Pastes, viscosity/consistency

Surimi is fish paste from deboned fish used to make simulated crab legs and other seafood. For preservation the paste is blended with cryoprotectants, such as sucrose, sorbitol and phosphates, and frozen. To make the final product, the frozen paste is thawed, blended with starch and extruded as a film onto a belt. The belt takes the film into an oven that heat-denatures the fish protein and cooks the starch. The film is then rolled to form striations, shaped, colored and cut. Depending on the required distribution, the product is frozen or refrigerated. Potato and tapioca starch were used in surimi products 400 years ago, since they provided a cohesive, elastic matrix consistent with seafood. Frozen distribution has made the use of highly-stabilized, moderately crosslinked tapioca starch popular, alone or with native tapioca starch. Modified waxy maize products are used, as is unmodified com starch, for increased cuttability. Kim188 reported that the gel strengthening ability of starch correlates with starch paste viscosity. 

In some cases, an extrudable and injectable paste may consist of 65% vol. ceramic powder and 35% vol. polymeric binder. In others, an extrudable paste may consist of a highly loaded aqueous suspension of clay particles such that its rheology is plastic. Hie low shear (i.e., <100 sec ) viscosity of such a paste is between 2000 and 5000 poise at ambient temperature. Highly nonlinear stress strain curves are typical of ceramic pastes, as well as time dependent thixotropy. In many cases, pastes behave like visco-elastic fluids. This complex rheological behavior of ceramic pastes has made theoretical approadies to these problems difficult. For this reason, the discussion in this chapter is limited to Newtonian fluids where analytical solutions are possible, with obvious consequences as to accuracy of these equations for non-Newtonian ceramic pastes. 

Viscosity Consistency—Related but quite different rheological (pertaining to flow) terms. A metal ball will penetrate a viscom substance (asphaltum) no matter how long it takes, but may stay on top of a weak but highly consistent semisolid (starch paste) for an indefinite period. 

The adhesive should be supplied in liquid form and should mix readily to a smooth, paste-like consistency of initial viscosity between 20 and 150 Pa at a shear rate of 10" s, and have a yield stress of at least 20 Pa at 20 °C suitable for spreading both on vertical and horizontal surfaces(2). The resin and hardener should be of dissimilar colour to aid thorough mixing. The mixed material should be free of lumps, and the components, including filler, should not settle out or separate during the pot life of the adhesive. 

Aqueous pastes, also called hydrophilic pastes, consist of a hydrophilic base with 40-60 % solid substance. This type of paste may consist of water only, made viscous by a viscosity enhancer (see Table 12.41) or by the addition of a hydrophilic cream or emulsion. They are supposed to have a good absorptive capacity and are therefore used in the treatment of wetting skin disorders [59b]. 

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. 

There are four principal classes of printing ink, which vary considerably in physical appearance, composition, method of appHcation, and drying mechanism. These also fall into two general types of consistency or viscosity, paste and Hquid. The classes are letter press and Hthographic (Utho) inks, which are called paste inks, and flexographic (flexo) and rotogravure (gravure) inks, which are called Hquid inks (1). 

The liquid crystal polymers consist of rod-like molecules which, during shear, tend to orient in the direction of shear. Because of the molecular order the molecules flow past each other with comparative ease and the melts have a low viscosity. When the melt is cooled the molecules retain their orientation, giving self-reinforcing materials that are extremely strong in the direction of orientation. 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

Consistency, working time, setting time and hardening of an AB cement can be assessed only imperfectly in the laboratory. These properties are important to the clinician but are very difficult to define in terms of laboratory tests. The consistency or workability of a cement paste relates to internal forces of cohesion, represented by the yield stress, rather than to viscosity, since cements behave as plastic bodies and not as Newtonian liquids. The optimum stiffness or consistency required of a cement paste depends upon its application. 

Resistance functions have been evaluated in numerical compu-tations15831 for low Reynolds number flows past spherical particles, droplets and bubbles in cylindrical tubes. The undisturbed fluid may be at rest or subject to a pressure-driven flow. A spectral boundary element method was employed to calculate the resistance force for torque-free bodies in three cases (a) rigid solids, (b) fluid droplets with viscosity ratio of unity, and (c) bubbles with viscosity ratio of zero. A lubrication theory was developed to predict the limiting resistance of bodies near contact with the cylinder walls. Compact algebraic expressions were derived to accurately represent the numerical data over the entire range of particle positions in a tube for all particle diameters ranging from nearly zero up to almost the tube diameter. The resistance functions formulated are consistent with known analytical results and are presented in a form suitable for further studies of particle migration in cylindrical vessels. 

The consistency or fluidity of the concrete will be a function of the viscosity of the cement paste. 

A viscometer can be used to study the yield stress and viscosity of cement pastes (Section 1.3.1). This is carried out by plotting the shear rate against shear stress as shown in Fig. 2.4 for cement pastes of various water cement ratios. These cement pastes are generally considered to exhibit Bingham plastic behavior where the yield value is the intercept on the shear stress axis and is related to cohesion, and the slope of the line is the apparent viscosity which is related to the consistency or workability of the system. The following general observations can be made ... 

The large differences, however, can largely be attributed to differences in viscosities of the different pastes at the same water-cement ratio. When the pastes were prepared to the same consistencies, similar levels of air entrainment were obtained. 

In the past, electron beam radiation was applied to produce PSA exclusively however, recent improvements in UV curing technology (precise UV dose control, suitable photoinitiators) permit UV to be used to produce pressure-sensitive adhesives. PSA formulations can vary in consistency from low-viscosity liquids up to solids melting at 80°C (176°F). Therefore, applications may vary from screen printing to roll coating to melt extrusion. Coat weights for most PSA materials vary from 1 to 10 g/m. ... 

---