Stabilizer additives in ionizing radiation environments under oxidizing conditions

Very little work has been done with energy-scavenger additives in radiation environments under oxidizing conditions. In this study, the stabilizing effect of an energy scavenger (pyrene) and a radical scavenger (hindered phenol), which were incorporated in a polychloroprene rubber, has been evaluated in a γ-radiation environment in air under conditions which gave rise to oxidation throughout the sample. Both compounds were found to significantly inhibit changes in macroscopic tensile properties of the materials. The pyrene enhanced the material's radiation resistance when used alone and also when used in conjunction with the hindered phenol. The rate of radiation-induced chemical destruction of the two additives was monitored by gas chromatography after solvent-extraction from degraded samples. Following a dose of 3 × 10⁵ Gy, only a few per cent of the hindered phenol remained, whereas more than 90% of the pyrene was still present. The rate of disappearance of the hindered phenol was slower in samples which also contained the pyrene.     

Comparison of antioxidants for combined radiation and thermal aging and superposition of radiation and thermal aging for EPR and XLPE

Thermal and combined thermal and radiation aging of low voltage EPR and XLPE cable insulation with Agerite MA antioxidant and with the ZMTI/Aminox antioxidant system was examined to compare the relative effectiveness of the antioxidant and polymer systems. All provided significant stability with no clear choice of any particular combination being superior to the others. A comparison of degradation from thermal and radiation aging with degradation from combined thermal/radiation aging showed that the damage from the individual aging effects was superposable. This indicates that synergistic effects have little importance for the EPRs and XLPEs tested under the aging conditions observed.     

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.     

Solar ultraviolet-B radiation in urban environments: the case of Baltimore, Maryland

Ultraviolet-B radiation (UV-B, 280-320 nm) has important effects in urban areas, including those on human health. Broadband UV-B radiation is monitored in Baltimore, MD, as part of the Baltimore Ecosystem Study, a long-term ecological research program. We compare broadband UV-B irradiance in Baltimore with UV-B at two nearby locations: a more rural station 64 km southeast and a suburban station 42 km southwest. The monitoring station in Baltimore is on the roof of a 33-m-tall building; there are no significant obstructions to sky view. The U.S. Department of Agriculture UV-B Monitoring and Research Program provided all sensors, which were calibrated at the National Oceanic and Atmospheric Administration Central UV Calibration Facility. UV-B irradiances at the three sites generally were similar. Over all conditions, Baltimore and the suburban site measured 3.4% less irradiance than the rural site. This difference is within the anticipated +/-3% calibration uncertainty of the pyranometers. On 59 days with cloud-free conditions at all three sites, average differences in measured UV-B among the three sites were even smaller; Baltimore measured 1.2% less irradiance than the rural site. High aerosol optical thickness strongly reduced daily UV-B dose, whereas [SO2] had no influence. Surface O3 increased with increasing UV-B dose when [NO2] exceeded 10 ppb.     

Prediction of paper permanence by accelerated aging II. Comparison of the predictions with natural aging results

Accelerated aging tests are credible and useful to predict paper permanence only if such tests can be shown to correlate with natural aging. In the first part of this study, a kinetic model was developed based on the accelerated aging results. In this report, we have shown that this kinetic model can indeed predict the natural aging results of lignin-free sheets with a statistical confidence. This is the first quantitative comparison of accelerated aging with natural aging.     

Evaluation of vinylidene fluoride polymers for use in space environments: Comparison of radiation sensitivities

Poly(vinylidene fluoride) and copolymers of vinylidene fluoride with hexafluoropropylene, trifluoroethylene and chlorotrifluoroethylene have been exposed to gamma irradiation in vacuum, up to doses of 1 MGy under identical conditions, to obtain a ranking of radiation sensitivities. Changes in the tensile properties, crystalline melting points, heats of fusion, gel contents and solvent uptake factors were used as the defining parameters. The initial degree of crystallinity and film processing had the greatest influence on relative radiation damage, although the cross-linked network features were almost identical in their solvent swelling characteristics, regardless of the comonomer composition or content.     

Further results on a family of generalized radiation integrals

In this paper we continue the investigation by Kalla et al. (1991) of the family of generalized radiation integrals defined by

, where Re(γ) > Re(β) > 0; −1 < λ < 2 − 2μ > −1; μ > −1; p, a, b > 0; 0 < a b < ∞. Several recurr ence relations are presented. By differentiation of these integrals with respect to the parameters λ and μ we obtain also various integrals that include the logarithmic function in the integrand. Finally, we propose an algorithm for numerical evaluation of the generalized radiation integrals and illustrate it by tables of their values computed for selected values of the parameters.     

Nylon 6.6 accelerating aging studies: II. Long-term thermal-oxidative and hydrolysis results

Long-term (greater than 5 year exposures), low-temperature (as low as 37 °C) accelerated oven aging results were obtained for Nylon 6.6 fibers under thermo-oxidative conditions (air aging with an oxygen partial pressure of 13.2 cmHg in Albuquerque). To assess the importance of humidity on aging, experiments were also conducted under a combination of 100% RH plus 13.2 cmHg of oxygen partial pressure at temperatures ranging from 138 °C to 64 °C plus an additional experiment at 70% RH and 80 °C. The low-temperature tensile strength results showed that the Arrhenius activation energy under the pure oxidative degradation conditions dropped from ∼96 kJ/mol above ∼100 °C-∼30 kJ/mol below this temperature, indicative of a transition in the oxidative chemistry at low temperatures. Earlier work by our group on the same material concluded that hydrolytic degradation effects dominated oxidation effects at higher aging temperatures. However, the current long-term, low-temperature comparisons lead to the conclusion that humidity is not an important aging factor below ∼50 °C. By extrapolating time-temperature superposed oxidative degradation data using the low-temperature activation energy, we obtain predictions at 21 °C. At this temperature, we estimate that a tensile strength loss of 50% takes on the order of 70 years. The 21 °C predictions are shown to be reasonably consistent with long-term (up to 38 year) ambient results on similar Nylon materials removed from field-aged parachutes. Although the estimated average exposure temperature varies from parachute to parachute, the highest average temperature is estimated to be on the order of 21 °C.     

Gamma radiation interacts with melanin to alter its oxidation-reduction potential and results in electric current production

The presence of melanin pigments in organisms is implicated in radioprotection and in some cases, enhanced growth in the presence of high levels of ionizing radiation. An understanding of this phenomenon will be useful in the design of radioprotective materials. However, the protective mechanism of microbial melanin in ionizing radiation fields has not yet been elucidated. Here we demonstrate through the electrochemical techniques of chronoamperometry, chronopotentiometry and cyclic voltammetry that microbial melanin is continuously oxidized in the presence of gamma radiation. Our findings establish that ionizing radiation interacts with melanin to alter its oxidation-reduction potential. Sustained oxidation resulted in electric current production and was most pronounced in the presence of a reductant, which extended the redox cycling capacity of melanin. This work is the first to establish that gamma radiation alters the oxidation-reduction behavior of melanin, resulting in electric current production. The significance of the work is that it provides the first step in understanding the initial interactions between melanin and ionizing radiation taking place and offers some insight for production of biomimetic radioprotective materials.     

Asymmetric environments in encapsulation complexes

Symmetrical, self-assembled capsules capable of surrounding two guests offer a new approach to enantioselection through coencapsulation: when one guest is chiral, the space remaining is also chiral. This notion is explored within a cylindrical capsule. The dimensions of the capsule select appropriately sized combinations of guests, the shape of the capsule prevents tumbling of rigid molecules, and the chemical surface of the capsule orients polar functions within. Chiral carboxylic acids such as mandelic acid and alpha-Br-butyric acid are identified as promising compounds for this purpose, but diastereoselection is modest (<25% de).     

Block polyelectrolytes in aqueous environments

Analogous to the self-assembly of low-molecular-weight amphiphiles in aqueous solutions, the formation of spherical micelle-like aggregates has been observed in systems of amphiphilic block copolymers in water. The aggregates, often called micelles due to structural similarities with surfactant associates, are found to exist above the critical micelle concentration (cmc). The micellization of amphiphilic block copolymers has been investigated using a wide range of techniques, such as size-exclusion chromatography (SEC), static and dynamic light scattering (SLS and DLS), small-angle x-ray scattering (SAXS), small-angle neutron scattering (SANS), transmission electron microscopy (TEM), viscometry, and steady-state fluorescence spectroscopy. The present lecture is a review of recent work in our laboratory concerning the micellization of ionic block copolymers. These high-molecular-weight amphiphiles may contain one or more of a variety of ionic blocks, such as poly(4-vinylpyridinium alkyl halides), poly(metal acrylates), poly(metal methacrylates) and sulfonated polystyrene. In water, such polymers are referred to as block polyelectrolytes, as they combine the colloidal behavior of block copolymers with the long-range electrostatic interactions of polyelectrolytes. Early work in this field has been reviewed by Selb and Gallot.¹     

Thermal analysis in industrial environments

Common industrial procedures often expose polymers to liquids and vapors which may affect their thermal properties. By carrying out thermoanalytical measurements under simulated process conditions, the scientist can investigate environmental effects on the properties of the material. Such experiments can also be used to demonstrate the nature of processes taking place. This work illustrates case studies regarding the application of thermogravimetry, thermomechanical, dynamic mechanical and dielectric measurements to monitor the behaviour of fibers, films and adhesives under such conditions.     

Evaluation of piezoelectric PVDF polymers for use in space environments. III. Comparison of the effects of vacuum UV and gamma radiation

Films of piezoelectric PVDF and P(VDF‐TrFE) were exposed to vacuum UV (115–300 nm VUV) and γ‐radiation to investigate how these two forms of radiation affect the chemical, morphological, and piezoelectric properties of the polymers. The extent of crosslinking was almost identical in both polymers after γ‐irradiation, but surprisingly, was significantly higher for the TrFE copolymer after VUV‐irradiation. Changes in the melting behavior were also more significant in the TrFE copolymer after VUV‐irradiation due to both surface and bulk crosslinking, compared with only surface crosslinking for the PVDF films. The piezoelectric properties (measured using d₃₃ piezoelectric coefficients and D‐E hysteresis loops) were unchanged in the PVDF homopolymer, while the TrFE copolymer exhibited more narrow D‐E loops after exposure to either γ‐ or VUV‐radiation. The more severe damage to the TrFE copolymer in comparison with the PVDF homopolymer after VUV‐irradiation is explained by different energy deposition characteristics. The short wavelength, highly energetic photons are undoubtedly absorbed in the surface layers of both polymers, and we propose that while the longer wavelength components of the VUV‐radiation are absorbed by the bulk of the TrFE copolymer causing crosslinking, they are transmitted harmlessly in the PVDF homopolymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3253–3264, 2006     

Environment-sensitive fluorophore emitting in protic environments

The unusual fluorescence properties of 8-methoxy-4-methyl-2H-benzo[g]chromen-2-one (1) are described. The fluorophore 1 is almost nonfluorescent in aprotic solvent (e.g., fluorescence quantum yield Phi(f) < 0.0003 in n-hexane), whereas it strongly fluoresces at long wavelengths (>450 nm) in protic solvent (e.g., Phi(f) = 0.21 in methanol). The fluorophore 1 also shows good applicability in developing a new fluorogenic (fluorescent "off-on") sensor. [structure: see text]     

Orientational environments in liquids and solids

Local orientational order in liquids and solids is examined using the average “distribution of cosines” of angles subtended at particles in the liquid by pairs of nearest-neighbour particles, a techninue first introduced by Scott and Mader. This average information is compared with recent, more complete theories and simulations of orientational order. The lowest-order approximation to the “distribution of cosines” is shown to fail for dense systems, but the true distribution may measured very efficiently in computer simulators.     

Photophysics of melatonin in different environments

Melatonin (N-acetyl-5-methoxytryptamine) is a naturally occurring hormone with a structural resemblance to tryptophan. Its fluorescence is solvent dependent and strongly quenched in the presence of oxygen. Fluorescence quenching in aqueous and organic solutions have been studied in the presence of different quenchers. A 1: 1 inclusion complex formation between melatonin and different cyclodextrins in aqueous solution has been observed. The effects of microheterogeneous media provided by cyclodextrins, micelles and reverse micelles on the fluorescence characteristics of melatonin have been investigated. The water-soluble inorganic quenchers like KBr and KI were unable to quench the fluorescence of melatonin dissolved in a microemulsion consisting of surfactant + cyclohexane + 1-propanol + water, whereas the organic quenchers like CCl4 and CHCl3 were able to quench the fluorescence with a rate constant (on the order of 10(8) dm3 mol-1 s-1) quite similar to that in neat organic solvents.     

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.     

Accelerated aging and aging mechanism of acrylic sealant

One-component acrylic latex sealants with different moduli were subjected to accelerated aging tests, heat aging at different temperatures, xenon arc and UV exposure. The effects of various aging factors on sealant properties were studied by reviewing the appearance and mechanical properties. The aging mechanism was evaluated by thermal analysis, FTIR and solvent swell. The results show that a good relation exists among mechanical properties after aging; mild accelerated aging conditions and short aging times are enough to lead to obvious degradation of acrylic sealants with low modulus. Fillers with ingredients within high modulus may resist UV radiation effectively and enhance their durability but the quantity must be proper. The degree of chain breaking caused by UV is larger than that by heat aging. The study also shows that there are the same or similar aging mechanisms for different aging conditions and that the cure reaction by water volatilisation dominates the initial period of aging while degradation will be induced when aging time is longer and it can result in changes of molecular structure.