Preparation and characterization of liquid natural rubber through oxidative degradation with phenyl hydrazine and benzoyl peroxide-oxygen

The aim of this research was to synthesize liquid natural rubber (LNR) from Natural rubber (SIR-20) by chain scission method in the presence of oxygen gas and difference of peroxides, phenyl hydrazine and benzoyl peroxide. The chain scission reaction was conducted in solution of xylene in close system. SIR-20 was diluted xylene before flushing with oxygen and the addition of the peroxide. The degradation oxidation by the oxygen and the peroxides was processed at 60°C for 24 hours. The degradative oxidation product was re-precipitated by adding the excess of methanol and filtrated before dried in vacuum oven 60°C for 24 hours. The dried product was characterized by Fourier Transform Infra Red (FTIR). It was found that the liquid natural rubber product successfully degraded by chain scission process as shown the change of the peak area intensity of infrared absorption. It was showed the peaks area intensity of O-H and carbonyl group of liquid natural rubber spectra increased.

Different Curing Systems for Ethylene-Propylene Elastomers

The scope of this paper is to review peroxide curing of ethylene-propylene elastomers, where apart from crosslinking, other reactions occur, such as chain scission, which account for the drop in physical properties. The results are compared with an alternative crosslinking system which is being assessed in our laboratory. With the new curing agent the chain-scission reaction is not present and the physical properties of the vulcanizates are improved. The effects of the different crosslinking systems on the reaction transitions are studied with the aid of dynamic mechanical analysis.

PHOTOLYSIS OF POLYACRYLONITRILE

The photolysis of polyacrylonitrile has been investigated in solution (80:20 by weight of ethylene carbonate to propylene carbonate), in powder form, and as a film. Experiments have been carried out in vacuum and in the presence of oxygen. All polymer samples were exposed to ultraviolet light of wavelength 2537 Å. In solution a random chain-scission reaction with a small quantum yield (ca. 10−4 main chain bonds broken/quantum absorbed) takes place; at the same time, photochemical reactions occur involving side groups of the polymer chains, as evidenced by the change in the ultraviolet spectra of the polymer solutions. It seems likely that double-bond formation, both conjugated and isolated, and cyclization are responsible for the change in spectra. Infrared spectra do not show any change. Exposed polymer films become cross-linked and exposed powder also cross-links and evolves small molecules, such as nitriles, particularly HCN. Degradation in oxygen suppresses what is believed to be double-bond formation as evidenced by ultraviolet absorption spectra. The photolysis of glutaronitrile as a model substance has also been studied.

Chain Scission Efficiency in the Oxidation of Natural Rubber Vulcanizates

Natural rubber when it degrades under the action of oxygen does so primarily by a chain scission reaction. A technique has been developed by Tobolsky and coworkers for assessing the magnitude of this chain scission reaction in vulcanizates by means of continuous stress relaxation measurements. Since the result of the oxygen attack on the rubber is a chain scission reaction, the question of the efficiency of the reaction comes to mind. The influence of antioxidants is of importance in this regard as is the type of vulcanizate or network structure. This paper describes some measurements of the chain scission efficiency of a simple benzothiazolyl disulfide (MBTS) vulcanizate and the effect of several antioxidants on this chain scission reaction. Some of the complications inherent in this type of measurement are discussed. The first published report on the efficiency of the chain scission reaction in vulcanizates was given by Tobolsky, Metz, and Mesrobian in 1950. A more recent publication is that of Baxter, Potts, and Vodden in 1955. Tobolsky has interpreted the stress decay of gum vulcanizates at elevated temperatures as a direct consequence of the cutting of polymer chains of the network. The reduced stress is postulated as being equal to the chain density ratio:

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

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.