Ageing of radiation-sterilized polypropylene: changes in semicrystallinity (original) (raw)
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Effect of radiation sterilization and aging on ultrahigh molecular weight polyethylene
Journal of Biomedical Materials Research, 1981
The mechanical properties of polyethylene components used in prosthesis are altered after fabrication by the sterilization procedure and by the environmental and mechanical aging which occurs after implantation. To assess the importance and extent of these alterations, ultrahigh molecular weight polyethylenes from two sources, Hercules 1900 and RCH 1000C, were subjected to gamma-ray irradiation and aging in serum and argon environments. Changes induced by these treatments in the structural and mechanical parameters have been determined by a variety of experimental techniques. The effect of irradiation is to introduce crosslinks and to increase the degree of crystallinity resulting in changes in the tensile properties in the direction of higher stiffness and reduced ductility. Aging for six months produced similar changes with subtle differences. The results suggest that the initially low degree of crystallinity which results from the sluggish mobility of the very long chain molecules offers the opportunity for significant changes in the properties during use in the long run.
Abstract The present work is aimed to study the changes in crystallinity and morphology of biomedical polyolefins after γ-sterilization. The isotactic polypropylene (iPP) films were sterilized by γ-radiation and the changes were characterized by differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and atomic force microscopy (AFM) techniques. The effect of dose rate on crystallinity changes also was focused. It was found that crystallinity and morphology significantly changed with γ-irradiation. There was an increase in crystallinity up to 10 kGy accompanied with decrease in viscosity and mechanical properties. The changes in crystallinity mainly depend on γ-irradiation dose and dose rate. The rise in crystallinity was attributed to chain scission and forming new perfect lamellae. However, the reduction was accompanied by occurrence of γ- and β-phases and crosslinking. Possible explanations for transitions in phases are also discussed based on our results.
EVALUATION OF ENVIRONMENTAL AGING OF POLYPROPYLENE IRRADIATED VERSUS PRISTINE
Polypropylene (PP) is the most common thermoplastic resin of the plastic market due to its very interesting physical, chemical and processing properties at very low market price, however after its use the resin does not degrade in the environment or it degrades at very low rate. This study has the objective of comparing the environmental exposure of PP irradiated with 20 kGy and pristine PP. Dumbbell samples were manufactured by injection molding and exposed to the environment during 90 days; another one set was subjected to gamma irradiation at 20 kGy total dose and exposed at the same conditions too. The samples were characterized by mechanical testing, visual inspection, infrared spectroscopy (IR), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The irradiated samples, after environmental aging, showed oxidation and presence of cracks in samples of the PP 20 kGy. Introduction Polypropylene is a thermoplastic polyolefin obtained by polymerization of propylene monomer. PP is of great importance to the market due to its versatility, easy processing, and good mechanical properties, at very low market price and up to 20% lighter than other polymers [1]. The structural changes, typically chain scission and crosslinking, induced by ionizing radiation cause changes in the physical properties of irradiated polymers. Although the degradation reactions of the type crosslinking or chain scission occur simultaneously during irradiation of the polymer, one of these processes is generally dominant, depending on the chemical structure of the polymer, dose, dose rate, type of radiation and the conditions of the environment (pressure and temperature) in which the material is irradiated. The primary process due to irradiation of the polymer is the generation of excited species and free radicals resulting from the breakage of chemical bonds [2, 3]. Environmental aging is widely used in research and industrial assessment and strictly depends on geographical variables and atmospheric exposure conditions. Reasons for degradation may result from the combination of these variables such as heat, ultraviolet light, moisture, oxygen and pollutants. PP is a sensitive polymer due to the presence of tertiary carbon in their structure and suffers the effects of degradation and alteration of mechanical and aesthetic properties [4-7]. The energy to break the tertiary carbon may also be provided by gamma irradiation, which will modify the chemical structure of the polypropylene by increasing the content of carbonyl and hydroxyl groups suggesting the occurrence of oxidative degradation [8].
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2010
The purpose of this research was to investigate the effects of electron beam irradiation on the properties of polylactic acid (PLA) composites reinforced with bioactive Mackerel-derived hydroxyapatite (MHAp), used for promoting the application in bone regeneration field. The PLA were compounded with 2-10 phr MHAp and were exposed to electron beam irradiation dosages of 10-30 kGy. The results from tensile test and X-ray diffraction analyses indicated that the reinforcing and nucleating effect of MHAp on PLA matrix effectively improved the tensile strength and crystallinity of pristine PLA. Without the addition of MHAp, an increase in irradiation dosages (>10 kGy) gradually decreased the mechanical properties and gel content of unfilled PLA. In contrast, the increasing irradiation dosage up to 20 kGy enhanced the mechanical properties and gel content of MHAp added PLA composites. Such improvement effect is also found to be more significant for samples containing higher loading levels of MHAp that could promote the formation of higher degree of crosslinking network. By this higher crosslinking effect on restricting the polymer chains mobility for extendability and crystallization, the subsequent reduction in the elongation at break and crystallinity of PLA/MHAp samples is thus more prominent for higher loading level of MHAp (�6 phr) under the increasing irradiation dosages. However, the crystallinity could be increased by further increasing irradiation dose up to 30 kGy due to the occurrence of recrystallization as promoted by the nucleating effect of MHAp particles.
Journal of Materials Science: Materials …, 1998
The effects of four sterilization treatments (gamma radiation in nitrogen, electron-beam radiation, ethylene oxide gas, and no sterilization) on the structure and morphology of ultrahigh molecular weight polyethylene (UHMWPE) were monitored as a function of ageing time in air for a period of 1.5 y. Characterization techniques employed include differential scanning calorimetry, density gradient column, transmission electron microscopy, and small-angle X-ray scattering. Ethylene oxide gas does not affect the structure of the polymer. Both forms of radiation lead to measurable alterations of the material's structure, including an increase in crystallinity, an increase in density, and the enhancement of lamellae crystalline stacking. Most changes in structure occur in the first few months with little differences observed upon subsequent ageing in air. The sharpness of the crystalline-amorphous boundaries decreases with time for irradiated UHMWPE and is believed to be linked to the oxidation of the polymer.
Change of Physico-Mechanical Properties of Polyethylene During Radiation-Induced Aging
The changes in the physicomechanical properties of LDPE are considered in the kinetic and diffusion regimes of radiation oxidation. In the kinetic regime fall in the instant modulus and strength at low doses is due to degradation of the macromolecules and at high doses to accumulation of oxygen-containing groups. In the diffusion regime strength is determined both by the size of the absorbed dose and depending on the dose rate by the ratio of the thicknesses of the internal crosslinked and external oxidized layers. In the heterogeneous sample the presence of a crosslinked layer does not lead to rise in the strength of the material as compared with that of the uniformly oxidized film.
Rheological study of polypropylene irradiated with polyfunctional monomers
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2007
The aim of this paper is to investigate the rheological properties of polypropylene (PP) modified by ionization radiation (gamma rays) in the presence of two different monomers. The samples were mixed in a twin-screw extruder with ethylene glycol dimethacrylate (EGDMA) or trimethylolpropane trimethacrylate (TMPTMA) with concentration in the range of 0.5-5.0 mmol. After that, they were irradiated with 20 kGy dose of gamma radiation. The structural modification of polypropylene was analyzed in the melt state by measuring melt flow rate (MFR), g* (complex viscosity) and G 0 (storage modulus) in the angular frequency range of 10 À1 to 3 • 10 2 rad s À1. From the oscillatory rheology data, one could obtain the values of g 0 (zero shear viscosity) that would be related to the molar mass. All results were discussed with respect to the crosslinking and degradation process that occur in the post-reactor treatment to produce controlled rheology polypropylene. The resulting polymeric materials were submitted the cytotoxicity in vitro test by neutral red uptake methodology with NCTC L 929 cell line from American Type Culture Collection bank. All modified PP samples presented no cytotoxicity.
Sameh AS Alariqi Ph.D. Thesis, 2006
ABSTRACT Synthetic polymers are ubiquitous in our world, finding diverse applications in many fields because of their useful properties and inexpensive and contribute to enhancement of comfort and quality of life in our modern industrial society. The properties of polymers like durability, resistance to weathering and photo-degradation as well as biological attack and hydrophobicity, have contributed to their skyrocketing utility in different applications. Polyolefins which have a wide range of applications from rigid thermoplastics to soft elastomers by varying ethylene-propylene monomer ratio are finding various applications in biomedical field depending upon their properties. In biomedical field, they have been the choice of materials for medical supplies such as syringes, catheters, vials, blood transfusion bags, dialyzers for blood purification etc. These medical devices are mostly sterilized by γ-radiation which is used when materials are sensitive to the high temperature of autoclaving but compatible with ionizing radiations. The most commonly validated doses used to sterilize medical devices is 25 KGy. This process is replacing the hazardous and environmentally destructive use of ethylene oxide/CFC mixtures, which are vented into atmosphere after use and the residue of ethylene oxide is even carcinogenic. However, sterilization of biomedical polymers using -radiation is also known to result in physical changes including embrittlement, stiffening, softening, discoloration, odour generation and decreases in molecular weight. In literature, no report has been made on the effect of -irradiation at low doses (at the range of sterilization dose) on the crystallinity and morphology of PP, HDPE and EP copolymers at the same irradiation conditions (dose rate, doses, irradiation atmosphere and temperature) and correlation with each other properties during irradiation and post-degradation. Thus, it is planned to investigate the effect of the -sterilization / irradiation on iPP, HDPE and EP copolymers under same irradiation condition which will enable to compare their durability, mechanisms and correlate the chemical and physical properties and to study the changes in above said properties in similar irradiation condition and to correlate with each other for polyolefins (PP, EP and PE). The effect of ethylene composition is also another parameter to be considered. For stabilization against γ-sterilization, it has been established that there have been always some drawbacks in using single stabilizer system. The samples of iPP, HDPE and EP coplymers were hot-molded as films by hydraulic press. They have been -sterilized / irradiated in a well type 60Co for -radiation allowing uniform exposure. The changes in functional groups, crystallinity, thermal properties, morphology, phase structure, mechanical properties and viscosity were followed by FTIR spectroscopy, WXRD, DSC, AFM, Universal testing machine (Instron) and Ublehode viscometer. The durability of -sterilized polyolefins under biological environment was studied in composting and microbial culture test methods. Stabilization was characterized in terms of changes in functional groups, tensile properties, yellowing and surface morphology, by FTIR spectroscopy, instron, colorimetry (reflectance) and scanning electron microscopy (SEM) respectively. Results were discussed by comparing the stabilizing efficiency of mixtures with and without phenol system. Under biological environment, in general, a decrease in intrinsic viscosity and increase in chain scission were also observed with the increasing dose of -sterilization and time of incubation in compost. Polypropylene was more susceptible than high-density polyethylene to microbial attack in neat and sterilized samples. The higher weight losses of -sterilized samples with lower intrinsic viscosity suggested that chain scission and radio-oxidized functional groups were important units in the bio- / γ- degradation of polymers. The variation in degradation behavior of EP copolymers (EPM and EPZ) suggested that the copolymer composition and distribution of comonomers have significant effects on both -degradation and biodegradation. This part of study suggests that the pre-treatment of -sterilization can also accelerate significantly the biodegradation of neat polymer matrix in biotic conditions. Irradiation of HDPE was found to produce an initial drop in crystallinity followed by an increase with continuous irradiation. No significant changes in morphology have been occurred with irradiation. The drop in crystallinity is related to the formation of crosslinks. Significant morphological transitions in PP and EP copolymers were observed during γ-irradiation. DSC and X-ray studies reveal the transition from α-modification to β- and γ-phases. However, DSC, X-ray and AFM studies indicate that the reduction in crystallinity was accompanied by simultaneous decline in α-phase and formation of γ-α and α-α branching (cross-hatching), β-phases and crosslinking. Crystallinity and morphology highly effected by ethylene content and distribution. However, the increase in the ethylene comonomer content favors the formation of γ- and β-phases crystals. The rise in crystallinity of iPP, HDPE and EP copolymers is attributed to the ‘chemi-recrystallization’ of the shorter chains, which are produced by chain scission of the tie-molecules located in the amorphous region forming new and perfect lamellae leading to an increase in the degree of crystallinity. Crystallinity and morphological changes caused by irradiation govern the mechanical properties. The improvement of Young’s modulus was found to be attributed to the increase in the degree of crystallinity. The enhancement in crystallinity causes mechanical failure. This study suggests that radiation-induced degradation of PP, HDPE and ethylene-propylene (EP) copolymers occur exclusively in the amorphous region. The results indicate that the γ-sterilization/irradiation affect both molecular structure of the chains and the crystallinity of solid samples. We conclude from this investigation that crystallinity changes mainly depends on chain scission, crosslinking, oxygen containing groups, irradiation dose and polymer structure. Investigation of the effect of γ-irradiation dose rate on chemical and physical changes of PP films was carried out. FT-IR coupled with chemical derivatization (NO, SF4), and measurements of elongation at break, toughness and molecular weight modifications were employed to describe the effects of dose rate on PP. Three different 60Co sources were used with dose rate 600, 660 and 780 Gy h-1. It is found that the G values of γ-products formation were very dependent on the dose rate. However, ketone kinetic accumulation was very sensitive to the dose rate. Our findings clearly established that the γ-radiation degradation depends on the dose rate: the degradation was much more at low dose rate than at high dose rate in terms of mechanical properties. Study indicates that the lower the dose rate, the higher the degree of oxidation in term of γ-products formation. The Comparison of the G value ratio of different γ-products revealed stoechiometry differences. It wss found that the changes in crystallinity are significantly effected by the changes of irradiation dose rate. The results showed that the lower the dose rate, the higher the degree of crystallinity. The stabilization efficiency in terms of tensile properties, discoloration, evaluation of carbonyl groups and surface morphology of one stabilizer was observed to be affected by another stabilizer. Among phenol containing systems where the discoloration was observed due to the oxidation of hindered phenol, the combination of secondary HAS, tertiary HAS, organo-phosphite and hindered phenol exhibited improved stabilization efficiency than single or binary additive systems. The mixture of secondary HAS and tertiary HAS, has shown antagonistic effect of stabilization whereas their combination with organo-phosphite has exhibited synergistic effect of stabilization even at higher doses of -sterilization. The combination of secondary and tertiary HASs with peroxide decomposer reduced the consumption of stabilizer imparting stability against higher doses of -irradiation. Surface morphological micrographs have also shown the similar results. The effects have been explained through the interaction between the stabilizers. It was also found that combinations of secondary and tertiary HASs based primary antioxidants with organo-phosphite based hydroperoxide decomposers have (secondary antioxidants) exhibited synergistic effect of stabilization. It can be inferred from results that the efficiency of the stabilizer for which the presence of the stabilizer in active form is necessary for long-term stability was found to affect in single additive systems esp. in organo-phosphites because of consumption which depends on molecular weight, miscibility, volatility and compatibility. This part of study suggest that stability of polyolefins against γ-sterilization can also be improved by blends of different stabilizers which protect via different mechanism (at different stages synergistically) as well as the synergistic mixtures of the stabilizers which are approved for food-contact applications can be used to perform the stabilization against γ-radiation during sterilization.