Increase in viscosity

The viscosity of a paraffinic crude increases rapidly with decreasing temperature on the other hand, for the naphthenic crudes, the increase in viscosity is more gradual. 

Consider that at low temperatures, a lubricant is a poor solvent for polymer chains. When the temperature increases, interactions between polymer chains decrease the space occupied by the polymer ball takes on greater volume and consequently, the viscosity decrease due to the lubricant temperature increase is compensated by the unfolding of the polymer chain and the result is a reduction of the difference between the viscosities at low and high temperature, and therefore an Increase in viscosity index. 

Dilatant fluids (also known as shear thickening fluids) show an increase in viscosity with an increase in shear rate. Such an increase in viscosity may, or may not, be accompanied by a measurable change in the volume of the fluid (Metzener and Whitlock, 1958). Power law-type rheologicaJ equations with n > 1 are usually used to model this type of fluids. 

If this approach is to have any success, the weighting factors Cj must also decrease with increasing i to avoid a catastrophic increase in viscosity due to the proposed web of entanglements. We shall not detail the entire derivation of these C values as developed by Bueche but shall only note the following points ... 

This produces even greater increases in viscosity, with the attendant increase in the difficulty of heat removal and processing. 

These oligomerization steps result in a continuous increase in viscosity of the desired isocyanate and ultimately cause sohdification. 

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

The physical and chemical properties of chlorinated paraffins are deteanined by the carbon chain length of the paraffin and the chlorine content. This is most readily seen with respect to viscosity (Fig. 1) and volatiUty (Fig. 2) increasing carbon chain length and increasing chlorine content lead to an increase in viscosity but a reduction in volatiUty. 

Fluid or Pour-Tjpe Resins. Fluid or pour-type resins are modified acryHc systems that can be cured chemically. A fine-particle-size polymer powder consisting mostly of high molecular weight material is preferred to prevent a rapid increase in viscosity during mixing and pouring. Polymerization occurs in flexible... 

On homogenization, the lysate may drastically increase in viscosity due to DNA release. This can be ameliorated to some extent using multiple passes to reduce the viscosity. Alternatively, precipitants or nucleic acid digesting enzymes can be used to remove these viscosity-enhancing contaminants. 

Install automatic or manual activation of bottom discharge valve to drop batch into a dump tank with diluent, poison, or inhibitor, or to an emergency containment area (May not be effective for systems such as polymerization reactions where there is a significant increase in viscosity.)... 

Whilst temperature rises at constant pressure cause a decrease in viscosity, pressure rises at constant temperature cause an increase in viscosity since this causes a decrease in free volume. It is in fact found that within the normal processing temperature range for a polymer it is possible to consider an increase in pressure as equivalent, in its effect on viscosity, to a decrease in temperature. 

With further heating there is an increase in viscosity and the syrup sets to an insoluble and irreversible gel which eventually converts, with the evolution of water and formaldehyde to a hard, colourless, transparent and infusible mass. 

Pot life is several hours versus several days for conventional non-reactive hot melts. A good reactive urethane is one which exhibits a viscosity rise of less than 10%/h. The slow increase in viscosity with urethane adhesives is due to chain extension via the slow reaction of the active hydrogen of the urethane groups with... 

For all its benefits, the water/isocyanate reaction can be troublesome as well. All raw materials that go into a one-component moisture-cured adhesive must have very low moisture content, usually less than 0.05% water. If higher water levels are present, the adhesive can start curing in the reactor, causing an increase in viscosity, or, in extreme cases, gelation. 

Most moisture-curing liquid adhesives utilize poly(oxypropylene) (PPG) polyols, as shown above. These raw materials produce among the lowest-viscosity prepolymers but may not have sufficient modulus at higher temperatures for some applications. A certain percentage of polyester polyols may also be utilized to boost performance, but these may cause a large increase in viscosity, and so they are more often used in conjunction with polyether polyols to provide a high-performance adhesive with workable viscosities. Poly(butadiene) polyols may be utilized for specific adhesion characteristics. 

Phase III in Fig. 5 is marked by a cooling of the bond line, which causes the bulk viscosity of the adhesive to rise. During phase IV, another sharp increase in viscosity is observed. This is caused by the re-crystallization of the polymer... 

Once the crosslinker is added, it is important to apply the adhesive and dry off the water. Most of the commonly used crosslinkers will react with water over a period of time and lose effectiveness. In some two-component PUD s, the system may increase in viscosity and even gel, giving the user some idea of when the useful life of the crosslinker is approaching its end. In other instances, no viscosity increases or other visible indications signal that the crosslinker has reached the end of its useful life. The improvements in physical properties, solvent resistance, and water resistance normally provided by a crosslinked PUD adhesive would not be fully realized, in this case. 

From this relatively simple test, therefore, it is possible to obtain complete flow data on the material as shown in Fig. 5.3. Note that shear rates similar to those experienced in processing equipment can be achieved. Variations in melt temperature and hypostatic pressure also have an effect on the shear and tensile viscosities of the melt. An increase in temperature causes a decrease in viscosity and an increase in hydrostatic pressure causes an increase in viscosity. Topically, for low density polyethlyene an increase in temperature of 40°C causes a vertical shift of the viscosity curve by a factor of about 3. Since the plastic will be subjected to a temperature rise when it is forced through the die, it is usually worthwhile to check (by means of Equation 5.64) whether or not this is signiflcant. Fig. 5.2 shows the effect of temperature on the viscosity of polypropylene. 

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