Flow retardation

Let us consider now the effect of flow retardation in extension under constant force 25). The above-mentioned experiments wer,e carried out with low-density polyethylene at 125 °C and with polyisobutylene 11-20 at 44 °C. It should be noted right away that at the above-specified temperatures these melts have approximately similar characteristics under shear strain maximum viscosity is t) 3x 105 Pa s and relaxation time is 0 102 s. Flow curves in the investigated region of shear strain velocities also did not differ significantly from one another. 

Sevruk VD, Prokunin AN (1980) Manifestations of flow retardation effect in extension of molten polyethylene at constant force. 2nd All-Union Symposium Theory of mechanical processing of polymer materials. Perm, pp 165-166... 

Rex = UaoX/v being the local Reynolds number. Here, the wall friction (resulting from the momentum flow retarded in the boundary layer) may be evaluated from ... 

Onwulata et al. (1995) studied the effect of flow conditioners calcium stearate, aluminum silicate, and silica added at different concentrations on bulk density, flow and mechanical properties of lactose, sucrose and modified cornstarch, as well as milk fat encapsulated in the same materials. Each flow conditioner was effective in reducing compressibility (C2) of the powders when applied at 1% concentration. Compressibility of the remaining unencapsulated powders continued to decrease with added flow conditioner. However, in the case of encapsulated butter-oil powder, the only effective additive was silica (2%), which resulted in a 35-70% decrease in compressibility. The most notable effect was observed with butter-oil powder encapsulated in lactose where reductions in compressibility resulted and an increase in powders flowability. The stearate resulted in flow retardation of all powders studied. 

Many modifications to the original van Deemter plate height eqnation have appeared in the literatnre (28-32). Some account for mass transfer in the gas phase (28,29), and other modifications have been made for velocity distribntion because of flow retardation of interfacial resistance (30,31). Improvements were attempted, usually stochastic theories based on random-walk theory (32). However, we elaborated on the work of Giddings (33), who described plate height contributions as a fnnction of the diffnsional character of zone broadening by accounting for local noneqnilibrium. 

The last term acconnts for the resistance to mass transfer in the gas phase. Low-loaded liquid coatings cause the Cg term to be significant. Equation 2.46 was further extended to account for velocity distributions due to gas flow retardation in the layers C and the interaction of the two types of gas resistance C2. ... 

Slibar, A. and Paslay. P. R, 1959. Retarded flow of Bingham materials. Trans. ASME 26, 107-113. 

A nitro group behaves the same way m both reactions it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution) Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution) By being aware of the connection between reactivity and substituent effects you will sharpen your appreciation of how chemical reactions occur... 

Filtration of viscose is not a straightforward chemical engineering process. The solution of cellulose xanthate contains some easy-to-deal-with undissolved pulp fibers, but also some gel-like material which is retarded rather than removed by the filters. The viscose is unstable and tends to form more gel as it ages. Its flow characteristics make the material close to the walls of any vessel or pipe move more slowly, get older, and gel more than the mainstream viscose. So while filtration can hold back gels arising from incomplete mixing, new gels can form in the pipework after the filters. 

TetrabromobisphenoIA. Tetrabromobisphenol A [79-94-7] (TBBPA) is the largest volume bromiaated flame retardant. TBBPA is prepared by bromination of bisphenol A under a variety of conditions. When the bromination is carried out ia methanol, methyl bromide [74-80-9] is produced as a coproduct (37). If hydrogen peroxide is used to oxidize the hydrogen bromide [10035-10-6] HBr, produced back to bromine, methyl bromide is not coproduced (38). TBBPA is used both as an additive and as a reactive flame retardant. It is used as an additive primarily ia ABS systems, la ABS, TBBPA is probably the largest volume flame retardant used, and because of its relatively low cost is the most cost-effective flame retardant. In ABS it provides high flow and good impact properties. These benefits come at the expense of distortion temperature under load (DTUL) (39). DTUL is a measure of the use temperature of a polymer. TBBPA is more uv stable than decabrom and uv stable ABS resias based oa TBBPA are produced commercially. 

Phosphoms-containing additives can act in some cases by catalyzing thermal breakdown of the polymer melt, reducing viscosity and favoring the flow or drip of molten polymer from the combustion zone (25). On the other hand, red phosphoms [7723-14-0] has been shown to retard the nonoxidative pyrolysis of polyethylene (a radical scission). For that reason, the scavenging of radicals in the condensed phase has been proposed as one of several modes of action of red phosphoms (26). 

Several commercial polyester fabrics are flame retarded using low levels of phosphoms additives that cause them to melt and drip more readily than fabrics without the flame retardant. This mechanism can be completely defeated by the presence of nonthermoplastic component such as infusible fibers, pigments, or by siUcone oils which can form pyrolysis products capable of impeding melt flow (27,28). 

This force acts in the —x direction and retards the flow. The rate at which the gas does work in pushing itself against this force is... 

Injection-Molded Products. Numerous housings, electrical enclosures, and cabinets are injection-molded from rigid PVC. These take advantage of PVC s outstanding UL flammabiUty ratings and easy mol ding into thin-waHed parts. PVC has developed melt flow capabiUties to the point where it competes with essentially any other flame-retarded engineering thermoplastic and molds easier than most. 

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