The Possibility of using Complex Anti-Agers for Rubber Compounds Based on Polyisoprene Rubbers and Operating under Extreme Conditions

When a polymeric material is exposed to service factors, a normal service regime and a regime typical of extreme conditions can be distinguished. The normal regime is characterised by a relatively slow and smooth change in indices during service. Here, prevailing factors (temperature, concentration of corrosive medium, and so on) lie at levels far from the fitness-for-purpose limits of the materials.

For extreme service conditions, the prevailing factors approach the fitness-for-purpose limits or even fall into the region where the material is no longer ft for purpose. The material in such a regime is capable of catastrophic failure. In a number of cases, changes in the indices are described by extremal dependences. Here, the properties of the material can be improved considerably by introducing into its composition functionally active components ensuring that the material will be protected during service.

A typical example is strengthening of rubber compounds at the initial stage of ageing if the rubbers contain polymer-forming compounds or structure-forming additives, in particular additives containing ∊-caprolactam [1–5].

It is known that normal service conditions of natural rubber compounds, like SKI-3 isoprene, are generally limited to a temperature range of 60–100°C. The question arises as to whether by introducing additives it is possible to influence structural transformations without changing the nature of the polymer, thereby postponing catastrophic failure of the isoprene rubber compound.

The aim of the present work was to investigate the effect of the complex anti-ager PRS-1N, which is a complex salt containing ∊-caprolactam and N-isopropyl-N-phenyl-p-phenylenediamine (IPPD) in its ligand sphere. In earlier studies [5,6] it was shown that anti-agers of this type are superior to IPPD primarily from the viewpoint of their prolonging effect on the process of thermooxidative ageing. Long retention of the physicomechanical properties of rubber compounds is ensured by synergy in the protective action of IPPD and ∊-caprolactam. The latter shows itself to be an anti-ager that transforms polymer peroxides into relatively neutral products, i.e. behaves like a preventive anti-ager [6].

The cited studies largely present the results of investigating rubber compounds based on polyisoprenes, obtained at a temperature not exceeding 100°C. Rubber specimens aged at this temperature do not undergo catastrophic failure either in static or in dynamic tests. For example, for specimens of rubber compounds of this type, the dynamic strength is of the order of 90 000–110 000 cycles, but the same rubber compounds aged for 24 h at 100°C are capable, without failing, of withstanding dynamic loads after 30 000–40 000 loading cycles. It follows that there is no dramatic fall in dynamic strength. Catastrophic failure begins after ageing at higher temperatures.

To investigate the effect of ageing on the mechanical properties, we used SKI-3-based compounds of the following composition: 100.00 parts rubber, 1.00 part sulphur, 0.60 parts dibenzothiazyl disulphide, 3.00 parts diphenylguanidine, 1.00 part technical stearic acid, 5.00 parts zinc white, and 30.00 parts N330 carbon black (introduced into the master batch). Compositions 1 and 2 contained 1 part IPPD and 1 part PRS-1N respectively. The rubber mixes were prepared on a 320 160/160 mill using the standard technology; they were vulcanised at 150°C for 30 min. The physicomechanical properties of the vulcanisates were determined in accordance with the GOST 270–75 standard.

Figure 1 presents the dependences of the nominal tensile strength, f_t, on the time of thermooxidative ageing, τ. It can be seen that rubbers placed in a thermostatically controlled chamber for 48 h at 80 and 100°C did not lose their initial strength to any significant extent: f_t decreased from 28–29 MPa to 22–27 MPa. If these indices are extrapolated to normal service conditions, then obviously loss of fitness for purpose in these rubber compounds cannot be expected. At the same time, exposure of rubber compounds to the same conditions but at 120 and 140°C leads to their sudden (catastrophic) failure. As follows from Figure 1 , sudden failure of rubbers can occur much earlier, for example after ageing for 30 h, when the nominal tensile strength has remained at a level of 7–8 MPa, which is inadmissible during the service of, for example, heavy-duty mechanical rubber goods.

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Figure 1

The dependence of the nominal tensile strength of rubber compounds, f_t, on the time of preliminary thermooxidative ageing, τ, for rubbers containing PRS-1N (1, 2, 3, 4) and IPPD (1′, 2′, 3′, 4′) at different temperatures: 1, 1′ – 80°C; 2, 2′ – 100°C; 3, 3′ – 120°C; 4, 4′ – 140°C. Vertical axis: f_t, MPa; Horizontal axis: τ, h

However, rubber specimens pre-aged in air at 140°C for 24–30 h do not undergo sudden failure in dynamic tests ( Figure 2 ). Sudden failure is observed in these specimens after 48 h in these conditions. Here, failure can occur when specimens are being prepared for testing on an MRS-2 machine (for example, during checking of their static and dynamic strains). Heat treatment at elevated temperature affects numerous processes determining the dynamic strength of rubber compounds, and at critical high temperatures (for isoprene rubber this is 140°C) they proceed particularly intensely. As is known, under extreme conditions, systems are particularly unstable and readily undergo structural and chemical changes [7]. At the same time, with the optimum heat treatment time, these processes, in particular degradation, seem to develop too rapidly.

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Figure 2

The dependence of the dynamic strength, N, on the time of preliminary thermooxidative ageing at 140°C, τ, for rubbers containing PRS-1N (1) and IPPD (2). Vertical axis: N, cycles; Horizontal axis: τ, h

Similar results are observed in wear resistance tests. Table 1 gives data on the abrasive wear of rubber compounds, α, obtained on a Grasselli machine. Testpieces are pre-aged in an air oven at 140°C. From Table 1 it can be seen that the wear of aged specimens is greater than the wear of initial, unaged specimens (α < 80 m³/TJ [8]).

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Table 1

The results of the abrasive wear of rubber compounds ^a

Rubbers (ageing time, temperature)	Stages a	A, J	α, m3/TJ
Composition 1 (24 h, 140°C)	100	2378	120
	100/200	2405	190
	100/300	2431	152
Composition 2 (24 h, 140°C)	100	2378	160
	100/200	2431	186
	100/300	2431	70
Composition 1 (48 h, 140°C)	100	1657	240
	100/200	2378	170
	100/300	2298	150
Composition 2 (48 h, 140°C)	100	2458	150
	100/200	2378	50
	100/300	1924	112

a

A – work of friction; α – abrasion.

b

In the case of several stages: numerators – number of revolutions; denominators – total number of revolutions.

In wear tests of aged specimens, a sharp difference in data appears for rubbers containing IPPD and PRS-1N (mixes 1 and 2 respectively), even more appreciable than for unaged specimens. It is quite probable that, after its heating, IPPD hardly remains in the surface layers of the specimen as a result of sublimation. PRS-1N, the diffusion activity of which is much weaker, is capable of reducing to some degree the abrasive wear of specimens aged for 48 h at 140°C. Thus, comparatively low values of α (50 m³/TJ) after 200 disc revolutions can be attributed to the possible penetration of low- molecular-weight fractions from mix 2 into surface defects of micrograins of the abrasive, thereby smoothing out their sharp faces. It is quite likely that the depth to which grains of the abrasive material are filled will depend on the affinity of the rubber macromolecules and their low-molecular-weight adducts. Thus, the presence in PRS-1N of ∊-caprolactam and its subsequent reaction with peroxide macroradicals [9] leads to the formation of polar oligomers and, in particular, their derivatives – hydroxylamines. It is natural that the compatibility of rubber macromolecules with such substances will turn out to be at a lower level than the compatibility only of hydrocarbon macromolecules and oligomers obtained by extracting rubber compounds with IPPD.

As shown by data of sol–gel analysis ( Figure 3a ), there are certain prequisites for such assumptions. The sol fraction (SF) – the extracted portion of the vulcanisate during its swelling (in our case, in toluene; tests were conducted in accordance with GOST 424–41) – comprises a resinous substance. The average molecular weight of the sol fractions, M_av, was determined by viscometry [10]. For this, the intrinsic viscosity, [h], was found as the dependence of reduced viscosity on concentration. The average molecular weight was calculated by means of the Mark–Houwink equation:

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Figure 3

The dependences on the ageing time at 140°C, τ, for rubbers containing PRS-1N (1) and IPPD (2) of (a) the amount of sol fraction and (b) the equilibrium degree of swelling. Vertical axis of (a): SF, wt%; Vertical axis of (b): Q_equ, wt%; Horizontal axes: τ, h

[ η ] = k . m a v α

where k and α are empirical constants [10]. The mean value of M_av was (6–12) × 10 ³ .

From data in Figure 3a it can be seen that the dependence of the SF values on the ageing time at 140°C is, like the dependence of N (see Figure 2 ), extremal in nature. Only the extremum in this case is a minimum, and it is observed at a greater heating time. Here there is a distinct difference between the data for rubber compounds containing IPPD and PRS-1N (mixes 1 and 2 respectively), which seems to ensure readier entry of oligomer fractions into the grains of the abrasive material in the case of rubber with PRS-1N. Note that, by the time the maximum dynamic strength, N, is reached (in Figure 2 this is 24 h), the amount of sol fraction has increased slightly.

Data on the equilibrium degree of swelling, Q_equ, given in Figure 3b and determined as a function of the ageing time, correlate with data of quantitative determination of SF. The dependences of Q_equ on ageing time are, again, extremal in nature. Here, the lowest value of Q_equ corresponds to the greatest release of sol fraction from a specimen with PRS-1N after ageing for 48 h, which, it appears, can be considered to confirm the above explanation of the increased wear resistance of specimens with PRS-1N. In our opinion, there are sufficient grounds for asserting that, after removal of the solvent, the resin chiefly consists of oligomeric products of degradation of polyisoprene macromolecules. Here we cannot rule out the presence in the resin (SF) of low-molecular-weight products that take part in the vulcanisation process but are not chemically bound with rubber macromolecules.

This is indicated by the IR spectra of the resins, which are fairly complex and saturated with different absorption bands. The IR spectra were obtained using a Nicolet-6700 Fourier IR spectrometer. Figure 4 gives the spectrum of the SF of rubber with IPPD, aged at 140°C for 72 h. The spectrum of the resin of rubber with PRS-1N is practically identical to the spectrum of the resin of rubber with IPPD. In the given spectrum there are stretching vibrations of the C–H groups of hydrocarbons of the resins in the region 2800–3000 cm^–1, of oxygen-containing (carbonyl, hydroxyl, and carboxyl) groups in the regions 1660 and 3300 cm^–1 (in the latter region, the superimposition of bands of the –COOH and –OH groups occurs), belonging to oligomeric products of degradation, and also in the 740 cm^–1 region – stretching vibrations of sulphide groups of the C–S type [11].

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Figure 4

IR spectrum of sol fraction of rubber with IPPD after heating at 140°C for 72 h (characteristic absorption bands are noted on the spectrum). Vertical axis: I; Horizontal axis: cm^–1

To a certain extent, an explanation of the abnormal behaviour of rubbers under different test conditions is given by morphological investigations. Electron microscopy of the surface of the brittle cleavage face of vulcanisates was conducted using an EM-14 microscope with transillumination of hydrocarbon replicas. Initially, a double replica was obtained: a primary gelatine replica and a secondary carbon replica. For contrasting of the structure, before the removal of replicas, specimens were subjected to ionic etching for 60 min. Ageing at 140°C for 48 h leads to the appearance of a more pronounced surface relief of the cleavage face of specimens, which may indicate the concentration in individual regions of oligomeric degradation products.

Comparing the results of static and dynamic tests, and also the results of abrasive wear, we can conclude that complex anti-agers with pronounced synergy in protective action are capable of postponing the catastrophic failure of rubber compounds. This occurs, for example, if the rubber is operating under conditions of uniaxial dynamic loading at a strain amplitude not exceeding the static elongation at break of rubber aged under the same conditions.

As regards the question as to the possible prevention of catastrophic failure by means of a particular anti-ager, this is unclear, as indicated by experimental data. Thus, if catastrophic failure occurs under static loading, then, as can be seen in Figure 1 , we should not expect the nature of the anti-ager to have any significant influence on the kinetics of change in nominal strength. The same effect is observed under conditions of uniaxial dynamic loading ( Figure 2 ). The effect of the complex anti-ager on the process of abrasive wear is more significant ( Table 1 ).

Thus, the heat treatment of vulcanisates under critical temperature conditions for the optimum time makes it possible to improve considerably the dynamic strength and wear resistance of rubber compounds through the physicochemical processes occurring under these conditions. The use of complex anti-agers with synergy in protective action enables the time of operation of an article before the development of the process of catastrophic failure of the rubber to be increased. Here, a special preventive role, necessary to ensure synergy, is played by ∊-caprolactam.