Long-termflexuralstrength of glass ionomercements G.J. Pearson and AS. Atkinson Department of Biomaterials Science, Institute of Dental Surgery, University of London, Grays inn Road, London, UK (Received 10 February 1990; revised 20 March 1990; accepted 20 June 1990)

The flexural strength of five glass ionomers was measured at intervals ranging from 1 h to 3 months after mixing. The pattern of change in flexural strength is material-specific with two water-activated materials having significantly higher long-term flexural strengths than the others. This may be attributable to the different polyacrylic acids used or to changes in the glass formation in the different materials. Keywords: Glass ionomer cement, mechanical properties, time dependance

The setting of glass ionomer cements has been described as being in two stages’. The initial stage which produces the clinical set occurs within the first 1O-20 min after mixing; the second stage, involving the exchange of calcium and aluminium ions within the matrix, is thought to continue for up to 24 h. After this further maturation has been reported’. Early work on the original glass ionomer, ASPA, by Crispeta/. indicated that there was a continuing increase in the compressive strength of this material for up to 1 yr. They also showed that there was a variation in the rate of increase of strength, depending on the storage medium. Long-term storage in paraffin rather than water resulted in a much higher strength. Materials with faster setting reactions were thought to be more likely to show this property. Williams and Billington4, 5 have demonstrated that with the recently developed materials, there is a reduction in compressive strength in some cases with time. Smith ef a/.6 have observed that the bond strength between composite and glass ionomer increases over the first 6 d, but after this period starts to decline. They infer that this may be due to hydrolysis of the glass ionomer cement matrix. This paper examines the variations in flexural strength with respect to time of five glass ionomer-based cements. Flexural strength was selected as a demanding but clinically relevant property. It is also sensitive at determining mechanical changes attributable to the alteration in the cement matrix.

MATERIALS

AND

METHODS

Five glass ionomer cements were selected for evaluation: three (Rexodent (R), Opusfil (OP) and Chemfil II (CH)) based on polyacrylic acid and the remaining two (Ketac Fil (KF) and Ketac Silver (KS)) based on a 1 : 1 polymaleic/polyacrylic copolymer (Table 7). Each material was mixed according to

the manufacturer’s instruction, packed in a split mould 25 X 2 X 2 mm and a slide placed over the exposed surface of the material. The assembly was then clamped and allowed to set in a thermostatically controlled oven at 37°C. After 10 min, each specimen was removed from the mould and two coats of varnish applied. The specimen was then replaced in water at 37 “C until required for testing. On removal from the water, the varnish was peeled off and the dimensions of the specimen recorded. A four-point bend test was carried out using a Hounsfield HK25K universal testing machine with a cross-head speed of 0.5 mm/min. The peak force applied to each specimen was noted and the flexural strength calculated from the formula: 6x1 F’hd2 where x is the peak force applied, b is the width, d is the depth and I is the length of specimen between the points of application of the force. A minimum of five specimens were tested at each of the following time intervals after mixing: I,3 and 6 h, 1,2,4, 5, 6, 7 d and 3 months. The flexural strengths of the samples of the same material at the different time intervals were compared statistically using the Mann-Whitney test. A comparison of the flexural strengths of the five different materials at these Table 1

Cements used in this study

cement

Manufacturer

Batch no.

Chemfil II Opusfil Rexodent Ketac Fil Ketac Silver

De Tray Dentsply, Konstanz, Germany Davis Schottlander Et Davis, London, UK Rexodent, London, UK ESPE, Seefeld, Germany ESPE. Seefeld, Germany

CF 87/06 KAB7/10 1o-87-20 Ch P143 Ch 30083

Correspondence to Dr G.J. Pearson. 0 199 1 Butterworth-Helnemann 658

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Flexural strength of glass wnomers: G.J. Pearson and A.S. Atkrnson

Table 2

Flaxural strength of hve glass ionomer restorafive materials at ten time intervals: I, 3, 6 and 24 h, 2, 4, 5, 6 and 7 d and 3 manths

Time

Flexural strength

lh 3h 6h Id 2d 4d 5d 6d 7d 3 months (

(MPa)

Chemfil

Opusfil

Rexodent

Ketac Silver

Ketac FII

30.819.1)

15.0(1.7) 23.9(7.1) 29.0(6.2) 36.3(7.0) 33.4(9.5) 32.3j10.5) 41X%(10.8) 37.1(10.3) 46.9(23.0) 59.4(9.0)

16.1t6.7) 23.3(6.2) 20.9(5.2) 39.1 (6.7) 49.2( 15.8) 43.617.5) 30.0(9.9) 37.4(8.6) 69.8(22.6) 55.6(6.7)

12.8(4.6) l&5(3.6) 21.5(6-O) 30.0(5.3) 21.7(2.5) 29.9( 10.0) 27.613.9) 37.7(4.7) 27.7(6.3) 30.5(6.7)

14.8135) l&0(2.4) 24.013.9) 29.1 ( 1.7) 27.0(4.2) 35.4(6.3) 34.2182) 43.6 (10.81 29.5(2.5) 31.2f4.1)

30.7f7.3) 23.9(9.8) 29.0(9.0) 24.2(7.0) 52.4(8.9) 53.6( 15.7) 38.211 1.3) 36.3 ( 10.4) 38.2(10.8)

) = Standard devrations.

intervals was also carried out using the same statistical analyses.

RESULTS Table 2 and Figures l-3 set out the changes in flexural strength of the five materials with respect to time. Three distinct patterns of change in strength were observed. The two materials based on the polymale~c/polyacrylic copolymer, KS and KF, showed a slow rise in flexural strength over the first 24 h after mixing (Figure I). After this time, the strength remained relatively constant for up to 3 months. Variations occurred between 5 and 7 d (points A and 8) but these appeared to be transitory. Material CH showed a rapid increase in strength and the 1 h value is comparable with that observed at 24 h

I

A..24

168

Log time, Figure 2

1 (h)

Graph of flexural strength verws log of time- Chemfil.

(Figure 2). Between 2 and 5 d (points X and Y), there is a significant rise in strength, but after this the flexural strength is not significantly different from that noted at 24 h. The third group of materials (R and 0) exhibited a steady increase in flexural strength which continued after 24 h and even at the 3 month time interval the trend appears to be upwards (Figure 3). Statistical comparison of the materials shows that material CH is significantly stronger than the other materials at 1 h (P = 0.01) but by 24 h there was no signifjcant difference between the strengths observed for any of the materials. All materials underwent changes in the flexural strength in the 4-6 d period. At 7 d (168 h), material R was significantly stronger than materials CH, KS and KF. This trend was continued at 3 months when material Rand 0 had flexural

strengths

the others

which

were very significantly

Pooling all the results of the polyacrylic and

higher than

(P = 0.01).

comparing

these

polymaleic/polya~ryiic materials

were

materials

based

with

the

materials,

significantly on polyacrylic

pooled

acid materials results

the polymaleic

weaker

of

the

acid-based

in flexure

than

those

acid (P = 0.01).

DISCUSSION

($2.--L-

’

1

Log

KS and KF materials,

I III

24

168 time,

f (h)

Figure I Graph of flexural strength versus tog of time: faj Ketac Fil and ib] Ketac Silver.

donor,

show

having a copolymer

a substantially

compared

to

correlation

between

these materials.

material

CH.

acting as the proton

slower

build-up

of properties

There

appears

to

this ard the measured

Oilo7 showed

working

be

some

times of

that both these materials

Bfomaterrals 799 1. Vo! 12 September

have

659

Flexural strength of glass ionomers: G.J. Pearson and AS. Atkinson

The exceptions to this occurrence were the two newer water-mixing cements 0 and R. Both these materials show a moderately rapid initial increase in strength, although this appears slower than material CH. However, the 24 h strength is significantly less than that at 3 months. It seems that there may be some technological improvements in the glass or acid used. Wilson’ has inferred that there are substantial areas of development possible with the components of the glass ionomer materials and it appears that the five materials studied are examples of the stages of development. It is interesting to note that while there is an overall greater consistency in the test results obtained from the encapsulated materials, this is not as marked as expected. A contributory factor may be the fact that only one operator mixed the materials or the test specimens. L

oa

ob

I

I

I

I

I

I

24

I

I

I 24 Log

I time,

II 168

I I II 168

I

I

t (h)

Figure 3 Graph of flaxural strength versus log of time: (a) Rexodentl and (b) Opusfil.

working and setting times about 33% longer than material CH. The lower flexural strength observed throughout the experiment for these polymaleic-based materials may be attributed to the copolymer and to the different type of glass used for these materials. Oilo showed that the glass used for these materials does not contain sodium and it is possible that the different glass composition and firing may contribute to the difference in the properties. The material CH showed a very similar pattern of maturation to the original ASPA3, but with peak strength reached after only 1 h. After a further rise up to 4 d, there was a definite fall-back to the initial 24 h strength. This phenomenon has not been previously reported, but testing at these time intervals is rare. The long-term pattern is as reported by Crisp et al.3, suggesting that this fall-off in strength may be a result of hydrolysis of the matrix.

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Biomaterials

199 1, Vol 12 September

CONCLUSIONS All glass ionomer cements tested reached an initial peak strength by 24 h after mixing, suggesting that the first two stages of the setting reaction had already occurred. All materials exhibited a further slow rise for at least 3 d. At this point, the flexural strength of three long-established materials reached a plateau and then fell slightly, so that at 3 months the flexural strength was similar to that observed at 24 h. The two more recently produced materials showed a slow increase in flexural strength with time. Given the identical test conditions, this difference in behaviour may be attributed togreater stability in these, because of the reduced tendency for hydrolysis of the matrix. It is also apparent that those materials using the polymaleic/polyacrylic copolymer have a lower flexural strength than those based on polyacrylic acid alone.

REFERENCES 1 2 3 4

5

6

7 8

Crisp, S and Wilson, A.D., Reactions in glass ionomer cements I., J. Dent. Res. 1974, 53, 1408-1413 Crisp, S. and Wilson, A.D., Reactions in glass ionomer cements Ill., J. Dent. Res. 1974, 53, 1420-l 424 Crisp, S., Lewis, E.G. and Wilson, A.D., Characterisation of glass ionomer cements, J. Dent. 1976, 4. 162-t 66 Williams, J.A. and Billington, R.W.. Increase in compressive strength of glass monomerrestorative materials with respect to time periods of 1 day to 4 months J. Oral Rehabil. 1991, 18. 163-l 68 Williams, J.A. and Billington. R.W., Increase in compressive strength of glass ionomer restorative materials with respect to time, J. Oral Rehabil. 1989, 16, 475-479 Smith, D.C., Ruse, D.N. and Zuccolin, D., Some characteristics of glass ionomercement lining materials, J. Canadian Dent. Assoc. 1988,54. 903-906 Oilo, G., Characteristics of glass ionomer filling matertals, Dent. Mater. 1988,4, 129-l 32 Wkon, A.D.. Ch 2, Composition, in Glass lonomers (Eds A.D. Wilson and J.W. McLean) Qumtessence Int.. 1989 pp. 21-42