Biblioteca Digital

A avaliação de uma estrutura danificada por incêndios usualmente inicia por observações visuais do concreto, incluindo sua mudança de cor. A mudança de cor observada no concreto exposto a altas temperaturas persiste após seu resfriamento, constituindo-se em uma ferramenta importante para uma avaliação preliminar do nível de deterioração do concreto, pois essas mudanças podem ser relacionadas com as temperaturas as quais o mesmo foi exposto. Com o objetivo de avaliar as mudanças de cor e a degradação térmica de concretos potencialmente encontrados em estruturas no estado, foram estudados concretos de resistência convencional, moldados com materiais usualmente empregados no RS. Foram utilizadas três relações água/cimento, empregando-se o cimento CPIV, agregado graúdo granítico e basáltico. Os espécimes moldados foram submetidos às temperaturas de 200°C, 400°C, 600°C e 900°C, utilizando-se como base uma taxa de aquecimento baseada na curva padrão “Temperatura-Tempo”. Após o aquecimento, os espécimes foram resfriados lentamente, no interior do forno, e bruscamente, com aspersão de água. A degradação térmica do concreto foi avaliada através de ensaios de resistência à compressão, módulo de elasticidade e ultra-som. Para a avaliação das mudanças de cor foi utilizada metodologia colorimétrica...

The non-isothermal data given by TG curves for poly(3-hydroxybutyrate) (PHB) were studied in order to obtain a consistent kinetic model that better represents the PHB thermal decomposition. Thus, data obtained from the dynamic TG curves were suitably managed in order to obtain the Arrhenius kinetic parameter E according to the isoconversional F-W-O method. Once the E parameters is found, a suitable logA and kinetic model (f(alpha)) could be calculated. Hence, the kinetic triplet (E +/- SD, logA +/- SD and f(alpha)) obtained for the thermal decomposition of PHB under non-isothermal conditions was E=152 +/- 4 kJ mol(-1), logA=14.1 +/- 0.2 s(-1) for the kinetic model, and the autocatalytic model function was: f(alpha)=alpha(m)(1-alpha)(n)=alpha(0.42)(1-alpha)(0.56).

HDPE and PVC geomembranes are sensitive to changes in their properties when in contact with high temperatures. The effects of hot temperature on polymeric geomembranes are assessed by the ASTM D794 and ASTM D5721.This paper brings an analysis of degradation of the Poly Vinyl Chloride (PVC) and High Density Poly Ethylene (HDPE) geomembranes when exposed to conventional and air oven after specific periods.. Mechanical and physical properties were evaluated. OIT tests were also performanced to evaluate the level of oxidation degradation occurred on the HDPE geomembranes. Geomembranes of two thicknesses were tested: 1.0, 2.0 nun (PVC) and 0.8, 2.5 mm, (HDPE). The results obtained show, for example, that after the last period of exposure, the PVC geomembranes (1.0, 2.0 mm) were more rigid and stiffer than fresh samples. The HDPE geomembranes, on the other hand, when exposed to heat presented increases in deformation. OIT tests showed efficient to detect some level of degradation on the HDPE geomembranes.

Latex collected from natural rubber trees forming membranes can be used as biomaterials in several fields being the temperature a key parameter. Thermogravimetry (TG) coupled to Fourier transform infrared spectroscopy (FTIR) is a useful technique to investigate the thermal degradation of both latex and cast films (membranes), wich were obtained from Hevea brasiliensis (RRIM 600 clone) and used without stabilization. The membranes were prepared by casting the latex onto a glass substrate at 65°C for 6 h. The thermal degradation was followed by FTIR spectra acquisition along the process, allowing the identification of the gaseous components evolved upon the thermal treatment. According to TG measurements, the main processes of thermal degradation of the latex and membranes occur at three temperature intervals for both. © 2008 Springer Science+Business Media, LLC.

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Processo FAPESP: 05/54358-7; Processo FAPESP: 08/00171-1; The thermal degradation of the polyamide 6,6 (abbreviated henceforth as PA 6,6) reinforced with different concentrations of carbon nanotubes (CNTs) was investigated by means of thermal analysis. In this study, the nanostructured composites were produced using 0.1, 0.5 and 1.0 wt% of CNT. X-ray diffraction analyses were performed in order to evaluate the crystallographic properties of nanostructured composite. The degradation kinetics of PA 6,6/CNT nanostructured composites were measured by thermogravimetric analysis at different heating rates under nitrogen flow. TGA experiments were performed to elucidate the thermal behavior and supply the data that characterize the degradation kinetic. The degradation parameter kinetics was determined using the Ozawa-Wall-Flynn (O-W-F) methods, which do not require knowledge of the reaction mechanism. In this work, the results show that the addition of CNT up to the amount of 0.5wt% increases the thermal stability of PA 6,6.

The thermal and hydrolytic degradation of electrospun gelatin membranes cross-linked with glutaraldehyde in vapor phase has been studied. In vitro degradation of gelatin membranes was evaluated in phosphate buffer saline solution at 37 °C. After 15 days under these conditions, a weight loss of 68% was observed, attributed to solvation and depolymerization of the main polymeric chains. Thermal degradation kinetics of the gelatin raw material and as-spun electrospun membranes showed that the electrospinning processing conditions do not influence polymer degradation. However, for cross-linked samples a decrease in the activation energy was observed, associated with the effect of glutaraldehyde cross-linking reaction in the inter- and intra-molecular hydrogen bonds of the protein. It is also shown that the electrospinning process does not affect the formation of the helical structure of gelatin chains

Sucrose thermal degradation is an important reaction in the food industry. When in concentrated and neutral solutions, pure sucrose presents a lag phase in the reaction. This work aimed at: (i) modelling sucrose thermal degradation autocatalytic behaviour and (ii) studying the concentration and temperature effects on kinetic parameters. Isothermal experiments were conducted at temperatures ranging from 100 to 180 C, using solutions with varying water content (3.58–30.03 (% w/w)). The logistic and Gompertz sigmoidal equations were modified and reparameterised, in order to describe degradation behaviour with kinetic parameters with physical meaning (maximum reaction rate, kmax, and lag time, k). In both models these parameters presented an Arrhenius type dependence on temperature. Following a mixed model effect methodology, the concentration dependence was observed on the Arrhenius parameters. This concentration effect was included in the proposed kinetic models, which were able to successfully describe experimental data.

The thermal degradation kinetics of both components of vitamin C, ascorbic acid (AA) and dehydroascorbic acid (DHAA), were determined in a nectar of Cupuacßu (Theobroma grandi¯orum) with 25% of pulp and 15% of sugar in water. AA was assayed by HPLC and the results showed that AA degraded into DHAA. A reversible ®rst order model described well the AA degradation data, with an activation energy of 74 5 kJ/mol and k80 C 0:032 0:003min ÿ1. DHAA kinetic behavior suggested a consecutive ®rst order reaction where DHAA was the intermediate product of AA degradation. A mechanistic model was derived to predict DHAA concentration. Rate constants were replaced by the Arrhenius equation in the model to evaluate the temperature dependence and the kinetic parameters for AA degradation, previously determined, were used. An activation energy of 65 9 kJ/mol and a k80 C of 0:013 0:003 min ÿ1 were estimated. The present ®ndings will help to predict the best Cupuacßu nectar processing conditions that minimize degradation of an important quality factor such as vitamin C.

Biocomposites of polymers with vegetal fibers have a broad spectrum of applications due to their high specific properties in comparison to their counterparts made with fiberglass. Polypropylene, PP, composites with curaua fiber compatibilized with different concentrations of maleic anhydride grafted polypropylene, PP-g-MA, were characterized according to their mechanical properties, morphologies and thermal stabilities in oxidative and inert atmospheres. The composites were prepared by single screw extrusion and injection molded specimens were used for testing. The composite with 20 wt % of curaua fiber with and without compatibilizer presented improved mechanical properties compared to pure PP. The use of PP-g-MA as a compatibilizer significantly increased fiber/matrix adhesion, however, the mechanical properties were only slightly improved in comparison with composites without compatibilizer. We observed an improvement in thermal stability of the composites, compared to that expected from the weighted average of the individual components, both under inert and oxidative atmospheres. Furthermore, the thermal stability improved under inert atmosphere as a function of the concentration of compatibilizer. In this situation, indeed, there was a different shift of the weight loss processes owing to the presence of the compatibilizer.

In this study, it is investigated the synthesis and characterizations of polymerizable vinyl sugars. Carbohydrate containing polymers were synthesized via free radical polymerization. Thermal behavior of polymer derivatives was analyzed by using DSC and TG. Molecular weight dispersion of polymer derivatives was also analyzed with GPC. Molecular structures were analyzed by FT-IR and 1H-NMR spectrophotometer. We found that molecular weight of copolymers could effect to the thermal stability. According to TG data related to the copolymers, molecular weight of polymers increased while the thermal stability decreased. Thermogravimetric analysis of polymers also investigated. The apparent activation energies for thermal degradation of carbohydrate containing polymers were obtained by integral methods (Flynn - Wall - Ozawa, Kissinger - Akahira - Sunose, and Tang).

The bulk recombination lifetime of multicrystalline silicon wafers is found to initially increase with phosphorus gettering, but then, for low resistivity wafers, to decrease after a certain optimum gettering time. This peak is attributed to a trade-off between the competing mechanisms of lifetime improvement through impurity removal, and lifetime reduction due to ‘thermal degradation’. Such thermal degradation is found to be more pronounced in low resistivity samples. The physical cause of the thermal degradation has been attributed by some to the dissociation of impurity precipitates, resulting in a greater concentration of interstitial impurities. However, we find that a variable temperature gettering method, designed to circumvent the deleterious effects of precipitate dissociation, does not result in an increase in recombination lifetime, but instead results in a significant decrease. An observed increase in the dislocation density of low resistivity samples upon high temperature treatment is suggested as an alternative explanation for the thermal degradation effect.; no

Although controlled/living radical polymerization processes have significantly facilitated the synthesis of well-defined low polydispersity polymers with specific functionalities, a detailed and systematic knowledge of the thermal stability of the products-highly important for most industrial processes-is not available. Linear polystyrene (PS) carrying a trithiocarbonate mid-chain functionality (thus emulating the structure of the Z-group approach via reversible addition-fragmentation chain transfer (RAFT) based macromolecular architectures) with various chain lengths (20 kDa ≤ Mn,SEC ≤ 150 kDa, 1.27 ≤ Crossed D sign = Mw/Mn ≤ 1.72) and chain-end functionality were synthesized via RAFT polymerization. The thermal stability behavior of the polymers was studied at temperatures ranging from 100 to 200 C for up to 504 h (3 weeks). The thermally treated polymers were analyzed via size exclusion chromatography (SEC) to obtain the dependence of the polymer molecular weight distribution on time at a specific temperature under air or inert atmospheres. Cleavage rate coefficients of the mid-chain functional polymers in inert atmosphere were deduced as a function of temperature, resulting in activation parameters for two disparate Mn starting materials (Ea = 115 ± 4 kJ·mol-1...

Degradation kinetics and the thermal stability of Invar 36 powder injection moulding feedstocks (PIM) based on cellulose acetate butyrate (CAB) and polyethylene glycol (PEG) binders were investigated using simultaneous thermogravimetric analysis (STA) and differential scanning calorimetry (DSC). The initial decomposition temperature (IDT) and the integral procedure decomposition temperature (IPDT) were used to analyse the thermal stability of the binder system as a function of the solid loading content and powder particle size. The degradation kinetics was studied, and the process apparent activation energies were assessed using isoconversional methods. All the methodologies revealed changes in the thermal degradation behaviours of the feedstocks for solid loadings that were previously determined to correspond to optimal solid loadings using other experimental procedures. The studies also contrast previous similar findings with a ceramic powder. Therefore these results strengthen the proposal of thermodynamic degradation studies of feedstocks as an alternative or complementary technique to determine optimal solid loading contents in metal injection moulding (MIM).; The authors a wish to thank GUZMÁN GLOBAL S.L. and MIM TECH ALFA for their collaboration on the ECOPIM project (ref. IPT 2011 0931 20000) that was funded by the Spanish Ministry of the Economy and Competitiveness. Furthermore...

Degradation kinetics and the thermal stability of zircon powder injection moulding feedstocks (PIM) based on cellulose acetate butyrate (CAB) and polyethylene glycol (PEG) binders were investigated using simultaneous thermogravimetric analysis (STA). The initial decomposition temperature (IDT) and the integral procedure decomposition temperature (IPDT) were used to analyse the thermal stability of the binder system as a function of the solid loading content. The degradation kinetics was studied, and the degradation activation energy was assessed for varying zircon powder contents using isoconversional methods. All the methodologies revealed changes in the thermal degradation behaviours of the feedstocks for solid loadings that were previously determined to correspond to optimal solid loadings using other experimental procedures. These results may promote the proposal of thermodynamic degradation studies of feedstocks as an alternative or complementary technique to determine optimal solid loading contents in powder injection moulding (PIM). The studies in this paper also examined PIM process operation temperatures for zircon feedstocks.; The authors also wish to thank GUZMÁN GLOBAL S.L. and MIMTECH ALFA for their collaboration on the ECOPIM project (ref. IPT-2011-0931-20000) that was funded by the Spanish Ministry of the Economy and Competitiveness. Furthermore...

Antocianinas (ACYS) são corantes naturais que conferem cor a folhas, flores e frutas, são derivados glicosilados do cátion flavílio, da classe dos flavonóides e apresentam potencial para uso como corante, além de atividade antioxidante e terapêutica. Essas características estimulam buscar formas de viabilizar a utilização desses corantes na indústria de diversos segmentos, além de fomentar pesquisas acerca de sua estabilidade. Nesse trabalho, realizou-se o estudo da degradação térmica de extratos de ACYS, obtidos de uva (Vitis vinifera L. `Brasil´) e jabuticaba (Myrciaria Cauliflora). Os padrões das ACYS majoritárias para cada fruta malvidina-3-glicosídeo e cianidina-3-glicosídeo foram utilizados como referência. Foram realizados ensaios de estabilidade, para extratos obtidos a 25, 55 e 85 °C, com pH ajustado para 3,0 e mantido natural, armazenados sob-refrigeração ou a temperatura ambiente. Além disso, foram realizados ensaios de degradação acelerada a 55 e 85 °C com monitoramento espectrofotométrico e análises dos extratos e padrões iniciais e degradados por UHPLC-MS. Os ensaios de estabilidade dos extratos foram monitorados por até 170 dias. Foi verificado o efeito da baixa temperatura de armazenamento...

The thermal degradation of miscible and immiscible poly (3-hidroxy butyrate) PHB/ poly (ethylene terephthalate) sulphonated (PETs) blends was investigated using thermogravimetric analyses. Model-free kinetic analysis, Vyazovkin and Flynn-Wall-Ozawa's methods, were used to determine the apparent activation energy in the whole interval of degradation of the pure polymers, immiscible blends, and miscible blends. The thermal stability of both polymers in their blends is higher when compared to the pure polymers. The synergistic effect in the thermal stability in the blends is higher for the miscible blend where the formation of the specific interaction between PHB and PETs occurs. The apparent activation energy of the individual polymers is higher in PETs/PHB blends, and this effect is potentiated by the miscibility of the blend.

Samples of cottonseed oil, partially hydrogenated cottonseed oil and their blends, with iodine values between 60 and 110, tocopherol-stripped or not by aluminium oxide treatment, were submitted to thermal oxidation, at 180°C, for 10 hours. Samples were collected at 0, 2, 5, 8 and 10 hours, for the determination of dimers and polymers (degradation compounds) and of tocopherols. The influence of the degree of hydrogenation on the formation of dimers and polymers and the role of originally present tocopherols in the protection of fats and oils against thermal degradation was verified. The degradation curves for tocopherols showed a fast destruction rate for the tocopherols present in cottonseed fats and oil (α and γ-tocopherols), with residual levels close to zero after 10 hours under thermal oxidation conditions. Nevertheless, samples with their natural tocopherols presented a slower rate of thermal degradation. The unsaturation degree was apparently more important in the protection against thermal degradation than the content of tocopherols.; Muestras de aceite de algodón, aceite de algodón parcialmente hidrogenado y sus mezclas, con índices de yodo de 60 a 110, tratadas o no con óxido de aluminio, fueron sometidas a termoxidación...

[ES] En este trabajo se ha estudiado la degradación térmica de nanocomposites resol-alcóxido (TEOS o APS) preparados mediante el proceso sol-gel. Esta degradación se ha caracterizado tanto mediante estudios cinéticos (Energía de Activación, orden de reacción y factor pre-exponencial) como por estudios degradativos (IPDT, IDT). En todos los casos la base de estudio han sido los termogramas realizados a distintas velocidades. Los valores de las energías de activación encontrados han sido similares a nanocomposites recogidos en la bibliografía (con valores comprendidos entre 100 y 200 kJ.mol-1). Por otro lado, los órdenes de reacción han estado comprendidos entre 1,5 y 3,0 dependiendo del método utilizado y del nanocomposite analizado. Los valores de IPDT para todos los nanocomposites han sido superiores a 500 ºC llegando en algunos casos a los 650 ºC. Finalmente se han calculado los valores del tiempo de vida media para una temperatura de 505 ºC y una degradación del 95% del nanocomposite, habiéndose alcanzado valores superiores a los 1100 minutos. Todos estos resultados se han correlacionado con los espectros IR en los que se ha observado que las mayores resistencias se alcanzan en aquellos nanocomposites en los que la resina interacciona con las partículas del alcóxido.; [EN] The thermal degradation of TEOS-resol and APS-resol nanocomposites prepared by the sol-gel process has been studied in this work. Kinetic studies based on activation energies...

The thermal degradation of blends of poly(vinyl chloride) ( PVC ) with poly(vinyl acetate) (PVA), and two ethylene-vinyl acetate copolymers containing 70 weight percent vinyl-acetate (EVA70) and 45 weight percent vinyl acetate (EVA45) were studied by thermogravimetric analysis and direct-pyrolysis mass spectrometry. The PVC, PVA, EVA45 and EVA70 degrade in two-step processes for each polymer. The PVA/PVC , EVA45/PVC and the EVA70/PVC blends do show some inter action during the first step of the degradation process while the respective second steps are unaffected. The acid produced by the degradation of one polymer causes accelerated loss of the other acid in the blend. The extent of the interaction is a function of the miscibility of the blends.

The thermal degradation of blends of phenoxy with polycaprolactone , poly ( 1 , 4-butylene adipate), poly(l,4-cyclohexane-dimethylene succinate), poly( 2 , 2-dimethyl-l , 3-propylene succinate), and poly( hexamethylene sebacate) has been studied using thermogravimetric analysis, direct-pyrolysis mass spectrometry, and infrared spectroscopy. For each blend, the polyester componenent was less thermally stable in the blend compared to the pure state. The thermal stability of the phenoxy was not affected by the presence of the polyesters in the blends. Blends of phenoxy with polycaprolactone and with poly ( 1 , 4-butylene adipate) degrade in two-step mechanisms where the amount of weight loss of each step was proportional to the composition of the blends. Directpyrolysis mass spectrometric analysis showed larger amounts of monomeric caprolactone evolved from polycaprolactone in the blend compared to pure polycaprolactone. Activation energies were calculated for these phenoxy/ polyester blends from the thermogravimetric analysis data using the Flynn-Wall method. Mechanisms are presented in explanation of the observed destabilization of the polyesters.