Tear strength of elastomeric impression materials Thomas W. Herfort, B.S.E., M.Sc.,* William W. Gerberich, Ph.D., M.S.,** Christopher W. Macosko, Ph.D., M.S.,*** and Richard J. Goodkind, D.M.D., M.S.**** University of Minnesota, Minneapolis, Minn.
Impressions are not always successful due to fracture of the impression rubber at the margins (Fig. 1). This difficulty is especially encountered in multiple preparation impressions. Tensile strength and tear resistance of the elastic impression materials are important factors in determining whether an impression is a Success or failure. The use of stronger impression rubbers would presumably lead to higher success rates, resulting in better impressions, reduced patient discomfort, and reduced injury to the gingival tissues. Rivlin and Thomas ~ developed the criteria that are currently used to study tear strength. They introduced the simple extension tear test piece (Fig. 2) which was later adapted to the investigation of dental impression materials by Webber and Ryge. 2 The simple extension tear test piece was employed by Braden 3 to evaluate the tear strengths of a silicone, a polysulfide, and an irreversible hydrocolloid impression material. His results showed that the polysulfide rubber was twice as strong as the silicone system, which in turn was twice as strong as the irreversible hydrocolloid. He also found that increased tear rates resulted in greater tear strengths. Crosslinking kinetics for the polysulfide and silicone systems have been studied by Elliot and Braden?' 5 Their results, coupled with laboratory determinations, showed that these materials reach a
Read before the Midwest Academy of Prosthodontics, Chicago, Ill. *Dental student, University of Pennsylvania; formerly graduate student, Department of Chemical Engineering and Materials Science. **Professor, Department of Chemical Engineering and Materials Science. ***Associate Professor, Department of Chemical Engineering and Materials Science. ****Professor and Director of Graduate Prosthodontics, School of Dentistry.
THE JOURNAL OF PROSTHETIC DENTISTRY
maximum elastic modulus 10 to 15 minutes after ~ mixing. I t was not clear whether tear strength reached a similar maximum within such a period. This investigation was proposed to evaluate tear resistance of commercial elastomeric impression materials and to introduce guidelines for their use so that maximum tear strengths will be obtained in practice. MATERIALS A N D M E T H O D S Three synthetic types of impression materials are currently marketed for use in dentistry: polysulfide, silicone, and polyether rubbers. The compositions of these products were characterized by Skinner 6 and Braden?. 5 The filler materials were determined by Herfort and associates. 7 In all cases commercial samples were mixed according to the manufacturers' recommendations. Modulus measurements during crosslinking. All base and catalyst materials were weighed and spatulated before testing procedures commenced. Mixing was done at 25 ~ C. A two-plate assembly was used with a Rheometrics mechanical spectrometer*; the upper spindle was driven and the lower spindle rotated freely. To simulate actual curing conditions the test was carried out in an oven set at 37 ~ C. Spatulation for 45 seconds was followed immediately by placing of the mixture in the test fixture. Data were recorded on a chart recorder from which shear modulus vs. time could be deduced. Tear testing. Commercial samples were mixed by weight according to the manufacturers' recommendations. A 1/3z-inch-thick copper sheet with a 3/4 • 6 inch cutout was placed between two glass sheets to serve as a mold for each specimen. A silicone spray was applied to the glass slab to facilitate release of the test specimens upon their polymerization. Samples were spatulated for 45 seconds and trans*Rheometries, Inc., Union, N. J.
59
HERFORT ET AL
FORCE
OF
to I n s t r o n Load Ceil
REMOVAL
b i
~\\\\\\\\\\\\\\xx..~
TRAY f
iMPRESSION CROWN AREA
MATERIALS
Environmentally Controlled Temperature -Chamber
PREPARATION OF
MARGINAL
. . . . . . . . . . . . . .
Clevis
TEARS \'w~4~,l I~I1'1I~,~
Tear Specimen -
.-4 Grips ._I~--Knurled
Fig. 1. Marginal tears: Cross-section of an impression m the oral cavity.
Thermometer
-
F ! to C r o s s - h e a d s
Fig. 3. Tear test piece in the lnstron tester.
L
Shear Modulus During Cure
Fig. 2. Simple extension tear test piece. ferred to the mold. T o s i m u l a t e c o n d i t i o n s in the m o u t h the specimens were cured in a 37 ~ C water bath. L o a d i n g the m o l d took 2 minutes, a n d curing in the w a t e r b a t h was c o n t i n u e d for 6 minutes. T h e time from initial s p a t u l a t i o n to testing was r e c o r d e d for later analysis. All specimens were simple extension tear test pieces e x t e n d e d at a rate of 2 inches per m i n u t e in an Instron* test a p p a r a t u s (Fig. 3). Extension r a u o s were d e t e r m i n e d b y h a n g i n g a weight e q u i v a l e n t to the tear force onto one of the legs. Thicknesses were m e a s u r e d with a m i c r o m e t e r . All calculations of tear energy were based on the equation2: T ( F / t ) (2t + t) where T - tear energy, t = s a m p l e thickness, a n d -the extension ratio. Test t e m p e r a t u r e s were m a i n t a i n e d at 37 ~ C in an oven to c o r r e s p o n d to conditions in the m o u t h (since, as d e t e r m i n e d b y Braden, 3 tear energy is t e m p e r a t u r e d e p e n d e n t ) . -
-
RESULTS T h e d a t a d e p i c t i n g the increase in shear m o d u l u s d u r i n g cure are given in Fig. 4. A l t h o u g h the results *Instron Corporation, Canton, Mass.
60
P
J
/ l
o
/
4 :E
Materials, 9 eoe-flex Light 9 Xantooren Blue
/"
o
O0
2
a
6
B
I
0
12
r
t
14
Time. minutes
Fig. 4. Increase in shear modulus during cure. b e y o n d 15 m i n u t e s are n o t shown it is a p p a r e n t from the e x p e r i m e n t a l d a t a t h a t the m o d u l u s for the two elastomers reached a p e a k after 15 m i n u t e s of curing. After this time the increase in strength was insignificant, being on the o r d e r of 1 or 2 percent. T h e tear strength d e t e r m i n a t i o n s for the c o m m e r cial materials tested are shown in T a b l e I. T h e results show t h a t the polysulfide r u b b e r s tested were considerably stronger t h a n the silicones. T h e polyether h a d a tear strength similar to t h a t of the silicone specimens. A l t h o u g h no m e n t i o n of t i m e is m a d e in T a b l e I the tear tests were timed, a n d it was f o u n d t h a t the tear strength for each m a t e r i a l tested r e a c h e d a
JANUARY 1978
VOLUME 39
NUMBER 1
TEAR STRENGTH OF ELASTOMERIC IMPRESSION MATERIALS
Table
I. Results of tear testing of c o m m e r c i a l products (simple e x t e n s i o n tear test piece)*
Material
Sulfides Coe-flex light Coe-flex regular Coe-flex heavy Omniflex Permlastic light Permlastic regular Permlastic heavy Silicones Syringe Elasticon Heavy bodied Elasticon Citricon Wash Xantopren blue (light bodied) Xantopren green (medium bodied) Polyether Impregum
No. of samples
Mean tear energy (ergs/cm 2)
S.D.
Coefficient of variation
8 7 6 7 6 7 7
2.63 x 2.97 x 6.8 x 6.4 X 1.09 x 1.54 x 2.22 x
106 106 106 106 106 10~ 106
0.18 X 106 0.11 X 106 1.00 X 106 0.88 X 106 0.06 X 10o 0.12 X 10~ 0.14 X 106
0.068 0.037 0.147 0.137 0.055 0.078 0.063
7 7 7 7 7
0.39 x 1.15 x 0.44 x 0.52 x 0.50 x
106 106 100 106 106
0.30 X 0.17 X 0.24 X 0.52 X 0.56 X
10~ 106 105 105 10~
0.077 0.148 0.055 0.100 0.112
7
0.64 x 106
0.62 • 10~
0.097
*Tear propagation rate = 1 inch per minute; test temperature = 37~ C. m a x i m u m u p to 10 m i n u t e s after the i n i t i a t i o n of mixing. This suggests that the tear strength of a n impression material reaches its m a x i m u m at approximately the same time as the shear m o d u l u s . M o d e r n theories of r u b b e r strength support this evidence. DISCUSSION It is evident from Fig. 4 that the m a n u f a c t u r e r s ' r e c o m m e n d a t i o n s c o n c e r n i n g the c u r i n g time for the two materials tested were sound. These elastomers h a d essentially reached their m a x i m u m shear m o d u l u s after 10 m i n u t e s of c u r i n g time, with negligible increase after this time. It is also a p p a r e n t that the polysulfide sample had nearly twice the shear m o d u l u s of the silicone a n d that the sulfides exhibited higher shear m o d u l i t h a n the silicones. Braden a n d associates s reported that the polyether material they tested had a shear m o d u l u s nearly four times as high as that of the silicone they tested. T h e y also reported that shear m o d u l u s increased r a p i d l y for the polysulfides in this sequence: light bodied to regular bodied to heavy bodied. It has been postulated that there is a direct relationship between shear m o d u l u s a n d difficulty in removing a n impression from the m o u t h . This idea is suggested by elastomer theory. O n these grounds it was d e t e r m i n e d that the ease of r e m o v i n g a n impression from the m o u t h would be in the following order: light bodied silicones, the easiest; then light bodied polysulfides, heavy bodied silicones, heavy bodied polysulfides, a n d polyethers.
THE JOURNAL OF PROSTHETIC DENTISTRY
Fracture mechanics theory suggests t h a t failure of a n impression is due to a complex network of stresses s u r r o u n d i n g a weak p o i n t in the elastomer. A material that requires a greater force to remove it from the m o u t h would necessarily have higher stresses in critical areas a n d would tend to fail more readily. Therefore m o d u l u s of elasticity has a n effect on clinical resistance to the i n i t i a t i o n of tearing. T h e small s t a n d a r d deviations shown in T a b l e I suggest that the d a t a is reliable a n d that some of the differences between the materials tested are significant. In general the polysulfides were three to six times as strong in resisting tearing as the silicones. Coe-flex light* showed a strength seven times greater t h a n that of Syringe Elasticon.'~ Impregum:~ demonstrated a slightly higher tear strength t h a n the light bodied silicones b u t was weaker t h a n heavy bodied Elasticon.J" T h e Coe-flex polysulfides displayed nearly twice the tear strength of the Permlastic types. This difference is a t t r i b u t e d to differences in filler types, which were m e n t i o n e d in a previous s t u d y / C o e - f l e x heavy a n d Omniflex* showed the highest tear strengths of all the materials tested. T h e strength of Coe-flex heavy is due to its high filler content, which also tends to increase its initial viscosity to such a point that its use in syringing a p p l i c a t i o n s is *Coe Laboratories, Inc., Chicago, Ill. tKerr Mfg. Co., Romulus, Mich. :~Premier Dental Products Co., Philadelphia, Pa.
61
HERFORT ET AL
contraindicated. T h e high tear strength of O m n i f l e x is a t t r i b u t e d to a t i t a n i u m dioxide filler a n d a n u n k n o w n filler type which also gave it u n i q u e viscosity characteristics. Some dentists use the regular bodied polysulfides for syringing applications. A previous investigation7 disclosed the high viscosity of Coe-flex regular w h e n compared with Coe-flex light impression material. This idea. coupled with the increased shear m o d u l u s of the regular base product, led to the prediction that m a n i p u l a t i o n of the regular base p r o d u c t would be more difficult t h a n m a n i p u l a t i o n of the light base: a n d yet the tear strength was e n h a n c e d by only 13%. Therefore the use of regular base to replace light base for increased strength in syringing applications is not recommended. T h e silicone materials tested were of similar tear strength, although the heavy bodied Elasticon showed nearly a three-fold increase in tear strength over the lighter based silicones. H e a v y bodied Elasticon was m u c h too viscous to be applied with a syringe. T h e differences in tear strengths of the various light bodied silicones tested are not considered clinically significant. It was stated in a previous investigation7 that I m p r e g u m ta polyether) h a d excellent viscosity characteristics for m a n i p u l a t i o n as a n elastomeric impression material. It was found, however, that it had a high shear m o d u l u s a n d mediocre tear resistance. This suggests that it could exhibit low clinical tear values w h e n c o m p a r e d to the other materials. CONCLUSIONS
2. T h e polysulfide materials showed three to six times the tear strength of the silicones. 3. T h e polyether material displayed a tear resistance slightly higher t h a n that of the silicones b u t one third to one fifth as high as that of the polysulfides. 4. T h e clinical significance of the differences in tear strength was u n d e t e r m i n e d since other factors (such as adhesion) were not considered. REFERENCES 1. Rivlin, R, S., and Thomas, A. G.: Rupture of rubber 1. Characteristic energy for tearing. J Poly Sci 10:291, 1953. 2. Webber, R. L., and Ryge, G.: The determination of tear energy of extensible materials Of dental interest. J Biomed Mat Res 2:231, 1968. 3. Braden, M.: Characterization of the rupture properties of impression materials. Dent Prac 14:67, 1963. 4. Braden,M., and Elliot, J. C.: Characterization of the setting process of silicone dental rubbers. J Dent Res 45:1016, 1966. 5. Braden,M.: Characterization of the setting wocess in dental polysulfide rubbers. J Dent Res 45:1065, 1966. 6. Skinner, E. W.: The properties and manipulation of Mercaptan and siliconebase impressionmaterials. Dent Clin North Am, Nov. 1958, p 685. 7. Herfort,T. W., Gerberich, W., Macosko, C., and Goodkind, R.: Viscosity of elastomeric impression materials. J PROSTHETDENT38:396, 1977. 8. Braden, M., Causton, B., and Clarke, R. L.: A polyether impression rubber. J Dent Res 51:889, 1972. Reprint requests to:
DR. RICHARDJ, GOODKIND 9-176C HEALTHSCIENCESBLDG.,UNITA SCHOOLOFDENTISTRY,GRADUATEPROSTHODONTICS UNIVERSITYOF MINNESOTA MINNEAPOLIS,MINN. 55455
1. U n d e r simulated clinical conditions the maxim u m tear strength of those materials tested was reached in 10 to 15 minutes.
62
JANUARY 1978
VOLUME 39
NUMBER 1
Impressions are not always successful due to fracture of the impression rubber at the margins (Fig. 1). This difficulty is especially encountered in multiple preparation impressions. Tensile strength and tear resistance of the elastic impression materials are important factors in determining whether an impression is a Success or failure. The use of stronger impression rubbers would presumably lead to higher success rates, resulting in better impressions, reduced patient discomfort, and reduced injury to the gingival tissues. Rivlin and Thomas ~ developed the criteria that are currently used to study tear strength. They introduced the simple extension tear test piece (Fig. 2) which was later adapted to the investigation of dental impression materials by Webber and Ryge. 2 The simple extension tear test piece was employed by Braden 3 to evaluate the tear strengths of a silicone, a polysulfide, and an irreversible hydrocolloid impression material. His results showed that the polysulfide rubber was twice as strong as the silicone system, which in turn was twice as strong as the irreversible hydrocolloid. He also found that increased tear rates resulted in greater tear strengths. Crosslinking kinetics for the polysulfide and silicone systems have been studied by Elliot and Braden?' 5 Their results, coupled with laboratory determinations, showed that these materials reach a
Read before the Midwest Academy of Prosthodontics, Chicago, Ill. *Dental student, University of Pennsylvania; formerly graduate student, Department of Chemical Engineering and Materials Science. **Professor, Department of Chemical Engineering and Materials Science. ***Associate Professor, Department of Chemical Engineering and Materials Science. ****Professor and Director of Graduate Prosthodontics, School of Dentistry.
THE JOURNAL OF PROSTHETIC DENTISTRY
maximum elastic modulus 10 to 15 minutes after ~ mixing. I t was not clear whether tear strength reached a similar maximum within such a period. This investigation was proposed to evaluate tear resistance of commercial elastomeric impression materials and to introduce guidelines for their use so that maximum tear strengths will be obtained in practice. MATERIALS A N D M E T H O D S Three synthetic types of impression materials are currently marketed for use in dentistry: polysulfide, silicone, and polyether rubbers. The compositions of these products were characterized by Skinner 6 and Braden?. 5 The filler materials were determined by Herfort and associates. 7 In all cases commercial samples were mixed according to the manufacturers' recommendations. Modulus measurements during crosslinking. All base and catalyst materials were weighed and spatulated before testing procedures commenced. Mixing was done at 25 ~ C. A two-plate assembly was used with a Rheometrics mechanical spectrometer*; the upper spindle was driven and the lower spindle rotated freely. To simulate actual curing conditions the test was carried out in an oven set at 37 ~ C. Spatulation for 45 seconds was followed immediately by placing of the mixture in the test fixture. Data were recorded on a chart recorder from which shear modulus vs. time could be deduced. Tear testing. Commercial samples were mixed by weight according to the manufacturers' recommendations. A 1/3z-inch-thick copper sheet with a 3/4 • 6 inch cutout was placed between two glass sheets to serve as a mold for each specimen. A silicone spray was applied to the glass slab to facilitate release of the test specimens upon their polymerization. Samples were spatulated for 45 seconds and trans*Rheometries, Inc., Union, N. J.
59
HERFORT ET AL
FORCE
OF
to I n s t r o n Load Ceil
REMOVAL
b i
~\\\\\\\\\\\\\\xx..~
TRAY f
iMPRESSION CROWN AREA
MATERIALS
Environmentally Controlled Temperature -Chamber
PREPARATION OF
MARGINAL
. . . . . . . . . . . . . .
Clevis
TEARS \'w~4~,l I~I1'1I~,~
Tear Specimen -
.-4 Grips ._I~--Knurled
Fig. 1. Marginal tears: Cross-section of an impression m the oral cavity.
Thermometer
-
F ! to C r o s s - h e a d s
Fig. 3. Tear test piece in the lnstron tester.
L
Shear Modulus During Cure
Fig. 2. Simple extension tear test piece. ferred to the mold. T o s i m u l a t e c o n d i t i o n s in the m o u t h the specimens were cured in a 37 ~ C water bath. L o a d i n g the m o l d took 2 minutes, a n d curing in the w a t e r b a t h was c o n t i n u e d for 6 minutes. T h e time from initial s p a t u l a t i o n to testing was r e c o r d e d for later analysis. All specimens were simple extension tear test pieces e x t e n d e d at a rate of 2 inches per m i n u t e in an Instron* test a p p a r a t u s (Fig. 3). Extension r a u o s were d e t e r m i n e d b y h a n g i n g a weight e q u i v a l e n t to the tear force onto one of the legs. Thicknesses were m e a s u r e d with a m i c r o m e t e r . All calculations of tear energy were based on the equation2: T ( F / t ) (2t + t) where T - tear energy, t = s a m p l e thickness, a n d -the extension ratio. Test t e m p e r a t u r e s were m a i n t a i n e d at 37 ~ C in an oven to c o r r e s p o n d to conditions in the m o u t h (since, as d e t e r m i n e d b y Braden, 3 tear energy is t e m p e r a t u r e d e p e n d e n t ) . -
-
RESULTS T h e d a t a d e p i c t i n g the increase in shear m o d u l u s d u r i n g cure are given in Fig. 4. A l t h o u g h the results *Instron Corporation, Canton, Mass.
60
P
J
/ l
o
/
4 :E
Materials, 9 eoe-flex Light 9 Xantooren Blue
/"
o
O0
2
a
6
B
I
0
12
r
t
14
Time. minutes
Fig. 4. Increase in shear modulus during cure. b e y o n d 15 m i n u t e s are n o t shown it is a p p a r e n t from the e x p e r i m e n t a l d a t a t h a t the m o d u l u s for the two elastomers reached a p e a k after 15 m i n u t e s of curing. After this time the increase in strength was insignificant, being on the o r d e r of 1 or 2 percent. T h e tear strength d e t e r m i n a t i o n s for the c o m m e r cial materials tested are shown in T a b l e I. T h e results show t h a t the polysulfide r u b b e r s tested were considerably stronger t h a n the silicones. T h e polyether h a d a tear strength similar to t h a t of the silicone specimens. A l t h o u g h no m e n t i o n of t i m e is m a d e in T a b l e I the tear tests were timed, a n d it was f o u n d t h a t the tear strength for each m a t e r i a l tested r e a c h e d a
JANUARY 1978
VOLUME 39
NUMBER 1
TEAR STRENGTH OF ELASTOMERIC IMPRESSION MATERIALS
Table
I. Results of tear testing of c o m m e r c i a l products (simple e x t e n s i o n tear test piece)*
Material
Sulfides Coe-flex light Coe-flex regular Coe-flex heavy Omniflex Permlastic light Permlastic regular Permlastic heavy Silicones Syringe Elasticon Heavy bodied Elasticon Citricon Wash Xantopren blue (light bodied) Xantopren green (medium bodied) Polyether Impregum
No. of samples
Mean tear energy (ergs/cm 2)
S.D.
Coefficient of variation
8 7 6 7 6 7 7
2.63 x 2.97 x 6.8 x 6.4 X 1.09 x 1.54 x 2.22 x
106 106 106 106 106 10~ 106
0.18 X 106 0.11 X 106 1.00 X 106 0.88 X 106 0.06 X 10o 0.12 X 10~ 0.14 X 106
0.068 0.037 0.147 0.137 0.055 0.078 0.063
7 7 7 7 7
0.39 x 1.15 x 0.44 x 0.52 x 0.50 x
106 106 100 106 106
0.30 X 0.17 X 0.24 X 0.52 X 0.56 X
10~ 106 105 105 10~
0.077 0.148 0.055 0.100 0.112
7
0.64 x 106
0.62 • 10~
0.097
*Tear propagation rate = 1 inch per minute; test temperature = 37~ C. m a x i m u m u p to 10 m i n u t e s after the i n i t i a t i o n of mixing. This suggests that the tear strength of a n impression material reaches its m a x i m u m at approximately the same time as the shear m o d u l u s . M o d e r n theories of r u b b e r strength support this evidence. DISCUSSION It is evident from Fig. 4 that the m a n u f a c t u r e r s ' r e c o m m e n d a t i o n s c o n c e r n i n g the c u r i n g time for the two materials tested were sound. These elastomers h a d essentially reached their m a x i m u m shear m o d u l u s after 10 m i n u t e s of c u r i n g time, with negligible increase after this time. It is also a p p a r e n t that the polysulfide sample had nearly twice the shear m o d u l u s of the silicone a n d that the sulfides exhibited higher shear m o d u l i t h a n the silicones. Braden a n d associates s reported that the polyether material they tested had a shear m o d u l u s nearly four times as high as that of the silicone they tested. T h e y also reported that shear m o d u l u s increased r a p i d l y for the polysulfides in this sequence: light bodied to regular bodied to heavy bodied. It has been postulated that there is a direct relationship between shear m o d u l u s a n d difficulty in removing a n impression from the m o u t h . This idea is suggested by elastomer theory. O n these grounds it was d e t e r m i n e d that the ease of r e m o v i n g a n impression from the m o u t h would be in the following order: light bodied silicones, the easiest; then light bodied polysulfides, heavy bodied silicones, heavy bodied polysulfides, a n d polyethers.
THE JOURNAL OF PROSTHETIC DENTISTRY
Fracture mechanics theory suggests t h a t failure of a n impression is due to a complex network of stresses s u r r o u n d i n g a weak p o i n t in the elastomer. A material that requires a greater force to remove it from the m o u t h would necessarily have higher stresses in critical areas a n d would tend to fail more readily. Therefore m o d u l u s of elasticity has a n effect on clinical resistance to the i n i t i a t i o n of tearing. T h e small s t a n d a r d deviations shown in T a b l e I suggest that the d a t a is reliable a n d that some of the differences between the materials tested are significant. In general the polysulfides were three to six times as strong in resisting tearing as the silicones. Coe-flex light* showed a strength seven times greater t h a n that of Syringe Elasticon.'~ Impregum:~ demonstrated a slightly higher tear strength t h a n the light bodied silicones b u t was weaker t h a n heavy bodied Elasticon.J" T h e Coe-flex polysulfides displayed nearly twice the tear strength of the Permlastic types. This difference is a t t r i b u t e d to differences in filler types, which were m e n t i o n e d in a previous s t u d y / C o e - f l e x heavy a n d Omniflex* showed the highest tear strengths of all the materials tested. T h e strength of Coe-flex heavy is due to its high filler content, which also tends to increase its initial viscosity to such a point that its use in syringing a p p l i c a t i o n s is *Coe Laboratories, Inc., Chicago, Ill. tKerr Mfg. Co., Romulus, Mich. :~Premier Dental Products Co., Philadelphia, Pa.
61
HERFORT ET AL
contraindicated. T h e high tear strength of O m n i f l e x is a t t r i b u t e d to a t i t a n i u m dioxide filler a n d a n u n k n o w n filler type which also gave it u n i q u e viscosity characteristics. Some dentists use the regular bodied polysulfides for syringing applications. A previous investigation7 disclosed the high viscosity of Coe-flex regular w h e n compared with Coe-flex light impression material. This idea. coupled with the increased shear m o d u l u s of the regular base product, led to the prediction that m a n i p u l a t i o n of the regular base p r o d u c t would be more difficult t h a n m a n i p u l a t i o n of the light base: a n d yet the tear strength was e n h a n c e d by only 13%. Therefore the use of regular base to replace light base for increased strength in syringing applications is not recommended. T h e silicone materials tested were of similar tear strength, although the heavy bodied Elasticon showed nearly a three-fold increase in tear strength over the lighter based silicones. H e a v y bodied Elasticon was m u c h too viscous to be applied with a syringe. T h e differences in tear strengths of the various light bodied silicones tested are not considered clinically significant. It was stated in a previous investigation7 that I m p r e g u m ta polyether) h a d excellent viscosity characteristics for m a n i p u l a t i o n as a n elastomeric impression material. It was found, however, that it had a high shear m o d u l u s a n d mediocre tear resistance. This suggests that it could exhibit low clinical tear values w h e n c o m p a r e d to the other materials. CONCLUSIONS
2. T h e polysulfide materials showed three to six times the tear strength of the silicones. 3. T h e polyether material displayed a tear resistance slightly higher t h a n that of the silicones b u t one third to one fifth as high as that of the polysulfides. 4. T h e clinical significance of the differences in tear strength was u n d e t e r m i n e d since other factors (such as adhesion) were not considered. REFERENCES 1. Rivlin, R, S., and Thomas, A. G.: Rupture of rubber 1. Characteristic energy for tearing. J Poly Sci 10:291, 1953. 2. Webber, R. L., and Ryge, G.: The determination of tear energy of extensible materials Of dental interest. J Biomed Mat Res 2:231, 1968. 3. Braden, M.: Characterization of the rupture properties of impression materials. Dent Prac 14:67, 1963. 4. Braden,M., and Elliot, J. C.: Characterization of the setting process of silicone dental rubbers. J Dent Res 45:1016, 1966. 5. Braden,M.: Characterization of the setting wocess in dental polysulfide rubbers. J Dent Res 45:1065, 1966. 6. Skinner, E. W.: The properties and manipulation of Mercaptan and siliconebase impressionmaterials. Dent Clin North Am, Nov. 1958, p 685. 7. Herfort,T. W., Gerberich, W., Macosko, C., and Goodkind, R.: Viscosity of elastomeric impression materials. J PROSTHETDENT38:396, 1977. 8. Braden, M., Causton, B., and Clarke, R. L.: A polyether impression rubber. J Dent Res 51:889, 1972. Reprint requests to:
DR. RICHARDJ, GOODKIND 9-176C HEALTHSCIENCESBLDG.,UNITA SCHOOLOFDENTISTRY,GRADUATEPROSTHODONTICS UNIVERSITYOF MINNESOTA MINNEAPOLIS,MINN. 55455
1. U n d e r simulated clinical conditions the maxim u m tear strength of those materials tested was reached in 10 to 15 minutes.
62
JANUARY 1978
VOLUME 39
NUMBER 1
