Durability of radiation-sterilized polymers III. Oxidation layer determined by chemiluminescence

Chemiluminescence (CL) analysis was used for determining the oxidation layer formed by the irradiation of polypropylene for medical supplies. The depth of the oxidation layer from the surface depended on dose rate and increased with decreasing dose rate. The oxidation occurred remarkably at a region near the surface area of the film where the diffusion of oxygen is more sufficient. On the contrary, there was very little oxidation in the interior portion. The oxidation layers of polypropylene samples irradiated with electron beam showed U-shaped profiles in the cross-section of film as did as a sample irradiated with γ-rays. However, the degree of oxidation by irradiation with electron beam was very small; CL intensity at the surface area was only one-third that for the γ-irradiated samples.     

Durability of radiation-sterilized polymers,XIII. The effects of nucleating agent on the oxidative degradation of poly(propylene-co-ethylene)

The radiation stability of the random and block poly(propylene-co-ethylene) (CP, copolymer with 6% ethylene units) with and without nucleating agent (NA) was compared in relation to radiation sterilization of medical supplies. In both cases, it was found that the radiation stability of CP in the presence of NA was lower than that of CP without NA. Addition of NA to CP did not improve the transparency but the crystallization occurred at higher temperature. Thus, adding NA to CP has the advantage of shorter moulding time in the production of medical supplies. The radiation instability of the CP with NA was due to a different crystal structure, because the spherulite shape differs from that of CP without NA. Furthermore, by the chemiluminescence analysis, a higher oxidative degradation was observed in the sample with NA.     

The oxidative degradation of imide polymers. I: Ozonolysis of a model compound,N-phenylphthalimide

The reactions of N-phenylphthalimide films with ozone have been studied under a variety of conditions: dry ozone, moist ozone, ozone plus ultraviolet light, as well as ozone plus water vapor plus ultraviolet light. This compound is a conveniently available model for imide polymers. These reaction conditions represent simulation of certain aspects of the space environment. For each set of reaction conditions the reaction products were determined by combined gas chromatography/mass spectrometry. Semiempirical molecular orbital computations were made of the relative energetics of possible reaction pathways consistent with the generation of the observed products. The reaction products found indicate fragmentation of the imidic moiety for which the energetics are favorable.     

Oxidative degradation of polymers. II. Thermal oxidative degradation of atactic polypropylene in solution under pressure

Atactic polypropylene was oxidized in solution in the presence and absence of radical initiator over the temperature range of 140° to 200°C under oxygen pressure of 3.3 to 12.4 kg/cm², and the effects of metal catalyst and additives on the rate, products, and change in molecular weight distribution were measured. The synergistic effect was observed with cobalt and managanese salts. The deactivation of the metal catalyst was suppressed by the addition of acetic acid and acetic anhydride as solvent. The rate of oxidation increased with increasing temperature, but the decrease in molecular weight was nevertheless not so significant as to give many low-boiling products. However, the production of mixtures of acid, ketone, alcohol, and ester with molecular weights of several hundreds was promising. The refractive index of polypropylene decreased markedly as the oxidation proceeded, and the complications involved in the determination of the change in molecular weight distribution measured by gel permeation chromatography are discussed.     

The mechanisms of oxidative degradation of biomedical polymers by free radicals

Degradation is an essential factor in polymer biocompatibility. The physiological environment of the human body can be aggressive to polymers. Most implanted polymers suffer degradation and the kinetics and mechanisms of the processes can be significantly affected by various biologically active species, especially enzymes, lipids, peroxides, free radicals, and phagocytic cells. Iron enhances the toxicity of oxygen free radicals. Superoxide and hydrogen peroxide can interact to form the very toxic hydroxyl radical in the presence of iron. The data have shown that the hydroxyl radical is likely to be one of the main causes of polymer degradation in implantable devices. © 1994 John Wiley & Sons, Inc.     

Oxidative degradation of polyolefins. I. Thermal oxidative degradation of atactic polypropylene in solution

Atactic polypropylene was oxidized in solution with radical initiator in the absence and presence of metal catalyst to considerably high conversion over the temperature range of 70° to 125°C, and oxidation products and the change in molecular weight distribution were measured. The unoxidized polypropylene had no ultraviolet absorption at 253.7 nm, whereas the oxidized polypropylene showed distinct absorption over a wide range of molecular weights. It was found that oxygen was incorporated into the polymer chain as hydroperoxide, acid, carbonyl, and hydroxy groups. Much of the absorbed oxygen was found to be involved in smaller fraction of low molecular weight products. Although the average molecular weight of the oxidized polypropylene decreased significantly, the formation of low-boiling products was quite small.     

Mechanisms of oxidative degradation. I. Oxidation of synthetic rubbers catalyzed by metallic ions

The oxidative stability of rubbers depends on the structure of the rubber and the impurities contaminating the rubber. It is known that the oxidation of natural rubber is accelerated by the presence of metallic impurities. Besides GR-S rubber and nitrile rubber, the effects of metallic impurities on the oxidation of new rubbers, especially those introduced after the discovery of Ziegler-Natta catalyst, have not been reported. In the first part of this paper, the theoretical background on the mechanisms of metalcatalyzed oxidation is given. Most of the early work was carried out for the oxidation of hydrocarbons in the liquid phase. The difficulties in applying the liquid-phase results to the oxidation of high polymers in the solid phase are mentioned. The new rubbers used for this study are polyisoprene, polybutadiene, cis-poly-1,3-butadiene, styrenebutadiene copolymer, butyl rubber, ethylene-propylene terpolymer (EPT), propylene oxide rubber, polyacrylic rubber, and butadiene-acrylonitrile rubber. The effects of stearic acid and various stearates on the oxidation of these rubbers are presented. The number of electrons transferred by the metal ion during redox reactions was found to be related to the effectiveness of the ion as a catalyst. Both the high and low oxidation states of the metal ion were shown to be active catalysts, supporting the accepted theory of metal catalysis through a hydroperoxide decomposition mechanism.     

The mechanism of oxidative degradation of ABS resin. Part I. The mechanism of thermooxidative degradation

In this work the early stages of the thermal oxidation of ABS resin films for commercial utilization and purified samples were studied by means of infrared spectroscopy. The film samples were submitted to thermal oxidation in a Gear oven at 70–90°C. During the aging in the Gear oven the nitrile and phenyl absorbance did not change, but oxidative products containing hydroxyl or carbonyl groups were produced rapidly, and the butadiene content decreased gradually. Two parts of the oxidative reaction were easily distinguished by analysis of the degradation procedure in the differential method and in the half-time method. An activation energy of 10 kcal./mole or less was obtained for the initiation process, involving mainly the formation of hydroperoxides induced by radical formation, which was apparent in the commercial samples, whereas a value of 10–20 kcal./mole was obtained for the subsequent process, initiated by the decomposition of the hydroperoxides, which induces chain-carrier reactions. A reaction scheme is proposed, and kinetic expressions are developed therefrom. These expressions explain the experimental results very well.     

Thermal degradation of polymers. Part I. Thermogravimetric and differential thermal analysis studies of atactic poly-m-aminostyrene and related polymers

Thermogravimetric and differential thermal analysis have been employed to study the thermal degradation patterns of poly-m-aminostyrenes and poly-m-acetamidostyrenes of different molecular weights and also the corresponding 2:1 copolymers with styrene. The thermal degradation routes have also been compared and contrasted with that of polystyrene. Experiments have been carried out in static air and dynamic nitrogen, and the different behavior exhibited by these polymers has been reconciled in terms of their differing structures and different mechanisms for pyrolytic degradation.     

Study on ozone treatment of water-soluble polymers. I. Ozone degradation of polyethylene glycol in water

Polyethylene glycol (molecular weight 8000) was degraded by ozone in 1% aqueous solution of pH 12. Chemical oxygen demand of the solution decreased with increasing ozone consumed. Intrinsic viscosity of the solution lowered exponentially as a result of the ozonization. The number of breaks calculated from the viscosity indicated that two molecules of ozone were consumed for one cleavage of the polymer chain. The molecular weight distribution obtained by gel permeation chromatography was very broadened and molecular weight was lower as well, and the polymer chain was found to be cleaved randomly by ozone. The production of formic ester, ethylene glycol, diethylene glycol, triethylene glycol, and hydrogen peroxide was confirmed by IR, NMR, gas chromatography, and chemical analysis. These observations could be accounted for by electrophilic attack of ozone on the ether bond.     

Characterization of polyethylene cable insulation by chemiluminescence measurements. I. Method development

Chemiluminescence (CL) measurements have been performed on polyethylene electrical cable insulation. Matched pairs of unaged and aged materials were used where possible. The polyethylene materials investigated included crosslinked polyethylene, low density polyethylene, and ethylene-propylene rubber. Various experimental techniques were examined to see if any distinctions could be made between unaged and aged samples. Chemiluminescence spectra were obtained for the first time for these materials. The spectra show differences between materials and also between unaged and aged samples.     

Applications of molecular dynamics to the study of thermal degradation in aromatic polymers. I: Polystyrene

This paper reports recent progress in the development of a generic molecular dynamics model that accounts for the major chemical reactions involved in the thermal degradation of polymers. The strategy employed in the design of the most recent version of this model was to interface our code for performing reactive dynamics on simple vinyl polymers with a commercial molecular dynamics package (Discover 95). The expanded range of applicability of the integrated model is illustrated by applying it to the study of thermal degradation in atactic polystyrene.     

Photoinitiated degradation of polymers by metal salts-recent developments

This paper reexamines the role of transition metal salts (mainly CuCl₂ and FeCl₃) in the photooxidative degradation of polymers. Metal salts belong to external impurities originated in polymers from synthesis, processing, storage, contacts with metal corrosion, etching reactions, etc. Transition metal salts in the presence of water become very reactive species. In such metal chlorides as CuCl₂ and FeCl₃ UV and/or visible light irradiation causes an electron-transfer from Cl^? to metal cation. The produced chlorine radical can abstract hydrogen atom from a polymer molecule giving polymer alkyl radical. which is easily oxidized in air. This process is accompanied with a chain scission reaction which is responsible for the degradation of a polymer. Mechanisms of these reactions are discussed in detail.     

XPS study of dimethylolurea polymers and hollow fibers modified by these polymers. I. XPS study of dimethylolurea polymers

Polycondensation of dimethylolurea leads to the formation of a gel or a film according to the experimental conditions. Both forms of the polymer would be characterized by a specific chemical structure, i.e., dimethylene ether or diaminomethylene, respectively. We studied both forms of dimethylolurea polymer by XPS and have shown that it is possible to distinguish them using this technique. The study of the carbon peak shape attributed mainly a dimethylene ether structure to the gel and a diaminomethylene structure to the film. © 1994 John Wiley & Sons, Inc.     

The thermo–oxidative degradation of acrylonitrile–butadiene–styrene copolymers during processing as studied by chemiluminescence

The glow curve obtained upon processing acrylonitrile–butadiene–styrene copolymers (ABS), through various machines, reaches a peak at 180°C. The proper assignment of that peak has required the study of the chemiluminescence (CL) shown by related polymers such as: polybutadiene (PB), styrene–acrylonitrile copolymer (SAN), and polyacrylonitrile (PAN). Three hydroperoxide types associated with the structural units, that is, 1, 2, and cis- and trans-1,4, exhibiting CL peaks at 180, 240, and 340°C, respectively, have been identified in the PB sample. The activation energy (E_a), recorded for the hydroperoxides thermal decomposition, was 15.0 ± 1.0, 17.85 ± 0.9, 20.7 ± 0.8 kcal/mol. PAN shows a CL peak at 180°C. Its occurance is related to the color developed during the thermal treatment. That PAN peak has been attributed to the hydroperoxides generated on the acrylonitrile units neighboring the azomethinic structures. The corresponding E_a is 23.3 ± 1.0 kcal/mol. The same peak (having an identical position and E_a) has been identified with processed ABS and SAN copolymers. As is evident by CL studies, the processing induced oxidation mainly occurs within the SAN phase of the ABS copolymers, though it was also noted within 1,2 units of the PB phase.     

Durability and environmental degradation of glass-vinylester composites

FRP materials are emerging as cost-effective and high-performing materials in various structural applications ranging from aerospace and defense to automotive, sporting goods, construction and transportation. These breakthroughs were achieved through better design practice and innovative manufacturing processes in addition to an understanding of the mechanisms governing the performance of these materials. Much of the developed expertise was confined to one composite material system, i.e., epoxy-based composite materials. For emerging applications such as the rehabilitation of civil engineering structures and the oil industry, this material system is not cost-effective when compared to conventional materials. Glass-vinylester composites are considered a potential material system offering enhanced mechanical properties at a competitive cost. However, there is very little long-term durability data related to their environmental performance and their resistance to corrosive fluids. In this paper, durability and environmental degradation of glass-vinylester composites are evaluated when exposed to high temperature, moisture, seawater, and corrosive fluid. Polym. Compos. 25:609–616, 2004. © 2004 Society of Plastics Engineers.