Effects Of Chemical Degradation Research Articles

Experimental study of mechanical behaviour of cement paste under compressive stress and chemical degradation

This paper presents experimental investigations of mechanical behaviour of a pure cement paste subjected to compressive stresses and chemical degradation. Two series of laboratory tests have been performed: decoupled and coupled chemical-mechanical tests. Hydrostatic and triaxial compression tests have first been realized respectively on sound and chemically degraded samples. The obtained results allow the characterization of basic mechanical responses of the tested cement paste and the identification of chemical degradation effects on the mechanical behaviour. In the coupled tests, the samples are simultaneously subjected to deviatoric stresses and chemical leaching by aggressive solution flow. Variations of deformation of cement paste samples are measured during chemical degradation process. The results obtained in these tests can be used for the validation of chemo-mechanical constitutive modelling.

Modelling a continuous devulcanization in an extruder

An engineering kinetic model is applied to EPDM devulcanization in an extrusion process. Parameters that were included in the model are crosslink density, devulcanizing agent concentration, time, shear rate, and temperature. The extruder is considered as a series of plug flow and stirred tank reactors. The residence time is calculated using solid flow or liquid flow model, depending on the degree of decrosslinking. The model can accurately predict the decrosslinking degree inside the temperature boundary defined by the applied experimental conditions. Outside this boundary, the effect of chemical degradation on the measured conversion becomes more significant.

Chemical degradation of an uncrosslinked pure fluororubber in an alkaline environment

AbstractThe chemical degradation of an uncrosslinked pure fluoroelastomer (FKM; Viton A) in an alkaline environment (10% NaOH and 80 °C) was investigated. Scanning electron microscopy images showed that on a microscopic level, significant degradation substantially increased the surface roughness after prolonged exposure (e.g., 12 weeks). The molecular mechanisms of the chemical degradation processes at the surface were evaluated with X‐ray photoelectron spectroscopy and attenuated total reflectance/Fourier transform infrared spectroscopy. The results revealed that the early degradation proceeded primarily via dehydrofluorination reactions, creating double bonds in the rubber backbone. This further accelerated the degradation after longer exposure times. Furthermore, the resulting double bonds underwent nucleophilic attack by an aqueous NaOH solution to form several oxygenated species. All these species ultimately recombined to form crosslinks, as evidenced by the increase in the gel fraction and surface hardness (Shore A). The pronounced effect of chemical degradation through a reduction in the thermal stability of the pure FKM rubber upon exposure was also evident from thermogravimetric analysis and differential thermogravimetry. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6216–6229, 2004

Thermal aging of undoped polyaniline: Effect of chemical degradation on electrical properties

AbstractAging experiments, with a special emphasis on the atmosphere effect, have been carried out on undoped polyaniline, in its half‐oxidized state, namely emeraldine base (EB). The polymer has been aged under vacuum and in air atmosphere. The chemical degradation has been analyzed by several complementary techniques such as viscosity measurements, FTIR, XPS, and UV‐Vis‐nIR spectroscopy. We show that emeraldine base exhibits two different degradation mechanisms. The first one is intrinsic to the polymer and occurs independently on aging conditions (vacuum or air). It consists of crosslinking via tertiary amine groups created from imine nitrogen via double‐bond breaking. The second mechanism is extrinsic and occurs concomitantly with the first one upon aging in air. It consists of oxygen incorporation in a form of carbonyl groups and chain scission. Both degradation pathways result in a decrease of the electrical conductivity of the polymer due to the lowering of the average conjugation length. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 395–404, 2002

Mechanical effects of chemical degradation of bonded geomaterials in boundary value problems

ABSTRACT The paper presents a constitutive approach to describe the effects of rock weathering processes in boundary value problems. The term rock weathering is used to refer to a number of chemical and physical phenomena that continuously transform a rock mass into a granular soil. From an engineering point of view rock weathering can be interpreted as a generalised decay of the mechanical properties of the original material. It acts at a constitutive level essentially by reducing the strength-of the bonds joining the grains together. Such a material degradation can occur in a time scale which is comparable to the average life of engineering structures. Weathering can induce subsidence on shallow foundations resting on soft rocks layers or it can be crucial for what concerns the stability of slopes or abandoned underground mines. In the first part of the paper, it is shown how the progressive destruction of the intergranular bonds due to weathering has been modelled satisfactorily by extending a strain h...

Adsorption of Poly(ethylene oxide)–Poly(lactide) Copolymers. Effects of Composition and Degradation

The effect of chemical degradation of two diblock copolymers of poly(ethylene oxide) (E) and poly(lactide) (L), E39L5 and E39L20, on their adsorption at silica and methylated silica was investigated with in situ ellipsometry. Steric stablization of polystyrene dispersions was investigated in relation to degradation. Hydrolysis of the poly(lactide) block of the copolymers was followed at different temperatures and pH by using HPLC to measure the occurrence of lactic acid in solution. The block copolymers were quite stable in pH-unadjusted solution at low temperature, whereas degradation was facilitated by increasing temperature or lowering of the pH. Lower degradation rates of E39L20 where observed at low temperature in comparison with those of E39L5, whereas the degradation rates of the copolymers were quantitatively similar at high temperature. The adsorption of the copolymers at methylated silica substrates decreased with increasing degree of degradation due to the reduction in the ability of hydrophobic block to anchor the copolymer layer at the surface. At silica the adsorption initially increased with increasing degradation, particularly for E39L20 due to deposition of aggregates onto the surface. After extensive degradation the adsorption of the copolymers at both silica and methylated silica resembled that of the corresponding poly(ethylene oxide) homopolymer. Overall, it was found that the eventual reduction in adsorption occurred at a lower degree of degradation for E39L5 than for E39L20. Mean-field calculations showed a reduced anchoring for the block copolymers with decreasing poly(lactide) block length at hydrophobic surfaces. In accordance with this finding, it was observed that polystyrene dispersions were stabilized by E39L20 or E39L5 in a way that depended on both the lactide block length and the degree of degradation. Upon degradation of the hydrophobic block, stabilization of the polystyrene dispersions was maintained initially, but eventually degradation resulted in destabilization. The average residual copolymer concentration required for stabilization of the polystyrene dispersions was much higher than the corresponding concentration of intact copolymer required for stabilization.

Chemistry of uranium compound doping in (Bi,Pb) 2Sr 2Ca 2Cu 3O x/silver superconducting tapes

Equilibrium stable compounds U(Ca,Sr) 2O 5 and U(Ca,Sr) 3O 6, have been identified in the U Bi Sr Ca Cu O system. These compounds have been synthesised and added to Bi-2223/Ag composite tapes in progressively greater amounts. Bi-2223/Ag doped with these compounds using compositions closer to the equilibrium composition has been shown to have superior superconducting properties to that doped with Uranium oxide compounds. The effects of chemical degradation by Uranium compound dopants via Strontium and Calcium removal from the Bi-2223 structure have been indirectly determined by I c measurements. Losses in I c can be minimised by stabilising Uranium into appropriate compounds.

The influence of combustion-derived pollutants on limestone deterioration: 1. The dry deposition of pollutant gases

This study concerns quantification of the relative chemical degradation effects of dry deposition of combustion-derived atmospheric pollutants, HCl, SO 2 and NO 2, on Portland and Monks Park limestones, employing laboratory exposure chambers for periods of 30 days. Using presentation rates up to 40 × 10 −4 μgcm −2s −1, the pollutants were assessed individually and in various combinations, along with ozone as oxidant, at 84% RH with dry or water-wetted surfaces. The degradation was followed by analysis of exposed stone, for Cl −, SO 4 2− and NO 3 −, and of run-off solution for Ca 2+, in addition to the anions. From these data, the total calcium released from limestone to reaction ions and products and the percentages of each pollutant reacted (and so the deposition velocity ( V D)) in each exposure regime were calculated. HCl acted independently of the presence of other pollutants, showed 40 and 100% reaction (mass transport control) and V D values of 2.2 and 5.3 mm s −1, at 84% RH and on wetted surfaces, respectively. The reactivity of SO 2 rose from 2.5 to 35%, and V D from 1 to 2.9 mm s −1, from conditions of 84% RH to those with the presence of water, NO 2 and O 3. Similarly, the reactivity of NO 2 rose from 3 to 40% at 84% RH in the presence of SO 2 and O 3, with a V D at 3 mm s −1, but decreased in the presence of run-off solution. A limited study was performed on smoke-coated and naturally exposed, reaction-product-coated, samples. In addition to obvious factors such as deposited mass, chemical constitution and transfer, smoke particle size distribution should also be considered in any possible action on limestone. Concerning naturally exposed samples, non-wetted surface reaction products tended to reduce the attack of pollutants, but had no resisting effect when wetted.

Effect of chemical degradation on superconducting properties of the HTSC ceramics

The effect of humid atmosphere (59 and 86%) on phase composition and properties of YBa2Cu3O7−y powder and ceramics prepared by hot pressing has been studied. It has been found that with increase of water content in the powder to 1·6 mass.% the superconducting properties of the corresponding ceramics gradually improve; on further increase of the water content the ceramics degrade resulting in a complete loss of superconductivity. The observed phenomenon can be explained by a plastifying effect of water on the powder by hot pressing on the one hand and by a solid-phase reaction of the products of decomposition (1-2-3) at a high temperature annealing, on the other.

Preparation of oxidatively stable cation-exchange membranes by the elimination of tertiary hydrogens

Model compounds and radiatively grafted membranes were prepared in order to study the effect of tertiary hydrogens on the stability of membranes used in oxidatively aggressive environments. Sulfonated isopropylbenzene, which contains a tertiary hydrogen, yielded substantial amounts of degradation products when exposed to a strong oxidizing solution. Sulfonated t-butylbenzene, which is identical to sulfonated isopropylbenzene except that the tertiary hydrogen is replaced by a methyl group, yielded only minute amounts of degradation products when exposed to the same oxidizing solution. Styrene (contains a tertiary hydrogen) and α-methylstyrene (tertiary hydrogen has been replaced by a methyl group) were radiatively grafted onto poly (tetrafluoroethylene) films and sulfonated to form cationic membranes. The styrene-prepared membrane exhibited several times more weight loss during exposure to an oxidizing aging solution than did the α-methylstyrene prepared membrane. The energy efficiencies of styrene-grafted membranes decreased rapidly to less than 60% after 60 or 80 cycles in a cycling test cell. The energy efficiencies of α-methylstyrene grafted membranes were 82.6% and 83.0% for 700 test cycles and showed no effects of chemical degradation during this period.