Off-line analysis in the manganese catalysed epoxidation of ethylene-propylene-diene rubber (EPDM) with hydrogen peroxide.

The epoxidation of ethylene-propylene-diene rubber (EPDM) with 5-ethylidene-2-norbornene (ENB) as the diene to epoxidized EPDM (eEPDM) creates additional routes to cross-linking and reactive blending, as well as increasing the polarity and thereby the adhesion to polar materials, e.g., mineral fillers such as silica. The low solubility of apolar, high molecular weight polymers in the polar solvents constrains the catalytic method for epoxidation that can be applied. Here we have applied an in situ prepared catalyst comprising a manganese(ii) salt, sodium picolinate and a ketone to the epoxidation of EPDM rubber with hydrogen peroxide (H2O2) as the oxidant in a solvent mixture, that balances the need for polymer and catalyst/oxidant miscibility and solubility. Specifically, a mixture of cyclohexane and cyclohexanone is used, where cyclohexanone functions as a co-solvent as well as the ketone reagent. Reaction progress was monitored off-line through a combination of Raman and ATR-FTIR spectroscopies, which revealed that the reaction profile and the dependence on the composition of the catalyst are similar to those observed with low molar mass alkene substrates, under similar reaction conditions. The combination of spectroscopies offers a reliable method for off-line reaction monitoring of both the extent of the conversion of unsaturation (Raman) and the extent of epoxidation (FTIR) as well as determining side reactions, such as epoxide ring opening and further, aerobic oxidation. The epoxidation of EPDM described, in contrast to currently available methods, uses a non-scarce manganese catalyst and H2O2, and avoids side reactions, such as those that can occur with peracids.

Recyclability of Photoinduced Cross-Linked EPM Rubber with Anthracene-Grafted Groups: Problems and Their Solutions.

In this paper, we present the formation of reversible covalently cross-linked networks in ethylene propylene rubber with grafted anthracene groups (EPM-g-AN) based on the principles of photoinduced anthracene dimerization. First, an industrial-grade EPM rubber grafted with maleic anhydride functional groups (EPM-g-MA) was modified with 9-anthracenemethanol. By irradiating EPM-g-AN with UV light (365 nm), the anthracene moieties dimerize via [4 + 4]cycloaddition, forming a covalent network. The network cleavage proceeds at high temperatures (>170 °C), even if with considerable (chemical) degradation. Furthermore, one of the degradation routes has been identified by 1H NMR to occur via the ester bond cleavage releasing 9-anthracenemethanol. Nevertheless, the reversibility of cross-linking has been achieved by performing the reverse reaction in decalin. The UV-vis spectroscopy clearly shows that the de-cross-linking process in these conditions is due to the anthracene dimer cleavage. Although the recovery in mechanical properties upon recycling is yet to be optimized, the disclosed results pave the way toward the use of anthracene chemistry in thermally reversible networks with possible industrial perspective applications.

Deformation mechanisms of sub-micrometer thermoplastic vulcanizates obtained by reaction-induced phase separation of miscible poly(ε-caprolactone)/dimethacrylate systems.

The micromechanical deformation mechanisms of sub-µm thermoplastic vulcanizates (TPVs) based on poly(ε-caprolactone) (PCL) and cross-linked methacrylate rubbers were studied by time-resolved small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) measurements in order to better understand the underlying deformation mechanisms responsible for the high elongation at break and good elastic recovery of sub-µm TPVs. It is demonstrated that, in contrast to neat PCL, the interlamellar void formation in the PCL matrix and subsequent coalescence of voids is suppressed by the presence of the rubber (nano)particles in these TPVs. The deformation of the TPVs under tensile conditions is dominated by yielding of the PCL matrix, which is initially localized at the equatorial regions of the rubber particles and progresses towards the polar regions at higher strains. Re-ordering of the crystal structures is both time and stress dependent, and stress relaxation of the TPV under tension is primarily governed by the break-up of the crystal lamellae at the equatorial regions of the rubber particles. This study demonstrates that the rubber particle size as well as chemical grafting of thermoplastic polymer chains onto the surface of cross-linked rubber particles are important parameters to control the mechanical deformation behavior of TPVs.

Morphological explanation of high tear resistance of EPDM/NR rubber blends.

The fatigue properties of cross-linked blends of ethylene propylene diene rubber (EPDM) with low natural rubber (NR) content and reinforced with carbon black (CB) are studied. It is found that such EPDM/NR compounds have superior crack growth resistance and fatigue lifetime. For low NR contents, transmission electron microscopy reveals that the NR phase forms small droplets of 20-50 nm. Remarkably, these droplets are even smaller than the primary CB particles. Atomic force microscopy shows that the the NR phase droplets have a higher loss factor and a smaller elastic modulus than the surrounding EPDM matrix. Rheometer measurements are used to study the effect of the phase morphology on the rubber mechanical properties. These rheological data are compared with the prediction of the Eshelby model describing the effect of elastic inclusions on solids. A complex interplay between the rubber phase morphology and the solubility of both the sulfur cross-linking system and CB is observed, which cannot be predicted theoretically. It is proposed that the soft NR droplets effectively inhibit the crack propagation in the EPDM matrix.

Thermoreversible Cross-Linking of Furan-Containing Ethylene/Vinyl Acetate Rubber with Bismaleimide.

A proof of principle for the use of Diels⁻Alder (DA) chemistry as a thermoreversible cross-linking tool for ethylene⁻vinyl acetate (EVA) rubber is demonstrated using two differently prepared amorphous furan-functionalized EVA rubbers. The first is an EVFM terpolymer of ethylene, vinyl acetate, and furfuryl methacrylate. The second is an EVA-g-furan product, resulting from the reaction of maleated EVA with furfurylamine. Both furan-containing EVA rubbers have been cross-linked with bismaleimide (BM) via a DA coupling reaction to yield final products with similar cross-link density. The BM cross-linked EVFM terpolymer products display rubber properties similar to the ones of peroxide-cured EVA rubbers with similar cross-link densities, whereas the rubber properties of the BM cross-linked EVA-g-furan correspond to those of a rubber with a higher cross-link density. The preparation of the EVA-g-furan was up-scaled to a small internal mixer, which also allowed compounding with carbon black and mineral oil in the same step. Compounding with carbon black results in reinforcement of the EVA rubber (i.e., enhanced strength), and does not interfere with the reprocessing via the retro DA reaction.

The Preparation and Properties of Thermo-reversibly Cross-linked Rubber Via Diels-Alder Chemistry.

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial ethylene-propylene rubber, grafted with maleic anhydride, is thermo-reversibly cross-linked in two steps. The pending anhydride moieties are first modified with furfurylamine to graft furan groups to the rubber backbone. These pendant furan groups are then cross-linked with a bis-maleimide via a Diels-Alder coupling reaction. Both reactions can be performed under a broad range of experimental conditions and can easily be applied on a large scale. The material properties of the resulting Diels-Alder cross-linked rubbers are similar to a peroxide-cured ethylene/propylene/diene rubber (EPDM) reference. The cross-links break at elevated temperatures (> 150 °C) via the retro-Diels-Alder reaction and can be reformed by thermal annealing at lower temperatures (50-70 °C). Reversibility of the system was proven with infrared spectroscopy, solubility tests and mechanical properties. Recyclability of the material was also shown in a practical way, i.e., by cutting a cross-linked sample into small parts and compression molding them into new samples displaying comparable mechanical properties, which is not possible for conventionally cross-linked rubbers.

EPR and modelling studies of hydrogen-abstraction reactions relevant to polyolefin cross-linking and grafting chemistry.

EPR spectroscopy has been employed to study directly the selectivity of hydrogen-atom abstraction by some alkoxyl radicals from a variety of linear and branched alkanes, as well as linear alkenes, chosen as models for low molecular-weight polyolefin cross-linking systems. In situ thermal and photolytic approaches, as well as spin-trapping, have been employed to provide information relating to an accessible temperature range of 233-453 K. in part to mimic conditions relevant to melt processing of polyolefins. Rate constants (in the range 3 x 10(3)-3.7 x 10(5) dm3 mol-1 s-1 per hydrogen) have been determined for C-H abstraction at room temperature. Radical selectivity is largely governed by enthalpic effects (modelled via bond dissociation energy calculations and kinetic analysis). Direct evidence has been obtained for lack of reactivity, as a result of unfavourable steric interactions, for the secondary and tertiary C-H bonds in 2,4-dimethylpentane and 2,4,6-trimethylheptane, models for polypropylene. This has been rationalized via free-energy calculations using DFT.