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The abrasion resistance of dental stone
Surface Technology, 22 (1984) 377 - 380
377
THE ABRASION RESISTANCE OF DENTAL STONE J. A. yon F R A U N H O F E R and R. R. SPIERS
Department of Restorative Dentistry, School of Dentistry, University of Louisville, Louisville, K Y 40292 (U.S.A.) (Received January 27, 1984)
Summary The resistance to abrasion by a hardened steel sphere of three high strength dental stones was studied. At 1 h, there was no significant difference in the abrasion resistance of the materials. At 24 h, however, a dental stone of improved mechanical properties was f o u n d to have a significantly greater resistance to abrasion than the other dental stones.
I. Introduction
In dental technology, impressions are taken of the hard and soft tissues (teeth and gums) of the m o u t h using a suitable impression material. A cast or model of the teeth and gums is then prepared by pouring a slurry of dental stone into the impression. Prosthetic teeth, crowns and bridgework are then prepared by molding wax to edentulous areas (those w i t h o u t teeth) and trimming-carving the wax to the appropriate shape and dimensions. Clearly, the abrasion characteristics of the dental stone substrate are important factors in the stability of the model for any dental laboratory procedure. The abrasion resistance of dental stones is an important characteristic, in order that loss of detail does n o t occur during wax carving and trimming. Over the years, there have been progressive improvements in the mechanical properties o f dental stones, notably compressive strength and setting time as well as setting expansion, but there have been no standardized evaluations of the resistance to abrasion. Abrasion resistance is a difficult property to assess, and there have been m a n y different attempts to determine it, including surface hardness studies, abrasion with abrasive paper and resistance to blade scraping [1 - 5]. None of these approaches has proved wholly satisfactory to date, notably because it has been difficult to quantify the results of the studies. In this paper quantitative abrasion studies with a novel technique are reported, and significant improvement in the abrasion resistance of a recently introduced dental stone is demonstrated. 0376-4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
378 2. M eth o d Three high strength dental stones (Table 1) were prepared in accordance with the manufacturer's specified water-to-powder ratios and were set in a steel mo ld to p r o d u c e 65 m m × 25 m m × 2 m m specimens (a minim u m n u m b e r o f 30 of each material). 20 specimens were tested after 1 h setting time and the remainder after 24 h.
TABLE 1 Abrasion resistance of dental stone Material a
Water-to-powder ratio
Abrasion resistance (Xl011 erg cm -3) for the following setting times
(ml (100 g)-') Quickstone
28.6
Silky Rock Prima Rock
22.2 18.2
1h
24h
1.08 + 0.33 (30.6%)b 1.19 -+0.27 (22.7%)b 1.02 + 0 . 2 9 (28.4%)b
1.55 + 0.41 (26.5%).b 1.80 + 0.43 (23.9%)~ 2.50 +- 0.48 (19.2%)b
Mean values together with standard deviations. awhip-Mix Corporation, Louisville, KY. b Coefficient of variation.
A commercially available abrasion tester (RE L abrasion tester, Research E q u i p m e n t ( L o n d o n ) Ltd.) was used for this study. The device consists of a reciprocating arm, m o u n t e d on the end of which is a pivoted pl at form , with a hardened spherical abrader (6.35 mm in diameter) m o u n t e d in a post welded to the platform. The upper surface o f the platform a c c o m m o d a t e s weights f o r th e c o n t r o l o f the abrasive force. The arm stroke is 50.5 mm, the rate being 48 strokes min -1 and a mechanical c o u n t e r records the n u m b e r of arm movements, each stroke giving t w o abrades (one each on the forward and return arm movements). The weight o f t h e platform assembly is 126 gf, and an overweight of 300 gf was used f or this study. Each specimen was subjected to 200 abrasion cycles (400 abrades). The abrasion resistance, expressed as the energy required to remove a unit volume of material, was determined by measuring the width and length o f the abraded area with a traveling microscope (Griffin linear Vernier microscope, Griffin and George Ltd., L o n d o n ) .
3. T h e o r y o f abrasion testing The volume o f material lost through abrasion by a spherical object of radius r is a segment o f a circle with width w subtended by an angle 0. The
379
area of the abraded segment is given by the area of the entire segment less that of the subtended triangle:
A-
7rr20
wr cos(0/2)
360
2
The width w of the abrade is given by w = 2r sin(0/2), so that 0 = For an abrade of length L, the volume lost for an abrader of radius r is given b y
2 sin-l(w/2r).
AV=AL =L[ zrr2(2sin-l(w/2r)~--360wrc°s~si21(w/2r)~]-
am3
The energy input for volume loss AV is 2mgNL for an abrade of length L, a mass m (g) on the abrading sphere and N arm movements (each corresponding to t w o abrades). Consequently, the energy E for loss of unit volu m e of material is given by E -
2mgNL AV
erg cm -z
The above expression does not include factors of frictional forces. Frictional forces are not involved since Coulomb's law of friction establishes that, with all ordinary surfaces, sliding friction is independent of the area of contact (within wide limits) and independent of speed (within limits). It follows that sliding friction is negligible until very deep abrades are produced, i.e. a depth of abrade approaching r/2. For materials of widely different surface characteristics, sliding friction will obviously vary, b u t the difference will be negligible for materials of similar composition and surface state. It is possible to report abrasion resistance as energy for unit volume loss, and as (volume loss)/(unit length), when the mass m, the abrade length L and the number N of arm movements are constant. For convenience, the former system is used, although (volume loss)/(unit abrade length) may have advantages under certain circumstances.
4. Results
The abrasion resistance of the three high strength dental stones at 1 and 24 h after mixing are given in Table 1. Statistical analysis (Anova and the T u k e y - K r a m e r honestly significant difference test [6]) showed that there was no significant difference (p > 0.05) in the abrasion resistance of Prima Rock, Silky R o c k and Quickstone at 1 h. At 24 h, the abrasion resistance of Prima R o c k was significantly greater (p < 0.01) than that of Quickstone and Silky Rock, b u t there was no significant difference (p > 0.05) between the abrasion resistance of Quickstone and Silky Rock.
380
5. Discussion
The abrasion test reported here uses a commercially available abrader that is marketed as a qualitative device for comparative evaluation of paint coatings. Preliminary studies using this device with commercial Plexiglas (poly(methyl methacrylate)) resin gave an abrasion resistance value of (1.46 -+ 0.29) × 1013 erg cm -3, with a coefficient of variation (19.9%) comparable with those found for high strength dental stone (Table 1). Attempts to correlate the calculated loss of material with the actual loss were not successful because of the small a m o u n t of material removed from the relatively large mass of test specimen necessary to ensure stability of the abrading surface relative to the moving arm. However, the comparability of the coefficients of variation suggests that the abrasion test used in this study is highly applicable to testing a variety of materials. The data obtained from the high strength dental stones indicate that all three showed a significant increase in abrasion resistance over 24 h, the increase being greatest for the new material, Prima Rock. In addition to the improved mechanical properties, Prima Rock [7] has an abrasion resistance significantly greater than those for compounds previously introduced. The findings indicate that this greater abrasion resistance is attained only after a period of several hours, and therefore to derive maximum benefit it would be advisable to delay any wax pattern carving or other operations on the cast for 24 h.
References 1 J. F. M c C a b e a n d B. H. S m i t h , A m e t h o d for m e a s u r i n g t h e wear o f r e s t o r a t i v e m a t e r i a l s in vitro, Br. Dent. J., 151 ( 1 9 8 1 ) 123 - 126. 2 G. S. Wilson, E. H. Davies a n d J. A. y o n F r a u n h o f e r , Abrasive wear c h a r a c t e r i s t i c s o f a n t e r i o r r e s t o r a t i v e materials, Br. Dent. J., 151 ( 1 9 8 1 ) 335 - 338. 3 K. D. J ~ r g e n s e n , R e s t o r a t i v e resins: a b r a s i o n vs. m e c h a n i c a l p r o p e r t i e s , Scand. J. Dent. Res., 88 ( 1 9 8 0 ) 557 - 568. 4 J. C. T h o m s o n , A t t r i t i o n o f acrylic t e e t h , Dent. Pract., 15 ( 1 9 6 5 ) 233 - 236. 5 W. B. Eames, C. E. E d w a r d s , Jr., a n d W. C. Buck, Scraping resistance o f d e n t a l die m a t e r i a l s : a c o m p a r i s o n o f b r a n d s , Oper. Dent., 3 ( 1 9 7 8 ) 68 - 72. 6 M. R. S t o l i n e , T h e s t a t u s o f m u l t i p l e c o m p a r i s o n s : s i m u l t a n e o u s e s t i m a t i o n s of all pairwise c o m p a r i s o n s in o n e - w a y A n o v a designs, A m . Star., 35 ( 1 9 8 1 ) 134 - 140. 7 J. A. y o n F r a u n h o f e r a n d R. R. Spiers, S t r e n g t h t e s t i n g o f d e n t a l s t o n e : a c o m p a r i s o n o f compressive, tensile, a n d shear s t r e n g t h tests, J. Biomed. Mater. Res., 17 ( 1 9 8 3 ) 293 - 299.
377
THE ABRASION RESISTANCE OF DENTAL STONE J. A. yon F R A U N H O F E R and R. R. SPIERS
Department of Restorative Dentistry, School of Dentistry, University of Louisville, Louisville, K Y 40292 (U.S.A.) (Received January 27, 1984)
Summary The resistance to abrasion by a hardened steel sphere of three high strength dental stones was studied. At 1 h, there was no significant difference in the abrasion resistance of the materials. At 24 h, however, a dental stone of improved mechanical properties was f o u n d to have a significantly greater resistance to abrasion than the other dental stones.
I. Introduction
In dental technology, impressions are taken of the hard and soft tissues (teeth and gums) of the m o u t h using a suitable impression material. A cast or model of the teeth and gums is then prepared by pouring a slurry of dental stone into the impression. Prosthetic teeth, crowns and bridgework are then prepared by molding wax to edentulous areas (those w i t h o u t teeth) and trimming-carving the wax to the appropriate shape and dimensions. Clearly, the abrasion characteristics of the dental stone substrate are important factors in the stability of the model for any dental laboratory procedure. The abrasion resistance of dental stones is an important characteristic, in order that loss of detail does n o t occur during wax carving and trimming. Over the years, there have been progressive improvements in the mechanical properties o f dental stones, notably compressive strength and setting time as well as setting expansion, but there have been no standardized evaluations of the resistance to abrasion. Abrasion resistance is a difficult property to assess, and there have been m a n y different attempts to determine it, including surface hardness studies, abrasion with abrasive paper and resistance to blade scraping [1 - 5]. None of these approaches has proved wholly satisfactory to date, notably because it has been difficult to quantify the results of the studies. In this paper quantitative abrasion studies with a novel technique are reported, and significant improvement in the abrasion resistance of a recently introduced dental stone is demonstrated. 0376-4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
378 2. M eth o d Three high strength dental stones (Table 1) were prepared in accordance with the manufacturer's specified water-to-powder ratios and were set in a steel mo ld to p r o d u c e 65 m m × 25 m m × 2 m m specimens (a minim u m n u m b e r o f 30 of each material). 20 specimens were tested after 1 h setting time and the remainder after 24 h.
TABLE 1 Abrasion resistance of dental stone Material a
Water-to-powder ratio
Abrasion resistance (Xl011 erg cm -3) for the following setting times
(ml (100 g)-') Quickstone
28.6
Silky Rock Prima Rock
22.2 18.2
1h
24h
1.08 + 0.33 (30.6%)b 1.19 -+0.27 (22.7%)b 1.02 + 0 . 2 9 (28.4%)b
1.55 + 0.41 (26.5%).b 1.80 + 0.43 (23.9%)~ 2.50 +- 0.48 (19.2%)b
Mean values together with standard deviations. awhip-Mix Corporation, Louisville, KY. b Coefficient of variation.
A commercially available abrasion tester (RE L abrasion tester, Research E q u i p m e n t ( L o n d o n ) Ltd.) was used for this study. The device consists of a reciprocating arm, m o u n t e d on the end of which is a pivoted pl at form , with a hardened spherical abrader (6.35 mm in diameter) m o u n t e d in a post welded to the platform. The upper surface o f the platform a c c o m m o d a t e s weights f o r th e c o n t r o l o f the abrasive force. The arm stroke is 50.5 mm, the rate being 48 strokes min -1 and a mechanical c o u n t e r records the n u m b e r of arm movements, each stroke giving t w o abrades (one each on the forward and return arm movements). The weight o f t h e platform assembly is 126 gf, and an overweight of 300 gf was used f or this study. Each specimen was subjected to 200 abrasion cycles (400 abrades). The abrasion resistance, expressed as the energy required to remove a unit volume of material, was determined by measuring the width and length o f the abraded area with a traveling microscope (Griffin linear Vernier microscope, Griffin and George Ltd., L o n d o n ) .
3. T h e o r y o f abrasion testing The volume o f material lost through abrasion by a spherical object of radius r is a segment o f a circle with width w subtended by an angle 0. The
379
area of the abraded segment is given by the area of the entire segment less that of the subtended triangle:
A-
7rr20
wr cos(0/2)
360
2
The width w of the abrade is given by w = 2r sin(0/2), so that 0 = For an abrade of length L, the volume lost for an abrader of radius r is given b y
2 sin-l(w/2r).
AV=AL =L[ zrr2(2sin-l(w/2r)~--360wrc°s~si21(w/2r)~]-
am3
The energy input for volume loss AV is 2mgNL for an abrade of length L, a mass m (g) on the abrading sphere and N arm movements (each corresponding to t w o abrades). Consequently, the energy E for loss of unit volu m e of material is given by E -
2mgNL AV
erg cm -z
The above expression does not include factors of frictional forces. Frictional forces are not involved since Coulomb's law of friction establishes that, with all ordinary surfaces, sliding friction is independent of the area of contact (within wide limits) and independent of speed (within limits). It follows that sliding friction is negligible until very deep abrades are produced, i.e. a depth of abrade approaching r/2. For materials of widely different surface characteristics, sliding friction will obviously vary, b u t the difference will be negligible for materials of similar composition and surface state. It is possible to report abrasion resistance as energy for unit volume loss, and as (volume loss)/(unit length), when the mass m, the abrade length L and the number N of arm movements are constant. For convenience, the former system is used, although (volume loss)/(unit abrade length) may have advantages under certain circumstances.
4. Results
The abrasion resistance of the three high strength dental stones at 1 and 24 h after mixing are given in Table 1. Statistical analysis (Anova and the T u k e y - K r a m e r honestly significant difference test [6]) showed that there was no significant difference (p > 0.05) in the abrasion resistance of Prima Rock, Silky R o c k and Quickstone at 1 h. At 24 h, the abrasion resistance of Prima R o c k was significantly greater (p < 0.01) than that of Quickstone and Silky Rock, b u t there was no significant difference (p > 0.05) between the abrasion resistance of Quickstone and Silky Rock.
380
5. Discussion
The abrasion test reported here uses a commercially available abrader that is marketed as a qualitative device for comparative evaluation of paint coatings. Preliminary studies using this device with commercial Plexiglas (poly(methyl methacrylate)) resin gave an abrasion resistance value of (1.46 -+ 0.29) × 1013 erg cm -3, with a coefficient of variation (19.9%) comparable with those found for high strength dental stone (Table 1). Attempts to correlate the calculated loss of material with the actual loss were not successful because of the small a m o u n t of material removed from the relatively large mass of test specimen necessary to ensure stability of the abrading surface relative to the moving arm. However, the comparability of the coefficients of variation suggests that the abrasion test used in this study is highly applicable to testing a variety of materials. The data obtained from the high strength dental stones indicate that all three showed a significant increase in abrasion resistance over 24 h, the increase being greatest for the new material, Prima Rock. In addition to the improved mechanical properties, Prima Rock [7] has an abrasion resistance significantly greater than those for compounds previously introduced. The findings indicate that this greater abrasion resistance is attained only after a period of several hours, and therefore to derive maximum benefit it would be advisable to delay any wax pattern carving or other operations on the cast for 24 h.
References 1 J. F. M c C a b e a n d B. H. S m i t h , A m e t h o d for m e a s u r i n g t h e wear o f r e s t o r a t i v e m a t e r i a l s in vitro, Br. Dent. J., 151 ( 1 9 8 1 ) 123 - 126. 2 G. S. Wilson, E. H. Davies a n d J. A. y o n F r a u n h o f e r , Abrasive wear c h a r a c t e r i s t i c s o f a n t e r i o r r e s t o r a t i v e materials, Br. Dent. J., 151 ( 1 9 8 1 ) 335 - 338. 3 K. D. J ~ r g e n s e n , R e s t o r a t i v e resins: a b r a s i o n vs. m e c h a n i c a l p r o p e r t i e s , Scand. J. Dent. Res., 88 ( 1 9 8 0 ) 557 - 568. 4 J. C. T h o m s o n , A t t r i t i o n o f acrylic t e e t h , Dent. Pract., 15 ( 1 9 6 5 ) 233 - 236. 5 W. B. Eames, C. E. E d w a r d s , Jr., a n d W. C. Buck, Scraping resistance o f d e n t a l die m a t e r i a l s : a c o m p a r i s o n o f b r a n d s , Oper. Dent., 3 ( 1 9 7 8 ) 68 - 72. 6 M. R. S t o l i n e , T h e s t a t u s o f m u l t i p l e c o m p a r i s o n s : s i m u l t a n e o u s e s t i m a t i o n s of all pairwise c o m p a r i s o n s in o n e - w a y A n o v a designs, A m . Star., 35 ( 1 9 8 1 ) 134 - 140. 7 J. A. y o n F r a u n h o f e r a n d R. R. Spiers, S t r e n g t h t e s t i n g o f d e n t a l s t o n e : a c o m p a r i s o n o f compressive, tensile, a n d shear s t r e n g t h tests, J. Biomed. Mater. Res., 17 ( 1 9 8 3 ) 293 - 299.