γ-Ray-induced degradation: A comparative study for homo- and copolymers of polypropylene

Disposable medical supplies are currently made of polypropylene, which are conveniently sterilized by exposure to γ-rays. However, the homopolymers (HP) and random copolymer (CP) of polypropylene, when exposed to high-energy γ-irradiation, undergo oxidative degradation and loss in mechanical properties. In the current article, a comparative study of the effect of γ-irradiation at different irradiation doses and the postirradiation oxidation on both the polymers is reported. The loss in mechanical properties was monitored by the bend angle and tensile measurements. The yellowness index measurements were carried out as a function of the irradiation dose and postirradiation storage time. The changes in the molecular weight (MW) and molecular weight distribution (MWD) were monitored using melt index (MI) measurements. The changes in chemical structure was determined using infrared spectroscopy. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2715–2720, 1997     

The effect of γ-irradiation on optical and electrical properties of polycarbonate

Poly-(bisphenol A carbonate) (PC) samples were exposed to different doses of γ-rays from a ⁶⁰Co source (from 0.05 to 0.3 M rad) at room temperature. Conductivity and dielectric constant as well as optical measurements were carried out. It was noticed that almost no work has been reported on the electrical conductivity of polycarbonate at high temperatures. γ-irradiation of polycarbonate is believed to create free radicals which increases the state of disorder and perturb the orientation of dipoles inside the polymer. At about 95°C, the electrical conductivities of irradiated samples were found to increase by nearly one order of magnitude with respect to the unirradiated one. The dielectric constant, ?, of fresh polycarbonate was nearly temperature independent over a range of ～ 70°C, then it increased with temperature, but for γ-irradiated samples (from 0.05 to 0.3 M rad) the dielectric constant increased continuously with temperature and also with respect to the fresh sample what confirms the effect of γ-irradiation in perturbing the orientation of the dipoles in the polymer. It was found that the UV absorption spectrum has a broad band at 0.280 μm and the absorption intensity was strongly dependent on the dose of γ-irradiation.     

γ-ray-induced degradation in ethylene–propylene copolymer

The stabilization of polypropylene-disposable medical syringes against the γ-ray-induced degradation has been the subject of active research during the last decade due to the large-scale use of γ-irradiation for sterilization purposes. Ethylene–propylene (EP) copolymers have been suggested as suitable alternatives to polypropylene for these purposes. In this article, we investigated the effect of irradiation dose and postirradiation time on the melt index and mechanical properties of an EP copolymer. We also determined the chemical changes occurring during irradiation and storage using IR spectroscopy and have made an attempt to co-relate the changes in chemical structure to the changes in mechanical properties and melt index measurements. © 1996 John Wiley & Sons, Inc.     

Electrical conductivity of γ-irradiated native and mercerized cotton celluloses

In the present study the electrical conductivity (σ) of ball-milled and NaOH-treated cotton fibers were measured in vacuum. The induced electrical conductivity and the activation energy ΔE of γ-irradiated samples were measured as a function of radiation doses and at various temperatures. It was found that γ-irradiation of cotton fibers either decreases or increases its electrical conductivity. Thus, it was found that γ-irradiation of cotton reduces the conductivity of the samples having particle sizes ranging from 0.07 to 0.11 mm and the magnitude of the reduction diminishes as the particle size increases. The results have also shown that, for any given concentration, γ-irradiation of NaOH-treated samples causes an increase of the measured electrical conductivity.     

Reactivity of poly(vinyl chloride) containing N-methyl dithiocarbamate toward metal ions in heterogeneous system and effect of γ-irradiation on the reactivity

The reaction of poly(vinyl chloride) containing N-methyl dithiocarbamate (PMD) with metal ions in a heterogeneous system and the effect of γ-irradiation on the reactivity were studied. PMD powder suspended in aqueous solutions did not show any reactivity toward metal ions. However, the PMD powder suspended in a methyl alcohol solution formed a chelate with various metal ions. The effects of the reaction time, pH, and the initial concentration of the metal ion were examined. PMD irradiated in methyl alcohol with γ-irradiation doses up to 28 Mrad was compared with the original PMD. The reactivity of the irradiated PMD toward the metal ions did not show any apparent decrease.     

A 13C-n.m.r. study of radiation-induced racemization in isotactic polypropylene

Unexpectedly large changes in the stereochemistry of isotactic polypropylene have been observed by ¹³C-n.m.r. following γ-irradiation at 25°C up to 250 kGy. The observed G-value of 64 for the loss of isotactic triads is much higher than previously reported G-values for scission, crosslinking or hydrogen formation in irradiated polypropylene. A mechanism of initial chain scission, racemization and subsequent recombination is indicated. The pattern of pentad sequences is inconsistent with the occurrence of isolated random racemization events; this suggests that the energy released on recombination promotes a second racemization within a range of 1–3 monomer units.     

Chemische charakterisierung von elektronen-bestrahltem polypropylengranulat

The application of electron irradiated polypropylene granulate as a nucleating agent in extrusion has recently been suggested. This article discusses the chemical characterisation of such an electron treated granulate. Since the irradiation is carried out under atmospheric conditions, the relationship between decomposition, crosslinking and oxidation products has been determined. Oxidation is confined principally to the outermost molecular layers and leads to the formation of carbonyl groups via peroxide intermediates. As well as specific group tests, ESCA has been used to measure the degree of oxidation and for indirect determination of vinyl groups after bromination.     

Reactivity of poly(vinyl chloride) containing N-methyl-N-carboxymethyl dithiocarbamate toward metal ions in heterogeneous system and effect of γ-irradiation on the reactivity

The reaction of poly(vinyl chloride) containing N-methyl-N-carboxymethyl dithiocarbamate (PSDC) with metal ions in a heterogeneous system and the effects of γ-irradiation on the reactivity were studied. Slurried PSDC has an affinity for a number of metal ions in aqueous solutions. On the basis of analyses for nitrogen and sulfur and on infrared spectra, a possible chelating structure for PSDC was deduced. The gaseous products evolved during the gamma radiolysis of PSDC under vacuum were measured. Although the dithiocarbamate group bonded to the main chain gave some protection against the dehydrochlorination of poly(vinyl chloride) component, a small amount of the carboxymethyl group was decomposed. It was found that with γ-irradiation doses up to 30 Mrad, only about 1 mole-% out of 17.6 mole-% of N-methyl-N-carboxymethyl dithiocarbamate in the polymer was decomposed. It was shown that the reactivity of the irradiated PSDC in water toward metal ions was almost the same as that before irradiation.     

Radiation degradation of poly(styrene-co-methylmethacrylate). 2. Protective effects of styrene on volatile products,chain scission and flexural strength

The volatile products from γ-irradiation of poly(styrene-co-methylmethacrylate)s at 30°C are found to be the same as from PMMA. The G values are substantially below the values corresponding to linear relationships between the homopolymers. This is attributable to an intramolecular protective effect by the styrene units. Net G(scission) values have been derived from viscosity measurements on the irradiated copolymers after different doses. They also show a protective effect by styrene. The radiation dose (in vacuum) for a 50% reduction in flexural strength increased linearly with mole fraction of styrene from (x_s)_p = 0 to 0.7, above which it increased more rapidly.     

The influence of the draw ratio of melt-spun polyethylene fibres on γ-irradiation induced effects as observed by ESR

The influence of draw ratio on free radical behaviour in melt-spun polyethylene fibres has been examined using ESR spectroscopy. The stability of free radicals produced by γ-irradiation is greatest some where between draw ratio 1 and 10. The general trend of radical stability has been found to correlate with the trend of gel content with draw ratio. The possible link between the extent of crosslinking and the production of stable radicals has been discussed in terms of morphological aspects.     

Dynamic-mechanical relaxations in high and low density polyethylenes. Effects of irradiation and annealing

A study of the dynamic-mechanical relaxation spectrum in a series of commercial high and low density polyethylenes (Dow Chemical), irradiated as well as unirradiated, and subjected to different annealing process, has been performed. The effect of 20-Mrad dose of irradiation on the chemical structure has been analyzed and an increase in the number of aldehyde, ketone, and transvinylene groups and a decrease in the number of vinyl and vinylidene groups has been observed. The dynamic-mechanical spectrum of irradiated and unirradiated high and low density polyethylenes contains the γ_II-, γ_I-, β-, α_I-, α_II-, and α_III- relaxations, in order of increasing temperature. It has been observed that γ-irradiation followed by annealing modified the intensity and the position of relaxations in these polyethylenes.     

Characterization of degradation of polypropylene nonwovens irradiated by γ‐ray

Polypropylene is a leading commercial, fiber‐forming polymer due to its low cost and potential for making high strength fibers. As the polymer of choice in the biomedical field, polypropylene contains only two elements, namely carbon and hydrogen. As a result, it is very hydrophobic and bio‐inert lacking biodegradability in the landfill. Meltblown and spunbond polypropylene nonwovens were exposed to γ‐radiation doses up to 25 kGy. The changes in morphology, chemical, thermal, and tensile properties were characterized by various analytical techniques. Following γ‐radiation, the FTIR spectrum illustrated an increase in carbonyl groups suggesting radio‐oxidation. Additionally, there was a decrease in thermal and tensile properties indicating deterioration of the polymer. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39917.     

EPDM recycling assisted by γ-processing

The integration of waste ethylene–propylene–diene terpolymer (EPDM), containing carbon black into pristine EPDM can be achieved by γ-irradiation as a versatile procedure to process ethylene–propylene elastomers. The presence of acrylic acid in the material formulation allows the formation of intermolecular bridges by threefold increase in gel content. The possibility of achieving greater stability by the addition of acrylic acid in EPDM systems was analyzed. The start materials were EPDM containing 30 and 50 phr of EPDM powder loaded with 40 phr of carbon black aged by pre-exposure to electron beam irradiation. The advanced γ-irradiation exceeding 100 kGy represented the optimal radiation processing condition. Two procedures of chemiluminescence under isothermal and non-isothermal regimes for the evaluation of radiation stability were applied on γ-irradiated samples. The thermal strength of irradiated samples was characterized based on the radiolysis mechanism of EPDM. The variation in the activation energy required for the thermal oxidation of these samples and the modification in gel contents due to the gelation action of acrylic acid were presented for the validation of proposed recycling radiochemical technique. Charlesby–Pinner representation provided different values for the ratios between radiochemical yields of cross-linking and scission, proving that the presence of acrylic acid promoted the conversion of EPDM wastes into valuable materials.     

Characterization of irradiated PEs/PA6 blends

Summary In this work, changes caused by γ-radiation on binary blends of different PEs (high, low and linear low-density polyethylenes) with PA6 were studied. The structure and property changes in these samples induced by γ-irradiation were characterized by the ESR, DSC and FTIR techniques. These techniques provide relevant information concerning the reaction mechanisms of the structural modifications that take place during the irradiation of polymers. The experimental results suggest that allyl, alkyl and polyenyl radicals are generated in the materials by the irradiation process. The dynamics dominating the process kinetics seem to be crosslinking, chain scission and a coexistence of both, depending on the sample composition and the absorbed dose. The degradation of the materials produced a disruption in the ethylene sequences in polyethylenes thus decreasing their melting point. Bands attributed to carbonyl groups (C=O) were observed in FTIR spectra after the materials were irradiated.     

Radiation crosslinking of bisphenol-a polycarbonate in the presence of bisphenol-a dimethacrylate and triallyl cyanurate

Summary Radiation crosslinking of bisphenol-A polycarbonate (PC) was carried out by the γ-irradiation of the polymeric films containing 5 and 2 wt % of bisphenol-A dimethacrylate (BPDMA) or triallyl cyanurate, respectively, as well as by the BPDMA grafting from acetone solution onto PC preirradiated in air. The modified samples were analyzed for the sol/gel content, and the dependences of gel fraction yield of crosslinked polymer on monomers concentration and absorbed dose were found. The radiation-chemical yields of crosslinking and degradation as well as gelation doses were determined for the modified PC. Molecular weights of the starting and irradiated pure PC were detemined by the GPC method, and radiation-chemical yield of PC degradation was calculated. It has been found essential difference in efficiency of PC crosslinking depending on monomers and doses used as well as on methods of modification. Effects of crosslinking agents distribution in the PC matrix and simultaneous processes of crosslinking and degradation in the polymer-monomer compositions on efficiency of PC crosslinking have been discussed. Received: 22 March 2000/Revised version: 26 June 2000/Accepted: 26 June 2000     

Effect of γ-radiation on synthetic fibres-II irradiation in the presence of chemicals

Polyester and polyamide yarns have been irradiated under controlled doses of γ-rays in presence of chemicals like gaseous ammonia and chlorine as also methyl methacrylate, vinyl acetate, and acrylonitrile monomers. The irradiated monomers have been tested for some of the more important physico-chemical and mechanical properties. The results indicate that the improvements obtained in various desirable properties are of much higher order when irradiation is carried out in presence of the chemicals employed.     

Comparison of the effect of ethylene oxide and γ-irradiation on selected tyrosine-derived polycarbonates and poly(L-lactic acid)

Tyrosine-derived polycarbonates are a new class of degradable polymers that have possible biomedical applications. In this study, the effect of the two most common sterilization techniques, ethylene oxide and γ-irradiation (0.3, 1.1, 3.9, 6.4, 10.6 Mrad), was evaluated for a family of four structurally related tyrosine-derived polycarbonates and for poly(L-lactic acid) (PLLA). The four polycarbonates were poly(DTE carbonate), poly(DTB carbonate), poly(DTH carbonate), and poly(DTO carbonate) and differed only in the length of the pendent chain. Ethylene oxide exposure had little effect on molecular weight, surface composition, mechanical properties, or degradation rate of all test polymers except for poly(DTO carbonate). Poly(DTO carbonate) was unique since following ethylene oxide exposure it degraded faster than did the nonsterilized control. γ-Irradiated tyrosine-derived polycarbonates retained over 81% of their initial molecular weight when exposed to a clinically relevant dose of 3.9 Mrad and retained still 58% of the initial molecular weight when exposed to the highest test dose of 10.6 Mrad. No changes in surface composition and only slight changes in yield strength and the Young's modulus were detected for any of the tyrosine-derived polycarbonates following γ-irradiation. In vitro, irradiated films of poly(DTE carbonate), poly(DTB carbonate), and poly(DTH carbonate) degraded at approximately the same rate as did the nonsterilized films regardless of irradiation dose. Only poly(DTO carbonate), irradiated at high doses, degraded faster than did the control. Medical-grade PLLA was tested under identical conditions. Ethylene oxide exposure of PLLA did not affect the molecular weight, surface composition, mechanical properties, or in vitro degradation rate. However, upon irradiation at 10.6 Mrad, PLLA retained only 29% of its initial molecular weight; a dose of 3.9 Mrad resulted in retention of 49% of the initial molecular weight. In correspondence with earlier publications, irradiation of PLLA induced significant losses in the Young's modulus, % strain at break, and changes in the postirradiation rate of degradation in some specimens. Compared to PLLA, tyrosine-derived polycarbonates are significantly more stable to γ-irradiation and can be sterilized by conventional γ-sterilization techniques. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1499–1510, 1997     

Kinetic effects of silica nanoparticles on thermal and radiation stability of polyolefins

The effects of γ-irradiation as the accelerated degradation procedure were analyzed for the evaluation of material stability. Low density polyethylene, polypropylene and ethylene–propylene terpolymer were studied in formulations with SiO₂ nanoparticles (2 and 5 wt%) or as neat materials. High energy irradiation (up to 100 kGy) has revealed a faster increase in the absorption of carbonyl band in comparison with the corresponding change in hydroperoxide band. The three studied materials present increased oxidation rates as the received energy transferred from incidental rays is enhancing, because the thermal regime of degradation depends on the structural characteristics, namely initial number of tertiary carbon atoms and unsaturation level. The fate of hydroperoxides as oxidation initiators is discussed.     

Thermal stability evaluation of polypropylene protected with grafted amine

Summary Polypropylene containing a grafted amine as oxidation protector was subjected to γ-irradiation for the evaluation of thermal stability. Several formulations were prepared containing hindered amine, Sanduvor PR 31 (0.1, 0.2 and 0.3% w/w) and CaCO₃ (0.6% w/w). The stability assessment was carried out in air by oxygen uptake procedure under isothermal and isobaric conditions (165 °C and normal pressure, respectively). The exposure doses were placed in the range from 0 to 70 kGy. The changes appeared in the main kinetic parameters (oxidation induction time and oxidation rate) evaluated for thermal degradation of irradiated PP specimens are presented and the stability analysis is explained starting from the behaviour differences of various formulations of radiation processed polypropylene. On the low dose range, the drop in thermal stability of modified polypropylene films occurs more abrupt than it was observed for the samples irradiated at higher doses.     

Electron spin resonance studies of γ-irradiated cellulose. I. Free radicals in decrystallized cellulose

The types of free radicals formed in decrystallized cellulose prepared from cellulose I and II after γ-irradiation in nitrogen atmosphere at room temperature were studied by ESR spectroscopy. X-Ray diffraction revealed that decrystallized cellulose I and II have the same microstructure. The ESR spectra obtained with the γ-irradiated decrystallized samples are simple. By contacting the irradiated sample with moisture in nitrogen atmosphere, the ESR spectrum changed to a narrow singlet, which gradually decreased in intensity until the spectrum completely disappeared. It was found that the types of free radicals generated in the decrystallized cellulose by γ-irradiation consist of the overlap of singlet and doublet. The singlet spectrum is mainly attributed to alkoxyl radical formed by the rupture of glycosidic linkage at the C 1 or C 4 position, and the doublet spectrum is ascribed to radical formed by hydrogen abstraction from the C 1 position in cellulose molecule.