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Significance of filler content to properties of silicone impression materials
Significance of filler content to properties of silicone mpress on materials
W. J. Finger Department of Prosthetic Dentistry and Dental Materials Science, Dental School of the RWTH, Aachen, FRG
Finger WJ. Significance of filler content to properties of silicone impression materials. Dent Mater 1988: 4: 33-37. Abstract - The effect of filler content on handling characteristics, mechanical properties and accuracy of addition curing silicones was investigated on 7 sets of experimental materials with filler contents between 0 and 61.5% by weight. The consistency increased linearly with the amount of filler; working and setting times were consistent irrespective of the filler content. Exponential relationships were found between filler loading on the one hand and Shore A hardness and stiffness on the other. The linear free curing contraction was approximately 0.05% for all materials tested while the coefficients of thermal contraction linearly decreased with increasing filler content. Cooling of impressions of a standardized metal preparation from 37 ~ to 23~ had a tremendous impact on accuracy. Adequate die accuracy was obtained only when impression and model were produced at the same temperature.
The amount of inert filler added to elastomeric impression materials affects handling characteristics of the pastes, mechanical properties and dimensional stability of the cured elastomer. Addition curing silicones are accepted by the profession particularly due to the excellent dimensional stability of the impressions. The degree of filler content is a variable often used by manufacturers mainly to control the consistency of the pastes and to adjust elastic parameters of the cured material. The purpose of this study was to investigate systematically the effect of filler content on basic properties of modern a-silicones and particularly on the accuracy of dental impressions. Material and m e t h o d s
Seven sets of base and catalyst pastes of addition curing silicones were studied.* The blue base paste contained the cross-linking agent and the white paste both an inhibitor and the Pt-catalyst. The filler contents of the 7 pairs of pastes in weight % were 0, 8.0, 16.3, 30.4, 42.5, 52.9 and 61.5 with identical amounts of filler included in corresponding sets of base and catalyst pastes. The mixing ratio was 1:1 by weight and the mixing time used throughout the study was 30 s. Unless * The experimental materials were made available by Bayer Dental, FRG. 3 DentalMaterials4:1, 1988
otherwise specified the experimental work was performed in a standard climate of 23___1~ and 50+5% relative humidity. The following material parameters and characteristics were studied: 1. The consistency of the individual pastes was determined according to the testing procedure described in A D A specification No. 19 (1). All mixes were loaded by 0.5 kg to make the results comparable within the series. The disk diameters of the mixes are given as the average of three tests and recorded to the nearest millimeter. 2. The working and setting times of the pastes were determined at 23 ~ and at 37~ by an oscillating rheometer as described previously by Finger and Ohsawa (6). Immediately after mixing, the materials were transferred to the rheometer; the reciprocating rotation and recording of the track were activated one minute after start of mixing. The instrument used works opposite to Wilson's rheometer (12) which means that for the uncured paste a straight line is registered only. Working and setting times are arbitrarily defined as the times after start of mixing when the recorded deflections reach 5% and 95%, respectively, of the final width of the trace (6, 11). For each material and each testing temperature 3 tests were performed. 3. The Shore A hardness numbers were determined as previously described (5) and following the procedure for preparation of speci-
Key words: impression materials; addition curing silicones. Werner J. Finger, Department of Prosthetic Dentistry, Dental School of the RWTH, PauwelsstraSe, D-5100 Aachen, FRG.
Received February 23; accepted April 23, 1987.
mens (20 mm ~ , 7 mm height) in A D A No. 19. The specimens were removed from the 32~ water bath after 3.5 min dwell time and stored at 23~ until testing was performed 60 min after start of mixing. The Shore A figures were read after 3 s loading time (Zwick 3114). For each material 5 indentations were made on each of 5 specimens. 4. The relative stiffness of the materials was determined from compression testing of 5 cylindrical specimens each (12.5 mm in diameter and 20 mm in height) prepared under the same conditions as the specimens for Shore A hardness testing. The cylinders were loaded after 60 min from start of mixing in a universal testing machine (Zwick 1774) at a rate of 62.5 mm per min. The relative stiffness was calculated from the slope of the initial, approximately straight part of the stress-strain-curve recorded. 5. The curing contraction was measured by means of the mercury bath method as modified by J6rgensen (7, 10). The Hg-bath is mounted on the traveling table of an Olympus microscope. Immediately after mixing the impression material is floated on the mercury forming a tongue-shaped specimen which is retained over the sharp edge of the rectangular jaw's wall. A thin metal mark is placed on the free end of the specimen giving a well defined reference point, the movement of which is recorded. For the calculation of the percentage linear contraction of the
34
Finger
test material a fiducial reading is taken when the reference mark stops moving because of liquid flow of the impression material. The reading accuracy of the microscope was 2 ~tm. For each of the 7 experimental materials the contraction of 5 specimens was recorded after 24 h. 6. The thermal expansion and contraction coefficients of the 7 materials were measured by a Mikrokator (C. E. Johansso~a, Sweden, 904-4) under a measuring load of 1 g. Cylindrical specimens (6 mm ~3 and 12 mm h) were produced in a split mold at 37~ The cylinders were removed 15 min after start of mixing and stored for 24 h at 37~ in air before being cooled to room temperature. For testing, the specimens were placed on the table of the Mikrokator centrally under the ball tip of the gauge. The top surface was fitted with a microscope cover glass (0.15 g). The assembly was placed in a thermostate and the temperature was monitoted by a thermocouple with the tip approximately 1 mm from the specimen's cylinder surface. The fiducial reading was taken when within 15 min the variation in temperature was less than 0.5~ and in dial gauge reading less than 0.5 ~tm. Then the temperature in the thermostat was raised within 60 min to 45 ~ approximately and another temperature and Mikrokator reading were taken after a holding time of at least 15 min, before the assembly was allowed to cool to room temperature for a final reading. The contribution of the thermal expansion and contraction of the assembly was determined by measurements on cylindrical specimens of steel and quartz glass with known thermal coefficients of expansion and with the same dimensions as the elastomeric test specimens. The calculations of the thermal expansion and contraction coefficients were adjusted accordingly. For each material 5 specimens were measured. 7. The accuracy of impressions was determined as previously described by Finger and Ohsawa (3). Impressions were taken from a truncated chromium, steel cone (8 mm base diameter and height, convergence angle 10~) in circular effectively perforated impression trays made from 0.75 mm thick stainless steel with a height of 13 mm and an internal diameter of 16 mm. Cone and tray were mounted in a stand giving 4 mm distance between the prominence line of the cone and the tray and 2 mm distance between the top surface of the die and the tray. Impressions were taken 1 min after start of
mixing either at 23~ or at 37~ and removed after another 14 min. After 24 h storage at room temperature the impressions were poured with vacuum mixed stone (Geostone, batch No. 52116 X, Bayer Dental, FRG; W/P ratio 0.23), the linear setting expansion of
which was 0.1%, measured by a dial gauge method (8). The stone dies were removed after one hour and their accuracy was determined by measuring with a displacement transducer the occlusal discrepancy in 4 places 90~ apart between the die surface and a steel ring
DISK )IAMETER mm
mm
40
40
36
36
32
32
28
28
24
24
i lO
i 20
i 30
i 40
i 50
i 60
%
FILLER Fig. 1. Linear relationship between filler content and disk diameter (r2 = 0.99). OR
MIN
0
MIN
0
o
0
-
-
0
6
6
-,5
5 0
4
-3
3 IL
2
9
9
9
0"~
9
-2 li------Ii____
9 . . . . . .
1
;/ I
0
I
I
I
I
I
I
10
20
30
40
50
60
%
FILLER Fig. 2. Relation between filler content and working (dotted lines) and setting times (full lines) as determined by the oscillating rheometer (OR). The full and open circles represent mean values (x3) of determinations at 37~ and 23~ respectively.
Filler content o f silicone impressions fitting accurately on the master steel cone. F r o m the m e a n value of these 4 readings the diameter deviation from the master die was calculated. For each impression material and each of the 2 curing temperatures 5 dies were produced.
Results
A linear relationship was found between the filler content of the experimental elastomeric materials and their consistencies. The coefficient of decision for the regression line is 0.99 (Fig. 1). The working and setting times of the mixes are shown i n Fig. 2 as dotted and full lines, respectively. Full circles give the times when tested at 37~ and open circles w h e n determined at 23~ Each circle is the m e a n value of 3 measurements. The m a x i m u m standard deviation a r o u n d an individual m e a n value found was + 0.35 min, the m e a n standard deviation + 0.15 min. The lines are drawn after linear regression analyses. The relationship between filler content and Shore A hardnesses is described by a curve constructed after exponential regression analysis (Fig. 3). The r2-value is 0.999 and statistically highly significant. The m a x i m u m standard deviation found for an individual m e a n value (xs) was + 1 Shore A number. Fig. 4 illustrates the exponential regression line for the relation of filler content vs. stiffness (r 2 = 0.99). All individual standard deviations were smaller than + 0.4 MPa; the m e a n standard deviation was + 0.1 MPa. Fig. 5 illustrates the linear free curing contraction after 24 h. Each circle is the m e a n value of 5 measurements, while the vertical bars give the standard deviation. W h e n tested by analysis of variance the individual mean values are not significantly different (p > 0.5). The central horizontal line is the m e a n of the 7 m e a n values; the thin lines describe the m e a n standard deviation around this m e a n value. In Fig. 6 the linear regression lines display the relations between a m o u n t of filler and coefficients of thermal expansion (lower line) and thermal contraction. The r2-values are 0.97 and 0.99. The m e a n standard deviations for the coefficients of expansion and contraction are 8 and 11.10 -6 K -1. Fig. 7 shows the linear relationships between filler content and accuracy of 3*
impressions. The lower line (r ~ = 0.73) shows the A d-values when impression taking and die pouring were done at 23~ the upper line (r 2 = 0.99) when the impressions were taken at 37~ and the dies poured at room temperature. The m a x i m u m a n d m e a n standard deviations for the m e a n values displayed
35
as full circles are + 7 and __+ 5 ~tm, for the open circles _+ 10 and + 7, respectively. When tested by linear regression analysis a correlation (r 2 = 0.996) is found between the coefficients of thermal contraction and the accuracy of the impressions.
SA 70
70
O
60
60
5O
50
40
40
30
30
20
20
j/I 0
I 10
I 20
I 30
I 40
I 50
I 60
%
FILLER Fig. 3. Relationship between filler content and Shore A hardness (SA). The decision coefficient of the exponential regression line is 0.999. STIFFNESS MPal
MPa o
5
5
4
o
4
3
-3
2
-2
1
-
'~1 0
I
I
I
I
I
I
10
20
30
40
50
60
1
%
FILLER
Fig. 4. Relationship between filler content and relative stiffness. The decision coefficient of the exponential regression line is 0.99.
Finger
36
%
% -0.03
- 0.03
-0.04
- 0.04
-0.05
- 0.05
- 0.06
-0.06
- 0.07
-0.07
_,,,,,, II 0
I
I
I
I
I
I
10
20
30
40
50
60
%
FILLER.
Fig. 5. Relation between filler content and linear free curing contraction. The central horizontal line is the mean contraction calculated from the seven individual means. The thin horizontal lines describe the interval of the mean standard deviation.
910-6K-1
9 10-6K-,
300
3O0
260
260
220
220
180
180
140
140
S, /I
//0
I
I
I
I
I
I
10
20
30
40
50
60
%
FILLER
Fig. 6. Linear relationships between filler content and coefficient of thermal expansion (full circles) and coefficient of thermal contraction (open circles)9 The r
It is reasonable and expected that the consistencies of the impression pastes linearly increase and that the coefficients of thermal expansion and contraction linearly decrease with increasing filler content. The coefficients of thermal contraction are consistently higher than the expansion coefficients. This is in agreement with J6rgensen's
findings (9). A tentative explanation for this phenomenon is creep of the specimens during the heating and cooling cycles rather than continued curing reaction, since repetitive measurements done on a few individual specimens did not reveal evidence for the latter hypothesis. The highly significant correlations found for the exponential relationships
between filler content on the one hand and Shore A hardness and relative stiffness on the other should be considered with regard to their clinical implication. The higher the stiffness of an impression material the greater is the force of removal required to pull the impression free from undercut tooth areas. Thus any increase of filler content which might be motivated with regard to the consistency of the paste results in an exponential increase of stiffness and thereby of force of removal (5). The rather uniform working and setting times recorded over the range of filler contents studied could be expected since the ratio of base and catalyst was consistent for the seven experimental materials 9 The present data as well as previous reports (6, 11) indicate that the oscillating rheometer offers a simple and practically sufficiently suitable means of determinating setting and working times. The linear free curing contractions registered were very low and rather consistent. For theoretical reasons the curing shrinkage will decrease with increasing filler content. However, the present data do not show this tendency. Possible reasons are: 1. The reading accuracy of the microscope was 2 ~m, which means 0.005% of a specimen typically 40 mm long; 2. the temperature in the laboratory was 23+1~ which means that possible deviations in temperature at the times when the fiducial and the final readings were taken, can have a tremendous impact when the coefficients of thermal expansion are considered. The most important and clinically relevant information is the A d-figures proving that the thermal contraction of the elastomeric materials when cooling from 37~176 is the dominating factor for the die inaccuracy. Even the highest filler loaded paste investigated did not result in clinically acceptable die accuracy. This indicates that in spite of the progress achieved with modern addition curing silicones the clinically desirable accuracy of dental impressions is not reached unless a reheating technique is used, as previously recommended by de Araujo and J6rgensen (2) and Finger and Ohsawa (4). Alternatively, the development of elastomerle impression compounds with a dramatically reduced coefficient of thermal contraction in combination with insignificant curing contraction would be a highly desirable breakthrough.
Filler c o n t e n t o f s i l i c o n e i m p r e s s i o n s
Ad t~m 140
37
References
t~m _
140
o
120
120
100
100
80
-
80
-
60
40
-
40
20
-
20
o
60
7//I 0
I 10
t 20
I 30
I 40
I- ~ 50
I-60
%
FILLER Fig. 7. Linear relationships between filler content and accuracy of impressions registered as
base diameter deviations A d of stone dies from steel master die. Full circles give mean A dvalues (xs) when impressions and dies were produced at 23~ open circles (,%) when impressions were produced at 37~ and dies poured at 23~ The r2-values are 0.73 and 0.99, respectively.
1. American Dental Association. Revised specification No. 19 for non-aqueous elastomeric dental impression materials. J A m e r Dent Assoc 1977: 94: 733--41. 2. de Araujo PA, J6rgensen KD. Improved accuracy by reheating additionreaction silicone impressions. J Prosthet Dent 1986: 55: 11-2. 3. Finger W, Ohsawa M. Accuracy of stone-casts produced from selected addition-type silicone impressions. Scand J Dent Res 1983: 91: 61-5. 4. Finger W, Ohsawa M. Accuracy of cast restorations produced by a refractory die-investing technique. J Prosthet Dent 1984: 52: 800-3. 5. Finger W, Komatsu M. Elastic and plastic properties of elastic dental impression materials. Dent Mater 1985: 1: 12%34. 6. Finger W, Ohsawa M. Effect of mixing ratio on properties of elastomeric dental impression materials. Dent Mater 1986: 2: 183-6. 7 . J6rgensen KD. Thiokol as a dental impression material. Acta Odontol Scand 1956: 14: 313-34. 8. J6rgensen KD. Study of the setting expansion of gypsum. Acta Odontol Scand 1963: 21: 227-54. 9. JOrgensen KD. Thermal expansion of addition polymerization (Type II) silicone impression materials. Aust Dent J 1982: 27: 377-81. 10. Ohsawa M, J6rgensen KD. Curing contraction of addition-type silicone inapression materials. Scand J Dent Res 1983: 91: 51-4. 11. Ohsawa M, Finger W. Working time of elastomeric impression materials. Dent Mater 1986: 2: 179-82. 12. Wilson HJ. A method of assessing the setting characteristics of impression materials. Br Dent J 1964: 117." 536--40.
W. J. Finger Department of Prosthetic Dentistry and Dental Materials Science, Dental School of the RWTH, Aachen, FRG
Finger WJ. Significance of filler content to properties of silicone impression materials. Dent Mater 1988: 4: 33-37. Abstract - The effect of filler content on handling characteristics, mechanical properties and accuracy of addition curing silicones was investigated on 7 sets of experimental materials with filler contents between 0 and 61.5% by weight. The consistency increased linearly with the amount of filler; working and setting times were consistent irrespective of the filler content. Exponential relationships were found between filler loading on the one hand and Shore A hardness and stiffness on the other. The linear free curing contraction was approximately 0.05% for all materials tested while the coefficients of thermal contraction linearly decreased with increasing filler content. Cooling of impressions of a standardized metal preparation from 37 ~ to 23~ had a tremendous impact on accuracy. Adequate die accuracy was obtained only when impression and model were produced at the same temperature.
The amount of inert filler added to elastomeric impression materials affects handling characteristics of the pastes, mechanical properties and dimensional stability of the cured elastomer. Addition curing silicones are accepted by the profession particularly due to the excellent dimensional stability of the impressions. The degree of filler content is a variable often used by manufacturers mainly to control the consistency of the pastes and to adjust elastic parameters of the cured material. The purpose of this study was to investigate systematically the effect of filler content on basic properties of modern a-silicones and particularly on the accuracy of dental impressions. Material and m e t h o d s
Seven sets of base and catalyst pastes of addition curing silicones were studied.* The blue base paste contained the cross-linking agent and the white paste both an inhibitor and the Pt-catalyst. The filler contents of the 7 pairs of pastes in weight % were 0, 8.0, 16.3, 30.4, 42.5, 52.9 and 61.5 with identical amounts of filler included in corresponding sets of base and catalyst pastes. The mixing ratio was 1:1 by weight and the mixing time used throughout the study was 30 s. Unless * The experimental materials were made available by Bayer Dental, FRG. 3 DentalMaterials4:1, 1988
otherwise specified the experimental work was performed in a standard climate of 23___1~ and 50+5% relative humidity. The following material parameters and characteristics were studied: 1. The consistency of the individual pastes was determined according to the testing procedure described in A D A specification No. 19 (1). All mixes were loaded by 0.5 kg to make the results comparable within the series. The disk diameters of the mixes are given as the average of three tests and recorded to the nearest millimeter. 2. The working and setting times of the pastes were determined at 23 ~ and at 37~ by an oscillating rheometer as described previously by Finger and Ohsawa (6). Immediately after mixing, the materials were transferred to the rheometer; the reciprocating rotation and recording of the track were activated one minute after start of mixing. The instrument used works opposite to Wilson's rheometer (12) which means that for the uncured paste a straight line is registered only. Working and setting times are arbitrarily defined as the times after start of mixing when the recorded deflections reach 5% and 95%, respectively, of the final width of the trace (6, 11). For each material and each testing temperature 3 tests were performed. 3. The Shore A hardness numbers were determined as previously described (5) and following the procedure for preparation of speci-
Key words: impression materials; addition curing silicones. Werner J. Finger, Department of Prosthetic Dentistry, Dental School of the RWTH, PauwelsstraSe, D-5100 Aachen, FRG.
Received February 23; accepted April 23, 1987.
mens (20 mm ~ , 7 mm height) in A D A No. 19. The specimens were removed from the 32~ water bath after 3.5 min dwell time and stored at 23~ until testing was performed 60 min after start of mixing. The Shore A figures were read after 3 s loading time (Zwick 3114). For each material 5 indentations were made on each of 5 specimens. 4. The relative stiffness of the materials was determined from compression testing of 5 cylindrical specimens each (12.5 mm in diameter and 20 mm in height) prepared under the same conditions as the specimens for Shore A hardness testing. The cylinders were loaded after 60 min from start of mixing in a universal testing machine (Zwick 1774) at a rate of 62.5 mm per min. The relative stiffness was calculated from the slope of the initial, approximately straight part of the stress-strain-curve recorded. 5. The curing contraction was measured by means of the mercury bath method as modified by J6rgensen (7, 10). The Hg-bath is mounted on the traveling table of an Olympus microscope. Immediately after mixing the impression material is floated on the mercury forming a tongue-shaped specimen which is retained over the sharp edge of the rectangular jaw's wall. A thin metal mark is placed on the free end of the specimen giving a well defined reference point, the movement of which is recorded. For the calculation of the percentage linear contraction of the
34
Finger
test material a fiducial reading is taken when the reference mark stops moving because of liquid flow of the impression material. The reading accuracy of the microscope was 2 ~tm. For each of the 7 experimental materials the contraction of 5 specimens was recorded after 24 h. 6. The thermal expansion and contraction coefficients of the 7 materials were measured by a Mikrokator (C. E. Johansso~a, Sweden, 904-4) under a measuring load of 1 g. Cylindrical specimens (6 mm ~3 and 12 mm h) were produced in a split mold at 37~ The cylinders were removed 15 min after start of mixing and stored for 24 h at 37~ in air before being cooled to room temperature. For testing, the specimens were placed on the table of the Mikrokator centrally under the ball tip of the gauge. The top surface was fitted with a microscope cover glass (0.15 g). The assembly was placed in a thermostate and the temperature was monitoted by a thermocouple with the tip approximately 1 mm from the specimen's cylinder surface. The fiducial reading was taken when within 15 min the variation in temperature was less than 0.5~ and in dial gauge reading less than 0.5 ~tm. Then the temperature in the thermostat was raised within 60 min to 45 ~ approximately and another temperature and Mikrokator reading were taken after a holding time of at least 15 min, before the assembly was allowed to cool to room temperature for a final reading. The contribution of the thermal expansion and contraction of the assembly was determined by measurements on cylindrical specimens of steel and quartz glass with known thermal coefficients of expansion and with the same dimensions as the elastomeric test specimens. The calculations of the thermal expansion and contraction coefficients were adjusted accordingly. For each material 5 specimens were measured. 7. The accuracy of impressions was determined as previously described by Finger and Ohsawa (3). Impressions were taken from a truncated chromium, steel cone (8 mm base diameter and height, convergence angle 10~) in circular effectively perforated impression trays made from 0.75 mm thick stainless steel with a height of 13 mm and an internal diameter of 16 mm. Cone and tray were mounted in a stand giving 4 mm distance between the prominence line of the cone and the tray and 2 mm distance between the top surface of the die and the tray. Impressions were taken 1 min after start of
mixing either at 23~ or at 37~ and removed after another 14 min. After 24 h storage at room temperature the impressions were poured with vacuum mixed stone (Geostone, batch No. 52116 X, Bayer Dental, FRG; W/P ratio 0.23), the linear setting expansion of
which was 0.1%, measured by a dial gauge method (8). The stone dies were removed after one hour and their accuracy was determined by measuring with a displacement transducer the occlusal discrepancy in 4 places 90~ apart between the die surface and a steel ring
DISK )IAMETER mm
mm
40
40
36
36
32
32
28
28
24
24
i lO
i 20
i 30
i 40
i 50
i 60
%
FILLER Fig. 1. Linear relationship between filler content and disk diameter (r2 = 0.99). OR
MIN
0
MIN
0
o
0
-
-
0
6
6
-,5
5 0
4
-3
3 IL
2
9
9
9
0"~
9
-2 li------Ii____
9 . . . . . .
1
;/ I
0
I
I
I
I
I
I
10
20
30
40
50
60
%
FILLER Fig. 2. Relation between filler content and working (dotted lines) and setting times (full lines) as determined by the oscillating rheometer (OR). The full and open circles represent mean values (x3) of determinations at 37~ and 23~ respectively.
Filler content o f silicone impressions fitting accurately on the master steel cone. F r o m the m e a n value of these 4 readings the diameter deviation from the master die was calculated. For each impression material and each of the 2 curing temperatures 5 dies were produced.
Results
A linear relationship was found between the filler content of the experimental elastomeric materials and their consistencies. The coefficient of decision for the regression line is 0.99 (Fig. 1). The working and setting times of the mixes are shown i n Fig. 2 as dotted and full lines, respectively. Full circles give the times when tested at 37~ and open circles w h e n determined at 23~ Each circle is the m e a n value of 3 measurements. The m a x i m u m standard deviation a r o u n d an individual m e a n value found was + 0.35 min, the m e a n standard deviation + 0.15 min. The lines are drawn after linear regression analyses. The relationship between filler content and Shore A hardnesses is described by a curve constructed after exponential regression analysis (Fig. 3). The r2-value is 0.999 and statistically highly significant. The m a x i m u m standard deviation found for an individual m e a n value (xs) was + 1 Shore A number. Fig. 4 illustrates the exponential regression line for the relation of filler content vs. stiffness (r 2 = 0.99). All individual standard deviations were smaller than + 0.4 MPa; the m e a n standard deviation was + 0.1 MPa. Fig. 5 illustrates the linear free curing contraction after 24 h. Each circle is the m e a n value of 5 measurements, while the vertical bars give the standard deviation. W h e n tested by analysis of variance the individual mean values are not significantly different (p > 0.5). The central horizontal line is the m e a n of the 7 m e a n values; the thin lines describe the m e a n standard deviation around this m e a n value. In Fig. 6 the linear regression lines display the relations between a m o u n t of filler and coefficients of thermal expansion (lower line) and thermal contraction. The r2-values are 0.97 and 0.99. The m e a n standard deviations for the coefficients of expansion and contraction are 8 and 11.10 -6 K -1. Fig. 7 shows the linear relationships between filler content and accuracy of 3*
impressions. The lower line (r ~ = 0.73) shows the A d-values when impression taking and die pouring were done at 23~ the upper line (r 2 = 0.99) when the impressions were taken at 37~ and the dies poured at room temperature. The m a x i m u m a n d m e a n standard deviations for the m e a n values displayed
35
as full circles are + 7 and __+ 5 ~tm, for the open circles _+ 10 and + 7, respectively. When tested by linear regression analysis a correlation (r 2 = 0.996) is found between the coefficients of thermal contraction and the accuracy of the impressions.
SA 70
70
O
60
60
5O
50
40
40
30
30
20
20
j/I 0
I 10
I 20
I 30
I 40
I 50
I 60
%
FILLER Fig. 3. Relationship between filler content and Shore A hardness (SA). The decision coefficient of the exponential regression line is 0.999. STIFFNESS MPal
MPa o
5
5
4
o
4
3
-3
2
-2
1
-
'~1 0
I
I
I
I
I
I
10
20
30
40
50
60
1
%
FILLER
Fig. 4. Relationship between filler content and relative stiffness. The decision coefficient of the exponential regression line is 0.99.
Finger
36
%
% -0.03
- 0.03
-0.04
- 0.04
-0.05
- 0.05
- 0.06
-0.06
- 0.07
-0.07
_,,,,,, II 0
I
I
I
I
I
I
10
20
30
40
50
60
%
FILLER.
Fig. 5. Relation between filler content and linear free curing contraction. The central horizontal line is the mean contraction calculated from the seven individual means. The thin horizontal lines describe the interval of the mean standard deviation.
910-6K-1
9 10-6K-,
300
3O0
260
260
220
220
180
180
140
140
S, /I
//0
I
I
I
I
I
I
10
20
30
40
50
60
%
FILLER
Fig. 6. Linear relationships between filler content and coefficient of thermal expansion (full circles) and coefficient of thermal contraction (open circles)9 The r
It is reasonable and expected that the consistencies of the impression pastes linearly increase and that the coefficients of thermal expansion and contraction linearly decrease with increasing filler content. The coefficients of thermal contraction are consistently higher than the expansion coefficients. This is in agreement with J6rgensen's
findings (9). A tentative explanation for this phenomenon is creep of the specimens during the heating and cooling cycles rather than continued curing reaction, since repetitive measurements done on a few individual specimens did not reveal evidence for the latter hypothesis. The highly significant correlations found for the exponential relationships
between filler content on the one hand and Shore A hardness and relative stiffness on the other should be considered with regard to their clinical implication. The higher the stiffness of an impression material the greater is the force of removal required to pull the impression free from undercut tooth areas. Thus any increase of filler content which might be motivated with regard to the consistency of the paste results in an exponential increase of stiffness and thereby of force of removal (5). The rather uniform working and setting times recorded over the range of filler contents studied could be expected since the ratio of base and catalyst was consistent for the seven experimental materials 9 The present data as well as previous reports (6, 11) indicate that the oscillating rheometer offers a simple and practically sufficiently suitable means of determinating setting and working times. The linear free curing contractions registered were very low and rather consistent. For theoretical reasons the curing shrinkage will decrease with increasing filler content. However, the present data do not show this tendency. Possible reasons are: 1. The reading accuracy of the microscope was 2 ~m, which means 0.005% of a specimen typically 40 mm long; 2. the temperature in the laboratory was 23+1~ which means that possible deviations in temperature at the times when the fiducial and the final readings were taken, can have a tremendous impact when the coefficients of thermal expansion are considered. The most important and clinically relevant information is the A d-figures proving that the thermal contraction of the elastomeric materials when cooling from 37~176 is the dominating factor for the die inaccuracy. Even the highest filler loaded paste investigated did not result in clinically acceptable die accuracy. This indicates that in spite of the progress achieved with modern addition curing silicones the clinically desirable accuracy of dental impressions is not reached unless a reheating technique is used, as previously recommended by de Araujo and J6rgensen (2) and Finger and Ohsawa (4). Alternatively, the development of elastomerle impression compounds with a dramatically reduced coefficient of thermal contraction in combination with insignificant curing contraction would be a highly desirable breakthrough.
Filler c o n t e n t o f s i l i c o n e i m p r e s s i o n s
Ad t~m 140
37
References
t~m _
140
o
120
120
100
100
80
-
80
-
60
40
-
40
20
-
20
o
60
7//I 0
I 10
t 20
I 30
I 40
I- ~ 50
I-60
%
FILLER Fig. 7. Linear relationships between filler content and accuracy of impressions registered as
base diameter deviations A d of stone dies from steel master die. Full circles give mean A dvalues (xs) when impressions and dies were produced at 23~ open circles (,%) when impressions were produced at 37~ and dies poured at 23~ The r2-values are 0.73 and 0.99, respectively.
1. American Dental Association. Revised specification No. 19 for non-aqueous elastomeric dental impression materials. J A m e r Dent Assoc 1977: 94: 733--41. 2. de Araujo PA, J6rgensen KD. Improved accuracy by reheating additionreaction silicone impressions. J Prosthet Dent 1986: 55: 11-2. 3. Finger W, Ohsawa M. Accuracy of stone-casts produced from selected addition-type silicone impressions. Scand J Dent Res 1983: 91: 61-5. 4. Finger W, Ohsawa M. Accuracy of cast restorations produced by a refractory die-investing technique. J Prosthet Dent 1984: 52: 800-3. 5. Finger W, Komatsu M. Elastic and plastic properties of elastic dental impression materials. Dent Mater 1985: 1: 12%34. 6. Finger W, Ohsawa M. Effect of mixing ratio on properties of elastomeric dental impression materials. Dent Mater 1986: 2: 183-6. 7 . J6rgensen KD. Thiokol as a dental impression material. Acta Odontol Scand 1956: 14: 313-34. 8. J6rgensen KD. Study of the setting expansion of gypsum. Acta Odontol Scand 1963: 21: 227-54. 9. JOrgensen KD. Thermal expansion of addition polymerization (Type II) silicone impression materials. Aust Dent J 1982: 27: 377-81. 10. Ohsawa M, J6rgensen KD. Curing contraction of addition-type silicone inapression materials. Scand J Dent Res 1983: 91: 51-4. 11. Ohsawa M, Finger W. Working time of elastomeric impression materials. Dent Mater 1986: 2: 179-82. 12. Wilson HJ. A method of assessing the setting characteristics of impression materials. Br Dent J 1964: 117." 536--40.