- Email: [email protected]
Early shear and tensile strength of composite to etched alloy
Early shear and tensile strength of composite to etched alloy*
P, S. H a s i a k o s 1, J. J. KoelbP, L. W. L a u b 2, J. L. Sandrik 2 Departments of ~OperativeDentistry, 2Dental Materials, Loyola University of Chicago, School of Dentistry Maywood, USA
Hasiakos PS, Koelbl J J, Laub LW, Sandrik JL. Early shear and tensile strength of composite to etched alloy. Dent Mater 1986: 2: 284287. Abstract. - Design of test configurations to measure interracial strengths between composite luting cement and etched alloys in shear and in tension, was the major objective of this work. Previous investigations of bond strengths in these systems have been primarily restricted to tensile testing. The test systems developed in this study were used to evaluate bond strengths between two brands of composite luting cement and one non-precious casting alloy. No statistically significant difference was found between the combination of products tested in either the shear or tensile modes.
Rochette (1) described a method for splinting mandibular anterior teeth by bonding a cast alloy framework to enamel with unfilled acrylic resin. Bonding was achieved by mechanical retention of the resin to perforations in the alloy framework, and to acid-etched tooth enamel. Howe and Denehy (2) replaced missing anterior teeth with a perforated cast framework. They used a self-curing composite resin to attach the prosthesis to abutment teeth. Because of perforations in the framework, resin was exposed to the oral environment. Subsequent resin wear and fracture led to failure at the resin-alloy interface. Dunn and Reisbick (3) bonded ceramic coatings to electrochemically etched Co-Cr alloy surfaces. Based on this work, Livaditis and Thompson (4) developed methods for electrochemically etching non-precious (Ni-Cr) alloys to enhance the composite-alloy bond. Composite resin would bond to an etched alloy surface in a manner analagous to bonding of composite to etched enamel. Using this technique, a solid appliance could be fabricated; the major drawback of the Rochette design, a perforated framework, was overcome. In addition, the solid framework could be made much thinner. The clinical technique for fabricating etched cast alloy retainers was further devel* Presented, in part, at the 63rd General Session, International Association for Dental Research, Las Vegas, Nevada, March 1985.
oped and described by Simonsen, Thompson, and Barrack (5). Success of an etched cast fixed retainer depends on design, bond strength to etched enamel, and bond strength to etched alloy. Zidan (6) reported composite-alloy tensile bond strength as related to observed relief patterns after electrolytic etching of several base metal alloys. The purpose of this study was, first, to design test configurations to measure interracial strengths between composite luting cement and etched alloys in shear and in tension, and second, to use these new test fixtures in evaluation of bond strengths of commercially available products. Material and m e t h o d s
Interfacial testing in shear and in tension was performed between a nickelchromium alloy, Litecast B (Williams Gold Refining Co., Buffalo, NY) and two composite luting cements, Comspan (L. D. Caulk Co., Milford, DE) and Conclude (3M, Minneapolis, MN). Specimen preparation
Patterns having dimensions 6.0 mm diameter and 12.0 mm long, with a hole drilled at 5.0 mm from one face, were prepared from Plexiglas| rod. A phosphate-bonded investment was used (Hi-Temp, Whip Mix Co., Louisville, KY). The alloy was melted with a propane-oxygen gas mixture, and cast centrifugally.
Key words: bonding, dental; composite resins; dental alloys Dr. Peter S. Hasiakos, Department of Operative Dentistry Loyola University School of Dentistry 2160 South First Avenue, Maywood, IL 60153, USA
Received February 19, 1986; accepted May 23, 1986.
Any specimens with voids, or other imperfections on the face to be etched were discarded; any blebs on the lateral surface were removed. All new alloy was used for each casting. Specimens were placed in a holder so that the face would remain perpendicular to the long axis of the cylinder during fine grinding through 600 grit SiC paper. Specimens were etched according to the alloy manufacturers' instructions (5: p. 133). A current density of 200 ma/cm 2was maintained for 6 rain, using an electrolyte of 10% sulfuric acid with absolute methanol (9 parts acid: 1 part methanol). A Micromet Etcher (Model 70-1740, Buehler Co., Lake Bluff, IL) was utilized. The etching set-up is depicted in Fig. 1. The cathode was a section of 0.072-inch diameter A.I.S.I. Type 302 stainless steel rod, bent at 90 degrees. Sticky wax was placed around the circumference of the cylinder to isolate the surface to be etched. The specimen surface, which served as an anode, was positioned perpendicular to the cathode at a distance of 1-1.5 cm. Use of a magnetic stirrer reduced the accumulation of bubbles on the surface to be etched. After etching, the specimen was rinsed with water, and agitated in 18% hydrochloric acid for 10 min in an ultrasonic cleaner. The specimen was next rinsed with water, air dried, then chilled in ice water to facilitate removal of the sticky wax from the periphery. To ensure that the surfaces were properly etched, selected specimens
Early shear and tensile strength of composite to etched alloy
285
Cast specimen with sticky wax on periphery
I
00[ Magnetic
I
Q
Stirrer
o
Electrolytic
9 Etcher
Fig. 1. Apparatus for electrolytic etching of the alloy. were viewed under a scanning electron microscope. In addition, all specimens were viewed under a metallographic microscope at 100• and 400• magnification. Any specimen which did not have a surface exhibiting sufficient three-dimensional relief was rejected. Shear testing
A specimen holder was designed to facilitate forming the composite-alloy interface and to align the sample in a universal testing machine. The stainless steel (A.I.S.I. Type 303) holder, 12 mm diameter by 3 cm long, includes a hole which is slightly larger than the specimen diameter (Fig. 2). The test spedmen is secured by a retaining pin which passes through the holder and the hole
Cast specimen
~
--Loose
~Set
pin
screw
Fig. 2. Alloy specimen positioned in the holder.
[__.;// Specimen holder
X
I
//:/ I
I
in the test specimen, then tightened by a set screw; the end of the specimen extends 2 mm beyond the holder. A Teflon | mold, consisting of a split ring with an outer retaining ring, is placed over the specimen (Fig. 3). The mold has a 6 mm inside diameter to prevent flash from forming around the circumference of the alloy specimen. Composite is then mixed and placed according to manufacturers' instructions. After the composite sets, the Teflon | mold is removed, producing an interfacial bond which is perpendicular to the long axis of the test specimen. A fixture (Fig. 4), designed to secure the sample holder in the lower member of a universal testing machine (Model 1130, Instron Corp., Canton, MA) by means of a retaining pin, was constructed from stainless steel. The fixture positions the composite-alloy interface parallel to an applied load, which is the configuration needed to test the shear strength of the specimen. The alloy-composite interface is accurately placed 2 mm from the face of the fixture (Fig. 5). Force is applied through a freely hanging stainless steel ring, 3 mm thick, with a knife edge at its center. By using a 1 mm thick shim, the knife edge is predictably and con-
S p l i t ring
Fig. 3. Mold design for placement of composite: a) cross-sectional view; h) top view.
Fig. 4. Fixture for securing the shear test specimen as positioned in the universal testing machine. sistently placed 0.5 mm from the alloycomposite interface on the composite. The ring is attached to the upper member of the universal testing machine by a chain, which aliows for self-alignment. Twelve specimens of each alloycomposite combination were tested at 8 min from the start of composite mixing until failure occurred, using a strain rate of 0.5 mm/min (0.02 in/rain). Tensile testing
Tensile test specimens were prepared by placing composite between the etched faces of 2 alloy samples. Each of a pair of alloy samples was placed in a holder (Fig. 2), as described above, and positioned 2 mm apart using the split Teflon | ring. The volume of composite needed to fill the space between alloy samples, 6 mm diameter by 2 mm thick, was inserted. Finger pressure held the ring together until the composite cured. One sample holder was mounted firmly to the lower member of the universal testing machine by a retaining pin. The other holder was attached to the upper member of the universal testing machine by means of a freely hanging chain which assured vertical alignment (Fig. 6). The test configuration was then loaded in tension with generation of tensile forces perpendicular to the etched alloy-composite interfaces. Twelve samples of each alloy-composite combination were tested at 8 rain from the start of composite mixing
286
Hasiakos et al.
II -~
~
3mm
lmm
Composite
'li'
[
2mm
/ /
--1ram 9
6mm
I
Fig. 5. Configuration for interracial testing in shear: a) right side view; b) front view in cross-section; c) detail of placement of the knife edge.
until failure occurred, using a strain rate of 0.5 ram/rain. (0.02 in/rain). Results
Bond strength is determined as the load to cause failure of the test specimen divided by the interfacial crosssectional area. Three modes of failure were observed: adhesive failure at the
composite-alloy interface, cohesive failure through the composite, and mixed adhesive-cohesive failure. The mean value (12 specimens) of interfacial strength in shear for Comspan-Litecast B is 6.63 + 0.94 MPa (961 _+ 137 psi), and for Conclude-Litecast B is 5.55 + 1.91 MPa (805 + 277 psi). See Table 1. There is no significant difference (p < 0.05) between the two composite-alloy combinations. The mean value (12 specimens) of interfacial strength in tension for Comspan-Litecast B is 6.97 + 2.19 MPa (1010 + 318 psi) and for Conclude-Litecast B is 8.32 + 2.38 MPa (1207 _+ 345 psi), See q2able f. There is no significant difference (p < 0.05) between the 2 composite-alloy combinations.
Discussion ~ - - - -
Cast specimen
To be effective in the mouth, an ap-
pliance which includes a resin b o n d e d to etched alloy c o m p o n e n t should have an interracial strength greater than at the composite-tooth interface. T h e n the bond strength at the compositetooth interface would be the limiting factor in the success of such an appliance. Clinical restorations are subject to shear, tensile, and compressive forces; usually a mixture of these. In this study, interfacial strengths were measured in shear and in tension. Several designs for interracial testing in tension were evaluated before the current test configuration was finalized. In an early design, a 360 ~ swivel was embedded into the composite during setting. This allowed the sample to be aligned in a universal testing machine through a chain attachment to the swivel. However, failure consistently occurred at the composite-swivel interface. Consequently, the load at failure
Composite
Table 1. Interfacial strength at 8 rain: composite luting cement to etched alloy C
a
s
t
specimen
Fig. 6. Configuration for interfacial testing in tension.
Interface
Shear Strength ~ + (s)* (MPa) (psi)
Tensile Strength ~ _+ (s)* (MPa) (psi)
Comspan ~- Litecast W Conclude3 - Litecast B
6.63+0.94 5.55_+1.9l
6 . 9 7 _ + 2 . 1 9 1010_+318 8 . 3 2 _ + 2 . 3 8 1207_+345
L. D. Caulk Co., Milford, DE 2 Williams Gold Refining Co., Buffalo, NY 3 3M, Minneapolis, MN * Mean value for 12 specimens
961_+137 805_+277
Early shear and tensile strength o f composite to etched alloy
was not a measure of composite-alloy interface bonding. For the tensile test configuration reported in this study, composite was bonded between two etched alloy cylinders. Attachment to the universal testing machine occurred through the alloy and not the composite, reducing the non-axial loading of the bonded interfaces. In previous investigations, tensile bond strength was determined using two configurations: composite bonded to a single alloy specimen (5, 7), and composite bonded between a pair of alloy specimens (6, 8). Thompson (5: Addendum) reported a tensile bond strength of 32.4 + 7.0 MPa (4700 +_ 1020 psi) between Cornspan | and Litecast | B. Details of alloy preparation were not given; it is assumed that the as-cast surface was air abraded with 50 micron alumina particles, and the alloy was subjected to oxidation and simulated porcelain firing cycles. The etcbing conditions: electrolyte, current density, and etching time, were the same conditions used in the present study. Composite bonded to single alloy specimens were thermally cycled (1000 cycles between 5-60~ and, at a n unspecified time after the initial composite mixing, were loaded until failure at a strain rate of 1 mm/min. Several parameters differed in the present study from the conditions described above which do not permit a direct comparison of tensile bond strength values. These differences include: test configuration, alloy surface roughness, alloy conditioning prior to application of composite, thermal cycling, time of testing after applying
composite, and strain rate. In the present study the test configuration used was a 2 mm thickness of composite placed between pairs of etched alloy specimens. Each alloy surface was finished using 600 grit silicon carbide metallurgical paper without undergoing a simulated sequence of porcelain firing cycles. Specimens were not thermally cycled prior to loading. Specimens were tested at 8 min from the start of composite mixing, using a strain rate of 0.5 mm/min. These test conditions may tend to give an apparent low value for tensile bond strength due to: 1) a relatively smooth and uniform alloy surface which minimizes potential mechanical retention; 2) a relatively thick layer of luting cement compared to a clinical situation where a luting cement should have a film thickness tess than 25 ~t; 3) testing at a time interval which is consistent with the manufacturers' directions for clinical placement of an appliance, but before the diametral tensile strength of the composite has reached its greatest value; and 4) testing at a low strain rate. For the thickness of luting cement used in this study, complex stress distributions may be generated through the cement layer when the specimen is loaded in tension. Regions of high stress concentrations may result in failure at low applied loads. In future work, a more clinically significant film thickness can be achieved by eliminating use of the split ring when preparing tensile specimens. Diametral tensile test results on cylinders of composite luting cement performed 8 min after mixing at 23~ show values approxi-
287
mately 50% lower than tests at 24 h and 37~ Comspan | 19.5 _+ 2.98 MPa (2830 _+ 432 psi) and 40.1 _+ 1.49 MPa (5810 _+ 216 psi), respectively; Conclude | 27.3 _+ 5.60 MPa (3960 + 812 psi) and 50.6 + 5.54 MPa (7340 _+ 803 psi), respectively. Acknowledgment - The authors thank Mr.
Zaker Sabri, Lombard, IL, for fabrication of test fixtures used in this study.
References 1. Rochette AL. Attachment of a splint to enamel of lower anterior teeth. J Prosthet Dent 1973: 30: 418-423. 2. Howe DF, Denehy GE. Anterior fixed partial dentures utilizing the acid-etch technique and a cast metal framework. J Prosthet Dent 1977: 37: 28-31. 3. Dunn B, Reisbick MH. Adherence of ceramic coatings on chromium-cobalt structures. J Dent Res I976: 55: 328-332. 4. Livaditis GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin-bonded retainers. J Prosthet Dent 1982: 47: 52-58. 5. Simonsen R, Thompson V, Barrack, G. Etched cast restorations: clinical and laboratory techniques. Chicago: Quintessence. 1983. 6. Zidan O. Etched base-metal alloys: comparison of relief patterns, bond strengths and fracture modes. Dent Mater 1985: 1: 209-213. 7. Thompson VP, Del Castillo E, Livaditis GJ. Resin-bonded retainers. Part h Resin bond to electrolytically etched nonprecious alloys. J Prosthet Dent 1983: 50: 771-779. 8. Thompson VP, Grolman KM, Liao, R. Bonding of adhesive resins to various non-precious alloys. J Dent Res 1985: 64: 314. Abst No. 1258.
P, S. H a s i a k o s 1, J. J. KoelbP, L. W. L a u b 2, J. L. Sandrik 2 Departments of ~OperativeDentistry, 2Dental Materials, Loyola University of Chicago, School of Dentistry Maywood, USA
Hasiakos PS, Koelbl J J, Laub LW, Sandrik JL. Early shear and tensile strength of composite to etched alloy. Dent Mater 1986: 2: 284287. Abstract. - Design of test configurations to measure interracial strengths between composite luting cement and etched alloys in shear and in tension, was the major objective of this work. Previous investigations of bond strengths in these systems have been primarily restricted to tensile testing. The test systems developed in this study were used to evaluate bond strengths between two brands of composite luting cement and one non-precious casting alloy. No statistically significant difference was found between the combination of products tested in either the shear or tensile modes.
Rochette (1) described a method for splinting mandibular anterior teeth by bonding a cast alloy framework to enamel with unfilled acrylic resin. Bonding was achieved by mechanical retention of the resin to perforations in the alloy framework, and to acid-etched tooth enamel. Howe and Denehy (2) replaced missing anterior teeth with a perforated cast framework. They used a self-curing composite resin to attach the prosthesis to abutment teeth. Because of perforations in the framework, resin was exposed to the oral environment. Subsequent resin wear and fracture led to failure at the resin-alloy interface. Dunn and Reisbick (3) bonded ceramic coatings to electrochemically etched Co-Cr alloy surfaces. Based on this work, Livaditis and Thompson (4) developed methods for electrochemically etching non-precious (Ni-Cr) alloys to enhance the composite-alloy bond. Composite resin would bond to an etched alloy surface in a manner analagous to bonding of composite to etched enamel. Using this technique, a solid appliance could be fabricated; the major drawback of the Rochette design, a perforated framework, was overcome. In addition, the solid framework could be made much thinner. The clinical technique for fabricating etched cast alloy retainers was further devel* Presented, in part, at the 63rd General Session, International Association for Dental Research, Las Vegas, Nevada, March 1985.
oped and described by Simonsen, Thompson, and Barrack (5). Success of an etched cast fixed retainer depends on design, bond strength to etched enamel, and bond strength to etched alloy. Zidan (6) reported composite-alloy tensile bond strength as related to observed relief patterns after electrolytic etching of several base metal alloys. The purpose of this study was, first, to design test configurations to measure interracial strengths between composite luting cement and etched alloys in shear and in tension, and second, to use these new test fixtures in evaluation of bond strengths of commercially available products. Material and m e t h o d s
Interfacial testing in shear and in tension was performed between a nickelchromium alloy, Litecast B (Williams Gold Refining Co., Buffalo, NY) and two composite luting cements, Comspan (L. D. Caulk Co., Milford, DE) and Conclude (3M, Minneapolis, MN). Specimen preparation
Patterns having dimensions 6.0 mm diameter and 12.0 mm long, with a hole drilled at 5.0 mm from one face, were prepared from Plexiglas| rod. A phosphate-bonded investment was used (Hi-Temp, Whip Mix Co., Louisville, KY). The alloy was melted with a propane-oxygen gas mixture, and cast centrifugally.
Key words: bonding, dental; composite resins; dental alloys Dr. Peter S. Hasiakos, Department of Operative Dentistry Loyola University School of Dentistry 2160 South First Avenue, Maywood, IL 60153, USA
Received February 19, 1986; accepted May 23, 1986.
Any specimens with voids, or other imperfections on the face to be etched were discarded; any blebs on the lateral surface were removed. All new alloy was used for each casting. Specimens were placed in a holder so that the face would remain perpendicular to the long axis of the cylinder during fine grinding through 600 grit SiC paper. Specimens were etched according to the alloy manufacturers' instructions (5: p. 133). A current density of 200 ma/cm 2was maintained for 6 rain, using an electrolyte of 10% sulfuric acid with absolute methanol (9 parts acid: 1 part methanol). A Micromet Etcher (Model 70-1740, Buehler Co., Lake Bluff, IL) was utilized. The etching set-up is depicted in Fig. 1. The cathode was a section of 0.072-inch diameter A.I.S.I. Type 302 stainless steel rod, bent at 90 degrees. Sticky wax was placed around the circumference of the cylinder to isolate the surface to be etched. The specimen surface, which served as an anode, was positioned perpendicular to the cathode at a distance of 1-1.5 cm. Use of a magnetic stirrer reduced the accumulation of bubbles on the surface to be etched. After etching, the specimen was rinsed with water, and agitated in 18% hydrochloric acid for 10 min in an ultrasonic cleaner. The specimen was next rinsed with water, air dried, then chilled in ice water to facilitate removal of the sticky wax from the periphery. To ensure that the surfaces were properly etched, selected specimens
Early shear and tensile strength of composite to etched alloy
285
Cast specimen with sticky wax on periphery
I
00[ Magnetic
I
Q
Stirrer
o
Electrolytic
9 Etcher
Fig. 1. Apparatus for electrolytic etching of the alloy. were viewed under a scanning electron microscope. In addition, all specimens were viewed under a metallographic microscope at 100• and 400• magnification. Any specimen which did not have a surface exhibiting sufficient three-dimensional relief was rejected. Shear testing
A specimen holder was designed to facilitate forming the composite-alloy interface and to align the sample in a universal testing machine. The stainless steel (A.I.S.I. Type 303) holder, 12 mm diameter by 3 cm long, includes a hole which is slightly larger than the specimen diameter (Fig. 2). The test spedmen is secured by a retaining pin which passes through the holder and the hole
Cast specimen
~
--Loose
~Set
pin
screw
Fig. 2. Alloy specimen positioned in the holder.
[__.;// Specimen holder
X
I
//:/ I
I
in the test specimen, then tightened by a set screw; the end of the specimen extends 2 mm beyond the holder. A Teflon | mold, consisting of a split ring with an outer retaining ring, is placed over the specimen (Fig. 3). The mold has a 6 mm inside diameter to prevent flash from forming around the circumference of the alloy specimen. Composite is then mixed and placed according to manufacturers' instructions. After the composite sets, the Teflon | mold is removed, producing an interfacial bond which is perpendicular to the long axis of the test specimen. A fixture (Fig. 4), designed to secure the sample holder in the lower member of a universal testing machine (Model 1130, Instron Corp., Canton, MA) by means of a retaining pin, was constructed from stainless steel. The fixture positions the composite-alloy interface parallel to an applied load, which is the configuration needed to test the shear strength of the specimen. The alloy-composite interface is accurately placed 2 mm from the face of the fixture (Fig. 5). Force is applied through a freely hanging stainless steel ring, 3 mm thick, with a knife edge at its center. By using a 1 mm thick shim, the knife edge is predictably and con-
S p l i t ring
Fig. 3. Mold design for placement of composite: a) cross-sectional view; h) top view.
Fig. 4. Fixture for securing the shear test specimen as positioned in the universal testing machine. sistently placed 0.5 mm from the alloycomposite interface on the composite. The ring is attached to the upper member of the universal testing machine by a chain, which aliows for self-alignment. Twelve specimens of each alloycomposite combination were tested at 8 min from the start of composite mixing until failure occurred, using a strain rate of 0.5 mm/min (0.02 in/rain). Tensile testing
Tensile test specimens were prepared by placing composite between the etched faces of 2 alloy samples. Each of a pair of alloy samples was placed in a holder (Fig. 2), as described above, and positioned 2 mm apart using the split Teflon | ring. The volume of composite needed to fill the space between alloy samples, 6 mm diameter by 2 mm thick, was inserted. Finger pressure held the ring together until the composite cured. One sample holder was mounted firmly to the lower member of the universal testing machine by a retaining pin. The other holder was attached to the upper member of the universal testing machine by means of a freely hanging chain which assured vertical alignment (Fig. 6). The test configuration was then loaded in tension with generation of tensile forces perpendicular to the etched alloy-composite interfaces. Twelve samples of each alloy-composite combination were tested at 8 rain from the start of composite mixing
286
Hasiakos et al.
II -~
~
3mm
lmm
Composite
'li'
[
2mm
/ /
--1ram 9
6mm
I
Fig. 5. Configuration for interracial testing in shear: a) right side view; b) front view in cross-section; c) detail of placement of the knife edge.
until failure occurred, using a strain rate of 0.5 ram/rain. (0.02 in/rain). Results
Bond strength is determined as the load to cause failure of the test specimen divided by the interfacial crosssectional area. Three modes of failure were observed: adhesive failure at the
composite-alloy interface, cohesive failure through the composite, and mixed adhesive-cohesive failure. The mean value (12 specimens) of interfacial strength in shear for Comspan-Litecast B is 6.63 + 0.94 MPa (961 _+ 137 psi), and for Conclude-Litecast B is 5.55 + 1.91 MPa (805 + 277 psi). See Table 1. There is no significant difference (p < 0.05) between the two composite-alloy combinations. The mean value (12 specimens) of interfacial strength in tension for Comspan-Litecast B is 6.97 + 2.19 MPa (1010 + 318 psi) and for Conclude-Litecast B is 8.32 + 2.38 MPa (1207 _+ 345 psi), See q2able f. There is no significant difference (p < 0.05) between the 2 composite-alloy combinations.
Discussion ~ - - - -
Cast specimen
To be effective in the mouth, an ap-
pliance which includes a resin b o n d e d to etched alloy c o m p o n e n t should have an interracial strength greater than at the composite-tooth interface. T h e n the bond strength at the compositetooth interface would be the limiting factor in the success of such an appliance. Clinical restorations are subject to shear, tensile, and compressive forces; usually a mixture of these. In this study, interfacial strengths were measured in shear and in tension. Several designs for interracial testing in tension were evaluated before the current test configuration was finalized. In an early design, a 360 ~ swivel was embedded into the composite during setting. This allowed the sample to be aligned in a universal testing machine through a chain attachment to the swivel. However, failure consistently occurred at the composite-swivel interface. Consequently, the load at failure
Composite
Table 1. Interfacial strength at 8 rain: composite luting cement to etched alloy C
a
s
t
specimen
Fig. 6. Configuration for interfacial testing in tension.
Interface
Shear Strength ~ + (s)* (MPa) (psi)
Tensile Strength ~ _+ (s)* (MPa) (psi)
Comspan ~- Litecast W Conclude3 - Litecast B
6.63+0.94 5.55_+1.9l
6 . 9 7 _ + 2 . 1 9 1010_+318 8 . 3 2 _ + 2 . 3 8 1207_+345
L. D. Caulk Co., Milford, DE 2 Williams Gold Refining Co., Buffalo, NY 3 3M, Minneapolis, MN * Mean value for 12 specimens
961_+137 805_+277
Early shear and tensile strength o f composite to etched alloy
was not a measure of composite-alloy interface bonding. For the tensile test configuration reported in this study, composite was bonded between two etched alloy cylinders. Attachment to the universal testing machine occurred through the alloy and not the composite, reducing the non-axial loading of the bonded interfaces. In previous investigations, tensile bond strength was determined using two configurations: composite bonded to a single alloy specimen (5, 7), and composite bonded between a pair of alloy specimens (6, 8). Thompson (5: Addendum) reported a tensile bond strength of 32.4 + 7.0 MPa (4700 +_ 1020 psi) between Cornspan | and Litecast | B. Details of alloy preparation were not given; it is assumed that the as-cast surface was air abraded with 50 micron alumina particles, and the alloy was subjected to oxidation and simulated porcelain firing cycles. The etcbing conditions: electrolyte, current density, and etching time, were the same conditions used in the present study. Composite bonded to single alloy specimens were thermally cycled (1000 cycles between 5-60~ and, at a n unspecified time after the initial composite mixing, were loaded until failure at a strain rate of 1 mm/min. Several parameters differed in the present study from the conditions described above which do not permit a direct comparison of tensile bond strength values. These differences include: test configuration, alloy surface roughness, alloy conditioning prior to application of composite, thermal cycling, time of testing after applying
composite, and strain rate. In the present study the test configuration used was a 2 mm thickness of composite placed between pairs of etched alloy specimens. Each alloy surface was finished using 600 grit silicon carbide metallurgical paper without undergoing a simulated sequence of porcelain firing cycles. Specimens were not thermally cycled prior to loading. Specimens were tested at 8 min from the start of composite mixing, using a strain rate of 0.5 mm/min. These test conditions may tend to give an apparent low value for tensile bond strength due to: 1) a relatively smooth and uniform alloy surface which minimizes potential mechanical retention; 2) a relatively thick layer of luting cement compared to a clinical situation where a luting cement should have a film thickness tess than 25 ~t; 3) testing at a time interval which is consistent with the manufacturers' directions for clinical placement of an appliance, but before the diametral tensile strength of the composite has reached its greatest value; and 4) testing at a low strain rate. For the thickness of luting cement used in this study, complex stress distributions may be generated through the cement layer when the specimen is loaded in tension. Regions of high stress concentrations may result in failure at low applied loads. In future work, a more clinically significant film thickness can be achieved by eliminating use of the split ring when preparing tensile specimens. Diametral tensile test results on cylinders of composite luting cement performed 8 min after mixing at 23~ show values approxi-
287
mately 50% lower than tests at 24 h and 37~ Comspan | 19.5 _+ 2.98 MPa (2830 _+ 432 psi) and 40.1 _+ 1.49 MPa (5810 _+ 216 psi), respectively; Conclude | 27.3 _+ 5.60 MPa (3960 + 812 psi) and 50.6 + 5.54 MPa (7340 _+ 803 psi), respectively. Acknowledgment - The authors thank Mr.
Zaker Sabri, Lombard, IL, for fabrication of test fixtures used in this study.
References 1. Rochette AL. Attachment of a splint to enamel of lower anterior teeth. J Prosthet Dent 1973: 30: 418-423. 2. Howe DF, Denehy GE. Anterior fixed partial dentures utilizing the acid-etch technique and a cast metal framework. J Prosthet Dent 1977: 37: 28-31. 3. Dunn B, Reisbick MH. Adherence of ceramic coatings on chromium-cobalt structures. J Dent Res I976: 55: 328-332. 4. Livaditis GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin-bonded retainers. J Prosthet Dent 1982: 47: 52-58. 5. Simonsen R, Thompson V, Barrack, G. Etched cast restorations: clinical and laboratory techniques. Chicago: Quintessence. 1983. 6. Zidan O. Etched base-metal alloys: comparison of relief patterns, bond strengths and fracture modes. Dent Mater 1985: 1: 209-213. 7. Thompson VP, Del Castillo E, Livaditis GJ. Resin-bonded retainers. Part h Resin bond to electrolytically etched nonprecious alloys. J Prosthet Dent 1983: 50: 771-779. 8. Thompson VP, Grolman KM, Liao, R. Bonding of adhesive resins to various non-precious alloys. J Dent Res 1985: 64: 314. Abst No. 1258.