Aging Process Effects on the Characteristics of Vacuum Residue Oxidation Products with the Addition of Crumb Rubber

This paper considers the effect of aging processes on viscoelastic characteristics of vacuum residue oxidation products modified with crumb rubber. Viscoelastic properties were compared to original bitumen raw material-vacuum residue and vacuum residue oxidation products during short-term and long-term aging. The complex shear modulus of the vacuum residue and its oxidation products decreased with an increase in temperature. Short-term aging resulted in increased shear modulus for all samples.The vacuum residue oxidation product modified with crumb rubber had the maximum values of the rutting parameter and fatigue parameter. There was an expansion of the temperature range of plasticity: for the vacuum residue oxidation product with crumb rubber, its value was 67.2 °C. The curves of the black diagram of the modified vacuum residue oxidation product are shifted towards smaller phase angles with the increase in the shear modulus, which indicates the increase in the stiffness and elasticity of the rubber bitumen binders. The vacuum residue oxidation product modified with crumb rubber corresponded to the rubber bitumen binder of the grade RBB 60/90, according to its physical and mechanical indicators.     

Kinetic modelling of the thermal oxidation of polyisoprene elastomers. Part 1: Unvulcanized unstabilized polyisoprene

The thermal oxidation of an unvulcanized, unstabilized polyisoprene rubber (IR) has been studied in the 40-140 °C temperature range. Ageing was monitored by FTIR determination of double bonds and carbonyl groups, mass uptake measurement, and weight average molar mass determination. A mechanistic scheme based on the standard scheme for radical chain oxidation, but taking into account the diversity of initiation processes and the existence of inter- and intramolecular radical additions to double bonds, was built. The kinetic model derived from this scheme is composed of seven differential equations to be solved in discrete thickness layers to take into account the kinetic control by oxygen diffusion. This system was numerically solved using a Matlab program dedicated to stiff systems of differential equations. The elementary rate constants and other kinetic parameters were then determined from experimental data, using an inverse approach. A set of physically reasonable parameter values was obtained, thus allowing us to envisage lifetime predictions at low temperature (long term). The results led to observations difficult to make from classical analytical studies, for instance the predominance of bimolecular hydroperoxide decomposition among other initiation modes or the competition between intermolecular hydrogen abstraction and intramolecular addition of peroxy radicals to double bonds.     

Improving thermal and ozone stability of skim natural rubber by diimide reduction

The physical, chemical and thermal properties of diene-based polymers are improved by a chemical modification method such as hydrogenation. Skim natural rubber (SNR) which is mainly comprised of cis-1,4-polyisoprene was hydrogenated by diimide reduction in latex form, using hydrazine and hydrogen peroxide with copper sulfate as catalyst. The effect of various parameters on the level of hydrogenation calculated from proton nuclear magnetic resonance spectroscopy (¹H NMR) was investigated. The kinetic results indicated that the diimide hydrogenation of skim natural rubber latex (SNRL) exhibited a first order behavior with respect to the CC concentration. The apparent activation energy of the catalytic and non-catalytic hydrogenation of SNRL was calculated as 9.5 and 21.1 kJ/mol, respectively. From the TEM micrograph of hydrogenated SNRL particles, non-hydrogenated rubber core and hydrogenated rubber layer were observed according to a layer model. The results from thermal analysis confirmed that thermal stability of hydrogenated SNR was improved compared with the starting SNR. In addition, the thermal aging and ozone resistance of vulcanized hydrogenated SNR blends were also investigated.     

Rubber aging in tires. Part 1: Field results

Oxidative aging of skim and wedge rubber inside the tire results in a loss of peel strength and tensile properties of these rubber materials, which has been found to increase the likelihood of tread separations in certain tires. In order to develop an accelerated laboratory tire aging test, we have carried out extensive field and laboratory studies of rubber property change in tires. This paper describes the analysis of rubber oxidation in a specific set of tires collected from the field. In particular, we determine the rate of property loss under worst-case environmental conditions and analyze the implications of variability in aging results. The analysis is used in a companion paper to develop acceleration factors for different laboratory tests.     

Characterization of effects of thermal-oxidative aging on styrene-butadiene rubber/silica composites with vitamin C-lanthanum complex

A novel rubber antioxidant, vitamin C-lanthanum complex, was prepared and applied in styrene-butadiene rubber (SBR)/silica composites. The anti-aging behavior of SBR/silica composites with vitamin C-lanthanum complex was systematically investigated by mechanical property retention after aging, oxidation induction time, exothermic enthalpies of thermal oxidation, and thermo-oxidative degradation kinetics. The highlight of this work lies in the fact that several thermal analysis techniques were successfully applied to fully evaluate the thermal-oxidative aging of SBR/silica composites and the vitamin C-lanthanum complex was found to endow SBR/silica composites with better protection against aging than commercial antioxidants, which may be beneficial for better characterization of rubber aging and fruitful for the preparation of highly aging-resistant rubber composites, respectively.     

A new approach on the thermal stability of SDS copolymer for HMPSA, Part I: Oxidation kinetics for the whole process

The poor aging property of the styrene–diene–styrene (SDS) triblock elastomer copolymer hot-melt pressure-sensitive adhesives (HMPSAs) has raised the importance of research on the aging and anti-aging properties of SDS triblock copolymers, such as styrene–butadiene–styrene (SBS) and styrene–isoprene–styrene (SIS). A mechanistic scheme based on the standard scheme for radical chain oxidation, but taking into account the decomposition of the oxidation-induced peroxide, was built. The kinetic equation of oxygen uptake was deduced from the proposed mechanism, which is composed of a set of reasonable parameters. The relationships among the parameters to induction time, maximum oxygen uptake and the maximum oxidation rate were examined. Numerical simulation methods were established to obtain parameters from the experiment data, by which most kinetic equations of oxygen uptake of the ingredients for HMPSAs were numerically fitted and the relativity of the model to the data was discussed. The study results should prove useful for future studies on the anti-aging performance of other materials.     

Further study on the prediction of isothermal induction time by dynamic differential scanning calorimetry

To test the applicability of dynamic DSC for predicting isothermal oxidation induction time, further study was carried out on butadiene rubber samples. Similar to the natural rubber samples, various Arrhenius plots of the butadiene rubber samples could be superimposed to form a single plot using a shift factor dependent on the oxidation peak temperature obtained from a dynamic DSC test. The superimposed plot can then be used to predict the oxidation induction times of other butadiene rubber samples from dynamic DSC test results.     

Effect of organo-modified clay on accelerated aging resistance of hydrogenated nitrile rubber nanocomposites and their life time prediction

Organically modified clay - reinforced hydrogenated nitrile rubber vulcanizate was subjected to accelerated heat aging to estimate its long-term thermo-oxidative stability and its useful lifetime was compared with that of the virgin polymer for the first time. Changes in technical properties such as tensile strength, modulus and elongation at break were studied as a function of time and temperature of aging. The infrared spectroscopic analysis of the degraded products revealed that under aerobic hot aging conditions, hydrogenated nitrile rubber (HNBR) compounds undergo cross-linking reactions that lead to embrittlement and ultimately failure. Incorporation of clay filler, however, resulted in significant improvement of the degradation profile of the nanocomposite at elevated temperatures. Loss of ductility during aging of the nanocomposite was also milder, relative to the unfilled polymer, indicating a restricted degradation by the clay filled rubber, thus prolonging the durability. From the scanning electron microscopy and atomic force microscopy studies, it was found that nanofillers protected the elastomer from surface rupture that took place on oxidation. Life prediction of both virgin elastomer and the nanocomposite indicated a three-fold increase in the effective service temperature range of the HNBR using 8 parts organically modified nanoclay.     

Rubber aging in tires. Part 2: Accelerated oven aging tests

The kinetics of oxidation of wedge and skim rubber from tires aged at different oven temperatures with various fill gases have been measured for 5 different tires and compared to field results. We demonstrate that oven aging tires mounted on wheels and inflated to the maximum sidewall pressure closely reproduce the aging behavior measured for tires collected after customer use. Temperatures as high as 70 °C can be used to accelerate aging. Use of 50/50 blend of N₂/O₂ as a fill gas accelerates the oxidative aging by 30-40% relative to air. By combining elevated temperatures with oxygen enriched fill gas, it is possible to oven age tires to an age equivalent to 6 years in Phoenix in 8 weeks or less.     

Kinetic modelling of the thermal oxidation of polyisoprene elastomers. Part 3: Oxidation induced changes of elastic properties

This paper is the third of a series elaborating a non-empirical kinetic model for the thermal oxidation of a sulfur vulcanized polyisoprene. Here, we try to identify kinetic parameters for post-crosslinking and reversion (“decrosslinking”) from torsion measurements, under nitrogen at temperatures ranging from 100 to 160 °C. The kinetic parameters relative to oxidative reversion (selective scissions on sulfur crosslinks) are also determined. Then a system of 13 differential equations is derived from the mechanistic scheme composed of 15 elementary reactions. Diffusion and reactions are coupled in the balance equation of oxygen in order to establish the degradation thickness profiles from which it is possible to determine the modulus profiles. The latter are used, through a composite mechanics model, to predict the global torsion stiffness of a rubber barrel. The results obtained at 100, 110, 130, 140, 150 and 160 °C are in good agreement with experimental data.     

Oxidation profiles of thermally aged nitrile rubber

The thermal degradation of a commercial, stabilized, unfilled nitrile (Buna-N) rubber material was investigated at temperatures in the range 85–140 °C. The resulting heterogeneous oxidation, due to diffusion limitations in oxygen availability, was studied using infrared microscopy and modulus profiling. Degradation-related spectral changes were observed primarily in the hydroxyl, carbonyl and ester regions; quantitative analysis revealed identical oxidation profiles for these chromophores. These chemical oxidation profiles (carbonyl formation) were correlated with mechanical modulus (hardness) profiles. Degradation of the sample proceeds via a linear increase in the carbonyl concentration, but an exponential increase in the modulus with time. It is concluded that the profile development and aging behavior can be described by a diffusion-limited autoxidation mechanism which can be modeled computationally. The results are compared to those of a previously studied carbon-black-filled material.     

The effect of aging during storage on the performance of a radial lip seal

Since aging is an inevitable phenomenon during storage of a radial lip seal, this study aims to investigate the effect of aging on the seal performance. An air oven aging test is conducted to produce aged rubber samples and seal products. An aging life prediction model is constructed to predict the equivalent material properties at any storage temperature. The material property parameters of aged rubber samples are measured in a uniaxial compression experiment. The pumping rate and friction torque of the seal are calculated through numerical simulation by using a mixed elastohydrodynamic lubrication (EHL) model. A bench test is used to verify the aging life prediction model and the simulation results of the EHL model. Finally, the verified EHL model is used to study how aging during storage affects the lip seal performance, as characterized by the leakage and friction torque.     

Accelerating effect of poly(methyl methacrylate) on rubber oxidation, Part 1: A chemiluminescence study

The oxidative stability of blends of poly(methyl methacrylate) (PMMA) with four types of rubber, poly(ethylene-stat-propylene-stat-5-methylene-2-norbornene) (EPDM), poly(ethylene-stat-vinyl acetate) (EVA), dicarboxy terminated poly(acrylonitrile-stat-butadiene) (NBR) and poly(1,3-butadiene-stat-styrene) (SBR), has been investigated as models for rubber-toughened PMMA. Chemiluminescence was used to monitor the oxidation of the rubber in the blends, revealing an unexpected accelerating effect of PMMA on the oxidation of all the rubbers investigated. The effect varies according to the type of rubber and the temperature. The thermo-oxidative stability of PMMA has also been found to decrease in the presence of the rubber as proved by thermogravimetry, confirming mutually antagonistic effects of PMMA and rubber in the blend oxidation. On the basis of results from all techniques, including identification of oxidation products by FTIR spectroscopy, a possible mechanism is suggested, involving the formation of mobile radicals in PMMA induced by rubber oxidation. These radicals are suggested to play a crucial role in enhancing rubber phase oxidation.     

Processing characteristics and thermal stabilities of gel and sol of epoxidized natural rubber

The processing performances and aging behaviours of gel and sol separated from epoxidized natural rubber (ENR) using organic solvents were studied by using rubber processing analyzer (RPA), thermogravimetric analysis (TGA) and difference FT-IR method. As the gel with intermolecular ether structure is formed by the ring-opening reactions of a part of epoxy groups during the preparation and storage of ENR, the molar percentage of epoxy groups of gel is lower than that of sol. The gel shows higher elastic moduli to temperature and frequency responses, lower tan δ to frequency and strain response and higher elastic torque to strain response as comparing to those of sol. Therefore, the formation of gel will reduce processing performance of ENR. When the aging behaviours of gel and sol were analyzed by different methods, the aging reactions and their degrees were different because of the differences of aging conditions. Compared to gel, the sol is easy to be crosslinked, leading to a higher Δtan δ from the RPA analysis and it is also easy to be oxidized into small molecules, leading to lower onset temperature and apparent activation energy when being analyzed by TGA. However, the variations of functional groups of gel are more obvious than those of sol when analyzed by difference FT-IR method. As the mechanical properties of ENR will be varied with the variations of molecular structures, the formations of gel during preparation and storage of ENR will reduce the oxidative aging resistance of ENR.     

Formation of Peracetic Acid upon Aging of Perborate in Acetic Acid. Kinetics of the Oxidation of S-Phenylmercaptoacetic Acids

 Upon aging, perborate in glacial acetic acid generates peracetic acid and thus oxidizes S-phenylmercaptoacetic acid rapidly. Perborate dissolved in ethylene glycol, however, does not show the aging effect, and the corresponding oxidation proceeds smoothly. The oxidation is of second order and not acid catalyzed. Boric acid and borate do not influence the oxidation. In the smooth oxidation, is the reactive species. The oxidation of some para-substituted S-phenylmercaptoacetic acids conforms to the Exner relationship, indicating operation of a common mechanism. Also, the oxidation obeys the Hammett equation with a negative reaction constant. However, the oxidation of p-nitro-S-phenylmercaptoacetic acid follows a different kinetic pathway.     

Thermal analysis of lanthanide(III) and Y(III) complexes with 4-hydroxy-3-methoxybenzoic acid

Summary A procedure for the extrapolation of accelerated thermo-oxidative ageing tests to lower temperatures is proposed. The procedure involves a deconvolution of the global process into high- and low-temperature components where the extrapolation to low temperatures is carried out using the low-temperature component. The method was tested on stabilized and unstabilized polyisoprene rubber and was found to produce realistic estimations of the length of the induction period of oxidation so giving a more accurate estimation of the service life. However, to obtain the low-temperature values of the adjustable kinetic parameters, very low heating rates are required (0.04 K min^-1, 0.1 K min^-1) making the measurement process time consuming. Using this method, more realistic estimates of the durability of a material are obtained.     

Oxidative degradation products analysis of polymer materials by pyrolysis gas chromatography–mass spectrometry

Pyrolysis gas chromatography–mass spectroscopy (PGC–MS) has been proved to be a powerful method to analyze both the volatile additives and the macromolecular structure of polymer materials. In this paper, flash evaporation technique was used to analyze the volatile degradation products of polymer materials during natural and artificial aging. In high density polyethylene (HDPE) composites, mainly n-alkanes with carbon number from 14 to 29 were detected after natural aging, while no oxidative product was found. Different composites have different n-alkane distributions. In contrast, various oxidative products including ketones, alcohols, esters and unsaturated species could be found in aged polypropylene (PP) nanocomposites. Nanoparticles accelerated the chain scission of PP and increased the formation of oxidative products significantly. During thermal oxidation of nitrile rubber (NBR) seal rubbers, heat/oxidation-induced extra crosslinking predominated and no volatile degradation products was detected. The main change happened in the volatiles is the decrease of additives, especially paraffins, antioxidant RD and hindered phenol. This resulted in the hardening of the rubber and the weakening of the protection from oxidation. Furthermore, the additive distribution along the depth was investigated, showing different migration speeds of different additives. From the additive levels remained in the NBR rubber, it is possible to predict the degradation status. In summary, PGC–MS can supply abundant information of polymer degradation and is helpful for mechanism research.     

Multiscale physicochemical characterization of a short glass fiber–reinforced polyphenylene sulfide composite under aging and its thermo‐oxidative mechanism

In this paper, the thermo‐oxidation for a short glass fiber–reinforced polyphenylene sulfide (PPS/GF) composite was experimentally and theoretically studied by a wide range of physicochemical and mechanical techniques. The accelerated thermal aging temperatures were fixed at 100°C, 140°C, 160°C, 180°C, and 200°C. Firstly, the results of weight loss under aging indicate the formation of volatile products because of chain scission of end groups. Also, Fourier‐transform infrared spectroscopy (FTIR) results suggest that the formation and accumulation of carbonyl group arising from the formation of hydroperoxides in oxidative propagation process. In all cases of different thermal oxidation temperatures, it is hard to observe some significant change about the concentration of carbonyl group during the induction time. This induction time depends inversely on the oxidation temperature. Moreover, the cross‐linking and chain scissions exist together according to the results of rheological results and it is easier to see the cross‐linking phenomenon at the beginning of oxidation while the chain scissions are more pronounced, with the oxidation process developing further. In aspect of mechanical properties, σ_max increases at the beginning of oxidation because of cross‐linking, and subsequently, the σ_max always decreases because of thermo‐oxidation of the PPS matrix. In addition, the detailed thermo‐oxidation processes are fully discussed in the end of this study. A mechanistic schema has been proposed to present different oxidation reactions of PPS polymer and then a kinetic model has been extracted from this mechanism. Afterwards, the model has been verified by experimental results at different temperatures.     

Prediction of thermal stability of fresh and aged parchment

The hyphenated thermal analysis-mass spectrometry technique (TA-MS) was applied for the investigation of the thermal behavior of reference and aged parchment samples. The kinetic parameters of the process were calculated independently from all recorded TA and MS signals. The kinetic analysis showed the distinct dependence of the activation energy on the reaction progress. Such behavior is characteristic for the multistage mechanism of the reaction. The comparison of the kinetic parameters calculated from the different signals i.e. TG, DSC, MS for H₂O, NO and CO₂, however, indicated that they were differently dependent on the aging of the sample. For the parchment samples, the aging almost does not change the kinetics of the decomposition calculated from the DSC data: the influence of aging seems to be too negligible to be detected by these techniques. On the other hand, the much more sensitive mass spectrometric technique applied to the kinetic analysis allowed monitoring of visible changes in the thermal behavior of the parchment samples due to the aging process. The influence of aging was especially visible when the MS signals of water and nitric oxide were applied for the determination of the kinetic parameters. The applied method of the kinetic analysis allowed also the prediction of the thermal behaviour of reference and aged parchment samples under isothermal and modulated temperature conditions. Presented results have confirmed the usefulness of thermoanalytical methods for investigating behaviour of such complicated systems as leather or parchment.     

Lifetime prediction of ABS polymers based on thermoanalytical data

The service life of ABS polymer, stabilized by 2-(3,5-di-tert-butyl-4-hydroxyanilino)-4,6-bis(octylthio)-1,3,5-triazine and containing 50% of a modifying rubber component, was estimated from oxidative induction times measured by DSC in isothermal mode in the temperature interval 140–170°C. The lifetime of ABS powder at the actual temperature of drying was predicted by linear extrapolation according to Arrhenius. However, the extrapolated value was much longer than the real lifetime determined from the long-term oven aging tests at 70 and 90°C, simulating the industrial drying process. The effect of changes in the apparent activation energy of oxidation due to antioxidant consumption during polymer aging is discussed.