A comparative evaluation of the microhardness, water solubility and water absorption of fissure sealants

A comparative evaluation of the microhardness, water solubility and water absorption of fissure sealants B. Williams, BDS, FDS RCS J. A. von Fraunhofer, MSc, G. B. Winter, MB, BDS, FDS.

PhD. ARK.

AIM

DCH

Institute of Dental Surgery (Eastman Dental Hospital), ABSTRACT The microhardness. water absorption and solubility characteristics of five fissure sealant materials have been investigated. The sealants studied consist of three polymeric materials and two quasiinorganic cements. The two classes of material, namely polymers and cements, differed markedly in their behaviour. The polymers were broadly similar in their water solubilities and absorption characteristics but one material was less soluble and absorbed less water than the other two. The microhardness of the polymeric materials appeared to increase with time but this effect was offset by the plasticizing action of absorbed water during storage. The cement behaviour reflected differences in their chemical structure-the glass-ionomer cement decreased in hardness whilst a polycarboxylate cement showed an increase; furthermore differences in water solubility and absorption for the

two materials were also apparent.

INTRODUCTION measurement of microhardness, water and water solubility is well absorption established in the assessment of the physical properties of dental materials (Peyton and Craig, 1971; Phillips, 1973). These measurements, however, do not appear to have been undertaken to date for fissure sealants. In assessing the durability of fissure sealants on enamel surfaces. many factors must be

THE

University of London

considered, notably adhesion, wear resistance and dissolution. The short and long term bond strengths of fissure sealants have been reported previously (Williams et al., 1974). Studies on the microhardness, water absorption and water solubility, which are important parameters in wear and dissolution, are presented here. The evaluation of these properties is important in assessing the long term suitability of fissure sealants since they should indicate the physical (and possibly the chemical) changes occurring within the materials.

MATERIALS Three polymeric fissure sealants, a dental cement claimed to have adhesive properties and a glass-ionomer cement were investigated. The supplier, type of materials and curing method of the tive materials are summarized in Table I. It should be noted, however, that although the ESPE 717 material has very recently been superseded by ESPE 717-29, the latter was not available at the start of the investigation.

Microhardness The fissure sealants were placed into wells (10 mm diameter ~0.5 mm) cut in Perspex blocks (Fig. I) so that fixed and reproducible specimens were prepared. The wells were made

2

Journal of Dentistry, Vol. ~/NO. 1

with an undercut at their base so that the fissure sealant formed a ‘retention lug’ when it set and was thereby stabilized. The ESPE 717 and Nuva Seal specimens were prepared by filling the wells with the polymers and then curing by exposure to an perspex block

retentionlug central well wnm diameter Dr;zI;;;

0.5 mm depth

Fig. 1.-Diagram Tab/e I.-Details Material

of microhardness

block.

diamond finish on a Knuth rotary grinder and then immersed in water. This polishing procedure was necessary because these materials presented as-set surfaces that were too rough for the microhardness indents to be measured accurately. Five specimens of each material were prepared and six indentations were made on each specimen at intervals of 1 hour, 1 month, 3 months and 6 months. The mean microhardness values were calculated from the lengths of the diagonals of the six indentations. The microhardness measurements were made with a Reichert microhardness attachment on a

of the fissure sealant materials studied Supplier

Availability

Type

Curing method

ASPA

Laboratory of the Government Chemist

Glass-ionomer cement

Research

Acid

: base reaction

Poly F

Amalgamated Co., Ltd

Cement

Commercial

Acid

: base reaction

Polymeric

Commercial

Chemical, benzyol acrylate

Epoxylite

9075

Dentaleez

Dental

Ltd. UK

probably peroxide

Nuva Seal

L. D. Caulk Co. Ltd

Polymeric

Commercial

Ultraviolet-lightsensitive catalyst 2 per cent benzoin methyl ether

ESPE 717

Espe GmbH, Germany

Polymeric

Research

Ultraviolet-lightsensitive catalyst 1 per cent benzoin methyl ether

ultraviolet light source.* The Epoxylite 9075 specimens were prepared by applying the primer fluid to the wells and drying for 2 minutes. Equal quantities of base and catalyst fluids were poured into the well to fill it and then stirred for 10 seconds. This ensured a more uniform mix in the well. The two cements, ASPA and Poly F, were mixed on a slab using P/L ratios of 3 :l and 2:l w/w respectively. All the materials were allowed to set for 2 minutes and then the specimens of ESPE 717 and Nuva Seal were immersed in tap water at 37&l% The Epoxylite 9075, ASPA and Poly F specimens were ground to 6 pm

Reichert MeF2 Research microscopet, indentations being made with a 20-g load for 10 seconds.

Water solubility Discs of each material, 1 cm diameter x0.5 mm, were prepared by allowing the specimens to set in stainless steel split rings. The polymeric materials, ESPE 717, Nuva * BLE spectroline, Black Light Eastern Division of Spectronics Corporation, New York. 7 C. Reichert Optische Werke AG, Vienna, Austria.

3

Williams et al. : Some Properties of Fissure Sealants

difference between the means. In the case of the microhardness measurements, however, the statistical analysis was performed on the actual indentation diameters rather than from their derived microhardness values to avoid gross distortions (Staheli and von Fraunhofer, 1971).

Seal and Epoxylite 9075, were allowed to cure in situ, whilst Poly F and ASPA were mixed on

a slab and then transferred to the rings. The same P/L ratios were used as for the micro-hardness study. Flat surfaces on the cements and Epoxylite 9075 were achieved by covering the rings with a glass slab, whilst the ultraviolet light-curing materials set with a smooth surface without the need for a cover. Six specimens of each material were prepared. The specimens were weighed and then immersed in a near vertical position in water at 37f 1°C. The water was changed weekly. After 1 month, 3 months and 6 months the specimens were removed from the water and dried by storing in a desiccator containing silica gel until constant weight was achieved, the specimens then being replaced in the water. The solubility of the materials at each time period was determined by taking the mean of the difference between the initial weights and Tab/e //.-Mean

120 110

0

I hr

m

3 mrh

?? I mth

loo %I

so $60 50 40 30 20 >

IO 70 : ASPA’

poly

F

Epxylite’

Nuva Seal

‘See text

Fig.

2.-Histogram sealants.

of microhardness of fissure

microhardness values (kg/mm2)

Material ASPA Poly F Epoxylite 9075 Nuva Seal ESPE 717

1 Hour

1 Month

114.66* 59.05 26.85* 12.06 12.48

105.32 60.47 17.74 15.85 15.05

3 Months

6 Months

93.80 64.47 14.14 17.85 17.44

77.14 84.02 14.00 19.43 15.42

* Figures for 24 hours. Tab/e ///.--Student’s

Material ASPA Poly F Epoxylite 9075 Nuva Seal ESPE 717

t tests comparing microhardness at different times 7 hr: 1 mth

7 mth: 3 mth

Comparison 3 mth: 6 mth

P.S. N.S. H.S. H.S. H.S.

H.S. H.S. H.S. H.S. H.S.

H.S. H.S. N.S. H.S. H.S.

N.S., Not significant (P>O.O5). P.S., Probably significant (P~0.05). H.S., Highly significant (P
the constant weights after desiccation divided by the surface area. The latter was calculated using the relation, surface area = 2++2mh, where r is the disc radius and h the thickness. Student’s t test was applied to the results to determine the statistical significance of the

1 hr: 3 mth H.S. H.S. H.S. H.S. H.S.

1 hr: 6 mth H.S. H.S. H.S. H.S. H.S.

1 mth: 6 mth H.S. H.S. H.S. H.S. N.S.

S., Significant (P~0.01).

Water absorption Discs identical to those used for assessing water solubility were employed for the measurement of water absorption. Six specimen discs of each material were prepared. The specimens were weighed and then immersed in a near

Journal of Dentistry, Vol. ~/NO. 1

4

vertical position in distilled water at 37f 1“C, necessary to take account of the mass of material lost by dissolution. Accordingly the the water being changed weekly. After 1 month, 3 months and 6 months the specimens were weight loss recorded at each time interval, removed from the water, the excess surface _the water solubility, was to be added to the water was removed by blotting and the discs apparent water absorption figure to obtain the were then weighed. The apparent water true water absorption. Table IV.-Intermaterial

comparisons of microhardness at 1 month and 6 months

Material

ASPA

1 Month ASPA Poly F Epoxylite 9075 Nuva Seal ESPE 717

Poly F

-

H.S. -

H.S. H.S. H.S. H.S.

6 Months ASPA Poly F Epoxylite 9075 Nuva Seal ESPE 717

Epoxylite 9075

H.S. H.S. -

H.S. H.S. H.S.

H.S. H.S.

-

H.S. H.S. -

H.S. H.S. H.S.

H.S. H.S. H.S.

H.S. H.S.

N.S., Not significant (P>O.O5). P.S., Probably significant (P~0.05). H.S., Highly significant (P
Tab/e V.-Water Material ASPA Poly F Epoxylite 9075 Nuva Seal ESPE 717

solubility (mg/mm*X 7 Month

3 Months

IO-‘) 6 Months

0.0917 0.0958 0.1595 No results, all specimens fractured 0.1135 0.1323 0.1323 0.1225 0.1323 0.1427 0.0436 0.0716 0.0589

Table V/.-Water solubility per month (mg/mm*X 10e2) Material ASPA Poly F Epoxylite 9075 Nuva Seal ESPE 717

1 Month

3 Months

6 Months

0.0917 0.0319 0.0266 No results, all specimens fractured 0.1135 0.0431 0.0220 0.1225 0.0431 0.0238 0.0436 0.0239 0.0093

absorption at each time period was determined by taking the mean of the difference between the initial weight and the weight after water immersion and blotting divided by the surface area, the latter being calculated as before. In order to obtain the true water absorption it is

-

Nuva Seal

ESPE 77 7

H.S. H.S. H.S. -

H.S. H.S. H.S. P.S. -

P.S.

H.S. H.S. H.S. H.S.

H.S. H.S. H.S. H.S. -

S., Significant (P
Student’s t test was applied to the results to determine the statistical difference between the mean values obtained for each material.

RESULTS Microhardness The results of the microhardness tests are given in Table II and Fig. 2. The mean values only are presented in this table but the statistical analysis of the values at different times is given in Tables ZZI and IV. It should be noted that the figures for ASPA and Epoxylite 9075 were obtained at 24 hours, and not 1 hour, owing to the initial resilience of the material which prevented clear indentations being obtained at an earlier stage. The five materials differed in the change in their hardness upon water immersion. Nuva Seal exhibited a progressively increasing hardness over 6 months, whilst ESPE 717 showed an increase in hardness up to 3 months which decreased at the end of 6 months to that found at 1 month. ASPA had progressively decreasing

Williams

et al.

: Some Prooerties of Fissure Sealants

5

microhardness values over the test period, whereas a ‘limiting’ hardness value was achieved by Epoxylite 9075 after 3 months. Poly F, however, showed no change over the

Water solubility The results of the water solubility tests, expressed as mean weight losses, are given in Tables Vand VZand Fig. 3, whilst the statistical

0.20

: s

0.18

loo

0.16

90

0.14

T80 2 70

0.12

x60

x 0.10 “E E oa

“E

50 40

E” 0.06

E” 30

0.04

20

0.02

IO ASPA

Epoxylite

3.-Histogram sealants.

Fig.

Tab/e

V//.-Student’s

Material ASPA Poly F Epoxylite 9075 Nuva Seal ESPE 717

Nuva

Seal

t tests comparing

Material 1 Month ASPA Epoxylite 9075 Nuva Seal ESPE 717 6 Months ASPA Epoxylite 9075 Nuva Seal ESPE 717

ASPA

water

solubility

Comparison 1 mth: 3 mth 3 mth: 6 mth S. No results, P.S. N.S. S.

N.S., Not significant (P>O.O5). S., Significant (P~0.01). H.S., Tab/e VU/.--Intermaterial

Erpe

of solubility in water of fissure

H.S. all specimens N.S. N.S. N.S.

times

1 mth: 6 mth

N.S. N.S. P.S.

of solubility

at 1 month

Epoxylite 9075

N.S. N.S. H.S.

N.S., Not significant (P~0.05). S., Significant (P~0.01). H.S.,

Erpe 717

H.S.

N.S. N.S. H.S.

S. N.S. H.S.

Seal

fractured

-

-

at different

Nuvr

of water absorption of fissure

P.S., Probably significant (P~0.05). Highly significant (P
comparison ASPA

Epoxylite

4.-Histogram sealants.

Fig.

S. N.S. H.S.

and 6 months

Nova Seal

N.S. N.S. H.S.

N.S. N.S. H.S.

Espe 717

H.S. H.S. H.S. -

H.S. H.S. H.S. -

P.S., Probably significant (PiO.05). Highly significant (P
first month, but then the hardness increased over the remainder of the test period. All the changes in microhardness were found to be highly significantly different (P-=~O~001,Tables ZZZandZV).

analysis of the results is presented in Tables VZZand VIII. No solubility figures could be obtained for Poly F since all the specimens fractured during testing. Progressive increase in the net weight

Journal of Dentistry, Vol. ~/NO. 1

6 Table /X.-Water Material ASPA Poly F Epoxylite 9075 Nuva Seal ESPE 717

Table X.-Water IO-‘) Material ASPA Poly F Epoxylite 9075 Nuva Seal ESPE 717

1 Month

3 Months

ASPA Poly F Epoxylite 9075 Nuva Seal ESPE 717

loss

6 Months

0.7567 0.8183 0.9186 No results, solubility specimens disintegrated during drying phase 0.1885 0.1625 0.1385

0.2160 0.2148 0.1535

0.2019 0.1765 0.1452

absorption

per month (mg/mm’

1 Month

3 Months

Water absorption x

6 Months

0.7567 0.2728 0.1537 No results, solubility specimens disintegrated during drying phase 0.1885 0.1625 0.1385

0.0720 0.0716 0.0511

0.0278 0.0296 0.0242

Table X/.-Student’s t tests comparing absorption at different times

Material

was found for ASPA and Nuva Seal, although the dissolution rate decreased over the test period for both materials, The ESPE material exhibited a continuing weight loss up to 3 months followed by weight loss at 3 months with no further dissolution occurring over the remainder of the test period.

x lo-‘)

absorption (mg/mm2

Comparison 3 mth: 6 mth

1 mth: 3 mth

water

1 mth: 6 mth

N.S.

N.S. Not available

N.S.

N.S. N.S. N.S.

N.S. N.S. N.S.

N.S. N.S. P.S.

N.S., Not significant (P>O.O5). P.S., Probably significant (P~0.05). Tab/e X/L-Intermaterial Material 1 Month ASPA Epoxylite 9075 Nuva Seal ESPE 717 6 Months ASPA Epoxylite 9075 Nuva Seal ESPE 717

The results of the water absorption, expressed as mean weight gains, are given in Tables IX and X and Fig. 4, whilst the statistical analysis of the results is presented in Tables XI and XII. Again no figures for the water absorption of Poly F were obtained owing to the disintegration of the specimens during the drying stages of the test procedure.

DISCUSSION The fissure sealants studied here fall into two categories, namely the polymeric materials Epoxylite 9075, ESPE 717 and Nuva Seal, whilst ASPA and Poly F are quasi-inorganic materials. Consequently, direct comparison of the behaviour of the two classes of material is not strictly justified. Over the 6-month test period the microhardness of Nuva Seal was found to increase steadily. This is probably due to a continuing polymerization reaction within the material. Over the same period the solubility of the materials also increased. However, the water absorption of Nuva Seal increased up until 3 months and then started to decline, suggesting

comparison of water absorption at 1 month and 6 months ASPA

Epoxylite 9075

-

H.S.

H.S. H.S. H.S.

N.S. P.S.

H.S. H.S. H.S.

Muva Seal

ESPE 717

H.S. N.S. -

H.S. P.S. N.S. -

N.S.

H.S. -

H.S. N.S.

N.S. S.

LS.

N.S., Not significant (P>O.O5). P.S., Probably significant (P
H.S. S. P.S. -

Williams et al. : Some Properties of Fissure Sealants

7

that water ingress had reached a steady state and possibly that the water was being incorporated in some form within the material. The behaviour of ESPE 717 would appear to be broadly similiar to that of Nuva Seal. Probably polymerization of this material ceases at an earlier stage than with Nuva Seal, because the microhardness increased up to 3 months and then started to decline. The water

that a steady state could be achieved correspondingly earlier. The continuing polymerization of these materials increases the microhardness, but this is probably offset to a certain extent by the plasticizing effect of water that is known to occur on the surface of plastics (von Fraunhofer and Suchatlampong, 1975). The microhardness of the polymeric materials would only

bis-GMA CH2

C,H3

CH2

\‘C-CO-O-CH2-CH-CH2-O~CC)uC~-~H-~-O-co-c~

CI;J

6H

CHJ

OH

‘CH3

bis-PMA

CHJ C-CO-O-CnHzn-0 -o;&o-0-Cn

q2 4, Fig. L-Chemical

ti2n-0-CO-Cy LE

formulas of bis-GMA and bis-PMA

solubility and water absorption of this material was lower than that found for Nuva Seal, and this reflects the differing natures of the side chains of the two materials (Fig. 5). The side chains of ESPE 717 should be more hydrophobic than those of Nuva Seal (and Epoxylite 9075) owing to the lower concentration of oxygen-containing groups. The water solubility and water absorption of Epoxylite 9075 are similar to those of Nuva Seal, but Epoxylite 9075 reaches a steady state more quickly than Nuva Seal. The microhardness of the material, however, is different in that after a very high initial value the hardness of this material declines. Other work (von Fraunhofer and Williams, 1974) has shown that there is a much greater heat evolution over a shorter period of time in the chemical-curing Epoxylite system than is found with the ultraviolet-light-cured Nuva Seal. This higher heat evolution would affect the incidence of porosity within the material and therefore might affect the rate of water ingress into the material so

start to decrease as soon as polymerization is balanced by the rate of water absorption and attendant softening effect. With the second category of materials, it is probable that leaching of polyacrylic acid occurs. The leached acid will contribute to the solubility figures, but the ingress and retention of water within the cement mass will tend to offset this loss of weight. The microhardness of Poly F increases over the test period whilst that of ASPA decreases. These differences are thought to be due to differences in the chemical structure of the two materials. Measurements of microhardness became increasingly difficult over the observation period since the surface finish deteriorated. The large increase in solubility and absorption of ASPA between 3 and 6 months might suggest that pieces of material were breaking off from the specimens. Since the microhardness decreased over this time period, it is possible that any such pieces lost were harder ceramic particles in the cement. Clinically,

Journal of Dentistry, Vol. ~/NO. 1

8

fracture of pieces of material from the main mass of the cement, rather than a gradual diffusion of ions, should have little effect on the efficacy of the material as a fissure sealant, viz., either it would be there in the fissures or it would be lost. The unfortunate break-up of the Poly F specimens does not allow further comparisons to be made between the cements. Further work, however, using dye penetration and zero resistance ammetry techniques indicates that both ASPA and Poly F are freely permeable to water and ions (Williams et al., 1975). The high water absorption characteristics of ASPA suggest that materials such as fermentable carbohydrates could also become incorporated within the body of the material. In this way leakage both around and through the material might occur and increase the incidence of caries. The results of a clinical trial indicate that there are differences at the 5 per cent statistical level in caries incidence when comparing ASPA and Nuva Seal and this would support the above contention since Nuva Seal has been shown to leak only around the edge and not through the body of the material, the edge leakage, however, being of short duration (Williams et al., 1975). The temporary beneficial effect that would be obtained through the use of a covering varnish on ASPA at the time of application would not prevent the absorption effect in this material since the absorption process would commence as soon as the varnish was lost.

CONCLUSIONS Alternating periods of hydration and dehydration adversely affect the cement Poly F, so the use of this material on premolar teeth in a ‘mouth breather’ may be a problem. The high water absorption characteristics of ASPA suggest that materials such as fermentable carbohydrates could become incorporated

in the body of the sealant in use and lead to leakage both through and around the material, thus facilitating the initiation of caries. Water absorption and water solubility characteristics of the polymeric materials appear closely connected. The leaching of soluble constituents from these polymeric materials is not excessive. Continuing polymerization of these materials increases the microhardness of the surface, but this is probably offset to a certain extent by the plasticizing effect of water that is known to occur on the surface of plastic materials. The microhardness of these materials only starts to decrease when polymerization is balanced by the rate of water absorption. The microhardness of the two cements is affected by differences in their chemical structure such that Poly F tended to become harder with time whilst ASPA tended to become softer.

REFERENCES VONFRAUNHOFERJ. A. and SUCHATLAMPONG C. (1975) The surface characteristics of denture base polymers. J. Dent. 3. In the press. VON FRAUNHOFERJ. A. and WILLIAMSB. (1974) Heat liberation during the setting of four fissure sealants. Br. Dent. J. 136, 498499. PEYTONF. A. and CRAIG R. G. (1971) Restorative Dental Materials, 4th ed. St Louis, Mosby, pp. 58-88. PHILLIPSR. W. (1973) Skinner’s Science of Dental Materials, 7th ed. Philadelphia, Saunders, pp. 28-54. STAHELIP. J. and VON FRAUNHOFERJ. A. (1971) Microhardness and compressive strength of preamalgamated and non-preamalgamated conventional and spherical amalgam alloys. Br.

Dent. J. 131, 145. WILLIAMSB., VONFRAUNHOFERJ. A. and WINTER G. B (1974) Tensile bond strength between fissure sealants and enamel. J. Dent. Res. 53,

23. WILLIAMSB., VON FRAUNHOFERJ. A. and WINTER G. B. (1975) In preparation.