australian dental journal article

8/13/2019 Australian dental journal article

1/6

Effect of G-Coat Plus on the mechanical properties of glass-ionomer cements

R Bagheri,* NA Taha,† MR Azar,* MF Burrow‡

*Department of Dental Materials, Biomaterial Research Centre, Faculty of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran.†Department of Conservative Dentistry, Faculty of Dentistry, Jordan University of Science and Technology, Irbid, Jordan.‡Oral Diagnosis and Polyclinics, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, Hong Kong SAR.

ABSTRACT

Background: Although various mechanical properties of tooth-coloured materials have been described, little data havebeen published on the effect of ageing and G-Coat Plus on the hardness and strength of the glass-ionomer cements (GICs). Methods: Specimens were prepared from one polyacid-modified resin composite (PAMRC; Freedom, SDI), one resin-modified glass-ionomer cement; (RM-GIC; Fuji II LC, GC), and one conventional glass-ionomer cement; (GIC; Fuji IX,GC). GIC and RM-GIC were tested both with and without applying G-Coat Plus (GC). Specimens were conditioned in37 °C distilled water for either 24 hours, four and eight weeks. Half the specimens were subjected to a shear punch testusing a universal testing machine; the remaining half was subjected to Vickers Hardness test. Results: Data analysis showed that the hardness and shear punch values were material dependent. The hardness andshear punch of the PAMRC was the highest and GIC the lowest. Applying the G-Coat Plus was associated with a signifi-cant decrease in the hardness of the materials but increase in the shear punch strength after four and eight weeks.Conclusions: The mechanical properties of the restorative materials were affected by applying G-Coat Plus and distilledwater immersion over time. The PAMRC was significantly stronger and harder than the RM-GIC or GIC.

Keywords: G-Coat Plus, glass-ionomer cements, mechanical properties.

Abbreviations and acronyms: ANOVA = analysis of variance; GIC = glass-ionomer cement; PAMRC = polyacid-modified resin com-posite; RM-GIC = resin-modified glass-ionomer cement; VHN = Vickers Hardness Number.

(Accepted for publication 5 March 2013.)

INTRODUCTION

Conventional glass-ionomer cements (GICs) areformed by an acid-base neutralization reactionbetween an aqueous polyalkenoic acid and an alumi-nosilicate glass powder, which results in a relativelybrittle material compared to resin composite. Sincethe end of the 1980s, more developed GICs such as

resin-modified glass-ionomer cements (RM-GICs) havebecome available. Stronger and less brittle hybridmaterials have been produced by the addition of water-soluble polymers to create a light-curing GICformulation.1 The aim of introducing RM-GIC was tomaintain the desirable properties of GIC and over-come the disadvantages such as moisture sensitivityand poor early mechanical strength.2 In their simpleform, RM-GICs are water-hardening cements with theaddition of a hydrophilic monomer, 2-hydroxyethylmethacrylate (HEMA).3

A recently introduced tooth-coloured restorativematerial that claimed to have the benefits of the GICs

and resin composite filling materials is the polyacid-modified resin composite (PAMRC), commonly called‘compomer’. PAMRCs were introduced in approxi-mately 1994.4 Commercially, the term ‘compomer’(composite-ionomer) is widely used to reflect its resincomposite and glass-ionomer derivation. PAMRCscombine glass polyalkenoic components with poly-merizable resin constituents such as dimethacrylates.

PAMRCs do not have an auto-setting acid-base reac-tion as in GICs but rely on resin polymerization tocreate a ‘set’ functional restorative material.2

When selecting a material to restore teeth, one of the main considerations is its mechanical properties.A restorative material used to replace missing toothstructure needs to be strong enough to withstand theforces associated with mastication and other possibleloading. Hardness and shear punch strength are twotests that can be used to evaluate the mechanicalproperties of a filling material. The material hardnesscan be defined as its resistance to surface indenta-tion.5,6 The Vickers Hardness test is a method used

448 © 2013 Australian Dental Association

Australian Dental Journal 2013; 58: 448–453

doi: 10.1111/adj.12122

Australian Dental JournalThe official journal of the Australian Dental Association


2/6

for brittle materials6 in which a pyramidal indentationis made using a specified load and application time,the resultant hardness number being independent of the applied load.7

The shear punch test has been used to determineproperties of clinical significance such as occlusal or

incisal forces that occur during mastication.8 It hasbeen used as a simple, reliable technique for assessingthe mechanical properties of resin-based materials.9

Both hardness and shear punch strength of tooth-col-oured materials have been evaluated to predict theirdurability, and a relationship between these mechani-cal properties to material filler content, filler size andsilane coupling agent has also been demonstrated.10,11

The objectives of the present study were to placeGIC, RM-GIC and PAMRC in distilled water for upto eight weeks at 37 °C and determine: (1) the resul-tant surface hardness and shear punch strength; (2)

the effect of ageing on the surface hardness and shearpunch strength; and (3) the effect of a recently intro-duced resin surface coating (G-Coat Plus) on the hard-ness and shear punch strength of the GIC andRM-GIC. The null hypotheses are that there is no dif-ference among the materials; that the ageing does notaffect mechanical properties; and that the surfacecoating does not affect the mechanical properties of GICs.

MATERIALS AND METHODS

Specimen preparationThe materials used in the study are listed in Table 1.For Freedom a total of 9 and for the GIC andRM-GIC, a total of 36 disc-shaped specimens forhardness testing (10.0 mm diameter 9 1.5 mm thick)were prepared. For shear punch testing, 18 and 72disc-shaped specimens (10.0 mm diameter 9 0.7 mmthick) were prepared for Freedom, and the GIC andRM-GIC respectively (n = 6). All materials wereplaced in the appropriate plastic mould and pressedbetween two plastic matrix strips and glass slabsunder hand pressure to extrude excess material. The

glass slabs were removed and the light-cured materials

were cured according to the manufacturers’ instruc-tions on each side using an LED curing light with awavelength range of 440 – 480 nm at an output of 1500 mW/cm2 (Radii plus LED, SDI, Bayswater, VIC,Australia). Specimens were removed from the mouldand excess material around the mould was removed

by manual gentle wet grinding both sides of the speci-mens in a circular motion with a sequence of 1000-,1500-, 2000-grit silicon carbide papers. Each speci-men was washed in an ultrasonic bath between eachgrinding. Specimens were randomly divided into fivegroups for each test (Tables 2 and 3); each group was

Table 1. Materials

Name Manufacturer Material type Filler/resin type Batch#

Freedom SDI, Vic, Australia Polyacid-modified resin composite strontium fluoroaluminium silicate/urethanedimethacrylate based

121314

Fuji IILC

GC Corporation,Tokyo, Japan

Resin-modifiedglass-ionomer cement

Aluminium-fluoro-silicateglass/Poly-HEMA 1202221

Fuji IX GC Corporation,Tokyo, Japan

Self-cure (conventional)glass-ionomer cement

Aluminium-fluoro-silicate glass 1110191

G-CoatPlus

GC Corporation,Tokyo, Japan

Nanofilled self- adhesive light- curedprotective coating

Table 3. Mean Vickers Hardness Number (VHN;

MPa) and standard deviations () of the materialsfollowing times interval (n = 3 discs 3 5 indentation= 15)

Materials 24 hours 4 weeks 8 weeks

Freedom aA87.6 (3.5) aB66.3 (3.5) aB77.4 (6.1)Fuji II LC bA 49.3 (2.4) bB 40.4 (4.7) aA 49.1 (2.4)Fuji II LC+ cB 27.8 (1.5) bA 43 (1.7) bB 28.4 (1.0)Fuji IX dA 15.5 (0.5) cB 11.8 (0.7) cA 13.2 (1.7)Fuji IX+ dA 15.4 (0.5) cB 7.3 (0.3) cB 8.2 (1.8)

– Determines control group (not coated by G-Coat Plus).+Determines treatment group (coated by G-Coat Plus).Means with the same upper-case letter in each row are not signifi-cantly different (p > 0.05). Means with the same lower-case letter

in each column are not significantly different (p > 0.05).

Table 2. Mean shear punch strength (MPa) andstandard deviations () of the materials followingtimes interval (n = 6)

Materials 24 hours 4 weeks 8 weeks

Freedom aA47.8 (4.0) aA47.2 (2.2) aB111.6 (5.1)Fuji II LC aA45.5 (7.5) aA40.3 (2.3) bB80.6 (6.5)Fuji II LC+ aA40.0 (6.6) cC59.3 (7.6) cB95.2 (5.0)Fuji IX bA31.7 (2.3) bA35.9 (3.8) dB59.9 (6.2)Fuji IX+ cA25.0 (1.1) aA42.56 (3.0) bB76.2 (3.4)

– Determines control group (not coated by G-Coat Plus).+Determines treatment group (coated by G-Coat Plus).Means with the same upper-case letter in each row are not signifi-cantly different (p > 0.05). Means with the same lower-case letterin each column are not significantly different (p > 0.05).

© 2013 Australian Dental Association 449

Mechanical properties of restorative materials


3/6

subdivided to three groups and conditioned in distilledwater at 37 °C for 24 hours, four and eight weeks.

G-Coat Plus was applied on the specimens in thecoated group by placing a thin coat of G-Coat Pluson the top surface of the specimen with a micro-brush, then gently air blown for 5 seconds and light-

cured for 20 seconds according to the manufacturer’srecommendation. Specimens were stored, and testedafter 24 hours, four and eight weeks immersion in dis-tilled water at 37 °C. The water was changed weeklyfor each of the time periods. Measurements of VickersHardness and shear punch strength were carried outas described below.

Shear punch strength

The thickness of each specimen was measured with adigital micrometer, positioned in the shear punch jig

(Fig. 1) and held in place by gently tightening therestraining screw. The shear punch jig was aligned tothe loading axis of the universal testing machine(Zwick/Roll Z020, Zwick GmbH & Co, Germany).A flat-ended 3.2-mm diameter stainless steel rod wasused to punch out a disc through the centre of eachspecimen at a crosshead speed of 1 mm/min, and themaximum load recorded. Shear punch strength (MPa)

was calculated using the following formula:

load (N)

specimen thickness (mm) punch circumference (mm)

Vickers HardnessEach disc was subjected to five indentations with35 lm apart across the specimen surface by applyinga load of 300 g for 15 seconds using a Digital Hard-ness Tester (Buehler, Chicago, USA) (n = 3 discs 9 5indentation = 15). The Vickers Hardness Number(VHN) was determined by dividing the load (kgf) bythe surface area (mm2), and the resulting valueconverted to MPa multiply by 9.807 (MPa = Kgf 99.8/m2 9 106).

Data analysis

To evaluate the interaction between material andimmersion time as well as the effect of G-Coat Plus ineach of the two tests, two-way analysis variance(ANOVA) was carried out. To determine inter-mate-rial differences for each test, data were analysed usingone-way ANOVA and Tukey’s test at a significancelevel of 0.05. A Pearson Correlation test was alsoconducted to determine if a relationship could beobserved between hardness and shear punch strength.

RESULTS

Shear punch strength

The means and standard deviations are shown inTable 2 and Fig. 2. The base line strength for thePAMRC was slightly greater than the RM-GIC, butsignificantly greater than the GIC (p < 0.05). Allmaterials showed a significantly higher shear punchvalue after eight weeks immersion in distilled water.G-Coat Plus coated specimens showed an increase in

Fig. 1 A schematic representation of the shear punch jig.

12024 hours 4 weeks 8 weeks

100

80

60

40

S h e a r P u n c h S t r e n g t h ( M P a )

20

0

Freedom Fuji II LC - Fuji II LC + Fuji IX - Fuji IX +

Fig. 2 Shear punch strength (MPa) versus time interval for all materialsin distilled water.


R Bagheri et al.


4/6

shear punch strength for the RM-GIC and GIC(p < 0.05) after four and eight weeks immersionrespectively compared to the non-coated specimens.

The Pearson Correlation test showed over each testperiod (24 hours, four and eight weeks) no correlationcould be determined between shear punch strength

and hardness. The correlations were 0.297, 0.175and 0.23 for each of the time periods respectively.Irrespective of time, the correlation was calculated at0.052.

Vickers Hardness

The means and standard deviations are shown inTable 3 and Fig. 3. With respect to the materialtested, regardless of time, the hardness of the PAMRCwas the highest and GIC the lowest. There was a sig-nificant difference between all materials at baseline

VHN (p < 0.05). Ageing in distilled water for most of the materials showed significantly lower VHN afterfour weeks but then an increase after eight weeks thatremained lower than at baseline. Tukey’s test showedG-Coat Plus exhibited a significant decrease in theVHN of the RM-GIC after 24 hours and eight weeks(p < 0.05), while it showed significant increase afterfour weeks immersion compared to that of 24 hours.

DISCUSSION

Roydhouse8 introduced the shear punch test as a prac-tical and reliable test for comparing dental cements.

The shear punch test was confirmed by Nomotoet al.,11 as a suitable method for standard specifica-tion testing across a broad range of restorative materi-als. Although flexural, compressive and diametraltests are the most commonly used, differences in spec-imen quality and stress concentration during loadingare common problems when comparing inter-labora-tory test results. In contrast to flexural, diametral andcompression testing, the shear punch test is not

particularly technique sensitive in terms of the qualityof the circumference edges of the disc. Therefore, sim-plicity of specimen preparation has been mentioned asthe main advantage of the shear punch test over theother tests.11,12 In the current study, specimens werepolished to obtain flat surfaces for uniform stress dis-

tribution around the punch circumference.The modified shear punch test jig (Fig. 1) intro-

duced by Nomoto et al.11 was used in the presentstudy. In this method, the specimen was restrainedduring shear punch testing by a screw clamp over thetop of the specimen, which has been advocated forthe prevention of specimen flexure during applicationof the force from the punch.11

In this study, in contrast to the effect of ageing onthe hardness of the tested materials, storage in dis-tilled water was associated with an increase in shearpunch strength (Fig. 2). A statistically significant

increase in shear punch strength was observed for allmaterials as the time interval increased with the great-est increase observed after eight weeks immersion indistilled water (Table 2). The shear punch value forthe specimens with the coating agent was more thanthe uncoated group. Coated specimens showed a sig-nificant increase after four weeks immersion in dis-tilled water and even more after eight weeks. It is apossibility that the coating agent exerted some localcontrol on the setting of the materials after four andeight weeks or reduced the effect of surface porosityand crack propagation. The rank order of increasingshear punch strength in this study (GIC


5/6

significantly lower surface hardness than compositescontaining zirconia/silica filler particles.

The hardness after 24 hours immersion in distilledwater showed a significantly greater value for theRM-GIC, approximately twice that of the GIC. Con-ventional GICs are formed by an acid-base reaction

between an aqueous polyacrylic acid and an alumino-silicate glass powder, which results in a relatively brit-tle material in comparison to RM-GICs. Higherhardness values for the RM-GIC can probably beattributed to the polymerized resin component, poly-HEMA,15 which may reinforce or stiffen the wholecement by creating a polymer structure throughoutthe set cement.

Regarding the effect of storage in distilled water onthe PAMRC and GIC, the hardness of all groupsdecreased as the time interval increased with thegreatest decrease observed after four weeks (Fig. 3).

This result is in agreement with the results of severalother studies.16,17 The reduction of hardness in thePAMRC due to ageing could be attributed to manyfactors, including water sorption by the resin compo-nent causing plasticization. Since hardness is a surfaceproperty, it is affected by water sorption.18,19 Watersorption is a complex phenomenon and is dependenton the matrix resin, the filler, and the properties of the interface between the matrix and the filler.20 Adecreased filler loading has been shown to result ingreater water sorption.21

As water is a poor solvent of dental composites,22

the water sorption is a process of slow diffusion.

Therefore, it would be expected that the storage timewill have an influence upon water sorption and conse-quently mechanical properties.23 It has been shownconsiderable time is needed for resin composite tobecome completely saturated by the water which maylead to a stabilization in hardness changes. Therefore,in order to compare the effect of water on the hard-ness of these materials more objectively, further long-term studies are needed.

Both the GICs and RM-GIC showed a significantlylower hardness after applying G-Coat Plus comparedto the uncoated groups. The results of our previous

study,24

the effect of coating on the fracture toughness(KIc) of GICs, revealed that coating with G-Coat Plusincreased the KIc of GIC significantly while it did notaffect that of RM-GIC. A recent study25 also reporteda similar finding. This study25 investigated the effectof G-Coat Plus on the fracture resistance of Fuji IXGP Extra after 14 days storage in distilled water. Thehighest reported fracture strength, 26.1 MPa, was forthe GIC coated before water contamination in com-parison with uncoated GIC and GIC coated afterwater contamination.25 According to the findings of Bonifacio et al.,26 Fuji IX GP Extra showed significantimprovement in wear resistance and flexural strength

when G-Coat Plus was applied. They observed micro-mechanical interlocking between the G-Coat Plus andthe GIC under SEM. The authors speculated thatG-Coat Plus is advantageous if used with Fuji IX GPExtra to decrease the early wear and increase its frac-ture strength.26

Hardness is a surface phenomenon, while fracturetoughness is an intrinsic characteristic of a materialrelated to energy needed for cracks to propagate andhow crack propagation may be prevented. Based onthe manufacturer’s claim, infiltration of G-Coat Plusgives internal protection against crack initiation andfills porosities, both of which may increase fracturetoughness and thus reinforcing and strengthening theGIC and RM-GIC. The self-adhesive coating bonds toGIC and provides a lamination effect that has beenshown to increase the fracture toughness.24 Its protec-tive effect from extrinsic water may also allow com-

plete maturation of the GIC reaction with delayedwater exposure, thus possibly creating a strongermaterial while it may not reinforce the surface of thematerial.

A recent clinical study trialled the use of the EQUIAsystem (GC Corp, Japan) in posterior teeth on occlu-sal and approximal restorations up to 24 months.27

The EQUIA system is Fuji IX GP Extra with G-CoatPlus. Their study concluded this system may be suit-able for long-term temporary and small permanentrestorations. However, the median age of the restora-tions was only 24 months and they failed to use acomparison material.

The results of the present study in the context of clinical usage show that all the GICs gain strengthsome time after the initial set. In addition, the use of G-Coat Plus seems to provide some benefit after theinitial set (>24 hours), i.e. enhancing the initialstrength. Hence, the longer term strength results lendsome support to the clinical study that GIC could pos-sibly be used in small load bearing restorations. Itwould also seem that the RM-GIC tested is approach-ing strengths of the PAMRC used. This is an interest-ing result that may indicate the placement of coatedRM-GIC restorations could be as successful as PAM-

RC for small restorations not exposed to a highdegree of load, e.g. anterior approximal or smallocclusal restorations and deciduous teeth. However,further work is needed to determine if the loss of thissurface coating due to occlusal wear will affect clini-cal survival.

CONCLUSIONS

Within the limitations of this study, the followingconclusions were drawn and the three hypotheseswere rejected. Ageing in distilled water affected thehardness and/or shear punch strength of all materials


R Bagheri et al.


6/6

to varying degrees. The effect of time and G-Coat Pluswas material dependent. In general, most of the mate-rials showed an increase in the shear punch strengthand decrease in the VHN after immersion in distilledwater, in comparison with the baseline. For both testresults, the ranking was consistent with the clinical

recommendation for the materials; the PAMRC wassignificantly stronger and harder than the RM-GIC,which in turn was significantly stronger and harderthan the conventional GIC. In this study, surface coat-ing of GIC and RM-GIC using G-Coat Plus was notfound to be effective in increasing the Vickers Hard-ness of the materials. However, it was effective on theshear punch strength of those materials. Coated FujiII LC and Fuji IX showed significantly higher shearpunch strength than uncoated groups after four andeight weeks immersion in distilled water while thecoated specimens of the 24-hour group exhibited

lower values than the uncoated groups. It must beemphasized that the results of the present study arevalid for the laboratory conditions used. Laboratorydata may provide an insight into clinical performance;however, a direct relationship between laboratory andclinical performance cannot always be assumed.

REFERENCES

1. Wilson AD. Developments in glass-ionomer cements. Int J Pros-thodont 1989;2:438 – 446.

2. Sidhu SK, Watson TF. Resin-modified glass ionomer materials.A status report. Am J Dent 1995;8:59 – 67.

3. Wilson AD. Resin-modified glass-ionomer cements. Int J Prosth-odont 1990;3:425 – 429.

4. McLean JW, Nicholson JW, Wilson AD. Proposed nomencla-ture for glass-ionomer dental cements and related materials.Quintessence Int 1994;25:587 – 589.

5. Craig RG, Powers JM, Wataha JC. Dental materials propertiesand manipulation. St. Louis: Mosby, 2000.

6. Anusavice KJ. Phillips’ Science of Dental Materials. St. Louis:Mosby, 2003.

7. Wassell RW, McCabe JF, Walls AW. Subsurface deformationassociated with hardness measurements of composites. DentMater 1992;8:218 – 223.

8. Roydhouse RH. Punch-shear test for dental purposes. J DentRes 1970;49:131 – 136.

9. Ikejima I, Nomoto R, McCabe JF. Shear punch strength andflexural strength of model composites with varying filler volumefraction, particle size and silanation. Dent Mater 2003;19:206 – 211.

10. McCabe JF, Wassell RW. Hardness of model dental compos-ites: the effect of filler volume fraction and silanation. J MaterSci Mater Med 1999;10:291 – 294.

11. Nomoto R, Carrick TE, McCabe JF. Suitability of a shearpunch test for dental restorative materials. Dent Mater2001;17:415 – 421.

12. Mount GJ, Makinson OF, Peters MC. The strength of auto-cured and light-cured materials: the shear punch test. Aust Dent

J 1996;41:118 – 223.

13. Bagheri R, Burrow MF, Tyas MJ. Comparison of the effect of storage media on hardness and shear punch strength of tooth-colored restorative materials. Am J Dent 2007;20:329 – 334.

14. Say EC, Civelek A, Nobecourt A, Ersoy M, Guleryuz C. Wear

and microhardness of different resin composite materials. OperDent 2003;28:628 – 634.

15. Kovarik RE, Muncy MV. Fracture toughness of resin-modifiedglass ionomers. Am J Dent 1995;8:145 – 148.

16. Ferracane JL, Berge HX, Condon JR. In vitro aging of dentalcomposites in water. Effect of degree of conversion, filler vol-ume, and filler/matrix coupling. J Biomed Mater Res1998;42:465 – 472.

17. Ravindranath V, Gosz M, De Santiago E, Drummond JL,Mostovoy S. Effect of cyclic loading and environmental agingon the fracture toughness of dental resin composite. J BiomedMater Res Part B Appl Biomater 2007;80:226 – 235.

18. Momoi Y, McCabe JF. Hygroscopic expansion of resin basedcomposites during 6 months of water storage. Br Dent J1994;176:91 – 96.

19. Sarrett DC, Ray S. The effect of water on polymer matrix andcomposite wear. Dent Mater 1994;10:5 – 10.

20. Beatty MW, Swartz ML, Moore BK, et al . Effect of microfillerfraction and silane treatment on resin composite properties. JBiomed Mater Res 1998;40:12 – 23.

21. Kim KH, Ong JL, Okuno O. The effect of filler loading andmorphology on the mechanical properties of contemporarycomposites. J Prosthet Dent 2002;87:642 – 649.

22. Mante F, Saleh N, Mante M. Softening patterns of post-cureheat-treated dental composites. Dent Mater 1993;9:325 – 331.

23. Ortengren U, Andersson F, Elgh U, Terselius B, Karlsson S.Influence of pH and storage time on the sorption and solubilitybehavior of three composite resin materials. J Dent2001;29:35 – 41.

24. Bagheri R, Azar MR, Burrow MF, Tyas MJ. The effect of agingon the fracture toughness of aesthetic restorative materials. Am J Dent 2010;23:142 – 146.

25. Lohbauer U, Kramer N, Siedschlag G, et al . Strength and wearresistance of a dental glass-ionomer cement with a novel nano-filled resin coating. Am J Dent 2011;24:124 – 128.

26. Bonifacio CC, Werner A, Kleverlaan CJ. Coating glass-ionomercements with a nanofilled resin. Acta Odontol Scand2012;70:471 – 477.

27. Friedl K, Hiller K-A, Friedl K-H. Clinical performance of a newglass ionomer based restoration system: a retrospective study.Dent Mater 2011;27:1031 – 1037.

Address for correspondence:

Rafat BagheriGhasrodasht, Ghomabad Street

Department of Dental MaterialsFaculty of Dentistry

Shiraz University of Medical SciencesShiraz

IranEmail: [email protected]

© 2013 Australian Dental Association 453

Mechanical properties of restorative materials

australian dental journal article

Documents