Plastic systems viscosity

Plastisols are often mixed and then stored rather than processed immediately (Fig. 5). It is of great importance in this case for the plasticizer to show htde or no paste thickening action at the storage temperature, and clearly it is not advisable to use a plasticizer of too great an activity, since grain sweUing, leading to plastisol viscosity increase, can occur at low temperatures for some active plasticizer systems. 

At the high polymer concentration used in plasticized systems the viscosity of amorphous polymer is given by the modified Rouse theory at low molecular weight, M - 2Mr [from equation (47)] and by the modified Doi-Edwards equation at high molecular weight. In the first case... 

In the plastisol propellant process, it is essential that the resin particles not solvate too rapidly at processing temperature since a rapid increase in viscosity of the propellant mix interferes with the mixing and casting operation. There must be adequate pot life of the mixed propellant. The resin-plasticizer system itself is the dominating influence on pot life, and for this reason certain combinations cannot be used in the plastisol process. 

Viscosity Measurements. Although in typical polymer-plasticizer systems, the polymer is the major component, it is possible to use the viscosity of dilute polymer solutions as a measure of the solvent power of the liquid for the polymer. Thus, liquids with high solvent power for the polymer cause a stretching out of the chain molecules, whereas a liquid of poor solvent power causes the chains to coil up. This is because, in the liquid with poor solvent power, the segments of the polymer chain (the monomer units) prefer to stay close to each other, while in a good solvent, interaction between polymer segments and solvent molecules is preferred. 

The complexity of formation of mesophase must not be underestimated. With the exception of a few model compounds, it is the industrial pitch which is the source of mesophase. Such materials contain thousands of reactive molecules and there is an interdependence in the carbonization system which currently is known to us but not analyzed in depth. This is an area for further research. Formation of mesophase is further complicated because it involves chemistry within a fluid/plastic system of increasing viscosity. And in the delayed coker, volatile release and liquid turbulence are yet additional factors in influencing final structure in mesophase. 

Metal sulfonate-containing ethylene-propylene-diolefin ter-polymers (EPDM) were plasticized with stearic acid and derivatives for the reduction of the melt viscosities of these ionomers through interaction with the very strong ionic associations. Substantial improvements in melt flow were achieved with stearic acid and the zinc, lead, and ammonium stearates, while other metal stearates were ineffective. Zinc stearate and lead stearate not only markedly improved melt flow but, remarkably, also enhanced the mechanical properties of the plasticized systems. These unique additives were fully compatible with the EPDM ionomers and provided thermoelastic systems with excellent physical properties and ready processability. 

The polymeric plasticizers are used where permanence is of prime importance. The molecular weights of these plasticizers vary quite extensively. Variations from under 850 to 8000 are not uncommon. The cost of polymeric plasticizers is usually in excess of that of the normal monomeric types. Resistance to extraction by soapy water, oils, and migration into nitrocellulose, polystyrene, and rubber are usually superior with polymeric plasticizer systems. Because of their higher viscosities, they are usually more difficult to handle. 

The 100% solid system of plastisols allows very thick coating weights to be applied. Because Increased plasticizer lowers viscosity and assists in coating application, very high plasticizer levels (for very soft films) are easily handled. The use of chemical blowing agents and mechanical air incorporation are easily adaptable to plastisol systems. 

It is possible that such inconsistencies reflect different interfacial interactions between plastics and cellulosic tiller. For example, loading of 50% (w/w) of wood flour to two different polyethylenes can increase the system viscosity 100-fold or only 25-fold. In the first case polyethylene was metallocene PE (MFI 4 g/10 min, MWD =... 

The equation reflects on the molecular mechanism of plasticization, as there is a correlation between viscosity of a plasticized system and the plasticizing action of a plasticizer. The greater is ATg, the greater is P (T) and the lower the viscosity. 

Selection of plasticizer or plasticizing system goes beyond lowering the viscosity. The amount of form coating, its nniformity, and appearance depend on rheological properties of plastisols. Forms are subjected to a controlled movement which has several steps ... 

There are two rather distinct types of shear-thinning behavior. Systems which show no flow (infinite viscosity) until a critical shear stress Oq, known as the yield stress, is exceeded are said to exhibit plasticity. The viscosities of such systems approach Newtonian limits as shear stress increases. In the other type of behavior, sometimes given the pejorative name pseudoplasticity, Newtonian behavior is approached at both high- and low-shear limits. Recent studiesyield stress, exhibiting Newtonian behavior with extremely high viscosities. Nevertheless, the concepts of plasticity and yield stress can be usefully applied to practical systems, including many adhesives. 

Fig. 7.42. Shear stress and viscosity versus shear rate for a pseudo-plastic system.

When that stress is exceeded, the shear rate grows. Further stress leads finally to linear (Newtonian) behaviour. Examples of plastic systems are chocolate, butter, cheese, various spreads and ice cream. In pseudoplastic systems the observed viscosity decreases with an increase in shear stress. An example of a pseudoplastic system is pudding. Dilatant systems resist deformation more than in proportion to the apphed force. The shear rate is growing much faster than that of Newtonian fluids and viscosity increases with an increase in shear stress. At low apphed forces, the system behaves as a Newtonian fluid. Examples of dilatants systems are honey with added dextran and a slurry of wet beach sand. Thixotropic systems become more fluid (they have lower viscosity) with increasing time of an apphed force. If the apphed force ceases to operate, the original viscosity of the system is restored due to a reversible transformation of the sol gel type. Examples of thixotropic systems are mayonnaise, ketchup, whipped and hardened fats, butter and processed cheeses. Rheopectic systems exhibit behaviour opposite to that of thixotropic systems. Their viscosity increases with increasing time of apphed force. An example is whipped egg white. 

The apparent viscosity that is to be used in the Reynolds number has to be measured in the viscometer at the pipeline shear rate. This can be obtained by fitting the flow curve to the power law relationship given by equation (4.25). In turbulent non-Newtonian flow the friction factor is a unique function of the Reynold s number. For Bingham plastic systems, the Reynold number is calculated by using the plastic viscosity since it remains constant with increasing shear rates. For pseudoplastic flow, the Reynold s number is calculated using an estimated apparent viscosity that is obtained by extrapolation to infinite shear rate. 

For a Newtonian system the viscosity at levels A and B is the same (by definition). For the pseudoplastic system the viscosity at A is greater than that at B. In a plastic system with a yield value between A and B, no settling would occur. Samyn s own discussion of these cases is given in the legend to the figure. 

As more plasticizer is added, the viscosity at a given shear rate drops. This is plotted in Figure 16C.11 for our example material from Figure 16C.1. For a particular polymer and plasticizer system, it should be fairly straightforward to run... 

Uses Dispersant, wetting agent for org. and inorg. pigments in nonaq. systems viscosity reducer/stabilizer for pigment dispersions in plasticizers such as DINP molybdenum dispersant for high pressure gear lubricants Features Heat stable... 

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