Viscosity increase

Let us consider the bimolecular reaction yielding the product AB from the colliding pair AB 

In the equilibrium or stationary states, the concentration of the colliding pair is given by the relation 

In a kinetically simple medium, the overall rate constant of product formation, kas, can be defined by the relations 

The equilibrium constant of AB formation from A and B is then defined as 

Kab must only depend on thermodynamic parameters so that the ratio kjka cannot be a function of such kinetic effects as, for example, the rate of diffusion. As ka includes the diffusion rate effects of both A and B, the same factor must also be included in kw. When the reaction between A and B is diffusion-controlled, the dissociation of AB must also be controlled by diffusion [41]. 

The ratio kjk is the equilibrium constant of collision pair AB formation for colliding uncharged molecules or radicals it can be approximated by the equation 

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The pour point is the lowest temperature at which an oil can still pour while it is cooled, without agitation, under standardized conditions. The pour point of paraffinic bases is linked to the crystallization of n-paraffins. The pour point of naphthenic bases is related to a significant viscosity increase at low temperatures. This property can be improved by additives. 

When fluid flow in the reservoir is considered, it is necessary to estimate the viscosity of the fluid, since viscosity represents an internal resistance force to flow given a pressure drop across the fluid. Unlike liquids, when the temperature and pressure of a gas is increased the viscosity increases as the molecules move closer together and collide more frequently. 

Unlike gases, liquid viscosity decreases as temperature increases, as the molecules move further apart and decrease their internal friction. Like gases, oil viscosity increases as the pressure increases, at least above the bubble point. Below the bubble point, when the solution gas is liberated, oil viscosity increases because the lighter oil components of the oil (which lower the viscosity of oil) are the ones which transfer to the gas phase. 

Oils and gums are nonhighly branched- or straight-chain polymers whose viscosity increases with the degree of polycondensation. 

The apparent viscosity, defined as du/dj) drops with increased rate of strain. Dilatant fluids foUow a constitutive relation similar to that for pseudoplastics except that the viscosities increase with increased rate of strain, ie, n > 1 in equation 22. Dilatancy is observed in highly concentrated suspensions of very small particles such as titanium oxide in a sucrose solution. Bingham fluids display a linear stress—strain curve similar to Newtonian fluids, but have a nonzero intercept termed the yield stress (eq. 23) ... 

Thickeners. These are used to kicrease viscosity of shampoos to achieve certain consistency characteristics ki the product, from a thickened Hquid to gels and pastes. Among the most important materials used for this purpose are the alkanolamides. The chain length of the amide alkyl group and its solubiHty ki the shampoo system are important aspects to be considered ki thek use for effects on viscosity. In general, as the chain length increases the viscosity response improves. The viscosity increase also is related to the water solubiHty of the amide the more water-soluble forms provide a lower viscosity response than the less soluble amides. 

Ucon HTF-500. Union Carbide Corp. manufactures Ucon HTE-500, a polyalkylene glycol suitable for Hquid-phase heat transfer. The fluid exhibits good thermal stabHity in the recommended temperature range and is inhibited against oxidation. The products of decomposition are soluble and viscosity increases as decomposition proceeds. The vapor pressure of the fluid is negligible and it is not feasible to recover the used fluid by distiHation. Also, because the degradation products are soluble in the fluid, it is not possible to remove them by filtration any spent fluid usuaHy must be burned as fuel or discarded. The fluid is soluble in water. 

Temperature control is important in the handling and storage of isocyanates. Storage at inappropriate temperatures can cause product discoloration, viscosity increases, and dimerization. Handling personnel should consult the technical data sheets for the recommended storage temperature of the specific isocyanate product. 

Viscosity (Viscosity-Index) Improvers. Oils of high viscosity index (VI) can be attained by adding a few percent of ahnear polymer similar to those used for pour-point depressants. The most common are polyisobutylenes, polymethacrylates, and polyalkylstyrenes they are used in the molecular weight range of about 10,000 to 100,000 (18). A convenient measure for the viscosity-increasing efficiency of various polymers is the intrinsic viscosity Tj, as given by the function... 

Ophthalmic ointments usually contain petrolatum as the base. The petrolatum is sterilized by dry heat and combined with the sterile dmg powder under aseptic conditions. Ophthalmic suspensions contain very fine (- 10 ji) particle sized soHds suspended in an aqueous vehicle. The vehicle is adjusted to isotonicity and viscosity-increasing excipients, chelating agents, and surfactants also may be needed. The aqueous vehicle in these cases is generally autoclaved and mixed with sterile dmg powder asceptically (30). 

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. 

Concentration and Molecular Weight Effects. The viscosity of aqueous solutions of poly(ethylene oxide) depends on the concentration of the polymer solute, the molecular weight, the solution temperature, concentration of dissolved inorganic salts, and the shear rate. Viscosity increases with concentration and this dependence becomes more pronounced with increasing molecular weight. This combined effect is shown in Figure 3, in which solution viscosity is presented as a function of concentration for various molecular weight polymers. 

Loss of elongation is the most sensitive criterion for aging measurement regardless of mechanism, and it is favored over measurement of tensile loss for cured compounds. In synthetic mbber production (SBR, in particular), viscosity increases with aging and can affect processing if not prevented. 

Bulk Polymerizations. In the bulk polymerization of vinyl acetate the viscosity increases significantly as the polymer forms making it difficult to remove heat from the process. Low molecular weight polymers have been made in this fashion. Continuous processes are known to be used for bulk polymerizations (68). 

The viscosity of an adhesive directly influences its penetration into a substrate as the viscosity increases, the penetrating power decreases. It also determines the amount of mileage or spread that can be obtained. An optimum viscosity exists for each substrate and each set of machine conditions and must be achieved in order to manufacture an efficient adhesive. Poly(vinyl acetate) emulsions are frequently too low in viscosity to be metered efficiently or to perform well as adhesives by themselves. They must be bodied to working viscosities, eg, by adding thickeners. 

Viscosity is an important property of calcium chloride solutions in terms of engineering design and in appHcation of such solutions to flow through porous media. Data and equations for estimating viscosities of calcium chloride solutions over the temperature range of 20—50°C are available (4). For example, at 25°C and in the concentration range from 0.27 to 5.1 molal (2.87—36.1 wt %) CaCl2, the viscosity increases from 0.96 to 5.10 mPa-s (=cP). 

Sodium carboxymethylcellulose is Hsted in the 1990 United States Pharmacopeia under the categories of pharmaceutic aid (suspending agent, tablet binder, viscosity-increasing agent), and cathartic tablets. 

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