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Effect of dry heat processing on the physical properties of acrylic crowns
Fig. I • A c r y lic resin jacket crow ns
E ffe c t o f d r y h e a t p r o c e s s in g on th e p h y s ic a l p r o p e r tie s o f a c r y lic c r o w n s
Gunnar R yge,* D .D .S ., M .S ., and Donald E. Foley , f D .D .S ., Milwaukee
In spite o f the many excellent character istics o f porcelain jacket crowns, acrylic resins have gained considerable accept ance as a material for com plete coronal restorations o f anterior teeth. A crylic resins were first used fo r jacket crowns in the late thirties, and since that time the materials have not changed very m uch. T h e basic ingredients are still methyl methacrylate m onom er with a small amount o f inhibitor such as hydroquinone, and methyl methacrylate poly m er with an initiator o f the peroxide type — most com m only benzoyl peroxide. T echnics have also been rather static. It was observed that a conservative ap proach to the preparation o f the tooth, with well-defined shoulder preparations, was equally advantageous fo r acrylic resin crowns as for porcelain, and the wellestablished technics for impression taking and fabrication o f dies developed for por celain work were adapted to the fabrica tion o f acrylic crowns w ithout m uch m odification. F or many years the conventional heatcuring process in a water bath has been em ployed, but in recent years several practitioners have advocated the use o f dry heat in the processing o f resin crowns.
Limited inform ation is available in the dental literature about the effect o f dry heat processing on the physical properties o f acrylic resin, but in spite o f the paucity o f quantitative data, it has been postu lated in the literature1 that dry heat proc essing results in increased hardness and density, increased resistance to wear and greater dimensional stability. Such claims are apparently based on the well-estab lished fact that hardness, density, abrasion resistance and dimensional stability are related to residual m onom er content— as shown by Caul and Schoonover2 and by several other investigators.3'5 M A T E R IA L S A N D M E T H O D S
Acrylic crowns for two upper cuspids and two upper lateral incisors were p roc essed by the dry heat compression m old ing technic by Charles L. Pincus, Beverly Hills, Calif. T h e dies were duplicated with rubber base impressions and a densite-type stone. Four duplicate crowns were carved in white w ax and sent to two independent laboratories to be processed by a routine compression m olding and boiling water technic. T w o o f Dr. Pincus’ crowns were made with porcelain par-
RYG E— F O L E Y . . . V O L U M E 66, M A Y 1963 • 95/673
tides in the polym er to make them more resistant to wear or abrasion (Fig. 1 ).
Table 2 • Microindentation hardness values Dry heat
Water Sorption Test • Because o f the size
and shape o f the crowns it was necessary to em ploy weight change determination as a m ethod o f studying water sorption. Since it is a com parative study o f crowns o f similar shape, one can obtain some inform ation by measuring the weight changes w ithout consideration o f the sur face area. Crowns were placed in a desiccator at room tem perature and weighed periodi cally until constant weight was obtained on two succeeding days with not more than 0.1 m g. variation. Four crowns were used: tw o control crowns and tw o cured by dry heat, on e o f which contained por celain particles in the polymer. A fter the dehydrated crowns were found to be o f constant weight, they were placed in dis tilled water at 37°C . for 48 hours. T hey were rem oved from the water, blotted with absorbent paper and weighed at one minute to the nearest 0.1 mg. T hey were weighed again at five minutes from the time o f rem oval from the water bath. T h e differences in weight between the dry and w et conditions o f the crowns were com puted. T h e tests were repeated and T able 1 gives the average o f three determinations for each crown. T h e re sults do not support the claim that dry heat processing results in lower water sorption values than the conventional processing m ethod. In fact, the crown processed with dry heat technic and with the porcelain particles in the polymer gave the highest water sorption values.
Mean Knoop hardness
Conventional
A
B
C
D
24
20
21
18
Hardness • T h e K entron Hardness Tester
was em ployed using a 100 Gm . load with 15 seconds time o f descent and 20 seconds dwell time. A n attempt was m ade to ob tain the surface hardness o f the specimen without preparation, but due to the sur face irregularities this m ethod was not satisfactory. M icrohardness testing re quires that the indentation surface be flat. Therefore, the jacket crowns were pre pared using 0, 2 /0 , 3 /0 and 4 /0 abrasive paper and the surfaces were finished with alpha alum ina abrasive on a metallurgical polisher. T e n acceptable indentations were obtained from each o f the specimens and the m ean values com puted for ten tests o f each crown. T h e specim en containing the porcelain particles gave slightly higher mean hard ness values than the others; this was per haps due to the fact that the diam ond indenter cam e in contact with the porce lain particles. It was noted that the in dentations in these specimens were slightly irregular com pared to the specimens that did not contain porcelain particles. M ean hardness values are given in T able 2. N o significant difference in hardness was noted between specimens processed with dry heat and those processed by the con ventional wet method.. Residual M on o m er * M olecu lar weight
Table 1 • W eight changes in per cent after 48 hours storage in water at 37°C. W eighed after remov al from water bath at
Dry heat
Conventional
A %
B %
C %
D %
1 min.
2.3
1.7
1.9
1.6
5 min.
1.9
1.4
1.8
1.1
determinations were carried out on four resin crowns. N o difference was found between the m olecular weight or residual m onom er content o f crowns processed by the dry heat m ethod and those processed by the conventional m ethod. Temperature Recordings • A fter these rather negative findings, it was decided
96/674 • T H E J O U R N A L O F T H E A M E R IC A N D EN T A L A S S O C IA T IO N
Fig. 2 • T e m p e ra tu re -tim e g ra p h
Table 3 • Temperatures recorded during tests O ven temperature
Time
Resin temperature
600°F.~ 700°F.
5 min.
208°F.
700 °F- 800°F.
10 min.
210°F.
800 °F- 900°F.
7
1000°F,
O O o o
o o
CN
to measure the temperatures at which the dry heat processing was actually tak ing place. In most technics advocated and used by clinicians and laboratories, the jacket crowns are first cured in boiling water for a m inim um o f 20 minutes, then rem oved from the water and immediately placed in an oven preheated to 600° to 800°F . for 15 minutes. It has been claim ed1 that this procedure w ould result in temperatures as high as 320°F. inside the resin crowns. In order to measure the actual tem perature reached by this procedure, sev eral jacket crowns were fabricated fo l low ing rigidly the dry heat technic d e scribed by Charles L . Pincus.1 T w o ther m ocouples were im bedded in each flask, with one placed in the thickest part o f the wax pattern and one centrally located in the stone m odel. Holes were drilled in the flasks to per mit the insulated therm ocouple wires to pass through. T h e thermocouples were o f the chrom el-alum el type and a calibrated Brown Potentiom eter was used to record the millivoltages and, thus, the tempera tures. Figure 2 shows typical results. T h e broken line indicates the temperatures measured in the stone, and the solid line, the resin temperature. It took about five minutes to get the internal temperature up to the transition temperature— the temperature at which
rapid polymerization starts. T h e letters Pi and P2 on the graph indicate the trial packing times, as indicated in the tech n ic described by Dr. Pincus,1 and not until about five minutes after the final packing does the temperature rise to ap proxim ately 212°F . A fter 20 minutes processing at this temperature, the flask was placed in the preheated oven and the temperature was recorded every two m in utes using the tw o thermocouples. After 15 minutes in the oven the highest tem perature recorded inside any resin crown in its thickest portion was 226°F ., and in most instances the temperature did not reach above 220°F . P rolonged heating was carried out at various temperatures, and T able 3 shows some o f the temperatures recorded. T h e oven was preheated to the temperatures listed in the first colum n, and it was set in such a m anner that the temperature increased during the time interval o f the dry processing to the second temperature listed. T h e highest internal temperatures recorded are listed in the last columns in Fahrenheit degrees. Even after 25 minutes at 1,000°F. the m axim al temperature in the heaviest portion o f the crow n was only 257°F . These findings were some what puzzling and called for an explana tion. Figure 3 shows a view o f the inside o f one o f the flasks used for the dry heat processing. T his flask was maintained in an 800°F . oven for one-half hour after the norm al curing in boiling water. T he dark ring inside the flask indicates an area o f the stone which is dehydrated— that is : T h e dihydrate has been converted
15 min.
219°F.
20 min.
241°F.
25 min.
257°F.
R Y S E — FO LEY . . . V O L U M E 66, M A Y 1963 • 97/4 75
Fig. 3 • Flask with d e h y d r a te d p e rip h e ra l area, a p p ro x im a te ly o n e q u a rte r inch w ide
back to hemihydrate. This was confirm ed by x-ray diffraction methods, and this reaction is one o f the reasons for the lim ited temperature changes inside the flask. A pparently, there is a large amount of water available in the stone after the cur ing in the boiling water. T h e dry heat first evaporates the excess water and then initiates a dehydration o f the stone, start ing at the periphery o f the flask. Both o f these processes consume a large amount o f heat, so that the temperature in the center o f the flask remains quite low. This, then, means that the internal tem perature never reaches a level at which on e w ould expect a considerable decrease in residual m onom er, and on this basis it is understandable that the hardness and water sorption determination d o not ex hibit significant variations. T h e next question which arises is obvi
ous: Is there any merit in the dry heat processing? T h e answer is no. T h e dry heat processing has not been shown to im prove the physical properties tested. Nonetheless, those w ho advocate this m ethod are quite convinced that their jacket crowns are superior, and one may tend to agree to some extent. T h e ex planation is to be found in the fact that the practitioners w ho utilize this technic are highly motivated and interested co l leagues w ho meticulously carry out excel lent preparations, take all precautions in impression taking, die preparation, wax up, and all other technical p roce dures, including a careful trial packingscheme. T hey are very capable operators w ho pay attention to all those m anipula tive variables which make for g ood den tistry— even with dry heat processing of acrylic resin crowns. 604 North 16th Street
This in vestigation was su ppo rte d by Research G ra n t D-1057 from the N a tio n a l Institute of Dental Research, U. S. Public H e alth Service. *P ro fe ssor and chairm an, de p artm e nt of dental m ate ri als, M a rq u e tte University, Sch oo l of Dentistry. fR esearch assistant, de p artm e nt of dental m aterials, M a rq u e tte University, Sch oo l of Dentistry. 1. Pincus, C . L. New con cepts in m od el techniques and high tem perature proce ssin g of a crylic resins for m axim um esthetics. J. South. C a lifo rn ia D. A . 24:26 Feb. 1956. 2. C aul, H . J., and Schoonover, I. C . A m ethod for de te rm ining the extent o f polym erization of a crylic resins and its a p p lic a tio n s for dentures. J. A . D. A . 39:1 Ju ly 1949. 3. C a u l, H . J.; Sweeney, W . T., a nd Paffenbarger, G . C . R elationship between residual m onom er and som e properties of self-cu ring dental resins. J. A . D. A . 53:60 Ju ly 1956. 4. Sm ith, D. C . A c ry lic denture base. R esidual m o n o mer. Brit. D. J. 105:86 A u g . 5, 1958. 5. A xelsson, B. D e gre e of polym erization of methyl m ethacrylate. O d o n t. Tskr. 63:395 N o v . 1955.
Fig. I • Illu strative e x h ib it o f d e n ta l in strum en ts a n d e a rlie r a d v e r tisin g p o ste rs a n d ca ric a tu re s
Fig. 2 • This e ig h t m od el series fo r d isse c te d skulls w as m a d e b y S ir J. F. C o ly e r at the R o y a l C o lle g e o f S u r g e o n s in Lon d on , E n g la n d
E ffe c t o f d r y h e a t p r o c e s s in g on th e p h y s ic a l p r o p e r tie s o f a c r y lic c r o w n s
Gunnar R yge,* D .D .S ., M .S ., and Donald E. Foley , f D .D .S ., Milwaukee
In spite o f the many excellent character istics o f porcelain jacket crowns, acrylic resins have gained considerable accept ance as a material for com plete coronal restorations o f anterior teeth. A crylic resins were first used fo r jacket crowns in the late thirties, and since that time the materials have not changed very m uch. T h e basic ingredients are still methyl methacrylate m onom er with a small amount o f inhibitor such as hydroquinone, and methyl methacrylate poly m er with an initiator o f the peroxide type — most com m only benzoyl peroxide. T echnics have also been rather static. It was observed that a conservative ap proach to the preparation o f the tooth, with well-defined shoulder preparations, was equally advantageous fo r acrylic resin crowns as for porcelain, and the wellestablished technics for impression taking and fabrication o f dies developed for por celain work were adapted to the fabrica tion o f acrylic crowns w ithout m uch m odification. F or many years the conventional heatcuring process in a water bath has been em ployed, but in recent years several practitioners have advocated the use o f dry heat in the processing o f resin crowns.
Limited inform ation is available in the dental literature about the effect o f dry heat processing on the physical properties o f acrylic resin, but in spite o f the paucity o f quantitative data, it has been postu lated in the literature1 that dry heat proc essing results in increased hardness and density, increased resistance to wear and greater dimensional stability. Such claims are apparently based on the well-estab lished fact that hardness, density, abrasion resistance and dimensional stability are related to residual m onom er content— as shown by Caul and Schoonover2 and by several other investigators.3'5 M A T E R IA L S A N D M E T H O D S
Acrylic crowns for two upper cuspids and two upper lateral incisors were p roc essed by the dry heat compression m old ing technic by Charles L. Pincus, Beverly Hills, Calif. T h e dies were duplicated with rubber base impressions and a densite-type stone. Four duplicate crowns were carved in white w ax and sent to two independent laboratories to be processed by a routine compression m olding and boiling water technic. T w o o f Dr. Pincus’ crowns were made with porcelain par-
RYG E— F O L E Y . . . V O L U M E 66, M A Y 1963 • 95/673
tides in the polym er to make them more resistant to wear or abrasion (Fig. 1 ).
Table 2 • Microindentation hardness values Dry heat
Water Sorption Test • Because o f the size
and shape o f the crowns it was necessary to em ploy weight change determination as a m ethod o f studying water sorption. Since it is a com parative study o f crowns o f similar shape, one can obtain some inform ation by measuring the weight changes w ithout consideration o f the sur face area. Crowns were placed in a desiccator at room tem perature and weighed periodi cally until constant weight was obtained on two succeeding days with not more than 0.1 m g. variation. Four crowns were used: tw o control crowns and tw o cured by dry heat, on e o f which contained por celain particles in the polymer. A fter the dehydrated crowns were found to be o f constant weight, they were placed in dis tilled water at 37°C . for 48 hours. T hey were rem oved from the water, blotted with absorbent paper and weighed at one minute to the nearest 0.1 mg. T hey were weighed again at five minutes from the time o f rem oval from the water bath. T h e differences in weight between the dry and w et conditions o f the crowns were com puted. T h e tests were repeated and T able 1 gives the average o f three determinations for each crown. T h e re sults do not support the claim that dry heat processing results in lower water sorption values than the conventional processing m ethod. In fact, the crown processed with dry heat technic and with the porcelain particles in the polymer gave the highest water sorption values.
Mean Knoop hardness
Conventional
A
B
C
D
24
20
21
18
Hardness • T h e K entron Hardness Tester
was em ployed using a 100 Gm . load with 15 seconds time o f descent and 20 seconds dwell time. A n attempt was m ade to ob tain the surface hardness o f the specimen without preparation, but due to the sur face irregularities this m ethod was not satisfactory. M icrohardness testing re quires that the indentation surface be flat. Therefore, the jacket crowns were pre pared using 0, 2 /0 , 3 /0 and 4 /0 abrasive paper and the surfaces were finished with alpha alum ina abrasive on a metallurgical polisher. T e n acceptable indentations were obtained from each o f the specimens and the m ean values com puted for ten tests o f each crown. T h e specim en containing the porcelain particles gave slightly higher mean hard ness values than the others; this was per haps due to the fact that the diam ond indenter cam e in contact with the porce lain particles. It was noted that the in dentations in these specimens were slightly irregular com pared to the specimens that did not contain porcelain particles. M ean hardness values are given in T able 2. N o significant difference in hardness was noted between specimens processed with dry heat and those processed by the con ventional wet method.. Residual M on o m er * M olecu lar weight
Table 1 • W eight changes in per cent after 48 hours storage in water at 37°C. W eighed after remov al from water bath at
Dry heat
Conventional
A %
B %
C %
D %
1 min.
2.3
1.7
1.9
1.6
5 min.
1.9
1.4
1.8
1.1
determinations were carried out on four resin crowns. N o difference was found between the m olecular weight or residual m onom er content o f crowns processed by the dry heat m ethod and those processed by the conventional m ethod. Temperature Recordings • A fter these rather negative findings, it was decided
96/674 • T H E J O U R N A L O F T H E A M E R IC A N D EN T A L A S S O C IA T IO N
Fig. 2 • T e m p e ra tu re -tim e g ra p h
Table 3 • Temperatures recorded during tests O ven temperature
Time
Resin temperature
600°F.~ 700°F.
5 min.
208°F.
700 °F- 800°F.
10 min.
210°F.
800 °F- 900°F.
7
1000°F,
O O o o
o o
CN
to measure the temperatures at which the dry heat processing was actually tak ing place. In most technics advocated and used by clinicians and laboratories, the jacket crowns are first cured in boiling water for a m inim um o f 20 minutes, then rem oved from the water and immediately placed in an oven preheated to 600° to 800°F . for 15 minutes. It has been claim ed1 that this procedure w ould result in temperatures as high as 320°F. inside the resin crowns. In order to measure the actual tem perature reached by this procedure, sev eral jacket crowns were fabricated fo l low ing rigidly the dry heat technic d e scribed by Charles L . Pincus.1 T w o ther m ocouples were im bedded in each flask, with one placed in the thickest part o f the wax pattern and one centrally located in the stone m odel. Holes were drilled in the flasks to per mit the insulated therm ocouple wires to pass through. T h e thermocouples were o f the chrom el-alum el type and a calibrated Brown Potentiom eter was used to record the millivoltages and, thus, the tempera tures. Figure 2 shows typical results. T h e broken line indicates the temperatures measured in the stone, and the solid line, the resin temperature. It took about five minutes to get the internal temperature up to the transition temperature— the temperature at which
rapid polymerization starts. T h e letters Pi and P2 on the graph indicate the trial packing times, as indicated in the tech n ic described by Dr. Pincus,1 and not until about five minutes after the final packing does the temperature rise to ap proxim ately 212°F . A fter 20 minutes processing at this temperature, the flask was placed in the preheated oven and the temperature was recorded every two m in utes using the tw o thermocouples. After 15 minutes in the oven the highest tem perature recorded inside any resin crown in its thickest portion was 226°F ., and in most instances the temperature did not reach above 220°F . P rolonged heating was carried out at various temperatures, and T able 3 shows some o f the temperatures recorded. T h e oven was preheated to the temperatures listed in the first colum n, and it was set in such a m anner that the temperature increased during the time interval o f the dry processing to the second temperature listed. T h e highest internal temperatures recorded are listed in the last columns in Fahrenheit degrees. Even after 25 minutes at 1,000°F. the m axim al temperature in the heaviest portion o f the crow n was only 257°F . These findings were some what puzzling and called for an explana tion. Figure 3 shows a view o f the inside o f one o f the flasks used for the dry heat processing. T his flask was maintained in an 800°F . oven for one-half hour after the norm al curing in boiling water. T he dark ring inside the flask indicates an area o f the stone which is dehydrated— that is : T h e dihydrate has been converted
15 min.
219°F.
20 min.
241°F.
25 min.
257°F.
R Y S E — FO LEY . . . V O L U M E 66, M A Y 1963 • 97/4 75
Fig. 3 • Flask with d e h y d r a te d p e rip h e ra l area, a p p ro x im a te ly o n e q u a rte r inch w ide
back to hemihydrate. This was confirm ed by x-ray diffraction methods, and this reaction is one o f the reasons for the lim ited temperature changes inside the flask. A pparently, there is a large amount of water available in the stone after the cur ing in the boiling water. T h e dry heat first evaporates the excess water and then initiates a dehydration o f the stone, start ing at the periphery o f the flask. Both o f these processes consume a large amount o f heat, so that the temperature in the center o f the flask remains quite low. This, then, means that the internal tem perature never reaches a level at which on e w ould expect a considerable decrease in residual m onom er, and on this basis it is understandable that the hardness and water sorption determination d o not ex hibit significant variations. T h e next question which arises is obvi
ous: Is there any merit in the dry heat processing? T h e answer is no. T h e dry heat processing has not been shown to im prove the physical properties tested. Nonetheless, those w ho advocate this m ethod are quite convinced that their jacket crowns are superior, and one may tend to agree to some extent. T h e ex planation is to be found in the fact that the practitioners w ho utilize this technic are highly motivated and interested co l leagues w ho meticulously carry out excel lent preparations, take all precautions in impression taking, die preparation, wax up, and all other technical p roce dures, including a careful trial packingscheme. T hey are very capable operators w ho pay attention to all those m anipula tive variables which make for g ood den tistry— even with dry heat processing of acrylic resin crowns. 604 North 16th Street
This in vestigation was su ppo rte d by Research G ra n t D-1057 from the N a tio n a l Institute of Dental Research, U. S. Public H e alth Service. *P ro fe ssor and chairm an, de p artm e nt of dental m ate ri als, M a rq u e tte University, Sch oo l of Dentistry. fR esearch assistant, de p artm e nt of dental m aterials, M a rq u e tte University, Sch oo l of Dentistry. 1. Pincus, C . L. New con cepts in m od el techniques and high tem perature proce ssin g of a crylic resins for m axim um esthetics. J. South. C a lifo rn ia D. A . 24:26 Feb. 1956. 2. C aul, H . J., and Schoonover, I. C . A m ethod for de te rm ining the extent o f polym erization of a crylic resins and its a p p lic a tio n s for dentures. J. A . D. A . 39:1 Ju ly 1949. 3. C a u l, H . J.; Sweeney, W . T., a nd Paffenbarger, G . C . R elationship between residual m onom er and som e properties of self-cu ring dental resins. J. A . D. A . 53:60 Ju ly 1956. 4. Sm ith, D. C . A c ry lic denture base. R esidual m o n o mer. Brit. D. J. 105:86 A u g . 5, 1958. 5. A xelsson, B. D e gre e of polym erization of methyl m ethacrylate. O d o n t. Tskr. 63:395 N o v . 1955.
Fig. I • Illu strative e x h ib it o f d e n ta l in strum en ts a n d e a rlie r a d v e r tisin g p o ste rs a n d ca ric a tu re s
Fig. 2 • This e ig h t m od el series fo r d isse c te d skulls w as m a d e b y S ir J. F. C o ly e r at the R o y a l C o lle g e o f S u r g e o n s in Lon d on , E n g la n d