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Thermal expansion and filler content of composite resins
FRIEDMAN
i 5
0
AND
HASSAN
Bassiouny, hl. A., and Grant, .I. .\, A visible light-c~urrd composite restorative. Br Dem J 145~327. 1978. Bassiouny, M. A., and Grant, A. ,4.: Physical properties 01 .+ visible lig-ht-currd composite resin. J P~c~u’rr-ib:rT)ENT 4.7:5 l(i 1980. Jordan, R. E.. Boksman, I,., Skinner, D. II.. Charles, D. H.. arltl Suzuki, Xl.: The new generation of comp&te materials Kk State I>rnc J 48219, 1982.
’
Thermal expansion and filler content of composite resins D. T. Hashinger, B.E.,* and C. W. Fairhurst, Ph.D.** Medical College of Georgia, School of Dentistry, Augusta, Ga.
L
inear coefficients of thermal expansion (a) of composite resins are usually reported as averages over certain temperature ranges.’ Studies of porcelain in our laboratory have demonstrated that information may be lost by reporting only average values. The thermal expansion of composite resins is primarily an additive property and is related to the filler/binder ratio and the composition of the filler and binder. The purpose of this study was to measure (Y of cured composite resins stored for periods up to 6 months, to ascertain the filler/binder ratio, and to measure the water absorption of the products. MATERIAL
AND METHODS
Thermal expansion measurements were made on nine commercial composite resins used as dental restorative materials. Gravimetric ashing analyses were performed on these and three additional available commercial composite resins. Water absorption data were obtained from eight of the products. Materials selected for study were those with a high filler content (regular) and low filler content (microfil). The composite resins and manufacturers are listed in Table I. Six cylindric specimens approximately 3 mm diameter x 25 mm length were prepared from each of nine composite resin products. The three light cured compos-
ite resins (Visio-Dispers, Visio-Fil, and Prisma-Fil) were inserted into glass tubing and polymerized by exposure to a visible light source (Espe Elipar, Espe Premier Sales Corp., and Prisma-Lite, L. D. Caulk Co.) for approximately 6 minutes from a distance of roughly 1 cm from various sides and angles. The two pastes of the chemically cured products were mixed as directed by the manufacturer, inserted into glass tubing, and left to cure for the recommended times. After the composite resins had polymerized, they were removed from the glass tubing and reduced in length to approximately 25 mm by grinding the ends flat and parallel with abrasive paper on a polishing wheel. Duplicate specimens of each composite resin were placed into a 37” C oven and stored dry for 1 day, 4 weeks, and 6 months before the thermal expansions were measured. The cy values of the composite resins were calculated from data obtained with a Research II differential dilatometer (Theta Industries Inc., Port Washington, N.Y.). The specimens were heated at a rate of 1’ C/minute from room temperature to 120” C in the dilatometer furnace. A push-rod force of approximately 90 gm was placed on the specimens. The standard for differential expansion measurements was a platinum cylinder (3.2 mm diameter X 25 mm long). The specimens were cooled after they reached 120” C by rolling the furnace away from the glass protection tube in which they rested. A second run was made on each specimen to evaluate the effect of heat treatment on a. The L-U values were calculated at 5” C intervals from heating curves of both runs and a comparison of (Y values was made. OCTOBER
1984
VOLUME
52
NUMBER
4
THERMAL
EXPANSION
AND
FILLER
CONTENT
OF RESINS
120
-.
3 a‘0
-I
100
-
l
Run
1
1 Day
0 h A . o
Run Run Run Run Run
2 1 2 1 2
1 1 1 6 6
Day Month Month Months Months
01 25
100
75
50 Temperature
125
25
75 Temperature
Fig. hour filler filler
1. LYvs. temperature curves for nonannealed
24specimens comparing high filler content, low content, and Espe products. The weight percent content is shown in parentheses.
Gravimetric ashing analyses were made on three disks of each of 12 composite resins. The disks were prepared by placing the composite resin into plastic rings 2 mm deep x 19 mm diameter that were pressed between glass slides. After the disks had polymerized either chemically or by exposure to light as previously described, they were placed in crucibles that were weighed after having been heated at least 30 minutes in a 600” C oven. The combined weights of crucible and specimen were recorded and the initial specimen weight calculated. After at least 60 minutes in a 600” C oven, the combined weights of the crucible and specimen were measured again. Water absorption data were obtained from the same cylindric specimens (3 mm diameter X 25 mm long) on which thermal expansion measurements had been made previously. The weights of three specimens of each of eight composite resins were recorded. The specimens were dried in a desiccator in a 37” C oven for 3 weeks until the weights stabilized. At this point the specimens were placed into bottles of 37” C distilled water and stored covered in a 37” C oven. Each time they were removed from the 37” C water for weighing, the specimens were quenched for 60 seconds in room temperature distilled water, blotted dry with a paper towel, and held in dry forceps while they were waved in room air for 15 seconds before the weights were measured. The composite resins were weighed at various time intervals. When they no longer appeared to exhibit a weight change, the specimens were removed from the water, blotted dry, and placed back into desiccators in the 37” C oven for drying. The measured weights of the OF PROSTHETIC
125
DENTISTRY
(“C)
Fig. 2. Effect of age and thermal history on
Table I. Composite
THE JOURNAL
100
(“C)
Trade name Adaptic Finesse lsopast
Isosit Miradapt P-10 Prisma-Fil Profile Silar Visio-Dispers Visio-Fil Vytol
resins studied Manufacturer
Johnson & Johnson Dental Products East Windsor, N.J. L. D. Caulk Co. Milford, Del. Vivadent (USA) Inc. Buffalo, N.Y. Vivadent (USA) Inc. Johnson & Johnson Dental Products Dental Products Div., 3M Co. St. Paul, Minn. L. D. Caulk Co. S. S. White Dental Product:; Philadelphia, Pa. Dental Products Div., 3M Co. Espe Premier Sales Corp. Norristown, Pa. Espe Premier Sales Corp. L. D. Caulk Co.
specimens were recorded at the end of 4 and 8 weeks in the oven. RESULTS All composite resins exhibited an increase in the value of the (Y from 30” C to 65” C. The rate of expansion decreased rapidly from 70” C to 100” C. A comparison of a! from 30” C to 65” C. The rate of expansion is shown in Fig. 1. To call attention to three groups within the overall group of composite resins studied, different symbols were used to represent: (1) five BISGMA composite resins with high filler contents, (2) two composite resins (one BIS-GMA and one diurethane) 507
HASHINGER
AND
FAIRHURST
1 0
20
WT.
3. Effect of age and thermal history on cyof Isopast (diurethane composite resin with low filler content).
Fig.
Table II. Thermal
name
.-- -----c_------~-a 37.5” C a 52.5” C
33.3 68.7 63.3 70.8 29.0 43.7 30.1 66.3 43.0
1 04 t 1.7 rt h.0 + 1.0 If- 1.1 t 0.6 ” 1.7 t 7.1 2 2.0
50.1 79.5 74.7 36.6 34.9 48.8 38.5 76.4 52.1
1 2 ? 2 -f z 2 f i
2.4 5.1 0.3 0.7 0.3 2.3 0.1 7.1 1.0
AL/r.
Avg %:/Ox 77.h 33 0 &364 2T 2 81.2
83.h 75.4 79 6 51 .o 62 3 81.4 79.5
52.5” c
8 6 .k 0.1 16.J -i 0.3 I-t.8 rt 0.4 7.3 + 0.2 : 0 i 0.2 10.2 I. 0.3 7.2 ?I 0.5 1.5.2 .t 1.1 JO.7 ? 0.1
Table III. Weight percent inorganic in composite resins Product
% Fuller
80
100
Content
Fig. 4. N at 37.5” C vs. weight percent inorganic filler content of BIS-GMA composite resins. The LYvalues are those on nonannealed 24hour specimens.
expansion values* for nonannealed composite resins 24 Hours
Product
60
40
-__--a 37.5” C
1 Month ..l_-__--.-~ a 52.5” c AL/L
___I52.5” C a 37.5” C
28.0 52.0 49.1 22.7 25.4 34.1 24.0 57.3 44.3
34.1 t 0.3 59.4 + 2.1 61.7 _+5.4 27 4 -r 1.0 27 2 -z 0.6 39.4 ir 2.1 26.1 t 1.6 64.4 I 4.9 51xJ: 2 7.3
_f t t i r T i i 2
filler MAD 01 0.0 0.5 0. I 0.1 0.0 0.0
0.1 0.0 01 0.1 0.0
I 0.4 t 2.8 f 5.1 i LO z 0.3 t 2.5 r 0.3 31 0.1 2: 5.2
6.5 Il.8 12.1 5.4 5.8 7.9 5.5 13.2 10.3
0.1 0.5 0.2 0.0 0.2 0.5 0.1 0.1 1.2
25.7 51.3 47.9 21.4 22.5 27.1 21.7 56.1 39.0
+ 0.6 + 3.4 r 1.3 + 0.3 -r 0.6 + 0.5 t 0.1 _+ 7.1 i‘
6 Months a 52.5” C
28.5 _t 55.4 i 59.3 i 22.8 i23.9 t 33.6 1 24.5 z 64.2 i39.1 t
AL/L
0.6 0.6 0.8 0.9 0.6 0.4 0.6 7.1
52.5” C
5.7 i 0.1 11,s t 0.7 11.9 i 0.1 4.8 -+ 0.1 5.2 -+ 0.2 6.7 t 0.1 5.0 i 0.1 12.7 i 1.1 8.6 t
with low filler contents, and (3) two Espe products of a different chemistry. As the age of the nonannealed specimens increased, the temperature at which the LYvalues began to drop from the maximum value also increased. Annealed specimens did not exhibit nonlinear expansion behavior; the cy values either increased gradually with increasing temperature or remained relatively constant over the temperature range studied. The effect of specimen age and thermal history on (Y values of Profile and Isopast can be seen in Figs. 2 and 3. respectively. In each case, the solid lines represent the N values obtained from the nonannealed specimens-ones which had not previously beenheatedabove 37” C. The dotted lines represent the LY values obtained from annealed
OCTOBER
1984
VOLUME
52
NUMBER
4
THERMAL
Table Days in water 1 8 35 60
EXPANSION
AND
FILLER
CONTENT
OF RESINS
IV. Water absorption*
Adaptic 0.11 0.40 0.52 0.61
2 0.01 f 0.09 t 0.10 f 0.10
Finesse 0.55 1.49 2.17 2.42
5 ? f +
Isopast
0.03 0.16 0.03 0.02
0.42 1.12 1.98 2.32
+ t f -t
0.02 0.02 0.02 0.02
Miradapt 0.13 0.41 0.68 0.68
+ f k f
0.00 0.04 0.02 0.03
Prisma-Fil 0.18 0.50 0.72 0.80
i 0.00 f 0.01 k 0.01
f 0.02
Profile 0.12 0.32 0.52 0.59
?I 0.01
+ 0.02 + 0.02 2 0.02
Visio-Dispers 0.20 0.54 0.55 0.54
Visio-Fil
t 0.01 k 0.01
0.10 * 0.02 0.18 + 0.01
f 0.03 k 0.04
0.20 ?I 0.02 C.25 + 0.01
*Values are in weight percent ? mean absolute deviation.
Table
V. Overall
Days dried
Adaptic
56
-0.04 + 0.02
weight
percent change*
Finesse
Isopast
-0.06 t- 0.01
-0.07 ? 0.02
‘After immersion in water until equilibrium
Miradapt -0.23 + 0.02
OF PROSTHETIC
DENTISTRY
-0.11
? 0.03
Profile -0.09 k 0.01
Visio-Dispers -0.22 ?. 0.04
Visio-Fil -0.07 k 0.01
was reached 8 weeks drying in oven. Values are in weight percent f mean absolute deviation.
specimens-ones which had been heated to 120” C in the dilatometer furnace prior to the second run. It can be seen from these plots that some annealing occurs during the initial run because the nonlinear expansion behavior is not repeated in the second run. The o values for all nonannealed composite resins aged 24 hours, 1 month, and 6 months are shown in Table II. The average values of two specimens and the mean absolute deviation are shown for each product listed. The weight of the inorganic filler (the residue from the ashing procedure) and its percent by weight were calculated for each composite resin disk analyzed. The mean and mean absolute deviation of the three values for each composite resin studied are shown in Table III. A Pearson product-moment correlation was determined for the filler content and a values at 37.5” C of BIS-GMA composite resins. A strong inverse correlation exists (R* = .94) between the filler content and (Y of the composite resins. In Fig. 4 a plot of (Y at 37.5” C vs. weight percent filler content of BIS-GMA composite resins is shown. Although a slightly higher R* might have been obtained with filler volume percent, uncertainties in several filler densities would have given rise to nonsystematic errors. Since the R2 = .94 explains 94% of the data, weight percents were deemed adequate. The percent weight gains for eight composite resin products after 1, 8, 35, and 60 days of water immersion are shown in Table IV. Because water is absorbed by the resin material rather than the filler in the composite resins, the two products Finesse and Isopast with low filler contents (33% and 36%, respectively) exhibited the greatest weight gain due to water absorption. At the end of 8 weeks in a desiccator in a 37” C oven, THE JOURNAL
Prisma-Fil
the measured weights of all specimens were less than their original weights. This is not surprising because some solubility and leaching from the composite resin in water is expected. The values for the overall weight percent change based on the original dry weight of the specimens are shown in Table V.
DISCUSSION In clinical use, composite resins probably do not reach the high temperatures required to bring about the annealing effect used in this study. Therefore, thermal expansion data from specimens not previously subjected to temperatures above 37” C are expected .to approximate in vivo expansion of composite resins more closely than data obtained from annealed specimens. The interpretation of the expansion coefficient results is not straightforward. The possibility of a glass transition temperature region existing at these temperatures is ruled out since the nonlinear expansion behavior is irreversible. At least two independent mechanisms may contribute to the increased volume expansion and expansion rate: 1. Continued polymerization of free radicals. The increase in volume and volume rate change could be due to the physical release of the low molecular weight materials thus allowing them to expand foRowed by a reduction due to polymerization volume rate change. 2. Polymerization stress relief. During initial polymerization of the composite resin, stress fields around individual filler particles must develop. During the annealing stages an initial volume increase may occur followed by decreased volume due to stress relief. In all likelihood some effects from both mechanisms occur to yield the total behavioral effect. The two BIS-GMA composite resins with low filler 509
HASHINGER
content, Finesse and Isopast, exhibit three to four times the water absorption of the regular BIS-GMA composite resins. This is to be expected because the water absorption occurs in the resin, which is present in the regular composite resin at the 20% to 25% level and in the Finesse and Isopast at the 60% to 65% level. The significance of these relationships is that the water absorption is not proportional to the filler surface area but to the volume of resin. It should be noted that the non-BIS-GMA composite resins, Visio-Dispers and Visio-Fil, exhibited lower water absorption for their filler levels than the BIS-GMA systems.
AND
FAIRHURST
nealed specimens at two temperatures approximately 37” and 50” C from heating curves rather than as an average value. The total expansion at the higher temperature would also be helpful in estimating thermal behavior. A strong inverse correlation exists (R’ = .94) between the filler content and cy at 37.5” C for the BIS-GMA composite resins. Water absorption is directly related to the volume fraction of resin and the type of resin in the composite. REFERENCE
CONCLUSIONS Thermal expansion data from nonannealed composite resin specimens are expected to approximate in vivo expansion of composite resins. To indicate the variation of cy over the temperature range, it is recommended that a be reported on nonan-
Compressive strength and surface hardness of type IV die stone when mixed with water substitutes R. L. Schneider, I ni\wsilv
W
D.D.S., M.S.,* and T. D. Taylor,
OF Iowa, (Mleqz
of Dentistrv.
ith numerous dental gypsum products available, dentists can be confused about which product will meet tbrir clinical needs. The type IV American Dental Association (ADA)-approved die stones offer a variety of information about compressive strength as it may t-elate to surface hardness. A harder die stone, because it is more resistant to abrasion. is theoretically a superior material. The purpose of this study was to investigate the six type IV ADA-approved die stones (Table I) by comparing their compressive strength and surface hardness when they are mixed with different solutitWY
D.D.S., M.S.D.*”
Iowa Gil);, Iowa
MATERIAL
AND METHODS
The liquid media used for testing were doubledistilled water, clear slurry water as described by Rudd and Dunn’ with double-distilled water as a base. Whip-Mix gypsum hardener, and Stalite. The die stone was divided from bulk into batches of 150 gm and mixed according to the manufacturers’ recommended water/powder ratio (Table II). If a range of liquid volumes was offered, a mean was used. The die stone was weighed, added to the liquid in a mechanical mixing bowl, hand spatulated for 10 to 15 seconds, and mechanically mixed in a Whip-Mix spatulator (Whip-Mix Corp.) at 450 rpm for an additional 15 seconds with a vacuum of 25 inches of mercury. Compressive strength was determined according to ADA specification No. 25 for gypsum products. A split aluminum mold was filled with stone under gentle vibration and c,avered on top and bottom with glass OCTOBER
1984
VOLUME
52
NUMBER
4
i 5
0
AND
HASSAN
Bassiouny, hl. A., and Grant, .I. .\, A visible light-c~urrd composite restorative. Br Dem J 145~327. 1978. Bassiouny, M. A., and Grant, A. ,4.: Physical properties 01 .+ visible lig-ht-currd composite resin. J P~c~u’rr-ib:rT)ENT 4.7:5 l(i 1980. Jordan, R. E.. Boksman, I,., Skinner, D. II.. Charles, D. H.. arltl Suzuki, Xl.: The new generation of comp&te materials Kk State I>rnc J 48219, 1982.
’
Thermal expansion and filler content of composite resins D. T. Hashinger, B.E.,* and C. W. Fairhurst, Ph.D.** Medical College of Georgia, School of Dentistry, Augusta, Ga.
L
inear coefficients of thermal expansion (a) of composite resins are usually reported as averages over certain temperature ranges.’ Studies of porcelain in our laboratory have demonstrated that information may be lost by reporting only average values. The thermal expansion of composite resins is primarily an additive property and is related to the filler/binder ratio and the composition of the filler and binder. The purpose of this study was to measure (Y of cured composite resins stored for periods up to 6 months, to ascertain the filler/binder ratio, and to measure the water absorption of the products. MATERIAL
AND METHODS
Thermal expansion measurements were made on nine commercial composite resins used as dental restorative materials. Gravimetric ashing analyses were performed on these and three additional available commercial composite resins. Water absorption data were obtained from eight of the products. Materials selected for study were those with a high filler content (regular) and low filler content (microfil). The composite resins and manufacturers are listed in Table I. Six cylindric specimens approximately 3 mm diameter x 25 mm length were prepared from each of nine composite resin products. The three light cured compos-
ite resins (Visio-Dispers, Visio-Fil, and Prisma-Fil) were inserted into glass tubing and polymerized by exposure to a visible light source (Espe Elipar, Espe Premier Sales Corp., and Prisma-Lite, L. D. Caulk Co.) for approximately 6 minutes from a distance of roughly 1 cm from various sides and angles. The two pastes of the chemically cured products were mixed as directed by the manufacturer, inserted into glass tubing, and left to cure for the recommended times. After the composite resins had polymerized, they were removed from the glass tubing and reduced in length to approximately 25 mm by grinding the ends flat and parallel with abrasive paper on a polishing wheel. Duplicate specimens of each composite resin were placed into a 37” C oven and stored dry for 1 day, 4 weeks, and 6 months before the thermal expansions were measured. The cy values of the composite resins were calculated from data obtained with a Research II differential dilatometer (Theta Industries Inc., Port Washington, N.Y.). The specimens were heated at a rate of 1’ C/minute from room temperature to 120” C in the dilatometer furnace. A push-rod force of approximately 90 gm was placed on the specimens. The standard for differential expansion measurements was a platinum cylinder (3.2 mm diameter X 25 mm long). The specimens were cooled after they reached 120” C by rolling the furnace away from the glass protection tube in which they rested. A second run was made on each specimen to evaluate the effect of heat treatment on a. The L-U values were calculated at 5” C intervals from heating curves of both runs and a comparison of (Y values was made. OCTOBER
1984
VOLUME
52
NUMBER
4
THERMAL
EXPANSION
AND
FILLER
CONTENT
OF RESINS
120
-.
3 a‘0
-I
100
-
l
Run
1
1 Day
0 h A . o
Run Run Run Run Run
2 1 2 1 2
1 1 1 6 6
Day Month Month Months Months
01 25
100
75
50 Temperature
125
25
75 Temperature
Fig. hour filler filler
1. LYvs. temperature curves for nonannealed
24specimens comparing high filler content, low content, and Espe products. The weight percent content is shown in parentheses.
Gravimetric ashing analyses were made on three disks of each of 12 composite resins. The disks were prepared by placing the composite resin into plastic rings 2 mm deep x 19 mm diameter that were pressed between glass slides. After the disks had polymerized either chemically or by exposure to light as previously described, they were placed in crucibles that were weighed after having been heated at least 30 minutes in a 600” C oven. The combined weights of crucible and specimen were recorded and the initial specimen weight calculated. After at least 60 minutes in a 600” C oven, the combined weights of the crucible and specimen were measured again. Water absorption data were obtained from the same cylindric specimens (3 mm diameter X 25 mm long) on which thermal expansion measurements had been made previously. The weights of three specimens of each of eight composite resins were recorded. The specimens were dried in a desiccator in a 37” C oven for 3 weeks until the weights stabilized. At this point the specimens were placed into bottles of 37” C distilled water and stored covered in a 37” C oven. Each time they were removed from the 37” C water for weighing, the specimens were quenched for 60 seconds in room temperature distilled water, blotted dry with a paper towel, and held in dry forceps while they were waved in room air for 15 seconds before the weights were measured. The composite resins were weighed at various time intervals. When they no longer appeared to exhibit a weight change, the specimens were removed from the water, blotted dry, and placed back into desiccators in the 37” C oven for drying. The measured weights of the OF PROSTHETIC
125
DENTISTRY
(“C)
Fig. 2. Effect of age and thermal history on
Table I. Composite
THE JOURNAL
100
(“C)
Trade name Adaptic Finesse lsopast
Isosit Miradapt P-10 Prisma-Fil Profile Silar Visio-Dispers Visio-Fil Vytol
resins studied Manufacturer
Johnson & Johnson Dental Products East Windsor, N.J. L. D. Caulk Co. Milford, Del. Vivadent (USA) Inc. Buffalo, N.Y. Vivadent (USA) Inc. Johnson & Johnson Dental Products Dental Products Div., 3M Co. St. Paul, Minn. L. D. Caulk Co. S. S. White Dental Product:; Philadelphia, Pa. Dental Products Div., 3M Co. Espe Premier Sales Corp. Norristown, Pa. Espe Premier Sales Corp. L. D. Caulk Co.
specimens were recorded at the end of 4 and 8 weeks in the oven. RESULTS All composite resins exhibited an increase in the value of the (Y from 30” C to 65” C. The rate of expansion decreased rapidly from 70” C to 100” C. A comparison of a! from 30” C to 65” C. The rate of expansion is shown in Fig. 1. To call attention to three groups within the overall group of composite resins studied, different symbols were used to represent: (1) five BISGMA composite resins with high filler contents, (2) two composite resins (one BIS-GMA and one diurethane) 507
HASHINGER
AND
FAIRHURST
1 0
20
WT.
3. Effect of age and thermal history on cyof Isopast (diurethane composite resin with low filler content).
Fig.
Table II. Thermal
name
.-- -----c_------~-a 37.5” C a 52.5” C
33.3 68.7 63.3 70.8 29.0 43.7 30.1 66.3 43.0
1 04 t 1.7 rt h.0 + 1.0 If- 1.1 t 0.6 ” 1.7 t 7.1 2 2.0
50.1 79.5 74.7 36.6 34.9 48.8 38.5 76.4 52.1
1 2 ? 2 -f z 2 f i
2.4 5.1 0.3 0.7 0.3 2.3 0.1 7.1 1.0
AL/r.
Avg %:/Ox 77.h 33 0 &364 2T 2 81.2
83.h 75.4 79 6 51 .o 62 3 81.4 79.5
52.5” c
8 6 .k 0.1 16.J -i 0.3 I-t.8 rt 0.4 7.3 + 0.2 : 0 i 0.2 10.2 I. 0.3 7.2 ?I 0.5 1.5.2 .t 1.1 JO.7 ? 0.1
Table III. Weight percent inorganic in composite resins Product
% Fuller
80
100
Content
Fig. 4. N at 37.5” C vs. weight percent inorganic filler content of BIS-GMA composite resins. The LYvalues are those on nonannealed 24hour specimens.
expansion values* for nonannealed composite resins 24 Hours
Product
60
40
-__--a 37.5” C
1 Month ..l_-__--.-~ a 52.5” c AL/L
___I52.5” C a 37.5” C
28.0 52.0 49.1 22.7 25.4 34.1 24.0 57.3 44.3
34.1 t 0.3 59.4 + 2.1 61.7 _+5.4 27 4 -r 1.0 27 2 -z 0.6 39.4 ir 2.1 26.1 t 1.6 64.4 I 4.9 51xJ: 2 7.3
_f t t i r T i i 2
filler MAD 01 0.0 0.5 0. I 0.1 0.0 0.0
0.1 0.0 01 0.1 0.0
I 0.4 t 2.8 f 5.1 i LO z 0.3 t 2.5 r 0.3 31 0.1 2: 5.2
6.5 Il.8 12.1 5.4 5.8 7.9 5.5 13.2 10.3
0.1 0.5 0.2 0.0 0.2 0.5 0.1 0.1 1.2
25.7 51.3 47.9 21.4 22.5 27.1 21.7 56.1 39.0
+ 0.6 + 3.4 r 1.3 + 0.3 -r 0.6 + 0.5 t 0.1 _+ 7.1 i‘
6 Months a 52.5” C
28.5 _t 55.4 i 59.3 i 22.8 i23.9 t 33.6 1 24.5 z 64.2 i39.1 t
AL/L
0.6 0.6 0.8 0.9 0.6 0.4 0.6 7.1
52.5” C
5.7 i 0.1 11,s t 0.7 11.9 i 0.1 4.8 -+ 0.1 5.2 -+ 0.2 6.7 t 0.1 5.0 i 0.1 12.7 i 1.1 8.6 t
with low filler contents, and (3) two Espe products of a different chemistry. As the age of the nonannealed specimens increased, the temperature at which the LYvalues began to drop from the maximum value also increased. Annealed specimens did not exhibit nonlinear expansion behavior; the cy values either increased gradually with increasing temperature or remained relatively constant over the temperature range studied. The effect of specimen age and thermal history on (Y values of Profile and Isopast can be seen in Figs. 2 and 3. respectively. In each case, the solid lines represent the N values obtained from the nonannealed specimens-ones which had not previously beenheatedabove 37” C. The dotted lines represent the LY values obtained from annealed
OCTOBER
1984
VOLUME
52
NUMBER
4
THERMAL
Table Days in water 1 8 35 60
EXPANSION
AND
FILLER
CONTENT
OF RESINS
IV. Water absorption*
Adaptic 0.11 0.40 0.52 0.61
2 0.01 f 0.09 t 0.10 f 0.10
Finesse 0.55 1.49 2.17 2.42
5 ? f +
Isopast
0.03 0.16 0.03 0.02
0.42 1.12 1.98 2.32
+ t f -t
0.02 0.02 0.02 0.02
Miradapt 0.13 0.41 0.68 0.68
+ f k f
0.00 0.04 0.02 0.03
Prisma-Fil 0.18 0.50 0.72 0.80
i 0.00 f 0.01 k 0.01
f 0.02
Profile 0.12 0.32 0.52 0.59
?I 0.01
+ 0.02 + 0.02 2 0.02
Visio-Dispers 0.20 0.54 0.55 0.54
Visio-Fil
t 0.01 k 0.01
0.10 * 0.02 0.18 + 0.01
f 0.03 k 0.04
0.20 ?I 0.02 C.25 + 0.01
*Values are in weight percent ? mean absolute deviation.
Table
V. Overall
Days dried
Adaptic
56
-0.04 + 0.02
weight
percent change*
Finesse
Isopast
-0.06 t- 0.01
-0.07 ? 0.02
‘After immersion in water until equilibrium
Miradapt -0.23 + 0.02
OF PROSTHETIC
DENTISTRY
-0.11
? 0.03
Profile -0.09 k 0.01
Visio-Dispers -0.22 ?. 0.04
Visio-Fil -0.07 k 0.01
was reached 8 weeks drying in oven. Values are in weight percent f mean absolute deviation.
specimens-ones which had been heated to 120” C in the dilatometer furnace prior to the second run. It can be seen from these plots that some annealing occurs during the initial run because the nonlinear expansion behavior is not repeated in the second run. The o values for all nonannealed composite resins aged 24 hours, 1 month, and 6 months are shown in Table II. The average values of two specimens and the mean absolute deviation are shown for each product listed. The weight of the inorganic filler (the residue from the ashing procedure) and its percent by weight were calculated for each composite resin disk analyzed. The mean and mean absolute deviation of the three values for each composite resin studied are shown in Table III. A Pearson product-moment correlation was determined for the filler content and a values at 37.5” C of BIS-GMA composite resins. A strong inverse correlation exists (R* = .94) between the filler content and (Y of the composite resins. In Fig. 4 a plot of (Y at 37.5” C vs. weight percent filler content of BIS-GMA composite resins is shown. Although a slightly higher R* might have been obtained with filler volume percent, uncertainties in several filler densities would have given rise to nonsystematic errors. Since the R2 = .94 explains 94% of the data, weight percents were deemed adequate. The percent weight gains for eight composite resin products after 1, 8, 35, and 60 days of water immersion are shown in Table IV. Because water is absorbed by the resin material rather than the filler in the composite resins, the two products Finesse and Isopast with low filler contents (33% and 36%, respectively) exhibited the greatest weight gain due to water absorption. At the end of 8 weeks in a desiccator in a 37” C oven, THE JOURNAL
Prisma-Fil
the measured weights of all specimens were less than their original weights. This is not surprising because some solubility and leaching from the composite resin in water is expected. The values for the overall weight percent change based on the original dry weight of the specimens are shown in Table V.
DISCUSSION In clinical use, composite resins probably do not reach the high temperatures required to bring about the annealing effect used in this study. Therefore, thermal expansion data from specimens not previously subjected to temperatures above 37” C are expected .to approximate in vivo expansion of composite resins more closely than data obtained from annealed specimens. The interpretation of the expansion coefficient results is not straightforward. The possibility of a glass transition temperature region existing at these temperatures is ruled out since the nonlinear expansion behavior is irreversible. At least two independent mechanisms may contribute to the increased volume expansion and expansion rate: 1. Continued polymerization of free radicals. The increase in volume and volume rate change could be due to the physical release of the low molecular weight materials thus allowing them to expand foRowed by a reduction due to polymerization volume rate change. 2. Polymerization stress relief. During initial polymerization of the composite resin, stress fields around individual filler particles must develop. During the annealing stages an initial volume increase may occur followed by decreased volume due to stress relief. In all likelihood some effects from both mechanisms occur to yield the total behavioral effect. The two BIS-GMA composite resins with low filler 509
HASHINGER
content, Finesse and Isopast, exhibit three to four times the water absorption of the regular BIS-GMA composite resins. This is to be expected because the water absorption occurs in the resin, which is present in the regular composite resin at the 20% to 25% level and in the Finesse and Isopast at the 60% to 65% level. The significance of these relationships is that the water absorption is not proportional to the filler surface area but to the volume of resin. It should be noted that the non-BIS-GMA composite resins, Visio-Dispers and Visio-Fil, exhibited lower water absorption for their filler levels than the BIS-GMA systems.
AND
FAIRHURST
nealed specimens at two temperatures approximately 37” and 50” C from heating curves rather than as an average value. The total expansion at the higher temperature would also be helpful in estimating thermal behavior. A strong inverse correlation exists (R’ = .94) between the filler content and cy at 37.5” C for the BIS-GMA composite resins. Water absorption is directly related to the volume fraction of resin and the type of resin in the composite. REFERENCE
CONCLUSIONS Thermal expansion data from nonannealed composite resin specimens are expected to approximate in vivo expansion of composite resins. To indicate the variation of cy over the temperature range, it is recommended that a be reported on nonan-
Compressive strength and surface hardness of type IV die stone when mixed with water substitutes R. L. Schneider, I ni\wsilv
W
D.D.S., M.S.,* and T. D. Taylor,
OF Iowa, (Mleqz
of Dentistrv.
ith numerous dental gypsum products available, dentists can be confused about which product will meet tbrir clinical needs. The type IV American Dental Association (ADA)-approved die stones offer a variety of information about compressive strength as it may t-elate to surface hardness. A harder die stone, because it is more resistant to abrasion. is theoretically a superior material. The purpose of this study was to investigate the six type IV ADA-approved die stones (Table I) by comparing their compressive strength and surface hardness when they are mixed with different solutitWY
D.D.S., M.S.D.*”
Iowa Gil);, Iowa
MATERIAL
AND METHODS
The liquid media used for testing were doubledistilled water, clear slurry water as described by Rudd and Dunn’ with double-distilled water as a base. Whip-Mix gypsum hardener, and Stalite. The die stone was divided from bulk into batches of 150 gm and mixed according to the manufacturers’ recommended water/powder ratio (Table II). If a range of liquid volumes was offered, a mean was used. The die stone was weighed, added to the liquid in a mechanical mixing bowl, hand spatulated for 10 to 15 seconds, and mechanically mixed in a Whip-Mix spatulator (Whip-Mix Corp.) at 450 rpm for an additional 15 seconds with a vacuum of 25 inches of mercury. Compressive strength was determined according to ADA specification No. 25 for gypsum products. A split aluminum mold was filled with stone under gentle vibration and c,avered on top and bottom with glass OCTOBER
1984
VOLUME
52
NUMBER
4