Physical and chemical stress relaxation of elastomers

In this paper we have developed a method whereby physical and chemical relaxation processes can be distinguished, using stress relaxation experiments as a function of temperature. We assumed that there exists some temperature range above the glass transition temperature over which the chemical effects can be neglected for the time scale of the experiments. The data in this low temperature range were then used to determine the WLF constants and other physical relaxation parameters. The physical component of the stress relaxation could then be subtracted from high temperature experiments in order to extract chemical kinetic information. Based on certain reasonable assumptions, an equation was developed for the relaxation modulus of a chemically reacting system. This equation could be used to determine the time dependence of the crosslink density, or conversely could be used to predict the long term relaxation modulus from an assumed kinetic mechanism. These calculations were demonstrated for ethylene propylene and butyl elastomers.     

Thermal stress relaxation creep and microstructural evolutions of nanostructured SiC ceramics by liquid phase sintering

We explored the thermal relaxation creep characteristics of nanostructured SiC ceramics by bend stress relaxation (BSR) method. The effects of the differences in microstructure and secondary phases by liquid phase sintering at 1800 or 1900 °C were especially discussed, based on microstructural evolutions during the creep. The creep was characterized by the BSR ratio (m) of ～0.80 up to 1200 °C, and the proportion of amorphous phase as a secondary phase was related to the creep resistance at 1300 °C. The microstructural evolutions during the creep consisted firstly in the re-distribution of amorphous phase, probably as a consequence of its viscous flow, and secondly in an extensive nucleation and growth of cavities. Furthermore, the former enhanced inter-diffusion of Al–Y among intergranular areas above the ternary eutectic temperature, which caused the significantly reduced creep resistance, and the latter reflected the crystalline YAG decomposition as another secondary phase near 1500 °C.     

Surface modified amorphous titanosilicate catalysts for liquid phase epoxidation

Titanium-containing mesoporous silica has been prepared by silylation of mixed oxides obtained via sol–gel method. These samples displayed enhanced activity in the epoxidation of styrene with Bu^tOOH in comparison to normal xerogels or other titanium-containing catalytic systems. The influence of the extension of the silylation reaction, as well as the use of different silylating agents, have been investigated. Samples were characterised by FTIR, XRF, ²⁹Si-MAS NMR, diffuse reflectance UV–Vis and N₂ adsorption–desorption isotherms, X-ray photoelectron spectroscopy (XPS) and FTIR of adsorbed deuteroacetonitrile. The improvement in the catalytic activity achieved has been related to the removal of OH groups by means of calcination or silylation treatments.     

Physical and chemical stress relaxation of a fluoroelastomer

Stress relaxation measurements were made at various temperatures on V-747-7, a commercial high-temperature rubber formulation from the Parker Seal Company. The data were analyzed by separating the chemical and physical relaxation processes by a method described in an earlier publication. The chemical relaxation process was found to be Arrhenius with an activation energy of 35.7 kcal/mole. The results allow us to predict the relative useful lifetimes of this material up to approximately 320°C.     

A modified allomone collecting apparatus

A modified allomone collecting apparatus was designed that could be used under water with a scuba tank. This apparatus provides a simple method of obtaining naturally secreted chemicals from benthic marine organisms at scuba depths, for the first time without the necessity of using a bilge pump. Organic material from Sep-paks in the allomone collector confirmed the release of secondary metabolites from a soft coral into the surrounding water.     

A simple model for non-linear stress relaxation

A simple model for the prediction of non-linear stress relaxation following the cessation of steady shear flow is proposed. The model allows the calculation of the shear and first normal stress difference components of the stress. The mathematical flexibility of the model is reduced to a minimum with the result that no adjustable parameters are employed and only linear dynamic deformation data are required to calculate the non-linear behaviour. Verification of the model was carried out with data available for two viscoelastic fluids and good agreement between the predictions and the experimental results was obtained for the range of shear rates examined (0.167 ? γ ? 16.7 sec^?1).     

On the separation of physical and chemical component of stress relaxation

This paper proposes a new method to separate physical and chemical components of stress relaxation. The stress relaxation measurements of tetrafluoroethylene-propylene rubber were carried out at various temperatures ranging from 200° to 310°C. A physical stress relaxation master curve could be generated from data of early period of time by the time-temperature superposition principle. The rate of physical stress decay at given temperatures was calculated from the master curve. The rate of chemical stress relaxation was given by subtracting the rate of physical decay from experimentally obtained rate at the corresponding temperatures. The activation energy was found to be 8.4 kcal mol^?1 for the rates of the calculated chemical stress relaxation, while it was found to be 5.7 kcal mol^?1 for the rates which were obtained in air. The results show that the physical component of stress decay should be subtracted from the measured stress relaxation curve to obtain the rate of chemical stress decay, especially at the low temperature.     

A one-dimensional numerical model to simulate liquid phase chemical reactions within radiation fog

A one-dimensional numerical model of the atmospheric boundary layer is coupled with a bulk phase aqueous chemistry model to calculate sulfate production, ionic balance, and pH-values during radiation fog events. The model can be used to investigate the influence of the initial aerosol composition and trace gas concentrations on sulfate production within the fog water. The calculated pH within the fog is between 3 and 4 in most cases of mature fog with much lower values during the initial and the dissipation stage. The turbulent transport of oxidants can have an influence on the production of sulfate within a fog layer. A considerable amount of aerosol material is removed by droplet sedimentation during episodes of radiation fog.     

液相化学氧化表面处理的研究进展

分别以无机型、有机型以及化学氧化聚合型3种化学氧化处理液分类介绍了液相化学氧化处理在材料表面处理方面的应用,并对液相化学氧化处理在处理表面隋性有机材料和含碳材料方面的研究进展进行了综述。     

Properties of bismaleimide resin modified with polyrotaxane as a stress relaxation material

In this study, 4,4′‐diphenylmethane bismaleimide (BMI)/2,2′‐diallylbisphenol A (DABPA) resin was modified with polyrotaxane (PR) as a stress relaxation material. Based on a dynamic mechanical analysis and various properties of the cured resin, the influence of the motion of PR on the cured properties of BMI/DABPA/PR alloy is discussed. The cyclic molecule α‐cyclodextrin (α‐CD) that is threaded onto the PR axis contains a methacryl group in its side chain that reacts with the allyl group of BMI/DABPA resin. The methacryl group of PR reacted with the allyl group of BMI/DABPA matrix resin to form a transparent and dense network structure, so that the glass transition temperature was increased with increasing PR concentration. In addition, the toughness, impact resistance and adhesiveness of BMI/DABPA resin were improved by modification with PR. These results indicate that the poly(ethylene glycol) chain, which is the axial polymer of the PR, moves in the space formed by the cavities of the threaded α‐CD and the surrounding BMI/DABPA resin matrix in the glassy state, thereby relaxing the internal stress applied to BMI/DABPA/PR resin. The range of applications of BMI/DABPA resin modified with PR may expand into fields requiring high heat resistance in addition to excellent toughness and adhesiveness. © 2018 Society of Chemical Industry     

H-MCM-22 zeolitic catalysts modified by chemical liquid deposition for shape-selective disproportionation of toluene

Catalysts were prepared by dealumination of H-MCM-22 zeolite with oxalic acid leaching, followed by shaping and liquid phase modification with siloxane. The catalytic performance was measured with selective disproportionation of toluene in a fixed-bed reactor. The unmodified H-MCM-22 zeolite exhibited much higher initial activity than H-ZSM-5 with a thermodynamic equilibrium distribution of xylene products. The oxalic acid treatment of H-MCM-22 improved the selectivity for para-xylene by 10–18%. The modification by siloxane of the oxalic acid treated H-MCM-22 turned out to be highly selective for para-xylene, however, with the expense of the conversion. It is proposed that the high para-selectivity was closely associated with the contraction of micropore windows and the elimination of acid sites at the external surface by the deposited silica. Therefore, the modified H-MCM-22 is a potential catalyst for the selective disproportionation of toluene with a high para-selectivity.     

H-MCM-22 zeolitic catalysts modified by chemical liquid deposition for shape-selective disproportionation of toluene

Catalysts were prepared by dealumination of H-MCM-22 zeolite with oxalic acid leaching, followed by shaping and liquid phase modification with siloxane. The catalytic performance was measured with selective disproportionation of toluene in a fixed-bed reactor. The unmodified H-MCM-22 zeolite exhibited much higher initial activity than H-ZSM-5 with a thermodynamic equilibrium distribution of xylene products. The oxalic acid treatment of H-MCM-22 improved the selectivity for para-xylene by 10–18%. The modification by siloxane of the oxalic acid treated H-MCM-22 turned out to be highly selective for para-xylene, however, with the expense of the conversion. It is proposed that the high para-selectivity was closely associated with the contraction of micropore windows and the elimination of acid sites at the external surface by the deposited silica. Therefore, the modified H-MCM-22 is a potential catalyst for the selective disproportionation of toluene with a high para-selectivity.     

兑卤在光卤石区反应平衡时液相组成的计算

在K~+、Na~+,Mg~++//Cl~--H_2O水盐体系中,利用E点和F点母液兑卤析盐过程计算平衡母液组成时发现,E、F点母液总物料中的MgCl_2与反应平衡时液相中MgCl_2的含量近似相等,掘此可以很快算出反应平衡的液相组成。在察尔汗盐湖进行的盐田兑卤生产光卤石的中试中,应用该方法计算的结果与实测的母液数据基本相符。该公式可以指导生产实践。