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Bench-curing acrylic resins
Bench-curing acrylic resins David N. Firtell, D.D.S., M.A.,* Larry L. Harman, D.D.S.,** and Assad E. Mora, D.D.S., M.S.D.*** University of California, School of Dentistry, San Francisco, Calif.
B
ench-curing heat-processed acrylic resins before processing is suggested for the purpose of equalizing pressure in the mold, releasing internal stress, dispersing monomer uniformly, and exposing resin teeth to the monomer.‘*2*+ Opinion varies on the exact amount of time a packed flask should bench-cure before being placed in a processing unit. Two hours is the most commonly quoted length of time.3 A text on dental materials refers to bench-curing only to state that manufacturer’s directions should be followed.’ Two manufacturers suggest bench-curing for 30 minutes.5~6 One states that their resin can be placed directly into water at processing temperature (163” F) immediately after bench-curing. 6 Another manufacturer suggests bench-curing for 20 minutes, but only when their material is to be placed directly into boiling water.’ The latter manufacturer cautioned against the procedures when processing “thick” sections of resin. Shari-y’ suggests a period of 1 to 4 hours without citing references, and Heartwell and Rahn* do not mention bench-curing. Boucher et al.9 suggest 30 to 60 minutes, and Hudis’O suggests bench-curing for 1 hour but adds that longer periods would not be detrimental. In a review of the literature for the Council on Dental Research on processing heat-cured denture base resins, Stanford and Paffenbarger” did not mention benchcuring. The intent of allowing the monomer to penetrate the polymer during bench-curing is to enhance the physical properties of the resin. In addition, residual islands of monomer can result in porosity within the cured resin. Usually, when a packed flask is placed in a curing unit, the water is assumed to be at room temperature (70” F) or at least well below processing temperature (160” to 163” F).3 Rapid polymerization of heat-cured acrylic resin does not begin until it reaches a temperature of
*Professor and Chairman, Removable Prosthodontics. **Assistant CLinical Professor, Removable Prosthodontics. ***Assistant Professor, Removable Prosthodontics. tHerigstad, G.: Personal communication, 1981.
THE JOURNAL
OF PROSTHETIC
DENTISTRY
140” F, at which point benzene peroxide breaks down and free radicals that cause rapid polymerization are released. At higher temperatures the breakdown is more rapid; therefore, the processing is more rapid.3 When resins are raised from room temperature to the temperature of rapid polymerization, a 70” F temperature rise can occur before processing begins. If a curing unit rises 1.8’ F/min, 40 minutes of benchcuring might occur in the unit before the resin begins to polymerize at an appreciable rate. Therefore, benchcuring need not be considered a separate step, but one that could take place in the processing unit while the temperature of the water bath is increased. If the cured resin were not homogeneous, or if islands of monomer had existed, the physical defects might be seen in the form of striations, lamination, or porosity that gives the resin a streaked or mottled appearance. The purpose of this study was to determine if homogeneity of the resin associated with bench-curing could be obtained in the processing unit as the water increased from room temperature to curing temperature. We used the criterion of visible defects in the resin to detect inadequate bench-curing.
MATERIAL
AND METHODS
Three series of specimens were used. To make the first series, a maxillary cast was duplicated 81 times and a wax baseplate 2 mm thick was formed on each cast. On the baseplate over the ridge where teeth are usually placed, a rim of wax 5 mm wide and 3 mm high was added. The rim simulated the average amount of wax around teeth on a denture wax-up. The waxed casts were divided into nine groups of nine each; each group was divided into three subgroups and marked for identification. Each cast was individually flasked, the wax was removed, the cast was painted with tinfoil substitute, and the flasks were prepared for packing in a standard manner. Each subgroup of three was packed with one of three different denture resins (Microlon, Hygenic Corp., Akron, Ohio; Lucitone, L. D. Caulk Co., Milford, Del.; and Compak 20, Dentsply International, Inc., York, Pa). Immediately after packing and
431
FIRTELL,
HARMAN,
AND
MORA
the first series and each subgroup was packed with one of the three dentnre resins used in the first series. The specimens of the second series were processed by being placed in a processing unit with room-temperature water and the unit turned on without additional bench-curing. They were recovered, polished, sectioned, and examined for defects in the same manner as the first series. A third series of specimens was prepared in the same manner as the second series, except that they were allowed to bench-cure for 60 minutes before being placed in the processing unit. RESULTS AND
Fig. 1. Simulated dentures were sectioned to look for internal porosity.
without bench-curing, the first group of nine was placed in an automatic processing unit (Teledyne Hanau, Buffalo, N.Y.) calibrated to raise its water temperature at 1.8” F/min. The water in the unit was at room temperature to begin, and the unit was started immediately to process for 9 hours and reach a maximum temperature at 163” F. This procedure was recommended by Peytor? to minimize porosity associated with processing and enhance the physical properties of the material. The remaining eight groups of nine each were treated in the same manner, except that bench-curing ranged from 15 minutes for the second group of nine to 120 minutes for the last group, increasing by a 15-minute increment for each group. After the heat-curing was complete, the flasks were removed from the water and cooled to room temperature. The simulated denture specimens were then recovered, polished, and examined externally for visible defects. The dentures were sectioned through the midsagittal and midfrontal planes, which exposed eight internal surfaces of the baseplates and occlusal rims (Fig. 1). The surfaces were polished and examined with a hand-held X10 lens for visible defects. A second series of specimens that consisted of blocks of baseplate wax 19.5 mm wide and 19.5 mm long was prepared. The blocks varied in height from 3 mm and increased in increments of 1.5 mm to a maximum of 19.5 mm for a total of 12 groups. In this series each group consisted of nine specimens with three subgroups for a total of 108 specimens, all marked for identification. The specimens were flasked and prepared as in
432
DISCUSSION
No visible defects were noted in any of the exposed surfaces of the specimens. No distinctions could be made between specimens processed after different periods of bench-curing time. It appears that long periods of bench-curing are not necessary for adequate processing of acrylic resins used in fabricating dentures. It also appears that benchcuring as a separate step in processing acrylic resins is not necessary, because bench-curing can take place in the processing unit. When this procedure is used, the water temperature should be low enough (70” to 80” F) to allow the monomer to penetrate the polymer before the water reaches the temperature where active polymerization occurs. Following manufacturers’ directions to determine the length of time for benchcuring is advisable. Placing the flasks in room-temperature water and then starting the processing unit should allow adequate time for bench-curing and produce an adequate resin. CONCLUSIONS Long periods of bench-curing are not necessary. Bench-curing can take place in the water bath of the processing unit. When this procedure is used, the water should be far enough below the temperature at which active polymerization of the resin proceeds to allow the bench-curing to occur. REFERENCES 1. Sharry, J. J.: Complete Denture Prosthodonrics. ed 3. New York, 1974, McGraw-Hill Co. 2. Peyton, F. A.: Packing and processing denturr base resins. J Am Dent Assoc 40~520, 1950. 3. Firtell, D. N.: A survey of bench curing procedures. (Unpublished data.) 4. Phillips, R. W.: Skinner’s Series of Dental Materials, ed 7. Philadelphia, 1973, W. B. Saunders Co. 5. ‘The Hygenic Corporation: Processing Instructions for MicroIon. Akron. Ohio.
MARCH
1984
VOLUME
51
NUMBER
3
BENCH-CURING
ACRYLIC
RESINS
6.
L. D. Caulk Co.: Directions for Processing Caulk CH Lucitone. Milford, Del. 7. Dentsply International, Inc.: Recommended Processing Instructions for Dentsply COMPAK 20. York, Pa., 1981. 8. Heartwell, C. M., and Rahn, A. 0.: Syllabus of Complete Dentures, ed 3. Philadelphia, 1980, Lea & Febiger. 9. Boucher, C. O., Hickey, J. C., and Zarb, G. A.: Prosthetic Treatment for Edentulous Patients, ed 7. St. Louis, 1975, The C. V. Mosby Co. 10. Hudis, M. M.: Dental Laboratory Prosthodontics. Philadelphia, 1977, W. B. Saunders Co.
11. Stanford, J. W., and Paffenbarger, G. C.: Processing denture base resins: Heat-curing types (prepared for Council on Dental Research). J Am Dent Assoc 53:72, 1956. Reprint requests to: DR. DAVID N. FIRTELL UNIVERSITVOF CALIFORNIA SCHOLLOF DENTISTRY SAN FRANCISCO,CA 94122
ARTICLES TO APPEAR IN FUTURE ISSUES Physical properties of a polycarboxylate tannin-fluoride preparation Mitsuhiro
Intrinsic
cement containing a
Tsukiboshi, D.D.S., D.Med.Sc., and Yoshiaki Tani, D.D.S., D.Med.Sc.
color of isophorone polyurethane
G. E. Turner, D.M.D., E. Lemons, Ph.D.
The distribution
prosthetics M.S.D., and J.
of the radius of the curve scribed during protrusion
Boyd B. Welsch, D.M.D.,
Arthrography
for maxillofacial
M.S.D., T. E. Fischer, D.D.S., D. J. Castleberry, D.M.D.,
M.S.
of the temporomandibular
joint
Per-Lennart Westesson, D.D.S., Odont.Dr.
Radiographic
assessment of the marginal fit of cast restorations
W. Weyns, L.D.S., and J. De Boever, L.D.S., D.M.D.,
Ph.D.
Implant-tissue interface of endosseous dental implants in dogs. Validity of clinical evaluation methods H. Wie, D.D.S., Dr.Odont., T. A. Larheim, D.D.S., Dr.Odont., K. Tolo, D.D.S., Dr.Odont., T. Solheim, D.D.S., 0. Faehn, D.D.S., H. R. Haanaes, D.D.S., M.D., Dr.Odont., J. Odegard, D.D.S., and B. Vorkinn
Use of spring-loaded dentures Susan M. Wright, M.D.&
THE JOURNAL
OF PROSTHETIC
DENTISTRY
attachments for the retention of removable partial F.D.S.R.C.S.(Eng.)
433
B
ench-curing heat-processed acrylic resins before processing is suggested for the purpose of equalizing pressure in the mold, releasing internal stress, dispersing monomer uniformly, and exposing resin teeth to the monomer.‘*2*+ Opinion varies on the exact amount of time a packed flask should bench-cure before being placed in a processing unit. Two hours is the most commonly quoted length of time.3 A text on dental materials refers to bench-curing only to state that manufacturer’s directions should be followed.’ Two manufacturers suggest bench-curing for 30 minutes.5~6 One states that their resin can be placed directly into water at processing temperature (163” F) immediately after bench-curing. 6 Another manufacturer suggests bench-curing for 20 minutes, but only when their material is to be placed directly into boiling water.’ The latter manufacturer cautioned against the procedures when processing “thick” sections of resin. Shari-y’ suggests a period of 1 to 4 hours without citing references, and Heartwell and Rahn* do not mention bench-curing. Boucher et al.9 suggest 30 to 60 minutes, and Hudis’O suggests bench-curing for 1 hour but adds that longer periods would not be detrimental. In a review of the literature for the Council on Dental Research on processing heat-cured denture base resins, Stanford and Paffenbarger” did not mention benchcuring. The intent of allowing the monomer to penetrate the polymer during bench-curing is to enhance the physical properties of the resin. In addition, residual islands of monomer can result in porosity within the cured resin. Usually, when a packed flask is placed in a curing unit, the water is assumed to be at room temperature (70” F) or at least well below processing temperature (160” to 163” F).3 Rapid polymerization of heat-cured acrylic resin does not begin until it reaches a temperature of
*Professor and Chairman, Removable Prosthodontics. **Assistant CLinical Professor, Removable Prosthodontics. ***Assistant Professor, Removable Prosthodontics. tHerigstad, G.: Personal communication, 1981.
THE JOURNAL
OF PROSTHETIC
DENTISTRY
140” F, at which point benzene peroxide breaks down and free radicals that cause rapid polymerization are released. At higher temperatures the breakdown is more rapid; therefore, the processing is more rapid.3 When resins are raised from room temperature to the temperature of rapid polymerization, a 70” F temperature rise can occur before processing begins. If a curing unit rises 1.8’ F/min, 40 minutes of benchcuring might occur in the unit before the resin begins to polymerize at an appreciable rate. Therefore, benchcuring need not be considered a separate step, but one that could take place in the processing unit while the temperature of the water bath is increased. If the cured resin were not homogeneous, or if islands of monomer had existed, the physical defects might be seen in the form of striations, lamination, or porosity that gives the resin a streaked or mottled appearance. The purpose of this study was to determine if homogeneity of the resin associated with bench-curing could be obtained in the processing unit as the water increased from room temperature to curing temperature. We used the criterion of visible defects in the resin to detect inadequate bench-curing.
MATERIAL
AND METHODS
Three series of specimens were used. To make the first series, a maxillary cast was duplicated 81 times and a wax baseplate 2 mm thick was formed on each cast. On the baseplate over the ridge where teeth are usually placed, a rim of wax 5 mm wide and 3 mm high was added. The rim simulated the average amount of wax around teeth on a denture wax-up. The waxed casts were divided into nine groups of nine each; each group was divided into three subgroups and marked for identification. Each cast was individually flasked, the wax was removed, the cast was painted with tinfoil substitute, and the flasks were prepared for packing in a standard manner. Each subgroup of three was packed with one of three different denture resins (Microlon, Hygenic Corp., Akron, Ohio; Lucitone, L. D. Caulk Co., Milford, Del.; and Compak 20, Dentsply International, Inc., York, Pa). Immediately after packing and
431
FIRTELL,
HARMAN,
AND
MORA
the first series and each subgroup was packed with one of the three dentnre resins used in the first series. The specimens of the second series were processed by being placed in a processing unit with room-temperature water and the unit turned on without additional bench-curing. They were recovered, polished, sectioned, and examined for defects in the same manner as the first series. A third series of specimens was prepared in the same manner as the second series, except that they were allowed to bench-cure for 60 minutes before being placed in the processing unit. RESULTS AND
Fig. 1. Simulated dentures were sectioned to look for internal porosity.
without bench-curing, the first group of nine was placed in an automatic processing unit (Teledyne Hanau, Buffalo, N.Y.) calibrated to raise its water temperature at 1.8” F/min. The water in the unit was at room temperature to begin, and the unit was started immediately to process for 9 hours and reach a maximum temperature at 163” F. This procedure was recommended by Peytor? to minimize porosity associated with processing and enhance the physical properties of the material. The remaining eight groups of nine each were treated in the same manner, except that bench-curing ranged from 15 minutes for the second group of nine to 120 minutes for the last group, increasing by a 15-minute increment for each group. After the heat-curing was complete, the flasks were removed from the water and cooled to room temperature. The simulated denture specimens were then recovered, polished, and examined externally for visible defects. The dentures were sectioned through the midsagittal and midfrontal planes, which exposed eight internal surfaces of the baseplates and occlusal rims (Fig. 1). The surfaces were polished and examined with a hand-held X10 lens for visible defects. A second series of specimens that consisted of blocks of baseplate wax 19.5 mm wide and 19.5 mm long was prepared. The blocks varied in height from 3 mm and increased in increments of 1.5 mm to a maximum of 19.5 mm for a total of 12 groups. In this series each group consisted of nine specimens with three subgroups for a total of 108 specimens, all marked for identification. The specimens were flasked and prepared as in
432
DISCUSSION
No visible defects were noted in any of the exposed surfaces of the specimens. No distinctions could be made between specimens processed after different periods of bench-curing time. It appears that long periods of bench-curing are not necessary for adequate processing of acrylic resins used in fabricating dentures. It also appears that benchcuring as a separate step in processing acrylic resins is not necessary, because bench-curing can take place in the processing unit. When this procedure is used, the water temperature should be low enough (70” to 80” F) to allow the monomer to penetrate the polymer before the water reaches the temperature where active polymerization occurs. Following manufacturers’ directions to determine the length of time for benchcuring is advisable. Placing the flasks in room-temperature water and then starting the processing unit should allow adequate time for bench-curing and produce an adequate resin. CONCLUSIONS Long periods of bench-curing are not necessary. Bench-curing can take place in the water bath of the processing unit. When this procedure is used, the water should be far enough below the temperature at which active polymerization of the resin proceeds to allow the bench-curing to occur. REFERENCES 1. Sharry, J. J.: Complete Denture Prosthodonrics. ed 3. New York, 1974, McGraw-Hill Co. 2. Peyton, F. A.: Packing and processing denturr base resins. J Am Dent Assoc 40~520, 1950. 3. Firtell, D. N.: A survey of bench curing procedures. (Unpublished data.) 4. Phillips, R. W.: Skinner’s Series of Dental Materials, ed 7. Philadelphia, 1973, W. B. Saunders Co. 5. ‘The Hygenic Corporation: Processing Instructions for MicroIon. Akron. Ohio.
MARCH
1984
VOLUME
51
NUMBER
3
BENCH-CURING
ACRYLIC
RESINS
6.
L. D. Caulk Co.: Directions for Processing Caulk CH Lucitone. Milford, Del. 7. Dentsply International, Inc.: Recommended Processing Instructions for Dentsply COMPAK 20. York, Pa., 1981. 8. Heartwell, C. M., and Rahn, A. 0.: Syllabus of Complete Dentures, ed 3. Philadelphia, 1980, Lea & Febiger. 9. Boucher, C. O., Hickey, J. C., and Zarb, G. A.: Prosthetic Treatment for Edentulous Patients, ed 7. St. Louis, 1975, The C. V. Mosby Co. 10. Hudis, M. M.: Dental Laboratory Prosthodontics. Philadelphia, 1977, W. B. Saunders Co.
11. Stanford, J. W., and Paffenbarger, G. C.: Processing denture base resins: Heat-curing types (prepared for Council on Dental Research). J Am Dent Assoc 53:72, 1956. Reprint requests to: DR. DAVID N. FIRTELL UNIVERSITVOF CALIFORNIA SCHOLLOF DENTISTRY SAN FRANCISCO,CA 94122
ARTICLES TO APPEAR IN FUTURE ISSUES Physical properties of a polycarboxylate tannin-fluoride preparation Mitsuhiro
Intrinsic
cement containing a
Tsukiboshi, D.D.S., D.Med.Sc., and Yoshiaki Tani, D.D.S., D.Med.Sc.
color of isophorone polyurethane
G. E. Turner, D.M.D., E. Lemons, Ph.D.
The distribution
prosthetics M.S.D., and J.
of the radius of the curve scribed during protrusion
Boyd B. Welsch, D.M.D.,
Arthrography
for maxillofacial
M.S.D., T. E. Fischer, D.D.S., D. J. Castleberry, D.M.D.,
M.S.
of the temporomandibular
joint
Per-Lennart Westesson, D.D.S., Odont.Dr.
Radiographic
assessment of the marginal fit of cast restorations
W. Weyns, L.D.S., and J. De Boever, L.D.S., D.M.D.,
Ph.D.
Implant-tissue interface of endosseous dental implants in dogs. Validity of clinical evaluation methods H. Wie, D.D.S., Dr.Odont., T. A. Larheim, D.D.S., Dr.Odont., K. Tolo, D.D.S., Dr.Odont., T. Solheim, D.D.S., 0. Faehn, D.D.S., H. R. Haanaes, D.D.S., M.D., Dr.Odont., J. Odegard, D.D.S., and B. Vorkinn
Use of spring-loaded dentures Susan M. Wright, M.D.&
THE JOURNAL
OF PROSTHETIC
DENTISTRY
attachments for the retention of removable partial F.D.S.R.C.S.(Eng.)
433