Analysis of Surface Degradation of Silicone Rubber Insulation Due to Tracking Under Different Voltage Profiles

Tracking is basically a carbonaceous process. In the present study, the tracking phenomenon in silicone rubber material has been studied under AC and DC voltage. The influence of applied voltage magnitude, conductivity, and flow rate of the contaminant on tracking, were analyzed. It is observed that the tracking is more severe under DC voltages. The tracking time is less under negative DC voltage compared to the positive DC voltage. The tracking mechanism is explained in detail. The leakage current during the tracking studies was measured and its trends were analyzed by using the moving average technique. The leakage current magnitude is high with thermally aged specimens when compared to the virgin specimen, irrespective of the type of applied voltage. It is realized that the tracking time and the leakage current magnitude shows an inverse relationship. In addition, the surface condition of the insulation material was analyzed with the water aged specimen and the diffusion coefficient of the material was calculated. The reduction in contact angle of the specimen has direct impact in the reduction of its tracking time. The tensile test and DMA test results indicate that thermal aging of the specimen alter the mechanical properties of the material. The activation energy of the material was calculated through DMA studies, indicating that materials with high activation energy show an increase in tracking time of the specimen.     

2，5-二烷氧基取代聚对苯撑热性能研究

采用DSC、TGA、WAXD研究了系列 2 ,5 二烷基取代聚对苯撑 (DAO PPP)的热稳定性、热氧化稳定性、相转变行为。DAO PPP的热分解由两步组成。在热分解温度超过 30 0℃时 ,首先开始发生侧链的断裂 ;温度超过 50 0℃时 ,聚合物主链才发生分解。实验结果表明 ,烷氧基的引入降低了聚合物的热稳定性 ,聚合物中的杂质和结构缺陷对其热稳定性也有一定的影响。在其DSC曲线上 ,没有观察到整个聚合物分子的熔点 ,但可观察到具有较长侧链的聚合物侧链的相转变行为。     

Gas permeability in the polystyrene films uniaxially drawn under CO2 plasticizing

 Gas permeability coefficients, Ps, of CO₂, O₂, and N₂ have been measured in four kinds of polystyrene (PS) films: an unconditioned film with no orientation (1), a high pressure CO₂-conditioned film with no orientation (2), an uniaxially hot-drawn film with some orientation (3), and a film uniaxially drawn under CO₂ plasticizing with some orientation (4). Wide angle X-ray diffraction (WAXD) scans for each sample have also been made. High-pressure CO₂-conditioning effect was confirmed from the result that P values for any permeation gas in sample 2 were larger than those in sample 1. Values of P for any gas in sample 3 were almost close to those in sample 1 and decreased slightly with Hermans orientation function, f. Gas permeability coefficients in sample 4 are larger than those in sample 3 over the whole range of f studied in this work and increased with f. These results were discussed from a viewpoint of inter-segmental distances for PS films. Received: 2 January 1997 / Accepted: 27 March 1997     

Comparative study of PHBV/TBP and PHBV/BPA blends

In order to clarify the effects of phenols on properties of polyesters, the blends of poly[(3‐hydroxybutyrate)‐co‐(3‐hydroxyvalerate)] (PHBV) with 4,4′‐dihydroxydiphenylpropane (BPA) and p‐tert‐butylphenol (TBP) were studied. The FTIR spectra revealed that there was strong hydrogen‐bond (H‐bond) interaction between PHBV and both phenols. By evaluating the fraction of H‐bonded C?O in the blend, it was concluded that BPA showed a stronger tendency than TBP to form H‐bonds with PHBV. Accordingly, BPA formed a stronger suppression than TBP on the crystallization of PHBV. When 30 wt% BPA or 50 wt% TBP were added into PHBV, the crystallization of PHBV was completely suppressed in the DSC cooling scan. As the phenol content was increased, the T_g of PHBV/TBP blend decreased while the T_g of PHBV/BPA blend increased. This difference indicated that TBP and BPA acted as plasticizer and physical crosslinking agent, respectively. Copyright © 2004 Society of Chemical Industry     

Properties,morphology and structure of BPDA/PPD/ODA polyimide fibres

Abstract

A family of random co-poly(amic acid)s containing 4,4′-oxydianiline (ODA) moiety were synthesised in N,N′-dimethylacetamide. The co-poly(amic acid) solutions were used as spinning dope for dry jet wet spinning process into as spun poly(amic acid) (PAA) fibres. The polyimide (PI) fibres were obtained from PAA fibres after being imidised and drawn in furnace. The processability and mechanical properties of the fibres were notably improved by incorporating ODA into 3,3′,4,4′-biphenyltetracarboxylic dianhydride/p-phenylenediamine (BPDA/PPD) backbone. The best strength and modulus of BPDA/PPD/ODA PI fibre (diamine mole ratio of PPD/ODA?=?85∶15) attained 2·25 and 96·5 GPa respectively, which were approximately three times the tenacity of the BPDA/PPD PI fibre. The SEM image showed that the cross-section of each stage fibres was round and void free. In addition, ‘skin–core’ and microfibrillar structure were not observed. The thermal properties of PI fibres were also investigated. The results showed that the PI fibres have excellent thermal stability; moreover, the dimensional stability and structural homogeneity of the fibres were significantly improved by heat drawn stage. T_g was found to be ～290°C by thermomechanical and dynamic mechanical analyses. The X-ray (wide angle X-ray diffraction and small angle X-ray scattering) experiments indicated that the ordering degree of longitudinal and lateral stacks, as well as the molecular orientation of PI fibre, was improved in the preparation process of fibres. Furthermore, the mechanical properties of fibres are profoundly affected by the heat drawn conditions.     

Characterisation of vulcanised natural rubber behaviour by monotonic and in situ cyclic X-ray scattering tests

Abstract

The effect of curing and loading conditions on the mechanical response of natural rubber are investigated by monotonic and in situ X-ray cyclic tensile tests. Tests are conducted on four samples, which differ by vulcanisation conditions. Samples are subject to two strain rates (2.7?×?10^??3 s^??1 and 16.66?×?10^??3 s^??1), and numerous imposed elongation levels range from 450 to 900%. The coupling between the strain rates and the elongation levels on the stress softening evolution resulting from strain induced crystallisation is investigated. In situ thermomechanical tensile cyclic test is performed in order to withdraw the effect of the strain induced crystallisation on the maximum stress decrease. The experimental results analysis shows that an optimum vulcanisation condition (150°C, 30?min) enhances the hardening process in the monotonic loading due to the strain induced crystallisation. However, under optimum curing conditions, cyclic loading induces a large hysteresis loss, a high stress softening and a high degree of strain induced crystallinity. The material softening sensitivity is controlled by coupled effect of strain ranges and elongation levels. This panoply of experimental measurements present a key information for material parameters identification that are useful to predict the lifetime of engineering components made of natural rubber such as racks, laminated rubber bearings and tires.     

Crosslinked chitosan membranes: characterization and study of dimethylhydrazine dehydration by pervaporation

Crosslinked chitosan membranes were synthesized using glutaraldehyde and characterized by infra‐red (IR) and wide‐angle X‐ray diffraction (WAXD) spectroscopic methods. The membranes were applied for the pervaporation‐based dehydration of the highly hazardous and hypergolic unsymmetrical dimethylhydrazine liquid propellant. The characterization techniques were an efficient tool in identifying polymer–liquid interaction sites and the separation mechanisms involved. The crosslinked polymer was found to have good potential for the separation of the aqueous azeotrope of the propellant (20 wt%) and its enrichment to >90% purity. An equilibrium sorption study examined the preferential affinity of the membrane amongst the two penetrating liquids. The pervaporation performance of the membrane was evaluated by varying the experimental parameters of feed composition, membrane thickness and permeate pressure, and found to be promising. © 2001 Society of Chemical Industry     

X射线衍射法测定氯磺化聚乙烯硫化胶的结晶度

根据Ｘ射线衍射强度的理论，应用图解分峰不，对氯磺化聚乙烯硫化胶的结晶峰及非晶峰的衍射强度进行校正，导出了氯磺化聚乙烯硫化胶结晶度的计算公式，并对同一配方不同硫化时间有胶料样品进行结晶度的测定，得出其结晶度ｘｃ与硫化时间ｔ的关系表达式。     

涤纶纤维晶胞参数的测定

用ｘ射线衍射法、透射技术，对八种不同来源的涤纶工业用纤维，进行赤道线（方位角为零）、方位角为４０°和子午线（方位角为９０°）扫描。并用分峰技术获得０１０、１１０、１００、０１１、１１１、１１２、１０３等晶面衍射峰的峰位，进而确定三斜晶系涤纶纤维的晶胞参数。结果表明不同工艺条件生产的涤纶工业用纤维的晶胞参数、晶胞体积、晶区的密度有差异。     

纤维的结构和性能（Ⅴ）

本文介绍了ＰＥＴ高速纺丝纤维的结晶结构与纺丝速度的关系，并根据广角Ｘ光衍射图、小角Ｘ光衍射图和结晶格子常数提出了ＰＥＴ纤维的结晶结构模型．