Shearing, orifice

Die swell is always present in polypropylene spinning because its melt is viscoelastic. The viscoelasticity also gives rise to melt fracture if temperatures are too low or if orifice shear rates are too high. Melt fracture must be avoided. 

Orifice viscometers should not be used for setting product specifications, for which better precision is required. Because they are designed for Newtonian and near-Newtonian fluids, they should not be used with thixotropic or highly shear-thinning materials such fluids should be characterized by using multispeed rotational viscometers. 

Preparation of Emulsions. An emulsion is a system ia which one Hquid is coUoidaHy dispersed ia another (see Emulsions). The general method for preparing an oil-ia-water emulsion is to combine the oil with a compatible fatty acid, such as an oleic, stearic, or rosia acid, and separately mix a proportionate quantity of an alkah, such as potassium hydroxide, with the water. The alkah solution should then be rapidly stirred to develop as much shear as possible while the oil phase is added. Use of a homogenizer to force the resulting emulsion through a fine orifice under pressure further reduces its oil particle size. Liquid oleic acid is a convenient fatty acid to use ia emulsions, as it is readily miscible with most oils. 

A capillary rheometer is another type of instmment, in which the uncured mbber is extmded through a small orifice and the change in dimensions of the extmdate is measured with a laser [2]. This instmment generates high shear rates, compared to Mooney rheometer. The capillary rheometer can thus represent flow of compounds on mbber processing machinery, such as injection molds. 

Zr(lV) has been used to crosslink HEC (64). Crosslinked CMHEC tends to show more shear degradation on passing through pumps and small orifices in downhole tools than crosslinked HPG fluids... 

Amidon and Houghton [48] completed a comparative study of several common methods of characterizing powder flow. Table 2 contains experimental results for a number of commonly used pharmaceutical excipients. Compressibility index, angle of repose, flow rate through an orifice, and shear cell data are presented. 

Angle of repose Flow rate through a 6-mm orifice Simplified shear cell ... 

Too little has been published about the flow properties of PET as a criterion for processing. The results of melt flow index (MFI) testing conditions do not correlate with the processing behavior in the case of PET. This may be caused by the discrepancy between the shear rates in testing and processing. MFI is defined as the amount of polymer melt (in g) extruded within 10 min through an orifice of specified diameter at a standard load and temperature. In the case of PET, this method was not very popular until recently due to the sensitivity of this material to hydrolytic degradation. 

The flow of solids through an orifice depends on the ability of the particles to dilate in the region of the aperture. Flow will occur if the shear force exerted by the superincumbent material exceeds the shear strength of the powder near the outlet. 

The flow-shear nebulizer consists of a spherical surface with a fine slot through which the argon gas passes horizontally, and creates an aerosol stream flowing normal to the tangent at the slot. The Babington flow-shear nebulizer has been used for FAA, by Fry and Denton(36). The Fry and Denton version requires a flow rate of nine to twelve liters per minute of nebulizing gas. It may be possible to select a proper orifice size to obtain adequate aerosol production with a nebulizing gas flow rate of one liter per minute which is more suited to most ICP systems. A peristaltic pump transfers the solution to the nebulizer. 

Solid shear. In this technique, a frozen cell paste at -20°C is forced through a narrow orifice under very high pressure. The shear forces exerted by the passage of the extruded paste is aided by ice crystal formation in the frozen paste. On the laboratory scale the Hughes press or X-press are used. 

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