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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
51

Estudo comparativo da degradação de poliestireno e de poliestireno de alto impacto por envelhecimentos natural e artificial. / Comparative study of natural and artificial weathering of polystyrene and high impact polystyrene.

Borrelly, Daniel Fernandes 27 March 2002 (has links)
O estudo do comportamento de materiais em relação a intempérie, esforços, tempo, temperatura é fundamental para a escolha de materiais para uma dada aplicação. O poliestireno é uma resina polimérica muito aplicada no mercado, pois é transparente e relativamente econômica, mas não é indicada para aplicações com muita exposição a radiações luminosas, já que é muito susceptível a intemperismos, não sendo recomendada para aplicações com excessiva exposição. A adição de borracha (copolímero de estireno e butadieno) melhora algumas de suas propriedades, como resistência ao impacto e ductilidade, mas piora outras características, como a resistência à tração. Em relação ao intemperismo, apesar da borracha degradar-se rapidamente, ela protege o poliestireno da radiação ultravioleta. Este estudo visa comparar as conseqüências da degradação por envelhecimento natural com o envelhecimento artificial acelerado do poliestireno e do poliestireno de alto impacto e a determinação da possível correlação entre eles. Foram utilizados equipamentos de envelhecimento artificial, de ensaio de tração, impacto IZOD, índice de fluidez, uma estação de envelhecimento natural e equipamento de análise térmica (DSC). Os resultados obtidos permitiram correlacionar os envelhecimentos natural e artificial em relação às propriedades medidas, sendo que para o poliestireno comum, o envelhecimento artificial acelerou a degradação em cerca de 4 vezes, enquanto que para o poliestireno de alto impacto, o fator de aceleração foi de cerca de 2 vezes. / The study of weathering materials regarding to climate, stresses, time, temperature, is essential to choose the materials for a purpose. Polystyrene is a polymeric resin with much usage in the market as it is transparent and relatively cheap, but it is not indicated for outdoor purposes with much actinic radiation exposure, as it is very sensitive to weathering, tough not recommended to purposes with much exposure. The addition of rubber (styrene-butadiene copolymer) improves some of its properties, like impact resistance and ductility, but decreases other properties, like tensile strength. Regarding the weathering, although the rubber degrades very quickly, it protects the polystyrene from the ultraviolet radiation. This study aims to compare the consequences of the degradation by natural weathering and accelerated artificial weathering and try to correlate their results. Equipments for artificial weathering, tensile strength, IZOD impact, melt flow, a natural weathering station and DSC thermal analysis equipment were used. The results allowed the correlation between the natural and artificial weathering regarding the measured properties, as for the common polystyrene the artificial weathering accelerated the degradation in approximately 4 times, while for the high impact polystyrene the acceleration factor resulted in approximately 2 times.
52

Factors governing spontaneous ignition of combustible dusts

Joshi, Kulbhushan Arvind 09 April 2012 (has links)
The problem of self-heating of combustible dusts accumulated on hot surfaces has caused several fires and dust explosions. The current test standards (ASTM E 2021, EN50281-2-1) used to ensure safe environment for a given dust, define a safe temperature of the flat hot surface for certain dust layer thickness. Since in these standards, measurement of temperature is taken along the centerline, they mainly represent a simplified scenario of one-dimensional heat transfer. A need to investigate behavior of spontaneous ignition in dust deposits in complex geometries forms the motivation of this work. The effect of hot surface geometry is experimentally studied by devising wedge-shaped configurations having angles of 60o and 90o. Results show that ignition always occurred around the top region in the case of 60o wedge, and in the top and middle regions in the case of 90o wedge. These trends are explained by investigating three parameters affecting the ignition behavior, namely, the heat transfer from the hot plate to the dust, the rate of heat transfer between different regions within the dust and the minimum volume of dust required to produce sufficient heat release. A mathematical method has been proposed to predict the ignition behavior of dust deposit subjected to any boundary conditions arising due to geometrical confinement. Further, numerical simulations have been carried out to simulate the conjugate heat transfer in the interface of dust surface and air. Both analyses, mathematical and numerical, compare well with the experimental data. Furthermore, in the standard test method, ASTM E- 2021, a metal ring is used to contain the sample dusts. It is observed from experimental and numerical simulations that the resultant temperature field is not one-dimensional as desired since the corner part ignites first due to heat transfer from both the bottom plate and the metal ring, which is at almost same temperature as that of bottom plate. Theories those describe the thermal ignition in these standard tests, use the assumption that the heat flow is unidirectional. Therefore, a better substitute to the metal ring has been proposed as a ring made out of an insulating material (having low thermal conductivity). This makes the heat transfer to the dust layer phenomenally one-dimensional. Another leg of the experiments have been carried out to investigate the effect of weathering of combustible dusts on their spontaneous ignition process. Two types of weathering methods, heat- and moisture-weathering are used. Sample preparation and weathering quantification methods follow the standard test procedure. Thermogravimetric analysis has been employed to understand the variation in weight loss of fresh, heat-weathered and moisture-weathered samples of coal and organic dusts. Preliminary results show that heat weathering increases the hazard level for organic (wheat) dust. In summary, the current research work mainly involves modification of the standard test method such as ASTM E-2021 to include an insulated ring instead of a metal ring to ensure one-dimensional heat transfer and extending the test method to include wedge-shaped geometries. The spontaneous ignition of combustible dust in the new setups is investigated thoroughly. Furthermore, mathematical and numerical models have been proposed to simulate the experimental tests. Finally, the effect of two types of weathering processes on the characteristics of spontaneous ignition has been studied. In all the cases, results are thoroughly discussed with the explanation of the physics involved.
53

A Physico-Chemical Study of Weathering Products in the Payson Soil Series

Douglas, Lowell A. 01 May 1959 (has links)
A knowledge of the clay minerals in a particular soil series would aid in the characterization and classification of that soil. The physical and chemical properties of a soil are largely controlled by the properties of the clay minerals in the soil.
54

The Improvement of Interfacial Bonding, Weathering and Recycling of Wood Fibre Reinforced Polypropylene Composites

Beg, Mohammad Dalour Hossen January 2007 (has links)
This study deals with medium density wood fibre (MDF) and Kraft fibre reinforced polypropylene (PP) composites produced using extrusion followed by injection moulding. Initially, composites were produced with MDF fibre using 10, 20, 30, 40, 50 and 60 wt% fibre, and 1, 2, 3 and 4 wt% maleated polypropylene (MAPP) as a coupling agent. A fibre content of 50 wt% with 3 wt% MAPP was found to be optimum. Alkali treatment of fibre was carried out to improve the interfacial bonding. After treatment, fibre surface charge was found to increase, but single fibre tensile strength (TS) and Young's modulus were (YM) decreased. Alkali treatment reduced composite TS but increased YM. The effects of hemicellulose and residual lignin content were assessed with Kraft fibre (subjected to different stages of a standard Kraft pulping process and therefore consisting of different hemicellulose and residual lignin contents). Fibre surface charge was found to increase with decreasing residual lignin content. Composites containing higher amounts of lignin lead to lower TS and lower thermal stability. Composites were subjected to accelerated weathering for 1000 hours. TS and YM were found to decrease during weathering, and the extent of reduction was found to be higher for composites with higher residual lignin. The reduction of mechanical properties was found to be due to degradation of lignin and PP chain scission as evaluated by increase in PP crystallinity after weathering. As low lignin (bleached) Kraft fibre composites were found to provide superior mechanical properties, as well as more stable during accelerated weathering, further study including optimisation of MAPP content, effects of fibre contents, fibre length, fibre beating, hygrothermal ageing and recycling were carried out with bleached Kraft fibre. MAPP contents of 1, 2, 3, 4, 5, 7 and 10 wt% were used in Kraft fibre reinforced PP composites, and 3-5 wt% was found to be most favourable. Composite fibre content was varied between 30-50 wt%, and 40 wt% found to provide the maximum TS. To investigate the effects of fibre length on composites, fibre fractions of different length distribution were separated using a pressure screen. TS, YM and impact strength were found to decrease and failure strain (FS) increased with decreasing fibre length. To improve the interfacial bonding, the fibre was treated by mechanical beater. Fibre beating increased the TS of composites up to a certain point, beyond which TS decreased. Hygothermal ageing of composites was carried out by immersing specimens in distilled water at 30, 50 and 70 C over an 8-month period. Equilibrium moisture content and diffusion coefficient increased with increased fibre content in composites as well as with increased immersion temperature. Composites without coupling agent showed higher water uptake and diffusion coefficient than that of with coupling agent. After hygrothermal ageing the TS and YM decreased but FS and impact strength were found to increase. An investigation into the effects of recycling was carried out with composites containing either 40 wt% or 50 wt% fibre (bleached Kraft) with 4 wt% MAPP, and recycled up to eight times. For composites with 40 wt% fibre, TS and YM were found to decrease with increased recycling by up to 25% for TS and 17% for YM (after being recycled 8 times). Although TS was lower for virgin composites with 50 wt% fibre than for those with 40 wt% fibre, this initially increased with recycling by up to 14% (after being recycled 2 times), which was considered to be due to improved fibre dispersion, but then decreased with further recycling, and an overall 11% reduction of TS was found after recycling 8 times compared to the virgin composites. YM was higher for virgin composites with 50 wt% fibre than those with for 40 wt% fibre, and also initially increased with recycling but decreased upon further recycling. Recycling was found to increase thermal stability. The TS of composites made by combining recycled with virgin materials was also assessed. Hygrothermal ageing behaviour of recycled composites was also investigated by immersing specimens in distilled water at 50 C over a 9 month period. It was found that the diffusion coefficient and the equilibrium moisture contents of composites decreased with increased number of times the materials were recycled. After hygrothermal ageing, TS and YM of composites were found to decrease. However, the extent of reduction was found to decrease with increased recycling.
55

Structural and determinative mineralogy with emphasis on layer silicates / Richard Anthony Eggleton.

Eggleton, Tony January 1998 (has links)
Includes bibliographical references. / 1 v. : / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / This thesis investigates the nature and origins of minerals of a group known as layer (sheet) silicates or phyllosilicates. It consists of a collection of research papers grouped into 3 categories: modulated layer silicates, clay silicates and the weathering of rocks and minerals, including the formation of clay minerals. Central to the research has been the use of X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). All three research categories were carried out simultaneously. / Thesis (D.Sc.)--University of Adelaide, Dept. of Geology and Geophysics, 1999
56

Berggrundsmorfologiska studier över kustslätten och Norrlandsterrängen inom delar av Umeå kommun, norra Norrland

Miškovský, Karel January 1982 (has links)
The aim of this studv is to investigate interrelationships betweenbedrock relief and geological material. The area of investigation covers ca 350 km and is located in the Precambrian bedrock in the central part of Umeå commune, in northern Norrland. The boundary between two morphologically different types of landscape, the coastal plain (kustslätten) and the Norrland terrain (Norrlandsterrängen), crosses this area in a north-westerly direction.The study area's Precambrian bedrock has been mapped in detail by the author and the results are presented in a map at the scale of 1:50,000 together with a description. The occurring rock types have been studied with regard to their resistance against physical and chemical weathering. By relating recent research experience concerning Fennoscandia's palaeogeography, palaeoclimatology and isostasy, an appreciation is given of the character and strength of the forces of disintegration. Accordingly, it seems probable that chemical weathering was the dominant phenomenon during Silurian, Devonian, and even Carboniferous and Permian, whereas when the climate worsened significantly after the Tertiary period Fennoscandia's landscape was resculptured by glacial forces.During Tertiary and Quarternary times, Pennoscandia was twice uplifted and once loweredStatistical correlations between landscape relief, bedrock structure and rock variations, show that the highest sections of the landscape are dominated by resistant older granitoids, granitic mobilizate and veined gneisses, whereas the low-lying valleys and plains are characterized by the easily weathered biotite-plagioclase schists. Much suggests that the distinctive morphology of the coastal plain and Norrland terrain was initiated by bedrock structure and rock variations. As in some earlier works on Fennoscandia, this study attributes great importance to selective weathering in shaping the landscape. / digitalisering@umu
57

Microstructural characterization of titanium alloys with fretting damage

Swalla, Dana Ray, January 2003 (has links) (PDF)
Thesis (Ph. D.)--School of Mechanical Engineering, Georgia Institute of Technology, 2004. Directed by Richard W. Neu. / Vita. Includes bibliographical references (leaves 269-278).
58

Exposed Memory: Weathering of Regional Architecture

Benninger, Cole Harris January 2010 (has links)
Weathering introduces a language of durability and change throughout time. Architecture and its materials are constituents of place, as is the way they weather and age. The intent of this research is to analyze regional weathering characteristics specific to the American Southwest as a reflection of a sense of belonging that evolves over time.
59

Coupled multicomponent NAPL dissolution and transport in the subsurface: analytic solutions and forensic aspects

Hansen, Scott 28 September 2012 (has links)
Dissolving multicomponent NAPL as a source of contamination in subsurface water is considered. In particular, two processes are analyzed with regard to how they alter inter-species concentration ratios at remote monitoring locations relative to inter-species molar ratios in the NAPL: nonlinear dissolution governed by Raoult’s Law and differential sorption during subsurface transport. An analytic solution for Raoult’s Law-governed dissolution is presented. Separately, it is shown how a variety of 1D analytic transport models for simple boundary conditions may be adapted to use arbitrary time-varying concentrations by use of some properties of Laguerre series. This is combined with the analytic solution so that Raoult’s Law-governed multicomponent NAPL dissolution may be employed as the boundary condition for analytic transport models. A new computer model implementing this technique in an environment of discrete, parallel fractures is presented, and its accuracy verified for specific conditions against an existing code. The new code is applied to a parametric study on the plume separation of PAH and phenolic compounds from the dissolution of creosote. Narrow fracture spacing as well as significant values of matrix organic carbon are seen as particularly conducive to separation of these types of plumes, which in some circumstances may be entirely disjoint. Concentration ratios downgradient are shown largely unrelated to concentration ratios at the source. Finally, a study of PCB speciation is undertaken in fractured rock with known parameters, for which a rigorous, least squares speciation approach is developed. Even at distances of 5 m from the source, given perfect information about the subsurface, it is found not possible to chemically fingerprint a source PCB mixture from a list of three absent a model of the weathering of the NAPL. Both the PCB and creosote studies demonstrate that forensic inference of source compositions from field data is unreliable and the need for coupled dissolution and transport models like the one developed here. / Thesis (Ph.D, Civil Engineering) -- Queen's University, 2012-09-25 21:43:29.04
60

Acidification and buffering mechanisms in soil ecosystems

McCourt, George H. January 1993 (has links)
The objectives achieved in this thesis are: (1) to develop a new method for measuring the most important acid buffering mechanism within a soil ecosystem-mineral weathering and (2) to test a soil acidification simulation model against actual field data to determine which soil acidifying or buffering mechanisms seem to be understood and which mechanisms need more detailed analysis. First, a new method was developed that allows for quantification of H$ sp+$ consumption due to weathering and H$ sp+$ consumption due to cation exchange. Initial results yield mineral weathering rates that are well within the range of results obtained by other workers. This technique permits the analysis of multiple soil samples in a relatively short time, allowing for better quantification of spatial variability of mineral weathering within a soil ecosystem. Secondly, it was demonstrated that the acid simulation model generally underestimates soil chemistry values for pH, base cation saturation and soil solution base cations, and overestimates soil solution nitrate concentrations. Problems with obtaining accurate measurements of atmospheric dry deposition, a lack of data on cycling of nutrient elements and the absence of a sub-model to deal with the accumulation and mineralization of organic matter are reasons thought to explain the differences between model and field results.

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