Stress relaxation of elastomers

Stress relaxation of elastomers

Stress relaxation of elastomers N. S. Salem, D. C. Watts, E. C. Combe Biomaterials Science Unit, Turner Dental School, Universityof Manchester, Engla...

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Stress relaxation of elastomers

N. S. Salem, D. C. Watts, E. C. Combe Biomaterials Science Unit, Turner Dental School, Universityof Manchester, England

Salem NS, Watts D C , C o m b e EC. Stress relaxion properties of elastomers. D e n t Mater 1987: 3: 37-39. Abstract - Stress relaxation studies have been carried out on fourteen elastomeric impression materials. A new m e t h o d of analysis of the data has been presented, and it was concluded that silicones (both addition and condensation-cured materials) and a polyether have superior elastic properties to the polysulphides.

Key words: elastomers, impression materials, stress relaxation Dr. E. C. Combe, Biomaterials Science Unit, Turner Dental School, University of Manchester, Manchester M16 6FH, England. Received March 31, 1986; accepted May 2, 1986

Elastomers have gained wide acceptance as impression materials. D i m e n sional accuracy and stability, and good elastic properties of the set material, are of prime importance. Four chemical types of elastomers are available for the dental practitioner, viz: Polysulphides, polyethers, and condensation and addition cured silicones. This latter type of elastomer has been developed to exhibit excellent dimensional stability (1). The present work was undertaken with two objectives: (i) to measure the stress relaxation properties of elastomers which are representative of the four chemical types, placing particular emphasis on the newer addition cured silicones. A high value of stress relaxation can be taken to indicate relatively poor elastic properties (1). The ideal set impression material should exhibit excellent elasticity; (ii) to present a new approach to the interpretation of stress relaxation data.

Material and methods Table 1 lists the fourteen materials which were studied, including batch n u m b e r , and base/catalyst ratios (by weight). The experimental m e t h o d employed is similar to that of Hertert et al (2), who reported on short-term stress-relaxation behaviour of non-metallic restoratives. The stress relaxation of the Instron* testing machine with time was determined by bringing the cross-head into contact with the lower platen carrying the compressive load cell, and when a * Instron, High Wycombe, Bucks, England

Table 1. Materials Material


Batch number

Base/catalyst (by weight)

Addition silicone Reprosil light Reprosil regular Reprosil heavy President light President regular President heavy President putty Reflect

De Trey AD International De Trey AD International De Trey AD International Coltene Inc Switzerland Coltene Inc Switzerland Coltene Inc Switzerland Coltene Inc Switzerland Kerr Mfg Co, Michigan

TM1, YE1 UA1, UA2 TM3, UM1 19903, 14808 20901 AG 018,05 1004793944

Condensation silicone Verone G light Verone regular Verone putty

Davis Schottlander and Davis, London 949, 950 Davis Schottlander and Davis, London 2294, 2301 Davis Schottlander and Davis, London 19379

Polysulphide Permlastic light Permlastic regular

Kerr Mfg Co, Michigan Kerr Mfg Co, Michigan

0531782118 0326792058

Polyether Impregum

Espe, Germany

BE 16621 CE153 AE 055

0.94:1 0.93:1 0.63:1 0.90:1 0.78:1 0.68:1 1:1 0.82:1 22.85:1 34:1 21:1 1:1 0.8:1 7.33:1

Table 2. Typical calculation to determine )~ and [3 (Reprosil light, 20 min) Time (s)

Load ( S t ) (kg)

0 12 24 36 48 60 72 84

15.48 14.90 14.71 14.60 14.50 14.42 14.37 14.32





In - St

In In - S,

In t


In S~

1.000 1.038 1.052 1.060 1.067 1.070 1.076 1.080

0 0.0379 0.0510 0.0588 0.0650 0.0700 0.0740 0.0770

-3.270 -2.976 -2.833 -2.727 -2.649 -2.602 -2.558

2.484 3.178 3.583 3.871 4.094 4.276 4.431

0 2.483 3.200 3.712 4.124 4.475 4.784 5.061

2.739 2.701 2.688 2.681 2.674 2.668 2.665 2.662

r = 0.997* 13 = 0.360 i = -4.15

r = ~).9992" k =0.015 i = 2.739

* r = correlation coefficient

S a l e m et al.


Rearranging we have:

F (kg) 16


In ~ -


= k tB





Taking natural logarithms of equation 3, we have:



In In 10






A linear regression analysis of equation 4 was performed, plotting In In So/S/t as a function of In t, to determine )/alues of the slope [5. T h e n e q u a t i o p / 2 was utilised, In S t being plotted as a function of t B, and the values of the slope ~. were obtained.


Results I




















Fig. 1 shows examples of experimentally obtained force-time curves. Table 2 is a typical calculation of the parameters ~. and [5 from the experimental data. Data for the stress relaxation parameters are given in Tables 3 and 4. In all instances, high values of r, the correlation coefficient were found.





Fig. 1. Experimental stress relaxation curves for Reprosil (20 min) and Permlastic (1 h).

certain load was applied, the crosshead m o v e m e n t was stopped. The materials to be tested were proportioned by weight and mixed thoroughly according to the manufacturer's instruction, and then packed into cylindrical brass moulds 13 mm long and 8 mm diameter. Care was taken to avoid the entrapment of air bubbles into the specimens. The two ends of each sample were flat and parallel to each other. The filled moulds were left to set under normal laboratory conditions of varying temperature and humidity for periods of 20 min and 1 h from the start of mixing. The sample was then removed from the mould and tested by axial compression on a constant strain rate testing machine* at a cross-head speed of 5 mm/min. W h e n a load of 16 kgf was attained, the m o v e m e n t of the cross-head was stopped. The relaxation of the strained specimen which followed, allowed a measaurable stress loss with no chgnge in strain, and this was recorded by means of the Instron recorder, using an attached tracer p e n and graph paper. A novel method of analysis of stress relaxation data has been applied, which is similar to the analysis of the dielectric relaxation behaviour of solid polymers by Williams and Watts (3) and Williams et al. (4). The proposed function for characterising stress relaxation behaviour is:

St = So e ~,B


where S t is the load observed at any time t, S Ois the m a x i m u m load applied, ~. is a parameter, and the values of [5 range from 0 < [3 --< 1. The terms S t and So could be changed into stresses by dividing the force value by the crosssectional area of the specimens. Taking natural logarithms of equation 1, we have: lnS,=lnSo-kt



The parameter ?~ is not by itself sufficient to characterise the stress relaxation behaviour of a material, since it is the slope of the curve based on t B. That is, two materials with identical )~ values would differ in the a m o u n t of stress relaxation unless the values of [3 were identical. It is essential to consider


Table 3. Values of stress relaxation parameters (materials at 20 min) Material

Correlation coefficient (r)




Reprosil light regular heavy

0.999 0.993 0.997

0.36 0.35 0.24

0.015 0.018 0.047

0.005 0.006 0.010

President light regular heavy putty Reflect

0.996 0.999 0.998 0.999 0.999

0.26 0.28 0.25 0.21 0.49

0.019 0.025 0.039 0.140 0.004

0.005 0.007 0.010 0.030 0.002

Verone light regular putty

0.999 0.999 0.999

0.53 0.33 0.25

0.009 0.027 0.081

0.005 0.009 0.020

Permlastic light regular Impregum

0.999 0.999 0.998

0.50 0.53 0.60

0.058 0.058 0.005

0.029 0.031 0.003

Stress relaxion of elastomers Table 4. Values of stress relaxation parameters (materials at 1 h) Material

Correlation coefficient (r)




0.999 0.999 0.999

0.27 0.26 0.30

0.022 0.034 0.033

0.006 0.009 0.010

0.997 0.997 0.997 0.999 0.999

0.25 0.33 0.34 0.23 0.25

0.020 0.030 0.035 0.130 0.020

0.005 0.010 0.012 0.030 0.005

Reprosil light regular heavy President light regular heavy putty Reflect Verone light regular putty Permlastic light regular

0.999 0.999 0.999

0.50 0.36 0.24

0.006 0.055 0.090

0.003 0.020 0.022

0.999 0.999

0.59 0.51

0.032 0.058

0.020 0.029







ence in behaviour between 20 min and 1 h; (ii) m o r e viscous forms of elastomers appear to show slightly greater stress relaxation then more fluid materials (see also Fig. 1); (iii) there is little or no difference in the stress relaxation behaviour of addition cured silicones, as c o m p a r e d to the condensation products; (iv) in general the polyether and silicones have much less stress relaxation (thus better elasticity) than the polysulphide materials.


some p a r a m e t e r which is a function of both L and [3. T h e form of the basic equation suggests that a product function between L and [3 is m o r e appropriate than an additive one. For this work values of the product k[3 have been reported; further work would be of value

to determine if a m o r e complex function of k and 13 would be meaningful. From the experimental data in Tables 2 and 3, s o m e conclusions about the behaviour of dental elastomers can be drawn: (i) most materials show little differ-

1. McCabe JF, Storer R. Elastomeric impression materials. The measurement of some properties relevant to clinical practice. Br Dent J 1980: 149: 73-79. 2. Hertert RS, Huget EF, Cosgrove JH. Short term stress-relaxation behaviour of non-metallic restoratives. J Dent Res 1975: 54: 1140-1144. 3. Williams G, Watts DC. Non-symmetrical dielectric relaxation behaviour arising from a simple empirical decay function. Trans Farad Soc 1970: 66: 80-85. 4. Williams G, Watts DC, Dev SB, North AM. Further consideration of non-symmetrical dielectric relaxation behaviour arising from a simple empirical decay function. Trans Farad Soc 1971: 67: 13231325.