Stress relaxation of elastomers Salem NS, Watts DC, Combe EC. Stress relaxion propert ies of elastomers. Dent Mater 1987: 3: 37-39.

Abstract - Stress relaxation studies have been carried out on fourteen elastomeric impression materials. A new method of analysis of the data has been presented, and it was concluded that silicones (both addition and condensat ion-cured materials) and a polyether have superior elastic properties to the polysulphides.

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

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 accep- tance as impression materials. Dimen- sional accuracy and stability, and good elastic propert ies of the set material, are of pr ime importance.

Four chemical types of elastomers are available for the dental practi- t ioner, viz: Polysulphides, polyethers, and condensat ion and addit ion cured silicones. This lat ter type of e las tomer has been developed to exhibit excellent dimensional stability (1).

The present work was under taken with two objectives: (i) to measure the stress relaxation propert ies of elastom- ers which are representat ive of the four chemical types, placing particular em- phasis on the newer addition cured sil- icones. A high value of stress relaxa- tion can be taken to indicate relatively poor elastic propert ies (1). The ideal set impression material should exhibit excellent elasticity; (ii) to present a new approach to the interpretat ion of stress relaxation data.

Table 1. Materials

Material Manufacturer Batch Base/catalyst number (by weight)

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

Condensation silicone Verone G light Verone regular Verone putty

Polysulphide Permlastic light Permlastic regular

Polyether Impregum

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 0.94:1 UA1, UA2 0.93:1 TM3, UM1 0.63:1 19903, 14808 0.90:1

- 0.78:1 20901 0.68:1 AG 018,05 1:1 1004793944 0.82:1

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

Kerr Mfg Co, Michigan Kerr Mfg Co, Michigan

22.85:1 34:1 21:1

0531782118 1:1 0326792058 0.8:1

Espe, Germany BE 16621 CE153 AE 055

7.33:1

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 exper imental method employed is similar to that of Her ter t et al (2), who repor ted on short- term stress-re- laxation behaviour of non-metall ic re- storatives.

The stress relaxation of the Instron* testing machine with t ime was deter- mined 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 2. Typical calculation to determine )~ and [3 (Reprosil light, 20 min)

Time Load ( S t ) S o S o S o (s) (kg) - - In - - In In - - In t t ~ In S~

St St S,

0 15.48 1.000 0 - - 0 2.739 12 14.90 1.038 0.0379 -3.270 2.484 2.483 2.701 24 14.71 1.052 0.0510 -2.976 3.178 3.200 2.688 36 14.60 1.060 0.0588 -2.833 3.583 3.712 2.681 48 14.50 1.067 0.0650 -2.727 3.871 4.124 2.674 60 14.42 1.070 0.0700 -2.649 4.094 4.475 2.668 72 14.37 1.076 0.0740 -2.602 4.276 4.784 2.665 84 14.32 1.080 0.0770 -2.558 4.431 5.061 2.662

r = 0.997* 13 = 0.360 i = - 4 . 1 5

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

* r = correlation coefficient

F (kg)

16

14

L R R E P R O S I L

_ H

12

10

Rearranging we have:

38 Sa lem et al.

P E R M L A S T I C

S o In ~ - = k t B (3)

Taking natural logarithms of equat ion 3, we have:

S o In In - l n k + ~ l n t (4)

St

A linear regression analysis of equat ion 4 was performed, plotting In In So/S/t as a function of In t, to determine )/alues of the slope [5. Then equa t iop /2 was utilised, In S t being plotted as a func- tion of t B, and the values of the slope ~. were obtained.

R e s u l t s

Fig. 1 shows examples of experimen- I I I I [ i I I [ I I I I I I I I

0 12 48 96 168 TIME (sec}

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

certain load was applied, the cross- head movement was stopped.

The materials to be tested were pro- port ioned by weight and mixed thor- oughly according to the manufacturer 's instruction, and then packed into cylin- drical brass moulds 13 mm long and 8 mm diameter. Care was taken to avoid the ent rapment of air bubbles into the specimens. The two ends of each sam- ple were flat and parallel to each other.

The filled moulds were left to set un- der normal laboratory conditions of va- rying temperature and humidity for pe- riods 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. When a load of 16 kgf was attained, the movement of the cross-head was stopped. The relaxation of the strained specimen which fol- lowed, allowed a measaurable stress loss with no chgnge in strain, and this was recorded by means of the Instron recorder, using an attached tracer pen and graph paper.

A novel method of analysis of stress relaxation data has been applied, which is similar to the analysis of the dielec- tric relaxation behaviour of solid poly- mers by Williams and Watts (3) and Williams et al. (4).

The proposed function for character- ising stress relaxation behaviour is:

St = So e ~,B (1)

where S t is the load observed at any time t, S O is the maximum 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 di- viding the force value by the crosssec- tional area of the specimens.

Taking natural logarithms of equa- tion 1, we have:

l n S , = l n S o - k t B (2)

tally obtained force-time curves. Table 2 is a typical calculation of the par- ameters ~. and [5 from the experimental data. Data for the stress relaxation par- ameters are given in Tables 3 and 4. In all instances, high values of r, the cor- relation coefficient were found.

D i s c u s s i o n

The parameter ?~ is not by itself suf- ficient to characterise the stress relaxa- tion 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 amount of stress re- laxation unless the values of [3 were identical. It is essential to consider

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

Correlation Material coefficient (r) [3 ~. kl3

Reprosil light 0.999 0.36 0.015 0.005 regular 0.993 0.35 0.018 0.006 heavy 0.997 0.24 0.047 0.010

President light 0.996 0.26 0.019 0.005 regular 0.999 0.28 0.025 0.007 heavy 0.998 0.25 0.039 0.010 putty 0.999 0.21 0.140 0.030

Reflect 0.999 0.49 0.004 0.002

Verone light 0.999 0.53 0.009 0.005 regular 0.999 0.33 0.027 0.009 putty 0.999 0.25 0.081 0.020

Permlastic light 0.999 0.50 0.058 0.029 regular 0.999 0.53 0.058 0.031

Impregum 0.998 0.60 0.005 0.003

Stress relaxion of elastomers 39

Table 4. Values of stress relaxation parameters (materials at 1 h)

Correlation Material coefficient (r) 13 ~ ~,f3

Reprosil light 0.999 0.27 0.022 0.006 regular 0.999 0.26 0.034 0.009 heavy 0.999 0.30 0.033 0.010

President light 0.997 0.25 0.020 0.005 regular 0.997 0.33 0.030 0.010 heavy 0.997 0.34 0.035 0.012 putty 0.999 0.23 0.130 0.030

Reflect 0.999 0.25 0.020 0.005

Verone light 0.999 0.50 0.006 0.003 regular 0.999 0.36 0.055 0.020 putty 0.999 0.24 0.090 0.022

Permlastic light 0.999 0.59 0.032 0.020 regular 0.999 0.51 0.058 0.029

Impregum 0.999 0.41 0.017 0.007

some parameter which is a function of both L and [3. The form of the basic equat ion suggests that a product func- tion be tween L and [3 is more appropri- ate than an additive one. For this work values of the product k[3 have been re- ported; further work would be of value

to determine if a more complex func- tion of k and 13 would be meaningful.

From the experimental data in Ta- bles 2 and 3, some conclusions about the behaviour of dental elastomers can be drawn:

(i) most materials show little differ-

ence in behaviour between 20 min and 1 h; (ii) more viscous forms of elas- tomers appear to show slightly greater stress relaxation then more fluid mate- rials (see also Fig. 1); (iii) there is little or no difference in the stress relaxation behaviour of addit ion cured silicones, as compared to the condensation prod- ucts; (iv) in general the polyether and silicones have much less stress relaxa- tion (thus better elasticity) than the polysulphide materials.

References

1. McCabe JF, Storer R. Elastomeric im- pression materials. The measurement of some properties relevant to clinical prac- tice. 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-sym- metrical dielectric relaxation behaviour arising from a simple empirical decay function. Trans Farad Soc 1971: 67: 1323- 1325.