good adhesion to both dentine and enamel and mnintained its surface integrity against attack by weak acids. In this form the glass-iononier cenient wits subjected to a two-year clinical trial to evaluate its efficiency for sealing and filling pits and fissures. Clinical indications reported by h4cLean and Wilson.? were favourable. For general use the shelf life of the liquids used for ASPA II proved to be limited varying froni obout 10-30 weeks depending on the batch. T h e thickening and gelation of poly (acrylic acid) Wilson, A. I>., nnd Crisp. S.--lonomer cements. Brit. Polym. J . . in press, Wilson, A. D.. Crisp. S.. mil Ferner. A . J.-Re:ictions in glass-ionorner cements: I V . The effect of chelating co-monomers on the setting hehaviour. J . D. Kes., in press. li Crisp, S.. and Wilson, A. I>.-Unpublished report. Laboratory of the Govcrnnient Chemist. 1974. IB

33

Australian I3ent;il Iournal. February. 1977 solutions of above 50 per cent concentrations i\ probably tluc t o an increase of between-chain hydrogen bonding 215 favourable molecular renrrangenients occur. Although gelation can be reversed by stirring o r warming such liquids remain inconvenient for general use. Methylation of some of the -COOH groups by methyl alcohol conferred stability o n t h e liquid but a t the price of certain clinical disadvantages. t h e principal one being increased uptake of stain o n the surface. T h e problem was solved finally by

the formulation of ii special co-polymer of acrylic and itaconic acids (3: 1 mole ratio). Aqueous solutions of this polymer proved t o be stable for at least two years under unfavourable conditions o f storage's. Its viscosity was also unusually low for a poly (carboxylic acid) solution which made i L particularly useful when mixing at low P / L ratios. Tn this form the glass-iononier cement was termed ASPA I V and was considered suitable for commercial production a s a fissure filling material ;ind for treatment erosion cavities". A fine grained version of ASPA I V has been developed as ;I luting agent. Microstructure and properties T h e microstructure of ASPA cement resembles that of a composite material (Fig. 2 ) : however there a r e unusual featuresl". T h e particulate matrix is it polysalt and the filler particles a r e to be distinguished by the presence of phase-separated droplets and consist o f an inert glassy core reacted * De l r e y ' s Aspn 13

Crisp. S., Lewis. H. G . , and Wilson, A . D-The gelation of poly (acrylic acid) aqueous solutions and the me:isurement of vircositv. I . D.- Kes.., 5 5 : 6 ., 1173-1175 ~.~~ (No\-.-Dec.) 1975. Barry T I Miller K. P. and Wilson, A. D.-Dental b k e d on' ion leachable glasses XI Conference on the Silicate Industry, Budapest, 1973, pp. 881-893. ~

1:'

~

cements

in ;I siliceous hydrogel depleted of cations. l h u s there is a graded structure between filler and matrix. Since both Lire involved in the reaction it is probable that interfacial stresses between chains a r e dissipated during the course of the cenientforming reaction. Such a material would be expected t o have good wear characteristics and this contention is supported by clinical observationsz. T h e physical and mechanical properties of ASPA IV filling cement4 a r e compared with those of :I silicate cement in Table 1. Compressive. antl tensile strengths are similar. Half the 24 hour strenglh of ASPA IV is developed in one hour'". T h e chief difference lies in the adhesive properties of ASPA. Also unlike t h e dental siliciite cenient

Fig. 3.-Comparison o f (he surfacc integrity of a dental silicate cement and glass-ionomer cement under various conditions.

the glassiononier ccnient maintains its surface integrity against attack by weak acids, such ;IS would be encountered in the mouth. This property is illustrated in Fig. 3 where the behaviour of the dental silicate cement antl the glass-iononicr cement towards aqueous and acid attack a r e compared. This difference in behaviour is to be explained in terms of the molecular structure and the bond types present in t h c matrices of these two cements. T h e matrix o f the dental silicate ccnient is composed of solated aluminium AI3+ and ortho phosphate, POI"-, (the P O 1 ions

Australian Dental Journal, February, 1977

34

ASPA 11:

Cement/

Property Powder/liquid ratio (s/ml) Consistency, (Disc Diam m m )

3.0 27 3.7s

Setting time, 37OC (niin) Compressive strength, 24h ( N / m m z ) Compressive strength, 7d (N/mm2) Tensile strength, 24h (N/mm‘)

I65

I4

Super”

3.5 31 4.0 I75 214

13

Aspa-‘

Syntrex

3.0

4.0

33

25

4.25

I40

14

Other‘; Silicates 3.2-4.3

25 2.X-5.2

226

175-250

247

-

I3

0.3- I .x

Water leachable material. 24h (% 1

0.4

0.4

0.3

0.5

Mod it1 us of elasticity, 24h ( K N / m m 2 )

-

9.0

-

I x.o

-

Modulus of elasticity, 7d ( K N / m m z )

-

-

24

-

Acid erosion, 7d, pH-4.0 (%)

I .?

I .2

-

5 .o

-

Opacity. C0.70

0.73

0.69

0.69

0.55

0.44-0.65

1

For test methods see ref. 1 I .

2

Powder batch PB5, Liquid batch LIT-w

:I

Powder hatch PBS, Liquid hatch L48Tab

‘ Powder batch

19

SE3, Liquid batch S A t 7

Powder hatch P42. Liquid hatch PA I8 I’

Current examples

a r e represented as tetrahedra in Fig. 421) which a r e bound together solely by ionic forces. Displacement o f A t ’ from the structure by hydrogen ions from the acid destroys the ionic bonding and leads to complete mechanical disruption of the cement matrix. T h e situation for the glass-iononier cement is different. Here the anion is ;I polynieric one, rather than a macronieric one its is orthophosphate, and the carboxyl (-COOH) functionnl groups a r e joined together by a covalently bonded carbon chain. T h e C-C bond is impervious to acid attack and consequently the polymer chain remains intact, in the presence of acids. Acids can

only destroy the ionic cross-links which hold these chains together and the loss o f some nietal cations can be sustained without causing mechanical disruption (Fig. 4 b ) . T h e properties of ASPA IV luting cement a r e compared with those of a polycarboxylate cement in Table 2. Adhesion

Glass-iononier cements when in the form of ;I fluid paste adhere to ii nuniber of substrates. probably because at this stage of the setting reaction many o f the acid C O O H groups are available for hydrogen bonding. Hydrosen bonding can

36

A u s t r a l i a n D e n t a l Journal, February, 1977

-_ I ABLE 2

T h e variotis physico-chemical ;itlhcsive bondsj including hydrogen hontling. hetween :I glass-iononier cement a n d tooth structure are depicted iii Fig. 5.

Properties of glnss-iorlotner orid zinc /~fllycclrl~o.rylnie Irri;,lg cerllerrt‘ Cement/

Glass-ionoiner

Properties I’owdcr/liquid ( g/ 1111)

I’olycarboxylatc

ASYA I V A I h i e l o n l’oly-I ratio

Consistency (Disc dianl. mm) Setting time, 37OC (niin)

Film thickness,

1.67

29 4.5

1.5 33

6.75

2.4 35 8.20

24

22

22

Compressive strength, 128 24h, (N/niniz)

79

89

Tensile strength, 2 4 h ( N / nini2)

12

13

(w)

Water le:ichable material, 2 4 h

X

0.2

0.9

0.1

0.7

1.0 (0.) 1.0 (0.)

(% 1 Opacity, (C0.70) I

22Og lo:id applied 2 minutes after start OF mixing.

snndwichetl between negatively charged o x i d e surfaces a n d COO- groups in the cement. U n l i k e composite resins, glass-iononier cements also adhere t o dentine. Collagen of dentine contains some pendant chains w h i c h contain COOH and NH, groups w h i c h w o u l d p r o v i d e sites for ndhesive ionic antl dipole interactions.

Opacity T h e opacity antl the gloss o f the gln:is-ionomer cements are slightly greater t h a n thosc of silicitte cenients ;ind ore ;I function of particle size a n d the disparity between the refractive indexes of the m a t r i x and the embedrled particles. Although the -iononier cements is similar to silicate cement, i t does not r i v a l that o f most m o d e r n composite filling materials a n d is therefore not yet completely suitable in this stage of its development for the aesthetic restoration o f large areiis of Inbial eniiiiie T h e most useful clinical applications for the gl -ionomer AS P A , ceiiien 1 w i l l be described i n Part I I o f this series.

.

Aclt now I ed g e m e nt T h e authors t h a n k the G o v e r n m e n t Chemist, Dr H. Egan, for permission to contribute this paper. C r o w n Copyright, reproduced b y permission of the C o n t r o l l e r of Her Britannic Majesty’s Stationery Office. I l c p a r t m e n t of Industry.

I_;iboratoiy of t h c G o v e r n m e n t Chemist, C o r n w a l l House S t a m f o r d Street. I-ontlon SE I 9NQ.

The clinical development of the glass-ionomer cements. i. Formulations and properties.

Austr;ilim Dental luurnal. February. I977 31 Volume 22. No. I The clinical development of the glass-ionomer cements. 1. Formulations and properties...
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