P. Keith Watson

The tensile strength of cohesive powders and its relationship to consolidation, free volume and cohesivity

The tensile strength of a powder is related to the interparticle force and to the free volume, which, in turn, are related to consolidation stress. The relationship between stress and free volume is described by the state diagram that has been measured at zero shear for a set of cohesive powders (xerographic toners) with a range of concentrations of a flow control additive. The toners are 12.7 μm diameter particles of styrene/ butadiene copolymer, and the surface additive is a submicron fumed silica that is used to control the interparticle forces. To overcome problems of sample non-uniformity, powder samples are initially fluidized and then allowed to settle under gravity. The tensile strenghts, σt, of these powders have been measured by means of a powder bed technique in which gas flow through the bed is increased until the bed fractures due to the tensile stress produced by the gas flow. The overpressure required to fracture the bed then provides a measure of σ1. The consolidation stress in the bed, σe, can be altered by varying the weight of the powder per unit area. Tensile strength is found to be linearly related to the consolidation stress in the limited range of stresses we have investigated, and the slope of this relationship is the same for all additive concentrations below 0.1%; above this concentration the slope decreases, consistent with a change from polymer-dominated to silica-dominated contacts between the particles. From the ratio σt/σe, we show that the contacts are fully plastic event at zero load, and that hardness of the contacts increases with increasing additive concentration.

An automated apparatus for measuring the tensile strength and compressibility of fine cohesive powders

This paper describes an apparatus based on a novel use of a powder bed, whereby the relationship between consolidation stress, tensile strength, and free volume of fine powder is measured. The powder to be tested is first initialized to a reproducible state. The initialized powder is next consolidated either beyond its own weight or below its own weight by means of a controlled flow of gas. An ultrasonic device measures the height of the bed, thus providing an average value of the powder free volume. Next the consolidated bed of powder is subjected to a slowly increasing gas flow, so directed as to put the powder under tension. The overpressure causing the powder to break provides a measure of the tensile strength of the powder, which in turn is a function of the consolidation and free volume. The relationship between consolidation stress, tensile strength, and free volume is related to powder flowability.

The Contact Electrification of Polymers and the Depth of Charge Penetration

Abstract Measurements of contact electrification have been made on polymer films charge by repeated contacts to mercury. In these studies the saturation surface potential of monodisperse polystyrene has been measured as a function of film thickness, and we show that charges appear to be trapped within about 30 nm of the surface. In a related series of experiments, using an electron beam to inject into the free surface of polystyrene film we, find that the electron range is several μm. A reason for this large difference in charge penetration distance is presented.

The tensile strength and free volume of cohesive powders compressed by gas flow

We report measurements of tensile strength and average free volume for a set of cohesive powders as a function of consolidation stress. Powders with average particle size between 7.8 and 19.2 μm were made from a styrene/butadiene copolymer, and were subsequently surface-treated with different concentrations of a submicron fumed silica. This silica acts as a flow control additive by controlling interparticle forces. The measurement technique consists of initialization of the sample by fluidization and subsequent consolidation by compression under a given gas flow while continuously monitoring the sample volume. By reversing the gas flow, a tensile stress is applied to the sample. For each consolidation state, we determine the tensile strength and the average free volume of the powder. We find that the relation between the free volume and the consolidation stress follows a logarithmic form. The magnitude of the interparticle forces is estimated from bulk measurements. At high consolidation stresses, the average tensile force per contact increases proportionally to the square root of the consolidation force per contact. Physical implications of these results are discussed.

The effect of particle size on interparticle adhesive forces for small loads

Abstract We have measured the tensile strength and average free volume of fine cohesive powders as a function of consolidation stress in the range of low consolidation stresses. All the measurements start by fluidizing the powder. Once the powder is homogeneously fluidized, the fluidizing gas is reduced to a value below the minimum fluidization flow. In this way, the powder bed settles down under a consolidation stress below its weight per unit area. The average free volume is calculated by measuring the powder bed height by means of an ultrasonic sensor. Then the gas flow is slowly increased until the powder breaks, enabling us to measure the powder tensile strength. This process is repeated for different values of the decompressing gas flow. We show that at low consolidations, there exists a linear relationship between the tensile strength and the consolidation stress. From the measured consolidation stress, tensile strength and average free volume, we estimate the interparticle consolidation force per contact, F c , and the interparticle tensile force per contact, F t . We show that F t increases linearly with F c and is independent of particle size. This result implies that, at low consolidations, the contact between fine cohesive particles is plastic.

EHD instabilities in the breakdown of point-plane gaps in insulating liquids

Our understanding of the breakdown process in insulating liquids has been advanced considerably by the use of pulsed Schlieren optical techniques to photograph the pre-breakdown events. Experimental work of this type began more than 25 years ago, and Farazmand was the first to publish Schlieren photographs which optically resolved pre-breakdown events of the type shown in Fig. 1.(1) This work was continued by Chadband and co-workers (2,3) who also used a high speed streak camera to check that the pre-breakdown events proceed in an orderly, not a discontinuous manner (without this evidence the work is open to the criticism that single photographs taken from a succession of events do not correctly represent the evolution of a single event).

The transport of low energy electrons in polystyrene

This is a continuation of earlier work (1, 2) in which an electron injection technique was used to study the transport of charge carriers in thin films of polystyrene. In these studies an electron beam is used to inject charge into the free surface of the polymer film, and a higher energy beam monitors the surface potential of the film. From the time dependence of the surface potential and its variation with temperature one should be able to distinguish between the various possible transport mechanisms in the polymer. In our work using a 2 keV beam for charge infection we consistently observe currents which decay as t−1 and this leaves an ambiguity of interpretation, as both trap-modified space-charge-limited tranpsort and beam-induced conductivity with bimolecular recombination can give rise to this same form of current decay. Repeating these charge decay measurements with low energy electrons (which should eliminate the contribution due to beam-induced conductivity) we observe charge decay characteristics which are similar to those using the 2 keV electrons. Thus we are able to distinguish between these two charge decay mechanisms: we conclude that beam induced conductivity effects are second order, and we are indeed observing electron transport in polystyrene, with electron mobility and range determined by the distribution of electron traps.

The transport of electrons in polystyrene

An electron injection technique has been used to study the transport of electrons in polystyrene. Two electron beams are used in the experiment: one beam injects charge into the free surface of the polymer film, the other beam monitors the surface potential of the film.1, 2 From measurement of the charge deposited and the potential of the surface one infers the average position of the injected charge. From the time dependence of the surface potential and its variation with temperature it is possible to distinguish between the various possible transport mechanism in the polymer.

Estimates of electron range and ion mobility in cholesteryl oleyl carbonate

Abstract An electron beam has been used as an ohmic contact to films of cholesteryl oleyl carbonate (COC). Incremental charging experiments show that the injected electrons propagate some tens of microns before becoming trapped to form stable, negative molecular ions. Although these ions approximately follow Waldens rule the magnitude of the ion mobility is lower than one would predict from the viscosity, and this is taken as evidence that the ions are accompanied by a molecular cluster. There is evidence of an abrupt increase in ion mobility near the cholesteric-to-isotropic liquid transition.