Polymerization
shrinkage
of index and pattern
acrylic
resins
Philippe Mojon, D.M.D.,* Jean-Pierre Oberholzer, D.M.D.,** Jean-Marc Meyer, Ph.D.,*** and Urs C. Belser, D.M.D.**** University of Geneva, School of Dentistry, Geneva, Switzerland; Columbia, School of Dentistry, Vancouver, B.C., Canada
and University
of British
Inadequate dimensional stability caused by polymerization shrinkage has been reported concerning the various applications of acrylic resins. The objective of the study was to evaluate dimensional changes of two self-curing acrylic resins marketed as pattern and index material. Early volumetric changes were measured with a dilatometer and late linear changes were recorded with an inductive transducer. After 24 hours the volumetric shrinkage was 7.9% for Duralay resin and 6.6% for Palavit G resin; 80% of the change appears before 17 minutes at room temperature. Shrinkage was significantly increased when the proportion of powder in the mix was diminished. Results suggest that these resins should be used with some method to compensate for the shrinkage, when used as index material. However, the dimensional change might provide significant advantages for intracoronal castings. (J PROSTHET DENT 1990;64:684-8.)
A
crylic resins have been used in dentistry for many purposes ranging from dentures to veneers. Self-curing acrylic resins have been advocated for direct post and core patterns, occlusal registration indexes, and soldering indexes for fixed partial dentures.ls6 They have numerous advantages over the waxes, impression compound, and plaster materials. For example, the initial viscosity can be adjusted by changing the proportion of the components. The wetting ability of the mix easily provides for reproduction of small details.7 The setting reaction is compatible with intraoral usage, and the set resin material is harder than wax and less brittle than impression compound. Because these resins are methylmethacrylate based, shrinkage occurs when they set. Shrinkage of pure methylmethacrylate is about 21% . Polymerization shrinkage of dental acrylic resins is less because one part of the material is already polymerized.8 The theoretical amount of shrinkage depends on the proportion of prepolymerized material and monomer. Inert particles, added to the powder by the manufacturer, also influence dental acrylic resin. Dimensional change has been evaluated for various types of acrylic
resins
by use of several
methods
of measure-
rnentg-16 Composite resins could have a volumetric polymerization shrinkage of as much as 7.1% ,l* while the linear dimensional change of custom tray materials is between 0.05%
and 0.38%.16
*Assistant Professor, Department of Clinical Dental Sciences, University of British Columbia, School of Dentistry. **Private practice, Geneva, Switzerland. ***Professor and Head, Department of Dental Materials, Ecole de Medecine-dentaire, Geneva, Switzerland. ****Professor and Head, Department of Fixed Prosthodontics, Ecole de Medecine-dentaire, Geneva, Switzerland. 10/l/21926
684
Table
I. Powder-to-liquid
ratio used in study Powder-to-liquid b3mhl)
Mix
Duralay
Standard Thick Thin
ratio
Palavit
LO/O.8 1.010.6 l.O/l.O
G
1.010.6 1.0/0.5 1.010.7
There have been few reports dealing with resins used for indexes and patterns. In 1953, Smith and Schoonover17 measured volumetric shrinkage of unfilled direct-filling resins that may be similar to indexing resins. Their results showed a volumetric shrinkage of 6 % to 8 % . If the amount of polymerization shrinkage were the same for resins used for indexes and patterns, the dimensional stability would be compromised and the accuracy of clinical records would be questionable. In 1970 Galan et a1.18found that a type of self-curing acrylic resin was more accurate than waxes but their indirect measurement did not elicit a definitive conclusion. There seems to be a need to evaluate the dimensional change of resins used for indexes and patterns and to weigh the possible clinical significance of the polymerization effect. This study evaluated the dimensional change of resins used for indexes and patterns, analyzed the influence of powder-to-liquid ratio on dimensional change, and compared two products available on the market.
MATERIAL Two self-curing
AND
METHODS
acrylic
resins marketed
as pattern
and
index material (Duralay, Reliance Dental Mfg. Co., Worth, Ill., and Palavit G, Kulzer Co., Bad Homburg, West Germany) were selected. In both products, the powder consists
DECEMBER
1999
VOLUME
64
NUMBER
6
POLYMERIZATION
SHRINKAGE
OF ACRYLIC
RESINS
%
t
PALAVIT 1 g/0.6ml
7.
a, :6. Y .E i 5.
G
.ou)4. L z 3 . E ;2. > I _
I
3
5
time
7
9 II in minutes
13
15
I7
t(min)
Fig. 1. Early volumetric shrinkage of Duralay resin standard mix, expressed as percent of initial volume.
of poly(methylmethacrylate) and initiator and the liquid contains monomer and activator. There are other substances present but not disclosed by the manufacturers. Because no instructions were available on the powder to liquid (P/L) ratio for either material, a standard mix was developed for each, to produce a consistency that was clinically manageable after 45 seconds of mixing. To evaluate the influence of P/L ratio on dimensional change, two other ratios were also tested for each resin. The ratios were varied by changing the amount of liquid, while the weight of powder remained constant. Therefore mixtures of different consistency (Table I) were used, each of which would have been clinically acceptable. Two types of tests were conducted because large changes were expected to occur initially and small changes later. The first measurements were made with a dilatometer and ended 17 minutes after the beginning of mixing. For the second measurement, a transducer recorded the linear change from 17 minutes to 24 hours or more. The timing of the two experiments was determined on the basis of preliminary trials. The dilatometer, which recorded volumetric changes, consisted of a glass specimen chamber (100 X 30 mm) connected to a 1 ml pipette filled with distilled water. The receptable was closed with a glass plate coated with white petroleum jelly. The specimen consisted of 4 ml of resin injected with a graduate syringe into the dilatometer. Change of water level in the pipette offered a view of the change in volume within the receptacle. The reading started 2 minutes after the beginning of mixing and continued for 15 minutes. The experiment was repeated five or six times. This method failed to demonstrate differences among mixes during preliminary trials, and only tests of standard mixes were completed. The equipment for the second experiment was initially developed to measure dimensional changes of amalgam.lg A micrometer was used to measure the initial length of a
TEE
JOURNAL
OF PROSTHETIC
DENTISTRY
I
3
5
7 time
9 II in minutes
I3
15
I7
t(min)
Fig. 2. Early volumetric shrinkage of Palavit G resin standard mix, expressed as percent of initial volume.
8-
6-
4-
2-
OFig. 3. Mean volumetric shrinkage at 17 minutes, expressed as percent of initial volume.
cylinder of resin, and change in dimension was recorded with a linear inductive transducer mounted on the other side of the cylinder. A graphic recorder was connected to the amplifier of the transducer. Accuracy of the measurement was in a range of 1 pm and stability of the equipment was checked for several days. The temperature around the sample was kept constant. Ten cylindrical samples, 8 mm long and 6 mm in diameter, were produced from each material. Testing began at 17 minutes, because resins were not hard enough before this time to prevent strain due to manipulation, and was continued for 24 hours.
685
MOJON
ET AL
igll.Oml lglO.Bml
PAL
lg10.7ml
DUR PAL
lglO.Bml lg10.6ml
PAL
lg10.5ml
I 2
4
6
8
/ IO time
12
14
I6
I8
20
22
24
t(h)
in hours
Fig. 4. Linear shrinkage from 17 minutes to 24 hours, expressed as percent of initial length. DUR, Duralay; PAL, Palavit G.
To relate the two experiments, a mathematical formula that transformed linear shrinkage to volumetric shrinkage was used. This formula is based on two assumptions: (1) The shape of the sample is a perfect cylinder; and (2) volumetric changes occur proportionately in the same amount in the three dimensions of space. Therefore, volumetric shrinkage at any time could be evaluated with the relation: cv = Cl x [Cl2 + 3 x (Cl + l)]
in which c1
It - Li =7
where Lt = Length of the sample at time t; Li = initial length of the sample; and cv = Sv/lOO
where Sv = Volumetric shrinkage expressed in a percent of initial volume.
RESULTS Volumetric changes of the standard mixtures of the two materials are shown in Figs. 1 and 2, The volumetric shrinkage is expressed as a proportion of initial volume of 4 ml and is time related. The peak of the reaction is indicated by a solid circle. Volumetric shrinkage at 17 minutes is shown in Fig. 3. Volumetric shrinkage reached 6.5% and 5.5% for Duralay and Palavit G resins, respectively. The Student t-test revealed no statistical difference between them.
Linear shrinkage results are expressed as a proportion of the initial length measured at 17 minutes. Fig. 4 represents the relationship between time and shrinkage for the two resins and the different mixes. Shrinkage occurred mainly during the first 3 hours (approximately 0.4% linear). A few samples were measured for 48 hours. Between 24 hours and 30 hours changes continued but their values were close to the accuracy limit of the equipment. No measurable change occurred after 30 hours. Fig. 5 illustrates the graphic relationship between linear shrinkage at 24 hours and the powder-to-liquid ratios. T bars represent one standard deviation. Duncan’s multiple range test was performed on the data. Black bars on the left join the data with no statistical differences at a 0.05 level. Linear shrinkage results of the standard mixes were then transformed in volumetric shrinkage and combined with dilatometric results. The total amount of polymerization shrinkage at 24 hours for the standard mixes was 7.9 % (SD 1.4 % ) for Duralay resin and 6.5 % (SD 0.5 % ) for Palavit G resin. Moreover, we could design the curves of the changes from 2 minutes to 24 hours (Fig. 6). Eighty-percent of all the shrinkage occurred before 17 minutes for both resins, 95% before 3 hours for Duralay and 2 hours for Palavit G resins.
DISCUSSION According to the curves of volumetric records and the few extended tests, there should be little change before 2 minutes or after 24 hours. These results are in accordance with
DECEMBER
1990
VOLUME
64
NUMBER
6
POLYMERIZATION
SHRINKAGE
OF ACRYLIC
RESINS
0.0 0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
ml of liquid / lg of powder Fig. 5. Linear shrinkage as function of powder-to-liquid ratio. Vertical bars on left indicate means that do not differ at 0.05 level of confidence.
time
in hours
Fig. 6. Volumetric shrinkage over 24 hours of standard mixes, expressed as proportion of initial volumes.
results previously reported on unfilled direct-filling resins.17 It has been shown that expansion due to water sorbtion of acrylic resins is approximately 1% over a period of hoursg~ l7 If the resin contacts water before a complete set, the expansion could be even larger. The water bath was
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
chosen for this experiment because it is assumed not to interfere with the chemical reaction and because of its therma1 stability. The water sorption might produce an underestimation of volumetric shrinkage, however. No statistical difference was found at 17 minutes between the two materials. At 24 hours, the linear shrinkage
687
MOJON
of Palavit G resin was significantly less than that of Duralay resin for similar mix consistencies. Duncan’s analysis revealed also statistically significant differences between thin and thick mixes. As expected theoretically, adding more liquid to the mix increased shrinkage. The relationship was not linear. The peak of the chemical reaction gave an indication of how fast the setting was. In both resins, the peak occurred at an average time of 10 minutes and 30 seconds, although Palavit G resin was less predictable than Duralay resin as shown in Figs. 1 and 2. Most of the changes (30%) occurred before 17 minutes for both resins. Inasmuch as the measurements were made at room temperature, it is likely that the setting time would be less if measured under clinical conditions. It is apparent from this study that these resins are not dimensionally stable. Nevertheless their use for direct dowel and core patterns is popular. If it can be assumed that the occlusal portion of the canal of an endodontically treated tooth is approximately 2 mm in diameter, the space, resulting from pattern shrinkage, between the casting and the canal wall would be 10 to 30 rm, dependent on the alloy and investment. This gap is in the same range as the film thickness of luting cements.20
CLINICAL
IMPLICATIONS
When used as indexes, these materials could also be distorted during polymerization in addition to dimensional change. This is because of unequal thickness along the index as well as the shrinkage “lakes” described by Smith and Schoonover.l’ Consequently, it might be advisable to reline indexes when almost all polymerization shrinkage has occurred. The use of a mix as thick as possible will also minimize the worst effects of polymerization.
CONCLUSIONS Under the condition of this study, the following conclusions can be drawn. Acrylic resins marketed as index and pattern materials have a polymerization shrinkage of 6.5% to 7.9%. Eighty percent of the change appears before 17 minutes at room temperature. Altering the powder-to-liquid ratio by adding more liquid significantly increases the shrinkage.
668
We gratefully Richter.
acknowledge
the editorial
assistance
ET AL
of Dr W. A.
REFERENCES 1. Bartlett SO. Construction of detached core crowns for pulpless teeth in only two sittings. J Am Dent Assoc 1968;77:843-5. 2. Dewhirst RB, Fisher DW, Shillingburg HT. Dowel-core fabrication. J South Cahf State Dent Assoc 196%37:444-g. 3. Mondelli J, Piccino AC, Berbert A. An acrylic resin pattern for a cast dowel and core. J PROSTHJZT DENT 1971;25:413-7. 4. Stern N. A direct pattern technique for posts and cores. J PROSTHET DENT 1972;28:279-83.
5. Hughes HJ. Two uses of acrylic copings Dent J 1973;4:102-4. 6. Patterson JC. A technique for accurate
in restorative soldering.
dentistry.
Au&
J PROSTHET DENT
1972;28:552-556.
7. Lens E. Werkst&kundliche Untersuchungen iiber die verwendarbeit schneIlh&tender Kunstsoffe fiir Gussmodelle. Dtsch Stomat 1964; 14:321-4. 8. Craig RG. Restorative dental materials. 7th ed. St Louis: CV Mosby, 1985469. 9. Sweeney WT. Denture base material: acrylic resins. J Am Dent Assoc 1939;26:1863-73. 10. Taylor PB. Acrylic resins: their manipulation. J Am Dent Assoc 1941;28:373-87. 11. Feilzer AJ, De Gee AJ, Davidson CL. Curing contraction of composites and glass ionomer cements. J PR~~THET DENT 1988;59:297-300. 12. Chevitarese 0, Craig RG, Peyton FA. Properties of various types of denture-base plastics . J PROSTHET DENT 1962;12:711-9. 13. Lee HL, Swarm MC, Smith FF. Physical properties of four thermosetting restorative resins. J Dent Res 1969;48:526-36. 14. Hegdahl T, Gjerdet NR. Contraction stresses of composite resin filling mat&da. Acta Odontol Stand 1977;35:191-5. 15. Penn RW. A recording dilatometer for measuring polymerization shrinkage. Dent Mater 1986;2:78-9. 16. Pagniano RP, Scheid RC, Clowson RL, Dagefoerde RO, Zardiackas LD. Linear dimensional change of acrylic resins used in the fabrication of custom trays . J PROS= DENT 1982;47:279-83. 17. Smith DL, Schoonover IC. Direct tilling resins: dimensional changes resulting from polymerization and water sorption. J Am Dent Assoc 1953;&540-4. 18. Galan J Jr, Mondelli J, Viera Fonterrada D. Comparative study of dimensional changes of castings obtained from wax or acrylic resin patterns. Their influence on the fit of castings. Estomat Cult 1970;4:157-74. 19. Perriard J, Meyer J-M, Hokz J. Quatre techniques de condensation d’amalgame soumis aux tests de laboratoire. Schweis Monatsschr Zahnmed 1986;96:709-23. 20. Eames WB. Techniques to improve the seating of castings. J Am Dent 1978:96:432-7. Reprint
requests
to:
DR. PHILIPPE MESON FACULTY OF DENTISTRY UNIVEBHTY OF BRITISH COLUMBIA VANCOWER, B.C. V6T 127 CANADA
DECEMBER
IBW
VOLUME
64
NUMBER
A
shrinkage
of index and pattern
acrylic
resins
Philippe Mojon, D.M.D.,* Jean-Pierre Oberholzer, D.M.D.,** Jean-Marc Meyer, Ph.D.,*** and Urs C. Belser, D.M.D.**** University of Geneva, School of Dentistry, Geneva, Switzerland; Columbia, School of Dentistry, Vancouver, B.C., Canada
and University
of British
Inadequate dimensional stability caused by polymerization shrinkage has been reported concerning the various applications of acrylic resins. The objective of the study was to evaluate dimensional changes of two self-curing acrylic resins marketed as pattern and index material. Early volumetric changes were measured with a dilatometer and late linear changes were recorded with an inductive transducer. After 24 hours the volumetric shrinkage was 7.9% for Duralay resin and 6.6% for Palavit G resin; 80% of the change appears before 17 minutes at room temperature. Shrinkage was significantly increased when the proportion of powder in the mix was diminished. Results suggest that these resins should be used with some method to compensate for the shrinkage, when used as index material. However, the dimensional change might provide significant advantages for intracoronal castings. (J PROSTHET DENT 1990;64:684-8.)
A
crylic resins have been used in dentistry for many purposes ranging from dentures to veneers. Self-curing acrylic resins have been advocated for direct post and core patterns, occlusal registration indexes, and soldering indexes for fixed partial dentures.ls6 They have numerous advantages over the waxes, impression compound, and plaster materials. For example, the initial viscosity can be adjusted by changing the proportion of the components. The wetting ability of the mix easily provides for reproduction of small details.7 The setting reaction is compatible with intraoral usage, and the set resin material is harder than wax and less brittle than impression compound. Because these resins are methylmethacrylate based, shrinkage occurs when they set. Shrinkage of pure methylmethacrylate is about 21% . Polymerization shrinkage of dental acrylic resins is less because one part of the material is already polymerized.8 The theoretical amount of shrinkage depends on the proportion of prepolymerized material and monomer. Inert particles, added to the powder by the manufacturer, also influence dental acrylic resin. Dimensional change has been evaluated for various types of acrylic
resins
by use of several
methods
of measure-
rnentg-16 Composite resins could have a volumetric polymerization shrinkage of as much as 7.1% ,l* while the linear dimensional change of custom tray materials is between 0.05%
and 0.38%.16
*Assistant Professor, Department of Clinical Dental Sciences, University of British Columbia, School of Dentistry. **Private practice, Geneva, Switzerland. ***Professor and Head, Department of Dental Materials, Ecole de Medecine-dentaire, Geneva, Switzerland. ****Professor and Head, Department of Fixed Prosthodontics, Ecole de Medecine-dentaire, Geneva, Switzerland. 10/l/21926
684
Table
I. Powder-to-liquid
ratio used in study Powder-to-liquid b3mhl)
Mix
Duralay
Standard Thick Thin
ratio
Palavit
LO/O.8 1.010.6 l.O/l.O
G
1.010.6 1.0/0.5 1.010.7
There have been few reports dealing with resins used for indexes and patterns. In 1953, Smith and Schoonover17 measured volumetric shrinkage of unfilled direct-filling resins that may be similar to indexing resins. Their results showed a volumetric shrinkage of 6 % to 8 % . If the amount of polymerization shrinkage were the same for resins used for indexes and patterns, the dimensional stability would be compromised and the accuracy of clinical records would be questionable. In 1970 Galan et a1.18found that a type of self-curing acrylic resin was more accurate than waxes but their indirect measurement did not elicit a definitive conclusion. There seems to be a need to evaluate the dimensional change of resins used for indexes and patterns and to weigh the possible clinical significance of the polymerization effect. This study evaluated the dimensional change of resins used for indexes and patterns, analyzed the influence of powder-to-liquid ratio on dimensional change, and compared two products available on the market.
MATERIAL Two self-curing
AND
METHODS
acrylic
resins marketed
as pattern
and
index material (Duralay, Reliance Dental Mfg. Co., Worth, Ill., and Palavit G, Kulzer Co., Bad Homburg, West Germany) were selected. In both products, the powder consists
DECEMBER
1999
VOLUME
64
NUMBER
6
POLYMERIZATION
SHRINKAGE
OF ACRYLIC
RESINS
%
t
PALAVIT 1 g/0.6ml
7.
a, :6. Y .E i 5.
G
.ou)4. L z 3 . E ;2. > I _
I
3
5
time
7
9 II in minutes
13
15
I7
t(min)
Fig. 1. Early volumetric shrinkage of Duralay resin standard mix, expressed as percent of initial volume.
of poly(methylmethacrylate) and initiator and the liquid contains monomer and activator. There are other substances present but not disclosed by the manufacturers. Because no instructions were available on the powder to liquid (P/L) ratio for either material, a standard mix was developed for each, to produce a consistency that was clinically manageable after 45 seconds of mixing. To evaluate the influence of P/L ratio on dimensional change, two other ratios were also tested for each resin. The ratios were varied by changing the amount of liquid, while the weight of powder remained constant. Therefore mixtures of different consistency (Table I) were used, each of which would have been clinically acceptable. Two types of tests were conducted because large changes were expected to occur initially and small changes later. The first measurements were made with a dilatometer and ended 17 minutes after the beginning of mixing. For the second measurement, a transducer recorded the linear change from 17 minutes to 24 hours or more. The timing of the two experiments was determined on the basis of preliminary trials. The dilatometer, which recorded volumetric changes, consisted of a glass specimen chamber (100 X 30 mm) connected to a 1 ml pipette filled with distilled water. The receptable was closed with a glass plate coated with white petroleum jelly. The specimen consisted of 4 ml of resin injected with a graduate syringe into the dilatometer. Change of water level in the pipette offered a view of the change in volume within the receptacle. The reading started 2 minutes after the beginning of mixing and continued for 15 minutes. The experiment was repeated five or six times. This method failed to demonstrate differences among mixes during preliminary trials, and only tests of standard mixes were completed. The equipment for the second experiment was initially developed to measure dimensional changes of amalgam.lg A micrometer was used to measure the initial length of a
TEE
JOURNAL
OF PROSTHETIC
DENTISTRY
I
3
5
7 time
9 II in minutes
I3
15
I7
t(min)
Fig. 2. Early volumetric shrinkage of Palavit G resin standard mix, expressed as percent of initial volume.
8-
6-
4-
2-
OFig. 3. Mean volumetric shrinkage at 17 minutes, expressed as percent of initial volume.
cylinder of resin, and change in dimension was recorded with a linear inductive transducer mounted on the other side of the cylinder. A graphic recorder was connected to the amplifier of the transducer. Accuracy of the measurement was in a range of 1 pm and stability of the equipment was checked for several days. The temperature around the sample was kept constant. Ten cylindrical samples, 8 mm long and 6 mm in diameter, were produced from each material. Testing began at 17 minutes, because resins were not hard enough before this time to prevent strain due to manipulation, and was continued for 24 hours.
685
MOJON
ET AL
igll.Oml lglO.Bml
PAL
lg10.7ml
DUR PAL
lglO.Bml lg10.6ml
PAL
lg10.5ml
I 2
4
6
8
/ IO time
12
14
I6
I8
20
22
24
t(h)
in hours
Fig. 4. Linear shrinkage from 17 minutes to 24 hours, expressed as percent of initial length. DUR, Duralay; PAL, Palavit G.
To relate the two experiments, a mathematical formula that transformed linear shrinkage to volumetric shrinkage was used. This formula is based on two assumptions: (1) The shape of the sample is a perfect cylinder; and (2) volumetric changes occur proportionately in the same amount in the three dimensions of space. Therefore, volumetric shrinkage at any time could be evaluated with the relation: cv = Cl x [Cl2 + 3 x (Cl + l)]
in which c1
It - Li =7
where Lt = Length of the sample at time t; Li = initial length of the sample; and cv = Sv/lOO
where Sv = Volumetric shrinkage expressed in a percent of initial volume.
RESULTS Volumetric changes of the standard mixtures of the two materials are shown in Figs. 1 and 2, The volumetric shrinkage is expressed as a proportion of initial volume of 4 ml and is time related. The peak of the reaction is indicated by a solid circle. Volumetric shrinkage at 17 minutes is shown in Fig. 3. Volumetric shrinkage reached 6.5% and 5.5% for Duralay and Palavit G resins, respectively. The Student t-test revealed no statistical difference between them.
Linear shrinkage results are expressed as a proportion of the initial length measured at 17 minutes. Fig. 4 represents the relationship between time and shrinkage for the two resins and the different mixes. Shrinkage occurred mainly during the first 3 hours (approximately 0.4% linear). A few samples were measured for 48 hours. Between 24 hours and 30 hours changes continued but their values were close to the accuracy limit of the equipment. No measurable change occurred after 30 hours. Fig. 5 illustrates the graphic relationship between linear shrinkage at 24 hours and the powder-to-liquid ratios. T bars represent one standard deviation. Duncan’s multiple range test was performed on the data. Black bars on the left join the data with no statistical differences at a 0.05 level. Linear shrinkage results of the standard mixes were then transformed in volumetric shrinkage and combined with dilatometric results. The total amount of polymerization shrinkage at 24 hours for the standard mixes was 7.9 % (SD 1.4 % ) for Duralay resin and 6.5 % (SD 0.5 % ) for Palavit G resin. Moreover, we could design the curves of the changes from 2 minutes to 24 hours (Fig. 6). Eighty-percent of all the shrinkage occurred before 17 minutes for both resins, 95% before 3 hours for Duralay and 2 hours for Palavit G resins.
DISCUSSION According to the curves of volumetric records and the few extended tests, there should be little change before 2 minutes or after 24 hours. These results are in accordance with
DECEMBER
1990
VOLUME
64
NUMBER
6
POLYMERIZATION
SHRINKAGE
OF ACRYLIC
RESINS
0.0 0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
ml of liquid / lg of powder Fig. 5. Linear shrinkage as function of powder-to-liquid ratio. Vertical bars on left indicate means that do not differ at 0.05 level of confidence.
time
in hours
Fig. 6. Volumetric shrinkage over 24 hours of standard mixes, expressed as proportion of initial volumes.
results previously reported on unfilled direct-filling resins.17 It has been shown that expansion due to water sorbtion of acrylic resins is approximately 1% over a period of hoursg~ l7 If the resin contacts water before a complete set, the expansion could be even larger. The water bath was
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
chosen for this experiment because it is assumed not to interfere with the chemical reaction and because of its therma1 stability. The water sorption might produce an underestimation of volumetric shrinkage, however. No statistical difference was found at 17 minutes between the two materials. At 24 hours, the linear shrinkage
687
MOJON
of Palavit G resin was significantly less than that of Duralay resin for similar mix consistencies. Duncan’s analysis revealed also statistically significant differences between thin and thick mixes. As expected theoretically, adding more liquid to the mix increased shrinkage. The relationship was not linear. The peak of the chemical reaction gave an indication of how fast the setting was. In both resins, the peak occurred at an average time of 10 minutes and 30 seconds, although Palavit G resin was less predictable than Duralay resin as shown in Figs. 1 and 2. Most of the changes (30%) occurred before 17 minutes for both resins. Inasmuch as the measurements were made at room temperature, it is likely that the setting time would be less if measured under clinical conditions. It is apparent from this study that these resins are not dimensionally stable. Nevertheless their use for direct dowel and core patterns is popular. If it can be assumed that the occlusal portion of the canal of an endodontically treated tooth is approximately 2 mm in diameter, the space, resulting from pattern shrinkage, between the casting and the canal wall would be 10 to 30 rm, dependent on the alloy and investment. This gap is in the same range as the film thickness of luting cements.20
CLINICAL
IMPLICATIONS
When used as indexes, these materials could also be distorted during polymerization in addition to dimensional change. This is because of unequal thickness along the index as well as the shrinkage “lakes” described by Smith and Schoonover.l’ Consequently, it might be advisable to reline indexes when almost all polymerization shrinkage has occurred. The use of a mix as thick as possible will also minimize the worst effects of polymerization.
CONCLUSIONS Under the condition of this study, the following conclusions can be drawn. Acrylic resins marketed as index and pattern materials have a polymerization shrinkage of 6.5% to 7.9%. Eighty percent of the change appears before 17 minutes at room temperature. Altering the powder-to-liquid ratio by adding more liquid significantly increases the shrinkage.
668
We gratefully Richter.
acknowledge
the editorial
assistance
ET AL
of Dr W. A.
REFERENCES 1. Bartlett SO. Construction of detached core crowns for pulpless teeth in only two sittings. J Am Dent Assoc 1968;77:843-5. 2. Dewhirst RB, Fisher DW, Shillingburg HT. Dowel-core fabrication. J South Cahf State Dent Assoc 196%37:444-g. 3. Mondelli J, Piccino AC, Berbert A. An acrylic resin pattern for a cast dowel and core. J PROSTHJZT DENT 1971;25:413-7. 4. Stern N. A direct pattern technique for posts and cores. J PROSTHET DENT 1972;28:279-83.
5. Hughes HJ. Two uses of acrylic copings Dent J 1973;4:102-4. 6. Patterson JC. A technique for accurate
in restorative soldering.
dentistry.
Au&
J PROSTHET DENT
1972;28:552-556.
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DR. PHILIPPE MESON FACULTY OF DENTISTRY UNIVEBHTY OF BRITISH COLUMBIA VANCOWER, B.C. V6T 127 CANADA
DECEMBER
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