G. D. Stafford, MSC, LDS J. F. Bates, BDS, MSC, DDS R. Huggett, LBIST, ARSH Department Cardiff
P.-O. Glantz, Department University
BDS, Odont Dr
of Dental Technology, Faculty of Odontology, of Gothenburg, Gothenburg, Sweden
ABSTRACT Creep behaviour of denture base resins is an important mechanical property that is not evaluated in standard dental materials testing. This paper outlines the development of a compressive creep test based upon a standard hardness testing machine that shows a good correlation to threepoint creep behaviour. Heat-polymerized, autopolymerized and pour type polymethyl methacrylate resins were tested and compared with glass-filled Nylon 12.
which is followed by a time-dependent tion which is the element known When the load is removed there is recovery, and then a time-dependent that may or may not be complete given time limit of a test (Fig. 1).
deformaas ‘creep’. an instant recovery within the Generally,
INTRODUCTION is important that in any practical use of plastics there should be resistance to change of shape when subjected to small stresses over short periods of time. One of the aspects of dimensional change which requires consideration in any structure in a plastic material is that of ‘creep’. This deformation is the time-dependent part of deformation undergone by a plastic material when loaded below its ‘elastic limit’. When a polymer is stressed below its elastic limit there is an immediate rapid instant deformation
* Based on a paper presented at the Annual Conference of the British Society for the Study of Prosthetic Dentistry in March 1974.
deformation curve lo illustrate the stages involved in load application and removal. 1, Instant deformation: 2, creep; 3, instant recovery; 4, time-dependent recovery (primary creep); 5, permanent set (secondary creep). provided that adequate time is allowed and temperature conditions are suitable, there could be full recovery, but there may be a measure of viscous flow that can be experienced by a specimen. This paper considers some aspects of’ this property in relation to denture base plastics and outlines the stages through which the work has progressed so far. Our aim is to produce a
laboratory test that can be carried out easily and rapidly and shows whether denture base polymers creep under clinical conditions. Earlier work (Glantz and Bates, 1973) has shown that in cantilever bending tests creep occurs in denture base polymers and that
40 Tfme (h)
Fig. 2.-Deformation of heat-polymerized polymethyl methacrylate resin in load application and removal.
Fig. 3.-Deformation of a pour type polymethyl methacrylate resin in load application and removal. Table batch
Kallodent 333 Orthocryl
Glass-filled nylon 12 (Grilamid)
170001 /I21/002/1 5363
L/L1 3/M.D.2 18212
Hawley, Russell 81 Baker Ltd, Potters Bar, Herts Vernon Benshoff, Albany, New York, USA Emser Werke, AG (Zurich). Sole UK distributors : Grilon Plastic Machinery Ltd. Dover
deformation remains in certain specimens even after a recovery period of 48 hours. The specimens were loaded in such a way as to impart a stress of 12 N/mm2 and this was maintained for 30 hours. The test was performed at a temperature of 37 “C in a relative humidity of 80 per cent. Fig. 2 shows the behaviour of a heat-polymerized acrylic resin. The amount of initial deformation, the large amount of recovery and the final deformation should be noted. Fig. 3 shows the behaviour of one of the pour type resins, and the greater amount of creep should be noted. The resulting deformation that remains after 48 hours’ recovery is also greater than that demonstrated by the heat-cured resins. Repeated loading experiments using a stress level of 6 N/mm2 show a similar pattern (Glantz and Stafford, 1973). Specimens were loaded for three periods of 6 hours, and dividing these periods and following the last one were three deloading periods of 18 hours. The amount of remaining deformation increases with each successive loading cycle. This work has been extended to compare preliminary results of transverse bending with compressive loading in an endeavour to find a better and easier test.
SPECIMEN Time (h)
Specimens were produced in polymers representative of heat-polymerized, autopolymerized and pour type polymethyl methacrylate resins and glass-filled nylon 12 as shown in Table I. The specimens were processed according to the manufacturers’ instructions, except that the nylon was processed using a method suitable for dental appliances (Hargreaves, 1971). After processing, the specimens were carefully handfinished to achieve a standard surface finish and size.
A series of transverse bend tests were carried out using a Zwick testing machine.* The * Zwick Universal Testing Machine, Type 1361, Zwick, West Germany.
et al. : Creep
specimens were 60 x 15 x 2 mm and the gauge length of the test equipment was 51 mm. The specimens were all measured individually, and the load arranged so that the stress applied was equivalent to 7.6 N/mm2. The deformation of the test pieces was determined by an electronic measurement of the deflections by a recording arm, which was brought into contact with the lower surface of the test piece beneath the central loading point; with this type of recorder the recorded deflection is magnified by 250 000 times. The test pieces were loaded for ten periods, each lasting for 10 minutes. Separating each of the loading periods were relaxation periods of 10 minutes. The time necessary for the application and removal of the load was approximately IO seconds. The deformation of the specimens was recorded immediately after loading, at the end Tab/e //.-Amount dental resins
of the loading period and at the beginning and the end of the relaxation period. The deformation values registered at these stages were used to determine both the magnitude of the increase in deformation during the loading and the amount of recovery between the beginning and the end of the relaxation periods. The results of repeated loading are shown in Table II. which lists the deformation measured through ten consecutive loading cycles. Column A shows the instant deformation on loading and is equivalent to distance 1 in Fig. 1. Column B shows the additional deformation after IO minutes of loading, and is represented by distance 2 in Fig. 1. Column C represents the time-dependent recovery as shown by distance 4 in Fig. I, and column D shows the deformation that remains after 10 minutes’ deloading. and is represented by distance 5 in Fig. 1. The recording arm of the testing machine returns stages
Amount Loading cycle
Column A Instant deformation
1 2 3 5 10
427.6116.8 418.8119.4 418~0118.9 418.0&l 8.3 418.4kl5.1
63.6& 62.01 59.6+ 58.4* 56.8~-
4.6 6.0 1.7 1.7 2.7
48.41 48.8? 51.2+ 47.61 51.6 -
4.3 3.3 5.0 4.3 3.8
22.1 .:I 135 11,6+3.6 8.4+5.5 8-O5.5 4.8 1 5.2
1 2 3 5 10
410.4136.9 394.0122.4 392.8k23.7 406.0146.0 396.8h24.4
43.2~ 42.4& 39.21 37.2:k 34.4*
4.4 4-6 3.9 3.3 5.0
40.4r 31.2*12.1 34.8+ 35.6&-41.24
30.0-c-25.4 16%k17.3 7.6-i 8.5 5.2 + 7.2 0.0 : 0.0
1 2 3 5 10
563.6h83.3 554.0182.1 529.2k76.2 528.0172.0 529.2*72.1
1 2 3 5 10
393.2&l 5.1 381.6114.0 378.8116.8 371.2&l 0.0 377.2+17.9
96.4121.4 108.0&15.4 96.8110.0 96.4+10.5 89.6$7.1 54.01 52.41 53.21 58.4kl8.5 50.0+
5.7 3.6 5.2 4.2
Column C Time-dependent recovery
75.6i 79.oi 78.41 80~0+10.8 77.2+ 45.6113.4 44.4; 41.65 46.8& 49.2:
7.3 7.3 11.1 4.3 9.0 9.5
51.2 : 12.9 21.0& 6.8 14.8-t 3.0 13.6 :- 4,3 10.0~ 6,O
5.5 7.7 I.8 3.6
25.2:+ 14.4+ 14.0-f 9.6-k 2.4
9 5 2.6 IO.4 3 6 3 6
N P a
0 1 5,015
I 605 recondr
of repeated loading and deloading
in compressive testing of some dental polymers. See Table ZZfor definitions of A, B, C and D.
behaviour of load applica-
tion and removal in compressive testing of some dental polymers. N, Nylon; P, Pronto 11; 0, Orthocryl; K, Kallodent. See Table ZZfor definitions of A, B, C and D. 180’
to zero at each successive cycle, and so the total
deflection of the second cycle is affected by the remaining deformation from the first cycle, and this is repeated through all successive cycles. In order to calculate the total deformations remaining after the tenth cycle, all deformations in each cycle should be added together.
Fig. 6.-Calculated amount of permanent set of some dental polymers following cyclic loading.
Compressive loading was carried out using the Wallace servo-operated micro-hardness tester. * By taking readings at intervals it is possible to relate the depth of penetration and recovery to time. With this machine the depth of indentation is measured in units of 1 x 1O-5 inches. The load applied in this study was 2.94 N with a contacting load of 0.01 N. The actual stress applied to the material varied, bearing in mind the surface area and shape of the pyramidal diamond, and was in the region of 125 N/mm2. The tests were carried out in air at a temperature of 23 “C, and the specimens had been stored in water at 37 “C until fully saturated, but were removed from the waterbath 2 hours prior to testing and transferred to water at room temperature. Fig. 4 shows the form of the results obtained, and the trace is characteristic of that shown earlier. The difference between B and A represents the amount of creep shown by the specimens. The Orthocryl specimens creep * Model H6B. Croydon, Surrey.
H. W. Wallace & Co Ltd,
et al. : Creep
more than any of the others, although the instant deformation in this particular loading was greatest with nylon. Fig. 5 shows the effect of repeating the cycle, and a similar behaviour obtains as found in the cantilever test, in that the resulting deformation is greater on the second loading cycle than on the first.
tests. Whilst it is clear from the tests carried out that the creep behaviour depends upon the type of test employed and the relationship between the load and the time, a comparison of the figures in this table between the percentage creep in the short and long term tests demonstrates a similar degree of ranking order. Table
An analysis of variance was performed on all the results and there were statistically significant differences between the materials in both transverse bend and compression tests at the 5 per cent level.
Orthocryl Pronto II
14.3 9.7 17.9 14.1
13.1 8.7 17.1 15.4
DISCUSSION All the tests showed that the polymers exhibited creep, and that the deformation on loading and the permanent set increased with each successive cycle; this behaviour was similar for all the tests described. The methacrylate polymers exhibited a characteristic behaviour of deformation as demonstrated in Fig. 6. The nylon did not follow that pattern but showed an initial remaining deformation which attained a limiting value during the test. This is likely to be due to the glass-fibre reinforcement phase in the material. Table III shows the percentage creep demonstrated by the transverse bend and compressive
The Wallace hardness testing system with the shorter load application time is more realistic clinically and, as the results seem to follow those in the other tests used, this system could provide a useful method for application in standards testing. REFERENCES
A. S. (1971) Nylon as a denture-base material. Dent. Puact. Dent. Rec. 22, 122-128. GLANTZ P.-O. and BATES J. F. (1973) Creep in some acrylic dental resins. Odont. Revy 24, 283292. GLANTZ P.-O. and STAFFORD G. D. (1973) Recovery of some acrylic dental resins after repeated loading. Swed. Dent. J. 66, 129-134. HARGREAVES