Crystallization and melting behaviour of photodegraded polypropylene - I. Chemi-crystallization

An investigation has been conducted into the effects of photodegradation on the crystallinity and melting behaviour of isotactic polypropylene (PP). PP samples having different structural characteristics were prepared and exposed to ultraviolet radiation (u.v.) in the laboratory for periods of up to 48 weeks. The changes in crystallinity during exposure were followed by X-ray diffraction and differential scanning calorimetry (d.s.c.), whereas the chemical degradation of the specimens sampled was evaluated by gel permeation chromatography (g.p.c.) and Fourier transform infrared spectroscopy (f.t.i.r.). An increase in fractional crystallinity during u.v. exposure was noted for all types of samples studied, and the gain in crystallinity was usually between 6% and 7% and was virtually independent of the initial structure of the polymer. Measurements conducted at different depths within the test bars indicated that the fractional crystallinity increased during u.v. exposure due to crystal growth using molecule segments released by the scission of molecules (probably taut molecules). This process, called chemi-crystallization, is restricted by the chemical defects introduced into the molecules by the photodegradation. Possible mechanisms for the chemi-crystallization process are discussed. The melting thermograms of most types of samples exhibited single peaks with melting range increasing with exposure time. The broadening of the melting thermograms occurred during the period between the commencement and the completion of the chemi-crystallization. Secondary crystallization proceeded much more slowly in the interior of the test bars because of the limited oxygen supply, but the final crystallinity was the same as the plateau value, which was attained more rapidly when the material in the interior was exposed by machining away the surface prior to u.v. exposure. © 1997 Elsevier Science Ltd.     

Electrochemical degradation of perchloroethylene in aqueous media: An approach to different strategies

An approaching study to the electrochemical degradation of perchloroethylene (PCE) in water has been carried out using controlled current density degradation electrolyses. The different electrochemical strategies to degrade perchloroethylene in aqueous media (i.e. cathodic, anodic and dual treatments) have been checked using divided and undivided configurations. The influence of the initial concentration, pH and current density on the general behavior of the system has been studied, and special attention was paid to the nature of the byproducts formed and to the analysis of the closed mass balance at the end of the reaction. Results from several analytical techniques have been compared. Undivided configuration provides the best results in these experimental conditions, with degradation percentages higher than 50% and with only 6% of the initial perchloroethylene concentration remaining in the system.     

Application of immobilized titanium dioxide photocatalysts for the degradation of creatinine and phenol, model organic contaminants found in NASA''s spacecrafts wastewater streams

In this project, immobilized titanium dioxide photocatalysis was utilized as a post-treatment technology for the destruction of model organic contaminants found in wastewater streams produced on-board during space exploration. Phenol, a known human carcinogen, and creatinine, a human metabolite found in urine, were the compounds tested in this study. Phenol and creatinine have cyclic structures consisting of six and five member rings, respectively. In addition, creatinine is a methyl guanidine derivative, with almost 40% (w/w) nitrogen. The degradation and carbon mineralization efficiencies of the target contaminants were investigated at different initial concentrations. Their photocatalytic degradation appears to follow pseudo-first-order reaction with phenol giving higher organic carbon reduction rates than creatinine. The presence and position of the functional groups of creatinine (amine, imine and peptide bond) are primarily responsible for the significantly slower mineralization. The degradation of creatinine was also tested at different pH_o values. Statistical analysis showed that there is an effect of pH on the treatment of creatinine. Besides the carbon mineralization, the extent of nitrogen mineralization and the mass balance of nitrogen were conducted for three pH values (pH_o 3.0, 6.2 and 11.0). Overall, the transformation of nitrogen was low, and the total maximum conversion (<20%) occurred at basic conditions.     

Microbial adaptation in the degradation of phenol byAlcaligenes xylosoxidans Y234

Microbial adaptation was performed to enhance the biological degradation of phenol. WhenAlcaligenes xylosoxidans Y234 was adapted with benzene, it could degrade phenol of 1,000 ppm completely in 60 hours while phenoladapted cell could not. This phenomenon was discussed in terms of intracellular enzyme activity and applied to the degradation of phenol in a packed-bed bioreactor.     

The plasticizer market: an assessment of traditional plasticizers and research trends to meet new challenges

Plasticizers have long been known for their effectiveness in producing flexible plastics for applications ranging from the automotive industry to medical and consumer products. The plasticizer industry has grown with the use of plastics worldwide. Recent plasticizer research has focused on technological challenges including leaching, migration, evaporation and degradation of plasticizers, each of which eventually lead to deterioration of thermomechanical properties in plastics. Human exposure to certain plasticizers has been debated recently because di(2-ethylhexyl) phthalate, used in medical plastics, has been found at detectable levels in the blood supply and potential health risks may arise from its chronic exposure. The current paper presents a brief history and an overview of the traditional plasticizers currently available in the world market, discusses some of the problems associated with the end uses of these plasticizers and reviews recent scientific approaches to resolve these problems. The definition of an ideal plasticizer changes with each application; thus, this paper addresses technical issues first from a broad perspective, and then with a focus on leaching, migration, evaporation and degradation issues. Several approaches to reduce leaching and migration of plasticizers are discussed, including surface modification of plasticized polymers and the application of alternative plasticizers and oligomers to meet technological requirements. New approaches to reduce evaporation and degradation of plasticizers are discussed, with the aim of formulating long-lasting flexible plastics and minimizing the ultimate environmental impact of these chemicals. The development of fire-retardant plasticizers and novel plasticizers for use in biodegradable plastics are also included.     

肉制品中亚硝酸盐降解方法、机理及研究进展

指明了肉制品生产中亚硝酸盐的投放量和残留量标准。对肉制品中降解亚硝酸盐机理的历史、现状和发展情况进行了论述。     

Pt/C/MnO2 hybrid electrocatalysts for degradation mitigation in polymer electrolyte fuel cells

Pt/C/MnO₂ hybrid catalysts were prepared by a wet chemical method. Pt/C electrocatalysts were treated with manganese sulfate monohydrate (MnSO₄·H₂O) and sodium persulfate (Na₂S₂O₈) to produce MnO₂. The presence of MnO₂ was confirmed by FTIR spectroscopy. Rotating ring–disk electrode (RRDE) experiments were performed on electrodes prepared using the hybrid electrocatalysts to estimate the amount of hydrogen peroxide (H₂O₂) formed during the oxygen reduction reaction (ORR) as a function of MnO₂ content. Pt/C/MnO₂ (5% by weight of MnO₂) hybrid electrocatalysts produced 50% less hydrogen peroxide than the baseline Pt/C electrocatalyst. The hybrid electrocatalysts were used to prepare membrane electrode assemblies that were tested at 90 °C and 50% RH at open circuit with pure hydrogen as fuel and air as the oxidant. The fluoride ion concentration was measured using an ion selective electrode. The concentration of F⁻ in the anode condensate over 24 h was found to be reduced by a factor of 3–4 when Pt/C/MnO₂ replaced Pt/C as the catalyst. Through cyclic voltammetry and RRDE kinetic studies, the lower ORR activity of the acid treated hybrid electrocatalysts was attributed to catalyst treatment with acid during MnO₂ introduction. The activity of the hybrid catalyst was improved by switching to a water-based synthesis.     

Resistance to peroxide degradation of Hyflon Ion membranes

Perfluorosulfonic acid (PFSA) membranes have been used for 40 years as solid electrolytes in low temperature fuel cells and are considered from the scientific community superior to other polymeric products due to their good combination between chemical resistance and proton conductivity. In recent years, development of the class of PFSA membranes known as ‘short side chain’ membranes has been restarted from Solvay Solexis (Hyflon^® Ion).