Combined TGA-MS kinetic analysis of multistep processes. Thermal decomposition and ceramification of polysilazane and polysiloxane preceramic polymers.

The polymer-to-ceramic transformation kinetics of two widely employed ceramic precursors, 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane (TTCS) and polyureamethylvinylsilazane (CERASET), have been investigated using coupled thermogravimetry and mass spectrometry (TG-MS), Raman, XRD and FTIR. The thermally induced decomposition of the pre-ceramic polymer is the critical step in the synthesis of polymer derived ceramics (PDCs) and accurate kinetic modeling is key to attaining a complete understanding of the underlying process and to attempt any behavior predictions. However, obtaining a precise kinetic description of processes of such complexity, consisting of several largely overlapping physico-chemical processes comprising the cleavage of the starting polymeric network and the release of organic moieties, is extremely difficult. Here, by using the evolved gases detected by MS as a guide it has been possible to determine the number of steps that compose the overall process, which was subsequently resolved using a semiempirical deconvolution method based on the Frasier-Suzuki function. Such a function is more appropriate that the more usual Gaussian or Lorentzian functions since it takes into account the intrinsic asymmetry of kinetic curves. Then, the kinetic parameters of each constituent step were independently determined using both model-free and model-fitting procedures, and it was found that the processes obey mostly diffusion models which can be attributed to the diffusion of the released gases through the solid matrix. The validity of the obtained kinetic parameters was tested not only by the successful reconstruction of the original experimental curves, but also by predicting the kinetic curves of the overall processes yielded by different thermal schedules and by a mixed TTCS-CERASET precursor.

[Is obesity a risk factor for complications, hospital admissions, and surgical cancellations in ambulatory surgery?]. / ¿Es la obesidad un factor de riesgo de complicaciones, ingresos y suspensiones quirúrgicas en cirugía mayor ambulatoria?

OBJECTIVES: To determine the incidence of outcomes, unanticipated admissions and cancellations in patients operated in an Ambulatory surgery unit, and to establish the relationships with their body mass index (BMI). SUBJECTS AND METHODS: An observational descriptive prospective study was conducted in the Ambulatory surgery unit of the University Hospital Virgen del Rocío of Seville, on ASA I or II adult patients proposed for day case surgery with loco-regional or general anesthesia. A cohort of 1,088 patients was classified according to their body mass index into four groups: no obesity (BMI<30), obesity i (BMI 30-34.9), obesity ii (BMI 35-39.9), and morbid obesity iii (BMI 40-49.9). Postoperative outcomes (48h), inpatient admissions, and cancellations where calculated. RESULTS: The obesity ii (BMI 35-39.9) group showed a higher incidence of postoperative complications (7.69%), unplanned admissions (7.69%), and surgical cancellations (4.87%), doubling, at least, the incidence of adverse events of the other study groups, even when no significant difference was found. Outcomes where similar in all study groups. CONCLUSIONS: The results of this study suggest that moderate and severe obesity should be a risk factor for postoperative complications, unplanned admissions, and cancellations in outpatient surgery. Adequate patient selection and preoperative evaluation, as well as strategies for the prevention and control of the most frequents complications in obese patients are the key factors for their integration in major ambulatory surgery programs.

Relevant influence of limestone crystallinity on CO2 capture in the Ca-looping technology at realistic calcination conditions.

We analyze the role of limestone crystallinity on its CO2 capture performance when subjected to carbonation/calcination cycles at conditions mimicking the Ca-looping (CaL) technology for postcombustion CO2 capture. The behavior of raw and pretreated limestones (milled and thermally annealed) is investigated by means of thermogravimetric analysis (TGA) tests under realistic sorbent regeneration conditions, which necessarily involve high CO2 partial pressure in the calciner and quick heating rates. The pretreatments applied lead to contrasting effects on the solid crystal structure and, therefore, on its resistance to solid-state diffusion. Our results show that decarbonation at high CO2 partial pressure is notably promoted by decreasing solid crystallinity. CaO regeneration is fully achieved under high CO2 partial pressure at 900 °C in short residence times for the milled limestone whereas complete regeneration for raw limestone requires a minimum calcination temperature of about 950 °C. Such a reduction of the calcination temperature and the consequent mitigation of multicyclic capture capacity decay would serve to enhance the efficiency of the CaL technology. On the other hand, the results of our study suggest that the use of highly crystalline limestones would be detrimental since excessively high calcination temperatures should be required to attain full decarbonation at realistic conditions.

The effect of polymer matrices on the thermal hazard properties of RDX-based PBXs by using model-free and combined kinetic analysis.

In this paper, the decomposition reaction models and thermal hazard properties of 1,3,5-trinitro-1,3,5-triazinane (RDX) and its PBXs bonded by Formex P1, Semtex 1A, C4, Viton A and Fluorel polymer matrices have been investigated based on isoconversional and combined kinetic analysis methods. The established kinetic triplets are used to predict the constant decomposition rate temperature profiles, the critical radius for thermal explosion and isothermal behavior at a temperature of 82°C. It has been found that the effect of the polymer matrices on the decomposition mechanism of RDX is significant resulting in very different reaction models. The Formex P1, Semtex and C4 could make decomposition process of RDX follow a phase boundary controlled reaction mechanism, whereas the Viton A and Fluorel make its reaction model shifts to a two dimensional Avrami-Erofeev nucleation and growth model. According to isothermal simulations, the threshold cook-off time until loss of functionality at 82°C for RDX-C4 and RDX-FM is less than 500 days, while it is more than 700 days for the others. Unlike simulated isothermal curves, when considering the charge properties and heat of decomposition, RDX-FM and RDX-C4 are better than RDX-SE in storage safety at arbitrary surrounding temperature.

Combined kinetic analysis of solid-state reactions: a powerful tool for the simultaneous determination of kinetic parameters and the kinetic model without previous assumptions on the reaction mechanism.

The combined kinetic analysis implies a simultaneous analysis of experimental data representative of the forward solid-state reaction obtained under any experimental conditions. The analysis is based on the fact that when a solid-state reaction is described by a single activation energy, preexponetial factor and kinetic model, every experimental T-alpha-dalpha/dt triplet should fit the general differential equation independently of the experimental conditions used for recording such a triplet. Thus, only the correct kinetic model would fit all of the experimental data yielding a unique activation energy and preexponential factor. Nevertheless, a limitation of the method should be considered; thus, the proposed solid-state kinetic models have been derived by supposing ideal conditions, such as unique particle size and morphology. In real systems, deviations from such ideal conditions are expected, and therefore, experimental data might deviate from ideal equations. In this paper, we propose a modification in the combined kinetic analysis by using an empirical equation that fits every f(alpha) of the ideal kinetic models most extensively used in the literature and even their deviations produced by particle size distributions or heterogeneities in particle morphologies. The procedure here proposed allows the combined kinetic analysis of data obtained under any experimental conditions without any previous assumption about the kinetic model followed by the reaction. The procedure has been verified with simulated and experimental data.

[Brain haemodynamics in obstructive sleep apnea syndrome]. / Hemodinámica cerebral en el síndrome de apnea obstructiva del sueño.

The obstructive sleep apnea syndrome (OSAS) is associated to a increased risk of cardiovascular diseases, including stroke. However, it is not known the cause of this last association. In this work the authors carry out a brief review of the theories that explain the relationship between OSAS and stroke and comment the potential alterations in cerebral hemodynamics of sleep apnea syndrome patients. The results of a recent case control study show that there are relevant differences in cerebral blood flow between OSAS patients and healthy subjects even in wakefulness. This fact could help to explain why OSAS causes a higher risk for suffering ischaemic strokes.

Hemodinámica cerebral en el síndrome de apnea obstructiva del sueño / Brain haemodynamics in obstructive sleep apnea syndrome

El síndrome de apnea obstructiva del sueño (SAOS) se asocia a un mayor riesgo cardiovascular en general, y también de isquemia cerebral. Sin embargo, aunque se han postulado diferentes mecanismos fisiopatológicos para explicar este hecho, no se conoce con certeza la causa de la asociación SAOS-ictus. En este trabajo se hace un breve resumen de las teorías etiopatogénicas más defendidas y se comenta con mayor detalle la posibilidad de que en los pacientes con SAOS existan alteraciones hemodinámicas cerebrales que los hacen más susceptibles de sufrir una isquemia cerebral. Los resultados de un estudio de casos y controles recientemente finalizado por nuestro grupo permiten comprobar que existen diferencias relevantes en las características del flujo cerebral de los pacientes con SAOS frente a las de los controles (AU)

The obstructive sleep apnea syndrome (OSAS) is associated to a increased risk of cardiovascular diseases, including stroke. However, it is not known the cause of this last association. In this work the authors carry out a brief review of the theories that explain the relationship between OSAS and stroke and comment the potential alterations in cerebral hemodynamics of sleep apnea syndrome patients. The results of a recent case control study show that there are relevant differences in cerebral blood flow between OSAS patients and healthy subjects even in wakefulness. This fact could help to explain why OSAS causes a higher risk for suffering ischaemic strokes (AU)

[Eales' disease with bilateral brain strokes and jaw-closing dystonia]. / Enfermedad de Eales con infartos cerebrales bilaterales y distonía de cierre mandibular.

Eales' disease is an idiopathic occlusive vasculopathy of the retina, which is characterized by extensive peripheral non-perfusion, perivascular sheathing, and neovascularization. It is associated with recurrent vitreous hemorrhages. Both eyes are affected consecutively in 80% to 90% of the patients. In spite of the multiple theories that have been proposed, it continues to have unknown origin and its diagnosis relies on exclusion of other causes of retinal vasculopathy. In some occasions, these patients develop complications of the central nervous system, above all brain infarcts. We present the case of a 38 year old woman with Eales' disease who developed bilateral brain infarcts associated with occlusion or stenosis of the middle cerebral arteries. The cerebral angiography showed beading of the Silvian arteries, suggestive of an underlying inflammatory disorder. Early corticosteroid therapy could avoid contralateral retinal involvement and neurological complications. The patient also presented delayed jaw closing dystonia secondary to basal ganglia infarct which followed a benign course with spontaneous resolution in a few days.