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Degradation of EPDM and FPM elastomers irradiated at very high dose rates in mixed gamma and neutron fields
Abstract
Elastomers are widely used in radioactive environments, where ionizing radiations induce a deterioration of their properties due to degradative phenomena occurring in the polymer structure. Their radiation resistance is usually assessed using γ‐rays and relatively low dose rates, but in actual applications, they are often exposed to mixed radiation fields and higher dose rates. Ethylene propylene diene monomer (EPDM) is known for its excellent resistance to γ‐rays but absorbs a larger dose by neutron interactions than fluoroelastomer (FPM). In this work, EPDM and FPM were irradiated in mixed neutron and gamma fields, using high dose rates (from 22 to 700 kGy h−1) and total absorbed doses between 0.2 and 3.5 MGy. The effects of irradiation were assessed by swelling tests, differential scanning calorimetry analysis and dynamic mechanical thermal analysis, and tensile tests. The results show that, even if irradiations were carried out in air, degradation took place under nonoxidative conditions owing to the high dose rates employed. Under such conditions, crosslinking is the dominant radiation‐induced reaction in both elastomers. Moreover, material degradation seems to be influenced mostly by the total absorbed dose and not by the type of radiation. Contrary to what observed at the lower dose rates employed with γ‐rays, major dose rate effects are not observed. POLYM. ENG. SCI., 2019.
... EPDM had the highest irradiation resistance, which depended on the material's inorganic content. Battini et al. [14] and Ferrari et al. [15] conducted several studies on the degradation of EPDM elastomers irradiated at high dose rates in mixed neutron and gamma fields and found that material degradation was mainly influenced by the total absorbed dose as opposed to the irradiation type and irradiation dosage. ...
... Polymers 2023,15, 3073 ...
In this work, a model for predicting the leakage rate was developed to investigate the effect of irradiation on the sealing performance of ethylene propylene diene monomer (EPDM) O-rings. The model is based on a mesoscopic interfacial gap flow simulation and accurately predicts the sealing performance of irradiated and non-irradiated materials by utilizing the gap height as an indicator in a mechanical simulation of the O-ring under operating conditions. A comparison with vacuum test results indicates that the model is a good predictor of leak initiation. The positive pressure leakage of the O-rings was investigated numerically. The results show the following. The sealing performance of the non-irradiated O-ring is much better than that of the irradiated one. The sealing performance is the worst at 0. 713 MGy and the best at 1.43 MGy, and the seal is maintained at an absorbed dose of 3.55 MGy. A theoretical analysis of the non-monotonic variation using the proposed model shows that the leakage behavior of the O-rings depends not only on the material properties but also on the roughness and prestressing properties. Finally, a method was proposed to classify the sealing performance, using the maximum allowable leakage rate as an indicator.
... The mechanical properties of these studied samples are tightly related to the level of γ-irradiation [43], whose values are the combination between this scission process and a certain degree of crosslinking [65]. The illustration of this overlapping is based on the extended durability that occurs in nuclear power plants [40,66]. ...
This study presents the functional effects of BaTiO3 powder loaded in ethylene–propylene–diene rubber (EPDM) in three concentrations: 0, 1, and 2.5 phr. The characterization of mechanical properties, oxidation strength, and biological vulnerability is achieved on these materials subjected to an accelerated degradation stimulated by their γ-irradiation at 50 and 100 kGy. The thermal performances of these materials are improved when the content of filler becomes higher. The results obtained by chemiluminescence, FTIR-ATR, and mechanical testing indicate that the loading of 2.5 phr is the most proper composition that resists for a long time after it is γ-irradiated at a high dose. If the oxidation starts at 176 °C in the pristine polymer, it becomes significant at 188 and 210 °C in the case of composites containing 1 and 2.5 phr of filler, respectively. The radiation treatment induces a significant stability improvement measured by the enlargement of temperature range by more than 1.5 times, which explains the durability growth for the radiation-processed studied composites. The extension of the stability period is also based on the interaction between degrading polymer substrate and particle surface in the composite richest in titanate fraction when the exposure is 100 kGy was analyzed. The mechanical testing as well as the FTIR investigation clearly delimits the positive effects of carbon black on the functionality of EPDM/BaTiO3 composites. The contribution of carbon black is a defining feature of the studied composites based on the nucleation of the host matrix by which the polymer properties are effectively ameliorated.
... The most promising commercial EPDM O-ring in terms of radiation tolerance, general mechanical properties and price was selected. Two different experiments were carried out [8]. ...
Lubricants and O-rings are necessarily used for the construction of many accelerator-driven facilities as spallation sources or facilities for the production of radioactive isotopes. During operation, such component will absorb high doses of mixed neutron and gamma radiation, that can degrade their mechanical and structural properties. Experimental radiation damage tests of these components are mandatory for the construction of the facility. Methodologies for irradiation in nuclear reactor mixed fields and post-irradiation examination of lubricating oils, greases and O-rings were developed and are here presented. Samples were characterized with standard mechanical and physical-chemical tests. Parametric studies on the dose rate effects have been performed on O-rings. A case studies for a specific O-ring application in a gate valve has been developed. Some of the tested samples showed a dramatic change of their properties with dose, while others remain stable. Results were collected on nine commercial greases, on one oil and on four commercial elastomeric O-rings. The most radiation resistant among the selected products are now considered for application in facilities under construction. The main mechanisms of neutron and gamma radiation damage on these polymers were investigated at the mechanical and structural level.
Acrylonitrile‐butadiene rubber (NBR) and ethylene‐propylene‐diene monomer (EPDM) gaskets are used as seals in plates heat exchangers due to their elasticity and resilience. However, contact with fluids, oxygen, ozone, and heat lead to permanent deformation. This study investigates the degradation of gaskets submitted to similar service conditions: compressed in a groove for up to 360 h at 100 and 120°C. The analysis was carried through compression set (CS), compression stress relaxation, surface morphology, Shore A hardness, indentation modulus profile, crosslink density, and Fourier transform infrared region with attenuated total reflection analysis. For EPDM, in thermo‐oxidative aging there is a counterbalance between chain scission and crosslinking, which increases CS while hardness and crosslink density remain low. For NBR, diffusion‐limited oxidation causes heterogeneous oxidation, creating a crosslinking network near the surface. The values for CS, Shore A hardness, and crosslink density raised to 50%, 10%, and 75%, respectively. Furthermore, the side thermo‐oxidation was reduced due to the presence of the groove, which reduces oxygen access. Using FITR analysis, a thermo‐oxidative interaction mechanism was proposed for the EPDM and NBR gaskets. It was concluded that EPDM presented a more reliable behavior for the gasket and seal applications under the analyzed conditions. Graphical abstract of the steps involved in this study.
Using ionizing radiation to sterilize medical and health care products is a widespread and well established approach, as products can be treated in their final shipping container without any residuals. During irradiation, the product materials will undergo some changes due to the absorbed energy, which have to be assessed during product qualification in order to specify a maximum allowable dose for the product. However, there is a lack of a systematic approach studying polymers in regard to dose ranges encountered and all types of ionizing radiation. Therefore, the aim of this work was to investigate the influence of radiation dose and source on the resulting properties of various polymers, which were already brought into final shape via injection molding, and to compare these effects found in the mechanical and rheological properties as well as their appearance. We found, that the mechanical properties of most investigated polymers are not influenced, except for polypropylene, but that yellowing was a pronounced effect in all the cases. The differences in polymer properties due to the radiation sources are small, only some materials exhibit more degradation by gamma irradiation due to the lower dose rate applied in comparison to x‐ray and e‐beam irradiation.
The Isotope Separation On-Line (ISOL) method for the production of Radioactive Ion Beams (RIB) is attracting significant interest in the worldwide nuclear physics community. Within this context the SPES (Selective Production of Exotic Species) RIB facility is now under construction at INFN LNL (Istituto Nazionale di Fisica Nucleare Laboratori Nazionali di Legnaro). This technique is established as one of the main techniques for high intensity and high quality beams production. The SPES facility will produce n-rich isotopes by means of a 40 MeV proton beam, emitted by a cyclotron, impinging on a uranium carbide multi-foil fission target. The aim of this work is to describe the most important results obtained by the study of the on-line behavior of the SPES production target assembly. This target system will produce RIBs at a rate of about 10¹³ fissions per second, it will be able to dissipate a total power of up to 10 kW, and it is planned to work continuously for 2 week-runs of irradiation. ISOL beams of 24 different elements will be produced, therefore a target and ion source development is ongoing to ensure a great variety of produced isotopes and to improve the beam intensity and purity.
Dynamic mechanical experiments performed on ethylene–propylene–diene (EPDM) terpolymer, cross linked to different extents, evidenced three relaxation processes. Besides the glass transition (Tg) and the melting of the ethylene crystals formed in the ethylene-based EPDM, a new transition appeared at a temperature higher than the glass transition temperature (so-called tan δgh) and decreased dramatically in amplitude with increase in the degree of cure. The new transition (tan δgh) was also above the melting point. The viscoelastic properties of the rubber with various crosslinking degrees are described. Based on the behavior of tan δgh and the curing properties of EPDM, the tan δgh peak is primarily attributed to the motion of non-elastic network chains loosely attached to the three-dimensional network, including free chains.
The SPES Radioactive Ion Beam (RIB) facility at INFN-LNL is in the construction phase. It is based on the ISOL method with an UCx Direct Target able to sustain a power of 10 kW. The primary proton beam is delivered by a high current Cyclotron accelerator, with energy 35-70 MeV and a beam current of 0.2-0.5 mA. Neutron-rich radioactive ions will be produced by proton induced Uranium fission in the UCx target at an expected fission rate in the order of 10 13 fissions per second. The exotic isotopes will be re-accelerated by the ALPI superconducting LINAC at energies of 10A MeV and higher, for masses in the region A=130 amu at expected rate on the secondary target of 10 7 – 10 9 pps. The SPES project has the aim to provide high intensity and high-quality beams of neutron-rich nuclei as well as to develop an interdisciplinary research center based on the cyclotron proton beam.
Fluorocarbon elastomers are specified for a number of applications where excellent high temperature and chemical resistance is required. To ensure that a fluoroelastomer with the required properties for a particular application is used, characterization techniques that allow the positive identification of the elastomer are required. In this paper the characterization of four fluoroelastomer formulations – a vinylidene fluoride/hexafluoropropene (VDF/HFP) dipolymer, a vinylidene fluoride/hexafluoropropene/tetrafluoroethylene terpolymer, and two vinylidene fluoride/perfluoro(methyl vinyl ether)/tetrafluoroethylene (VDF/PMVE/TFE) tetrarpolymers – is described. The characterization techniques included pyrolysis gas chromatography/mass spectrometry (py-GC/MS), Fourier transform infrared (FT-IR) spectrometry, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Py-GC/MS was the only characterization technique that could identify the four formulations unambiguously. The positive identification was based on differences in the pyrolytic degradation products of the flouroelastomer formulations.
Rubber elasticity is an important sub-field of polymer science. This book is in many ways a sequel to the authors' previous, more introductory book, Rubberlike Elasticity: A Molecular Primer (Wiley-Interscience, 1988), and will in some respects replace the now classic book by L.R.G. Treloar, The Physics of Rubber Elasticity (Oxford, 1975). The present book has much in common with its predecessor, in particular its strong emphasis on molecular concepts and theories. Similarly, only equilibrium properties are covered in any detail. Though this book treats much of the same subject matter, it is a more comprehensive, more up-to-date, and somewhat more sophisticated treatment.
The knowledge of neutron flux parameters, both in terms of value and gradient, in irradiation channels of nuclear reactors is required to avoid biased results when applying Neutron Activation Analysis. To this aim, the AueZr bare triple-monitor method with the addition of Lu as a temperature monitor was used to collect experimental data in irradiation channels of three facilities at the TRIGA Mark II reactor of Pavia. Twenty-four monitors were placed in as many positions to determine the vertical variations of the neutron flux parameters. The results obtained in three different activation experiments pointed out a decrease of the conventional fluxes up to 70% in some cases. Nevertheless, the epithermal neutron flux correction factor, the thermal to epithermal neutron flux ratio and the modified spectral index were constant throughout the entire length of the channels within the core whereas trends were identified in channels outside the core.
EPDM O-rings of gate valves employed for the construction of a second-generation accelerator for the production of neutron-rich Radioactive Ion Beams were studied in order to assess their sealing performance both during the facility service time and the post-service storage phase. Several O-ring specimens were at first exposed to different dose levels of mixed neutron and gamma radiations. Correspondent modifications of physical and mechanical properties of the material were investigated by means of uniaxial tensile tests, dynamic mechanical analyses, aging, compression set and vacuum leak tests. A hyperelastic strain energy function was adopted to fit the mechanical response of the material as a function of the absorbed dose. The minimum squeeze degree that guarantees O-ring sealing efficiency at different irradiation levels was determined by varying the interference between O-rings and grooves. A finite element model of the vacuum leak test was then set up to assess the contact pressure level required to ensure sealing. Numerical simulations of the gate valve main O-ring were subsequently carried out. By comparison of the predicted contact pressure and strain levels with experimental results, a life prediction map, as function of the service time, the storage time and the O-ring squeeze degree, was proposed.
Materials and components employed in the presence of intense neutron and gamma fields are expected to absorb high dose levels that may induce deep modifications of their physical and mechanical properties, possibly causing loss of their function. A protocol for irradiating elastomeric materials in reactor mixed neutron and gamma fields and for testing the evolution of their main mechanical and physical properties with absorbed dose has been developed. Four elastomeric compounds used for vacuum O-rings, one fluoroelastomer polymer (FPM) based and three ethylene propylene diene monomer rubber (EPDM) based, presently available on the market have been selected for the test. One EPDM is rated as radiation resistant in gamma fields, while the other elastomers are general purpose products. Particular care has been devoted to dosimetry calculations, since absorbed dose in neutron fields, unlike pure gamma fields, is strongly dependent on the material composition and, in particular, on the hydrogen content. The products have been tested up to about 2 MGy absorbed dose. The FPM based elastomer, in spite of its lower dose absorption in fast neutron fields, features the largest variations of properties, with a dramatic increase in stiffness and brittleness. Out of the three EPDM based compounds, one shows large and rapid changes in the main mechanical properties, whereas the other two feature more stable behaviors. The performance of the EPDM rated as radiation resistant in pure gamma fields does not appear significantly better than that of the standard product. The predictive capability of the accelerated irradiation tests performed as well as the applicable concepts of threshold of radiation damage is discussed in view of the use of the examined products in the selective production of exotic species facility, now under construction at the Legnaro National Laboratories of the Italian Istituto Nazionale di Fisica Nucleare. It results that a careful account of dose rate effects and oxygen penetration in the material, both during test irradiations and in operating conditions, is needed to obtain reliable predictions.
The mechanical properties of EPDM polymers, degraded as a result of extensive thermal and radiochemical aging treatment, are studied. The focus is given to dose rate effects in polymer insulation materials extracted from industrial cables in use in Belgian nuclear power plants. All studied mechanical characteristics such as the ultimate tensile stress, the Young's modulus, and the total elongation (or elongation at break) are found to be strongly affected by the irradiation dose. The ultimate tensile stress and Young's modulus are clearly exhibiting the dose rate effect, which originated from oxidation mediated interplay of polymer cross-linking and chain scission processes. The change of crossover between these two processes is found to be gradual, without critical dose rate or temperature values. On the contrary, the total elongation is observed not to be sensitive neither to irradiation temperature nor to the dose rate. Both cross-linking and chain scission seem to affect the total elongation in a similar way by reducing the average polymers chain length. This idea is confirmed by the model which shows that all total elongation data as a function of irradiation time can be reproduced by varying a single parameter, the pre-exponential factor of the irradiation rate constant.
It has been known that curatives, fillers, resins, and processing oils are the major variables which determinine the modulus level of elastomers by changing the degree of crosslinking and reinforcement. Attempts have been made to relate the tensile strength of elastomers to the network structure and the molecular characteristics. Unique empirical relations are found to exist between the extension ratio at break and the crosslink density in the absence and in the presence of gross strain-induced crystallization. With black-filled elastomers, the modulus at 100% strain is proportional to the crosslink density over a wide temperature range. The proportionality constants can be used as convenient measures of the extensibility and in material evaluated on a constant modulus basis.
In 1940, the first all-hydrocarbon elastomer containing limited olefinic functionality, butyl rubber, was announced. Because of its resistance to em-brittlement on aging, cracking on exposure to ozone, its impermeability to gases, and high damping capacity, this elastomer found use in inner tubes, cable insulation and miscellaneous hose, grommet, and gasket applications, generally lumped together as mechanical goods. It was the chemist's desire to prepare elastomers from other simple, inexpensive olefins, varying the structure to meet property requirements and using limited functionality to accommodate vulcanization. But no such preparative scheme was found, and, instead, attention was turned to modifications of known, substantially saturated hydrocarbon polymers to effect elastomeric properties and vulcanizability or to prepare elastomers from other raw materials, typical examples being Hypalon (chlorosulfonated, low density polyethylene) and polymers derived from epoxides and acrylates. However, K. Ziegler and coworkers pursued the matter of dimerization or oligomerization of olefins with metal alkyls until a breakthrough was made, and the catalyst system now known as Ziegler or Mulheim catalysts was discovered. This basic discovery, initially applied to the polymerization of ethylene, was expanded by Natta and his group and others to preparation of entirely new polymer structures. And, while attention was initially devoted to the preparation of miscellaneous stereoregular, crystalline homopolymers from simple olefins, it was virtually axiomatic that it would inevitably turn to the preparation of elastomeric copolymers. So, like the development of butyl rubber from polyisobutylene, ethylene/propylene elastomers (EPM) were followed by their olefin-containing terpolymers (EPDM), all of which are commercially prepared today. The Nobel Prize Award addresses by each of these aforementioned men trace the history of this important step forward in organometallic catalysis and polymer science.
Fluoroelastomer is a polymer usually used as a sealing material due to some excellent properties comparing to other elastomers, such as good resistance to high temperatures and to the attack of chemical substances. The fluoroelastomer used in this work was a commercial product obtained from two monomers, vinylidene and hexafluoropropylene, containing also carbon black and inorganic fillers. The aim of this work was to study the effect of the ionizing radiation of electron beam (EB) on the tensile, hardness and thermal properties of this sealing material obtained by a conventional curing process. The overall doses applied were 10, 25, 50, 75, 100, 125, 150, 175, 200 and 250 kGy. Tension tests showed that the tensile stress at break increases 34 % in the range of radiation dose applied. On the other hand, the total strain decreases considerably, from 347 % to 109 %, with the increase of the radiation dose. Hardness Shore A values increase 15 % in the range of radiation dose studied. Thermogravimetric curves showed that there are no considerable variations on the onset temperatures for all samples in the range of radiation doses applied. These results indicate that EB radiation produces modifications on the fluoroelastomer mechanical properties, but without promoting considerable chain scission. The modifications on the mechanical properties can be related to a better adhesion, induced by radiation, between the fluoroelastomer and the fillers.
This review covers some of the pioneering work of the radiolysis of fluoropolymers, which started in the mid 1950s and has continued up to the present day. In recent years, there has been renewed interest in the radiolysis of fluoropolymers—extending from the standpoint of radiation degradation in, for example, space environments and nuclear facilities, to the deliberate use of radiation for the modification of the fluoropolymers. In compiling this literature review, a fact that stood out was the dearth of information regarding the mechanisms for chain scission and especially crosslinking reactions. The lack of current, fundamental knowledge of the radiolysis of fluoropolymers, in the authors opinion, has limited the use of radiation in many applications, which are only now being realised, e.g. exciting new applications of crosslinked poly(tetrafluoroethylene).
We have previously presented a quantitative model that combines diffusion and kinetic expressions with chemical/mechanical property relationships to predict the time development of diffusion-limited oxidation profiles in polymers. This model was shown to be applicable to thermal aging environments and should be generally applicable to environmental stresses for which total damage to the material is directly proportion to total exposure time. In this work, we use a fluorocarbon elastomer to demonstrate applicability of the model to ionizing-radiation environments.
The article studied the methods how to shorten the irradiation time by increasing dose rate without changing the relationship between dose versus properties of degraded samples. The samples used were nine kinds of EPDM which have different compounding formula. The different dose of Co-γ ray was exposed to the samples. The maximum dose was 2MGy.The reference condition to be compared with two short time test conditions is irradiation of 0.33kGy/h at room temperature. Two methods shown below were studied as the time-accelerate irradiation conditions.(1)Irradiation of 4.2kGy/h in 0.5MPa oxygen at room temperature.(2)Irradiation of 5.0kGy/h in air at 70°C.After irradiation the mechanical properties of samples were measured at room temperature. The changes in 100% modulus suggest that irradiation in 0.5MPa oxygen increases slightly scission reaction and irradiation at 70°C increases slightly crosslinking, compared with the results obtained under low dose rate irradiation (the reference condition). The deviation was mostly in ±0.25 for 100% modulus and was ±0.5 for ultimate elongation throughout the all doses, where the value obtained at the reference condition referred to as 1.0. Thus, it was found out that two methods mentioned above are available as time accelerated irradiation conditions.
Cable ageing under the nuclear power plant (NPP) conditions must be effectively managed to ensure that the required plant safety and reliability are maintained throughout the plant service life. Ionizing radiation is one of the main stressors causing age-related degradation of polymer-based cable materials in air. For a given absorbed dose, radiation-induced damage to a polymer in air environment usually depends on the dose rate of the exposure. In this work, the effect of dose rate on the degradation rate has been studied. Three types of NPP cables (with jacket/insulation combinations PVC/PVC, PVC/PE, XPE/XPE) were irradiated at room temperature using 60Co gamma ray source at average dose rates of 7, 30 and 100 Gy/h with the doses up to 590 kGy. The irradiated samples have been tested for their mechanical properties, thermo-oxidative stability (using differential scanning calorimetry, DSC), and density. In the case of PVC and PE samples, the tested properties have shown evident dose rate effects, while the XPE material has shown no noticeable ones. The values of elongation at break and the thermo-oxidative stability decrease with the advanced degradation, density tends to increase with the absorbed dose. For XPE samples this effect can be partially explained by the increase of crystallinity. It was tested by the DSC determination of the crystalline phase amount.
EPDM-type elastomers (70% ethylene; 28% propylene; diene monomer: 2% norbornene) were characterised before and after irradiation using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Three formulations were studied: pure elastomer, cross-linked elastomer with dicumyl peroxide and protected with a phenolic-type antioxidant agent. Two irradiation methods were used: γ radiation, under oxygen, at a fixed dose rate of 1 kGy/h, for total integrated doses ranging from 50 to 450 kGy and electron irradiation, under inert atmosphere, at a dose rate of 500 kGy/h, for doses of 100 and 500 kGy. For both types of irradiation, for the three EPDM formulations and for any total integrated dose, DSC and DMA show that a cross-linking phenomenon predominates. Cross-linking increases with the total integrated dose. It is also observed that cross-linking is stronger with the γ irradiation than with the electron irradiation, which confirms the influence of the oxidising atmosphere.
Irradiation effects of fast neutrons on polystyrene were compared with those of 60Co γ-rays from the viewpoint of linear energy transfer (LET). For in vacuo irradiations, the G values of cross-links and main-chain scissions, G(X) and G(S), are 0.036 and 0.022, respectively, for γ-rays of low LET (ca. 0.02 eV/Å). Fast neutrons of high LET (ca. 3.7 eV/Å) increase not only the G(X) value to 0.14-0.26, depending on the initial molecular weight of polystyrene, but also the G(S) value to 0.22. For in air irradiations, oxygen increases the G(S) value by a factor of about 7 without affecting the G(X) value appreciably for γ-rays, whereas for fast neutrons oxygen decreases both the G(X) and G(S) values to about 1/3 and 1/2 those obtained in vacuo, respectively. As to the change in the molecular weight distribution by irradiation, the Mz/Mw ratio of polystyrene irradiated with fast neutrons in vacuo increases much more rapidly than that with γ-rays as the number of cross-linked units increases, suggesting the nonrandom cross-linking at high LET. All of these findings are explained qualitatively by a difference in the spatial distribution of microscopic energy deposition and the resulting reactive species between fast neutrons and γ-rays. The computer simulation shows that nonrandom cross-linking actually takes place at high LET, and it is caused by the microscopic inhomogeneity of energy deposition.
Irradiation effects of fast neutrons on poly(methyl methacrylate) were compared with those of 60Co γ rays from the viewpoint of linear energy transfer (LET). The G value of main-chain scissions, G(S), and the change in the molecular weight distribution of irradiated polymers were determined by means of gel-permeation chromatography. The LET dependence of G(S) is definite for in-vacuo irradiations [Gγvacuo(S) = 1.54 ± 0.11 for 60Co γ rays (LET: ca. 0.02 eV/Å) and Gnvacuo(S) = 1.04 ± 0.13 for fast neutrons (LET: ca. 3.7 eV/Å)], but the dependence is not so definite for in-air irradiations [Gγair(S) = 0.73 ± 0.06 and Gnair(S) = 0.66 ± 0.10]. The plot of the Mw/Mn ratio vs. the number of main-chain scissions deviates from that predicted by the random-degradation theory. The deviation is more distinct for fast-neutron than for γ-ray irradiations. The reduced G(S) value and the enhanced deviation from the random degradation at high LET are attributable to the increased inhomogeneity of microscopic energy deposition. A parameter expressing the microscopic inhomogeneity, ΔD/x, is introduced in the computer simulation of the nonrandom degradation and is adjusted so as to give the best fit between the observed and simulated plots of the Mw/Mn change. The computer simulation gives the ΔD/x value of 1.54 and 0.52 Mrad for fast-neutron and γ-ray irradiations, respectively, indicating that the ΔD/x ratio (ca. 3) is strikingly small compared with the LET ratio (ca. 180).
Linear energy transfer (LET) effects in polymers provide an indication of the reaction mechanisms involved, as well as a basis for the prediction of property changes in a radiation field. Polystyrene specimens irradiated in γ-sources and in a nuclear reactor have been compared by solution viscosity, gel content, and solvent-swelling measurements. The cross-linking rate was two- to threefold higher for the reactor by all three methods. For fast neutrons (LET ∼ 3 e.v./Å), G(cl) = 0.096 ± 0.015; for 60Co γ-radiation (LET ∼ 0.02 e.v./Å), G(cl) = 0.034 ± 0.002. The ratio of scission to formation of cross-linked units was 0.14 for both cases. The increased yields of cross links at high LET can be accounted for by enhancement of second-order processes in overlapping spurs at the expense of competing first-order reactions. Several reaction mechanisms can meet these general requirements.
The morphology and the mechanical properties at room temperature of crosslinked EPDM irradiated or not have been studied. It has been shown that these materials are composed of two phases: semicrystalline zones with a crystallinity ratio of 20% and mainly amorphous zones. The semicrystalline zones make a continuous path through the film and therefore control the mechanical properties of the material below the melting temperature. As irradiation (in the tested range of irradiation dose) and crosslinking degree have no significant influence on the arrangement and proportion of the crystalline lamellae, all samples have nearly the same mechanical behavior at small strains. At large strains, the interactions between amorphous and crystalline parts in semicrystalline zones play the main role in the mechanical response; irradiation, by degradation of these interactions, leads to a smaller hardening phenomenon and a decrease in elongation at break. From an application point of view, in spite of the low crystallinity fraction of these materials, the presence of an important number of crystallites, as evidenced by SAXS measurements, strongly limits the consequences of irradiation on the mechanical properties. However, the mechanical reinforcement strongly depending on the presence of these crystallites, it is therefore highly sensitive to temperature: this can be an important issue for the applications of these materials since their use temperature is close to the crystallite melting temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 97–105, 2010
In the presence of oxygen, polymeric materials may undergo diffusion-limited heterogeneous degradation with significant oxidation occurring only near the surfaces. It is important in studying polymer degradation (and in designing accelerated aging experiments) to be able to determine under what conditions heterogeneous degradation occurs, and to estimate the extent of significant oxidative penetration under those conditions. We describe here the use of several techniques for identification of oxidative degradation gradients. A rapid determination of oxidation depth is accomplished by optical examination of metallographically polished cross-sectioned samples; oxidized and nonoxidized regions are distinguished by differences in surface reflectivity. A more detailed determination of the shapes of degradation gradients is accomplished by performing a series of sensitive determinations of relative hardness changes across the surface of cross-sectioned, polished samples. Typical oxidation depths for the commercial polymers examined are on the order of fractions of millimeters over a dose-rate range of 104–106 rad/h. Significant variations among different materials are found, as would be expected given differences in oxygen consumption and permeation rates. A detailed example is given of the tensile property behavior of a Viton material over a range of dose rates where the degradation is seen to change from strongly heterogeneous at high dose rates to homogeneous as the dose rate is lowered. Degradation differences in this material are very pronounced: at high dose rates the polymer undergoes primarily crosslinking to give a hard brittle material, whereas under lower dose rates, where oxygen permeation is complete, the polymer undergoes predominantly scission to yield a soft, easily stretchable material.
On the basis of simple assumptions, equations are derived for predicting the specific volume-temperature-molecular weight (v-T-M) relationships for a homologous series of polymers from the v-T curves for the monomer and the polymer of infinite molecular weight. Analogous v-T-ρ relationships (where ρ is the degree of crosslinking) are similarly obtained. The result, that v decreases linearly with decreasing 1/M and with increasing ρ, conforms with the picture that either the removal of two chain ends (as M increases) or the introduction of a crosslinkage involves, between two polymer segments, the exchange of a van der Waals bond for a shorter covalent bond. The validity of these equations is illustrated by comparison with the observed data for polymers covering a wide range in M, T, and chemical type. For several polymer series the extrapolated value of v at 0°K (v0) is the same for all members of the series, indicating that at this temperature the average length of the van der Waals and covalent bonds between polymer segments are the same. Assuming that the form of the linear relationship observed for polystyrene between the specific volume (vg) at the glass temperature and the value (Tg) of the latter is valid for other polymer series, equations are derived which predict Tg as a function of M from the limiting value of the glass temperature for a chain of infinite length, and from the v-T curves in the liquid state for the monomer and for the polymer of M = ∞. Analogous Tg-ρ relationships are similarly obtained. According to these equations, a linear relationship between 1/Tg and 1/M should be obtained, which reduces at high M to a linear dependence of Tg on 1/M. Similarly, a linear dependence of Tg on ρ is predicted at low ρ. These predictions are in satisfactory agreement with data on polystyrene fractions and on crosslinked copolymers of styrene with divinylbenzene.
The tensile strength and ultimate elongation properties of any given amorphous elastomer can be described by a characteristic failure curve. It is shown in this paper that the failure curve can be predicted from a knowledge of the creep curve of the elastomer together with the data from a Mooney‐Rivlin plot. The theory relating the ultimate properties to the viscoelastic properties of the elastomer is based upon the idea of a propagating crack, the rate of propagation being limited by viscoelastic mechanisms. Data for the failure curves and creep response for EPR and SBR elastomers are presented and shown to support the theory. Literature data for butyl rubber are also shown to confirm the theory.
The dynamic-mechanical properties of some fluoroelastomers were determined as a function of composition at low frequency (≈ 1 Hz), by means of a free oscillation torsion pendulum, between −180°C and the softening point. Vinylidenefluoride (VDF)-hexafluoropropene (HFP) copolymers of molar composition 0–39% HFP and terpolymers of VDF and HFP with up to 30mol% tetrafluoroethylene (TFE) and a constant VDF to HFP molar ratio of 3.4 were considered. Two relaxation processes typical of the amorphous phase were found. The first, located at about −87°C, is related to local motions and the transition temperature was found to be independent of composition for copolymers, while it depends on TFE molar content for terpolymers. The second is related to the glass transition and the transition temperature depends on the composition. However, for semicrystalline copolymers the double glass transition phenomenon was observed. When crystallinity goes to zero at about 20 mol% HFP, only one transition is observed. It was also found that ordered structures can take place for terpolymers when TFE molar concentration exceeds 20%. The crystal disorder transition of pure PVDF (75°C) is observed also for low HFP concentrations but the transition temperature is strongly reduced. Analogies between the VDF-HFP and E-P systems are also discussed.
A series of radiation tests have been carried out on halogen-free cable-insulating and cable-sheathing materials comprising commercial LDPE, EPR, EVA and SIR compounds. samples were irradiated at five different radiation sources, e.g. a nuclear reactor, fuel elements, a 60Co source, and in the stray radiation field of high-energy proton and electron accelerators at CERN and DESY. The integrated doses were within 50–5000 kGy and the dose rates within 10 mGy/s–70 Gy/s. Tensile tests and gel-fraction measurements were carried out. The results confirm that LDPEs are very sensitive to long-term ageing effects, and that important errors exceeding an order of magnitude can be made when assessing radiation damage by accelerated tests. On the other hand, well-stabilized LDPEs and the cross-linked rubber compounds do not show large dose-rate effects for the values given above. Furthermore, the interpretation of the elongation-at-break data and their relation to gel-fraction measurements show that radiation damage is related to the total absorbed dose irrespective of the different radiation types used in this experiment.
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