绝缘流体流动静电起电速率实验测量

静电积聚是绝缘性易燃易爆流体的输运和存储最严重的潜在危险之一。研究流体与固体材料摩擦产生静电的速率是揭示流体静电积聚特性的关键问题。本文设计并搭建了一套旋转圆盘静电起电和测量系统,对常见的绝缘性油品如变压器油等多种流体进行了不同工况下的实验测试,获得了线速度、温度、流体黏度等因素对于起电速率的影响规律。实验结果表明,流体起电速率基本上均与流动线速度呈正比,而与温度的关系相对复杂,在一定的温区内存在最低起电速度所对应的温度。也定量地验证了绝缘性更强的流体起电速率更大这一定性特征。     

Particle Charge Distribution Measurement for Commonly Generated Laboratory Aerosols

ABSTRACT

An improved particle charge analyzer system has been developed to measure the absolute charge distribution of common generated laboratory aerosols. The charge analyzer system consists of an integral cylindrical mobility analyzer used in conjunction with an optical aerosol spectrometer, with computer assisted operation and data reduction. The charge analyzer collects aerosol particles over an absolute electrical mobility range from 4.2*10^?4 to 400 cm²/(stat · Volt second) and flow rates that can vary from 0.3 to 30 liters per minute. The charge analyzer has been used to investigate the nature of spray and contact electrification during aerosol generation by measuring the residual charge distribution on the liquid and solid generated particles. In addition, the neutralization of charged particles by bipolar ions also was studied using conventional neutralizers that use ionizing radiation from alpha and beta sources. Charge distribution measurements were performed on alumina dust (Al), Arizona road dust (ARD), potassium chloride (KCl), sodium chloride (NaCl) and di-octyl sebacate (DOS) liquid particles. Aerosol generation devices include a Collison atomizer, a condensation aerosol generator and a fluidized bed dust generator. Our work provides experimental charge distribution data for comparison with simple models of electrification theory. Experimental results showed that charge levels of atomized KCl and NaCl particles were high and decreased as the dissolved ion concentration increased. DOS particles generated by evaporation-condensation were both neutral and moderately charged. These conclusions support the existence of a dipole layer at the liquid-gas interface that interacts with dissolved particles and changes their charge state. Alumina and ARD generated by the fluidized bed disperser are highly charged due to strong contact electrification during dispersion. In most cases, the charge on generated aerosols could be reduced to Boltzmann charge equilibrium conditions by commonly used radioactive neutralizers.     

Melt fracture behavior of polypropylene‐type resins with narrow molecular weight distribution. II. Suppression of sharkskin by addition of adhesive resins

Standing on a hypothesis that the sharkskin of a polymer with a narrow molecular weight distribution at extrusion processing originates from a stick‐slip of the polymer at the die wall, the suppression of the sharkskin was tried by means of suppressing the slip by the addition of adhesives. To polypropylene (PP)‐type resins with narrow molecular weight distributions such as a PP‐type thermoplastic elastomer, PER and a controlled rheology PP were added small amounts of adhesives such as maleated PP, maleated PER, reactive polyolefin oligomers, ethylene/ethylacrylate/maleic anhydride (MAH) copolymer, ethylene/vinyl acetate copolymer, and styrene/MAH copolymer, and their melt fracture behaviors at capillary extrusion were observed. It was found that the sharkskin of the PP‐type resins with narrow molecular weight distributions was suppressed by the addition of the adhesive resins with good adhesion to metal. The suppressive effect of the sharkskin was generally the more remarkable by the higher loading of the adhesives with the higher MAH content. This is the direction of increasing adhesion. From this fact, it was assumed that the sharkskin of the PP‐type resins with narrow molecular weight distribution does not originate from a periodic growth and relaxation of tensile stress at the extrudate surface but from a stick‐slip at the die wall. Based on this mechanism, it may be said that the sharkskin can be suppressed by both ways of directions of promoting and suppressing the slip at the die wall. The former way is the previously known method, and the latter way is the method proposed in the present study. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2120–2127, 2002     

Optimization of the Electret State of Polymer Fibres

The effect of the electrification parameters and industrial pneumatic extrusion spinning on the electret properties of polypropylene fibres. In a strong enough corona discharge field, many factors can cause the formation of an electret, manifested as both injection and dipole polarization. Increasing the intensity of the process and polarization conditions and repeated processing of the material can potentiate the charge state of the fibres.     

阳离子絮凝剂PDA的合成与应用——对再生造纸废水的污泥脱水处理

研究了阳离子聚丙烯酰胺絮凝剂PDA对一种废纸再生造纸废水中的污泥的絮凝脱水处理过程，探讨了絮凝剂PDA的投加量，电荷密度，平均相对分子质量对絮凝脱水效果的影响，并比较了各种不同絮凝剂的处理效果，研究结果表明，絮凝剂PDA的电荷密度越大，相对分子质量越高，其絮凝脱水效果越好，在与无机絮凝剂PAC复配使用的相同条件下，实验室自制的絮凝剂PDA的絮凝脱水效果优于其他作对比的市售多种阳离子聚丙烯酰胺类絮凝剂。     

Measurement of static stresses on the wall of a cylindrical container for particulate solids

Measurements of the stresses acting on the base and wall of a cylindrical container were made in a 100-mm dia. steel tube containing a granular material before downward flow of the charge had commenced. The load cells used were designed for this purpose and have high sensivity and thermal stability, permitting the measurement of very small forces.

The results show that the ratio of the normal stresses on a horizontal and a vertical plane at any point in the container is substantially constant and has a value that can be predicted by a formula derived in the paper.

Drawing the Mohr circles for stress conditions in the material in contact with the wall suggests that friction at the wall is fully mobilised, the coefficient of wall friction varying slightly with depth.

The normal stress on the base does not increase as an exponential function of the bed depth and does not appear to approach a limiting value.

A small shear stress on the base of the container was detected, but friction was very far from being fully mobilised.

The results showed that deformation of the base of the container due to the weight of the charge may cause a large increase in the normal stress on the wall.     

The onset of wall slip and sharkskin melt fracture in capillary flow

The capillary die flow of high density and linear low density polyethylenes is simulated under slip conditions to investigate the origin of sharkskin melt fracture. As suggested in the literature, it is shown that sharkskin originates at the exit of the die and is due to the acceleration (high stretching rate) of the melt as it exits the die. It is also shown that both adhesion and slip promoters eliminate surface defects by decreasing the stretching rate of the polymer melt at the exit region of the die. The effect of length-to-diameter ratio of the die on the sharkskin melt fracture is also examined. It is found that sharkskin is more pronounced in short dies which is in accord with experimental observations. Finally, it is suggested that applied pressure at the capillary exit suppresses surface defects.     

Effect of wall slip on blown film thickness distribution

One of the most important materials for blown film is high‐density polyethylene (HDPE) with wide molecular weight distribution. First, we computed a wall stress at the entrance of a spiral groove in a particular die during blown film processing on a particular condition, to which a similar condition is widely utilized in a film works. The computed value is about 170 kPa, while the HDPE melt slips at die wall at stresses above approximately 50 kPa. The stress of 170 kPa is sufficiently large for the slip occurrence of the melt. Then, we investigated the effects of wall slip and melt visosity on film thickness distribution in the circumferential direction; the distribution tends to decrease with decreasing wall slip and melt viscosity. This tendency is explained by considering flow distribution in a spiral mandrel die and polymer melt flow characteristics.     

Mechanism and control of electrification in pneumatic conveying of powders

This paper describes the mechanism and control of electrification in pneumatic conveying of powders by both numerical and experimental approaches. A three-dimensional discrete element method (DEM) was used to analyze the particle movement (collision velocity, number of collision, etc.) in a pneumatic conveying process under various operating conditions. A simplified model that assumed the electrification of a single particle to be proportional to the vertical collision velocity and the number of collision against the pipe wall was proposed and the electrification during conveying of powders was numerically computed. The electrification of particles during the pneumatic conveying was also measured and then compared with the calculated results to confirm the validity of the proposed model. A novel electrostatic charge control system comprised corona discharge neutralizer, electrostatic filed strength sensor, and computer control system was developed and applied to the powder pneumatic conveying process. Dynamic characteristics of electrostatic charge and its elimination process through the corona discharge neutralizer were analyzed. Based on these characteristics, a simplified transfer function composed of first-order lag element including dead time was proposed and optimal control parameters for digital PID (proportional-integral-differential) control was determined. Performance of the control system was also investigated experimentally. It was found that the electrostatic charge in the pneumatic conveying process was completely self-controlled.     

Wall slip of molten polymers

There is considerable experimental evidence that the classical no-slip boundary condition of fluid mechanics is not always a valid assumption for the flow of high molecular weight molten polymers. In fact, molten polymers slip macroscopically at solid surfaces when the wall shear stress exceeds a critical value. Moreover, for linear polymers there exists a second critical wall shear stress value at which a transition from a weak to a strong slip occurs. These two modes of slip (weak and strong) are due to flow-induced chain detachment/desorption at the polymer/wall interface and to chain disentanglement of the polymer chains in the bulk from a monolayer of polymer chains adsorbed at the interface. In this review, the two physical mechanisms of slip are discussed and validated on the basis of published experimental data. The slip velocity of molten polymers is a complex function and has been reported to depend on wall shear and normal stresses, temperature, and molecular characteristics of polymers (molecular weight and its distribution). Proposed slip models, static and dynamic, are also reviewed and their significance on the rheology and flow simulations of molten polymers is discussed.     

Multiple charging of ultrafine particles in a corona charger

The charge distribution on ultrafine aerosol particles in the size range below 35 nm has been measured using a corona-based unipolar charger, in which ion generation and aerosol charging take place simultaneously in the region around a sharp-point discharge electrode. The mean number of charges per particle predicted by Fuchs’ diffusion charging theory is in relatively good agreement with the experimental results, and this implies that diffusion charging is the predominant mechanism in spite that the electric field in the charging region is very high. Since a steady state is unattainable in unipolar charging, the charge distribution depends on the geometry and operating conditions of each particular charger. When the present device operates under the conditions (nt-product) which yield the maximum charging efficiency, double charge appears on particles with diameter as small as 15 nm. At larger values of nt, 32 nm particles can acquire up to five charges. The critical particle diameter above which multiple charging occurs is about four times smaller than for bipolar radioactive chargers. In order to use corona charging in aerosol particle size measurement by electrical methods, the required mobility data inversion is thus straightforward for particle diameter below about 15 nm, but becomes quite complex for larger sizes.     

Novel processing aids for extrusion of polyethylene

Commercially available thermoplastic elastomers (TPEs) based on block copolymers of diisocyanates and polyols (i.e., TPUEs) were used to delay sharkskin and stick‐slip instabilities in the extrusion of linear low density polyethylene (LLDPE). When TPUE is added in a small mass fraction to LLDPE, it deposits at the die surface during extrusion and may postpone the onset of sharkskin instability to a 12–20 times higher rate of extrusion. Substantial delay of sharkskin was also achieved under conditions without slip of molten PE inside the die coated by TPUEs. Efficiency to delay the instabilities depends on elasticity of TPUE at processing temperature. The TPUEs could be a cost‐effective substitution of fluorinated polymers such as processing aid, lubricant and release agent in the processing of polyethylene by extrusion, blow molding, and injection molding. J. VINYL ADDIT. TECHNOL., 11:127–131, 2005. © 2005 Society of Plastics Engineers     

Electric field-induced aligned multi-wall carbon nanotube networks in epoxy composites

CVD-grown multi-wall carbon nanotubes were dispersed as an electrically conductive filler in an epoxy system based on a bisphenol-A resin and an amine hardener. The application of both AC and DC electric fields during nanocomposite curing was used to induce the formation of aligned conductive nanotube networks between the electrodes. The network formation process and resulting network structure were evaluated by in situ optical microscopy and current density measurements as a function of curing time. Parameters such as field strength and nanotube weight fraction were varied. The carbon nanotube agglomeration mechanism was dominated by the electric field-induced forces acting on the nanotubes, which have a negative surface charge after processing in the epoxy. The network structure formed in AC fields was more uniform and more aligned compared to that in DC fields. The specific bulk composite conductivity of fully processed composite samples reflected the differences in the nanotube network structure. Perhaps surprisingly, the network efficiency was not enhanced by this processing method, although the approach does offer the possibility of achieving bulk conductive nanotube-polymer composites with anisotropic electrical properties and a degree of optical transparency.     

Stick—slip friction as a method of powder flow characterization

We have made use of a previously ignored quantity, stick—slip friction, in determining powder flow properties. Stick-slip friction can easily be measured with a Table Model Instron or similar device. The stick—slip friction of most powders can be measured with greater accuracy and precision than any type of angle of repose measurements. Stick—slip friction values for a set of plastic powders were related to the particle size, flow properties and tensile strength. While stick—slip friction may not be universally relatable to flow behavior, it does hold promise for more limited sample tests.     

Electrophoresis of a soft toroid coaxially along the axis of a cylindrical pore

The electrophoresis of a soft toroid, which comprises a rigid core and a porous layer, coaxially along the axis of a cylindrical pore is analyzed under the conditions of low surface potential and weak applied electric field. The geometry considered simulates the electrophoresis of various types of biocolloid such as bacterial DNA, plasmid DNA, and anabaenopsis, in a confined space. We show that under typical conditions the electrophoretic behavior of a soft toroid can be different from that of a rigid toroid. For instance, the effect that the presence of a boundary can raise the mobility of a rigid toroid may not be present for a soft toroid. Several interesting behaviors are also observed. For example, if the friction coefficient of the membrane layer of a toroid is small, the larger the friction coefficient the greater is the effect electroosmotic flow, and therefore, the larger the mobility, but the reverse trend is observed if the friction coefficient is sufficiently large.     

Effects of pressure on poly(ether-ether-ketone) (PEEK) sintering mechanisms

This study deals with the understanding of the sintering mechanisms that occur during consolidation of an ultra-high-performance polymer: poly(ether-ether-ketone). In particular, we investigated the effects of uniaxial pressure during spark plasma sintering (SPS) processing. Glass-transition temperature (T_g) measurements under loading, stress–strain curves and scanning electron microscopy analysis allowed us to determine the role of pressure intensity and temperature of application on macromolecular chain mobility in both the bulk and the surface of the particles. First, a loss of chain mobility in the bulk of the particles was observed under high pressure. Second, it was shown that high pressure applied at low temperature leads to friction effects between the particles which enhances chain mobility in the particle shell through a local melting phenomenon. These experimental conditions favor the healing of particles and high cohesion was then observed in the resulting sintered samples. The level of friction is enhanced when pressure is applied at a low temperature since the powders are still in a glassy state. Finally, the achievement of cohesive grain boundaries was found to be related to the location and conformation of chain ends. Good healing between particles can only occur if a sufficient number of chain ends are available at the surface of the particle shell. We showed that the native powder state plays a significant role. The direct use of as-received powder leads to final material exhibiting good cohesion whereas pretreatments of the native powder are highly detrimental. It should be noted that this processing does not affect the high initial degree of crystallinity because no bulk melting is observed during consolidation by sintering. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47645.     

Solid-State Extrusion of Isotactic Polypropylene Through a Tapered Die. I. Phenomenological Analysis

Abstract

The solid state extrusion of isotactic polypropylene through a tapered die was conducted under various extrusion conditions using a piston-cylinder type extruder. The equation for evaluating the extrusion pressure was derived based on the plasticity theory as a function of die geometry and frictional coefficient between die wall and material. The integrand of the equation includes the yield stress as a function of true strain, which was given by the tensile properties of the same polymer. The data of lower degree of processing were fairly well described by the equation, but the estimation was not so accurate at high extrusion ratio. The molecular weight dependence of extrusion pressure was very small in solid state extrusion compared with the case of melt viscosity. This fact showed that the local inter-molecular interaction is a predominant factor in solid state extrusion. The effect of frictional resistance on a cylinder part was found to be negligible in our experimental conditions.     

Melt fracture of HDPEs: Metallocene versus Ziegler–Natta and broad MWD effects

The melt fracture of high-density polyethylenes (HDPEs) is studied primarily as a function of molecular weight and its distribution for broad molecular weight distribution metallocene and Ziegler–Natta catalyst resins. It is found that sharkskin and other melt fracture phenomena are very different for these two classes of polymers, although their rheological behaviors are nearly the same for many of these. Moreover, the metallocene HDPE shows significant slip at the die wall without exhibiting stick-slip transition. Important correlations are derived between the critical conditions for the onset of melt fracture and molecular characteristics.     

Induction Charging and Electrostatic Classification of Micrometer-Size Particles for Investigating the Electrobiological Properties of Airborne Microorganisms

Our earlier studies have shown that the electrostatic collection technique, a potentially "gentle" bioaerosol collection method, allows for efficient collection of airborne bacteria, but sensitive bacteria such as Pseudomonas fluorescens ( P. fluorescens ) lose their culturability during collection. We hypothesized that excessive stress was imposed on the sensitive bacteria by the sampler's conventional corona charging mechanism. In this research, we developed and built an experimental setup that allows us to analyze electrobiological properties of airborne microorganisms. In this experimental system, we imparted electric charges on airborne biological and nonbiological particles by aerosolizing them in the presence of an electric field. The charged P. fluorescens test bacteria and NaCl test particles were then channeled into a parallel plate mobility analyzer, which we have designed so that bacteria and inert particles carrying specific charge ranges can be extracted and made available for further analysis. When testing the experimental system, we related the extracted particle concentrations to the total particle concentration and obtained the charge distributions of these particles at different charging conditions. Our results have shown that even without charging, aerosolized P. fluorescens bacteria have a net negative charge and can carry up to 13,000 elementary charges per bacterium. In contrast, the NaCl particles were found to carry very few electric charges. We concluded that the electric charge carried by a bacterium consists of 2 components: its own natural charge, which can be high, and the charge imposed on it by the dispersion process. Our experiments have shown that the charge distributions on biological and nonbiological particles can be effectively manipulated by varying the external electric field during their aerosolization. Since airborne microorganisms may carry high internal electric charges, their collection by electrical field forces may be possible without first electrically charging them.