Mathew Celina

Characterisation and degradation studies of peroxide and silane crosslinked polyethylene

Abstract Crosslinked polyethylene (XLPE) used as an insulation material is increasingly employed in aerial bundle cable systems for domestic power supply. This has resulted in the need to investigate the long term performance and properties of the polymer under thermal and UV ageing conditions. In the current study it has been attempted to characterise various silane and peroxide crosslinked low density polyethylene samples and to investigate the fundamental processes occurring during the thermal and UV degradation of these materials. The technique of solvent extraction used to determine the gel content and solvent swelling of the XLPE was shown to be ideally suited to characterise and to monitor the degradation of the XLPE. The two materials differ in relation to the mechanical properties, melting characteristics and network properties of the XLPE, with silane XLPE being generally less homogeneously crosslinked than peroxide XLPE. Various degradation studies revealed that this fundamental difference may also influence the mechanism of the degradation. Silane XLPE was shown to be more sensitive to the degradation and may degrade in a more heterogeneous manner than the peroxide XLPE.

Heterogeneous and homogeneous kinetic analyses of the thermal oxidation of polypropylene

Abstract The evidence from chemiluminescence analysis for the heterogeneous model for the oxidation of polypropylene powder and film is re-examined. In this model, the polymer contains a small number of localized zones in which oxidation occurs at a high rate and from which it then spreads. Stabilizer, if present, prevents the spreading for a period of time (the induction period) and also lowers the rate of subsequent spreading through the polymer. By considering a model of progressive ‘infection’ of the unoxidized polymer fraction a sigmoidal chemiluminescence growth curve is predicted and depends on several parameters including the initial oxidized fraction and the spreading coefficient. It is shown that a homogeneous kinetic analysis based on an autoaccelerating free radical reaction initiated by trace hydroperoxides fails to provide a good fit to the observed CL data. The activation energies for the chemiluminescence at the onset of oxidation, the maximum in the sigmoidal profile, and the induction period are reinterpreted with consideration given to the activation energy for the chemiluminescence quantum yield, as well as the influence of instrumental sensitivity on measured values. From these results and the observation that the extent of oxidation at the end of the induction period is not a constant, independent of temperature, it is concluded that a homogeneous kinetic model cannot be applied.

Thermal Degradation Studies of A Polyurethane Propellant Binder

The thermal oxidative aging of a crosslinked hydroxy-terminated polybutadiene (HTPB)/isophorone diisocyanate (IPDI) based polyurethane rubber, used as a polymeric binder in solid propellant grain, was investigated at temperatures from 25 C to 125 C. The changes in tensile elongation, polymer network properties and chain dynamics, mechanical hardening and density were determined with a range of techniques including modulus profiling, solvent swelling, NMR relaxation and O{sub 2} permeability measurements. We critically evaluated the Arrhenius methodology that is commonly used with a linear extrapolation of high temperature aging data using extensive data superposition and highly sensitive oxygen consumption experiments. The effects of other constituents in the propellant formulation on aging were also investigated. We conclude that crosslinking is the dominant process at higher temperatures and that the degradation involves only limited hardening in the bulk of the material. Significant curvature in the Arrhenius diagram of the oxidation rates was observed. This is similar to results for other rubber materials.

Real-time analysis of the thermal oxidation of polyolefins by FT-IR emission

Quantitative FTIR-emission spectroscopy was applied to investigate the real-time thermal oxidation of polyolefin samples and shown to be a useful method for studying polymer degradation. The high sensitivity of the technique was demonstrated by studying the oxidation of individual pressed polypropylene (PP) and high-density polyethylene (HDPE) reactor powder particles and microtome cuttings of crosslinked polyethylene (XLPE). A detailed analysis of the growth in carbonyl bands during the degradation showed no changes in the product distribution with time. Secondary oxidation products, such as γ-lactones in the case of polypropylene, were observed from the onset of the degradation, which is consistent with a model of oxidation occurring in localized zones with high reactivities. Differences in the oxidation sensitivity of these materials were identified and attributed to variations in the stability of the PP particles, catalyst residues in the HDPE and the degree and method of crosslinking of the XLPE.

Attenuated total reflectance infrared microspectroscopy of aged carbon-filled rubbers

A number of different carbon-filled rubbers have been studied using infrared microspectroscopy with an attenuated total reflectance objective incorporating a silicon internal reflectance element (IRE). Excellent spectra were obtained of these difficult samples in a reasonable measurement time. The IRE had a contact face of 100 μm diameter which could be further apertured to improve the spatial resolution, but at the expense of the signal-to-noise ratio of the spectra. The minimum aperture size, which still produced an acceptable spectrum, was 40 μm. The spatial resolution was less than the aperture size because of the effect of the refractive index of the IRE. The method was applied to thermally aged rubbers by measuring spectra at intervals across a sectioned surface. Oxidation profiles were derived from the absorbances of oxygenated functional groups in the spectra and were found to agree with modulus profiles previously published. The IR-ATR microspectroscopic method is shown to be simple and effective because the measurement is made directly on the sectioned surface of the rubber without sacrificing the sample.

Morphology changes during radiation-thermal degradation of polyethylene and an EPDM copolymer by 13C NMR spectroscopy

The γ radiation induced degradation of an EPDM copolymer was compared to that of a 13C enriched polyethylene at exposure temperatures of 22 and 80°C. Morphological changes were measured by MAS 13C NMR spectroscopy and DSC. By first examining the high quality and less complex spectra of 13C enriched polyethylene, a protocol for the interpretation and deconvolution of the more complex EPDM spectra was developed. The morphological changes during ambient temperature γ irradiation were similar for the two materials while the morphological changes during γ irradiation at 80°C were notably different. The polyethylene exhibited a substantial increase in crystallinity as a function of aging time at both temperatures. The EPDM copolymer exhibited a substantial increase in crystallinity at 22°C but only a minor increase in crystallinity at 80°C. The reduction in the growth of the crystalline phase was attributed to the melting of a significant fraction of the smaller crystallites at the 80°C irradiation temperature. When the sample was returned to ambient temperature these crystallites were unable to reform due to the crosslinks introduced during the radiation-thermal exposure.

Selection and Optimization of Piezoelectric Polyvinylidene Fluoride Polymers for Adaptive Optics in Space Environments

Various piezoelectric polymers based on polyvinylidene fluoride (PVDF) are of interest for large aperture space-based telescopes. Dimensional adjustments of adaptive polymer films depend on charge deposition and require a detailed understanding of the piezoelectric material responses which are expected to deteriorate owing to strong vacuum UV, γ -, X-ray, energetic particles and atomic oxygen exposure. We have investigated the degradation of PVDF and its copolymers under various stress environments detrimental to reliable operation in space. Initial radiation aging studies have shown complex material changes with lowered Curie temperatures, complex material changes with lowered melting points, morphological transformations and significant crosslinking, but little influence on piezoelectric d33 constants. Complex aging processes have also been observed in accelerated temperature environments inducing annealing phenomena and cyclic stresses. The results suggest that poling and chain orientation are negatively affected by radiation and temperature exposure. A framework for dealing with these complex material qualification issues and overall system survivability predictions in low earth orbit conditions has been established. It allows for improved material selection, feedback for manufacturing and processing, material optimization/stabilization strategies and provides guidance on any alternative materials.

Polymer materials and component evaluation in acidic-radiation environments

Abstract Polymeric materials used for cable/wire insulation, electrical connectors, O-rings, seals, and in critical components such as motors, level switches and resistive thermo-devices were evaluated under accelerated degradation conditions in combined radiation-oxidative elevated-temperature acidic-vapor (nitric/oxalic) environments relevant to conditions in isotope processing facilities. Experiments included the assessment of individual materials such as PEEK, polyimides, polyolefin based cable insulation, EPDM rubbers, various epoxy systems, commercial caulking materials as well as some functional testing of components. We discuss how to conduct laboratory experiments to simulate such complex hostile environments, describe some degradation effects encountered, and evaluate the impact on appropriate material and component selection.

Piezoelectric PVDF materials performance and operation limits in space environments.

Piezoelectric polymers based on polyvinylidene fluoride (PVDF) are of interest for large aperture space-based telescopes. Dimensional adjustments of adaptive polymer films are achieved via charge deposition and require a detailed understanding of the piezoelectric material responses which are expected to suffer due to strong vacuum UV, gamma, X-ray, energetic particles and atomic oxygen under low earth orbit exposure conditions. The degradation of PVDF and its copolymers under various stress environments has been investigated. Initial radiation aging studies using gamma- and e-beam irradiation have shown complex material changes with significant crosslinking, lowered melting and Curie points (where observable), effects on crystallinity, but little influence on overall piezoelectric properties. Surprisingly, complex aging processes have also been observed in elevated temperature environments with annealing phenomena and cyclic stresses resulting in thermal depoling of domains. Overall materials performance appears to be governed by a combination of chemical and physical degradation processes. Molecular changes are primarily induced via radiative damage, and physical damage from temperature and AO exposure is evident as depoling and surface erosion. Major differences between individual copolymers have been observed providing feedback on material selection strategies.