Gamma-irradiated Low-density Polyethylene Research Articles

Surface Roughness and Raman Spectra of Gamma-Irradiated Low-Density Polyethylene

Surface Roughness and Raman Spectra of Gamma-Irradiated Low-Density Polyethylene

X-ray Diffraction Study of the Structure of Gamma-Irradiated Low-Density Polyethylene

X-ray Diffraction Study of the Structure of Gamma-Irradiated Low-Density Polyethylene

Positron irradiation effect on positronium formation in gamma-irradiated LDPE and unplasticized PVC

Positron irradiation effects on positronium formation in low-density polyethylene (LDPE), gamma-irradiated LDPE and unplasticized PVC (UPVC) are studied. At least in one of the three different measurements, i.e., prolonged positron annihilation measurement at room temperature, low temperature in darkness and subsequent measurement under light, changes in o-Ps intensity are observed in non-irradiated LDPE and gamma-irradiated LDPE. While in UPVC, change in o-Ps intensity is hardly observable in all the above-mentioned three measurements. Reduction of o-Ps intensity by light indicates that positronium formation via the recombination of a positron and a trapped electron exists in LDPE and gamma-irradiated LDPE. The absence of light bleaching effect, together with the fact that the value of o-Ps intensity in heating and cooling process of a thermal circle is nearly the same, indicates that in UPVC, positronium can not be formed through trapped electron mechanism. This study highlights the speciality of positronium formation in UPVC, positronium is formed exclusively by the recombination of electron-positron pairs with short separations.

Isothermal crystallization of gamma irradiated LDPE in the presence of oxygen

Abstract This work is focused on the study of the effect of oxygen on the isothermal crystallization process of gamma irradiated low density polyethylene (LDPE). The induction time increased with the dose indicating a retarding effect. On other hand, at the same dose, this parameter decreased with the augment in the oxygen content. The classical Avrami equation was used to analyze the crystallization kinetic of these materials. n values suggested that both, the dose and the oxygen content, did not affect the mechanism of crystals growth. An Arrhenius type equation was used for the rate constant (k). Used models correctly reproduced the experimental data. TTT diagrams of studied materials were constructed and also reflected the effects of the doses and the oxygen content.

Gamma irradiated LDPE in presence of oxygen. Part I. Non-isothermal crystallization

Non isothermal crystallization of low density polyethylene (LDPE) samples irradiated under three different atmospheres was analyzed. The used doses were between 33 and 222kGy and the atmospheres content 0, 21 and 100% of oxygen. At a given radiation dose, the amount of produced gel is higher when the oxygen content present in the radiation process is lower.The temperature and the enthalpy of crystallization of the modified polymer decreased almost linearly with the radiation dose. Different classic models were used to predict the development of the relative degree of crystallinity (in the range 0.01–0.3 crystallinity fraction) as a function of temperature and several parameters were analyzed. The values of half-time of crystallization (t1/2) and the parameter Zc, from Avrami's method which characterize the kinetics of non-isothermal crystallization, showed that the crystallization rate, at the same cooling rate, decreased with increasing radiation dose and, at the same dose, with the decrease in the oxygen content. The activation energy for crystallization increased with the radiation dose and with the reduction of the oxygen content, indicating that the increment in molecular crosslinking restricts the crystallization process.

Influence of gamma irradiation on surface potential decay of corona charged polyethylene

In the present paper the surface potential decay after dc corona charging of gamma-irradiated low-density polyethylene (LDPE) has been studied. The irradiation was carried out in air, nitrogen gas and vacuum to investigate the effect of irradiation environment on the surface potential of LDPE. Samples with different absorbed doses were obtained by exposing samples to 60Co for various times at room temperature. It has been found that there is a significant change in the surface potential decay characteristics after different irradiation dose and the surface potential decay pattern depends also on irradiation environment. The possible surface potential decay mechanisms responsible for the above were discussed. The results are useful in the context of long-term performance of electrets.

Thermal Stability of Gamma Irradiated Low Density Polyethylene Films Containing Hindered Amine Stabilizers

AbstractSummary: The thermal stability of gamma irradiated low density polyethylene (LDPE) films in presence of various structures of hindered amine stabilizers (HAS) was investigated by the oxygen uptake method under constant temperature of 180 °C and normal pressure conditions. The thermo‐oxidation was run using air environment. The sigmoidal dependency of oxygen uptake allows the calculation of the main kinetic parameters: oxidation induction time and oxidation rate on the propagation step. The various steps involved in the thermal degradation process were detected by the derivative procedure applied to the dependencies of oxygen uptake on time. It was found that the kinetic parameters of the thermal degradation process, determined by oxygen uptake, revealed the antioxidant role of HAS in γ‐irradiated LDPE films by providing better stability, when compared with the unstabilized samples. Moreover, the results indicated that the dependencies of oxygen uptake on thermal degradation time involved two degradation stages: the former occurring in the ungrafted moiety and the latter taking place after the antioxidant depletion is achieved. The stabilization efficiency of these oxidation inhibitors provides satisfactory thermal resistance to LDPE films, especially those based on an alkoxyamine structure.Dependencies of oxygen uptake on time at the thermal degradation of LDPE irradiated at 812 kGy. (‐□‐) free of additive; (‐○‐) Tinuvin 123; (‐▵‐) Sanduvor PR 31; (‐⋄‐) Uvasil 299.magnified imageDependencies of oxygen uptake on time at the thermal degradation of LDPE irradiated at 812 kGy. (‐□‐) free of additive; (‐○‐) Tinuvin 123; (‐▵‐) Sanduvor PR 31; (‐⋄‐) Uvasil 299.

The influence of oxidation on space charge formation in gamma-irradiated low-density polyethylene

The research presented in this paper investigates the role of oxidation in the formation of space charge in gamma-irradiated low-density polyethylene after being electrically stressed under dc voltage. Polyethylene plaques both with and without antioxidant were irradiated up to 500 kGy using a 60Co gamma source and space charge distributions were measured using the piezoelectric induced pressure wave propagation method. It has been found that a large amount of positive charge evolved adjacent to the cathode in the sample without antioxidant and was clearly associated with oxidation of the surface. The amount of charge formed for a given applied stress increased with the dose absorbed by the material. A model has been proposed to explain the formation of space charge and its profile. The charge decay after the removal of the external applied stress is dominated by a process being controlled by the cathode interfacial stress (charge injection) rather than a conventional RC circuit model. On the other hand, space charge in a sample containing antioxidant under the same applied electric stress was negligible even in the sample exposed to 500 kGy. The main process to form space charge is via charge injection rather than charge separation in the sample without antioxidant.

Effect of ageing on the dielectric relaxation of oriented and gamma irradiated LDPE

The influence of accelerated ageing on the dielectric relaxation of drawn and gamma irradiated low density polyethylene (LDPE) was studied through dielectric loss (tan δ) analysis in the temperature range from 25 to 325 K. The accelerated ageing was performed under 0.5 MPa pressure of oxygen at an elevated temperature (80°C). The radiation induced and the post-irradiation oxidation were observed using IR spectroscopy. The intensity and the position of the γ and β dielectric relaxation maxima (tan δ max) were found to be strongly dependent upon the changes in the microstructure of the amorphous phase induced by crosslinking and oxidation.

Charge trapping in gamma irradiated low-density polyethylene

The influence of gamma irradiation in two different atmospheres, air and distilled water on charge trapping in LDPE has been studied through dielectric loss (tan δ) analysis in the temperature range from 25 to 325 K and using thermally stimulated discharge current (TSDC) measurements. The radiation induced oxidation was observed using IR spectroscopy. The intensity and the position of the γ and β dielectric relaxation maxima were correlated with maxima of TSDC measurements, and found to be strongly dependent upon the changes in the microstructure of the amorphous phase and on the surface of the crystallites induced by oxidation and crosslinking.

Analysis of volatile radiolysis products in gamma-irradiated LDPE and polypropylene films by thermal desorption–gas chromatography–mass spectrometry

Low-molecular-weight (‘volatile’) radiolysis products of low-density polyethylene (LDPE) and polypropylene (PP) films were investigated by thermal desorption–(TDS)–GC–MS after absorbed doses of up to 25 kGy. The films produce fingerprint chromatograms with highly characteristic patterns of groups of radiation-induced peaks; these are mainly hydrocarbons, aldehydes, ketones, and carboxylic acids with concentrations (after 25 kGy) ca one order of magnitude below that of the residual hydrocarbons (oligomers). PP additionally produces very substantial amounts of three degradation products of phenol-type antioxidants. The low molecular-weight (MW) radiolysis products are retained for considerable times in LDPE films and they are retained in PP films much longer than had been expected. Besides product identification, the following topics are addressed: Effects of the absorbed dose and the desorption temperature; comparison of several commercial (proprietary) films; high-temperature thermal desorption; the question whether TDS analyzes radiation-induced artifacts rather than genuine products; the possible existence of cyclic radiolysis products; the possibility of identifying an LDPE film as irradiated after a dose of only 1 kGy; and atypical trace fragments of antioxidants. Finally, the geometry and efficiency of the thermal desorption system is briefly discussed, and the implications of our findings for irradiation detection and for the safety of irradiated materials are considered.

Investigation of the distribution of space charge in gamma-irradiated low-density polyethylene

The deterioration of electrical insulation during its service life remains a problem. Accordingly, investigations of the behaviour of insulation in its working environment and methods to monitor its condition continue to be actively pursued. One of the most deleterious working environments for polymer dielectric materials is where there is nuclear radiation. In this paper space charge distributions in gamma-irradiated LDPE have been monitored using the LIPP and PEA techniques. It has been found that the charge distributions are related to both sample thickness and irradiation environment.

Fatigue crack propagation behavior of gamma irradiated LDPE films

In the present study, thin films of low density polyethylene (LDPE) were subjected to gamma radiation of 50 and 250 kGy. The tensile properties, fatigue life, and fatigue crack propagation (FCP) rate of the irradiated samples were compared with unexposed LDPE. The irradiated samples showed a decrease in fatigue life which was dose dependent. The fatigue life of LDPE-0, LDPE-50 and LDPE-250 was determined to be 4058, 250, and 58 cycles, respectively. The FCP rate was found to increase with dose. The changes in fatigue behavior was directly attributed to the crosslinking of molecular chains upon exposure to gamma radiation.

Thermal expansion behavior of drawn and gamma irradiated low-density polyethylene

The thermal expansion behavior of gamma irradiated low-density polyethylene has been measured over the temperature range from −100 °C to +10 °C. The influence of drawing on the gel content and thermal expansion behavior has been related to the radiation effects in low-density polyethylene.

Stability of the interphase interaction and changes in the microdestruction mechanisms in low density polyethylene on high energy irradiation

The effect of gamma irradiation on non-isothermally crystallized low density polyethylene (LDPE) has been studied by thermal-mechanical analysis (TMA) and transmission and scanning electron microscopy (TEM and SEM, respectively). Analysis of the TMA curves of the irradiated samples shows that the radiation-induced crosslinking of the macromolecular chains in the dose range 0–10 Mrad causes an increase, decrease and partial freezing of the mechanical microstresses located at the phase surfaces in the bulk of the polymer. These effects are explained in terms of the physical kinetics and macromolecular mobility. At temperatures above 120 °C., gamma-irradiated LDPE with doses above 5 Mrad develops a physical state corresponding to a high elastic state with thermotropic mesophases. Below 5 Mrad, controllable changes of the melting entropy and enthalpy of the crystalline regions and the free energy of the crystalline lamellae can be achieved. The TEM and SEM study shows an increase in the degree of co-operation of the sublevels of the superstructural molecular organization with the irradiation dose, even in areas that undergo plastic microdestruction.

Dry lubricant for mould release

Positron irradiation effects on positronium formation in low-density polyethylene (LDPE), gamma-irradiated LDPE and unplasticized PVC (UPVC) are studied. At least in one of the three different measurements, i.e., prolonged positron annihilation measurement at room temperature, low temperature in darkness and subsequent measurement under light, changes in o-Ps intensity are observed in non-irradiated LDPE and gamma-irradiated LDPE. While in UPVC, change in o-Ps intensity is hardly observable in all the above-mentioned three measurements. Reduction of o-Ps intensity by light indicates that positronium formation via the recombination of a positron and a trapped electron exists in LDPE and gamma-irradiated LDPE. The absence of light bleaching effect, together with the fact that the value of o-Ps intensity in heating and cooling process of a thermal circle is nearly the same, indicates that in UPVC, positronium can not be formed through trapped electron mechanism. This study highlights the speciality of positronium formation in UPVC, positronium is formed exclusively by the recombination of electron-positron pairs with short separations.

Positronium formation: Interrelation with radiation chemistry

Positron irradiation effects on positronium formation in low-density polyethylene (LDPE), gamma-irradiated LDPE and unplasticized PVC (UPVC) are studied. At least in one of the three different measurements, i.e., prolonged positron annihilation measurement at room temperature, low temperature in darkness and subsequent measurement under light, changes in o-Ps intensity are observed in non-irradiated LDPE and gamma-irradiated LDPE. While in UPVC, change in o-Ps intensity is hardly observable in all the above-mentioned three measurements. Reduction of o-Ps intensity by light indicates that positronium formation via the recombination of a positron and a trapped electron exists in LDPE and gamma-irradiated LDPE. The absence of light bleaching effect, together with the fact that the value of o-Ps intensity in heating and cooling process of a thermal circle is nearly the same, indicates that in UPVC, positronium can not be formed through trapped electron mechanism. This study highlights the speciality of positronium formation in UPVC, positronium is formed exclusively by the recombination of electron-positron pairs with short separations.