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Creep and hardness testing of some denture base polymers
Creepand hardness testing of some denture base polymers G. Derek Stafford, M.Sc., L.D.S,,* and Robin HuggeR, C.G.I.A., s
A.R.S.H.**
Dental School, Welsh National School of Medicine, Cardiff, Wales, United Kingdom.
C r e e p in plastics is one aspect of their viscoelastic nature, and information concerning the creep behavior of polymers is important as a guide to service performance. Polymers used in the construction of denture bases are subjected tO loading by the masticatory apparatus during function. Thus it is important that there should be no Change in dimension during chewing. In general terms the apparatus for creep testing is simple, and uniaxial tension has been a common technique experimentally. Compressive creep, flexural creep, and shear creep techniques are also used. T w o main requirements are necessary for accurate uniaxial tensile experiments? First, the force must be applied symmetrically along the geometric axis, and second, the extensometer must record the deformation in the specimen and yet not restrict it. The requirements for uniaxial compressive creep testing are similar. Thomas ~ emphasizes that, although true uniaxial compression is more difficult to achieve than true uniaxial tension because the frictional effects between the compression plates and the Specimen ends leads to the development of restrictive forces, The type of experimental method Used in this w o r k attempts to overcome this problem. In addition, the type of test is more of an intermittent stress test, which is nearer to the clinical behavior in chewing, biting, and swallOwing. When polymers are subjected to cOnstant loads they demonstrate a time-dependent strain. These applied stresses may be lower than those which would cause failure of the material in direct loading. In this type of test the application of a small load causes an initial rapid, time-dependent deformation (Fig 1, A) that is related to the rate of loading, the environment, and the nature of the material. This is *Senior Lecturer and Consultant in Restorative Dentistry. **Senior Instructor in Restorative Dentistry.
682
JUNE1978
VOLUME39
NUMBER6
followed b y a slower time-dependent deformation (Fig, 1, B), which is the creep. When the load is ~moved, recovery takes place following a similar , but reversed pattern (Fig. 1, C and D). This recovery can be complete, provided the conditions of time and temperature are suitable. An idealized drawing demonstrating this behavior is shown i n Fig ! " The developmen t of this test has been reported by Huggett? The basic deformation of a denture in function is complex but in terms of molar-to-molar deformation may be more ideally considered as defomaation o f a simple beam. It was therefore necessary to consider Whether this compressive test has relevance to flexural creep testing. A series of experiments w a s c0mpletedlby Stafford and associates4 in which it was shown that this compressiv e test correlated with beam testing; it can be proposed that this compressive test is an ideal alternative. The test machine used for the creep testing was also used for the hardness measurements and for a study" of cyclic loading.
METHOD Tests were completed upon the materials listed in Table I, and the key names given in that table are Used fordescrlption. These materials Were selected as being examples of a standard heat-cured denture base material (Kallodent*) a pour, type denture base material ( T r u p o u r t and P0ur-nTCure:~), a denture base without cross-linking agents (Resin 1"), and a denture base that had been shown experimentally by Gauston 5 to have a greater flexural strength through the addition o f methacrylic acid (Resin 2*)- A polymer containing rubber (Impactw was also tested. *British Drug Houses Ltd., Poole, Dorset, England. ~-Dcntsply International, York, Pa. :~Coe Laboratories, Inc., Chicago, Ill. w Mfg. Go., Romulus, Mich.
0022-3913/78/0639-0682500.60/0 9 1978 The C. V. Mosby Co.
CREEPAND HARDNESSOF POLYMERS
Test specimens were produced while ensuring that the method of preparation to size and surface finish was carefully standardized. The specimens were produced in moulds made by investing 38 • 38 X 1.6 m m pattern blocks of perspex into either gypsum, gypsum and reversible hydrocolloid, or gypsum and modified investment, using the investing technique appropriate to the particular polymer. The method of processing conformed to the manufacturers' instructions. Where manufacturers offered alternative curing cycles the longer cycle was employed. Resin 1 and Resin 2 were cured at 70 ~ C for 14 hours. After processing, a standard finish was obtained using successively finer grades Of silicon carbide paper (grades 280 to 600). The size of the finished specimens was 38 m m X 38 mm X 1.32mm __+ 0.03 ram. T h e specimens were conditioned by storing in a water bath at 37 ~ C for 1 month, thus ensuring that the specimens were fully saturated. This was determined by weighing at suitable intervals. In addition, this long period of storage in water allowed any residual monomer to fully leach out. 6 All measurements were made in air at 23 ~ ___ 2 ~ C. The hardness measurements were made according to the procedures laid down in B.S.I. Specification 3990. 7 The apparatus used, shown in Fig. 2, is a Wallace servooperated microhardness tester.* It measured the depth of the indentation produced in units of 0.0002 ram. This machine utilizes a diamond indentor of pyramidal shape with an apical angle of 136 degrees. The instrument measures the difference between the depth of penetration of the indentor under an initial load of 1 gm and a major load of 300 gm. A detailed description of the apparatus has been given by Huggett s but has not been published. The principle of the Wallace microindentation tester is that it measures the depth of penetration of an indentor into a material under a known dead load. The depth of penetration is indicated directly on a dial gauge and recorded on a flat-bed recorder. The instrument consists of an indentor (A) and a method (B) of applying a contact load and secondary load ((7,) to the indentor. The specimen (1)) is mounted on the table (E), and the indentation is measured by raising this table until the indentor is returned to the same position it occupied before the secondary load was applied. The distance the table has to be raised is measured and displayed by dial gauge (F). T h e *Wallace Tester Model H6B/SA/(2: H. W. Wallace & (20. Ltd., Croydon, Surrey, England.
THE JOURNALOF PROSTHETICDENTISTRY
mmmmmmmmm~+||l
m~m~mm 8
c
A
D
Fig. I. Idealized deformation curve illustrates the st~iges involved in load application and removal. (A) Initial deformation, (B) Creep, (C) Initial recovery/ (D) Slow recovery. Table I. Materials examined Name used
Polymer type
Material
in study
Manufac~rer i
Heat-polymerized Kallodent 60 dough-molded Autopolymerizing Dentsply pour-type Trupour Autopolymerizing Pour-n-Cure pour-type Heat-polymerized Impact dough-m01ded Heat-polymerized Kallodent 333 dough-molded polymer with pure liquid monomer* Heat-polymerized Kallodent 333 dough-molded polymer with pure liquid
Kallodent Dental Mfg. Co.
Trupour Pour-nCure Impact
Dentsply International Coe Labs, Inc. Kerr Mfg. Co.
Resin 1
Dental Mfg. Co. for polymer
Resin 2
Dental Mfg. Co. for polymer
monomer*
20% EGDMt and 1.75% methacrylic acid~ *Monomer: British Drug Houses Ltd., Poole,"Dorset, England. ~'EGDM:Ethylene GlycolDimethacrylate, Koch-Light Laboratories Ltd., (2olnbrook,Bucks, England. :~Methacrylic acid: Koch-Light Laboratories Ltd., (2olnbrook, Bucks, England.
position occupied by the indentor before and after the secondary load is applied is termed the "null" position. When the null point is achieved a zero reading is shown on the "null" meter (G). The primary load and secondary load are applied automatically by the pneumatic load applicator, which is controlled from the control box. This control box actuates the load application by operating the individual buttons or by a semiautomatic sequence. When the semiautomatic sequence is used the time intervals between the application and removal of the
683
STAFFORD AND HUGGETI"
Fig. 2. Wallace Hardness Tester H6B/SAC. (+4)Indentor. (B) Method of applying load. (C) Secondary load. (D) Specimenl (E) Specimen table. (F) Dial gauge. (G) "Null" meter. (H) Control box. (]) (K) (L) Electric timers. (M) Linear potentiometer. (N) Chart recorder.
Table II. Creep test results Indentation at 5 sec (mm • 10 4) (n ffi 10)
I n d e n t a t i o n at 180 sec
(mm • 10-+) (n ---- 10)
Material
Mean
SD
CV (%)
Mean
SD
CV (%)
Percent creep
Kallodent Trupour Pour-n-Cure Impact Resin 1 Resin 2
260.6 291.0 271.5 279.9 272.0 260.2
3.8 3.3 2.7 1.5 2.9 5.4
1.50 1.10 0.99 0.50 1.10 2.10
294.6 351.0 317.2 319.5 306.8 295.4
3.5 7.5 3.8 2.5 5.2 7.6
1.2 2.1 1.2 0.8 1.7 2.6
13.1 20.6 16.8 14.2 12.8 13.5
GV: Coefficient of variation.
primary and secondary loads are determined by preset electric timers (1t, J, and K). By using the third timer (K), continuous application and removal of the secondary load may be automatically undertaken. When the load is applied to the indentor the capacitor comparator becomes unbalanced, and this electrical imbalance provides a potential to an electric moto r t h a t is geared to move the specimen table (E) up or down depending upon the sense of imbalance. The motor returns the table, specimen, and indentor to the null position. This movement, in addition to being measured directly on the dial gauge, is recorded on the chart recorder, thus enabling continuous recording of indentation and recovery characteristics to be monitored. This facility is effected by a linear potentiometer (L) linked to the servosystem by an electromagnetic clutch. The reproducibility of the apparatus has been reported by Huggett. '~ It is a particularly sensitive
684
apparatus (sensitivity _ 0.0015 m m ) and is capable of detecting small variations in hardness. The specification of the machine conforms to B.S. 3990. 7 Each creep test was repeated 10 times on each specimen and the mean calculated. Creep is expressed in this study as the difference between the initial indentation at 5 seconds and the indentation 180 Seconds later. Repeated cycle tests were also carried out. The loading and unloading cycles were each of 60 seconds duration. Hardness measurements were also repeated 10 times on each specimen, and the mean of the indentations at 15 seconds was calculated. RESULTS The results of the creep tests are given in Table II. The results of the hardness tests are given in Table III. The results of the cyclic loading are g i v e n in Table IV and are shown graphically in Fig. 3. The creep and hardness tests were analyzed for comparative significance using Students t-test. T h e
JUNE 1978
VOLUME 39
NUMBER 6
CREEP AND HARDNESS OF POLYMERS
B] A] . _ . ~ ~ ~ . . . , . : . . ~ 300 9
-----"
Trupour
~
Pour-n-Cure
. . . . . * Impact mmmm Kallodent
:a ..-.........
: 1 i=72;
3 ~.oo
3
~
100.
4 4 49
4
D
60 5
60 5
60 5
60
TIME (Seconds)
Fig. 3. Cyclic loading of materials. Table III. Hardness test results Indentation at 15 sec (mm X I 0 -~) (n --- I0)
Material
Mean
KaUodent Trupour Pour-n-Cure Impact Resin 1 Resin 2
272.2 306.5 287.8 292.8 282.4 271.8
SD i
i
3.3 4.1 3.4 2.6 3.0 4.0
Coefficient of variation (%) I
j
1.2 1.3 1.2 0.9 1.1 1.5
creep tests showed that there was no significant difference (p > .05) in behavior between Kallodent, Impact, Resin 1, and Resin 2. Thus there was no change in this property in this test in the non-crosslinked or methacrylic acid-added materials when compared with a standard denture base material. Neither did the addition of rubber, as in Impact, alter this property, Trupour showed the greatest amount of creep, and this was very significantly different from the creep of all the other materials (p <.001). Trupour was also the softest material tested, the difference from all the other materials was significantly different. Impact also gave a high softness
THE JOURNAL OF PROSTHETIC DENTISTRY
value that was very significantly different from the softness of Kallodent, Trupour, Resin 1, and Resin 2 and was significantly different from Pour-n-Cure (p < .01). The addition of the methacrylic acid a n d the cross-linking increased the hardness when compared with Resin 1, but this resin was very similar to Kallodent. DISCUSSION The difference in viscoelastic behavior of heatcured and autopolymerizing resin bases has been noted by 8~rensen and Ryge,10 Glantz and Bates, 1~ Glantz and Stafford, 12 and Braden. 13 Loading of polymer strips to examine the creep and recovery was carried out by S~rensen and Ryg& ~ using transverse-bend test specimens. They loaded the specimens to produce stress levels of 7.1 M N / m 2, 17.1 M N / m 2, 29,4 M N / m ~, and 47.1 M N / m 2 and examined behavior at 21 o G, 34 ~ G and 58 ~ C, using A.D.A. Specification test No. 12. However, they allowed a 24-hour recovery period between each loading. Glantz and Stafford ~2 loaded cantilever specimens at a stress level of 5,8 M N / m 2 at 37 ~ G, Specimens were loaded for 6 hours, and recovery periods were 18 hours. This latter work showed good agreement with S~rensen and Ryge, ~~ and this present study agreed
685
STAFFORD A N D HUGGETT
KALLODENT 300
250
1
200
150
100
50
30 secs.
Time
Fig. 4. Repeated loading of Kallodent for a number of cycles. This behavior is representative of the pattern showed by all the materials tested. T a b l e IV. Depth of indentation ( m m x 10 -4) (n = 5) Cycle I
C y c l e II
Load o n
5 sec A
Load off
L o a d off
Load on
60 sec
5 sec
60 sec
5 sec
60 sec
5 sec
60 sec
B
C
D
A1
B1
C1
D1
Material
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Kallodent Trupour Pour-n-Cure Impact Resin 1 Resin 2
262.6 292.6 292.6 281.9 273.2 259.2
2.8 3.3 2.1 2.5 2.2 1.7
283.9 330.2 325.1 304.2 298.4 284.4
2.7 5.9 1.7 2.1 1.6 2.6
128.5 176.7 161.5 142.2 106.8 123.2
5.5 9.6 2.2 4.0 4.6 6.2
103.1 148.3 127.5 116.8 77.2 89.6
5.8 5.2 1.1 3.5 5.9 5.2
268.2 313.4 304.8 291.5 283.6 268.8
3.8 5.8 1.8 5.1 1.6 2.6
288.5 337.8 332.2 311.4 303.2 287.6
4.2 6.7 2.1 4.5 1.0 2.6
148.3 204,2 185.4 165.6 135.6 144.4
2.2 4.2 1.7 2.1 1.6 2.6
124.9 174.7
8.4 4.1 2.2 3.3 1.6 2.8
with the earlier work of Glantz and Stafford 12 and Stafford and associates? The actual stress varied due to the shape of the indentor and was in the region of 125 M N / m 2. The cyclic tests (Fig. 3) show that the trace is characteristic of Fig. 1. The initial deformation (Fig. 3, A) and the amount of creep (Fig. 3, A and B) of the pour-type resins is greater than that of the other resins. The recovery (Fig. 3, B and D) is also iess. T h e same ranking pattern is obtained on the second loading cycle. In this second cycle the depth of indentation of both A 1 and B 1 is greater than at A and B, and at C1 and D I i t is greater than at C and D. The greater depth of indentation in the second cycle
686
153.4 142.7 106.4 114.0
is due to the combined effects of that particular loading and the lack of recovery of the material due to the previous loading. I m p a c t occupies an intermediary position between the heat-cured and pour-type resins. Kallodent, Resin 1, and Resin 2 showed similar behavior, so that neither the loss of crosslinking agent nor addition of methacrylic acid had a marked effect on the pattern of behavior. SOrensen and Ryge 1~ suggested that the use of cross-linked monomer increased the deflection, although Jagger and Huggett 14 did not find any significant difference in creep or hardness when added to the a m o u n t of 10%. Fig. 4 shows the effect of repeated loading cycles;
JUNE 1978
VOLUME 39
NUMBER 6
CREEP AND HARDNESS OF POLYMERS
it is interesting to note that the resulting deformation is greater from each subsequent loading and that the amount of recovery is less. The same pattern exists for all the materials tested. SUMMARY
5.
6. 7. 8.
A method of measuring creep, recovery, hardness, and the effect of cyclic loading on denture base polymers using a modified Wallace hardness tester is described. This apparatus is particularly sensitive and is capable of detecting very small variations in these properties of the materials. A representative group of denture base polymers that include heatcured, pour-type, and other modified materials was tested. The results obtained are in good agreement with those shown by other workers.
10.
The authors gratefully acknowledge the technical assistance of Mrs. A. Shipper in preparing the test specimens.
13.
REFERENCES 1. Haward, R. N.: The Strength of Plastics and Glass, ed 1. London, 1949, Interscience. 2. Thomas, D. A.: Uniaxial compressive creep studies. Plastics and Polymers, 37:485, 1969. 3. Huggett, R.: Studies on the measurement of hardness of some denture base resins. Denny Award Thesis, British Institute of Surgical Technicians, London, 1975. 4. Stafford, G. D., Bates, J. F., Huggett, R., and Glantz, P.-O.: Creep in denture base polymers. J Dent 3:193, 1975.
THE JOURNAL OF PROSTHETIC DENTISTRY
9.
11. 12.
14.
Causton, B. E.: The physical and chemical properties of some polymeric materials. Thesis, University Of London, 1972. Walter, J. D., and Glaysher, J. K.: The properties of self curing denture bases. Br Dent J 132:223, 1972. British Standards Institution: British Standards Institution, Specification 3990, amendment slip No. 1, Aug. 1968. Huggett, R.: An evaluation of recently developed denture base materials. City and Guilds of London Institute Insignia Award Thesis in Dental Technology, 1976. Huggett, R.: Studies on the measurement of hardness of some dental acrylic resins. J Br Inst Surg Techns 1:36, 1975. S~rensen, S. E., and Ryge, G.: Flow and recovery of denture plastics. J PROSTnETDENT 12:1079, 1962. Glantz, PI-O., and Bates, J. F.: Creep in some acrylic dental resins. Odont Revy 24:283 , 1973. Glantz, P.-O., and Stafford, G. D.: Recovery of some acrylic dental resins after repeated loading. Swed Dent J 66:129, 1973. Braden, M.: Scientific aspects of dental materials. In J. A. yon Fraunhofer (editor), ed 1. Butterworths, London, p 433. Jagger, R. G., and Huggett, R.: The effect of cross-linking on the indentation resistance, creep and recovery of an acrylic resin denture base material. J Dent 3:15, 1975.
Reprint requests to: DR. G. D. STAFFORD DENTAL HOSPITAL
HEATH CARDIFFCF4 4XY WALES
687
A.R.S.H.**
Dental School, Welsh National School of Medicine, Cardiff, Wales, United Kingdom.
C r e e p in plastics is one aspect of their viscoelastic nature, and information concerning the creep behavior of polymers is important as a guide to service performance. Polymers used in the construction of denture bases are subjected tO loading by the masticatory apparatus during function. Thus it is important that there should be no Change in dimension during chewing. In general terms the apparatus for creep testing is simple, and uniaxial tension has been a common technique experimentally. Compressive creep, flexural creep, and shear creep techniques are also used. T w o main requirements are necessary for accurate uniaxial tensile experiments? First, the force must be applied symmetrically along the geometric axis, and second, the extensometer must record the deformation in the specimen and yet not restrict it. The requirements for uniaxial compressive creep testing are similar. Thomas ~ emphasizes that, although true uniaxial compression is more difficult to achieve than true uniaxial tension because the frictional effects between the compression plates and the Specimen ends leads to the development of restrictive forces, The type of experimental method Used in this w o r k attempts to overcome this problem. In addition, the type of test is more of an intermittent stress test, which is nearer to the clinical behavior in chewing, biting, and swallOwing. When polymers are subjected to cOnstant loads they demonstrate a time-dependent strain. These applied stresses may be lower than those which would cause failure of the material in direct loading. In this type of test the application of a small load causes an initial rapid, time-dependent deformation (Fig 1, A) that is related to the rate of loading, the environment, and the nature of the material. This is *Senior Lecturer and Consultant in Restorative Dentistry. **Senior Instructor in Restorative Dentistry.
682
JUNE1978
VOLUME39
NUMBER6
followed b y a slower time-dependent deformation (Fig, 1, B), which is the creep. When the load is ~moved, recovery takes place following a similar , but reversed pattern (Fig. 1, C and D). This recovery can be complete, provided the conditions of time and temperature are suitable. An idealized drawing demonstrating this behavior is shown i n Fig ! " The developmen t of this test has been reported by Huggett? The basic deformation of a denture in function is complex but in terms of molar-to-molar deformation may be more ideally considered as defomaation o f a simple beam. It was therefore necessary to consider Whether this compressive test has relevance to flexural creep testing. A series of experiments w a s c0mpletedlby Stafford and associates4 in which it was shown that this compressiv e test correlated with beam testing; it can be proposed that this compressive test is an ideal alternative. The test machine used for the creep testing was also used for the hardness measurements and for a study" of cyclic loading.
METHOD Tests were completed upon the materials listed in Table I, and the key names given in that table are Used fordescrlption. These materials Were selected as being examples of a standard heat-cured denture base material (Kallodent*) a pour, type denture base material ( T r u p o u r t and P0ur-nTCure:~), a denture base without cross-linking agents (Resin 1"), and a denture base that had been shown experimentally by Gauston 5 to have a greater flexural strength through the addition o f methacrylic acid (Resin 2*)- A polymer containing rubber (Impactw was also tested. *British Drug Houses Ltd., Poole, Dorset, England. ~-Dcntsply International, York, Pa. :~Coe Laboratories, Inc., Chicago, Ill. w Mfg. Go., Romulus, Mich.
0022-3913/78/0639-0682500.60/0 9 1978 The C. V. Mosby Co.
CREEPAND HARDNESSOF POLYMERS
Test specimens were produced while ensuring that the method of preparation to size and surface finish was carefully standardized. The specimens were produced in moulds made by investing 38 • 38 X 1.6 m m pattern blocks of perspex into either gypsum, gypsum and reversible hydrocolloid, or gypsum and modified investment, using the investing technique appropriate to the particular polymer. The method of processing conformed to the manufacturers' instructions. Where manufacturers offered alternative curing cycles the longer cycle was employed. Resin 1 and Resin 2 were cured at 70 ~ C for 14 hours. After processing, a standard finish was obtained using successively finer grades Of silicon carbide paper (grades 280 to 600). The size of the finished specimens was 38 m m X 38 mm X 1.32mm __+ 0.03 ram. T h e specimens were conditioned by storing in a water bath at 37 ~ C for 1 month, thus ensuring that the specimens were fully saturated. This was determined by weighing at suitable intervals. In addition, this long period of storage in water allowed any residual monomer to fully leach out. 6 All measurements were made in air at 23 ~ ___ 2 ~ C. The hardness measurements were made according to the procedures laid down in B.S.I. Specification 3990. 7 The apparatus used, shown in Fig. 2, is a Wallace servooperated microhardness tester.* It measured the depth of the indentation produced in units of 0.0002 ram. This machine utilizes a diamond indentor of pyramidal shape with an apical angle of 136 degrees. The instrument measures the difference between the depth of penetration of the indentor under an initial load of 1 gm and a major load of 300 gm. A detailed description of the apparatus has been given by Huggett s but has not been published. The principle of the Wallace microindentation tester is that it measures the depth of penetration of an indentor into a material under a known dead load. The depth of penetration is indicated directly on a dial gauge and recorded on a flat-bed recorder. The instrument consists of an indentor (A) and a method (B) of applying a contact load and secondary load ((7,) to the indentor. The specimen (1)) is mounted on the table (E), and the indentation is measured by raising this table until the indentor is returned to the same position it occupied before the secondary load was applied. The distance the table has to be raised is measured and displayed by dial gauge (F). T h e *Wallace Tester Model H6B/SA/(2: H. W. Wallace & (20. Ltd., Croydon, Surrey, England.
THE JOURNALOF PROSTHETICDENTISTRY
mmmmmmmmm~+||l
m~m~mm 8
c
A
D
Fig. I. Idealized deformation curve illustrates the st~iges involved in load application and removal. (A) Initial deformation, (B) Creep, (C) Initial recovery/ (D) Slow recovery. Table I. Materials examined Name used
Polymer type
Material
in study
Manufac~rer i
Heat-polymerized Kallodent 60 dough-molded Autopolymerizing Dentsply pour-type Trupour Autopolymerizing Pour-n-Cure pour-type Heat-polymerized Impact dough-m01ded Heat-polymerized Kallodent 333 dough-molded polymer with pure liquid monomer* Heat-polymerized Kallodent 333 dough-molded polymer with pure liquid
Kallodent Dental Mfg. Co.
Trupour Pour-nCure Impact
Dentsply International Coe Labs, Inc. Kerr Mfg. Co.
Resin 1
Dental Mfg. Co. for polymer
Resin 2
Dental Mfg. Co. for polymer
monomer*
20% EGDMt and 1.75% methacrylic acid~ *Monomer: British Drug Houses Ltd., Poole,"Dorset, England. ~'EGDM:Ethylene GlycolDimethacrylate, Koch-Light Laboratories Ltd., (2olnbrook,Bucks, England. :~Methacrylic acid: Koch-Light Laboratories Ltd., (2olnbrook, Bucks, England.
position occupied by the indentor before and after the secondary load is applied is termed the "null" position. When the null point is achieved a zero reading is shown on the "null" meter (G). The primary load and secondary load are applied automatically by the pneumatic load applicator, which is controlled from the control box. This control box actuates the load application by operating the individual buttons or by a semiautomatic sequence. When the semiautomatic sequence is used the time intervals between the application and removal of the
683
STAFFORD AND HUGGETI"
Fig. 2. Wallace Hardness Tester H6B/SAC. (+4)Indentor. (B) Method of applying load. (C) Secondary load. (D) Specimenl (E) Specimen table. (F) Dial gauge. (G) "Null" meter. (H) Control box. (]) (K) (L) Electric timers. (M) Linear potentiometer. (N) Chart recorder.
Table II. Creep test results Indentation at 5 sec (mm • 10 4) (n ffi 10)
I n d e n t a t i o n at 180 sec
(mm • 10-+) (n ---- 10)
Material
Mean
SD
CV (%)
Mean
SD
CV (%)
Percent creep
Kallodent Trupour Pour-n-Cure Impact Resin 1 Resin 2
260.6 291.0 271.5 279.9 272.0 260.2
3.8 3.3 2.7 1.5 2.9 5.4
1.50 1.10 0.99 0.50 1.10 2.10
294.6 351.0 317.2 319.5 306.8 295.4
3.5 7.5 3.8 2.5 5.2 7.6
1.2 2.1 1.2 0.8 1.7 2.6
13.1 20.6 16.8 14.2 12.8 13.5
GV: Coefficient of variation.
primary and secondary loads are determined by preset electric timers (1t, J, and K). By using the third timer (K), continuous application and removal of the secondary load may be automatically undertaken. When the load is applied to the indentor the capacitor comparator becomes unbalanced, and this electrical imbalance provides a potential to an electric moto r t h a t is geared to move the specimen table (E) up or down depending upon the sense of imbalance. The motor returns the table, specimen, and indentor to the null position. This movement, in addition to being measured directly on the dial gauge, is recorded on the chart recorder, thus enabling continuous recording of indentation and recovery characteristics to be monitored. This facility is effected by a linear potentiometer (L) linked to the servosystem by an electromagnetic clutch. The reproducibility of the apparatus has been reported by Huggett. '~ It is a particularly sensitive
684
apparatus (sensitivity _ 0.0015 m m ) and is capable of detecting small variations in hardness. The specification of the machine conforms to B.S. 3990. 7 Each creep test was repeated 10 times on each specimen and the mean calculated. Creep is expressed in this study as the difference between the initial indentation at 5 seconds and the indentation 180 Seconds later. Repeated cycle tests were also carried out. The loading and unloading cycles were each of 60 seconds duration. Hardness measurements were also repeated 10 times on each specimen, and the mean of the indentations at 15 seconds was calculated. RESULTS The results of the creep tests are given in Table II. The results of the hardness tests are given in Table III. The results of the cyclic loading are g i v e n in Table IV and are shown graphically in Fig. 3. The creep and hardness tests were analyzed for comparative significance using Students t-test. T h e
JUNE 1978
VOLUME 39
NUMBER 6
CREEP AND HARDNESS OF POLYMERS
B] A] . _ . ~ ~ ~ . . . , . : . . ~ 300 9
-----"
Trupour
~
Pour-n-Cure
. . . . . * Impact mmmm Kallodent
:a ..-.........
: 1 i=72;
3 ~.oo
3
~
100.
4 4 49
4
D
60 5
60 5
60 5
60
TIME (Seconds)
Fig. 3. Cyclic loading of materials. Table III. Hardness test results Indentation at 15 sec (mm X I 0 -~) (n --- I0)
Material
Mean
KaUodent Trupour Pour-n-Cure Impact Resin 1 Resin 2
272.2 306.5 287.8 292.8 282.4 271.8
SD i
i
3.3 4.1 3.4 2.6 3.0 4.0
Coefficient of variation (%) I
j
1.2 1.3 1.2 0.9 1.1 1.5
creep tests showed that there was no significant difference (p > .05) in behavior between Kallodent, Impact, Resin 1, and Resin 2. Thus there was no change in this property in this test in the non-crosslinked or methacrylic acid-added materials when compared with a standard denture base material. Neither did the addition of rubber, as in Impact, alter this property, Trupour showed the greatest amount of creep, and this was very significantly different from the creep of all the other materials (p <.001). Trupour was also the softest material tested, the difference from all the other materials was significantly different. Impact also gave a high softness
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value that was very significantly different from the softness of Kallodent, Trupour, Resin 1, and Resin 2 and was significantly different from Pour-n-Cure (p < .01). The addition of the methacrylic acid a n d the cross-linking increased the hardness when compared with Resin 1, but this resin was very similar to Kallodent. DISCUSSION The difference in viscoelastic behavior of heatcured and autopolymerizing resin bases has been noted by 8~rensen and Ryge,10 Glantz and Bates, 1~ Glantz and Stafford, 12 and Braden. 13 Loading of polymer strips to examine the creep and recovery was carried out by S~rensen and Ryg& ~ using transverse-bend test specimens. They loaded the specimens to produce stress levels of 7.1 M N / m 2, 17.1 M N / m 2, 29,4 M N / m ~, and 47.1 M N / m 2 and examined behavior at 21 o G, 34 ~ G and 58 ~ C, using A.D.A. Specification test No. 12. However, they allowed a 24-hour recovery period between each loading. Glantz and Stafford ~2 loaded cantilever specimens at a stress level of 5,8 M N / m 2 at 37 ~ G, Specimens were loaded for 6 hours, and recovery periods were 18 hours. This latter work showed good agreement with S~rensen and Ryge, ~~ and this present study agreed
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STAFFORD A N D HUGGETT
KALLODENT 300
250
1
200
150
100
50
30 secs.
Time
Fig. 4. Repeated loading of Kallodent for a number of cycles. This behavior is representative of the pattern showed by all the materials tested. T a b l e IV. Depth of indentation ( m m x 10 -4) (n = 5) Cycle I
C y c l e II
Load o n
5 sec A
Load off
L o a d off
Load on
60 sec
5 sec
60 sec
5 sec
60 sec
5 sec
60 sec
B
C
D
A1
B1
C1
D1
Material
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Kallodent Trupour Pour-n-Cure Impact Resin 1 Resin 2
262.6 292.6 292.6 281.9 273.2 259.2
2.8 3.3 2.1 2.5 2.2 1.7
283.9 330.2 325.1 304.2 298.4 284.4
2.7 5.9 1.7 2.1 1.6 2.6
128.5 176.7 161.5 142.2 106.8 123.2
5.5 9.6 2.2 4.0 4.6 6.2
103.1 148.3 127.5 116.8 77.2 89.6
5.8 5.2 1.1 3.5 5.9 5.2
268.2 313.4 304.8 291.5 283.6 268.8
3.8 5.8 1.8 5.1 1.6 2.6
288.5 337.8 332.2 311.4 303.2 287.6
4.2 6.7 2.1 4.5 1.0 2.6
148.3 204,2 185.4 165.6 135.6 144.4
2.2 4.2 1.7 2.1 1.6 2.6
124.9 174.7
8.4 4.1 2.2 3.3 1.6 2.8
with the earlier work of Glantz and Stafford 12 and Stafford and associates? The actual stress varied due to the shape of the indentor and was in the region of 125 M N / m 2. The cyclic tests (Fig. 3) show that the trace is characteristic of Fig. 1. The initial deformation (Fig. 3, A) and the amount of creep (Fig. 3, A and B) of the pour-type resins is greater than that of the other resins. The recovery (Fig. 3, B and D) is also iess. T h e same ranking pattern is obtained on the second loading cycle. In this second cycle the depth of indentation of both A 1 and B 1 is greater than at A and B, and at C1 and D I i t is greater than at C and D. The greater depth of indentation in the second cycle
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153.4 142.7 106.4 114.0
is due to the combined effects of that particular loading and the lack of recovery of the material due to the previous loading. I m p a c t occupies an intermediary position between the heat-cured and pour-type resins. Kallodent, Resin 1, and Resin 2 showed similar behavior, so that neither the loss of crosslinking agent nor addition of methacrylic acid had a marked effect on the pattern of behavior. SOrensen and Ryge 1~ suggested that the use of cross-linked monomer increased the deflection, although Jagger and Huggett 14 did not find any significant difference in creep or hardness when added to the a m o u n t of 10%. Fig. 4 shows the effect of repeated loading cycles;
JUNE 1978
VOLUME 39
NUMBER 6
CREEP AND HARDNESS OF POLYMERS
it is interesting to note that the resulting deformation is greater from each subsequent loading and that the amount of recovery is less. The same pattern exists for all the materials tested. SUMMARY
5.
6. 7. 8.
A method of measuring creep, recovery, hardness, and the effect of cyclic loading on denture base polymers using a modified Wallace hardness tester is described. This apparatus is particularly sensitive and is capable of detecting very small variations in these properties of the materials. A representative group of denture base polymers that include heatcured, pour-type, and other modified materials was tested. The results obtained are in good agreement with those shown by other workers.
10.
The authors gratefully acknowledge the technical assistance of Mrs. A. Shipper in preparing the test specimens.
13.
REFERENCES 1. Haward, R. N.: The Strength of Plastics and Glass, ed 1. London, 1949, Interscience. 2. Thomas, D. A.: Uniaxial compressive creep studies. Plastics and Polymers, 37:485, 1969. 3. Huggett, R.: Studies on the measurement of hardness of some denture base resins. Denny Award Thesis, British Institute of Surgical Technicians, London, 1975. 4. Stafford, G. D., Bates, J. F., Huggett, R., and Glantz, P.-O.: Creep in denture base polymers. J Dent 3:193, 1975.
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9.
11. 12.
14.
Causton, B. E.: The physical and chemical properties of some polymeric materials. Thesis, University Of London, 1972. Walter, J. D., and Glaysher, J. K.: The properties of self curing denture bases. Br Dent J 132:223, 1972. British Standards Institution: British Standards Institution, Specification 3990, amendment slip No. 1, Aug. 1968. Huggett, R.: An evaluation of recently developed denture base materials. City and Guilds of London Institute Insignia Award Thesis in Dental Technology, 1976. Huggett, R.: Studies on the measurement of hardness of some dental acrylic resins. J Br Inst Surg Techns 1:36, 1975. S~rensen, S. E., and Ryge, G.: Flow and recovery of denture plastics. J PROSTnETDENT 12:1079, 1962. Glantz, PI-O., and Bates, J. F.: Creep in some acrylic dental resins. Odont Revy 24:283 , 1973. Glantz, P.-O., and Stafford, G. D.: Recovery of some acrylic dental resins after repeated loading. Swed Dent J 66:129, 1973. Braden, M.: Scientific aspects of dental materials. In J. A. yon Fraunhofer (editor), ed 1. Butterworths, London, p 433. Jagger, R. G., and Huggett, R.: The effect of cross-linking on the indentation resistance, creep and recovery of an acrylic resin denture base material. J Dent 3:15, 1975.
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