Method of investigating thermal and thermal-oxidizing decomposition of vulcanizates under static and dynamic loads

METHOD OF INVESTIGATING THERMAL AND THERMAL-OXIDIZING DECOMPOSITION OF VULCANIZATES UNDER STATIC AND DYNAMIC LOADS* S. M. KAvu~, M. M. PODKOLTA~Aand Z. 51. TAXASOVA Scientific Research Institute of the Tyro I n d u s t r y

(Received 17 December 1971) D~,co~POSI~ION processes in vulcanization networks taking place under thermal and thermaloxidizing conditions consist of breakage reactions of molecular chains and crosslinks. This results in the loss of numerous technical properties of rubbers (fatigue) and reduces the service life of products made from them. Chemical stress relaxation, first proposed by Tobol'skii [1] and further developed b y other authors [2-4] enables the thermal and thermal-oxidizing decomposition of vulcanization network to be examined under constant deformation. A s t u d y of the effect of dynamic stress on the rate of decomposition of the vulcanization • network is of particular interest. Recording stress in vuleanizates subjected to dynamic (cyclic a n d repeated) stress, in spite of the apparent complexity, is not a fundamental problem. The use of various strain gauges with subsequent amplification of the electrical signal emitted enables this process to be carried ou~ continuously using electronic or loop oscillographs. As the dynamics stress component at fairly high temperatures conforms to the same regularities as the static component, these measurements of "dynamic relaxation", in principle, assist in calculating the rate of decomposition of unfilled vulcanizate networks. A complicating factor of investigations under dynamic conditions is the rapid accumulat i o n in the vulcanizate of residual deformation and therefore testing dynamic creep is a more ~onvenient method [5]. Earlier studies [6, 7] in which dynamic creep of rubbers was examined only provided qualitative information and the devices used did not facilitate quantitative evaluation t o be made of the effect of dynamic stress on the rate of chemical decomposition of vulcani~ates. D y n a m i c stress, in principle, may firstly accelerate thermal decomposition of vulcani-zation networks. I n addition, dynamic creep of rubbers in the presence of atmospheric oxygen m a y also be accelerated b y reactions of oxidizing decomposition. To establish which stages of chemical degradation of vnlcanizates are affected by dynamic stress, investi~gations should not only be carried out in air, b u t also in v a c u u m over a fairly wide range o f temperature (80-170°). Existing constructions of apparatus used do n o t facilitate this as elements of electrical and mechanical systems--electric motors, reductors, relays, solenoids, e t c . - - a r e arranged in thermostatically controlled compartments. These disadvantages were avoided in the construction proposed. Specimen 1 placed in thermostatically controlled chamber 2 is fixed with clamps 3 a n d 4 using rods 11 and 6. The lower rod 6 is joined to the cam mechanism of vibrator 7 with evenly changing eccentricity. The eccentric mechanism is equipped with drive 8. To transmit * Vysokomol. soyed. A14:No. 9, 2113-2117, 1972. 2372

T h e r m a l a n d t h e r m a l - o x i d i z i n g d e c o m p o s i t i o n o f vulcanizates RATE CONSTANTS OF STATIC (k,t) AND DYNAMIC (kayn) CREEP OF N ~ V U ' L O ~ T E S S U L P ~ u J ~ I N V A C U U M A T 110 °

Indices

2373 CONTAINING

Composition o f vulcanizates, wt. Yo per 100 wt.Yo r u b b e r tetrathiodiphenylguanidine : S a n t o c u r e : sulphur dimorpholine : sul ~hur 1.5:1

0-8:2

1:1

1:2

3.6

2-8 6-3

6.0 13.5

7.0 19.3

9.2 27.6

5.0 7.9

2.2

2.2

2.8

3.0

1.5

kstX 103 m i n kdyn X 103 m i n Activation coemcient kdy,,/k,t

v i b r a t i o n f r o m t h e c a m gear to t h e lower clamp m a n y gas a t m o s p h e r e a n d v a c u u m , r o d 6 was h e r m e t i c a l l y sealed w i t h a v a c u u m r u b b e r hose, as described p r e v i o u s l y [8]. R o d 5 of t h e u p p e r c l a m p is h i n g e d to mobile d y n a m o m e t e r 9, which is a fiat steel spring w i t h wire strain gauge 10 fixed on it. T h r o u g h rod 11 also a r r a n g e d ir~ r u b b e r hose 12 the d y n a m o m e t e r is c o n n e c t e d to a s]iding n u t 13 m o v e d b y screw 14, w h i c h is rigidly joined to t h e shaft of t h e reversible m o t o r 15. Strain gauge 10 is joined to an 8-ANCh amplifier, at t h e o u t -

.24

'7

!

[=..l~fZz

1~

4,

.........

/Fzs J 2l/

~-] I

5------¸ 3~

9 6--~_

E0 F-

FIG. 1. L a y o u t of t h e device used for m e a s u r i n g d y n a m i c creep.

2374

S . M . K A v u N e~ a/.

p u t of which a potential difference develops, proportional to the mechanical stress in the specimen. Voltage from the output of the tensometrie amplifier is transmitted through microswiteh 17 to differential voltmeter 18 (N-373 millivoltmeter with zero indication in the centre of the scale).

~0

..

I

I "~

/

I

0 Time

Time, rain

FIG. 2 FIG. 3 FIG. 2. Dependence of deformation (a) and stress (b) on time: a - - a m p l i t u d e of the dynamic component of deformation ~a = eonst; b-- average static load f0 = const. FIG. 3. Kinetics of static (1) and dynamic (2) creep of a N R vuleanizate containing sulphur: ~0=200; An= 100K. During each cycle of elongation voltage is transmitted to a differential voltmeter by a microswitch from the output of the strain gauge amplifier at the m o m e n t corresponding to average load on the specimen (static component) a n d is compared with a given constant voltage 19 which is proportional to the static component. To record the connection of the microswitch at the moment of average elongation of the specimen, it is moved in relation to plunger 20 which moves to and from in phase with the elongation (deformation) of the specimen. To m a i n t a i n these conditions it is necessary to control the average load f0 per cycle. An auxiliary device is therefore provided to the microswitch to be set in a position correspondLug to closure at the m o m e n t of average load in the specimen per cycle. The equipment incorporates a low-frequency oscillograph (ENO-1 type) connected to the output of amplifier 16 and the tumbler switch, which connects the contacts of the microswitch to the i n p u t of the oscillograph. As the period of closure of the microswitch is 0.03-0.05 sec (the stress cycle lasting 0.1-0.4 scc), a sine curve is observed on the screen which is interrupted by discontinuities corresponding to moments of closure of the microswiteh. To observe requisite conditions, discontinuities should be on the axis of the sine curve observed on the screen, which is achieved by moving the microswiteh in relation to plunger 20 b y a microscrew, followed by fixing. After installing the microswitch and switching it to measuring position, voltage pulses will be transmitted to the i n p u t of the differential voltmeter which are equal to the difference b e t w e e n a given voltage and the voltage at the output of amplifier 16, corresponding to avers ge load in the specimen. I f there is an imbalance between the voltage given and the voltage corresponding to average load in the specimen, the indicator of the differential voltmeter moves in one of the directions at the moment of comparison according to the sign of the imbalance. Contact lines with an indicator are arranged on the scale of the differential voltmeter on both sides of the zero mark. The contact lines are connected to "electronic key" 21, the anede circuits of which are connected to a high-resistance relay, the connection of which causes rotation of reversible motor 15, which actuates dynamometer 9 in the requited direction until the imbalance of differential voltmeter 18 is eliminated. Under conditions of imbalance the needle of the voltmeter having a frequency equal to the frequency of

Thermal and thermal-oxidizing decomposition of vulcanizates

2375

the specimen appears to strike against the right or left contact lines. The electronic key is so designed t h a t immediately after the first closure of a n y contact pair the relay system disconnects voltmeter 18 from the measuring circuit and corrects 2~ within a few seconds. As average load per cycle is maintained constant in the specimen studied the specimen is stretched all the time. This value of dynamic creep is recorded by potentiometer 22 conneeted in the diagonal of Wheatstone bridge 23, one arm of which is a rectilinear rheoehord 24; the mobile contact of rheochord 17 is connected with sliding n u t 13. The displacement of the latter in absolute value is equal to the increase in the static component of deformation of the specimen during the experiment. The reduction in amplitude of the dynamic voltage component can be recorded b y a loop or electronic oscillograph 25. The thermostatically controlled hermetic glass compartment 2 is connected to the v a c u u m apparatus (pre-vacuum and diffusion oil pumps) to enable tests to be carried out in v a c u u m or in inert atmosphere. The temperature a n d temperature control of the specimen are maintained b y a chromel-copel thermocouple which, if necessary, can be brought into contact with the specimen. The thermocouple is connected to a n E P V electronic recording potentiometer. Figure 2 shows the time dependences of voltage and deformation in a specimen subjected to fatigue under conditions of dynamic creep. I t can be seen t h a t the fatigue conditions selected are similar to conditions of static creep, the only difference being in the superimposition of the dynamic component of deformation. As with static creep, dynamic creep is due to breakdown of the three-dimensional network of the vlucanizate. According to the classical ratio of the theory of high-elasticity, voltage in a n nnfilled vu]canizate sample elongated 2 times in relation to the initial section f is determined from the ratio f = 3N J~T (2t-- 1/21), (1) where Nt is the concentration of elastically active chains of the vulcanization network, /~ is Boltzman~'s constant, T - - a b s o l u t e temperature and index t refers to the time from the beginning of the experiment. U n d e r conditions of static creep f remains constant a n d the length of the specimen increases as a consequence of decomposition (reduction in 2V~). Using ratio (1) Tobolskii pointed out [9] that chemical creep data, similarly to results of chemical stress relaxation, can be used to determine the decomposition rate constant of the vulcanization network. Therefore, Tobolskii introduced the chemical creep function (CCF)

C C F = 20--20-* 2,--2-~ '

(2)

where index 0 indicates the beginning of the experiment. I f the network decomposes by a first order reaction the semi-logarithmic dependence of CCF on time helps to derive the rate constant of this reaction log 2°--2°m N, 2t__2~ = l o g --=N0 --/c't.

(3)

I n practice, there are at least two limitations of the use of this relation. Equation (3) does not allow for the inhibition of creep as a consequence of secondary processes resulting in structure formation. The equation of state for unfilled vulcanizates is described by a semi-empirical formula proposed by Mooney and Rivlin [10, 1 l] and not by theoretical dependence (1)

f:2Ao(Cl+-~)

( a ' - - 1/2),

where A 0 is the initial section of the specimen, C1 has a classical interpretation ( C 1 : while the physical meaning of empirical constant C~ up to now is not quite clear.

(4)

NkT/2),

2376

S . M . ]~AVUN e~ a~.

According to some assumptions, parameter C2 is related with intermolecular repulsion to "non-intersecting" macromolecules [12] which is not taken into account in the theory of high-elasticity. I n spite of these restrictions it is evident that they should have the same effect on const~nts of static and dynamic creep because deformation conditions, under which these experiments are carried out, are essentially the same. The only difference is in the superimposition of a relatively small sinusoidal load component. Although the quantitative interpretation of constants determined from equation (3) is thus difficult, their comparison, in principle, helps to answer the problem whether dynamic conditions of stressing influence the rate of decomposition of the vulcanization network. To illustrate this Fig. 3 compares the kinetics of dynamic and static creep of N R vlucauizates containing sulphur at 100 ° in vacuum (the ratio of Santocure a n d sulphur in the vulcanizate being 1.5 : 1). I t can be seen t h a t in the initial sections of kinetic curves (up to 10 rain) a rapid reduction is observed in log CCF-time coordinates, which is followed b y a linear relation for 50 rain. The process then gradually slows down which, apparently, is due to secondary crosslinking. Rate constants of thermal decomposition of N R vulcanizates containing sulphur (in vacuum) under conditions of static and dynamic creep are tabulated; it follows from the Table that repeated dynamic stressing accelerates (activates) thermal decomposition of the vulcanization network 1.5-3-fold, according to the structure of the vulcanizate. The construction of the apparatus proposed in this paper for measuring dynamic creep enabled a differentiated study to be made of the role of dynamic stress a n d chemical reac Lions taking place while rubbers are subjected to fatigue, obsel-cation of mechanical activation of decomposition and an examination to be made of the type and role of this activation in fatigue.

CONCLUSIONS A method is proposed for the measurement of dynamic creep in rubber, which enables us to examine the breakdown of vulcanization network under thermal and thermal-oxidizing action and dynamic stress. I t was found that the measurement of dynamic creep provides quantitative information concerning the effect of repeated cyclic stressing on the rate of chemical decomposition of vulcanizates due to fatigue. Translated by E. S:EI~ERE t ~-

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

REFERENCES A. V. TOBOLSKY, J. B. PRETYMAN and J. H. DILLON, J. AppI. phys. 15: 309, 1944 P. J. FLORY, Trans. F a r a d a y Soc. 56: 722, 1960 A. S. LYKIN, Kolloidn. zh. 26: 697, 1964 J. SCANLAN, Trans. F a r a d a y Soc. 57: 839, 1961 D. DILLON, Sb. Ustalost' vysokopolimerov (Fatigue in High Polymers). Goskhimizdat, 1957 V. G. EPSHTEIN, Sb. Starenie i utomlenie kauchukov i rezin (Ageing and Fatigue in Rubbers). Goskhimizdat, 1952 B. A. DOGADKIN, K. G. KUANYSHEV and V. Ye. GUL', Kolloidn. zh. 27: 182, 1965 A. GENT, J. Polymer Sci. 6: 497, 1962 A. V. TOBOL'SKII, Svoistva i struktura polimerov (Properties and Structure of Polymers). Izd. "Khimiya", 1964 M. MOONEY, J. Apply. Phys. 11: 582, 1940 S. M. GAMBRELL, L. MULLINZ and R. S. RIVLIN, Trans. Faraday Soc. 43: 1095, 1953 L. S. PRIGS, Zh. tekhn, fiziki 28: 636, 1958