Degradation Of Natural Rubber Research Articles

Effect of treating latex with some metal ions on storage hardening of natural rubber

Storage hardening of natural rubber is a complex phenomenon involving crosslinking reactions due to certain abnormal groups on the rubber chains. The various mechanisms put forward can each account for certain of the experimental observations. Recently, an ionic crosslinking mechanism involving carboxylate and metal ions was proposed. It is being investigated by treating latex with a series of selected cations. While monovalent cations are found to have no effect on the crosslinking reaction, some divalent cations are able to reduce the storage hardening. Certain transition metal ions catalysed the degradation of natural rubber. Infra-red spectra show that the main chain structure remains the same after the degradation, and the abnormal groups appear unaffected.

Some aspects of stabilization and degradation of nigerian natural rubber (II): The environmental degradation of DBHBT‐modified unprocessed, unvulcanized nigerian natural rubber

Abstract3,5‐Di‐tert‐butyl‐4‐hydroxybenzyl thioglycollate (DBHBT) was bound to the natural rubber (Hevea Brasilenisis) latex using 4,4'‐azobiscyanovaleric (AZBN) acid‐ferrous sulfate redox system. The degradation of various dilutions of the master batch as cast films under environmental influence has been investigated using spectroscopic method. Latex‐bound DBHBT protects natural rubber latex against environmental degradation with an optimum concentration around 50%. The formation of C0 groups by some pathways independent of OH groups and allylic hydrogen atoms has been suggested in the natural weathering of DBHBT‐modified and unprocessed natural rubber.

Effect of iron oxides on the thermal degradation of natural rubber from Manihot glaziovii

AbstractThe effect of iron oxides particles mixed in natural rubber (NR) or aged natural rubber (ANR) from Manihot glaziovii were investigated. IR spectroscopy showed that α‐Fe2O3 inhibits or retards the thermal degradation (200°C) of ANR, but it does not inhibit or retard the thermal degradation of NR. A retardation effect is observed using iron (III) oxide in both ANR and NR. Using both oxides, longer autoxidation induction periods of ANR and NR are observed (150°C). The periods were extended for a higher content of rubber hydroperoxide concentration. The inhibition or retardation of autoxidation using Fe3O4 in both ANR and NR is not evident. We suggest an interaction between hydroxyl groups of hydroperoxide, mainly present in aged natural rubber, and the iron oxide surface.

Kinetic Study of Oxidative Scission in Natural Rubber by Wallace Plasticity Measurements

Abstract The kinetics of thermal oxidative degradation of natural rubber (NR) have been studied by Wallace plasticity measurements. On the basis of the Kotliar's model of random chain scission, an expression was derived to interpret the kinetic data collected over a range of temperatures from 80 to 160°C. It has been shown that the kinetic parameter resulting from the present analysis, indeed refers to a diffusion-controlled reaction with an overall activation energy estimated to be 135 ± 6 kJ · mol−1. In addition, the implications of plasticity retention index, which measures the oxidizability of NR, are discussed.

Studies on the effect of blend ratio and cure system on the degradation of natural rubber—ethylene-vinyl acetate rubber blends

The effects of blend ratio and type of cross-link system on thermal ageing, γ-radiation and ozone resistance of blends of natural rubber and ethylene-vinyl acetate (EVA) rubber have been evaluated. The morphology of the blends is such that the EVA forms a continuous phase when its proportion in the blend is 40% or more. The resistance of the blends to thermal ageing, γ-radiation and ozone attack is better for those which contain a higher proportion of EVA. These properties are also highly dependent on the type of cure system used.

Natural rubber pyrolysis: Study of temperature-and thickness-dependence indicates dimer formation mechanism

The technique of pyrolysis—gas chromatography (Py—GC) has been used to study the thermal degradation of natural rubber. The monomer/dimer (M/D) ratio has been measured over the temperature range 300 to 500°C, using sample sizes of the order of 0.3 mg. The M/D ratio has also been measured as a function of sample thickness, using samples in the range 30 nm to 3 μm (0.1 μg to 10 μg). The dependence of M/D on temperature may be interpreted in terms of (a) different activation energies for depropagation versus intramolecular cyclisation, (b) dissociation of dimer to monomer at higher temperatures, or (c) reduced residence time of monomer in the melt at higher temperatures allowing less opportunity for its recombination. The results for the dependence of M/D on thickness (at constant temperature) indicate that interpretation (c) is the most probable. The present work therefore suggests that monomer recombination, possibly by a Diels-Alder mechanism is an important contributor to dimer formation in rubber pyrolysis.

Some aspects of stabilization and degradation of nigerian natural rubber. I. Photostabilization of natural rubber with bound di‐tert‐butyl‐4‐hydroxybenzyl thioglycollate (dbhbt)

Abstract3,5‐Di‐tert‐butyl‐4‐hydroxybenzyl thioglycollate (DBHBT) has been reacted with Nigerian Natural rubber latex at 65 ± 1°C using 4,4‐azobiscyanovaleric (AZBN) acid‐ferrous sulfate redox system to obtain a binding efficiency of 13%. The bound DBHBT was investigated as a UV stabilizer in unprocessed and unvulcanized rubber using spectroscopic method. With respect to the CO index, the masterbatch diluted to 56% gave the optimum activity and higher concentrations showed significant pro‐oxidant activity. The CC bonds in particular, showed greater stability which is a function of DBHBT concentration. The methylenic and the allylic hydrogen atoms, the most susceptible, and probably the sites of the primary attack, were not protected by DBHBT.

Kinetic study of random chain scission by viscometry

AbstractBased on the Saito's model for random chain scission, a novel procedure is developed to determine the activation energy of polymer degradation process. Basically, this is a simple viscometric technique which yields reliable results. In fact, it is superior to the existing viscometry which has been widely used to study the kinetics of random scission. The present method is successfully applied to the thermal degradation of natural rubber, polycarbonate, and poly(tetramethylene oxide) in bulk.

Synthese d'elastomeres photoreticulables par modification chimique du caoutchouc naturel liquide—I. Introduction de l'anhydride maleique sur les structures polyisopreniques

With the aim of preparing photocrosslinkable elastomers, partial chemical modification of liquid natural rubber (LNR) by reaction with maleic anhydride has been studied. LNR was obtained by controlled degradation of natural rubber in latex state with the phenylhydrazine/oxygen system. As a first approach, 4-methyl-4-octene was studied as a model for the 1,4-polyisoprene structure. This led to the choice of a thermal fixation process, called the “ene reaction”, which is more selective than the radical reaction system using initiators. A reliable method for determining the modification level by maleic anhydride was provided by titration of the formed acid functions which result from the succinic ring opening reaction by the action of methanol. With the exception of the partial succinic ring opening which occurs during the maleic anhydride addition process, a good match was observed between the maleinization study on LNR and those on liquid synthetic 1,4-polyisoprene. For a theoretical partial modification fixed at 20%, a maximum yield of 70% was achieved under optimum reaction conditions.

Metathesis degradation of natural rubber

Metathesis reaction of natural rubber was carried out using tungusten hexachloride and tetramethyl tin in chlorobenzene. The rubber had to be purified by several times precipitations before the reaction. Molecular weight decreased very much with the progress of reaction and with the increase of temperature. However, NMR studies indicated considerable decreases of carbon-carbon double bonds, which can not be explained by simple metathesis reaction. Some other reactions which consumed unsaturation have to be taken into account to elucidate the exact structure of resultant oligomers.

Studies relating to the oxidative degradation of natural rubber. The autoxidation of 1-methylcyclohexene: primary product analysis, allylic hydroperoxide isolation, and regiochemistry of the initial events

The autoxidation of 1-methylcyclohexene has been examined; the regiochemistry of the initial events has been defined by direct examination and isolation of the first-formed products and a synthetically useful method has been developed in which t-butyl hydroperoxide-loaded mixtures undergo rapid, low temperature autoxidation.

Production of hydroxyl‐terminated liquid natural rubber—mechanism of photochemical depolymerization and hydroxylation

AbstractPhotochemical degradation of natural rubber yielded hydroxyl‐terminated liquid natural rubber (HTNR) when carried out in solution in presence of H2O2. Ultraviolet radiation from a medium pressure mercury vapor lamp and sunlight were found to be almost equally effective in bringing about the depolymerization and hydroxylation of natural rubber. The variations in the composition of the reagents and exposure time on the extent of depolymerization was conducted, and a suitable procedure for the large scale preparation of HTNR was described. A probable mechanism leading to the formation of HTNR as well as the side products is discussed based on the analytical data.

The formation of trans‐epoxides during the photochemical oxidation of cis‐polyisoprenes to liquid rubbers

AbstractPhotochemical hydrogen peroxide catalyzed degradation of both natural rubber and synthetic cis‐polyisoprene is shown to result in the formation of a low‐molecular‐weight oil functionalised with hydroxyl, carbonyl and epoxide groups. Interestingly, the epoxide groups are shown to be of a trans‐configuration. A scheme is proposed to account for the formation of trans‐epoxide as well as concomitant mainchain isomerization. The preferential formation of trans‐epoxide in free radical oxidations strongly suggests that the naturally occurring cis‐epoxide, present in natural rubber, is due to a stereospecific enzymatic process, and is not the byproduct of radical oxidation.

Why does the degree of cracking of rubber decrease with increasing strain? (An extended note to the paper by A.A. Katbab and G. Scott)

Mechanical data by Katbab and Scott have been re-evaluated. It has been demonstrated that an anomalous decrease in the degree of cracking and an increase in the time-to-break with increasing strain reflect the heterogeneous character of degradation of natural rubber in an atmosphere of ozone. Macroscopic effects and structural mechanisms of equivalence between degradation and incision are briefly discussed.

Degradation of Natural Rubber byFungi imperfecti

Fungi degrading natural rubber were isolated from soil and deteriorated tyres. The products of degradation were estimated by gel permeation chromatography and viscosity measurement.

Degradative Effects of Guayule Resin on Natural Rubber. II. Kinetic Studies

Abstract Qualitative studies reported earlier and the kinetic studies reported here indicate that the unsaturated fatty acids in guayule resin are primarily responsible for the ability of guayule resin to accelerate the oxidative degradation of natural rubber. These unsaturated fatty acids apparently oxidize to form peroxide compounds which function as initiators of rubber oxidation.

Degradation of natural rubber by fungi imperfecti.

Fungi degrading natural rubber were isolated from soil and deteriorated tyres. The products of degradation were estimated by gel permeation chromatography and viscosity measurement.

Degradative Effects of Guayule Resin on Natural Rubber

Abstract This study indicates that the degradative effects of guayule resin on natural rubber are due to the presence of unsaturated fatty acids in the resin. Since linoleic acid is, by far, the most abundant unsaturated fatty acid in guayule resin, it appears that linoleic acid is primarily responsible for the degradative effects of guayule resin on natural rubber. A possible mechanism to explain the acceleration of natural rubber degradation caused by linoleic acid can be developed by fitting linoleic acid into the Bolland oxidation mechanism for hydrocarbon polymers.

Inverse gas chromatography in the study of polymer degradation. Part I. Oxidation of squalene as a model for the oxidative degradation of natural rubber

Inverse gas chromatography with an olefinic stationary phase and air as carrier gas has been found to yield characteristic shifts of retention data. The resulting plots of reten index versus reaction time have been found to be similar to those given by the existing methods for monitoring oxidation reactions available for use in rubber technology. Furthermore, the expected dependence upon temperature and presence of metal ions of these chromatographic ocidation curves have been observed. The chromatographic method is shown to be complementary to the existing technological systems.

Effect of mastication on molecular weight and molecular weight distribution of EPDM polymer and SBR

AbstractEPDM and SBR were masticated on an open mill. The temperature range of mastication for EPDM was 68–480°F. SBR was milled at 170–200°F. The gel‐permeation chromatography analyses were made on the masticated samples. For EPDM at 68°F, molecular weight decreases and molecular weight distribution narrows with mastication time; the degradation process is nonrandom. At constant mastication time between 182 and 315°F, there is little change in molecular weight. Mastication for 18 min at 480°F broadens the molecular weight distribution; the degradation is random. For SBR at 170–200°F, molecular weight decreases and molecular weight distribution narrows with mastication time; the degradation process is also nonrandom. Nonrandom degradation for both EPDM and SBR results in a narrowing of the molecular weight distribution, without build‐up of low molecular weight molecules, and without a shift in the peak molecular weight. This is contrary to nonrandom degradation of natural rubber where a shift in the peak molecular weight occurs with mastication time.