Rheological characterisation of hydroxyapatite filled polyethylene composites. Part 2 – Isothermal compressibility and wall slip

Abstract

Rheological characterisation of hydroxyapatite–high density polyethylene (HA–HDPE) composites has been performed in terms of isothermal compressibility and wall slip. Addition of HA to the polymer melt decreases the compressibility of the melt. The unfilled HDPE was found to exhibit wall slip at shear stresses as low as 0·10 MPa. The flow curves of the composites showed three distinct regions: a gradient at low shear rates; a plateau region; and a gradient at higher shear rate. An increase in rheometer pressure seems to suppress the slip in composites. The 40 vol.-% HA–HDPE composite exhibited two critical shear stresses, one corresponding to wall slip, which occurs in the lower shear rate region of the flow curve, and the other corresponding to a plateau, which is identified with the stick–slip behaviour of unfilled HDPE reported in the literature. The plateau shear stress increased with filler volume fraction and this effect is attributed to the decreased compressibility of the melt. A good correlation with a negative correlation coefficient was found to exist between compressibility and shear stress in the plateau region. The slip observed in unfilled HDPE and at low shear rates in the 40 vol.-% HA–HDPE systems has been explained in terms of a low molecular weight polymer layer formed at the melt/wall interface. The large interfacial slip observed in the plateau region is attributed to complete disentanglement of adsorbed chains from free chains at the melt/wall interface at and beyond the plateau region.     

Slag-coated wall structure technology for entrained flow gasifiers

An extensive research and technology base for utilization of slag as an active protective coating on wall structures has been developed. This work was aimed initially at MHD topping cycle components. Duration testing under conditions very severe relative to those considered for gasifier operation has shown that negligible wall damage occurs over hundreds of hours of flow. Complete and uniform slag coverage of properly designed water wall structures is obtained, resulting in operation in regimes heretofore inaccessible because of refractory dissolution or wall erosion. The active slag coating serves as an effective erosion barrier, with particle impact and gas flow shear load absorbed by the flowing self-renewing slag coating. Multiple shutdowns and restarts, including in-situ development of a slag coating on a bare wall, have been demonstrated.Improved durability in high temperature reaction zones is projected, using this technology. An advanced technology two stage entrained flow gasifier, utilizing high peak temperature and velocity to achieve high carbon conversion by pyrolysis, and low overall oxygen consumption, is outlined.     

克劳斯工艺防腐蚀涂层数值模拟

燃料、烟气中的高浓度硫化氢以及液滴对克劳斯工艺中燃烧设备有严重的腐蚀和冲蚀作用,很容易引发大型生产安全事故。液滴的冲蚀在金属受热面壁面产生细微裂纹是化学腐蚀的起点。涂层能够防止化学腐蚀,本文通过建立数学模型准确描述了克劳斯硫黄回收工艺中液滴撞击防腐蚀涂层的过程,运用离散迭代方法对整个计算区域进行求解,得到了撞击过程中防腐蚀涂层内部、涂层与固体黏结面以及固体内部的应力分布,确认了涂层方案能够在克劳斯硫黄回收工艺中解决金属受热面液滴冲蚀的问题;同时研究了不同厚度的涂层对应力分布的影响,得出了量纲为1最大应力与撞击速度之间的经验公式,提出采用20μm厚的氮化硅涂层是经济高效的克劳斯工艺壁面保护措施。     

Effect of water absorption on the behavior of E-glass fiber/nylon-6 composites

The effect of water absorption on the stress transferability across E-glass fiber/nylon 6 interface has been studied using the embedded single fiber composite technique. The behavior of silane coated fiber and untreated fiber composites after periods of water immersion were compared. The silane coating provided both higher interfacial shear stress transferability and protection from permanent water damage in the interphase region. It was found that water absorption in the nylon matrix reduced the shear stress transferability through plasticization of the matrix, weakening of the interface, and the development of tensile swelling stresses at the phase boundaries. In untreated materials the shear stress transferability was limited by decoupling of the matrix from the broken fiber ends by either interface slippage or local plane strain fracture in the interphase region near the fiber end. In the silane treated materials the shear stress transferability was limited by constrained yielding of the polysiloxane/nylon interphase at the fiber end, thus indicating plasticization of the matrix was the primary factor. After 20 days of water immersion, there was permanent deterioration of stress transferability in the untreated samples, but very little permanent damage in the treated samples.     

不同冲击条件下微晶玻璃涂层冲蚀应力的有限元模拟

材料表面应力状态对材料的冲蚀磨损行为起着重要作用。本文利用有限元方法探讨了冲蚀角度、冲蚀速度、磨料粒径对微晶玻璃涂层冲蚀应力的影响,利用常温冲蚀试验机对微晶玻璃涂层在不同冲蚀角度下的体积磨损率进行了测定。结果表明,随冲蚀角度、冲蚀速度、磨料粒径增大,涂层的冲蚀应力均逐渐增大。在相同的冲蚀角度和冲蚀速度条件下,磨料粒径对涂层冲蚀应力的提升效果显著。微晶玻璃涂层冲蚀磨损率随冲蚀角度的增大而增加,其变化规律与冲蚀应力变化趋势基本一致,从而验证了有限元应力模拟的可靠性。     

喷射精炼过程炉衬侵蚀模拟研究──Ⅳ.从炉衬的机械冲蚀研究复吹转炉的合理炉型

在复吹转炉所受剪切应力和冲击应力分布的模型实验研究基础上，本文研究了不同炉型的炉衬所受剪切应力分布，为合理选用炉型提供依据.实验结果表明，底吹对延长转炉炉衬寿命有利，转炉下部形状的合理性可以减少流动死区和降低液面波动；还表明在复吹转炉内接近钢液面的炉衬所受侵蚀最为严重．在受损最严重的地方可以采用特殊材质的炉衬或使炉壁向上呈有扩张趋势的梨状炉型，来达到减少炉壁受侵蚀的速度.同时还讨论了转炉大小与底吹供气制度的关系问题并提出了最佳炉型的建议     

Flow birefringence of poly(n-hexyl isocyanate) solutions

A correction for optical quantities measured by a slit rheometer is proposed in order to obtain the corresponding optical properties at wall. This correction together with the Rabinowitch correction enables studies of the relation between the optical and the mechanical properties. A simple optical flow cell of a slit geometry was designed for the simultaneous measurements of the mechanical and the optical properties of flowing birefringent materials. The setup and the correction were applied to investigate the flow birefringence of the isotropic solutions of a liquid crystalline polymer, poly(n-hexyl isocyanate). The apparent birefringence defined as the refractive index difference between the ordinary light and the extraordinary light was found to increase with the increasing shear rates and was higher for solutions of high concentrations at a given shear rate. The stress optical law was checked. The ratio of the apparent birefringence to the shear stress was found to be independent of the shear rate at concentration of 10 wt%. For solutions of 15 and 20 wt%, the ratios increase with the shear rate when the shear rate is small, and they reach a maximum then decrease with increasing shear rate. The optical-stress behavior of PHIC solutions qualitatively follows the theoretical prediction of the Doi theory.     

管道内多巴胺超疏水涂层的制备

超疏水涂层在表面自清洁、流体减阻、防雾防冰冻和微流控等领域具有巨大的应用潜力,而工程应用中圆管内表面超疏水涂层微结构的调控具有一定的挑战性。利用电化学沉积法,在不同的剪切应力下将聚多巴胺 （PDA） 涂层制备到不锈钢圆筒内壁,并将正十二硫醇（NDM）修饰到PDA表面,进而制备出PDA/NDM超疏水涂层。采用场发射扫描电镜（SEM）、接触角测试仪（CA）、傅里叶变换红外光谱（FT-IR）、X射线衍射测试仪（XRD）进行分析和表征。结果表明PDA沉积过程可分为两个阶段,第一阶段为溶液中的PDA颗粒在不锈钢基底上面团聚;第二阶段以PDA颗粒团聚体为基础,PDA进行原位生长,并且生长过程受到剪切应力的控制。不同剪切应力最终生长的形貌有所不同,当剪切应力为1.85 mPa时,涂层表面呈“珊瑚状”小球,粒径大小约15~24 μm;而当剪切应力为7.41 mPa时,涂层表面呈“片状”结构,粒径大小约1~4 μm。所制备出PDA/NDM涂层润湿角均大于150°,属于超疏水,且涂层具有良好的化学稳定性、耐热性、耐磨性以及耐腐蚀性。本工作对圆管内表面涂层的制备和表面纳微结构的调控具有一定的指导意义。     

Analysis of the kinetics of agglomerate erosion in simple shear flows

A model for the erosion kinetics of particle agglomerates in simple shear flows has been developed. The erosion rate is taken to be proportional to the difference between hydrodynamic and cohesive forces and to the rotation velocity of the dispersing agglomerate. For dispersion under identical hydrodynamic conditions, the model predicts faster erosion for larger agglomerates. Moreover, at equivalent hydrodynamic stress, erosion is enhanced at higher shear rates. Dispersion experiments using silica agglomerates of various densities and liquid low molecular weight polymers (e.g., poly(dimethyl siloxane)) of different viscosities were conducted in an oscillatory shear flow device. The experimental results validate the erosion model; the general shape of the erosion kinetic curve, and the effect of viscosity, shear rate and agglomerate size on dispersion kinetics are well predicted. The model can also predict erosion for agglomerates with fractal structure.     

The effect of teflon™ coatings in polyethylene capillary extrusion

Two LLDPE resins were used in this work to determine the critical conditions for the occurrence of wall slip and melt fracture in capillary extrusion. It was found that the polymer-metal interface fails at a critical value of the wall shear stress of about 0.1 MPa and, as a result, slip occurs. At values of wall shear strees of about 0.18 MPa the extrudate surface appears to be matte, while small amplitude periodic distortions (sharkskin) appear on the surface of extrudates at wall shear stresses above 0.25 MPa. Using a special slit die, the polymer–wall interface was coated with Teflon? in order to examine the effect of this coating on the processability of polyethylenes. It was found that use of Teflon? promotes slip, thus reducing the power requirement in extrusion and, most importantly, eliminates sharkskin at high extrusion rates. © 1995 John Wiley & Sons, Inc.     

HVOF喷涂用于提高锅炉换热面耐磨损耐腐蚀性能综述

火力发电是我国的主要发电方式,在燃用煤、生物质等固体燃料时会面临锅炉换热面的冲蚀磨损或腐蚀问题,导致管道失效停炉,严重影响了电厂的安全稳定运行。超声速火焰（HVOF）喷涂作为热喷涂工艺的一种,可以通过在换热管道表面添加防护涂层来缓解磨损或腐蚀问题。因其制备的涂层具有与基体结合强度高、孔隙率低等优异的特点,在锅炉换热面的耐磨损耐腐蚀方面研究及应用前景广阔。综述了HVOF喷涂的发展、工艺流程以及涂层的特性,并重点总结了用于提升锅炉换热面耐磨损耐腐蚀性能的HVOF涂层材料,以及不同材料应用时需要考虑的环境因素。最后从工艺优化、材料进步以及实验方法创新三个方面对HVOF工艺在锅炉换热面上的应用做出展望。     

High shear rheology of paper coating colors – more than just viscosity

Capillary viscometry is used to characterize viscosity, entrance pressure loss and apparent wall slip of paper coating colors at high shear rates. Special emphasis is laid on the dependence of these phenomena on solids content in order to account for changes in the rheology due to the dewatering of the color during the coating process. Coating colors with substantially different runnability have been investigated. Differences in apparent wall slip and high shear viscoelasticity (manifesting itself in extremely high entrance pressure losses) are observed at increased concentration, even if these phenomena do not show up at the initial solids content. Poor runnability is observed when viscosity, entrance pressure loss and wall slip increase strongly with increasing solids content. But all rheological features change simultaneously with the coating color recipe and it is not possible to separate out the contribution of the particular rheological features on the runnability of the coating colors or to correlate the runnability to a single rheological prorameter. Future work will have to focus on a numerical analysis of the blade coating process taking into account all the rheological features described here. First simulations including slip at the color/blade interface indicate that wall slip may cause severe runnability problems, at least when the apparent slip velocity exceeds the web velocity.     

纸桨泵叶轮中纤维悬浮湍流的流动特性(英文)

A numerical method for predicting fiber orientation is presented to explore the flow properties of turbu-lent fiber suspension flowing through a stock pump impeller. The Fokker-Planck equation is used to describe the distribution of fiber orientation. The effect of flow-fiber coupling is considered by modifying the constitutive mode. The three-dimensional orientation distribution function is formulated and the corresponding equations are solved in terms of second-order and fourth-order orientation tensors. The evolution of fiber orientation, flow velocity and pressure, additional shear stress and normal stress difference are presented. The results show that the evolutions of fiber orientation are different along different streamlines. The velocity and its gradient are large in the concave wall region, while they are very small in the convex wall region. The additional shear stress and normal stress difference are large in the inlet and concave wall regions, and moderate in the mid-region, while they are almost zero in most downstream regions. The non-equilibrium fiber orientation distribution is dominant at the inlet and the concave wall regions. The flow will consume more energy to overcome the additional shearing losses due to fibers at the inlet and the concave wall regions. The change of flow rates has effect on the distribution of additional shear stress and normal stress difference. The flow structure in the inlet and concave wall regions is essential in the resultant rheological properties of the fiber suspension through the stock pump impeller, which will directly affect the flow efficiency of the fiber suspension through the impeller.     

The effect of re-entrainment on particle deposition

The effect of particle removal on observed deposition rates is considered by combining previous sublayer analyses. Wall shear stress is shown to be a controlling parameter; below a critical value where removal can occur, deposition varies linearly with time but above this value the observed deposition rate is dependent on the choice of time interval over which measurements are made. For monodisperse particles, a discontinuity in deposition plot is predicted although this may be smoothed out for particles with a wide size distribution. It is suggested that the measurement of the critical wall shear stress may be useful for estimating adhesion forces under dynamic conditions such as occur for redeposition problems.The functional dependence of deposition on wall shear stress for combined deposition and removal has been derived for the cases where diffusional, inertial and gravitational forces are dominant. Comparison with data from the literature confirms the general trends but precise matchings have not been attempted.     

Influence of non-uniform distribution of shear stress on aerobic biofilms

This work deals with the detachment of biofilm subjected to a shear stress. Biofilms are developed on plates, under very low shear stress for one month and then subjected to an erosion test for 2 h in a Couette-Taylor reactor (CTR). During the erosion test, the plate was fixed on the external cylinder of the CTR. The presence of the plate modifies the velocity field in the CTR. A first zone close to the facing step region is characterized by the detachment of the stream lines. A second zone, downstream, is characterized by a pure shear flow: the distribution of the shear stress is uniform; the residual biofilm mass was measured and the detachment can be classically related to the magnitude of shear stress. In the first zone, the recirculating flow induces a strong non-uniform distribution of shear stress. The residual biofilm mass was also measured and found to be much lower than in the uniform shear stress zone, whereas the magnitude of shear stress is of the same order or even smaller. The assumption of elastic rheology for the biofilm enables the strong detachment observed in the region subjected to non-uniform shear stress to be explained.     

A model viscoelastic fluid

Shear stress and first normal stress difference data are presented for materials which exhibit a constant viscosity and yet at the same time exhibit elasticity levels of the same order as polymer melts. Flow pattern observations in circular die entry flows in conjunction with independent shear and normal stress measurement strongly suggest that these fluids would make excellent model fluids for melt studies. Studies in which the influence of elasticity in the absence of shear thinning and fluid inertia can easily be made. Furthermore it is clearly shown that a realistic solution to the die entry flow problem is not obtained using second order flow theory. In the second order region the secondary cell is observed to be almost identical in size to the cell observed for an inelastic Newtonian fluid in creeping flow. Marked growth in the secondary cell as a function of elasticity is not observed until the shear rates exceed the region of second order behavior. This growth in cell size as a result of elasticity is followed at higher shear rates by a spiraling flow instability like that observed for some polymer melts.     

纳米环氧树脂复合涂层耐冲蚀磨损性能研究

研究了固化剂低分子聚酰胺、有机蒙脱土和纳米Al2O3的用量对环氧树脂胶粘涂层拉伸剪切强度和耐冲蚀磨损性能的影响。试验表明,有机蒙脱土和纳米Al2O3能有效提高涂层拉伸剪切强度和耐冲蚀磨损性能,在最佳配方下,涂层的耐冲蚀磨损性能是Q235钢的10.83倍。     

Effect of five kinds of pores shape on thermal stress properties of thermal barrier coatings by finite element method

Thermal ablation is a very important technique to characterize the thermal properties of coating systems. On account of the concentration of thermal stress, thermal barrier coatings (TBCs) often break off from the substrate partly or completely during the thermal erosion. In this paper, the thermal erosion simulation of finite element geometric models based on the possible pore shapes were implemented, especially, the influence of pore shapes on the distribution of coating temperature, X component of stress, Y component of stress, XY-shear stress and von-Mises stress were focused on. The effects of the different porosity of square pore coatings on thermal insulation properties and thermal stresses were discussed in term of the simulation results. The simulation results indicate that different shape pores not only affect the thermal stress distribution above the contact area between the bond coating and top coating surface, but also affect the plastic deformation behavior of TBCs. The micromechanism was discussed in details in this study. The computed results verified that, the computational method can successfully predict thermal shear, crack initiation and normal failure mode of the studied TBCs. All the results are in good agreement with the corresponding experimental findings. The failure mechanism factors in this paper are of great importance to explain the failure micro-mechanism of TBCs.     

Capillary rheometry of raw elastomers,butadiene-acrylonitrile copolymers

Capillary rheometry of four butadiene-acrylonitrile copolymer raw elastomers was performed at 125°C over a wide shear rate range. The data were corrected for the effect of pressure on viscosity, for pressure loss in the barrel and at the capillary entrance, for the non-Newtonian velocity profile (Rabinowitsch correction) and for the temperature rise at high shear rates (shear heating). The corrected results were compared to the tensile stress strain data obtained at comparable strain rates. The agreement between the capillary flow data and the tensile stress strain data was quite good, however, some slight systematic differences were noted. The difference was interpreted as the consequence of slipping of materials at the capillary wall. Die swell values were measured and presented as functions of shear rate and capillary length. Extrudate distortions are described.     

INSTANTANEOUS VELOCITY PROFILE MEASUREMENTS IN A TURBULENT BOUNDARY LAYER

Instantaneous wall shear stress and streamwise velocities have been measured simultaneously in a flat-plate, turbulent boundary layer at moderate Reynolds number in an effort to provide experimental support for large eddy simulations. Data were obtained using a buried-wire, wall shear gage and a hot-wire rake positioned in the log region of the flow. Fluctuations of the instantaneous U⁺ versus Y⁺ profiles about a mean law of the wall are shown to be significant and complex. Peak cross-correlation values between wall shear stress and the velocities are high, and reflect the passage of a large structure inclined at a small angle to the wall. Estimates of this angle are consistent with those made by other investigators. Conditional sampling techniques were used to detect the passage of various sizes and types of flow disturbances (events), and to estimate their mean frequency of occurrence. Events characterized by large and sudden streamwise accelerations were found to be highly coherent throughout the log region and were strongly correlated with large fluctuations in wall shear stress. Phase randomness between the near-wall quantities and the outer velocities was small. The results suggest that the flow events detected by conditional sampling applied to velocities in the log region may be related to the bursting process.