Characterization of glass-ionomer cements

Journal of Dentistry, 4, No. 6, 1976, pp. 287--290, Printed in Great Britain

Characterization of glass-ionomer cements 2. Effect of the powder : liquid ratio on the physical properties S. Crisp, BSc,PhD, CChem, FRIC B. G. Lewis A. D. Wilson, DSc,CChem, FRIC /.~h~,=tn,v-

nt_Cherni . . . .

artment of Industrv, London

ABSTRAC" An investigation has been made of the relationship between the powder:liquid ratio and the properties o f glass-ionomer cements. Four types of this cement were studied. An increase in the powder : liquid ratio led to increases in stiffness Of mix, setting rate, compressive strength, superficial hardness and resistance to aqueous attack. It is concluded that the highest powder:liquid ratio compatible with convenient mixing and adequate working should be used.

INTRODUCTION The first paper in this series (Crisp et al., 1976) reviewed the literature on the development, chemistry, structure, properties and clinical applications of glass4onomer cements. This paper reports the effect of varying the powder: liquid ratio on the properties of the glass4onomer cement variants ASPA III and ASPA IV. Some limited points of comparison with earlier variants ASPA I and ASPA II are made.

EXPERIMENTAL Materials The glass-ionomer formulations comprise a powder and a liquid; a complete description of these is to be found in Crisp e t al. (1975). All the varieties employ the same powder, based on the aluminosilicate glass G 200i The same batch of powder which had been sieved through a 45-/~m

mesh was u ~ d throughout the experiments and was supplied by the Amalgamated Dental Company. The four polyacid liquids were prepared in this laboratory. Not more than tWO batches of each were used to prepare the cements.

Methods All the cements were mixed at 21 °C and 50 per cent relative humidity by spatulation on a glass block using a known powder: liquid (P: L) ratio (g/ml). About two-thirds of the powder were incorporated in the liquid in the first 15 seconds and the remainder in the next 15 seconds. Total mixing time was 1 minute. The determination of consistency, setting time, compressive strength and solubility was done in accordance with BS 3365.* Compressive strength determinations were made on specimens which had been stored either in water or in paraffin. Working time was assessed as the time at which a 28.3-g Gillmore needle, diameter 1.05 mm, just failed to penetrate to a depth of 0.5 mm in 5 seconds. Hardness was measured by a Wallace Microindentation Tester and the method has been described (Wilson et al., 1976). In this test the

* British Standard 3365/1:1969. Specification for dental silicate cement and dental silicophosphate cement. Part 1. Dental silicate cement.

Journal of Dentistry, Vol. 4/No. 6

288

Table L ~ P r o p e r t i e s of 91ass-ionomer cement types

Cement

ASPA ASPA ASPA ASPA

Consistency (disc diameter, ram)

Working time 21 °C (rain)

P:L 2.0

P:L 2.0

! II I!f IV

P:L 3.0

37-5 39 38 --

24.5 27 27 33.5

4.0 5.0 5.0 7.0

P:L 3.0 2.25 2-25 2.75 3.25

indentation of a Vickers diamond under a load of 300 g is measured in units of 10"s inches and is the Wallace indentation number (1). This is an inverse function of hardness, A direct measure of hardness is given by the Woxen hardness number (WHN). These two parameters are related by the expression:

WHN = 1.532 × 106 N/mm2.

/2 RESULTS AND DISCUSSION The influence of P : L ratio on the properties of ASPA III and IV is shown in Figs. 1-6. In addi~ tion, the results o f a limited study on four glassionomer cements (only two P: L ratios were used) are presented in Table 1. The variation of cement consistency, expressed as the diameter of a pressed-out disc of cement,is shown i n Fig. 1. There is an inverse linear rela-

Setting time 37 °C (rain) P:L 2.0 9-0 5-25 6.0 6-0

P:L 3.0 5.5 3.75 3.75 4.25

730(2.9) 340(13) 310(16) 300(17)

320(15) 148(70) 205(36) 150(68)

tionship between disc diameter and P : L ratio. The slopes of the two lines are similar, but at corresponding P : L ratios ASPA IV yields pastes which are more fluid and which have disc diameters 7 - 9 mm greater than those recorded for ASPA III. Looking at the results for all four variants (Fig. 1, Table I) it can be seen that an increase in P : L ratio of 1 g/ml gives rise to decreases in disc diameters of 13, 12, 11 and 13.5 mm for ASPA I, II, Ill and IV respectively. Thus, the effect of varying the P: L ratio is generally similar for all ibur variants. However, ASPA I produces the stiffest paste and ASPA IV, which utilizes a low viscosity liquid, the most fluid cement pastes.

,0! 94

si .!

7i

44-

Hardness 15 rain (I, 10~ in, with WHN, N/mm =, in parentheses) P:L 2.0 P:L 3.0

ASPAIV '¢~

vASPAIV 40-

E ~u

36'

~, 32

3 .~ 28

2-

d

ASPAIII

2+

~~ASPA 204 . . . . . . . . .

0

o:s

i~o

I.s

2:0

z:s

P: L ratio(g/mJ)

3.0

3:s

4.0

--- 0:s

t:o

i:S~2:0

2~s

3:0

3:s

4.o

P : L ratio (g/ml)

Fig. /.---Variation of the consistency disc dia-

Fig. 2.--Variation of working time of ASPA III

meter of ASPA III and IV with P : L ratio.

and IV with P : L ratio.

Crisp et a'l.: Glass-ionomer Cements

289

As expected, the working times of all four types decrease with increasing P: L ratio (Fig. 2, Table 1). The relationships for ASPA III and IV cements assume the form of an S-shaped curve. Thus, loss o f working time is sharpest at intermediate P : L ratios, namely, between 2.0 and 2.5 g/ml for ASPA III and between 2.5 and 3.0g/ml for ASPA IV. At corresponding P : L ratios ASPA IV has the greatest .working time and ASPA I the least.

~100 90

ASPA IV

80 7c E Z ~ Z I

60

j/

SC

PA II1

l

40

30

20 I0

~4 SPA IV

0

0'.s

~'.0

J:s

Y,s

z~O-- ~'.s

st0

3'.s

~0

P : L ratio (g/ml)

Fig. 4.--Variation of Woxen hardness number at 15 minutes of ASPA III and IV with P: L ratio.

_

O

1

i

0"5

1.0

•

_

1

~

2-5 P : L ratio (g/ml)

I "5

2'0

i

3"0

L

3"5

_.A

4'0

Fig. 3. ~ V a r i a t i o n of setting time of ASPA III and IV with P ' L ratio,

Setting time decreases as the P : L ratio is increased (Fig. 3, Table/), a property the glassionomer cement has in c o m m o n with all dental cements. At higher P: L ratios cement pastes cont ~ n a greater specific surface of powder per unit volume of paste and thus there is a corresponding increase in the rate o f reaction: However, the extent o f the effect o f P: L ratio differs between ASPA types. An increase in the P : L ratio from 2.0 t o 3.0 g/ml results in decrease's ha setting times of 3.5, 1.5, 2.25 and 1.75 minutes for ASPA I, II, III and IV respectively: Although the relationship for ASPA III is approximately linear, the situation for ASPA IV is more complex. The effect o f P : Lratio becomes much less when the P" L ratio exceeds 2.5 g[ml.

The early hardness (WHN) of both ASPA III and IV increases linearly with P : L ratio (Fig. 4), although it is possible that the relationship for ASPA IV may have the form of an S-shaped ASPA iV (paraffin-stored)

220 200 .-, . . . . . . . . .

a ~ °"

.+ ASPA I11 (paraffin-stored) +..S~ ~ ' ~ ASPA IV (water-scored)

as0 ~ ~60 ~ ~40 ~*~z0.

~/ ./

F

ASPA Iit (water-stored)

j~ =

•

.~ ~oo ~ 80 ~ 60 4o 20 0

Fig. 5.

0.5

1.0

1.5 2.0 2.5 P : L ratio (glml)

3.0

3-5

4.0

Variation of compressive strength of ASPA III and IV with P : L ratio.

290

Journal o f D e n t i s t r y , V o l , 4 / N o . 6

curve. ASPA II and IV develop the greatest early hardness values, the effect of the P: L ratio being somewhat more pronounced in the case of ASPA II (Table 10.

An increase in P:L ratio decreases the amount of material that can be water-leached from ASPA III and IV (Fig. 6). This behaviour is a reflection of the increase in the rate of setting with P: L ratio for reasons which have been given already.

CONCLUSIONS

2.2

Glass-ionomer dental cements used for fillings should be mixed at the highest P: L ratio possible in order to produce cement pastes which have rapid hardening rates, high strengths and greatest resistance to the effect of moisture. The qualifying consideration is that of working time, which is reduced as the P: L ratio increases. The required working time will depend on clinical considerations appertaining to the treatment being given.

2.0

1.8

1.6

~, 1.4

,~ l.o

A

A

Acknowledgements

m 0.8

0.6 0.4 0.2

d.s

J'.0

i'.s ~'.0

2'.s

3'.0

3'.s

4!0

The authors thank the Government Chemist, Dr H. Egan, for permission tO publish this paper. This work was carried out on behalf of the National Research Development Corporation. Crown Copyright. Reproduced by permission of the Controller of Her Majesty's Stationery Office.

P: L ratio (g/ml)

Fig. 6. wVafiation of solubility of ASPA III.and IV with P : L ratio.

The compressive strength of both ASPA III and IV increase continually with P:L ratio (Fig. 5). Although there is some fall off in :the rate of increase at the upper end of the P: L ratio range, a maximum plateau is not attained, as in the ease of dental silicate cements (WilSon andBatchelor, 1967). Still less is there a decrease in strength after an optimal P : L ratio has been reached, as is the case with thezinc polycarboxylate cements (Smith, 1971). The behaviour of the glass, ionomer cements is advantageous for the clinician who can mix them as thickly as is compatible with adequate manipulative properties and sufficient working time, without fear of loss of strength.

REFERENCES

Crisp S., Ferner A. J., Lewis B. G. and Wilson A. D. (1975)Properties of improved glassionomer cement formulations. J. Dent. 3 , 1 2 5 130. Crisp S., Lewis B. G. and Wilson A. D. (1976) Charactefiz~ition of glass-ionomer cements. 1. Long term hardness and compressive strength. J. Dent. 4, 162-166. Smith D. C. (1971) A review of the zinc polycarboxylate Cement. Jo Can. Dent. Assoc. 37, 22-29. Wilson A. D. and Batchelor R. F. (1967) Dental silicate cements. II: Preparation and durability. J. Dent, Res. 46,~ 1425-1432. Wilson A. D., Crisp S. and Ferner A. J. (1976) Reactions in glass-ionomer cements: IV.. Effect of Chelating ,co,monomers on the setting reaction. J. Dent. Res, 5 5 , 4 8 9 " 4 9 5 .