yp

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ndthe die stones. Specimens coated with die hardener had

lower hardness values (P\.001 in all cases)

Conclusions. The 3 types of die stones evaluated in this study did not differ significantly in surfacemicrohardness. Under these conditions, die hardener coatings reduced the surface hardness of the gypsummaterial. (J Prosthet Dent 2004;92:35-8.)

CLINICAL IMPLICATIONS

The die hardeners evaluated did not increase the surface hardness of the gypsum materials butmay have played a role in preventing brittle fracture of the die margin.

Dental stones occupy an important role in thefabrication of indirect dental prostheses. Casts poured indental stones should be accurate in every respect,dimensionally stable over time, hard enough to with-stand the fabrication process, resistant to the inadvertentabrasions caused by fabrication, and have a surfacewettability compatible with the waxing process.Laboratory technicians and dentists depend on thesecharacteristics to predictably fabricate accurate, preciseprostheses. ANSI/ADA Specification No. 251 providesguidelines governing characteristics that group dentalstones into 4 classes of quality based on use and a rangeof physical properties. Types I-IV are defined, re-

spectively, as impression plaster, plaster, dental stone,and high-strength dental stone.2 Each type is equal in itschemical composition, but differences exist in theprocessing of each type to affect setting expansion,compressive strength, and reproduction of detail.

Dies used to fabricate dental prostheses are often castin Type IV or high-expansion Type V gypsum materialsto produce a hard, accurate surface on which to make thewax pattern for the prosthesis. These materials are oftenthought to differ significantly in their hardness and othercharacteristics. To increase surface hardness, surfacecoatings or various treatments have been recommendedto improve the stones hardness or abrasion resistance.3

Contemporary methods for hardening dies havechanged over the years, from soaking or boiling dies indifferent materials4-7 to coating them with variousagents, coupled with different methods for thinningsuch agents. These methods include air thinning, brushthinning, or shaking.8 For the most part, these coatingshave not been shown to alter the dimensions of the

aPrivate Practice, Logan, Utah.bOral Surgery Resident.cAssociate Professor, Prosthodontic Department.dCentennial Fund Professor, Director of Clinical Research, Dental

Clinical Research Center (Dows Institute for Dental Research) andDepartment of Prosthodontics, University of Iowa.Alterations of surface hardness with g

Paul E. Harris, DDS,a Scott Hoyer, DDS,b TerClark M. Stanford, DDS, PhDd

University of Iowa, College of Dentistry, Iowa

Statement of problem. Die stones require abrassurface wettability material properties.

Purpose. The purpose of this study was to compargypsum materials with and without surface die harde

Material and methods. Materials used were a TypeRock, and ResinRock). Die hardener was cyanoacrylwere hand mixed with distilled room temperature wdirections. Five cylinders (15 3 15 mm) per grouppositioned on top of a glass slide. The face of each spface of each of 5 specimens/material was coated wiClear Coat, air thinned and dried; and 5 specimens/mmm2) readings were made on each face (5 readings/tprocedure with post hoc Tukey tests were performed

Results. Microhardness did not vary between 3 asignificantly lower surface hardness (P\.0001) thanJULY 2004sum die hardeners

J. Lindquist, DDS, MS,c and

ity, Iowa

resistance, dimensional stability with time, and high

the surface microhardness (Knoop) of 4 contemporaryr.

II stone (Microstone) and 3 die stones (Die-Keen, Silky-(Permabond 910) or Clear Coat. Specimens of stone

er and vacuum spatulated according to manufacturersere poured, using vibration, into phenolic ring moldsmen was polished with 2400-grit Al2O sandpaper. Onecyanoacrylate; 5 specimens/material were coated withterial had no treatment (control). Knoop hardness (kg/e point) 3, 12, and 24 hours after pouring. An ANOVAa=.05).

24 hours for any material (P[.05). Microstone hadTHE JOURNAL OF PROSTHETIC DENTISTRY 35

THE JOURNAL OF PROSTHETIC DENTISTRY HARRIS ET ALfinished die appreciably.8 Some materials, such ascyanoacrylate, die sealants, and resins, have been foundto increase surface hardness.5,6,8 Each of these studiesused similar testing methods. The results showedincreased strength was dependent on specimen thick-ness5,6 and film thickness of the applied surfacehardener.8 Polystyrene applied to the surface and surfacehardeners mixed with the stone have not been found toimprove hardness.9,10 Epoxy resin has been shown tosoften gypsum surfaces.9

It is commonly thought that these coatings ortreatments are important to reduce surface abrasionand surface fracture, especially at critical margin areas ofthe dies. In a recent study, the impact of surface coatingmaterials on improving surface abrasion and preventingwater sorption was shown.11 The purpose of this studywas to determine the effect on surface microhardness of4 contemporary stone materials with and without diecoating (cyanoacrylate or a ketone-based resin). The useof surface coatings or hardeners may provide resistanceto surface abrasion and, therefore, provide a more sealedgypsum surface for use in the dental laboratory setting.

MATERIAL AND METHODS

Four stone materials were included in this study:a high-expansion Type V die stone material (Die-Keen;Heraeus Kulzer, Armonk, NY), a Type III stone material(Microstone; Whip Mix Corp, Louisville, Ky), and 2Type IV die stone materials (Silky-Rock and ResinRock;Whip Mix Corp). The stone used was provided inprepackaged envelopes (Die-Keen, 120 g/25 mL water,Microstone, 140 g/40 mL water, Silky-Rock, 140 g/32mL water, and ResinRock, 70 g/14 mL water). Volumeand weight were verified to ensure that stones weremixed according to manufacturers directions. Volumeof stone for a given weight may differ depending ontemperature, settling of the powder in its container, andhumidity of ambient air.2,13

Two die coatings or hardeners were evaluated:a cyanoacrylate (Permabond 910; PermabondInternational, Bridgewater, NJ) and a blend of ketone-based resins (Clear Coat; American Dental Supply,Easton, Pa). Both agents utilize an acetone solvent thatevaporates when left exposed to air; new bottles of eachsurface hardener were therefore used to minimize theeffect of viscosity due to desiccation.

The stone was hand mixed with distilled, roomtemperature water until each material was wetted, thenvacuum spatulated in a 500-mL bowl at 400 rpm for thespecified amount of time, according to manufacturersrecommendations. Mixed specimens were poured im-mediately under vibration into 15 3 15-mm phenoliccylinders on top of a glass specimen slide and set aside toharden (258C, ambient humidity). Fifteen cylinders

were made for each gypsum material group. Five

36gypsum specimens were evaluated with no die hardener(control), 5 were coated with Permabond 910, and 5were coated with Clear Coat.

Specimens were allowed to polymerize for 45minutes before being removed from the phenoliccylinder forms. The flat faces were then dry polishedwith 2400-grit aluminum oxide sandpaper. Althoughthis step removed the surface layer of stone, it facilitatedthe subsequent measurement of the Knoop diagonal(microhardness measurements necessitate a smooth,highly polished surface for accurate stylus indentationand visual measurement). After polishing, 5 specimensof each stone were coated with Permabond 910, and 5were coated with Clear Coat, which was air thinned andallowed to dry.

Trials for each stone were conducted by measuringKnoop microhardness (Micromet II; Buehler, Ltd, LakeBluff, Ill, expressed as kg/mm2 or HK) of each specimenface at 3, 12, and 24 hours after initial pour. Hardnesswas determined by loading each specimen face 5 times,in areas 600 to 1400 mm apart, for 15 seconds witha force of 25, 50, or 100 g. The determinant of theamount of force used was defined by the length of thelong diagonal of the Knoop indenter. Since the largestwidth of the indentation is 250 mm, the load wascustomized to provide indentations of 60 to 250 mm(the limits of measurement). Knoop microhardness wascalculated with the following formula:

HK 142303Fd2

where HK is Knoop Hardness, F is the force used to loadthe instrument in grams, and d is the length of theimprinted diagonal in micrometers.

Five indentations were recorded for each face, withthe same operator making all measurements. Eachspecimen group was analyzed by group measurementsmade for each stone, each die coating, and each time ofmeasurement. One-way ANOVA, 2-way ANOVA, and3-way ANOVA and post hoc Tukey tests constituted thestatistical analysis. In addition, individual stones wereanalyzed for the difference between die coatings andbare stone by the Bonferroni (Dunn) t test. Significancewas assumed at a Type I error rate of .05 or a=.05.

RESULTS

Data are presented in Figures 1 through 4. ANOVAstatistical tests were employed comparing the KnoopHardness associated with each of the following variables:product (type of stone specimen used), coating(with without die coating), and time (amount of timeelapsed from initial pour3, 12, or 24 hours).

No significant interaction was observed between the

length of time elapsed from the initial pour and the type

VOLUME 92 NUMBER 1

THE JOURNAL OF PROSTHETIC DENTISTRYHARRIS ET ALof die coating (P=.6988, F=0.55, df=4). The 3-wayANOVA between product, time, and coating did notshow significant interaction (P=.0992, F=1.59, df=12).

At 3 hours, microhardness did not vary betweenstones. Two-way ANOVA showed that the type of stonedid not have an effect on hardness (P=.0765, F=2.43,Dfn=3, Dfd=48), while die coating did have a significanteffect on hardness (P\.0001, F=32.56, Dfn=3,Dfd=48). At 3 hours, there was no significant differencebetween the hardness of any of the stones given each typeof coating, while between coating groups, there weresignificant differences in hardness. Figures 1 through 4show that die-coated specimens had significantly lowerKnoop Hardness scores than their noncoated coun-terparts. All of the noncoated stone specimens werestatistically similar in hardness at 3 hours, as were all ofthe specimens coated with Permabond and all of thespecimens coated with Clear Coat.

At 12 hours, there was a significant interactionbetween the type of stone and type of coating used(P=.01, F=3.2, Dfn=6, Dfd=48). The type of stone hadan effect on hardness (P=.0067, F=4.58, Dfn=3,

Dfd=48), and die coating also had a significant effecton hardness (P=.0002, F=10.46, Dfn=2, Dfd=48). By12 hours, uncoated gypsum materials differed signifi-cantly from each other in hardness. The same is true forstones coated with either Permabond or Clear Coat.

At 24 hours after initial pour, the same trend wasobserved as at 12 hours. There was a significant in-teraction between type of stone and coating used(P=.0241, F=2.71, Dfn=6, Dfd=48). The type of stonesignificantly influenced hardness (P =.0002, F=7.84,Dfn=3, Dfd=48), and the type of coating significantlyinfluenced hardness (P=.0014, F=7.6, Dfn=2, Dfd=48).

Between 3 and 12 hours, the initial polymerization ofthe stones resulted in significant differences in hardness.Stones coated with a die coating produced consistentlylower Knoop hardness values at each time periodthrough 24 hours, and for every type of stone exceptfor ResinRock.

A comparison of the stones coated with eitherPermabond or Clear Coat showed stone-specific dif-ferences in surface hardness. For Silky-Rock, Die-Keen,

Fig. 3. Microhardness of Silky-Rock die materials. Gypsumcoated with Permabond Cyanoacrylate resin or Clear Coat.Specimens (n=5/group) evaluated at 3, 12, and 24 hours.

Fig. 4. Microhardness of Microstone Type III Gypsummaterials. Gypsum coated with Permabond Cyanoacrylateresin or Clear Coat. Specimens (n=5/group) evaluated at 3,12, and 24 hours. Both coating materials reduced surfacemicrohardness.Fig. 1. Microhardness of resin-reinforced ResinRock diematerial. Gypsum coated with Permabond Cyanoacrylateresin or Clear Coat. Specimens (n=5/group) evaluated at 3,12, and 24 hours.JULY 2004Fig. 2. Microhardness of Die-Keen die materials. Gypsumcoated with Permabond Cyanoacrylate resin or Clear Coat.Specimens (n=5/group) evaluated at 3, 12, and 24 hours.37

and Microstone, specimens with no die coating were CONCLUSION

THE JOURNAL OF PROSTHETIC DENTISTRY HARRIS ET ALsignificantly harder than specimens with either diecoating (P=.0013, F=7.84, df=2), (P\.0001,F=21.77, df=2) and (P=.0052, F=5.96, df=2) respec-tively. For ResinRock, the die coating did not makea significant difference in hardness (P=.5447, F=0.62,df=2).

DISCUSSION

Knoop hardness has been used to evaluate very hard,brittle materials with a low modulus of elasticityincluding enamel, amalgam, gypsum, and porcelain.14

Other hardness scales, such as the Brinnell and Vickers,are more commonly used in evaluating viscoelasticmaterials with a high modulus of elasticity, such as metalalloys.

In a previous study, low variances were associatedwith measures of surface microhardness.15 In thepresent study, within the same specimen face, Knoophardness differed across the surface of the specimen.This may be attributed to subsurface porosities in onesite versus another, or to specific sites in the crystallinematrix that are weaker than others and tend to breakdown more quickly, resulting in a lower hardness value.For example, the nucleus of crystallization may beharder than the outer fringes where one crystal meetsanother. Other limitations of the present study were thesmall sample size and the necessity of polishing thesurface layer to facilitate the measurement of the Knoopdiagonal.

Previous studies have differed as to the effect of diecoatings on the surface hardness of dental stones. Afterthe treatment or application of surface hardeners, somematerials have shown an increase in hardness,5,6,8 whileothers have demonstrated no effect,9,10 and still othershave shown a reduction in hardness.9 This may be due,in part, to differences in measurement technique sincehardness is an operationally defined physical property ofmaterials. The impact of surface coatings is potentiallymore important as a means to reduce surface abrasionand surface water absorption of die material, as shown byLindquist et al.11 Abrasion resistance can be improvedby the impregnation of a supportive resin that acts tobind the gypsum matrix, filling subsurface voids andsealing the gypsum surface. Impact fracture and loss ofsurface material is thereby reduced by having reinforce-ment, both at the surface and within the material, due tosurface penetration of the resin. The resin film thickness,measured by conventional microhardness (Knoopmeasurements) as in the present study, may thereforebe lower due to the nature of the surface hardnessmeasurement of the coating film itself, and not the film/gypsum matrix.38Based on the results of this study, the followingconclusions are made:

The use of air-thinned cyanoacrylate (Permabond910) and Clear Coat die coatings on Silky-Rock, Die-Keen, and Microstone decreased the surface hardness(P=.0013). At 24 hours, Microstone, a Type III dentalstone, did not differ in hardness from the other Type IVdental stones evaluated (P=.0002).

REFERENCES

1. ADA Council on Scientific Affairs. ANSI/ADA Specification No.25 (2000),

an adoption of ISO Standard 6873: 1998 for Dental Gypsum Pro-

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denmat.asp. Accessed March 28, 2004.

2. Craig R, Craig RG, Powers JM. Restorative dental materials. 11th ed.

St. Louis: Elsevier; 2001. p. 373, 392-404.

3. Rosenstiel SF, Land MF, Fijimoto J. Contemporary fixed prosthodontics.

3rd ed. St. Louis: Elsevier; 2000. p. 433.

4. Skinner EW, Gordon CC. Some experiments on the surface hardness of

dental stones. J Prosthet Dent 1956;6:94-100.

5. Sanad ME, Combe EC, Grant AA. The effect of model sealant solutions on

the properties of gypsum. J Dent 1980;8:152-7.

6. Sanad ME, Combe EC, Grant AA. Hardening of model and die materials

by an epoxy resin. J Dent 1980;8:158-62.

7. Peyton FA, Liebold JP, Ridgley GV. Surface hardness, compressive

strength, and abrasion resistance of indirect die stones. J Prosthet Dent

1952;2:381-9.

8. Fukui H, Lacy AM, Jendresen MD. Effectiveness of hardening films on die

stone. J Prosthet Dent 1980;44:57-63.

9. Bajada SB, Makinson OF. The effect of some surface treatments to dental

modelling stones. Aust Dent J 1974;19:118-21.

10. Fan PL, Powers JM, Reid BC. Surface mechanical properties of stone,

resin, and metal dies. J Am Dent Assoc 1981;103:408-11.

11. Lindquist TJ, Stanford CM, Knox E. Influence of surface hardener on

gypsum abrasion resistance and water sorption. J Prosthet Dent 2003;90:

441-6.

12. Kaiser DA, Nicholls JI. A study of distortion and surface hardness of

improved artificial stone casts. J Prosthet Dent 1976;36:373-81.

13. Mahler DB. Hardness and flow properties of gypsum materials. J Prosthet

Dent 1951;1:188-95.

14. Naylor WP, Munoz CA, Goodacre CJ, Swartz ML, Moore BK. The effect of

surface treatment on the Knoop hardness of Dicor. Int J Prosthodont 1991;

4:147-51.

15. Duke P, Moore BK, Haug SP, Andres CJ. Study of the physical properties of

type IV gypsum, resin-containing, and epoxy die materials. J Prosthet Dent

2000;83:466-73.

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Copyright 2004 by The Editorial Council of The Journal of ProstheticDentistry

doi:10.1016/j.prosdent.2004.04.002VOLUME 92 NUMBER 1

Alterations of surface hardness with gypsum die hardenersMaterial and methodsResultsDiscussionConclusionREFERENCES