Aging Of Rubber Research Articles

Polymerization products of p-benzoquinone as bound antioxidants for SBR. Part II—The antioxidizing efficiency

The antioxidizing efficiency of the polymeric products of p-benzoquinone as bound antioxidants for SBR has been investigated by measuring the percentage deterioration of tensile strength and ultimate elongation for the thermally aged rubber samples after up to 14 days in an air oven at 100°C. The results indicate the remarkable antioxidizing efficiency of these products for SBR compared with the common industrial antioxidants. The higher efficiency is attributed to the ability of the polymeric antioxidants to react with the radical intermediates of the oxidative degradation process, thus bonding them chemically to the rubber chains. The effect of the polymeric antioxidants on the rate of sulfur vulcanization has also been studied. The results reveal that, depending on the quinone/hydroquinone ratio of the product, the polymeric antioxidants may accelerate or slightly retard the rate of vulcanization. The results also indicate that blending the polymeric quinones in equal proportions with an amine antioxidant 6PPD leads to a pronounced improvement in the antioxidizing efficiency of the resulting stabilizer without affecting the rheological or other properties of the rubber. A mechanism for the antioxidant action is proposed.

Effect of ageing on critical cut length and morphology of fracture surface in tensile rupture of natural rubber

Variation of tensile strength with flaw sizes has been studied both for unaged and aged natural rubber (NR) gum vulcanizates (aged up to 150° C). A precut of varying lengths is given at the centre of the tensile specimens. The morphology of the fracture surfaces has also been reported. A critical cut length is observed for NR vulcanizates. There is an increase in the critical cut length (lc) on ageing. The sharp fall of tensile strength at the critical cut length, however, gradually diminishes. On prolonged ageing, no critical cut length is observed. A mathematical model has been made to explain the behaviour of the critical cut length of NR with ageing time/temperature. Scanning electron microscopic studies support the prediction of Thomas, that there is a change in the mechanism of rupture above thelc. Below thelc. it is a cut growth process and fracture is originated from natural flaws/nicks and proceeds towards the precut at the centre. However, for samples with precut greater than thelc, the fracture is mainly a tearing phenomena initiating from the given precut. A quantative correlation between the tensile strength and the distance between crack lines/tear lines has been found.

Antioxidants and stabilizers, 81. Transformations of N,N′‐diphenyl‐ and N‐phenyl‐N′‐isopropyl‐1,4‐benzoquinonediimines on silicagel

AbstractTransformations of N,N′‐disubstituted 1,4‐benzoquinonediimines have been investigated as a means for the elucidation of processes occurring in hydrocarbon polymers stabilized with amine antidegradants and of transformations accompanying the analyses of extracts of aged rubbers. N‐Phenyl‐N′‐isopropyl‐1,4‐benzoquinonediimine (1b) is quickly hydrolyzed on a silicagel surface, giving rise to N‐phenyl‐1,4‐benzoquinoneimine (3). The hydrolysis of N,N′‐diphenyl‐1,4‐benzoquinonediimine (1a) proceeds more slowly under the same conditions, but the released aniline quickly reacts with the initial (1a), giving rise to N,N′‐diphenyl‐1,4‐phenylenediamine(2a), 2‐(phenylamino)‐N,N′‐diphenyl‐1,4‐benzoquinonediimine (4) and azophenine (5). The Reaction is catalyzed by the weakly acid surface of silicagel. Compound (4) is readily disproportionated thermally to 1,2,4‐tris(phenylamino)‐benzene (8) and 2, 10‐dihydro‐10‐phenyl‐2‐(phenylimino)phenazine (9), even in the absence of a catalyst. A mechanism of the formation of compounds arising in the reaction has been suggested. Diimine (1a) possesses a higher oxidation potential than all the products derived from it, so that it is quickly reduced by these products to (2a). The gradual formation of (2a) may explain the retardation effects of (1a) in the autooxidation of hydrocarbon substrates.

Aluminum Block Heater for Aging Rubber and Rubber Compounds at High Temperatures

Abstract A description is given of the design and construction of an aluminum block heater, drilled with holes to contain test tubes for the aging of rubber or other organic materials at temperatures up to 800° F.

Infrared studies on natural rubber and aged natural rubber

AbstractIn the infrared absorption spectra of solutions of natural rubber in carbon tetrachloride, two new absorptions are observed, one at 6.52 μ and another at 8.2 μ. These bands disappear when films from the solution are examined. Possible origins of these bands are discussed and it is suggested that the absorption at 6.52 μ may be due to a perturbed double bond in a loose complex formed between rubber and carbon tetrachloride. The infrared absorption spectrum of rubber aged in solution also shows these absorptions. However, in the film from aged rubber the 8.2 μ absorption is found to persist. Chemical evidence for the loss in unsaturation is advanced.

Chlorination of Natural Rubber in Solution with Gaseous Chlorine

Abstract The chlorination of rubber solutions by gaseous chlorine was followed by isolating the partially chlorinated products and preparing their ozonides. The ozonides were hydrolyzed, and the acids and aldehydes formed on hydrolysis were determined. By a comparison with the amounts of acids and aldehydes obtained from ozonides of unreacted rubber, the amount of residual isoprenic double bonds present was found. The loss of double bonds attending the introduction of chlorine atoms into the molecule of rubber indicates four definite stages in chlorination : (1) initial substitutive attack by chlorine, with concomitant cyclization, resulting in a loss of one double bond between two isoprenic units, (2) substitution, (3) additive reaction, and (4) essentially substitution. Chlorination of aged rubber solutions differs from the above in that the cyclization reaction (stage 1) seems to be absent.

Aging of GR-S Vulcanizates. III. Some Effects of Oxygen and Temperature on Aging

Abstract Experiments at high temperatures (135° to 153° C) show that the fall in tensile strength and elongation and the increase in modulus of GR-S vulcanizates are accelerated by the presence of oxygen. At 70° C an increase of oxygen concentration from 0.2 atm. (air) to 20 atm. likewise promotes both loss of tensile strength and stiffening (increase of modulus), unless the rubber is under-vulcanized, when the increase of oxygen concentration often results in less stiffening. The increase of oxygen concentration also causes the aged rubber to retain more set after stretching. At 80° C, increasing the oxygen concentration from 0.2 to 20 atm. does not promote stiffening even in well vulcanized rubbers, and may even lead to less stiffening. These results indicate the following general conclusions: (1) oxidation, in addition to promoting loss of strength, is one of the causes of the stiffening of GR-S during aging, presumably due to oxygen-bridging between the molecules; (2) oxidation can also cause softening, presumably by chain scission, which further results in a change in the shape of the stress-strain curve and an increased set after deformation; (3) at high temperatures (80° C or above) increase of oxygen concentration, as in the bomb test, favors the softening reaction relative to the stiffening reaction, but at 70° C (and presumably below) this effect is not evident; (4) undervulcanized GR-S is especially susceptible to the softening (chain scission) reaction; (5) the fact that the softening reaction is favored by increase of oxygen concentration at a high temperature casts doubt on the value of a high-temperature oxygen bomb test for simulating the natural aging of GR-S (see Part IV for an analogous conclusion on other accelerated tests). The above conclusions agree with and extend those of Shelton and Winn. Prolonged (60-day) air-aging tests at 100° C confirm a previous suggestion that the tensile strength of GR-S vulcanizates reaches a constant value or even increases eventually; however, they become progressively stiffer and less extensible. Thus, under hot service conditions GR-S is more likely to fail through inability to stretch than through loss of strength.

Studies of Hard Rubber Reactions. VI. Liberation of So-Called Free Sulfur and Changes in the Acetone Extract of Vulcanized Rubber by Repeated Extraction and Heating

Abstract 1. True Free Sulfur.—In the case of the pure rubber-sulfur compound, as shown in the graph, the quantity of true free sulfur liberated from the thermally active hard rubber product by 120 minutes' cure was generally a little greater than that from the thermally nonactive sample with 300 minutes' cure. During the whole course of the extraction and heating, there occurs a lowest point in the true free sulfur curve, which increases again. In view of this fact, liberation of free sulfur can not be attributed to the insufficiency of the acetone extraction but is probably attributable to depolymerization of the vulcanizate or the like. 2. Acetone Extract.—Although purified rubber was used, the corrected acetone extract due to the formation of resinous substances from the depolymerized or aged hard rubber product showed relatively high values at the beginning of extraction and heating. To some extent, the change in acetone followed a course similar to that of the true free sulfur.

Mechanism of Rubber Aging

Abstract The position of the diffraction lines obtained on x-ray photographs of rubber does not change upon aging the rubber in the oven at 70° C. for 10 days. However, the intensity of the lines decreases as the aging time is increased. The increased elongation needed to produce fibering in aged rubber is considered necessary to expel the increased concentration of lower polymers, introduced by depolymerization during aging, from the crystalline aggregates or fibers allowing the latter to diffract the x-rays. Oxidation might also produce the same effect by the formation of obstacles to the stretching out of tangled molecules or micelles. The rate of depolymerization upon aging vulcanized rubber in the oven at 70° C. is studied, and a curve of the elongation at which the crystalline modification first appears is plotted against the time of aging. The same general curve is obtained for rubber both with and without phenyl-β-naphthylamine, indicating that an antioxidant such as phenyl-β-naphthylamine does not affect the rate of depolymerization of rubber or the elongation necessary to produce fibering in aged rubber.

Mechanism of Rubber Aging

Abstract The position of the diffraction lines obtained on x-ray photographs of rubber does not change upon aging the rubber in the oven at 70° C. for 10 days. However, the intensity of the lines decreases as the aging time is increased. The increased elongation needed to produce fibering in aged rubber is considered necessary to expel the increased concentration of lower polymers, introduced by depolymerization during aging, from the crystalline aggregates or fibers allowing the latter to diffract the x-rays. Oxidation might also produce the same effect by the formation of obstacles to the stretching out of tangled molecules or micelles. The rate of depolymerization upon aging vulcanized rubber in the oven at 70° C. is studied, and a curve of the elongation at which the crystalline modification first appears is plotted against the time of aging. The same general curve is obtained for rubber both with and without phenyl-β-naphthylamine, indicating that an antioxidant such as phenyl-β-naphthylamine does no...

Studies on Rubber in Solution. I. Studies on the Aging of Rubber Solutions

Abstract It is well known that raw rubber in colloidal solutions with different dispersion mediums shows the general characteristics of aging such as a decrease in its viscosity, whether aged by natural or artificial methods, and especially when exposed to ultra-violet light. Whitby and Jane (Colloid Symposium Monograph, Vol. 2, page 16) pointed out this phenomenon, and Asano (Mem. Coll. Eng. Kyoto Imp. Univ., Vol. 3, page 267) noticed that dispersed rubber sols decreased in viscosity and the rubber depolymerized and became insoluble by exposure to ultra-violet light. This effect was particularly noticeable with light of 2250 A. U.; however, light of longer wave lengths caused oxidation of the rubber and formed a transparent insoluble substance. Recently Hada (Rubber Ind., 1931, 147) demonstrated the curves of decrease in viscosity of rubber sols by exposure to ultra-violet light, and pointed out that the sols reached a definite viscosity after long exposure. The theory generally accepted to explain the mechanism of the lowering of the viscosity of rubber sols on aging is that depolymerization results from the oxidation of rubber molecules, and thereby increases the degree of dispersion. The process as a whole involves the formation of a dispersed phase from the absorption of the dispersion medium, and after passing the stages of solvation the viscosity of sols thus formed decreases on aging. The principal cause of these phenomena is considered to be due to a change in the structure of the dispersed particles by a chemical change of the dispersed phase itself. Whether or not it is possible to obtain, in the laboratory, a stable rubber sol by the process of long aging is not certain. It is, however, no exaggeration to say that the oxidation of the dispersed phase causes a decrease in its affinity for the dispersed medium, i. e., the oxidation changes the emulsoid to a supersoid or, in other words, there is a decrease in the size of the particles in the dispersed phase if the theory of the aging of rubber sols is that there is an increase in the dispersity by depolymerization. The author subjected rubber sols to long exposure to ultra-violet light and measured the lowering of their viscosity. The results show that (1) rubber sols in benzene reached a constant viscosity, and (2) the use of different solvents causes marked differences in rubber sols. The Brownian movement of the particles in aged rubber sols was observed with a cardioid ultra-microscope. These experiments were made as a preliminary test to explain the mechanism of dispersion of raw rubber in solvents.

The Effect of Temperature on the Stress-Strain Properties of Vulcanized Rubber

Abstract A method has been devised for testing rubber at various temperatures by putting a simple attachment onto a commercial testing machine. Wide variations have been found in the stress-strain curves on the same stock at different temperatures. The stress-strain properties at different temperatures vary for different rubbers. The state of cure causes a wide variation in tests at various temperatures. The amount of sulphur used is a factor in the stress-strain relationship at different temperatures. Successive stresses on the same piece of rubber show large decreases after the first or second stress. Stripping tests on frictions show much lower strength at 100° C. than at 0° C. Overcures are indicated prominently when stocks are tested at 100° C. Artificially aged rubber tested under these conditions shows a very marked deterioration which may be offset by anti-oxidants.