Stress Relaxation in Rubber EVALUATION OF ANTIOXIDANTS H. W. H. ROBINSON AND H. A. VODDEN Monsanto Chemicals,Ltd., Ruabon, N . Wales, Great Britain
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TUDY of stress relaxation in rubbers and other elastomers, at constant elongation, has received much attention. The relaxation of stress in an elastomer can occur through viscoelastic flow in the material, or may arise from scission of the hydrocarbon chains supporting the stress. Simultaneously with the rupture of primary bonds, new secondary cross linkages may be formed, causing hardening of the material. A number of authors have discussed the significance of these effects, principally Tobolsky and his coworkers (4-8, 10). During stress relaxation a t elevated temperature under oxidation conditions, oxygen attack causes chain scission, thus causing degradation; the reversible visco-elastic effect is generally of short duration a t these elevated temperatures. Tobolsky, Meta, and Mesrobian (9) have discussed the relationship between stress relaxation and the number of primary bonds cut and have determined the number of oxygen molecules absorbed per chain scission, Since degradation of the rubber is caused by chain scission, the stress relaxation should give a fairly direct measure of the degradative effect of the absorbed oxygen. If antioxidant chemicals are incorporated in the elastomer, then the relative rates of stress relaxation will indicate the relative efficiencies of the antioxidants. This will apply whether the antioxidant inhibits the absorption of oxygen or directs the absorbed oxygen into nonactive channels ( 2 ) . This method of evaluation has been suggested by Tobolsky, Prettyman, and Dillon ( I O ) , and has been
Figure 1.
applied to the comparison of antioxidant chemicals in GR-S stocks by Mesrobian and Tobolsky (S). This article describes the development of apparatus suitable for the investigation of stress relaxation in rubber and shows how the method can be applied to the evaluation of antioxidant chemicals. A number of stocks have been compared using this technique and the conventional air-oven and oxygen bomb testa, and it is suggested that more rapid and accurate estimates'of the antioxidation efficiencies are possible with this than with the conventional methods. APPARATUS
The apparatus has been designed to measure the relaxation of stress in rubber samples held under constant elongation, a t elevated temperatures, in air. In the apparatus provision is made for six samples, each housed in a separate compartment, to be measured simultaneously; the six stress-relaxation curves are recorded on a six-point chart recorder. The stress measurements are made electrically using resistance wire strain gages. Bonded-type gages were found to be unsatisfactory for this work and unbonded gages were designed and constructed for the purpose. The design and construction of a suitable gage have been described (1). The general layout of the apparatus is indicated in Figure 1.
Figure 2.
General layout of apparatus
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Apparatus
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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R22
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Figure 3. Ri, Rz, R3, R4
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strain gage elements 1000 ohmsa R24 RPS Rza Rz7 Rzs R29 R3o Rai Raz Ras
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The six compartments which house the rubber samples are formed by the six vertical tubes, A (1 X 12 inch), which run through the tank, B. They are heated by circulating a heat exchange liquid from a well-thermostated reservoir. The tank, B, is coupled to the reservoir via flexible pi es and is free to slide vertically on the guide rods, C. These ro& form part of a framework which supports the six strain gages, D, which, in turn, carry thin connecting rods leading to the six steel pulleys which hold the rubber samples, E. The lower ends of the rubber samples are held by six additional pulleys, mounted on stout rods, F , which can be positioned to give any desired extensions. During a test, tank B is raised so that the top pulleys are some 2 inches inside the tubes. When this is done, loose fitting disks cover the tops of the tubes. Tests have shown that the temperatures inside the tubes are uniform (10.4' C.) over the regions occupied by samples. Figure 2 is a general view of the apparatus from the front. The strain gages, each of which consists of four elements of 15 strands of 46 English standard wire gage constantan wire, arranged as a Wheatstone bridge network, are designed to withstand a tension of about 1.5 kg. Complete details of construction and design have been published ( 1 ) . The gages are energized with alternating current a t 50 cycles per second. This is derived from a 4-volt winding on a power transformer. The small out-ofbalance potential difference across each bridge network is amplified before being fed to a recording potentiometer The amplify-
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Unless otherwise noted.
ing circuit is shown in Figure 3. The twin-valve balance circuit has been employed to minimize interference from power circuits; this is advisable since the power frequency is used for operation of the bridge networks. Negative feedback is introduced by the network C7, R41, and Rls to stabilize the gain. Each gage is brought into circuit every 30 seconds by a bank of relays operated from a rotating switch synchronized to the six points of the recording potentiometer. The recorder is an Elliotronic recorder, and since it employs a low impedance magnetic amplifier as a preamplifier, the input to the recorder is connected across a low resistance, RO( 5 ohms), in series with the 0 t o 1 milliampere instrument meter, M , fitted to the output of the main amplifier. Full scale deflexion on this meter corresponds to full scale input to the recorder. The bridge networks are balanced, when they are subject to zero load, by means of radio-type potentiometers, R6, shown in Figure 3. All the electrical equipment, including the heaters and thermostat controls, is supplied via an electromechanical voltage stabilizer, which is stable to within 0.5%. Preparation of SampIes. The apparatus was designed t o hold ring-shaped sample specimens about 50 mm. in diameter X 4 mm. in thickness and somewhat less than 1 mm. in radial width. So that direct comparisons could be made with conventional tests, the samples were cut from the same test sheets. These test
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
sheets were 4 mm. in thickness and, for conventional tests, rings with inner diameter of 44.6 mm. and radial width of 4 mm. were cut with a rotary cutter. By suitably modifying the clamping system, removing the inner blade, and displacing the outer blade 1 mm. outward, it was found possible to cut satisfactory ring samples from the material that remained after removal of the conventional test specimens. The rubber samples were carefully prepared, and after preliminary experiments had indicated the necessity for standardization of the procedure, the technique of milling a large masterbatch of basic composition, to which the antioxidants and sulfur were added subsequently under well-controlled conditions, was adapted. This procedure led to improved reproducibility of results and enabled comparisons to be made between the various chemicals added to the masterbatch.
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each set of stocks contained a control, usually a stock with no antioxidant, a stock containing an antioxidant of known activity, and two or three containing experimental antioxidants. The 1mm. sample rings were used for stress relaxation measurements and the 4-mm. samples were subjected to air-oven and oxygen bomb aging. Tension strength and 300% modulus measurements were made after 0 and 12 days aging in the air oven at 70" C. and after 0 and 6 days aging in the bomb at 70" C. under 300 pounds per square inch oxygen pressure. Test procedures were made in accordance m-ith the details outlined in BS903-1950.
EXPERIMENTAL
A study has been made of the effects of various antioxidants on the stress relaxation in natural rubber a t elevated temperatures. It is clear from fundamental reasoning that if, as is generally postulated, the relaxation of stress a t elevated temperatures is caused primarily by scission of the hydrocarbon chains by oxygen attacks, incorporation of antioxidants will reduce the rate of relaxation. The efficiency of the protective action of the antioxidant should, therefore, be related to the rate of relaxation.
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Figure 4.
Typical stress-relaxation curves
Throughout the work described in this article the following base formulation was used. Pale crepe Precipitated barium sulfate (Blanc Fixe) Zinc bxide Titanium dioxide Stearic acid Sulfur Tetramethylthiuram disulfide (Thiurad) Antioxidant
Parts 100.0 50.0 5.0 5.0 1 .o 2.0 0.375 1.0
Stocks were cured for 20 minutes at 126" C. and molded to sheets 4 mm. thick. Rings of nominal 4- and I-mm. radial thicknesses were cut from these sheets and, until required, were stored in the dark a t a temperature of 0" C. The antioxidants studied were not chosen specifically for this investigation, but they were part of an applicational research program being carried out a t the time in the technical service laboratory of Monsanto Chemicals, Ltd. From each masterbatch about five different stocks were derived;
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Comparison of antioxidants A, B, C, and D, with control stock
The stress relaxation measurements were made at 100O, 110", and 120' C. Normally, three samples from each stock were measured a t each temperature. The thermostat temperature was adjusted to the appropriate value; time was allowed, in each case, for the temperature to reach equilibrium. With the tank in its loR-est position the rubber samples were looped lightly over the pulleys. The tank was then raised BO that the samples were enclosed in the heated tubes. Meanwhile, the recording potentiometer drive was started, thus initiating the appearance of the six "zero" traces. Any necessary adjustments to the zeros were made during this time. The samples were left in the unstrained positions for 10 minutes to ensure temperature equilibrium; the extension rods were then pulled down, and the catches automatically held them in their new positions. Throughout the investigation the extensions were adjusted to 100%. The stress versus time curves for the samples were drawn automatically on the 10-inch-roll chart of the recording potentiometer. The absolute values of the stresses could have been found from calibrations of the strain gages, but for this work such a calibration was unnecessary, since only relative measurements were required. It was verified that the recorder output was directly proportional to the force applied. RESULTS
Initially, all chart records were transcribed onto logarithmic graph paper, in order to simplify the study of the form of the stress-relaxation curves. Typical examples of such curves are shown in Figure 4. I n nearly all cases the log stress versus time plot showed a rapid initial drop followed by a less rapid linear portion. T o a first approximation, therefore, it was assumed that the stress relaxations were exponential and could thus be characterized by a single parameter. The time taken for the stress to relax to half its initial value-the half life-is a convenient parameter to use. The value of this can, of course, be taken directly from the recorder chart, and this procedure was adopted during many of the later measurements. I n Figure 5 are shown diagramatically the half-life periods of a number of different stocks a t a temperature of 110" C. These
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diagrams show how, in one masterbatch, the antioxidant efficiencies can be compared. The reproducibility is good and the discriminating power seems satisfactory. For comparison with the conventional evaluation methods, the stress-relaxation half lives have been plotted as ordinates with either per cent tension strength retained or per cent modulus retained as abscissas. Figure 6 shows stress relaxation results at 110" C. plotted against per cent tension strength retained after 12 days oven aging, while
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CONCLUSIONS
While it is not possible to say that stress-relaxation measurements can completely replace more conventional methods for evaluation of rubber chemicals and, in particular, antioxidants, it is probable that the method will find increasing and useful application in research and development of such chemicals. For the rapid screening of large numbers of compounds it will be of great value. Furthermore, in conjunction with conventional modulus and tension-strength measurements, the stress-relaxation characteristics will provide additional information to enable the relative importances of cross linking and chain scission to be predicted.
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