Roger L. Clough

High-energy radiation and polymers: A review of commercial processes and emerging applications

Abstract Ionizing radiation has been found to be widely applicable in modifying the structure and properties of polymers, and can be used to tailor the performance of either bulk materials or surfaces. Fifty years of research in polymer radiation chemistry has led to numerous applications of commercial and economic importance, and work remains active in the application of radiation to practical uses involving polymeric materials. This paper provides a survey of radiation-processing methods of industrial interest, ranging from technologies already commercially well established, through innovations in the active R&D stage which show exceptional promise for future commercial use. Radiation-processing technologies are discussed under the following categories: cross-linking of plastics and rubbers, curing of coatings and inks, heat-shrink products, fiber–matrix composites, chain-scission for processing control, surface modification, grafting, hydrogels, sterilization, natural product enhancement, plastics recycling, ceramic precursors, electronic property materials, ion-track membranes and lithography for microdevice production. In addition to new technological innovations utilizing conventional gamma and e-beam sources, a number of promising new applications make use of novel radiation types which include ion beams (heavy ions, light ions, highly focused microscopic beams and high-intensity pulses), soft X-rays which are focused, coherent X-rays (from a synchrotron) and e-beams which undergo scattering to generate patterns.

An ultrasensitive technique for testing the Arrhenius extrapolation assumption for thermally aged elastomers

Abstract We present a general approach for more confidently correlating accelerated aging results with aging under service conditions using the Arrhenius methodology. We first show that, as a result of complex diffusion-limited oxidation effects, time/temperature correlations may occur for some properties but not for others. To rigorously extrapolate high temperature results to low temperatures, we sought an ultrasensitive technique correlated to macroscopic degradation and capable of measurements at or near service temperatures. We achieved this objective by monitoring oxygen consumption rates at high (accelerated) temperatures, to establish the necessary correlation, and at low temperatures (down to 23 °C), to determine their temperature-dependence in the extrapolation region. Because easily measurable oxygen consumption rates of 10 −13 mol/g s correspond to decades of predicted lifetime for most elastomers, this approach increases confidence in long-term predictions and therefore provides a means of testing Arrhenius extrapolations.

Low-density, microcellular polystyrene foams☆

Abstract Numerous applications have been identified for low-density, microcellular, polymeric foams. In this paper the authors describe a general technique to produce foams of this type with organic-soluble polymers, in particular polystyrene. Open-celled polystyrene foams have been developed with densities of 0.02–0.2 g cm−3 and uniform cell sizes of 1–20 μm. By using well-characterized polymers the authors have related form morphology to the phase diagram of the polymer/solvent system employed.

Quantitative model for the time development of diffusion-limited oxidation profiles

We combine diffusion and kinetic expressions with chemical/mechanical property relationships to develop a complete, quantitative model with no arbitrarily adjustable parameters for the time development of diffusion-limited oxidation profiles. Excellent agreement between experimental and theoretical modulus profiles for two materials (a nitrile and a neoprene) confirms our understanding of diffusion-limited oxidation and its time and temperature dependencies. The simple assumption of no time dependencies in any of the model parameters is adequate for predictions relevant to elastomer service lifetimes.

Correlation of chemical and mechanical property changes during oxidative degradation of neoprene

The thermal degradation of a commercial, stabilized, unfilled neoprene (chloroprene) rubber was investigated at temperatures up to 140 C. The degradation of this material is dominated by oxidation rather than dehydrochlorination. Important heterogeneous oxidation effects were observed at the various temperatures investigated using infrared micro-spectroscopy and modulus profiling. Intensive degradation-related spectral changes in the IR occurred in the conjugated carbonyl and hydroxyl regions. Quantitative analysis revealed some differences in the development of the IR oxidation profiles, particularly towards the sample surface. These chemical degradation profiles were compared with modulus profiles (mechanical properties). It is concluded that the profile development is fundamentally described by a diffusion-limited autoxidation mechanism. Oxygen consumption measurements showed that the oxidation rates display non-Arrhenius behavior (curvature) at low temperatures. The current results, when compared to those of a previously studied, clay-filled commercial neoprene formulation, indicate that the clay filler acts as an antioxidant, but only at low temperatures.

Time-temperature-dose rate superposition: A methodology for extrapolating accelerated radiation aging data to low dose rate conditions

Abstract Time-temperature superposition is an empirical approach which has been used in polymers for more than 30 years to make thermal aging predictions at experimentally inaccessible times. Given its historical success, we have expanded this approach to combined radiation-thermal environments, yielding an empirical time-temperature-dose rate shifting procedure. The procedure derives an isothermal curve for a given amount of material damage versus dose rate at a selected reference temperature by finding the Arrhenius activation energy which causes higher-temperature, dose-rate data to superpose when shifted to the reference temperature. The resulting superposed curve extends to much lower dose rates which, in effect, are experimentally inaccessible due to the long time periods which would be required. This procedure therefore allows meaningful predictions to be made for long-term, low dose rate, radiation aging conditions. We have successfully applied the time-temperature-dose rate superposition approach to four different materials. For two of these materials, extrapolated predictions based on the superposed data were found to be in excellent agreement with 12 year, low dose rate aging results. Additional confidence in the approach results from the observation that the empirically-derived activation energies for all four materials can be quantitatively rationalized.

Polymer recycling: potential application of radiation technology

Abstract Management of solid waste is an important problem, which is becoming progressively worse as a byproduct of continuing economic growth and development. Polymeric materials (plastics and rubbers) comprise a steadily increasing proportion of the municipal and industrial waste going into landfill. Development of technologies for reducing polymeric waste, which are acceptable from the environmental standpoint, and which are cost-effective, has proven to be a difficult challenge due to complexities inherent in the reuse of polymers. Establishing optimal processes for the reuse/recycling of polymeric materials thus remains a worldwide challenge as we enter the new century. Due to the ability of ionizing radiation to alter the structure and properties of bulk polymeric materials, and the fact that it is applicable to essentially all polymer types, irradiation holds promise for impacting the polymer waste problem. The three main possibilities for use of radiation in this application are: (1) enhancing the mechanical properties and performance of recovered materials or material blends, principally through crosslinking, or through surface modification of different phases being combined; (2) treatment causing or enhancing the decomposition of polymers, particularly through chain scission, leading to recovery of either low molecular weight mixtures, or powders, for use as chemical feedstocks or additives; (3) production of advanced polymeric materials designed for environmental compatibility. This paper provides an overview of the polymer recycling problem, describes the major technological obstacles to the implementation of recycling technologies, and outlines some of the approaches being taken. A review of radiation-based recycling research is then provided, followed by a discussion of future directions where irradiation may be relevant to the problems currently inhibiting the widespread recycling of polymeric materials.

General solution for the basic autoxidation scheme

Abstract The basic autoxidation scheme (BAS), first used by Bolland and Bateman, has been widely applied to oxidation studies for almost 50 years. Steady-state kinetic analysis of this reaction scheme has always depended upon making the simplifying assumptions of long kinetic chain length and/or a specific ratio of termination rate constants. We present and discuss a generalized steady-state kinetic solution to the BAS that is derived without any simplifying assumptions. We also combine this generalized solution with diffusion expressions to derive theoretical oxidation profiles appropriate to diffusion-limited oxidation situations. The modeling predicts conditions yielding unusual dependencies of the oxidation rate on oxygen concentration and on initiation rate, as well as conditions yielding some unusual profile shapes.

Oxygen diffusion effects in thermally aged elastomers

Abstract We have obtained detailed, time-dependent modulus profiles on 2·2-mm thick samples of Neoprene and styrene-butadiene rubber (SBR) that were subjected to aging at temperatures of 80–150°C in air. The materials undergo strongly heterogeneous oxidation throughout this temperature range, with degradation much higher near surface regions than in the sample interior. The samples exhibit a latent onset of heterogeneous effects, particularly at the lower temperatures. As aging proceeds, the modulus in the exterior regions rises rapidly, whereas modulus changes in the interior region may slow down or stop. Numerous experiments with Neoprene formulations employing hindered phenol stabilizers of very different molecular weights demonstrate that heterogeneous oxidation is not the result of selective depletion of antioxidant from the surface regions of the sample. The primary cause of the incipient heterogeneous oxidation effects is a dramatic decrease in oxygen permeability coefficient as aging proceeds.

Rigorous experimental confirmation of a theoretical model for diffusion-limited oxidation

Abstract A rigorous test of theoretical treatments for diffusion-limited oxidation was completed by conducting an extensive series of radiation-initiated oxidation experiments on a commercial EPDM material. Oxidation profiles were monitored from density changes; profiles were obtained versus sample thickness, radiation dose rate and surrounding oxygen partial pressure. The resulting profile shapes and magnitudes could be quantitatively fit with a two-parameter theoretical treatment based on oxidation kinetics containing unimolecular termination reactions. The theoretical parameters derived from fitting allowed quantitative confirmation of a governing theoretical expression relating these parameters to independently measured values for the oxygen consumption and permeation rates.