Processing via the Viscous State

Several different processes can be used to produce a laminate. In what is called the hand application process, for example, glass-fiber mats are impregnated with unsaturated polyester resins. The impregnated mats are then removed from the mold by hand and pressed between rollers. The final molding is effected by cold pressing. The method is suitable for small numbers of objects with large surface areas (e.g., boat hulls). 

To produce what are called prepregs, mats are repeatedly dipped, compressed, prehardened in an oven, and then processed by compression-cavity molding. 

Particularly thin articles (e.g., rubber gloves) are produced by dipcoating. In this case, the mold negative is dipped into a latex (a dispersion) or a paste for as long and/or as often as is necessary to obtain the desired thickness. The latex viscosity should be less than 12 Pa s the flow limit as low as possible. Latices of natural rubber poly(chloroprene), and silicones, as well as PVC pastes, are processed in this way. 

For spray-coating, the viscosity should be less than 7 Pa s. The latices, plastisols, or solutions should have little or no dilatancy. One can describe this as injection casting. 

In spread coating (laquering), solutions of film-forming polymers are applied and the solvent allowed to evaporate. Metallic and nonmetallic components and films of cellulose or aluminum are lacquered. Lacquered cellulose films are less permeable to water vapor, but poly(ethylene)-bonded films compete with these on the market (Section 12.2.6). Paints and lacquers 

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Figure 12.1. Processing via the viscous state (schematic). (I) Molding or casting, (II) laminating, (III) compression molding (cavity molding), (IV) direct injection molding or injection casting.

Figure 12-2. Processing via the elasto-viscous state (schematic). (I) Injection molding (with torpedo), (II) extrusion, (III) rolling or milling. In injection molding, the granulate is plasticized by heat and then injected into the mold by a torpedo (or with a screw). When the molded article is cold, the mold is removed and the article ejected. In extrusion, the heated material passes through a metal screen (filter) placed in front of the nozzle.

The modeling and control of movements in this chapter relates to external control of muscles via so-called functional electrical stimulation. Macroscopic viscoelastic models started from the observation that the process of electrical stimulation transforms the viscoelastic material from a compliant, fluent state into the stiff, viscous state. Levin and Wyman [35] proposed a three-element model— damped and undamped elastic element in series. Hill s work [36] demonstrated that the heat transfer depends upon the type of contraction (isometric, slow contracting, etc). The model includes the force generator, damping and elastic elements. Winters [37] generalized Hill s model in a simple enhancement of the original, which... 

Reactive polymer processing modifies or functionalizes the macromolecular structure of reactor polymers, via chemical reactions, which take place in polymer processing equipment after the polymer is brought to its molten state. The processing equipment then takes on an additional attribute, that of a reactor, which is natural since such equipment is uniquely able to rapidly and efficiently melt and distributively mix reactants into the very viscous molten polymers. The operation is shown schematically in Fig. 1.3. 

The parameter pco characterizes a short-time, high frequency viscosity and models viscous processes which require no structural relaxation, like in the general case (15). Together with F, it is tire only model parameter affected by Hl. Steady state shear stress under constant shearing, and viscosity then follow via integrating up the generalized modulus ... 

Reversible and irreversible processes. Classify each of the following processes as reversible (in the thermodynamic sense), nearly reversible, or irreversible. If the process is irreversible, devise a reversible or nearly reversible process by which the system could be brought from the same initial state to the same final state, (a) A bottleful of water evaporates very slowly through a small leak around the stopper, (b) A compressed gas expands in a cylinder fitted with a piston. The expansion is slow because the piston is lubricated by a viscous oil. (c) A gas expands into an evacuated space via a very small hole. 

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